Advances in Photochemistry, Volume 18

ADVANCES IN PHOTOCHEMISTRY Volume 18

ADVANCES IN PHOTOCHEMISTRY Volume 18 Editors

DAVID H. VOLMAN Department of Chemistry, University of California, Davis, California

GEORGE S. HAMMOND Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio

DOUGLAS C. NECKERS Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio

A WILEY-INTERSCIENCE PUBLICATION

JOHN WILEY & SONS, INC. New York

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This text is printed on acid-free paper. Copyright 0 1993 by John Wiley & Sons, Inc. All rights reserved. Published simultaneously in Canada. Reproduction or translation of any part of this work beyond that permitted by Section 107 or 108 of the 1976 United States Copyright Act without the permission of the copyright owner is unlawful. Requests for permission or further information should be addressed to the Permissions Department, John Wiley & Sons, Inc., 605 Third Avenue, New York, NY 10158-0012. Libmry of Congress Cataloging in Publication Data: Library of Congress Catalog Card Number: 63-13592 ISBN 0-471-59133-5

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CONTRIBUTORS Andre M. Braun Lehrstuhl fur Umweltmesstechnik Engler-Bunte-Institut Universitat Karlsruhe 7500 Karlsruhe, Germany George S. Hammond Center for Photochemical Sciences Bowling Green State University Bowling Green, OH 43403 Graham Hancock Physical Chemistry Laboratory Oxford University South Parks Road Oxford OX1 342,United Kingdom Dwayne E. Heard School of Chemistry Macquarie University Sydney, NSW 2109 Australia Laurent Jakob Lehrstuhl fur Umweltmesstechnik Engler-Bunte-Institut Universitat Karlsruhe 7500 Karlsruhe, Germany

Claudio A. Oller do Nascimento Escola Politechnica da Univesidade de S5o Paulo 01OOO S5o Paulo, SP, Brasil Douglas C. Neckers Center for Photochemical Sciences Bowling Green State University Bowling Green, OH 43403 Esther Oliveros Lehrstuhl fur Umweltmesstechnik Engler-Bunte-Institut Universitat Karlsruhe 7500 Karlsruhe, Germany V. Ramamurthy Central Research and Development Experimental Station The Du Pont Company Wilmington, DE 19880-0328 Oscar M. Valdes-Aguilera Center for Photochemical Sciences Bowling Green State University Bowling Green, OH 43403 Richard G. Weiss Department of Chemistry Georgetown University Washington, D C 20057

PREFACE

Volume 1 of Advances in Photochemistry appeared in 1963. The stated purpose of the series was to explore the frontiers of photochemistry through the medium of chapters written by pioneers who are experts. As editors we have solicited articles from scientists who have strong personal points of view, while encouraging critical discussions and evaluations of existing data. In no sehse have the articles been simply literature surveys, although in some cases they may have also fulfilled that purpose. In the introduction to Volume 1 of the series, the editors noted developments in a brief span of prior years which were important for progress in photochemistry: flash photolysis, nuclear magnetic resonance, and electron spin resonance. Since then two developments have been of prime significance: the emergence of the laser from an esoteric possibility to an important light source; the evolution of computers to microcomputers in common laboratory use for data acquisition. These developments have strongly influenced research on the dynamic behavior of excited state and other transients. With an increased sophistication in experiment and interpretation, photochemists have made substantial progress in achieving the fundamental objective of photochemistry: Elucidation of the detailed history of a molecule which absorbs radiation. The scope of this objective is so broad and the systems to be studied are so many that there is little danger of exhausting the subject. We hope that the series will reflect the frontiers of photochemistry as they develop in the future. DAVIDH. VOLMAN GEORGE S. HAMMOND C . NECKERS DOUGLAS Davis, California Bowling Green, Ohio Bowling Green, Ohio

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CONTENTS

Time-Resolved FTIR Emission Studies of Photochemical Reactions GRAHAM HANCOCK AND DWAYNE E. HEARD A Model for the Influence of Organized Media on Photochemical Reactions V. RAMAMURTHY,RICHARDG. WEISS, AND GEORGE s. HAMMOND

1

67

Up-Scaling Photochemical Reactions AND& M. BRAUN,LAURENT JAKOB, ESTHER OLIVEROS, AND CLAUDIO A. OLLER DO NASCIMENTO

235

Photochemistry of the Xanthine Dyes DOUGLAS c.NECKERS AND OSCAR M. VALDES-AGUILERA

315

Index

395

Cumulative Index, Volumes 1-18

402

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Advances in Photochemistry, Volume18 Edited by ,David H. Volman, George S. Hammond, Douglas C. Neckers Copyright © 1993 John Wiley & Sons, Inc.

TIME-RESOLVED FTIR EMISSION STUDIES OF PHOTOCHEMICAL REACTIONS Graham Hancock and Dwayne E. Heard Physical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom

CONTENTS

I. Introduction 11. Fundamentals of Fourier transform spectroscopy 111. A practical guide to time-resolved FTIR emission studies A. Stop-scan (SS) time-resolved Fourier transform spectrometers 1. Historical development 2. The Oxford stop-scan time-resolved FTIR emission spectrometer B. Continuous-scan (CS) time-resolved Fourier transform spectrometers C. Comparison of S S and CS time-resolved FTIR spectrometers IV. Applications A. Internal state distributions of the fragments of molecular photodissociation B. Product state distributions from laser-initiated bimolecular reactions

2 5 10 10 10 12 22 28 31

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Advances in Photochemistry, Volume 18, Edited by David Volman, George S . Hammond, and Douglas C. Neckers ISBN 0-471-59133-5 Copyright 1993 by John Wiley & Sons, Inc.

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G . HANCOCK AND D. E. HEARD

C . Kinetic studies of chemical reactions and energy transfer processes Conclusions Acknowledgments References

INTROD ICTIO

48 57 58 58

i

Fourier transform infrared (FTIR) emission spectroscopy is now the method of choice for observation of weak sources of infrared radiation in a wide variety of applications. The advantages of Fourier transform techniques [11, namely, that all light frequencies are observed simultaneously at the detector (the multiplex or Fellgett advantage [2]), and that the optical throughput is greater than for a dispersive instrument with light-confining slits (Jacquinot advantage [3]), have enabled high-resolution spectra to be obtained routinely in short periods of time with excellent sensitivity. In the field of chemical reaction dynamics, the nascent product state distributions of many bimolecular chemical reactions have been measured by FTIR analysis of the infrared chemiluminescence from vibrationally excited products [4-91. In most of these studies reaction has been initiated by simple mixing of the reagents. The nascent product state distributions are obtained free of the effects of collisional relaxation by extrapolation of the distributions observed at various positions in a flow-tube (measured relaxation [lo, 111) or by carrying out the reactions at low total pressure such that the excited species are completely quenched at the walls before any relaxation occurs (arrested relaxation [lo, 123). Many of the fundamental concepts on which molecular dynamics is based, for example, understanding the partitioning of the available energy into the internal degrees of freedom of the reaction products, or the efficacy of reagent vibrational versus translational energy in overcoming barriers in the potential energy surface, have emerged from such experiments [13,14]. Conventional FTIR instruments, in which the interferometer mirror is translated at a constant velocity, are ideally suited to the analysis of steady state infrared emission. However, time resolution of the infrared emission is required in many applications, such as the measurement of absolute rate constants for the formation or subsequent relaxation of a vibrationally excited species. It is then necessary to follow the intensity of the emission (at a particular wavenumber if state-specific rate constants are required) as a function of time. For continuous-wave experiments, crude time resolution

TIME-RESOLVED FTIR EMISSION STUDIES

3

can be obtained by varying either the distance between the point of mixing of the reagents and the FTIR observation point [lo, 11,151 or the gas flow rate for a fixed mixing-spectrometer separation [161. Analysis of the transformed spectra at different reaction times allows the populations to be monitored with time and, if desired, extrapolated to obtain the initial distribution [lo]. A more desirable approach is to take advantage of the short pulse duration, high energy, and high repetition rates of pulsed laser sources to cause photolytic production of one of the reagents, allowing exact specification of zero time for the reaction. This method is standard for studies of molecular dynamics in which products are observed by laser-induced fluorescence [17], but has been little used for monitoring products by IR emission because it is difficult to obtain good signal-to-noise ratios for spectrally resolved emission from specific vibration-rotation states as a function of time. Interference filters are able to isolate spectrally the emission from one particular molecule (or one vibrational mode within a polyatomic molecule), and in several experiments the temporally resolved filtered emission has been studied under varying conditions of pressure and temperature in order to measure the rates of bimolecular reactions or energy transfer processes [18-22]. Early time-resolved experiments employed circular variable filters with slightly improved but still poor spectral resolution (3060 cm- l ) to measure the vibrational distribution and relaxation of molecular fragments excited by UV photodissociation [23] or translational-tovibrational energy transfer [24]. However, the time dependences at any given wavenumber under these conditions correspond to emission from a wide range of excited levels. Time-resolved experiments using dispersive grating spectrometers to analyse pulsed emission from laser-initiated reactions [25] again only allowed low (11-22 cm-') spectral resolution owing to optical throughput problems. Individual rovibronic spectral features could not be resolved, and vibrational distributions were deconvoluted from the data by spectral fitting routines. The use of FTIR techniques in studies of time-resolved IR emission has been a relatively recent development, and two of the major practitioners in the field have provided excellent reviews of progress up to 1989 [26,27]. This chapter does not attempt a historical survey of the method, but instead describes progress since 1989, suggests possible further areas of study, and most importantly tries to provide the experimentalist with a practical guide to the use of the technique for studying a wide range of photochemical systems. Commercial Fourier transform spectrometers operating at moderate resolution (1 cm-l) require fractions of seconds to complete a scan of the interferometric mirror (scans may only take tens of milliseconds if only low spectral resolution is required). A new strategy must now be used to study the

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G. HANCOCK AND D. E. HEARD

transient IR emission signals originating from the products of laser-initiated reactions or photofragmentations which typically decay in tens or hundreds of microseconds. A number of early experiments which employed timeresolved FTIR methods are documented [28-361, but during the last six years we have seen extensive development of fast time-resolved FTIR spectrometers to tackle problems in gas-phase reaction dynamics and molecular photodissociation. Two fundamentally different approaches have been employed. The first, referred to as the stop-scan (SS) method, records the entire temporal evolution of the IR emission while the interferometric mirror is held stationary at each of its sampling positions. The key feature is that the complete temporal evolution of every wavenumber in the product emission spectrum is obtained from only one scan of the interferometric mirror [37]. A SS instrument was developed in the authors’ laboratory [37-381 to study the IR vibrational fluorescence of the products of atom-radical reactions and molecular photodissociations, and the modification of a low-cost teaching interferometer for use as a fast time-resolved instrument is described in detail in later sections. Free radicals with low internal energy were generated by IR multiple photon dissociation (IRMPD) [39], and their reactions with atomic partners were studied in a discharge flow system. For example, nascent vibrational distributions in CO(u’) and HF(o’), generated in the exothermic reaction between oxygen (3P) atoms and the monofluorocarbene radical CHF, were measured together with kinetic parameters from the temporal evolution of the emission spectrum [40]. The second approach is referred to as the continuous-scan (CS) method, in which the interferometric mirror is never stationary throughout the duration of the IR transient. The photolysis laser is triggered when the continuously moving mirror reaches each interferogram sampling point, and the subsequent pulse of IR radiation is digitized at some user-defined delay following the laser pulse. It is assumed that the IR signal is constant during digitization (a very short sample-andhold gate width is used) and hence following one sweep of the mirror an interferogram (and therefore a FT spectrum) corresponding to one time point in the transient is obtained. The application of the CS method to reaction dynamics was introduced by Sloan and co-workers in 1985 [41]. Electronically excited O(’D,) was generated from the 248-nm photolysis of ozone, and nascent OH(u’, N ’ ) distributions were measured after reaction with a variety of hydrogen-containing molecules [7,41-451. The same strategy has been successfully applied to time-resolved observations of nascent fragments from the U V photolysis of a variety of molecules by Leone and co-workers [46501. The high resolution of the CS method has allowed emission from polyatomic fragments to be analyzed to obtain vibrational distributions in electronically excited states formed by U V photolysis of suitable precursors [49, SO]. Recent modifications to the technique allow the observation of

TIME-RESOLVED FTIR EMISSION STUDIES

5

transient signals which do not conveniently decay before the next sampling position is reached [Sl]. The chapter is set out in the following way. Section I1 contains elements of the theory of Fourier transformations which, rather than being exhaustive (and exhausting), aims to cover the details and limitations of the technique which are of importance for the experimentalist to understand. Section I11 contains descriptions and comparisons of the S S and CS methods and outlines the advantages and pitfalls of each, together with recommendations for their suitability for specific applications. Section IV presents recent results from time-resolved FTIR emission experiments, emphasizing photochemical applications.

11. FUNDAMENTALS OF FOURIER TRANSFORM SPECTROSCOPY Fourier transform methods have revolutionized many fields in physics and chemistry, and applications of the technique are to be found in such diverse areas as radio astronomy [52], nuclear magnetic resonance spectroscopy [53], mass spectroscopy [54], and optical absorption/emission spectroscopy from the far-infrared to the ultraviolet [55-571. These applications are reviewed in several excellent sources [l, 54,581, and this section simply aims to describe the fundamental principles of FTIR spectroscopy. A more theoretical development of Fourier transform techniques is given in several texts [59-611, and the interested reader is referred to these for details. For a conventional dispersive spectrometer operating between 400 and 4000cm-' at a resolution of 1 cm-', only 0.03% of the radiation that enters the instrument reaches the detector at any one time. Such losses can be disastrous if weak emissions are to be observed in the infrared, where detectors are limited by background noise from the black-body surroundings and their own thermal energy. The time taken to record a high signal-to-noise ratio (SNR) spectrum at high spectral resolution becomes prohibitively long. In Fourier transform spectrometers the incident beam of collimated IR radiation is passed into a Michelson interferometer [62,63]. A beamsplitter divides the beam into two equal parts which are reflected by two mutually perpendicular plane mirrors, one of which can be translated along the optical axis. Following recombination at the beansplitter, the two spatially coherent beams interfere to give a stationary pattern of interference fringes. If the radiation is monochromatic this will occur for all values of the translating mirror, but if it is broad band a pattern is only seen in the vicinity of zeropath difference between the two mirrors. If a detector is placed at the center of

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G. HANCOCK A N D D.E. HEARD

the interference pattern (which consists of circular rings for a monochromatic source) and the path difference between the two beams is precisely varied by translating one mirror, the resulting interferogram has encoded information about the source of radiation. The multiplex or Fellgett advantage resulting from this approach [2] gives a superior SNR of M”’ over a dispersive instrument, where M is the number of resolution elements in a spectrum (defined as the spectral bandwidth divided by the resolution). The same SNR can thus be achieved in a considerably shorter measurement time. The Fellgett advantage is only realized in spectral regions where detectors are background-noise limited, and is lost for detectors which are shot-noise limited (e.g., photomultipliers). The throughput or Jaquinot advantage [3] of an interferometer is typically a factor of 100 or more [64], and is realized for all spectral regions. The signal seen at the detector for a given value of the optical path difference (OPD),given by the symbol 6, is dependent upon the wavelengths, amplitudes, and phases of the components of the radiation. Constructive interference for all components occurs only at 6 = 0, where the maximum signal is observed (often referred to as the centerburst or central maximum). The signal that is seen at the detector as a function of 6, I ( 6 ) for an ideal interferometer, is given by

::j

Z(6) = -

B(V){l + cos 27cV6)dV

where B(S)dS is the intensity of the spectral component in the wavenumber range V to 5 dS. The first term of the expression is constant and is subtracted from the interferogram before computation of the spectrum. The spectrum B(V)is then calculated from an inverse Fourier transformation of the modulated part of Eq. (1):

+

B(V)=

s_a

1(6)cos(2nV6)d6

The cosine Fourier transform given by Eq. (2) is only applicable if the interferogram is perfectly symmetrical about 6 = 0. In practice additional wavenumber-dependent phase shifts are present, owing to beamsplitter characteristics or refraction effects, and cause the interferogram to appear partially asymmetric. The modulated part of Eq. (1) then becomes

:J:-

I(6) = -

B(V)COS(27cV6

+ O(ii))dV

(3)

TIME-RESOLVED FTIR EMISSION STUDIES

7

where O(V) is a wavenumber-dependent phase error. The interferogram now contains some sinusoidal character, and a complex Fourier transformation is required:

Z(6) exp{ -2ni56)dd

(4)

yielding a spectrum consisting of real and imaginary parts, representing contributions from the cosine and sine components of the interferogram respectively. The integral expressions (3) and (4)cannot be realized in practice for two reasons: 1. The optical path difference over which the interferogram can be digitized is limited by the dimensions of the interferometer. 2. Digitization of Z(6) can only be performed at finite intervals of 6.

The integrals are thus replaced by summations: B(V)= A6

m= +M m= -M

I(mA6)exp( -2niVmA6)

The interferogram is digitized at a total of 2M + 1 points with a sample interval of A6 and computation now involves evaluation of a sum over the 2M + 1 values of 6 for each value of V in the spectrum. A Fourier transform instrument is able to record an interferogram corresponding to a spectrum with comparable SNR and spectral resolution to a dispersive instrument in a fraction of the time. However the time advantage for very high resolution studies was not realized until the advent of fast digital computers, which, using the “Fast Fourier Transform” algorithm developed by Cooley and Tukey [65], could calculate the Fourier transforms of interferograms with very large numbers of M very quickly. Replacement of Fourier integrals by summations has two important ramifications of practical importance: 1. The resolution of the spectrum is reduced, as no data points are taken in the interferogram beyond ,a,, = iMA6; higher resolution requires greater mirror travel. At the extrema of the mirror travel there is a discontinuity in the value of the interferogram (it suddenly becomes zero), and this is equivalent to multiplying it by a boxcar function D(6) given by

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G. HANCOCK AND D. E. HEARD

One of the fundamental theorems of Fourier transforms states that multiplying two functions in one Fourier domain is equivalent to convoluting the two functions in the other domain [60]. The FT spectrum thus has a lineshape corresponding to the Fourier transformation of D(d), which is the sinc function B(5) = 2d,,,

sin(2~5d,,,) = 26,,, 27158,,

sinc(2~5d,,,)

(7)

where d,,, is the maximum mirror travel. The complex nature of the spectral lineshape [given by Eq. (7)] resulting from Fourier transformation with boxcar truncation has some interesting properties. The full width at half maximum (FWHM, often used as a definition of resolution) is given by 0.6034/6,,,, and is hence inversely proportional to the maximum mirror travel. Lines in the transformed spectrum appear with subsidiary side lobes or “feet,” an inconvenient distortion which may be mistaken for other spectral features or even swamp a genuine weak feature. This problem is circumvented by “apodization” (Greek for removal of the feet) of the interferogram. New improved lineshapes with suppressed feet are obtained by multiplying the interferogram by an apodization function prior to Fourier transformation. The “weighted” interferogram now reaches zero at fd,,, rather more gradually but, as information at the extreme ends of the interferogram is being thrown away, the spectral resolution is degraded in the form of a broader FWHM. Triangular apodization is popular (reduced sidelobes, FWHM = 0.88/6,,,), and gives the same sinc’ lineshape as encountered in a diffraction-limited grating spectrometer. Commercial instruments employ sophisticated apodization functions, optimizing the FWHM and suppression of sidelobes, and are specific to the SNR available [ 5 5 , 5 9 ] . 2. For a finite sampling interval Ad, more than one superposition of cosine/sine waves can give rise to the recorded interferogram. For the transformed spectrum to be unique, the sampling interval Ad must be sufficiently small to detect modulations in the interferogram due to the shortest wavelength present in the spectrum, the so-called Nyquist criterion [66]:

Here V,, is the maximum wavenumber present (Nyquist wavenumber). must be removed by suitable optical filtering or Radiation 5 above V,, additional features and noise will be folded back onto the spectrum. Aliasing, as this phenomenon is called, places constraints on the operation of real

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9

interferometers. Experimentally precise sampling at Ad intervals is performed using the sinusoidal interferogram of a monochromatic source. Helium-neon lasers are used for this purpose to provide an accurate reference of the optical path difference, and values of A6 [and hence V,, from Eq. (S)] may be chosen to be 0.5nAH,,,, where n is an integer. Sampling in S S and CS interferometers is discussed in Section 111. If lines in an aliased spectrum do not overlap those in the normal spectrum, more efficient “undersampling” may be used, giving a greater mirror travel and hence enhanced resolution for the same number of data points [67]. A consequence of using a discrete FT is a spectrum with data points equally placed in wavenumber, with a spectral spacing determined by the Nyquist wavenumber and number of interferogram points. It is stressed that the spectral spacing is not equal to the resolution of the instrument, which depends not only upon d,, but also upon the apodization used. As the interferograms contain some asymmetry, data should be recorded on both sides of the centerburst. Collection of a complete double-sided interferogram (whose N data points are real numbers), gives the complex Fourier transform spectrum:

+

forming a Hermitian sequence, that is, B ( N / 2 K ) is the complex conjugate of B(K).The imaginary part would be zero for a symmetric interferogram. The power spectrum is then given by

A disadvantage of calculating the power spectrum is that the spectral information and noise are computed to have positive values (noise is generally randomly positive and negative) due to the squared terms in Eq. (lo), and the height of the baseline may increase above its true value for a noisy spectrum. Recording a single-sided interferogram and performing a cosine Fourier transformation would obviate the need to calculate a power spectrum, and would immediately improve the spectral resolution by a factor of 2 for the same number of data points. However, distortions in the spectrum will result unless the phase error e(5) in Eq. (3) is known for each 5 [SS]. ‘The term e ( i ) can be found by recording a very short double-sided interferogram and calculating

e(v) = tan- lCT,in(v)/T,os(S)I

(11)

Phase correction algorithms can then use O(V) to correct the asymmetry in a

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G. HANCOCK AND D. E. HEARD

much longer single-sided interferogram [68,69]. The discrete nature of interferogram sampling ensures that no single data point will exactly correspond to 6 = 0, generating an additional phase error. Commercial instruments employ a white light reference beam, whose sharp centerburst indicates 6 = 0 precisely, which, together with phase correction packages, allow single-sided data to be taken. In practice, a number of experimental factors degrade the resolution from that of an ideal interferometer. Emission sources are finite in size, and hence the beam entering the interferometer is slightly divergent. A critical angle of divergence for a given resolution and Nyquist wavenumber can be calculated [ S S ] , and for large area sources, an aperture (or Jacquinot stop) may be required to increase the quality of collimation, reducing the throughput advantage. Typically emission occurs only from the region of a laser focus in photochemical applications, and a Jacquinot stop may only be required at very high resolution. Mirror misalignment during a scan can also degrade resolution [ S S ] , and for high resolution work, commercial instruments employ dynamic alignment of the moving mirror (the reference laser forms a two-dimensional image) during a scan. Unapodized resolutions of <0.002 cm-' are obtained in this way. Sampling errors, such as missed or extra points, or variable sample interval Ad, degrade the resolution and increase the noise level, the effect being most severe for errors near to the centerburst.

111. A PRACTICAL GUIDE TO TIME-RESOLVED FTIR EMISSION STUDIES In this section it is hoped to provide useful experimental details which demonstrate the simplicity of both the time-resolved FTIR technique and its incorporation into an experiment. Two major implementations of the technique have emerged, namely stopped and continuous-scan; they are dealt with separately below in Sections 1II.A and 1II.B. A detailed comparison of the two methods is then presented in Section 1II.C.

A. Stop-Scan (SS) Time-Resolved Fourier Transform Spectrometers 1. Historical Development, In this category of FT spectrometer the complete time-evolution of the IR transient is digitized whilst the interferometric mirror is held stationary at each sampling point. The transient can be initiated repeatedly and signal averaged to achieve an adequate SNR. The

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TIME-RESOLVED FTIR EMISSION STUDIES

stop-scan (SS) idea is by no means a new one, and its potential for the study of short-lived phenomena was first proposed over 20 years ago [70]. Early implementations of the technique are reviewed by Sloan [27] and are only briefly considered here. The first demonstrations by Murphy and co-workers used pulsed electron bombardment to investigate the production and relaxation of vibrationally excited molecules [30,35,70]. Pulses of energetic electrons lasting several milliseconds and repeated at 80 Hz impinged on slowly flowing mixtures of gases at reduced pressures. A pulse of IR emission was generated, and decayed completely between electron pulses. Many pulses were averaged and digitized with about 50-ps temporal resolution before the moving mirror was translated to the next sampling position using a reference laser interferogram and a servo feedback mechanism. After rearrangement of the data, interferograms were constructed as a function of time, which, after Fourier transformation, gave a series of time-resolved spectra. Modulated discharges have also been employed to excite atomic and molecular emissions in the infrared, generating time-resolved spectra at 4 cm- resolution [29]. Some quasi stop-scan experiments have employed a very slowly moving mirror, as opposed to a stationary one, with the entire emission pulse digitized and signal averaged many times between two adjacent reference laser sampling points [36,71-731 again using electron bombardment as the excitation source. An assumption was then made that all electron beam pulses occurring between sampling points have the same mirror position, that is, it was stationary. However, results from quasi SS spectrometers, some of which have appeared very recently [72,73], may be subject to errors as discussed in reference 27. The location of the moving mirror is only precisely known at the HeNe zero crossings, and also the temporal behavior of the IR transient will change as the mirror is translated between these. Palmer et al. have very recently modified a commercial FTIR spectrometer (IBM 44 FT-IR) for stop-scan operation [74,75]. The moving mirror position is controlled with a feedback loop using path difference modulation of the reference laser intensity, together with lock-in amplifiers to detect the mirror position. The interferogram can be sampled at intervals as small as %AHeNe, and hence V,, is 31,600 cm-'. The software developed only allows data collection at approximately 1.6Hz, but with a mirror settling time of < 20 ms much more efficient data collection should be possible. The stability of the moving mirror imposes an upper limit on the SNR of the measured spectra and is f 1 5 n m . A number of applications have been reported in photoacoustic and photothermal spectroscopy [75-771 and twodimensional FTIR correlation spectroscopy [78], although so far the timeresolved examples show only low resolution emission spectra (100 cm-', 2 ms) [79,80]. In another example, Siebert et al. recently described the modification of a Bruker IFS 88 FTIR spectrometer for use as a stop-scan

'

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G. HANCOCK AND D. E. HEARD

time-resolved instrument [8 13. Absorption spectra at moderate resolution (4 cm-') were taken following laser initiation of a biological system, with a good temporal resolution of l o p . Signal averaging was performed at each interferogram sampling point. In our laboratory a low-cost teaching interferometer has been modified for use as a S S time-resolved instrument to study the infrared vibrational chemiluminescence arising from a variety of photochemical processes [37,38, 40, 82, 831. The entire temporal evolution of the emission spectrum is obtained from a single interferometric scan, with spectral resolution of 2 cm-' and temporal resolution down to 10 ns. One of the major difficulties encountered with S S interferometers is that of keeping the mirror stationary during the signal averaging of the IR transient. Small position inaccuracies will, after Fourier transformation, manifest themselves as decreased SNR [27]. Elaborate stabilization electronics and position-sensing techniques have been used [29], but as described in the next section, spectra of excellent SNR have been straightforwardly obtained at this resolution without the need for such expensive instrumentation.

2. The Oxford StopScan Time-Resolved FTIR Emission Spectrometer. Earlier studies in this laboratory [84] showed that infrared radiation was emitted following the IRMPD of a number of halocarbons in the presence of oxygen atoms in a low-pressure discharge flow system. Attempts to resolve spectrally the emission with a dispersive scanning monochromator were unsuccessful because of throughput losses. Kinetic studies of the total unresolved emission revealed in some cases more than one emitting species, but their identities remained speculative. A time-resolved interferometer was designed and constructed to resolve the pulse of weak IR chemiluminescence, enabling identification, energy partitioning, and kinetic measurements to be made. The instrument is described in detail in references 37 and 38. The aims of this section are to highlight the insights into time-resolved interferometry that are gained from practical use of the technique, and to demonstrate that an inexpensive instrument is capable of solving important questions in chemical physics. The interferometer is a modified Michelson teaching instrument (Ealing Optics) and is shown schematically in Figure 1, and follows the basic design previously used to study CW emissions in the visible and near IR [67]. The beamsplitter and compensator plates were constructed of CaF, ; the latter compensates the phase differences between the radiation transmitted and reflected by the beamsplitter. One face of the beamsplitter was coated with a 4-nm thick layer of gold, deposited by ion sputtering, to give approximately equal reflection and transmission in the midinfrared. The NaCl collimation and collecting lenses were used with a variety of detectors (InSb, HgCdTe,

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TIME-RESOLVED FTIR EMISSION STUDIES

Pulsed photolysis

I

I I

i Fixed i mirror I I

ctor laser

detector Figure 1. Schematicdiagram of the Michelson stop-scan interferometer used for timeresolved FTIR emission studies. Reproduced with permission from Ref. 38.

Ge, Si depending on the spectral region studied), suitably filtered to limit the spectral bandwidth in order to prevent aliasing errors. The entire interferometer was housed in an air-tight aluminum box, mounted on a steel baseplate to ensure mechanical stability, and was flushed with N, to prevent atmospheric absorption of IR radiation. The box is portable, and has been readily interfaced to a number of experimental configurations (including a reactive ion plasma etching chamber) with a minimum of alignment. A helium-neon laser with an interference filter to cut out all emission lines except 1 = 632.8 nm provides the reference interferogram, and passes through the center of the main optics and is detected by a photodiode. The scanning mirror is moved via a micrometer drive allowing approximately 8 nm mirror travel per step. The micrometer has a tungsten-carbide tip and is connected to the stepper motor via a rubber belt to ensure smooth mirror travel. A Zenith 2-158 PC computer (8 MHz 20 MB, 640 kB RAM 8087 coprocessor) is interfaced to the experiment and controls the interferometer sampling, data acquisition, and subsequent sorting/Fourier transformation of the data to produce time-resolved spectra. Two digitizers were employed to record the temporal evolution of the IR emission and were fully dedicated

14

G. HANCOCK AND D. E. HEARD

to the computer. The first was home-built, and incorporated a 10-bit 2.2-ps conversion speed A-D chip, with 8192 channels. The second was a Biomation 8100 transient recorder, with only 8 bit digitization but a faster temporal resolution of 10 ns per channel. Also interfaced to the computer is a standard stepping motor controller, a 60-ps A-D converter for recording the energy of the photolysis laser, and an average-crossing detector which enables the computer to identify the correct position for sampling of the IR interferogram. On stepping the mirror, a sinusoidal voltage is produced at the helium-neon laser detector. The detector output is fed to a low-pass filter which, after a few He-Ne cycles, produces a DC voltage which is an average of the oscillating laser interferogram. Once the average is established, the subsequent incoming sinusoidal He-Ne laser signal at the photodiode is compared with this average, and standard TTL high or low pulses are sent to the computer depending on whether the reference signal is above or below the average value. Each time the mirror is stepped by 158.2 nm (corresponding to a change in the optical path difference of 316.4nm or one half of a laser wavelength), the square-wave output undergoes a TTL high-low logic change, thus allowing the computer to sample the IR interferogram accurately. Figure 2 shows a block diagram of the hardware, illustrating the control structure of the instrument. In a recent development the He-Ne laser signal is digitized by a slow A-D converter, and the computer can directly decide when the average value is reached. The average is updated throughout a scan. Figure 1 shows the interferometer sampling IR radiation from a flow tube in which atoms (e.g., 0,N) react with free radicals (e.g., CF,, CHF, NCO) following pulsed initiation of the process. Atoms were formed in a microwave discharge of a diatomic precursor diluted in argon and then mixed with the radical precursor in a conventional flow system. Radicals were formed by IRMPD of the precursor, using a pulsed line-tunable CO, laser (Lumonics K 103,0.5 Hz, 6 J per pulse, or Laser Applications Ltd., 10 Hz, 6 J per pulse). The IRMPD production of free radicals ensures relatively low internal excitation [39] compared with ultraviolet photolysis methods. Following laser photolysis, the radicals react with the atoms to produce a pulse of product IR chemiluminescence at the laser focus, a region approximately 2mm in diameter at the center of the flow tube, which is imaged into the interferometer. Time-resolved chemiluminescence spectra are obtained as follows. For a given optical path difference, the entire temporal profile of the pulse of product IR chemiluminescence is recorded at the detector, amplified, digitized (up to 1011s resolution) and stored directly on hard disc for a preset number of photolysis laser shots, the number depending on the SNR of the system. The CO, laser energy for each shot is recorded by a pyroelectric

15

TIME-RESOLVED FTIR EMISSION STUDIES ZERO CROSSING REFERENCE LASER

IU

A°C

I

ZENITH z-158 PC

1 1 INTERFEROMETER^ I

I

TRANSIENT

MIRROR DRIVE

I

COOLED TO 77K PULSE

Figure 2. Block diagram illustrating the control structure of the stop-scan instrument

hardware.

joulemeter and stored for subsequent normalization-this also allows rejection of a laser shot if its energy is below a predetermined value. The internal reference of the He-Ne laser is then used to step the moving mirror on to the next sampling position (the path difference is incremented by n/2 He-Ne wavelengths, where n is an integer). The pulse of chemiluminescence following C 0 2 photolysis is again recorded, digitized, and stored for this optical path difference. The temporal shape of the emission changes as a function of path difference, as shown in Figure 3, because the component spectral frequencies in the emission, originating from a variety of transitions in the excited molecules, have different time dependences and, as the optical path difference is changed, will interfere differently when recombined at the

16

G. HANCOCK AND D. E. HEARD

0 Figure 3. The overall temporal profile of the infrared emission as seen by the detector at four positions of optical path difference 6, in the vicinity of the position of zero optical difference 6 = 0. The data shown are for emission from highly vibrationally excited CO,, taken with a temporal resolution of 3 ps and with one shot of the CO, laser per mirror position, and illustrate how both the intensities and the time profiles of the emission, arising from the production and decay of many vibrational levels, change as a function of 6. Reproduced with permission from Ref. 37.

beamsplitter. The degree of change between temporal profiles at different positions of the mirror is most noticeable for a system containing more than one emitting species which are formed and removed at different rates. Intuitively it is obvious that temporal profiles at different values of 6 must be different in order for the interferograms and hence spectra to change with time. The sequence described above is repeated for all the required sampling points in the interferogram. The experimental spectral resolution depends only on the maximum path difference, 6,,,, travelled from the position of the zero path difference. Double-sided interferograms are taken to avoid phase errors, and as the mirror travel is limited in this instrument to kO.25 cm-' from the centerburst, this results in d,, = 0.5cm and thus an unapodized resolution limit of 1.22 cm-'. The number of points required in an interferogram is 4V,,,6,,, . Thus, for example, if sampling is performed every second zero-crossing of the laser reference, giving V, = 7901 cm-l, and an unapodized spectral resolution of 3 cm- is required ,a(, = 0.2 cm), then the

'

TIME-RESOLVED FTIR EMISSION STUDIES

17

number of interferogram points is 6320. The laser must be fired 6320n times to achieve a SNR improvement of nli2 from that of a single shot scan. Typically in these experiments n varied from 1 to 5 when using the 0.5-Hz laser, but considerably more averaging was performed on weaker emissions with the 10-Hz laser. A large computer disc space is needed to store such a time-resolved scan; for example, if 150 digitized time points are used for each temporal decay, and 6000 sampling points are used in the interferogram, the required memory space is 1.72 MBytes. After the scan is finished the raw data are in the form shown in Figure 4,

1 Pho to1y si s Laser

Time/psec

Figure 4. Three-dimensional representation of the time-resolved data near S = 0 as they appear following one interferometric scan. The sampling interval employed was 1.2656pm, corresponding to a Nyquist wavenumber of 3950.7 cm-'. Selection of an interferogram at any time delay following the photolysis laser pulse is possible, and is shown here for t = 150 p s . Reproduced with permission from Ref. 37.

18

G. HANCOCK AND D. E. HEARD

that is, as a two-dimensional file of sequential temporal decays as a function of optical path difference. The data are then sorted to give interferogramo as a function of time following the COz photolysis pulse. During the recording of an interferogram, the CO, laser energy values followed a roughly normal distribution, with a FWHM of 5% of the mean value. Over this small range of laser energies, the intensities of the component spectral features were found to have very similar dependences on the laser energy, as illustrated in Figure 5 for COz and H F emissions observed when CF,HCl is photolyzed in the presence of oxygen atoms. Hence, the variation of the total emission as a function of laser energy (which for this almost saturated IRMPD process is less than linear) can be used to normalize the interferograms for energy output variations. Although corrections are small, the variation in the interferogram away from the centerburst are also small, and the quality of the spectra are improved by this normalization. Figure 6 illustrates interfer-

-

'1

0 -.

I

1

7 -

,

I

1

Figure 5. Variation of the infrared emission intensity for vibrationally excited CO, and HF as a function of the CO, laser fluence. Both display similar functional forms over the spread of values around the normal operating value, and these data were used to normalize the interferograms for the 5% variation in laser fluence during the course of an experiment.

19

TIME-RESOLVED FTIR EMISSION STUDIES

I .-

_ _ _ _ ~ I -0.04

Unnorrnalised

,

0

P a t h difference I

t

/

cm

0.04

Normalised

Figure 6. Interferograms before and after normalizing for the variations of CO, laser fluence with functional form similar to that shown in Figure 5. The data shown correspond to emission from vibrationally excited HF, generated from the IRMPD of CH,F,, and are for a delay of 20ps after photolysis, taken with one laser shot per interferometric mirror position and a Nyquist wavenumber of 7901.4 cm- '.

ograms from the same data before and after the normalization procedure, demonstrating the emergence of interferometric structure previously obscured by laser energy fluctuations (the data correspond to emission from highly vibrationally excited HF, generated from the IRMPD of CH,F,, and are at a delay of 20 p s following photolysis). Apodization is then applied to each interferogram (the triangular function giving sine' lineshape was most commonly used) before a fast Fourier transform [63] is performed to give chemiluminescent spectra for all the successive time intervals. The spectra are corrected for the instrument response as a function of wavenumber, which was obtained by measuring the emission spectrum of a black body at a known temperature. The data can then be sorted again to give the time dependence at each wavenumber in the chemiluminescence spectrum. Figure 7 presents a flow chart summarizing the batch processing of raw interferometric data that is required to obtain time-resolved spectral information. As an example of the reactions studied using this instrument we consider that between O(3P) atoms and the C H F (%'A') radical [40]. Atoms are formed in a microwave discharge of O,/Ar, and C H F by IRMPD of

20

G. HANCOCK AND D. E. HEARD Interferometric scan (Input recording

Sequential temporal decays as a function of path difference

dependence

(1) Normalisation (2) Dc subtraction (3) Apodisation Time-resolved 'manipulated' interferograms

(1) Fast Fourier Transform (2) Instrument response

correction

DYNAHICS

Time-resolved spectra

KKNETICS

Temporal profiles of component wavenumbers rhru.rrJrur*

Figure 7. Flow chart summarizing the steps needed to produce time-resolved emission profiles at different wavenumbers from the raw data, the time-resolved emission profiles as a function of path difference.

difluoromethane. Several product channels have been suggested, one of which is highly exothermic: o ( ~ P+ ) CHF(PA') +CO(X'Z++)+HF(X'X+)

AH:

=

-795 kJ mol-'

(12)

+CO(X1X+)+H(2S)+F(2P) AH:

=

-231 kJ mol-'

(13)

AH: = -333 kJ mol-'

(14)

-,HCO(?A~)

+F ( ~ P )

21

TIME-RESOLVED FTIR EMISSION STUDIES

and we have observed IR emission from both vibrationally excited CO and H F products (emission is also seen from H F formed directly from the photolysis of the precursor). Figure 8 shows a three-dimensional representation of time-resolved spectra in the region 1840cm-' (the InSb detector cut-off)-5000 cm-' taken in 30-ps slices over a timescale of 3 ms [40]. Both H F and CO emission were clearly identified, each with different temporal behavior. Analysis of the spectra at early times (spectra were taken at 394s intervals for this purpose) enabled nascent vibrational distributions of both CO and H F to be obtained, and the populations could be monitored as a function of time. Kinetic measurements, in which rate constants are obtained from the temporal dependence of specific regions of the spectrum at varying oxygen atom concentrations (measured by titration) are discussed in Section 1V.C. The formation and relaxation rates of specific rotational levels can readily be studied by this technique. Figure 9 compares the time dependence of emission from the P(3) lines of u = 2 and 1 of vibrationally excited H F [38,85] at 3834 and 3667 cm-' respectively. The decays were sorted from a two-dimensional file of 150 spectra at 5 p s intervals, and clearly show the lower rising and falling rates for u = 1. The ability of the S S technique to obtain complete time histories (hundreds of points with microsecond resolution) of specific features from only one scan is a clear advantage for kinetic studies. For comparisons of the S S and CS methods, including the

3

I. loo0

2000

3000

4000

Wavenumber / cm-1

5000

Figure 8. Three-dimensional representation of the time evolution of the IR chemiluminescence spectra following the IRMPD of CH2F2 in the presence of 0 atoms. Conditions were 28.5 mTorr CH,F,, 12.0 mTorr 0 atoms, 5.09 Torr total pressure, unapodized FWHM resolution of 6.04 cm-', Nyquist wavenumber 7901.4 cm-' with the signal obtained for 1 shot per sampling point. The data were digitized at 30 ps resolution, but are shown here with 150 ps between spectra and have been corrected for the instrument function. Emission from H F near 4000cm-' and C O near 2000 cm- is clearly seen. Reproduced with permission from Ref. 40.

22

G.HANCOCK AND D. E. HEARD

1.0 0.8

0.6 0.4 0.2

0

0

200

400

600

T i me l u s e c

8 3

Figure 9. Temporal behavior of two wavenumbers corresponding to the P(3) lines of the v' = 2 1 and u' = 1 -+ 0 transitions of HF produced under the same conditions as for Figure 8. The increased rate of vibrational relaxation for u' = 2 relative to v' = 1 --f

is evident.

errors that may arise for example if the intensity of the transient source changes during a scan, the reader is directed to Section 1II.C.

B. Continuous-Scan (CS) Time-Resolved Fourier Transform Spectrometers In all CS interferometers the mirror moves smoothly and fairly rapidly during data acquisition. Time resolution can be implemented in several ways and depends upon the lifetime of the IR emission and the repetition rate at which the emission can be initiated. If the process under study has a relatively slow time dependence, the interferometer mirror can be scanned as fast as possible and an entire interferogram (over a limited path difference) can be collected before the signal changes. Successive scans are then repeated throughout the evolution of the transient. The rapid scan method is limited to low-resolution studies and has a best time resolution of several milliseconds, but if the high velocity requirements of the interferometer mirror are met, it can be performed with commercial instrumentation. Although many interesting processes have relatively slow time dependences, very few fall into the category of photochemical reactions (whose lifetimes are typically on the microsecond scale) and are not considered further here. Sloan [27] has reviewed the many applications of rapid scan techniques, which include

23

TIME-RESOLVED FTIR EMISSION STUDIES

polymer-stretching studies, kinetics of slow gas-phase reactions, reflectance measurements, and gas chromatography. For transient IR emissions whose lifetimes are orders of magnitude less than the time for even the most rapid interferometric scan, another strategy must be used. As the mirror continuously scans, the zero crossings of the laser reference interferogram are used to initiate the time-dependent infrared signal. The transient is then digitized at one fixed time point t in its decay. If the IR emission decays completely between zero crossings, the same time delay can be introduced before digitization for every sampling point in the interferogram. Fourier transformation gives a spectrum of the emission at any single time t after initiation. The continuous-scan/single time-delay technique [27] was pioneered by Sloan and co-workers [41]. It has been used by his group to observe time-resolved infrared emission from the products of reactions between photolytically generated O('D,) and various gases at typical pressures of several mTorr, conditions such that the emission intensity reaches a maximum about 20-40 ps after the laser pulse [7,41-451. Leone and co-workers have also enjoyed considerable success with the technique, using it to look at vibrationally excited photofragments following UV photodissociation [26,46-501 and again at low total pressure, such that vibrational, and in some cases rotational, distributions are not collisionally relaxed prior to collection of the first interferogram. In both groups a high repetition excimer photolysis laser was multipassed through the reaction cell to increase the degree of photodissociation. The subsequent IR emission was collected with good efficiency by a White cell mounted with its axis perpendicular to the multipass laser mirrors, and focused through low f number optics into a commercial Fourier transform spectrometer (capable of 0.02 cm-' resolution), The laser beam was collimated into a sheet, and, with proper alignment of the White cell, a collection region of 10 x 4 cm in the horizontal plane, and -0.5 cm in the vertical plane could be established [47]. With such an arrangement pulsed emission could be observed from as few as lo9 emitting molecules per quantum state per cubic centimeter. The temporal synchronization of the photolysis laser to the interferometric mirror sweep of the FTIR instrument is shown schematically in Figure 10. The FTIR spectrometer uses a single-mode He-Ne laser to measure accurately the position of the moving mirror. The He-Ne detector produces a sinusoidal output as the mirror is continuously swept, the periodicity of which is determined by the wavelength of the laser and the mirror sweep velocity. At each positive zero crossing of the laser reference fringes the spectrometer provides an output pulse which starts a delay generator. The pulsed excimer laser is triggered after some short delay, t , (see Figure lo), and after another longer delay, t , , the signal at the infrared detector is sampled (with a gate width of 1 p).In this way the entire interferogram is obtained

-

-=

24

G . HANCOCK AND D. E. HEARD

-1 1

c

I

t2+

tl

1'

1

1

IR Signal digitized

Laser trigger

"

+

+ c Timing pulses

At = tl - t 2 = Time delay at which emission is observed

Zero crossing initiates one data acquisition sequence

Figure 10. Schematic diagram for the synchronization of the laser pulse to the mirror sweep in a CS FTIR instrument. The pulse repetition rate of the photolysis laser is determined by the velocity of the moving mirror, and the interferogram is composed of data points which have all been collected at the same delay time (= t l - t z )after the laser pulse. Reproduced with permission from Ref. 48.

from 1 to 160 ,us after the excimer pulse. Cooled interference filters are used to eliminate some of the blackbody radiation falling onto the detector. After the raw spectrum is obtained, the background blackbody emission, measured by taking a spectrum while no precursor molecules are flowing, is subtracted. Then the spectrum is normalized for the instrument response as a function of wavenumber obtained by calibration with a known high-temperature blackbody source. The flow rate is such that a fresh sample is photolyzed with each laser pulse. To achieve the goal of obtaining the nascent energy distributions under collisionless conditions, a low total pressure must be maintained, resulting in low signal levels. Variations in the intensity of the excitation laser lead t o fluctuations of 5-10% in the amplitudes of the transients, and without normalization it proved necessary to record and add an appreciable number of scans (of the order of 1000 [27]) of the interferometer mirror to obtain an adequate SNR for spectral analysis. The maximum mirror retardation per

TIME-RESOLVED FTIR EMISSION STUDIES

25

scan, and hence spectral resolution, is limited severely by the very long data acquisition times necessary. Since there is a one-to-one correlation between zero crossings and data collection, the only temporal jitter is that introduced by the delay generator (- 100 ns). Even though fluctuations in the speed of the moving mirror do occur in the course of a scan of data collection, no uncertainties in the temporal resolution of the experiments are introduced because the zero crossings, laser pulse, and data acquisition are all tightly coupled in time. Operation of the instrument in this configuration has the drawback that only one temporal data point is taken per excitation event (only one interferogram and hence chemiluminescence spectrum is obtained per scan of the instrument). All interferograms are taken single-sided and are phase corrected (see Section 11) prior to Fourier transformation. The mirror sweep velocity was set to the lowest available value of 0.01 cm s - l , below which the mirror moves too slowly to permit smooth scanning, and results in the He-Ne zero crossings generated at 316 Hz. When the photolysis excimer laser is triggered at this frequency, its output energy per shot decreases and shot to shot fluctuations increase compared with operation at lower repetition rates. Poorer data resulted at the end of runs requiring many interferograms to be co-added, which together with maintenance of mirror and window cleanliness, represents the most serious limitation of the technique as it stands. The excimer laser gives increased output energy per pulse and is more stable over long times if operated at lower repetition rates. The majority of pulsed laser systems have an optimum repetition rate of less than 50 Hz, and hence at a sampling rate of > 300 Hz more than six scans of the mirror would be required to obtain a complete interferogram at a particular time delay. However, as the sampling rate can usually be made larger than the decay rate of the IR emission, a different technique of observing time-resolved spectra can be used, and is based on firing the photolysis laser after some multiple of reference fringes has occurred [27]. A schematic of the timing sequence of this “interleaved” sampling scheme is shown in Figure 11. The rate at which zero crossings occur is determined solely by the mirror sweep velocity. For a mirror velocity of, for example, 1 cm s-l (commercial instruments are more stable at higher velocities), the delay between positive zero crossings is 31.6 ps, and can be used as a clock to trigger the time resolved software of the instrument and the photolysis laser. A datum point is then acquired at some selected delay and stored in an array for interferogram number one. At the next positive zero crossing the photolysis laser is not triggered, but a second datum point is acquired and stored away as a point for interferogram number two. This sequence is repeated for up to N time points. For a laser repetition rate of 100 Hz, there are 316 crossings between laser triggers and hence the mirror now scans a further 316-N fringes before triggering the photolysis

-

G. HANCOCK AND D.E. HEARD

26 HeNe REFERENCE LASER

I

TIME RESOLVED

SR0:G'G"E"R"'

-

LASER TRIGGER ADC SAMPLING PULSES INTE RFEROGRAM NUMBER

I ARBITRARILY SELECTED DELAY

2

3

4

5

6

7

8

Figure 11. The "interleaved" sampling scheme used to increase the efficiency of data acquisition in time-resolved CS spectrometer experiments (see text for details). Reproduced with permission from Ref. 86.

laser again, and the sequence described above is repeated. At the end of one complete scan, partial interferograms at N times after photolysis are recorded. Hence 316 sweeps of the mirror, each time incrementing the trigger point of the laser by one fringe, are required to generate one complete set of interferograms at N time points. If a large number of coadditions is required for adequate SNR, the time required for a complete experiment can become prohibitive. However, this method does allow time-resolved data to be taken at sensible laser repetition rates and stable scanning speeds, and with transients that decay only after many zero crossings. The data points for a given delay time are spread throughout the series of interferograms, and clearly a sorting of the data is required to generate complete interferograms which correspond to a common time after photolysis. Following Fourier transformation, a series of N time-resolved spectra (at 31 ps intervals) are obtained. A number of examples of the interleaved method of sample collection are

27

TIME-RESOLVED FTIR EMISSION STUDIES

documented. For the simple case of N = 1, Leone et al. have measured nascent spectra at 5 p s delay of the near infrared emission from NH,(A 'A,) fragments generated by 193.3 nm photodissociation of NH, [ S O ] . Other studies, with N = 8-10, measured time-resolved spectra of the products from laser-initiated chemical reactions, namely from vibrationally excited HCl formed in the chain reaction initiated by the photolysis of C1, in the presence of C2H, [86] as shown in Figure 12, and CO (v = 1-8) from the reaction of 0 with CH, [Sl].

I

2450

I

I

I

I

cm- 1

I

I

I

3150

Figure 12. A series of time-resolved spectra of HCI emission taken after initiation of a chain reaction by laser photolysis of C1, in the presence of C,H,. At the earliest time delay shown here HC1 is highly excited, and relaxes by collisional deexcitation at later times. Reproduced with permission from Ref. 86.

28

G. HANCOCK AND D. E. HEARD

There is a difficulty reported with the interleaved method of sampling in that it is prone to generate spectral artefacts which are indistinguishable from genuine features. The origins of such artefacts, which were observed in the very early investigation using CS interleaved sampling [30-32,871 but only explained later [33,34], are considered in detail by Sloan [27] and will not be dwelt on here. Briefly, they are created if the amplitude of the IR emission source changes between scans or over the course of many scans. Even though the drift may be small, it will have an increasing effect as each interferogram is recorded, and will manifest itself as additional modulations superimposed upon the correct interferogram for a given time delay. Although there are few documentations of these resultant spectral artefacts [33,34,88], care should be taken in assigning unknown spectra recorded using this technique. These artefacts do not appear in the absence of the sorting step as in the CS/single time-delay technique, and are unlikely to occur with the SS technique, as only one scan with no interleaving is required to obtain the time-resolved data.

C. Comparison of SS and CS Time-Resolved FTIR Spectrometers All commercial interferometers are continuously scanning and hence the CS technique can be performed with only minor modifications. For instruments designed for absorption measurements the broad-band IR source (such as a globar) is replaced by the emission source under study, which must be f matched to the instrument. Many vendors now supply a time-resolved option, with the appropriate software and hardware to provide timing pulses for slaving the external photolysis source to the movement of the mirror. The SS method requires specialized hardware, as the mirror is stationary for much of the time. Stop-scan modifications to commercial instruments have been made [74], and include an electronic position control circuit (in conjunction with a reference laser), mechanical viscous damping of the moving mirror assembly, and software to control stop-scan data collection and manipulation. A modular design was used so that both SS and rapidscan operation are possible with the same instrument. The instrument developed in our laboratory was based on a low-cost teaching interferometer, and required only minor mechanical modifications and some software development to operate as a versatile (but low-resolution) SS instrument. Data acquisition in CS instruments requires trigger delays and sample-andhold to digitize the transient at one particular point. The SS method on the other hand requires a transient recorder, as the entire temporal evolution of the transient is digitized per excitation event, and must be interfaced to the computer. Normalization of the data due to fluctuations in the laser intensity are

TIME-RESOLVED FTIR EMISSION STUDIES

29

much simpler to apply in the SS case. One additional datum point, corresponding to the laser energy, is stored together with the digitized temporal profile, for each mirror position. In the data-processing stage this value can be used to correct a given interferometric datum point at all times. For CS data acquisition a different laser energy value has to be stored for every time delay of each interferometric datum point, effectively doubling the storage capacity of the computer that is required. The temporal resolution of both methods is limited by the risetime of the IR detectors and preamplifiers, rather than the delay generators (for CS work) or transient recorders (SS) used to acquire the data, and is typically a few hundred nanoseconds. For experiments at low total pressure the time between gas-kinetic collisions is considerably longer, for example, approximately 8 ps for self-collisions of H F at 10 mTorr. Nascent rotational and vibrational distributions of excited fragments following photodissociation can thus be obtained from spectra taken at several microseconds delay, subject to adequate SNR at the low pressures used. For products of chemical reactions, the risetime of the IR emission will depend upon the rate constant, and even for a reaction that proceeds at the gas-kinetic rate the intensity may not reach its maximum for tens of microseconds. Although the products may only have suffered one or two collisions, and the vibrational distribution is still the initial one, rotational distributions may be partially relaxed. The spectral resolution of an FTIR instrument is one of the key considerations. Commercial CS FTIR instruments, with accurate sampling and dynamic alignment of the moving mirror during a scan, are now routinely able to record spectra with <0.005 cm-' resolution. The instruments that have been used for CS time-resolved studies are capable of typically 0.02cm-' resolution [47], but owing to the large number of co-added scans required to achieve adequate SNR, only resolutions of 0.2 cm- at most have geen realized. Spectral resolution of rotational lines in species in low vibrational levels is facile with such instruments, and has been observed in time-resolved studies for molecules with small rotational constants, for example, C O [48] (B = 1.9 cm-') and more recently the polyatomic radicals CCH [49] and NH, [SO]. Co-adding of successive spectra, which is not necessary for SS instruments, requires that the absolute, as well as the relative, optical path difference is known precisely, and serious errors will result if successive interferograms do not begin exactly at the same point (nanometer accuracy is required after scans of several centimeters). The resolution of the S S instrument developed in this laboratory is limited by the = 0.5 cm and a mirror travel, & 0.25 cm from the centerburst, giving ,,a, theoretical unapodized resolution of 1.22cm-l. All data were obtained double sided, but if phase correction algorithms were used to correct singlesided interferograms, the resolution could be improved to 0.61 cm-'. It is

'

30

G . HANCOCK AND D. E. HEARD

doubtful, even with a larger mirror travel, that higher resolutions could be obtained with such a simple S S instrument because of the problem of keeping the mirror absolutely stationary during data acquisition over the entire scan. Rotational resolution of hydride molecules was readily demonstrated with this instrument, but individual rotational lines of CO and CO, could not be distinguished. The majority of studies observed IR emission from atomradical reactions in a discharge-flow system at a pressure of several Torr [38, 40, 82, 83). Under these conditions, collisions with the Ar bath gas ensure that all emitting species are rotationally relaxed to a room-temperature Boltzmann distribution, even in the spectrum taken at the earliest delay of a few microseconds following photolysis (rotationally resolved spectra of hydrogen halide molecules at these early delays gave a temperature near 300 K). Only vibrational distributions were sought, and these were obtained by spectral fitting techniques and the assumption of a Boltzmann temperature of 300 K, not requiring rotational resolution. Assignment of unknown spectra demands the highest resolution possible, and the commercial CS instruments clearly have the edge over SS design for this. If, however, temporal resolution is required, the S S method, with complete digitization of the IR transient at each sampling point (with any desired time base) has the advantage. The interleaved sampling CS method increases the duty cycle somewhat, but at the typical mirror velocities used, the transient is effectively sampled at intervals of 30ps or more, and this is insufficient for kinetic measurements of fast chemical reactions. Continuousscan techniques that require more than one interferometric scan to assemble an interferogram at a given time delay are subject to spectral artefacts as discussed above. If the excitation of the emission can keep up with the sampling rate and the transient completely decays between zero crossings, then the consequences of a variation in the intensity of the emission during one scan are the same as for the SS method. A change in the intensity during a scan is mathematically the same as multiplying the correct interferogram by a function f(6), depending on the optical path difference 6. The convolution theorem of Fourier transforms [ S S ] then states that the spectrum obtained will be a convolution of the real spectrum with the Fourier transform of the function f(6). As long as f ( 6 ) varies slowly with 6, the effect of any intensity charge over a scan is not serious. If f ( 6 ) is periodic in 6, more worrying errors are introduced into the spectrum, manifesting themselves as additional peaks, similar in appearance to other features, but shifted by unpredictable amounts. The final point of comparison between the two techniques concerns the time required to record the data. A decent time resolution on the signal requires data to be taken at perhaps 100 time points following photolysis, and this is done in a facile manner by use of a transient digitizer, as described for

TIME-RESOLVED FTIR EMISSION STUDIES

31

the SS instrument in Section 1II.A. So far CS spectrometers have been largely employed to measure initial quantum state distributions, for which such numbers of time-resolved data points are not required, and so the reported software limited number of 16 such points for each set of interleaved scans has not been a problem. Extension of this range to larger numbers of points if required would not be difficult. For a given laser repetition frequency and number of laser shots to be averaged, the differences in the data acquisition times for the two instruments depend thus upon the “dead time” of the spectrometers. Contributions such as data transfer to computer memory are common to both instruments; differences come in the time needed to move the mirror and for it to equilibrate to a new position in the SS method (between 0.1 and 0.5 s in the simple design discussed above), and the time between scans for the’CS system. Spectral acquisition time may thus be straightforwardly calculated, but it should be remembered that considerable data storage and manipulation is required for both methods, and this may make a significant contribution to the experimental time scale.

IV. APPLICATIONS The following sections are not intended to give an exhaustive account of the application of time-resolved FTIR emission spectroscopy, but highlight particular problem areas in chemical physics to which the technique is particularly suited and focus on photochemical reactions, and results which have emerged since the previous reviews on the subject [26,27].

A. Internal State Distributions of the Fragments of Molecular Photodissociation Photodissociation dynamics [89,90] is one of the most active fields of current research into chemical physics. As well as the scalar attributes of product state distributions, vector correlations between the dissociating parent molecule and its photofragments are now being explored [91-931. The majority of studies have used one or more visible or ultraviolet photons to excite the molecule to a dissociative electronically excited state, and following dissociation the vibrational, rotational, translational, and fine-structure distributions of the fragments have been measured using a variety of pumpprobe laser-based detection techniques (for recent examples see references 94-100). Vibrationally mediated photodissociation, in which one photon

32

G. HANCOCK AND D . E. HEARD

prepares a vibrational state which a second photon dissociates, can selectively cleave one bond in a molecule, and has been demonstrated for HOD by monitoring the products [1011. Two-dimensional imaging of state-selected photofragments has also been used to determine product translational energy distributions [102,1031. A large number of energy-partitioning measurements have been performed for fragments of IRMPD of molecules in the ground electronic state (comprehensive reviews may be found in references 39, 104, and 105).The majority of IRMPD investigations have again used laser-based detection of the photofragments, particularly LIF, and more recently time-resolved diode laser absorption spectroscopy [1063. Many photofragments, particularly polyatomics and stable diatomic molecules such as hydrogen halides, are not readily amenable to laser-based detection, but (except for ground-state vibrational levels) are straightforwardly probed by infrared emission. Continuous-scan (CS) time-resolved FTIR spectroscopy has been used with considerable success by Leone and co-workers to study the UV photodissociation dynamics of a wide number of molecular species. The first systems investigated were hydrogen halide eliminations from halogenated ethylenes [46,47]. Time-resolved spectra for the vibrationally excited states of HCl produced in the photodissociation of 1,2-trans-dichloroethylene(t-DCE) at 193 nm were readily assigned up to u = 3 [46]. The spectra were taken at about 1 Torr total pressure in argon, so that the rotational manifolds were completely relaxed but the vibrational states were not. Typically 5C100 interferograms were collected and co-added at 1 cm-’ resolution. One spectrum at a particular time was then acquired in about 1 hour for photolysis with a 400-Hz ArF excimer laser. The intensities of the lines in the spectra were measured and corrected for the infrared emission line strengths to determine the vibrational distributions. That for tDCE was found to be u = 1,2,3,4 = 0.49, 0.32, 0.19, 0.0. The monotonically decreasing vibrational distribution is typical of those for HX eliminations from halogenated ethylenes [46, 1071 and closely fits a modified statistical calculation which uses one adjustable parameter, namely the vibrational frequency of the HX stretch in the complex [45]. A more detailed study was performed on 1,l-chlorofluorethylene,the H F product following 193-nm photolysis being produced only by @elimination across the double bond [47]. Again the vibrational distribution was well described by the modified statistical model in which the energy is partitioned into the H F product before its frequency has reached that of the free molecule, that is, near the transition state. The distribution was approximately Boltzmann, with a “temperature” of 28,000 K. Figure 13 shows part of the high-resolution emission spectrum taken under pressure-time conditions such that the rotational distributions start to become close to nascent. Measurements were possible after times corresponding to an average of 0.2 gas-kinetic collisions,

-

33

TIME-RESOLVED FTIR EMISSION STUDIES

HF

(54)

r-rl-Tl

I

2000

2500

3000

I

3500

Frequency/(cm-’)

(1.0)

4000

4500

Figure 13. Infrared emission following the dissociation of CH, = CFCI. (a) A spectrum taken at lops after 193-nm photolysis of 3-mTorr precursor by the CS method [47]. (b)Emission produced following IRMPD of the precursor, in this case at l o p delay at a pressure of 40mTorr using the SS method [lOS]. Emission from HF (u’ = 1-5) is observed, and in both sets of measurements (with lower SNR than shown in the figure), data were taken at a combination of pressure of precursor and delay times such that rotational, as well as vibrational, relaxation was negligible. Figure 13a reproduced with permission from Ref. 47.

with the distributions showing evidence of interactions taking place as the products separate. Figure 13 also shows a spectrum of H F emission from the same photofragmentation process, but now carried out by IRMPD [lOS]. Parent molecules are now prepared with far less excess energy than by 193-nm photolysis, and the vibrational energy is correspondingly lower (at a “temperature” of 6500 K). Furthermore, the fraction of available energy partitioned into vibration is lower than in the 193-nm photolysis, an effect consistent with the modified statistical theory and showing that specific partitioning of energy into vibration as the products separate is not taking place. Nascent rotational distributions taken under identical conditions to

-

34

G. HANCOCK AND D. E. HEARD

those in the 193-nm experiments showed again that rotational energies were considerably lower than vibrational. Figure 14 shows emission from other products of the IRMPD process in CH,CFCl. From the ratio of intensities of the HCl and H F fundamental bands it can be seen that the H F channel predominates (preliminary results suggest that the total H F to total HCl ratio is approximately 17 : l), and this is borne out by the observation of emission from the HCCCl coproduct as shown in the figure. Although the formation of HCl is a more endothermic process than for HF, the barrier height for the former process is believed to be lower [47]. Clearly the IRMPD process surmounts the barriers for both elimination steps, and the marked H F propensity may well reflect the relative densities of states for the two transition states accessed. This example is taken CHXCL CHXF

I

I

'

1800

I

2000

'

1

I

2200

'

I

Z

2400

l

'

l

2600

2800

3000

~

3200

~

3400

Frequency/ (cm-') Figure 14. Spectra of the products of IRMPD of CH,CFCI taken under conditions of higher decomposition yield than in Figure 13, brought about by addition of 1.6Torr Ar to 40 mTorr of the precursor. At the time of observation, 45 p s after photodissociation, considerable rotational relaxation has occurred but extensive vibrational excitation in the HCI product is observed (relative intensities between 2800 and 3200cm-' are uncorrected for the effect of a cut-off filter in this region). From the intensities of HCl emission relative to that of H F it is seen that the latter dissociation channel dominates: emission from the CHCCl coproduct can also be observed near 2100 cm- with CHCF emission far weaker.

',

'

TIME-RESOLVED FTIR EMISSION STUDIES

35

to illustrate the detail that can be obtained for the photodissociation process by both the CS and SS techniques, in this case for different methods of photoexcitation. The photolysis of acetone by a single photon at 193 nm occurs primarily by the breaking of both C-C bonds to give two methyl radicals and CO [1091. Early IR emission studies by Leone employed circular variable filters at a low resolution of 30 cm-’ [23], and identified vibrationally excited CO and CH, fragments, but it was not possible to quantify the C O rotational distribution. The three-body dissociation of acetone may occur in a precisely concerted fashion or there may be some degree of sequential breaking of the bonds. The symmetry of the acetone molecule ensures that concerted breaking of the two bonds would impart no rotational angular momentum to the C O fragment. If, on the other hand, one methyl radical were to depart first, it would leave a highly excited bent acetyl radical fragment, the breakup of which would result in high rotational excitation of CO. With the CS timeresolved method, Leone et al. were able to record sequences of spectra of the vibrationally and rotationally excited C O fragment from acetone, at various pressures and times after the ArF photolysis pulse [48]. The detection time of 4-6 p s required pressures of below 10 mTorr in order to observe products before collisions. Most of the C O is born in u = 0, and many scans were co-added (- 150) at a resolution of 1 cm-’ (0.25 cm-’ scans were recorded at higher pressures). Rotational levels as high as J = 50 were detected in several vibrational states of CO. The vibrational distribution of the C O fragment was closely approximated by a temperature of 2000 K, whereas that of the rotational excitation was higher at 3350 K. Both were successfully modelled by a “pure impulsive” mechanism for the fragmentation of an initial bent acetyl fragment. The methyl fragment has also been observed at high resolution with the FTIR instrument [86], and a rotationally relaxed but vibrationally nascent spectrum of the weak v 3 mode around 3150 cm-l has been assigned. Two final examples from the UV photodissociation work of Leone et al. concern emissions from excited polyatomic photofragments. In the first the formation of CCH is observed following the 193-nm photolysis of acetylene [49]. The CCH radical has a low-lying electronic state, and a rotationally resolved emission spectrum of CCH A(OlO)-%(OOO) was recorded at a high resolution of 0.2 cm-’ following photolysis. A delay of 7 p s and a pressure of 1 mTorr ensured collision-free conditions, and the CCH radical was found to be rotationally colder (225 K) than the parent acetylene (300 K). One mechanism for rotational cooling comes about from the loss of angular momentum due to the departing H atom from an initially rotating acetylene. Although the H atom is light compared to the C-C framework, its departure can account for the cooling if the molecule remains linear. A second

36

G . HANCOCK AND D. E. HEARD

mechanism considers the dissociation of acetylene in a trans-bent configuration; the departing H atom can impart a kick which acts either in the same direction or in the opposite direction to the initial acetylene rotation. The rotation of the C-C framework can be enhanced substantially or slowed to a point where the initial rotation changes direction and leaves the polyatomic fragment rotationally cool. Energy and angular momentum constraints sometimes prevent dissociation of the trans-bent acetylene (the state that is populated following absorption at 193 nm) in which the direction of the departing H atom would add significantly to the initial rotational energy. The second example is a recent photodissociation study of ammonia at 193 nm [SO]. The rovibrational state distribution of the nascent NH& 'A,) fragments was determined from an analysis of the NH,(A 'A, + 2 'B,) near infrared emission spectrum between 6000 and 13,000cm-l. The interleaved CS method of sampling (see Section 1II.B) was used to obtain a 1.0-cm-l resolution spectrum at 5 p s following photolysis of a 5-mTorr sample of NH, at room temperature, such that the NH,(A) fragments suffered, on average, 0.2 gas-kinetic collisions before observation. The fragments are formed predominantly in their zero-point vibrational level, with substantial rotational excitation about their a internal axis up to the limit of the available energy, but with little excitation about the other axes. The source of this rotation is traced back to the out-of-plane bending vibration excited in the NH,(A) parent molecules by the photoexcitation process. The SS interferometer described in Section 1II.A has been used in our laboratory to measure the vibrational and in some cases rotational distributions of the fragments produced by IRMPD of several molecules in addition to CH,CFCl already discussed. Dissociation from the ground electronic state occurs over a wider range of available energies within the molecule than for single photon UV excitation because of the nature of the multiple photon absorption process. The available energy for partitioning into photofragments is thus not precisely known, but is generally assumed to be that corresponding to several infrared photons above the barrier to dissociation. In small molecules a bottleneck in the excitation ladder is often encountered owing to the limited number of sequential resonant transitions following absorption of one or two infrared photons [39]. A collisional process known as rotational hole-filling [l lo], whereby rotationally inelastic collisions with added buffer gas molecules repopulate levels depleted by excitation, helps to give a much higher dissociation yield. Rotational equilibration within excited vibrational levels assists the excitation of the molecule. Studies of the nascent product state distributions of fragments following IRMPD under collisionless conditions are thus difficult to study, as the fragment yield may be extremely small.

TIME-RESOLVED FTIR EMISSION STUDIES

37

Vibrational distributions of the H F (a strong emitter) product of IRMPD of fluorinated ethenes and ethanes have been reported under collisional conditions which nevertheless do not perturb the initial vibrational populations [111-113], but at resolutions of -60-80cm-' limited by the monochromator or circular variable filter employed. In a recent series of experiments we have studied the vibrational and rotational distributions in the H F produced by IRMPD of CF,HCH, [lo81 (in order to compare these results with the vibrational distribution measured previously by Quick and Wittig [ll 11). Again, the energy partitioning into rotational degrees of freedom of H F was substantially smaller than vibrational. For the series of halogenated methanes CH2F2, CF,HCI, and CH,FC1 it has not proved possible to measure nascent rotational distributions, as the multiple photon up-pumping process is considerably enhanced by collisions (i.e., the IR emission signal does not increase linearly with pressure) and collision-free conditions for rotational measurements produced signals too small for interferometric measurements. Vibrational populations however were able to be extracted. The halid H F was observed from CH,F,, HCl alone from CF,HCI, and both from CH,FCl [l08].

B. Product State Distributions from Laser-Initiated Bimolecular Reactions Sloan and co-workers were the first to apply fast time-resolved FTIR emission spectroscopy to study the products of chemical reactions [27], by means of both the single time delay and interleaved sampling forms of the CS method. In all cases reactions of O('D,) were studied, the atom generated by photolysis of 0, by a KrF laser operating at 330Hz in a multipass cell. Reactant gases include H, [41,42], CH, [43], CHCl, [7], CHF, [7], CH,CI [7], HC1 [44], and H,S [45], and all react with O('D,) at approximately the gas kinetic rate, so that at a pressure of a few mTorr the total emission intensity reaches a maximum at 20-40 p s after the laser pulse, and the earliest time-delayed spectra are collected then. The excited products have undergone 1 or 2 collisions, so although the observed vibrational distribution is nascent, the observed rotational distribution will have suffered some relaxation. The O('D,) + H2+0H(X211,u', N') H reaction creates an initial vibrational distribution which is broad and slightly inverted [42,114]. It is not a statistical distribution, as would be predicted by the decomposition of a strongly bound H,O intermediate. The O('D,) + HCI reaction [44] generates both OH(o' < 4) and HCl(u' < 5) emission, the former by a reactive channel, the latter by E -+ V energy transfer. From the relative intensities of emission from the two species, the cross section for the reactive channel giving O H was

+

38

G . HANCOCK AND D. E. HEARD

found to be about 20 times larger than the (spin-forbidden) E + V cross section [44]. The lower vibrational levels of O H decay more slowly, as they are being repopulated from higher levels in a cascade relaxation process. The nascent O H vibrational distribution was highly inverted, peaking at 0' = 3, whereas HCl produced peaked at 0' = 4, indicating similar vibrational energy distributions for both products. The results suggest that the dynamics are viewed as a slow 0-C1 collision during which the rapid motion of the H atom permits the intersection between the first excited singlet and triplet surfaces of HOCl to be crossed many times. The time evolution of both O H and HCl vibrational distributions indicated that V-V energy transfer from OH(v) to the HCl(v = 0) reagent was occurring as a secondary reaction. The dynamics of the O('D2) H2S+OH(v')+ HS reaction have recently been investigated. Time-resolved spectra at 0.4 cm- resolution were recorded at 40-ps intervals, beginning at 20 p s and continuing until 540 p s after the laser pulse. The time-dependent OH vibrational populations recorded in this experiment are shown in Figure 15. The rotational distributions in all vibrational levels at all observation times could be fitted by near roomtemperature Boltzmann distributions. The vibrational distribution obtained at the earliest time (corresponding to approximately two gas-kinetic collisions after the reaction) was strongly inverted [45]. The LIF measurements

+

t

0 .-Y

100-

6 I

8

a

v'=3

50-

v'=2

r

v'= 1 v'=4

'0

2 50 200 250 300 350 100

150

Time/microseconds

Figure 15. Time-dependent behavior of OH(d = 1-5) observed following production of the radical by the O('D) + H2S reaction. The data, taken at 50-ps intervals, clearly show the effects of vibrational cascade by collisional deexcitation of the initially produced inverted distribution. Reproduced with permission from Ref. 45.

TIME-RESOLVED FTIR EMISSION STUDIES

39

performed elsewhere [115] show that [OH(u’ = O)]/[OH(u’ = l)] is greater than 1, and hence the overall vibration distribution is bimodal, suggesting the existence of two dynamically different reaction channels. The PES permits both a direct H atom abstraction, predominantly populating O H (0’> l), and an insertion (forming a long-lived HSOH or H,SO intermediate) which populates O H (u‘ c 1). The S S implementation of time-resolved FITR spectroscopy has been used to measure the initial vibrational distributions of a number of reactions [37]. Atom plus radical reactions have been little studied by infrared emission because of the difficulties of producing sufficient concentrations of two unstable species in the low-pressure fast-flow systems generally employed [ 116-1251, The PES for bimolecular radical-radical reactions often possess three features which are of major influence on their chemical dynamics [126]. First they have small or negligible activation energies, so that the dynamics are not dominated by entrance-channel effects. Second, the initially formed adduct is often bound with respect to products, and the influence of this resultant potential well may be reflected in the degree of statistical partitioning of the available energy in the products. Third, the reactions are often highly exothermic, and can lead to multiple reaction pathways. In this laboratory, free-radical reagents are formed by IRMPD of a suitable precursor molecule, and subsequently react with an atomic reagent in a discharge flow system [38]. This method overcomes many of the problems encountered when the polatomic free radical of interest is formed in a preceding abstraction reaction of the atomic reagent with a larger precursor molecule, as the prereaction itself in many cases generates infrared chemiluminescence from the same emitting species as the product of the atomradical reaction of interest, and can complicate the product population analysis. In addition, IRMPD generates free-radical reagents with low internal excitation [39,105], in contrast to UV production methods which often form electronically and highly vibrationally excited products, complicating the study of the ground state species [127]. The reaction between ground state oxygen atoms O(3P)and the monofluorocarbene species CHF(2’A’) possesses all three features of the PES discussed above. The reaction proceeds at almost gas-kinetic rate at room temperature [128,129], and the reaction channel (12) to produce C O and H F products in their ground electronic states (in a spin-forbidden process) is one of the most exothermic bimolecular reactions known, and several other product channels, such as reactions (13) and (14) as well as the production of electronically excited states, can occur. Pulsed IR chemiluminescence was observed following IRMPD of 10-40 mTorr of CH,F, in the presence of 0 atoms (5-25 mTorr, and measured by titration), and was passed through the S S interferometer and recorded by one of three detectors: InSb (1840-

40

G . HANCOCK A N D D. E. HEARD

8000 cm-'), Ge (6OOO-12,000 cm-'), or Si (9000-15,000 cm-') [40]. Timeresolution was generally 3ps, and the spectra were taken at an apodized resolution of 3 cm- or higher. A three-dimensional representation of the IR chemiluminescence spectra was shown in Figure 8 and emission from vibrationally excited C O and H F was readily assigned. Emission spectra in the 2 0 0 0 - ~ m -region ~ at long times consisted of the resolved P and R branches of the 1,0 transition in CO only, and the presence of this transition at all times was confirmed by the reduction in intensity seen when a cell containing a low pressure (5 Torr) of CO was placed in the beam path. The proportion of the emission removed by CO(u = 0) increased with time and was entirely consistent with an initial population of vibrationally excited CO cascading eventually to u = 1 by collision deactivation. A plot of the rising rate of the total emission in the 2000-cm- region against [O] was linear with a slope of (1.1 & 0.25) x lo-'' cm3 molecule-' s-', a value consistent with previous measurements [128,129] for the rate constant of the O + C H F reaction. Figure 16 shows the emission near 2000 cm- summed up to 25 ps after the CO, laser pulse, together with a fit of a synthetic spectrum to the data. For the fit a rotational temperature of 300 K was assumed, spectral constants [130] and Einstein A coefficients [125] were taken from the literature, and the instrument's spectral response was included from a previous blackbody calibration. An iterative least-squares fitting routine was used [131], with values of the populations in vibrational levels 1-12 taken as the variables (the InSb detector cut-off marked on the figure means that emission from levels u 13 is not observed) and the resultant populations are plotted in Figure 17. Spectra analyzed out to 150 ps showed very little change in the vibrational distribution, implying the 25-ps spectrum is the nascent one, The HF distribution at early times, with the contribution from that produced in the photolysis step subtracted, is shown in Figure 18, and is compared to previous experimental measurements [117,132]. Near IR emission spectra were taken at lower resolution, and features due to H F overtones and electronic transitions of CO in the Asundi (a'3X+- a 3 n ) and Triplet (d3A- a 3 n ) bands were readily assigned [40]. The formation of a' and d states implies some high excitation in the C O product, but the complex kinetic behavior of these species and of the CO(A'n) observed by vacuum UV emission in this reaction [128,133] has so far not allowed unambiguous identification of the steps producing electronically excited CO in process (12). Addition of D, to the reaction mixture quantitatively converts F atoms produced in the process (13) to DF, and the nascent CO and D F emission intensities were used to establish the ratio of channels producing CO [(12) and (13)] to those producing F [(13) and (14)]. The nascent CO: F ratio was established as 2.2 f0.2: 1, and hence the channels (12) and (13) are formed in the ratio (12):(13) 2 1.2. Figure 17 indicates that the vibrational distribution

'

'

'

41

TIME-RESOLVED FTIR EMISSION STUDIES

13

t

1800

5

9

r

19aJ

2003

1 I

2100

v-v-1

I

2200

2300

Wavenum ber/cm-l

+

Figure 16. CO(X'Z+) emission from the O(3P) CHF(2'A') reaction in the 2000 cm-' region, showing a fit of a synthetic spectrum (---) to the data -( 1. Data were summed up to 2 5 p after the IRMPD pulse under the following conditions: [O] = 24 mTorr, [CHzFz] = 32 mTorr, total pressure 4.5 Torr, unapodized FWHM 9.31 cm-'. Nyquist wavenumber 3950.7 cm-', 4 shots per point, fluence 82 Jcm-'. The positions of the CO u, 0-1 band origins are shown together with the detector cut-off point at 1840 cm-'. Reproduced with permission from Ref. 40.

of C O is close to that statistically predicted for a ratio of channels (12) and (13) = 1.2: 1, consistent with the channel leading to F('P) HCO(g2A) products being negligible. The H F distribution indicates it is formed with less than a statistical share of the available energy from process (12). Results are consistent with the initially formed 3 H F C 0 adduct (which correlates adiabatically with reagents) crossing to a 'HFCO surface, but with trajectories not heavily influenced by the 'HFCO potential well, which is weakly bound with respect to the reaction exothermicity. In a closely related system the S S instrument has observed emission from highly vibrationally excited C O , , formed in the reaction sequence

+

+ o ( ~ P )+ FCO(PA~) -,coz(?tlz+) + F('P) o ( ~ P+ ) CF,(PA,) -,FCO(%~A') F('P)

(15)

(16)

42

G. HANCOCK A N D D. E. HEARD

I

-0-

Thls wcrk

---- Lin et a1

\ I I 1

I I

Figure 17. Plot of the relative vibrational populations of CO produced in the 0 + CHF reaction on a logarithmic scale obtained from the fit shown in Figure 16 (a), with the error bars showing one standard deviation obtained from the fitting routine. (---)results of Lin and co-workers [132] with both sets of experimental data equated at v = 1. -( ) is a prior distribution calculated for reactions (12) and (13) in the ratio 1.2: 1 as discussed in the text, with the prior populations summed to the same total CO population as in the experimental data. Reproduced with permission from Ref. 40.

under conditions such that, although the rotational relaxation was complete, the nascent vibrational distribution was essentially preserved [38,82,83]. Reaction was initiated by the IRMPD of CF2HCl in a flow of 0 atoms and Ar diluent, and the IR emission from C 0 2 radiating in the Av, = - 1 bands near 2000 cm- was resolved with an unapodized spectral resolution of 3.18 cm- and a temporal resolution of 3 ps. Figure 19 shows FTIR spectra taken at 50,200, and 500 p s following photolysis. At long times the emission consists solely of the P and R branch transition of the (0,0 , l ) -+ (0,0,O) band of CO,, with its origin at 2349 cm-'; this long-time emission was completely removed by admitting air to the interferometer. The change of spectral features with time is consistent with the formation of an initially vibrationally excited set of emitters cascading to lower levels as time increases, with an increase in emission wavenumber due to anharmonicity. Although emission is assigned to the Av, = - 1 bands of C 0 2 , a unique vibrational distribution cannot be obtained from the data because bands of the type

'

TIME-RESOLVED FTIR EMISSION STUDIES

43

Figure 18. Distributions of HF (plotted as N u on a linear scale) as a function of lo, the fraction of available energy from process (12) appearing as a vibration in HF with the HF vibrational quantum numbers u given in the upper scale. (0) early time distribution obtained as described in [40]; (0)distribution from Lin and co-workers [132]; (0)distribution from Klenerman and Smith [117]; (-) a prior distribution for reaction (la). All distributions have been normalized so that the sum of their populations in u = 1-3 are equal. Reproduced with permission from Ref. 40.

(vl, v2, v3) -+(vl, v,, v 3 - 1) with different combinations of vl, v2, and v 3 can

occur at the same wavenumber. Observation of rotational structure, which could in principle solve the problem, requires a resolution of at least 5x cm-' [134]: a broad unresolved emission, similar to that seen in the present experiments, was also observed at a far higher resolution, 0.1 cm-', in studies of CO, chemiluminescence produced by the recombination of 0 and CO at a Pt surface [135]. If CO, was formed in the reaction sequence (15), (16) with excitation only in the asymmetric stretch (i.e., v1 = v 2 = 0), then the substantial emission below 2000 cm-' at early times (see Figure 19) could only be explained if

44

G. HANCOCK AND D. E. HEARD

1875

2150

Wavenumber

/

an-1

2325

Figure 19. Emission spectra at 50,200, and 500 ps following the IRMPD of CF,HCl (24 mTorr) in the presence of 0 atoms (11.3mTorr) and Ar (4.05Torr), obtained from FTIR emission experiments. Data are unapodized with resolution (FWHM) 3.18 cm-', Nyquist wavenumber 3950cm-', and were obtained at one CO, laser pulse per sampling point. The P, R envelope of the (0,0,1) + (O,O,O) transition of C 0 2 is clearly seen in the 500-ps data. Reproduced with permission from Ref. 82.

100% of the exothermicity of reaction (15) were deposited in internal energy of F C O to be utilized in reaction (16), an unlikely dynamical outcome. Furthermore, simulations of the emission in v3 for v1 = v, = 0 gave more structure than experimentally observed. (A search was made for the LIF spectrum of the FCO radical [136] in the wavelength regions 278-282 and 288.5-294 nm, but only LIF from vibrationally excited CF, was observed.) Partitioning of the energy into one mode of vibration represents one extreme of the reaction dynamics, that of high selectivity in product-state formation. The other extreme is a statistical partitioning of energy, such as would occur if an FC0, complex were formed, living long enough to enable energy randomization to take place, and decomposing with no exit-channel barrier. A prior distribution with the rigid-rotor harmonic oscillator approximation [137] was performed at three values of the available energy for partitioning into the three modes of CO, [83]. Spectra were simulated with these distributions, and are shown in Figure 20 together with an experimental spectrum at 35 p s after photolysis, at which time no appreciable vibrational

TIME-RESOLVED FTIR EMISSION STUDIES

1600

1800 2000 Wavenurnber /c

45

rn-1

Figure 20. Comparison of the “early-time”(35 p s ) spectrum with three prior distributions (dashed curves) calculated for three values of the available energy for the 0 FCO reaction: (a) 392 kJ mol-’, the exothermicity of reaction (16); ( b ) 451 kJmol-’, the exothermicity of reaction (16) when the upper limit to the recommended range of values of AH: of FCO is used; (c) 549kJ mol-’, the exothermicity of reaction (16) if FCO contains all of the energy released in its

+

formation step, reaction (15).

relaxation has occurred. The data adequately fit a prior distribution, but only with an available energy some 59 kJ mol-’ or 15% above that released in reaction (16). This extra energy could be accounted for either if the heat of formation of the FCO radical is at the upper limit of its recommended values, or if some internal energy is available in the FCO as a result of its formation in the 0 + CF, reaction. Alternatively, specific partitioning of the available energy in vibrational degrees of freedom of CO, would be consistent both with the data and with the reaction pathways expected from calculations on the FCO, system [138]. Recently the high resolution capabilities of diode lasers have been utilized with considerable success by Flynn and co-workers to investigate energy partitioning within the internal modes of CO, following a variety of processes [1391. Individual rotational lines were readily isolated, and examples of studies using this technique include V-V, R energy transfer following collisions with polyatomics [140-1421 and excitation by collisions with hot atoms [143-1471 and electrons [148]. The technique has also been applied to measure the vibrational state distribution of CO, produced in the reaction between OH radicals and CO [149]. Results showed an absence of any population in excited states with v 3 > 0, and hence FTIR emission spectroscopy could not be used to measure spectra in the 2000 cm-’ region for this reaction.

46

G. HANCOCK AND D. E. HEARD

A number of studies of the kinetics of the NCO radical have been previously carried out in this laboratory [150-1521, and its reaction with the N O molecule

+

NC0(gZII) NO(X211)-,N,0(X'C')

+ CO(XIC+)

AH:-278 kJmol-' +N2(X1C+)+CO,(X'C;)

(17)

AH: - 643 kJ mol-

(18)

+N2(X'Xl)

+ CO(X'Z+) + O(3P)

AH: - 111 kJ mol-'

(19)

has been recently studied by time-resolved FTIR emission spectroscopy [lOS]. Although the rate constant is well known over a wide temperature range [150-1541, the relative importance of the various reaction channels is still the source of some debate. The reaction is of combustion importance, as it contributes to the removal of N O in the RAPRENO, process, in which the cyclic trimer of cyanuric acid, (HOCN),, is added to combustion flue gases. Reaction (17) is believed to be responsible for the high levels of N z O observed in experimental RAPRENO, reactors [l55], and has been assumed to be the most likely pathway, with the formation of CO, being less favored owing to the 1-3 oxygen migration necessary following formation of the ON-NCO intermediate [1541. However, diode laser absorption measurements have recently shown, surprisingly, that the C 0 2 product dominates, with k , , :k , , : k , , = 0.44:0.33 : 0.23 [156]. Infrared emission experiments can add to this body of data, but care must be taken in the interpretation of the results, as both RAPRENO, reactor and diode laser experiments probe ground states of product molecules, whereas IR emission only refers to vibrationally excited states. Three methods of NCO formation have been used to study IR emission from reactions (17)-(19), namely IRMPD of C,H,NCO, IRMPD of o-CF3C,H,-NCO, and reaction (20)

the C H F being formed from IRMPD of CH,F, as described earlier. Figure 21 shows the spectral region between 1800-2800cm-' measured at a resolution of 9 cm- '. The significant feature, common to all NCO formation methods, is that an initially broad distribution between 1900 and 2300 cm-' relaxes collisionally to produce N 2 0 (001) as the dominant emitting species. Both CO, and CO are also seen but the former, despite having a larger

TIME-RESOLVED FTIR EMISSION STUDIES

47

N,0(001-000)

1800

2300

2800

Wavenumberlcm-1 Figure 21. Emission in the 1800-2800cm-' region following the IRMPD of CH,F, (25 mTorr) in the presence of NO (100 mTorr) and Ar (10 Torr). Emission from N,O (001) and C O , (001) are seen at long times following the vibrational cascade of highly excited species; their presence at early times [together with CO(v = l)] was established by cold gas filter experiments. N,O is the dominant emitter, and is believed to originate from the reaction sequence (20), (17).

Einstein A coefficient in its (001-OOO)transition at 2349 cm-' than that for the corresponding band at 2223 cm-' in NzO, appears always to be the minor triatomic product emitting in this range. Although these results are presently preliminary (and their interpretation may need to be revised if, for example, rates of relaxation of the excited products are distorting the partially relaxed vibrational distributions), they seem incompatible with the diode laser observations unless reaction (18) produces CO, substantially in vibrational levels with v 3 = 0. Further experiments are in progress. Atom-radical reactions have recently been investigated using the CS timeresolved instrument (with interleaved sampling) in Leone's laboratory [Sl, 861. Infrared emission from HCl was observed in the chain chlorination of C,H,, initiated by 351-nm photolysis of C1, in the presence of the alkane [86]. Interferograms were recorded at 34-ps time intervals (fixed by the sampling rate of the HeNe crossings) at a resolution of 0.36 cm-' as shown in

48

G. HANCOCK AND D. E. HEARD

Figure 12. An initial distribution of HCl(u = 1-3) was obtained, and is believed to originate from the reaction of C1 atoms with the ethyl radical, although several reactions can generate vibrationally excited HC1. Very recently 193-nm ArF excimer laser photolysis of Me1 or acetone and SO, mixtures was employed to generate 0 atoms and CH, radicals, and their reaction was monitored using the product CO(u’) chemiluminescence [S 11.

C. Kinetic Studies of Chemical Reactions and Energy Transfer Processes Measurement of the rates of formation of vibrationally excited molecules and the subsequent relaxation is a natural extension of the time-resolved FTIR technique. No other method exists for measuring the time-dependent population of all the excited rotational-vibrational levels of any emitting species simultaneously. Both CS and S S methods can generate the temporal evolution of a given spectral feature, and observation of the behavior as a function of reagent pressures enables kinetic information to be gained. However, one of the disadvantages of FTIR spectroscopy for kinetic studies is that it is not possible to “sit” on a spectral feature and acquire temporally resolved data for a given number of laser shots. An entire interferogram must be scanned before any spectral information is obtained. To avoid prohibitively long data acquisition times, time-resolved measurements obtained by systematically varying the experimental parameters are usually limited to short scans and hence lower resolution. It must be stressed however that spectra at high resolution are first required to identify unambiguously the emitting species and to measure the degree of internal excitation. An ensemble of vibrationally excited molecules formed, for example, by chemical reaction, will relax via a variety of collisional and radiative processes, the rates of which will depend upon the vibrational level, and will result in strongly wavenumber dependent kinetic behavior in emission. Only the highest vibrational level formed will have a rise which is nofslowed by cascade of population from higher levels. This is clearly illustrated in Figure 22, where the temporal behavior of CO, formed vibrationally excited in the 0 + FCO reaction (16) (see Section 1V.B) is shown for Av, = - 1 emission at four different spectral wavenumbers. The temporal resolution was 3 ,us, and the decays were obtained from sorting the time-resolved spectra, three of which were shown in Figure 19. The shift in emission to higher wavenumbers at longer times is consistent with an initially vibrationally excited set of emitters cascading to lower levels mainly by collisions, with the consequent shift in emission wavenumber being due to anharmonicity. The rising rate is very nonexponential for the higher wavenumbers.

TIME-RESOLVED FTIR EMISSION STUDIES

49

Figure 22. Time resolved behavior at four wavenumbers for C 0 2 produced in the reaction sequence (15,16). The peaking of the emission at longer times with increasing wavenumber is characteristic of vibrational relaxation of the emitting CO, molecules. The traces were obtained by sorting time-resolved data such as that shown in Figure 19.

We give the following description of the kinetics of the emission from CO, formed in the reaction sequence (15), (16) as a case history showing the information that can be extracted for IR emission studies, and how, after identification of the emitting species by means of a complete FTIR emission spectrum, the kinetic behavior can be more easily studied by means of narrow band filter experiments. It was first established that the photolysis product CF, and not vibrationally excited precursor CF,HCl, which will undoubtedly be present in higher concentrations (because of the CO, laserinduced photolysis step) is responsible for the observed IR emission. The total CO, chemiluminescence was a linear function of CF2HCl pressure; rising and falling rates were independent of [CF,HCl], but increased markedly with [O] over a wide range of CF,HCl pressures. For the CF, radical to be involved, the IR chemiluminescence intensity should, for a given set of reagent pressures, track the CF, concentration as measured by LIF. Both these quantities were varied by changing the CO, laser fluence, and despite the very nonlinear variation of each of these quantities with CO, laser fluence, they were found to be linearly related [82]. The fluence dependence of the excitation of CF,HCl would be markedly different from that for production of CF,, and this rules it out as a source of the CO, emission. With the time dependence of the emission spectrum thus established by the SS FTIR instrument, kinetic measurements were then carried out using a series of narrow band interference filters, whose transmitted wavenumber ranges

50

G . HANCOCK A N D D. E. HEARD

encompass the spectral features shown in Figure 19. The temporal behavior was identical to that observed at a given wavenumber from FTIR measurements (see Figure 22) but was achieved in a far shorter acquisition time. With a filter centered at 2326 cm-', radiation from the CO2(O,0,1)-+ (O,O,O) transition (among others) was transmitted, and the kinetics of this upper level alone were observed by recording the difference between the emission passed by the filter with and without a cell containing 10 Torr of C 0 2 in the path of the beam. Cold gas filters have often been used to isolate emission from the first vibrationally excited level in kinetic studies using IR emission [157, 1581. In this case C 0 2 absorbs the resonant (0,0,1) + (O,O,O) emission (nonresonant absorption was shown to be negligible at this pressure), and the difference gives the time history of CO,(O,O,l) level. A marked induction time for formation of CO2(O,0,l) was observed. Figure 23 shows the temporal behavior of the emission transmitted by filters centered at 1873 and 2066 cm-' respectively, at a variety of oxygen atom pressures. In both cases the rising and falling rates increase with [O], and the timescale is compressed for the lower wavenumbers. Cascading of population from higher levels complicates the analysis of data such as those of Figure 23b, but this problem is minimized at the lowest wavenumber emission. A simple reaction scheme applied to the data of Figure 23a [82] produced a rate constant for reaction (15) of (2.3k0.2) x lo-'' cm3 molecule-' s-l, in excellent agreement with previous measurements [129,159,160]. The scheme also resulted in a rate constant of (1.8k0.2) x lo-" cm3 molecule-' s-' for quenching of the emitter at this wavenumber, highly vibrationally excited C 0 2 , by 0 atoms, a value almost 100 times faster than that for quenching the CO2(O,0,l) level [161]. Emission near 1873 cm-' is not due to a unique vibrational state of CO,, and has substantial excitation in all three modes. The rapid quenching may involve loss of energy in the lower-frequency v1 and v2 modes, with the magnitudes of their anharmonic couplings to v3 ensuring that the relaxed molecule emits outside the observed range of the filter. Extraction of V-V or V-T rate constants as a function of vibrational level would require significant modelling of the complex cascade processes in the triatomic product. A similar procedure to this was used to measure rate constants for the 0 CHF and NO + NCO reactions [40,108]. Electron bombardment of gas mixtures, although not strictly a photochemical process, has been used in conjunction with time-resolved SS interferometers to obtain rate constants for the vibrational relaxation of highly excited molecules [30, 35,36,71-731. Murphy et al. [35,71] observed the production and relaxation of vibrationally excited N O and the (O,O,l -+ O,O,O) bands of N 2 0 and NO2 following excitation of N 2 / 0 2 mixtures with a pulsed electron gun. The infrared emission created by the electron beam decayed completely between pulses and the complete temporal

+

51

TIME-RESOLVED FTIR EMISSION STUDIES

[ O]/m Torr

la)

e \

5

0

0

100

200

Time / microseconds

[ O l l m Torr

0

rn

Time

/

400

microseconds

400

300

Ib)

800

Figure 23. Variation of the intensities and time profiles of the emission from CO, produced in the sequence (15,16) passed through filters centered at 1873 cm-' (a) and at 2066cm-'(b) as a function of oxygen-atom pressure marked on the diagrams in mTorr. [CF,HCl] = 10.4 mTorr, total pressure 4 Torr, 3 p s time resolution. (a) Data were taken over 200 laser shots at 68 J cm-'. (b)Data for 100 laser shots at 59 J cm-'. Note the different timescales for the two sets of traces. Reproduced with permission from Ref. 82.

52

G . HANCOCK AND D. E. HEARD

evolution of 50 excitation cycles was recorded and averaged for each interferometric mirror position (or between adjacent sampling positions for a quasi-SS scan). The strongest emission was from NO(u’ = 1-10), generated from the reactions of N(,D) and N(,P) with 0,. The N,O (0,0,1+O,O,O) emission around 2200cm-’ was thought to be due to V-V exchange with excited N, or N,O, or to a three-body process. Time-resolved spectra of the NO(Au’ = - 1) emissions were recorded at 100-ps intervals during and after the beam pulse, with a resolution of 10cm-’. No rotational structure was resolved, and relative vibrational populations as a function of time were obtained from fits to the spectra [71]. The observed temporal histories of the vibrational populations were interpreted in terms of quenching by the reactant gases N,, O,, and the beam-created species 0, and by radiative decay. The master equations which couple together all the quenching processes of all vibrational levels were solved to give vibrational relaxation rates as a function of u’. From time-resolved spectra taken at different 0, pressures, the room temperature rate constants for the processes NO(u’) 0, +NO(u’- 1 ) + 0 , , u’ = 1-7, increased monotonically with 0’. In a related study Caledonia et al. [36] determined the vibrational level dependent quenching rates of CO(u = 1-16) by CO, with a quasi-SS instrument (see Section III.A.1). The CO production mechanism, predominantly dissociative recombination of CO:, was found to produce CO up to u = 19 following electron bombardment of Ar/CO, mixtures. Time-resolved spectra at lOcm-’ resolution were taken at 200-ps intervals, and both CO and CO,(Av, = - 1) features were observed. Quenching of CO occurred primarily by collision with CO,, and spectral fitting was again used to obtain relative vibrational populations as a function of time. At later times a population inversion occurred between u1 = 5 and u1 = 2-4 as a result of a bottleneck in which the quenching of u’ = 5-9 is slower than that of the higher and lower levels. The complex vibrational relaxation was modelled, and including quenching by CO,, V -+ V energy transfer with CO(u = 0), quenching by Ar, and radiative loss in the fundamental and overtone bands. Spectra taken at different CO, pressures enabled the rate constants for CO(u’ = 2-15) quenching by CO, to be extracted from a Stern-Volmer analysis [36], and were in good agreement with previous CW infrared chemiluminescence depletion experiments [162]. Very recently the formation and vibrational relaxation of OH(X211i, u = 1-9) by 0, and CO, [73] and NH(X3X-, u = 1-3) by N,, Ar, and H, [72] has been investigated in electron-irradiated gas mixtures. OH(u + u - 1, u - 2) and NH(u +u - 1) IR emission was observed using a quasi-SS time-resolved FTIR spectrometer, averaging about 100 samples to characterize the IR transient for each of lo00 sampling positions. Timedependent populations are shown for OH(u = 1-6) in Figure 24, taken from

+

53

TIME-RESOLVED FTIR EMISSION STUDIES

0.0

0.5

1.5

1.o

Time/(ms)

2.0

Figure 24. Vibrational populations of OH(o = 1-6) following electron beam irradiation of a mixture of Ar (lOTorr), O2 (0.3 Torr), H2 (90mTorr) and an estimated 4 mTorr of 03.Relaxation rate constants were obtained from model calculations fitted to the data and shown by the solid lines. Reproduced with permission from Ref. 13.

Ref. [73]. Note the large number of temporal data points that are generated in this experiment for each vibrational level, enabling accurate fits to a kinetic model to be made. Population data were analyzed using a single-quantum relaxation model, but also considered possible effects of multiquantum relaxation. Derived values for relaxation of OH(u) by both 0, and CO, increase rapidly with u (by a factor of 25 for CO, from u' = 1-4). The NH emission [72] enabled a measurement of the nascent vibrational state distribution of the atom-radical reaction:

-

N(,D)

+ H2(X1Zl)

+ NH(R3Z-,

u)

+ H('S)

(21)

to be made, and the fraction of the available product energy in vibration was found to be (f,) = 0.44.[The NH(u = O)/NH(v = 1) ratio was obtained from a linear surprisal plot.] The evidence suggests that the reaction proceeds by direct H atom abstraction rather than the formation of a long-lived H-N-H intermediate [72].

54

G . HANCOCK AND D. E. HEARD

Sloan and co-workers have used their CS time-resolved FTIR spectrometer to determine rates of formation and subsequent V + T and V + V relaxation of OH(u') [45, 1631. In one study vibrationally excited OH in u' = 1-4 was generated by the reaction O('D)

+ H,(X'E:

-+

OH(X21'I,V' = 0-4)

+ H('S)

and time-resolved spectra were taken at high spectral resolution [42]. The vibrational populations in v' = 1-4 were obtained at three time delays for several sets of experimental pressures. Forty V-T and V-V rate constants for the disappearance of vibrationally excited OH in collisions with O 3 and OH were determined by an analytical technique based on a complete solution of the master equation [163]. In a recent study of the O('D) H2S + OH(u') HS reaction [45], time-dependent populations of OH u' = 1-5 were obtained. A numerical integrator was used to model the production of OH and its subsequent vibrational relaxation by H,S and 0, using known or assumed rate constants. The time dependence of the highest accessible vibrational level OH(u' = 5) was calculated, and agreed with the experimental data. Finally, some S S time-resolved FTIR studies in this laboratory of the collisional deactivation of highly vibrationally excited polyatomic molecules are considered. The rates of uni- and termolecular reactions in the gas phase are controlled by the collisional deactivation of highly vibrationally excited reaction complexes, and energy transfer information is often derived from the pressure dependence of thermally or chemically activated unimolecular reactions. The energy transfer efficiencies of various collider gases are often expressed as (AE), the average energy transferred per collision, and have been measured as a function of the total energy within the polyatomic molecule using several methods. Troe and co-workers [164- 1681 prepared highly vibrationally excited molecules by U V laser excitation (one photon absorption to an electronically excited state followed by rapid internal conversion to the ground electronic state), and monitored the collisional energy loss by transient U V absorption spectroscopy (UVA). Barker and coworkers used infrared fluorescence (IRF) to monitor the time-dependent relaxation of excited aromatics [169-1751 following absorption of a single visible or U V photon. The IRF measurements have now been extended to observe excitation of COz by V-V energy transfer from the initially photoexcited molecule [1761, measurements complementary to the timeresolved diode laser studies of Flynn and co-workers [140-142,1771. Both the U V excitation and IRF methods depend critically upon calibration curves to determine the average vibrational energy ( E ) within the molecule. Despite much theoretical and experimental effort by both sets of

+

+

TIME-RESOLVED FTIR EMISSION STUDIES

55

workers to demonstrate the consistency of the respective calibration techniques, there remains a discrepancy as to the functional dependence of ( A E ) on ( E ) , although agreement is found for the magnitudes of ( A E ) for a variety of bath gases at specific values of ( E ) . In a more recent development Luther and co-workers selectively detected molecules from a predetermined narrow range of energies within a quasi-continuum of rovibrational states using MPI. The method of “kinetically controlled ionization” uses the strong energy dependence in the kinetics of competitive intramolecular channels to give some energy selectivity in the ionization process [178]. For a given laser wavelength only molecules with a certain range of ( E ) are ionized and hence detected, and the temporal evolution of different total energies is straightforwardly obtained by tuning the laser. Measurements at different pressures of added buffer gas enable energy transfer parameters to be found. The IRF technique so far applied measures the total IR fluorescence within the spectral region corresponding to a given IR active mode, for example, a C-H stretch. The intensity of the emission is related to the average energy content ( E ) via a model calculation: an extension of this technique using the time-resolved FTIR method would be to observe the emission decay in a spectrally resolved manner. Molecules containing different ( E ) will emit at different wavenumbers, and providing some identification of the dependence of the emission spectrum on ( E ) can be found, a measurement of the dependence of ( A E ) on ( E ) can be, in principle, extracted. The method essentially adds another observational parameter to the IRF method applied extensively by Barker and co-workers [169-1751. As an example of the feasibility of this method, we illustrate emission observed during studies designed to create NCO radicals by IRMPD of a precursor C,H,NCO [SS]. Figure 25 shows spectra near 2200 cm-’ taken at early (6 p s ) and late (100 p)times following the irradiation of 70 mTorr pure C6H5NC0, and Figure 26 gives a three-dimensional representation of the temporal behavior of the spectrum, obtained from a single scan of the S S instrument. The spectrum alters markedly with time, with the peak shifting to higher wavenumbers at longer times, indicative of vibrational cascade of a nascent population of vibrational levels. Time-resolved spectra in the presence of 5 Torr Ar were similar, but the emission was much shorter lived. Potential photofragments of the IRMPD of C6H,NC0 were ruled out as the source of the emission on spectroscopic grounds, and on the marked differences between the fluence dependences of this emission and that of the formation of photofragments [SS]. A low-pressure gas phase IR spectrum of C,H,NCO revealed a strong absorption at 2278 cm-’ (-NCO stretch) which coincides well with the emission observed at long times. At early times the fluorescence may originate from both high and low levels within the emitting mode, but is also consistent with low excitation in this mode with significant

2-

I-

=rn -

0 - r--l---r---

2000

I800

2200

Wavenurnber

I

/

cm-I

2400

' I 2600

Figure 25. FTIR emission spectra at two times following the IRMPE of 70mTorr PhNCO. Unapodized FWHM 3.18 c n - Nyquist wavenumber 3950.7 cm-l; the spectrum is corrected for the instrument response function and the maxima of both spectra have been scaled to unity.

',

1

2000

I

I

2200

I

Wavenurnber/cin-l

1 24w

Figure 26. Three-dimensional representation of the time evolution of the PhNCO fluorescence following IRMPE, showing a spectral shift to higher wavenumbers at longer times as a result of vibrational relaxation. Spectral and temporal resolution of 3.18 cm-' and 3 p s respectively. 56

TIME-RESOLVED FTIR EMISSION STUDIES

57

cross-anharmonic couplings with other excited modes causing substantial shifts of the emission to lower wavenumbers. No emission was observed from other modes, which are all weak in absorption compared to the -NCO stretch at 2278 cm-'. The temporal decay of each wavenumber (decaying more rapidly at lower wavenumbers) was found to follow single exponential behavior, except near 2280 cm- '. The interpretation of these data to extract collisional quenching parameters is not straightforward, as it requires a correlation between emission wavenumber and ( E ) . This would be straightforward if the emission was purely from a cascading ensemble of molecules excited in only one mode, the vibrational anharmonicity then allowing identification of emission at a given wavenumber as originating from a molecule with given ( E ) . This is undoubtedly not the case for the C6H,NC0 molecule, and thus the precise correlation of ( E ) with wavenumber is not unambiguous. Future ways forward can be suggested. Nascent IR spectra should be taken following monochromatic UV excitation at a variety of irradiation frequencies (i.e., values of ( E ) ) of a molecule which undergoes rapid internal conversion. If these spectra differ sufficiently, then their absolute intensities and distributions could be used to fit the collisionally evolving spectra taken as a function of time following monochromatic UV excitation. To this end we have recently managed to resolve spectrally the nascent emission in the 3000cm-' region following the 266-nm excitation of azulene, a molecule whose collisional properties have been previously investigated by the (unresolved) IRF method [169-1731. Although collisional evolution of the spectrum was observed, it has so far proved impossible to measure spectra from excitation at other (longer) wavelengths, the signals being extremely weak: a stronger emitter is needed. Another use for infrared spectra as a measure of ( E ) values would be in attempts to deconvolute spectra such as those shown in Figures 25 and 26, observed following infrared multiple photon excitation (IRMPE), to obtain a measure of the distribution of energies in such systems. Little is known of the population distribution of molecules formed by IRMPE [39, 112, 179, 1801, and even qualitative information in this area would be most useful.

V. CONCLUSIONS This review has attempted to highlight recent progress in the development and use of time-resolved FTIR emission in three areas of chemical kinetics and dynamics, namely, product state distributions in photodissociation processes, product state distributions in chemical reactions, and rate processes involving the formation and loss of the internally excited species

58

G. HANCOCK AND D. E. HEARD

responsible for the emission. Applications in these areas will clearly increase, and that is the only safe prediction of future uses for the SS and CS instruments described in Section 111. We also hope that the method will be applied more to one of the most important and difficult areas in chemical kinetics-quantitative measurement of quantum yields of product channels in reactive processes, particularly those of combustion and aeronomic importance. The following caveat applies: emission fails to measure population in the lowest vibrational level, and for relative quantum yields this must be estimated, for example, by information-theoretic analysis of vibrationally excited populations, and extrapolation of these to v = 0. The limitations of the method are clear. We finally come full circle. The greatest advance influencing the practical use of interferometers has undoubtedly been the development of computers able to perform the Fourier transforms rapidly and efficiently. Computers rely on integrated circuits, now almost all of which are produced by processes involving the gas phase plasma etching of semiconductor surfaces. Plasmas glow, and infrared emission is readily seen [181-1841. Modulation of the plasma current can be used in conjunction with time-resolved measurements to separate the emission which arises from species produced and destroyed during the plasma current cycle from the blackbody background, and emission from species such as CO,, CO, HF and highly excited atoms has been seen with the SS instrument [184]. Emission on or near the etched surface also occurs [185], and we hope that studies of this may aid in understanding the complex etching processes which are used in the production of the fast Fourier transform integrated circuit.

ACKNOWLEDGMENTS This work would not have been possible without the initial support of the United States Army European Research Office and, more recently, the SERC. We thank our colleague Richard Wayne for introducing us to the benefits of interferometry, and to the other members of our group who have labored with an often cantankerous but generally rewarding interferometer to generate the results presented here.

REFERENCES 1. P. R. Griffiths, Transform Techniques in Chemistry, Heyden, London, 1978. 2. P. B. Fellgett, J . Phys. Radium 19, 187, 237 (1958). 3. P. Jacquinot, J . Opt. SOC.Am. 44, 761 (1954).

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4. N. Jonathan, C. M. Melliar-Smith, S. Okuda, D. H. Slater, and D. Timlin, Mol. Phys. 22, 561 (1971). 5. J. G. Moehlmann, J. T. Gleaves, J. W. Hudgens, and J. D. MacDonald, J . Chem. Phys. 60, 4790 (1974). 6. J. C. Polanyi and W. J. Skrlac, Chem. Phys. 23, 167 (1977). 7. P. M. Aker, B. J. Niefer, J. J. Sloan, and H. Heydtmann, J . Chem. Phys. 87, 203 (1987). 8. K. Tamagake and D. W. Setser, J . Phys. Chem. 83, 1000 (1979). 9. K. Dehe, H. Heydtmann, and V. Schwante, Chem. Phys. Lett. 169, 603 (1990). 10. K. G. Anlauf, P. J. Kuntz, D. H. Maylotte, P. D. Pacey, and J. C. Polanyi, Discuss. Faraday SOC.44, 183 (1967). 11. P. D. Pacey and J. C. Polanyi, J . Appl. Opt. 10, 1725 (1971). 12. R. G. MacDonald and J. J. Sloan, Chem. Phys. 31, 165 (1978). 13. J. C. Polanyi, Acc. Chem. Res. 5, 161 (1972). 14. J. J. Sloan, J . Phys. Chem. 92, 18 (1988). 15. J. Habdas and D. W. Setser, J . Phys. Chem. 93, 229 (1989). 16. B. S. Agrawalla and D. W. Setser, J . Phys. Chem. 90, 2450 (1986). 17. R. B. Bernstein, Chemical Dynamics via Molecular Beam and Laser Techniques, Clarendon Press, Oxford, 1982. 18. C. C. Mei and C. B. Moore, J . Chem. Phys. 70, 1759 (1979). 19. D. A. Dolson and S. R. Leone, J . Chem. Phys. 77,4009 (1982);J . Phys. Chem. 91, 3543 (1987). 20. E. Wurzberg and P. L. Houston, J . Chem. Phys. 72, 5915 (1980). 21. E. Wurzberg, A. J. Grimley, and P. L. Houston, Chem. Phys. Lett. 57, 373 (1978). 22. P. L. Houston, Ado. Chem. Phys. 47, 625 (1981). 23. D. J. Donaldson and S. R. Leone, J . Chem. Phys. 85, 817 (1986). 24. S . R. Leone, Acc. Chem. Res. 16, 88 (1983). 25. L. L. Feezel and D. C. Tardy, J . Phys. Chem. 93, 3124 (1989). 26. S. R. Leone, Acc. Chem. Res. 22, 139 (1989). 21. J. J. Sloan and E. J. Kruus, in Advances in Spectroscopy (Time Resolved Spectroscopy), R. J. H. Clark and R. E. Hester, Eds., Wiley, Chichester, 1989. 28. B. D. Moore, M. Poliakoff, M. B. Simpson, and J. J. Turner, J . Phys. Chem. 89, 850 (1985). 29. W. Barowy and H. Sakai, Infrared Phys. 24, 251 (1984). 30. H. Sakai and R. E. Murphy, Appl. Opt. 17, 1342 (1978). 31. A. W. Mantz, Appl. Opt. 17, 1347 (1978). 32. A. W. Mantz, Proc. SPIE 82, 54 (1976). 33. A. A. Garrison, R. A. Crocombe, G. Mamantov, and J. A. de Haseth, Appl. Spectrosc. 34, 399 (1980). 34. J. A. de Haseth, Proc. SPIE 289, 34 (1981). 35. R. E. Murphy, F. H. Cook, and H. Sakai, J . Opt. SOC.Am. 65, 600 (1975).

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36. G. E. Caledonia, B. D. Green, and R. E. Murphy, J . Chem. Phys. 71,4369 (1979). 37. P. Biggs, G. Hancock, D. E. Heard, and R. P. Wayne, Meas. Sci. Technol. 1,630 (1990). 38. G. Hancock and D. E. Heard, Chem. Phys. Lett. 158, 167 (1989). 39. G. Hancock and M. N. R. Ashfold, in Gas Kinetics and Energy Transfer, Royal Society of Chemistry Special Report, Vol. 4 (1981), p. 73. 40. R. A. Brownsword, G. Hancock, and D. E. Heard, J . Chem. SOC.Faraday Trans. 87, 2283 (1991). 41. P. M. Aker and J. J. Sloan, in Time-Resolved Vibrational Spectroscopy, Springer Proceedings in Physics, Vol. 4, A. Laubereau and M. Stockburger, Eds., Springer Verlag, Berlin, 1985, p. 6; J. J. Sloan, P. M. Aker, and B. I. Niefer, Proc. SPIE 669, 169 (1986). 42. P. M. Aker and J. J. Sloan, J . Chem. Phys. 85, 1412 (1986). 43. P. M. Aker, J. J. A. O’Brien, and J. J. Sloan, J . Chem. Phys. 84, 745 (1986). 44. E. J. Kruus, B. I. Niefer, and J. J. Sloan, J . Chem. Phys. 88, 985 (1988). 45. E. J. Kruus, B. I. Niefer, and J. J. Sloan, J . Photochem. Photobiol A , Chem. 57,419 (1991). 46. D. J. Donaldson and S. R. Leone, Chem. Phys. Lett. 132, 240 (1986). 47. T. R. Fletcher and S. R. Leone, J . Chem. Phys. 88, 4720 (1988). 48. E. L. Woodbridge, T. R. Fletcher, and S. R. Leone, J . Phys. Chem. 92, 5387 (1988). 49. T. R. Fletcher and S. R. Leone, J . Chem. Phys. 90, 871 (1989). 50. E. L. Woodbridge, M. N. R. Ashfold, and S . R. Leone, J . Chem. Phys. 94,4195 (199 1). 51. P. W. Seakins and S . R. Leone, J . Chem. Phys., submitted. 52. J. Connes and P. Connes, J . Opt. SOC.Am. 56, 896 (1966). 53. R. Freeman, A Handbook of Nuclear Magnetic Resonance, Longman, Harlow 1987. 54. M. B. Comisarow, Fourier Transform Ion Cyclotron Resonance Spectroscopy, in Transform Techniques in Chemistry, P. R. Griffiths, Ed., Plenum, New York, 1978. 55. P. R. Griffiths, Chemical Infrared Fourier Transform Spectroscopy, Chemical Analysis, Vol. 43, Wiley, New York, 1975. 56. E. C. Richard, C. T. Wickham-Jones, and V. Vaida, J . Phys. Chem. 93, 6346 (1989). 57. N. Oliphant, A. Lee, P. F. Bernath, and C. R. Brazier, J . Chem. Phys. 92, 2244 (1990). 58. R. P. Wayne, Chem. Br. 23, 440 (1987). 59. J. Chamberlain, The Principles of Interjerometric Spectroscopy, Wiley, Chichester, 1979. 60. R. N. Bracewell, The Fourier Transform and its Applications, McGraw Hill, New York, 1978.

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61. R. J. Bell, Introductory Fourier Thnsform Spectroscopy, Academic, New York, 1972. 62. A. A. Michelson, Phil. Mag. Ser. 5 31, 351 (1891). 63. A. A. Michelson, Phil. Mag. Ser. 5 34, 280 (1892). 64. P. R. Griffiths, H. J. Sloane, and R. W. Hannah, Appl. Spectrosc. 31,485 (1977). 65. J. W. Cooley and J. W. Tukey, Math. Comput. 19, 297 (1965). 66. A. W. Green, I A G A News (Nov. 1984). 67. P. Biggs, F. J. Holdsworth, and R. P. Wayne, J. Phys. E . Sci. Instrum. 20, 1005 (1987). 68. L. Mertz, Infrared Phys. 7, 17 (1967). 69. M. L. Forman, W. H. Steel, and G. A. Vanasse, J. Opt. SOC.Am. 56, 59 (1966). 70. R. E. Murphy and H. Sakai, Proceedings of the Aspen International Conference on Fourier Spectroscopy, G. A. Vanasse, A. T. Stain, D. J. Baker, Eds., Air Force Cambridge Research Laboratory Special Report No. 114, 1970, p. 301. 71. B. D. Green, G. E. Caledonia, R. E. Murphy, and F. X. Robert, J. Chem. Phys. 76, 2441 (1982). 72. J. A. Dodd, S . J. Lipson, D. J. Flanagan, W. A. M. Blumberg, J. C. Person, and B. D. Green, J . Chem. Phys. 94, 4301 (1991). 73. J. A. Dodd, S. J. Lipson, and W. A. M. Blumberg, J . Chem. Phys. 95,5752 (1991). 74. C. J. Manning, R. A. Palmer, and J. L. Chao, Reo. Sci. Instrum. 62, 1219 (1991). 75. M. J. Smith, C. J. Manning, R. A. Palmer, and J. L. Chao, Appl. Spectrosc. 42,546 (1988). 76. R. M. Dittmar, J. L. Chao, and R. A. Palmer, Appl. Spectrosc. 45, 1104 (1991). 77. C. J. Manning, R. M. Dittmar, and R. A. Palmer, Infrared Phys. in press. 78. R. A. Palmer, C. J. Manning, J. L. Chao, I. Noda, A. E. Dowrey, and C. Marcott, Appl. Spectrosc. 45, 12 (1991). 79. R. A. Palmer, C. J. Manning, J. A. Rzepiela, J. M. Widder, and J. L. Chao, Appl. Spectrosc. 43, 193 (1989). 80. V. S . Gregorion, J. L. Chao, H. Toriurni, and R. A. Palmer, Chem. Phys. Lett. 179, 491 (1991). 81. W. Uhmann, A. Becker, C. Taran, and F. Siebert, Appl. Spectrosc. 45,390 (1991). 82. G. Hancock and D. E. Heard, J . Chem. SOC.Faraday Trans. 87, 1039 (1991). 83. G. Hancock and D. E. Heard, J. Chem. SOC.Faraday Trans. 87, 1045 (1991). 84. D. E. Heard, Part I1 Thesis, Oxford University, 1986. 85. D. E. Heard, D. Phil. Thesis, Oxford University, 1990. 86. E. L. Woodbridge, Ph.D. Thesis, University of Colorado, Boulder, 1991. 81. A. W. Mantz, Appl. Spectrosc. 30, 459 (1976). 88. D. A. Mantell, S. B. Ryali, and G. L. Haller, Chem. Phys. Lett. 102, 37 (1983). 89. S. R. Leone, in Dynamics of the Excited State, K. P. Lawley, Ed., Advances in Chemical Physics, Vol. 50, Wiley, New York, 1982, p. 255.

62

G. HANCOCK AND D. E. HEARD

90. K. P. Lawley, Ed., Advances in Chemical Physics, Vol. 60, Wiley, New York, 1985. 91. G. E. Hall and P. L. Houston, Ann. Rev. Phys. Chem. 40, 375 (1989). 92. R. N. Dixon, Acc. Chem. Res. 24, 16 (1991). 93. P. L. Houston, J . Phys. Chem. 91, 5388 (1987). 94. S. W. Novicki and R. Vasudev, J . Chem. Phys. 95, 7269 (1991). 95. C. G. Hickman, A. Bricknell, and J. G. Frey, Chem. Phys. Lett. 185, 101 (1991). 96. D. J. Anderson and R. N. Rosenfield, J . Chem. Phys. 94, 7857 (1991). 97. G. E. Hall, V. Bout, and T. J. Sears, J . Chem. Phys. 94, 4182 (1991). 98. C. E. M. Strauss, S. H. Kable, G. K. Chawla, P. L. Houston, and I. R. Burak, J . Chem. Phys. 94, 1837 (1991). 99. L. Schnieder, W. Meier, K. H. Welge, M. N. R. Ashfold, and C. M. Western, J . Chem. Phys. 92, 7027 (1990). 100. P. T. A. Reilly, Y. Xie, and R. J. Gordon, Chem. Phys. Lett. 178, 511 (1991). 101. R. L. Van der Wal, J. L. Scott, F. F. Crim, K. Weide, and R. Schinke, J . Chem. Phys. 94, 3548 (1991). 102. D. P. Baldwin, M. A. Buntine, and D. W. Chandler, J . Chem. Phys. 93, 6578 (1990). 103. T. Suzuki, V. P. Hradil, S. A. Hewitt, and P. L. Houston, Chem. Phys. Lett. 187, 257 (1991). 104. P. A. Schultz, A. S. Sudber, D. J. Krajnovich, H. S. Kwok, Y. R. Shen, and Y. T. Lee, Ann. Reo. Phys. Chem. 30, 379 (1979). 105. D. S . King, in Dynamics of the Excited State, K. P. Lawley, Ed., Advances in Chemical Physics, Vol. 50, Wiley, New York, 1982, p. 105. 106. P. Dietrich, M. Quack, and G. Seyfang, Chem. Phys. Lett. 167, 535 (1990). 107. E. Zamir and R. D. Levine, Chem. Phys. 52, 253 (1980). 108. R. A. Brownsword, D. G. Weston, and G. Hancock, unpublished results. 109. P. D. Lightfoot, S. Kirwan, and M. J. Pilling, J . Phys. Chem. 92, 4938 (1988). 110. F. M. Lussier, J. L. Steinfeld, and T. F. Deutsch, Chem. Phys. Lett. 58,277 (1978). 111. C. R. Quick, Jr. and C. Wittig, J . Chem. Phys. 72, 1694 (1980). 112. J. M. Zellweger, T. C. Brown, and J. R. Barker, J . Chem. Phys. 83, 6251 (1985). 113. J. M. Zellweger, T. C. Brown, and J. R. Barker, J . Chem. Phys. 83, 6261 (1985). 114. J. E. Butler, D. J. Donaldson, R. G. Macdonald, and J. J. Sloan, Chem. Phys. 95, 183 (1983). 115. S. Klee, K.-H. Gehricke, and F. J. Comes, Chem. Phys. Lett. 118, 530 (1985). 116. S. J. Wategaonkar and D. W. Setser, J . Chem. Phys. 90, 251 (1989). 117. D. Klenerman and I. W. M. Smith, J . Chem. SOC. Faraday Trans. II 83, 243 (1987). 118. D. J. Donaldson and J. J. Sloan, J . Chem. Phys. 77, 4777 (1982). 119. D. J. Donaldson and J. J. Sloan, J . Chem. Phys. 82, 1873 (1985).

TIME-RESOLVED FTIR EMISSION STUDIES

63

D. J. Donaldson, J. J. Sloan, and J. D. Goddard, J . Chem. Phys. 82,4524 (1985). J. J. Sloan and D. G. Watson, J . Chem. Phys. 74, 744 (1981). J. J. Sloan, D. G. Watson, and J. S . Wright, Chem. Phys. 63, 283 (1981). J. J. Sloan, J. M. Williamson, and J. S. Wright, J . Chem. Phys. 75, 1190 (1981). M. G. Moss, J. W. Hudgens, and J. D. McDonald, J . Chem. Phys. 72,3486 (1980). G. Hancock, B. A. Ridley, and I. W. M. Smith, J . Chem. Soc. Faraday Trans. II 68 2117 (1972). 126. I. W. M. Smith, J . Chem. SOC.Faraday Trans. 87, 2271 (1991). 127. T. K. Minton, P. Felder, R. S. Scales, and J. R. Huber, Chem. Phys. Lett. 164, 113 (1989). 128. G. Hancock, G. W. Ketley, and A. J. MacRobert, J . Phys. Chem. 88,2104 (1984). 129. C. Tsai and D. L. McFadden, J . Phys. Chem. 94, 3298 (1990). 130. K. P. Huber and G. Herzberg, in Molecular Spectra and Molecular Structure IK Constants of Diatomic Molecules, Van Nostrand, New York, 1979; R. A. Toth, R. H. Hunt, and E. K . Plyler, J . Mol. Spectrosc. 32, 85 (1969). 131. W. H. Press, P. B. Flannery, S. A. Tenkolsky, and W. T. Vetterling, in Numerical Recipes, The Art of Scientllfic Computing, Cambridge University Press, Cambridge, 1986. 132. D. S. Y. Hsu, R. G. Shortridge, and M. C. Lin, Chem. Phys. 38, 285 (1979). 133. C. J. Astbury, Part I1 Thesis, Oxford University, 1985. 134. D. Bailly, R. Farrenq, G. Guelachvili, and C. Rossetti, J . Mol. Spectrosc. 90, 74 (1981). 135. M. Kori and B. L. Halpern, Chem. Phys. Lett. 110, 223 (1984). 136. G. Hancock and D. E. Heard, J . Photochem. Photobiol. A Chem. 60, 265 (1991). 137. J. T. Muckermann, J . Phys. Chem. 93, 179 (1989). 138. J. S . Francisco and A. Ostefin, J . Phys. Chem. 94, 6337 (1990). 139. G. W. Flynn, Science 246, 1009 (1989). 140. A. J. Sedlacek, R. E. Weston, Jr., and G. W. Flynn, J . Chem. Phys. 94,6483 (1991). 141. J. Z. Chou, S. A. Hewitt, J. F. Herschberger, and G. W. Flynn, J . Chem. Phys. 93, 8474 (1990). 142. J. Z . Chou, S. A. Hewitt, J. F. Herschberger, B. B. Brady, G. B. Spector, L. Chia, and G. W. Flynn, J . Chem. Phys. 91, 5392 (1989). 143. S. A. Hewitt, J. F. Herschberger, J. Z . Chou, G. W. Flynn, and R. E. Weston, Jr., J . Chem. Phys. 93, 4922 (1990). 144. L. Zhu, S. A. Hewitt, and G. W. Flynn, J . Chem. Phys. 94, 4088 (1991). 145. T. G. Kreutz and G. W. Flynn, J . Chem. Phys. 93,452 (1990). 146. F. A. Khan, T. G. Kreutz, G. W. Flynn, and R. E. Weston, Jr., J . Chem. Phys. 92, 4876 (1990). 147. J. F. Herschberger, S. A. Hewitt, S . K. Sarkar, and G. W. Flynn, J . Chem. Phys. 91, 4636 (1989). 120. 121. 122. 123. 124. 125.

64

G. HANCOCK AND D. E. HEARD

148. S . A. Hewitt, L. Zhu, and G. W. Flynn, J . Chem. Phys. 92, 6974 (1990). 149. M. J. Frost, J. S. Salh, and I. W. M. Smith, J . Chem. SOC.Faraday Trans.87,1037 (1991). 150. J. L. Cookson, G. Hancock, and K. G. McKendrick, Ber. Bunsenges. Phys. Chem. 89, 335 (1985). 151. G. Hancock and K. G. McKendrick, Chem. Phys. Lett. 127, 125 (1986). 152. G. Hancock and G. W. Ketley, J . Chem. SOC.Faraday Trans. 2 78, 1283 (1982). 153. R. A. Perry, J . Chem. Phys. 82, 5485 (1985). 154. B. Atakan and J. Wolfrum, Chem. Phys. Lett. 178, 157 (1991). 155. J. A. Miller and C. T. Bowman, Int. J . Chem. Kinet. 23, 289 (1991). 156. W. F. Cooper and J. F. Herschberger, J . Phys. Chem. 96, 771 (1992). 157. I. W. M. Smith and J. F. Warr, J . Chem. SOC.Faraday Trans. 87, 807 (1991). 158. I. W. M. Smith and X. F. Yang, Chem. Phys. Lett. 181, 335 (1991). 159. K. R. Ryan and I. C. Plumb, Plasma Chem. Plasma Proc. 4, 271 (1984). 160. G. Hancock, P. D. Harrison, and A. J. MacRobert, J . Chem. SOC.Faraday Trans. 2 82, 647 (1988). 161. M. I. Buchwald and G. J. Wolga, J . Chem. Phys. 62, 2828 (1975). 162. G. Hancock and I. W. M. Smith, Appl. Opt. 10, 1827 (1971). 163. H. Teitelbaum, P. M. Aker, and J. J. Sloan, Chem. Phys. 119, 79 (1988). 164. H. Hippler, J. Troe, and J. Wendelken, J . Chem. Phys. 78, 6709, 6718 (1983). 165. M. Heymann, H. Hippler, and J. Troe, J . Chem. Phys. 80, 1853 (1984). 166. H. Hippler, L. Kindemann, and J. Troe, J . Chem. Phys. 83, 3906 (1985). 167. H. Hippler, B. Otto, and J. Troe, Ber. Bunsenges. Phys. Chem. 93, 428 (1989). 168. M. Damn, F. Deckert, H. Hippler, and J. Troe, J . Phys. Chem. 95, 2005 (1991). 169. J. R. Barker, J . Phys. Chem. 88, 11 (1984). 170. M. J. Rossi, J. R. Pladziewicz, and J. R. Barker, J . Chem. Phys. 78, 6695 (1983). 171. J. Shi, D. Bernfeld, and J. R. Barker, J . Chem. Phys. 88, 6211 (1988). 172. J. Shi and J. R. Barker, J . Chem. Phys. 88, 6219 (1988). 173. J. R. Barker and R. E. Golden, J . Phys. Chem. 88, 1012 (1984). 174. M. L. Yerram, J. D. Brenner, K. D. King, and J. R. Barker, J . Phys. Chem. 94, 6341 (1990). 175. B. M. Tosselli, J. D. Brenner, M. L. Yerram, W. E. Chin, K. D. King, and J. R. Barker, J . Chem. Phys. 95, 176 (1991). 176. B. M. Tosselli and J. R. Barker, J . Chem. Phys. 95, 8108 (1991). 177. W. Jalenak, R. E. Weston, Jr., T. J. Sears, and G. W. Flynn, J . Chem. Phys. 83, 6049 (1985). 178. H. G . Lohmannsroben and K. Luther, Chem. Phys. Lett. 144, 473 (1988). 179. J. M. Zellweger, T. C. Brown, and J. R. Barker, J . Chem. Phys. 90, 461 (1986). 180. A. L. Malinovsky, E. A. Ryabov, and V. S. Letokhov, Chem. Phys. 139, 229 (1989).

TIME-RESOLVED FTIR EMISSION STUDIES

65

181. J. C. Knights, J. P. M. Schmitt, J. Perrin, and G. Guelachvili, J . Chem. Phys. 76, 3414 (1982). 182. M. Betrencourt, D. Boudjaadar, P. Chollet, G. Guelachvili, and M. MorillonChapey, J . Chem. Phys. 84, 4121 (1986). 183. M. Elhanine, R. Farrenq, and G. Guelachvili, Appl. Opt. 28, 4024 (1989). 184. G. Hancock and J. P. Sucksmith, unpublished results. 185. T. J. Chuang, Phys. Rev. Lett. 42, 815 (1979).

Advances in Photochemistry, Volume18 Edited by ,David H. Volman, George S. Hammond, Douglas C. Neckers Copyright © 1993 John Wiley & Sons, Inc.

A MODEL FOR THE INFLUENCE OF ORGANIZED MEDIA ON PHOTOCHEMICAL REACTIONS V. Ramamurthy Central Research and Development, Experimental Station, The Du Pont Company, Wilmington, Delaware Richard G. Weiss and George S. Hammond Department of Chemistry, Georgetown University, Washington, DC

CONTENTS I. Introduction 11. A brief introduction to organized media A. Organic inclusion hosts B. Silica, clay, and zeolite surfaces C. Micelles, monolayers, and LB films D. Liquid crystals 111. The concept of reaction cavity A. Reaction cavity defined by boundary, size, and shape B. Concept of free volume: Stiff and flexible reaction cavities C. Active and passive reaction cavities D. Microheterogeneity in organized media E. Limitations and conclusions

69 70 70 78 83 86 88 91 96 97 100 103

Advances in Photochemistry, Volume 18, Edited by David Volman, George S. Hammond, and Douglas C . Neckers ISBN 0-471-59133-5 Copyright 1993 by John Wiley & Sons, Inc.

67

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IV. Reaction cavity model applied to examples A. Consequences of an enclosure B. Reaction cavity and free volume C. Location and directionality of free volume D. Magnitude of free volume V. Reaction cavities as “templates” highlighted with examples A. Reaction cavities with “active walls” as templates: Photodimerization B. Reaction cavities with “passive walls” as templates VI. Anisotropy (microheterogeneity) in organized media VII. Norrish I1 photochemistry of ketones in media affording reaction cavities with little or no constraints A. Photophysical characteristics of ketones B. Photoreactions of simple ketones other than Norrish I1 processes C. Norrish I1 processes 1. Medium effects on Norrish I1 reactivity and photoproduct selectivity VIII. Norrish I1 reactions in organized media A. Neat crystalline phases 1. Reaction cavities of alkanones in neat solid phases 2. Reaction cavities of alkanophenones in neat solid and liquid-crystalline phases B. Reaction cavities with very stiff walls and preformed shapes and sizes: Silica gel and zeolites C. Reaction cavities with some wall flexibility: Solid inclusion complexes D. Reaction cavities with strong external medium influences: Cyclodextrin complexes and their aqueous solutions E. Reaction cavities with walls of variable flexibility and strong external medium influences: Aqueous microheterogeneous complexes F. Reaction cavities with walls of variable flexibility 1. Polymer matrices 2. Thermotropic liquid crystalline and solid matrices IX. Perspectives on future research Acknowledgment References

105 105 109 117 126 132 133 150 153 162 163 164 165 169 171 171 173 177 186 195 200 204 210 210 212 220 22 1 22 1

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69

I. INTRODUCTION Two striking biological phenomena which serve as a source of inspiration to photochemists and which illustrate what can be achieved in a confined environment are photosynthesis and visual signal transduction. In the former, the medium (protein) is able to organize a number of active elements so as to predispose them to a desired physical process; in the latter, the medium (rhodopsin) restricts the rotational mobility on certain parts of a single molecule, retinal. Being inspired by and having realized the complexity of natural systems, chemists have utilized a number of organized media to study the photochemical behavior of molecules [l-lo]. Examples of organized media which have been investigated include molecular crystals, inclusion complexes (both in the solid and solution states), liquid crystals, micelles and related assemblies such as vesicles, microemulsions, and membranes, monolayers, Langmuir-Blodgett (LB) films, surfaces (silica, clay, and zeolites) and more recently natural systems such as proteins and DNA [l 1691. These media have been referred to in the literature under various names-organized, confined, constrained, ordered, restricted, microheterogeneous, nonhomogeneous and anisotropic media and nanoscopic reactors. However, most of these media fit only some of the categories listed above. Photochemical and photophysical studies carried out in these media can be classified under the following categories: Influence of media on the photochemistry and photophysics of molecules. Photochemistry and photophysics as a tool to understand the media. Influence of photochemistry and photophysics on the media. Application studies directed toward devices, solar energy, and biological implications. Progress has been made in each category and the results have been the object of a large number of recent reviews and’monographs [l-691. This chapter provides an overview of the activities carried out under the first category and, almost as an inescapable consequence, aspects of the second category are provided. It is not intended to be a summary of the activities in the field. Recent reviews and monographs should be consulted for extensive coverage of the literature [l-691. Kuhn, in his book entitled The Structure of Scientijic Revolutions, points out that in order for science to progress, two types of scientific activities are required [70]. He states, “It is necessary for normal science to be to a large extent uncritical. If all scientists were critical of all parts of the framework in

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V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

which they worked all of the time then no detailed work would every get done.” At the same time, “If all scientists were and remained normal scientists then a particular science would become trapped in a single paradigm and would never progress beyond it.” With these words of wisdom in mind, this chapter attempts to provide a minimal conceptual model for the manner by which organized media influence the photochemical and the photophysical behavior of organic molecules incorporated within them. Concomitantly, we take heed of the words of Chalmers [71] that “a paradigm will always be sufficiently imprecise and open-ended to leave plenty of work to be done.” Our presentation has benefited from the attempts of others to develop unified descriptions for photochemical and photophysical phenomena in organized media [13,72]. A conceptual model which is the centerpiece of this chapter is developed in Section 111. This is preceded (Section 11) by a brief introduction to various organized media. The validity and generality of the model is examined by two approaches. In the first (Sections IV-VI), selected photochemical reactions belonging to various classes and chromophores are presented as supporting examples. In the second (Sections VII and VIII), a critical reevaluation of the results reported on Norrish I1 reactions in a number of organized media is made on the basis of the model. However, although we examined the literature examples on the basis of our model, we often have deviated from the initial explanations offered by the authors.

11. A BRIEF INTRODUCTION TO ORGANIZED MEDIA To appreciate the model discussed in this chapter it is essential to have some knowledge of the characteristics of various organized media. A large number of monographs are available on this topic [73-911 and readers should consult them for detailed understanding. For the sake of completeness, we provide below a very brief summary of the structures and characteristics of the relevant media.

A.

Organic Inclusion Hosts

An inclusion compound is composed of two or more distinct molecules held together by noncovalent forces in a definable structural relationship. Hosts can contain cavities that are rigid or that are developed by reorganization of the hosts during the process of complexation. Inclusion compounds may be subclassified as (1) the true clathrate type in which the guest molecules are

71

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

imprisoned in discrete cavities or cages; (2) the channel type in which the guest molecules are accommodated in continuous canals running through the crystal; or (3) the layer type in which the guest component is situated between bands of the host structure. Urea, deoxycholic acids, tris-ortho-thymotide, Dianin's compound, perhydrotriphenylene, and cyclodextrin are a few examples of organic hosts. Clays and zeolites are a few of the well-known inorganic host systems. The urea inclusion compounds generally crystallize in long, hexagonal prisms or occasionally as hexagonal plates. X-ray structural analysis of the urea-n-hydrocarbon complex shows clearly the existence of a central channel the diameter of which is about 5.3 A (Figure 1). The arrangement of thiourea molecules in rhombohedra1 crystals is similar to that of urea in urea inclusion compounds where the channel size is about 6.1 A. Urea and thiourea channel-type inclusion complexes are generally stabilized by van der Waals forces between the host and the guest and by hydrogen bonding between the host molecules. Deoxycholic acid (DCA), apocholic acid (ACA), and cholic acid (CA) form channel-type inclusion compounds with a wide variety of organic molecules. Of these DCA has been extensively investigated.

HO*'

CA

DCA

ACA

Deoxycholic acid complexes can be grouped into three crystal forms: orthorhombic, tetragonal, and hexagonal. In the more common orthorhombic structures, a two-dimensional bilayer motif with axial dimensions of b 13.6 A and c 7.2 A is observed. These bilayers contain grooves parallel to the c axis which induce DCA to form channel inclusion complexes (Figure 2). These channels have a variable size and shape depending on the mutual positions of two adjacent bilayers along the b axis and this attribute accounts for the ability of the DCA host lattice to accommodate guest molecules of very different dimensions. The dimension (between 2.6 x 6.0 8, and 5 x 7 A) and the length of the channel vary depending on the size of the guest. Perhydrotriphenylene (PHTP) is a chiral molecule. The equatorial isomer of PHTP gives rise to a wide variety of inclusion compounds with different kinds of molecules, ranging from those with a nearly spherical or planar

-

-

-

-

(4

(b)

Figure 1. (a) End-view cross section of the urea-hydrocarbon complex. (b)View of the hexagonal PHTP inclusion compound in the ab plane. PHTP inclusion compounds are composed of infinite stacks of host molecules, repeating at about 4.78 A, parallel to the molecular threefold axes. The regular packing of the stacks gives rise to parallel channels. Channel cross section in both cases is 5 A.

-

(4

(d 1

Figure 2. Deoxycholic acid packing illustrating the flexible size of the channel cross section. Packing viewed along the c axis: (a) no guest, (b) phenanthrene, (c) norbornadiene, and (d)acetone as guests. The channels have variable size and shape depending on the mutual positions along the b axis of two adjacent bilayers. (Reproduced with permission from E. Giglio in Inclusion Compounds, Vol. 2, J. L. Atwood, J. E. D. Davies, and D. D. MacNicol, Eds., Academic Press, New York, 1984, p. 215.) 72

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

73

shape to linear ones. All the investigated adducts have a channel-like structure with the PHTP molecules arranged in infinite stacks whose axes are parallel to the threefold axis of the molecule (Figure 1). The diameter of the channel is about 5 A and it is slightly flexible. Guest molecules are held within the hydrophobic channels through van der Waals interactions.

Dianin’s compound, (1)

PHTP

The general crystal structure of the Dianin’s compound (1) lattice consists of hexamers of Dianin’s compound held together by a ring of hydrogen bonds involving the phenolic hydroxy groups. The monomeric units of the hexamers form chains of hourglass-shaped cages. As illustrated in Figure 3, the upper half of each cage consists of three molecules of 1 from one hexamer, and the lower half is three molecules of 1 from another hexamer. The cage is held together at the roof and floor by the hydrogen bond network and at the waist by van der Waals forces between molecules. These cages are quite large, as

H:.

R

RY+(, R‘

(4

(b)

(4

Figure 3. The shape and size of the cavity/channel of Dianin’s compound. (a) The cage formed by two sets of hexamers. (b)The top or bottom half of the cage, hexamers are held together by hydrogen bonds. (c) Schematic illustration of the cage/channel. The stacking is along the c axis. (Reproduced with permission from D. D. MacNicol in Inclusion Compounds, Vol. 2, J. L. Atwood, J. E. D. Davies, and D. D. MacNicol, Eds., Academic Press, New York, 1984, p. 1.)

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V. RAMAMURTHY, R. G. WEISS AND G. S . HAMMOND

shown in Figure 3, and can accommodate even bulky molecules. Each one of these hourglass-shaped cages is connected on both the top and the bottom to yet another cage and thus this arrangement gives rise to a column consisting of bulged channels. Every column is ideally infinite in extent, runs parallel to the c axis, and is surrounded by (but not interconnected with) six other identical columns related by a threefold screw axis. Tris-ortho-thymotide (TOT) forms two types of crystalline inclusion complexes: in one, the guest molecules are enclosed in discrete closed cavities, in the other, the guest molecules are accommodated in continuous linear channels running through the crystal along a crystallographic axis. Cage-type complexes are formed with guests of length less than -9 A and channel-type structures are formed with long chain-like molecules. The cage in the former type of structure is comprised of eight TOT molecules related pairwise about a crystallographic twofold axis. The average diameter of the ellipsoid cage is about 1 2 A (Figure4). These cages deform to a limited extent to accommodate molecules of different dimensions.

Channel-type structures are formed in the case of trans-stilbene and benzene as guests. In these cases at least two independent channels are

Figure 4. Stereoview of the cage in the TOT complex. [Reproduced with permission from R. Gerdil, Topics Curr. Chem. 140, 71 (1987).]

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75

present, one along the a and the other along the b axis, as illustrated in Figure 5. The channel along the a axis is of fairly uniform cross section, whereas the one along the b axis presents a succession of bulges and constrictions. The TOT cages and channels lack any specific binding sites and the host-guest complexation therefore is essentially controlled by van der Waals’s forces. Cyclodextrins (CD), one of the most commonly used host systems, possess hydrophobic cavities that are able to include, in aqueous solution, a variety of organic molecules. Internal diameters and depths of cyclohexaamylose (aCD), cycloheptaamylose (p-CD), and cyclooctaamylose (y-CD) provide cavities of different sizes (Figure 6). The oligosaccharide ring forms a torus with the primary hydroxyl groups of the glucose residues lying on the narrow end of the torus. The secondary glucopyranose hydroxyl groups are located on the wider end. Inclusion complexes of known ratio can be precipitated from aqueous solutions of C D when an excess of guest is added. Such precipitates contain the guest accommodated within the cavities of CD. In addition to the local structure, the global structure of the solid is determined by how these individual complexes are arranged in the solid state. Based on

(b)

Figure 5. Stereoview of the two channels present in TOT-benzene complex. (a)View along the a axis and (b)viewed down the b axis. One channel runs along the a axis and the other along the b axis. [Reproduced with permission from R. Gerdil, Topics Curr. Chem. 140, 71 (1987))

V. RAMAMURTHY, R. G . WEISS AND G . S. HAMMOND

76

G6

ci,

d

P Y

5.6 6.0

4.2 5.6

0.0 10.8

7.0

0.0

6.0

12.0

7.8

0-5

7.8

Figure 6. Shape and structure of cyclodextrin cavity.

the overall appearance, these are described as cage- or channel-type structures (Figure 7). 1,1,6,6-Tetraphenylhexa-2,4-dyne1,6-diol(2) and 1,6-bis(o-chlorophenyl)1,6-diphenyl-2,4-dyne-1,6-diol (3) function as hosts to a number of organic

2

3

molecules. One of the advantages of using these hosts is the flexible packing they provide. When the size of the guest molecule is on the order of the size of the host molecule, the host molecules accommodate themselves to form a channel. When the guest molecule is smaller than the host, the latter forms a cage with a cross section enclosing the guest molecules in an antiparallel fashion. Two such structures are shown in Figure 8. In both of these structures, hydrogen bonding between the host and the guest plays an important role in guest inclusion. The ability of chiral host 3 to form channel inclusion complexes has been utilized to resolve optical isomers and to conduct asymmetric photochemical transformations.

a

C

Figure 7. Schematic representation of the packing arrangement in cyclodextrin complexes: (a) channel type, (b) cage or herringbone type, and (c) brick type. (Reproduced with permission from W. Sanger in Inclusion Compounds, Vol. 2, J. L. Atwood, J. E. D. Davies, and D. D. MacNicol, Eds., Academic Press, New York, 1984, p. 231.)

Figure 8. A stereoscopic view of the packing in the inclusion compound of 1,1,6,6tetraphenylhexa-2,4-dyne-1,6-diol,2, with (a) chalcone and (b) 9-anthraldehyde. 9Anthraldehyde gives cage-type and chalcone gives channel-type structures. [Reproduced with permission from F. Toda, Topics Curr. Chem. 140,43 (1987).] 77

78

V. RAMAMURTHY. R. G. WEISS AND G. S. HAMMOND

B. Silica, Clay, and Zeolite Surfaces Silica gel and porous silica are rigid three-dimensional networks of silica particles. They are constituted from basic tetrahedral SiO, units. Their porous, sponge-like structure results in large surface areas (100-600 m2 g-') which correspond primarily to areas on the internal pore walls. Pore sizes can range from micropores of < 20 A to macropores of > 2000 A, although values from 20 to 150 A are typical. Porous glass (commonly called porous Vycor) is also structurally similar to the other porous silicas but has a composition of -97% SiOz and -3% B,O,. The surface of porous silica consists of a network of siloxane (Si-0-Si) and silanol (Si-OH) linkages and physically adsorbed water molecules. There are generally 4-5 silanols in each square nanometer of area and these may be isolated, geminal, or vicinal (hydrogen bonded to water or to each other), as illustrated in Figure 9. Surface adsorption can occur via dispersion forces arising from induced dipole interactions, induction forces, and chargetransfer interactions such as hydrogen bonding. Adsorption of most polarizable organic molecules is generally accepted to involve interaction with the surface hydroxyls. It has been suggested that the vicinal silanols provide active sites for adsorption. Adsorbed molecules will usually interact with several surface silanols because of their relatively large size.

H

D

E

Figure 9. Schematic representation of active groups on silica surfaces: (A) isolated silanols; (B) siloxane bonds; (C) geminal silanols; (D) hydrogen-bonded silanols; ( E ) hydrogen-bonded water. (Reproduced with permission from L. Johnston in Photochemistry in Organized and Constrained Media, V. Ramamurthy, Ed., VCH, New York, 1991, p. 359.)

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

79

The term clay generally refers to sheet aluminosilicates with particle sizes in the micron range. These consist of layered structures, some of which are capable of expanding and intercalating a variety of guest molecules (swelling clays are also known as smectites). Clays are composed of two distinct types of layers consisting of [SiOJ tetrahedra and [M(O,OH),] octahedra with M being, in general, A13+, MgZ+,or Clays of principal interest result when one octahedral layer is sandwiched between two tetrahedral layers (Figure 10). In addition to substitution in the octahedral layers, the tetrahedral layers may also be partially substituted by ions of lower valency. Substitution of this type produces a negative charge on the layers which is balanced by interlamellar exchangeable cations. Two of the most commonly used clays are montmorillonite and hectorite (whose synthetic form is known

Al (with some Mg replacement)

Figure 10. Schematic illustration of the structure of a montmorillonite like clay. Two layers of silica-oxygen tetrahedra are joined together by an octahedrally coordinated aluminium-oxygen/hydroxide layer to give an individual clay sheet. (Reproduced with permission from W. Jones in Photochemistry in Organized and Constrained Media, V. Ramamurthy, Ed., VCH, New York, 1991, p. 387.)

80

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

as laponite). In montmorillonite, octahedral substitution consists of A13 and MgZ', whereas in laponite it is primarily Mg2+ (with the charge compensating cation Li'). Clays swell in the presence of water as a result of the increased uptake of interlayer water. To a considerable extent the interlayer water may be replaced by other (neutral) guest molecules (Figure 11). The +

- - - _ _

-

-

-

-

I

ONE, LAYER HYDRATE

I

TWO.LAYER HYDRATE

(4

-I

THREE.LAYER HYDRATE

a

& \

wm (b)

Figure 11. (a) For certain sheet silicates (e.g., montmorillonite), several layers of guest may be intercalated in a gallery region. The spacings illustrate the values obtained with water as the guest molecules. (b) Idealized diagram illustrating the three possible arrangements of intercalated n-hexylamine molecules inside a sheet silicate. (Reproduced with permission from W. Jones in Photochemistry in Organized and Constrained Media, V. Ramamurthy, Ed., VCH, New York, 1991, p. 387.)

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

81

hydrated nature of the interlayer region tends to make the uptake of nonpolar molecules more difficult. However, once an organic “surface” is generated the sorption properties will change. Pyrene, for example, is adsorbed little by a clay until detergent molecules are also sorbed. Zeolites may be regarded as open structures of silica in which aluminum has been substituted in a fraction of the tetrahedral sites. The frameworks thus obtained contain pores, channels, and cages. Substitution of trivalent aluminum ions for a fraction of the tetravalent silicon ions at lattice positions results in a network that bears a net negative charge which must be compensated for by other counter ions. Such ions are mobile and may occupy various exchange sites depending on their radius, charge, and degree of hydration. They can be replaced, to varying degrees, by exchange with other cations. If zeolite water is removed, many other organic and inorganic molecules can be accommodated in the intracrystalline cavities. The topological structure of X- and Y-type zeolites (also known as faujasites) consists of an interconnected three-dimensional network of relatively large spherical cavities termed supercages (diameter of about 13 A; Figure 12). Each supercage is connected tetrahedrally to four other superCATION LOCATION INSIDE FAUJASITE CAGES

Figure 12. Supercage structure, cation location (I, 11,111or 1, 2, 3 ) within X- and Ytype zeolites. Bottom portion shows the reduction in available space (relative) within the supercage as the cation size increases.

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V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

cages through 8-A windows or pores. The interior of zeolites X and Y also contains, in addition to supercages, smaller sodalite cages. The windows to the sodalite cages are too small to allow organic molecules access. Chargecompensating cations present in the internal structure occupy three different positions (Figure 12) in the zeolites X and Y. Only cations of sites I1 and I11 are expected to be readily accessible to the adsorbed organic molecule. Among the medium-pore sized zeolites, perhaps the most studied are the pentasil zeolites, ZSM-5 and ZSM-11 (Figure 13). These zeolites also have three-dimensional pore structures; a major difference between the pentasil pore structures and the faujasites described above is that the pentasil pores do not link cage structures as such. Instead, the pentasils are composed of two intersecting channel systems. For ZSM-5, one system consists of straight channels with a free diameter of about 5.4 x 5.6 8, and the other consists of sinusoidal channels with a free diameter of about 5.1 x 5.5 di. For ZSM-11, both are straight channels with dimensions of about 5.3 x 5.4 8,. The volume at the intersections of these channels is estimated to be 370A3 for a free

/

5.1 x 5.5

A

(b)

Figure 13. Channel structure of ZSM-5. Note the presence of two types of channels, dg-zag and straight. (A)Viewed through the straight channel. (B) Schematic drawings of both zig-zag and straight channels; channel dimensions are noted.

83

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

diameter of about 8.9 A. Other zeolites of interest for photochemical studies include the LZ-L, mordenite, offretite, omega, and beta (Table 1).

C. Micelles, Monolayers, and LB films The term micelle denotes an assembly formed by aggregation of surfactant molecules, consisting of long hydrocarbon chains and ionic or nonionic polar head group, in water. Hydrophobic repulsion between the hydrocarbon chain and water is responsible for the aggregation of surfactant molecules in water. There is a concentration value (critical micelle concentration, CMC) below which aggregation of surfactant molecules is absent or occurs among a few molecules (premicellar aggregation) and above which association leads to TABLE 1 Size of Pore Openings and Dimensionality of the Pore System for Selected Medium- and Large-Pore Molecular Sieves'

Molecular Sieve Name Faujasite (X and Y type) Omega Linde Type L Mordenite Offretite ZSM-34 ZSM-11 ZSM-5 Theta-1 4A

Pore (Window) Size

(A)

Channel/Cage Size

-

Three-dimensional channel with a cage (d 12A) Two noninterconnected 7.5 (3.4 x 5.6) channels Single channel with a lobe 7.1 (d 7.5A) 7.0 x 6.7 and (2.6 x 5.7) Two interconnected channels Two interconnected 6.7 and (3.6 x 4.9) channels Two interconnected 6.7 and (3.6 x 4.9) channels Two interconnected 5.3 x 5.4 channels 5.3 x 5.6 and 5.1 x 5.5 Two interconnected channels Single channel 4.4 x 5.5 4.2 Three-dimensional channel with a cage (d 12A) 7.4

-

-

"W. M. Meier, and D. H . Olson, in Atlas of Zeolite Structure Qpes, 2nd ed., Butterworths, Cambridge, 1987; D. W. Breck, Zeolite Molecular Sieoes: Structure, Chemistry, and Use, Wiley, New York, 1974.

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V. RAMAMURTHY, R. G. WEBS A N D G. S. HAMMOND

micelles. The number of detergent molecules that aggregate to form micelles is called the aggregation number. Structure, aggregation number, and CMC are all dependent on the nature of the hydrocarbon chain, head groups, counterions, temperature, and added electrolytes. The currently accepted model of an ionic micelle consists of a core where the radius approximates the length of the fully extended alkyl chain of the detergents (15-30 A), surrounded by the Stern layer containing water, head groups, and more than half of the counter ions and Gouy-Chapman layer, extending to several hundred angstroms, containing water and remaining counter ions (Figure 14). Micelles are dynamic in nature and exist in equilibrium with monomers and other forms of aggregates. Organic substrates, owing to their hydrophobicity, tend to be solubilized inside micelles which provide a restricted hydrophobic space in an aqueous environment. The site of solubilization may be either the micellar interior or the micellar surface. Surfactants commonly used to form micelles are sodium dodecyl sulfate (SDS),cetyl trimethylammonium chloride (CTAC), cetyl trimethylammonium bromide (CTAB), and Triton X-100. Some amphiphilic molecules such as oleic acid and hexadecyl alcohol containing an alkyl chain and a polar head group form monolayers on the surface of water. The polar head groups of these molecules are attracted to and are in contact with water while their hydrocarbon tails protrude above it (Figure 15). The term monolayer implies the presence of a uniform monomolecular film on the surface of water. Monolayer films can be classified as gaseous, liquid, or solid depending upon the degree of compression and the effective area per molecule. Clearly the liquid phase of a monolayer film and, more so, the solid represent constrained environments for individual molecules of amphiphiles. Monolayers, just like micelles, are dynamic species.

ebon 'tor

exta: r i o r

-+-+-+*+Micellar c o r e Gouy-Olaprnan layer

k

r

n layer

Figure 14. A cartoon representation of the structure of a micelle.

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

85

X. Deposition

Y . Deposition

Z - Deposition

(b) Figure 15. ( a ) A schematic representation of organic amphiphilic molecules at airwater interface. (b) Schematic representation of the various deposition modes for monolayer films and the resulting L-B assemblies. (Reproduced with permission from H. Kuhn, D. Mobius, and H. Bucher, Physical Methods of Chemistry, Vol. I, Part IIIB, A. Weissberger and B. W. Rossiter, Eds., Wiley, New York, 1972, p. 577.)

Stepwise transfer of a monolayer film to a rigid support results in a Langmuir-Blodgett (L-B) assembly (Figure 15). Transfer can be accomplished only for those monolayer films that compress to a tightly packed solid phase. Although it is frequently assumed that the transfer of a monolayer film from the air-water interface to a rigid support is accomplished with minimal reorganization of the molecular arrangement of the monolayer film, this is certainly not always the case. In L-B assemblies, molecules are held in a rigid and well-defined geometry. Since the monolayer deposition occurs in steps, the interlayer distances may be controlled by varying the number of spacer layers, the surfactant chain length within spacer layers, or the mode of deposition. One of the commonly used spacers is arachidic acid.

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V. RAMAMURTHY, R. G . WEISS A N D G. S. HAMMOND

D. Liquid Crystals Liquid crystals, as the name implies, are condensed phases in which molecules are neither isotropically oriented with respect to one another nor packed with as high a degree of order as crystals: they can be made to flow like liquids but retain some of the intermolecular and intramolecular order of crystals (i.e., they are mesomorphic). Two basic types of liquid crystals are known: lyotropic, which are usually formed by surfactants in the presence of a second component, frequently water, and thermotropic, which are formed by organic molecules. The thermotropic liquid-crystalline phases are emphasized here; they exist within well-defined ranges of temperature, pressure, and composition. Outside these bounds, the phase may be isotropic (at higher temperatures), crystalline (at lower temperatures), or another type of liquid crystal. Liquid-crystalline phases may be thermodynamically stable (enantiotropic) or unstable (monotropic). Because of their thermodynamic instability, the period during which monotropic phases retain their mesomorphic properties cannot be predicted accurately. For this reason it is advantageous to perform photochemical reactions in enantiotropic liquid crystals. Thermotropic liquid-crystalline phases can be grouped according to their microscopic organization into four major classes: nematics, smectics, cholesterics, and discotics (Figure 16). Molecules forming nematic, smectic, and cholesteric phases are typically rod-like. They usually consist of at least one rigid group and at least one flexible chain at a molecular extremity. Plate-like molecules (or molecular aggregates that adopt this shape) are found in discotic phases that include a rigid central core from which several flexible chains emanate. The structures and acronyms for some molecules that form liquid crystalline phases are shown below.

BS

n-Butylstearate (hexatic B phase)

BCCN

(stnectic and nematic phases) OR I

Cholesteryl chloride (cholesteric phase)

I

OR

(discotic phase)

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

87

The directors (long molecular axes) of the constituent molecules in nematic phases are parallel to one another on average. This is the only order present in nematic liquid crystals, which are the most fluid type of liquid-crystalline phase. Molecules that form cholesteric phases must be optically active or contain an optically active dopant. As the phase name implies, the constituent molecules are frequently steroids and most commonly are cholesteric esters or halides. A conceptual model of the cholesteric phase includes “layers” of molecules in nematic-like positions, each layer being twisted slightly with respect to the ones above and below it. When the phase consists only of optically active molecules, the angle of twist between “layers” is typically less than one degree. Several subclasses of discotic phases exist. In all, the molecular planes of the constituent molecules are parallel. However, the discs can pack in nematic-like arrangements (N,) or in columns that are internally ordered (Do) or disordered (Dd)and may be stacked vertically,

nematic

cholesteric

columnar discotic

Srnecljc A structure

Snit.Lltc C structure

Figure 16. Idealized cartoon representation of the molecular shapes and orientations of the major liquid-crystalline phase types. (Reproduced with permission from R. G. Weiss in Photochemistr>y in organized and constrained media, V. Ramamurthy, ed., VCH, New York, 1991, p. 603).

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V. RAMAMURTHY, R. G.WEISS AND G. S. HAMMOND

tilted, or twisted. In smectic phases, the constituent molecules are arranged in layers with their long axes parallel to one another. A number of subclassifications of smectics are known. Each differs from the others based on the packing arrangement of molecules within a layer (hexagonal, orthorhombic, nematic, etc.) and the angle of tilt between the long molecular axes and the layer plane (Figure 16). Two of the most common ones are smectic A and smectic B types. In these, the long molecular axes of the constituent molecules are in a perpendicular orientation with respect to the layer plane.

111. THE CONCEPT OF REACTION CAVITY We believe that the manner in which organized media, in general, control or modify the reaction course followed by organic molecules included in them can be viewed on the basis of a simple qualitative model. One can envision reactions taking place in an organized medium as occurring within enclosed surroundings. This enclosed space we call the “reaction cavity.” Semantically “enclosed” suggests that there is a physical boundary around the guest reactants, and in this sense it may be a misleading term. One should think of a reaction cavity as the space in which excited state reactant molecules and their preproduct intermediates are confined (restricted or mobilized) during their lifetimes. Unlike isotropic solution media where molecules have limited but equal mobility and conformational flexibility in all dimensions, in an organized medium their mobility and flexibility is restricted or constrained in one if not all three dimensions. The dimension or dimensions along which translational and conformational changes of the reactant molecules are constrained determine the ability of the reaction cavity to influence reactivity. Terms such as “restriction” and “confinement” are integral parts of the description of a reaction cavity. This description is elaborated below with an idealized model shown in Figure 17. Imagine a molecule tightly enclosed within a cube (model 10). Under such conditions, its translational mobility is restricted in all three dimensions. The extent of restrictions experienced by the molecule will decrease as the walls of the enclosure are removed one at a time, eventually reaching a situation where there is no restriction to motion in any direction (i.e., the gas phase; model 1). However, other cases can be conceived for a reaction cavity which do not enforce spatial restrictions upon the shape changes suffered by a guest molecule as it proceeds to products. These correspond to various sit ations in isotropic solutions with low viscosities. \ We term all models in Figure 17 except the first as “reaction cavities” even

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

Model

89

Equivalent organized spaces

1 No Enclosure

2

Single plane

3

Two parallel planes

4.

Two perpendicular planes

5

Three adjacent sides oDen cubes

6.

Three adjacent sides open cubes

7.

Two adjacent sides open cubes

8

Two opposite sides open cubes

9.

One side oDen cube

Cavitands

10

Closed cube

Crystals, Zeolite cages, TOT cages

Silica surface air-water interface Clays, Liquid crystals

Zeolite, Urea channels, Cyclodextrin Liquid crystals

Figure 17. An idealized representation of a cubic model. Restriction imposed on a ball trapped within a cube with different faces lacking. In models 1-10 various faces of the cube are absent. Such an absence increases the freedom the trapped ball experiences as one moves from model 10 to 2. The similarity between this situation and a guest within a restricted space is highlighted.

though the dimensionality of their inhibition of guest motions differ. Although it is difficult to find examples for all the models, several can be related to the reaction cavities afforded by the organized media discussed here; they are as noted. Reaction cavities of some media, like micelles and liquid crystals, may be categorized more precisely as model 2 or 10 depending upon the nature of the motions required to effect the reaction in question. Thus, to which class a medium belongs will be a function of the guest molecule and the nature of the reaction it undergoes. According to this model, the restrictionJconfineVentJinhibitionof motion experienced by the guest depends on the particulat case and may be usefully represented in terms

90

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

of angstroms (one dimension inhibited), square angstroms (two dimensions inhibited) or cubic angstroms (three dimensions inhibited). The term “reaction cavity” was originally used by Cohen to describe reactions in crystals [13]. He identified the reaction cavity as the space occupied by the reacting partners in crystals and used this model to provide a deeper understanding of the topochemical control of their reactions. Selectivity seen for reactions in crystals, according to this model, arises because of lattice restraints on the motions of the atoms in reactant molecules within the reaction cavity. In other words, severe distortion of the reaction cavity will not be tolerated and only reactions that proceed without much distortion of the cavity are allowed in a crystal (Figure 18). Crystals possess timeindependent structures; the atoms that form the walls of the reaction cavity are fairly rigid and exhibit only limited motions (e.g., lattice vibrational modes) during the time periods necessary to convert excited state molecules to their photoproducts. Therefore, in the Cohen model, the space required to accommodate the displacement of reactant atoms from their original positions during a chemical reaction must be built largely into the reaction cavity. Packing of polyatomic molecules in crystals leaves some distances between neighboring nonbonded atoms greater than the sum of their van der Waals radii. This creates a certain amount of free volume which may be so disposed as to allow the atomic motions required to effect a reaction. In the usual case, a reaction product will also place some stress on the host crystal, as is evidenced crystals such as those studied by Schmidt and Cohen, which are usually reduced to powders as the reaction progresses [l]. Can we extend the reaction cavity concept, which emphasizes the shape

REACTANTS

TRANSITION STATE

---

PRODUCTS

Figure 18. The reaction cavity model as presented by Cohen. Reaction cavity before and after the reaction is shown as full lines. Transition state requirements for a reaction are shown as broken lines. Case I represents a favorable and case I1 an unfavorable reaction. [Reproduced with permission from M. D. Cohen, Angew. Chem. Int. Ed. Eng. 14, 386 (1975).]

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

91

changes that occur as the reactant guest transforms itself to the product, so that we can understand and predict the photobehavior of guest molecules included in organized media in general? We believe that such an extension should be possible with some limitations. The concept of the reaction cavity will serve well as a vehicle for the discussion of results obtained in media in which organized structures of hosts have significant effects on the photochemical response to excitation of guests. The cavities may change their size and shape over time periods which may or may not be important depending on the transition times involved in the reactions. Reaction cavities will also vary in the stiffness and chemical activity of their walls. Finally, the relative sizes and functionalities of the guest molecule and its reaction cavity can influence the model of Figure 17 into which a system will fall.

A. Reaction Cavity Defined by Boundary, Size, and Shape The boundaries of the enclosed space (reaction cavity) in an organized medium may be physically realistic as in the case of some zeolites or may be more nebulous and transitory as in the case of liquid crystals. Although the presence of an enclosure comprised of a network of atoms or molecules around reactant molecules is common in all condensed media, the size, shape, and nature of the reaction cavity differs significantly from one medium to the other. Such factors, we believe, are responsible in large part for the different influences exerted by organized media on the course of reactions of included guest molecules. The size of the reaction cavity may vary from being very small (as in crystals, cyclodextrins, etc.) to being potentially very large (as in micelles) or unbounded in at least one dimension (as with surfaces of silica). The exact size of the enclosure can be estimated easily for systems such as crystals and some inclusion complexes where the guest molecules are totally immobilized within a single site (model 10). Unlike these systems, wherein individual reaction cavities are physically separated and the migration of reactant molecules from one reaction cavity to any other is arrested, the regions in which potential reactant molecules are accommodated on surfaces of silica, between the layers of clay, or within zeolites are interconnected and migration from one region to the other may not be inhibited (models 2-9). When guest molecules are able to explore more space during their transformation to products than is available in the cavity in which they are accommodated at the time of excitation (initial reaction cavity),their behavior may depend upon the “effective space” explored. The effective reaction cavity, the space explored, will depend on the lifetime of the excited state, the nature of the mobility, and the structure of the guest molecule and the intermediate(s) derived therefrom. The initial and effective reaction cavity

I

92

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

sizes and shapes for molecules included within spaces corresponding to model 10 are expected to be the same if the volume of the guest molecule and its cavity are similar; this may not be so in micelles, for instance, which provide a potential volume for reaction which exceeds the size of most organic molecules. However, when the molecules are included in a cavity/cage/surface which is continuous through interconnections, such as those corresponding to models 2-9, the initial and effective reaction cavity sizes and shapes need not be the same. Since the effective reaction cavity size depends on the structure and on the lifetime of the species involved, these sizes may be different for the excited state of guest molecules and the intermediates derived from them. Also, two intermediates with different structures and lifetimes may have different effective reaction cavity sizes. Consider a guest molecule within the supercages of a faujasite. In the absence of mobility, the initial enclosure size would be the size of the supercage (i.e., -800A3). Since the supercages in X and Y zeolites are interconnected, the guest molecules may migrate from one cage to the other. Assuming that the guest is able to scan n cages within its lifetime, the effective enclosure size would be n x 800 A3, where the value of n depends upon the diffusion rates and the lifetimes of the migrating transients (e.g., the electronically excited state of guest molecules and reaction intermediates derived from them). For instance, a molecule with a long excited state lifetime and a high diffusion rate might travel through a number of supercages giving it a very large effective reaction cavity. This is illustrated in Figure 19. Consider another example of a molecule adsorbed in a pore of diameter 100 A on a silica surface. The “enclosure” in which the molecule resides may be thought of as a cylinder with lo4 A2 cross-sectional area and undefined height. Our model would indicate that the volume of the cavity is potentially infinite since there is no boundary in one dimension. However, many molecules adsorbed on silica exhibit photochemical behavior far different from that found in isotropic liquids [31,46,50]. In fact, molecules adsorbed on silica are not able to explore the full extent of their reaction cavity since they are tethered to the surface of the pore through strong interactions; translational motion in the open dimension is restricted. For the same reason, we cannot assume that an electronically excited guest molecule can freely explore even lo4 A2 crosssectional area. However, we can assert that diffusion and changes in molecular structure in these two dimensions are definitely limited. An illustration of the different space explored by two radicals generated from the same reactant is provided by the photobehavior of l-phenyl-3-(0tolyl)propan-2-one (4) adsorbed onto zeolite ZSM-5 [42]. ZSM-5 is a narrow pore zeolite which is known to admit p-xylene but not o-xylene. On this basis, 4 is expected to adsorb only on the outer surface of ZSM-5 (Figure 20). Upon photolysis, of the two radicals produced, benzyl (A) and 0-xylyl (B), the latter

-

-

-

-

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

6 a

b

c

o* a

b

I

c

d

e

93

a, b, c and d correspond

to different interconnected reaction cavities.

hu

d

e

Excited guest, confined to cvaity 'a' upon excitation, may travel between cages

'a' and Id'.

Excited guest reactant

h*

a

b

0 Reactant

c

I

d

e

t ran s fo r ins to products wlien located in cage Id'.

U Product

a, b, c, d, e correspond to the cavity volumes of various cages. Initial Reaction Cavity = a Effective Reaction Cavity = a+b+c+d Final Reaction Cavity = d

Figure 19. An illustration of three possible reaction cavities as the reaction occursinitial, effective, and final reaction cavities.

is too bulky to enter the channels of ZSM-5. Therefore, the effective space explored is expected to be different for the two radicals A and B; radical A will remain essentially on the outer surface and radical B can also explore the interior of the zeolite. The actual difference is reflected in the product

4

94

V. RAMAMURTHY, R. G. WEISS A N D G. S. HAMMOND

-

I -co

L

AA

LL-

A -

-A

suc

Figure 20. Different spaces explored by two radicals of different sizes, generated from the same molecule. Because of its large size, radical A stays on the surface of ZSM-5 whereas radical B enters the channels. Products formed are controlled by the different

spaces explored by the two radicals.

distribution of the photolysis. While a statistical mixture of AA, AB, and BB is obtained in solution, only AA and BB are formed in ZSM-5. Our concept of a reaction cavity in organized media has been considerably modified from that of Cohen [13]. It requires the inclusion of more factors to be used effectively, but it provides a base for discussion of a myriad of reaction environments. It is very important to note that the exact size and shape of a reaction cavity (initial, effective, and Jinal) that control the excited state behavior of guest reactants will depend on the particular reaction as well as on the guest and intermediate(s) themselves. Whether the information regarding the space explored (effective reaction cavity) by the excited molecule will be registered in the distribution or stereochemistry of the products will depend on the nature of the mechanism involved in the product formation. In some cases, explorations over a larger space by excited state species and their intermediates may not be germane to the distribution and types of products formed. In certain cases, especially those that involve the probability of encounters, all of the space excited molecules and their intermediates explore before they yield final products may be important. In cases for which the distribution of specific product types is being probed, only the site in which

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

95

decay of the final intermediates to products occurs may define the reaction cavity. If this reaction cavity is different from the initial one, the space within which the final reaction occurs we call final reaction cavity. An illustrative example of this phenomenon is provided by the photobehavior of a-alkyl dibenzylketones, 5, included in the supercages of faujasites (Scheme 1; for details see Section 1V.D)[92,93]. Irradiation of these ketones results in a-cleavage (in addition to y-hydrogen abstraction). Subsequent decarbonylation will yield the geminate radicals A and B within a supercage. Recombination of radicals A and A can yield only coupling products (AA), whereas recombination of A and B or B and B can yield both coupling (AA, AB, and BB) and disproportionation products, toluene, and olefins (Scheme 1).The ratio of total products (coupling and disproportionation products) obtained by encounters of A and A, A and B, and B and B will depend upon the total space explored by A and B. That is, the effective reaction cavity for encounter of radical pairs is a function of the total volume explored by the two radicals from the moment of their creation until they combine. O n the other hand, the ratio of coupling to disproportionation products from each of the A and B and B and B encounter partners will not depend on the total volume explored by the two radicals. The specific nature of the products from those encounters will depend only upon the nature of the enclosure which contains them at the moment when they establish ultimate contact and collapse to even electron species (final reaction cavity). It is clear from the example given above that the cage effect, which is a measure of products from the combination of A and A, B and B, and A and B, provides information regarding the space the individual radicals A and B explored from the time of their inception to the time of their destruction, whereas the individual products obtained from the combination of either A and B or B and B do not carry the memory of their path into the final products.

Scheme 1.

%

V. RAMAMURTHY, R. G . WEISS AND G. S. HAMMOND

In this example, we have assumed that the cages in which the two radicals are generated and finally collapse to products have the same size and shape. In some media, the shape and size of individual sites may differ (see Section D) so that predictions concerning the excited state behavior of guest molecules require knowledge only of the characteristics of the final reaction cavity. Thus, we envision three possible types of reaction cavities (Figure 19): the initial reaction cavity, defined by the space in which the excited states of reacting molecules are generated; the effective reaction cavity, which encompasses the space the excited states and the intermediates explore from the time of their inception to the moment of their final product formation, and the jinal reaction cavity, which includes only the space in which the product determining steps occur. Whether they are different depends upon the nature of the guest molecule, its photochemical reactions, and the medium in which the transformation occurs.

B. Concept of Free Volume: Stiff and Flexible Reaction Cavities The concept of free volume has been introduced into the reaction cavity model to accommodate the shape changes that occur as the reactants transform themselves to products. Free volume exists in all condensed media, with the possible exception of perfect crystals of monatomic solids. Normally, there is more free volume in a liquid than in the crystalline state of the same substance, as evidenced by the decrease in density of most solids on melting (with ice as a familiar exception). In isotropic liquids, free volume is highly mobile because it wanders about by virtue of motions (translation, rotation, vibration, internal rotation, etc.) of the molecules that constitute the liquid. In organized media, the free volume may be essentially immobile, as in crystals, or have mobility ranging from that of crystals to that of isotropic liquids. The motions of potentially reactive molecules will be subject to gross one-, two-, or three-dimensional constraints as discussed above (see also Figure 17), to the fluctuating availability of appropriately disposed free volume and to the flexibility (response to stress) of the constraining structures. As indicated in Figures 1-7, there exists a well-defined free space in a number of host systems where the guest molecules can be accommodated. In the case of inclusion complexes, depending on the size of the guest molecule, there will be some free volume within the host channel/cage/cavity. Since surfaces of silica, the interplanar regions of clay, and the interiors of zeolites, possess timeindependent structures like crystalline materials (i.e., their relaxation times are much longer than the periods necessary to transform a reactant molecule to its products), the free volume needed to accommodate shape changes which occur during the course of a reaction must be present intrinsically within the fixed structure. Reaction cavities of such media possess stif walls.

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

97

Distinct differences exist between the rigidly organized structures discussed above and media such as micelles, microemulsions, molecular aggregates, and liquid crystals. In the latter, guest reaction cavities may contain minimal intrinsic free volume at the time of photoexcitation. However, since the molecules that constitute these organized assemblies are mobile, the reaction cavity can respond to shape changes as the reaction occurs. How much and how readily each medium accommodates shape changes is an important determinant of the selectivity obtained. In our nomenclature, these media possess Jlexible reaction cavities. How easily a medium responds to shape changes that occur during the course of a reaction depends on the microviscosity of the medium and on the extent of cooperative motions involving guest and host molecules. For a guest molecule to react in the environment provided by a restrictive host, the medium must make necessary adjustments within the time frame determined by the rate-limiting spatial decay paths leading to the preproduct transition state@). If the host medium cannot respond in a “timely” fashion, no reaction will occur. The time scale for common photochemical and photophysical processes spans many orders of to 10 s. The dynamics of significant magnitude, from approximately relative movement of the various hosts also span a very wide time range. Thus the boundaries of the enclosures in organized media may be of two types: they may be stty (i.e., none of the guest molecules can diffuse out and the walls do not bend), as in the case of crystals and some inclusion complexes, or Jlexible (i.e., some of the guest molecules may exit the cavity and the walls of the cavity are sufficiently mobile to allow considerable internal motion of the enclosed molecules), as in the case of micelles and liquid crystals. In these two extremes, free volume needed for a reaction is intrinsic (built into the reaction cavity) and latent (can be provided on demand).

C. Active and Passive Reaction Cavities In discussions to this point, no significant interaction between a guest and its medium has been considered. This is probably the case in the reaction cavity model of Cohen [13] as well, since product selectivity was attributed mainly to the presence or absence of free volume within the cavity. The analogy of guests in hosts to balls in boxes is very deficient, but is really not different from the situation in the kinds of crystal systems which first inspired the Cohen nomenclature. Interatomic attraction and repulsion was important in analyzing those systems and was even critical to the “crystal engineering” used to assemble some of the systems used in the studies by Schmidt and his co-workers [l, 48,891. In addition to being stiff or flexible, cavity walls must

98

V. RAMAMURTHY, R. G. WEISS AND G. S . HAMMOND

be characterized as active or passive (i.e., are the wall interactions with guests attractive, repulsive, or benign?) (Figure 21). When the interaction between a guest molecule and a wall of the cavity is attractive or repulsive, the cavity wall is termed active and it may serve as a template for the guest as it proceeds to products; when there is no significant interaction it is considered to be passive. There will always be a net attractive action between guest and host if the system is at equilibrium, or the guest would not be there! However, different parts of a guest molecule may be strongly bound to some parts of the host and other parts of the two may be repulsive. When the walls of the cavity are passive, predictions concerning the excited state behavior of guest molecules can be made on the basis of size, shape, and flexibility of the reaction cavity. Active walls require possible wall-guest interactions to be considered also. The walls of some active cavities bear functional groups which are inhomogeneously distributed and which interact noncovalently with specific functional groups of guest molecules and their intermediates. If sufficiently attractive or repulsive, the interactions will influence the locations or conformations of guest molecules in the cavity. Additionally, the interactions must persist for times at least comparable to those required for the reaction to occur if they are to have a discernible influence on the course of the transformations. Although it is easy to recognize the presence of specific binding or repulsion between the ground state guest molecules and host framework, it is important to note that new potentially stronger or weaker interactions may develop between the walls and functional groups created in intermediates during the course of a reaction. Interactions may vary from weak van der Waal’s forces to hydrogen bonds to strong electrostatic forces

Active Walls

Passive Walls

Figure 21. A cartoon representation of active and passive walls of a reaction cavity. Active wall is characterized by inter guest-wall interactions. Such is absent in the case of passive walls.

99

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

between charged centers. For example, a number of hosts capable of forming inclusion complexes (such as cyclodextrin, urea, and deoxycholic acid) possess functional groups which can form hydrogen bonds with guest molecules. Silica surfaces possess silanol groups which may orient the adsorbed molecules through hydrogen bonding. Surfaces of clays and zeolites often carry a large number of cations that can interact electrostatically with guests. Micellar, monolayer, and related assemblies have interfaces at which guest molecules can be oriented through hydrophobic-hydrophilic interactions. Most reaction cavity walls may in fact be active to some extent. As an illustration of the phenomena involved, consider the photocycloaddition of fumaronitrile to 5-X adamantanone, 6 (Scheme 2; X = F, C1, Br, OH, Ph, or t-Bu) [94]. In isotropic solvents, different quantities of adducts to the two carbonyl faces are formed. When 6 is complexed by p-CD, the intrinsically more reactive face of the carbonyl group becomes more hindered toward attack by fumaronitrile than the less reactive one. As a result of this attractive interaction (NB, hydrogen bonding between the carbonyl oxygen and a hydroxyl on the p-CD torus), the distribution of photoadducts is reversed. In this example, p-CD serves the function of a reaction cavity with “active” walls ke., a template). N

X

6

5 - X . Anti

5-X-AD X= F. CI.

N

Br

5-

(favored in solution)

x - Syn

(favored in p-CD)

\

\

m-

-&-0

I1

X

In Solution

In p-CD Solution

Scheme 2.

100

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

D. Microheterogeneity in Organized Media In most noncrystalline phases, guest molecules reside in a distribution of reaction cavity types. Fluid solutions allow reacting molecules to experience an average microenvironment by virtue of fast relaxation time of the solvent and/or high mobility of the reactant molecules. However, in organized and very viscous media, solvent relaxation and rates of guest diffusion may be slower than the time period of a photoreaction, leading to reaction occurring in a variety of reaction cavities (“sites”).On the other hand, even if there is no migration of molecules between sites, one could imagine a situation in which all reacting molecules experience identical environments because all sites are equivalent. However, this is not expected to be the case in most organized media. Even perfect crystals have at least two types of reaction cavities corresponding to molecules in the interior and on the surface. Normally, crystals have defects wherein the molecules may react still differently. Diffusion of molecules among these site types is not expected to occur as rapidly as single molecules are transformed to photoproducts; from a mechanistic point of view at least three different reaction cavity types should be expected in crystalline media. By contrast, interconversion between molecules present at interfacial and interior sites of a micelle may be comparable to the time scale of a photoreaction. Generally, the relationship between the rates of hopping by molecules among various sites and the rates of photoreactions will determine the importance of multiple sites on the transformation of single molecules and, therefore, the selectivity of their product distribution. Scheme 3 summarizes this problem with a minimum number of sites and competing processes. In this scheme, two sites, square-well type (X) and spherical-well type (Y), are available for the residence of reactant molecules (A). For the sake of convenience, molecules residing at sites X and Y are labeled A, and A,. Excitation of these molecules gives rise to A; and A t . Photoreactivity of molecules excited in each site will be identical if they equilibrate between X and Y before becoming photoproducts. In media with time-independent structures, such as crystals, equilibration requires diffusion of molecules of A; in media with time-dependent structures, such as micelles and liquid crystals, equilibration can be accomplished via fluctuations in the microstructure of the reaction cavities as well as translational motion of A (Scheme 4). An additional mechanism for site selective reactions or equilibration of A; and A t molecules can be achieved via energy migration (e.g., energy hopping, exciton migration, or Forster energy transfer). Three limiting dynamic situations can be envisioned in Scheme 3: (1) the rate of excited state reaction is slower than the rate of interconversion between A; and A? (equilibrium is established between A; and A t before

lhv k*,,

1 op’

-

lhu

-uScheme 3.

(I) Translational of guest from site X to site Y

(Ii) Fluctuationsof the media (Time dependent structures-shapes of sites change with ttme)

lJ-(J--QJlJx

*J-f

Y

-

(Hi) Energy transfer between molecules present at sites X and Y

Scheme 4.

101

102

V. RAMAMURTHY, R. G. WEISS A N D G. S . H A M M O N D

decay; kfy and k& >> k;, and k;,); (2) the rate of excited state reaction is faster than the rate of interconversion between A$ and A;: (3) the rate of interconversion between the Af and A: and the rates of reaction (decay) of excited guest molecules at these sites are comparable (assume for instance, k;, >> ktx and kfY >> k;, and k:,), Mechanism 1 corresponds to the situation in a fluid isotropic solution where a single effective reactive cavity should exist. It should also apply to micellar solution. Under such conditions single exponential decay would be expected for the guest excited states and the photoreactivity would be predictable on the basis of a single effective reaction cavity. In mechanism 2 there should be two kinetically distinct A* in two noninterconverting sites. Double exponential decay will be expected for the excited state of A. The quantum efficiency of product formation and the product distribution may depend upon the percent conversion. An elegant example of mechanism 2 is available from the studies of Pokkuluri et al. in crystalline media (Scheme 5) [95]. The ratio of products 8,9, and 10 from 7 depends on the crystal size (i.e., the ratio of the number of molecules present in the bulk to those on the surface). According to mechanism 3, even if the concentration of A in one of the two sites (Y) is much smaller than the amount in X sites, appreciable

r-.

CH3

L

i

-

* hv

7

i

CH3

via s1

CH3

i E

El

E

9

i

-A.

E=C02CH3

CH3

i

E

& 8

CH3

Scheme 5.

10

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

103

photoreaction from A, may occur through A$ -,A t energy transfer as outlined in Scheme 4 or if the quantum yield of reaction from A? is much larger than that from A;. An example in which mechanism 3 operates is the photodimerization of 9-cyanoanthracene in the crystalline state presented above (Scheme 6) [96]. On a statistical basis, many more molecules within the crystalline bulk phase are expected to be excited than those at defect sites. However, the reaction cavities capable of supporting reaction are specific to defect sites. Efficient photodimerization is believed to occur owing to exciton migration from the inert bulk sites to the defect sites.

Adjocent molecules in bulk

@P NC

i

8 CN

hr,

CN

9 cyono onthroceno Rote of r e a c t i o n Vs. Rote of energy tronsfer

1

Adjocent niolecules in structural fault

Mirror symmetric dimer not formed

Scheme 6.

Terms such as microheterogeneous, anisotropic, and nonhomogeneous media used to describe a number of organized media derive from the unique features described above (multiple sites) they exhibit.

E. Limitations and Conclusions Thus far, attention has been focused on the guest molecules in their ground states. This is so because it is relatively easy to predict and visualize the geometry and orientation of molecules within reaction cavities based on attractive and repulsive interactions between ground state guest molecules and the host structure. However, electronic excitation frequently lead to changes in molecular geometry and polarizability [97]. For example, it is well known that formaldehyde becomes pyramidal upon excitation and the C-0

104

V. RAMAMURTHY, R. G. WEISS A N D G. S. HAMMOND

group dipole moment is greatly reduced. Such differences in geometry and polarizability between the ground and the reactive excited state are expected to have subtle consequences on the interaction between the excited guest molecules and their neighbors. Even more complications arise when intermediates intervene between the excited states of reactants and their photoproducts. Clearly, consideration of interactions of reaction cavity with the ground state, excited states, and reaction intermediates of a guest at the atomic level will be required to achieve reliable predictability concerning the specific influence of aniostropic environments on the course of photoreactions. Additionally, an understanding of the response of the medium during transformation of a guest molecule will also be necessary. Both are lacking presently in all but the most elementary systems. The following points with respect to our model emerge from the presentation given above: (1) A reaction cavity is an enclosure that reduces the mobility of reactant molecules in at least one dimension and provides a boundary which reactant molecules may not cross without overcoming an energy barrier. (2) The sizes and shapes of the reaction cavities among organized media may vary. (3) Free volume within a reaction cavity is an important parameter; it is the shape, size, location, and directionality of free volume and the dynamics of their change that control in large part the extent to which the medium influences a photoreaction. (4) When the atoms/molecules that constitute the walls of the reaction cavity are stationary (i.e., possess time-independent positions on the time scale of the guest reaction), the free volume necessary to convert a guest molecule to its photoproducts must be built into the reaction cavity. On the other hand, in systems where the walls are flexible, the free volume may become available during the course of a reaction. For these media, the free volume content in a reaction cavity cannot be readily represented by static molecular models, since it is a property of the macro medium and exhibits structural fluctuations. ( 5 ) The reaction cavity may contain specific functional groups or atoms which may strongly interact attractively or repulsively with either a guest molecule and/or the transition state and/or the intermediate that is formed as the guest proceeds to products. Such specific interactions may lead to unique product selectivity and either enhance or decrease the quantum yields for reactions. (6) Unlike in fluid isotropic solutions, there may be more than one type of reaction cavity in which guest molecules reside in an organized medium. If interchange of molecules among different types of reaction cavities is slow on the time scale of excited state processes, prediction of quantum yields and product distributions requires detailed structural and dynamic knowledge of the system, especially the nature and relative abundance of the different reaction cavities.

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

105

IV. REACTION CAVITY MODEL APPLIED TO EXAMPLES In this section several photoreactions from the literature are examined in terms of the reaction cavity concepts outlined above. Examples have been so chosen that only the particular aspect highlighted is the major influencing factor. However, in certain cases there may be more than one factor responsible for the changes observed. To establish the generality of the proposed model, examples have been chosen from a number of different organized media. In this section features relating to enclosure and free volume are discussed.

A. Consequences of an Enclosure When reacting molecule(s) are confined within a small enclosure, both uni and bimolecular reactions would be expected to be affected (in comparison to behavior in isotropic solution or in vapor phase). It is easy to visualize that the probability of collisions between two molecules or fragments, generated from a molecule, will be higher when the space within which these are enclosed is small. On this basis one would predict that a common characteristic of organized media would be to provide cage effects that are larger than those commonly observed in solvents of high fluidity. Also the efficiency of bimolecular reactions would be enhanced when more than one molecule is confined within a single enclosure. Both of these aspects have received attention. Cage effects in organized media have been extensively investigated by Turro and co-workers using the photobehavior of parent and unsymmetrically substituted dibenzylketone (DBK) as a model [98-1201. Photolysis of DBK has been so thoroughly investigated that its behavior can be used as a test to explore the existence and nature of the enclosure in an organized medium. Irradiation of DBK in fluid solutions leads to the formation of benzyl radicals following a-cleavage and decarbonylation processes. The termination process of these radicals is generally the uncorrelated (random) coupling to yield 1,2-diphenylethane. Photolysis of unsymmetrically substituted derivatives (represented as ACOB) results in the formation of three radical coupling products, AA, AB, and BB, in a ratio of 1:2:1 (Scheme 7). If the volume of the space in which the radical fragments A and B are generated is too large, separated radical pairs will tend to lose their correlation and yield noncage products. In contrast, when separation of the radicals is impeded by confining environments, the radical recombination probability is modified by

106

*

V. RAMAMURTHY, R. G. WEISS AND G. S . HAMMOND

&”’+ /

-

Me

hv

Me

{*TB]

Me

’

/

Me

12

+

11

Me

AA

AB

BB

Scheme 7.

favoring the preferential formation of geminate products of the AB type. The cage effect, defined as Cage effect =

+ + +

AB -(AA BB) (AA AB BB)

is a measure of restriction and is calculated from the yields of diary1 ethanes AA, AB, and BB. It has also been observed that recombination of the primary radical pair can sometimes occur before decarbonylation (or separation), giving rise to rearranged ketone photoproducts (ortho and para isomers) and recovered starting material (Scheme 7). Recombination of the primary radical pair generates 1-phenyl-ortho-methyl acetophenone (11) and l-phenyl-paramethyl acetophenone (12) by ortho and para coupling, respectively, in the case of DBK. Thus, in using unsymmetrical DBK as a probe one measures the yields of diphenylethanes and rearranged products and, in certain cases, the I3C isotope enrichment factor (which is not the subject of discussion here). The cage effects measured in various media are compiled in Table 2. Results clearly show that all of the organized media listed in the table have a cage effect larger than is observed in benzene (-0%). Also note that the magnitudes of the cage effect and the yields of the rearrangement product depend on the medium (probably a reflection of the differences in the characteristics of the reaction cavity in various media). The importance of the size of the enclosure (reaction cavity) on a reaction course has been a subject of investigation in several laboratories. On the basis of the proposed mechanistic scheme for DBK fragmentation and on the basis of the “effective reaction cavity” model presented in Section 111, the following predictions can be made: (a) a relationship will exist between the cage effect and the reaction cavity size; (b) the cage effects observed for singlet and triplet benzyl radical pairs will be different (assuming very similar diffusion rates)

107

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

TABLE 2 Representative Examples of the Product Distribution from Photolysis of Dibenzyl Ketones in Various Environments‘

Percentage Yield ~~

Media Benzene

Crystals Silica surface (22 A) Zeolite Na X p-Cyclodextrin complex Aqueous complex Solid complex HDTCI-Micelles Liquid crystals CCI/CNd,Chol‘ CCI/CN, Isof

~

Reactant

DPE

pMeDBK a-MeDBK pMeDBK DBK-d,

100 100

100 55

DBK a-MeDBK DBK r-MeDBK DBK p-MeDBK

19 40 100 98 98 100

p-MeDBK

-

100 100

RP

C.Eb

-

0

100

80 60 2

-2

-

22 22

-

80 100 96 33‘ 59’ 18 14

“Adopted from Ref. 70. bCage effect (%). ‘Cage effect measured by quenching of the escape radicals with CuCI,. dCholesteryl chloride/cholesteryl nonanoate, 35/65, wjw. ‘Chloesteric phase. ’hotropic phase.

since the lifetimes can be very different. Indeed these predictions have been verified. A relationship between the cage effect and the pore size on silica surfaces has been established by Turro and co-workers with the photobehavior of DBK-d, and unsymmetrical DBK as probes [101-1041. An inverse relationship between the cage effect and the pore size has been observed at all coverages as summarized in Table 3. de Mayo et al. estimated the cage effects for singlet and triplet benzyl radical pairs generated independently by photolysis of benzyl phenyl acetate, 13a, and dibenzyl ketone 13b, respectively on the surface of silica (Scheme 8) [121, 1221. This study presumes that the excited ketone crosses rapidly to the triplet state and that the excited ester does not. The cage effect measured by monitoring the yields of diarylethanes (AA, AB, and BB) from the singlet pair was approximately twice that from the triplet radical pair (Table 4).This is consistent with the postulate that the lifetime difference between the singlet and the triplet radical pair gives rise to a difference in size of the “effective” reaction cavity. The

108

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

t

AA

+

AB

t

BB

OCH,

Scheme 8.

longer lifetime of the triplet allows the radical pair to explore more space and thus favor noncorrelated products. Studies in micellar media provide some elegant examples of the influence of the reaction cavity size on reactions carried out in micelles. One can alter the micellar size by changing the length of the constituent detergent molecules, the aggregation number, and the concentration of the detergent and by the addition of salts. Turro and Weed, utilizing p-methyl DBK as the probe, showed that the measured cage effect depends linearly on the chain length of the detergent that forms the micelle [123]. Further studies from the laboratories of Turro established that the exit rate of the benzyl radical pair from micelles depends inversely on the size of micelles [ 117, 1181. Exit rates of TABLE 3 Cage Effects for Triplet 4-MethylbenzylBenzyl Radical Pairs on Silicas with Various Pore Sizes at Room Temperature [102,461 Pore Size

Coverage

Cage

22

16 3 1 25 4 1

18 23 28 6 14 21

(4

95

(%I

(%I

109

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

TABLE 4 Cage Effects for Singlet and Triplet Benzyl Radical Pairs Produced from Dibenzyl Ketones and Benzyl Phenylacetates on Silica at 20°C (see Scheme 8) 1121, 1221

Coverage

Radical Pair

Precursor

(%I

Cage Reaction (%) ~

4-CH,-C,jH,CH,/ 4-CH3O-C,jH,CH,

Triplet

4-CH3-C,jH,CH,/ 4-CH30-C,jH,CHZ

1 10 50

25 23 21

Singlet

1 10 50

51 39 32

benzyl radicals from micelles of C,,, Cl2, and C , , sulfates were determined to be 2.7,1.8, and 1.2 x lo6 s-’, respectively. Quantitative parameters, such as radical exit rate, rate of coupling of radical pairs within the cage, and fluorescence quenching rate, have been measured for a number of systems and correlated with micellar size. Evans, Scaiano, and Ingold showed that both the rate of cage reaction and the exit rate of the triplet radical pair derived from photoreduction of butyrophenone by cl-tocopherol depend on the micellar size [124]. Both decrease with the increasing size of the micelle. de Schryver and co-workers have shown that the rate of fluorescence quenching of 1-methylpyrene by N-methyl-N-decylaniline and alkylpyridinium chloride (both quencher and quenchee being present within a micelle) is micellar size dependent [125]. The rate decreases with the increasing size of the micelle.

B. Reaction Cavity and Free Volume Over the last decade, a large number of examples (in the crystalline state) have corroborated the reaction cavity model due to Cohen and have brought out elegantly the need to have free volume within reaction cavities for the occurrence of solid state reactions. Even quantitative correlations have been attempted. Scheffer, Trotter, and co-workers have examined free volumes within reaction cavities to gain insight into the mechanism of intramolecular photorearrangements of enones [126,1273. They have shown that “the course of a solid state reaction is influenced profoundly by certain specific

110

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

steric interactions which develop between the reacting molecules and their stationary lattice neighbors.” This approach has been termed steric compression control. Gavezzotti has applied theoretical analyses in terms of empty and filled volume spaces in the crystal to understand the photodecomposition of a number of peroxides and azo compounds originally investigated by McBride [66, 128, 1291. He concluded that “a prerequisite for a crystal reactivity is the availability of free volume around the reaction site.” Ramamurthy, Venkatesan, and co-workers have utilized similar ideas to understand the photobehavior of a number of olefins which do not follow Schmidt’s original postulates [ 1301. In this context, lattice energy calculations involving van der Waals attractive and repulsive terms were performed for incremental changes in the orientation of reactive pair along the dimerization coordinate. Zimmerman and Zurcaw [131,1321 have utilized molecular volume calculations to understand the solid state reactivity of a number of systems potentially capable of undergoing di-n-methane rearrangement. In this approach quantitative prediction of solid state reactivity was based on the superimposability of the molecular structures of the product(s) or intermediate(s) into the reaction cavity of the reactant in the crystal lattice. The studies mentioned above serve as a basis for extension of the free volume concept to several other media considered in this chapter. In this as well as in the following sections free volume includes both typesobvious and concealed corresponding to stiff and flexible reaction cavities. We illustrate in this section with a number of examples how the presence or absence of free volume within a reaction cavity determines the feasibility of a reaction in organized media. Presence of free volume alone may not be sufficient to effect a reaction within a reaction cavity. Its location and its directionality (presence of free volume in the critical dimension) are extremely important, as revealed by a number of examples discussed in Section D. Zimmerman and Zuraw [131,132] have illustrated with a number of examples that the product selectivity in the solid state can be predicted on the basis of whether intermediate(s) and/or resultant product(s) from a reaction will fit within the reaction cavity in which reactant molecules are accommodated. In other words, intermediates whose sizes are larger than the reaction cavity will not be favored-the larger the reaction cavity the poorer the selectivity. One example from their studies is discussed below. Tetraphenyldicyanotriene, 14, upon irradiation in solution yields products resulting from di-n-methane rearrangement (Scheme 9). Irradiation in the solid state, surprisingly, gave only a tri-n-methane rearrangement product, and none of the solution products were formed. This unique preference has been rationalized on the basis of molecular volumes of the reactive intermediates involved in the di-n-methane and tri-n-methane rearrangements (Table 5). As reflected in the values of A V and AS (Table 5), diradical intermediates 15,16,

t

A

Ph

rh

Ph

14 -

Ph

CN

Ph

Ph

CN

Ph, CN

Ph

Ph Ph Ph

t "

1 1 5

dc:

Ph

CN

Ph

Ph

hv

ph& CN

CN Ph

gph CN

CN

Ph

Crystal)CN

Ph

Ph

Ph

CN Ph

Ph

p

CN

18 -

14

:

Ph

L

Ph

Ph

h

CN

Scheme 9.

TABLE 5 Calculated Overlap Parameters for the Diradical Intermediates During the di-n Methane and tri-n-Methane Rearrangements of 14 (see Scheme 9)" Diradical Intermediates 15 16

17 18

AMb 0.78 1.87 0.53 0.24

A V'

ASd

15 26 15 6

5 10 2 1

(%I

(%I

"Adopted from Refs. 131 and 132. *Average straight line atomic movement from reactant to the intermediate diradical. 'Volume of superimposed species not in common with reactant. dOverlap of the inserted diradical species with neighboring molecules.

111

112

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

and 17 are too large to fit within the reaction cavity of crystalline tetraphenyldicyanotriene.On the other hand, diradical intermediate from tri.n-methane rearrangement, 18, is small enough to fit within the reaction cavity. A comparison of the photobehavior of two crystals, 7-methoxycoumarin (19) and methyl m-bromo-cinnamate (20), exemplifies the importance of the need for the existence of free volume close to the reaction site [133-1351. Neither of these molecules is topologically oriented for efficient reaction in the crystal, meaning that the reactive pairs need to undergo some intermolecular realignment before they can dimerize. COOMe

Me0

Br

19

20

In spite of the unfavorable topochemical arrangement (the reactive double bonds are rotated by 65" with respect to each other and the center-to-center double-bond distance is 3.83 A, see Figure 22), photodimerization occurs in crystals of 19 to give the syn head-tail isomer. On the other hand, 20 which also has nonideal topological arrangement of double bonds in the crystalline state (the distance between the centers of adjacent double bonds is 3.93 .$; the double bonds are rotated and make an angle of 28" when projected down the

Figure 22. Packing arrangement of 7-methoxycoumarin in the crystals-a unit cell representation. Note the two reaction centers (darkened circles) are twisted with respect to each other.

113

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

line joining the centers of the bonds) does not undergo photochemical dimerization in the crystalline state. This difference in behavior has been rationalized on the basis of lattice energy calculations. The lattice energy increase upon reorientation of the reactive pair of 19 in the crystal lattice was estimated to be roughly of the same order of magnitude as for many photoreactive crystals with favorably oriented pairs. On the other hand, the lattice energy increase necessary to align the molecules parallel to each other in a geometry suitable for dimerization is enormous in the case of 20. Such a large increase in the lattice energy indicates that the cavity wall will resist the reorganization required for the photodimerization of 20. To rephrase this conclusion in terms of the reaction cavity model, the reaction cavity occupied by a pair of 7-methoxycoumarin molecules contains a large amount of free volume than the ones present in the crystals of methyl rn-bromo-cinnamate. Because of the larger free volume, the reaction cavities in 19 crystals are tolerant toward large motions executed by a pair of 19 molecules during dimerization. The importance of free volume within the reaction cavities in the case of inclusion complexes has also been shown by several examples. Lahav, Leiserowitz et al. have observed that irradiation of the inclusion complexes of acetophenones with deoxycholic acid yields an addition product enantiomerically pure in each case (Scheme 10) [136]. Supported by a detailed

a : X=H

b:X=F

Scheme 10.

crystallographic study, they have proposed a mechanism in which a net rotation of 180" of the C-C bond specifically along one direction connecting phenyl and carbonyl carbon atoms precedes the final addition product (Scheme 11).Such a specific rotation is shown to occur only under conditions wherein short contacts between the host framework and the rotating guest molecules do not exist. In other words, where there is void volume around the reaction center the rotation in a specific direction is favored. In the propiophenone-DCA complex where there is minimal free volume near the reaction center, both optical isomers were obtained (Scheme 12). They are

114

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

1

-c-

Scheme 11.

Scheme 12.

proposed to arise from guests which add to the host steroid without the rotation of the C-C bond connecting phenyl and carbonyl carbon. The importance of free volume and the influence on a reaction of variation in the size, shape, and mobility of free volume in various organized media will be illustrated in the following paragraphs by examining the geometric isomerization of olefins conducted in various media. In fact, as pointed out in Section I, the primary photochemical process of visual pigments is geometric isomerization of a specific double bond in retinyl polyene. This occurs within the cavity defined by the protein opsin to which the polyene is bound. The rapidity (less than one picosecond) and the specificity with which the isomerization occurs suggest that the reaction cavity in these protein helices probably has some free volume surrounding the Cll-C,, bond of retinyl chromophore. Although the photoisomerization of retinal has not been

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

115

investigated in a wide variety of organized media, geometric isomerization of the derivatives of cinnamic acids, stilbenes, and stilbazoles has been examined. The ease with which the isomerization occurs is a reflection of the medium’s accommodation to shape changes that occur during the transposition of the groups attached to the double bond. Photolysis of alkyl cinnamates and cinnamic acids (21) has been carried out in aqueous media, in micelles, in liquid crystalline and crystalline media. In the crystalline state, trans-cinnamic acid has been reported not to isomerize to the cis isomer. Dimerization does occur. However, there are a few examples of cis-cinnamic acids and cis- 1,2-di(1-naphthy1)ethylenes undergoing photochemical isomerization to the trans forms in the crystalline state [I 1,137, 1381. Although the exact mechanism by which such an isomerization occurs is debatable, free volume near the C-C double bond has been identified in the crystal structures of the isomerizable compounds. A recent report concerning P-nitrostyrene (22) also highlights the importance of free volume near the C-C double bond [139]. Irradiation of trans-22 crystals gives an anomalous dimer which is suggested to result via the cis isomer. Geometric isomerization of alkyl trans-cinnamates readily occurs in the micellar phase of sodium dodecyl sulfate, in the aqueous phase (where cinnamates are proposed to exist as molecular clusters) and in the liquid crystalline smectic phase of n-butyl-stearate [140-1421. This is not surprising considering the fact that the walls of the reaction cavities of these media are fairly mobile and the volume demand during isomerization of these cinnamates is not excessively high. However, when the volume demand during isomerization is high and when the medium is highly ordered, isomerization is inhibited as in the case with 5a-cholestan-3/.?-yl transcinnamate (23) [143]. Upon irradiation of 23 in its crystalline and in cholesteric liquid crystalline phases, no isomerization was detected although this process occurs in isotropic media. Although the reported lack of cis isomer may be due to its very low concentration at the small conversion of trans which were affected, the propensity to dimerize is clearly greater in the ordered phases. The photochemistry and photophysics of trans-stilbene derivatives (24) have been utilized by Whitten and co-workers to understand the relaxation characteristics of media such as micelles, monolayers, and LB films [144, 1451. For example, the CD,,, to cis for stilbene derivatives show the following trend: solvent system methylcyclohexane > SDS micelle >> multilayer assemblies (with arachidic acid). In fact, no isomerization is observed in multilayer assemblies. This is the trend expected on the basis of how readily the media can respond to stilbene shape changes during isomerization process. The importance of free volume has been demonstrated further in L-B

116

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

L f P 23 21

22

24

assemblies by Whitten and co-workers [146-1481 through investigation of the isomerization of thioindigo dyes 25 and 26 in L-B assemblies. In solution, the reversible photoisomerization of both thioindigo isomers is well known. Irradiation of cis-thioindigo in L-B assemblies leads to rapid, irreversible cis to trans isomerization. On the other hand, no reaction was observed when trans-thioindigo L-B assemblies were irradiated over prolonged periods. The fact that cis-thioindigo isomerizes and trans-thioindigo does not was attributed to rotational constraints imposed by the rigid L-B assembly and the larger volume occupied by the cis isomer (from the isotherm data it was inferred that the cis-thioindigo required a larger area per molecule than the trans isomer). Studies by Nishiyama and Fujihara [1491 utilizing azobenzene derivative (27) as isomerizable chromophores have demonstrated the importance of reaction cavity free volume in L-B films. The L-B films of amphiphilic derivative 4-octyl-4’-(3-carboxytrimethyleneoxy)-azobenzene (27) upon irradiation was found to be stable, no geometric isomerization of the azobennzene moiety occurred. This compound forms L-B films with water soluble polyallylamine 28 at an air-water interface. Reversible cis-trans photoisomerization occurs in the film containing 28. The reversible photoisomerization reaction in polyion complexed films is thought to occur because of the increased area per molecule provided in the film. The cross sections of molecule 27 in the pure film and in film containing 28 were estimated to be 0.28 and 0.39nm2. Such an increased area per molecule

117

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS 0

R)pJ$JR'

R'

/

0

0

R 0

25

W U ~ SR = n-C6H130; R = H b trans R = H; R = n-C4H9 tram R = H; R = r u t - m y 1

27

28

guarantees a large reaction cavity, as illustrated in Figure 23. The surface area per molecule was found to be controlled by varying the size of the monomer unit chosen in the ionic polymer.

C. Location and Directionality of Free Volume Consideration of overall free volume within a reaction cavity may not always help in understanding or predicting the photobehavior of guest reactant

(4

(b)

Figure 23. (a) Schematic representation of an anionic surfactant azobenzene derivative monolayer film at the air-water interface. (b) Schematic representation of the stable monolayer film formed from the polyion complex of anionic surfactant azobenzene derivatives with a cationic polymer. Note the difference in free volume around the reactant chromophores in the two monolayers.

118

V. RAMAMURTHY. R. G. WEISS A N D G. S. H A M M O N D

molecules in organized media (Figure 24). We show below that preference for one reaction among the many possible ones is directly related to the presence of free volume near one part of the molecule and absence of it near the other. Further, directionality of free volume within a reaction cavity is an important parameter.

-

(I) Overaii free volume is large, but not in the required place

Reactant

’

Free Volume

Product

(ii) Large free volume is in the wrong place

(iii) Directionaiity of free volume

Reaction is along this axis

Figure 24. A relationship between free volume and the feasibility of a reaction in an organized media. Filled areas correspond to the shapes and sizes of reactants and products. Note the shape changes between the reactant and the product. Free volume around a reaction center is represented as unfilled regions. In all three cases shown here the total free volume present is much larger than needed for a reaction to occur, but it is not present at the correct location. Importance of location and directionality of free volume highlighted.

119

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

An illustrative example is available from the studies of Scheffer et al. on enones [126,127]. Enones of general structure 29, when irradiated as neat solids, can undergo either a or /J hydrogen abstraction to yield products shown in Scheme 13. Scheffer et al. noted that irradiation of 29 in the solid state gave exclusively products arising from a-hydrogen abstraction. The preference for a-hydrogen over p- has no topochemical origin. Based on lattice energy calculations, they showed that pyramidalization of the pcarbon following hydrogen abstraction is not favored by the nearest neighbors. On the other hand, similar pyramidalization of the a-carbon does not pose any steric compression and is expected to cause no increase of lattice energy (Figure 25). The steric compression between the nearest neighbors and the reactive center arises because of the absence of free volume. The selectivity obtained is therefore the result of the presence of free volume near the a-site and absence of it near the p-site.

/ * \ Path A hu

29 -

Path C

Me R4

R3

Me

Scheme 13.

Pokkuluri et al. have recently addressed the question of the directionality of free volume in their studies on the solid state di-x-methane rearrangement of methyl 2-benzoyl-1,4-dihydrol,4-ethenonaphthalene-3-carboxylate, 30 [1501. The title compound gives two different sets of products upon solution and solid state irradiation, both resulting from di-x-methane process involving 1,3-diradical intermediate (Scheme 14). Strikingly, the solid state products derive via less stable 1,3-diradical intermediates, 31 and 32 whereas the

120

V. RAMAMURTHY, R. G. WEISS A N D G. S. H A M M O N D

Methyl Group from Neighboring Molecule

(4

(b)

Figure 25. (a) Steric compression accompanying pyramidalization at C,. Note the interaction between methyl groups of adjacent molecules as the reaction occurs. Substituents have been omitted. (b)Stereoview of the same phenomenon in a molecule

carrying OAc group at y position. [Reproduced with permission from S. Ariel, S. Askari, S.V. Evans, C . Hwang, J. Jay, J. R. Scheffer, J. Trotter, L. Walsh, and Y. F. Wong, Tetrahedron 43, 1253 (1987).] solution products, as expected, derive via the most stable intermediates, 33 and 34. No understanding of the preference for 31 and 32 over 33 and 34 was offered by the estimated molecular volumes, as per the methods of Zimmerman and Zuraw [131,132], of the potential intermediates. Molecular volumes of reactive intermediates 31 and 32 differed only by 2 A3. This approach suggests that the reaction cavity is large enough to accommodate either one of these intermediates. However, lattice energy calculations revealed that motions required for the formation of more stable 1,3-diradical

-

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ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

-b

B E

0 II

B=-C-Ph

34

-C

32 -

-d

\--

0 II

E= - C - OMe

Scheme 14.

intermediates was not favored by the lattice whereas formation of the less stable intermediates did not result in lattice energy increase. This suggests that the free volume, the presence of which was indicated by the calculations of Zimmerman and Zuraw, is located specifically in certain regions around the reactant molecule and that dictates the course of reaction. Location of free volume has also been shown to be important in bimolecular reactions [130,1341. Irradiation of crystalline 7-chlorocoumarin yields a single dimer (syn-head to head). The packing arrangement shown in Figure 26 reveals that there are two potentially reactive pairs of 7chlorocoumarin molecules in a unit cell. One pair being translationally related, has a center-to-center distance of 4.54 8, (favored to yield syn headto-head dimer). The other pair, being centrosymmetrically related, has a center-to-center distance of 4.12 A (favored to yield anti head-to-tail dimer). In spite of a favorable arrangement, the latter does not react; dimer is

122

V. RAMAMURTHY, R. G. WEISS A N D G. S. H A M M O N D

a :4 . 4 5 4

p

I91.21’

i

b

=

30.488

c

:5 . 6 8 4

2 :4

Viewed down a - a x i s

Figure 26. Packing arrangement of 7-chlorocoumarin in the crystalline state. Note the presence of two pairs of reactive 7-chlorocoumarin. In one pair the molecules are related by a centrosymmetry and in the other by a mirror symmetry. On this diagram mirror symmetric molecules appear one on top of another. Centrosymmetric molecules appear to be displaced with respect to the other.

obtained only from the translationally related pair. Lattice energy calculations reveal that the relative increase in lattice energy were the centrosymmetrically related pair to start reacting, would be much higher than when the translationally related pair reacts. The calculations given above indirectly point out the presence of free volume near the translationally related pair and its absence near the centrosymmetrically related pair. Another set of examples consists of enantioselective photocyclizations reported by Toda, Kaftory, and co-workers [56-58, 151-1541. Upon excitation, a-tropolone alkyl ethers and pyridones undergo intramolecular cyclization to the corresponding bicyclic products by disrotatory ring closure. Depending on the direction of disrotation, two optical isomers may be formed. Racemic products are obtained in solution, but irradiation of crystalline inclusion complexes of a-tropolone alky ethers and pyridones with 1,6-bis (2-chlorophenyl)-l,6-diphenylhexa-2,4-diyne-1,6-diol (3) gave cyclic products of 100% optical purity (Scheme 15). The X-ray structure of the 1 : 1 a-tropolone ethyl ether complex with 3 shows that the guest molecule is held by hydrogen bonds to two host molecules as illustrated in Scheme 16 (note the reaction cavity walls are “active” and the interaction between the walls and the reactant molecules is not the determining factor for selectivity). On the basis of crystal packing, Kaftory reasons that the enantiomeric control results from the chiral environment provided by the host and from the differences in short contacts that develop between the alkoxy group of the

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

123

0

-

OMe

hu

OMe

ee

=

100%

'Me

Scheme 15.

(lS,5R)-(-)

(1R.55)- ( + I

Scheme 16.

guest and the aryl group of the host during the two directions of rotation. In other words, presence of free volume on one face of the molecule allows the rotation in that direction and absence of it on the other face forbids the rotation along that side. On the basis of the crystal structure of the 1 : 1 complex of 4-methoxy-1-methylpyridonewith 3, Fujiwara et al. have reached similar conclusions in the case of pyridones [lSS]. These two examples

124

V. RAMAMURTHY, R. G . WEISS AND G. S. HAMMOND

abundantly make it clear that it is important to note the absence of free volume in some places as it is to note its availability in others. Irradiation of the cr-cyclodextrin complexes of rn-alkoxyphenyl ally1 ethers (35) gave only a single ortho isomer in each case, although in solution each ether gave two ortho and one para isomers via photo-Claisen rearrangements (Schemes 17 and 18) [156,157]. This can be attributed to the difference in free volume available within the cavity near the two ortho centers and to the difference in accessibility of the radical fragments to them. In support of this conclusion, irradiation of the /J-cyclodextrin complexes of 35 results in no

q2

hlJ +

OR

Scheme 17.

(p(3s

a

I

+

OR

&QH

OR

OR

OR

8-co

OR QLCYD

R = Methyl

62

30

0

100

Propyl

51

49

5

95

Hoxyl

24

76

12

88

Oclyl

10

90

12

a8

Dodecyl

25

75

36

64

Scheme 18.

OR

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

125

significant product selectivity (Scheme 18). The P-cyclodextrin cavity, being larger, probably provides adequate free volume around the two ortho centers for easy attack by the ally1 radical. The tight fit necessary for achieving selectivity in P-cyclodextrin can apparently be provided simply by increasing the space filling capacity of the substrate (e.g., by adding a long alkyl chain, Scheme 19).

R = CH3

R = octyl

R = dodecyl

Scheme 19.

Elegant illustration of the need for free volume along a specific direction/dimension is provided by studies on the photochemical geometric isomerization of stilbenes included in tris-onho-thymotide (TOT) and with in zeolites [ 158,1591.Presence of free volume along the perpendicular axis is essential for geometric isomerization of stilbene caged in a small reaction cavity since such an isomerization process involves shape changes only along the axis perpendicular to the long molecular axis. The photochemistry of stilbene and methyl cinnamate in TOT has been subjected to extensive investigation by Arad-Yellin et al. [1581. Although isomerization of cisstilbene to trans-stilbene occurs readily in the TOT matrix, the trans to cis isomerization does not take place. The difference in behavior has been attributed to, among other factors, the amount of void volume present near the central double bond in these two isomers when they are enclosed within the channels formed by TOT molecules. On the basis of X-ray structures and computer calculations, the authors have estimated the void volume near the olefinic double bond to be 56 and 103 A3 for trans and cis stilbene complexes, respectively. Although there is very large void volume within the stilbeneTOT cages (826 for trans and 887 A3 for cis) apparently it is not present where it is required. Sharp difference in void volume near the isomerizable double bond (56 vs. 103 A3) is probably responsible for the difference in behavior between the trans and the cis isomers. Similar studies in zeolites having different sizes of channels/cages also reveal the importance of the directionality of free volume within the reaction cavity [159]. Both the photochemical and the photophysical behavior of trans-stilbene and longer all trans-a,o-diphenyl polyenes critically depend on the zeolite in which they are included. In pentasil zeolites (ZSM-5, -8, and

126

V. RAMAMURTHY, R. G . WEISS AND G. S. HAMMOND

-ll), wherein these polyenes are tightly held with no free volume near the double bonds, no photochemical isomerization occurs. In the faujasites, which contain larger chambers, the photoisomerization does take place. Such a dramatic difference in influence on the rotational motions of the guest by the zeolites is also reflected in the photophysics of these molecules. In the extremely confining space of the pentasil channels, all of the polyenes examined exhibit enhanced fluorescence and singlet lifetimes which are significantly longer than in fluid solution or in the supercages of faujasites. It is important to note that in the channels of ZSM-5, -8, and -1 1 there is a large amount of free volume along the long axis of the polyenes, but that does not help the isomerization since for this process to occur, as pointed out earlier, the free volume is needed along the axis perpendicular to the molecular long axis (Figure 27).

D. Magnitude of Free Volume Consider reactant molecules or intermediates being caged within a reaction cavity with limited free volume. A preference might be envisioned in the reactions these reactant molecules or intermediates undergo, if the competing reactions require different amounts of free volume for shape changes that take

:Fh\9 \ /

1 1,

n

Figure 27. Inclusion of trans-stilbene in X type and in ZSM-5 zeolites. Required free volume for geometric isomerization is present in supercages of X zeolite and such is absent in ZSM channels. Extensive free volume in ZSM-5 channel is present along the molecular axis, but that is of no use for the reaction to occur.

127

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

place during the conversion of the reactant to products. Indeed several such examples are available in the literature. An a-alkylbenzyl radical pair, as discussed in Section III.A, can undergo both coupling and disproportionation processes (Scheme 1). In solution, coupling is predominant; on surface of silica or within zeolites or micelles, disproportionation also occurs. Within the supercages of X and Y zeolites, two benzylic radicals generated from a-alkyldibenzyl ketones (5)react by both coupling and disproportionation processes, the latter yielding the olefins (Table 6 ) [92,93,160]. The ability of the disproportionation reaction, a minor process in solution, to become the major process within faujasite supercages and on silica surfaces is attributed to an inhibition of certain specific motions of the radicals in the restricted environments (Scheme 20). Relatively larger overall motion would be required to bring the two benzylic radical fragments together for head-to-head coupling than to move an alky group of one of the benzylic radical fragments so that one of its methylene hydrogens is in a suitable position for abstraction by the other benzylic carbon radical. A model developed on the assumption that the radical pair would prefer the pathway of “least volume” when the free volume around it is small, would predict that there should be an inverse linear relationship between the reaction free volume and the yield of the disproportionation products. Indeed this has been observed. Free volume can be controlled by altering the cations (Li, Na, K, Rb, and Cs) present within the supercages of X and Y zeolites (Table 6). The yield of disproportionation products indeed increases with the TABLE 6 Product Distribution Upon Photolysis of a-Alkyl Dibenzylketones in Zeolites Yields of Olefin, Coupling Product (AB) and Rearrangement Product 7 (see Scheme l)n,b

5b

5a Zeolite

Ole

AB‘

7

Ole

AB

7

Na-X K-X Rb-X

16.8 26.6 50.1 55.9

18.2 18.9 12.5 10.2

62.5 54.5 37.4 33.6

26.3 36.3 50.6 64.7

23.1 28.5 23.8 20.8

50.5 34.6 25.3 14.4

cs-x

Supercage Free Volume

(A3) 852 800 770 132

-

“Adopted from Ref. 93. benzene 5 gave AA ( - 20%), AB (-4079, BB ( - 20%), and type I1 products (DBK, 5% and cyclobutanol, 5%). Neither rearrangement product 7 nor disproportionation product olefin were obtained. ‘In zeolites no AA and BB were obtained.

-

128

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

Scheme 20.

decrease in the size of the supercage free volume (i.e., with the increase in the size of the cation). Values of k,/k, of 0.055 and 0.28 (k, is the rate constant for the disproportionation process and k, is that for the coupling process) were measured for cumyl radicals (36)in benzene and on silica, respectively (Scheme 21) [1611. Similarly, increases in the yields of disproportionation products have been observed for a-methylbenzyl radical pairs at low temperature on porous glass and several types of silica [loll. For example, k,/k, ratios of 0.054 and 1.33were obtained in pentane and on porous glass at

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

129

36

Scheme 21.

- 77°C. The increasing order of preference for disproportionation follows the same increasing trend in restriction experienced by the guests/intermediates and the available free volume within the reaction cavity: isotropic solution, silica surface, and zeolite, the last being the medium providing very high restriction and smallest free volume. Another example of the impact of the size of free volume within the supercages of faujasites on the course of a reaction is provided by the pathways undertaken by the primary triplet radical pairs, 40, generated by the a-cleavage of the a-alkyl dibenzylketones (9,a-alkyl deoxybenzoins (37), and benzoin ethers (38) [160, 162, 1631. In zeolites, owing to the cage effect, the primary triplet radical pair primarily undergoes cage recombination to yield a rearrangement product, 39, in addition to the starting ketone (Scheme 22). Examination of the results obtained with a number of a-alkyl dibenzylketones and benzoin ethers and a-alkyl deoxybenzoins reveals that although the rearrangement takes place in all X and Y zeolites, its yield is dependent on the cation and decreases as the cation size is increased from Li’ to Cs’. As the available free volume inside the supercage is decreased by the increase in the size of the cation, the translational and rotational motions required for the rearrangement process become increasingly hindered (Scheme 23). Under these conditions, the competing path, coupling to yield the starting ketone, a process which require “less volume and less motion,” dominate. This pathway eventually leads to products resulting via type I1 reaction. The pathways followed by radical pairs, 42 (disproportionation or coupling with rearrangement) resulting from the type I cleavage of 2phenylcycloalkanones (41) are influenced by cyclodextrin complexation [164]. The product ratio depends both on the size of the cyclic ketone and on

+

130

+

131

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

Scheme 23.

the diameter of the CD cavity, as illustrated in Scheme 24. The relationship between the sizes of the guest and the a-, p-, and y-CD cavity points out that the reaction selectivity depends on the tightness of guest inclusion (Scheme 25, the tighter the fit, the smaller the free volume).

0

41

C

E

= 10 Methanol a-CD complex p-CD complex y-CD complex

E/C 0.12 3.4

II

1.3 0.5 Scheme 24.

132

V. RAMAMURTHY, R. G. WEISS AND G. S . HAMMOND

-

Closed chain form (Tight complex)

Biradical separation (Loose complex)

42

\

p-coupling

lntremolecular hydrogen transfer

Enal

para-Cyclophane

Scheme 25.

V. REACTION CAVITIES AS “TEMPLATES” HIGHLIGHTED WITH EXAMPLES The strategy by which reactant molecules are predisposed toward a particular reaction coordinate we call a template efect. In this approach the products are prepared from the “mold” or “structural pattern” imposed on the reactant molecule(s) by their reaction cavity. The template efect involves orientation and maintenance of reactant molecules in a particular arrangement (relative orientation in the case of bimolecular reactions and conformation in the case of unimolecular reactions) till the reaction is completed. Orientation of guest reactant molecules can be achieved in a predictable manner when the walls of the reaction cavity are active, i.e., they contain functional groups which can interact specifically with specific parts of potential reactant molecules. If the reaction cavity is “stiff’ and small (e.g., in crystals), even weak wall-guest interactions may be sufficient to reduce the mobility of guest molecules and, thus, maintain their preorganization. On the other hand, if the reaction cavity is “flexible” and large (e.g., in micelles), the preorientation may be disturbed upon excitation and stronger wall-guest interactions may be needed to maintain the orientation throughout the transformation of reactants to products.

133

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

A. Reaction Cavities with “Active Walls” as Templates: Photodimerization In this section we explore a number of photodimerizations in organized media in which the results can be interpreted on the basis of control by template effects. Crystals. Photodimerization in the crystalline state has been known for over a century. Based on the pioneering crystallographic and photochemical investigations on cinnamic acids and a large number of its derivatives (which exhibit a rich variety of polymorphic forms and photochemical reactivity patterns), Schmidt and co-workers formulated an important set of “topochemical rules” connecting the crystal structures of the reactants and the configurations of the products (Figure 28) [l]. In almost all cases, the stereoand regiochemistry of product dimers can be predicted on the basis of molecular orientation in the reactive crystal. This clearly suggests that once the reacting pair is preoriented, it dimerizes upon excitation to a predictable dimer. Mobility of molecules in the crystalline state is highly restricted since the reaction cavities are small and stiff. Orientation of the reacting pair is achieved through intermolecular interactions. Crystal packing is controlled by a large number of repulsive and attractive forces of which some are not often considered important by chemists utilizing isotropic solvents as reaction media. These include C-H...O, X...X (where X is a halogen), S...S, S...X, and C=O...X interactions [87-891. Examples of [2+ 23 photodimerizations reported in the literature may COOH

1 ’ coon

d - form Doub!: bond s e p a r a t i o n : 3.6 -4.1.A N r a r r a l nrighbour relation; Centric

p-

f o r m (3.9-4.1.A;

Tranalation I

’

coow ~ - T R U X I L L I C ACID

coow J3-TRUXINIC

ACID

Figure 28. Topochemical rules are based on the photochemical behavior of cinnamic acids in three crystallographic modifications.

134

V. RAMAMURTHY, R. G. WEISS AND G. S . HAMMOND

provide the impression that it is fairly straightforward to orient molecules in crystals. This is certainly not true. In the absence of an a priori recipe several empirical strategies under the title of “crystal engineering” have been employed [48,89]. One of the crowning achievements in the field, the asymmetric photodimerizations of chiral and achiral crystals, have been achieved by this strategy (Scheme26) [20,21]. A number of reviews and a monograph of a collection of selected articles by Schmidt should be consulted for details concerning photodimerization in crystalline media [l, 15, 48, 49, 601.

X

S,single crystal

AX

R, single crystal

SOLID STATE CHIRAL SYNTHESIS

Scheme 26.

Inclusion Complexes. Within the reaction cavities of inclusion complexes, prealignment can be achieved through specific and well-defined (geometrydependent) interactions between the host and the guest. One of the illustrative hosts of this type are the diacetylene diols 2 and 3 that form crystalline inclusion complexes with a variety of small molecules [165]. In both, hydrogen bonding plays an important role in orientating the guest with

135

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

respect to its host [166]. In essence, the host acts as a template to preorient the reactive guest molecules. Irradiation of powdered complexes of benzylidene acetophenone 43 included in the nonchiral2 yields a single photoproduct, syn head-tail dimer 44 (Scheme 27) [167,168]. Irradiation of 43 in solution gives a mixture of cis Solution

7 Solid State

Polymer

Complex mixture

Complex with 2

Bz

43 Ar

Scheme 27.

and trans isomers of 43 and a polymer, and irradiation of 43 in pure crystals produces a mixture of stereoisomeric photodimers in low yields. Host 3 brings pairs of 43 molecules close to each other (3.86 A) and orients them in a head-tail fashion, as shown in Figure 8. This is distinctly different from the situation in pure crystals of benzylidene acetophenone wherein the potentially reactive molecules are farther apart (> 4.8 A). Other examples of this effect include [4+ 41 dimerization of 9-anthraldehyde and of 2-pyridone [1661. In these two cases, also, hydrogen bonding between the host and the guest is responsible for orientation of the reactive pair in the proper geometry for dimerization. One should not assume that any olefin can be made to dimerize by including it within the crystalline matrices of diols 2 and 3. It has been found that inclusion complexes of coumarin, 7-methoxycoumarin, and 7methylcoumarin in chiral3 form the mirror image symmetric syn head-head dimers [169]. However, there is a large number of coumarins which fails to dimerize as complexes of 2. Similar observations have been made in the case of benzylidene acetophenone derivatives. Although some of them dimerize when included in 3, many others are inert. In the absence of crystal structures, no correlations between reactivity and molecular structure and the disposition of molecules in the complexes can be made. Clearly, the templates can deactivate as well as activate their guests. Tamaki et al. were the first to explore the utility of the cavities of CD’s in effecting regio- and stereoselective dimerization of guest molecules (Scheme 28) [170-1721. The CDs in aqueous solutions may be thought of as affording “active” reaction cavities which inhibit guest motion rigorously in two dimensions, but only moderately along the third (defined by the axis of C D symmetry) owing to the hydrophobicity of the guests and the mobility of

136

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

p - cyclodextrin

y - cyclodextrin

46

47

48

49 -

Scheme 28.

the capping water molecules. Upper and lower rims of CD’s carry a large number of hydroxyl groups which can interact as templates with guest molecules and hold them in a specific geometry. Photodimerization of 2anthracenesulfonate (45) in aqueous solution produces four photodimers: anti-head-tail(46), syn-head-tail(47), anti-head-head (48), and syn-head-head (49) in the ratio 1 :0.8 :0.4: 1.1. Photolysis of a 2 :2 complex of 45 with p-CD yields exclusively 46. By contrast, photodimerization of 45 included in y-CD (2: 1 complex) gives almost the same relative yields of the photodimers as those in the host-free solution. Also a l-anthracenesulfonate (50) forms only 1 : 1 complexes with p-CD and 2 : 1 complexes with y-CD but regioselective photodimerization was not observed in either case. Although interactions between the p-CD cavity and 45 must be responsible for the photodimer selectivity, no comprehensive guiding principle has emerged which allows one to predict the orientation of guest molecules within a CD cavity, or which explains fully the quantitative changes in photoproduct yields which accompanies complexation. These templates can operate in a myriad of ways.

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

137

so; I

50

45

Surfaces. Preference in orientation of the substrates on the surfaces of silica, due to cavity wall-guest interactions, may also lead to some regio- and stereoselectivity. Examples of photoreactions of steriodal enones provided by de Mayo and co-workers illustrate the principles involved [173-1751. In these examples, the surface serves as a template when guest molecules are adsorbed on it. Preferential adsorption from the less-hindered face of the steriodal enones, 51, in Figure 29 should force addition of an olefin (from gas or solution phase) (ii) In solution: (more hindered) p face

-

I

J

Preferred

a-face (less hindered)

(ii) Silica surface: Preferred

I

Silica surface J

Figure 29. A schematic illustration of the face preference for addition of olefins to a steroidal enone in solution and on silica surface. In solution the less-hindered CI face is preferred. On silica surface the molecule adsorbs from the less-hindered face exposing the more-hindered face for attack by an olefin.

138

V. RAMAMURTHY, R. G . WEISS A N D G . S. H A M M O N D

to occur from the more-hindered face of the enone. Selectivity should be large when the template surface prefers to bind one face of the molecule and when the adsorbed molecules do not undergo motions that would permit the reagent to attack both faces with equal facility. In the examples shown in Scheme 29 a preference for the attack from the more-hindered side is commonly observed, although mixtures of products are typical. Adsorption on the silica surface leads not only to reduced attack from the less-hindered aside, but also to an increase in the barrier to inversion of the 1,4-diradical required for the formation of the trans-fused 4a,5p adduct. The preference for reaction on the more-hindered side is consistent with adsorption of the lesshindered side to the template surface. The primary mode of binding interaction is surely hydrogen bonding of surface hydroxyls to the oxygen atom of the carbonyl group. However, if that was the only attachment, the steroidal molecule would be free to assume an orientation approximately normal to the average surface plane. The stereochemical result implies that there is also attractive interaction between the surface and at least one point of the molecule, probably by polarization of the x system of the conjugated enone system. The results suggest a methodology for further study of the nature of the interactions of adsorbed species on solid template surfaces. O n the surfaces of clay, much stronger electrostatic interactions are

__*

0

CH 2=CH2

51

La 5a

LP 5p

MeOH

-78°C

82

12

6

SiO2

-78°C

42

50

-

~ a 5 p

OCOEt

LU 5d MeOH 5iO2

-78'C

-70'C

53 32

Scheme 29.

4p58

La5p

41 57

6 10

139

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

utilized to orient a photoactive species. The spatially controlled photodimerization of stilbazolium cations 52 inside saponite and montmorillonite clays has been reported: UV irradiation of the intercalate resulted in the generation of the syn head-tail as the dominant dimer (Scheme 30) [1761791. Selective dimer formation has been rationalized on the basis of an Py+

Ar

L-\ PY+

hdPyrex clay

+y+

Ar

+

A+y

+

Ar

PY+

PY+

52 b A r = e C H 3

c A r = e C N

Scheme 30.

arrangement shown in Figure 30, wherein unsaturated molecules are packed alternately in an antiparallel alignment. Such an arrangement is facilitated by the interaction between the stilbazolium cations and the anionic framework

Figure 30. Schematic illustration of the packing of stilbazolium cations inside a magnesium-rich saponite clay. Stereoselectivedimerization takes place to give the syn head-to-tail dimer. [Reproduced with permission from K . Takagi, H. Usami, H. Fukaya, and Y. Sawaki, J . Chem. Soc., Chem. Commun. 1174 (1989).]

140

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

of the clay. Among the stilbazoles investigated in saponite clay, the p cyanophenyl derivative gave a considerable amount of syn head-head dimer along with syn head-tail dimer. This is suggested to be the result of competition between the two types of interactions, the cyano group with sodium ions and pyridinium cation and framework anions. In any case it is important to note that strong ionic interactions with clay surface may result different template effects for the reacting olefins.

Micelles and LB Films. In micellar media, especially for photodimerization of amphiphilic molecules, the reaction cavity is primarily the interface ik., the poorly defined region where surfactant head groups and water molecules mingle most; and not the micelle itself. The ability of micelles to solubilize substrates and to orient them at the interfaces can often lead to regioselective photocycloadditions. de Mayo et al. have shown that micellar preorientation has a profound influence on the photochemical dimerization of 3-alkylcyclopentenones (53) [1803. Efficient dimerization in potassium dodecanoate micelles was observed for 3-n-butyl- and 3-n-decylcyclopentenones with a reversal in regiochemistry compared with that in organic solvents (Scheme 31). The almost exclusive formation of head-head dimers in micellar solution was attributed to orientation of cyclopentenone with its carbonyl oxygen at the interface and the remaining hydrophobic portion oriented

9% Benzene

91 OIo

50% Methanol

50% R-nCbHg

53

A

\

KDC Micelle ~

@Ao+ R

2 Yo Cyclohoxane

' Scheme 31.

0

4:l

1:19

R R 98%

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

141

toward the more hydrophobic interior of a micelle. Similar template like orientation of isophorone (Scheme 31) and coumarins (Scheme 32) in micellar solution can be invoked to explain the high regioselectivities of both photodimerizations [181-1861.

Syn-HH

(0.02 M I

Anti-HH

Solvent

Product

Methanol

Anti-HH

Benzene

Anti H H

<

Water

Syn-HH

-3 2 x 10

S D S

Syn-HH

2

Syn-HH

0.3

Syn-HH

1

CTA

B

Triton X

- I00

Q u a n t u m yield

< 10+

-

Scheme 32.

Micellar preorientation can also explain variation in regiochemistry in

[4+ 41 photodimerizations of 2-pyridones (Scheme 33) [187]. In homog-

eneous solutions, the major photoproducts of 2-pyridones are trans dimers and no dependence of the cis/trans dimer ratio on the substituent chain length could be detected. However, in hexadecyl trimethylammonium bromide micellar solution (HDTBr), variation in the chain lengths of R, and R, lead to changes in the &/trans photodimer ratio. With long chain lengths the cis dimer was obtained as the major product. Orientational effect of micellar aggregates can also explain the formation of the preferred photodimers of 2-substituted naphthalenes 54 and 9-substituted anthracenes 55

54

55 R = CH3

= CHIOH = COOH = CHzCHzCOOH

142

V. RAMAMURTHY, R. G. WEISS A N D G. S . H A M M O N D

trans - anti R1 :( C H Z I ~ C O O H EtOH R2= H )H20

85'10

CTAB

R1 :(CH2),0 COOH EtOH R2 = H >H20 CTAB R1 :(CH2)2 COOH

R 'C3H7

}

trans -syn

c i s - anti

cis - syn

150/0 8

0% 13

51

0 010 0 0

23

26

81 66 70

0 0 0

19 18 15

0 16 1s

79

EtOH

100

-

-

-

H20

48

24

14

14

0

0

CTAB

65

35

Scheme 33.

[188-1911. In both cases, polar substituents are needed if the aromatics are to interact in a head-to-head to fashion (i.e., aligned at the interface). Regioselectivity has been induced by micelles not only in photodimerization reactions but also in photocycloaddition reactions of olefins to 3alkylcyclopentenones [192,1931. Photocycloaddition of 3-butylcyclopentenone to 1-hexene and 1-octene in organic solvents results in two adducts 56 and 57 (Scheme 34). The ratio, which is slightly solvent dependent, is roughly 1 : 1. However, irradiation in potassium dodecanoate micelles yields 56 preferentially. These results are consistent with a simple model in which the cyclopentenone is oriented in the micelle with the polar carbonyl function at the interface and the hydrophobic butyl chain is directed toward the micelle interior. (Figure 31). When the orientation of the olefinic reaction partner was made more pronounced with respect to the micellar interface by the introduction of an acetoxy group, regioselectivity increased. For example, 1-heptenyl acetate gave 59 exclusively in organic solvents and only 58 in micelles. There are several reports wherein head-head dimers were not formed preferentially in micellar solutions. For example, the syn head-tail simers, 61, the same ones obtained in solution formed when a number of 7-alkoxycoumarins (60) (alkoxy group chain length was varied between methoxy to octadecyloxy) were irradiated in SDS and HDTCl micelles [194,195]. The same head-tail dimers, obtained in solution, were formed when 9-methylanthracene and 9-anthracenecarboxylic acid were irradiated in CTAB and

143

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

u

Medium

1- Hexene R'' C6H13 1- Octene

&+ nBu

R1 = H

Vinyl acetate

R' :CSHll

n Bu

nEu

R' = Ch Hg

I-Heptenyl acetate

cyclohexane diethyl ether K D C micelle

51 57 78

cyclohexane diethyl ether K D C rnicelle

47 88

56

49 43 22

57

47

53

53 12

h3 n Bu cyclo hexane diethyl ether K D C rnicelle Methanol cyc lo hexane K D C rnicelle

25 26

58

n Bu

51

75 74 49

0

100

0 70

59

100

30

Scheme 34.

Figure 31. Representation of the relative orientation of an enone and an olefin on a micellar surface; polar groups orient themselves toward the external aqueous phase. The broken line represents the interface between the aqueous exterior and largely hydrocarbon interior of a micelle. [Reproduced with permission from P. de Mayo and L. K. Sydnes, J . Chem. SOC.,Chem. Commun. 994 (1980).]

144

V. RAMAMURTHY, R. G. WEISS A N D G. S. HAMMOND

SDS micelles [188-1901. These results may be a consequence of a lack of template-induced orientation or of the orientational forces being too weak to overcome the orientational preferences between an excited and a ground state molecule. It is certainly the case in all of the micellar examples cited that the solvent relaxation times should allow molecules to reorient themselves at the interface (should they so choose) on timescales which are comparable to those necessary for an excited molecule to form its photoproducts.

RO

60

RO

61

R = CHz(CH2),CH, n = 0 to 16

The L-B films offer some advantages over aqueous-hydrocarbon interfaces of micelles and the related assemblies discussed above in terms of the magnitude of their orienting ability and the ease of interpretation of selectivity in photoreactions conducted in them. Molecules in the films have very little freedom of motion (stiff reaction cavities), their interfaces are very well defined, and therefore the alignment of reactant molecules can be readily expressed in the products. Photodimerization of stilbazole derivatives 62, N octadecyl-l-(4-pyridyl)-4-(phenyl)-l,3-butadiene, (63), surfactant styrene derivatives 64 and 65, and cinnamic acids have been carried out in L-B films [18, 196-2001. In all cases, single isomeric head-head dimers are obtained. Geometric isomerization of olefins has not been observed in competition with photodimerization. Independent of the location of the chromophore (i.e.,

62

63

64

65

145

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

whether at the hydrophilic end as in the case of 62 and 63 or at the hydrophobic end as in the case of surfactant styrene derivatives 64 and 65), preferential orientation of the reactive 7c-bonds is achieved. Thus, the L-B reaction cavities consist of both a directing template and a very restrictive space with little free volume which orient guest molecules very strongly. Liquid Crystals. Weiss and co-workers have carefully probed the use of liquid crystalline media to orient molecules for dimerization reactions. They have succeeded in identifying reactivity patterns with a limited number of examples [142, 143,2011. In isotropic solutions, the major photochemical processes of 5u-cholestan3p-yl trans-cinnamate (trans-23) are photoelimination to yield trans-cinnamic acid and cholestene and trans s cis isomerization (Scheme 35) [143]. In the R

( 2 5 ' or 166.5'C)

liq. crysf . o r solid ( 2 7 - 29.C) R

Ph

R

/-7 +

Ph

R

I -COZ-cholcstanyl

Scheme 35.

Ph

/+

Ph

+ Cholestene h

146

V. RAMAMURTHY, R. G. WEISS A N D G. S. H A M M O N D

neat ordered solid and cholesteric phases of trans-23, the a-truxillate diester (anti head-tail isomer, 66) was the only isolated photodimer. Formation of the a-truxillate dimer was suggested to be the result of an antiparallel pairwise alignment as shown in Figure 32. Such an arrangement would be favored by the individual dipoles of cinnamate groups. To examine whether such an arrangement is general (note the possibility of another arrangement in which head-head dimer will result, Figure 32), irradiation of n-alkyl trans-cinnamates (trans-21) was carried out in crystalline, hexatic B, and isotropic liquid-crystalline mixtures of butyl stearate (BS) [1421. The alkyl chains were chosen to make the trans-21 molecules slightly shorter than, the same length as, and slightly longer than the extended length of BS. In all irradiations, only one head-to-tail dimer (67) and one head-to-head dimer (68)were observed at low percent conversions (Scheme 35). Elimination and isomerization were competing processes whose importance increased with irradiation time. The results in Table 7 demonstrate that the 67/68 ratios are very large in the hexatic and solid phases of BS; they are much smaller in the isotropic phase. Results are consistent with the conclusion that the cinnamate esters prefer to orient in these liquid-crystalline media in a head-to-tail fashion as shown in Figure 32. Thus, dipolar interactions between individual groups can be utilized to orient molecules in a liquid-crystalline phase. Absence of any selectivity in isotropic solutions suggests that this arrange-

(b)

Figure 32. Head-head and head-to-tail orientation of photodimerizable molecules in hexatic BS phase. Head-head dimer results from reactions between molecules present within the same layer whereas head-tail dimer results from molecules present in different layers. [Reproduced with permission from V. Ramesh and R. G. Weiss, J . Org. Chem. 51, 2535 (1986).]

147

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

TABLE 7 Photodimer Ratio from t-21 in Various Phases of BS (see Scheme 35) 11421 t-21 a

wt%

Phase (7; "C)

Conversion (%)

20

Isotropic (32) Hexatic (18) Solid (8) Isotropic (32) Hexatic (18) Isotropic (32) Hexatic (18) Solid (8) Isotropic (32) Hexatic (18) Isotropic (32) Hexatic (18)

41

40

b

20 40

C

10

12 9

17 11 40 12 12 19 14

53 37

*

67/68

2.5 0.9 8.0 & 0.6 8.0 k 0.6 2.3 k 0.1 9.4 f 0.4 3.3 0.7 > 20 > 20 2.7 0.1 > 20 No dimer detected 9.4 0.2

+

ment is not preferred strongly (in liquid crystalline media), owing to dipolar interactions alone, but depends upon the same multitude of factors that determine crystalline packing. Unlike isotropic solutions, liquid crystalline media help to maintain the specific orientations by curtailing the motions of the reactant molecules in at least one dimension. The reaction cavity of trans21 in the liquid-crystalline phase of BS, especially, must not be limited to the cylindrical volume of two solvent molecules in the layered lattice; the shape of the cavity is determined primarily by the substrates, and the liquid crystal plays only a secondary role in securing the ground state orientations. In another case, where packing preferences within the liquid-crystalline matrix are not well defined, the course of dimerization has not been influenced (Scheme 35) [201]. Thus the chemical and physical interactions between pairs (and larger aggregates) of cholesteryl4-(2-anthryloxyl)butyrate (69) have been investigated in its various phases. There is little difference between the head-headlhead-tail and synlanti head-tail dimer ratios from irradiation of 69 in its neat cholesteric, liquid-crystalline, gel, and neat isotropic phases (no dimer was detected from irradiation of solid 69). The distribution obtained from irradiation of the isotropic toluene solution of 69 reflects the orientational preferences of 69-69 partners when no constraints are imposed by the ordered phases. Since it differs only slightly from the other ratios, they seem to reflect marginal orientational restrictions of anthracenyl groups by the liquid crystalline medium. This appears to be a case in which intermolecular interactions between ground and excited state molecules are more important than the forces responsible for phase packing arrangement.

148

V. RAMAMURTHY, R. G . WEISS AND G. S . HAMMOND

Molecular Aggregates. Association of nonpolar solutes in water is now an accepted phenomenon, although the origin of such a behavior is yet to be fully understood [202]. Such aggregates are expected to possess an amphiphilic character, such that polar groups would expose themselves to the aqueous phase and nonpolar portions of the molecules congregate as much as possible away from water. The size of these aggregates, in general, is fairly small compared to micelles. The role of aggregates in the photochemical dimerization of thymine, uracil, and their derivatives, all having considerable solubility in water, has been investigated by Morrison and co-workers (Scheme 36) [203,204]. Osmometric determination of apparent molecular

Water CHQCN

27.8% 24.9

CH30H

31.4

L

syn h-t

63.1% 68.2 68.6

+

syn h-h

+

anti h-t

9.1% 6.7

-

M=0.015

M=0.0047 #=0.004

+ anti h-h (dlrners)

Water

38

39

CH3CN

37

49

18 11

6 4

# = 0.014 #=0.0015

CH3OH

42

37

18

3

# = 0.002

Scheme 36.

weights of solutes as a function of concentration in aqueous media has established the existence of ground state aggregates. As a consequence of preassociation, dimerization quantum efficiencies for these substrates are considerably higher in water than in organic solvents. However, there is no specificity in the product distribution due to preassociation and, in general, the product dimer proportions are in line with the expectations based on polarity considerations. Similar observations have been made with coumarins [lSl]. The effect of association on photochemical reactivities of stilbenes and alkyl cinnamates having poor solubilities in water has been demonstrated recently (Schemes 37 and 38) [140,205,206]. Even at concentrations of transstilbene in water as low as M, dimerization occurs efficiently. The ratios of dimers were similar to those obtained in benzene when the initial stilbene concentration is high. However, in organic solvents, geometric isomerization is the only reaction observed at low stilbene concentrations. Similar behavior

149

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

Q @ + G&+P@ x

-

t

x

Ph

ArAr

X=H X CH3 X-F X=CN

Product distribution after 24 hrs. of Photolysis XsH

Benzene Water Water + LiCl Water + Guanidinium chloride

4 33 25 53

04 11 7 11

12 33 27 21

-

12 25 0

-

10 17 6

Scheme 37.

6 - truxinote p-cyclodextrin or Water- S D S <S>-l

OR

Scheme 38.

has been reported for alkyl cinnamates. In aqueous media, dimerization is able to compete with geometric isomerization, the only reaction in nonaqueous solution. Even in this case, no clear preference for a single isomer of the dimer was evident. These results are consistent with the conclusion that the organic molecules tend to cluster in water, but these clusters are internally disorganized. Thus, water forms reaction cavities for hydrophobic solutes that are as large as the solute aggregates but the cavity wall-guest interactions are not strong enough either to orient or to maintain the orientation of reacting molecules.

150

V. RAMAMURTHY, R. G. WEISS AND G. S . HAMMOND

General Observations. In the studies discussed above, the amount of photoproduct selectivity depends both on the medium and the reactant molecules. Of all the media, the highest specificity is generally observed in the crystalline state and the smallest selectivity is found in organic clusters in water; silica and micellar media showed intermediate behavior. Preorganization of reactant molecules in all media discussed above is achieved through cavity wall-guest interactions. Excepting in crystals and inclusion complexes, no quantitative information is available regarding the effectiveness of the preorganization. When there is a lack of selectivity, it may be possible to improve the preference for one reaction over others by utilizing stronger wallguest interactions. In fact, any host-guest interaction which can attenuate translational and rotational motions of the reactant molecules should increase selectivity in a system that preorients molecules in their ground states. Therefore, the strongest “template effects” will be observed in media that provide strong wall-guest interactions and have “stiff’ and small reaction cavities. These can be achieved by modifying a medium and the reactant molecules. Complimentary functional groups of the medium and the reactant are essential to effect specific orientations and to maintain them throughout the lifetime of excited state species.

B. Reaction Cavities with “Passive” Walls as Templates Specific orientations and conformations can be achieved if guest molecules fit snugly within reaction cavities. In the absence of interactive sites (active walls) which are complimentary to functional groups of the reactant molecules, this may be the only manner by which a desired geometry within reaction cavities having passive walls may be attained. “Tight fit” of guest molecules within a reaction cavity not only restricts the orientations that the guest molecules can adopt, but also ensures that they do not undergo large translational and rotational motions which may lead to loss of organization. In this context, “tight” denotes at least several points of approach of the reaction cavity walls and the guest molecules to within the distances calculated by van der Waals’ radii; closer approach will lead to significant repulsive interactions. When the fit is “loose” or when the medium is very “flexible,” preorganization of reactant molecules is expected to be difficult in “passive” reaction cavities; another contributor to loss of selectivity should be unrestricted translational and rotational motions of guest molecules. Depending on the nature of the host-guest fit, the amount of free volume within reaction cavities may vary. Such variations may also influence the selectivity. The “template effect,” as we define it, arises from factors that differ from those related to free volume in the following manner: although the “free volume” allows reactants and inter-

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

151

mediates to occupy certain regions of space if they so chose, template effects preclude certain motions of the reactant and intermediates even if they prefer them in isotropic media. Some of the best examples of “prealignment” in a narrow cavity are provided by y- or X-ray radiation-induced polymerization of olefins included in the channels of urea, thiourea, cyclophosphazines, cyclodextrins, deoxycholic acid (DCA), and apocholic acid (APA) (see Figures 1-7 for details on the structures of hosts) [207-2121. In these cases, the hosts form essentially long channels which enforce restrictions on guest molecules in two dimensions (cross sections only) (Figure 33). No important reaction cavity wallguest attractive interactions need be involved in the orientation of the guest molecule; orientation is governed by repulsive forces. A brief summary of the results is provided below. When butadiene and 2,3-dimethylbutadiene are included in the channels of urea and thiourea, respectively, 1,4 addition invariably results to yield polymers with chemical and stereo regularities (Scheme 39). Note that addition in the 1,2 fashion is prevented sterically by the narrow channel. Similarly, high selectivity was obtained when butadiene, vinyl chloride, and styrenes were polymerized in the channels of cyclophosphazenes. Syndiotactic polymer alone is obtained from vinyl chloride included in urea channels; this is apparently the first example of inclusion polymerization of a vinyl polymer in which control is exerted over the steric configuration of the developing tetrahedral carbon atom (Scheme 39). Highly isotactic polymer is obtained from 1,3-pentadiene when it is included in a perhydrotriphenylene matrix (Scheme 39). Note that addition could occur at either end (i.e., C , to

Polmeriratlcm

Figure 33. Polymerization of 2,3-dimethylbutadienein the thiourea channel. Note the relative arrangement of reactive monomers.

152

V. RAMAMURTHY, R. G. WEISS A N D G.S. HAMMOND

Urea-

~

C

-

I

Y’mY

Scheme 39.

C1, C1to C4, and C4 to C4), but the channel directs the addition selectively only to C1 to C4. Polymerization of these dienes takes place in DCA and ACA channels, but with less selectivity. In all of these cases, it is the tight fit of the monomer within the cross section of the channels that enforces the geometry of inclusion and subsequently the structure of the polymer. The fact that the selectivities noted above correlate well with the channel crosssectional area (lower selectivity obtained in the broader channels of DCA and ACA compared to urea and thiourea) is consistent with the hypothesis that alignment is controlled primarily by repulsive forces. Photochlorination of n-alkanes in solution results in substitution at both primary and secondary carbon atoms [213]. When n-alkanes included in the channels of zeolite LZ-105 (structure very much like ZSM-5 with channel diameter 5.5 A) were photochlorinated, selective substitution at the primary carbon atom occurred; also multiple chlorination was avoided. The high selectivity can be attributed to the template effect of the zeolite. Zeolite LZ-105 includes n-alkanes into its channels and holds them in such a way that only the primary (terminal) carbons are exposed to the attacking chlorine atom (Figure 34). It is the tight fit of the alkanes in the channels that forbids both access of chlorine atoms to secondary carbon atoms and folding of the alkane chain, which would permit secondary carbons to be exposed to the reagent. These results suggest that tight fit is needed to orient molecules within reaction cavities having passive walls. Too tight a fit will leave no free volume within a reaction cavity that would be needed to accommodate displacement of atoms during the course of a reaction. This limits the number of transformations that can be achieved within a reaction cavity wherein the reactants are held tightly.

-

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

SIDE VIEW

153

AXIS VIEW

Figure 34. Side view and axial end-on view representations of dodecane adsorbed in the straight channels of zeolite LZ-105. Note that the end CH3 alone are exposed to the chlorine atoms reaction from outside the surface of the zeolite. [Reproduced with permission from N. J. Turro, J. R. Fehlner, D. P. Hessler, K. M. Welsh, W. Ruderman, D. Firnberg, and A . M . Braun, J . Org. Chem. 53, 3731 (1988).]

VI. ANISOTROPY (MICROHETEROGENEITY) IN ORGANIZED MEDIA We noted earlier (Section 1II.D) that there can be more than one type of reaction cavity in an organized medium. If the interconversion between molecules experiencing different environments of sites of an organized medium is slow on the timescale of excited state processes, then the excited state behavior of reactant molecules must be considered in terms of several reaction cavities accessed. Studies from several laboratories have shown that site inhomogeneity in organized media is more common than site homogeneity. We highlight this point with a few illustrative examples below.

Crystals. In Section III.D, we noted that the photobehavior of molecules on the surface and in the interior of crystals need not be the same. In addition the arrangement and excited state energy of molecules at defect sites are often not the same as in the bulk of the crystal. Indeed, there are a number of examples wherein the stereochemistry of photoproducts cannot be predicted on the basis of the molecular alignment of molecules in the bulk of a crystal [214].

154

V. RAMAMURTHY, R. G. WEISS A N D G. S. HAMMOND

For instance, head-head photodimers are predicted from the crystal structures of 9-cyanoanthracene and 9-anthraldehyde, but the head-tail isomer is produced. Craig and Sarti-Fantoni and later others found that photoreactions of 9-cyanoanthracene and 9-anthraldehyde take place at defect sites [96,215]. Systematic photochemical and crystallographic studies by Schmidt and co-workers uncovered many cases of substituted anthracenes which behave in an unexpected fashion (Scheme 40) [216,217]. Examples shown in Scheme 40 clearly illustrate that, unlike cinnamic acid derivatives, the stereochemistry of the product dimer from anthracenes cannot be predicted on the basis of crystal packing. An example from the laboratories of Venkatesan is noteworthy in this context [218]. Irradiation of crystals of 7-

I -Q,4-dichlorophenoxycar bony11 I,5-dichloro --%Head-to- head dimer 9 -Cl hV 4Head-to-tail 9-CHO dimer 9 -CN 1 ,lO-dichloro) 9 -Br

9 - C02Me 9-CO2H

t

J

hV

Lightstable

d- type I-CI

9-CI

hv

d

9-Me 9 - CONH2 Scheme 40.

Head-to-tail dimer

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methoxy 4-methyl coumarin gave both syn head-head and syn head-tail dimers. The former, on the basis of the X-ray crystal structure, is predicted to arise from the bulk of the crystal by a topochemical process; the latter is proposed to originate at defect sites. Occasionally, long-range disorder and/or different phases may coexist within a crystalline material. Arrangement of molecules in the different regions will necessarily be different in at least some respects. One of the earliest reports of invocation of this phenomenon involves the photodimerization of anthracene in the crystalline state [219]. In the crystal structure of anthracene, the faces of no molecules are separated by < 4 8. Yet upon irradiation, a dimer is readily formed. Thomas, Jones, and co-workers used electron microscopy to reveal the coexistence inside “normal” anthracene crystals of regions of a metastable phase. In the minor phase (space group Pl), the Cg...C,, distance is 4.2 8,whereas in the stable crystal it is 4.5 8. The dimerization is proposed to originate in the minor phase of the crystal. Surfaces-Silica, Clay. Dramatic evidence for inhomogeneities in the surfaces of silica, alumina, and clay is provided by the common observation of multiexponential decays of the excited states of adsorbed probe molecules. It is, in fact, very unusual to observe a single exponential decay when probe molecules like naphthalene, pyrene, methylviologen, and ruthenium tris(bipyridine) salts are adsorbed on these surfaces. Careful time-resolved study of pyrene adsorbed on silica surfaces have been conducted by a number of groups [220-2301. Several differ on details, but they all agree that pyrene molecules adsorbed on the surface of silica experience more than one environment. Although some of the studies attempt to fit the decay to a double or triple exponential function, Ware and co-workers argue that it is best fitted by a distribution function [226]. Further evidence for multiple sites on the surfaces of silica come from time-resolved emission and excitation spectra. The observations that the excitation spectra of the monomer and the excimer emissions are slightly different and that the excimer emission has no rise time have prompted the suggestion that the associated molecular structures are preformed on the surfaces of silica. Interestingly, the sites responsible for the multiplicity of behavior on these surfaces have not been clearly identified. Zeolites. Unlike silica and clay, zeolites possess interior structures that are uniform and well defined in shape and size. In spite of this, inhomogeneity in the microenvironment around a guest included in a faujasite zeolite may arise for two reasons: variation in the occupancy number within a cage and the presence of sites of varying microenvironment. Even at low loading levels, the

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cages may not be uniformly occupied (i.e., some may be singly occupied, some multiply occupied, and others not occupied at all). A similar situation also arises in channel-forming pentad zeolites which can be occupied at both intersections and main channels. Studies based on IR, Raman, UV-diffuse reflectance, NMR, neutron diffraction, and small angle neutron scattering reveal the presence of a number of different sites within faujasite zeolites [231]. For example, at high loadings three distinct types of benzene molecules, located within the supercages (one at the cation site I1 or 111, one at the 12-ring window site, and the last corresponding to benzene clusters within the cage), can be detected (Figure 35). In the case of pentad zeolites (e.g., ZSM-5), similar studies point to the intersection between channels as the preferred site at low loading levels. At higher levels of loading, both sinusoidal and straight channels are also occupied by the guests. The influence of the nonuniform character of the interior of zeolites on the photophysics of adsorbed guest molecules has been observed. Pyrene molecules included in zeolite faujasites show both monomer and excimer emission [232,233]. As in the case of silica surfaces, the excitation spectra of the emission corresponding to the monomer and the excimer differ (Figure 36), suggesting that there are at least two independent sites, each responsible for monomer emission and excimer emission. Time-resolved emission studies of pyrene included in Na+-X and Na+-Y ( ~ 0 . molecule 1 per cage) indicate Cation Site

Window Site

Figure 35. Two sites, the window and the cation sites, in which benzene molecules may be present within Na'X. Sites have been identified by neutron diffraction studies. [Reproduced with permission A. N. Fitch, H. Jobic, and A. Renouprez, J . Chem. SOC. Chem. Commun. 284 (1984).]

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Excimer (480 nm)

I 280

>

,

320

360

I

400

I

4 D

Nanometers

Figure 36. Diffuse reflectance and excitation spectra of pyrene included in Na’X. Note the monomer and excimer emissions possess different excitation spectra. This difference may be the result of nonuniform distribution of pyrene molecules within cages.

that the pyrene excimers are formed within IO-’s of excitation. Similar observations have been made with pyrenealdehyde [233]. Absence of any rise time for excimer formation suggests that one of the two sites mentioned above must have two molecules of pyrene adjacent to one another at the moment of excitation. Based on steady-state and time-resolved emission studies, Scaiano and coworkers have concluded that silicalite (a pentad zeolite) provides at least two types of sites for guest molecules [234-2361. The triplet states of several arylalkyl ketones and diary1 ketones (benzophenone, xanthone, and benzil) have been used as probes. Phosphorescence from each molecule included in silicalite was observed. With the help of time-resolved diffuse reflectance spectroscopy, it has been possible to show that these triplet decays follow complex kinetics and extend over long periods of time. Experiments with benzophenone and arylalkyl ketones demonstrate that some sites are more easily accessed by the small quencher molecule oxygen. Also, diffuse reflectance studies in Na+-X showed that diphenylmethyl radicals in various sites decay over time periods differing by seven orders of magnitude (z varies between 20ps and 30min) [237]. Caspar et al. have also observed multiexponential decay for naphthalene triplets in M+-X (M = K, Rb, and Cs) zeolites [238,239]. Interestingly at

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V. RAMAMURTHY, R. G. WEISS AND G. S . HAMMOND

temperatures below 150 K the decay was single exponential. However, at higher temperatures, the lifetime was determined by at least two independent first-order decays. This and different time-resolved emission spectra of naphthalene in Cs+-X for the slow and long-decaying components suggest the presence of at least two independent sites for naphthalene in M + - X zeolites. Site inhomogeneity in zeolite may also arise because of the restrictions in rotational freedom of molecules. Such behavior has been noted in the case of trans-stilbene [240]. The dependence of phosphorescence spectra of transstilbene included in Tlf-X on excitation wavelength is shown in Figure 37. This dependence can be attributed to the presence of various rotational (phenyl ring rotated with respect to the double bond) conformers of transstilbene within the supercage. trans-Stilbene dissolved in an organic solvent has a single minimum energy conformation. Inclusion Complexes. The excited state behavior of organic molecules included within cyclodextrin (in aqueous media) is often considered in terms of a single time-averaged structure. However, clear indications for the presence of more than one noninterconverting (within the excited state lifetime of guest molecules) structure in aqueous solutions of C D complexes has come to light recently. For example, Turro and co-workers noted that the excited singlet state of trans-stilbene included in fi-cyclodextrin exhibits a double exponential decay whose decay constants are 35 and 450 ps [241,242]. They proposed that the decay components are due to loose and tight complex structures (70 and 71).

1.600e+07-

0.00000

500

550

600 650 700 WAVELENGTH (nm)

750

800

Figure 37. Phosphoresence emission spectra of trans-stilbene included in TI'X. Emission maxima are excitation wavelength dependent.

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

70 -

159

71

fert-butyl-in 72 -

73 -

s), the presence of two slowly interconEven on the esr timescale verting p-cyclodextrinldiphenylmethyl-tert-butylnitroxide structures have been proposed [243, 2441, with “phenyl in” (72) or “tert-butyl in” (73) structures. Recently Bright et al. [245,246] have examined the fluorescence decay of systems consisting of several anilinonaphthalene sulfonates (ANS) included into p-CD. For 2,6-ANS, 74, they found that discrete one- or twocomponent rate laws failed to provide acceptable fits to their experimental data. Both unimodal Gaussian or Lorentzian distributions gave improved fits as compared with the discrete model, but it was not possible to differentiate between the two types of distributions. The authors argued that a reasonable picture of the system involves the ANS probe included in p-CD with an ensemble of different conformations, all in coexistence with each other. Figure 38 illustrates a recovered distribution and the suggested range of conformations of the inclusion complex.

74

75

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V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND i ,200 ~

- Gaussian - - Lorentzion

0.900--

0.600 --

0

2

4

6

8

Lifetime (ns) Figure 38. Recovered unimodal Gaussian (-) and Lorentzian (- - - - -) lifetime distributions for 10-5 M 2,6-ANS in 1OmM p-CD at 25°C. The inclusion complex structures shown on the diagram represent possible conformations responsible for the lifetime distribution process [Reproduced with permission F. B. Bright, G. C. Catena, and J. Huang, J . Am. Chem. SOC. 112, 1343 (1990).]

In addition to 1 : 1 (host-guest) complexes with different structures, a number of complexes with different host-guest ratios exist in equilibrium when aromatic molecules are included into either CI- or p-CD. For example, sodium l-pyrenebutyrate (75)/a or 8-cyclodextrin complexes in solution form equilibrium mixtures of a 1 : 1 and 2 : 1 complexes and form 1 : 1 and 2 : 2 complexes with y-CD (Figure 39) [247]. A consequence of the occupation of more than one site in solid inclusion complexes is illustrated with a single example, although many more are available in the literature. Detailed study of arylalkyl ketones included in deoxycolic acid (DCA) channels have been carried out by Lahav, Leiserowitz,

Figure 39. Various modes of complexation of guests within cyclodextrin. Possible structures of 1: 1, 1 : 2, and 2 :2 complexes are shown.

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and co-workers [136]. On the basis of the structures of a 3 : 8 stoichiometric complex of DCA with p-fluoroacetophenone it was expected that it would form two diastereomeric photoaddition products (Schemes 10-12). Unirradiated crystals of the 3 : 8 DCA-p-fluoroacetophenone contain two independent guest molecules G and G in the DCA channel. The guest molecule G' exposes the re face of its acetyl group to a steroid C,-H bond, the G molecule exposes mainly the si face of its acetyl group to a steroid C,-H bond (re and si corresponds to prochiral arrangement). Such an arrangement should yield a diastereomeric mixture from the addition of both G and G' to DCA framework. However, UV irradiation of the inclusion complex yields only one diastereomeric photoaddition product with the chirality S. X-Ray structural investigation of the irradiated crystal revealed that only the G' molecules had reacted. Were the G molecules to react they would force impossibly short contacts with a neighboring G molecule (Figure 40). This (b)

\

G'

G reached Droduct

G reached product

Figure 40. (a) Central picture showing fluoroacetophenone guest triplet G'GG' spanned by DCA steroid molecules. (b) Hypothetical arrangement were steroid S, to react with G. (c) Hypothetical arrangement were steroid S2 to react with G. Note the presence of two types of reactive fluoroacetophenone (G and G')in the DCA complex. In these crystals only G molecules react. Reaction of G molecules with either of the DCA molecules S, or S2 will result in short contacts. (Reproduced with permission from M. Vaida, R. Popowitz Biro, L. Leiserowitz and M. Lahav in Photochemistry in Organized and Constrained Media, V. Ramamurthy, Ed., VCH, New York, 1991, p. 247.)

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example illustrates the difference in reactivity between two crystallographically symmetry-unrelated molecules. Micelles. Traditionally the photophysical behavior of pyrene and other probes incorporated into micelles has been interpreted as if these molecules were embedded into a homogeneous phase [2,5,27]. For example, the singlet decay of pyrene in micelles is analyzed in terms of a single exponential. This approach has neglected the environmental heterogeneity within a single micelle as well as the presence of a distribution of micelles with different aggregation numbers. Although considerable amounts of water are present at the ill-defined region called micellar interface (or the head group region), the micellar core is largely water free. Further, at higher concentrations of surfactants the shape of the micelle can change from spherical to rod-like structures with widely varying aspect ratios [248,249]. Also, micelles containing a fluorescent probe can potentially contain zero, one, or more quencher molecules distributed theoretically according to Poisson statistics; the occupancy number of probes themselves would follow Poisson statistics [250]. Only recently, Ware and co-workers have addressed the structural heterogeneity that pyrene molecules must be exposed to in micelles [251,251]. In general, they find that the fluorescence decays are better fitted with a distribution function rather than by a single or two exponentials. Their work provides evidence for heterogeneity in guest/micelle interactions.

VII. NORRISH I1 PHOTOCHEMISTRY OF KETONES IN MEDIA AFFORDING REACTION CAVITIES WITH LITTLE OR NO CONSTRAINTS In previous sections we developed a model to describe how constraining environments may influence the course followed by molecules undergoing a variety of photochemical reactions. Examples that demonstrate the salient features of the model have been taken from a wide spectrum of reaction types in order to show the breadth of potential applications. In Section VIII we apply the same model in greater detail to Norrish I1 reactions of ketones. By doing so, a set of similar mechanistic criteria can be viewed in many constraining environments, allowing a more systematic picture of the model to be drawn. To place in perspective the Norrish 11 reactions of ketones in constrained

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environments, we shall first provide a very brief introduction to the photophysical properties of ketones and their possible common photoreactions. Thereafter, Section VIII is limited to examples in which reactions competing with Norrish I1 processes are unimportant. The description of Norrish I1 processes in environments which are expected to afford little or no external constraints on the internal motions of electronically excited ketones and their intermediates provide a basis for comparison of reactivity and selectivity in the more constraining and anisotropic media.

A. Photophysical Characteristics of Ketones Aliphatic ketones show broad, low-intensity absorption maxima in the vicinity of 280 nm which are a result of n + n* transitions. By use of the Stark effect, Freeman and Klemperer estimated that the dipole moment in the iz, n* singlet state is reduced to 1.48 D from its ground state value of 2.34 D [253]. In the vapor phase, the emission from excited acetone has been shown to be a mixture of fluorescence and phosphorescence [254]. The estimated lifetime of the excited singlet state is 10 ns, a figure commonly accepted as a reasonable approximation to the excited singlet to triplet (S, + T,) transition time in aliphatic ketones. The overlap of the fluorescence and phosphorescence spectra reflects the fact that the energetic separation between the lowest n, n* single and triplet states is small, at least in comparison with the S , - T, splitting between lowest excited n, n* singlet and triplet states. Alkyl aryl ketones and diary1 ketones have n --+ n* absorptions shifted to longer wavelengths than those of alkanones: acetophenone has Amax at 3 10330 nm and benzophenone exhibits Amax at 345 nm in cyclohexane solution. These absorption bands are considerably overlapped with transitions at shorter wavelengths attributed to n + n* transitions. The limited mixing of n, n* and n, n* states has two consequences. First, the lifetimes of the lowest excited single states of aryl ketones are very short (e.g., for S, +T1 in benzophenone, k,,, x 10-l' sec-I). Second, in many aryl ketones, a triplet state having essentially n, n* configuration may be lower in energy than the n, n* triplet. Consequently, reactivity associated with n, n* excited states may not be observed because the decay,

-

allows rapid reversion of the excited state to a n,n* configuration which frequently has entirely different reactivity.

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V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

B. Photoreactions of Simple Ketones Other than Norrish I1 Processes A recent, extensive review by Wagner and Park on photoreactions of ketones has appeared [255]. Our discussion is limited to simple ketones in which the excitation energy is localized primarily in the carbonyl group, allowing the n,n* configuration to be assigned in first approximation to the lowest excited singlet and triplet states. In such ketones and some others in which n,n* excited states appear to be lowest in energy, cycloaddition to other unsaturated molecules (Paterno-Buchi reaction) and hydrogen abstraction from molecules bearing suitably activated R-H bonds (photoreduction) can be important bimolecular processes. In both cases, the initial chemical changes appear to emanate from interactions with the electron-deficient oxygen atom of the carbonyl group. In alkyl aryl ketones with very short singlet lifetimes, virtually all reaction occurs from the triplet manifold; in aliphatic ketones with longer singlet lifetimes, reaction can be traced frequently to both singlet and triplet states. The net course of simple examples of both reactions are shown in Eqs. 2a and 2b.

Both of these bimolecular reactions depend upon diffusion and, frequently, additional (energetic) constraints. As a result, suitably designed ketones can be made to undergo unimolecular photochemical reactions to the near exclusion of the bimolecular ones. The most common of the unimolecular reactions are called Norrish I (involving cleavage of a C-C bond alpha to the carbonyl group) and Norrish I1 (involving abstraction by oxygen of a hydrogen atom on a carbon located gamma to the carbonyl group [256]) processes. The initial step in each reaction is shown in Eqs. 3a and 3b.

R" OH

*/

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165

Whether a ketone can undergo a-cleavage depends on a complex function of the dissociation energy of the bond being broken and the excitation energy of the excited state, and competition with other physical and chemical decay processes available to the excited states. Suitably designed ketones which eschew Norrish I reactions in favor of the Norrish I1 pathways are plentiful and their photochemical behavior has been studied in depth.

C. Norrish I1 Processes Norrish discovered the cleavage reactions (E of Eq. 4) of excited state ketones to methyl ketones and olefins [257]. It was later recognized by Yang and Yang [258] that the biradical intermediates (BR) can also cyclize (C) to yield cyclobutanols.

R

UR!’ I

E

L C

There has been a tendency to call both the E and C pathways as Type I1 or Norrish I1 reactions since it is well established that they emanate from common intermediates. Wagner has suggested that the C pathway be called more appropriately the “Yang reaction” [255]. Whether this is followed will depend upon the tolerance of photochemists for the proliferation of “name reactions.” We group the two processes together while recognizing the fundamental work of Yang. The major features of the Norrish I1 reactions which are germane to this chapter are included in Scheme 41. Note that each structure in Scheme 41 represents a family of conformers which are related by similarities in both structure and reactivity. A ketone molecule in collision-free space can exist in a variety of conformations produced by rotation around single C-C bonds. In a linear alkanone, the conformation of lowest energy is all-trans although local minima can exist where there is one or more gauche arrangements.

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R-YoH+

I

I

LA R

Scheme 41.

Internal vibrations and rotations can combine to bring the molecules into orientations necessary to accomplish photoreactions such as y-hydrogen abstraction to form LBR, the first step after ketone excitation. In the vast majority of examples, excited states with n, n* configurations participate in the process shown in Eq. 3b [255]. However, there are some notable exceptions [259]. In alkanones, both excited singlet and triplet n, n* states may be involved (owing to the relatively long lifetime of the former) so that the i-BR may be singlets or triplets also. The exceedingly rapid intersystem crossing rates of excited singlet states of alkyl aryl ketones limit reaction to the triplet manifold. If we accept the generalization that kS+T of all simple alkanones is lo8 s-', the component of y-hydrogen abstraction occurring from the singlet state must proceed at a comparable rate since the two are competitive. In isotropic solvents of low viscosity, equilibration of all conformers is assumed to be complete within 10 ns at room temperature. Consequently, the inferred reaction rate constant k, is a composite figure, reflecting both variation in rates and equilibrium populations of various conformations. In

-

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167

linear alkanones, the extended all-trans conformations of the alkyl chains are believed to be the most highly populated. Logic dictates that they are unreactive. Therefore, those conformations that are well prepared to enter the six-membered transition state required for hydrogen transfer [256] must have reaction rate constants much higher than the average value. On the other hand, because both the C=O. ..H7distance and the relative orientation of the C=O and C-H bonds are believed to have somewhat specific optima (i.e,, for which d < 3 A and z and A of Figure 41 are near 0 and 90°, respectively) [49, 255, 2601, the y-abstraction process will, in some cases, involve geometric parameters which are not “ideal.” Some of the unfavorable aspects of nonideal geometries can be offset if long residency times in a few conformations are imposed on the molecule either by its own structure or by the nature of the medium. Thus, if the ketones are held preferentially or rigidly in discrete conformations, y-hydrogen abstraction may be either very rapid or very slow. We can express these ideas via the simplified mathematical formalism for k , in Eq. 5 which assumes the importance of only one excited state type:

where

ki = the specific rate constant for reaction in configuration i ti = the time spent by the excited molecule in configuration i X ti = the total lifetime of the excited state The k , values at room temperature in a series of eight substituted valerophenones have been measured by various authors. They range over two orders of magnitude but the majority are between 2 and 8 x 10’ sec-’ (i.e., of the same order of magnitude as was inferred for reactions of excited singlets of alkanones).

Figure 41. Distance and orientational parameters associated with the efficiency of y-hydrogen abstraction by a carbonyl oxygen atom. (Copied with permission

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V. RAMAMURTHY, R. G. WEISS AND G. S . HAMMOND

The concept of discrete configurations from which reactions may emanate loses much of its clarity in liquids of high fluidity. A large, but definable number of configurations of the excited molecules can be formulated if one considers only the staggered orientations about each of the single bonds in acyclic alkanones. The number becomes very large and undefined if account is taken of all configurations through which the molecule will pass while exploring all of the local energy minima; yet reaction may occur from such transitory arrangements also. The discussion has been overly simplified because some rotational motions may be regarded as components of the reaction coordinate for chemical reaction from configurations regarded as local energy minima. Introduction of such refinements will gain us nothing since the simplest picture is already more complex than we can manage in an analytical mode. Thorough analysis of the possibilities, which is further complicated with alkanones, because they undergo Type I1 reactions from both excited singlet and triplet states, is not feasible, but we will consider photochemical results under a wide variety of conditions within the conceptual framework. Alkyl aryl ketones such as alkanophenones, have been studied much more than alkanones and most of our quantitative knowledge of the dynamics of BR formation and decay come from such systems. In spite of the simplifications introduced by the mechanistic importance of only one alkanophenone excited state, the triplet, and presumably the formation of only triplet biradicals from it, a rigorous solution to Eq. 5 in virtually all real systems remains not possible. It is highly probable that most of the conformational exploration of the B R s occurs while they are triplets. Ultimately, the triplets will convert to singlets which probably react (by cyclization, fragmentation, or reverse hydrogen transfer to the parent molecule) before significant conformational change occurs. The lifetimes of the BRs are of critical importance to any attempt at quantitative analysis of the factors which will determine quantum yields and product distributions (E/C and t/c ratios) in Type I1 reactions of ketones under various reaction conditions. Virtually all information about lifetimes is derived from study of triplet BRs and much of it has been provided, and reviewed, by Scaiano [261]. There are many interesting reactions, both bimolecular and unimolecular, which occur at only one of the radical centers but they have little relevance to this chapter and are not discussed here. BR triplets derived from alkanophenones have lifetimes of 25-50 ns in hydrocarbon solvents. They are lengthened several fold in t-butyl alcohol and other Lewis bases capable of hydrogen bonding to the OH groups of the BRs. The rates of decay are virtually temperature independent but are shortened by paramagnetic cosolutes such as O2 or NO. The quenchers react with the BRs

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169

to form products, but in insufficient amount to account for the decreases in lifetime. It seems that paramagnetic quenching of triplet (to form singlet) BRs must occur. Since the product distribution between fragmentation and cyclization is changed, Scaiano posits that the relative rates of intersystem crossing of the various triplet BR conformers are altered by paramagnetic catalysis. The important question now arises as to whether uncatalyzed intersystem crossing rates are different for different conformers. If they are the same, the product distributions reflect the relative population of the conformers which we believe to be essentially equilibrated during triplet BR lifetimes for simple structures in isotropic liquids with high fluidity. Simple structural considerations dictate that t-BR is the precursor of fragmentation products while c-BR, and c-BR, yield trans and cis cyclobutanols, respectively, and possibly some fragmentation products depending upon the nature of the initial ketone and the reaction medium. Because of the near orthogonality of its singly occupied orbitals, i-BR can only retransfer a hydrogen atom to reform reactant ketone molecules. In media that force the intramolecular motions of BRs to be slower than their rates of collapse to products, the smallest motion pathway from i-BR to a preproduct conformer involves primarily rotation by -90" about the C-C bond alpha to the COH radical center. In such systems, very low E/C product ratios may be predicted. Usually, elevated values of E/C are found in viscous anisotropic media! If the rates of intersystem crossing vary with conformation, the problem of correlating photoproduct distributions and the relative abundances of equilibrated BR conformers is far more complex. Scaiano favors this view and theoretical considerations offer some support for it [262]. However, Caldwell and co-workers [263] have collected inferential experimental evidence to the contrary. Wagner [264] has advanced the notion that bond formation and intersystem crossing are not discrete steps, but are, in fact, coupled in one process. The question remains unsettled but renders tentative any rationalization of the effects of confining media on Type I1 reactions offered by ourselves or others.

1. Medium Effects on Norrish I1 Reactivity and Photoproduct Selectivity. In condensed (i.e., liquid or solid state) media, the energetic relationships among the conformations and the rates of passage among them may be modified owing to bulk influences and specific interactions of the solvent with functional groups on the ketone molecules. Norrish I1 reactions have been used as an indicator of such effects since the conformational requirements for initiation of reaction via abstraction of y-hydrogen atoms and ring closure of the intermediate biradicals [49, 255, 2601 to form cyclobutanols are rather specific. The degree to which fragmentation of BRs is able to compete with the cyclization mode appears to depend upon internal (the location and

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V. RAMAMURTHY, R. G. WEISS A N D G. S. H A M M O N D

nature of pendant groups on the ketone molecule) and external (solvent interactions with the BRs analogous to those suffered by the ketones) considerations. The influence of these various effects may be manifested in measurable parameters of the reaction like the overall quantum yields ((DII) and the photoproduct ratios for fragmentation to cyclization (E/C) and for trans to cis cyclobutanol formation (t/c) as shown in Scheme 41. The values of these quantities and their variations as the media are ehanged can provide comparative information concerning the relative importance of solvent anisotropy on Norrish I1 reactions, also. Specifically, they reveal characteristics of the activity of the walls and the size, shape, and rigidity of the reaction cavities occupied by electronically excited ketones and their BR intermediates. Simple expressions for these three reaction parameters are given in Eqs. 68. Subscripts specify the multiplicity of the excited states. The expression for (D,, includes the singlet component 41as well as the triplet one which is given by the product of the intersystem crossing quantum yield 413and the fraction of triplets which eventually result in products 43. As mentioned previously, the singlet related terms in the three equations are considered negligible when alkyl aryl ketones are the reactants.

Some interactions with the extremely flexible and nonhomogeneous reaction cavities of even isotropic solvents can affect Norrish I1 reactions. For instance, values of E/C from some classes of ketones tend to be higher in polar solvents than in nonpolar ones as a result of template effects related to the association of hydroxyl groups of BR intermediates with dipolar groups which are a part of a cavity wall. In nonpolar nonviscous liquids, the quantum yields and photoproduct ratios of n-alkanones vary remarkably little. This is most readily interpreted as meaning that the conformational equilibria of the molecules in their ground states and the rates of exploration of various conformations are independent of the molecular structure of the solvents. Since alkyl aryl ketones react only from their triplet states, it is not surprising that their photoproduct ratios vary only slightly in nonpolar, nonviscous solvents, also. However, from the fact that quantum yields and photoproduct ratios do change markedly when some alkanones and alkyl aryl ketones are irradiated in organized and viscous media, we infer that in

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171

those cases the reaction cavities restrict motions and shape changes of the ketone and its intermediates more than do fluid isotropic media. Another factor which should influence only minimally Norrish I1 reactions in fluid isotropic media is the size and shape of the photoproducts relative to each other and to the reactant ketone. However, in media that provide reaction cavities with stiff walls, this factor may be of paramount importance. As shown in Figure 42 for the Norrish I1 reactions of a simple ketone, 2nonanone, not only do the shapes of the products differ from those of the reactant, but so do their molecular volumes [265]. Interestingly, the volume of the fragmentation products, 1-hexene and 2-hydroxypropene (which ketonizes to acetone), are closer in volume to 2-nonanone than is either of the cyclization products. They are also capable of occupying more efficiently the shape allocated by a stiff solvent matrix to a molecule of 2-nonanone in its extended conformation; the cross-sectional diameter of either of the cyclobutanols is much larger than that of extended 2-nonanone or the fragmentation products when spaced end-on. Both of these considerations should favor fragmentation processes if isomorphous substitution for the precursor ketone in the reaction cavity is an important requirement for efficient conversion to photoproducts.

VIII. NORRISH I1 REACTIONS IN ORGANIZED MEDIA A. Neat Crystalline Phases The initial (and, perhaps, effective) reaction cavity provided to carbonyl and adjacent groups by the matrix of like ketone molecules in their solid state is perhaps more readily characterizable than any other if the crystal structure is known or can be inferred by reasonable analogy. The number of cavity types is most frequently one (and almost never exceeds two or three). If the fixed configurations of the ketones in their normal cavity sites do not allow any reactions during the excited state lifetimes, as mentioned previously (Sections 1II.D and VI), some may still occur at crystal surfaces, edges, and defect sites. In experiments in which predominant surface excitation is avoided by irradiation with light in the red edge of the absorption band of the sample, reactions occurring exclusively at “abnormal” sites will be signaled by lower overall quantum yields than those measured upon excitation with radiation of wavelength near an absorption maximum. If reactions at defect sites are important photoproduct ratios are usually not very different from those measured after irradiation of isotropic solutions. However, the absence of

M O ~VOL: .

0

~

~

/

~

1

cis M O ~voi.: .

81.6 ~ m ~ / ~ 0 1 Cross section: 5.7 x 6.5 cm2 Length: 11.2 cm

~

Mol. Vol.: 82.3 cm3/m01 Cross section: 5.7 x 6.5 cm2 Length: 11.2 cm

trans

51.8 cm3/mO1 Cross section: 5.0 x 3.2 cm2 Length: 9.8 cm

M O ~voi.: .

Figure 42. Molecular volumes of 2-nonanone and its initial Norrish I1 photoproducts from MNDO-optimized geometries [265a] using the method of Bondi [265b].

91.6 ~

MO~.VO~.: 36.7 cm3/mO1

-

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

173

defect sites does not ensure exceptional photoproduct ratios or quantum yields [48,88,266]. 1. Reaction Cavities of Alkanones in Neat Solid Phases. The early report that irradiation of crystalline 7-tridecanone at 10°C does not result in discernible photoreaction [267] has been corroborated subsequently with other solid symmetrical n-alkanones [268]. However, careful scrutiny of the irradiated ketone reveals traces of Norrish I1 products in ratios which are very close to those found from photoreactions in solution. On this basis, it was concluded that the source of the photoproducts is reactions occurring at crystal defect sites. Available powder diffraction X-ray data on symmetrical di-n-alkyl ketones [269] demonstrate that the molecules pack in layers in completely extended conformations, Since the group volume of a carbonyl (11.75 cm3 mol-') is calculated to be only 15% greater than that of a methylene group (10.22 cm3 mol-') [270], it is reasonable that a carbonyl and its neighboring methylene groups located in the middle of a closely packed layer are unable to attain the bent geometry necessary for initial y-hydrogen abstraction during the singlet and triplet excited-state lifetimes of the ketone; it is known from spectroscopic studies with solid n-alkane phases [271] that the region of greatest molecular stiffness is the middle of a layer. The inflexibility of the walls and the very small free volume around the carbonyl groups limit severely any deviations from the extended geometry. By contrast, 2-alkanones in their neat solid phases do react with appreciable (but unmeasured) quantum efficiencies [272]. The photoproduct ratios decrease with increasing percent of conversion, indicating that the reaction cavity is disturbed significantly by the presence of photoproducts in the alkanone matrices. The ratio remains invariant with percent conversion when irradiations are conducted in the liquid (melt) phase. At
174

V. RAMAMURTHY, R. G. WEISS A N D G. S. HAMMOND

potential importance of hydrogen-bonding interactions and template effects along the active walls of the reaction cavity are only conjectural in this case. The distribution of photoproducts from two perfluoroalkyl alkyl ketones 76 in their neat solid and isotropic phases have been examined also [273,274]. Although the packing arrangements of the crystals are unknown, they are likely to be layered, but probably not exactly like the solid phases of the simple alkanones. The larger cross-sectional area of a CF, group (versus a CH, group) may force the 76 molecules to interdigitate [275] or their alkyl portions to kink within a layer. Regardless, the placement of the carbonyl group in the two 76 homologues near their central carbon atom forces the reaction cavity to be near the middle of a solid layer. The consequences of the potential disturbance to that cavity by the factors alluded to above are evident in the photoproduct ratios. As seen in Table 8, the E/C ratios in the solid phases are not very large when compared to those from other solid 2alkanones, although the t/c ratios are larger than those detected from irradiation of solid 2-alkanones [272]. Since the values of the product ratios from the neat isotropic phase and either hexane or perfluorohexane are very similar [273,274], the source of the higher photoproduct selectivities in the solid phase must be the same reaction cavity factors cited for the alkanones. It is noteworthy that the solid-state photoproduct ratios reported in Table 8 decrease upon further irradiation, as do the 2-alkanones.

m, n = 7 , 8 and 9 , l O TABLE 8 Photoproduct Ratios from Irradiation of 76 in Their Neat Isotropic and Solid Phases [273, 2743 76 (m,4 (7, 8)

(9, 10) "i

=

Conversion T("C)

Phase"

(%I

E/C

t/c

25 1 50 20

i k i k

< 15 < 15

3.7 k 0.4 10.9 k 2.5 3.5 k 0.3 31

3.4 & 0.3 3.9 & 0.5 3.4 k 0.2

isotropic and k

=

crystalline.

<5

< 10

18

175

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

Scheffer, Trotter, and co-workers have provided elegant demonstrations of the distance and angular dependence between a carbonyl oxygen atom and a gamma hydrogen atom on the ease of initial abstraction [276]. Their singlecrystal X-ray structural information allows the various courses of Norrish I1 reactions of neat solids to be understood in the context of topochemical control [ l l , 131. For instance, they have noted that the cyclic diones 77 ( n = 7,8,10, and 12) follow different Norrish I1 pathways depending upon the conformations of the individual molecules in their crystals (Eq. 9) [277].

\

0

As discerned from Table 9, the photoproduct ratios from irradiation of neat 77 vary drastically among homologues and with temperature for the n = 7 member. Unlike the n-alkanones discussed previously, the cyclic diones must be intrinsically bent as shown in the ORTEP drawings in Figure 43. Since the C=O...H, distance in each case is c3.0& excitation of the molecules should result in formation of hydroxy-1,4-biradicals. The fate of those radicals depends upon the flexibility of the reaction cavity and its free volume. The results indicate that sufficient free volume is present in the reaction cavities to allow topochemical conversion to photoproducts, but not TABLE 9 Dependence of Distributions of Photoproducts on the Phase of 77 12773 Relative Yields 77

.-

n=

Phase"

7

k k Hexane solution k k Plate-k Needle-k

8 10 12

"k = crystal.

T("C) 20

40 40 20 20 RT RT

78

79

89 17 19 3

10 20 30 84

4

91 <5 >90

>95 5-10

80 1 63 51

13 5 <1 <1

176

V. RAMAMURTHY, R. G . WEISS AND G. S . HAMMOND

(4

(b)

Figure 43. ORTEP structures of 77 (n = 12) in its plate (a) and needle (b) dimorphs. Only one half of the atoms is numbered since each structure has a molecular center of symmetry. (Copied with permission [277b].)

enough to permit solution-like conformational changes. In fact, the walls of the reaction cavity must be quite rigid since the major cyclobutanol stereoisomer in each case can be predicted from the ORTEP drawings. Even the small amount of 78 from irradiation of the needle crystals of 77 (n = 12) can be rationalized by the nonequivalent local environments of the two carbonyl groups in each molecule and the expected greater difficulty of one of them to abstract a gamma hydrogen. As with 77 (n = 12), the change in the photoproduct ratios from 77 (n = 7) at 20 and 40°C is due to two solid phases rather than a strong temperature dependence. The lack of any discernible fragmentation (80) from the crystals of 77 (n = 12) and a 1% fragmentation yield which are from a dimorph of 77 (n = 7) are very suggestive that the c-BRs (which are topochemically favored) do not fragment in this environment. In fact, this may be a property of this type of cisoid biradical rather than its environment.

177

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

2. Reaction Cavities of Alkanophenones in Neat Solid and Liquid-Crystalline Phases. As mentioned previously, solid-state studies on the Norrish I1 processes of alkyl aryl ketones are unambiguous with respect to the triplet multiplicity of the reactive excited state. On the other hand, a bulky aryl auxochrome can create complications during the transformation of the excited triplet states to photoproducts in neat anisotropic phases. An even more dramatic example of the potential lack of selectivity afforded to the Norrish I1 reactions of ketones by supposedly very ordered systems than that described in the 76 systems is provided by neat samples of the mesomorphic alkanophenones (81) [278]. These molecules are capable of existing in nematic and smectic B mesophases (see Figure 16) as shown in Scheme 42. The instability of the monotropic smectic B phase of 81a and smectic B phase of 81b did not allow their photoreactions to be examined; these smectic phases became solids soon after the initiation of irradiation. 81a:

1

38.4 ( 4 3 )

solid I < _________ t solid I 1>-

60.7 (56)

58.0 ( 2 , 3 )

34.0 ( 3 5 ) 52.5 ( 1 3 )

smectic B- < &b:

solid I smectic

81c: -

A

solid I

35.8 ( 4 4 )

_________ > solid

t

50.5 ( 2 . 0 )

II

nematic

40,2 (11.7)

1

isotropic nematic

56.5 ( 5 3 )

64.0 ( 1 . 8 )

nematic 64,3 (1.9)

isotrooic

70,O ( 4 4 . 5 ) > smectic B

solid I 1 <

43.8 ( 4 4 , 2 )

TI

(20.0) (20.8) 7 1 - 2 1171.5

isotropic

ad:

solid>-

70.2 (52,2)

4

smectic B

80.1 ( 2 8 . 4 )

isotropic (27.5)

smectic E c

43,O

smectic B

Phases, transition temperatures ("C),and heats of transition (J/g) in parentheses from heating end cooling of 81.

Scheme 42.

178

V. RAMAMURTHY, R. G. WEISS A N D G. S. HAMMOND

However, the monotropic nematic phase of 81a was sufficiently stable to allow its Norrish I1 reactions to be examined. Similarly, Norrish I1 product ratios from irradiation of the smectic B phases of 81c and 81d were easily measured. Although the packing of molecules in the solid phases of the 81 homologues is unknown, inferential evidence supports their being layered also.

m

81

5 7

a b C

7 7

d

The changes in E/C ratios from the four neat 81 homologues versus temperature (phase) are presented in Figures 44-47. The corresponding t/c ratios are not included since the diastereomeric cyclobutanols could not be separated analytically. As can be seen, there is no apparent variation in E/C

Yw

T1

28 -

24

K I1

KII * KI

36 32 -

20 -

N

I

1

16-

128-

4-

0

5

1

'

"

15

'

'

25

T

35

I

,

,

45

, T I

Temperature

55 ('C

T,

,

65

,

,

75

,

85

)

Figure 44. Plot of the E/C product ratios from irradiation of neat 81a versus temperature ("C) on cooling from the isotropic phase ( 0 )and on heating from the solid I phase (a). Arrows indicate phase transitions on cooling from the isotropic phase.

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

"

'

1

I

90 80 70

O0

179

60 .

50 40 30

1

7

-

10 -

0'

"

'

I

1015 20

- I

0 "

25

'

I

30

'

T , , , , ,

35 40

45

Temperature

0 ,'

,Y ,

,

50 55 60 6 5 7 0 75 80 (-c)

Figure 45. Plot of the E/C product ratios (0)from irradiation of neat 81b versus temperature ("C), and (0)data from irradiations in dilute benzene solutions. Arrows indicate phase transitions.

Temperature

C

O

C

1

F w e 46. Plot of the E/C product ratios from irradiation of neat 81c versus temperature. Arrows indicate phase transitions.

180

V. RAMAMURTHY, R. G. WEISS A N D G. S. H A M M O N D

~ e m p e r a t u r e ("C>

Figure 47. Plot of the E/C product ratios from irradiation of neat 8ld versus

temperature. Arrows indicate phase transitions.

ratios from irradiations in the nematic, smectic A, smectic B, or isotropic phases. The larger E/C ratios in the monotropic temperature ranges of the smectic phases can be attributed to rapid phase transitions initiated by small amounts of photoproducts and subsequent solid phase reactions [278]. On this basis, the reaction cavities in the mesophases must be larger than the van der Waals volume of the part of the BR which is central to reaction and/or have very flexible walls. We suspect both in spite of the fact that 'H NMR studies with 81 deuteriated at the methylene carbons alpha to the carbonyl groups indicate that this region of the ground state molecule in the liquidcrystalline phases is quite constrained conformationally. Even if some interdigitation of the alkyl chains were present in the anisotropic phases of these molecules, the volume they would occupy does not equal the volume potentially available if the cyclohexylphenyl core groups of 81 limit the intermolecular distances; these phases are well ordered macroscopically, but some parts of the molecules must be much less organized. The solid phases of 81 are also well ordered macroscopically and their higher E/C ratios require that the hydroxy- 1P-biradical be in rather inflexible reaction cages with little excess free volume. Hydrogen bonding to neighboring ketone molecules may be partially responsible for the high photoproduct ratios found upon collapse of the biradicals in the solid phases, but the size, shape, and flexibility of the reaction cavity are clearly the more important factors. The highest E/C ratios observed in the second solid phase of 81a

181

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

exceed 100, indicating that the ability of its polymethylene chains to bend is severely compromised. On the other hand, the quantum yields are not inconsequential so that it must be possible for a reasonable fraction of the molecules to reach a conformation in which y-hydrogen abstraction can occur during the triplet lifetime of 81. Since “melting” of the alkyl chains of 81 is probably a key factor in converting a solid to a liquid crystal, it is not unreasonable to assume that the reaction cavities might differ in the two phases. Another important difference between them is that molecules of the crystal are translationally fixed within a lattice while they are free to diffuse anisotropically in the liquid crystal and, in some cases, cooperatively with their neighbors. Thus, during its lifetime, a BR in liquid-crystalline 81 may migrate to places where the free volume is greatest or wait until adequate free volume diffuses to it before decaying to products. As mentioned previously, the concept of a “reaction cavity” as a discrete, definable entity becomes tenuous in this case. Again, Scheffer and co-workers have provided the most extensive and intensive studies concerning the Norrish I1 reactions of alkyl aryl ketones in neat crystals [49,260,276]. Their analyses, which assume that the reaction cavity is very stiff (i.e., that ground state ketone molecules surrounding a hydroxy-1,4-biradical intermediate remain in almost the same conformations and positions as in the pristine crystal) and which do not consider intermolecular hydrogen bonding between the carbonyl oxygen of neighboring ketone molecules and the hydroxyl proton of a BR, have provided important insights into how neat solvent order influences Norrish I1 processes. For example, they find that initial hydrogen abstraction by the triplet states of crystalline cr-cyclohexyl para-substituted acetophenones (82a-c) occurs from boat-like transition states [279]. The fate of the so-formed triplet BRs is similar to that experienced by them in acetonitrile solutions and the photoproduct ratios are constant over a wide temperature range. Although there is a somewhat greater preference for fragmentation (83) in the solidstate irradiations, the 84/85 ratio from each of the three ketones is virtually invariant in its solid and in acetonitrile solution (Scheme 43). Crystal structures show that all three ketones adopt very similar conformations in the crystal which should favor the abstraction of an equatorial y-hydrogen atom of a cyclohexyl ring on the basis of the C=O...H distance ( 2.6 A versus 3.8 A for axial y-hydrogen abstraction) and orientation (see Newman projection of 82 and its biradical in Figure 48). The biradicals, once formed, are poorly shaped to provide any of the Norrish I1 products. Since extensive motions are required to bring the 1,4-biradicals to preproduct conformations [279], the reaction cavity must be flexible and capable of either rearranging or adding an appreciable amount of free volume during the short triplet biradical lifetimes. An intriguing possibility is that the elusive singlet

-

N

182

Ar

52

L

ArCOCH3

83 a, Ar

-

V. RAMAMURTHY, R. G. WEISS A N D G. S . HAMMOND

= p-tolyl;

b, Ar

Ar

";o""'U3 84

= p-chlorophenyl;

HA

c, Ar

55

= p-methoxyphenyl.

Scheme 43.

biradicals [280] are formed (via intersystem crossing of the triplets) long before they collapse to the products. The fragmentation route of hydroxy-1,4-biradicals could be effectively removed as a viable possibility by replacing the cyclohexyl rings of 82 with a 1 -adamantyl group; the l-adamantene which would be formed violates

%Ax

X

Figure 48. Newman projections along the equatorial carbon-carbon bond of 82, its biradical, and its cyclization product (Copied with permission [279].)

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

183

Bredt's rule. In this way, 1-adamantyl p-methoxyacetophenone 86a was forced to yield only cyclobutanols 87a and 88a as photoproducts 12811. Whereas (benzene) solution phase irradiations of 86a yielded a 2.6 ratio of 87a/88a, the solid state photoproduct ratio, 0.5, favors the more sterically hindered cyclobutanol. X-Ray diffraction studies predict a chair-like yhydrogen abstraction pathway for 86a (in contrast to the boat-like transition states of 82) in which the C=O...H, distance is 2.67 A. Other abstractable hydrogens (HJ are at least 0.3 A farther from the carbonyl oxygen (Scheme 44). If 1-BR has a conformation which mimics that of the ketone, its least motion pathway favors formation of the more sterically hindered cyclobutanol 88a.

86 a ) Ar b ) Ar

88

=

pmethoxyphenyl p-chlorophenvl

87

Scheme 44.

What is as noteworthy as the excess of 88a from the solid-state irradiation of 86a is the presence of any 87a at all. Its formation requires either that one of the more distant y-hydrogen atoms be abstracted also or that the reaction cavity suffer some dramatic changes in shape and size while the hydroxy-1,4biradical is present. Based upon the lattice properties of 86a, the latter seems the more probable scenario; without the movement of groups on adjacent

184

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

ketone molecules or extensive molecular motion of the 1,4-biradical within a unit cell, no 87a should be possible. Rather phase-insensitive Norrish I1 photoproduct ratios are reported from irradiation of p-chloroacetophenones with a-cyclobutyl, a-cyclopentyl, acycloheptyl, a-cyclooctyl, and a-norbonyl groups [282]. In each case, the E/C and cyclobutanol photoproduct ratios are nearly the same in neat crystals as measured in benzene or acetonitrile solutions. On this basis, we conclude that the reaction cavity plays a passive role in directing the shape changes of these hydroxy-1,4-biradicals. As long as the initial ketone conformation within the cavity permits y-hydrogen abstraction (and these ketones may be able to explore many conformations even within their triplet excited state lifetime), the cavity free volume and flexibility allow intramolecular constraints to mandate product yields. These studies demonstrate that the reaction cavity provided by a neat crystal may or may not remain passive and stiff during the transformation of a ketone to its Norrish I1 photoproducts. Significant flexibility of the reaction cavity can accompany the transformations. In fact, Norrish I1 photoproducts form in crystals where the C==O...H, distance exceeds the sum of the van der Waals radii of 0 and H (2.7A) [282]. (This observation is somewhat deceptive since it is known that the C-0 bond length of ketones is longer in n, IT*states than in the ground state and the carbon atom may not be planar [282].) On the other hand, as witnessed by the lack of reactivity of the solid sym-alkanones and the almost complete stereospecificity exhibited by Norrish I1 rearrangements of the dimorphs of 77 (n = 12), the reaction cavity of neat crystals can provide rigidity and shape which make the motions necessary for some photoproducts to form so energetically disfavored that they cannot compete with others. Examples of this extreme stereoselectivity and specificity have been found by Scheffer and Trotter in crystals of a-adamantyl p-chloroacetophenone (86b), also [282]. Figure 49 shows ORTEP-like drawings of the conformations in the dimorph plate and dimorph needle crystals of 86b as obtained upon recrystallization from different solvents. A major difference between the two is the cant of the aryl ring (and the carbonyl group) with respect to the adamantyl moiety. The selectivity afforded upon irradiation of the plate dimorph of 86b can be traced to the cant of the aryl rings and the specific disposition of the carbonyl group in the molecule (Table 10). Since one of the ortho aromatic hydrogen atoms is between an adamantyl methylene and the methylene alpha to the carbonyl group, the ring is impeded intramolecularly from rotating without prior motions of the bulky adamantyl group. Although formation of 87b is quite feasible from a BR of 86b in this conformation, 88b is not. We conjecture that the cavities of the biradicals in the plate and needle dimorphs

185

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

x

h

needles

P I atG

Figure 49. Structures of 86 in its dimorphic crystals. (Copied with permission [282].)

are equally flexible and contain about the same free volume. The difference between them lies in the initial disposition of the groups and the degree to which specific motions can occur within the defined shapes of the two cavities. Thus, the ortho hydrogen atom on the aromatic ring in the needle dimorph of 86b is far removed from the impeding atoms and its biradical has greater conformational flexibility. In fact, the solution phase cyclobutanol ratios indicate that the hydroxy-l,4-biradical from the needle dimorph is able to equilibrate its conformations over the same space as it requires in a fluid isotropic solvent. This conclusion must be tempered, however, by the observation that photoreaction occurs preferentially near crystal surfaces and the photoproducts are not isomorphous with 86b [282c]; the Norrish I1 reactions of needle dimorphs may not be topochemically controlled. The substitution of a methyl group at carbon 3 of the adamantyl group of 86b (designated 89)lowers the molecular symmetry and makes conceivable the formation of three cis and three trans cyclobutanol photoproducts. Of these, only two cis (90 and 91) and two trans (92and 93)isomers could be TABLE 10 Requisite Geometric Factors and Norrish I1 Product Ratios from 86b I2821

86b Phase

44

Plates Needles Benzene solution

2.53 2.78

Abstraction Geometry 43 62

92 77

Chair Chair

87b/88b

9911

74/26 73/27

186

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

detected after irradiation of 89 in solution or its neat crystalline state (Eq. 10) [281b].

90

92

i)H

& J

91

hr

Al

(1 H

nr

93

Presumably, the same steric factors that discourage formation of the other two isomers in solution are operative in the solid state. In addition, the reaction cavity of the solid places clear conformational constraints upon which y-hydrogen atom can be abstracted: 91 and 93 have a common BR precursor; 90 and 92 emanate from a different BR conformer. The control is selective, but not specific. Unlike 86b, 89 crystallizes in only one form. However, it is chiral [281b,284]. The major isomer of the cyclobutanol mixture, 90, has been isolated from irradiation of a single crystal of 89 and shown to have a > 80% enantiomeric excess. Inversion twinning in the crystals, disruption of the crystalline lattice due to the presence of photoproducts [281b], possible preferential reaction at surface and defect sites, and comparable activation energies for abstraction of enantiotopic y-hydrogen atoms on different methylenes of the adamantyl group in the undisturbed lattice may explain the lack of total enantiomeric purity. Again, even in this well-controlled system, the results point to a reaction cavity which is not totally rigid or which contains adequate free volume to permit rather large conformational changes of molecules during the course of the reaction. In subsequent sections we shall see that in extreme cases, the control provided by reaction cavities in foreign solid hosts can be as great or greater than those provided by the neat crystals.

B. Reaction Cavities with Very Stiff Walls and Preformed Shapes and Sizes: Silica Gel and Zeolites Silica gel surfaces (Figure 9) offer an anisotropic environment to adsorbed ketone molecules in which motions are restricted (in a crude sense) to two dimensions. Under the best of circumstances, a distribution of site sizes and

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

187

activities is present so that the photochemical parameters associated with the Norrish I1 reactions of adsorbed ketones are best described by expressions much more complex than Eqs. 6-8. De Mayo and Ramnath [285] have reported that irradiation of ketone (94) adsorbed on dehydrated silica (heated at 200°C under vacuum) and in methanolic solutions at 25°C results only in Norrish I1 isolated photoproducts (Eq. 11) whose E/C ratio is 2.5-3. Thus, the silica surface under these conditions affords reaction cavities which allow 94 to react as though it were in a fluid isotropic solution.

-

94 -

95 -

96

Using silica of a different source, Turro [46] again found that the E/C ratios from irradiation of adsorbed alkanophenones 97 (n = 4,7,10) at room temperature (3.7-3.9) are close to the values obtained from irradiations in tbutyl alcohol (4.2-6). Consistent with the expected reduction in mobility of adsorbed molecules on silica surfaces at lower temperatures, only cyclization products were isolated from irradiations of 97 on silica at - 125°C. At these

97 very low temperatures, the distribution of ground-state ketone conformers may be nearly static and only those initially bent to allow y-hydrogen abstraction after electronic excitation may be able to react. Under conditions of very slow shape changes, the favored photoproducts from the biradicals are cyclobutanols and they should form more quickly than in solution. On these bases, we conclude that there is no compelling evidence for specific control over Norrish 11 photoreactions of ketones in rigid reaction cavities on silica surfaces. However, this does not mean that the rates of conformational changes or the distribution of conformers of 97 and its biradical intermediates cannot be altered by adsorption onto a silica surface. Evidence for this is found in diffuse-reflectance flash photolytic studies of valerophenone (97, n = 4) adsorbed onto silica (average pore diameter = 255 A) [286]. Whereas in fluid solution the triplet lifetime of molecules like 97 is < 5 ns

188

V. RAMAMURTHY, R. G. WEISS AND G. S . HAMMOND

-

[287], it is found to be 310 ns for the adsorbed valerophenone. Furthermore, the triplet biradical of valerophenone, whose lifetime in methanol is 100 ns [287,288], cannot be detected upon irradiation of the adsorbed species due either to an enormous decrease in the lifetime or, more likely since the photoproduct ratios are like those found in isotropic solutions [286], to its very low concentration. Zeolites are somewhat like silica in their surface characteristics. Ketones and hydroxy-1,4-biradicals have very polar groups which can interact favorably with metal cations located along zeolite walls. The potential effect of the metal ions on the position of the reacting ketones is twofold. First, the cations may force a ketone molecule into a conformation or a site which it would normally not occupy based solely upon free-volume considerations. Second, the diffusion coefficient of a ketone or a hydroxy-l,4-biradical is probably much more than an order of magnitude smaller than that of benzene [289] so that the residence time of a ketone and its Norrish I1 intermediates in a zeolite site with at least one metal ion is expected to be closer to 100ns than to 1 ns. These numbers should be compared with the lifetimes of excited states and BR triplets derived from ketone guests before interpreting the influence of zeolite reaction cavities on the course of the Norrish I1 reactions of guest molecules. Since the biradical lifetimes are usually tens of nanoseconds and triplet excited state lifetimes may vary from < 5 ns to hundreds of nanoseconds, it seems safe to assume that a ketone guest molecule will be able to sample no more than one or two cages, cavities, or channels in a zeolite during the period that separates the moment of electronic excitation and eventual photoproduct formation. Presumably, the time spent in transit is much shorter than the time spent at preferred guest sites. In spite of this, the normally transitory positions of a guest molecule can become the primary ones of intermediates and products whose shapes or polarities differ significantly from those of their precursor. Thus, although the sites at which guest molecules reside in zeolites may be well defined (see Figures 12 and 13), the location at which they react may be less clear in cases that allow initial cation-carbonyl interactions to be interrupted. In the absence of specific evidence for such changes, we will assume that the reactive ketones remain within their original cage and, possibly, explore a portion of a neighboring channel if a long alkyl chain is present. Since the diameter of a phenyl ring is 6 A, aromatic ketones can enter sites within a zeolite whose constrictions at all points from the surface are no smaller than that dimension (Table 1). A clear indication that zeolites can control the Norrish I1 selectivities of aromatic ketones (97) has been provided by Turro and Wan [290] and by Ramamurthy and co-workers [291]. They find extremely diverse photoproduct yields for different members of the 97 series in zeolites whose pore sizes and shapes differ (Table 11). The absence of selectivity from irradiations in

-

~~~

---

~

4 8 8 8 8 8 7 6 5.5 5.5 5.5 -7.5-8.0

(4

"R.J. Ward, J . Catal. 10, 34 (1968).

~

Benzene solution Na-A Li-X Na-X K-X Rb-X cs-x Na-Mordenite Silicalite Na-ZSM-5 Na-ZSM-8 Na-ZSM-11 Na-Zeolite$

Zeolite

Pore Diameter

0.6 0.95 1.33 1.48 1.69

(8)"

Cation Radius

873 852 800 770 732

(A3)

Cage Free Volume

73 82 56 0.54

3.9 2.7 3.3 1.9 2.3

6.2

n =3 EJC

E/C

2.6 2.2 2.6 2.6 1.1 1.6 3.2 2.3

tJc

n=7

3.0 1.9 2.7 2.3 1.6 1.3 1.6 1.1 1.9 1.1 1.9 1.6 1.9 1.3 2.2 2.5 > 50 > 50 > 100 > l o o > 100 >loo > 100 >loo 1.3 0.62

n =4 EJC

0.68

> 100 > 100 > 100

0.6 0.6 0.5 0.7 1.2

1.2

EJC

> 100 > 100 > 100 0.31

0.4 1.4 1.5 1.9 1.7

2.7

2.4 1.4 1.4 1.1 4.2 6.7

EJC

2.1 0.8 1.3 2.2 5.1

2.4

tJc

n=13

tJc

n=ll

TABLE 11 Dependence of Photoproduct Ratios from 97 on the Nature of Zeolite Hosts 1290, 2911

0.48

> 100 > 100 > 100

1.8 1.3 2.0 3.4 4.9

2.7

2.7 2.5 2.7 4.3 6.2 6.8

t/c

EJC

n=17

190

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

Na-A may be explained easily as a consequence of the aromatic rings of 97 being unable to enter the pores. Photoreactions are forced to occur at the surface in a two-dimensionally restrictive environment reminiscent of that provided by silica. The absence of selectivity upon irradiation of 97 in the NaZeolite-/l or in Na-mordenite follows from the relatively large size of their pores and channels which impose few conformational restrictions on BRs in the two dimensions that affect the ease of c-BR formation. Whereas a small increase (over the values in isotropic media) is found in the E/C and t/c ratios from the longest 97 in the X faujasites with the largest metal ions, no discernible differences could be found for any of the 97 in any of the Y faujasites. Apparently, all of the ketone molecules can be accommodated into Y supercages which impose few constrictions to the internal motions BRs. When the number of space-filling large ions per cage is increased, the hydroxy-1,4-biradicals with the longest tails sense the restrictions imposed by the limited volume. Those restrictions, somewhat surprisingly, appear to favor the BR conformations which are most extended, implying that the long alkyl tails of 97 (n = 11,13,17), especially, are forced from one cage into a neighboring one. In this way, the reaction cavity may be thought of as offering constraints in two dimensions, but not in the third (along which the alkyl chains lie). Selectivity is apparent, however, from irradiations conducted in silicalite or the Na-pentads (ZSM-5). The interior of the silicalite is rather hydrophobic and should provide a nonpolar environment to reacting ketones. More importantly, its relatively small pores can provide snug fits to aromatic rings in the channels. Adequate mobility must exist to permit the bent ketone conformations necessary for y-hydrogen abstraction, but the dearth of cyclobutanol photoproducts indicates that the stiff walls of the reaction cavity repel attempts by the hydroxy-1,4-biradicals to adopt cisoid conformations whose cross-sectional areas are larger than that of the pores. As shown in Scheme 45, the very stiff walls of the narrow channels prohibit the BR precursors to the cyclobutanols from forming, but allow even the longest 97 to be fully extended [291]. It should be noted in this regard that the circumference of a c-BR is larger than that of the corresponding cyclobutanol since the carbon atoms bearing the odd electrons are within the sum of their covalent radii in the product; in the BR, the radical centers can be no closer than the sum of their van der Waals radii. Additional spectroscopic information which supports these conclusions is available [292]. For instance, the diffuse reflectance absorption maxima of valerophenone (97, n = 4) in Li-X and Cs-X zeolites are shifted only slightly from each other and both are similar to the spectrum obtained in methanol solution (Figure 50). Although the absorption spectrum of valerophenone in the less polar ZSM-5 (Si/Al -490) also resembles that in hexane, the

191

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

R=OH

-

Scheme 45.

spectrum in ZSM-5 (%/A1 24) is much more like the one in methanol. Both spectra in the ZSM-5 zeolites exhibit broadened n -+ TC* transitions which may be due to the aromatic rings being in a variety of site types. The corresponding phosphorescence spectra at 77 K also exhibit intensities, shapes, and wavelength maxima that depend upon the zeolite host [292]. Transient absorption spectra from valerophenone in Na-ZSM-5 and CsZSM-5 could be assigned to triplet-triplet transitions; no spectra which can be attributed to the hydroxy-1,4-biradicals were detected. Furthermore, the decay of the transient signals could not be fit to either a single or double exponential expression, and samples prepared under apparently identical conditions exhibited half-lives that varied by a factor of 2 [292]. All of these spectroscopic observations suggest that the valerophenone molecules reside in a distribution of sites within a zeolite and migration among them under the experimental conditions is slow. Thus, the Norrish I1 photoproduct ratios must be interpreted in terms of not only the relative populations of alkanophenones at each site type, but also the quantum efficiencies of each and the conformational preferences of the intermediate BR in each environment. In fact, the rates of y-hydrogen abstraction in 97 must be attenuated severely in the restrictive channels of zeolites. Even in Na-X and Na-Y, where virtually no photoproduct selectivity from valerophenone is observed, its 1 ,us (vs. 5 ns in fluid isotropic solvents), The triplet state half-life is narrower channels of Na-ZSM-5 increase the triplet half-life to 15 p ! Narrow zeolite channels that normally either exclude or provide a very tight fit for alkanophenones due to the much larger cross-sectional area of an

-

N

-

1.2 -

1

Nanometers

(4

'\

Y

ZSM-5 (SiAI-24)

ZSM-5 (SVAI-490) ZSM-5 (SiAI-490)

n

a

0.0 220

260

300 Nanometers

340

(4 Figure 50. Absorption spectra of valerophenone in zeolites and isotropic solutions [2923. 192

193

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

aromatic ring include n-alkanones. On the other hand, n-alkanones are unable to fill as well the supercages of X and Y zeolites, allowing much greater conformational motions and much lower selectivity in the Norrish I1 processes. These expectations have been borne out by experiment [293]. For instance, 2-tridecanone gives rise to E/C ratios in Na-X (1.0) and Na-Y (0.9) zeolites which are very similar to those found from valerophenone in the same media. However, whereas only fragmentation products were detected from valerophenone irradiated in ZSM-5 and ZSM-11, the E/C ratios in these zeolites with narrower channels were 4.3-4.5 from 2-tridecanone. Clearly, the channel cross sections of these zeolites (>28A2) can accommodate the c-BR conformations necessary to form cyclobutanols. Closer examination of the cyclobutanol t/c ratios from homologous and isomeric n-alkanones in the same zeolites reveals some interesting trends (Table 12). The t/c ratios from all of the alkanones in the Na-X and Na-Y zeolites are as expected from reaction in a large or flexible reaction cavity which is very polar (like that provided by an alcohol solution). The t/c ratios in the ZSM zeolites indicate that selectivity depends upon the total ketone length and the position of the odd electron centers of the BR along the chain. Ratios of 60 or more are reported from 4-alkanones with 9 or more carbon atoms; the t/c ratios from 4-octanone in ZSM-5 and ZSM-11, 15 and 18 respectively, mark 8 carbon atom chains as being critical to a c-BR occupying TABLE 12 The t/c Ratios from Irradiation of Alkanones in Zeolites and in Hexane Solution 12931 Alkanones 4-Nonanone 4-Undecanone 4-Dodecanone 4-Tridecanone 4-Tetradecanone 4-Decanone 3-Decanone 2-Decanone 3-Octanone 4-Octanone 2-Octanone 2-Heptanone 3-Heptanone 2-Hexanone

Hexane

NaX

NaY

ZSM-5

ZSM-11

1.8 1.8 1.7 1.7 1.7 1.8 1.8 1.5 1.8 1.8 1.4 1.7 1.5 1.5

0.6 0.4 0.7 0.7 0.8 0.4 0.4 0.6 0.7 0.7 0.8 0.8 0.6 0.8

1.3 0.7 0.9 1.1 1.1 0.7 0.9 1.0 0.8 1.3 1.2 0.9 0.9 1.3

60 60 65 70 72 60 16 6.0 20 15 8.0 3.8 2.8 2.4

60 60 70 68 66 60 14 6.5 18 18 7.1 4.1 2.6 2.7

194

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

fully the cross-sectional area of a ZSM zeolite. Furthermore, the t/c ratios from 2-, 3-, and 4-decanone in the ZSM illustrate the shape differences that accrue in the c-BRs from isomeric alkanones. As shown in the cartoon of Figure 51, the family of c-BR, conformational precursors of a cis cyclobutanol is bulkier and not amenable to accommodation by a 28 A' cross section when 4-decanone is the ketone since both R, and R, are of equal length and are projected in directions which are unlikely to alleviate the steric crowding that ensues. The fact that the reaction cavity allows virtually unlimited expansion along a channel length is of no use to a biradical whose width along any axis is greater than the diameter of the channel.

t-BR

#

C

- BR,

C - BR,

Figure 51. Cartoon representations of hydroxy-1,4-biradicals from 2- and symalkanones. The methylene chains (solid lines) are assumed to be extended and the sizes of the atoms between the odd-electron centers and the hydroxy groups are depicted as circles and filled dots, respectively.

195

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

C. Reaction Cavities with Some Wall Flexibility: Solid Inclusion Complexes Comparatively, the walls of a reaction cavity of an inclusion complex are less rigid but more variegated than those of a zeolite. Depending upon the constituent molecules of the host lattice, the guest molecules may experience an environment which is tolerant or intolerant of the motions that lead from an initial ketone conformation to its Norrish I1 photoproducts and which either can direct those motions via selective attractive (NB, hydrogen bonding) and/or repulsive (steric) interactions. The specificity of the reaction cavity is dependent upon the structure of the host molecule, the mode of guest inclusion, and the mode of crystallization of the host. Several interesting examples of solid inclusion complexes with ketone guests which undergo the Norrish I1 reactions have been examined. They illustrate the breadth of reaction cavity types and resultant selectivities that can be expected in such systems. For instance, 5-nonanone has been included into channels of solid urea crystals and irradiated [294]. The conformation associated with a yhydrogen being within an abstractable distance from a carbonyl oxygen atom is possible, but, as was shown in Figure 50 for alkanophenones in zeolites, some motions of the BRs necessary to form cis-cyclobutanols, especially, will be inhibited. Other conformational changes may be strongly directed by hydrogen bonding of the hydroxy group to the urea. In a 5-nonanone/urea complex whose molar ratio suggests that the channels of the host are nearly filled with guest molecules [294], it was found that the time required to effect photoconversion is similar to that required in solution; consistent with 5-8, diameter channels (see Figure 1) attenuating only slightly the motions necessary for the initial y-hydrogen abstraction, the total quantum efficiency in the complex does not appear to be reduced appreciably. However, the photoproduct ratios clearly indicate that the fate of the hydroxy-1,4-biradicals is controlled by the urea channels. Only Norrish I1 photoproducts could be isolated, but they did not include the cis cyclobutanol (Eq. 12).

b

complex

bII \\ T

')' '

-4TH u

Since the volume of a methyl group is slightly larger than that of a hydroxyl, the lack of the cis cyclobutanol may result from an inability of its precursor

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

1%

(which is larger in diameter than the cis cyclobutanol), c-BR,, to fit within a 5-A diameter channel or to specific BR-cavity wall interactions (i.e., template effects). The E/C ratio from irradiation of 5-nonanone in methanol was measured to be about 3; in the complex, the ratio was about 1.5. Since E/C ratios from irradiations of alkanones in fluid isotropic media are somewhat smaller when the environment is more polar, the lower ratio in the urea complex indicates that local polarity effects alone cannot explain the course of this photoreaction. A similar study of Norrish I1 reactions has been conducted on complexes of aryl ketones in Dianin’s compound 1 [295], a nonpolar host whose channels are effectively truncated at each 11 A of length by a 2.8-A constriction from 6 hydrogen-bonding hydroxyl groups (see Figure 3) [296]. Table 13 summarizes the results from complexes with ketones expected to undergo primarily the Norrish I1 reactions [297]. As befits the rather large (and mostly) nonpolar reaction cavities, the E/C and t/c ratios in Table 13 provide evidence for relatively little control by the channels of Dianin’s compound over the fate BRs. Even in the most selective case from 5-methylTABLE 13 Photoproduct Ratios from Alkanophenones in Benzene and Complexes of Dianin’s Compound (2971 Guest Ketone 0

II

(PhCR) R= -(CH2)3H

+CHZ),H +CH,),H +CHZ)CH(CH3)2 -CH2CH(CH3)2 -CH,-cyclohexyl

-CH,-cyclopent yl

Medium“ Benzene complex (5-7) Benzene complex (5-7) Benzene complex (18-22) Benzene complex (9) Benzene complex (20-26) Benzene complex (6) Benzene complex (9)

“Host/guest ratio in parentheses. bOnly traces of cyclobutanols detected.

E/C

8.0 8.4 4.0 3.7 4.9 5.3 7.3 27.6 7.3 6.1 0.5 0.9 12.7 Largeb

t/c

2.5 1.6 3.4 2.8

2.2 1.9

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

197

I-phenylpentanone, the E/C ratio from Dianin’s complex is only a factor of 3 more than the value observed in benzene. Additionally, the relative reactivities of the ketones in benzene and in the complexes are reported to be similar. The constancy of quantum yields indicates that even the bulkiest ketone in Table 13 has little trouble in finding appropriate conformations during its triplet lifetime to permit y-hydrogen abstraction and for the hydroxyl,4biradicals to adopt all of the preproduct conformations that they would in fluid isotropic media. The walls of the channels in Dianin’s complex are probably more flexible than those in urea complexes (owing to the saturated nature of several bonds) and the potentially active hydroxyl groups of 1 are networked with each other and in a location which makes it improbable that the hydroxyl group of a BR could reach them. Even if it could, the arrangement shown in Figure 3 for the hydroxyl groups of 1 should inhibit their acting as a template for guest reactions. In essence, the channels appear analogous to the “supercages” of a zeolite, but have walls that are less rigid. However, it should be possible to observe selectivity from irradiation of ketone molecules complexed to Dianin’s compound if the shape of the guest and its intermediates match more closely the size and shape the host channel [298]. Several attempts have been made to engineer host molecules for clathrate formation [298]. Among the most successful of these efforts for the investigation of the photochemical reactions of guest molecules has been the studies of Toda and co-workers with 1,1,6,6-tetraarylhexa-2,4-diyne-1,6-diols (2and 3)1561. Pertinent guest molecules in the complexes are N,N-dialkyl-aoxoamides (98) whose photochemistry in the neat solids and solutions has been investigated by Aoyama and co-workers [299] and by Ackerman and Johansson [300]. For instance, irradiation of 98a in either phase yields the two cyclization products shown in Eq. 13, including both diastereomers of each.

198

V. RAMAMURTHY, R.G. WEISS A N D G. S. H A M M O N D

Although all the products can be rationalized on the basis of y-hydrogen abstraction followed by cyclization or rearrangement-cyclization of 1,4biradical intermediates, the mechanism has been shown to involve analogous zwitterionic intermediates [299b, 3011. Although they are not strictly Norrish I1 reactions, transformations of 98 will be considered so for the purposes of discussion since the photoproducts and the mechanisms of their formation are very similar to those expected of Norrish I1 processes. The clathrate inclusion complex of 98a with 2 is somewhat disordered; two conformations of 98a exist but in both, there are hydrogen bonds to only one of the two carbonyl groups on a molecule of 98a [166]. As shown in Figure 52, the major conformer of 98a in its clathrate complex with 2 is nearly the same as in the neat crystal. However, only 99a was obtained as a photoproduct from irradiation of the clathrate. This result can be explained readily since the carbonyl group alpha to the nitrogen atom is hydrogen bonded to 2, making the tautomerization from 101 to 102 unfeasible. No hydrogen bond donors exist in the neat crystal and tautomerization of the intermediates can occur. Both butyrophenone and valerophenone form 1 :2 host :guest complexes with 2 [302]. Although crystal structures to reveal the mode of guest incorporation are unavailable, it is likely that the two hydroxyl groups of each molecule 2 are occupied by hydrogen bonds to the carbonyl groups of two guest molecules. Given the similarity between the structures of the guests and the stoichiometries of their complexes, it is surprising that the butyrophenone complex is reported to be nonreactive while the one with valerophenone produces E/C and t/c ratios similar to those found from isotropic media [302]. Since the more voluminous guest is able to attain the requisite conformation for y-hydrogen abstraction and its BR is able to explore a wide variety of preproduct conformations during its triplet lifetime

3

(4 (b) Figure 52. ORTEP molecular structures of 98a (a) in its neat crystal and (b)the major conformer in its clathrate complex with 2. (Copied with permission [166].)

199

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

in the reaction cavity of 2, one might expect that butyrophenone would also be photoreactive. Given the similarity between the electronic and structural natures of the two ketones, we suggest that the role of the reaction cavity of 2 in the inhibiting Norrish I1 reactions of butyrophenone merits reexamination. Complexes of 98a-d have been prepared with optically active 3, also [56, 152, 154, 3031. Their irradiation yields only 99 as photoproducts, as before, but in many cases they have a large enantiomeric excess. The data are summarized in Table 14. Presumably, the same hydrogen bonding between 2 and 98a which was cited as suppressing the formation of 99 is responsible in the clathrates of 3 for stabilizing the 101 intermediates so that they may equilibrate between their diastereomeric conformers which precede formation of the cis and trans p-lactam photoproducts [1521. Enantioselectivity is highest in the complex of 98a (and perhaps comparable to that of 98b), but is clearly very low with 98c. A reasonable interpretation of this result is that the reaction cage is enlarged by the presence of two guests and that they may reside in a twinned crystal. Single crystal X-ray structure determination would be very helpful in elucidating the reasons for the lack of appreciable chiral induction from 98c. Figures 53 and 54 show the structure of the 31981 complex as it exists in the unit cell [154,303]. Unlike the complexes with 98a-q the 98d complex has both hydroxyl groups of one 3 hydrogen bonded to both carbonyl groups of one molecule of 98d. As a result, the diyne backbone is curved (Figure 53) [154,303]. There is no reason to believe that the walls of the reaction cavity experienced by 98d or by transients, lOld and 102d derived from it, in optically active 3 complexes are any more rigid or contain less free volume than do the other complexes. The enantiomeric purity of the product must result from specific attractive host-guest interactions retained along the

TABLE 14 Photoproduct Ratios from 98 in Complexes with Optically Active 3 1152, 154, 3031 Enantiomeric Excess (%)

98

Stoichiometry (Host : Guest)

translcis 99

cis

43/57 22/78 51/49

62.5 55.8 11.2

trans ~

a b c d

1 : 1 (+one molecule of benzene) 1:l 1:2 1:l

"Purity unknown; [a],-48.7"

-

_

95 a

0 100

_

_

200

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

Figure 53. Structures of 3 and 98d in their solid complex. (Copied with permission ~561.)

reaction coordinate to 99,the crystalline matrix of 3 supplies a cavity which is better described as a well-defined template than a rigid cage whose active walls preclude the motions necessary to form both enantiomers of 99 or even 100.

D. Reaction Cavities with Strong External Medium Influences: Cyclodextrin Complexes and Their Aqueous Solutions Solid inclusion complexes of photoreactive ketones with cyclodextrins as hosts (Figure 6 ) provide interesting examples of how a fairly stiff, somewhat heterogeneous (in terms of polarity, size, and guest orientation) reaction

H'

(-)-rn

Figure 54. Conversion of the solid 98d/3 complex to (-)-99d. (Copied with permission ~561.1

201

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

cavity can influence the Norrish I1 processes. Single crystal X-ray diffraction studies to obtain details of a ketone guest's structure and orientation within a C D torus have been unsuccessful owing to disorder of the complexes. Reddy et al. have examined the influence of p-CD on a series of alkanophenones 97 and 103 for which Norrish I reactions in solution do not

103 103

R

R'

H H H o-Me o-Me m-Me m-Me p-Me p-Me

compete with Norrish I1 processes [304]. All of the complexes had 1 :1 stoichiometries as solids; aqueous solutions were prepared by dissolving the preformed complexes in water. For the most part, E/C ratios were measured. They and K , , the dissociation constants of the complexes in solution, are included in Table 15. In the solid p-CD complexes where escape from the torus is not possible during the period between excitation of 97 or 103 and formation of its photoproducts, all of the E/C ratios resemble those from the nonpolar solvent benzene more than the polar solvent t-butyl alcohol. This is in spite of the very high polarity estimated for the interior of a p-CD. Evidence that polarity is not the dominant product-determining factor in these systems is found in the t/c ratios from 97 ( n = 4) and 97 ( n = 13): whereas P-CD/97 ( n = 4)yields a mixture of trans and cis cyclobutanols whose ratio is like that found from benzene, the P-CD/97 (n = 13) complex yields only the trans isomer; benzene solutions of 97 ( n = 13) produce both cis and trans cyclobutanols. If 97 ( n = 13) (and other 97 with long alkyl chains) is held as shown in Figure 55, with the alkyl chain helping the aromatic ring to fill the torus, the motions of the derived hydroxy-1,4-biradical may be expected to be limited sterically and by hydrogen-bonding interactions with rimhydroxyl groups of the CD. It would appear, however, that this form of N

103 a b c d e f g h i

3 4 7 9 13

R H H H o-Me o-Me m-Me m-Me p-Me p-Me

97 n =

Alkanophenone

~~

~

1.9 0.8 1.6 3.5 3.2 1.9 3.1 2.6 2.5

6.5 3.0 1.2 2.5 1.6

Benzene

~~~~

2.9 1.1 2.1 5.1 4.9 3.4 4.7 4.7 3.6

8.5 4.2 2.5 3.3 2.9

t-BuOH

-

~-

1.7 0.4 1.1 2.2 0.8 1.3 0.7 2.2 1.6

3.5 2.7 0.8 0.7 0.4

fi-CD Solid

-

-

~-

~-

3.6 1.0 2.7 2.3 2.1 2.5 2.1 2.5 2.4

3.8 3.8 1.8 1.6 1.4

8-CD Aqueous

~

d

(0.52)

(4.81)

(2.07)

M) ,

-

(1.1) (0.77) (0.6) (0.17)

(103 ~

_ _

TABLE 15 E/C Photoproduct Ratios from 97 and 103 in Various Media, Including Aqueous and Solid fl-CD Complexes 13041

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

203

(trans)

Figure 55. Representation of possible modes of inclusion of 97 (n = large), its c-BRs, and the corresponding photoproducts in a B-CD torus [304].

complexation would favor formation of the cis cyclobutanol isomer instead of the observed trans. If the rim hydroxyl groups act as a template, other factors must override their influence. The data in Table 15 reveal that the values of E/C ratios from the solid pC D complexes are always lower than from benzene solutions. The degree to which the cis cyclization products are favored over the trans increases from 103a to 103e as expected from the model in Scheme 46. Also, the longer homologue of each ortho-, meta-, and para-methylated 103 yields the greater amount of cyclobutanols when irradiated in solid p-CD complexes. Thus, Scheme 46 explains qualitatively the photoproduct ratios from solid p-CD. In the aqueous p-CD complexes, the E/C ratios remain generally lower than those found in t-butyl alcohol. A specific exception to this is the three ethers 103a-c whose relative yields of fragmentation products in the p-CD solutions are as high as, or higher than, in t-butyl alcohol. If the ether oxygens permit the BRs to protrude farther into the aqueous medium than the BRs from 97, the higher E/C ratios are to be expected. The hydrophobic shielding nature of the alkyl chains is again demonstrated by the E/C ratios of 97. As the tails increase in length, the degree to which the molecules are bound to pC D increases (& decreases) and the E/C ratios decrease; the longer alkyl chains may shield the BRs somewhat from water, making their local environment appear less polar, or force a smaller fraction of the reaction to occur in the aqueous bulk. The progression of Kd from 103e to 103f and to 103h is as anticipated. The para isomer is bound most strongly since its methyl group does not inhibit

204

V. RAMAMURTHY, R. G. WEISS AND G. S . HAMMOND

the aromatic ring from entering the torus. However, the effect of greater binding (and probably deeper penetration into the torus) of the para isomer is not seen in the E/C ratios. It should be remembered that the Norrish I1 product ratios depend more upon the lability and location in a torus of the hydroxy-1,4-biradicals (i.e., the final reaction cavity) than of the ketone precursors (i.e., the initial reaction cavity). These results suggest again that the biradicals probably migrate somewhat from their initial positions, when yhydrogen abstraction occurs, to locations which may be quite different from those experienced by the ketones. Moreover, in both the solid complexes and in aqueous solutions, the effective reaction cavities cannot be defined exclusively by considering the shape of the torus; water molecules near the rims, the hydroxyl groups at the rims of the host, and the guests themselves will all contribute to the definition of the free volume, the shape, and the polarity of the reaction cavity in these systems. Clearly, the nature of the reaction cavity provided by a cyclodextrin in solution (where movement of the guest into and out from the torus are facile and where water molecules can vitiate selective hydrogen bonding interactions between the hydroxyl groups on the C D rims and functional groups of the guest) and in the solid state are very different for several mechanistically important reasons. Furthermore, escape from either C D rim by a guest ketone is thwarted in a solid complex by the proximity of other species (unless the mode of crystallization forms channels as shown in Figure 7a and neighboring C D molecules are free of guests); the inhibition of dissociation of a complex in solution is largely due to the necessity to bring water molecules into a torus as the guest leaves and for the (usually) nonpolar guest molecule to be in a highly polar, aqueous environment outside of the cage. Thus, depending upon the binding constant for a complex in solution, a significant fraction of ketone guest molecules may exist outside the torus in the aqueous bulk and react there. This requires that complicating factors not included in Eqs. 6-8 be considered when discussing microheterogeneous reaction cavities like those of C D toruses in solution.

E. Reaction Cavities with Walls of Variable Flexibility and Strong External Medium Influences: Aqueous Microheterogeneous Complexes Some of the complexities mentioned above have been encountered by Hui et al. who examined the Norrish I1 reactivity of two benzoyl carboxylic acids 104a and 104b in aqueous solutions of carboxymethylamylose (CMA) [305]. Like a C D torus, CMA can form “pockets” where guest molecules like 104 may reside. The CMA can adopt helical conformations whose cross-sectional

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

205

areas may vary within limits to accommodate the size of the guest molecules. The factors reponsible for ketone complexation in a CD torus should operate as well with CMA with some modification because the latter contains carboxyl groups (0.12 per glucose residue) in addition to the hydroxyl moieties. By determining the fraction of 104a and 104b complexed by the relatively

104

a)n=3; R = H b) n = 10; R = H

c)n=14; R=C%

hydrophobic pockets of aqueous CMA and correlating those numbers with the quantum yields for the Norrish I1 reactions, it was possible to extract the relative @,I values for totally bound (0.14) and totally free x 1) guest molecules. This reduction of reactivity of the complexed molecules cannot be attributed to polarity changes alone since the @,I values of molecules like 104 in aromatic or aliphatic hydrocarbons are 0.25-0.40. Thus, the reaction cavities experienced by excited 104 are envisioned to be rather small and stiff in order to restrict in some way the motions that lead to products. Since it is likely that hydroxyl groups of the CMA project from the active cavity walls, they may also direct the course and efficiency of the reaction. Unfortunately, the distribution of photoproducts from the study is unavailable. In an investigation which compares the fragmentation component of 011 for 104c in a variety of media, Winkle et al. found that monolayer assemblies (Figure 15) of 104c and an equimolar amount of arachidic acid are essentially nonreactive [306]. In t-butyl alcohol, the relative fragmentation quantum yield is reported to be near unity (Table 16). In each of the microheterogeneous assemblies examined, the carboxylate groups of 104c are anchored at or near the aqueous/surfactant interface. The reactive benzoyl group then is located at positions which are determined by the order and polarity of the surfactant assembly. Between the two extreme values from the assemblies and t-butyl alcohol, (fragmentation) varies according to polarity and the type of microheterogeneous solvent aggregate. The stiffness of the polymethylene chains of the surfactant molecules and the availability of water molecules in the vicinity of the reactive aroyl group and the BRs derived from it appear to act in tandem to moderate mI1:the stiffer the alkyl chains, the more closely packed and better aligned they will be; the more

-

206

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

TABLE 16 Relative Quantum Yields for Fragmentation of 104 in Various Media 1305, 3061 -

Medium

104 a or b C C C

C C

C

@,I

CMA/H,O/NaCl t-Butyl alcohol Sodium dodecylsulfate (SDS) micelles/H,O Cetyltrimethylammoniumchloride (CTAC) micelles/H,O

(Fragmentation)’

l~,,,ax(nm)

0.14

Benzene

Dioctadecyldimethylammonium chloride (DODAC) vesicles/H,O Arachidic acid assemblies < O.OOlb

1.02 0.81

255

0.72

256

0.27 0.22

252

257

”Relative to @,I (fragmentation) of butyrophenone in t-butyl alcohol. ’@ for disappearance of 104c was 60.06 (no Type I), suggesting some intermolecular reactions.

closely packed the chains, the more difficult water penetration and bending necessary for y-hydrogen abstraction will be. Since alldecreases as solvent polarity decreases and as the reaction cavity becomes smaller and its walls become stiffer, the two factors act synergistically. Clear evidence for the existence of more than one type of ketone reaction cage in SDS micelles (see Figure 14), perhaps the most flexible and least “organized” of the microheterogeneous media in Table 16, was obtained upon laser flash photolyses of 104c [306]. In the presence of 5.4 x M 2,5dimethyl-2,4-hexadiene, an efficient (diffusion-controlled) quencher of excited triplet states of 104c which is not soluble outside the micelles, the intensity of the BR transient absorption was reduced by 55% but its lifetime was virtually unchanged. Addition of 1.4 x M Eu3+,a diffusion-controlled quencher of 104c triplets which is only water soluble, resulted in a 59% depletion of the biradical signal and a small decrease in its lifetime. These results indicate that during the excited triplet lifetime of 104c ( 15 ns), the reactive groups are either in a distribution of sites which are available to the hydrophobic tails of the SDS micellar molecules or the Eu3+ ions at the Stern layer (i.e., the aqueous interface), or they migrate rapidly between the two. In fact, a distribution of final reaction cavities in micelles may lie between the hydrophobic interior and the aqueous exterior since the biradicals from alkanophenones have lifetimes which are long enough to allow them to explore large effective reaction cavities [307]. The highly flexible nature of the micellar reaction cavities has been N

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demonstrated by Turro, Liu, and Chow [308] who measured the E/C and t/c product ratios and CD,, from octanophenone 97 (n = 7) and valerophenone 97 (n = 4) in hexadecyltrimethylammonium chloride (HTAC) micelles. In contrast to the efficient quenching of the Norrish I1 reactions of 104c in anionic SDS micelles by Eu3+ [306], 0.005 M of the same ion caused no discernable quenching of the Norrish I1 reactions of valerophenone in cationic HTAC micelles. The same concentration of Eu3+ in water without surfactant molecules suppresses more than one-third of the Norrish I1 product formation. The difference between the influence of Eu3 + on ketone photoreactions in the two micelles is easily explained: since the cationic head groups of HTAC repel the metal ions electrostatically, they shield even those ketone triplets which reside near the Stern layer; the head groups of SDS attract Eu3 +,making their local concentration higher than in the bulk. Pertinent data concerning the photoreactions are summarized in Table 17. Quantum yields are for loss of starting ketone and are relative to irradiation of valerophenone in t-butyl alcohol. Both the values of CD,I(rel)and t/c from the micelles resemble more closely those from the t-butyl alcohol solution than from the nonpolar benzene. Coupled with the data of Winkle et al. [306], these results indicate that the alkanophenones reside primarily within the hydrophobic interiors of the micelles, but that the BRs migrate to waterenriched environments at or near the nebulous micellar surfaces. As such, even the template effect on the BRs is nonselective and probably allows all of the conformational changes which occur in a polar isotropic environment. In another set of surfactant systems comprised of 50% potassium stearate (KS) and water, 50% potassium palmitate (KP) and water, and 50% 1 : 1 KS/l-octadecanol (KSO) and water, the Norrish I1 reactions of the homologous series of 2- and sym-alkanones (2-105 and s-105, respectively, with n as the total number of carbon atoms in the alkanone) have been investigated [309].

TABLE 17 Relative Quantum Yields and t/c Ratios from 97 in Isotropic and Micellar Media I3081 PhCO(CH,),H 97

n=4 n=7

Medium

@11(rel)

t/c

t-Butyl alcohol Benzene HTAC micelles t-Butyl alcohol Benzene HTAC micelles

1.00 0.33 1.06 1.oo 0.29 0.71

1.5 3.6 1.9 1.1 4.7 1.2

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V. RAMAMURTHY, R. G. WEISS A N D G. S.HAMMOND

In the gel phases of these systems [310], the surfactant molecules are in extended conformations and packed hexagonally in bilayers. A cut-away cartoon representation of the layers is presented in Figure 56: the KS and KP gel layers are nearly completely interdigitated; the KSO gel layers are without interdigitation, and KS and 1-octadecanol molecules alternate within a hexagonal layer. All of the detected photoproducts from s-105 irradiated in the gel phases

2-105 (n = m+2)

S-105 (n = 2m

+ 1)

can be rationalized as being of Norrish I1 origin. About 10% of the photoproducts from 2-105 are from Norrish I reactions. It is likely that CI-

'M!' Figure 56. Stylized representation of the molecular order in layers of the hexatic B and solid phases of BS and gel phases of KS, KP, and KSO. The dark circles and the straight lines represent the carboxyl(ate) groups and the methylene chains of the

molecules, respectively. The dark triangles represent hydroxyl groups.

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209

cleavage occurs to some extent in the s-105 systems, but that the radical pairs combine before cage escape allows them to yield Norrish I products [311]. 2H NMR studies on a-deuteriated alkanones in the gel phase indicate that the guest molecules substitute for a KP or KS molecule within a bilayer with varying degrees of disturbance to the local environment [309]. Those 105 whose lengths match that of the polymethylene portion of the surfactant molecules appear to fit best into a gel matrix; the s-105 whose length approaches twice that of a KS molecule, s-105 (n = 35), also seems to fit well. The model described above of incorporation places the relatively polar carbonyl groups of 2-105 molecules at or near an aqueous interface, but allows the carbonyl groups of s-105 which are no longer than a KP or KS molecule to be buried in the hydrophobic portion of a gel layer. As expected from this model, the t/c ratios from 2-105 (n = 13) to 2-105 ( n = 21) are always significantly less than 1 when they are irradiated in KS or KP gels at 25°C; in KSO gels at 38"C, the t/c ratios from 2-105 (n = 13, 15, and 17) are near unity and the E/C ratios vary between 5.0 and 6.9 (vs. 0.9 to 2.6 in KP and KS gels) [309]. Even in a polar solvent like t-butyl alcohol, the t/c ratios are greater than 1. This suggests that the reaction cavity experienced by BRs from 2-105 in the gel phases (F in Figure 57) is even more polar and probably dominated by the presence of disordered water molecules at a layer boundary (except, perhaps, in the case of the KSO gels). Presumably, migration of molecules in the gel layers is much slower than in micelles, and the initial and final reaction cavities are almost the same. The photochemical behavior of the s-105 in the gel phases is very different. All of the homologues whose length are no more than 2 carbon atoms longer than the polymethylene portion of a surfactant host yield values for the E/C ratios which are like those from fluid solutions; however, the corresponding

R

D

Figure 57. Stylized representation of the possible solubilization sites for 2- and s-105 in KS and KP gels.

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V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

t/c ratios and the quadrupolar splitting parameter (Av90) from the 2H NMR spectra are maximized for the s-105 homologue which is expected to fit best into a gel matrix. As the length of the s-105 becomes significantly longer than that of a surfactant molecule, the values of the t/c ratio become very small, as though the carbonyl groups (and the corresponding hydroxy- 1,Cbiradicals) are no longer shielded in the hydrophobic middle of a bilayer. Pertinent t/c and Av90 data are shown in Figures 58 and 59 for the KS and KSO gels. Based upon these data, the longer s-105 appear to bend into a hairpin conformation which places the carbonyl groups at the water interface of a layer and buries the flanking polymethylene chains within it (D in Figure 57). Thus, the reaction cavity in the gel systems are fairly uniform for each solute, but can change enormously depending upon the length of the alkanone homologue. As with the CD complexes, the guest molecules can also exert a large influence on the nature of their reaction cavity by influencing its local anisotropy, flexibility, and polarity. Even under the most favorable conditions, the reaction cavities afforded by the gel matrices are not very restrictive when compared to those clathrate and zeolite complexes in which guests occupy nearly fully the channel cross sections; the walls of the gel reaction cavities give every evidence of being relatively flexible and maleable.

F. Reaction Cavities with Walls of Variable Flexibility 1. Polymer Matrices. An excellent example of systems in which there is potentially a wide distribution of reaction cavity volumes and for which

Figure 58. The t/c cyclobutanol ratios (0) and

'H NMR Avg0 values ( 0 )versus chain length for s-105 in KS gels [S0/50 (w/w)H,O/KS] at 38°C.

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211

12 10

a Ic 6 4

2 17

25

33

cartxn c h k length

Figure 59. The t/c cyclobutanol ratios (0,38°C) and ’H NMR Avg0 values (0,70°C) versus chain length for s-105 in KSO gels [S0/50 (w/w) H,O/KSO].

translational diffusion should be relatively slow during the excited state lifetime of a guest molecule is Norrish I1 reactions in amorphous polymer matrices. However, as pointed out by Heskins and Guillet [312], rates of diffusion of small guest molecules in a polymer matrix, although several orders of magnitude lower than when in a low viscosity solvent, may be severely underestimated by the Debye equation [313] which predicts an inverse relationship between macroscopic viscosities and microscopic selfdiffusion rate constants. Equally important is the discussion of diffusion of free volume in polymers. It, too, may migrate through a polymer matrix allowing electronically excited ketones and hydroxy- 1,4-biradicals to explore temporarily conformations not normally permitted by their environment. In these cases, the efficiency and selectivity of Norrish I1 processes of guest molecules will be dominated by “solvent” motions [314]. In polymers with units capable of undergoing the Norrish I1 processes, migration of excitation energy from sectors of the polymer which are crystalline or lacking in free volume adequate to allow reaction may skew the photochemical results so that they do not reflect the distribution of reaction cavities in the polymer; those chromophores at sites with the greatest free volume and flexibility will react preferentially and may even use excitation energy originally delivered elsewhere in the sample. Many of these concepts have been discussed in some detail especially by Guillet [315]. Unfortunately, it has been assumed in many of the studies on the irradiation of polymer-doped or polymer-based ketones that cyclization processes are the fate of only a small fraction of the BRs. Although this may

212

V. RAMAMURTHY, R. G. WEISS AND G. S . HAMMOND

be true in some instances, it is not generally the case. The lack of analyses for cyclobutanols in the polymer studies compromises their utility. 2. Thermotropic Liquid Crystalline and Solid Matrices. A related thermotropic liquid-crystalline medium, the hexatic B (meso)phase of n-butyl stearate (BS) [316], consists of layers like those of the gel phases with the exception that the “head group” of each molecule is a carbobutoxy group instead of a carboxylate anion and the hydrophobic layers are in a continuous assembly (Figure 16). The solid phases of BS are also packed in layers with individual molecules in extended conformations and normal to the layer planes. The orthorhombic (instead of hexagonal) arrangement in the solid is related to the inability of individual molecules to rotate about their long axes. The magnitudes of the largest of the E/C values from the 2-105 (n = 17) and 2-105 (n = 18) in BS at 0°C are very close to the E/C ratios found upon irradiation of neat 2-105 ketones in their layered solid phases [272]. Again, the t/c ratios are about the same as in the isotropic phase of BS [272]. Detailed investigations of the Norrish I1 reactions of 2-105 in the meso and solid phases of BS show no exceptional values for t/c ratios but do indicate values of E/C ratios which are dependent on the ketone chain length [268]. The maximum ratios are found when the 2-105 have n between 17 and 19 (Table 18). This is significantly shorter than a BS molecule (with 22 carbon atoms and one oxygen atom in its main chain). The model which has been proposed substitutes a 2-105 molecule for a BS molecule in a layer [317] (analogously to the mode shown in E or F of Figure 57). When n = 17-19, the BRs from 2-105 are situated in such a way that they can donate hydrogen bonds to the carboxylate groups of neighboring BS molecules. Longer BR homologues cannot hydrogen bond without extending beyond a layer boundary or making a kinked conformation; shorter homologues are free to hydrogen bond, but they introduce greater disorder in their somewhat flexible reaction cavity by not occupying the total volume allocated to a stearoyl group. The Norrish I1 reactions of the s-105 homologues vary in a somewhat different fashion [268]. Of the ketones examined, both the E/C and t/c product ratios are largest for the same homologues, n = 19 and 21 (Table 18); the product ratios from n = 23 and n = 25 could not be measured. Even with this limitation, it is clear that the Norrish I1 reactions of s-105 and 2-105 in BS ordered phases are sensitive to different guest length considerations. Apparently, this is due to the inability of the BRs from the s-105 homologues to hydrogen bond to the carboxyl groups of BS molecules when they are extended in the layers. A second consequence of the inability of the biradicals from s-105 to hydrogen bond to solvent molecules can be seen in the

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213

TABLE 18 Photoproduct Ratios From 105 in the Isotropic, Hexatic, and Solid Phases of BS 12681 2-105

n 15 17 18 19 20 21

S-105

T("C)"

E/C

t/c

30 20 0 30 20 0 30 20 0 30 20 0 30 20 0 30 20 0

3.6 11.7 20.5 3.7 15.1 40.0 3.7 14.8 42.3 4.1 16.5 29.4 3.9 10.4 18.5

1.3 1.3 1.6 1.5 1.9 2.0 1.1 1.6 1.7 1.3 2.4 2.5 1.0 1.5 1.5

E/C

t/c

1.9 2.3 3.6 2.2 3.2 10.0

2.4 2.8 3.5 2.2 10.8 6.8

1.8 6.2 15.9

13.3 4.2

2.0 5.5 > 15

2.5 14.1 6.0

"30°C,isotropic; 20°C, hexatic B; O T , second solid phase.

magnitudes of the E/C ratios which, at their largest, are less than one-half of the maximum values from 2-105. The dependence of the magnitude of the Norrish I1 photoproduct ratios on the total ketone length and the position of the BR within the ketone in the ordered phases of BS has been examined in some detail using and p-alkyl alkanophenones (106) [3 181 and alkanophenones (97) [319]. No difference

106

between the E/C ratios from 97 (n = 4) or 97 (n = 10) in the hexatic B phase and isotropic melt phases of BS could be discerned. However, the longer 97

214

V. RAMAMURTHY. R. G. WEISS AND G. S . HAMMOND

(n = 17, 19, 21) exhibit high E/C ratios due to their ability to fill more efficiently the volume allocated by the cylindrical reaction cavity (Table 19). As shown in Figure 60, the temperature profile of the E/C ratio from 97 (n = 10) remains isotropic-like in the hexatic B phase whereas that of 97 TABLE 19 E/C Photoproduct Ratios from 97 in the Isotropic and Hexatic B Phases of BS" I3191

4

10 17 19

21

30 20 30 20 30 20 30 20 30 20

3.2 3.3 1.7 2.0 3.0 21 19 15

1.3 9.5

"30"C, isotropic; 20°C, hexatic B.

Figure 60. Temperature dependence on the ratio of elimination to cyclization and 97 ( n = 19) (A) in n-butyl products for photolysis of ketones 97 (n = 10) (0) stearate.

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

215

(n = 19) responds to changes in the anisotropy and flexibility of the reaction cavity around it in a very sensitive fashion. As noted before with the s-105 [268], the rise in E/C values in the solid phases of BS is not continuous. The discontinuity corresponds roughly to the temperature reported for the transition between the first and second solid phases of BS [316]. The dependence of the photoproduct ratios on the location of the oddelectron centers along a polymethylene chain of a BR derived from 106 has been examined also in the ordered phases of BS [318]. Starting with m = 5 and allowing n to vary from 11 to 18, it was established that the largest E/C and smallest t/c values were observed when n = 16; the onset of these excursions occurs at the isotropic-hexatic B transition temperature of BS. Since this homologue is longer than a BS molecule, the cylindrical reaction cavity must be somewhat distorted. The bend imposed on BRs by the aromatic ring along a chain (Figure 61) must also contribute to the distortion and to the optimal 106 length being longer than a BS molecule; when the aromatic ring is at a molecular terminus of 97, the largest E/C ratios are observed when the total molecular length is at least four carbon atoms shorter than the optimal 106. Apparently, the 97 are able to fit into a lessdistorted cylinder than 106 of the same length, but one which places the oddelectron BR centers in a more flexible environment near a layer (cylinder) end. Starting with 106 for which m n = 21, the relative wall stiffness and

+

\

hw

hw

hv

hw

BS Figure 61. Representation of extended transoid hydroxy-1,4-biradicalsfrom selected 97 and 106 (m n = 21) and their orientations in a BS layer according to the solubilization model. Note the approximate depths of the hydroxyl groups of the biradicals and their relationship to the carboxyl of BS. The approximate layer

+

boundaries are shown as shaded areas.

216

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

constraints imposed by various portions of the distorted cylindrical reaction cavities in hexatic B and solid phases of BS were explored [318]. As shown in Figure 62, extraordinarily high E/C ratios result from irradiation of 106 (m,n = 3,18) in the anisotropic BS phases. Both values are similar in magnitude to the ratios found from irradiation of alkanophenones in ZSM type zeolites. Although both media may be thought of as having cylindrical (channel-like) reaction cavities, those from the zeolites have very rigid walls and uncapped ends; those from BS have somewhat flexible walls which can both expand and compress the cross section of BRs, but offer little room for longitudinal expansion. As a result, the BS reaction cavities provide the greatest influence on the fate of the BRs from 97, 105, and 106 homologues which have very specific lengths. Furthermore, Figure 60 suggests that only a few of the p-alkyl alkanophenone isomers with m n = 21 can be arranged in the BS reaction cavities so that the hydroxyl group of their BRs will be hydrogen-bonded to a BS carboxyl group. Of these homologues, only the ones with the shorter m (e.g., m, n = 3,18) will provide, the two radical centers with relatively stiff walls. The isomers with longer m (e.g., m, n = 15,6) must be arranged in the reaction cavity so that the odd-electron centers are adjacent to a very flexible and easily distorted section of the reaction cavity. Other isomers which allow the biradical centers to reside at the most ordered middle of a BS layer (e.g., m, n = 10,ll) and, thereby, experience the stiffest walls of the reaction cavity do not permit hydroxyl-carboxyl hydrogen bonding. Thus, for the 106 with m + n = 21, the most restrictive reaction cage is one that promotes hydrogen bonding [318] and places the radical centers nearer a layer middle; its walls

+

0

--

100.0

0 80.0--

--

0 0

0

60.0 EIC

40.0--

0 0

0

m

Figure 62. E/C ratios from 2% (w/w)106 (m + n = 21) in the solid (lO°C, O),hexatic B (20°C, a), and isotropic (30°C, 0)phases of BS. n = 21 - m on the abscissa.

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

217

act as a template while affording minimal flexibility to the locus of reaction. Evidence of the existence of the purported hydrogen-bonding interactions was provided with 107, which mimics the structures of the BRs from 106, in the ordered phases of BS [318].

107

The general model which emerges for the reaction cavity of BS is a cylinder whose dimensions are approximately the same as the van der Waals size of an extended BS molecule and whose wall stiffness is influenced by the size and shape of the reacting guest molecules [317]. At one point along the length of the cylinder is a cross-sectional segment of much higher polarity than the rest, where the carboxyl groups reside and which is capable of acting as a hydrogen-bond acceptor. Thus, the walls of the reaction cavity in hexatic or solid phase BS may be either active or passive depending upon the location of the hydroxyl group in a BR from an alkanone or alkanophenone. Since cis cyclobutanols can form in low yields even in the most ordered solid BS phases, some flexibility in even the stiffest reaction cavities must remain (or, perhaps, the cyclobutanols are produced at defect sites?). The lifetime of BR generated from 97 (n = 19) was found to be 64 k 5 and 70 A 5 ns in the isotropic and hexatic B phases of BS [319]. The lack of influence on the lifetime of the biradical by the hexatic phase when the E/C ratios are clearly affected is at first puzzling. However, it can be cited as evidence that the T -,S rate is independent of the conformation in which a BR is held [263]. Note that the BR from 97 (n = 21) as shown in Figure 60 has its hydroxyl group far removed from the cross-sectional segment of the BS-provided reaction cavity cylinder which is quite polar. In any case, the long lifetime of the BR found in hexatic BS and its near equivalence to that in the isotropic phase indicate that the various biradical conformers have equilibrated in the cylindrical reaction cavity prior to collapsing to products. Removal of the carboxyl group from a BS molecule leaves heneicosane (C21) which exhibits no liquid-crystalline phase, but does form two layered solid phases in which the C21 molecules are fully extended with the molecular axes normal to the layer planes [320]. In the higher temperature solid (Phase 11),the molecules are hexagonally packed within and execute hindered rotations about their long axis. In the lower temperature solid (Phase I), the rotational motions are damped and the molecules are orthorhombically packed within a layer. Gauche defects in the preferred all-transoid conformations occur more frequently near layer ends than near the middle of layers

218

V. RAMAMURTHY, R. G. WEISS AND G.S. HAMMOND

[271]. Thus, the ordered phases of BS and C21 are quite analogous, except that C21 lacks a polar carboxyl group along its stiffer molecular backbone. Exploration of the modes of solubilization of 2- and s-105 in the solid phases of C21 has led to many unexpected and complicating observations [321]. First and foremost among these is that only guest alkanone molecules whose lengths are very near that of C21 can be incorporated isomorphously into its solid phases. Thus, C21 is a much more demanding host matrix than BS; the dimensions and free volume content of the reaction cavities it provides have a very narrow distribution and their walls are probably less flexible than those in many neat crystals. Using differential scanning calorimetry and 'HNMR spectroscopy, it was possible to ascertain that at least 1 wt% of 105 with it = 18-22 is solubilized into the solid phases of C21. Representative results from irradiations of these solid solutions are collected in Table 20. They and other results from irradiations in the solid phases of eicosane (C20) [321] indicate that the highest photoproduct ratios are observed when the alkanone is one carbon atom shorter than the alkane host. As in many other ordered systems, the t/c ratios are probably mechanistically meaningless when the E/C ratios are very large since a very small fraction of the BRs cyclize and those that do may

TABLE 20 Photoproduct Ratios from 105 in the Isotropic and Solid Phases of C21 13211 S-105"

2-105"

n 18 20 21 22

45 35 25 45 35 25 45 35 25 45 35 25

E/C

t/c

4.0 3.9 14 4.9 18 165 2.4 6.0 46 4.0 8.4 29

1.5 4.6 2.4 1.6 4.7 18 1.0 2.5 3.6 1.7 2.8 4.0

E/C

t/c

1.8 3.5 >69

2.5 25 0.9

"Error limits, which are large in the case of high ratios, have been deleted for the sake of clarity. b45"C,isotropic; 3 5 T , Phase 11; 25°C Phase I.

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

219

react in cavities formed at defect sites. Regardless, the E/C ratios show that the motional restrictions experienced by alkanones in the C21 solid phases can be exceedingly high in spite of the passive walls of the reaction cavities (i.e., in the absence of a group which can act as a hydrogen-bond acceptor for the BRs). Comparison between the results from irradiations of common alkanones in C21 and BS suggest strongly that the degree of flexibility and the amount of free volume in a reaction cavity are more important factors in directing Norrish I1 transformations than are template effects. Other comparisons of this sort should be made before a definitive conclusion is reached. Regardless, the fit between the ketone and the cylindrical reaction cavity of the C21 phase must be very tight if its influence on photoproduct ratios is to be manifested. Apparently, the stiffness of the reaction cavity walls in Phase I, even near a layer end, is sufficient to inhibit selective motions of the BRs of 2105. However, the reaction cavities provided by Phase I1 have walls which are somewhat more flexible as witnessed by the relatively large fraction of transcyclobutanol produced from s-105 (n = 21) and the low E/C and t/c ratios from 2-105 (n = 21).

An extreme example of the flexibility which can be incorporated into a cylindrical reaction cavity is given by the two smectic G (or J) phases [275] of perfluorodecyldecane (FlOH10). In these smectic phases, the long axes of individual FlOHlO molecules are tilted with respect to the layer planes and are packed pseudohexagonally within a layer. An interesting characteristic of FlOHlO is that its two parts are incompatible both in terms of size and solubility: the molar volumes of hexane and perfluorohexane are 131 and 201 cm3, respectively, at 25°C [322]; although the dielectric constants of hexane (1.89) and perfluorohexane (1.69) are similar, that of perfluoropropylpropane (5.99) is much larger [322] and indicates the predictable polarization along the chain. Thus, in its isotropic phase, FlOHlO may also segregate itself into regions that are respectively methylene and difluoromethylene enriched, although the two halves of the molecule behave very differently than they do as a combined unit. Even in the smectic phases, NMR evidence suggests that the polymethylene portions of the FlOHlO molecules retain a great deal of flexibility [273,322]. This assertion is borne out by the small deviations from fluid-isotropiclike values of the photoproduct ratios found in the Norrish I1 reactions of the two alkyl perfluoroalkyl ketones 76 (with m,n = 7,8 and 9,lO) in the (macroscopically) highly ordered smectic phases of FlOHlO [273,274]. As

220

V. RAMAMURTHY, R. G. WEISS A N D G. S. H A M M O N D

TABLE 21 Protoproduct Ratios from Neat Melted and Solid 76 and Their Isotropic and Srnectic Phase Solutions in FlOHlO [273,274] 76

T("C)

m, n

=

m, n

= 9,

7, 8

10

65 48 26 25 1 65 48 26 50 26

Phase" Isotropic FlOHlO Smectic I1 FlOHlO Smectic I FlOHlO Neat isotropic Neat solid Isotropic FlOHlO Smectic I1 FlOHlO Smectic I FlOHlO Neat isotropic Neat solid

E/C

t/c

1.9 1.4 1.5 3.7 10.9 1.5 0.7 1.0 3.5 31

3.5 9.4 6.2 3.4 3.9 2.7 4.5 12.2 3.4 18

"Smectic I1 and I are the higher and lower temperature smectic phase of FlOH10, respectively.

seen in Table 21, only the t/c ratios from the smectic phases are elevated with respect to the isotropic phase values. The reaction cavity afforded by smectic FlOHlO to the BRs from the two partially fluorinated ketones does influence the conformational preferences which determine the fragmentation and trans cyclobutanol products, but modestly retards formation of the cis cyclobutanols and their precursor, c-BR, , due to its large cross-sectional area. By contrast, in the neat solid phases of the two ketones where carbonyl groups on neighboring molecules may function as hydrogen bond acceptors for BR intermediates, the E/C ratios are high; they increase relative to the isotropic phase values more than the t/c ratios [273]. Since the exact packing arrangement of each of the neat ketones in its solid phase is unknown, it is not possible to provide more than speculative explanations at this time. However, it is clear that the free volume of the reaction cavities and/or flexibility of their walls in neat solid 76 are much greater than that in the neat crystalline phases of the analogous s-105 which are virtually photoinert.

IX. PERSPECTIVES ON FUTURE RESEARCH The last decade or so has witnessed a great deal of activity to explore the influences of anisotropic environments on photochemical reactions. Results from this research have led to a rudimentary understanding of how photochemical processes may be directed by media in individual cases, but

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221

the general principles which underlie the field do not seem to have been collected into a cohesive model. Possible reasons for this include the interdisciplinary nature of the research and the disparate ways in which the participating researchers have approached the problem and the interpretation of their data. To provide a common basis for discussion, we have attempted in this chapter to provide a workable, rudimentary model which includes the more important identifiable parameters of anisotropic media responsible for directing photochemical processes. In spite of the many examples cited during its development and the many more which have not been, it is clear that the model requires a great deal more experimental testing and refinement if it is to become a paradigm. We hope that the model will be tested, criticized, and refined in the future. The words of Chalmers [69], that “a paradigm will always be sufficiently imprecise and open-ended to leave plenty of work to be done,” have not been forgotten.

ACKNOWLEDGMENTS The authors gratefully acknowledge the intellectual and experimental contributions of their colleagues whose names appear in the references. Without their efforts, the authors’ portion of this chapter could not have been written. We thank Dave Eaton, Bob Liu, John Scheffer, Nick Turro, K. Venkatesan and C. L. Khetrapal for useful discussions and helpful comments. The National Science Foundation is also thanked for its support of the research of RGW related to this chapter.

REFERENCES 1. For a collection of selected papers by Schmidt see G. M. J. Schmidt et al., Solid State Photochemistry, D. Guinsburg, Ed., Verlag Chemie, Weinheim, 1976. 2. J. K. Thomas, Chemistry of Excitation at the Interfaces, American Chemical Society, Washington, DC, 1984. 3. M. A. Fox, Ed., Organic Phototransformations in Non-homogeneous Media, American Chemical Society, Washington, DC, 1985. 4. V. Ramamurthy, J. R. Scheffer, and N. J. Turro, Eds., Organic Chemistry in Anisotropic Media, Tetrahedron Symposia in Print # 29, Tetrahedron 43, 11971746 (1987). 5. K. Kalyanasundaram, Photochemistry in Microheterogeneous Systems, Academic Press, New York, 1987.

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V. RAMAMUBTHY, R. G . WEISS A N D G . S. HAMMOND

6. T. Matsuura and M. Anpo, Eds., Photochemistry on Solid Surfaces, Elsevier, Amsterdam, 1989. 7. V. Balzani and F. Scandola, Supramolecular Photochemistry, Ellis Horwood, New York, 1991. 8. H. J. Schneider and H. Durr, Eds., Frontiers in Supramolecular Organic Chemistry and Photochemistry, VCH, Weinheim, 1991. 9. V. Ramamurthy, Ed., Photochemistry in Organized and Constrained Media, VCH, New York, 1991. 10. K. Honda, Ed., Photochemical Processes in Organized Molecular Systems,NorthHolland, Amsterdam, 1991. 11. G. M. J. Schmidt, Pure Appl. Chem. 27, 647 (1971). 12. C. H. Nicholls and P. A. Leermakers, Ado. Photochem. 8, 315 (1971). 13. M. D. Cohen, Angew. Chem. Int. Ed. Eng. 14, 386 (1975). 14. D. G. Whitten, F. R. Hopf, F. H. Quina, G. Sprinschnik, and H. W. Sprinschnik, Pure Appl. Chem. 49, 379 (1977). 15. J. M. Thomas, S. E. Morsi, and J . P. Desvergne, Ado. Phy. Org. Chem. 15, 63 (1977). 16. J. K. Thomas, Acc. Chem. Res. 10, 133 (1977). 17. K. Kalyanasundaram, Chem. SOC.Rev. 453 (1977). 18. D. G. Whitten, D. W. Eaker, B. E. Horsey, R. H. Schmehl, and P. R. Worsham, Ber. Bunsenges. Phys. Chem. 82, 858 (1978). 19. D. G. Whitten, Angew. Chem. Int. Ed. Eng. 18, 440 (1979). 20. B. S. Green, M. Lahav, and D. Rabinovich, Acc. Chem. Res. 12, 191 (1979). 21. L. Addadi and M. Lahav, Pure Appl. Chem. 51, 1269 (1979). 22. R. Popovitz-Biro, H. C. Chang, C. P. Tang, N. R. Shochet, M. Lahav, and L. Leicerowitz, Pure Appl. Chem. 52, 2693 (1980). 23. N. J. Turro, M. Gratzel, and A. Braun, Angew. Chem. Int. Ed. Eng. 19,675 (1980). 24. N. J. Turro and B. Kraeutler, Acc. Chem. Res. 13, 369 (1980). 25. J. R. Scheffer, Acc. Chem. Res. 13, 283 (1980). 26. N. J. Turro, M. Gratzel, and A. Braun, Angew. Chem. Int. Ed. Eng. 19,675 (1980). 27. J. K. Thomas, Chem. Rev. 80, 283 (1980). 28. J. H. Fendler, Acc. Chem. Res. 13, 7 (1980). 29. N. J. Turro, J. Mattay, and G. Lehr, in Inorganic Reactions in Organized Media, S. L. Holt, Ed., American Chemical Society, Washington, DC, 1981, p. 19. 30. N. J. Turro, Pure Appl. Chem. 53, 259 (1981). 31. P. de Mayo, Pure Appl. Chem. 54, 1623 (1982). 32. D. G. Whitten, J. C. Russel, and R. H. Schmehl, Tetrahedron 38, 2455 (1982). 33. N. J. Turro, Proc. Natl. Acad. Sci. U S A 80, 609 (1983). 34. J. M. McBride, Acc. Chem. Res. 16, 304 (1983). 35. J. H. Fendler, J . Chem. Educ. 60,872 (1983).

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

223

M. Hasegawa, Chem. Rev. 83, 507 (1983). I. R. Gould, N. J. Turro, and M. B. Zimmt, Adv. Phys. Org. Chem. 20, 1 (1984). G. Desiraju, Proc. Ind. Acad. Sci. 93, 407 (1984). N. J. Turro, G. S . Cox, and M. A. Paczkowski, Topics Curr. Chem. 129, 57 (1985). N. Ramanath, V. Ramesh, and V. Ramamurthy, J . Photochem. 31, 75 (1985). V. Ramamurthy, Tetrahedron 42, 5753 (1986). N. J. Turro, Pure Appl. Chem. 58, 1219 (1986). J. M. McBride, B. E. Segmuller, M. D. Hollingsworth, D. E. Mills, and B. A. Weber, Science 234, 830 (1986). 44. V. E. Shklover, T. V. Timofeeva, and Y. T. Struchkov, Rum. Chem. Rev. 8, 55 (1986). 45. B. S . Green, R. Arad-Yellin, and M. D. Cohen, Top. Stereochem. 16, 131 (1986). 46. N. J. Turro, Tetrahedron 43, 1589 (1987). 47. S . Devanathan, M. S. Syamala, and V. Ramamurthy, Proc. Indian Acad. Sci. (Chem. Sci). 98, 391 (1987). 48. V. Ramamurthy and K. Venkatesan, Chem. Rev. 87, 433 (1987). 49. J. R. Scheffer, M. Garcia-Garibay, and 0. Nalamatsu, Org. Photochem. 8, 249 (1987). 50. P. de Mayo and L. Johnston, in Preparative Chemistry Using Supported Reagents, P. Laszlo, Ed., Academic Press, San Diego, 1987, Chapter 4. 51. J. R. Scheffer and J. Trotter, in The Chemistry of the Quinonoid Compounds, Vol. 2, Part 2, S . Patai and Z. Rappoport, Eds., Wiley, New York, 1988, p. 1199. 52. J. K. Thomas, Acc. Chem. Res. 21, 275 (1988). 53. V. Ramamurthy and D. F. Eaton, Acc. Chem. Res. 21, 300 (1988). 54. R. G. Weiss, Tetrahedron 44, 3413 (1988). 55. G. Bunan and T. Wolff, Adv. Photochem. 14, 273 (1988). 56. F. Toda, Top. Curr. Chem. 149, 210 (1988). 57. F. Toda, Mol. Cryst. Liq. Cryst. 161, 355 (1988). 58. F. Toda, J . Incl. Pheno. Mol. Recog. 7, 247 (1989). 59. S . Basu, D. Nath, and M. Chowdhury, Proc. Ind. Natl. Sci. Acad. 54A, 830 (1988). 60. C. R. Theocharis, in The Chemistry of Enones, S . Patai and Z. Rappoport, Eds., Wiley, New York, 1989, p. 1133. 61. N. J. Turro and Z. Zhang, in Photochemistry on Solid Surfaces, M. Anpo and T. Matsuura, Eds., Elsevier, Amsterdam, 1989, p. 197. 62. N. J. Turro, in Molecular Dynamics in Restricted Geometries, J. Klafter and J. M. Drake, Eds., Wiley, New York, 1989, p. 387. 63. N. J. Turro, in Ultrastructure Processing of Advanced Ceramics, J. D. Mackenzie and D. R. Ulrich, Eds., Wiley, New York, 1989, p. 603. 64. N. J. Turro, in Inclusion Phenomena and Molecular Recognition, J. Atwood, Ed., Plenum Press, New York, 1990, p. 289. 36. 37. 38. 39. 40. 41. 42. 43.

224

65. 66. 67. 68. 69. 70. 71. 72. 73.

74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90.

V. RAMAMURTHY, R. G . WEISS AND G. S. HAMMOND

F. Toda, MoZ. Cryst. Liq. Cryst. 187, 41 (1990). M. D. Hollingsworth and J. M. McBride, Ado. Photochem. 15, 279 (1990). N. J. Turro, J. K. Barton, and D. A. Tomallia, Acc. Chem. Res. 24, 332 (1991). V. Ramamurthy, D. F. Eaton, and J. V. Caspar, Acc. Chem. Res. 25, 299 (1992). V. Ramamurthy, Chimia 46, 359 (1992). T. S . Kuhn, The Structure of Scientijc Revolutions, The University of Chicago Press, Chicago, 1962. A. F. Chalmers, What is this Thing Called Science, University of Queensland Press, New York, 1982. N. J. Turro and M. Garcia-Garibay, in Photochemistry in Organized and Constrained Media, V. Ramamurthy, Ed., VCH, New York, 1991. p. 1. M. Hagan, Clathrate Inclusion Compounds, Reinhold Publishing, New York, 1962; L. Mandelcorm, Ed., Non-stoichiometric Compounds, Academic Press, New York, 1964; J. L. Atwood, J. E. D. Davies, and D. D. MacNicol, Inclusion Compounds, Vols. 1-3, Wiley, New York, 1984. C. D. Gutsche, Calixarenes, Royal Society of Chemistry, London, 1989. J. L. Atwood, Ed., Inclusion Phenomena and Molecular Recognition, Plenum Press, New York, 1990. J. L. Atwood, J. E. D. Davies, and D. D. MacNicol, Eds., Inclusion Compounds, Vols. 4 and 5, Oxford University Press, New York, 1991. F. Vogtle, Supramolecular Chemistry, Wiley, Chichester, 1991. M. S. Wittingham and A. J. Jacobson, Eds., Intercalation Chemistry, Academic Press, New York, 1982. R. K. Iler, The Chemistry of Silica, Wiley, New York, 1979. J. H. Fendler and E. J. Fendler, Catalysis in Micellar and Macromolecular Systems, Academic Press, New York, 1975. J. H. Fendler, Membrane Mimetic Chemistry, Wiley, New York, 1982. D. W. Breck, Zeolite Molecular Sieves: Structure, Chemistry and Use, Wiley, New York, 1974. A. Dyer, An Introduction to Zeolite Molecular Sieves, Wiley, Bath, UK, 1988. G. Roberts, Ed., Langmuir-Blodgett Films, Plenum Press, New York, 1990. G. W. Gray, Molecular Structure and Properties of Liquid Crystals, Academic Press, New York, 1962. S. Chandrasekhar, Liquid Crystals, Cambridge University Press, Cambridge, 1977. J. D. Wright, Molecular Crystals, Cambridge University Press, Cambridge, 1987. G. R. Desiraju, Ed., Organic Solid State Chemistry, Elsevier, Amsterdam, 1987. G. R. Desiraju, Ed., Crystal Engineering: The Design of Organic Solids, Elsevier, Amsterdam, 1989. M. Pierrot, Ed., Structure und Properties of Molecular Crystals, Elsevier, Amsterdam, 1990.

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

225

91. J. Klafter and J. M. Drake, Eds., Molecular Dynamics in Restricted Geometries, Wiley, New York, 1989. 92. B. Nageswer Rao, M. S. Syamala, N. J. Turro, and V. Ramamurthy, J . Org. Chem. 52, 5517 (1987). 93. V. Ramamurthy, D. R. Corbin, D. F. Eaton, and N. J. Turro, Tetrahedron Lett. 30, 5833 (1989). 94. W. S . Chung, N. J. Turro, J. Silver, and W. J. le Noble, J . Am. Chem. SOC.112, 1202 (1990). 95. P. R. Pokkuluri, J. R. Scheffer, and J. Trotter, Tetrahedron Lett. 30, 1601 (1989). 96. D. P. Craig and P. Sarti-Fantoni, J . Chem. SOC., Chem. Commun. 742 (1966). 97. N. J. Turro, Modern Molecular Photochemistry, University Science Books, Mill Valley, CA, 1991; H. Shimamori, H. Uegaito, and K. Houdo, J . Phys. Chem. 95, 7664 (1991). 98. N. J. Turro and W. R. Cherry, J . Am. Chem. SOC. 100, 7431 (1978). 99. G. F. Lehr and N. J. Turro, Tetrahedron 37, 3411 (1981). 100. N. J. Turro and J. Mattay, J . Am. Chem. SOC. 103, 4200 (1981). 101. B. H. Baretz and N. J. Turro, J . Am. Chem. SOC. 105, 1309 (1983). 102. N. J. Turro, C. C. Cheng, and W. Mahler, J . Am. Chem. SOC.106, 5022 (1984). 103. N. J. Turro, C. C. Cheng, P. Wan, C. J. Chung, and W. Mahler, J . Phys. Chem. 89, 1567 (1985). 104. N. J. Turro, M. A. Paczkowski, and P. Wan, J . Org. Chem. 50, 1399 (1985). 105. N. J. Turro, K. C. Waterman, K. M. Welsh, M. A. Paczkowski, M. B. Zimmt, and C. C. Cheng, Langmuir 4, 677 (1988). 106. V. Pushkara Rao, M. B. Zimmt, and N. J. Turro, J . Photochem. Photobiol. A: Chem. 60, 355 (1991). 107. B. Nageswara Rao, N. J. Turro, and V. Ramamurthy, J . Org. Chem. 51, 460 (1986). 108. D. A. Hrovat, J. H. Liu, N. J. Turro, and R. G. Weiss, J . Am. Chem. SOC. 106,5291 (1984). 109. N. J. Turro and P. Wan, J . Am. Chem. SOC. 107, 678 (1985). 110. N. J. Turro and Z. Zhang, Tetrahedron Lett. 28, 5637 (1987). 111. N. J. Turro, C. C. Cheng, L. Abrams, and N. J. Turro, J . Am. Chem. SOC. 109, 2449 (1987). 112. N. J. Turro, X. Lei, C. C. Cheng, D. R. Corbins, and L. Abrams, J . Am. Chem. SOC. 107, 5824 (1985). 113. N. J. Turro, C. C. Cheng, X. Lei, and E. M. Flanigen, J . Am. Chem. SOC.107,3740 (1985). 114. N. Han, X. Lei, and N. J. Turro, J . Org. Chem. 56, 2927 (1991). 115. M. A. Garcia-Garibay, Z. Zhang, and N. J. Turro, J . Am. Chem. SOC.113, 6212 (1991). 116. N. J. Turro and Z. Zhang, Tetrahedron Lett. 30, 3761 (1989).

226

V. RAMAMURTHY, R. G. WEISS A N D G. S. H A M M O N D

117. N. D. Ghatlia and N. J. Turro, J . Photochem. Photobiol. A: Chem. 57, 7 (1991). 118. I. R. Gould, M. B. Zimmt, N. J. Turro, B. H. Baretz, and G. F. Lehr, J . Am. Chem. SOC.107, 4607 (1985). 119. M. B. Zimmt, C. Doubleday, and N. J. Turro, J . Am. Chem. SOC.106,3363 (1984). 120. I. R. Gould, C. Tung, N. J. Turro, R. S. Givens, and B. Matuszewski, J . Am. Chem. SOC.106, 1789 (1984). 121. D. Avnir, L. J. Johnston, P. de Mayo, and S. K. Wong, J . Chem. SOC.Chem. Commun. 958 (1981). 122. B. Frederick, L. J. Johnston, P. de Mayo, and S. K. Wong, Can. J . Chem. 62,403 (1984). 123. N. J. Turro and G. Weed, J . Am. Chem. SOC.105, 1861 (1983). 124. C. H. Evans, J. C . Scaiano, and K. U. Ingold, J. Am. Chem. SOC.114,140 (1992). 125. S. Reekmans and F. C. De Schryver, in Frontiers in Supramolecular Organic Chemistry and Photochemistry, H. J. Schneider and H. Durr, Eds., VCH, Weinheim, 1991, p. 287. 126. S. Ariel, S. Askari, J. R. Scheffer, J. Trotter, and L. Walsh, in Organic Phototransformations in Hom-homogeneous Media, M. A. Fox, Ed., ACS Symposium Series 278, American Chemical Society, Washington, DC, 1985, p. 243. 127. S. Ariel, S. Askari, S. V. Evans, C. Hwang, J. Jay, J. R. Scheffer, J. Trotter, L. Walsh, and Y. F. Wong, Tetrahedron 43, 1253 (1987). 128. A. Gavezzotti, J . Am. Chem. SOC.105, 5220 (1983). 129. A. Gavezzotti, Tetrahedron 43, 1241 (1987). 130. G. S. Murthy, P. Arjunan, K. Venkatesan, and V. Ramamurthy, Tetrahedron 43, 1225 (1987). 131. H. E. Zimmerman and M. J. Zuraw, J . Am. Chem. SOC.111, 2358 (1989). 132. H. E. Zimmerman and M. J. Zuraw, J . Am. Chem. SOC.111, 7974 (1989). 133. L. Leisorowitz and G. M. J. Schmidt, Acta Crystallogr. 18, 1058 (1965). 134. K. Gnanaguru, N. Ramasubbu, K. Venkatesan, and V. Ramamurthy, J . Org. Chem. 50, 2337 (1985). 135. M. M. Bhadbhade, G. S. Murthy, K. Venkatesan, and V. Ramamurthy, Chem. Phys. Lett. 109, 259 (1984). 136. For a review: M. Vaida, R. Popovitz-Biro, L. Leiserowitz, and M. Lahav, in Photochemistry in Organized and Constrained Media, V. Ramamurthy, Ed., VCH, New York, 1991, p. 247. 137. J. Bergman, K. Osaki, G. M. J. Schmidt, and F. I. Sonnatag, J . Chem. SOC.2121 (1964). 138. S. M. Aldoshin, M. V. Alfimov, L. 0. Atovmyan, V. F. Kaminsky, V. F. Razumov, and A. G. Rachinsky, Mol. Cryst. Liq. Cryst. 108, 1 (1984). 139. V. R. Pedireddi, J. A. R. P. Sarma, and G. R. Desiraju, J . Chem. SOC.Perkin Trans. 2 311 (1992).

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

221

140. S. Devanathan and V. Ramamurthy, J . Photochem. Photobiol. A: Chemistry 40, 67 (1987). 141. Y. Nakamura, J . Chern. SOC.Chem. Commun. 477 (1988). 142. V. Ramesh and R. G. Weiss, J . Org. Chem. 51, 2535 (1986). 143. V. Ramesh and R. G. Weiss, Mol. Cryst. Liq. Cryst. 135, 13 (1986). 144. B. R. Suddaby, P. E. Brown, J. C. Russel, and D. G. Whitten, J . Am. Chem. SOC. 107, 5609 (1985). 145. W. F. Mooney, P. E. Brown, J. C. Russel, S. B. Costa, L. G. Pederson, and D. G. Whitten, J . Am. Chem. SOC.106,5659 (1984). 146. D. G. Whitten, J . Am. Chem. SOC.96,594 (1974). 147. L. Collins-Gold, D. Mobius, and D. G. Whitten, Langmuir 2, 191 (1986). 148. D. G. Whitten, L. Collins-Gold, T. J. Dannhauser, and W. F. Mooney, in Biotechnological Applications of Lipid Microstructures, B. P. Gaber, J. M. Schnur, and D. Chapman, Eds., Plenum, New York, 1988, p. 291. 149. K. Nishiyama and M. Fujihara, Chem. Lett. 1257 (1988). 150. P. R. Pokkuluri, J. R. Scheffer, J. Trotter, and M. Yap, J . Org. Chem. 57, 1486 (1992). 151. F. Toda and K. Tanaka, J. Chem. SOC.Chem. Commun. 1429 (1986). 152. F. Toda, K. Tanaka, and M. Yagi, Tetrahedron 43, 1495 (1987). 153. F. Toda and K. Tanaka, Tetrahedron Lett. 29, 4299 (1988). 154. M. Kaftory, M. Yagi, K. Tanaka, and F. Toda, J . Org. Chem. 53, 4391 (1988). 155. T. Fujiwara, N. Tanaka, K. Tanaka, and F. Toda, J . Chem. SOC.,Perkin Trans. 1 663 (1989). 156. M. S. Syamala, B. Nageswer Rao, and V. Ramamurthy, Tetrahedron 44, 7234 (1988). 157. M. S. Syamala and V. Ramamurthy, Tetrahedron 44, 7223 (1988). 158. R. Arad-Yellin, S. Brunie, B. S. Green, M. Knoossow, and G. Tsoucaris, J . Am. Chem. SOC.101, 7529 (1979). 159. V. Ramamurthy, J. V. Caspar, D. R. Corbin, D. F. Eaton, J. S. Kauffman, and C. Dybowski, J . Photochem. Photobiol., Chem. A 51, 259 (1990). 160. V. Ramamurthy, D. R. Corbin, and D. F. Eaton, J . Org. Chem. 55, 5269 (1990). 161. J. E. Leffler and J. J. Zupancic, J . Am. Chem. SOC.102, 259 (1980). 162. D. R. Corbin, D. F. Eaton, and V. Ramamurthy, J . Am. Chem. SOC.110, 4848 (1988). 163. D. R. Corbin, D. F. Eaton, and V. Ramamurthy, J . Org. Chem. 53, 5384 (1988). 164. V. Pushkara Rao, N. Han, and N. J. Turro, Tetrahedron Lett. 31, 835 (1990). 165. F. Toda and K. Akagi, Tetrahedron Lett. 3605 (1968). 166. M. Kaftory, Tetrahedron 43, 1503 (1987). 167. K. Tanaka and F. Toda, J . Chem. SOC., Chem. Cornmun. 593 (1983). 168. M. Kaftory, K. Tanaka, and F. Toda, J . Org. Chem. 50, 2154 (1985).

228 169. 170. 171. 172. 173.

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

J. Narasimha Moorthy and K. Venkatesan, J . Org. Chem. 56, 6957 (1991). T. Tamaki, Chem. Lett. 53 (1984). T. Tamaki and T. Kokubu, J . Inch. Phenom. 2, 815 (1984). T. Tamaki, T. Kokubu, and K. Ichimura, Tetrahedron 43, 1485 (1987). R. Farwaha, P. de Mayo, and Y. C. Toong, J . Chem. SOC.Chem. Commun. 739

(1983). 174. R. Farwaha, P. de Mayo, J. H. Schauble, and Y. C. Toong, J . Org. Chem. 50,245 (1985). 175. V. Dave, R. Farwaha, P. de Mayo, and J. B. Stothers, Can. J . Chem. 63, 2401 (1985). 176. K. Takagi, H. Usami, H. Fukaya, and Y. Sawaki, J . Chem. SOC.,Chem. Commun. 1174 (1989). 177. H. Usami, K. Takagi, and Y. Sawaki, J . Chem. SOC.,Perkin Trans. I I 1723 (1990). 178. H. Usami, K. Takagi, and Y. Sawaki, J . Chem. SOC.,Faraday Trans. 88,77 (1992). 179. C. A. Backer and D. G. Whitten, in Photochemistry on Solid Surfaces, T. Matsuura and M. Anpo, Eds., Elsevier, Amsterdam, 1989, p. 216. 180. K. H. Lee and P. de Mayo, J . Chem. SOC., Chem. Commun. 493 (1979). 181. K. Muthuramu and V. Ramamurthy, J . Org. Chem. 47, 3976 (1982). 182. K. Muthurams and V. Ramamurthy, Ind. J . Chem. 23B, 502 (1984). 183. R. S. Fargues, M. T. Maurette, E. Oliveros, M. Riviere, and A. Lattes, J . Photochem. 18, 101 (1982). 184. R. S. Fargues, M. T. Maurette, E. Oliveros, M. Riviere, and A. Lattes, Noun J . Chim. 3, 487 (1979). 185. R. S. Fargues, M. T. Maurette, E. Oliveros, M. Riviere, and A. Lattes, Tetrahedron 40, 2381 (1984). 186. H. Amarouche, C. Bourayne, M. Riviere, and A. Lattes, C. R. Acad. Sci. Paris, Series I I 298, 121 (1984). 187. Y. Nakamura, T. Kato, and Y. Morita, Tetrahedron Ld G . T/: Bonau, J . Photochem. 22, 61 (1983). 190. T. Wolff and N. Muller, J . Photochem. 22, 131 (1983). 191. V. Ramesh and V. Ramamurthy, J . Org. Chem. 49, 536 (1984). 192. P. de Mayo and L. K. Sydnes, J . Chem. SOC.,Chem. Commun. 994 (1980). 193. N. Berenjian, P. de Mayo, M. E. Sturgeon, L. K. Sydnes, and A. C. Weedon, Can. J . Chem. 60, 425 (1982). 194. K. Muthuramu, N. Ramanath, and V. Ramamurthy, J . Org. Chem. 48, 1872 (1983). 195. N. Ramanath and V. Ramamurthy, J . Org. Chem. 49, 2827 (1984). 196. F. H. Quina and D. G. Whitten, J . Am. Chem. SOC. 97, 1602 (1975). 197. F. H. Quina and D. G. Whitten, J . Am. Chem. SOC. 99, 877 (1977). 198. F. H. Quina, D. Mobius, F. A. Carroll, F. R. Howhitten, J . Am. Chem. SOC.99, 877 (1977).

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

229

198. F. H. Quina, D. Mobius, F. A. Carroll, F. R. Hopf, and D. G. Whitten, 2.Phys. Chem. 101, 151 (1976). 199. D. G. Whitten and P. R. Worsham, Org. Coatings Plastics 38, 572 (1978). 200. I. Weissbuch, L. Leiserowitz, and M. Lahav, J . Am. Chem. SOC.113, 8941 (1991). 201. Y. C. Lin and R. G. Weiss, Liq. Cryst. 4, 367 (1989). 202. R. Brewlow, Acc. Chem. Res. 24, 159 (1991). 203. R. Klopfer and H. Morrison, J . Am. Chem. SOC.94, 255 (1972). 204. J. G. Otten, C. S. Yeh, S. Byrn, and H. Morrison, J . Am. Chem. SOC.99, 6353 (1977). 205. M. S. Syamala and V. Ramamurthy, J. Org. Chem., 51, 3712 (1986). 206. Y. Ito, T. Kajita, K. Kunimoto, and T. Matsuura, J . Org. Chem. 54, 587 (1989). 207. M. Farina, in Inclusion Compounds, Vol. 3, J. L. Atwood, J. E. D. Davies, and D. D. MacNicol, Eds., Academic Press, New York, 1984, p. 297. 208. J. F. Brown and D. M. White, J . Am. Chem. SOC.82, 5671 (1960). 209. H. R. Allcock, W. T. Ferrar, and M. L. Levin, Macromolecules 15, 697 (1982). 210. M. Farina, G. Audisio, G. Allegra, and M. Loffelholz, J . Poly. Sci. C16, 2517 (1967). 211. M. Miyata and K. Takemoto, J . Polym. Sci., Polym. Lett. Ed. 13, 221 (1975). 212. K. Takemoto and M. Miyata, Mol. Cryst. Liq. Cryst. 186, 189 (1990). 213. N. J. Turro, J. R. Fehlner, D. P. Hessler, K. M. Welsh, W. Ruderman, D. Firnberg, and A. M. Braun, J . Org. Chem. 53, 3731 (1988). 214. W. Jones and W. Thomas, Prog. Solid State Chem. 12, 101 (1979). 215. M. D. Cohen, Z. Ludmer, J. M. Thomas, and J. 0. Williams, Proc. R. SOC., London, A 324,459 (1971). 216. J. C. J. Bart and G. M. Schmidt, Isr. J . Chem. 9, 429 (1971). 217. E. Heller and G. M. J. Schmidt, Zsr. J . Chem. 9, 449 (1971). 218. J. N. Moorthy and K. Venkatesan, J . Mater. Chem., in press. 219. G. M. Parkinson, M. J. Goringe, S. Ramdas, J. 0.Williams, and J. M. Thomas, J . Chem. SOC.,Chem. Commun. 134 (1978). 220. R. K. Bauer, P. de Mayo, W. R. Ware, and K. C . Wu, J . Phys. Chem. 86, 3781 (1982). 221. D. Avnir, R. Busse, M. Ottolenghi, E. Wellner, and K. A. Zachariasse, J . Phys. Chem. 89, 3521 (1985). 222. C. Francis, J. Lin, and L. A. Singer, Chem. Phys. Lett. 94, 162 (1983). 223. T. Fujii, E. Shimizu, and S . Suzuki, J . Chem. SOC.Faraday Trans. 184, 4387 (1988). 224. R. K. Bauer, R. Borenstein, P. de Mayo, K. Okada, M. Rafalska, W. R. Ware, and K. C. Wu, J . Am. Chem. SOC.104, 4635 (1982). 225. P. de Mayo, L. V. Natarajan, and W. R. Ware, J . Phys. Chem. 89, 3526 (1985). 226. D. R. James, Y. S. Liu, P. de Mayo, and W. R. Ware, Chem. Phys. Lett. 120,460 (1985).

230

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

227. J. K. Thomas, Acc. Chem. Res. 21, 275 (1988). 228. V. G. Kuykendall and J. K. Thomas, J . Phys. Chem. 94, 4224 (1990). 229. A. Habti, D. Keravis, P. Levitz, and H. Van Damme, J . Chem. Soc., Faraday Trans. 2 80, 67 (1984). 230. V. Joshi and P. K. Gosh, J . Am. Chem. Soc. 111, 5604 (1989). 231. For a summary of results see V. Ramamurthy, in Photochemistry in Organized and Constrained Media, V. Ramamurthy, Ed., VCH, New York, 1991, p. 429. 232. X. Liu, K. K. Iu, and J. K. Thomas, J . Phys. Chem. 93, 4120 (1989). 233. V. Ramamurthy, D. R. Sanderson, and D. F. Eaton, Photochem. Photobiol., in press. 234. H. L. Casal and J. C. Scaiano, Can. J . Chem. 63, 1308 (1985). 235. J. C. Scaiano, H. L. Casal, and J. C. Netto-Ferreria, ACS Symposium Series, 278, 211 (1985). 236. F. Wilkinson, C. J. Willsher, H. L. Casal, L. B. Johnston, and J. C. Scaiano, Can. J . Chem. 64, 539 (1986). 237. G. Kelly, C. J. Willsher, F. Wilkinson, J. C. Netto-Ferreira, A. Olea, D. Weir, L. J. Johnston, and J. C. Scaiano, Can. J . Chem. 68, 812 (1990). 238. J. V. Caspar, V. Ramamurthy, and D. R. Corbin, Coord. Chem. Rev. 97, 225 (1990). 239. V. Ramamurthy, J. V. Caspar, E. Kuo, D. F. Eaton, and D. R. Corbin, J . Am. Chem. Soc. 114, 3882 (1992). 240. V. Ramamurthy, J. V. Caspar, and D. R. Corbin, Tetrahedron Lett., 31, 1097 (1990). 241. G. L. Duveneck, E. V. Sitzman, K. B. Eisenthal, and N. J. Turro, J . Phys. Chem. 93, 7166 (1989). 242. M. S . Syamala, S. Devanathan, and V. Ramamurthy, J . Photochem. 34, 219 (1986). 243. Y. Kotake and E. G. Janzen, J . Am. Chem. Soc. 111, 5138 (1989);Y. Kotake and E. G. Janzen, J . Am. Chem. Soc. 111, 3699 (1989). 244. Y. Kotake and E. G. Janzen, J . Am. Chem. Soc. 114, 2872 (1992). 245. F. B. Bright, G. C. Catena, and J. Huang, J . Am. Chem. Soc. 112, 1343 (1990). 246. J. Huang and F. B. Bright, J . Phys. Chem. 94, 8457 (1990). 247. W. G. Herkstroeter, P. A. Martic, T. R. Evans, and S. Farid, J . Am. Chem. Soc. 108, 3275 (1986). 248. G. G. Warr and F. Griese, J . Chem. Soc., Faraday Trans. I 82, 1813 (1986). 249. G. G. Warr, F. Griese, and D. F. Evans, J . Chem. Soc., Faraday Trans. 182,1829 (1986). 250. M. van der Auweraer and F. C. de Schryver, in Structure and Reactivity in Inverse Micelles, M. P. Pileni, Ed., Elsevier, Amsterdam, 1989, p. 70. 251. A. Siemiarczuk and W. R. Ware, Chem. Phys. Lett. 160, 285 (1989). 252. A. Siemiarczuk and W. R. Ware, Chem. Phys. Lett. 167, 263 (1990).

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

253. 254. 255. 256.

257. 258. 259.

260.

261. 262.

263. 264. 265. 266.

267. 268. 269.

270.

271. 272.

231

D. E. Freeman and W. Klemperer, J . Chem. Phys. 40, 604 (1964). J. Heicklen and W. A. Noyes, Jr., J . Am. Chem. SOC.16, 223 (1948). P. Wagner and B.-S. Park, Org. Photochem. 11, 227 (1991). For some theoretical treatments, see: (a) R. R. Sauers and S.-Y. Huang, Tetrahedron Lett. 31, 5709 (1990); (b) A. E. Dorigo, M. A. McCarrick, R. J. Loncharich, and K. N. Houk, J . Am. Chem. SOC.112, 7508 (1990); (c) M. J. S. Dewar and C. Doubleday, J . Am. Chem. SOC. 100,4935 (1978). R. G. W. Norrish, Trans. Faraday SOC.33, 1521 (1939). N. C. Yang and D.-H. Yang, J . Am. Chem. SOC.80, 2913 (1958). See for example: (a) Y . Ito, in Photochemistry on Solid Surfaces, M. Anpo and T. Matsuura, Eds., Elsevier, Amsterdam, 1989, p. 469; (b) Y . Ito and T. Matsuura, Tetrahedron Lett. 29, 3087 (1988). J. R. Scheffer and P. R. Pokkuluri, in Photochemistry in Organized and Constrained Media, V. Ramamurthy, Ed., VCH Publishers, New York, 1991, Chapter 5. J. C. Scaiano, E. Lissi, and M. V. Encina, Rev. Chem. Intermed. 2, 139 (1978). (a) D. H. R. Barton, B. Charpiot, K. U. Ingold, L. J. Johnston, W. B. Motherwell, J. C. Scaiano, and S. Stanforth, J . Am. Chem. SOC.107, 3607 (1985); (b) M. B. Zimmt, C. Doubleday, Jr., I. R. Gould, and N. J. Turro, J . Am. Chem. SOC.107, 6724 (1985). R. A. Caldwell, S. N. Dhawan, and T. Majima, J . Am. Chem. SOC.106, 6454 (1984). P. J. Wagner, Acc. Chem. Res. 22, 83 (1989). (a) C. Cui, Georgetown University, unpublished results; (b) A. Bondi, J . Phys. Chem. 68,441 (1964). K. Venkatesan and V. Ramamurthy, in Photochemistry in Organized and Constrained Media, V. Ramamurthy, Ed., VCH Publishers, New York, 1991, Chapter 4. J. A. Slivinskas and J. E. Guillet, J . Polym. Sci., Polym. Chem. Ed. 11,3043 (1973). R. L. Treanor and R. G. Weiss, Tetrahedron 43, 1391 (1987). (a) P. K. Sullivan, J . Res. ( N B S ) 78A, 129 (1974); (b) A. V. Bailey, D. Mitcham, and E. L. Skau, J . Chem. Eng. Data 15, 542 (1970); (c) W. B. Saville and G. Shearer, J . Chem. SOC. 127, 591 (1925). (a) G. Herzberg, Electronic Spectra of Polyatomic Molecules, Van Nostrand, Princeton, NJ, 1967; (b) L. E. Sutton, Ed., Tables of Interatomic Distances and Configurations in Molecules and Ions, The Chemical Society, London, 1965;(c) L. E. Sutton, Ed., Tables of Interatomic Distances, Special Publication No. 11, The Chemical Society, London, 1958; (d) A. I. Kitaigorodsky, Molecular Crystals and Molecules, Academic, New York, 1973, p. 21. M. Maroncelli, H. L. Strauss, and R. G. Snyder, J . Chem. Phys. 82,2811 (1985). R. G. Weiss, S. Chandrasekhar, and P. M. Vilalta, Coll. Czech. Chem. SOC.,in press.

232

V. RAMAMURTHY, R. G. WEISS AND G. S . HAMMOND

273. P. M. Vilalta, P b D . Thesis, Georgetown University, Washington, DC, 1992. 274. P. M. Vilalta, G. S . Hammond, and R. G. Weiss, Photochem. Photobiol. 54,563 (1991). 275. (a) W. Mahler, D. Guillon, and A. Skoulios, Mol. Cryst. Liq. Cryst. Lett. 2, 111 (1985);(b) C. Viney, T. P. Russell, L. E. Depero, and R. J. Twieg, Mol. Cryst. Liq. Cryst. 168,63 (1989). 276. J. R. Scheffer,in Organic Solid State Chemistry,G. R. Desiraju, Ed., Elsevier, New York, 1987, Chapter 1. 277. (a) T. J. Lewis, S . J. Rettig, J. R. Scheffer,J. Trotter, and F. Wireko, J . Am. Chem. SOC.112,3679 (1990);(b) T. J. Lewis, S . J. Rettig, J. R. Scheffer, and J. Trotter, J . Am. Chem. SOC. 113,8180 (1991). 278. (a)I. Furman and R. G. Weiss, J . Am. Chem. SOC.114,1381 (1992).(b) I. Furman, R. J. Butcher, R. M. Catchings, and R. G. Weiss, J . Am. Chem. SOC.,114,6023 (1992). 279. S. Ariel, V. Ramamurthy, J. R. Scheffer, and J. Trotter, J . Am. Chem. SOC.105, 6959 (1983). 280. L. J. Johnston and J. C. Scaiano, Chem. Rev. 89, 521 (1989). 281. (a) S . Evans, N. Omkaram, J. R. Scheffer, and J. Trotter, Tetrahedron Lett. 26, 5903 (1985); (b) S. V. Evans and J. Trotter, Acta Crystallogr. B45, 5 0 0 (1989). 282. (a) J. R. Scheffer, J. Trotter, N. Omkaram, S . V. Evans, and S . Ariel, Mol. Cryst. Liq. Cryst. 134, 169 (1986); (b) S. Ariel, S. V. Evans, M. Garcia-Garibay, B. R. Harkness, N. Omkaram, J. R. Scheffer, and J. Trotter, J . Am. Chem. SOC.110, 5591 (1988); (c) S . V. Evans and J. Trotter, Acta Crystallogr. B45, 159 (1989). 283. See for instance: V. T. Jones and J. B. Coon, J . Mol. Spectrosc. 31, 137 (1969). 284. S . V. Evans, M. Garcia-Garibay, N. Omkaram, J. R. Scheffer, J. Trotter, and F. Wireko, J . Am. Chem. SOC.108,5648 (1986). 285. P. de Mayo and N. Ramnath, Can. J . Chem. 64,1293 (1986). 286. N. J. Turro, I. R. Gould, M. B. Zimmt, and C.-C. Cheng, Chem. Phys. Lett. 119, 484 (1985). 287. J. C. Scaiano, Acc. Chem. Res. 15,252 (1982). 288. R. A. Caldwell, T. Majima, and C. Pac, J . Am. Chem. SOC.104,629 (1982). 289. B. Zibrowius, J. Caro, and H. Pfeifer, J . Chem. SOC.Faraday Trans. 184,2347 (1988). 290. N. J. Turro and P. Wan, Tetrahedron Lett. 25, 3655 (1984). 291. V. Ramamurthy, D. R. Corbin, and D. F. Eaton, J . Chem. SOC.,Chem. Commun. 1213 (1989). 292. V. Ramamurthy, D. R.Corbin, and L. J. Johnston, J . Am. Chem. SOC.114,3870 (1992). 293. V. Ramamurthy and D. R. Sanderson, Tetrahedron Lett. 33, 2757 (1992). 294. H. L. Casal, P. de Mayo, J. F. Miranda, and J. C. Scaiano, J . Am. Chem. SOC.105, 5155 (1983). 295. A. P. Dianin, J . Russ. Phys.-Chem. SOC.46, 1310 (1914).

ORGANIZED MEDIA IN PHOTOCHEMICAL REACTIONS

233

296. (a) D. D. MacNicol, J. J. McKendrick, and D. R. Wilson, Chem. Soc. Rev. 7 , 65 (1978); (b) J. L. Flippen, J. Karle, and I. L. Karle, J . Am. Chem. Soc. 92, 3749 (1970). 297. P. C. Goswami, P. de Mayo, N. Ramnath, G. Bernard, N. Omkaram, J. R. Scheffer, and Y.-F. Wong, Can. J . Chem. 63, 2719 (1985). 298. (a) I. Goldberg, Topics Curr. Chem. 149, 1 (1988);(b) Topics in Current Chemistry, Vol. 140, E. Weber, Ed., Springer-Verlag, Berlin, 1987. 299. (a) H. Aoyama, T. Hasegawa, and Y. Omote, J . Am. Chem. SOC.101,5343(1979); (b) H. Aoyama, M. Sakamoto, K. Kuwahara, K. Yoshida, and Y. Omote, J . Am. Chem. Soc. 105, 1958 (1983). 300. B. Ackerman and N. G. Johansson, Tetrahedron Lett. 5, 371 (1969). 301. C. A. Chesta and D. G. Whitten, J . Am. Chem. SOC. 114, 2188 (1992). 302. J. Vincens, Mol. Cryst. Liq.Cryst. 187, 115 (1990). 303. M. Kaftory, F. Toda, K. Tanaka, and M. Yagi, Mol. Cryst. Liq. Cryst. 186, 167 (1990). 304. G. D. Reddy, B. Jayasree, and V. Ramamurthy, J. Org. Chem. 52, 3107 (1987). 305. Y. Hui, J. R. Winkle, and D. G. Whitten, J . Phys. Chem. 87, 23 (1983). 306. J. R. Winkle, P. R. Worsham, K. S . Schanze, and D. G. Whitten, J . Am. Chem. SOC.105, 3951 (1983). 307. J. C. Scaiano and J. C. Selwyn, Photochern. Photobiol. 34, 29 (1981). 308. N. J. Turro, K.-C. Liu, and M.-F. Chow, Photochern. Photobiol. 26, 413 (1977). 309. R. L. Treanor and R. G. Weiss, J . Am. Chem. SOC. 110, 2170 (1988). 310. J. M. Vincent and A. Skoulios, Acta Crystallogr. 20, 432, 441, 447 (1966). 311. D. A. Hrovat, J. H. Liu, N. J. Turro, and R. G. Weiss, J . Am. Chem. Soc. 106,5291 (1984). 312. M. Heskins and J. E. Guillet, Macromolecules 3, 224 (1970). 313. (a) P. Dbeye, 7kans. Electrochem. Soc. 82, 265 (1942); (b) A. D. Osborne and G. Porter, Proc. Roy. SOC.(London) 284A, 9 (1965). 314. H. A. Kramers, Physica 7 , 284 (1940). 315. (a) J. Guillet, Polymer Photophysics and Photochemistry, Cambridge University Press, Cambridge, 1985; (b) J. Guillet, Adu. Photochem. 14, 91 (1988). 316. (a) D. Krishnamurti, K. S . Krishnamurthy, and R. Shashidhar, Mol. Cryst. Liq. Cryst. 8, 339 (1969); (b) K. S . Krishnamurthy and D. Krishnamurti, Mol. Cryst. Liq. Cryst. 6,407 (1970); (c) J. S. Dryden, J . Chem. Phys. 26, 604 (1957);(d) K. S . Krishnamurthy, Mol. Cryst. Liq.Cryst. 132, 255 (1986). 317. R. G. Weiss, R. L. Treanor, and A. Nunez, Pure Appl. Chem. 60, 999 (1988). 318. Z. He and R. G. Weiss, J . Am. Chem. SOC. 112, 5535 (1990). 319. D. A. Hrovat, J. H. Liu, N. J. Turro, and R. G. Weiss, J . Am. Chem. SOC.106,7033 (1984). 320. (a) M. G. Broadhurst, J . Res. ( N B S )66A, 241 (1962) and references cited therein; (b) M. Maroncelli, S . Ping Qi, R. G. Snyder, and H. L. Strauss, J . Am. Chem. Soc. 104, 6237 (1982).

234

V. RAMAMURTHY, R. G. WEISS AND G. S. HAMMOND

321. (a) A. Nunez and R. G . Weiss, J. Am. Chem. SOC. 109,6215 (1987); (b) A. Nunez and R. G. Weiss, Boll. SOC. Chilena Quim. 35, 3 (1990); (c) A. Nunez, G. S. Hammond, and R. G. Weiss, J . Am. Chem. Soc., in press. 322. W. Mahler, private communication of unpublished results.

Advances in Photochemistry, Volume18 Edited by ,David H. Volman, George S. Hammond, Douglas C. Neckers Copyright © 1993 John Wiley & Sons, Inc.

UP-SCALING PHOTOCHEMICAL REACTIONS Andre M. Braun, Laurent Jakob, and Esther Oliveros Lehrstuhl fur Umweltmesstechnik, Engler-Bunte-Institut, Universitat Karlsruhe, 7500 Karlsruhe, Germany Claudio A. OIler do Nascimento Escola Politecnica da Universidade de SBo Paulo 01000 SBo Paulo, SP, Brasil

CONTENTS

I. Introduction 11. Parameters for qualitative reactor design A. Reaction system 1. Gas phase reactions 2. Liquid phase reactions 3. Solid-liquid heterogeneous reaction systems 4. Gas-solid heterogeneous reaction mixtures 5. Liquid-liquid microheterogeneous reaction systems 6. Reaction mechanism and reactor design 7. Reaction system and safety requirements B. Light sources 1. Point sources 2. Extended light sources 3. Excimer lamps 4. Safety requirements

236 239 239 239 239 243 243 244 245 248 25 1 255 256 259 262

Advances in Photochemistry, Volume 18, Edited by David Volman, George S. Hammond, and Douglas C. Neckers ISBN 0-471-59133-5 Copyright 1993 by John Wiley & Sons, Inc.

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111. Concepts of qualitative reactor design A. Reactor design and light source B. Light distribution IV. Qualitative up-scaling rules and parameters A. Rate of production B. Sensitized oxidations C. Photocatalyzed reactions V. Mathematical modelling of photochemical reactors A. Radiation models 1. Incidence models 2. Emission models B. Coupling of mass and light energy balances VT. Modelling and optimization of pilot results A. Optimal experimental design 1. Fundamentals 2. Example of application: TiO, photocatalyzed oxidative degradation B. Neural networks 1. Fundamentals 2. Example of application: Large-scale actinometry VI. Conclusions Acknowledgments References

264 264 268 272 272 274 277 282 282 283 286 29 1 29 1 292 292 295 302 302 304 305 307 307

I. INTRODUCTION Industrial preparative photochemistry has been ill-famed for many decades [l], and successful processes and production units are kept out of publicity, because invested know-how is of considerable value. It is, hence, quite difficult to collect and review data on preparative photochemical technology and to discuss the evolution in this field [2,3]. For a few years, interest in photochemical reactions of potential preparative interest has however been revived. Reasons may be found for instance in: Constraints due to regulations concerning environmental protection, such as limitations in the choice of solvents, waste disposal, and work-up procedures.

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0

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The evolution of teaching programs in academia and, therefore in a larger awareness of research chemists and engineers for the potential of photochemical processes. The development of photochemical reactor engineering.

Rapidly outgrowing the number of photochemical reactions of potential interest for industrial production, photochemical oxidative degradation processes will most probably find applications in the chemical treatment of polluted surface and ground waters, as well as of (industrial) waste waters containing nonbiodegradable or highly bactericidal pollutants [4]. In implementing these photochemical or photocatalyzed methods of oxidative degradation, the same or similar concepts of photochemical reactor engineering are used as for preparative purposes. This chapter does not deal with the technical developments in copying, isotope separation, microelectronics, medicine, nonlinear optics, photography, sculpturing, and surface coating, domains that are still neglected by a large part of the chemical industry and, certainly, by the majority of academic teachers. General views on the different fields of applied photochemistry have been published for the first two “Schools of Industrial Photochemistry” which were held in 1988 and 1990 at the ENSIC-INPL in Nancy [S, 6 ) . The Chapter does, however, address problems linked to the technical development of photochemical processes with a potential interest in chemical synthesis or large-scale photolysis. It describes therefore the different approaches of photochemical engineering which comprise the fundamentals and the developments of methods and procedures used to implement and optimize large-scale photochemical reactions. The technical development of photochemical reactions involves specific problems due to the necessity to activate by electronic excitation one of the substrates present in a reaction mixture. Thus in addition to the rather wellmastered mass and energy balances in conventional process development, optimal light distribution must be achieved, a condition difficult to satisfy when taking into account synthetically meaningful substrate concentrations. Difficulties encountered in up-scaling photochemical reactions are then mainly due to the consequences of the inhomogeneity between irradiated and nonirradiated reaction volumes which are created by the reaction systems particular absorption characteristics and by the light distribution of the chosen light source. Even in a well-mixed reactor, where a thermal reaction would proceed in all parts of the reactor, a photochemical reaction will only take place within the volume where light is absorbed, and no means of mixing will be able to move electronically excited states within their lifetime into the nonirradiated volume. The selection of an appropriate light source and the design of a reaction specific photochemical reactor are, hence, the major tasks

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IPhotochemical Engineering

Figure 1. Interactive domains in photochemical engineering (see also [7]). of photochemical engineering and require experience in mechanistic and analytical chemistry, chemical engineering, and photochemical technology (Figure 1). Among the few academic and industrial groups working in the field of photochemical engineering, two different strategic lines of research and development can be distinguished:

1. A pragmatic but mostly qualitative approach is using a check list of concepts, such as a. Specific excitation b. Spatial separation between light source and reaction mixture c. optimal gas saturation d. Convergence of light source and reactor geometry e. Fixed exitance* during up-scaling and optimizing a concept related reactor module in accord with the absorption parameters and the kinetics of the photochemical reaction. 2. A more quantitative approach is investing in a. A precise description of light distribution within the reactor volume and, hence, in the calculation of the irradiated volume and its shape. b. A quantitative combination of this spatially defined energy uptake with ideal classical mass and energy system balances which in turn may then yield reaction specific reactor geometries and operational limits for optimal reaction conditions. Although the first approach has been surprisingly successful in most of the recent industrial development projects, accurate models of light distribution *Exitance: power radiated in all directions per unit area of an extense light source [2,3].

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and propagation would be definitely needed to overcome some of the disadvantages of the thumb-rule designed reactor modules, where professional experience is the main asset.

11. PARAMETERS FOR QUALITATIVE REACTOR DESIGN A. Reaction System The reaction system is one of the most important parts of choosing a basic module for the design of a photochemical reactor. This might be explained by enumerating some of the corresponding parameters and describing their impact on reactor geometry and operational conditions: Physical state of substrates, intermediates, and products Absorption characteristics of substrates and products Reaction mechanism Thermodynamic and kinetic parameters and chemical yield Quantum yield Safety requirements.

1. Gas Phase Reactions. Substrates in the gas phase show low absorption cross sections, and reactor geometries providing long optical paths are usually required for efficient operation (e.g., tubular photochemical reactor designed for irradiation by a laser beam; see Figure 2). However, annular reactor geometries with light sources placed in their axis have proven to be extremely useful for reactions exhibiting high quantum yields, as for example in photochlorinations of partially fluorinated hydrocarbons of low molecular weight (Figure 3). Gas phase reactions yielding liquid or solid products might be realized in vertically mounted tubular reactors operating at wall temperatures that provoke product condensation. The tube may be fitted with a device to rinse off the reaction products (see Figure 7). 2. Liquid Phase Reactions. Photochlorination of low molecular weight hydrocarbons, such as methane or ethane, can be achieved in liquid phase using pressurized reactors or an inert solvent (e.g., CC1,) [9,10]. Photolysis of chlorine saturated, liquid phase substrates is the most common procedure reported in research and patent literature [2,3]. Absorption cross sections of saturated chlorine solutions allow optical pathlengths of a few centimeters. Organic radicals generated by hydrogen abstraction (Eqs. 1 and 2) react with molecular chlorine (Eq. 3) at high efficiency, and secondary reactions, such as

jacket

Figure 2. Diagram of a tubular photochemical reactor irradiated by a laser beam [2, 3, 71 (dimensions in mm). Coolant outlet

$?

Product

t 1. f

CH3

- CHFz-

CH, - CHF2

I

C'2

source Coolant

Figure 3. Conceptual diagram of an annular photochemical reactor for the production of 1-chloro-l,l-difluoroethanein the gas phase with introduction of 1,ldifluoroethane at different levels [2, 31. 240

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241

dismutation reactions (Eq. 4), are almost negligible. Under those conditions, immersion-type reactors provide most economic operation units, although the lamp wells must be periodically cleaned from polymerized secondary products.

c1, 42C1'

-

+ RH, --+ HR' + Clz

C1'

2HR'

(1)

+ HCl HRCl + C1' HR'

R+RH,

A similar mechanism has been proposed for photonitrosylations, with the difference that the reaction of the alkyl radical with NOCl (Eq. 5 ) is not competitive (absence of chain reaction) and that the rate of reaction 6 cannot prevent dismutation (Eq. 4) and subsequent radical polymerization of unsaturated hydrocarbons (e.g., cyclohexene in the case of caprolactam synthesis). HR'

+ NOCl

HR' + N O '

__+

HRNO HRNO

+ C1'

(5)

(6)

Photonitrosylations show an extremely high tendency for filming, and the difficulties in implementing the caprolactam synthesis on an industrial scale are also linked to reactor designs not taking into account the competitive secondary reactions leading to polymerized material. Liquid phase photochemical reactions may show in most cases filming effects [2,3]. This is due to usually high-absorption cross sections of the substrates dissolved up to concentrations of preparative importance, simple calculations on the basis of Beer-Lambert's law showing that optical paths might be as short as a fraction of a millimeter (Figure 4) [2,3]. Under these conditions, high local concentrations of electronically excited molecules and intermediates are produced within the irradiated reaction volume and cannot be transfered into nonirradiated zones within the time frame of diffusion controlled primary reactions. For optimal reactor operation, extremely inhomogeneous distributions of reactive states within the reaction volume must be avoided, a task which can be achieved by either reducing the absorbance of the reaction mixture by diminishing the concentration of the light-absorbing substrate, by reducing the radiant power, or by changing the light distribution in modifying the design of the photochemical reactor. Extremely inhomogeneous conditions are found in oxidative degradation processes induced by vacuum ultraviolet (VUV) irradiation of aqueous reaction systems. In fact, the absorption cross section of water for an almost monochromatic excitation at 172 nm (Xe excimer lamp, vide infra) being very

242

A. M. BRAUN, L.JAKOB, E. OLIVEROS AND c. A. OLLER DO NASCIMENTO

0

0.004

0.008

0.012

0.016

0.02

1 [cml

Figure 4. Relative number of photons transmitted as a function of distance 1 traversed by the light (Beer-Lambert law); E of reactant at the wavelength of irradiation: lo4 ~.mol-'.crn-'; concentration of reactant: (I) c = mo1.L-'; (2) c = lo-' rno1.L-I [2,3].

efficient [ll], light is totally absorbed within an optical path of less than approximately m [121. Combining this very small irradiated reaction volume with a high irradiance leads in accord with Eq. (7) to large local HO' radical concentrations. Under conditions of residual (industrial) waters containing more than lo3 ppm of total organic carbon (TOC), subsequent hydrogen abstraction (Eq. 8) leads to organic radical concentrations which will almost instantaneously deplete the irradiated volume of dissolved oxygen (Eq. 9). This latter reaction is, however, of primary importance, not only for initiating some thermal chain reaction of oxidative degradation, but also for preventing the deposit of polymerized material at the lamps surface.

+ HO' + H,O + RH' hv

H,O + H'

+ RH, RH' + 0, -+

HO'

RHO,' +

-

(7)

(8) (9)

Assuming chemical inertness of sensitizers, immersion-type reactors apply practically without restriction to sensitized reactions, and by varying the concentration of the sensitizer, radiant power and irradiated volume of the reaction system can be controlled rather easily. Secondary reactions usually occur with low efficiencies, for example, in sensitized oxidations they may include electron transfer reactions [13] and singlet oxygen reactions with and photolysis of the sensitizer molecule, the latter due to insufficient light stability. Changes of the absorption spectrum and, hence, of the absorbed photon rate might also occur as a consequence of the changing composition of the reaction mixture in the course of the reaction. It is for instance well

UP-SCALING PHOTOCHEMICAL REACTIONS

243

known that role bengal solutions decolorize in the presence of Brernsted or Lewis acids. To avoid purification of the reaction product from the (colored) sensitizer or its oxidation and photolysis products, the use of insoluble sensitizers has been proposed, in particular for sensitized oxidations [141. Whereas sensitizers adsorbed on solid supports, such as ion exchange resins, silica, or alumina [l5], show considerable leading rates [16] and must be discarded, potential application might be found for sensitizers that are chemically bound to inert surfaces [14-171. However, a loss of efficiency of at least 50% has to be taken into account when comparing overall quantum yields with those determined in homogeneous reaction systems [171.

3. Solid-Liquid Heterogeneous Reaction Systems. For solid-liquid heterogeneous reaction systems, immersion-type reactors provide good technical and economical results. Transition to fluidized bed reactors would be continuous, but high contents of solid particles is severely limiting reactor dimensions. For Ti0,-photocatalyzed oxidative degradation, multilamp immersion-type photoreactors have been used to offset the problem of limited light penetration due to diffusion of light (vide infra) [l2, 181. However, alternative reactor geometries may be developed using fixed photocatalysts [191 and consequently taking into account smaller efficiencies.

4. Gas-Solid Heterogeneous Reaction Mixtures. Gas-solid heterogeneous reaction mixtures may be advantageously irradiated in annular (immersiontype) photochemical reactors. Again, the content of solid particles is limiting the size and the productivity of the reactor system. This is of particular importance when the solid support is used to specifically adsorb substrates or products of the photochemical reaction; the first to enhance specificity of radical substitution reactions [20], the latter to reach better photostability and to ensure optimal purity. Particular ingenious reactor geometries have been developed for the photochlorination of PVC [2,3]. By exploiting the fact that PVC particles swell in liquid chlorine, the reaction has been carried out at temperatures below the condensation temperature of Cl,. Starting from experiments with tubular photochemical reactors, where light sources were installed at the outside wall of the reactor to ensure efficient mixing, a continuous process has been developed in which the stirrer was replaced by a helical stirring blade (Figure 5) [21]. This production unit may be relatively easily optimized in varying the principal parameters, such as number, mounting places and electrical power of a given type of light sources, reactor diameter, feeding speed, and particle charge.

244

A. M. BRAUN,L. JAKOB, E. OLIVEROS AND c. A. OLLER DO NASCIMENTO

Porous plate

CPVC

Figure 5. Tubular reactor equipped with a helical agitator for the photochlorination of PVC in liquid chlorine [2,3,21].

5. Liquid-Liquid Microheterogeneous Reaction Systems. Several authors have shown that liquid-liquid microheterogeneous reaction systems may be advantageous for overall chemical yield and positional and stereochemical specificity of photochemical reactions [22]. Ionic interphases may for instance assist in differentiating between reactive intermediates and thus enhance reaction specificity and chemical yield. Sensitized oxidations may often include singlet oxygen as well as superoxide or hydroperoxide reactions depending on the redox potentials of ground and excited states of the sensitizing molecule, as well as of the substrate to be oxidized. In fact, besides energy transfer to oxygen (Eq. lo), electron transfer to oxygen (Eq. 11) or to singlet oxygen (Eq. 12) may occur ~ 3 1 : 3Sens 3Sens

+ 0, + 0,

'O,+RH, '0,

+R

__+

Sens

+ '0,

Sens+'

+ 0;'

0; + R H l '

-

R0, ++

(10) (11) +

(12) (13)

In contrast to singlet oxygen, charged intermediates cannot penetrate ionic liquid-liquid interphases (ionic micelles or microemulsions) owing to charge repulsion or attraction [23]. Sensitizer and substrate solubilization in different phases may therefore be of interest for an improvement of chemical yield.

UP-SCALING PHOTOCHEMICAL REACTIONS

245

Charged interphases may also be exploited to create high local concentrations of electron acceptors which affect the rate of electron transfer reactions confined within these restricted reaction volumes and diminish considerably the efficiency of the corresponding back-transfer [24]. These results have been primarily applied in photochemical conversion projects [22,25], but technically more interesting applications may be found in their use for the development of new specific analytical procedures (e.g., optical or photoelectrochemical probes). High local concentrations are also of considerable interest in the optimization of photochemical dimerization reactions [22], as the rate of bimolecular reactions between excited and ground state molecules confined in an extremely restricted reaction volume (microreactor) will be considerably enhanced. In addition, spatial gradients of polarity may lead to preferential structures of the solvated substrate and, hence, to the synthesis of specific isomers [24,22,26]. Similar selectivities have been found when monomolecular photochemical or photoinduced reactions [2,3] are made via inclusion complexes [27,28]. Micelles are of no practical interest as far as preparative chemistry and photochemistry is concerned, their capacity of substrate solubilization being far too restricted. On the other hand, microemulsions can dissolve rather large concentrations of starting materials and provide, because of their transparency, an excellent environment for photochemical reactions on a preparative scale [26, 28, 291. However, pseudo-ternary phase diagrams of clearly defined microemulsions are certainly altered by adding rather high concentrations of substrate or product, and care must be taken that microheterogeneity is not lost during the course of the reaction owing either to overloading or incompatibility between solubilizates and environment. Secondary reactions with the components of the environment or changes in temperature which may reduce the microheterogeneous area in the pseudoternary diagram are additional constraints for the use of microemulsions that may cause severe turbidity of the reaction mixture, aggregation, or precipitation of its components. Technical development of such preparative procedures appears in general to be rather difficult, and, as in the case of liquid phase homogeneous reaction mixtures, the use of immersion-type photochemical reactors cannot be recommended.

6. Reaction Mechanism and Reactor Design. The impact of reaction mechanism and rates of competitive steps of reactions have already been mentioned in order to explain the phenomenon of filming (polymerization of secondary products at the irradiated reactor surface). Even without filming, photochemical reactions might come to a halt, if secondary reactions counteract with high efficiency. The fact that the first step of the propagation in photobrominations with Br, is reversible (Eq. 15 [2,3]) has for a long time

246

A. M. BRAUN, L. JAKOB, E. OLIVEROS A N D C. A. OLLER DO NASCIMENTO

been neglected, treating corresponding photochlorinations and photobrominations without differentiation. Br,

Br' HR'

hv

+ RH,+===HBr + Br,

2Br'

+ HR'

+ HRBr

+ Br'

(16)

In batch procedures, liquid phase photobrominations halt in general at conversions between 50 and 70%, the concentration of HBr reaching a level where the back reaction (Eq. -15) becomes more efficient than the productforming step (Eq. 16). Particular reaction conditions and, hence, particular reactor geometries and/or accessories are then required to remove continuously the HBr produced. Photobrominations are preferentially developed at boiling point temperatures, as has been beautifully demonstrated in the industrial synthesis of 1-bromo-diethylcarbonate (Eq. 17) [30]. Br,

-

hv + CH,-CH2-OCOO-CH,-CH, CHBr + CH,-CHBr-OCOO-CH,-CH,

(17)

Photochemical reactors designed for this purpose may either be linked to (flash) distillation columns or be part of the distillation column itself, the latter being an advantageous solution for continuous production units. Bromination of allylic positions cannot be achieved specifically by using elementary bromine, unless electrophilic addition to the z-bond (Eq. 18) is unfavorable because the substituents have a high negative inductive effect. Efficiency of electrophilic addition of Br, may also be diminished by steric effects. In addition to these secondary thermal reactions, the specificity of photochemical brominations of allylic positions using Br, will also suffer from the competing (radical) addition of Br' to the double bond (Eq. 19) [31].

Br'

+

Rl

qR3 -R')d,R3 Br,

Ri

*R3

Br

Br

+

Br'

247

UP-SCALING PHOTOCHEMICAL REACTIONS

Rather specific thermal or photochemical bromination of allylic positions is, however, possible by using N-bromosuccinimide as brominating agent. Both procedures produce, however, variable quantities of Br, depending on reaction conditions [32]. In thermal procedures, the concentration of intermediate Br2, and, hence, the importance of secondary addition products, can be controlled by the relative quantity of radical initiator (e.g., AlBN) and by reaction temperature. The appearance of addition products in a photochemical procedure would be evidence for the mechanism proposed by Adam et al. [2, 3, 331 which includes the intermediate production of Br, (Eq. 21).

Br,

+

+

Br'

In the photochemical procedure, addition product can be minimized in keeping the relative NBS concentration as small as possible. In addition, substrate concentrations should be optimized with regard to the exitance of the chosen light source to avoid secondary recombination reactions. Under these conditions 4-bromomethyl-5-methyl-1,3-dioxol-2-one can be prepared with only minor impurities (bromine addition and multiple allylic bromination reactions (Eq.23)) [34].

Br2

+

)=( OKo 0

B

hv

r

w

OKo 0

WBr B

+

OKo 0

+

r

x

B

r

O K 0 0

(23)

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A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A. OLLER DO NASCIMENTO

7. Reaction System and Safety Requirements. For most photochemical reactions, both chemical yield and quantum yield of product formation depend primarily on substrate concentration, reaction environment, temperature, mixing, and specific excitation. In addition, the rate of the photochemical reaction is strongly affected by the incident photon rate and the absorption cross section of the reaction mixture. In designing and up-scaling photochemical reactors, combinations of some of those parameters have to be taken into account to comply with safety requirements. Leaving aside those safety rules that concern light sources and their operation, decisions on reactor geometry and accessories to be mounted are also affected by experimental and calculated results concerning, for example, exothermicity as a function of incident photon rate and substrate concentration, reactive gas consumption and pressure regulation, reaction volume, temperature, and inert gas saturation. As an example of safety requirements linked to the reaction system, the problem of oxygen present in photohalogenation may be mentioned. Except in the very few cases of applied photochemical oxychlorination [2,3], the presence of oxygen should be eliminated in photohalogenations. The presence of oxygen may be difficult to avoid in photochlorination units where production rate is primarily controlled by chlorine feed and consequently operating conditions may include periods during which less pressure is created in parts of the feeding installation. In fact, organic radicals generated by hydrogen abstraction (Eqs. 2 or 15) react efficiently with oxygen (Eq. 9) to form peroxyl radicals and subsequent products. This situation is exploited for the synthesis of trichloroacetyl chloride from 1,1,1,2-tetrachloro-ethane [Eqs. 2, 9 (R = CC1,-CC1) and 241,

2 CC13-CHC100'

+ C1,

2 CCl3-COCl

+ 2 HCl + 0

2

(24)

from pentachloro-ethane [35], as well as from tetrachloro-ethylene [36]. The overall reaction scheme of the synthesis of trifluoro acetylchloride from 1,ldichloro-2,2,2-trifluoro-ethane [37] includes a similar sequence of reactions as postulated in Ref. 35. All procedures produce or may produce phosgene as a secondary product either by radical fragmentation of the peroxyl radical CCl,-CHC100' (Eq. (25), CC13-CHC100'

+ COCl,

+ HCl + CO + C1'

by reaction of carbon monoxide with chlorine (Eq. 26),

co + c1,

+ coc1,

UP-SCALING PHOTOCHEMICAL REACTIONS

by fragmentation of CCl,-CCl,OO'

cc1,-cc1,00'

-

249

(Eq. (27), 2COC1,

+ C1'

(27)

or light-induced fragmentation of the same intermediate radical (Eq. 28).

CC~,-CC~,OO*

4CC~,O'+ COC~,

The last reaction has also been postulated in the course of the trifluoro acetylchloride synthesis [38] and is of general importance, as there is no qualitative restriction of this rearrangement to a-trihalogenomethyl groups. Trichloromethyl-chloroformate has been proposed as a relatively safe phosgene source for small-scale diisocyanate and polyurethane production units. Phosgene is produced upon fragmentation of this compound on a hot iron contact. ClCOOCH,

+ 3c1, 4ClCOOCCl, + 3HC1

(29)

Trichloromethyl-chloroformate is produced by exhaustive photochlorination of methyl-chloroformate (Eq. 29) [39], and, although so far never observed, phosgene could be an additional hazard in case of an equipment breakdown. In up-scaling the preparative procedure, care has to be taken that A compact production unit (Figure 6) can be installed in an area isolated from general ventilation system. This area is equipped with a fire-extinguishing installation (CO,). Cooling water circuits can be controlled from an outside stand. The lamp (Figure 6: 16) is mounted in a double jacket which would withstand mechanical strain due to explosion of the light source (e.g., upon rupture of the inner tube of protection). Addition of gaseous chlorine involves a drying device (Figure 6: 1) and buffer volume (Figure 6: 2) and can be controlled from an outside stand. Exiting gaseous HCl is checked for unreacted C1, (Figure 6: 5 ) before neutralization [pressure variations between neutralization tank (Figure 6: 6) and active carbon filter (Figure 6: 9) are balanced in an intermediate buffer system (Figure 6: 8)]. The operator has at his disposal a personal gas mask (active carbon filter) as well as an operator's mask (compressed air) to be used in case of product spills.

Cl,

!i

cIcooccI,

CICOOC%

electricity

H,O

20% NaOH

P

.

*

drain

*

waste

Figure 6. Block drawing of the pilot installation for the production of trichloromethyl chloroformate by exhaustive photochlorination [39]; 1: Dryer for gaseous C1, (H,SO, conc.). 2 Safety tank. 3: Thermoregulated immersion-type photochemical reactor. 4: Raschig column. 5: C1, detection system (1,2,4-trichlorobenzene).6: Neutralization tank (20% NaOH). 7: Reservoir of 20% NaOH. 8: Buffer to atmospheric pressure (20% NaOH). 9: Active carbon filter. 10: Reservoir of crude trichloromethyl chloroformate. 11: Buffer to normal atmosphere via CaCl, filter and direct entry for trichloromethyl chloroformate to be distilled. 12: Distillation flask with Vigreux column. 13: Exit to vacuum pump. 14: Solid NaOH filter before pump. 15: Cooling water alarm linked to power supply of the light source. 16: Medium pressure mercury arc. 17: Heater for distillation apparatus. 18: Magnetic stirrers. I,: thermometer; I,: manometer. 250

UP-SCALING PHOTOCHEMICAL REACTIONS

251

More recent developments favor bis(trichloromethy1)-carbonate, which is produced by exhaustive photochlorination of dimethyl carbonate. Singlet oxygen reactions with unsaturated hydrocarbons are rather specific and lead to endoperoxides (by 2 + 4 cycloaddition from 1,Cdienes) or hydroperoxides (by ene reaction from acyclic allylic systems) [2,3]. Whereas for ene reactions protic solvents may be used, such a reaction medium could lead in the case of endoperoxides to substitution reactions and thus direct the synthesis to partially oxidized products rather than cis-diols (Eq. 30). The hazard of explosion of intermediate hydroperoxides is usually avoided by in situ reduction by Na,SO,, NaHSO,, or reducing solvents like methanol [2,3]. In apolar and aprotic solvents, thiourea may be used for the same purpose, but reduction may be less efficient, and peroxide intermediates may accumulate. Under these conditions, the reaction should be carried out at low temperature with a workup procedure including a progressive temperature increase, after the reducing agent has been added.

A

OOH

OH

RO

0

B. Light Sources Besides the system parameters mentioned above, emission characteristics and geometry of the light source are decisive for the design of a photochemical

252

A. M. BRAUN, L. JAKOB, E. OLIVEROS A N D C. A. OLLER D O NASCIMENTO

reactor. In fact, light sources with defined emission characteristics exist either as point sources or as extended light sources. For instance,

0

Low-pressure mercury and sodium arcs, as well as fluorescent tubes [2,3] of technical importance are necessarily extended light sources because their mode of operation is incompatible with compression of the source to a point without alteration of their emission spectra. Technical limits of exitance also impose minima for the dimensions of medium-pressure light sources (length of arc, length and diameter of bulb), for example, medium-pressure (doped) mercury arcs, highpressure sodium vapor lamps, and incandescent lamps.

If the size of the production unit requires higher radiant power than can be provided, for technical reasons, by one lamp, clusters of light sources may be installed, which, consequently will alter the diameter or the height of the inner core of, for example, an annular photochemical reactor. However, following the check list of concepts (vide supra), optimal reaction conditions will in most cases limit the size of the photochemical reactor, and the planned rate of production may require several reactor units installed in a parallel mode (batch process) or in series (continuous process). Basically, the light source is chosen for its emission characteristics in order to achieve optimal selectivity of excitation. Spectral selectivity may be enhanced by using special glasses for the lamp well. On the other hand, aqueous solutions of transition metal salts [40] fulfill reasonably well their double task as IR and selective UV-VIS filters. They operate on a closed circuit including a heat exchanger; these installations are normally less expensive than glass filters in investment and maintenance. This is particularly true with respect to colored glasses, the light stability of which is in most cases not satisfactory. In a second step, several options on light sources must be evaluated with respect to their impact on production rate and experimental conditions imposed by the reaction system. Many technical applications require high radiant power, which cannot be furnished by an operationally reasonable number of lamps having otherwise optimal emission characteristics. Examples of this situation are mostly found in applications of.the 254-nm line, where a number of low-pressure mercury lamps may be replaced by one medium-pressure mercury arc. This substitution represents a compromise where spectral selectivity and energy wasting (VIS and IR radiation) is traded against a compact production unit which is less expensive (number of reactors, quartz, safety requirements) and easier to operate (number of reactors, space, and overview). Absorption conditions of the reaction system or spectral selectivity of

UP-SCALING PHOTOCHEMICAL REACTIONS

253

excitation imposed by the reaction mechanism may call for spatial separation of consecutive steps of chemical transformation. The photochlorination of a substrate absorbing in the same spectral region as the dissolved elemental chlorine is a typical example. Excitation of C1, (Eq. 1) under these conditions is far from being efficient, since the substrate to be chlorinated is acting as an inner filter. On the other hand, direct excitation of the substrate should be avoided as it leads to secondary products, both upon direct excitation and subsequent to chlorination. This rather difficult task in photochemical engineering may be solved with a reactor configuration as shown in Figure 7 Gas

I

Gas introduction tube

Cooling coil

Central tube

Reactant liquid (or solution)

4

t Light beam

Figure 7. Falling film tubular photochemical reactor, for reactions requiring excitation of a reactive gas [2,3].

254

A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A. OLLER DO NASCIMENTO

[2], where a narrow diameter light beam traverses the reactor from bottom to top and excites gaseous Cl,, which is introduced at the top of the reactor in the region of the central tube. The generated chlorine atoms propagate in turn a chain reaction at the gas-liquid interface of the falling film (liquid substrate, substrate solution) which is produced by an overflow at the top of the central tube. Another means of selectively exciting a gas phase initiator (e.g., chlorine) could be conceived in developing the idea of a partitioned parallel plate reactor [41]. The reactor volume is divided into two separate compartments by a grid (e.g., of Teflon or titanium), and the reactant gas is introduced into the irradiation compartment (A, Figure 8) at a pressure sufficient to prevent the liquid contained in compartment B from leaking across the grid. Several authors gave, in fact, evidence that chlorination in the liquid phase (B, Figure 8) takes place after chlorine atoms are generated in A and pass through the grid separating the different reactor compartments 142-441. The grid may be designed in such a way as to prevent electronic excitation in compartment B. In addition to specific excitation, this design may also be of potential use for the differentiation between radicals of different reactivity. Lucas has proposed an annular reactor with concentric irradiation and reaction zones for an improvement of the photochemical synthesis of cyclohexanone oxime, caprolactam precursor, (Eqs. 31-33) [45]. The design would prevent filming [by radical polymerization of unsaturated hydrocarbons (Eq. 4)] and reduce chlorinated secondary products (Eq. 3), as the

Gas

Products

I

I

Gas

Reactant liquid (or solution)

faces and two compartments [2,3,41]. A : Region of gas excitation. B : Region of radial reaction with a liquid (or gaseous) substrate.

UP-SCALING PHOTOCHEMICAL REACTIONS

255

probability of recombination of chlorine atoms (Eq. 34) in the reaction compartment would be considerably reduced.

However, radical generation and transfer efficiency and/or chain length of the radical reaction must be sufficiently high to ensure reaction rates of technical importance. Radical transfer efficiency is highly dependent on the thickness of compartment A, irradiance, and flux of added reactant gases. By varying the irradiance in an experimental design, keeping the geometry of the reactor module and working conditions constant, Tournier et al. obtained similar results for the photochlorination of toluene as in chlorine-saturated liquid phase conditions [43]. On the other hand, Lucas [46] and Richard and Lenzi [44] reported increasing reaction rates in maintaining irradiance but increasing the flux of the reactant gases by decreasing the thickness of the initiation compartment. A general verification of Lucas' theoretical model [46] seems timely, even if there is no immediate industrial potential, given the generally low specificity of radical reactions and the technical obstacles at the present state of experience. 1. Point Sources. Point sources are only applied in preparative photochemistry, when excitation is required in a restricted area (particular angular distribution of light intensity due to the shape of the light source or the optics applied [2,3]). This concerns then in most cases the generation of light beams which may be used for gas phase reactions in reactor geometries providing long optical paths (Figure 2). Usually, excitation in a narrow spectral (band pass or interference filters) or spatial domain (Figure 7) also calls for a beam geometry. Narrow band emission light sources may also be built using the absorption and emission characteristics of fluorescent dyes. Actinometry of polychromatic light using fluorescent substances makes use of a photophysical process (fluorescence) whose quantum yield is in most cases independent of the wavelength of the incident radiation within the spectral domain examined [2,3]. A common fluorescent substance often used is Rhodamine B, and Amrein et al. have constructed an integrating actinometer using a combination of this fluorescent dye and a photodiode as a detector system [47]. Since the incident photons of different wavelengths are completely absorbed by the fluorescent substance, a narrow-band fluorescing dye can in

256

A. M. BRAUN, L. JAKOB, E. OLIVEROS A N D C. A. OLLER DO NASCIMENTO

fact be used for the construction of a fairly monochromatic light source. Such a set-up has been devised by AndrC and Viriot [48] using a mercury arc immersed into a solution of a fluorescent substance. Under conditions of total reflection of the emitted light at the outer wall of an annular reactor, a relatively high photon rate can be measured at the surface of an optical window, to which the emitted light is directed (Figure 9). The efficiency of light harvesting by this “optical fiber effect” is approximately 100% [49]. However, limited photostability of the fluorescent dye, temperature effects on fluorescence emission, and the already mentioned disadvantages of external light sources and attached parallel plate photochemical reactors will most probably restrict the use of such “fluorescent relays” to laboratory investigations. Recent developments make use of laser excitation where, besides excellent monochromacy, high irradiance may be used for changing the course of a photochemical reaction [SO]. Extremely high irradiances would cause, however, rapid filming of liquid reaction systems in tubular reactors (e.g., Figure 2), and preparative application became only possible after spatial separation between irradiated reactor surface and reaction system had been achieved [Sl]. Filming may also be avoided by using pulsed light sources, as enough time between pulses may be allowed in order to change the irradiated reaction mixture in a highly turbulent system (e.g., a jet-injection photochemical reactor [2]). 2. Extended Light Sources. As mentioned above, extended light sources emit, with the exception of low-pressure mercury and sodium arcs, polychromatic light, which can be restricted to some spectral areas by liquid filter solutions or special glass tubes. It is, for instance, advantageous to use a potassium chromate filter in combination with a medium pressure mercury arc for dye-sensitized oxidations, the products of which absorb at wavelengths longer than 320nm. Optimal results are usually obtained, if a highpressure sodium vapor lamp is used, however up-scaling of the latter fluorescent emission

I

optical wlndow

I

photochemical reactor ercury low pressure arc fluorescent dye solution

Figure 9. “Fluorescent relay” photochemical reactor [49].

UP-SCALING PHOTOCHEMICAL REACTIONS

257

procedure is very difficult, as sodium lamps of high electrical power are not available commercially and therefore large multilamp reactors have to be built. The use of doped medium-pressure mercury arcs [2,3] is another means of concentrating the emitted light to a more or less restricted spectral area. Doped mercury arcs with up to tens of kilowatts of electrical power are now available. Extended light sources may be installed around a tubular reactor or in the axis of an annular irradiated reaction volume. In the first case, an annular (or coaxial) radiation field focalized on the axis of the tubular reactor is created (Figure lo), and, in reaction mixtures of very low absorbance, irradiance as a function of the radius of the cylindrical reactor shows highest values in the axis of the reactor (positiue geometry of irradiation, Figure 11 [2,3]). Calculated values approach those obtained by actinometry, if an empirical relation taking into account a fraction of the reflected light is used (Eq. 35): E(r) = r R M[lO-EC('R-r) + 0.5 x 10-&C(rR+*)] r

(35)

where E(r) = irradiance in the interior of a cylindrical reactor at a distance r from its axis r R = radius of a cylindrical reactor M = radiant exitance of the light source used, assuming that the irradiance at the reactor surface is equivalent to the exitance of the lamp in a given sector E = molar absorption coefficient c = concentration of the dissolved substrate

(4

(b)

Figure 10. Cross section of a cylindrical reactor with a coaxial radiation field: (a)cross section perpendicular to the axis of the reactor; (b)cross section along the axis of the reactor (rR = reactor radius) [2, 31.

258

A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A. OLLER DO NASCIMENTO

Reactor axis

Figure 11. Relative irradiance along the diameter of a cylindrical photochemical reactor surrounded with many tubular light sources; transmittance ( T )of the reaction mixture through 1 cm: (1) T = 0.7; ( 2 ) T = 0.65 [ 2 , 3, 521.

Cylindrical photochemical reactors placed in coaxial radiation fields are often used for carrying out photochlorination reactions, but their most important application remains in the area of water sterilization [52-541. We use the same reactor geometry for photochlorinations because, given the high quantum yields of these chain reactions, irradiance must not be optimized to achieve good productivity. These reactors may then be equipped with fluorescent tubes for which no external cooling and no water filters are needed. Since a positive irradiation geometry cannot be successfully exploited in applications where high substrate concentrations and absorbances are used, corresponding reactor geometries should be avoided, although investment costs might be lower than for more complicated reactor designs. In addition, operational difficulties might arise from the control and maintenance of lamp clusters, corresponding mirrors, and cooling and IR-filter installations (e.g., when medium-pressure mercury arcs are used). The extended light source may also be placed at the axis of a reactor composed of two coaxial cylindrical tubes (Figure 12). The emitted radiant power is absorbed by the reaction system contained in the annular reactor volume. Irradiance diminishes in a filled reactor with increasing radius (Eq. 36), this geometry is called the negative geometry of irradiation [2,3].

UP-SCALING PHOTOCHEMICAL REACTIONS

259

Linear light source

Exterior tube of reactor

I Interior tube of reactor

Figure 12. Cross section of an ideal annular photochemical reactor along the plane perpendicular to its axis (symbols, see Eq. 36) [2, 31.

where ER = irradiance at the outer reactor wall E, = irradiance at the inner reactor wall ri = reactor inner tube radius rR = reactor outer tube radius, 1, = r R - ri = thickness of the annular reactor

This geometry of irradiation makes the most efficient use of the light emitted by an extended light source. In fact, this geometry is used in all immersiontype photochemical reactors, and most industrial photochemical production units are based on this design.

3. Excimer Lamps. Recently, the group of extended light sources could be opened to the new class of excimer lamps. These new light sources are unique for their relatively good monochromacy of the emitted light. Even more important is that for the first time in photochemical engineering, the geometry of a light source can be adapted to optimal reaction (and reactor) conditions. Excimer formation in certain gases and vapor mixtures is known from modern UV lasers; typical examples are excited complexes between rare gas

260

A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A. OLLER DO NASCIMENTO

atoms (excimers) [55] and between rare gas and halogen atoms (exciplexes) [56]. Unlike the excitation technique used in excimer lasers, dielectric-barrier discharge, typically applied in ozone generators, is used for excimer formation in these new light sources [57]. In a simple configuration, the lamp consists of a hermetically sealed annular quartz cell containing the gas and of electrodes located at the inner and outer wall, outside the cell and, hence, separated from the discharge plasma. The lamp may be used as any other extended light source, if the outer electrode is made of a wire mesh or a transparent conductive layer (Figure 13), allowing the emitted light to radiated into an annular reaction volume. In applying this configuration to problems of preparative photochemistry and large-scale photolysis, we discovered that direct contact between the outer electrode and the reaction mixture can be avoided by taking the latter as part of the dielectric [12]. The outer electrode may then be placed outside an annular reactor of limited thickness (Eq. 36) depending on the chemical properties of the reaction system (Figure 14) [12,58,59]. The simple configuration principle of the excimer lamp combined with the possibility of including the reaction system among the dielectrics yield unprecedented freedom in light source and reactor design. For instance, an ideally irradiated tubular reactor with positive irradiation geometry (Figure 15a) may be built with combined negative and positive irradiation geometries, where the reaction system is between two discharge layers (Figure 15b) [60]. Early experiments with planar electrodes (Figure 16a)lead to “car battery” assemblies of radiating walls, where the reaction mixture is circulating in between the lamps (Figure 16b) [60-621.

hV

Figure 13. Cross section of a cylindrical excimer lamp along the plane perpendicular to its axis; see also [57].

Quartz

Reaction mixture Figure 14. Cross section along the plane perpendicular to the axis of an annular reactor with a cylindrical excimer lamp mounted in its axis [12, 58, 591.

(4

' Reaction mixture

Quartz

'

Reaction mixture

(b)

Figure 15. Cross section along the plane perpendicular to the axis of a photochemical reactor using (a) an annular excimer lamp of coaxial (positive) irradiation geometry and (6) of a combination of cylindrical and annular excimer lamps (see also [60].

261

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A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A. OLLER D O NASCIMENTO

Reaction mixture

/ \

Figure 16. Cross section along the plane perpendicular to (a) the planar electrodes of a flat excimer lamp and (b) a corresponding multilamp assembly (see also [60-621.

The excimer light sources are driven with frequencies of about 50 kHz, and voltage amplitudes of 5-10 kV are required to ignite the discharge. Under those conditions, bursts of light pulses with a repetition frequency of about 100 kHz are produced which provide this light source, in addition to its largely variable geometry, with unique advantages-narrow spectral bandwidth of emission, short pulse duration, high peak intensity, and high repetition frequency. Table 1 shows the potential wavelengths of emission (see also [57]), of which some have already been used in projects of metal vapor deposition [63], photopolymerization [64], oxidative degradation of organic materials [l2, 58, 591, and preparative photochemistry [65]. The narrow bandwidth emission is shown for the Xea excimer lamp (172 f 12nm, Figure 17). The monochromacy of the light source is a great advantage in many preparative applications [2, 3, 661 and facilitates radiant power and irradiance measurements and calculations [2, 31 in up-scaling projects. Eximer light sources may require water cooling for optimal operation. Their life times have not yet been determined, but excimer lamps (see Table 1) have been operational for more than 3000 hours with radiant efficiencies of approximately 6%. 4. Safety Requirements. Besides the potential danger of radiation for eyes (UV/VIS) and skin (UV), high-pressure light sources require special pre-

TABLE 1 Potential Peak Wavelengths of Excimer Light Sources‘ Peak Wavelength (nm)

Gas

126 146 152 165 172 175 188 193 207 222 249 253 259 283 289 308 334 342 351 443 503 558

Art Kr T

FT

ArBrf Xer ArClT KrJT ArFT KrBrt KrClr KrFT XeJT Clt XeBrt Br; XeClT HgT

Jt

XeFz HgJT HgBrz HgC1;

References

12, 58, 59

59

63, 64 65

“See also Ref. 57.

1 ,o

Intensity [arbi!raw unils]

0,5

-

0100

‘

150

1

200

250

300

nm

Figure 17. Emission spectra of a Xe excimer lamp operated at 15OW of electrical power [12] (see also [57]). 263

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A. M. BRAUN, L. JAKOB, E. OLIVEROS AND c. A. OLLER DO NASCIMENTO

caution against the risk of explosion. Lamps should be mounted in accord with the manufacturers instructions and mechanical strain as well as contact with cold surfaces must be avoided. Safety requirements also include protection against risks of electrocution, in particular in applications requiring cooling water (excimer lamps). Special care in designing photochemical production units at all levels must also be applied to avoid contact of explosive gas mixtures with the heated lamp.

111. CONCEPTS OF QUALITATIVE REACTOR DESIGN A. Reactor Design and Light Source As indicated earlier, positioning a light source to obtain a negative geometry of irradiation provides the most economical result in most cases of industrial preparative photochemistry and photolysis. Extended light sources of a given length and diameter, contained in a corresponding protection and filtering/cooling system, may be immersed into a reaction mixture of minimum volume required to absorb entirely the emitted light. However, as many negative experiences demonstrated over a period of about 50 years, this simple design cannot cope with the consequences of a strongly inhomogeneous distribution of activated species and is in general a complete failure. To maintain the negative geometry of irradiation, we must ensure filming prevention: 1. During photonitrosylation of cyclohexane (caprolactam synthesis, Eqs. 31-33 [2,3]), the formation of deposits on the cooling jacket of the medium-pressure or doped mercury arc could apparently be prevented by introducing a liquid-liquid phase barrier. Intermittent additions of concentrated sulfuric acid along the wall of the cooling jacket [67] create a strongly hydrophilic phase within the hydrocarbon bulk solution. This prevents penetration by relatively apolar intermediates and products from which precursors of the polymeric deposits originate. 2. Deposits at the lamp jacket can also be prevented in bubble photochemical reactors, where small rising gas bubbles of nearly uniform size and distribution create strong turbulences [ 2 , 3, 18, 681. This reactor is usually conceived as an annular photochemical reactor of relative small thickness (IR, Eq. 36). In principle, its use with reactive gas is rather limited, as the turbulence provoked by the bubbles must be maintained over the entire height of the reactor [69,70]. Bubble characteristics may also change as a function of the flux of the substrate solution.

UP-SCALING PHOTOCHEMICAL REACTIONS

265

3. The concept of spatial separation between light source and liquid reaction mixture is by its simplicity highly advantageous in technical development work. In general, it can be achieved by generating thin layers of substrate solution on a surface placed parallel to and at optimal distance from the surface of the lamp jacket. In early proposals, photochemical reactors comprised moving parts, such as rotating reactor walls on the inner surface on which the liquid to be irradiated could be spread [2,3]. None of these designs found technical application because of severe deficiencies due to material strain, leaks of liquids and gases, and difficulties in placing such reactors into large batch or continuously operating production units.

Nevertheless, subsequent reactor designs reflect clearly some of the ideas pioneered in earlier constructions. The falling film photochemical reactor first published by Tarkoy and Campana (Figure 18) [2, 3, 711 may be seen as an important step in the technical development of the rotating reactor originally designed by the photochemistry unit of the Max Planck Institut at Mulheim [72]. The falling film is generated in the later design by dispersion of the liquid reaction mixture at the top of an inverted immersion-type photochemical reactor and covers the entire inside surface of the outer reactor wall. A modified version of this design is one of our basic reactor modules [2,3]. It is mainly chosen for the development of photochemical processes in which reaction mixtures of high-absorption cross sections are used. The design is also advantageous in cases where pronounced photoreactivity of compounds produced in a single pass might generate secondary products and, consequently, diminish chemical yield. The short residence time limits such secondary reactions, and very low ratios of reactor to reservoir volumes (vide infra) ensure almost instantaneous high dilution of such photoreactive compounds at the elevated flow rates required. For processes requiring medium- or high-pressure arcs, the bottom-up mounting of the light source is leading to up-scaling limitations due to problems of heat dissipation. Other means of cooling or an improved design of the lamp jacket would be needed if technical development were to require reactor units of more than 5 kW of electrical power. Another module adapted to photochemical processes working inversely at low absorption cross sections and at rather limited flow rates is pictured in Figure 19 [2,3]. Again, this design can be seen as the subsequent development of an earlier design by de Meijere et al. [73,74] which, because of its complicated construction, became inflexible and in most cases inefficient. The later design works at higher ratios of reactor to reservoir volumes (vide infra), the thickness of the annular reactor volume depending on the absorption cross section of the substrate solution. This has been achieved by substituting

266

A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A. OLLER DO NASCIMENTO Liquid distributor

Light source

Falling film

Coolant

Reaction mixture

i

1

Reaction mixture

Cleaning solvent for cooling jacket

Figure 18. Annular falling film photochemical reactor [2, 3, 711.

the original double wall with a more efficient serpentine thermoregulating system. In addition, the new design exhibits an exchangeable inner reactor tube and may, hence, accommodate light sources of different diameter. Most important is the entry point for the substrate solution, which is located at the bottom of the reactor, thus ensuring a better mixing of the reaction system. A falling film is created as the reaction mixture is flowing over the rim of the inner reactor tube, its thickness being limited by the spatial separation between this tube and the lamp jacket. This design is complementary to the one shown above (Figure 18) and has found successful application in

267

UP-SCALING PHOTOCHEMICAL REACTIONS Inert gas (around lamp) /

d i gaseous reactant

Liquid level

Falling film -

Coolant

I

Reaction mixture

Reaction mixture

Figure 19. Annular falling film photochemical reactor [2,

31.

photoinitiated reactions (photohalogenations) and sensitized dimerizations and oxidations. In general, these are processes where low absorption cross sections are the rule, and secondary photochemical reactions are practically nonexistent. So far, no technical limits in up-scaling this reactor module have been encountered. In summary, spatial separation of reaction mixtures from the lamp jacket is one of the most efficient means of preventing filming (see also earlier comments on laser photolysis). In addition, as shown in Figure 7, the same principles of design may be used for solving problems created by inner filters. Experience with falling film photochemical reactors lead us to the

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A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A. OLLER DO NASCIMENTO

application of the concept of convergence of light distribution and reactor geometries. Traditional reactor design placed the entire reaction volume of a batch operation into the photochemical reactor, although absorbance calculations could show that in most cases only an extremely small volume fraction would be irradiated (vide supra). In contrast, modern reactor design aims at reactor geometries adapted to the size of the light source and would limit their size in accord with the spatial distribution of the emitted light which might be either measured or calculated for a given reaction system. This concept allows for: Batch processes using a number of photochemical reactors of optimal size in a parallel arrangement with a central unit in which optimal reaction conditions (temperature, gas saturation, mixing, etc.) can be maintained and classical operations of product separation can be performed (Figure 20). Such a production unit also permits maintenance of photochemical reactors without interruption or strong disturbance of an ongoing production or photolysis process. Continuous processes if photochemical (and thermal) reactors are installed in series. Such an arrangement could be useful in the production of previtamin D (PD), where tachysterol (T) produced as a secondary product (Figure 21) may be recovered by a subsequent triplet sensitized cis-trans isomerization [2, 3, 75-77].

B. Light Distribution The spatial distribution of the emitted light within a certain reactor geometry may be directly determined by radiometric measurements. For this purpose,

Figure 20. Batch process design using several photochemical reactors (hv) in a parallel arrangement linked to a central reservoir (R) [18].

269

UP-SCALING PHOTOCHEMICAL REACTIONS

T

Figure 21. Principal photochemical reactions of the previtamin D synthesis (for a complete scheme of photochemical and thermal isomerizations, see [2, 31).

a light-sensitive probe is placed at different locations within the reactor, its position being always defined by x, y, and z axes, the origin of which may be placed at the center of the light source. Probes may be directive or consist of an integrating sphere (Figure 22). The first are mainly used to determine

(4

(b)

Figure 22. Directed (a) and spatially integrating (b) probes for the measurement of irradiances at any point within a photochemical reactor [Z, 3, 6, 78-80].

270

A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A OLLER DO NASCIMENTO

irradiances (of point sources) in the function of distance and attenuating or amplifying effects by transparent materials and mirrors. For such tasks, optical fibers with defined apertures or small bolometers placed behind a focusing cone (Figure 22a) have been used [2,3]. As reflection and refraction of light must be taken into account in a photochemical reactor irrespective of its geometry, the incident photon rate at a given point in the reactor originates from any direction. It is thus best measured by an integrating sphere attached to an optical fiber, which in turn is connected to an appropriate diode, bolometer, or photomultiplier (Figure 22b) [2, 3, 6, 78SO]. The irradiance measured by such an integrating device is related to the radiance of a light source by

where Ee,sph= irradiance measured by a spherical probe Le = radiance of the light source R = solid angle By definition, for a given wavelength, the photon (Gp,J and radiometric (GeJ spectral quantities are related by Eq. (38),

Since in industrial photochemistry mostly polychromatic light sources are used, photon quantities are relatively difficult to calculate and require knowledge of the spectral distribution of the radiometric quantity measured. Assuming on the other hand that the radiometric measurements do not need to be corrected for the spectral response of the probe, the photon irradiance at a given point within the reactor volume would then be given by Eqs. (39) and (40), respectively.

UP-SCALING PHOTOCHEMICAL REACTIONS

271

where ,Il, 2, are the spectral limits of (specific) excitation.

Under conditions equivalent to spectrophotometric measurements and for a solution of a light-absorbing compound of known c Aand concentration, the thickness of the system traversed by light ( I ) is given by the Beer-Lambert law (Eq. 41):

where

T = internal transmittance (or transmission factor) P = measured (transmitted) radiant flux Po = incident radiant flux cA = molar absorption coefficient of the light-absorbing substance at wavelength 2 1 = thickness of the solution traversed by light c = concentration of the light-absorbing substance In taking into account the concept of convergence of light distribution and reactor geometries, the thickness 1, (Eq. 36) of an irradiated solution in a negative irradiation geometry should not exceed considerably the limit defining total absorption. However, under conditions where irradiation of a given unit of volume is arriving from many directions, the outer diameter of a photochemical reactor is difficult to calculate and empirical measurements are by far more efficient [2, 3, 6, 78-80]. Tournier et al. [78] measured the irradiance with a spherical probe within an immersion-type photochemical reactor at different distances from the surface of the light source and attempted to correlate the results to different emission models of extended light sources (vide infra). In fact, none of the chosen models would fit the experimental results, the best fits being obtained by introducing a correction factor into an equation similar to Eq. (36):

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A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A. OLLER DO NASCIMENTO

where k is a correction factor, which is found to be 1.14 under the experimental conditions described in [78].

IV. QUALITATIVE UP-SCALING RULES AND PARAMETERS

A. Rate of Production The principal aim of an up-scaling project is to increase production volume, and thus the production rate, of a given preparative procedure without major changes of previously determined reaction conditions. In our qualitative approach to industrial photochemistry, optimal conditions would have clearly been determined on a laboratory scale by using a specific reactor module. Following the qualitative concepts of photochemical reactor design, the volume of the photochemical reactor is reduced as much as possible, ideally to the limits of the irradiated reaction volume, which itself depends on the geometry of the light source, the spatial distribution of the emitted light, and the geometry of the chosen reactor module. It should satisfy the condition of total absorption of the emitted light at the wavelengths of (specific)excitation. For a given unit (light source and reactor module), the irradiated reaction volume is best determined experimentally (vide supra). Evaluations of model calculations have also been published for simple reactor geometries (spectroscopic cell and immersion-type geometries) and where changes of reaction parameters in the course of the ongoing transformation need not be taken into account [78]. The concept of convergence of light distribution and reactor geometries would facilitate such calculations, as mixing between irradiated and nonirradiated reaction volumes within the photochemical reactor does not practically enter into consideration. The rate of a photochemical reaction can be defined as

where S = substrate (I-s = quantum yield of substrate consumption Pa = absorbed photon rate = Po(l- 1 0 - A ~ ) Po = incident photon rate A , = substrate absorbance

(44)

UP-SCALING PHOTOCHEMICAL REACTIONS

273

and it was shown above that calculation of Pa in accord with the BeerLambert law (Eq. 44) can only serve as an estimate, since the incident photon irradiance arriving at a given point of the irradiated reaction volume cannot be calculated precisely [78]. Up-scaling of the light source and, hence, of a given reactor geometry is therefore easier if the radiant power but not the exitance of the (same kind of) light source is varied. Under otherwise unchanged conditions, the irradiance and consequently the rate of reaction per unit of irradiated reaction volume would remain unchanged. In the case of batch processes, where multiple passes of the reaction mixture are needed to reach the required percentage of transformation, the rate of production is then increased by enlarging the irradiated reaction volume, and the up-scaling factor corresponds to the factor by which the radiant power of the light source at the wavelengths of (specific) excitation has been increased. Variations may show toward the end of a given photochemical reaction, where the rate of photochemical transformation slows down owing to lower absorbance (Eq. 44). Within the technical limits implied (limited irradiated reaction volume) and given the required rates of production at an industrial level, production units usually comprise a number of photochemical reactors mounted in parallel (Figure 20). If the number of photochemical reactors is reduced (e.g., for technical or economical reasons), a lower rate of production results. During an up-scaling procedure, it might become necessary for technical reasons (availability of light sources) to vary the exitance of the lamp. Since the rate of production is linearily related to the absorbed photon rate (Eq. 43), the up-scaling factor remains predictable within the conditions given above. However, if the exitance is increased, higher rates of photochemical reaction per unit of irradiated reaction volume may result in an accumulation of intermediates or products which may lead by subsequent photochemical or thermal reactions to disproportionate increase in the yield of secondary products. In this case, the geometry of the reactor would have to be changed. For continuous processes, all substrate molecules entering the irradiated reaction volume should be transformed during the residence time. This residence time is defined as the time after which a reaction volume corresponding to the irradiated reaction volume has been completely replaced by fresh input material. With the exception of some chain reactions and applications of UV irradiation to sterilization purposes, radiant power is usually not high enough to complete the photochemical reaction in one pass. In any case, conditions of very high radiant power might imply considerable energy wasting, since the absorbed photon rate could decrease rapidly as the substrate molecules react during residence time. Continuous production units may be conceived advantageously as a number of photochemical reactors mounted in series, each adapted to a given range within the variation

274

A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C . A. OLLER DO NASCIMENTO

of reaction parameters that results from the progressing level of transformation. Under these conditions, best results have been achieved in sequential up-scaling, which as far as the photochemical transformation is concerned, does not present any additional problems to those already evoked for batch processes.

B. Sensitized Oxidations In sensitized or photocatalyzed reactions, conditions of total or constant absorbance can easily be controlled by the concentration of the sensitizer or photocatalyst added. In addition, experience has shown that the concept of spatial separation between the light source and the reaction mixture is in general not required. Dragoco uses immersion-type annular reactor geometries [2, 3, 69, 701 for the production of (-)-rose oxide by rose bengal sensitized oxidation of ( -)-citronello1 (Eqs. 45-48, Figure 23). RB 'RB

3RB

+ O2

4'RB __+

__+

(45)

3RB

(46)

RB

(47)

+ '0,

On the other hand, sensitized oxidation of highly concentrated sensitizersubstrate mixtures have been successfully developed in falling film reactors (for example, the synthesis of 2-hydroxy-5H-furanone (Eq. 48 [82, 831) [12]. hv QCHO

Sens. i 0, I H,O

w

HO

For the sensitized oxidation, the rate of production of the oxidized substrate, that is, the rate of the chemical reaction of singlet oxygen, depends on the efficiencies of singlet oxygen production and singlet oxygen reaction (Eqs. 49 and 50).

where AO, = product of singlet oxygen reaction DA= quantum yield of singlet oxygen production 4r= efficiency of singlet oxygen reaction

UP-SCALING PHOTOCHEMICAL REACTIONS

275

k, = rate constant of singlet oxygen reaction (Eq. 51) singlet oxygen acceptor (substrate) k, = rate constant of physical deactivation of singlet oxygen by A (Eq. 5% kd = rate constant of physical singlet oxygen deactivation by the solvent (Eq. 53) A

=

lo2+ A*

lo, + A 4A + 0 10,

hv

(51)

'402

40

I

2

2

(52)

(53)

Sens./O,

4 O H

Figure 23. Synthesis of (-)-rose oxide by sensitized oxidation of (-)-citronello1 [2, 3, 811.

276

A. M. BRAUN, L. JAKOB, E.OLIVEROS AND c. A. OLLER DO NASCIMENTO

Therefore, technical means are required to keep oxygen concentration at its highest level [2,3,69]. Up-scaling limits are known for cases where irradiated reaction volumes have been practically depleted of oxygen by applying too high radiant powers, mostly in combination with very high concentrations of sensitizer. Work at high acceptor concentrations is advantageous (Eq. 50). The rate of reaction following pseudo-zero order kinetics for approximately 90% of transformation, deviation from optimal conditions during up-scaling is easily detectable. In most cases, work-up procedures are most economical at levels of transformation close to loo%, but long irradiation times might be necessary due to depletion of the substrate toward the end of the reaction. The efficiency of the chemical reaction of singlet oxygen depending primarily on k , (Eq. 53), the choice of a solvent exhibiting weak interaction with the excited oxygen species, is advantageous. However, most sensitized oxidation reactions terminate by a sequence of thermal reactions of the intermediate product A 0 2 in which the solvent may play a decisive role as far as the specificity and thus the chemical yield of the sensitized oxidation are concerned. The intermediate endoperoxide of furfuryl aldehyde (Eq. 48) reacts preferentially by homolysis of the 0-0 bond in apolar solvents, yielding a complex mixture of products due to subsequent radical reactions (Eq. 54).

In water, however, the endoperoxide is exclusively transformed by S,2 reaction with the solvent into the corresponding ring-opened hydroperoxide (Eq. 5 9 , which then undergoes intramolecular fragmentation to the desired product [12]. Limitations in the choice of the solvent may in turn restrict the choice of the sensitizer for reasons of solubility and photochemical (and thermal) stability. As a result, experimental conditions for an optimal chemical yield may call for a sensitizer exhibiting a relatively small Q,. As indicated above, considerable efforts in research and development have been deployed into the development of insoluble singlet oxygen sensitizers. Although, rose bengal chemically bound to chloromethylated copolymer of styrene and divinylbenzene [14,841 (and commercialized as “Sensitox”) has shown excellent results as far as its light stability in solid matrices is concerned [ 8 5 ] , the sensitizer is suffering, in addition to the drawbacks of the

UP-SCALING PHOTOCHEMICAL REACTIONS

+

H20

271

+ HCOOH

chromophore (e.g., pH dependence of the absorption spectrum and reactivity in acid solutions), from a relatively labile chemical fixation (solvolysis) and a severe loss of efficiency due to the solvent-dependent large particle diameter [16,17]. The technical and economical advantages of a workup procedure involving a heterogeneous reaction system are nevertheless very substantial, and more work in this area might be triggered with research on spectral sensitization in photocatalysis.

C. Photocatalyzed Reactions Applications of photocatalyzed reactions are actually focused on the oxidative degradation of organic material dissolved or suspended in aqueous systems [86]. For practical reasons, TiO, is taken most frequently as a photocatalyst, its absorption spectrum showing a strong band gap edge at about 360nm with an exponential tail dropping to zero at about 400nm. These absorption characteristics might prove extremely advantageous for the chemical treatment of industrial waste water, where other photochemical degradation processes (e.g., H20,/UV or 03/UV) are severely limited by the UV absorption characteristics of the substrates to be degraded. In addition, TiO,, absorbing at the onset of the sea level solar spectrum, industrial waste water treatment, or chemical treatment of polluted surface waters driven by solar light, has initiated some interest in research and development [S7]. Research on spectral sensitization (sensitizer-semiconductor combinations) [SS] and catalyzed photoelectrochemical reactions (semiconductor-catalyst

278

A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A. OLLER DO NASCIMENTO

combinations) [89] has shown very promising results in the domain of photochemical solar energy conversion. In relation to degradation of organic material in aqueous systems, two mechanisms of photocatalyzed oxidation have found experimental confirmation so far. Promotion of an electron from the valence to the conduction band upon electronic excitation leads to charge separation (Eq. 56), the negative charge being readily transferred to, for example, oxygen dissolved in the aqueous system (Eq. 57). Depending on the pH of the system, superoxide anion and its protonated form (Eq. 58) rapidly dismutate to yield H 2 0 2(Eqs. 59 and 60), the decomposition of which is in turn catalyzed by TiO, surfaces (Eq. 61). hv

TiO, + TiO,(e-

+ h')

+ h') + 0, + TiO,(h+)+O;' O;'+H,O+ * HO; + H,O O;*+HO; + HO; + 0, HO; + H,O++ H,02 + H,O

TiO,(e-

HO;

+ H,O,

O;'+HO'+H,O

(56)

(57) (58) (59) (60) (61)

The valence band edge of TiO, exhibits in aqueous suspensions a potential of more than +2V vs. SCE and is capable of oxidizing hydroxide anions or water molecules adsorbed to the semiconductors surface (Eq. 62 and 63) producing hydroxyl radicals [90]. Hydroxyl radicals, when desorbed from the surface, may react with organic substrate [91] by hydrogen abstraction (Eq. 64) or by electron transfer (Eq. 65), thus initiating oxidative degradation which leads to CO, (mineralization). TiO,(h')

+ HO-

+ H,O HO' + RH,

TiO,(h+)

HO'

+ RH,

+ HO' + TiO, + H O ' + H 3 0 + H,O + H,O + RH' HO- + RH: --+ TiO,

(62)

-

(63) (64) (65)

A second mechanism proposes electron transfer between the photogenerated hole (h') and an adsorbed organic substrate (Eq. 66) [92].

TiO,(h+)

+ RH,

--+

TiO,

+ RH:

(66)

UP-SCALING PHOTOCHEMICAL REACTIONS

279

Because of the highly positive valence potential of TiO,(h+), electron transfer from some of the halogenated (chlorinated) pollutants (e.g., tetrachloroethylene) might be possible, thus initiating the degradation of compounds which cannot be oxidized by hydrogen abstraction. However, under the realistic conditions of waste or surface water treatment, reaction 66 seems of lesser importance. This is due to the high dilution of the pollutants and their usually rather weak adsorption to the semiconductor surface with respect to water or hydroxide ions. Photocatalyzed degradation may therefore be classified among those mineralization procedures in which oxidation is initiated by hydrogen abstraction and/or electron transfer reactions by hydroxyl radicals generated as intermediates [93]. As stated earlier, the concept of spatial separation between light source and reaction mixture does, in general, not apply to photocatalyzed processes. However, similar to the results obtained from a series of experiments in the domain of sensitized oxidations using insoluble sensitizers, particle size and mobility are among the decisive parameters as far as quantum efficiency of TiOz photocatalyzed oxidative degradation is concerned. Degussa P-25 TiO, is a powder of relatively small particle size distribution (diameter: 50 I 20nm). This particle size offers optimal conditions for electronic excitation in heterogeneous (solid-liquid) phase [17], but seems counterproductive in up-scaling procedures owing to technical difficulties in separating and recycling the photocatalyst. On the other hand, reactors utilizing fixed photocatalysts [19, 94-96] are, similar to those constructed for singlet oxygen generation, of distinctly lower efficiency or, when conceived as irradiated coated fibers [97], difficult to install even at a pilot level. Precipitation of the photocatalyst can be provoked, in particular in largescale degradation processes, by taking advantage of the increasing ionic strength of the irradiated system [l2]. In designing modules of mono- or multilamp immersion-type photochemical reactors, again the concept of convergence of light distribution and reactor geometries is followed, and knowledge of light penetration in a suspension of optimal photocatalyst concentration is therefore essential. Optimal thickness of annular irradiated reaction volume is best determined by a spherical probe under conditions where only absorption by the photocatalyst has to be taken into account [12, 78, 98, 991. The radiant power P = f ( r ) within the limits of ri and rR,respectively, has been simulated by the Monte Carlo method on the basis of

where coefficient K depends on the nature and the concentration of the suspended photocatalyst.

280

A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A. OLLER DO NASCIMENTO

In contrast to the investigations made by Andre et al. [79, 98, 991, these evaluations should be made without regard to any absorption due to dissolved or suspended organic pollutants. However, absorption of the suspended photocatalyst should be taken into account, as described by Yokata et al. in the case of suspended sand particles [loo]. Efficient mineralization requires efficient trapping by oxygen of free organic radical intermediates, the resulting peroxyl radicals and (hydro)peroxides initiating themselves chains of thermal oxidation reactions. Consequently, optimal saturation of the reaction system with oxygen or air is a necessity, and on a pilot level, air or oxygen bubbles may be injected, which serve at the same time as a means of maintaining high concentrations of photocatalyst suspended [121. Model calculations of three-phase heterogeneous reaction systems have been published in the domain of photocatalyzed water decomposition [1011, and design studies for oxidative degradation reactions and fluidized bed conditions are known mainly from the collaborative effort between the groups of Yue and Rizzuti [e.g., 1021. As these investigations concern particular reactor geometries, a factor of K = 1.2 has been chosen for our module of a multilamp immersion-type reactor (Figure 24) in order to take into account additional reflection and transparence by gas bubbles of different sizes [l2]. The resulting 1, is large enough to provide an optimal absorption cross section under conditions of high levels of mineralization. Even though an increasing number of publications now deal with quantum efficiencies of photosensitized or photocatalyzed reactions in heterogeneous media, the determination of quantum yields under conditions where light is absorbed by solid surfaces remains, for mostly technical reasons, impossible. Good approximations of the absorbed radiant power are possible with, for example, equipment like that described by Iatridis et al. [102], but a rectangular photochemical reactor would certainly not be the best choice for further technical development. In addition, in the potential applications of the photocatalyzed oxidative degradation, one does not know precisely the composition of the mixture of organic compounds dissolved or suspended in water. In general, under those conditions, only the efficiency of total mineralization, that is, the production of CO,, has a real meaning [lZ, 93, 1031. It seems therefore reasonable to determine the degree of mineralization from the results of TOC (total organic carbon) or DOC (dissolved organic carbon) analyses performed on samples of the reaction mixture at different times of irradiation. In parallel, energy consumption of the light source(s) might be measured, permitting us to determine an “energy efficiency” of mineralization in ppmC.kWh-’ or mg C.L-’.kWh-’. Maintaining equal photocatalytic and thermal parameters, rates of mineralization of different types of organic compounds vary considerably [e.g., 1031. Conse-

UP-SCALING PHOTOCHEMICAL REACTIONS

air

281

n

Figure 24. Scheme of multilamp immersion-type photochemical installation for the photocatalyzed oxidative degradation of industrial waste water [121. A: Bypass circuit. B: Reactor circuit. 1: Gas-liquid mixture and injection. 2: Reservoir. 3: Pump (ceramics).4: Water pump. 5: Heating circuit. 6: Cooling circuit. hv: Medium pressure mercury lamps (Pyrex).T: Thermometers.

quently, the performances of different modules of photochemical reactors can only be compared if the same model compound or mixture of substrates is used. In addition, the ratio of irradiated to total reaction volumes varying between different experimental designs, such “energy efficiencies” of mineralization must be corrected for the total reaction volumes implied. The resulting volume-corrected efficiencies (mgC * kWh - ’) are in contrast to what was proposed earlier (ppmC.kWh-’.L-’ or mgC.kWh-’.L-2 c93, 1043). At present, only results of the mineralization of industrial waste water containing relative high concentrations (ca. 5000 ppm C) of mainly nitroxylenes [12] and of phenol and salicylic acid as model compounds at initial concentrations of approximately 100 ppmC [ 1041 are available. Volumecorrected “energy efficiencies” of 142 and 820 mgCekWh-’ for the waste water and the aqueous solution of phenol, respectively, would indicate that the tubular photochemical reactor with negative irradiation geometry used by Matthews and McEvoy is more efficient by a factor of 5.8 than the multilamp module shown in Figure 24. However, the two experiments cannot be compared, since the rate of TiO, photocatalyzed mineralization of phenol

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is certainly higher than that of nitro aromatics, and Matthews et al. used a low-pressure mercury arc (Aexc:254 nm), reporting that excitation at higher photon energy is increasing overall mineralization efficiency by a factor of approximately 2 [1041. Interesting comparisons of reactor module designs must, hence, be postponed, until a set of standard model compounds and mixtures fits the interests of all research units working in this field. Up-scaling a photochemical reactor module for photocatalyzed oxidative degradation follows the rules indicated above for batch operations of photochemical and sensitized reactions, the volume-corrected “energy efficiency” providing the means to keep the decisive parameters in an optimal range (vide infra). However, a main concern for the technical development of any light-induced oxidative degradation technique, and of the Ti0,photocatalyzed oxidative degradation in particular, is the limited number of photons available (limited radiant power). This results in a rate of mineralization which is too small to cope with large volumes (> lo3m 3 . d - l ) of weakly contaminated surface waters as well as with restricted volumes (ca. 10, m3.d-’) of industrial waste water of high TOC. The process is, however, accelerated upon addition and electronic excitation of H,O, [SO, 1051, and research on combined thermal and photochemical catalysis shows quite interesting results [106].

V. MATHEMATICAL MODELLING OF PHOTOCHEMICAL REACTORS The mathematical modelling of chemical reactors is based on the coupled heat and mass system balances and on the kinetics of the chemical reaction [7]. Its progress is linked to the evolution of digital computers and powerful mathematical methods. Modelling of photochemical reactors presents an additional complication, as the light energy balance has to be taken into account. This new coupling between mass and light energy balances, the latter depending on the concentrations of reactants and products, incident light distribution, and photon concentration, presents additional difficulties which are not yet resolved.

A. Radiation Models Calculations taking into account the strong coupling between the radiant energy conservation equation and the momentum, mass, and heat balances call for a model describing this interaction as a function of space and time.

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Such radiation models have been in permanent development over the last 30 years, and the published results may be classified in two main categories: incidence models which may be characterized by mathematical models assuming the existence of a given radiant energy distribution in the vicinity of the reactor, and emission models in which lamp characteristics, reaction, and flow processes are taken into account. 1. Incidence Models. First attempts to describe the radiant power distribution in photochemical reactors can be summarized under the heading of the RI model (radial incident model, Figure 25a). This model is based on the hypothesis of a radial radiation field [2, 31, that is, that all the light striking the reactor wall will be directed radially inward. Corresponding profiles of radiant power or of irradiance are strongly dependent on the radius of the cylindrical reactor (Eq. 68).

where qA= photon flux density or photon irradiance ( E , [2, 31) qA,w= photon flux density or photon irradiance at the wall R , = radius of a cylindrical photochemical reactor (see Figure 25) r = radial coordinate p = attenuation coefficient

Among the first to use the RI model were Gaertner and Kent, who studied the

(4 (b) (4 Figure 25. Characteristics of two-dimensional incidence models: (a) radial, (b) partially diffuse, (c) diffuse [lo71 (see also [2, 31).

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uranyl nitrate sensitized photolysis of oxalic acid (Eq. 89) in a tubular flow reactor [l08], but the pioneering work of Hill and Felder [l09], Cassano and Smith [llO], Jacob and Dranoff [lll], and Santarelli and Smith El121 should also be mentioned. These authors made important contributions to today’s experience as far as applicability and limitations of this model are concerned. The RI model in particular has been tested in an ideal elliptical photochemical reactor (Figure 26), where the cylindrical reactor tube and the lamp considered as a linear light source are located at the foci of an elliptical reflector [2, 3, 1131. In actual practice, any tubular light source will have a finite diameter and will not behave as a true line source. Radiation from an extended light source will emanate from points displaced from the lamp’s axis, causing the lamp to appear rather like a diffuse light source. In addition, imperfections in the

Elliptical reflector

Light source

Cylindrical reactor

I

‘V

Light rays

Figure 26. Diagram of a photochemical apparatus with an elliptical reflector [2, 3, 1131.

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reflector surface will result in a more or less strong diffusion of the light, which is assumed to be radially emitted. The DI model (diffuse incident model, Figure 25c) is thought to take into account the abovementioned inadequacies. The model in which profiles of radiant power or of irradiance are independent of the radius of the cylindrical reactor was originally proposed by Huff and Walker [114] and has been tested by Jacob and Dranoff [ll 11 using sensor equipment. Their results show that radius-independent radiant power or irradiance distribution can only be found for radii of less than 0.5in. in their particular equipment (Figure 27). In the RI model, all incident rays intersect at the center axis of the reactor tube, and Eq. 68 produces an infinite value of irradiance as r 0. The DI model, on the other hand, proposes parallel layers of rays which are wider than the diameter of the tubular reactor and which ‘traverse the reactor perpendicularly to its axis from all directions with equal probability. The calculated results of both models are far from reality, as found in industrial size photochemical reactors. Matsuura and Smith [1071 proposed an intermediate model (PDI model, partially diffuse model, Figure 25b) in which parallel layers of rays are assumed, and the width of each is smaller than the diameter of the tubular reactor. These two-dimensional bands form by themselves radial arrangements, the center ray of each band intersecting the

Reactor axis Figure 27. Profile of radiant power in a tubular reactor [ill].

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A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A. OLLER D O NASCIMENTO

axis of the reactor tube. Based on this hypothesis, the profile of radiant power or irradiance within a tubular reactor can be calculated by developing Eq. 69:

where

p = sin-' p1 = sin-' R, R,

= halfwidth

(?)

(2)


(72)

of the parallel layers of rays

x1 = - r cos p + ( R f - r 2 sin2p)'l2

(73)

p +(Rf- r 2 sin2fl)1/2

(74)

x1 = r cos

where 0 < r < R , . If R , + 0, then the conditions on which the RI model is based are reproduced, and Eq. 69 becomes identical to Eq. 68. On the other hand, if R , + R , , and using Eq. 75, a mathematical expression is found which describes the profile of radiant or photon power in accord with the DI model (Eq. 76).

Note that for calculations with RI and DI models one adjustable parameter (qn;w)and for the PDI model two adjustable parameters (qn;w and R,) are

needed.

2. Emission Models. Incident models do not make use of all the operating variables, such as radiant power or radiant exitance, diameter, and length of the light source, or dimensions and reflection coefficient of the elliptical cavity mentioned above. Models that describe the space of irradiation on the basis of geometry and process variables are known as emission models. The simplest emission model is the line source with parallel plane (LSPP) emission model. In this model the lamp is considered to be a linear source in

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which each point emits radiation in parallel planes perpendicular to the lamp axis (Figure 28). Combining this hypothesis with the Beer-Lambert equation, the integration of the differential equation describing the radiant power or irradiance as a function of the optical path yields the profile of radiant power or irradiance (Eq. 77 [18, 1151).

where is the photon rate of the light source per unit length, and Ri is the inner radius of an annular photochemical reactor. Harris and Dranoff [ll5] appear to have been the first to use the LSPP model to study the performance of a photochemical reactor for scale-up purposes. Experimentally, two sizes of perfectly mixed photochemical reactor were used for the decomposition of hexachloroplatinic acid in dilute aqueous solution, and the result of the theoretical analysis was acceptable in comparison with the experimental data. A very important extension of the LSPP model was developed by Jacob and Dranoff who, for the first time, took into account the three-dimensional

Figure 28. Scheme for the LSPP model [llS].

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nature of the light emission process [116]. In the resulting line source with spherical emission (LSSE) model, the lamp is still considered to be linear, but each of its points emits radiation isotropically and in all directions (Figure 29). The three-dimensional profile of the radiant power or irradiance is given by Eq. (78) [116], the empirical factorf(r, z ) having been introduced to correct errors due to the inadequacy of the modelled linear light source and to the consequences of reflection and refraction of the incident light crossing several interfaces.

where p = spherical coordinate pa,g= attenuation coefficient of glass walls dw = thickness of glass walls

(for rectangular Cartesian and spherical coordinates, see also Figure 29). Experimental testing of the LSPP and LSSE models (Eqs. 77 and 78, respectively) was performed by Jacob and Dranoff [1161 by using an annular reactor filled with either air or water, and the corresponding results showed that the predictions of the LSPP model are not satisfactory for the reasons already mentioned. Good agreement was found between experimental and calculated data by using the LSSE model after optimization of the empirical light source

/

annular reactor

A-

I

I

/

r

light source

Figure 29. Scheme for the LSSE model [116].

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289

factor. However, this optimization procedure may restrict the usefulness and application of this emission model. Further development of the emission models was made by Irazoqui et al. who introduced the three-dimensional nature of the extended light source [117]. Hence, the most significant feature of the extense source with volumetric emission (ESVE) model is the inclusion of a radiant energy source with finite spatial dimensions. In fact, the lamp is considered to be a perfect cylinder, the boundaries of which are represented by a mathematical surface of zero thickness (Figure 30). The mathematical expression of the three-dimensional profile of radiant power or irradiance calculated in accord with the ESVE model is given in Eq. (79) [117].

where

v = frequency of light N , = numbers of emitters per unit volume of the light source P(v) = probability density distribution function of emission per unit time and per unit frequency h = Planck's constant 4 , O = spherical coordinates

L = reactor length

01(4)= tan-' O,(#J) = tan-'

r cos

4 - [r2(cos24 - 1) + 7-3l l 2 L-z

r cos

4 - [r2(cos24 - 1) + r:]'i2 -Z

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A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A. OLLER DO NASCIMENTO

($ Figure 30. Scheme for the ESVE model [117].

Equations (79) and (80) describe the radiant exitance at any point in space and originating from the total volume of the light source. By using geometrical concepts to describe a radiation field in an empty annular reactor (Figure 30), integration limits for the spherical coordinates (p, O , $ )are given by Eqs. (81)-(84). Based on the experimental data of Jacob and Dranoff [lll], Cerda et al. [11 31 evaluated the LSSE and ESVE models and found for the latter a much better agreement between experimental and calculated data. However, Tournier et al. state that none of the above mentioned models could be used to interpret all the experimental results obtained from radiant power measurements in an immersion-type photochemical reactor using a lamp jacket equipped with a compartment for filter solutions [78]. Within the general concept of the ESVE models, Alfano et al. conceived a model for the radiant power profile of a tubular light source located in the focal axis of a parabolic reflector in order to analyze the design of a cylindrical photochemical reactor irradiated from the bottom [1 181. Differences between experimental and calculated (ESVE) results were always less than 15%. Other emission models for photochemical reactors include the extense source with superficial emission (ESSE) model in which the light source is assumed to be a surface [119], and the equivalent extense source with difuse superficial emission (ESDSE) model which has been developed to calculate the radiant power profile generated by several superficial light sources [1201. Radiation field modelling for photochemical reactors has been extensively reviewed by Cassano et al. [121, 1221.

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By comparing the reactor designs employed in these modelling studies with those actually used for pilot and production scale operations, the gap between concepts imposed by the present limitations of the mathematical tools and those imposed by pragmatic rules of large-scale chemistry is obvious. Enormous efforts will be needed before up-scaling of photochemical reactors will be at least partially possible with the help of such modelling techniques. On the other hand, developments in mathematical models of emission will have a progressive impact on the design of new photochemical reactors and their optimization (vide infra) by facilitating the isolation of decisive design parameters.

B. Coupling of Mass and Light Energy Balances Given the extreme difficulties of modelling the emission of light in photochemical reactors of actual design, a different approach to developing quantitative concepts of up-scaling photochemical reactors would consist of focusing on the coupling of mass and light energy balances. This work consists of adapting well-known mathematical descriptions of classical reactor engineering to the particular conditions of reactions governed by light absorption [1231. It is not surprising that under realistic absorption conditions, the sophistication of the emission model is of lesser importance and that good convergence between experimental and calculated results has been found in studies implying simple two-dimensional emission models (radial and parallel propagation of light [124, 1251).

VI. MODELLING AND OPTIMIZATION OF PILOT RESULTS There is no doubt that experimental conditions for a given photochemical process have to be optimized before decisions concerning a potential technical development can be made. During the procedure of up-scaling, and in particular at a pilot level where small-scale production activities may have started, optimization of the experimental conditions should be repeated. Among the many methods of modelling and optimization, we would like to describe the application of the methodology of optimal experimental design and the potential of artificial neural networks in the field of photochemical technology.

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A. Optimal Experimental Design 1. Fundamentals. Given the particular complexity of most photochemical processes, the importance of implementing a properly planned set of experiments in order to find optimal experimental conditions is necessary. Indeed, even if mechanistic, kinetic, and radiation data of small-scale laboratory experiments are available, further experimentation is in most cases essential in order to determine the influence of different experimental factors (input variables which can be controlled by the operator), and, consequently, to predict the best processing conditions at a technically more important (production) level. The complexity of optimizing experimental conditions increases considerably if a large number of factors is likely to affect the process and if, moreover, these factors are not intrinsically independent (i.e,, the effect of a given factor depends on the setting of another one and vice versa). The method that consists of adjusting the level of one variable (setting or value) at a time, while maintaining the other variables at fixed levels, is completely inappropriate for several reasons [126):

The number of experiments to be carried out becomes rapidly unreasonable and unrealizable, in particular at a pilot level. Despite a large number of experiments, this procedure will in most cases fail to reach a true optimum, as the different factors interact with one another, that is, they are not independent. Conclusions drawn from such experiments may be completely misleading, as they do not give any information about how such interactions may affect the process. Furthermore, it has been clearly pointed out [127] that, once a large number of experiments of more or less arbitrary design have been performed, a posterior use of statistical methods of data treatment can no longer guarantee a result of reliable value, because

0

Even with the most powerful statistical tools, it is not possible to extract from measurements more information than they contain. Often and in spite of a considerable number of experiments, essential information is still missing. The quality of information contained in experimental results is not automatically related to the number of experiments, but depends very much on the whole experimental design (set of experiments).

Strategies and procedures for ensuring successful experimentation have

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been proposed for quite some time [127-1301. Our purpose is to outline some of their important characteristics and advantages, and to guide the interested reader toward the specialized literature. A methodological approach to experimentation not only improves efficiency in collecting information, but is absolutely essential when objectives must be reached with a minimum number of trials and/or at minimum costs [127,131]. An efficient experimental strategy, based on multivariate methods, may be designed to: Reaching the objective@)as quickly as possible. Avoiding loss of time incurred by carrying out experiments which are either useless or which yield little or poor information. Providing the highest possible accuracy of the results. Making data analysis an easier task. Allowing modelling and optimization. A prerequisite to designing the optimal program of experimentation (actual set of experiments to be carried out) is to formulate the problem clearly by defining precisely the objective(s), assessing existing data and the consequences of a wrong decision, and listing the constraints (costs, time, etc.). The responses of interest, which are assumed to be affected by changing the values of the factors, must also be defined (yield of a product, percentage of an isomer, cost price, etc.). All factors, qualitative or quantitative (temperature, pressure, stirring, concentrations of reactants, irradiation conditions, pH, nature or origin of a catalyst, gas, etc.), which might affect the response(s) must be listed, because noncontrolled variations of influencing factors not included in the experimentation can cause systematic errors. The experimental region of interest (e.g., range of variation of the levels of the different factors) must be determined, and the reproducibility of the experiments checked. The most adequate and economical program of experimentation may then be established. In this procedure, the most appropriate experimental matrix (set of experiments) or sequence of several experimental matrices is usually chosen among the classical types (vide infra), which have been designed using well-known mathematical and statistical techniques (matrix algebra, multiple regression analysis, least squares, variance analysis, etc.). In fact, a basic strategy may be the following [127]: 1. Determination of the factors that actually influence the response(s). Screening methods exist which require a number of experiments not much higher than the number of factors and allow a relatively large number of factors to be sorted and roughly ordered according to their influence (weight) on the response(s) investigated.

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2. Quantification of possible interactions between the influencing factors by eliminating hypotheses of independence of the factors and of the additivity of their effects postulated in the screening step. 3. More detailed investigations by modelling and searching for the optimum (maximum or minimum) of one or several responses. The response surface methodology (RSM [130, 132-1341) offers a set of techniques for quantifying the relationship between the settings of the experimental factors and the responses presumed to be affected by the variations of these factors. As mechanisms involved in a process are most often not completely understood, a deterministic model can rarely be applied. Therefore, the mathematical model is usually empirical and depends on the type of problem (polynomials are most often used). Such a model allows us to contour plots (lines or curves of constant response value) and, once tested, to predict the values of the response(s) at any point in the experimental region of interest. Note that the accuracy of the prediction does not depend on the experimental results, but is, apart from the experimental error, very much dependent on the structure of the experimental matrix chosen.

A large number of classical experimental designs, adapted to various types of problems, are available, for example, Hadamard or Plackett-Burmann matrices [1351 (screening), complete and fractional factorial designs [130, 1361 (main effects of and interactions between factors), simplex [137] (optimization of one response at a time without mathematical model), equiradial or composite matrices [ 130, 1341 (RSM), and Doehlert’s uniform arrays [134, 1381 (RSM). Modern methods for designing experimental matrices have also been developed; they are based on exchange algorithms and are more flexible than the classical matrices for adapting the strategy to unforseen problems (e.g., an experiment of a classical matrix cannot be carried out, a new factor must be taken into account during experimentation, or a change of mathematical model is required) [127, 139, 1401. The methodology outlined above is flexible: experiments may be performed in successive steps (sequentially) and the information acquired from a series of experiments used to plan more adequately the next step. This approach applies to the different stages of the scaling-up procedure. It would in fact be unrealistic to begin optimization of a process only at the production level, as a great part of the necessary experiments can be performed at the pilot (or even at the laboratory) level. However, final refinements of the settings should be done at the last stage. In the following section, an example of how to evaluate the influence of different factors on the photocatalyzed oxidative degradation of organic pollutants in an aqueous system is presented.

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2. Example of Application: TiOz Photocatalyzed Oxidative Degradation. As mentioned in Section IVC, applications of photocatalysis (e.g., with TiO,) in the domain of environmental chemistry have developed considerably in the last decade, particularly in the area of water purification [141]. However, pilot or larger scale experiments remain scarce [142], despite the large number of basic papers published on the oxidative degradation of model compounds [103, 143). We have been interested in investigating the efficiency of the photocatalyzed oxidative degradation of pollutants contained in heavily contaminated industrial waters, using a multilamp immersion-type pilot reactor, the scheme of which is shown in Figure 24 [12]. Preliminary Experimental Planning. The purpose of this project was to analyze the performance of a multilamp immersion-type photochemical reactor at pilot level, conceived in accord with the qualitative design concepts shown above, in a project to apply the TiO, photocatalyzed oxidative degradation of pollutants to the purification of an industrial waste water. The water, provided by a chemical company specializing in the synthesis of organic intermediates, contained a mixture of organic compounds, predominantly nitro aromatics showing relatively slow rates of biodegradation [dissolved organic carbon (DOC) of about 4500ppm1, as well as nitric and sulfuric acid (pH of about 1.7). Since the exact composition of the mixture was not known, the same stock solution was used for the entire series of experiments to avoid reproducibility problems. Likewise, TiO, of the same origin was used in all runs. Degradation of the organic pollutants upon irradiation was monitored by measuring the DOC content [144] of samples taken at regular intervals. At the same time, the energy consumed by the lamps was measured using a kWh-meter. The factors likely to influence the rate of degradation and their range of variation (experimental region) were selected on the basis of literature data and of a few preliminary experiments (e.g., for evaluating the minimum flow of liquid and gas in order to maintain the TiO, particles in suspension, technical limits to control adequate mixing in the reactor and to solve problems of thermoregulation). The initial concentration of pollutants (DOCo) was varied from 4500 ppm (concentration of the stock solution) to a minimum of 900 ppm, a realistic range from a practical and industrial aspect. In principle, each factor can be defined by one or several variables; the factor “catalyst” might, for instance, be characterized by the three variables: nature, concentration, and granulometry. However, in the defined case to be presented, each factor was characterized by one variable (Table 2), and both terms are used indistinctly. Five natural variables (denoted U , - U , , respectively), which could affect significantly the degradation rate, were

2%

A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A. OLLER DO NASCIMENTO

TABLE 2 Variables and Experimental Region" for the Investigation of the Pilot Level TiO, Photocatalyzed Oxidative Degradation of Organic Pollutants Contained in an Industrial Waste Water [12]

Natural Variable ( Ui) U1: DOCo (initial concentration of DOC) U,: Flow of solution in the reactor

U,:Nature of the gas U,: TiOz concentration U5:Temperature

Experimental Regionb 900-4500 ppm 0.5-3 L.min-' Air or oxygen 0.5-5 g'L-' 50-80°C -1 +1

Coded variables (Xi)

"The following factors have been kept constant at this step of the investigation: reactor size, solution volume (20L), lamps electrical power (about 3.5 kW),gas flow (940L.h-'). 'Experiment at the center of the experimental region: X i = 0, that is, when meaningful, U i = (Urnin UmaX)/2.

+

selected, taking into account previous knowledge about the reaction mechanism [86] and the characteristics of our system (Table 2):

U , : initial concentration of D O C (DOCo) U,: waste water flow in the reactor U , : nature of the gas (air or oxygen) U,: TiO, concentration U : temperature Since different variables are usually expressed in different units and may be qualitative (e.g., type of gas) or quantitative, their effects can only be compared

Experimental Design. The first objective of the experiment was to determine which of the five factors listed above were of decisive importance, and to quantify their effects and their eventual interactions. The number of variables ( k = 5 ) being limited, a screening was not required, and a 2k factorial design was chosen [130,136]. In this type of design, all experiments are performed with variables set at two different levels, which correspond to the limits of the experimental region and which are coded (- 1) and ( + 1) (coded variables are denoted X i ) (Table 2). Complete factorial designs 2k require a minimum of 2' experiments, which

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correspond to all possible combinations of the k variables set at the levels (- 1) and (+ 1). However, the number of experiments can be considerably reduced if, at least in a first approach to the problem, high-order interactions, and even some first-order interactions (between two factors), can be assumed to be zero or negligible. In such a case, fractional factorial designs 2 k - pcan be used. Formally, they are associated with a response function Y represented, when only first-order interactions are taken into account, by a polynomial model of the form

Y = bo

+ 11 biX,+ 2 k

i=

k

c bijXiXj k

i=l j=1

where i#j b, = average value of the experimental responses bi = main effect of variable i bij = first-order interaction between variables i and j ( = bji)

The experimental matrix corresponding to a given design determines the settings of the variables [( - 1) or (+ l)] for each experiment. Once the series of experiments has been carried out, estimates of the coefficients b,, bi, and bij are calculated from the observed response values Y using the method of least squares for fitting the data. In the absence of interactions and if a main effect is positive (negative), the value of the response Y increases (decreases) when the corresponding variable changes from the level (- 1) to the level ( + 1). However, if interactions do exist, this simple reasoning can no longer be applied, and the main effects alone do not indicate the true variation of the response. Note that Eq. 85 can only be used to predict a response within the experimental region if the validity of the model is tested. This is usually done by performing an experiment at the center of the experimental region and comparing the experimental value of the response (Yo)to that of b,.

First Experimental Matrix. In the present case we planned to determine the main effects of 5 variables (b,-b5, Table 2), and the following first-order interactions were considered most probable: Interaction between DOC, and TiO, concentration (b14) because of a possible competition between the different pollutants and saturation effects at the surface of the catalyst.

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A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A. OLLER DO NASCIMENTO

Interaction between DOC,, and type of gas (b13) because the oxygen concentration is important for the rate of photocatalyzed and subsequent thermal oxidation steps (Eqs. 63, 64, 9). Interaction between temperature and type of gas (b3,) since oxygen solubility in water is temperature dependent. Interaction between TiO, concentration and temperature (b,,), the adsorption kinetics, for example, depending on the temperature. Therefore, 10 coefficients (including b,) had to be calculated, and a fractional factorial design 25-' (16 experiments), or at least 3/4 of such a design (12 experiments) [130, 1361, would have been necessary to determine these 10 coefficients independently. Limited in the total volume of stock solution available, we used a sequential approach and planned a factorial fractional design 2 5 - 2 (8 experiments). In such a design, interactions b,, and b45 could not be determined independently, and only their sum would be known. In fact, only one experiment from this matrix [ ( X , = X , = X , = X , = - 1 and X , = 1; i.e., DOC, = 900ppm, air bubbling, [TiO,] = 0.5g.L-', temperature = 50°C and flow of solution = 3 L emin-', respectively] was carried out. The results obtained for this experiment indicated that two factors among the five selected had to be set at a constant value. In fact, oxidative degradation proceeded very slowly, and more than 80 hours of irradiation were needed to mineralize the whole organic matter (Figure 3 1). Hence, oxygen could not be used as a saturating gas for economical reasons.

+

0

20

40

60

80

100

Irradiation time (h)

Figure 31. Relative DOC decrease as a function of irradiation time for the four experiments at low DOCo [experiments 1 (A),3 (a), 5 (A), and 7 (a), Table 31 and control experiments [thermal reaction: experiment 10 (O), and direct photolysis: experiment 11 (n)][12].

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Moreover, when the flow of solution was lower than 3 L.min-l, the minimum temperature of 50°C could not be maintained, even with maximum cooling. Second Experimental Matrix. For the technical reasons mentioned above, a complete factorial design 23 was chosen for investigating the effects of the three factors: DOC,, TiOz concentration, and temperature (Table 3). An additional experiment at the center of the experimental region ( X , = X4 = X , = 0; i.e., DOC, = 2700 ppm, [TiO,] = 2.75 g . L-', temperature = 65°C) was repeated twice for testing the model (Eq. 85). Note that experiment # 1 (Table 3) is the same as the one already performed for the fractional factorial design (vide supra). Experiment # 7 was repeated three times for controlling the reproducibility of the results and estimating the standard deviation of the response. Control experiments showed that

TABLE 3 Experimental Matrix for Investigating the Influence of Three Factors on the TiOz Photocatalyzed Oxidative Degradation of Waste Water Pollutants in a Pilot Reactor: Complete Factorial Design 23, Control Experiments"(Natural and Coded Variables are Indicated), and Values of the Experimental Response Y

1 2 3 4 5 6 7' 8 9d 10 11

900 (-1 4500 (+) 900 (-1 4500 (+) 900 (-1 4500 (+) 900 (-1 4500 (+) 2700 (0) 900 (-1 900 (-1

0.5 (-) 0.5 (-) 5 (+I 5 (+I 0.5 (-) 0.5 (-) 5 (+I 5 (+I 2.15 (0) 0 0

3.88 1.19 4.11 3.23 4.01 4.20 5.51 7.09 2.84 e

2.09

"Signs (+) and (-) represent the settings of the coded variables at the limits of the experimental region, (0) represents the center; X , = 1 (flow of solution equal to 3 L'min-') and X , = - 1 (air) in all experiments. 'Standard deviation calculated from experiments 7 and 9: & 0.39 ppm. kWh'Experiment repeated three times. dExperimentat the center of the experimental region, repeated twice. 'Control experiment in the absence of irradiation and TiO,, almost no thermal degradation (see Figure 31).

+

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stripping of organic compounds from the photochemical reactor was negligible. In fact, less than 0.2% of the DOC, was stripped due to air blown into the reaction mixture. Furthermore, controls also indicated that there was no degradation in the absence of irradiation; however, the reaction mixture absorbed between 300 and 450 nm, and direct photolysis made a significant contribution to the decrease of DOC. All other experimental conditions being kept constant, degradation by direct photolysis remained slower than that photocatalyzed by Ti02. The decrease of DOC as a function of irradiation time is shown in Figure 31 for experiments performed at low DOC, and for control experiments (see also Table 3). The average degradation rate (in ppm.kWh-') was chosen as an experimental response ( Y ; Table 3; see also Section 1V.C)for the calculation of the main effects as well as of the first- and second-order interactions. The results are given in Table 4.

Discussion of the Results. Table 4 shows that first-order interactions between the three factors investigated are almost as important as their main effects. Hence, conclusions can only be drawn if the influence of these factors on the response is studied by considering them two by two. In fact, this is a typical

TABLE 4 Influence of DOC,, TiOz Concentration, and Temperature on the Photocatalyzed Oxidative Degradation of Waste Water Pollutants in a Pilot Reactor: Coefficients (Main Effects and Interactions) Calculated from the Experimental Results of a z3 Factorial Design (Table 3) Coefficient

Value"

Average

bo

4.16

Main Efect

.0.23 0.83 1.06

DOCo [Ti021 Temperature Interaction

DOCo-[Ti02] DOCo- temperature [TiOJ -temperature Second order

b14

b,S b4S b145

0.41 0.66 0.26 -0.05

'Toefficients expressed in ppm/kWh; standard deviation: +0.14pprn.kWh-'.

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example of a situation where the main effect of a factor alone does not indicate its real effect on the response. The main effect of DOC, ( b , = -0.23, standard deviation: k0.14)seems to indicate that the degradation rate is nearly insensitive to the variation of DOC,. However, this is only true at high TiO, concentration (Figure 32), whereas at low TiO, concentration the rate of degradation is decreasing markedly with increasing DOC,. Figure 32 shows a three-dimensional graphic representation of the variation of the degradation rate as a function of DOC, and catalyst concentration. Note that the drawn surface is not a true response surface, but is a convenient means to represent and visualize the results obtained by performing experiments at the limits of the experimental region. This approach assumes a firstorder linear model (Eq. 85) and does not take into account possible curvatures (vide infra). Figure 32 also shows that at low DOC,, the TiO, concentration has little effect and consequently low catalyst concentrations can be used. In contrast, for optimal conditions, the catalyst concentration must be increased if waste waters of high DOC, are to be treated. The DOC,-[TiO,] interaction (bI4) probably results because the waste water solution absorbs in the same spectral region as TiO,, and, hence, at low TiO, concentrations and at high DOC,, competition for light absorption occurs. As direct photolysis is slower than photocatalyzed degradation, a high DOC,, induces an inner filter effect that slows down the process; this effect is decreasing as the reaction proceeds.

DOC,

-1

Figure 32. Three-dimensional representation of the variation of the degradation rate ( Y ) as a function of DOC, and TiOz concentration (temperature: 50"C), obtained from the results of a 23 factorial design (Table 3) [12].

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The influence of temperature on the degradation rate appears to be most important, and this factor also interacts with the two others. Without discussing all the details, the results show that a temperature increase is generally favorable, which is certainly the consequence of a simultaneous increase of the rate of the secondary thermal reactions. This effect is logically more pronounced at high DOC,. The rather large differencebetween the experimental response at the center of the experimental region (Yo= 2.84 ppm.kWh-', experiment #9) and the corresponding value calculated on the basis of the results of the 2' factorial design (b, = 4.16ppm. kWh-', when Xi = 0) indicates that a linear model is not adequate for representing the response in the experimental region. A correct modelling would require further experimentation, using a seconddegree polynomial. However, in the next experimental design, the temperature should be set at its high level (80°C, thermal energy provided by the lamps), where strong cooling is not required. Thus, a response surface involving DOCoand catalyst concentration could be drawn by performing the necessary additional experiments (six experiments in the case of a Doehlert's array, for example). Using such a response surface, the optimal concentration of Ti02 (between 0.5 g L-' and 5 g L-l) and the optimal DOC0 (between 900 a nd 4500 ppm) could be predicted. At present, the fastest degradation rate was observed at 80°C, with 5 g . L - ' of TiOz and a DOCo of 4500ppm (nondiluted stock solution), and the whole organic matter contained in the reaction system could be mineralized. However, this degradation rate is still relatively slow from an industrial point of view (vide supra).

B. Neural Networks 1. Fundamentals. Modelling photochemical processes by solving the radiant energy conservation equation coupled to the momentum, mass, and heat balances is a very difficult task, because research on mathematical descriptions of radiation profiles remains far behind the empirical development of new reactor designs. By using a completely different approach, artificial neural networks [1451 might provide suitable simulations for upscaling chemical and photochemical processes which otherwise would prove to be extremely difficult to model by transport phenomena equations [146,147]. Neural networks have the ability to "learn" and record linear and nonlinear behaviors of a system from a set of experimental data [e.g., processing conditions and corresponding response(s)]. They require neither the knowledge nor the resolution of mathematical equations characteristic of the system. However, a good simulation certainly depends on a judicious

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choice of experimental conditions and on a good knowledge of the factors influencing the process investigated. Typically, a neural network consists of three layers of neurons, input, hidden and output layers, and of information flow channels between the neurons called interconnects (Figure 33). The neurons in the input layer (boxes, Figure 33) store the independent variables (xi) which are characteristic of the system investigated. Each processing neuron (circles, Figure 33) in the hidden and output layers carries out a local computation (transfer function) which converts neuron inputs into neuron outputs. Each neuron output is connected to all neurons of the following layer and each connection is weighed. The network output consists of a value (or a series of values), o k , which represents the response variable (or response variables) calculated by the network. In the learning phase, the network compares the calculated response value (0,)with the experimental response (q)and corrects the weights at each connection in order to reach a convergence between 0, and q.For this purpose, most neural networks use the back-propagation algorithm [145]. In this procedure, each neuron j of the hidden layer calculates a weighed sum Sj, N

where N is the number of input variables xi, scaled between 0 and 1, and wij are the weights associated to each input i in the neuron j .

Output layer

Hidden layer

Input layer

Figure 33. Three layer neural network.

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A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A. OLLER DO NASCIMENTO

Then, the output of the neuron j , Oj,is calculated as a function of S j ,

Once the outputs O j of the neurons from the hidden layer have been calculated, they are transfered (with associated weights wjk)to the output layer. The neurons of this layer carry out similar calculations to those given by Eqs. (86) and (87), and the final outputs of the network, O,,are obtained. The system “learns” by changing successively the weights at the interconnects. At the beginning of the process, the weights are selected randomly. Assuming that there are R input-output pairs ( X , Y ) available for training the network, X and Y being the input vector of dimension N and the output vector of dimension P (response vector), respectively, the criterion used for improving the convergence between experimental responses ( y k ) and corresponding calculated outputs (0,) is to minimize the mean-square error E defined as

In the method using the back-propagation algorithm (see [145] for details), the training pairs are presented successively, and for each of them the weights at the interconnects between the hidden and output layers are adjusted before those between the input and hidden layers. Once a good fit between experimental and calculated data has been obtained, the model function given by the neural network may be tested and, if adequate, used for simulation and prediction purposes.

2. Example of Application: Large-Scale Actinometry. Neural network modelling was applied to large-scale actinometry in a continuous elliptical photochemical reactor with a concentric annular reaction chamber [2,3, 108, 1481. Uranyl oxalate was used as an actinometer, which is based on the photosensitized decomposition of oxalate ions (Eq. 89) [2, 31; the experimental data were taken from the literature [1081. HZCO,

kv(Sens)

h

H,O

+ CO, + CO

(89)

One of the main problems in modelling a photochemical process by neural networks is the correct choice of input and output variables to accurately describe the process. In the particular application presented here, the

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available information was composed of the light source, the initial concentration of reactants, the flow rate of the actinometric solution, the temperature, the dimensions of the photochemical reactor (length of the irradiated section: 3.05 m, inner diameter of the outer tube: 2.499 cm, the outer diameter of the inner tube and, hence, the volume of the reactor could be varied), and the conversion rate (output variable). The following input variables were chosen: flow rate, diameter of the inner tube, and reaction volume (or reactor length). The only variable affecting the light distribution is the diameter of the inner tube, and this information is essential to simulate variations of the radiation field. The learning set given to the neural network consisted of the experimental data for a series of three runs among the four available. A run was characterized by a given diameter of the inner tube (run 1: 2.171 cm; run 2: 1.861 cm; run 3: 1.546cm; run 4: 1.234cm). For each run, conversion rates were known at 10 different flow rates. The number of neurons in the hidden layer was 15. The data from runs 1-3 were fed to the network until the convergence of the learning procedure was achieved. About 30,000 presentations of the set of experimental data (34 pairs) were necessary to obtain an acceptable low value of the residual sum of squares ( E , Eq. 88). The final simulations of the network for runs 1-3 (learning set) and the resulting fit between experimental and simulated data are shown in Figure 34a. The most interesting result is the rather good prediction for run 4 (Figure 34b). This run did not belong to the learning set and represented an extrapolated case (larger reactor). As extrapolation (scaling-up) is one of the most important challenges in reactor design, further investigation into the ability of neural networks to predict the behavior of a given process outside the experimental (learning set) region seems worthwhile.

VI. CONCLUSIONS Despite the many failures encountered over the last decades in up-scaling photochemical reactions, industrial preparative photochemistry has not lost its image as an attractive tool for the synthesis of fine chemicals. Reaction conditions (solvent, temperature, work-up procedures) are, in general, quite flexible and facilitate implementation of such processes on an industrial scale in terms of environmental protection. In addition, light-induced oxidation processes exhibit a very promising potential for the chemical treatment of contaminated surface and ground waters, as well as for liquid and gaseous industrial waste. On the pragmatic, conceptual, and theoretical modelling level, important

12 -

-8

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8-

v

a, + K

.-0

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a,

6-

c

0

0

42-

01 0.0

I

1.o

I

2.0

3.0

flow rate (I/rnin)

12

-

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10-

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8-

.-0 2 a, >

6-

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v

a, L

c

c

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flow rate (I/rnin) Figure 34. Experimental points and neural network simulated curves for a large scale actinometry. (a)Learning set: runs 1 (a),2 (A), and 3 (m). (b) Extrapolated case: run 4 (0) (experimental data from [lOS]) [148].

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progress has been made, not only in inventing and improving new reactor designs, but also in the mathematical description of radiant energy profiles, and their coupling with momentum, mass, and heat balances. By joining the experience of the specialists of these two domains, photochemical reactor design and technical development of photochemical processes in the chemical and environmental sector will enter a new era and receive general acceptance. It should however be noted that sophistication is not a guarantee for success, and that a successful implementation of photochemical processes is necessarily linked to a teaching curriculum that goes beyond ground state chemistry.

ACKNOWLEDGMENTS The photochemical and computational work mentioned has been performed at the photochemical laboratory of the Institut de Chime Physique, Ecole Polytechnique FCdCrale de Lausanne. Besides the valuable information obtained from the companies cited in Refs. 2 and 3, cooperation with Atochem Elf-Aquitaine (Paris and PierreBenite, France), E. I. DuPont de Nemours & Company (Wilmington, Delaware, USA) and Ems-Dottikon AG (Dottikon, Switzerland) in the domain of industrial photochemistry, as well as with Asea Brown Boveri AG (Baden, Switzerland) for the technical application of excimer light sources in photochemistry, provided many interesting discussions, new insights in problems of industrial chemistry, and valuable new tools for their solution. Financial support of the Commission for the Encouragement of Scientific Research (Bern, Switzerland), project nr 1823.1, is gratefully acknowledged. We thank the FundaGiio de Amparo a Pesquisa do Estado de Siio Paulo (FAPESP) for the grant enabling Prof. C. A. Oller do Nascimento to spend his sabbatical year in Lausanne.

REFERENCES 1. R. Roberts, M. A. Muradaz, and R. F. Cozzens, Photochemistry in the Chemical Process Industries, Mitre Corp. McLean, VA, 1982. 2. A. M. Braun, M.-T. Maurette, and E. Oliveros, Technologie photochimique, Presses Polytechniques Romandes, Lausanne, 1986. 3. A. M. Braun, M.-T. Maurette, and E. Oliveros, Photochemical Technology, D. F. Ollis, N. Serpone, translators, Wiley, Chichester, 1991. 4. J. R. Bolton, E P A Newsl. 43, 80 (1991).

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A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A. OLLER DO NASCIMENTO

5. J. C. Andre and M. L. Viriot, Eds., Photochimie Industrielle, CPIC-ENSIC,

Nancy, Volumes 1 and 2, 1988. 6. M. L. Viriot, J. C. Andre, and A. M. Braun, Eds., Industrial Photochemistry, CPIC-ENSIC, Volumes A and B, Nancy, 1990. 7. J. C. Andre, M.-L. Viriot, N. Midoux, and C. Roizard, “Why not Industrial Photochemistry?“ in Industrial Photochemistry, Vol. A, M. L. Virior, J. C. Andre, and A. M. Braun, Eds., CPIC-ENSIC, Nancy, 1990, p. 1. 8. T. Shirotsuka and M. Sudoh, Waseda Diagaku Rikogaku Kenkyusho Hokoku 78, 43 (1977). 9. C. S. Lian and S. Y. Ho, Proc. Natl. Sci. Counc. Republ. China 2, 4 (1978). 10. For example, US Patent 2906681, 29/9/1959, F. Passler (Hoechst). 11. J. L. Weeks, G. M. A. C. Meaburn, and S. Gordon, Radiat. Res. 19, 559 (1963). 12. L. Jakob, Thesis, Ecole Polytechnique FBdCrale de Lausanne, Switzerland, 1992. 13. P. Murasecco-Suardi, E. Gassmann, A. M. Braun, and E. Oliveros, Helv. Chim. Acta 70, 1760 (1987). 14. US Patent 4,315,998 (Wayne State University, University of New Mexico, Research Corporation, A. P. Schaap, E. C. Blossey and D. C. Neckers). 15. A. P. Schaap, A. L. Thayer, K. A. Zaklika, and P. C. Valenti, J . Am. Chem. SOC. 101, 4016 (1979). 16. G. A. Graf, Thesis, Universite de Haute Alsace, Mulhouse, France, 1978. 17. E. Gassmann, Thesis, Ecole Polytechnique Federale de Lausanne, Switzerland, 1984. 18. A. M. Braun and C. A. 0. do Nascimento, Nachr. Chem, Tech. Lab. 39, 515 (1991). 19. H. Al-Ekabi, A. Safarzadeh-Amiri, J. Story, and W. Sifton, Advanced Technology for Destruction of Organic Pollutants by Photocatalysis, Nulite, London, Ontario. 20. N. J. Turro, J. R. Fehlner, D. P. Hessler, K. M. Welsh, W. Ruderman, and A. M. Braun, J . Org. Chem. 53, 3731 (1988). 21. Belgian Patent 889968, 13/8/1981, R. G. Parker (B. F. Goodrich Company). 22. K. Kalyanasundaram, Photochemistry in Microheterogeneous Systems, Academic Press, Orlando, 1987. 23. M.-T. Maurette, E. Oliveros, P. P. Infelta, K. Ramsteiner, and A. M. Braun, Helv. Chim. Acta 66, 722 (1983). 24. A. M. Braun and E. Oliveros, “Local Concentrations Effects on Oxidation Reactions in Microheterogeneous Media,” in Frontiers in Supramolecular Organic Chemistry and Photochemistry, H.-J. Schneider and H. Diirr, Eds., VCH Verlagsgesellschaft, Weinheim, 1991, p. 393. 25. M. Gratzel, Heterogeneous Photochemical Electron Transfer, CRC Press, Boca Raton, FL, 1988.

UP-SCALING PHOTOCHEMICAL REACTIONS

309

26. R. Fargues, M.-T. Maurette, E. Oliveros, M. Riviire, and A. Lattes, Noun J. Chim. 3, 487 (1979). 27. N. J. Turro and M. Garcia-Garibay, “Thinking Topologically about Photochemistry in Restricted Spaces,” in Photochemistry in Organized and Constrained Media, V. Ramamurthy, Ed., VCH Verlagsgesellschaft, Weinheim, 1991, p. 1. 28. D. F. Eaton, “Application of Organized Media-Examples”, in Photochemistry in Organized and Constrained Media, V. Ramamurthy, Ed., VCH Verlagsgesellschaft, Weinheim, 1991, p. 841. 29. E. Oliveros, P. Pheulpin, and A. M. Braun, Tetrahedron 43, 1713 (1987). 30. UK Patent 3236758, 22/2/1982, (Astra Lakemedel). 31. Japanese Patent 58-152879, 10/9/1983, Y . Takeda (Mitsubishi Kasei Kogyo K.K.). 32. H. J. Dauben and L. L. McCoy, J. Am. Chem. SOC.81, 4863 (1959). 33. J. Adam, P. A. Gosslain, and P. Goldfinger, Nature 171, 704 (1953). 34. A. M. Braun, M. El Bouamri, H.-J. Federsel, A. Stihlberg, and U. Stenhede, in preparation. 35. H.-J. Schumacher and W. Thiirauf, Z . Phys. Chem. A189, 183 (1941). 36. German Patent 2118540, 29/11/1979, J. Dahlberg and C. A. G. Oestlund (Uddeholms AB). 37. US Patent 3883407, 20/4/1973, A. L. Dittman (Halocarbon Products Corp.). 38. R. N. Hazeldine and F. Nyman, J. Chern. Soc. 387 (1959). 39. J. M. Curchod and A. M. Braun, Report, EPF Lausanne, 1985. 40. J. F. Rabek, Experimental Methods in Photochemistry and Photophysics, Part 2, Wiley, Chichester, 1982, p. 887. 41. US patent 3853729, 10/12/1974, G. Lucas (Socikte Nationale des Petroles d’Aquitaine). 42. D. Dworkin and J. S . Dranoff, AZChE J. 24, 1134 (1978). 43. A. Tournier, X. Deglise, and J. C. Andre, J. Photochem. 22, 313 (1983). 44. M. Richard and M. Lenzi, AlChE J. 30, 838 (1984). 45. German patent 2209626, 15/6/1978, G. Lucas (SNEA). 46. G. Lucas, Transfer Theory for Trapped Electromagnetic Energy, Wiley, Chichester, 1986. 47. W. Amrein, J. Gloor, and K. Schaffner, Chimia 28, 185 (1974). 48. J. C. Andre and M. L. Viriot, J. Photochem. Photobiol. A50, 343 (1990). 49. A. Sayd, M. C. Carre, M. L. Viriot, and J. C. Andre, J. Photochem. Photobiol. A61, 253 (1991). 50. R. M. Wilson, W. Adam, and R. Schulte, The Spectrum 4, 8 (1991). 51. R. M. Wilson, K. A. Schnapp, K. Hannemann, D. M. Ho, H. R. Memarian, A. Azadnia, A. R. Pinhas, and T. M. Figley, Spectrochim. Acta 46A, 551 (1990).

310

A. M. BRAUN, L. JAKOB, E.OLIVEROS AND c. A. OLLER DO NASCIMENTO

52. G. 0. Schenck, “Einsatz der Ultraviolett-Bestrahlung von Wasser in der pharmazeutischen Industrie”, in Technologie der Wasseraufbereitung f u r pharmazeutische Zwecke, Concept, Heidelberg, 1980, p. 5. 53. G. 0. Schenck, “Ultraviolet Sterilization”, in Handbook of Water Purijcation, W. Lorch, Ed., McGraw Hill, London, 1981, Chapter 10, p. 363. 54. German patent 2119961, 2/11/1972, G. Horstmann (Wedeco). 55. Y. Tanaka, J . Opt. SOC.Am. 45,710 (1955). 56. K. L. Kompa and H. Walther, Eds., High Power Lasers and Applications, Vol. 9, Springer Series in Optical Science, Springer, Berlin, 1987. 57. B. Gellert and U. Kogelschatz, Appl. Phys. B52, 14 (1991). 58. L. Jakob, T. M. Hashem, M. M. Kantor, and A. M. Braun, Oxidative Degradation Processes by Vacuum Ultraviolet ( V U V ) Photolysis, Division of Environmental Chemistry, ACS Meeting, Proceedings, San Francisco, 1992. 59. 0. Legrini and A. M. Braun, unpublished results. 60. German patent application DE 4022279 A l , 12/7/1990, B. Eliasson, B. Gellert, and U. Kogelschatz (Asea Brown Boveri AG). 61. US Patent 4837484,6/6/1989, B. Eliasson, P. Erni, M. Hirth, and U. Kogelschatz (BBC Brown, Boveri AG). 62. B. Eliasson and U. Kogelschatz, Appl. Phys. B46, 299 (1988). 63. H. Esrom and U. Kogelschatz, M a t . Res. SOC.Symp. Proc. 158, 189 (1990). 64. ABB Infocom, New Excimer UVSourcesfor Industrial Applications, Publication CH-E 3.30833.0 E. 65. M. Wohlers, Thesis, Ecole Polytechnique FCderale de Lausanne and Universitat Karlsruhe, in preparation. 66. A. Bouchy, J. C. AndrB, E. George, and M. L. Viriot, J . Photochem. Photobiol. A 48, 447 (1989). 67. US patent 3090739, 21/5/1963, Y. Ito (Toyo Rayon). 68. K. H. Pfortner (F. Hoffmann-La Roche und Co., AG, Basel, Switzerland), personal communication. 69. See Ref. 2, p. 460 and Ref. 3, p. 477. 70. W. Rojahn and H.-U. Warnecke, Dragoco Report 27, 159 (1980). 71. Swiss patent 493268, 31/8/1970, N. Tarkoy and C. Campana (Ciba AG). 72. Gerstel Labormechanik, D-4330 Mulheim/Ruhr, Rotations-Photoreaktor. 73. W. Spielmann, H.-H. Fick, L.-U. Mexyer, and A. de Meijere, Tetrahedron Lett. 405 (1976). 74. NORMAG, Otto Fritz GmbH, Normschliff und Aufbaugerate, D-6238 Hofheim im Taunus, Falling-film Photoreaktor. 75. A-E. C. Snoeren, M. R. Daha, J. Lugtenburg, and E. Havinga, Rec. Trav. Chim. Pays-Bas 89, 261 (1970). 76. M. Denny and R. S. H. Liu, Nouu. J . Chim. 2,637 (1978).

UP-SCALING PHOTOCHEMICAL REACTIONS

311

77. K.-H. Pfortner and H.-J. Hansen (Hoffmann-LaRoche), personal communication. 78. A. Tournier, X. Dkglise, J. C. Andre, and M. Niclause, J . Photochem. 18, 47 (1982). 79. J. C. Andre, M. Roger, A. Said, and J. Villermaux, Entropie 137/138, 87 (1987). 80. J. C. Andre and M. L. Viriot, J . Photochem. Photobiol. A 50, 343 (1990). 81. G. Ohloff, E. Klein, and G. 0. Schenck, Angew. Chem. 71, 578 (1990). 82. G. 0. Schenck and K. Ziegler, Naturwiss. 32, 157 (1954). 83. French patent 2111119, 3/4/1980, G . Bolz, W. Wiersdorf, W. Wielant, A. Bachmeier, and R. Depold (BASF). 84. E. C. Blossey, D. C. Neckers, A. L. Thayer, and A. P. Schaap, J . Am. Chem. SOC. 95, 5820 (1973). 85. E. Oliveros, M.-T. Maurette, E. Gassmann, A. M. Braun, V. Hadek, and M. Metzger, Dyes and Pigments 5, 457 (1984). 86. N. Serpone and E. Pelizzetti, Photocatalysis, Fundamentals and Applications, Wiley, New York, 1989. 87. D. F. Ollis, “Solar-Assisted Photocatalysis for Water Purification: Issues, Data, Questions” in Photochemical Conversion and Storage of Solar Energy, E. Pelizzetti and M. Schiavello, Eds., Kluwer Academic Publishers, Dordrecht, 1991, p. 593. 88. M. Gratzel, Comments Inorg. Chem. 12, 93 (1991). 89. V. N. Parmon and K. I. Zamaraev, “Photocatalysis in Energy Production”, in Photocatalysis, Fundamentals and Applications, N. Serpone and E. Pelizzetti, Eds., Wiley, New York, 1989. 90. E. M. Cereas, L. Burlamacchi, and M. Visca, J . Muter. Sci. 18, 289 (1983). 91. C. D. Jaeger and A. J. Bard, J . Phys. Chem. 83, 3146 (1979). 92. M. A. Fox, R. B. Draper, M. Dulay, and K. O’Shea, “Control of Photocatalytic Oxidative Selectivity on Irradiated TiO, Powders: A Diffuse Reflectance Kinetic Study” in Photochemical Conversion and Storage of Solar Energy, E. Pelizzetti and M. Schiavello, Eds., Kluwer Academic Publishers, Dordrecht, 1991, p. 323. 93. A. M. Braun, “Progress in the Application of Photochemical Conversion and Storage” in Photochemical Conversion and Storage of Solar Energy, E. Pelizzetti and M. Schiavello, Eds., Kluwer Academic Publishers, Dordrecht, 1991, p. 551. 94. E. Borgarello, N. Serpone, P. Liska, W. Erbs, M. Gratzel, and E. Pelizzetti, Gazz. Chim. Ital. 115, 599 (1985). 95. R. W. Matthews, J . Phys. Chem. 91, 3328 (1987). 96. H. Al-Ekabi and N. Serpone, J . Phys. Chem. 92, 5726 (1988). 97. R. Marinangeli and D. F. Ollis, AIChE J . 26, 1000 (1980). 98. C. Braun and J. C. Andre, J . Photochem. 28, 13 (1985). 99. J. C. Andre and J. F. d’Allest, J . Photochem. 36, 221 (1987).

312

A. M. BRAUN, L. JAKOB, E. OLIVEROS AND C. A. OLLER DO NASCIMENTO

100. T. Yokata, Y. Takahata, H. Nanjo, and K. Takahashi, J . Chem. Eng. Jpn. 22,537

(1989). 101. L. Rizzuti and A. Brucato, “Recent Developments in Heterogeneous Photoreactor Modelling,” in Photochemical Conversion and Storage of Solar Energy, E. Pelizzetti and M. Schiavello, Eds., Kluwer Academic Publishers, Dordrecht, 1991, p. 561, and literature cited therein. 102. D. Iatridis, P. L. Yue, L. Rizutti, and A. Brucato, Chem. Eng. J . 45, 1 (1990). 103. R. W. Matthews, “Environment: Photochemical and Photocatalytic Processes. Degradation of Organic Compounds,” in Photochemical Conversion and Storage of Solar Energy, E. Pelizzetti and M. Schiavello, Eds., Kluwer Academic Publishers, Dordrecht, 1991, p. 427. 104. R. W. Matthews and S. R. McEvoy, J . Photochem. Photobiol. A, 64, 231 (1992). 105. K. Tanaka, T. Hisanaga, and K. Harada, New J . Chem. 13, 5 (1989). 106. US patent application, 1991, M. Gratzel (Ems Inventa AG, file number OZ 675A). 107. T. Matsuura and J. M. Smith, AIChE J. 16, 321 (1970). 108. R. F. Gaertner and J. A. Kent, Ind. Eng. Chem. 50, 1223 (1958). 109. F. B. Hill, and R. H. Felder, AIChE J . 11, 873 (1965). 110. A. E. Cassano and J. M. Smith, AIChE J. 12, 1124 (1966). 111. S. M. Jacob and J. S. Dranoff, AIChE J . 15, 141 (1969). 112. F. Santarelli and J. M. Smith, Chem. Eng. Commun. 1, 297 (1974). 113. J. Cerda, H. A. Irazoqui, and A. E. Cassano, AIChE J . 19, 963 (1973). 114. J. E. Huff and C. A. Walker, AIChE J . 8, 193 (1962). 115. P. R. Harris and J. S. Dranoff, AIChE J. 11, 497 (1965). 116. S. M. Jacob and J. S. Dranoff, AIChE J. 16, 359 (1970). 117. H. A. Irazoqui, J. Cereda, and A. E. Cassano, AIChE J . 19, 460 (1973). 118. 0.M. Alfano, R. L. Romero, and A. E. Cassano, Chem. Eng. Sci. 40,2119 (1985). 119. C. Stramigioli, G. Spadoni, and F. Santarelli, Appl. Sci. Res. 33, 23 (1977). 120. T. Yokota, T. Iwano, and T. Tadaki, Kagaku Kogaku Ronbunshu 2,298 (1976). 121. M. 0.Alfano, R. L. Romero, and A. E. Cassano, Chem. Eng. Sci. 41,421 (1986). 122. A. E. Cassano and 0. M. Alfano, “Photoreactor Design” in Handbook of Heat and Mass Transfer, Vol. 111, N. Cheremisinoff, Ed., Gulf Publishers Corp., 1989, Chapter 16. 123. N. Midoux, C . Roizard, and J. C. AndrC?, J . Photochem. Photobiol. A58, 71 (1991). 124. J. C. Andrt, M. Niclause, A. Tournier, and X. Dkglise, J. Photochem. 18, 57 (1982). 125. J. C. Andre, A. Tournier, and X. Dtglise, J . Photochem. 22, 7 (1983). 126. R. Carlson, T. Lundstedt, R. Phan-Tan-Luu, and D. Mathieu, Nouv. J . Chim. 7, 315 (1983).

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127. E. Fargin, M. Sergent, D. Mathieu, and R. Phan-Tan-Luu, Bio-Sciences 4, 77 (1985) and references cited therein. 128. C. A. Bennet and N. L. Franklin, Statistical Analysis in Chemistry and the Chemical Industry, 3rd ed., Wiley, New York, 1963. 129. 0.L. Davies, Ed., The Design and Analysis of Industrial Experiments, Longman, London, 1978. 130. G. E. P. Box, W. G. Hunter, and J. S. Hunter, Statisticsfor Experimenters: An

Introduction to Design, Data Analysis and Model Building, Wiley, New York,

1978. 131. D. Mathieu and R. Phan-Tan-Luu, l n t Chim. 292, 171 (1988). 132. G. E. P. Box and N. R. Draper, Empirical Model Building and Response Surfaces,

Wiley Series on Probability and Mathematical Statistics, Wiley, New York,

1987. 133. A. I. Khuri and J. A. Cornell, Response Surfaces, Designs and Analyses, Marcel Dekker, ASQC Quality Press, New York, 1987. 134. D. Feneuille, D. Mathieu, and R. Phan-Tan-Luu, Mdthodologie de la Recherche

135. 136. 137. 138. 139. 140. 141. 142.

143. 144. 145.

146. 147. 148.

Expdrimentale: Etude des Surfaces de Rdponse, L.P.R.A.I., Universitk d’AixMarseille, 1983. R. L. Plackett and J . P. Burmann, Biometrika 33, 305 (1946). J . Goupy, La mdthode des plans d’expdriences, Dunod, Paris, 1988. J. A. Nelder and R. Mead, Computer J . 7 , 308 (1965). D. H. Doehlert, Appl. Stat. 19, 231 (1970). V. V. Fedorov, Theory of Optimal Design, Academic Press, New York, 1972. M. Chariot, G . A. Lewis, D. Mathieu, R. Phan-Tan-Luu, and H. N. E. Stevens, Drug Dev. Pharm. 14, 2535 (1988). M . Schiavello, Ed., Photocatalysis and Environment, Trends and Applications, NATO AS1 Series, Kluwer Academic Publishers, Boston, 1988. D. F. Ollis, E. Pelizzetti, and N. Serpone, “Heterogeneous Photocatalysis in the Environment: Application of Water Purification,” in Photocatalysis, Fundamentals and Applications, Wiley, New York, 1989, p. 603. H. Yoneyama, S. Haga, and S . Yamanaka, J . Phys. Chem. 93,4833 (1989). J. H. Stachka and F. W. Pontius, J . Am. Waterworks Assoc. 76, 73 (1984). D. Rumelhart and J. McClelland, Parallel Distributed Processing: Explorations in the Microstructure of Cognition, Vol. 1, MIT, Cambridge, MA, 1986, Chapter 8. N. V. Bhat, P. A. Minderman Jr., T. McAvoy, and N. S. Wang, IEEE Control Systems Magazine 4, 24 (1990). N. V. Bhat and T . McAvoy, Comp. Chem. Eng. 14, 573 (1990). C. A. 0. Do Nascimento, E. Oliveros, and A. M. Braun, to be published.

Advances in Photochemistry, Volume18 Edited by ,David H. Volman, George S. Hammond, Douglas C. Neckers Copyright © 1993 John Wiley & Sons, Inc.

PHOTOCHEMISTRY OF THE XANTHENE DYES* Douglas C. Neckers and Oscar M. Valdes-Aguilera Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio

CONTENTS I. Introduction 11. Fluorescein A. Background B. Spectroscopy 111. Eosin A. Background B. Eosin as a photoinitiator IV. Erythrosin V. Rose bengal A. Background B. Absorption and emission spectra C. Chemical reactivity 1. Substitution at C-6 2. C - 2 substitution 3. Effects of substitution on spectra: Solvent effects 4. Aggregation effects

316 317 317 321 323 323 326 3 50 351 351 353 354 354 356 356 357

*Contribution # 144 from the Center for Photochemical Sciences. Advances in Photochemistry, Volume 18, Edited by David Volman, George S. Hammond, and Douglas C. Neckers ISBN 0-471-59133-5 Copyright 1993 by John Wiley & Sons, Inc.

315

316

D. C. NECKERS AND 0. M. VALDES-AGUILERA

D. Quenching of rose bengal singlets and triplets: Energy transfer E. Dismutation by electron transfer: Dye oxidation and dye reduction F. Decarbox y lation G. Photobleaching and photofading H. Pho topolymerization I. Polymeric rose bengals J. Photobiological applications VI. Hydroxyfluorones VII. Summary Glossary Acknowledgments References

357 360 361 370 371 37 1 373 376 379 379 380 38 1

I. INTRODUCTION The xanthene dyes are among the oldest and most commonly used of all synthetic dyestuffs. As a class they include the fluorenes or aminoxanthenes, the rhodols or aminohydroxyxanthenes, and the fluorones or hydroxyxanthenes. This chapter would become far too long if we included all classes of xanthenes, which would include the laser dyes, such as Rhodamine 6G, so we have arbitrarily limited our coverage to the hydroxyxanthenes and, more specifically, to fluorescein, Eosin, Erythrosin, and Rose Bengal and certain new derivatives of these synthesized recently in our laboratories. In the older literature the hydroxyxanthenes are also referred to as fluorans. A huge number have been synthesized [l]. The general structures trace to Baeyer [Z], who synthesized fluorescein along with phenolphthalein and gallein from ZnC1,-catalyzed condensation with resorcinol, phenol and pyrogallol respectively. In 1877 Baeyer dumped fluorescein (10 kg!) into the headwaters of the Danube to trace the connection of the Danube to the Rhine through the Alps. Sixty hours later when the characteristic fluorescence of the dye was observed in a small river having no obvious connection to the Danube, but feeding Lake Constance which, in turn, flows into the Rhine, the existence of the connection between the Danube and the Rhine was confirmed. To this day, “trace water streams” remains a use in the Colour Index of Baeyer’s fluorescein, and the EPA uses it to trace waste streams. The preparation and characterization of Eosin, tetrabromofluorescein, constituted the work Emil Fischer presented for his Ph.D. dissertation [3] at Strasbourg under Baeyer’s

PHOTOCHEMISTRY OF THE XANTHENE DYES

317

direction in 1871. Baeyer and Fischer were sent to Strasbourg in 1871 at the conclusion of the Franco Prussian War to start a chemical laboratory at the university. Baeyer returned to Berlin in 1875; Fischer assumed the chair in Munich in 1874. An immediate subject of controversy between Baeyer and his own mentor, Hoffmann [4], was generated when a sample of Fischer’s first preparation of Eosin was sent to Car0 in Ludwigshafen for evaluation as a dye. (It was used as a dye for cloth for a while, but, like fluorescein, was not a very good one.) A sample had been sent to Hoffmann’s laboratories in Berlin by Martius; Hoffmann proved the structure to be tetrabromofluorescein and published his results. This prompted Baeyer’s claim that Hoffmann had the history of Eosin wrong, since Fischer was its real progenitor. Bayer’s paper was entitled “Zur Geschichte des Eosins.” The structures of a phthalein, phenolphthalein, and two xanthenes, fluorescein and Eosin Y, are shown along with three rhodamines, for comparison, in Figure 1. (We capitalize the names of Eosin, Rhodamine, Rose Bengal, and Erythosin because they are trade names. The evolution of the word fluorescence, hence fluorescein, is uncertain.) Baeyer’s papers on the structure of phenolphthalein [51 are clearly among the intellectual landmarks of this early period, and from them developed an entire thought process of reasoning by analogy [6].

1

Xanthene (Gr., yellow) (1) is the parent heterocycle of fluorescein and its derivatives [7]. The compounds to be discussed in this chapter, plus some related ones, are given in Table 1. The Colour Index is the source of the early, definitive information on dyestuffs. Chemical Abstracts uses it as its chief reference source for the chemical constitution of trade-named dyes.

11. FLUORESCEIN IS] A. Background

Fluorescein is still made commercially by the Baeyer synthesis and is very useful because of its fluorescence properties. Called uranine as the disodium salt, it is a yellow dye of low tinctorial power. Commercial suppliers call the

Phenolphthalein

\

I&a+

Nat; 0

/

I

Eosin

Fluorescein

0

,&a+ \

\ I

I

0

Nat,’O

/

I

Erythrosin

\

0

\ I

L

I

Rose Bengd

H3C

H

H

Rhodamine B

Rhodarnine 6G

H

H

Rhodamine 123 Figure 1. Examples of the most common xanthenes.

318

o

W, \o

Br H C1 H

I

Br

C1

I Br I Br

X

~

I Br I Br CI Br H H C1 H H

Y

C1 C1 H H H Br H H H H H

Z

~

-~

~~

549.0 538.5 526.5 517.0 510.0 540.0 506.5 504 502.5 502.5 490.0

nmin H,O

~

I,,, __ ~

513 510 51 1 51 1 499

557.0 547.5 532 523 520

nminEtOH

L

~

0.33 0.32 0.04 0.04 0.03

0.86 0.40 0.69 0.32 0.05

EtOH

dJb

0.48 0.42 0.07 0.07 0.03

0.75 0.65 0.63 0.57 0.05

-

H,O

~

~

3.69 4.38

-

~

3.75

~

3.72 3.69 4.18 3.80

pK,‘

4.9 1 6.99

4.75

-

-

-

pK,

4.9

5.1 5.3 1.8

535 535 555

528

1.5

10:Fd

560

I,

560

560 600 600

575

AT-T

0.9

5.0 2.7 2.6

1.2

I($T

400

430 400

1 2 4

TR

“Most of the data in this table is from E. Gandin, Y. Lion, and A. Van de Vorst, Photochem. Photohiol. 37, 271 (1983). bSinglet oxygen formation. ‘Data from I. M. Issa, R. M. Issa, and M. M. Ghoneim, 2. Phys. Chem. 250, 161 (1922). dLifetimesare in seconds; measurements are in 0.1 N KOH. This data is from A. W. Elcov, N. P. Smirnova, A. I. Ponyeva, W. P. Martinova, R.Schutz, and H. Hartmann, Luminescence, in press. By permission of the author. ‘Eosin Y stands for Eosin Yellowish and it is the disodium salt of tetrabromofluorescein. There are a number of other fluorescein derivatives which are also referred to as Eosins. Eosin B or bluish is 4,5-dibromo-3,7-dinitrofluorescein.The investigator is best advised to buy dyes on the basis of structure rather than name. In the case of the other dyes in the table the literature confusion is less imposing.

Fluorescein

Rose Bengal Phloxin Erythrosin Eosin Yd

~~~

Compound

TABLE 1 The Xanthenes”

320

D. C. NECKERS AND 0. M. VALDES-AGUILERA

lactone fluorescein. The disodium salt, uranine, is sometimes called “water soluble fluorescein.” The disodium salt is the fluorescent dye. The lactone has only UV absorption and the concomitant fluorescence. The Chemical Abstracts name for fluorescein is spiro[isobenzofuran-1(3H),9‘-[9H]xanthen3-one,3’6’hydroxy-.This name derives from the lactone form, rather than the more common quinoid form, of the dye. In editions of Chemical Abstracts prior to 1971, fluorescein, disodium salt was also given as an alternative name. In either case, “fluorescein” is a descriptive rather than a systematic name; and spiro[isobenzofuran-1(3H),9’-[9H]xanthen-3-one,3‘6’hydroxy-is such a long name that it’s essentially useless save in Chemical Abstracts indices, so we have chosen the convention of using simpler rather than systematic names throughout this chapter. Biologists find fluorescein useless as a stain [9], and it is useless as a dye for cloth because it is fugitive-it does not stick. It remains, however, one of the most fluorescent of all known organic compounds, its yellow-green fluorescence being detectable at very high dilution. This fluorescence is even more pronounced under ultraviolet light, which also makes it useful in fluorescence microscopy. Fluorescence with visible radiation gives rise to fluorescein’s principal uses as an optical brightener, for fluorescence depolarization immunoassay, and as a molecular probe. The crystal structure of the acetone complex of fluorescein lactone has been reported [lo]. Many derivatives of fluorescein containing a reactive group at the C-5’ position are commercially available [111. Fluorescein isothiocyanate, for example, is widely used as protein tag [12]. These substances have essentially the same spectroscopic properties as the parent compound with the additional capability of binding covalently to proteins. Because of their high emission quantum yields, fluorescein conjugates are extensively used as tracers for microinjection in living cells to gather information on the structure and function of cells, localization of proteins, and cell-to-cell and intracellular diffusion [13- 171. The xanthenes exist in solution in several different forms depending on pH, as shown in Figure 2 and Table 1 [18]. The emission quantum yield of fluorescein depends on the acidity of the solution, the fluorescence intensity decreasing as the protonated forms of the dye come to predominate with decreasing pH. This pH sensitivity allows fluorescein derivatives to be employed as pH indicators, to measure the pH inside living cells [19-221, at water-lipid interfaces [23], and in the interior of phospholipid vesicles [24]. The sensitivity of fluorescein emission to the pH of the medium has also been used to measure lateral proton conductances at water-lipid interfaces [25281 and proton translocation across phospholipid vesicles [29] and to determine the electrostatic potential of macromolecules [30,3 11. The pheno-

PHOTOCHEMISTRY OF THE XANTHENE DYES

321

H

H

H

H

Figure 2. Acid base equilibria-fluorescein.

lic group at C-6 is much more acidic than the carboxylic acid function at C-2' (pK, vs. pK,) (Table 1).

B. Spectroscopy The absorption and emission spectra of the hydroxyxanthenes are strongly solvent dependent [32]. Under conditions where the only species present is the dianion (Figure2), the I,,, of fluorescein shifts from 484nm in CF,CH,OH ( E = 81,000) to 520nm in DMSO ( E = 113,000) and the fluorescence maxima shift similarly from 500 to 550 nm. The quantum yields of fluorescence decrease from unity to 0.60, though there is little apparent change in the radiative lifetimes. The shift in absorption maximum to shorter wavelengths with the hydrogen bonding capacity of the solvent indicates that the ground state is more stabilized by hydrogen bonding than is the excited state. Like all phenols, fluorescein is a stronger acid in the excited state than in the ground state [33]. The emission of fluorescein is strongly self-quenched at higher concentrations ( > M) of the dye. Thus the yellow-green fluorescence gives way to a dull orange solution at high concentrations. Transport of solutes across a membrane is accompanied by a change in volume. Thus Chen et al. [34] and

322

D. C. NECKERS AND 0. M. VALDES-AGUILERA

Kim et al. [35] have studied the kinetics of transport across membranes by measuring the variation in emission intensity of the entrapped fluorophore. Polarization of fluorescein fluorescence provides useful information on the rotational mobility of the fluorophore. Binding to a macromolecule decreases mobility of the fluorophore and it is possible to differentiate between bound and free fluorophore by measuring the depolarization of fluorescence that results. The fluorophore is thus used as a probe for macromolecular binding. This is the basis for Fluorescence Depolarization Immunoassay, a diagnostic technique which, along with other optical methods, is rapidly replacing Radioimmunoassay in clinical applications. In the technique the difference in the polarization of fluoresced light of a fluorescein analogue functionalized with an antitoxin for the drug is quantitated as a function of the concentration of drug in serum. This can lead to a direct measure of the amount of drug in the serum at picomolar concentrations. Steady-state measurements of the fluorescence anisotropy of fluorescein derivatives form the basis of a sensitive analytical technique also used to detect and quantitate proteins [36], steroids [37-391, therapeutic drugs, and narcotics [40-421. In a different approach, the anisotropy of the fluorescein conjugate is measured as a function of the medium viscosity to determine the segmental mobility of the chains that cover the binding site [43-451. Conformational changes of a protein to which a fluorophore is bound alter the microenvironment of the probe and can be followed by measuring the changes in the fluorescence. Thus spectral shifts, changes in the emission intensity and singlet lifetimes have all been employed to study the conformational changes produced as a consequence of protein-protein association [46], association of receptors to cells [47], ligand binding [48-521, and ternary complex formation [53]. Fluorescein-labeled proteins are also used to measure the translational mobility of proteins and lipids by the Fluorescence Recovery After Photobleaching technique [54-591. The uniformly labeled fluorescent sample is flashed with an intense light source to bleach a spot, thus producing a concentration gradient. The rate of recovery of fluorescence in that bleached area is measured and used to calculate the diffusion coefficient of the probe dye into the bleached zone. Such diffusion coefficient measurements have been used to determine the association constants of proteins in cells [60], to measure the exchange of tubulin between the cytoplasm and the microtubules [61,62], to study the polymerization-depolymerization process of actin [63651 and to monitor the changes that occur upon cell maturation [66,67]. Fluorescein is an energy acceptor for chromophores such as naphthalene and anthracene and acts as energy donor toward Eosin and Rhodamine, so derivatives have been used for singlet-singlet energy transfer studies. According to Forster’s theory [68] the rate constant for energy transfer increases

PHOTOCHEMISTRY OF THE XANTHENE DYES

323

with the overlap between the donor emission and acceptor absorption spectra, and decreases with the sixth power of the spatial separation of the donor and acceptor. Thus, provided that energy transfer occurs by the Forster’s mechanism, the distance between donor and acceptor can be estimated. Recent studies using fluorescein derivatives include the determination of intramolecular distance between binding sites [69-761 and between active sites and the surface of the membrane [77,78], study of the intercalation of ethidium in RNA [79], and use of energy transfer for determination of human lactoferrin [SO]. The kinetics of fusion of vesicles [Sl], polymerization of myosin, and the rate of exchange between filaments [82,83] have been followed in situ by monitoring the efficiency of energy transfer as a function of time. The changes in energy transfer efficiency observed upon addition of electrolytes to a solution of histone H4 have been attributed to an electrolyteinduced conformation change of the enzyme [84]. Energy transfer measurements have been performed to study aggregation of proteins [85-871 and the formation of polymeric clusters of protein complexes in membranes [88]. By measuring the efficiency of energy transfer as a function of protein concentration, with appropriate control experiments, Podgorski et al. [89,90] obtained the equilibrium constants for association of spectrin with band 4.1 and band 2.1 proteins in aqueous solution.

111. EOSIN A. Background

Eosin, tetrabromofluorescein, derives its name from EOC (eos-Gr., dawn). It has been used since the 19th century as a histological stain, being employed to observe certain components of the leucocytes (Eosinophiles), and as a reagent for the spectrophotometric determination of silver [9l]. Its photochemistry is of particular interest because its maximum absorption is near the lLemissions of the Ar(+) laser. The ionization of Eosin in solvent mixtures of differing compositions demonstrates, as do the other xanthenes, the effect of hydrogen bonding (see Figure 2) [92]. Several derivatives of Eosin have been prepared and employed to study biological systems. Their main applications are as singlet energy acceptors and as triplet probes [93-971 to measure the rotational mobility of virus particles [98] and proteins in membranes and in solution. Examples of proteins studied using Eosin derivatives include myosin [99,100], band 3 protein [lOl, 1021, pyruvate dehydrogenase [103,104], and Sarcoplasmic

324

D.C. NECKERS AND 0. M. VALDES-AGUILERA

Reticulum ATPase [l05, 1061. Owing to the long-lived nature of the triplet state, Eosin derivatives are suitable to study protein dynamics in the microsecond-millisecond range. Rotational correlation times are obtained by monitoring the time-dependent anisotropy of the probe’s phosphorescence [107-1121 and/or the recovery of the ground state absorption [1131181 or fluorescence [119-1221. The decay of the anisotropy allows determination of the mobility of the protein chain that cover the binding site and the rotational diffusion of the protein, the latter being a function of the size and shape of the protein, the viscosity of the medium, and the temperature. Eosin triplet is efficiently quenched by oxygen and its lifetime depends on the access of oxygen and other quenchers to a given binding site. Therefore, Eosin triplet lifetime is sensitive to both the proximity of the dye to the bulk medium and to the flexibility of the protein chains that cover the binding site wherein quenchers might reside. It is then possible, by measuring the triplet lifetime, to discriminate between binding sites at the protein surface, or within a flexible protein, and those inside a rigid protein matrix [123,124]. Eosin generates singlet oxygen upon irradiation with visible light. Its ability to act as a photodynamic sensitizer has been known for many years [125]. Irradiation of Eosin in the presence of oxygen is found to deactivate viruses [126] and yeasts [127] and to damage the photosystem I1 of leaf tissue [128,129]. Eosin covalently immobilized in polystyrene beads has been used to remove E. coli from drinking water [130]. The localization of the sensitizer has a significant effect on the photodynamic activity. Eosin is scarcely taken up by cells, and Pooler and Girotti [131] report that Eosin isothiocyanate is 50-100 times more efficient for inducing photohemolysis of human erythrocytes. The spectral properties and quantum yields for singlet oxygen production are identical for both compounds. Therefore, they attribute the difference in photodynamic activity to the ability of the isothiocyanate derivative to bind covalently to band 3 protein. Singlet oxygen produced by the triplet state of the sensitizer is commonly assigned as the exclusive toxic intermediate during photodynamic inactivation. However, Kepka and Grossweiner [132] present evidence that photoinactivation of lysozyme occurs by both singlet oxygen-dependent and independent mechanisms. Electron transfer to Eosin triplet from tryptophan and other amino acids has been studied in detail [133-1351, charge separation being evidenced by the appearance of semireduced Eosin. Furthermore, fluorescence measurements and flash photolysis of the lysozyme-Eosin complex show that both the singlet and triplet states of Eosin are quenched by complex formation with the enzyme [136]. Koloczec and Vanderkooi [137] studied the structure flexibility of

PHOTOCHEMISTRY OF THE XANTHENE DYES

325

rhodanese in aqueous solution by measuring the phosphorescence quenching of the Eosin label. Using quenchers of different sizes, iodide ions, thiosulfate, methylvinyl ketone, and the spin label reagent 2,2,6,6-tetramethyl-lpiperidinoloxy (TEMPO), they were able to determine the amplitude of the motion for the protein conformational changes. Determination of translational diffusion rates of proteins requires measurements at longer timescales, one-tenth of a second to several minutes. Eosin derivatives are also commonly used to measure translational diffusion coefficients using the Fluorescence Recovery After Photobleaching technique [138-1411. Eosin emission characteristics depend strongly on the solvent. Specifically transfer from aqueous solution to a nonaqueous solvent shifts the emission of Eosin toward longer wavelengths and increases the emission intensity. Wang and Cheung [142] have used the fluorescence enhancement of the Eosin label to study the association of troponin I with troponin C. Similarly Skou and Esmann [143] and Helmich de Jong et al. [144] have used Eosin itself as a fluorescent probe to study the conformational changes of enzymes involved in ionic transport. The photochemistry of Eosin under both reductive and oxidative conditions has been studied by several groups [145-1511. Photoreduction by amines such as tribenzylamine ( R = CH,$, R” = 6) produces two leuco analogues, the dihydro derivative, and the cross-coupled product formed from the amine radical and the dye radical anion (2) [152]. In addition, debromination of Eosin is reported during photobleaching with amines and phenols. The reader however is referred to the extensive studies of Rose Bengal dehalogenation by Paczkowski and Neckers [1531. Radiolysis of Eosin in methanol shows that debromination is a consequence of the photochemical decomposition of semireduced Eosin [1541.

br

I r

2

Photosensitized electron injection in colloidal TiOz has been reported by Moser and Gratzel as part of a scheme to photoreduce water [155]. Moser et al. [156] and Rosetti and Brus [157] saw the formation of semioxidized Eosin by monitoring its absorption in the visible and by Raman spectroscopy. No oxidation of the triplet of Eosin adsorbed in TiO, is observed, the lifetime

326

D. C. NECKERS AND 0. M. VALDES-AGUILERA

being the same as in pure water. Electron injection occurs exclusively from the excited singlet with a rate constant of 9.5 x lo8 s - l at pH 3 and 25°C.

B. Eosin as a Photoinitiator Electron transfer to and from the xanthenes and other dyes has been employed for a number of years to initiate polymerization of a variety of monomers in solution. The excited state of Eosin serves both as an oxidizing agent [l5S] and a reducing agent [159]. Though generally used for acrylate polymerization [1601, it has also been used-with ascorbic acid-for vinyl acetate [1611, with triethanolamine and fumed silica for unsaturated polyesters [162,163], alone with acrylonitrile [164], in combination with UV initiators (where it is claimed to sensitize in the visible) and aromatic sulfides for acrylates [165], pressure-sensitive adhesives [166], and acrylonitrile [167]. In combination with methylene blue, refractive index patterns have been recorded by means of a He-Ne laser into thin PMMA films with styrene as the polymerizable monomer [168]. In color imaging systems Eosin is used as a green (530 nm) sensitive charged, photoconductive toner [169], as well as in Fourier imaging systems [ 1701. Reports on the kinetics and mechanisms of photopolymerization are numerous as well as controversial. One of the issues of substantial disagreement is the effect of oxygen on the rate of the reaction. For example, Chaberek and Allen [171] found oxygen to be an inhibitor at any level, whereas Yang and Oster [172,173] report the polymerization will not proceed in carefully degassed solutions and a small amount of oxygen is required for polymer formation. At the core of the discussion is whether the initiating radicals are produced in the photochemical step from the dye (D; (Eq. 1) or whether the photochemical reaction forms a product, such as the leuco dye (DH,), that reacts with oxygen to generate the initiating radicals (Eqs. 2 and 3).

D D

+ reducing agent + hv-

+ reducing agent + hvDH,

+ 0,-

radicals

(1)

leuco dye (DH,)

(2)

radicals

(3)

Oster [174] proposed the second hypothesis to explain his results on the photopolymerization of acrylonitrile in aqueous solution, buffered at pH 7.0, and sensitized by xanthene dyes and riboflavin using ascorbic acid as the reducing agent. Whereas the monomer is efficiently polymerized when the solution is illuminated in the presence of oxygen, irradiation in its absence leads to photoreduction of the dye to its leuco form but no polymer is formed. Therefore, the author suggests that the leuco dye reacts with atmospheric

PHOTOCHEMISTRY OF THE XANTHENE DYES

327

oxygen to generate hydroxyl radicals that can initiate polymerization (Eqs. 4 and 5).

+ 302-

DH, OH'

DH'

+ OH'

+ monomer --+polymer

(4) (5)

Evidence supporting this mechanism is presented for the case of acrylamide polymerization sensitized by riboflavin, but not for the case of fluorescein and its halogenated derivatives. Irradiation with a millisecond flash in the presence of air leads to polymer formation after an induction period of one hour. In contrast, when the irradiation is carried out with degassed solutions, polymerization starts only after the sample is exposed to atmospheric oxygen. Pemberton and Johnson [175,176] have performed a thorough study of the photopolymerization of vinyl acetate in aqueous methanol sensitized by ethyl Eosin and ascorbic acid. In agreement with Oster's results, these authors find that polymer is not formed in the absence of oxygen. In addition, dye photobleaching without polymer formation is observed when water is excluded. Polymer formation starts after an induction period that depends on the light intensity and ascorbic acid concentration. During this period the dye is completely bleached at a rate that varies linearly with light intensity. The rate of polymerization, measured after the induction period, is independent of light intensity and dye concentration, indicating that the initiating species is not produced in the photochemical step. They detect hydrogen peroxide as a product formed during dye photobleaching and propose that the initiating radicals are generated by reaction between hydrogen peroxide and excess ascorbic acid. Control experiments show the system hydrogen peroxideascorbic acid is an efficient initiator of vinyl acetate polymerization. In contrast to the proposal by Oster that radicals are produced by oxidation of the leuco dye, Pemberton and Johnson propose that hydrogen peroxide is formed by oxidation of the semireduced Eosin radical, as shown in Eqs. (6)-(10) (AH, is ascorbic acid; AH' is the semioxidized ascorbic acid radical), and that the initiating radicals are produced by thermal reaction D

+ AH, + hv-

DH'

DH'

DH,

+ AH'---+

DH'+O,-HO;+D HO; DH'D

+

2HO;-

+ AH'

+ dehydroascorbic acid

+ H202 H 2 0 2+ 0,

(6) (7) (8) (9)

(10)

328

D. C. NECKERS AND 0. M. VALDES-AGUILERA

between hydrogen peroxide and ascorbic acid. Our examination of the reported values for the induction period reveals that its reciprocal is proportional to the square root of the light intensity (Figure 3). This intensity dependence, combined with the known mechanism of bleaching for the xanthenes [177], provides support for the suggestion that the initiating radicals are generated as shown above rather than by oxidation of the leuco dye. In particular, if Eqs. (8)-(10) are replaced by Eq. (ll), the rate of DH, generation and the reciprocal of the induction period are predicted to increase linearly with light intensity. DH,

+ 02+

radicals

(11)

A more conventional mechanism appears to be operative in the photopolymerization of ethyl acrylate 11781 and methyl methacrylate [179] in aqueous solution, sensitized by fluorescein and Erythrosin, respectively. Ascorbic acid is the reducing agent in both cases and it is observed that the reaction does not proceed in the absence of buffer, usually phosphate buffer pH 6 . Polymer formation starts after an induction period but its dependence on light intensity and ascorbic acid concentration has not been determined. The rate of photopolymerization is proportional to the monomer concentration and to the square root of the light intensity, dye, and ascorbic acid concentration. The authors report the order with respect to the monomer as 3/2. However, from our analysis of the data for fluorescein, which are more

I

.oe-2 8.0e-3 -

4.0e-3 6.0e-3

0.0

0.2

0.4

0.6

(Relative I)

0.8

1:0

0.5

Figure 3. Variation of the reciprocal of the induction period with light intensity for the polymerization of vinyl acetate in aqueous methanol. Data from D. R. Pemberton and A. F. Johnson, Polymer 25, 536 (1984).

PHOTOCHEMISTRY OF THE XANTHENE DYES

329

extensive than for erythrosin, we conclude that the rate of photopolymerization increases linearly with the monomer concentration. These results indicate that the initiating radicals are generated in the photochemical step and that the termination reaction is bimolecular. The lack of polymerization observed in the absence of buffer is explained by postulating the formation of a complex between the buffer and ascorbic acid. The complex reacts with the excited state of the dye to produce the radicals (Eqs. 12 and 13).

+ buffer t-l(AH,-buffer) R' + DH' + buffer D(T) + (AH,-buffer)AH,

(12) (13)

However, no independent evidence is presented for the existence of this complex. In our view a simpler explanation for the lack of reaction in unbuffered solution is that the nondissociated form of ascorbic acid is a chain terminator. At the typical concentrations employed, 1-10 mM, more than 90% of ascorbic acid (pK = 4.1) is not dissociated when dissolved in pure water. At pH 6.0 only 1% is present as the protonated form. Furthermore, in their study of the photopolymerization of methyl methacrylate initiated by acriflavine-ascorbic acid, Lenka and Mohanty [180] report the rate of polymerization reaches a maximum when the ascorbic acid concentration is approximately 10 mM. The decrease in polymerization rate at higher concentrations suggests ascorbic acid participates in chain transfer and/or termination reactions. Photoreduction of Eosin, Erythrosin, and Rose Bengal with amines has been employed for a number of years to initiate polymerization of monomers in the absence of solvent. Examples of recent applications include the production of volume holograms [l8l], color images on plain paper [182], printing plates [183-1851, and, in the presence of suitable peroxides, manufacture of coatings for glass fibers [186]. Photooxidation of Eosin with periodate ion has been used to initiate the polymerization of acrylonitrile in aqueous solution [1871. Addition of acrylonitrile to a periodate solution shifts the absorption maximum from 220 to 280nm. This spectral change is interpreted as being due to complex formation between the monomer and oxidizing agent. The rate of photopolymerization increases linearly with the absorbed light intensity and monomer concentration. The observed intensity dependence indicates the main chain terminator is not produced photochemically. Polymer is not formed when the concentration of periodate ion is lower than 0.5 mM and the rate of polymerization is independent of its concentration for higher values.

330

D. C. NECKERS AND 0. M. VALDES-AGUILERA

The mechanism suggested by the authors is shown in Scheme 1.

-

+ HJO; ?==?.complex D + hv D(S) D(T) OH-M' + D" + H2104 + H 2 0 Complex + D(T)M

M;+M-M; Mi-l Mi

+ M+

+ complex-

Mi polymer

+ HJO; + H 2 0

Scheme 1.

The complex formed between the monomer and oxidizing agent acts both as the source of initiating radicals, by reacting with the triplet of the dye, and as a chain terminator. In both cases the net effect is transfer of a hydroxyl group. The monomer concentration employed is typically 300 times higher than periodate ion and the concentration of free periodate ion is too low to participate in the termination reaction. The IR spectrum of the isolated polymer shows a strong absorption at the O H stretching frequency (3600 cm- I), providing support for the proposed mechanism. Photopolymerization of multifunctional monomers occurs rapidly under UV irradiation, even in the presence of oxygen, to form highly crosslinked insoluble polymers. These reactions have found many industrial applications [1881, including the production of fast drying protective coatings, printing inks, and adhesives. Lasers have played an important role in the continuing search for faster polymerizing systems and in the use of photopolymerization in the fields of microelectronics, holographic recording, and the reproduction of three-dimensional objects. The high intensity of the laser allows completion of the photopolymerization in the millisecond timescale and the small beam diameter makes it possible to write on photosensitive plates with high spatial resolution. Extension of laser initiated polymerization to the visible range depends on the existence of sensitizers which absorb at the wavelengths of available lasers as well as a chemical reaction able to produce free radicals from the sensitizing dye. One such system, sensitive to the wavelengths of the Ar' laser, is the photoreduction of Eosin. The photopolymerization of pentaerythritol triacrylate sensitized by Eosin and several amines has been studied by Chesneau and Fouassier [189]. Samples containing the dye, amine, and UV initiator which consumes oxygen to produce a tack free surface are irradiated in thin films w was detected with superoxide dismutase and an oxygen electrode in water in the presence of sulfite. Rodgers and Lee later confirmed

331

PHOTOCHEMISTRY OF THE XANTHENE DYES

this result quantitatively [259] in water where 25% superoxide was observed. Superoxide formation may also require something other than direct electron transfer from Rose Bengal triplet, though this is surely not the case in the time-resolved measurements. Under the conditions used in the time-resolved experiments, Rose Bengal triplet produces 75% singlee bond conversion at the end of the exposure is approximately 25%. Examples of UV photoinitiators employed as additives are alkyl ethers of benzoin, benzophenone, thioxanthones, and the oxime ester (PDO, 3). Of course most of these UV photoinitiators decrease the rate of photopolymerization sensitized by Eosin and N-methyldiethanolamine (MDEA) likely by serving as radical traps. The exception is PDO which, at a concentration of 3%, increases the photospeed by a factor of 2.

0-C -OC,H, II

0

3

The photospeed increases linearly with incident light intensity (Figure 4). The linear relationship between the photospeed and the incident light intensity is explained by considering that in this viscous monomer the main termination mechanism is radical occlusion instead of bimolecular reaction between macroradicals. We will comment more extensively on the intensity 1.1

0.83

0.55 028 0.0 0

50

100

150 2M) I (mWlcm2)

250

300

Figure 4. Intensity dependence of the photospeed. Eosin (OD=0.3 at 488 nm), 10% PDO, 5% MDEA, pentaerythritol triacrylate. Data from E. Chesneau and J. P. Fouassier, Angew. Makromol. Chem. 135, 41 (1985).

332

D. C. NECKERS AND 0. M. VALDES-AGUILERA

dependence when we describe our photopolymerization results on a similar system. The effect of the structure of the coinitiator is studied using 3% PDO and 5% electron donor. The measured photospeed increases in the order NN,N dimethylbenzylisopropylamine < dibenzylamine < triethylamine amide < N-benzylethanolamine < MDEA. However, a reactivity order for the electron donors can not be inferred from these data because the molar concentration is not the same for the different coinitiators. As shown for MDEA in Table 2, the variation of the photospeed with the amine concentration is not simple. The photospeed increases with increasing amine concentration up to 5%, higher concentrations resulting in lower polymerization rates. At the high MDEA concentration employed, quenching of Eosin singlet state becomes important, and part of the observed inhibition is certainly due to nonreactive quenching of the singlet. Amines are known to quench the fluorescence of xanthene dyes with an efficiency that increases with decreasing ionization potential of the amine [190]. However, the rate of photoreduction of Eosin [191] and Rose Bengal [192] by amines in solution starts decreasing at amine concentrations high enough to quench more than 10% of the fluorescence intensity. Therefore, although the dependence of quenching efficiency on the ionization potential suggests the dye singlet is quenched by a charge transfer reaction, it appears that radicals are not generated because of incomplete charge transfer or the singlet radical pair (Dye'- Am") decays preferentially by black electron transfer to the original reactants. Our work on the photopolymerization of multifunctional monomers has been concentrated on the development of faster initiators to be used in the

-

TABLE 2 Effect of MDEA Concentration on Photospeed" MDEA

(%I

Relative Photospeed

1

0.33

5

1.00 0.50 0.33

3

10 15

0.67

"Eosin (OD=O.lS at 488nm); 10% PDO, pentaerythritol triacrylate; I,,,= 488 nm, I=2S W cm-', Ar* laser. Data from E. Chesneau and J. P. Fouassier, Angew. Makrornol. Chem. 13541 (1985).

PHOTOCHEMISTRY OF THE XANTHENE DYES

333

reproduction of three-dimensional objects with the recently developed technology of stereolithography. Stereolithography is a new technology linking the power of computer graphics to the rapid formation of a solid, shaped object. Patented originally by Chuck Hull [193] and assigned to 3D Systems, stereolithography converts CAD/CAM/CAE generated solid or surface model data to a three-dimensional real part synthesized, via photopolymerization, from a photosensitive monomer such as an acrylate. Cured by a laser beam directed across an x,y surface, a liquid monomer mix is converted to a solid plastic object, point by point, layer by layer, true as allowed by the photopolymer in all three dimensions x, y , and z (Figure 5). Stereolithography is simple in concept and it provides great economies for the design lab as well as for the modeling process. It also provides previously unrecognized challenges for the polymer photochemist, for it is entirely a laser-initiated technology, and the polymerization reactions take place to depths below a finitely thin surface layer. Initiator systems used to initiate photopolymerization with UV light generate radicals mostly by homolytic dissociation. Ketone acetals (e.g., Irgacure 651) absorb in the mid-range UV, dissociating to a benzoyl radical which initiates acrylate polymerization. The benzoyl group remains as part of the formed polymer (Eqs. 14 and 15).

-

monomer

elevator

~~

STEREOLITHOGRAPHY APPARATUS (SLA)

Figure 5. Diagram of the stereolithography apparatus.

334

D. i. 7;ZCKERS AND 0. M. VALDES-AGUILERA

Ultraviolet lasers such as the He-Cd laser used in 3D Systems' SLA 1 and SLA 250 have a number of drawbacks. The power available is somewhat limited and the lifetime of the laser tube tends to be uneconomical. Since high power, stable visible lasers are available and convenient, targeting stereolithographic systems to them is an obvious goal. In recent years visible photoinitiators for the formation of polymers via a radical chain reaction have also been developed. These absorb light which is blue, green, or red and also cause the polymerization of polyolacrylates, in some instances, such as encapsulated systems, with speed which is near photographic. Some of these photoinitiators provide the photochemical backbone of the nonsilver, near-photographic speed, imaging processes such as the CycolorTMprocesses invented by the Mead Corporation. CycolorTM initiators are cyanine dye, borate ion salts (4)-so-called (+,-) ion pair

4

initiators-which utilize single electron transfer to the excited singlet state of the light-absorbing dye from the gegen or partner ion as the initial step. Thus the cyanine dye, in its excited state, accepts an electron from the partner borate, producing a neutral boranyl radical (Ar,BR'), which subsequently fragments, producing an alkyl radical (R') which is the real polymerization initiator. Other initiators active in the visible have been invented by us. These are xanthene dye, onium salt (-,+), ion pair photoinitiators (e.g. 5, a xanthene bis iodonium salt), which function by oxidation of the excited state of the dye rather than by reduction.

5

PHOTOCHEMISTRY OF THE XANTHENE DYES

335

As part of our interest in visible light photoinitiators, we have been particularly interested in developing systems which are active at the emissions of convenient lasers such as the argon ion laser (488 and 514 nm) or the helium neon laser (632 nm) and which also allow the formation of thick films. In the process of studying photoinitiating systems based on Eosin and other dyes, it became apparent that a previously unrecognized component of the photoinitiation process, namely the photobleaching of the dye initiator, has an extraordinary impact on the overall polymerization process when lasers of moderate intensity are used as the initiating radiation source. The photobleaching of the dye initiator increases the laser penetration depth. As a consequence, polymer formation is not constrained to a thin film near the surface and the reaction can occur to a depth which is controllable by the dye concentration, irradiation time, and laser power (Figure 6). In stereolithography the positions of polymerization x and y are controlled by a mirrored scanner which reflects the laser onto the surface of the to-be-polymerized monomer at a point x, y and the z dimension is determined by the position of the elevator, as shown in Figure 5. During the polymerization z, the depth to which reaction occurs, is held constant through the use of a UV photoinitiator that bleaches either not at all or very slowly, relative to the rate of polymerization. Our use of bleachable photoinitiators to carry out polymerization at depth opens the possibility of controlling the vertical dimension photochemically rather than mechanically. We have used the photoreduction of Eosin by triethanolamine to sensitize the polymerization of multifunctional acrylates to demonstrate the principle. Irradiation is carried out at 514 nm with an Ar+ laser having a beam diameter of 1.4 mm. The volume of sample irradiated is a small fraction of the total, simulating the conditions found in stereolithography. Because of bleaching of the photoinitiator, the irradiation generates

polymerized solid object

Figure 6. Polymerization at depth.

336

D. C. NECKERS AND 0. M. VALDES-AGUILERA

a spike the dimensions of which depend on the irradiation time and the laser power (Figure 7). As a measure of the photospeed we determine the length of the formed spike or, if the strength of the polymer permits, we isolate the spike and estimate the photospeed gravimetrically. These methods are an extension to thick samples of the sensitometric techniques used to evaluate the photospeed in thin films [194]. A typical composition contains Eosin or Eosin lactone (Eosin lactone is neutralized by the amine to form the bisammonium salt of Eosin in situ) at a concentration of 1.3 x M, triethanolamine (0.12 M), trimethylolpropane triacrylate, and 5% vinyl pyrrolidone as reactive diluent. Table 3 summarizes our results obtained with Eosin lactone using different exposures and incident laser intensities. These results indicate that the rate of growth of the spike increases linearly with the incident intensity. The data show that the polymerization depth depends on the total incident energy and not on the rate at which the energy is delivered. The system is said to obey reciprocity since irradiation with a high intensity for a short period of time produces the same effect as irradiation with a low intensity for a long time, provided the total energy delivered is the same in both cases. From Table 3 we calculate the average polymerization depth per unit exposure, that is, the sensitivity, at 0.30 k 0.03 cm3 J-'. Decreasing the concentration of triethanolamine to 0.06 M changes the sensitivity to 0.46 cm3 J- Fluorescence measurements in ethyl acetate/20% methanol indicate triethanolamine quenches the emission intensity of Eosin with a Stern-Volmer constant equal to 2.2 M - '. From this value we conclude that the lower sensitivity at the higher amine concentration is probably due to quenching of the Eosin singlet. As we discussed above, this quenching does not produce free radicals and the net effect is to decrease the quantum yield of intersystem crossing.

Figure 7. Schematic representation of spike formation upon laser irradiation.

PHOTOCHEMISTRY OF THE XANTHENE DYES

337

TABLE 3 Average Depth of Polymerization"

I (w cm-') 1.62 3.25 3.25 6.50 16.2 16.2

E (J cm-2)

Polymerization Depth (cm)

0.20 0.21 0.41 0.81 1.08 2.03

0.060 0.066 0.125 0.206 0.360 0.540

"Photopolymerization of trimethylolpropane triacrylate/5% vinyl pyrrolidone. M, triethanolamine 0.12 M; li,,=514nm; Ar' laser, Eosin lactone 1.3 x beam diameter = 1.4 mm.

Using Eosin instead of the lactone as the reactant increases the sensitivity by approximately 40%. Because of the high triethanolamine concentration employed (0.12 M), it is unlikely that the lower sensitivity in the former case is due to incomplete neutralization of the lactone. More likely it is due to quenching of the Eosin singlet and/or triplet by the ammonium ion. Fluorescence studies with several Rose Bengal derivatives in nonpolar solvents indicate that ammonium ions quench the singlet of Rose Bengal by a combination of hydrogen bonding and tight ion pair formation [195]. We, as well as Chesneau and Fouassier, find that the photospeed increases linearly with light intensity. From this observation one can conclude that chain termination reaction is not the usual interaction between two macroradicals. We have measured the initial rate of photopolymerization using thin foil calorimetry and find a linear relationship between the rate of photopolymerization at low conversions (less than 15%) and the absorbed light intensity. Using the same monomer but with a different photoinitiator (to be discussed in detail later) we observe an order of one half with respect to light intensity both by thin foil calorimetry and by measuring the polymer spike. Therefore we conclude that the linear dependence observed for the Eosintriethanolamine system is real and not an artifact of the technique employed to determine the photospeed. It has been known for some time that UV photopolymerization of multifunctional monomers does not obey the classical rate expression, R, proportional to Z0.5, but follows an approximately first-order relationship [196,1971. These results have been explained by postulating that, in these viscous monomers, radical occlusion competes with bimolecular termination.

338

D. C. NECKERS A N D 0 . M. VALDES-AGUILERA

Scheme 2 presents a general mechanism and the intensity dependence derived for the limiting cases. Initiator + hv +radicals R'+M+

initiation

Mi

M;+M---+M;

propagation

Mi + M i -----+ polymer M n ' j inert

bimolecular termination radical occlusion

Scheme 2.

Intensity dependence: (a) Termination exclusively by bimolecular reaction: R , is proportional to 10.5

(b) Termination exclusively by radical occlusion: R, is proportional to I . When both types of termination occur simultaneously, the observed order with respect to light intensity depends on the relative contributions of radical occlusion and bimolecular reaction to the termination process. An example of this condition is reported by Bellobono and co-workers for the case of diallyloxydiethylene dicarbonate [1981. They measure the intensity dependence of the rate of photopolymerization at different conversions and find that the apparent order with respect to light intensity increases with the conversion at which it is determined. At the initial stage of the reaction the photopolymerization follows the classical rate law, indicating that bimolecular termination is predominant at low conversion. The dependence of the reaction order on the percent of double bonds reacted is indicative of increasing contribution of radical occlusion to the overall termination at the higher conversions. Recent work by Decker and Moussa [199,200] on the photopolymerization of acrylic polyurethane resins in the presence of monoand diacrylate diluents indicates bimolecular termination reactions predominate at all conversions in formulations containing equal parts of resin and diluent. Postpolymerization effects give an estimate of five seconds for the lifetime of the macroradicals in these monomer systems. It is reasonable to expect that in a viscous monomer such as trimethylolpropane triacrylate (q = 65 cp), bimolecular termination reactions proceed more slowly than in monofunctional monomers. However, considering the long lifetime observed for the polymer radicals in these monomers, caution must be exercised in the interpretation of the linear intensity dependence. Long-lived radicals are more likely to terminate by chain transfer and

PHOTOCHEMISTRY OF THE XANTHENE DYES

339

inhibition even at relatively low concentrations of transfer agents. Chen [201,202] has presented evidence that the dimers of methylene blue are the main chain terminators when this dye is used, with triethanolamine, to sensitize the photopolymerization of acrylamide in aqueous solution and it is possible that a similar degradative chain transfer process is occurring with Eosin under our conditions. Eosin dimerizes in water and in organic solvents, the equilibrium constant being equal to 91 M-' in glycerol [203]. For the dye concentration we have M, the concentration of dimer is calculated employed in our work, 1.3 x M. Using a 5-sec lifetime estimate for the macroradicals, the to be 1.5 x rate constant for the reaction shown in Eq. (16) needs to be 1.3 x lo5 M-' S-' to compete with the reaction in Eq. (17) and would be the predominant mode of termination if the rate constant equals 7 x lo5 M-'s-'. Since the rate con-

MI, + (D)2+inert polymer MI, + Mk-

inert polymer

stant for a diffusion controlled reaction in the monomer is 1.7 x 108M-'s-', we may have a situation in which the aggregates are present at a concentration high enough to dominate the termination process, but too low to be detected spectroscopically. Flash photolysis of Eosin in the presence of amines in aqueous solution has been performed by Kasche and Linsqvist [204] and by Rizzuto and Spikes [205]. The photooxidation of phenol and derivatives of phenol has been studied by Zwicker and Grossweiner [206,207] and by Chrysochoos and Grossweiner [208]. The rate constants for quenching of Eosin triplet by ferricyanide ion, p-phenylenediamine, and phenol are 2.5 x lo8, 1 x lo1', and 1.5 x 105 M-' s-' respectively. In the absence of oxidizing or reducing agents the triplet of Eosin decays by mixed first- and second-order processes. The triplet lifetime depends on the energy of the flash and the concentration of Eosin ground state. Scheme 3 has been proposed to account for the observations.

-

D(T)-

D(T) + D

D

k,

2D

k2

D(T) + D +D"

D(T)+ D(T) +2D D(T)+ D(T)

D(T)+Am-

D"

+ D'-

k, k4

+ D'-

D'- +Am'+

Scheme 3.

k5 k6

D.C. NECKERS AND 0. M. VALDES-AGUILERA

340

D represents Eosin ground state, D(T) the dye in its triplet state (A, = 540 nm, E,,, = 4.4 x lo4 M-' cm- '), D" semioxidized Eosin radical (A,, = 462 nm, E,,, = 6 x lo4 M-' cm-' ), D' - semireduced Eosin radical (A,,, = 405 nm, E,,, = 4 x lo4 M-' cm-' ), Am represents amine. The rate constants obtained by Kasche and Lindqvist in aqueous solution, with Am = p-phenylenediamine as donor, are summarized in Table 4. We have performed laser flash photolysis experiments in ethyl acetate/20% methanol under conditions in which triplet-triplet annihilation is not important. We achieve these conditions by using Eosin concentrations higher M and low pulse energies in order to obtain clean exponential than 1 x decays for the triplet. Determination of the triplet lifetime at different Eosin and amine concentrations allows us to obtain the rate constants collected in Table 5. A summary of our observations is as follows: 1. In the absence of amine both D" and D'- appear simultaneously with, the decay of the triplet. The absorptions due to the radicals decay to zero within one millisecond. 2. In the presence of triethanolamine at concentrations higher than 2 x lo-' M only the semireduced Eosin radical is formed and its absorption decays in a few minutes. M is used to quench the triplet only 3. When triphenylamine 3 x the semireduced Eosin radical is formed and its absorption decays to zero within one millisecond.

-

TABLE 4 Triplet Decay Rate Constants for Eosin in Aqueous Solution" Rate Constant

kl k*

Value

k3

540 s - l 3 . 0 10' ~ M-'s-' 7.0 x lo7 M-'s-'

k4

1.1 x 109 ~

k5

k6

- 1 s - 1

1.0 x lo8 M-'s-' 1 . 0 10" ~ M-'s-'

"See Scheme 3 for reactions. Am represents p phenylene diamine. Data from V. Kasche and L. Lindqvist, Photochem. Photobiol. 4, 923 (1965).

PHOTOCHEMISTRY OF THE XANTHENE DYES

341

TABLE 5 Triplet Decay Rate Constants for Eosin in Ethyl Acetate/20% Methanol" Rate Constant kl

k2

Value 3.4 x 1 0 3 s - 1

6.0 x 107

+k 3

6.ox 105 2.3 x 107

k6 k6

~ - 1 s - 1 ~ - 1 s - l b ~ - 1 s - 1 ~

"See Scheme 3 for reactions. 'For Am = triethanolamine. 'For Am = triphenylamine.

The stability of the semireduced Eosin radical produced by reduction with triethanolamine is similar to that reported for the radical in aqueous solution [209]. Our results indicate that the lifetime is substantially shorter when it is generated by reduction with triphenylamine. We attribute this difference to the ability of the triethanolamine radical cation to form the a-amino radical by loss of a proton, as shown below, thus preventing the reaction of the radical by reverse electron transfer (Eq. 18).

+'N(CH,CH,OH),

I

CH,CH,OH

-1

N(CH,CH,OH), CHCH,OH

+H+

(18)

It has been suggested in the literature that the a-amino radical is the species that initiates polymerization [210]. This view is supported by our observation that, in spite of the relatively high quenching rate constant of Eosin triplet by triphenylamine (Table 5), the system Eosin-triphenylamine does not sensitize the photopolymerization of multifunctional acrylates. Thus, it is necessary that the amine contains a hydrogen at the a-carbon to be released as a proton after oxidation of the amine by the dye triplet. This deprotonation prevents the back electron transfer and forms a carbon radical that is sufficiently long-lived to be captured by the monomer. We have also used the Eosin-triethanolamine system to design a twophoton system based on visible laser-induced photopolymerization followed by UV-induced crosslinking as a means of building a three-dimensional network structure aimed at three-dimensional imaging. The key to our design, shown in Scheme 4 below, is incorporation of an acrylate monomer

342

D. C. NECKERS AND 0. M. VALDES-AGUILERA

with UV sensitive pendant group as a comonomer and/or as a reactive diluent along with di- and trifunctional acrylate monomer compositions.

/3D UV laser

Object

Polyfunctional UV sensitive Visible laser Polymer with UV / + acrylate P acrylate Polymerization sensitive pendant groups

Scheme 4.

Acrylate derivatives of various oximes have been selected for this purpose for the following reasons: (1) A wide range of oxime acrylates having desired absorption maxima in the UV range may be prepared by a one-step procedure from corresponding oximes. (2) Polymers bearing 0-acyloxyimino groups are reported by Tanaka et al. [211] and Hong et al. [212] and by Ghogare and Kumar [213,214] to be UV-sensitive and participate in photocrosslinking or photodegradation. (3) 0-acetyl oxime esters have been used as photoinitiators, but the photopolymerizability of acrylates with pendant oxime groups has not been studied in detail. Photopolymerization studies involving these nitrogen-containing monomers is of interest. (4) Selection of monofunctional, low-viscosity UV sensitive monomers as reactive diluents introduces a new formulation latitude in image forming. The acyl oximes introduced by Delzenne et al. [215] in 1970 have proven to be highly effective in photogenerating free radicals (quantum yields of radicals are of the order of 90%). Delzenne et al. and more recently Li, Liang, and Reiser [216] have investigated the photofragmentation of linear polymers containing acyloxime moieties in the main chain and in the crosslinks, respectively. Their respective goals were to use the acyloximes as a means of photodepolymerization and eventually to provide the basis of new positive photoresists. Preliminary work by Tanaka et al. and Hong et al. shows that acyl oxime pendant groups in the side chain of linear polymers undergo photocrosslinking or photodegradation depending on the structure and physical state of the polymer and nature of oxime. Part of their success is based on the fact that primary bond scission is followed in these systems by a rapid second fragmentation step (Scheme 5). This puts the final radicals at least 4.5 A apart outside the primary cage. As a result the radical pairs do not recombine. In looking for reactive structures that could serve as photolabile side groups, this was an important consideration because cage recombination is an important factor in viscous media such as those used in the polymerization of polyfunctional acrylates.

343

PHOTOCHEMISTRY OF THE XANTHENE DYES

I Scbeme 5.

Another important consideration was the fact that the absorption maxima of oxime acrylate monomers could be tuned to the desired wavelength by the selection of desired ketone or aldehyde chromophore from whence the oxime was originally synthesized. In our design, UV photocrosslinking followed dye-sensitized visible polymerization which formed a soft gel. It is very important to be able to choose the absorption window for the crosslinking reaction to prevent interfering absorption, if any, from the residual dye initiator. Acyl oximes as part of the backbone of a photodegrading polymer chain have been studied by Smets [217]. Table 6 lists the absorption characteristics of the series of UV-sensitive monomers investigated [218]. Aliphatic oxime acrylates show absorption maxima below 230 nm, while benzophenone oxime acrylate shows an absorption maximum at 252nm. It is important to note that while benzophenone has absorption at 360nm owing to the n-z* transition, the corresponding oxime has no strong absorption above 300 nm. Pyrene-lcarboxaldehyde oxime acrylate (POA) and 9-phenanthrenecarboxaldehyde TABLE 6 Spectral Characteristics of Oxime Acrylates *ma,

Compound

(nm) 364 290 316 280 252 228

Pyrene oxime acrylate (POA) Phenanthrene oxime acrylate (PhOA) Benzophenone oxime acrylate (BOA) 2,3 Butanedione monooxime acrylate (BMA) Cyclohexanone oxime acrylate (COA)

I

<220

-

E,,,

IR (cm- I)

27,605

1740,810

11,353

1740,810

14,500 13,300

1740,810 1730,810

1730,810

344

D. C. NECKERS AND 0. M. VALDES-AGUILERA

oxime acrylate (PhOA) exhibit strong maxima at 325 and 364nm, respectively, which are due to the n-n* transition, and the formation of oxime changes this absorption very little. POA, PhOA, and benzophenone oxime acrylate (BOA) are yellow solids, whereas the aliphatic oxime acrylates are low-viscosity liquids which can serve as reactive diluents. The solubility of POA and PhOA was limited in the monomer mixture, hence small amounts of N-vinyl pyrrolidone or chloroform were necessary to make the to-bepolymerized monomer mixture homogeneous. Irradiations were carried out with the Argon ion laser tuned to its emission line at 514 nm. The formulation consisted of Eosin 1 x M, triethanolamine (0.05-0.1 M), a monomer mixture, and a reactive diluent such as N-vinylpyrrolidone included at 5% by weight where necessary. The monomer mixture normally consisted of equal parts of acyloxime acrylate and a multifunctional acrylate. Where the laser light strikes the monomer solution, solid polymer is formed and the length of the spike so obtained in the direction of the laser beam increases with irradiation time. In all cases, the experiments were carried out in duplicate and the average value of the weight of the polymer was taken to measure the rate of polymerization. The nature of the polymer varied depending on the selection of the monomer, initiator, and the extent of polymerization. The solid polymer still contained some unreacted double bonds as determined from the IR absorption peaks at 810 cm-'. The extent of polymerization was dependent on the functionality of the monomer. In general, as the concentration of the monofunctional oxime acrylate is increased, the rate of photopolymerization is correspondingly reduced. Below 10% by weight, monofunctional acrylates did not influence polymerization rates significaxtly. The dye-sensitized homopolymerization of one monomer, cyclohexanone oxime acrylate (COA), did not result in polymer spike formation, whereas 2,3-butanedione monooxime acrylate (BMOA) formed a spike under the same conditions. Likewise, a comparison of copolymerization behavior of BMOA and COA indicates faster conversions with BMOA which may be due to the participation of its pendant groups as radical sites for chain growth. Copolymerization studies involving POA and PhOA could not be carried out at higher concentrations because of poor solubility of these monomers in the polyfunctional acrylates. To demonstrate the UV crosslinkability of the polymers formed having pendant acyloxyimino groups, copolymers of bisphenol diacrylate and 1,6 hexanediol diacrylate were prepared by thermal methods with 5% (by weight) of pyrene oxime acrylate and phenanthrene oxime acrylate, both of which have considerable absorption in the region of 320-360 nm. The polymerization was stopped before the gel point and the prepolymer solutions were then irradiated with light from a monochromator at the maximum of

345

PHOTOCHEMISTRY OF THE XANTHENE DYES

the corresponding oxime acrylate (364nm). The solution exposed to UV radiation crosslinked within a few minutes, forming a pale yellow film on the wall of the cuvette. The weight of the film was taken as an index for the gelation (Table 7). These experiments demonstrate the intrinsic photosensitivity of polymers containing pendant oxime groups in the absence of any dye absorption that might interfere in the case of dye-sensitized copolymer samples. Parallel sets of polymerization experiments were carried out using the monomer mixture described above with an Eosin-triethanolamine initiator system irradiated (514 nm) for short periods of time. The resulting prepolymer (or partly polymerized) solution was then exposed to UV radiation, as above, and the extent of crosslinking was measured. Since the polymerization at 514nm was not carried out to completion, bleaching of the dye was incomplete at the concentration of the initiator used. Film formation was observed, but only after 3 minutes of irradiation. The lower levels of crosslinking can be attributed to partial masking of the oxime absorption by the residual initiator. Like experiments with a He-Cd laser (325 nm) produced gel along the laser path. We have explored the possibility of combining visible laser-induced polymerization with UV induced crosslinking as a means of building a threedimensional network structure aimed at three-dimensional imaging. Our investigations have established the possibility of incorporating photosensitive monomers with desired absorption maxima ranging from near UV to deep UV and their polymerizability with other acrylate monomers. Although the relative quantum yields of chain scission and crosslinking are difficult to measure because of the presence of dye molecules in the polymer matrix, gelation due to ultraviolet exposure (both He-Cd laser and medium pressure mercury lamp) indicates that the photocrosslinking is facile in these systems. TABLE 7 UV Crosslinking of Copolymers with Pendant Groups

% Gel Composition 1,6HDA-POA copolymer ( 9 5 : 5 ) 1,6HDA-PhOA copolymer (95: 5)

Time (min)

Sample I

Sample I1

2 5 10 2 5 10

10 30 60

3 10 20

10 25

10

5

4

346

D. C. NECKERS AND 0. M. VALDES-AGUILERA

As shown by Tanaka et al., chain photodegradation might be more significant in linear polymers than crosslinking, but in the case of branched or crosslinked networks, photocrosslinking seems to be the dominant reaction. We believe that the crosslinking following scission of acyl oxime might proceed by two mechanisms (Scheme 6). Intermolecular attack of carboxy radicals, generated in the polymer side chains because of cleavage of the -C=N-0bonds on unreacted double bonds of another chain is a possibility. In other words, the macroradical generated on the polymer acts as a grafting site leading to intermolecular crosslinks. Alternatively, carboxy macroradicals might decarboxylate and then react with other carboxy radicals leading to crosslinking (Scheme 6).

Scheme 6.

Recently Fouassier and Chesneau [219] studied the photochemistry of the system Eosin-PDO-MDEA in aqueous acetonitrile using steady-state irradiation and laser flash photolysis. The photopolymerization of methyl methacrylate (MMA) sensitized by the photoreduction of Eosin is investigated in acetronitrile to understand the mechanism of initiation and the enhancement in the rate of polymerization caused by the presence of PDO, 3. Rates, quantum yields of photopolymerization, and number average molecular weights of the polymer are determined with MMA (7M), Eosin (3 x M), and MDEA (0.1 M) in the presence and in the absence of 2 x 10-3 M PDO. Steady-state irradiation of Eosin and PDO leads to the formation of ethyl benzoate, benzil, and benzoyloxycarboxylate. Energy transfer from Eosin to P D O is unfavorable and the decomposition of P D O is most likely sensitized by electron transfer. In addition, at the concentration of PDO employed the fluorescence of Eosin is not significantly quenched and it is concluded that the excited state involved is the triplet of Eosin.

PHOTOCHEMISTRY OF THE XANTHENE DYES

347

Irradiation in the presence of MDEA completely inhibits the formation of products. The amine quenches the fluorescence of Eosin with a rate constant of 8 x 10' M - l s - l and quenches the Eosin triplet with a rate two orders of magnitude lower. A summary of rate constants for the decay of the triplet is presented in Table 8. In addition to the reactions shown in Scheme 3, with Am = N-methyl diethanolamine, the rate constants for reaction of PDO with Eosin triplet and semioxidized Eosin radical in aqueous solution (Eqs. 19 and 20) are included in the table. D(T) + PDO D"

-

+ PDO-

quenching

(19)

D

(20)

+ PDO"

The photopolymerization of 7 M methyl methacrylate in acetonitrile was studied. Measurements of the quantum yield of photopolymerization and the molecular weight of the formed polymer indicate PDO increases the quantum yield of initiation and decreases the rate of termination. The data are shown in Table 9 with the quantum yield of initiation, $ i , and k i / k , reported in arbitrary units. To explain the reduced rate of termination observed in the presence of PDO, the authors propose that one of the termination reactions involves the protonated form of the semireduced Eosin radical which, by reaction with PDO, is reduced to the leuco dye (E,qs. 21 and 22). DH'

+ Mn'-

inert polymer + D

DH'+PDO-DH-+PDO'+

(22)

TABLE 8 Rate Constants Obtained in Aqueous Solution" Rate Constant

kl k,+k, k6

k, k8

Value

6.0 x lo3 s - l 8.0 x 10' M-'s-' 4.3 x lo6 M-.' S-1 9.ox 104 ~ - . l s - l 1.5 x lo6 Mi's-'

"See Scheme 3 for reactions. Am represents N-methyl diethanolamine. k , is the rate constant for D(T) PDO+quenching. k , is the rate constant for D" + P D O + D + PDO'+. Data from J. P. Fouassier and E. Chesneau, Makromol. Chem. 192, 245 (1991).

+

(21)

348

D. C. NECKERS AND 0. M. VALDES-AGUILERA

TABLE 9 Photopolymerization of Methyl Methacrylate Sensitized by Eosin"

Coinitiator None

MDEA MDEA PDO

+

Molecular Weight

4,

R,(M-'s-') 0 6.8~10-~ i . 9 x 10-4

0 15 42

-

17,500 12,700

6i(au)

k,2/kt(au)

0

-

38

1.2 2.4

150

"Methyl methacrylate, 7 M; eosin (OD=0.15 at 547 nm); solvent, acetonitrile; MDEA, 0.1 M, PDO, 2 x M; Z o = 1 x 10l6 photons s-' cm-*. Data from J. P. Fouassier and E. Chesneau, Makromol. Chem. 192, 245 (1991).

In the presence of MDEA the decomposition of PDO does not occur, which suggests the amine is reacting with oxidized PDO, preventing its cleavage and increasing the yield of initiator radicals as shown in Eqs. (23) and (24), where a-Am' represents the a-amino radical of MDEA. PDO" +AmAm"

-

PDO +Am"

(23)

a-Am'

(24)

+H+

This sequence predicts that the presence of PDO will increase the yield of initiating radicals by a factor of 2. Because the quantum yield of initiation increases by a factor of approximately 4 (Tableg), it is proposed that an additional increase in a-Am' is achieved by the sequence in Eqs. (25)-(27). D"

+ PDO-

PDO" +Am+

+ PDO"

(25)

PDO +Am'+

(26)

D

Am" +a-Am'

+H+

(27)

In the presence of MDEA most of the triplets decay by reduction with the amine, and the yield of D" is very low. Therefore the authors conclude that the efficiency of the dissociation in Eq. (28) must be low enough to produce free radicals with a yield comparable to that obtained by reduction of D" as shown in Eqs. (25)-(27).

From our analysis of the reported data for the photopolymerization and the flash photolysis results, we conclude that the explanation offered by

PHOTOCHEMISTRY OF THE XANTHENE DYES

349

Fouassier and Chesneau [219] is not consistent with the experimental observations. From the values of the rate constants of triplet decay presented in Table 8, and taking into account that k3/k2 = 0.23 (as determined by Kasche and Lindqvist), we calculate the quantum yield of D" under the polymerization conditions. For Eosin (3 x l o w 5M) and MDEA (0.1 M) the yield of semioxidized Eosin radical is 4 x M in the presence or in the absence of 2 x M PDO. From the values for the quantum yield of photopolymerization and the molecular weight in the absence of PDO we calculate a quantum yield of initiation between 0.086 and 0.17, the actual value depending on the mode of termination. Therefore, we conclude that formation of a-amino radicals according to Scheme 10 represents only a minor contribution to the quantum yield of initiation observed in the presence of PDO. Based on the reported results, our calculation of the yield of semioxidized Eosin radical, and the fact that reduction of free semireduced Eosin radical by PDO accounts, at most, for one half of the increase in $ i , we propose that an additional source of initiating radicals involves reduction of DH' in the radical pair, as shown in Eq. (29), in combination with the reactions represented in Eqs. (23) and (24). {DH' a-Am')

+ PDO

-

DH-

+ a-Am' + PDO"

(29)

A more efficient photoinitiator has been designed by combining the Eosin-MDEA system with diphenyliodonium salt. The quantum yield of photopolymerization of methyl methacrylate (7 M) in acetonitrile is 30 with MDEA (0.1 M) and diphenyliodonium (0.05 M). The molecular weight of the isolated polymer is 55,000. In the absence of 421+the values are 15 and 17,500 for the quantum yield and molecular weight, respectively. Thus, the presence of diphenyliodonium decreases the quantum yield of initiation by approximately 40% and increases the value of k,2/k1by a factor of 6. The decrease in the termination rate is attributed to the efficient capture of semireduced Eosin radical by diphenyliodonium. The rate constant of the reaction shown in Eq. (30) was determined by laser flash photolysis as 1 x 10" M-'s-'.

Diphenyliodonium, at a concentration of 0.05 M, does not quench the Eosin triplet. It decreases the fluorescence intensity by a factor of 20 and shifts the fluorescence wavelength 10nm toward the red. The calculated rate constant for singlet quenching is 5 times larger than for a diffusion controlled

350

D. C. NECKERS AND 0. M. VALDES-AGUILERA

reaction. These results are explained by proposing the formation of a complex between diphenyliodonium and Eosin in the ground state (Eq. 3 1).

D

+ 421+ Icomplex

(31)

Addition of MDEA to a solution containing Eosin and q521+ increases the fluorescence intensity and shifts the spectrum toward the blue. Similarly, the fluorescence quenching by 421+is less efficient in the presence of the MDEA, indicating that the amine destroys the complex between Eosin and diphenyliodonium.

IV. ERYTHROSIN Erythrosin (Gr; &p$zqoo-red), tetraiodofluorescein, was synthesized first by Gnehm [220] and named by him. It has been used as a food coloring for many years and is the red dye of maraschino cherries, Tylenol capsules, and bright red luncheon meats. It is known as FD&C {Food Dyers and Colorists} # 3 and has received substantial attention from the FDA over the last 10 years since it causes bladder cancer when ingested in huge amounts by rats. It has recently been banned as a food dye in the United States, but is a source of dietary iodine for the Japanese [221]. It is also used as a stain. Like Rose Bengal, Erythrosin is an excellent singlet oxygen sensitizer (Table l), though not used as frequently for that purpose [222]. Photodynamic action is a term developed by physiologists to explain the effect of a dye, light, and oxygen on a living organism or an enzyme in vitro. In recent years photodynamic effects have usually referred to the effects of singlet oxygen on biological systems. There are a number of reports of photodynamic effects of Erythrosin, which are attributed to singlet oxygen, as might be expected [223]. The relative rates of photoinactivation of acetylcholinesterase increase in precisely the order of the singlet oxygen quantum yields obtained from the respective dyes in solution; fluorescein < Eosin < Erythrosin < Rose Bengal. Complexation of the enzyme by the dye was essentially the same in the case of Rose Bengal and Erythrosin [224]. Erythrosin bleaches in the presence of electron donors and this has, on occasion, been incorrectly ascribed to an effect of singlet oxygen [225]. Singlet oxygen formation and dye bleaching are only related to the extent that the products of oxidation are oxidants. According to an international patent, the photobleaching of Erythrosin in a polymer matrix has a quantum yield of about Deiodination occurs from constant exposures to visible radiation with relative yields of 4.6% in polyvinylpyrrolidone, 4.7% in

PHOTOCHEMISTRY OF THE XANTHENE DYES

351

cellulose acetate, 3.1% in polyvinyl acetate, and 0.6% in polyvinylbutyral [226], and this reaction has been used as a system for the visual monitoring of exposure to UV radiation [227]. Electron transfer processes involving Erythrosin in the case of transfer to metal ions form the basis for photoinitiation of polymerization [228]. The efficiency of charge separation after transfer of an electron to methylviologen is solvent dependent and yields decrease in the order CH,CN > acetone > dioxane > EtOH > DMSO [229]. Erythrosin is said to exchange iodines with iodide ion in EtOH [230]. Erythrosin has also been used as a photoinitiator with reducing agents such as ascrobic acid and triethanolamine and in the presence of light-sensitive diazosulfonates for the polymerization of N,N-dialkylacrylamides and in the production of a calcium acrylate-vinyl sulfonate copolymer [231]. The principle is that Erythrosin is sensitive to green light and hence responsive to wavelengths ranging from 490 to 535nm. Erythrosin is used for photolithography in combination with interactive dyes which form complexes absorbing at 600 nm, such as N-phenylthioacridone, presumably as active initiators at the wavelengths of the He-Ne laser [232]. Methyl methacrylate was grafted onto nylon 6 in citrate buffer with Erythrosin [233] serving as the photoinitiator. In similar experiments PVC was degraded using Erythrosin-initiated reactions and green light [234]. The yield of radicals from the photoreduction of Erythrosin by methyl viologen is increased by an external magnetic field, but decreased upon electron transfer to a viologen in a high electric field [235]. There are a large number of spectroscopic reports involving Erythrosin, particularly from Russian laboratories. The near-IR absorption spectra of both Eosin and Erythrosin triplets and their dianion radical cations have been reported [236], the pK’s of both species measured [237], and the influence on radiative and radiationless transitions by external heavy atoms such as iodide reported [238]. The nonradiative decay of Erythrosin in water at room temperature has also been studied by photoacoustic spectroscopy [239].

V. ROSE BENGAL A. Background Rose Bengal, 2,4,5,7-tetraiodo-3’,4’,5’,6’-tetrachlorofluorescein, was originally synthesized by Gnehm [240] as a fabric dye to mimic the red colors in “bengalis.” Bengalis was a widely recognized pattern used for cotton. Its name is connected to the red symbolic spot worn at the part of the hair by

352

D. C. NECKERS A N D 0. M. VALDES-AGUILERA

Bengali women to symbolize marriage. Only Bengali women wear the red spot at the center of the forehead. The decorative spot worn in the middle of the forehead by other Indian women is-today-pasted on. Much of the history of Rose Bengal has been reviewed elsewhere [241]. Citations to Rose Bengal have increased in the last three or so years, and the diversity of its application has greatly expanded. Many of the Rose Bengal derivatives listed are now commercially available from either Molecular Probes or Aldrich. Rose Bengal has a large absorption in all media in which it is soluble and this absorption corresponds almost exactly to the emission wavelengths of common tungsten sources. Its intersystem crossing yield ((Disc)is high, but not unity, and all Rose Bengal derivatives show some fluorescence. Its triplet is quenched by oxygen at % 1/9 the diffusion-controlled rate in methanol. Its spectrum is highly dependent on its immediate environment. It undergoes slow photochemical bleaching in protic, polar solvents. It is a photodynamic sensitizer. Its singlet may be quenched by strong oxidizing agents, in some cases with formation of radicals. Rose Bengal, disodium salt, is the commercial dye and this was Gnehm’s original dyestuff. The salt is difficult to purify and the commercially available materials are not seriously purified after synthesis. The commercial material frequently contains inorganic salts either incorporated during the synthesis or added as part of an old farbstof protocol. Dye purity is reported relative to Colour Index methods [242,243] which are often old. Rose Bengal has been reportedly purified by chromatography, though these methods often accomplish little other than making the laboratory table a big red mess. Rose Bengal is best purified by conversion to the lactone [244] or to the lactone diacetate [245], with subsequent recrystallization and stoichiometric reconversion to the disodium salt (Figure 8). The crystal structures of the lactone and the bis-triethylammonium salts have been reported [246]. The lactone crystals include solvent, which in the case studied were two molecules of dioxane per molecule of dye. The included solvent slowly evaporates from the crystal, causing a color change. Perfectly

lactone

lactone diacetate

Figure 8. Rose Bengal lactone and Rose Bengal lactone diacetate.

PHOTOCHEMISTRY OF THE XANTHENE DYES

Rose Bengal Lactone

353

Anion of Rose Bengal, bis Triethylammonium salt

Figure 9. Crystal structures of Rose Bengal lactone and bis-triethylammoniumsalt.

dry Rose Bengal lactone (and the same is true for the other xanthene lactones) is almost colorless. A small amount of solvent, however, causes the color of the dye to appear. The crystal structure clearly shows that the lactone ring is almost perpendicular to the average plane of the xanthene core and that the latter is rather nonplanar. The crystallographer suggested that the lactone ring is sort of “nodding” toward the deformed xanthene. The lactone oxygenC-9 bond is shorter than that reported for fluorescein lactone and nearly the same as those in six-membered ring lactones or an epoxide. The oxygen of the xanthene ring is not fully sp2 hybridized since the central ring of the xanthene is in the shape of a deformed boat. The benzyl ester (not shown) does not include solvent in the crystal because it is more tightly packed and the plane of the benzene ring lies at about a 15” angle to the average plane of the xanthene ring. The ammonium salts crystallize well enough for structures of the anion to be obtained (gegen ion not shown). The xanthene ring is, as expected, nearly planar, and the aromatic tetrachlorophenyl lies almost at a right angle to the central ring system. Three-dimensional drawings simulated from the actual crystal data are shown in Figure 9. The ammonium salts, phosphonium salts, iodonium salts, and other organic derivatives are easily prepared from the lactone by neutralization with the appropriate Lewis base. These organic ion salts are relatively easy to purify though they are not-for the most part-soluble in water.

B. Absorption and Emission Spectra All Rose Bengal derivatives retain their general xanthene color and spectra if ionization of the -OH is supported by the solvent. The xanthene ring is symmetrical in the ionized form and we have assigned the ionized phenol the

354

D. C. NECKERS AND 0. M. VALDES-AGUILERA

6 position merely for reference (Figure2). The important forms of Rose Bengal are analogous to those of fluorescein and are the C-6 monoanion (Figure 2, bottom left) and C-2, C-6 dianion (Figure 2, bottom right). The fully protonated C-2(H), C-6(H) (Figure 2, top left) undergoes immediate ring closure to the lactone. The pK, is that of the C-6 protio form and has been measured in water as 3.72. The ionization constant of the COOH group at C2’ in Rose Bengal has not been reported. Based on the value measured for related xanthenes such as diiodofluorescein, the pK, value would be about 4.75. Rose Bengal differs in reactivity from fluorescein and Eosin because of the iodines at C-2, C-4, C-5, and C-7 which differentiate the reactivity of the phenoxide from the carboxylate and make substitution at C-2’ possible in lieu of substitution at (2-6. Essentially three different kinds of Rose Bengal derivatives have been synthesized. The most common include derivatives with gegen ions other than sodium at both C-2‘ and C-6. Many esters of C-2‘ have been synthesized and compounds with a number of different gegen ions at C-6 have been reported. The rarest derivatives of Rose Bengal are the C-6 ethers, of which only three have been reported.

C. Chemical Reactivity 1. Substitution at C-6. Because of the sterically imposing iodines ortho to the phenoxide, the reactivity of Rose Bengal dianion as a nucleophile is essentially entirely at the C-2’ position, and most of the known Rose Bengal derivatives which have been prepared take advantage of this reactivity. Derivatives with a nonionizable substituent at C-6 are rare and only three C6 ethers are known. All are C-2’ esters, C-6 ethers, 6. CI

6

Ethers at C-6 are characteristically orange (resembling the nonionized form of the dye in spectra) and show a typical absorption spectrum: 1,,,, = 494 3 nm ( E x 10,000); I,,, = 404 nm 3 nm ( E x 10,000). This absorption spectrum also pertains to compounds that have ionic substituents at

PHOTOCHEMISTRY OF THE XANTHENE DYES

355

C-6 in solvents which do not support ionization. A typical example of the latter behavior is that of 7, the C-2’ benzyl ester, C-6 protio, which is red in methanol and orange in methylene chloride. CI

7

In the latter solvent the spectrum of 7 is identical to that reported for the orange C-6 ethers. The shorter wavelength A,, at 404 nm is a particularly characteristic absorption since its presence distinguishes the nonionized form of the dye from species having anomalous spectra, such as those introduced at relatively higher concentrations by dye aggregation. Substituting carbon groups at the C-6 oxygen requires special conditions, and once the substituted C-6 derivative has been prepared, special care in handling is necessary. The C-2’ esters with an OH group at C-6 and a number of derivatives of the lactone (of which the lactone diacetate is an example) are also known [247]. To the best of our knowledge there are no cases of pure compounds having been isolated in which (2-2‘ is ionic with C-6 substituted with a nonionizable group. The one compound that has been reported, C-6 M e 0 and C-2’ COY, has approximately the proper spectrum [AmaxEtOH = 493 nm (5850), 402 nm (6610); &,axEtOH,acetone = 486 nm (5430), 394nm (5930)l but has not yet been isolated as a pure solid. Attempts to hydrolyze C-2’ ester, C-6 ethers with several different bases all resulted in concomitant cleavage of the ethers. The driving force for xanthene aromatization is very large. To a greater or lesser degree the spectra of all C-6 ionic salts, whether they are substituted with a covalent group at C-2’ or not, are comprised of a mixture of the spectra of C-6 ionic and C-6 covalent forms. Differences among the spectra of the compounds result from secondary effects on the structure of the bare anions such as hydrogen bonding and dimerization due to hydrophobic dye/dye molecular interactions. This is particularly obvious when the fluorescence spectra of the ammonium salts at room temperature of the C-2’, C-6 bis trimethylammonium salt shows a triad of broad peaks centered at 583 nm. At reduced temperature the same compound shows three sharper peaks at 538, 583, and 605nm. The shortest wavelength of these

356

D. C. NECKERS AND 0. M. VALDES-AGUILERA

emissions is due to the contact ion pair, whereas the higher wavelength pair of peaks are the result of the totally dissociated dianion. Specific examples illustrate that similar principles affect the absorption spectra. For example, as we have pointed out above, the neutral form of ,the C-2’ benzyl ester is red in MeOH and orange in methylene chloride. Thus it has the spectrum of the ionized form in the polar, protic solvent and of the nonionized form in the nonpolar solvent methylene chloride [248]. The tributyl ammonium salt of the C-2’ octyl ester is soluble in solvents ranging from ethanol-water to toluene. Its spectrum in an essentially nonionizing solvent such as toluene is that of the ionized xanthene [249]. The spectrum of the pyrillium salt in ethanol is concentration dependent. In dilute solution the compound is totally ionized and is red, whereas in concentrated solution the compound is not fully ionized and the orange form predominates, as predicted by the law of mass action.

2. C - 2 Substitution. In solvents that support the complete ionization of an ionic group at C-6, C-2’ substitution has a relatively small effect on the spectrum. Thus the I,,, of C-2’ esters is shifted only 5-7nm to the red relative to that of dianion, and this relative shift is essentially the same regardless of the solvent. This is probably the consequence of interaction of the negative carboxylate group with C-9, a center of relatively low electron density in the ion. There is little difference in the extinction coefficients of Rose Bengal (2-2’ esters and the disodium salt (Figure 10).

3. Effects of Substitution on Spectra: Solvent Effects. Solvent effects on the absorption spectra can be summarized as follows: if the compound is soluble in water, alcohols, and polar, protic solvents such as DMSO, DME, and DMF, the I,,, is most red shifted in polar, nonprotic solvents. Compounds that are soluble in nonpolar solvents such as CH,Cl, are generally not soluble in water, and their absorption lies at about the same place in both alcohols and methylene chloride, but is shifted to the red in polar, nonprotic solvents. The value of A,, also reflects the hydrogen bonding ability of the

‘0 6

I

6

0 0 0

I

Figure 10. Rose Bengal C-2’ ester.

PHOTOCHEMISTRY OF THE XANTHENE DYES

357

solvent-the shorter the wavelength of the maximum absorption, the stronger the hydrogen bond to the solvent from the xanthene. The same is true of the fluorescence maxima. 4. Aggregation Effects. Rose Bengal, disodium salt is soluble in water up to a concentration of approximately M. The spectrum in water, however, follows Beer’s law only below M. At higher concentrations subtle perturbations of the observed spectrum begin to appear, and the ratio of the two absorption maxima (A, /&) decreases. The accepted explanations attribute the effect to formation of aggregates of the dye in water, with aggregation being driven by hydrophobic effects [250,25 11. Though “aggregation” has been proven to really mean “dimerization” in very few systems of which we are aware, the terms aggregate and dimer are generally used interchangeably in the literature. The observed effect on the absorption spectrum of Rose Bengal of higher concentration of the dye is that the absorption at 550 nm decreases whereas the absorption at 525 nm remains the same or increases slightly. This results because the aggregate absorbs at the shorter wavelengths and the longer wavelengths with about the same extinction coefficient. Therefore, as the concentration of aggregate increases the longer wavelength absorption is sacrificed in favor of the shorter wavelength absorption. This accounts for the marked change in ratio of the longer wavelength absorption to that at shorter wavelength [EA,/EA~] as a function of concentration in aqueous solution described by early workers in the field [252].

D. Quenching of Rose Bengal Singlets and Triplets: Energy Transfer The triplet of Rose Bengal dianion is “quenched” by oxygen in polar solvents producing both singlet oxygen and superoxide radical ion [253]. Rate constants for oxygen quenching are dependent on oxygen pressure and are approximately 1/9 the diffusion-controlled limit under conditions of air saturation in all solvents. The details of this energy transfer have been described elsewhere but the quantum yields of singlet oxygen formation, fluorescence, and intersystem crossing in MeOH at 20” are compared for four of the xanthenes in Table 10. Singlet lifetimes, fluorescence quantum yields, and singlet oxygen quantum yields are compared for Rose Bengal, Erythrosin, Eosin, and fluorescein in a number of different solvents in Table 11. In virtually every case water differs from the other solvents studied. Singlet oxygen yields for several monomeric derivatives of Rose Bengal

358

D. C. NECKERS AND 0 . M. VALDES-AGUILERA

TABLE 10 Quantum Yield of Fluorescence, Triplet Formation, and Singlet Oxygen Formation for Simple Xanthenes"

Compound Rose Bengal, dianion Erythrosin, dianion Eosin, dianion Fluorescein, dianion

4f

4,

0.08 0.08

0.76 0.62 0.28

0.63 0.93

0.03

402a'b 0.76 0.62 0.39 0.09

"Singlet oxygen reactions measured at [dye] = 3.0 x using 2,5dimethylfuran as the singlet oxygen trap; solvent MeOH at 20". bQuantum yields are a function of solvent, and are also likely a function of dye concentration.

have been measured by Linden and Neckers [254]. Because Rose Bengal, disodium salt is only soluble in polar solvents, certain of the oxidation products formed in singlet oxygen reactions may be consumed by these solvents in secondary reactions. Classic examples include 2,5-dimethylfuran [255], which produces the hydroperoxide hemiacetal rather than the subsequently isolated intermediate ozonide [256,257]. In polar solvents the values for each of the compounds do not vary much. In nonpolar solvents, however, other processes, which are discussed later, come into play. The formation of superoxide radical anion (0;') from Rose Bengal was first reported by Srinivasan et al. [258]. 0;' was detected with superoxide dismutase and an oxygen electrode in water in the presence of sulfite. Rodgers and Lee later confirmed this result quantitatively [259] in water where 25% superoxide was observed. Superoxide formation may also require something other than direct electron transfer from Rose Bengal triplet, though this is surely not the case in the time-resolved measurements. Under the conditions used in the time-resolved experiments, Rose Bengal triplet produces 75% singlet oxygen and 25% superoxide when it is quenched by oxygen. The energetics allow electron transfer between the triplet and the ground state, followed by electron transfer from the semireduced Rose Bengal radical to dioxygen. The mechanism, however, is bimolecular in Rose Bengal and suggests that the 0;' yield should increase with the concentration of the dye. The quantum yield of singlet oxygen formation from Rose Bengal varies from 0.76 to 0.86 in water depending on who measures it. There is a 10-fold difference in dye concentration in the experiments by the Schenck and Gollnick, and Lion, Gandin, and van de Vorst groups, however, which may account for the difference [260]. Rose Bengal singlets are quenched by other electron acceptors, including several which are singlet oxygen traps. Davidson and Trethewey [261] have

W

01

w

Rose Bengal dianion Erythrosin dianion Eosin dianion Fluorescein dianion

Compound

-

0.77 [0.14]{0.73} 0.55 [0.10]{0.65} 3.15 [0.68]{0.39} ~

0.71 [0.13]{0.74} 0.55 [0.10]{0.65} 3.21 [0.71]{0.40}

~ _ _ _ _

0.69 C0.l ll(0.78) 0.55 [0.10]{0.65} 3.34 [0.72]{0.43} 4.48 [1.0]{0.07)

-~

0.44 [0.08]{0.62} 2.92 [0.63]{0.39} 4.16 [0.93]{0.09}

0.44 [0.08]{0.76}

MeOH

TABLE 11 Singlet Lifetimes, Fluorescence Quantum Yields, and Relative Singlet Oxygen Yield as a Function of Solvent Water

0.05 [O.Ol] 0.1 1 CO.021 0.51 [O.ll] 3.72 C0.831

~~

360

D. C. NECKERS AND 0. M. VALDES-AGUILERA

shown that a number of typical singlet oxygen acceptors, for example, pcarotene, certain anthracenes, and 2,5-diphenylisobenzofuran, interact with and quench both the singlet and the triplet of Rose Bengal [262,263]. Anthracene quenching of the triplet is endothermic and reversible [264]. In the case of Erythrosin the transfer is almost thermally neutral and the quenching constant is 1.7 x lo9 M-’s-’ . The reaction is reversible and the rate of the reverse reaction, triplet anthracene to Erythrosin, is found to be about twice the rate of the forward reaction. Therefore, triplet energy must not be the only factor affecting the transfer. Interaction of the singlet state is shown by the fluorescence quenching by many typical singlet oxygen quenchers as well as by a number of anions such as the iodide and azide of Rose Bengal [265]. Rodgers and Burrows [266] looked at energy transfer in reverse micelles. A reverse micelle allows for the dispersion of a polar substance in a nonpolar solvent. Thus singlet energy transfer from fluoranthene to Rose Bengal was reported at different concentrations in the solvent mixture cyclohexane/Manoxol OT [267]/water. The aromatic hydrocarbon was excited by a pulsed N, laser at 337 nm. The decay rate of the fluoranthene singlet was followed (460nm). In the absence of Rose Bengal, k , = 17.8 x lo6 sec-’. In the presence of Rose Bengal the decay rate decreased. In such systems the donor and the acceptor are compartmentalized in defined regions and molecular mixing is essentially prevented. It is suggested that energy transfer occurs by a long-range mechanism between excited donor in the hydrocarbon phase and acceptor present in the central region of the reversed micelles which contain “water pools.”

E. Dismutation by Electron Transfer: Dye Oxidation and Dye Reduction In a review, Eaton reported that couples consisting of many xanthene dyes and various reducing agents have been used as initiators of vinyl polymerization. Presumably these are triplet reactions [268]. In nonpolar solvents the iodonium salts of the dyes, whether esterified at C-2’ or not, bleach extremely rapidly. The gegen ion, 4J+, is easily reduced (Ered= -0.30 V vs. SCE) by the excited singlet state of the dye. Therefore the bleaching occurs by oxidative electron transfer from the dye to the gegen ion. Methylene chloride solutions of the iodonium salts of Rose Bengal bleach from the singlet state in a few seconds in room light. The resulting phenyl radicals have been reported to initiate acrylate polymerization [269]. Other onium salts of Rose Bengal bleach much more slowly. Xanthene dyes have also been reported as sensitizers for the reduction of

PHOTOCHEMISTRY OF THE XANTHENE DYES

361

water as models for solar energy conversion. In systems which are sacrificial, H + ions are reduced by electrons derived from an added donor. Such systems also include a colored photosensitizer, an electron-transfer agent, and an electron transfer catalyst [270]. Colored dyes can be crucial to the chemistry, and a number of groups have now considered such systems. Johansen et al. compared fluorescein, Eosin, Rose Bengal, and Rhodamine B. The system included the electron acceptor, methyl viologen, mv2+ which does not oxidize the dyes (nor are there dye-mv2+ complexes involved) but which reacts with the semireduced radicals formed by reduction of the dyes. The reaction scheme, in the presence of a platinum catalyst, is shown in Eqs. (32)-( 35). D + hv+

D(T) + AmD-' mv"

D-'

+ mv2+

--+

+ H+--Pt+

D(S)+

D(T)

+ Am+'

D + mv"

mv2+ + 1/2H2

(32) (33) (34) (35)

Rose Bengal was used as a sensitizer in a system utilizing anthracene-9carboxylic acid to accelerate the photoreduction. Flash photolysis studies indicate that, in basic solution, the triplet of the anion of anthracene-9carboxylate [271] is sensitized by transfer of triplet energy from Rose Bengal. Mau claims that energy transfer is much more efficient than electron transfer under the conditions of the experiment, and has demonstrated that this is the case by flash photolytic observation of the formation of anthracene-9carboxylate triplet from Rose Bengal triplet in the absence of EDTA. This energy transfer leads to a greatly improved quantum yield of hydrogen production because the anthracene-9-carboxylate triplet reduces mv+' more efficiently than does the semireduced radical derived from the dye. The watersplitting scheme is drawn in Figure 11. In the Mau scheme fluorescein, Eosin, and Rose Bengal are used in combination to collect the light [272]. In a related process developed by Gratzel, microparticles of collodial platinum are used as the reducing surface [273].

F. Decarboxylation One of the limitations in using the xanthene dyes as photoinitiators is their relatively low solubility in nonpolar media. Solubility considerations become important in dye-sensitized photopolymerization of multifunctional acrylates in the absence of solvent. In the search for faster initiators for three-

362

D. C. NECKERS AND 0. M. VALDES-AGUILERA

Eosin Rose Dengal-

X=510

amwvwww+

h = 550

Figure 11. Mau water-splitting scheme. The xanthene dyes serve as the antenna molecules collecting the solar energy, transferring it to aa-, which reduces methyl viologen (mv), which in turn reduces H'. EDTA reduces the oxidized aa to complete the cycle.

dimensional imaging applications, we [274] have developed a new group of visible light photoinitiators which, being uncharged, are more soluble than the parent xanthene dye in acrylate monomers. The reader may, perhaps, recognize 8 as being a derivative prepared from Rose Bengal which is first decarboxylated and then acetylated. We refer to 8 as RBAX. RBAX is a xanthone, analogous to the thioxanthones which are common commercial UV photoinitiators. The latter are used with typical

thioxanthone

363

PHOTOCHEMISTRY OF THE XANTHENE DYES

tertiary amine accelerators and therefore can be described, as can benzophenone/Michler's ketone, as electron transfer UV photoinitiators. In other words, the first step in the mechanism of their initiating polymerization is electron transfer from a donor, such as a tertiary amine, to the excited state of a thioxanthone. Based on the known photoreduction chemistry of Rose Bengal [275], one would anticipate that electron transfer would reduce the xanthene skeleton of RBAX and that the radical anion thence formed might decay by the elimination of an acetyl radical. Acetyl is totally analogous to benzoyl, the radical that initiates chains in the case of most Norrish type I UV photoinitiators, that is, benzoin ethers or acetophenone acetals. The putative scheme is shown in Scheme 7.

CH3C 0

p&J0

-0

I

I

Scheme 7.

One of the experimental limitations on the electron donor is that it must not catalyze the hydrolysis of the RBAX. It cannot, therefore, be a highly basic amine. We have chosen, therefore, to study the photoreduction of RBAX in the presence of a soluble nonhydrolytic donor, triphenyl n-butyl borate. To study the effect of substituents on the photochemistry and photopolymerization rate we prepared a number of derivatives related to RBAX. The general structure of these compounds is shown in Figure 12, where RBAX corresponds to R = Me. M in Figure 13 presents the absorption spectrum of RBAX, 1.19 x ethyl acetate. The molar absorptivities at 384 and 486 nm are 1.63 x lo4 M-' cm-l and 1.03 x lo4 M - ' cm-', respectively. The spectral shape as well

364

D. C. NECKERS AND 0. M. VALDES-AGUILERA

Derivative

R Me C6H5

CGH,(OMe)3

[3,4,5 Methoxyl

OCHpC6Hs OEt OCH(Me),

Figure 12. Structures of visible light photoinitiators.

as the molar absorptivities are similar to those reported for nonionic derivatives of Rose Bengal vide supra [276,277]. Irradiation of the solution einsteins/l x s) produces no change for 20 minutes at 514 nm ( I , = 1.26 x in the absorption spectrum. M) in the presence of triphenyl n-butyl Irradiation of RBAX (1.19 x M) results in the bleaching of RBAX and the borate ion (1.89 x

350

400

450 500 Wavelength (nm)

550

Figure 13. Visible absorption spectrum of RBAX (1.19 x

600

M) in ethyl acetate.

365

PHOTOCHEMISTRY OF THE XANTHENE DYES

appearance of an absorption peak at 560 nm which corresponds to I,,, for decarboxylated Rose Bengal (RB). In addition, fluorescence measurements show that the excitation spectrum is identical to the absorption spectrum of decarboxylated Rose Bengal. The bleaching products have not been isolated, but based on the known products from bleaching of Rose Bengal under reductive conditions [278], and the fact that oxidation of triphenyl n-butyl borate ion generates butyl radical [279], we expect the formation of the coupling product between RBAX and the butyl group. Therefore, the reaction can be represented in Eq. (36). RBAX + +,B(n-bu)- -+ Bleaching product + RB + 43B hv

(36)

The quantum yields of RBAX bleaching and of RB generation were determined by measuring the decrease in the absorption at 486 nm and the increase in absorption at 560 nm as a function of light intensity and borate concentration. The rates of RBAX bleaching and generation of decarboxylated Rose Bengal vary linearly with absorbed light intensity, whereas is independent of light the ratio +(RB)/$(-RBAX) is (8.6+ 1.2)x intensity, borate concentration (1.0-24 mM), and the presence of air. The effect of electron donor concentration on the quantum yield for photoreduction of RBAX in aerated solutions could be fitted by the equation describing saturation kinetics with a quantum yield of 0.39 at infinite borate concentration [274]. In argon-purged solutions the value obtained was 0.37. In view of the low borate concentrations required to observe the photoreduction, we conclude that the reactive excited species is the triplet state of RBAX. Measurements of the lifetime of RBAX triplet by laser flash photolysis yield values of 3.64 p s in argon-saturated solution and 288 ns in the presence of air. These lifetimes are independent of the energy of the laser pulse and of the concentration of RBAX. A mechanism consistent with these observations is presented in Scheme 8, where, n-bu' and Ac' represent n-butyl and acetyl hv

RBAX +RBAX(T) RBAX(T) +RBAX +RBAX + 0, RBAX(T) + 0, RBAX(T) 43B(n-b~)---+ RBAX $,B(n-bu)RBAX(T)+ 43B(n-b~)---+ {RBAX'- n-bu'} + 43B {RBAX'- n-bu') +Bleaching product {RBAX'- n-bu'} +RBAX'- n-bu' RBAX'RB Ac'

+

-

+

+

Scheme 8.

+

Ia4T kl k2

k3

k, k5 k6 k7

366

D. C. NECKERS A N D 0. M. VALDES-AGUILERA

radicals, respectively. The observation that the rate of RB generation varies linearly with the absorbed light intensity is evidence that the radicals RBAX' - and n-bu' do not undergo significant recombination after dissociation of the geminate pair. The independence of 4(RB)/4(-RBAX) on the presence of air indicates that RBAX'- is not significantly quenched by oxygen during the lifetime of the radical. Assuming that oxidation of RBAX'- by oxygen is diffusion-controlled, we obtain a minimum value for k, of 4 x 10' s-'. Deactivation of the geminate pair by oxygen would affect the photoreduction quantum yield at infinite borate concentration. Our results are evidence that the geminate pair is not deactivated by oxygen under our conditions. Table 12 presents the quantum yields of photoreduction and RB generation in aerated ethyl acetate for the six derivatives shown in Figure 12. The ratio q5RB/4-dye is independent of the identity of the radical generated by cleavage of the semireduced dye. The free energy of formation of these radicals in the gas phase [280] ranges from 3.1 kcal/mol for benzoyl to - 67 kcal/mol (calculated) for isopropoxycarbonyl. We would expect the ratio 4RB/4-dye to depend on the nature of the radical released if cleavage of the semireduced dye occurred in the geminate pair, since it would compete with the formation of bleaching product. However, as we show later in our description of photopolymerization in thick samples, the nature of this radical has a moderate though significant effect on the rate of photopolymerization. Photopolymerization in thin films was carried out at 514 nm, the rate of heat evolution being measured by thin-foil photocalorimetry. The monomer formulation consisted of 85% TMPTA and 15% HDDA, giving a concen-

TABLE 12 Photoreduction Quantum Yields for Derivatives in Figure 12 in Aerated Ethyl Acetate" Derivative

($-dye

0.132 0.115 0.135 0.153 0.113 0.129

4RB

1.07 x lo-' 9.31 x 9.56 x 1.38 x 9.69 x 1.08 x lo-'

&B/6dye

8.1 x 8.1 x 7.1 x 9.0 x 8.6 x 8.4 x

lo-' lo-' lo-' lo-' lo-'

"Dye and borate ion concentrations are 0.15 and 2.1 mM respectively. Data from 0.Valdes-Aguilera, C. P. Pathak, J. Shi, D. Watson, and D. C. Neckers, Macromolecules 25, 541 (1992).

PHOTOCHEMISTRY OF THE XANTHENE DYES

367

tration of double bonds of 10.8 M. Irradiation was carried out with the argon ion laser, the beam being expanded to a circle of 1.1 cm diameter. The conversion of double bonds was measured by IR spectroscopy. Determination of the absorbance at 810cm-' before irradiation and after cessation of the reaction yields a value for the maximum conversion of 45%, or 4.86 M. Heat evolution starts after an induction period, the reciprocal of which increases linearly with the light intensity and is independent of the borate concentration. The initial rate of photopolymerization is proportional to the square root of the light intensity, and at constant intensity the rate of photopolymerization is independent of borate concentration. Based on these results we write Eq. (37) for the initial rate of photopolymerization, where [MIo is the initial concentration of double bonds (10.8M, Ri the rate of R, = [M]oR~.5k,/k~~S

(37)

initiation, and k, and k, the rate constants for propagation and bimolecular termination, respectively. The rate of initiation, equal to twice the rate of RB generation, is given by Eq. (38) (see Scheme 8):

Measurements of the lifetime of RBAX triplet in the monomer formulation, but in the absence of borate, yields values of 3.2 and 1 . 6 ~ in s argonsaturated samples and in the presence of air, respectively. Our results indicate that the photopolymerization order with respect to 6,B(n-bu)- is zero, implying that (k3 + k4) is greater than 3 x lo9 M-'s-'. The rate constant calculated for a diffusion-controlled reaction [28 11 in this monomer formulation is 1.7 x lo* M-'s-' , and we conclude that under these conditions quenching of the RBAX excited state occurs, at least partially, by a static mechanism not requiring diffusion. The quantum yield of photopolymerization, as well as the kinetic chain length (+,/&J, decrease with the square root of the absorbed light intensity. For an absorbed light intensity of 1.06 x einsteins/l x s and borate concentration of 10 mM the photopolymerization quantum yield is 1420. The quantum yield for radical generation is 0.067 [274], giving a kinetic chain length of 2.1 x lo4, which compares favorably with the value of 2.9 x lo4 reported for the UV photopolymerization of epoxy diacrylate/TMPTA in the presence of air [282]. Photopolymerization of the system we have studied appears to proceed by the common mechanism in which termination occurs by reaction between two macroradicals. Analysis of the photocalorimetry traces at different light intensities for our initiator-monomer system shows no evidence for a

368

D. C. NECKERS AND 0. M. VALDES-AGUILERA

conversion-dependent order with respect to light intensity up to conversions approaching 40%, indicating that bimolecular termination is predominant even at relatively high conversions. Photopolymerization rates in thick samples were determined gravimetrically. Irradiation was carried out in glass cuvettes with the argon ion laser, the beam diameter being 1.4 mm. The volume of sample irradiated is a small fraction of the total, simulating the conditions found in stereolithography. The polymer spike (Figure 7) is isolated, washed with acetone, and dried until a constant weight is obtained. To prevent losses during the isolation step, the solid must be strong and this limits the kind of formulation we can study using this methodology. An appropriate monomer formulation is 90% dipentaerythritol hydroxypentaacrylate (DPHPA) and 10% l-vinyl-2pyrrolidinone (VP). Experiments performed with different power densities in the range from 3 to 12 W cm-' indicate that the rate of photopolymerization varies with the square root of the laser intensity. Table 13 presents the rate of photopolymerization measured for the six derivatives shown in Figure 12 and the values of k,/kp.5 (relative to RBAX) derived by combining the rate of photopolymerization with the quantum yields of RB generation presented in Table 12. Because the rate constant of propagation is independent of the identity of

TABLE 13 Photopolymerization Rates and Relative Values of k,kF5 for the Compounds in Figure 12"

Dye 1 2 3 4 5 6

Rate (mg s-') 22.9 26.1 31.1 23.1 31.0 29.6

k,/kp.5 1 1.22 1.43 0.89 1.42 1.28

'Monomer formulation is 90% DPHPA, 10%VP. Dye and N P G concentrations are 0.5 and 50 m M respectively. Laser power density=7.3 W cm-* (514nm). Data from 0. Valdes-Aguilera, C. P. Pathak, J. Shi, D. Watson, and D. C. Neckers, Macromolecules 25, 541 (1992). bRelative to RBAX.

PHOTOCHEMISTRY OF THE XANTHENE DYES

369

the initiating radical, the variation of k,/k;.’ shown in Table 13 must be due to different rate constants for termination. Because the rate constant for termination between two macroradicals is independent of the initiating radical, we interpret these results as evidence that small initiating radicals are involved in the termination process. However, the dependence of k, on the identity of the initiator does not exclude a contribution of bimolecular termination between two polymer radicals. To estimate this contribution we have synthesized an ester derivative of RB containing a polystyrene fragment of molecular weight equal to 90,000 (R = polystyrene in Figure 12). The details of the synthesis and the kinetics of photopolymerization using this initiator will be published in another journal [283]. Of relevance to the results presented in Table 13 is the observation that with this polymeric initiator the rate of polymerization varies linearly with the laser intensity, indicating that bimolecular termination between polymer radicals is too slow to be competitive with radical occlusion. Therefore we conclude that in our systems, for which we observe one-half order with respect to light intensity even at high conversions, termination occurs not by reaction between two polymer radicals but between one macro and one small radical, the latter being produced by cleavage of the semireduced dye initiator. Scheme 9 presents a general mechanism consistent with the photopolymerization results we observe with our photoinitiators and electron donors. Table 14 shows relative values of termination rate constants for the radicals generated by cleavage of semireduced dye. Most of the radicals are less effective terminators than acetyl with the exception of

TABLE 14 Relative Rate Constants of Termination Between a Polymer Radical and R;“ R;

CH,C=Of$C=O-

(MeO),dC=O* f$CH,OC=OEtOC==O* (Me),CH0(3=0-

4 (R;)lk,(CH3 c = 0 9 1.oo 0.67 0.48 1.26 0.50 0.61

“See Scheme 9. Data from 0. Valdes-Aguilera, C. P. Pathak, J. Shi, D. Watson, and D. C. Neckers, Macromolecules 25, 541 (1992).

370

D. C. NECKERS AND 0.M. VALDES-AGUILERA

benzyloxycarbonyl, which can easily lose carbon dioxide to generate benzyl radical. The radicals shown in Table 14 are expected to be good initiators. We would expect them to add to unreacted monomer as soon as they are produced and, as a consequence, not to be available for termination when the photopolymerization is carried out under steady-state irradiation. However, our results indicate that these radicals are the main chain terminators in our systems. We postulate that this effect is partially due to RBAX'- being a longlived species in the monomers we have employed in this work. From our bleaching results we estimate a lifetime shorter than 3 ns for RBAX'- in ethyl acetate. It ispagate to a considerable extent before the terminator radical is generated. D + Donor

hv

D' - +(Donor)"

(Donor)" +R; +acid

D'--

R',+RB

R;+M---+M;

-

M;+M-M; M;-,+MM i + R;

propagation

Mi

inert polymer

termination

Scheme 9.

R', = acetyl when D = RBAX (see Figure 2), acid = q53B when Donor = triphenyl n-butyl borate ion, acid = H + when Donor = N-phenylglycine.

G. Photobleaching and Photofading It seems superfluous to mention that dyes were originally synthesized as color formers for cloth. None of the xanthenes in this chapter are worth much in this arena because they are fugitive or fade, that is, bleach [284]. Rhodamine 6G is still used, however, and has a popular brilliant red-purple color. The products from leuco Rose Bengal prepared under reducing conditions have been isolated by Zakrzewski [285]. The primary photoreduction product of Rose Bengal is the dihydro compound (reduction of the quinomethine) though under harsher reducing conditions it has been reported by earlier workers to produce tetrachlorofluorescein [286].

PHOTOCHEMISTRY OF THE XANTHENE DYES

371

H. Photopolymerization At concentrations of dye > M in water and ethanol, triplet reaction of the triplet with ground state dye is a primary reaction, and this results in electron transfer forming the semireduced and semioxidized radicals [287]. At lower dye concentrations, triplets may be intercepted by reducing agents [288] and this is a primary method for forming radicals from xanthene dyes [289]. Electron transfer to the xanthenes, particularly reduction with amines, has been used for a number of years to initiate acrylate polymerization. A typical system is that reported to form volume holograms-lithium or zinc acrylate, triethanolamine and Eosin, Erythrosin, or Rose Bengal [290]. Similar mixtures are used to form printing plates: photoreducible dye, phenylacridine, and acrylate monomer [292]. A recent patent application discloses aryl iodonium salts, Rose Bengal, and oxidizable triazines such as 2-methyl4,6-bis(trichloromethyl)-s-triazineto polymerize acrylates [292].

I. Polymeric Rose Bengals Polymer Rose Bengal, synthesized by Neckers’ group in 1972, was the first successful polymeric sensitizer [293] though Petterson had suggested using solid Rose Bengal previously [294]. This work was based on the early experiments of Kautsky and de Bruihn [295] who concluded that when “two dyes separately absorbed on a surface, one a sensitizer and the other an acceptor, were irradiated at the absorption of the sensitizer such that the acceptor bleached, a metastable state of oxygen was involved.” Polymer Rose Bengal was synthesized using Merrifield chemistry [296] from chloromethylated polystyrene beads [297] and has quantum yields of singlet oxygen formation about the same as does Rose Bengal in similar solvents. It does not bleach, does not undergo self-quenching, and therefore seems not susceptible to electron transfer between dye molecules, a phenomenon induced by dye site isolation on the backbone of the polymer. Many polymeric Rose Bengals have been studied in detail by Paczkowski and Neckers [298]. In addition to its role as a sensitizer for singlet oxygen formation, Rose Bengal also serves as a probe of dye/dye proximity. Because of intramolecular energy transfer, the effect of one dye interacting with another dye on a polymeric support is to decrease energy transfer to oxygen. This has been demonstrated in noncrosslinked polystyrenes to which increasing quantities of Rose Bengal have been attached. The quantum yield of singlet oxygen formation rises to a maximum at a loading of about 0.39 mg of Rose Bengal and drops from this maximum regularly as the loading

372

D. C. NECKERS AND 0. M. VALDES-AGUILERA

increases beyond that point. This drop in quantum yield of singlet oxygen formation is accompanied by a regular increase in the extent of aggregation of the dye on the polymer backbone as assessed from the changes occurring in the absorption spectra. Rose Bengal on Merrifield resins which are crosslinked and hence maintain separation of dye units does not change the singlet oxygen yield much as a function of loading. The dyes are essentially site isolated to the point that no matter how high one loads them on the crosslinked beads they do not come close enough to interact. Luttrull et al. assessed the extent of dye interaction in a series of rationally synthesized dimeric Rose Bengals (Figure 14). Based on extensive studies of the spectra of these compounds in ethanol and ethanol-water, these workers concluded that the compounds exist in an open conformation in EtOH, but that the extent of dye/dye interaction increases as the solution becomes more aqueous. Thus the extinction coefficient at the maximum decreases as the amount of ethanol in the solution decreases. The more hydrophobic the groups at C-6 (changing, for example, from Na to tributylammonium), the more changing the solvent toward an increasing water content decreases the extinction coefficient, that is, enforces aggregation. The ratio of the extinction coefficients in water-ethanol is a function of the length of the chain, n. Thus Ewater/EEtOH is 0.725 for Rose Bengal ethyl ester, C6 Na, 0.430 for the dimeric compound, disodium salt (n = 5); 0.254 for the dimeric compound, disodium salt (n = 10); and 0.492 for the dimeric compound (n = 16) [299]. One significant change that can modify the efficiency of singlet oxygen formation in an immobilized Rose Bengal is a change in the gegen ion at (2-6. As the gegen ion becomes of increasing hydrophobicity, the immobilized dyes become more compatible with the organic solvents in which they are generally used, and the quantum yield of singlet oxygen production increases. All of this has been detailed in the references [300]. Rose Bengal has been immobilized on a number of other supports. In the original patent (Neckers, Blossey, and Schaap) a number of these are listed.

R

19

Figure 14. Rose Bengal dimers.

R

PHOTOCHEMISTRY OF THE XANTHENE DYES

373

Among the more interesting applications of polymer Rose Bengal is that of a sensitizer in studying the oxidation of other polymeric substrates [301]. Rose Bengal immobilized on Sepharose has been reported as a sensitizer for protein photooxidation [302]. The oxygen uptake by the amino acids cysteine, hisitidine, methionine, tryptophan, and tyrosine was reported to be about 20% as much from the immobilized dye as from the free dye in aqueous solution. Silica gel as a support for Rose Bengal was reported by Tamagaki, Liesner, and Neckers [303]. A similar system had been reported previously with the sensitizer methylene blue [304]. Rose Bengal immobilized to poly(2hydroxyethyl methacrylate) has also been reported by Schaap [305].

J. Photobiological Applications The literature on the photobiology of Rose Bengal is enormous. Like Eosin it was used as a biological stain early on and, until the mid-l960s, to assay for liver function. It has been used to screen for methamphetamine in urine [306]. It remains in use today to assess Vitamin A status in undernourished children [307]. Rose Bengal has also been used to study damage mechanisms such as those caused by singlet oxygen in the eye [308]. It was used as a food coloring in the past and has been tested for the induction of thyroid tumors in mice [309]. Our purpose is to outline the areas of research which it has affected, but we could not hope to consider, even superficially, every publication. There are more biological reviews to which the reader interested in more detail is referred [310]. Singlet oxygen was discovered originally by Raab who first reported the photodynamic effect of dyestuffs, light, and oxygen [311]. The biomedical implications of this discovery have only recently been made clear, particularly by Dougherty and his co-workers, who have demonstrated the implications of the photodynamic effect in tumor therapy [312]. The mechanisms by means of which visible light, oxygen, and Rose Bengal cause photodynamic damage of microorganisms was first outlined by Krinsky [313] and Foote [314]. Singlet oxygen formation is one; the other is rather loosely defined as “hydrogen atom/electron transfer from sensitizer to substrate, from which free radicals form which react with oxygen.” Rose Bengal is the most effective photodynamic singlet oxygen sensitizer because it is water soluble and has the highest triplet yield of the xanthene dyes [315]. The target of singlet oxygen in vivo has been the subject of a huge number of studies as it has the potential to change chemical reactivity, and hence biological function, at many sites. Unsaturated fatty acids, for example, are excellent targets [316] and peroxidation of cell membranes is probably

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D. C. NECKERS AND 0.M. VALDES-AGUILERA

important 1317,3181. Carotenes appear to protect against damage either by quenching the sensitizer or, more likely, by quenching singlet oxygen [3 191. Histidine and tryptophan are the amino acids most susceptible to singlet oxygen [320,321], Rose Bengal having been introduced as a sensitizer for same by Westhead [322]. Studies of enzyme inactivation and modification [323] are complicated by dye binding [324] (the anionic dye is strongly bound to many enzymatic sites). The suggestion that Rose Bengal has more specificity for certain amino acid centers in enzymes than do other singlet oxygen sensitizers is complicated as a result [325]. Histidine is, for example, said to “protect” against damage because it quenches singlet oxygen, and histidine has been of particular interest in Rose Bengal sensitized photodynamic studies [326-3281. Polymer Rose Bengal specifically sensitizes histidine [329]. Specific binding has the advantage of targeting the singlet oxygen only to certain amino acid centers in the enzyme. This strategy was first intimated in the late 1960s [330], and subsequent application of the strategy led to elucidation of the mechanism by which Rose Bengal mediates the photoinactivation of 6-phosphogluconate dehydrogenase-it binds to the active site of the enzyme [331] (photooxidation of ribonuclease, for example [332]). This strategy was immediately adopted by others [333-3351. The reaction with acetylcholinesterase has been studied in some mechanistic detail [336]. Kochevar and her co-workers claim that excitation of Rose Bengal with a nitrogen laser at 308 nm resulted in 50% greater inhibition of red blood cell acetylcholinesterase than did excitation with the 514-nm line of an Argon (+) laser. Both reactions were quenched by sodium azide and the rates reduced in the presence of argon. The rate of enzyme inhibition was essentially reduced by the same amount by quenchers at both wavelengths. Pulsed irradiation (532 nm) resulted in a decrease in inhibition with increasing intensity, suggesting that ground state depletion by triplet-ground state reactions was responsible for the lower enzyme inhibition. The authors suggest that population of a higher singlet state results in an enhanced sensitizing efficiency. A number of workers have looked at the effect of photooxidation and photodynamic sensitizers on DNA. Rose Bengal photosensitizes strand breaks in double-stranded, supercoiled, pBR322 DNA; the effect follows firstorder kinetics with respect to light fluence and dye concentration. The reaction is substantially more efficient in the absence of oxygen, but the quantum yield of strand breaks in air is only lo-*. The results are consistent with the initiation of chain scission by Rose Bengal triplet, with some additional mechanism coming into play in the presence of oxygen. The cytotoxic and photodynamic inactivation of microorganisms by Rose Bengal is an important line of research [337]. Rose Bengal causes ATP depletion [338] in killing Escherichia coli B/r [339]. Since E . coli is also killed

PHOTOCHEMISTRY OF THE XANTHENE DYES

375

by irradiation of Polymer Rose Bengal [340] in an oxygen stream, singlet oxygen is obviously implicated in the killing event [341]. Polymer Rose Bengal is said to have bactericidal action, and cell sensitization has been observed with both the immobilized and free forms [342]. The target, in early studies, was suggested to be RNA polymerase, to which Rose Bengal binds rather strongly [343]. Rose Bengal is also said to affect DNA polymerase [344], inhibiting it reversibly in the light and irreversibly in the dark [345]. Among the more important observations, and one which clearly fits with everything else we know about Rose Bengal photochemistry, is that oxygen independent damage of DNA (and backbone damage) can occur [346]. The mechanism of the photoinactivation of bacteria by Rose Bengal [347] is discussed by Dahl, Midden, and Neckers, who studied the photodynamic inactivation and the difference in penetration of the cell envelopes of grampositive and gram-negative bacteria by Rose Bengal [348]. These workers found that the gram-positive types (Bacillus subtilis, Staphylococcus aureus, Streptococcus faecalis, and Streptococcus salivarious) were inactivated about 200x more quickly than a Salmonella typhimurium wild-type strain. A lag time was observed with the Salmonella bacteria, and this was suggested to be the result of the failure of the dye to immediately enter the cell envelope. The results obtained are notably similar to results obtained with externally generated singlet oxygen, which suggests the mechanism [349]. Subsequent studies determined that the binding site of Rose Bengal in Salmonella locates the dye in a nonaqueous environment upon binding, and that the site is in the outer membrane. Time-dependent sensitizer locale follows time-dependent changes in the effectiveness of killing. The results provide clues as to the factors that determine photodynamic sensitivity. The photoinactivation of viruses, for example, in the treatment of waste waters, is one of those areas about which scientists dream. Through a sun-activated dye in the pond, and while it bleaches in sunlight, it kills noxious bacteria and organisms [350]. Organic wastes in streams are not completely removed by sunlight, but photodynamic action is one way in which they can be reduced. An early review of the subject is that of Sargent and Sanks [351]. Williams holds a patent on heterogeneous photosensitizers for such a purpose [352]. Viruses have been the subject of a number of investigations [353] and singlet oxygen from sensitized reactions plays a prominent role, in theory at least [354]. Studies, in the main, use azine dyes as sensitizers except for recent targeting studies reported by the Midden-Neckers group [355]. Another use of Rose Bengal as a sensitizer is in identifying reaction processes as being caused by singlet oxygen. In a study of aerobically perfused rat hearts, the in situ photoactivation by very low levels of Rose Bengal was shown to lead to rapid development of electrocardiogram abnormalities and arrhythmias [356]. This effect was light dependent and

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D. C. NECKERS AND 0. M. VALDES-AGUILERA

caused irreversible damage to the heart. The workers suggest that the formation of short-lived reaction intermediates from Rose Bengal, oxygen, and light cause the damage, and imply that reperfusion injury to the heart muscle upon treatment for heart abnormalities may be caused by the same reaction intermediates. Rose Bengal/light effects on frog cardiac muscle suggests that they are similar to the effect of xanthine-xanthine oxidase or hydrogen peroxide [357,358]. In a study of rabbit skin, Rose Bengal was used to induce damage-with light. Nanomolar concentrations, when irradiated with visible light for 30-120 minutes, caused an increase in vasular permeability and the accumulation of polymorphonuclear leucocytes. The authors claim that 10 picomolar Rose Bengal had a discernable effect [359]. Rat pancreas cell membrane becomes more permeable when irradiated with Rose Bengal in oxygen-an effect of singlet oxygen [360]. Among the more interesting applications of Rose Bengal sensitized photochemical processes is its use as an insecticide. Heitz and his co-workers have reported a number of insects which can be “singlet oxidized to their eternal reward.” Among them are the housefly (Musca domestica) [361], face fly (Musca autumnalis) [362], Trypanosoma cruzi [363], Staphylococcus aureus [364], Culex pipiens quinquelasciatus [365], larvae of the cabbage looper, corn earworm, and pickle looper [366], boll weevil [367], fire ant [368], and, when fed, to photosensitize cockroaches to death [369]. Rose Bengal has even been fed to cattle for “insect control.” It is also said to participate in the degradation of wood surfaces by “generating singlet oxygen” [370], to affect the growth of duckweed [371] and the red-tide organism, and even to sterilize medical and surgical instruments [372].

VI. HYDROXYFLUORONES The literature of the xanthenes provides many examples of compounds with various substituted aromatic nuclei at C-9, and other examples with different substituents on the xanthene ring. The basic skeleton, however, has not been modified in recent years. It is apparent that the crowded aromatic subunit will slow reactions involving the formation of intermediates involving reaction at C-9 and may retard the subsequent reactions of the intermediates as well. This observation predicts slower rates of electron transfer, for example, and subsequent photoreduction than would be expected for unsubstituted xanthenes. Our interest was to synthesize a new series of dyes without substituents at that position and to investigate the effect of C-9 substitution on photochemical and photophysical properties.

377

PHOTOCHEMISTRY OF THE XANTHENE DYES

Recently, several novel xanthene dye derivatives with H and CN groups at the 9 position have been synthesized [373]. Their general structure is 9, with

9

M = H or CN and X = H, Br, or I. The following notation is used for these derivatives: M

X

Abbreviation

H H H CN CN CN

H Br I H Br I

HF TBHF TIHF HCF TBHCF TIHCF

Comparison of these new compounds with the common xanthene dyes reveals that substitution of H at C-9 has only a minor influence in the absorption and emission spectra. Table 15 summarizes the spectral properties of HF, TBHF, and TIHF as well as the acid dissociation constants and electrochemical potentials. The color, fluorescence quantum yield, apTABLE 15 Properties of Novel Xanthene Dyes Derivatives: H-Substituted Compounds"

Property 1nIa,,

nm

41, nm 4fl

Tfll

ns

pK,(10% MeOH/H,O) E,,, V (vs. SCE) Ere,,,V (vs. SCE)

HF

TBHF

TIHF

504 513 0.95 3.58 5.97 1.04 - 0.95

530 539 0.52 2.33 3.29 1.09 -0.95

536 548 0.13 0.65 4.08 1.34 -0.99

"Unless otherwise indicated, the solvent employed is ethanol.

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D. C. NECKERS AND 0 . M. VALDES-AGUILERA

pearance of fluorescence, lack of phosphorescence, and electrochemical potentials of H F are virtually identical to fluorescein. TBHF resembles Eosin in virtually every regard and TIHF resembles Erythrosin. The cyano sugstituent at C-9 gives the compounds entirely different properties. The 9-CN group allows the formation of a n-electron system highly delocalized through the whole molecular structure. That leads to a red shift of about 100 nm in the absorption and emission spectra (Table 16). Laser flash photolysis has been performed with the halogenated derivatives in ethanol [374]. Measurements of the triplet lifetimes as a function of dye concentration, laser power, and in the presence of electron donors allowed us to determine the rate constants collected in Table 17. For the case of TBHF and TIHF, the triplet lifetime is strongly dependent not only on the concentration of the dye, which indicates the important role of the self-quenching process, but also on the concentration of triplets. At high laser dose and low dye concentration the triplet decay trace includes a second-order component that could be attributed to triplet-triplet annihilation. Unlike TBHF and TIHF, the dyes containing the CN-group at C-9 position give triplets which decay by first-order kinetics not influenced by dye concentrations or laser dose. Thus we observed no triplet-triplet annihilation process. This observation may be due to insufficiency of the absorbed energy (optical densities for the CN-containing dyes are much less at A,, than for Hdyes at the same concentration), but we had to carry out our experiments using low dye concentrations in order to avoid the aggregation processes which are well known for xanthene dyes. Varying the ground-state concentration does not have a strong effect on the triplet lifetime, though the value of the self-quenching rate constants are rather high (Table 17). The nonradiative decay of the triplet state (first reaction, Scheme 3) begins to play an important role since k, is comparable to (k, + k3)x [D] or even higher, depending on ground state concentration. Since the triplet lifetimes are the same as dye recovery lifetimes, we can TABLE 16 Spectral Properties of Novel Xanthene Dyes Derivatives: CN-Substituted Compounds" Property

"The solvent is ethanol.

HCF

TBHCF

TIHCF

594 608 0.38

626 638 0.20

638 654 0.02

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PHOTOCHEMISTRY OF THE XANTHENE DYES

TABLE 17 Laser Flash Photolysis Results with Novel Xanthene Dyes Derivatives" TBHF

TIHF

TBHCF

TIHCF

2.4 x 104 2.0 108

1.5 x 105 1.0 x 109

1.5 x lo9

8.9 x lo8

~~

k,, s-' (kZ+k3), M-'s-' (k4+k5), M - l s - ' k6, M-'s-'

0.9 x 3.7 x 1.7 x 1.5 x

10' 108 109 107

2.0 x 1.4 x 1.2 x 2.4 x

103 109 10'0 107

-

-

"The solvent is ethanol. See Scheme 3 for reactions. Am = diphenylamine.

conclude that there is no disproportionation (third reaction, Scheme 3) for CN-substituted derivatives. Indeed, disproportionation of the CN-dyes with cation-radical formation does not occur because of the strong electron acceptor properties of CN-substituted derivatives. The difficulty of electron removal from dyes of this kind was confirmed by electrochemical experiments (cyclic voltammetry and square wave voltammetry), in which no peaks corresponding to oxidation of the dyes were observed.

VII. SUMMARY This chapter has concentrated on the photochemistry, photophysics, and numerous applications of the hydroxyxanthenes, among the most versatile of the synthetic dyes. The literature concerning Rose Bengal seems to have grown most rapidly in recent years, in large measure because of its numerous applications. As we have studied these systems over many years, we find that their chemistry is rational, though complicated, and their spectroscopy, rich with information, is rewarding. We have tried to cover the subject broadly, but there is no possibility that we could do so exhaustively. We hope that readers will bear that in mind, and share additional information about the xanthenes and related systems with us as it comes to their attention.

GLOSSARY AH' AH2 Am Am"

Semioxidized ascorbic acid radical Ascorbic acid Amine Amine radical cation (radical center on nitrogen)

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D. C. NECKERS A N D 0. M. VALDES-AGUILERA

a-Am' Ar,BRAr,BR' BOA BMOA n-bu' COA D D(S) D(T) D+' D-' DH' DH, DPHPA HDDA 1, kP

k,

M M;, MDEA MV' MV" NPG PDO POA PhOA R' RB +

4i 4P

4 T

4 3

@,B(n-bu)-

a-Amino radical (radical center on a-carbon atom) Aryl alkyl borate ion Aryl alkyl boranyl radical Benzophenone oxime acrylate 2,3-Butadione monooxime acrylate n-Butyl radical Cyclohexanone oxime acrylate Dye in the ground state Dye in the lowest excited singlet state Dye in the lowest excited triplet state Semioxidized dye radical Semireduced dye radical Protonated semireduced dye radical Leuco dye Dipentaerythritol hydroxypentaacrylate 1,6-Hexanediol diacrylate Absorbed light intensity Rate constant of propagation Rate constant of termination Monomer n-Mer radical derived from M N-Methyldiethanol amine Methyl viologen (1,l'-dimethyl- 4,4'bipyridinium) Semireduced methyl viologen radical ion N-Phenylglycine Oxime acrylate Pyrene-1 -carboxaldehyde oxime acrylate 9-Phenanthrenecarboxaldehydeoxime acrylate Ra dica1 Decarboxy Rose Bengal Quantum yield of initiation Quantum yield of polymerization Quantum yield of triplet formation Triphenylboron Triphenyl n-butyl borate ion

ACKNOWLEDGMENTS This work has been supported by the National Science Foundation, the National Institutes of Health, and Mead Imaging-a division of Mead Corporation. The authors are indebted to these contributors to their

PHOTOCHEMISTRY OF THE XANTHENE DYES

381

program. The authors are pleased to make Chemical Abstracts Literature Searches on the dyes discussed in this chapter available to readers at cost.

REFERENCES 1. J. Gronowska and H. Dabkowska-Naskret, Polish J . Chem. 55, 2151 (1981; J. Gronowska, H. Dabkowska, and H. Walerys, ibid. 53, 403 (1979). 2. A. Baeyer, Chem. Ber. 4, 555 (1871). 3. A. Baeyer, Chem. Ber. 8, 146 (1875). 4. A. von Hoffmann, Chern. Ber. 8, 55 (1875). 5. A. Baeyer, Annalen 202, 36; ibid. 153 (1880). 6. L. F. Fieser and M. Fieser, Organic Chemistry, Heath, Boston, 1943, pp. 847. 7. C. Graebe, Ann. 238, 318 (1881). 8. A. Baeyer, Chem. Ber. 4,662 (1871);A. Baeyer, Annalen 183,2 (1876);ibid. 202, 36 (1880); ibid. 212, 347 (1882); ibid. 372, 107 (1910), and E. Fischer, Chem. Ber. 7, 1211 (1874). 9. Conn’s Biological Stains, 9th ed. Williams and Wilkins, Baltimore, MD, 1982, Chapter 13. 10. J. P. Dubost, J. M. Leger, J. C. Colleter, P. Levillain, and D. Fompeydie, C. R. Acad. Sci., Paris 292, 965 (1981). 11. R. P. Haughland, Handbook of Fluorescent Probes and Research Chemicals, Molecular Probes, Inc., Eugene, Oregon. 12. Fluorescent Protein Tracing, Williams and Wilkens, Baltimore, MD., 1962. 13. R. V. Stirling, Trends in Neurosci. 147 (1986). 14. K. Wang, J. R. Feramisco, and J. F. Ash, Methods Enzymol. 85, 514 (1982). 15. D. L. Taylor and Y. L. Wang, Nature 284, 405 (1980). 16. T. E. Kreis and W. Birchmeier, Int. Rev. Cytol. 75, 209 (1982). 17. A. L. Zimmerman and B. Rose, J . Membr. Biol. 84, 269 (1985). 18. R. Markuszewski and H. Diehl, Talanta 27, 937 (1980). 19. M. L. Graber, D. C. Dilillo, B. L. Friedman, and E. Pastoriza-Munoz, Anal. Biochem. 156, 202 (1986). 20. R. F. Murphy, S. Powers, and C. R. Cantor, J . Cell. Biol. 98, 1757 (1984). 21. M. J. Geisow, Exp. Cell Res. 150, 29 (1984). 22. D. F. Babcock and D. R. Pfeiffer, J . Biol. Chem. 262, 15041 (1987). 23. P. Soucaille, M. Prats, J. F. Tocanne, and J. Teissie, Biochim. Biophys. Acta 939, 289 (1988). 24. M. Thelen, G. Petrone, P. S. O’Shea, and A. Azzi, Biochim. Biophys. Acta 766, 161 (1984). 25. M. Prats, J. F. Tocanne, and J. Teissie, Eur. J . Biochem. 162, 379 (1987).

382

D. C. NECKERS AND 0. M. VALDES-AGUILERA

26. M. Prats, J. Teissie, and J. F. Tocanne, Nature 322, 756 (1986). 27. M. Prats, J. F. Tocanne, and J. Teissie, Eur. J . Biochem. 149, 663 (1985). 28. J. Teissie, M. Prats, P. Soucaille, and J. F. Tocanne, Proc. Natl. Acad. Sci. 82, 3217 (1985). 29. M. Thelen, P. S. O’Shea, G. Petrone, and A. Azzi, J. Biol.Chem. 260,3626 (1985). 30. K. Friedrich, P. Woolley, and K. G. Steinhauser, Eur. J . Biochem. 173,233(1988). 31. K. Friedrich and P. Woolley, Eur. J . Biochem. 173, 227 (1988). 32. M. Martin, Chem. Phys. Lett. 35, 105 (1975). 33. M. Rozwadowski, Acta Phys. Polon. 20, 1005 (1961); H. Leonhardt, L. Gordon, and R. Livingston, J . Phys. Chem. 75, 245 (1971). 34. P. Y. Chen, D. Pearce, and A. S. Verkman, Biochemistry 27, 5713 (1985). 35. Y. K. Kim, N. P. Illsley, and A. S. Verkman, Anal. Biochem. 172, 403 (1988). 36. K. J. Schray and P. G. Artz, Anal. Chem. 60,853 (1988). 37. M. T. Shipchandler, J. R. Fino, L. D. Klein, and C . L. Kirkemo, Anal. Biochem. 162, 89 (1987). 38. A. A. K. Al-Ansari, M. Massoud, L. A. Perry, and D. S. Smith, Clin. Chem. 29, 1803 (1983). 39. A. A. K. Al-Ansari, D. S. Smith, and J. Landon, J . Steroid. Biochem. 19, 1475 (1983). 40. M. Lu-Steffes, G. W. Pittluck, M. E. Jolley, H. N. Panas, D. L. Olive, Chao-Huei, J. Wang, D. D. Nystrom, C. L. Keegan, J. P. Davis, and S. D. Stroupe, Clin. Chem. 28, 2278 (1982). 41. D. L. Colbert, D. S. Smith, J. Landon, and A. M. Sidki, Clin. Chem. 30, 1765 (1984). 42. D. L. Colbert, G. Gallacher, and R. W. Mainwaring-Burton, Clin. Chem. 31,1193 (1985). 43. S. H. Grossman, J . Neurochem. 41, 729 (1983). 44. Y. G. Chu and C. R. Cantor, Nucl. Acids Res. 6, 2363 (1979). 45. P. K. Lambooy, R. F. Steiner, and H. Sternberg, Arch. Biochem. Biophys. 217,517 (1982). 46. J. K. Abrahamson, T. M. Laue, D. L. Miller, and A. E. Johnson, Biochemistry 24, 2692 (1985). 47. K. Sugiura, M. Inaba, H. Ogata, R. Yasumizu, K. Inaba, R. A. Good, and S. Ikehara, Proc. Natl. Acad. Sci. 85, 4824 (1988). 48. R. S. Zukin, P. R. Hartig, and D. E. Koshland, Jr., Proc. Natl. Acad. Sci. 74, 1932 (1977). 49. R. S. Zukin, P. R. Hartig, and D. E. Koshland, Jr., Proc. Natl. Acad. Sci. 74, 1932 (1977). 49. R. S. Zukin, P. R. Hartig, and D. E. Koshland, Jr., Biochemistry 18, 5559 (1979). 50 R. S. Zukin, Biochemistry 18, 2139 (1979). 51. J. G. Kapakos and M. Steinberg, J. Biol. Chem. 261, 2084 (1986).

PHOTOCHEMISTRY OF THE XANTHENE DYES

383

52. J. G. Kapakos and M. Steinberg, J . Biol. Chem. 261, 2090 (1986). 53. H. J. Adkins, D. L. Miller, and A. E;. Johnson, Biochemistry 22, 1208 (1983). 54. W. M. Saxton, D. L. Stemple, R. J. Leslie, E. D. Salmon, M. Zavortink, and J. R. McIntosh, J. Cell Biol. 99, 2175 (1984). 55. B. Babbitt, L. Huang, and E. Freire, Biochemistry 23, 3920 (1984). 56. A, J. Jesaitis and J. Yguerabide, J. Cell Biol. 102, 1256 (1986). 57. M. Foley, A. N. MacGregor, J. R. Kusel, P. B. Garland, T. Downie, and I. Moore, J . Cell Biol. 103, 807 (1986). 58. B. Schulz and R. Peters, Biochim. Biophys. Acta 930, 419 (1987). 59. M. Noda, K. Yoon, G. Rodan, and D. E. Koppel, J . Cell Biol. 105, 1671 (1987). 60. L. Plank and B. R. Ware, Biophys. J . 51, 985 (1987). 61. T. Scherson, T. E. Kreis, J. Schlessinger, U. Z. Littauer, U. Z. Borisy, and B. Geiger, J . Cell Biol. 99, 425 (1984). 62. P. Wadsworth and E. D. Salmon, J . Cell Biol. 102, 1032 (1986). 63. E. A. Waling, G. A. Krafft, and B. R. Ware, Arch. Biochem. Biophys. 264, 321 (1988). 64. L. Plank and B. R. Ware, Biochem. Biophys. Res. Commun. 140, 308 (1986). 65. A. Mozo-Villarias and B. R. Ware, Bio hemistry 24, 1544 (1985). 66. Y. Kawasaki, N. Wakayama, and A. Seto-Ohshima, FEBS Lett. 231,321 (1988). 67. I. V. Zlatanov, M. Foley, J. Birmingham, and P. B. Garland, FEBS Lett. 222,47 (1987). 68. T. Forster, Disc. Faraday SOC.27, 7 (1959) and references therein. 69. C. Wolff and C. S . Lai, Biochemistry 27, 3483 (1988). 70. T. G. Squier, D. J. Bigelow, J. Garcia dos Ancos, and G. Inesi, J . Biol. Chem. 262, 4748 (1987). 71. M. Miki, C. G. dos Remedios, and J. A. Barden, Eur. J. Biochem. 168,339 (1987). 72. P. D. Chantler and T. Tao, J. Mol. Biol. 192, 87 (1986). 73. H. C. Cheung, F. Gonsoulin, and F. Garland, Biochim. Biophys. Acta 832, 52 (1985). 74. T. L. Scott, J . Biol. Chem. 260, 14421 (1985). 75. R. E. Hirsch, R. S . Zukin, and R. L. Nagel, Biochem. Biophys. Res. Commun. 138, 449 (1986). 76. M. Miki and C. G. dos Remedios, J . Biochem. 104, 232 (1988). 77. B. S. Isaacs, E. J. Husten, C. T. Esmon, and A. E. Johnson, Biochemistry 25,4958 (1986). 78. B. Baird and D. Holowka, Biochemistry 24, 6252 (1985). 79. B. Ferguson and D. Yang, Biochemistry 25, 5298 (1986). 80. K. Nithipatikom and L. McGown, Anal. Chem. 59, 423 (1987). 81. J. G. Comerford and A. P. Dawson, Biochem. J . 249, 89 (1988). 82. A. D. Saad, D. A. Fischman, and J. D. Pardee, Biophys. J . 25, 140 (1986).

384

D. C. NECKERS AND 0.M. VALDES-AGUILERA

83. A. D. Saad, J. D. Pardee, and D. A. Fischman, Yroc. Natl. Acad. Sci. 83, 9483 (1986). 84. D. G. Chung and P. N. Lewis, Biochemistry 25, 5036 (1986). 85. H. R. Trayer and I. P. Trayer, Biochemistry 27, 5718 (1988). 86. S. Papp, S . Pikula, and A. Martonosi, Biophys. J . 51, 205 (1987). 87. M. H. Fagan and M. H. Dewey, J. Biol. Chem. 261, 3654 (1986). 88. K. H. Cheng, T. Wiedmer, and P. J. Sims, J . Immunol. 135, 459 (1985). 89. A. Podgorski, P. Alster, and D. Elbaum, Biochemistry 27, 609 (1988). 90. A. Podgorski and D. Elbaum, Biochemistry 24, 7871 (1985). 91. Eosin, Y., 11,983-0 (Aldrich) lists a number of such uses. 92. N. 0. Mchedlov-Petrosan, Ukr. Khim. Zh. 53, 1304 (1987); CA 109, 230132t (1987); N. 0. Mchedlov-Petrosan, M. I. Rubtsov, L. L. Lukatskaya, T. A. Chernaya, and A. Pereverzev, Yu., Dokl. Akad. Nauk SSSR 299, 921 (1988); CA 109, 28341e (1987); J. Polster, A . Naturforsch. A : Phys. Sci. 42, 636 (1987); G. A. Kedtsle, L. V. Levshin, G. V. Mel'nikov, and A. M. Saletskii, Zh. Prikl. Spektrosk. 46, 746 (1987). 93. N. E. Geacintov and H. C. Brenner, Photochem. Photobiol. 50, 841 (1989). 94. P. Johnson and P. B. Garland, Biochem. J. 203, 313 (1982). 95. P. B. Garland and C. H. Moore, Biochem. J . 183, 561 (1979). 96. R. J. Cherry, Methods Enzymol. 54, 47 (1978). 97. E. A. Nigg and R. J. Cherry, Biochemistry 18, 3457 (1979). 98. M. E. Bayer, I. Morrison, and R. Cherry, FEBS Lett 175, 329 (1984). 99. T. M. Eads, D. D. Thomas, and R. H. Austin, J . Mol. Biol. 179, 55 (1984). 100. S. Ishiwata, K. Kinosita, H. Yoshimura, and A. Ikegami, J . Biol. Chem. 262,8314 (1987). 101. E. A. Nigg and R. J. Cherry, Proc. Natl. Acad. Sci. 77, 4702 (1980). 102. I. G. Macara and L. C. Cantley, Biochemistry 20, 5095 (1981). 103. A. J. Visser, W. H. Scouten, and D. Lavalette, Eur. J . Biochem. 121, 233 (1981). 104. J. P. Harrison, R. J. Cherry, L. C. Packman, and R. N. Percham, Biochem. SOC. 7'rans. 14, 1284 (1986). 105. A. Baba, T. Nakamura, and M. Kawakita, J . Biochem. 100, 1137 (1986). 106. A. Burkli and R. J. Cherry, Biochemistry 20, 138 (1981), and references therein. 107. R. H. Austin, S . S . Chan, and T. M. Jovin, Proc. Natl. Acad. Sci. 76, 5650 (1979). 108. M. Bartholdi, F. J. Barrantes, and T. M. Jovin, Eur. J. Biochem. 120, 389 (1981). 109. T. M. Eads, D. D. Thomas, and R. H. Austin, J. Mol. Biol. 179, 55 (1984). 110. S. Ishiwata, K. Kinosita, H. Yoshimura, and A. Ikegami, J . Biol. Chem. 262,8314 (1987). 111. T. M. Jovin, M. Bartholdi, and W. L. C. Vas, Ann. N Y Acad. Sci. 366, 176 (1981). 112. R. D. Ludescher and D. D. Thomas, Biochemistry 27, 3343 (1988). 113. R. Wagner, N. Carrillo, W. Junge, and R. H. Vallejos, Biochim. Biophys. Acta 680, 317 (1982).

PHOTOCHEMISTRY OF THE XANTHENE DYES

385

114. C. S . Andreo, A. A. Iglesias, F. E. Podesta, and R. Wagner, Biochim. Biophys. Act 870, 292 (1986). 115. T. M. Eads, D. D. Thomas, and R. H. Austin, J . Mol. Biol. 179, 55 (1984). 116. K. Kinosita, S. Ishiwata, H. Yoshimura, H. Asai, and A. Ikegami, Biochemistry 23, 5963 (1984). 117. M. E. Bayer, I. Morrison, and R. Cherry, FEBS Lett. 175, 329 (1984). 118. A. Burkli and R. J. Cherry, Biochemistry 20, 138 (1981) and references therein. 119. T. M. Yoshida, F. Zarrin, and B. G. Barisas, Biophys. J . 54, 277 (1988). 120. P. Johnson and P. B. Garland, FEBS Lett 132, 252 (1981). 121. A. F. Corin, E. Blatt, and T. M. Jovin, Biochemistry 26, 2207 (1987). 122. T. M. Jovin, M. Bartholdi, and W. L. C. Vas, Ann. N Y Acad. Sci. 366,176 (1981). 123. R. Wagner, N. Carrillo, W. Jundge, and R. H. Vallejos, Biochim. Biophys. Acta 680, 317 (1982). 124. C. S. Andreo, A. A. Iglesias, F. E. Podesta, and R. Wagner, Biochim. Biophys. Acta 870, 292 (1986). 125. L. I. Grossweiner, Radiat. Res. Rev. 2, 345 (1970). 126. Y. Tano, S. ‘Kinosita, K. Kishida, J. Hara, K. Sato, and R. Manabe, Jpn. J . Ophthalmol. 21, 392 (1977). 127. G. E. Cohn and H. Y. Tseng, Photochem. Photobiol. 26, 465 (1977). 128. J. P. Knox and A. D. Dodge, Planta 164, 22 (1985). 129. J. P. Knox and A. D. Dodge, Planta 164, 30 (1985). 130. A. Savino and G. Angeli, Water Res. 19, 1465 (1985). 131. J. P. Pooler and A. W. Girotti, Photochem. Photobiol. 44, 495 (1986). 132. A. G. Kepka and L. I. Grossweiner, Photochem. Photobiol. 18, 49 (1973). 133. F. Rizzuto and J. D. Spikes, Photochem. Photobiol. 18, 49 (1973). 134. A. G. Kepka and L. I. Grossweiner, Photochem. Photobiol. 14, 621 (1971). 135. L. I. Grossweiner and E. F. Zwicker, J . Chem. Phys. 39, 2774 (1963). 136. G. J. Fisher, C. Lewis, and D. Madill, Photochem. Photobiol. 24, 223 (1976). 137. H. Koloczec and J. Vanderkooi, Biochim. Biophys. Acta 916, 236 (1987). 138. R. Peters, J. Peters, K. H. Tews, and W. Bahr, Biochim. Biophys. Acta 367, 282 (1974). 139. D. E. Golan and W. Veatch, Proc. Natl. Acad. Sci. 77, 2537 (1980). 140. R. Peters, Naturwiss. 70, 294 (1983). 141. I. V. Zlatanov, M. Foley, J. Birmingham, and P. B. Garland, FEBS Lett 222, 47 (1987). 142. C. K. Wang and H. C. Cheung, Biophys. J . 48, 727 (1985). 143. J. C. Skou and M. Esmann, Biochim. Biophys. Acta 647, 232 (1981). 144. M. L. Helmich de Jong, J. P. M. van Duynhoven, F. M. A. H. Schuurmans Stekhoven, and J. J. H. H. M. De Pont, Biochim. Biophys. Acta 858,254 (1986). 145. M. Koizumi, Mol. Photochem. 4, 57 (1972) and references therein.

386

D. C. NECKERS AND 0. M. VALDES-AGUILERA

J. Bellin and G. Oster, J . Am. Chem. SOC. 79, 2461 (1957). G. Oster and A. H. Aldeman, J . Am. Chem. SOC.78, 913 (1956). G. Oster, G. K. Oster, and G. Karg, J . Phys. Chem. 66, 2514 (1962). R. F. Bartholomew and R. S. Davidson, J . Chem. SOC. C 2347 (1971). H. Misawa, A. Wakisaka, H. Sakugari, and K. Tokumaru, Chem. Lett. 293 (1985). 151. I. H. Leaver, Aust. J . Chem. 24, 891 (1971). 152. K. Phillips and G. Read, J . Chem. SOC. Perkin Pans. I 6 7 1 (1986). 153. J. Paczkowski, B. Paczkowska, and D. C. Neckers, J . Photochem. Photobio. A: Chem. 61, 131 (1991). 154. K. Kimura, T. Miwa, and M. Imamura, Bull. Chem. SOC. Jpn. 43, 1329 (1970). 155. J. Moser and M. Gratzel, J . Am. Chem. SOC.106, 6557 (1984). 156. J. Moser, M. Gratzel, D. K. Sharma, and N. Serpone, Helu. Chim. Acta 68, 1686 (1985). 157. R. Rosetti and L. Brus, J . Am. Chem. SOC. 106,4336 (1984). 158. With triethanolamine; M. T. Carver, U. Dreyer, R. Knoesel, and F. Candau, J . Polym. Sci., Part A., Polym. Chem. 27,2161 (1989);D. C. Neckers, K. S. Raguveer, and 0. Valdes-Aguilera, Polym. Muter. Sci. Eng. 60, 15 (1989). 159. With periodate; T. P. Sarathy, P. Frinivas, K. N. Rao, B. Sethuram, T. N. Rao, J . Macromol. Sci., Chem. A26(9), 1347 (1989); with diphenyl iodonium; J. P. Fouassier, E. Chesneau, and M. Lebaccon, Macromol. Chem. Rapid Comm. 9,223 (1988). 160. G. W. Collins and J. R. Costanza, J . Coat. Technol. 51, 57 (1979);S. Maiti, M. K. Saha, and S. R. Palit, Makromol. Chem. 127, 224 (1969). 161. D. R. Pemberton and A. F. Johnson, Polymer 25, 529, 536 (1985). 162. R. C. Patel, Eur. Pat. Appl. EP 97,012 (1983). 163. H. Nakayama, N. Kida, and K. Asada, Japan 7231,713 (1972). 164. I. Katime and 0. J. Perez, Ajnidad 40, 421 (1983). 165. G. L. Collins, A. B. Conciatori, and J. R. Costanza, US Patent 4, 207,156 (1980). 166. N. R. Lazear and R. W. Stackman, US Patent 4,150,170 (1979). 167. D. J. Nortantz and C. S. Bilen, Lasers Chem. Proc. Con$ 282-7, M. A. West, Ed., Elsevier, Amsterdam, 1977. 168. H. Franke, Polymer 28, 659 (1987). 169. H. Watanabe, J. Seto, K. Suzuki, and T. Fukuma, US Patent 4,542,084 (1985). 170. S. Ishizaka, T. Urano, and K. Tokumaru, Chem. Lett. 10, 1705 (1984). 171. S. Chaberek and R. J. Allen, J . Phys. Chem. 69, 647 (1975). 172. N. L. Yang and G. Oster, J. Phys. Chem. 74, 856 (1970). 173. G. Oster and N. L. Yang, Chem. Rev. 68, 125 (1968). 174. G. Oster, Nature 173, 300 (1954). 175. D. R. Pemberton and A. F. Johnson, Polymer 25, 529 (1984). 176. D. R. Pemberton and A. F. Johnson, Polymer 25, 536 (1984). 146. 147. 148. 149. 150.

PHOTOCHEMISTRY OF THE XANTHENE DYES

387

177. M. Koizumi and Y. Usui, Mol. Photochem. 4, 57 (1972) and references therein. 178. S. Lenka, P. L. Nayak, and K. M. Mishra, J . Polym. Sci., Polym. Chem. Ed. 19, 2671 (1981). 179. S. Lenka and I. B. Mohanty, Polym. Photochem. 7, 447 (1986). 180. S. Lenka and I. B. Mohanty, J . Polym. Sci., Part A: Polym. Chem. 24,2729 (1986). 181. J. L. Cawse, P. J. Harris, and M. V. Cwykla, Eur. Pat. Appl. EP 321, 286 (1989). 182. N. Kobayashi, Jpn. Kokai Tohhyo Koho J P 0377,952 (1991). 183. K. Rode, J. Gerstorf, D. Mohr, and W. Frass, Eur. Pat. Appl. EP 321,286 (1989). 184. K. Kitamura, Jpn. Kokai Tokkyo Koho J P 0123,251 (1989). 185. R. Zertani, D. Mohr, and K. Rode, Ger. Offen DE 3,824,903 (1990). 186. Y. Tomura and S. Tomono, Jpn. Kokai Tokkyo Koho J P 0297,503 (1990). 187. T. P. Sarathy, P. Srinivas, K. N. Rao, B. Sethuram, and T. N. Rao, J . Macromol. Sci., Chem. A26, 1347 (1989). 188. C. G. Roffey, Photopolymerization of Surface Coatings, Wiley, New York, 1982. 189. E. Chesneau and J. P. Fouassier, Angew. Makromol. Chem. 135, 41 (1985). 190. R. S. Davidson and K. R. Trethewey, J . Am. Chem. Soc. 98, 4008 (1976). 191. G. Oster and A. H. Adelman, J . Am. Chem. SOC.78, 913 (1956). 192. R. S. Davidson, J . Chem. SOC.Perkin II 173 (1977). 193. C. Hull, US Patent 4, 575, 330 (1986). 194. A. Reiser, Photoreactive Polymers, Wiley, New York, 1989. 195. J. Paczkowski, J. J. M. Lamberts, B. Paczkowska, and D. C. Neckers, J . Free Rad. Biol. Med. 1, 341 (1985). 196. C. Decker and K. Moussa, Makromol. Chem. 189, 238 (1988). 197. C. Decker and K. Moussa, J . Appl. Polym. Sci. 34, 1603 (1987) and references

therein.

198. I. R. Bellobono, E. Selli, and L. Righetto, Makromol. Chem. 190, 1945 (1989). 199. C. Decker and K. Moussa, Makromol. Chem. 191, 963 (1990). 200. C. Decker and K. Moussa, Eur. Polym. J . 26, 393 (1990). 201. C. S. H. Chen, J . Polym. Sci. Part A 3, 1107 (1965). 202. C. S. H. Chen, J . Polym. Sci. Part A 3, 1155 (1965). 203. K. K. Rohatghi and A. K. Mukhopadhyay, J . Indian Chem. SOC.49,1311 (1972). 204. V. Kasche and L. Lindqvist, Photochem. Photobiol. 4, 923 (1965). 205. F. Rizzuto and J. D. Spikes, Photochem. Photobiol. 18, 49 (1973). 206. L. I. Grossweiner and E. F. Zwicker, J . Chem. Phys. 34, 1411 (1961). 207. E. F. Zwicker and L. I. Grossweiner, J . Phys. Chern. 67, 549 (1963). 208. J. Chrysochoos and L. I. Grossweiner, Photochem. Photobiol. 8, 193 (1968). 209. K. Hashimoto, T. Kawai, and T. Sakata, Nouv. J . Chim. 8, 693 (1984). 210. D. Eaton, Adv. Photochem. 13, 427 (1986) and references therein. 211. M. Tsunooka, S. Imono, K. Nakayama, H. Kuwabara, and M. Tanaka, J . Polym. Sci., Chem. Ed. 24, 317 (1986).

388

D . C. NECKERS A N D 0. M. VALDES-AGUILERA

212. S. Hong, T. Kurosaki, and M. Okawara, J . Polym. Sci., Chem. Ed. 12, 2553 (1974). 213. A. Ghogare and G. S . Kumar, J . Chem. Soc., Chem. Commun. 1533 (1989). 214. A. Ghogare and G. S . Kumar, J . Chem. SOC.,Chem. Commun. 134 (1990). 215. D. A. Delzenne, U. Laridon, and H. Peeters, Eur. Polym. J. 6, 933 (1970). 216. M. Li, R. C. Liang, and A. Reiser, Macromolecules 23, 2704 (1990). 217. G. Smets, Polym. J. 17, 153 (1985). 218. G. S. Kumar and D. C. Neckers, Macromolecules 24, 4332 (1991). 219. J. P. Fouassier and E. Chesneau, Makromol. Chem. 192, 245 (1991). 220. R. Gnehm, Schultz’s “Farbstofftabellen,” Akademisch Verlagsgesellschaft m. b. H., Leipzig (1885). 221. S. Katamine, Y. Mamiya, K. Sekimoto, N. Hoshino, K. Totsuka, U. Naruse, A. Watabe, R. Sugiyama, and M. Suzuki, J . Nutr. Sci. Vitaminol. 32, 487 (1986). 222. Y. Usui, Chem. Lett. 743 (1973). 223. E. K. Matthews and D. E. Mesler, Br. J. Pharmacol. 83, 555 (1984). 224. G. Tomlinson, M. D. Cummings, and L. Hryshko, Biochem. Cell Biol. 64, 515 (1986). 225. J. P. Dubosc, C. Mercier, and J. Bourdon, Bull. SOC. Chim. Fr. 9, 3286 (1971). 226. H. Boettcher, H. Buchhammer, L. Kniipfer, and F. W. Miiller, 2. Chem. 26,443 (1986). 227. K. B. Mullis, P C T Int. Appl. (1988). CA, 111, 205548 (1988). 228. A. K. Chibisov and G. V. Zacharova, J . Inf: Rec. Muter. 15, 395 (1987). 229. Y. Nishimura, H. Misawa, H. Sakurag, and K. Tokumaru, Chem. Letts. 9, 1555 (1989). 230. C. A. Marcopoulos, M. Pertessis-Keis, and K. Georgopoulos, 2.Phys. Chem. (Leipzig) 249, 21 (1972). 231. S. Levinos, Ger. Offen: 2,125,457 (1970). 232. J. Bendig, D. Dreysig, and K. Nakamura, Z . Chem. 25, 70 (1985). 233. S. Lenka and M. B. Indu, Angew. Makromol. Chem. 140, 171 (196). 234. W. Sobek, Acta Polym. 138 (1987). 235. E. S. Klimchuk, U. V. Khudyakov, L. A. Margulis, and V. A. Kuz’min, lau. Akad. Nauk. SSSR, Ser. Khim. 12,2736 (1988);CA, 111,6754 (1988);P. P. Levin, Y. Ya. Shafirovich, E. W. Klimchuk, V. A. Kuz’min, and V. E. Maier, ibid. 1436; CA, 110, 57038 (1987). 236. G. V. Zakharova and A. K. Chibisov, Khim. Vys. Energ. 18,552 (1984):CA, 102, 35648 (1985). 237. G. G. Kozlova, E. E. Sagalovich, I. S. Rudkovskii, D. S. Umreiko, and L. N. Vasilevskaya, Zh. Prikl. Spektrosk. 38, 647 (1983): CA, 99, 39793 (1983). 238. L. V. Levshin, L. K. Sokolova, G. A. Ketsle, and V. V. Brukhanov, Vestn. Mosk. Univ., Ser 3: Fiz., Astron. 27, 41 (1986): CA, 102, 161326 (1986). 239. W. W. A. Keller, W. Schubert, R. Germer, and E. Strauss, J. Phys. Colloq. C6, 397 (1983).

PHOTOCHEMISTRY OF THE XANTHENE DYES

389

240. D. C. Neckers, J . Chem. Ed. 64,649 (1987). 241. J. Paczkowski, B. Paczkowska, and D. C. Neckers, J . Free Radicals Biol. Med. 1, 341 (1985). 242. The Colour Index, 2nd Ed., 1956. 243. G. 0. Schenck and K. Gollnick, Forsch. des Landes Nordrhein-Westfalen, Nr. 1256, Koln und Opladen (1963). 244. J. J. M. Lamberts and D. C. Neckers, J . Am. Chem. SOC.105, 7465 (1983). 245. G. 0. Schenck and E. Z . Koch, Electrochem., Ber Bunsenges. Physik. Chem. 64, 170 (1960). 246. J. A. Janakiraman, Ph.D. thesis, Iowa State University (1988). 247. D. Xu, A. Van Loon, S. M. Linden, and D. C. Neckers, J . Photochem. 38, 357 (1987). 248. J. M. Lamberts and D. C. Neckers, Zeit. 1: Naturf. 39b,474 (1984). 249. D. K. Luttrull, 0. Valdes-Aguilera, S. M. Linden, J. Paczkowski, and D. C. Neckers, Photochem. Photobiol. 47, 551 (1988). 250. D. Xu and D. C. Neckers, J . Photochem. 40,361 (1987). 251. 0. Valdes-Aguilera and D. C. Neckers, J. Phys. Chem. 92,4286 (1988). See also K. K. Rohatgi and A. K. Mukhopadhyay, Photochem. Photobiol. 14,551 (1971). 252. 0. Valdes-Aguilera and D. C. Neckers, Acct. Chem. Res. 22, 171 (1989). See an earlier review; V. I. Yuzhakov, Russ. Chem. Rev. 48, 1076 (1979). 253. A. Jodlbauer and H. von Tappeiner, Arch. klin. Med. 82, 520 (1905); H. von Tappeiner, Arch. klin. Med. 86, 478 (1906). 254. S. M. Linden and D. C. Neckers, J . Am. Chem. SOC. 110, 1257 (1988). 255. “Singlet Molecular Oxygen,” Benchmark Papers in Organic Chemistry, Vol. 5, A. P. Schaap, Ed. Dowden, Hutchinson and Ross, Inc., Stroudsburg, PA, 1976; “Singlet Oxygen,” H. H. Wasserman and R. W. Murray, Eds., Academic Press, New York, 1979; C. S. Foote and G. Uhde, Org. Photochem. Synth. 1,60 (1971). 256. K. Gollnick and A. Griesbeck, Tetrahedron Lett. 4921 (1984). 257. S. M. Linden and D. C. Neckers, J . Am. Chem. SOC.110, 1257 (1988). 258. V. S.Srinivasan, D. Podolski, N. Westrick, and D. C. Neckers, J . Am. Chem. SOC. 100, 6513 (1978). 259. M. A. J. Rodgers and P. Lee, J . Am. Chem. SOC.submitted. 260. J. J. M. Lamberts and D. C. Neckers, J . Am. Chem. SOC. 106, 5879 (1984). 261. R. W. Davidson and K. R. Trethewey, J.C.S.,Perkin I1 169 (1977);J . Chem. SOC., Chem. Comrnun. 674 (1975); ibid. 178 (1976); R. S. Davidson and R. F. Bartholomew, J . Chem. SOC., C 2347 (1971). 262. R. W. Davidson and K. R. Trethewey, J.C.S., Perkin I1 173 (1977). 263. R. H. Young, D. Brewer, R. Kayser, R. Martin, D. Feroisi, and R. A. Keller, Can. J . Chem. 52, 2889 (1974); R. H. Young, R. H. Martin, R. H. Feriozi, D. Brewer, and R. Kayser, Photochem. Photobiol. 17, 233 (1973); N. R. K. Raju, M. Santhanam, B. R. Sethuram, and T. Vananeeth, Indian J . Chem. 12,422 (1974). 264. K. Kikuchi, H. Kokubun, and M. Liozumi, Bull. Chem. SOC.Jpn. 43 2732 (1970); this reference deals with Eosin and Erythrosin.

390

265. 266. 267. 268. 269. 270.

271. 272. 273. 274. 275. 276. 277. 278. 279. 280. 281. 282. 283. 284. 285. 286. 287. 288.

D. C. NECKERS AND 0. M. VALDES-AGUILERA

L. Kraljic and L. Lindqvist, Photochem. Photobiol. 20, 351 (1974). M. A. J. Rodgers and H. D. Burrows, Chem. Phys. Lett. 66, 238 (1979). Di-octyl ester of sulfosuccinic acid. D. F. Eaton, Advances in Photochemistry, Vol. XIII, Chapter 4, D. Volman, J. N. Pitts, and G. S. Hammond, Eds., Wiley, New York, 1986. D. C. Neckers and S. M. Linden, US. Patent applied for. 0. Johansen, A. Launikonis, J. W. Loder, A. W. H. Mau, W. H. F. Sasse, J. D. Swift, and D. Wells, Aust. J . Chem. 34, 981 (1981); J. K. Kiwi, K. Kalyanasundaram, and M. Gratzel, Struct. Bond. 49, 37 (1982); K. P. Ghiggino, J. M. Brown, A. Launikonis, A. W. H. Mauand, and W. H. F. Sasse, Aust. J . Chem. 41, 9 (1988);K. Hashimoto, T. Kawai, and T. Sakata, Nouv. J . Chim. 8,693 (1984);A. V. Elcov, N. P. Smirnova, M. N. Bobrovnikov, E. A. Stratonnikova, and G. V. Sacharova, Zh. Org. Khim. 58, 635 (1988). G. A. Getsie, L. V. Lveshi, and L. K. Sokolva, Opt. Spectrosc. U S S R 47, 494 (1979). A. W. Mau, 0. Johansen, and W. H. F. Sasse, Photochem. Photobiol. 41, 503 (1985). M. Gratzel, Ed., Energy Resources through Photochemistry and Catalysts, Academic Press, NY, 1983, p. 629. 0. Valdes-Aguilera, C. P. Pathak, J. Shi, D. Watson, and D. C. Neckers, Macromolecules 25, 541 (1992). D. C. Neckers, J . Photochem. Photobiol. 44, 1 (1989). J. J. M. Lamberts, D. R. Schumacher, and D. C. Neckers, J . Am. Chem. SOC.106, 5879 (1984). J. Paczkowski, J. J. M. Lamberts, B. Paczkowska, and D. C. Neckers, J . Free Rad. Biol. Med. 1, 341 (1985). A. Zakrzewski and D. C. Neckers, Tetrahedron 43, 4507 (1987) and references therein. S. Chatterjee, P. Gottschalk, P. D. Davis, and G. B. Schuster, J . Am. Chem. SOC. 110, 2326 (1988). S. W. Benson, Thermochemical Kinetics, Wiley, New York, 1976. S. L. Murov, Handbook ofPhotochemistry, Marcel Dekker, New York, 1973. C. Decker and T. Bendaikha, Eur. Polym. J . 20, 753 (1984). J. Paczkowski, B. Paczkowska, and D. C. Neckers, J . Polym. Sci., Chem. Ed., in press. See, for example, N. Kuramoto and T. Kitao, Dyes and Pigments 3,49 (1982). A. Zakrzewski and D. C. Neckers, Tetrahedron 43, 4507 (1987). G. Oster, G. K. Oster, and G. Kang, J . Phys. Chem. 66, 2514 (1962). M. Koizumi and Y. Usui, Mol. Photochem. 4, 57 (1972); V. Wintgens, J. C. Scaiano, S. M. Linden, and D. C. Neckers, J . Org. Chem. 54, 5242 (1989). R. Chapelon, G. Perichet, and B. Pouyet, J . Photochem. 6, 43 (1976).

PHOTOCHEMISTRY OF THE XANTHENE DYES

289. 290. 291. 292. 293.

294. 295. 296. 297. 298.

299. 300. 301. 302. 303. 304. 305. 306. 307.

308.

309. 310.

391

G. Oster, G. K. Oster, and G. Karg, J . Am. Chem. Soc. 66, 2514 (1962). J. L. Cawse, Ph. J. Harris, and M. V. Cwykla, Eur. Pat. Appl. EP 297,050 (1988). K. Rode, D. Johr, and W. Frass, Ger. Offen. DE 3,710,281 (1987). M. C . Palazzotto, A. F. Ubel, J. D. Oxman, and M. Ali, Eur. Pat. Appl. EP 290, 133 (1988). E. C. Blossey, D. C. Neckers, A. M. Thayer, and A. P. Schaap, J . Am. Chem. Soc. 95, 5820 (1973); D. C. Neckers, E. C. Blossey, and A. P. Schaap, US. Patent 4,315,998; A. P. Schaap, A. M. Thayer, E. C. Blossey, and D. C. Neckers, J . Am. Chem. Soc. 97, 3741 (1975). R. C. Petterson, S. M. Kalbag, and C. S. Irving, Ann. N Y Acad. Sci. 171, 133 (1970). H. Kautsky and H. de Bruijn, Naturwiss. 1043 (1931). R. B. Merrifield, J . Am. Chem. Soc. 85, 2149 (1963). Crosslinked with 2% or so divinylbenzene; so-called Biobeads. J. Paczkowski and D. C. Neckers, Macromolecules 18, 1245 (1985); B. Paczkowska, J. Paczkowski, and D. C. Neckers, Photochem. Photobiol. 42, 603 (1985); B. Paczkowska, J. Paczkowski, and D. C. Neckers, Macromolecules 19, 863 (1986). D. D. Luttrull, 0. Valdes-Aguilera, S. M. Linden, J. Paczkowski, and D. C. Neckers, Photochem. Photobiol. 47, 551 (1988). D. C. Neckers, Reactive Polym. 3, 277 (1985). J. F. Rabek and B. Rinby, Photochem. Photobiol. 27,547 (1978);H. W. Ng and J. E. Guillet, Photochem. Photobiol. 28, 551 (1978). N. M. C. Kaye and P. D. J. Teizman, FEBS Lett. 62, 334 (1976). S. Tamagaki, C. Liesner, and D. C. Neckers, J . Org. Chem. 45, 1573 (1980). P. Brkic, P. Forzatti, I. Pasquon, and F. Trifiro, J . Mol. Catal. 3, 173 (1977). A. P. Schaap, A. L. Thayer, A. K. Zakalika, and P. C. Valenti, J . Am. Chem. Soc. 107, 4016 (1979). M. Syoyama and T. Sano, Eisei Kagaku 31,410 (1985);CA 105 05036955 (1985). R. M. Favaro, M. V. De Sousa, H. Vannucchi, I. D. Desai, and J. E. D. Oliveira, Am. J . Clin. Nutr. 43,940 (1986);K. Vijayaraghavan, K. V. R. Sharma, V. Reddy, and P. Bhaskaram, Am. J . Clin. Nutr. 31, 892 (1978). D. S. Hull, K. Green, and L. Laughter, I n n Ophthalmol. Visual Sci.25,455 (1984); H. M. Jernigan, H. N. Fuku, J. D. Goosey, and J. H. Kinoshita, E x p . Eye Res. 32, 461 (1981); I. Memon, B. Aravind, K. Prasanta, S. D. Persa, S. Rosatone, and J. D. Wilshire, Exp. Eye Res. 49, 67 (1989). A. Ito, H. Watanabe, M. Naito, H. Aoyama, Y . Nakagaw, and N. Fujimoto, J. Natl. Cancer. Inst. 77, 277 (1986). J. D. Regan and J. A. Parrish, Ed. The Science ofPhotomedicine, Plenum, NY, 1982; J. A. Parrish, M. L. Kripke, and W. L. Morison, Photoimmunology, Plenum, NY, 1983.

392

D. C. NECKERS A N D 0. M. VALDES-AGUILERA

311. 0. Raab, 2. Biol. 39, 524 (1900). 312. T. J. Dougherty, J. E. Kaufmann, A. Goldfar, K. R. Weishaupt, D. Boyle, and A. Mittleman, Cancer Res. 38, 2628 313. N. I. Krinsky, Trends Biochem. Sci. 2, 35 (1976). 314. C. S . Foote, Free Radicals in Biology and Medicine, Vol. 2, W. A. Pryor, Ed., Academic Press, NY, pp. 85-133. 315. A. D. Dodge, Human Welfare and the Environment, UIPAC Pesticide Chemistry, Ed. J. Miyamoto and P. C. Kearney, Eds., Pergamon, Oxford, 1983, pp. 59-66. 316. M. B. Korycka-Dahl and T. Richardson, CRC Crit. Rev. Food Sci. Nutr. 10,209 (1978). 317. N. I. Krinsky, Pure Appl. Chem. 51, 649 (1979). 318. S. H. Moss and K. C. Smith, Photochem. Photobiol. 33, 303 (1981). 319. W. A. Maxwell, J. D. Macmillan, and C . 0.Chichester, Photochem. Photobiol. 5, 567 (1966). 320. D. T. Harris, Biochem. J. 20, 288 (1926); H. Graffon, Biochem. J. 179, 157 (1926); F. Lieben, Biochem. J. 184,453 (1927). 321. C. Tanielian, H. Muller, and L. Golder, Oxygen Radicals Chem. Biol. Proc. Int. C o n t 3 r d 551 (1984). 322. E. W. Westhead, Biochemistry 4, 2139 (1965). 323. N. Minagawa and A. Yoshimoto, Agric. Biol. Chem. 49, 277 (1985). 324. L. Brand, J. R. Gohlke, and D. S . Rao, Biochem. 6, 3510 (1967). 325. A. P. Harrison, Annu. Rev. Microbiol. 21, 143 (1967); J. D. Spikes, Photophysiology 3, 33 (1968). 326. 0. Groundinsky, Eur. J . Photochem. 18,480 (1971). 327. L. Hederstedt and Y. Hatefi, Arch. Biochem. Biophys. 247, 346 (1986). 328. U. Pathre, A. K. Singh, and P. V. Sane, Indian J . Biochem. Biophys. 26, 136 (1989); CA, 111, 53090 (1989). 329. F. I. Llorca, J. L. Iborra, and J. A. Lozano, Photobiochem. Photobiophys. 5, 105 (1983). 330. M. Rippa and S . Pontremoli, Arch. Biochem. Biophys. 133, 112 (1969). 331. M. Rippa, C. Picco, and S . Pontremoli, J . Biol. Chem. 19, 4977 (1970). 332. K. Takahashi, J . Biochem. (Tokyo) 69, 331 (1971); ibid. 67, 833 (1970). 333. M. J. Wade and J. D. Spikes, Photochem. Photobiol. 14, 221 (1971). 334. E. Scoffone, G. Galiazzo, and G. Jori, Biochem. Biophys. 38, 16 (1970). 335. A. N. Glazer, Proc. Natl. Acad. Sci. U S A 65, 1057 (1970). 336. E. N. Fluhler, J. K. Hurley, and I. E. Kochevar, Biochem. Biophys. Acta 990,269 (1989). 337. J. G. Banks, R. G. Board, J. Carter, and A. D. Dodge, J . Appl. Bacteriol. 58, 391 (1985). 338. J. G. Banks, R. G. Board, and J. Paton, Lett. Appl. Microbiol., 1, 7 (1985). 339. R. A. Cerione and T. Chase, J . Protein Chem. 2, 383 (1983). 340. A. Savino and G. Angeli, Water Res. 19, 1465 (1985).

PHOTOCHEMISTRY OF THE XANTHENE DYES

393

341. S. A. Bezman, P. A. Burtis, T. P. J. Izod, and M. A. Thayer, Photochem. Photobiol. 28, 325 (1978). 342. A. Ito and T. Ito, Photomed. Photobiol. 6, 41 (1984). 343. C. W. Wu and F. Y. H. Wu, Biochemistry 12,4349 (1973). 344. S. K. Srivastava and M. J. Modak, Biochemistry 22, 228 (1983); ibid. 21, 4633. 345. A. W. Stern, V. D’Aurora, and D. S. Sigman, Arch. Biochem. Biophys. 202, 525 (1980). 346. M. J. Peak, J. G. Peak, C. S. Foote, and N. I. Krinsky, J . Photochem. 25, 309 (1984). 347. T. Ito and K. Kobayashi, Photochem. Photobiol. 26, 581 (1977). 348. T. A. Dahl, W. R. Midden, and D. C. Neckers, Photochern. Photobiof. 48, 607 (1988). 349. W. R. Midden, T. A. Dahl, and P. E. Hartman, in New Directions in Photodynamic Therapy, D. C. Neckers, Ed., SPIE, Bellingham, WA, 1987, 122126. 350. J. L. Melnick, C. Wallis, and C. A. Phillips, Am. J . Epiderniol. 81, 222 (1965). 351. J. W. Sargent and R. L. Sanks, J . Enuiron. Eng. Div. Am. SOC.Ciu. Eng. 102, 879 (1976). 352. J. R. Williams, U.S. Patent 4,008,136, February 15, 1977. 353. C. W. Wallis and J. L. Melnick, Photochem. Photobiol. 4, 159 (1965); H. Fujita and K. Suzuki, Radiat. Res. 11, 9 (1970); H.-E. Jacob, Photochem. Photobiol. 19, 133 (1974); J. S. Bellin, L. Lutwick, and B. Jonas, Arch. Biochem. Biophys. 132, 157 (1969); J. D. Spikes, Photodynamic Reactions in Photomedicine, in J. D. Regan and J. A. Parrish, Eds., 1982, The Science of Photomedicine, Plenum, NY, pp. 113-144; J. L. Melnick and C. Wallis, Chapter 19, op. cit.; M. Jarratt, W. Hubler, and W. Panek, Chapter 20, op. cit.; F. Rapp and J.-L. H. Li, Chapter 21, op. cit. 354. T. Ito, Photochem. Photobiol. 28,493 (1978);W. I. Poppe and L. I. Grossweiner, Photochem. Photobiol. 22, 217 (1975). 355. S. Devanathan, W. R. Midden, and D. C. Neckers, Proceed. Nat. Acad. Sci., U S A 87, 2980 (1990). 356. Y. Kusama, M. Bernier, and D. J. Hearse, Circulation 80, 1432 (1989); D. J. Hearse, Y. Jusama, and M. Bernier, Circ. Res. 65, 146 (1989). 357. M. Tarr and D. P. Valenzeno, Mol. Cell. Cardiol. 21, 549 (1989). 358. M. Netto and H. Martins-Ferreira, Photochem. Photobiol. 50, 229 (1989). 359. N. S . Ranadive, I. A. Menon, S. Shirwadkar, and S. D. Pesad, Inflamation ( N Y ) 13, 483 (1989). 360. E. K. Matthews and Z. J. Cui, FEBS Lett. 256, 29 (1989). 361. J. M. Wages and J. R. Heitz, Arch. Inset Biochem. Physiol. 5, 24 (1987); H. Sakurai and J. R. Heitz, Enuiron. Entomol. 11, 467 (1982). 362. T. E. Fairbrother, H. W. Essig, R. L. Combs, and J. R. Heitz, Enuiron. Entomol. 10, 506 (1981). 363. F. S. Cruz, L. A. V. Lopes, W. De Souza, N. J. Moreno-Silvia, R. P. Mason, and R. Docampo, Acta Trop. 41, 99 (1984).

394

D. C. NECKERS A N D 0. M. VALDES-AGUILERA

364. M. Kanai, Nippon Saikingaku Zasshi 38, 645 (1983); CA 09909067450. 365. T. L. Carpenter and J. R. Heitz, Enuiron. Entomol. 9, 467 (1980). 366. C. S. Creighton, T. L. McFadden, and J. M. Schalk, J . Ga. Entomol. SOC.15, 66 (1980). 367. M. F. Callaham, J. R. Broome, W. E. Poe, and J. R. Heitz, Enuiron. Entol. 6,669 (1977). 368. J. R. Broome, M. F. Callaham, L. A. Lewis, C. M. Ladner, and J. R. Heitz, Copmp. Biochem. Physiol. C . 51, 117 (1975). 369. J. B. Ballard, A. D. Vance, and R. E. Gold, J . Econ. Entomol. 81, 1641 (1988). 370. D. N. S. Hon, T. Chang-Shang, and W. C. Feist, Wood Sci. Technol. 16, 193 (1982). 371. D. F. Martin and C. D. Norris, J . Enuiron. Sci. Health, Part A A23,765 (1988);B. F. Martin and D. F. Martin, ibid. 757. 372. D. Lerner, N. Didier-David, and J. C. Breux, Eur. Pat. Appl. EP 286,524 (1988). 373. J. Shi, X. Zhang, and D. C. Neckers, J . Org. Chem. 57,4418 (1992). 374. E. Klimchuk, M. A. J. Rodgers, and D. C. Neckers, J. Phys. Chem., in press.

Advances in Photochemistry, Volume18 Edited by ,David H. Volman, George S. Hammond, Douglas C. Neckers Copyright © 1993 John Wiley & Sons, Inc.

INDEX

Absorption spectra, Rose Bengal, 353-354 Acrylamide, polymerization, 327 Acrylonitrile, photopolymerization, 326-327 Actinometry, large-scale, 304-306 Acyl oximes, 342-343 5-X Adamantanone, fumaronitrile addition, 99 a-Adamantyl p-chloroacetophenone, 184- 185 1-Adamantyl p -methoxyacetophenone, 183-184 n -Alkanes, photochlorination, 152- 153 Alkanones: irradiation in zeolites, 193 Norrish I1 reactions, 207-210.212-213 reaction cavities, 173-176 Alkanophenones: P-cyclodextrin effects, 201-203 EIC ratios, 178-180 mesomorphic, 177-181 Norrish I1 reactions, 213-215 photoproduct ratios, 196-197 reaction cavities, 177-186 7-Alkoxycoumarins, 142, 144 p-Alkyl alkanophenones, 213-216 Alkyl cinnamates, 115-116, 148-149 3-Alkylcyclopentenones, 140 N-Alkyl trans-cinnamates, 146-147 a-Alkyl dibenzylketones, 95, 127 Alkyl perfluoroalkyl ketones, Norrish I1 reactions, 219-220 Anilinonaphthalene sulfonates, 159

Anthracenes, 9-substituted, 141 Anthracenesulfonates, 136-137 9-Anthraldehyde, 154 Apocholic acid, 71 Apodization, 8 Atom reactions: Cl(’P), 47-48 F(2P), 41-42 N(~D) 52 , N(2P),52 O(ID), 4,23, 37-38, 54 O(3P), 4, 19-20,39-41,48 Azobenzene derivative monolayer film, air-water interface, 116-1 17 Beer-Lambert law, 271 Benzoyl carboxylic acids, Norrish I1 reactivity, 204-207 Benzylidene acetophenone, 135 Benzyl radical pairs, singlet and triplet, cage effects, 107, 109 1,6-Bis(o-chlorophenyl)- 1,6-diphenyl-2,4dyne-1,6-diol, 76 Blackbody emission, 5,24 Borate ion salts, 334 I-Bromo-diethylcarbonate, synthesis, 246 3-Butylcycloalkylcyclopentenones, photocycloaddition, 142-143 n-Butyl stearate. hexatic B mesophase, 2 12-2 14 Carboxy radicals, crosslinking, 346

395

396

INDEX

7-Chlorocoumarin, packing arrangement, 121- 122 Sa-Cholestan-3P-yl trans-cinnamate, 115-116, 145-146 Cholesteryl4-(2-anthryloxyl)butyrate, 147 Cholic acid, 71 Cinnamic acids, 115-1 16, 133 (-)-Citronellol, oxidation, 274-275 Clathrate inclusion complex, 198 Clay surfaces, 79-80 Coumarins, orientation, 141 Crystals: defect sites, 155 microheterogeneity, 153-155 photodimerization, 133-134 9-Cyanoanthracene, 154 Cyclic diones, 175-176 Cyclodextrin complexes: guest complexation, 160 irradiation, 124-125 reaction cavities with strong external medium influences, 200-204 -Cyclodextrinldiphenylmethyl-tetf-butyl nitroxide, structure, 159 Cyclodextrins, 75-77 a-Cyclohexyl para-substituted acetophenones, 181- 182 Deoxycholic acid, 71-72, 160-167 NN-Dialkyl-a-oxoamides, 197-200 Dianin’s compound, 73, 197 Dibenzylketone, 103-109 irradiation, 105-106 photolysis, 106-108 Diffuse incident model, 283,285 Di-n methane, rearrangements, 110-1 11 1,2-Dimethylbutadiene, polymerization, 151 Diphenyliodonium, 349-350 Diradical intermediates, overlap parameters, 110-1 11 Emission models, photochemical reactors, 286-291 Emission spectra, Rose Bengal, 353-354 Enones, irradiation, 119 Eosin, 323-350 derivatives, 323-324 emission characteristics, 325 photochemical decomposition, 325 as uhotodvnamic sensitizer. 324 as photoinitiator, 326-350

a-amino radical, 341, 348-349 dimerization, 339 flash photolysis, 339-341 free semireduced radical, 349 oxime acrylates, 342 oxygen effects, 326-328 photooxidation, 329-33 1 polymerization at depth, 335-338 polymerization extent, 344 rate constants in aqueous solutions, 347 spike formation, 335-336 termination, 337-339 termination rate, 347 triplet decay rate constants, 340-341 W photocrosslinking, 342-346 triplet, 324 Eosin-PDO-MDEA system, 346-349 Eosin-triethanolamine system, 341-342,345 Erythrosin, 350-351 Ethyl acrylate, photopolymerization, 328-329 Excimer lamp, 241-242 photochemical reactor, design, 259-263 Extense source with volumetric emission model, 289-290 Faujasite supercages, 81-82 a-alkyl dibenzylketone photolysis in, 127-128 free volume size impact, 129-131 guest molecule in, 92 microheterogeneity, 156-157 Fellgett advantage, 2,6 Flash photolysis: Eosin, 339-341 novel xanthene dye derivatives, 378-379 Fluorescein, 3 17,320-323 Fluorescence, laser-induced, 3 Fourier transform spectroscopy, fundamentals, 5-10 Free volume, reaction cavity, magnitude, 126-132 FTIR emission, see Time-resolved FTIR emission Fumaronitrile, addition to 5-X adamantanone, 99 Gas-solid heterogeneous reaction mixtures, photochemical reactor design, 243-244 Heneicosane. 217-219 Heterogeneous reaction systems, 280

INDEX

y-Hydrogen abstraction: by carbonyl oxygen atom, 167 reaction cavities with very stiff walls, 190- 191 Hydroxy-l,4-biradicals, extended transoid, 215 Hydroxyfluorones, 376-379 2-Hydroxy-SH-furanone, synthesis, 274 Incidence models, photochemical reactors, 283-286 Inclusion complexes: microheterogeneity, 158-162 Norrish 11 reactions, 195-200 photodimerization, 134- 137 Inclusion hosts, organized media, 70-77 Infrared emitting species: CCH, 29,35 CH,, 35 C,H,NCO, 55-56 CO, 4, 21,27,29, 3.5, 39-40, 46, 52, 58 C02,41-44,46-47,48,53,58

DF, 40 F, 40 HCl, 27, 32, 38, 47 HF, 4,21,37,39-40,42-43,58 NH, 52-53 NH,, 27,29, 36 NO,, 50 NzO, 46-47,50,52 OH, 4,37-39, 52 phase correction, 29-30 product distribution, 37-48 atom-radical reactions, 47-48 CFzHCI, 42-44 chemical reactions, 37 energy transfer, 37-38 H F distributions, 41, 43 rate constant, 40 spot-scan implementation, 39 statistical energy partitioning, 40-41 Isophorone, orientation, 140-141 Jacquinot advantage, 2,6 Ketones, see also Norrish I1 reactions photophysical characteristics, 163 photoreactions other than Norrish I1 processes, 164-165 Langmuir-Blodgett films, 85

397

photodimerization, 144-145 reaction cavity free volume, 116-117 Light measurement, spatial distribution, 237, 241,268-272,279,285,305 Light sources, 241-242 extended, 284 photochemical reactor, design, 251-264 qualitative, 264-269 Line source with parallel plane emission model, 286-288 Line source with spherical emission model, 288 Liquid crystals, 86-88, 145-147 Liquid-liquid microheterogeneous reaction systems, 244-245 Mau water-splitting scheme, 361-362 7-Methoxycoumarin, 112-1 13 7-Methoxy 4-methyl coumarin, 155 Methyl 2-benzoyl- 1,4-dihydro 1,4ethenonaph thaleneJ-carboxyla te, di-n-methane rearrangement, 119-121 4-Methylbenzyl-benzyl radical pairs, triplet, cage effects, 107-108 Methyl rn-bromo-cinnamate, 112-113 N-Methyldiethanolamine, 331-332 Methyl methacrylate, photopolymerization, 328-329,347-348 Micelles, 83-84 enone and olefin orientation, 142-143 microheterogeneity, 162 photodimerization, 140-144 preorientation, 141- 142 reaction cavity, flexibility, 206-207 Michelson interferometer, 4-5, 12-13 Microheterogeneity, organized media, 100-103 Microheterogeneous complexes, aqueous, reaction cavities, 204-210 Molecular aggregates, p hotodimerization, 148-149 Molecular sieves, pore opening size and dimensionality, 83 Monolayers, 84-85 Naphthalenes, 2-substituted, 141 Neural networks, photochemical reactors, 302-306 fundamentals. 302-304 large-scale actinometry, 304-306 P-Nitrostyrene, 115-1 16

398

INDEX

2-Nonanone, molecular volume, 171-172 5-Nonanone/urea complex, 195-196 Norrish I1 reactions: ketones, 165-171 aromatic, selectivity, 188, 190 in benzene and Dianin’s compound, 196- 197 BR lifetimes, 168-169 conformation studies, 168 E and C pathways, 165 y-hydrogen abstraction, 167 liquids of high fluidity, 168 major features, 165-166 medium effects, 169-171 photoproduct selectivity, 169-171 Yang reaction, 165 organized media, 171-220 alkanone reaction cavities, 173-176 alkanophenone reaction cavities, 177-186 neat crystalline phases, 171-186 reaction cavities with some wall flexibility, 195-200 reaction cavities with strong external medium influences, 200-204 reaction cavities with very stiff walls and preformed shapes, 186-194 reaction cavities with walls of variable flexibility, 210-220 reaction cavities with walls of variable flexibilihi and strong external medium influences, 204-210 Nyquist criterion, 8

-

N-Octadecyl-l-(4-pyridyl)-4-(phenyl)-1,3butadiene, 144 4-0ctyl-4-(3-carboxytrime thy1eneoxy)azobenzene, 116-1 17 Olefin addition, face preference, 137-138 Organized media, 67-221. See also Norrish I1 reactions, organized media; Reaction cavity future research, 220-221 inclusion hosts, 70-77 subclassification, 70-71 Langmuir-Blodgett films, 84-85 liquid crystals, 86-88 micelles, 83-84, 162 microheterogeneity, 100-103, 153-162 crystals, 153-155 inclusion complexes, 158-162

micelles, 162 surfaces, 155 zeolites, 155-158 monolayers, 84-85 photochemical and photophysical studies, classification, 69 silica, clay, and zeolite surfaces, 78-83 Tris-Onho-thymotide, 74-75, 125 Oxidation, sensitized, 274-277 Oxidative degradation, 278-280 photocatalyzed, TiO,, 295-301 Oxime acrylates, 342-344 Partially diffuse model, 283,285 Pentasil zeolites, 82 Perfluoroalkyl alkyl ketones, 174 Perhydrotriphenylene, 7 1-73 2-Phenylcycloalkanones, 129, 131 1-Phenyl-onho-methyl acetophenone, 106 1-Phenyl-para-methyl acetophenone, 106 I-Phenyl-3-(o-tolyl)propan-2-one, 92-93 Photobleaching, Rose Bengal, 370 Photobrominations, 245-247 Photocatalyzed reactions, 237 photochemical reactors, 277-282 Photochemical reactor: bubble, 264 cylindrical, 257-258 falling film, 265-267 falling- film tubular, 253-254 “fluorescent relav”. 256 immersion-type, 241,272,279 mathematical modeling: coupling of mass and light energy balances, 291 emission models, 286-291 incidence models, 283-286 radiation models, 282-291 photocatalyzed reactions, 277-282 rate of production, 272-274 sensitized oxidations, 274-271 tubular, 281,284-286 up-scaling, 237 Photochemical reactor design: experimental, 292-303 experimental matrix, 297-300 fundamentals, 292-294 neural networks, 302-306 strategy, 293-294 TiO, photocatalyzed oxidative degradation, 295-302

INDEX

399

Polymer matrices, reaction cavities with gas phase reactions, 239-240 walls of variable flexibility, 210-212 gas-solid heterogeneous reaction mixtures, Polymers: 243-244 with chemical and stereo regularities, light sources, 251-264 151-152 emission characteristics, 252 diffusion of free volume, 21 1 excimer lamps, 259-263 Previtamin D, synthesis, photochemical extended, 256-259 reactions, 268-269 negative geometry of irradiation, Pyrene, diffuse reflectance and excitation 258-259 spectra, 156-157 point sources, 255-256 2-Pyridones, [4+4] photodimerizations, safety requirements, 262,264 141-142 liquid-liquid microheterogeneous reaction systems, 244-245 Radiation models, photochemical reactors, liquid phase reactions, 239,241-243 282-291 qualitative, 238,264-272 Rate constants, 2 filming prevention, 264-265 Reaction cavity, 88-104 light distribution, 268-272 active and passive, 97-99 light source, 264-269 alkanones, 173-176 spatial separation of light source and alkanophenones, 177-186 reaction mixtures, 265-267 cage effects, 105 quantitative, 238 Cohen’s model, 90 reaction mechanism, 245-247 defined by boundary, size, and shape, safety requirements, 248-251 9 1-96 solid-liquid heterogeneous reaction definition, 90 systems, 243 effective, 91-92 up-scaling, 249 enclosure consequences, 105- 109 Photochlorination, liquid phase reactions, final, 94-95 239,241 free volume, 90,96-97, 109-117 Photo-Claisen rearrangements, 124 C-C bond rotation, 113-1 14 Photodimerization: C D cavity size, 131 crystals, 133-134 di- and tri-n methane rearrangements, inclusion complexes, 134-137 110-1 11 L-B films, 144-145 disproportionation products, 128-129 liquid crystals, 145-147 enantioselective photocyclizations, 122 micelles, 140-144 impact of size in faujasite supercages, molecular aggregates, 148-149 129-131 surfaces, 137-140 inclusion complexes, 113 Photofading, Rose Bengal, 370 lattice energy calculations, 110 Photohalogenation, 248 L-B assemblies, 115-116 Photonitrosylation, 241,264 location and directionality, 117-126 Photopolymerization: tris-ortho-thymotide, 125 acrylonitrile, 326-327 photoClaisen rearrangements, 124 ethyl acrylate, 328-329 photodimerization, 112 methyl methacrylate, 328-329,347-348 photolysis, 115-116 Rose Bengal, 371 reaction feasibility, 117-1 18 thick samples, 368 steric compression accompanying thin films, 366-367 pyramidalization, 119-120 vinyl acetate, 327-328 idealized model, 88-89 Point sources, 255-256 initial, 91-92 Polymeric Rose Bengal, 371-373 limitations, 103-104 Polymerization, laser initiated, 330-33 1

400

INDEX

Reaction cavity (Continued) microheterogeneity, 100- 103 multiple sites, 103 with some wall flexibility, 195-200 steric compression control, 110 stiff and flexible, 96-97 with strong external medium influences, 200-204 as “templates”, 132-153 prealignment, 150-151. See also Photodimerization types, 93,96 very stiff walls and preformed shapes, 186-194 with walls of variable flexibility, 210-220 with walls of variable flexibility and strong external medium influences, 204-210 Reaction mechanism, photochemical reactor, design, 245-247 Response surface methodology, 294 Retinal, photoisomerization, 114-1 15 Rose Bengal, 351-376 absorption and emission spectra, 353-354 acetylcholinesterase reaction, 374 chemical reactivity, 354-357 crystal structures, 353 cytotoxic and photodynamic inactivation of microorganisms, 374-375 decarboxylation, 361-370 electron donor concentration, 365 photopolymerization rate, 368 photoreduction quantum yields, 366 relative rate constants of termination, 369-370 thick sample photopolymerization, 368 thin film photopolymerization, 366-367 visible absorption spectrum, 363-365 xanthene skeleton reduction, 363-364 dimers, 372 dismutation by electron transfer, 360-361 lactone, 352-353 photobiological applications, 373-376 photobleaching and photofading, 370 photopolymerization, 371 polymeric, 371-373 quenching of singlets and triplets, 357-360 Safety requirements: light sources, 262,264 photochemical reactor, design, 248-251

Silicalite, 157, 190 Silica surfaces, 78-80, 186-188 Solid-liquid heterogeneous reaction systems, photochemical reactor design, 243 Spectroscopy, fluorescein, 321-323 Stereolithography, 333-335 Stilbazolium cations, 139-140 Stilbenes: derivatives, 115-1 16 photochemical reactivity, 148- 149 in X type and in ZSM-5 zeolites, 126 Surfaces: microheterogeneity, 155 photodimerization, 137-140 Template effect, 132 Tetrabromofluorescein, see Eosin Tetraiodofluorescein, 350-351 2,4,5,7-Tetraiodo-3’,4‘,5’,6’-tetracholorfluorescein, see Rose Bengal 1,1,6,6-Tetraphenylhexa-2,4-dyne1,6-diol, 76-17 Thermotropic liquid crystalline and solid matrices, reaction cavities with walls of variable flexibility, 212-220 Thioindigo dyes, isomerization, 116-1 17 Thioxanthone, 362 Time-resolved FTIR emission, 1-58 apodization, 8 atom reactions: C1(2P),47-48 F(2P), 41-42 N(*D), 52 N(2P), 52 O(’D), 4, 23, 37-38, 54 O(3P), 4, 19-20,39-41,48 beamsplitter, 5 comparison of SS and CS spectrometers, 28-3 1 continuous-scan method, 4 continuous-scan spectrometers, 22-28 digitization, 7 energy transfer, 48-57 chemical reactions, 53 C02,48-53 cold gas filter, 50 electron bombardment, 50, 52 IRMPD, 55 rate constant, 52 HCCC1,34 interleaved sampling, 47

INDEX

IRMPD, 33-34,36-37 IR multiple photon dissociation, 4 normalization, 28-29 Nyquist criterion, 8 optical path difference, 6-7 phase correction, 9-10 photodissociation dynamics, 31-37 power spectrum, 9 resolution, 29 signal-to-noise ratios (SNR), 3, 6-7 spectrum resolution, 7-8 stop-scan spectrometers, 4, 10-22 applications, 39 historical development, 10-12 IRMPD production of free radicals, 14, 18-19 normalization, 15, 18-19 phase correction, 16 pulsed electron bombardment, 11 resolution, 16 Ti02. photocatalyzed oxidative degradation, 295-301 Trichloroacetyl chloride, synthesis, 248 Trichloromethyl-chloroformate, 249-250 Tri-n methane, rearrangements, 110-1 11 Uranine, 320 W photodissociation, 3.23, 32, 35

401

Valerophenone, 190-192,207 Vinyl acetate, photopolymerization, 327-328 Visual pigments, photochemical process, 114-115 Xanthene bis iodonium salt, 334 Xanthene dyes, 315-379. See also Eosin; Rose Bengal erythrosin, 350-351 fluorescein, 317,320-323 hydroxyfluorones, 376-379 novel derivatives, 378-379 oxidation and reduction, 360-361 physical data, 319 quantum yield of fluorescence, triplet formation, and singlet oxygen formation, 358-359 structures, 317-318 Zeolites: free volume directionality, 125-126 microheterogeneity, 155-158 photoproduct ratios, dependence on, 188-190 reaction cavities with very stiff walls, 188- 194 site inhomogeneity, 158 X- and Y-type, 81-82 Zeolite surfaces, 81-83

Advances in Photochemistry, Volume18 Edited by ,David H. Volman, George S. Hammond, Douglas C. Neckers Copyright © 1993 John Wiley & Sons, Inc.

CUMULATIVE INDEX VOLUMES 1-18

Addition of Atoms to Olefins, in Gas Phase (Cvetanovic) . . . . . . . . . . . . . . . . . Alcohols, Ethers, and Amines, Photolysis of Saturated (von Sonntag and Schuchmann) Alkanes and Alkyl ................................. Effects of (Rabinovi Alkyl Nitrites, Decomposition of and the Reactions of Alkoxyl Radicals in Chromophorically Substituted (Becker) .......................... Aromatic Hydrocarbon Solutions, Photochemistry of (Bower) . . . . . . . . . . . . . Atmospheric Reactions Involving Hydrocarbons, FTIR Studies of (Niki and Maker) . . Benzene, Excitation and Deexcitation of (Cundall, Robinson, and Pereira) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biochromophoric Systems, Excited State Behavior of Some (De Schryver, Boens,andPut) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cancer Treatment, Photochemistry in (Dougherty) ....................... Carbonyl Compounds, The Photocycloaddition of, to Unsaturated Systems: The Syntheses of Oxetanes (Arnold) ............................... Cobalt (111) and Chromium (111) Complexes, the Photochemistry of, in Solution (Valentine, Jr.) ........................................... Complexes, Photoinitiated Reactions in Weakly Bonded (Shin, Chen, Nickolaisen, Sharpe, Beaudet, and Wittig) .......................... Cyclic Ketones, Photochemistry of (Srinivasan) ......................... Cyclobutanones, Solution Phase Photochemistry of (Morton and Turro) . . . Cyclometallated Complexes, Photochemistry and Luminescence of (Maestri, Balzani, Deuschel-Cornioley, and von Zelewsky) . . . . . . . . . . . .

402

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3

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1s

69

10

147

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359

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275

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301

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16 1 9

249 83 197

17

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403

CUMULATIVE INDEX VOLUMES 1-18 VOL. 8

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16

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3 8

241 1

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13 6 11

237 425 489

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137

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11 2

375 263

Imaging Systems, Organic Photochemical (Delzenne) .................... Intramolecular Proton Transfer in Electronically Excited Molecules (KlBpffer) Ionic States, in Solid Saturated Hydrocarbons, Chemistry of (Kevan and Libby) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Isotopic Effects in Mercury Photosensitization (Gunning and Strausz)

11 10

1 31 1

2 1

183 209

Ketone Photochemistry, a Unified View of (Formosinho and Amaut) . . . . . .

16

67

a-Dicarbonyl Compounds, The Photochemistry of (Monroe) . . . . . . . . . . . . . . Diffusion-Controlled Reactions, Spin-Statistical Factors in (Saltiel and Atwater) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electron and Energy Transfer, Mimicking of Photosynthetic (Gust and Moore) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electron Energy Transfer between Organic Molecules in Sol (Wilkinson) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electronically Excited Halogen Atoms (Hussain and Donovan) . . . . . . . . . . . Electron Spin Resonance Spectroscopy, Application of to Photochemistry (wan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........ Electron Transfer, Photoinduced in Organic Systems, Control of Back Electron Transfer of (Fox) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............... Electron Transfer Luminescence in Solution (Zweig) Excimers, What’s New in (Yakhot, Cohen, and Ludmer) . . . . . . . . . . . . . . . . . . Excited Electronic States, Electronic and Resonance Raman Spectroscopy Determination of Molecular Distortions in (Zink and Shin) . . . . . . . . . . Free Radical and Molecule Reactions in Gas Phase, Problems of Structure and Reactivity (Benson) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FTIR Emission Studies, Time Resolved, of Photochemical Reactions (Hancock and Heard) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gas Phase, Addition of Atoms of Olefins in (Cvetanovic) . . . . . . . . . . . . . . . . . Gas Phase Reactions, Photochemical, in Hydrogen-Oxygen System (Volman) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gas Phase Reactions Involving Hydroxyl and Oxygen Atoms, Mechanisms and Rate Constants of (Avramenko and Kolesnika) . . . . . . . . . . . . . . . . . . Halogenated Compounds, Photochemical Processes in (Major and

..................................

Hydrogen-Oxygen Systems, Photochemical Gas Phase Reactions in (Volman) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydroxyl and Oxygen Atoms, Mechanisms and Rate Constants of Elementary Gas Phase Reactions Involving (Avramenko and

Mechanisms of the Reactions of (Atkinson, Darnall, Winer, Lloyd, and ........ .................................. Pitts) Hypophalites, Developments in Photochemistry of (Akhtar)

404

CUMULATIVE INDEX VOLUMES 1-18

Mechanism of Energy Transfer, in Mercury Photosensitization (Gunning andstrausz) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanistic Organic Photochemistry, A New Approach to (Zimmerman) Mercury Photosensitization, Isotopic Effects and the Mechanism of Energy Transfer in (Gunning and Strausz) ................................. Metallocenes, Photochemistry in the (Bozak) ........................... Methylene, Preparation, Properties, and Reactivities of (De More and Benson) . . . . . . . ..................................... . . . . . . . . . . . . Molecular Distortions in Excited Electronic States, Electronic and Resonance Raman Spectroscopy Determination of (Zink and Shin) , . .

VOL.

PAGE

1 1

209 183

1 8

209 227

2

219

16

119

10 5 13 6

1 157 vii 193

14

135

17 1

313 323

8

315

5 2

21 263

12

201

18

67

10

221

I 17 2 9 8 4

57 69 305 311 161

4 7

25 373

4 4 4

81 195 113

Neutral Oxides and Sulfides of Carbon, Vapor Phase Photochemistry of the

....................................................

Nitric Oxide, Role in Photochemistry (Heicklen and Cohen) . Noyes, W.A., Jr., A Tribute (Heicklen) . Nucleic Acid Derivatives, Advances in t

..........

Olefins, Photolysis of Simple, Chemistry of Electronic Excited States or Hot ............ Ground States? (Collin) 'Onium Salts, Photochemistry fson, and Sahyun) .................................................... Organic Molecules, Photochemical Rearrangements of (Chapman) .. Organic Molecules in Adsorbed or Other Perturbing Polar Environments, Photochemical and Spectroscopic Properties of (Nicholls and Leermakers) ..................................................... Organic Molecules in their Triplet Stat (Wagner and Hammond) . . . . . . . ...... Organic Nitrites, Developments in Photochemistry of (Akhtar) . . , . , . . , , . , . Organic Photochemical Refractive-Index Image Recording Systems (Tomlinson and Chandross) ....................................... Organized Media on Photochemical Reactions, A Model for the Influence of (Ramamurthy, Weiss, and Hammond) ........................... Organo-Transition Metal Compounds, Primary Photoprocesses of (Bock andvon Gustorf) ................................................ Perhalocarbons, Gas Phase Oxidation of (Heicklen) ..................... Phenyl Azide, Photochemistry of (Schuster and Platz) . . . . . . . . . . . . . . . . . . . Phosphorescence and Delayed Fluorescence from Solutions (Parker) . . . . . . Phosphorescence-Microwave Multiple Resonance Spectroscopy (El-Sayed) Photoassociation in Aromatic Systems (Stevens) ......................... Photochemical Mechanisms, Highly Complex (Johnston and Cramarossa) Photochemical Oxidation of Aldehydes by Molecular Oxygen, Kinetics and Mechanism of (Niclause, Lemaire, and Letort) ...................... Photochemical Reactivity, Reflections on (Hammond) . . . . . . . . . . . . . . . . . . . Photochemical Rearrangements of Conjugated Cyclic Ketones: The Present State of Investigations (Schaffner) ................................. Photochemical Transformations of Polyenic Compounds (Mousseron) . . . . . Photochemistry of Conjugated Dienes and Trienes (Srinivasan) . . . . . . . . . . .

1

405

CUMULATIVE INDEX VOLUMES 1-18 VOL. 12

PAGE 97

12 9

1 369

11 1 1

305 1 275

8

109

13

329

2

385

11 4 3 13 8

207 225 83 1 427 245

16

1

15

229

14

91

9

147

Quantum Theory of Polyatomic Photodissociation (Kreslin and Lester) . . . .

13

95

Radiationless Transitions, Isomerization as a Route for (Phillips, Lernaire, Burton, and Noyes, Jr.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Radiationless Transitions in Photochemistry (Jortner and Rice) . . . . . . . . . . .

5 7

329 149

17

145

15 7 17 11

279 311 217 105

4 13

49 1

2

183

Photochemistry of Rhodopsins, The (Ottolenghi) ........................ Photochemistry of Simple Aldehydes and Ketones in the Gas Phase (Lee and Lewis) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photochemistry of the Troposphere (Levy) .............................. Photochemistry of Vitamin D and Its Isomers and of Simple Trienes (Jacobs and Havinga) . ............................ Photochemistry, Voc on, Hammond) Photochromism (Dessauer and Paris) ...................... Photo-Fries Rearrangement and Related Photochemical (1.j) Sh (j = 3,5,7) of Carbonyl and Sulfonyl Groups (Bellus) . . . . . . . . . . . . . . . . . Photography, Silver Halide, Chemical Sensitization, Spectral Sensitization, Latent Image Formation (James) ............................. Photoionization and Photodissociation of Aromatic Molecules, by Ultraviolet Radiation (Terenin and Vilessov) ........................ Photoluminescence Methods in Polymer Science (B Phillips) ................................. Photolysis of the Diazirines (Frey) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photooxidation Reactions, Gaseous (Hoare and Pearson) . . . . . . . . . . . . . . . . . Photooxygenation Reactions, Type 11, in Solution (Gollnick) . . . . . . . . . . . . . . Photopolymerization, Dye Sensitized (Eaton) Photosensitized Reactions, Complications in ( Photosynthetic Electron and Energy Transfer, Mimicking of (Gust and Moore) ................................................ ysics and Photochemistry of (Sc Phytochrome and Holzwarth) ............................... Polymers, Photochemistry and Molecular Motion in Solid Amorphous (Guillet) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Primary Proceses and Energy Transfer: Consistent Terms and Definitions (Porter, Balzani and Moggi) . . . . . . . . . . . . . . . .

6

Silver Halides, Photochemistry and Photophysics of (Marchetti Peroxides (Hollingsworth and McBride) ............................ Singlet Molecular Oxygen (Wayne) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Singlet Molecular Oxygen, Bimolecular Reactivity of (Gorman) Singlet Molecular Oxygen, Physical Quenchers of (Bellus) . . . . . . . . . . . . . . . . Singlet and Triple States: Benzene and Simple Aromatic Compounds (Noyes andUnger) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Small Molecules, Photodissociation of (Jackson and Okabe) . . . . . . . . . . . . . . Solid Saturated Hydrocarbons, Chemistry of Ionic States in (Kevan and Libby) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

406

CUMULATIVE INDEX, VOLUMES 1-18

Solvation, Ultrafast Photochemical Intramolecular Charge Transfer and ... Excited State (Barbara and Jarzeba) . . . . . . . . . . . . Spin Conservation (Matsen and Klein) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stilbenes, Bimolecular Photochemical Reactions of (Lewis) . . . . . . . . . . . . . . . .. Sulfur Atoms, Reactions of (Gunning and Strausz) Sulfur and Nitrogen Heteroatomic Organic Compou Reactions of (Mustafa) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surfactant Solutions, Photochemistry in (von Biinau and Wolff) . . . . . . . . . . . Theory and Applications of Chemically Induced Magnetic Polarization in Photochemistry (Wan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transition Metal Complexes, Primary Processes in (Forster) . . . . . . . . . . . . . . .. Triatomic Free Radicals, Spectra and Structures of (Herzberg)

VOL.

PAGE

15 7 13 4

1 1 165 143

2 14

63 273

12 16 5

283 215 1

3

157

Ultraviolet Photochemistry, Vacuum (McNesby and Okabe) . . . . . . . . . . . . . . Ultraviolet Radiation, Photoionization and Photodissociation of Aromatic Molecules by (Terenin and Vilessov) ............................... Up-Scaling Photochemical Reactions (Braun, Jakob, Oliveros, Oller do Nascimento) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2

385

18

235

Weakly Bonded Complexes, Photoinitiated Reactions in (Shin, Chen, Nickolaisen, Sharpe, Beaudet, and Wittig) . . . . . . . . . . . . . . . . . . . . . . .

16

249

18

315

Xanthine Dyes, Photochemistry of the (Neckers and Valdes-Aguilera)

,,

., . .