Advances in Photochemistry. Volume 15

ADVANCES IN PHOTOCHEMISTRY Volume 15

ADVANCES IN PHOTOCHEMISTRY Volume 15 Editors

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

GEORGE S. HAMMOND Department of Chemistry, Georgetown University, Washington, D.C.

KLAUS GOLLNICK lnstitut fur Organische Chemie, Universitat Munchen, Munchen, Federal Republic of Germany

A WILEY-INTERSCIENCE PUBLICATION

John Wiley & Sons, Inc. NEW YORK I CHICHESTER I BRISBANE / TORONTO I SINGAPORE

3 1730 00693 3225

Copyright

0 1990 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. Library of Congress Cataloging in Publication Data:

Library of Congress Catalog Card Number: 63-1 3592 ISBN 0-471-63289-9 Printed in the United States of America 10 9 8 7 6 5 4 3 2 1

CONTRIBUTORS

Paul F. Barbara Department of Chemistry University of Minnesota Minneapolis, MN 55455

Wlodzimierz Jarzeba Faculty of Chemistry Jugiellonian University 3 Karasia, 30-060 Krakow Poland

Hans-Dieter Becker Department of Organic Chemistry Chalmers University of Technology and University of Goteborg S-412 Goteborg, Sweden

J. Michael McBride Department of Chemistry Yale University New Haven, CT 06511

Silvia E. Braslavsky Max-Planck-Institut fur Strahlenchemie Stiftstrasse 34-36 D-4330 Miilheim a.d. Ruhr Federal Republic of Germany

Paul D. Maker Scientific Research Laboratory Ford Motor Company Dearborn, MI 48121

Mark D. Hollingsworth Department of Chemistry University of Alberta Edmonton, Alberta Canada T6G 2G2

Hiromi Niki Department of Chemistry and Canadian Center for Research in Atmospheric Chemistry York University North York, Ontario Canada M3J 1P3

Alfred R. Holzwarth Max-Planck-Institut fur Strahlenchemie Stiftstrasse 34-36 D-4330 Mulheim a.d. Ruhr Federal Republic of Germany

Kurt Schaffner Max-Planck-Institiit fur Strahlenchemie Stifstrasse 34-36 D-4330 Mulheim a.d. Ruhr Federal Republic of Germany

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 sense have the articles been simple 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 KLAUSGOLLNICK Davis, California Washington, D.C. Miinchen, Federal Republic of Germany

vii

CONTENTS

Ultrafast Photochemical Intramolecular Charge Transfer and Excited State Solvation PAUL F. BARBARA AND WLODZIMIERZ JARZEBA Atmospheric Reactions Involving Hydrocarbons: FTIR Studies HIROMINIKI AND PAUL D. MAKER Excited State Reactivity and Molecular Topology Relationships in Chromophorically Substituted Anthracenes HANSDIETERBECKER Photophysics and Photochemistry of Phytochrome KURTSCHAFFNER, SILVIAE. BR~SLAVSKY, AND ALFREDR. HOLZWARTH

1

69

139

229

Photochemical Mechanism in Single Crystals: FTIR Studies of Diacyl Peroxides MARKD. HOLLINGSWORTH AND J. MICHAEL MCBRIDE

279

Index

381

Cumulative Index, Volumes 1-15

387

ix

Advances in Photochemistry, Volume15 Edited by David H. Volman, George S Hammond, Klaus Gollnick Copyright © 1990 John Wiley & Sons, Inc.

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE AND EXCITED STATE SOLVATION Paul F. Barbara and Wfodzimierz Jarzeba Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455

CONTENTS I. Introduction A. Overview B. Solvation C. Charge transfer 11. Solvation A. Basic solvatochromism 1. The relationship of C(t) to the solvent coordinate 2. A simple model for C(t) 3. Debye-Onsager model for C(t) and the longitudinal relaxation time z, B. Solvation probes C. Ultrafast fluorescence instrumentation D. Time resolved spectra, solvation dynamics and 2, 1. The practical determination of C(t) 2. Summary of published C(t) measurements 3. An evaluation of the Debye-Onsager model E. Modern theories of solvation 1. Onsager theory for C(t) for non-Debye solvents 2. Onsager inverted snowball F. Solvation dynamics in water 1

2

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

111. Excited state charge transfer

A.

Electronic states and the solvent coordinate 1. A prototype charge transfer molecule: Bianthryl 2. Electronic states in the gas phase 3. Torsional angle 4. The solvent coordinate 5. Vibrational dependence on LE and CT 6. Diabatic charge transfer 7. An adiabatic potential for S , 8. The probability distribution function and spectra B. Theory of dynamics 1. Solvent controlled electron transfer 2. Excited state charge transfer 3. Simulation of the charge transfer dynamics of bianthryls 4. The charge transfer rate constant C. Experiments on 9,9’-bianthryl 1. Time resolved spectra D. ADMA E. DMABN F. Other molecules and related theoretical results IV. Conclusions Acknowledgments References

I. INTRODUCTION

A. Overview The study of the structure and dynamics of electronically excited polar aromatics has been an internationally active area of research for over three decades. Two related phenomena have been at the center of this field, namely: (i) the sensitivity of fluorescence spectra of aromatic solutes to solvation and (ii) the appearance of dual fluorescence bands due to intramolecular adiabatic, excited state charge transfer reactions. The latter process was discovered in 1961 by Lippert et al. [l], who observed dual fluorescence bands for paracyano-N,N-dialkylanilines in polar solvents. Later Grabowski et al. [2] assigned the existence of two fluorescence bands to an excited state torsional (twisting)isomerization, involving a “twisted” intramolecular charge transfer

INTRODUCTION

3

(TICT) mechanism, i.e., both torsional motion and charge transfer occur during the interconversion of the two excited state species. In the intervening years TICT compounds and related examples of excited state charge transfer have been examined from many perspectives, including a fundamental investigation of the molecular interactions responsible for the TICT phenomenon, photodynamic mechanistic studies of the kinetics of TICT, and more recently, the use of TICT (and related dual fluorescence) compounds as model systems for the investigation of ultrafast charge transfer reactions in solution. This last topic is at present an extremely active area of research, and it is emphasized in this review. This paper is concerned with a special type of charge transfer reactions in which both the precursor and product of charge transfer process are emitting species (dual fluorescence).We will not discuss intramolecular charge transfer between weakly coupled donor and acceptor groups of the type that produces nonemitting charge transfer intermediates. Many excellent reviews and well-referenced papers have considered excited state charge transfer with dual fluorescence, and transient solvation. The broad range of chemical systems that exhibit bond torsion “twisting” comcomitant with charge transfer was summarized in 1986 by Rettig [3]. In the same year Kosower and Huppert published a review that emphasized their influential work on the role of solvation dynamics in excited state charge transfer [4]. An extensive and in-depth review on many aspects of the photophysics of internal twisting was published in 1987 by a group of experts in this field i.e., Lippert, Rettig, BonaEiC-Kouteck?, Neisel, and Miehe [ S ] . Recent reviews on excited state charge transfer and related excited state isomerizations have also been published [6-81. The other major subject of this review is the study of excited state solvation of polar aromatics that do not undergo charge transfer. The understanding of excited state solvation was greatly advanced over 30 years ago by the seminal papers of Lippert [9] and Mataga’s group [lo], which were concerned with static (nontime-resolved) fluorescence solvatochromism [9,10]. In 1960s and 1970s, the foundations of the study of transient soluatochromism were pioneered by Bakshiev, Mazurenko, and their coworkers [ll-141. Major progress on transient solvation has been made in just the last few years as a result of new ultrafast spectroscopic methods and the advent of new theoretical approaches. Presently, photodynamic studies on excited state solvation is leading to unprecedented knowledge on the microscopic motion of polar liquids. The recent progress in this area, and its relationship to excited state charge transfer, is extensively reviewed in this article. Very recently, Simon briefly reviewed publications up to 1987 on solvation and some of the recent research on ultrafast spectroscopic measurements on charge transfer [lS].

4

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

The present review article is primarily concerned with the enormous recent theoretical and experimental progress made in the study of excited state dynamics, both solvation and charge transfer. Over 50 new references not previously reviewed can be found in 1987 and 1988. A few new directions have emerged. Ultrafast (femtosecond and subpicosecond) time-resolved fluorescence spectroscopy has allowed for the unprecedented measurements of transient dipolar solvation of excited molecules in common solvents. Coordinated ultrafast experiments on the charge transfer dynamics of a number of molecules has opened the door to careful analysis of the fundamental aspects of the role of molecular motion in excited state charge transfer. At the same time, intense theoretical activity on solvation dynamics and ultrafast charge transfer has made substantial progress in our understanding of these phenomena. Indeed, many important comparisons of theory have been made.

B. Solvation Time-resolved fluorescence spectroscopy of polar fluorescent “probes” that have a dipole moment that depends upon electronic state has recently been used extensively to study microscopic solvation dynamics of a broad range of solvents. Section I1 of this paper deals with the subject in detail. The basic concept is outlined in Figure 1, which shows the dependence of the nonequilibrium free energies ( F g and F,) of solvated ground state and electronically excited probes, respecitvely, as a function of a generalized solvent coordinate. Optical excitation (vertical) of an equilibrated ground state probe produces a nonequilibrium configuration of the solvent about the excited state of the probe. Subsequent relaxation is accompanied by a timedependent fluorescence spectral shift toward lower frequencies, which can be monitored and analyzed to quantify the dynamics of solvation via the empirical solvation dynamics function C(t), which is defined by Eq. (1).

Here v(O), v(t), V ( Q ) represent the frequency of the intensity maximum of the fluorescence spectrum immediately after photon excitation, at some time t after excitation, and at a time sufficiently long to ensure the excited state solvent configuration is at equilibrium. Until recently, due to the lack of sufficiently short time-resolution, C(t) measurements were limited to slowly relaxing and associated viscous solvents, typically at low temperature [16-29). In the last 2 years, the first C(t)

INTRODUCTION

5

I

\ LL

s P

a C w

al

E

LL

~

xgq

xzq

Solvent Polarization, X

Figure 1. Nonequilibrium free energy as a function of instantaneous solvent polarization for the ground electronic state So and the excited state S , of an ideal probe. In this example the equilibrium solvent polarization in S1 ( X t q )is larger than in So ( X i q ) because the dipole moment is larger in S , than So. Reprinted from Ref. 31 with permission, from J . Chem. Phys. 88, 2372 (1988). Copyright 1988, American Physical Society.

measurements of the solvation of ordinary, nonviscous room temperature liquids have been made by state-of-the-art (subpicosecond and femtosecond) fluorescence spectrometers [22, 30-331. Very recently, the first report of a C(t) measurement of the solvation dynamics of water has been published [33]. A few papers have dealt with the potential sources of errors in C(t) measurements, particularly with respect to the properties of the fluorescent probes [22, 23, 311. Theoretical activity on solvation dynamics has also blossomed in the recent past [34-50). Traditionally, solvation dynamics have been described in terms of a simple continuum model [Sl, 521, which treats the solvent as a uniform dielectric medium with exponential dielectric response [52]. The associated dielectric parameters are E ~ and , zD,which are the infinite frequency dielectric constant (approximately n2), the static dielectric constant, and the dielectric relaxation time, respectively. According to the simple continuum model, the microscopic solvation function C(t) should decay exponentially with a time constant that is

6

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

approximately given by

where z, is the so-called longitudinal dielectric relaxation time [1 1- 15, 53561. Considerable progress has been made in going beyond the simple Debye continuum model. Non-Debye relaxation solvents have been considered. Solvents with nonuniform dielectric properties, and translational diffusion have been analyzed. This is discussed in Section 11. Furthermore, models which mimic microscopic solute/solvent structure (such as the linearized mean spherical approximation), but still allow for analytical evaluation have been extensively explored [38,41-431. Finally, detailed molecular dynamics calculations have been made on the solvation of water [57, 58, 711. Analysis of the experimental measurements of transient solvation (primarily C(t)) in terms of contemporary theoretical models has led to several conclusions [lS, 22-26,30-33,411, which are reviewed in detail in Section 11. Continuum treatments are seen to fail in several cases, but are remarkably predictive considering the simplicity of the model. Qualitative features predicted by theories that go beyond the simple continuum model are borne out in experiment, although the agreement is qualitative at best.

C. Charge Transfer Much of the recent activity in the study of the excited state charge transfer reactions stems from the impact of these studies on the understanding of the role of solvent dynamics in charge transfer reactions of many types, including examples in biology, inorganic chemistry, and organic chemistry. There is a growing appreciation that fast electron transfer reactions are not well characterized by traditional models (such as Marcus-Hush theory or related nonadiabatic theories), which are based on quasi-equilibrium approximations for the population of thermally excited states of the solute/solvent systems [59-611. Contemporary theories go beyond and treat solvation dynamics in detail. In Section 111 we review many recent papers in this field [62-73,136-1421. A key result is that the rate of a charge transfer reactions should be a function of the microscopic dynamics of the specific solvent. In fact, in the case of very small intrinsic charge transfer activation barrier, the rate is predicted to be roughly equal to the rate of solvation (i.e., z'; for a solvent with a single relaxation (rD) time). This result was first derived over 20 years ago by

SOLVATION

7

Mozumder for the neutralization of an isolated ion pair in polar media [74]. The predictions are considerably more complex for solvents with a distribution of relaxation times and barrier energies that are comparable or larger than the available thermal energy. Intramolecular vibrational effects can also be important. Recent progress in evaluating the new theoretical models by comparing predictions to ultrafast transient fluorescence data [4, 30,75-881 on excited state charge transfer of a variety of molecules is also reviewed in Section 111. The first observation correlating k,, with t;' was made by Huppert and Kosower [4]. Many of the theoretical predictions of new models are indeed observed experimentally, supporting the need for the new theoretical models. Unfortunately, many aspects of the experiments remain poorly defined, so the comparison to theory is often qualitative at best. In other cases, the complexity of the solvation dynamics (nonexponential)and the complexity of the solute solvent interactions make it difficult to make an unambiguous interpretation of the ultrafast data. A number of potential new directions for future experiments on charge transfer that could circumvent the present problems are discussed in Section 111.

11. SOLVATION

A. Basic Solvatochromism The photodynamics of polar fluorescent molecules, has been the subject of a number of theoretical studies. The earliest studies were from Bakshiev and Mazurenko [12- 141. Contemporary treatments that take advantage of recent theoretical advances have been published by a number of groups [53561. The formalism of Van der Zwan and Hynes [54] is particularly convenient and we employ it here, as we review the derivation of some of the basic equations of dynamic solvatochromism. To begin with we consider a fluorescent probe molecule that has ideal properties: it is not polarizable, it does not undergo excited state chemical reactions of any sort (including charge transfer) and it does not interact with the solvent by specific interactions such as hydrogen bonding. Furthermore, we assume that the ground state dipole moment of the probe pr is in the same direction as the excited state dipole moment p e . Assuming further that linear response theory for the solute/solvent system applies, the dependence of the nonequilibrium free energies of the system (in the ground F , and excited F, states) are portrayed in Figure 1 as a function of the electrical polarization of the solvent (see below). In a transient fluores-

8

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

cence experiment the probe/solvent system is in an equilibrium configuration Xi9 of the ground state prior to photoexcitation. Immediately after FranckCondon (vertical) excitation, only the electronic polarization of the solvent has adjusted to the new enlarged dipole p e . Subsequently, the solvent reorients, and the Franck-Condon (vertical)emission energy shifts to lower frequency (larger wavelength). As stated above, the quantity that is typically used to characterize the evolution of the fluorescence spectrum is C(t)which is defined in Eq. (1). In the remaining part of Section 1I.A we review the formal relationship of C(t) to fundamental quantities in the statistical mechanical description of solvation. The derivation we review is adopted from the work of Van der Zwan and Hynes. A useful result of the derivation is that a physical basis for the solvent coordinate in Figure 1 is established [54]. The reader is referred to papers by Bagchi et al. [53], and Sumi and Marcus [54] for related treatments. 1. The Relationship of C(t) to the Solvent Coordinate. The fluorescence frequency of a solvating probe molecule whose spectrum is dominated by solvent interactions (see below) can be expressed by the following equation h ~ ( t=) AU(t) = F,(t) - F,(t)

(3)

Here F e ( t ) and F,(t) are the time-dependent nonequilibrium Helmholtz free energies of the e and g states, respectively. The energy difference AU(t)can be replaced by a free energy difference due to the fact that the entropy is unchanged in a Franck-Condon transition [Sl]. Free energies in Eq. (3) can be represented [54] by a sum of an equilibrium value Feq and an additional contribution related to nonequilibrium orientational polarization in the solvent. Thus for the free energy in the excited state F,(t) we have

The first term in Eq. (4), Feq, is given by

where Uz is the gas phase electronic energy of the excited state. The second term $Bop + B,,)p: is the equilibrium solvation energy according to linear response theory. The excited state dipole moment is represented by pc, and B,, and Bop are the orientational and optical components of the “force constant” of solvation, respectively.

SOLVATION

9

The physical significance of these variables is apparent when they are evaluated in the Onsager cavity description of solvation, which treats the solute as a sphere (which we will assume here is unpolarizable) of radius a. The solvent is modeled as a uniform dielectric medium with a static dielectric constant E~ and an optical dielectric constant E , ~ . The following relationships apply in the Onsager cavity description B

2 & -1 a3 2Eop+ 1

=-= OP

In simple models the orientational component is associated with the orientations of the dipole moments of the solvent, but in general other nuclear displacements may play a role. The optical component Bop is associated with the instantaneous electronic response. copis often assumed to be equal to the square of the refractive index n. The nonequilibrium term in Eq. (4)which is responsible for the time evolution of the fluorescence frequency can be expressed as

where p(t) is one possible representation of the solvent coordinate (i.e., orientational displacement of the solvent). p(t) is denoted by the term “instantaneous effective dipole moment.” It is a hypothetical dipole moment that “would be in equilibrium with the actual instantaneous orientational polarization of the solvent” [54]. It is interesting to note that the nonequilibrium free energy term is analogous to the equilibrium term if p, - p(t) is replaced by pe. To further develop the notion of a solvent coordinate associated with Figure 1 and the solvent time-dependent reorientation, it is useful to introduce the dimensionless solvent coordinate z(t), which is defined by

Figure 2 shows the dependence of the nonequilibrium free energy of the excited state F,(z(t)) and ground state F,(z(t)). The expressions for the two curves in Figure 2, Eqs. (10) and (11) follow from the derivation outlined

10

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

2

Figure 2. Schematic illustration of nonequilibrium free energies F in the ground and excited electronic states versus the solvent coordinate. Absorption and prompt fluorescence are illustrated, as is relaxed fluorescence. From Ref. 54 with permission, from J . Phys. Chem 89, 4181 (1985).Copyright 1985, American Chemical Society.

above, with an analogous treatment for the ground state.

+

F,(z(t)) = F:q

+

F,(z(t)) = F:

+5 (Ap)’[~(t)]~ 2

(Ap)’[l - ~(t)]’

Here A p = p, - pS. The solvent coordinate z(t)is dimensionless; it is zero when p(t) is equal to pLsand it is unity when p(t) is equal to p e . This representation of the solvent coordinate leads to a compact form for some of the key equations of solvation and electron transfer (see Section 1II.B) in solution. An expression that relates z(t) and C(t)can easily be derived by recalling the definition of C(t) (Eq. (l)),the result for h d t ) (Eq. (3)) and Eqs. (10) and (1 1). C(t)= 1 - z(t)

(12)

SOLVATION

11

Thus it becomes clear that C(t)in fact measures the evolution of the solvent coordinate as it evolves from its initial (Franck-Condon) nonequilibrium displacement in the excited state, to its final equilibrium value, i.e., C(00) = 0 = 1 - z(o3). A related view of the physical basis of the solvent coordinate is the wellknown reaction field R, which is the field located on the solute dipole due to the dipole induced solvent polarization. For example, the reaction field for the equilibrated excited state dipole R:q is given by

We can also consider a time-dependent reaction field (Eqs. (14) and (15)) which includes an orientational term R,, and an instantaneous (optical) term Ro, *

Thus the optical part of the reaction field responds instantaneously to the change of the molecular dipole moment when the probe is optically excited. In contrast, the orientational component relaxes as z(t) evolves. In summary, in this subsection we have attempted to give a physical basis of C(t)in terms of different definitions of the solvent coordinate. We have not yet described how the time evolution of C(t)is related to dynamical properties of the medium, which is the subject of the next two subsections and Section ILE of this review. 2. A simple model for C(t). In this subsection we explore the relationship of C(t) to dynamic properties of the solvent, in terms of the Onsager cavity description, following the work in the literature on this subject [12- 14, 53571. Theories that go beyond the Onsager model are described in Sections 1I.E and 1I.D. The Onsager cavity description of solvation treats the solvent as a dielectric continuum. The dielectric dynamics of the solvent is typically characterized by the frequency-dependent complex dielectric constant 2(w). The measurement of 2(o) for a neat solvent is conventionally called a dielectric dispersion measurement. Several authors have discussed how C(t) can be calculated from the Onsager cavity model. Briefly, we need to consider the time-dependent reaction field, which was related above (Eqs. (12) and (15)) to C(t). For simplicity we consider the case of a probe with pg = 0 and p e # 0. If the probe

12

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

is excited at t = 0, then it can be shown that

rr R(t) = ,ue J dt’r(t’) ~

0

where r(t) is the so-called pulse response function of the medium above the cavity. It is the inverse Laplace transform of the frequency-dependent response, ?(a)that is defined by

2 ;(a)- 1 a3 2i(w) E,

30) = -

+

E,

+2 3

where E, is the dielectric constant of the cavity which is often assumed to be negligible ( E , x 0). Thus by combining Eqs. (12), (15)-(17), a procedure exists for calculating C(t) i j the dielectric response ?(w) of the medium is known. 3. Debye-Onsager Model for C(t) and the Longitudinal Relaxation Time TI. The physical meaning of the relationship described in the previous subsection becomes apparent when we consider the popular special case of the Onsager cavity model that arises if we assume that the solvent’s dielectric properties are well described by a Debye form. qw) = E,

Em + 16s+-iwr,

Here zD is the so-called Debye dielectric relaxation time. One could view tD as a phenomenological time constant which applies to dielectric relaxation measurements, or alternatively for simple causes, involving dielectric relaxation of weakly interacting dipoles, zD is related to the reorientation time constant of the solvent dipole in the laboratory frame. Following the analysis of the previous section it has been shown that C(t) is given by Eq. (19) for a Debye solvent C(t) = exp( - t/zF)

where rF is given by

where z, is the longitudinal relaxation time defined by Eq. (2).

(19)

SOLVATION

13

According to Eq. (19), tIis the time scale for excited state solvation for a Debye solvent. In fact, it is the time scale for both excited state and ground solvation of dipolar solutes and ionic solutes. t l also plays a role in a broad range of reactive (Section 111) and nonreactive charge transfer processes in solution. It is clearly worthwhile to establish a physical picture for this important variable. The solvation time zI is considerably shorter than tD for many solvents. For example for water E , = 4.84, E,, = 79.2 and tD = 8 . 7 2 ~ s[33]. Thus in water tI= 0.59~s.Why is the time scale for solvation of a dipole so much shorter than t,? Why are there apparently two characteristic times (tland T ~ ) for a dielectric medium? Friedman [SS] suggested two simple thought experiments to resolve the paradox of two times. The relevant theory of dielectrics was described in the 1940s by Frohlich [89]. The first thought experiment corresponds to dielectric measurements. It involves applying a voltage to a capacitor containing a dielectric medium at t = 0, and then holding the voltage constant at t > 0. The dependent variable is the time-dependent current which decays as dielectric relaxation of the medium occurs. From the current, the characteristic relaxation time of the time-dependent displacement (D(t)))field can be calculated. The time is t,. This is essentially a time domain analog of 2(0) dielectric measurements. The second thought experiment resembles transient solvation. At t = 0, a certain amount of charge is put on the capacitor plates. This charge jump (D field jump) is analogous to the photon induced change of the dipole moment in the fluorescence solvation experiment. Subsequently (t > 0), the decay of the voltage on the capacitor due to dielectric relaxation of the medium is measured. Note the capacitor in this experiment is not connected to an external power supply for t > 0. The characteristic relaxation time for the decay of the voltage (and electric field E) is tI. According to Friedman’s analogy, t, is much shorter than tD because much less charge is carried in the second experiment (charge jump) than the first experiment (field jump), because the power supply was working during the first experiment while it was disconnected in the second experiment. In other words, in the first experiment,as the dielectric medium relaxes (creating a polarization which tends to lower the electric field E), the power supply compensates in order to keep the voltage (and E ) constant by changing the charge on the capacitor. In the second experiment, as the polarization develops, the E field decreases, so that the relaxation is sensitive to the history (correlations)of the dielectric medium. A microscopic model for coupling of a small number of dipoles that emphasizes the dipolar correlation picture of r1 has been developed by Berne et al. [90]. Section 1II.D describes experiments to test the applicability of the t, concept to real measurements of solvation dynamics.

14

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

B. Solvation Probes The analysis of the transient fluorescence spectra of polar molecules in polar solvents that was outlined in Section LA assumes that the specific probe molecule has certain ideal properties. The probe should not be strongly polarizable. Probe/solvent interactions involving specific effects, such as hydrogen-bonding should be avoided because specific solute/solvent effects may lead to photophysically discrete probe/solvent complexes. Discrete probe/solvent interactions are inconsistent with the continuum picture inherent in the theoretical formalism. Probes should not possess low lying, upper excited states which could interact with the first-excited state during the solvation processes. In addition, the probe should not possess more than one thermally accessible isomer of the excited state. The spectral shape of the emission band should be dominated by probe/solvent interactions as opposed to vibronic activity in intramolecular modes. Rigid molecules with large A p = p e - pg come closest to having a probe/solvent dominated spectrum [23]. Excited state (adiabatic) chemical reactions, such as charge transfer (Section 111), cis/truns isomerization [6-8, 91-93], and proton transfer [S] complicate the C(t)analysis, by producing additional emission bands and by leading to a time-dependent pe. In addition, useful dynamic solvation probes should have the following properties: 1. They should be soluble in a broad range of solvents 2. They should not have very rapid excited state radiationless decay mechanisms which would compete with fluorescence 3. They should have large radiative rate constants (kOJ so that emission can be observed on the femtosecond time scale.

There are a number of empirical tests for the near ideality of potential probes. Ideal probes should exhibit a solvent dependence of the fluorescence Stokes-shift (hi,,, - hi,,) that is in account with the Lippert-Mataga equation [9, lo].

In other words, a plot of the Stokes shift versus [(cO - 1)/(2&,+ l)] - [ ( E , - 1)/(2~, + l)] should be linear. Alternatively, a plot of the Stokes shift versus empirical measures of the solvent polarity, such as E , [30] or TL* may be a more realistic test of the ideality of a specific probe [22,23]. Again, a

SOLVATION

15

linear plot represents ideal behavior. In contrast, nonideal effects such as excited state charge transfer can lead to a plot of the type just mentioned that is significantly curved [23]. For ideal probes the shape of the fluorescence spectrum should be structureless and smooth without vibronic features in a range of solvents of different polarity. Another important observable is the radiative rate constant (Eq. (22)) where 4 is the absolute quantum yield and zobs is the fluorescence lifetime. This measure of the spectroscopic properties of the probe should not be strongly dependent on solvent polarity. A strong dependence would suggest that the electronic character of the excited state is strongly solvent polarity dependent, a situation expected for molecules capable of excited state charge transfer [23,30,82], a nonideal effect. A variety of probe molecules have been used in studies of microscopic solvation dynamics, such as: 1-aminonaphthalene [29,94], 4-aminonaphthalimide [23,95,96], coumarins 102 [30, 311, 152 [32], 153 [22,97], 311 [31], 343 [98], 7-dimethylamino-4-coumarin-acetic acid (DMACAA) [33], MPQB [24] and rhodamine 6G [26] (see Figure 3). In many solvents, many of these probes exhibit the empirical behavior expected for ideal probes. Figure 4 is an example of the usual trend observed for the absorption maxima (hvabs), emission maxima (hv,,) and the Stokes shift (ha,- hv,,) for a polar probe, in this case 4-aminophthalimide. The solvents employed in Figure 4 are all nonhydrogen bond donating solvents. Hydrogen bond donating solvents have a large specific interaction with 4-aminophthalimide [23,96]. The probes C311, C152, and DMACAA lack the bridging alkyl groups found in C102, C153, and C343. Coumarins without bridging alkyl groups have been reported to undergo excited state charge transfer associated with twisting motion about the single bond that connects the amino group to the aromatic ring [99]. However, linear Stokes shift versus solvent polarity plots are observed for these compounds. Furthermore, the fluorescence spectrum is not obviously due to two emitting species (the initial excited state and a charge transfer form). For example see absorption and emission spectra for C152 in Figure 5. The degree of the excited state charge transfer character for the emitting state of these probes must be very small, perhaps negligible for the use of these compounds as solvation probes. An additional group of molecules that have been used as transient solvation probes actually rely on charge transfer to produce the necessary dipole moment charge for solvation measurements. Examples here are 4-(9anthry1)-N,N-dimethylaniline (ADMA) [23,75] and bis(4-

16

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

m I-AN

H ,C\ N I CH 3

4-AP

0

cc$Lo

C311

Go

c102

C153

p

-

0

"

"

(CH312

c343

DMACAA

Figure 3. Molecular structures for many of the probes that have been used in excited state solvation measurements.

17

SOLVATION L

28

-

24

-

20

,*

6.4

6.0

5.6

-,

;/

-

w

5.2

*

I

30

L

40

50

to),

Figure 4. Absorption maxima (e), static emission maxima and Stokes shift (V) for 4AP as a function of solvent E,(30). Vertical bars indicate the uncertainty in the measurements. The lines are linear least-squares fits to the data. From Ref. 23 with permission, from J . Chem. Phys. 86, 3187 (1987). Copyright 1987, American Physical Society.

dimethylaminopheny1)sulfone (DMAPS) [15,251. These molecules are dual fluorescent in nonpolar solvents due to emission from the initial FranckCondon state and from a charge transferred form (see Figure 6 and Section 111). In Figure 6, R denotes reactant (the initial excited state) and P denotes product of the charge transfer state. Normally, charge transfer would be an unwanted effect, but in these compounds the charge transfer time is much faster than the average solvation dynamics and the ratio of concentrations of the charge transfer state to the initial state rapidly becomes >> 1. One must be careful when using probes of this nonideal character to measure early time solvation dynamics because electron transfer may not be more rapid than the fastest components of solvation; thus inaccurate results will be obtained (see Section 1II.C).

300

400

500

600

WAVELENGTH(nm1 Figure 5. Electronic absorption of coumarin 152 in methanol (---), and emission spectra in ethyl acetate (----), propylene carbonate (.-.-.-.-), and methanol (.---.-- ). The fluorescence spectrum red shifts with increasing solvent polarity; there also appears to be an additional red shift in the presence of hydrogen bonding. The absorption spectrum does not change significantly with change of solvent. From Ref. 32 with permission, from J . Chem. Phys. 90,153 (1989). Copyright 1989, American Physical Society.

Figure 6. Nonequilibrium free energy as a function of both internal coordinates and instantaneous solvent polarization for the ground electronic state So (F,) and the excited state SI ( F J . From Ref. 23 with permission, from J. Chem. Phys. 86,3183 (1987). Copyright 1987, American Institute of Physics. 18

Solvent Polarization and Internal Coordinates

SOLVATION

19

C. Ultrafast Fluorescence Instrumentation The investigation of solvation dynamics in typical solvents at room temperature requires time-resolved fluorescence measurements with picosecond to femtosecond resolution depending on the solvent. There are several emission techniques available in the picosecond time range. Time correlated single photon counting [1001 offers high sensitivity and time resolution near 20 ps. Streak cameras [1001 can measure fluorescence transients with time resolution close to 1 ps. Resolution on the order of picoseconds can be obtained using laser spectrometers with optical Kerr shutter [loll. But, only the fluorescence upconversion technique has been demonstrated to have sufficiently short time resolution to study C(t) for common, nonviscous liquids. Castner, Maroncelli, and Fleming used fluorescence upconversion with subpicosecond resolution to study the dynamics of a visible light absorbing probe, LDS-750 [22]. Our group used ultraviolet, subpicosecond resolved fluorescence upconversion to study the solvation dynamics of the nearly ideal coumarin probes [30,31]. Recently, we made the first femtosecond resolved solvation dynamics measurements [32, 33, 97, 98, 1021 using an improved version of our apparatus. The fluorescence upconversion apparatus recently built in our laboratory [lo21 can measure fluorescence following ultraviolet excitation with time resolution near 50fs. In this section we describe this apparatus in order to demonstrate the principles of the fluorescence upconversion experiment. Figure 7 shows the schematic of the fluorescence upconversion technique. Fluorescence at frequency wf, following excitation with an ultrafast, ultraviolet laser pulse is mixed with another laser pulse at frequency w , . The two beams are focused into a nonlinear optical crystal where light at the sum frequency ws = wfl w1 is generated. The intensity of the generated light is proportional to the fluorescence intensity. Changing the time delay between the fluorescence and gating laser pulse with optical delay stage, we can optically gate different parts of the fluorescence transient so we can record

+

m2

n

w

Sample

fl

Nonlinear crystal

Figure 7. Schematic of the fluorescence upconversion apparatus. The notation is defined in the text. From Ref. 6 with permission, from J . Irnag. Sci . 33, 53 (1989). Copyright Society for Imaging Science and Technology.

20

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

fluorescence intensity versus time. In our apparatus the excitation pulse is the second harmonic of an amplified ultrafast dye laser and the probe pulse is the fundamental of the same laser. Time resolution in this method is limited by the width of the laser pulses used for excitation and for optical gating, and by optical effects that produce time broadening [102,103], such as the frequency dependence of the speed of the fluorescence light as it travels through the apparatus. The laser source in our spectrometer is an amplified femtosecond dye laser with a much larger repetition rate than many of the existing amplified laser systems used for femtosecond spectroscopy. The amplification is necessary to improve the signal intensity which actually depends on roughly the third power of the laser intensity. The large repetition rate helps average over pulse-to-pulse fluctuations of the laser. The instrument consists of three sections-a synchronously pumped linear cavity femtosecond dye laser, a dye laser amplifier which amplifies individual pulses several thousand times, and the upconversion system where the fluorescence intensity versus time is measured using sum frequency generation. Figure 8 shows the schematic of the femtosecond linear cavity dye laser, built following the design of Dawson et al. [104]. The laser is synchronously pumped at 76 MHz by a mode locked frequency doubled, Nd :YAG laser. With the dye combination styryl 8 as a gain dye and HITCI as a saturable absorber, the laser produces 70 fs pulses (FWHM) at 792 nm. The average output power is 25mW which corresponds to energy 0.3 pJ/pulse. To amplify the dye laser pulses we use a copper vapor laser (CVL) pumped amplifier. The design of the amplifier based on the approach of Knox et al. [lo51 is shown in Figure 9. The amplifying medium is a dye jet pumped by CVL at 8kHz with 15ns pulses and average power 20 W. The CVL pulses are synchronized with dye laser train. The pumping laser beam is 3 . 7 5 c m R.C.

5 c m R.C.

HlTCl 790

nm

PUMP B E A M

500

-

700 mW. 80 p8

Figure 8. Schematic of the femtosecond linear cavity dye laser (see [lo21 and [104]).

21

SOLVATION

Figure 9. Design of the dye laser amplifier. Ultrafast laser pulses are amplified roughly 10,000 times by seven passes through a dye jet pumped by a copper vapor laser.

focused to a 1 mm spot size in a 1 mm thick dye jet of ethylene glycol/styryl 8 solution. The ultrafast dye laser pulses are amplified several thousand times passing seven times through the dye jet. The energy of the output pulses is usually in the range 2-3 pJ. Figure 10 shows the schematic of the fluorescence upconversion section. The amplified femtosecond laser pulses with a repetition rate 8 kHz and frequency w1 are used to generate second harmonic (396 nm) in a ' l mm KDP crystal with conversion efficiency of ~ 2 0 %The . second harmonic (wz)and residual fundamental (wl) laser beams are separated using a dielectric mirror. The laser beam at w 2 passes through a variable optical delay stage and excites a sample, which flows in 0.5 mm thick cell. The residual fundamental at w1 is used as the gate beam for upconversion. The fluorescence from the sample is collected and collimated with a microscope objective. It is focused and combined linearly or nonlinearly with the residual fundamental into a KDP crystal. The KDP crystal is angle tuned to phase match the sum frequency The light at the sum frequency is separated from generation, ws = wf, ol. residual fluorescence, second harmonic, and fundamental using a prism or a spatial filter, for the linear and nonlinear geometries, respectively. The signal is measured with a photomultiplier, read in and stored by a microcomputer. A plot of the fluorescence intensity versus time is obtained scanning the optical path length for ultraviolet excitation beam. A time response function of the apparatus can be measured by upconversion of the excitation beam. The width of such measured instrument response function is 280fs (FWHM). Comparing this result with the width of the autocorrelation function of the dye laser llOfs we observe -17Ofs broadening of the instrument response function due to group velocity

+

N

22

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

input

Upconversion

E

Sample

..

BS

KDP

Pol

Figure 10. Schematic of the upconversion: A/2, halfwave plate; BS,dichroic beamsplitter; POI, polarizer; M, monochromator; P, photomultiplier; and KDP, potassium dihydrogen phosphate, nonlinear optical crystal.

dispersion and group velocity mismatch 11021 in this system. Therefore, this apparatus with different nonlinear crystals (0.1mm BBO) and reflective optics for the fluorescence collection can still have two times better resolution. Figure 11 shows the instrument response function and fluorescence transients measured for DMACAA in water [97], for which femtosecond measurement was critical (see below for a full discussion of the probe in water.) Some ultrafast measurements presented in Section I11 were obtained with 200 fs instrument response function. The better time resolution of the instrument was obtained by replacing the fluorescence collecting microscope objective by a quartz lens. The sensitivity of the apparatus is cut in half due to a less well-focused fluorescence spot.

D. Time Resolved Spectra, Solvation Dynamics and TI 1. The Practical Determination of C(t). The time-dependent fluorescence Stokes shift of the spectrum should manifest itself as (i) a rapid decay in the fluorescence intensity on the blue edge of the fluorescence spectrum, (ii) a

23

SOLVATION

i -1.0

time

(PSI

-

4.0

Figure 11. Fluorescence transients of 7-(dimethylamino)coumarin-4-acetateion in water recorded at 445 nm (upper), 483 nm (middle), and 509 nm (lower).The solid line through the points is a fit of the data to a multiexponential decay. The peak near zero time in the upper panel is instrument response function (280fs fwhm). From Ref. 33 with permission, from J . Phys. Chem. 93, 7040 (1988). Copyright 1988, American Chemical Society.

rapid rise in the intensity at the red edge of the spectrum, and (iii) no fast transient at wavelengths near maximum of the spectrum (assuming that total shift is small). Figure 11 shows just this result for DMACAA in water [97]. The recorded wavelengths are 445 nm, 483 nm, and 509 nm at the blue edge, center, and red edge of the fluorescence spectrum, respectively. Note that the decay of population of the excited state of the molecule (roughly 1 ns) is not evident on this time scale.

24

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

Time-resolved fluorescence spectra have usually been measured by recording several transients at different wavelengths, a technique which is denoted by spectral reconstruction. The intensity from each of the transients is adjusted to correct for the wavelength-dependent sensitivity of the apparatus by setting the time integral of the transient intensity equal to the intensity from the static fluorescence spectrum. The corrected transients can then be used to reconstruct fluorescence spectra at different times after excitation. Usually, to obtain better time resolution, deconvoluted multiexponential functions are used for reconstruction instead of the experimental transients (see Figure 11). Following a procedure of Maroncelli and Fleming [22], the reconstructed time-resolved fluorescence spectra are fitted to log-normal line shape function [22,106]. Figure 12 shows examples of time-resolved reconstructed spectra and log-normal fits for DMACAA in water. The Stokes shift correlation function C(t) can be now calculated (Eq. (1)) using spectra maxima or their first moments from the log-normal function. The calculated function C(t)for DMACAA in water is shown in Figure 13. Unfortunately, the procedure just described to determine C(t) can consume many hours of spectrometer time, since several transients must be acquired and processed. Recently, an alternative timesaving procedure for measuring C(t)was developed [23,31]. The procedure, which is approximate, requires a single emission transient and certain photophysical data on the probe. It is based on a simple photodynamic model, in which it is assumed that the spectrum of the probe is a simple function of a single solvent parameter, X, denoted as the solvent polarization. During the solvation process, X is time-dependent, such that C(t) = [X(t) - X(co))]/ CX(0) - X(w)l. The instantaneous emission intensity at a certain detection wavelength ;Iis given [23] by

. Figure 12. Reconstructed fluorescence spectra of 7-(dimethylamino)-coumarin-4acetate ion 0.1 and Ips after excitation. The solid line represents the best fit of the log normal distribution function to the data. From Ref. 33 with permission, from J . Phys. Chem. 93, 7040 (1988). Copyright 1988, American Chemical Society.

.-c>. 03 c

Q)

c

w

1

1

1

1

1

1

1

25

SOLVATION

0.0

time (ps)

5.0

Figure 13. The upper section of the figure is a superposition of an experimentally determined C(t) function for the fluorescing probe 7-(dimethylamino)coumarin-4acetate ion in the water and biexponential fit of C(t).The lower section in the figure portrays the difference between the experimental C(t) and the biexponential fit on a scale that is expanded by a factor of 3 in the Y direction. From Ref. 33 with permission, from J . Phys. Chem. 93,7040 (1988). Copyright 1988, American Chemical

Society.

where C is an instrument constant, k&(X)is a fluorescence radiative rate constant (in the general case solvent polarity dependent), g ( X , A) is a spectral shape function that describes the (X) (polarization) dependent shape of the fluorescence spectrum and S&) is the population of the excited state e. The , in Eq. polarity dependence of the spectral density function k:, [ X ( t ) ] g [ X ( t )A] (23) can be estimated empirically from the static fluorescence spectroscopy and excited state population lifetimes of the chosen probe molecule in various quickly relaxing solvents [23] of known solvent polarity. Such measurements give us values of kF,[X(t = 00)]g[A,X ( t = co)] where the equilibrium excited state solvent polarity, X ( t = 00) varies from solvent to solvent. To estimate an empirical E,(30) scale [lo71 or A* scale the solvent dependent X ( t = a), [lo81 can be used. Alternatively, the solvent dependence of X ( t = co) can be estimated from the equilibrated fluorescence maximum hv,, of the probe in each solvent. Thus the dependence of the photophysical properties of the probe, on its fluorescence maximum hv,,, can be established. This is demonstrated in Figure 14. The usefulness of the curves in Figure 14 is clear if one considers Eq. (23) and the fact that in the subpicosecond solvation experiments, the excited state population S, can be assumed to be a constant during the solvation process. It follows that the curves in Figure 10 represent how the fluorescence intensity at different wavelengths should change as the emission maximum

26

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

0

390 nm

a 400 nm

4 1 0 nm

2.3

~~

2.4

Wavenumber, 104Cm-’

2.5

Figure 14. The spectral density function versus the frequency that corresponds to the fluorescence maximum in various solvents for coumarin 311. The points for each wavelength correspond to different solvents. Reprinted from Ref. 31 with permission, from J . Chem. Phys. 88, 2372 (1988). Copyright 1988, American Physical Society.

shifts, e.g., during the transient solvation process. The curve for 400nm is particularly important. At this emission wavelength, the change in intensity is directly proportional to the frequency of the fluorescence maximum. It follows from Eqs. (1) and (23) and Figure 14 that the measured fluorescence intensity at 400 nm is directly proportional to correlation function C(t) for this probe! In summary, to obtain C(t), one merely has to measure the fluorescence transient at the wavelength where the intensity is linearly dependent on the fluorescence maximum. This variation of the single wavelength method should be denoted by the linear wavelength method. The linear wavelength method has been used to study the solvation dynamics of numerous solvents employing several coumarin probes [31, 32, 97,981. The results are in reasonable agreement with those obtained through the spectral reconstruction method. The linear wavelength method is experimentally much easier than spectral reconstruction, but it also has some disadvantages. For some probes a linear wavelength may not exist. In such a case, a more complex variation of the single wavelength method can still be used [23].

27

SOLVATION

2. Summary of Published C(r) Measurements. Transient solvation dynamics of a variety of excited state probe molecules have been studied in the last few years. Table 1 summarizes the measurements that have been made at room temperature. Table 2 gives a brief summary of the low temperature results. For each entry in Table 1, the measured C(t) was fit by either a single exponential function [exp( - t/z,)] or a biexponential function [A, exp( - t / z J + A, exp(-t/z,)]. For example where a single exponential fit was satisfactory, a dash is found in the zt column. On the other hand, when an observed C(t) was better fit by a biexponential form, the best fit times r 1 and T, and their relative amplitudes (as a percentage) are listed in Table 1. The average relaxation time ( r s ) is the zero moment of the fitted C(t)(i.e., the amplitude weighted average of the two times). For some measurements, the biexponential fit was only a slight improvement over the single exponential form. In these cases two entries are given for the same measurement. There is reasonably good agreement between the measurements made by spectral reconstruction and the linear wavelength method. This supports the validity of this method which takes considerably less instrument time (see above). Some of the measurements in Table 1 were made with 500 fs resolution. Other results (from [32,97,98]) were made with 50 fs time resolution. The shorter time resolution measurements tend to exhibit short components (<4OOfs) at early times which were not resolved in the subpicosecond measurements. As a result the 50fs resolved measurements tend to be more obviously biexponential and tend to have shorter average relaxation times. The results for different probes in the same solvent are in good agreement, especially if it is taken into account that the 50fs measurements tend to give shorter times because of better time resolution, as mentioned above. The experimental measurements clearly demonstrate that solvation dynamics are often poorly described by a single exponential decay at room temperature. Low temperature results on alcohols, D M F and propylene carbonate have also been observed to be poorly represented by a single exponential (see references in Table 2). The presence of nonmonoexponential relaxation dynamics greatly impacts fast electron transfer reactions in these solvents (see Section 111).

- -

3. An Evaluation of the Debye-Onsager Model. The simplest treatment for solvation dynamics is the Debye-Onsager model which we reviewed in Section 1I.A. It assumes that the solvent (i) is well modeled as a uniform dielectric continuum and (ii) has a single relaxation time (i.e., the solvent is a “Debye solvent”) zD (Eq. (1 8)). The model predicts that C(t)should be a single

sw sw sr sw sw sw sw sr sw sw

c102 C311 c102 C152 c102 C311 c102

PrCN

BuCN Benzonitrile

PrOAc BuOAc DMSO

EtOAc

MeOAc

C311 c102 c102 LDS-750 C152 C153

sw sw sr sw sw sw sr sw sr sw sw sw sw

c102 C311 LDS-750 C152 c102 C311 C152

McCN

EtCN

Methodb

Probea

Solvent

-

0.9 0.7 0.4 0.56 1.5 1.1 0.31 (48) 0.33 (68) 1.5 2.1 1.6 3.6 2.1 (39) or 4.7' 1.5 1.8 2.7 2.6 2.3 4.0 6.6 3.1 0.33 (57) 0.33 (441

(%I

51 (PSI

(PS)

(%I

T2

-

0.9 0.7 0.4 0.56 1.5 1.1 0.85 0.7 1.5 2.1 1.6 3.6 4.5 4.7 1.5 1.8 2.7 2.6 2.3 4.0 6.6 3.1 1.2 1.4

(5)

31 31 22 32 31 31 32 32 31 31 31 31 97 97 31 31 31 31 31 31 31 22 97 97

Reference

TABLE 1 Experimentally Observed Solvation Dynamics at 298K Determined by the Correlation Function C(t); q ,r2 Relaxation Times, (TS) Average Relaxation Time (First Moment of C(t))

sw

C153 c102 C152

Propylene carbonate

C153 LDS-750 DMAPS ADMA DMACA c343

n-Propanol

n-Butanol Ethylene glycol Glycerol triacetate water sr sr sw

et

sr wr

sr

sr sr sw sr

sr sw sr sw

0.40 (55) 0.75 (55) or 1.4' 0.31 (47) or 0.70' 0.83 4.9 0.43 (46) 0.64 (58) 0.48 (50) 0.60 (40) 3.3 1.2 (40) 1.3 (60) 9.3 (32)* 14 (30) or 33' 61 100 820' 0.16 (33) 0.25 (50) 0.29 (51) 1.2 (67) 0.96 (50) 1.1 (49)

-

9.6 (60) 8.4 (40) 78.8 (68) 40 (70)

-

4.1 (54) 4.8 (42) 6.2 (50) 4.6 (60)

I

-

0.99 (53)

-

-

1.7 (45) 2.5 (45)

1.0 1.5 1.4 0.67 0.70 0.83 4.9 2.4 2.4 3.4 3.0 3.3 6.2 4.1 57 33 33 61 100 820 0.86 0.61 0.54 97 97 97 97 97 97 31 32 32 97 97 22 32 32 22 97 97 22 15 30,23 33 98 98

'C, coumarin; BA, bianthryl; DMACA, 7dimethylaminocoumarin-4-acetate-ion; DMAPS, 4,4'ddimethylamino) phenyl sulfone; ADMA, 4-(9-anthryl)-N,N-dimethylaniline. bsr, spectral reconstruction; sw, single wavelength method; et, electron transfer process. 'For this case, observed solvation dynamics can also be reasonably well represented by single exponential decay. dTemperature = 295 K. 'Temperature = 284 K.

LDS-750 C152

Methanol

C153

sw

C152

Acetone

sw

sw

sw

C152 C153

DMF

Ethanol n-Propanol 2-Propanol n-Butanol Propylene carbonate N-Methylpropylamide Glycerol triacetate

Solvent

Temperature Range (K) 130-273 222-295 253 190; 253 203 -258 244; 273 275-290

Probes' DMAPS, C153, R6G C153 C153 C153, 1NA C153, MPBQ C153 ADMA

Range (ps) 2.5 x 104-50 1.3 x 103-59 488 1.0 x lo4; 479 470-50 545; 125 1500-690

(7)

25,22,26 22 22 22 22,24 22 23

Reference

TABLE 2 Low Temperature Solvation Dynamics Determined from Time-Dependent Fluorescence Stokes Shift Measurements

31

SOLVATION

exponential with a decay time T, (Eq. (20)). Many of the solvents in Table 1 have been studied by dielectric relaxation methods and the observed &a) values can be well fit by a single relaxation time Debye form. z1 values are therefore available for comparison with experiment. In most cases, however, dielectric measurements have not been made at sufficiently high frequency to test whether potentially important, additional relaxation components with short time scales are present for these solvents. We will return to this important issue later. Ignoring the potential limitations of the dielectric data, we can evaluate the Debye-Onsager model for a number of apparently roughly Debye solvents, like propylene carbonate, the alkyl nitriles, the alkyl acetates, and other solvents. First of all, C(t) is often strongly nonmonoexponential, in contradiction to the theoretical prediction. Second, the observed average solvation time ( T ~ )is often much different from T ~ . Figure 15 from a paper by Maroncelli and Fleming shows a comparison of T~ and rD for measurements from several laboratories [44]. This figure was made before the new 50 fs resolution data were available, which we discuss below. The data in Figure 15 show that ( T ~ )is usually equal to or larger than

-

-G

>

-c

0,

1.1 0.8 0.5

0.2

0

J-o.

I

-0.4 1

0

.

.

.

1

.

0.4

.

.

1

3. . 0.0

Log

'

1

'

"

1.2

1

'

I.6

(Eo / E l )

Figure 15. Average response times (z)/zL versus E&,. The numbered points are experimental data from various groups denoted according to solvent and probe molecule: 1 = alcohols/Cul53; 2 = ethanol/DMAPS; 3 = methanol, n-butanol/LDS750; 4 = NMP/Cul53; 5 = PC/Cu153; 6 = PC/CulO2; 7 = nitriles/Cul02, Cu311; 8 = acetates/Cul02, Cu311; 9 = dimethylsulfoxide/LDS-750.The solid curve shows the MSA result for parameters p = 1 and E , = 5. The dashed line represents the continuum prediction. From Ref. 44 with permission, from J . Chern. Phys. 89, 879 (1988). Copyright 1988, American Physical Society.

32

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

It is always much smaller than zD. Furthermore, there is an apparent trend that as the ratio of E,JE, increases the deviation from the Debye model becomes more severe (the solvent propylene carbonate [ 5 ] at low temperature shows an especially big deviation from r,). This trend is consistent with theories that go beyond the continuum model (see Section 1I.E). It is interesting to consider how the new, 50 fs measurements affect the observation that (z,) is often much longer than z1 for very polar solvents. Looking at Table 1, one can see the -5Ofs results give shorter (zs) values which are much closer to r, than the previous subpicosecond results would have indicated. A similar conclusion can be made for propylene carbonate at lower temperatures. Previous C(t) measurements [22] using time-correlated single photon counting gave longer (z,) values than measurements of C(t) with better time resolution [97]. Nevertheless, it would be a mistake to assume that the newer, better resolved measurements necessarily contradict or diminish the trends that have been observed between q and (z,), because, in most cases, the dielectric measurements i ( w ) have themselves been made with limited time resolution (actually in most cases frequency resolution). We assume that if many of the liquids of interest, such as propylene carbonate, were studied by higher frequency 2(w) measurement techniques, new, high frequency components would be discovered which would account at least partially for the short time scale dynamics we see in the solvation C ( t ) data. Indeed, the apparent observation of a single Debye time is inconsistent with theories of liquids that take into account dipole-dipole interactions (see Kivelson [109]). Furthermore, some of the liquids studied have extraordinarily large apparent infinite frequency dielectric constants E , (e.g., E , = 10 for propylene carbonate at lower temperatures [22]). A very large E , value can be a harbinger of unresolved, high frequency relaxation components [l lo]. 7,.

-

E. Modern Theories of Solvation A number of theoretical models for solvation dynamics that go beyond the simple Debye-Onsager model have recently been developed. The simplest is an extension of Onsager model to include solvents with a non-Debye like E(w). In other regards that approach still assumes the solvent is a uniform dielectric continuum and the probe can be represented by a spherical cavity. Newer theories allow for nonspherical probes [46], a nonuniform dielectric medium C4.51, a structured solvent represented by the mean spherical approximation [38-431, and other approaches (see below). Some of these are discussed in this section. Attempts are made where possible to emphasize the comparison between theory and experiment.

33

SOLVATION

1. Onsager Theory for C(t) for Non-Debye Solvents. Generally solvents have more complex dielectric responses than described by the Debye equation (Eq. (18)). To obtain the time dependence of the reaction field R from Eqs. (12, (15), (16) and (7) an appropriate model for dielectric behavior of a specific liquid should be employed. One of the most common dielectric relaxation is given by the Debye-type form, which is applicable to normal alcohols.

If there is no overlap between different regimes, j , then the pulse response function r(t) and accordingly C(t)decays as a sum of n exponents [22,53]. C(t) =

1 uj exp(-t/zFj) n

j=l

with TFj =

+ +

2Emj

E,

2EOj

E,

7Dj

and relative amplitudes

uj x

Eoj - E m j

(2Em j

+ ~c ) ( ~ Ej o+ ~

c )

(27)

Bagchi et al. have derived analogous equations for a solvent with two Debye times associated with two overlapping dispersion regimes [53]. These results allow a test of the Onsager cavity model for a uniform dielectric continuum solvent with a dielectric response that is well modeled by Eq. (24). Our group recently tested this model for methanol. In this case, both high frequency S(w) data (see Barthel et al. [lll]) and short time resolution C(t) data [32] exist. The experimental C(t) curve (solid line) and the predicted C(t) curve (dashed dot line) are in remarkable agreement (Figure 16), as are the predicted and observed (zs) values. The agreement is also excellent for npropanol [97]. In fact, good agreement between the continuum model and C(t) for alcohols has been noted in several cases. The dielectric dispersion for some solvents is poorly modeled by a multiple Debye form. Alternative, E(w) distributions such as the Davidson-Cole equation or the Cole-Cole equation are often more appropriate. The Davidson-Cole equation for S(o)is S,(o)

= Em

+ (1 +-iwto)p' Eo

Em

0
34

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

... *.....

.......

-------0.0

50.0

Time (ps)

Figure 16. Experimental methanol C(t) function (-) with continuum (-), MSA ion (-----I, and the MSA dipole (-.--) calculations.The time scale is 0.0-50.0~s.A two Debye relaxation function was used for the solvent dielectricrelaxation. From Ref. 32 with permission, from J . Chem. Phys. 90, 153 (1989). Copyright 1989, American

Physical Society.

The Cole-Cole equation is given by

Both forms have a distribution of relaxation times about zo which contribute to i((w). The Laplace transform can be obtained numerically, and in this case C(t)can be well fit to a stretched exponential function [46]: C(t) = exp[-(t/$],

0 < c1 < 1

(30)

Employing the Davidson-Cole model for propylene carbonate and the Cole-Cole model for propionitrile with the appropriate dielectric parameters from the literature, we have predicted C(t) for these polar aprotic solvents according to the dielectric continuum model. The agreement between the predicted and observed C(t)is not nearly as good as the alcohol examples (see below). 2. Onsager Inverted Snowball. It was suggested by Onsager that the molecular structure of the solvent would cause deviations [34] from the simple uniform picture, such that the relaxation time in the vicinity of the

SOLVATION

35

probe should be -zD, while the relaxation time far from the probe should be -zl. Hence relaxation starts far from the probe and proceeds toward the probe (the inverted snowball). In simple terms, the liquid far from the probe is like the bulk, hence the solvation time should be zl, while the solvent near the probe is more like single molecules relaxing, hence zD is more relevant close to the probe. The first qualitative treatment of Onsager’s prediction can be found in the work of Hubbard and Onsager in 1977 who showed that relaxation of an ion’s mobility in polar liquids occurs with a distribution of time scales from z1 to zD [35]. Calef and Wolynes in 1983 looked in detail at the solvation problem and verified that a distribution of time scales are relevant by numerically solving the Smoluchowski equation for the solvent orientational relaxation [37]. An important advance in the understanding of microscopic solvation and Onsager’s snowball picture has recently been made through the introduction of the linearized mean spherical approximation (MSA) model for the solvation dynamics around ionic and dipolar solutes. The first model of this type was introduced by Wolynes who extended the equilibrium linearized microscopic theory of solvation to handle dynamic solvation [38]. Wolynes further demonstrated that approximate solutions to the new dynamic MSA model were in accord with Onsager’s predictions. Subsequently, Rips, Klafter, and Jortner published an exact solution for the solvation dynamics within the framework of the MSA [43]. For an ionic solute, the exact results from these author’s calculations are in agreement with Onsager’s inverted snowball model and the previous numerical calculations of Calef and Wolynes [37]. Recently, the MSA model has been extended by Nichols and Calef and Rips et al. [39-431 to solvation of a dipolar solute. It is interesting to compare the MSA theory with the experimental results. Independently, Maroncelli and Fleming [MI, Rips et al. [43], and our group [32] have noticed that the MSA model is qualitatioely in accord with experiment, both in average solvation time (zs) and the shape of C(t). However, both the MSA ion and MSA dipole theory, tend to predict longer relaxation times from experiment for the long time decay of C(t). This is demonstrated in Figure 17 for the solvent propionitrile. The C(t)theoretical predictions for MSA ion (dashed line) and MSA dipole (dotted line) both decay more slowly than experiment (solid line) at long times. Indeed, the average solvation times (zS) for experiment and theory further demonstrate that the MSA based theories tend to decay more slowly than experiment, as shown in Table 3. Recent work on the theory of solvation dynamics has attempted to go beyond the linearized MSA model of Wolynes, which considers the rotational dynamics of the solvent as the only relaxation mechanism. Certain translational and hydrodynamic-like motions of the solvent are neglected.

36

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

0.0

Time

(PSI

8.0

Figure 17. Experimental propionitrile C(t)function (-)

with dielectric continuum MSA ion (-----), and MSA dipole (-) calculations. From Ref. 32 with permission, from J . Chem. Phys. 90,153 (1989). Copyright 1989, American Physical Society.

(.-.-.),

Bagchi and co-workers [47-501 have explored the role of translational diffusion in the dynamics of solvation by employing a Smoluchowski-Vlasov equation (see also Calef and Wolyness [37] and Nichols and Calef [42]). A significant contribution to polarization relaxation is observed in certain cases. It is found that the Onsager inverted snowball model is correct only when the rotational diffusion mechanism of solvation dominates the polarization relaxation. The Onsager model significantly breaks down when there is an important translational contribution to the polarization relaxation [47-501. In fact, translational effects can rapidly accelerate solvation near the probe. In certain cases, the predicted behavior can actually approach the uniform continuum result that T~ = 7,.

F. Solvation Dynamics in Water Besides the obvious practical importance of solvation dynamics in water, from a fundamental standpoint, water offers a unique opportunity to evaluate theoretical models for solvation. Only for water have extensive molecular dynamics simulations been accomplished (see below). Also semi-empirical models for solvation dynamics, such as MSA, can be carefully examined for water because the necessary information on the dielectric dispersion 2(0) of water is available at a very high level of accuracy. The results of the solvation

w

4

6.2

2.4

0.85

0.56

Methanol

Propylene carbonate

Propionitrile

Acetonitrile Cole-Cole

Debye

Cole-Cole

Cole-Davidson

Debye

Two component

Dielectric Model ES

= 32.47 = 5.47

TD, =

E , ~=

3.68 51.02 PS T D = ~ 3.29 PS % = 66.4 E , = 3.9 to = 46.2 PS /? = 0.91 % = 28.6 E m = 2.0 50 = 4.7 ps a = 0.09 % = 36.23 E , = 2.49 TD = 5.9 % = 36.23 E , = 1.81 T~ = 3.8 a = 0.14

&,I

Parameters'

"Dielectric data are from references found in Kahlow et al. [32].

(4

Solvent

0.30

0.53

0.44

2.7

6.1

(PS)

0.47

0.8 1

0.63

4.6

8.8

(PSI

('MSA;)

1.42

1.95

1.70

11.2

17.8

(PSI

(5MSA,+)

TABLE 3 Comparison of Calculated Average Solvation Times from Dielectric Continuum and MSA Models with Experimental Results

38

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

TABLE 4 Comparison of Experimental and Theoretical Values for Solvation Times in Aqueous Solutions"

Method Experimental

DC model, single Debye DC model, Cole-Cole MSA ion MD simulations (MCY water) (ST2 water) (TIP4P water)

0.16 (33) 0.25 (50) 0.29 (51) 0.59 (100) 0.48 (96) 0.59 (82)

G0.l 0.025 (50) ~

1.2 (67) 0.96 (50) 1.1 (49) -

1.40 (4) 3.20 (18) 1.o 0.3-1.0 (50)

-

0.86 0.61 0.54 0.59 0.52 1.05 -

0.09-0.33b 0.3-1.0

33 98 98 33 33 33

112

44 58

"Dielectric data for Debye ( T =~ 8.72 ps, E~ = 79.2, and E , = 4.84) and for Cole-Cole 8.7ps, E , = 4.2, and a = 0.013) distribution functions 1331. bThe average time constant was evaluated by integrating the decay from 0.0 to 1.Ops.

( T = ~

measurements for two probe molecules DMACAA and coumarin 343 are summarized in Table 4. Both molecules are organic acids and in the experiment they were used as salts. Therefore, solvation of the excited state of the anion form was observed in measurements. The results in Table 4 clearly indicate that solvation in water is nonexponential. Two examples of the uniform dielectric continuum (DC) estimates are given, both of which give approximately the correct average solvation time. The first is the result of a calculation using a single dispersion region fit to the main dispersion region of water; the second uses a Cole-Cole distribution, which more faithfully represents the frequency-dependent dielectric constant of water. The solvation dynamics predicted by these two models do not show the strongly multiexponential behavior observed in the experimental results [33]. Models for solvation in water that allow for a structured solvent do indeed predict a multiexponential response. For instance, the dynamical mean spherical approximation (MSA) for water solvation predicts that solvation of an ion in water is well represented by two characteristic times [38J Nonetheless, the specific relaxation times differ substantially from the observed behavior [33]. Recently, several authors have studied solvation dynamics of aqueous solutions using molecular dynamics (MD) computer simulations [36, 57, 58, 1121. The simulations offer a detailed molecular approach to interpreting the experimental results, as they focus particularly on the microscopic, molecular aspects of the solvation process.

39

SOLVATION

Engstrom et al. [112] used molecular dynamics simulations to study quadrupole relaxation mechanism for Li', Na', and C1- ions in dilute aqueous solutions. They found that NMR relaxation rate for these ions was determined by the relaxation of water molecules in the first solvation shell. The simulations show nonexponential solvation dynamics which can be modeled by two relaxation time constants T~ < 0.1 ps and z2 = 1 ps (see Table 4). Maroncelli and Fleming [57] studied the time dependence of solvation dynamics of monoatomic ions immersed in large spherical clusters of ST2 water. The simulations for solutes of different size and charge predict nonexponential hydration dynamics with a very short component (10 - 20 fs) due to librational motions of water molecules and a longer nonexponential component with average time constant of a few hundred femtoseconds. Our measurements would not be able to resolve the 10-20fs component. The results indicate the importance of the solvation process in the nearest neighborhood of the solute molecule. Figure 18 shows an example simulation

u

I1

\

'1 0

0.5

Time

1

(ps)

Figure 18. (a) (6V6V(t)). The solid curve at the bottom is the total time correlation function and the curves labeled 1-3 are the contributionsof individual shells 1-3. The cnrve marked p is the single-particledipole time correlation function (C,,(t) shown for comparison. From Ref. 57 with permission, from J . Chem. Phys. 89, 5044 (1988). Copyright 1988, American Physical Society.

40

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

of the time-dependent solvation energy with contributions from different solvation shells (1, 2, 3). The 16 water molecules in the first solvation shell account for 85% change in solvation energy. The simulations show that response of the first shell is much faster than that of the shells 2 and 3. The hydration dynamics were also studied by Karim et al. [58]. These authors used the TIP4P model of water in their molecular dynamics simulations. The observed hydration dynamics was nonexponential with average time constant in the range of 0.4-0.7~s. In this case simulated relaxation of the first solvation shell was also faster than that of the other shells. Table 4 compares experimental results of hydration dynamics obtained in our laboratory with M D simulations and calculations using D C and MSA models. Generally, the presented molecular dynamics simulations are in qualitative agreement with experiment, predicting nonexponential hydration dynamics with an average time constant below 1 ps. The dielectric continuum model gives a single exponential spectrum relaxation with a time constant close to the average observed experimentally. The MSA ion model predicts non-exponential relaxation, with little longer average time constant. The long time component is, however, not observed experimentally.

111. EXCITED STATE CHARGE TRANSFER A. Electronic States and the Solvent Coordinate During the nearly three decades since the discovery of the dual fluorescence of para-cyano-N,N-dialkylanilines in polar solvents [11, many other examples of dual fluorescence as a consequence of charge transfer have been discovered. Indeed, new molecular examples of these phenomenon are being discussed at a rate more rapid than ever. In this paper we do not attempt to exhaustively review the phenomenology of charge transfer induced dual fluorescence. Other reviews on this subject can be found ekewhere [3-51. Instead, we emphasize recent progress on the fundamental understanding of the dynamics of charge transfer, both from a theoretical and experimental standpoint. In particular, recent picosecond and femtosecond experiments receive a lot of attention. Our review emphasizes three molecular systems: 9,9’-bianthryl (BA) [30, 82, 88, 113- 1213, 4-(9-anthryl)-N,N-dimethylaminoaniline (ADMA) [1221301, and to a lesser extent the well known p-N,N-dimethylaminobenzonitrile (DMABN), a compound in the para-cyano-N,N-dimethylaminoaniline class [l-5, 75-81, 1313. We only briefly mention the arylaminonaphthalenesulfonates, which have recently been reviewed by Kosower and Huppert [4]. Brief mention is also made of picosecond experiments on diaminophenylsulphones and a number of other systems (see Section 1II.F).

41

EXCITED STATE CHARGE TRANSFER

L 00

1

500

hlnm

-

600

Figure 19. Corrected fluorescence spectra (room temperature) of BA, BACI, and C9A in nonpolar and polar aprotic solvents: Hexane (-), ethyl ether (--), tetrahydrofuran (-----), acetonitrile (-----). From Ref. 116 with permission from Ber. Bunsenges. Phys. Chem. 87, 1143 (1983). Copyright 1983, Verlag Chemie.

1. A Prototype Charge Transfer Molecule: Bianthryl. The phenomenon, charge transfer induced dual fluorescence, has been particularly extensively investigated for the molecule BA by a number of research groups [30,82,88, 113-121, 132,1333. This molecule is used as a prototype throughout Section 111. The fluorescence spectrum of BA is strongly solvent polarity dependent as shown in Figure 19 (from a paper by Rettig and Zander [116]). In polar solvents the emission can be roughly interpreted as being due to the sum of bands from two isomers of S,: a nonpolar LE (locally excited) form and CT (charge transfer) from, as proposed over a decade ago by Nakashima et al. [115] and Grabowski et al. [2].

CT'

LE

CT

Figure 19 also portrays analogous emission spectra for the molecules bianthryl-10-carboxaldehyde (BACI) and 9-anthrylcarbazole (C9A) whose chemical structures are shown in Figure 20 (see caption).

42

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

5OOJ

1

L50.

E

UMT ' Loo.

350.

Figure 20. Ground and excited state twist potentials in kJ/mol for BA, BAA, and C9A in the QCFF/PI framework. Ground and TICT states are labeled (-), the other states with a transition moment parallel to the linking bond are labeled (----), those with a transition moment perpendicular to it (-----). From Ref. 116 with permission from Ber. Bunsenges. Phys. Chem. 87, 1143 (1983). Copyright 1983, Verlag Chemie.

2. Electronic States in the Gas Phase. As a starting point for understanding the nature of the LE and CT forms it is useful to examine the gas phase electronic calculations of Rettig and Zander [116] on the ground and excited states of BA, and the related BAA and C9A (see Figure 20). For the case of BA, the first excited state in Figure 20 is particularly important. It corresponds to the so-called locally excited state mentioned above. Emission from this state to the ground state is responsible in simple terms for the fluorescence of BA in nonpolar solvents (e.g., hexane) in Figure 19. The LE emission is also present on the short wavelength edge of BA emission in the more polar solvents. The highest energy two states in Figure 20 for BA are the so-called charge transfer resonance states. They correspond to symmetrized combinations of radical cations (one ring) and radical anions (other ring) of BA. These CT states are strongly solvent dependent and are responsible for the interesting charge transfer behavior of BA (see below).

3. Torsional Angle. 4 in Figure 20 is the torsional angle between the aromatic rings of BA and the other molecules. Much of the research on molecules in this class has emphasized how this angle can modulate the critical vibronic mixing between LE and CT configurations [2,116]. The

EXCITED STATE CHARGE TRANSFER

43

importance of this angle is often emphasized by denoting the CT form as a twisted intramolecular charge transfer state (TICT). Our own work on BA [30, 82, 132, 1331 seems to indicate that while the torsional angle is certainly an important variable in determining the energy and mixing of the various states, it does not seem to be necessary to consider torsional dynamics in detail when modeling and analyzing results on the dynamics of LE to CT conversion in polar solvents (see below). We do not, therefore, emphasize the role of the torsional coordinate in this review and, rather, refer the interested reader to other papers concerned with various aspects of the photodynamics of the torsional degree of freedom in the excited state structure and dynamics of polar compounds [5, 120, 121, 1341.

4. The Solvent Coordinate. In order to explain the polarity dependence of the fluorescence of BA it is necessary to consider the effect of solvent polarity on the energy of the LE and CT states. For simplicity we will consider three elementary electronic configurations (Figure 21) resulting from hypothetical excitations of the highest occupied molecular orbital HOMO and lowest unoccupied molecular orbital LUMO on each of the anthracene rings of bianthryl. The LE state is an excitonic doublet of two nonpolar configurations which we model as a single configuration. The two charge transfer forms, CT and CT’, have large dipole moments that are in directions opposite to each other. r i ngl

LUMO -

inil

i ng2

I

I

i ngl

I

i ng2

I

+ -

;

HOMOfi

I

I

LUMO

;+

I

HOMO

I

ring1 ring2

i ni2

I

ingl

I

ing2

LUMO EOMO-

i;

Figure 21. Rudimentary electron configurations for BA. For each of the two anthracene rings (ring 1 and ring 2) the molecular orbitals are simply represented by HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital). Electrons are represented by vertical lines. All other terms are defined in the text.

44

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

LE

-1

0

1

Reaction coordinate(z1

Figure 22. Theoretical estimates for the zero order free energies for the So, LE, CT, and C T states as a function of the solvent coordinate z. The parameters have been adjusted in order to bring agreement between predicted and observed spectra for BA in the polar aprotic solvent, propylene carbonate. See text for further details.

The effect of solvent on the total free energy of a BA/solvent system is represented in Figure 22 for a polar solvent such as propylene carbonate. A corresponding plot for BA in a nonpolar solvent is given in Figure 23. These zero order (uncoupled) curves for the various configurations correspond to the following equations. F,, = FL'$

+ -21 BorzZ

Here pCTand pCT,are the dipole moments of the CT and CT' forms of the intramolecular charge transfer. They have the same magnitude but opposite

EXCITED STATE CHARGE TRANSFER

1

I

1

-1

0

1

45

Reaction coordinate(z1

Figure 23. Theoretical estimates for the zero order free energies for the So, LE, CT, and C T states as a function of the solvent coordinate z. The parameters have been adjusted in order to bring agreement between predicted and observed spectra for BA in the nonpolar solvent, hexane. See text for further details.

direction. z is the solvent coordinate which physically corresponds to the dielectric polarization of the solvent that is the result of reorientation of the solvent molecules. The remaining symbols are completely analogous to Eqs. (4) and (5). The actual values for the parameters employed in constructing Figure 22 were determined empirically as described elsewhere [132, 1331. The energy curves in Figure 22 are closely related to the Marcus-Hush theory for electron transfer. The formalism we employ emphasizes a dipole model for the solute solvent interaction, i.e., an Onsager cavity model. However, a Born charge model based on ion solvation as something in between [135] would be essentially equivalent because we do not attempt to calculate Bopand B,, but rather determine them empirically. It is obvious from Figure 22 that for a particular configuration of the solvent (particular z value), except for z = 0, the CT and C T states are of unequal energy. This effect is an example of local symmetry breaking which was recognized as important for BA several years ago [115].

46

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

5. Vibrational Dependence on LE and CT. Before demonstrating how the information in Figure 22 can be used to model the static and dynamic spectroscopy of BA it is important to explore how the vibrational degrees of freedom of the BA molecule might alter the simple energetics in Figure 22 that are derived considering the electronic energy and solute-solvent interactions only. Based on a Huckel model for the anthracene fragments of BA (which is an even alternant hydrocarbon) one can show that the delocalized LE state and the CT states should all have the same n bond order. This in turn implies that the vibrational modes and frequencies should be similar for LE, CT, and C T . Thus the vibrations of bianthryl can be ignored in the energy dependence on z (even for highly polar solvents). An additional vibrational coordinate to consider is the torsional angle cp. As stated above, dynamic experiments on BA and related compounds, strongly suggest that the torsional coordinate is essentially stationary (or at least changes very little) during the process of charge transfer. It should be noted that the strong repulsion.between nearby hydrogen atoms of BA tend to keep the rings near 90" in So and at large angles >60" in the LE state [120,121], in the gas phase. 6. Diabatic Charge Transfer. The various energy curves in Figure 22 ignore mixing (configuration interaction) between the various zero order states. Mixing turns out to be nonnegligible for BA because of the close proximity of the two (donor/acceptor) rings for the charge transfer reaction. Nevertheless, it is interesting to explore how the charge transfer of S , BA would be described in terms of a zero order picture. Before photon excitation the system would be distributed according to a Boltzmann distribution in the ground state So. Photon excitation would primarily produce population in the LE state due to the vertical excitation principle and the much stronger transition oscillator strength that connects So -+ LE compared to So -+ CT [132, 1331. Subsequent to photon excitation, the population would transfer diabatically from LE to CT and CT. In this limit the charge transfer process would correspond to the diabatic limit for charge transfer processes (see Section 1II.B).

7. An Adiabatic Potential for S1. In fact, the LE/CT mixing is quite substantial and the adiabatic energy curves of Figure 24 are a more accurate representation of BA than are the diabatic curves of Figure 22. These curves come from mixing the zero order curves of Figure 22 assuming two independent matrix elements: (LeIH'ICT) = (LEIH'ICT') = 2 kcal/mol and (CTIH'ICT) = 0. More details are given elsewhere [133].

41

EXCITED STATE CHARGE TRANSFER

r. 0 L

al

C

al

al

al L

U

-2

-1

0

1

2

Reaction coordinate(z1

Figure 24. Theoretical estimates for the adiabatic energies for the ground (So)and first excited singlet state (S,) of BA in the solvent propylene carbonate. The equilibrium distribution function for each state is denoted by psq.

In terms of the adiabatic curves of Figure 24, the electronic wave function of S, BA is represented as follows

The coefficients c , , c2, and c3 and the energy of S1 E,, are functions of z that are found by diagonalizing the 3 x 3 Hamiltonian of zero order energies and off diagonal mixing between LE, CT, and C T at each z. For this adiabatic model, the LE to CT interconversion occurs on a single potential energy surface. The methods for describing charge transfers of this type have recently made major progress (see Section 1II.B). 8. The Probability Distribution Function and Spectra. A key variable for the BA problem is the normalized probability distribution function p(z, t). Its equilibrium value p(z, t = 00) can be found from the Boltzmann distribution

48

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

on the S , surface (see Figure 24). All the physical properties associated with S , can be calculated from p(z, t). Indeed, charge transfer dynamic simulations result in the full time dependence of p(z, t), and consequently many features of the charge transfer dynamics (see Section 1II.B).The physical significance of Figure 24 is firmly established by considering how emission spectra are calculated from this model. First it can be assumed that the solvent is stationary on the time scale of emission so that for each specific value of z we can make the Franck-Condon approximation for the emission intensity.

(XIX’) is the vibrational overlap integral. The electronic part

Pel is given by

Here pLEIso, pcTIso,pCTtlso are the z independent transition moment matrix elements in terms of zero order states, but IpCT/sol = IpCT,lsol.The FranckCondon factors in Eq. (35) are assumed to be z independent since LE and CT have similar vibrational spectra. It follows simply that each z value contributes the following element to the spectrum for an arbitrary distribution P(Zlt),

dl(w, t )

gCwo(z),

0

- wo(z)lP:1p(z, t )dz

(37)

where wo is the origin of the electronic state, i.e., oo= [E,,(z) - Eso(z)]/h and g(wo,o - wo) is the normalized shape of the spectrum from the FranckCondon factors shifted to the appropriate oo(z) value. The total spectrum is the integral

We have been able to reproduce in Figure 25 the equilibrated emission spectra of BA remarkably well by using Eq. (38), the potential in Figure 24 and p(z,m) in Figure 24. The emission shape g(wo,w - oo)function was determined empirically by assuming that it is simply equal to the emission spectra of BA in hexane where the S , of BA is dominated by LE in the relevant region of the potential and oois not a function of z because LE and So are both nonpolar (see Figure 23). The band positions and band widths agree qualitatively well with experiment. The theoretical results also correctly predict that there is not a strong solvent effect on the absorption spectra of BA [133].

EXCITED STATE CHARGE TRANSFER

5000

F r e q u e n c y ( c m- ' 1

49

20000

Figure 25. Theoretical simulated spectra for BA in a nonpolar, moderately polar, and very polar solvent, see text and Ref. 132 for details.

In summary, an adiabatic model for the first excited state of BA is able to account well for the equilibrium absorption and emission properties of BA. It seems reasonable to assume that many of the dual fluorescent molecules would be well described by an adiabatic model of this type. In the following section we show the dynamic properties of BA are also well described by an adiabatic model. Indeed, the results are revealing from the standpoint of understanding small barrier charge transfer reactions in general.

B. Theory of Dynamics The theory of charge transfer, i.e., electron transfer, has been an active area of research for nearly three decades [59-611. Recent work on this problem has emphasized the dramatic role of solvation dynamics in certain cases, especiallysmall barrier electron transfer (ET) reactions [62-73, 136- 1423. A common starting point for theories of ET is a two-well model that is closely analogous to the diabatic representation that was invoked in the previous section for BA (see Figures 22 and 23). Figure 26 shows a two-well model for an excited state charge transfer between an LE and CT form. This might apply to a molecule like C9A for which one of the CT states is too high in energy to be important (see Figure 20).

50

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

I

Solvent Polarization

Figure 26. Schematic representation of the nonequilibrium free energy model. LE and CT denote locally excited and charge transfer states respectively. From Ref. 82 with permission, from J . Phys. Chem. 92, 6801 (1988). Copyright 1988, American Chemical Society.

Electron transfer (ET) is of course accompanied by rearrangement of the solvent as shown by the horizontal displacement in Figure 26. Tradiational theories for ET fall into two cases. In the nonadiabatic case it is assumed that the rate of ET is controlled by the process of crossing from one electronic state (e.g., LE) to the other (eg., CT) [60,61]. Alternatively in the weakly adiabatic case, it is assumed that the solvent polarization is always in equilibrium with the changing charge distribution. For this latter case transition state theory is applicable [59]. 1. Solvent Controlled Electron Danger. Recent research on electron transfer theory has explored the possibility that the motion of the solvent may be rate limiting in certain limiting cases. A number of very important contributions have been made in this field by many researchers [62-73,1361421. The reader is referred to this interesting body of work which has revolutionized the understanding of electron transfer. For the sake of brevity we will only consider the simplest results of this research. Many of the theoretical treatments predict that the electron transfer rate constant should be of the form

k,, = Aexp(-AG$/k,T)

(39)

EXCITED STATE CHARGE TRANSFER

51

where AGS = (A - AG0)2/41(see Figure 26). The preexponential factor A is a function of matrix element coupling the reactant and product (e.g., LE and CT), the zero order potential parameters, the temperature and the solvent dynamics. In a certain limit the reaction becomes particularly sensitive to the solvent configuration and for a Debye solvent A at;‘

(40)

This result simply shows that solvation dynamics can indeed affect the rate of ET reactions. 2. Excited State Charge Transfer. Our goal here is to discuss aspects of ET theory that are most relevant to the charge transfer processes of excited molecules. One important point is that often the solvent relaxation is not well modeled with a single t1but rather a distribution of times apply. This subject has been treated by Hynes [63], Nadler and Marcus [65], Rips and Jortner [66], Mukamel [67], Newton and Friedman [68], Zusman [62], Warshel [71], and Fonseca [139]. We also would like to study ET in the strongly adiabatic regime since experimental results on BA indicate this is the correct limit. Finally, we would like to treat the special case of three-well ET, which is the case for BA. 3. Simulation of the Charge Transfer Dynamics of Bianthryls. Recently, our group collaborated with Fonseca on a theoretical model for charge transfer in BA and related molecules employing an approach that allowed for each of the special .characters of these reactions, i.e., strong adiabaticity, nonmonoexponential solvation dynamics, and multiple well charge transfer [132, 1333. Our goal was to calculate p(z, t), the time-dependent distribution function for the solvent coordinate, in order to model the extensive new data that are available on the time and wavelength resolved fluorescence of excited state charge transfer molecules (see Sections 1II.C and 1II.D). The starting point is the well-known generalized Langevin equation (GLE) as adopted for stochastic motions involving coupling to a solvent coordinate. We employ the notation of Hynes [63] which is consistent with the discussion of solvation in Section 11.

The GLE is a stochastic equation of motion for the coordinate z (see Figure 24). The left-hand side of Eq. (41) is the inertial force along z in terms of the effective polarization mass m of the solvent and the acceleration z. The term

52

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

- d V [ z ( t ) ] / d z is associated with the force due to the potential V ( z )= E,,(z). The term F(t) is a random force due to fluctuations of the solvent. The remaining term of the right-hand side of Eq. (41) accounts for the friction (or retarding motion) along the reaction coordinate due to the lag of the solvent motion. The quantity q(t) is the time-dependent friction kernel. It characterizes the dissipation effects of the solvent motion along the reaction coordinate. The dynamic solute-solvent interactions in the case of charge transfer are analogous to the transient solvation effects manifested in C(t)(see Section IT). We assume that the underlying dynamics of the dielectric function for BA and other molecules are similar to the dynamics for the coumarins. Thus we quantify q(t) from the experimental C(t) values using the relationship discussed elsewhere [139]. The solution to the GLE is in the form of p(z, t), the probability distribution function. Figure 27 shows p(z, t) calculated for BA with the appropriate static and dynamic parameters for BA in the polar aprotic solvent, propylene carbonate. The results show how the charge transfer in S , occurs. At early times p(z, t ) is highly peaked near zero z, where the LE probability is high. As time progresses, the charge transfer occurs and the system approaches the equilibrium configuration (see Section 1II.A).

.4-

m

C a3 0 C

0 .c

Q

3

P

Figure 27. The time-dependent probability distribution function p(z, t ) for the excited state charge transfer of BA from a GLE simulation (See Refs. 132 and 133). The S1 potential employed in the simulation is shown in Figure 24.

0

-1

0

1

R x n coordinate(z1

2

53

EXCITED STATE CHARGE TRANSFER

In Section 1II.C we compare predicted time and wavelength resolved fluorescence spectra for BA (calculated from p(z, t)) to experimental results. Indeed there is reasonable agreement between experiment and theory. 4. The Charge Transfer Rate Constant. It is interesting at this point, however, to consider how the dynamics of p(z,t) is related to the usual measure of kinetics, the reaction rate constant. The key quantity to consider is the survival population S(t),

which is proportional to the concentration on the well centered at z = 0, i.e., the LE well in Figure 24. z, is the position of the maximum of the small barrier separating the LE and CT regions of S,. S ( t ) is shown in Figure 28 for BA in propylene carbonate calculated from the p(z, t) in Figure 27. The decay is nonmonoexponential with an average decay time of 1.7 ps. This is in fact very close to the average solvation time (zs) of the C(t) that was used to calculate &), see below. Thus, as expected from the simple theoretical models, the reaction time (j? s(t)dt) is indeed very close to the average of the solvation times from C(t). In other words the GLE simulations support the notion that the charge transfer of BA is controlled by solvation!

0.0

7.25

Tirne(ps)

Figure 28. Simulated time-dependent survival probability of the LE form calculatea from Eq. (42) and p(z,t) shown in Figure 27.

54

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

For other molecules, simulations and theory show a different behavior. If the barrier is comparable or greater than kBT the rate is of course partly controlled by thermal activation (Eq. (41)).On the other hand, if the barrier is zero and the reaction is very exoergic, then the average relaxation time can be much shorter than the average solvation time [139], as is the case for the molecule ADMA. which is discussed in Section 1II.D.

C. Experiments on 9,9’-bianthryl The time and wavelength resolved fluorescence dynamics of bianthryl has been investigated by several groups [30,82, 132, 133, 115, 1161. In addition, this molecule has been studied by picosecond absorption spectroscopy [115], electric field induced fluorescence anisotropy measurements [1171 and optically induced dielectric absorption (microwave) measurements [118, 1191. The results are generally in accord with the theoretical model presented in Sections 1II.A and 1II.B. One of the challenges of studying the photodynamics of BA is that the LE and CT interconversion is so rapid (i.e., on the time scale of solvation) that it is necessary to employ ultraviolet subpicosecond and even femtosecond fluorescence spectroscopy which has only recently become available 130, 82, 132, 1331. 1. Time Resolved Spectra. We have studied the time evolution of the fluorescence of BA in acetone as shown in Figure 29. The results are in qualitative agreement with the simulated time-dependent spectra using the simulated p(z, t ) and Eq. (38) (see Figure 30). This strongly supports the validity of the adiabatic GLE model for the charge transfer of S , BA. Many of our measurements on BA have been confined to single emission wavelength near the short wavelength edge of the BA fluorescence. The emission at these wavelengths monitors population in the LE region of Figure 24. The results show that population in this region evolves with nonmonoexponential charge transfer kinetics due to solvent kinetic control, and in turn, the underlying nonmonoexponential solvation dynamics. Fitting parameters for the emission dynamics are compared in Table 5 with the fitting parameters of C(t) measured with coumarin probes in the same solvent. It is remarkable how closely the population dynamics of LE parallel dynamics of C(t).Thus experimental and theoretical data strongly indicate that the excited state LE/CT interconversion of BA is controlled by solvation dynamics and that the barrier for the reaction is very low compared to KBT The relationship between the intensity dynamics near the peak wavelength of LE (- 420 nm) and the conventional description of charge transfer rate constants is clearly established with GLE simulations. We observe that the

I

I

2.8

I

1

2.4

2.0

,

Wavenumber(x lo3)

Figure 29. Time-resolved fluorescence spectra of BA in acetone at ambient temperature. The points in the figure are experimentaldata. The curves through the data are log normal function [lo61 fits to the data.

13800

Fr e q u e n c y ( c m-

29000

)

Figure 30. Simulated time resolved fluorescence spectra for BA in propylene carbonate. See text for further details. 55

TABLE 5 A Comparison of the Average Electron Transfer Times ( r e t ) of 9,9-Bianthryl in Various Polar Solvents to the Average Solvation Times ( 7 s ) of Coumarin Probes

~~

~~

Propanol(273 K) Butanol (273K) Pentanol (273 K) Pentanof' Acetone Benzonitrile Dimethyl sulfoxide Propylene carbonate

~~

178 +_ 8 306 5 326 +_ 38 188 17 0.86 5.4 1.2 1.7

138 & 21

-

142 5 2 0.83 5.9 1.2 2.4

"For alcohol solvents, measurements were made with timecorrelated single photon counting. The remaining measurements were made with the fluorescence upconversion system. The transients in alcohol solvents were fitted with a single exponential kinetic function. The kinetics in acetone is also well described by a single exponential, but in benzonitrile, dimethylsulfoxide, and propylene carbonate the kinetics were modeled with a biexponential decay. "Average solvation times obtained with several coumarin probes (see Section 11). 'Room temperature, if not specified.

I

.-c

to E

0)

c. C

-

0.0

7.25

Time(ps1

Figure 31. Simulated intensity vs. time transients for BA at 420nm in propylene carbonate, see text and Ref. 132 for further details. 56

EXCITED STATE CHARGE TRANSFER

57

simulated intensity dynamics for a given solvent (Figure 31) are nearly identical to the simulated “survival” dynamics (Figure 28), which supports the notion that the intensity dynamics on the short wavelength edge of the spectrum does indeed give an accurate measure of the underlying charge transfer dynamics. Some of the most elegant work on bianthryl has been published by Baumann et al. [177], who studied the effect of external fields on the anisotropy of the emission and the absorption of BA. The external field aligns the S1 molecules due to per. The results give clear evidence of the role of solvent in symmetry breaking. Another interesting type of experiment that directly probes the dipole moment of S1 is the optically modulated microwave experiment of the Dutch group [118] and Toublanc and Fessenden [1191. The latter results, which are time-resolved in the nanosecond time scale, have apparently detected and determined the rate of the CT -, LE -,CT’ double electron transfer reaction. These exciting results are made possible because the applied external field lifts the degeneracy of CT and CT. Experiments with applied external fields hold great promise for future work on BA and related S , charge transfer reactions. A number of analogous compounds to BA have been reported, including 5,5’-dibenzo-[a]-pyrenyl (BBPY) [116). These compounds exhibit emission spectra similar to BA. It would be interesting to explore the ultrafast dynamics of BBPY in order to test the generality of the GLE model. It would also be interesting to study the femtosecond dynamics of BA as a function of applied pressure. Static experiments on the emission of BA, reported by Hara et al. [123], demonstrate that in low viscosity solvents an increase of pressure affects the emission similarly to an increase of solvent polarity. As the pressure is increased, however, the LE/CT interconversion is slowed down. It would be interesting to measure C(t)in these environments and compare the solvation dynamics with LE/CT dynamics, in order to test the generality of the GLE dielectric friction model.

D. ADMA The excited state charge transfer of the S1 of 4-(9-anthryl)-N,Ndimethylaniline (ADMA) has been widely investigated [122, 130, 132, 133, 143). Kinetic investigations demonstrate that a CT band is found soon after excitation in polar solvent. The dipole moment, polarizabilities, and fluorescence behavior has been studied in detail. The excited state equilibrium constant lies more closely to the CT form than BA in a particular solvent. Excited state transient absorption studies show clear evidence of the charge transfer processes. Reports of more than one CT state have been made. The

58

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

short time scale dynamics in alcohols and polar aprotics have been studied by ultrafast fluorescence spectroscopy. We recently undertook the first investigation of the emission dynamics of ADMA with femtosecond resolution [132, 1331. An example of an emission transient from this work is shown in Figure 25. Table 6 compares the emission dynamics of ADMA at various wavelengths to the parameters of the dynamics of solvation C(t)of the particular solvent. It is quite apparent that for the charge transfer of S , ADMA (which is much more exoergic than that for BA) the decay time for population of the LE form is much shorter than the average solvation time (zS). This is well supported by analytical theory [I 391 and simulations [132,133] for the LE/CT interconversion of S , ADMA. The results are in accord with the general result that for strongly exoergic, barrier free electron transfer in solvents with more than one relaxation time (z,,), the average electron transfer time, will be much shorter than the average solvation time (zs) [132, 133, 1391. The especially short charge transfer dynamics of S , ADMA can lead to confusion on the interpretation of time resolved data on this compound. The emission dynamics are strongly emission wavelength dependent as shown in Table 6. Of course the different dynamics reflect the differing population dynamics at various values of the solvent coordinate. This is a complex effect which must be modeled in detail, along the lines of Section III.B, employing a GLE and a determination of p(z, t). Qualitatively, emission at the short wavelength edge more closely measures the dynamics of charge transfer, while emission near the middle wavelength tends to more closely measure solvation dynamics in the CT, well in analogy to the experiments of Section 11. Recent reports of (z,,) for ADMA being close to (zJ were probably measuring the solvation dynamics of the CT rather than the LE/CT interconversion [1433. Although, it should be emphasized that charge transfer and solvation are not completely distinct processes since the LE/CT interconversion is probably strongly electronically adiabatic, as in the case of BA (see Section 1II.A). TABLE 6 Best-Fit Decay Times for the Fluorescence Decays of ADMA in Propylene Carbonate at Several Wavelengths Emission Wavelength (am) 433 445

470

(PS)

0.4 1 0.52 1.50

59

EXCITED STATE CHARGE TRANSFER

E. DMABN The excited state LE/CT interconversion of p-dimethylaminobenzonitrile (DMABN) has been the most extensively investigated compound exhibiting charge transfer induced dual fluorescence. Much of the earlier work on this compound has been reviewed in detail. Time-resolved experiments on DMABN have been made by many groups, including Struve and Rentzepis [77], Rettig [78], Rentzepis et al. [75], Eisenthal et al. [79], Heisel and Miehe [76], Su and Simon [80], and others [4,5]. In addition, many interesting static experiments have been made on DMABN including extreme environments such as seeded supersonic expansions, electric fields, and derivatives of DMABN. But, these results are too extensive to be described in detail. Many different mechanistic proposals on this compound (and compounds in this class) have been made. The rate limiting process to LE/CT interconversion has been attributed to twisting motion around the bond connecting the nitrogen to the benzene ring [S], a thermally activated barrier crossing [79], dielectric friction due to solvation dynamics [4,51, and intramolecular vibrational effects [SO]. It may be that each of these mechanisms have validity depending on solvent, temperature, viscosity, and substituent. Eisenthal’s reports that the solvent dependence of the kinetics of the LE/CT interconversion of DMABN should be attributed predominantly to polarity-induced barrier height changes rather than viscosity (friction) changes, has received a lot of recent attention. Su and Simon have explored the role of intramolecular vibrational fluctuations in the LE/CT kinetics of DMABN [SO]. This mechanism is outlined in the following subsection, where the same mechanism has been invoked to understand the photodynamics of bis-(N,N-dimethylaminopheny1)-sulfone.Research on the DMABN class of molecules continues to the very active. It will be interesting to see how the various mechanisms and apparently contradictory arguments are reconciled in the future.

F. Other Molecules and Related Theoretical Results The compound bis-(4,4’-dimethylaminophenyl)-sulfone(DMAPS) and related compounds show multiple fluorescences in polar solvents due to excited state charge transfer (Rettig and Chandross [144]). Su and Simon [84,85] have examined the intramolecular electron transfer reaction in DMAPS, in alcohol solution over the temperature range from - 50°C to 30°C. They observe that the decay of the local excited state is nonexponential and significantly faster than the longitudinal relaxation time of the solvent. In addition, they observed that the emission spectrum of the TICT state

+

60

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

undergoes a time-dependent Stokes shift, indicating that significant solvent restructuring occurs following the electron transfer. The electron transfer rates are discussed in terms of the theory of Nadler and Marcus, which addresses the role of vibrational fluctuations as well as solvent motion determining the reaction rate constant [65]. In Figure 32,log(k,z,) is plotted as a function of log(k,q) for the electron transfer dynamics of DMAPS in series of linear alcohols at several temperatures. If solvent diffusion were controlling the observed dynamics, the experimental data should fall on a line of unit slope, the solid line in the figure. As contributions from the intramolecular modes increase, Ai > A,, the slope is predicted to decrease. In the limit of AJA, .+ 03, solvent diffusion does not contribute to the reaction process at all and the reaction rate ( w 7,) will become independent of change in 7,. From the correlation observed in Figure 32, they conclude that fluctuations in the intramolecular modes of the molecule make the dominant contribution to the reaction process. Su and Simon have reported similar

Figure 32. The kinetic data for the intramolecular charge transfer reaction of DMAPS in alcohol solutions, k,z, is plotted as a function of the solvent relaxation k e q . These data span the temperature range from -50°C to +3o"C. The solid line corresponds to the case where T~ = q, the expected result for a solvent controlled chemical reaction. The solvents plotted are ethanol (+), propanol (*), butanol( x ), pentanol (O),and hexanol(0). From Ref. 87 with permission from Chem. Phys. Lett., in press.

CONCLUSIONS

61

results for the intramolecular charge transfer reactions of DMABN and DEABN in low temperature alcohol solutions [SO]. Another interesting class of molecules are stilbene derivatives with charge donating groups. These compounds offer the opportunity to explore the role of polar solvation dynamics (dielectric friction) in cisltrans isomerization. Interesting papers on this subject have been published by Waldeck et a]. [I451 and Rulliere et al. [146], Other well-studied polar excited state isomerization examples include pinacyanol, l,l'-diethyL4,4'-cyanine, and crystal violet, which have been studied by Sundstrom, Gilbro and their coworkers [148]; and Ben-Amotz and Harris [148] and others who are referenced in these papers [148,149]. One of the most important class of excited state charge transfer molecules is the alkyloaminonaphthalenesulfonates,which have been employed by Kosower and Huppert to make some of the early observations of the role of solvation dynamics in fast charge transfer reactions. As stated previously compounds in this class have been reviewed recently [4]. The reader is also referred to the innovative nonphotochemical electron transfer studies of Weaver et al. [147]. These authors have been exploring dynamical solvent effects on ground state self-exchange kinetics for organometallic compounds. This work has explored many aspects of solvent control on intermediate barrier electron transfer reactions, including the effect on a distribution of solvation times. The experimental C(t) data on various solvents have been incorporated into the theoretical modeling of the ground state electron transfer reactions studied by Weaver et al. [147]. One of the most important new areas of theory of charge transfer reactions is direct molecular simulations, which allows for an unprecedented, molecular level view of solvent motion during reactions in this class. One of the important themes for research of this type is to ascertain the validity at a molecular level of the linear response theory estimates of solvent interactions that are inherent in Marcus theory and related approaches. In addition, the importance of dynamic solvent effects on charge transfer kinetics is being examined. Recent papers on this subject have been published by Warshel [71], Hynes [141] and Bader and Chandler [137, 138).

IV. CONCLUSIONS An international effort over the last decade on the photophysics of polar aromatic molecules has led to a detailed, and in some cases microscopic, understanding of these systems. One area, the study of the excited state solvation of nearly ideal probe molecules has advanced to the point where

62

ULTRAFAST PHOTOCHEMICAL INTRAMOLECULAR CHARGE

measurements of this type are leading to valuable data on the motion of complex solutions. The impact of this work goes far beyond photophysical interests alone. In particular, important new theoretical research on the dynamics of liquids has been strongly influenced by the availability of the dynamic solvation data. Photodynamic studies on excited state charge transfer has advanced to the point where some of the simpler prototype molecules are now well understood. The charge transfer dynamic measurements have also been centrally important in the great progress being made on understanding of the solvent’s role in photochemical and nonphotoinduced fast charge transfer processes in solution. Future prospects on the study of intramolecular photoinduced charge transfer and solvation is bright and will undoubtedly involve many new challenges as researchers examine molecular systems with more and more rich photochemical behavior.

ACKNOWLEDGMENTS We would like to thank following individuals for stimulating and useful discussions, and/or important preprints of papers, relating to this review: B. Bagchi, B. J. Berne, W. Baumann, B. Brunshwig, E. W. Castner, D. Chandler, R. I. Cukier, M. Fayer, R. W. Fessenden, G. R. Fleming, T. Fonseca, H. L. Friedman, D. Huppert, R. M. Hochstrasser, J. T. Hynes, J. Jortner, D. Kelly, J. Klafter, M. Kreevoy, R. A. Marcus, M. Maroncelli, N. Mataga, J. Michl, S. Mukamel, M. 0. Newton, W. Rettig, C. Rulliere, J. D. Simon, H. P. Trommsdorff, D. Waldeck, A. Warshel, M. Weaver, and P. T. Wolynes. Our contribution would not have been possible without our co-workers: A. M. Brearley, V. Nagarajan, A. E. Johnson, M. A. Kahlow, T. J. Kang, G. C. Walker, and others who collaborated on this research. Our research discussed in this review was generously supported by the National Science Foundation. PFB was a visiting Assistant Professor at the University of Grenoble I for some of the time during which this review was written, and would like to thank the CNRS for support.

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89. H. Frohlich, Theory of Dielectrics, Oxford University Press, London, 1949. 90. B. J. Berne, J. Chem. Phys. 62, 1154 (1975). 91. T. J. Kang, T. Etheridge, W. Jarzeba, and P. F. Barbara, J . Phys. Chem. 93, 1876 (1 989). 92. S. R. Flom, V. Nagarajan, and P. F. Barbara, J. Phys. Chem. 90, 2085 (1986). 93. A. B. Brearley, S. R. Flom, V. Nagarajan, and P. F. Barbara, J . Phys. Chem. 90, 2093 (1986). 94. E. W. Castner, Jr., B. Bagchi, M. Maroncelli, S. P. Webb, A. J. Rugiero, and G. R. Fleming, Ber. Bunsenges. Phys. Chem. 92, 363 (1988). 95. Y. T. Mazurenko, Opt. Spectrosc. 36,283 (1974) [Opt. Spektrosk. 36,491 (1974)]. 96. S. W. Yeh, L. A. Philips, S. P. Webb, L. F. Buhse, and J. H. Clark, in Ultrafast Phenomena I D. A. Auston and K. B. Eisenthal, Eds., Springer, Berlin, 1987, p. 359. 97. W. Jarzeba, G. C. Walker, A. E. Johnson, and P. F. Barbara, in preparation (C 153). 98. G. C. Walker, W. Jarzeba, A. E. Johnson, and P. F. Barbara, in preparation (C 343). 99. G. Jones 11, W. R. Jackson, and A. M. Halpern, Chem. Phys. Lett. 72,391 (1980); G . Jones 11, W. R. Jackson, C. Choi, and W. R. Bergmark, J . Phys. Chem. 89,294 (1985);W. Rettig and A. Klock, Can. J. Chem. 63, 1649 (1985). 100. G. R. Fleming, Chemical Applications of Ultrafast Spectroscopy, Oxford, New York, 1986. 101. M. M. Malley and G. Mourou, Opt. Commun. 10, 323 (1974); E. Gilabet, A. Declemy, and C. Rulliere, Rev. Sci. Instrum. 58, 2049 (1987). 102. M. A. Kahlow, W. Jarzeba, T. P. DuBruil, and P. F. Barbara, Rev. Sci. Instrum. 59, 1098 (1988). 103. J. Shah, IEEE J. Quantum Electron. 24, 276 (1988) 104. M. D. Dawson, T. F. Boggess, and A. L. Smirl, Opt. Lett. 12,254 (1987); M. D. Dawson, T. F. Boggess, D. W. Garvey, and A. L. Smirl, Opt. Commun. 60, 79 (1986). 105. W. H. Knox, M. C. Downer, R. L. Fork, and C. V. Shank, Opt. Lett. 9,552 (1984). 106. D. B. Siano and D. E. Metzler, J . Chem. Phys. 51, 1856 (1969). 107. C. Reichardt, Solvent Eflects in Organic Chemistry, Verlag Chemie, New York, 1979, Chap. 7. 108. M. J. Kamlet, J. L. M. Abbound, and R. W. Taft, Prog. Phys. Org. Chem. 13,485 (1981). 109. D. Kivelson and P. Madden, Mol. Phys. 30, 1749 (1975). 110. R. L. Fulton, MoZ. Phys. 29, 405 (1975). I 1 1 . J. Barthel, K. Bachhuber, and R. Buchner, Chem. SOC. Faraday Div. General Discuss., Vol. 85, Durham, England, 1988 J. Barthel and F. Feurlin, J. Solution Chem. 13, 393 (1984).

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112. S. Engstrom, B. Jonsson, and R. W. Impey, J. Chem. Phys. 80, 5481 (1984). 113. F. Schneider and E. Lippert, Ber. Bunsenges. Phys. Chem. 72,1155 (1968);ibid. 74, 624 (1 970). 114. E. M. Kosower and K. Tanizawa, Chem. Phys. Lett. 16,419 (1972). 115. N. Nakashima, M. Murakawa, and N. Mataga, Bull. Chem. SOC.Jpn. 49, 854 (1976). 116. W. Rettig and M. Zander, Ber. Bunsenges. Phys. Chem. 87, 1143 (1983); M. Zander and W. Rettig, Chem. Phys, Lett. 110, 610 (1984); N. Mataga, H. Yao, T. Okada, and W. Rettig, J. Phys. Chem. 93, 3383 (1989). 117. W. Baumann, E. Spohr, A. Bischhof, and W. Liptay, J. Luminescence 37, 227 (1987). 118. R. J. Visser, P. C. M. Weisenborn, P. J. M. van Kan, B. H. Hvizer, C. A. G. 0. Varma, J. M. Warman, and M. P. de Haas, J. Chem. SOC.,Faraday Fans. 2,81, 689 (1985). 119. D. B. Toublanc and R. W. Fessenden, and A. Hitachi, J. Phys. Chem. 93,2893 (1989). 120. K. Yamasaki, K. Arita, 0. Kasimoto, and K. Hara, Chem. Phys. Lett. 123, 277 (1986). 121. R. L. Khundkar and A. H. Zewail, J. Chem. Phys. 86, 1302 (1986). 122. T. Okada, N. Mataga, W. Baumann, and A. Siemiarczuk, J. Phys. Chem. 91,4490 (1987); A. Siemiarczuk, J. Koput, and A. Pohorille, Z. Naturforsch. 37a, 598 ( 1982). 123. K. Hara, T. Arase, and J. Osugi, J. Am. Chem. SOC.106, 1968 (1984). 124. T. Okada, M. Kawai, T. Ikemachi, N. Mataga, Y. Sakata, S. Misumi, and S. Shionoya, J . Phys. Chem. 88, 1976 (1984). 125. W. Baumann, B. Schwager, N. Detzer, T. Okada, and N. Mataga, J . Phys. Chem. 92, 3742 (1988). 126. T. Okada, T. Fujita, and N. Mataga, Z. Phys. Chem. ( N e w Folge, Bd.) 101, 57 (1976). 127. W. Baumann, F. Petzke, and K. D. Loosen, Z. Naturforsch. 34a, 1070 (1979). 128. A. Siemiarczuk and W. R. Ware, J . Phys. Chem. 91, 3677 (1987). 129. A. Siemiarczuk, Chem. Phys. Lett. 110, 437 (1984). 130. A. Siemiarczuk, Z. R. Grabowski, A. Krowczynski, M. Asher, and M. Ottolenghi, Chem. Phys. Lett. 51, 315 (1977). 131. T. Okada, N. Mataga, and W. Baumann, J . Phys. Chem. 91, 760 (1987) and numerous references therein. 132. W. Jarzeba, T. J. Kang, T. Fonseca, and P. F. Barbara, to be submitted. 133. T. J. Kang, W. Jarzeba, T. Fonseca, W. Rettig, and P. F. Barbara, to be submitted. 134. V. Bonacic-Koutecky, J. Koutecky, and J. Michl, Angew. Chem. Int. Ed. Engl. 26, 170 (1987).

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135. B. S. Brunschwig, S. Ehrenson, and N. Sutin, J . Phys. Chem. 91, 4714 (1987). 136. D. Y. Yang and R. I. Cukier, preprint. 137. J. S. Bader and D. Chandler, preprint. 138. D. Chandler, preprint. 139. T. Fonesca, J . Chem. Phys. 91,2869 (1989). 140. W. Nadler and R. A. Marcus, Chem. Phys. Lett. 144, 24 (1988). 141. E. A. Carter and J. T. Hynes, J. Phys. Chem. 93, 2184 (1989). 142. I. Rips, K. Klafter, and J. Jortner, in Proc. IPS-7, Chicago, Elsevier Science Publishers, Amsterdam, 1988. 143. D. Huppert and P. M. Rentzepis, J . Phys. Chem. 92, 5466 (1988). 144. W. Rettig and E. A. Chandross, J. Am. Chem. SOC. 107, 5617 (1985). 145. D. M. Zeglinski and D. H. Waldeck, J . Phys. Chem. 92,692 (1988);N. Sivakumar, E. A. Hoburg, and D. H. Waldeck, J . Chem. Phys. 90, 2305 (1989). 146. E. Gilabert, R. Lapouyade, and C. Rulliere, Chem. Phys. Lett., in press. 147. R. M. Nielson, G. E. McManis, and M. J. Weaver, J. Phys. Chem. 93,4703 (1989). McManis, M. N. Golovin, and M. J. Weaver, J . Phys. Chem. 90,6563 (1986) and references therein; M. J. Weaver, G. E. McManis, W. Jarzeba, and P. F. Barbara, J . Phys. Chem., submitted. 148. G. E. McManis, R. M. Nielson, A. Gochov, M. J. Weaver, J. Am. Chem. SOC.111, 5533 (1989). 149. E. Akesson, V. Sundstrom, and T. Gillbro, Chem. Phys. Lett. 121, 513 (1985); E. Akesson, H.Bergstrom, V. Sundstrom, and T. Gillbro, Chem. Phys. Lett. 126,385 (1986). 150. D. Ben-Amotz and C. B. Harris, Chem. Phys. Lett. 119, 305, (1985).

Advances in Photochemistry, Volume15 Edited by David H. Volman, George S Hammond, Klaus Gollnick Copyright © 1990 John Wiley & Sons, Inc.

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS: LONG PATH-FTIR STUDIES Hiromi Niki Department of Chemistry and Centre for Atmospheric Chemistry, York University, North York, Ontario, Canada M3J 1P3

Paul D. Maker Scientific Research Laboratory, Ford Motor Company, Dearborn, Michigan 48121, U.S.A.

CONTENTS I. Introduction 11. Experimental methods A. Preparation of reactive species B. Measurement of rate constants C. Determination of reaction mechanism 111. Atmospheric oxidation of hydrocarbons A. Methane 1. CH,O, reaction 2. HCHO oxidation 3. Kinetics and mechanism for HO CO 4. Kinetics and mechanism for HO CH,OOH B. Ethane and higher alkanes 1. C,H,02 reaction 2. CH,CHO oxidation and CH,C(O)O, reaction 3. R 0 2 (R 2 C,) reaction

+ +

69

70

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

C . Alkenes and alkynes 1. HO-initiated reaction 2. 0,-initiated reaction 3. NO,-initiated reaction D. Aromatics 1. HO-initiated oxidation 2. Unsaturated 1,4-dicarbonyl reaction Acknowledgments References

I. INTRODUCTION In recent years it has been well established that a large variety of volatile organic and inorganic compounds introduced into the atmosphere from both natural and anthropogenic sources undergo chemical transformations induced either by direct photochemical action of the solar ultraviolet (UV) radiation or by reactive photochemical products [1-71. Notably, recent advances in our understanding of the relevant chemical reactions have led to major progress in the development of highly sophisticated, comprehensive computer kinetic models of urban, regional and global atmospheres [8-121. These advances can be attributed largely to recent improvements in experimental methods for obtaining laboratory data on reaction kinetics and mechanisms associated with atmospherically important chemical systems. In this regard, Fourier Transform Infrared Spectroscopy (FTIR) has emerged in a relatively short period as a uniquely powerful analytical tool for both laboratory and field studies of atmospheric chemistry [13-18]. Laboratory studies of atmospheric chemistry demand analytical methods and instrumentation having extremely high performance capabilities, i.e., unambiguous, universal, multicomponent in situ quantitative analysis with high detection sensitivity, and data acquisition speed fast enough to resolve concentration-time profiles of reactants and products. There are two primary requirements for such laboratory studies; firstly, since reactions of atmospheric interest are often sensitive to the pressure of diluent gases and molecular oxygen [6], it is imperative to study them in the presence of atmospheric pressures of air; secondly, many of the atmospherically important molecular products are physically and/or chemically unstable and are difficult to sample and gas-handle for chemical analysis. These two requirements demand the use of an in situ detection method operative under atmospheric conditions. Among various optical detection methods, mid-IR (approx. 500-4000 cm- ') absorption spectroscopy of molecular vibrational-

INTRODUCTION

71

rotational transitions offers the most nearly universal finger-printing capability. UV/VIS spectroscopy is highly useful for selective, sensitive detection, but is limited to those compounds possessing strong, discrete vibronic transitions in that region. Also, note that vacuum UV spectroscopy will suffer from interference from the strong absorption by atmospheric pressures of O,, and that p-wave and far-IR spectroscopies are not compatible with in situ analysis of high pressure samples because of overwhelming pressure broadening of rotational lines. With its inherent capabilities as mentioned above, IR spectroscopy has served atmospheric chemists for many years as a general analytical tool. For instance, it played a key role in the unique identification of O3 and in the discovery of peroxyacetyl nitrate (PAN, CH,C(O)OONO,) over 20 years ago [19,201. However, conventional scanning monochromators do not realize the full potential of the IR method for such applications. Their deficiency arises from the severe limitations imposed by the “trade-off between instrumental resolution and detection sensitivity. Namely, numerous molecules of interest here exhibit rotation-vibrational fine structure of GO.1 cm- spectral width in ambient air. A correspondingly high resolving power is required to record their absorption spectra faithfully and thereby maximize the information content of the spectra for the purpose of quantitative spectroscopic characterization and chemical analysis [131. It has been recognized for some time that these shortcomings are greatly mitigated by use of the Fourier transform-Michelson interferometric method, by virtue of its high optical throughput and its multiplexing advantage [21]. These advantages as well as others (e.g., accuracy of frequency measurement, ease of data manipulation because of its digitized form) have been amply demonstrated by its applications to astrophysical observations and high resolution spectroscopy [21]. However, its full application to routine laboratory analysis of trace gases in the air was not feasible prior to the recent arrival of commercial rapid scan Michelson interferometers with sensitive IR detectors and powerful mini-computers for performing the data acquisition, processing and analysis [13, 18,221. The foregoing considerations dictate the need for a photochemical reactor that provides a clean, controllable chemical environment and simultaneously a long IR absorption path. Several such facilities have recently been described [23-26). That used in our laboratory [18] is quite simple, suited to rapid change of the chemical mixture, and to the conduction and recording of many experiments in a single day. Others are much more complex. That shown in Figure 1 illustrates a fully implemented facility designed specifically for a long path-FTIR instrument system, providing carefully controlled photolysis illumination, temperature control, sample mixing, and diluent-air preconditioning. While the FTIR instrument system as a whole is still evolving, it

Figure 1. Schematic diagram of a reactor designed for long path-length interforometricabsorption measurements upon a photochemically induced reaction system (Courtesy of K. H. Becker).

EXPERIMENTAL METHODS

73

already fulfills virtually all of the performance requirements for studies of atmospheric reactions, and to a high degree. In this chapter, recent laboratory results are highlighted to illustrate the many important advances in the understanding of atmospheric transformations of hydrocarbons achieved through use of the FTIR method.

11. EXPERIMENTAL METHODS Effective utilization of a long path-FTIR spectroscopic facility for studies of atmospheric reactions requires appropriate design of chemical systems to match the analytical capabilities of the instrument (e.g., data acquisition speed and detection sensitivity), and characterization of the large reactors frequently used (e.g., photochemical versus thermal reactor, photolysis wavelength, homogeneity of irradiation, mixing time of reactants in atmospheric pressures of air, diffusion time to reactor surfaces, and various associated heterogeneous processes). Experimental parameters such as initial reactant concentrations, decay rate of reactants and extent of conversion of reactants to primary and secondary products all need to be carefully controlled and fine-tuned together with the optimization of instrumental performance. Some of the experimental techniques commonly employed with the long path-FTIR method for generating reactive species and for obtaining kinetic and mechanistic data are described below.

A. Generation of Free Radical Reactants Atmospheric molecules such as O,, O,, NO and NOz are inherently reactive because of the “free radical” nature of their electronic structures. In addition, there are literally hundreds of free radical species produced in the atmosphere via either photochemical or dark reactions of various hydrocarbons [1,2,27]. Clearly, an important prerequisite to laboratory studies of atmospheric chemistry is the ability to generate key free radical species in a “clean” fashion. Some representative techniques for generating the major free radical reactants, i.e., HO, HOO, R, RO and ROO (R = alkyl or other organic group), in combination with a long path IR absorption cell-chemical reactor are described below. Many of the atmospheric reactions leading to the formation of HO and HOO radicals have been adopted for laboratory FTIR-based studies. Namely, HO radicals are produced photochemically in the atmosphere from 0, in the presence of water vapor, and also from atmospheric products such

74

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

as H,02 and HONO [1,2]: 0 3

+ hv(<310nm)+02 + O(lD)

Followed by O('D) + H,O

-+

+

2H0

H 2 0 2 hv( < 350 nm) -, 2 H 0 HONO

+ hv( <390 nm)

-+

HO

+ NO

An important additional atmospheric source of HO radicals is the reaction of the HOO radicals with NO: H02

+ NO + HO + NO2

HOO radicals are formed, in turn, by the 0,-reactions of H atoms and organic free radicals [l, 23:

+ O,( + M) + HOO'( + M) H e 0 + 0, -+ HOO' + CO kH,OH + 0, -,HOO' + HCHO RO' + O2 + HOO + carbonyl products

H

Among the direct photochemical sources mentioned above, HONO has the largest absorption cross-section in the solar spectral region [ 5 , 6 ] and serves as an efficient HO source for the long path-FTIR studies. However, there is a drawback to using HONO as the HO-radical precursor due to its chemical instability. Namely, it undergoes a reversible reaction 2HONO-NO + NO, + H,O heterogeneously, and cannot be well controlled [17,283. Therefore, many trials are usually required to successfully prepare and gas-handle HONO samples in a relatively pure form at predetermined concentrations. More convenient methods involve the generation of HOO or its precursor radicals, e.g., H, CHO, RO (R = alkyl group) first, and the subsequent HOO-to-HO conversion by NO. One of the chemical schemes used successfully for this purpose is based on the chlorinephotosensitization technique. Namely, C1, molecules can readily be photodissociated in the near UV region, and the resulting CI atoms are highly reactive with hydrogen-containing compounds (and importantly, are inert towards 0,) [5, 61: C1,

+ hv( > 300 nm) + 2C1

C1+ H2 + H

+ HCl

EXPERIMENTAL M E T H O D S

75

C1+ HCHO + CHO + HC1 C1+ RH + R' R'

+ HCl

+ O,( + M) -+

(R

+


=

ROO'( M)

+ NO + RO' f NO, RO' + 0, + HOO' + carbonyl product

ROO'

Another highly suitable method involves the photochemical generation of RO radicals from alkylnitrites (RONO), especially methyl and ethyl nitrites [29,30]:

RONO

+ hv .--, RO' + NO

The alkylnitrites have large UV absorption cross-sections, comparable to that of HONO [31]. These compounds can be prepared in pure form and are reasonably stable both in the vapor and liquid phases when kept in the dark. As will be illustrated later, the selection of a specific RONO for a given experiment is based largely on the consideration of potential chemical and/or analytical interference problems arising from the accompanying carbonyl products. Also, a novel nonphotolytic method for generating HO, and HO radicals has been successfully used for FTIR-based kinetic and mechanistic studies [32,33]. This method involves the use of peroxynitric acid (HOONO,, PNA) as a thermal source of HO, radicals which in the presence of NO react rapidly to give HO radicals: HOONO,

c*

HOO'

+ NO,

The unimolecular decay time of PNA at room temperature is of the order of 10 s [32]. However, because of the rapid establishment of a thermodynamic equilibrium, its actual lifetime in large reactors can be hours. PNA can be prepared in sufficiently pure form by reacting concentrated HzOz with NO, at 0°C [32]. It should be noted that both the photochemical and dark HOgeneration methods described above are, by design, applicable only to chemical systems containing NO and the product NO, as major reactive species. Generation methods which will provide improvements over the slow near-UV photolysis of O3 or H,O, are needed for studying numerous atmospherically important HO and HOO-reactions occurring under NOfree conditions. The simple R, RO and ROO radicals, e.g., R = C < 4 alkyl group, can be generated using the photochemical and dark methods described above for HO and HO,. In particular, the C1,-photosensitization method can be

76

ATMOSPHERIC REACTEONS INVOLVING HYDROCARBONS

extended to the generation of a large variety of ROO radicals (cf. Section 1II.B). In some instances, C1, can be replaced by Br, to minimize undesirable side reactions [34]. At present, there are no well established methods for generating the R, RO, and ROO species where R is other than a simple alkyl group. In many cases, the identity of free radical and molecular intermediates occurring in the atmospheric oxidation of hydrocarbons remains unknown. Quite often, no independent methods are available for generating the desired radical species because they are unique to particular reactions occurring in the atmosphere. Thus, FTIR studies can be aimed at identifying these unknown free radical reactants and products based on the observation of molecular products. As described later, some free radicals can be “trapped” for identification purposes by appropriate molecular or free radical species to yield unique products, e.g., ROO + NOz --+ ROONOz and ROO + HOO + ROOH + 02.

B. Measurement of Rate Constants The analytical capabilities of long path-FTIR spectroscopy combined with the reactant generation methods described above can be effectively utilized for the determination of rate constants for numerous unimolecular and bimolecular reactions under atmospheric conditions. Among various established kinetic methods, those which are based on the measurements of molecular species at a typical sampling rate > 1 s are generally applicable to this experimental technique. In particular, there are a large number of reactions involving molecular species which are difficult to monitor by other analytical techniques but are well suited for the FTIR method. Although free radicals are not generally detectable by this technique when reactions are carried out under simulated atmospheric conditions, several indirect methods can be used for their kinetic studies. For instance, large RO radicals (R = 2 C4 alkyl group) undergo unimolecular dissociation and isomerization in competition with bimolecular reactions with O2 [2,27]. It is important to accurately determine the relative rather than absolute values of these uni- and bimolecular rate constants. Ratios of these rate constants can be determined from the corresponding product yields. For free radical reactions involving hydrocarbons, the bimolecular rate constants can be derived from measurements of their decay rates relative to those of a reference compound for which the rate constant has been well established: -d[RH]/dt

= fk~”[x][RH]

-d[Ref]/dt

= kRcf[X][Ref]

EXPERIMENTAL METHODS

77

where [RH], [Ref] and [XI are the concentrations of the reactant hydrocarbon, reference compound and a free radical reactant, respectively, and kR, and kRef are the corresponding bimolecular rate constants. When the decay rates of both RH and Ref are measured in a chemical system containing X, the standard integrated relative rate expression yield the rate constant ratio kRH/kRef:

where tl and t 2 are the two reaction times at which both [RH] and [Ref] are measured. An underlying assumption for this method is that both RH and Ref are removed solely by the free radical X. In cases where X is HO or NO,, most of the chemical systems used are well characterized and the experimental conditions can be optimized to test and ensure the applicability of this assumption. Also, commonly encountered first-order loss processes such as heterogeneous or photolytic decay can be accurately taken into account as correction factors to the above expression [30]. For bimolecular reactions involving two molecular reactants, e.g., 0, alkenes, concentration-time profiles of both reactants can often be recorded simultaneously, and the data can be treated using the integrated second-order rate expressions to derive the corresponding rate constants:

+

and

where [0lelto and [Ole],, are the alkene concentrations at reaction times kale and k,, are the apparent rate constants derived from the decay rates of the olefin and 0 3 ,respectively. These expressions are required in order to treat the kinetic data obtained with mixtures containing approximately equimolar concentrations of 0, and the reactant alkene. Such experimental conditions can be utilized very effectively by the FTIR technique to obtain both kinetic and mechanistic information from the same data set. More commonly, for the purpose of kinetic measurements alone, large values of the reactant mixing ratios, i.e., 10-100, are selected to reduce these cumbersome integrals to simple pseudo firstt = to and t,, respectively, and

78

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

order kinetics:

where [O,],, and [Olelsv are the concentrations of the larger components averaged over the reaction time period (t, - tl), and are very close to their initial concentrations because of their small fractional consumption. Often, the values of ko, and kale thus determined are not identical due to the occurrence of secondary reactions consuming either the reactant alkene in the former or 0, in the latter case. In such cases, the smallest k,, and kale values determined can be tentatively assigned to the bimolecular rate constant for the primary reaction step [35]. Clearly, a complete mechanistic knowledge of the reaction system is required to confirm the validity of such kinetic measurements.

C. Determination of Reaction Mechanism The long path-FTIR spectroscopic method has a great advantage over more selective detection methods for carrying out thorough product analyses to obtain mechanistic information. Simultaneous in situ measurements of all the molecular reactants and products, and thereby the determination of a complete material balance, can be made with this technique. In many cases, individual species appearing in composite product spectra can be uniquely identified and quantitatively analyzed using a computer-based library search of suitable reference spectra recorded beforehand [13,141. In addition to gaseous species, aerosols arising from the condensation of less volatile molecular products have also been observed in the form of the corresponding liquid or solid-phase absorption spectra [36]. Furthermore, a large number of “new” or “as-yet unidentified products have been detected by the FTIR method in complex atmospheric reactions. Since the authenticated pure samples required for recording standard spectra are often not available, tentative identification of unknown species has to be based on kinetic and spectroscopic evidence. Through systematic product studies starting from relatively simple chemical systems and increasing the degree of complexity in a step-wise fashion, there is a good chance for gaining a better understanding of atmospheric reactions involving hydrocarbons as the primary reactants. Some important mechanistic considerations for designing experiments are described below.

EXPERIMENTAL METHODS

79

In the atmospheric free radical reactions involving hydrocarbon species, molecular products of interest are formed via either radical chain propagation or termination steps. Chain Propagation

HCO + 0, + HOO'

+ CO RO' + 0, + HOO' + aldehydes and ketones Chain Termination

+ NO -,RON02 ( b C 4 alkyl nitrates) HOO' + HOO' 0, + H 2 0 z ROO' + HOO' + O2 + ROOH (alkyl peroxides) ROO'

-P

RC(0)OO' RC(0)OO'

+ NOz -+

RC(O)OONO,

(peroxyacyl nitrates)

+ HOO' + 0, + RC(0)OOH

(peroxy acids)

With the FTIR spectroscopic method, these free radical reactions cannot be studied individually under completely isolated conditions since competing side reactions and also secondary reactions involving the molecular products must be taken into account. These mechanistic complications can be greatly reduced by appropriate selection of the method of free radical generation, as described in Section 1I.B. In general, to minimize the occurrence of secondary reactions, the conversion of the molecular reactants, and consequently the product yields, have to be kept as small as is permissible in order to obtain accurate concentration measurements. Also, the reaction time required for such chemical analysis must be kept as short as possible to minimize photochemical and heterogeneous losses of labile products. To optimally utilize the product data for mechanistic interpretations, consideration should be given to the data analysis methods. Although in some instances conventional steady-state kinetic analyses can be performed in order to derive analytical expressions for the temporal relationships among the various observed products, these treatments are not generally applicable to complex chemical systems. Recent progress in numerical analysis methods and the ready accessibility of powerful minicomputers have made it possible to routinely carry out numerical simulations of the experiments to preselect appropriate experimental parameters as well as to evaluate the sensitivity and validity of a reaction mechanism against data bases. It should be noted in this regard that computational techniques dealing with hundreds of reaction steps and chemical species are being

80

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

employed for the development and testing of detailed chemical mechanisms for use in atmospheric models [37-481. Many, but not all, of the experimental results described in the following section involve a relatively small number of reaction steps and species, and are designed to uniquely characterize as many relevant elementary reactions as possible in a given chemical system.

111. ATMOSPHERIC OXIDATION OF HYDROCARBONS Analogous to high temperature combustion processes, atmospheric oxidation of hydrocarbon compounds ultimately leads to the formation of CO, and H 2 0 via free radical reactions involving a large variety of oxygenated intermediate products. Some of the representative products and their free radical precursors are illustrated schematically in Figure 2. As stated in the preceding section, many of the molecular products formed from individual hydrocarbons are still poorly characterized, primarily because of the difficulties associated with their detection. Nevertheless, numerous significant findings have been made in recent years largely as a result of advances in FTIR spectroscopic instrumentation. Some of the reactions described below are not important in the atmosphere per se but serve as reference reactions whose knowledge is essential for a better understanding of the atmospheric chemistry of hydrocarbons.

Figure 2. Schematic illustration showing the atmospheric transformation undergone by hydrocarbons.

81

ATMOSPHERlC OXIDATION OF HYDROCARBONS

A. Methane The chemical fate of CH, in the troposphere is governed solely by the reaction with HO radicals [1,301.The HO-radical initiated oxidation of CH, involves the CH,OO radical as the precursor for oxygenated products such as CH,OOH and HCHO, i.e., R = CH, in Figure 2. Both of these products can, in turn, undergo photodissociation and/or reaction with HO radicals to eventually yield CO and CO,. Illustrated in Figure 3 are the IR absorbance spectra of some of the CH,-derived products in the frequency region of 6003300 cm - I recorded at 0.1 cm - resolution in the presence of 760 Torr of air. All these compounds are seen to exhibit sharp, distinct vibrational-rotational bands which can be used for unique identification.

1745.8

1800

1033.4

1700

I100

1000

(C) CH300H

3601.3 3595.5

A-

2963.8

1 3 3 2 37 . 9 821.I

(D) C H 3 0 0 C H 3

3000

103I

2000

1000

I / A ( c m-' 1

Figure 3. IR absorbance spectra of CH,-derived products: (A) HCHO, (B) CH,OH; (C) CH,OOH; and (D) CH,OOCH,.

82

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

1. CH,OO Reaction

+

CH,OO CH,OO. Reactions of the C H 3 0 0 radicals in air under NO, free conditions have been studied using the long path-FTIR method by Kan, Calvert, and Shaw (KCS) and by the authors' group (NMSB) both in the photooxidation of CH3N2CH3(KCS, NMSB) and in the C1-atom initiated oxidation of CH, (NMSB) [49, SO]. These product studies have provided evidence for the occurrence of the following three reaction channels, suggested earlier by Heicklen and co-workers, for the self-reaction for the CH30, radicals [Sl]:

+ 0, + HCHO + CH30H + 0, CH302CH3 + 0 2

2CH300' -+ (adduct) + 2CH,O'

+

(la) (1b)

(W

followed by the two dominant secondary reactions

+ 0, + HCHO + HOO' C H 3 0 0 ' + HOO' CH,OOH + O2 CH30'

-.

(2) (3)

The rate of reaction (2) is significant in ambient air, even though its rate constant is only 1.5 x 10-"cm3 molecule-'s-' at 298K [5,6]. Also, the self-reaction of the radical product H 0 2 [6], i.e., 2H0, --* H,02 + 0,, could be kept negligible in these experiments, so that the relative importance of the three primary reaction channels could be derived from the product distribution

[HCHoI/[CH30Hl

= (2klo

[CH300H]/[CH,0H]

[CH30,CH31/[CH30Hl

+ klb)/klb

= 2kl,/kl, = klc/klb

Based on measurements of these product ratios, KCS [49] reported the relative rate constants k , , / k , b = 0.75 k 0.10 and k l c / k l b < 0.14. The corresponding values reported by NMSB [SO] were 0.55 L 0.07 (0) and 60.13, respectively. In both studies, a weak, structureless IR absorption band centered at approximately 1030cm-' in the C-0 stretching region was detected but not positively identified. Some portion of this band overlapped with one of the bands of CH,0,CH3 (cf. Figure 3), from which the indicated upper limit values for k l c / k l b were derived.

ATMOSPHERIC OXIDATION OF HYDROCARBONS

83

Prior to these two FTIR studies, Nangia and Benson advanced thermochemical arguments in favor of an alternative H-atom transfer mechanism to account for then available experimental results [52]: 2CH300’ -,CH,OOH

+ kH,OO’

(4)

followed by

CH,OO* + CH,OO.

-+

HCHO + CH,O’

+ 0,

(5)

where CH,OO was assumed to be a zwitterion originally postulated by Criegee as an intermediate in the O3 + C2H, reaction [53]. However, this scheme does not yield CH30H as a primary product, contrary to the observation. Also, experiments designed to examine the presence of CH,OO were negative [49, SO]. Another important implication of the results of these product studies is that many of the previously measured second-order rate constants for the CH302decay should be greater than the true value of k, by as much as 30% depending on the extent of the secondary reactions (2) and (3) in these kinetic measurements [6,54]. Also, reaction (1) presumably has a slightly negative activation energy [S, 61, but the temperature effect on the primary reaction channels remains unknown at present. CH,O, + NO,@ = 1-2). The kinetics and mechanism for the two crucial radical propagation steps in the atmospheric oxidation of CH,, i.e., CH,OO + NO -+ CH,O + NO, and CH,O + 0,-+ HCHO + HOO have been well established by other experimental techniques [.5,63, and these reactions are frequently employed for HO-radical generation in FTIR-based studies. Another potentially important atmospheric reaction of CH,OO is that with NO,. The first positive identification of peroxynitric acid, formed via HOO + NO, + HOONO,, made by the authors’ group using the FTIR method, suggested the probable occurrence of its higher homologues, i.e., RO, NO, -+ ROONO, [SS]. The peroxy nitrates with R = alkyl or haloalkyl group up to C, have been identified in a series of studies from this laboratory [56,57]. For comparative purposes, the characteristic absorption bands of RNO,, RONO, RONO, and ROONOz for R =C,H, are illustrated in Figure 4. As can be seen in this figure, the ROONO, spectra resemble, in general, those of RONO, whose major “nitrate (ON0,)-type” bands occur at around 850 (N-0 stretching), 1290 (symmetric NO, stretching) and 1670 cm-’ (asymmetric NO, stretching), However, the corresponding frequencies for “peroxynitrate (-OON0,)-type” bands are seen to be shifted to 800, 1300,

+

84

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

R-NOz

I576

k-(& \ /

R-ON0

1391 1363

\ /

,166 I

R-ONO,

kJ,

R-OONOZ ' 7I/

I

2000

I

I

1000

I / X (ern-')

Figure 4. IR absorbance spectra of RNO,, RONO, RONO,, and ROONO,

(R = CZH5).

and 1720 cm- '. Both the intensities and shapes as well as frequencies of these bands are distinguishable for the peroxyalkyl nitrates containing small R groups ( S C J but become virtually identical for larger R groups. These peroxyalkyl nitrates are thermochemically unstable at room temperature and readily undergo dissociation back to their radical precursors [27], thus, these compounds have to be produced at a detectable level in the shortest possible reaction time. The main radical-generation scheme employed for their observation involved the C1-atom initiated oxidation of the corresponding hydrocarbon [56]. However, in the case of CH,, its concentration had to be adjusted upward by a factor of lo00 relative to those of

85

ATMOSPHERIC OXIDATION OF HYDROCARBONS

other hydrocarbons because of its low reactivity toward C1 atoms [6]. This resulted in nearly total attenuation of the incident radiation from the TR source in the frequency regions corresponding to the two major bands of CH, at 1300 and 3000cm-’. Thus, the presence of a high concentration of CH, prevented the reliable analysis of one of the “peroxynitrate-type”bands occurring in the vicinity of 1300cm- To minimize such spectral interferences, an alternative scheme was also used whereby the C H 3 0 0 radicals were produced from CH,OOH by C1 CH,OOH + CH,OO HC1 [56]. When CH,OONO, was prepared in this manner in the presence of excess NO,, its observed lifetime became longer than 30min after many repeated runs, presumably due to conditioning of the reactor walls. However, to date there have been no reported experimental data on the thermal dissociation of CH,OONO,. In the case of C,H,OONO,, Edney, Spence, and Hanst used the photolysis of C1,-C,H,-NO,-air mixtures for its generation and, by the FTTR method, determined its dissociative lifetime to be 0.5s at 300K ~581.

’.

+

+

2. HCHO Oxidation

HO + HCHO. Despite the well-recognized, critical role of the HO + HCHO reaction in atmospheric chemistry [1, 113, considerable uncertainty existed until recently concerning both the rate constant and the mechanism operative under tropospheric conditions. Namely, of the two exothermic reaction channels (6a) and (6b), HO

+ HCHO -+ H e 0 + H,O + HC(0)OH + H

(64 (6b)

the H-atom abstraction channel was commonly assumed to be the dominant path, although the possible occurrence of the H-atom displacement channel with a rate constant ratio k6b/k6a < 2 had been reported in the literature [59]. Since reaction (6b) proceeds via a chemically activated adduct, pressure dependence in the rate constant was also expected. In fact, an earlier determination of k(H0 HCHO) (i.e., k,, in 700Torr of air made by the authors’ group) gave a value approximately 50% higher than those obtained directly by Atkinson and Pitts [60] and Stief et al. [61] using the flashphotolysis-resonance fluorescence technique in the presence of diluent Ar at <40Torr. This high value was obtained by the FTIR-competitive kinetic method based on measurements of two rate constant ratios k(H0 HCHO)/k(HO C2D4) and k(H0 C,D,)/(HO C2H4),in combination with the literature value of k(H0 C2H4)[62]. In this experiment, HONO was used as a photochemical source of HO radicals. The two-step

+

+

+

+ +

+

a6

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

+

+

procedure for deriving k(H0 HCHO)/k(HO CzH4) in this fashion was employed to circumvent interferences in the rate measurements from the product HCHO formed in the HO-initiated oxidation of C2H,. Since the value of k ( H 0 + HCHO) thus determined could suffer from propagation of errors, its accuracy was re-examined in a later study [63]. In this follow-up experiment, the H O radicals were generated by the photooxidation of RON0 (R = CH, and C,H,) (cf. Section ILA), and 13Clabeled formaldehyde H13CH0 was employed as the reactant. Illustrated in Figure 5, for comparison, are the two most conspicuous portions of the spectra of H ” C H 0 and H13CH0. Both the -0 stretching region (16001850 cm- ’) and the C-H stretching region (2600-3100 cm- ’) displayed in this figure provided unique finger-printing features for these compounds. Similarly, two prominent spectral regions, i.e., C-0 and C=O stretching modes, of H”C(0)OH and H13C(0)OH are shown in Figure 6. The rate constant ratio k(H0 H13CHO)/k(H0 C2H4) for the HO-reactions of the reactant HI3CHO and the reference compound C2H4 was determined to be 0.99 k O.O6(3u).The reference rate constant k(H0 CzH4) required for calculating k(H0 HI3CHO) from this relative value is known to be pressure dependent, and the most appropriate value for this purpose was judged to be that of Atkinson et al. obtained in air by using a competitive kineticmethod, i.e., C8.48 & 0.60(3a)] x 10- l 2 cm3 molecule-’ s - l [62]. The value of k(H0 + H13CHO) thus derived is C8.4 k 1.1(3a)] x lo-’’ cm3

+

+

+

+

I

- 1745.8

12

CH, 0

- 1707.7

3000

2600

I

I

1800

A ( cm-‘

I

1600

Figure 5. IR absorbance spectra of ”C- and 13C-labeled formaldehyde in the frequency regions of 1600- 1850 and 2600-3050 cm-

’.

87

ATMOSPHERIC OXIDATION OF HYDROCARBONS

1095.5

1736.7

I

I

1105.0

1776.4

i

I

I800

I

1

i

12

CH ( 0 ) OH

I

I

I700

I I50

x

(cm-')

I

1050

Figure 6. IR absorbance spectra of 12C-and 13C-labeledformic acid in the frequency regions of 1025-1175 and 1675-1825 cm-'.

molecule-' s-'. The kinetic isotope effect for the HO-reactions of H"CH0 and H13CH0 is expected to be much less than the indicated experimental uncertainty [64]. This value for k(H0 + HCHO) is in reasonable agreement with those measured at low pressures by Atkinson and Pitts, and Stief et al., cm3 rnolecule-'s-I, reand (9.86 k 1.13) x i.e., (9.4 5 1.0) x spectively [60,61]. Thus, there appears to be no pressure dependence in k(H0 + HCHO). This notion is further supported by the mechanistic results described below. The relative importance of the two possible channels (6a) and (6b) was readily determined from an analysis of '3C-containing products arising from the HO + H13CH0 reaction, in the photolysis of mixtures containing RONO, NO, and H13CH0 at ppm levels in an atmosphere of air [63]. NO was added as an initial reactant to ensure that the HOO radicals formed by the RO + O2 reaction were efficiently converted to HO radicals by HOO + NO + HO + NO,. The reactant H13CH0 was shown to produce 13C0 quantitatively, and the H13C(0)OH expected from reaction (6b) was only a very minor product, with an upper-limit of 2% for its yield. The observation of 13C0 could be attributed to the predominant occurrence of reaction (6a) followed by the fast secondary reaction H i 3 C 0 + O2 + 13C0 + HOO. Thus, both the kinetics and mechanism for the HO + HCHO

88

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

reaction can be considered to be well established under atmospheric conditions. HOO + HCHO. One of the long-standing questions concerning the photo and thermal oxidation of HCHO in NO-free systems is the mechanism for the copious production of HC(0)OH [65,66]. The results described above rule out the possibility of its formation as a primary product in the HO + HCHO reaction. Furthermore, the 0,-reaction of the ensuing radical product HCO has been shown to yield CO and HOO rather than the adduct radical HC(0)OO which could, otherwise, be a likely candidate for a radical precursor of HC(0)OH. In contrast to the RC(0)OO radicals (R = alkyl group), the HC(0)OO radical is presumably thermochemically unstable in atmospheric pressures of air. Recently, the reaction of HOO with HCHO has been demonstrated to be sufficiently fast to contribute to various aspects of HCHO oxidation including the formation of HC(0)OH [67-701. Su et al. first reported experimental evidence for the HOO + HCHO reaction by the FTIR method in the C1-atom initiated oxidation of HCHO [67,68].

HOO

+ HCHO

(HOOCH20') c* HOCH2OO'

c-)

(7, - 7)

These investigators detected an unstable product which was tentatively identified as HOCH,OOH on the basis of kinetic and spectroscopicevidence. Perhaps the most notable feature of reaction (7) is the unimolecular rearrangement of the primary adduct to a thermodynamically more stable peroxy radical HOCH,00. This peroxy radical was suggested to undergo redissociation (reaction ( - 7)), bimolecular reaction with HOO and also selfreaction leading ultimately to HC(0)OH; i.e., HOCH,OO'

+ HOO'

2HOCH2OO' HOCH,O'

+

HOCH,OOH

+0 2 HC(0)OH + HOO'

+ 2HOCH20'

+ 0, -+

+ O2

(8) (9) (10)

The IR spectrum of the intermediate product in the HOO + HCHO reaction shows peaks characteristic of 0-0 stretching ( x 800 cm - I ) , C-0 stretching ( x1050 cm-'), and C-H stretching ( x2900 cm- '). Note that the presence of an -OH and -0OH in this compound should give rise to two OH stretching bands. Confirmation of the presence of two weak 0-H stretching bands near 4000 cm-' was made by enhancing spectroscopic performance with a CaF, beam splitter and an TnSb photo-voltaic detector [71]. Figures 7 and 8 show the spectral data from which a high-quality

ATMOSPHERIC OXIDATION OF HYDROCARBONS (A)

89

Clz (50ppm) -CH20 (20ppm 1 -Q( - 100 Torr 1-H2(600Torr) Before Irradiation

( B ) After 3min Irrodialion

3800

3500

3000 I/X (cni')

2600

+

Figure 7. Spectral data for the HOO HCHO reaction in the frequency region of 2600-3800cm-1. (A) CH,O (20ppm)-C1, (50ppm)-O, (100Torr)-H, (600Torr) before irradiation; (B) after 3 min irradiation; (C) reference spectra of HzO, and HCI; and (D) reference spectrum of HCOOH. Signals were truncated at 80% absorption, i.e., lJ1, = 0.2.

spectrum of this compound was extracted. Figures 7A and 7B are the absorption spectra in the frequency region 2600-3800 cm- ' recorded before and after 3-min UV irradiation of a mixture containing C1, (50 ppm), HCHO (20 ppm), O 2 (100 Torr) and H, (600 Torr). The very high concentration of H, had to be used in order to optimize the formation of the HOO radicals via C1 + H, H + HCl followed by H + O 2( + M) -+ HOO (+ M). Figure 7A corresponds to the spectrum of pure HCHO in the C-H stretching region (2600-3OoO cm- l ) and the C=O overtone region ( z 3500 cm- '). Inspection of Figures 7A and 7B reveals substantial spectral changes in the vicinity of 3600cm-' as well as consumption of the HCHO. Figure 7C shows reference spectra of H,O, (0-H stretch) at about 3600cm-' and HCl in the region 2600-3100cm-'. In Figure 7D,both the C-H and 0-H stretching bands of HCOOH are displayed. These compounds plus CO (not shown here) represent all the known products in this

90

-

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS ( A ) Residual Spectrum

3647.5

HC I

2989.5

( C ) CzH5OH 3676.2

I

3800

3500

I / X (cm-')

3000

2600

Figure 8. Analysis of the product spectrum in Figure 6. (A) residual spectrum from Figure 6B (see text); (B) reference spectrum of C,H,OOH; (C) reference spectrum of C,H,OH. Signals in (A) were truncated at 20% absorption.

reaction system. The residual spectrum shown in Figure 8A was obtained by subtracting appropriate fractions of the reference spectra of H,O, and HCOOH from Figure 7B. Two bands with sharp Q-branches centered at 3598.3 and 3647.5 cm-' and a broad band in the vicinity of 2900 cm-' are clearly seen in this spectrum. The high frequency bands fall in the O-H stretching region common to alkyl hydroperoxides and alcohols, as illustrated in Figure 8B and 8C for C z H 5 0 0 Hand C,H,OH, respectively. It can be noted from a comparison of these reference spectra that the O-H stretching band associated with an OOH group is significantly down-shifted in frequency from that of an OH group. Thus, the two bands in the residual spectrum, Figure 8A, are indicative of the presence of an OH and an OOH group. Further spectroscopic characterization of this compound was made using an isotope-labeling technique [71]. In the reaction of HOz with DCDO, the two O-H stretching bands were shifted only slightly to 3558.8 and 3649.2cm-', while the C-H stretching band was replaced by a C-D stretching band at about 2200cm-'. These results further support the

ATMOSPHERIC OXIDATION OF HYDROCARBONS

91

formation of HOCD,OOH via the proposed reaction mechanism. This compound was found to be relatively stable ( x 1 h) in the dark after thorough conditioning of the reactor walls. Further corroborative evidence for reaction (7) was obtained by the authors' group [72] and by Barnes et al. [73] from the observation of a transient product identified as HOCH,OONO, formed via photolysis the HOCH,OO + NO, reaction in a CI,/HCHO/O,/NO, system and a HOzNOz/HCH0/O2/NO, dark reaction system, respectively. By computer simulation of the concentration-time profiles of the reactants and products, Su et al. [67,68] derived approximate values for the rate constant k, = 1 x 10-'4cm3 molecule-' s - l and the dissociative lifetime k-, = 1 s(25"C,700Torr air). The more recent, extensive study by Barnes et al. [73] reported an order of magnitude larger values for both rate constants. Thus, the HOO + HCHO reaction represents the fastest known reaction of HOO with organic molecules at large and also a potentially important source of HC(0)OH in the atmosphere.

3. Kinetics and Mechanism for HO + CO. Atmospheric interest in the H O + C O reaction stems from its well-recognized role as the sole process for converting CO to CO,. Although this reaction is among the most extensively studied gas phase free radical reactions, a detailed understanding of its kinetics and mechanism is still lacking. Presumably, it is not a simple elementary reaction but most likely involves an addition complex which can yield HOO and CO, in the presence of O,, e.g.,

+ CO + HOCO* HOkO* + H + CO,

( 12a)

HOkO*

( 12b)

HO

+ M +H o e 0 H O k O + 0, CO, + HOO' +

(1 1)

(13)

where asterisks denote chemically activated states. Accordingly, the rate constant for the HO C O reaction has been shown to exhibit complex pressure and temperature dependence [6]. Prior to the most recent direct kinetic measurements by Hynes et al. [74], there have been considerable uncertainties in the values of these parameters, particularly under atmospheric conditions. FTIR studies of this reaction in air were made by Chan et al. [75] and by the authors' group [76]. The former group employed HONO photolysis as the H O source and i-C4HIoas the reference compound. Based on the observed concentrations of the corresponding products, i.e., C4H,0N0, for the i-C4Hlo, and the CO, for C O and (CH,),CO literature value of k ( H 0 i-C4Hio) = 2.4 x lo-'' cm3 molecule-'s-'

+

+

+

92

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

+

[30], they derived an average value of k(H0 CO) = (3.1 k 0.2) x 1 0 - l ~cm3 rnolecule-'s-'. Experimental approaches adopted in the latter study were similar to those of Chan et al. [75] but had the following features in an attempt to optimize the precision and accuracy of the kinetic measurements: (1) use of 13C'60 and 12C'80as the reactant C O and analysis of the corresponding isotopelabeled CO, products; (2) the HO C2H4 reaction as the reference; (3) photochemical production of HO radicals from R O N 0 (R = CH, or C2H,); and (4) accurate spectral recording at sufficiently high resolution (1/16 cm- ') in 700 Torr of air. Feature (1) was necessitated because of the instability in the background IR absorption signals of 12C'60 present as an impurity in the reactor. Spectra of isotope-labeled CO, are shown in Figure 9. One potential complication in this experiment was examined and was shown to be unimportant. Namely, Kurylo and Laufer have shown qualitatively that isotope exchange reactions such as H"0 CI6O2+ C ' 6 0 1 8 0+ H 1 6 0 and H"0 C i 6 0 + C"0 H160 can occur at low diluent pressures but not in the presence of 760 Torr of SF, [77]. In agreement with these results,

+

+

2385

+

+

I/X

2300

(crn-'1

2235

Figure 9. IR absorbance spectra of 12C160 12C160180 , and 13C1602in the frequency region of 2235-2385 cm-'. 2 9

ATMOSPHERIC OXIDATION OF HYDROCARBONS

93

no evidence for the occurrence of these reactions was obtained in 700 Torr of air under the experimental conditions employed for the kinetic studies. On the basis of measurements of the rates of C,H, decay and formation of isotope-labeled CO, products, k(H0 + C,H,)/k(HO + 13C160)= 35.95 + 0.95(a) and k(H0 C,H,)/k(HO + 12C180)= 36.30 k 0.84 were determined. These values combined with the value of k(H0 + C2H4) mentioned earlier give k(HO+ '3C'60)=(2.36+0.12) x and k(HO+ 12C'80) =(2.34+0.12) x cm3 molecule-'s-'. These results are in excellent agreement with those of the most recent direct measurement by Hynes et al. cm3 molecule-'s-'. However, the detailed [74], i.e., (2.35 & 0.20) x kinetics and mechanism of the elementary reactions involved in CO, formation, e.g., reactions (12a), (12b) and (13) remain to be determined.

+

+

Kinetics and Mechanism for HO CH,OOH. Methyl hydroperoxide is an important atmospheric constituent formed in the photooxidation of CH, and other organic compounds [ l l , 78,793, i.e., C H 3 0 0 + HOO + C H 3 0 0 H + 0, (reaction (3)). To date, only one quantitative study of the HO-reaction of C H 3 0 0 H has been reported in the literature [78]. Numerous difficulties associated with sample preparation, gas handling, identification and calibration of the C H 3 0 0 H were encountered in an earlier qualitative FTIR study by the authors' group [56]. These experimental problems have been largely overcome after many attempts over the course of several years [SO]. The HO + C H 3 0 0 H reaction has two possible H-abstraction channels, in one instance from the CH, group and in the other from the HO group, i.e., reactions (14a) and (14b)

4.

HO

+ CH,OOH -,kH,OOH + H 2 0 C H 3 0 0 ' + H,O --*

(144 ( 14b)

The primary radical CH,OOH is expected to dissociate unimolecularly to regenerate the HO radical, i.e., reactions (15). k H 2 0 0 H --* HCHO

+ HO'

(15)

The relevant reactions of the C H 3 0 0 radical are reasonably well understood, as discussed before. The overall rate constant k(H0 + CH,OOH) for reactions (14a)and (14b) was determined by the competitive decay method. C2H4 served as a convenient reference compound, especially since the relative decay rates of CH,OOH and C2H, were found to be comparable. Also, CH3CH0 could

94

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

be used as the reference when HO radicals were generated from C H 3 0 N 0 but not from C,H,ONO-the carbonyl product from the latter nitrite is CH3CH0. Values of k(H0 + CH,OOH)/k(HO + CzH4) and k(H0 + CH,OOH)/k(HO + CH,CHO) thus determined were 1.20 f 0.09 and 0.68 k 0.07, respectively, from which an absolute value of k(H0 + CH,OOH) = 1.0 x lo-" cm3 molecule-' s-' was derived. An ear~ on~an assumed ~ , lier estimate for k(H0 CH,OOH), specifically l ~based similarity to the HO H,Oz and HO C,H, reactions is a factor of 20 smaller than this experimental value [8 11. Clearly, much more extensive kinetic data are needed to apply such empirical relationships reliably. It should also be noted that because of the possible regeneration of HO radicals by reaction (15), the value of k , , cannot be measured directly from the decay measurements of the HO radicals alone. Thus, the determination of the relative importance of reaction (14a) versus reaction (14b) requires mechanistic knowledge based on product studies. In order to establish the mechanisms for reactions (14a) and (14b), experiments were designed to uniquely identify C H 3 0 0 and HCHO as the primary products. For this purpose, HO radicals could be generated from C,H,ONO but not from CH,ONO, since the latter nitrite produced HCHO as a major product. Also, the CH,OO radical could lead to the formation of HCHO via CH,OO + NO -+ C H 3 0 + NO, followed by CH,O + O2 + HCHO + HOO. However, it was possible to adjust the 0, pressure and NO, concentrations so that the C H 3 0 radicals reacted competitively with O,, NO and NO,:

+ +

CH30' CH,O' CH,O'

+

+ NO (+ M) + CH,ONO (+ M) + NO, (+ M) CH,ONO, (+ M) + 0, -,HCHO + HOO' 4

(W (16b)

(W

For instance, the relative rates of reactions (16a) and (16c), k16c[02]/k16a[N0] vary from <0.2 to x 1 with [NO] = 10 ppm and an 0, pressure ranging from 20 to 140Torr [82-851. The percentage yields of CH,OOH and CH30, were derived from those of the corresponding products, ([HCHO] + [CO]) and ([CH,ONO] + [CH,ONO,]), extrapolated to zero 0, pressure. On this basis, a value of k14a/k14b= 0.77 was determined with a conservative error limit of +20%. Thus, both reaction channels were shown to be operative. Perhaps, the most significant aspect of these results is that the atmospheric oxidation of CH, can lead to the formation of HCHO, CO and eventually CO, without the intervention of NO, species.

ATMOSPHERIC OXIDATION OF HYDROCARBONS

95

B. Ethane and Higher Alkanes 1. CzH500 Reaction. In the atmosphere the C 2 H , 0 0 radicals formed by HO + C2H6 -+ C2H5 + H,O followed by C2H, + 0, -,C,H,OO are removed primarily by NO and HOO via mechanisms analogous to those of the CH,OO radicals (cf. Figure 2). For laboratory studies of these bimolecular reactions the self-reaction of this radical serves as a reference. The mechanism of this elementary reaction has been studied using the FTIR method in the photooxidation of C2H,N2C,H, and also in the C1-atom initiated oxidation of C2H6 [86] in 0,-N2 mixtures at 700 Torr and 298K. On the basis of the mechanism for the CH,02 + CH,O, reaction described in the preceding section, the products from the self-reaction of the C,H,O, radicals are expected to be CH,CHO, C2H,0H, and possibly C,H,O,C,H,, i.e.,

2C,H,OO'

+ 0, + CH,CHO + C2H,0H + 0,

+ (adduct) + 2C2H,0'

+ C2H,02C2H,

+ O2

(174 (17b) (174

followed by the dominant secondary reactions (18) and (19)

C2H50'+ 0, + CH,CHO C,H,OO'

+ HOO'

+ HOO' -+ C2H,0,H + 0,

(18) (19)

In these experiments, the major products CH,CHO, C,H,OH and C 2 H , 0 0 H and several minor secondary products were quantitatively analyzed. However, upon subtraction of the spectral contributions from these known compounds, there still remained a residual spectrum. This spectrum but its most intense band at approximately resembled that of C2H502C2H5, 1040cm - was slightly broader than the corresponding band of reference spectra of C,H502C,H,. This product was seemingly stable and did not noticeably decay during aging in the dark for 15 min. An upper limit for the C2H,02C,H, yield was derived from these data. Another interesting finding of this study was the observation of CzH4 as a very minor product. Its formation was attributed to the occurrence of the H-atom transfer reaction C,H, + 0, -+C2H4 + HOO at a rate up to 0.1% of the corresponding addition reaction. On the basis of the observed product yields [86], the relative rate constants for reactions (17a)-(17c) were determined to be k17a/k17b= 1.3 (+ 12%) and k17c/k17b< 0.22. The branching ratio k17a/k17b= 1.3 is signifi-

96

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

cantly greater than the corresponding value of 0.54 for the CH,OO -k C H 3 0 0 reaction [49, SO]. This trend is to be expected for these competitive dissociation channels of the R 0 4 R (R = alkyl group) adduct. For example, the dissociation path for the CH30,CH3 adduct may be visualized schematically by the following states I and II. H

H

H

I

I

H- C -O-:-O-O-+O-

C -H

I .:.

HtC-H

.I

0''

H

H

* I

H-C-H

1

I

H

I

0

I I ................... I I 0

0 -

I

I1

State I represents a precursor to the dissociation to two C H 3 0 radicals and O2 and state I1 to the formation of CH30H, HCHO and 0,. Since state I1 involves H-atom migration via a six-membered ring, a bulkier alkyl substitution is likely to reduce the efficiency of this channel because of steric factors. Notably, the branching ratio for the self-reaction of i-propylperoxy radicals, (CH,),CHOO, has been determined using GC-product analysis by Kirsch et al. to be 1.39 & 0.05 [87]. 2. CH,CHO Oxidation and CH,C(O)O, Reaction. The atmospheric photooxidation of C,H, exhibits a major departure from that of CH, in the subsequent fate of the carbonyl product CH,CHO [88]. Namely, the acetyl radical CH,CO formed in the HO-reaction of CH,CHO adds to O,, reaction (20), to yield the peroxyacetyl radical CH,C(O)OO, whereas the HCO radical in the CH, oxidation undergoes an H-atom transfer reaction with O,, i.e., HCO + 0 , -P CO + HOO. CH3b

+ 0, ( + M)

+

CH,C(O)OO' (+ M)

(20)

CH&(O)OO -k NO, The subsequent reactions of the CH,C(O)OO radicals with NO, have been the subject of extensive studies and are now reasonably well established [2,6,27,88].

CH3C(0)OO' + NO -P kH3 + C O , CH,C(O)OO'

+ NO2

+ NO, + CH,C(O)OON02

(21)

(PAN)

(22)

Peroxyacetyl nitrate (PAN) was first identified in the 1950s by long-path IR spectroscopy [89,90]. Interest in PAN has been renewed as a result of recent

ATMOSPHERIC OXIDATION OF HYDROCARBONS

97

recognition of its potential role as a “green-house’’ gas, and also as a NO, reservoir for the global troposphere [l]. A number of FTIR studies have dealt with photochemical preparation of PAN and its homologues both for spectroscopic characterization and for kinetic measurement of their thermal dissociation rates. Gay et al. [91] first reported the in situ synthesis of PAN in the Cl-atom initiated oxidation of CH3CH0 in the presence of NO,. The C1 + CH3CH0 reaction was later shown in this laboratory to be very fast, 7.6 x lo-” cm3 molecule-’ s-l at 298K, and to quantitatively yield CH,CO radicals [92]. However, depending on the mixing ratios of these reactants, the reactions leading to the formation of FAN are often accompanied by numerous side reactions which potentially complicate a photochemical system such as this. Namely, upon UV irradiation in the wavelength region of 3Oo-400 nm, not only C1, but also CH,CHO and NO, can undergo photodissociation and subsequent secondary reaction [7,27]. In addition, the Cl-atom reactions with NO, forming ClNO, and ClONO can interfere with the quantitative preparation of PAN [93]. It was shown that these problems can be largely circumvented by an alternative method based on the Br-atom reaction of CH,CHO [34].

+ hv(approx. 350-600 nm) .+2Br Br + CH3CH0 C H 3 k 0 + HBr Br,

+

(23) (24)

Since Br, photodissociates efficiently at >400 nm [94], the photolysis of both CH3CH0 and NO, can be avoided by using a visible light source. Also, the Br + NO, reaction was shown to yield negligibly small concentrations of BrNO, and BrONO, presumably due to the photochemical instability of these products. The rate constant for the Br-CH,CHO reaction was determined to be 3.7 x lo-’’ cm3 molecule-’ s-l, from the decay rates of CH3CH0 relative to those of a reference compound HCHO in the VIS photolysis of mixtures containing Br, and these reactants in 700Torr of N, [34]. Figure 10 illustrates typical spectral data obtained in the photolysis of mixtures containing Br,, CH,CHO, and NO, at ppm levels in 700 Torr of air. Figures 10A and 10B show the spectra in the frequency region of 5003200 cm- recorded before and after 30 s of irradiation, of Br, (20 ppm), CH3CH0 (4.88ppm), and NO, (3.25 ppm). The spectral changes during the irradiation are more clearly seen in the difference spectrum, Figure lOC, obtained by subtracting Figure 10A from Figure 10B. The extent of the consumption of both CH,CHO (1.66 & 0.10 (reading accuracy)) and NO, (1.78 & 0.04) appear in this spectrum as negative signals, while the positive signals are mainly those belonging to HBr and PAN as indicated. Minor components observed were C 0 2 (0.09), BrNO, (0.06),NOBr (0.01), HNO,

’,

98

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

(A) t = O s NO2 (3.25) CH3CH0 (4.88)

I

(6)t.30 s

DIFFERENCE SPECTRUM

PAN

[(B 1- ( A)]

3000

2000

I / X (cm-'1

1000

Figure 10. Spectral data in the frequency region of 500-3200 cm-' resulting from the photolysis of a mixture containing Br, (20 ppm), CH,CHO (4.88 ppm) and NO, (3.25 ppm) in 700 Torr of air. Values in parentheses are concentrations in ppm (1 ppm = 2.46 x 1013 molecule cm-').

(0.02), and HONO (0.09) in ppm units. The PAN spectrum derived from Figure 10B is shown in Figure 11A in the frequency region of 5002000 cm-'.All the band features seen in this spectrum are in good agreement with those reported earlier for PAN by Stephens [89] and by Bruckman and Willner [95]. Absorptivities for these bands were determined from several runs similar to that illustrated in Figure 10A, and the peak values for prominent bands are given in Table 1 together with the literature values. Figure 11B shows the spectrum of ISN-labelled PAN obtained in a manner analogous to that described above for the 14N-labeled PAN. Substantial isotope shifts are seen to occur for the vibrational modes

99

ATMOSPHERIC OXIDATION OF HYDROCARBONS

TABLE 1 IR Absorptivities of Gaseous PAN"

Peak Frequency (crn- ')

Stephens' Bruckrnann and Willnerd Niki et al.'

1841.35

1740.75

1302.28

1163.32

793.75

10.0 12.4

23.6 32.6

11.2 13.6

14.3 15.8

10.1 13.4

10.2

31.0

11.3

14.5

11.5

"In the units of (base 10, ppm-' m-' x lo4). bEstimated uncertainties are +0.15 cm-' for the 1841.35 cm-' band, and kO.05 for all other bands. c(Ref.89) PAN in 730Torr of air; resolution > 5 an-'. d(Ref.95) Pure PAN at 3Torr; resolution = 1.2cm-'. '(Ref. 34) PAN in 700 Torr of air; resolution = 0.06 cm- '.

associated with the -0ON0, group (Figure 11C). Similar observations were made by Varetti and Pimentel for matrix isolated spectra of PAN-14 and -15 in solid oxygen at 20K [96], although virtually all the spectral bands in the solid phase were somewhat down-shifted in frequency as compared with those shown in Table 1 for gaseous PAN. For example, the PAN-14 bands corresponding to those in Table 1 were reported to occur at 791, 1155, 1300, 1735, and 1835 m-' with frequency accuracy of & 1 cm-'. These quantitative spectra of PAN-14 and -15 permitted a kinetic study of the thermal dissociation of PAN, described below. It has been established previously that in ambient air, PAN thermally decomposes predominantly via reaction (25) and that this reaction is in equilibrium with the reverse reaction (22). CH3C(0)OON02--t CH3C(0)OO'

+ NO2

(25)

The most direct evidence for this mechanism has been obtained by Hendry and Kenley [97] from an IR spectroscopic observation of the formation of I4NO2 and PAN-15 in mixtures containing PAN-14 (80-180 ppm) and I5NO2 at [1sN02],/[PAN-14], = 1-3. Based on the conversion rates of PAN-14 to PAN-15, the lifetime of PAN, 1/kz5, was determined to be 42 ( f20) min at 25°C. These investigators also derived k 2 5 from the decay rates of PAN in excess NO over the temperature range of 25-40"C, i.e. loglO(kz5/s-') = 16.29 f (26,910 f 900)/0 (0= 2.303RT in cal/mol), corresponding to l/k25 = 45 (f 10%)min at 25°C. These results of Hendry and Kenley have been verified using the FTIR method, and the above-mentioned

100

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

( A ) PAN -14

( 6 ) PAN

- 15

(C) DIFFERENCE SPECTRUM [(B) - ( A ) ]

2000

1500

I / X (cm-1)

1000

500

Figure 11. IR absorbance spectra of isotope-labeled PAN CH3C(0)OO'4N0,and CH,C(0)OOISNO,in the frequency region of 500-2000 cm- '. Values in parentheses in (C) are approximate frequency shifts in cm-' for 15N-labeled PAN. Band assignments taken from Ref. 95. in situ generation of PAN, from the observation of the conversion of PAN-15 to PAN-14 in excess I4NO2 in air [34]. An average value of l/k25 at 23.9"C was determined to be 51 min with maximum observed error limits of 56%. This value is in excellent agreement with 54 ( f 10%)min at 24°C derived from the temperature-dependent expression of k,, given by Hendry and Kenley C971. CH~(0)OO + HOO. The possible occurrence of the two reaction channels (26a) and (26b) was borne out by a preliminary FTIR study of the C1-atom initiated oxidation of CH,CHO in NO,-free conditions, in which

ATMOSPHERIC OXIDATION OF HYDROCARBONS

101

CH,C(O)OH and CH,C(O)OOH as well as 0, were detected among the products [92]. HOO'

+ CH3C(0)OO'

+ -+

CH,C(O)OOH CH,C(O)OH

+ 0,

+ 0,

(26a) (26b)

However, the formation of the HOO radicals in such a system involves a long series of reactions triggered by the self-reaction of CH,C(O)OO, i.e., reaction (27), and is not easily controlled independently from the generation of CH,C(O)OO. 2CH,C(O)OO'

-+

(adduct) + 2kH3 + 2C02 + O2

(27)

Addison et al. [98] have studied reaction (27) in the modulated photolysis of the Cl,-CH,CHO-0, and CH,C(O)C(O)CH,-0, systems. These investigators verified the formation of 0,reported much earlier by HaagenSmit et al. [99] and have tentatively attributed it to a minor channel of reaction (27). In a later study conducted by the authors' group [92], positive evidence for reactions (26a) and (26b) was obtained from the observed effect of added HCHO on the product distribution in the photolysis of C1,-CH,-0, mixtures. Namely, the HOO radicals were generated from the CI-initiated oxidation of HCHO, and concomitantly, the occurrence of the self-reaction of CH,C(O)OO, reaction (27), was minimized. IR absorbance spectra of CH,C(O)OH and CH,C(O)OOH are illustrated in Figure 12 together with that of CH,OOH for comparison. The product distribution was measured in the photolysis of mixtures containing C1, (20 ppm) and CH,CHO (5 ppm) typically in 700 Torr of air with HCHO added at up to 20ppm. It was established that both O3 and CH,C(O)OH yields were comparable for all runs at approximately 1/3 of that of the CH,C(O)OOH, and that their absolute yields per [CH,CHO] increased with added HCHO. Furthermore, none of these products exhibited any indication of heterogeneous formation or decay under the experimental conditions employed for this study. These results can, therefore, be taken as evidence for the occurrence of reactions (26a) and (26b) possibly via formation of a common intermediate adduct CH,C(O)OOOOH which undergoes subsequent rearrangement and unimolecular dissociation. In any case, it is clear that the HOO-reaction of ROO when R = organic group other than small alkyls is not a simple H-atom transfer reaction.

3. RO and RO, (R 2 C,) Reaction. It is presently well established that the only significant gas phase loss process for the alkanes in the troposphere is reaction with HO radicals [88]. In general, the reaction mechanism for the

102

(-"I

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

1790.2

0 II

642.2

3581.2

0 II

1243.6

(6)CH3COOH

(

I

1451.6

C 1 CH3 OOH ~2963.7

1320.7

I

1

3601.2 I

3000

2000

820.7

to00

~(cm-')

Figure 12. IR absorbance spectra of CH,OOH, CH,C(O)OH and CH,C(O)OOH in the frequency region of 600-3700 cm-

'.

HO-initiated oxidation of the alkanes become complex with increase in carbon number. Namely, a large variety of alkyl radicals can be produced by the H-atom abstraction from the primary, secondary and tertiary C-H bonds in the parent alkane [88]. The resulting ROO ( 2C4) radicals have been shown by Atkinson et al. to yield RONO, as well as RO + NO, upon reaction with NO [100-102]. A major complication in the alkane oxidation mechanism arises from the variety of competitive reaction channels that RO radicals can undergo, e.g., 0,-reaction, unimolecular dissociation and internal isomerization. There have been a number of experimental and theoretical studies of these reactions [31,88]. Carter et al. first pointed out the significant occurrence of internal isomerization of the RO radicals by a 1,5-hydrogen shift via a low-strain six-

103

ATMOSPHERIC OXIDATION OF HYDROCARBONS

membered ring intermediate under atmospheric conditions [1033. The nbutoxy radical is the smallest R O radical that can undergo such an isomerization, i.e., H.

CH,CH,CH,CH,O’

/ ..-. H2C 0 + I - - 1- H,C

\ /

CHI

-+CH,(OH)CH2CH2CH,

(28)

CH,

An alternative fate of this radical is to undergo intermolecular reaction with O,, reaction (29). CH,CH,CH,CH,O‘

+0 2

--*

CH3CH2CH2CHO

+ HOO’

(29)

On the basis of thermochemical arguments, Carter et al. [lo31 and Baldwin et al. [1041 estimated the ratio of (isomerization/O, reaction), k28/k29[02] = 100-300 with an uncertainty of a factor of approximately 100. From a GCanalysis of the CH,CH,CH,CHO yields in the photolysis of n-ButaneNO,-air, Carter et al. [lo51 and Cox et al. [lo61 derived rate constant ratios k,,Jk,, of 1.7 x 1019 and 1.5 x 1019molcmW3,respectively. An FTIR study was made by the authors’ group to determine the relative importance of reactions (28) and (29) in a more direct fashion, and also to obtain spectroscopic evidence for the formation of HO-containing products in the subsequent reactions of the CH2(0H)CH2CH,CH2radicals [1071. For this purpose, n-butoxy radicals were generated directly by the photodissociation of n-butyl nitrite and product analyses were made over a range of experimental parameters, e.g., reactant conversion, 0, pressure, and addition of “HO-radical scavengers”. Based on the n-C,H,CHO yield observed as a function of the 0, pressure, the rate constant ratio kzs/k,, was determined to be (1.9 f 0.2(a)) x 1019mol cm-,. This value is in excellent agreement with those obtained by Carter et al. [105) and Cox et al. [1061. On the other hand, the thermochemical estimate is greater than the experimental values by a factor of 100. The major source of this discrepancy appears to be in the estimated rate for the isomerization, since the value of k29 was based on experimental values for analogous reactions involving other alkoxy radicals [l08]. The spectral data obtained from the photolysis of n-C,H,ONO (5.7 ppm) in 1 Torr of 0,and 700 Torr of N, are illustrated in Figure 13. The low 0, pressure used in this run was chosen to make reaction (28) dominant over

104

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

reaction (29), yet be sufficient to sustain reaction (30).

CH,(OH)CH,CH,kH2

+ O2 + CH2(OH)CH2CH2CH,00'

(30)

The residual spectrum shown in Figure 13C is a result of spectral subtraction of all the identified products. This residual spectrum suggests the presence of several functional groups, i.e., HO (3647.2 cm-'), CHO (2718 cm-I), C=O (1824, 1750cm-'), OONO, (1740cm-'), and C(0)OH (1050, llOOcm-'). Many of these broad bands appear to consist of at least two contributing bands. The three bands indicated by asterisks in Figure 13 correspond to "nitrate (ON0,-type)" compounds formed during "aging" of the unknown products in the dark. Although the relative intensity distribution among the ( A ) t = 0 min

(B)t

I

= 5 min ( 35 70 Decay 1

( C ) Residual Spectrum ( x 5 )

1740

I

2000

3000 I/

X (ern-')

10T3.9

1000

Figure 13. Spectral data resulting from the photolysis of n-C,H90N0 (5.7 pprn) in 1 Torr of O2and 700 Torr of N,. (A) before irradiation;(B) after 5 min irradiation; and (C) residual spectrum from (B) (see text). The absorbance scale for (C) was expanded by a factor of 5 as compared with those for (A) and (B).

ATMOSPHERIC OXIDATION OF HYDROCARBONS

105

various absorption bands changes somewhat depending on the particular experimental conditions used, these spectral variations were not sufficiently distinct to permit the desynthesis of this composite residual spectrum into individual components. Nevertheless, the intense bands corresponding to stretching (1ooOO-H stretching (approx. 3650 cm- I ) and C-OH 1100cm - I ) modes provide evidence for the occurrence of isomerization, reaction (28). Baldwin et al. [lo41 and Carter et al. [lo51 suggested several possible subsequent reactions for the CH,(OH)CH2CH,CH202 radicals and the corresponding HO-containing products, e.g. HOCH,CH,CH,CHO, CH(OH),CH,CHO, and HOC(O)CH,CHO, and (HO),CO. However, residual spectra such as that in Figure 13C do not on their own allow unique distinction among all these possible products and their nitrates. Clearly, one of the important remaining tasks is to generate appropriate reference spectra for such positive identification.

C. Alkenes and Alkynes 1. HO Reaction

Alkenes. The available kinetic and mechanistic data show that under atmospheric conditions the reaction of HO radicals with alkenes proceeds predominantly via addition of the HO radical to the carbon-carbon double bond(s) [30]. The energized HO-adducts which result are rapidly thermalized to yield HO-substituted alkyl radicals which, in turn, undergo subsequent free-radical reactions leading to the formation of molecular products. Thus, the possible reactions in the HO-initiated oxidation are, in many respects, analogous to those of alkyl radicals described in the preceding section. Product studies on these reactions have mainly been made by the FTIR method [109-1 123. In an earlier long-path FTIR study of the photolysis of the HONO-NO-C2H,-0, system conducted in this laboratory [l09], the reaction of HO radicals with C2H4was shown to yield two HCHO molecules up to at least 80% of the time. These results suggest the occurrence of the following series of elementary reactions.

+ C2H, CH2(OH)kH2+ 0, HO

CH,(OH)CH,OO'

+

+ NO

CH,(OH)kH, CH,(OH)CH,OO'

(32)

CH2(0H)CH,O'

(33)

+

+

(3 1)

CH2(OH)CH20'--+ HCHO

+ NO2

+ CH20H

CH,OH + 0, -,HCHO + HOO-

(34)

(35)

106

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

The key reaction step for the formation of HCHO in this reaction scheme is the unimoiecular dissociation of the oxy-radical CH,(OH)CH,O, i.e., reaction (34). In this experiment, products arising from as much as 20% of the C2H4reacted could not be fully characterized due to experimental difficulties encountered in the product analysis due to the presence of large amounts of H 2 0 impurity in the HONO samples employed [l09]. Notably, thermochemical estimates for the unimolecular reaction (34) indicated the predominant occurrence of the competing bimolecular reaction with 0, under atmospheric conditions [104,108,113].

CH,(OH)CH,O'

+

0 2 + CHZ(0H)CHO

+ HOO'

(36)

However, it is difficult to explain the observed high yield of HCHO if reaction (34) is of negligible importance in the photooxidation of C,H4. In a subsequent study, the product characterization was greatly improved by the use of R O N 0 as a photochemical source of HO radicals [llO]. Several characteristic bands belonging to glycolaldehyde, CH,(OH)CHO, were positively identified in the product spectra recorded in this experiment. The fraction of the HO-C2H, reaction leading to the formation of CH,(OH)CHO was measured as a function of O2 pressure and corresponded to 21 f 4% at 700Torr air and 298K. These experimental results clearly disagree with the thermochemical prediction, but the reason for this discrepancy is not clear. For the HO-substituted alkoxy radicals formed from the higher alkenes, Atkinson et al. [112], Akimoto et al. [lll], and this group [log] have shown that unimolecular dissociation, analogous to reaction (34),is dominant over reaction with 0,, leading to the formation of aldehydes and HOO. For instance, the reaction sequence for the HO-initiated oxidation of 2-butene in the presence of NO is shown below.

HO

+ CH3CH=CHCH3 -+

CH,CH(OH)kHCH,

+ 0, -+ CH,CH(OH)CH(OO')CH, CH,CH(OH)CH(OO')CH, + NO -+ CH,CH(OH)CH(O')CH, + NO2 CH,CH(OH)kHCH,

(37)

(38) (39)

CH,CH(OH)CH(O')CH, + CH,CH(OH) CH,kH(OH)

+ CH,CHO

+ 0, -+CH,CHO + HOO'

(40)

(41)

The predominance of 0,-reactions for the a-hydroxy radicals, e.g., reactions (35) and (41), has been verified by Ohta et al. [1141, and by the authors' group

107

ATMOSPHERIC OXIDATION OF HYDROCARBONS

[lo91 using the C1 + alcohol reaction to generate the a-hydroxy radicals. Also, Atkinson et al. [112] have shown that for the CH3CH2CH(OH)CH20' radical formed from the internal addition of an HO radical to 1-butene, decomposition dominates over isomerization. Alkynes. The kinetics and mechanism for the HO-initiated oxidation of acetylene, propyne and 2-butyne under atmospheric conditions have been studied recently by Hatakeyama, Washida, and Akimoto using the FTIR detection method [llS]. HO radicals were generated either from H,O, at 254 nm using germicidal lamps or alkyl nitrites at >300 nm using black light lamps in an 11-L cylindrical quartz reactor equipped with multireflection mirrors to give a path length of 40 m. In these experiments deuterated R O N 0 (R = CD, or C2D,) was also used to generate DO radicals in order to distinguish the products of alkyne oxidation from the secondary products of the R O reactions. The rate constants at atmospheric pressure and 297 K were determined to be (8.8 & 2.0) x (5.71 If: 0.18) x lo-", and (3.01 & 0.28) x 10-'2cm3 molecule-' s-' for acetylene, propyne, and 2butyne, respectively, by the competitive decay method using cyclohexane as a reference compound (k(H0 cyclohexane) = (7.57 -t 0.05) x lo-'' cm3 molecule-' s-', determined by Atkinson et al. [116]). The rate constant for acetylene is in good agreement with the literature values, in particular with those measured directly by Wahner and Zetzsch [117] using the laser photolysis-laser absorption technique at high pressure, e.g. (8.3 & 0.6) x cm3 molecule-' s-' at 749 Torr and (8.1 k 0.7) x cm3 molecule- s-' at 771 Torr. The reported pressure dependence of this rate constant is consistent with an addition mechanism for the HO C,H, reaction. Hatakeyama et al. [115] have also carried out detailed product studies in the HO-initiated oxidation of the alkynes both in the presence and in the absence of NO,. The major products consisted of a-dicarbonyl compounds, i.e., HC(0)CHO from acetylene, CH,C(O)CHO from propyne and CH,C(O)C(O)CH, from 2-butyne, as well as HC(0)OH from acetylene and propyne and CH,C(O)OH from 2-butyne. The formation of these products was attributed to 0,-reactions of the hydroxyvinyl radicals resulting from the addition reaction of HO with the alkynes, e.g.,

+

+

HO + C2H, + (HO)CH=kH (HO)CH--~'H

+ 0,

(HO)CH=CHOO'

+

(HO)CH=CHOO-

+ HC(0)OH + HC(0)CHO + HO

+H

e0

(42) (43) (444 (44b)

108

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

Reaction (44a) is analogous to the 0,-reaction of the vinyl radical leading to HCO and HCHO as reported by Gutman and co-workers [lls, 1191. Evidence for the occurrence of reaction (44b) was based on the observation of a-dicarbonyl products in the absence of NO. Schmidt et al. [120] have recently observed the regeneration of HO from the HO + C2H2reaction by means of a laser fluorescence technique. The branching ratios for the two unimolecular dissociation channels typified by reactions (44a) and (44b) were estimated to be 0.4 f 0.1 and 0.7 f 0.3 (acetylene), 0.12 & 0.02 and 0.53 & 0.03 (propyne), and 0.12 _+ 0.87 f 0.07 (2-butyne), respectively. In any case, the formation of acidic products as well as the a-dicarbonyl products from these reactions is of potential importance in the atmosphere. 2. 0, Reaction. Amongst the various classes of organic compounds present

in the atmosphere, the alkenes are unique in exhibiting significant reactivity toward 0, as well as toward the HO and NO, radicals (cf. Figure 2). The available experimental data on the kinetics and mechanism for the 0,4- alkene reactions have been discussed in detail by Calvert et al. [121], Herron et al. [122], Niki et al. [123] and Atkinson and Carter [35]. Recent mechanistic studies under atmospheric conditions have been made mostly by means of the FTIR method, and the major findings of these results are highlighted below. A general scheme for the 0,-alkene reactions, which emerged over the years, can be represented by reaction (45) followed by reactions (45a)-(45d).

0

\ / \I C=c +o,+ c-c / \ /

\ .C-00 /

\. /

C-OO

molozonide

0

I/

\

-b

\. \ c-oo+ c=o /

/

Criegee intermediate

decomposition (e.g. CO, CO,, RH, R 0 2 , RCO, etc.) + isomerization -+

+ ( H 2 0 , SO2,NO) -+ products

ATMOSPHERIC OXIDATION OF HYDROCARBONS

109

The primary ozonide initially formed in reaction (45) decomposes rapidly to the Criegee intermediate [53] and the accompanying carbonyl product. The Criegee intermediate may isomerize to other more stable configurations prior to unimolecular and bimolecular processes, i.e., reactions (46a)-(46d). Thus, numerous competing reactions involving the Criegee intermediate and the ensuing secondary free radicals can give rise to the formation of a large variety of products. For the majority of the alkenes, these products are still poorly characterized. Ethylene. Reactions occurring in mixtures containing O 3 and C2H, in the ppm concentration range in air have been examined by Su et al. [123] and by the authors' group [124]. A major product, previously unidentified (compound X), was detected, and the kinetic and spectroscopic characterization of this compound was attempted. The representative spectral data and the results of the computer-aided data analysis are illustrated in Figures 14-16. CzH, (9.46)

( A ) t = O min

03(10.10) A 03

+-

( E ) t = 15min c02

el I

HCHO

r-.

HCHO

3000

I/

2000

1000

I / X,crn-' Figure 14. Product spectra in the 600-3200cm-' frequency region from the O3 CzHo reaction. Values in parentheses are concentrations in ppm. Bands labeled (a) and (b) are discussed in text.

+

110

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS ( A ) Difference Spectrum

( 9 ) Residual Spectrum From ( A )

1767

II05

I

( C )Formic Anhydride (CHO-O-CHO)(.35ppm)

I / A , cm-’

Figure 15. Analysis of the product spectra in Figure 14. (A) difference spectrum of Figures 14B and 14C; (B) residual spectrum obtained from (A) above; and (C) reference spectrum of formic anhydride. Values in parentheses are concentrations in ppm. Values without parentheses are frequencies.

Figures 14A-C illustrate typical time-resolved spectra recorded for the 0,-C2H,-air system. In addition to the sharp bands belonging to CO, CO,, and HCHO, broad bands indicated by A and B are seen in Figures 14B, C. Band A increases in height between reaction times of 15 and 40 min, while band B remains virtually unchanged. This temporal behavior is readily discernible in a scale-expanded display of the difference spectrum, Figure 15A, derived by subtracting Figure 14B from Figure 14C. The residual spectrum, Figure 15B, was obtained from this difference spectrum by removing contributions from known products. This spectrum matches that of formic anhydride (CH0)20shown in Figure 15C.

111

ATMOSPHERIC OXIDATION OF HYDROCARBONS

( A ) Residual Spectrum 1116 1117 11x4

I760

I

,o- 0,

( B ) EthyleneOzonide ( H2C,0,CH2)

1081,8 I

(C)Glycol Aldehyde ( H CO-CH2(OH))

860.5

1754.I I

n(. -

2000

1275.3

I500 I/

A , cm-'

I

1114.6

I

I

-PCI

P

1000

Figure 16. Analysis of the product spectra in Figure 14. (A) residual spectrum of Figure 14B;(B) and (C) are reference spectra of ethylene ozonide and glycolaldehyde.

The spectrum shown in Figure 16A was derived from Figure 14B by removing the spectra of the reactants and all the known products including (CH),O. This species (compound X) is neither of the two suspected products, ethyleneozonide or glycolaldehyde. Glycolaldehyde is a known product in the HO-initiated oxidation of C,H, as discussed previously [llO]. Note in Figure 16A that compound X exhibits two overlapping bands at 1737 and 1760cm-' in the C=O stretch region and three bands at approximately 1044, 1116, and 1170cm-' in the neighborhood of the C-0 stretch region. Also, in the high frequency region (not shown), two 0-H stretch bands were observed at 3405 and 3583cm-'. Su et al. [123] tentatively identified compound X as CH,(OH)-0-CHO. This being the case, the observed spectrum suggests the possible formation of two isomeric forms, as shown

112

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

schematically by the following configurations:

0---H

H

H-C .

\-

/” 0-CH,

H-C

\

cis form

HOCH,

//

\ /

0

0

trans form

The following simplified reaction scheme represents the formation of compound X via the reaction of HCHO with the thermally stabilized entity of the Criegee intermediate CHzOO [123]:

0, + C2H4 --* CHZ00*

+ HCHO

C H 2 0 0 * -+ dissociation products (z60%) CH200*+ M-+Y

+M

+ HCHO + CH,(OH)-0-CHO (compound X) CH,(OH)-0-CHO + wall -,(CHO),O + H, Y

(47) (48) (49) (50)

(51)

The identity of species Y remains uncertain. It was observed that compound X was formed in significant amounts even at air pressures as low as z 10 Torr [124]. Thus, a large fraction of the CH,OO appears to be produced initially “cold without sufficient internal excitation to induce unimolecular dissociation. Interesting observations were also made concerning the reactions of the CH,OO entity with CH,CHO and with SO,. The results obtained by adding an excess concentration of CH,CHO to the 03-C2H4 system are illustrated in Figures 17 and 18. The extent of the reactant consumption and product formation is indicated in Figure 17C and Figure 18 in the form of a difference spectrum. The most notable effect of the added CH3CH0 is the formation of prolylene ozonide presumably via reaction (52).

ATMOSPHERIC OXIDATION OF HYDROCARBONS

113

( A ) t = O min

CH, CHO

( B ) t 16 min

I

(C)DifferenceSpectrum[(R)-IA)] 1 x 2 )

I

3000

03(-.92)

1

I

2000

1000

-

I / X , cm-' Figure 17. Effects of added CH,CHO on the 0,+ C2H,reaction: Spectral data in the frequency region of 600-3200cm- (A) initial mixture containing 0, (15.7 ppm), C,H, (4.3ppm) and CH,CHO (13.1 ppm); (B) 6min after mixing; and (C) difference spectrum of (B) and (A) above.

':

The effect of added SO, on the product distribution has been reported by Su et al. [123], Hatakeyama et al. [125], and the authors' group [36]. The formation of compound X was suppressed to the extent to which SO, was converted to aerosol product. To illustrate, Figure 19 shows the results from a mixture initially containing 0, (5 ppm), C,H, (10 ppm), and SO2 (5 ppm) in 700 Torr of air. The difference spectrum (Figure 19A) represents the extent of the reaction during the course of 40-min reaction time. The material balance calculated for sulfur and carbon from this spectrum is seen to be poor, presumably because the missing products were incorporated into the unidentified compound(s) represented by the residual spectrum shown in Figure 19B. This residual spectrum exhibits two broad peaks at approximately 1180 and 1383 cm-I (-SO, symmetric and asymmetric stretching), one peak at

114

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

I

3000

I

2000

-._I-

1000

I / A , cm-'

Figure 18. Analysis of the product spectrum in Figure 17. Asterisks indicate composite bands used for the product analysis. Numbers in parentheses represent PPm.

955 cm-' (S-OH stretching?) and an extremely broad band extending from 2000 to 3500cm-' (hydrogen-bonded liquid phase 0-H stretch). In addition, a weak band is seen at about 1750cm-' in the C=O stretching region. Therefore, the chemical composition of the aerosol product(s) remains uncertain. However, from the aerosol products deposited on the reactor walls, Hatakeyama et al. [126] identified sulfuric acid but no other sulfurcontaining compounds, by means of GC and GC/MS. Shown in Figure 20 are a residual spectrum recorded in the 03-C3H,-S0,-air system and the H2S04aerosol spectrum generated in the HO-S02-NOx-air system. These two spectra are seen to be nearly identical. However, it was noted that the three conspicuous bands centered at 970, 1170 and 1375 cm-' in Figure 20 became much broader and diffuse with aging of the aerosol samples, presumably due to coagulation and incorporation of H 2 0 . Also, in the latter system aerosol products could be produced in a much shorter period than in system. Thus, the aerosol spectrum obtained from the the O3--C2H4-S0, 03-C2H4-S02 system (Figure 19B),may be indicative of the aging of pure, dry H2S04 aerosols produced initially.

ATMOSPHERIC OXIDATION OF HYDROCARBONS

( A ) D i f f e r e n c e Spectrum (

Af= 40-2

115

min 1

i:

CH O ( 3 . 0 )

(€3)

R e s i d u a l Spectrum ( x 2 )

2000

3000

1000 -I

Frequency (cm )

+

Figure 19. Effects of added SO, on the 0, C2H, reaction. (A) initial mixture contained O3 (Smppm), C,H, (10ppm) and SO, (5 ppm) in air at 700Torr; and (B) residual spectrum of (A) above.

Hatakeyama et al. [126] also determined the product distribution in 0,-C,H,-S02 mixtures at air pressures ranging from 10 to 1140 Torr. On the basis of the observed effects of added SO, on the yields of HCHO and HC(0)OH and sulfuric acid, the following mechanism for the reaction of the Criegee intermediate with SO2 was proposed. H CH2OO + SO2 -+

\ /

0-0

\

S=O

(adduct)

(53)

H adduct + HC(0)OH + SO,

(54)

adduct

(55)

+ SO2 -,HCHO + SO, + SO,

116

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

( A ) R e s i d u a l S p e c t r u m From 0,-C,H,-SO,

' (B)

HS , O,

f

II

Spectrum From HO-S02-N0,

I

I

3000

2000

O I

970

I

1000

Frequency (em-')

Figure 26. Residual spectrum from an ozone-propylene-SO, reaction and H ,SO, aerosol spectrum from the HO-initiated oxidation of SOz (see text).

The SO3 is readily converted to sulfuric acid in the presence of water vapor [6,7]. An experiment employing '*O, provided evidence for acyclic adduct formation as originally proposed by Martinez et al. [127]. The ratio k,,/k,, was calculated to be (4.9 4 2.0) x cm3 molecule-'. At any rate, the reaction of C H 2 0 0 with SOz does not appear to be a simple bimolecular 0atom transfer reaction. Among remaining questions concerning the potentially important atmospheric role of the 0, + C2H, reaction is the bimolecular reaction of CHzOO with HzO. Cox and Penkett [128] found that the SO, oxidation in cis-2-butene-03-air system was greatly reduced when water vapor existed in the system. Calvert et al. [ l Z l ] argued that the effect might be due to the occurrence of the reaction of the Criegee intermediate with H 2 0 . CH,CHOO

+ H,O --+ [CH,CHO.H,O]

+ CH,C(O)OH

+ H,O

(56)

117

ATMOSPHERIC OXIDATION OF HYDROCARBONS

Akimoto et al. [111] observed a marked increase in the HC(0)OH yield in photooxidation of the propylene-NO,-air system upon addition of H 2 0 and suggested that the effect may in part be explained by a reaction analogous to reaction (56). C H 2 0 0 + H 2 0 + [CH200-H20] -+ HC(0)OH + H,O

(57)

These investigators have obtained further evidence for reaction (57) in the near-UV photolysis of CH2C=0 and CH2N2[129]. Namely, the C H 2 0 0 O2 + M + CH,OO + M was shown formed in the reaction of CH,(,B,) to react with H 2 I 8 0 to produce "0-labeled formic acid: C H 2 0 0 + H2"0 -+ HC("O)OH, HC(O)I80H + H 2 0 .

+

+

Tetramethylethylene. Very recently, product studies on the 0, tetramethylethylene (TME) reaction have been made by the authors' group in attempts to probe various reaction channels operative for the dimethylsubstituted Criegee intermediate (CH,),C-00 under atmospheric conditions [1301. Among the potentially important reaction channels suggested by previous theoretical studies [131] and experimental results in the gas- and solution-phases [122,1323 are the bimolecular reaction with aldehydes (reaction (46c)) and the following unimolecular processes: CH,

CH,

\ c-00 /

-

CH,

dissociation

-

CH,

CH,

isomerization

\ c=o + o(3~) / \ / /'\

CH3

0'

0 -+

0'

II

CH,COCH:

4

products (CH,; CH,O; CH,CO; CO; C 0 2 ) OOH H migration

I

+ CH,=C-CHf

(59)

0

II

-+ CH,CCH,

+ HO

(60)

The intermediate species indicated by the asterisks are in chemically activated states which might be collisionally stabilized, in part, by atmospheric pressure. Reference spectra generated for the expected CH,C(O)containing products in the 0,-TME reaction are illustrated in Figure 21. It can be noted that reactions (58)-(60) all yield relatively simple and

118

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

(A)

0 0 II II

1733

Ct$-C-CH

I

L L 1.

0

(El

OH II I CH3-C-CHz

(C)

CH3-C-O-CH3

1741.5 I

0 II

1107.5 I I

1770 ['24g'8

1231.1

/

1070.9

1386.0-

L

3600

I

I

I

2000

3000

I/

x

(cm")

1000

I 600

Figure 21. IR absorbance spectra of CH,C(O)X (X = CHO, CH,OH, and OCH,) and isobuteneozonide. Approximate frequencies are indicated for the characteristic bands of each X group.

distinct free radicals whose subsequent oxidaion reactions and products are relatively well characterized, as described earlier. Thus, information concerning the role of these dissociation channels could be deduced to a large extent from a knowledge of the reaction stoichiometry and product distribution in reaction. Also, several experiments have been designed to the 0,-TME obtain more direct evidence for particular secondary reactions. For instance, the use of "0,in the 0,-TME reaction in air could lead to the formation of 1 6 0 3 if reaction (58) is operative, i.e., "0 1 6 0 2 -+ l6O -t 160180 followed by l60 l6OZ(+M) + 1 6 0 3 (+M). Also, in the presence of added NOz, some of the free radical products R from reactions (59) and (60) may be detectable in the form of the corresponding ROONO,. Finally, attempts

+

+

119

ATMOSPHERIC OXIDATION OF HYDROCARBONS

were made to characterize the secondary reactions expected to follow from reaction (60) by using photochemical generation of the HO and CH,C(O)CH, radicals. On the basis of the results, described briefly below, it was concluded that approximately 25% of the (CH,),COO moiety formed was thermally stable, that the remainder dissociated largely via reaction (60) to yield CH,C(O)CH, and HO radicals, and that the HO-initiated oxidation of TME accounted for the reaction stoichiometry as well as some of the major products observed. Displayed in Figure 22 are typical spectral data from this system in the frequency region of 900-3700cm-'. Figure 22A is a synthesized spectrum ( A ) 0,&3ppm)+TME ( 7 p p m ) + A I R ( 7 0 0 torr)

(C ) Residual x 5

I

I

I

3700

3000

2000

I/

X

I

1000

(cm-')

Figure 22. Spectral data in the frequency region 900-3700cm-1 from a mixture containing 0, (3 ppm) and tetramethylethylene (7 ppm) in 700 Torr of air. (A) synthesized spectrum corresponding to the initial reactant mixture prior to the reaction; (B) recorded after approximately 1 min of mixing; and (C)residual spectrum from (B) above.

120

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

corresponding to a mixture containing 0,(3 ppm) and TME (7 ppm) in 700 Torr of air prior to initiation of the reaction, and Figure 22B was actually recorded after approximately 1 min of mixing. The stoichiometry [TME]/[O,] was determined to be 1.76, and the reaction products included CO (6%), COz (53%), HCHO (73%), HC(O)OH (<2%), CH3OH (7%), CH,OOH (16%), CH,C(O)OOH (373, (CH3),C0 (168%), CH,C(O)CHO (15%), and CH,C(O)CH,(OH) (1 l%), where the percentages represent yield relative to the amount of O3 consumed. Also, CH,C(O)OCH,, which is a possible product arising from the collisional stabilization of the intermediate in reaction (59), could not be positively identified and its yield was estimated to be < 3% of A[03J. The residual spectrum in Figure 22C was derived from Figure 22B by subtracting the spectral contributions of the reactants and all these identified products. Tentative assignments of the observed bands are C-0 stretching (1165 cm-I), C-CH3 bending (1375 cm-’), C-0 stretchstretching (2990 cm-’) and 0-H stretching ing (1745 cm-’), C-H (3600 cm- I). In several runs on this system carried out at different reactant concentrations, the relative intensities of all these bands excepting the c----O band remained essentially unchanged. Therefore, the C=O band must belong to a minor carbonyl product. Approximately 75% of the TME consumed could be accounted for by the identified products, and thus, the remainder is accounted for by the product corresponding to the residual spectrum. This spectrum was shown later to belong to an ROOH species formed by the ROO + H 0 2 reaction, where R was the HO-TME adduct. Analyses of products from experiments performed with isotopically labeled “0,showed that the CO and CO, produced were predominantly ( > 90%) Cl6O and C ’ 6 0 1 8 0 ,and must therefore have been formed via free radical reactions involving 1 6 0 2 rather than directly from reaction (59). Also, no formation of I6O3was observed, ruling out reaction (58) as well. It therefore seems likely that the excess stoichiometry A[TME)/A[O,] = 1.7 was caused by the consumption of TME in a secondary reaction most likely involving the HO radical formed in reaction (60). In addition, many of the observed products could be explained by reactions originating from the HO radical and the accompanying radical product CH,C(O)CH,. Spectroscopic evidence for the formation of the latter radical specie was obtained by observation of the corresponding peroxy nitrate in the 0,-TME-NO,-air system. The residual spectrum shown in Figure 23A was obtained from such a run by subtracting the spectral contributions of the reactants and all the products mentioned earlier. This spectrum exhibits the characteristic “peroxynitrate (-OONO,)” bands (approx. 790, 1300, and 1723 cm-I). The “nitro (-NO,)” bands (857, 1297, and 1655 cm-I) did not belong to the same product but were due to the occurrence of the sidereactions NO, + 0,+ NO3 + NO, and NO3 + TME + NO,-containing

121

ATMOSPHERIC OXIDATION OF HYDROCARBONS

-

(A) Residual 0C , I) -TME (2) NO,( 10)

I297

II

I

( B) Residual O& I ) -TME (2)"NO,(

0 OONO,

CH3-C-hH2

I

10)

o

II

(C) Residual CI2(15)- MEK ( 5 ) -NO2 ( 5 ) I726

- H V (45 sec)

CH,-c

I

2000

I/

0

I

I

1500

x ( cm-'1

0 0 ~ ~ ~ 0 , I

C - CH,

CH3-

00N02

- LH - CH,

1000

I

700

Figure 23. Comparison of residual spectra containing c--O and OONO, bands from the 0,-TME-NO,-air system. (A) 0,-TME- 14N02 system; (B) 0,-TME''NO2 system; and (C) photolysis of a mixture containing CI,, methyl-ethyl ketone, and NO, in air. Tentative identification of the product species is indicated.

product, as described later. This compound should also exhibit a C=O band in the vicinity of the - 0 O N 0 , band centered at 1723 cm-'. Indeed, a C==O band could be observed at 1747 cm- when the - 0 0 N 0 , band was down shifted by '5N-substitution (cf. Figure 23B), thus in the I4N-system, this c=O band is masked by the prominent -0ON0, band. To obtain further evidence that the observed compound was indeed (CH3)C(0)CH200N02, an attempt was made to generate this compound in situ photochemically by the C1-atom initiated oxidation of (CH,),CO in the presence of NO, [130]. The C1 + (CH3),C=0 reaction turned out to be too cm3 molecule-' s-', to obtain a sufficiently high slow, i.e., approx. 1 x

122

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

yield of this labile peroxynitrate for a reliable spectral characterization. On the other hand, the next higher alkyl ketone CH,C(0)CH2CH, (MEK) was found to react rapidly with C1 atoms, i.e., 3.8 x lo-" cm3 molecule-' s-', and led to the formation of the expected carbonyl-peroxynitrate (cf. Figure 23C). This spectrum exhibits an -0ON0, band analogous to those in Figure 22A, and upon ',N-labeling, revealed a C=O band at 1738cm-'. Results of detailed product studies in the C1-initiated oxidation of both acetone and MEK have provided further evidence that reaction (60) is the predominant unimolecular reaction in the 0, + TME reaction [133].

3. NO, Reaction. The NO, radical has been identified as an important reactant in the night-time chemistry of the atmosphere and has been the subject of a number of laboratory and field studies [2,134-1381. This radical was first shown to react rapidly with alkenes and aldehydes in this laboratory using a long path-grating IR system over a decade ago [134,135]. In these studies, NzO, was employed as a source for the NO, radicals and the presence of NO, was inferred from the equilibrium relationship N 2 0 , -NO, + NOz [6,7]. More recently, Pitts's group have measured relative reactivities of a large number of organic compounds toward NO, by GC, using a competitive method [136], and Ravishankara and Mauldin [137] determined directly the absolute rate coefficient for the NO3 + trans-2butene reaction, (3.78 f 0.41) x lo-', cm' molecule-' s-' at 298K. The NO, radical is now known to be two to four orders of magnitude more reactive than 0, toward alkenes, tending to react electrophilically by adding to the carbon-carbon double bond. The available data on detailed reaction mechanisms for the NO,-initiated oxidation of alkenes are still very limited. However, it is evident that the primary addition product, i.e., -0N0, substituted alkyl radical, governs the subsequent reactions leading to the formation of the observed products. ONO,

NO,+

\ / I . C S -+-CX/ \ / \ ON0200'

ONOz

I

.

4-c/

\

+

0 2

I

-b

-c-c-

I

\

Namely, in the N,O,-C,H,-air system, Japar and Niki [135] observed an IR absorption band near 1670 cm-' corresponding to a product containing the -ONOz group. Akimoto and co-workers positively identified propanediol dinitrate (PPDN, CH,CH(ON02)CHz(ON0,)) as a major

ATMOSPHERIC OXIDATION OF HYDROCARBONS

123

product in this system [139], and also, as a minor nitrogen-containing photooxidation product in their smog chamber study of the C3H6NO,-air system [lll]. In the C3H6-N,O,-0,-N, system, these investigators later observed new IR bands consistent with the formation of nitroxyperoxy nitrate (NPPN, CH,CH(ONO,)CH,(OONO,) and/ or CH,CH(OON02)CH2(ON02)) and nitroxypropyl nitrite (NPN, CH,CH(ONO,)CH(ONO) and/or CH,CH(ONO)CH,(ONO,)) in the presence and absence of 02,respectively [140]. These two products were suggested to be formed in the NO,-reactions of the ROO (reaction (62)) and the R (reaction (61)) radicals.

ONO, OONO,

ONO, 00'

I

I

- C d -

/

\

I

/

ON02

I

-42-C-

.

/

\

I

+ NO2 -4d\

ONO, O N 0

+ NO2

I

-P

I

4-C-

/

\

NPPN should be in fast equilibrium with its precursor ROO, as given -y the forward reaction (63) and the reverse reaction (- 63). Based on the analogous reactions of ROO radicals (R = alkyl group), these investigators estimated an 1 x loi2 molecule cm-3 at 300K. equilibrium constant K = k 3 6 / k - 3 6 NPPN was shown to decay rapidly to yield nearly equimolar concentrations of HCHO, CH,CHO and PPDN. A plausible reaction scheme for the formation of these products is the self-reaction of R 0 2 , to yield the corresponding RO which, in turn, undergoes both unimolecular dissociation and bimolecular reaction with NO,, e.g.,

ON02 0'

I

I

C H 3 C H 4 H 2+ CH,CHO ONO, 0'

I

I

+ NO + HCHO

(65)

ONO,ONO,

I

C H 3 C H 4 H 2 + NO2 + CH3C-CH2

I

1-Methylethyl nitrate CH,CH(ONO,)CHO arising from another expected reaction channel, RO + 02,was tentatively identified by GC-MS and FTIR, but its yield was seemingly very low because of the dominant occurrence of reaction (64) at the high NO, concentrations employed in the studies by Akimoto and co-workers. Indeed, Shepson et al. [141] recently employed

124

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

much lower ( x 1/10) NOz concentrations in this system, and observed the expected a-(nitrooxy)acetone as a major product. Very recently, the NO, + tetramethylethylene (TME) reaction was studied by this group in the NO,-TME-air system [130]. The rate constant for this reaction is known to be nearly collisional and four orders of magnitude greater than that for the corresponding reaction of C,H, [136]. As a result, the expected unstable ROONOZ, i.e., [(CH3),C(0N0,)-C(CH,),-00N0,], could be formed rapidly as the predominant product without interferences from its decay products. Also, since TME is a symmetrically substituted alkene, only one form of ROONOz (A)

N205 (1.4)-N02(4.0)-HN0,(0.7)

( ppm)

( 8 ) Plus T M E ( 2 p p m ) , t " 2 m i n

( C ) Residual ( X 3 )

1295 (1281 1

2000

1500

I/X(cm-')

1000

Figure 24. Product spectra from the dark reaction of NO, with TME in the presence of NOz and air. (A) mixture containing N,O, (1.4 ppm), NO, (4.0 ppm), and HNO, (0.7 ppm); (B)addition of TME (2 ppm) to (A) above; and (C)residual spectrum from (B).

ATMOSPHERIC OXIDATION OF HYDROCARBONS

125

is possible. A residual spectrum attributable to this compound was observed in mixtures containing N,O,, NO,, and TME in air, as illustrated in Figure 24. Namely, in an N205-N0, mixture (Figure 24A), NO, was formed through the equilibrium N 2 0 , t,NO, + NO, and upon addition of TME, N,O, was consumed completely in less than 2 min due to the reaction between NO, and TME (Figure 24B). The residual spectrum (Figure 24C) was derived from Figure 24B by subtracting all the species indicated in Figure 24A. The spectrum exhibits the characteristic - 0 0 N 0 2 bands (789, 1295, and 1722cm-'), and the -0N0, bands (856,1295, and 1722cm-l). Frequencies given in parentheses are those observed by using 15N205and 15N02as the reactants, and the magnitudes of the frequency downshifts are as expected for these groups. Also, this compound was found to be unstable, decaying to acetone and NO, by reactions analogous to reaction (65).

D. Aromatics 1. HO-Radical Initiated Oxidation. Aromatic hydrocarbons are important constituents of the troposphere, particularly in urban atmospheres [30]. However, the reaction pathways occurring during their atmospheric photooxidation are still incompletely understood, primarily because of a lack of knowledge of the relevant reaction products [23, 30,44, 1411. Experimental and modeling studies suggest that reaction with the HO radical is the sole gas-phase chemical process governing the atmospheric fate of aromatic hydrocarbons. According to recent product data obtained by the FTIR method [143-1461, the HO + aromatic reactions proceed via two pathways, namely, H-atom abstraction from the substituent alkyl groups and HO radical addition to the aromatic ring, e.g., for toluene (&CH,)

The HO-toluene adduct corresponds to the methyl-substituted 0-, m- and phydroxycyclohexadienyl radical isomers. The kinetics of formation and of unimolecular dissociation of HO-aromatic adducts have been studied extensively [30]. The benzyl radical &CH, is converted to the aldehydic product +-CHO in the presence of NO via a series of reactions analogous to those involving simple alkyl radicals. Bandow et al. 1144-146) have determined the yields of aromatic aldehydes to be < 12%of the overall reactions of toluene, xylenes, and trimethylbenzenes, and thus, the H-atom abstraction channel is relatively small but significant. In the case of $J-CHO, the

126

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

subsequent HO-initiated oxidation can give rise to peroxybenzoyl nitrate (PBzN, 4-C(0)O0NO2)[9l, 1471and phenyl radical [4'] via reactions (69)(71).

+ +CHO &CO' 4-CO + 0, (b-C(O)OO. @2(0)00* + NO + 4' + C02 + NO2 4-C(O)OO' + NO2 4-C(O)OONO2

HO

(68)

4

(69)

4

+

(70) (PBzN)

(71)

Not much is known about the gas-phase oxidation of the phenyl radical. In the CI-atom initiated oxidation of &CHO in the presence of NO, Niki et al. [1481 identified o- and p-nitrophenol (HO-&NO2) as the major products. The most likely precursor for HO-+NO2 appears to be the phenoxy radical 4-0which is formed by:

4. + 0

4-00. 4-00. + NO 4-0. + NO2 2 --t

(72) (73)

In contrast to the simple alkoxy radicals, the 4-0 radical is seemingly quite In addition, the odd electron in the 4stable against further oxidation by 02. 0 is delocalized by resonance, and can facilitate o- and p-addition of NO, to the benzene ring. The phenolic H-0 bond of p-nitrophenol must be formed intermolecularly,whereas an intramolecular H-atom transfer cannot be ruled out for the formation of o-nitrophenol. The detailed mechanism for these final steps is presently unknown. Major kinetic and mechanistic uncertainty remains concerning the subsequent reaction steps for the HO-aromatic adducts under atmospheric conditions. It has been proposed that the HO-aromatic adduct can react with O2 by two routes [2,30,149,150]. HO-&CH3 + 0, + HO-4-CH3

+ HOO'

+ HO-+(CH&OO'

(74) (75)

For toluene, Atkinson et al. [142] have obtained an o-cresol yield of 13 _+ 7"/,, independent of total pressure from 62 to 740Torr of air. When combined with the o-cresol/total cresol formation ratio of approx. 0.8 [149,150], this leads to a total isomeric cresol yield of approx. 16 f 8%. The observation of the a-dicarbonyls glyoxal (CH(O)CHO), methylglyoxal (CH,C(O)CHO), and biacetyl (CH,C(O)C(O)CH,) in significant

ATMOSPHERIC OXIDATION OF HYDROCARBONS

127

yields from methyl-substituted aromatic hydrocarbons indicates that ring cleavage is also an important overall reaction pathway [Z]. Thermochemically feasible reaction sequences for the HO-aromatic-0, adducts (as formed in reaction (75)) leading to the formation of these a-dicarbonyls and accompanying products have been discussed in detail by Atkinson et al. [30], Killus and Whitten [lsl], and Atkinson and Lloyd [27]. Shown in Figure 25 is a reaction scheme for the ring cleavage of the HO-toluen-0, adduct via cyclization as proposed by Atkinson et al. [143]. Recently, the yields of the a-dicarbonyls from benzene, toluene, the xylenes and the trimethylbenzenes have been determined quantitatively by Bandow et al. [144-1461 using the FTIR method, and by Tuazon et al. [152,153] using the FTIR method in combination with UV differential optical ab-

a:., CH3

1

00 I1 II

00 II II

HCCH

CH3CCH

HOO

HOO

+

+

Figure 25. A reaction scheme for the HO-toluene-0, adduct (see text).

128

ATMOSPHERIC REACTIONS INVOLVlNG HYDROCARBONS

sorption spectroscopy (DOAS) and gas chromatography. In these studies, the observed a-dicarbonyl yields had to be corrected for reaction with HO radicals and photolysis in order to derive the formation yields of these compounds. The results reported by both groups are generally in good agreement and are, in most cases, within the combined stated error limits. For example, in the case of toluene, the formation yields of HC(0)CHO and CH,C(O)CHO were (15 k 4)% and (14 k 4)% 11521, and (10.5 & 1.9)% and (14.6 & 0.6)% [153], respectively. The overall a-dicarbonyl yields were -20% for benzene, 25-30% for toluene, 30-50% for the xylenes, and 60% for the trimethylbenzenes. According to the reaction scheme shown in Figure 25, unsaturated 1,4dicarbonyls, i.e., (CHO)CH=CH(CHO), (CH,CO)CH=CH(CHO), and (CH,CO)CH=CH(CH,CO), should be formed as counterpart products of the a-dicarbonyls in the ring cleavage reactions of aromatic hydrocarbons. In the HO-initiated oxidation of p-xylene and 1,2,4-trimethylbenzene,Takagi et al. [154] earlier reported the observation of the expected 1,4-diacarbonyl (CH,CO)CH=CH(CH,CO) by the GC/MS method among the products collected at liquid N, temperature. In subsequent FTIR studies of these reaction systems by both Bandow et al. [145] and Tuazon et al. [153], broad absorption bands in the region of skeletal vibrations (1100- 1400 cm- ') in the residual spectra suggested the possible formation of (CH,CO)CH=CH(CH,CO), the absorption features being consistent with the presence of more of the cis form than of the trans form. However, estimates of yields could not be made reliably due to the presence of interfering absorptions from unidentified products. Recent mass spectrometric product studies by Shepson et al. [l55] and Dumdei and OBrien [156] revealed the formation of a large number of polyfunctional products including those arising from ring-opening, e.g., CH,C(O)-CH=CH-CH=CH-CHO from toluene [1561. Thus, a reaction scheme other than that in Figure 24 may account for the poor material balance, e.g., approx. 50% for toluene. Clearly, further quantification of reaction products by means of various complementary analytical tools is urgently needed to establish the reaction pathways occurring in the HOinitiated oxidation of aromatic hydrocarbons under atmospheric conditions.

-

-

-

2. Unsaturated 1,4Dicarbonyl Reaction. Bandow et al. [144,145] observed the formation of maleic anhydride from benzene, toluene, and o-xylene possibly arising from the HO-initiated oxidation of the expected 1,4dicarbonyl (CHO)CH=CH(CHO), i.e.,

129

ATMOSPHERIC OXIDATION OF HYDROCARBONS

O=CH

HO

\

+

/

/

c=c

H O=CH

\

/

H

CH=O

\

CO'

O=CH +

H

H O=CH

+o,+ H

OCH H

/

'c=c' /

\

H

c=o

\

\

H

\ c=c / \

\=*/c=o

H

\

+NO,

(78)

H

/"\ /

(77)

H

H

O=C +02+

(76)

H

/

H

+NO+

+ H,O

/c(0)OOa

O=CH

C=C

CO'

\

c=c

O S H

H

\ / c=c / \

+ HOO'

(80)

H

Tuazon et al. [1571 prepared cis- and trans-(CH,CO)CH=CH(CH,CO) samples and recorded their absorption spectra by FTIR spectroscopy. A standard spectrum of the trans-isomer was also obtained by Bandow et al. [145]. The latter investigators attempted syntheses of other 1,4-dicarbonyls. However, these compounds were readily polymerized and could not be purified. Tuazon et al. [157] also studied the photolyses of cis- and trans(CH,CO)CH=CH(CH,CO) and the reactions of these isomers with HO radicals and O3 at 298 f 2 K. For the photolysis experiments, the radiation from a 25 kW xenon arc solar simulator was filtered by passage through a Pyrex pane to eliminate radiation at <300 nm and the resulting spectral distribution approximated that of the lower tropospheric solar spectrum

130

ATMOSPHERIC REACTIONS INVOLVING HYDROCARBONS

[158,1591. Under these irradiation conditions, photoisomerization to the corresponding cis or trans isomer accounted for 2 80% of the total photolysis rate (approx. 1 x s- l), with a photoequilibrium [trans]/[cis] ratio of 0.55 f 0.05. The rate constants for the cis and trans isomers have been determined to be, respectively, (1.8 f 0.2) x lo-" and (8.3 k 1.2) x 10-'*crn3 molecule-' s - ' for reaction with O3 and (6.3 k 0.6) x lo-'' and (5.3 _+ 0.5) x lo-" an3molecule-' s - l for reaction withHO radicals [157]. On the basis of these results, these investigators concluded that under atmospheric conditions reaction with HO radicals would be competitive with, or possibly more important than photodissociation. Tuazon et al. [157] also reported the detection of as yet unidentified absorption bands due to the products arising from the photolysis of the (CH,CO)CH==CH(CH,CO) isomers and their reaction with HO radicals. For the HO radical reactions, an alkoxy radical is expected to be formed by the sequence of reactions (15)-(17).

HO

+ (CH,CO)CH=CH(CH,CO) + (CH,CO)CH(OH)CH(CH,CO)

(CH,CO)CH(OH) kH(CH,CO)

+ 0,

(81) +

(CH3CO)CH(OH)CH(CH3CO)OO'

(CH3CO)CH(OH)CH(CH3CO)OOa + NO+

(CH3CO)CH(OH)CH(CH3CO)O'+ NO,

(82)

(83)

On the basis of analogous reaction systems, it would be expected a priori that decomposition of this alkoxy radical to methylglyoxal CH,C(O)CHO would predominate [27]. CH,C(0)CH(OH)CH(O')C(O)CH3+ (CH,CO)kH(OH) CH,C(O)eH(O)

+ 0,

+

CH,C(O)CHO

+ CH,C(O)CHO

+ HOO'

(84)

(85)

However, CH,C(O)CHO was not observed during these irradiations, and these investigators suggested the dominant occurrence of other reaction pathways such as isomerization or 0,-reaction, e.g., CH,C(O)CH(OH)CH(O')C(O)CH,+ CH,C(O)CH(OH)CH(OH)C(O)CH, (86)

REFERENCES

+0

CH,C(O)CH(OH)CH(O')C(O)CH,

131

2+

CH,C(O)CH(OH)C(O)C(O)CH,+ HOO'

(87)

With current estimates of the rates of alkoxy radical reactions [27], isomerization is likely to be the more important of these latter two processes. Clearly, a better understanding of these reactions is required before their role in the atmospheric degradation of aromatic hydrocarbons can be assessed.

ACKNOWLEDGMENTS We wish to thank the following researchers who generously provided preprints and unpublished materials: H. Akimoto (National Institute for Environmental Studies, Japan); K. H. Becker (Bergische University, W. Germany); J. G. Calvert (National Center for Atmospheric Research); C. J. Howard (NOAA) and S. P. Sander (JPL). We are grateful to C. M. Savage (Ford), P. B. Shepson, Carol Francis and G. Yarwood at York University for giving us assistance in completing the manuscript. The financial support to one of us (H.N.) by the Natural Sciences and Engineering Research Council of Canada and by the Atmospheric Environmental Service of Canada is greatly appreciated.

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132. P. S. Bailey, Ozonation in Organic Chemistry, Vol. 1, Academic Press, New York, 1978. 133. Unpublished results obtained by the authors’ group. 134. E. D. Morris, Jr., and H. Niki, J . Phys. Chem. 78, 1337 (1974). 135. S. M. Japar and H. Niki, J. Phys. Chem. 79, 1629 (1975). 136. R. Atkinson, C. N. Plum, W. P. L. Carter, A. M. Winer, and J. N. Pitts, Jr., J. Phys. Chem. 88, 1210 (1984). 137. A. R. Ravishankara and R. L. Mauldin 111, J. Phys. Chem. 89, 3144 (1985). 138. D. Perner, A. Schmelterkopf, R. H. Winkler, H. S. Johnston, J. G. Calvert, C. A. Cantrell, and W. R. Stockwell, J. Geophys. Res. 90, 3807 (1985). 139. M. Hoshino, T. Oka, H. Akimoto, G. Inoue, F. Sakai, and M. Okuda, Chem. Lett. 1367 (1978). 140. H. Bandow, M. Okuda, and H. Akimoto, J. Phys. Chem. 84, 3604 (1980). 141. P. B. Shepson, E. 0.Edney, T. E. Kleindienst, J. H. Pittman, and G. R. Namie, Environ. Sci. Technol. 19, 849 (1985). 142. R. Atkinson, W. P. L. Carter, and A. M. Winer, .I. Phys. Chem. 87, 1605 (1983). 143. R. Atkinson, W. P. L. Carter, K. R. Darnall, A. M. Winer, and J. N. Pitts, Jr., Int. J . Chem. Kinet. 12, 779 (1980). 144. H. Bandow and N. Washida, Bull. Chem. SOC. Jpn. 58,2549 (1985). 145. H. Bandow and N. Washida, Bull. Chem. SOC.Jpn. 58,2541 (1985). 146. H. Bandow, N. Washida, and H. Akimoto, Bull. Chem. SOC.Jpn. 58,2531 (1985). 147. J. M. Heuss and W. A. Glasson, Environ. Sci. Technol. 2, 1109 (1968). 148. H. Niki, P. D. Maker, C. M. Savage, and L. P. Breitenbach, in Nitrogenous Air Pollutants, Chemical and Biological Implications, D. Grosjean, Ed., Ann Arbor Science, Ann Arbor, MI, 1979, p. 1. 149. M. Hoshino, H. Akimoto, and M. Okuda, Bull. Chem. SOC. Jpn. 51, 718 (1978). 150. R. A. Kenley, J. E. Davenport, and D. G. Hendry, J . Phys. Chem. 85,2740 (1981). 151. J. P. Killus and G. Z. Whitten, Atmos. Enuiron. 16, 1973 (1982). 152. E. C. Tuazon, R. Atkinson, H. MacLeod, H. W. Biermann, A. M. Winer, W. P. L. Carter, and J. N. Pitts, Jr., Enuiron. Sci. Technol. 18, 981 (1984). 153. E. C. Tuazon, H. MacLeod, R. Atkinson, and W. P. L. Carter, Enuiron. Sci. Technol. 20, 383 (1986). 154. H. Takagi, N. Washida, H. Akimoto, and M. Okuda, Spectrosc. Lett. 15, 145 (1982). 155. P. B. Shepson, E. 0.Edney, and E. W. Corse, J . Phys. Chem. 88,4122 (1984). 156. B. E. Dumdei and R. J. OBrien, Nature (London) 311, 248 (1984). 157. E. C. Tuazon, R. Atkinson, and W. P. L. Carter, Environ. Sci. Technol. 19, 265 (1985). 158. A. M. Winer, R. A. Graham, G. J. Doyle, P. J. Bekowies, J. M. McAfee, and J. N. Pitts, Jr., Adu. Enuiron. Sci. Technol. 10, 461 (1980). 159. C. N. Plum, E. Sanhueza, R. Atkinson, W. P. L. Carter, and J. N. Pitts, Jr., Enuiron. Sci. Technol. 17, 479 (1983).

Advances in Photochemistry, Volume15 Edited by David H. Volman, George S Hammond, Klaus Gollnick Copyright © 1990 John Wiley & Sons, Inc.

EXCITED STATE REACTIVITY AND MOLECULAR TOPOLOGY RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES Hans-Dieter Becker Department of Organic Chemistry, Chalmers University of Technology and University of Gothenburg, S-412 96 Gothenburg, Sweden

CONTENTS I. Introduction 11. Substituent effects on excited state properties of

nonconjugatively linked anthracenes A. Di-9-anthrylmethane derivatives B. 1,2-Di-9-anthrylethanesand related two- and three-atom linked anthracenes C. Triplet state isomerizations 111. Excited state properties of 9-anthrylalkenes A. Molecular topology and photochemical reactions of cistrans isomeric 1,2-di-9-anthrylethylenesand related di-9anthrylalkenes B. Geometrical isomerization of 9-anthrylalkenes C. Fluorescence properties of 9-anthrylalkenes and 1,2-di-9anthrylethylenes IV. Solvent-assisted intramolecular anthracene :arylcarbonyl interactions 139

14

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

V. Adiabatic photochemical cycloreversions involving anthracenes A. Photolytic adiabatic fragmentation of anthracene adducts B. Excited state properties of lepidopterenes Epilogue References

I.

INTRODUCTION

A wealth of fundamental photochemical knowledge has been derived from the study of aromatic hydrocarbons whose excited state properties are characterized by a variety of well-defined deactivation processes such as fluorescence, valence isomerization, and cycloaddition [13. In particular, anthracene and many of its derivatives have been investigated extensively, as their availability, and their absorption and emission spectral properties in conjunction with simple and measurably accessible chemical transformations are experimentally attractive features [2,3]. The energy diagram relating ground state and excited states of anthracene is known in detail [4-61, and for many photoexcited 9-substituted anthracenes in dilute solution the sum of quantum yields of fluorescence and intersystem crossing to the triplet state is near unity, i.e., radiationless deactivation of the excited singlet state to the ground state is a negligible process [7]. The chemical deactivation of photoexcited anthracenes by dimerization usually proceeds by 4n + 4n cycloaddition [S]. However, exceptions to this rule have become known in recent years [S], and a multitude of steps, including the formation of metastable intermediates such as excimers, may actually be involved in a seemingly simple photochemical reaction such as the dimerization of 9-methylanthracene [9, lo]. Moreover, substitution of the anthracene chromophore may affect and alter its excited state properties in a profound manner for a variety of reasons. For example, in 9-tertbutylanthracene the aromatic ring system is geometrically distorted [ l l , 121 and, consequently, photoexcitation results in the formation of the tert-butylsubstituted Dewar anthracene [13- 151. The analogous photochemical isomerization of decamethylanthracene [16] probably is attributable to similar deviations from molecular planarity. When the anthracene chromophore is substituted at the 9-position by a nsystem, coplanarity of the two n-systems will be impaired for steric reasons, and so will be electronic conjugation. For example, in ground state 9phenylanthracene the angle between the plane of the anthracene and that of the phenyl group is about 60" [17,18]. Likewise, in 9-acetylanthracene, the

SUBSTITUENT EFFECTS ON EXCITED STATE PROPERTIES

141

plane of the carbonyl group is virtually perpendicular to the plane of the anthracene moiety [191.However, in their electronically excited state, both 9phenylanthracene and 9-acetylanthracene assume conformations in which the degree of conjugation of the n-systems increases, as can be deduced from fluorescence studies [20,21]. Large differences between the geometry of the ground state and the geometry of the fluorescent state are borne out in large Stokes shifts [22]. The magnitude of the Stokes shift may depend on solvent polarity, and solute-solvent interactions can enhance the formation of fluorescent exciplexes by way of state switching. In symmetrical biaryls such as 9,9'-dianthryl, where the dihedral angle between the planes of the two nsystems is about 90",the nonplanar molecular geometry is a prerequisite for the solvent-induced formation of the twisted intramolecular charge transfer (TICT) state, which is characterized by structureless emission and large Stokes shifts [23-251.Fluorescence spectroscopy in conjunction with excitation spectra and lifetime measurements has also revealed the existence of nonequilibrated excited rotamers (NEER), e.g., in the case of 2-vinyl- and 2styrylanthracenes [26-321. The present contribution deals mainly with novel 9-substituted anthracenes in which the substituent either incorporates or by itself represents a n-system, and whose effect on the overall molecular shape is such as to have major photochemical and photophysical repercussions [33]. Not discussed are anthracenophanes [34] and various types of bichromophoric anthracenes whose excited state properties have been reviewed previously [8,25,35].Considered beyond the scope of this contribution are the photochemistry and photophysics of anthraceno crown ethers and cryptands [36381, and of intramolecular exciplexes derived from anthracenes linked to aromatic amines [39-411.

11. SUBSTITUENT EFFECTS ON EXCITED STATE

PROPERTIES OF NONCONJUGATIVELY LINKED ANTHRACENES A. Di-Qanthrylmethane Derivatives

In numerous investigations during the past two decades, the excited state properties of various types of linked anthracenes such as 1,2-di-9anthrylethane have been assessed in terms of fluorescence and isomerization quantum yields [8,42]. Although details of the mechanism of the isomerization by cycloaddition may still be a subject of discussion [43-451, effects of substitution of the anthracene chromophore are clearly noticeable

142

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

TABLE 1 Deactivation of Photoexcited Dianthrylmethane Derivatives by Fluorescence ( 4 ~and ) Cycloaddition ( 4 ~ )

Compound

4JF

Di-9-anthr ylmethane la

Di-9-anthrylmethanol l b Methyl 9-anthrilate 3 Di-9-anthryldimethylsilane Di-9-anthryl ketone 5

4JR

Addition Mode

Solvent

Ref.

0.06

0.15

4x + 4n

Benzene

c421

0.02

0.29

4n + 4n

Benzene

c421

0.18

0.45

4n + 271

Toluene

C601

0.45

0.05

4n + 28

Me-Cyclohexane

[48]

0.0003

4n+2x

Benzene

c5li


in the varying quantum yields of fluorescence and cycloaddition, as well as in the mode of isomerization. Results for di-9-anthrylmethane derivatives are summarized in Table 1. Thus, di-9-anthrylmethane l a and di-9anthrylmethanol Ib, both characterized by fluorescence quantum yields of less than lo%, undergo photochemical intramolecular 4a +4n cycloaddition to give 2a and 2b with quantum yields of 0.15 and 0.29, respectively. The 10methoxy derivative of Ib, i.e., 1-( l0-methoxy-9-anthryl)-9-anthrylmethanol, whose fluorescence quantum yield (& = 0.02) is the same as that of la, isomerizes by 4n +4n cycloaddition with the reduced quantum yield of 0.05, suggesting that steric factors may affect the efficiency of the cycloaddition.

1; 2

a b

X

m la-b

hi-

2a-b

The importance of molecular topology for the course of photochemical isomerizations is evident from the different modes of cycloaddition for seemingly similar linked anthracenes. For example, the photochemical isomerization of methyl 9-anthrilate 3, different from that of Ib, proceeds with

143

SUBSTITUENT EFFECTS ON EXCITED STATE PROPERTIES

R : OH R': COOCH,

3

4b

4a

+

high quantum yield (4R= 0.45) exclusively by 4n 271 cycloaddition, and only one of two possible stereoisomers, namely 4a is formed [60]. Significantly, exclusive formation of 4a also occurs by thermal reaction of 3. Ground state intramolecular interactions between the two aromatic nsystems in 3 are indicated by the broadness of its ultraviolet (UV) absorption in comparison to that of l b (see Figure 1). Single-crystal X-ray diffraction 20L

-1

Figure 1. UV absorption spectra of di-9-anthrylmethanol l b (-----) and methyl 9anthrilate 3 (-) in dichloromethane.

144

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

analysis [47] reveals that the ground state geometry of 3 indeed is such as to favor the observed Diels- Alder reaction. The distance between C-9 of the anthracene serving as the diene, and the dienophile terminus C-1' is as short as 2.86 A. Similar suitable topological predisposition of interacting chromophores probably accounts for the photochemical 4n + 271 cycloadditions of di-9-anthryldimethylsilane [48], di-9-anthryldimethylgermane [49], and [3.3]-paracyclo-(9,lO)-anthracenophane derivatives [SO].

6

5

Di-9-anthryl ketone 5 isomerizes photochemically also by 4n + 271 cycloaddition, but the quantum yield for the formation of 6 is as low as 0.0003 [Sl]. Since dianthryl ketone is nonfluorescent, and the quantum yield for its triplet-sensitized intramolecular 471 + 211 cycloaddition is considerably higher (0.005), deactivation of the excited singlet state by internal conversion must be an important process for photoexcited 5. As for the ground state geometry of dianthryl ketone [ S Z ] , the two anthracene moieties are twisted out of the plane of the carbonyl group by 42.8"and 52.2"so that the angle between the aromatic n-systems is 87.3", and the intramolecular distance between the Diels-Alder termini C-9 and C-1' is 3.1 1 A. Significantly, similar intramolecular contact distances between the corresponding carbon atoms are found in crystalline di-Panthrylmethanol [52]. Consequently, the modes of isomerization by cycloaddition, viz. 4n + 471 for dianthrylmethanol 1b, versus 4n+ 2n for dianthrylketone 5, are probably governed by the multiplicity of the excited state involved in the reactions (cf. Section II.C), rather than by molecular geometry.

B. 1,2-Di-9-anthrylethanes and Related Two- and Three-Atom Linked Anthracenes For a series of substituted 1,2-di-9-anthrylethanes 7, the quantum yields of fluorescence and isomerization to give 8 by 4n 4n cycloaddition are summarized in Table 2 [ 5 3 ] . By and large, steric impairment of cycloaddition

+

SUBSTITUENT EFFECTS ON EXCITED STATE PROPERTIES

145

TABLE 2 Excited State Properties of 1,2-Di-9-

anthrylethanes 7. Quantum Yields of Deactivation by Fluorescence (QF)in Cyclohexane, and 4n + 4n Cycloaddition (QR)in Benzene Solution 1531

R

7

H H

i

k

H H H Methyl Methox y Acetoxy Acetoxy n-Butyl Phenyl

R H Methyl Methoxy Acetoxy Phenyl Methyl Methox y Methoxy Acetoxy n-Butyl Phenyl

4F

0.20" 0.15 0.14 0.40 0.61 0.16 0.30 0.45 0.74 0.70 0.79

0.26" 0.26 0.24 0.14 0.034 0.14 0.1 1 0.068 0.015 ~0.015 <<0.015

"From Ref. 42.

due to substitution is borne out in reduced reaction quantum yields paralleled by increased quantum yields of fluorescence. Thus, the parent compound 7a isomerizes with the optimal quantum yield of 0.26. The quantum yields for the isomerization of 10,lO-di-substituted dianthrylethanes can be considerably lower, depending on the spatial demand of the substituents (see Table 2).

The effect of substitution of the ethane linkage on the isomerization quantum yields is less dramatic, though the course of the photochemical reaction can be affected by the nature of the substituents (cf. also Section 1I.C). Both 1,2-di-9-anthrylethanol (& = 0.04) and its acetate (& = 0.03) undergo photochemical intramolecular 471+ 471 cycloaddition with a quantum yield of 0.35 [Sl]. However, irradiation of meso-1,2-di-9-

146

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

anthrylglycol 9 in toluene solution gives as main product not the expected cis-1,2-cyclobutanediol derivative 10 but the sterically less congested transisomer 12. The formation of 12 suggests that photoexcited 9 undergoes stereoisomerization to give the d,l-l,2-di-9-anthrylglycoI11 by way of photolytic cleavage of the central ethane single bond. Since irradiation of the diacetate of the meso-diol 9 exclusively gives the diacetate of the cis-1,2cyclobutanediol 10, the photolytic cleavage reaction appears to be associated with the 1,Zdihydroxy substitution of the ethane bond [46]. A

= 9-ANIIIRYL

. 10

. 11

12

Although isomerizations of singlet-excited dianthrylethanes 7 typically proceed by 4n + 4n cycloaddition to give cyclomers 8, intramolecular 4n + 2n cycloaddition can be a competing low-quantum yield process, as has been established for the 10,lO-dimethylderivative 71 [54]. Due to the thermolytic cycloreversion of the 4x + 411 cycloadduct Sf, (its half-life in solution at 25°C is only 33min), irradiation of 7f at elevated temperature actually leads to the formation of 13 as the main product. It is of interest in this context that spectroscopic evidence for dynamic conformational flexibility, and for the existence of distinct conformers in solution has been obtained for substituted dianthrylethanes 7i and 7k [55,56]. Two structured emission spectra, separated by about 10nm, and associated with two structured excitation

147

SUBSTITUENT EFFECTS ON EXCITED STATE PROPERTIES

-

he.

382

I

---

445 429

...... 382

I

I

350

I

1

400

I

450

I

I

500 nm

Figure 2. Wavelength-dependent emission and excitation spectra of diacetoxydianthrylethane 7i in decalin at 77 K.

spectra are evident for dianthrylethane 7i in decalin at 77 K, suggesting the presence of two conformers (see Figure 2). The short-wavelength and longwavelength spectra have been attributed to the anti and gauche conformers, respectively. It is conceivable that the isomerization of dianthrylethane 7f by 471+ 271 cycloaddition involves deactivation of the photoexcited gauche conformer (see Figure 3). Intramolecular interactions of dianthrylethanes are evident from excimer luminescence detected under various conditions [8,57591.

hu

8f

I

7f

13

148

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES ENERGY

1

,

I

r-.

\

\

w‘,

I

.-

~

ANTI

GAUCHE

increasing degree of overlap

Figure 3. Schematic representation of anti and gauche conformations of 1,2-di-9anthrylethanes, and the corresponding ground state and excited state potential energy curves.

+

Photochemical isomerizations by intramolecular 4a 4n cycloaddition of carbon oxygen linked bichromophoric anthracenes to give oxetane derivatives have not been reported yet. Upon irradiation (A > 400 nm) in either toluene or ethyl acetate, the methoxycarbonyl substituted carbon oxygen linked bichromophoric anthracene 14 indeed isomerizes smoothly and efficiently (4 = 0.45). However, the two products, obtained in an approximate ratio of 5: 1, are anthrone derivatives 15 and 16 whose formation can be rationalized by migration of the anthryloxy moiety [60]. An analogous photolytic rearrangement has been found for 9-anthryloxy substituted dianthrylethylenes (see Section 1II.A).

H-C-R

I

0

0

-

0

+

hu

H-C-R

R

H/c,

R = C02CH3

14

15

16

Separation of two aromatic ring systems by a three-atom link, as in 1,3diphenylpropane, is associated with a propensity for excimer formation, since the possible interplanar spacing of about 3.5 8, between overlapping

SUBSTITUENT EFFECTS O N EXCITED STATE PROPERTIES

149

chromophores is such as to facilitate excited state interaction [61]. However, for propano linked anthracenes, chemical deactivation of the excited state usually competes efficiently with excimer emission [35,62]. In the case of 1,3di-9-anthrylpropane 17a and various of its chain-substituted derivatives such as 1,3-dianthryl-l-propanol17b, excimer emission has not been observed under ambient conditions [62,63]. However, evidence for intramolecular excimer formation has been obtained by time-resolved fluorescence spectroscopy on asymmetric 1,3-dianthrylpropanes for which perfect overlap of aromatic n-systems is impaired for structural reasons [64]. According to a recent investigation [65], fluorescence quantum yields of 1,3-dianthrylpropane derivatives vary greatly, and so do the quantum yields for the isomerization by 4n+4n cycloaddition (see Table 3). Thus, fluorescence and cyclization quantum yields for the parent compound 17a are 0.47 and 0.14, respectively, and similar values characterize the reactivity of 1,3-di-9-anthryl- 1-propanol 17b whose isomerization affords cycloadduct 18b. Dianthrylisopropanol17d also has a fluorescence quantum yield of close to SO%, but its cyclization quantum yield is only 0.046. Dianthrylacetone 19a, by contrast, cyclizes to give 20a with a quantum yield of 0.25, while its

TABLE 3 Deactivation of 1,SLinked Bichromophoric Anthracenes by Fluorescence (QF)in Cyclohexane and 41r 4n:Cycloaddition (QR)in Benzene Solution

+

Compound

4F

0.47" 1,3-Di-9-anthrylpropane 17a 1,3-Di-9-anthryl-l-propanol 17b 0.27 1,3-Di-9-anthryl-l-acetoxypropane 17c 0.20 1,3-Di-9-anthryl-2-propanol 176 0.48 1,3-Di-9-anthryl-2-methyl-2-hydroxypropane 17e 0.24 (1R,3S/lS,3R)- 1,3-Di-9-anthryl-1-butan01 17f 0.15 (1R,3R/lS,3S)-1,3-Di-9-anthryl-1-butanol 17g 0.07 1,3-Di-9-anthryl-2-propanone 19a (degassed) 0.0085 1,3-Di-9-anthryl-2-propanone 19a (air-saturated) 1,3-Di-9-anthryl-2-butanone 19b (degassed) 0.007b 1,3-Di-9-anthryl-2-butanone 19b (air-saturated) 0.00026 1,3-Di-9-anthryl-1-propanone 2la (degassed) 1,3-Di-9-anthryl-l-propanone 21a (air-saturated) 1,3-Di-9-anthryl-1-butanone 21b (degassed) < o.oO01 1,3-Di-9-anthryl-l-butanone 21b (air-saturated) 1,3-Di-9-anthryl-2-methyl-l-propanone 2lc (degassed) < 0.001

4R 0.14" 0.14 0.20 0.046 0.021 0.23 0.23 0.25 0.22 0.22 0.22 0.65 0.015 0.40 0.048 0.72

"In methylcyclohexane. bBroadness of fluorescence spectrum suggests contribution of excimer emission.

Ref.

19

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

fluorescence quantum yield is as low as 0.0085. Similar excited state properties are exhibited by 1,3-di-9-anthryl-2-butanone19b. It has been ascertained by oxygen-quenching experiments that the cyclization of dianthrylacetone, analogous to that of dianthrylisopropanol, proceeds predominantly in the excited singlet rather than the triplet state. Therefore, it is conceivable that the enhanced cyclization efficiency of dianthrylacetone is attributable to a more favorable ground state conformation.

X-C-H I

X

Y

17/18

R

a b

H H H H O H H H OAc H H H H O H H Me Me OH H Me OH H

C

d e f

g

19/20 a b

Y-c--z I R-C-H

Z H H H

&

H 17a-g OH H (lR, 3S/1S, 3R) H ( l R , 3R/lS, 3s)

hi

2

18a-g

R H methyl

19a-b

21/22

-

R

R'

20a-b

c=o I

a b C

H H methyl

H methyl H

R -C-H

R)-;--H

21a-c

22a-c

151

SUBSTITUENT EFFECTS ON EXCITED STATE PROPERTIES

Among the 1,3-linked bichromophoric anthracenes listed in Table 3, 1,3di-9-anthryl-1-propanone 21a,1,3-di-9-anthryl-l-butanone 21b,and 1,3-di-9anthryl-2-methyl-1-propanone 21c are exceptional because their photochemical isomerization by intramolecular 471+ 4~ cycloaddition to give 22 is characterized by high quantum yields, viz. 0.65, 0.40, and 0.72, respectively. For photochemical cycloadditions of linked anthracenes, the quantum yield of 0.72 is the highest ever observed. Oxygen quenching and sensitization experiments indicate that 21a, 21b, and 21c undergo the 47t +4x cycloaddition in the excited triplet state (see Section 1I.C). The excited state properties of bis-a-9-anthrylmethyl ethers 23 closely resemble those of 1,3-di-9-anthrylpropane derivatives. The photoexcited parent compound 23a deactivates by fluorescence (4 = 0.03) from the locally excited state only, and it isomerizes by intramolecular 47t + 4x cycloaddition with a quantum yield of 0.32 [66]. By contrast, excimer emission (see Table 4) does characterize the excited state properties of the 10,lO-diphenyl derivative 23b,which does not undergo intramolecular cycloaddition for steric reasons [66,67].

23

R

H

a b

H phenyl

I

I

I

H

H

23a-b

TABLE 4 Deactivation of Bis-9-anthrylrnethyl Ethers by Fluorescence (QF)and 4n: 4n: Cycloaddition (QR)

+

Compound Bis-9-anthrylmethyl ether 23a Bis- 10-phenyl-9-anthrylmethyl ether 23b d,l-Bis-cr-9-anthrylethyl ether 24 rneso-Bis-a-9-anthrylethylether 24

4F

0.03" 0.55a*b 0.014' -

&

Ref.

0.32"

[66] C661 0.25d [68] 0.25d [68] -

"In methylcyclohexane. methylcyclohexane; sum of excimer emission (4 = 0.34) and emission from the locally excited state (4 = 0.21). % cyclohexane; mainly excimer emission (cf. Figure 4). benzene.

152

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

a-Substitution of bis-9-anthrylmethyl ether 23a leads to diastereomeric meso and d,l ethers 24. Both the meso and racemic diastereomers in the case of R = methyl and R = phenyl have been found to undergo photochemical intramolecular 47c 4n cycloaddition to give isomers 25 with identical quantum yields (see Table 4). However, stereochemical differences between the meso-form and the racemate are borne out in their different electron

+

H M E S O

24a-b 24/25

a b

R A C E M A T E

F O R M

25a-b

R methyl phenyl

spectral properties [68]. Thus, the meso compounds, which may assume a molecular geometry of parallel overlapping aromatic ring systems, are distinguishable from their d,l-isomers by the broadness of their UV absorption (see Figure 4a), and by being nonfluorescent. (According to singlecrystal X-ray diffraction analyses, the ground state conformation of the meso

.*

I

M

. I . .

. . .m. . . . . 3y) . . . . . a. .r .m . I .

. .400 ..

b

I

.

.

.

.

.

, .500 . . . . . . . . . 600 . . .nrn. .

Figure 4. Absorption spectra (a) of diastereomeric bis-9-anthrylethyl ethers 24a, and dual emission spectra (b) of racemic substituted bis-9-anthrylmethyl ethers 24 in cyclohexane at room temperature. (Reprinted with permission from Ref. 68. Copyright 1982 American Chemical Society).

SUBSTITUENT EFFECTS ON EXCITED STATE PROPERTIES

a: meso-form

153

b: d , l - f o r m

Figure 5. Schematic view of parallel aligned anthracene moieties in (a) meso and (b) d,l-bis-9-anthrylmethylethers 24.

compounds is such as to have the two anthracene moieties in an orthogonal arrangement, while the corresponding angle for the racemate was found to be 120" [69].) The d,l compounds in solution are characterized by dual fluorescence, i.e., emission From both the locally excited state and an excimer state is observed (see Figure 4b). As intramolecular parallel overlap of the anthracene moieties in the d,l compounds is sterically impaired (see Figure 5), it is probable that the geometry of the luminescent excimer deviates from perfect sandwich arrangement. Similar arguments have been advanced to rationalize the fluorescence properties of stereoisomeric 1,4-bis-9-anthryl-2,3diazabutadienes (9-anthraldehyde azines). In that case, the EE and EZ isomers are nonfluorescent, while the Z Z isomer is characterized by dual fluorescence [70]. (For a recent discussion of conformational aspects on intramolecular excimer formation of diastereomeric diary1 compounds see L-711).

C. Triplet State Isomerizations Biacetyl sensitized photoisomerization of 1,2-di-9-anthrylethane 7a does not lead to the 471 + 47c cyclomer 8a but yields exclusively the 47c 2n cycloadduct 26 with a quantum yield of 0.1 [72]. Since the phosphorescence of biacetyl is quenched by dianthrylethane 7a at nearly diffusion controlled rate, the photochemical Diels- Alder reaction is explicable by triplet energy transfer from biacetyl to 7a. The photochemical isomerization of lO-benzoyl-1,2-di-9anthrylethane 27 also proceeds exclusively by 47c 271 cycloaddition and gives cycloadduct 28 with a quantum yield of 0.005 [73]. The low fluorescence quantum yield of 27 (& = 0.002) suggests that the formation of 28 proceeds from the excited triplet state. Biacetyl sensitization of 27 leads to 28

+

+

154

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

with a quantum yield of 0.007. Worth noting is the regiospecificity of the photochemical Diels-Alder reaction of 27, in which the 1,Zbond of the benzoyl-substituted anthracene moiety reacts exclusively as dienophile.

ho

Biuetyl

26

7s

27

28

The different modes of isomerization observed for singlet and triplet excited chloro-substituted 1,2-di-g-anthrylethanes (DAE) 29a and b indicate that the molecular geometry of the ground state can be of major importance for the course of the reaction [72]. Both upon direct photoexcitation and by biacetyl sensitization (see Table 5), 1,l’-dichloro-DAE 29a and 1,1’,5,5’tetrachloro-DAE 29b isomerize by 4n + 47t cycloaddition to give regioisomeric photoproducts 30/30. By contrast, 4,4,5,5’-tetrachloro-DAE 32 undergoes intramolecular 4n 4n cycloaddition to give 31 only upon direct excitation, while triplet sensitization leads to the 471 2n cyclomer 33. Ground state conformational differences between 1,l’-chloro-substituted dianthrylethanes 29 and DAE 32 are noticeable in their electronic absorption spectra (see Figure 6). The broadness of absorption in the case of chlorosubstituted dianthrylethanes 29 is indicative of partially overlapping

+

+

1,1’-5,5’-Tetrachloro-DAE 29b 1,3-Di-9-anthryl-l-propanone 21a 4,4‘,5,5’-Tetrachloro-DAE32

Di-9-anthryl ketone 5 1,2-Di-9-anthrylethane7a 10-Benzoyl-DAE27 1,l’-Dichloro-DAE29a

Compound
4F O.ooOo3 0.26 0.005 0.35 0.39 0.65 0.16

Direct

4 R

4a 2n 4n + 4n 48 + 2n 4x + 4n 4n + 4n 4x + 4n 47C 48

+

Addition Mode

0.005 0.1 0.007 0.67 0.43 0.65 0.02

Sens

4It

4n+2n 47t+2R 4n+2n 4n+4n 4n+4x 4n+4n 4n+2n

Addition Mode

TABLE 5 Deactivation of Singlet and Triplet Excited Linked Anthracenes. (Fluorescence Quantum Yields QF in Cyclohexane. Reaction Quantum Yields QR in Benzene)

[Sl] [72] [72] [74] [72] [Sl] [72]

Ref.

1%

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

Figure 6. The effect of substituents on the absorption spectra of 1,2-di-9anthrylethanes.

anthracene moieties. By comparison, the well-resolved spectrum of 4,4,5,5'tetrachloro-DAE 32 resembles that of unsubstituted dianthrylethane 7a, and so does its photochemistry in terms of multiplicity-dependent isomerization modes. The quantum efficiencies of triplet state 4n + 4n cycloadditions of chloro-substituted dianthrylethanes are comparatively high (see Table 5).

R 29

R

a b

C1

H

2%-b

157

SUBSTITUENT EFFECTS ON EXCITED STATE PROPERTIES

32

31

33

Other examples of linked anthracenes for which triplet state photochemistry has been established by sensitization or quenching experiments are di-9anthryl ketone 5 (see Section LB), 1,3-di-9-anthrylpropanone21a, 1,3-di-921c (cf. Table anthrylbutanone-1 2lb, 1,3-di-9-anthryl-2-methyl-l-propanone 3), and 1,2-di-9-anthrylethanone34.The photochemical isomerization of 21a proceeds by 4n 4n cycloaddition to give 22a both upon direct excitation and by biacetyl sensitization. Significantly, dianthrylpropanone 21a isomerizes in both cases with identical quantum yields, namely 0.65 [Sl]. Quenching experiments reveal that the isomerization by direct excitation of 21a solely involves the excited triplet state. The fact that the triplet state isomerization of dianthrylpropanone 21a, different from that of dianthryl ketone 5, leads to a 4n+4n rather than 4n+2n cyclomer suggests that the molecular geometry of the ground state plays an important role in governing the mode of cycloaddition. In the case of dianthrylethanone 34, the course of the photochemical isomerization was found to be concentration-dependent (cf. Table 6). Triplet state reaction products are the 4n + 2n cyclomers 35 and 36, which are formed mainly (80%) by irradiation of 34 at low concentration [Sl]. The predominant (98%) formation of the singlet state product, ie., 4n +4n cycloadduct 37, is observed when dianthrylethanone 34 at high initial concentrations is

+

TABLE 6 Concentration Dependent Product Composition (yo)in the Photochemical Isomerization of 1,2-Di-9-anthrylethanone 34 in Benzene

Concentration (M) 0.00003 0.00013 0.001 0.0075

+

471 272 Cyclomer 35

+

471 2n Cyclomer 36

60

20 14 4

44 12

4n -k 4n

Cyclomer 37

20 42 84 98

158

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

irradiated, so that triplet quenching can become an important process (cf. Table 6). The quantum yield of intersystem crossing to the triplet state of dianthrylethanone was estimated to be 0.59 [Sl].

111. EXCITED STATE PROPERTIES OF 9-ANTHRYLALKENES

A. Molecular Topology and Photochemical Reactions of cis-trans Isomeric 1,2-Di-9-anthrylethylenes and Related Di-9-anthrylalkenes In view of their molecular topology, 1,2-di-9-anthrylethylenes 38/39 are of both photophysical and photochemical interest [75-771. As for cisdianthrylethylene 38a, its nonplanar ground state geometry should be characterized by spatial proximity of overlapping anthracene n-systems. In the case of trans-isomer 39a, the molecule evades steric interaction between the ethylene hydrogens and H-l/H-8 of the anthracenes by either conrotatory or disrotatory twists about the 9-anthryl-ethylene single bonds. Consequently, in ground state trans-dianthrylethylene 39a, the two anthracene planes may be aligned either parallel, or be in an angular arrangement. It is known from a series of X-ray structure analyses that the dihedral angle between the plane of the ethylene and the plane of the anthracene n-systems is about 5560” for trans-dianthrylethylenes which are not additionally substituted at the ethylene double bond [78-801. The angle increases considerably by double bond substitution [SO].

159

EXCITED STATE PROPERTIES OF 9-ANTHRYLALKENES

R

38/39 R a b c d e

f

H H H H Me Me0

H CHO MeOCO Me0 Me0 Me0

A

\C/

R

II

C A/

\C/ II

R ..

'R'

38s-f

A

A

R'

A

39a-f

= 9-ANTHRYL

The molecular geometry of the parent cis-1,2-di-9-anthrylethylene38a has not been established by X-ray diffraction, but crystal structure analyses of several 1,2-substituted cis-dianthrylethylenes 38 are available. Depending on the spatial demand of the substituents R and R', the planes of the anthracene moieties are twisted out of the plane of the ethylene by 59-84' [80]. For both cis- and trans-dianthrylethylenes, the degree of deconjugation, which parallels the degree of deviation from coplanarity of the ethylene and anthracene n-systems, is borne out in the shape of the electronic absorption spectra. Thus, the long-wavelength absorption of cis-dianthrylethylene 38a is characterized by the fine structure pattern which is typical of the anthracene chromophore, while the bathochromically shifted spectrum of the transisomer 39a is virtually structureless (see Figure 7). Substitution of the ethylene double bond is absorption spectroscopically noticeable for the cisisomers 38b-f by distortion of the anthracene absorption. The absorption

in C y c l o h e r a n e

Ex lor A ,300 nm E x 10-4 for A < 300 nm

300

400

(A.nm)

300

400

(A:nm)

Figure 7. Absorption spectra of dianthrylethylenes 38a/39a and 38e/39e.

160

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

spectra of 1,2-substituted trans-isomers 39b-f are hypsochromically shifted relative to that of the parent compound 39a, and they increasingly assume the anthracene fine structure. Thus, trans-l-methoxy-2-methyl-1,2-di-9-anthrylethylene 39e is almost colorless, while the corresponding cis-isomer 38e is yellow (see Figure 7).

e

H

38a

40

41

As far as the excited state chemistry of cis-dianthrylethylene 38a is concerned, in cyclohexane solution, the quantum yield for its isomerization by 471 271 cycloaddition to give 40 is as low as 0.0007, and the concomitant isomerization by 471+4n cycloaddition to give 41 proceeds with a quantum yield of <0.00007. Both cyclomers 40 and 41 smoothly undergo photolytic cycloreversion in cyclohexane to give cis-dianthrylethylene 38a with quantum efficiencies of 0.61 and 0.20, respectively [76]. Unexpectedly, also the quantum yield for geometrical isomerization of 38a to give 39a was found to be remarkably low in cyclohexane (#J = 0.0007), though it increases in more polar solvents such as benzene (4 = 0.007) or chloroform (4 = 0.015). (It is not understood why the quantum yield of cis-+trans isomerization in cyclohexane exceeds that of the intramolecular 4n + 271 cycloaddition when cis-dianthrylethylene 38a is irradiated in the presence of suspended crystalline 39a). Attempts to bring about the photochemical trans-wis isomerization of dianthrylethylene have not been successful [76], i.e., cis-dianthrylethylene 38a belongs to that group of anthryl-substituted ethylenes which undergo “one-way” isomerization (vide infra; cf. [8 1-83]). However, substitution of the ethylene double bond in 1,2-dianthryIethylenes 38/39 markedly enhances

+

161

EXCITED STATE PROPERTIES OF 9-ANTHRYLALKENES

TABLE 7 Photochemical Isomerizations of cis-trans Isomeric Dianthrylethylenes 38 and 39 in Cyclohexane. (Excitation wavelength 366 nm) Reaction Quantum Yields

38/39

R

R’

39

-+

38

Major Mode 38 -+ 39 38 -+ Cyclomer of Addition

Ref. ~

a b C

d e f

H H H H Me Me0

H CHO MeOCO Me0 Me0 Me0

<<0.0001 0.22 0.20 0.088 ?

0.26

0.0007 0.015 0.015 0.053 <0.01 0.049

0.0007

<0.001 <0.001 0.0012 0.20 0.0025

4n+2n 4n+2n 4n+2n 4n+4n 4n+47r 4n+4n

[76] [84] [84] [84]

[84] [85]

the photochemical reactivity in terms of quantum yields for both geometrical isomerization and cycloadditions. Thus, substituted trans-dianthrylethylenes 39b-f do undergo trans-wis isomerization, and quantum yields as high as 0.26 have been observed (see Table 7). Moreover, the cis isomers 38b-f are favored at the photostationary state. Photochemical isomerizations by cycloaddition for carbonyl-substituted 38b and 3&, conceivably involving the excited triplet state, proceed with low quantum efficiency as regioselective Diels- Alder reactions, in which the more electron deficient anthracene moiety reacts as the dienophile. Thus, 3& affords cycloadduct 42. For the methoxy-substituted derivative 38d, the photochemical cyclomerization by 4n + 4n cycloaddition leading to 43 clearly is favored (by 4 : 1) over that by 4n + 2n cycloaddition leading to 44. It is worth noting that the photochemical Diels- Alder reaction of 3&d proceeds regioselectively, as it is the methoxyvinyl-substituted anthracene which functions as diene. The effect of substitution and, presumably, molecular geometry on excited state reactivity of cis-dianthrylethylenes is most pronounced in the photochemical isomerization of 38e, which proceeds with drastically enhanced quantum yield ( = 0.20) exclusively by 471+471cycloaddition to give 45. The photochemical isomerization of the trans-dianthrylethenol 46, uniquely, does not result in the formation of a cycloadduct but leads to the 9anthryl-substituted enone 48 (4 = 0.18), whose formation may be rationalized by two consecutive hydrogen migration steps involving intermediate 47 [84]. The formation of 48 from 46 suggests that the nature of the substituent attached to the ethylene double bond can govern the course of the excited state chemistry of dianthrylethylenes. This assumption is indeed borne out in the photochemical properties of cis-l-(9-anthryloxy)-2-methoxy1,2-di(9-anthryI)ethylene49. The quantum yield for its disappearance in toluene solution is 0.13; however, the expected isomer 51, resulting from

162

A

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

=

9-ANTHRYL

42

k C 0 H

II

A’

C %OCH, 38d

ho

As 400 nm

43 44

45

intramolecular 4n + 4n cycloaddition of the cis-dianthrylethylene moiety, is formed in about 12% yield only. The major product, isolated in 65% yield, is the allene derivative 50 (see Figure 8) whose formation involves a photolytic rearrangement whereby the 9-anthryloxy group migrates and is converted into a 9-anthronyl moiety [86]. Thus, photoexcited trichromophoric 49 reacts mainly as a carbon-oxygen linked bichromophoric anthracene (cf. Section II.B), rather than a cis-dianthrylethylene. Also the chemistry of photoexcited di-9-anthryl-cyclopropenone52 is not that of a cis-dianthrylethylene, as decarbonylation to give di-9-anthrylacetylene 53 proceeds

163

EXCITED STATE PROPERTIES OF 9-ANTHRYLALKENES

Figure 8. Absorption spectra of trichromophoric 49 and its photoisomers 50 and 51 in dichloromethane.

CHa

II

46

hu

41

48

cleanly (see Figure 9) with a quantum yield of 0.14 in cyclohexane [84]. Considerably lower quantum yields are observed in toluene (4 = 0.02) and dichloromethane (4 = 0.01). Excited state properties of 1,3-di-9-anthrylpropnes 54/55 in terms of fluorescence and isomerization quantum yields are summarized in Table 8. Photochemical trans-wis isomerization, in general, is an inefficient process,

Figure 9. Absorption spectral changes associated with the photochemical decarbonylation of dianthrylcyclopropenone 52. Also shown (. .. .) is a spectrum at partial conversion.

6% 8 C

II

II

+

\'

/

'/

\ '

0

49

A

A

A

hv_

52

164

51

50

A-CSC-A

53

+

co

A

=

9-ANTHRYL

165

EXCITED STATE PROPERTIES OF 9-ANTHRYLALKENES

TABLE 8 Deactivation of Photoexcited 1,3Dianthrylpropenes 54/55 by Fluorescence (in Cyclohexane) and Intramolecular 4u + 4u Cycloaddition (in Benzene) 1,3-Di-9-anthrylpropene

54a; R = H S4b R = methoxy SSa; R = H SSb R = methoxy 5%; R = formyl

dJF

dJR

Ref.

0.13 0.26 0.06 0.006 0.0017

<0.001 0.015 0.25 0.09 0.13

[63] [87] [87] [87] [87]

and chemical deactivation of photoexcited cis- 1,3-dianthrylpropenes 55 by intramolecular 471 471 cycloaddition proceeds with far higher quantum yields than by geometrical isomerization. However, the cis-isomers 55 may be obtained by thermal cycloreversion of adducts 56, as their direct photochemical preparation from the trans-isomers is experimentally not feasible. Selective excitation of the trans-isomer is difficult because both isomers have strikingly similar absorption spectra in the long-wavelength region. In the

+

-

a

hv

hv

hv

54

0

57

H

&

55

n

1 58

59

166

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

Figure 10. Absorption and emission spectra of trans- and cis-di-9-anthrylpropenes 54a/55a in cyclohexane.

short-wavelength region around 250 nm, the spectra of dianthrylpropenes 54/55 are characterized by two maxima. For the trans-isomers, the highenergy maximum generally is of lower intensity. The opposite applies to the cis-isomers. Emission spectroscopically, the trans-isomers differ from their cis-isomers by noticeably higher fluorescence quantum yields and larger Stokes shifts (see Figure 10). trans- 1,3-Di-9-anthrylpropenone57 is nonfluorescent, and it isomerizes to give the 4n+4n cyclomer 59 with a quantum yield of <0.005 [63]. Although cis-dianthrylpropenone 58 is not detectable during the course of the reaction, the formation of cyclomer 59 by photoexcitation of the transisomer 57 is not to be interpreted in terms of an adiabatic photoreaction, but is explicable by a stepwise process in which inefficient geometrical isomerization to the cis-isomer 58 is followed by intramolecular cycloaddition of far higher quantum yield. Photoexcitation of trans,trans-l,5-di-9-anthrylpentadienone60 gives isomer 63 whose formation by two consecutive photoreactions, i.e., geometrical isomerization to give the cis-trans-isomer 61, followed by geometrically favored intramolecular Diels- Alder addition, has been established by quenching experiments. In the presence of molecular oxygen, the photochem-

167

EXCITED STATE PROPERTIES OF 9-ANTHRYLALKENES

ical geometrical isomerization of 60 proceeds smoothly to give both 61 and the cis,cis-isomer 62, while the formation of the Diels- Alder adduct 63 is suppressed. The result suggests that geometrical isomerization of 60,61, and 62 involves the excited singlet state, while isomerization of 61 by intramolecular 4n + 2n cycloaddition is a triplet state reaction 1631. Both upon direct photoexcitation and by biacetyl sensitization, the n-chromophorically substituted anthracene 63 rearranges to product 64 [88].

60

61

63

/hu

62

64

B. Geometrical Isomerization of 9-Anthrylalkenes The results summarized in Table 7 indicate that the quantum yields for the geometrical isomerization of dianthrylethylenes 38/39 are markedly affected by substitution of the ethylene double bond. For trans-substituted di-9-

168

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

anthrylalkenes such as propene 54 or propenone 57, the quantum yields for geometrical isomerization are low by comparison with those for intramolecular cycloadditions of the corresponding cis-isomers. trans-9-Anthrylalkenes, in which the double bond is substituted by an acyl group, do undergo fairly efficient geometrical isomerization to give the corresponding cis-alkenes. Thus, trans- 1-(9-anthryl)-2-benzoylethylene 65a in dichloromethane isomerizes upon irradiation to give the cis-isomer 66a with a quantum yield of 0.28. The reverse reaction, i.e., the photoinduced geometrical isomerization of 66a proceeds with a quantum yield of 0.1, and the absorption spectral differences between the two isomers are such as to facilitate preparative trans-wis conversions by selective excitation of the trans-isomer. Similar results have been obtained for the geometrical isomerization of photoexcited 1-(9-anthry1)-2-acetylethylenes 65b/66b (see Table 9). n

R

In a low quantum yield process (4 = 0.0008;see Figure 1 l), singlet excited cis-l-(9-anthryl)-2-benzoylethylene 66a isomerizes by a skeletal rearrangement to give furano-annelated SH-dibenzocycloheptene 67 in excellent chemical yield. Various derivatives of 65a/66a, in which either the phenyl ring or the anthracene moiety are substituted, have been found to rearrange TABLE 9 Deactivation of 1-(9-AnthryI)-2-acylethylenes 65/66 by Fluorescence ($F) in Cyclohexane and Geometrical Isomerization ($1) in Dichloromethane 1631 Compound

65a 65b 66a 66b

R Benzoyl Acetyl Benzoyl Acetyl

4F

$1

0.0025 0.0024 0 0

0.28 0.39 0.1 0.32

EXCITED STATE PROPERTIES OF 9-ANTHRYLALKENES

,,,I

2 x 10

-5

curve M i n degassed benieiie

360 nm c u t - o f f

min

0

filter

8'<

169

0

1

7

2

266

5

1548

-

1.0

\'

"

300

. I ,

350

I

I

*

1

I

400

.

I

>

1 . .

1

450 nrn

*

I

Figure 11. Absorption spectral changes associated with the photochemical isomerization of 1-(9-anthryl)-2-benzoylethylene65a (----) into dibenzocvclohetxene derivative 67 (curve 3). (Reprinted with permission from Ref. 89. Copyright 1985, Pergamon Press PLC).

photochemically in the same fashion [89]. The formation of dibenzocycloheptene 67 is suggested to proceed by way of an intramolecular 4 ~ + 2 n cycloaddition of photoexcited &a, as outlined on page 170. For steric reasons, the cis-l-(9-anthryl)-2-benzoylethylene66a has to assume the depicted s-cis conformation. Consequently, the contact distance between the bond-forming atoms, i.e., the carbonyl oxygen and C-9 of the anthracene ring, should be about 3 A. The effects of solvent polarity and of substituents on the quantum yields of geometrical isomerization are borne out strikingly in the photochemical properties of 1-(9-anthryl)-2-phenylethylenes (9-styrylanthracenes) 68/69. The deactivation of the photoexcited parent cis-compound 68a by geometrical isomerization to give the trans-compound 69a proceeds in cyclohexane, dichloromethane, and acetonitrile with quantum yields of 0.22,0.31, and 0.41, respectively. By contrast, trans-9-styrylanthracene 69a upon irradiation in cyclohexane disappears slowly (4 = 0.0014) to give some undefined product, but evidence for geometrical isomerization to give the cisisomer 68a has not been obtained [88]. In that latter respect, trans-9-

170

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

ho

67

styrylanthracene 69a resembles trans-dianthrylethylene 38a. For various 2anthrylethylenes undergoing triplet-sensitized “one-way” cis+ trans isomerization, the reaction has been established to proceed adiabatically [81-831. The observed quantum yields exceeding unity have been rationalized by a quantum chain mechanism in which the triplet-excited cis-isomer leads to the triplet-excited trans-isomer which, in turn, deactivates to ground state by way of triplet energy transfer to a ground state cis-isomer [81,82]. Thus, the potential energy curve for the excited triplet state of these anthrylethylenes is

R

68/69

a

R

H

b

N,N-dimethylamino

decf

methyl phenyl methoxy chloro

g

cyano benzoyl formyl 4phenylsulfonyl nitro

h i j

k

&H

/ 68a-k

69a-k

171

EXCITED STATE PROPERTIES OF 9-ANTHRYLALKENES

TABLE 10 Quantum Yields of Geometrical Isomerization of 1-(9-Anthryl)-2phenylethylenes 68/69 in Cyclohexane and in Acetonitrile 195) trans +cis

&-+trans

68/69 a b

k

R

Cyclohexane

Acetonitrile

Cyclohexane

Acetonitrile

H N,N-Dimethylamino Methox y Methyl Phenyl Chloro Cyano Benzoyl Formyl 4-Phenylsulfonyl Nitro

0.22" 0.18

0.41" 0.40

<0.01" 0.24

0.003" 0.22

0.17 0.2 1 0.28 0.47 0.45 0.70" 0.73"

0.25 0.36 0.46 0.46 0.46 0.32 0.35" 0.36"

0.02 <0.01 <0.0 1 <0.01 <0.01 0.20 0.02" 0.06"

0.33 0.18 0.02 0.13 0.4 1 0.26 0.38" 0.37"

0.51

0.05

0.13

0.02

0.19

"From Ref. 90.

characterized by the virtual absence of a minimum at an ethylene double bond twist of 90". In 9-styrylanthracenes 68b-k/69b-k, para-substitution of the phenyl ring by either electron-donating or electron-withdrawing groups drastically affects the efficiency of their photochemical isomerization, as is evident from recent results summarized in Table 10. Both the nature of the substituent and the solvent polarity are found to govern the ck+trans isomerization of 9styrylanthracenes 68 in an apparently distinct fashion. Thus, for 68b-e, characterized by electron-donating substituents, the quantum yields are noticeably higher in acetonitrile than in cyclohexane solution. By contrast, for 68h-k carrying electron-withdrawing substituents, the quantum yields for cis-mans isomerization in acetonitrile solution are lower than they are in nonpolar cyclohexane. More important, all of the 4-substituted trans-9-styrylanthracenes69b-k undergo photochemical trans-wis isomerization. The effect of solvent polarity is obvious from the greatly enhanced quantum yields of trans-wis isomerization observed for most substituted 9-styrylanthracenes in acetonitrile solution. In some cases, the cis-isomers are actually favored at the photostationary state (see Table 10). Exceptional photochemical properties are exhibited by the N , N dimethylamino- and nitro-derivatives 69b and 69k, as both undergo

172

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

trans-wis isomerization, even with rather high quantum yields, in cyclohexane. Moreover, for 69k, the trans-cis isomerization quantum yield decreases markedly in acetonitrile solution. As for the unexpectedly efficient trans-rcis isomerization of the benzoyl derivative 69h in both cyclohexane and acetonitrile, it is conceivable that the reaction involves intramolecular triplet sensitization, since the benzoyl n - n* absorption of the ketone overlaps with the anthracene spectrum around 380 nm. The photochemical properties of 9-styrylanthracenes 70/71, all of them substituted by polar groups in the 10-position of the anthracene, closely resemble those of the parent compounds 68a/69a insofar as trans+& isomerization is either nondetectable or inefficient in both cyclohexane and acetonitrile solution (see Table 1I). The lack of photochemical trans-tcis isomerization of the 10-formyl and benzoyl derivatives 71b, 71c and 71d is of particular interest in view of the substituent effect which both the benzoyl and formyl groups exert on the photochemical trans-wis isomerization of 9styrylanthracenes 69h and 69i (cf. Table 10). By introducing the formyl group into the 10-position of benzoyl-substituted 9-styrylanthracene 69h, trans-9styrylanthracene 71d becomes photochemically inert, even in acetonitrile solution. Conceivably, the anthraldehyde chromophore in 71c and 71d acts as an energy sink. 70171 a b c

d e

R

R

R'

cyano benzoyl formyl formyl methoxy

70a-e

71ae

Various mechanisms for the photoinduced geometrical isomerization of 9styrylanthracenes have been proposed recently [90-931. From steady-state and time-resolved fluorescence measurements on the parent 9-styrylanthracenes 68aJ69a and their formyl and 4-phenylsulfonyl derivatives (68i/69i; 68j/69j), it has been concluded that the geometrical isomerization is an adiabatic process which occurs both in the excited singlet and triplet states [90]. Thus, for the conversion of the parent cis compound 68a into 69a, 85%

173

EXCITED STATE PROPERTIES OF 9-ANTHRYLALKENES

TABLE 11 Quantum Yields of Geometrical Isomerization of 10-Substituted 1-(9-Anthryl)-2-phenyIethylenes 70/71 in Cyclohexane and in Acetonitrile 1951 cis+ trans

70171 a

b

c d e

R’

R

Cyano Benzoyl Formyl

H H H

Formyl Benzoyl Methoxy H

trans-xis

Cyclohexane Acetonitrile Cyclohexane Acetonitrile 0.09 0.23 0.41 ? ?

0.21

0.41 0.34 ? ?

0 0 0 0 0

0.06 0 0 0 0

of the isomerization proceeds as triplet state reaction. Polar solvents, and polar substituents in the para-position of the phenyl moiety, enhance the adiabatic cis+trans isomerization on the surface of the excited singlet state. For the conversion of 6% into 69i (R = CHO), 80% of the isomerization proceeds in the excited singlet state [90]. (For a discussion of other mechanisms for the geometrical isomerization of 9-styrylanthracenes, see [32, 91-93]). The results summarized in Tables 10 and 11 suggest that solvent-solute interactions, in which the substituents play a major role, may affect the photochemistry of diarylethylenes in several ways. For the geometrical isomerization of both formyl-, methoxy-, and N,N-dimethylaminosubstituted styrylpyrenes, whose trans-wis isomerization quantum yields are markedly higher in acetonitrile than in hexane solution, the involvement of solvent-induced electron transfer processes has been proposed [94]. As for substituted 9-styrylanthracenes 68/69, the apparently synergistic effect of polar solvent and polar substituent on the reaction quantum yields suggests that the excited state species undergoing geometrical isomerization is polar in character, and that 9-styrylanthracenes are bichromophoric compounds in which intramolecular electron transfer processes also may be feasible. It is worth noting in this context that the excited state properties of 3’-methoxy-9styrylanthracenes 72 and 73, for which dipolar resonance contributors involving both the methoxy group and the anthracene n-system are not possible, clearly differ from those of the isomeric 4-methoxy compounds 68c/69c. In principle, the photochemical properties of 72/73resemble those of unsubstituted 9-styrylanthracenes 68a/69a, since geometrical isomerization of the cis-isomer 72 to give 73 in cyclohexane and acetonitrile proceeds smoothly with quantum yields of 0.27 and 0.33, respectively, but trans-wis isomerization of 73-+72 is virtually undetectable in both solvents [95].

174

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

73

12

Polar solvents also may affect the geometrical isomerization of ethylenes by affecting the efficiency of intersystem crossing [96,97]. Thus, even in the absence of polar substituents, solvent polarity can be of importance in the photochemical isomerization of aryl-substituted ethylenes. For example, cis1-(9-anthry1)-2-(1-naphthy1)ethylene 74 in cyclohexane isomerizes to the trans-isomer 75 with a quantum yield of 0.05, while the quantum yield in acetonitrile solution is 0.31. Any trans-wis isomerization 75474 in cyclohexane has not been observed, but the reaction does proceed with a preparatively useful quantum yield of 0.05 in acetonitrile [98].

14

15

In analogy to double bond substituted trans-dianthrylethylenes discussed in the preceding section, photoexcited “trans-9-styrylanthracenes”73 and 79 in which the ethylene moiety is substituted by methoxy and nitro, respectively, smoothly undergo geometrical isomerization to give the corresponding “cis”-compounds 76 and 78 [99]. The effect of solvent polarity on the quantum yields of isomerization appears negligible (see Table 12). Both TABLE 12 The Effect of Solvent on the Isomerization Quantum Yields of Substituted BStyrylanthracenes 76/77and 18/79 Solvent Cyclohexane

Benzene Dichloromethane

76-77

77+76

78479

79-78

0.23 0.22 0.17

0.23 0.21 0.23

0.14 0.15 0.10

0.045 0.028 0.013

175

EXCITED STATE PROPERTIES OF 9-ANTHRYLALKENES

76

77

79

78

from an electronic and structural point of view, the nitro derivatives 78/79 have very little in common with the parent 9-styrylanthracenes 68a/69a, as is evident from a comparison of their electronic absorption spectra (cf. Figures 12 and 13), and as has been revealed by their single-crystal X-ray diffraction analyses. It is the spectrum of the "trans-9-styrylanthracene" 79 which resembles that of the cis-9-styrylanthracene 68a insofar as it is characterized by anthracene-like fine structure. In crystalline 79, the planes of the anthracene and the ethylene moiety are found to be in virtual orthogonal arrangement (86.8").The corresponding angle in crystalline 78 is 66.7" [ 1001. For 1-(9-anthryl)-2-nitroethylenes80a/81a and the next higher homologue pair 80b/81b, the quantum yields of geometrical isomerization are markedly solvent dependent, and it is the cis-isomer which in both cases is favored at the photostationary state (see Table 13). The drastic increase in trans-wis isomerization quantum yield in benzene compared to that in cyclohexane solution suggests unique solvent-solute interactions. 80/81

R

a

H

b

methyl

R

/

80s-b

81a-b

/

176

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

Figure 12. Absorption and emission spectra of cis-9-styrylanthracene68a (----) and its trans-isomer 69a in cyclohexane. (Reprinted with permission from Ref. 63. Copyright 1983 American Chemical Society).

300

400

500 nm

Figure 13. Absorption spectra of E- and Z-2-(9-anthryl)-l-nitro-l-phenylethylenes 78/19 in cyclohexane.

EXCITED STATE PROPERTIES OF 9-ANTHRYLALKENES

I77

TABLE 13 Solvent Dependence of Quantum Yields for the Geometrical Isomerization of trans- and cis-l-(9-Anthryl)-2nitroalkenes 80/81

R

R=H

Solvent Benzene Cyclohexane Dichloromethane

80a -+ 81a 81a --* 80a 80b

0.20 0.017 0.008

0.021 0.01 1 0.005

-+

=

Methyl

81b 81b + 80b

0.14 0.038 0.055

0.026 0.11

0.004

Both trans-9-styrylanthracene 69a and 9-nitrovinylanthracene 80a dimerize photochemically by 4n + 2n and 6 n + 6n cycloaddition, i.e., reactions which involve both the anthracene and the ethylene n-systems [SS, 1011. The observed regioselectivity of the dimerization of 9-styrylanthracene 69a leading to dimers 82-84 is worth noting (see page 178). An analogous regioselectivity characterizes the photochemical dimerization of 9phenylethynylanthracene 85 leading to 86, and suggests the involvement of a reaction complex of optimal n-orbital overlap [1021.

C. Fluorescence Properties of 9-Anthrylalkenes and 1,2-Di-9-anthrylethylenes Geometrical differences between cis- and truns-isomers of 1,2-diarylethylenes are typically borne out in differences between their absorption spectra as well as in drastically different radiative properties. Symmetrical cis- 1,2diarylethylenes in solution at room temperature usually are nonfluorescent, unless the central carbon-carbon double bond is part of a rigid ring structure [1). trans-1,2-Diarylethylenes are fluorescent, and large Stokes shifts in conjunction with deviations from the mirror image relationship between absorption and fluorescence spectra are indicative of large differences between their geometries in the electronic ground state and in the emitting excited state [22, 33,77,103,104]. As for cis- and trans-l-(9-anthryl)-2-phenylethylenes (9-styrylanthracenes) 68a and 69a, differences in their ground state molecular geometry in terms of angles between the planes of the three n-systems, viz. anthracene, ethylene, and phenyl, as revealed by X-ray diffraction analyses, are relatively small (see Table 14). The major geometrical difference between the two isomers is found in the angle between the anthracene and ethylene n-systems.

178

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

Q

&

hv [BENZENE 1 1

>

4 4 0 nm

69a

82

+

83

Q Q

84

I:.'

'

I

.-.', :;

P I

& cC

- hr

85

86

TABLE 14 Angle (degrees) Between Planes in 9-Styrylanthracenes [[lo51 1051 Compound EthyleneIAnthracene 68a 69a 86

78.8 65.5 81.9

Ethylene/Phenyl 7.0 6.8 39.1

AnthraceneIPhenyl 76.8 12.4 84.4

179

EXCITED STATE PROPERTIES OF 9-ANTHRYLALKENES

In the cis-isomer 68a, the ethylene system is twisted out of the plane of the anthracene by 78.8", but the trans-isomer 69a also deviates from molecular planarity considerably, namely by as much as 65.5". cis- 1-(9-Anthryl)-2-phenyIethylene 68a, in contrast to both cis-stilbene 38a, is fluorescent (4= 0.16) at room temperand cis- 1,2-di-9-anthrylethylene ature in cyclohexane. In its rather structureless shape and Stokes shift of 4000 cm- ',the emission spectrum closely resembles that of the trans-isomer 69a, whose emission (4 = 0.46) is characterized by a Stokes shift of 4600 cm- (see Figure 12). For the structurally rigid cis-9-styrylanthracene 86, the emission (4 = 0.50) is structured, and is associated with a Stokes shift of only 850 cm ' (see Figure 14). In acetonitrile, the fluorescence properties of 86 are virtually the same as in cyclohexane. By contrast, the fluorescence quantum yield of cis-9-styrylanthracene 68a is markedly affected by solvent polarity, decreasing from 0.16 in cyclohexane to 0.04 in acetonitrile. Timeresolved fluorescence measurements have revealed that photoexcitation of cis-9-styrylanthracene 68a not only leads to emission from the locally excited state but also to emission from electronically excited trans-isomer 69a. The formation of singlet excited trans-isomer 69a by photoexcitation of 68a has been rationalized in terms of an adiabatic photoreaction. Polar solvents and polar substituents, as in cis-9-styrylanthracenes 68i (R = formyl) and 68j

'

~

300

400

500

800 nm

Figure 14. Absorption and emission spectra of cis-9-styrylanthracene68a (-) and rigid styrylanthracene 86 (----) in cyclohexane. (Reprinted with permission from Ref. 90).

180

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

(R = p-phenylsulfonyl) have been found to enhance the proposed adiabatic isomerization [90,93]. The hypothetical existence of two distinct conformers of cis-9-styrylanthracenes has also been invoked in order to explain the solvent-dependent fluorescence properties of cis-9-styrylanthracenes C92-J. The effects of substitution and solvent polarity on the fluorescence properties of trans-9-styrylanthracenes 69a-k in terms of Stokes shift and fluorescence quantum yields have been summarized in Table 15. The fluorescence quantum yields in cyclohexane solution generally are about 0.5, exceptions with lower quantum yields (0.27) being the N,N-dimethylamino and nitro derivatives. For nonpolar substituted trans-9-styrylanthracenesin acetonitrile solution, the quantum yields are of the same order of magnitude as in cyclohexane. By contrast, the fluorescence quantum yields for trans-9styrylanthracenes substituted by polar groups are drastically reduced in acetonitrile, as would be expected for bichromophoric excited state species of polar character (cf. Section 1II.B).

87a.e

88

89

EXCITED STATE PROPERTIES OF 9-ANTHRYLALKENES

181

TABLE 15 Fluorescence Properties of 9-Styrylanthracenes 68/69 in Cyclohexane and Acetonitrile 190, 951

Compound

68a 68i 68j 69a 69b 69c 69d 69e 69f 69g 69h 69i 69j 69k

R H Formyi 4-Phen ylsulfonyl H N,N-Dimethy lamino Methoxy Methyl Phenyl Chloro Cyano Benzoyl Formyl 4-Phen ylsulfon yl Nitro

Stokes Shifts (cm- I ) in Cyclohexanel Acetonitrile

Fluorescence Quantum Yields in Cyclohexanel Acetonitrile 0.1610.040 0.3 3/0.005 0.3 1/0.002 0.4610.45 0.28/0.0076 0.49/0.0044 0.4610.38 0.5410.3 5

0.4710.39 0.48/0.024 0.41/0.10 0.5110.045 0.51/0.026 0.27/nonfl.

The effects of limited molecular flexibility and increasing deviation from coplanarity of the anthracene and ethylene n-systems on the radiative properties have been assessed in a series of symmetrically 2,2-substituted 1-(9anthry1)ethylenes 87 [63]. As for structurally rigid 9-anthrylethylenes 88 and 89, for which rotation about the anthryl-ethylene single bond is not possible, and in which the ethylene double bond has been forced to be coplanar with the anthracene x-system, their fluorescence quantum yields in cyclohexane are exceptionally high, i.e., 0.94 and 0.96, respectively, and the Stokes shifts are less than 200cm-' (see Figure 15). For nonplanar 9-vinylanthracene 87a and its dimethyl derivative 87b,whose ethylene double bond may be twisted out of the plane of the anthracene by about 60", the quantum yield is 0.63, and the Stokes shifts are around 1OOOcm-' (see Table 16). The fluorescence properties of 2,2-diaryl-substituted l-(9-anthryl)ethylenes 87c-e differ markedly from those of 87a,b by a decrease in the quantum yields of emission, and by the loss of vibrational fine structure of the emission spectra, which is associated with a dramatic increase of the Stokes shifts. For the 2,2-diphenyl derivative 87c in cyclohexane solution, the quantum yield is 0.29, and the Stokes shift is 5600cm-'. For 9-anthrylethylene 87e, in which the formal conjugation has been extended by a terminal methylene group, the quantum yield in cyclohexane is as low as

Figure 15. Absorption (-) and emission spectra (----) of 9-anthryl-substituted ethylenes in cyclohexane. (Reprinted with permission from Ref. 63. Copyright 1983 American Chemical Society). TABLE 16 Absorption and Emission Spectral Data For 9-Anthrylethylenes 87a-e in

G ? X

87

(nm)

(nm)

(cm-’)

C,HI2

CH,CN

a b

386 386 388 388 390

405

1200 900 5600 8100 10700

0.63 0.64 0.29 0.0003 0.015

0.72 0.50 0.00033
C

d e

182

4F

Av’

;labs mar

400

495 565 670

EXCITED STATE PROPERTIES OF 9-ANTHRYLALKENES

183

0.015, and the Stokes shift as large as 10700cm-'. The sterically more congested fluorenylidene compound 87d is characterized by a fluorescence quantum yield of only 0.0003, and a Stokes shift of 8 100cm (see Figure 15). Moreover, the solvent dependence of the emission quantum yields of 87c-e is worth noting. The fluorescence quantum yield of the diphenyl derivative 87c decreases from 0.29 in cyclohexane to 0.00033 in acetonitrile. Both 87d and 87e are nonfluorescent in acetonitrile solution. Conceivably, intramolecular electron transfer is responsible for the drastically reduced emission quantum yields (see Table 16) of 2,2-diaryl-substituted 1-(9-anthry1)alkenes 87c-e in acetonitrile solution. Because of the virtually orthogonal arrangement of the anthracene and 2,2-diarylethylene n-systems, 87c-e probably exhibit the excited-state properties of bichromophoric compounds [25, 33, 631. Exceptionally large Stokes shifts indicative of remarkably large differences between the geometry of the ground state and that of the emitting excited state also characterize the room temperature fluorescence of trans-1,2-di-9anthrylethylene 39a and its derivatives in cyclohexane or toluene. Both the temperature and the solvent viscosity affect the quantum yield and the Stokes shift of emission. For example, for 1,2-di(10-acetoxy-9-anthry1)ethylene 90 in cyclohexane at room temperature, the fluorescence quantum yield is 0.002, and the Stokes shift is 9700 cm-'. In a glassy matrix at 77 K, by contrast, the Stokes shift is 3100crn-', and the quantum yield is as high as 0.8 [75,77]. Thus, upon increasing the solvent viscosity, the rate of conformational relaxation is reduced, and geometrical differences between the geometry of the light-absorbing ground state and that of the fluorescent state decrease [22,77]. In line with this interpretation is the observed dependence of emission spectra on solvent viscosity and excitation wavelength dependence for trans- 1,2-di-9-dianthrylethylene 39a (see Figure 16). ~

R

k 90; R = acetoxy

'

184

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES ~~

1: A 2: A

ex

1614

-

10

---

---

/

\

.

/--

3: A

exc

exc

383 nm, in di(2-ethylhexy1)phthalate

4 2 1 nm, in di(2-ethylhexy1)phthalate 4 2 1 nm, in toluene 2

-

1

350

400

450

500

550

600

650 nm

Figure 16. Absorption spectrum (----) and excitation wavelength- and viscositydependent emission spectra of trans-1,2-di-9-anthrylethylene39a at 0°C. (Reprinted with permission from Ref. 33).

The emission spectral properties of trans-dianthrylethylenes 39a-f, whose molecular geometry has been discussed in Section III.A, are summarized in Table 17. The noticeable decrease of the Stokes shift for substituted ethylenes 39c and 39d, which is paralleled absorption spectroscopically by an enhancement of vibrational fine structure [84], suggests that conformational relaxation at room temperature is sterically impaired. The emission spectral properties of trans-dianthrylethylenes 39e are particularly interesting, not only because of the drastically decreased Stokes shift of 3500cm-' in cyclohexane solution, but also because of the unique dependence on solvent polarity (see Table 17). In dichloromethane and in acetonitrile, the Stokes shifts are 7000, and 7700cm- ', respectively. Moreover, in these two solvents, a low-intensity component with a maximum around 420 nm is apparent (see Figure 17), whose excitation spectrum agrees with the absorption spectrum of 39e. If photoexcited trans-dianthrylethylene 39e indeed gives rise to dual emission, its fluorescence properties would resemble those of 9,9'-bianthryl, i.e., the short-wavelength emission could originate from the locally excited state, while the structureless emission around 550nm is attributable to a twisted intramolecular charge transfer (TICT) state [23-25).

185

EXCITED STATE PROPERTIES OF 9-ANTHRYLALKENES

TABLE 17 Emission Spectral Data for trans-Dianthrylethylenes 39a-f I841

a

b c

d e

f

R'

R

H

Solvent

H

Toluene Dichloromethane H CHO Cyclohexane Dichloromethane H MeOCO Cyclohexane Dichloromethane H Me0 Cyclohexane Dichloromethane Me Me0 Cyclohexane Dichloromethane Acetonitrile Me0 Me0 Cyclohexane Dichloromethane

A,,

(nm)

(nm)

Stokes shift (cm- ')

400 400 387 389 388 390 389 391 389 392 389 389 391

650 660 610 610 590 590 525 565 450 540 555 535 565

9600 9800 9400 9300 8800 8700 6600 7900 3500 7000 7700 7000 7900

,labs

39

4F 0.0042 0.0041 0.0021 0.00026 0.0018 0.0004 0.0022 0.0009 0.0014 0.0016 0.0013 0.15" 0.0077'

"From Ref. 85.

ABSORPIION

400

500

600

nm

Figure 17. Absorption and solvent-dependent emission spectra of transdianthrylethylene 39e. (Reprinted with permission from Ref. 84. Copyright 1987 American Chemical Society).

186

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

b ~~10-3 (M-1 cm-1)

EMISSION

ABSORPTION

C

I

/-

I

10 I

I

I

I

/

/

I I

/

5

/

I

I

IN CYCLOHEXANE

0 I

,

I

,

,

,

,

L

,

,

I

1

,

,

1

,

,

,

,

1

,

,

,

,

1

400 500 600 nm Figure 18. Molecular shape, and absorption (-) and emission spectra (----) of cisdianthrylethylene 38e in cyclohexane. (Reprinted with permission from Ref. 84. Copyright 1987 American Chemical Society).

Exceptional fluorescence properties also characterize the cis-isomer 38e. Unsubstituted cis- 1,2-di-9-anthrylethylene 38a and its monosubstituted derivatives such as 38b are nonfluorescent at room temperature. By contrast, cis-dianthrylethylene 38e does fluoresce with quantum yields of 0.0018, 0.0042,and 0.0064 in cyclohexane, dichloromethane, and acetonitrile, respectively. The emission is structureless (see Figure 18),and is associated with a solvent-independent Stokes shift of about 6000 cm- '. As the molecular geometry of 38e is characterized by overlapping anthracene systems [SO], the structureless emission may be attributable to an intramolecular excimer state. As for trans-1,2-dimethoxy-1,2-di-9-anthrylethylene 39f, its fluorescence quantum yield, uniquely, is strongly solvent dependent. In cyclohexane the quantum yield is as high as 0.15, but in dichloromethane the quantum yield has decreased to 0.0077.

187

SOLVENT-ASSISTED INTRAMOLECULAR INTERACTIONS

IV. SOLVENT-ASSISTED INTRAMOLECULAR ANTHRACENE:ARYLCARBONYL INTERACTIONS The fluorescence properties of carbonyl-substituted anthracenes depend both on the position of substitution, and on the nature of the carbonyl substituent [1063.9-Keto derivatives of anthracene, such as 9-acetylanthracene, in which the plane of the carbonyl group is twisted out of the plane of the aromatic Rsystem [191, generally are nonfluorescent at room temperature in solution because of efficient intersystem crossing to the triplet state. By contrast, both 1-aceanthrenone 91 and 2-aceanthrenone 92, whose carbonyl groups are virtually coplanar with the anthracene ring system, do fluoresce at room temperature in cyclohexane solution (see Figure 19), albeit with greatly different quantum yields, viz. 0.0004 and 0.025, respectively [1071.9-Anthroic acid and its ester and amide derivatives, in which the carbonyl group is substituted by electron donating groups, exhibit fluorescence which is attributed to the involvement of dipolar resonance structures [ 1081.

92

91

"/ABSORPTION

AND EMISSION SPECTRA IN C

Figure 19. Absorption (-) in cyclohexane.

Y

C

L

O

H

E

X

A

N

E

1

1

7

and emission spectra (---) of aceanthrenones 91 and 92

188

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

TABLE 18 Fluorescence Quantum Yields of 9-Anthryl Substituted Carbonyl Compounds in Cyclohexane and Dichloromethane Solution [1071

Compound 1-Aceanthrenone91 2-Aceanthrenone 92 9-Anthrylacetaldehyde 93 9-Anthrylacetone 94 1-(9-Anthryl)-3-butanone95 9-Phenacylanthracene96

Cyclohexane

4F

Dichloromethane

4F

0.015 0.66 0.52

0.0004 0.025 0.38 0.49 0.39 0.0026 0.082

w-9-Anthrylpropiophenone 97b

0.53 0.54

0.0008 0.062

Interactions between anthryl-substituted carbonyl compounds and polar solvents are borne out in the solvent dependence of both fluorescence quantum yields and Stokes shifts. Solvent polarity may affect the relative disposition of excited singlet and triplet levels and, consequently, govern the fluorescence quantum yield by affecting the rate of intersystem crossing. Thus, drastic differences in fluorescence properties are observed for aceanthrenones 91 and 92 in cyclohexane and in dichloromethane solution. In the polar solvent, the fluorescence quantum yields of 91 and 92 increase by one order of magnitude. For 9-anthrylalkyl substituted aliphatic carbonyl compounds such as 9-anthrylacetaldehyde 93, 9-anthrylacetone 94, and 9anthrylbutanone 95, whose fluorescence quantum yields in cyclohexane are between 0.38 and 0.49, the increase in quantum yield in dichloromethane is only moderate (see Table 18).

93

94

C=0

95

SOLVENT-ASSISTED INTRAMOLECULAR INTERACTIONS

189

The effect of solvent polarity on the fluorescence quantum yield of 9phenacylanthracene 96 differs profoundly from that observed for its aliphatic analogue anthrylacetone 94. The fluorescence quantum yield of 96 in cyclohexane is only 0.0026 and, remarkably, it does not increase in dichloromethane, but decreases to 0.0002. In view of the absorption spectral overlap of the anthracene and benzoyl chromophores in 96, the low fluorescence quantum yield in cyclohexane may be understood in terms of an intramolecular triplet sensitization. As for the role of solvent polarity, solvent-solute interactions in dichloromethane most likely involve hydrogen bonding to the carbonyl oxygen. Consequently, not only will the disposition of the energy levels for the lowest excited singlet states in bichromophoric 96 be affected, but fluorescence quenching may also involve intramolecular charge transfer interaction because the “protonated” carbonyl moiety could be a better electron acceptor. From a systematic study of bichromophoric compounds 97-99, the importance of substituents and solvent polarity in intramolecular deactivation processes of photoexcited anthracenes by nonconjugatively tethered, and spatially separated, aromatic ketones in their electronic ground state is apparent. For 97a-d, in which the electron acceptor properties of the aromatic ketone moiety have been varied by appropriate p-substitution of the phenyl ring (R is methoxy, H, phenyl, and acetyl, respectively), the longestwavelength absorption maximum band lies at 388 nm, i.e., any ground state effects of substitution are not detectable by UV spectroscopy. Also, the fluorescence spectra of 97a-d in cyclohexane are all related to the absorption spectra by mirror symmetry. However, the fluorescence quantum yields for 97a-d in cyclohexane dramatically are substituent dependent (see Table 19), ranging from 0.20 for the methoxy derivative to 0.00059 for the acetyl compound [33,109].

91 a b c

d

R methoxy H phenyl acetyl

CH,

I c=o

91a-d

190

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

TABLE 19 The E5ect of Solvents and Substituents on the Quantum Yields of Emission from the Locally Excited State of w-9-Anthrylpropiophenones 97 1109]

Solvent

97a

97b

H

Phenyl

97c

97d Acetyl

0.20 0.19 0.15 0.12

0.082 0.10 0.062 0.052 0.016

0.03 1 0.0088" 0.0036" 0.0036" 0.0014"

0.00059 0.000 10" 0.00008" 0.00010" 0.00008"

R: Methoxy

Cyclohexane Chloroform Dichloromethane Acetone Acetonitrile

0.075

"Exciplex emission noticeable.

An explanation for this seemingly remote substituent effect in compounds 97a-d can be deduced from the effect of substituents on the electronic absorption of the carbonyl chromophore. In the spectra of 97a-d, the carbonyl n-n* absorption is hidden under the strong absorption of the anthracene chromophore. However, the position and energy of the n-n* transition can be assessed from the spectra of the correspondingly substituted acetophenones (see Figure 20). Thus, the onset of absorption and, consequently, the excited singlet level, shifts towards lower energy in the same order as the fluorescence quantum yield decreases (i.e., methoxy, H, phenyl, acetyl). This finding suggests that endothermic intramolecular singlet energy transfer from the anthracene to the ketone, followed by intersystem crossing to the triplet state (see Figure 21) may be operative in the quenching process. Intramolecular downhill singlet energy transfer between aromatic hydrocarbons and carbonyl chromophores has been studied extensively [l lo]. It is also worth noting that emission spectroscopic evidence has recently been presented for intramolecular singlet energy transfer from the anthracene chromophore along a linearly conjugated polyene chain to a tetraphenylporphyrin n-system [ill]. In polar solvents such as chloroform, dichloromethane, acetone, and acetonitrile, the fluorescence quantum yields of 97a-d decrease by varying degrees (see Table 19). Moreover, in the case of the phenyl and acetyl derivatives 97c and 97d, the rather drastic decrease of the structured fluorescence from the locally excited anthracene is associated with the appearance of a structureless, red-shifted emission which is attributable to an intramolecular exciplex. For 97d, in which the electron acceptor properties of the aromatic carbonyl moiety are enhanced by p-acetyl substitution, exciplex emission is dominant even in toluene solution (see Figure 22).

Figure 20. n-x* Absorption of substituted acetophenones in cyclohexane.

I

S,

acetophenone

I

anthracene

I

anthrone

Figure 21. Schematic representation of intramolecular energy transfer processes in bichromophoric arylcarbonyl-substituted anthracenes. (Reprinted with permission from Ref. 33). 191

192

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

Figure 22. Absorption and solvent-dependentemission spectra of 976.

The effects of substituents and solvent polarity on the luminescence properties also have been evaluated for of a series of bichromophoric anthronyl-substituted anthracenes 98 and 99. It can be concluded from the quantum yield data summarized in Table 20 for spiro-substituted compounds 98a-e that, dependent on solvent polarity, two different modes of intramolecular interactions between the electronically excited anthracene chromophore and the ground state ketone typically are operative, and both types of interaction result in fluorescence quenching. In nonpolar solvents, fluorescence quenching apparently involves endothermic intramolecular R

98 a

H

b c d

CH,COO CH, CH,O (CH,),SiO

e

p \

98a-e

SOLVENT-ASSISTEDINTRAMOLECULAR INTERACTIONS

193

TABLE 20 The Effect of Solvents and Substituents on the Quantum Yields of Emission from the Locally Excited State of 98a-e [lo91 R: Iz,,, (nm): E," (kcal/rnol): Cyclohexane Toluene Chloroform Dichloromethane Acetone Acetonitrile

98a H 396

98b Acetoxy

72.2

70.8

0.037 0.014

404

0.34 0.20 0.083b 0.0021' 0.0005Sb 0.017' 0.00031' 0.0057' 0.00016b 0.00046'

9& Methyl 407 70.3

0.34 0.20 0.00093b 0.00029b 0.00022' O.OOOIOb

98d Methoxy 410 69.8 0.47 0.35 0.00093b 0.00026b 0.00022' 0.00009'

98e Trimethylsiloxy 422 67.8

0.66 0.66 0.00051' 0.00012b 0.00015b 0.00006

"Energy of the singlet excited state of anthracene moiety. bExciplex emission not included.

energy transfer from the anthracene to the carbonyl chromophore, as the quantum yields of fluorescence are related to the energy gap between the excited singlet levels of the two chromophores (see Figure 21). As shown in Table 20, by substituting the anthracene chromophore with acetoxy, methyl, methoxy, and trimethylsiloxy, the energy level of the excited singlet state can be lowered from about 72 kcal/mol in 98a, to about 68 kcal/mol in 98e, while the position of excited singlet energy level at about 77 kcal/mol for the anthrone should remain unchanged. The increase in energy gap is borne out in the increase of the fluorescence quantum yield from 0.037 [112] for the parent compound 98a (R = H), to 0.66 for the trimethylsiloxy derivative 98e. In agreement with this explanation of fluorescence quenching due to uphill singlet energy transfer, the vinyl conjugated spiro-benzanthracene Wa ( E , = 66.5 kcal/mol) and its methoxy derivative Wb are not subject to the substituent effect in cyclohexane solution, and both are characterized by high fluorescence quantum yields of about 0.70 (see Table 21). 99

R

?

Ha-b

430

E,”

66.5 64.4

(kcal/mol)

0.71 0.65

Cyclohexane

0.64 0.64

Toluene

0.0057b 0.0027b

Chloroform

“I,,, and E , refer to position and energy of the longest wavelength absorption maximum. bDominant exciplex emission not included.

444

H

Me0

(nm)

R

0.00073’ 0.00039’

Acetone

0.00012’ 0.00009b

MeCN

TABLE 21 Solvent Dependence of Quantum Yields of Emission from the Locally Excited State of Spire Substituted Benzanthracenes 99s and 99b 11091

SOLVENT-ASSISTED INTRAMOLECULAR INTERACTIONS

195

In polar solvents, the quantum yields for the emission from the locally excited state of anthronyl-anthracenes 98 and 99 decrease drastically (see Tables 20 and 21), and a structureless, red-shifted exciplex emission is observed (see Figure 23). For the parent compound 98a in dichloromethane, for example, the quantum yield of emission from the exciplex state is 0.012, but that of emission from the locally excited state has decreased to 0.00058 (cf. Tables 20 and 22). Thus, intramolecular exciplex formation between the photoexcited anthracene moiety and the aromatic ketone in its electronic ground state represents the major mode of deactivation in polar solvents. Concerning the interaction of solvents with bichromophoric compounds such as 98, solvent polarity affects mainly the carbonyl chromophore, rather than the anthracene n-system, and alters its electronic character. This conclusion can be drawn from the solvent dependence of the n-n* absorption of the model compound 10,lO-diethylanthrone. Increased solvent polarity, (though not necessarily increased dielectric constant alone) lowers the energy of n-n* transition and concomitantly raises the energy of n-n* transition (see Figure 24). As a result, the lowest excited state of the carbonyl chromophore in polar solvents will no longer be dominantly n-n* in nature,

4nax

tbF

(nm) 520

0.032

Chloroform 0.012 535

Dichloromethane 0.019 510

Benzotrifluoride

0.0032 560

Acetone

0.0011 585

Acetonitrile

TABLE 22 Exciplex Luminescence Data for Spiro-Anthronyl Substituted Anthracene 98a in Polar Solvents I1 121

1

197

SOLVENT-ASSISTED INTRAMOLECULAR INTERACTIONS

'

I

I

:

I

!

in cyclohexane in t o l u e n e

I+----

I

I 1

:

!

,

100

!

'*

t

I

r-

I

.

\ \

320

.\.

.' .

\. ,-. '..

\

x.

\

0

.-.-. - in rnethylene chloride

+ .\

I

1

I

I

3hO

I

I

360

I

I

380

1

7

L OC

Figure 24. The effectof solvent on the n-n* transition of 10,lO-diethylanthrone.

but it will partly assume n-n* character, so that intramolecular excited state interactions in bichromophoric compounds like 98 become feasible upon photoexcitation of the anthracene n-system. From a structural point of view, the anthracene :anthrone exciplex emission from compounds 98 and 99 is of particular interest because the spiro junction precludes a charge-transfer complex geometry in which donor and acceptor are aligned parallel. The X-ray diffraction analysis [113] of 98a reveals that the distance between anthracene and anthrone moiety is as short as 2.5& and that the donor and acceptor n-systems are in an orthogonal arrangement (see Figure 25). In that latter respect, the molecular geometry of spiro compounds like 98 thus resembles that of the formally conjugated but twisted benzanthronyl substituted anthracene 100. In the case of 100, the weakly structured fluorescence spectrum in cyclohexane is associated with a Stokes shift of 4300cm- I , while the structureless emission in dichloromethane solution is characterized by a Stokes shift of 8500cm-' (see Figure 26). Specific solvent-solute interactions are indicated by the slight but noticeable red-shift of about 300cm-', with concomitant decrease in emis-

100

0

Figure 25. Molecular geometries of anthronyl-substituted anthracenes 98a, lola, and 101b. 198

199

SOLVENT-ASSISTED INTRAMOLECULAR INTERACTIONS

300

350

,

,

,

l

400

,

,

,

,

l

450

,

,

,

,

l

(nm) 500

,

,

,

,

l

550

,

,

,

,

l

600

,

,

,

,

,

,

650

Figure 26. Absorption spectrum of benzanthronyl-substituted anthracene 100 in dichloromethane (----), and emission spectra in cyclohexane (---), and dichloromethane (-).

sion intensity, brought about by addition of small amounts of methanol to the dichloromethane solution [115]. As for the efficiency of solvent-assisted charge-transfer interaction between photoexcited anthracenes and ground state ketones, spatial proximity apparentIy enhances the formation of luminescent exciplexes. For exampIe, exciplex luminescence quantum yields for methylene-linked anthronylanthracene lOla (see Figure 27) exceed those of its next higher homologue lOlb (cf. Table 23). Exciplex formation for lOla was found to be pronounced even in toluene (& = 0.016). The ground state molecular geometry is known for both lOla and lOlb from X-ray diffraction analyses [114]. In the crystalline state, 101b assumes a conformation in which the hydrogen atoms of the ethano linkage are staggered, and intramolecular contacts between the anthrone and the anthracene moieties all exceed 4 A. In the methano-linked compound lola, by contrast, intramolecular repulsion of the inner four perihydrogen atoms results in twists about the anthronyl-methyl and anthrylmethyl single bonds by 46" and 6", respectively (see Figure 25). Consequently, intramolecular contact distances between the anthracene and the anthrone 7 ~ systems in lOla are as short as 3 . 3 k

200

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

I

EMISSION SPECTRUM IN ETHYL ACETATE CHLOROFORM ACETONE

LOO

L50

500

600

550

---- -- - -0-1-1-

ACETONITRILE

350

~

~

ABSORPTION AND EMISSION SPECTRA IN CYCLOHEXANE

-- -

650

nm

Figure 27. Absorption and solvent-dependent emission spectra of 10la. (Reprinted with permission from Ref. 33).

The excitation energy and electron acceptor properties of the 9-anthrone chromophore also are affected by substitution of the 10-position, as can be concluded from a comparison of the absorption spectra of 9-anthrone, 10,lOdiethylanthrone, and 10-hydroxy-10-methylanthrone(see Figure 28). The 10hydroxy substituted anthrone, having the onset of its absorption at lowest energy, appears to be the better electron acceptor. Thus, solvent-assisted quenching of the fluorescence from the locally excited state in.conjunction with intramolecular exciplex formation is more pronounced for hydroxysubstituted 102c than for its analogue 102b (see Table 24). TABLE 23 Solvent Dependent Quantum Yields of Exciplex Emission for 1Ola and lOlb 11091

Compound

Chloroform

Dichloromethane

Acetone

0.27

0.26

0.15

0.063

0.04 1

0.017

Acetonitrile ~~

lOla lOlb

~~

0.1 1 0.0018

201

SOLVENT-ASSISTED INTRAMOLECULAR INTERACTIONS R

101

n

m@3

a

b

CHa I

(CH,)"

;@

0

1018-b

@ , \

R

102

X

a

H

H

bc

methoxy

OH H

0

10284

The geometrical requirements for intramolecular exciplex formation involving through-space anthracene :carbonyl charge-transfer interactions may be less stringent than they are for anthracene :olefin exciplexes discussed in the following section, but certain spatial arrangements, more severe than those for intramolecular energy transfer, appear to be essential. For example, the low fluorescence quantum yield (0.047)observed for the cyclopentenonesubstituted anthracene 63 relative to that of cyclopentanone-substituted

Figure 28. n-n* Absorption of substituted 9-anthrones in benzene.

202

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

TABLE 24 Solvent Dependent Quantum Yields of Emission from the Locally Excited State of 102a-c [lo91 102

a

b

c

R

X

H Methoxy Methoxy

H H OH

ma, E, (nm) (kcal/mol) 388 403 403

73.7 71.0 71.0

Cyclohexane

Toluene

Chloroform

0.075 0.27 0.14

0.095 0.24" 0.022b

0.0042b 0.0026 0.00052b

"Including overlapping exciplex emission. bExciplex emission not included.

63

103

anthracene 103 (& = 0.81) in cyclohexane solution is indicative of intramolecular triplet sensitization in the case of 63 (cf. the triplet-state photochemistry of 63 described in Section 1II.A). This conclusion is supported by the more favorable spectral overlap of the cyclopentenone n-z* absorption with the anthracene absorption, as is apparent from the comparison of the UV spectra of corresponding model ketones shown in Figure 29. As for the fluorescence properties of anthryl-cyclopentenone 63 in dichloromethane

Figure 29. n-n* Absorption of the cyclopentanone and cyclopentenone chromophores.

*-.......

0-

300

350 ,

,

,

,

,

,

I

1

400 nm

203

ADIABATIC PHOTOCHEMICAL CYCLOREVERSIONS

solution, exciplex emission has not been observed. Nevertheless, solvent interactions with the carbonyl group are indicated by an increase of the fluorescence quantum yield from 0.047 in cyclohexane to 0.28 in dichloromethane.

V.

ADIABATIC PHOTOCHEMICAL CYCLOREVERSIONS INVOLVING ANTHRACENES

A. Photolytic Adiabatic Fragmentation of Anthracene Adducts Adiabatic photoreactions, by definition [1161, are characterized by the formation of an electronically excited product P* from an electronically excited reactant R*, so that the P* configuration corresponds to a minimum on the excited state potential energy surface (see Figure 30). Most photochemical reactions of organic molecules proceed diabatically, i.e., they do not give products in their electronically excited state [117, 1181. However, the formation of an excimer or exciplex (AB)* by photoexcitation of an anthracene A and subsequent reaction with a ground state molecule B exemplifies an adiabatic photochemical reaction in which the electronically excited product P*, though metastable, is detectable by its radiative deactivation:

+ hv+A* A* + B -+ (AB)* (AB)* -+ A + B + hv' A

R-P

hv

R-P

hv

R-

hv

P

Figure 30. Schematic representation of potential energy curves for adiabatic (a, b) and diabatic (c) photoreactions. (Reprinted with permission from Ref. 33).

204

-

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

When the concept of adiabatic photoreactions was first tested on the photolytic cycloreversion of the 9-methylanthracene photodimer 104, the formation of singlet excited 9-rnethylanthracene A*(R =methyl) according to

hv

- & + A

R

A2

104

(AA)"

A*

was found to proceed with a quantum yield of as low as 0.0004 [119]. The preponderance of the diabatic pathway of this photodissociation has been confirmed in a more recent investigation [lo]. However, at low temperature and high solvent viscosity, excimer formation by photodissociation of 4.n + 471 cyclomers, particularly of those derived from linked anthracenes, is enhanced [cf. 57-59]. Thus, the quantum yield of excimer luminescence in the photolysis of the intramolecular 4n 4n cyclomer of 1-(9-anthryl)-3-(1naphthy1)propane at 120K in highly viscous media is close to unity [120]. Evidence for adiabatic photolytic cycloreversions at room temperature has been obtained more frequently in recent years [121,122]. The adiabatic generation of singlet oxygen by photochemical cycloreversion of the anthracene and 9,lO-dimethylanthracene endoperoxides 105 and 106 proceeds with wavelength-dependent quantum yields of 0.22 and 0.35, respectively, and involves the second excited singlet state of the endoperoxides [1231. Photodissociation of the 1,Cendoperoxide from 1,4-dimethyl-9,10diphenylanthracene was found to yield both fragments, i.e., molecular oxygen and 1,4-dimethyl-9,10-diphenylanthracene,in their electronically excited state [124].

+

105 R = H 106: R =methyl

ADIABATIC PHOTOCHEMICAL CYCLOREVERSIONS

205

As for photodissociations of heterodimers leading to two different hydrocarbon fragments, the efficiencies of adiabatic cycloreversions vary greatly and are seemingly unpredictable. The quantum yield of emission from adiabatically formed tetracene by photodissociation at room temperature of the anthracene-tetracene 4rc+4rc cycloadduct 107 can be as high as 0.05, depending on the wavelength of excitation [125]. The cycloadduct 108 from 9-cyanoanthracene and 1,3-cyclohexadiene undergoes adiabatic photolytic cycloreversion with a quantum yield of larger than 10% [126]. However, the adiabatic photolytic cycloreversion of adducts 109 and 110 from 1,3cyclohexadiene and 9,lO-dichloroanthracene (see Table 25) is much less efficient [1271.

107

109

108

110

TABLE 25 Quantum Efficiencies of Adiabatic Photolytic Cycloreversions Cycloadduct

Singlet Excited Product

L a b .

Oxygen Oxygen Tetracene 9-Cyanoanthracene 9,lO-Dichloroanthracene 9,lO-Dichloroanthracene Anthracene Anthracene

0.22 0.35

~

105 106 107 108 109 110 111 112

0.05

>0.10

0.007-0.015 0.004-0.0068 0.80 0.08

Ref.

206

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

The photolysis of anthracene-benzene adducts 111 and 112 has been studied in detail [128]. Photodissociation of 111 was found to give electronically excited anthracene with a quantum yield of 0.80, but the isomeric 4n + 27c adduct 112 photodissociates mainly diabatically, leading to electronically excited anthracene with a quantum yield of 0.08. The different efficiencies of adiabatic cycloreversions have been rationalized by correlation diagrams involving doubly excited states. Evidence for biradicals as intermediates in the photolyses of 111 and 112 has not been obtained.

111

112

B. Excited State Properties of Lepidopterenes The name lepidopterene* refers to the hydrocarbon 113 (L) whose butterflylike molecular shape was first revealed by X-ray diffraction analysis [129,1301. The structured electronic absorption spectra of lepidopterenes around 270 nm closely resemble that of 9,lO-dihydroanthracene (see Figure 3 1). However, in terms of excited state properties, lepidopterenes have very little in common with 9,10-dihydroanthracene, which in solution fluoresces with a quantum yield of 0.16. By contrast, photoexcitation of lepidopterenes leads mainly to intramolecular exciplexes of n-chromophorically substituted anthracenes in an adiabatic process, for which both the molecular topology *“Lepidoptera” defines the class of insects which includes the butterflies, and which are characterized by having four wings. Etymologically, lepidoptera originates from both Latin (lepis = flake, scale) and Greek (pteron = wing).

ADIABATIC PHOTOCHEMICAL CYCLOREVERSIONS

113

207

114

mentioned above, and the ground state chemical properties of lepidopterenes described below play an important role [131-1351. In solution, lepidopterene 113 (L) is in temperature dependent equilibrium with its cycloreversion product 114 (A). The equilibrium ratio [L]/[A] at room temperature in toluene is 630, and the regeneration of L from A proceeds by an intramolecular Diels- Alder reaction which is associated with an activation energy of 17 kcal/mol [13 13. Monosubstituted lepidopterenes 116 can give rise to two different cycloreversion products, 115 (A-1) and 117 (A-2). When R is methyl, formyl, benzoyl, and cyano, the cycloreversion involves mainly the A-1 isomer, and the [L]/[A] equilibrium ratios at 25°C in toluene are 37,000, 1500, 33, and 22, respectively [73]. Presumably, the formation of the A-1 isomer is favored for steric reasons over that of A-2.

Figure 31. Absorption spectra of lepidopterene 113, its dimethyl derivative 118 (Y = methyl), and 9,lO-dihydroanthracene (DHA) in cyclohexane. Also shown is the emission spectrum of DHA (----).

208

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

\--I A- 2 A-1

115

116

117

However, in the case of 116 when R = Br, the presence of the A-2 cycloreversion product in equilibrium with A- 1 has been established by chemical means [63]. Symmetrical bridgehead substitution of the lepidopterene skeleton may drastically increase the equilibrium ratio (see Table 26). For dimethyllepidopterene 118 (L; Y = methyl), the [L]/[A] ratio is > lo’, i.e., the cycloreversion product 119 (A; Y = methyl) is not detectable by absorption spectroscopy.

xL*

hvi

Y

E*

h v’

L 118

A*

h’J\

E

1

A 119

hv

209

ADIABATIC PHOTOCHEMICAL CYCLOREVERSIONS

TABLE 26 Ground State and Excited State Properties of Lepidopterenes 118 at 298 K

Y CLMAI" L* 4 F b E*4Fb E*l,,(nm)b L c (nmIb

=H

630 0.005 0.58 597 273

Methyl

Ethyl

Benzyl

Phenyl

P-Styryl

>lo7

> 2 x 106 0.025 0.28 573 273

3 x 105 0.003 0.34 584 273

2x104 0.003 0.52 607 213

>lo6

0.013 0.54 573 273

-

0.07 608 210

"In toluene. "In cyclohexane.

Photoexcitation of lepidopterene 118 (L; Y = H in cyclohexane solution results in cycloreversion and gives the electronically excited product E* whose deactivation to ground state is characterized by the structureless emission around 600nm (see Figure 32). The quantum yield of the E* emission is 0.58 (0.80),while that of the emission from the locally excited state L* is only 0.005 (0.016). (The lower quantum yield data have been reported by

L*

hv] ihv'

1

hv'

hvl

ihv

/

t I

I

I

I

I

I

c _ L

I

L

E

i

* I l /

I

I

I

I

Figure 32. Absorption (-) and triple emission spectra (----) of lepidopterene in cyclohexane. (Reprinted with permission from Ref. 33).

210

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

TABLE 27 Adiabatic Photolytic Cycloreversion of Lepidopterene in Methylcyclohexane/Isopentane upon Excitation at 274 nm. Temperature Dependent Fluorescence Quantum Yields and Lifetimes 11341" Quantum Yields of Emission from Temperature (K) 295 250 203 170

I55

130

Lifetimes (ns)

L*

E*

L*

0.016 0.033 0.096 0.18 0.21 0.22

0.80 0.75 0.50 0.17 0.076 0.015

2 6 15 16 19

E* 27 26 26 26 25

'Reprinted with permission. Copyright 1984 American Chemical Society.

Becker et al. [133] for cyclohexane solutions, while the higher values in parentheses refer to measurements in methylcyclohexane/isopentane, reported by Ferguson et al. [134]; cf. Tables 26-28). The highly efficient spontaneous regeneration of L subsequent to the deactivation of E* leads to the conclusion that the geometry of the photolytically generated exciplex E* is such as to have the exocyclic ethylene double bond and the anthracene moiety aligned at a distance favorable for bond formation, i.e., the exciplex E* geometry closely resembles the geometry of L*. The photolytic cycloreversion of lepidopterene to give E* occurs at room temperature with a quantum yield of 90%. Upon lowering the temperature, TABLE 28 Adiabatic Photolytic Cycloreversion of Dimethyllepidopterene (DM-L) in Methylcyclohexane/Isopentane: Temperature Dependent Fluorescence Quantum Yields and Lifetimes 11341"

Temperature (K) 295 250 203 170

Quantum Yields of Emission from

Lifetimes (ns)

DM-L*

DM-E*

DM-A*

DM-L*

DM-E*

0.033 0.08 1 0.2 1 0.26

0.80 0.62 0.21 0.047

0.01 0.0004

2 4 12 14

24 24 24 24

'Reprinted with permission. Copyright 1984 American Chemical Society.

211

ADJABATIC PHOTOCHEMICAL CYCLOREVERSIONS

the quantum yield for the E* emission decreases while that for the emission from the locally excited state L* increases (see Table 27). On the other hand, the efficiency of adiabatic conversion of L* into E* increases upon excitation at shorter wavelengths. Thus, the quantum yield of photolytic cycloreversion at 230 nm remains close to unity down to a temperature of 90 K, suggesting that cycloreversion from higher vibrational levels can compete efficiently with usually fast internal conversion [134]. Photoexcitation of lepidopterene in solution also gives rise to a structured emission of low intensity around 400 nm. This emission is attributable to the deactivation of the locally excited state of the E* rotamer A*, formed mainly by inadvertent direct excitation of the ground state cycloreversion product 114 [131]. The absorption and emission spectra of 114 are typical of the anthracene chromophore (see Figure 33). Selective excitation of 114, experimentally possible because of the suitable ground state &]/[A] equilibrium ratio, gives rise to locally excited A*, which in cyclohexane solution at room temperature has a fluorescence quantum yield of 0.84 [131]. The adiabatic conversion of A* into E* is difficult to detect because it proceeds at 298K

I

A W

U

z 4

m a

-m >

m 0

1

c

m

z

4

W

c

z

I

W U

z

yl

m U W

a

0 3 2

U.

i

I

350

Figure 33. Absorption (-) A in cyclohexane.

400

450

500

550 nm

and emission spectra (----) of the cycloreversion product

212

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

OICHLORO-L

TETRACHLORO-L

120

121

PERI-TETRACHLORO-L

122

with a quantum yield of only 0.005, and is associated with an activation energy of 10.3kcal/mol [133]. For dichlorolepidopterene 120 and tetrachlorolepidopterene 121, whose equilibrium ratios (see Table 29) are also such as to make selective excitation of the corresponding cycloreversion products A possible, the activation energies for the adiabatic conversion of A* into E* are 5.7 and 4.0 kcal/mol, respectively, and the formation of E* by excitation of A is easily detectable (see Figure 34). Concerning the molecular geometry of the cycloreversion product 114, molecular mechanics calculations ideally arrive at a conformation "A60" for the 1,2,2-substituted ethane which is characterized by a torsion angle of 60" about the ethane bond which links the anthracene and the 1,l-diarylethylene chromophores [135]. Single-crystal X-ray diffraction analysis of the structurally related 10-(9-anthryImethyl)anthronelOla (see Section IV) suggests that the actual molecular geometry of 114 is governed by intramolecular hydrogen atom repulsions, and that "A60 assumes an energetically favorable conformation by twists (of 46" and 6" in the case of the carbonyl derivative TABLE 29 Ground State and Excited State Properties of Chloro-Lepidopterenes at 298 K

CLI/CAI" L*4Fb E*&(Aexc 280nm)b E*&(Aerc 230 nm)b E*A,, (nm)b "In toluene. cyclohexane.

Dichloro-L 120

Tetrachloro-L

peri-Tetrachloro-L

121

122

63
790 < 0.0001

0.15

0.17

628

0.016 628

0.0003 0.002 0.39 0.39

610

213

ADIABATIC PHOTOCHEMICAL CYCLOREVERSIONS

-

A*

1

E*

I:

5;

I

z

c'

l

*

h

350 *

1

8

4

I

400 I

I

1

1

I

4 50 1

1

1

1

'

1

1

500

1

'

1

550 1

1

1

1

1

I

600

I

I

I

f

i

I

t

8

650 nm

Figure 34. Absorption spectrum (--.-) and dual emission spectrum (-) tetrachloro-A in toluene at 323 K.

8

1

of

101a) about its two essential carbon-carbon single bonds. The formation of a second A-conformer "A1 80*" has been detected in the photolysis of lepidopterene in solution at low temperature and at high solvent viscosity. It has been suggested that the geometry of this conformer is that of the exciplex, but differs from E*-geometry by both butterfly angle and intramolecular spacing between the two n-systems. A schematic correlation of ground state and excited state processes of lepidopterene is shown in Figure 35 [135]. The adiabatic photochemical cycloreversion of dimethyllepidopterene (DM-L) 118 (Y = methyl) is of particular interest because the exceptionally large [L]/[A] equilibrium ratio of > lo7 precludes direct photoexcitation of DM-A 119 (Y = methyl). However, the radiative deactivation of DM-A* to DM-A can be detected by steady state emission spectroscopy, indicating that electronically excited DM-E* not only deactivates by luminescence with high quantum yield (0.54 in cyclohexane; 0.80 in methylcyclohexane/isopentane), but also undergoes adiabatic conversion into its rotamer DM-A*. The rotamerization is a thermally activated process with an activation energy of 9.3 kcal/mol (see Figure 36 and Table 28). The activation energy for the conversion of L* into E* increases from 3.6 kcal/mol for lepidopterene to 4.2 kcal/mol for its dimethyl derivative. The slight increase in DM-E*

DECREASING DEGREE

OF

T-SYSTEM OVERLAP I N

v

A

Figure 35. Potential energy diagram relating ground-state and excited-state cycloreversion processes of lepidopterene (adapted with permission from Ref. 135. Copyright 1986 American Chemical Society).

3;x -

2-

-

-

.

. j

--

CH,

:

.;

.. . .._.. ._ ... .:: .: ::., :.:: .. .. .. .. ..

:

:,...

t

, ............

:

9 . j kc:il/mol

EMISSION

25 oc

-ABSORPTION: :

48 OC

fi!

(I,,,

273 n m )

----_______

._ .*

0-

.......

300

I N CVCLOHEXANE

400

500

600 nm

Figure 36. Absorption (.-) and triple emission spectra of dimethyllepidopterene at 298 K (-) and 321 K (----). (Reprinted with permission from Ref. 133).

ADIABATIC PHOTOCHEMICAL CYCLOREVERSIONS

215

emission energy, relative to that of the parent E*, has been attributed to a destabilizing steric effect of bridgehead substitution on the E* exciplex state [134]. In principle, substituted lepidopterenes 118 (see Table 26) undergo photolytic cycloreversion in the same fashion as discussed for the parent compound 113 and its dimethyl derivative, and as outlined on page 208. However, in chlorolepidopterenes 120-122, the pattern of substitution does affect the efficiency of adiabatic excited state cycloreversion. Thus, the quantum yields for the E* emission are markedly reduced for both dichloroL 120 and tetrachloro-L 121 (see Table 29). Conceivably, the close spatial proximity of the chloro-substituents and the aromatic rings induces intersystem crossing to the triplet state. Since the quantum yields for the E* emission increase upon photoexcitation at shorter wavelengths, adiabatic photolytic cycloreversion from higher vibronic levels and radiationless deactivation of L* are two competing processes. As for peri-tetrachloro-L 122, in which the substitution pattern is such as to preclude steric interaction between the chlorine atoms and the aromatic n-systems, the quantum yield of photolytic cycloreversion was found to be both fairly high (0.39) and independent of the wavelength of excitation (see Table 29). The photophysical properties of rr-chromophorically substituted lepidopterenes are typically affected by the nature of the substituent. Selective photoexcitation of the substituent is experimentally possible because the absorption spectra of chromophorically substituted lepidopterenes are superpositions of the component spectra. For example, excitation of the naphthalene chromophore in di-l-naphthylmethyl-L 118 (Y = 1naphthylmethyl) at 285 nm leads to naphthalene fluorescence with a quantum yield of 0.29. (The fluorescence quantum yield of l-methylnaphthalene is 0.21). Photoexcitation at wavelengths < 275 nm, however, not only leads to naphthalene fluorescence, but also results in photolytic cycloreversion of the lepidopterene skeleton and population of the E* state, which is characterized by emission around 577 nm. In addition, the formation of an A* rotamer is indicated by the structured emission around 420 nm (see Figure 37). As for the E* emission from di-l-naphthylmethyl-L, its quantum yield is dependent on the wavelength of excitation. Excitation of the L-chromophore at 273nm leads to E* luminescence with a quantum yield of 0.002. Interestingly, photoexcitation at 210 nm, where the absorption of light involves the S, state of the naphthalene moiety, the quantum yield for the E* emission is far higher, viz. 0.16. The observed wavelength dependence of photolytic cycloreversion of di-l-naphthylmethyl-L 118 suggests that intramolecular energy transfer from the naphthalene to the lepidopterene chromophore, as schematically outlined in Figure 37, is an efficient process.

I1b

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

EXCITATION S Y C T R A

EMIS510N SWClRUM

Y

u W

8

3

, m

300

400

500

m

m

Figure 37. Triple emission spectrum of di-( 1-naphthylmethy1)-lepidopterene upon excitation at 210nm. Also shown are the excitation spectra for the naphthalene emission around 330 nm (----),and for the E* emission around 577 nm (-). The inset is a schematic representation of absorption, emission, and energy transfer processes. (Reprinted with permission from Ref. 133).

E*-exciplex formation from lepidopterenes is not limited to solutions but also characterizes their crystal state. Moreover, in the case of polymorphic lepidopterenes, the metastable phases are found to be distinguishable from their thermodynamically favored crystal modifications by a noticeable red shift of emission. It has been concluded from the evaluation of single-crystal structure analyses of polymorphic lepidopterenes that the decrease in exciplex emission energy may be attributable to smaller angles between the upper and lower butterfly wings [136]. Table 30 summarizes the molecular geometries of stable and metastable crystal modifications of lepidopterene and tetrachlorolepidopterene and, for comparison purposes, the corresponding data for dimethyllepidopterene. Characteristic of all lepidopterenes analyzed thus far is the length of the central carbon-carbon single bond which exceeds 1.6A (cf. also [1371). What makes photoexcited lepidopterene and its derivatives undergo adiabatic cycloreversion with so high quantum efficiency?The answer to this question must be linked with fact that the formation of lepidopterene from its cycloreversion product A is a highly efficient ground state process, viz. an intramolecular Diels-Alder reaction, which is symmetry-allowed by Woodward-Hoffmann rules. By the same token, the excited state 4a+2n cycloreversion of lepidopterene L is a symmetry-forbidden process. Thus, it is

_I

54.8 54.8 1.636 540

53.5 52.1 53.5 52.1 1.639 1.645 560

L 48.4 45.5 1.65

L-1 42.2 55.9 1.61 580

L-2

X

45.1 54.4 1.65

L-3

54.8 54.8 1.66 575

CI,-L

D M L : CH,

51.3 56.3 51.3 56.3 1.67 1.67 590

C14-L

*Explanatory comments on crystal structures of lepidopterenes: dimethyllepidopterene, and the stable phase of tetrachlorolepidopterene are centrosymmetric, i.e., the asymmetric unit consists of a molecular half. The stable phase of L, and the metastable phase of tetrachlorolepidopterene also are centrosymmetric, but the asymmetric unit consists of two half-molecules. The asymmetric unit of the metastable phase of lepidopterene consists of the three independent, complete molecules L-1,L-2,and L-3. bReprinted with permission from Ref. 33.

L I (nm)

[(A)

A/C (deg) B/D (deg)

DM-L

X

TABLE 30 Crystal Luminescence of Lepidopterenes: Butterfly-Angles Between Upper (A; B) and Lower (C; D ) Aromatic Rings, and Length ( I ) of Central Single Bond of Dimethyllepidopterene (DM-L), Lepidopterene (L), and Tetrachloro-Lepidopterene (CI4-L).vb (cf. Ref. 136)

218

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

a higher excited state of L* which correlates with the ground state of the cycloreversion product “E”. Therefore, the adiabatic formation of E* is favored over a stepwise photodissociation of L* leading to biradical intermediates and diabatic deactivation to ground state products [134,1381. The molecular geometry of the lepidopterene skeleton remains a feature of unique importance for the observed photochemical reaction. The photolytic 471 + 271 cycloreversion generates an electronically excited product in which the diene and dienophile moieties are bound to face each other in an arrangement which, subsequent to deactivation to the Franck-Condon ground state, is an ideal one for bond formation.

EPILOGUE The framework for the present contribution was provided by photochemical investigations of the past decade, and most of the chromophorically substituted anthracenes studied in our laboratory first became known during that period. A growing interest in the synthesis of anthracenes originated from a seemingly unrelated earlier photochemical study which dealt with the formation of 4-hydroxybenzyl radicals by hydrogen atom transfer from diphenylhydroxymethyl radicals to p-quinone methides [139, 1403. Thus, methylene anthrone 123 was found to undergo reductive dimerization, leading to previously unknown 1,2-di(10-hydroxy-9-anthry1)ethane124, from which 1,2-di-9-anthrylethane 7a as well as many of its derivatives listed in Table 2 became easily accessible [141,1423. The acid-catalyzed dimerization of methylene anthrone, investigated for comparison purposes [ 1431, became essential for the synthesis of spiro-anthronyl substituted anthracenes such as 98 whose solvent-assisted intramolecular exciplex luminescence is discussed in Section IV. The dehydrogenation of 1,2-di(10-hydroxy-9-anthry1)ethane 124 afforded dianthronylidene ethane 125 [1421, which attracted my attention because it formed crystal phases of unique luminescence properties [144,1451. Moreover, reduction of 125 by diphenylhydroxymethyl radicals led to the previously unknown 1,2-di(lO-hydroxy-9-anthryI)ethylene126 [1421. When the stereochemistry of 126 and its derivatives was to be established by absorption and emission spectroscopy, the cis- and trans-isomers of unsubstituted 1,2-di-9-anthrylethylene, i.e., compounds 38a/39a of Section 111 were needed for spectral comparison. cis- and trans-1,2-Di-9-anthrylethylene are reported in the literature [146]. More important, the electron spectral properties of both geometrical isomers had been the subject of several photophysical studies published in 196311964

219

ADIABATIC PHOTOCHEMICAL CYCLOREVERSIONS

YH + 2PhzC.

-

OH

XYLENE

-Ph2CD

123

dH

&

+ 2 Ph2C.4”

XYLENE

124

OH

I;H

@

-Ph2C0

0

OH

125

126

[cf. 1463, and the trans-isomer 39a was described to be nonfluorescent, while the cis-isomer 38a reportedly exhibited blue fluorescence with a lifetime of 13.3 ns at room temperature. trans-Dianthrylethylene 39a was synthesized in our laboratory and found to be fluorescent. The Stokes shift of about 10,000cm (see Figure 16) was of interest, and so was the fact that photoexcited trans-dianthrylethylene 39a did not undergo geometrical isomerization. As for the literature cis- 1,2-di-9anthrylethylene, which was prepared by pyrolysis of 9-anthraldehyde azine, it was indeed fluorescent as described, but it turned out to be 9cyanoanthracene. Unexpectedly, an impulsive attempt to publish a brief note entitled “The Structure of the So-called cis-1,2-Di-9-anthrylethylene”was unsuccessful. The Editor of the Journal oforganic Chemistry found it appropriate to adopt a referee’s opinion that publication was not justified, “at least not until the compound in question has been synthesized and properly characterized.” Consequential to this editorial decision, some remarkable results were obtained during the ensuing efforts to synthesize and properly characterize the compound in question. As trans-1,2-di-9-anthrylethylene resisted photochemical geometrical isomerization, other synthetic avenues to the cis-isomer were explored. One of ~

220

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

Cn, I

128 127

38s

I

-4 0

41

these routes involved catalytic hydrogenation of bis-9-anthrylacetylene which we tried to prepare from a 12-dibromo- 1,2-di-9-anthrylethane described in the literature. However, we found that this dibromo compound of the literature was 1,2-di(lO-bromo-9-anthryl)ethane [78]. An esoteric attempt to synthesize cis-dianthrylethylene 38a via the cyclobutanol derivative 128 involved as starting material 1,2-di-9anthrylethanol 127, which was reportedly formed in good yield in a one-pot reaction by lithium aluminum hydride reduction of 9-anthraldehyde 129. Again, we had to abandon this route to cis-dianthrylethylene when the dianthrylethanol of the literature was found to be the 9,lO-dihydroanthracene derivative 130. However, “dianthrylethanol” 130 did form the acetate 131, which upon treatment with base afforded, via elimination product 114, hydrocarbon 113, viz. lepidopterene [1471, whose structure, fortunately, had been established in 1975 [129,130]. When I observed the “E*” luminescence shown in Figure 32, the emission not only aroused interest and excitement but also confusion, because lepidopterene, prior to 1979 always obtained as by-product in the synthesis of its isomer 1,2-di-9-anthrylethane 7a, was described in the literature as “nonfluorescent”. Today, one may speculate as to why the E* exciplex luminescence discussed in Section V.B has evaded earlier discovery. With the benefit of hindsight, I presume that lepidopterene previously never got photoexcited. Its electron spectrum (see Figure 32) simply precludes the absorption of light through ordinary laboratory glassware.

ADIABATIC PHOTOCHEMICAL CYCLOREVERSIONS

&

Li AIH,

221

~

THF

129

130

K-tert-butaxide

“114”

*

113

Eventually, cis-l,2-di-9-anthrylethylene was prepared, as originally planned, by catalytic hydrogenation of bis-9-anthrylacetylene 53 [148]. Also, thanks to the appearance of a suitable oxidant for the conversion of primary and secondary alcohols into aldehydes and ketones [149], the synthesis of 1,2-di-9-anthrylethanol 127 has been accomplished [Sl]. Numerous other chromophorically substituted anthracenes discussed in Sections I1 and 111 then became available as a matter of consequence. In retrospect, I feel gratefully indebted not only to my co-workers for their collaboration, but also to those numerous authors from whose earlier published efforts in the area of anthracene chemistry we have benefitted in our photochemical

222

RELATIONSHIPS IN CHROMOPHORICALLY SUBSTITUTED ANTHRACENES

studies. Moreover, I feel obliged to acknowledge my very first encounter with organic photochemistry, which happened to involve formation and reactions of diphenylhydroxymethyl radicals [150).

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Advances in Photochemistry, Volume15 Edited by David H. Volman, George S Hammond, Klaus Gollnick Copyright © 1990 John Wiley & Sons, Inc.

PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME Kurt Schaffner, Silvia E. Braslavsky and Alfred R. Holzwarth Max-Planck-Institut fur Strahlenchemie, D-4330 Miilheim a.d. Ruhr, West Germany

CONTENTS I. Introduction A. Discovery, isolation, and characterization of phytochrome B. The structure of the visible-light absorbing P, and P,, chromophores 11. Photophysical properties of phytochrome A. The excited singlet P, bilatriene chromophore B. The “anomalous blue” emission of P, C. The tryptophane fluorescence in P, and P,, D. The photophysics of P,, 111. The light-induced P, + P,, transformation A. Primary photoreactions and the first thermal steps B. Comparison of the 64-kDa and 114/118-kDa P, samples with free and liposome-bound 124-kDa P, C. The terminal reaction steps of the P,, formation D. The influence of protein stabilization by ethylene glycol and ubiquitin E. The chemical nature of the individual reaction steps IV. The light-induced Pf, -+ P, transformation V. Conclusion Acknowledgments References 229

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PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME

I. INTRODUCTION Light serves living organisms as a source of either information or energy, or both. Animals use light predominantly as a source of information. Position and movement of objects are recorded, and their form and color are recognized in all details. Most of the animal behavior is determined by such visual contacts and prompt responses, in search of food, of shelter and of partners for reproduction, and at flight from enemies. Higher plants cannot profit of such vision coupled with immediate response which most often results in motion of some kind. They are bound to a particular site for their lifetime, depending on whichever energy sources are available locally, and they have to respond to any environmental, including adverse, changes without the option to move elsewhere. Lacking mobility, they are instead forced to resort to physiological regulatory responses, like, e.g., adapting their growth parameters, their shape, etc., to the conditions imposed by the environment [l]. In order to do this, higher as well as more primitive plants must be able to sense-to “see”-their surroundings. However, their light monitoring system is of a basically different and more varied nature than the animal eye, since the plants’ “vision” has to fulfill still another very important function: in addition to providing information on the environment and on the time of the day, it also comprises, in photoautotrophic organisms, light receptors designed to harvest the light energy needed in photosynthesis. The light fluence, its angular distribution, and spectral composition are therefore of critical importance. It is not surprising, therefore, that the light perception of animals and of plants have developed in entirely different directions. These are manifest in the plant kingdom at large by such diverse response categories as photomorphogenesis, photokinesis (i.e., photoperiodism, phototaxis, phototropism), etc. These light responses are coupled to several different light receptor systems, in contrast to the visual process in the animal eye.

A. Discovery, Isolation, and Characterization of Phytochrome The presently best known light receptor in all higher green plants, certain mosses, liverworts and algae is phytochrome. It absorbs visible light both in the blue and red regions (with maxima at around 380 nm and at wavelengths longer than 650 nm). The energy of the absorbed light is then converted into signals which exert photomorphogenic controls (i.e., controls of plant growth and development by light independent of photosynthesis), primarily as a function of the spectral composition of the absorbed light [2- lo]. While

Figure 1. Oat (left) and pea (right) seedlings which have been grown simultaneously in the dark (etiolated plant at left in each picture) and in normal daylight (green plant at right). The pale yellow colour of the etiolated chlorophyll-free plants is due to anthocyanin dyes. Note that only under daylight conditions thc lcaves developed normally, chlorophyll synthesis had set in, and thc growth of the roots, stalks and internodes was reduced.

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INTRODUCTION

phytochrome action mostly responds to the red portion of the visible spectrum, other-still less well defined-receptors selectively absorb in the blue spectral range. Flavins, carotenoids, and certain porphyrins are believed to be the chromophores of such blue-light receptors (so-called cryptochromes). In certain cases phytochrome appears to coact with blue-light receptors. (For comprehensive surveys of this field of photoreceptors see Senger [Ill] and Mohr [8h, 121.) The influence of light on growth and development of plants is particularly manifest with seedlings which have been grown in the dark from the beginning of germination. Such etiolated plants are whitish and possess relatively long stalks and internodes, either without leaves or with underdeveloped leaves only. When exposed to light, the plants turn green, the excessive longitudinal growth is reduced, and the leaves develop normally (Figure 1). But also outdoors, the vertical growth is usually more rapid in the shadow of trees and bushes than in open areas and on exposure to full daylight. This effect is mainly a response to the quality, i.e., spectral composition, of the light rather than to its quantity. Figure 2 illustrates the 6.0

-

2.0

-

,

,

,

I

I

,

,

l

,

,

,

-

400

500

600

I

700

Wavelength (nm)

,

- 0 800

Figure 2. Top: Spectral photon distribution of daylight on a sunny day. The distribution remains qualitatively similar on cloudy days with the total photon irradiance (i.e., E,) in the region 400-800 nm reduced by a factor of up to 10. Bottom: The spectral photon distribution under a dense patch of ivy leaves near to the ground surface, measured on the same day as the spectrum on top. Not only is the total photon irradiance ( E , ) reduced by a factor of approx. 100 by the leaves, but also the spectral distribution is markedly altered, in particular at around 700 nm and above. (Both spectra from Smith [13].)

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considerable spectral change which is undergone by daylight when passing through a canopy of green leaves. The blue-green-red range of the visible spectrum is much more strongly attenuated than the far-red region above 700nm, and it is notably the intensity ratio of red (655-665 nm) to far-red (725-735 nm) light which governs the photomorphogenic processes in plant development. At a very early stage, action spectra (i.e., the wavelength dependence) of the induction of such diverse responses to light as the germination of seeds [14171, the opening of hooks of etiolated seedlings [18], and blooming [19,20], had indicated that a common regulator might control these processes, and that this regulator is activated by red light and inactivated by far-red light. A particularly impressive illustration of the reversibility of the red/far-red control is provided by the following experiment 114- 171. Lettuce seeds, kept in the dark at otherwise optimal humidity and temperature conditions for germination, are subjected to series of alternating red and far-red light flashes, each of several minutes duration. Germination is only observed when the last light flash was red, and it is invariably prevented when the last flash was far-red. The wavelength dependence of this behavior is exemplified in Figure 3. When the reversible red/far-red light effect on a wide array of photomorphogenic patterns was fully recognized, the action spectra were soon attributed, as a working hypothesis, to the absorption spectra of the physiologically inactive P, (red absorbing) and active P,, (far-red absorbing) forms of a single “photomorphogenic pigment” called phytochrome. The absorptions pointed to one or several bilatrienes ( =linear tetrapyrroles) as possible chromophore(s) of the regulating system. In a spectacular effort of only a few hours, Butler et al. [21] in 1959 succeeded in characterizing this pigment in crude extracts by in vivo absorption spectroscopy, and to establish its proteinic nature (for a retrospect see Butler [22]). The subsequent isolation of the pure native chromoprotein proved considerably more tedious, as witnessed by the 24 years it took to subsequently reach this goal. The first phytochrome preparation which exhibited the anticipated P, % P,, photochromism had a molecular weight of about 64000 (= 64 kDa [23]); initial estimate 60kDa [24]). It later proved to be a product of endogenous proteolysis during the isolation procedure [25], which had just about halved the protein. Subsequent preparations of 114/118 kDa were still found to have undergone proteolytic degradation, with 6- and 4-kDa fragments missing from the N-terminus of the polypeptide chain [23,26,27]. Only when rapid isolation procedures are employed in the presence of protease inhibitors and under reducing conditions minimizing the oxidation of the aromatic amino acids, can undegraded phytochrome of 2 120 kDa molecular weight be obtained from etiolated (dark-grown) seedlings. The

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INTRODUCTION

100

c

0 .c

2

._

aJ

0

50

s

0

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600

Wavelength (nm)

800

1000

Figure 3. The effect of wavelength on the germination of lettuce seeds which were initially irradiated to establish 50% germination. Additional red light (600-700 nm) further promotes germination most efficiently, whereas far-red light (700-750 nm) inhibits germination. (Action spectrum after Flint and Alister [151.)

native origin of such preparations was established when Vierstra and Quail [27] as well as Lagarias and co-workers [28] found them to have the same molecular weight as the cell-free translation product of mRNA from oat [29]. The size of phytochrome varies slightly with the plant source; i.e., the molecular weights are 120 (from zucchini), 121 (from peas), 124 (from oat and rye), and 127kDa (from corn) [30]. In the case of, e.g., phytochrome from oat this corresponds to a polypeptide chain of 1128 amino acids ( = 124.9 kDa), the sequence of which has been deduced from cDNA and a genomic clone sequence analysis 1311. The isolation of phytochrome from green (lightgrown) plants is more cumbersome ( = type-I1 phytochrome*). According to Pratt and Cordonnier [32] the yield of pure undegraded phytochrome is 10100 times lower than from etiolated tissue. For example, in the case of lightgrown oat seedlings, the yield of type41 phytochrome amounts to approx. 0.0004% of the fresh weight (corresponding to a mere 2 3 x lo-* M in vivo concentration!). Moreover, the chlorophyll present in green tissue renders spectral controls during the isolation procedure considerably more difficult. *The terms type4 and type41 phytochrome have been recommended for native phytochrome from etiolated and light-grown plants, respectively, by the European Symposium on Photomorphogenesis in Plants, Freiburg i. Br., 1989.

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All detailed investigations of the P, % P,, transformation have to this date been carried out with phytochrome material from etiolated plants. It is interesting in this connection to recall immunological evidence that-at the least-the amino acid sequence of the phytochromes from etiolated and green plants may vary somewhat [32]. Further caution is appropriate also with regard to the identity of molecular mechanisms and reactivities of phytochromes isolated from different plant species, which has not yet been established in any case. One may also view these circumstances in connection with the fact that, in addition to regulating the development of higher plants as a function of the spectral characteristics, phytochrome in certain organisms also registers other properties of the incident light. In the case of the alga Mougeotiu, for instance, it monitors the polarization direction of the electrical light vector. It can thus control intracellular movement of chloroplasts [33--351. The photochromic P, % Pf, equilibrium is consonant with the absorption spectra of the two phytochrome forms (Figure 4). Since at 2 730 nm only P,, is absorbing, the equilibrium is entirely shifted to the P, side upon irradiation with wavelengths in this region. The reverse phototransformation with shorter wavelengths is not as quantitative in favor of P,, because of the 0.3

p:c

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1

1

1

1

1

1

1

1

I

0.2

CI

gul n

a 0.1

0

300

400

500 600 Wavelength (nm)

700

800

Figure 4. Absorption spectra of phytochrome (124 kDa) from oat. Before the was measured, the sample was irradiated with far-red light spectrum of P, (-) (Airr = 738 nm). The spectrum of the PI + P,,mixture (---) corresponds to the redlight adapted (Airr = 648 nm) equilibriumof approx. 12% PI 88% Pr,,and Pf:lCd(-) is the spectrum calculated for 100% Pfr.(Spectra after Lagarias et al. [36].)

+

INTRODUCTION

235

overlapping absorptions of the two forms, and pure P,, is therefore not accessible at all by photochemical means. An enrichment of only 87% P,, at best can be obtained by saturating irradiation with red light of 660nm [7,36]. An immunopurification procedure using monoclonal antibodies specific for P, and resulting in a considerable further enrichment of P,, has recently been developed [37,38]. However, the applicability of this method is still quite limited since only very small amounts of P,, are produced in this way. The phytochrome molecule possesses a single chromophore absorbing in the red visible region. This chromophore is covalently bound to the sulfur of cystein-321 (Figure 5 ) and is contained in a hydrophobic pocket in the Nterminal domain largely shielded from contact with the external medium. The presence of only one such chromophore in the large protein was unequivocally ascertained when the proteolytic degradation of phytochrome afforded the chromophore attached to the undecapeptide shown in Figure 5: the amino acid sequence of this fragment is unique in intact phytochrome [31].

I

Pro -His-Ser /

Ala I

Arg I Leu

- Cys (321)-His- Leu -Gln-Tyr -.... I

S

I

+ ....

H 0

COzH

Figure 5. The red-light absorbing chromophore of P, phytochrome. A “stretched conformation with Z,Z,Zconfiguration of the exocyclic double bonds was chosen in analogy to that of the phycocyanobilin chromophores a-84 and 8-84 in C-phycocyanin of cyanobacteria [52]. For other proposals of conformation based on spectroscopic comparisons, see Scheer [47] and Rudiger [48], and for the protonated form of the pyrrolenine nitrogen in P,, see text, and Lagarias and Rapoport [41] and Fodor et al. [53].

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PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME

Electron micrographs have recently shown [l58] that in solution the chromoprotein combines to form a Y-shaped dimer even at below the presumed in vivo concentrations [Sc] (cf. also Section 1.B). The N-terminal domains have been located in one Y branch each, and the carboxyl group domains were found to be associated to form the third branch.

B. The Structure of the Visible-Light Absorbing P, and Pfr Chromophores Constitution and (in part) configuration of the chromophore absorbing the visible light were elucidated in the period 1980-1983 by teams in Berkeley (Lagarias and Rapoport), Braunschweig/Berlin (Gossauer) and Munich (Rudiger and Schneider) for both the P, [39-411 and the P,, forms [42,43] from etiolated plants. Figure 5 shows that it is indeed a bilatriene derivative. The parent compound, called phytochromobilin, possesses a vinylidene group at C(3) of ring A. The covalent attachment to the apoprotein is effected by the thioether bond formally resulting from addition of the cystein-321 thiol group across this vinylidene double bond. The chromophores of P, and P,, are stereoisomeric at the C( 15) double bond: P, possesses the 15Z and P,, the 15E configuration. The configurations of the C(4) and C(9) double bonds are still unknown in both isomers. It should be noted that compounds of this kind can adopt many different conformations. In addition to the configurational double-bond isomerism, formal rotation around the C-C single bonds of the methine bridges provides for further conformational variability [44]. In view of certain absorption characteristics-the very large molar absorption coefficient in the visible (&668 = 1.32 x lo5 M - ’ cm-’ for oat P, [36]) and the low oscillator strength ratio of the absorption bands in the UV and the visible region [45,46] (Figure 4)-the P, and P,, bilatriene chromophores have been proposed to adopt in the native protein one of several possible “stretched” conformations each [47,48] (in contrast to the helicaloid “cyclic” all-syn-Z,Z,Z form which constitutes the crystal lattice of, e.g., the model compound biliverdin dimethyl ester [49], and which also prevails in solutions of such bilatrienes in organic solvents [SO, 511). In this connection the in vivo conformations of the three phycocyanobilin chromophores in C-phycocyanin, a protein of the light-harvesting antennae ( = phycobilisomes) of cyanobacteria and red algae are noteworthy. These chromophores are closely related constitutionally to phytochromobilin: they differ only with regard to the C, substituent in ring D, which is a vinyl group in phytochromobilin and an ethyl group in phycocyanobilin. Their biological function, however, is quite different. They do not trigger by photochemical

INTRODUCTION

237

induction an energy .+ signal transformation, but rather serve to absorb and efficientlytransfer light energy to the photosynthetic reaction center complex. The refined X-ray diffraction analysis of C-phycocyanin by Huber and coworkers [52] has established that two chromophores have Z,Z,Z configuration and “stretched” anti,syn,anti conformations rigidly incorporated into the apoprotein frame. Interestingly, in the third phycocyanobilin chromophore (8-155) the C(15) double bond is strongly twisted and in fact assumes an almost orthogonal arrangement. The anti,syn,anri conformation with Z,Z,Z configuration, which has arbitrarily been chosen in Figure 5 for the stretched P, chromophore, resembles the arrangement of the first two C phycocyanin chromophores, a-84 and p-84. Another facet of the C-phycocyanin chromophores, i.e., the possible protonation of the pyrrolenin nitrogen, has not been included in Figure 5, although Lagarias and Rapoport [41] had previously suggested that this is also the case for the phytochrome chromophore, and Mathies [53] has recently presented resonance Raman data in favor of the pyrrolenin nitrogen being protonated. Since protonation of bilatriene model compounds causes considerable bathochromic shifts in absorption relative to that in organic solvents [54,55], the strong perturbation of the phytochrome chromophore by the protein, which amounts to a red shift of approx. 50 nm, could indeed be readily rationalized by such a protonation. Moreover, the shape of the absorption bands can be taken as an additional argument in favor of a protonated chromophore. Figure 6 compares the phytochromobilin-type red P, absorption with that of the bilatriene chromophores in C-phycocyanin hexamer [56], and of a phycocyanobilin derivative in neutral and acidic solutions. The Gaussian curve analysis of the envelope absorption of the three chromophores in the phycocyanin hexamer, which are presumed to be protonated [52], indicates that the individual contributions are relatively narrow and somewhat structured bands. Clearly, the protonated phycocyanobilin model exhibits narrower and more structured bands than the neutral form, and the spectra of both phytochrome and of C-phycocyanin appear to be more similar to the former. As mentioned above, the photochemically induced P,+ P,, transformation manifests itself in the bilatriene chromophore, when still attached to the undecapeptide fragment, solely by the Z,E isomerism of the C(15) double bond in the P, and P,, froms (cf. Figure 5) [42,43]. It is not yet known whether the chromophore in the intact P,, chromoprotein differs additionally from the P, chromophore with regard to the configuration of any of the other two exocyclic double bonds and/or the conformation. The overall P, + P,, reaction is associated with a change in the dichroic orientation of the long-wavelength transition moment [33,58-631. It has been interpreted to reflect the overall orientational effect composed of the

238

20000

PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME

15000

Wavenumbers (cm-')

Figure 6. Long-wavelength regions of the absorption spectra of 3'-ethylthio-3,3'-dihydrophycocyanobilin dimethyl ester [57] in (A) neutral and (B) acidified chloroform solutions, and of (C) P, phytochrome (124kDa) and (D) C-phycocyanin hexamer (both in buffer solutions of pH 7.8). Although the P, band (C)is possibly a composite of the absorptions of several (stretched) conformers owing to the greater conformational mobility of the phytochrome protein, the full bandwidth at half maximum of the principal Gaussian component is just as narrow as that of the absorptions of the N-protonated stretched Z,Z,Zphycocyanobilins a-84 and 8-84 [52,56] which have been extracted by Gaussian curve analysis from the envelope absorption of the C-phycocyanin hexamer (D; for 8-155 see text; the broad band at the shortest wavelength is composed of contributions of all three chromophores), viz., approx. 810cm-' at I,,, 665 nm (C), 9 0 0 m - ' at I,,, 620nm (a-84), and 630cm-' at I,,, 635nm (8-84). The spectra A and B have conclusively been shown to arise from conformer mixtures [57], with the protonated phycocyanobilin derivative (B) comprizing more stretched species [cf. 50, 51, 551.

Z + E configurational change of the chromophore and the subsequent conformational changes of the protein [63]. However, in view of the considerable uncertainties about the size of the orientation angles and about the conformations of the P, and P,, chromophores, definitive conclusions on this point would be premature at the present. In addition to the configurational Z --* E double-bond isomerization of the chromophore, the P, -+ P,,transformation also involves some changes in the secondary and possibly also tertiary structures of the protein. The elucidation of such structural differences has been addressed by circular dichroism spectroscopy [28,64,65], linear dichroism [62], size exclusion chromatography [66], proteolytic degradation experiments [23,27,31,66691, quasi-elastic light scattering [70], hydrogen-tritium exchange [71,72], exogenous chemical probes [73], and tryptophane and tyrosine phosphorescence [74]. In summary [6,8b, 8c, 751, the numerous results pertain to a modification of the selectivity in proteolytic fragmentation, to a better accessibility of the N-terminal 6-kDa polypeptide segment of P, to specific antibodies, and t o a larger number of exchangeable protons and a greater reactivity of the chromophore with certain reagents in P,, than in P,. They can be understood in terms of a local conformational reorganization of the

PHOTOPHYSICAL PROPERTIES OF PHYTOCHROME

239

protein (cf. Section IILE), with the shielding of the chromophore against the environment being generally less effective in P,, than in PI. The shielding is also more effectivein the native chromoprotein than in the partially degraded species.

11. PHOTOPHYSICAL PROPERTIES OF

PHYTOCHROME

The prime goal of our own investigations has been the mechanistic elucidation of the molecular aspects of the P r + P f , transformation in vitro at physiological temperatures. The analytical requirements were in part such that some, notably time-resolved, spectroscopic methods first had to be improved or newly developed. They are described wherever appropriate. In order to qualitatively and quantitatively evaluate the various deactivation channels of the electronically excited bilatriene chromophore in PI, the photophysical and photochemical parameters of this chromophore were determined by stationary fluorescence [76] and time-resolved spectroscopic methods (for in-house developments and first applications of fluorescence and optoacoustics see [77-801 and [81,82], respectively) [2, 9,10,83,84]. Furthermore, a time-resolved fluorescence study has been carried out also with the selectively excited tryptophane residues of PI and P,. The results are discussed in Sections 1I.A-C and 1II.A. Unless specified otherwise, the experiments were performed in vitro with 124-kDa phytochrome isolated from etiolated oat seedlings (Auena satiua L.), and the measurements were carried out with ethylene glycol-containing phosphate buffer solutions at > 275 K.

A. The Excited Singlet P, Bilatriene Chromophore The stationary red fluorescence (versus blue fluorescence; see below) and fluorescence excitation spectra of P, [76] are characterized by relatively narrow long-wavelength emission and excitation (=absorption) bands, a small blue/red intensity ratio of the excitation maxima (1380/Z666= 0.5), and a small Stokes shift between the absorption and emission 0-0bands of 180 kJ mol-' (Figure 7 top; Hendricks, Butler and Siegelman [ 8 5 ] were the first in 1962 to report on the fluorescence of 64-kDa P,). All three criteria are appropriate to differentiate between the helicaloid and the diverse stretched conformations of linear bilatrienes [44,47]. They confirm the proposed stretched alignment of the bilatriene chromophore in P, (see Section 1.B). The

240

PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME

1

Agxc

380 nm

"'A =

520 nrn

I

300

350

400

500

600

700

Wavelength (nm)

Figure 7. Corrected fluorescence and fluorescence excitation spectra of the bilatriene chromophore of P, phytochrome (124 kDa) at 275 K. Top: Uncontaminated red fluorescence (I,,, = 640 nm) and excitation (Ae, = 720 nm). Bottom (spectra after figure in Colombano et al. [87]): Fluorescence obtained with I,,, = 380nm; the anomalous blue emission between 400 and 550nm marked by V originates from contaminants absorbing at 380 nm (cf. Holzwarth et al. [93] for spectra of degraded P, preparations). Excitation: A,, = 520 nm.

spectral shape of the fluorescence and excitation bands of the stretched biliverdin dimethyl ester, in conjunction with a relatively long (nanosecond) lifetime, has been interpreted as an indication of an inherent greater rigidity than in the helicaloid arrangement, owing to stabilization by strong intramolecular hydrogen bonding [SO, 511. The spectral similarity of P,suggests a similar situation for the stretched chromophore in the chromoprotein.

PHOTOPHYSICAL PROPERTIES OF PHYTOCHROME

241

A single-photon-timing (SPT) set-up has served for kinetic fluorescence measurements. It consists of a mode-locked and cavity-dumped dye laser system synchronously pumped by a mode-coupled argon ion laser. Using either conventional or microchannel-plate photomultipliers and appropriate deconvolution methods, lifetimes as low as 5- 10 ps could be measured [77791. This time resolution is two to three times better than that of comparable set-ups using conventional photodetectors. The SPT system is sufficiently sensitive even at extremely low laser flash intensities of 1 x 10" photons/cm2 and a repetition rate of 800 kHz, as they are often prerequisite in photobiological studies. It is also unsurpassed with regard to the wide dynamic range of a few picoseconds to tens of nanoseconds, and to the optimal signalto-noise ratio. Component lifetimes of multiexponential decay curves were determined by an iterative convolution procedure. Quality criteria for the fitting of the calculated and the experimental decay functions were reduced x2 values and plots of weighted residuals (cf. Figure 8). The analysis of the complex fluorescence decay curves of P, was further improved with the help of an algorithm for global data analysis, which had been developed for application in the picosecond kinetics of fluorescence and transient absorption [78,80]. This global data analysis furnishes time-resolved decayassociated spectra, i.e., it affords the resolution of spectra and lifetimes of multiple kinetic components even in cases where the wavelength ranges of these spectra strongly overlap (cf. Figure 9). The decay of the red fluorescence of P,, measured with this SPT equipment, follows a triexponential rate law (Figure 8) [76]. The decayassociated fluorescence spectra of the three components are only very slightly shifted from each other (Figure 9). Together with the excitation spectrum around 665 nm (Figure 7 top), the spectral characteristics of all three fluorescence decay components thus are those of phytochromobilin-type chromophores. Assuming that the corresponding ground state species have similar absorption coefficients (hence radiative lifetimes), the relative amplitudes (RY-3)tof the fluorescence decay components (P:*, approx. 91%; P:*, approx. 8%; P,"', approx. 1%;$ cf. also Table 1) also are a measure of the composition of the ground state phytochrome. The amplitudes of these components should reflect the expected photostationary concentrations upon phototransformation to mixtures of P, Pfr. It is therefore an important result to note that only the two fluorescence components with the

+

tElectronically excited states are denoted by * for the sake of clarity wherever desirable. $Comparable data for phytochrome preparations of the same molecular weight (see Tables 1,3, and 4 and the text) may show small deviations, which are insignificant considering the fact that these values have often been measured at different times with samples obtained from different isolation runs.

242

PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME

&

z

-5.0 4

a

c

ri(ns): 1.071 R;: 0.010

ti(ns): 0.184 R;: 0.078

3 -

r;(ns): 0.048 Rj: 0.912

3 C

8

Z L

0 0

- 1 -

0

0

1

2 3 Time (ns)

4

5

Figure 8. Fluorescence decay of P, phytochrome (124 kDa); excitation at A,,, = 640 nm, emission measured at A,, = 680 nm. The semilogarithmic plots of the measured decay (curve with signal noise) and the decay function calculated from bestfit kinetics parameters obtained by single-decay analysis (thin line superimposed on measured decay) are shown. In the inset the calculated lifetimes ~ f and - relative ~ amplitudes RT- of the decay components are given. On top, a weighted residuals plot (sigma) indicates the deviations of these computer-fitted parameters from the measured decay, with the value of the squared reduced error ( x 2 ) in the inset. The fluorescence decay of the red-light adapted P, + PI, mixture exhibited a comparable triexponential behaviour. (After Figure 4 in Holzwarth et al. [76].)

larger relative amplitudes and shorter lifetimes (7: x 44 ps, ~f x 163ps) possess the P,sPP,,-photochromicproperties characteristic of phytochrome. The smallest P, component with the longest fluorescence lifetime (7: x 1 ns) is not “photoreversible.” The criterion of photochromicity therefore qualifies only P,‘ and Pf as functional phytochrome species, whereas no physiological function can be attributed as yet to the P? component. The contribution of fluorescence to the deactivation of the excited singlet state of the phytochromobilin chromophores in P, and P,, (see Section 1I.D) is negligible in quantitative terms. The total fluorescence quantum yield of the photochromic P,‘ and P: components amounts only to cDf x 2.6 x 10-3 (from Tables 1 and 4). This means that deactivation proceeds predominantly through nonradiative channels, i.e., via internal conversion back to the electronic ground state of P, and via primary photoreaction(s). Nevertheless, the fluorescence efficiency suffices to serve as a sensitive tool to monitor certain aspects of the competing primary reaction(s) of P: (see Sections 1II.A and 1II.C).

PHOTOPHYSICAL PROPERTIES OF PHYTOCHROME

1

t

I 650

243

1

675

1

700

Wavelength (nm)

I

725

Figure 9. Time-resolved decayassociated spectra of the fluorescence components z:-~ with the relative amplitudes Rf-3of P, phytochrome (124 kDa). The spectra were obtained by global data analysis of the singlephoton-timing measurements (i.e., by calculating simultaneously decay amplitudes and lifetimes with variable I,,,s and constant AcXcs, and by assuming the lifetimes to be independent of &,) (cf. Figure 8, and Tables 1 and 4). (Spectra after Figure 2 in Brock et al. [86].)

B. The “Anomalous Blue” Emission of P, A comparison of the quantum yields of the red P, fluorescence upon excitation into the Soret band at 380 nm and the longest-wavelength band at 640nm shows that higher excited states of the bilatriene chromophore populated with the shorter wavelength are exclusively deactivated via internal conversion to the lowest-lying emitting singlet state (S,) of the same chromophore [87,93]. In particular, the apparently lower fluorescence yield at the shorter excitation wavelength appears to be solely due to the additional absorption at this wavelength of contaminants which are responsible for an emission in the range 400-550 nm, i.e., blue-shifted from the accompanying P, fluorescence (Figure 7 bottom). This fluorescence has been termed “anomalous” or “blue.” Contrary to early speculations it is not emitted from a higher excited singlet state, as is known, e.g., for azulene systems. The blue emission has been found in phytochrome samples of different origins and molecular sizes [87,88,93,95]. The molecular entity responsible for this emission presumably plays no physiological role. It presumably arises from a dipyrromethenone structure (bilirubin-type partial chromophore) generated by nucleophilic addition (Figure 10A, nucleophile X)at the central methine carbon C(10). The nucleophile could either be a proteinic group, a solvent molecule, or a reagent used in the isolation procedure (e.g., dithiothreitol which serves as an antioxidant). Processes judged to be analogous on the basis of the fluorescence and fluorescence excitation characteristics of

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PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME

TABLE 1 Comparison of the In Vitro Photophysical and Photochemical Parameters of the Excited Singlet State of 64 kDa, 114/118 kDa and 124 kDa P, Phytochrome from Oat at 275-278 K in Potassium Phosphate Buffer Solution Containing Ethylene Glycol 114/118 kDa Parameter Radiative lifetime ??ad (ns) Measured lifetime r: (PSI 7: (PS) 7: (PSI Calculated fluorescence quantum yield @T(calcd) lo3 a Measured fluorescence quantum yield, corrected for emission from impuritiesb @f(cxptl,corr)

x lo3

Uncorrected prompt heat release from P: and ac Quantum yields of the phototransformation @r+,oo @r+fr @fr-r

64 kDa

(%I

14

14

45f10

-

124 kDa (%)

Ref.

14

[86, 881

-

48flq-91) 4 4 f 3 (-91) [76, 86, 891 290 f30 ( 8) 160f 15 ( 8) 1400f140 900+50(-1) (- 1)

3.2 f0.4

3.2 f0.4

3.4f0.4

[76, 891

1.5& 0.3

2.0 f0.2

2.9 f0.4

[76, 891

0.90

__

0.9 1f0.02

[82, 901

-

-

0.2 1 0.14

0.17 0.17

>0.5 0.15 0.06

[82] [36, 91, 921 [36]

a@f(calsd) = T&ptl)/Tzd.

- -

bSee r931 for an additional fluorescence at shorter wavelengths (attributed to a phytochromobilin nucleophilically substituted at C-lo), [94] for a comparable value of 114/118 kDa P,,, and [84] for a discussion of digressing literature data. 'cf. Eq. (1); values measured at I,,, = 660 nm and effective acoustic transit times of T: = 400 ns (for 64 kDa phytochrome) and 532 ns (124 kDa).

the products, have also been observed in vitro with bilatriene derivatives. Electron delocalization through the hydrogen bridge between the nitrogens of rings B and C renders C(10)the position of lowest electron density in these model compounds. The central carbon is therefore susceptible to strong interaction with external electron donors and, ultimately, to facile nucleophi-

PHOTOPHYSICAL PROPERTIES OF PHYTOCHROME

A

245

protein

-/-

S

HOZC

COzH

0 Me02C

Figure 10. The origin of the contaminants responsible for the “anomalous” blue emission in phytochrome(A) and in the model compound biliverdin dimethyl ester (B) by the addition of a nucleophile at C(10) (A, X; B, EtOH).

lic substitution [96- 1011. Biliverdin dimethyl ester, as one example, in ethanol solution at room temperature reversibly adds a solvent molecule. The resulting new emission [88] was recognized as the fluorescence of 10ethoxybilirubin ester (Figure 10B) [96]. The groups of Falk [102, 1031 and Rudiger and Scheer [lo41 elegantly exploited these properties of C(l0) in bilatrienes and effected selective photochemical double-bond isomerizations of the type observed in phytochrome, i.e., 2 -+ E inversions of the lateral double bonds C(4,5) and/or C(l5,16). For instance, photochemical conversions into the thermally stable E,Z,Z and Z,Z,E diastereoisomers were achieved with Z,Z,Z model chromophores when either adsorbed on alumina or in solution in the presence of strong electron donors. (Bilatrienes in organic solvents otherwise photoisomerize preferentially around the central double bond to form Z,E,Z isomers which are not stable at room temperature in the dark and revert back to the Z,Z,Z configuration [44].) A stepwise sequence was demonstrated for a 2,3-dihydrobilatriene: by way of deliberate and reversible addition of mercaptoethanol to C(lo), the dipyrromethenone partial structure could be selectively Z -P E photoisomerized.

246

PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME

It is noteworthy that the quantum yield of the blue emitting chromophore in phytochrome, x [87], is of the same order of magnitude as that of dipyrromethenones and bilirubin associated with human serum albumin [l05], which corresponds to an increase of more than 20 times over the solution Of values of the latter compounds. The similarity of the values suggests that the radiationless decay processes of the bilirubin-type chromophore are less efficient owing to interaction with the phytochrome apoprotein.

myue

C. The Tryptophane Fluorescence in P, and Pf, Time-resolved fluorescence of aromatic amino acid residues is a sensitive probe for environmental and conformational changes in proteins [106,1071. For any such investigation of phytochrome, the fluorescence from tryptophane (Trp), the most efficient emitter among such amino acid residues, is the obvious target. The presence of altogether IOTrp residues in 124-kDa phytochrome [31] is both an advantage and a disadvantage for the analysis of the protein fluorescence kinetics. The relatively large number offers the opportunity to probe a wide range of protein domains while, at the same time, it may prove difficult to discriminate between so many individual emitters. Clearly only the most powerful measuring and data analysis techniques, i.e., an SPT investigation of the Trp fluorescence kinetics of P, and P,, combined with global data analysis, should be employed in such a study in order to ascertain an optimum of information [78, 1081. The steady-state UV (protein) fluorescence spectra of P, and of the redlight adapted P, P,, mixture (Figure 11) reveal no significant difference at A,, = 295 nm, a wavelength which preferentially excites Trp and minimizes the excitation of tyrosine residues. Fluorescence decays as shown in Figure 12 were obtained in the range 320-350 nm, which covers the central fluorescence range of Trp residues in proteins. A combined global analysis of the complex decay patterns showed that four kinetic components are necessary to satisfactorily fit the entire wavelength-time data surface of both P, and red-light equilibrated P, + PI, mixtures. Surprisingly, the four component lifetimes of P, and P,, are very similar (Table 2), as are also their decay-associated spectra with respect to shape and relative and absolute intensities (Figure 13; amplitude values +lo%). Both spectral positions and lifetimes of the four components are distinctly different. They correspond to four classes consisting of one, two, three and four Trp residue@). Two lifetimes are in the picosecond range, which is unusually short for Trp residues and thus provides particularly useful clues about the environments of their loci. Assignments of the four

+

PHOTOPHYSICAL PROPERTIES OF PHYTOCHROME

320

350

400

450

247

500

Wavelength (nm)

Figure 11. Corrected stationary UV (protein) fluorescence spectra of 124-kDa P, phytochrome and of the red-light adapted mixture P, + P,, at 275 K; A,,, = 295 nm (Holzwarth et al. [lOS]).

lifetime classes to specific Trp residues are difficult at present. The short fluorescence lifetimes undoubtedly are due to efficient quenching by the sulfhydryl groups of close-lying cystein residues. The emission wavelengths respond primarily to the hydrophilicity and/or polarity of the environment, i.e. the vicinity of hydrophobic and hydrophilic amino acid residues and the degree of exposure to the protein surface. Furthermore, energy transfer from the two shortest-lived Trp classes to the bilatriene chromophore cannot be excluded, while any-a priori possible-contribution to the shortest-lived emission by the 19 tyrosine residues of phytochrome was judged to amount to no more than 10% in relative amplitude [78,108]. Surprisingly, the four components lifetimes of P, and P,, are very similar (Table 2), as are also their decay-associated spectra with respect to shape. Furthermore, the relative and absolute intensities exhibit no major differences. (Figure 13; amplitude values k 10%).(For conclusions derived from these findings see Section 1II.E.) A digressing result concerning the Trp fluorescence decay has been reported by Sarkar and Song [lo91 for 114/118-kDa phytochrome: the decay at 293 K was found to be monoexponential with a nanosecond lifetime in the case of P,, and biexponential with lifetimes around 2 and 5 ns in the case of Pfr.Since the degraded phytochrome possesses only eight Trp residues [l lo],

248

PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME

'

-0.1L

3.7 I

I

I

41

"

0

2

4

6

8

10

12

Time (ns)

+

Figure 12. UV (protein) fluorescencedecay of the red-light adapted mixture P, P,, (124 kDa) at 275 K; a,,, = 295 nm, A,, = 330 nm. Inset: calculated lifetimes r;Trp), -4 and relative amplitudes R;Trp)l- of the decay components calculated by single-decay analysis. Top: weighted residuals plot and autocorrelation function of the residuals. The fluorescence decay of pure P, exhibited a comparable tetraexponential behaviour (Holzwarth et al. ClO8-J).

the lifetime distribution could in fact be significantly simpler and kinetic differences easier to determine. However, since phase fluorimetry with only a few modulation frequencies was employed, which is not a very reliable method to differentiate between mono- and multiexponential kinetics, the lifetimes found could well cover more complex kinetics.

D. The Photophysics of Prr No emission at all has been detected to date from PI,.The failure to detect, by stationary fluorescence spectroscopy, any contribution from Pfr limits the emission quantum yield to < lop6.The fluorescence lifetime of P,, can be

TABLE 2 Lifetimes and Approximate Maxima of TimeResolved UV (Protein) Fluorescence Spectra of 124-kDa P, Phytachrome from Oat and of the Red-Light Adapted P, PfrMixture at 275K 178, 1081

+

Lifetime (ps) Component

PI

pr

+ Pr,‘

4k(nm)

45*5

40k4

320,340

64Ok 70

680 f 70

325-330

ZXIP)3

2310+ 120

2430k 120

340

z:,r,,4

5760k300

6130_+300

350

TXIP),

Z&P,Z

‘After saturating irradiation at Air* = 630 nm.

I

I

I

I

Pr

/

/

320

/

/

/ /

/

/

/

/’

e’

330

340

350 320

330

340

350

Wavelength (nm)

Figure 13. Time-resolved decay-associated spectra of the UV (protein) fluorescence components T & ~ ~ )of~ P,- ~phytochrome (124kDa) and of the red-light adapted P, + PI, mixture obtained by global analysis. The dashed line corresponds to the stationary fluorescence spectrum obtained by A,,, = 295 nm (cf. Figure 11). The amplitudes of the two sets of spectra can be compared on an absolute basis (Holzwarth et al. [IOS]). 249

250

PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME

estimated to be considerably shorter than the 44 ps of the main component of P: since the amplitudes of T: and 7: decrease to the expected extent while the lifetimes remain essentially unchanged when P, is in part converted to P,, upon irradiation with red light [76]. Further studies of the excited-state properties of P,, phytochrome have been impaired so far by the lack of emission and by the fact that pure P,, (i.e., without accompanying P,) can only be prepared by immunopurification using monoclonal antibodies and therefore is presently not available in sufficient quantities for spectroscopic studies (see Section 1.A).

111. THE LIGHT-INDUCED Pr --+ Pfr TRANSFORMATION The P, -, P,, transformation has been discussed during the earliest phase of investigations to proceed at least via two spectroscopically identifiable intermediates: pr-,

I700

(= “lumi-R”)

-,

I,,

(= “meta-R”)

-., p,,

The formal terminology which had been used for the intermediates is analogous to those of bacteriorhodopsin. The following results of our own investigations at >,275 K are discussed in terms of the working hypothesis shown in Figure 14, with mention of critical and uncertain points in this scheme whenever appropriate. Studies at relatively low temperatures have led to a similarly complex reaction picture, although the transformation may not always proceed to completion under such conditions [8b]. Low-temperature data have served to identify spectral properties of intermediates, such as the absorption coefficients which have been calculated by Eilfeld and Rudiger [l 113 from measurements in the temperature range 108-273 K with 124-kDa P, in buffer solution containing 66% glycerol.

A. Primary Photoreactions and the First Thermal Steps Flash photolysis with nanosecond laser excitation, monitoring absorption [112- 1141 and optoacoustic signals [82,90,115] of the transients, showed that in the primary reaction of P:, two photoproducts are formed, presumably simultaneously in parallel pathways (see below and [136] for a caveat), with a total quantum yield of 2 0.5: 38% of I+oodecaying with a

7

THE LIGHT-INDUCED P,+ P,, TRANSFORMATION

P:.*

200 @S

Z+E

isomerization of 15,16 C=C

251

\

JPf'

A

conformation relaxations of chromophore protein solvent assisted

Lifetimes: 275 K

Figure 14. Working hypothesis of the mechanism of the P, -+ P,, transformation. The number of steps between I;, and P,, (---) is still unknown, and the thermal restitution of P, from the intermediates is not included. For critical evaluations see in particular Sections IILA, C, and E.

lifetime of ~ i ( 7 0 0 x ) 21 ps, and 62% of I;oo decaying with tZ(700)x 2 0 0 p at 275 K in ethylene glycol-free buffer solution (Table 3). The transient absorption maximum at 695nm, which is also observed in vivo at low temperature [116], had initially been termed "lumi-R and was recognized in the first flash photolytic studies of 64-kDa P, at room temperature by Linschitz [117,118] to arise from two components (for subsequent investigations of 124-kDa P, and of the degraded forms see also [ l l l , 1191271). The energy released as heat in the course of the nonradiative decay of P: to the ground state and detected as a pressure wave by laser-induced optoacoustic spectroscopy (LIOAS) exhibits positive deviations (i.e., 0:> 1; cf. Eq. (1)) from the values which were calculated on the basis of the absorption spectrum of P, alone (Figure 15) [90,115]. This indicates that already within the 15-11s duration of the excitation flash, one or several intermediates must have been formed. These in turn, within the same interval, may again absorb light from an intense laser flash and (at least in part) dissipate heat upon their return to the ground state of the same species (internal conversion) and/or to P, (photochemical back reaction). The formation of primary photoproducts within the nanosecond flash duration was of course to be expected in view of the much shorter lifetimes of the photochromic fluorescence decay compo-

I

I

1.6 -

d

1.4

-

1.2

-

I

-I

1

-

-

.....I

0.8

0.6

- Figure 15. Optoacoustic action spec-

-

-

-

I

I

I

600

650

700

trum of the heat release (a, A-A) from the P,* (64 kDa) and Iyoo states, relative to the light energy absorbed by P, (calibration with CuCI, (.--)). (Spec-

trum after Figure 5 in Jabben et a]. ~901.)

Wavelength (nm)

TABLE 3 Lifetimes, Decay Activation Parameters and Distribution of the Primary Photoproducts I+ooand I:oo of Free (64, 114/118 and 124kDa) and LiposomeBound P, Phytochrome From Oat

Iio0 of free P, at 275 K"

Parameter 'l(700)

@s)

Ea. 1

(kJ x mol-')

64 kDa ~1131

114/118 kDa [121]

21 f 4

15

60+2

logA1

16.2 0.4

*2(700) (ps)

230+30

Ea.2

(kJ x mol- ')

217

63+3

log A ,

15.7 f0.3

at/@, +%)e

0.38f0.04

0.38

124kDa [ll3, 1141

21 +_3 25 3d 54f2 54 f2d 15.1 k0.3 14.9 f0.2d 200f20 240 f 20d 58f 1 58fld 14.7f0.2 14.6f0.1d 0.38f0.02 0.20 f0.02d

+

Iio0 from liposome-bound P, (124 kDa) at 273 K" SYL b

DOL'

[128]

c w

19k2

22k2

59f 10

50f2

16.0+ 1.7

14.3f0.3

226 rfr 23

196f 20

58f3

54+3

14.7f0.5

14.1 f0.2

0.43f0.01

0.43f0.01

data were obtained also for SYL-boundb P, "Potassium phosphate buffer solution. Similar in tris(hydroxymethy1)aminomethane buffer solution. bUnilamellar liposomes of 500-600 A diameter from soybean lecithin. 'Unilamellar lyposomes of .500-600 A diameter from dioleyl lecithin. dPotassium phosphate buffer solution with 20% (v/v) ethylene glycol. 'cf. Eq. (2).

252

THE LIGHT-INDUCED P, + P,, TRANSFORMATION

253

nents P:* and Pf*. The deviations of the a values show a positive maximum around 695 nm characteristic of absorbing I,,,-type intermediates. This was independently confirmed by the observation that during the flash the transient absorption at 695 nm increased by approx. 25%, whereas the P, absorption at 660 nm was bleached during the same period by a comparable percentage [82]. Furthermore, the optoacoustic results also imply that the dynamic photoequilibrium P,esI$Go is established within the 15 ns period (cf. Figure 16), since the thermal forward reaction of I;,, (-+I;,) is considerably slower than the photochemical back reaction to P, at sufficiently high laser intensity [125]. A similar conclusion has been drawn from in vivo experiments with nanosecond laser flashes [34,126,129- 1321. Under natural conditions the equilibrium P, k I:<, is therefore not attained normally. Nevertheless, the in vivo photochemical recovery of P, from the 1700 intermediates could be demonstrated with lettuce seeds: the percentage germination was higher with laser flashes at 620nm than at 690 nm, since at the latter wavelength the ITo0 intermediates photoreverted more efficiently to the physiologically inactive P, [1291. In order to gain a direct experimental approach to probe the internal conversion of P:, LIOAS was further developed to afford time-resolved measurements. For this purpose B-polyvinylidene difluoride foil was introduced [ S 11 as a broad-frequency band piezoelectric detector-instead of the previously used ceramic transducers-which permitted for the first time the A

1.2.

I700

$

I

P,"' Figure 16. State and reaction diagram of the system P,% I;,, --t Ifil including the experimentally verified routes of energy conversion via chemical reaction (---), fluorescence (-.-) and nonradiative internal conversion (--). See text for AE; the value A E of Ibl has not yet been determined. (Diagram after Figure in Heihoff et al. C821.1

254

PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME

direct time-resolved measurement of internal conversion in a system of complex molecular structure. The advantage of this detection device is that it is nonresonant. The signal form is therefore no longer governed by ringing but rather reflects the real-time course of the pressure wave caused by the heat release [133, 1341. Kinetic (yields and lifetimes) and calorimetric LIOAS measurements have thus become possible with a time resolution from about 30 ns to 2 ps. Measurements of phytochrome with this time-resolved LIOAS technique [82] allowed the quantum yield Qr+700 of the photoreaction P, --f I!,oo to be calculated using Eq. (1) [135] (and neglecting the small contribution of fluorescence [76]).

The expression Qr+700AEis the energy stored in the ground and excitedstate I;OOS for longer than the effective acoustic transit time (30-530ns), N A is the Avogadro number, h the Planck constant, v the laser excitation frequency, and a the fraction of the total absorbed energy released as heat within the transit time. At the shortest possible transit time, a mean c1 value of approx. 0.5 was recorded for the heat emission by P:, after correction for the contributions by the I ~ o o sto heat dissipation (cf. Figure 17). Since the reaction P,*+r700 is most likely exothermic, AE (the energy content of the

100

300

500

Effective Acoustic Transit Time (ns)

Figure 17. Semilogarithmic plot of the heat (1 -a) stored by P: and by the ground and excited-state p,oos versus the effective acoustic transit time at two excitation wavelengths: A,,, = 660nm (0-0) and 695 nm (M). Data corrections for the contribution of the 1700s to the heat emission: - 30%at A,,, = 660nm (0---0) and -60% at A,,, = 695 nm (0---0). (Plots after Figure 8 in Heihoff et al. [82].)

THE LIGHT-INDUCED P,-+ P,, TRANSFORMATION

255

products relative to ground-state P,; cf. Figure 16) should be smaller than the 0-0 energy difference between the excited and ground states, P,* - P,. Both this difference (see Section 1II.A) and the stored energy N,hv (calculated for A,,, = 660 nm) amount each to 180 kJ x mol-'. A value of 20.5 is thus derived from Eq. (1) for the quantum yield @,+700 [82], which is more than three times greater than the total yield (cf. Table 1). This means that P, is partly recovered from intermediates such as 1700,I;, etc. via unknown thermal routes. Flash photolysis has shown that the back reaction to P, is sufficiently slow for any recovery of P, absorption to escape detection on a time range up to 2ms. The time-resolved difference spectra (see Figure 19 for an example), obtained from the decay curves of the difference absorptions after laser excitation of P, (cf. Figure 18), reveal that the depleted P, absorption was not recovered within 2 ms, although the newly grown-in absorption of I\oo was bleached within 1 ms [1 12,1133. The disappearance of this absorption can be fitted, for a period of up to 2 ms, to a time law composed of two exponential decay functions and a constant according to Eq. (2). It is appropriate, however, to caution that a build-up of complex equilibria on a Pr+Pf, sequential reaction path (rather than parallel pathways) might also appear as a biexponential decay.

In this equation a, and a2 are the amplitudes of the two decay components with the corresponding first-order rate constants ( l/q1700)). The amplitude a3 designates a residual absorption remaining after 1 ms, which possibly arises from one or several as yet unidentified low-absorption intermediates. Figure 18 illustrates the good agreement of the decrease in Ibo0 absorption with Eq. (2) in the wavelength regions with both positive (at 410 and 695nm) and negative AA values (at 645 nm; the F700 absorption is overcompensated here by the greater loss of P, intensity). The intermediate(s) responsible for the residual absorption a3 could either be additional primary photoproduct(s) or secondary product(s), such as I;,, which have already been formed but are further transformed much more slowly [121, 1231 than the I+oo and I$oospecies possessing microsecond lifetimes. A more accurate evaluation of the data was not possible since very low absorbances had to be monitored and the decay time of a3 was beyond the nano- to millisecond resolution of the experimental set-up used [113]. The existence of parallel pathways (cf. Figure 14) from P, to Itoo and I:o~ has not yet been established beyond doubt. It is compatible with the dependence of the amplitude ratio of the two intermediates (Eq. (2)) on both the solvent and the temperature [113,114].

256

PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME

-1.00 c)

0

7 -? Q

-3.00

-5.00 -7.00

0.30 N

0

7 K

a

0.20 0.10

0

200 400

600 800

Time ( ~ s )

Figure 18. Decay curves of the absorption at 695 nm, 645 nm, and 410nm after excitation of P, (124kDa) with a 15ns-laser flash (A,,, = 650 nm)at 275 K.The experimental values have been fitted with the sum of two exponential decays and a constant (cf. Eq. (2)). On top of each decay curve a weighted residuals plot indicates the deviations of these computer-fitted parameters from the measured decay. (Curves from Figure 4 in Ruzsicska et al. [113].)

Which are the alternatives to the model of two competing parallel primary photoreactions, P : + I+oo+ I$oo, shown in Figure 14? Sequential steps such as P, -,I!joo+ I:oo + 6,have been suggested by Eilfeld et al. [127]. This proposal has not found any independent support yet. In fact, the time resolution in the flash photolysis experiments of these authors does not permit any discrimination of intermediates in the microsecond lifetime range. A differentiation between parallel and sequential mechanisms is possible on examining the influence of temperature in the light of Eq. (3). This expression has been derived El131 to evaluate the relative pre-exponential coefficients of

THE LIGHT-INDUCED P,4P,, TRANSFORMATION

-

257

1.o

T

a

v

PI

Figure 19. Time-resolved difference spectra of the absorption decay curves of I\oo (e.g., see Figure 18) at several monitoring wavelengths, measured 1 p s (M, AA) and l m s ,-.( A---A) after

0.6

U

PI ?

g n

c0

.c

0.2

o

a -0.2

8 In

n Q

- 0.6 620

700 Wavelength (nm)

740

excitation of free and liposome-boundP, (124kDa) with a 15-ns laser flash (A,,, = 650nm) at 275 K. The AA values are normalized to the 1-ps signal amplitude at 695 nm. (Spectra from Figure 2B in Ruzsicska et al. [113] and Figure 1 in Krieg et al. [128].)

Eq. (2) for the case of a sequential transformation of I$oo

to I;oo and ILl (with rate constants k, and k2, respectively). The ratio $ ~ ~ ) / (-E EL]) ~ ~ will ~ remain constant in the temperature range from 273 to 298 K, and the activation energies for the decay of both intermediates are very similar (Table 3). Consequently, the coefficient a2/(al+ u2) should not vary with temperature for the case of a sequential mechanism. This is not in agreement with the finding that the fraction of Z$oo decreases, e.g., in the case of liposome-bound P, [1281 (cf. Section III.B), from (43 k 3)% at 273 K to (32 k 3)% at 297 K. This decrease appears to be much more important than the negligibly small temperature dependence of the fluorescence decay components [78,108], unless the-as yet unknown-quantum yields of each of the proposed primary photochemical steps and the number of P,+I -b 700 cycles performed during a single 15-ns excitation flash (the accumulation of which might reinforce the temperature effect) should eventually be found to correlate with the temperature response. In so far there is no evidence yet pertaining to the question whether I!,oo and I:oo arise from one excited P, form or from two different P, species (e.g., P:* and P,"') with different photochemical quantum efficiencies. The latter possibility in fact represents two independent sequential mechanisms, P :' + I$oo-+ I;, and P:' + I:oo -+ I;], which may or may not merge at the I,, stage and which presently cannot be distinguished experimentally from a parallel reaction scheme. In picosecond transient absorption experiments Lippitsch et al. [1361

(EL] - E

258

PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME

found a delay between the decay of degraded P, from rye and the appearance of the I;,, absorption. They suggest the formation of a pre-17,, intermediate as a primary photoproduct. This certainly deserves further experimental scrutiny. In particular, the questions have to be addressed whether there is a mechanistic discrepancy between the two P,s differing with regard to both their plant origin and proteinic integrity, and/or whether a pre-17,, intermediate also occurs in 124-kDa P, from oat. The answer may ultimately introduce an extension of the mechanistic scheme shown in Figure 14. Finally, Song et al. [1371 have advanced a model including some pre-17,, intermediate which was proposed to equilibrate with P,*.The proposal was based on the lifetime dependence of both the shortest- (photochromic 7:) and the longest-lived (nonphotochromic zf) of the three fluorescence decay components on the buffer viscosity. The viscosity was varied by adding up to 50% (v/v) ethylene glycol or glycerol to the buffer. We have tested Eq. (4) with our own fluorescence data, using the quantum yield of Q r + 7 0 0 2 0.5 and the fluorescence lifetimes and relative amplitudes of the two shorter-lived 7: and z: components in the temperature range 274.3-293.7 K in buffer solutions with 25% (v/v) and without added ethylene glycol [78]. This version is more realistic since the z: and :T amplitudes show the expected photochromic changes, while the amplitude of the long-lived zs component taken into account by Song et al. [1371 does not. We found that under all reasonable conditions, approx. 99% of the thermal back reaction of pre-17,, to P, proceeded directly (k4) rather than via P,*( k - 2 and kl). While this essentially invalidates either of the proposed-a priori unlikely-mechanisms involving the formation of an electronically excited photoproduct and excitedstate % ground state product equilibration, it also shows that the model of Figure 14 is possibly not an adequate description of the detailed excited-state reaction.

The question whether one or two primary photoproducts are formed from

P,is further complicated by the fact that there are two different excited-state species, Pf' and Pf: both of which are potential precursors of either I;,, or

I:,, or both. No correlation between the individual P, and 1700components has as yet been established. One possible model takes the dimeric nature of phytochrome into account. The individual phytochromobilin pockets in any dimeric aggregate could be diastereoisomeric to each other, depending on several still unknown factors such as the local structure of the associative

THE LIGHT-INDUCED P, -+ PrrTRANSFORMATION

259

arrangement, the (non)identity of the contact sites of both monomer components, etc. (cf. Section 1.A). This pertains to [Pr-P,] homodimers and [Pr-Ib1] etc. heterodimeric derivatives, and thus might their [P1-I7,,], provide for ample diversity to accommodate, for the time being, any correlation between the different PI and I,,, components, and indeed the subsequent composite intermediates, Ibl and I i , may just as well fall in line with this hypothesis. The unresolved complexity of this structural aspect adds to the mechanistic diversity of signal generation by P,, which may arise from the dimer heterogeneity, [Pr-P,], [P,-Pf,], and [Pfr-Pfr] [cf. 8e, 31, 1381. The present state of investigations does not provide further insight into the first thermal steps as defined in Figure 14, except for a kinetic analysis of the P,, formation in degraded pea phytochrome. Cordonnier et al. r1211 and Furuya and co-workers [1201 have presented indications for this phytochrome that there are most probably three I,,-type species with millisecond lifetimes.

B. Comparison of the 64-kDa and 114/11&kDa P, Samples with Free and Liposome-bound 124-kDa P, The three forms of PI differing in molecular weight exhibit very similar photophysical and photochemical properties [7,113,114], regarding the shape of the stationary red fluorescence and excitation spectra, the triexponential emission decay function and its component composition, the emission parameters (cf. zt, Q)f(expt,.eorr); cf. also Aussenegg et al. [139]), the heat release (a)by the Pr and I:, species, and the total photochemical quantum yield (Q)r+f,) (Table 1). The similarity extends also to the I;,, products of 64-kDa and 124-kDa P, and their conversion into Ibl, the thermal decay kinetics and activation parameters of I$oo and I;oo of both molecular weights being essentially the same throughout a period of 2ms (Table 3). Obviously, the proteolytic elimination of the 6-, 10- and 60-kDa fragments affects neither photophysics nor photochemistry of PI, nor the thermal reactivity of the I,,, intermediates. This is particularly remarkable since the endogenous proteolyses which eliminate the 6- and 10-kDa fragments occur in the phytochromobilin-binding domain near the N-terminus [1401. The surroundings of the chromophore are modified by this degradation to the extent that its ground-state reactivity in the P, form with external reagents is enhanced, and the absorption spectrum is changed in the P,, form. The insensitivity towards partial protein degradation suggests that the processes involved in the steps of the sequence P, -P I\oo -tI:, -i Ibl are confined to the chromophore and its close surroundings (cf. Figure 14). These findings are

260

PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME

Figure 20. Covalent binding of P, (124 kDa) to the functionalized liposome (cf. Krieg et al. [141]).

further supported by the properties of 124-kDa P, which is covalently attached to the surface of unilamellar lipid vesicles ( = liposomes) (Figure 20) [128,141]. (The binding of phytochrome to liposomes has also been carried out in view of considerations that the initiation of physiological signals by phytochrome may occur in interactions with membrane surfaces [8d, 8g, 142, 1431.) The photochromicity of such liposome-bound P, samples was fully preserved irrespective of the membrane stability (liquid crystal versus gel phase), and the partition into I;,, and I:,,, the lifetimes of these intermediates and the thermodynamic activation parameters of the step I;,, + Ibl in different ethylene glycol-free buffer solutions remained unchanged (Table 3) (see also Section 1II.D). The insensitivity of so many of the photophysical and photochemical properties to proteolytic degradation down to about half of the original molecular weight is remarkable. It is particularly noteworthy when the location of the bilatriene chromophore near the N-terminus-one of the sites of preferential endogenous proteolytic attack-is taken into account and also the fact that the photochromic behavior of chromopeptides obtained by further protein degradation progressively deteriorates and is eventually lost C8bI.

C. The Terminal Reaction Steps of the Pa Formation In order to study the PI, formation, the P, form in ethylene glycol-free phosphate buffer solution was subjected to white-light flashes of 7-ms duration. The growing-in of the absorption at wavelengths 2720nm was monitored in periods from 10 ms to several seconds after each flash with a fast spectrophotometer equipped with a rotating grating sweeping 500-nm ranges in approx. 30ms [144]. The absorption increase in the range of 720 to 750 nm (cf. also [123]) proceeded in two phases according to Eq. ( 5 ) which comprises

THE LIGHT-INDUCED P, -* P,, TRANSFORMATION

261

two monoexponential grow-in functions and a constant a,. The simplest interpretation of this result is-in accordance with previous conclusions by Linschitz and Kasche [118] and Pratt et al. [123]-that there are two immediate precursors of P,, (I; and I;). However, an unequivocal evaluation of the kinetic data has not yet been possible. There is still room for alternative interpretations, in particular with respect to an ultimate differentiation between two competing parallel steps as in Figure 14, which is presently favoured by ourselves, and a sequence 1: + 1; -, P,,, which has been proposed by Eilfeld et al. [159]. A kinetic analysis of these precursors in the shorter time range (below approx. 20 ms) was quite difficult, however, since one or several preceding transients were formed within a few milliseconds. These transients absorb at 720 nm with a coefficient smaller than the coefficients of P, and P,, combined at this wavelength. The lifetimes of the 12’ precursors, formed in an approximate ratio of 1 : 1, were about 1 s and 10-25 ms, respectively, at 275K. The ratio was temperature dependent, with the longer-lived I, exhibiting an activation energy of approx. 40kJ/mol. The data for the shorter-lived species were not sufficiently precise to permit any thermodynamic evaluation. The 12’ intermediates might well be identical with “meta-R,” observed as a thermally stable species at low temperature and high glycerol content [l 111. AAA= ab - a; exp( - k , t ) - a; exp( - k2t)

(5)

The change from H 2 0 to D 2 0 buffer solutions affected the formation of Pfr only very slightly, which excludes any rate-determining H/D effect. It appears premature to decide whether the observable small H/D effect operates at the stage of the formation of the two Ix intermediates or at the stage of their parallel conversions to P,,.

D. The Influence of Protein Stabilization by Ethylene Glycol and Ubiquitin Addition of 20-25% (v/v) ethylene glycol, known to stabilize native protein structures, has often been found to be a convenient measure also for phytochrome. Absorption spectroscopy showed that the additive does not cause any loss of P, P,, photochromicity, and the lifetimes and amplitudes of the Pr components [lo81 as well as the kinetic parameters of the absorption decays of Itoo and I$oo [113] at 272-297 K (Table 3) were hardly affected either. This is in accord with a confinement of the sequence P, -,I,,, + I,,, to the phytochromobilin-binding protein domain without

242

PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME

any far-reaching assistance by other protein domains (Section 1II.B). The increase in amplitude of the nonphotochromic P:' component by high glycerol concentrations, as reported by Song et al. [137], is not expected to be of any consequence for the P, -+P,, transformation. The absence of any influence by ethylene glycol on the properties of Pf," and I:f0 does not exclude, however, that differential influences on the kinetics of individual reaction steps may be operative. And indeed, the ratio I:oo :I:oo proved to be clearly independent of temperature in the presence of ethylene glycol, whereas it changed with temperature otherwise (Figure 21). The additive evidently exerts some control in the first step, possibly by interfering in the phytochromobilin-binding protein domain which determines the rate of formation, and hence the ratio, of the two primary photoproducts. It should be noted that this interference does not change the total photochemical conversion of the P: components. The subsequent reaction steps to I:, are not measurably affected by the addition of ethylene glycol. A search for other agents to modify the dynamics of the P, +Pfr transformation, and in particular to influence differentially the reaction intermediates, focused on cellular constituents which presumably interact in vivo with phytochrome. Ubiquitin, an 8.5-kDa protein claimed to bind covalently in vivo to P,, [160] has now also been found to interact in vitro with P, in the absence of any other cellular constituent [161]. The protein

I

270

280

,

1

,

290

T (K)

Figure 21. Temperature dependence of the amplitudes a, and a, of the I:oo and I+oo absorption decay components (cf. Eq. (2)). A, 0, 4 = free P, (124kDa) and 124-kDa P, bound to two different liposomes in ethylene glycol-free potassium phosphate buffer; A---A = free P, (124kDa)in potassium phosphate buffer with added 25% (v/v) ethylene glycol. (From Figure 4 in Krieg et al. [128].)

THE LIGHT-INDUCED P,-+ P,, TRANSFORMATION

263

dimer is dissociated and ubiquitin-P, complexes are formed in which the PI monomer is associated with ubiquitin without covalent binding. These complexes are still fully photoreversible, the PI absorption spectrum is unchanged, and gel electrophoresis under denaturing conditions smoothly restores the band of uncomplexed 124-kDa phytochrome. In addition to the monomerization effect, complexation with ubiquitin shifts the 695-nm absorption maximum of the ,!I intermediates differentially to shorter wavelengths by approx. lOnm, with the shorter-lived I:, being affected at a lower ubiquitin : PI ratio (5 : 1) than If,, (at x 20 : 1). Also, the bleaching of the PI absorption around 660 nm, which is normally maintained for as long as 2 ms after excitation (cf. Figure 19), is recovered on the microsecond time-scale in the presence of ubiquitin. The results altogether point to a reversible interaction of ubiquitin with the protein pocket housing the bilatriene chromophore. Evidently this constitutes a possibility-in contrast to previous transformations with non-biological reagents-to monitor differentially the complex reaction dynamics without a lasting chemical change in the domains involved directly.

E. The Chemical Nature of the Individual Reaction Steps When the pyrrolenin nitrogen of the bilatriene chromophore is assumed not to be protonated, the constitutional identity of the P, and P,, chromophores limits the a priori possible choice of primary photochemical processes of PIto a one-step 2 + E double bond isomerization and a proton shift. The latter could in turn lead to a 2 + E configurational change as well via a stepwise thermal reaction sequence. Figure 22A illustrates the two mechanistic routes for a double bond isomerization in dipyrromethene moieties. Photochemical proton shifts of this kind have been proposed to be responsible, e.g., for the ultrafast nonradiative deactivation of biliverdin dimethyl ester, a model compound for the phytochrome chromophore [SO, 511. In the case that the pyrrolenin nitrogen of the bilatriene P, chromophore should be “tightly” protonated, only a direct 2 + E isomerization of the charge-delocalized system (Figure 22B) could be expected as the primary photoreaction. It is conceivable, however, that charge redistribution in either ground as excited state facilitates bond rotation sufficiently to render 2 + E isomerization possible in either state. The primary photochemical reaction of PI is not subject to kinetic hydrogen isotope (H/D) control. Evidence regarding a kinetic H/D effectand hence a rate-determining proton shift-could only be obtained by fluorescence lifetime measurements in the red region in H 2 0 and D,O buffer solutions. An attempt to resort to stationary fluorescence for this purpose

264

grQ

PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME

X

X

Z

hv

Il.-*

Y

Y

xq E

Y

Figure 22. Possible mechanisms for the configurational change of the exocyclic C=C double bond. A (nonprotonated chromophore): direct E % Z isomerization (hv) or coupling of two proton shifts ([HI; formal 1,5-H shifts) with the rotation around a C - 4 single bond ( r ) ) ; B (protonated chromophore): direct E % 2 isomerization (either photochemical or thermal).

[145) had ignored the individual contributions of the fluorescence components and the fact that about 30% of the total integrated P, emission is to be attributed to the minor components Pf and P)' as a consequence of their relatively long lifetimes. Photochromicity as well as lifetimes, relative amplitudes and quantum yields of P:* and P:', which account for 299% of the total amplitude of the fluorescence decay, were the same in H 2 0 and D 2 0 at 275 K (Table 4) and

THE LIGHT-INDUCED P,

-

P,, TRANSFORMATION

265

TABLE 4 Fluorescence Lifetimes, Relative Amplitudes and Normalized Fluorescence Quantum Yields of 1 2 4 kDa P, Phytochrome from Oat in HzO- and D2OBased Buffer Solutions 1861 In PPEG-H,Oa

7: 7:

7:

R: R: R:

( A ) Lifetimes ( p s ) 40f4 197 & 10 1110f 60

In PPEG-D,Oa 41 f 4 210f 10 1460f 60

(B) Relatioe amplitudes (%) 93.0f0.7 93.0f0.7 6.2 f 0.6 6.1 f 0.4 0.8 f0.2 0.8 f0.2

(C)Normalized relative (my) and total quantum yields (@!)

@?

@Y

@ ;

@ ;

0.65 f 0.05 0.20 f0.02 0.15f0.01 1

0.58 f0.05 0.19 f0.02 0.19 f0.01 0.96 f0.07

"PPEG= potassium phosphate buffer solution with 25% (v/v) ethylene glycol in H,O and ethylene glycol-D, in D,O, respectively.

293 K [86]. Only the lifetime of the nonphotochromic longest-lived P,"'was significantly prolonged in DzO. The transient difference absorption attributed to the Isoo products did not reveal either any kinetic H/D effect on the efficiency of the primary photoreaction (Figure 23) [1141. Barring the possibility of accidentally compensating factors, there is evidently no ratedetermining proton transfer involved in neither the primary photoreaction nor any other deactivation channel of P,"" (Figure 14). Similar conclusions have also been drawn from low-temperature studies [127]. The interpretation of the H/D effect on the rate constants of the first thermal reaction steps, Iio0 + I;,, is less straightforward. Hardly recognizable in Figure 24, the rate constants were reduced by about 20% upon changing from H,O to D,O [1141. This is considerably less than the factor of 1.4 expected for kH/kD at room temperature [146]. The result does not yet rigorously exclude a proton transfer. However, the measured value may just as well arise from a solvent-assisted H/D effect on reaction rate constants which are not determined by a proton transfer. A similar situation is encountered with regard to the small H/D effect on the ratio of the precursors

266

PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME

I

I

I

I

I

10 20 30 Laser Energy (m3)

-

Figure 23. Difference absorption of p,oo at 696 nm immediately after laser excitation of P, (124kDa), as a function of the laser flash energy: W in H 2 0 , 0---0 in D20. (From Figure 2 in Aramendia et al. [114].)

1000/T(K-')

Figure 24. Arrhenius diagram for the rate constants of the transformation of the I+oo and I:oo intermediates to I;, in H 2 0 (A,A) and D,O (0,O) buffer solutions. (Diagram from Figure 1 in Aramendia et al. [114].)

of P,,. Finally, a primary kinetic H/D effect was also excluded for the two final parallel steps of P,, formation [la]. The working hypothesis shown in Figure 14 presumes therefore that the P, + P,, transformation does not involve any rate-determining proton transfer. The simplest conclusion is that the primary photoreaction is Z --t E isomerization of the C(15) double bond. However, more complex mechanisms are not rigorously excluded. For instance, it has been pointed out in Section 1.B that the configuration of neither of the other two exocyclic double bonds in P, and in P,, is known. It is therefore still conceivable that one of these double bonds might Z + E photoisomerize in the primary step. The configurational change around the C(15) double bond could subsequently be brought about in one of the thermal steps. Apart from this possibility, the steps I+$, + Ibl might comprise other relatively rapid conformational re-

THE LIGHT-INDUCED P,, --t P, TRANSFORMATION

267

laxations of the bilatriene chromophore. This would leave room in the still insufficiently explored “grey” zone between I;, and P,, for relatively slow reorganizations of the protein structure. In terms of the forward reaction to P,, these processes are remarkably inefficient, allowing for the recovery of up to 85% P, from I:, and/or subsequent intermediates which had initially been formed. This recovery of P, involves the thermal reversion of the photochemical 2 -+ E double bond isomerization. The similarity of the Trp fluorescence lifetimes and amplitudes of P, and P,, (Section ILC) reveal a further interesting facet of the overall P, + P,, transformation which bears on the protein dynamics involved in the overall reaction: they strongly suggest that the tertiary apoprotein structure does not undergo any lasting extensive reorganization as a whole in the phototransformation. This is definitely so with regard to surface exposure and/or conformation of the relatively wide central region between positions 366 and 788 which is probed by the fluorescence of the 10 Trp residues. These conclusions are still consistent with the finding that significant conformational differences between P,and P,, do in fact exist ([65,147]; for reviews see [8c, 1483). They can be rationalized-albeit not with conclusive rigour-by a conformational adaptation of the apoprotein part located around the bilatriene-binding pocket, following the Z -,E photoisomerization of the chromophore. This local change then should suffice to determine through bilatriene chromophore-protein interactions the spectroscopiccharacteristics of the chromophore as well as stability and reactivity of the two photochromic forms of phytochrome. It is also remarkable that the addition of ethylene glycol has no major effect on the thermal reorganizations of chromophore and protein (Section IILD), although both exhibit solvent-assisted H/D effects. Rate-determining effects of ethylene glycol appear to be limited to the control of the two primary photoreaction channels and the protein relaxation in the last step. The restriction of stabilization effects by ethylene glycol to the site of the phytochromobilin attachment may again be a consequence of a high protein flexibility.

IV. THE LIGHT-INDUCED Pfr+ P, TRANSFORMATION There is much less known about the photoconversion of the physiologically active Pfr form to the dormant P, phytochrome. Quantum yields of = 0.06 and 0.08 have been reported for the 124-kDa phytochromes of oat and rye, respectively [36]. The values for the 64- and 114/118-kDa phytochromes of oat are 2-3 times higher (Table 1).

268

PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME

The P, % P,, phototransformation evidently revolves in a cycle. At low temperature two new thermally stable transients (“lumi-F and “meta-F) have been observed to be consecutivelyproduced on the Pfr+ P, pathway [S, 117, 122, 124, 149-1511. Their absorption maxima (A, = 673 and 660nm, respectively)differ from those of all intermediates registered for the P, -+ P,, conversion under any experimental condition. The absorption maxima of the intermediates in the P,, + P, pathway overlap more strongly than do those of the reverse transformation, a complication which adds to the difficulty that the pure Pfrform has so far not been available in sufficient amounts (see Section LA). Mechanistic photochemical studies have therefore been limited to work with red-light equilibrated P, + Pfrmixtures.

V. CONCLUSION The structure-function relationship of phytochrome occupies a particularly interesting position among the diverse light receptor systems in plants and animals. It is a biliprotein as are also the proteins of the light-harvesting antennae of, e.g., photosynthetic cyanobacteria and red algae [152], the chromophores of both systems being practically the same. In both biliprotein systems conformational mobility of the chromophore(s) correlates very well with reactivity and radiationless deactivation of the electronically excited states. This behavior illustrates especially well a concept, formulated probably for the first time by Delbruck [153], that the flexibility of such light receptor proteins determines the nature of physiological reaction modes. The bilatriene-type light-harvesting chromophores are rigidly attached to the antenna apoproteins. Their only function is to transfer the energy of the absorbed light to the photosynthetic reaction center. This is effected essentially without energy loss, since a sufficiently close positioning of several chromophores per monomeric protein unit ascertains a smooth energy flow. Moreover, competing modes of excited-state deactivation are inefficient: the quantum yield of fluorescence is negligibly small and, in vivo, the chromophores do not seem to undergo any photochemical reaction. In contrast, the single chromophore of phytochrome is conformationally more loosely embedded in a more mobile and large apoprotein. The isolation of the chromophore within a relatively large proteinic volume prevents energy transfer to similar chromophoric groups, and the less restricted conformational mobility allows for a defined primary photoreaction to occur. Thus, light absorption in either of the stable forms P, and Pfrinitiates a series of events which ultimately lead to a local rearrangement of the protein

CONCLUSION

269

domain around the chromophore. One of these forms, i.e., P,,, is then capable of generating a physiological signal by a hitherto unknown mechanism. The other form represents the physiologically dormant state (P,). Quantitative exploitation of the absorbed light is not as critical for the generation of physiological signals controlling photomorphogenic functions as it is in the antenna pigments which harvest light as the principal energy source. Hence, a substantially smaller than unity quantum yield of the light-induced overall (,,, 0.15, Q,,,, 0.06;Table 1) is transformations of phytochrome (i.e., ,D tolerable for an information-processing pigment like phytochrome. Analogies to the molecular aspects of the functioning of phytochrome are found in structurally unrelated light receptor proteins. Thus, a basically similar mechanism of the initiation of signal transduction chains play a role in the rhodopsin of vertebrates [154-1561 and in the sensory rhodopsin of halobacteria [1571. In both types of chromoprotein, photochemically induced reaction sequences bring about substantial conformational changes of the chromophore and, by way of dynamic chromophore-protein interactions, changes in the apoprotein, which then turn on physiological signal transduction. For the present state of the investigations, the molecular mechanism of the P, -+P,, transformation has been studied in far greater detail than the reverse process. The results include the work by a large number of biological, physical and organic chemical research groups, achieved over many years. Figure 14 may serve as a means to summarize the experimental observations and to mechanistically interpret them within the hypothetical framework. Both some fundamental aspects and a number of details of what has been unravelled of this complex reaction sequence require further experimental scrutiny-and eventually partial revision of the scheme. Within the context of our own work, which has been presented in Sections I1 and 111, most of the following points appear reasonably well established and in accord with the mechanism of the P, -+ P,, transformation summarized in Fig. 14. 1. The excited singlet P, is composed of three species possessing bilatrienetype chromophores; approx. 91% are undoubtedly the functional component, P:', with a lifetime of 44ps, approx. 8% represent a close to 200-ps component, P:', which still exhibits the essential photochromic properties of the active chromoprotein, and finally a very minor component, P:*, which is not photochromic and is classified as an impurity. 2. P,* is converted to P,, in (at least) two parallel pathways each comprising several consecutive steps. The primary photoreaction affords in a quantum yield of 20.5 two products, I$oo and I:oo, with microsecond lifetimes. These photoproducts form a photoequilibrium with P, within the

270

PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME

15-ns duration of the excitation flash and they are thermally transformed via at least one set of intermediates, I:, to two immediate precursors of P,,, 1: and I:, with lifetimes of around 1 s and somewhat below. (However, see [I361 and Section 1II.A concerning potentially controversial aspects of the primary photoreaction(s). Concerning a disagreement about the issue of parallel versus sequential formation of I:& and I?, see Section I1I.A and [127] and Section II1.C and [159], respectively.) as well as the first set of thermal 3. The two parallel steps (P: -+ reactions (I:$, 3 I:,) are confined to the chromophore and its immediate surrounding proteinic domain. In other words, they proceed without extensive participation (reorganization) of the protein, although protein stabilization with ethylene glycol influences the relative efficiencies of the photochemical processes leading to I;oo and I;oo. 4. Neither the primary photoreaction itself (either to I,,, or to a pre-I,oo) nor any of the subsequent thermal steps which have so far been accessible to experiment are subject to a rate-determining H/D effect, although the change from H,O to D,O does exert some slight influence in terms of solvent assistance. 5. Evidence from protein fluorescence indicates that the equilibrium conformation of the central regions of the polypeptide backbone, which are amenable to probing by emission from the tryptophan residues, is not changed overall to any major extent in the P, 4P,, transformation. The conformational reorganization of the protein, which is induced by the 2 -+ E isomerization (which in turn is presumed to represent the primary photoreaction of the overall P, -+ P,, transformation), appears to be confined mostly to the domain housing the bilatriene chromophore.

Among the still unanswered questions of the scheme in Figure 14 are the number of steps between I:, and I;’, a definitive characterization of the chemical nature of all reaction steps, and the elucidation of the relatively slow processes which thermally regenerate P, from the intermediates and thus ,?,, and cause the difference of 20.35 between the quantum yields of @ Or+,,. An independent in vitro determination of the number of I;! intermediates (there are possibly three [120,121, 1231) is missing as is a correlation of the P,’ and P; (and eventually Pl‘, and Pfr) components with individual intermediates (I:$,, I:, , etc.).

REFERENCES

271

ACKNOWLEDGMENTS Since the first report in 1980 on our own work on phytochrome El121 many more dedicated young scientists have been engaged in the work presented here. We should like to acknowledge with gratitude the association with all of them and the profit from their enthusiasm and scientific competence; uiz., Cecilia Bonazzola and Dres. Tatsuo Arai (Alexander-von-Humboldt postdoctoral fellow), Pedro F. Aramendia, Helmut Brock (Reimar-Lust postdoctoral fellow), Claudio G. Colombano, Stephen Culshaw, Klaus Heihoff (Alfried-Krupp-von-Bohlen-und-Halbach doctoral fellow), Merten Jabben, Peter Klein-Bolting, Jan de Kok, Marianne Krieg (Alexander-von-Humboldt postdoctoral fellow),J. Ioan Matthews, Bela P. Ruzsicska, Giuliana Valduga, and Joachim Wendler (Otto-Hahn awardee). Complementary, and indeed often prerequisite, to their relentless efforts have been those of our technical staff, especially of Heike Deinert, Hans J. Herbert, Andrea Keil, Gulumse Koq-Weier, Dagmar Kreft, Dagmar Lenk, Iris Martin, Ulrich Paul, Willi Schlamann, Michael Schlusen, Wolfgang Schuster, Hans-Volker Seeling, Gerda Wojciechowski, and Ottmar Wolff. Last but not least we should like to acknowledge the collaboration with Dr. R. Scheuerlein in in vivo projects, fruitful contacts with Professors W. Rudiger, E. Schafer, H. Scheer, and C. J. P. Spruit, and the offer by L. Lindgens, GmbH & Co., Miilheim a.d. Ruhr, to use their facilities to grow etiolated oat seedlings.

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PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHYTOCHROME

7. J. C. Lagarias, Photochem. Photobiol. 42, 81 1 (1985). 8. Contributions in Ref. lc: by (a) L. 0.Bjorn, p. 3. (b) W. Rudiger, p. 17. (c) R. D. Vierstra and P. H. Quail, p. 35. (d) L. H. Pratt, p. 61. (e) E. Schafer, K. Apel, A. Batschauer, and E. Mosinger, p. 83. (f) G. H. M. Kronenberg and R. E. Kendrick, p. 99. (g) S. J. Roux, p. 115. (h) H. Mohr, p. 547. 9. K. Schaffner, Rheinisch- Westfalische Akademie der Wissenschaften (Diisseldorf): VortrageJNatur-, Ingenieur- und Wirtschaftswissenschaften N 362,47 (1988). 10. S. E. Braslavsky, in Photochromic Materials: Theory and Application, H. Diirr and H. Bouas-Laurent, Eds., Elsevier, Amsterdam, in press (1990). 11. H. Senger, Ed., Blue Light E’ects in Biological Systems, Springer, Berlin, 1983. 12. H. Mohr and H. Drumm-Herrel, Physiol. Plant. 58, 408 (1983). 13. H. Smith, in Photomorphogenesis in Plants, R. E. Kendrick and G. H. M. Kronenberg, Eds., Martinus Nijhoff, Dordrecht, 1986, p. 187. 14. L. H. Flint and E. D. McAlister, Smithsonian Misc. Collect. 94, 1 (1935). 15. L. H. Flint and E. D. McAlister, Smithsonian M i x . Collect. 96, 1 (1937). 16. H. A. Borthwick, S. B. Hendricks, M. W. Parker, E. H. Toole, and V. K. Toole, Proc. Natl. Acad. Sci. USA 38, 662 (1952). 17. W. Shropshire Jr., W. H. Klein, and V. B. Elstad, Plant Cell Physiol. 2,63 (1961). 18. R. B. Withrow, W. H. Klein, and V. B. Elstad, Plant Physiol. 32,453 (1957). 19. M.W. Parker, S. B. Hendricks, H. A. Borthwick, and N. J. Scully, Bot. Gaz. 108,l (1946). 20. H. A. Borthwick, S. B. Hendricks, and M. W. Parker, Bot. Gaz. 110, 103 (1948). 21. W. L. Butler, K. H. Norris, H. W. Siegelman, and S. B. Hendricks, Proc. Natl. Acad. Sci. USA 45, 1703 (1959). 22. W. L. Butler, in Photoreceptors and Plant Development, Proc. Ann. Eur. Photomorphogenesis Symp., J. de Greef, Ed., Antwerpen University Press, 1980, p. 3. 23. A. M.Jones, R. D. Vierstra, S. M. Daniels, and P. H. Quail, Planta 164, 501 (1985). 24. F. E. Mumford and E. L. Jenner, Biochemistry 5, 3657 (1966). 25. G. Gardner, C. Pike, H. V. Rice, and W. R. Briggs, Plant Physiol. 48,686 (1971). 26. L. H. Pratt, Annu. Rev. Plant Physiol. 33, 557 (1982). 27. R. D. Vierstra and P. H.Quail, Proc. Natl. Acad. Sci. USA 79, 5272 (1982); Biochemistry 22, 2498 (1983). 28. J. C. Litts, J. M. Kelly, and J. C. Lagarias, J . Biol. Chem. 258, 11025 (1983). 29. G. W. Bolton and P. H. Quail, Planta 155, 212 (1982). 30. R. D. Vierstra, M.-M. Cordonnier, L. H. Pratt, and P. H. Quail, Planta 160, 521 (1984). 31. H. P. Hershey, R. F. Barker, K. B. Idler, J. L. Lissemore, and P. H. Quail, Nucl. Acids Res. 13, 8543 (1985).

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Advances in Photochemistry, Volume15 Edited by David H. Volman, George S Hammond, Klaus Gollnick Copyright © 1990 John Wiley & Sons, Inc.

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS: FTIR STUDIES OF DIACYL PEROXIDES Mark D. Hollingsworth Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada J. Michael McBride Department of Chemistry, Yale University, Box 6666, New Haven, Connecticut 06511, U.S.A.

CONTENTS 1. Background and scope of this review 11. Studying photochemical mechanisms in crystals A. What is a Mechanism? 1. Traditional: sequence of differently bonded intermediates 2. Ideal: atomic trajectories 3. The post-photochemical sequence of rapid, productdetermining reactions B. How crystals help 1. By stabilizing intermediates 2. By providing uniform orientation for anisotropic diffraction or spectral studies 3. By providing uniform mechanisms 4. Allowing deeper consideration of “solvent effects” C. Special considerations for solid-state reactions 1. Time scale of motion 2. Physical reaction steps 279

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D. Limitations 1. Reaction type 2. Conversion limitation a. Stress b. Change in optical properties 3. Sample preparation 4. Light penetration 5. Migration of excitation 6. Additional information requires additional work 7. Missing tools a. Concentration, quenching, competition b. Substituent effects c. Limitation to trace conversion 111. Mechanistic tools for studying crystal reactions A. X-Ray diffraction B. Computer simulation C. Microscopic studies of damage and etching D. Electronic spectroscopy E. Solid-state NMR F. EPR G. Vibrational spectroscopy: IR/Raman IV. Reporter probes for FTIR A. Concept B. Desirable features 1. Generated by reaction, easy to label 2. High epsilon 3. Windows 4. Understandable polarization 5. Sensitivity to interesting aspects of environment 6. Quantitative interpretability C. C 0 2 as a probe 1. Frequencies and intensities 2. Interpretable polarization 3. Sensitivity to the environment a. Stress b. Complexation and charges c. Polarizability d. Other virtues 4. Fingerprinting 5. Alternative probes

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

V. Experimental techniques A. Crystal mounting B. Temperature control C. Decomposition limits and throughput D. Polarized IR spectroscopy VI. Peroxide case studies A. Motivation for research on solid peroxides B. Selection of topics for this article C. The systems studied 1. Acetyl benzoyl peroxide (ABP) a. EPR studies b. Infrared studies 2. Diundecanoyl peroxide (UP) a. EPR reaction.scheme b. IR reaction scheme and peak notation 3. Derivatives of UP VII. Obtaining information from infrared spectroscopy A. Reaction sequence 1. Identification of intermediates: comparison with EPR results 2. Real-time kinetics with IR spectroscopy 3. Measurement of reversible interconversion 4. Processes that EPR cannot detect a. Ultimate loss of CO, b. Decarboxylative chain decomposition B. Measurement of stress 1. Observed versus intrinsic frequencies 2. Excluding alternative explanations 3. Magnitude of stress 4. Long-range effects within and between molecules a. Intramolecular effects b. Intermolecular effects C. Structural measurements by FTIR spectroscopy I. Determining the orientation of individual molecules a. By polarization b. By isotopic labeling 2. Coupling of molecular pairs a. Intermolecular vibrational coupling b. CO, pairs with toluene in acetyl benzoyl peroxide c. Pair 1 of 1 1-bromoundecanoyl peroxide (11-BrUP)

281

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d. Determining the structure of Pair A in undecanoyl peroxide (UP) D. Environment of C 0 2 VIII. Other aspects of solid-state behavior revealed by FTIR A. Change in product distribution with extent of conversion B. Steric isotope effects on reaction pathways C . Perturbation and additivity of substituent effects IX. Formulation of a mechanism Acknowledgments References

I. BACKGROUND AND SCOPE OF THIS REVIEW Photochemical reactions in organic solids are important in practical fields as diverse as photography, biology, photoresist technology, polymerization,and the decomposition and stabilization of dyes, energetic materials, pharmaceuticals, and polymers 113. They have been equally important in basic research, particularly for preparing matrix-isolated reactive intermediates for spectroscopic investigation [2]. Despite the intrinsic importance of chemical reactions in organic solids, most often they have been investigated in order to shed light on their fluidphase analogs. This goal is realistic, because many factors that govern reactivity do not depend on the phase. Other factors, particularly those that have to do with molecular motion, can be quite different between solids and fluids. There has been a great deal of research over the past 30 years aimed at discovering, understanding, and exploiting these differences. The most detailed studies have involved reactions in crystals, and since Cohen and Schmidt’s pioneering work on cinnamic acid cyclodimerization [3], photochemistry has played a dominant role in studies of solid-state organic chemistry. A number of fine reviews of photochemistry in organic crystals have appeared in recent years [4], so this review makes no attempt to cover the field generally. It focuses more narrowly on three topics: (i) refining the concept of mechanism for reactions in organic crystals; (ii) outlining some techniques that are available for studying these mechanisms; and (iii) describing the unique information, beyond simple identification of chemical intermediates, that can be supplied by Fourier-transform infrared spectroscopy (FTIR) of single-crystal samples. Most of the examples will be taken from our own work, in which we have used the FTIR spectrum of C 0 2 to study the decomposition of fatty acid diacyl peroxides [SJ.

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11. STUDYING PHOTOCHEMICAL MECHANISMS IN

CRYSTALS

A. What is a Mechanism? 1. Traditional: Sequence of Differently Bonded Intermediates. Organic chemists have traditionally considered a reaction mechanism, in its most primitive form, to consist of a sequence of direrently bonded intermediates on the path between starting materials and products. In these terms, a mechanism may be considered understood once these chemically distinct species have been correctly identified. For purposes of understanding reaction rates and stereochemistry, it is necessary to expand this set of metastable reaction intermediates to include transition structures at the saddle points between intermediates on a potential energy surface. For photochemistry one must also consider transitions between potential energy surfaces. 2. Ideal: Atomic Trajectories. One could hope for a much more detailed understanding of mechanism, which would include not only changes in chemical connectivity but the actual three-dimensional trajectory that each atom follows as all atoms move simultaneously from their positions in the starting material to those in the product. For reactions in fluids or in glassy solids this sort of picture is unrealistically precise. At this level of detail there is no unique reaction mechanism, because there are usually many different trajectories that pass through the same family of metastable intermediatestwo reactant molecules can associate in somewhat different relative orientations, a flexible molecule can react in different conformations, and even when a reaction is unimolecular and the molecule is rigid, surrounding solvent molecules can be arranged differently. Thus one must usually draw the curtain of statistical averaging over a host of different detailed pictures and be content with a fuzzier view of mechanism. The existence of a broad family of trajectories has been particularly important for kinetic studies of reactions in glassy matrices. Processes that would be first-order, or pseudo first-order, in fluids typically show nonexponential behavior in rigid matrices, as shown in Figure 1. This phenomenon, often referred to as dispersive kinetics [ 6 ] , occurs when there are many related mechanisms with rate constants that differ because of subtle differences in the local arrangement of reagents or solvent. In glasses, the equilibration of local structures can be slow compared to the reaction under investigation. Under these conditions, the different reaction paths do not compete but operate in parallel, since any one molecule is predestined by the structure of its environment to react by a particular path. Thus one sees a superposition of independent first-order processes (Figure 2a). As compared

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PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

I

I

10

20

I

1

4

30

Time imini

Figure 1. Nonexponential decay of methyl benzoyloxyl radical pairs in a single crystal of acetyl benzoyl peroxide after long photolysis at 77 K.The initial rapid decay has an effective rate constant of 1.1 min-', while the later decay has an effective rate constant of 0.06 min- Shorter photolysis gave clean exponential decay indicating a more uniform radical-pair structure (see Refs. 16b and 66).

'.

d(A

+

B)/dt

I

klA

+

k2B

k2

c

d(A

k

+

= (k 1 A

B)/dt = k(A

+

8)

k 2 B ) / (A

+

8) = C o n s t

+

Figure 2. (a) Independent first-order reactions giving nonexponential decay; (b) competitive first-order reactions giving exponential decay.

STUDYING PHOTOCHEMICAL MECHANISMS IN CRYSTALS

285

to a single exponential, the overall decay is fast at the outset and becomes slower after the rapidly reacting population has been depleted. In fluids, structural equilibration is typically rapid and the reaction paths are competitive. In this case the overall decay is exponential with an effective rate constant which is the weighted average of the individual rate constants (Figure 2b). The weighting factors are the steady-state populations of the equilibrating reactant structures. Phenomena such as nonexponential decay in glasses have given solid-state processes the reputation of being too complex to study effectively. But it is important to note that even in fluids, many slightly different processes compete, although the range of rates of those that contribute significantly is narrower than in a glassy matrix. In a glass, the differences among related paths are obvious; in a fluid, they are ignored. If one has a taste for mechanistic detail, the glassy system might be experimentally preferable to the fluid. The case for crystals is considered below. 3. The Post-Photochemical Sequence of Rapid, Product-Determining Reactions. In the systems to be discussed in this chapter, we will focus attention on reaction steps that involve atomic motion without bonding change. Typically these involve the sequence of rapid, product-determining motions of free radicals which follow their photochemical generation. Since no bonding change occurs in most of these steps, they might be considered to be physical rather than chemical transformations. Still they are as much a part of the reaction mechanism as any other steps. Since we are interested in details of atomic motion, our concept of mechanism is closer to that of specified atomic trajectories than to the fuzzier statistical picture. That steps involving atomic or molecular motion can be rate determining, even in fluids, is well known through diffusion limited reaction rates and the solvent cage effect. In solids, motion more subtle than translational diffusion can be influential, and cases of rotational diffusion control are familiar [7].

B. How Crystals Help Crystalline samples in general, and single crystals in particular, are well suited for the type of mechanistic study discussed above. There is a close parallel between how single crystals facilitate detailed mechanistic investigations and how they facilitate detailed structural investigation by X-ray diffraction. Crystals also share important experimental virtues with amorphous rigid matrices.

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1. By Stabilizing Intermediates. Like amorphous matrices, crystals provide for the isolation of reactive intermediates from other species that might destroy them in bimolecular processes. They also allow work at very low temperature to suppress unimolecular reactions. Other things being equal, a crystal should be even more rigid than an amorphous matrix, because there is a lower density of the structural defects that make plastic deformation possible. Paradoxically, with respect to prolonging the lifetime of reactive intermediates by isolation, solids are more like low-pressure gases than like liquids. 2. By Providing Uniform Orientation for Anisotropic Diffraction or Spectral Studies. An important advantage of single crystals, which makes X-ray diffraction useful for structural studies, is the uniformity of molecular structure and orientation. In a few cases, X-rays have been used to determine the structure and orientation of a product in situ in a reactive single crystal [S]. Reactive intermediates are almost always too dilute for diffraction studies, but single crystal samples can provide advantages for spectroscopic studies of mechanism which are analogous to those they provide for X-ray studies of structure. Fluids generally give simpler spectra than rigid matrices, because rapid molecular reorientation reduces the influence of anisotropic interactions or eliminates it altogether. When one is interested only in chemical connectivity patterns, as in most solution NMR or EPR studies, the simplicity can be helpful, but for other applications the richness of solid-state spectra can be indispensible. Many spectroscopic phenomena depend to first order on symmetric second-rank tensors, that is, on six independent numbers [9]. The information contained in these six numbers, and its experimental accessibility in different media are illustrated in Figure 3. In fluids, five of these numbers are usually inaccessible to experiment, and the surviving isotropic value of

Isotropic

Value

Fluid -

Anisotropy (Range of Values)

Powder or Glass

Orientation

-

Single Crystal Figure 3. Amount of spectral information for second-rank tensorial properties using different types of sample.

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the phenomenon, useful as it may be, conveys only 1/6 of the total information. In rigid glasses or crystalline powders, two more numbers become available. From a powder pattern one can obtain maximum and minimum values of a property (a chemical shift for example) as well as its average or isotropic value. The three numbers that constitute the remaining half of the information describe orientation and become available in single crystal spectra. Some aspects of this information are available through polarized photoselection in glassy matrices [lo]. For purposes of developing a detailed picture of atomic trajectories during reaction, the orientational information, which is available only from single crystals, can be the most valuable.

3. By Providing Uniform Mechanisms. In the same way that a crystalline environment can dictate a single molecular conformation for structural investigation, it can dictate a single sequence of arrangements of reaction intermediates for mechanistic investigation. In contrast to reactions in fluids and glasses where there are many parallel reaction paths that may or may not compete kinetically, we have found that reactions in crystals pass through a very well-defined sequence of metastable intermediate structures. The surrounding undamaged molecules are typically as effective in ordering a set of reaction intermediates as they were in ordering the precursor molecule. Occasionally processes compete so that there are two coexisting reaction paths, but rarely more, unless there has been enough reaction to destroy the integrity of the crystalline matrix. Sometimes EPR linewidths for free radical intermediates indicate that for an entire population of radicals, position in the unit cell is uniform within less than 0.01 A, and angular orientation within 0.2” [l 13. This range is less than vibrational amplitudes about the mean atomic positions. There is a detailed mechanism to be found. From a structural point of view, mechanism in a single crystal can be much closer to a set of identical atomic trajectories than to the kind of fuzzy statistical average with which one must be content in solution. It is not surprising that with this kind of structural uniformity the “site problems” that plague kinetic studies in rigid glasses disappear. Adherence to first-order rate laws can be as close in single crystals as it is in fluids, and equally valid activation parameters can be obtained for thermal unimolecular reactions of reaction intermediates [121. In the sequence of metastable intermediates between starting materials and products of the free-radical reactions we have studied in crystals, many structures differ only in the arrangement of an identical set of molecular fragments. The “physical” reactions that connect them involve motion without a change in chemical bonding, but these steps are as well-defined kinetically and as important to the overall mechanism as chemical steps.

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4. Allowing Deeper Consideration of “Solvent Effects.” Because the arrangement of molecules surrounding a molecule to be photolyzed in a pure crystal can be determined in detail by X-ray diffraction, and because their arrangement seems to change relatively slightly during photolysis [S, 131, pure crystals are well suited for detailed investigation of intermolecular, or solvent, effects on certain chemical reactions. An advantage of partial photolysis of a pure crystal over complete photolysis of a dilute dopant in a host crystal is that one can be more confident of knowing the initial structure of the reacting molecule and its environment. With a well-defined reaction environment one can speculate more deeply about constraints imposed by the “reaction cavity” [141, or about cooperative motion of adjacent molecules [15- 171.

C. Special Considerations for Solid-state Reactions 1. Time Scale of Motion. Much of the difference between fluids and glassy matrices can be attributed to the differing time scales for molecular motions, but there is an additional significant difference between either of these amorphous phases and a crystalline solid. Over very short distances, structure in a fluid or glass can be more or less well defined, but within a few molecular diameters structural correlation is lost, as is shown by the decay of radial distribution functions to a constant value reflecting the average density of a medium. In defect-free regions of crystals, however, structure is much more highly cooperative and persists to longer range. This suggests that when intermolecular potentials are similar, motion in crystals should be opposed by longer-range restoring forces than in glasses, and that modest structural changes should be more difficult. Thus it is not obvious that all conclusions based on mechanistic studies in crystalline systems will be generalizable to glassy matrices, or vice versa.

2. Physical Reaction Steps. An important difference between fluids and solids as reaction media is the possibility of sustaining strong intermolecular repulsions in the latter, even in the absence of external constraints. Experiments discussed below (Section V1I.B) suggest that photolyzing molecules in pure molecular crystals can generate local stress fields equivalent to pressures greater than 20 kbar. Pressurizing typical molecular crystals to this extent requires energies of about 5 kcal/mol, so such stress can have substantial mechanistic significance [161. Since intermolecular forces on the atoms within a crystal are a com-

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Figure 4. Representations of the potential energy surface for a gas-phase reaction, indicating regions where increases in relative energy on passing to the solid state would have different effects on the reaction rate. From Ref. (17) with permission from the American Chemical Society.

plicated function of atomic positions, such forces can influence reaction trajectories in a variety of ways. Walter and McBride illustrated the variety of these effects by classifying some of them according to which part of the potential energy surface suffers most from intermolecular repulsions [171. The solid line in Figure 4 represents a portion of the potential energy surface for a one-step reaction in the gas phase. In condensed phases the surface is lowered by intermolecular attraction. Nonpolar reactions in fluids are often rather insensitive to phase, suggesting that stabilization by attraction is uniform across the surface, lowering it without changing its shape. Repulsions are typically very weak in fluids, but in crystals they can be strong and localized in certain portions of the potential energy surface. Thus repulsions can alter the shape of the surface. Repulsions that raise the energy of the starting material, A, would accelerate reaction, while repulsion in the region of the transition state, B, would retard it. Selective destabilization of product C, would affect the equilibrium constant, but not the rate. All of these effects influence the thermodynamics of bond making or breaking, and thus might be classified as “chemical” modes of lattice influence. Selective destabilization in regions D-F is less familiar. D and F involve new transition states in which there is no change in bonding. They can be classified as “physical” effects. In these cases, repulsions make atomic motion, rather than bonding change, rate-limiting. Destabilization in the region E can influence the reaction in two ways. By limiting the region of the potential energy surface accessible to starting material, it accelerates reaction by decreasing the entropy of the starting material. In addition, by cutting off access of starting material to competing chemical transition states, it favors the reaction in a competitive sense even without increasing its rate. This latter type of lattice influence is common and includes most cases of topochemical control. The various effects are summarized in Table 1. Reference 17 gives examples and a fuller discussion of this classification scheme.

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TABLE 1 Influence of Selective Destabilization on Solid-state Reactions

Destabilized Region A

B C D E

F

Mechanism of Influence

Influence on Bonding Change?

Accelerates reaction by decreasing AH’ Retards reaction by increasing AH*; decreasing AS’ (difficult to distinguish experimentally from D and F) Does not normally influence forward rate; reduces K,, for nearly thermoneutral process Retards reaction by creating a new “physical” transition state with larger AH* or lower AS: (diffusion control) (1) Accelerates reaction through lowered AS* achieved by restricting configurational space of starting materials (2) Increases yield without acceleration by inhibiting alternative reactions; (normal topochemical control) Retards reaction by creating new “physical” transition state; (difficult to distinguish from D)

Yes Yes Yes No

Yes

No

D. Limitations Reactions in crystals provide a convenient window for detailed investigation of mechanism and for discovering what factors might make chemistry in the “real” world, which so often occurs in rigid or structured media, different from chemistry in the laboratory. Unfortunately there are substantial limitations on the kind of studies that may be conducted. 1. Reaction Type. It is often difficult to study thermal reactions in simple molecular crystals, because so often the sample melts at the necessary reaction temperature. This problem is exacerbated when small amounts of product lower the melting point of a crystal substantially. For this reason photolysis, especially at low temperature, has been a preferred method of initiation. As the sample is warmed from cryogenic temperature, subsequent thermal reactions of the photolytic intermediates can be studied with less danger of melting. The use of ionizing radiation has been popular for some types of spectroscopic investigation, but it typically gives rise to a complicated mixture of various radical and ionic intermediates, which makes it difficult to understand the overall chemistry.

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Since it is hard to make a crystal grow with a particular desired arrangement of neighboring molecules, it is difficult to study bimolecular reactions. Sometimes it has proven possible to “engineer” the packing of pure crystals or solid solutions to give controlled chemistry, most notably in the case of 2 + 2 cycloaddition [lS], but at present there are not many reliable, general tools for crystal design [19]. It is generally much easier to study unimolecular than bimolecular processes. It should be noted in this connection that some reactions that are bimolecular in solution are unimolecular in crystals. For example, the coupling or disproportionation of two radicals generated in a crystal cage from a single precursor molecule is a first-order reaction of a radical pair, not a second-order reaction of independent radicals. 2. Conversion Limitation. A serious limitation on the ability of crystals to provide a uniform reaction environment is that as conversion proceeds, there is an increasing chance that reaction will occur in the disturbed region near previously reacted molecules. A severe disturbance might change the local structure, but even a modest disturbance could change the intermolecular forces on individual atoms. Since molecules typically contact as many as 12 neighbors, the chance of having a previously decomposed molecule in the first coordination shell is already very high at 5% conversion. Furthermore, as we discuss in Section VII.BA.b, significant influence of such defects can extend beyond the first coordination shell and appear at conversions as small as 0.06%. Sometimes solid-state product distributions seem not to be very sensitive to lattice destruction [20,15]. In other cases the sample may anneal during reaction, preserving the integrity of the reactant solid by segregating decomposed molecules into a separate phase [21]. There are rare cases where the change in overall shape between starting material and products is so slight that the lattice survives reaction [S]. Survival is particularly plausible when only a few atoms of a very large molecule are involved in reaction, which makes the study of guests in molecular inclusion compounds appealing [22]. But even the hemoglobin molecule, with its 10,000 atoms, changes its crystal packing on the addition of just four molecules of oxygen [23]. Usually conversion in a pure reactive crystal must be held to less than 1% to assure the kind of uniformity necessary for establishing a detailed mechanism. This poses obvious problems for classical chemical analysis of product mixtures. For spectroscopic investigationswe typically limit conversion to less than 0.1%. a. Stress. The accumulation of stress in a crystal as some of its molecules are converted to products can cause it to fracture. Sometimes pieces fly away

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to a distance of several centimeters. After a number of months on a benchtop in room light a crystallizing dish containing crystals of azoisobutyronitrile was surrounded by fine fragments of fractured crystals. In experiments with single crystals, fracture creates obvious problems for the mechanical and orientational integrity, thermal conductivity, and optical perfection of the sample.

b. Change in Optical Properties. Reaction can change a crystal’s optical properties by creating or destroying chromophores. Filtering of incident light by products formed near the surface can be especially troublesome in single crystals. Even more subtle changes in crystal optics may also occur. Photolysis of optical quality single crystals of dibenzoyl peroxide makes them become milky, but the milkiness is selective with polarized light. Crystals become polarizing filters upon reaction, transmitting light polarized in one direction and scattering, rather than absorbing, light with the orthogonal polarization [24]. The reason for this behavior is that the peroxide is strongly birefringent, while the fluid product mixture (biphenyl, phenyl benzoate, carbon dioxide) is isotropic and has a refractive index close to two of the indices of the nearly uniaxial peroxide crystal. Light polarized in the direction where the solid and fluid indices differ is scattered, but other light is transmitted normally. After slight photolysis, crystals give normal transmission, but they become polarizing filters upon subsequent warming in the dark. Here the amount of product is too small to give a significant effect, because its uniform distribution causes its scattering to be incoherent. As the crystal anneals, the fluid products collect in pockets large enough to give coherent scattering. 3. Sample Preparation. In our experience the most troublesome obstacles to this type of research involve growing single crystals suitable for different types of investigation: small chunky crystals for X-ray diffraction, larger chunks with well-defined facets for EPR, optical quality plates of specified thickness for FTIR spectroscopy, crystals weighting hundreds of milligrams for NMR. The microscopic counterpart of this problem is to prepare the substance with a unit cell packing of appropriate symmetry. The frustration that comes from failure to grow a suitable crystal after tedious but successful preparation of a substance of appropriate molecular structure, is similar to that of a synthetic chemist who is unable to complete the final step of a total synthesis. From one point of view crystallization is a problem in synthesis, where the challenge is not to assemble atoms into a given molecular structure, but to assemble molecules into a given supramolecular structure. In the last few years significant progress has been made toward transforming the growth of molecular crystals from an art to a science. The

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contributions of Addadi, Lahav, Leiserowitz and their collaborators at the Weizmann Institute have been particularly important [25]. But this field is still in its infancy. If pure crystals and solid solutions were easier to design and prepare, much of the work now done in amorphous matrices would be done in single crystals. 4. Light Penetration. Achieving uniform photolysis is much more challenging for crystalline samples than for fluids or matrices. A substance with an extinction coefficient of 30M-' cm-' and a molar volume of 100cm3 absorbs 90% of incident radiation within a layer only 0.03 mm thick. Thus surface molecules are exposed to much more radiation, and secondary photolysis can occur near the surface before any photolysis at all occurs in the interior. This is a special problem when one needs to generate a significant amount of product without undue damage to the surrounding lattice. If reaction is confined to a thin surface layer, this may be impossible. More even illumination throughout the sample can be achieved with a powder that is tumbled during photolysis or stirred while suspended in a liquid in which it is insoluble [26]. Although these techniques have proven useful for product studies, they are obviously inappropriate for obtaining orientational information. Furthermore, light scattering in powders makes both photolysis and optical spectroscopy problematic. Most troublesome, we have found that FTIR spectra of powdered samples are much less well resolved than analogous spectra of single crystals. For example, a photolyzed powder of diundecanoyl peroxide at 90 K gave several small CO, stretching peaks with 1 cm- linewidth, but they were superimposed on a structureless mound 20 cm-' wide which accounted for more than 95% of the total C O , absorption. Under analogous conditions all CO, peaks in a photolyzed single crystal were less than 1cm- wide. This suggests that the density and severity of structural defects is higher in powdered samples. Another way to achieve uniform illumination is to lower the optical density of the sample by increasing the effective molar volume. Solid solution of an absorbing guest in a transparent host in either a normal crystal or a molecular inclusion compound can achieve this end; so can using a pure crystal of a much larger molecule which includes the chromophore of interest [27]. While it is harder to reach high dilution with the latter approach, it has the virtue that the initial compound is a pure crystal and thus better suited for definitive X-ray investigation. Irradiating in the long wavelength tail of an appropriate chromophore is a useful technique for decreasing the optical density, as long as the light still results in photolysis. In our work with diacyl peroxides we typically use 313 nm radiation from a high pressure mercury arc, which has an extinction

'

'

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PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

of only 2.9cm-l. Illumination is practically uniform since a typical single crystal, 0.3 mm thick, absorbs only 20% of the incident light. If long wavelength absorption fails to give reaction, one can sometimes resort to analogous approaches such as using light of a polarization that is only weakly absorbed or using two-photon excitation by a laser source at twice the desired wavelength [28].

A different sort of problem is posed by the optical anisotropy of crystals. Incident light is propagated within a typical birefringent crystal as independent optical disturbances polarized along the major and minor axes of an elliptical optical section. In general these ordinary and extraordinary rays diverge from one another as they pass through the crystal. Usually one ray is more strongly absorbed than the other with obvious implications for the spatial distribution of product. Whereas an isotropic medium allows polarized photoselection for any orientation, most organic crystals transmit linearly polarized light without changing its polarization only if the light is polarized along one of the three principal axes of the crystal's electric polarizability tensor. These and related considerations make some otherwise attractive experiments impossible, and require careful design of others. The same problems can complicate spectroscopic product analysis. For example Beer's Law does not hold for unpolarized light in a birefringent sample, because the two polarized components have different extinction coefficients. 5. Migration of Excitation. When chromophores are in intimate contact or have very high oscillator strengths, excitation can migrate from molecule to molecule and result in reaction far from the site of initial absorption [29]. In a perfect crystal this might not be very troublesome, but in an imperfect crystal the excitation can be trapped, or reaction may be more rapid, in sites of unusual and unknown structure. This makes it difficult to understand ensuing reactions. A classic case in point is the photodimerization of 9cyanoanthracene [30]. On the basis of crystal packing one would predict head-to-head dimers. The observation of head-to-tail dimers was attributed to exciton migration to defects where neighboring molecules had the reactive head-to-tail relationship, although subsequent studies by transmission electron microscopy failed to support an earlier assignment of the defect structure [31].

6. Additional Information Requires Additional Work. Although much more information is supplied by crystalline than by amorphous systems, more work is required to collect and interpret it. Typically, under conditions where spectra for many different orientations are necessary to obtain the ani-

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sotropic information from a single crystal, one spectrum contains all the information available for an amorphous system. For careful analysis we often collect EPR spectra in more than 100 different crystal orientations. This apparent overkill is useful because overlap makes assignment difficult or impossible in many of the individual spectra. Since FTIR intensities depend on orientation of the chromophore, spectra measured at normal incidence to a crystal plate do not give complete information about its orientation or concentration. Not only must polarized spectra be measured, but the crystal must also be tilted to allow measurement of the third component of absorption. Although integrating the information from many spectra measured in different crystal orientations provides much more information than is available from a single spectrum of a fluid or glassy sample, an individual single-crystal spectrum can be even less informative. It can have the virtue of less spectral overlap than in a superposition of powder patterns, so that minor components are easier to see than in glasses, but features that depend on anisotropic properties (such as line positions in EPR and intensities in FTIR) can be very difficult to interpret in an individual single crystal spectrum. Orientational or temporal averaging in glasses and fluids obscures some information, but what remains can be easier to interpret. Obviously the additional work at every stage of a complete single-crystal investigation cannot be justified for every reaction that one might think of studying. Care is required to choose situations where detailed study of one compound, or of a few related compounds, can supply more insight than less detailed studies on a much larger number of systems. 7.

Missing Tools

a. Concentration, Quenching, Competition. Many of the most useful tools for determining reaction mechanisms in solution are not available for studying reactions in crystals. For example, it is not possible to vary concentrations in pure crystals. This generally makes it difficult or impossible to use such solution methods as excited state quenching, free radical scavenging, or any other competitive technique for identifying intermediates or determining kinetics. One could imagine that with considerable effort and good luck it might be possible to build a quencher, radical trap, or other competitor into the molecule under study, but it would have to be suitably positioned to function, and it must not change the crystal packing in such a way that the packing itself would interfere with the process under investigation. One might imagine designing a separate molecule that can compete and form a solid solution with the substance under investigation. This type of approach has been successful in a few cases [32], but it can by no

2%

PHOTOCHEMICAL MECHANISM IN S I N G L E CRYSTALS

means be considered general, and the parallel with free variability of concentrations in solution is questionable. b. Substituenr Efects. Substituent effects are also very difficult to employ in studying crystal reactions. Although changing a substituent may influence reaction in a crystal even more than in solution, interpreting the influence is usually impossible, because even very modest changes in molecular structure typically change crystal packing profoundly. It is then hard to know whether the substituent influenced the process under study directly, as in solution, or indirectly by changing intermolecular contacts in the vicinity of the reaction. Of course this is not usually a problem for isotopic substitution. Non-isotopic substituent modification has been used to good effect in solid-state mechanistic studies in cases where crystal packing is not seriously perturbed [16,33], but it is a much less general tool than in solution. c. Limitation to Trace Conversion. It is usually possible to perform product analysis with the same techniques used for solution reactions by dissolving the sample after photolysis. But analysis is more challenging because in careful mechanistic studies conversion is kept to 1% or less in crystal reactions. Not only are impurities in the starting material more troublesome than after high conversion in solution, but the large excess of residual starting material may be converted in part to the same or other products under conditions of sample work-up. This problem is less acute for solid solutions and inclusion compounds than for one-component crystals, because solute reaction can be carried to high conversion without destroying the crystal.

111. MECHANISTIC TOOLS FOR STUDYING CRYSTAL

REACTIONS

A. X-Ray Diffraction Although many solution mechanistic tools are inappropriate for reaction studies in solids, their absence is more than compensated by the availability of other techniques that are unique to single crystals. Perhaps the most significant is X-ray diffraction, which can establish precise atomic coordinates not only for the starting material, but also for the environment in which reaction occurs. Availability of this kind of information puts discussions of mechanisms and “solvent effects” on a completely different footing from those for fluid reactions. Since X-ray usually involves extensive temporal and spatial averaging, it is

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difficult to study reaction intermediates directly. Temporal averaging is not an intrinsic feature of the method, as it is in magnetic resonance spectroscopy, but results only from the time required to collect data at an adequate signalto-noise ratio. Using synthrotron or flash sources and rapid data collection, progress is being made toward studying reacting systems in the subsecond regime [34], but this is by no means a straightforward task. Spatial averaging means that minor components of the crystal appear superimposed on the host structure, so that it is quite challenging to pick out reaction intermediates or products in concentrations of a few percent, even if they are all identically oriented. In some cases product structures have been determined in situ, but often the mechanistic connection between initial and final geometries is not obvious, especially if the shape or symmetry of the unit cell changes [35]. A special kind of dynamic information can be supplied by careful analysis of anisotropic displacement parameters. Dunitz and others have used the ‘‘thermal” parameters from X-ray to provide valuable information on intraand intermolecular mobility [36].

B. Computer Simulation Recent success in modeling the conformational behavior of simple organic molecules and the packing of molecular crystals using molecular mechanics and atom-atom potential methods suggests the possibility of studying reaction pathways in organic crystals by computer simulation [37]. For purposes of computation, reaction in a crystal has a great advantage over reaction in solution in that initial atomic coordinates of the reagents and their environment are available from X-ray diffraction. In appropriate cases spectroscopic investigation of reaction intermediates can give highly precise information about their orientation and position in the lattice. Although such information is usually not sufficient to establish the reaction mechanism in full detail, there can be more than enough to discriminate between a realistic and an unrealistic computer simulation. The problem of searching for metastable geometries and the transition structures between them is formidable for the number of degrees of freedom involved in a system consisting of several molecular fragments and the number of surrounding molecules that are likely to undergo coordinated motion. Despite potential complications, simple simulations that hold the lattice molecules stationary have yielded results in very good agreement with experimental observations [ 13,371. Even simpler calculations which only locate empty regions in the crystal lattice that could accommodate reactant

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motions have proven very helpful [38]. There can be no doubt that more sophisticated computational modeling will play an increasingly important role in investigations of solid-state reactions.

C. Microscopic Studies of Damage and Etching. Optical and electron microscopy provide information about crystal reactions at a more macroscopic level. They are particularly good at revealing when reaction is favored near pre-existing lattice defects rather than occurring uniformly through the bulk of the crystal. Sometimes reaction products can be observed directly; other times their presence is revealed by chemical etching, fluorescence, or the development of lattice strain [39].

D. Electronic Spectroscopy Various kinds of visible and ultraviolet spectroscopy-absorption, excitation, emission-are well suited for monitoring certain reaction intermediates. These techniques are complementary to X-ray in that they provide excellent sensitivity and time resolution but low structural precision. Pure and doped crystals allow investigation of chromophore interactions and exciton motion. Polarized spectroscopy can be uniquely useful for assigning spectral features and determining orientations of intermediates or products in single crystals, but birefringence often makes the technique less versatile than might have been imagined (Section II.D.4). Birefringence itself can provide a novel mechanistic tool. Reduction in the symmetry of tetragonal crystals due to selective formation of product in one orientation can be observed with a polarizing microscope, even when neither starting material nor product has an absorption in the visible spectrum. This technique has been used to show confinement of reaction within individual lamellae of a layered crystal [40]. By using dispersion rather than absorption, this technique extends the range of reactions that can be studied optically.

E. Solid-state NMR Solid-state NMR with magic angle spinning has been used to good effect in studying organic crystals and their dynamics [41]. The powdered samples used in such studies can stabilize reaction intermediates or products and provide them with a uniform molecular environment. In order to simplify the spectra, this technique intentionally discards the information contained in

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anisotropic magnetic interactions. There is increasing interest in exploiting anisotropies for understanding the electronic structure of molecules [42] and for studying molecular orientation and reorientation [41]. Deuterium quadrupole splitting is particularly useful for studying the dynamics of reorientation through lineshape analysis [43]. Almost all of this work has been conducted using powdered samples. Single crystal samples could have some advantages. They would yield absolute orientational information, provide increased sensitivity and resolution as compared to static powders, and allow more confident spectral assignment, particularly for minor components. Difficulty in preparing single crystals of suitable size (several hundred milligrams would be desirable) is a major obstacle to this type of study.

F. EPR Increased sensitivity makes EPR spectroscopy of single crystals easier than NMR, especially for studying reaction intermediates. That EPR is applicable only to paramagnetic species is an advantage for studying reaction mechanisms involving free-radicals or triplet-states. Since there is no background from diamagnetic starting materials or products, and sensitivity is high, it is easy to study trapped intermediates at the ppm level (for favorable cases even ppb) in crystals less than 0.5 mm on an edge. When spin-polarized transients are formed photochemically, sensitivity increases by several more orders of magnitude [44]. Anisotropy due to nuclear-electron hyperfine splitting, to spin-orbit coupling (y-factor), and particularly to strong electron-electron dipolar splitting in molecular triplets and radical pairs, provides a great deal of orientational information. It also makes it possible to shift peak positions in the spectrum, creating new windows through which to observe minor components that would be completely obscured in powder spectra.

G. Vibrational Spectroscopy: IRIRaman Vibrational spectroscopy in rigid matrices has been used extensively for studying species that would be transient in fluid media [45]. Quenching of rotational motion at low temperature typically gives lines much sharper than those observed in fluids. Studies of organic mechanisms by this technique have tended to focus on using spectra to identify the chemical structure of trapped intermediates or products.

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The uniformity of environment provided by crystalline samples makes lines still sharper than in amorphous solids. Polarized IR spectra allow definition of orientation and assist in peak assignment. A less complete orientational study is possible in glasses when the species are generated using polarized photoselection [lo]. Raman spectroscopy can be used in similar ways to study transient intermediates, but it can also play a different role in mechanistic studies of crystals. Prasad and his collaborators have used phonon spectra of the lattice itself to probe solid-state reactions [46]. They have used this technique to study whether reaction occurs uniformly throughout the solid, whether phase transformation accompanies reaction, and whether the starting lattice allows easy deformation in a direction that could lead toward reaction. The idea of using spectroscopy of the environment to probe reaction leads to the main topic of this chapter, which is using spectroscopy of reaction products to probe the environment in which reaction occurs.

IV. REPORTER PROBES FOR FTIR A. Concept During 20 years of using the EPR spectra of radical pairs to study reactions in single crystals of photolyzed azoalkanes and diacyl peroxides, we became familiar with the strengths and limitations of this technique. In addition to its high sensitivity, low background, and specificity for paramagnetic reaction intermediates, it allows studying of the reorientation and relative translation of the radicals and thus probing the mechanical properties of their environment [9, 16,471. In particular it provides strong, if indirect, evidence for high local stress accompanying solid-state reaction. EPR provides no means of quantifying this stress, however, and is blind to diamagnetic intermediates and products [16]. In searching for a spectroscopic means of measuring reaction-generated stress, which is confined within the immediate vicinity of reacted molecules, we were reminded of the work of Riepe and Wang, who looked for shifts in the TR stretching frequency of C 0 2 in the active site of carbonic anhydrase as an indication that the molecule is activated, at least in part, by mechanical stress [48]. Analogous work by Belasco and Knowles [49], and the observation of curious frequencies for COz molecules generated together with cyclobutadiene in rigid matrices [SO], confirmed the idea of using the IR spectrum of a molecule generated by reaction, not to identify the molecule but rather to probe the state of stress in its environment.

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Since beginning this investigation we have measured many thousands of FTIR spectra of reaction-generated CO, under a variety of conditions in many crystals, and have come to see a parallel between this technique and NMR. In both cases, interest focuses not on what the spectra tell about the chromophore itself, but on what shifts in peak positions or intensities tell about the environment in which the chromophore is found.

B. Desirable Features By considering the properties which might be desired in an IR spectroscopic probe of the reaction environment, one can appreciate the suitability of CO, for this purpose. 1. Generated by Reaction, Easy to Label. The best spectroscopic probe of a reaction-generated crystal defect is a molecule that is generated by the reaction itself. Otherwise very few of the probe molecules would be found near defects and large shifts in frequency would be necessary to resolve their signals from the much stronger signals of molecules in undamaged regions of the sample. Since frequency shifts are expected, it is also helpful if crystals that yield isotopic variants of the probe molecule are easy to prepare. Isotopic molecules help in assigning bands definitively and in explaining band shifts. 2. High Epsilon. In order to study reactions at less than 0.1% conversion, it is important that modes of the probe molecule have very high oscillator strength. This is particularly true if one wishes to study isotopic variants at natural abundance. To a limited degree, one may compensate for a weak oscillator strength by using a thicker sample, but this is often impossible because of increased background absorption by the host crystal.

3. Windows. Even for strongly absorbing probes, it is important that the band falls in a window of the spectrum of the host crystal. In pure crystals 0.1-0.5 mm thick, even very weak bands of the host can absorb so much light that even very strong absorption by the probe makes no measurable difference in the amount of transmitted light. Sometimes fortuitous narrow windows in otherwise congested regions of the host spectrum can be useful, but the 1800-2500cm-' region between carbonyl and CH stretching absorptions is more predictably available for monitoring probe absorptions. Having such a broad window is especially convenient for studying isotopic

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shifts. It is worth remembering that isotopic substitution changes the location of host windows as well as of probe absorptions. 4. Understandable Polarization. To use polarized IR absorptions in order to infer orientation of the probe, it is important that polarization of the molecular modes be well understood. This usually means that the strongly absorbing modes must be simple ones. Intramolecular coupling among local modes can be particularly troublesome if it depends on conformational details (although it might ultimately allow determining conformation). Polarized absorptions from a small, conformationally rigid probe are simplest to interpret. In the context of interpreting polarized absorptions it is worth noting a fundamental ambiguity. In a crystal with no more than inversion symmetry, one can in principle determine the absolute orientation of an IR chromophore by measuring peak intensities for three different directions of IR polarization. Most crystals have additional symmetry elements, however, and since symmetry related chromophores have the same frequency, one can measure only the sum of their intensities. With polarization in special directions, for example, along or perpendicular to particular symmetry elements, one can infer the absorptivity of the individual chromophores because contributions from those related by symmetry are equivalent. However, for other polarization directions, separation is not possible. This means, for example, that one can determine the magnitude of the transition moment components along each of the principal axes of an orthorhombic crystal, but not their relative signs. This creates an eight-fold ambiguity in the orientation of the chromophore, since by changing relative signs one can make the vector lie in any octant. Sections VII.C.1.a and VII.C.2.d describe a method of resolving this ambiguity.

5. Sensitivity to Interesting Aspects of Environment. Of course the spectrum of the probe must be sensitive to the environmental property under investigation. Such properties could include polarity, polarizability, compressibility, stress, and ability to form a specific chemical complex with the probe. In order to simplify interpretation, an ideal probe should be much more sensitive to the property of interest than to other properties.

6. Quantitative Interpretability. Finally there must be a reliable empirical or theoretical method for quantitative interpretation of environmentally induced changes in the probe’s spectrum. This condition is best met by very simple molecules which have been extensively studied spectroscopically and are subject to treatment by high level theory.

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REPORTER PROBES FOR FTIR

C. CO2 as a Probe 1. Frequencies and Intensities. Although our ongoing research on photoreactions of solid diacyl peroxides dictated the initial choice of CO, as an environmental probe, it soon became obvious that the choice was particularly fortunate. Photochemical generation of CO, and radicals occurs smoothly even at very low temperature (Eq. (l)), and synthesis of isotopically labeled peroxides is straightforward. Information from EPR about the radical pair intermediates in diacyl peroxide photolysis is nicely complemented by information from the IR spectrum of CO,, which, unlike EPR, can reveal diamagnetic intermediates. The extinction coefficient for C0,’s asymmetric stretching mode (V3) is extremely high, while that for the bending mode (9,) is also very high [Sl]. The first ranks with the most intense modes known (and makes CO, a major contributor to the “Greenhouse Effect”). In several instances, peroxide crystals photolyzed to less than 0.1% conversion gave reasonably strong bands from C ’ 8 0 1 6 0in 0.4% natural abundance! 0

hv 0

-20 K

R

co, co, .R

--

Products

In addition to having high extinction coefficients,both V3 and V, fall in IR windows of the peroxide crystals. Figure 5 shows absorbance spectra for a crystal of diundecanoyl peroxide before (a) and after (b) photolysis, and their difference,(b) - (a),(c).In most regions of the IR spectrum the crystal absorbs more than 99.9% of the light, so that differential absorbance becomes meaningless. However, absorbance subtraction in window regions reveals 5,, V,, V l + V,, and 5, (13C02) bands near 660, 2350, 3700, and 2280cm-’, respectively (see arrows). The remaining CO, fundamental, the symmetric stretching mode (FJ, is not infrared active. In peroxides containing aryl groups, the ii, region may be obscured. 2. Interpretable Polarization. Since CO, is a linear molecule, the IR polarization of its asymmetric stretching mode is particularly easy to interpret. In the absence of resonant interactions with modes of neighboring molecules (see below),it is safe to assume that the “oriented gas” model holds, and that the absorption of polarized light is maximum in the direction of the long axis [52]. The polarization of the bending mode can also be interpreted, although we have not used it as much.

304

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

P

lrl W

2

6L"

0

m

9

I

I no

Figure 5. Spectrum of single crystal of diundecanoyl peroxide (UP) before and after photolysis (4000-400cm-'): (a) UP at approx. 20 K before photolysis; (b) after photolysis;(c) difference. Note that spectral windows exist near 3700,2350,2280, and 660 cm- l , and that COz bands are easily visible in absorbance subtraction.The scale in (a) and (b) is 5/2 that of (c).

3. Sensitivity to the Environment Stress. Apart from yielding orientation information, the infrared modes of CO, are useful because of their sensitivity to several properties of the environment. For our purposes, it was most important that either G3 or V2 be sensitive to stress or compression, since we were seeking direct evidence for the stress that had been implicated in several EPR studies of peroxide a.

REPORTER PROBES FOR FTIR

305

reactions. Soon after we started measuring spectra of COz, Hanson and Jones reported high pressure spectra of solid CO, showing that both Vz and 3, shift substantially as pressure is increased from 6 to 120 kbar [53]. As shown in Figure 6, the asymmetric stretching mode shifts to higher frequencies as pressure is applied, with a slope close to 0.4 cm-'/kbar. The influence of compression on V,, the asymmetric stretching of CO,, is readily interpreted in terms of the anharmonicity of bond stretching. Vibrational frequency depends on a mode's force constant, the curvature in the plot of potential energy versus distortion at the point of minimum energy. Even a very large force will not affect the frequency unless it changes this curvature. In a condensed medium the effective potential is a sum of intramolecular and intermolecular contributions. Intermolecularforce constants are much smaller than intramolecular ones and thus make little contribution to the total curvature of the potential function at a given geometry. However, a sufficiently repulsive intermolecularforce may shift the position of the energy minimum, and thus the point at which the intramolecular force constant is to be evaluated. Compressing the C=O bond

Pressure I Kbar I

Figure 6. Pressure dependence of the asymmetric stretching frequency of CO,. Experimental frequencies were derived from combination bands and Fermi resonance doublet frequencies. The theoretical line was derived from a mechanical anharmonicity model with force constants from Ref. 73. From Ref. 53 with permission from the American Institute of Physics and the authors.

306

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

length increases the effective force constant by shifting the energy minimum toward the repulsive inner wall of the anharmonic intramolecular bonding potential. In Figure 6, agreement between this theory and experiment is quite good; the small deviation of the calculated line from the experimental one probably reflects contribution from an intermolecular force constant. In Section VII.C.1.b we describe another case where this simple theory breaks down because of contributions from intermolecular force constants, as well as intermolecular forces. The bending frequency of CO, (V,) decreases modestly with increasing pressure, in good agreement with mechanical anharmonicity theory using the normal coordinate system. Between 6 and 120 kbar, the frequency shifts from approx. 655 to 645 cm-'. Unfortunately, the physical picture that emerges for the bending mode's response to pressure is not as straightforward as that for the stretching mode. In fact, one obtains opposite results with curvilinear and normal coordinate systems, indicating that further work is necessary to reconcile these approaches [53]. In our studies of crystalline peroxides, the bending mode was extremely sensitive to environmental changes in the reaction cavity, but the large shifts that we observed were not readily interpretable in terms of simple axial compression.

b. Complexation and Charges. Apart from their response to compression, both V2 and 5, reflect other aspects of their environment, such as polarity, polarizability, and the presence of complexing groups. For different relative orientations of CO, and an electron donating group (such as a free radical), one might expect changes in V, and V,. Matrix isolation studies of H,N...CO, indicate that complexation of a Lewis base to the carbon of CO, gives large red shifts in the in-plane bending of CO,, but only small blue ones in the out-of-plane component [54]. Similar complexation of CO, to either H,O [ S S ] or cyclobutadiene [ S O ] shifts one V, band substantially to the red, while having much smaller effects on the second V, band. In the complexes of CO, with H 2 0 and NH,, and with analogous complexes of CO, with rare gases [56], only small effects were observed in Y,. Electron acceptors in the vicinity of the oxygen of CO, are known to raise vg. CO, adsorbed on cationic centers within X and Y zeolites has shown hypsochromic shifts as large as 30 cm- [57]. In the zeolite study, fields of 2.0 V/A were necessary to raise the CO, frequency to 2350 cm ',which is the highest (intrinsic) frequency observed in the peroxide study. Such high fields are over an order of magnitude larger than those expected in nonionic media. In our studies, weak complexation of the CO, oxygen to radicals is probably responsible for some of the extremely low frequencies that we observe for 5,. This sort of interaction is also thought to be responsible for very high intensities of 5 , in these same sites. Combined with polarized IR spec~

REPORTER PROBES FOR FTIR

307

troscopy, these measurements provide information about the positions of C0,’s in the reaction cavity (Section V1I.D). c. Polarizability. Solvent and matrix effects show that polarizable environments shift v 3 to lower frequencies C58, 51. In fact, “complexation” between CO, oxygens and free radicals may be viewed simply as the interaction of a polarizable group with the CO,. Although 7, is not related in any straightforward way to the dielectric constant of the solvent, studies of CO, in halogenated solvents suggest an inverse relationship between solvent polarizability and frequency shift. In studies of certain halogenated peroxides, the ultimate movement of C0,’s to the vicinity of the halogens has been demonstrated by comparison of V3 of CO, in different peroxide crystals [59]. Small, but measurable frequency shifts have also been observed for s1, in selectively deuterated peroxide crystals. Although it is not clear whether these perturbations arise from size or polarizability differences between C-H and C-D, their existence allows us to identify contacts between CO, and certain methylene groups in nearby peroxides and radicals.

d. Other Ertues. A particular advantage of generating CO, probe molecules by diacyl peroxide photolysis is that they are generated in pairs. Besides enhancing the sensitivity of the IR experiment, the large transition moments of the CO, modes give rise to electrostatic coupling between vibrations of adjacent C0,’s. This coupling is manifested as a divergence in frequencies and intensities from uncoupled or “intrinsic” values. Both stretching and bending modes show significant intermolecular coupling in certain CO, dimers, but very little in others. I3C labeling experiments have revealed coupling constants as large as 9.1 cm- for V, and 4.5 cm-’ for V,. A simple theory based on interactions of electrostatic multipoles describes coupling of stretching modes reasonably well. As discussed below, the coupling of V, yields valuable information about the pair-wise structure of the dimers. Describing the coupling of bending modes is much more difficult. In each molecule there are two orthogonal bends, and in the solid state these are rarely degenerate. Depending on the relative orientation of two CO,s, it is possible for a bend in one molecule to couple with both bends in the second. In many cases, however, it has been possible to assign the coupling in the bending region to two independent pairs of coupled modes. Introduction of “0 into one oxygen also allows one to probe the symmetry of the site. When moderate differences exist between the environare split into ments at the two ends of the CO,, bands from ‘80=C=’60 doublets if the CO, can undergo end-for-end rotation, so that l8O may be found in either environment.

’

308

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

4. Fingerprinting. In crystalline peroxides at low temperature, where rotation is quenched, both V, and V, give extremely sharp peaks. The width at half height is often less than 0.5 cm-'. Peak positions are reproducible from sample to sample within 0.1 cm-'. Since the range of environmental shifts is large (25cm-' for 5 , and 40cm-' for V,), each species gives a reliable fingerprint, especially when it consists of a pair of coupled C0,s. In some cases, it has been possible to identify as many as six separate CO, pairs in one spectrum.

5. Alternative Probes Even though CO, possesses a broad range of special virtues, other small molecules could also be used as reporter probes. Once the technique has been refined using CO,, photofragmentation reactions that yield N,, CO, HCN, SO,, 0,, NO, NO, or any of a host of other small molecules should be amenable to similar mechanistic studies using Raman or FTIR spectroscopy.

V. EXPERIMENTAL TECHNIQUES A. Crystal Mounting To achieve reasonable sensitivity at low conversion and to minimize background absorption, we used large, flat peroxide crystals of high optical purity. Crystals grown to a size of about 5 mm on each edge and a thickness of about 0.5 mm were held with their flat faces perpendicular to the IR beam. In some polarized IR experiments the flat face was tilted 35-40" about a crystallographic axis, which remained perpendicular to the incident beam. With a small amount of thermally conducting grease on each edge the crystals were mounted on a thin copper mask, which contained a hole slightly smaller than the crystal. A mask made of aluminum tape was then used to cover the crystal, giving a sandwich that was mounted in an optical ring attached to a closed cycle refrigerator. The orientation of the crystal was determined with a polarizing microscope [60]. Holding the crystals free, rather than mounting them on a cooled salt plate, optimized the optical throughput.

B. Temperature Control Because the crystals were cooled from their edges, temperature gradients were significant. This problem was especially severe when a crystal cracked

EXPERIMENTAL TECHNIQUES

309

during cooling. Because of temperature gradients and long acquisition times, it was not generally possible to measure real-time kinetics; instead, the temperature was cycled, and almost all spectra were collected at low temperature. A background spectrum of the unphotolyzed crystal was taken at the same low temperature, and absorbance subtraction gave very flat baselines in regions where as much as 98% of incident light was absorbed in the background. With proper filtering of the mercury arc lamp, heating during photolysis was not usually a significant problem, although in certain types of crystals, photolysis generated intermediates that were not usually formed until the crystal had been warmed by 24 K from its original temperature. Obviously, it is important that the excess photolytic energy of 82 kcal/mol is quickly dissipated from the reaction center.

C. Decomposition Limits and Throughput Although the C 0 2 modes are very intense, signal-to-noise was a severe problem because of the necessity to limit conversion. With some peroxides, spectra taken after conversions higher than 0.06% gave more complicated spectra than those taken after 0.03%. To keep the spectra simple, we used crystal plates that were 0.3-0.7 mm thick and 20-40 mm2 in area. With this thickness one could decompose less than 0.06% of the crystal and still observe CO, with intensities close to 0.6 absorbance units. Unfortunately, such crystals absorbed virtually all of the IR light in most regions, and gave fairly high absorptions, even in the spectral windows. To achieve tolerable signalto-noise in difference spectra, it was often necessary to collect several thousand scans over periods of 3-30 h. Given that the sharp lines required high resolution (nominally 0.24 cm- I), some experiments would last up to 10 days. During these long experiments, the crystal would slowly cloud, most likely because solid nitrogen would accumulate on the surface and scatter the incident light. Data collection became a race against time. Polarized IR studies were especially challenging, since the polarizer attenuates the incident light by approximately 60%. In these experiments, the dynamic ranges of both the instrument and the experimentalist were often pushed to their limits!

D. Polarized IR Spectroscopy For polarized IR spectroscopy, one must consider crystal optics. If the crystal is birefringent, the optical disturbance is usually perturbed; in some cases it is not even perpendicular to the direction of propagation. This could pose severe problems with some crystals, but our measurements of refractive index

310

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

in long-chain peroxides indicate that these crystals are not appreciably birefringent in infrared regions of low absorbance. Since refractive indices were the same along two of the three crystallographic axes, we assumed that it was the same along the third axis as well. This assumption rests on work by Henniker, in which it was shown that for biaxially stretched films of polyacrylonitrile, poly(viny1) alcohol and polyamide 6, birefringence is immeasurably small in regions of low absorbance [61]. Thus, even though their space group makes them biaxial, certain long-chain peroxide crystals may be treated as isotropic, and the polarization of V3 should reflect the orientation of the CO,, once refractive index corrections have been applied. We were also concerned about the possibility of infrared rotatory dispersion (IRD). Although this is a potential problem with any chiral crystal, the literature suggests that this effect is negligible in the infrared region of interest [62]. In fact, the vanishingly small magnitude of this effect has made it very difficult to measure it directly in any system.

VI. PEROXIDE CASE STUDIES A. Motivation for Research on Solid Peroxides The FTIR studies discussed in the balance of this chapter constitute part of a larger research effort that has been in progress for some 20 years. The goal has been to use a variety of techniques to establish a picture, which is as detailed and reliable as possible, of the photoinitiated reaction sequence in each of a small number of different organic crystals. It was hoped that such pictures would be useful in understanding reaction mechanisms and solvent effects in solution, but even more that they would provide insight into the factors that make reactions in rigid, organized molecular assemblies different from analogous reactions in fluid media. Both of these goals have to some extent been achieved, and progress has been summarized in several reviews C161. The studies have depended on integrating results from many different methods. The most important have been X-ray diffraction and EPR spectroscopy of radical and, particularly, of radical-pair intermediates. Initial studies centered on azoalkanes, but diacyl peroxides were found to give more varied behavior and have been the focus of most of our recent work. Diacyl peroxides proved to be a fortunate choice, because when other techniques suggested that reaction-induced lattice stress played an important role in solid-state reactions, FTIR spectroscopy of the C 0 2product provided a method for measuring the stress. In the process of applying this method we

PEROXIDE CASE STUDIES

311

were continually surprised by the amount of additional mechanistic and structural information that a pair of CO, molecules could report through their asymmetric stretching bands.

B. Selection of Topics for this Article In choosing which aspects of the peroxide work to present in this review our goal has been neither to describe the reaction scheme in complete detail nor to present the experimental basis for its validity. Rather, we have selected a few experiments to illustrate the various FTIR techniques that were developed for, or used in, this work and which should be applicable for other solidstate mechanistic studies. We also discuss several cases where FTIR has revealed new principles that should be important in understanding and predicting differences between solid-state and solution reactions.

C. The Systems Studied In our FTIR work, we have concentrated on three different peroxide systems. All three classes had been studied carefully by EPR spectroscopy; in every instance, infrared spectroscopy has revealed phenomena that could not be observed with EPR. Before describing the FTIR results in greater detail we present a general description of the reaction sequence after photolysis for each of these systems. 1. Acetyl Benzoyl Peroxide (ABP) a. EPR Studies. Radical pairs were first observed in crystalline ABP by Lebedev [63]. The first peroxide investigationsin our laboratory involved the isotopic cross-over studies by Karch, who established that toluene and methyl benzoate were formed during photolysis of solid ABP by radical-pair coupling rather than by a radical-chain process [64,65]. Subsequent studies by Whitsel [66,9,16], Pankratz [67], Merrill [68,69], and Kearsley [13] established a mechanism for this reaction. Whitsel’s EPR studies provided the general kinetic scheme shown in Figure 7. Photolysis of crystalline ABP with light of 300-400nm yields methyl-benzoyloxyl (MB) radical pairs, which can either collapse to give methyl benzoate, or decarboxylate thermally or photochemically to give methyl-phenyl (MP) radical pairs. At 65 K, visible light decarboxylates the MB radical pair, converting it quantitatively to an MP pair. Under the same photolysis conditions at 77 K, the thermal decay of MB to M P constitutes

312

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

ABP

/

hv OR A

lAo

oc‘o’c

OCH3

II

+ 1co,

+ 2c0,

Figure 7. Photolytic and thermal decomposition pathways in crystalline ABP. Initial photolysis at 300-400 nm gives methyl-benzoyloxyl (MB) radical pairs, which can either collapse to give methyl benzoate, or decarboxylate thermally or photochemically to give methyl-phenyl radical pairs.

some 15% of the total decay of the MB pair. Since thermal decay of MB is slow below 77 K, one expects toluene to be the major product of photolysis at 65 K. Using isotope dilution analysis Pankratz showed that toluene is indeed the major product at very low conversion (0.016-0.041%), but at higher conversion (0.82-7.75%) the yield of methyl benzoate is as high as 46%, even when photolysis is carried out at 65 K. At 77 K, where thermal collapse of MB to ester is faster, the ratio of ester to toluene increases to 2: 1. Possible explanations for the formation of ester at 65K upon higher conversion are that thermal collapse of MB to ester accelerates as the lattice is damaged, or that its photolysis is retarded. Whitsel tested these hypotheses by studying the radical-pair kinetics after longer photolysis. MB photolysis was slightly retarded, but contrary to expectation, the thermal decay rates of both MB and M P pairs decreased even more dramatically with increasing conversion. Furthermore, the kinetics became dispersive (Figure 1). Both species included a very slowly decaying component in addition to a modestly

PEROXIDE CASE STUDIES

313

retarded version of the normal decay observed after low conversion. With longer photolysis times, the proportion of very slow decay increased. EPR spectra showed that, despite the kinetic variation, there were only small changes in the structure of the radical pairs after long photolysis. Although thermal decay rates slowed by more than an order of magnitude, the radical-radical distances changed by less than 0.1 A or 2%. Whitsel therefore proposed that the pressure generated by bond-breaking at one decomposition site can restrict the motion of fragments at nearby sites, even though it does not significantly perturb their structure. Low-temperature formation of ester remains a mystery, the only suggestion being that if stress generated by high conversion can dramatically retard collapse of some radical pairs, it might also dramatically accelerate collapse of others, so that some MB pairs might collapse to ester before they can be observed by EPR.

h. Infrared Studies. Infrared studies of ABP were conducted before we had low temperature capability, so that spectra were measured above 90 K and only after radical pairs had collapsed. By using 13C-labeledABP (acetoxy or benzoyloxy labeled) we could distinguish CO, in sites containing toluene and two C0,s from those in methyl benzoate sites, where all COz was derived from the acetoxy position. The IR spectra revealed the existence of two types of toluene sites and one for methyl benzoate. EPR had shown only one structure for MP, the precursor of toluene. While this observation did not explain the anomalous product distribution at high conversion, it did suggest that FTIR might distinguish between subtly different species when EPR cannot. FTIR at lower temperatures may help resolve the anomaly.

2. Diundecanoyl Peroxide (UP) a. E P R Reaction Scheme. Segmuller determined the crystal structure of diundecanoyl peroxide (UP) and used EPR to study the motions of pairs of decyl radicals, which were generated by its photolysis [60]. Her crystal structure of UP [33] shows that these long-chain peroxides pack in sheets, with the chains tilted from the layer normal by 50" (Figure 8). At the interface between sheets, packing of the terminal methyl groups is relatively inefficient [70]. These crystals grow rapidly in the plane of the sheets but slowly along the normal to form thin plates. By measuring zero-field splitting (zfs) from electron-electron dipolar coupling in triplet radical pairs and proton hyperfine splitting (hfs), Segmuller developed a detailed picture of the motions of these radicals. The sequence of

314

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

Figure 8. Lamellar stacking in U P viewed along the crystallographic (I axis, the shortest axis in the C222, unit cell. The c axis is vertical. Molecules drawn with heavy lines lie in grooves formed by their nearest neighbors, which lie behind them and are drawn with light lines. Reproduced from Ref. 33 with permission from Gordon and Breach Science Publishers S. A.

radical pair intermediates is depicted below:

-

33K hv 53K 134K Pair B -Pair C UP,wPalr A t1 =Imin* 683i tlzirnin 7-68A tq=imin --20K 6.06A Z 2

Products

Irradiation of crystalline UP at or below 20 K yields Pair A, a pair of decyl radicals in which the trivalent carbons are 6.06 A apart. At 33 K, Pair A undergoes clean first-order decay to Pair B, in which the radical-radical distance is somewhat larger (6.63 A). At 53 K, Pair B undergoes a similar irreversible decay to Pair C, in which the radicals are still further apart. At 134 K, the EPR signal from Pair C decays, and products (primarily eicosane) are formed [71]. Single crystal EPR of radical pairs affords not only the distance between radical pairs, but also their relative orientations. By measuring the zfs at differentcrystal orientations, the inter-radical vector can be determined, since

PEROXIDE CASE STUDIES

315

the splitting is maximized when the vector is parallel to the magnetic field. Although only relative positions can be determined by this technique, it is possible to interpret this information in terms of absolute positions as shown in Figure 9. With the assumption that the radicals move one at a time (see below), one obtains the structure for Pair A shown in Figure 9a. The first motion of the radical on the right breaks the local site symmetry; this new arrangement begins to accommodate the two new COz molecules. During the relaxation leading to Pair B, the same radical moves again, while the radical on the left again remains in its original position. After this second motion, the trivalent carbon occupies the position originally held by its neighbor in the peroxide. Apparently each successive carbon replaces its neighbor by a screw motion of the chain, except for the last carbon, which is driven into the inefficiently packed region between molecular layers. The stress generated by bond homolysis is thus relieved by expansion into the most loosely packed region of the crystal.

PAIR C

Figure 9. Radical pair motion in photolyzed crystal of UP. Open circles denote oxygens. The squares denote positions of the trivalent carbons in successive intermediates while the dashed lines connecting them represent the inter-radical vector as determined from zfs. In Pair C, arrows denote the directions of the C,-C, bonds, as determined by hfs anisotropy.

316

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

In the transition from Pair B to Pair C, the overall symmetry of the site is restored by an analogous 180" rotational translation of the second radical. In Pair C there is rapid internal rotation about the C,-C, bond, and the anisotropy of hyperfine splitting by the alpha hydrogens establishes the direction of the first C-C bonds in the radicals. This direction confirms the supposed screw motion. The breaking of symmetry in the first step, its subsequent restoration in the third, and the overall similarity in the motions of the two radicals give support to the hypothesis that the radicals move one by one. Infrared evidence discussed in Sections IX and VII-B.4.a confirms this inference. b. I R Reaction Scheme and Peak Notation. FTIR spectra reveal a more complex reaction scheme than that obtained from EPR. Instead of three intermediates on a single pathway leading to products, eight intermediates on two convergent pathways were observed:

Here, A24, B14, and C,, are the CO, dimers assigned to sites containing radical pairs normally observed in Segmuller's EPR studies of deuterated peroxides. The other intermediates are not detected by EPR spectroscopy, because of an isotope effect on the partitioning between the two pathways, as discussed below. In discussing the intermediates in photolyzed UP, we must distinguish particular CO, molecules and their IR bands. To do so we have used subscripted letters, which, when isotopes are involved, are followed by a pair of numbers in parentheses. The initial letter denotes position in the kinetic sequence of intermediate structures (A, R, S, T, B, C), while the subscripts denote band positions, from high to low frequency, in the spectrum of an unlabeled sample. Thus, Al denotes the highest frequency asymmetric stretching peak in the first intermediate from UP. It also denotes the COz molecule that contributes most strongly to that absorption. A, denotes the CO, pair that gives the first and third peaks of the first spectrum, and A24 is the differently arranged C O , pair that is generated simultaneously with A,, and gives the second and fourth peaks. When isotopes are relevant, the numbers in parentheses give the carbon atomic weights for the pair of CO, molecules in the same order as the subscripts. For a sample of U P with partial 13Clabeling in the carbonyl positions, there are four types of pairs for each unsymmetrical C 0 2 dimer. For example, for Pair AI3,the four pairs are

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

317

A,,(l2,12), A13(12, 13), A,,(13,12), and A1,(13,13). An individual peak, such as A,(13,12) is the A, band in a pair in which A, is 13C02and A, is I2CO2.

3. Derivatives of UP. Much recent work has involved interpreting substituent effects on the solid-state reactions of UP, and more than 60 derivatives have been prepared for this purpose. Some substitutions drastically alter the packing pattern within crystal layers, which makes their influence difficult to interpret. This is particularly true when the parity of the carbon chain length of the constituent fatty acids is changed from odd to even. In the discussion below we focus on the following substitutions of UP which give the same layer structure and are thus easier to understand: 1. Terminal halogenation (e.g. bis-(I 1-bromoundecanoyl)peroxide or 11BrUP). 2. Double homologation (bis(tridecanoy1)peroxide or TP). 3. Specific introduction of a lattice vacancy (solid solution of decanoyl undecanoyl peroxide (DUP) as guest in UP as host). 4. Specific deuteration within the incipient decyl radical (alpha and beta positions). 5. Deuteration in adjacent molecules (solid solution of '3C-labeled UP as guest in deuterated U P as host).

Except for the halogenated peroxides, all of these crystals undergo the same sort of reactions that occur in pure UP, and the substituents can be treated as modest perturbations. With 1I-HaloUPs, however, the reaction pathway is different, even though the crystal structure in the immediate vicinity of the peroxide functionality is unchanged. In the halogenated crystals, motion of the radicals is more difficult, because packing at the interface between layers is tighter, and radical-molecule chain reactions occur. Many of the high-temperature intermediates have been characterized by EPR [72], and a free-radical chain reaction that produces large amounts of C 0 2 was revealed by infrared spectroscopy.

VII. OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

A. Reaction Sequence 1. Identification of Intermediates: Comparison with EPR Results. The most straightforward application of FTIR spectroscopy in studying the decomposition of UP is to identify reaction intermediates and track their intercon-

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PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

version. FTIR is less sensitive than EPR, and the asymmetric stretching peak of CO, is in many ways less informative than the highly structured EPR spectra. However, FTIR provides an indispensible supplement to EPR because it gives only one or two sharp peaks per intermediate, and it shows all intermediates, not just paramagnetic ones. Thus a one-to-one correspondence between FTIR and EPR on the number of intermediate structures and their interconversions would go far toward confirming the mechanism revealed by EPR. Figure 10 shows the V3 region of CO, spectra from a single crystal of U P photolyzed at 20 K and warmed in stages through the temperatures at which EPR spectra showed the intermediates depicted in Figure 9. The final spectrum (10d) was taken after warming to 140 K to destroy the radical pairs and then recooling to 20K. The correspondence between EPR and IR spectral changes demonstrates that both techniques reveal the dominant reaction pathway. However, stages A and B both show four peaks from G3, even though a single pair of C 0 2molecules can give only two such lines. Thus A and B must each have two different CO, pair structures, where EPR had shown a single radical pair structure for each. Two independent techniques demonstrated that the peaks labeled A, and A, represent one kind of CO, pair, while A, and A, represent a second. One technique involved introducing 3 C 0 2 into the pairs to break vibrational coupling within pairs (see Section VII.C.2.d). The other involved using substitution to change the environment subtly so as to alter the ratio in which the two pairs were formed (see Section V1II.B). These pairs of bands, and the structures they represent, are named AI3 and A,, respectively. Species AI3 and A,, grow in parallel during photolysis at 20 K, and do not interconvert at that temperature. In UP and TP, they do interconvert just below the transition from A to B, but in some deuterated peroxides, they seem to decay at slightly different rates. In the same way it was shown that in Species B, B, and B, constitute one pair (B14), while B, and B, constitute a second (B23). These species interconvert above 28-31 K. Because EPR is very sensitive to minor changes in radical pair structure, we were initially surprised that the IR studies showed two CO, pair structures in Species A while EPR revealed only one radical pair structure. The infrared spectra demonstrate that two pathways leading to products actually exist, and that they converge on forming Species C. The EPR studies do not show doubling because the deuteration used to simplify the EPR spectra biased the partitioning between the two pathways, and gave only a single structure for Pair A (Section V1II.B). With Pair B, only one structure was observed by EPR because B,, has a much higher population than B,, in the deuterium labeled peroxide. The simple, sharp V, lines of the FTIR

-

Figure 10. C 0 2 asymmetric stretching region in FTIR absorbance subtraction of lightly photolyzed crystals of UP. (a) Species A, after photolysis at 20 K ( - 0.05% conversion);(b),(c), and (d)are representative of species formed after warming through temperatures where EPR spectra show transitions.

319

T R A N S I T I W FlDY A TO I IN UP

IZUI SfllEfl

2360

2365

2360

2340 2335 U~VENUP(BER5

2345

2340

2325

23 I

Figure 11. CO, asymmetric stretching bands in transition from A to B in UP, showing the existence ofthree species (R,,, S , , , and TI2)not observed by EPR.

320

E0.0345

A: 0.6447 0:0.1034

At0.3959 0Z0.4435

AZO.3543

0Z0.4700

A:0.1071 0:0.6195

II 4x

n

4x

; WFIVENUMBERS

0

Figure 12. Absorbance subtraction of Y3 to show intermediates on pathway from A to B, as well as extent of reaction. The numbers indicate the fraction of A and B subtracted.

321

322

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

spectra allow more sensitive detection of minor constituents than EPR. Although EPR gave no evidence for any intermediates between A and B, the infrared spectra reveal the presence of as many as 14 different C0,s during this transition (Figure 11). Eight of the bands represent the species already discussed, A13, A24, BI4, and B23. The other six represent three new species, which are named R,,, S12,and TI2. These minor intermediates are not observed in the EPR spectra because they never constitute more than 20% of the mixture. Even with deuteration, the EPR spectra are so complicated that these minor species cannot be detected by EPR, but the high dispersion of the infrared spectra of C 0 2 made them visible. Since the spectra in Figure 11 were all taken at the same temperature, and with the crystal in the same position, it is possible to obtain spectra of the intermediates R, S, and T by subtracting the spectra of A and B from the spectra of the mixtures (Figure 12). By adjusting the scale factors for the subtraction of A and B to cancel the peaks from those species, it was possible to determine the extent of conversion from A to B. Thus, in Figure 12c, 64% of A had decayed, 18% of B had formed, while the remainder (17%) was composed of a mixture of R,S, and T. That this procedure gives a flat baseline shows that both A,, and A24 decay at the same rate. Spectral overlap and the small amount of B23 make it dificult to tell if B,, and B23 grow at exactly the same rate. By plotting peak intensities during the conversion of A to B, one observes sequential growth of Species R12, S12, and T12; in another set of spectra, an induction period is clearly evident in the formation of Species S12. 2. Real-time Kinetics with IR Spectroscopy. The type of temperature cycling experiment described above cannot distinguish mechanisms involving irreversible decay through the chain of intermediates from those involving reversible interconversion between Species A and the less primitive intermediates. For example, the Species S that is formed with an apparent induction period in the temperature cycling experiment may instead represent an equilibrium concentration formed at high temperature and frozen in as interconversion slows during cooling to the temperature for measuring the spectrum. To distinguish between reversible and irreversible processes, realtime kinetics are needed. When the crystal is cooled from its edges, as in our experiment, temperature gradients make this experiment very difficult, since the rates in different regions of the crystal vary substantially. Small temperature differences are particularly significant at cryogenic temperatures. At any given temperature setting in the transition region, there is an immediate burst of reactivity followed by almost none at all; thus an experiment of this sort would take weeks, with most of the reaction occurring in the first few hours. Even then, absorbance subtraction could not be used to simplify the spectra since the

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

323

spectrum of the precursor would have been taken at a different temperature. Despite these challenges, such an experiment was attempted. Temperature gradients made it necessary to raise the temperature setting by 1 K twice during the transition. Figure 13 shows the results of this experiment, in which 34 spectra were collected over a period of 67 h. In this plot each species is measured by its peak height, normalized to the largest height the peak reaches during the transition. The reaction sequence is the same as in the temperature cycling experiments, as expected for a series of irreversible steps. Species T and B exhibit distinct induction periods in their growth curves. It is difficult to tell if the formation of Species S is preceded by an induction period, since the first spectrum was taken after a substantial amount of a possible precursor, Species R, had already formed. However, S is formed more slowly than R, and in light of the equivalent real-time decay rates of A,, and A,, (not shown), it probably derives from Species R. In this case the infrared technique has revealed not only the presence of minor intermediates, but also their order of appearance, as well as information about energetics of the transition from A to B. This type of information is not unique to single crystals, but the reproducibility of this

. 1 . 1 . . . . .

0

.

SO0

.

-

1600 2400 . TIME IMlNl

3200

4000

Figure 13. Real-time kinetics of the transition from A to B in UP. Relative rates are formulated as the percent of the largest peak height reached during the transition. After 446 and 1216 min, the temperature setting was raised by 1 K (see arrows).

324

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

process from crystal to crystal makes it possible to compare the results of several experiments with high confidence. 3. Measurement of Reversible Interconversion. A process that is not often observed in matrices and glasses is the reversible interconversion of species that differ only in their positions or orientations. Because of the high resolution of the IR spectra of CO,, we have been able to document several cases of reversible interconversion of intermediates. While some of these have been observed by EPR, spectral complexity precluded its use in observing others. Temperature dependence studies of irradiated UP showed that B,, and B,, interconvert reversibly above about 30 K, with B,, dominating at all temperatures at which Species B is stable. In Figure 14, a plot of In K versus 1/T gives a straight line for spectra taken above 30K, but more or less random values for spectra taken below that temperature. The abrupt change near 30 K shows that below that temperature, interconversion is slow, and nonequilibrium populations are obtained. If this interconversion were being measured in a matrix isolation experiment, one might simply use band intensities to measure the equilibrium constant. With an oriented crystal, however, correction must be made for the

35K

2.3”

2.1..

Y

C

1.92

TEMPERATURE DEPENDENCE OF BZ3

= El4 I N 0 - U P

A H = -255cal/mol

AS =

1.51

0.026

0.027

-

0.028

0.029

- 3.5 eu

0.030

0.031

0.032

0:033

0.034

0.035

Vf

Figure 14. Temperature depencence of BZ3= B I 4 in 3,3’,4,4‘-d,-UP (,%UP).Since cooling below 30 K traps a nonequilibrium population ratio, only points to the left of T = 31 K were used to calculate the least squares line. Point 1 (low temp.) is not shown.

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

325

polarized absorption by the oriented molecules. Such a correction requires consideration of resonant coupling within CO, dimers. Since orientation and coupling can be measured directly, it is a fairly simple manner to make these corrections. Ordinarily, one might expect the molar absorptivity to be the same for different CO, bands, but our studies have shown that they can vary by as much as 80% from one species to another. Fortunately, there exists a fairly good inverse correlation between absorptivity and frequency for the CO, bands studied in this work (see Section V1I.D). Since some experiments gave B,, with very little B23, the absorptivities of B, and B, relative to other species such as C,, are known. Although B23 never dominated over B,,, the absorptivity of B2 can be approximated assuming the correlation between absorptivity and frequency holds for this band.

4. Processes that EPR Cannot Detect a. Ultimate Loss ofCO2. In addition to revealing minor intermediates on the reaction pathway, the infrared experiments supplement EPR by revealing processes that involve no change in the EPR spectrum, or occur after all radicals have reacted. Once the radical pairs have collapsed in UP, the CO, molecules escape the vicinity of the reaction center and are slowly lost from the crystal (see Section VII.B.4.b). In heavily photolyzed crystals, and in solid solutions containing structural defects, the rate of CO, loss is much greater than in lightly photolyzed ones. Although CO, escape is not a “reaction” that photochemists would ordinarily monitor, the extent of annealing that it represents has a profound influence on reaction during subsequent photo1ysis. h. Decarboxylative Chain Decomposition. In crystals of 11-HaloUP, infrared studies have revealed a radical chain decomposition. EPR spectra of these crystals showed that annealing in the vicinity of 230 K converted secondary radicals c1 to the carbonyl group into secondary radicals located further down the hydrocarbon chain [72]. Infrared spectra taken after such annealing showed a dramatic increase in the CO, absorption (Figure 15), indicating that this transformation involves a decarboxylative chain reaction. Although it was not possible to measure the intensity of v 3 directly, the intensity of the band from naturally abundant 13C02indicated that the C 0 2 concentration increased more than four-fold, and in some cases perhaps 10or even 100-fold (Dr. T. C. Semple, personal communication) during this process. At the same temperature, bands most likely from terminal olefins, alkyl cyclopropanes, and primary alkyl radicals were formed. The mechanism of this chain reaction remains uncertain.

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

Figure 15. Y3 region of C 0 2 in 11-BrUP at -20K after photolysis and annealing. Near 240 K, a decarboxylative chain mechanism occurs.

B. Measurement of Stress 1. Observed versus Intrinsic Frequencies. The initial goal of our FTIR work was direct measurement of the high local stress that had been implicated in the motions of radicals generated in crystalline peroxides. The best single line of evidence for high local stress, and the most direct measure of its magnitude come from the frequencies of the CO, 5 , bands. In all cases studied thus far (UP, 11-BrUP, and ABP), spectra of the most primitive species contain bands at higher frequencies than those of the most relaxed species. In photolyzed UP, the frequency shift of Y3 with site relaxation is dramatic; the highest frequency bands in spectra of the most primitive and most relaxed species differ by almost 20cm-' (Figure 10). While Species D absorbs at a

327

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

frequency normally observed for CO, in hydrocarbon solvents, the frequencies of A, and B, are even higher than that for gaseous CO,! Since local stress is highly anisotropic and 7, is primarily sensitive to stress along the long axis of COz, the infrared spectra give only a lower limit to the maximum component of stress. This anisotropy also makes the relationship between frequency and site relaxation far from monotonic, but the general trend from high frequency to low is as expected for relaxation. Before using these frequency shifts to infer local stress, one must exclude other possible explanations. Among the most likely alternatives is coupling between 5 , of a CO, and some other nearby oscillator, in particular the corresponding mode of the CO, molecule with which it is paired. To distinguish between “intrinsic’@ frequency shifts caused by an unusual local environment and those caused by resonant coupling with vibrations in an adjacent molecule, one may compare the frequencies of bands from 12C02 and those from naturally abundant 13C02. If there is significant coupling with the second CO, in the reaction site, then pairs containing ‘zC02 and 13C02 will couple much less than ‘2C02-’zC0z pairs, since the nearresonance ordinarily found in “homoisotopic” pairs is disrupted. In unlabeled UP, almost all 13C02molecules exist in “heteroisotopic” pairs, and the W O , frequencies are very close to their intrinsic values. Frequency shifts due to coupling can be understood quantitatively using first-order perturbation theory, and the intrinsic frequency for an uncoupled CO, can be calculated. For each intermediate species a self-consistent set of frequency shifts was observed in spectra of 13C labeled UP, where all four possible isotopic combinations could be observed (see Section VII.C.2).This consistency also excluded coupling with other surrounding molecules, since the influence of such coupling would be different for ’2C0, and 13C0,. Table 2 lists the observed and intrinsic frequencies for each of the C 0 2

TABLE 2 Observed and Intrinsic Asymmetric Stretching Frequencies for C02 species in UP (in c r n - ’ ~ Species

Observed V

Intrinsic S

Species

Observed I,

~~

A1

A2

B1

BZ

c1 D

2352.40 2346.36 2351.49 2344.26 2342.38 2333.23

2350.81 2346.30 2350.19 2342.96 2342.07 2333.23 ~

‘Gas phase at 2349.16 an-’ (see Ref. 73).

A3 A4 B4 B3 c2

~

2336.83 2332.98 2329.61 2334.49 2331.35

Intrinsic i ~~

2338.42 2333.04 2330.92 2335.84 2331.68

328

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

species observed in UP. The difference between the highest and lowest intrinsic frequencies is greater than 17cm-'. If this shift were due simply to changes in local stress, one would infer from Hanson and Jones' results [53] that the maximum stress has a lower limit equivalent to a pressure of 4045 kbar. 2. Excluding Alternative Explanations. Other environmental factors could account for at least some of the frequency shift between A, and D. It is important to exclude or at least assess other possible explanations for the range of frequency shifts. Several possibilities can be discounted fairly readily. High quality force fields for CO, show that distortion from linearity lowers 5, instead of raising it [73], so this cannot explain the anomalously high frequency of Al. In zeolites, high charges can raise V3 by as much as 30 cm-', but an electric field of 2.0 VIA would be necessary to raise 5, to 2350 cm [57]. Such a high field is more than an order of magnitude larger than the ones expected in a non-ionic medium such as an organic peroxide. Complexation of a CO, carbon with a free radical is a possibility, but intrinsic frequencies as high as 2346.6 cm- have been observed in spectra of ABP at temperatures where radical pairs have already collapsed. This does not disprove the existence of complexation in Species A,, for example, but it does show that high frequency bands can occur in the absence of radicals. Spectra of 5, provide some evidence for complexation, but analogous effects on V, spectra have not been observed. For example, Semple has found an abnormally low bending frequency in early spectra of bis(1l-chloroundecanoyl) peroxide, suggesting complexation between the carbon of CO, and a primary radical, but the stretching band shows no unusual shift to high frequency (T. G. Semple, personal communication). These observations are commensurate with measurements on H,O and NH, complexes of CO, discussed in Section IV.C.3.b. ~

3. Magnitude of Stress. We suspect that sources besides stress may, in the aggregate, account for as much as half of the observed spread in V3, so that the most highly stressed CO, experiences the equivalent of at least 20kbar of pressure. Support for the inference of high local stress comes from a survey of the temperature dependences of the bands observed in 24 different reaction site environments. Since a crystal expands as it warms, one can make an analogy between temperature and pressure. When the temperature is raised, the crystal lattice expands, and the average force constant between stressed molecules decreases [74]. The effect of warming on the 5, bands may be understood by considering various contributions to its frequency in condensed media. Classic experi-

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

329

ments along similar lines were conducted by Wiederkehr and Drickamer in the late 1950s using V2 of CH3CN [75]. They showed that in various halogenated solvents, contributions from dispersion, electrostatic and induction all lowered the CN stretching frequency, while only repulsion raised it. In this study, which yielded' good agreement between theory and experiment, CN stretching frequencies were measured over pressures of 1- 10,OOO atm (Figure 16). At 1atm, all solvents produced red shifts, as might be expected for these polarizable media, in which the excited vibrational state of CH3CN is stabilized by its surroundings relative to the less polar ground state. As pressure was applied to most solutions, the frequency initially dropped, and

r

1.o

1 1.1

I 1.2

RELATIVE DENSITY

IP I P o I

I 1.3

Figure 16. Pressure dependence of CN stretching frequency of CH3CN. Pressures range from 1 to 10kbar, with the largest red shifts found between 1 and 3 kbar in all solvents except CCI,. Adapted from Ref. 75 with permission from the American Institute of Physics and the authors.

330

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

then rose. The minimum in the frequency versus pressure curve occurred between 1 and 3 kbar. At low pressures, dispersive interactions dominated and the frequency dropped, but as more pressure was applied, repulsive interactions began to dominate, and the frequency rose. This is thought to be a general phenomenon, but the pressure at the minimum frequency can vary considerably with solute and solvent [76]. For V3 of CO, one might expect similar contributions to the frequency shift. Inductive and electrostatic effects should be somewhat less important than with CH,CN, since the time-averaged asymmetric stretching mode is more nearly symmetric with respect to its environment. Thus, dispersive interactions should again dominate the frequency shift at low pressure. This is borne out in studies of solvent shifts on V,, which show the largest negative shifts for the most polarizable solvents. Even more importantly, recent work by Buback, Schweer and Tups shows that the second overtone of the asymmetric stretching mode of CO, (3V3) behaves much like V, of CH,CN [77]. As pressure is applied to CO,, 39, drops by -21 cm-', and then starts to rise with a turning point near 2.3 kbar (Figure 17).These large shifts are in accord with Buckingham's theory of solvent-induced frequency shifts, which predicts 37, to have a shift that is three times greater than that of 9, for a given environmental perturbation [78]. For CO, trapped in peroxide crystals, the local stress is very anisotropic, and a wide range of frequencies have been observed. Presumbly, the repulsive contribution to the frequency shift is small for CO, absorbing at low frequencies, and expansion of the lattice should raise the frequency by diminishing the dispersive stabilization that the excited vibrational state gains from its surroundings. The opposite should hold for CO, in tight cage environments [79], where expansion of the crystal should relieve stress on the molecule, and lower the frequency of V,. Thus, high frequency CO, bands should shift down with warming, while low frequency bands should shift to higher frequencies [80]. Since many of the CO, molecules exist as dimers, it was necessary to consider the influence of coupling, which itself might be temperature dependent, in order to compare the influence of temperature on intrinsic frequencies. Figure 18 shows the temperature dependence of the intrinsic frequencies of 24 different V3 bands in several different types of peroxide single crystals. The spectra were taken over a wide temperature range, with the temperature differences ranging from 5 K to 85 K for any given band. In all but three cases (see asterisks) the influence of coupling could be excluded either through the use of heteroisotopic pairs or because the coupling constant was very small. Except for a narrow transition region from 2341 to 2343cm-', a clear distinction can be made between high and low frequency bands. All of the

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

331

Figure 17. Density dependence of the band maximum position of the 33, mode (V,,) in pure C 0 2 at temperatures between 298 and 500 K (with Vg = 6972.6 cm- I). Below the critical density (0.45 g cm-3), P and R branches were observed, and V, is taken as the arithmetic mean of the P and R branch maxima. Points on the lower left represent hot bands. Reproduced from Ref. 77a with permission from Verlag der Zeitschrift fur Naturforschung.

bands above 2343 cm-', but none of those below 2341 cm-', shift to lower frequency with warming. Two of the bands below 2341 cm-' show no shift, but in each of those cases the band was very weak and hard to measure, or the temperature could not be varied by more than a few degrees. For the high frequency bands, shifts less than 0.2 cm- ' (short arrows) occurred when the temperature could not be varied by more than 6". This dichotomous behavior provides strong evidence that repulsive terms dominate the frequency shift for species that absorb above 2343 cm-'. Although complexation with adjacent functional groups cannot be excluded in all cases, the relief of stress provides a simpler, more general explanation of this phenomenon. Clearly, the high frequency bands represent C 0 2 molecules in a wide range of sites, and it would be very coincidental if all the shifts arose from different degrees of complexation.

ww

2352

t

*

'

*

' no detectable shilt

2348

2346

t

2344

A

B2

A

Frequency

2342

b

2340 icm ' 1

C1 BrUP

*

2338

2336

BrUP 22

2334

D

Figure 18. Effect of warming on intrinsic frequencies of V3 of CO, in UP, 11-BrUP and ABP.

2350

A2

measured frequency with Questionable coupling

: presence of coupling

-

2332

BrUPBq

23

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

333

Comparison of this behavior with that shown for fluid CO, under high pressure suggests that in the peroxides, CO, molecules absorbing near 2342 cm- experience stresses roughly equivalent to the pressure that gives the minimum in the frequency versus pressure curves for 35, of CO,. We find it striking that the minimum in the 3V, versus pressure curve is 21 cm-l below the gas phase value, while the crossover point in Figure 18 occurs 7 cm-’ below the gas phase value of 2349.1 cm-’. Since Buckingham’s theory of solvent shifts in infrared spectra predicts that the solvent-induced shift of 35, will be three times greater than that of V3 itself, we think this is more than a numerological artifact. If repulsion dominates the frequency shift beyond 2342cm-’, and if v j increases by 1 cm-’ for every 2.7 kbar of stress, as in crystalline CO, [57], then the C 0 2 molecules that give the highest frequencies are under approximately 24 kbar more stress than the ones at 2342 cm- Thus, it is likely that the stresses on A, and B1 are equivalent to those in a solution under approximately 25-28 kbar of pressure. This is roughly one-third the pressure required to convert graphite into diamond [81] and many times larger than pressures ordinarily used to influence organic reactions [82], so it is not surprising that generation of such high local stress can have dramatic chemical consequences. We think that large local stresses play dominant and controlling roles in many solid state reactions, especially ones involving bond cleavage.

’

’.

4. Long-Range Effects Within and Between Molecules. An intense stress field might exert mechanical influence at a distance from the location where bond-breaking generated the stress [83]. If the medium were an unstructured isotropic continuum, this stress should fall off with the cube of the distance from the reaction center, while the strain or displacement should drop off with the square of that distance [84]. Thus, if a group at a distance of one molecule from the defect were displaced by 1 A, a group ten molecules away would be displaced by only 0.01 A. Likewise, a stress equivalent to 25 kbar at a distance of one molecule from the reaction site would fall to 25 bar at a distance of ten molecules. This equilibrium gradient of elastic strain and stress should quickly be established by the same mechanism which propagates sound in the medium. a. Intramolecular Efects. The most notable difference between a molecular crystal and an isotropic continuum is that the crystal is discontinuous and highly anisotropic. Most of the elastic “give” comes between molecules rather than within them. In polyethylene, for example, compression perpendicular to the chain direction is 70 times easier than along the chain, because there are more intermolecular gaps in the former direction [85]. In long-chain

334

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

diacyl peroxide crystals, stress and strain should be transmitted more efficiently and over longer distances along the molecular chains than in any other direction. A second difference from the continuum model is that large stresses near the reaction center should undergo thermally activated relaxation. According to the molecular mechanism of stress relaxation proposed above, such irreversible, or plastic, deformations occur in UP when the two decyl radicals back away from the reaction center by rotational translation along their long axes. In the process of making more room for the two new CO, molecules, each radical chain is driven into the adjacent interface between two layers of peroxide molecules. Introduction of a defect or a "hole" at the end of the peroxide chain should facilitate this motion and allow efficient relaxation of the stress. The prediction was tested by introducing holes into the crystal structure of UP in specific positions. This was achieved by growing solid solutions of UP containing small amounts of decanoyl undecanoyl peroxide (DUP) in which one alkyl chain is shorter than those of the host by one methylene group [86]. In order to distinguish the IR signals from C 0 2 molecules generated next to short chains, the guest molecules were labeled with 13C. In UP crystals containing 11.8% DUP (Figure 19), 5.9% of the chains are one carbon short,

\ ; h h h

Figure 19. Schematic diagram of crystal packing in solid solutions of DUP/UP.

Large open circles highlight the absence of a methylene in the short chain of DUP. Small open circles denote the position of 13Cin DUP*"/UP, while the solid circles show the position of ''C in DUP*"/UP.

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

335

leaving holes at their ends. With this composition, peaks due to l3COZfrom DUP "guest" sites should be more than ten times larger than naturally abundant ' T O 2 of reaction sites in the host. By labeling the carbonyl group in either the ten carbon chain (DUP*") or the. eleven carbon chain (DUP*"), separate experiments were used to deduce the origin of the C 0 2 s in guest sites. Thus, signals from COz molecules next to nonyl and decyl radicals in DUP sites should be distinct unless they exchange positions. For these experiments it was important to establish that solid solutions were formed, and that the crystals had structures that were isomorphous with that of pure UP. The crystals were shown to be single by examination with a polarizing microscope, and to have lattice constants within 0.1% of pure UP by X-ray diffractometry. The presence of a single melting endotherm upon differential scanning calorimetry showed that the DUP molecules had not segregated into a separate phase. Certain spectroscopic similarities between DUP and U P sites also indicate that the DUP is included within the crystal lattice of UP. Figure 20 shows the asymmetric stretching bands in the spectrum of a crystal of DUP*" in UP after photolysis at approximately 20-30 K. Because of large temperature gradients, the 12C02 spectrum shows eleven absorptions from 2329.3 to 2352.4cm-'. Ten of these can be assigned unambiguously to the first four stages of relaxation previously observed in UP. The new peak at 2330.9cm-' was assigned to the l2COz member of a 12C02-13C02pair from DUP*". In the l3COz region (Figure 20b), which has been shifted and scaled so that peaks of equal force constant are displaced vertically from those in Figure 20a, the single band at 2265.3 cm- is due to the other member of the CO, pair from guests. An analogous spectrum of DUP*ll in U P confirmed this assignment. In this sample the 12C02peak shifted down by 0.5 cm-' to 2330.5 cm-', while the I3CO2peak shifted up by 0.5cm-' to 2265.8cm-', showing that the sites adjacent to the nonyl and decyl radicals give slightly different C 0 2 frequencies. Thus, under conditions in which C 0 2 pairs from UP are undergoing the transition from A to B, those from included DUP have already relaxed into a single structure in which the COz molecules are relatively unstressed and do not exchange positions over a period of hours. The initial 13C02band at 2265.3cm-' (Figure 20b) was assigned to a DUP site in which the nonyl radical has undergone a full rotational translation more easily than the Pair A transforms to Pair B in the pure host crystal. Further warming causes the I3CO2 band at 2265.3cm-' to shift to 2266.5cm-' at a rate somewhat slower than the transformation of Species B to Species C in UP. This retardation may be due to a lower driving force for motion of the second chain when the first chain has fallen into a hole rather than being squeezed into the interface. The low frequencies of the CO, bands in guest sites and

B

WVENUnBERS

0

1'1

.

'5 1

Figure 20. Solid solution of DUP*''/UP after photolysis at 20-30K. (a) "C02 asymmetric stretching. The arrow marks the '*CO, peak from DUP; (b) 13C0, asymmetric stretching band from DUP. The frequency scale of spectrum b ("CO,) has been shifted and scaled so that peaks with the same force constant are displaced vertically between a and b.

336

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

337

their relaxation properties demonstrate that a lattice vacancy at a distance of 14.2A can have a profound influence on stress relaxation. In an isotropic continuum one might expect a defect at a distance of 14 A to have a much smaller influence on stress relaxation. This suggests that the observed influence is relayed specifically through the carbon chain of the free radical. If the mechanism were less specific, the holes should perturb reaction centers at similar distance in other directions. In the herringbone packing of crystalline UP there are 42 peroxide groups of neighboring molecules that are closer to the terminal methyl group than its own peroxide group; the closest is only lOA away. Statistical analysis of a random solid solution with 5.9% methyl defects shows that 74% of the host CO, pairs should have as many methyl defects within 14.2A as 74% of the guest pairs have. Since spectral bands from hosts show nothing but a very slight broadening relative to pure crystals, and guest spectra show a profound influence from the defect, mechanical stress must be transmitted specifically through the radical chains. These observations confirm Segmuller’s step-wise mechanism of stress relaxation in crystals of UP (Section VI.C.2.a). For DUP guests, in which the terminal methyl is replaced by a hole, the initial rotational translation of the nonyl radical is achieved at the lowest experimental temperatures, whereas the same motion is complete only after the first three or four relaxation steps in UP. After the first rotational translation motion of the nonyl radical, stress is accommodated in the reaction center, and subsequent motion of the decyl radical away from the reaction center is slower than in the pure crystal. The very low frequency of the initial band in defect sites might suggest even greater retardation in motion of the second radical, but it is important to remember that frequency shifts show only the component of stress along the long axis of CO,. Given an opportunity, CO, should reorient so that its long axis lies in the direction of least stress [79]. The motion of the second radical in DUP sites is almost certainly driven by stress, which is underestimated by IR frequency shifts. 6. Intermolecular Efects. The experiments with solid solutions of DUP in UP demonstrate long-range mechanical effects transmitted within molecules. Analogous intermolecular effects could be important in solid-state reactions in which significant decomposition occurs, since the stress fields from nearby reaction sites should influence the mechanical properties of a center undergoing reaction [83]. Depending on the amount of environmental distortion and the degree to which the crystal lattice can accommodate this distortion, the course of a solid-state reaction should change more or less dramatically during decomposition.Since product distributions are commonly interpreted in terms of ideal crystal structures, it was important in at least one case to establish the conversion limit beyond which reaction-induced changes in crystal properties would begin to complicate chemical behavior.

338

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

When crystals of U P are photolyzed at low temperature to conversions greater than about 0.06%, the IR spectra show numerous q3 bands that are not observed after less extensive photolysis. The onset of this conversion threshold is sharp, suggesting cooperative behavior of several defects. The shift of these bands to high frequency suggests the influence of stress fields

7 a M I N h v AT 901

230 HRS AT 1781

WARMED TO 1bIK

14 HRS AT 205K

COOLED TO 901

COOLED TO W K

A

e

78MIN h Y AT 9OK WARMED TO 239K[lOMll

WARMED TO 1 4 2 1

COOLED TO W K

COOLED TO 9 0 K

70 HRS AT 279K

8 HRS AT 29bK

WARMED TO 2 5 6 1

COOLED TO 9OR

COOLED TO 9 0 1

13 0 M l N hYAT SOK

WARMED T O 1 4 2 1

COOLED TO 901

I

2360

2340 2390 YAVENUtlBERS

2330

2380

Z3$0 2340 2390 URVENUIlBERS

2: 10

Figure 21. C 0 2 stretching during three successive cycles of photolyzing and annealing a single UP crystal. All spectra have the same scale and were measured at the photolysis temperature of 90 K. After the first and third 78-min photolyses (a and f) many extra bands accompany the Species D singlet. After the second 30-min photolysis (d) they do not appear, although the singlet is nearly as large. Reproduced from Ref. 83 with permission from Gordon and Breach Science Publishers S.A.

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

339

generated by distant reaction sites. The influence becomes more pronounced upon warming to 140K,but disappears after annealing above 280K. This suggests that stress measured at a certain distance from the reaction center increases with each successive plastic deformation between the stress nucleus and the site of measurement, but then it decreases as the relaxation front passes this site and stress becomes more evenly distributed throughout the crystal. The sequence of photolysis and annealing of Figure 21 shows this more clearly. Isotopic labeling studies show that the observed frequency shifts of abnormal bands are caused primarily by site differences, and not by changes in resonant coupling. In a spectrum of Species C from UP with 25% 13Clabel, for example, the new peaks from '3C0, generated after long photolysis had mass-adjusted frequencies that were very close to frequencies of analogous peaks in the " C 0 2 region. The small differences that were observed could be attributed to modest changes in coupling, but the major source of frequency shift was a change in environment, not coupling (Figure 22). The absence of pronounced vibrational coupling may argue against the view that the abnormal bands are due to local pockets containing several decomposed molecules formed by some nonrandom decomposition process. Since photodecomposition appears to occur randomly throughout the UP crystals, the influence of cooperative intermolecular effects at conversions as low as 0.06% indicates that reactivity in single crystals can be controlled by defects far beyond the first coordination sphere of the reaction center, and that caution must be used when interpreting product ratios, even after low levels of decomposition. The threshold behavior for such influence also warms against extrapolation of product ratios at finite decomposition levels to those expected after infinitesimal amounts of conversion. Although the estimates of range (more than five molecular distances) and cooperativity (three or four sites), which were developed in a semiquantitative treatment of long-range stress, must be considered highly approximate, the danger for simplistic interpretation of solid-state reaction mechanisms and the power of IR spectroscopy for studying such complications were clearly demonstrated [83].

C. Structural Measurements by FTIR Spectroscopy Information about the actual arrangement of CO, molecules in decomposition sites is valuable for many reasons. Interpreting local stress and its anisotropy, and predicting how it should influence reaction dynamics, require some knowledge of how the CO, probe molecules are arranged in the radical pair cage. By tracking the motions of the CO, molecules in parallel with

I

J

/ I

2950 2340 2330 WAVENUHELRS

2330

WAVENUHEERS

i5

Figure 22. Comparison of CO, V3 for Species C in 24.7% l3C-1-UP (random label) during very long photolysis at 86.5 K. Reproduced from Ref. (83) with permission from Gordon and Breach Science Publishers S.A.

340

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

341

those of the free radicals, it should be possible to develop a clearer understanding of the cooperative mechanisms by which stress is relieved. Even though we cannot hope by EPR and FTIR to establish precise experimental atomic coordinates for the set of reaction fragments in each stage of relaxation, every item of structural information is valuable. Kearsley has begun simulating the intermediate structures and the low energy paths among them using molecular mechanics calculations [131. The combined spectral data will probably be more than suflicient to guide these computations, excluding otherwise plausible but incorrect mechanisms and confirming the correct one. 1. Determining the Orientation of Individual Molecules a. By Potarization. The most useful structural information about CO,

comes from polarized IR spectra of oriented peroxide crystals. Combined with an analysis of the coupling of the V3 vibrations of CO, dimers, this technique has provided orientations of most CO, molecules to within approximately 5". In the orthorhombic crystals of UP, all unique information obtainable by polarization studies is contained in the absorption measured along each of the crystal axes. This illustrates a fundamental difference between optical and magnetic resonance spectroscopy.In EPR spectroscopy, transition energies depend on molecular orientation because of anisotropic interactions, but absorption strength is insensitive to molecular orientation (it does depend on the direction of the microwave magnetic field with respect to the applied magnetic field, which is routinely fixed at 900). In IR, on the other hand, frequencies are independent of molecular orientation, while absorption strengths vary according to the angle between the molecular transition moment and the electric field of the IR beam. Orientational information in EPR comes from spectral frequencies,while in IR it comes from intensities. In EPR even symmetry-related species give different peaks for a general mounting of a crystalline sample, and one can determine the orientation of individual molecules. In IR, symmetry-related species contribute to the same peak so that one cannot measure individual intensities and orientations. Although the transition moments of individual molecules may have arbitrary orientation, the absorption tensor for a set of molecules displays the symmetry that relates them. Thus in an orthorhombic crystal the absorption tensor is diagonal in the unit-cell axis system. All molecular transition moments in the symmetry-related set have the same projection on each of the cell axes, but further information about the orientation of individual moments must come from other lines of evidence, as discussed below.

342

PHOTOCHEMICAL MECHANISM 1N SlNGLE CRYSTALS

To obtain projections on all three axes, spectra from at least two crystal mountings are necessary. For the flat crystals of UP the most convenient mounting was with the large ab or (001) face perpendicular to the incident IR beam. In this orientation, light that propagates along the c axis is resolved into components along a and b. Thus, alignment of the polarizer along either a or b yields the two absorbance components in this plane. The ratio of the square roots of these absorbances measures the tangent of the angle between the b axis and the projection of the CO, axis on the ab plane. Figure 23 shows polarized and unpolarized spectra of Species A in a UP crystal with this orientation. All four C 0 2 molecules have somewhat larger components along b than along a. Their projections were found to make angles with the b axis that range from 27.9" to 39.7",with estimated errors of about 1". Determining the c component of the long axis of CO, requires measuring a crystal in a different orientation. Ideally one would measure spectra with the ac or bc plane perpendicular to the incident beam, but with flat plates this was not possible. Instead the crystal was tilted about the horizontal b axis, so that the IR beam and its electric vector had both a and c components for vertical polarization of the IR beam. Tilting by as much as 35-40" still allowed measuring accurate polarized spectra. This kind of tilting does not influence the b orientation of the electric vector of horizontally polarized light, but the electric vector of vertically polarized light changes direction on entering the crystal because of refraction. Refractive indices along both a and b were measured to be about 1.50, and we have assumed that, for purposes of calculating the change in direction, the crystal may be treated as isotropic, even though its space group makes it biaxial. Snell's Law gives the direction of propagation of the optical disturbance through the crystal,

sin 8' =

sin 0

(4)

where 0 is the angle between the c axis and the incident beam, 8' is the angle between c and the beam within the crystal, and q is the refractive index. In this orthorhombic crystal absorbance of light polarized in the ac plane can be treated as the sum of an a and a c component. The a component of absorbance varies as cos' 8', while the c component varies as sin' 8'. Absorbances from tilted and untilted crystals can be compared by normalizing the absorbance obtained with vertical polarization to that obtained for horizontal (b) polarization, which is the same in both experiments. For normalized a and a + c absorbances, the absorbance along c can be reckoned. The square roots of the three absorbance components give the projection of the long molecular axis of CO, on each of the crystallographic

0

-

e

J Q

a

a

3

0'1 I------

m

2' 1

m - l

W Q r t -

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3-0

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Figure 23. Polarized and unpolarized FTIR spectra of V, of COz in Species A of UP. The crystal was oriented with its large ab face (001) perpendicular to the incident beam.

W N

343

344

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

axes. Because accurate spectra cannot be measured with a tilt angle greater than 40", error in the c component is substantially larger than for a and b. Still, in most cases orientations are reliable to within 5" or less. These orientations have been instrumental in analysis of the reaction pathway, because they allow tracking individual C0,s from one intermediate to the next by assuming minimal reorientation. The foregoing analysis is adequate for isolated normal modes, but in photolyzed UP most of the CO, molecules exist as dimers, with vibrations that couple more or less strongly, depending on the relative positions of the molecules. Although the frequency shifts from coupling do not depend on IR polarization, the changes in intensity do, since orientations of the C0,s in a pair generally differ. For example, in Figure 24, the in-phase vibrations reinforce one other in polarization 2, but tend to cancel in polarization 1. Thus for a given crystal orientation, the polarized absorbances must be corrected for coupling in order to calculate transition moments for the individual C0,s. For a given coupling constant there is a simple relationship between individual and coupled absorbances in a pair of oscillators. Since the total absorbance for the two modes is independent of their coupling, one can easily obtain the corrected absorbance for each molecule. This value is then used to calculate the orientation according to the analysis given above. Although this type of analysis has yielded magnitudes of the Cartesian components for almost all of the C 0 2 species discussed above, it does not give their signs. As in the case of many other measurements on symmetric crystals, the determination is precise, but ambiguous. Unless a CO, lies on a crystallographic axis or plane, it could have any of four orientations corresponding to different choices of relative signs for the components. The ambiguity is appropriate, because the symmetric crystal includes molecules in each of the orientations. Even if the symmetry were broken by selectively decomposing a single orientation of starting material, it would be impossible with polarized IR to distinguish among the four possible product orientations in cases where the crystal birefringence destroys off-axis polarization. For a molecule less symmetrical than CO,, the long axis could be pointing in any of eight directions. This "octant problem" is inherent in optical spectroscopy when symmetry-related species absorb at the same frequency.

2*

Figure 24. Polarization effect on intenscoupled asymmetric stretching

E.i;f

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

345

In magnetic resonance spectroscopy the octant problem does not arise, although ambiguity in assigning each symmetry-relatedtensor to the appropriate symmetry-related reaction site persists. A way to resolve the octant problem for pairs of CO, molecules with coupled vibrations is discussed in Section VII.C.2.d. In general the ambiguity must be resolved by other means, including packing analysis and spectral perturbations from nearby molecules or functional groups. b. By Isotopic Labeling. Since the oxygens in CO, come from different atoms in the precursor, it might be possible to distinguish them by labeling the peroxide with l80.Carefully measured spectra of photolyzed UP show V3 bands from naturally abundant 180==C='60 after less than 0.1% decomposition. For several species these bands are doubled, as shown for Species C, in Figure 25, where for comparison purposes the bands from '80=C=160 have been shifted and scaled by the ratio of gas-phase frequencies. After correction for resonant coupling and reduced mass differences, the two

2319.06

II

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.

SPECIES C,

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.

2315.09

231310

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'

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Figure 25. Comparison of 180=C=='60 and I2CO2 bands from C2 in UP. Bands (----) have been shifted and scaled by the ratio of the gas phase from C1*OL60 frequencies for 1 2 C 0 2 and C ' 8 0 1 6 0from Ref. 99.

346

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

frequencies for 180==C=='60 come 0.98 cm- ' higher and 0.77 cm-' lower than the corresponding unlabeled C, band. band would not be doubled in this In the gas phase, the 180=C='60 way. The only effects of isotopic modification would be to make the symmetric stretch (V1) weakly allowed, and to have slightly more stretching of C=l6O in V,, and slightly less in 5 , . In an asymmetric environment, intermolecular forces on the two ends of the CO, molecule must balance, but the intermolecular force constants can differ. Since in V,, the vibrational amplitude of the C=l60 bond is slightly greater than of the C="O bond, the mode is more sensitive to the intermolecular force constant at the l6O terminus. When l60is on the end with a higher force constant, the ii, frequency is shifted to higher frequency, while the opposite is true when the l 6 0 is on the other end. The splitting in Figure 25 shows that both orientations of 1e0=C='60 are present in the crystal. The separation of the peaks is a measure of the asymmetry of the environment, while the shift of their average from the value expected from unlabeled CO, is probably related to the average strength of the intermolecular force constants. Besides revealing the asymmetry in peroxide reaction sites, studies of peroxides labeled specifically with '*O should allow measuring very slow end-for-end rotation of CO, molecules, or demonstrating its absence. In labeled acetyl benzoyl peroxide, for example, end-for-end rotation was evident in the spectra of CO, measured at 90 K after radical pair collapse. In the process of labeling these compounds with 13C,a small amount of l80had been incorporated specifically into the carbonyl oxygens. In spectra taken a few hours after photolysis, the two peaks in each of three different doublets were the same size, indicating that end-for-end rotation had occurred on this time scale for all three species. Experiments specifically designed to study rotation at lower temperature should yield more interesting results. Since deuterium in neighboring molecules influences intermolecular force in constants on CO,, a series of peroxides with both deuterium and specified positions could in principle allow tracking the motion of an individual oxygen of COz from starting material through successive radical pair structures, until end-for-end rotation scrambles the oxygens. 2. Coupling of Molecular Pairs. There are several allusions above to the perturbation of IR frequencies and intensities of CO, pairs by resonant coupling between vibrations of the two molecules. For example, determining the orientation of individual molecules by use of the intensity of polarized spectra usually requires correction for the effect of coupling. Far from being an annoyance, however, such coupling has proven to be a valuable independent source of structural information. In some cases coupling has been used

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

347

simply to demonstrate the proximity of two CO,s, or to help unscramble an overlapped spectrum from several different coexisting CO, dimers. In other cases coupling has been used, alone or in conjunction with polarized IR studies, to help define the arrangement of CO, dimers. When coupling is unusually strong, it even suggests the existence of high local stress which drives the molecules together. After several generalizations about vibrational coupling we discuss three cases in which coupling has provided the types of information mentioned above. The theory of coupling is developed in greater detail in Ref. 5. a. Intermolecular Vibrational Coupling. Although intermolecular resonant vibrational coupling is less familiar than intramolecular analogs, such as the creation of symmetric and asymmetric normal modes from coupling the two C=O stretches of CO,, the principles involved are identical. If asymmetric stretching of one CO, molecule influences the energy generated by asymmetric stretching of another CO,, the modes of the two molecules will couple to give in-phase and out-of-phase dimer modes [87]. It is useful to relate the observable dimer frequencies to the hypothetical intrinsic frequencies (V;, ):C that individual C0,s in the two different sites would display in the absence of coupling. If we let r~ be the shift of the observed absorptions from the position of the lower intrinsic frequency (V:), the secular equation for the problem yields two roots rJ=

A

*

J 2

m

(5)

where C is the strength of the coupling and A = v; - Vl;. When C is much smaller than A, the lower dimer frequency is approximately C 2 / Abelow the lower intrinsic monomer frequency, and the higher dimer frequency is the same amount above the higher intrinsic monomer frequency. Measurement of the shift allows determination of the coupling constant C. Coupling also influences the relative intensity of the absorptions by mixing vibrational excitations of the two molecules (first-order perturbation theory gives a mixing coefficient of C/A). If M denotes the hypothetical intrinsic intensity ratio of the individual molecules (a function of IR polarization), and r denotes the observed intensity ratio, the following relationship allows more sensitive determination of small coupling constants.

+JMr

C=(A+o)( J M -J r 1

)

348

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

Related equations are involved in correcting observed intensity ratios for coupling in the polarization studies. At first glance the equations above appear useless because in addition to measured quantities they require either intrinsic frequencies(or intensities) or coupling constants, neither of which is available in a standard spectrum. The equations become useful for comparing the spectra of normal dimers to heteroisotopicdimers in which one C O , is replaced by I3CO2.This change in reduced mass lowers the intrinsic V, frequency to 0.97204 of its former value [73], and the dramatic increase in A substantially reduces the influence of coupling. If C is 4cm-', the A of about 65 cm-' between 12C02and 13C02 means that the observed peaks are shifted by only 0.25 cm- from their intrinsic values, although observed and calculated intensity ratios would still differ by about 25%. Measuring the shift of a particular "CO, peak between homo- and heteroisotopic pairs gives a good first approximation of the coupling constant. If more precision is necessary, the calculation may be iterated using this approximate coupling constant to correct for the residual effect of coupling in the heteroisotopic pair. A similar procedure may be applied to infer coupling constants from band intensities. Independent determinations of coupling from frequencies and from intensities provide a check on the validity of the treatment. One might imagine that the coupling arises primarily from steric repulsion during the collision of vibrating molecules, but in fact the oxygen atoms, which determine the borders of linear CO,, move very little in V,, only about one-third as much as the carbon atom. The dominant mechanism of intermolecular coupling is instead electrostatic, involving the same transition dipoles that couple molecular vibration to the IR field. Coupling between asymmetric stretching modes of adjacent C 0 2 molecules can be strong for the same reason that these modes give intense IR absorption. From IR oscillator strengths one can estimate a maximum coupling of about 6cm-', which would occur for parallel molecules in side-to-side van der Waals contact. As discussed in Section VII.C.2.d, this theoretical maximum is almost certainly underestimated.The positive sign of the coupling means that potential energy increases as the molecules vibrate in phase, generating repulsion between parallel electrostatic dipoles. Out-of-phase vibration would be favored by the same amount as the in-phase vibration is disfavored. In practice, molecular transition dipoles are inadequate for precise interpretation of the electrostatic interaction between neighboring molecules, and even atom-centered multipole expansions are not perfect [SS]. Still, such analysis provides a semiquantitative guide to understanding dimer structure which is analogous to the use of anisotropic magnetic dipolar couplings in EPR or solid-state NMR. Sensitivity of the coupling to relative orientation and to the inverse cube of distance is particularly valuable.

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

349

b. CO, Pairs with Toluene in Acetyl Benzoyl Peroxide. Photolysis of ABP at 90 K yields toluene and methyl benzoate, according to the scheme shown in Figure 7. Although the spectrum of unlabeled ABP was complex, separate experiments with ABP labeled with -92% I3C in acetoxy (ABP**) or benzoyloxy (ABP*B)positions allowed complete assignment of the spectra. Figure 26 shows the "CO, and ',CO, V, spectra of ABP*Aand ABP*Bafter a small amount of photolysis at 90K. The 13C02spectra are shifted and scaled by the factor of 0.97204 to correct for the differencein reduced mass, so that, in the absence of coupling, corresponding peaks would be displaced vertically from spectrum to spectrum. Beside each spectrum is a formula of the precursor, with an asterisk showing the original position of the label, and an arrow pointing to the incipient CO, responsible for the particular spectrum. Thus, spectrum b is the 12C02spectrum of ABP*B,which is due mostly to the C 0 2 that originated in the acetoxy position, although incomplete labeling gives rise to some residual intensity from "benzoyloxy" 'TO,. Although this series of spectra looks complex at the outset and contains seriously overlapped bands in the central region, an analysis of resonant coupling allows easy assignment of each band. In the '2C02 spectra b and d, the outermost bands (dashed lines 1 and 6) are weak, but they absorb at the same positions as strong bands in unlabeled ABP. The stronger bands 2 and 5 (corresponding to bands at slightly lower mass-adjusted frequency in the I3CO2spectra) are not present in unlabeled ABP. Since 13C labeling is incomplete, bands 1 and 6 are assigned to the small fraction (8%) of '2C02-'zC02 (homoisotopic) pairs in sites containing toluene. Bands 2 and 5 in spectra b and d arise from I2CO2in 12C02-13C02 (heteroisotopic)pairs in the same types of sites. Bands 2 and 5 absorb close to their intrinsic positions since vibrational coupling between '?CO2and I3CO2 is small compared to the isotopic frequency difference. Intermolecular vibrational coupling makes the frequencies of the vibrations in I2CO2l2C0, pairs diverge much more than in 12C0,-13C02 pairs. Residual coupling within heteroisotopic pairs is large enough to shift both 2 and 5 slightly. Since the I3CO, bands in spectra a and c are the lower-frequency members of heteroisotopic pairs, coupling shifts them slightly to low frequency from their intrinsic positions, while in spectra b and d the analogous bands from I2CO2 in the same environments are the higherfrequency members of heteroisotopic pairs, and are shifted slightly to high frequency. Except for the small frequency shifts, which can be calculated to within experimental accuracy from Eq. (5), the outer peaks in the I2CO2and I3CO2 spectra are the same and are due to a single C 0 2 pair. Since a pair of C 0 2 molecules must be formed together with toluene, this pair is called T14 T,

55.1

0

Figure 26. Comparison of '2C02 and 13C02V3 bands in labeled ABP. 13C02spectra have been shifted and scaled as in Figure 16.

350

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

351

and T, are the first and fourth bands from COz in toluene sites in the unlabeled crystal of ABP. There is another toluene-containing site with bands T, and T,. Similarity in the T, :T4intensity ratio between spectra a and c shows that by 90 K the CO, molecules of TI, have reached equilibrium with respect to exchange of positions, since the initial position of 13C was different for the two samples. The exchange seems plausible, since, as discussed below, the strong vibrational coupling indicates that the CO, molecules are in contact and closer to being side-by-side than end-to-end. Such an arrangement should favor exchange of positions. This structure of TI4 was inferred not from polarization, but from vibrational coupling. Iterative application of Eq. (5) using the observed "CO, and 13C02 band positions for TI, yielded a coupling constant of 5.06 cm - This value indicates a strong interaction between the vibrating molecules. The sign of the coupling constant, which is determined from the intensities of the bands, has substantial geometric significance. For the idealized case in which two COz molecules are placed side-by-side and parallel to each other, coupling should enhance the intensity of the in-phase mode at the expense of the out-of-phase mode, as shown in Figure 27. The in-phase mode should absorb at high frequency, since the unfavorable interaction between parallel side-by-side dipoles would raise the energy of deformation, The weaker, outof-phase mode should absorb at lower frequency. An intensity pattern in which the more strongly absorbing in-phase mode comes at high frequency, and the weaker out-of-phase mode comes at low frequency, indicates a positive coupling constant. If the molecules are arranged end-to-end, the in-phase mode would absorb at lower frequency and the opposite intensity pattern would occur, indicating a negative coupling constant, Applying this argument to nonparallel molecules requires a convention as to what constitutes in- and out-of-phase vibration. We define as in-phase, the mode in which the coupled transition moments make an angle of less than 90" with one another. To be rigorous, it is the trace of the

6

6e

&

6t

0-c-0 0-CEO

a

6

O-c-0 bt

o=C-o b

6

Figure 27. Dipolar coupling of asymmetric stretching in side-by-side CO, dimer. For this configuration, unfavorable electrostatic interaction shifts the intense in-phase mode of a degenerate pair (a) to higher frequency. At the same time, the weak out-of-phase mode (b) shifts to lower frequency. From Ref. 87 with permission from Elsevier Science Publishers.

352

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

absorbance tensor that obeys the intensity relationship described, so that the opposite result could be found for a single measurement with special polarization and nonparallel molecules. In the CO, spectra of ABP, bands from heteroisotopic TI, pairs (2 and 5 ) have intensities that represent the squares of the projections of the long molecular axes on the (001) face of the crystal, which was held perpendicular to the incident beam. In each case, band 5 was more intense than band 2, suggesting a larger molecular axis component on the (001) plane for the CO, that gave absorption 5. In the homoisotopic pairs of spectra b and d, where coupling has a greater influence on intensity, band 1 is stronger than band 6, indicating that coupling enhances the high frequency band at the expense of the low frequency one. According to the electrostatic coupling model, the molecules are therefore much closer to side-by-side than end-to-end. Because of fortuitous overlap, the bands in the middle of the spectrum (bands 3 and 4) are more difficult to analyze. However, comparison of F3 bands of C O , in spectra of ABP** and ABP*B, as well as kinetic and spectroscopic evidence from other vibrational modes, show that both the single CO, in the presence of methyl benzoate and a differently structured CO, pair in the presence of toluene are found in this region. Band 3 consists of contributions from both types of sites (methyl benzoate and toluene), while band 4 is the lower frequency member of the CO, pair in the presence of toluene. The band from CO, in the presence of methyl benzoate is labeled M. The bands from the CO, pair in this toluene site are named T2 and T3, because in the spectrum of unlabeled ABP, they are the second and third bands from toluene sites. Analysis of the bending modes shows that CO, in methyl benzoate sites does not originate from the benzoyloxy position; that is, methyl benzoate does not result from capture of the “acetoxy” CO, by a methyl-phenyl radical pair. Thus, spectra a and b show CO, from both M and T sites, while c and d show CO, from T sites only. The absence of very much site exchange in T,, is shown by the lack of significant intensity in band 4 of spectra a and b. Had site exchange been occurring, the intensity of band 4 would have been much greater in these spectra. Although the CO, molecules in TI, exchange positions readily at 90 K, C0,s originating from acetoxy and benzoyloxy positions in T23 remain distinct until the crystal is warmed to 165 K. The inference that CO, sites in T23 are isolated from one another is reinforced by the absence of resonant coupling for this pair of CO, molecules. Presumably, the two CO, molecules move in different directions during product formation, and these sites are physically isolated from each other. Pair 1 of 11-Bromoundecanoyl Peroxide (11-BzUP). The C 0 2 spectrum of the first intermediate from low temperature photolysis of 11-BrUP shows

c.

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

353

extremely strong coupling in a C 0 2 dimer. This coupling is much larger than the maximum calculated on the basis of molecular transition dipoles [89], but it is less than the upper limit set by more recent calculations using atomcentered multipole expansions for the transition moments [S]. These spectra also illustrate the use of naturally abundant I3CO2in determining coupling constants. Photolysis of a single crystal of 11-BrUP at 20 K gives the spectrum shown in Figure 28, where the frequency scale for naturally abundant 13C02has been shifted and scaled to correct for changes in reduced mass. Although the 12C0, spectrum contains many bands from less primitive species, the bands marked 11(12,12) and 12(12, 12) can be assigned to the in- and out-of-phase modes of a strongly coupled C 0 2 dimer by reference to natural abundance I3CO2absorptions [87]. In the 13C02region, which shows only heteroisotopic pairs, both frequencies and intensities are much closer to their intrinsic values than those of homoisotopic pairs. By iterative use of Eq. ( 5 ) the

2350

2340

2330

I

2280

2270

WAVENUMBERS

'

2i60

Figure 28. Y3 of CO, near 20K from lightly photolyzed crystals of 11-BrUP. (a) "CO, asymmetric stretching region. Unmarked bands are from less primitive intermediates; (b) bands from naturally abundant 13C0,. The frequency scale of b has been shifted and scaled as in Figure 16. From Ref. 87 with permission from Elsevier Science Publishers.

354

PHOTOCHKMICAL MECHANISM IN SlNGLE CRYSTALS

frequency shifts of the coupled bands from their intrinsic positions yield a coupling constant of 9.03 cm- '. The extremely high coupling constant for this pair suggests that the CO, molecules are close to side-by-side and parallel. The similarity between the intrinsic transition dipoles (p(l1)/p(lZ)= 1.15) is also consistent with a dimer in which the CO, molecules are almost parallel. A plausible structure would have the parallel CO, molecules held between the primary radicals with their long axes more or less perpendicular to the inter-radical vector. This would explain the absence of high intrinsic frequencies in this pair: CO, molecules with their long axes perpendicular to the inter-radical vector are not aligned to receive the stress from the free radicals. Instead the stress is manifested as a high coupling constant for V, of the CO, molecules, which are squeezed together between free radicals that have been shown by EPR to be only 5.98 A apart. d. Determining the Structure of Pair A in Undecanoyl Peroxide ( U P ) . We have studied the most primitive CO, pairs generated in UP more extensively than any other pair. Since coupling is easily measurable in Species A,,, and since high quality polarized IR spectra have been taken of this intermediate, it is well-suited for detailed study. Figure 29 compares the spectra of 24.7% ',C-l-UP (random label) with that of unlabeled UP after slight photolysis at 15-20 K. In the labeled crystal, the following pairs were generated: 57% '2C02-12C02,37% '2C0,-'3C0 2 3 and 6% 13C02-13C02.In the presence of intermolecular coupling the differences between the heteroisotopic pairs and homoisotopic pairs are instructive. Pair A,, shows very little coupling, as evidenced by the absence of frequency differences between homoisotopic and heteruisutopic pairs. The. small difference in intensity ratio of A2 to A, between "CO, and 13C0, regions reveals a weak coupling of about 0.5 cm-' for this pair. For Pair A13, strong coupling is demonstrated in the "CO, region of Figure 29b by the presence of satellite bands inside the A,-A, doublet that is formed in unlabeled UP. These satellites are the "CO, components of pairs that contain both I2CO2and I3CO2(see brackets). Since the near-resonance is broken in these heteroisotopic pairs, the bands lie close to their intrinsic, uncoupled positions. In the 13C02region of Figure 29b, the frequencies of A1(13,12) and A3(12,13) match the frequencies of the naturally abundant CO, found in unlabeled UP. The bands marked A,(13,13) and A,(13, 13) flank their heteroisotopic counterparts because of their stronger resonant coupling. Iterative application of Eq. (5) to the frequency shifts in ',C02 and w O , regions gives a coupling constant of 4.71 cm-' for 12C02-'zCOz pairs and 4.68 cm- for 13C02-13C02pairs. The difference is consistent

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

355

WAVENUMBERS

Figure 29. Y3 of CO, near 15-20 K in lightly photolyzed UP crystals. (a) Unlabeled UP. In the lzCO, region, brackets connect bands from CO, in coexisting pairs A,, and AZ4; (b) 24.7% randomly labeled 13C-1-UP (UP*). For Species A,, brackets connect bands from isotopically distinct pairs. From Ref. 87 with permission from Elsevier Science Publishers.

with the small decrease in transition dipole due to reduced vibrational amplitude of the heavier isotope. Comparison of intensities of homoisotopic and heteroisotopic pairs shows that coupling enhances the high frequency bands at the expense of the low frequency ones. In both A,,(l2,13) and A,,(13,12), the relative intensities of A, and A, are very similar, but in A,,(l2,12) and AI3(13,13), the high frequency band is enhanced, while the low frequency one is attenuated, as in the example shown in Figure 27. The sense of the intensity pattern shows that the C0,s are closer to side-by-side than end-to-end, and further analysis will show that the long axes of A , and A, differ in direction by 29.5". The foregoing examples show how the resonant coupling can be used to deduce proximity of CO, molecules, and in favorable cases, a reasonably adequate description of their relative positions. In fact, many vibrational spectroscopists who study van der Waals dimers in matrices rely almost entirely on the intermolecular coupling to assess structure [89]. Our

356

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

experience suggests that in the absence of extremely large coupling constants, as in Pair 1 of 11-BrUP, such assessments must be treated with caution, since large families of relative orientations can yield the same coupling constants. A much more powerful method of structural assessment combines the information from coupling and polarization studies. Taken alone, the polarized IR experiments yield information about the orientations of individual CO, molecules, but cannot fix the relative orientations of the members of a pair because of the octant problem. Very strong positive or negative coupling constants can establish some limits on possible pair structures, but usually these limits are broad. Taken together, the polarization and coupling results can narrow the range of possible structures dramatically. Knowledge of the coupling constant is of course necessary to derive intrinsic molecular absorbances from polarized spectra of dimers, but coupling can also resolve the octant problem for a dimer and suggest relative positioning, as well as orientation, within it. The polarized IR studies yield the magnitudes of the components of a CO, molecule’s long axis on each of the crystallographic axes, but they cannot be used to deduce the relative signs of the long-axis components for two molecules in a given CO, pair. Depending on the relative signs, the angle between the long axis directions of C0,s in a pair may take on any one of four values. As a further complication, the coupling constant can be either positive or negative, and depending on which sign is used, different long-axis vectors are calculated from the observed polarized absorptions. This ambiguity further increases the number of possible intermolecular angles by a factor of two. To see how coupling information can solve this problem, one might imagine that for a crystal partially labeled with 13C,the response to polarized light would be different for the homoisotopic and heteroisotopic pairs. The transition moments of CO, in heteroisotopic pairs should be relatively independent of the pair-wise orientation of the C0,s in the dimer, while those of the homoisotopic pairs would be composed of the sum and difference of the transition moments for coupled CO, molecules. With the proper polarizer orientation, one should be able to determine the sign of the coupling constant and the relative signs of the components of the two molecules on the different crystallographic axes. Figure 24 illustrates this approach. Such a labeling experiment is not necessary, since results from separate polarized IR studies and 13C-labeling experiments can provide the same information. For each possible sign of the coupling constant, there are four possible angles that describe the relative orientations of the two CO, molecules in a pair; these depend on the relative signs of the Cartesian components of the long axes of the molecules. For each pair of CO, orientations, and for a given polarization and crystal orientation, one can

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

357

predict the ratio of intrinsic absorbances for the two molecules and the ratio of absorbances for the coupled modes. This can be compared with the ratio that is observed experimentally. For all pairs in which the coupling constant was greater than 1.5 cm-', we were able to make a clear choice between the eight possible sets of orientations. With pair AI3, for example, only six relative orientations were possible, since with a positive coupling constant A, had a c component of zero, removing one sign ambiguity. For these six possibilities the predicted absorbance ratios were 2.35,0.81,5.16, 10.4, 12.6, and 9.27 for IR polarization along the crystal's a axis (assuming that the intrinsic oscillator strengths are the same for the two molecules in the pair). The ratio that comes closest to the observed ratio of 2.71 is 2.35, which was predicted with a positive coupling constant and with identical signs of a and of b for the two molecules (c = 0 for A Since differences in polarizability of their environments should give the C0,s slightly different intrinsic oscillator strengths, this agreement is quite good, and the relative orientation is fixed for this pair. For all CO, pairs to which this type of analysis could be applied, the long axes were found to lie in the same octant. Knowledge of the coupling allows more complete interpretation of the polarized absorbance data, and provides relative individual orientations of the molecules in a pair. In the same way, knowledge of the relative orientations allows more complete interpretation of the coupling, and thus provides information about the arrangement, as well as the orientation, of molecules in the dimer. The strength of electrostatic coupling between two CO, molecules depends on seven structural variables: the distance and direction between them (three variables) and their individual orientations (four variables). If one has determined the individual orientations by the above method and assumes that the coupled molecules are in van der Waals contact, only two structural variables remain, and knowledge of the coupling constant can narrow the range of possible structures dramatically. In order to use observed couplings for this purpose, one must be able to calculate the expected electrostatic coupling strength for a given structure of the COz pair. At such short distances errors can be quite large in calculations of coupling constants based on interaction of transition dipoles, but an analysis based on atom-centered multipole moments of the asymmetrically stretched CO, molecules should avoid the most serious problems. This approach was used to calculate coupling constants for a wide variety of dimer structures. In the process two quite different partitionings of the charge distribution of stretched CO, into atom-centered charges, dipoles, and quadrupoles were compared. These were supplied by Stone [88] and by Bader [90J Although the individual moments differed dramatically, the sets predicted very similar coupling constants for any particular pair structure.

358

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

The variation of coupling constant with molecular arrangement is most easily visualized with a contour plot. To generate this kind of plot coupling constants were calculated with one molecule held fixed and the second oriented with its axis at a given angle from the axis of the first, but moved around the first so as to keep the minimum contact distance at an assumed van der Waals contact distance of 3.6a. Figure 30 shows such a plot for Pair A13,in which the CO, axes are 29" from each other, and the coupling constant is 4.74cm-'. For this plot, A, was placed at the origin of polar coordinates in the orientation determined by polarized IR studies. In the orientation determined by polarized IR and in van der Waals contact with A,, A, was initially set on the polar axis, and its position was varied in 10" increments of longitude (0) and latitude (4).The electronic distribution, calculated with a 6-31G* basis set, predicts a transition moment of 13.6 D/A, which is slightly larger than the experimental gas phase value [Sl]. Since the transition dipole of polycrystalline C O , is much smaller (7.84D/&, the calculation probably overestimates the

e

Figure 30. Coupling constant contour plot for Species A,, using electrostatic interactions derived from atom-centered multipoles. The contour level is 1 m-*,with dotted contours ranging from - 7 to - 1 and solid contours ranging from 1 to 12cm-'.

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

359

electrostatic contribution to the coupling constant for a given structure of the pair. Since the concentration of CO, in the peroxide crystal was not determined independently, an experimental transition moment is not available, but the method is probably reliable for excluding structures that yield calculated coupling constants below 3cm-’ and above 7 or 8cm-l. The map shows that these limits exclude large portions of the available space. Ultimately, as with many aspects of the intermediate structures, these results will be most useful in validating or excluding models based on computer simulation of packing and dynamics in the crystal defects.

D. Environment of COz The discussion thus far has emphasized sensitivity of the frequency of C0,’s V3 mode to local stress, sensitivity of its absorption intensity to IR polarization, and sensitivity of both properties to resonant coupling of dimers. For the type of crystals under consideration, which consist mostly of simple hydrocarbon groups, these factors probably dominate in determining the IR spectral characteristics.Other factors can be involved, however, and although they can make simple interpretation of the spectra more problematic, they can also provide additional information about the environment of the CO, probe molecule. The following examples illustrate how one can track the motion of C 0 2 over distances of I - l S A by observing its proximity to free radical centers or to halogen or deuterium substituents in the crystal lattice. This information complements the previously discussed structural studies, which related to structure within the dimer rather than to the location of the CO, in the crystal matrix. Although few studies have been conducted on the influence of environment on absolute intensity of infrared bands, it seems clear that a polarizable group at the end of a CO, molecule should increase the absorptivity of V,, since its effective transition dipole would be lengthened. A polarizable environment should also lower the absorption frequency by stabilizing the vibrationally excited state, which has higher transient polarity. We have not measured absolute molar absorptivities, but careful polarization and coupling studies of a sequence of structures for a single set of CO, molecules undergoing stepwise thermal relaxation have provided reliable relative absorptivities for the most prominent C 0 2 species in UP and TP. The plot in Figure 3 1 demonstrates a modest inverse correlation between absorptivity and frequency. The variation of molar absorptivity by a factor of 1.8 over this range of frequencies is striking, and suggests significant interaction between the C 0 2 molecules and their environment. It is not clear if the extremely low intensities of the high frequency bands are due to

360

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

8

c2

A

c2

%

05 2352

2346

2344

INTRINSIC

2340

2336

2332

2328

FREQUENCY ICW”1

Figure 31. Correlation between absorptivity and intrinsic frequency. Squares show relative absorptivities in TP while triangles show those in p-UP.

“intrinsic” differences in the polarizability of the neighboring groups or to changes induced by stress. No systematic studies have been done on the effect of high pressure on intensities of infrared bands, although the work of Buback, Schweer and Tups shows a small decrease in the intensity of 35, of CO, as pressure is applied [77]. It is suggestive that the very high intensity bands come from C O , molecules that have been shown by polarized IR and packing analysis to point toward free radical centers. Such centers could well be more polarizable than other functional groups present in the crystals. It may be noteworthy that C0,s with very similar orientations (e.g. A, and B,) have very similar absorptivities. Studies of variously halogenated analogs of 11-BrUP have provided evidence about the motion of CO, during a complex reaction sequence [59]. Early intermediates involve radical pairs and resonantly coupled CO, dimers, but subsequent hydrogen transfer reactions convert radical pairs to

OBTAINING INFORMATION FROM INFRARED SPECTROSCOPY

361

isolated radicals, and IR spectra cease to show coupling of the (20,s. An obvious question is where the C0,s are located. It seems plausible that they should start between the radicals in the midst of the hydrocarbon layer and ultimately move to the interface between two lamellae of tightly packed, parallel chains. Changing one or both of the terminal bromines of 11-BrUP to chlorine or iodine had very little influence on the frequency of the V, bands of early intermediates. For five different compounds the frequency range of corresponding peaks was less than 1.1 cm-’. However, the spectrum observed after annealing at 200 K showed frequencies characteristic of the end groups. Crystals containing chlorine, bromine, or iodine showed peaks near 2342, 2338, or 2344 cm- respectively. Crystals with different halogens showed two peaks. This confirms the hypothesis that the CO, molecules move some 10A from the middle of the layer, where they are insensitive to terminal halogen, to the interface, where they are in contact with it. When one of the brominated chains of 11-BrUP is replaced by an alkane chain one carbon shorter, initial relaxation at 160 K generates a peak at 2339 cm- (bromine interface) and a peak at 2334 cm- (methyl interface). On continued annealing, the former peak disappears and the latter grows, suggesting that C0,s migrate from the bromine interface to the more loosely packed methyl interface. A more subtle application of the same technique involves isotopic substitution. Because most CO, frequencies are reproducible from crystal to crystal to within 0.07 cm- ’,it is possible to demonstrate small, but significant shifts of V3 when certain hydrogen atoms are replaced by deuterium. Analogous “orientation effects” have been reported for mixed crystals of isotopically substituted benzene, where isotopic differences in adjacent molecules gave rise to splittings as large as 4.7 cm- [91]. The most informative experiments involved isotopically mixed crystals of UP. Guest molecules contained carbonyl I3Cand hosts contained deuterium, so that the l3CO2spectrum would show deuterium influence from neighboring U P molecules, while the l2CO, spectrum,would show influence both from neighboring molecules and from the adjacent radicals. Shifts were measured relative to peak positions in an undeuterated crystal. For example, in crystals of a 1 : 4 solid solution of 13C-1-UP(92%singly labeled) in a-UP (90% deuterium in all a positions) Species B, showed only small shifts (-0.09 cm- I ) for the ‘,CO, bands, but significant shifts (-0.50 cm- ’) for the ‘ 2C02bands. This shows that alpha deuterium in the radical, not that in neighboring U P molecules, is responsible for the shift, and suggests that this particular CO, touches at least one a-hydrogen of the radical. A similar pattern from a mixed crystal in which the host was deuterated in both a and /I positions, showed that /?-hydrogens do not influence V3. Similar conclusions

’,

362

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

can be drawn for C 0 2 sAqr T2, and C1.Other C02s, such as C1, are affected by deuterium in the alpha position of adjacent UP molecules. Both positive and negative shifts have been observed with deuterium substitution, and although the directions of the observed frequency shifts are not readily understood, it is significant that CO, molecules that have similar orientations and spectroscopic features show similar shifts with deuteration.

VIII.

OTHER ASPECTS OF SOLID-STATE BEHAVIOR REVEALED BY FTIR

A. Change in Product Distribution with Extent of Conversion Section VI.C.l.a, above, mentioned the EPR and product studies of Whitsel and Pankratz concerning prolonged photolysis of crystalline ABP. They showed that changes in the temperature of photolysis and in the extent of conversion had large effects on the ratio of methyl benzoate to toluene in the product mixture, presumably because photochemical decarboxylation of the benzoyloxyl radical competes with thermal collapse of the methyl/ benzoyloxyl radical pair, which is accelerated by warming but retarded by extensive decomposition. Since our IR experiments were conducted at higher temperatures than the product studies (90 K versus <78 K), thermal decay of MB pairs should be more prominent, and the fraction of methyl benzoate should be larger than in the product studies. One would expect that after extensive photolysis at 90 K, the ratio of methyl benzoate to toluene should decrease, since photochemical decarboxylation in MB pairs should compete more effectively with the slowed thermal collapse of that pair. Although we have not determined the orientation and absorptivity of the different CO, species in ABP, it is easy to show that the relative amounts of methyl benzoate and toluene change in the expected way during long photolysis. Figure 32 shows a spectrum of ABP*Aduring a short photolysis and after a longer one. The peaks from C 0 2in most toluene sites increased by 34-37% after longer photolysis, but the peak from 13C02in the presence of methyl benzoate increased by only 5% during the same period. It is curious that this peak did not show more growth, because some of it must be due to 13C02 in the presence of toluene (Section VII.C.2.b). Some growth most likely appears as an increase of linewidth. Measuring the ratio of toluene to methyl benzoate sites, rather than just changes in their ratio, will require determining orientation of their C 0 2 s by polarized IR spectroscopy.

OTHER ASPECTS OF SOLID-STATE BEHAVIOR REVEALED BY FTIR N

?

E F F E C T OF PnOTOLYsts LEYGTU ON Y &NO T

1 I

INWER L I N E ’ 26.39.5 W I N

OUTER LIWE: 8 0 4 UIW

Figure 32. Effect of long photolysis on species M and T in ABP**. The integrated intensity ratio of MjT dropped by a factor of -1.35 as the length of photolysis increased by a factor of -2.5.

B. Steric Isotope Effects on Reaction Pathways Steric isotope effects are well documented in the literature of organic chemistry, and have been measured in several reactions in which severe crowding occurs. Mislow and co-workers first demonstrated such effects by determining racemization rates of I, where R = H or D [92]. The inverse

364

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

isotope effect (k,,/k, = 1.17 at 292.5 K) indicated a 50 cal/mol reduction in the enthalpy of activation per deuterium. The presumed involvement of four hydrogens in the transition state was subsequently confirmed by MM2 calculations of Allinger and Flanagan [93]. Other workers have reported steric isotope effects on equilibrium and kinetic processes. These span the range of 24- 112 cal/mol of deuterium [94].

CR3 CRs

In every case C-D bonds were observed to be effectively shorter than C-H bonds, even though the mean bond length would be the same in a harmonic approximation [95]. The isotope effects arise because the mean squared vibrational amplitude of a C-D bond is smaller than that of a C-H bond by a factor of 0.734, which amounts to a shortening of 0.012A in the rms amplitude. C-D bonds spend less time with the particularly long distances that dominate the average energy when there are strong steric interactions. Beyond this change in the mean squared vibrational amplitude, anharmonicity shortens the average C-D bond by some 0.005 A relative to the C-H bond, as shown by Bartell and Higgenbotham's electron diffraction studies of ethane [96]. Since the dependence of a reaction rate on enthalpy of activation scales exponentially with inverse temperature, it should not have been surprising to find substantial deuterium isotope effects during photolysis of UP at cryogenic temperatures. An effect of 100cal/mol at 15 K is as significant as one of 2 kcal/mol at room temperature. Furthermore, severe crowding in crystal reactions enhances the energetic significance of isotope effects on molecular dimensions. Differential crowding in the transition states that lead to the two structures of Species A in photolyzed U P generates large isotope effects on the partitioning between them. Experiments with isotopically mixed crystals show that deuterium in the alpha and beta positions both of the radicals and of the adjacent peroxide molecules changes the A,, :AZ4ratio by substantial amounts. Figure 33 shows infrared spectra of CO, V, in peroxide crystals with the following composition: a. "C-l-UP (24.7%of the carbonyl carbons labeled at random). b. 20% of 13C-1-UP(92% of the carbonyl carbons labeled) in 80% a-UP (90% D in the a positions). c. 20%of 13C-1-UP(92%of the carbonyl carbons labeled) in 80% a,/?-UP (-90% D in both a and /? positions).

.30

.2s

.s

0

Jc:n I

0

N W

I. I

2.10

.IS

.3

ABSORBANCE .20

ABSORBANCE

.2 N.1

.4 .3

0

m

0

N

tlI

;I I

RBSORBANCE .2

.a10

0

N N

0

N

UIN

3

mI 0 n

C

S O

N N

-3014

5C::m.

CIBSORBCINCE -.000 -.On2 .004

lo.

N.0 .22

.I6

.lo

ABSORBANCE

.OL,

n.02

3

0

3

B

2 0

E'

P

s

p.,

Figure 33. Comparison of host and guest spectra of Species A in 24.7% "C-1-UP, l3C-1-UP/cr-UP,and '3C-1-UP/r,b-UP.Looking across rows two and three show the effect of intramolecular deuterium on the CO, spectra. Looking down the second column shows the effect of deuterium in adjacent peroxides. These spectra demonstrate the importance of both intra- and intermolecular isotope effects on the partitioning between A,3 and A2&

365

366

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

In the column on the right, the spectra of 13C02represent the guest sites, for which deuterium is found only in the surrounding peroxide molecules. For the 'zCOz spectra in the left column, there exists the added influence of deuterium in the radical that is formed during photolysis. Thus, looking from right to left across each row, one sees the effect of deuterium in the incipient radicals. Looking down the right column, one sees the effect of deuterium in the surrounding molecules. From considering only the spectra shown in rows I and 3 (undeuterated versus a and p deuterated host), one might conclude that the isotope effect is caused primarily by deuterium in the incipient radical. The l3COZspectrum of guests in a,p-UP is very similar to the analogous one in undeuterated 13C1-UP, while the '2COz spectrum from deuterated host shows much less A13 than that from unsubstituted UP. The spectrum of mixed crystals of "C-1-UP and a-UP in the second row of Figure 33 shows that the above conclusion is mistaken. In the "C02 region, the ratio of A13 to A,, with the a-deuterated host is 2.60, not 0.54 +_ 0.02 as it is with the undeuterated or the a,S-deuterated host. The l3COZspectra alone make clear that a and fl deuteria in adjacent peroxide molecules have strong, but offsetting effects on the partitioning between A,, and A24. Since intermolecular isotope effects were unprecedented in solid-state chemical reactions, a number of experiments were required to show that the results are reproducible and that differences could not be attributed to the history or condition of the crystals, the temperature or extent of photolysis, or differences in the orientation, coupling, absorptivity, or state of complexation of COz dimers between labeled and unlabeled crystals. Success in excluding these alternative interpretations [S] underlines the power of the infrared method, and the special virtues of studying single crystals. The large intermolecular isotope effects must have a steric origin. Since deuterium atoms in adjacent molecules do not make chemical bonds with the reacting fragments, the rate determining step for the partitioning must involve a physical process, as in the Type D and F effects of Figure 4. Since A13 and A2, do not interconvert at the temperature of photolysis the isotope effects must be kinetic, not equilibrium ones. Since these isotope effects have been interpreted in terms of a physical transition state, it is instructive to contrast this phenomenon with studies in which the absence of an isotope effect was used to demonstrate a physical rate determining step [97]. The most relevant example is rotational-diffusion control of radical disproportionation in the solid-state photolysis of azobisisobutyronitrile (AIBN). Since there is normally a primary isotope effect on the disproportionation of cyanoisopropyl radicals to methacrylonitrile and isobutyronitrile, the absence of such an effect in the solid-state photolysis of

OTHER ASPECTS OF SOLID-STATE BEHAVIOR REVEALED BY FTIR

367

AIBN showed that the disproportionation was limited by diffusion, not by bond breaking. In A I B N photolysis, as in most solid-state product studies, the reaction was carried out at much higher temperature (200 K), where it is probably safe to assume that steric isotope effects are unimportant. At cryogenic temperatures, however, they should be important whenever strong steric interactions influence the rate of reaction. We have observed steric isotope effects at higher temperatures in the transitions between A and B ( 30 K) and between B and C ( 50 K) in mixed crystals of l3C-l-UP and deuterated peroxides. By observing the differences in the rates of these transitions for hosts and guests, we could assess the effect of deuterium in the ct and /3 positions of the radicals themselves. For instance, the transition from A to B shows a substantial difference in the rates of decay of A24 in hosts and guests of '3C-1-UP/a,/3-UP, but no noticeable difference between hosts and guests in the decay of A13.Figure 34e shows the "CO, and I3CO2 spectra of '3C-1-UP/ct,/3-UP after briefly warming the crystal to the transition temperature, and then cooling. Comparison of this spectrum with that of Species A (Figure 34a) shows that the guest fragments have proceeded through this transition more rapidly than host fragments. In this transition, A,, and A24 decayed at different rates for hosts, but at the same rate for guests. It is conceivable that the rates are identical in the absence of deuterium because there is an interconversion of A,, and AZ4, which does not occur when the radicals are deuterated. Subtraction of different fractions of spectrum a from spectrum e allows measuring the differences between A, 3 and A,, and between hosts and guests. Careful examination of a series of spectra with different scale factors for subtraction shows that 34% of the guest sites had reacted in the same time that 13% of A,, from hosts and 33% of A13 from hosts had reacted. Thus, k d k D = 3 1 for A24 and 1.0 t- 0.6 for A13. Comparison with the results of analogous experiments on 13C-1-UP/a-UP suggest that /3 deuteria have a larger effect both in Figure 34 and in the transition between B and C , where the effect is somewhat smaller (AAGZ = 45 versus 65 cal/mol). Since the radical motions are unlikely to involve changes in bonding, the effects of deuterium in the /3 positions are almost certainly steric, but the sense of the effect (kH k,) is opposite the steric isotope effects that are normally observed. Given the standard interpretation of steric isotope effects, which assumes that the transition state survives several periods of normal vibration 1981, the present results imply that the smaller size of deuterium reduces the driving force for radical motion. That is, the fi methylene group is under more stress in Species A,, than in the transition state leading to the next intermediate structure.

-

-

-

=-

Figure 34. Isotope effects in the transition from A to B of "C-l-UP/a,B-UP. Traces b, c, and d are difference spectra in which a certain fraction (scale factor) of trace a (before warming) has been subtracted from trace e (after warming) so as to measure and remove the remaining contribution of A,, or A,,+. Note that the rates of decay of A,, and A2, are the same for the undeuterated guest fragments. However, for the deuterated host fragments, A,, decays more quickly than A,,, giving a negative A, peak in b and positive A,, peaks in c. For A,,, kJkD = 3. 368

OTHER ASPECTS OF SOLID-STATE BEHAVIOR REVEALED BY FTIR

369

C. Perturbation and Additivity of Substituent Effects Structural modification of a molecule usually has profound and complex influence on its crystal's packing and mechanical properties. This type of behavior is so pervasive that one usually despairs of being able to apply the kind of linear free energy relationships that are so useful in solution chemistry to studies of solid-state mechanisms. When structural modification is sufficiently subtle, however, analogous techniques can be helpful. Homologation of most long-chain compounds by an even number of carbon atoms is a subtle substitution. Tridecanoyl peroxide (TP), the homologue of U P in which each alkyl chain is extended by two methylene groups, packs in the same pattern as UP (space group C222,) with nearestneighbor distances changed by less than 0.025 A. Thus one could hope that effects due to such homologation would lend themselves to simple analysis. EPR investigations by Drs. Segmuller [60,16] and Feng [72] have shown that the radical-radical vector of radical Pair A in a,P-d,-TP differs from that in a,b-d,-UP by only 0.1 A in distance and 4" in direction. This structural similarity is also apparent in the FTIR spectra of the CO, pairs that are lodged between the radicals as shown in Table 3, which presents data for species A,, and A,, in UP, TP, and their d8 analogs. The similarity in most properties among these four differently substituted samples contrasts with the differences in the transition state energies leading to them, as revealed by the range of over 1000-fold in the ratio of species A,, and A2,. It is clear that there are dramatic substituent effects on the product ratio both from homologation and from deuterium substitution. It is particularly notable that the substituent effects on activation energy are additive, that is, deuteration lowers the ratio by a factor of about 25 in both UP and TP. The situation is quite different when comparing different deuterium TABLE 3 Effects of Homologation and Deuteration on Frequencies and Population ratios in Pairs A13 and A24

UP dB-UP TP d,-TPa

2352.4

2336.8

-

-

2352.4 2352.8

2337.0 2337.2

"Unpublishedwork of Dr. T. C. Semple. bFrequencies in cm-

'.

2346.4 2346.7 2348.6 2348.9

2333.0 2332.6 2331.9 2332.0

0.54 (2)b 0.025(5) 38 (3) 1.29 (4)

370

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

substitution patterns, where a-deuteration increased the ratio by 20% for unsubstituted UP, but decreased the ratio by a factor greater than 2 when the UP was already 8-deuterated. The influences of a- and 8-deuteration on the activation energies are clearly not additive. It is easy to see why effects are additive in one case and not in the other. Homologation and deuteration are structural modifications that occur in widely separated regions of the crystal. Thus their influences are independent. The steric effects of a- and 8-deuteration, however, occur in the same region of the crystal, so they may influence one another. One can easily imagine that how much easier it is to squeeze a deuterium than a hydrogen through a hole depends on how large the hole is, that is, on whether it is bordered by hydrogen or deuterium. Since the deuterium isotope effects operate by several intra- and intermolecular mechanisms simultaneously, realistic detailed interpretation of this interaction is not possible at present. Thus, although quantitative interpretation of substituent effects on reaction rates is possible in some cases, it is much less generally applicable in crystals than in fluids.

IX. FORMULATION OF A MECHANISM This review has illustrated the utility of infrared spectra of CO, as a probe of solid-state peroxide reactions. Although we originally hoped to use the asymmetric stretching mode solely as a probe of local stress, it soon became obvious that carefully measured IR spectra are rich in many kinds of information about these reactions. Not only did they provide the expected evidence and numerous independent confirmations of high local stress, they also supply the following: 1. a much more detailed kinetic scheme than had been available from EPR spectroscopy, although the EPR results were in general confirmed; 2. more subtle and informative methods for studying structural influence on reaction rates in solids; and 3. direct evidence on details of molecular orientation and reorientation during reaction.

Several different sorts of observations provide information about the arrangement of CO, molecules in the successive intermediates in photolyzed UP: 1. Polarized IR spectra give the orientation of most C0,s to within 5".

FORMULATION OF A MECHANISM

371

2. Intensity changes from resonant coupling indicate the relative signs of the Cartesian components for the long axes of C0,s in a pair. 3. Theoretical treatment of electrostatic coupling narrows the choice of arrangements for CO, dimers. 4. Influence of halogen and deuterium on frequencies and linewidths identify contacts between C 0 2 and nearby groups. 5. CO, band intensities, which vary by a factor of 1.8 in UP, probe the polarizability of the environment. In several cases, high intensities for CO, 5, bands indicate contacts between the oxygens of CO, and the free radicals, in agreement with the isotope effects on frequencies and linewidths mentioned above. 6. Frequency shifts indicate the amount of stress along the long axis of CO,, and thus probe the shape of the environment. 7. I8O isotope effects on CO, frequencies allow studying slow end-for-end rotation of the molecule.

Although the structural evidence is formidable, no single structural model for the reaction pathway has yet emerged, in part because the symmetry properties of the crystal create ambiguities that inhibit direct inference of structures. Plausible pathways must be generated independently, probably by computer simulation, for testing against the experimental information. Since several structural features can be inferred directly, it is appropriate to close by presenting a pictorial model that conveys some of the information revealed by the infrared spectra, while at the same time showing which structural features require further analysis. Although by no means definitive, this model should serve as a starting point for computer simulations of the pathway. Section VI1.C showed how polarized IR spectra and resonant coupling information may be used to show that the signs of the different molecular components are the same for the two molecules in each pair of C0,s with a coupling constant greater than 1 cm- I . This restricts the inter-vector angles to the smallest ones possible for Pairs A,,, R,,, BI4, and C12.For AZ4,S,,, and T,, the coupling constants were too small to determine relative signs of the components. Because large deuterium isotope effects suppress formation of A I 3 in a,@-UP,Segmuller’s EPR studies gave the structures only of radical pairs on the pathway containing Subsequent studies by Feng [72] using a,@-TPsuggest that the radical positions are not significantly different in the pathway containing A13. Since IR spectra have provided much more detailed informacion about the A13 pathway, this discussion will focus on A13,R,,, BI4, and C12,and assume that the radical pair motions are essentially the same as in Segmuller’s mechanism for the pathway beginning with A2&

372

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

Figure 35 presents the motions of C0,s and radicals in the pathway containing A,, through a schematic representation which is consistent with the IR and EPR observations. The view shows projections of fragments on the plane parallel to the a axis and 40.3" from the 6 axis of the crystal. The crystallographic Q axis is vertical, so that this view is equivalent to the one shown in Figure 9. Unresolved ambiguities regarding position and orientation of the C0,s are discussed below. Polarized IR spectra show that orientations of A,, R,, and B, differ by less than 2". By contrast, the transition from A3 to R, and thence to B4 involves reorientation by 10" and then 30". Much of this motion is in and out of the plane of Figure 35, and therefore is not apparent in this projection. One ambiguity due to crystal symmetry involves associating a particular CO, dimer orientation with the appropriate radical pair orientation. In constructing Figure 35 it was assumed that the CO, which undergoes greater motion is adjacent to the moving radical. In the transition from B14 to C,,, both high-

2

/*?

/h I

Figure 35. Schematic depiction of COz and radical motions on pathway containing AI3, R,,, B,,, and Clz. The curved arrow indicates motion of the a-carbon from its position in the starting peroxide to that in A13. The question mark in R l z is a reminder that radical positions were not determined experimentally for this structure, because its EPR spectrum was obscured. The vertical arrows emphasize that the radical on the right is involved primarily in first two rearrangements, while the radical on the left moves in the third.

FORMULATION OF A MECHANISM

373

and low-frequency CO, molecules rotate significantly, with B, rotating by

40" and B, by -20". That the the CO, next to the moving radical again

undergoes greater reorientation tends to support the previous assumption. The discrete, step-wise motions of the CO, molecules observed by IR correspond to those of the radical pair, which Segmuller observed by EPR. This correspondence confirms Segmuller's proposed mechanism as the dominant process in the photolyzed crystal. An additional consistency is that the CO, molecule that undergoes the most reorientation in the first two steps is the one that absorbs at lower frequency. This suggests that motion is freer in the site where stress has been reduced by radical motion. One might have imagined that, other things being equal, the COz molecule under greater stress would have more driving force for reorientation, but in this case, the tighter environment seems to constrain it from moving at all. Constancy in the values of the axial components of the high frequency CO, through these transformations suggests that their signs are also constant, that is, that the C0,'s long axis does not reorient from one octant to another. If this is so, the axis of the low frequency CO, also stays in the same octant, since its relationship to its partner is established by the influence of coupling on absorptivity. The implied parsimony of motion from A, to R, to B, is consistent with other spectroscopic similarities among them; for example, their frequencies are perturbed by the same deuterium atoms in nearby molecules. Given these similarities, the alternative of octant-to-octant reorientation by 73" for A, can be excluded. In Figure 35, five of the COz molecules have been drawn with uniform lines, instead of in perspective. In each case, the out-of-plane component is less than 12" for this projection. Indeterminacy in the signs of the long-axis vectors makes it conceivable that the molecule should be rotated by some 90" about the vertical a axis, so that it would point mostly in and out of the page. However, spectral perturbations from nearby deuterium atoms and radicals suggest that the projection of Figure 35 is correct. This is most evident for A,, B,, C,, and C,. In each case, i 3 is shifted by presence of deuterium in the radical, and the relative intensities of these bands are extremely high, presumably because the CO, points in the general direction of a polarizable free-radical center. If it had pointed in and out of the page, this would not have been possible. Although this type of information can narrow the range of possible structures, several uncertainties remain. It is not certain whether the C 0 2 molecules should be pointing toward the upper right or the upper left in Figure 35. Since radical pairs A and B do not share the two-fold symmetry of the site, it is also not certain whether the top right portions of A,, R,, and B, should be pointing into or out of the plane of projection. Translational positioning of the C 0 2 molecules is speculative; positions within the pair

314

PHOTOCHEMICAL MECHANISM IN SINGLE CRYSTALS

were chosen so that the observed coupling constants would be roughly approximated, but the range of relative positions that yield the same observed coupling constants prohibits a confident assessment. Recent advances in the application of “surface walking” algorithms to atom-atom potential calculations provide hope for a complete simulation of this pathway, including structures of the intermediates and the minimum energy pathways connecting them. Experimental constraints, such as zerofield splitting, hyperfine splitting, and g tensors, has been successfully imposed on such calculations to predict radical pair structures in ABP, even without relaxation of surrounding molecules [13]. Although the problem is much more complex for long-chain peroxides, because of the large number of intramolecular degrees of freedom, efforts are underway to simulate the pathway in U P and its derivatives. It seems likely that developing a reliable, coherent picture for this reaction mechanism and others will draw heavily on the information and experimental techniques described in this chapter.

ACKNOWLEDGMENTS We are grateful to acknowledge the contributions of graduate students and postdoctoral colleagues whose names are mentioned in the references. In particular, we acknowledge the unpublished work of Drs. Bonnie L. Whitsel, Richard A. Pankratz, Thomas C. Semple and Xu-Wu Feng. This work was supported by the N.S.F. Division of Materials Research (Grant DMR8203662) and by a Dox Fellowship to MDH. MDH acknowledges support from the Central Research Fund of the University of Alberta and the Natural Sciences and Engineering Research Council of Canada during preparation of the manuscript. JMM acknowledges the Office of Naval Research (N0001487-K-0437) for support during the same period.

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Advances in Photochemistry, Volume15 Edited by David H. Volman, George S Hammond, Klaus Gollnick Copyright © 1990 John Wiley & Sons, Inc.

INDEX

Alkoxy radicals, 75 peroxyalkyl radicals reaction with, 101 photochemical preparation from alkyl nitrites of, 75 9-Acetylanthracene topology, 140 Anthracene-arylcarbonyl intramolecular solvent-assisted interactions, 187 1-aceanbenone, 187 acetophenones. 190 anthracene:anthrone exciplex, 197 anthracene:carbonyI charge transfer, 20 I anthronyl-anthracene exciplex, 199 anthrylacetone, 189 10. 10-diethylanthrone, 195 energy transfer reactions, 190 exciplex emission, 195 9-phenylanthracene, 189 spiro compounds, 197 Atmospheric oxidation, 80 Atmospheric oxidation of acetaldehyde, 96 Atmospheric oxidation of ethane, 95 perhydroxyethane from, 95 reactions of, 95 Atmospheric oxidation of formaldehyde, 85 hydroperoxyethane from, 90 hydroxyl in, 85, 91 methanol from, 90 perhydroxyl in, 88 Atmospheric oxidation of methane, 81 dimethyl peroxide from, 81 formaldehyde from, 81 formic acid from, 81

IR absorbance spectra of products from, 81 methanol from, 8 1 peroxyalkyl nitrates from, 84 Atmospheric oxidation of peroxyacetyl radical, 96 peroxyacetyl nitrate, PAN, from, 96 Azoethane photooxidation, 95 Bilatrene chromophore, excited singlet Pc, 239 Bilatrenes, 232, 236, 260 Bilirubin, 243 Biliverdin, 236, 245 C-phycocyanin, 236 Crystal reaction study mechanistic tools, 296 computer simulation, 297 electronic spectroscopy, 298 electron microscopy, 298 electron paramagnetic resonance (EPR), 299 nuclear magnetic resonance (NMR), 298 Raman spectroscopy, 299 Crystal reaction study techniques: crystal mounting, 308 decomposition limiting, 309 polarized IR spectroscopy, 309 temperature control, 308 Cycloreversions, adiabatic photochemical involving anthracenes, 203 excited state properties of lepidopterenes, 206 fragmentation of anthracene adducts, 203 anthracene-benzene adducts, 206

381

382

INDEX

Cycloreversions (Conr.) endoperoxides, 204 9methylanthracene photodimer, 204 1.2-Di-9-anthrylethane and related derivatives topology, 144 9-anthraldehyde azines, 153 anthrone derivatives, 148 bis-a-9-anthrylmethyl ethers, 151 dianthry lacetone, 149 1,3-di-9-anthryl-I-butanone,151 1.2-di-9-anthrylethanol.145 1,3-di-9-anthrylpropane,149 dianthrylisopropanol, 149 Di-9-anthrylmethane derivatives topology, 141 di-9-anthryl ketone, 144 di-9-antrylmethane, 142 di-9-antrylmethanol, 142 photochemical intramolecular 4 a 4 ~ r cycloadducts, 142

+

Etiolated plants, 231, 233 Excited state charge transfer dual fluorescence, 40 adiabatic, 46 ADMA experiments, 57 BA experiments, 53 diabatic, 46 DMABN experiments, 59 electronic states related to, 42 Franck-Condon emission in, 48 probability distribution function in, 47 prototype molecules for, 40, 59 4-(9-anthryl)-N,N-dimethylaminoaniline (ADMA), 40 9,9’-bianthryl (BA), 40 p-N,N-dimethylaminobenzonitrile (DMABN), 40 solvent coordinate related to, 43 Born charge model, 45 Marcus-Hush theory, 45 Onsager cavity model, 45 theory of, 49, 59 rate constant of dynamics in, 53 simulation of dynamics of bianthryls, 51 solvent controlled, 50 Excited state properties of 9-anthrylalkenes, 158 di-9-anthrylcyclopropenone,162 cis-dianthrylethylene, 160

I ,2-di-9-anthrylethylenes,158 tram-dianthrylethanol, 161 1.3-di-9-anthrylpropenes. 163 rrans-l,3-di-9-anthrylpropenone, 166 Fluorescence instrumentation, 19 femtosecond, 19 picosecond, 19 Fluorescence of P4phytochrome: decay of red components of, 241 excitation spectra of, 239 Fluorescence properties of 9-anthrylalkenes. 177 adiabatic photoreaction effect on, 179 Stokes shift, 179 Fluorescence properties of 1,2-di-9anthrylethelenes, 177 trans-dianthrylethylenes,184 symmetrical cis- 1,2-diarylethelenes, 177 twisted intramolecular charge transfer (TICT) state, 184 Fluorescence spectra, 22 spectral density function for, 25 spectral shape function for, 25 time resolved, 22 reconstruction of, 24 Stokes shift in, 22 Fourier transform infrared (FTIR) spectroscopy, 70 long path system for, 73 photochemical reactor for, 71 rate constant measurements by, 76 reaction mechanism from, 78 atmospheric, 79 models of atmospheric, 79 Fourier tranform, Michelson interferometric method, 91 FTIR information on crystals, 317 absortivity and frequency correlation, 359 atom-centered multipole expansion, 348, 353 Buckingham theory and, 330 chain decomposition, 325 cooperative behavior of defects, 338 deuterium shifts, 361 dispersive interactions, 330 elastic strain, 333 electrostatic coupling, 352 exchange of positions, 351 heteroisotopic pairs, 348

383

INDEX

identification of intermediates, 317 comparison with EPR, 317 intrinsic oscillator strength, 357 lattice vacancy hole, 334, 335 local stress, 326 molar absortivity, 325 molecular orientation, 341 octant problem, 344, 356 real-time kinetics, 322 reversible introconversion, 324 rotation of COz, 346 solid solutions, 335 steric isotope effects, 363 intermolecular, 366 rotational diffusion and, 366 substituent effect additivity, 369 temperature dependence of frequency, 330 tight cage, 330 vibrational coupling, 318, 327, 344, 346, 349 ITIR reporter probe, 300 carbon dioxide as a, 303 anharmonicity and, 305 bending frequency and, 306 electrolytic coupling and, 307 intermolecular forces and, 305, 346 intrinsic frequency of, 306 ' 8 0 C ' 6 0 isotopic, 307 peroxide related to, 303 polarizability and, 307 windows and. 301 Gaussian curve analysis, 237 chromophore absorption by phycocyanin hexamer, 237 Geometrical isomerization of 9-anthrylalkenes, 167 cis-trans adiabatic, 173 cis-l-(9-anthryl)-2-(1-naphthyl)ethylene, 174 1-(9-anthryl)-2-nitroethylenes,175 1-(9-anthryl)-2-phenylethylene, 169 9-phenylethynylanthracene,177 Global data analysis, 241, 246 P, fluorescence decay, 241 Hydroxyl radicals, 73 atmospheric sources of, 73 photochemical sources of, 74 thermal from peroxynitric acid (PNA), 75

Hydroxyl reactions in the atmosphere, 9 1 acetic acid, 93 acetylene, 107 alkanes, 102 aromatic hydrocarbons, 125 toluene, 125, 127 trimethylbenzenes , 125 xylenes, 125 2-butene, 106 2-butyne, 107 carbon monoxide, 91 ethene, 92 glycolaldehyde from, 106 propyne, 107 I,

(lumi-R) intermediates, 250, 253, 255, 269 I,, (meta-R) intermediates, 250, 255, 270 I, intermediates, 259, 261, 270 Internal conversion in phytochromes, 242

Laser, 241 mode-coupled argon ion, 241 mode-locked cavity-dumped dye, 241 Laser-induced optoacoustic spectroscopy (LIOAS), 251 time resolved, 253 Light-harvesting antenna chromophore, 236 Liposomes, 260 Lippert-Mataga equation, 14 Lumi-F intermediate in Pf, -+ P, transformation, 268 Lumi-R intermediate in P, + Pfr transformation, 250 Meta-F intermediate in P, -+ P, transformation, 268 Meta-R intermediate in P, + P, transformation, 250 Nitrogen trioxide in atmospheric reactions, 122 trans-2-butene with, 122 nitrogen pentoxide as source of, 122 oxidation of alkenes by, 122 tetramethyl ethylene (TME) with, 124 Nonequilibrated excited rotamers (NEER), 141 Nonradiative deactivation of P, and P,,, 242 Onsager inverted snowball theory, 34 linearized mean spherical approximation in, 35

384 Onsager inverted snowball theory (Con?.) relation to Smoluchowski equation in, 35 relaxation time by, 34 rotational diffusion and, 36 Ozone in the atmosphere, 108 alkene reactions with, 108 Crigee intermediate from, 108 molozonide from, 108 ethylene reaction with, 109 acetaldehyde effect on, 113 formic anhydride from, 110 sulfur dioxide effect on, 113 sulfuric acid aerosols from, I14 infrared detection of, I08 tetramethylethylene (TME) reaction with, 117 Perhydroxyl radical, 75 thermal generation from PNA of, 75 Peroxy radical generation, 75 Peroxide crystal photoinitiated reactions, 310 acetyl benzoyl peroxide (ABP), 31 I radical pairs in, 311, 313 stress generated in, 3 13 diundecanyl peroxide (UP), 3 I3 derivatives of, 3 I7 EPR reaction scheme for, 313 IR reaction scheme for, 3 I6 zero field splitting of, 313 Peorxyacetyl nitrate (PAN), 7 1 , 96 CH,C(0)00 radical from, 96 ethane oxidation formation of, 96 IR spectroscopy detection of, 71, 96 perhydroxyl radical formation of, 96 synthesis of, 97 Peroxyalkyl nitrates, 83 IR absorption spectra of, 83 preparation of, 85 Peroxymethyl reactions, 82 Photochemical mechanisms in crystals, 283 atomic trajectories in, 283 Beer's law and, 294 bimolecular processes in, 291 concepts of, 283 crystal growing for study of, 292 excited state quenching and, 295 exciton migration and, 294 fracture related to, 292 free radical scavenging in, 295

INDEX

glassy matrices relative to, 283, 288 nonexponential behavior in, 283 dispersive kinetics in, 283 lattice destruction and, 29 1 optical properties relative to, 292 birefringence, 292, 294, 298, 309 filtering of incident light, 292 scattering, 292, 293 physical effects in, 288 physical transformation and, 285, 287 plastic deformation and, 286, 334, 339 polarized spectra and, 295, 302, 303, 341 potential energy surfaces relative to, 283, 289 reaction cavity and, 288 substituent effects in, 296 surface photolysis related to, 293 tensors for, 286 Photochromism, P, = Pfr equilibrium 232, 234 Photomorphogenesis, 23 1 Phycocyanobilin. 237 Phytochrome, 230 molecular weight of, 232 native undegraded, 232 P, (far-red absorbing form), 236 bilatrene chromophore structure of, 236 photophysical properties of, 236 P,, + P, transformation, 234 P, + Pi, transformation of, 234 chemical nature of individual reaction steps of, 263 first thermal steps in, 250 primary photoreaction in 242, 250, 257, 261 protein stabilization of, 261 terminal steps of the P, formation in, 260 P, (red absorbing form) of, 232 anomalous blue emission of, 243 bilatrene chromophore structure of, 239 excited singlet bilatrene chrornophore in. 239 liposome-bound, 257, 259 photophysical properties of, 239 preparation of, 232 partially degraded, 232 structure-function relationship, 236 tryptophane and protein fluorescence of, 246 Phytochromobilin, 236, 241

INDEX

Single photon-timing (SPT), 241 Solvating probe molecule fluorescence, 8 Franck-Condon transitions in, 8 Lippert-Mataga equation in, 14 nonequilibrium Helmholtz energies in, 8 Onsager cavity description of, 9 spectral shape of, 14, 25 Stokes shift in, 14 Solvation dynamics function, C(t), 4 Debye-Onsager model of, 12, 27 fluorescence related to, 25 measurements of, 27 linear wavelength, 27 spectral reconstruction of, 27 model for, 11 Onsager cavity description of, 11 relation to solvent coordinate of, 4 Stokes shift and, 24 theories of, 32 Cole-Cole equation and, 33 Davidson-Cole equation and, 33 non-Debye extension of Onsager model, 32 Solvation dynamics in water, 36 mean spherical approximation of, 38 semi-empirical model of, 36 time dependence of, 39

385

two probe model of, 38 Solvation force constant, 8 optical component of, 8 orientation component of, 8 Solvation probes for transient fluorescence, 14 molecules for dynamic studies of, 15 Solvatochromism, 7 model of, 7 Stokes shift in fluorescence, 17 excited singlet P, bilatrene chromophore, 239 time dependent, 22 Triplet state isomerizations, 153 biacetyl sensitized, 154 dianthrylethanone, 157 direct photooxidation induced, 154 Vision, 230 contrast of animal and plant, 230 plant (“vision”), 230 photokinesis, 230 photomorphogenesis, 230

Z,E isomerism in phytochrome P, and P, forms, 237

Advances in Photochemistry, Volume15 Edited by David H. Volman, George S Hammond, Klaus Gollnick Copyright © 1990 John Wiley & Sons, Inc.

CUMULATIVE INDEX, VOLUMES 1 - 15

Addition of Atoms to Olefins, in Gas Phase (Cvetanovic) . . . . . Alcohols, Ethers, and Amines, Photolysis of Saturated (von Sonntag and Schuchmann) . . . . . . . . . . . . . . . . . . . . . . . . . . . Alkanes and Alkyl Radicals, Unimolecular Decomposition and Isotope Effects of (Rabinovitch and Setser) . . . . . . . . . . . . . . . . Alkyl Nitrites, Decomposition of and the Reactions of Alkoxyl Radicals (Heicklen) . . . . . . . . . . . . . . . . . . . . . . . . . . Anthracenes, Excited State Reactivity and Molecular Topology Relationships 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) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bichromophoric Systems, Excited State Behavior of Some (De Schryver, Boens and Put) . . . . . . . . . . . . . . . . . . . . . . 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.) . . . . . . . . . . . . . . . . . . . . . . . Cyclic Ketones, Photochemistry of (Srinivasan) . . . . . . . . . . . . Cyclobutanones, Solution Phase Photochemistry of (Morton and Turro) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a-Dicarbonyl Compounds, The Photochemistry of (Monroe) . . . . Diffusion-Controlled Reactions, Spin-Statistical Factors in (Saltiel and Atwater) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

VOL. 1

PAGE 115

10

59

3

1

14

177

15 1

I39 23

15

69

10

147

10

359

6

30 1

6 1

123 83

9

197

8

77

14

1

387

388

CUMULATIVE INDEX

Electron Energy Transfer between Organic Molecules in Solution (Wilkinson) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electronically Excited Halogen Atoms (Husain 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) . . . . . Free Radical and Molecule Reactions in Gas Phase, Problems of Structure and Reactivity (Benson) . . . . . . . . . . . . . . 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 (Majer and Simons) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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 Kolesnikova) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydroxyl Radical with Organic Compounds in the Gas Phase, Kinetics and Mechanisms of the Reactions of (Atkinson, Darnall, Winer, Lloyd and Pitts) . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hypophalites, Developments in Photochemistry of (Akhtar) . . . . Imaging Systems, Organic Photochemical (Delzenne) . . . . . . . . Intramolecular Proton Transfer in Electronically Excited Molecules (Klopffer) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ionic States, in Solid Saturated Hydrocarbons, Chemistry of (Kevan and Libby) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Isotopic Effects, in Mercury Photosensitization (Gunning and Strausz) Mechanism of Energy Transfer, in Mercury Photosensitization (Gunning and Strausz) . . . . . . . . . . . . . . . . . . . . . . . . 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) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 8

24 1

9

I

13 6 11

237 425 489

2

1

1

115

1

43

1

2

25

2

137

1

43

2

25

11 2

375 263

11

1

10

311

2 1

183 209

1

209

1

183

1 8

209 227

2

219

389

CUMULATIVE INDEX

Neutral Oxides and Sulfides of Carbon, Vapor Phase Photochemistry of the (Filseth) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nitric Oxide, Role in Photochemistry (Heicklen and Cohen) . . . . Noyes, W.A., Jr., A Tribute (Heicklen) . . . . . . . . . . . . . . . . Nucleic Acid Derivatives, Advances in the Photochemistry of(Burr) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Olefins, Photolysis of Simple, Chemistry of Electronic Excited States or Hot Ground States? (Collin) . . . . . . . . . . . . . . . . . . . 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 States, Properties and Reactions of (Wagner and Hammond) . . . . . . . . . . . . . . . . . . . . . . . Organic Nitrites, Developments in Photochemistry of (Akhtar) . . Organic Photochemical Refractive-Index Image Recording Systems (Tomlinson and Chandross) . . . . . . . . . . . . . . . . . . . . . Organo-Transition Metal Compounds, Primary Photoprocesses of (Bock and von Gustorf) . . . . . . . . . . . . . . . . . . . . . . . . Perhalocarbons, Gas Phase Oxidation of (Heicklen) . . . . . . . . . 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) . . 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, Vocabulary of (Pitts, Wilkinson, Hammond) . . . Photochromism (Dessauer and Paris) . . . . . . . . . . . . . . . . . . Photo-Fries Rearrangement and Related Photochemical (1 .j) Shifts of (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) . . . . . . . . . . .

10 5 13

157 vii

6

193

14 I

135 323

8

315

5 2

21 263

12

20 I

10

22 1

7 2

57 305

9 8

31 1 161

4

1

4

25 373

7

4

1

4 4 12

81 195 113 97

12 9

369

11 1

1

I

305 1 275

8

109

13

329

2

385

390

CUMULATIVE INDEX

Photoluminescence Methods in Polymer Science (Beavan. Hargreaves and 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 (Engel and Monroe) . Phytochrome. Photophysics and Photochemistry of (Schaffner. Branslavsky. and Holzwarth) . . . . . . . . . . . . . . . . . . . . Polymers. Photochemistry and Molecular Motion in Solid Amorphous (Guillet) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Primary Processes and Energy Transfer: Consistent Terms and Definitions (Porter. Balzani and Moggi) . . . . . . . . . . . . .

11

4 3 6 13 8

207 225 83

1

427 245

1s

229

14

91

9

147

Quantum Theory of Polyatomic Photodissociation (Kreslin and Lester)

13

95

Radiationless Transitions. Isomerization as a Route for (Phillips. Lemaire. Burton and Noyes. Jr.) . . . . . . . . . . . . . . . . . . Radiationless Transitions in Photochemistry (Jortner and Rice) . .

5 7

329 149

15 7 11

279 311 105

4 13

49

2

183

1s 7 13

1 1

Single Crystals. Photochemical Mechanism in: FTIR studies of Diacyl Peroxides (Hollingsworth and McBride) . . . . . . . . . . . . . Singlet Molecular Oxygen (Wayne) . . . . . . . . . . . . . . . . . . . Singlet Molecular Oxygen. Physical Quenchers of (Bellus) . . . . Singlet and Triple States: Benzene and Simple Aromatic Compounds (Noyes and Unger) . . . . . . . . . . . . . . . . . . . . . . . . . . Small Molecules. Photodissociation of (Jackson and Okabe) . . . . Solid Saturated Hydrocarbons. Chemistry of Ionic States in (Kevan and Libby . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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 Compounds. Photochemical Reactions of (Mustafa) . . . . . . . . . . . . . . . . . . . . . . . . Surfactant Solutions. Photochemistry in (von Biinau and Wolf0 Theory and Applications of Chemically Induced Magnetic Polarization in Photochemistry (Wan) . . . . . . . . . . . . . . . . . . . . . . . Triatomic Free Radicals. Spectra and Structures of (Herzberg) . . Ultraviolet Photochemistry. Vacuum (McNesby and Okabe) . . . . Ultraviolet Radiation. Photoionization and Photodissociation of Aromatic Molecules by (Terenin and Vilessov) . . . . . . . . .

1

4

165 143

2 14

63 273

12 5

283 1

3

157

2

385