New Developments in Construction and Functions of Organic Thin Films (Studies in Interface Science)

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S T U D I E S IN I N T E R F A C E SCIENCE

New Developments in Construction and Functions of Organic Thin Films

STUDIES

IN I N T E R F A C E

SERIES D. M 6 b i u s

SCIENCE

EDITORS and R. M i l l e r

Vol. I

Dynamics of Adsorption at Liquid Interfaces

Theory, Experiment, Application by S.S. Dukhin, G. Kretzschmar and R. Miller Vol. z An Introduction to Dynamics of Colloids by ].K.G. Dhont Vol. 3 Interfacial Tensiometry by A.I. Rusanov and V.A. Prokhorov Vol. 4

New Developments in Construction and Functions of Organic Thin Films edited by T. Kajiyama and M. Aizawa

New Developments in Construction and Functions of Organic lhin Films Edited by T I S A T O KAJ I Y A M A

Department of Chemical Science and Technology Kyushu University 6-10-1 Hakozaki Higashi-ku, Fukuoka 812 Japan MAS U O AI Z A W A

Department of Bioengineering Tokyo Institute of Technology Nagatsuta, Midori-ku Yokohama 226 Japan

x996 ELSEVIER Amsterdam

- Lausanne

- New York-

Oxford - Shannon

- Tokyo

ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 211, iooo AE Amsterdam, The Netherlands

ISBN: o 444 81956 8

9 I99 6 Elsevier Science B.V. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science B.V., Copyright & Permissions Department, P.O. Box 52I, I OOO AM Amsterdam, The Netherlands. Special regulations for readers in the U . S . A . - This publication has been registered with the Copyright Clearance Center Inc. (CCC), 222 Rosewood Drive, Danvers, MA, oi923. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the publisher. No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. This book is printed on acid-flee paper. Printed in The Netherlands.

PREFACE This book deals with organic thin films by a variety of techniques. The component molecules are relatively simple ones with self-organizing properties, that is, ordered molecular assembly characteristics. The contents of this book are arranged in the order from the fundamental concepts of molecular assembly of self-organizing molecules to the potential biological applications of protein assemblies, supramolecular species. Though the many promising applications for new electric, magnetic or optical devices, biomimetic membranes and so on have been attractively investigated recently, the fundamental studies on molecular assembly characteristics and functions for monolayers, bilayers and multilayers, LangmuirBlodgett films are indispensable to future technological innovations for molecular electronic devices, biological sensors and so on. A Priority Area Research Program for "New Functionality Materials: Design, Preparation and Control" was organized by Professor Teiji Tsuruta, Tokyo Science University in the fiscal years from 1987 to 1992 under the support of the Ministry of Education, Science and Culture, Japan. The main studies on the contention of this book was enthusiastically carded out by the research group for design of functionality of materials with supramolecular structure, design of functionality of materials composed of oriented molecules and information transmission functions of biofunctionality materials. This book is timely in view of the recent surge of interest and effort in "New Developments on Construction and Functions of Organic Thin Films". Tisato Kajiyama Faculty of Engineering, Kyushu University April, 1996

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~

VII

Contributors Masuo Aizawa Tokyo Institute of Technology, Yokohama, Japan Masanao Era KyushuUniversity, Kasuga-shi, Japan Masamiehi Fujihira Tokyo Institute of Technology, Yokohama, Japan Tisato Kajiyama KyushuUniversity, Fukuoka, Japan Masakazu Makino Universityof Shizuoka, Shizuoka, Japan Toshihiko Nagamura Shizuoka University, Hamamatsu, Japan Hiroo Nakahara Saitama University, Urawa, Japan Yushi Oishi Saga University, Saga, Japan Yoshio Okahata Tokyo Institute of Technology, Yokohama, Japan Kenji Okuyama Tokyo University of Agriculture and Technology, Tokyo, Japan Shogo Saito Kyushu University, Kasuga-shi, Japan Masatsugu Shimomura HokkaidoUniversity, Sapporo, Japan Tohru Takenaka ScienceUniversity of Okayama, Okayama, Japan Tetsuo Tsutsui KyushuUniversity, Kasuga-shi, Japan Junzo Umemura Kyoto University, Kyoto, Japan Kenichi Yoshikawa NagoyaUniversity, Nagoya, Japan

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CONTENTS Preface

..............................................

v

1 Novel Concepts of Aggregation Structure of Fatty Acid Monolayers on the Water Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ttsato KAJIYAMA and Yushi OISHI 2 Crystal Engineering of Synthetic Bilayer Membranes . . . . . . . . . . . . . . Kenji OKUYAMA and Masatsugu SHIMOMURA

39

3 Control of Molecular Orientation and Packing in Monolayer Assemblies. 9 71 Hiroo NAKA HARA 4 In situ Characterization of Langmuir-Blodgett Films by using a Quartz Crystal Microbalance as a Substrate . . . . . . . . . . . . . . . . . . . . . . . . . Yoshio OKAHA TA

109

5 Application of Vibrational Spectroscopy to the Study of StructureFunction Relationship in Langrnuir-Blodgett Films . . . . . . . . . . . . . . . Tohru TAKENAKA and Junzo UMEMURA

145

6 Construction of Well Organized Functional Langrnuir-Blodgett Films by Mimicking Structures and Functions of Biological Membranes . . . . . Masamichi F UJIHIRA

181

7 Nonlinear Characteristics of Thin Lipid Films . . . . . . . . . . . . . . . . . . . Masakazu MAKINO and Kenichi YOSHIKA WA

211

8 Molecular Control of Photoresponses of LB Films Containing Redox Chromophores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Toshihiko NA GAMURA

247

9 Design of the Non-Linear Optical Films by Langmuir-Blodgett Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Masanao ERA, Tetsuo TSUTSUI and Shogo SAITO

287

10 Protein Assemblies for Information Transduction . . . . . . . . . . . . . . . . Masuo AIZA WA

323

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New Developments in Construction and Functions of Organic Thin Films T. Kajiyama and M. Aizawa (Editors) 9 1996 Elsevier Science B.V. All rights reserved.

N o v e l c o n c e p t s of a g g r e g a t i o n structure o f fatty acid m o n o l a y e r s on the w a t e r surface T. Kajiyama and Y. Oishi* Department of Chemical Science and Technology, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812, Japan. 1. E V A L U A T I O N O F M E L T I N G AND C R Y S T A L L I N E R E L A X A T I O N T E M P E R A T U R E S O F F A T T Y ACID M O N O L A Y E R S ON T H E W A T E R SURFACE

1.1 Monolayer preparation and modulus measurement of monolayer. Myristic(C14), palmitic(Cl6), stearic(C18) and arachidic(C20)acids(chromatographic reference quality) were used without further purification. Benzene with spectroscopic quality was used as solvent. Benzene solutions of myristic, palmitic, stearic and arachidic acids were prepared with concentrations of 3.7x10 -3, 2.8x10 -3, 2.2x10 -3 and 3x10 -3 mol-1-1, respectively. The subphase water was purified by the Milli-QII| system(Millipore Co.,Ltd.). The subphase temperature, Tsp was varied in a temperature range of 274-321 K by circulating constanttemperature water around an aluminum support of a trough. The accuracy of Tsp was _+1 K, which was evaluated by using a thermocouple positioned ca. 1 mm below the water surface. And also, the room temperature was adjusted to the same temperature as Tsp by using an airconditioner and three infrared radiation lamps. Each monolayer was compressed to a given surface pressure at a barrier speed of 48 mm. min- 1. Pressure-area (~t-A) isotherms were obtained at various Tsps with a microprocessorcontrolled film balance system. The static elasticity, KS of the monolayer on the water surface was evaluated from the ~-A isotherm by using the following equation[ 1-3 ]. Ks = -A(dJt/dA)

(1)

Figure 1 shows the ~t-A(solid line) and the logKs-A(broken line) isotherms for the stearic acid monolayer at Tsp of 293 K. LogKs(max) was defined as the maximum of logKs which corresponded to -the collapsing point. At this point, even though the collapsed monolayer fragments were observed as an appearance of patchy pattern on the base monolayer(the substrate monolayer on the water surface), molecules in the base monolayer were packed most densely and homogeneously. Therefore, it is reasonable to consider that homogeneous compression force was transmitted throughout the monolayer. Then, the temperature dependence of logKs(max) was adopted for determination of the melting temperature, Tm and the crystalline relaxation temperature, Tac of the monolayer on the water surface. The hydrophilic SiO substrate(static water contact angle:0=30 ~ was prepared by vapordeposited SiO onto a Formvar substrate[4], with which an electron microscope grid(200-mesh) was covered. The relatively hydrophobic siliconized substrate(0=90 ~ was also prepared by surface siliconized treatment; a collodion-covered electron microscope grid was dipped into an aqueous solution of silane coupling agent. Bright field electron micrographs and electron diffraction (ED) patterns were taken with a Hitachi H-500 electron microscope, which was operated at an acceleration voltage of 75 kV and *Present address: Department of Applied Chemistry, Saga University, 1 Honjo-machi, Saga 840, Japan.

7060-

Stearic Acid Tsp- 293 K 1.11"-A isotherm 2.LOGKs-A isotherm

E50 ~'40 E 30 20 10 0o

'-,n0

area~

0.1

"7

E 2.0 ~" E 1.0 o

0 _.I

[\

0.2 0.3 0.4 A/nm2.moleeule -1

3.0

o .5

Fig. 1. The ~t-A and log Ks-A isotherms of the stearic acid monolayer at Tsp of 293 K. a beam current of 2.5 laA. Electron microscopic observations were carried out at the same temperature as Tsp at which the monolayer was prepared on the water surface, by using a thermostating apparatus. Pt-carbon was vapor-deposited onto the monolayer samples with a shadowing angle of 25 ~ for the bright field electron microscopic observation. Figure 2 shows the ED patterns of arachidic acid monolayers transferred onto a SiO substrate(a) and a siliconized one(b) by an upward drawing method with a drawing speed of 60 mm-min-1 at Tso=293K and ~t=25 mN-m-], respectively. The hydrophilic part of the monolayer contacts with die substrate surface by the upward drawing method. As shown in Figure 2, the crystalline structure of the monolayer depends on the hydrophilic or hydrophobic characteristics of the substrate, maybe due to the difference of interracial interaction. The monolayer transferred on the hydrophilic substrate such as SiO shows that the crystal system of the monolayer is hexagonal. The diffraction spot of 0.42 nm spacing is assigned to the (10) reflection of two-dimensional hexagonal crystal. This hexagonal crystal system agrees with that of the arachidic acid monolayer on the water surface, which was confirmed by a grazing incidence in-plane X-ray diffraction[5]. Since the hydrophilic group of the monolayer contacts

Fig. 2. The ED patterns of the arachidic acid monolayers transferred onto (a) SiO and (b) siliconized substrates.

with the hydrophilic SiO substrate during transfer of the monolayer, this interfacial condition is in a similar fashion to that of the monolayer on the water surface with respect to the magnitude of interfacial free energy between the hydrophilic(polar) group of monolayer and the hydrophilic substrate surface. Therefore, it is reasonable to consider that the hexagonal crystal system of the monolayer on the water surface is transferred and stably maintained on the hydrophilic substrate. On the other hand, the ED pattern of Figure 2(b) shows that the crystal system of the arachidic acid monolayer transferred on the siliconized hydrophobic substrate is face-centered rectangular. The diffraction spots of 0.42 and 0.38 nm spacings are assigned to the (1 I) and (20) reflections of the two-dimensional rectangular crystal, respectively. The hexagonal crystal system on the water surface could not be transferred and maintained on the siliconized hydrophobic substrate because the surface energy of the substrate was quite different from that of the water surface. In this case, since the interfacial interaction between the hydrophilic(polar) group of the monolayer and the hydrophobic substrate surface becomes weaker with an increase in the surface energy difference, fatty acid molecules form their inherent crystalline structure. Then, it seems reasonable to conclude from Figures 2(a) and 2(b) that the crystalline structure of fatty acid monolayers on the water surface can be transferred onto a hydrophilic substrate by the upward drawing method without any change of crystallographical system. Also, since the surface of the hydrophilic SiO substrate in an amorphous state is quite smooth, the SiO substrate is suitable for the electron microscopic observation of the monolayer without any change of the crystallographical structure of the monolayer on the water surface. Then, in order to investigate the thermal behavior of the monolayer structure on the water surface, the monolayer was transferred onto the hydrophilic SiO substrate by the upward drawing method with the drawing speed of 60 mm- min- 1 at various Tsps and at a certain surface pressure where each monolayer was morphologically homogeneous[6,7]. 1.2. Determination of melting temperature of m o n o l a y e r on the water surface. Figure 3 shows the Tsp dependences of logKs(max) for stearic acid monolayer on the water surface and the ED patterns oI the monolayer transferred onto the hydrophilic substrate at the surface pressure of 20 mN-m-1. The homogeneous monolayer was formed on the water surface at this magnitude of surface pressure. The ED patterns were taken at the same temperature as

Fig. 3. The Tsp dependences of log Ks(max) and ED patterns of the stearic acid monolayers.

Fie. 5. The Tsn dependences of log Ks(max), bright field electron micrographs and ED patterns of~he palmitic~cid monolayers. Tsp at which the monolayer was prepared. The magnitude of logK_Sma:'(x) started to apparently decrease at ca. 298 K and3 i7 K. The ED patterns at 313 K and 319 K were a crystalline Debye ring and an amorphous halo, respectively. Therefore, Figure 3 indicates that a fairly remarkable decrease of logKs(max) at around 317 K corresponds to the melting behavior of stearic acid monolayer on the water surface. Also, Tms of palmitic and myristic acid monolayers on the water surface were similarly estimated to be 301 K and 278 K, respectively, on the basis of the Tsp dependence of logKs(ma.x) and ED patterns as shown in Figures 4 and 5. As mentioined above, Tm of the fatty acid monolayer on the water surface is successfully evaluated from the Tsp dependences of static elasticity and ED pattern.

2

350

A

Z~

1 9

,t,,s

C 0 300 a. m~,,,

O1 C O

o 3

o

o

1 9 Fatty Acid (monolayer) 2 z~ F,~j Acid (bulk)

250

3 o n-Paraffin (Broadhurst eq.)

12 '1'4' 16 ' 1 8 . .20 . . . . .22 24 26 Number of Carbon Fig. 6. Variation of Tm with alkyl chain length for (1) fatty acids in a monolayer state, (2) fatty acids in a three-dimensional crystalline state, and (3) normal paraffins in a three-dimensional crystalline state. 0.32 0.30

Stearic Acid(C~ 8)

~0.28

~0.26 E 0.24 0.22

c

o

T~c(298 K)

T

0.18 28O

i

I

290

~

I

300

i

|

310

Tsp / K

,

I

320

Fig. 7. The Tsp dependences of the limiting area for the stearic acid monolayers. Figure 6 shows variation of Tms for the fatty acid monolayers on the water surface(I), three-dimensional crystals(2) and normal paraffins(3)[8] with the number of corresponding carbon atom in an alkyl chain. Tms of myristic(Cl4), palmitic(C16), stearic(C18) and arachidic (C20) acid monolayers are ca. 278 K, 301 K, 317 K and 331 K, respectively. It is difficult to evaluate Tms of fatty acid monolayers with alkyl chain longer than Coo due to an experimental limitation with respect to increasing Tsp(heating the LB trough) and also, heating the monolayer samples in an electron microscope. Tms of the fatty acid monolayers are much lower than those of corresponding three-dimensional crystals(2"bulk) because the m o n o l a y e r is thermodynamically less stable than the three-dimensional crystal. However, the magnitude difference between Tms for monolayer and bulk crystal becomes smaller with an increase in alkyl chain length. This indicates that intermolecular aggregation force in a monolayer state increases with increasing alkyl chain length[9|. As mentioned in Figure. 7, Tm was also confirmed from the limiting area vs. Tsp curve since the magnitude of the limiting area jumps in the temperature range of Tm due to a remarkable increase of the molecular occupied area in a melt state.

1.3. New Concept on Crystalline Relaxation Process in Monolayer. Figure 3 shows that the slope of logKs(ma.x) vs. Tsp curve for the stearic acid monolayer slightly decreased at ca.298 K which was denoted by Tctc. In a Tsp range below 298 K, the ED pattern of the stearic acid monolayer exhibited sharp crystalline spots with spacings of 0.42 and 0.24 nm. Since the ratio of reciprocal spacings is 1:31'2, stearic acid molecules are packed in a hexagonal unit cell and these diffraction spots are assigned to (10) and (11) reflections, respectively. In contrast, in a Tsp range above 298 K, the ED spots tended to be more arc along an azimuthal direction and also, to be broader with an increase of Tsp, resulting in the Debye ring. This indicates that the orientation of stearic acid molecules and/or crystalline domains become broader or less regular with an increase in Tsp above ca. 298 K. In the case of the palmitic acid monolayer, similar behaviors with respect to the Tsp dependence of logKs(ma.x) and ED patterns were observed in Tsp ranges below and above 291 K, as shown in Figure. 4. Figure 8 shows the Tsp dependences of the (10) spacing evaluated from the ED patterns of the stearic and the palmitic acid monolayers. Hereupon, it should be reminded that the ED pattern was taken at the same temperature as Tsp, by warming up the sample chamber in an electron microscope. The (10) spacing vs. Tsp curve exhibited a distinct break at ca. 298 and 291 K for the stearic and the palmitic acid monolayers, respectively, though the values of the (10) spacing were fairly scattered. Then, the temperature range of the break point on the (10) spacing vs Tsp curve apparently corresponds to that for an apparent decrease of logKs(max) as shown in Figures 3 and 4. The Tsp dependence of logKs(max) is very similar to that of the crystalline relaxation behavior of which the mechanical crystalline relaxation mechanisms were extensively studied for a single crystal mat and a spherulitic film of high density polyethylene(PE)[ 10-13]. The lateral attractive force between PE chains in the crystal lattice is fairly weaker compared with the covalent bonding force along a PE chain. Therefore, the rotational oscillation about a PE chain readily occurs across the intermolecular energy barrier in a higher temperature range at which the oscillations of neighboring chains are incoherent. This causes an increase of anharmonicity for the intermolecular potential energy, which was confirmed by an increase in Griineisen constant evaluated from the pressure dependence of sound velocity and compressibility [ 14,24]. Therefore, a remarkable increase in anharmonicity on intermolecular potential energy causes an increase in viscous contribution to the viscoelastic characteristics in a crystalline region and also, an apparent change on the thermal expansion of the lattice spacing. This kind of viscoelastic characteristics in a crystalline r e , o n was designated the crystalline relaxation behavior. Consequently, it is reasonable to conclude from Figures 3, 4 and 8 that the viscoelastic crystalline relaxation behaviors were newly confirmed even in the crystalline stearic and palmitic acid monolayers.

l"m ( C 1 6 ) ~

0.425

E 0.420 ~

18)

T

-o 0.415

0.410

T(zc 1C181

28o

1. 9Stearic Acid (C18) T~c(C16) 2_oPalmitic Acid (C16)

2~o

36o Tsp/K

3io

3~o

Fig. 8. The Tsp dependences of the (10) spacing of the stearic and the palmitic acid monolayers.

Table 1. Melting temperature, Tm, and crystalline relaxation temperature, Totc, of fatty acid monolayers on the water surface. Fatty acid Myristic acid (C 14) Palmitic acid (C16) Stearic acid (C18) Arachidic acid (C20)

Ta c / K 291 298 308

Tm / K 278 301 317 331

Figure 7 shows the Tso dependence of the limiting area for the stearic acid monolayer. The limiting area was obtained -from the extrapolation of the z~-A isotherm to z~=0, as shown in Figure. 1. The limiting area vs. Tso curve exhibited a slight increase at around 298 K and a remarkable one at about 317 K. These temperature ranges well corresponded to Tac and Tm for the stearic acid monolayer on the water surface, respectively, as shown in Figures. 3, 6 and 8. It may be considered that increases of limiting area at around 298 and 317 K occur in accordance with increases in the thermal expansion coefficient of the crystal lattice and also, in the molecular occupied area due to the solid-liquid phase transition, respectively. Therefore, the Ts p dependence of the limiting area of Figure. 7 is a supplementary support for determination of Tc~c and Tm for the monolayer on the water surface on the basis of the Tsp dependences of logKs(max) and ED pattern as shown in Figure. 3. Then, it seems reasonable to conclude that the counter-part investigation on the Tsp dependence of logKs(ma.x) and also, ED pattern for the monolayer which was transferred onto a hydrophilic substrate is quite reliable to determine the magnitudes of Tac and Tm of the monolayer on the water surface. 2. M O R P H O L O G I C A L AND S T R U C T U R A L S T U D I E S O F C R Y S T A L L I N E AND A M O R P H O U S M O N O L A Y E R S O N T H E W A T E R S U R F A C E 2.1. M o n o l a y e r p r e p a r a t io n a n d electron m i c r o s c o p i c o b s e r v a t i o n . Myristic, palmitic and stearic acids(chromatographic reference quality) were used without further purification. Benzene with spectroscopic quality was used as solvent. Concentrations of benzene solutions for myristic, palmitic and stearic acids were 4.4, 4.0 and 3.5x10 -3 mol-1-1, respectively. Subphase water was purified by the Milli-Q| system. The trough dimensioins were 502 x 150 x 5 mm. Every monolayer was compressed to a given surface pressure at an area change rate of 2x10 -3 nm2-molecule-1- sec -1. Tsp was varied in a temperature range of 283303 K. Table 1 shows Tms and Tacs of the fatty acid monolayers on the water surface[2,3].The authors reported that fatty acid monolayers on the water surface could be transferred onto a hydrophilic substrate by the upward drawing method without any change of crystal system[3,15| as mentioned above. The hydrophilic SiO substrate(water contact angle:0--30 ~ was prepared by vapor-deposition of SiO onto CaF2 plate for infrared measurements. Also the surface of hydrophilic SiO substrate was confirmed to be smooth and amorphous, based on morphological and ED studies, respectively. Therefore, the crystallographic study of the monolayer and the surface characteristics of the substrate indicate that the hydrophilic SiO substrate is suitable for the electron microscopic morphological and structural investigations on the monolayer on the water surface. Then, the monolayer was transferred onto the hydrophilic SiO substrate by the upward drawing method[3,15] at a transfer rate of 60 mm-min- l at various Tsps and pressures, except at the surface pressure of 0 mN-m -1. The monolayer at 0 mN-m -~ can be transferred only by a horizontal lifting method[ 161. Bright field electron micrographs and ED patterns were taken with an electron microscope, which was operated at an acceleration voltage of 75 kV and a beam current of 2.5 ~A. The

electron microscopic observations were carried out at a corresponding temperature to Tsp at which the monolayer was prepared. Pt-carbon was vapor-deposited onto the monolayer samples with a shadowing angle of 23 ~ for the bright field image observations. In order to investigate the surface structure of the monolayers by the decoration method[ 17,18], gold was evaporated onto the monolayer samples.

2.2. Electron microscopic studies of aggregation structure in monolayer. Figure 9 shows the ~-A isotherm for the palmitic acid monolayer spread on the water surface at Tsp of 283 K below Tm of 301 K(and also Tac of 291 K), as well as the bright field images and ED patterns of the monolayers which were transferred onto the hydrophilic SiO substrate at surface pressures of 0, 20 and 30 mN-m-1. The ~-A isotherm showed a sharp rise of surface pressure with decreasing surface area without any appearance of a plateau region. The limiting area and collapse pressure for the monolayer were 0.23 nm2-molecule -l and 54 mN-m -1, respectively. At 0 mN-m -l, many isolated domains were observed in a bright field image. The bright field image of the monolayer transferred at 20 mN. m -1 exhibited a fairly uniform, smooth and continuous morphology. Also, the bright field image at 30 mN.m -1 showed heterogeneous aggregation which was composed of partially patched domains due to the collapse of the monolayer. This partial collapse at a lower surface pressure than the collapsed pressure generally defined as the peak of the ~t-A isotherm were also reported for the barium stearate monolayer[ 19]. The ED pattern of the palmitic acid monolayer transferred at 0 mN-m-1 exhibited a crystalline Debye ring. Therefore, the monolayer domains observed in the bright field image at 0 mN-m -1 are in a crystalline state and also, orient randomly their crystallographic axes. On the other hand, the ED patterns at 20 and 30 mN-m -l showed crystalline hexagonal spots. This indicates that crystalline domains in a fairly wide area(corresponding to an electron beam diameter of 2 lam) were fused or recrystallized at the interfacial lateral surfaces owing to sintering behavior caused by higher surface pressure, resulting in formation of a large area monodomain monolayer. It has been confirmed from synchrotron X-ray diffraction studies that lead stearate[20] and arachidic acid[5] monolayers were in a crystalline state at every surface pressure as well as 0 mN-m -1 at Tsp of 293 K below Tm of the monolayer on the water surface. Therefore, the bright field images and the ED patterns in Figure 9 reveal that two-dimensional crystalline domains were grown right after spreading a solution on the water surface and also, gathered during a process of surface compression. Finally, all crystalline domains in a fairly wide area in comparison with the

Fig. 9. ~t-A isotherm, electron micrographs, and ED patterns of palmitic acid monolayer at Tsp of 283 K.

Fig. 10. ~t-A isotherm, electron micrographs, and ED patterns of palmitic acid monolayer at Tsp of 303 K. electron beam diameter of 2 Iam form crystallographically homogeneous single crystal. At a higher surface pressure of 30 mN-m -l, it can be expected that sintering corresponding to fusion or recrystallization at the interface among the monolayer domains proceeds, though the sintering behavior or mechanism are under investigation at a present time. The monolayer as shown in Figure 9 was designated the crystalline monolayer. On the other hand, at Tsp of 293 K above Tac and below Tm of palmitic acid monolayer, a surface pressure-induced crystallographic orientation was not observed even at higher surface pressure[2,3]. The crystalline relaxation process corresponds to a change from elastic to viscoelastic characteristics in a crystalline phase due to a considerable contribution of anharmonic thermal molecular vibration. At Tsp above Tc~c, sintering among crystalline domains was prevented by the anharmonic thermal molecular vibration, resulting in random orientation of crystalline domains along their crystallographic axes. Figure 10 shows the zt-A isotherm for the palmitic acid monolayer spread on the water surface at Tsp of 303 K above Tm of the monolayer as well as the bright field images and the ED patterns of the monolayers being transferred on the hydrophilic SiO substrate at surface pressures of 0, 5, 20 and 30 mN-m -1. A plateau region of the ~t-A isotherm was observed in a range of 0.35-0.50 nm2-molecule -1. The limiting area and collapse pressure for the monolayer were 0.28 nm2-molecule -1 and 38.0 mN-m -l, respectively. These values of the limiting area and the collapse pressure were larger and smaller than those for the crystalline palmitic acid monolayer shown in Figure 9, respectively. The bright field image and the ED pattern of the monolayer even at the surface pressure of 0 mN-m -1 showed island fragments and an amorphous halo, respectively. The bright field images clearly show the gathering process of these domains and the appearance of partial collapse with an increase in surface pressure, whereas each ED pattern remained an amorphous halo. Then, Figure 10 indicates that amorphous domains formed right after spreading a solution on the water surface aggregate into a large amorphous monolayer during compression. Even though the monolayer was compressed up to the collapsing pressure at Tsp above Tm, the monolayer was still in an amorphous state without a pressure-induced crystallization. This type of monolayer was designated the amorphous monolayer. Next, the morphological and structural variations of the monolayers on the water surface were investigated by using two kinds of fatty acids with different Tms, in order to confirm the Tsp dependence of the morphology and structure for the monolayer, as discussed in Figures 9

10

Fig. 11. rt-A isotherms, electron micrographs, and ED patterns of (a) stearic and (b) myristic acid monolayers at Tsp of 293 K. and 10. Figure 11(a) shows the ~t-A isotherm for the stearic acid monolayer spread on the water surface at Tsp of 293 K below Tm. The bright field images and the ED patterns of the stearic acid monolayer on the hydrophilic SiO substrates at 0, 24 and 40 mN-m -1 were also shown in Figure 1 l(a). A plateau region on the :t-A isotherm was not observed for the stearic acid monolayer. The ED pattern of the monolayer at 0 mN-m -1 was a crystalline Debye ring and those at 24 and 40 mN-m -l crystalline hexagonal spots. The bright field images exhibited the gathering process of crystalline domains formed right after spreading a solution. This molecular aggregation of Figure 1 l(a) is similar to that of crystalline palmitic acid monolayer, shown in Figure 9. Figure 1 l(b), also, shows the ~t-A isotherm for the myristic acid monolayer formed at Tsp above Tm, as well as the bright field image and the ED patterns. In this case, a distinct plateau region on the ~t-A isotherm was observed in an area range of 0.24-0.32 nm2- molecule -l and the ED pattern at 0, 14, 18 and 25 mN-m -l were amorphous halos, in a similar fashion to the amorphous monolayer of palmitic acid shown in Figure 10. Also, the bright field images showed the gathering process of amorphous domains formed right after spreading a solution. This molecular ag~egation of Figure 11(b) is also similar to that of the amorphous palmitic acid monolayer shown in Figure 10.

11 a)

(--: h~~"-~~1' t'

Crystalline

state

(Tsp
Amorphous state (Tsp>Tm)

•

A Fig. 12. Schematic representation for the molecular aggregating process of (a) crystalline monolayer and (b) amorphous monolayer. It is, therefore, clearly concluded from Figures 9-11 that in the case of conventional fatty acids such as myristic, palmitic, stearic and so on, the crystalline or amorphous phase of monolayer completely depends on the relative magnitude of Tsp to Tm of the monolayer, being independent of the magnitude of surface pressure. The fatty acid monolayers do not show any pressure-induced crystallization during compression of the monolayer on the water surface. The crystalline and amorphous monolayers are schematically summarized in Figure 12.

2.3. Surface aggregation structure of the monolayer based on gold decoration technique Next, we investigated the surface ag~egatioin structure of the crystalline and amorphous monolayers by using a gold decoration technique[ 17,18]. The size of gold colloidal particles on a material surface depends on the surface roughness, that is, the gold particles aggregate to be larger as the material surface is smoother or flatter. Figure 13(a) shows the gold decoration patterns of the crystalline monolayer at the surface pressures of 0 and 24 mN-m- ~ and also, the schematic representation of the molecular ag~egation state, including the average dimension of monolayer domain or mosaic structure which was evaluated from the number of gold particle in a unit area. Solid and dashed lines represented in Figure 13 indicate the domain boundary and the smooth surface r e . o n one in domain, respectively. The gold-decorated pattern at 0 mN- m-1 showed the two distinct regions, the surbstrate and the two-dimensional crystalline domains of which the average colloidal particle size was smaller than that on the SiO substrate. This indicates that the monolayer surface at 0 mN-m- 1 or low surface pressures was rougher than the substrate surface. At higher surface pressure of 24 mN-m -l, a distinct boundary between the crystalline domains was not observed on the gold-decorated micrograph, resulting in the apparently homogeneous monolayer being formed by sintering at the interfacial lateral region of crystalline domains. This gold-decorated micrograph corresponds to the appearance of homogeneous monolayer as speculated from the bright field image shown in Figures 9 and 1 l(a). The average dimension of the homogeneous monolayer block mosaic structure at higher

12

surface pressure was about 20 nm, which became larger in comparison with that of 15 nm at lower surface pressure. This may arise from the progress in sintering at the interface between lateral surfaces of crystalline domains and/or in molecular packing with an increase in surface pressure. Figure 13(b) shows the gold decoration patterns of the amorphous monolayer at different surface pressures of 0, 14 and 18 mN-m -1 and the schematic representation of the molecular aggregation state. The low density region of gold particles was observed in the case of low surface pressure. Then, this indicates that the fairly flat aggregate in comparison with the roughness of the SiO substrate may correspond to the amorphous domains in a gel state. It is reasonable to say from the gold particle density that there exist two kinds of amorphous aggregates whose surface are rougher and smoother than the substrate surface in a plateau region. The rougher surface region may be consisted of molecules which straightforwardly aligned on the water surface as schematically represented by the lower ag~egation model. Since molecules are packed densely even though in an amorphous state at fairly higher surface pressure(a straight-up region on the zt-A isotherm), the molecular surface became rougher than that at lower surface pressure due to the pseudo phase transition from gel-like to solid-like, as schematically shown by the molecular aggregation model in the lower part of Figure 13(b). In comparison with the gold decoration patterns for the crystalline and amorphous monolayers at fairly high surface pressure as shown in Figure 13(a) and i3(b), it can be concluded that the crystalline and amorphous monolayers are in a similar molecular aggregation state. Since it seems a reasonable assumption that the amorphous monoiayer is in a more homogeneous state

Fig. 13. Gold decolation patterns of (a) crystalline monolayer and (b) amorphous monolayer.

13

rather than the crystalline monolayer(in other words, more defect-diminished state), it is expected that a defect-diminished crystalline monolayer can be prepared by crystallization of the amorphous monolayer. We have reported that a large two-dimensional single crystal with a small number of defect can be prepared by cooling the amorphous monolayer down to a temperature range below Tm of the monolayer and by further crystallizing the monolayer for a long time at that temperature[21,22].

2 . 4 . Infrared spectroscopic analyses on molecular conformation in crystalline and amorphous monolayers Transmission infrared spectra were recorded on a Nicolet model 510 Fourier transform infrared, FT-IR spectrophotometer equipped with a liquid-nitrogen-cooled HgCdTe MCT detector. Three-thousand interferograms were coadded, apodized with the Happ-Genzel function, and Fourier-transformed with one level of zero filling to yield spectra with a resolution of 2 cm -1. Peak frequencies were determined with an error of _+0.05 cm -1 by statistical procedure using a spline function. As a reference, stearic acid single crystal with all trans conformation was grown on CaF2 plate from a n-hexane solution of stearic acid at a concentration of 4.0x10 -1 mol-1-1 through isothermal crystallization at 308 K[23 ]. As another reference, a CC14 solution of myristic acid in a molecular dispersion state was prepared at a concentration of 4.0x 10-4 mol. 1-1. A remarkable increase in the frequency at the infrared absorption maximum for the CH 2 asymmetric and symmetric stretching bands at about 2920 and 2850 cm -1, respectively, was observed at the phase transition from crystalline state to liquid crystalline one for lipids in bilayers[25]. Hence, this frequency shift have been used to determine thermodynamic phase in various hydrocarbon systems[26-28]. Snyder et al evaluated the force constant of CH stretching which was in relation to trans or gauche conformation of adjoining C-C bonds in n-alkanes on the basis of the normal coordinate analysis. According to Snyder's analysis, a frequency increase in the CH 2 asymmetric and symmetric stretching bands was due to an increase of gauche conformation along the hydrocarbon chain[29,30]. Therefore, the molecular conformational change in the crystalline and amorphous monolayers during a process of surface compression can be investigated on the basis of the frequency shift of CH 2 stretching band. Figure 14(a) shows the n-A isotherm and the frequency maximum of the CH 2 asymmetric stretching band for the crystalline stearic acid monolayer which was transferred onto the hydrophilic SiO substrate at different surface pressures and at Tsp of 293 K below Tm of the monolayer. The frequency maximum of the CH 2 asymmetric stretching bands for the stearic acid single crystal in a trans-zigzag conformation was 2917.1 cm -1. This value was slightly smaller compared with 2917.7 cm -1 measured at higher surface pressure for the crystalline stearic acid monolayer. Thus, the trans conformational order or fraction for hydrocarbon chains in the crystalline stearic acid monolayer at higher surface pressure is comparable to that in the stearic acid single crystal. The frequency maximum of the CH 2 asymmetric stretching band decreased very slightly with increasing surface pressure. It is, therefore, reasonable to consider that the fraction of trans conformation in the crystalline monolayer does not vary remarkably during compression. Figure 14(b) shows the n-A isotherm and the frequency maximum of the CH 2 asymmetric stretching band for the amorphous myristic acid monolayer which was transferred onto the SiO substrate at different surface pressures and at Tsp of 293 K above Tm of the monolayer. The frequency maximum of the CH,~ asymmetric stretching band at lower surface pressure was 2923.8 cm-l, while that of myristic acid molecules in a CCI 4 solution was 2927.3 cm-1. As mentioned above, the frequency maximum of the CH~ asymmetric stretching band for the trans conformation is 2917.1 cm -1. Therefore, the magnitude of the frequency maximum of the CHo asymmetric stretching band indicates that the conformational order of hydrocarbon chains in the amorphous myristic acid monolayer at lower surface pressure is quite similar to that for myristic acid molecules in a molecular dispersion state, rather than that for the crystalline monolayer. In other words, the gauche conformation is predominant in the case of the amorphous monolayer in a lower surface pressure region than that for the plateau one. The frequency maximum of the

14 CH 2 asymmetric stretching band for the amorphous monolayer strikingly decreased in a plateau r e , o n of the ~c-A isotherm with an increase in the surface pressure. This suggests a remarkable increase in the conformational order from gauche to trans in the amorphous monolayer during a process of surface compression at a plateau region of the ~c-A isotherm. This IR investigation agrees with the speculative conclusion based on the gold-decoration technique shown in the lower part of Figure 13(b). Furthermore, it is reasonably considered that appearance of a plateau r e , o n on the ~c-A isotherm may be strongly associated with dissipation of compression energy induced during a surface compression. Figure 12 shows the schematic representation of the aggregation structure of the crystalline and the amorphous fatty acid monolayers at different surface pressures, that were concluded on the basis of the bright field and gold decoration EM observations, ED study and IR investigation. Figure 12(a) shows the aggregation process of the crystalline monolayer at Tsp below Tm of the monolayer. There is no plateau r e , o n on the 7c-A isotherm. The isolated twodimensional crystalline domains are formed right after spreading a solution on the water surface. During compression on the water surface, these crystalline domains aggregate together concurrently with a little increase in the conformational order, resulting in the formation of morphologically homogeneous monolayer. A large-area homogeneous monolayer may be induced from lateral sintering or fusion at the interface among monolayer domains. Also, the aggregating process of the amorphous monolayer at Tsp above Tm of the monolayer is shown in Figure 12(b). There is a plateau region on the ~c-A isotherm. Many gel-type domains in which the molecules aggregate randomly on the water surface are formed in a low surface pressure

40

E

"7"

(a)CrystaUine monolayer

:=)

30

.

2918 ~E

.

X

~E

Z

~: 20 r-

% ~

>..

Sinale

Z LtJ

2917 D O i

1

o.2

LIJ riLL

i

o.2 A / nm2.molecule -1

0.26 2930

30 (b) Amorphous rnonolayer

"7

Solution" "7

2O

E

u

~r

2925~-

E

x

E

:E >-

f- lO

Z I.iJ

29zo8

fo

0.20

0.30

A / nm z. molecule-1

0.40

Fig. 14. Surface area dependences of surface pressure and frequency maximum of the CH2 asymmetric band for (a) crystalline stearic acid monolayer and (b) amorphous myristic acid monolayer.

15

region. With an increase in surface pressure, the amorphous domains aggregate concurrently with an apparent increase in the conformational order, such as the conformation change from trans to gauche form. Appearance of the plateau region on the ~-A isotherm may correspond to the conformational variation in the amorphous monolayer during surface compression. Finally, all molecules are considerably well aligned on the water surface at higher surface pressure, in spite of an amorphous state. Thus, the crystalline or amorphous structure of fatty acid monolayers on the water surface is fundamentally determined by the relative magnitude of Tsp to Tm of the monolayers. 3. T H E E F F E C T OF IONIC R E P U L S I O N A M O N G H Y D R O P H I L I C GROUPS ON T H E A G G R E G A T I O N S T R U C T U R E OF M O N O L A Y E R 3.1. Aggregation structure of arachildic acid monolayer in a dissociated state. Arachidic acid monolayers were prepared from a benzene solution on the water subphase of pH5.8(pure water) and 12.6(adjusted by addition of NaOH) at Tsp of 303 K below Tm(-328 K) of the monolayer [31 ]. The ionic dissociation state of hydrophilic group was estimated on the basis of the stretching vibrations of carbonyl and carboxylate groups by Fourier transforminfrared attenuated total reflection, FT-IR ATR measurements. 70 arachidic acid monolayers were transferred on germanium ATR prism, resulting in the formation of the multi-layered film. Transfer on the prism was carried out at surface pressures of 25 or 28 mN-m -l. Infrared absorption measurements revealed that almost carboxylic groups of arachidic acid molecules did not dissociate on the water subphase of pH5.8, whereas all carboxylic groups dissociated as carboxylate ions on the water subphase of pill2.6. Figures 15 (a) and (b) show the zt-A isotherms for the arachidic acid monolayers on the water surface of pH5.8(pure water) and of pill2.6 at Tsp of 303 K, respectively, as well as the

Fig. 15. ~t-A isotherms and ED patterns of arachidic acid monolayers at a Tsp of 303 K on the water subphase of pH 5.8 (a) and pH 12.6 (b).

16 ED patterns of the monolayers at several surface pressures. In the case of a neutral state of arachidic acid(pH5.8), the ~-A isotherm showed a sharp rise of surface pressure with decreasing surface area without any appearance of plateau region. The ED patterns at surface pressures of 0 and 25 mN-m -1 showed a crystalline arc and crystalline spot, respectively, indicating the formation of "the crystalline monolayer"[6,7]. Kjaer et al.[5] also reported from synchrotron X-ray diffraction studies that the arachidic acid monolayer on the pure water surface revealed the crystalline phase of the monolayer at various surface pressures. The change of the ED pattern from the crystalline arc to the crystalline spot suggests that crystalline domains were fused or recrystallized at the monolayer domains interface owing to sintering behavior caused by surface compression, resulting in the formation of larger two-dimensional crystalline domains[2,3]. In the case of a dissociated state of arachidic acid on the water subphase of pill 2.6 at Tsp of 303 K, a plateau region of the ~-A isotherm was observed in a range of 0.3-0.5 nm2-molecule -1. The ED pattern at5 mN-m -z showed an amorphous halo, whereas those at 12 and 28 mN-m -1 exhibited crystalline arc or spot. Therefore, Figure 15(b) indicates that the arachidicacid monolayer is crystallized by compression on the water surface of pill2.6. This type of monolayer has been classified as "the compressing crystallized monolayer"[32,33]. It is clearly concluded from Figure 15 (a) and (b) that amphiphile molecules form the crystalline monolayer and the compressing crystallized monolayer at Tsp below Tm in the cases of a neutral state(maybe the low degree of ionic dissociation) and a highly dissociated state of polar groups, respectively. Figure 16 shows the classification based on the aggregation structure of monolayers with respect to thermal(Tsp, Totc, Tm) and chemical(the degree of ionic dissociation of hydrophilic group) factors. This figure is divided into the four quadrants by the two axes of Tsp and the repulsive force among hydrophilic groups. In the case of amphiphiles with nonionic hydrophilic group(corresponding to the third and fourth quadrants), isolated domains grown fight after spreading a solution on the water surface are gathered to be a morphologically homogeneous monolayer by compression. Then, at Tsp below Tm (the third quadrant), the monolayer is in a crystalline phase which is designated "the crystalline monolayer". The crystalline monolayer is further classified into the two types: crystalline domains are assembled as a large homogeneous crystalline monolayer due to a surface compression-induced sintering at interfacial region among monolayer domains at Tsp below Tcxcand also, crystalline domains are gathered without any si:~ecial orientation among domains above Tac[3 ]. The crystalline relaxation phenomena Ionic repulsion among hydrophilic groups

quadrant l Crystalline Monolayer, Compressing Cry,stallized Monolayer

12nd

"

'i I

A

/',

: ,

A

TK.-KH :

I 3rd quadrant

'i

A

i

' "Tsp

To~ i

,i

A

A

~

]1st quadrant, ]

Amorphous Monolayer

!

J A

j

A

A

, Fusing-oriented .RandomlyAssembled Crystalline Monolayer Amorphous Monolayer 14th quad'rant I

Fig. 16. Classification of the aggregation structure of a monolayer on the water surface.

17

1. 2CnSNa (Anionic Amphiphile) 0 II

CH3(CH2)n_IOC(~H2 C H3(C H2)n. iOICICH--SO~ Na+ O n=12, Tc=293 K (wet) n=14, Tc=317 K (wet) n=16, Tc=328 K (wet) 2. PEI (Cationic Polymer) H2N (CH2C H 2~)x--(CH2C H2NH)~-y

CaeCneN( Mw=40,000-50,000 Amino Groups: primary:secondary:tert iary=1:2: I N + /N=75% (pH=3.2)

Fig. 17. Chemical structures of anionic amphiphile sodium 1,2-bis((tetradecyloxy)carbonyl) ethane- 1-sulfonate (2C 14SNa) and poly(ethylenimine)(PEI). correspond to a change from elastic to viscoelastic characteristics in a crystalline phase due to a remarkable increase from anharmonic thermal molecular vibration[ 10-13]. At Tsp above Tm(the fourth quadrant), the monolayer is in an amorphous phase which is designated "the amorphous monolayer". In the case of amphiphiles with ionic hydrophilic group(the first and second quadrants), a distinct domain structure is not formed at lower surface pressure owing to an electrostatic repulsion among polar head groups. At Tsp below Tm (the second quadrant), amphiphile molecules form a large homogeneous crystallized monolayer(Tsp
18 observations were carried out at a corresponding to Tsp at which the monolayer was prepared. For the observation of bright field image, Pt-carbon was vapor-deposited onto the monolayer samples with a shadowing angle of 23 ~ Figure 18 shows the n-A isotherm for the anionic amphiphile(2C14SNa) monolayer on the pure water surface, as well as the bright field images and ED patterns of the monolayers at surface pressures of 0, 10, 17, 22 and 35 mN-m -l. A plateau region on the ~t-A isotherm was observed between 17 and 22 mN-m-1. At 0 mN. m -1, the bright field image and the ED pattern of the monolayer showed a homogeneous morphology and an amorphous halo, respectively. At this pressure, the monolayer may be in a fluid state and not form the distinct domain structure because an electrostatic repulsion among the hydrophilic groups of 2C14SNa prevents the ag~egation of the molecules. The ED pattern at 17 mN-m-1 at the be~nning of the plateau zone exhibited crystalline arc. This result indicates that 2C14SNa monomeric monolayer is crystallized by the compression on the water surface(a pressure-induced crystallization). Therefore, it is apparent that a considerably high surface pressure is required for the crystallization of 2C 14SNa monomeric monolayer because of the strong repulsion among the hydrophilic groups of 2C 14SNa. The ED pattern changed to a sharper hexagonal spot along an azimuthal direction in a surface pressure range from 22 mN-m-1 at the end of the plateau zone to 35 mN.m -1, indicating that crystalline domains in the fairly wide area orient in the crystallographically same direction at a higher surface pressure. The rearrangement of crystalline domains aligning along the crystallographically same direction may associate with the sintering behavior caused by the fusion or recrystallization at the boundary surfaces among crystalline domains and the pressure-induced rearrangement acceleratedly proceeds owing to the compression beyond the plateau re,on[3]. Figure 19 shows the schematic representation of the aggregation structure of the 2C14SNa monolayer during the compression. At lower surface pressure, in other words, larger occupied area per one molecule, the monolayer was formed in an amorphous state because the free energy for the monolayer in an amorphous state was lower than that for the monolayer in a crystalline state owing to the fairly strong repulsion among negative charges of hydrophilic groups in anionic amphiphile. When the free energy for the monolayer in an amorphous state increased by the compression, finally the crystallization occurs. The plateau region may be a coexistence region of crystalline and amorphous phases at which the two-dimensional crystallization proceeds. The monolayer formed by a pressure-induced crystallization is desi~maated as "the compressing crystallized monolayer".

Fig. 18. ~t-A isotherm for the anionic amphiphile(2Cl4SNa) monolayer on the pure water surface, and the bright field images and ED patterns of the monolayers.

19 Figure 20 shows the ~-A isotherm for the 2C14SNa amphiphile spread on the cationic polymer(PEI) solution subphase, as well as the bright field images and ED patterns of the monolayers at 0, 10 and 30 mN-m -l. The :t-A isotherm showed a rapid rise of surface pressure with decreasing surface area without any appearance of a plateau on the isotherm. Both crystalline Debye ring and clear domain structure were observed even at 0 mN-m -l in the ED pattern and bright field image, respectively. This result indicates that the 2C ]4S/PEI complex monolayer can be crystallized without compression owing to a weakened repulsion among the negative hydrophilic groups by neutralizing effect of cation in PEI or the bridging effect between hydrophilic group of amphiphile and cation group of PEI. During a compressing process of the polyion complex monolayer, the bright field images of the monolayer represented the aggregating process of crystalline domains, whereas ED pattern changed into crystalline spots. These results indicate that two-dimensional crystalline domains grown with random crystallographic orientation fight after spreading a solution on the PEI solution subphase are rearranged to orient in the crystallographically equivalent direction owing to the pressureinduced fusion or sintering behavior at the boundaries among crystalline domains. Figure 21 shows the schematic representation for the aggregation structure of the 2C14S/PEI complex monolayer during the compression. The free energy for the monolayer in an amorphous state is higher than that for the monolayer in a crystalline state at 0 mN-m-1 Compressing

Crystallized

Monolayer

A Fig. 19. Schematic representation for the aggregation structure of 2C 14SNa monolayer during the compression. 2C14SNa/PEI Tsp=293K

.:

I 3ot- ,".

• 40

200nm

~....

10 0/

v0

I .....

0.20

O~

"/

0.42nm

/ I k

I

" -'l

I

1

I

0.40 0.60 0.80 1.00 1.20 1.40 A/nm2.motecule -1

Fig. 20. ~t-A isotherm for the 2C14SNa amphiphile spread on cationic polymer(PEl) solution subphase, and the bright field images and ED patterns of the monolayers at 0, 10, and 30 mN.m-1.

20

Compressing Oriented Crystalline Monolayer [~t

2C14S/PE!

N Fig. 21. Schematic representation for the aggregation structure of 2C14S/PEI complex monolayer during the compression. owing to the weakened repulsion among the hydrophilic groups by the neutralizing or the bridging effects by cationic group in PEI. This results in the growth of crystalline domains right after spreading a solution. During the compression up to higher surface pressures, the sintering behavior at the interfaces among crystalline domains proceeds to form larger two-dimensional crystalline domains oriented in the crystallographically equivalent direction. This molecular aggregation structure was designated "the fusing-oriented crystalline monolayer". 5. C O N S T R U C T I O N O F D E F E C T - D I M I N I S H E D C R Y S T A L L I N E MONOLAYER.

5.1. Construction of high-quality monolayer by crystallization on the water surface. A benzene solution of stearic acid of 3.5 x 10-3 mol'L -1 was prepared as a spreading solution. Scheme 1 shows the preparation process for crystallized monolayer. The amorphous monolayer was prepared on the pure water surface at Tsp of 320 K above Tm of 317 K [2,3 ] and then, was compressed to the surface pressure of 15 mN-m -1. With maintaining the surface pressure at 15 mN-m -l , Tsp was reduced to the temperature of 303 K below Tm at the speed of 40 K-h -! and then, the monolayer was further crystallized for 3 h on the water surface. This monolayer was again compressed to the surface pressure of 26 mN-m -1, at which the stearic acid monolayer was confirmed to be morphologically homogeneous [6,7]. The monolayer constructed by this method was designated the crystallized monolayer. In order to compare the distortion and continuity of the crystallized monolayer with those of the crystalline monolayer, the crystalline monolayer was prepared at Tsp of 293 K below Tm and 26 mN-m -1. These two kinds of monolayer were transferred onto collodion-covered electron microscope grids by a vertical dipping method. The transfer ratio was unity. ED patterns were taken with a transmission electron microscope. Crystallographical continuity, Llat and crystallographical distortion in a direction along the monolayer surface, Dlat were evaluated by a modified single line method based on the Fourier analysis of ED profiles [9,37,38]. Table 2 shows the values of Dlat and Llat for the crystalline and crystallized monolayers of stearic acid. The magnitude of Dlat and Llat for the crystallized monolayer was much smaller and larger than those for the crystalline monolayer, respectively. Though the magnitude of Dlat for the crystallized monolayer is comparable with that of high density PE single crystal or spherulite, Llat of the crystallized monolayer is 5-8 times as large as that of high density PE [ 11 ]. Therefore, it is clear that the monolayer with crystallographical sintering (fusion) at the

21

Stearic acid C18 (Tin =317K) 320K(>Tm)I ][ I c~176 40K/hr J~ T(
compression i T ]transfer onto ~26mN/m ] substrate ~

T[

crystallized monolayer

Scheme 1. Flow sheet describing the construction for crystallized monolayer.

Table 2. Crystallo~aphical distortion and continuity for crystalIine and crystallized monolayer.

Crystallographical distortion, Dsat / %

Crystallographical continuity, Llat /nm

Crystalline monolayer (26 mN/m, 293 K)

4.9

6.4

Cooling-crystallized monolayer (15 mN/m, 303 K)

1.5

1.2x10 2

Polyethylene ( Single crystal )

< 2.0

.

.

.

.

.

.

30 ~ 60 .

.

.

.

boundary surfaces among crystallites did not occur by the compression, which suppressed the formation of the structural defect-diminished monolayer. In conclusion, crystallization of the amorphous monolayer on the water surface is remarkably effective to construct a twodimensional crystallized monolayer with large crystalline size and small fraction of crystalline defects.

5.2. Construction of defect-diminished crystalline monolayer by multi-step creep method. A benzene solution of stearic acid was spread on the pure water surface at Tsp of 293 K. Since Tsp is below TeLc and Tm of the stearic acid monolayer (T~c=298 K, Tm=317 K), stearic acid molecules form a fusing-oriented crystalline monolayer [3]. Scheme 2 shows the preparation process of the stearic acid monolayer by the multi-step creep method. The stearic acid monolayer was compressed to a surface pressure of l0 mN-m -1. The variation of the monolayer area was measured, while maintaining the monolayer at the surface pressure of 10 mN-m -j. Then, the monolayer was further compressed to 13 mN-m -l and again, maintained the monolayer at 13 mN-m -] to measure its area-creep behavior. By stepwise compression, the monolayer was finally compressed to 23 mN-m -1. The monolayer on the water surface was transferred onto a hydrophilic SiO substrate for TEM observation and a freshly cleaved mica for atomic force microscope (AFM) observation, where the monolayer was transferred without any change of crystallographic structure [3]. The bright field image and ED pattern of the monolayer were taken with TEM and also, the AFM image of the monolayer was obtained with a SFA300 (Seiko Instruments, Inc.). A digital filtering treatment for the Fourier-transformed image was carried out in order to reduce the noise component.

22

Spreading

monolayer on the

water surface ~ Compression (120 m m 2- sec -1) Area-creep (10 m N - m "1 ) ~ Compression (6 m m 2o sec "1) Area-creep (13 m N 9m "1 ) ~ Compression (6 mm2- sec "1) Area-creep (15 m N - m "1 ) ~ Compression (6 m m 2o sec -1) Area-creep (17 m N 9m "1 ) Compression (6 mm 2osec -1) Area-creep (18.5 m N 9m 1 ) Compression (6 mm 2- sec -1) Area-creep (20 m N . m 1 ) ~ Compression (6 rnm2o sec-1) Area-creep (21.5 m N 9m "1 ) ~ Compression (6 m m 2 ~ sec-1) Area-creep (23 m N o m "1 ) /

Transferred ofnto hydrophilic SiO substrate

Scheme 2. Flow sheet describing the preparation of monolayer by multi-step creep method. Figure 22 shows the time dependence of molecular occupied area in the stearic acid crystalline monolayer at various surface pressures during the multi-step creep. The molecular occupied area in the monolayer at each surface pressure became fairly constant after long time area-creep. Therefore, it seems from Figure 22 that the area-creep of the monolayer at each surface pressure was almost finished. Figure 23 shows the area-creep behaviors and the bright field images of the stearic acid crystalline monolayers which were prepared at 23 mN-m -1 after the multi-step creep method (1) and a continuous compression method (2). The continuous compression method is the conventional one to prepare the monolayer on the water surface, the compression of monolayer to the desired surface pressure at a constant rate. In the case of the continuous compression method, the surface area of the monolayer decreased remarkably with creep time and the monolayer morphology was heterogeneous after the area-creep. Hence, this remarkable decrement of the surface area might be ascribed to the localized collapse of the monolayer. In contrast, the surface area of monolayer which was prepared by the multi-step creep method decreased very slightly with creep time and remained almost constant after long time area-creep. The magnitude of surface area of the monolayer after the multi-step creep was comparable to that of the calculated surface area of the monolayer on the basis of its ED profile, indicating the closed packing of molecules in the monolayer. Further, the electron micrographs of the monolayer which was prepared by the multi-step creep method exhibited that the monolayer was still morphologically homogeneous after long time area-creep at 23 mN-m ~ Thus, the multi-step creep provides the crystalline monolayer which is not collapsed even by the long time area-creep at higher surface pressure. Table 3 shows the crystallographical regularity, that is, crystallographical distortion (Dlat) and continuity (Liar), of the monolayer compressed to 23 mN-m -l by the multi-step creep method and the continuous compression method. Dlat and Llat were evaluated by a single line method based on the Fourier analysis of ED profiles [9,37,38]. The magnitudes of Dlat and Llat of the monolayer which was prepared by the multi-step creep method were smaller and larger than those of the monolayer being prepared by the continuous compression method, respectively. Further, the crystallographical regularity of the monolayer by the multi-step creep method was close to that of polyethylene single crystal. Thus, the crystallographical regularity of the crystalline monolayer being prepared by the multi-step creep method was remarkably progressed, compared with that of the monolayer without the area-creep process.

23

Fig. 23. Area-creep behaviors and bright field images of stearic acid crystalline monolayer prepared by multi-step creep method and continuous compression method. Figure 24 shows a filtered AFM image of the stearic acid crystalline monolayer which was prepared by the multi-step creep method with a scan area of 20 x 20 nm 2. A higher region in the AFM image represents an individual methyl group of stearic acid molecule, because the hydrophilic part of stearic acid molecule in the monolayer on a mica surface was oriented toward air by transferring the monolayer onto a hydrophilic mica surface by a vertical dipping method. The AFM image exhibits that stearic acid molecules are regularly arranged with a hexagonal array. The regularly periodic hexagonal array was extended over the scan area of 20 x 20 nm- which was comparable to the magnitude of crystallographical continuity shown in Table 3. Though scanning was done repeatedly, the stearic acid crystalline monolayer prepared by the multi-step creep method was not apparently damaged by a tip. On the other hand, the monolayer which was prepared by the continuous compression method was collapsed by a tip on scanning. This collapse may be caused by the stress concentration at vacancies among crystalline domains in the monolayer which was prepared by the continuous compression method. Thus, the monolayer prepared by the multi-step creep method was mechanically stable without stress concentration because the monolayer was fully relaxed during its preparation process. In conclusion, the multi-step creep provides the crystalline monolayer which was remarkably progressed in the crystallographical regularity, compared with the monolayer

24

Table 3.Crystallographical distortion and continuity of stearic acid monolayer prepared by multi-step creep method and continuous compression method. Crystallographical distortion, Dtat / %

Crystallographical continuity, Llat / nm

5.0

6.2

Multi-step creep method

3.1

22.9

Polyethylene ( Single crystal )

< 2.0

30 ~ 60

Preparation method ( Stearic acid Tsp = 293 K) Continuous

compression

method

Fig. 24. A filtered AFM image of a stearic acid monolayer on a scan area of 20x20 nm 2. The stearic acid molecules were regularly arranged in a hexagonal array over 20x20 nm2.

without area-creep. Also, the monolayer after multi-step creep was morphologically homogeneous and stable even at higher surface pressures, while the monolayer prepared by the continuous compression method was easily collapsed. Therefore, the multi-step creep method is useful for construction of the defect-diminished monolayer. 6. D I R E C T O B S E R V A T I O N O F M O L E C U L A R A R R A N G E M E N T S IN F A T T Y ACID M O N O L A Y E R S W I T H AN A T O M I C F O R C E M I C R O S C O P E .

6.1. Monolayer preparation and AFM imaging. Benzene solutions of lignoceric (CH3(CH2)22COOH) and stearic (CH3(CH2)16COOH) acids with concentrations of 1 x 10.3 and 3 x 10 .3 mol-L-l, respectively, were spread on the pure water surface at Tsp of 293 K. Since Tsp is below Tm of the lignoceric acid (Tm - 347 K) and the stearic acid (Tm = 317 K) monolayers [31 ] those monolayers are in a crystalline state. The subphase water was purified with the Milli-QII system. The lignoceric acid monolayer was prepared at a surface pressure of 5 mN-m-1 by a continuous compression at a rate of 1.7 x 10.3 nmZ-molecule-l-s -1. The stearic acid monolayer was prepared at 23 mN-m -! by the continuous compression method or the multi-step creep method [391. The multi-step creep method is a monolayer preparation method for which the monolayer is stepwisely compressed up to a

25

higher surface pressure by alternating the compression and area-creep. This procedure causes rearrangement of molecules in the crystalline monolayer and/or filling of the vacancies in the interfacial regions among crystalline monolayer domains, which releases the stress concentration in the monolayer. Therefore, the multi-step creep method provides a mechanically stable and defect-diminished monolayer. Each monolayer was transferred onto a freshly cleaved mica by a vertical dipping method. The transfer ratio for each monolayer was unity, which implies that a mica substrate is completely covered with each monolayer. The AFM images of the monolayers were obtained with a SFA300 (Seiko Instruments, Inc.) in air at room temperature, using a 0.8-~tm scanner and a silicon nitride tip on a cantilever with a small spring constant of 0.027 N-m -1. Images were recorded within 20 s in the "constant-height" mode; that is, feedback electronics and software were used to keep the sample height constant and measure the cantilever deflection. The applied force on imaDng was evaluated to be about 10-l~ N in an attractive force re,on, from the magnitude of cantilever deflection. This attractive force may not be the actual applied force between the tip and sample but an apparent composite force (applied force between the tip and sample, adhesion force by the water molecule at sample surface, etc.).

6.2 AFM observation of lignoceric acid crystalline monolayer prepared by a continuous compression method. Figure 25(a) shows a nonfiltered AFM image of the lignoceric acid crystalline monolayer, which was prepared at 5 mN-m -1 by the continuous compression method, on a scan area of 4 x 4 nm 2. The AFM image is given in a top-view presentation in which the brighter and darker portions correspond to higher and lower regions of the monolayer surface, respectively. Though scanning was done repeatedly, the monolayer was not damaged by a tip. However, a hole could be artificially pierced through the monolayer with a stronger applied force than 10-9 N. The hole was about 3 nm deep, being comparable with the calculated molecular length based on the CPK model, in other words, the thickness of the lignoceric acid monolayer. It is reasonable to expect that the brighter portion in the AFM image represents the single methyl group of the lignoceric acid molecule, because the hydrophobic part of the lignoceric acid molecule was oriented towared air by the vertical dipping method. The AFM image exhibits that lignoceric acid molecules are regularly arranged with a hexagonal array. Then, in order to clarify the molecular arrangement in the monolayer, a two-dimensional fast Fourier-transform (2D-FH ~)treatment was carried out.

Fig. 25. (a) Nonfiltered AFM image ofa lignoceric acid monolayer on a scan area of 4x4 nm2. The monolayer was prepared at a surface pressure of 5 mN-m -1 by a continuous compression method. This image was not changed even by repeated scanning. Note the periodic arrangement of the molecules with a hexagonal array. (b) 2D-FFF spectrum of (a). The numbers area Miller indices of (hk). The spectrum exhibited a hexagonal pattern with the (10) spacing of 0.43 nm.

26 Figure 25(b) shows the 2D-FFI" spectrum of the image shown in Figure 25(a). The bright spots in the 2D-PTT spectrum exhibit a hexagonal pattern with the (10) spacing of 0.43 nm. (10) represents the two-dimensional lattice plane with Miller indices of h = 1 and k = 0. The magnitude of (10) spacing which was evaluated from the 2D-FFI' spectrum agrees well with the spacing of 0.43 nm which was estimated from the ED pattern of the lignoceric acid monolayer [31], and also this magnitude is quite different from the 0.46 nm spacing of a mica substrate calculated from the AFM image (Figure 26). Moreover, the molecular occupied area of the lignoceric acid molecule in the monolayer which was evaluated from the AFM image and the ED pattern was 0.21 nm2-molecule-1. This magnitude was close to 0.25 nm2-molecule-1 being evaluated on the basis of ~-A isotherm measurements. Therefore, it is reasonable to conclude from Figures 25 and 26 that the brighter portion in the AFM image of Figure 25(a) represents the single methyl group of the lignoceric acid molecule in the monolayer and also that lignoceric acid molecules are regularly arranged with a hexagonal array. Figure 27 shows a nonfiltered AFM image of the lignoceric acid monolayer on a large area scan of 9 x 9 nm 2. A regularly periodic hexagonal army in the AFM image was extended over

Fig. 27. Nonfiltered AFM image of a lignoceric acid monolayer on a scan area of 9x9 nm2. Note a two-dimensional periodic structure with locally disordered molecular arrangements which is marked by a circle.

27 about 10 nm. The range of the periodic hexagonal array was comparable to the magnitude of crystallographical continuity which was evaluated by a single line method based on Fourier analysis of the ED profile of the lignoceric acid monolayer. As shown by the circle in Figure 27, the hexagonal array of lignoceric acid molecules was locally disordered. Thus, the molecular-resolution AFM image of the lignoceric acid monolayer was nondestructively obtained and exhibited a two-dimensional periodic structure with locally disordered molecular arrangements. The nondestructive AFM observation of the lignoceric acid monolayer was successful, as shown in Figures 25 and 27. However, it was impossible to obtain a molecular-resolution AFM image of the lignoceric acid monolayer which was continuously compressed up to higher surface pressures than 10 mN-m -1 because of the monolayer destruction during the AFM scan. In the case of the crystalline monolayer, the crystalline domains grown right after spreading a solution are assembled into a morphologically homogeneous monolayer during compression [3,6,7]. However, the vacancies at a molecular level may remain at the interfaces among the crystalline domains in the monolayer when the monolayer is prepared by the continuous compression method. These vacancies cause the stress concentration in the monolayer under a continuous compression. Since large stress locally concentrates in the monolayer at higher surface pressures, such a monolayer is mechanically unstable. This mechanical unstability may be the reason why the monolayer which was continuously compressed up to a high surface pressure, for example, 10 mN-m -~, was easily collapsed by the applied force on the AFM scan. This argument was justified in the nondestructive AFM image of the monolayer which was prepared at a fairly higher surface pressure, for example, 23 mN-m -1, by the multi-step creep method [39], because stress concentration in the monolayer was almost completely released.

6.3. AFM observation of stearic acid crystalline monolayer prepared by the multi-step creep method. Figure 28(a) shows a nonfiltered AFM image of the stearic acid crystalline monolayer which was prepared by the multi-step creep method with a scan area of 5 x 5 nm 2. Though scanning was done repeatedly on the stearic acid monolayer prepared at the high surface pressure of 23 mN-m -1 by the multi-step creep method, the monolayer was not damaged by the

Fig. 28. Nonfiltered AFM image of a stearic acid monolayer on a scan area of 5x5 nm2. The monolayer was prepared at 23 mN-m -1 by a multi-step creep method. This image was not changed even by repeated scanning. (b) A filtered AFM image of (a). For (b), a digital filtering treatment for the Fourier-transformed image was carried out by keeping only the spatial frequencies corresponding to the six spots of the Fourier-transformed image of (a). The stearic acid molecules were regularly arranged in a hexagonal array with the (10) spacing of 0.42 nm.

28 tip. On the other hand, the monolayer which was prepared at the same surface pressure by the continuous compression method was easily collapsed by the tip on scanning. The collapse may be caused by the stress concentration at vacancies among crystalline domains in the monolayer. In order to reduce the noise component in Figure 28(a), a digital filtering treatment for the Fourier-transformed image was carried out by keeping only the spatial frequencies corresponding to the six spots of the Fourier-transformed image. Figure 28(b) shows a filtered AFM image of the monolayer. A higher region in the AFM image represents the single methyl group of the stearic acid molecule in the monolayer. The AFM image of Figure 28(b) indicates that stearic acid molecules are regularly arranged with a hexagonal array with the (10) spacing of 0.42 nm. This magnitude agrees with the spacing of 0.42 nm which was estimated from the ED pattern of the stearic acid monolayer [3,7]. When the area scan was enlarged, the regularly periodic hexagonal array was extended over 20 x 20 nm 2, as shown in Figure 24. This magnitude was comparable to the magnitude of crystallographical continuity evaluated on the basis of the single line method. Further, no distinct molecular disordered region was observed in the scan area of 20 x 20 nm 2 at the AFM image. The larger crystallographical continuity and extended regular molecular arrangement of the stearic acid monolayer are ascribed to effective sintering at interfaces among crystalline domains by the multi-step creep method [3].

6.4 Direct observation of edge dislocation in lignoceric acid monolayer based on atomic force microscopy. In order to obtain an AFM molecular image of the fatty acid monolayer, it is indispensable to prepare the monolayer with a high mechanical stability [40,41 ]. In the case of a continuous compression method, the mechanically stable fatty acid monolayer can be prepared at a low surface pressure. Mechanical stability of the monolayer depends also on the degree of thermal molecular motion in the monolayer because a molecular aggregation strength increases with decreasing the d e ~ e e of thermal molecular motion. Then, the mechanical stable monolayer used in this study was prepared as follows. A benzene solution of lignoceric acid with a concentration of 1 x 10 -5 mol-L -1 was spread on the water surface at Tsp of 293 K. Since Tsp is below T~c and Tm of the lignoceric acid monolayer (Tc~c = 329 K, Tm = 345 K), the monolayer is in a crystalline state with a low degree of thermal molecular motion[3,31 ]. The lignoceric acid monolayer was prepared at a low surface pressure of 5 mN-m -l by a continuous compression methodat a rate of 1.7 x 10-3 nm2-molecule-l-s -1 and then, transferred onto a freshly cleaved mica by a vertical dipping method. The transfer ratio was unity, implying that mica substrate was completely covered with the monolayer. The AFM image of the monolayer was obtained with a SFA300 (Seiko Instruments, Inc.) in air, using a silicon nitride tip on a cantilever with a spring constant of 0.022 N-m -1. The applied force on scanning was about 10-10 N. Figure 29(a) shows the AFM image for the lignoceric acid monolayer on a scan area of 13.5 x 13.5 nm 2. The Fourier-transform spectrum of the AFM image revealed that the brighter portions in Figure 29(a) were arranged in a hexagonal array with the (10) spacing of 0.43 nm. This magnitude agrees with the spacing of 0.43 nm which was estimated from the ED pattern of the lignoceric acid monolayer and also, is quite different from the 0.46 nm spacing of a mica substrate [40,41 ]. It is, therefore, reasonable to conclude that the brighter portions in the AFM images represent the individual methyl group of lignoceric acid molecule in the monolayer and also, that lignoceric acid molecules are regularly aligned with a hexagonal array. A periodic hexagonal array was extended over ~10 nm. The image in Figure 29(b) corresponds to a magnification (3 x 3 nm 2) of the marked zone shown in Figure 29(a). Figure 29(b) apparently exhibits a discontinuous molecular array in the crystal lattice. That is, a typical edge dislocation can be observed in the center portion of the image, in which an additional molecular array is inserted between two molecular arrays coming down, as schematically shown by opened and closed circles in Figure 29(c). The crystal defect such as edge dislocation in the lignoceric acid monolayer was directly observed with an AFM. The direct observation of edge dislocation in the monolayer was succeeded in for the first time, resulting from the possible preparation of the mechanically stable monolayer [42].

29

Fig. 29. AFM images of lignoceric acid monolayer on scan area of (a) 13.5x13.5 nm2, (b) 3x3 nm2 and (c) a schematic representation of (b).

7. CONSTRUCTION OF DEFECT-DIMINISHED LITHIUM 1 0 , 1 2 HEPTACOSADIYNOATE MONOLAYER BY MEANS OF CRYSTALLIZATION ON THE WATER SURFACE. 7.1. Monolayer preparation and determination of melting temperature 10,12-Heptacosadiynoic acid (chromatographic reference quality) was used without further purification. Benzene with spectroscopic quality was used as a spreading solvent. A benzene solution of 10,12-heptacosadiynoic acid was prepared with the concentration of 2.0 x 10-3 mol-L -1. The subphase water was purified by a Milli-QII system. The trough dimensions were 502 x 150 • 5 turn. Tsp was varied in a temperature range of 283-308 K by circulating constant-temperature water around the aluminum support of the trough. The control accuracy of Tsp was + 1 K which was evaluated by using a thermocouple positioned ca.1 mm below the water surface. The monolayer was prepared by spreading the benzene solution of 10,12heptacosadiynoic acid on the water subphase containing 2.0x10 -3 mol-L -1 of LiOH [43,44]. Monolayer was compressed to a given surface pressure at the area change rate of 2 x 10-3 nm 2" molecule- 1. s-1. Figure 30 shows the Tsp dependences of log Ks(max) for the lithium 10,12heptacosadiynoate monolayer on the water surface and also, ED patterns of the monolayer

"7, E Z E

x

E

2.8

C 14(6-C) 2 6 8 6 0 0 " Li +

~=20mN'm-1

2.6 2.4 2.2

,,,,,,, 2.0 o

--,.,.

1.8 . 6

280

V

~

~

.1_

..

290

"~

.....

l ........

"~l

.... l

.

300

~

J

310

Temp./K Fig. 30. Tsp dependence of logKs(ma.x) and ED patterns of lithium heptacosadiynoate monolayer.

30 transferred onto the hydrophilic substrates at the surface pressure of 20 mN-m -1 , at which the lithium 10,12-heptacosadiynoate monolayer was confirmed to be morphologically homogeneous. The ED patterns were taken at the same temperature as Tsp at which the monolayer was formed. With an increase in Tsp, the magnitude of log Ks(max) increased and started to decrease at around 293 K. The ED patterns at 298 and 303 K were a crystalline Debye ring and an amorphous halo, respectively. Therefore, it is apparent from Figure 30 that the monolayer on the water surface melts in a temperature re#on above 298 K. On the other hand, in a low temperature region below 293 K, the magnitude of log Ks(max) decreased with a decrease in Tsp, and the monolayer on the water surface was in an amorphous state at 283 and 288 K as shown by the ED patterns. This may indicate that the monolayer at Tsp below 288 K is in a glassy state, because lithium 10,12-heptacosadiynoate molecules are quenched maintaining the conformational misalignment along a hydrocarbon chain owing to strikingly fast evaporation of solvent. In order to clarify this quench effect for the monolayer on the water surface, the Tsp dependences of both log Ks(max) and ED pattern were investigated by using the monolayer prepared by cooling the amorphous monolayer down to Tsp of 283 K from Tsp of 303 K at the cooling rate of 15 K-h-land also, at a surface pressure of 12 mN-m-1. Figure 31 shows the Tsp dependences of log Ks(max) and ED pattern of the monolayer prepared by the process mentioned above. This monolayer preparation corresponds to the case without a quench effect, because the amorphous monolayer was slowly cooled down to the temperature of 283 K. The ED pattern at 283 K exhibited a crystalline triclinic spot which was apparently different from the ED pattern of an amorphous halo, as shown in Figure 30. The magnitude of log Ks(max) started to decrease apparently at ca. 300 K without any expression of the maximum log Ks(max)- Since the ED patterns at 298 and 303 K were a crystalline triclinic spot and an amorphous halo, respectively, Tm of lithium lO,12-heptacosadiynoate monolayer on the water surface was evaluated to be around 300 K. Tm of the monolayer on the water surface is much lower than that of three-dimensional crystal of 10,12-heptacosadiynoic acid (Tm=342 K). This is reasonable, because the monolayer is thermodynamically less stable than its three-dimensional crystal.

Fig. 31. Tsp dependence of logKs(max) and ED patterns of lithium heptacosadiynoate monolayer without quench effect.

31

7.2. The aggregation structure of monolayer during a compressing process at Tsp below and above Tin. Figure 32 shows the zt-A isotherm at Tsp of 283 K which was much lower than Tm of the monolayer and also, the bright field images and the ED patterns of the monolayers being transferred onto the hydrophilic SiO substrate at surface pressures of 0 and 17 mN-m -1. The 7tA isotherm showed a steep rise of surface pressure with decreasing surface area without any plateau region. At 0 mN. m -l, many isolated domains were observed in the bright field image, as shown by an inserted photograph in Figure.32. The bright field image of the monolayer at 17 mN-m -1 exhibited the fairly uniform, smooth and continuous morphology. The ED patterns of the monolayer being transferred onto the substrate at 0 and 17 mN-m -1 exhibited a crystalline triclinic spot. The distinctly sharp ED spot indicates that the dimension of crystalline domains is fairly large in comparison with an electron beam diameter of 2/~m, even at a surface pressure of 0 mN-m 1. Also, the ED pattern makes us expect that a large area monodomain monolayer may be formed due to a sintering or fusion at the interfaces among the domains during a continuous compression on the water surface [3]. Figure 33 shows the 7t-A isotherm for the monolayer at Tsp of 303 K above Tm of the monolayer, together with the bright field images and the ED patterns of the monolayer being

Fig. 33. zt-A isotherm, electron micrographs arrd ED patterns of lithium heptacosadiynoate monolayers at Tsp of 303 K.

32 transferred on the hydrophilic SiO substrate at 0, 5 and 20 mN-m -1. A plateau region on the ~tA isotherm was observed in a surface area range of 0.50-0.30 nm2-molecule -1. At 0 mN-m -1, many isolated domains with round shape were observed in the bright field image. The bright field images showed that morphology of the monolayer became uniform with increasing surface pressure. The ED patterns were amorphous halos, being irrespective of the surface pressure. Therefore, Figure 33 indicates that amorphous domains being formed already at 0 mN-m -1 are aggregating during a continuous surface compression, without any phase transition such as from an amorphous phase to a crystalline one. Figures 12(a) and (b) show the schematic representations for the aggregation structure of lithium 10,12-heptacosadiynoate monolayers at Tsp below and above T m , respectively. On the basis of the morphologically structural studies mentioned above, the monolayer on the water surface is classified into a crystalline monolayer and an amorphous one depending on Tsp below and above Tm of the monolayer, respectively. In the case of Tsp below Tm ( Figure 12 (a)) , there is no plateau region on the zt-A isotherm. The isolated two-dimensional crystalline domains are formed on the water surface at 0 mN-m -1. During compression on the water surface, these crystalline domains aggregate, resulting in the formation of a morphologically homogeneous monolayer. This type of monolayer was designated the crystalline monolayer [6,7]. In the case of Tsp above Tm ( Figure 12 (b)), there is a plateau region on a ~t-A isotherm. Many gel-type domains, in which lithium 10,12-heptacosadiynoate molecules aggregate randomly on the water surface, are formed in a low surface pressure region. With an increase in surface pressure, the amorphous domains aggregate, resulting in the formation of a morphologically homogeneous monolayer. This type of monolayer was designated the amorphous monolayer [6,7]. It seems reasonable to consider that there is no apparent boundary surfaces (interface) among the amorphous domains in the aggregated homogeneous state due to gel-like interface. Therefore, the amorphous monolayer is thought to be in a more homogeneous state on a microscopic level rather than the crystalline monolayer. It is apparently expected that a large two-dimensional single crystal with a small number of defect can be prepared by coolingcrystallization of the amorphous monolayer.

7.3. Construction of defect-diminished 1O,12- hep tac osadiyn oate.

crystallized

monolayer

of lithium

Scheme 3 shows the preparation process for the crystallized monolayer of lithium lO,12heptacosadiynoate. The amorphous monolayer was prepared on the water surface at Tsp of 303 K above Tm( ca. 300 K ) and then, was compressed to the surface pressure of 12 mN-m -1.

Amorphous monolayer 303 K (>Tm) 12 mN-m-1 I cooling (15 K-hr "1) 283 K (
33

Table 4. Crystallographical distortion and continuity for crystalline and cooling-crystallized monolayer. Crystallographicai distortion, Dlat / %

Crystallographical continuity, L tat / nm

Crystalline monolayer

6.7

1.8

Cooling-crystallized monolayer

3.1

20.4

< 2.0

30 ~ 60

Polyethylene ( Single crystal ) .

.

.

.

.

.

.

.

.

,

With maintaining this surface pressure, Tsp was reduced to the temperature of 283 K below Tm with the cooling speed of 15 K-h-1 and finally, the monolayer was further crystallized for 20 min on the water surface. This monolayer was again compressed to the surface pressure of 20 mN-m -1, at which the lithium 10,12-heptacosadiynoate monolayer was confirmed to be morpholo~cally homogeneous. The monolayer constructed by this method was designated the cooling-crystallized monolayer. In order to compare the crystallographical distortion and continuity of the cooling-crystallized monolayer with those of the crystalline monolayer, the crystalline monolayer was prepared at Tsp of 293 K below Tm and also, at a surface pressure of 20 mN-m -I. Crystallographical distortion and continuity in a direction along the monolayer surface were quantitatively evaluated by a modified single line method based on the Fourier analysis of ED profiles [9,22,37,38]. Table 4 shows the values of crystallographical distortion and continuity for the crystalline and the cooling-crystallized monolayers of lithium 10,12-heptacosadi ynoate as well as those for polyethylene single crystal. The magnitudes of crystallographical distortion and continuity for the cooling-crystallized monolayer were much smaller and larger than those for the crystalline monolayer, respectively. Furthermore, it can be considered that the crystallo~aphical regularity characteristic of the cooling-crystallized monolayer is comparable with that of high density polyethylene single crystal which was prepared under suitable conditions of crystallization as shown in Table 4. Therefore, it is clear that the monolayer of lithium 10,12-heptacosadiynoate with erystallographically superior quality can be constructed by cooling-crystallization of the amorphous monolayer on the water surface. On the other hand, in the case of the crystalline monolayer, crystallographical sintering (fusion) at the boundary surfaces among crystallites did not occur sufficiently by the compression, which suppressed the formation of the structural defect-diminished monolayer [221. 8. STRUCTURAL R E G U L A R I T Y - W A V E G U I D E P R O P E R T Y R E L A T I O N S H I P S OF L A N G M U I R - B L O D G E T T FILMS A benzene solution of stearic acid was spread on the water surface at Ts,, of 293 K. Since Tsp is below Tctc and T m of the stearic acid monolayer (Tctc=298 K, Tm=31 ~ K [2,3]). stearic acid molecules form a fusing-oriented crystalline monolayer [3,32]. Stearic acid monolayers were compressed to a surface pressure of 24 mN-m -] at a compression speed of 120 mm2. sec -1 by the continuous compression or the multi-step creep methods [39], and then, 150 layers were transferred onto the substrate by the vertical dipping method to construct the Langmuir-Blodgett (LB) films. The multi-step creep is the preparation method for the mechanically stable and defect-diminished monolayer by the stepwise alternating compression and area-creep. The surface morphology of LB films was investigated with an AFM(Seiko SPI3700). The

34

Fig. 35. AFM images for the LB films on a scan area of 20x20/~m2. propagation loss of the LB film waveguides was measured on the basis of the scattered light intensities from the waveguide. Figure 34 shows the blockdiagram of a home-made instrument for propagation loss measurements. Light of a He-Ne laser was coupled into the waveguide by prism. We use a substrate which is equipped with a prism for coupling the light into the waveguide in one body. The scattered light intensity from the waveguide was measured by a movable photodiode along the light propagation direction. Figure 35 shows the AFM images for the LB films prepared by the continuous compression (a) and the multi-step creep methods (b) on a scan area of 20 X 20 tam2. The surface morphology of the LB film prepared by the multi-step creep method was fairly homogeneous, while that of the LB film prepared by the continuous compression method was heterogeneous. This morphological difference on the top surface of each LB film might be caused by the difference in the preparation processes of monolayers on the water surface. Figure 36 shows the schematic representation for the aggregation process of the crystalline monolayer during the compression and the AFM images of LB films. In the case of the continuous compression method (Figure 36(a)), the crystalline domains in a monolayer state were gathered together by compression at a constant speed to a high surface pressure. Thus, since the compression at such a constant and high speed can not give a sufficient time for a full structural relaxation, some vacancies among the crystalline domains remain in the monolayer. Since the existence of vacancies causes a stress concentration in the monolayer, the monolayer prepared by the continuous compression method is mechanically unstable. It seems reasonable to consider that the multilayered LB film composed of mechanically unstable monolayers is easily collapsed resulting in the formation of fairly rough surface. On the other hand, in the case of the multi-step creep method(Figure 36(b)), the monolayer is firstly compressed to a low surface pressure. Though, in this case, vacancies among the crystalline domains and stress

35

Fig. 36. Schematic representation for the aggregation process of the crystalline monolayer during compression and AFM images of LB films.

concentrations exist in the monolayer, the surface pressure is so low that the magnitude of stress concentration may be insufficient to collapse the monolayer. These vacancies in the monolayer are homogeneously filled up owing to the structural relaxation by the area-creep. Furthermore, the stepwise alternating compression and area-creep of the monolayer can provide the mechanically stable and morphologically homogeneous monolayer even at a high surface pressure because of the sufficient structural relaxation attributed to the area-creep during the compressing process. Hence, the monolayer prepared by the multi-step creep method has no stress concentration and is mechanically stable. Therefore, it can be concluded that LB films with a homogeneous surface morphology can be prepared by transferring a mechanically stable monolayer onto the substrate. Figure 37 shows the scattered light intensities from the stearic acid LB films prepared by the continuous compression and the multi-step creep methods. The scattered light intensity of LB film prepared by the continuous compression method remarkably decreased with the waveguide length, while that of the LB film prepared by the multi-step creep method slightly decreased. It can be considered that the degree of a decrease in the scattered light intensity with the propagation length results from that in the propagated light intensities. Thus, the propagation loss of LB film waveguide was evaluated from the slope of the scattered light

36

0 ~ - 10 L -20

Stearic acid LB film waveguide 150 layers Tsp---293 K ---

-

_o -30 ~

9-

-40 -50 -60

1 Co 2 ----- Multi-stepcreep 0

" " " ' " '

5

....

10

" " "

15

Waveguide

20

.... ' .... ' .... 25 30 35

length / mm

Fig. 37. Scattered light intensities from the LB films with the waveguide lenTh. Table 5. Propagation loss of LB films prepared by continuous compression and multi-step creep method.

Preparation method Continuous compression m e t h o d Multi-step creep m e t h o d

Propagation loss dB .cm -~

8.4 2.5

intensity with propagated length [45], as given in Eq. (2), --10 N [ d B - c m "1 ] =

Iog{l(x2)/ l(x 1)} x 2 2. Xl

(2)

where I(Xl) and l(x 2) are the scattered light intensities at the detector positions, x] and x2, respectively. Table 5 shows the propagation losses of the LB films prepared by the continuous compression and the multi-step creep methods. The propagation loss of the LB film prepared by the multi-step creep method was much smaller than that of the LB film prepared by the continuous compression method. Consequently, it is reasonable to consider from Figure 35 and Table 5 that the propagation loss of the LB film waveguide is strongly related to the flatness of the surface morphology of waveguide. In conclusion, LB film with a homogeneous surface morphology can be constructed by the mechanically stable monolayer being formed with sufficient structural relaxation. The propagation loss of the LB film waveguide was increased by the surface roughness of the LB film. Therefore, the LB film waveguide with a low propagation loss should be composed of the monolayer prepared by the structural regularization method such as the multi-step creep method [46].

37

REFERENCES 1. Y. L. Chen, M. Sano, M. Kawaguchi, H. Yu, and G. Zografi, Langmuir, 2 (1986) 349. 2. T. Kajiyama, N. Morotomi, M. Uchida, and Y. Oishi, Chem. Lett., (1989) 1047. 3. T. Kajiyama, Y. Oishi, M. Uchida, N. Morotomi, J. Ishikawa, Y. Tanimoto, Bull. Chem. Soc. Jpn., 65 (1992) 864. 4. S. Fereshtehkhou, R. D. Neuman, and R. Ovalle, J. Colloid Interface Sci., 109 (1986) 385 5. K. Kjaer, J. Als-Nielsen, C. A. Helm, P. Tippman-Krayer, and H. Mohwald, J. Phys. Chem., 93 (1989) 3200. 6. T. Kajiyama, Y. Tanimoto, M. Uchida, Y. Oishi, and R. Takei, Chem. Lett., (1989) 189. 7. T. Kajiyama, Y. Oishi, M. Uchida, Y. Tanimoto, H. Kozuru, Langmuir, 8 (1992) 1563. 8. M. G. Broadhurst, J. Res. Natl. Bur. Stand., Sect. A, 70 (1966) 481 . 9. T. Kajiyama, I. Hanada, K. Shuto, and Y. Oishi, Chem. Lett., (1989) 193. 10. M. Takayanagi and M. Matsuo, J. Macromol. Sci., Phys., B 1 (1967) 407. 11. T. Kajiyama, T. Okada, A. Sakoda, and M. Takayanagi, J. Macromol. Sci., Phys., B7 (1973) 583. 12. T. Kajiyama, T. Okada, M. Takayanagi, J. Macromol. Sci. Phys., B9 (1974) 35. 13. T. Kajiyama, M. Takayanagi, J. Macromol. Sci. Phys., B10 (1974) 131 . 14. T. Kijima, K. Koga, K. Imada, and M. Takayanagi, Polym. J., 7 (1975) 14. 15. J. Ishikawa, M. Uchida, Y. Oish, T. Kajiyama, Rept. Prog. Polym. Phys. Jpn., 33 (1990) 235. 16. I. Langmuir, V. J. Schaefer, J. Am. Chem. Soc. 60 (1938) 1351 . 17. G. A. Bassett, Philos. Mag. 3 (1958) 1042. 18. F. Kumamaru, T. Kajiyama, M. Takayanagi, J. Macromol. Sci. Phys. B15 (1978) 87. 19. T. Kajiyama, K. Umemura, M. Uchida, Y. Oishi, R. Takei, Bull. Chem. Soc. Jpn., 62 (1989) 3004. 20. P. Dutta, J. B. Peng, B. Lin, J. B. Ketterson, M. Prakash, Phys. Rev. Lett., 58 (1987) 2228. 21. T. Kajiyama, K. Umemura, M. Uchida, Y. Oishi, R. Takei, Chem. Lett., (1989) 1515. 22. K. Kuriyama, T. Kajiyama, Bull. Chem. Soc. Jpn., 66 (1993) 2522. 23. K. Sato, M. Kobayashi, H. Morishita, J. Cryst. Growth., 87 (1988) 236. 24. T. Kijima, K. Koga, K. Imada, and M. Takayanagi, J. Macromol. Sci. Phys., B 10 (1974) 709. 25. H. L. Casal, D. G. Cameron, H. C. Jarrell, I. C. P. Smith, H. H. Mantsch, Chem. Phys. Lipids., 30 (1982) 17 . 26. T. Kawai, J. Umemura, T. Takenaka, Langmuir, 4 (1988) 449. 27. M. L. Mitchell, R. A. Dluhy, J. Am. Chem. Soc., 110 (1.988) 712. 28. R. D. Hunt, M. L. Mitchell, R. A. Dluhy, J. Mol. Struct., 214 (1989) 93. 29. R. G. Snyder, H. L. Strauss, C. A. Elliger, J. Phys. Chem., 86 (1982) 5145. 30. R. A. MacPhail, R. G. Snyder, H. L. Strauss, J. Chem. Phys., 77 (1982) 1118. 31. Y. Oishi, H. Kozuru, K. Shuto and T. Kajiyama, Mat. Res. Soc. Symp. Proc., in press. 32. T. Kajiyama, Y. Oishi, M. Uchida and Y. Takashima, Langmuir, 9 (1993) 1978. 33. T. Kajiyama, L. Zhang, M. Uchida, Y. Oishi, A. Takahara, Langmuir, 9 (1993) 760. 34. T. Kunitake, Y. Okahata, Bull. Chem. Soc. Jpn., 51 (1978) 1877. 35. C. J. Bloys van Treslong, A. J. Staverman, Recl. Trav. Chim. Pays. Bas. 93 (1974) 171 36. K. Fukuda, H. Nakahara, T. Kato, J. Colloid Interface Sci., 54 (1976) 430. 37. D. Hofmann and E. Walenta, Polymer, 28 (1987) 1298. 38. Y. Oishi, F. Hirose, T. Kuri, and T. Kajiyama, J. Vac. Sci. Technol. A, 12 (1994) 2971 . 39. T. Kuri, F. Hirose, Y. Oishi, and T. Kajiyama, Rept. Prog. Polym. Phys. Jpn., 36 (1993) 209. 40. T. Kajiyama, Y. Oishi, F. Hirose, K. Shuto and T. Kuri, Chem. Lett., (1993) 1121 . 41. T. Kajiyama, Y. Oishi, F. Hirose, K. Shuto, T. Kuri, Langmuir, 10 (1994) 1297. 42. T. Kajiyama, Y. Oishi, K. Suehiro, F. Hirose, T. Kuri, Chem. Lett., (1995) 241 . 43. D. Day and J. B. Lando, Macromolecules, 13 (1980) 1478 and 1483.

38 44. K. Miyano and A. Moil, Thin Solid Films, 168 (1989) 141. 45. W. Hickel, G. Appel, D. Lupo, W. Prass and U. Scheunemann, Thin Solid Films, 210/211 (1992) 182. 46. T. Kuri, N. Honda, Y. Oishi and T. Kajiyama, Chem. Lett., (1994) 2223.

New Developments in Construction and Functions of Organic Thin Films T. Kajiyama and M. Aizawa (Editors) 9 1996 Elsevier Science B.V. All rights reserved.

39

C r y s t a l e n g i n e e r i n g of s y n t h e t i c bilayer m e m b r a n e s K. Okuyama a and M. Shimomura b aDepartment of Biotechnology, Tokyo University of Agriculture & Technology, Koganei, Tokyo 184, Japan bResearch Institute for Electronic Science, Hokkaido University, Sapporo 060, Japan A senes of aggregation structures of bilayer forming azobenzene amphiphiles, CnAzoCmN+Br -, both in single crystals and cast films was determined by the X-ray diffraction method and uv-visible absorption spectroscopy. From the relationship between chemical structures and their two-dimensional supramolecular structure, factors determining the molecular orientation in bilayer structure were discussed. Some unique properties based on two-dimensional molecular ordering were also discussed. 1. I N T R O D U C T I O N The preparation of designed molecular assemblies is attracting considerable attention. I_angmuir-Blodgett technique has been proved quite effective in prepanng two-dimensional supramolecular assemblies which are applicable to molecular electronics, optical devices, sensing devices, and so on. Bilayer membranes are another two-dimensional supramolecular assemblies spontaneously organized in water. It has been shown that the bilayer membrane is formed from a large variety of amphiphiles which are not directly related to the structure of biolipid [ 1-4]. Many unique properties of synthetic bilayer membranes, e.g. ordered molecular onentation [5], dynamic thermal phase transition [6], and controllable molecular distribution [7], etc. have been discovered. Many efforts have been made to use synthetic bilayer membranes as novel functional molecular materials. Immobilization of the aqueous bilayer membranes as thin solid films with keeping the unique properties of the aqueous solution are required together with giving mechanical property, to the films. A simple solvent cast method has been proved successful in preparing the immobilized bilayer membranes as self-standing solid films [8]. Structural characterizations of the immobilized bilayer assemblies are essential for the molecular design of the functional materials. On the bases of the systematic crystallographic investigation of single crystals of double-chain ammonium amphiphiles [9], Okuyama wrote a computer simulation program for the calculation of bilayer structures in cast bilayer films and bilayer thicknesses estimated from the repeating period in the X-ray diffraction data have been exclusively used for structural discussions [ 10,11 ]. In this paper, UV-visible absorption spectra and X-ray diffraction experiments of single crystals and solvent cast films of the azobenzene amphiphiles, CnAzoCmN+Br -, were systematically investigated. Structural charactenzation of the cast bilayer films are discussed in comparison with aqueous solutions and single crystals. Some novel functional properties of the cast films are descnbed, too. We also emphasize that the two-dimensional molecular assemblies, cast films and crystals of bilayer-forming amphiphiles, are suitable candidates for "crystal engineenng" because of their simple structures compared with usual three-dimensional molecular crystals.

40 2. S P E C T R A L P R O P E R T I E S O F A Z O B E N Z E N E A M P H I P H I L E S 2.1. UV-visible absorption spectrum in an aqueous bilayer solution Bilayer formation and spectral properties of the azobenzene amphiphiles in aqueous solutions were extensively investigated by Shimomura and Kunitake [5]. On the basis of systematic organic synthesis and spectral investigation of twenty-one azobenzene amphiphiles, UV-visible absorption spectra of the aqueous azobenzene bilayer membranes were found to be strongly affected by the chemical structure and temperature. Figure 1 briefly summarizes absorption spectra of the azobenzene amphiphiles in water. The absorption maximum of the aqueous solution of C6AzoC2N+Br- is very close to that of the isolated azobenzene chromophore (absorption maximum; 355 nm), so there may not be any interaction between adjacent azobenzene chromophores. On the other hand, an aqueous bilayer solution of CsAzoC10N + Br- showed a large hypsochromic shift (absorption maximum; 300 nm) relative to the monomeric azobenzene chromophore. Aqueous bilayer solutions of CnAzoCsN § Brhaving a long alkyl tail (rL>_7) showed a large bathochromic shift (absorption maxima; 375 nm and 390 nm) relative to the monomeric azobenzene chromophore. The spectral versatility was ascribed to Davydov splitting due to strong intermolecular interaction in the ordered molecular assemblies. Semi-quantitative calculation based on Kasha's molecular exciton theory [12] predicted that hypsochromic shift and bathochromic shift in the absorption spectrum observed in the aqueous bilayer solutions was attributed to the side-by-side (Fig.2 model a) and the headto-tail (model b) orientation of the azobenzene dipole moments, respectively. In the field of spectroscopy, these types of chromophore arrangement are known as "H-aggregation" and ".laggregation", respectively.

Ci'I~CH2)n.1-0

CHs , CH2)m-N+-" CH2CH20H CH3 Br"

=N

C n A z o C m N + BrScheme 1. Ammonium amphiphile having an azobenzene chromophore.

~,

..

200

_

c

-,;

;~',

,

.

/

300 400 Wavelength (nm)

u

500

Figure 1. UV-visible absorption spectra of azobenzene bilayer membranes in water. curve a: C8AzoCIoN+ Br-, curve b: CnAzoC5N+ Br-, curve c: C6AzoC2N+Br -.

41

b

a )

J-type orientation

H - t y p e orientation

I1

K

excited state

_

_

_

A |

I I i

ground state .... monomer

i

monomer

dimer

dimer

head-to-tail

side-by-side N molecules , ~

i

A / /

,

,

,/

r

Figure 2. Molecular orientation model of bilayer membranes and schematic explanation of Kasha's molecular exciton t h e o ~ (see equation (2)).

2.2. U V - v i s i b l e a b s o r p t i o n s p e c t r u m of s o l v e n t cast films Optically transparent films were prepared on quartz plates by casting of water or ethanol solutions at room temperature. Typical absorption spectra of the cast films are shown in Figure 3. Two strong absorption bands attributed to a-:t* electronic transition are observed in the UVvisible region. The absorption band located around 250 nm is attributed to a transition dipole moment along a short axis of the azobenzene chromophore [13]. The long axis transition at lower photo excitation energy (300-400 nm) was found to be strongly affected by the chemical structure of the amphiphile in cast films as well as in aqueous bilayer solutions. Absorption spectra of the cast films are classified into following six groups (Table 1).

42

~

a

,

i,

3 o.6

o02

.

<

0

200

0.4

300 400 Wavelength (nrn)

0.8

b

(1) (.1 C m .Q

ITI

,.0.2

0 o3 .Q

<

J

500

I

I.. cr

0.6o

o

al

I

0 4 9 ~, !.-

~v ,-,\

O w ,Q

0.2< 200

L _ . , ~

300 400 Wavelength (rim)

0

500

Figure 3. UV-visible absorption spectra of cast films. (a) group I (C~zoCsN+Br-), group II (CloAzoCIoN+Br-), group V (C7AzoC6N+Br-), group VI (C14AzoC5N+Br-). (b) group III (CTAzoCTN+Br-), group IV (C8AzoC4N+Br-). Group I" Casting from ethanol solutions of amphiphiles with the structural relation of m-n >__ 2 shows a sharp absorption at ca. 300 nm attributed to a hypsochromic shift typical to the sideby-side chromophore orientation. A small red shift with sharp double peaks in the short axis transition is characteristic of this group. Group II" Absorption maximum of the group II locates around 320 nm. Spectral red shift and two sharp shoulders in the short axis transition are found in the UV region. Structural

43 requirements of the group II are m-n _<1 and the total alkyl chain length : (m + n) > 18, except m=5 series included in the group VI. Group III and Group IV: Spectral shape and absorption maximum in the visible region of these two groups are almost identical. Four vibronic structures are included in the visible absorption band (ca.345nm, 355nm, 370nm and 410nm). The second vibronic transition at 355nm is in some case more intense than the first transition at 345nm (e.g. C12AzoC4N+Br-). Discrimination criterion of these two groups is the spectral shape in the ultraviolet region (Figure 3b). The group III shows sharp shoulders attributed to the vibronic transitions, while the group IV has a broad band without splitting. Six amphiphiles listed in the middle part of Table 1 are classified in the group III and three amphiphiles with m--4 are in the group IV. Group V: All amphiphiles with m=6 and C4AzoCsN+Br - belong to the group V. An ethanol-cast film of the shortest alkyl chain derivative, C3AzoC5N+Br -, which can not form stable bilayer assembly in water and its spectral shape of water solution is very similar to that of an ethanol solution, is classified in this group, too. Interestingly, a water-cast film of C3AzoCsN+Br - shows a typical group I spectrum (see chapter 4). Grot:p VI: A typical bathochromic shift attributed to the head-to-tail chromophore orientation shown in Figure 1 is found in cast films of most amphiphiles of m=5 (except n=3 and 4). Crystallographic investigations on single crystals of the m=5 series has been systematically studied by Okuyama et al. and the amphiphiles are packed laterally with tilting by about 26 ~ to the bilayer surface. The calculated orientational angle of the azobenzene chromophore estimated from Kasha's equation (equation (2)) based on spectral Davydov splitting was very, consistent with that of the single crystal. Table 1 Spectral classification and absorption maxima (nm) of cast bilayer films tail n

spacer m

3

4

5

6

7

4

5

10

11

12

IV

IV

V

347

344

355

13

14

Ia,V b

V

VI

VI

VI

VI

VI

VI

VI

VI

VI

362

375

372

372

373

375

375

374

374

373

7 8 9

12

9

300a363 b

6

10

8

V

V

V

V

V

367

362

365

364

360

I

III

III

III

304-

346

345

344

I

I

III

III

III

II

304

306

340

342

336

325

I

I

I

303

302

305

I

I

I

I

I

I

II

II

II

304

300

301

301

305

302

317

317

325

I

I

303

305

a: water-cast film" b: ethanol-cast film

44

3. X-RAY STRUCTURAL ANALYSES OF AZOBENZENE AMPHIPHILES 3.1. X - r a y analyses of single crystals Recently, Okuyama et al. succeeded to prepare single crystals of some azobenzene amphiphiles and decided molecular and aggregation structure of single crystals [14-19]. The spectral prediction of the chromophore orientation in the bilayer assemblies were very consistent with the X-ray structural analyses of the single crystals. Single crystals of CnAzoCsN+Br -, in which azobenzene chromophores are in the Jaggregation state, were obtained by a solvent evaporation method from a mixture solution of water and ethanol (n=9, 10 [15], 11, 12 [14])or ethanol and benzene (n=6, 7, 8 [15]). Seven crystals with various tail lengths (n) belonged to the same crystal system and showed only one significant difference in the dimension of a-axis, which corresponds to the direction of the bilayer thickness. As expected from the similarity in crystal data, the molecular and crystal structures of CnAzoCsN+Br - were very similar to each other. The packing structure of C8AzoC5N+Br - are shown in Figure 4a as a t3"pic~ example of J-aggregates. The crystal structure can be considered as a structure regularly stacked with bimolecular layers along the a-axis. Within the bimolecular layer, two molecules related by inversion symmetry face each other in the tail-to-tail fashion with their molecular axes inclined by about 26 ~ to the bilayer surface. This inclination enables the head-to-tail arrangement of azobenzene chromophores as expected from the spectroscopic study.

a

b

Figure 4. Packing structure of (a) C8AzoCsN+Br - and (b) C6AzoC8N+Br -. Single crystals of H-aggregates were prepared from four compounds with the relation mn = 2 . The crystallization of these compounds was very difficult and took several months to grow single crystals suitable for X-ray analysis. Various solvents were examined for crystallization. Among them, single crystals were obtained from the mixture of ethanol and

45

nitrobenzene for CsAzoCTN+Br - and C6AzoC8N+Br - and that of ethanol and trichloromethane for C3AzoC5N+Br - and C8AzoC10N+Br -. The molecular packing structure of C6AzoCsN+Br - [ 17] is shown in Figure 4b. In contrast to the bimolecular layer of the J-aggregates, molecules pack laterally to form a monomolecular layer, so that two neighboring molecules are arranged in the anti parallel fashion and mutually interdigitated. The azobenzene chromophores are aligned at the center of the hydrophobic layer and inclined about 65* to the neighbonng chromophores. Although the molecular arrangement obtained from the single crystal analysis was not a bilayer structure as the one expected, the side-by-side parallel packing of azobenzene chromophores was observed in these H-crystals as expected. Since the largest part of the hydrophobic chain is a phenyl moiety, the cross-section of the hydrophobic part, Sc, could be estimated as 0.23nm 2 by using van der Waals radii 0.18 and 0.12nm for atoms C and H, respectively. On the other hand, the cross-section of the hydrophilic part can be represented as Sm.cos~, here, Sm is the molecular area at the bilayer surface (Figure 5). Furthermore, it is possible to calculate the molecular area, Sm, from the lattice constants: Sm =bc-sina, and it was found to be 0.50nm 2. In order to make an energetically stable structure, it is important to balance the cross-section of the hydrophilic part and that of the hydrophobic part. That is, Sm-cos0=Sc. By substituting the above values, a chain tilt angle of 62 ~ (0=cos-i(Sc/Sm)) will be obtained. Thus, the azobenzene chromophores are obliged to form head-to-tail J-like aggregation state. So far, we have considered cross-section balance between one hydrophilic part and one hydrophobic part. We can also consider a different type of cross-section balance between one hydrophilic part and two hydrophobic parts. This is possible ~ u s e the molecular area of CnAzoCmN+Br - is almost equal to twice that of the chain cross-section. In this case, we will obtain a tilt angle of 0_3~ from the relation of Sm-cos0=2-Sc. This is another stable state of CnAzoCmN+Br, known as the interdigitated H-aggregation state which is observed in several compounds with m-n_~_2.

GI.! e

hainli,!

./~O~SC

ChQi. . . . . .

ion

LQyer vnter~clce moteculClr QrGQ

0./,9? nml

Figure 5. Cross-section balance between the hydrophobic and hydrophilic parts. Now we shall elucidate factors which determine the J- and H-aggregation state. At first, we shall consider the H-aggregation state obtained from structure analyses of single crystals. In the layered structure, the methyl moiety at the end of the hydrophobic chain is in contact with

46

the bromide anion with a length of 0.41nm (Figure 4b), which is very close to the van der Waals distance between these atoms (0.39nm) so that further interdigitation is prevented by the short interaction between them. This fact shows that the difference in the number of carbon atoms between the spacer part and the tail part becomes a very important factor of packing in the H-crystals. That is, from the geometrical point of view, the difference in the number of carbon atoms (m-n) must be 2 or larger than 2 when the azobenzene chromophores duster together in the H-aggregation state. In the case of m-n>2, however, the vacant space makes the packing structure unstable. Therefore, we were only able to obtain single crystals with the Haggregation state for those compounds in which m-n=2. When m-n>2, H-aggregation states appeared only in cast films in which three-dimensional repetitions are not so stnct as those for single crystals. Next we shall consider the J-aggregation states. Although we observed J-aggregation states even for the H-group compounds, single crystals were obtained only from those compounds with a spacer length of 5. The number of spacer carbon atoms seems to be very important for single crystals of J-compounds. This was explained by considering the packing density. That is, since the cross-section of the azobenzene moiety is larger than that of the alkyl chain, total packing energy will become low when the length of the alkyl spacer is similar to that of azobenzene moiety. Since the approximate length of the azobenzene moiety corresponds to that of five spacer carbon atoms, only the compounds of CnAzoC5N+Br - were able to form stable single crystals with the J-aggregation state of chromophores. 3.2. X - r a y D i f f r a c t i o n s o f C a s t F i l m s . Electron microscopy easily yields structural images of cast bilayer films. Figure 6 shows a scanning electron microscope (SEM) image of the cross section of the bilayer film of CsAzoCloN+Br - prepared by the simple casting of water solution. From the presence of well developed layers parallel to the film plane, it can be assumed that the cast film was composed from multiple highly oriented bilayers.

Figure 6. SEM image of cast bilayer film of CsAzoCloN+Br -.

47 X-ray diffraction from cast films provide useful information of bilayer structure. Periodic peaks in small and middle-angle diffraction from cast films on glass plates are attributed to the reflections from (h, 0, 0) planes of the multiple lamella structure. The spacing of higher order reflections (h > 1) satisfies with numerical relation of 1 / h of the long period calculated from the first order reflection(h = 1), which is equivalent to the bilayer thickness. Every cast film measured in this experiment showed more than 6 reflection peaks.

a

2

CsAzoCeN*

4,,* i m

g~

l:

41

er m

4 j

1

.......... 1.5 .........

5

6

~

J~_ . _ A ..... L_,. lO . . . . 20 ( d e g . )

b

20

C14AzoC5N+ 1

....,

In c:

(I) r

=am

1.5

9

,

2 A_

,

3 ~

9

4

/%_[

5 ~,

10 26 ( d e g . )

6 ,A

,

7 .._

,

8 20

Figure 7. X-ray diffraction of cast films. (a) CsAzoCsN+Br -. (b) C 14AzoCsN+Br -. The diffraction profiles of cast films are roughly grouped into two types. Ever3' cast film of the group I and II showed a diffraction pattern with the second order reflection as the strongest peak(Figure 7a). The films of the group IV, V, and VI showed the strongest first order reflection with weaker higher order reflections (Figure 7b). The bilayer thicknesses estimated as the long period of the cast films are summarized in Table II. Since the length of the longest

48

axis in the unit cell of a single crystal is very similar to the long period of the cast film, aggregation structure in the cast film is assumed to be almost same as that in the single crystal.

Table II. Long periods (nm) of cast films calculated from X-ray diffraction experiment. tail n

3

spacer ITI

4

5

6

7

4

I;. VIb

VI

VI

2.6a.3.2 b (2.5866)

(2.7446)

(2.8570)

6 7

8

9

10

11

12

IV

IV

IV

2.8

3.0

3.3

VI

VI

31 (3.0098) (3.1125)

VI 3.2 (3.233)

VI

VI

3.3 34 (3.3136) (3.4243)

13

14

VI

VI

37

3.7

v

V

3.9

4.1

III

I

2.9

4.3

(2.9500)

I

I

8

3.3

34 (3.4O4)

9

3.s

10

3.9

I I

I 35

I

I

I

II

II

4.0

3.9

3.9 (3.881)

4.7

4.8

( ): length of the longest axis in the unit cell of single crystals. a: water-cast f i l m b: ethanol-cast film Figure 8 shows relation between the long period (d) and the total number of methylene unit in the chain (n + m). Two linear relations are found in Figure 8a where d values of the cast films in the group V and VI are plotted against the total chain length. Bilayer thicknesses of these cast films simply increase linear with increasing total chain length of the azobenzene amphiphile. Assuming that the molecule extends linearly with the tilt chromophore orientation m the cast film as well as in the single crystal, the tilt angle to the bilayer surface a (see Figure 2) is roughly estimated from equation (1), d - 2 1 sin et

(1)

where 1 is molecular length calculated from the CPK molecular model. Estimating the molecular length as 3.59 nm and 3.97 nm for CTAzoCTN+Br - and C12AzoC5N§ -, the tilt angle of the group V and VI is calculated to be 36 ~ and 26 ~ respectively. The later value is just same as that of the single crystal. Spectral shift (in wave number;Av) of the azobenzene chromophore caused by intermolecular interaction can be estimated by using Kasha's equation (2), which is a function of the transition moment (~t), distance between dipoles (r), and number of interacting molecules in bilayer assemblies(N).

49

Av-

9_2_ (_N_zL-) ~2 ( i - 3 cos 2 a) hc

N

(2)

r3

Equation (2) indicates that an increase of the tilt angle from 26* to 36 ~ results a small blue shift of the absorption maximum (in wavelength) in the visible region. Spectral observation of the group VI (375 nm) and V (360 nm) is v e ~ consistent with the structural estimation from the Xray diffraction experiments.

''O / "~

3"5/

/.l

~n---5,m---6, (n=14.m=5/~ /

<":~"~

7

9

11

13

15

17

(n--8. m=lC BE J i B

(n=7.m=/

~*1

<.:~,, m:~/,g:,z ,,:4~

= , = I/~.=~,,-:~) . (._:,o,y~~-

2.5

(n--6, m=lOi (n=5. m = l O i i

19

I

i

"o 3 . 0

21

n+m

2.0

."

7

'

'

9

"" " ' L I

11

(n=5, m=7)

i , , ,

13

15

I

i

17

,

19

n+m

Figure 8. Plots of total alkyl chain length (n+m)and long period of cast film. (a) group IV, V and VI. (b) group I.

The d values for the group IV amphiphiles are plotted in Figure 8a, too. The correlation of the group IV is very similar to the group VI. Taking the spectral difference between group VI and IV into account, the aggregation structure in the group IV must be considerably different from those in the group VI. Since the molecular length of CsAzoC10N+Br - estimated from CPK model (4.10nm) is almost same as the long period, CsAzoC10N+Br - molecule are assumed to be packed laterally and interdigitated in cast films as well as in single crystals shown in Figure 4b. Plots of long period versus total chain length of the group I are shown in Figure 8b. These points can be classified into two groups. The d value increases with increasing alkyl chain length when the experimental data of the homologous series of m-n=2 are connected. An odd-even effect on the long period was recently found by Xu in c~stalline samples of these amphiphiles [19]. Another correlation is found from connecting the points of the same spacer series, e.g. m=10. The long period of this series is almost identical. Two types of aggregation structure are proposed for the group I films (I and I' in Figure 9).

3.3. Aggregation Structure in Cast Fihns. X-ray diffraction studies strongly indicate that the films of the azobenzene amphiphiles simply cast on solid substrates are composed from highly oriented multiple stacked bilayer

50 structures parallel to the substrate surface. A repeating penod consistent with the bilayer thickness in cast film is found to be almost same as that of the single crystals whose bilayer structures have been completely determined. On the basis of spectral observation and X-ray diffraction experiments, we propose structural models of the six spectral groups (Figure 9). 9

9

9

.i.Ii..IiI I'

9

9

9

V

9

9O o O o O O 0

I i

300

II i

.... I

lll,lV l

!

I.

350

VI I

I

I

i

I

400

Wavelength (nm) of Absorption Maximum Figure 9. Schematic models of bilayer structures of azobenzene amphiphiles in cast films.

4. S T R U C T U R A L

POLYMORPHISM

OF CAST FILMS.

4.1. Structural diversity of the group I and II We have already reported that a water-cast film of CsAzoC10N+Br - showed two endothermic peaks in differential scanning calorimetry (DSC) at 115~ and 177~ corresponding to a solid-solid and a solid-isotropic phase transition, respectively [20]. On cooling a third solid state was formed (Figure 10a). The X-ray diffraction studies suggested that this third solid phase in the annealed film is a metastable state at room temperature and is transformed to the high temperature solid state on heating to 60~ (Figure 10b). In addition, the absorption spectrum of the cast film was strongly affected by the peculiar solid-solid phase transition, too [20,21]. An as-cast film, i.e. the initial solid state, showed a typical group I spectrum having absorption maximum at 302 nm. Absorption maximum shifted to 360 nm upon heating above the first phase transition and moved to 370 nm on cooling to room temperature. Spectral shape of the high temperature solid state and the third solid state are ve~' similar to the group V and VI, respectively (Figure 1 la). Eve~' cast film of the group I and II showed similar spectral transition after annealing above their phase transition temperature [22].

51

(1) I i' (2) ._o

L=3.90nm

t

,

L---4.2Onrn

(1)

@ J~

2rid hea~n9

"O e-

I/J

115

9

(31

20

60

-.

~

(2)

8O

140

2OO

o

Temperature (oc)

lO

20

28 (deg)

Figure 10. DSC thermogram(a) and X-ray diffraction(b) of C8AzoCIoN+Br - cast film.

b 1.5

(2)anneeled at 130"C j ~

(3)room temperature

(1) H-aggregate

ng

C:

1.0

0

~ 0.5

0

(llbefore luting (41 after moisten

2O0

wavelength (nm)

l (3) J-aggregate

Figure 11. Spectral change of C8AzoC10N+Br - cast film by thermal phase transition and proposed schematic model of the phase transition. Interesting finding was a reversible spectral change coupled with moistening of the cast film [20]. Figure 12a shows spectral change of the annealed film of C8AzoC10N+Br - kept in 62% humidity at room temperature. The Wpe VI absorption gradually shifts to the type I spectrum that is identical to the as-cast film. As Shown in Figure 12b, a transition rate of the isothermal spectral change strongly depends on the relative humidity. Spectral transition is found to be accelerated in a moist atmosphere. This is concluded that the type VI state is a metastable state in the annealed film in low humidity.

52

1.0 o

c .(3 ,- 0 . 5 0 w .13 <

0

300

200

500

400

W a v e l e n g t h (nm)

b 1.0

!

o

<

,'=43%

o.s

~,~e %.,.,,.,,,,,,,,,,~ e .....

0.0

0

5

10 15 Time (mln)

20

25

Figure 12. Spectral change of the annealed film by moisture treatment. (a) C8AzoC10N+Br film was sealed in a quartz cell with 62% humidity after annealing. The type V spectrum (broken line) immediately moved to the t3"pe VI spectrum and then shifted to the type I absorption. (b) Humidity effect on time courses of the spectral change. A0 and At are absorbance of 370 nm immediately and t min after stored in a sealed quartz cell, respectively.

Another spectral change was found when a cast film of C6AzoC8N+Br - was annealed and then moistened (Figure 13). The type I spectrum of the as-cast film changed to the type V spectrum (the broken-line spectrum in Figure 13) after heating above its phase transition (115~ and then immediately shifted to the type III spectrum within 30 sec in a 75% humidity condition at room temperature. The type III state seemed to be another metastable state in the annealed film because the moisture induced isothermal transition from the type III to the type I required a long period (e.g. 13 hours even at 75% humidity).

53

i ~_-k~j3ose c _

0.3 o.,,

n t

7

~ 0.1 0

"---

200

"""

~

300

400

500

W avelength (nm)

Figure 13. Spectral change of the annealed film (C6AzoC8N +) by moisture treatment. The broken line spectrum of the annealed film was stable in a dry condition but immediately shifted to the type III spectrum within 30 sec in a humid atmosphere.

5.0 4.5

-

_

~ ( n ~ - - 7 . m=9) ',.y'(n---6,m=9)

~.=7,, = 7 ~ ~ .

t

,~9~

~..) (n=S, 111--8)

~" 4.0

""" ype v i //

"~ 3.5 '

~

3.0 /

2.5

A,f~(n=11, m=5) _ jl"a(n=lO,

VI !

11

m=5)

I

13

I

I

15

I

I

17

I

I

19

I

I

21

I

23

n+m

Figure 14. Plots of the total alkyl chain length (n+m) and long period of annealed cast film (type III and VI) of the group I and II. An as-cast film of CTAzoCTN+Br - of the group III is plotted as a full circle. Ever3" cast film classified in the group I and II shows one of these two types of spectral change with annealing and moistening. Tow metastable spectral states in the annealed films, 1.e. the type VI and III, were stable enough during the X-ray diffraction measurement in our experiments. As shown in Figure 14, two correlations are given from plots of the total chain length and long period of the annealed films. These correlations consist of annealed films

54

showing the type III and VI spectrum, respectively. The correlation line of the group VI shown in Figure 8a is plotted again in this Figure. The aggregation structure of the metastable state VI thermally formed in the annealed film is assumed to be very similar to that in the as-cast film of the group VI. The d value of the as-cast film of CTAzoC7N+Br - classified in the group III fits on the correlation curve of the type III films thermally formed from the group I and II.

4.2. Effect of casting solvent on structural p o l y m o r p h i s m The azobenzene chromophore was found to be a good spectral probe for bilayer formation because its absorption spectrum is strongly affected by chromophore interaction resulting from molecular aggregation. As summarized in Figure 1, the molecularly dispersed azobenzene chromophore has an absorption maximum at 355 nm and aggregated chromophores show large spectral shifts induced by strong intermolecular coupling in the ground state. Spectral splitting (~.max = 310 and 355 nm) of C3AzoC10N+Br - observed in an aqueous solution is ascribed to a monomer-aggregate equilibrium in water. Kunitake et al. noted that short alkyl tail (n _< 6) of the single-chain amphiphile having a rigid aromatic segment was not sufficient for forming a stable bilayer membrane in water [23]. Shimomura et al. judged stable bilayer formation of the azobenzene amphiphiles by using some criteria based on spectral properties and thermal behaviors (e.g. crystal-liquid crystal phase transition) [5]. It was concluded that a stable bilayer membrane could be hardly formed from some azobenzene amphiphiles if the absorption spectrum of water solution was different from that of the cast film. The casting solvent is a determining factor of the spectral shape of the cast film. The amphiphiles sufficient for stable bilayer formation in water never show significant solvent effect on spectral shape of cast film. But for some amphiphiles a remarkable solvent effect on the absorption spectrum was found. Water-cast films of C3AzoC10N+Br - , ~ z o C 1 0 N + B r - , and CsAzoC12N+Br - showed similar absorption spectra of their water solutions (ca. 360nm), while ethanol-cast films of these amphiphiles gave a large blue shift to 300 nm. The aggregation structure formed in water is assumed to be kept in the water-cast films. The amphiphiles molecularly dispersed in ethanol aggregate to form the most stable twodimensional crystalline assemblies during casting process. A peculiar solvent effect was found for the amphiphile C3AzoCsN+Br - which satisfies structural requirements of the group I (m-n > 2) and VI (m=5). The water- and ethanol-cast film of C3AzoCsN+Br - showed group I and V spectrum, respectively. Thus the group V is defined as an analogue of the group VI (see Figure 9).

5. Crystal engineering based on two-dimensional molecular assemblies. 5.1. Design of charge transfer c o m p l e x e s in bilayer assemblies Taking the balance of the cross sectional area of the hydrophilic head and hydrophobic part in consideration, molecular packing in the two-dimensional bilayer assembly is designed. Since the cross sectional area of the hydrophilic head is about twice as large as that of the aromatic moie~', two types of the most densely molecular packing models are proposed as plausible structural models of bilayer assembly: (1) tilt molecular orientation (model VI in Figure 9) and (2) interdigitated orientation (model I). The homologous series of m=5 (except n=3 and 4) form bilayer structure VI with the tilted molecular orientation in cast films as well as in single crystals. Structural model V is described as an analogous bilayer structure of the model VI and formed from amphiphiles with veD, similar chemical structure (m=4 and 6) as the amphiphiles of m=5. A striking example for the application of these results is the formation of an intermolecular charge transfer (CT) complex in a bilayer membrane. In order to veri~' Okuyama's prediction on molecular orientation in bilayer assemblies, azobenzene amphiphiles having a viologen moiety as a hydrophilic head group, CnAzoCmV 2+ 2Br-, were newly prepared. Bathochromic shift to 390 nm in the visible absorption band of the

55

azobenzene chromophore, as same as that of the ammonium bilayer C 12AzoC5N+ Br, strongly indicates that the tilted chromophore orientation is also formed in the bilayer membrane of C12AzoC5 V2+ 2Br. As was expected, the water solution of C8AzoClO V2+ 2Br- shows a blue shifted azobenzene absorption similar to the bilayer membrane of the ammonium amphiphile C8AzoC10N + Br-. Spectral finding suggests that the "crystal engineering" based on the ammonium bilayer membranes is applicable to the viologen bilayer membranes. These results indicate that the alkyl chain length is an important structural factor of molecular orientation in the bilayer membrane of the viologen amphiphile as well as the ammonium amphiphile, too.

CHa(CH2)n-.t-O ~

(CH2)m-~N~N- C2Hs C n A z o C m V 2+ 2Br2Br

CH3(CI"12) n_.l-O~ -

=N~-

(CH2)m-N~N- C2H5

C n B p h C m V 2+ 2Br-

2Br

Scheme 2. Viologen amphiphiles having rigid aromatic segments.

Is the concept of "crystal engineering" applicable if the azobenzene segment is replaced with another rigid segment ? A biphenyl group is chosen as the rigid segment because its cross sectional area is similar to that of the azobenzene. Two viologen amphiphiles having the biphenyl chromophore, C12BphC5 V2+ 2Br-and C8BphClo V2+ 2Br-, were newly prepared. Absorption spectra of the viologen group and the biphenyl chromophore in ethanol are located in uv region. Absorption spectra of C8BphC 10V2+ 2Br- in water is very similar to that of in ethanol. On the other hand, a water solution of C12BphC5V2+ 2Br- shows a peculiar absorption band at around 460nm, while an ethanol solution has no absorption band in this spectral region (Figure 15). We assume that the newly appeared absorption band in the visible region is ascribed to a charge transfer interaction between the viologen and the biphenyl groups. Viologen derivatives are known to be a strong electron acceptor of a CT complex. Owing to its higher n-electron density" of a phenyl ring, a biphenyl group is expected to be a electron donor of the viologen group. If C 12BphC5 V2+ 2Br- forms a bilayer membrane with the tilted molecular orientation, as well as the azobenzene-containing viologen amphiphile C12AzoC5 V2+ 2Br-, the viologen head must be closely located to the hydrophobic biphenyl segment of the adjacent molecule in the two-dimensionai molecular arrangement. So an intermolecular CT interaction is strongly expected in the tilted molecular orientation of the biphenyl bilayer. The colored solution was never formed from CsBphC10 V2+ 2Br- in water. When CsBphClo V2+ 2Br- forms the interdigitated orientation in bilayer membranes, CT interaction must be depressed because of the spatially separation of donors and acceptors.

56

I

!

12"C,15"C, 20"C, 25"C

0.4

O C: t~ .Q lh,,.

O

<

0.2

5 0

..

350

400

t

500

600

Wavelength (nm)

Figure 15. Temperature dependence of absorption spectrum of the viologen bilayer membrane having biphenyl chromophore, C I2BphC5 V2+ 2Br- in water.

5.2. Temperature dependence of CT interaction in bilayer membranes Thermally induced crystal-to-liquid crystal phase transition is one of fundamental properties of bilayer membranes in water. Restricted molecular ordering of the aromatic segment is abruptly loosen by the thermally induced fluidization of the alkyl chain above the phase transition temperature. The visible absorption band sharply decreases when the aqueous solution of C12BphC5V2+ 2Br- is heated above 30"C and appears again on cooling (Figure 15). Assuming that the temperature is a crystal-to-liquid crystal phase transition temperature of the bilayer membrane, the CT interaction is weakened in the fluid bilayer membrane. A probable spatial arrangement and strictly fixed molecular orientation of the viologen head and the biphenyl segment are strongly required for the CT complex formation in the crystalline phase of the aqueous bilayer membrane. Coupled cycles of heating and cooling of the water solution gave reversible spectral change of the CT band. Thermochromic property of the CT absorption based on the dynamic characteristics of molecular membranes has been found by Fuhrhop et al, too [24]. The visible absorption band vanished when the viologen amphiphile was diluted in the mixed bilayer membrane with dioctadecyl-dimethylammonium bromide, too. The mixing experiment indicates that the observed band is attributed to the intermolecular CT interaction, not to the intramolecular interaction.

57

5.3. Photochemically induced charge separation from the CT complex by visible light irradiation. Effective coupling of photochemically induced charge separation and successive electron transfer is required for construction of artificial photosynthesis based on supramolecular assemblies. Photoionization processes of viologen derivatives in bilayer membranes were intensively studied by Kevan et al [25]. Matsuo et al. found that the bilayer membrane formed from the double-chain viologen amphiphile act as an electron pool of photochemically formed viologen radicals and the life time of the radical was extremely prolonged in the bilayer membrane [26]. Efficient electron transport along the electrochemically active surface of the monolayer assembly formed from viologen amphiphiles were demonstrated by Bourdillon and Majda [27].

|

i i

500

600

700

800

900

Wavelength (nm) i

I

i

I

I

1

2

I

'1

Flash

E 0 0 r tl;l r t15 0 CO

<

i

0

.,

1.,,

3

!

4

Time (msec)

Figure 16. Flash photolysis of the viologen bilayer membrane having biphenyl chromophore. [C12BphCsV2+ 2Br- ] - 10mM. a; transient absorption spectrum 50msec after visible light irradiation, b; decay of 600nm absorbance after photolysis.

58

Taking advantages of efficient electron migration through the two-dimensional viologen array, our viologen bilayer membrane including CT complexes is expected to be an effective charge separation device for artificial photosynthesis. Viologen radical was generated when the CT band was excited in a flash photolysis experiment. The transient absorption spectrum at 50msec after visible light irradiation is shown in Figure 16a. Absorption spectrum having 630 nm maximum is ascribed to the viologen radical formed by photochemical reduction. A time course of 600nm absorption after photolysis clearly shows that the radical gradually decays in over 4 msec (Figure 16b). Color of the water solution turned from dark red to purple when a visible light longer than 440 nm from a 150W Xe lamp was continuously irradiated to the aqueous bilayer solution of C 12BphC5 V2+ 2Br- in the presence of EDTA as a sacrificial electron donor. A new absorption band at 560 nm, suggesting dimer radical formation [28], gradually increased during the continuous visible light irradiation (Figure 17). The spectral change suggests that the photogenerated electrons from the excited CT complex migrate along the two-dimensional viologen array and are accumulated as a stable dimer radicals. CT complex formation is essential for the photoinduced radical formation because the visible light irradiation to an aqueous micellar solution of octadecylviologen dibromide was not effective for the color change due to the radical formation, even in the presence of sacrificial electron donors.

~

/,,oo,.

//::~

.~o.+

.

it t,

b.

"~t

200

. . . . . . .

9

300

__

I

i

/

It

240

-

."

_

400 $00 600 Wavelength(rim)

700

800

Figure 17. Formation of viologen radicals on visible light irradiation in the presence of EDTA as a sacrificial electron donor.

CT complexes are often used as a charge separation device in artificial photosynthesis or photoconductive materials. Formation of the long lived viologen radical in C 12BphC5 V2+ 2Brbilayer membrane via CT complex excitation suggests a very fast electron exchange process among the viologen groups oriented in the two-dimensional molecular arrangement. We are currently investigating an application of the CT bilayer membrane as a photochemical electron pool and an electron mediator for an enzyme reaction.

59 6. U t i l i z a t i o n o f t w o - d i m e n s i o n a l m o l e c u l a r assembfies as novel functional molecular materials 6.1. M o l e c u l a r r e c o g n i t i o n The balance of the chain length between the spacer group (m) and the alkyl tail (n) is another determining factor of aggregation structure. Structural analysis of the single crystals indicates that hydrophilic ammonium groups tightly bind with bromide ions to form two-dimensional charge network at the bilayer surface. A free space larger than the van der Waals volume of bromide anion (closed circle in structural models of Figure 9) is required between ammonium ions. If the amphiphiles with the structural relation of m-n=2 are packed as the structural model I, the distance between the terminal methyl group of the alkyl tail and the bromide anion of the adjacent molecule (0.41 nm) is very close to their van der Waals distance (0.39 nm). In the case of the packing model I, molecules are densely packed with full n-electron overlap of the azobenzene chromophores. If the difference of the chain length (m-n) is smaller than two, the adjacent molecules must slide out to secure enough spaces of the bromide ions (structural model II in Figure 9). Then only partial overlapping of azobenzene chromophores is allowed in the molecular packing of the model II. The difference between the chromophore overlapping in the model I and II is assumed to be reflected in spectral difference of these groups (I: kmax = 300 nm, II: Z.max - 320 nm). Amphiphiles of m-n>2 are classified in the group I, too. A large hypsochromic shift to 300nm in the absorption spectrum indicates side-by-side molecular packing with full overlapping of the azobenzene chromophores. The long period of these cast films is independent of the total chain length (see m=10 series in Figure 8b). A modified molecular packing model proposed as the structural model I' is consistent with experimental results of Xray diffraction and spectroscopy. A peculiar void space under bromide counter ion is characteristic for this structural model.

0.5 ~

c

~

0

3

(1:1)

8

"~,

00.1

200

J/

""

~:

. C4S03 "

300 400 Wavelength (nm)

500

Figure 18. Absorption spectra of CsAzoC10N+Br - cast film containing alkylsulfonate. Spectral experiments on incorporation of sodium all~'lsulfonates as guest molecules into the host bilayer assembly strongly suggest formation of the void space expected in the structural model I'. Cast films of 1:1 mixture of CsAzoCIoN+Br - and sodium n-butylsulfonate (~.max = 303 nm) or sodium propylsullonate (~.max = 302 nm) showed a similar absorption spectrum of pure CsAzoC10N+Br cast film (~.max = 301 nm). Longer guest molecules; sodium npentylsulfonate (Z.max = 326 nm), n-hexylsulfonate (z.max = 329 nm), and n-heptylsulfonate

60 (~.max = 337 nm), disrupted the original spectrum of the host bilayer assembly (Figure 18). If a host cast film has no void space, e.g. C8AzoC10N+Br -, absorption maximum of the azobenzene chromophore does not shift even in the presence of a long chain guest sulfonate (Figure 19). Schematic models of the host-guest interaction is summarized in Figure 20.

E = E

350

-

340

-

O

C5 AzoC

N+

9CSAzoC

N+

330

o/

320 310 300 290

~ -

-" 2

-

' 4

n

'

' 6

'

' 8

10

(CnSO3-)

Figure 19. Absorption maxima of CsAzoCloN+Br - cast film (open C8AzoC 1oN § Br- film (square) incorporating guest alkylsulfonate molecules.

a

circle)

and

b Smaller size Guest

rU[rlr~

IGuest ncorporat Mol(~:eculioenfs~

Kmax = 300rim Incorporation of Guest Molecules

of

CsAzoC10N +

CsAzoC10N +

Zmax = 300nm

kmax = 300am

hmax = 30Ohm Larger size Guest

bnax = 340am

Figure 20. Schematic models of host-guest interaction. Host membranes are (a) CsAzoC10N+Br - and (b) C8AzoCloN+Br - , respectively.

61

6.2. P h o t o c h e m i c a H y i n d u c e d p h a s e t r a n s i t i o n o f cast b i l a y e r m e m b r a n e s Structural polymorphism has been already reported as a peculiar solid-solid phase transition with a large spectral shift in the cast film of CSAzoC10 N+ B1- (chapter 4). The type I spectrum was thermally transformed to the type VI spectrum and then backed to the type I by the isothermal moisture treatment. The reversible spectral change between the type I and VI is a good experimental evidence of Okuyama's prediction on the molecular packing. Since the type VI state is assumed to be a metastable state, the isothermal phase transition to the type I state is expected to be induced by some external stimuli. Water molecules adsorbed to cast bilayer films might act as an accelerator of the phase transition. Organic compounds which show reversible color change by a photochemical reaction are potentially applicable to optical switching and/or memory materials. Azobenzenes and its derivatives are one of the most suitable candidates of photochemical switching molecular devices because of their well characterized photochromic behavior attributed to trans-c/s photoisomerization reaction. Many works on photochromism of azobenzenes in monolayers LB films, and bilayer membranes, have been reported. Photochemical isomerization reaction of the azobenzene chromophore is well known to trigger phase transitions of liquid crystals [2931 ]. Recently we have found the isothermal phase transition from the state VI to the state I of the cast film of CsAzoC10N + Br- induced by photoirradiation [32]. A typical photochemical isomerization of the azobenzene amphiphile was found in an ethanol solution. A trans isomer converted to a cis isomer with ultraviolet irradiation. Back reaction from cis to trans was accelerated when a weak n-n* absorption band of the cis isomer at ca.450nm was excited (Figure 21a). An alternative irradiation of uv and visible light to the ethanol solution gave reversible changes of the n-~t* transition between 355nm and 325nm attributed to the trans and cis isomers, respectively.

1.0

EtOH

Cast film

/

~ 3 ) Visible /: : ~

k 0.5 ~o

0.4

1) before irradiation irradiation

(lOmi.) t_.

0.2 5 rain ~- I0

t0.0

200

..:::~,"

_.

rain

,.,

400

Wavelength i nm

600

o'L 20O

30O

4O0

50O

Wavelength I nm

Figure 21. Spectral change of azobenzene amphiphile CsAzoC10N+ Br- on photoirradiation. (a) ethanol solution. (b) cast film at 50"C in dry condition. Photoisomerization of the azobenzene amphiphile was found to be strongly affected by molecular packing and orientation in the aqueous bilayer solutions. A rate constant of trans to cis isomerization was extremely faster in the liquid crystalline state than in the crystalline bilayer membrane [33]. Photoreaction of the aqueous bilayer membrane of C8AzoCIoN + Br- was

62

completely depressed in the crystalline state below phase transition temperature. The cast film of CsAzoCIoN + Br- in the spectral state I did not show photochemical response on uv irradiation, too. A large spectral change similar to that of the moisture induced isothermal phase transition was found when the annealed film was photoirradiated at room temperature in a dry atmosphere (Figure 21b). Irradiation with uv light from a 150W Xe lamp equipping a UV-D 35 cut filter decreased a large absorption band located at 370 nm attributed to a n-n* transition of the trans isomer concurrently with increase of 300 nm absorption, whose spectral shape was different from that of the cis isomer generated in the ethanol solution. Absorption band at 450nm attributed to the n-n* transition of the cis isomer increased at the first stage of the photoirradiation and then decreased gradually. The spectral change completed within 20 min in this experimental condition. After uv light irradiation, only a small amount of the trans isomer at 370 nm was regenerated with irradiation of the visible light passed through a Y-46 cut filter. The spectral shift due to uv light irradiation is ascribable to an isothermal phase transition triggered by a photochemical isomenzation reaction. Photoinduced spectral change was commonly found in the cast film of every amphiphile in the group I. Alternative irradiation with uv and visible lights gave us useful information. Semiquantitative estimation of the isomenzation reaction and the induced phase transition is given by the alternative irradiation experiments. At the initial stage of the photoirradiation, a net amount of the trans isomer transforming to the spectral state I is estimated by an alternative irradiation of uv and visible light. Absorption change in the first uv irradiation includes the spectral change resulted from the trans-to-cis isomerization and that from the induced phase transition. After successive visible light irradiation, only reversible spectral restoration caused by the cisto-trans isomerization is observed (Figure 22). About 45% trans isomer is converted to cis isomer in the first uv irradiation at 64"C. In the second uv irradiation, the conversion decreases to 2 5 % . If the alternative irradiation experiments is conducted in the ethanol solution, the conversion of cis isomer is constant because the isomerization reaction of azobenzene is completely reversible. The residual cis isomer gradually decreases with repeating the alternative irradiation.

~

0.4

2 (lst. visible)

i/~~4

( 2rid. visible)

0.2

200

0 (before in-ad)

300

400

,.,,,)

500

wavelength (nm) Figure 22. Spectral change of CsAzoC10 N+ Br- cast film by alternative irradiation of uv and visible light on the cast film at 80~ in dry condition. An irradiation interval is 1 rain for each light.

63

a

1.0

0.8 0

< <

i CD

0.6 0

0.4

13 8o~ 9 ioo-c A

0.2 0.0_---~_ 0

4o% 9 6o*c

9

, 5

, 10

120"(2 140~

, 15

Time (min)

b 1.0 0.8 o

<

0.6

<

i

c

<

0.4 0.2

o ~ 0.0 20

, 80

O 140

200

Temperature (~ Figure 23. Temperature dependence of the photoinduced phase transition. (a) time courses of absorbance change at various temperature. (b) temperature dependence of apparent initial rate of phase transi tion.

64 Kinetics of the photoinduced spectral change was found to be affected by the irradiation temperature. Figure 23a shows time courses of the spectral change of cast films on uv light irradiation at various temperature conditions. A0 and At are absorbance at 370nm before photoirradiation and after irradiation for t min, respectively. The absorbance ratio, (A0 - At) / A0, is a kinetic probe of the photoinduced spectral change. An apparent initial rate constant estimated from the absorbance ratio after 1 min irradiation, (A0 - A1) / A0, increased with increasing temperature up to ca. 70"C and then decreased (Figure 23b). Owing to higher molecular mobility, the isomerization reaction is assumed to be preferable with increasing temperature. With rising temperature more, however, the backward thermal reaction from cis to trans should depress the spectral change. Since the crystalline state I can not exist above phase transition temperature, no spectral shift to 300nm was observed at higher temperature than 115"C. The photochemically generated cis isomer, which enhances the instability of the metastable state VI, could trigger the isothermal phase transition to the stable state I. As shown in a schematic model of the phase transition (Figure 24), reversible spectral switching conducted by the alternative combination of heating and uv-irradiation could be successfully repeated for many times as well as by the coupling of heating and moistening. Since the spectral change induced by the peculiar phase transition is "one-way" direction, the immobilized bilayer membrane will be available to an erasable memory material based on the phase transition triggered by thermal and photochemical processes. Detailed mechanism of photo- and moisture-induced phase transition is now investigating.

moistening

State I <

J cis

hv

State VI hv (UV) l

~

heating Figure 24. Schematic model of photoinduced phase transition of the cast bilayer film.

7. Immobilization of bilayer membranes as ultra thin polymer films 7.1. Preparation of polymeric bilayer cast films by polyion complex technique Immobilization of the bilayer membranes as thin solid films is required when the bilayer membranes are used as novel functional materials. Casting method is a simple way to immobilize the bilayer membrane on a solid support from an aqueous solution by d~ing. Polymer film is easily prepared when the cast film of polymerizable bilayer membrane is polymerized. A free standing polymer film prepared by photo polymenzation of the cast film of diacetylene amphiphiles was reported by O'Bnen and co-workers [34]. Composition with macromolecular materials is another way of polymer film preparation. Bilayer membranes are immobilized as polymer composites by the following physical methods; (1) Porous polymers, e.g. a membrane filter or a nylon capsule [35], are soaked in organic solvents containing bilayer forming amphiphiles and then dried.

65 (2) Bilayer-forming amphiphiles are cast with hydrophobic polyrner(e.g, poly(vinyl chloride)) from organic solvents. A large cluster of the bilayer membrane are formed as phase separated micro domains in the polymer matrix [36]. (3) Cast with water soluble polymer from an aqueous solution. Poly(vinyl alcohol) is a suitable inert matrix for supporting bilayer membranes [37]. Water solubility of the films composed with poly(vinyl alcohol) can be lowered by coating with celluloseacetate [38] and closs-linking of polymer [39]. (4) Polyion complex technique [40] is a unique method for immobilization of bilayer membranes with polymers. Water-insoluble complex is precipitated as the polyion complex when an aqueous solution of the charged bilayer membrane is mixed with a water solution of the counter charged polyelectrolyte. Stoichiometric ion pair formation is often found. Aging of the precipitate in a hot mixture kept above phase transition temperature of the bilayer membrane completes the ion exchange reaction [41]. Chloroform solution of the polyion complex is washed by water several times to remove water soluble components [42].

--+

OI

,so3" K*

0SO3" K+

OR .Jn R--SOaNa

1

2

3

"-'(" CH2IH -)n----(-CH2-TH-')--m so3- K* CONH2

CH2OH

Ro/~

q

-

O

c,

O

OH

R=SOaNa 6

__•

H2OR

o,

CO01~a I~~NHC 0 I_

soa" K*

_

R

~O OR R=SOaNa

I~//~ H2OR n

_3~'~1:R

7

~~-O OR R=CH2COONa

n

8

Scheme 3. Anionic polymers used for polyion complex formation.

The fundamental bilayer characteristics (two-dimensional ordering, phase transition, and phase separation, etc.,) are mostly maintained in the immobilized films with and without polymers. Aging or thermal treatment on the as-cast films improve the film properties because

66 structural defects are often formed in the as-cast films. Enhancement of the layer correlation after thermal treatment is reflected in number and sharpness of the X-ray reflections [43-45]. Taguchi [43] reported the annealing effect on phase transition property of the polyion complex film prepared from dihexadecyldimethyl ammonium and poly(styrene sulfonate). In their experiment, the as-cast film showed no transition peak in the differential scanning calorimetry (DSC). After annealing of the complex film in a hot water (60"C) for 10min a sharp endothermic peak (at 28"C) caused by phase transition was dearly observed in DSC. The advantage of the polyion complex technique is combination varsatilities of polymers and amphiphiles. Charged polymers used as counter ions are shown in Scheme 3. The bilayer characteristics should be occasionally affected by electrostatic interaction with polymers. It is well known that the bilayer characteristics are often disturbed when the bilayer membrane polymerizes itself or combines with some polymers [46]. Ringsdorf and co-workers showed that introduction of the hydrophilic spacer groups into the polymer main chain effectively reduced the stenc influence of the polymer main chain on ordered molecular packing of bilayer membranes [47]. Increased flexibility of the polymer main chain of the copolymer might allow the higher ordenng of the alkyl chain packing of the bilayer forming amphiphiles [48]. Similar effect of the spacers introduced in the copolymer on the molecular ordering is found in the immobilized cast films by the polyion complex technique [32,49]. Absorption maximum of the cast film of C8AzoC10N§ B r complexed with poly(styrene sulfonate) (polymer 3 in scheme 3) was located at 323nm. The red shift from the original cast film (300nm) without polymer suggested that the chromophore packing in the complex film was loosen due to the steric influence of the polymer main chain. The long spacing of the complex film calculated from X-ray diffraction was slightly longer than the original film but the layer correlation was very poor. Spectral shift from 323nm to 347nm induced by thermal phase transition of the complex film was also different from that of the original film. Transition temperature of the complex film was lower than the original one. Complex film with copolymer of acrylamide and styrene sulfonate (polymer 5) showed similar spectral property of the onginal film. The layer correlation of the complex film with the copolymer was better than that of the complex film with the homopolymer. The layer correlation of the polyion complex film of homo- and copolymer are schematically represented in Figure 25.

horrr~tyrr~

Figure 25. Schematic models of spacer effect of polymer main chain on molecular ordering of polyion complex films.

67 7.2. S p e c t r a l c h a n g e of polyion p h o t o c h e m i c a l p h a s e transition.

complex

films

due

to

thermal

and

Electrostatic interaction between polyanions and bilayer membranes often seems to disturb molecular ordenng of the bilayer assembly. Judging from the results of the spectral experiments and X-ray diffraction studies, the H-aggregate is basically kept in the polyion complex film. The peculiar thermal transition in the absorption spectrum are also observed in polyion complex films. The temperature of the spectral change and spectral shape are strongly dependent on the chemical structure of anionic polymers. Spectral shift to 360rim on heating is found for the complex film with copolymer of vinylsulfonate and acrylamide 4. While in the case of the complex film with poly (vinyl sulfate) 1, xmax moves to 340nm; type III or IV specmnn (Figure 26). The results indicates that the fine tuning of the molecular assemblage can be controlled by the polyion complex technique. As was described in section 7.1., spacer effect of polymer chain on molecular packing of bilayer membrane is also observed in this figure.

a

b

1.5-

~

,~

,,

,

,

0.8

.1

a/ter annealed

1.2 1.0 0.8

~.O-~O'C

/

'

/

.,,I~>-v.O'C

1-

~0.6

.

,

,

,

t

as cast

c

,~o.6

|

after ar.~-aled

80"C XtC ~C

tn z3

<0.2 300

400

500

wavelerNth(nm)

0

--,

90"C

"~0.4

200

,

' 400 wave(ength(n m)

2oo

36o

S00

Figure 26. Temperature dependence of absorption spectrum of CsAzoC10 N+ B r complexed with anionic polymer, (a) polymer 4 and (b) polymer 1.

. . . . . .

L

.

.

.

.

.

.

:

- -

7 1.0

0

2

4

6

8

10

Time (min) Figure 27. Polymer effect on isothermal spectral transition induced by moisture treatment of polyion complex films. (relative humidity = 62%)

68 The moisture induced isothermal transition are also observed in the complex films. The speed of the isothermal transition are also strongly dependent on the chemical structure of the counterpart polymer (Figure 27). For example, the J-aggregate in the complex film with polymer 2 is more stable than in the original cast film without polymer. Photochemically induced phase transition is also found in the polyion complex films. Transient behavior of the cis isomer formation is more apparent in the complex films with polymer 6 and 7.

1.2

a

b 9

~

.

.

/,~~0 rain.

a8 i

[~

,I,_~0

min,

Jo

2oo-3oo'

4bo

wavelength (rim)

0200

3oo

4oo

soo

wavelength(nrn)

6o0

Figure 28. Spectral change of annealed polyion complex films of CsAzoC10N + Br- by photoirmdiation. (a) with polymer 7, (b) with polymer 6. Photochemical switching of the phase transition is also found in the polyion complex film. Figure 29 shows reversible cycles of the absorption at 370nm by the coupling of the thermal and photoinduced phase transition of the complex film with carboxymethylcellulose 8. In conclusion, we indicate that the immobilized bilayer membranes containing the azobenzene chromophore are available to the erasable memory materials based on the phase transition triggered by thermal and photochemical processes. The polyion complex technique is clearly shown to be a very useful method for materialization of the immobilized bilayer membranes.

CONCLUSION We systematically investigate spectral properties and aggregation structure of solvent cast films of forty-three azobenzene amphiphiles. It can be concluded that the alkTl chain length of the amphiphile is a definitive determining factor for the molecular orientation in the cast bilayer film. Our findings are one of successful examples of structural prediction and design of two-dimensional pseudo-cry'stals, the so-called "crystal engineering". On the basis of the "crystal engineering" of the two-dimensional molecular assemblies, we are developing cast bilayer films as novel molecular materials potentially used as charge transfer, molecular recognition, and optical memory devices. Tailored charge transfer complexes in the viologen bilayer assembly effectively produce long lived viologen radicals on visible light irradiation. The peculiar void space formed in the aggregation structure of the model I' could recognize size and shape of guest molecules. Taking advantages of the reversible spectral switching by heating and photoirradiation and a large spectral bistabilit3" due to the structural polymorphism, the cast bilayer films of the azobenzene amphiphiles are applicable as an optical memory device.

69

~"

0.3

:~

0.1

0

10

repealing

20

30

Figure 29. Reversible cycles of spectral change of polyion complex film by coupling of heating (higher absorbance) and photoirradiation (lower absorbance).

REFERENCF~

1. T.Kunitake, Angew.Chem.Int.Ed.Engl., 31 (1992) 709. 2. H.Ringsdorf, B.Schlarb, J.Venzmer, Angew.Chem.Int.Ed.Engl.,27 (1988) 113. 3. J.H.Fendler, Membrane Mimetic Chemistry, Wiley-Interscience: New York, 1982. 4. M.Shimomura, Prog.Polym.Sci., 18 (1993) 295. 5. M.Shimomura, R.Ando, T.Kunitake, Ber.Bunsenges.Phys.Chem., 87 (1983) 1134. 6. Y.Okahata, R.Ando, T.Kunitake, Ber.Bunsenges.Phys.Chem.,85 (1981) 787. 7. M.Shimomura, T.Kunitake, Chem.Lett., (1981) 1001. 8. T.Kunitake, M.Shimomura, T.Kajiyama, A.Harada,K.Okuyama, M.Takayanagi,Thin Solid Films. 121 (1984) L89. 9. (a) K.Okuyama, Y.Soboi, K.Hirabayashi, A.Harada, A.Kumano, T.Kajiyama, T.; Kunitake, M.Takayanagi, Chem.Lett., (1984) 2117. (b) K.Okuyama,Y.Soboi, N.Iijima, K. Hirabayashi, T.Kunitake,T.Kajiyama, Bull.Chem.Soc.Jpn.,61 (1988) 1485. (c) K.Okuyama, N.Iijima, K.Hirabayashi, T.Kunitake, M.Kusunoki, Bull.Chem. Soc.Jpn., 61 (1988) 2337. 10. S.Asakuma, H.Okada, T.Kunitake,J.Am.Chem.Soc., 113 (1991) 1749. 11. A.Harada,K.Okuyama, A.Kumano, T.Kajiyama, M.Takayanagi, T.Kunitake, Polym.J., 18 (1986) 281. 12. M. Kasha, Radiat. Res., 20 (1963) 55. 13. D.Beveridge, H.Jaff6,J.Am.Chem.Soc., 88 (1966) 1948. 14. K.Okuyama, H.Watanabe, M.Shimomura, K.Hirabayashi, T.Kunitake, T.Kajiyama, N.Yasuoka,Bull.Chem.Soc.Jpn., 59 (1986) 3351. 15. K.Okuyama, C.Mizuguchi, G.Xu, M.Shimomura, Bull.Chem. Soc.Jpn., 62 (1989)3211. 16. G.Xu, K.Okuyama, M.Shimomura, Bull.Chem.Soc.Jpn., 64 (1991) 248. 17. G.Xu, K.Okuyama, M.Shimomura, Mol.Cryst.Liquid.Cryst., 213 (1992) 105. 18. G.Xu, K.Okuyama, M.Shimomura, Bull.Chem.Soc.Jpn., 66 (1993) 2182. 19. G. Xu, K.Okuyama, K.Ozawa, M.Shimomura, Mol.Cryst.Liq. Cryst., 237 (1993) 207.

70 20. K.Okuyama, M.Ikeda, S.Yokoyama, Y.Ochiai, Y.Hamada, M.Shimomura,Chem.Lett., (1988) 1013. 21.M.Shimomura, Y.Hamada, N.Tajima, K.Okuyama,J.Chem.Soc., Chem.Commun., (1989) 232. 22. K.Okuyama, NFormation of Bimolecular Films and Crystal Structure" in "Reactivity in Molecular Crystals", Ed. Ohashi, Y. Kodansha VCH, (1993) pp. 299. 23. T.Kunitake,Y.Okahata, M.Shimomura, S.Yasunami, K.Takambe,J.Am.Chem.Soc., 103 (1981) 5401. 24. J.H.Fuhrhop, D.Fritsch, J.Am.Chem.Soc., 106 (1984) 4287. 25. M.Sakaguchi, M.Hu, L.Kevan, J.Am.Chem.Soc., 106 (1990) 4287. 26. T.Nagamura, N.Takeyama, T.Tanaka, T.Matsuo, J.Phys.Chem., 90 (1989) 2247. 27. C.Bourdillon, M.Majda, J.Am.Chem.Soc., 112 (1985) 1795. 28. E.M.Kosower, J.L.Cotter, J.Am.Chem.Soc., 86 (1964) 5524. 29. E.Sackmann, J.Am.Chem.Soc., 93 (1971) 7088. 30. K.Ichimura, Y.Suzuki, T.Seki, A.Hosoi, K. Aoki, Langmuir, 4 (1988) 1214. 31. T.Ikeda, T. Sasaki, K. Ichimura, Nature, 361 (1993) 428. 32. M.Shimomura, N.Tajima, K.Kasuga, J.Photopolym.Sci.Tech., 4 ( 1991) 267. 33. M.Shimomura, T.Kunitake, J.Am.Chem.Soc., 109 (1987) 5175. 34. T.Kuo, D.F.O'Brien, J.Am.Chem.Soc., 110 (1988) 7571. 35. Y.Okahata, Acc.Chem.Res., 19 (1986) 57. 36. T.kajiyama, A.Kumano, M.Takayanagi, Y.Okahata, T.Kunitake, Chem.Lett., (1979) 645. 37. S.Hayashida, H.Sato, S.Sugawara, Chem.Lett., (1983) 625. 38. M.Shiomomura, T.Kunitake, Polym.J., 16 (1984) 583. 39. N.Higashi, T.Kunitake, Polym.J., 16 (1984) 583. 40. T.Kunitake, A.Tsuge, N.Nakashima, Chem.Lett., (1984) 1783. 41. Y.Okahata, G.En-na, J.Phys.Chem., 92 (1988) 4546. 42. M.Shimomura, K.Utsugi, J.Horikoshi, K.Okuyama, O.Hatozaki, N.Oyama, Langmuir, 7 (1991) 760. 43. K.Taguchi, S.Yano, K.Hiratani, N.Minoura, Nippon Kagaku Kaishi, (1990) 1373. 44. M.Shimomura, H.Ihara, J.Photopolym.Sci.Tech., 4 ( 1991) 199. 45. T.Seki, K.Ichimura, Macromolecules, 20 (1987) 2958. 46. R.Btischl, H.Ringsdorf, Makromol.Chem., Suppl. 6 (1984) 245. 47. A Laschewsky, H.Ringsdorf, G.Schmidt, J.Schneider, J.Am.Chem.Soc., 109 (1987) 788. 48. J.Schneider, H.Ringsdorf, J.F.Rabolt, Macromolecules, 22 (1989) 205. 49. K.Kasuga, M.Inaba, M.Shimomura, Polym.Prep.Jpn., 40 ( 1991) 964.

New Developments in Construction and Functions of Organic Thin Films T. Kajiyama and M. Aizawa (Editors) 9 1996 Elsevier Science B.V. All rights reserved.

71

Control of molecular orientation and packing in monolayer assemblies Hiroo Nakahara Department of Chemistry, Faculty of Science, Saitama University, Shimo-okubo 255, Urawa, Saitama, 338 Japan Molecular aggregations and domain structures in monolayers on the water surface were observed by a multichannel photodetector and a Brewster angle microscope in situ, and the monolayer assemblies were constructed by the Langmuir-Blodgett method as well as the horizontal lifting technique to give a well-defined organization of amphiphilic functionalized molecules in ultrathin films. The physico-chemical behaviours such as photoinduced electron transfer and polymerization reaction can be investigated in relation to the molecular arrangements in organized thin films to clarify the effects of orientation and packing of the molecules upon some functions.

1. I N T R O D U C T I O N Prior to fabrication of functionalized LB films, control of molecular orientation and packing as well as domain structures and sizes in monolayers on the water surface should be required. Recently, new optical instruments such as multichannel photodetector (MCPD) [1], fluorescence microscope [2] and Brewster angle microscope (BAM) [3], have been invented to observe some physical states of the monolayers in situ. In the present report, for examples, using merocyanine and squarilium dyes control of molecular aggregations such as J- and H-forms has been demonstrated in the monolayers under different conditions such as surface pressures and molecular enviroments, and further some phase transitions and the heterogeneous domain structures in the monolayers have been exhibited for the BAM images. Various amphiphilic functionalized molecules were synthesized in order to control orientation and packing of the functional groups in organized thin films by changing the substituents, their number and positions. Adsorption and/or molecular recognition of functional molecules by the spread monolayers at the air/water interface are very useful for controlling the molecular orientation. The monolayer assemblies with various ordered and well-defined structures have been fabricated by the conventional Langmuir-Biodgett (vertical dipping) method as well as the horizontal lifting technique [4], and Some physico-chemical behaviours such as electronic conduction, photoinduced electron and energy transfer and polymerization reactions in the molecular organized films, have been investigated to clarify effects of the molecular orientation and packing upon several functions in

?2 the ultrathin layered structures. These studies are considered to be essential to deduce some biological and artificial functions of molecular organized systems for developements of future life and material science.

2. E X P E R I M E N T A L 2.1. M a t e r i a l s

Samples of immunoglobuline G (IgG) unlabelled and labelled with fluorescence isothiacyanate (FITC) were purchased from Cappel Laboratories Inc. Long-chain merocyanines (MC), oxyocyanine and thiacyanine dyes were purchased from Japanese Research Institute for Photosensitizing Dyes, Co. (Okayama, Japan) and various squarylium dyes substi- tuted with different lengths of alkyl chains were synthesized by L.S.Pu et al. of Fuji Xerox Co. [5]. Amphiphilic cyclodextrins were synthesized by W.Tagaki et al. of Osaka City University [6]. Several quinacridone derivatives with four alkyl chains and phthalocyanines with eight alkyl chains were synthesized by H.Nishi, et al. of Saitama University [7, 8]. Other common fatty acids and their derivatives, various glycerol derivatives and phospholipids were obtained commercially and purified by recrystallization. Naphthylalanine and pyrenylalanine were synthesized as reported previously [9], and octadecylesters of amino acids were obtained by esterification with octadecanol in the presence of p-toluenesulfonic acid, followed by desalting with ammonium gas.

2.2. P r o c e d u r e s

Monolayers were spread onto the distilled water surface or the aqueous subphase containing 3 x 10-4 M CdCI2 and 5 x 10-5 M KHCO3 (pH 6.3) from chloroform, benzene or toluene solutions and surface pressure - area isotherms were measured by a Langrnuir-type film balance (Lauda, FW-1) or a Fromherz type multicompartments trough with a Wilhemy film balance. The morphology of a monolayer on the water surface was obtained in situ by a Brewster angle microscope (BAM 1, Nanofilm Technologie GmbH, FRG) and the monolayer spectra at the air/water interface were measured by a multichannel photodetector (Otsuka Electronics) with reference to a spectrum obtained without the monolayer. The monolayers were transferred onto various solid substrates such as glass, quartz, CaF2 and Si plates by the Langmuir-Blodgett method as well as the horizontal lifting technique. Mass changes during the deposition process were monitored by frequency of Pt electrode (0.196 crn2 in area) on the oscillating quartz crystal plate (9 MHz, AT-cut). Molecular orientations in the monolayer assemblies were examined by polarized UV-Vis. and IR spectra, using a Hitach spectrophotometer (model 340) and a JEOL FT-IR spectrometer (JIR-100), respectively. Fluorescence spectra were measured by a Hitachi MPF-3 fluorescence photometer and the fluorescence lifetimes were obtained by a picosecond time-correlated single photon counting system. Measurements of ultraviolet photoelectron spectra were carried out using an angle-resolved photoelectron spectrometer combined with a plane-grating monochrometer at the

?3 UVSOR storage ring in the Institute for Molecular Science. Circular dichroism for LB films was measured by a JASCO spectropolarimeter (J-600) with rotating the film sample to remove the influence of linear dichroism.

3.

RESULTS

3.1.

AND DISCUSSION

observation of monolayers of fatty acids on the water surface by a Brewster angle microscope, Although a monolayer on the water surface has been generally considered to have various phases such as gas, liquid expanded, intermediate, liquid and solid condensed, two-dimensional solid domains were observed in some monolayer systems even at the zero surface pressure right after spreading by using a fluorescence microscope [10, 11]. The domains are accumulated together to form a condensed monolayer during the compression. Recently, Kajiyama and co-workers [12, 13] have suggested that the physical states of fatty acid monolayers on the water surface can be classified into the crystalline and amorphous states, which depended on the difference of the aqueous subphase temperature (Tsp) from the melting temperature (Tin). On the basis of both apparent decrease in the monolayer modulus v.s. Tsp curve and the change of the electron diffraction pattern from a crystalline Debye ring to an amorphous halo for the transferred monolayers. Values of Tm for myristic, palmitic and stearic acids were evaluated to be 278, 301 and 317 K, respectively. The authors determined directly these melting points of fatty acid monolayers on the water surface by using a Brewster angle microscope [14]. Figure 1 shows, as an example, BAM pictures of the melting process in a stearic acid monolayer during increasing Tsp up to 50 ~ The domains are accompanied with a solid boundary at the Tsp below 25 ~ When the Tsp increased gradually, the shape of the domains flexible, and the reflectivity of the monolayer reduced, although they still have the clear boundaries (Figure 1 (a)), which might be called as a soft domain. After the Tsp is above 45 ~ the domains become smaller and the boundaries become vaguer, some part of the monolayer material is melted, as shown in Figure 1 (b), where both soft domain and the molten monolayer phases are coexisted. The two dimensional domains are dissolved completely and the monolayer becomes dark and homogeneous above 47 ~ as shown in Figure 1(c). Figure 1 (d) shows that the sizes of the domains I n situ

Figure 1. Melting process of a stearic acid monolayer at the air/water interface. The BAM images of the domain structures: (a) 41 ~ (b) 45 oC, (c) 47 ~ and (d) at 20 ~ after melting.

74 become uniform and their distribution becomes homogeneous after a stearic acid monolayer is cooled down back to the room temperature after melting. Therefore, the following melting model of a fatty acid monolayer could be suggested during increase of the temperature: Solid domains- =-Soft domains Two co-existed phases between domains and molten monolayer - ~ completely molten monolayer, as shown in Figure 2.

.

.

.

.

q

SOlid domain

" - - ' - - ' - -

- - - - - .

Soft domain

- - - - -

~

. - - - . . . -

9

. _ _ _

Two-phases co-existed

Molten monolayer

Figure 2. The schematic pictures of the monolayer melting process. Collapsing and nucleation processes of a stearic acid monolayer have been also studied on the different aqueous subphases by a Brewster angle microscope [15]. Both of the temperature and the pH value of the aqueous subphase can influence the nucleation process and produce very different collapsing patterns.

Figure 3. The nucleation process of the collapsed monolayer of stearic acid on acidic subphase at 15 ~ during over-compression process from (a) 19.5 to (c) 18.5 ~2/molecule. In the case of an acidic subphase at the temperature below 20 ~ both the number of the nuclei and their size increase during over-compression, as shown in Figures 3(a) and (b). If the m o n o l a y e r w a s compressed further, some neighbouring nuclei tend to unite with each other, and become large irregular clump-like nuclei shown in Figure 3(c). The lower temperature is, the more important the number-increase nucleation process is. Slight increase of the pH value of the subphase has the similar effect to the nucleation process. Moreover, it can be expected that a slow compression makes a larger nucleus in the monolayer, which has been supported by the BAM observation [16].

3.2. BAM observation for monolayers of tetraphenyiporphyrins substituted with four fluorocarbon chains. Recently, the selforganization amphiphilic porphyrins with alkyl chains at the

75 air/water interface has been studied by BAM [1 7]. LB films containing fluorocarbon chains have received much attention because of characteristic properties such as low friction, excellent insulation, remarkable durability and soon. The morphology change in monolayers of tetraphenylporphyrin derivatives with four fluorocarbon substituents (TFPPs) has been monitored by visualization with a BAM to obtain informations for nucleation and growth of two-dimensional crystallites in s i t u [18]. Monolayers of the tetraphenylporphyrin derivatives containing fluorocarbons with different chain lengthes, as shown in the insert of Figure 4, were spread from the chloroform or toluene solutions in concentration of 10 -5 M onto the twice distilled water surface. As shown in Figure 4, surface pressure - area isotherms at 20 ~ indicate that TFPP-2 and 3 have a transition from the liquid-expanded to condensed states, whereas TFPP-1 gives a stable condensed monolayer without any transition. Figure 5 shows the BAM images for TFPP-1,2 and 3 monolayers at zero mN/m and the molecular area about 400 ,~,2. It is considered that the bright parts are due to crystalline states and the lower reflectance implies amorphous or thin70 R ner monoayers. . X More h o m o g e n e 60 ous domains reguE -x 50 larly formed in the 'FFPI ~ R E condensed phase .,-'X\\ x ~ x: -oc.~ ~ 40 with some ordered T F P P- 2 , R "dots" (highly refTFPP-I: -O(2112(CF2)41l 3: 3O 'N \ " ~ X TFPP-2:-OClh(CF2s lective parts) were ~ % ~ R TI:PP-3:-OCt12(CF2)8It o b s e r v e d in t h e ~ 20 T F P P- 3 t" ~ ~ , ~ monolayer of TFPPr.~ 1, as compared with 10 those of the TFPP-2 and 3 in which the heterogeneous 40 80 120 160 200 240 domain structures Area (~2/molecule) were formed immeFigure 4. Surface pressure - area isotherms of diately after spreaTFPP monolayers at 20 ~ ding. The highly crystalline states u c'J

"N~

t

I

I,

9

Figure 5. BAM images at zero mN/m, T = 20 ~ for a) TFPP-1, b) TFPP-2, and c) TFPP-3.

?6 seem to be due to stronger interactions between the fluorocarbon chains existed in TFPP-2 and 3 comparing with those in TFPP-1, which is unfavorable to obtain a m o n o - d i s p e r s e d monolayer. It has been clearly demon- strated that the homogeneity of the TFPP monolayers depends on the substituent chain length. A similar behaviour was found for the monolayers at 2 mN/m.

Figure 6. Nucleation process of TFPP-1 monolayer spread from chloroform (A) and toluene solution )B) at zero mN/m, T = 20 ~ (a) after spreading (t = 0) and (b) t = 60 min.; (c) t = 120 min.; (d) t = 180 min. Figure 6 shows BAM images during the nucleation process of TFPP-1 monolayer at zero mN/m, spread from both chloroform and toluene solutions. In the primary stage, ripple structures were observed, which seems to be due to crystallization on the solvent evaporation. Some "dots" in the domain of the TFPP-1 monolayer as spread at zero pressure, grow for one hour to form a fine order structure and nucleation proceeds for 3 h to the brightened monolayer. Essenentially the similar results were observed for the TFPP-1 monolayer spread from the toluene solution. Since the evaporation rate of toluene is slower than that of chloroform and there is difference in polarity of these solvents, a relatively slower appearance of nuclei was observed in the early step for the TFPP-1 monolayer spread from the toluene solution.

Figure 7. Nucleation process of TFPP-1 monolayer on compression from zero to five mN/m within 30 min.(a - d). And further, this nucleation growing process was promoted by compression up to 5

77 mN/m for the TFPP-1 monolayer spread from the chloroform solution, as shown in Figure 7. However, heterogeneous monolayers were only formed for TFPP-2 and 3 and any nucleation or growing peocess of crystallites could not be observed at both zero and five mN/m.

3.3. Control of oriented antibody layer by monolayer assemblying technique. Immunosensors constructed by the Langmuir-Blodgett method were reported by using amperometric or potentiometric, optical interference, and radioactivity measurements [19 - 22]. The surface density and orientation of antigen or antibody molecules can be controlled by the LB techiques, and there is a possipossibility of realizing a well-designed interface for immunosensors. We have investigated immunological activity of the oriented IgG in the LB films. Firstly, adsorption of IgG molecules to the spread monolayers of various lipids were examined under different conditions. When the aqueous subphase contains IgG, the surface pressure of a lipid monolayer at a constant area increases with time, suggesting the adsorption of IgG. After sufficient time (usually I to 3 hours) the

E

lgG

z6 ~

(3

I

w

b

:::) CO

2s ,;

.

.

w

rr 4 0 i n

o3

.

A

co

tll (D <[: LL 0::

.

oA

20 I I

OL 0

I i i t

I

i

L

40 60 80 AREA (A2/molecule) Figure 8. Surface pressure - area isotherms for stearylamine before (a) and after (b) adsorption of IgG at pH 8, 20 ~ and schematic representation for IgG molecule adsorbed to stearylamine monolayer. 2O

78

lipid monolayer involving IgG was replaced onto the buffer solution, and the surface pre- - - - C~8N1-12 ssure - area isotherm was mea----anlibody sured. Figure 8 shows the pressure - area isotherms for stearyl)~---anlige n amine m o n o l a y e r before and Z3 d after the adsorption of IgG at pH C2oA >-8. Limiting areas obtained by pextrapolation of the linear parts to c0 zero pressure are increased from uJ 20 to 43/~,2/molecule. The optikz cally measured thickness of the uJ monolayers transferred onto the solid plates were also increased L_ LLI from 25 to 68,~. From these 0 u0 results, it can be supposed that LLI Q~ the rather hydrophobic tail (Fc) of 0 Y-shaped IgG molecule pene_J t r a t e s into the s t e a r y l a m i n e LL ;@ monolayer as shown in the insert of Figure 8. From measurements of the fluorescence spectra for the deposited films of the FITC J , l I l I I I , ' ' ' l I labelled IgG a d s o r b e d to the 500 550 600 monolayers of various lipids WAVELENGTH (nm) under different pH values of the Figure 9. Immunological activity of the aqueous subphase, it has been oriented igG films detected by found that the highest surface density fluorescence measurements.

/

1 0 -9 ! ....

~.

CONC.

OF ANTIGEN

1 0 -8 I

'

'

(mol/I)

10 -~ I

10 -6 I

i'

>c/)

=

LIJ

/ "

/

5oo

100

I--

=

.

50

o z m 0

m w 0

=

10 5

~

,

0.5

,

1 CONC.

f

I

5

10

OF ANTIGEN

,,

I

50 (p.g/ml)

I

100

,

500

Figure 10. Fluorescence intensities plotted against the concentralJon of antigen, using double layers of the oriented IgG immobilized in the LB films.

?9 of IgG can be obtained by adsorption to the stearylamine monolayer on 0.1 M borate buffer of pH 8 at the lower initial surface pressures, followed by later compression, which was also supported from observation by the fluorescence microscopy. The IgG-lipid complex monolayer was transferred under the compression at 30 mN/m onto a quartz plate by the vertical dipp-ing (conventional LB) or the horizontal lifting method. On a down-trip of the plate or by the horizontal lifting, the rather hydrophilic active site (Fab) of IgG exposed under the lipid monolayer will be faced toward the outer side, whereas on an up-trip of the plate the tail part (Fc) embedded in hydrocarbon chains of stearylamine will be oriented to the outor side. Thus, it is expected that opposite orientations of the antibody molecules can be achieved in the deposited films. The immobilized antibody films with different orientations were brought into contact with the aqueous solution of corresponding antigen labelled with FITC for 20 to 30 minures. After rinsing with the buffer solution, the amounts of specifically combined antigen were determined by fluorescence intensity. Figure 9 shows the immunological activity of the oriented igG films detected by fluorescence measurements. These results clearly indicate that the reactivity of the antibody layers immobilized by the LB techniques depends upon the orientation of the IgG molecules and the number of layers. In the case of a single layer, the activity of the immobilized IgG film transferred by the down-trip process of the horizontal lifting (B) is about two times greater than that by the uptrip (D), according to the expectation from the above-mentioned orientations of IgG molecules. The double layers of IgG obtained by the down and up-trips of the plate (A) was more effective in an additive manner. On the other hand, covering by additional deposition of an arachidic acid monolayer onto the immobilized IgG films (C) depressed considerably the immunological activity. The sensitivity of the immobilized IgG films was estimated in the antigen solutions of different concentrations for the reaction time of 20 minutesat 15 ~ As shown in Figure 6, by using the double layers of IgG films, the corresponding antigen could be detected in the concentration range from 10-8 to10 -6 M or I to 100 ~g ml-1.

3 . 4 . Selective inclusion of naphthalene derivatives by monolayers of amphiphilic cyclodextrins at the air/water interface [23]. It is well-known that cyclodextrins have a unique cylindrical structure of which the cavity can include various guest molecules in solutions [24]. Inclusion of guest molecules with oriented cyclodextrins at the interface has been scarecely studied. Previously, we have reported that cyclodextrin derivatives (CDs) with long alkyl chains form stable condensed monolayers on the water surface and their multilayers can be deposited on solid substrates by the Langmuir-Biodgett method, in which the CD rings are oriented parallel and the alkyl chains perpendicular to the surface [25]. The inclusion of CDs at the air/water interface seems to be concerned with molecular recognition of the CD membranes. The LB films of CDs are expected to incorporate functional molecules without any long-chain into the cavities with an ordered structure. The reversible photoisomerization of azobenzene derivatives incorporated in the LB films of the amphiphilic 13-CDs was observed by alternate photoirradiations with different wavelengths, and the host-

80

guest interaction between the CDs monolayers and azobenzene derivatives (methyl red) was examined at the air/water interface [26 - 28]. Heptakis(6-dodecyis ulphinyl-6-deoxy-)2, Cyclodextrin deriv.(Host) 3-d i-O-acetyl)-13-cycl I/ CHzR 'h I R=SOCI2H25 X=COCH3 H J-I (p-ACCDSOC~2H2~) odextrin [13-AcCDSO C12H25] and hepta~L--J\UX .H/L-O 2 R=NHC~zH~5 X=H (p-CDNHC12H25) kis(6-dodecylamino6-deoxy-)-13-cycloNaphthalene deriv.(Guest) dextrin ~-CDNH NHz SO3Na C12H25], as shown in Figure 11 together with naphtha1-NaphNH2 2-NaphNH2 1-NaphS 9 2-NaphS 9 lene derivatives for guest molecules, were Figure 11. Amphiphilic cyclodextrins (Host molecules) synthesized as and naphthalene derivatives (Guests). reported previously by W.Tagaki et al. [6]. Induced circular dichroisms for the cyclohexane solutions of I%AcCDSOC 12H25 and NaphNH2 mixtures (molar ratio of 1 : 10) and the ,, ethanol solutions of I~-CDNHC12H25 and NaphSO3Na (1 : 10) mixtures were observed at each 1Bb transition band along __o the long-axis of naphthalene, whereas it was not the case for the solutions of I~-AcCDSO C12H25 mixed with NaphSO3Na or oo10

6ott-

13-CDNHC12H25 with NaphNH2. These facts indicate that the 0 100 200 300 400 host-guest complex formation of AREA (A2/molecule) the CDs depends upon not Figure 12. Surface pressure - area isotherms only the cavity size of CDs but also the coulombic interaction between (20 ~ of I}-CDNHC12H25 monolayers the substituents of naphthalene included and/or adsorbed 1-NaphSO3 at the and the polar groups connecting air/aqueous solution interface under the the alkyl substituents with the CD different initial surface pressures: rings. A(30 mN/m), B(20 mN/m), C(10 mN/m) and D(5 mN/m), as compared with that of ~-CDNHC12H25 alone (. . . . . ). !

,,

I

81

The monolayers of I}-AcCDSOC12H25 including 1- or 2-NaphNH2 could be transferred onto solid plates at 30 mN/m by the LB method to give the alternating Y-type films. Polarized UV absorption spectra of these LB films suggested that the long-axis of naphthalene is oriented rather parallel to the film plane for 1-NaphNH2 in the CD cavity, whereas it tends to be nearly vertical for 2-NaphNH2. In fluorescence spectra of the LB films ~-AcCDSOC12H25 including NaphNH2 molecules irradiated by 300 nm light, an intense monomer band of naphthalene was observed around 400 nm, while the LB films obtained from the mixture of 13-CDNHC12H25 and NaphSO3Na showed only a very weak fluorescence. On the other hand, inclusion and/or adsorption of NaphSO3Na molecules from the aqueous subphase to the spread monolayer of I%CDNHC12H25 were examined by using the multicompartment trough. When the I}-CDNHC12H25 monolayer spread on the distilled water surface was compressed to the prescribed initial surface pressures of 5, 10, 20, and 30 mN/m and transferred onto the aqueous subphase containing 10 -3 M NaphSO3Na, the surface pressure increased with time, air/aqueous solution interface under the suggesting the adsorption of NaphSO3Na molecules by the CD monolayer. The Cd arachidate surface pressure increments were O 13-CDNHC,.H., ," / ' ~ I more significant in the case of 600' t 9~CDNHC,'H2, ,/ lower initial pressures of 5 and 10 / + 2.NaphSqNa ,' J / / I mN/m, as compared with those for ~ the higher initial pressures of 20 500 1.N~~;s;~N.~,.' and 30 mN/m. When the 13-AcCDSOC12H25 mono- layer was kept on the aqueous subphase containing iq" 40C 10 3 M Naph- SO3Na, the surface LL pressure was almost unchanged : // irrespective of the initial pressure. ,i Freq. After the 13-CDNHC12H25 monoL , /// t o,i:, J / layer was kept on the NaphSO3Na 20Or ," d// Q",,,; I / aqueous solution for sufficient time (usually 30 min.), the CD monolayer involving NaphSO3Na w a s re100 "'1 placed onto the distilled water surface to rinse the excess NaphSO3Na, and the surface pressure 0 4 8 12 Number of layers area isotherms were measured after Figure 13. Frequency changes of the quartz expansion to the zero pressure. crystal microbalance with number of Figure 12 shows the surface deposition of CD monolayers at 30 mN/m. pressure - area isotherms for the monolayers of I~-CDNH-

7001

i

///I

+

+ool-

/

,,

,

//

1'i,? l

/

/

82 C12H25 including 1-NaphSO3Na under the different initial pressures at 20 ~ In the case of lower initial pressures,NaphSO3Na molecules insert among the CD rings at the air/water interface, resulting in the expansion of the apparent molecular area, while under the higher initial pressures the isotherms are slightly changed accompanying with less compressibility, suggesting the inclusion of NaphSO3Na molecules in the CD cavities. Deposition of the 13-CDNHC12H25 monolayers including NaphSO3Na molecules (under the initial pressure of 30 mN/m) onto Pt electrode of the quartz crystal microbalance by the LB method at 30 mN/m was monitored by frequency changes, as shown in Figure 13, in comparison with those of the 13-CDNHC12H25 alone and cadmium arachidate monolayers. Taking the molecular areas of 210 ,~2 for I~-CDNHC12H25 and 20.5 .~2 for cadmium arachidate in the films, the mass change was estimated to 1.09 ng per Hz, and hence the composition of the host-guest complex between I~-CDNHC12H25 and NaphSO3Na molecules was considered to be approximately 1 ' 1. Figure 14 shows the circular dichroism spectra for the LB I I I I J I films of I~-CDNH C12H25 including Naph..., /~ p-CDNHC,2H~s ~-\ ,/ "X / / ~, / +2-NaphSO3Na SO3Na m o l e c u l e s ' " 1" ~" J '~ // hX so,-, under the initial surface pressure of 30 ~3 . . . . . . . . . . . '~'' i' )~ " ~ ~ mN/m. Different indu"": 9 \"::,"-- .... '/!,, ..... ..... ced circular dichroisms are clearly ob11 \ /, '~" ~ p-CDNHC12H2s served at 1Bbband of -e+1-NaphSO3Na < SO~'q= naphthalene, depending on the substituted position; the negative and positive ,! ! , I [ Cotton effects occur 200 25O for 1- and 2-NaphWAVELENGTH (nm) SO3Na included in the cavity of the CD Figure 14. Circular dichroism spectra for the LB films of films, respectively. And ~-CDNHC12H25( ......... ), and prepared from the CD further, from polarized UV monolayers spread on the aqueous solution of absorption spectra for 1-NaphSO3Na( . . . . . ), 2-NaphSO3Na(------) and those equimolar mixture( ) at 10- 3 M. these LB films, it was found that the long-axis of naphthalene tended to nearly parallel orientations to the film plane for 1-NaphSO3Na, whereas it was preferably vertical for 2-NaphSO3Na.

83 When the m o n o l y e r of 13-CDNHC12H25 compressed to the initialsurface pressure of 30 mN/m on the distillled from the CD monolayers spread on the aqueous water was transferred onto the aqueous subphase containing equimolar mixture of 1- and 2-NaphSO3Na (total 10-3 M), the obtained LB film of the CD including the guest molecules gave the circular dichroism and the polarized UV absorption spectra characteristic to those of the CD film including 2-NaphSO3Na alone, as shown by the solid line in Figure 14. Similar inclusion of NaphNH2 molecules in the aqueous subphase to the 13-AcCDSOC12H25 monolayer was observed. From these facts that the 2-substituted naphthalene is preferentially included into the cavity of the amphiphilic I~-CD closely packed monolayer, it should be noted that a selective inclusion of the naphthalene derivatives, i.e., a molecular recognition by the 13-CD monolayer at the air/water interface occurs depending upon the substituted positions in the guest molecule. Photoinduced excited states of the naphthalene derivatives included in the amphiphilic 13-CD LB films were found to be stablized by measurements of the fluorescence lifetimes and the excimer formation of the naphthalene derivatives adsorbed by the CD monolayer occured mainly between the adjacent layers [29].

3.5. S p e c t r o s c o p i c studies on aggregation of amphiphilic dyes in m o n o l a y e r s on the water surface.

Molecular orientation and packing in monolayers on the water surface can be easily controlled by compression and expansion in a two-dimensional way. Spectroscopic measurements in UV-visible region for monolayers of amphiphilic dyes on the water surface provide a valuable information on orientation and aggregation of the chromophores. Several J-aggregates o f c y a n i n e and squarylium dyes have been interested in view of photosensitizing function and photovoltaic effects [30 - 31]. J-aggregates are characterized by an intense narrow absorption band shifted to longer wavelength relative to the monomer band, and also by a strong emission band with a nearly zero Stokes' shift. Previously, we reported~ J-aggregates of a long-chain merocyanine dye [MC18] in mixed monoand multilayers with cadmium arachidate [AA] and methyl arachidate [MA], and applying an extended dipole model to the two-7dimensional arrangement of the transition moments, the geometry and aggregation number of the chromophore in the J-aggregate were clarified [32, 33]. The process of the J-aggregate formation in the monolayers on the water surface could be controlled by adding non-polar hexadecane [HD] and compressing at a constant surfcae pressure to form the aggregate mcnolayers homogeneously distributed. Figure 15 shows spectral change with time for the mixed monolayers of [MC18] : [AA] : [HD] and [MC18] : [MA] : [HE)], respectively, spread on the aqueous subphase containing CdCI 2 and KHCO3 and compressed at 15 mN/m. In the early stages the dimer band around 500 nm was intense and decresed with time accompanying with the enhanced J-band at 610 nm, of which absorbance was a maximum at 40 rain. for the mixed monolayer with [MA] and [HD]. On the other hand, squarylium dyes have an interesting chromophore which are

84

c.-

Nt C,,H37 9

1"~

"

i ~-Om n

.................. IiI i . . \ L.......... N

Mcl

il-~

:i i i,!i/ou

..................... i:.l

NXCH2COOH i

ii~

,5rain

...................

[MCI8]

r~

MC18 MA:HD, .............. 1

,I

O

,o I

.......

................... i5 ...............i...................

I

,o

3O ,40 /

~.so/

O

:: 8 "AA:HD 1"1-1

!:,

"

O

500 600 W A V E L E N G T H (nm)

500 600 W A V E L E N G T H (nm)

Figure 15. Spectral change with time for the mixed monolayers of [MC18] ' [MA] [HD] 9 and [MCl 8] "[AA] [HD] 9 on the aqueous subphase containg CdCI2 and KHC03 on compression at 15 mN/m. o 04H9\ ~ N / C 4 H 9 04H9/N \C4H 9 O" 1

I

- 5~

mN/m

--12.0

0.06

..........14.2

Q) U c-c~

----14.5

.--2_o.o4

O ..Q <

0.02 V

~

.

.... 16.9

I

a

(:,~ I,i i '\.J.l.-""-.\\, I~^ ., . . . . . \../..."),',.;\.X, ~ii~ ,~............ ../..::-----:.-;)'~'~"~-~W.......',/

//,':---"

500

"-i

~",-'"

600 700 Wavelength (nm)

800

Figure 16. Reversible change of monolayer spectra among J-, H-aggregates and monomeric species of squarylium with four butyl groups by compression.

85 known to be a highly sensitive carrier generator of organic photoconductors for use in electrophotography [34, 35]. Research groups of the Fuji Xerox Co. have synthesized a wide variety of squarylium dyes substituted with different lengths of alkyl chains. For the squarylium dye substituted with four butyl groups the monolayer spectra on the distilled water surface changed markedly depending on the surface pressure and the temperature without any matrix molecules. As shown in Figure 16, a reversible change of absorption spectra among J-, H-aggregates and monomeric species with a clear isosbestic point, was observed by compression and expansion at the subphase temperature of 5 ~ while at 20 ~ almost pure H-aggregate could be obtained [36].

3.6. Photophysical behaviours of J-aggregate LB films.

For a long-chain merocyanine substituted with chlorine at 5-position [CI-MC18] we could obtained the Jaggregate mono- and multilayers mixed with various glycerol derivatives such as ~ ~ ~ = CH--CH:=~~~H monostearin [MS], distearin [DS], and tristearin [TS], R \ 2COOH C,8H3~ 0 phospholipids such as diR=CI. CI-MC palmitoylphosphatidylethaol amine [PE] and choline MC-PE-I -I [PC], and cholesterol [Ch]. MCChI 9I Figure 17 shows absorption F__~L spectra for the built-up LB tvIC'MS-1" 2 MC'DS-I" I MC'TS=3"2

LLJ O Z <~

M C" P C - I

O3 rv O Lf') s

soln.

"I

,,",, !

_

400

500

600

WAVELENGTH (nm)

700

Figure 17. Absorption spectra for the LB films of [CI-MC] mixed with various matrices at different compositions, together with fluorescence spectra.

films of [CI-MC18] mixed with the various matrices at different compositions accompanying with the fluorescence spectrum, as compared with the solution absorption spectrum. The excited states and the exciton-phonon interaction in the J-aggregate LB films have been investigated by applying the Urbach rule [37] to temperature dependence of the J-absorption bands in the range of 296 -136 K. A good l i n e a r relationship between the logarithmic absorption coefficient and the incident photon energy was observed for the low-energy tail of the J-bands,

86

T

E

v

r.D

l-Z LLI (D

MC:L-MS= 1:1

136K 164K 185K 205K 296K

LogK=-35.13+19.75E LogK=-34.02+19.22E LogK=-33.41+18.93E LogK=-32.15+18.33E LogK=-29.41+17.01E

0.533 0.626 0.695 0.746 0.999

6.08)

ii- 6.0 LL LU

0 L.) Z

I!~:i~ .III,

o 5.5 t-EL

IT" 0 CO ED < 5.0

136K 164K-~X~,

O

:s

185K'~I\\

"lp-n-"

2O

<~ 9 0 ..J

296K

1

2.15

1

2.10

PHOTON

I

2.05

.I

2.00

E N E R G Y (eV)

Figure 18. Plots of logarithmic absorption coefficient to the incident photon energy for the low-energy tail of the J-band of [CI-MC] with [L-MS], at various temperatures. with [L-MS], at various temperatures.

as an example of [CI-MC] mixed with [L-MS] shown in Figure 18. The low-energy tail of the J-bands can be expressed by the Urbach rule with a steepness parameter (s') of 1.34 ~ 1.49. Since the exciton - phonon coupling (g) is given by the ratio of steepness index (S) and s', the v a l u e s of S = 1.24 for the two-dimensional system [38] lead to g < 1. Consequently, for the J-aggregates in LB films it has been f o u n d that the exciton - phonon coupling is weak and the fluorescence occurs by a radiative annihilation from the free exciton state. For fluorescence decay curves of the J-aggregate LB films of [CI-MC] mixed with various matrix agents, measured with a picosecond time-resolved single photon counting system, three components of the the lifetimes fitting to exponential terms in the following equation:

F(t) = A1 exp (- t/1:1) + #,2 exp (- t/'[2) + A3 exp (- t/1:3) have been summarized in Table 1. The fluorescence lifetimes (1:1) observed at 610 nm for a single layer of the J-aggregates were obtained to be 5 - 11 ps in a large portion above 95 percent, irrespective of the matrix agents. The other slow components (~2 and 1:3) are minorities less than 5 percent. In the time-resolved fluorescence spectra for the J-aggregate LB film, as shown in Figure 19, the fluorescence band at 610 nm decreases with time and its intensity is reduced to one-half the initial value at about 1 ns, while the 570 nm band due to the monomer is grown up. In the case of LB systems containing the J-aggregate ([CI-MC18] : [AA] = 1 : 2) as a donor combined with long-chain bipyridinium as an acceptor

87 Table 1. Fluorescence lifetimes for J-aggregate LB films of [CI-MC] mixed with various matrices. -

,

,l

l

. . . . . .

l

II

Films I

~ _

II

I

i

' ~ I C : A A = 1:2

MC:DL-MS

l

ii

Ill

l

.

.

III

At

9

II

ii

"172 ( p s ) I

0.995 .

.

.

= 1:2 8.01

m

84.43

0.005

0.990

76.07

0.010

.

.

% (ps)

A 2

I

i

6.48 -

A 3

i i

.

350.3

0.000

M C ' L - M S = 1:2

5.88

0.995

84.35

0.005

M C ' D S = 1"1

6.66

0.979

51.65

0.020

213.5

0.001

M C : T S = 3:2

7.97

0.980

62.06

0.019

261.7

0.001

M C : P E = 1" 1

7.62

0.971

50.48

0.027

195.6

0.002

M C : P C = 1"1

11.11

0.951

50.32

0.046

188.8

0.003

0.014

216.3

M C : C h = 1"1 _1

I I

~t (ps)

i

i

7.15 ii

i

0.985 Ul

i

i

9

51.01 i

ii

iii

i

_

0.001 ii

i ill

Acceptor H37C~e-N

_~N-C~aN37 -(Ct04")2

Acceptor

(BPy)

Donor C14-22

957-1026ps "-

~

h

13ps

10 4

410-479ps

r

215 244ps

E

, "'.,. ' / ~ , ~'"'.,.,

/MC'AA=I:2

49ps Ops

Z

57O 600 650 O (D WAVELENGTH (nm) Figure 19. Time-resolved fluorescence spectra for J-aggregate LB film of [CI-MC] : [AA] = 1 : 2.

1 !

10 2

10

\'~

1

t, '

l

exc) .

.

0

.

.

"

c.

,,~

~1~

I

.

500 1000 T I M E ( ps )

Figure 20. Fluorescence decay curves of J-aggregate ([CI-MC] [AA] 9 = 1 2) 9 in LB films combined with spacer and acceptor layers.

88

>.I--,_.., th

~,~i

MCIC~41BPy

Z LLI I-Z ,,_,.

~ ~

MCIC161BPy

40ps

~

,, MCIC~81BPy

W Z W ~.~ bq W O~ 0

j)

_.J LL

570

600

650

WAVELENGTH (nm)

.....

570

600

650

WAVELENGTH (rim)

570

600

650

WAVELENGTH (nm)

Figure 21. Time-resolved fluorescence spectra of J-aggregate ([CI-MC] [AA] 9 = 1 2) 9 in LB films combined with spacer (Cn) and acceptor layers. layer at a distance of 20.2 - 30.2 ,~, which was controlled by the insertion of a single layer of cadmium myristate (C14), palmitate (C16), stearate (C18), arachidate (C20), or behenoate (C22), the fluorescence decay curves of the J-aggregate are shown in Figure 20. It has been found that the fluorescence of the J-aggregate decays more rapidly with the shorter spacing, as compared with no acceptor layer. From the time resolved fluorescence spectra of these systems as shown in Figures 21 (a) - (c), it can be seen that the fluorescence band at 610 nm disappears and the 570 nm band occurs more rapidly as the spacing between the donor and the acceptor layers decreases. From these results, it is considered that the electron transfer from the J-aggregate in the composite LB films to the acceptor occurs with significantly fast less than a few picoseconds [39].

3.6. V a r i o u s Orientation of functional groups in m o n o l a y e r assemblies and its effect upon some functions.

3.6.1. Quinacridone derivatives with four alkyl chains. Previously, we have reported that various types of orientation of chromophores such as anthraquinone and azobenzene [40] can be controlled in the monolayer assemblies by changing the number and positions of substituted alkyl chains, and also reflected in some characteristics of spectroscopic properties. Recently, we have found that the quinacridone derivatives withfour alkyl chains take different orientation and packing in the multilayers, depending on the chain length of the alkyl substituents [41]. Polyhetrocyclic compounds which are interesting for functional pigments are almost insoluble in usual organic solvents. By introducing alkyl chains to chromophores, the soluble derivatives can be obtained and spread as monolayers on the water surface. Quinacridones are well known chemically stable pigments which are expected to have photovoltaic

89

functions. Figure 22 shows 2,5, 9,12-tetraaikyiquinacridones with CsH17 o different substi- tuents together with their abbreviations. From the surface pressure - area isoC18H3 7 " '~ therms for the monolayers on the CsH17 C12H2 5 0 water surface and the dichroism 2H2 5 in polarized UV-visible spectra C 1 2 N C 12 ~ ~ C1 for the multilayers deposited at C12H2 5 ~~ .4~ ~ 30 mN/m by the horizontal lifting C12H2 5 method, it is suggested that the C8 HI 7 O l! C sHI 7 long-axis of the chromophore is lying nearly flat for C12NC12 in CSN C8 ~~'~/~"Y~, . ~ ' ~ \ N/ ~ C8 H 1 the f i l m , w h e r e a s t h o s e of o cs%7 C18NC8 and C8NC8 have rather C8H17 O oblique orientation even though C6FI3NC8 the parallel one for C18NC8 is at lower surface pressures. Further, C6FI 3 ~ i7 ~ O CsH] 7 from the incident angle dependence of intensities of the uc=o Figure 22. Quinacridone derivatives with four bands in the polarized IR spectra alkyl chains and their abbreviations. for the multilayers, it has been found that the chromophore is oriented with the short axis lying nearly flat to the film plane for C18NC8 and C6F13NC8, whereas the short axis is rather perpendicular to the surface for C8NC8 and C12NC12. Taking account of the results of X-ray diffractions together with the above results for the multilayers of the quinacridone derivatives, the most probable molecular orientation in the films can be deduced, as illustrated schematically in Figure 23. Abrrev.

~~N~

l C18NC8

%F13

i

C12NC12

C6F13NC8

C8NC8

Figure 23. Schematical illustrations for molecular orientation of the quinacridone derivatives in the films. It should be noted that for C6F13NC8 a remarkably anisotropic orientation of the chromophore in the film plane was observed in the polarized UV-vis. spectra at the normal incidence using the rays with electric vectors perpendicular and parallel to

90 the monolayer compression on deposition, as shown in Figure 24 (a). The in-plane anisotropy seems to be caused by the narrow trough (6 x 180 cm) with the molecular flowing on spreading and compression of the relatively rigid monolayer due to the fluorocarbon substituents. These results were supported by the scanning electron microscopy (SEM), as shown in Figure 24 (b), which shows the fibril structures with their axes almost parallel to the direction of the monolayer compression, that is, the moving direction of a barrier, probably accom- panying with flow of the aqueous subphase in the narrow trough.

Figure 24 (a) Polarized UV-vis. spectra for C6F13NC8 films at the noemal incidence: electric vectors perpendicular ( - - - - - ) and parallel ( - - - - - ) to the monolayer compression and (b) the SEM picture (Arrow : direction of the monolayer compression). On the other hand, from measurements of the cyclic voitammogram for these quinacridone derivatives in dichloromethane solution, it was found that the first reduction potential was 0.62 V vs. Fc/Fc+ [Fc = Ferrocene], being smaller than those of naphthoquinone and anthraquinone in the same cell system containing 0.1 M tetrabutylammoniumperchlorate (TBAP) as an electrolyte and the scanning rate of 100 mV/s. Electrochemical behaviours in the solutions of the quinacridone derivatives used in this work are considered to be almost same since these quinacridones have the similar alkyl substituents at the same positions of the chromophore. In order to examine the effect of the molecular orientation upon the photoinduced electron transfer in the layered structure, as shown in the insert of Figure 25, the monolayer assemblies containing a long-chain thiacyanine dye mixed with cadmium arachidate (1 : 10 in the molar ratio) as a donor (D) and the above quinacridone derivatives with different orientations of the chromophore as an acceptor (A), were constructed in separating the D layer from the A layer at a distance distance of 20.2 ~- 30.2 ,~, which was controlled by the insertion of a single

9]

layer of cadmium myristate (C14), palmitate (C16), stearate (C18), arachidate (C2o) or behenoate (C22). The values of these spacings obtained by X-ray diffractions were 20.1 A for C14, 22.65 ,~ for C16, 25.15 ,~, for C18, 27.75 .~, for C20 and 30.2 ,~ for C22. With a first approximation for the nonradiative relaxation process from the photoinduced excited states, it can be considered that the direct electron transfer from the D to A is a fast event as well as the resonance energy transfer. By measuring the donor fluorescence intensity of Io in the absence of the acceptor layer and that of I for the D layer combined with the spacer (Cn) and the A layers, excited at 410 nm, the values of (Io - I)/I can be presumed a measure of the rate of the nonradiative electron transfer. Figure 25 shows plots of the values of Iog(Io/I 1) as a function of a distance between the D and the A layers at room temperature. 0.8

~

C--le1-137 C,eH3T

%

"\,

0.6

0.4 I

"~\\

[ clsNc8

[C6FI3NC8 (--~-.)

~,, '\

\ o

(-.--)

C12NC12

,R'

o

o

.'

L-b&,

o

0.2 i

E 'Xl-0.2

C14 ,I 20

C16 l

\'" Cl8

,l

C20 l

r22

, ~" .

25 30 DISTANCE(A) Figure 25. Plots of the Iog(Io/I - 1) against the distance between the donor and the acceptor layers. Structure of the composite LB films with the donor - spacer- acceptor layers in the insert.

92 From the fact that these plots produce nearly straight lines, it is suggested that the rate constant of the electron transfer follows an exponential distance dependence at the D - A distance of 20 - 30 A [42]. The slopes of these lines have implied the barrier height for the tunneling event. For the C18NC8 and C6F13NC8 acceptors with the quinone moiety parallel to the film plane, the barrier is relatively large, whereas the lower barrier is observed for the C12NC12 acceptor with the quinone -D-Cn-C12NC12

~

f D-Cn-CISNC8

I oooo,

,

-'

(a)

.

.

.

:

0onor

i.~.~.,"

C 2 0 ~ C16 014

I

I {

.

(b)

o'nlv i

,,. C2C

i

I

+ c~6 C~4

i

!

1'

1 I

0.0

l

.....

[

I

t

i

I

0.5

1.0 1.5 2.0 2.5 0 . b - ~ O . 5 1.0 1.5 2.0 2.5 Time(ns) Time (ns) Figure 26. Fluorescence decay curves of the donor layer separated by the spacer (Cn) from the acceptor:(a)C12NC12 and (b)C18NC8, observed at 470 nm.

Table 2. Fluorescence lifetimes, Xn(n = 1,2, 3) and their amplitudes, An for the donor- spacer(Cn) - acceptors (C12NC12 and C18NC8) films, together with the averacle lifetimes < 1; > = Z 1: An . ,,

m,

,1

,

,

Donor only

C1~2NC12 .

C22 C20

"C1 (ps) A1 11~z (ps) Az

,,

,

.

cisNc8 .

C16 C14

.

.

.

.

.

C22 C20

C18 C16 C14

1343 1149

1101 1018 1035

1465

1715 1529 1018 1294

0.253

0.054 0.058 0.070 0.060 0.144 0.101 0.121 0,060 0.044

536 0.392

1[:3 (ps) A3

9

AccePtor

67 0.355

< 1[; > (ps)

258

255

202

214

238

209

214

173

182

0.207 0.200 0.253 0.193 0.281 0.286 0.283 0.225 0.196 33

31

30

29

33

31

29

27

26

0.739 0.742 0.677 0.639 0.575 0.613 0.596 0.715 0.762 171

162

143

129

279

258

212

142

105

..

93 orientation vertical and the medium one for C8NC8. The slope in the case of the C18NC8 acceptor is similar to that for the long-chain bipyridinium acceptor in the composite LB systems with the same donor layer [42]. In addition, Figure 26 shows the fluorescence decay curves of the D layer for the composite LB films containing the C12NC12 or the C18NC8 as the A layer at a distance of 20 - 30/~, in comparison with that for the D layer alone, which decays approximately with a single exponential way [43]. In the presence of the acceptor layer the donor fluorescence decays more rapidly and the decay curves depend upon the spacers of C n. In a conventional way, these curves have been fitted with three components of the lifetimes (1;n) and amplitudes (An). Table 2 summarizes the values of t n and An, in which 1.0 - 1.7 ns, 170 - 260 ps, and 26 ~ 33 ps were obtained for n = 1,2, and 3 of 1;n, respectively. From the values of amplitudes, it can be seen that the quick component of 1;3 is major for the composite systems, as compared with 1;1 and 12 . Approximately, assuming that the change of the lifetime 1;3 can be ascribed to the electron transfer from the donor to the acceptor layers through the hydrocarbon spacer, the reciprocal of 1;3 indicates the rate of the electron transfer. Figure 27 shows plots of of the values of - log O ] T3 a g a i n s t the disIO.60 I 0 . 0 / ~ / ~ / tance, d, between the ~ ~ j ~~~Cl,NCS | D a n d A l a y e r s , using the C12NC12 and C18NC8 acceptors, in which a linear relation10.55 ship between logarithms of the rate of the electron transfer and the D - A distance was I / -~.... ",,._~,_~ "--I T obtained. As an alter,I 1 native treatment of the 10.50 lifetimes, an average lifetime, e.g. < 1; > = Z 1;i O Ai are indicated in the bottom row of Table 2, and also the values ofC,6 C,a C2o C22 IT 10.4" Ct4 It6 ,ira i20 (i2z log < 1; > are plotted 20 25 30 against the distance, d, D I S T A N C E (A) in Figure 27. The Figure 27. Plots of -log ~ ( - - - - - and - - - ---) and relative values of the

-]

~

]

4 9.8~'

t

C12NC12

~~l

- log< 9> ( and -- - - - ) against the distance between the donor and the acceptor layers. 9

I

slopes for these lines are found to be very similar to the results obtained from

94 the statical quenching of the donor fluorescence for the same composite LB systems, as shown in Figure 25. These results indicate the effects of the orientation of the acceptor molecules upon the barrier height for the electron tunneling in the layered structures. 3.6.2. Orientation control of organometaric compounds. Ferrocene derivatives are interested for electrochemical mediators and used to modify some organic compounds such as proteins and lipids. For amphiphilic ferrocene and biferrocene derivatives [44, 45] from the surface pressure - area isotherms of the rnonolayers on the water surface and the polarized UV-visible and IR spectra of the multilayers on solid plates, cyclo- pentadienyl rings of the ferrocene derivatives with one alkyl chain and 2,10-bissubstituted biferrocence are oriented parallel to the surface, whereas those of the ferrocene derivative with two chains on different rings and l',6'-bis-substituted biferrocence are vertical, as shown in Figure 28. Charge transfer complexes of the amphiphilic ferrocene and biferrocene with iodone or a long-chain TCNQ derivative and also the salts with BF4-form stable condensed monolayers, in the absorption spectra for mono- and multilayers of the complexes or salts, the charge transfer or the ferricenium cation

0

Fe

I

0

( < <

> > >

<

>

I

'>

(

>

o d :~

( co

\

< <

< < < <

Fe

II

Fe

III

IV

Fe

V

Figure 28. Molecular orientation of ferrocence derivatives in LB films. band was observed in the longer wavelength region. And also, electrochemical oxidation and reduction of the ferrocene derivative in the LB films on an indium-tin oxide (ITO) electrode were realized reversibly. The electrical conductivities for the multilayer of the biferrocene derivative, deposited on aluminum electrodes arranged in a surface cell and a sandwich cell, were measured in the directions parallel (o//) and perpendicular (O.L) to the film plane, respectively, by analyzing Lissajous's figures taken at 10 mHz. The normal conductivity (O.L) was about 10-13 S cm- 1 while the lateral one (o//) was in the order of 10- 9 S cm- 1. The latter

95

value increased up to 10-6 S cm -1 with the film of the BF4- salt. This can be ascribed to the mixed valency between the neutral biferrocene and oxidized ferricenium in the two-dimensional array, which was confirmed by the shift of the binding energy of iron atom in the ESCA spectra for the LB films. Phthalocyanines (Pc) are attractive materials for their potential functions including the semiconductive behaviours in addition to the thermal and chemical stabilities. In particular, control of orientation of the Pc macrocycles in thin films is expected to provide novel molecular electronic devices. Previously, we have found that copper tetrakis(butoxycarbonyl) Pc is oriented nearly perpendicular to the surface and also the dipping direction in the LB films [46], while octa-alkyl Pc derivatives [H2Pc(R)8, CuPc(R)8 : R = CnH2n+1, n=7, 9, 11] take the orientation with Pc macrocycles nearly parallel to the plane of films deposited by the horizontal lifting method to form a non-alternating X-type film [47], as illustrated schematically in Figure 29. For the latter case, it is considered that the semiR conducting ~-electron systems are separated by insulating hydrocarbon spacers, resulting _,~--NH N~-~J, in a l t e r n a t e thin l a y e r s of N ,~N organic s e m i c o n d u c t o r and insulator in these monolayer assemblies. The direct current voltage (I - V) characteristics w e r e m e a - s u r e d f o r the F~= - C n H2n § I multilayers H2Pc(R)8 and Cu( n = 7 - 11 ) Pc(R)8 in directions perpendicular and parallel to the film plane. In both cases, the linear I - V relationships of Ohm's law were observed at low electric field and obtained DC conductivities are summarized in Table 3. The normal conductivity (oj.) w e r e ca. 10-13 S cm- 1, while the lateral ones (o/i,) were 3.4 x 1O-7 and 9.9 x 107 S c m- 1 for films of the metal-free and copper Pc derivatives, respectively. The former (o.L) t e n d e d to decrease slightly with increase of Figure 29. Schematical illustration of the substituent alkyl chain length, molecular arrangements of octa-alkyl while the latter (o//) were nearly Pc derivatives in the films. constant. On the other hand, at high

R~~~LN~R R R

-

lit l/ I/I/I/I/"

-

-

"

C

96

Table 3. DC conductivities of octa-aikyl Pc derivatives in the films. ,

.=

,l

l

,

,

,,

,,

o,,

(S/cm) 2H

Cu

2H

(S/cm) Cu

CTHls

1.68 x 10 -13 2.43 x 10 -13

3.42 x 10 -7 9.85 x 10 7

C9H17

1.51 x 10 -13 2.31 x 10-13

3.43 x 10 .7 9.94 x 10 .7

C11H23

1.42 x 10 "13 2.17 x 10 -13

3.38 x 10 .7 9.92 x 10 .7

-7

-8 IR

R

R = -CTH Is

-9 7

E

b

electric field the I - V characteristics followed log I=r Vn with n = 0.7 and 0.3 for directions perpendicular and parallel to the surface, respectively. Figure 30 shows frequency dependence of the AC conductivities for the H2Pc(R)8 multilayers in directions parallel (o/,,,) and

perpendicular (Ol - )to the film plane. The 0 -II normal conductivity (a_L) was significantly increased from 10"13 to 1 0-10 S c m 1 with -12 applied frequencies from 1 mHz to 100 Hz, while the lateral one -13 (o~.) was kept in the range of 10- 8 .. 10 7 1 S c m l . it should be - '1. . . . . 0 I 2 noted taht the fre-:3 ~ -'2 log f(Hz) quency dependence of Figure 30. Frequency dependence of AC conductMties the normal conductivity of octa-alkyl Pc derivatives in the films. of the Pc multilayers was I

. . . .

I

9? enhanced by a decrease in the substi- tuent alkyl chain length. The capaci- tances were almost constant irrespective of the frequencies. These results indicate that electronic hopping conduction across the hydrocarbon chains between the Pc layers is predominant along the perpendicular direction, whereas a considerable increment of the lateral conduction can be ascribed to greater mobility of carriers transportyed through the adjacent ~-electron systems of the Pc macrocycles accompanying the electronic hopping through the lateral domains of the Pc molecules. It is considered that the disperse conductivities can be ascribed to a distribution of jumping frequencies due to difference in the relative positions of the mobile carriers, resulting in a widely dispersed distribution of carrier relaxation time [48]. 3 . 6 . 3 . Molecular organized thin films containing thiophene derivatives.

In a series of researches on functional molecules conaining chalcogens and on their molecular organized thin films, previously electronic structures of oligothiophenes with 3 - 8 thiophenen rings were revealed by ultraviolet photoelectron spectroscopy (UPS) together with the calculated geometrical optimization [49], and the rod-like oligothiophenes were found to be well incorporated in the mixed monolayers with fatty acids [50]. Recently, 2,5-bis-(diarylmethylene)-2,5-dihydrothiophene, furan, selenophene and N-methyl- pyrole analogues were synthesized by Ishii et al. [51]. The 2,5-dirnethylene- 2,5-dihydrothiophene derivatives have been attractive materials for the amphoteric properties of electron donating and accepting in dependence on the substituents at the exocyclic double bonds. We have investigated for effects of the substituted awl-groups on the electronic structures of the 2,5-dihydro-thiophene and selenophene derivatives, as shown in Figure 31, by UPS spectra of the vacuumdeposited films together with crystallographic analysis of the single crystals [51]. Figure 32 shows the skeleton structures of 2,5-bis(diphenylmethylene)- and 2,5-bis(dithienylmethylene)-2,5-dihydrothiophenes projected in [1] X = S, Y = phenyl the a-c plane and the b-c plane respectively. [2] X = S, Y = thienyl And also the dihedral angles between the [3] X = Se, Y = p h e n y l central thiophene ring and the substituted [4] X = Se, Y = t h i e n y l phenyl or thienyl rings together with the relative Figure 31.2,5-Dihydro-thiophene electron density of each HOMO are indicated. and selenophene derivatives. In both cases, the substituted aryl-groups are twisted largely out of the plane of the central ring (~) in the order of the rings ( ~ , ( ~ , ( ~ , and (~). The electron density is scarecely found in the ring ( ~ with the largest angle of the twisting. In comparison with the substituted thienyl groups, the phenyl groups with larger steric hinderance have greater torsion and less electron density except for the ring ( ~ . Therefore, the electron is localized in the central thiophene ring substituted with the phenyl groups rather than that with the thienyl groups. This fact seems to reflect on the electronic absorption spectra of the solution, in which the absorption maximum

98

o

S 9

"

Dihedr31 a.ns

C H

(o)

@ | -

Electron dcn=ify (%) 43

@ @ @ @ @

26.1

75.2

12

I

56.1

38.4

4

11

-

15

-

15

@ | 37

|

14.6 6 8 . 7 15 3

@ @ @ | 48.8 5

40.2 16

12

12

Figure 32. Skeleton structures of [A] 2,5-bis(diphenylmethylene)- and [B] 2,5bis(dithienylmethylene)-2,5-dihydrothiophenes, and the dihedral angles between the central and the substituted rings together with the relative electron density. ' ' v' ' ' ' ' ' ' ' groups with larger steric hinderh ,~= 4 5~e ance have greater torsion and less electron density except for the ring 5 . Therefore, the electron is localized in the central thiophene ring substituted with the phenyl groups rather than that with the thienyl groups. This fact seems to reflect _ on the e l e c t r o n i c a b s o r p t i o n C6H6 ~= , A spectra of the solution, in which the 8' L~ L___ absorption maximum was a shorter [P/eV 16 12 _~ wavelength of 414 nm for the former 03 = in comparison with 472 nm for the latter. F i g u r e 33 s h o w s the UPS spectra of 2,5-bis(diphenylmethylene)-2,5-dihydrothiophene and di' ' ~ThiopheY" ' ~ v ~ ~" '~~,A , hydroselenophene, as compared with that of 2,5-bis(dithienyl methyI, I 9 lene)-2,5-dihydrothiophene, togeI P / d 77 i l l ,. ! I I I I r z ~ therwith the spectraof benezene 20 10 Eb(Relative to Evac)/eV and thiophene for references. Figure 33. UPS spectra of 2,5-bis(diphenylirrespective of the central ring with methylene)-2,5-dihydrothiophene, dihydrosulphur or selenium the UPS selenophene and 2,5-bis(dithienylmethyspectra are found to be almost lene)-2,5-dihydrothiophene, as compared same. However, the spectra were with those of benzene and thiophene, significantly different with changing the substituent rings at the exocyclic double bonds from the phenyl to the thienyl groups. In both the phenyl and the thienyl substituents, the spectra are very similar to those of benzene and thiophene

99 respectively, except for the bands at the lowest binding energies. Furthermore, these observed UPS spectra were compared with the calculated ones which were simulated by broadening the delta function located at the MNDO orbital energies with each Gaussian function. The simulated spectra were well correspondence to the observed ones. The band at the lowest binding energy could be mainly ascribed to the electronic structures of the central dihydrothiophene ( ~ with the double bonds (E) and ( ~ , and the bands in the ranges of 6 ~ 16 eV and 20 - 22 eV were considered to be due to those of the phenyl substituents ( ~ , ( ~ , (E) and (~). And it was found that the electronic structures of dihydrothiophene substituted with thienyl groups were more delocalized around the HOMO in comparison with that having phenyl groups. From preliminary m e a s u r e m e n t s of cyclic voltammograms for 2,5-bis(dithienylmethylene)-2,5-dihydrothiophene and dihydroselenophene, 2,5-bis(diphenylmethylene)-2,5-dihydrothiophene, and octadecyl tetracyano- quinodimethane (C--,18TCNQ) in acetonitrile solutions, the oxidation and reduction potentials were obtained to be +0.21, +0.28, +0.39 and -0.23 V respectively, from the potentials of Fc/Fc + as a standard. These potentials seem to reflect the electronic structures deduced from the UPS measure- ments. 80 From thses results, it is considered that the dihydrothiophene substituted with C~5 ~ thienyl groups is more favorable for 70 the donor to the acceptor of C18TCNQ rather than the other dihydrothiophene .-.60 2 and selenophene. E By refluxing the a c e t o n i t r i l e or z ethanol solution of 2,5-bis(dithienylE 50 methylene)-2,5-dihydrothiophene and Ill t rr C18TCNQ(1 : 1 - - 1 : 2 in the molar u)40 CJ) ratio) for six hours and cooling the LB solution at room temperature, the m I( molecular complex was precipitated w 3O as black crystals. The composition of t~ the molecular complex was found to n.D20 ~..- izl-be 1 : 2 by the 1H_NMR spectrum and the absorption bands at 743 nm and 847 nm due to the TCNQ radical anion I0 w e r e o b s e r v e d in t h e s o l u t i o n spectrum. As shown in Figure 34, the 00 complex gave a typical condensed 30 60 90 120 I50 AREA ( ~ 2 / m o l e c u l e ) monolayer and from observation by a Figure 34. Surface pressure - area isofluorescence microscopy it was found therms for monolayers of C18TCNQ (a), that the more homogeneous film could be obtained by spreading the monothe mixture of the dihydrothiophene and layer on the aqueous LiTCNQ (10-5 M) C18TCNQ (b), and the complex (c), subphase, as shown in Figure 35. The spread on distilled water, as compared monolayers of the complex were transwith that on the aqueous subphase with 10.5 M LiTCNQ (c'). ferred onto solid plates at 30 mN/m

100 by the horizontal lifting method and the film thickness measured by an o p t i c a l i n t e r f e r e n c e technique was 4 9 A per layer. This value is considered to be reasonable for the molecular model of a pair of C18TCNQ (32 A) anchored on the d i h y d r o thiophene molecule (16 A), as shown schematically in the insert of Figure 30. The electrical conductivities for the multilayers of the complex in the directions parallel (o~.) and perpendicular (o.L) to the film plane, were 8.9 x 10 -8 - 8.3 x 10-9 S cm- 1 and 1.0 x 10-14 S cm-1, respectively. The former value increased up to 1.1 x 10 -7 S cm -1 with the homogeneous film. Thus, it has been found that the monolayer assembly of the novel complex of the dihydrothiophene substituted with thienyl groups and C18TCNQ (1 : 2) exhibits the remarkably anisotropic conductivity, that Figure 35. Fluorescence microscopic pictures is, the lateral conductivity is about for LB films of a mixture and the complex 107 times larger than the normal one. of 2,5-dihydrothiophene and C18TCNQ.

3.8. Polycondensation of long-chain esters of amino acids containing aromatic rings in the monolayer assemblies. It is interesting subject to clarify effects of molecular arrangements of some chemical reactions in molecular assemblie. Previously, we have found that the polycondensation reaction of long-chain esters of a-amino acids proceeds easily in the LB films at room temperatures without any initiation [52 - 54]. On the other hand, we have reported the molecular arrangements and the photophysical behaviours of N-octadecanoyI-L-lysyI-L-l-naphthylalanine methylester in the LB films [53], in which different orientations and packing of the chromophore were obtained depending on the deposition conditions. In this report, we have synthesized octadecylesters of L-l-naphthylalanine (L-NaphAla-C18) a n d L-l-pyrenylalanine (L-PyrAla-C18), as shown in the top of Figure 35, and examined the molecular orientation and packing by circular dichroism together with UV a b s o r p t i o n and f l u o r e s c e n c e s p e c t r o s c o p i e s . Further, the polycondensation of these amino acid derivatives and the mixtures with octadecylester of alanine (L-Aia-C18) in the LB films has been studied.

101

Figure 36 shows the surface pressure area isotherms for the (20 ~ 40 long-chain esters of H2N-tCH-COOCle H~;, L-alanine containing n a p h t h a l e n e and p y r e n e as a s i d e E group, indicating H2N-CH-C 0 0 Cle Hzz stable condensed v monolayers with the l i m i t i n g a r e a s of L-PyrAia-Cl8 about 54 and 47 A2/ I/3 molecule for L-Naph~zo Ala-C18 and L - P y r NapAla-C~s O. Ala-C18, respectively. Taking into account tO 13 the molecular dimenL. sions of naphthalene ~I0 or) (7.2 x 9.1 x 3.6 A) and pyrene (9.1 x 9.5 x 3.6 .~,) from crystallographic data [56], it is considered that the ~ ,o 2o 30 ;o 50 60 70 80 naphthyl group lying A r e a (/~21molecule) flat in the expanded Figure 36. ufface pressure - a r e a isotherms of the region of the monoayer monolayers of L-NaphAia-C18 and L-PyrAla-C18. stands up upon compression, whereas the pyrenyl group is oriented rather vertical to the water surface even at lower pressures as well as at higher pressures. These monolayers could be transferred onto the solid plates at 10 - 30 mN/m by the horizontal lifting method to give the nonalternating X-type films, and above 20 mN/m the alternating Y-type films were obtained by the vertical dipping (LB) methods. Polarized UV absorption spectra for the multilayers supported the molecular orientations mentioned above. Figures 37 shows circular dichroisms for the multilayers of L-NaphAla-C18. A positive Cotton effect was observed at 225 nm in the solution of L-NaphAla-C18, which can be ascribed to dipole-dipole interactions for the 1Bb transition along the long-axis of naphthalene. A negative Cotton effect was obtained at 207 - 213 nm in the X-type films, which seems to be associated with the amino group and the 1Bb transition, while the Y-type film prepared at the higher surface pressure exhibited a strong negative Cotton effect due to the interaction of the chromophores. Thus, the circular dichroisms for the films depend significantly upon the deposition methods and the surface pressures. The similar results were obtained for the LB films of L-PyrAla-C18. Therefore, the strong Cotton effect in the Y-type films involves a contribution of the interlayer interaction of the chromophores in

po

102 addition to the intralayer interaction. These results were reflected to the efficiency of excimer 20 m N / m X film-..... ~> , L - N a p A l a - C~8 formation in the LB films. Figures 38 (a) and (b) show fluorescence ...,,...,,..o, so,.. spectra for the multilayers of / i \ \io ,..~.,.,." \ L-NaphAla-C18 and L - P y r A l a C18, respectively, prepared at 20 mN/m. In the both cases, the monomer bands are predominant in the X-type films where the layers of the chromophores are s e p a r a t e d by h y d r o c a r b o n chains, while the excimer bands are enhanced in the Y-type films where the dimer formation of the chromophores can occur preferably in the head to head arrange200 250 ments of the bilayer sequence. WAVELENGTH (nm) These facts are in well correspondence to the fluorescence Figure 37. Circular dichroism spectra for spectra for the solutions of different the films of L-NaphAla-C18 prepared concentrations and the excimer bands at different conditions. are enhanced in the higher concentration. Fluoreasence decay curves of the excimer observed at 480 nm for the multilayers of L-PyrAla-C18 also depend upon the deposition types and the surface I . . . . . . . .

I

9

I

/7"

/ L

9

I

--

,,

2

-

I

I

I

HzN-CH-CO0ClaH3.t

! ~•

.

Ex

I

HzN-i_m H-COOC C

=285nm

H37

Ex =310nm

db E / ~

/

~

i lm

9

Mr'/

300

f

~-i

/

l

I

,

\\

i

|

I

\

J

I

!

I

I

4OO 500 400 500 600 WAVELENGTH (nm) WAVELENGTH (nm) Figure 38. Fluorescence spectra for the X- and Y-types of multilayers of L-NaphAla-C18 and L-PyrAla-C18.

103

pressures, as shown r in Figure 39. In the case of Y-type films prepared at 2 0 raN/m, a rise of a Ex-310 nm, Em=480nm 4 360 ps component ZLgj0 suggest a process of 3 6 0 p s r J S e \i I.-the excimer formaz ~ . , ~ . ~ 20 mN/m, tion b e t w e e n the "%~.;,..... ..... " - ' ~ ~ Y film O adjacent layers. The .~. ~ . 9 ......... .,:...-,..~..._:.?,,.., (..) main components z '.'. ""~"'.. 20 mN/m[ for lifetimes of the C::) 10 ? , ': ~ ~ ' ~ X film excimer in the films 20 P~"' ~ ~ " " ::. : """:s > O 2E were obtained to be EL "" """ :'~.: :': .':'. .. 10 m N / m , about 800 ps and 5 ns, which are very >-.. short as compared "- "" "" "...'- ": . " - ' 2 I--.- 0~ NJ 000p, " ::? with those in the solution (200 ~ 400 z uJ ns). The more rapid HzN--?H--COOC18H3z z component of 120 ps is found to be c3 tlJ added for the X-type 1",4 1 film prepared at the lower surface pressure. The different oro arrangements of the z chromophore have o been well reflected _10 in therelaxation pro.t. 2 3 4 5 6 7 8 g J0 c e s s of p h o t o TIME / NANOSECONDS excited states in the Figure 39. Fluorescence decay curves of the excimer layered system. observed at 480 nm for the multilayers of L-PyrAlaPolycondensation C18 prepared at different conditions. reactions in the LB films of L-NaphAla-C18 were followed by monitoring change of the IR spectra, as shown in Figure 40. From diminishing the ester band (1735 cm-1) and appearance of the amide I bands (1690 - 1650 cm-1), it can be found that the polycondensation occurs in the LB films. In the X-type film, however, the conversion was very low at 40 ~ This is probably due to the reason that the intralayer reaction resulting in a helical structure favorable to polyalanine [56] is inhibited. For the Y-type film, in the first step for the oligomer was obtained at 25 ~ and then the poly-amino acid in a random coil conformation was produced through increasing the temperature up to 40 oC, as suggested by the amide I bands at 1690 and 1650 cm-1, respectively. It is supposed that the interlayer reaction propagates by "sewing up" the amino and 14.17

~n

O2.tU -4

! |

.

17 t

,

,

.

.

.

,

_ _

",

b

j

I

I

.,

,

K2)

!

I

I

i

I

.....

I

I

_

I

,

104

J

{I ~c~:o'17!5

'P CH2

2921

'

ester A amideI

~~,'=,~o-~:~-

.-~ ,~.-~ ~

Xtype 2!.,~-,

28b0` '; 18(X) ' WAVENUMBER ( cm -~)

14~3

CH2

2920

')C:O omido I (oligomer)

1690{rondom co~} PC:~er 1735 I 1650

850

V'\.H C/\N. 6 ~ o 0 H 0 H ~\/~-H .~\ /~-H O~ HC o~ Hc,

"-

~.v'\~ oc/ .. I

I I i I oc%/NH 0C%/NH

HC Ho. HC

1

40~

t

_f

Y type !

!

....

I~,

I

,,

I

1400 2800 1800 WAVENUMBER ( cm-1) Figure 40. Change of IR spectra for X- and Y-type LB films of UNaphAla-C18 together with the reaction mechanisms.

3200

105

the ester groups facing each other at the adjacent layers in the Y-type films. A similar spectral change was observed for the LB films of L-PyrAla-C18. The percent conversion of the monomer to the polypeptide can be estimated by the quantity 100 ( A o - At) / Ao, where Ao and At are the integrated intensities of the ester band at the start and time t, respectively. At 40 ~ it tended to saturate at about 30 % for the Y-type films of L-NaphAla-C18, and L-PyrAla-C18, although the conversion for the LB film of long-chain ester of alanine (L-Ala- C18) reached to 90 % [52]. This difference is considered to be due to a larger steric hindrance of the aromatic rings. The equimolecular mixture of L-NaphAla-C18 and L-Ala-C18 forms the stable condensed monolayer on the aqueous sub- phase of pH 8 and can be deposited onto solid plates at 30 mN/m and 10 ~ as Y-type films by the vertical dipping m e t h o d . F i g u r e 41 s h o w s plots of the conversion against the reaction time for the - A l a - C ; . , 40 ')C m i x e d LB f i l m , in comparison with that f o r t h e LB f i l m of L-Ala-C~s : L - N a p A l a - C ; ~ = I : 1 L-NaphAla-C18 a lo n e 45 ~ at 40 ~ A consider40"C ~ O~"s"l:l~~ able increase of the 35 ~ conversions was observed for the mixed LB film, although the freaction rates were 0 smaller than that in the L~ pure L-Aia-Ct8 f i l m L. N a p A l a - C t . , 40 ~ > reported in the prec o vious paper [53]. This 0 result indicates that by mixing the ester of amino acid containing the aromatic ring with L-Ala-C18, the steric h i n d r a n c e can be partly reduced. With the temperature elevation from 35 to 45 ~ o ' s'o ;6o =5o the initial reaction rates Time (hr) were somewhat enFigure 41. Plots of the percent conversion against hanced, while the final the reaction time for LB films of L-NaphAla-C18, conversions were not L-Ala-C18 and their equimolar mixture. significantly changed. Further, the circular dichroisms

S

106

for the mixed LB film were changed during the progress of the polycondensation and the Cotton effect due to the interaction of the 1Bb transition of naphthalene was enhanced. It is expected that the polycondensation in LB films of long-chain esters of amino acids with aromatic rings provide an interesting step to control arrangements of n-electron systems in the layered structures, depending upon conformations of the resultant polypeptides [57].

4. C O N C L U S I O N By applying the monolayer assemblying methods to the amphiphilic functionalized molecules, the layered structures with a well-defined orientation and packing of the functional groups could be obtained, in which various molecular orienations and packings could be demonstrated, depending on the chemical modification such as kinds, number and positions of the substituents. Some physical and chemical processes such as relaxation of the excited states and polycondensations in the films, can be investigated in relation to the molecular arrangements to clarify the biological and artificial functions of the molecular organized systems. These studies are expected to be developed in future aspects for molecular electronics.

REFERENCES 1. N.Kimizuka and T.Kunitake, Colloid Surfaces, 19 (1989) 301. 2. M.Losche and H.Mohwald, Rev. Sci. Instrum., 55 (1984) 1968. 3. D.Honig and D.M6bius, J. Phys. Chem., 95 (1991) 4590. 4. H.Nakahara and K.Fukuda, J. Colloid Interface Sci., 69 (1979) 24; 93 (1983) 530. 5. S.Kim, H.Tanaka, and L.S.Pu, Japanese Patents L.O., 60-128453, 60-130558, 1985. 6. H.Takahashi, Y.Irinatsu, M.Tsujihashi, S.Kozuka, and W.Tagaki, Nippon KagakuKaishL (1987) 293. 7. H.Nishi, T.Kawashima, and K.Kitahara, Nippon KagakuKaishL (1990) 1162. 8. H.Nishi, N.Azuma, and K.Kitahara, J. Heterocycle. Chem., 29 (1992) 475. 9. M.Sisido, S.Egusa, and Y.Imanishi, Macromolecules, 18 (1985) 882. 10. K.Kjaer, J.AIs-Nielsen, C.A.Helm, P.Tippman-krayer, and H.MSwald, J. Phys. Chem., 93 (1989) 3200. 11. C.Knobler, Science, 249 (1990) 870. 12. T.Kajiyama, N.Morotomi, M.Uchida, and Y.Oishi, Chem. Lett., (1989) 1047. 13. T.Kajiyama, Y.Oishi, M.Uchida, N.Morotomi, J.Ishikawa, and Y.Tanimoto, Bull. Chem. Soc. Jpn., 65 (1992) 864.

107

14. Z.-h.Lu and H.Nakahara, Chem. Lett., (1995) 117. 15. Z.-h.Lu and H.Nakahara, Chem. Lett., (1994) 2005. 16. S.Henon, D.Honig, D.Vollhardt, and D.M6bius, J. Phys. Chem., 96 (1992) 8157. 17. K.Kobayashi, M.Takasago, Y.Taru, and K.Takaoka, Thin Solid Films, 247 (1994) 248. 18. W.Liang and H.Nakahara, Chem. Lett., (1995) 973. 19. T.Moriizumi, Thin Solid Films, 160 (1988) 413. 20. T.Katsube, T.Yaji, K.Kobayashi, T.Kawaguchi, and T.Shiro, Appl. Surface Sci., 33/34 (1988) 413. 21. T.Kawaguchi, T.Shiro, and K.Iwata, Thin Solid Films, 191 (1990) 369. 22. I.V.Turko, I.S.Yurkevich, and V.L.Chashchin, Thin Solid Films, 210/211 (1992) 710. 23. H.Nakahara. H.Tanaka, K.Fukuda, M.Matsumoto, and W.Tagaki, Thin Solid Films, in press. 24. M.L.Bender and M.Komiyama, Cyclodextrin Chemistry, Springer-Verlag (1978). 25. Y.Kawabata, M.Matsumoto, T.Nakamura, M.Tanaka, E.Manda, H.Takakashi, S.Tamura, W.Tagaki, N.Nakahara, and K.Fukuda, Thin Solid Films, 159 (1988) 353. 26. A.Yabe, Y.Kawabata, H.Niino. M.Tanaka, A.Ouchi, H.Takahashi, S.Tamura, W.Tagaki, H.Nakahara, and K.Fukuda, Chem. Lett., (1988) 1. 27. A.Yabe, Y.Kawabata, H.Niino, M.Matsumoto, A.Ouchi, H.Takahashi, S.Tamura, W.Tagaki, H.Nakahara, and K.Fukuda, Thin Solid Films, 160 (1988) 33. 28. M.Tanaka, R.Azumi, H.Tachibana, T.Nakamura, Y.Kawabata, M.Matsumoto, T.Miyasaka, W.Tagaki, H.Nakahara, and K.Fukuda, Thin Solid Films, 244 (1994) 832. 29. H.Nakahara, K.Fukuda, I.Yamazaki, M.Matsumoto, and W.Tagaki, unpublished data. 30. R.Steiger, R.Kitzing, and P.Junod, Photographic Sensitivity (ed. Cox,R.J.), Academic Press, London (1973) 221. 31. S,Kim, M.Furuki, L.S.Pu, H.Nakahara, and K.Fukuda, Thin Solid Films, 160 (1988) 303. 32. H.Nakahara and D.M6bius, J. Colloid Interface Sci., 114 (1986) 363. 33. H.Nakahara, K.Fukuda, D.MSbius, and H.Kuhn, J. Phys. Chem., 90 (1986) 6144. 34. R.O.Routfy, C.K.Hsiano, and R.M.Kazmaier, Photohr. Sci. Eng., 27 (1983) 5. 35. K.Y.Law, Chem.Rev., 93 (1993) 449. 36. K.Fukuda and H.Nakahara, Colloids and Surfaces, A102 (1995) 57. 37. F.urbach, Phys. Rev., 92 (1953) 1324. 38. M.Schreiber and Y.Toyozawa, J. Phys. Soc. Jpn., 51 (1982) 1544. 39. H.Nakahara, H.Uchimi, K.Fukuda, N.Tamai, and I.Yamazaki, Mol. Cryst. Liq. Cryst., 183 (1990) 345.

108

40. H.Nakahara and K.Fukuda, J. Colloid Interface Sci., 69 (1979) 24; 93 (1983) 530. 41. H.Nakahara, K.Kitahara, H.Nishi, and K.Fukuda, Chem. Lett., (1992) 711. 42. H.Kuhn, Light-Induced Charge Separation in Biology and Chemistry, (eds. Gerischer, H. & Katz,J.J.), Verlag Chemie, Weinheim (1979) 151. 43. H.Nakahara, Y.Sano, I.Yamazaki, T.Yamazaki, K.Kitahara, H.Nishi, and K.Fukuda, J. Phys. Chem., to be submitted. 44. H.Nakahara, M.Sato, and K.Fukuda, Thin Solid Films, 133 (1985) 1. 45. H.Nakahara, T.Katoh, M.Sato, and K.Fukuda, Thin Solid Films, 160 (1988) 153. 46. K.Ogawa, S.Kinoshita, H.Yonehara, H.Nakahara, and K.Fukuda, J. Chem. Soc., Chem. Commun., (1989) 477. 47. H.Nakahara, K.Fukuda, K.Kitahara, and H.Nishi, Thin Solid Films, 178 (1989) 361. 48. H.Nakahara, K.Z.Sun, K.Fukuda, N.Azuma, H.Nishi, H.Uchida, and T.Katsube, J. Mater. Chem., 5 (1995) 395. 49. H.Fujimoto, U.Nagashima, H.Inokuchi, K.Seki, Y.Cao, H.Nakahara, J.Nakayama, M.Hoshino, and K.Fukuda, J. Chem. Phys., 92 (1990) 4077. 50. H.Nakahara, J.Nakayama, M.Hoshino, and K.Fukuda, Thin Solid Films, 160 (1988) 87. 51. A.ishii, Y.Horikawa, I.Takaki, J.Shibata, J.Nakayama, and M.Hoshino, Tetrahedron Lett., 32 (1991) 4313. 52. H.Nakahara, A.Nagasawa, A.Ishii, J.Nakayama, M.Hoshino, K.Fukuda, K.Kamiya, C.Nakano, U.Nagashima, K.Seki, and H.Inokuchi, Mol. Cryst. Liq. Cryst., 227 (1993) 13. 53. K.Fukuda, Y.Shibasaki, and H.Nakahara, J. Macromolecular Sci., Chem. Ed., A15 (1981) 999; Thin Solid Films, 160 (1988) 43. 54. K.Fukuda, Y.Shibasaki, H.Nakahara, and H.Endo, Thin Solid Films, 179 (1989) 103. 55. H.Nakahara, H.Endo, K.Fukuda, T.Ikeda, and M.Sisido, Mol. Cryst. Liq. Cryst., 218 (1992) 177. 56. Ralph W.G.Wyckoff, Crystal Structure, vol.6, Pt.2, Intersci. Pub., New York, (1971) 383, 511. 57. H.Nakahara, K.Hayashi, Y.Shibasaki, K.Fukuda, T.Ikeda, and M.Sisido, Thin Solid Films, 244 (1994) 1055.

New Developments in Construction and Functions of Organic Thin Films T. Kajiyama and M. Aizawa (Editors) 9 1996 Elsevier Science B.V. All rights reserved.

Characterization by using a Quartz Substrate

In

situ

Yoshio

of L a n g m u i r - B i o d g e t t Crystal Microbalance

109

Films as a

Okahata

Department of Biomolecular Engineering, Tokyo Institute Technology, Nagatsuda, Midori-ku, Yokohama 226, Japan

of

We review useful usages of a quartz crystal-microbalnce (QCM) as tool of in situ characterization of Langmuir-Blodgett (LB) films" transfer ratio and water incorporation during a transfer process, swelling behavior in water subphase, and detachment at the airwater interface. LB films of cadmium octadecanoate and other amphiphiles were transferred on a quartz-crystal microbalance (QCM, 9 MHz, ATcut) as a substrate with a vertical dipping method. Frequencies of the QCM substrate were followed with time in air, after the QCM was raised from the interface. From the time courses of these frequency changes at each dipping cycle, the transfer amount of dry LB films (W1), the incorporated amount of water (W2), and its evaporation speed (v) could be obtained in nanogram level. The QCM can be used as a monitor of hydration behavior of phospholipid LB films in water. The hydration rate and amount were obtained from time-courses of frequency changes of the QCM deposited with LB f i l m s of various phospholipids. The phosphatidylethanolamino (PE) LB films showed the large hydration (water penetration) only around their phase transition temperatures (To). The LB film of DPPC and DPPG having relatively hydrophilic head groups gradually flaked from the substrate in the fluid state above their To. On the other hand, the LB film of DPPS having relatively water-unacceptable head groups hardly hydrated at any temperatures both below and above T~.

110

The QCM can be used as a monitor of detachment of LB films from the QCM substrate passed through the air-water interface. The removal of LB films mainly occurred when the substrate was lowered slowly into a pure water surface, but not in the raising process. 1. I N T R O D U C T I O N Interest in Langmuir-Blodgett (LB) films is wide spread and formation of ordered thin organic films by transforming lipid monolayers from a water surface is well known [1]. The characterization of LB multilayer films has been studied usually in the dry state by various methods such as FT-IR spectroscopy, X-ray diffraction, ellipsometry, and X-ray photoelectron spectroscopy [2-4]. Recently, the direct observation of the monolayer on the water subphase has been given by fluorescent microscopy [5-7], X-ray diffraction, and electron microscopy techniques [8,9]. However, the in situ evaluation of LB films during a transfer process, stability in water or at the air-water interface has not been fully explored. In this review, we describe our recent results of in situ characterization of LB films during vertical transfer process, swelling or hydration behavior in water subphase, and detachment at the airwater interface during pass through the interface by using a quartzcrystal microbalance (QCM). QCMs are known to provide very sensitive mass measuring devices because their resonance frequency changes upon the deposition of a given mass on the electrode [10]. QCMs are widely employed as a sensor device such as gas sensing [11], trace ion detection [12], detection of odor compounds and other bioactive compounds [13], immunoassay [14], DNA hybridization [15], enzyme reaction [16], surface analysis [17], gelation monitoring [18], liquid chromatographic detection [19], and electrochemical analysis [20-22]. However, only a few preliminary uses of the QCM plate to analyze the transfer processes of LB films in situ [23]. 2. EVALUATION OF LB FILMS DURING A T R A N S F E R PROCESS: T R A N S F E R RATIO AND WATER I N C O R P O R A T I O N Although characterization of LB films after deposited on a substrate or monolayers at the air-water interface have been studied in detail. However, characterization of LB films during a transfer

111 process such as water incorporation between layers and drying time has not been fully given in the conventional methods. When a monolayer moves from a water subphase to a solid substrate, behavior of water might be the key to control the film quality. It has been reported that subtle change of the thin water layer under the phospholipid monolayer during the transfer process caused the crystallization [24]. The polarized micrograph of the 22-tricosanoic acid LB film showed anisotropy due to water flow [25]. Water content must affect the electrical and the optical properties of LB films. For a practical problem, drying time of LB films in air during transfer processes has been determined with our own experience and feeling. Quartz-crystal

microbalance

(QCM) r

Osc,,,a,,ng L I counter F--quenc,.H ecomputer ona, circuit ] - --

1

LB films LB film-forming a m p h i p h i l e s (CH3(CH 2)n.2.CO O " )Cd2+

n = 16, 18, 20, 22

CH3(CH2)lsCOOH CH3(CH2)lsCOOCzHs CH3(CHz)16CONH2 CH3(CH2)lT-OH

Figure 1 E x p e r i m e n t a l set-up for vertical dipping processes of monolayers on a quartz-crystal microbalance (QCM) substrate.

In this chapter, we characterize in situ vertical transfer process of LB films of cadmium octadecanoate and other amphiphiles by using a QCM as a dipping substrate. Experimental set-up is shown in Figure 1. A transfer amount of dry LB films ( W ~ / ng), an incorporated amount of water during a lifting process (W2/ ng), and

112 an evaporation speed of the water under drying in air ( v / ng s-l) are obtained from time courses of the frequency change of the QCM at each dipping cycle. Effect of transfer conditions such as surface pressure, number of layers, and dipping speed, and chemical structures of amphiphiles on these values (W1, W2, and v ) a r e studied.

2.1

Experimental

Measurements of pressure-area (n-A) isotherms and transfers of monolayers on a substrate were carried out by using a computercontrolled film balance system (San-Esu Keisoku, Co., Fukuoka, FSD20). Maximum surface area on the trough was 475 X 150 mm2. The trough surface and the moving barrier were coated with Teflon, and the subphase was temperature-controlled with a thermostat (20 + 0.5 ~ The concentration of lipid solutions was 1 mg/ml and the spreading amount of lipid solutions was 50 - 150 g l. After solvent evaporation, the monolayer was compressed at the speed of 0.60 cm2 s-l. Measurements of n-A isotherms and transfers of monolayer on a QCM substrate were performed automatically with the usual manner [26,27]. AT-cut, 9 MHz quartz-crystal oscillators were purchased from Kyushu Dentsu, Co., Tokyo, in which Ag electrodes (0.238 cm2) had been deposited on each side of a quartz-plate (0.640 cm2). A homemade oscillator circuit was designed to drive the quartz at its resonant frequency both in air and water phases. The quartz crystal plates were usually treated with 1,1,1,3,3,3-hexamethyldisilazane to obtain a h y d r o p h o b i c surface unless otherwise stated [28]. Frequencies of the QCM was followed continuously by a universal frequency counter (Iwatsu, Co., Tokyo, SC 7201 model) attached to a microcomputer system (NEC, PC 8801 model). The following equation has been obtained for the AT-cut shear mode QCM [10]:

zlF = -2F2 Am A~/pqUq

(1)

where A F is the measured frequency shift (Hz), 17o the parent frequency of the QCM (9 X 106 Hz), Am the mass change on the electrode (g), A the electrode area (0.238 cm2), pq the density of quartz (2.65 g cm-3), and ~Zq the shear modulus of quartz (2.95 X 1011

113 dyne cm-2). Thus, the frequency decreases linearly with increasing the mass on the electrode area of the QCM. Calibration of the QCM used in our experiment by a polymer-casting or LB film-depositing method gave the following equation [13,26,27]. Am

=

(2)

- (1.27 + 0.01) x 10-9 AF

It is close to the theoretical equation calculated 1.30 X 10-9 AF). The stability of the QCM examined. The standard deviation of frequencies ng) and no frequency-drift was confirmed by with 95% confidence.

from eq. (1) (Am = frequency was also was ca. 0.5 Hz (0.6 a statistical method

A 0 N "r -

-

cll

200

ID

c t~ e-o

-400

slope,

9

v i/

-600

111111

r

c o" u.

-800 -I000

o '2'o'2o-6'o

1

8o

.

,;o

.

1

1

Time / min Figure 2 Frequency changes of the QCM substrate in air during 4 cycles of vertical dipping processes of cadmium octadecanoate LB films (surface pressure" 20 mN m-l, 20 ~ dipping speed" 100 mm min-~).

Process of LB Films on a QCM Typical time courses of frequency changes of the QCM substrate in air during 4 cycles of vertical dipping processes are shown in Figure 2. The QCM was lowered into the subphase at point A and raised in air at point B with a dipping speed of 100 mm min-~ through the cadmium octadecanoate monolayer (20 mN m-~, 20 ~ The frequency of the QCM in air gradually increased with time and 2.2

Transfer

114 reached a constant value in 15 min at point C. From the decrease of frequencies of 183 + 3 Hz from points A to C, the increased mass with each cycle was calculated to be W1 = 232 + 3 ng according to eq. (2). This value was consistent with the theoretical mass of four dry monolayers (two layers on each side) of cadmium octadecanoate (225 ng) on the Ag electrode of the QCM, which was calculated from the average area per molecule in the monolayer (0.237 nm2 from a rt-A isotherm) and the area of Ag electrode (0.238 cm2). Thus, the frequency decease is affected only with the mass on the electrode area of the quartz plate. The gradual frequency increase from points B to C is explained by the mass decrease due to the evaporation of water deposited b e t w e e n layers from the subphase. The amount of incorporated water (W2 / ng) and its evaporation speed (v / ng min-1) were calculated from the frequency change and the initial slope of the time course between points B and C, respectively. The cadmium octadecanoate LB films were observed to incorporate W2 = 209 + 5 ng of water with 4 layers of LB films (WI = 232 + 3 ng) at the first d i p p i n g cycle: c a d m i u m o c t a d e c a n o a t e were transferred on a substrate with the water of almost the same mass of LB films. Figure 2 also indicates that we should wait ca. 15 min to get the dry LB films during transfer processes in this conditions. When the next deposition was carried out before the complete evaporation of water, a transfer ratio of LB films was gradually decreased from 0.9 to 0.7 with increasing dipping cycles. 2.3

T r a n s f e r R a t i o of LB F i l m s The transfer process was continued at least 10 times at different surface pressures (20, 10 and 5 mN m-~). The total transferred weight (ZW~) of dry cadmium octadecanoate LB films was plotted against the number of transfer cycle as closed circles in Figure 3. The amount of transferred films was also estimated from the conventional method calculated from a moving area of a barrier in kept the surface pressure constant, and plotted as closed triangles in Figure 3. Straight lines indicate the theoretical mass of Y-type two layers on each side of the QCM. At the high surface pressure of 20 mN m-~, the transferred w e i g h t obtained from both the frequency changes of the QCM s u b s t r a t e and the barrier m o v e m e n t was almost equal to the

115 theoretical line, and the obtained transfer ratio was 1.01 + 0.02. McCaffrey et. al. had reported the d e p o s i t i o n of c a d m i u m octadecanoate LB films on a QCM plate and obtained the similar linear correlation between the frequency change and the number of transfer cycle [23]. However, the mass associated with each layer was 20% larger than the theoretical mass of the dry LB films, probably because they deposited LB films with the water lifted still incorporated in them. In this study, the weight calculated from the QCM method was also in accord with the value from the conventional barrier movement, which means all the disappeared monolayers from the air-water interface were transferred on the substrate under this condition.

:L

(a)

20 m N

m "t

(b)

10 m N

rn "1

(c) 5

mN

m-I

2.0 o&

.c I.S

o&

ID

O&

"~ 1.0 ~D (I)

"~ 0.5 c-

~-

~ 2 ~ o 8~oo2

~. ~ 8 ~ b o

2 ~, 6 ~ ~b

Number of Transfer Cycle

Figure 3 The total transferred weight ( E W e ) of cadmium octadecanoate LB films calculated from the QCM method ( 0 ) and the barrier movement ( & during repeated depositions at the surface pressure of (a) 20 mN m-l, (b) 10 mN m-l, and (c) 5 mN m-~. Solid lines represent the theoretical value calculated from the n-A isotherms (dipping speed: 100 mm min-~, 20 ~

At the low surface pressures of 5 and 10 mN m-i, the observed values showed the deviation from the theoretical lines and the transfer ratios were c a . 0.9 - 0.7. The transferred mass obtained from the barrier movement was smaller than that from the QCM method under these conditions. This means that the barrier motion on the surface cannot compensate the disappeared area of the

116 transferred monolayer enough at the low surface pressure. Thus, the QCM method is useful to estimate in situ the real transfer weight on the substrate in comparison with the conventional method even at the low surface pressure. 2.4

Effect of Surface of the S u b s t r a t e and N u m b e r of Layers The amount of incorporated water (W2) and its evaporation speed (v) were obtained as a function of dipping cycles in the transfer of cadmium octadecanoate LB films and the results are shown in Figure 4. Both relatively hydrophilic and hydrophobic surfaces of the QCM were employed as a substrate, in which the former was a bare Ag electrode (contact angle for water: 50 +_ 5 o) and the later was prepared by treating with 1,1,1,3,3,3hexamethyldisilasane (contact angle for water: 110 + 5 o)[28]. The transfer ratio of LB films ( W : ) w a s 0.98 + 0.05 for each dipping cycle on both hydrophilic and hydrophobic substrates. The monolayer could be transferred even on the hydrophilic bare electrode for the first downward process, because the Ag surface is not so hydrophilic (contact angle: 50 + 5 ~

600 c-

60 . m

400 200

E

40

20

00246810 Number of Layers

Number of Layers

Figure 4 Effect of number of layers on the amount of incorporated water (W2) and the evaporation speed of the water (v) in the transfer of cadmium octadecanoate LB films. The hydrophilic (O) and hydrophobic ( 0 ) s u b s t r a t e s were used in this e x p e r i m e n t (surface pressure" 20 mN m-~, dipping speed" 100 mm min-~, 20 ~

117 The amount of incorporated water (W2) and its evaporation speed (v) decreased as the number of layers increased in both hydrophilic and hydrophobic surfaces of the QCM. W2 and v values were particularly large at the first cycle in the case of the hydrophilic surface. It has been shown that several layers on a substrate are disordered by the influence of the substrate surface and this effect disappears as the number of layers increases [29,30]. The first few layers seem to incorporate the large amount of water in the defects of LB films and the water easily evaporates through the disordered monolayer. The hydrophobic alkyl chains contact with the substrate surface at the first down stroke. Such a contact has a disadvantage in energy in the case of the substrate having a hydrophilic surface, then the first monolayer on the hydrophilic surface was particularly disordered and shows the large W2 and v values. The dependency of the evaporation speed (v) on the number of layers seemed to be larger than that of the amount of incorporated water (W2), which means v values are more sensitive parameters reflecting the film disorder than the W2 values. When the larger number of layers was deposited, the longer drying time was required due to the slower evaporation speed of water.

4O

400

,,.,,,. !

._~ 30 E

30O ~

g2o

200 ~",,

> 10

100

06

fo

Sudace PressureImN m"

Figure 5 Effect of surface pressures on the !ncorporated water (W2) and the evaporation ~n the transfer of cadmium octadecanoate (dipping speed" 100 mm min-~, 20 ~ at the cycle).

amount of speed ( v ) LB films 5th transfer

118 E f f e c t of Surface Pressure on Water I n c o r p o r a t i o n The W2 and v values at the 5th dipping cycle were obtained at various surface pressures, and the results are shown in Figure 5. Both W2 and v values increased with decreasing the surface pressure. At the low surface pressures below 20 rnN m-~, the LB films having many defects were transferred with a low transfer ratio below 1.0, and the large amount of water was incorporated in these defects and its evaporation speed was fast through the disordered film. 2.5

2.6

Effect of Dipping Speed W 2 and v values at the 5th transfer cycle of c a d m i u m o c t a d e c a n o a t e at 20 mN m-~ as a function of dipping speed of the QCM substrate are summarized in Table 1. The transfer ratio of LB films (W j) was 0.95 + 0.05 at the dipping speeds of 40 - 100 mm min-~. W h e n the dipping speed was decreased, the evaporation speed (v) was decreased: the well-oriented LB films could be obtained at the low dipping speed. This is consistent with the report by Pitt, et. al. that the lower transfer speed was favorable to obtain the higher quality LB films in the first 10 layers [31]. Table 1 Effect of dipping speeds octadecanoate LB filmsa dipping speed/mm min-1 100 80 60 40

on

the

transfer

of cadmium

W2/ng Wswen/ng W2-Wswell/ng v/ng min-X 209 186 212 262

40 50 66 95

169 136 146 167

15.8 14.2 10.0 7.7

a Surface pressure: 20 mN m -1, 20 ~ at the fifth dipping cycle. The incorporated amount of water (W2) seems to increase with decreasing the dipping speed. Since the W2 value may include both the really i n c o r p o r a t e d water and the swelling with water when the substrate exists in the water subphase, the effect of dipping speed on W2 values should be divided in two factors. We have already reported

119 that cadmium octadecanoate LB films swelled largely in the subphase, compared with other LB films [26]. The swelling amount (W,,,,u)was separately obtained from the frequency decreases (mass increase) when the LB film-deposited QCM was soaked for the time in the subphase calculated from each dipping speed. The (W,w,l~- W z) value reflects the true amount of water pulled into the outer layer during the lifting-up process and was almost independent of dipping speeds of 40 - 100 mm min-~ (see Table 1). The W2 value seems to increase with decreasing the dipping speed due to the swelling amount. In the case of the transfer of octadecanol monolayers, the different results were obtained as shown in Table 2. The (W,,,,l~-W 2) value decreased largely with decreasing the dipping speed for the octadecanol LB films. The (W,~,t~-W 2) value for octadecanol LB films was more than two times larger than those for cadmium octadecanoate (369 ng and 169 ng at the dipping speed of 100 mm min-~, respectively), which indicates the OH head groups interact with water strongly compared with the CO0head groups (see the latter section). Therefore, the effect of dipping speed on the incorporated amount of water depends on the hydrophilic head groups of LB films. Table 2 Effect of dipping speeds octadecanoate LB filmsa dipping speed/ram min -1 100 80 60 40 20 5

W2/ng 393 267 184 111 81 62

on

the

transfer

Wswen/ng 24 25 30 36 44 56

of

cadmium

W2-W~,en/ng 369 242 154 75 37 6

a Surface pressure: 20 mN m -1, 20 ~ at the fifth transfer cycle.

2.7

Effect

of

Lipid

Structures

The W2 and v values were obtained for various single-chain a mp h ip h iles with the same chain length (C18) and the different

120 hydrophilic head groups, and the results are summarized in Table 3. All LB films could be transferred with the transfer ratio of 1.0 _+ 0.1 in these conditions. The octadecanol LB film showed particularly large W2 and v values, which means that much amount of water was incorporated into the octadecanol LB film and the water was easily evaporated. The OH groups of octadecanol seem to form the hydrogen bond network in the subphase. The LB films transferred with much amount of water might have defects and disorders in LB films, which make the evaporation speed large. The LB films except octadecanol showed the tendency that the W2 depended on their hydrophilicity of head groups and the v value was independent of the head groups.

Table 3 Effect of hydrophilic head single-chain amphiphilesa

groups

on

the

transfer

amphiphiles

W2/ng

v/ng min -1

CH3(CH2)I7--OH [CH3(CH2)16-COO-] Cd2+ CH3(CH2)I6--COOH CH3(CH2)16--CON'I-I2 CH3(CH2)16-COOC2H5

389 155 52 33 42

19.7 4.27 3.74 4.24 2.93

of

a Surface pressure: 30 mN m -1, 20 ~ at the fifth dipping cycle.

The W2 and v values for LB films prepared from various aliphatic acid cadmium salts having different alkyl-chain lengths (C16- C22) were also obtained and the results are shown in Figure 6. All LB films could be transferred with the transfer ratio of 1.0 _+ 0.1 in these conditions. The W2 value was constant and independent of the chain length, but the v value decreased with decreasing the chain length. This indicates that the chain length of lipids mainly affects the evaporation speed of water in LB interlayer and has no effect on the amount of the incorporated water. Thus, the incorporated water seems to exist near the hydrophilic head groups, so that the W2 value depended on their hydrophilicity, but not on the alkyl chain length. On the contrary, the evaporation speed v

121

depended on the alkyl chain length, but not on the hydrophilic head groups, since the rate-limiting process of evaporation of water is to pass through the hydrophobic alkyl-chain part.

,ol

9-, , m

E

,A

-

=

f-

5

&

r

--

400 200

Q

A

> ,

~

16

9

18 2 0

!

22

Acyl Chain Length i l

i

1

|

,

|

-

-

16 18 20 22

Acyl Chain Length n

Figure 6 Effect of alkyl-chain length of aliphatic acid cadmium salts monolayers, [CH3(CH2),_2COO-]2 Cd2+, on the amount of incorporated water (W2) and the evaporation speed of the water (v). The broken line is calculated from eq. (3). (surface pressure" 30 mN m-l, dipping speed: 100 mm min-l, 20 ~ at the 5th dipping cycle).

The evaporation speed is expressed in the following equation according to Fick's low, where the water evaporation is supposed to occurs only from the outer layer, but not from the side part of LB films

[32]-

v

-

60

=

D

x

A

W-Wo

x

~

d

(3)

where v / 6 0 (g s-l) is the evaporation speed, D the diffusion coefficient of water in LB film, A the cross-sectional area of diffusion (the electrode area), w o the vapor pressure of water outside, w the pressure of water vapor equilibrating with balk water in the LB film, and d the thickness of the monolayer. The W o and w values can be calculated with 60% in

122 relative humidity (outside) and 100% (inside, saturated) at 25 ~ The dependency of the film thickness d on acyl chain length n is expressed in the following equation on the assumption that the chain forms a t r a n s zig-zag conformation with the 2.54 A C-C spacing. d (cm) = 1.27 • 10 -8 (n - 2)

(4)

The curve fitting for the observed value was done with the least square method, and the obtained curve is shown as a broken line in Figure 6. This model seems to reproduce the experimental value qualitatively. From this curve fitting, the apparent diffusion coefficient D = 9.7 x 10-12 cm2 s-~ was obtained. Since the diffusion coefficient of free water is 2.6 X 10-1 cm2 s-~ at 25 ~ LB films of the fatty acid cadmium salts are calculated to have the 1/(2.7 X 1011) free space for water diffusion to whole area. The experimental value had the sharper slope than that calculated from the model, which means the diffusion coefficient D decreased as chain length increased. The change in quality depending on the chain length might also affect the water evaporation. According to eq. (3), the evaporation speed depends on the humidity in air, and is independent of the amount of water remaining in LB films. Thus, the water amount in LB film should decrease by zero-order with the amount of water. This is why the frequencies increase due to the water evaporation was linear in Figure 2 (points from B to C). 2.8

Conclusions LB films of various lipids were transferred on a QCM as a substrate under various conditions. The mass of the transferred film (W l, the transfer ratio), the amount of incorporated water (W2), and the evaporation speed (v) were evaluated from the frequency changes of the QCM during transfer processes in situ. We could estimate the deposition state and structures of LB films from these values. When the LB films deposited at the lower surface pressure and at the higher dipping speed on the more hydrophilic surface, the smaller transfer ratio and the larger amount of incorporated water, and the larger evaporation speed of the water were observed, which indicates the deposition of the disordered LB films. On the contrary, when the wellpacked LB films are obtained, the good transfer ratio (W~), the small W2

123 and v values are observed. The W2 and v values also reflects the hydrophilic head groups and alkyl-chain length of amphiphiles, respectively. A QCM system will become a useful sensor system to evaluate LB films during a transfer process in situ. 3. H Y D R A T I O N

B E H A V I O R OF LB FILMS IN W A T E R S U B P H A S E

Hydration of phospholipid head groups is essential properties not only for stabilizing bilayer structures in an aqueous environment, but also for fusion or endocytosis of biological membranes including protein transfers [33-35]. Hydration or swelling behavior has only been studied by indirect methods such as X-ray diffraction [36],differential scanning calorimetry (DSC) [37], and 2H-NMR [38,39]. m

lPersonal [computer,] 9

,

_

{Frequency[ counter ~

,o

mometer

J I_

i

Quartz-Crystal Microbalance

! Oscillating I ,,, |circuit 1111

,_

r--

(QCM)

O

DLPE (n = 12) DMPE (n = 14) DPPE (n = 16) CH3-(CH2) f-4-.-COO ~----] CH3.(CHz)fZ.COO~ DPPC

O',

^ . . +it';N3

O

CH3

C H3-(C H2)/-4,'-COO----."] CH3.(CH2)t.rCOO-- I HO

rol

O" /~. / ~

'--o-P-o y OH

DPPG

0

OH

C H 3-(C H2)f-a.--COO -"'-! C,3.(CH2),T.COO--. ~ DPPS

O" ~,,,~NH3. I.

o-~-o- T0

COO"

Figure 7 Apparatuses for frequency measurements of the LB film-deposited QCM in water and structures of lipid molecules

124 In this chapter, we determined directly the hydration behavior and stability (flaking) of LB films of various naturally-occurring lipids such as dilauroyl-, dimyristoyl-, dipalmitoylphosphatidylethanolamine (DLPE, DMPE, and DPPE, respectively), dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG), dipalmitoylphosphatidylserine (DPPS) and cholesterol (see Figure 7). The frequency of the QCM deposited with phospholipid LB films is expected to decrease (mass increase), when hydration occurred at head groups of LB films on the QCM in water. The initial hydration rate and the hydration amount were obtained at various temperatures below and above Tr changing acyl-chain length and hydrophilic head groups in phospholipids. Twodimensional morphology of phospholipid monolayers during hydration was also observed by fluorescent microscopy.

er

N

500-

--5(30

D

0

1000 <E

14.

"q -1000

C 0

-1500 Initial Rate

2000 ~

Vo Time / hour

Figure 8 Typical time-courses of frequency changes when the QCM deposited with 10 layers of DMPE (T~ = 49 ~ LB films on each side is immersed in water (a) at 25 ~ (b) at 70 ~ and (c) at 50 ~

3.1

Hydration

of

Phosphatidylethanolamine

LB

Films

Ten layers of dimyristoylphosphatidylethanolamine (DMPE) LB films were deposited on the QCM. The QCM was immersed into temperaturecontrolled water phase, and the frequency was followed with time. Typical time-courses of frequency changes arc shown in Figure 8. The phase transition temperature from solid to liquid crystalline states of

125 DMPE membranes was determined to be Tc = 49 ~ from differential scanning calorimetry. The frequency hardly changed at 25 ~ (below To) and at 70 ~ (above T~). On the contrary, the frequency largely decreased at 50 ~ (near To), which indicates that the mass increase due to hydration around head groups of phospholipid LB films. After reaching the equilibrium of the frequency decrease, the QCM was picked up to air phase and dried. The frequency was gradually reverted to the original mass of the dry LB films before immersing in water due to the evaporation of water in air. Thus, the frequency decrease indicates simply the mass increase due to hydration on LB films. The resonance frequency has been reported to be also affected with the change of viscoelasticity of the membrane on the QCM, especially when the membrane is thick and fluid [26,40,41]. The resonance resistance reflecting viscoelasticity of the membrane was measured by an impedance analyzer to be constant (800 +_ 50 f~) during the large resonance frequency change (AF - -1400 _+ 10 Hz) at 50 ~ (near T~). We [26] and Muramatsu e t . a l . [40] have already mentioned that the viscoelasticity change does not affect the frequency change when the membrane is very thin such as 10 layers of LB films even in the fluid state, because the energy loss in the thin film is very small. Therefore, the frequency decrease of 10 layers of DPPE LB films around their Tc shown in curve (c) in Figure 8 is attributed mainly to the mass increase due to the hydration (swelling) of lipid membranes on the QCM. Twenty layers (1130 _ 5 ng) of DMPE LB films (each 10 layers on both sides of QCM electrodes) were observed to be hydrated with 1 7 8 0 _ 20 ng of water near T~ at 50 ~ 61 mol of H20 per 1 tool of DPPE. It has been reported that DPPE has 7-8 mol of non-frozen water molecules per lipid even in the dry state [44]. The initial hydration rate V o and the equilibrium hydration amount W** were obtained as parameters reflecting the hydration behavior of LB films (see Figure 8). Temperature dependencies of the hydration behavior (voand W~,) of 10 layers of DMPE (To = 49 ~ LB films are shown in Figure 9. Large W.~ and v o values were observed only around the phase transition temperature (T~) of DMPE membranes. Thus, DMPE LB films were hydrated only near the To, but not in the solid state below the T~ and in the fluid state above the T~. This indicates that the

126

coexistence of two phases (solid and fluid domains) near the T~ caused the large hydration rate and amount in the LB films.

2500

600 "7.

~2000

S00 E e-

400 ~

1500

300 :~

looo

200 ~

500 0

0

A - -

20

30

.. w

w

40

50

60

Temperature

I

70

~

Figure 9 Effect of temperatures on the hydration amount W** ( 9 and the initial hydration rate Vo ( ~ of 10 layers of DMPE LB films

r

'r /,00 E

4OOO

,,.,...

300 Q.=

2000

:~ 200 tO

looo "~

"~

A

0

o

_

T

100

=

zo

" 30

~

~

2-0

30

Number of Layers Figure 10 Effect of the number of layers of DMPE LB films on each side of the QCM on the swelling behavior (W.~ and V o ). [3 at 20 ~ (below T~), 0 at 50 ~ (near T~), z~ at 65 ~ (above Tr

127

Figure 10 shows effects of the membrane thickness of DMPE LB films on the h y d r a t i o n behavior at three different temperatures. The hydration amount (W**) increased linearly with increasing the number of layers of LB films only around T~, but not temperatures below and above To. This indicates that water molecules deeply penetrate into LB layers around T~. The hydration rate ( v o) was very large and hardly depended on the membrane thickness around T~. This means that water can penetrate from the top surface of the membrane, but not from the side part of LB films.

o~ c

near Tr

2500

700 600 '7c:

o

500 E

0

e-

400 ~

g soo

300 | q,.

~ 1000

200 N 100 --~ 0

9

C12

DLPE

C14

C16

0

D M P E DPPE

Acyl Chain Length Figure 11 Effect of acyl chain length of phospholipid (PE) LB films (10 layers on each side) on the swelling behavior (W.~ and v o ) around each T~ (DLPE: Tc = 30 ~ DMPE: T~ = 49 ~ and DPPE: T~ = 63 ~

Figure 11 shows effect of acyl chain length of phospholipid on the hydration behavior around the each Tc of 10 layers of LB films of DLPE (C12, Tc = 30 ~ DMPE (CI4, Tc = 49 ~ and DPPE (Cl6, Tc = 63 ~ The h y d r a t i o n amount W~. was constant and independent of acyl chain length, which i n d i c a t e s that the hydration water exists around

128 hydrophilic head groups, but not in hydrophobic acyl chain region. On the other hand, the hydration rate Vo decreased sharply as the chain length increased. This means that water molecules penetrate through the defects coexisting solid and fluid states in the acyl chain regions near the T~ and the hydration rates decreased largely in the long chain lipid membranes, and water molecules exist in the hydrophilic regions between layers.

500

at 50 *C -500

250

-250

at 25 *C 0 9 0

. . . -!. . I

! 2

! 3

I , 4

,-! 5

Time / hour Figure 12 Frequency changes of the QCM deposited with 10 layers of DPPC LB films, when the QCM is immersed in water at 25 ~ (below T~ ) a n d at 50 ~ (around the T~ of DPPC).

3.2

Hydration of Other phospholipids and Cholesterol Ten layers of DPPC (T~ = 42 ~ LB films were deposited on a QCM plate by a horizontal lifting method on one side and immersed in water. Typical time-courses of frequency changes of the DPPC-deposited QCM are shown in Figure 12. At 25 ~ in the solid state below the T~, DPPC LB films were stable and hardly swelled in water. However, the frequency gradually increased (mass decreased) at 50 ~ (above the T c) and reached equilibrium at AF = 450 + 50 Hz (-Am = 575 + 50 ng), which is equivalent to the loss of 10 layers mass of the dry LB films. Frequency

129 measurements after drying in air indicated that most of LB films flaked from the QCM plate into water. The similar flaking behavior in water at temperatures above the Tc was also observed in the LB film of DPPG (T~ = 42 ~ Thus, LB films of DPPC and DPPG having relatively hydrophilic head groups such as phosphocholine and phosphoglycerol are easily hydrated and then flaked in the fluid state above their Tc in water.

2500

600

2000

500 E

,") e-

L,0O ~

1500

30o

0

E ~ 1000 "o

:~ (3)

200 ~:

500

~

I(X)

"~

@

0 -~- 20

" 30

/~0

.

.

50

Temperature

.

.

/

60

.

70

~

Figure 13 Effect of temperature on the hydration amount W** (C)) and the initial hydration rate V o ( ~ of 10 layers of DPPS LB films on each side of the QCM

The hydration behavior of DPPS (To = 55 ~ LB films is shown in Figure 13. The DPPS LB film having phosphoserine head groups little hydrated ( A m = 600 + 10 ng, 20 mol of water per lipid) even near the phase transition temperature. Hydration ability has been reported from adsorption experiments of water vapor to lipid powder to be in the order of PC > PE > PS lipids [43]. It has been determined from calorimetry that the amount of non-frozen water around lipid molecules is 10 mol, 7-8 mol, and 0 mol for 1 mol of PC, PE, and PS lipids, respectively [44]. This tendency is consistent with our results that PC molecules are easily hydrated and flaked from the substrate, and PE and

130

PS lipids are hydrated with 61 and 20 mol of water per lipid around their T~, respectively.

250O

600

t,,-

2000

500

~= 1500,

z.O0

0

~,

'=::

,'-

R=,.

E I:::

3OO r----

soo

200

lOO ~

20

9

A "F

30

60

Temperature

/

7b

~

Figure 14 Effect of temperature on the hydration amount W.o ( O ) and the initial hydration rate V o ( ~ of 10 layers of Cholesterol LB films on each side of the QCM

The hydration behavior of LB films of cholesterol is shown in Figure Interestingly both the hydration amount W** and the hydration rate V o decreased with increasing temperatures. Since cholesterol molecules are very hydrophobic and are thought to be hardly hydrated, the incorporated water might exist in the structure defects near the hydrophilic OH groups of LB films and these defects could disappear at high temperature by annealing effect. After aging of cholesterol LB films at 70 ~ for 1 h in water, cholesterol membranes are hardly hydrated at all temperatures (W,, = 50 + 10 ng, 3-4 mol of H20 per cholesterol). 14.

3.3

Observation of LB films by Fluorescence Microscopy In order to confirm the large hydration behavior of phospholipid LB films only around their T~, two-dimensional m o r p h o l o g y of DMPE monolayers was observed by a fluorescence microscope. The two-

131 dimensional morphology of transferred monolayers was observed by using a fluorescent microscope (Olympus, Co., Tokyo, model BSH-RFK). The fluorescence image was detected with the high sensitive SIT camera (Hamamatsu Photonics, Co., Tokyo, model C2741) and the image processor (Hamamatsu Photonics, Co., model DVS-1000) [42].

Figure 15 Fluorescent images of the DMPE monolayer on a slide glass before and after aging at different temperatures.

A DMPE monolayer containing 2 mol% of octadecylrhodamine as a fluorescent dye was transferred onto a slide glass and aged in water at

132 different temperatures for 10 minutes. After drying, the fluorescent image was observed and photographs are shown in Figure 15. A dark region represents the crystalline phase, because the fluorescent dye is refused from the crystalline domain and exists in the disordered domain. The 1 0 - 20 ~tm size of crystalline domains were observed in the samples without aging and aged at 20 ~ (below Tc~. The sample after aging at 60 ~ (above T~) showed large crystalline (dark) domains due to an annealing effect. On the other hand, when the sample aged at 50 ~ (around T~ very small crystalline domains were observed, which indicates the microscopic coexistence of crystalline and liquid crystalline phases near To. The total area of the disordered region (white region) after aging near T~ was larger than those at any temperatures above and below To. These observations are consistent with the large hydration behavior of DMPE LB films only around T~, which may occur in the defects between two domains. We observed also the morphological pictures of cholesterol LB films. The small disordered (white) area was frequently observed in the prepared cholesterol LB films and they were largely decreased after aging at 70 ~ for 1 h (photographs are not shown). This is consistent with the hydration behavior that the hydration of cholesterol LB films was largely decreased with increasing temperatures (see Figure 14). 3.4

Conclusion From the frequency measurements of the LB-film-deposited QCM plate in water, the behavior of phospholipid LB films can be classified into three types: (i) phospholipids having relatively hydrophilic head groups such as DPPC and DPPG are hydrated and then easily flaked from the substrate in the fluid liquid crystalline state a b o v e To; (ii)DPPE and DPPS having the less hydrophilic head groups are hydrated only n e a r their T~; (iii) cholesterol LB films show relatively large hydration behavior even at low temperatures due to the water penetration into the structure defects in the membrane. This is the first example to demonstrate directly and quantitatively the hydration behavior of phospholipid membranes in water. The combination of the QCM and the LB method is a useful tool for characterization of lipid membranes in water.

133 4. D E T A C H M E N T OF LB FILMS F R O M A QCM SUBSTRATE AT THI AIR-WATER INTERFACE In this chapter, we focused on the stability of LB films at the airwater interface: the detachment of LB films from a substrate was followed from the frequency increase (mass decrease) of the QCM that was passed through the air-water interface (see Figure 16). It is necessary to know the stability (detachment) of LB films at the airwater interface for the application of LB films in aqueous systems. We have quantitatively obtained the detached amount of LB films at the air-water depending on dipping (lowering or rising) speed of a substrate, conditions of water surface (surface tension or temperature), chemical structures of amphiphiles (chain length or hydrophilic groups), and structures of LB films (monomeric or polymeric). LB film-forming amphiphiles (CH 3(CH 2),.2.CO O " )Cd2+

n = 16, 18,20,22

OI i

CH3(CH 2)n-l--O-C-~ H--NH3+

CI

CH 3(CH =).. ~ --O ~-(CH2)~ O 2Cn-glu-NH3 +

n = 16, 18

o CH3 CH 3(CH 2)--1 --O --C --~H --NHCO -CH2 -~1, -CH 3 "O3S CH 3(CH 2).- 1 --O-C-1CH2)2

CH3

0

2Cn-glu-N "3C 1 / PSS "

n = 16, 18

OEt O I CH 3(CH 2)18~ N C II ~ / C O N H , ~ ~ / S i - O E t | CH 3(CH 2)18 /

OEt 2 C 18-Si

Figure 16 Measuring system for detachment of LB films from a QCM substrate at the air-water interface, and chemical structures of LB film-forming amphiphiles.

LB films

134

1Ol.

.Jill

N

e=

-r ',2,,

200

150

150

u}

ca 100

14. m ..I q~

100

0

r'--

~ , 50 0

5O

c:

~.

m

E .=.

0

0

0 c 0

E <. ,~

L_

IJ..

@ W

0

,1

, I

L

!

l

1

2

3

4

5

Number of Dipping Cycles

Figure 17 Frequency increases (mass decreases) of the QCM deposited with cadmium octadecanoate LB films (10 layers on each side, 1130 + 5 ng), when the QCM plate was lowered and raised repeatedly at arrows with a rate of 100 mm min-~ through the air-water interface at 20 oC

Detachment of LB Films at the Air-Water Interface When the LB film-deposited QCM was lowered and raised through the air-water interface at 20 ~ the frequency of the QCM was observed to increase (the mass decrease) compared with that before soaking into the water phase. This is due to the detachment of LB films from the QCM substrate during dipping processes. Figure 17 shows the typical frequency increase (mass decrease) of the QCM deposited with cadmium octadecanoate LB films (10 layers on each side of the QCM, 1130 + 5 ng), when the QCM substrate was lowered and raised repeatedly through the interface at a rate of 100 mm min-~. Any increase in surface pressure wasn't observed during the experiments. The frequency of the QCM was

4.1.

135 measured each time the QCM was raised and dried in air. The detached amount was calculated according to eq. 2. The mass of LB films on the Q C M substrate decreased 37 + 4 ng with one cycle of lowering and raising, which is consistent with c a . 3.0 wt% of the total LB films or ca. 16 w,% of the surface layer. The d e t a c h e d a m o u n t increased as dipping cycles and almost one layer of the LB film was peeled at the 4th cycle. T h e same e x p e r i m e n t s w e r e c a r r i e d out with and w i t h o u t an o s c i l l a t i o n of the QCM during the t r a n s f e r process. The difference b e t w e e n two cases was w i t h i n e x p e r i m e n t a l error, w h i c h means an oscillation of a QCM has no effect on the detachment of LB films. Table 4 Effect of lowering and raising speeds on the detachment o f c a d m i u m o c t a d e c a n o a t e LB f i l m s f r o m a Q C M substrate at the air-water interface (20 ~ Dipping speed (mm min -~) No. 1

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Lowering

Raising

Detached amount b (ng)

100 80 60 40 20 5 100 100 100 100 100 80 60 40 20 5

100 80 60 40 20 5 80 60 40 20 5 100 100 100 100 100

37 42 47 49 64 75 37 41 44 43 42 42 50 51 62 79

Ten layers on each side were deposited on the QCM substrate ( 1130+5 ng). b The amount detached in the first dipping cycle.

136

4.2.

Effect

of

Lowering

and

Rinsing

Speeds

The effect of the lowering and raising speeds of the cadmium o c t a d e c a n o a t e LB films-deposited QCM substrate on the detached amount at the first dipping cycle was summarized in Table 4. When both lowering and raising speeds are the same, the smaller dipping speed showed the larger detached amount (Runs 1 - 6). When the raising speed is reduced from 100 to 5 mm min-~ and the lowering speed was constant at 100 mm min-~, the detached amount was almost constant and hardly depended on the raising speed (Runs 1 and 7 - 11). On the other hand, the detached amount increased in two times with decreasing the lowering speed from 100 to 5 m m min-~ at the constant raising speed (Runs 1 and 12 - 16). These results suggest that the LB film peeled off easily when the substrate was lowered slowly from the air phase to the water phase. Since the dipping speed was controlled by changing the arm speed of the QCM substrate, the reducing of the dipping speed also affects the existent time in the water phase as well as the time passing through the interface. The effect of the time at the interface and the time in water on the detached amount was examined separately to clarify whether LB films peeled off mainly at the air-water interface or in the water phase, and the results are summarized in Table 5. Runs 1-8 show the flaking amount of cadmium octadecanoate LB films with the constant dipping speed (the c o n s t a n t time at the i n t e r f a c e ) a n d t h e different time in water. The results indicate the detached amount was independent of the time in water. On the contrary, when the time at the interface was increased and the time in water was kept constant, the detached amount of the LB films increased about two times larger (Runs 9 - 15). From these findings, It can be concluded that the LB films peeled off at the air-water interface but not in the water phase when the substrate is lowered slowly with the long contact time at the interface.

4.3.

Effect

of Water

Surface

The detached amounts of cadmium octadecanoate LB films at the water surface with various temperatures are shown in Figure 18. The d e t a c h e d a m o u n t increased linearly with increasing the subphase temperature. The detachment of LB films is concerned with equilibrium spreading pressure (ESP), which represents the equilibrium between bulk lipid crystals and a lipid monolayer on the water surface [45]. ESPs

137

o f fatty a c i d s [46], their esters [47], g l y c e r i d e s [48], and p h o s p h o l i p i d s w e r e r e p o r t e d to increase as t e m p e r a t u r e i n c r e a s e d b e l o w t h e i r p h a s e t r a n s i t i o n t e m p e r a t u r e s , w h i c h m e a n s lipid m o l e c u l e s e a s i l y m o v e f r o m crystals to w a t e r surface at the high t e m p e r a t u r e . That is the r e a s o n the l a r g e d e t a c h e d a m o u n t w a s o b s e r v e d at the h i g h t e m p e r a t u r e o f the water surface. T o m i n i m i z e the d e t a c h m e n t , LB films s h o u l d be s o a k e d into the w a t e r p h a s e at the low t e m p e r a t u r e . Table 5 E f f e c t of time at the interface and time in w a t e r on the d e t a c h m e n t of cadmium octadecanoate LB films from a Q C M substrate at the air-water i n t e r f a c e (20 ~

No. 1

2 3 4 5 6 7 8 9 10 11

12 13 14 15

Dipping speed b (mm min -t)

Time at interface c (s)

Time in water d (s)

Detached amounV (ng)

100 100 100 100 100 100 100 I00 80

9.6 9.6 9.6 9.6 9.6 9.6 9.6 9.6 12

9.6 12 16 24 48 100 150 192 192

60

16

40 20 10 7 5

24 48 96 137 192

192 192

37 40 36 40 40 37 43 42 51 54 58 64 67 70 75

192 192 192 192

a Ten layers on each side were deposited on the QCM substrate ( 1130+__5 ng). b Lowering and raising speeds are the same. c The time when the substrate passes through the interface, which is calculated from the dipping speed. d The time when the substrate exists in water between lowering and raising processes. e The detached amount at the first dipping cycle.

138

=

120

.__E100 IZl J

80

0

=

.S

60

0

E 40 w '~ m

20

UL

~

lb2'o

3'0 4'o 5'0

Temperature / ~

Figure 18 Effect of water temperature on the detached amount of LB films at the first dipping cycle (10 layers of cadmium octadecanoate on each side of the QCM; dipping speed, 100 mm min-~). =

120

w

E 100

=,m I==

El j

80

C13H27COOEt,k,

~1~

o

-e~

60

0

E 4(1 ._~ __v m

,-r

2C 0 Surface pressure by C13H27COOEt / mN m "1

Figure 19 Effect of the surface pressure on the detachment amount of LB films at the first dipping cycle. The QCM deposited with 10 layers of cadmium octadecanoate on each side was lowered and raised at 100 mm min-] through the interface existing with ethyl tetradecanoate at 20 ~

139 The d e t a c h m e n t experiments were carried out at the different surface t e n s i o n of the water surface. The surface pressure was controlled in the range of 0-10 mN m-~ with ethyl t e t r a d e c a n o a t e m o n o l a y e r that was confirmed not to be transferred onto a substrate under the e x p e r i m e n t a l conditions. The d e t a c h m e n t a m o u n t s of cadmium octadecanoate LB films at various surface pressures are shown in Figure 19. At the high surface pressure (the low surface tension) the detached amount became negligibly small, which means the surface tension of water is the driving force of the detachment of LB films at the air-water interface.

er

"

E

120

-7- 1 0 0 III .J

-0

80

c

= 60 0 E

20

Number of layers Figure 20 Effect of number of layers of c a d m i u m octadecanoate LB films on the QCM on the detached amount. (at 20 ~ dipping speed, 100 mm min-~).

4.4.

E f f e c t of LB Film S t r u c t u r e s Figure 20 shows the effect of the number of layers of cadmium octadecanoate LB films (2 - 10 layers) on the detachment. The detached amount was hardly dependent on the number of layers, which indicates the LB film is peeled only from its surface layer. The d e t a c h m e n t of the LB films transferred at various surface pressures was also examined. Relation between detached amounts and transfer conditions of cadmium octadecanoate LB films are summarized

140 in Table 6. The LB films transferred at the lower surface pressure showed the lower transfer ratio and the larger detachment. Because, the LB films transferred at the low surface pressure have defects in the membrane and show the low stability as two-dimensional crystals. To make sure of this point, the detachment of both the LB film and the cast film of octadecanoic acid was compared under the same condition. The cast film expected to contain a large amount of defects was peeled off largely (18 wt% of the total LB films) at the one dipping cycle compared with the LB film (5.7 ~t%).

Table Effect films water

6 of transfer condition of cadmium octadecanoate LB on detachment from the QCM substrate at the airinterface (20 ~ Transfer process

Surface pressure (mN m -~)

Transfer ratio

Detached amountb (ng)

40 30 20 10 5

1.10 1.13 1.04 0.88 0.77

3.0 3.0 3.3 4.7 5.9

~ layers on each side were deposited on the QCM substrate (1130+5 ng). b The QCM plate was lowered and raised through the interface in one cycle at the rate of 100 mm min -~.

4.6. C O N C L U S I O N Several aspects of the detachment of LB films from a substrate at the air-water interface were found out by using a QCM technique. The conditions and the molecular design to suppress the detachment of LB films are summarized as follows. As conditions; i) LB films have to be introduced into water phase as fast as possible.

141 ii) Temperature of water surface should be low. iii) Surface tension of water should be lowered, if possible. As molecular designs; i) Well oriented, defect-free LB films should be prepared. ii) Amphiphiles having the longer alkyl chains are better. iii) Amphiphiles having the smaller equilibrium spreading pressure and the smaller free energy of compression are better. iv) Polymerized LB films are the best.

REFERENCES

(a) K. B. Blodgett, J. Am. Chem. Soc., 57 (1935) 1007. (b) K. B. Blodgett, I. Langmuir, Phys. Rev., 51 (1937)964. 2 J . D . Swalen, J. Mol. Electronics, 2 (1986) 155. 3 Y. Sasanuma, Y. Kitano, A. Ishitani, H. Nakahara, and K. Fukuda, Thin Solid Films, 190 (1990)325. 4 M. Uchida, T. Tanizaki, T. Oda, and T. Kajiyama, Macromolecules, 24 (1991) 3238. 5 (a) H. Mtihwald, Angew. Chem. Int. Ed. Engl., 27 (1988) 728. (b) M. L~Ssche and H. M6hwald, Eur. Biophys. J., 11 (1984) 35. 6 R. Weis, and H. McConnell, J. Phys. Chem., 89(1985) 4453. 7 (a) M. Shimomura, K. Fujii, P. Karg, W. Frey, P. Meller, and H. Ringsdorf, Jpn. J. Appl. Phys., 27 (1988) L1761. (b) M. Shimomura, K. Fujii, T. Shimomura, M. Oguchi, E. Shinohara, Y. Nagata, M. Matsubara, and K. Koshiishi, Thin Solid Films, 210/211 (1992) 98. 8 (a) T. Kajiyama, K. Umemura, M. Uchida, Y. Oishi, and R. Takei, Chem. Lett., 1515 (1989). (b) T. Kajiyama, I. Hanada, K. Shuto, and Y. Oishi, Chem. Lett., 193 (1989). (c) T. Kajiyama, Y. Oishi, M. Uchida, N. Morotomi, J. Ishikawa, and Y. Tanimoto, Bull. Chem. Soc. Jpn., 65 (1992) 864. 9 T. Kato and K. Ohshima, Jpn. J. Appl. Phys., 29 (1990) L2102. 1 0 G. Sauerbrey, Z. Phys., 155 (1959) 206. 1 1 (a) W. H. King, Jr, Anal. Chem., 36 (1964) 1735. (b) J. Hlavay and G. G. Guilbault, Anal. Chem., 49 (1977) 1890. 1 2 T. Nomura and M. Iijima, Anal. Chim. Acta, 131 (1981) 97. 1

142 1 3 (a) Y. Okahata, H. Ebato, and K. Taguchi, J. Chem. Soc., Chem. Commun. (1987) 1363. (b) Y. Okahata and O. Shimizu, Langmuir, 3, (1987) 1171. (c) Y. Okahata, G. En-na, and H. Ebato, Anal. Chem., 62 (1990) 1431. (d) Y. Okahata and H. Ebato,Trend in Anal. Chem., 11 (1992) 344. (e) Y. Ebaraand Y. Okahata, Langmuir, 9 (1993) 574. 1 4 M. Thompson, C. L. Arthur, and G. K. Dhaliwal, Anal. Chem., 58 (1986) 1206. 1 5 (a) Y. Okahata, Y. Matsunobu, K. Ijiro, M. Mukai, A. Murakami, and K. Makino, J. Am. Chem. So c., 114 (1992) 8299. (b) Y. Okahata, K. Ijiro, and Y. Matsuzaki, Langmuir, 9 (1993) 19. (c) S. Yamaguchi, T. Shimomura, T. Tatsuma, and N. Oyama, Anal. Chem., 65 (1993) 1925. (d) R. C. Ebersole, J. A. Miller, J. R. Moran, and M. D. Ward, J. Am. Chem. Soc., 112 (1990) 3239. 1 6 Y. Okahata, and Y. Ebara, J. Chem. Soc., Chem. Commun., (1992) 116. 1 7 M. Yang, M. Thompson, Langmuir, 9 (1993) 1990. 1 8 H. Muramatsu, M. Suzuki, E. Tamiya, and I. Karube, Anal. Chim. Acta, 215 (1988) 91. 1 9 P. L. Konash, and G. L. Bastiaans, Anal. Chem., 52 (1980) 1929. 20 (a) S. Bruckenstein and M. Shay, J. Electroanal. Chem. Interfacial Electrochem., 188 (1985) 131. (b) S. Bruckenstein, C. P. Wilde, M. Shay, A. R. Hillman, and D. C. Loveday, J. Electroanal. Chem. Interfacial Electrochem., 258 (1989) 457. 2 1 (a) R. Ebersole and M. D. Ward, J. Am. Chem. Soc., 110 (1988) 8623. (b) M. D. Ward and D. A. Buttry,Science, 249 (1990) 1000. 2 2 M. R. Denkin and D. A. Buttry, Anal. Chem., 61 (1989) 1147. 23 R. R. McCaffrey, S. Bruckenstein, and P. N. Prasad, L a n g m u i r , 2 (1986) 228. 24 (a) H. Riegler and K. Spratte, Thin Solid Films, 210/211 (1992) 9. (b) H. Riegler, J. D. LeGrange, Thin Solid Films, 185 (1990) 335. 2 5 I. R. Peterson, Thin Solid Films, 116 (1984) 357. 26 (a) Y. Okahata, K. Kimura, and K. Ariga, J. Am. Chem. Soc., 111 (1989) 9190. (b) Y. Okahata and H. Ebato, Anal. Chem. 61 (1989) 2185. (c) H. Muramatsu, K. Kimura, Anal. Chem., 64 (1992) 2502. 27 (a) Y. Okahata and K. Ariga, L a n g m u i r , 5 (1989) 1261. (b) Y. Okahata and K. Ariga, Thin Solid Films, 178 (1989) 465. 2 8 G. Farriss, J. Lando, S. Rickert, J. Materials Sci., 18 (1983) 2603. 2 9 T. Tachibana and K. Fukuda, Bull. Chem. Soc. Jpn., 24 (1951) 4. 3 0 T. Isemura, Bull. Chem. Soc. Jpn., 15 (1940) 467.

143 31 32 33 34 35 36 37 38 39 40 41 42

43 44 45 46 47 48

C. W. Pitt and L. M. Walpta, Thin Solid Films, 68 (1980) 101. I. Langmuirand V. J. Schlaefer, J. Framklin Inst., 235 (1943) 119. R. P. Rand, Annu. Rev. Biophys. Bioeng., 10 (1981) 277. T. J. Mclntosh, and S. A. Simon, Biochemistry, 25 (1986) 4058. G. Cevc and D. Marsh, Biophys. J., 47 (1985) 21. M. J. Ruocco and G.G. Shipley, Biochim. Biophys. A c t a , 691 (1982) 309. D. Chapman, Forms and Function of Phospholipids, Elsevior Scientific, Amsterdam, 1933, pp 117. E. G. Finer and A. Darke, Chem. Phys. Lipids, 12 (1974) 1. F. Borle and J. Seelig, Biochim. Biophys. Acta, 735 (1983) 131. H. Muramatsu and K. Kimura, Anal. Chem., 64 (1992) 2502. H. Muramatsu, E. Tamiya, and I. Karube, Anal. Chem., 60 (1988) 2142. M. Shimomura, K. Fujii, T. Shimamura, M. Oguchi, E. Shinohara, Y. Nagata, M. Matsubara, and K. Koshiishi, Thin Solid Films, 210/211 (1992) 98. G. L. Jedrasiak and J. H. Hasty, Biochim. Biophys. Acta, 337 (1974) 79. B. D. Ladbrooke and D. Chapman, Chem. Phys. Lipids, 3 (1969) 304. G. L. Gains Jr., Insoluble Monolayers at Liquid-Gas Interface, Wily, New York, p21, 1990. I. M. Jalal, G. Zografi, A. K. Rakshit, andF. D. Gunstone, J. Colloid. Interface Sci., 76 (1980) 146. A. Cary and E. Rideal, Proc. Roy. Sci. (London) Ser. A., 169 (1980) 318, 331. M. C. Phillips and H. Hauser, J. Colloid Interface Sci., 49 (1974) 31.

This Page Intentionally Left Blank

New Developments in Construction and Functions of Organic Thin Films T. Kajiyama and M. Aizawa (Editors) 9 1996 Elsevier Science B.V. All rights reserved.

145

A p p l i c a t i o n of vibrational s p e c t r o s c o p y to the study of structure-function r e l a t i o n s h i p in L a n g m u i r - B l o d g e t t films T. Takenaka a and J. Umemura b ~Department of Chemistry, Science University of Okayama Ridai-machi 1-1, Okayama 700, Japan blnstitute for Chemical Research, Kyoto University Uji, Kyoto-Fu 611, Japan

The FT-IR technique using reflection-absorption ( RA ) and transmission spectra to quantitatively evaluate the molecular orientation in LB films is outlined. Its application to some LB films are demonstrated. In particular, the temperature dependence of the structure and molecular orientation in alternate LB films consisting of a phenylpyrazinecontaining long-chain fatty acid and deuterated stearic acid (and of their barium salts) are described in relation to its pyroelectricity. Pyroelectricity of noncentrosymmetric LB films of phenylpyrazine derivatives itself is represented, too. Raman techniques applicable to structure evaluation of pyroelectric LB films are also described.

1. I N T R O D U C T I O N Recently, much attention has been devoted to Langrnuir-Blodgett (LB) films because of their good possibilities for ultrathin functionality elements. Concomitantly, there has been an increased need for establishment of characterization methods of LB film structure. Among various analytical methods, vibrational spectroscopy such as infrared and Raman spectroscopy has been known to be the most useful and direct tools to obtain information about the structure and orientation of constituent groups in molecules. Recent progress of Fourier transform infrared (FT-IR) spectrophotometer and of highefficiency multichannel detectors for Raman measurements has increased the sensitivity sufficiently so as to observe reliable vibrational spectra at single monomolecular levels. Improvements in novel reflection techniques for infrared and Raman measurements of ultrathin organic films at various interfaces have also contributed to the development of this field. In order to evaluate quantitatively the molecular orientation in thin LB films, several attempts have been made by using infrared reflection-absorption (RA) technique [ 1,2]. Recently, we have proposed a useful and convenient method combining infrared

146 RA and transmission techniques [3], and applied it to the studies of LB films of cadmium stearate [3], azobenzene-containing long-chain fatty acids and their barium salts [4], dipalmitoylphosphatidylcholine (DPPC) [5], and polyion complexes [6]. Furthermore, we explored the relationship between the molecular orientation evaluated by this method and pyroelectricity in alternating (noncentrosymmetric) Y-type LB films consisting of a phenylpyrazine-containing long-chain fatty acid and deuterated stearic acid and of their barium salts [7]. In this article, we will outline infrared RA technique, our method for quantitative evaluation of the molecular orientation [3], and results obtained for thin LB films of {CH3(CH2) 11-O~N

-- N - ~ O - ( C H 2 ) 5-COO} 2Ba

(abbreviated as C12AzoCs-Ba) [4]. Furthermore, we will deal with details of the study on the structure-pyroelectricity relationship in the alternating LB films [7].

2. INFRARED REFLECTION-ABSORPTION (RA) METHOD When infrared beam incidents upon thin organic films extended over a plain metal surface (Figure 1), there usually occurs a standing wave electric field on the metal surface as a consequence of the interference between the incident and reflected beams. Since the standing wave may interact with, i. e. be absorbed by, molecules in the organic film, the reflectance of the beam from the metal surface is reduced. Generally, upon reflection from the metal surface, the beam changes its phase depending upon the angle of incidence 0 and the polarization of the beam. Figure 2

AIR

'l I I I I

u_

~r to

-0

Z ~ -90 Ow

X~

ANGLE OF INCIDENCE(DEGREES) 45 I

90

'l

w I CLW

THIN FILM

0

P

~o

S- P O L A R I Z A T ~ w r

-180

METAL

Figure 1. Optical path in the reflectionabsorption measurement of thin film.

Figure 2. Phase shift for p- and s-polarized beams reflected from metal surface

[8].

147

Incidenl

radiation/

/ e

~

"

c

t

e

'/ /-

/ ........ , .... / oo,, ,.

/

radiation

(A)

../

/'~

(B)

Figure 3. Interference between the incident and reflected beams on metal surface. (A) s-polarized beam. (B) p-polarized beam.

F2

60. 0

~P /.o, E~]cosO

1'o

3'o

8

so

70

9o*

Figure 4. Incident angle dependence of E i and E L2/COS/9 on highly reflecting metal surface [9].

148 shows typical phase shift of the p- and s-polarized beams reflected from the metal surface [8]. The phase shift for the s-polarized beam is roughly -180* irrespective of the angle of incidence. This means that the reflected beam is in out-ofphase to the incident beam as shown m Figure 3A, and there fore no electric field occurs. The behavior of the p-polarized beam is quite different. The phase shift in this case changes rapidly at large angles of incidence. Figure 3B represents the geometry of the perpendicular electric vector generated at a large angle of incidence where the phase shift is near 90 ~ Figure 4 illustrates the incident-angle dependence of the amplitude of the produced electric vector E.L [9]. The maximum amplitude is obtained at ca. 80 ~ The intensity of the absorption by molecules depends on E_0and on the area over which the electric field is exerted. As the area intercepted by the p-polarized beam varies as 1/cos& the intensity of a particular band may depend on E~/cos0 as shown in Figure 4 [9]. This function has the sharp maximum at ca. 88* for highly reflecting metals such as Cu and Ag [8], and is more than an order of magnitude greater in a single reflection than in transmission through the equivalent film isolated from the metal substrate and at the normal incidence. Here, it is to be emphasized that the generated electric vector is perpendicular to the film surface, and therefore only absorption bands which have the transition moments perpendicular to the film surface can be detected with a large intensity enhancement due to the interference between the incident and reflected beams from the metal surface (the surface selection rule). This may allow us to discuss the molecular orientation in thin organic films.

L B films

z/

.. -:.

P

9" . . ' -

9" . : . : : .

~l~

.:.':

...'.,...

. . . . . . . . . . . . . . .................... . . .

I

(A)

!. . . . .

LB films

, / ".",'d...

""

Ag

-L,~Glass

(B)

Figure 5. Comparison between infrared transmission (A) and reflection-absorption (B) methods for obtaining spectra of LB films.

149 3. QUANTITATIVE EVALUATION OF MOLECULAR ORIENTATION In the normal-incident transmission measurements of LB films deposited on transparent substrates, the electric vector of the infrared beam is parallel to the film surface (Figure 5A). Therefore, only absorption bands which have the transition moments parallel to the film surface can be detected by this method. On the other hand, in the above-mentioned RA measurements, in which the p-polarized infrared beam is incident upon the LB film prepared on Ag-evaporated substrates at a large angle of incidence, we have a strong electric field perpendicular to the film surface as shown in Figure 5B. Therefore, in this case, only absorption bands which have the transition moments perpendicular to the film surface can be detected with a large intensity enhancement. Thus, if the molecules are highly oriented in the LB films, the peak intensities of particular bands should be different between the transmission and RA spectra. Figure 6 shows the FT-IR transmission spectra of 1- to 11- monolayer LB films of CI2AzoC5-Ba deposited on ZnSe plates. Figure 7 is the FT-IR RA spectra of the same LB films deposited on Ag-evaporated glass plates. It is to be noted that high quality spectra could be obtained even for the 1-monolayer films in Figures 6 and 7. Apparently, the peak intensities of the respective bands are quite different between the corresponding transmission and RA spectra. The antisymmetric and symmetric CH 2 stretching bands (2919 and 2849 cm -~) of the methylene chains, the antisymmetric CO0- stretching band (1533 cm ~) of the carboxylate group and the ~-H out-of-plane bending band (843 crn-~) of the benzene ring are much stronger in the transmission spectra than in the RA spectra. But, the in-plane vibration bands (1605, 1586, 1502 and 1478 cm ~) of the benzene ring, the symmetric CO0- stretching band (1433 cm -~) of the carboxylate group, and the ~ - O stretching band (1252 cm -~) are much stronger in the RA spectra than in transmission spectra. If the molecule of C 12AzoCs-Ba with the all-trans methylene chain is oriented perpendicularly to the film surface, the CH 2 stretching vibrations, the antisymmetric C O 0 stretching vibration, etc. which have the transition moments parallel to the film surface should be observed strongly in the transmission measurements. On the other hand, the in-plane vibration of the benzene ring, the symmetric CO0- stretching vibration, etc. which have the transition moments perpendicular to the film surface should be observed strongly m the RA measurements. Since these expectations were fully realized in Figures 6 and 7, we can conclude that the molecular axis is highly perpendicular to the film surface. On the basis of these experimental results and discussion, we tried to establish a convenient and useful method for quantitative evaluation of the molecular orientation in thin LB films using FT-IR transmission and RA spectra. Here, we assume a uniaxial orientation of the transition moment with an angle ~ around the normal axis, Z, to the LB film surface i. e. the XY plane (Figure 8). In this case, the ratio of the absorbance of a particular band in the transmission spectrum, A r, to that of the same band in the RA spectrum, AR, is given by

150

O.O Ii LU (O Z < 133 nO cO <

~11

[_

i

.'3000

I

Jk

i

i

2800 1800

i-

I

1600

-i

~

1400

i

I

1200

~

i

1000

i

i

800

WAVENUMBER / cm -1

Figure 6. FT-IR transmission spectra of 1- to 11- monolayer LB films of C12AzoCs-Ba deposited on ZnSe plates.

I

I O.O5

uJ z rn r~ 0 u9 ~

5

I

3000

1

I

zaoo

t

aaoo

1

1

~eoo

1

I

~4oo

I

1

1

,2oo

!

,ooo

t

t

aoo

WAVENUMBER / cm -~

Figure 7. FT-IR RA spectra of 1- to 11- monolayer LB films of C12AzoCs-Ba deposited on Ag-evaporated glass plates. The angle of incidence is 85*

151

Z

i

9

e

momen'l

Y

X Figure 8. Uniaxial orientation model of the transition moment.

20 s

1.6 1.5

-

S~4

0.2

15

o.1

_]:

10

5 t

4000

.

i

3000

I

.I

2000

WAVENUMBER / cm -1

1000

7O0

Figure 9. Wavenumber dependences of the enhancement factors m z and m x for LB films with n2=1.4, 1.5, and 1.6; k2=0.1; h2=17.5 nm [3]. Substrates for transmission and RA measurements are ZnSe and Ag, respectively. The angle of incidence in RA measurements is 85*.

152

Table 1 Calculation of the ~ and 1, values for 11-monolayer LB film of C12AzoCs-Ba v/cm

-1

assignment

A T

Ale

AT/A R

mz

mx

2919

Va(CH 2)

0.01121

0.02232

0.50

8.8

0.15

ct=72~

2849

Vs(CH 2)

0.00574

0.01530

0.38

9.0

0.15

13=70~

1533

Va(COO-) 0.00446

0.00105

4.25

12.7

0.06

85 ~

1433

Vs(COO-) 0.00037

0.03097

0.012

13.0

0.05

29 ~

1252

v(~-O)

0.00370

0.11653

0.032

13.5

0.04

43 ~

843

~:(~-H)

0.00689

0.00472

1.46

14.4

0.02

81"

AT'_

-

A R

Kx

}

'~=27 ~

(1)

mzKz+ mxKx

Here, K x and K z are the X and Z components of the absorption coefficient, and those of the intensity enhancement factor in the RA measurements due to the presence of the Ag film. The second term in the denominator of the right hand side of Eq. (1) was added to take into account the slight contribution of the electric field parallel to the film surface in the RA measurements. After a simple calculation under the condition of the uniaxial orientation, we have

m x a n d m z are

A---T-r= A R

sin2O

(2)

2mzCOS2t)+mxsin2~

The enhancement factors mz and m x can be calculated exactly by Hansen's formulas for optics of thin multilayer film [ 10]. The results of the calculation for our experimental systems are shown in Figure 9 as a function of wavenumber. Three lines for the m z values and those for the m x values refer to the refractive indices of the LB film, 1.4, 1.5, and 1.6. The m x values are very small and about one percent of the m z values. This means that the electric field generated by the RA measurements is practically perpendicular to the film surface, as was mentioned above. Then, we calculated the ~ values for the transition moments of the major bands of the 11-monolayer LB film of C12AzoCs-Ba by Eq. (2) from the observed absorbance ratios between the transmission and RA spectra. The results are shown in Table 1. The angles (ct and 13 ) of the transition moments of the antisymmetric and symmetric CH 2 stretching vibrations are 72" and 70 ~ , and those of the antisymmetric and symmetric C O 0 stretching vibrations are 85 ~ and 29 ~ , respectively. Furthermore, those of the

153

Table 2 Orientation angles (degree) of the hydrocarbon chain axis 7 and the transition moment of the ~ - H out-of-plane bending vibration = ( ~ - H ) around the surface normal in 11-monolayer LB films of CmAzoC n and CmAzoCn-Ba [4]. rn

n

8

3

8 12 12

5 3 5

7

~(~-H)

acid

33

63

Ba salt

23

79

acid

50

56

Ba salt

18

80

acid

29

80

Ba salt

27

81

acid

36

70

Ba salt

27

81

~ - O stretching and the ~ - H out-of-plane bending vibrations are 43 ~ and 81~ respectively. As can be understood from the nature of the function ~) in Eq. (2), experimental errors in the intensity ratio measurements have smaller effect for the r values larger than 70 ~ but have larger effect for the ~) values smaller than 30 ~ As to the total accuracy of the ~ value, it can be determined within an error less than + 2 ~ for the larger ~ values and + 5 ~ for the smaller ~) values. Since the transition moments of the antisymmetric and symmetric CH 2 stretching vibrations and the methylene chain axis are mutually perpendicular, the average orientation angle 7 of the hydrocarbon chain axis around the surface normal is obtained to be 27 ~ by the orthogonal relation 2 2 2 cos o~ + cos [3 + cos Y = 1

(3)

by using ~ = 72 ~ and 15=70 ~ These results are schematically illustrated in Figure 10. The same studies have also been performed for 11-monolayer LB films of the analogous series of barium salt films, CsAzoC3-Ba, C8AzoCs-Ba and C12AzoC3-Ba, as well as of the corresponding acid films C8AzoC 3, C8AzoC 5, C12AzoC 3 and C12AzoCs. The results of the orientation angles of the hydrocarbon chain axis 7 and the transition moment of the ~ - H out-of-plane bending vibration ~ (~-H) around the surface normal are summarized in Table 2 together with those for the C 12AzoCs-Ba film [4]. It can be understood that the smaller 7 value and nearer rt (~-H) value to 90 ~ mean the higher degree of the molecular orientation. From these stand points, it is generally seen that the degrees of molecular orientation of barium salts are higher than those of the corresponding acids. This tendency is most remarkable between CsAzoCs-Ba and

154 CsAzoC 5 films. The molecular axis of CsAzoCs-Ba is most perpendicularly oriented of all the molecules examined, but that of CsAzoC 5 is least. Further, it is to be noted that both orientation angles T and ~ (@-H) of the latter molecule are near 54.7* which is so called the magic angle meaning the average of the angles between the randomly oriented axes and the surface normal. Previously, we recorded UV absorption spectra of the same molecules in spread monolayers on the water surface and in 11-monolayer LB films on solid substrates as shown in Figure 11 [ 11]. In the case of CsAzoC 5, the absorption band due to the ~-~* transition of the azobenzene moiety in the spread monolayer is largely shifted to

CsAZOC 5

Z k

r

l~.x i/~i

. ."-. /.:......\

..J

'

/

27"

{

,"

h

o o3

S

Awc" .-.. .......

1

I

1

1

. . . .

/~x 1/11

I

~[0.002

~-.8 f i ~

~ ''" 9

-

,

/-- / ~\ solution

-\- \ . , . ' / / / /'-.... 't. \ \\ - \

// 9

.

9 9

Spread mono layer

\

9

."

\

~

/ : \

I

_I

."../

",

,Q'-..,<....

..

9

... .-" -......"

250 Figure 10. Schematic illustration of molecular orientation of C12 AzoC 5Ba in the LB film.

/

C8AzoC5-Ba

43 ~

Substra~e

x

-...... .,<,

<

//////////7111/////////7

I .""'__ LB film _ /--,~

.:.

\I

(D r C O tm

I

Spread

monolayer

"-aa-:cu~.

, , ,

I

500 350 Wavelength / nm

I

400

Figure 11. UV absorption spectra of CsAzoC 5 and CsAzoCs-Ba in mono-layers on water surface, LB films, and C H C I 3 solution [11].

155 shorter wavelength side from that of the randomly oriented molecule in the chloroform solution at 355 nm, suggesting a formation of high degree of H-aggregates. When, however, the monolayers is transferred from the water surface onto the solid substrate, the band is shifted back to longer wavelength side attaining that in the chloroform solution, indicating a breakdown of the H-aggregates. In the case of CsAzoCs-Ba, on the other hand, the band is shifted further to shorter wavelength side during the transfer of the monolayer from the water surface onto solid substrate. This reveals a further devel-opment of the molecular aggregation, resulting in a very high H-aggregates in the LB film. Since the higher H-aggregation means the higher degree of orientation of the long axes of the chromophores (as well as the larger number of the aggre-gated molecules), there is a good consistency between the results shown in Table 2 obtained by the infrared spectroscopy and those obtained by the UV spectroscopy.

4. PYROELECTRICITY AND LB FILMS If the crystal possesses center of symmetry, it has no spontaneous polarization. Among 32 crystallographic point groups, 10 groups coveting Cn (n=1,2,3,4 and 6), C s, and Cnv (n=2, 3, 4 and 6) may possess spontaneous polarization under no pressure. In such a crystal with spontaneous polarization, the direction or the magnitude of polarization changes with temperature. Therefore, the induced surface charge which is neutralized with bulk polarization changes by thermal stimulations. This is the pyroelectric effect. In particular, if the direction of spontaneous polarization is inverted by the external electric field, the crystal is called ferroelectric. When the temperature is raised, the crystal loose its spontaneous polarization at the transition temperature called Curie temperature. The mechanism of the pyroelectric effect is schematically illustrated in Figure 12. At a constant temperature, the surface charge is neutralized with that within the sample, so that no current flows between the top and bottom electrode (Figure 12A). However, if the temperature is raised by dT, the spontaneous polarization P(T) changes and the surface excess charge is produced (Figure 12B). Therefore, the induced pyroelectric current I expressed by the next equation flows in the external circuit. I = A d P = A dP d T = A p d T

dt

dT

dt

(4)

dt

Here, A is the area of the electrode, dT/dt the rate of the temperature change, and p ~ d P / d T is called the pyroelectric coefficient. For practical use of pyroelectric elements as infrared sensors and so forth, the induced voltage V is an important quantity. From the relations concerning surface charge, dQ=Idt=

pAdT

(5)

156

(A)

-@|174174174174 dT

9169169 :,,

:-',1

:",

:",

(B)

~-@@@ Figure 12. Schematic illustration of pyroelectric effect.

(A)

(B) CHOPPER CONTROLLER

"ale

)

SO0 "K BLACKBODY

APERTURE

I I

!

LOC~ IN N~PLI FIER

I I i i

I LB FILH I~,,, "OPTICAL CHOPPER

BLFFER AHPLIFIER

Heat Sink ( Water Cooled)

Figure 13. Schematic illustration of pyroelectric measurement; (A) static method; (B) dynamic method [ 13 ].

1

157 and

dQ=CdV=

Aedv (6) d where C, d, and e are electric capacitance, thickness, and dielectric constant of the pyroelectric film, we can get dV

= d p (7) dT Thus, p/e is also a significant parameter for evaluating pyroeleetrie materials. In the most common LB films with the Y-type structure, the center of inversion exists, and hence they are not suitable for pyroelectric usages. On the other hand, since LB films with X- or Z-type structure have no center of symmetry, it is possible to construct the polar pyroelectric film with permanent dipoles pointing toward one direction. Similar structures can also be formed m hetero LB films with two different amphiphiles stacked altematingly. The first report on the pyroelectric LB film with Xor Z-type structure appeared in 1982 by Blinov et al. [12]. It was followed by those of the alternate LB films by Smith et al. [13] and Christie et al. [14]. The polarized structure of the fabricated LB film can be checked by the surface potential measurements using the Kelvin probe [ 15], the Stark effect measurements [ 12], or the sign inversion of the induced current between heating and cooling processes. For measurements of the pyroelectric coefficient, there are two methods; the static (Figure 13A) and dynamic (Figure 13B) methods. In the static method, the temperature of the whole system, substratre/bottom electrode/LB film/top electrode is changed at a constant rate dT/dt, and the pyroelectric current I is measured. Then, we can obtain the pyroelectric coefficient p by using Eq. (4). In the dynamic method, the modulated infrared beam by a mechanical chopper is directed to the pyroelectric system coated with a black light-absorbing layer (bismuth etc.). The resulting pyroelectric voltage is detected, amplified, and displayed by a phase-sensitive lock-in amplifier. The pyroelectric coefficient is given by [ 14] Ipl

= 2~'2Vm~/%Kpc

(8)

W ~TBNI where vm is the measured root-mean square pyroelectric voltage, 0~0 the modulation frequency, W the incident radiation power per unit area, ~B the emissivity of the blacking layer, N the number of monolayers, I the thickness of a monolayer, K, p, and c thermal conductivity, density and specific heat capacity of the substrate, respectively. Therefore, m order to obtain the pyroelectric coefficient, we have to get the information of these necessary parameters beforehand. Pyroelectric coefficients of LB films obtained to date is in the order of l.tCm-2Kl [16], as compared with 300 (triglicine sulfate, TGS) and 30 ~tCm-2K~ (polyvinylidenefluoride, PVF2). The small E value (2-3) keeps p/e in a similar order (0.2-2.2 ~tCm-2K-~), while it is reduced to 6 and 3 ~tCm-2K-1 in TGS and PVF 2. Dielectric

158 loss coefficients, tanS, of LB films are very small (0.003-0.008). This helps to reduce the dielectric noise of the pyroelectric device since it is proportional to the root square of tan& However, we have to improve the p/e value of LB films at least one order larger, to cope with the prevailing pyroelectric devices. If we consider the mechanism of the pyroelectric effect in the microscopic level, the spontaneous polarization P is given by [ 17] P = < n/'to h > = n ~0 < costp >

(9)

where n is the volume concentration of molecules, ~t0 the molecular dipole moment in the ground state, h a unit vector of the surface normal, and tp the angle between I.to and h. Therefore, p -

dP _/-to ~T + n d~0 ~ + n ~ d dT

(10)

The first term represents the temperature dependence of the film density, being directly related to the thermal expansion coefficient of the film, and also indirectly related to that of the substrate. Generally, the thermal expansion coefficient of the organic substance is much larger than that of the inorganic substance like glass. In the case of the alternate LB film of fatty acid/alkylamme or fatty acid/alkylanilme, the pyroelectric coefficient p decreased as the thermal coefficient of the substrate increased [ 18]. The second term in Eq. (10) originates from the change in dipole moment itself with temperature, and the third term from that in the orientation angle with temperature. These microscopic problems can be well investigated by vibrational spectroscopy, as demonstrated in the following sections.

5. R E L A T I O N S H I P B E T W E E N M O L E C U L A R O R I E N T A T I O N AND PYROE L E C T R I C I T Y IN ALTERNATING LB FILMS

5.1. Temperature dependence It has been known that alternating LB films consisting of two different amphiphiles have noncentrosymmetric structures, and therefore, are expected to provide piezoelectric, pyroelectric and nonlinear optical properties. Therefore, we prepared alternating LB films consisting of 5-(p-dodecyloxyphenyl)pyrazine-2-carboxylic acid (DOPC)

C,2H25 0

C0 0 H

synthesized by Takehara et al. [ 19] and deuterated stearic acid (St-d35). Hereafter, we will symbolize DOPC by P and deuterated stearic acid by S. The alternating LB film

159 was prepared by n-time depositions of the unit PS bilayer on the first monolayer of S (Figure 14). We designate this structure by S(PS)n. Then, we measured pyroelectricity and temperature dependence of the molecular orientation in these films using the above-mentioned method [3]. The same studies were also performed for the alternating LB films of their barium salts designated by S(PS)n-Ba. For pyroelectric measurements, we used two AI electrodes on both sides of the alternating LB film as shown in Figure 14. The electric current generated on linearly heating the LB film was measured by a picoammeter in the temperature range from -30 ~ to 60 ~ The pyroelectric coefficient p is calculated from the observed pyroelectric current/by

(dT) -1

(11)

P= /'dt"

Here, A was 170 nm 2 and dT/dt was 2.2 K/min. Figure 15 shows the current-temperature (I-T) curves for the alternating S(PS) 9 and S(PS)9-Ba films [7]. For reference, the I-T curve for the homogeneous (centrosymmetric) SPIs-Ba film which consists of 18-monolayers of DOPC-Ba on the first monolayer of St-d35-Ba is also shown in Figure 15. For both alternating LB films, the negative current increases on heating above 0 ~ After passing through the minimum points around 40 ~ the curves rise rapidly on further heating. For homogeneous SP18-Ba film, on the other hand, almost no current is obtained up to 40 ~ and then the positive

jAL II / ./!11/

., / /

/'1/

//13

I

I

I

I

t

t

I

I

I

I

I

I

i

I

i

I

I

i

I

I

,

i

!

,

|

i

I

|

,

I

i

i

I

!

I

i i i

p

Picoammeter

' ! !

S

'

P } (PS)~

,.

--I l l

/ i i Z I

I / i l l

iI'.I

tl

GLassslide Figure 14. Alternating LB films S(PS). and the electric circuit for pyroelectric measurements [7].

160 current increases. Therefore, the negative current observed only for the alternating films can be regarded as pyroelectric currents which may be due to changes in spontaneous polarization in the alternating films. Maximum pyroelectric coefficient, pmax, calculated by Eq. (5) is 1.8 ~tCm2K -~ at 43 ~ for the S(PS)9-Ba film and 1.2 lxCm2K1 at 37 ~ for the S(PS) 9 film. These values are comparable to previously reported data for some alternating LB films by other investigators. The positive currents observed above 50 ~ for all LB films examined will be discussed later in connection with the molecular orientation. Figure 16 shows thermal stability of the pyroelectricity for the S(PS)9S4-Ba film [7]. After the current was measured on the heating process to 40 ~ (the curve a), the sample was cooled to -30 ~ and then the current was measured again m the second heating process to 60 ~ This I-T curve (the curve b) is almost identical with the first one. However, the third and fourth I-T curves (the curves c and d) which were obtained after the repeated heating of th sample to 60 ~ show smaller negative currents than the previous ones. These results indicate that the heating of the sample

20

< r

*E

10

SPIe-Bo

0

-

|

/

I._

L_ =

S(PS)9

-

1~=:l.2pCm'tK ''

(j

-lO

-

-40

!. -20

S(PS)9-Ba pNx=l.g/JCm-=E' ,.! I .... 0 20

Temperature

~ I

I

40

/

I ! 60

,

80

~

Figure 15. Current-temperature curve of alternating S(PS) 9 and S(PS)9-Ba films and homogeneous SPls-Ba film [7].

161 above 60 ~ induces the depolarization of the alternating LB films and consequently reduces the pyroelectricity. Figures 17 and 18 represent FT-IR transmission and RA spectra, respectively, of the alternating S(PS)9-Ba films at various temperatures from 0 ~ to 120 ~ [7]. Two intense bands at 2919 and 2852 cm -~ are the antisymmetric and symmetric CH 2 stretching bands of DOPC, and two bands at 2192 and 2088 cm ~ are the antisymmetric and symmetric CD 2 stretching bands of St-d35, respectively. Apparently, all these bands decreases their intensities with the increase in temperature in Figure 17. At the same time, intensity differences of the respective bands are evident between the transmission and RA spectra. From these data, we calculated temperature dependence of the orientation angle T of the hydro-carbon chain axes of the constituent molecules in the alternating S(PS) 9 and S(PS)9-Ba films using Eqs. (2) and (3). The results are shown in Figure 19 [7]. Apparently, the Y values of the respective constituents in the S(PS)9-Ba film are much smaller than those of the corresponding molecules in the S(PS) 9 film. This reveals that the barium salt molecules are more highly oriented as compared with the

20

~176

I-

~176

I:

.~

,,~

I: I:

10

~176

d ,,e-,

fi

0

oo

(13 i.. L_ :3

~

~:

"~.~..'~.....

~176176 ~

(.)

~~

-... -10

-40

I

,,1/

C

-~

a

~

~ i

t

b 1

-20

1

0

I

20

Temperature

,

I

I

40

I

60

,L

.

80

~

Figure 16. Current-temperature curve of alternating S(PS)4-Ba film [7]; (a) first curve measured on heating to 40 ~ (b) second curve measured on heating to 60 ~ after the first measurement and cooled to -30 ~ (c) third curve measured on heating to 60 ~ after the second measurement and cooled to -30 ~ (d) fourth curve measured on heating after the third measurement and cooled to -30 ~

162

ILj~// V,(CH2) \

V,(Cl-.~)

v,(cl::m)

~ V,(CD2)

~2~176~

. ~

80 *

uJ ro z

V,(CO0")

c

T

J

~

60 *C

<

133

_ .

0 CO CO

<

.

.

.

40 _

*C

_

2

.'5000

2500 2000 WAVENUMBER /

1500

1000

cm "~

Figure 17. Infrared transmission spectra of the alternating S(PS)9-Ba film at various temperatures [7].

163

u|

V=(CO0") 'V,(CI~)

\ ,,.!~~

0.02 ]

4L_j

V,(CD~)

~ V,(CD~)

120 *C

/

80 *C Lt..I 0 Z ,<

60 *C

0 03 cn

40 *C

20 *C

4

0 *C _

3OOO

1

I

2500-2000 WAVENUMBER

-~_~

/

1

1500 cm-'

.,

|

1000

Figure 18. Infrared RA spectra of the alternating S(PS)9-Ba film at various temperatures [7].

164 acid molecules as m the case of stearic acid and its cadmium salt [3]. For the S(PS)9-Ba film, the T values for both constituent molecules are almost constant up to ca. 45 ~ and then gradually increase on further heating. For the S(PS) 9 film, the "~values for both constituents are also almost constant up to ca. 35 ~ and then increase rapidly with increasing temperature. It is to be noted that these temperatures, 45 and 35 ~ at which the T values start to increase for the alternate S(PS)9-Ba and S(PS) 9 films, respectively, are in good agreement with the above-mentioned temperatures (43 and 37 ~ at which the positive currents start to increase for the same films (Figure 15). Since it is reasonable to consider that the increases in the T value at higher temperatures are mainly due to the increase in the conformational disorder of the hydrocarbon chains, the agreements of these temperatures suggest that the positive current can be

60

-

9

DOPC I !

!

I

L_ Q~

-o

)...,, c~ D tO

jt

S(PS)9

4O '-"

/

s

,___...o_i:~,~~Si'-d~5

...

50

I

e

DOPC-BG

w

_

,i

( 50

S(PS)9-Ba

o -I-. t--

St-dz~-Ba

O

t "r' 0

I 30

I

60

Temperature

.

.I

,, I

90 /

_

120

*C

Figure 19. Temperature dependence of orientation angle 7 of the methylene chain axes of the constituent molecules in the alternating S(PS)9-Ba ( ) and S(PS) 9 films ( ..... ) [7].

165 ascribed to depolarization by the increase in the conformational disorder and thermal motion of the hydrocarbon chains at higher temperatures. Negative currents ob-served only for the alter-natmg films around 40 ~ are regarded as the pyroelectric currents due to the changes in spontaneous polarization as described above. In this temperature region, the higher molecular orientation is kept unchanged for all the consti-tuents as seen in Figure 19. However, we observed the frequency increase of the antisymmetric COO-stret-chmg (v a (COO-)) bands around 1516 cm I and the corresponding frequency decrease of th symmetric COO- stretching (Vs(COO)) bands of St-d35-Ba around 1430 cm 4 and of DOPC-Ba around 1410 cm~ on heating of the barium salt films (Figure 20) [7]. If we consider the dipole-dipole interaction

(o) St-d35-Ba, V,(CO0") z~

1518 j,p

1516 1514 ?

E

(J

(D .,a

E

t-" a,} t~

~:

(b) S'l'-d35-Bo, Vs(CO0") 1452

_

--,,.

1430 1428

\

1426 (c) DOPC-Bo. Vs(CO0")

1412

1410

-

~

1408

0

..

I

50

I

1

60

Temperelure

i

90

I

120

*C

Figure 20._ Temperature dependence of frequencies of (a) the Va(COO ) and (b) vs(COO ) bands of St-d35-Ba and (c) the Vs(COO ) bands of DOPC-Ba in the S(PS)9-Ba film [7].

166 between the two C-O bonds of the carboxylate group [20], the increase in the v a (COO-) frequency and the decrease in the v s (COO-) frequency above-observed can be understood by the widening of the O-C-O angle of the carboxylate group. This widening reduces a local dipole moment of the carboxylate groups along their bisectors and consequently diminishes the overall polarity of the molecules along their long axes. This may be one of the possible reasons for the changes in the spontaneous polarization and, therefore, the generation of pyroelectric activity in the alternate S(PS)9-Ba film on heating. Thus, the pyroelectricity in this film can be ascribed to the structure modification of the carboxylate groups of the constituent molecules with the highly oriented all-trans hydrocarbon chains. In the case of the acid film, we also observed temperature dependence of intensities and frequencies of the C = O stretching bands for both constituents. Since, however, there were mutual overlapping between these bands, the situation was not so clear as in the case of the barium salt films. Relationship between the pyroelectricity and structure of the carboxyl group in the acid films may be a subject of further studies. For the alternating LB films consisting of long-chain fatty acids and amine derivatives, Davies et al. [21,22] have reported a relationship between pyroelectricity and degree of proton transfer from acid to amine head groups. Therefore, the head group structure seems to be one of the most important factors for pyroelectricity of the alternating LB films.

"7

A \

ol

S(PS) n

\

2.0-

\

L)

\

::L

,/ N

\

"~

~.5-

",,A

_____z_____%-o \

u

o

xa

S(PS)n-Ba

!

1

I

5

_,

1

_1

i

5

7

9

Number of unit bilayers, n Figure 21. Pyroelectric coefficients of alternate S(PS)n-Ba (------) and S(PS)n films ( ..... ) at 30 ~ as a function of the number of unit bilayers n [7].

167 5.2. Effect of the n u m b e r o f unit PS bilayers, n

Now, we consider the effect of the number of unit PS bilayers n of the alternating S(PS)n-Ba and S(PS) n films on their pyroelectricity and mole-cular orientation at room temperature [7]. In Figure 21, the pyroelectric coefficients for both alternating films at 30 ~ are plotted against the n value. The coefficients of the S(PS) n films apparently decrease with the increase in the n value. The value at n = 9 (1.1 txCm2K-1) is half of that (2.1 IxCm2K1) at n = 3. However, the coefficients of the S(PS)n-Ba films are almost constant (ca. 1.3 ~tCm2K ~) within the experimental error. Figure 22 is a plot of the orientation angle T of the hydrocarbon chain axis of DOPC in the S(PS) n film and of DOPC-Ba in the S(PS)n-Ba film as a function of then n value. For the S(PS) n films, the T values increase with increasing n value. But, for the S(PS)n-Ba films, the T values are almost independent of the n value. These results correspond well with those for the pyroelectric coefficients, and indicate that if the molecular orientation is unchanged, the pyroelectricity is also unchanged, but if the molecules are more inclined, the pyroelectricity is decreased. The increase in inclination

5O

DOPC in S(PS)n

/"

b

f

r

f

40

9/I J t

~

0

"~

3o-

j

,,,

o

DOPC-Ba in S(PS)n-Ba 20-

101

1

, I

3

I

1

I

5

7'

9

Number of unit bilayers, n Figure 22. Orientation angles T of the methylene chain axes of DOPC-Ba in S(PS)n-Ba film ( ) and of DOPC in S(PS) n film (..... ) at 30 ~ as a function of the number of unit bilayers n [7].

168 of the molecule may give rise to the decrease in polarity along the normal direction to the film surface and, therefore, may result m the decrease m pyroelectricity. Furthermore, we compare the pyroelectric activities between the acid and barium salt films. As is seen in Figure 22, the molecular orientations of DOPC in the S(PS) 3 film and that of DOPC-Ba in the S(PS)3-Ba film are almost the same. However, the pyroelectric coefficient of the S(PS) 3 film is about twice as large as that of the S(PS)-Ba film (Figure 21). It can, therefore, be concluded that if the molecular orientations are in the same order, the acid film has larger pyroelectricity than the corresponding barium salt film. In the former film, DOPC and St-d35 in the adjacent monolayers form the ring dimer through two hydrogen bondings. In the latter film, on the other hand, DOPC and St-d35 anions in the adjacent monolayers form ionic bindings through the divalent barium ion. This may be a reason of the above-mentioned statement that the acid films are thermally less stable than the barium salt films. Therefore, the head group structure is more easily changeable on heating, and then pyroelectric activity becomes larger in the acid film as compared to that in the barium salt film.

CCD Detector

SPEX Triplemate

~

Raman Scattering

s-Polarized Laser Beam Qum'tz/~_.___._~ Prism Figure 23. A total reflection method for measurements of Raman spectra of thin LB films.

169

5.3. Nonresonance Raman study of the order-disorder transition of the hydrocarbon chains at high temperatures In a previous section, we considered that the increase in the ~fvalue of the alternating S(PS) 9 and S(PS)9-Ba films at higher temperatures above 40 ~ in Figure 19 is due to the increase in the conformational disorder of the hydrocarbon chains and therefore that the rapid increase in the positive current in the same temperature range in Figure 15 can be ascribed to depolarization by the increase in the conformational disorder and thermal motion of the hydrocarbon chains. In order to confirm these considerations, we measured the temperature dependence of nonresonance Raman spectra in the CH stretching region of DOPC in the S(PS)ll film and of DOPC-Ba in the S(PS)11 -Ba film using a high-sensitivity charge-coupled device (CCD) detector [23]. Nonresonance Raman spectra of the alternating LB films were measured by a total reflection method shown in Figure 23. The films were deposited on quartz prisms. The s-polarized beam of 647.1 nm from a Kr + laser was incident upon the interface between the quartz and film at an angle of 45 ~ from the quarz side, and totally reflected. Raman line scattered from the film in the direction of 45 ~ from the surface was measured through a Spex Triplemate by a Photometrics PM512 CCD detector with 512><512 pixels operated at -125 ~ The spectral resolution was about 5 -1 cm

.

Nonresonance Raman spectra of the CH stretching region of DOPC-Ba in the S(PS)ll-Ba film thus obtained is shown in Figure 24. Apparently, intensities of the 2880- and 2850-cm l bands which are assigned to the antisymmetric and symmetric CH 2 stretching vibrations, respectively, decrease with the increase in temperature. Gaber and Peticolas [24] have reported that the intensity ratio I (2880) / I (2850) is a sensitive measure of regularity of the hydrocarbon chain and that its value decreases from 1.5 to 0.7 when the hydrocarbon chains change from the vibrationally decoupled all-trans structure to the completely melted state. Therefore, the I (2880) / I (2850) ratios obtained from Figure 24 are plotted in Figure 25 together with the results for the DOPC in the S(PS)11 film as a function of temperature. The ratios are ca. 1.2 for both DOPC and DOPC-Ba in S(PS)ll and S(PS) 11-Ba films at low temperatures. However, it starts to decrease at ca. 35 ~ for DOPC and at ca. 45 ~ for DOPC-Ba. It is to be noted that these temperatures are in good agreement with those pointed out in Figures 15 and 19 as the changing points of the pyroelectric current and ~tvalue, respectively. These facts indicate that the hydrocarbon chains of DOPC and DOPC-Ba in respective films are in a partly disordered state at low temperatures. But the irregularity of the hydrocarbon chains starts to increase at ca. 35 and 45 ~ for DOPC and DOPC-Ba, respectively. Thus, the previous statement that the rapid increase in the positive current observed for the alternating S(PS) 11 and S(PS) 11-Ba films at higher temperatures above 40 ~ (Figure 15) is due to depolarization by the increase in the conformational disorder and thermal motion of the hydrocarbon chains is confirmed also by Raman spectroscopic measurements.

170

100~ ~

~

r e-t.e-t'r

E rr"

I

3000

.

I

I

i

2900 Wavenumber

/

cm

,.

I

2800

-1

Figure 24. Nonresonance Raman spectra of the CH stretching region of DOPC-Ba in S(PS)ll-Ba film [23].

171

1.25

i-----m,r~_1.20

C) tr oo t"q

0 oo oO C'4

O

-

1.15

DOPC-Ba in S(PS)I1-Ba

~

~

\},., ra,,, o . O\ \0 \ in S(PS)II \

-

DOPC

0

1.10

1..-.,4

1.05

1.00

. 0

,. . . .

20

l ..... 40

l 60

....

Temperature

l 80

, , ...... 100 120

/

~

Figure 25. 1(2880)/I(2850) of DOPC and DOPC-Ba in S(PS)I1and SfPS),,-Ba films, respectively, as a function of temperature [23].

5.4. S u m m a r y The electric current generated on linearly heating the alternating S(PS) 9 and S(PS)9-Ba films was measured in the temperature range from -30 to +60 ~ At the same time, the temperature dependence of the orientation of the constituent molecules in both alternating LB films was quantitatively measured by combining infrared transmission and RA spectroscopy. Furthermore, the effect of the number of unit PS bilayers n of the alternating S(PS) n and S(PS)n-Ba films on the molecular orientation and electric current was also studied. Finally, nonresonance Raman spectra in the CH stretching region of DOPC and DOPC-Ba in the respective alternating LB films were observed by a high-sensitivity CCD detector at various temperatures from 0 to 120 ~ A detailed comparison of these data leads us to conclusions shown below. (1) Maximum pyroelectricity can be obtained around 40 ~ i. e. the highest temperature, below which DOPC can maintain its highly ordered molecular orientation. (2) The pyroelectricity can be ascribed to the change in the spontaneous polarization

172

(3)

(4)

(5)

due to a structure modification of the polar head group. The positive current which increases rapidly on heating above 40 *C may be caused by the increase in the conformational disorder and thermal motion of the hydrocarbon chains. The alternating LB films with more highly oriented constituent molecules give rise to larger pyroelectricity. If the molecular orientations are in the same order, the acid alternating film has larger pyroelectricity than the barium salt film. This may be due to a weaker intermolecular interaction in the former film than in the latter film.

6. P Y R O E L E C T R I C I T Y OF N O N C E N T R O S Y M M E T R I C LB F I L M S PHENYLPYRAZINE DERIVATIVES

OF

To investigate the pyroelctricity of noncentrosymmetric LB films in more details, we measured the pyroelectricity of several kinds of alternating LB films composed of phenylpyrazine derivatives and stearic acid [25]. Furthermore, we examined the effect of packing density of molecules in the film and that of thermal expansion of the substrate on the pyroelectricity. From the results obtained, mechanisms of pyroelectricity of the LB film were discussed. In this study, monolayers of three types of phenylpyrazine derivatives (Compounds I, II, and IU shown in Figure 26) and stearic acid were used for fabricating alternating LB films. Note that Compound I is DOPC in the previous section. Each monolayer of phenyl-pyrazine derivatives and stearic acid was spread from a chloroform solution (1.00 mg/ml ) on double-distilled water containing 2• 10"4 mol/1 BaC12 and buffered with NaHCO 3 to pH 7.2, and compressed to the surface pressure of 30 mN/m at 20 ~ In the case of fabricating alternating LB films of polyion complexes, polyallylamine hydrochloride (PAA) or polydimethyldiallylammonium chloride (PDDA), purchased from Nittobo Co., was dissolved in distilled water, instead of BaC12. The monolayer of phenylpyrazine derivatives spread on water containing PDDA was compressed to 40 mN/m in order to improve the transfer

cl2. q3 _%coo.

IIl Figure 26. Molecular Formula of Compounds I, II, and III.

173 ratio. The monolayers of stearic acid were transferred upon withdrawal and those of phenylpyrazine derivatives upon dipping of solid substrates. The pyroelectric current of the film, which flowed through the circuit from the bottom electrode to the top electrode in Figure 14 was measured using a Keithley Model 617A picoammeter with the heating and cooling apparatus for the sample film. The heating and cooling rates were controlled to 4 ~ by an Ohkura Model EC56 temperature controller. Temperature was monitored by a copper-constantan thermocouple. Relative dielectric constant ~ and dissipation factor D of the film are given by E

_

(12)

Cd

~r ~- ~'-~--~0 A

D = G an7

(13)

where C and G are the capacitance and conductance of the film, respectively, measured by a GenRad 1621 precision capacitance measurement system, d the film thickness, E0 the dielectric permittivity of a vacuum, and r the angular frequency. 6.1. Nature of pyroelectric currents at various temperatures Figure 27 shows the pyrodectric current trace and the temperature cycle around 20 ~ applied to the pyroelectric measurement of the alternating film consisting of the barium salt of Compound II and stcaric acid. As the temperature increased, positive current flowed through the circuit to reach an almost constant value. When the temperature started to decrease, the current immediately changed its flow direction to negative until it reached an almost constant value. This square wave pattern of the current flow did not vary during continuous one-hour measurement. Similar current

4

,-.9

0

= 0

-2

.o

1i -3O 0

5

1 Time

/

1

20

min

Figure 27. Pyroelectric current trace and temperature cycle around -20 ~ alternating LB film of Ba salts of Compound II and stearic acid.

for the

174

Table 3. Dielectric constants, dissipation factors, and pyroelectric coefficients of the alternating LB films comoosed of Ba salts of ohenvltwrazine derivatives. Pyroelectric coefficient Sample

e r

D

30 ~ Heating

Cooling

(-0.36)

(0.090)

Compound I

2.78

0.024

Compound H

2.97

0.028

0.33

Compound Ill

2.95

0.1340

-0.45

~ Cm-2K I

-20 *C nearing

-60 *C

Coolh2g

nearing

CooUng

0.14

-0.092

0.10

-0.093

-0.56

0.43

-0.40

0.43

-0.43

0.14

-0.081

0.090

0.12

-0.064

patterns were also observed for other sample films except for some cases as mentioned later. The same measurements were made on a 13-monolayer barium stearate film as reference; however, almost no current could be observed m any of the temperature ranges used. Therefore the current with square wave pattern can be identified as the pyroelectric current. Pyroelectric coefficients were calculated using Eq. (11). Table 3 shows relative dielectric constant e~, dissipation factor D, and pyroelectric coefficient p at various temperatures for the alternating LB films. The relative dielectric constants of Compounds I, II, and III were about 2.9 at 1 kHz and room temperature, being consistent with those reported for alternating LB films of fatty acid and fatty amine [13]. The dissipation factors at 1 kHz were on the order of 3-4x 10 -2 for these compounds, which were fairly low and desirable for the application of these films to pyroelectric devices. The film composed of Compound II had larger absolute pyroelectric coefficients than others at all temperatures except for 30 ~ which may be due to a

/ /r

'

ID 0 e-

-e 0

...-

.--.

~

250

..

-

\

',

.=

%.....

i \

.

300

,

350

400

Wavelength /

450

500

nm

Figure 28. UV spectra of chloroform solution of Compounds I (solid line), II (broken line), and III (dotted line).

175

conjugated ~-electron system extending from the benzene to pyrazine rings through the C=C bond in Compound II. UV spectra of each compound in chloroform solution are shown m Figure 28. The maximum absorption wavelength of Compound U is the longest (383 nm) among the three compounds, suggesting that the chromophore of Compound II has the most extended ~-conjugation structure. It is also likely that Compound II has the largest dipole moment along the long molecular axis, and therefore gives the largest spontaneous polarization with its alternating LB film. A different pattern of current flow was observed for the alternating LB film consisting of the barium salt of Compound I and stearic acid in the high-temperature range. Typical pyroelectric currents flow at an almost constant value with opposite signs on heating and cooling as seen in Figure 26. This square wave pattern of the current flow was observed in the low-temperature range for this film. However, the sawtooth wave pattern, in which positive current increased with cooling of the film and negative current increased with heating, was observed around 30 ~ as shown in Figure 29. It is suggested that the pyroelectric response of this film is not sharp in the high-temperature range. The pyroelectric coefficients of this case are designated in parentheses in Table 3. Heretofore, some researchers estimated the pyroelectricity, as a function of temperature, by simply heating the sample and obtained the largest pyroelectric effect around room temperature for noncentrosymetric LB films [26-28]. We also showed that the largest pyroelectric coefficient was obtained at 43 *C for the alternating film consisting of the barium salt of Compound I and stearic acid in the previous section. However, the above results indicate that the simple heating method is not adequate to precisely estimate pyroelectricity. Furthermore, as seen in Table 3, the pyroelectric effects of these alternating LB films showed temperature dependence. In the low-

F 30 i---

20 0

5

10 Time

/

15

20

min

Figure 29. Pyroelectric current trace and temperature cycle around 30 ~ for the alternating LB film of Ba salts of Compound I and stearic acid.

176

Table 4. Pyroclectric coefficients of the alternating LB films composed of several complexes of the compound II. Pyroelectric coefficient Sample

Substrate

~C.m-:.K-I

-20"C Heating

-60"C Cooling

Heating

Cooling

Barium salt

Glass slide

0.43

-0.40

0.43

-0.43

Barium salt

Polypropylen

0.55

-0.41

0.55

-0.32

PAA complex

Glass slide

0.50

-0.48

0.60

-0.55

PDDA complex

Glass slide

0.20

-0.063

0.22

-0.097

temperature range (around -60 ~ all films had positive pyroelectric coefficients on heating. Around -20 ~ however, the film containing Compound III had a negative value on heating. In the high-temperature range (around +30 ~ negative pyroelectric coefficients were observed for the films of Compounds I and III on heating. This means that the pyroelectric current flowed in the positive direction on heating in the low-temperature range and flowed in the negative direction on heating in the hightemperature range. The temperature at which the direction of current flow was changed was dependent on the constituent compounds. This is an interesting phenomenon, but at present, we cannot explain it. The anomalous phenomenon that the sign of pyroelectric coefficient changes on heating has also been found for polytrifluoroethylene [29]. The present alternating LB films had pyroelectric coefficients on the order of 10 .7 ~tC 9m -2 9K l . Smith et al. proposed that the pyroelectric coefficient of this order was due to the thermal expansion of the sample [30]. If the spontaneous polarization of the film was only produced by the permanent dipole moment of phenylpyrazine chromophore, then the pyroelectric current had to flow in the negative direction upon the thermal expansion of the sample in this arrangement of pyroelectric measurements (Figure 14). However, the signs of pyroelectric coefficients were positive on heating in the low-temperature range and changed to negative between -60 ~ and +30 ~ This phenomenon cannot be explained by the simple thermal expansion mechanism. Other mechanisms such as certain changes of the electron distribution in the chromophore or the molecular structure with temperature have to be considered. In order to investigate the mechanism of pyroelectricity, thermal expansion of the sample should always be considered in spite of the above results. We thought that the thermal expansion of these samples was dependent on the substrate rather than the LB film itself under the assumption that the thickness of the film might be too thin to give several bulk properties [30]. Then, the influence of the substrates on pyroelectricity

177 was examined. The alternating LB film of barium salt of Compound II was deposited on polypropylene substrate of 1 mm thickness which had a thermal expansion coefficient one order larger (10 -4 K") than the glass slide used here (10SK"). The pyroelectric coefficients of this sample are shown in Table 4. The film deposited on the polypropylene substrate showed slightly larger pyroelectricity than that on the glass slide substrate, but did not give a large difference as expected. If the pyroelectricity of the sample is mainly governed by the thermal expansion of the substrate, then the pyroelectric coefficient of the sample on polypropylene should be much larger than that on the glass slide. Smith and Evans indicated that heat capacity of the substrate was effective in rendering the quick response of the devices for pyroelectric measurements utilizing the dynamic method [31]. In this static measurement, however, the dependence of pyroelectricity on the heat capacity of the substrate was scarcely observed. Thus, the thermal expansion of the substrate may not be essential for obtaining large pyroelectric effects. To obtain large pyroelectricity, the spontaneous polarization should be largely changed by thermal stimulation. The spontaneous polarization of these alternating LB films is governed by the molecular structure of the film. Here, we thought that the molecular packing density of the film should have an effect on pyroelectricity, because the film which has a smaller packing density can be expected to have much more spatial freedom and be more flexible to structural or orientational changes caused by thermal stimulation. Thus, we fabricated alternating LB films consisting of polyion complexes of phenylpyrazme derivatives and cationic polymers (PAA and PDDA). The LB films of polyion complexes are expected to have mechanical stability and spatial freedom due to the introduction of the polyion linings [6]. Figure 30 shows the 60 ...-...

E 50 Z E 40 O L

m 30

_

t

-

II) t..._

13. |

O

20

i

"-.

1::: r~ lO

9

! - - " '-.." "-......

9

0

1

0.2

....... I

0.4

I

I

0.6

Area per Molecule

0.8 /

1.0

nm 2

Figure 30. The zc-A isotherms of Compound II (solid line 9barium salt, broken line" polyion complex with PAA dotted line" polyion complex with PDDA).

178 ~t-A isotherms of the monolayers of Compound II in the form of barium salt, polyion complex with PAA, and that with PDDA. Values of the mean area per molecule in the monolayer were estimated to be 0.26 (barium salt), 0.30 (polyion complex with PAA) and 0.44 nm 2 (polyion complex with PDDA). Thus, it was revealed that Compound II had a more sparse molecular packing in the monolayer of polyion complexes than in the monolayer of barium salt. The pyroelectric currents for alternating LB films of polyion complexes flowed through the circuit to reach an almost constant value with opposite signs on heating and cooling, the patterns of which were almost similar with that shown in Figure 27. The pyroelectric current caused by the cationic polymer is negligible because almost no current flowed through the circuit for the homogeneous LB films of each polyion complex of stearic acid in any of the temperature ranges used. Table 4 includes the pyroelectric coefficients of the alternating LB films composed of various ion complexes of Compound II. The p values in the high-temperature range around 30 ~ are not shown because of their poor reproducibility. As the smaller density leads to a smaller spontaneous polarization, the polyion complexes seem to lower the pyroelectricity if other factors are precisely kept constant. The pyroelectric coefficients of the alternating films of PAA and PDDA polyion complexes should be smaller than that of the barium salt by factors of 0.87 and 0.59, respectively, due to expansion of the mean molecular area. The alternating film of polyion complex with PAA, however, showed a larger pyroelectricity and that with PDDA showed a smaller pyroelectricity than expected. The following effects on pyroelectricity should be considered in the fabrication of alternating LB films of polyion complexes. (1) Dipole moments of phenylpyrazine chromophores are altered because of the different polar head groups. (2) Spontaneous polarization of the film is decreased by the low degree of molecular orientation in the film, which may lead to smaller pyroelectricity. (3) Spontaneous polarization is largely varied with thermal stimulation owing to the increase of spatial freedom, which may be effective to show large pyroelectricity. Unfortunately, as it is difficult to make an experimental estimation of each effect independently, we cannot discuss the precise mechanisms of pyroelectricity. Here we regard that the dipole moments of Compound II are the same among respective ion complexes. It can be considered that the large pyroelectricity of the PAA complex is caused by the large ratio of polarization change with temperature owing to the spatial freedom of the constituent molecules in the film. On the other hand, the alternating LB films of PDDA complex have an extremely sparse molecular packing as mentioned before, and thus the strong intermolecular interaction within the same monolayer which governed molecular orientation in the film [33] cannot be expected. Therefore, an extreme spatial freedom of the constituent molecules may lead to a low degree of molecular orientation in the film, resulting in smaller spontaneous polarization of the film and thus in smaller pyroelectricity.

179 6.2 Conclusion Pyroelectricity of several kinds of alternating LB films consisting of phenylpyrazine derivatives and stearic acid was measured by the static method at various temperatures. Effects of thermal expansion and molecular packing density of the film on pyroelectricity were also examined. The following conclusions were derived. (1) The film composed of compounds with highly conjugated re-electron systems showed larger pyroelectricity. (2) Pyroelectricity of the film is highly dependent on the range of temperature applied to the sample film. The LB film of Compound I showed dull pyroelectric response in the high-temperature range. The signs of pyroelectric coefficients were changed at a temperature characteristic of the film. (3) Thermal expansion of the substrate is not essential for large pyroelectricity. (4) LB films which have smaller packing density give larger pyroelectricity. However, the film which has an extremely sparse molecular packing shows small pyroelectricity because of its low degree of molecular orientation. The results obtained may contribute to the development of more efficient pyroelectric devices on the basis of thin-film techniques.

REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

P.-A. Chollet, J. Messier and C. Rosillio, J. Chem. Phys., 64 (1976) 1042. D.L. Allara and R. G. Nuzzo, Langmuir, 1, 45 (1985) 52. J. Umemura, T. Kamata, T. Kawai and T. Takenaka, J. Phys. Chem., 94 (1990) 62. T. Kawai, J. Umemura and T. Takenaka, Langmuir, 6 (1990) 672. E. Okamura, J. Umemura and T. Takenaka, Can. J. Chem., 69 (1991) 1691. J. Umemura, Y. Hishiro, T. Kawai, T. Takenaka, Y. Gotoh and M. Fujihira, Thin Solid Films, 178 (1989) 281. T. Kamata, J. Umemura, T. Takenaka and N. Koizumi, J. Phys. Chem., 95 (1991) 4092. R.G. Greenler, J. Chem. Phys., 44 (1966) 310. P. Hollins and j. Pritchard, Vibrational Spectroscopy of Adsorbates, Ed. R. F. Willis, p. 125, Springer-Verlag, Berlin (1980). W.H. Hansen, J. Opt. Soc. Am., 58 (1968) 380. T. Kawai, J. Umemura and T. Takenaka, Langmuir, 5 (1989) 1378. L.M. B linov, N. N. Davydov, V. V. Lazarev and S. G. Yudin, Sov. Phys. Solid State, 24 (1982) 1523. G.W. Smith, M. F. Daniel, J. W. Barton and N. Ratcliffe, Thin Solid Films, 132 (1985) 125. P. Christie, C. A. Jones, M. C. Petty and G. G. Roberts, J. Phys. D.: Appl. Phys., 19 (1986) L167. P. Christie, G. G. Roberts and M. C. Petty, Appl. Phys. Lett., 48 (1986) 1101.

180 16. G.G. Roberts, Langmuir-Blodgett Films, Ed. G. G. Roberts, Chap. 7, Plenum Press, New York, 1990. 17. L. M. B linov, L. V. Mikhnev, C. P. Palto and S. G. Yudin, Proc. 4th Intern. School Condens. Matter Phys.: Molecular Electronics, p.575, World Scientific, 1987. 18. C.A. Jones, M. C. Petty and G. G. Roberts, Thin Solid Films, 160 (1988) 117. 19. K. Takehara, H. Niino, Y. Oozono, K. Isomura and H. Taniguchi, Chem. Letters, (1989) 2091. 20. J.O. Hirschfelder, C. F. Curtiss and R. B. Bird, Molecular Theory of Gas and Liquid, John Wiley & Sons, New York, 1954. 21. G.H. Davis and J. Yarwood, Mikrochim. Acta, (1988) 305. 22. G.H. Davis, J. Yarwood, M. C. Petty and C. A. Jones, Thin Solid Films, 159 (1988) 461. 23. J. Umemura, T. Kamata, T. Takenaka, K. Takehara, K. Isomura and H. Taniguchi, Bull. Inst. Chem. Res., Kyoto Univ., 71 (1993) 120. 24. B.P. Gaber and W. L. Peticolas, Biochim. Biophys. Acta, 465 (1977) 260. 25. T. Kamata, J. Umemura, T. Takenaka, N. Koizumi, K. Takehara, K. Isomura and H. Taniguchi, Jpn. J. Appl. Phys. Part 1, 33 (1994) 1074. 26. C.A. Jones, M. C. Petty, G. Davies and J. Yarwood, J. Phys. D: Appl. Phys., 21 (1988) 95. 27. T. Sakuhara, H. Nakahara and K. Fukuda, Thin Solid Films, 159 (1988) 345. 28. G.G. Roberts, Ferroelectrics, 91 (1989) 21. 29. Y. Oka and N. Koizumi, Jpn. J. Appl. Phys., 22 (1983) L281. 30. G.W. Smith, N. Ratcliffe, S. J. Roser and M. F. Daniel, Thin Solid Films, 151 (1987) 9. 31. G.W. Smith and T. J. Evans, Thin Solid Films, 146 (1987) 7. 32. F. Kimura, J. Umemura and T. Takenaka, Langmuir, 2 (1986) 96. 33. T. Kamata, J. Umemura, T. Takenaka, K. Takehara, K. Isomura and H. Taniguchi, J. Mol. Struct., 240 (1990) 187.

New Developments in Construction and Functions of Organic Thin Films T. Kajiyama and M. Aizawa (Editors) 9 1996 Elsevier Science B.V. All rights reserved.

181

C o n s t r u c t i o n of well o r g a n i z e d f u n c t i o n a l L a n g m u i r - B l o d g e t t films b y m i m i c k i n g s t r u c t u r e s a n d f u n c t i o n s of biological m e m b r a n e s Masamich~ Fujihira Department of Biomolecular Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan

In a series of our recent studies, the simulation of the light harvesting and the succeeding charge separation processes with Langmuir-Blodgett films were examined. These molecular devices can be used for photo-electric conversion and are called molecular photodiodes. The size of the molecular devices is, however, molecular only in the thickness, where the lateral dimension is macroscopic. Phase separation phenomena in mixed monolayers of hydrocarbon and fluorocarbon amphiphiles were studied with various scanning probe microscopes (SPM). We propose that such a phase separation can be used as one method to decrease the size of these molecular devices as well as micro processing by SPM. 1. INTRODUCTION

The highly oriented molecules in thin organic films such as Langmuir-Blodgett (LB) films and self-assembled monolayers (SAM) [1] are essential for some molecular functions. Non linear optical and opto-electronic properties are two of the most important and interesting functions of these molecular assemblies. In the past more than thirteen years, simulation of the primary process of photosynthesis using such molecular assemblies and its application to molecular photodiodes [2,3] have been one of the main subjects of our laboratory. In the natural photosynthetic reaction center, solar energy is first captured by light harvesting antenna pigments, then excited energy harvested by the pigments is funneled to a special pair of the charge separation unit by energy migration and energy transfer, and finally an excited electron-hole pair in the special pair can be separated via electron and hole transfer by electron acceptors and donors located appropriately across the membrane. This molecular mechanism of photo-electric conversion has been simulated by using an artificial mixed monolayer consisting of light harvesting H and A-S-D triad amphiphilic molecules [2-4] as shown in Fig. 1 [2,4]. A, S, and D stand for electron acceptor, sensitizer, and electron donor moieties of the triad, respectively. The sensitizer moiety of the triad can absorb light energy by itself, but also can accept the excited energy of H molecules in the mixed monolayer. The A, S, and D moieties are linked covalently in the triad and

182

expected to be positioned in this order across the monolayer due to an amphiphilic property of the triad. Upon excitation of the S moiety followed by intramolecular electron and hole transfer to the A and D moieties, respectively, the charge can be separated unidirectionally across the monolayer in the same way as in the natural photosynthetic reaction center.

(a)

(r

Lateral Energy Transfer

[IFI ll rl Illl

V

{b) C'kX X ~ X

~xxxx:~

X-X'~ ]CX)

..

~xxxxd

c]xx~r~

(~XXXXXXX (3~(X X X X X X] CX ~ X X X X X ) ~

(XX~,,IcX X]M[~ CX) CX.X.X.x..x.x.XX

Electron Transfer across the Membrane

0 0 ....:::~i!~i!ii!!!i~!i~::::

Figure 1. Schematic representation of the artificial photosynthetic reaction center by a monolayer assembly by A-S-D triad and antenna molecules for light harvesting (H), lateral energy migration and energy transfer, and charge separation across the membrane via multistep electron transfer: (a) Side view ofmonolayer assembly, (b) top view of a triad surrounded by H molecules, and (c) energy diagram for photo-electric conversion in a monolayer assembly. The LB technique is suitable for malting such highly oriented molecular assemblies. But, detailed structures of LB films had not been clarified until these films were studied recently by scanning probe microscopes (SPMs) [2,5-8]. The SPMs, such as scanning tunneling microscope (STM) [9], atomic force microscope ( F M ) [10], friction force microscope (FFM) [11-13], a Kelvin probe force microscope [14,15], and scanning near-field optical microscope (SNOM) [16,17], are useful to characterize the organic thin films on sub-micron to molecular scales. In order to realize true molecular devices, we have to reduce the size of our LB photodiodes not only in thickness (z-direction) but also in the x-y plane. Micro processing by AFM was applied previously [5,18,19] to fabricate molecular devices in nanometer scales in lateral dimensions (x-y plane) as shown in Fig. 2a [3].

183

Figure 2. Compartmentation of the LB films for molecular photodiodes by (a) micro processing with an AFM tip and by (b) m ~ use of phase separation of the mixed monolayer of HC-FC amphiphiles. In the present review, first we will describe how to fabricate artificial photosynthetic reaction center in nanometer scales by m a l ~ g use of phase separation in mixed monolayers of hydrocarbon (HC) and fluorocarbon (FC) amphiphiles [2,5,20-26] as shown in Fig. 2b [3]. The phase separated structures were studied by SPMs such as AFM, SSPM, and scanning near-field optical/atomic force microscopy (SNOAM) [27-33] as well as a conventional local surface analysis by SIMS [3,5]. The model anionic and cationic HC amphiphilic

184

compounds were used for the present study of phase separation for simplicity in place of the H light harvesting antenna and the A-S-D charge separation triad. The intense fluorescence due to cationic cyanine dyes with two long alkyl chains observed from HC islands by a conventional fluorescence microscopy and SNOAM indicates that the HC dyes preferentially dissolved in the HC islands in the phaseseparated system. The success in fluorescence measurements of the nanostructure demonstrates also that SNOAM can be used to access nano-devices from macro-scale external devices in optical modes.

2 Layers

30 Layers

30 Layers

5 Layers 3 Layers

:::!::(: 9 :::::::::::::::::::::::::: ::.::~:::. :::.::!:.ii!ili.:: :::::::::::::::::::::::..~

(a)

i:i:ii!iiii!i i i!!iiii ii!ilili

(b)

Figure 3. Unidirectionally oriented A-S-D triads in LB films containing 15 alternate bilayers consisting of a triad-CaT (1:5) and a pure CaT monolayer. In type a (a), the D tails of A-S-D triads orient towards the air, while in type b (b) the tails orient towards the substrate plate.

185 We will also describe observation of charge separation in LB films under laser light illumination by SSPM [3,5,22,34,35]. In the LB films, linear A-S-D triad amphiphilic molecules oriented unidirectionally in two ways with the D tail of the triad towards the air or towards the substrate plate as shown in Figs. 3a and 3b [3], respectively. The A, S, and D moieties are expected to be located in this order or reversed order across each alternate monolayer in the LB films. By photoexcitation of the S moiety, the electron and hole can be separated unidirectionally, but with opposite directions in the LB films. The resulting photo-induced changes in dipole moments of the highly oriented triad molecules were detected as the surface potential changes of gold electrodes covered with the LB films by SSPM as shown in Fig. 4 [3]. The results described here indicate that SSPM can be used as one of electronic access tools to the nano-devices from the macro-scale external circuit.

Figure 4. Detection of the change in photo-induced surface dipole moments in highly oriented A-S-D triads in artificial photosynthetic reaction centers as the local surface potential change in nano-domains measured by SSPM.

2. MATERIALS Amphiphilic compounds and other chemicals used are shown in Figs. 5 [3,4] and 6 [36] together with their abbreviations. The synthetic procedures for A-S-D triad, A-S dyad, and H light harvesting antenna molecules in Fig. 5 were described previously [3,37,38]. A cationic cyanine dye with two long alkyl chains (CD) in Fig. 5 was purchased from Japanese Research Institute for Photosensitizing Dyes,

186 Okayama, Japan. The synthetic procedures for S-D dyads and their reference compounds shown in Fig. 6 are illustrated in Fig. 7, where a naphthalene and a ferrocene moiety are used as an S and a D moiety, respectively. For comparison of photoinduced electron transfer rates between a single alkyl chain and a triple alkyl chain as the spacers of the S-D dyads with the same length of four-carbons, S-D dyads with a rigid spacer with a bicyclo[2.2.2]octane were synthesized [39]. The synthetic procedure for the S-D dyads with the rigid spacer is also shown in Fig. 8 [39]. Other amphiphilic compounds and chemicals were commercially available. 0

BF

18H37 71aH37 CIOZ

B~+

.-.~ :e'N H N....~ I~r~"r" -)-c=c-c=~ r, J~ ~ o H H O~ CD

A-S-D

8rBr'

C9F19C2H4OC2H4COOH

PFECA

A-S

PVA ~

O

H ( H2C=CH (CH2)20COO-)2-Ca 2+ CaT

CH3(CH2)lsCOOH SA

O Fe

CH3(CH2)I7N(CH3)3CI

ODTMAC

O DAFc

Figure 5. Structural formulae of amphiphilic compounds and other chemicals used for photo-electric conversion and their abbreviations.

187

Fe

@

@

Fe

S4DTCOOH

0

Q F.O

0

D4STCOOH

OH

O

~

OH

4S7COOH

PAA

NH~.] n NH2

ODA

Figure 6. Structural formulae of S-D amphiphilic compounds and other chemicals used for S-D monolayers for comparison of photo-induced electron transfer rates between a single alkyl chain and a triple alkyl chain as the spacers of the S-D dyads with the same length of four-carbons. In these S-D dyads, a naphthalene and a ferrocene moiety are used as an S and a D moiety, respectively. S-D dyads with a rigid spacer consisting of a bicyclo[2.2.2]octane are used as dyads with a triple alkyl chain.

188 0

oo§ SOCI2 =

0

0

Et3SiH

A'c"~ oO~176

~~..~(~~1~0 Cl

[~~o/V~o

,,.._

OH

TFA

Fc _ 0C10~oo~

AICI3

OEt AICi3 0

c,~-~.~.o~t Lo~~ 0 /

Fe

t~

Et3SiH

0

o~h--~-~~

Et3Si

FA

Q

Fe

~

0

OEt

KOH

KOH

Q OH

Figure 7. Synthetic routes for S-D dyads with a single alkyl chain.

189

CI

Fe

C! BF3.Et20

+ ~

AIC~

+

=

OCH3

1

2a

2b

J[

"J

"1

3a

31)

Figure 8. Synthetic routes for S-D dyads with a rigid spacer consisting of a bicyclo[ 2.2.2 ]octan e.

3. METHODS 3.1. SPMs, f l u o r e s c e n c e microscopy, and SIMS of HC-FC phase-separated m i x e d monolayers HC-FC phase-separated mixed monolayers of a partially fluorinated anionic surfactant (PFECA, Asahi Glass Co.) and stearic acid (SA) containing a cationic surfactant, i.e. octadecyltrimethylammonium chloride (ODTMAC) or the cationic cyanine dye (CD), were formed on an aqueous subphase containing 0.04 mM PVA (monomer unit) and 0.05 mM NaHCO 3 in a Langmuir trough equipped with an electronic microbalance and a glass Wilhelmy plate (Kyowa Interface Science). The mixed monolayers were deposited on oxidized Si(100) wafer [21], slide glass, and cover glass plates for SSPM, fluorescence microscopy, and SNOAM, respectively. The mixed monolayer deposited on the Si(100) wafer was also used for SIMS. The temperature for LB film depostion was 15 ~ SPMs such as AFM, FFM, and SSPM were performed with a SEIKO SPA-300 unit together with an SPI-3700 control station. Details for fluorescence microscopy by SNOAM based on a modified SEIKO SPA-300 unit with an SPI3700 control station were reported previously [27-33]. Conventional fluorescence microscopy was carried out with a Nikon XF-EFD2 fluorescence microscope [40]. SIMS was performed with a Perkin Elmer PHI model 6600 SIMS system with a Ga liquid metal ion source (beam diameter: ca. 80 nm). For mapping of F" negative secondary ions, a width of ca. 50 ~m was scanned with 256 lines.

190 3.2. SSPM of unidirectionally oriented A-S-D triads in a l t e r n a t e monolayers for detection of photo-induced charge separation LB films with the A-S-D triad for charge separation measurements were deposited by an San-esu Keisolm alternate deposition system. As the subphase, an aqueous solution containing 0.3 mM CaC12 and 0.05 mM NaHCO 3 at 15 ~ was used. The LB films resulted in the structures of type a and type b as shown in Figs. 3a and 3b, respectively. The substrate electrodes were prepared by gold vepor deposition on slide glass plates in ultra high vacuum [41,42]. The substrate electrodes were first covered with 3 and 5 layers of calcium o~-tricosenoate (CAT) monolayers for type a and type b, resepectively. Then, 15 alternate bilayers consisiting of a triad-CaT (1:5) mixed monolayer and a pure CaT monolayer were deposited f m ~ e r with the D tails towards the air and towards the substrate electrode in type a and type b, respectively. In type a, another bilayer of pure CaT was deposited on the active bilayers to stabilize them. For comparison, similar LB films containing A-S dyads in place of the A-S-D triads were also prepared. As a blank, the substrate covered with 35 pure CaT monolayers was prepared. For UVvisible absorption spectroscopy, LB films with the same structures as those for SSPM were deposited on quartz plates without gold. For improvement of type b deposition on quartz, quartz plates were chemically modified with octadecyltrichlorosilane [25,26,43] in place of the first CaT monolayer. The UV-visible spectra of the LB films and ethanol solutions were recorded by a Hitachi UV 220A spectrophotometer. SSPM of the samples described above was performed with a modified SEIKO SFA-300 AFM unit with an SPI-3600 control station described previously [34]. The sample was placed on a quartz prism installed on a tube scanner of the AFM unit. Immersion oil (R. P. Cargille Laboraotories Inc., Cedar Grove, NJ, USA) with almost the same refractive index as quartz was used between the sample plate and the prism surface to avoid light scattering. The LB film on the quartz plate was irradiated from the back side by a Kimmon He-Cd laser beam of 441.6 run in a total reflection mode as shown in Fig. 4. Photo-induced surface potential change was recorded under step-functional irradation of the laser beam by using a component of Maxwell stress detected by a cantilever with a conducting tip of SSPM under a constant 2o~-component condition [34].

4. PHASE-SEPARATED HC-FC MIXED MONOLAYERS 4~1. Phase separation in polyion complexed mixed monolayers containing HC cationic a n d anionic s u r f a c t a n t s and a FC ~nionic s u r f a c t a n t studied

by SPMs and SIMS To clarify the effect of addition of a cationic HC surfactant on phase separation behavior in the mixed monolayers of anionic HC and FC surfactants polyion complexed with cationic polymers, the mixed monolayers containing three omphiphilic components complexed with PVA were transferred on various substrate plates and studied by AFM, FFM, SSPM, and SIMS. As a cationic surfactant, ODTMAC was examined.

191

Figure 10. (a) AFM, (b) FFM, and (c) SSPM images of an SA-PFECA-ODTMAC (1:1-1) mixed m.onolayer polyion complexed with PVA deposited on Si(100).

In Figs. 9 and 10 are shown AFM, FFM, and SSPM images of mixed monolayers of SA-PFECA (1:1) and SA-PFECA-ODTMAC (1:1:1), respectively. The AFM and FFM images were recorded simultaneously on the same surface areas, while the SSPM images were recorded on the different areas from those for AFM and FFM but on the same samples. It was found from comparison of these images that the average size of the HC islands was increased dramatically, but height difference between the HC islands and the FC sea level was unchanged by addition of the cationic HC surfactant. The latter fact immediately leads a conclusion that the

192 p h a s e separation proceeded into an on-top structure [2,5,21,22] even in the presence of ODTMAC. Schematic illustrations of a side-by-side and an on-top strucutre are shown in Figs. l l a and llb, respectively. Recently, we found p h a s e separation into a side-by-side structure in the mixture of trichlorosilanes with a HC or a FC chain [25,26,43]. The result suggests t h a t a hydrocarbon part of polymer cations plays a role of hydrophobic layers. In other words, the polyion complexed monolayers can be regarded as a kind of ready-made paired-bilayers. Taking the previous pH effect on the phase separation behavior [21,24], it is most likely t h a t the increase in the size of the islands by addition of the cationic surfactant can be interpreted in terms of contribution of electrostatic force to the phase separation energetics. Details of the effect is now under investigation.

!:i:!:i:i:i:i:i:!:!:i:i:!:!:i:!:!:i:!:i:i:i:i:i:!:i:i:!:i:!:iSi(100)!:!:i:i:!:!:!:i:i:i:iiiii!i!iiiii!iiiiiii:ii!ii!ii!iiiii!i::!iii:::::

(a)

:.: :: : . - : .: :. : :.- ..... i :: : . :: : 9: . " .:Si(100). :::: 9

.

.

.

-.

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:

..

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9

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..

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.

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:

.

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.

9

.

.

.

.

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.

.

.

.

.

(b) Figure 11. Schematic illustration of (a) a side-by-side and (b) an on-top structure of HC-FC mixed monolayers. Dark squares stand for HC tails of a HC amphiphile and a HC main chain of a cationic polymer, and open squares stand for FC tails of a FC amphiphile.

193

Figure 12. Lateral distribution of F" negative secondary ion intensity measured by scanning SIMS of the same SA-PFECA-ODTMAC (1:1:1)mixed monolayer as that shown in Fig. 10.

Nevertheless, the increase in the average size enabled us to perform conventional local analyses of the phase separated monolayers. From the view point of lateral resolution, we examined Auger electron spectroscopy (AES) and SIMS [3,5]. To lower sample damage by AES, an electron beam with 3 keV and 0.2 nA was used, but an Auger peak due to F (647 eV) was not detected probably due to the damage. Contrary, mapping of F was successfully achieved by measuring lateral distribution of F" negative secondary ion intensity by scanning SIMS as shown in Fig. 12. It is clear from the SIMS image that the F" intensity is much stronger in the sea area than in the islands. This SIMS result again confirms the assignment of the sea area as the FC phase. If an island part were removed by ion sputtering, the FC phase underneath the island would exposed and the presence of F would be detected by SIMS. The sputtering was not attempted, however, and instead an island was removed by the AFM tip to confirm the presence of the FC phase underneath the island by SSPM. As we would expect, the exposed surface after scratching showed the same surface potential as that of the FC sea area. This result consistents with the previous results obtained by FFM [21]. If each surfactant does not dissolved into the other phase, e.g. the HC surfactant into the FC phase, the smaller total area of the HC islands than the FC sea area in the SA-PFECA-ODTMAC (1:1:1) mixed monolayer cannot be rationalized, because the molar ratio of HC to FC surfactants is 2:1 and the molecular area of PFECA is less than twice of those of SA and ODTMAC. The AFM image in Fig. 10 suggests substantial dissolution of HC surfactants into the

194 underneath FC sea phase. The SIMS image in Fig. 12, however, indicates that PFECA does not dissolved into the HC islands so much as the HC surfactants into the FC phase. The dissolution of HC into the FC phase was confirmed by the careful SSPM study of the three component mixed monolayers [44]. It is interesting to note from the FFM images in Figs. 9b and 10b that, in spite of the partial dissolution, the FC sea area again shows higher friction than the HC islands in the same way as those observed previously for two surfactant components systems [20-24].

Figure 13. (a) AFM and (b) FFM images and (c) a conventional fluorescence micrograph of a mixed monolayer of SA-PFECA-CD (1" 1"1/40) polyion complexed with PVA deposited on a slide glass plate.

4.2. Fluorescence microscopy of phase separated mixed monolayer by a convnetional fluorescence microscope and SNOAM In the next step, a small amount of CD was added to see whether this HC fluorescence dye is preferentially dissolved in the HC island phase. In Fig. 13 are shown F M and FFM images and a conventional fluorescence micrograph of almost the same region of a mixed monolayer of SA-PFECA-CD (1:1:1/40) polyion

195 complexed with PVA. Size of islands changes from place to place on the.same monolayer, but on average the size was again larger than that of the SA-PFECA mixture. In Fig. 13, the sizes of islands range from 2 to 4 ~m in diameter. Friction in the FC sea again is higher than that observed in the HC islands. The fluorescence micrograph in Fig. 13c sb~ws that fluorescent CD with two long HC chains is preferentially dissolved in the HC islands, although CD is also partially dissolved in the FC sea area as is clear from the appreciable fluorescence intensity.

Figure 14. (a) A cyclic contact AFM and (b) a fluorescence micrograph measured simultaneously with SNOAM of a mixed monolayer of SA-PFECA-CD (1:1:1/100) polyion complexed with PVA deposited on a cover glass plate.

When CD concentration was decreased from 1/40 to 1/100 in the SA-PFECACD mixed monolayers, the size of islands was decreased on average. A typical cyclic contact AFM and and a fluorescence SNOAM image of the mixture simultaneously recorded are shown in Figs. 14a and 14b, respectively. In the AFM image obtained by cyclic contact (i.e. tapping) mode, topographic image between the islands and the sea is reversed in contrast with those obtained by AFM in contact mode. Although difference in elastic properties between these two areas may be responsible to the reversal, origin of the change in contrast has not been clarified yet and is now under intensive study. The fluorescence SNOAM image in Fig. 14b clearly shows that the present SNOAM can be used to observe fluorescence images of islands less than 0.3 ~tm. In other words, the SNOAM can be used to irradiate a single island domain. The image also confirms the preferential dissolution of CD in the HC islands. It is concluded from these observations that the cationic A-S-D triad for charge separation will be preferentially dissolved in islands consisting of light harvesting pyrene derivatives

[45].

196 5. O R I E N T E D A-S-D TRIADS 5.1. SSPM of a l t e r n a t e LB films containing unidirectionally o r i e n t e d A-S-D triads One of our final goals of the present molecular photodiodes is that detection of the photo-induced surface potential change in a single HC island domain of an artificial photosynthetic reaction center embedded in the FC sea by SSPM as illustrated in Fig 4. The reaction center island consisting of a few A-S-D triads and a lot of H antenna molecules can also sit on the FC sea in the on-top structure (Fig llb). Prior to this goal, we examined recently [3] whether charge separation in triads unidirectionally embedded in alternate monolayers in LB films in Fig 3 can be detected as the surface potential change by SSPM as we would expect from orientational directions. In Fig. 15 are shown UV-visible absorption spectra of type a and type b of 15 bilayers consisting of an alternate A-S-D triad-CaT (1:5) and a pure CaT monolayer and type a of 15 bilayers containing A-S dyads in place of the triads. The LB films of type a were deposited on quartz plates, but the LB film of type b on a chemically modified quartz plate by octadecyltrichlorosilane. An absorption peak around 450 nm can be assigned to the acylated perylene moieties for the dyad and

I

atYpe A'-S'D b type A-S-D a type A-S

0.2

,

,

?,

,

I

'/3 I

"

O

<0.1 ~ ~l

'\.l

i

200

I

\'~ 300

400

500

600

Wavelength/nm Figure 15. UV-visible absorption spectra of types a (solid line) and b (dashed line) of 15 alternate bilayers of a triad-CaT (1:5) and a pure CaT monolayer and type a of 15 alternate bilayers containing dyads in place of triads (dot-dash line).

197 the triad, but peaks observed between 200 and 300 rim are assigned to absorption by the A, S, and D moieties [4]. Substantial difference in absorbances in this wavelength region between the triad and the dyad can be interpreted by the absence of the diacylated ferrocene D moiety in the dyad.

o.8 I

0.6

A-S-~I

-DAFc

0.4

0.2

0 200

300

400

500

600

Wavelength/nm Figure 16. UV-visible absorption spectra of A-S-D triad (solid line) and DAFc (dashed line) in 10 ~M ethanol solutions.

A UV-visible absorption spect~lm of a 10 ~M ethanol solution of a model compound (DAFc in Fig. 5) for the D moiety is shown together with that of the A-SD triad in Fig. 16. The absorption bands between 200 and 300 nm can be clearly seen for DAFc. Qualitatively, absorption spectra for A-S-D triads in the LB films and in the 10 ~M ethanol solution are similar. It is interesting to note, however, that the relative intensities of the acylated perylene band around 450 nm against the 200-300 nm UV absorption bands are different between the LB films and the ethanol solution. This difference can be attributed to orientation of the perylene moiety in the LB films in the same way as in the antenna LB films reported previously [38]. The change in surface potentials of the LB films of types a and b under step illumination with He-Cd laser light of 441.6 nm was measured by SSPM with a gold

198 coated Si3N 4 tip and the results are shown in Fig. 17 together with t h a t of a blank LB film. As shown in Figs. 17a and 17b, the increase and the decrease in the surface potentials were observed continuously during illumination for ca. 2 s on the LB

-.,---ON 10

o

(a)

~-

20

~

=~

30

~ ~

40

/

/

/

./

60 70

0

eo

~. (b)

-70

I\.~_.--OFF

-60

[\

I

I

20 30 Time / s

I

40

I

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~=~ -40

.~---.

._~ -30

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t"

./ON

i

I

!

40

50

20 30 OFF Time/s

40

50

0 -~, 0 ~10

u

20 30 Time. / s

.-4

Figure 17. The change in the surface potential by step illumination for ca. 2-3 s with a He-Cd laser of 441.6 nm in a total reflection mode on (a) a type a alternate and (b) a type b alternate LB film containing 15 unidirectionally oriented A-S-D triad monolayers, and (c) a black LB film.

199 films containing the A-S-D triads in type a and type b, respectively, but the created surface potentials decayed almost exponentially to the original surface potentials within several ten seconds. The potentials created varied from place to place even on the same sample, but the direction was always the same. Namely, on type a was observed a positive change in the surface potential corresponding to the increase in dipole moments in the triads whose positive ends were d i r e ~ towards the air, while the opposite surface potential change was observed on type b. On the blank LB film, much smaller changes v ~ from place to place were observed, but almost always the shi~ was positive in the same direction as that for type a. The gradual changes during the illumination correspond to the increase in the number of the charge separated species, while the exponential decays after shutting the illumination down reflect the decrease in the number of the separated charges by recombination. If the charge separation and recombination proceed only within the triads, i.e. via intramolecular processes, it seems that the observed recombination rates were too slow in comparison with the intramolecular recombination rates for folded type triads determined by a transient photocurrent measurements with a transmission line with a time resolution of 25 ps [46]. Even if we take the difference between the present linear triads and the folded type triads for the transient methods into account, the difference in decay times between ca. 10 s and ca. 100 ns is too large. Therefore, the much slower recombination can be interpreted by ass**ming further charge separation via the mechanism by lateral diffusions of photocreated radical anions and cations as illustrated in Fig. 18.

o il o

Dll o+ slow

.

[ 8

i

l, _!:

....

......

9

Figure 18. Schematic illustration of slow charge recombination via lateral diffusion of electrons and holes in the A and the D layers, respectively, in the A-S-D triad monolayer. Radical anions and cations on A and S moieties were created by photoexcitation of the S moieties followed by the charge separation.

200 5.2. S S P M of a l t e r n a t e LB films

u n i d i r e c t i o n a l l y o r i e n t e d A-S

containing

dyads

To confirm the long-lived charge separation by the lateral diffusion mechanism, we also measured the surface potential change on alternate LB films containing AS dyads similar to types a and b. To increase singnal intensities, both types of LB films contained 30 bilayers of the alternate monolayers. As shown in Figs. 19a and -10 ON

,

~

't2 ffl

(a)

.~_ _

F

IDA

0

-10

i

,

l

10

-f.,

{D 0 t~

(b) .~__ ~ ~ 10 x::N

Time / s

I

I

I

I

1 20

t

20

!

30

OFF

F':,~

{/3

!

ON

0,",

i

0

I

10

1

Time / s

20

I

30

-10 ID s

-c: ID

(c)

0

.=--

(Dt~

OFF 0

_

I

I 10

I

Time / s

1 30

Figure 19. The change in the surface potential by step illumination for ca. 2 s with a He-Cd laser of 441.6 nm in a total reflection mode on (a) a type a alternate and (b) a type b alternate LB film containing 30 unidirectionally oriented A-S dyad monolayers, and (c) a corresponding black LB film.

201 19b, it is clear from comparison with the change observed on a corresponding blank (Fig. 19c) that, even on the LB films of unidirectionally oriented A-S dyads, the photo-induced surface potential changes were clearly observed with signs expected from the orientations. Since the intramolecular recombination of the dyad can be expected to proceed in a ns time scale [2], the observed change in the surface potentials and the slow decays are again rationalized in terms of the lateral charge separation mechanism shown in Fig. 20. The charge separation via the lateral diffusion mechanism is expected to be enhanced by the increase in concentrations of the dyad and the triad in the monolayers, but the effect of concentration will be studied in the near future.

!

~

r e c o m b i n a f i o n ~ e- V'x~_ ,

!

hv

Figure 20. Schematic illustration of slow charge recombination via lateral diffusion of electrons and holes in the A and the S layers, respectively, in the A-S dyad monolayer. Radical anions and cations on A and S moieties were created by photoexcitation of the S moieties followed by the charge separation.

6. S-D (Np-Fc) DYADS 6.1. S-D dyads with a rigid s p a c e r As described above, the arrangement of the various fimctional moieties was controlled spatially across the films at molecular dimensions in the form of LB films. In a series of folded type of sensitizer (S) and electron-donor (D) dyads in a previous work, however, the dyad molecules in the LB films can take m a n y conformations due to flexibility of the longer alkyl chain of the dyads so that clear dependence of the photoinduced electron transfer rate on the alkyl chain length, i.e. S-D distance, was not observed [2]. By this reason, we are studying the chain length dependence by using a series of linear type S-D dyads, in which the S and D moieties were linked by a single alkyl chain. In the closely packed LB films, the alkyl chain was considered to be extended and the distance between S and D to be

202 controlled merely by changing the alkyl chain length [36]. In order to confirm the reasonable rates of electron transfer in such ideal S-D systems, it is interesting to have an standard S-D dyad in which these chromophores were separated at a fixed distance. In addition, precise comparison of the photoinduced electron transfer quenching rates between S-D dyads linked with either an extended four-carbon alkyl chain spacer (Figs. 6 and 7) or a rigid bicyclo[2.2.2] unit with almost the same spacer length (Figs. 6 and 8) may provide us with knowledge of the effect of the number of paths in the through-bond mechanism of electron transfer. For this purpose, as described in section 2, bicyclo[2.2.2]octane was used as a rigid spacer between two chromophores [47-50] that satisfy the requirements mentioned above. The resulting rigid dyad molecule contains a naphthalene and a ferrocene moiety, as S and D, respectively. Friedel-Crafts alkylation was used for attaching these functional moieties directly to the bicyclooctane spacer as illustrated in Fig. 8. Selective alkylation was achieved by altering the alkylation catalyst, because boron trifluoride was known to act more effectively toward ethers and alcohols than toward halides [51]. As we would expect, the bicyclo[2.2.2]octane possessing both ether and halide groups reacted selectively with naphthalene and ferrocene at different bridgehead positions by making use of the appropriate catalysts. This method will be applicable widely to prepare the rigid compounds containing two kinds of aromatic moieties as their functional groups. S-D dyads 3a and 3b were synthesized by the reaction sequence shown in Fig. 8. 1-Chloro-4-methoxybicyclo[2.2.2]octane 1 was synthesized from 4-methoxybicyclo[2.2.2]octane-l-carboxylic acid [52] by adopting the procedure of Grob et al. [53]. Using a boron trifluoride ethyl ether complex as a Lewis acid catalyst, the Friedel-Crafts alkylation of naphthalene with 1 afforded regioisomeric naphthyl compounds 2a and 2b. The Rigid S-D dyads 3a and 3b were obtained from 2a and 2b by Friedel-Crafts alkylation of ferrocene using all~mi,mm chloride as a catalyst. For the reference compounds of the rigid S-D dyads, methyl group was used instead of ferrocenyl donor moiety. These reference compounds were obtained from 1methoxy-4-methylbicyclo[2.2.2]octane [54] by Friedel-Crafts reaction with naphthalene catalyzed by the boron trifluoride diethyl ether complex. 1H NM:R spectra revealed that the products by boron trifluoride were the mixture of two regioisomers linked at 1 and 2 position of the naphthyl moiety. The reference compound mixture could be separated into two pure isomers by using reversed phase HPLC. On the other hand, separation of the S-D dyad mixture was difficult because the ferrocenyl moiety of the dyad is easily decomposed by oxygen in water that is one of the solvent generally used for reversed phase HPLC. The reversed phase HPLC without oxygen is now under investigation. Therefore, separation of the S-D dyads was carried out with normal phase HPLC, although it is not an effective method for the separation of such a mixture of regioisomers. Two fractions of different molar ratio of isomers (1-S-D : 2-S-D = 1:1 and 2:1) were obtained with the HPLC. The ratios of the 1- and 2-naphthyl isomers in these fractions were determined quantitatively by 1H NMR [51]. The absorption spectra of the mixed S-D dyads (1:1) and the reference compound (2-S) in cyclohexane are shown in Fig. 21 together with the absorption

203

spectrum of butylferrocene (Bu-Fc). The superposition of the spectra of 2-S and Bu-Fc is almost identical to the spectrum of S-D. The emission spectrum of the SD dyads in cyclohexane was similar to those of the reference compounds, but the intensity of the former was two orders of magnitude lower than that of the latter as shown in Fig. 22. These spectral data indicate that the n-electronic structure of the naphthyl moiety was not changed by introduction of the ferrocene moiety, while the excited state of the naphthyl moiety was quenched effectively by the ferrocene donor moiety. A study of the dynamics of the S-D dyads and the reference compounds, using picosecond pulsed laser light, is currently in progress.

0.4 I

(a)

0.3 I to c

"0

u)

0.2

,<

(c)

0.1

0.0

240

260

280 300 ;L/nm

320

Figure 21. Absorption spectra of (a) S-D dyads (3a:3b = 1:1), (b) 1-methyl4-(2-naphthyl)bicyclo[2.2.2]octane (2-S), and (c) butylferrocene (Bu-Fc) in cyclohexane.

340

204

(a)

1 100

(b)

o==,

rr

m

c

,,,,..=,

n,-'

300

340

;L/nm

380

420

Figure 22. Fluorescence spectra of (a) 1-methyl-4-(2-naphthyl)bicyclo[2.2.2]octane (2-S) and (b) S-D dyads (3a:3b = 1:1) in cyclohexane.

6.2. Stable monolayers of S-D amphiphilic dyads with a single alkyl chain spacer containing a naphthalene and a ferrocene moiety In LB films [1], the arrangement of the various functional moieties is considered to be controlled spatially across the films at molecular dimensions as described above. As one of functional moieties, various polycyclic aromatic hydrocarbons were used [55,56]. Some of the compounds were mixed with arachidic acid to form stable monolayers [37], because the amphiphilc derivatives of aromatic hydrocarbons themselves often form unstable monolayers on the subphase [40]. For preparation of stable monolayers of polycyclic aromatic amphiphiles, Steven et al. investigated the effects of the length of alkyl chain and the composition and pH of the subphase [57].

205 Further alkylation of a pyrene moiety of 10-(1-pyrene)decanoic acid or 6-(1pyrene)hexanoic acid also stabilizes their Langmuir monolayers [38] because these dialkylated pyrene derivatives have lower melting points in addition to their higher hydrophobicity than those of the corresponding monoalkylated derivatives [40]. The lowering in the melting points due to mixture formation of regioisomers by dialkylation resulted in suppression of crystallization of the dialkylated Amphiphiles in the monolayer. Shimomura and Kunitake have reported that stable monolayers and LB films were obtained by electrostatic interaction of water soluble anionic polymers with cationic amphiphiles [58]. This polyion-complexation was also a useful method for stabilization of monolayers of unstable [59] or water soluble anionic surfactants [60]. Mixtures of water soluble cationic and anionic surfactants (1:1) also formed stable Langmuir monolayers at the air / water interface [60]. In this work, stabilization of Langmuir monolayers of newly synthesized S-D dyads and their reference compounds shown in Figs. 6 and 7 was studied. In these S-D dyads and reference compounds, a naphthalene and a ferrocene moiety act as the sensitizer and the electron-donor moiety, respectively. We can estimate the photoinduced electron transfer rate between a naphthalene and a ferrocene moiety bridged by a rigid bicyclo[2.2.2]octyl spacer from comparison of the fluorescence intensities between the S-D and the reference 2-S compound in Fig. 22 and the fluorescence lifetime of 2-S. To clarify the effect of the number of paths in the through-bond mechanism of electron transfer, it is interesting to know the corresponding photoinduced electron transfer rate between the same sensitizer and electron-donor pair bridged by a four-carbon single alkyl chain with almost the same distance as described above. The single alkyl chain spacer between these functional moieties is considered to be extended in closely packed LB films [2]. Thus, if stable LB films of the S-D amphiphilic dyads are obtained, the rates of photoinduced intramolecular electron transfer of the S-D dyads can be measured and will be compared to the rates with the S-D dyads bridged by a rigid bicyclo[2.2.2]octyl spacer. To obtain stable LB films containing the S-D and the reference S amphiphilic compounds, stability of Langmuir monolayers of these anionic compounds at the air / water interface was investigated [36] in terms of i) structure of the amphiphiles, ii) kinds of counter cations, such as Ca 2+ and a cationic polymer, iii)pH of the subphase, and iv) 1:1 mixed monolayer formation with a cationic surfactant. As a cationic polymer and a cationic amphiphile, poly(allylamJne hydrochloride) (PAA) and octadecylamine (ODA) shown in Fig. 6 were used, respectively. The stability of the monolayers of the anionic amphiphiles was increased by polyioncomplexation with PAA added in the aqueous subphase in comparison with Ca 2+ salt formation. Ion complexation (1-1) of each anionic amphiphile with ODA was also performed at the air-water interface by spreading a chloroform solution of a 1:1 surfactant mixture. From comparison of the ~-A isotherms obtained under the various conditions, it can be summarized that the 1:1 ion complexation of the anionic amphiphiles with the cationic surfactant can be used to form their stable monolayers. Especially, when the amphiphile is so soluble as S7COOH, the ion complexation with a

206 cationic surfactant is more effective than the polyion-complexation in order to suppress the dissolution. In other words, the ion complexation method by mixing with an amphiphile with a head group having opposite charge can be applied to other anionic or cationic surfactants to form their stable monolayers. In addition, the ion complexation is more advantageous than the polyion-complexation, when no additives in the subphase are required, because it is not necessary to add other cations in the subphase and pure water can be used as the subphase. The transfer of the stable monolayers onto substrate plates and the comparison of the photoinduced electron transfer rates between S-D dyads with a rigid and a single alkyl chain spacer is now under investigation. 7. CONCLUSION The sizes of the phase separated H C islands in the mixed monolayers of H C and F C anionic amphiphiles polyion complexed with cationic polymers were increased by addition of a H C cationic amphiph/le. The H C islands sat on the F C sea in a structure of on-top in a two story monolayer in the same way as two-component amphiphile systems. By S S P M and conventional and S N O A M fluorescence microscopies, the cationic H C amphiphile was found to dissolve preferentiallyinto the H C islands, although it also dissolved partiallyin the F C sea phase. The charge separation under laser light illumination was observed by S S P M on the L B films containing multilayers of alternate triad (or dyad) and inactive fatty acid monolyers. The signs of observed dipolem o m e n t changes were in accordance with the direction of charge separation in the unidirectionally oriented triad (or dyad) in the L B films. The slow decays of recombination of the separated charge in the L B films were interpreted in terms of the lateral diffusion of electrons and holes, among the A and the D (or S) moieties, respectively. S N O A M and SSPM, respectively, was found to be used as optical and electronic access tools from the macro-scale external systems to nano molecular devices. N e w S-D dyads were also synthesized. The dyads contain a naphthalene and a ferrocene moiety as the S and the D functional group. These two functional groups were linked with either a single four-carbon alkyl chain spacer or a rigid bicyclo[2.2.2] unit with almost the same spacer length. Distance between the S and D moieties is fixed in the S-D dyads with a rigidspacer, while that m a y change in the S-D amphiphiles with a single alkyl chain spacer. To fix distance between the S and D moieties in the S-D amphiphil/c dyads, it is important to form stable monolayers containing the S-D amphiphiles, because an alkyl chain for linkage is considered to be extended in the stable monolayers or L B films. To stabilize the monolayer, polyion complexation with polymer or ion complexation with an amphiphile having opposite charge was examined. Stabilized monolayers of the SD amphiphiles were formed by using these ion complexation methods. If the stable monolayers can be transferred onto the substrate plates by the L B method, the photo-induced electron transfer rates in the S-D dyads with a single alkyl chain spacer will be determined and compared with those for the S-D dyads with a rigid spacer. The results m a y provide us with knowledge of the effectof the number of paths in the through-bond mechanism of electron transfer.

207

Acknowledgments This research was supported by a Grant-in-Aid on Priority Areas (02204019), that for Scientific Research (06403020), and that on New Programs (03NP0301) from the ministry of Education, Science, and Culture and by a NEDO International Joint Research Grant for Pico-transfer team, 1992-1995.

REFERENCES 1. A. Ulman,Ultrathin Organic Films, Academic Press, San Diego, 1991. 2. M. Fujihira in A. Ulman (ed.), Thin Films, Volume 20, Academic Press, Boston, 1995, pp. 239-277; M. Fujihira, Advances in Chemistry Series, 240 (1994) 373. 3. M. Fujihira, 2]lin SolidFilms, 273 (1996), inpress. 4. M. Fujihira, M. Sakomura, and T. Kamei, 2~dn Solid Films, 180 (1989) 43; M. Fujihira, Mol. Cryst. Liq. Cryst. 183 (1990) 59. 5. M. Fujihira in H.-J. Gtmtherodt, D. Anselmetti, and E. Meyer (eds.), Forces in Scanning Probe Methods, NATO ASI Series, Series E: Applied Sciences, Vol.286, Kluwer Academic Publishers, Dordrecht, 1995, pp.567-591. 6. J. Frommer, Angew. Chem., Int. Ed. 31 (1992) 1298. 7. J.P. Rabe,Ultr~microscopy, 42-44 (1992) 41. 8. W. lVLHeckl,Thin Solid Films, 210/211 (1992) 640. 9. G. Binnig, H. Rohrer, C. Gerber, and E. Weibel, Appl. Phys. Lett., 40 (1982) 178. 10. G. Binnig, C. F. Quate and C. Gerber, Phys. Rev. Lett., 56 (1986) 930. 11. C. lVLMate, G. M. McClelland, R. Erlandsson, and S. Chiang, Phys. Rev. Lett., 59 (1987) 1942. 12. G. Meyer and N. M. Amer, Appl. Phys. Lett., 57 (1990) 2089. 13. O. Marti, J. Colchero, and J. Mlynek, Nanotechnology, 1 (1990) 141. 14. M. Nonnenmacher, M. O'Boyle, and H. K. Wickramsinghe, Appl. Phys. Lett., 58 (1991) 2921. 15. M. Nonnenmacher, M. O'Boyle, and H. K. Wickramsinghe, Ultramicroscopy, 42-44 (1992) 268. 16. Near Field Optics, D. W. Pohl and D. Courjon (eds.), NATO ASI Series E, Vol. 242, Kluwer Academic Publishers, Dordrecht, 1993. 17. Near Field Optics, M. Nieto-Vesperinas (ed.), NATO ASI Series, Kluwer Academic Publishers, Dordrecht, 1996, in press. 18. M. Fujihira and H. Takano, J. Vac. Sci. Tech., B12 (1994) 1860. 19. H. Takano andM. Fujihira, Thin Solid Films, 273 (1996), in press. 20. R. M. Overney, E. Meyer, J. Frommer, D. Brodbeck, R. Lathi, L. Howald, H.-J. G~nthemdt, M. Fujihira, H. Takano, and Y. Gotoh, Nature, 359 (1992) 133. 21. E. Meyer, R. Overney, R. L~thi, D. Brodbeck, L. Howald, J. Frommer, H.-J. G~mthemdt, O. Wolter, M. Fujihira, H. Takano, and Y. Gotoh, 2 ~ n Solid Films, 220 (1992) 132. 22. M. Fujihira and H. Kawate, ~ Solid Films, 242 (1994) 163.

208 23. M. Fujihira and H. Takano, 2bin Solid Films, 243 (1994) 446. 24. R. M. Overney, E. Meyer, J. Frommer, H.-J. Gtmthemdt, M. Fujihira, H. Takano, andY. Goto, L ~ u i r , 10 (1994) 1281. 25. T. Arai, D. Aoki, Y. Okabe, and M. Fujihira, 2 ~ n Solid Films, 2 73 (1996), in press. 26. M. Fujihira, D. Aoki, Y. Okabe, H. Takano, H. Hokari, J. Frommer, Y. Nagatani, and F. Sakai, submitted. 27. M. Fujihira, H. Monobe, H. Muramatsu, and T. Ataka, Chem. Lett., (1994) 657. 28. H. Muramatsu, T. Ataka, H. Monobe, and M. Fujihira, Ultramicroscopy, 57 (1995) 141. 29. M. Fujihira, H. Monobe, H. Muramatsu, and T. Ataka, T. Ultramicroscopy, 57 (1995) 118. 30. H. Muramatsu, N. Chiba, K. Homma, K. Nakajima, T. Ataka, S. Ohta, A. Kusumi, and M. Fujihira, Appl. Phys. Lett., 66 (1995) 3245. 31. M. Fujihira, Near Field Optics, M. Nieto-Vesperinas (ed.), NATO ASI Series, Kluwer Academic Publishers, Dordrecht, 1996, in press. 32. M. Fujil~a, H. Monobe, N. Yamamoto, H. Muramatsu, N. Chiba, K. Nakajima, and T. Ataka, Ultramicroscopy, (1996), in press. 33. M. Fujihira, L. M. Do, A. Koike, and E. M. Han, Appl. Phys. Lett., 68 (1996), in press. 34. M. Fujihira, H. Kawate, and M. Yasutake, Chem. Lett., (1992) 2223. 35. M. Fujihira and H. Kawate, J. Vac. Sci. Tech., B12 (1994) 1604. 36. H. Hokari and M. Fujihira, 2bin Solid Films, 273 (1996), in press. 37. M. Fujihira and H. Yamada, 2~dn Solid Films, 179 (1988) 125. 38. M. Fujihira, T. Kamei, M. Sakomura, Y. Tatsu, and Y. Kato, Thin Solid Films, 179 (1989) 485. 39. H. Hokari and M. Fujihira, J. Chem. Soc., Chem. Commun., (1995) 2139. 40. M. Fujihira, K. Nishiyama, Y. Hamaguchi, and Y. Tatsu, Chem. Lett., (1987) 253. 41. M. Fujihira, K. Nishiyama, andH. Yamada, 2bin Solid Films, 132 (1985) 77. 42. M. Fujihira, J. Electroanal. Chem., 130 (1981) 351. 43. M. Fujihira, D. Aoki, Y. Okabe, and J. Frommer, to be submitted. 44. M. Fujihira and D. Aoki, unpublished results. 45. M. Fujihira, T. Kamei, H. Takano, and Y. Gotoh, 9th International Symposium on Surfactants in Solution, June 10-15, 1992, Varna, Bulgaria. 46. E. G. Wilson, ~ Solid Films, 273 (1996), in press: M. Fujihira, M. Sakomura, R. V. Sudiwara, K. J. Donovan, and E. G. Wilson, to be published. 47. D. Joran, B. A. Leland, G. G. Geller, J. J. Hopfield, and P. B. Dervan, J. Am. Chem. Soc., 106 (1984) 6090. 48. B. A. Leland, A. D. Joran, P. M. Felker, J. J. Hopfield, A. H. Zewail, and P. B. Dervan, J. Phys. Chem., 89 (1985) 5571. 49. A. D. Joran, B. A. Leland, P. M. Felker, A. H. Zewail, J. J. Hopfield, and P. B. Dervan, Nature, 327 (1987) 508. 50. L.R. Khudkar, J. W. Perry, J. E. Hanson, and P. B. Dervan, J. Am. Chem. Soc., 116 (1994) 9700.

209 51. H.E. Zimmerman, T. D. Goldman, T. K. Hirzel and S. P. Schmidt, J. Org. Chem., 45 (1980) 3933. 52. W. Adcock andA. N. Abeywickrema, J. Org. Chem., 47 (1982) 2951. 53. K.B. Becker, M. Geisel, C. A. Grob and F. Kuhnen, Synthesis, (1973) 493. 54. W. Adcock, A. N. Abeywickrema, V. S. Iyer, and G. B. Kok, Magn. Reson. Chem., 24 (1986) 213. 55. R. A. Harm, in Langmuir-Blodgett Films, G. Roberts (ed.), Plenum, New York, 1990, pp.17-92. 56. A.-F. Mingotaud, C. Mingotaud, and L. K. Patterson, in Handbook of Monolayers, Volumes 1 and 2, Academic Press, San Diego, 1993. 57. J . H . Steven, R. A. Harm, W. A_ Barlow, and T. Laird, ~ i n Solid Films, 99 (1983) 71. 58. M. Shimomura and T. Kunitake, ~ i n Solid Films, 132 (1985) 243. 59. K. Nishiyama, M. Kurihara, and M. Fujihira, Thin Solid Films, 179 (1989) 477. 60. 1VLFujihira and Y. Gotoh, in Nanostructures Based on Molecular Materials, W. C~pel and Ch. Ziegler (eds.), VCH Weinheim, 1992, pp.177-193.

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New Developments in Construction and Functions of Organic Thin Films T. Kajiyamaand M. Aizawa (Editors) 9 1996 Elsevier Science B.V. All rights reserved.

Nonlinear

characteristics

211

of thin lipid films

M. Makino" and K . Y o s h i k a w a b* "Graduate School of Environmental Health Sciences, University of Shizuoka, 52-1 Yada, Shizuoka 422, Japan bGraduate School of Human Informatics, N a g o y a University, N a g o y a 464-01, Japan

1. I N T R O D U C T I O N Living organisms maintain their activity by the dissipation of energy. In other words, non-equilibrium is the foundation of life phenomenon. It is true that recent rapid development of the research on the detailed chemical structure of individual biomolecules such as DNA and generally proteins gives us deeper understanding of the basis of life. However, if one decomposes a living either multicellular organism or single living cell into individual molecules, most of the important aspects of life will disappear. The thinking processes in brain, beating of the heart, cellular development, and other spatio-temporal life p h e n o m e n a can not be generated on a level of single molecules. Recently there has been an increasing interest in self-oscillatory phenomena and also in formation of spatio-temporal structure, accompanied by the rapid development of theory concerning dynamics of such systems under nonlinear, nonequilibrium conditions. The discovery of model chemical reactions to produce self-oscillations and spatio-temporal structures has accelerated the studies on nonlinear dynamics in chemistry. The BelousovZhabotinskii(B-Z) reaction is the most famous among such types of oscillatory chemical reactions, and has been studied most frequently during the past couple of decades [1,2]. The B-Z reaction has attracted much interest from scientists with various discipline, because in this reaction, the rhythmic change between oxidation and reduction states can be easily observed in a test tube. As the reproducibility of the amplitude, period and some other experimental measures is rather high under a found condition, the mechanism of the B-Z reaction has been almost fully understood until now. The most important step in the induction of oscillations is the existence of auto-catalytic process in the reaction network. t To whom correspondence should be addressed.

212 In other words, rapid increase in the concentration of the intermediate species plays the most significant role in the self-oscillatory phenomena. Although the detail mechanism of the B-Z reaction has been clarified, it may be unrealistic to consider the possibility that autocatalytic reaction of intermediate radicals directly concern with various self-oscillatory phenomena in living organisms including the beating of the heart, nervous excitations, circadian rhythm, rhythms in cell division, etc. About a decade ago, we p r o p o s e d a working hypothesis that most of the rhythmic phenomena and also spatio-temporal structure formation in biology could be related to nonlinear kinetic process via biomembranes [3,4]. Actually, we have found that various kinds of artificial membranes, such as oil/water system, porous membrane doped with lipid, bilayer lipid membrane, show self-oscillatory phenomenon [3-10]. It has been indicated that the oscillatory phenomenon in these artificial membranes exhibits highly nonlinear characteristics of lipid membrane. In this chapter we would like to discuss the dynamic aspects of thin lipid films, with the special attention to their nonlinear properties.

2. C O O P E R A T I V E A G G R E G A T I O N A T AN A I R / W A T E R I N T E R F A C E

OF A M P H I P H I L I C

MOLECULES

As for the mechanical response of thin lipid films, surface p r e s s u r e ( n ) surface area(A) characteristics of lipid monolayer at air/water interface have been well studied under quasi-static conditions. It has been established that different phases are observed for the ensemble of lipid molecules in a twodimensional arrangement, similarly to the gas, liquid, and solid phases and some other intermediate phases as in three-dimensional molecular assemblies. In the experiments on the n-A characteristics, it has been usually assumed that thermal equilibrium will be attained easily if the experiment is performed using a slow rate of compression of thin film at the interface. Measurements under thermal equilibrium are, of course, the necessary condition to obtain the p h y s i c o - c h e m i c a l properties of the individual "phase" of the lipid ensemble. Contrary to the accumulated knowledge on the static or quasi-static characteristics of thin lipid films at air/water interface, less attention has been paid to the dynamical or nonequilibrium behavior of the film. Studies on the dynamical characteristics of thin lipid films may be quite important, because the life phenomena are maintained under nonequilibrium conditions. According to the modern biochemistry [11,12], thin lipid membrane in living cells is not a rigid wall but a thermally fluctuating barrier with high fluidity. In the present section, we will show that thin lipid film exhibits the various interesting dynamical n-A characteristics, such as the "overshoot hump", the "zero surface pressure", and the "flat plateau".

213

2.1. Characteristic change of the surface pressure under compression Langmuir-Blodgett(LB) technique is a useful method to construct organic thin film with high degree of orientation of amphiphilic molecules [13, 14]. In recent years, the LB films have attracted much attention for their ability of producing new optical and/or electronic devices [15-17]. Amphiphilic molecules containing conjugated diacetylene group have been examined as surfactants and as possible candidates increase the electronic conductivity of the LB films. Therefore, the n-A isotherms were measured for many amphiphilic molecules containing conjugated diacetylene group [18-21] or containing both phenyl and diacetylene groups [22]. During the extensive studies on the quasi-static rc-A characteristics, an interesting phenomenon of "an overshoot hump" has been encountered in the n-A "isotherms" at the onset of the LE/LC coexistence phase during the compression process [18,22]. The overshoot hump has also been reported for the rt-A "isotherms" of other amphiphilic molecules such as the bipolar substance [23]. It has been suggested that the appearance of the overshoot hump is dependent on the molecular structure of lipids [20,21], the ionic composition in the subphase [21,24] and the compression rate [21,25]. The origin of the overshoot hump has been ascribed to various factors such as (i) polymerization of amphiphilic lipids [26], (ii) change in steric configurations [21], (iii) lack of nucleation for the growth of domains through "nonequilibrium" compression process [25], or (iv) a kind of collapse of the monolayer [22]. In spite of these extensive studies, definite conclusion seems not to have been obtained on the mechanism of the overshoot hump generation in the rc-A "isotherm". Here, we will describe the overshoot hump observed in the rc-A characteristics of lipid molecules containing both phenyl and diacetylene groups [22]. Figure 1 shows the chemical structure of the fatty acid (14-phenyl-9,11-tetradecadidynoic acid; PhDA2-8) examined in the present study.

Figure 1. Chemical structure of PhDA2-8

214

2.2.Effect o f s u b p h a s e on the o v e r s h o o t h u m p The x-A curves were measured with a trough equipped with a moving blade and a piezoelectric device (Figure 2). Both the trough (286 mm long and 70 mm wide) and blade were coated with Teflon. The subphase temperature was kept within + 0.1 ~ by use of a water jacket connected with a thermostated circulation system, and the environmental air temperature was kept at 18 0(2. The surface tension was measured with a Wilhelmy plate made of filter paper (25 x 25 x 0.25 mm) using a piezoelectric device. The surface p r e s s u r e ( n ) is defined as; x = TO- 'Y

(2-1)

where Y0 is the surface tension of the subphase with absolutely pure water and 7 is the surface tension of the lipid thin film covering the subphase [25].

/~(G)

A)

kLaj

ki j

Figure 2. Schematic representation of the experimental apparatus used for measurement of the x-A curves of a thin film of PhDA2-8 molecules at the air/water interface. (A) piezoelectric device, (B) filter paper(Wilhelmy plate), (C) trough, (D) blade, (E) arms, (F) water circulating outlet, (G) screws to fix the cover, (H) cover.

215

The whole experimental devise was sealed from the outer environment as s h o w n in Figure 2, in order to avoid contaminants such as air dust and oil materials. In the measurement of n - A curves, the blade was driven at a constant rate using pair of mechanical side arms equipped with magnet.s. PhDA2-8 exposed to air atmosphere was readily polymerized which was accompanied by a notable color change. In order to avoid the polymerization, the ~-A curves of the PhDA2-8 were measured within 24 h after preparation of the developing solution. The developing solution of PhDA2-8 was diluted to 1.6 mM concentration with chloroform, which had been purified by three consecutive distillations.

~50

- 40 m30 20

,

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,

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I

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~ 20 0

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10 20 30 40 50 60 Surface Area [ .~z/molecule] Figure 3. Effect of subphase on the n-A curve of PhDA2-8 film. The subphase temperature was 5.0~ and the compression rate was 7.5(/~2/molecule)/min. The subphase composition was as follows; (a) potassium bicarbonate (5x10SM) and cadmium chloride (3x10aM), (b) potassium bicarbonate (5x105 M) and magnesium sulfate (3x10-4M), (c) potassium bicarbonate (5x10SM) and sodium chloride (3x104M), (d) cadmium chloride (3x104M), (e) potassium bicarbonate (5x10SM), (f) pure water.

216

Figure 3 shows the n-A curves of the PhDA2-8 thin film at the air/water interface. A remarkable overshoot hump was observed in the ~-A curves ((a), (b) and (d)). It is noteworthy that the overshoot hump was clearly observed with the subphase containing both CdCI2 and KHCO3(curve (a)). Curve (a) s h o w s a steep rise of the surface pressure up to 20 mN/m. Just after the m a x i m u m surface pressure, the curve shows a rapid decrease to 0 mN/m (zero surface pressure). The pressure again begins to increase at around A= 15/~2/molecule, which corresponds about half of the area at the starting point of the increase of the pressure before the hump. Such a large o v e r s h o o t hump, or zero surface pressure, was observed only for the subphase containing both CdC12 and KHCO3. This result suggests that the stabilization effect of cadmium ion is enhanced in the presence of KHCO3 due to its buffering capacity of the subphase. 2.3.Effect o f r e c o m p r e s s i o n on the o v e r s h o o t h u m p F i g u r e 4 shows the recompression curves of a PhDA2-8 thin film. These curves were measured under following conditions. In the course of the firsttime c o m p r e s s i o n process the blade was stopped, and then the surface area of the initial area was expanded before continuing the compression. After 15 rain, r e c o m p r e s s i o n of the PhDA2-8 thin film was started at the same rate as in the first-time compression. The recompression curves (a)-(f) were obtained with temporal stoppage of the blade at points (a)-(f), respectively(upper right of Figure 4), during the first-time compression. As the stop points were shifted from (a) to (f), the recompression n-A curve migrated to the left as a whole, and the o v e r s h o o t hump tended to disappear. It has been suggested that the initially formed monolayer of PhDA2-8 molecules collapses to a multilayer when the surface pressure passes through the maximum point of the hump. This explanation is consistent with our observation, indicating that the PhDA2-8 molecules suffer some loss, possibly due to formation of the multilayers during the hump formation. In addition, we assume that the multilayers are formed, by an irreversible process, through an unstable state of the monolayer which existed during the hump. This idea is supported by the fact that the zero surface pressure was maintained as long as the blade was stopped at position f in the Figure 4. The overshoot hump can be regarded as a p h e n o m e n o n similar to "supercooling" or "supersaturation" [25].

2 . 4 . E f f e c t o f s u b p h a s e t e m p e r a t u r e on the o v e r s h o o t h u m p Figure 5 shows the temperature dependence of the rc-A curve of the PhDA2-8 film on the subphase containing both CdC12 and KHCO3, indicating the appearance of the overshoot hump and the successive zero pressure region. The o v e r s h o o t hump becomes more remarkable with the decrease of the temperature from 30.0 ~ (f) to 5.0 ~ (a). The surface pressure reaches zero immediately after the overshoot hump formation in the curve (a) at 5.0 ~ while at the higher temperatures the pressure falls to non-zero minimum p r e s s u r e s of several dynes.

217

r-J~I

l ~

~=

t~ _ ? \ ~, ~

E//

\,

~l~ L~I~

i

.-, / \

\',,'~,9

10 zo 30 ~ so 6o SurfaceArea[ ~ / m o ~ ]

,, "-

Co)

\-- ',,

(e)

~#~

~" \x\

(d)

"..

(e)

',.

(f)

I

I

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10

20

30

40

50

60

Surface Area [~2/molecule] Figure 4. Recompression effects on PhDA2-8 rc-A curves over a subphase containing both cadmium chloride and potassium bicarbonate. The subphase temperature 5.0~ and the compression rate was 7.5 (A2/molecule)/min. For recompression the blade was stopped at each of the following the surface pressures: (a) 15 mN/m; (b) 16 mN/m; (c) 16.5 mN/m; (d) 17 mN/m; (e) 20 mN/m; (f) 0 mN/m.

2.5.Effect of compression

rate on the overshoot hump As has been indicated recently [27], the relaxation process during the compression of the monolayers of saturated fatty acids is rather slow and usually incomplete. Thus, the experimental rc-A curves obtained under the usual continuous compression include the nonequilibrium effects.

218

9 ~(a)

(b)

(c) I=t

(d) (e) (f) ,,

I

I

,I

,

i

.

l

10 20 30 40 50 60 Surface Area[ ~)/molecule] Figure 5. Effect of temperature change on the PhDA2-8 rc-A curve for the subphase with 3x10-4M CdC12, and 5x10-SM KI-ICO3. The compression rate was 7.5(AZ/molecule)/min. The subphase temperature was; (a) 5.0~ (b) 10.0~ (c) 15.0~ (d) 20.0~ (e) 25.0~ (f) 30.0~

F i g u r e 6 s h o w s the effects of compression rate on the rc-A curve for the PhDA2-8 thin film at air/water interface. Accompanied with the increase in the c o m p r e s s i o n rate, the hump becomes more significant and the maximum surface p r e s s u r e of the hump shifts toward the larger surface area. It is to be noted that the region with zero surface pressure appears only with appropriate c o m p r e s s i o n rates of 3 - - 7 . 5 (A2/molecule)/min as in (d), (e), and (f).

2 . 6 . T h e o r e t i c a l i n t e r p r e t a t i o n of the o v e r s h o o t hump As has been mentioned above, the ~-A curves of the PhDA2-8 thin film show the existence of the zero pressure region after the formation of the o v e r s h o o t hump. It is noteworthy that the remarkable overshoot hump and the s u b s e q u e n t zero surface pressure are observable only in a particular range of c o m p r e s s i o n rates, suggesting that the inclusion of the effect of non-

219

't _ ,\\,

,,

", \\-

~

(a)

- .......

c~'-

~c,

.~,:A I \ . . . . . . . . \\%! . \ (d) -, \~', : ', ! ~ "rx

I ",.Ik;

\ ,

(e) ",. . . . . . . . . .

"-

~o

',,

(h)

'~.

I

10 Surface

O)

, ..........

---/'_'/

!

20 30 40 50 60 Area [ ~.~/molecule]

Figure 6. Effect on the compression rate on the PhDA2-8 rc-A curve for the subphase with 3x 10-4M CdC12and 5x 10SM KHCO3. The subphase temperature was 5.0~ The compression rate[(/~2/molecule)/min] was; (a)18.0, (b)15.0, (c) 12.0, (d) 7.5, (e) 4.5, (f) 3.0, (g) 2.25, (h) 1.5, (i) 0.15.

equilibrium kinetics is essential. Since the characteristic change of the ~-A curves depends on the compression rate, we may expect the following kinetic effects to be important: (I) irreversible formation of a specific aggregated state of the surfactant molecules and (II) relaxation kinetics of spatial inhomogeneity of the surface concentration. The unique characteristics such as "overshoot hump", "zero surface pressure" and "flat plateau" seem to be originated from the competition between these different kinetic p r o c e s s e s .

220 Recently, it was found that stable planar bilayer of fatty acid containing diacetylene groups is formed spontaneously at an air/water interface when the surface pressure is maintained at 35 mN/m. It has also been observed [28,29] that, for a mixed monolayer composed of surface-active dye and fatty acid, the dye is squeezed out from the monolayer above a critical surface pressure and the bilayer is then formed under continuous compression process. These studies indicate that the lipid bilayer is formed only above a critical surface pressure. We can similarly expect that the compression process of the PhDA28 above the critical surface pressure of 20 mN/m correspond to the bilayer formation. The expected structure of the bilayer is given in Figure 7. Formation of the bilayer is supported by the fact that the surface area observed around the zero surface pressure is about one half of the surface area at the starting point of the increase of the pressure before the hump (see Figure 3). The aggregation process corresponding to the kinetic effect-(I) may include autocatalytic and cooperative effects. The decrease of surface pressure down to zero is rapid compared to the time required for the increase of the surface pressure up to the critical pressure, which may correspond to the kinetic effect-(II). Next, we discuss the mechanism of the characteristic features observed in the rt-A curve. When the PhDA2-8 thin film is compressed with relatively high speed, there will be a non-negligible effect of the special inhomogeneity over the lipid film, i.e., the surface pressure in the region near the blade becomes larger than that in the other region.

Au"

Water Surface vggggggg ~ggggggv

vgggggg'g~lgggggg

Aqueous Subphase Figure 7. Assumed structure for the bilayer formed at the zero surface pressure point (Z) in Figure 3.

221

By contrast, if the compression rate is sufficiently smaller than the rate of relaxation of the spatial inhomogeneity, there should be no effective surface pressure gradient at the surface, i.e., a spatially uniform thin film of the PhDA2-8 will be formed. Thus, with relatively large compression rate, the process of the change from the single layer to the bilayer will begin in the region near the blade prior to the other region, when the "observed" pressure with the Wilhelmy plate reaches the critical value 20 mN/m. If the compression rate is lower than the rate of formation of a specific aggregated state, the autocatalytic and cooperative aggregation becomes apparent. Thus, the rapid decrease of surface pressure becomes observable. In practice, zero surface pressure is attained only by appropriate compression rates between 3.0 and 7.5 (/~2/molecule)/min. At the higher compression rates, the aggregation process cannot proceed simultaneously whole over the surface. In this case only a part of the PhDA2-8 molecules on the surface exhibits cooperative aggregation and this induces a small drop in the surface pressure. With continuous compression, the surface pressure again increases. This cycle repeats again and again, leading to an oscillation of the surface pressure. H o w e v e r , in our measurements such an oscillatory phenomenon was not observed, possibly due to damping effects transmitted from the part of the thin film next to the blade to the other parts around the Wilhelmy plate. In our case, the widening of the crest of the hump (flat plateau) was observed. We believe, however, that the above mechanism provides an explanation for the essential aspect of the observations of the "overshoot hump", "zero surface pressure" and "flat plateau" present in the n-A curves measured in this work. Based on the above qualitative discussion of the characteristics of the n-A curves, we now provide a useful kinetic model for the molecular aggregation. Let [S] and [D] be the concentration of PhDA2-8 molecules in the "regular" and "aggregated" states, respectively. The "regular" state corresponds to the usual conformation of a surfactant molecule at the interface, i.e., the hydrophilic head faces the hydrophilic environment and the h y d r o p h o b i c tail is expelled from the interface towards the hydrophobic environment. Then the aggregation can be described as kl S+S

~

D k_l

This step corresponds to the formation of bimolecular aggregates of the PhDA2-8 molecules which leads to the construction of the bilayers. Because the above step takes place in the high surface pressure region c o r r e s p o n d i n g to the condensed layer, we may consider that the subsequent association process of each bimolecular aggregate is much faster. Therefore, this step is rate determining for the overall kinetics at the air/water interface. Based on this kinetic model, the time-dependence of [D] is given by the differential Equation 2-2

222

NDv A2

d[D]dt = kl[S]2 f(r0 - k.l[D] +

(2-2) where A is the surface area at time t, v the compression rate, k~ the aggregation rate constant, k_~ the dissociation rate constant, ND the number of bimolecular aggregates of PhDA2-8 molecules at the air/water interface and f(~) a discontinuous function of surface pressure(re). This function has been introduced to account for the first order like transition [30,31], which may play an important role in the autocatalytic and cooperative aggregation of monolayer molecules to the stable bilayers which proceeds beyond the critical surface pressure.

(A) f(x) iii

i

f(~)

, 1

= 17.5 x + 20.0 f(x)/

'

I '

1

1 1

""

I I

17.5 20.0

(B)

Ideal///// //

////

/

I i

/

/

/

/

F~.L

r~n(

A

i!

,

Figure 8. (A) Schematic representation of the shape of the function f(r0. The arrows represent the first order like phase transition effect. The two straight lines are fin) = 17.5n + 20.0 and fin) = 0.01re, respectively. (B) Schematic representation of the relationship between the surface pressure (n) and the effective concentration of surfactant at the air/water interface (Fae). The solid and dashed lines represent the expected and ideal relationships, respectively.

223 The function f(n) can be given in a concrete expression as "S"-shape nonlinear function, schematically shown on the left in Figure 8A. For the convenience of analysis we take the approximation to express the "S"-shape characteristics with the combination of two straight lines as shown on the right in Figure 8A. The third term of Equation 2-2 means the increment of [D] with compression at the air/water interface. To simplify the analysis, we further assume kl>> k.~. This assumption is consistent with the observed stability of the bilayers formed at the zero surface pressure point. The kinetics of [D] can be then e x p r e s s e d as NDv d[D_....~]dt= k1[S]2 f(x) + A----~ (2-3) and the kinetics of ND can be obtained finally from

dND =k~ NS2 dt -~-- fix)

(2-4)

where Ns is the number of PhDA2-8 molecules at the air/water interface. Now we consider the relationship between the effective concentration(F,ff) and the surface pressure(x) at the air/water interface. Ideally, the surface pressure is directly proportional to the concentration of surfactants. However, as the actual n-A isotherms show several specific effects, such as limiting area and points of inflexion, we shall assume the following relationships:

Fen = [S] + a[D],

f

(2-5)

Feff
x=0

Feff > Feff,L ~ ,

/1; or (Feff - Feff,L) m

where F is the surface concentration of lipid molecules and it is inversely proportional to A, and a is the correction factor representing the contribution of [D] (related to [S]) to the effective concentration, respectively. The effective concentration at the limiting area is expressed as Feff.L. Judging from the actual x-A isotherms, the relationship between n and Feff can be expressed in terms of a function of higher order. To simplify the analysis without losing generality, we put m equal to 2, as shown in Figure 8B. To illustrate the inhomogeneities of the surface pressure we adopt a rectangular cell model, as is schematically shown in Figure 9.

224

Cell Number 1

Compression

Figure 9. Schematic representation of the model adapted for the numerical simulation.

This model assumes that the air/water interface f r o m the blade to the Wilhelmy plate can be divided into a number of equal small cells. We apply a simple a r g u m e n t that the rate of mass t r a n s f e r by d i f f u s i o n is p r o p o r t i o n a l to the d i f f e r e n c e in concentration between the n e i g h b o r i n g cells, while the c o n c e n t r a t i o n and the surface p r e s s u r e within each cell are a s s u m e d homogeneous. To give a full interpretation of e x p e r i m e n t a l results o b t a i n e d in this work, we have to note that the i n h o m o g e n e i t y of the s u r f a c e p r e s s u r e at the air/water interface is one of the most important factors g i v i n g rise to the characteristic f e a t u r e s in the n - A curves. In the right-hand part of F i g u r e 10 are s h o w n simulation results obtained by using the above kinetic equations and the r e c t a n g u l a r cell model which divides the air/water interface into one h u n d r e d cells. In this simulation, the relative magnitudes of the rate of relaxation p r o c e s s e s and the rate of c o m p r e s s i o n were set up as f o l l o w s . : F i g u r e 10A: the rate of c o m p r e s s i o n > the rate of p r o c e s s (I) > the rate of p r o c e s s (II), F i g u r e 10F: the rate of process (I) > the rate of c o m p r e s s i o n > the rate of p r o c e s s (II), F i g u r e 10I: the rate of p r o c e s s (I) > the rate of p r o c e s s (II) >> the rate of compression. F i g u r e 10A shows the oscillation of the surface p r e s s u r e . H o w e v e r , if we include the damping factors and increase the n u m b e r of ceils in the simulation, the oscillation would be s m o o t h e d to a flat plateau such as it is r e p r e s e n t e d by the dotted line. It is evident that the above simulation results can r e p r o d u c e essentially all characteristic features such as "flat plateau", "zero s u r f a c e p r e s s u r e " , and " o v e r s h o o t hump" o b s e r v e d in the actual n - A curves. T h e s e properties are characteristic examples of nonlinearity in the n o n e q u i l i b r i u m state of a thin

225

film [32]. In the future, it will be necessary to obtain direct s p e c t r o s c o p i c i n f o r m a t i o n on the stable aggregates ( p o s s i b l y bilayers) o f P h D A 2 - 8 fatty acids. U t i l i z a t i o n of s p e c t r o s c o p i c methods, such as e p i f l u o r e s c e n c e m i c r o s c o p y [25, 3 3 - 3 6 ] , may give us direct information on the unique nature in the transition of the thin film.

ii

ill

E (a) inml~l~lmi~lml r~

n 4o

-.

10

20

30

10

I 20

i30

s so

I 60

I 50

I 60

-'-1

i

!

z.

| =

i-

i

ii

........

I 40

ii

I

I

I

I

iii

I,m

l..

8

qa

(i) {o}

I

I

1o

20

I 3O

I, 40

o,)

i ,, I

50 60 Surface Area [ ~.2/molecule]

I

I

i

Surface Area

Figure 10. Comparison between the experimental n-A curve(left) and the corresponding curve obtained by computer simulation(right). The left-hand parts of Figures 10(a), (f) and (i) are the same as in Figures 6. Parameters used in the simulation are; a = 0.60, k] = 0.75, (A) v = 1, (F) v = 0.5, (I) v = 0.01.

226 3. N O N L I N E A R VISCOELASTICITY AIR/WATER INTERFACE

OF

THIN

FILM

AT

In the preceding section we have shown several interesting phenomena, such as the overshoot hump and the zero surface pressure for a thin film with PhDA2-8, observed during the compression process. In order to evaluate nonlinear viscoelastic property in a quantitative manner, we have developed a method to observe the dynamic n-A characteristics with repeated cycles of c o m p r e s s i o n and expansion. Before we shall describe the m e t h o d o l o g y and results of the observation of the dynamic ~-A characteristics, we w o u l d like to discuss first the thermodynamic aspects of a lipid monolayer with special emphasis on the effects of the Coulombic interaction between the charged head groups of the lipid molecules. Inclusion of the Coulombic interaction enables to obtain a new insight into the origin of the nonlinear viscoelastic properties of lipid monolayer at the air/water interface.

3. l . I m p o r t a n c e e f f e c t s of C o u l o m b i c I n t e r a c t i o n s on the f o r m of the i s o t h e r m of the p h o s p h o l i p i d thin f i l m The ensemble of lipid molecules situated at the air/water interface can be regarded as composed of interacting "particles" in t w o - d i m e n s i o n a l space. Different types of physico-chemical interactions exist among the "particles", such as hydrophobic interaction, hydrogen bonding, van der Waals interaction, etc. Therefore we can expect that the essential feature of the thermodynamic state of the ensemble of the t w o - d i m e n s i o n a l "particles" can be interpreted similarly as in the model of t w o - d i m e n s i o n a l non-ideal gas. In other words, the isotherm of the lipid film at the interface, or the equilibrium pressure(rt)-concentration(l") characteristics, may be expressed by means of the virial expansion in a two-dimensional space. In the following we would like to discuss the actual isotherm of p h o s p h o l i p i d film, by comparing the standard virial expansion with a different type of expansion implying the specific aspect of Coulombic interactions. If we consider the thermodynamic behavior of t w o - d i m e n s i o n a l "particles", in the neighborhood of the phase transition region, the n-A isotherm of the film will behave as it is depicted by the line I in Figure 11. H o w e v e r , it is well known that in actual measurement, the surface pressure does not generally show the flat plateau, as in line II in Figure 11. Such a general experimental trend may be, at least to some degree, attributed to the nonequilibrium effect as is discussed t h r o u g h o u t this chapter. Actually, Mingins et al. has carried out a careful measurement of the equilibrium n-A characteristics for phospholipid molecules at an oil/water interface and observed nearly the ideal trace similar to a t w o - d i m e n s i o n a l van der Waals gas as it is slow by the line I(see Figure 11) [37]. Recently we have p r o p o s e d a theoretical model to interpret the re-A, or n-I', relationship.

227

fluid/solid coexistence

llI g I

A Figure 11. Schematic representation of the n-A relationship of a lipid monolayer

When one takes into account the effects of interaction between the polar head groups using similar degree of the approximation as in the Debye-Hiickel theory, the following relationship results [38]: n = R T F (1 - ~ F 1/2 + ctF),

(3-1)

where 0t is the second virial coefficient in the usual two-dimensional van der Waals equation, and ~ is the parameter proportional to the Debye-Htickel length around the charged group. Let us compare the theoretical isotherm of Equation 3-1 with the n-F relationship represented as the standard expansion with the second ot and third 13 virial coefficients as in rc = R T F

(1 + czF + [~1"2).

(3-2)

Both Equations 3-1 and 3-2 contain two "adjustable" parameters. Thus, the c o m p a r i s o n of these equations with the results of actual measurements will allow us to the determine the validity of the unique expansion in Equation 3-1. Equation 3-1 can be re-arranged into

228

f(r)

=

/r

RT1-'2

1

- --

= a-

~ r -~/2

F

.

(3-3)

Similarly, Equation 3-2 gives rc f(r')

=

1 - --

RTI ~

= a

+ I~r"

F

.

(3-4)

Using Equations 3-3 and 3-4, we can plot f(F) v s . F 1/2 or F based on the experimental data on the rc-F characteristics (see Figure 12) [39]. Figure 13 indicates that Equation 3-3 is more realistic than Equation 3-4. The correlation coefficient, R, for Equation 3-3 is nearly unity, suggesting that the assumption of the "Coulombic" interaction between the hydrophilic headgroups is essential in the interpretation of the isotherm, especially for the low F(or the large A) region. Equation 3-1 implies that the Ir-F relationship becomes "N"-shaped when becomes sufficiently large. Indeed, Mingins et al. have reported that at the oil/water interface the ~-A isotherms of the phospholipids with Cls, C20 and Czz alkyl chains show clear first-order transitions [37].

4o E z E

ii

5 30 2o IO

0.5

~.0

t5

z.o

z.5

A (nm2/moLecute)

Figure 12. n-A isotherm for 1,2-dimyristoyl lecithin at an n-heptane-aqueous NaC1 solution interface. The numerals on the curves are the temperatures given in Celsius.

229

oo

(A)

4

O3)

"7, 3.5 t

~ ~"* x.-'~ o. " .. >~" / ~" X."" A ~'tx

3.

AX-~

1

"o.

~""la.laA %

x~x,."..., "l.

2.

2.5

!

0.3 0.5 o.7 0.9 ~.1 ].3 ~.5 F x 1018

, ,

0.7

t

i

i

0.9

i

i

i

I

i

1.1

,

i , , ,

1.3 1

1"'~

|

1.5

1/,,~

,

.7

x 10.9

D Equation (3-3) E Equation (3-4)

(C) 1 0.98 ~0.96 0.94 0.92 0.9

5

10

15

20

Temperature Figure 13. (A) The plot of f(I') v s . F based on the data of Figure 12. 03) The plot of f(l") v s . F -1/2 for the following temperatures: O , 5 ~ ~ , 10 ~ X, 15 ~ 0 , 20 ~ (C) The comparison of the correlation coefficients, R, between the two different analysis.

3.2.Dynamic

surface

pressure

as a m e a s u r e

of the nonlinearity

In the above subsection it was demonstrated that the inclusion of electrostatic interactions into the pressure-area-temperature equation of state provides a better fit to the observed equilibrium behavior than the model with t w o - d i m e n s i o n a l neutral gas. Considering this fact, we would like to devote our attention now to the character of the lipid film under the dynamical, nonequilibrium conditions. In the following we shall describe the dynamical behavior of the p h o s p h o l i p i d ( 1 , 2 - d i p a l m i t o y l - 3 - s n - p h o s p h a t i d y l e t h a n o l a m i n e s ; DPPE) thin films in the course of the compression and expansion cycles at air/water interface. The dynamic surface pressure was measured with a trough equipped with a couple of moving Teflon blades and an electronic balance, CHAN/Ventron, Cerritos, CA, U . S . A . , similar to the apparatus describe by Mendenhall and Mendenhall [40], and by B i e n k o w s k i and Skolnick [41]. The trough coated

230 with Teflon was 116 mm long, 61 mm wide, and 14 mm deep; it was rinsed carefully with methanol and distilled water before each experimental run. The surface pressure was monitored as the tension of the piano steel wire connected to the platinum plate(9.8 x 20 x 0.03 mm), previously polished with 250-mesh emery, dipped one half into the aqueous subphase. DPPE was gently spread onto the aqueous phase(ca. 100 ml), containing various chemical species. The surface area was then changed successively between 1300 and 4600 mm z, by moving the Teflon blades with a cycle of 60 see. When the n - A curve began to trace a single closed-line after several cycles, the curve of the dynamic surface pressure was recorded. All measurements were performed at 2 0 + 1~ Figure 14 shows the dynamic surface pressure for the thin f i l m ' o f DPPE after several compression and expansion cycles. Figure 15 s h o w s the characteristic response of the dynamic ~-A curve to the chemical stimuli. It is apparent that the dynamic n-A curves traced closed-lines exhibiting hysteresis and the hysteresis loops varied with the addition of chemical c o m p o u n d s . The characteristic response may be explained taking into the consideration the specific effects of added chemicals on the dynamic behavior of the DPPE thin film. Nicotine is rather hydrophobic and is expected to change the manner of packing of alkyl-chains in the aggregates of the DPPE molecules. Sodium chloride changes the Debye length of the charge effect of the DPPE head group and, thus, changes the aggregation of the DPPE molecules.

(B)

(A)

8

r~

!k

! JLLA_J I

120

!

,

,

,

,

180

240

300

360

420

Time

,,.

|

,

,

,

,

,

-

.

_

480 20 30 40 50 60 70 80901 00

see Surface Area

Figure 14. (A) Time trace of surface pressure with periodic change of the surface area for the DPPE thin film at an air/water interface. (B) Dynamic n-A curve obtained after two cycles.

231

(A)

(B)

l, .

.

.

.

|

(C)

(D)

i|1

.|

r~

Surface Area Figure 15. Characteristic changes in the dynamic n-A curve caused by the addition of various kinds of chemical compounds to aqueous solution containing DPPE. (A) 0.05 mM nicotine, (B) 0.5 mM sodium chloride, (C) 0.1 M sucrose, (D) 0.5 mM citric acid.

S u c r o s e changes the dynamic structure of water molecules, which, in turn, affects the manner of aggregation of the DPPE. Citric acid changes the degree of dissociation of the head group of the DPPE molecules. It becomes, therefore, apparent that each chemical species affects the viscoelastic behavior of the lipid thin film in a characteristic manner. It is noted that, as is shown in Figure 15, the chemicals with different t a s t e - r e s p o n s e s show markedly different effects on the dynamic behavior of the p h o s p h o l i p i d film. Detail d i s c u s s i o n on the chemical r e s p o n s e in relation to the mechanism of taste sensation has already been given in a series of studies from our research group [ 3 , 4 2 , 4 3 ] .

3 . 3 . A n a l y s i s o f the d y n a m i c s u r f a c e p r e s s u r e After the a cco mp lis h men t of the above mentioned experiment on the nonlinear viscoelasticity of the DPPE thin film, we have tried to construct a new instrument for the m e a s u r e m e n t of the dynamic surface tension. We have noticed that, the blades used to change the surface area in the commercial instrument, did not show genuine triangle or sinusoidal trajectory but rather mathematically undefined. With our newly designed instrument, the time change of the surface area can be controlled according to a chosen function with the aid of a micro-computer. Experimental procedure for the preparation of the thin film was similar to that described in the preceding subsection.

232

(E)

tlIi

9

(c)

03)

Figure 16. Experimental arrangement used for the measurement of the dynamic behavior(n-A curve) of the DOPC thin film at an air/water interface. (A) trough, (B) blades, (C) platinum plate(Wilhelmy plate), (D) water circulating outlet, (E) electric balance, (F) blades controller, (G) data processor.

F i g u r e 16 s h o w s the experimental a r r a n g e m e n t for the m e a s u r e m e n t of the s u r f a c e p r e s s u r e . The trough (200 mm long, 50 mm wide and 10 mm deep) was coated with Teflon. The s u b p h a s e temperature was controlled within + 0.1 ~ by means of a jacket connected to a thermostated water circulator, and the e n v i r o n m e n t a l air temperature was kept at 18 ~ The surface tension was m e a s u r e d with a Wilhelmy plate of p l a t i n u m ( 2 4 . 5 x 10.0 x 0.15 mm). The s u r f a c e p r e s s u r e monitored by an electronic balance was s u c c e s s i v e l y stored in a m i c r o - c o m p u t e r , and then Fourier t r a n s f o r m e d to a frequency domain. The s u r f a c e area was changed s u c c e s s i v e l y in a sinusoidal manner, between 37.5 /~2/molecule and 62.5 /~2/molecule. We have chosen an u n s a t u r a t e d phospholipid(1,2-dioleoyl-3-sn-phosphatidyl-choline; DOPC) as a curious s a m p l e to m e a s u r e the dynamic surface tension with this novel instrument, as the u n s a t u r a t e d lipids play an important role in b i o m e m b r a n e s and, m o r e o v e r , such a "fluid" lipid was expected to exhibit marked dynamic, nonlinear characteristics. The spreading solution was 0 . 1 3 3 mM c h l o r o f o r m solution of DOPC. The c h l o r o f o r m was purified with three consecutive distillations. The experimental traces of the t i m e - d e p e n d e n t change of the surface p r e s s u r e and the dynamic n-time and n-A curves are shown in Figure 17.

233

03)

(A)

50

0

100

1 0 2 250 Time(sec)

300+

354045~0~5 Surface Area

(~k2/mole, culc)

Figure 17. (A) Time trace of surface pressure with periodic change of the surface area for the DOPC thin •rn at an air/water interface. (B) Dynamic n-A curve. After two or three cycles, the curve begins to trace a single closed loop.

(A) ~ !

~)

; - ......... ~_ . . . .

Ii

0

V

~,.

I4

coo 2 ~ o 3 ~ o 4 ~

,,

Frequency

354045 505560

,

,

,

Surface Area

(~/mol~l~)

Figure 18. (A) Fourier transformation spectra of the time trace of surface pressure for the steady loop (see Figure 17). Top: real part (elastic component), bottom: imaginary part (viscous component). (B) Inverted Fourier Spectra for the real and imaginary parts.

234

After several cycles of the compression and expansion, the dynamic n-A curve becomes a single closed loop, somewhat distorted from a genuine ellipsoid. In order to analyze the forms of the hysteresis loop under stationary conditions, we have measured the time trace of the dynamic surface pressure after five cycles of the compression and expansion, and then Fouriertransformed it to the frequency domain. The Fourier-transformation was adapted to evaluate the nonlinear viscoelasticity in a quantitative manner. The detailed theoretical consideration for the use of the Fourier transformation to evaluate the nonlinearity, are contained in the published articles [8,43]. F i g u r e 18A shows the Fourier spectra thus obtained. The real and imaginary parts correspond to the elastic and viscous components of the DOPC thin film, respectively. We can see that the spectrum is composed not only from the fundamental (tOo) but also from the higher (2o)0, 3~0, 4c00,..) harmonic components. Such a trend indicates that the DOPC thin film exhibits rather large nonlinearity in the viscoelastic characteristics. In other words, higher harmonics serve as a useful quantitative measure of the nonlinear characteristics of the thin film. Using the inverse Fourier transformation, the real and imaginary parts can be represented as the elasticity and viscosity of the film in a separate manner as is shown in Figure 18B.

3 . 4 . Theoretical interpretation of hysteresis loops Next, we discuss the physico-chemical meaning of the hysteresis or the imaginary components in the dynamic ~-A curve. Generally, DOPC molecules form a soluble monolayer at an air/water interface. When the monolayer is c o m p r e s s e d rapidly, the DOPC molecules are dissolved into the subphase and form micellar aggregates. If the aggregates are dissolved continuously into the subphase, in other words, if the total amount of the lipid present at the interface decreases with each cycle of the repeated compression and expansion, the dynamic n-A curve can not exhibit a single closed loop. In the actual experiment, we have observed a single closed line for the n-A curve (see Figure 17B), indicating that under our experimental conditions, DOPC molecules dissolve into the subphase in the course of the compression and then return into the interface. The proposed scheme for the cooperative adsorption/desorption of the DOPC molecules at the air/water interface is given in Figure 19. On the basis of the above discussions, we would like to propose the f o l l o w i n g kinetic model. Let [S] and [Dint] be the concentration of DOPC molecules in the "regular" and "aggregated" states at the air/water interface, respectively. The concentration of the aggregated state in the vicinity of the air/water interface(sublayer) is expressed as [Dsub].

235

kl ~ -" k_1

S+S

Din t

k2 S + Din t

"k.2

D sub

Based of this mechanism, the time course of c o n c e n t r a t i o n s is given by the f o l l o w i n g set of differential equations,

diS]

-

_ kl[S] 2 + k.l[Din t] - k2[S][Dint] + k_2[Dsub],

dt

(4-1)

d[DinO 9 = dt d[Dsub] dt

,,

=

kl[S] 2 - k-l[Din0 - k2[S][Dint] + k-2[Dsub],

(4-2)

k2[S][Dint] - k.2[Dsub], (4-3)

w h e r e k I is the aggregation rate constant, k.l the d i s s o c i a t i o n rate constant, k. 2 the a d s o r p t i o n rate constant, respectively, and the d e s o r p t i o n rate coefficient k 2 is represented by a nonlinear function of the surface p r e s s u r e , k2

=

F(rr).

(4-4)

Air-Water Interface

Sublayer

, , . . ~ . , .

.

.

.

.

. . , . . . . .

.

.

.

.

.

.

.

.

.

.

.

.

.

,

.

.

.

.

.

.

.

.

i

.

.

.

.

.

.

o

.

I

-

-

VA Subphase Figure 19. Schematic representation of the cooperative adsorption and desorption of DOPC molecules between an air/water interface and a sublayer.

236

(A)

r.~

I

I

(B) m|l

I,

I

I

'

35 40 45 50 55 60

.

.

.

.

Surface Area

Surface Area

(.~2/molecule)

Figure 20. Comparison between the experimentally observed dynamic surface behavior and the results obtained by computer simulation. The left-hand parts of Figure 20(A) is the same as in Figure 1703).

This function has been introduced to account for the first order like transition in the process of the compression of the film. The function F(rc) may be thus represented as an "S"-shape function (Figure 8) [30,31]. In analogy with the section 2, the time dependent changes of concentrations, [S], [Dint] and [Dsnb] are calculated from the above equations and the rectangular cell model based on division of the air/water interface into twenty cells. In the present work, we take the approximation that the dynamic surface pressure is directly proportional to [S] and [Dint] [44,45]. The simulation results are shown in Figure 20. We can observe that the simulation can reproduce the characteristic feature, the hysteresis loop, o b s e r v e d in the dynamic rc-A curves.

4.SPONTANEOUS AND INTERFACE SYSTEM

OSCILLATION T E N S I O N IN

OF E L E C T R I C A L POTENTIAL AN OIL-WATER-SURFACTANT

Recently we have found that sustained electrical oscillations are generated in a liquid membrane consisting of water/oil phase, where the aqueous phase contains surfactant and alcohol [3,10]. The difference between the concentrations in the aqueous and the oil phases forms the driving force of electrical oscillations. In the last section, we would like to describe this type of rhythmic oscillation from the point of view of the nonlinearity in the kinetics of transfer of lipid molecules through the oil/water interface. It is

237 closely related to the cooperative character of lipid molecules at the oil/water interface.

4.1.Oscillatory DPPE

pattern in liquid membrane with s o d i u m oleate and

Experiments for the measurement of oscillations were performed using a cell in the form of a concentric hollow cylinder shown schematically in Figure 21 [9]. Nitrobenzene solution (13 ml) of 0.5 mM tetraethylammonium bromide (TEAB) was placed at the base of the cell. TEAB was used to decrease the electrical resistance of the bulk organic phase. An aqueous solution (8 ml) containing both surfactant, sodium oleate or DPPE, and alcohol, butanol, was carefully introduced into the inner cylinder and, simultaneously, 0.5 M NaC1 aqueous solution (14 ml) was introduced into the outer cylinder. All measurements were carried out at 20 + 1 ~ The voltage across the aqueous/organic/aqueous phase, or the thin lipid film, was measured with a high-impedance voltmeter connected by two salt bridges to two Ag/AgC1 electrodes. Interracial tension was monitored by the Wilhelmy method [46] using a hydrophilic glass plate (15 x 15 x 1 ram).

(A)

I

(B)

~_~

i

", I

,

'

I

i~

i

30mm ---50mm

'

2mm

I .k.--~_4.: I_J ....

t Figure 21. (A) Side view of the experimental apparatus for the simultaneous measurements of electrical potential and interfacial tension. (a) High impedance voltmeter, (b) electronic balance, (c) salt bridge. 03) Bird-eye view of the apparatus.

238

(A) o

rO

:

t

20min

(B)

I

20rain

(C)

E

I

20rain

-t

Figure 22. Variations of the electrical potential and the interfacial tension. The aqueous phases in the inner cylinder are; (A) 0.1 mM sodium oleate plus 5 vol% butanol. 03) 0.07 mM DPPE plus 5 vol% butanol, and (C) 5 vol% butanol without surfactant.

239 Figure 22 shows the results of the simultaneous m e a s u r e m e n t of electrical potential and interfacial tension for the liquid membrane with an aqueous phase of (A) 0.1 mM sodium oleate plus 5 vol% butanol, (B) 0.07 mM DPPE plus 5 vol% butanol and (C) 5vo1% butanol. Rhythmic changes of the electrical potential (Figure 22A and 22B) continued for 1-2 h. The amplitude, frequency and wave form of the oscillations were found to be essentially the same for each experiment performed under fixed conditions. On the other hand, in the absence of the surfactant irregular pulses were o b s e r v e d (Figure 22C). Such an irregular change in the electrical potential in Figure 22C is attributable to the so-called Marangoni effect [46], i.e. spontaneous interfacial turbulence accompanied by mass transfer across the interface. Let us note that the electrical potential and the interfacial tension change in a synchronized manner as it is shown in Figures 22A and 22B, whereas no apparent change in the interfacial tension can be observed in Figure 22C. When experiments were performed with DPPE and without butanol, only a few pulses were observed and sustained oscillations were not generated. Thus, the main driving force for the oscillations appears to be the diffusion of alcohol from the bulk aqueous phase to the organic phase through the interface, and the surfactant molecules induce a simultaneous or cooperative change over the entier interface. As the interfacial tension is directly related to the concentration of surfactant molecules at the interface [46], the rhythmic changes in the interfacial tension suggest that the concentration of surfactant, sodium oleate in Figure 22A and DPPE in Figure 22B, at the oil/water interface changes repeatedly between high and low values.

State A

State B ,|

,,

WaterPhase FA

f

~

Oil Phase

~

~a~hol

i-' Oil Phase

~

Figure 23. Schematic representation of the repetitive formation and reconstruction of the lipid thin film.

240

Table 1. Repeated change of the interfacial state.

State m

-

'

'

~

B

AE

low

high

Force change on the glass plate

z~tF

low

high

Surface tension

A7

high

low

Concentration of the surfactant

AI-"

low

high

Electrical potential ,,

.

.

.

.

The increase in the concentration of the surfactant molecules at the interface corresponds to the growth of the monolayer, which increases the electrical potential of the electronic bilayer. The synchronized change is shown schematically in Figure 23 together with Table 1.

4.2.

T h e o r e t i c a l e x p l a n a t i o n of the o s c i l l a t i o n s

Based on the above results and discussion, the mechanism for the rhythmic oscillations at the oil/water interface with surfactant and alcohol molecules may be explained in the following way [3,47,48] with reference to Table 1. As the first step, surfactant and alcohol molecules diffuse from the bulk aqueous phase to the interface. The surfactant and alcohol molecules near the interface tend to form a monolayer. When the concentration of the surfactant together with the alcohol reaches an upper critical value, the surfactant molecules are abruptly transferred to the organic phase with the formation of inverted micelles or inverted microemulsions. This step should be associated with the transfer of alcohol from the interface to the organic phase. Thus, when the concentration of the surfactant at the interface decreases below the lower critical value, accumulation of the surfactant begins and the cycle is repeated. Rhythmic changes in the electrical potential and the interface tension are thus generated.

241

Aqueous phase

Interface ! !

Organic phase

! !

I i Negative Feed back

Xb "

9 :'%11

i ~.~Zi'

;- Bulk Organic Phase

~,y.d/~ ,

Y t --D

t

,

r

1

],

Scheme 1. The process of transfer of surfactant and alcohol from the aqueous phase to the organic phase through the interface.

The foregoing interpretation of the mechanism, although necessarily somewhat speculative, provides a useful kinetic model [10]. Let X, Y, and Z be the concentrations of the key chemicals; X i, the concentration of surfactant at and/or near the interface; Yi, the concentration of alcohol near the interface; Zi the concentration of the aggregate or complex of surfactant and alcohol at and/or near the interface. Scheme 1 can be considered as a possible explanation for the mechanism of oscillation in a liquid membrane. Xb and Yb are the concentration of the surfactant and alcohol, respectively, in the bulk aqueous phase. Scheme 1 is composed of the following steps (i)-(iv).

Dx Xb ~

Xi

(i)

Vi

(ii)

Dy Yb ~

Xi + Yi

Zi

~

k4

~

k3

Zi

bulk organic phase

(iii)

(iv)

242 Steps (i) and (ii) correspond to the migration of surfactant and alcohol from the bulk aqueous phase toward the oil/water interface. Step (iii) is the formation of aggregates of surfactant and alcohol at the interface, and is related to the construction of the monolayer associated with surfactant and alcohol. Step (iv) indicates the migration of the aggregates of surfactant and alcohol from the interface into the bulk organic phase, forming inverted micelles or microemulsions in the organic phase. It may be expected that the rates of transfer of surfactant and alcohol, Dx and De, are affected by the negative feedback of Zi. In other words, the diffusion rate, Dx, of surfactant from the aqueous phase to the interface may decrease with the net increase in the concentration of surfactant at the interface, Xi plus Zi. A similar situation may hold for the diffusion rate, Dr, of alcohol from the aqueous phase to the interface. Hence, the system kinetics may be considered under the following assumptions: (a) the concentration of surfactant and alcohol in the bulk aqueous phase, Xb and Yb, remain constant; (b) the rates of diffusion of surfactant alcohol from the bulk aqueous phase to the interface are expressed as Dx(Xb - Xi) and Dv(Yb - Yi), respectively; (c) the negative feedback of Zi on the diffusion of X and Y are given Yb- kl Zi and - k2Zi, respectively; (d) the rate of step (iv) is expressed as a function, F(Xi, Yi),with the rate constant k3; and (e) the rate of step (iv) is expressed as a function, G(Zi), with the rate constant k4. Under these assumptions, the kinetics of the migration of surfactant and alcohol are described by the differential equations:

dX i dt

-

Dx (Xb

-

X

-

D y (Yb

-

Yi)

dYi dt dZi dt

i)

-

k l Zi,

-

k2 Zi,

- k 3 F(Xi,Yi) - k 4 G(Zi),

(4-1)

(4-2)

(4-3)

F(Xi, Yi) may be given by (Xi + Yi), a simple form for the synergetic effect of Xi and Yi. which has the physical meaning that the monolayer is formed together with surfactant and alcohol. Self-oscillatory states can be obtained if G(Zi) has "N"-shape nonlinearity.

243

(A) o (Do_ I

i

Time

~

I

-~.o Zi

1.0

(B)

o (-9 o I

I

I

-1.0 Zi 1.0

"Time

(c)

9 T -

Time

,

-~.0 Zi

1.0

Figure 24. Computer simulation of the oscillation with variation of the nonlinear function G(Zi). Parameters; (A) Dx=0.1, Dy=0.025, Xb=0.4, Yb=2.56, k~--0.2, k2--0.05, k3=2.5, k4=2.5; (B) Dx--0.2, Dy=0.05, Xb=0.4, Yb=2.5804, k~=0.4, k2=0.1, k3=5.0, k4=5.0; (C) Dx--0.1, Dy=0.025, Xb=0.4, Yb=2.8, kl=0.2, k2=0.05, k3=2.5, k4=2.5. The shapes of G(Zi) are shown on the right side of Figure 24.

244 As has been discussed in the preceding sections, it is expected that the surfactant monolayer exhibits "N"-shape nonlinearity in its dynamic x - F characteristics. Thus, we would like to discuss the kinetics, assuming that G(Zi) is a cubic function. Figure 24 shows numerical results for the set of Equations 4-1, 4-2, 4-3. Comparison with the experiments, Figure 24A and 24B indicates that the oscillation patterns have been well reproduced. Thus it becomes evident that repetitive formation and destruction of the monolayer are the key steps for the rhythmic pulsing. In the right of Figure 24, the shape of the "N"-type nonlinearity, G(Zi) versus Zi. is shown. When G(Zi) is a smooth function as in Figure 24C, growing oscillations with regular bursting pulses are generated. On the other hand, the period between the bursting pulses becomes irregular when the derivative of G(Zi) is discontinuous as in Figure 23A and 23B. The steepness on the right-hand side of the "N"-shape function is the important factor to induce the small pulses between the great bursting pulses. In conclusion, we have shown that lipid film generally exhibits marked nonlinear characteristics. Nonlinear characteristics become particularly significant under the dynamic and/or nonequilibrium conditions. Further experimental and theoretical studies of nonlinear characteristics of lipid films are desirable.

ACKNOWLEDGEMENTS The authors express their sincere thanks prof. M. Marek (Prague Institute of Chemical Technology) to the helpful comments on the present articles. The authors thank Prof. H. Kawakami (Tokushima University), Dr. T. Ishii (Tsurumi University), and Dr. S. Nakata (Nara University of Education) for their helpful suggestions and to Dr. Y. Yoshioka (Kanegafuchi Chemical Industry Co., Ltd.) for the advice on the experimental technique of rc-A measurement of PhDA2-8.

REFERENCES 1. R. J. Field and M. Burger(eds.), Oscillations and Traveling Wave in Chemical Systems, John Wiley and Sons, New York, 1985. 2. G. Nicolis and I. Prigogine, Self-organization in Nonequilibrium Systems, John Wiley and Sons, New York, 1977. 3. K. Yoshikawa and Y. Matsubara, J. Am. Chem. Soc., 105 (1983) 5767. 4. K. Yoshikawa and Y. Matsubara, J. Am. Chem. Soc., 106 ( 1 9 8 4 ) 4 4 2 3 . 5. K. Yoshikawa, T. Omochi and Y. Matsubara, Biophys. Chem., 23 (1986) 211.

245 6. K. Yoshikawa, T. Fujimoto, T. Shimooka, H. Terada, N. Kumazawa and T. Ishii, Biophys. Chem., 29 (1988) 293. 7. K. Yoshikawa, S. Nakata, T. Omochi and G. Colaccico, Langmuir, 2 (1986) 715. 8. K. Yoshikawa, M. Shoji, S. Nakata, S. Maeda and H. Kawakami, Langmuir, 4 (1988) 759. 9. K. Yoshikawa and M. Makino, Chem. Phys. Let., 160 (1989) 623. 10. K. Yoshikawa, Excitable Liquid Membranes, in T. Araki and H. T s u k u b e ( e d s . ) , CRC press, 1990. 11. S . J . Singer and G. L. Nicolson, Science, 175 (1972) 720. 12. G . W e i s s m a n and R. Claiborne(eds.), Cell Membranes: Biochemistry, Cell Biology & Pathology, HP Publishing Co., Inc., New York, 1975. 13. I. Langmuir, Trans. Far. Soc., 15 (1920) 62. 14. K . B . Blodgett, J. Am. Chem. Soc., 57 (1935) 1007. 15. H . - J . Cantow(ed.), Polydiacetylenes, Advances in Polymer Science, New York, Vol. 63, 1984. 16. G. Wegner, Recent Progress in the Chemistry and Physics of Poly(diacetylenes), in W.E.Hatfield(ed.), Molecular Metals, Plenum, New York, 1979. 17. M. Brenton, J. Macromol. Sci. Chem., 21 (1981) 61. 18. A. Laschewsky, H. Ringsdorf, G. Schmidt and J. Schneider, J. Am. Chem. Soc., 109 (1987) 788. 19. B. Ostermayer, O. Albrecht and W. Vogt, Chem. Phys. Lipids, 41 (1986) 265. 20. B. Hupfe and H. Ringsdorf, Chem. Phys. Lipids, 33 (1983) 263. 21. D . J . Scoberg, D. N. Furlong, C. J. Drummond, F. Grieser, J. Davy and R. H. Prager, Colloids and Surfaces, 58 (1991) 409. 22. Y. Yoshioka, N. Nakahara and K. Fukuda, Thin Solid Films, 133 (1985) 11. 23. H. Matsuo, D. K. Rice, D. M. Balthasar and D. A. Cadenhead, Chem. Phys. Lipids, 30 (1982) 367. 24. B. Tieke and G. Lieser, J. Colloid Interface Sci., 88 (1982) 471. 25. S . W . Hui and H. Yu, Langmuir, 8 (1992) 2724. 26. D. Day and H. Ringsdorf, J. Polym. Sci.: Polym. Let. Ed., 16 (1978) 205. 27. T. Kato, Y. Hirobe and M. Kato, Langmuir, 7 (1991) 2208. 28. Y. Kawabata T. Sekiguchi, M. Tanaka, T. Nakamura, H. Komizu, K. Honda and E. Manda, J. Am. Chem. Soc., 107 ( 1 9 8 5 ) 5 2 7 0 . 29. T. Kurata, A. Tsumura, H. Fuchigami and H. Koezuka, J. Phys. Chem., 95 (1991) 8831. 30. N . L . Gershheld, Annu. Rev. Phys. Chem., 27 (1976) 349. 31. H . W . Horn and N. L. Gershbeld, Biophys. J., 18 (1977) 301. 32. M. Makino, M. Kamiya, T. Ishii and K. Yoshikawa, Langmuir 10 (1994) 1287. 33. A. Miller and H. M6hwald, J. Chem. Phys., 86 (1987) 4258.

246 34.

H . M . McConnell, D. Keller and H. Gaub, J. Phys. Chem., 90 (1986) 1717. 35. N. Kimizuka and T. Kunitake, J. Am. Chem. Soc., 111 (1989) 3758. 36. M. Shimomura, K. Fujii, T. Shimamura, M. Oguchi, M. Shinohara, Y. Nagata, M. Matsubara and K. Koshiishi, Thin Solid Films, 210/211 (1992) 98. 37. J. Mingins, J. A. G. Taylor, B. A. Pethica, C. N. Jackson and B. Y. T. Yue, J. Chem. Soc., Faraday Trans. I, 78 (1982) 323. 38. K. Yoshikawa, S. Maeda and H. Kawakami, Ferroelectrics, 86 (1988) 281. 39. K. Yoshikawa, M. Makino, $. Nakata and T. Ishii, Thin Solid Films, 180 (1989) 117. 40. R . M . Mendenhall and A. L. Mendenhall Jr., Rev. Sci. Instrum., 34 (1963) 1350. 41. R. Bienkowski and M. Skolnik, J. Coll. Interface Sci., 39 (1972) 323. 42. K. Yoshikawa, M. Shoji, and T. Ishii, Biochem. Biophy. Res. Commun., 160 (1988) 699. 43. M. Makino, K. Yoshikawa, and T. Ishii, Nippon Kagaku Kaishi 10 (1990) 1143. 44. S. Nakata, K. Yoshikawa, M. Shoji, H. Kawakami and T.Ishii, Biophys. Chem., 34 (1989) 201. 45. M. Makino, M. Kamiya, N. Nakajo and K.Yoshikawa, Progress in Anesthetic Mechanism, in press. 46. A . W . Adamson, Physical Chemistry of Surface, 5th ed., John Wiley & Sons Inc., New York, 1990. 47. M. G. Velarde(ed.), Physicochemical Hydrodynamics Interfacial Phenomena, Plenum Press, New York, 1988. 48. I. Langmuir, J. Am. Chem. Soc., 39 (1917) 1848.

New Developments in Construction and Functions of Organic Thin Films T. Kajiyama and M. Aizawa (Editors) 9 1996 Elsevier Science B.V. All rights reserved.

M o l e c u l a r control of p h o t o r e s p o n s e s of LB films containing

247

redox

chromophores Toshihiko Nagamura Crystalline Films Laboratory, Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432, Japan Langmuir-Blodgett (LB) films containing redox chromophores such as 4,4'bipyridinium, carbazole, aUoxazine, porphyrin, and Ru(II)-bipyridine complexes showed various photoresponses including photoinduced electrochromism, photocurrents, amplified fluorescence quenching, or photochemical modulation of second harmonic generation. The molecular control of these photoresponses in LB films was successfully achieved by controlling the molecular arrangements, molecular orientation, and/or the extent of aggregation of redox chromophores. The relationship between the structure of organized redox chromophores and the photochemical or photophysical properties will be discussed. 1. INTRODUCTION Organized molecular assemblies containing redox chromophores show specific and useful photoresponses which cannot be achieved in randomly dispersed systems. Ideal examples of such highly functional molecular assemblies can be found in nature as photosynthesis and vision. Recently the very precise and elegant molecular arrangements of the reaction center of photosynthetic bacteria was revealed by the X-ray crystallography [1]. The first step, the photoinduced electron transfer from photoreaction center chlorophyll dimer (a special pair) to pheophytin (a chlorophyll monomer without magnesium ion) separated by about 1.7 nm occurs in about 3 ps [2]. Then electrons are transported between precisely aligned redox chromophores in bilayer membrane with a rate about two orders of magnitude faster at each step than the corresponding reverse reactions. As can be found in this elegant model molecular design of functional compounds and molecular control of their arrangements will be essential to construct highly efficient photoresponsive systems. The Langmuir-Blodgett deposition is one of the best methods to prepare highly organized molecular systems, in which various molecular parameters such as distance, orientation, extent of chromophore interaction, or redox potential can be controlled in each monolayer. We have been studying

248 photophysical and photochemical properties of LB films in order to construct molecular electronic and photonic devices in the near future. In this review article recent results on structural properties, spectroscopic characterization and various photoresponses of organized redox chromophores in LB films will be presented mainly based on our studies. 2. SPECIFIC ORIENTATION AND INTERACTIONS OF CHROMOPHORES IN LB FR,MS The molecular orientation and interactions of redox chromophores are very important in controlling photoresponses at the molecular level. Absorption and fluorescence spectra will give important information on them. We have studied, photoresponses, specific interactions, in-plane and out-of-plane orientation of various chomophores in LB films composed of amphiphiles shown in Figure I [3-12]. 2.1. Specific absorption due to the interactions of chromophores in LB films Chromophores in LB films show specific absorption spectra reflecting their

DDA

RCN'~R2

~ c ~ O

CUA

~N C (.H2hoCOOH

BCZ~NC (.H~~ )oCO HO

PV2+

HV2+ AV2§ TFPB-

C19H39C~OH PA

AMP

DHA

R~=((CH~kO)..R2--(CHz), Re~=C~.. R.=C~.,c,N(c,eH~h, R2=C2Hs o

,,~'~

C15H31C00H

RuC18B

C16H33 Br

~16"33 ~~"~,0CI6 H33

0 II

~ , , , CNICH2)I?CH3

-~.'~K.~

H .2c~o2 ICIN(Ch]I?CH3 O

2C18NB c~{C~)r~,,:~,-cHs. e; CH3(CH~I7/"-, C.3 H

k

0

N

"-2~

n= 5 x 10~

Figure I Structuresand abbreviationsof compounds employed.

249 interactions due to dose packing. Absorption spectrum of four monolayers of 1,3-didodecylaUoxazine (DDA) deposited at 20 ~ and 20 m N o m -z by a vertical dipping method on a quartz plate coated with three monolayers of cadmium arachidate is shown in Figure 2 together with that in n-hexane solution (0.01 0.15 -

i

|'

i

1 ....

10.5

l

0.4

lL_

0.10

0.3~

o r .<

0.05 "

r~

o t/1

O.2 ~

9

0.1 0 200

300

0 500

400 W a v e l e n g t h /nm

Figure 2 UV/vis absorption spectra of DDA deposited four monolayers thick on each side of a quartz plate (solid line) and 0.01 mM of DDA in n-hexane (dotted line).

fififififis YYYYY

:'._" L:= N.

.....

/-

(a)

gO"

(b)

5,4.1"

c~

(a)

O*

(b)

Figure 3 (a) Simple and (b) alternate translational arrangement of chromophores (left) corresponding exciton band structures (right).

and

mM). Two bands at 378 and 318 nm were red-shifted and that at 252 run was blue-shifted in LB films as shown in Figure 2. The spectral shift due to the interactions of chromophores aligned parallel was explained by Eq. (1) according to the simplified exciton model proposed by McRae and Kasha [13] as a first approximation, Av=2(N-1)I~2 (1-3 Cos 2y)/(hcNr 3)

(1)

250 where h is Planck's constant, c is the velocity of light, N is the aggregation number of chromophores which are separated by the distance r, ~ is the transition dipole moment, and y is the tilt angle between the direction of transition dipole moment and the line of centers of aligned chromophores which are translationaUy equivalent as shown in Figure 3(a). This equation predicts that Av becomes negative (red-shi~) or positive (blue-shift) if y is smaller or larger than the magic angle (54.7~ as schematically shown in Figure 3(a). The transition dipole moment , in 10-18 esuocm unit was evaluated from the observed oscillator strength for absorption spectrum of DDA in n-hexane at 378, 318, and 252 nm as 3.18, 3.31, and 7.11, respectively. Following tilt angles were evaluated by employing r=- 4 A and N= 2 - oo; 41 - 48 ~ 40 - 47 ~ and 58 - 56 ~ from observed spectral shift of-548,-677, and +661 cm -1 for transition moments of 378, 318, and 252 nm band, respectively. Transition moments of these bands are parallel to the plane of an aUoxazine ring. These results indicated that the line of centers connecting alloxazine rings is equally tilted from both transition moments of 378 and 318 nm bands and is tilted more from a transition moment of the third band. Transition moments of aUoxazine were oriented approximately 60~, 100~, and 120~ counterclockwise from the line connecting central two nitrogen atoms for 378, 318, and 252 nm bands, respectively [14]. The larger tilt angle for a transition moment of the third band can well be explained by the direction of these calculated transition moments [3]. All absorption bands of carbazolyl groups in mixed LB films of 11-(9carbazolyl)undecanoic acid ( C U A ) a n d palmitic acid (PA) showed the red-shi~ [4,8]. The long and short axes of carbazolyl chromophores were thus shown to be oriented making 45 ~ from the line connecting the centers of chromophores. Other chromophores like anthraquinone, azobenzene, anthracene, naphthalene, squarylium showed spectral shifts in LB films showing interactions [15-19]. The spectral shift in most of these chromophores was, however, observed in an absorption band corresponding to a transition dipole moment along only one direction in contrast to aUoxazine or carbazolyl chromophores mentioned above. The random orientation was assumed for transition dipole moments of other absorption bands showing no shifts. Two transitions of excitons allowed in alternate translational arrangements as shown in Figure 3(b) were reported in LB films of fatty acid containing anthracene chromophore at its hydrophobic end[20].

2.2 In-plane and out-of-plane orientation of chromophores In LB films not only the interaction of chromophores but also their orientation can be controlled at the molecular level. Molecular orientation of chromophores has been determined by several methods including polarized UV/vis or IR absorption, second harmonic generation (SHG), Electron Spin Resonance (ESR), or resonance Raman scattering. We have measured the incident angle and polarization angle dependencies of polarized UV/vis absorption to study the molecular orientation of aUoxazine, porphyrin, and carbazolyl chromophores, or 4,4'-bipyridinium radical cations in LB films[3-12]. Usually in-plane components of transition dipoles of chromophores are

251 randomly distributed in LB films, which give no polarization angle dependencies. In LB films of DDA prepared by a vertical dipping method, a 387 nm band showed higher absorption by p-polarized light than by s-polarized light at normal incidence with a dichroic ratio (DR) of about 1.6, which indicated a preferred orientation of aUoxazine chromophores along the dipping direction [3]. Such anisotropy was not observed in LB films of DDA prepared by a horizontal deposition method. The in-plane orientation was thus most probably caused during the transferring process of alloxazine chromophores aligned at the surface of aqueous subphase as flat hydrophilic groups. Similar in-plane orientation of chromophores was observed in LB films containing flat and fairly large chromophores such as phthalocyanine, porphyrin, merocyanine, or cyanines [21-30]. Phthalocyanines have rather strong tendency to orient their rings perpendicular both to the dipping direction of a substrate and to the substrate plane. Highly in-plane orientation with DR> 6 was reported in phthalocyanine LB films [26]. Minari et al. [31] made a model calculation on the in-plane orientation of merocyanines and reported that it was caused by their flow during a transfer process from air/water interface to the substrate surface. Bird et al. [32] obtained a sort of epitaxial LB films by depositing 3,3'-dioctadecyl-9ethylthiacarbocyanine on the cleavage ac-face of gypsum. The amphipathic chromophores stand with long edge against the gypsum face, and with chromophore short axes nearly vertical. The polarized absorption spectra with polarization parallel and perpendicular to a-axis showed almost perfect orientation of long axes along a-axis with dichroic ratio higher than 20. Similar amphipathic thiacarbocyanine without 9-ethyl group showed little dichroism [32]. The out-of-plane orientation of chromophores can be more easily controlled in LB films as compared with the in-plane orientation. Many chromophores are known to show anisotropic orientation in the surface normal direction. The molecular structure of chromophores and their position in amphiphile molecules, the surface pressure, the subphase conditions are among those affect their out-of-plane orientation. The out-of-plane orientation has been studied by dichroic ratio at 45 ~ incidence, absorbance ratio at normal and 45~ incidence, and incident angle dependence of p-polarized absorption [3,4,27,33-41]. The evaluation of the out-of-plane orientation in LB films is given below using amphipathic porphyrin (AMP) as an example [5,10,12]. Mixtures of AMP and arachidic acid (AA) showed two solid condensed phases below and above about 30 mNom -1 as in Figure 4. The extrapolated limiting area of both solid condensed phases decreased with decreasing molar fraction of AMP. The molecular areas (Alow and Ahi~) occupied by AMP were estimated from the molecular area of cadmium arachidate (0.20 n m 2) and the molar fraction of AMP; Alow - 1.96 + 0.15 rtm2 and Ahi~ = 0.92 + 0.12 nm 2. These values correspond to the molecular area of a porphyrin ring of AMP (about 2 nm2), estimated from a Corey-Pauling-Koltun (CPK) molecular model, and to one-half of its value. Ruaudel-Teixier et al. [42] reported a value of 2.2 nm 2 for the molecular area of tetraarylporphyrin derivatives. Thus at surface pressures lower than 30 mNom -1, molecules of AMP are dispersed in mixed monolayers with their rings almost parallel to the water surface. The ~-A isotherms in Figure

252 4 indicated that the structural changes of surface monolayers should occur at about 30 m N , m q either by orientational change of a porphyrin ring from fiat to vertical or by the squeezing out of AMP or AA from mixed monolayers at the a i r / w a t e r interface. 60

'.

,

.

,

'

"',

,

"'

"r~: E " 40L,.

3

~-

:

b

a

cL 2 0 -

O u

t,.

U~ 0

I

0.2

I

0.4 Area

0.6

.

t

0.8

1.0

/ n m l - m o l e c u l e -!

Figure 4 Surface pressure-molecular area (~-A) isotherms for mixtures of AMP and AA at 18~ pH6.3. The molar ratios (AMP:AA) are as follows: (a) 1-.5, (b) 1:10, (c) 1:15, (d) 1".27.2. Z

M

J Figure 5 The incident ~ and polarization angle a dependence of polarized absorption. The y and z axes indicate the dipping direction and the surface normal. The transition dipole M and its xy projection makes # and 0 from the z and y axes, respectively. The angular dependence of the polarized absorption spectra of LB films containing AMP deposited at lower (20 mN*m -1) and higher (43 or 50 m N . m -1) surface pressures was studied to determine the molecular orientation of p o r p h y r i n s as schematically shown in Figure 5. No polarization angle (a) dependence was observed at normal incidence. This indicates that the projections of the transition dipole moments of the porphyrins are statistically

253 distributed in the film plane. If chromophores are homogeneously dispersed in the LB films, more chromophores are excited by increasing the incident angle as far as the width of illuminated LB films is smaller than that of a monitor light. The correction for this cannot be made directly by the incident angle 13 to the substrate because of the refractive index. The absorbance of s-polarized light with electric vector perpendicular to the incident plane should not depend on the incident angle if the same numbers of chromophores are excited. The absorbance of s-polarized light for LB films of a 1:5 mixture of AMP and AA increased with the incident angle in practice [5,10]. Since porphyrins were found to be homogeneously distributed in the film as mentioned above, this dependence was attributed to the increase of optical path with the incident angle. Then, the correction factor cos~' was estimated at each incident angle so as to make the spolarized absorbance constant [5,10]. The estimated values of cosl3' were smaller than those of cosl3 and corresponded with the result reported by Yoneyama et al [25] for phthalocyanine LB films. The dependence of the p-polarized absorption of AMP at 435 nm on the incident angle is shown in Figure 6 ( a ) a n d 6(b)for LB films from a 1:5 mixture deposited at 20 and 50 m N - m 4, respectively. In these figures the observed absorbance and that corrected by cosl3~ are shown by open and filled circles, respectively. Similar angular dependencies were observed for LB films from a 1:27.2 mixture deposited at 20 and 43 mNom 4. The maximum was observed at normal incidence in all LB films. A larger absorbance was observed in LB films deposited at higher surface pressures compared with those deposited at lower surface pressures in both mixtures. The full lines in these figures were calculated by the least-squares method from the corrected data using the Eq. (2) to describe the dependence of the absorbance on the incident angle, A(I3) = constant{ + cos2~ o(1 - 3)} (2)

.30 k) c-

cO .(3 K..

0 o1

.20

'

1

~

!

i

l

J

I

I

I

o O ~ O o -

.10 0

,

t_

~

I

-90 -60 -30

!

I

0

I

I

30

I

I

t

60

90

Incident angle, B / degrees Figure 6 Dependenciesof p-polarized absorbance at 435 nm on the incident an}~lefor six monolayer LB films of a 1".5mixture of AMP and AA deposited at (a) 20 and (b) 50 mNom-1. The full line is the calculated dependence.

254 It was derived for a model system in which the transition dipole M making an angle ~ from the surface normal as shown in Figure 5 are distributed in the zdirection. The projections of transition dipoles to the substrate surface are statistically distributed in the present LB films as mentioned above. is the mean value for the transition dipoles statistically distributed between two values of ch and ~ . Eq. (2)shows that the incident angle dependence of the ppolarized absorption will give a maximum or a minimum at normal incidence, depending on whether the mean value of ~ is larger or smaller than the magic angle. The following values for the distribution of ~ were obtained for a 1:5 mixture from the full line in Figure 6; 72~ ~ < 78~ in LB films deposited at 20 m N om 4 and 70~ # < 76~ those deposited at 50 mNom 4. For a 1:27.2 mixture, 68~ ~ < 72 ~ and 66~ ~ < 79~ were evaluated in LB films deposited at 20 and 43 m N o m 4, respectively. These results indicate that the porphyrin rings are oriented almost fiat in LB films with various molar ratios deposited from two solid condensed phases at lower and higher surface pressures than 30 mNom -1. The orientation of the porphyrin rings in the two solid condensed phases was concluded to be the same irrespective of the molar ratio. The structural change of mixed monolayers observed at about 30 mNom 4 as shown by the ~-A isotherms in Figure 4 was therefore not caused by orientational changes of porphyrin in mixed monolayers. It was most probably caused by the squeezing out of porphyrins at higher surface pressures. Several investigations have been reported on the formation of supermonomolecular (stacked) structures either at the air-water interface or on solid substrates by the squeezing out process [33,4345]. Most of these have been concerned with changes in the ~-A isotherms and the thickness of LB films. Very few detailed studies have been made of the structural properties, including the orientation of chromophores in the supermonomolecular structure. The supermonomolecular structure with "loosely stacked" porphyrins after the squeezing out at about 30 mNom 4 is strongly suggested by the decrease in the extrapolated area to almost one-half of a porphyrin ring and by the larger absorbance of AMP in LB films deposited at higher surface pressures with a similar orientation of porphyrins. The absorbance ratio of AMP in LB films deposited at higher and lower surface pressures increased with increasing molar ratio of AMP from 1.14, 1.45 to 1 . 8 6 for a 1:27.2, 1:10 and 1:5 mixture, respectively. Such tendency with increasing the molar ratio agreed with that expected for the squeezing out of porphyrins in the supermonomolecular structure at higher surface pressures. The absorption spectra of AMP in LB films of a 1:5 mixture deposited at 20 and 50 mN-m 4 are shown in Figure 7 together with that in dilute methanol solution. Similar absorption spectra were observed for LB films of a 1:27.2 mixture deposited at 20 and 43 mNom -1. The shght broadening and red shift of the absorption spectrum in LB films suggests an interaction between porphyrin chromophores. The shift and width of the absorption spectrum in LB films are not dependent on the surface pressure of deposition corresponding to the monomolecular and supermonomolecular

255 0.3

.!

U

IZ .J

,~ 0.2 /--

0 In X3 .<

C~

:.

b

0.1

~00

500

6O0 Wavelength /nm

700

Figure 7 Absorptionspectra of AMP in LB films (1:5mixture) deposited at (a) 20 and (b) 50 mN-m-1, (c) AMP in methanol solution (0.001 mM). structures mentioned above. This result strongly suggests that the interaction of the porphyrin rings in LB films with a supermonomolecular structure is almost the same as in the monomolecular structure. The presence of three phenyl groups and a pyridinium group at the meso positions, which are known to be almost perpendicular to the porphyrin ring, probably hinders the strong interaction of two porphyrin rings even in squeezed monolayers. It is thus strongly suggested that two porphyrin rings are "loosely stacked" in LB films deposited at higher surface pressures after squeezing out. In the case of porphyrins without phenyl rings at the meso positions such as mesoporphyrin IX dimethylester, the absorption spectra exhibit considerable shifts and broadening depending on the molar ratio of porphyrin and arachidic acid in LB films with monomolecular structure [12]. 2.3 Three-dimensional extended dipole model for interaction and alignment of chromophores The spectral shift observed in LB films has been explained using two models. One is that by McRae and Kasha [13] based on the nearest-neighbor point dipole interaction as mentioned above. Although this model has been widely used to get approximate information on the alignment, it cannot predict the orientation of chromophores with respect to the substrate. In addition, the point dipole approximation is too simple for chromophores in LB films, since the transition density of chromophores extends over most of their molecular length which greatly exceeds the molecular separation of aggregated chromophores[46]. The other is an extended dipole model proposed by Kuhn et al. [47] for a twodimensional arrangement of chromophores as an approximation of the quantum mechanical treatment. In this model a molecule is replaced by a dipole of length l and charges +~ and-~ and the transition dipole moment M is related by M= ~.l.

256 The interaction integral J12 is then given by summing all Coulomb interactions of translationally equivalent transition dipoles. The excitation energy AE' of the aggregated chromophores is approximately represented by that of the free chromophores AE and the interaction integral as AE'= AE + 2Z J12 (3) This model has been successfully applied to J aggregates of cyanine dyes in a brick stonework arrangement [47,48]. However, this model cannot explain the spectral shift of chromophores having transition moments in two or more directions as shown in Figure 8 for long-axis and short-axis transition dipoles of carbazolyl chromophores, nor it can predict the orientation of chromophores with respect to the substrate. In order to explain such spectral shifts and molecular orientation of aUoxazine and carbazolyl chromophores as mentioned above, we proposed a three-dimensional extended dipole model which takes a three-dimensional

-'1

0

a

b

\

t_ =,-4

7

m

tILl

-60

'

30

cr

60

90

-90

30

60 I degrees

90

Figure 8 The interaction energy of (a) long-axis and (b) short-axis transition dipoles in a twodimensional arrangement of carbazolyl chromophores calculated by Kuhn's extended dipole model[47] for the longer wavelength transitions at 291 and 342 nm. Z'

(p~q:r')

-6 (p,q,r)

r

+E

Figure 9 Schematic representations of the aligmnent of transition dipoles for the short and long axes of carbazolyl chromophores in a three-dimensional extended dipole model (left) and the interactions of two transition dipoles (p,q,r) and (p',q',r') (right).

257 arrangement of chromophores into consideration [8]. The alignment of the transition dipoles for the short and long axes of carbazolyl chromophores is shown schematically in Figure 9, in which the arrow showing the transition dipole is directed towards +E. The chromophores with long- and short-axis transition dipoles is denoted by an index (p,q,r). The negative end of the long-axis transition dipole (1,1,1) is set at the origin of a Cartesian coordinate system xyz as shown in Figure 9. The lines connecting the negative ends of the long axis transition dipoles are located in the xy plane, which makes an angle a with the y axis. The long-axis transition dipole makes an angle 0 with the z axis. The projection of the long-axis transition dipole in the xy plane makes an angle ~ with the x axis. The direction of the short-axis transition dipole is characterized by an angle ~ which is defined as the angle between the chromophore plane and the plane including the long-axis transition dipole and z axis. The line connecting the negative ends of the longaxis transition dipoles aligned in the z direction makes an angle y with the z axis. The distance between chromophores in the x, y and z direction is Ax, Ay, and Az, which is assumed to be larger than 0.35 run. The interaction energy J12 between the two transition dipoles (p,q,r) and (p',q',r') shown in Figure 9 is calculated by J12=~2(l/r++ + l/r__- l/r+_- l/r.+)/D= {M21(4acD~12)}(1/r++ + l/r__- l/r+_- l/r.,) (4) where D is the dielectric constant of the medium and r=0is the dielectric constant of vacuum. The Coulomb interaction energy is summed for all combinations of long-and short-axis transition dipoles in three-dimensional arrangements. The spectral shift for aggregated chromophores is related to the total interaction energy by :~X J12. The calculated interaction energies for the long- and shortaxis transitions are plotted against c~ in Figure 10(a) and (b) for some combinations of (p,q,r) with y= 70~ 0= 60~ @=40~ Ax= 0.6 nm, Ay= 0.45 nm, and Az= 0.45 nm. The minimum negative interaction energy corresponding to the -'

20

-, m0

I0

~

-2

_

"

3 0

0

~

, ~ 30 0r

_

_ 60 /degrees

90

"-

,

,

"

"

iq

c w

,

_100

9

9 , 30

, ~

, , I 60

....

90

/degrees

Figure 10 The calculated interaction energy for (a) the long-axis and (b) the short-axis transition plotted against a using the number of chromophores (p,q,r) as a parameter;, (4,3,2); (3,4,2); (3,3,2); (2,4,2).

258 observed spectral shift was obtained at c~= 50~ for both long- and short-axis transition dipoles with p--3, q=3, and r=2. Above-mentioned values of y, 0, and ~> were determined as the values giving minimum interaction energy. These results indicate that the three-dimensional extended dipole model can predict the molecular arrangement of carbazolyl chromophores in LB films with the "true" minimum energy. The projections of the long- and short-axis transitions in the xy and zy planes are shown in Figure 11 for the minimum interaction energy, which indicate that about 18 carbazolyl chromophores in hydrophobic region of two adjacent layers of LB films made interactions to give the spectral shift and specific orientation. Similar calculations were performed for aUoxazine chromophores in LB films, which exhibit absorption spectral shifts for three different transition moments as mentioned in section 2.1. Fairly good prediction was obtained with the interactions of about 32 alloxazine chromophores in hydrophilic region of two adjacent layers of LB films for observed spectral shi~ and orientation. I

.s x4

"

= I

I

I

i '

I

"

I ~

I

"

I

|

'

i

I

I

I

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5

-

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,,,

~

1

I

.

I

2

I

Y

i

3

/nm

I

4

~0

1

2

Y

3 Inm

Figure 11 Schematic representation of the alignment of the transition dipoles of carbazolyl chromophores in the xy and zy planes. 3. AMPLIFIED FLUORESCENCE QUENCtiING AND EFFICIENT ENERGY TRANSFER IN LB FILMS In addition to the change of absorption and fluorescence spectra, the interaction and organization of chromophores in LB films also give various physical properties which cannot be achieved in randomly dispersed system. Highly amplified fluorescence quenching due to efficient energy transfer and efficient photocarrier conduction mentioned in Section 4 are among those examples. The former was observed in LB films of CUA with and without acceptors. In this section, molar fraction dependent energy transfer and amplified fluorescence quenching of carbazolyl chromophores in mixed LB films of CUA and palmitic acid upon photoexdtation will be mentioned in connection with the molar fraction, temperature, and microenvironments [4,49-51]. 3.1 Molar fraction dependent efficient energy transfer in mixed LB films Many studies on the photochemical properties of carbazolyl chromophores

259 in polymers and biscarbazolyl alkanes have been made in relation to the photoconductivity of poly(N-vinylcarbazole) [52], which is one of a few organic materials practically applied in optical information processing. The efficient energy transfer among carbazolyl chromophores have been observed in crystals [53], polymers [54,55], and synthetic bilayer membranes [56] using a small amount of acceptors. The exciton diffusion was shown to be the main mechanism for the electronic energy transfer in molecular crystals of N-isopropyl carbazole or poly(N-vinylcarbazole) [53,54]. The energy transfer from carbazolyl groups in synthetic bilayer membranes of double chain ammonium amphiphiles in aqueous solutions to perylene or anthraquinone derivatives adsorbed on their surface was also analyzed by this mechanism [56]. The fluorescence intensity ratio between acceptors and donors depended linearly on the amount of acceptors in these systems [53-56]. The extent of exciton diffusion reflecting the chromophore aggregation was thus constant in these systems mainly depending on the chemical structure of polymers or amphiphiles. We found the molar fraction dependent efficient energy transfer in mixed LB films of CUA and longchain fatty acids using a small amount of benzocarbazolyl chromophores as an acceptor [4,50]. The molecular orientation and molecular interactions at the ground state of carbazolyl chromophores in mixed LB films of CUA with longchain fatty acids are shown in the previous section from the changes of absorption and fluorescence spectra [3,4,49]. A CUA sample (MP-CUA) containing 470 ppm of 5H-benzo[b]carbazolyl derivative (BCZ) was used to form mixed LB films with PA or AA. Mixed LB films with PA containing MP-CUA by a molar fraction (fc) of 0.05, 0.10, and 0.27 were deposited at 15~ and 20 mNom 4. A mixture of MP-CUA (fc= 0.20) and AA was deposited at 20~ and 20 mNom4. Figure 12 shows the fluorescence spectra (~.ex-- 296 nm) of MP-CUA observed under identical conditions for (a) n-hexane solution (4x10-7 M) and (b) one monolayer mixed LB film with PA (fc= 0.27). In addition to the red-shift of monomer fluorescence of carbazolyl chromophores .

,

.

,

,

o _ . .

o3 r_ O o G,

.,: "..,'..

1

lug 300

350

:"

,,"

"":.~:-',~: ":':"

9

I;..

.

400 450 Wavelength /nm

"

500

Figure 12 Fluorescence spectra of (a) MP-CUA in n-hexane (0.4 ~M, solid line) and (b) one mixed m o n o l a y e r of MP-CUA (fc= 0.27, dotted line) and PA at 20~ and ~ex = 296 nm.

260 observed at 350 and 366 nm in n-hexane by about 6-10 nm, additional fluorescence peaks at 417 and 442 nm were observed in LB films. The latter was assigned to the fluorescence of 5H-benzo[b]carbazole [58]. The excitation spectra monitored at 417 and 442 nm corresponded with the absorption spectra of pure CUA. The direct excitation of BCZ at 390 nm in LB films or in dilute n-hexane solution did not give any fluorescence most probably due to negligibly small absorbance (<1x10-6). These results indicated that a very small amount of BCZ was excited effectively by the efficient energy transfer from carbazolyl chromophores in LB films which was confirmed by the fluorescence depolarization [4]. Figure 13 shows the fluorescence spectra of mixed LB films of MP-CUA with PA (a) fc= 0.05 and (b) fc= 0.10. The fluorescence from BCZ is hardly detected for fc= 0.05. Similar fluorescence spectrum was reported for LB films of MP-CUA (fc= 0.015) and stearic acid [57]. An increase of the molar fraction of MP-CUA to 0.10 while keeping the content of acceptors constant (470 ppm) resulted in fairly strong acceptor fluorescence of BCZ as shown in Figure 13(b). The intensity ratio of the emission of BCZ at 417 nm (IA) and the m o n o m e r emission of carbazolyl groups at 360 nm (ID) is plotted against fc-values ( solid line) in Figure 14. The result indicates that the IA/ID ratio increased steeply with the molar fraction fc in LB films above about 0.05. At higher fc-values the IA/ID ratio increased more gradually. The dotted line shown in Figure 14 was calculated from F6rster energy transfer among randomly distributed donors and acceptors in LB films. It did not explain the observed molar fraction dependence of IA/ID ratio in mixed LB films. Then the singlet exciton migration model assuming that carbazolyl chromophores are aggregated in mixed LB films sufficiently to allow the delocalization of excitation energy. This model was originally proposed for the energy transfer in perylene-doped N-isopropyl carbazole crystal or poly(N-vinylcarbazole) [53,54]. According to the hopping model of exciton diffusion, the fluorescence intensity ratio of acceptors and donors is given by IA/ID = nc(1-F)cpA/q)D (5)

>,5~ "4

%

t..-

e3 u 1::

e2

u ol

*.. ;

-.

'- 1 0 :::}

)0

L

350

.

|

,

400 Wavelength

9

450

---

500

/nm

Figure 13 Fluorescence spectra of mixed LB films of MP-CUA and PA at 20~ for (a) 15 monolayers of a 1:19 mixture and (b) 11 monolayers of a 1:9 mixture, kex = 296 nm.

261

~

e t_

:,,1.0-

,

-

c"

._c t

uC o . 5

r u i/I r I-

0

U.

0

0

'

|

0.1 0.2 0.3 0.4 0.5 Molar fraction of MP-CUA

Figure 14 Fluorescence intensity ratio of acceptors at 417 nm and donors at 360 nm plotted against the molar fraction of MP-CUA in mixed LB films with (O) PA and (@) AA. The dotted line shows the calculated dependence by FOrster energy transfer. where c is the concentration of guest molecules (acceptors in this case), n is the number of jumps during the lifetime of donors in the absence of acceptors, and F is the probability that the exciton retun~ to its starting point at least once during its random walk. Using the values of ~A= 0.675 for BCZ, cI)D= 0.32 for carbazole, and c= 4.7x10-4, the value of n was evaluated from the observed fluorescence intensity ratio to be 1600, 1400, 700, and 45 for f,:= 0.27, 0.20, 0.10, and 0.05, respectively [49]. These n-values can be compared with that (about 1000) reported for perylene-doped poly(N-vinylcarbazole) [53,54] or in synthetic bilayer membranes of a m m o n i u m amphiphiles containing carbazolyl groups [56]. These results strongly supported the exciton migration model in the present mixed LB films. Much higher value of 4.1x104 was found in perylene-doped crystal [53]. The non-statistical aggregation of chromophores depending on the molar fraction

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Figure 15. Schematicrepresentation of mixed LB films including carbazolyl donors ( q ) and a small amount of acceptors ( I ~ ) with (a) higher and (b) lower fc values.

262

was also shown from these results. The aggregated chromophores in mixed LB films will form domains depending on the molar fraction due to the phase separation between MP-CUA and fatty adds. A schematic representation is given in Figure 15 for mixed LB films with (a) a higher fc and (b) a lower fc value according to these results and the orientation of carbazolyl groups mentioned in section 2.2. Both the intra- and inter-layer energy transfer will be possible between aggregated carbazolyl chromophores in LB films [50]. The extent of energy migration among aggregated carbazolyl chromophores can be controlled in LB films by molar fractions whereas it is almost constant in other systems reported so far [53-56].

3.2 Amplified fluorescence quenching in LB films Amplified photochemical quenching of carbazolyl fluorescence was observed in mixed LB films containing pure CUA and long-chain fatty acids [49,51]. A pure CUA was synthesized from 2-nitrobiphenyl and 11bromoundecanoic acid methyl ester as reported previously [49,50]. Two monolayers of mixtures of CUA (fc= 0.02 - 0.50) and PA were deposited on five monolayers of cadmium arachidate at 15~ and 20 m N * m -1 at pH 6.3. Figure 16 shows the fluorescence spectra of LB films containing CUA with fc= 0.25 during irradiation at 290 nm and 2.5 mW*cm -2 in argon atmosphere. It is clearly shown that the fluorescence of CUA decayed considerably and monotonously by irradiation even in argon. Irradiation of similar LB films in air caused much faster decay of fluorescence spectra [49]. No changes were observed in fluorescence spectra and intensities for CUA in solution similarly irradiated or for LB films of CUA stored in the dark. The extent of fluorescence quenching in mixed LB films of CUA and PA during irradiation at 290nm in air is shown in Figure 17 as a function of ft. The '

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....

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-

Time

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tn

c

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15

1-

225

L,,

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L'- 0

300

j

350--

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-

,

.

.

.

400 450 W a v e l e n g t h Inm

500

Figure 16 The fluorescence spectra of LB films with a CUA molar fraction of 0.25 during irradiation in argon at 290 run and 2.5 mW / cm2.

263 fluorescence intensity was normalized by the initial value. These results dearly show very rapid fluorescence quenching upon irradiation for a few minutes and very gradual quenching during later irradiation in LB films with fc> 0.05. Only gradual quenching was observed for a system with fc= 0.02. Similar results were observed for mixed LB films irradiated in argon [51]. The time dependencies are not expressed by a first-order nor a second-order kinetics, which will be explained below. No effects of the excitation wavelength on the fluorescence quenching behavior were observed.

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t. 0

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! 45

I o0

t ime/rain Figure 17 Time dependences of normalized fluorescence intensity at 355 nm for two mixed LBfilms of CUA and PA irradiated at 290 nm and 1.0 mW/cm2 in air at 15 ~ The fc values are 0.02, 0.05, 0.15, 0.25, 030 and 0.40, respectively, from the top. No changes in the absorption spectra of LB films were observed in argon during irradiation for up to 225 h. The decay of fluorescence during irradiation in argon was also found to depend on the temperatures. It became slower at lower temperatures and only 16% decayed at 80~ after 90 min irradiation, which indicated some contribution of thermal process to the fluorescence decay. From these results the fluorescence quenching in argon is most probably due to the efficient energy transfer to the nonradiative sites formed by some changes of aggregation structure of carbazolyl chromophores in LB films. Similar fluorescence quenching without changes of absorption spectra was reported in vacuum deposited films of pyrenecarboxylic acid upon irradiation by a Xe lamp or an excimer laser [59]. It was attributed to the structural changes of aggregates of pyrenyl chromophores to the non-fluorescent aggregate [60]. Meanwhile irradiation of LB films in air caused gradual spectral changes suggesting the photooxidation of carbazolyl chromophores [61]. The absorbance at 290 nm, however, decreased only 4% during irradiation for 90 min in air, where about 80% of fluorescence was quenched as shown in Figure 17. All these results indicated that the fluorescence was quenched with much higher extent than the changes of absorbance at the excitation wavelength. Such amplified fluorescence quenching in mixed LB films of CUA and PA was most probably

264 caused by the very efficient and molar fraction dependent energy transfer to a trace amount of nonradiative sites formed by photooxidation or changes of aggregation structure of carbazolyl chromophores during irradiation. The efficient energy transfer among carbazolyl chromophores occurred by the singlet exciton migration as mentioned above [50]. The mean displacement rh for randomly hopping exdtons corresponding to the number of jumps is 1.9 nm at fc= 0.05 and 20.0 nm at fr 0.27 [50]. Then the fluorescence from CUA molecules located in a circle with a radius of rh will be quenched if one CUA molecule within that circle would become nonradiative by photooxidation or changes of aggregation structure. The dependencies of fluorescence quenching on the irradiation time shown in Figure 17 and on the molar fraction can be explained by the mechanism that carbazolyl chromophores are distributed or aggregated inhomogeneously in mixed LB films and the extent of their aggregation varied with the molar fraction of CUA. The time dependencies of fluorescence quenching in LB films shown in Figure 17 can be understood as the result of dispersive electronic excitation transport, i.e. a set of single exciton hopping processes with different rates depending on the spatial distribution. As a small number of the energy trap sites are formed during irradiation, the rate of exciton transport slows because the density of carbazolyl chromophores available to accept exciton is reduced. The time dependence of such dispersive processes is known to be expressed by I(t) = I0-exp(-kt a) (0< cz <=1) (6) with the dispersion parameter cz being a measure of the deviation from pure exponential decay. Good linear relationships were obtained from logarithmic plots of IF(t)versus t ~ for fluorescence quenching in air and in argon with a = 0.20 except some deviation at the very early part of the decay in air [51]. This result strongly suggests that amplified fluorescence quenching is caused by almost the same dispersive energy transfer process among carbazolyl chromophores in both cases. The difference in air and in argon is most probably due to the formation rate of nonradiative sites. Similar dispersive electronic excitation transport was observed in polymeric solids, an organic glass and dye monolayers on crystals[62-65]. Figure 18 shows the schematic representation of the energy transfer to an energy trap site, formed by photooxidation or by the

high-"

.

m o l a r fraction of C U A

.,.

~ low

Figure 18 Schematic representation of the energy transfer to a trap site ( ~ ), formed during irradiation, depending on the molar fraction of CUA in LB films.

265 changes of aggregation structure, depending on the molar fraction of CUA. The phase separation of CUA and PA occurred in LB films at about fc= 0.02-0.05 as mentioned above. The number of aggregated CUA and the extent of aggregation will increase with the molar fraction fc. The larger the fraction of CUA in mixed LB films, the more fluorescence will then be quenched by the energy transfer to the trap site as shown in Figure 18. Similar amplified fluorescence quenching was also observed in squarylium dye LB film containing J-aggregates and was successfully used to detect NO2 gas at as low as a ppb (parts per billion) level [66,67].

4. TRANSIENT AND STEADY PHOTOELECTRIC PROPERTIES OF LB FILMS CONTAINING PORPHYRIN AND REDOX CHROMOPHORES The LB technique is relevant to fabricating molecular electronics devices (MED). Among possible applications of MED, the storage or processing of information using excess electrons in a molecular layer is very interesting. To achieve this, the control of slow interlayer and rapid intralayer electron transfer is required. LB films containing redox chromophores are a sort of organic multi-quantum well. Several reports have recently been made on transient photoelectric responses in LB films [68-80]. The mechanism of electron transfer in the LB films will be analyzed at the molecular level from such measurements. The rate of electron transfer is controlled by the distance, orientation of chromophores or height of multiple-quantum well. In this section the control of photocarrier conduction in LB fihns by incorporating redox chromophores and their interactions will be discussed. 4.1 Control of photocarrier conduction in LB films by redox chromophores We have first reported transient photocu~ents in LB films consisted of monolayers of amphipathic porphyrin (AMP) and monolayers of long-chain fatty acids[68]. From this result and steady-state photocurrent measurements it was shown that most photocarriers generated from excited porphyrins decayed in adjacent monolayers of fatty adds. Transient and steady photoelectric responses of LB films composed of AMP and redox chromophores such as 1,3dihexadecylaUoxazine (DHA) or CUA were then studied in an attempt to control photocarrier conduction at the molecular level [71,74,77]. On five monolayer of a 1:4 mixture of AMP and AA were deposited additional two monolayers of DHA or PA alone, mixtures of CUA and PA with fc= 0.02 - 0.25, or a mixture of DHA and AA with fc= 0.17, as schematically shown in Figure 19 for five AMP/AA and two CUA/PA (fc= 0.05) monolayers with base ITO and top A1 electrodes. These LB films are abbreviated to PCUA0, PCUA2, PCUA5, PCUA25, PD0, PD17, PD100 corresponding to the mol% of CUA or DHA. These LB films set in a cryostat were irradiated by a 150W Xe lamp through appropriate filters and a monochromator. Transient photocurrents upon excitation with a ns dye laser were measured with a 2-CH digital memory through a fast current amplifier. The action spectra of steady photocurrents corresponded well with the

266

absorption spectrum of AMP in LB films for all systems at about 350 - 700 nm except PD17 and PD100 which gave additional photocurrents below about 425 nm due to alloxazine chromophores [71,74]. Steady photocurrents in LB films containing redox chromophores depended on the nature and molar fractions of chromophores. They decreased in the order of PCUA5 > PCUA25 > PCUA2 > PCUA0 in LB films containing carbazolyl chromophores [74,77]. Meanwhile, they increased with increasing the molar fraction of DHA in the case of LB films containing alloxazine derivatives [71,77]. Transient photocurrents upon excitation of AMP in LB films with a pulsed dye laser showed a fast rise controlled by the time resolution (4-6 ~s) of a current amplifier and a slow decay in all LB films. No transient photocurrents were observed for LB films of PA or AA alone. The dependencies of transient photocurrents on the wavelength of a dye laser corresponded well with the absorption spectrum (Q-band) of AMP. These results clearly indicated that observed transient photocurrents were due to the movement of photocarriers generated by the excitation of porphyrin in LB films. The decay was found to depend on the bias voltage, the nature and fraction of chromophores in mixed monolayers adjacent to porphyrin LB monolayers. Logarithmic plots of transient photocurrents are shown in Figure 20 for short circuit photocurrents in (a) PD100, (b) PD17, and (c) PD0 systems, all of which showed an exponential decay. The decay became slower by incorporating DHA in cadmium arachidate matrix. In a PD100 system which contained two monolayers of pure DHA, the decay became markedly retarded. With increasing bias voltages, the decay became slower and the extent of changes depended on the molar fraction of DHA. Similar results were obtained for LB films containing CUA. These results cannot be explained by a simple tunneling mechanism in which no decay of carriers are assumed before reaching electrodes [68,71]. They

9

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=. C

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top A1

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ao Figure 19 Schematicrepresentation of LB films composed of chargegeneratingAMP and charge transporting redox monolayers sandwiched between two electrodes.

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....

o'.~

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Time / ms

Figure 20 Logarithmicplots of short circuit transient photocurrentsfor (a) PD100,(b) PD17, and (c) PD0 LB films excited at 525 nm laser.

267 strongly suggest that most photocamers are disappeared or are trapped on the way of transfer processes. As long as the relaxation time required for tunneling conduction which was estimated to be 1-10 ms for arachidate monolayers or their equivalents [81,82] is longer than the "lifetime" of the photocarriers determined by various decay processes, the apparent lifetime observed will increase as the probability of capture of photocarriers decreases. The increase of lifetime with bias voltages can be explained qualitatively by assuming that the increased velocity of photocarriers at high bias voltage decreased the capture probability of the photocarriers by recombination sites or "traps" [68]. Such decay of photocarriers observed by transient photocurrents was shown to control the steady photocurrents[68]. The incorporation of redox chromophores in LB films composed of AA or PA without changing the thickness of monolayers increased the lifetime of photocarriers as shown in Figure 20. Redox chromophores are thus concluded to effectively retard the decay of photocarriers during their transfer processes presumably by a superexchange mechanism which works in long distance electron (hole) transfer in rigid matrix or in biological systems [8385]. It is based on wave function propagation via nearest-neighbor exchange interactions. Aromatic chromophores containing x-electrons have been reported to facilitate the electron transfer reaction by a superexchange mechanism between donors and acceptors linked by them as a bridge [86]. Redox chromophores in LB films placed between carrier-generating porphyrin layer and electrode are expected to make the transfer of photocarriers by through-bond electronic exchange interactions and to form much less "deep" traps as would be expected in long-chain fatty acid monolayers alone. 4.2 Effect of supermonomolecular structure on photoelectric properties As discussed in section 2.2, a mixture of AMP and AA showed two solid condensed phases above and below about 30 m N - m q [5,10]. A loosely stacked structure of two porphyrins was proposed for LB films prepared at higher surface pressures than 30 mN-m -1, which was caused by squeezingout of a monomolecular structure formed at lower surface pressure [5,10]. In this section, photoelectric characteristics of LB films containing AMP and AA deposited at two solid condensed phases will be discussed in relation to multilayer structure and the anisotropic intermolecular tunneling rates [87]. Seven monolayers of 1:5 or 1:10 mixture of AMP and AA were deposited at 20 and 50 mNom -1 on an ITO plate at 18 ~ to form stable Y-type LB films. Aluminum was vacuum evaporated onto LB films as sandwich-type electrodes at 10-6 Torr. Steady photocurrents were measured in a similar manner as mentioned above. The dependence of steady photocurrents on excitation wavelength corresponded well with the absorption spectra of AMP in LB films as shown in Figure 7 independent of the molar ratio or the surface pressure of deposition. Much higher photocurrents were observed at the same bias voltage in LB films deposited at 50 m N . m -1 (film A) than those at 20 m N - m a (film B). Figure 21 shows a bias voltage dependence of the ratio of photocurrents at 560 nm for film A and film B, I50/I20, deposited from 1:5 and 1:10 mixtures. The ratio was

268 almost independent of mixtures and was much higher than the absorbance ratio, 1.86 and 1.45, respectively, as mentioned in section 2.2. It decreased with increasing bias voltages above -0.2 V and showed an asymptotic value of about 11 at higher bias voltages. The ratio of steady photocurrents in two LB films deposited at 50 and 20 mN~ can be written as follows based on anisotropic intermolecular tunneling in LB films proposed by Donovan et al. [80] who took into accounts of the intralayer bimolecular recombination of photocarriers and the interlayer tunneling conduction. 40 +

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l

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04 ==.,,

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-1.0

i

I

I

t"

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0.0

0.s

1.0

A p p l i e d bias v o l t a g e / V

Figure 21 The ratio of steady photocurrents for LB films of 1:5 (dosed circles) and 1:10 (open circles) mixtures deposited at 50 mN.m-1 to those at 20 mN.m-1 as a function of applied bias voltages. I5o/I2o =

(Qso/t)/(Q2o/t)=

(T,palS0/T,perS0)(A20/As0)(I"I50/H20)/(T,pal20/T,per20)

(7)

where t is the irradiation time, and I, O~ A and H are steady photocurrents, photoinduced charges, projected area of a molecule in the layer plane, and a geometric dielectric factor, respectively. The suffix, 50 and 20, denotes the surface pressure of deposition for film A and film B. T,pal and T-per are parallel and perpendicular intermolecular tunneling time, respectively. The Eq. (7)holds for the recombination of electrons and holes as a result of the intralayer diffusion simultaneously occumng with the interlayer transfer. In the present case, Hso is almost identical with H20, since the same monolayer was deposited as Y-type LB films at 50 mNom -1 and 20 m N - m -1 and similar molecular arrangement in each monolayer, except a loosely stacked structure as schematically shown in Figure 22, can be assumed from ~-A isotherms and polarized absorption measurement. A20/Aso is constant depending on the molar fraction of AMP and is two for the present mixtures as mentioned in section 2.2. The T,pa] will be independent of electric fields, since porphyrin rings lie almost fiat in both LB films. The bias voltage dependence of I50/I20 is then proportional to Zper20/T-perS0- Since the observed

269 ratio of I50/I20 was higher than 11 as shown in Figure 21, the ratio of ~er20/ZperSO was shown to be much larger than unity, which means that ~er50 was much smaller than ~er20. The perpendicular intermolecular tunneling rate (~er50-1) in LB films prepared from solid condensed phase at 50 m N . m -1 is thus shown to be much larger than that of LB films prepared from solid condensed phase

oooooo> oooooooooooo oooooo -

oooroooooo ooo oooooooooooo ooooooooo!ooooooo!,ooooooooo!ouooooo!

Figure 22 Schematic representation of LB films containing AMP deposited at (a) 20 and (b) 50 mN~ at 20 m N . m q. The larger absorbance and polarized absorption spectroscopy of mixed porphyrin LB films prepared at higher surface pressures than 30 m N . m q strongly suggested that porphyrins were squeezed-out and loosely stacked in LB films as mentioned in section 2.2. Then comparing the intermolecular tunneling rate (~hish-1) from a porphyrin pair p to a porphyrin pair q in LB films containing a loosely stacked pair as schematically shown i ! !

I! I o8~ 9

Ipairp~

"p~~

r,a~ f"

~

Figure 23 Schematicrepresentation of the intermolecular tunneling interaction between a pair p to a pair q in LB films containing loosely stacked porphyrins (ellipses).

270 in Figure 23 with that (Zlow-1) for monomolecularly dispersed porphyrins in LB films prepared at lower surface pressure, we obtain ZlowI'rhi~ = exp(l~((dx2+dy2) 1/ 2 _ (dx2+dy2+4_4dycosO+4dxsinO)l / 2)) + ~+Jay--j ~1t2 - (dx~dy2+4+4dycos0.4dxsin0)l/2)) + 2 exp(kb(( d x~

(8)

where rx=bdx and ry=bdy. From the spectroscopic results mentioned above, we can set 0= 0~ [10]. The calculated values of Tlow/Xhir,h for kb= 1.74, based on the damping constant for through-space tunneling (k= 0.58 A-l) and the typical distance (2b) between tetraphenyl-type porphyrin rings (about 6/k), varied depending on dxand dy values [87]. The minimum value of dy is 2 (ry= 6.0~) for porphyrin in adjacent (face-to-face arrangement) hydrophilic regions as schematically shown in Figure 22. The maximum value of dy is 14.3 for porphyrin pairs in hydrophilic regions separated by two arachidate monolayers as schematically shown in Figure 22. The former is expected to predominantly contribute to the observed photocurrents , since the probability of perpendicular intermolecular tunneling decreases exponentially with the distance. The observed value of Zper20/X~erS0 at higher bias voltages is estimated to be 5.5 from Figure 21 and eq. (7) as mentioned above. Although the distance between porphyrins in the parallel direction will be distributed over a wide range, we can estimate the average value of dx to be 5.4 and 6.9 for 1:5 and 1:10 mixtures, respectively, from the observed limiting area at both solid condensed phases and the molar ratio. The calculated values of Zlow/Zhighwith dy = 2 and dx = 57 corresponded well with the observed value of Zper20/X~er50 at higher bias voltages. This result indicates that the average distribution of porphyrins in LB films contributes to the photoconduction at higher bias voltages where intermolecular tunneling of photocarriers occurs rather easily because of a decreased tunnel barrier, as suggested by the Poole - Frenkel theory [27]. Meanwhile, the observed value of X?er20/ZperS0 at lower bias voltages corresponds with the calculated values of Xlow/Xhigh with dy = 2 and dx = 1 - 2. This result strongly suggests that at lower bias voltages only porphyrins located much closer than the average distribution value can contribute to the photoconduction, probably due to a larger intermolecular tunneling time because of a much less decreased barrier height. The present model of intermolecular tunneling is thus supported for photocurrents in LB films deposited from two solid condensed phases with different molar fractions of amphipathic porphyrins which were loosely stacked and monomolecularly dispersed depending on the surface pressure. Thus the supermonomolecular structure of porphyrin pairs in LB films are concluded to greatly facilitate intermolecular photoconduction. 5. PHOTOINDUCED ELECTROCHROMISM IN LB FILMS CONTAINING IONPAIR CHARGE-TRANSFER COMPLEXES Various photochromic systems employing polymeric thin films or LB

271 films have recently attracted much interest in view of their promising applicability to high-speed and high-density photon-mode optical memory. The photochromism reported so far involves changes of chemical bonds such as heterolytic cleavage of a pyran ring in spiropyrans or cis-trans isomerization in azobenzenes. Very recently we have reported novel photochromism (photoinduced electrochromism) in organic solutions [89,90], microcrystals [91,92], LB films [7,9,93,94], and polymer films [95-98] which was due only to the photoinduced electron transfer reaction via the excited state of specific ion-pair charge-transfer (IPCT)complexes [99,100] of 4,4'-bipyridinium salts with tetrakis[3,5-bis(trifluoromethyl)phenyl]borate [101] (abbreviated to TFPB-). The photochemical colouring and the thermal fading due to the reverse electron transfer were highly reversible in deaerated atmosphere in all systems [7,9,89-98]. The lifetime of coloured (blue) state was found to depend markedly on the microenvironments and temperatures. From steady and laser photolysis results it has been strongly suggested that 4,4'-bipyridinium radical cations escaped from the geminate reaction immediately after the photoinduced electron transfer in less than 20 ps [102] upon IPCT excitation became metastable owing to the bulk and chemical stability of TFPB-, to the restriction of molecular motion by the microenvironment, and also probably to the very high exothermicity of the reverse reaction in the Marcus inverted region [103]. Higkly sensitive detection of photoinduced electrochromism in ultra-thin LB and polymer films has also been achieved by the optical waveguide method [104,105]. Such photoinduced electrochromism may be thus applied to ultrafast photon-mode optical memory and to redox sensors. In this section photoinduced electrochromism and molecular control of orientation of photogenerated radicals in LB films will be discussed. 5.1 Steady photolysis and control of molecular orientation of radicals in LB films Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB-) salts of N,N'dihexadecyl-4,4'-bipyridinium (HV2+) and N-ethyl-N'-(2-ethylamide)-N",N"dihexadecyl-4,4'-bipyridinium (AV2+) were prepared from corresponding bromide salts and Na§ -. The structures of amphipathic 4,4'-bipyridinium ions are shown in Figure 1 together with that of TFPB-. Monolayer properties of several mixtures of AA with TFPB- salts of HV 2+ or AV2+ were studied on an aqueous subphase containing 0.25 mM CdC12 and 0.05 mM NaHCO3 pH 6.3 at 18~ LB films were deposited at 20 m N . m -1 and 18~ on a quartz plate for UV/vis or on a poly(ethyleneterephthalate) film for ESR measurements from 1:1 and 4:1 mixtures of AA and HV 2§ or AV2+. The deposition ratio was almost unity during 30 deposition cycles for all mixed monolayers. For steady photolysis these samples were irradiated in degassed condition by a Hamamatsu 150 W XeHg lamp equipped with a Toshiba L-39 cut off filter (~ex> 365 nm) and a 10 cm water filter to excite their IPCT absorption band alone. The incident angle dependencies of both s- and p-polarized absorption for 4,4'-bipyridinium radical cations were measured in degassed condition together with the polarization angle dependence at normal incidence.

272 The ~-A isotherms are shown in Figure 24 for three mixtures of AV 2+ and AA. The ~-A isotherms exhibited several transitions. A similar ~-A isotherm was observed for a mixture of HV 2+ ( 18.8 ~ ) and AA [94,95]. The apparent limiting area observed at each transition for mixtures of A V 2+ and AA corresponded well with the calculated values based on the molecular area of TFPB- (1.4 nm 2) and 4,4'-bipyridinium ion (0.82 nm 2) for a stepwise squeezingout of 4,4'-bipyridinium ion and TFPB-, which does not dissolve in water, as schematically shown in the inset of Figure 24. From an X-ray analysis on a single crystal of N,N'-dimethyl-4,4'-bipyridinium tetraphenylborate (TPB-), Moody et al. [106] reported that the 4,4'-bipyridinium ion was sandwiched between two TPBions. Ion-pair charge-transfer (IPCT) complexes of TFPB- salts were expected to have similar configuration from several spectroscopic data [89,92,107]. Such a structure of IPCT complexes corresponds well to that schematically shown in the inset (A) of Figure 24 based on the limiting area. AV 2+ and AA systems showed larger molecular areas than HV 2+ and AA systems in all corresponding mixtures. This result may reflect the different orientation of 4,4'-bipyridinium ions as mentioned below. ~

m

=40 Q.

20

b.

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~

9

0".2

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0.4 0.6 0.8 Molecular area l n m 2

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Figure 24 The a-A isotherms for mixtures of AA with AV2+ by a molar fraction of (a) 0.10, (b) 0.188, and (c) 0.50 at pH 6.3 and 18~ The inset shows the schematic representation of surface monolayers during compression processes (C)-> (B) -> (A). The circle and rectangle in the inset represent TFPB- and 4,4'-bipyridinium group of AV2+, respectively. Upon irradiation of an IPCT band in degassed condition (Xe• 365 nm), the colour of both LB films changed from pale yellow to blue. The UV/vis absorption spectrum after irradiation is shown in Figure 25, which is characteristic of 4,4'-bipyridinium radical cation monomer[108]. Coloured species photogenerated in mixed LB films of AV2+/AA or HV2+/AA systems decayed almost exponentially in the dark in vacuo with a lifetime of about 4 h at 20 ~ [93,94]. The lifetime of 4,4'-bipyridinium radical cations in LB films was almost

273

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Figure 25 Absorptionspectrum of mixed LBfilms (60x2) of HV2+ and AA (1:4) after excitation (>365 nm) in degassed condition at 20~ for 10 min using non-irradiated LBfilms ~ a reference. the same as that in microcrystalline films [91], which indicated the microenvironment around photogenerated radical cations in LB films is similar to that in microcrystals. Such photochemical colouring and thermal fading was repeated reversibly. A broad single line ESR spectmnn was observed upon irradiation of both LB films. In the HV2+/AA system it showed little anisotropy upon rotation around the dipping direction. In the AV2+/AA system, the spectral width (Amsl= 1.42 mT) for the magnetic field parallel to the film plane was larger by about 6% than that (1.33 roT) of the magnetic field perpendicular [9]. Polarized absorption spectra of photogenerated 4,4'-bipyridinium radical cations were measured in vacuo for LB films of HV2+/AA and AV2+/AA systems as a function of polarization angle and incident angle. The thermal decay of radicals during measurements of polarized absorption spectra was corrected by their lifetime [7,9,93,94]. The different optical path length in the incident angle dependence measurements was also corrected from an apparent incident angle dependence of s-polarized absorption in a similar way mentioned above for amphipathic porphyrin [5,10]. No polarization angle dependencies were observed at normal incidence in both LB films. The p-polarized absorption of 4,4'-bipyridinium radical cations at 400 nm, which corresponds to the shortaxis transition, are shown in Figure 26 for (a) HV2+(18.9 %)/AA and (b) A V2+(18.9 %)/AA. Figure 26 shows a minimum absorbance in HV2+/AA and a maximum in AV2+/AA at normal incidence. The solid lines in Figure 26 are calculated by the least square method taking the angle (~) distribution of the transition dipole moments with respect to the surface normal into account. The best fit curves gave following value of ~; 45 ~ < ~ < 46 ~ for HV2+ / A A and 89 ~ < ~ < 90 ~ for A V2+/AA systems, respectively. Similar incident angle dependencies were observed at 614 nm which is due to a long-axis transition of 4,4'-bipyridinium radical cations. From these results and the simulation of angular dependencies,

274

it was shown that both the long and short axes of 4,4'-bipyridinium radical cations lay almost fiat in LB films of AV2+/AA and inclined by about 46 ~ to the substrate surface in LB films of HV2+/AA as schematically shown in Figure 27.

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Figure 26 The p-polarized absorbance of 4,4'-bipyridinium radical cations in LB films at 400 nm after correction of the decay and optical path length for photoexcited (a) HV2+/AA and (b)AV2+/AA systems. The solid lines are calculated dependences.

Zy Figure 27 Schematic representation of the orientation of 4,4'-bipyridinium radical cations in (a)HV2+/AA and (b) AV2+/AA LB films. Counter anions (TFPB-) and AA are not shown for simplicity.

275 5.2 Optical waveguide detection of photoinduced electrochromism in ultra-thin films It is very interesting and important to observe colour changes in LB films of a single or a few monolayers thick in view of a sensing application with fast response, studying dynamics of photoinduced reactions between organized chromophores, or the easy preparation of good quality LB films. We have applied an optical waveguide (OWG) method for such purposes [104,105]. The electric fields of light propagating through the OWG layer have an exponentially decreasing value as evanescent waves beyond the surface of the OWG. Evanescent waves have been used to sensitively detect and characterize adsorbates and thin films on the OWG. Thin films (S) deposited on the surface of OWG were degassed by a rotary pump in a small chamber and irradiated with a Xe-Hg lamp through appropriate filters (~,ex > 365 nm) as shown schematically in Figure 28. A linearly polarized He-Ne laser (632.8 nm) was used as a monitor light. A 150-fold sensitivity of the OWG method as compared with the conventional method was demonstrated from the colour change measurement in about 180 nm thick film of pV2+CFFPB-)2 containing 4,4'bipyridinium groups as part of the main chain as shown in Figure 1. The absorbances calculated from the OWG signal, using that before irradiation as a reference, are plotted in Figure 29 against the irradiation time for pV2+Cl~FPB-)2 thin films of various thickness; (a) 10.0, (b) 40.4, (c) 64.9, (d) 95.5, and (e)179.6nm. It is clearly seen that the number of photogenerated 4,4'bipyridinium radical cations increased linearly with irradiation time. The rate of absorbance changes was proportional to the film thickness in the range studied. |o

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1000

2000

300U

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Figure 28 Schematic representation of the OWG system for detecting photoinduced electrochromism of ultrathin films (S) in the evacuation chamber shown in an inset.

Figure 29 Changes in the OWG absorbance of pV2+(TFPB-)2 thin films of various thickness during IPC~ excitation: (a) 10.0, (b) 40.4, (c) 64.9, (d) 95.5, and (e) 179.6 nm.

276 These results strongly suggested that pV2+(TFPB-)2 thin films of various thickness are homogeneous and that 4,4'-bipyridinium groups are distributed randomly throughout the polymer films. Photoinduced colour change in a single-monolayer LB film was successfully detected as shown in Figure 30(a) for the HV 2+ /AA system [105]. Comparison of this result with Figure 25 also demonstrated more than 120fold sensitivity of the OWG method. Changes of OWG absorbance are also shown for Y-type LB films deposited on glass slides covered with three monolayers of cadmium arachidate; (b) 2, (c) 4, and (d) 6 monolayers. The absorbance changes increased with the number of monolayers deposited. In contrast with the almost linear increase in absorbance of polymer fiJms as shown in Figure 29, the absorbances in LB films tended to saturate at longer irradiation times. The "saturated" absorbances increased almost proportional with the number of monolayers. Similar results were obtained for LB films of AV2+/AA systems. In LB films the 4,4'-bipyridinium ions are not distributed randomly but are confined to and aligned in a layer of a few tmgstr6ms thick periodically distributed in the direction of the surface normal. The long spacing of LB films of HV2+/AA was evaluated as 55 7~ by small angle X-ray scattering [93,95]. Such structural properties and much smaller thickness of LB films most probably contributed to the "saturation" tendency shown in Figure 30. It should be possible in principle to determine the orientation of chromophores in a single monolayer on an OWG by the absorption of transverse electric (TE, s-polarized) and transverse magnetic (TM, p-polarized) modes laser. Swalen et al. [109] reported that much stronger absorption was observed for a thin evaporated film of 4-dimethylamino-4'-nitrostilbene with the TM mode and for seven monolayers of cyanine dyes with the TE mode. These results corresponded .4

e o n~ J:3 o ul J3

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2400

3600

Time / s Figure 30 Changes in the OWG absorbance of LB films of HV2+/AA (1:4) with various numbers of monolayers during IPCT excitation in degassed condition: (a) 1, (b) 2, (c) 4, and (d) 6 monolayers.

277 with the predicted molecular orientation of the two dyes, perpendicular and parallel to the substrate surface, respectively [109]. 4,4'-Bipyridinium radical cations photogenerated in polymer thin films showed the same absorbance for both TE and TM modes. This result corresponded with the random orientation of radical cations, which is consistent with the result from the time dependence of photogeneration mentioned above that 4,4'-bipyridinium groups are randomly distributed throughout the polymer films. In LB films of HV2+/AA with 1-6 monolayers the OWG signals after photoexcitation displayed anisotropic absorption for TE and TM modes. Both the subsituents of the 4,4'-bipyridinium ions and the nature of the substrate surface were found to affect the anisotropic absorption in LB films by the OWG method. It is thus strongly suggested that photogenerated 4,4'-bipyridinium radical cations show specific orientation even in a single-monolayer LB films controlled at the molecular level as found in 120 monolayers LB films by a conventional method [7,9]. 6. PHOTOCHEMICAL MODULATION OF SECOND HARMONIC GENERATION IN LB FILMS CONTAINING METAL COMPLEXES Organic compounds with highly polarizable ~-electron systems have recently attracted much interest in view of their much larger optical nonlinearity compared with inorganic materials which are in practical use. The LB technique is suited to prepare a system without inversion symmetry which is another essential prerequisite to achieve second order nonlinear responses. Organic molecules with donor and acceptor groups at each end of molecular struct~es showing an intramolecular charge-transfer character have been extensively studied in search of large second order nonlinearity. Metal complexes showing a metal-to-ligand charge-transfer are also expected to have large hyperpolarizability, but only a few reports have been made on second order nonlinearity from them [110,111]. We have reported that ruthenium(II)-bipyridine complexes are capable of second harmonic generation (SHG) in alternate LB films and also reported that the SHG can be optically modulated in such LB films by an additional UV laser [112-116]. This is a sort of optical switching based on the changes of second order optical nonlinearity at the ground and excited states. The development of optical phenomena and materials to control light by light is very important for making ultrafast optical switches and phototransistors. Photobistability using third-order optical nonlinearity is one of the most promising phenomena for them, and many studies have been made. The basic concept of photobistability is to combine the photoinduced third-order refractive index change with fight feedback in the cavity. Transmitted light intensity from the cavity is controlled by input light intensity and an ultrafast optical switching is expected by this method if materials with sufficiently high this nonlinear optical coefficient can be developed. Our approach combining second order optical nonlinearity, the efficiency of which is much higher than the third-order one, with photoexcitation will give a new means for controlling light by light. The n-A isotherms for several mixtures of N,N'-dioctadecyl-4,4'-

278 dicarboxamide-2,2'-bipyridine)-bis(2,2'-bipyridine)-ruthenium(II) perchlorate (RuC18B) and dioctadecyldimethylammonium bromide (2C18NB) as shown in Figure I were observed at 14- 25 ~ on an aqueous subphase containing 0.25 mM dextranesulfate polyanion (Dex). A monolayer of a 1:4 mixture of RuC18B and 2C18NB was deposited alternately with that of 2C18NB alone at 25 ~ and 20 m N . m q with a deposition ratio of unity. Thirty-six alternate Y-type LB films were deposited on each side of a glass slide treated with a silane coupling agent. The second harmonic signal from LB films irradiated with an Nd:YAG laser at 1064 nm (100 mJ cm -2, 10 ns) was detected with a photomultiplier through an aqueous CuSO4 solution, two IR-cut filters and monochromator. In some cases LB films were excited by the third harmonic (355 nm, 0.1 - 10 mJ cm -2) of an Nd:YAG laser 10 ns before irradiation of a 1064 nm pulse. In the subpicosecond time-resolved measurement of optical modulation of SHG, 590 nm laser pulse having a temporal width of 2 ps was used as SHG probe pulse from LB films. UV laser pulses at 378 nm, 1 ~tJ, were used for the pump pulses that were generated by sum-frequency mixing of 590 nm and Nd:YLF 1053 nm laser pulses passing through the rhodium dihydrogen phosphate (RDP) crystal. The SH intensity from the LB film was detected while changing the delay time between the pump and probe pulses. No signal at 532 nm upon irradiation with a 1064 nm pulse was observed for usual Y-type LB films, whereas a strong signal with the same temporal width as an incident laser pulse was detected from alternate LB films. From these results, the latter was attributed to SHG from RuC18B. The intensity of second harmonic light (SHL) increased with increasing incident angle. A periodic fringe pattern was observed for LB films deposited on both sides of the substrate, while the intensity increased monotonously in LB films deposited on one side [112]. The absorbance at 480 nm of LB films containing RuC18B, which is attributed to the metal-to-ligand charge-transfer (MLCF) transition to the 2,2'-bipyridine moiety with two amide groups [117], increased with the incident angle for ppolarized light [112]. The observed SHG is thus most probably due to the MLCr transition to the 2,2'-bipyridine containing two amide groups [117]. The intensity of SHL observed from alternate LB films decreased upon excitation at 355 nm before the irradiation of a 1064 nm pulse. The SHG was varied reversibly many times as shown in Figure 31 without ( 0 ) and with (O) 355 nm laser excitation [113,114]. The extent of the decrease in SHG increased with the intensity of the UV laser, about 50% decrease at 10 mJ cm -2. The decrease of SHG upon UV excitation may be caused by various reasons such as the changes in the orientation of chromophores, the phase matching condition, and the refractive indices, by the thermal lens effect, or by the possible absorption of 532 or 1064 nm light at the excited state. Detailed investigation were made on them. The transient absorption of RuC18B upon excitation at 355 nm was resolved into absorption bands of the bipyridine anion radical and Ru 3+, the depletion of the ground state MLCT band, and luminescence above 600 nm [113]. These results clearly indicated the formation of a charge-separated excited state of metal complex [118,119]. Only very weak absorption was observed at 532 nm upon UV laser excitation, which was too weak to explain the observed changes in SHL. No

279

1.0

t-

355 nm off

O.5 355 nm on

U3 !

!

!

i

i

I

i

1 2 3 }, 5 6 ? 8 9 10

Times of excitation Figure 31 The intensity of SHL at 532 nm from alternate LB films of RuC18B and 2C18NB (O) with and (0) without excitationby a 355 nm pulse as a function of the number of irradiation of a 1064 nm pulse. absorption was detected in LB films at 1064 nm upon excitation at 355 nm. The possibility of the change in refractive indices for both 1064 and 532 nm light can also be excluded from these results. The change of orientation of chromophores and the thermal lens effect were excluded from experiments done at exactly the same conditions except the wavelength of an exciting laser (460 nm, 10 ns, 10 mJ cm -2) using alternate LB films containing 1-methyl-4-(4'-(N-octadecyl-Nmethylamino)styryl)pyridinium iodide (C18STZ). The intensity of SHL did not change in C18STZ LB films upon excitation with a 460 nm laser, while the excitation with a 355 nm laser at the same power density caused about 50% decrease in the intensity of SHL in RuC18B LB films. It was most probably due to the fact that the lifetime of the excited state is much longer in RuC18B (about 600 ns) than that of C18STZ (< 1 ns). The thickness of RuC18B LB films was confirmed to be much smaller than the coherent length, which led to the pseudophase matching. From these experimental results, we have excluded the abovementioned possibilities for the cause of the observed decrease in SHG. The observed photochemical modulation of SHG is most probably attributed to the changes of molecular hyperpolarizability upon formation of excited states, which is confirmed below by time-resolved measurements. From comparison with the SHG of hemicyanine and the UV laser power dependence we roughly estimated the molecular hyperpolarizability (~) at the ground and the excited state to be 70 x 10-30 esu and 36 x 10-30 esu, respectively [114]. The dynamics of the SHL intensity after subpicosecond UV laser excitation of RuC18B LB films is shown in Figure 321115,116]. The SHL intensity decreased to 70 % of its initial value upon excitation and returned to almost the initial value within several hundred picoseconds as shown by a bold line. The fluorescence decay of RuC18B LB films measured by the single photon-counting

280

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. . . .

.

.

.

.

.

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m

:

A

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0.8

(A)

i

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.

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-800

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:

:

-400

0

400

800

Delay time I ps

Figure 32 Temporal profile of relative SHL intensity from alternate LB films of RuC18B and 2C18NB with subpicosecond laser pulses at 378 nm: bold curve (A) experimental data, fine curve 03) calculated SHL intensity based on the excited-state lifetime of RuC18B in LB films. method was best fitted with three components of 178 ps, 3.9 ns and 63 ns. This result suggested that at least three kinds of molecular aggregates for RuC18B exist in the LB films due to the high concentration of chromophores. The fine line in Figure 32 shows the predicted time dependence of relative SH intensity resulting from the excited-state decay described by the following equations. The deviation of these equations is based on a kinetic analysis similar to that in ref. 120: ISH(eX)/IsH(0) = (1 + CF(t)) 2 where C = (r- 1)(1 - exp(-oD~))

and

F(t)= IF(t) / I~0)

(9) (10)

IsH(ex) and ISH(0) indicate the SH intensity with and without UV pump laser pulses, r = <~ex>/<~g> is the ratio of orientationaUy averaged molecular hyperpolarizabilities of RuC18B in the excited and ground states, o is the absorption cross section, and Dwr is the total photon dose from the UV laser pulse delivered within a volume of the probe pulse. F(t) is the temporal profile of the excited-state concentration defined by Eq. (10), where IF(t) is the fluorescence intensity at time t after the UV pump pulse. These equations are applicable in the case where there are no decay processes faster than the laser pulse width. The calculated curve using experimentally measured F(t) agrees well with the experimental time dependence of the SH intensity. The best fit was obtained for C=-0.16. This result suggested that the change of SH intensity with UV laser excitation was caused by the change of molecular hyperpolarizability between the ground and the excited states. The negative value of C indicated that <~ex> is smaller than <~s> or they are opposite sign. The former corresponds with the rough estimate of ~ by ns laser experiments as mentioned above. The

281 intensity of subpicosecond UV laser for modulating SHG is much higher than that for the fluorescence measurement, which may cause ultrafast processes such as $1-$1 annihilation within the pulse width of UV laser. The ultrafast dynamics will be necessary to accurately determine the C-value. The temporal profile of SHL intensity was found to change with time resolution of the system (about 2 ps), which dearly demonstrated the ultrafast optical modulation of SHG. In principle optical amplification of SHG will be also possible using the photoexcited state as predicted by calculations [121]. 7. CONCLUSIONS We have shown that redox chromophores organized in LB films with respect to their orientation, alignment, or electronic interactions make very useful and specific photoresponses such as amplified fluorescence quenching, photocurrents controlled at the molecular level photoinduced anisotropic electrochromism, and photochemically modulated second harmonic generation. These results may contribute to facilitate the design and construction of novel photonic devices in the near future.

Acknowledgments The author would like to acknowledge his collaborators and students for their contributions to these works. Financial supports from the Ministry of Education, Science and Culture, Japan, and several foundations are also gratefully acknowledged. REFERENCES 1. J. Deisenhofer, O. Epp, K. Miki, R. Huber, and H. Michel, J. Mol. Biol., 180 (1984) 385. 2. M.W. Windsor, J. Chem. Soc., Faraday Trans. 2, 82 (1986) 2237. 3. T. Nagamura, K. Matano, and T. Ogawa, Ber. Bunsenges. Phys. Chem., 91 (1987) 759. 4. T. Nagamura, K. Kamata, and T. Ogawa, Nippon Kagaku Kaishi, (1987) 2090. 5. T. Nagamura, T. Koga, and T. Ogawa, Denki Kagaku, 57 (1989) 1223. 6. T. Nagamura, Hyomen, 28 (1990) 1. 7. T. Nagamura, Y. Isoda, K. Sakai, and T. Ogawa, J. Chem. Soc., Chem. Commun., (1990) 703. 8. T. Nagamura and S. Kamata, J. Photochem. Photobiol. A: Chem., 55 (1990) 187. 9. T. Nagamura, Y. Isoda, K. Sakai, and T. Ogawa, Thin Sohd Films, 210/211 (1992) 617. 10. T. Nagamura, T. Koga, and T. Ogawa, J. Photochem. Photobiol. A: Chem., 66 (1992) 119.

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New Developmentsin Constructionand Functionsof OrganicThin Films T. Kajiyamaand M. Aizawa (Editors) 9 1996Elsevier ScienceB.V. All rights reserved.

287

Design of non-linear optical films by Langmuir-Blodgett technique Masanao Era, Tetsuo Tsutsui and Shogo Saito Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 816, Japan

1. S E C O N D - O R D E R N O N L I N E A R OPTICAL F ~ S Organic compounds containing delocalized n-conjugation system have a potential for exhibiting extremely high second-order optical nonlinearity due to their large molecular hyperpolarizability [1,2]. For the a t t a i n m e n t of highly efficient second-order nonlinear optical effect, however, assembling the organic molecules into highly ordered noncentrosymmetric structure is required. The Langmuir-Blodgett (LB) technique provides an unique method of fabricating organic thin films with noncentrosymmetric molecular orientation. Using the LB technique, one expects to obtain three types of noncentrosymmetric LB films: hetero Y-type, X-type and Z-type films (the molecular arrangements are shown in Fig.l). In addition, the film thickness is controllable at molecular level, within several nm, by the number of deposited monomolecular layers. The refractive index of the LB films can be also controlled by incorporating heavy metal ions as a counter ion into the films. These features are advantageous for the application to nonlinear optical devices, in particular, nonlinear optical waveguides. The first issue of the construction of noncentrosymmetric LB films with highly efficient optical nonlinearity is how one overcome the difficulty in realizing a high-degree orientational order of polar molecules, which possess high

hetero Y-type X-type Z-type g..1. Moclcular arrangement of noncentrosymmetric LB films; 9hydrophilic head, I I : chromophom, and 9alkyl chain.

288 molecular hyperpolarizability, in the noncentrosymmetric LB films. In the former part of this chapter, we demonstrate a molecular design for the construction of noncentrosymmetric LB films with high degree of molecular orientation and a novel technique to improve the orientational order of polar molecules in LB films using the molecular mixing between homologous Amphiphiles. For the practical application to nonlinear optics, further, noncentrosymmetric LB films are required to possess not only large nonlinear optical response but excellent optical quality and thickness appropriate to optical devices. In this study, a family of pyrazine derivatives was found to be an LB film-forming material applicable to waveguide devices. The optical nonlinearity in the pyrazine LB i'dms and the application of the pyrazine LB films to a frequency-doubling waveguide device is demonstrated in the latter part.

1.1. Design of a m p h i p h i l e s for noncentrosymmetric LB films with large nonlinear optical response For obtaining the information on fabrication of noncentrosymmetric LB films with highly efficient second-order optical nonlinearity, six azobenzene-linked amphiphiles were synthesized as a model compound, and their molecular hyperpolarizabilities ~, monolayer-formation at the air-water interface, and molecular orientation and second-order susceptibilities of the azobenzene-linked ~mphiphiles LB films were evaluated. The molecular structures of the azobenzenelinked omphiphiles are shown in Fig.2.

018H370. - O - N:N.-~)- COOH C18OAZOCOOH

C18H370- ~ N : N - ~

NO2

C18OAZONO2

Cl.H3,NH-O-NN-GOOOH Cl.H3,NHC}-N:N--{3 NO2 C18NHAZOCOOH (C18H37)eN--~~N=N--~COOH 2C18NAZOCOOH

Cl 8NHAZONO2 (018H37)2N-~N:N--.~NO 2 2C18NAZONO2

Fig.2. Molecular structures of azobenzene-linked amphiphiles used in this study.

1.1.1 Molecular h y p e r p o l a r i z a b i l i t y Molecular hyperpolarizabilities ~ of the azobenzene-linked amphiphiles were estimated using the following equation derived from the two-level model [3]:

289

Table 1. Molecular hyperpolarizabilities ~ of azobenzene-linked amphiphiles used in this study.

amphiphie

Eg (eV)

f

C18OAZOCOOH C18OAZONO2

3.43 3.26

0.57 0.64

4.5 5.4

14 24

C18NAZOCOOH C18NAZONO2

2.96 2.90

0.64 0.76

15 19

120 210

2C18NAZOCOOH 2C18NAZONO2

2.87 2.70

0.66 0.80

18 20

190 410

13(-2o~;t~, to)= 3e2~2 2m

F_~gf A~

A~ (debye)

13at 1064 nm (1030esu)

(1)

where E-g is the energy of the optical transition, f the oscillatorstrength, and A~ the difference between the excited state and the ground state dipole moment. The values of E-g and f were evaluated from the absorption spectra of the amphiphiles in chloroform. The A~t values were evaluated by the Stark effect measurement. The detailed evaluation procedure is describe in the section 1.2.2. The evaluated values of E.g, f, A~t and ~ are listed in Table 1. The value of I~is greatly enlarged by substituting with strong acceptor and donor groups; in particular, the substitution of alkylamino group is effective for the enhancement of ~ value. The enhancement is mainly due to the increase of A~t and the red-shift of the absorption band which originate from the intramolecular charge-transfer (CT) interaction induced by the substitution of the acceptor and donor groups. The compounds, therefore, have the ~ value/transparency trade-off relation, as observed in other organic compounds with an intramolecular CT resonance state. 1.1.2. M o n o l a y e r - f o r m a t i o n Figure 3 shows surface pressure-area isotherms of the azobenzene-linked amphiphiles. The amphiphiles with a nitro substituent as hydrophilic group did not form a stable monolayer. Further, the amphiphiles of C18OAZONO2 and 2C18NAZONO2 gave v a r y small limiting area, which were obtained by extrapolating the isotherms in a condensed region to a surface pressure of 0 mNm "1 : 10 ~2 molecule-1 for C18OAZONO2 and 17 A2 molecule-1 for 2C18NAZON02. The small limiting a r e a suggests t h a t the amphiphiles formed not a twodimensional monolayer but three-dimensional crystalline domains. Failure in

290 the formation of a stable monolayer of the amphiphiles with a nitro substituent is most likely to result from strong repulsive interaction between the polar molecules with a nitro group and deficient hydrophilicity of a nitro group. On the other hand, the amphiphiles with a carboxyl substituent formed a stable monolayer owing to high hydrophilicity of a carboxyl group. From the isotherms, the limiting area of C18OAZOCOOH, C18NAZOCOOH and 2C18NAZOCOOH were evaluated to be 26 A 2 molecule"1 , 49 A2 molecule"1 and 32 A 2 molecule"z , respectively. The values of limiting area demonstrate that the monolayer of 2C18NAZOCOOH was well condensed, in comparison with the monolayer of C18NAZOCOOH, although the amphiphiles are assumed to possess almost same magnitude of molecular dipole moment. The formation of a condensed monolayer of 2C18AZOCOOH is most likely to be due to the large attractive interaction effected by the two long atkyl chain linked to azobenzene chromophore. 60 "7'

,

i

i

i

i

i

6O

i

i

i

A

E

l

i

i

,,

i

6

Z E40

5

I..

3

4

0 I...

o 20

i

2

20

0

O

0 0

_11 ; 20 40 60 Area per molecule (A'2)

0 0

.'rr----20 40 60 Area per molecule (fit':)

Fig.3. Surface pressure-area isotherms of azobenzene-linked amphiphiles: 1" C18OAZONO2, 2; C 18NAZONO2, 3; 2C 18NAZONO2, 4; C 18OAZOCOOH,5; C18NAZOCOOH, and 6; 2C18NAZOCOOH. 1.1.3. M o l e c u l a r orientation and second-order nonlinearity in noncentrosymmetric LB films Using the alternating deposition of the amphiphiles with a carboxyl substituent and arachidic acid, noncentrosymmetric LB films (hetero Y-type) were prepared, and molecular orientation and second-order optical nonlinearity in the LB films were evaluated with the linear dichroism [4] and the second-harmonic generation (SHG) m e a s u r e m e n t s , respectively. The SHG m e a s u r e m e n t procedure is mentioned in the section 1.3. The values of the order parameter S, which describe the degree of molecular orientation to film normal, and effective second-order susceptibility at45"

291

Table 2 0 d e r parameter S and effective second-order susceptibilities X~ of the alternating LB films of azobenzene-linke~l'amphiphiles and arachidic acid. (2) LB film S Xeg (10"7 esu) C18OAZOCOOH

0.52

0.45

C18NAZOCOOH 2C18NAZOCOOH

0.08 0.48

1.1 16

* ; S = (3< cos2 r

-

1)/2, where ~ is molecular tilt angle to film normal.

incidence of Nd:YAG laser of the alternating LB films are listed in Table 2. The S value of an LB film of C18NAZOCOOH is very small, suggesting the small degree of molecular orientation to the film normal. As a results of the small degree of molecular orientation, the value of-~eff (2) in the LB film of C18NAZOCOOH is not so high; the ~eff .(2)value is comparable to that of C18OAZOCOOH, although C18NAZOCOOH possesses the large ~ value which is more than 8 times that of C18OAZOCOOH. It is thought that the small degree of molecular orientation in the C18NAZOCOOH LB film was caused by the strong repulsive interaction between the polar chromophores. On the other hand, the LB film of 2C18NAZOCOOH shows a high degree of molecular orientation and a large value of second-order susceptibility. Two alkyl chains linked to the chromophore of 2C18NAZOCOOH should provide strong molecular interaction which compensates with the repulsion between the polar chromophores. Accordingly, the molecules of 2C18NAZOCOOH formed predominant molecular orientation along the film normal, and highly efficient second-order optical nonlinearity was attained in the noncentrosymmetric LB film of 2C18NAZOCOOH. The-~eff (2) value of 1.6 x 10.6 esu is much large compared with those of the conventional second-order nonlinear materials (for example, LiNbOs). In conclusion, not only molecular hyperpolarizability but hydrophilicity of hydrophilic group, repulsive interaction between polar chromophores, and attractive interaction between alkyl chains are important points of molecular design for noncentrosymmetric LB films with highly efficient optical nonlinearity. In this study, substitution of dialkyl amino group as a hydrophobic and electrondonating group was demonstrated to provide high molecular hyperpolarizability and strong attractive interaction between molecules. As a result, highly efficient second-order nonlinearity was accomplished in the hetero Y-type film of a azobenzene-linked amphiphile containing a dioctadecylamino group.

292 1.2. Stark effect m e a s u r e m e n t s for determination of molecular orientation and second-order m o l e c u l a r hyperpolarizability 1.2.1. D e t e r m i n a t i o n o f n o n c e n t r o s y m m e t r i c m o l e c u l a r o r i e n t a t i o n in LB films by the linear Stark effect m e a s u r e m e n t For the establishment of a preparation method for LB films with well-defined noncentrosymmetric structure, quantitative evaluation of noncentrosymmetric molecular orientation is essential. The linear Stark effect, which is observed only in noncentrosymmetric materials, is expected to be helpful for the characterization of noncentrosymmetric molecular orientation in LB films [5]. In this section, we describe the quantitative evaluation of noncentrosymmetric molecular orientation in LB films by the linear Stark effect measurement. The Stark effect is electric-field-induced change in optical transition energy of materials, and the effect is observed as spectral change in absorption due to the energy shift. In the linear Stark effect, energy shift of optical transition Av in proportion to the electric field F is presented by hcAv = AlxF

(2)

where A~t is the difference between the molecular permanent dipole moment in the ground state and that in excited state of a considered optical transition, h is Planck's constant and c is the light velocity. The linear Stark effect is observed only in noncentrosymmetric molecular assemblies. In molecular assemblies with a macroscopic inversion center, the linear effect is not observed due to compensation of the contribution from individual molecules. If permanent dipoles in a sample are uniaxially oriented to the direction of the field, the electroabsorption ATZr due to the linear Stark effect is presented by AT =_ 2.303 dD ~h~lFI T he dv

(3)

where 0 is the angle between h~ and F. is the order parameter that characterizes the noncentrosymmetric molecular orientation in a uniaxially oriented molecular assembly. When v is replaced by wavelength ~ eq.(2) is reduced to

aT_ 2.303 T

hc

dK

b.I IFI

We used two types of LB films obtained by Z- and hetero Y-type deposition which were assumed to possess noncentrosymmetric molecular orientation. For comparison, an LB film with symmetrical structure obtained by Y-type deposition was also used. On these three types of LB films, the molecular orientation were determined by the linear Stark effect measurement [6]. Azobenzene-linked amphiphile C18OAZOSN employed for our experiments (Fig.4.) is an intramolecular charge-transfer compound with donor (alkoxy) and

293 acceptor (sulfonamide) groups. Because the direction of A~ is parallel to the long molecular axis in C18OAZOSN, the order parameter for the azobenzene chromophore can be correlated with that for the azobenzene-linked amphiphile molecules.

CI8H370-~N- N-'~SO2NH

C19H39COOH

CI8OAZOSN

arachidic acid

Fig.4. Molecular structures of azobenzene-linked amphiphile and arachidic acid

The hetero Y-type layers were prepared by an alternating deposition of compound C18OAZOSN and arachidic acid ( surface pressure = 25 mNm "1 for C18OAZOSN and 30 mNm 1 for arachidic acid, and subphase was 104 M aq. cadmium chloride solution). For the preparation of the Z-type layers, the monolayers of C18OAZOSN were deposited only during withdrawing substrates. The Y-type layers were prepared by the ordinary deposition procedure. The samples for the linear Stark effect measurement were prepared as follows. First, 9 monolayers of cadmium arachidate were deposited on fused quartz plates with semitransparent A1 electrodes. Next, test layers which contained 30 layers of compound C18OAZOSN were deposited. Then, further 10 layers of cadmium arachidate were deposited. Finally, the semitransparent top A1 electrodes were vacuum-deposited. The measurement of the Stark effect were carried out with the electric-field modulation technique at room temp. in vacuo (about 10 s torr). A sinusoidal ac voltage (500 Hz) was applied between the A1 electrodes. Then, the change in transmittance induced by the applied electric field were measured with a phasesensitive detector (NF Electronic Instruments LI-575A) at the fundamental frequency. Figure 5 shows the electroabsorption spectrum of the hetero Y-type film at applied ac field of 3.2 x 10SVcm ~. The AT/T spectr~m corresponds well to the firstderivative of absorption spectxmn of the hetero Y-type film, dD/dX. Moreover, the intensity of ATZF was increased linearly with the electric field strength. These results demonstrate that the electroabsorption is due to the linear Stark effect.According to eq.(3),the order parameter of the hetero Y-type film was determined to be 0.57 (the Art value of compound C 1 8 O A Z O S N , which is requisite for this calculation,was reported to be 3.5 debye by Blinov et al. [5]). The large value of demonstrates that high degree of noncentrosymmetric molecular orientation was attained in the hetero Y-type film.

294

4.0

!/"-.

_

9

:

3.0 ZO

60 -03

~ :

9 "

":

::

- 4.0 - 0.2

:

-

-

/-... 9 , o

1.0

1:3

hetero.Y ~

:

......... .

1.0 C)

~9 0

I--

-,

o ?j o

200 [~

k/nm

Z

y

._

"o

-I.0 !i

-" -2.0

-2.0 -3.0

- -4.0

Fig.5. Spectra of the Stark effect (solid line), the absorption (dotted line), and the first derivative dD/dZ. (broken line) for a hetero Y-tvoe LB film of C18OAZOSN.

Fig.6. Comparison of the Stark effect spectra of hetero Y-type, X- and Y-type LB films of C18OAZOSN.

Figure 6 compares the electroabsorption spectra of the three type of LB films in the wavelength range that corresponds to the absorption due to the transition moment in the direction of the long molecular axis. The applied field was 3.2 x 105 Vcm 1 in each case. In the Y-type deposition film, a small Stark signal is observed; nevertheless, the Y-type film is assumed to possess a symmetrical molecular orientation. The reason for this weak signal may be that the fluctuation of molecular orientation across the films induced a small asymmetry in the multilayer structure. The eleetroabsorption intensity of the Z-type deposition film is comparable to that of the Y-type film. The values of for the Z-type film and the Y-type film were estimated to be 0.10 and 0.04, respectively. The molecular orientation in the Z-type turn out to possess a small asymmetry, although the asymmetrical deposition methods was applied. We should, therefore, recognize that the structure of the Z-type film is close to that of the Y-type film; a turnover of molecules was believed to occur. The examination of the layer structure of the films by the X-ray diffraction supported the above discussion. The long spacings due to the layer structures of

295 the Y-type and the hetero Y-type films were determined to be 7.2 nm and 6.6 nm, and the molecular length of compound C18OAZOSN and arachidic acid were calculated to be 3.9 nm and 2.8 nm, respectively. The long spacing values were a little smaller than the calculated values from the molecular lengths: 7.8 nm for the Y-type film and 6.7 nm for the hetero Y-type film. This result indicates that the Y-type and the hetero Y-type layer structure are valid in those deposition types of films and that the molecules tilt within the layer planes. In the Z-type deposition film, however, the long spacing of 7.2 nm did not agree with the predicted value of 3.9 nm; rather, it was the same value as that of the Y-type deposition film. This result demonstrates that the Z-type film does not possess the Z-type layer structure but the Y-type layer structure. It should be assumed that the molecules were turned over in the deposition process and formed the Y-type layer structure, since the Z-type layer structure in which a hydrophilic group touches on a hydrophobic group is unstable. The conclusion from the examination of long spacings well supports molecular orientations in the LB films determined from the linear Stark effect measurements. From the linear Stark effect and the X-ray diffraction measurements, it is demonstrated that the hetero Y-type deposition method is useful for fabrication of stable noncentrosymmetric LB films. As mentioned in this section, the linear Stark effect measurement provides detailed information on noncentrosymmetric molecular orientation. This technique is very helpful for the advanced molecular design of noncentrosymmetric LB films for electro-optic and nonlinear optic applications.

1.2.2. Evaluation of second-order molecular hyperpolarizability using the quadratic Stark effect On the two level model [3], second-order molecular hyperpolarizability ~ is given by 13(-2~; to, co) - 3e2'fi2 2m

F_~gf A~t

where E.g and f are the energy and the oscillator strength of the optical transition concerning with the nonlinear process, respectively, and Art is the difference between excited-state and ground-state dipole moments. The values of E.g and f can be evaluated from the absorption spectra of test molecules dissolved in solvent. The A~ values of the molecules can be determined by the quadratic Stark effect in solid solution, in which the test molecules are dissolved, such as molecularly doped polymer films. Using the experimental values of E~, f, and A~t, we can estimated the ~ value. Under the following conditions; (i) the polarization vector of the probe light is perpendicular to the applied electric field, (ii) the molecular orientation in solid

296

1.0

'

o.-

...-

~

I

.o 9

"..

I

;..-"._.~ "_ "'''t s Sr

,[

""-.9......:.....t.... . . . . .

~o ~oo~/_~ ',,,oo~ I

~ZO -0.5

tO

..

,'."7~~176

-1.0

~

6oo o~

: ~

'E 'o

-0~

-1.0 ,2.0

Fig.7. Quadratic Stark effect spectrum of a poly(methylmetacrylate) film doped with an azobenzene-linked amphiphile C18OAZOCOOH (solid line). Dotted line, broken line, and dash and dotted line show an absorption spectrum of the film, its first derivative, and second derivative, resoectivelv. solution is i n d e p e n d e n t on the applied field, (iii) the direction of A~ and the difference between excited-state and ground-state molecular polarizability ha of the dissolved molecule coincide with transition moment ~%, from the Liptay's theory [7], the electroabsorption spect~lm due to the quadratic S t a r k effect in solid solution were described as

_

d

-

where F(o~) is effective electric field at a frequency of o~. Figure 7 shows the quadratic Stark spectrum of a poly(methyl metacrylate) film d o p e d w i t h a a z o b e n z e n e - l i n k e d a m p h i p h i l e , 4 - o c t a d e c y l o x y - 4 ' nitroazobenzene. Using eq. (5) and the most characteristic spectral point on the ATfP curves, where dD/d~ = 0 and d~D/d~2 = 0, the value of Alx was evaluated to be 5.4 debye. F u r t h e r , the [~ value of the azobenzene-linked amphiphile was calculated to be 24 x 10 3~ esu at a fundamental wavelength of 1064 nm. The [~ values of azobenzene-linked amphiphiles employed in this study were evaluated by the procedure mentioned here. The values are listed in Table 2 in the section 1.1.1.

297

1.3. M o l e c u l a r m i x i n g m e t h o d for i m p r o v e m e n t of t h e m o l e c u l a r orientation in m o n o m o l e c u l a r layer For the construction of LB films with highly efficient second-order optical nonlinearity, it is necessary to introduce high orientational order of polar molecules which possess high molecular polarizability(~) into noncentrosymmetric LB films. However, large dipole-dipole repulsiveinteractionbetween polar molecules makes it difficult to realize high orientational order of polar molecules in noncentrosymmetric L B films. In this section, we propose a novel technique to improve the orientational order of polar molecules in L B films utilizingmolecular mixing of a polar amphiphile with a homologous nonpolar amphiphile [8-10]. In several studies, it has been reported that specific aggregated structures, which can never attain in single-component LB films, were successfully attained in two-component LB films through the complete mixing between two amphiphile with utterly different molecular structures [11-13]. The molecular origin of complete mixing of two components in the mixed monolayers has been ascribed to the sophisticated close packing of the component molecules. When one considers the control of orientation of polar molecules in LB films by the mixing of a second component amphiphile, one anticipates that the simple close-packing principle may not operate on account of a molecular interaction, especially dipole-dipole repulsive interaction. Then, we focused our attention not only on the geometrical packing but on the molecular interaction and found that the fabrication of the LB films with high orientational order of polar molecules was possible, when we used an assisting second component which possessed small polarity but almost the same chemical structure as the polar amphiphiles. Two homologous azobenzene-linked amphiphiles C18OAZONO2 and C18OAZOCOOH were used in this study. The polar amphiphile C18OAZONO2 possesses a strong electron-withdrawing nitro group as a hydrophilic group. Due to strong repulsion between the polar molecules, polar molecules can not form vertical orientation in its monolayer, even though the molecules has rod-like shape favorable for the formation of card-pack structure called H-aggregation. On the other hand, homologous Amphiphile C18OAZOCOOH, with small polarity, has a tendency to orient vertically with respect to a film plane and form Haggregation in the LB film. When the polar amphiphile C18OAZONO2 is mixed with the homologous amphiphile C18OAZOCOOH, one expects that mixing of the two components at molecular level is attained in the mixed monolayer, because the electric repulsion between the polar molecules is markedly decreased without losing the close-packing condition. Further, it is expected that orientation of the polar molecules may be forced to improve owing to a strong tendency of vertical orientation of the less polar amphiphile. Consequently, appearance of highly efficient second-order optical nonlinearity in the mixed monolayer is expected. Two azobenzene-linked amphiphiles were dissolved in chloroform and mixture solution with various molar ratio were prepared. The mixed monolayers of C180AZONO2 and C18OAZOCOOH were spread on a 104 M aq. BaC12 solution

298 at 20 ~ from the chloroform solution. The monolayers were deposited on hydrophilic fused quartz substrates, which were treated with a KOH ethanol solution beforehand, by withdrawing a substrate from the subphase after the monolayers were compressed to a surface pressure of 15 mNm 1 . The absorption spectra of the mixed monolayers were measured with a spectrophotometer (Hitachi 330). Figure 8 shows the absorption spectra of the mixed monolayer. The spectra of single-component monolayers of C18OAZONO2 and C18OAZOCOOH are also shown in this figure. In the mixed monolayer with the composition of C18OAZONO2/C18OAZOCOOH~_3/1, the absorption peak was located at 305 rim, which was 65 nm blue-shifted from that of C18OAZONO2 in hexane. Further, absorption peak was not observed at 350 nm, where the peak due to the monolayer of C18OAZONO2 would be appear. The absorption spectra demonstrate that the polar amphiphile C18OAZONO2 was mixed at molecular level with the homologous amphiphile C18OAZOCOOH in the mixed monolayers, and that the H-aggregate was formed as a result of a vertical molecular orientation of the amphiphiles. On the other hand, the absorption peak appeared at 350 nm in the mixed monolayer with large fraction of C18OAZONO2, where C180AZONO2/C18OAZOCOOH_~4/1. The appearance of the peak at 350 nm indicates t h a t single-component domains of C18OAZONO2 were present in the mixed monolayers. The absorbance at 350 nm grew as the fraction of C18OAZONO2 increased, indicating the increase of the single-component domains of C18OAZONO2 in the mixed monolayers. 0.06 ! ! ! 0.04

t

"l

"

:~

0.02

,y i" 0 ......... 200

,

\,',

\\

, ....... ":"..,.-. . . . ~: .--~'~.____: 300 400 500 Wavelength (rim)

Fig.8. Absorption spectra of monolayers of C18OAZONO2 and C 18OAZOCOOH and their mixture: 1" C 18OAZONO2, 2 ;C 18OAZOCOOH, 3 ;C 18AZONO2:C 18OAZOCOOH=3:1, and 4; C 18AZONO2:C 18OAZOCOOH--4:1.

299

C180AZONO2 ~ C180AZOCOOH

C180AZONO2 j, arachidic acid T

incomplete mixing

complete mixing

Fig.9. Schematic representation of the difference between complete mixing and incomplete mixing. For comparison, azobenzene-linked amphiphile C18OAZONO2 was mixed with arachidic acid (molar ratio C18OAZONO2/arachidic acid = 1/3), and the absorption spectrum1 of the mixed monolayer was examined. The mixed monolayer gave the same spectn~m as the single-component monolayer of C18OAZONO2. It is implied that the aggregation and orientation structures remained unchanged, when C18OAZONO2 was mixed with arachidic acid. They formed the mixture of domains composed of each single-component monolayer in the mixed monolayer. Figure 9 schematically shows the difference of complete mixing, molecular mixing, in the C18OAZONO2 + C18OAZOCOOH system and incomplete mixing, domain formation, in the C18OAZONO2 + arachidic acid system. Second-order optical nonlinearity of the mixed monolayer was evaluated by the second-harmonic generation (SHG) measurement. The SHG m e a s u r e m e n t was performed using a Q-switched Nd:YAG laser (Spectron Laser System SL402), which has pulse energy of 200 mJ, pulse width of 20 nsec, and repetition frequency of 10 Hz. Laser pulses, the energy of which was attenuated to 10 mJ, were irradiated to the monolayers at incident angle b e t w e e n - 8 0 ~ and 80 ~ . The generated second-harmonic (SH) light was detected with a photomultiplier and a boxcar integrator (Stanford Research System SR250). We also evaluated molecular orientation in the mixed monolayers using the SHG measurement. When the fundamental light is irradiated to the film with uniaxial molecular orientation such as LB films at an incident angle of 0, the transmitted second-harmonic intensity P2 from the film is given by P2 - C

P21 t4 T2 (a33)2 (n~-n~)

p(O)2 sin=(i~(O~j

(7)

where t~ is the transmittance of the fundamental light; T~ is the second-harmonic Fresnel factor; dij are the second-harmonic coefficient tensor components; n 1 and

300 n~ are the refractiveindices of the film for the fundamental and the second-harmonic lights,respectively;and r = (n/2)( 4 ~ ) (n~cos0~-n#os0~.),with 01 and 0~ denoting the propagating angle of the fundamental and the second-harmonic lights in the film, respectively, and L denoting the film thickness [14]. The projection factor p(0) is a function of the propagating angles. For p-polarized fundamental light and p-polarized harmonic, p(O) = (acos201 + sin201)sin02 + 2a cosOlsinOlCOS02

(8)

and for s-polarized fundamental light and p-polarized harmonic, p(O) = a sin02

(9)

where a = d~/d~. The ratio a can be related to the measured ratio of the p-polarized secondharmonic intensities for s- and p- polarized fundamental lights, Pp/P, Assuming that n~-~n~ and L<< the coherence length, the relation between the ratio a and Pp/P, is described as the following equation; a =

sin201 f( 0 X Pp[Ps) 1/2 3 cos 201

(10)

_

where f(O) = ((COS O1 + COS O)(nlCOS 0 + cos 01)/2 (COS O + COS 01) (nlcos 01 + cos 0 ]

(11)

For monolayers and LB films, the distribution of the molecular orientation is expected to be sharply peaked at a tilt angle ~ between the molecular axis and the film normal. Using the tiltangle @, then, the ratio a is also presented by a

~ .

<sin 2 ~ cos ~> 2
--

sin2 _ tan2/2 2COS3 < ~ . . . . .

(12)

Using eqs (2)-(4) and the value of PJP,, accordingly, we can determine the tilt angle ~ in monolayers and LB films. Figure 10 shows the incident angular dependences of the p-polarized SH intensities, Pp and P, from the mixed monolayer (C18OAZONO2/C18OAZOCOOH = 1/1) for p- and s-polarized fundamental lights. One can see the fringing pattern of SH intensity. The pattern was caused by the interference of SH light from the monolayers deposited on both sides of a substrate. The envelope of the fringing SH intensity increases with increasing the incident angle. Further, the intensity of Pp is more than 37 times larger than that of P,. These results implies

301

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-.

9 ."

..

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.~

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9 . 9

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.

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9

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..

.

0 80 Incident angle (degrees) Fig.10. The dependence of SH intensity in the mixed monolayer of C18OAZONO2:C 118OCOOH=I" 1 on incident angle, a) and b) are p-polarized SH intensity when p- and s-polarized light was incident, respectively. -80

50 Molecular mixing ~40

80

< 60 ~,

_= 30 O

,-. 20 ~

"~ -e" _

m

0 0

,

, , i t 0.2 0.4 0.6 0.8 Fraction of C18OAZONO2

0 1

Fig. 11. The dependence of SH intensity (open circle) and molecular tilt angle 0 to film normal of the mixed monolayer on the fraction of C 18OAZONO2. t h a t the azobenzene-linked Amphiphiles were p r e d o m i n a n t l y oriented n o r m a l to the film plane. The v a l u e s of t h e envelopes of S H i n t e n s i t y at 45 ~ incidence in t h e m i x e d m o n o l a y e r s are plotted a g a i n s t t h e fraction of C18OAZONO2 in F i g . l l . The molecular tilt angle ~ e v a l u a t e d by the above-mentioned procedure is also shown in t h i s figure. T h e S H i n t e n s i t y of t h e s i n g l e - c o m p o n e n t m o n o l a y e r of C18OAZONO2 is very small. This result m e a n s t h a t the orientation of amphiphile

302 C18OAZONO2 is poorly asymmetric. On the other hand, the SH intensity of the single-component monolayer of C18OAZOCOOH is moderately large. In the range of the composition of C18OAZONO2/C18OAZOCOOH~_3/1, in which two amphiphiles were molecularly mixed, the SH intensity is greatly enhanced as the fraction of C18OAZONO2. In the mixed monolayers, further, the molecular tilt angle ~ ranges from 20" to 35" . These observations mean that we succeeded in the predominant orientation of the polar amphiphile C18OAZONO2 normal to the film plane by means of molecular mixing with the homologous amphiphile C18OAZOCOOH. Further increase of fraction C18OAZONO2, however, brought about rapid decrease of the SH intensity. The fraction of the single-component domains of C18OAZONO2 which possess minimal SHG activity increased with increasing fraction of C18OAZONO2, as demonstrated by the absorption spectra of the mixed monolayers. By comparison between the SH intensity of the mixed monolayer and octadecy stilbazium iodide monolayer (;~(2) = 0.51 x 10 ~ esu) [15], effective susceptibilities (2) xeff (2) of the mixed monolayers at 45 incidence were roughly estimated. The .~cff value of the mixed monolayer (C18OAZONO2/C18OAZOCOOH = 3/1), which showed most efficient SHG activity, was 1.1 x 10 .7 esu. In this study, we demonstrated that high orientational order of a polar azobenzene-linked amphiphile was introduced to monolayers by means of the molecular mixing with a homologous amphiphile. Further, enhancement of SHG was observed in the mixed monolayer owing to the improvement of orientation of polar molecules. The mixing technique is expected to develop the construction of noncentrosymmetric LB films with highly efficient optical nonlinearity. o

1.4. Fabrication of noncentrosymmetric LB films using two compartment trough For not only practical applications but advanced studies of noncentrosymmetric LB films, it is indispensable to establish a efficient preparation technique of noncentrosymmetric LB films. However, the LB technique using a conventional Langmuir trough usually provides symmetric LB films. Preparation of noncentrosymmetric LB film using the conventional Langmuir trough is very difficult. Then, we used two compartment trough with two flexible gates for the reliable and quick preparation of noncentrosymmetric LB films with good film quality. The two compartment trough is schematically shown in Fig.12. The trough is divided to two large compartment, where monolayer is spread, and a small buffer portion by two fixed barriers with a flexible gate. Compression ofmonolayers spread on each compartment is controlled independently by two movable polytetrafluoroethylene (PTFE) barriers. A substrate can move from a compartment to the other compartment by passing through the flexible gates

303 composed of abutting PTFE leaf springs. The small compartment between the two fixed barrier prevents cross-contamination caused by leaking of monolayer through the flexible gates. This trough allow the convenient and efficient p r e p a r a t i o n of noncentrosymmetric LB films. For the preparation of alternating LB films (hetero Y-type films), two different monolayers are spread on each compartment. After one monolayer is deposited on a substrate in down stroke, the substrate is transferred to the other compartment through the flexible gates. Then, another monolayer is deposited by withdrawing the substrate. One can obtain alternating LB films by repeating the deposition process. For the X-type or Z-type films, a monolayer is spread on one compartment and the other compartment is kept empty. The monolayers are deposited on substrate only in up or down stroke.

i

___

bstrate

J

flex~le

i i!

gate p-1

.....

surface pressure~~

~.

~

.....

11 lsubstrate

J

surface pressure

Fig.12. Two compartment Langmuir trough used in this study. 1.5. Second-order n o n l i n e a r optical properties of n o n c e n t r o s y m m e t r i c LB films composed of p y r a z i n e derivatives For the actual application of noncentrosymmetric LB films to second-order nonlinear optical devices, the LB films are required to possess the following properties: (i) high second-order susceptibility %~2), (ii) high optical quality and uniformity, in other words, high transparency in a wide range of wavelength, and (iii) thickness appropriate for application to optical devices, for example, a few ~tm for a nonlinear optical waveguide. To date, however, few studies have succeeded in providing LB films fulfilling the above requirements [16,17]. In Almost all of previous works, large second-order optical nonlinearity was observed

304 only in very thin LB films with a few tens of monolayers at most. The fabrication of thick LB films which preserved large nonlinearity has not realized. In this study, we found that a family of pyrazine derivatives (Fig.13) is a LB film-forming material applicable to waveguide devices [18,19]. The pyrazine derivatives have a large molecular hyperpolarizability comparable to pnitroaniline: 26 x 10 -30 esu for C12PPy, 31 x 10 -30 esu for C12OPPy, and 67 x 10.30 esu for C12SPPy. Moreover, the degree of molecular orientation in the LB films of pyrazine derivatives is very high. Thus, h i g h - p e r f o r m a n c e noncentrosymmetric LB films for nonlinear optics are expected to be fabricated using the pyrazine derivatives. By the alternating deposition of the pyrazine derivatives and arachidic acid, thick noncentrosymmetric LB films of more than 400 bilayers (thickness of more than 2 ~m) were successfully fabricated, as we expected. Further, the LB films were found to possess good optical quality and fairly large second-order optical nonlinearity. For the fabrication of noncentrosymmetric LB films, a two-compartment Langmuir trough, of which detail is given in the section 1.4, was used. Monolayers ofpyrazine derivatives and arachidic acid were spread on each compartmentalized subphase (BaCI~ or CdC12 aqueous solution, 2 x 10-4 M). The monolayers were compressed to 30 mNm 1, and then monolayers of pyrazine derivatives and arachidic acid were deposited fused quartz substrates in up stroke and down stroke, respectively. The aggregation structure in the LB films was studies by the absorption spectra and X-ray diffraction.

C12H2sO~ - N ~ COOH C12PPy NC,2H250- - - ~ OH:CH-~.-N--~,COOH C12OPPy NC,2H2sS" - - ~ CH:CH'-~.-~COOH C12SPPy Fig. 13. Molecular structures of pyrazine derivaives We successfiflly fabricated the alternating LB films with more than 400 bilayers. In all dipping stokes, the transfer ratio was almost unity. The LB films were uniform and light scattering was not observed with either the naked eye or a differential interference microscope. Figure 14 shows the absorption spectra of the LB film with 200 bilayer and C12PPy in chloroform. The film is transparent from visible to near infrared region; the electronic absorption due to ~-~* transition of C12PPy is located at 400 nm. The inset in Fig.14 shows fringing pattern in the absorption spectrum of the LB films. The fringing pattern is due to interference of a incident light and reflected light in the LB film, suggesting that optical

305

3.5

~176176

3.0

0.08[

2.5 2.0"

,.--,i O

C9

, '6;0' ' ' obo' Wavelength/nm

),..

~Q

ol.5

' ' 400

03

~.0~

r--i

1 LB film

0.5 0 200

..... I

400

,

I

r

in chloroform I

,

,

I

600 800 1 0 0 0 1200 Wavelength (nm)

0 1400

Fig. 14. Absroption spectra of an alternating LB film of C 12PPy and arachidic acid and C 12PPy in chloroform. The inset shows a fringing pattern in the transparent region of the LB film. quality of the LB films was high. These results give us the prospect that the LB films are applicable to nonlinear optics in a wide range of wavelength, in particular, a conversion of infrared laser light from a laser diode to blue laser light. Absorption peak of the C12PPy LB film locates at 290 nm. The wavelength is 50 nm blue-shifted from that of C12PPy in chloroform. The blue shift of the absorption band demonstrates the formation of H-aggregation in the LB film. The X-ray diffraction profile of the alternating LB film showed sharp diffraction peaks assigned to the (00n) plane in the small-angle region. From the diffraction peaks, the long spacing was calculated to be 5.0 nm. The value is slightly smaller than the sum of molecular length of each amphiphile (2.83 nm for C12PPy and 2.90 nm for arachidic acid). The good agreement demonstrates that C12PPy was predominantly oriented normal to the film plane in the LB films. Second-order nonlinearity and molecular orientation in the alternating LB films were evaluated by the SHG measurement using a Q-switched Nd:YAG laser. The detailed evaluation procedure was described in the section 1.3. In this measurement, we used the samples in which one of the LB films deposited on both surfaces of a substrate was removed by wiping it of with a cloth soaked with chloroform. The dependence of the SH intensity from the alternating LB film (30 bilayers) on the incident angle of laser light is shown in Fig.15. The SH signals for incident p- and s-polarization increases with increasing the incident angle of laser pulses, where as the diminution of the SH signal beyond 60" incidence was observed owing to the reflection of the fundamental and the harmonic lights at the boundaries of air/LBfilm/substrate/air system. The SH signal excited by the

306

lo 6

a) p-p 400 A

/:"..~:~.-,.,.%

u)

"E 200

lO~ -q,.

xi

0

"u} 20 e-

d

_0

20

b) s-p

!

!

40

80

cD .,..~

~

c

m

~

60

~10 3

9

..

102

I

g~.::~'"-." .::,a-":.

o9

10

0

"Wt

--

~'~.'~i" 9 ,

" ": " " o .

s.

-.

.....~ ~ ~ ~,-'-.:'/"

0

I

20

101

"1

I

40

I .

60

Incident angle / degrees

80

Fig. 15. The incident angle dependences of SH intensity from the alteanting LB films of C 12PPy; a) and b) are p-polarized SH intensities for p- and s-poalrized fundamental lights, respectively.

I0

9

I I t '''d

.(2)

Table 3. Effective second-order susceptibilities xe~ at 45 pyrazine. LB film

C12PPy

Absorption Edge (nm)

_

t

, ,t,u,|

,

,~,,ut

1 10 1O0 1000 Number of deposited bilayer Fig. 16. The dependence of SH intensity at 45 ~ incidence from alternating LB films of C12PPy on number of deposited bilayers.

o

incidence of

Long Spacing (nm)

X (2>e~(107esu)

Ba salt

400

5.0

1.0

C12OPPy Cd salt

460

4.5

2.3

Ba salt

450

5.3

0.81

C12SPPy Cd salt

450

5.6

1.1

Ba salt

450

5.6

0.69

307 p-polarized light is about 20 times as large as that excited by the s-polarized light. According the procedure mentioned in the section 1.3, the tilt angle of C12PPy molecules to the film normal was calculated to be 30" from the ratio of the SH intensities for incident p- and s-polarization. In this calculation, we assumed that the dispersion of the refractive index is very small, and that the values for the fundamental and harmonic lights are 1.5. The tilt angle of 30" gives the quantitative proof of high degree of the asymmetric molecular orientation in the LB film. Figure 16 shows the dependence of the SH intensity at 45" incidence of ppolarized fundamental light on the number of deposited bilayers. The SH intensity increases quadratically with the film thickness of up to 400 bilayers (2 ~tm), as predicted theoretically in the case of the nonlinear slab with a thickness much smaller than the coherence length. The quadratic dependence demonstrates that the highly ordered noncentrosymmetric molecular orientation, which was confirmed in the relatively thin LB film from the SHG and X-ray diffraction measurements, was preserved in the alternating LB film with the thickness enough to be applied to the nonlinear waveguide devices. By comparison with an octadecyloxy stilbazium iodide monolayer ()C(2~ = 0.51 x 10~ esu) [15], the value of the effective second-order susceptibility Xeff .(2) at 45" incidence was estimated to be 1.0 x 10 "7 esu. The value is fairly large, in comparison with those of the conventional nonlinear optical materials (e.g. LiNbOs). Other pyrazine derivatives (C12OPPy and C12SPPy) also gave thick noncentrosymmetric LB films with fairly large second-order nonlinearity by the alternating deposition with arachidic acid. The estimated Xeff (2) values of the pyrazine LB films are listed in Table 3.

1.6. Second-order nonlinear optical waveguide using noncentros y m m e t r i c LB film of pyrazine derivative For nonlinear optical applications, optical waveguides are promising because of the following advantages; (i) the strong beam confinement in the waveguides provides high power density over long propagation distance which can be achieved with only moderate input power. The high power density allow the highly efficient nonlinear optical response. (ii) The modal dispersion relation in the waveguides makes it possible to achieve phase matching by the selection of the propagation modes and the variation of the effective refractive index with the waveguide thickness and with the refractive indices of the cladding and the substrate. (iii) Waveguides possess the compatibility with the integrated optoelectronic circuits and fiber optics. To date, many organic materials have been applied to the waveguide devices, for example, poled polymer films [20,21], single-crystal films [22], and Langmuir-Blodgett films [23,24]. Using the LB technique, one can control the film thickness and the molecular arrangement along the film normal at molecular level. These features are very useful for the construction of sophisticated nonlinear waveguide with high

308

conversion efficiency;, one can fabricated waveguiding layer with well-defined thickness. Further, it is possible to improve the optical field overlap between the propagating fundamental and harmonic modes by modification of the molecular arrangement [25]. In this study, we fabricated Cerenkov type waveguide devices for the frequency-doubling using the pyrazine derivative (C12PPy) LB film, which possesses good optical quality and fairly large optical nonlinearity as mentioned in the section 1.5. First, we determined the refractive index dispersion of the pyrazine LB film, which is necessary for the design of the waveguide devices, by using the integrated optical technique with prism coupler [26]. The coupling angles V for TE and TM polarized light to the discrete modes in waveguide of a pyrazine LB film were determined by measuring reflected light intensity from the prism coupler as a function of incident angle. From the values of V, the effective refractive index of the waveguide mode 13was calculated according to the following equation: knpsinV = k ~

(13)

where np is refractive index of prism and k is wave vector in air. From the values of [3, the refractive indices and film thickness of the pyrazine LB film were evaluated using the waveguide mode equation: 2kW(n~ -

1~2) -1/2

-

2~10 -2~12 = 2m~t

(14)

where W and nr are the film thickness and refractive index, respectively. The quantities ~ and ~n are the phaseshit~s by the reflection of guided wave at two interfaces: film/cladding and film/substrate.

1.64~~k x {D

.-= 1.58,

._~

~176

~

1.5

n-

1.54 1.52 1.50

400

no

600

1

,.1

800 1000 1200 Wavelength / nm Fig.17. Wavelength dispersion of refractive index of a hetero Y-type LB film consisting of C12PPy and arachidic acid.

309

_

4000

ns(,~)ns(2~)

/

E

//

c \

_

c

._~ 2 0 0 0 ,,c

,

/

II

// .

/

~...," ' '

|

/

//

TM2,

t/

/

."

/

n~) ~,

~: /

"" Cde~kov

I ~/

~

~

I

,

i:1 t/ ,

/

i/:1

,/ |

o , ~

01.4

nf(~ ,

.

:

/ l

I

1.5 .rM~.rM~l.6 Effective index N Fig.18. TM mode dispersion curves of fundamental (broken line) and harmonic waves (solid line) in waveguide of a hetero Y-type LB film consisting of C 12PPy and arachidic acid.

The pyrazine LB films was expected to show uniaxial birefringence because the LB film possess uniaxial molecular orientation. Accordingly, the refractive index in the film plane n o and film thickness W was determined by the analysis of TE modes. Then, the refractive index perpendicular to the film plane n~ was obtained form the analysis of the TM modes. Figure 17 shows wavelength dispersion of refractive index of a hetero Y-type LB films consisting of a pyrazine derivative C12PPy and arachidic acid. The solid lines in this figure are fitting curves by a SeUmeier formula. As expected from the molecular orientation, large birefringence is observed. The large value of n~ compared with n o suggests t h a t the pyrazine molecules predominantly oriented normal to the film plane. Using the refractive index value of the pyrazine LB film, we calculated the mode dispersion curves of the TM fundamental and the TM second-harmonic waves in the waveguide device composed of a waveguiding pyrazine layer and a fused quartz substrate when Nd:YAG laser is used as a fundamental light (Fig.18). These curves show t h a t the Cerenkov type phase matching is possible in the range of the thickness from 410 nm to 510 nm. In the Cerenkov type waveguide devices with waveguide thickness from 450 nm to 520 nm, arched green emission due to the Cerenkov type phase-matching was observed when the Nd:YAG laser was coupled into the devices. The result is in good agreement with prediction from the mode dispersion curves, suggesting that the LB technique is very useful to fabricate waveguides with well-defined thickness. In these d evices, the maximum value of the conversion efficiency was 1.9x 10"11%W.

310 2. T H I R D - O R D E R N O N L I N E A R OPTICAL FILMS

Up to now, many conjugated polymers have been found to possess large and very rapid third-order nonlinear optical response, which originates from the one-dimensionally delocalized n-conjugation system along the polymer chain. Their application to the all optical signal processing devices has been expected. For the practical application, the n-conjugated polymers are required to possess not only large optical nonlinearity but excellent fdm processability. The via precursor method [26-30], is one of the most promising technique to obtain the high performance n-conjugated polymer thin films. The via-precursor method gives dense and tough n-conjugated polymer films with good transparency by the solid-state thermal conversion of processable precursor polymers. In the via precursor method, however, it is difficult to prepare the n-conjugated polymers with ideally developed n-conjugation system; the n-conjugated polymer chains contain many conformational defects because the n-conjugated chains are caused to develop from disordered precursor polymer, which form random coil conformation, in solid state. For the preparation of polymers with well-developed n-conjugation system by the via precursor method, accordingly, it is necessary to introduce orientational and conformational orderliness of the precursor polymers in the films. Polyarylenevinylene (PAV) expressed by the chemical formula of [-Ar-CH=CH-]n, where Ar is an arylene ring, is an attractive n-conjugated polymer family because of the following features; (i) by the thermal conversion from polyelectrolyte or organic-solvent-soluble precursors, one can obtain the PAV films which have large third-order susceptibility and excellent optical quality, and (ii) the band gap can be adjusted by suitable selection of the arylene rings.

~S-H2C,-Ar--CH2-S+~ 2Cimonomer

~CH:CH"~n

---(-Ar- .CH-CH-)--

PPV

OCH3

n

CHaOH

----~Ar- CH-OHn-)-~--OCH3 alkoxy pendant precursor

---(--Ar- CH- CH-)-.~-n polyarylenevinylene

H3CO/

MOPPV

rz--a

PTV

Fig. 19. Synthetic route and chemical structures of polyarylenevinylenes.

311 Here, we demonstrate t h a t oriented PAV films with well-developed x-conjugated system can be fabricated through the regulation of orientation of precursor polymer chains by use of the Langmuir-Blodgett technique, and that large and anisotropic third-order optical nonlinearity was observed in the oriented PAV films. 2.1. T h i r d - o r d e r n o n l i n e a r o p t i c a l effect in p o l y a r y l e n e v i n y l e n e c a s t films

prepared by v i a - p r e c u r s o r m e t h o d Three kinds of PAV films was prepared using methoxy pendant precursors. The chemical structures and synthetic route of the PAV films used in this study are shown in Fig.19. The details of synthesis of the methoxy pendant precursors have been described in refs. 29 and 30. The precursors were soluble in conventional organic solvents, for example, chloroform, dichloromethane, benzene and so on. The precursor polymer thin films were spin-coated on fused quartz substrates from the chloroform solutions. The precursor films were converted to PAV films by the heat-treatment at 250~C under a nitrogen flow with a slight amount of HC1 as a catalyst. This method provided high performance PAV films with excellent optical quality. Third-order susceptibilities of the PAV cast films were evaluated with the third-harmonic generation (THG) measurement [31,32]. The THG measurement was carried out at fundamental wavelength of 1064 nm and between 1500 nm and 2100 nm using difference-frequency generation combined with a Q-switched Nd:YAG laser and a tunable dye laser. From the ratio of third-harmonic intensities I3~ from the PAV films and a fused quartz plate ( 1 ~ n thick) as a standard, the value of x (3) was estimated according to the following equation derived by Kajzar et al. [33]: I3~ = CI3

~'(o~)2

~'(3o~)2

X(3) -

Alei~(eia~ - 1l ,

(15)

where q~= 3 ton(3~)c~ 03oL/c, Aq~= 3 o~(n(to)cos0co- n(3 to)cos03o~)I_/c,

(16) (17)

I~ is fundamental light intensity, L is film thickness, c is light velocity, ff is complex refractive index, and 0~ and 03~ are propagation angle for fundamental light and harmonic light, respectively. The dispersion of complex refractive index of PAV films, which is requisite for the calculation, was determined by the Kramers-Kronig analysis of the transmission spectra of the PAV films. Figure 20 shows the absorption spectra and the X(3) spectra of the fully converted PAV cast films. The PAV films exhibit a large absorption in the visible

312 region, which is associate with n-n* transition. The optical band gap values of PPV, MOPPV and PTV, which were determined from longer wavelength edge of the absorption band due to n-n* transition, were 2.4 eV, 2.1 eV and 1.8 eV, respectively. The X(s) values of the PAV cast films were greatly enhanced when the harmonic wavelength was in the absorption region due to n-~* transition. This results suggest t h a t the enhancement was caused by the three photon resonance. The m a r i m u m X(s) were 0.4 x 10 "1~ esu at 1475 rim, 1.6 x 10 "1~ esu at 1600 nm, 4.5 x 10"1~ esu at 1900 nm for PPV, MOPPV and PTV, respectively. The values show the tendency t h a t enhancement in Xcs) value is accompanied with decrease in band gap of n-conjugated polymer, as theoretically predicted by Wu et a1.[34]. Assuming the tensor component of X~x)xparallel to the polymer chain direction is dominant for the nonlinear optical response, one can use the following equation:

(18)

X(3x~ ) = X(3)(-30~; t0,t0,(0)/.

We assume that the chains were oriented parallel to the film plane but the chain direction were distributed randomly in the film plane. In this case, the correction factor for the chain orientation, , is 3/8. Consequently, the X~x)xvalue of the PTV cast film was calculated to be 1.2 x 10 .9 esu, which is comparable to that of the highly oriented vacuum-deposited polydiacetylene film at the resonance with the exciton absorption.

_

..e

o~"" "'"% .. I " " ~ ~o

0

./

0

_

~3

A

~2

J~

0

_

-rn

,

0 3q )0

400

.

?'...

,~ ~ , ~

-

_

~

..Q O ..Q

<

L,~

500 600 700 Harmonic wavelength (nm)

800

Fig.20. X(3)spectra of PAV cast films: open circles; PTV, open squares; MOPPV and open triangles; PPV. The lines shows absorption spectra of PAV cast films: solid line; PTV, dash and dotted line; MOPPV and dotted line; PPV. The X(3) spectra are plotted against harmonic photon energy.

313 2.2. P r e p a r a t i o n of o r i e n t e d p o l y a r y l e n e v i n y l e n e films by LB t e c h n i q u e Figure 21 shows three possible routes to obtain oriented PAV films by the LB technique. In these route, it is anticipated that orientational orderliness of precursor polymers is introduced in the precursor LB films through the formation of two-dimensionally oriented monolayer of a polyelectrolyte precursor-anionic amphiphile polyion complex at the air/subphase interface and orientation of the precursor monolayers along the dipping direction during the deposition process. As a result, it is expected to obtain oriented PAV LB films with well-developed ~-conjugation system. In this study, we successfully prepared oriented PAV films using two routes of them, b-1 and b-2 route [35-37]. The chemical structures of PAWs, their polyeleetrolyte precursors and an anionic amphiphile used in this study are shown in Fig.22. 2.2.1. P r e p a r a t i o n of o r i e n t e d poly(p-phenylenevinylene), PPV, LB film Using the b-1 route, PPV LB films were successfully fabricated [35]. The precursor polymer of PPV was prepared according to ref.27. Anionic Amphiphile 2C12SUC was purchased from Sogo Pharmaceutical Co. and used without further purification. An aqueous solution of the precursor polymer was added dropwise to an aqueous 2C12SUC solution. Then, the precursor-anionic amphiphile po]yion complex was precipitated. The precipitates were filtered and washed with deionized

~

o..p q..o ~

water

----o

precursor monomerformation of ion anionic arnphiphile complex monolayer ion complex (a) NN~

precursor polymeramphiphile polyion complex

/

(b-l)

~ random coiled percursor polymer

+

" amphiphile " precursor monomer " precursor polymer

. o owater "1 oriented precursor polymeramphiphile polyion complex monolayer

PAV Film

water

adsorption of precursor onto amphiphile monolayer (b~

Fig.21. Preparation routes of polyarylenevinylene PAV LB films.

314

R

--~ R

R

~1"i+--CH:z"~n n -'~ R ' - ~ ~

CH:CH"-~n

Ci-

PPV;R=H, MOPPV;R=CH30 O II

CH3(CH2)11 -O-C-CH 2

I

CH3(CHQ11 -O-C-CH-SO-3 Na+ II O anionic

amphiphile;2C12SUC

Fig.22. Chemical structures of PAV's, their precursor and an amonic amphiphile used in this study. water. The polyion complex was soluble in chloroform. The polyion complex monolayer was spread on deionized water (18 Mohm, from Millipore Milli-Q system) from the chloroform solution. The preparation of the LB films of the precursor-anionic amphiphile polyion complex was accomplished by using the conventional LB technique after the monolayer compressed to a surface pressure of 20 mNm z . More than 60 monolayers could be deposed on fused quartz substrates, which were made hydrophobic by the 5 layers deposition of cadmium arachidate. The deposition ratio was nearly unity in both of up and down stroke. Finally, the polyion complex LB films were converted to the PPV LB films by the heat treatment at 300~ for 1 hour in vacuo (about 10 -2 torr). The IR absorption spectra of the PPV precursor LB film before and after the heat treatment are depicted in Fig.23. After the heat treatment, absorptions assigned to trans-vinylene CH out-of-plane bending and trans-CH stretching are observed at 963 cm~ and around 3022 cm~ , respectively. The appearance of the modes is an evidence that the PPV structure was formed by the thermal e]imination of the sulfonium group in the LB film. On the other hand, absorptions at 2922 cm~ and 2854 cm "z, which are assigned to aliphatic chains of the anionic amphiphile, disappear after the heat treatment. It is turned out that almost all of the anionic amphiphile molecules were removed during the heat treatment. Figure 24 shows the absorption spectra of the PPV LB film measured with polarized lights parallel and perpendicular to the dipping direction at normal incidence. One should note that the absorption peak wavelength of the PPV LB films is about 40 nm longer than that of a PPV cast film obtained form the organic solvent soluble precursor and that the absorption band of the PPV LB film is sharp and well resolved. The result clearly demonstrate that extension of the mean n-conjugation length and narrowing of the distribution of n-conjugation lengths were attained in the LB film. The absorbance of the PPV LB film at peak wavelength measured with polarized light parallel to the dipping direction is about twice large that measured with polarized light perpendicular to the dipping direction. The anisotropy of absorbance

315

I ........... : b e f o r e h e a t - t r e a t m e n t : --------: a f t e r h e a t - t r e a t m e n t

|

I

11 i

o

U C

I

:

:

.......

...o

-:

: . ' " - . . . . ...... :" -.. !

4000

....

t

.

.,,

.,.

-..." ~ : v'-.;,.....

. . . . . ..~.. . . . . .

I

3000

:

..

i .... ~.... ,

,

,

I

,

,

2000 1500 Wave N u m b e r / c m -1

1000

Fig.23. IR spectra of a PPV precursor LBfilms before and after the heat treatmet.

1

r r c~ .Q O

<

I

I

!

....

300

400 500 61)0 Wavelength (nm) Fig.24. Absorption spectra of a PPV LB films (1, 2) and a PPV cast film (3). Spectra of 1 and 2 were measured with polarized light parallel and perpendicular to the dipping direction. indicates t h a t the PPV chains were highly oriented along the dipping direction. F u r t h e r , p l a n a r orientation of PPV chains in the film plane was d e m o n s t r a t e d from the linear dichroism; dichroic ratio Ap/As decreased from 2.1 to 1.2 when the incident angle of polarized light increased from 0 ~ to 45" , w h e r e Ap and As

316 are absorbances of p- and s-polarized light, respectively, and the incident plane coincides with the dipping direction.

2.2.2 P r e p a r a t i o n of o r i e n t e d poly(2,5-dimethoxy-p-phenylenevinylene), MOPPV, LB films The same way with PPV LB films was not applicable to MOPPV, because its polyion complex was unstable in solid state. The elimination reaction of a sulfonium leaving group in the polyion complex rapidly progressed in solid state even at room temperature and the complex consequently became insoluble in the conventional organic solvents. Then, there is no way to form the polyion complex monolayer at the air/water interface. In the preparation of the MOPPV LB films, the polyion complex monolayer were formed by the adsorption of the precursor onto the anionic amphiphile monolayer (route b-2 in Fig.5) [36,37]. The monolayer of 2C12SUC was spread on a aqueous solution of the MOPPV precursor polymer (repeating unit concentration about 104 M). The monolayer was allowed to stand for 4 hours to adsorb the precursor after being compressed to a surface pressure of 30 mNm 1 . The monolayer was deposited on fused quartz substrate by the LB technique. At last, the deposited polyion complex was converted to MOPPV by the heat treatment at 200 ~C in vacuo (about 10.2 torr). More than 300 layers of the polyion complex monolayer could be deposited by the above-mentioned procedure. After the heat treatment of the polyion complex LB films, we obtained dark red films with good optical quality. The IR spectra of the polyion complex LB film before and after the heat treatment show in Fig.25. In the spectrum after the heat treatment, absorption assigned to CH stretching of trans-vinylene is observed around 3000 cm "1, demonstrating the formation of MOPPV structure. On the other hand, absorptions around 2900 cm "~ assigned to CH stretching of aliphatic chains become very weak after the heat treatment. It is demonstrated that almost all of the amphiphile molecules were removed from the LB film after the heat treatment. Corresponding to the removal of the amphiphile, shrinking of the LB film thickness was observed; thickness of a polyion complex LB film (100 layers) decreased from 120 nm to 50 nm after the heat treatment. Figure 26 shows the absorption spectra of an MOPPV LB film and an MOPPV cast film obtained from the organic solvent soluble precursor. The absorption peak of the MOPPV LB film locates at a wavelength of 510 nm, which is 30 nm longer than that of the MOPPV cast film. The result demonstrates that mean ~-conjugation length in the LB film was extended, in comparison with the cast film. In other words, diminution of chemical and conformational defects in ~-conjugation system was attained in the LB films. The linear dichroism measurement demonstrated that MOPPV chains formed the planar orientation chains in the LBfilms; dichroic ratio Ap/As

317

i . . . . . before h e a t t r e a t m e n t i: ~aider h e a t t r e a t m e n t i b

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c

:" "..

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:

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", :.

',,)

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,', Ii

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,,,

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Fig.25. IR spectra of an MOPPV precursor LB film before and after the heat treatmet.

.$ ., .

.

.

.

.

.

.

.

.

.

.

:3

ii ~

8 400

500

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700

Wavelength / nm Fig.26. Absorption spectra of an MO PPV LB film (1, 2) and an MOPPV cast film (3). Spectra of 1 and 2 were measured with polarized light parallel and perpendicular to the dipping direction decreased from 3.0 to 1.9 with increasing incident angle of polarized light from 0 ~ to 45 ~ . The ratio of 3.0 at 0 ~ incidence also d e m o n s t r a t e s t h a t M O P P V chains were highly oriented along the dipping direction. The development of n-conjugation system and high degree of orientation in the PAV LB films arise from the orientational orderliness of the precursor chains in the LB films. The precursor polymers should form two-dimensional o r i e n t a t i o n

318 of expanded chains through the formation of the precursor-anionic amphiphile polyion complex monolayer at the air/water interface. In addition, the precursor polymers should be aligned along the dipping direction during the deposition process. Then, the polyion complex LB films, which consist the ordered polyion complex monolayers, would possess ordered aggregation structure of the precursor polymers. Conversion reaction should progress exquisitely in ordered precursor films such as the LB film. Consequently, the chemical and conformational defects are expected to be decrease in the PAV LB films. Beside, the ordered precursor polymer would bring about high degree of orientation of the PAV chains in the LB films. As described above, extension of the mean x-conjugation length, narrowing of the distribution of the n-conjugation length, and high degree of orientation of the n-conjugated chains are most likely to be realized in the LB films.

Third-order nonlinear poly(arylenevinylene) LB films 2.3.

optical

properties

of

oriented

Third-order optical nonlinearity of the oriented MOPPV LB film was evaluated by the THG measurement [30]. The detailed procedure is described in the section 2.1. Figure 27 shows spectra of third-order susceptibility X(3) of the oriented MOPPV LB ftim. Owing to the orientation of MOPPV chains, anisotropy of X(3) was observed. The 3~(3)values of the LB film, which is enhanced by the three photon resonance, are maximized at a fundamental wavelength of 1600 nm, as observed in a MOPPV cast film. The maximum values of ;~1~ 3) and ~•(3) are 3.2 x 1040 esu (3)

and 1.8 x 10 1~ esu, respectively, where ~4~3) and •• are third-order susceptibilities in the directions parallel and perpendicular to the orientation axis of the MOPPV LB film. The value of X~ ) is twice that of a MOPPV cast films (X(3) = 1.6 x 10 1~ esu at a fundamental wavelength of 1600 nm). At a f u n d a m e n t a l wavelength of 1064 nm, large and anisotropic optical nonlinearity was also observed. The values of X~ ) and X•(3) are 4.5 x 10.1o esu and 1.0 x 10 1~ esu, respectively. The large values are due to the two photon resonance, because the harmonic wavelength of 355 nm is near off-resonance region. From the two-photon fluorescence measurement, we confirmed t h a t a two-photon absorption band, which is origin of the enhancement effect, exist around 532 nm, half of the fundamental wavelength. The ~(~) value of the LB film is about 10 times larger than that of the MOPPV cast film. When we consider only the effect of orientation of the MOPPV chains on )~(3), the relation between X(3) of the films with perfectly aligned ~-conjugated chains and t h a t of unoriented film is presented by the eq.(18). The value of is 1/5 for three-dimensional random orientation and 3/8 for only in-plane

319 random orientation. Accordingly, it is expected that the ratio of XI~) of the M O P P V L B film and ;((3)of the cast M O P P V film should take a value less than 5. However, ~4c~) of the M O P P V L B film was more enhanced, although the uniaxial orientation in the L B film was not perfect. Therefore, we should consider not only the orientational effectbut also the enhancement effecton X (3)due to extension of m e a n x-conjugation length.

1.5 o

-

1.0

-

0.5

~3 A

~2

o

---lX- ~

300

"

"'Xlk

ZX/X

400 500 600 Harmonic wavelength (nm)

0 700

Fig.27. ;~3)and absorption spectra of an MOPPV LB film. Open and solid circles show value of ;~(3)of the MOPPV LB film in the direction parallel and perpendicular to the dipping direction. Open triangles show ;((3)of an MOPPV cast film. Solid and broken lines show the absorption spectra measured by the polarized light parallel and perpendicular to the dipping direction, respectively. Dotted line shows an absorption spectrum of the MOPPV cast film. In conclusion, the application of the LB technique to the via precursor method provided polyarylenevinylene films which possess well-developed n-conjugation system with less defects, high degree of orientation of the x-conjugated chains, and good linear and nonlinear optical properties. This procedure, we believe, is promising for the preparation of high performance x-conjugated polymer films for nonlinear optics.

R E F E R E N C E S

1. D.J.Williams, Angew. Chem. Int. Ed. Eng., 23 (1884) 690. 2. D.S.Chemla and J.Zyss, Nonlinear Optical Properties of Organic Molecules, Academic Press, Orland, 1987.

320 3. J.L.Ouder, J. Chem. Phys., 67 (1977) 446. 4. L.M.Blinov, N.D.Dubinin, V.G.Rumyantsev, and S.G.Yudin, Opt. Spectros., 55 (1983) 403. 5. L.M.Blinov, N.D.Dubinin, and S.G.Yudin, Opt. Spectros., 56 (1983) 173. 6. M.Era, M.Fukuda, T.Tsutsui, and S.Saito, Jpn. J. Appl. Phys., 26 (1987) L1809. 7. W.Liptay, Dipole Moments of Molecules in Excited States and the Effect of External Electric Fields on the Optical Absorption of Molecules in Solution, in Modern Quant, m Chemistry, ed by O.Subabiglu, Academic Press, New York, 1965. 8. M.Era, T.Tsutsui, and S.Saito, Langmuir, 5 (1989) 1410. 9. X.Xu, M.Era, T.Tsutsui, and S.Saito, Thin Solid Films, 178 (1989) 541. 10. K.Nakomura, M.Era, T.Tsutsui, and S.Saito, Jpn. J. Appl. Phys., 29 (1990) L628. 11 H.Kuhn, Thin Solid Films, 99 (1983) 1. 12. H.Buecher, H.Kubn, Chem. Phys. Lett., 6 (1970) 183. 13. X.Xu, M.Era, T.Tsutsui, and S.Saito, Thin Solid Films, 173 (1989) L135. 14. M.G.Kuzyk, K.D.Songer. J.E.Zahn, and L.~King, J. Opt. Soc. Am. B, 6 (1989) 742. 15. D.Lupo, W.Prass, U.Sheunemann, A.Laschewsky, H.Ringsdorf, and I.Ledoux, J. Opt. Soc. Am. B, 2 (1988) 300. 16. R.H.Tredgold, M.C.J.Young, R.Jones, P.Hodge, P.Kolinsky, and R.J.Jones, Electon. Lett., 24 (1988) 308. 17. G.J.Ashwell, EE.J.C.Dawnay, and A.P.Kuczynski, J. Chem. Soc., Chem. Commun., (1990) 1355. 18. M.Era, K.Nakamura, T.Tsutsui, S.Saito, H.Niino, K.Takehara, K.Isomura, and H.Taniguchi, Jpn. J. Appl. Phys., 29 (1990) L2261. 19. M.Era, H.Kawafuji, T.Tsutsui, S.Saito, Ka.Takehara, Ke.Takehara, K.Isomura, and H.Taniguchi, Thin Solid Films, 210/211 (1992) 163. 20. O.Sugihara, T.Kinoshita, M.Okabe, S.Kunioka, Y.Nonaka, and K.Sasaki, Appl. Optics, 30 (1991) 2957. 21. R.A.Norwood and G.Kanarian, Electron. Lett., 26 (1990) 2105. 22. O.Sugihara and K.Sasaki, J. Opt. Soc. Am. B, 9 (1992) 104. 23. Ch.Bosshard, M.Floersheimer, M.Kuepfer, and P.Guenter, Opt. Commun., 85 (1991) 247. 24. T.L.Penner, H.R.Motschmann, N.J.Armstrong, M.C.Ezenyilimba, and D.J.Williams, Nature, 367 (1994)49. 25. P.K.Tien, Appl. Optics., 10 (1971) 2395. 26. D.C.Bott, C.K.Chai, J.H.Edwards, W.J.Feast, R.H.Friend, and M.E.Horton, J. Phys. (Paris) Colloq. C3, 44 (1983) 1443. 27. I.Murase, T.Ohnishi, T.Noguchi, and M.Hirooka, Polym. Commun., 25 (1984) 327. 28. D.G.BaUard, A.Courtis, I.M.Shirley, and S.C.Taylor, J. Chem. Soc., Chem. Commun., (1983) 954. 29. T.Momii, S.Tokito, T.Tsutsui, and S.Saito, Chem.Lett., (1988) 1201. 30. S.Tokito, T.Momii, H.Murata, T.Tsutui, and S.Saito, Polymer, 31 (1990)

321 1137. 31. T,Kurihara, Y.Mori, T.Kaino, H.Murata, N.Takada, T.Tsutsui, and S.Saito, Chem. Phys. Lett., 183 (1991) 534. 32. H.Murata, N.Takada, T.Tsutsui, S.Saito, T,Kurihara, and, T.Kaino, Appl. Phys. Lett., 70 (1991) 2915. 33. F.Kajzar and J.Messier, Phys. Rev., A32 (1985) 2352. 34. C.-Q.Wu, X.Sun, Phys. REv. Lett., B42 (11990) 9736. 35. M.Era, H.Shinozaki, S.Tokito, T.Tsutsui, and S.Saito, Chem. Lett., (1988) 1097. 36. M.Era, K.KAmiyama, K.Yoshiura, T.Momii, H.Murata, S.Tokito, T.Tsutsui, and S.Saito, Thin Solid Films, 179 (1989) 1. 37. K.Kamiyama, M.Era, T.Tsutsui, and S.Saito, Jpn. J. Appl. Phys., 29 (1990) L840.

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New Developmentsin Constructionand Functionsof Organic Thin Films T. Kajiyamaand M. Aizawa (Editors) 9 1996 Elsevier Science B.V. All rights reserved.

323

P r o t e i n a s s e m b l i e s for i n f o r m a t i o n t r a n s d u c t i o n M.Aizawa D e p a r t m e n t of Bioengineering, Tokyo Institute of Technology Nagatsuta, Midori-ku, Yokohama 226, J a p a n Protein molecular organization on solid matrices has intensively been explored in various manners. Although most of these researchers have not paid attention on retainment of protein functions, this chapter focuses on artificially designed protein organizates which are capable of molecular recognition and molecular information transduction. Redox enzymes have been assembled in a monolayer on the solid surface by a potential-assisted self-assembling method as well as a thiol-gold selfassembling method. These enzymes are electronically communicated with the solid substrate through a molecular interface of conducting polymer and a covalently bound mediator. Electron transfer type of enzyme sensors have been fabricated by the self-assembling methods. Calmodulin has been hybridized with phospho diesterase with retaining its activity to respond to calcium ion in changing its conformation so as to activate phospho diestelase. Intelligent molecular materials have been designed using calmodulin. An ordered antibody array has also been assembled on the solid surface by a combination of Langmuir Blodgett (LB) film method and self-assembling method. An ordered monolayer of protein A is deposited on the solid surface by LB method, which is followed by self-assembling of antibody. Individual antigen molecules which are complexed with the antibody array have been q u a n t i t a t e d selectively by atomic force microscopy (AFM). 1. I N T R O D U C T I O N

Various information networks such as gene, intercellular, intracellular, sensory, and brain information networks are implemented in biological systems. The information transduction, conduction, and retreaval functions are integrated in these bioinforrnation networks. Every bioinformation network is totally consisted of organic molecules including protein. A keen interest has been focused on the molecular mechanisms of the information transduction by the biomolecular assemblies because of their excellent selectivity and sensitivity. This chapter concerns protein molecular

324 assemblies which are designed for information transduction.

1.1. Information Transduction in Biological Systems Molecular communication is the characteristic information system in the bioinformation networks. The endocrine system, which is one of intermolecular information networks, may represent the feature of molecular communication. The gland is a collection of specialized cells that synthesize, store, and release hormones. A hormone, molecular information, is released into the extracellular fluid and transported via the blood to two types of cells; target cells where the hormone acts, and other cells that degrade the hormone as schematically presented in Fig.1. In some systems the target cell and the degradation site are in the same organ or even the same cell. Both activities may even be located on the same plasma membrane. The receptor for the hormone is located on the surface of the plasma membrane. Another example of molecular communication is found in a neuronal synapse, which is a communication junction between two neurons as shown in Fig.2 . The presynaptic membrane releases the n e u r o t r a n s m i t t e r molecule t h a t is recognized and captured by the receptor located on the surface of the postsynaptic membrane.

Excretary cell

Presynaptic membrane ,Postsynaptic membrane ~ / / Receptor and

A Blood vessel

B C

iooo fr

P

!

: I

--e,

%

"i.

-'',

Na § :

Q

' "

t

Target cell Neurotransmitter vesicle

Fig.1 Schematic illustration of molecular communication in the endocrine system

A ~--~o^~

Fig.2 Molecular communication in the synapse

325 In these molecular communications an information molecule is selectively recognized and transduced by the corresponding receptor which is a characteristic protein assembly found on the cellular membrane surface as schematically illustrated in Fig.3. Major categories based on its constitutions are 1)Receptor/G-protein/Modulating protein, 2)Receptor/Ion channel, 3)Receptor/Protein kinase. For instance, insulin acts by binding to receptors in the plasma membrane of target cells. The insulin receptor is an integral membrane glycoprotein consisting of two a (135kd) and two 13 (95kd) chains joined by three disulfide bonds. The a chains are on the extracellular side of the membrane, whereas the ~ chains traverse the membrane. Each a~ unit of the receptor is derived from a single-chain precursor of 1382 residues. The precursor begins with a signal sequence, followed by the a subunit sequence and the 13 chains. The binding sites for insulin are on the a chains, on the extracellular side of the membrane. The binding of insulin switches on the tyrosine kinase activation of the receptor. The activated receptor then phosphorylates two tyrosine residues on the same chain as the catalytic site. It is amazing that these integrated molecular systems undergo information transduction by intermolecular conformation transfer but not by electron transfer. The receptor molecule induces the conformation change of an adjacent molecule when it recognizes the corresponding molecule. Individual molecules in the integrated molecular systems are assembled in such a manner as the conformation of each molecule is coordinated. This is the distinctive characteristics which are hardly mimicked by the synthetic molecular systems. In contrast to the information transduction mechanism based on conformational change, the information transduction may effectively be performed through the electron transfer if the integrated molecular systems are composed of electronically active molecules.

1.2. I n t e g r a t e d Molecular Systems for Information Transduction On the model of the receptors in the bioinformation networks, several types of molecular assemblies may be designed for molecular information transduction. The molecular assembly should contain at least one receptor molecular component that can recognize selectively a specific molecular information. The receptor component responds to a specific molecular information in changing in conformation and electron transfer, which results in information transduction as schematically shown in Fig.4. One category of the molecular assembly for molecular transduction is based on an electron transfer. The receptor molecule recognizes the specific molecular information, which accompanies an electron transfer between the

326 information molecule and the receptor molecule. Further electron transfer within the molecular assembly should be required for information transduction. To detect the molecular information the electron transfer should be monitored through an electron collector, electrode. The molecular assemblies may preferably be fabricated on the electrode surface. The other category of molecular assembly depends its molecular information transduction on the conformational change. The receptor molecule changes its conformation, when it recognizes the specific molecular information. No electron transfer is accompanied by this type of molecular recognition. The conformational change may be transferred within the molecular assembly. The molecular components should be assembled on the matrix surface with retaining their flexibility in conformational change.

Molecular c ~ P////'J -.-.~') Re cep~or Information - ~ O protein -

-

-

messenger ~ / / / / A E ffec to r

Molecular

Information

~

Molecular Information ~ (

Receptor

Ion Channel

V///A

,/////~ ~

Receptor Phosphorylation ofprotein

Fig.3 Schematic representation of molecular recognition by receptors embedded in cellular memoranes

Fig.4 Integrated molecular systems for information transductionthrough intermolecular electron transfer and conformation change

1.3. P r o t e i n Assembling Technology for I n t e g r a t e d M o l e c u l a r Systems There have been several methods for assembling protein molecules on the surfaces of various matrices. These include (1) covalent, (2) LangmuirBlodgett, and (3) self-assembling methods. The protein assembling by these methods are mostly accompanied by appreciable loss of biological activity. Two different types of self-assembling processes are described. The one is a potential-assisted self-assembly of redox enzymes on the surface of an electrode. The potential-assisted self-assembly is followed by electrochemical

327 deposition of conducting polymer which works as molecular interface to enable the redox enzyme to electronically communicate with the electrode. The other is a self-assembly of mediator-modified redox enzymes on the surface of a porous gold electrode. In addition, fabrication of antibody arrays on the solid surface by LB film technology coupled with self-assembly technology will also be presented. 2. R E D O X ENZYM]$ A S S E M B L I E S WITH E L E C T R O N T R A N S F E R ON ELECTRODE SURFACE 2.1. M o l e c u l a r I n t e r f a c e for E l e c t r o n T r a n s f e r of E l e c t r o d e - b o u n d Redox Enzymes One of the key technologies required for fabricating biomolecular electronic devices concerns with molecular assembly of electronic proteins such as redox enzymes in monolayer scale on the electrode surface. Furthermore the molecularly assembled electronic proteins are required to be electronically communicated with the electrode. Individual protein molecules on the electrode surface should be electronically accessed through the electrode. Redox enzymes catalyze either oxidation or reduction of corresponding substrates. However, these enzymes cannot be regenerated by themselves if neither electron acceptor nor donor is associated. An oxidase, for instance, accepts an electron from corresponding substrate to be oxidized. The enzyme remains in reduced form when the electron cannot be transferred to such an electron acceptor as oxygen. Redox enzymes are thus generally associated by either electron acceptor or donor for their regeneration as shown in Fig.5. f Electrode

~

Electrochemical oxidation of reduced enzyme

~~

Enzyme //

Enzymatic reduction of enzyme

Fig.5 Electrochemical communication of an oxidase on the electrode surface

328 In absence of any electron acceptors the reduced form of a redox enzyme can transfer an electron to an electrode of which potential is appropriately controlled, if the enzyme could be provided with an electron transport path between the redox center of the enzyme and the electrode surface. The pioneering works of Hill and Eddows have opened the way to realize fast and efficient electron transfer of enzymes at the electrode surface. They modified a gold electrode with 4,4'-bipyrydyl, an electron promoter, not a mediator since it does not take part in electron transfer in the potential region of interest, to accomplish rapid electron transfer of cytochrome [1].Their work has triggered intensive investigation of electron transfer of enzymes using modified electrodes [2]. Apart from electron promoters a large number of electron mediators have long been investigated to make redox enzymes electrochemically active on the electrode surface. In the line of this research electron mediators such as ferrocene and its derivatives have successfully been incorporated into an enzyme sensor for glucose [3]. The mediator was easily accessible to both glucose oxidase and an electron tunnelling pathway could be formed within the enzyme molecule [4].The present authors [5,6] and Lowe and Foulds [7] used a conducting polymer as a molecular wire to connect a redox enzyme molecule to the electrode surface. Mediator / / Enzyme

Bound mediator Enzyme

(a) Molecular interface of electron mediator Conducting polymer

Conducting organic electrode

11",/*'.

.

9

.

E n z y~ m e~~_s~i ( .

(b) Molecular interface of molecular wire Fig.6

(c) Molecular interface of organic electrode

Schematic concept of the molecular interface

329 These progresses in electron transfer of enzymes have led us to conclude that a molecular level assembly should be designed to facilitate electron transfer at the interface between an enzyme molecule and an electrode. Such a molecular level of assembly at the interface m a y be termed "molecular interfaces" [8-10]. There are several molecular interfaces for redox enzymes to promote electron transfer at the electrode surface (Fig.6). 1)Electron mediator: Either electrode of enzyme is modified by an electron mediator in various msnners. 2)Molecular wire: The redox center of an enzyme molecule is connected to an electrode with such a molecular wire as conducting polymer chain. 3)Organic salt and conducting polymer electrodes: The surface of an organic electrode m a y provide enzymes with smooth electron transfer.

2~2. P o t e n t i a l - a s s i s t e d Self-assembly of E n z y m e s on t h e E l e c t r o d e Surface The potential-assisted self-assembly has been proved feasible for fabricating a protein monolayer on the electrode surface. The schematic procedure is illustrated in Fig.7. A pair of electrodes are installed in an electrolytic cell along with a reference electrode. The electrolyte solution contains the objective protein to be deposited on the electrode surface. One of these two electrodes works as a working electrode on which the objective protein forms a monolayer. The potential of the working electrode is adjusted at constant with referring to the Ag/AgC1 electrode through a potentiostat. When the electrode potential is fixed positive, the negatively charged part of the protein molecule could be self-assembled on the positively charged electrode surface due to the electrostatic interaction. The potential-assisted self-assembly has been applied to several proteins including glucose oxidase and fructose dehydrogenase. Glucose oxidase was first adsorbed onto the platinum surface at a controlled potential. The protein adsorption varied depending on several factors as electrode potential, glucose oxidase concentration, pH and temperature. The effects of these factors on protein adsorption were carefully investigated. Glucose oxidase gradually adsorbed onto the platinum electrode at a potential of 0.5 V vs. Ag/AgC1 and pH5.5, reaching a saturated state within 60 rain. Adsorbed glucose oxidase was assayed for its enzyme activity and protein content. As shown in Fig.8, these results indicate that a monolayer of adsorbed glucose oxidase at a different surface coverage may be prepared under a controlled electrode potential. According to these characterizations, a glucose oxidase monolayer was prepared at a surface coverage of 60-70% on the platinum electrode surface. Fructose dehydrogenase (FDH) having pyrroloquinoline quinone (PQQ) as a prosthetic group is an redox enzyme to catalyze the oxidation of fructose. A

330 platinum electrode was soaked in an electrolyte solution containing FDH. The electrode potential of the electrode was set at a constant value to proceed potential-assisted self-assembled in monolayer on the electrode surface [11-13]. FDH was self-assembled in monolayer on the electrode surface.

Enzyme

~600

,oo Electrode

:.~~ f~

(~

O

AdsorptionTime / rain

Fig.7 Procedure of the potentiala s s i s t ~ self-assembly of enzyme molecules

250

Fig.8 Time course of the potentialassisted self-assembly ofglucose oxidase

2.3. P o l y p y r r o l e Molecular I n t e r f a c e for E l e c t r o n T r a n s f e r of Electrode-bound Enzymes The polypyrrole molecular interface has been electrochemically synthesized between the self-assembled protein molecules and the electrode surface for facilitating the enzyme with electron transfer to the electrode. Figure 9 illustrates the schematic procedure of the electrochemical preparation of the polypyrrole molecular interface. The electrode-bound protein monolayer is transferred in an electrolyte solution containing pyrrole. The electrode potential is controlled at a potential with a potentiostat to initiate the oxidative polymerization of pyrrole. The electrochemical polymerization should be interrupted before the protein monolayer is fully covered by the polypyrrole layer. A postulated electron transfer through the polypyrrole molecular interface is schematically presented in Fig.10.

331 The adsorbed glucose oxidase was in contact with a solution containing pyrrole after rinsing. The electrode potential was immediately controlled at 0.7 V vs. Ag/AgCI to initiate polymerization of pyrrole. Total charge passed during polymerization was controlled with a coulombstat. Polypyrrole deposited on the electrode surface so that the glucose oxidase intermolecular space could be filled.Polymerization was stopped at an estimated membrane thickness of 2 nm. Assuming that glucose oxidase is a globular protein with a diameter of 3.5 nm, the monolayer of glucose oxidase/polypyrrole on the electrode surface is schematically illustrated in Fig.10. Polypyrrole strongly adheres the electrode surface to prevent glucos', oxidase from leaching. Electron transfer of the glucose oxidase/polypyrrole on the electrode surface was confirmed by differential pulse voltammetry and cyclic voltammetry. The glucose oxidase clearly exhibited both reductive and oxidative current peaks in the absence of dissolved oxygen in these voltammograms. These results indicate that electron transfer takes place from the electrode to the oxidized form of glucose oxidase and the reduced form is oxidized by electron transfer to the electrode through polypyrrole. It m a y be concluded that polypyrrole works as a molecular wire between the adsorbed glucose oxidase and the platinum electrode.

/,/

Redox enzyme

Enzyme

Product

Po~-a~ed ~.ff-uu~mbly Electropolymerization

Polypyrrole

j~~.j

Substrate

Fig.lO Schematic illustration of the molecularly interfaced enzyme on the electrode surface

Pyrrol 9

Fig.9 Electrochemical polymerization of pyrrole on the electrode-bound redox enzyme oxidase

332 The FDH-modified electrode was transferred in an electrolyte solution containing 0.1M pyrrole and 0.1M KCI. The electrode potential was controlled by polymerization charge. A f a r washing with distilled water, the polypyrrole (PP)/FDH/Pt electrode was kept in Maclvaine buffer of pH 4.5. Almost all the FDH molecules on the electrode surface seemed to retain the enzyme activity because of the mild immobilization at less extreme potential. The enzyme activity of immobilized FDH was dependent on the thickness of polypyrrole membrane because a thicker membrane could prevent the enzyme substrate from diffusing into the membrane matrix. Therefore, it was very important to make the polypyrrole membrane as thin as possible to minimize the effect on substrate diffusion and to ensure the complete coverage of the enzyme layer. The polymerization charge was varied from zero to 20mC. The thickness of the polypyrrole membrane is directly proportional to the polymerization charge. The apparent activity of FDH sharply dropped with an increase in polymerization charge. The PP membrane thickness prepared by passing a polymerization charge of 2-3 mC correspoT, ds roughly to the monomolecular thickness of FDH. Electrochemical communication between electrode-bound enzyme and an electrode was confirmed by such electrochemical characterizations as differential pulse voltammetry. As shown in Fig. 11, reversible electron transfer of molecularly interfaced FDH was confirmed by differential pulse voltammetry. The electrochemical characteristics of the polypyrrole interfaced FDH electrode were compared with those of the FDH electrode. The important difference between the electrochemical activities of these two electrodes is as follows; by the employment of a conductive PP interface, the redox potential of FDH shif~d slightly as compared to the redox potential of PQQ, which prosthetic group of FDH and the electrode shuttling between the prosthetic group of FDH and the electrode through the PP interface. In addition, the anodic and cathodic peak shapes and peak currents of PP/FDH/Pt electrode were identical, which suggests reversibility of the electron transport process. One further point requires emphasis: the peak current of the PP/FDH/Pt electrode is about 8 times greater than that of the FDH/Pt electrode. The significant increase in redox peaks strongly supports our concept of the molecular interface. Due to the incorporation of FDH molecules in the conducting polymer on the surface of the electrode, the prosthetic PQQ electrochemically communicates with the base electrode through the electrode was enhanced significantly in the presence of the role works as an effective molecular interface for FDH on the electrode surface.

333

2.4. Self-Assembly of Mediator-Modified Redox Enzymes on the P o r o u s Gold E l e c t r o d e S u r f a c e In contrast to the molecular wire of molecular interface, electron mediators are covalently bound to a redox enzyme in such a manner as an electron tunneling pathway is formed within the enzyme molecule. Therefore, enzyme-bound mediators work as molecular interface between an enzyme and an electrode. Degani et al. proposed the intramolecular electron pathway of ferrocene molecules which were covalently bound to glucose oxidase [ 4 ]. However, few fabrication methods have been developed to form a monolayer of mediator-modified enzymes on the electrode surface. We have succeeded in development of a novel preparation of the electron transfer system of mediator-modified enzyme by self-assembly in a porous gold-black electrode as schematically shown in Fig.12 [14]. 150

Mediator

100

@

5O

, Covalent modification

-50

I 1 Mediator-modifie redox enzyme

-I00

-150

Q

9

@

0 o

9

Redox enzyme

9

|

0.0

.

|

o.2

.

9

o.4

Potential / V vs. Ag / AgCI

Serf-assembly

F i g . l l Differential pulse voltammogram of the molecularly interfaced fructose dehydrogenase (--) and simply adsorbed one

Fig.12 Scheme ofself-assembly of mediator-modified enzyme on the gold electrode surface

334 Glucose oxidase (from Asperigilus nigar) and ferrocene carboxyaldehyde were covalently conjugated by the Schiff base reaction, which was followed by NaBH 4 reduction. The conjugates were dialyzed against phosphate buffer with three changes of buffer and assayed for their protein and ion contents. Porous gold-black was electrodeposited on a micro gold electrode by cathodic electrolysis with chloroauric acid and lead acetate. Aminoethane thiol was self-assembled on a smooth gold disk electrode and a gold-black electrode in diameter). Ferrocene-modified glucose oxidase was covalently linked to either modified plain gold or gold-black electrode by glutaraldehyde as shown in Fig. 13.

Ferrocene~

I

~-'~--Ferrocene

J

N

N

fj

ft

CH

i (C~, I CH

II

N

I

I

9 -~'T~TDL,

:4

400 1

/

J (CI'l,), I

"

200 ;d

I

I

",.~] :~: : :?/:

Fig.13 Self-assembled linkage of ferrocene-modified glucose oxidase (GOD) on thiol monolayer

ee

"

/i

//

"

N

S

!:

/ l

/..'

L'H

II

I

/ ,'2"-

o ,,_

I

0

,,

i

_

9

t

0.4

9

It

Potential / V vs. Ag / AgC1 Fig.14 Cyclic voltammograms of ferrocene-modified glucose oxidase with (-- -) and without (--) glucose

It is noted that the anodic peak current prominently increases with an increase in the molar ratio of ferrocene to glucose oxidase whilst the amount of enzyme self-assembled on the electrode surface is fixed as presented in Figs. 14-16. This indicates that each modified ferrocene may contribute to electron transfer between the enzyme and the electrode in the case of gold-black electrode, the ferrocene-modified enzyme could form multi electron transfer paths on the porous gold-black electrode.

335

l

'

'

l

i

"

{

,

|

"v

'

i

r,.)

~"

0.5

m{

120-

[

"

!

"

9 ' "

i

l

9

0

"

o.4

=

80

~

60

"~

0.8

I

"/p~

ooO~176176- - ~

r

~

"

Glucose~ ..

W.,,

Pa

~

..oO

40" ~ ...... ~'" ~

I

,

'

8

-

I

,

"l"

I

10

,

o

l

8

s

.

,

.

1.~

1o

.

_L

Bonded ferrocene / GOD

Bonded ferrocene / GOD

Fig.15 Relationship between the n u m b e r ofbound ferrocenes per glucose oxidase and anodic peak current on the gold disk electrode

I

.0,o

Fig.16 Relationship between the number of bound ferrocenes per glucose oxidase and anodic peak current on the gold black electrode

Substrate concentration dependence of response current of the gold-black electrode was compared with that of gold disk electrode. The ferrocenemodified glucose oxidase which was used in this m e a s u r e m e n t had 11 ferrocenes per glucose oxidase. The electrode potential was controlled at 0.4 V vs. Ag/AgC1. The response current was recorded when the output reached at a steady state. The response current was enhanced when ferrocenemodified glucose oxidase was self-assembled on a porous gold-black electrode. The porous matrix of gold-black electrode has enabled ferrocene-modified glucose oxidase to perform the smooth electron transfer by means of easy access between self-assembled molecules and electrode surface. 3. MOLECULAR SENSING I N T E R F A C E S ENZYMES

DEVICES

WITH

MOLECUIJkR

3.1. E l e c t r o n T r a n s f e r T y p e of O x i d a s e S e n s o r s A biosensor consists, in general, of two functional parts for molecular recognition and signal transduction as schematically shown in Fig.17. Either biocatalyst or bioaffinity substance is used as major material for molecular recognition to attain extremely high selectivity. The signal transducing part involves typically an electrochemical, optical, thermal, and piezoacoustic devices. To design a highly sensitive biosensor, it is important to link efficiently, in function, a biological substance for molecular recognition with a signal transducing device [15].

336

Molecular recognition

C~

Signal transduction

O

Substrate (analyte)

O~

Oxidase

Electric signal

k~

H202

Products

Analyte

Amverometric determination

Fig.17

Biosensor construction

Fig.18 Reaction scheme of an oxidase enzyme sensor based on amperometric determination of hydrogen peroxide

Redox enzymes are recognized as major materials in constructing both biocatalytic sensors and bioaffinity sensors. Biocatalyic sensors for glucose, lactate, and alcohol utilize glucose oxidase, lactate dehydrogenase (lactate oxidase), and alcohol dehydrogenase (alcohol oxidase) and molecularly recognizable material. Since these redox enzymes are mostly associated with the generation of electrochemically active substances, m a n y electrochemical enzyme sensors have been developed by linking redox enzymes for molecular recognition with electrochemical devices for signal transduction. These enzymes, however, have been linked in an indirect manner with electrochemical devices, resulting in a loss in sensitivity as illustrated in Fig.18. The most successful approach to construct more practical devices has been to use an electron transfer mediator used as a shuttle to carry charge between the electrode and the active site of an enzyme [Fig.19]. Cass et al. [3] found that ferrocene and its derivatives are very efficient electron acceptors for glucose oxidase in their oxidized form. Cyclic voltammetric studies showed good, quasi-reverible electrochemistry of ferrocene in the presence of glucose alone. O n the addition of enzyme, however, a large catalytic current flows at oxidizing potentials. The ferrocene-mediated approach offered the change of developing a biosensor for glucose. It was 1987 that a disposable biosensor for glucose has been commercialized for self-monitoring by diabetic patients.

337

Oxidized ~z~vnne

Substrate ( a n a l y t e )

Pr od uc ts

Fig.19 Reaction electron transfer enzyme senser

A

hdlced ~nzyme

/

e-

tmperomtric determination

J

scheme of the type of oxidase

Electron mediators successfully used with oxidases include 2,6dichlorophenolindophol, hexacyanoferrate-(III), tetrathiafulvalene, tetracyanop-quinodimethane, various quinones and ferrocene derivatices. From Marcus' theory it is evident that for long-range electron transfer the reorganization energies of the redox compound have to be low. Additionally, the redox potential of the mediator should be about 0 to 100 mV vs. standard calomel electrode (SCE) for a flavoprotein (formal potential of glucose oxidase is about -450 mV vs SCE) in order to attain rapid vectrial electron transfer from the active site of the enzyme to the oxidized form of the redox species. Ferrocene-mediated enzyme sensors have usually been obtained by adsorption of the mediator onto the electrode surface because of this insolubility in aqueous solution. Due to the good solubility of ferrecinium cations in aqueous solution complications arise from leakage of the mediator from the electrode surface. To overcome the poor stability of ferrocene-mediated enzyme sensors, mediator-modified electrodes have been used. In the case of glucose oxidase, the cofactor FAD is deeply buried within the protein matrix. The depth of the active center is estimated to be 0.87 nm. Therefore, one cannot expect that the mediator covalently attached to the electrode surface via a short spacer retain the possibility of closely approaching the cofactor of the enzyme. Mizutani et al. [16] have demonstrated that ferrocene derivatives, attached by means of covalent bonds to the surface ofbovine serum albumin, have been able to mediate the electron transfer between the glucose oxidase and the electrode through the osemium complex. In contrast to the mediator-modified electrodes, Degani et al. modified glucose oxidase itself by means of covalently bound ferrocene [4]. After modifying enzymes with ferrocene carboxylic acid, they observed direct electron transfer from the active site of the enzyme to a gold or platinum

338

electrode at the potential given by the ferrocene redox couple. It has been claimed that the tunneling distance for electron transfer from the active site to the surface of the macromolecule could be drastically shortened after incorporation of about 12 ferrocebe carboxylic acid moieties between the two subunits of the enzyme, and that this might be possible through an electronhopping mechanisms between adjacent mediator molecules. Schuman et al. have synthesized ferrocene-modified glucose oxidase with the ferrocene derivatives bound via long and flexible chains directly to the outer surface of the enzyme [17]. A peripherally attached redox mediator may accept electrons through either an intramolecular or through an intermolecular process. Aizawa et al. [14] have immobilized mediator-modified glucose oxidase within micropores of a gold black electrode by self-assembling via the thiolgold interaction. A n electron transfer type of oxidase sensor has been fabricated by assembling mediator-modified glucose oxidase within the micropores of a gold black electrode or on the surface of a plain gold electrode [Fig.20]. The output currents of these biosensors at a constant potential of 0.4 V vs. Ag/AgCI are plotted against the glucose concentration in Fig 21. A n enhanced response current has been obtained by the mediator-modified glucose oxidase self-

9" - i

"'

~'

t .....

"1

=-

9 '~ . . . . .

I

9

9i,,,',-,

I

i

. . . . . . . .

F.~pym~

active site @

ferrocene Gold E l ~ x i r

100

tl

'_

.

-,J

~o-2

.......

J

~

i

| i --.-I

loo

J

......

,1

9 .....

Glucose concentration

Gold Black Electrode Fig.20 Postulated scheme of mediator-modified glucose oxidase within a pore of a gold black electrode

~l

~o2

mM

Fig.21 Glucose concentration dependence of response current on ferrocene-modified glucose oxidase self-assembledon the gold black (0) and plain gold (0) electrodes

339 assembled within matrix of a gold oxidase to perform the self-assembled

the micropores of a gold black electrode. The porous black electrode has enabled ferrocene-modified glucose smooth electron transfer by means of easy access between molecules and the electrode surface.

3.2. E l e c t r o n T r a n s f e r T y p e of D e h y d r o g e n a s e S e n s o r s To fabricate an enzyme sensor for fructose, we found t h a t a molecular interface of polypyrrole was not sufficient to realize high sensitivity and stability. We thus incorporated mediators (ferricyanide and ferrocene) in the enzyme-interface for the effective and the most sensitive detection of fructose in two different ways;(1) two step method: first, a monolayer FDH was electrochemically adsorbed on the electrode surface by electrostatic interaction, then e n t r a p m e n t of mediator and electro-polymerization of pyrrole in thin membrane was simultaneously performed in a separate solution containing mediator and pyrrole, (2) one-step method: co-immobilization of mediator and enzyme and polymerization of pyrrole was simultaneously done in a solution containing enzyme enzyme, mediator and pyrrole as illustrated in Fig.22.

Fig.22 Preparation of mediator/pp/enzyme electrodes If the potentials of the FDH-interfaced electrode are controlled to be more positive than the redox potential of PQQ (0.06 v), it is expected t h a t the reduced form of FDH (FDH-PQQH 2) will reoxidize to the active oxidized form (FDH-PQQ) by transferring two electrons to the electrode; thus a continuous flow of anodic current is observed upon the addition of fructose. At a lower potential such as 0.1 V the background current was cathodic and magnitude was very high as the rest potential of the electrode is around 0.35 V. To make

340 the residual current as low as possible and to have an anodic current the determination of fructose was performed at 0.4 V. Satisfactory selectivity toward fructose was obtained in both the mediated and nonmediated enzyme electrode. Thus, the mediator does not affect the specificity of the enzyme electrode. In both the cases, the response was very fast (3-5 sec) as the substrate diffusion problem was successfully overcome by reducting the membrane thickness. The most noticeable difference between the response of these two enzyme electrodes was t h a t the response current of the mediatorcontaining enzyme electrode (PP/FCN/FDH/Pt) was 5-6 times higher t h a n that without mediator (PP/FDH/Pt), showing the effectiveness of mediator for the smooth electrochemical regeneration of enzyme by rapid release of electrons from the reducted FDH and transfer of the electrons through PP matrix to the transducer electrode. The calibration curves for fructose were compared between PP/FDH/Pt and PP/FCN/FDH/Pt electrodes (prepared by the two-step method) [Fig.23]. In both cases, the response current was directly proportional to fructose concentration up to 10 mM and with a detection range of up to 30mM fructose. The m i n i m u m detection limit was 10 /~ M and 5 mM in the case of PP/FDH/Pt and PP/FCN/FDH/Pt electrodes, respectively. The slope of the linearity was about 80 nA/mM and 450 hA/raM, respectively.

Alcohol dehydrogenase

P l a t i n u m black

PP/FCN/FDH/Pt

9

5

9 o@ ~ 9

4;

M

2

PP/FDH/Pt

#p

./

m

0 ~ 0

5

lO 15 F r u c t e ~ / mM

20

Fig.23 Calibration curves for fructose with two different enzyme sensors

..~ Electrode 9 9/ I 9 Enzyme-bound meldora blue O NAD Fig.24 Electron transfer system constructed in a biosensor for ethanol

341 In the case of the sensor prepared by the two-step method, very fast (3-5 sec) and highly sensitive (minimum detection: 5 ~ M ) response was observed. But the response was highly unstable due to the easy leakage of the mediator because the membrane was ultrathin. On the other hand, in the case of sensors prepared by the one-step method, comparatively slower (8-10 sec) and less sensitive response (minimum detection: 50 /~M) was observed. But the stability of the sensors improved significantly. Of the two mediators investigated ferrocene derivative (FTAI) was found to be more effective t h a n ferricyanide with higher response and stability. We believe t h a t the stability of the sensor can be improved further by stopping leakage of mediators by covering the enzyme-membrane with some porous membrane, although this appears to result in lower sensitivity. It is difficult to incorporate dehydrogenases that are coupled with NAD(P) into amperometric enzyme sensors owing to the irreversible electrochemical reaction of NAD. We have developed an Amperometric dehydrogenase sensor for ethanol in which NAD is electrochemically regenerated within a membrane matrix. The amperometric dehydrogenase sensor for ethanol consists of a platinum electrode on the surface of which alcohol dehydrogenase (ADH), Meldra blue (MB) and NAD are immobilized with a conductive polypyrrole membrane as schematically illustrated in Fig.24. A platinum disk electrode was electrolytically platinized in a platinum chloride solution to increase the surface area and enhance the adsorption power. The platinized platinum electrode was then immersed in a solution containing 10 mg m1-1 ADH. 0.75 mM and 6.2 ~ NAD. After sufficient adsorption of these molecules on the electrode surface, the electrode was transferred into a solution containing 0.1 M pyrrole and 1 M KC1. Electrochemical polymerization of pyrrole was conducted at +0.7 V vs. Ag/AgC1. The electrolysis was stopped at a total charge of 1 C cm -2. An enzyme-entrapped polypyrrole membrane was deposited on the electrode surface. Cyclic voltammetry was performed with the ADH-NAD-MB/polypyrrole electrode in 0.1 M phosphate buffer (pH 8.5) at a scan rate of 5 mV s-1. The corresponding substrate of ADH caused the anodic current at +0.35 V vs. Ag/AgC1 to increase. These results suggest a possible electron transfer from membrane-bound ADH to the electrode through membrane-bound NAD and MB with the help of the conductive polymer of polypyrrole. The electrode gave a steady-state current when the electrode potential was maintained at +0.35 V vs. Ag/AgC1 in 0.1 M phosphate buffer. Addition of ethanol to the buffer solution resulted in an increase in the anodic current, which was attributed to the oxidation of membrane-bound NADH. A steady response was obtained within 40 sec. The increase in the anodic current was linearly correlated with the concentration of ethanol.

342 An electron transfer type of enzyme sensor was thus fabricated by a electrochemical process. Although no appreciable leakage of ADH and MB from the membrane matrix was detected, NAD leaked slightly. To prevent this leakage, the ADH-MB-NAD/polypyrrole electrode was coated with Nation. A calibration curve is presented in Fig.25 for ethanol determination in an aquous solution with the enzyme sensor. Ethanol is selectively and sensitively determined in the concentration range from 0.1 nM to 10 raM. In a further development, an ADH-MB-NAD/polypyrrole electrode, a platinum counter electrode and an Ag/AgCI reference electrode were assembled and covered with a gas-permeable polymer membrane to form an gaseous ethanol sensor. This appears to be the first time that a complete enzyme sensor for gaseous ethanol has been fabricated in such a manner with NAD incorporated in immobilized form. o

. 9

e

9

10

R e d o x pot. of e n z y m e substrate ......

~ ~

~

.

o

~ 9

~

.

.~

Redox pot. of e n z y m e ....

=5

Electrode pot. - - - 9 9

~

Molecular interface

,.

!

0 0

_

!

5

9

EtOH/mM Fig.25

~

~

10

Calibration curve for ethanol

Fig.26 Postulated potential profile of the molecularly interfaced enzyme on the electrode surface

4. E L E C T R I C M O D U I ~ T I O N O F BIOCATALYTIC ACTIVITY 4.1. P o t e n t i a l D e p e n d e n c y of M o l e c u l a r l y I n t e r f a c e d GOD Activity Enzymes possess two characteristic functions: molecular recognition and selective biocatalytic functions. The molecular recognition function is essentially i m p o ~ t for constructing biomolecular devices, especially molecular sensing devices, because their selectivity depends on the molecular recognition of enzymes. The biocatalytic function has potentiality to create novel devices. In the molecular sensing devices, the molecular information to be determined is transduced in an amplified m a n n e r into the output signal.

343 However, further investigation on implementation of the biocatalytic function is highly demanded or urgently needed. Investigation on the molecular interfacing of redox enzymes yields the following important findings. The molecular-interfaced redox enzymes showed the potential dependency of enzyme activity. The enzyme was inactive when the electrode potential was set below a certain threshold. In contrast, the enzyme activity increased with an increase in the electrode potential above the threshold. The activity of the molecular-interfaced enzyme is reversibly modulated by changing the electrode potential. The molecular-interfaced glucose oxidase (GOD) catalyzes the cycling reaction. The substrate molecule of glucose transfers an electron to the active site of G O D and is oxidized to gluconolactone. Since the prosthetic F A D is located at the active site of G O D , it is reduced to F A D H 2. It is necessary to regenerate the oxidized form of the prosthetic F A D to enable continuous catalytic reaction. If the electrode potential is lower (which means less negative) reduced form of F A D to the electrode through the molecular interface. The catalytic process could be cycled. Unless the energy balance is satisfied, the catalytic process could be inhibited as schematically illustrated in Fig.26. The glucose oxidase/polypyrrole membrane electrode was immersed in a p H 5.5 cirate buffer solution containing potassium choride with a counter electrode and an Ag/AgCI reference electrode. The solution was bubbled with N 2 gas for deoxygeneration prior to the experiment. The electrode potential was set 0.0 V vs. Ag/AgCl, which might be sufficient to oxidize the reduced form of F A D to the oxidized form. Al~r a steady anodic current was measured, glucose was added to the solution. The anodic current immediately increased with addition of glucose, which resulted from the oxidation of the reduced form of F A D due to the enzymatic oxidation of glucose in solution. These results show that the molecular function of glucose oxidase is transduced to the electron flow to the electrode with the aid of polypyrrole. The anodic current increase is caused by the electrochemical oxidation of the enzymatically generated reduced form of glucose oxidase, which corresponds to the enzyme activity. As shown in Fig.27, the enzyme activity of the molecularinterfaced glucose oxidase changed depending on the electrode potential. The enzyme activity instantly changed when the electrode potential was varied in the range from 0 V vs. Ag/AgCI. Such a change was completely reversible. These results lead us to conclude that the enzyme activity of molecuarinterfaced glucose oxidase can be controlled by the electrode potential. 4.2. E l e c t r i c a l A c t i v a t i o n a n d I n a c t i v a t i o n of F D H Potential dependency of the enzyme activity of the FDH/PP/Pt electrode is shown in Fig. 28. The dependency was investigated by adding 5ram fructose and the resulting current response was compared.

344 100 80 I000

60 800 t~

Q)

40

600 400

20 m

rJ~ v

-0.4

, - -

-0.2

0

0.2

Potential / V vs. AgAgCl

Fig.27 Potential dependency of the enzyme activity of the molecularly interfaced glucose oxidase

200

J t

0 -0.2

0

I

, ,,t ,

0.2 0.4 0.6

0.8

Potential /V vs. AgAgCI

Fig.28 Potential dependency of the enzyme activity of the molecularly interfaced fructose dehydrogenase

The curve in Fig.28 [12] might be divided into three parts. First, the potential range was less than the redox potential of FDH, 0.07V, where negligible response current generated because fructose was negligibly oxidized in this potential range. Secondly, in the potential range from 0.07 to 0.35V, the response current increased sharply around 0.1V and then gradually above 0.1V. Thirdly, a sharp increase in the response current was observed up to 0.6V. The response current extremely decreased in the potential range above 0.6V probably due to an irreversible inactivation of FDH. The possible masons for the inactivation at higher potentials might be (1) conformational change of FDH in such a m~nner as the enzyme loses its prosthetic group PQQ, and (2) a drastic change in pH of the molecular interface that causes the enzyme inactivated. It should be emphasized that the enzyme activity of the molecular interfaced FDH was reversibly controlled by electrode potential in the potential range from 0.1 to 0.6V. It is also no~d that the enzyme can be activated and inactivated at a threshold potential of 0.07V which corresponds to the redox potential of the prosthetic group PQQ. The FDH remained inactive in the potential range below the redox potential of the prosthetic PQQ of the FDH. When the electrode potential exceeded the redox potential of the prosthetic PQQ, the enzyme activity increased sharply

345 depending on the potential. In Fig.26, the energy correlation is schematically presented. The potentialcontrolled modulation of the molecular-interfaced enzymes may be interpreted by Fig.26. The enzyme and its substrate molecule have their intrinsic redox potentials. The redox potentials of oxidases and dehydrogenases are determined by an electron transferring molecule, i.e. a cofactor such as FAD, which is located at the active site of the enzyme. Due to potential gradient, an electron can be transferred from the substrate molecule to the active site of the enzyme, if the substrate molecule is accepted by the molecular space of the enzyme active site. However, the electron transfer between the active site of the enzyme and the electrode is regulated by the electrode potential, even if the molecule wire could be completed. It should be reasonable that the enzyme activity is electrically modulated at a threshold of the redox potential of the enzyme.

5. PROTEIN HYBRID ASSEMBLIES C O N F O R M A T I O N A L CHANGE 5.1.

Information

Processing

by

WITH

INTERMOLECD-LAR

Intermolecular

Conformational

Change The receptor protein assembly on the cellular membrane surface is one of the unique information systems in biological systems where molecular information is selectively and sensitively processed. The molecular information is efficiently processed through the intermolecular transfer of the conformational change within the molecular assembly. The receptor molecule of the assembly responds to the corresponding molecular information in changing its conformation, which is followed by the subsequent conformational change of adjacent molecules such as G-protein. On the model of the intermolecular transfer of the conformational change in biological systems, an integrated protein molecular assembly has been designed for molecular information processing.

5.2 C a l m o d u l i n - m o d u l a t e d P h o s p h o d i e s t e r a s e Calmodulin (CAM) undergoes drastic conformational change when it binds Ca 2+ and amphiphilic peptides such as mastoporan and endorphin, which results in the modulation of many important biochemical reactions. The Nterminal and C-terminal of rigid structured globular domains are bridged with a long flexible peptide of a-helical structure. Each domain binds two Ca 2+ ions to its hydrophobic sites. Phosphodiesterase (PDE) is one of enzymes that are modulated in activity by binding the Ca2+-bound state of CaM. The enzyme remains inactive in a Ca2+-free assembly. Our effort has been focused on synthesizing a protein hybrid of CaM and PDE which retains its information processing function in

346 native form as illustrated in Fig.29 [18]. Immobilized hybrid on Free l~rkl in solution DEAE-,.qephamee

tmg PDE Ca~ binding Effm:th~ CaM part

/[ (§

Fig.29 Schematic representation of self-modulated activity of CaM/PDE hybrid

Fig.30

Ca2§

~ (-)Cas"

(+)C~~

(-)C~s"

PDE activity modulated by

5.3. C o v a l e n t H y b r i d i z a t i o n of C a l m o d u l i n a n d P h o s p h o d i e s t e r a s e CaM was purified from porcine brain. The purity of proteins was examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and isoelectric focusing. CaM and PDE were cross-linked with 1-ethyl-3(3-dimethylaminopropyl) carbodiimide (EDC) or N-succinimidyl 3-(2-pyridyldithio)propionate (SPDP) in a buffer solution of 0.1 M HEPES (pH 7.1) in the presence of I mM CaC12. After a buffer solution containing 2 mM EGTA added into the reaction solution, the CaM-PDE hybrid was separated from other ingredients by gel chromatography on a Sepharose CL-6B solumn. The covalently bound hybrid was prepared in the presence of Ca 2+. Crosslinking experiments in the absence of Ca 2+ were also performed using CaM and PDE. However, we could not obtain the hybrid between CaM and PDE in the fraction pattern without Ca 2+. This finding suggests that the assembling of the present molecules requires that CaM is associated with the specific CaM binding domain of PDE when the two biomolecules are linked with remaining as much cooperatively as possible. The cross-linked CaM-PDE hybrid with EDC was activated in the presence of i mM GAG12, whereas native PDE was activated in the coexistence of I mM GaG12 and CaM. However, the cross-linked CaM-PDE hybrid with SPDP was inactivated in the presence of GaG12 and E G T ~ This result suggest that SPDP blocked information processing and actuating part of CaM and PDE

347 because of long spaser part. The activity of the CaM-PDE hybrid with EDC in the presence of Ca 2+ increased about 2.5 times that in a Ca2+-free buffer solution containing 1 mM EGT/~ In general, the activity of native PDE is increased by 15 to 18-folds by CaM in the presence of Ca 2+. On the other hand, the activity of the CaM conjugated PDE in the absence of Ca 2+ was about three times that of native PDE in a Ca2+-free buffer solution. From the fact that further addition of CaM and Ca 2+ to the conjugate resulted in no increase in PDE activity, unreacted PDE seems to be completely excluded by the chromatography. The activity modulation of CaM conjugated PDE was also reversibly observed in the presence and absence of Ca 2+. The remaining activity of the CaM-PDE hybrid in the absence of Ca 2+, presumably resulted from CaM-PDE hybrid with multipoint cross-linking. The cross-linking with EDC occurs only when the carboxyl group of one protein is very close to the amino group of the corresponding protein, therefore EDC has capability of multipoint cross-linking among the carboxyl groups and the amino groups of the concerned two biomolecules. In this coupling reaction one CaM is bound to one PDE with a few peptide bonds. Even after the multipoint cross-linking, the PDE moiety in the hybrid seemed active in conformational change and shows smaller enzyme activity. In this stage of investigation it is very difficult to separate denatured hybrids from multipoint cross-linked hybrids.

5.4. Calmodulin-Phosphodiesterase Conjugates I m m o b i l i z e d on Solid Matrix To elucidate the intramolecular interaction within the hybrid, CaM-PDE hybrid with EDC was immobilized onto DEAE-Sapharose, because immobilized hybrids were expected to have no interaction with each other (inter-hybrid interaction). The CaM-PDE hybrid was immobilized to DEAESepharose suspended in 100 mM glycylglycine buffer (pH 7.5). In the presence of Ca 2+, the CaM-PDE hybrid showed the activity change in its bound form. The activity of the CaM-PDE hybrid in the absence of Ca 2+ was ca.40% as compared to that in the presence of Ca 2+. The normalized modulation in the immobilized form is comparable to that in the free form, although the total modulated activity was smaller in the DEAE-Sepharose immobilized form as presented in Fig.30. These results indicate that the selfmodulation of CaM-PDE activity can be performed in its immobilized form, as this modulation was caused by the CaM moiety in the hybrid (intrainteraction). If CaM and PDE are independently present in solution, CaM has to randomly access to PDE to modulate the enzyme. However, in the case of CaM-PDE distinct CaM molecules modulates the corresponding PDE molecules in intra-hybrid interaction. In other words, CaM concentration

348 seemed to be effectively raised in the vicinity of PDE, by conjugating the Ca2+ stimulated association of two component should be specifically bound to another component for the assembly of intelligent molecular materials. 6. INTELI.IGENT MOLECULAR ASSEMBLIES I N T E R M O L E C I K A R CONFORMATION CHANGE

WITH

6.1. I n t e l l i g e n t M o l e m d a r Materials The design concept of intelligent materials has emerged from a view point of integration of sensing, information processing and actuating functionalities within material as schematically represented in Fig.31. The intelligent materials may be endowed with environmental adaptability, self-repair, selfdegradability, learning ability and other intelligent characteristics. The new technology will change the current philosophy of material design and usher in a new material age. The intelligent molecular materials are designed on the model of the biological systems which involve the molecular systems and structures with the intelligent characteristics such as environmental adaptability and selfrepair [19]. INPUT Ca~free

4Ca ~

..~,~SENSING UNIT PROCESSING UNIT ACTUATING UNIT

OUTPUT

Fig.31 Design concept intelligent material

Shrinked-state

of

the

Extended-state

Fig.32 Postulated scheme of the calci.m responsive CaM/BSA mon61ayer on water

349 6.2. P r o t e i n C o n j u g a t e M e m b r a n e In other experiments, the Ca2+-stimulated protein membrane was prepared by developing CaM and albumin on the water surface as shown in Figs. 32 and 33 [20]. Although the work presented here is the most simple approach towards a molecular intelligent material, the data obtained in this study will open new fields for assembling of new functional protein hybrids.

2

0

m

I

L

I

a

I

7

6

5

4

3

,,

i

2

pCa

Fig.33 Surface pressure of the CaM/BSA monolayer on water at different Ca 2§concentrations

We have observed that such proteins as CaM and bovine serum albumin (BSA) can be developed at the air-water interface to form monolayer protein films. In previous works, the developed BSA monolayer was stabilized by cross-linking with a biftmctional reagent immediately after the preparation of protein monolayer. The BSA thin film thus prepared can be employed as a passive material, e.g., an ultrathin protein film for a matrix of enzyme-linked immunosorvent assays. Here we describe a method of manufacturing an active material by conjugating a modulating protein with a matrix protein by using a multicompartment Langmuir-Blodgett trough of Fromherz type. The modulating protein is expected to play the role of a trigger in the regulation of the matrix protein (bovine serum albumin) as schematically postulated in Fig.32. The changing in surface pressure as a function of a molar mixing ratio was investigated for the B S A - CaM conjugated monolayer. Also in this

350 experiment, the surface pressure change was compared by transferring the conjugated monolayer from an ultrapure water trough to a 1 mM CaC12 containing trough and form the Ca~§ compartment to another ultrapure water compartment. In both cases maximum changes of surface pressure were obtained when CaM was mixed with an equimolar a m o u n t of BSA. The net change (increase) of surface pressure in the presence of Ca 2§ induced conformational change. When the mixing ratio of CaM to BSA wqs greater than 2 times the equimolar ratio, the change in surface pressure tended to fluctuate. The reason for this fluctuation may be ascribed to the instability of the resulting BSA-CaM conjugate where excess amount of CaM of lower molecular weight (16.7 kDa) was cross-linked with BSA of higher molecular weight (69.0 kDa). A more detailed role of Ca 2§ in the surface pressure change was further investigated from the viewpoint of Ca 2§ concentration. Crough et al. Have reported that a dynamic change in surface pressure of free CaM was observed in the Ca2+ concentration range from pCa 6 to 5. However, in the case of the conjugated BSA-CaM monolayer a big difference in surface pressure was obtained in the range from pCa 4 to 3, as shown in Fig.33. The / I F value was determined at 20 min a ~ r the transport from one trough of a fixed Ca 9§ concentration to another one. Another conjugated BSA-CaM monolayer, both protein components previously treated at 100~ for 5min and then developed at the air-water interface, had a dependency on Ca 2§ concentration. Although BSA was denatured in the heating condition, CaM underwent no denaturation in this heating process. Also in the combination of CaM and denatured BSA, a conjugated protein monolayer was manufactured t h a t retained the surface pressure dependency on Ca 2§ concentration. Therefore, one can understand that the conjugated protein monolayer works as a Ca 9§ driven ultrathin protein film whose extensibility or permeability may be controlled by the divalent cation. The Ca 2§ -responsive conformationally changeable membrane may be a good model of a new drug-delivery system, where calcitonin or parathyloid hormone can be released for the homeostasis of serum Ca 2§ level. 7. S E L F - A S S E M B L E D ANTIBODY ARRAY

7.1. A n t i b o d y Array Construction on Solid Surface Biosensors may be classified into two categories 9biocatalytic biosensors and bioaffinity biosensors. Biocatalytic sensors contain a biocatalyst such as an enzyme to recognize the analytic selectively. Bioaffinity biosensors, on the other hand, may involve antibody, binding protein or receptor protein, which form stable complexes with the corresponding ligand. An immunosensor in which antibody is used as the receptor may represent a bioaffinity biosensor. Advanced biotechnology and monoclonal antibody production have provided

351 strong support for bioalTmity biosensors, and various new principles of electrochemical and optical immunosensors have been proposed. Concentrated efforts have been sharply focused on the development of homogeneous immunosensors, which require no bound-free separation. Examples include an optical immunosensor based on surface plasmon resonance (SPR), an optical fibre immunosensor based on fluorescence determination using an evanescent wave and an optical fibre immunosensor based on electrochemical luminescence determination. These immunosensors are characterized by a single step of determination and high selectivity as well as high sensitivity. The responses of these immunosensors, however, result from averaging the physiochemical properties of the antibody-bound solid surface. W e have succeeded in fabricating an ordered array of antibody molecules on the solid surface and in quantitating individual antigen molecules that are complexed with the antibody array [15].

7.2. P r o t e i n A M o l e c u l a r O r g a n i z a t e s o n Solid S u r f a c e Protein A is a cell-wall protein of Staphylococcus aureus with a molecular weight of 42,000. Since protein A binds specifically to the Fc part of IgG from various animals, it has been widely used in immunoassay and affinity chromatography. We found that protein A could be spread over the water surface to form a monolayer membrane by the LB method [21]. On the basis of this finding, an antibody array on the solid surface can be obtained by the following two steps. The first step is fabrication of an ordered protein A array on the solid surface by the LB method. The second step is self assembly of antibody molecules on the protein A array by biospecific affinity between protein A and the Fc of IgG as shown in Fig.34.

Q

Fig.34 Schematic procedure of the Prot. A monolayer film on water, followed- by self-assembling of antibody and antigen

9

i

-'~"

352

A Fromhertz type of LB trough was used for fabrication of the protein A a r r a y on highly oriented pyrolytic graphite (HOPG) (15mm x 15ram x 2ram), Protein A was dissolved in ultrapure water to make a 0.1 X 10-6 g ml-1 solution. With a micropipette, 0.2 ml of protein A solution. With a micropipette, 0.2 ml of protein A solution was dropped on 150 cm 2 of the air/water interface of a compartment t h a t contained ultrapure water as subphase. The protein A layer was compressed at a rate of 10 ram2 s-1 with a barrier. Compression was stopped at a surface pressure of 11 mN m -1 and the monomolecular layer of protein A was transferred to an adjacent compartment containing 0.5% glutaraldehyde solution at a rate of 10 mm 2 s -1. The protein A layer was incubated for 1 h to be cross-linked by glutaraldehyde, which was followed by transfer to a compartment containing u l t r a p u r e water for rinsing. The protein A molecular membrane was then transferred onto the surface of an HOPG plate by the horizontal method. The molecular imaging of the preparation was obtained by AFM in solution. To prepare an antibody protein array, a monolayer of protein A, which was compressed at a surface pressure of I I mN m -1 was transferred to a c o m p a r t m e n t containing anti-ferritin antibody in 10 mM pH 7.0 phosphate buffer. The antibody molecules were self assembled onto the protein A layer. The protein A/antibody molecular membrane was transfered to a compartment containing ultrapure water for rinsing, and was then tra_nsfered onto the surface of an HOPG plate by the horizontal method. AFM measurements were m a d e in a pH 7.0 of 10 mM phosphate buffer solution. A F M imaging of the protein A array deposited on an HOPG plate showed an ordered alignment of protein molecules when the measurement was made in a pH 7.0 10raM phosphate buffer at a controlled force of 4 X 10-11 N and a scanning rate of 0.6 Hz. However, an ordered structure was not observed unless the protein A molecules were not cross-linked by glutaraldehyde.

7.3. S e l f - a s s e m b l i n g of A n t i b o d y on P r o t e i n A The antibody a r r a y t h a t was self-assembled on the protein A array was also visualized in molecular alignment by AFM. The antibody array was in contact with a pH 7.0, 10 mM phosphate buffer. The AFM m e a s u r e m e n t was conducted at a controlled force of molecular size of the antibody was estimated as 7 n m in diameter. The antibody a r r a y was soaked in different concentrations of ferritin solutions for 1 h, and was assayed for AFM imaging in solution. Ferritin molecules were recognized and fixed by the antibody array. The ferritin concentration was 10 ng m1-1. Individual ferritin molecules on the antibody a r r a y were selectivity quantitated by AFM imaging.

353 8. SUMMARY Several protein assemblies have successfully been fabricated on the solid surfaces after the bioinformation transduction. These include the following molecular systems ; ~ molecularly interfaced redox enzymes on the electrode surfaces, ~) calmodulin / protein hybrides, and @ ordered antibody array on protein .4, These protein assemblies find a wider application in various fields such as biosensors, bioreactors, and intelligent materials. REFERENCES

1. M.J.Eddows and H.&O.Hill, J.Chem.Soc., Chem.Commun., (1977) 771 ;J.Am.Chem.Soc., 101 (1979) 4461. 2. I~Nakamura, M.Aizawa and O.Miyawaki, Electroenzymology, Coenzyme Regeneration, Springer Verlag, Berlin (1988). 3. A.E.G.Cass, G.Davis, G.D.Francis, H.A.O.Hill, W.J.Aston, I.J.Hggins, E.V.Plothin, L.D.L.Scott and A.P.F.Turner, Anal.Chem., (1984) 1880. 4. Y . D e g a n i a n d A . H . H e l l e r , J . P h y s . C h e m . , 91, (1987) 1285. 5. M.Aizawa, S.Yabuki and H.Shinohara, Proc. 1st Internat.Symp. on Electroorganic Synthesis (S.Torii, ed.) Elsevier, Amsterdam, 1987, pp.353360. 6. S.Yabuki, H.Shinohara and M.Aizawa, J.Chem.Soc., Chem. Commun., (1989) 945. 7. N.C.Foulds and C.R.Lowe, J.Chem.Sos., Faraday Trans. 82 (1986) 1259. 8. M.Aizawa, S.Yabuki, H.Shinohara and Y.Ikariyama, in: Molecular Electronics: Science and Technology, (A~ Aviram, ed.), Engineering Foundation, New York (1989). 9. M.Aizawa, S.Yabuki, H.Shinohara, in:Molecular Electronics- Biosensors and Biocomputers (F.T.Hong, ed.) Plenum Press, New York, 1988, pp.269276. 10. M.Aizawa, G.F.Khan, H.Shinohara, and Y.Ikariyama, in- Chemical Sensor Technology Vol.5 (M.Aizawa, ed.), Kodansha, Tokyo (1994), pp.157-186. 11. G.F.Khan, H.Shinohara, Y.Ikariyama and M.Aizawa, J.Electroanal.Chem., 315 (1991) 263. 12. G.F.Khan, E.Kobatake, H.Shinohara, Y.Ikariyama, and M.Aizawa, Anal.Chem., 64 (1992) 1254. 13. G.F.Khan, E.Kobatake, Y.Ikariyama, and M.Aizawa, Anal.Chim.Acta, 281 (1993) 527. 14. M.Imamura, T.Haruyama, E.Kobatake, Y.Ikariyama, M.Aizawa, Sensors Actuators B, 24-25 (1995) 113. 15. M.Aizawa, K.Nishiguchi, M.Imamura, E.Kobatake, T.Haruyama, and Y.Ikariyama, Sensors Actuators B, 24-25 (1995) 1. 16. F.Mizutani and M.Asai, Denki Kagaku, 56 (1988) 1100.

354

17. W.Schumann, H.L.Schmidt and A,Heller, J.Am.Chem.Soc., 113 (1991) 1394. 18. T.Miwa, N.Damrongchai, H.Shinohara, Y.Ikariyama, and M.Aizawa, J.Biotechnol., 20 (1991) 141. 19. M.Aizawa, Trans. Mat.Res.Soc.Jpn., 15A (1994) 653. 20. T.Miwa, E.Kobatake, Y.Ikariyama, and M.Aizawa, Bioconjugate Chem., 2 (1991) 270. 21. ILOwaku, M.Goto, Y.Ikariyama, and M.Aizawa, Anal.Chem., 67 (1995) 1613.

355

Subject

Index

A Absorption -, p-polarized 253 - spectrum 41 AC conductivity 96 Acceptor 267 Action spectrum 265 Aggregation -, cooperative 221 - number 250 Air/water interface 212 Alcohol dehydrogenase 341 Alkyl chain length 121 Amid I band 103 Amphiphilic biferrocene 94 Amphiphilic ferrocene 94 Amphiphilic ~ C D 79 Amphiphile -, anionic 17 -, fluorocarbon 183 -, homologous 302 -, hydlocarbon 183 -, azobenzene 40 -, azobenzene linked 288, 301 Anisotropic intermolecular tunneling rate 267 Antenna molecule 182 Antibody array 351, 352 Area-creep 21 Artificial photosynthetic reaction center 182 Atomic force microscope 21, 25, 34 - imaging 352 Auger electron spectroscopy 193 Auto-catalytic process 211 Azobenzene 154 - chromophore 40 B

Belousov-Zhabotinskii reaction 211 Bias voltage 2 ~ Bicyclo[2.2.2]octane 187 Bilayer -, alternate 184 - membrane 39 - - , immobilization of 64 -, planar 220 -, viologen 55 Biosensor 335 -, bioaffinity 350 -, biocatalytic 350 -, disposable 336 Bioinformation network 323 Bipyridinium - radical cation 271 - salt 271 Blue-shift 250 Brewster angle microscope 73 Butanol 239

C Cadmium arachidate 276 Cadmium octadec.anoate 116, 119 Calmodulin 345 11-(9-Carbazolyl)undecanoic acid 250 Card-pack structure 297 Cationic polymer 177 Cerenkov type phase-matching 309 Change in surface potential 198 Change of aggregation structure of carbazolyl charge separation 181 Chemical factor 16 Cholesterol 130 Chromophore 263 - aggregation 259 -, in-plane orientation of Z51 -, interaction of 248 -, non-statistical aggregation of 261 -, redox 247, 248 Circular dichroism 82, 101 Collapsing pattern 74 Complex -, charge transfer 54 -, host-guest 80 -, lgG-lipid 79 -, ion-pair charge-transfer 271 -, polyion 64, 177 -, ruthenium(II)-bipyridine 277 Compression and expansion cycle 229 Conduction -, hopping 97 -, photocarrier 265 Conductivity -, disperse 97 -, lateral 94 -, normal 94 Conformation change 325 Continuous compression method 22, 24 Cooling crystallization 32 Cooperative adsorption/desorption 234 Corey-Pauling-Koltun molecular model 251 Coulombic interaction 226 Coulombs interaction energy 257 Covalently bound hybrid 346 Critical value 240 Cross-section balance 45 Crystal - engineering 54 -, perylene-doped 261 -, single 44 Crystalline relaxation 6 - temperature 1 Crystallographical continuity 20, 22, 33 Crystallographical distortion 20, 22, 33 Crystallographical regularity 22 CT interaction 56 Cyanine dye 185 D

Damping constanl 270 DEAE-Sapharose 347

356 De~ye-Hiickel theory 227 Dehydrogenase 341 Deposition -, alternating 290 Detachment 133, 139 1,3-Didodecylalloxazine 249 2,5-Dihy&o-thiophene 97 4-Dimethylamino-4'-nitrostilbene 276 1,2-Dioleoyl-3-sn-phosphatidyl-choline 232 1,2-Dipalmitoyl-3-sn-phosphatidylethanolamine 229 Dipalmitoylphosphatidylcholine 124, 128 Dipalmitoylphosphatidylethanolamine 124 Dipalmitoylphosphatidylglycerol 124,129 Dipalmitoylphosphatidylserine 124, 129 Dipping speed 118 Dipole -, long-axis and short-axis transition 256 model - -, extended 255 - - - , three-dimensional 255 moment - -, photo-induced surface 185 -, transition 252 - - moment 273 -

-

Disorder , conformational 164 Dispersive electronic excitation transport 264 5-(p-Dodecyloxyphenyl)pyrazine-2carboxylic acid 158 Domain -, amorphous 9, 12, 32 -, crystalline 8, 11, 32 -, soft 73 -, two-dimensional solid 73 Donor 267 Dyad 186 -, A-S 200 Dynamic and/or nonequilibrium condition 244 E

Edge dislocation 28 Effective concentration 223 Electrical activation 343 Electrical oscillation 236 Electroabsorption spectrum 294 Electrochemical communication 332 Electrochemical polymerization 330 Electrode -, gold-black 333 -, mediator-modified 337 Electron diffraction pattern 1 -, lateral diffusion of 199 mediator 328 - promoter 328 spin resonance 250, 271 transfer 331 Energy - transfer 264 - -, molar fraction dependent efficient 258 trap site 264 -

Enhancement factor 151, 152 Environmental adaptability 348

Enzyme -, mediator-modified 333 - sensor for fa'uctose 339 Equilibrium hydration amount 125 Ethyl tetradecanoate 139 Evanescent wave 275 Evaporation speed 112, 114 Excited state 279 Exciton - diffusion 259 -, fl'ge - state 86 - model (simplified) 249 phonon interaction 85 -, single - - hopping process 264 -, singlet migration model 260 Exponential distance dependence 92 -

-

- -

F Ferrocene 334 - modified glucose oxidase 338 Fick's low 121 Film -, cast 46 -, oriented polyarylenevinylene 313 -,X-type 102 non-alternating 95 -, Y-type 102 - - , hetero 293 -, Z-type 294 FITC label 78 Fluorescence -, amplified photochemical quenching of carbazolyl 262 - depolarization 260 - lifetime 86 - micrograph 194 - microscopy 99, 130, 194 quentching - -, highly amplified 258 - SNOAM image 195 - spectrum 102, 259 -, time resolved 88 Fourier-transformation 234 Friction force microscope 182 Fringing pattern 300 Fructose dehydrogenase 329 FT-IR 13, 15 F6rster energy transfer 260 - - ,

-

-

-

-

-

-

G Glucose oxidase 329 Gold decoration 11 H

H-aggregate 85 H-aggregation 40, 297

357

Harmonic generation - - , second 277, 290, 299 - -, third 311, 318 He-Cd laser 198 Higher harmonic 234 Horizontal lifting technique 71 Hormones 324 Hydration behavior 110, 123 Hydrophilic head 121 Hydrophilic headgroup 228 Hysteresis 230 I

Immobilized form 347 Immunoglobuline G (IgG) 77 Immunosensor 77, 351 In-plane anisotropy 90 Incident angle 253 Incorporated amount 111 Incorporated water 114 Information transduction 325 Infrared reflection-absorption - spectrum 163 - technique 145 Infrared transmission spectrum 162 Initial hydration rate 125 Intelligent material 348 Interlayer - tunneling conduction 268 Inlramolecular charge-transfer 277 Ionic repulsion 15 IR spectntm 103 Isothermal phase transition 61 J J-aggregate 83, 256 J-aggregation 40 K

Kelvin probe force microscope 182 L Langmuir-Blodgett - fdm 33,109,181,247,327 --, alternating 146, 159, 169, 277 --, noncentrosymmelric 302, 303 - method 71 -technique 287, 313 Limiting area 7, 272 Lipid structure 119 Long-chain - bipyridinium 86 - ester of a-amino acids 100 - fatty add 265 - TCNQ 94 Lowering ~ 136

Memory -, erasable 64 -, photon-mode optical 271 Merocyanine dye 83 Mesoporphyrin IX dimethylester 255 1- Methyl-4-(2-naphthyl)bicyclo[2.2.2]octane 203 Micro processing with AFM tip 183 Mixed valency 95 Mode dispersion curve 309 Molecular area 251 Molecular communication 324 Molecular control 247 Molecular electronic and photonic device 248 Molecular electronics device 265 Molecular hyperpoladzability 279, 288 Molecular information 326 Molecular interface 329 Molecular-interfaced FDH 344 Molecular-interfaced glucose oxidase 343 Molecular mixing 302 - - of polar amphiphile 297 Molecular occupied area 5 Molecular orientation 149, 305 -, noncentrosymmetric 292 Molecular photodiode 181 Molecular recognition 59, 335 Molecular wire 331 Monolayer 1, 350 -, amorphous 9, 12, 13, 17, 32 -, compressing crystallized 16, 18 -, crystalline 9, 13, 16, 25, 27, 32 --, fusing-oriented 16, 20, 21 --, randomly assembled 16 -, crystallized (cooling) 20 -, defect-diminished 21, 25, 32 -, lithium heptacosadiynoate 29 -, mechanically stable 28 -, mixed 190 - of protein A 352 -, stable 204 Multi-step creep 21, 24, 27, 34

M

N N-shape nonlinearity 242 NAD 341 Naphthalene 80 Nearest-neighbor exchange interaction 267 Negative feedback 242 Neurotransmitter 324 Noncentrosymmetric structure 287 Nonlinear characteristic 244 Nonlinear optical response 277 -, second order 277 -, third order 310 Nonlinear viscoelastic property 226 Nucleation process 76 Number of paths 202

Marangoni effect 239 Mean :z-conjugation length 319 Meldra blue 341 Melting temperature 1, 3, 29

O O c t a d ~ o l 119 Octadecyl tetracyano-quinodimethane 99

358

Octadecylester - of alanine 100 - of L-l-naphthylalanine 100 Octadecyltrimethylammonium ctdoride 189 Oil/water interface 237 Optical amplification 281 Optical band gap 312 Optical modulation 278 Optical nonlinearity 277 -, second-order 287, 299, 305 -, third-order 277, 318 Optical switching 277 Order parameter 290 Organic multi-quantum well 265 Organized molecular assembly 247 Orientation -angle 153, 167 -, anisotropic 89 -, out-of-plane 251 Overshoot hump 219 P

Partially fluorinated anionic suffactant 189 Phase separation 190, 265 Phase transition temperature 125 14-Phenyl-9,11-tetradecadidynoic acid 213 Phenylpyrazine derivative 172 Phosphatidylethanolamine 124 Phosphodiesterase 345 Photochromic system 270 Photochromism 271 Photo-induced charge separation 190 Photoinduced eleetrochromism 247, 271 Photoinduced electron transfer 90 Photonic device 281 Photooxidation 264 Photoresponse 247 Phthalocyanine 95 Plateau -, flat 219 - region 9, 12, 16, 32 Polyarylenevinylene 310 Polycondensation 100 Poly(2,5-dimethoxy-p-phenylenevinylene) 316 Polyion-complexation 206 Poly(p-phenylenevinylene) 313 Polypyrrole 330 Porphyrin -, amphipathic 265 -, loosely stacked 254, 255 - pair 269 - ring 252 Potential-assisted self-assemble 326 Pressure - concentration characteristic 226 -, equilibrium spreading 136 - induced crystallization 18 -, surface 115, 117, 118, 251, 349 - - area (x-A) isotherm 1, 112, 213, 251, 252, 289 . . . . dynamic 230

--, critical 222 - -, inhomogeneity of 223 - -, zero 219 Primary process of photosynthesis 181 Propagation loss 34 Protein - A351 - A/antibody molecular membrane 352 -, modulating 349

Pyrazine -derivative 288, 303 - LB fdm 308 Pyroelectric coefficient 155, 157, 159, 166, 176 Pyroelectric current 160, 165, 173, 178 Pyroelectricity 145, 155, 172 :t-conjugated polymer 310 n-electron 267

Q Q-band 266 Quadratic Stark spectrum 296 Quartz crystal-microbalance 82, 109 R

Raising speed 136 Raman spectrum 168 - -, nonresonance 169 Receptor 325 Recombination site 267 Rectangular cell model 224 Red-shift 250 Refractive index dispersion 308 Rigid spacer 189 Ripple structure 76 S S-D dyad 186 - with rigid spacer 201 S-polarized light 253 S-shape nonlinear function 223 Sandwich-type electrode 267 Scanning near-field optical microscope 182 Scanning near-field optical/atomic force microscopy 183 Scanning probe microscope 181 Scanning SIMS 193 Second-order molecular hyperpolarizability 295 Selective inclusion 83 Self-assembling 338 Self-oscillatory phenomenon 212 Sensor -, gaseous ethanol 342 - for ethanol 341 Signal transduction 335 Silicon(100) wafer 189 Simple tunneling mechanism 266 Single alkyl chain spacer 204 Single closed loop 234 Single line method 20, 33 Sintering 9, 11, 14, 20 Slow charge recombination 199

359 SNOAM 194 Sodium oleate 239 Spatio-temporal structure 212 Spiropyran 271 Squarylium dye 83 Squeezing out 254 SSPM 196 Stark effect -, linear 292 -, quadratic 295 Static elasticity 1 Steady-state photocurrent measurement 265 Stearic add 189 Stedc hindrance 105 Storage or processing of information 265 Stress concentration 23, 25, 27, 35 Stretching -, CH 2 asymmetric 13 -, CH symmetric 13 S t m c t ~ diversity 50 Structural polymorphism 54 Structural relaxation 34 Substrate -, hydrophilic 1, 3 -, hydrophobic 116 Supercooling 216 Superexchange mechanism 267 Supermonomolecular structure 254 Supersaturation 216 Surface -, effect of 116 - normal 273 roughness 36 Susceptibility -, second-order 306 -, third-order 311, 318 Synchronized change 240 -

T Taste-response 231 Tetraalkylquinacridone 89 Tetrakis[3,5bis(trifluoromethyl)phenyl]borate 271 Tetraphenylporphyrin 75 Thermal factor 16 Thermal stability 160 Thiol-gold interaction 338 Through-bond mechanism 202, 205 Through-space tunneling 270 Tilt angle 250, 300 Time - at the interface 136 in water 136 - resolved measurement 279 Transfer amount 111 - process 111, 113 110, 114, 304 Transient photoelectric response 265 Transition - moment 250 -

-

- r a t i o

-, solid-solid phase 61 Transmission electron microscope 1, 21 Transverse electric mode 276 Transverse magnetic mode 276 Trap 267 Triad -, A-S-D 181, 190 --, unidirectionally oriented 196 Two compartment trough 302 Two-dimensional crystallite 75 Two-dimensional van der Waals equation 227 Two level model 295 U Ultrafast process 281 UPS spectrum 97 Urbach rule 85 UV-visible absorption spectrum 197 V Vibrational spectroscopy 145 W Water incorporation 110 Waveguide 33 - device 304 -, optical 307 - - method 275 Wilhelmy plate 214 X X-ray analysis 44

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