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Interaction between a Liquid Crystalline Molecule (4′-Pentyl-4-cyanobiphenyl) and a Poly(vinyl alcohol) Derivative Containing an Azobenzene Group at the Air-Water Interface: Sum Frequency Generation Spectroscopic Study C. Ohe,† M. Arai,† H. Kamijo,† M. Adachi,† H. Miyazawa,† K. Itoh,*,† and T. Seki‡ Department of Chemistry, School of Science and Engineering, Waseda UniVersity, Shinjuku-ku, Tokyo 169-8555, Japan, and Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya UniVersity, Chikusa, Nagoya 464-8603, Japan ReceiVed: August 3, 2007; In Final Form: February 13, 2008
Sum frequency generation (SFG) spectroscopy was applied to study the structures of mixed monolayers consisting of a liquid crystalline molecule, 4′-pentyl-4- cyanobiphenyl (5CB), and a poly(vinyl alcohol) derivative containing trans- and cis-azobenzene groups (6Az10-PVA) at the air-water interface. The mixed monolayers of 5CB and trans-6Az10-PVA with mole ratios of 5CB and the azobenzene unit of 1:1 and 2:1 exist in a homogeneously mixed state at the air-water interface. The SFG spectra in the CH stretching vibration region showed that upon interaction with 5CB the alkyl group side chain of trans-6Az10-PVA is converted from an irregular structure containing gauche defects to a regular one consisting of all trans bonds. The line amplitude of the SFG band due to a CN stretching vibration of 5CN observed for the 1:1 and 2:1 mixed monolayers containing trans-6Az10-PVA is negligibly small for the mole fraction of 5CB, in contrast to the case of mixed monolayers with a mixing ratio larger than 2, where the amplitude increases with the mole fraction of 5CB. These results showed that the trans-azobenzene unit accommodates at most two 5CB molecules and most of the 5CB exist in a dimeric form with an inversion symmetry. In the case of mixed monolayers of 5CB and cis-6Az10-PVA the line amplitude increases in proportion with the mole fraction of 5CB, corroborating the conclusion from the compression isotherm measurement that the mixed monolayers exist in a phase-separated state consisting of monolayers of 5CB and those of cis-6Az10-PVA.
Introduction Recently we applied sum frequency generation (SFG) spectroscopy to a poly(vinyl alcohol) derivative containing an azobenzene group (6Az10-PVA; see Figure 1A) at the airwater interface and clarified the structural changes of the alkyl, azobenzene, and PVA moieties of the polymer associated with the trans-cis isomerization of the azobenzene group at various surface pressures.1 Especially, the analyses of the spectra in the backbone stretching vibration region performed by using a DFT calculation method gave clear-cut orientation changes of the trans- and cis-azobenzene group induced by the compression of the monolayers. The trans-cis isomerization of azobenzene groups forming surface layers has a variety of functionality including optical storage elements, light switching devices, nonlinear optical devices, and liquid crystalline alignment surfaces.2-6 The results of the study1 showed that SFG spectroscopy is one of the most efficient methods to optimize the functions. In the present work we applied SFG spectroscopy to study interactions of a liquid crystal (LC) molecule, 4′-pentyl-4cyanobiphenyl (5CB; see Figure 1B), with the 6Az10-PVA monolayer at the air-water interface. According to Ubukata et al.,7,8 mixed monolayers of 5CB and trans-6Az10-PVA (the PVA derivative with the trans-azobenzene group) with a mole ratio of 5CB to the azobenzene group (r ) [5CB]/[azobenzene]) * To whom correspondence should be addressed. E-mail:itohk@ waseda.jp. † Waseda University. ‡ Nagoya University.
Figure 1. Chemical structures of trans-6Az10-PVA (A) and 5CB (B). See the text.
less than or equal to 2 form a homogeneously mixed state without lateral phase separation at the air-water interface. The hybrid monolayer, in which the molecular axis of the LC molecule takes a more or less perpendicular orientation to the water surface as a result of a direct interaction with the transazobenzene unit, serves as an ideal model system to elucidate how the surface trans-cis isomerization of the azobenzene unit regulates the homeotropic and homogeneous alignments of LC
10.1021/jp076223u CCC: $40.75 © 2008 American Chemical Society Published on Web 04/02/2008
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Figure 2. Compression (π-A) isotherms of 5CB/trans-6Az10-PVA (A) and 5CB/cis-6Az10-PVA (B) mixed monolayers with various mixing ratios at 22 ( 1 °C. The vertical lines indicate the surface pressure at which the SFG mesurements were performed. See the text.
molecules in the bulk phase. Actually, the authors prepared Langmuir-Blodgett (LB) films of the hybrid monolayer to mimic the two-dimensional contact region of the LC-azobenzene interface and applied UV-vis and Fourier transform spectroscopies to indicate an induction of reversible perpendicular/ tilt orientation changes of both azobenzene groups and 5CB molecules upon alternate irradiation of 365 and 436 nm light.9 Kago et al.,10 who applied X-ray reflectometry to the 5CB/trans6Az10-PVA hybrid monolayer (r ) 1) at the air-water interface, gave a model in which the trans-azobenzene unit and 5CB are stretched out in a direction perpendicular to the water surface, being interdigitated with each other. In spite of these studies, the structures of the mixed monolayers of 5CB and the trans- and cis-6Az10-PVA molecules at the air-water interface have not been clarified enough at the molecular level. Experimental Section Materials. trans-6Az10-PVA was synthesized by a procedure already reported.11,12 5CB and 5CB-d11 (5CB with a perdeuterated pentyl group) were purchased from Kanto Chemicals Co., Inc. and CDN Isotopes (Quebec, Canada), respectively, and used without further purification. Water used in the experiment was purified by a Milli-A Plus (Millipore Inc.), and its resistivity was measured to be higher than 18 MΩ. Preparation of Monolayers. trans-6Az10-PVA and 5CB (or 5CB-d11) were dissolved in chloroform to prepare stock solutions; the concentration of trans-6Az10-PVA was 0.64 mg/mL and that of 5CB (or 5CB-d11) 0.75 mg/mL. Appropriate amounts of the stock solutions of trans-6Az10-PVA and 5CB (or 5CBd11) were mixed to prepare mixed solutions with mole ratios of 5CB (or 5CB-d11) and the azobenzene unit of 1:1, 2:1, 3:1, and 6:1. The corresponding mixed solutions containing cis-6Az10PVA were obtained by irradiating the solutions for 14 min with a 365 nm light (15 mW/cm2) from a 500 W high-pressure mercury lamp (Oriel Corp., model 68810) equipped with a bandpass filter (KENKO Optical Co., Ltd., U-360).1 Monolayers at the air-water interface were prepared by spreading the solutions of the pure and mixed samples on pure
water in a MiniMicro trough equipped with a Wilhelmy balance of KSV Instruments Ltd., which allows us to measure monolayer compression (surface pressure-molecular area, π-A) isotherms and to keep the pressure at a constant value by using a PC control system. Each monlayer was kept at a certain surface pressure (2 mN/m) for at least 15 min so it could be stabilized and used for SFG spectral measurements. SFG Spectral Measurements. SFG spectra were measured by using an apparatus which is virtually identical with that used in previous studies.1,13-15 The spectra were recorded by combining 3 ps pulses of an 800 nm light (1 kHz, ca. 20 µJ/pulse) and 3 ps pulses of infrared light (1 kHz, ca. 4 µJ/pulse at 3 µm) tunable from 3600 to 1100 cm-1 at the interface in a copropagating geometry with an angle of incidence of 43.1° from the surface normal for visible light and an angle of 53.2° for the IR beam. The sum frequency beam was introduced to a monochromator (Acton, SP-308) equipped with a liquidnitrogen-cooled charge-coupled device detector (Princeton Instruments, LN-CCD HRBN) to record SFG spectra. Data points for the spectra in the region from 3100 to 2700 cm-1 were taken in 4 cm-1 intervals, and each point was taken with 30 s of accumulation. Data points for the spectra in the region from 2300 to 2000 cm-1 were taken in 5 cm-1 intervals, and each point was taken with 60 s of accumulation. All spectra were measured at 22 ( 1 °C under the ssp polarization combination, which refers to the sum frequency, visible, and infrared beams, respectively. Results and Discussion π-A Isotherms of Mixed Monolayers of 6Az10-PVA and 5CB at the Air-water Interface. Figure 2A shows the π-A isotherms of mixed monolayers of 5CB and trans-6Az10-PVA with mixing ratios of 1:1, 2:1, and 6:1 and those of trans-6Az10PVA and 5CB monolayers measured at 22 ( 1 °C. The results are virtually identical with those observed by Ubukata et al.7 The isotherms of the 1:1 and 2:1 mixed monolayers are similar to the isotherm of the trans-6Az10-PVA monolayer, giving limiting molecular areas of ca. 44 and 58 Å,2 respectively. On
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Figure 3. SFG spectra in the 3100-2750 cm-1 region observed at 22 ( 1 °C for 5CB/trans-6Az10-PVA (A) and 5CB/cis-6Az10-PVA (B) mixed monolayers with various mixing ratios. The spectra, except for the bottom ones, are offset by certain values to exhibit the spectra clearly.
the other hand, the 6:1 mixed monolayer exhibits a plateau region at ca. 6 mN/m and a steep uprise region, which almost coincides with that of the 2:1 mixed monolayer at pressure higher than 30 mN/m. The isotherm of the 6:1 mixed monolayer can be interpreted as a sum of the isotherm of the 2:1 mixed monolayer and that of pure 5CB. On the basis of these results, Ubukata et al.7,9 concluded that the trans-6Az10-PVA monolayer can accommodate at most two 5CB molecules per azobenezene unit, forming a homogeneous phase, and that the mixed monolayers with a mixing ratio (r ) [5CB]/[azobenzene]) larger than 2 consist of a homogeneous phase of the 2:1 mixture and a phase of pure 5CB. As can be seen from Figure 2B, the π-A isotherms of mixed monolayers of 5CB and cis-6Az10-PVA are quite different from those of mixed monolayers containing trans-6Az10-PVA. The isotherms of the 1:1 and 2:1 mixed monolayers can be interpreted as a sum of the isotherm of the cis-6Az10-PVA monolayer and that of 5CB, indicating that the mixed monolayers are in a laterally phase separated state consisting of a pure cis monolayer and a 5CB monolayer. All SFG spectra of the mixed monolayers were measured at 2 mN/m, as indicated by the horizontal lines in Figure 2. SFG Spectra in the CH Stretching Vibration Region of 6Az10-PVA/5CB Mixed Monolayers at the Air-Water Interface. Parts A and B of Figure 3 exhibit the SFG spectra in the 3100-2750 cm-1 region observed for mixed monlayers containing trans-6Az10-PVA and those containing cis-6Az10PVA at the air-water interface, respectively, together with those observed for the monolayers of pure trans-6Az10-PVA (the bottom spectrum in Figure 3A), cis-6Az10-PVA (the bottom spectrum in Figure 3B), and 5CB (the top spectrum in Figure 3A) spread on the water surface. The bands observed around 2850, 2870, 2925, 2940, and 2970 cm-1 in the spectra of the trans-6Az10-PVA monolayer and the 1:1, 2:1, and 3:1 mixed monolayers in Figure 3A are ascribable to the CH2 symmetric stretching (νs(CH2)), CH3 symmetric stretching (νs(CH3)), CH2 asymmetric stretching (νas(CH2)), Fermi resonance (νF(CH3)), and CH3 asymmetric
stretching (νas(CH3)) bands of the alkyl groups, respectively.16,17 The similarity in the spectra between the pure trans monolayer and the mixed monolayers suggests that the conformation of the alkyl groups of trans-6Az10-PVA in the mixed monolayers is not strongly affected by the introduction of the 5CB molecules into the monolayer. The SFG bands observed near 3010 and 3080 cm-1 in the spectra of the monolayers of the trans-6Az10PVA monolayer in Figure 3A are due to CH stretching vibrations of the benzene rings.18 The appearance of these bands suggests that the azobenzene moiety in the monolayer takes a more or less ordered orientation relative to the water surface. Upon increasing the mole fraction of 5CB, the intensity ratio of the 3010/3080 cm-1 bands decreases appreciably, indicating that the azobenzene moiety changes its orientation upon interaction with the 5CB molecules. Ubukatat et al.7 reported that the intensity of the absorption band at 322 nm due to a π-π* transition (with the transition moment parallel to the long axis of the azobenzene group) of the azobenzene moiety of the trans-6Az10-PVA monolayer decreases appreciably upon introduction of 5CB (r ) 1), which the authors interpreted by considering that the introduction causes an orientation change of the long axis from a tilted to a less tilted one. The SFG spectrum of the 5CB monolayer shows the bands around 2850, 2880, 2930, and 2940 cm-1, which are ascribable to the νs(CH2), νs(CH3), νas(CH2), and νF(CH3) bands, respectively, and those around 3020 and 3070 cm-1 due to benzene ring CH stretching vibrations. It is noteworthy that the 2930 cm-1 band almost disappears in the spectrum of the 6:1 mixed monolayer. Presumably, this can be explained by considering a conformation and/or orientation change of the alkyl group of 5CB upon interaction with the trans-6Az10-PVA molecule. As can be seen from Figure 3B, the SFG spectra of the pure cis-6Az10-PVA monolayer and 1:1, 2:1, and 3:1 mixed monolayers give much weaker bands compared to those of monolayers containing trans-6Az10-PVA in Figure 3A, giving the νs(CH2), νs(CH3), νF(CH3), and νas(CH2) bands, which are situated around 2850, 2870, 2940, and 2950 cm-1, respectively. Comparing these spectra with that of the pure 5CB monolayer
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Figure 4. SFG spectra in the 3100-2750 cm-1 region observed at 22 ( 1 °C for 5CB-d11/trans-6Az10-PVA (A) and 5CB-d11/cis-6Az10-PVA (B) mixed monolayers with various mixing ratios. The spectra, except for the bottom ones, are offset by certain values to exhibit the spectra clearly.
(the top spectrum in Figure 3A), it can be recognized that the weight averages of the spectrum of the pure cis-6Az10-PVA monolayer and that of the pure 5CB monolayer approximate the weight averages of the spectra of the mixed monolayers, being consistent with the conclusion based on the π-A isotherms in Figure 2B; i.e., the mixed monolayers are in a phase-separated state consisting of a monolayer of cis-6Az10PVA and a monolayer of 5CB. The predominance of the νs(CH2) band ascribable to the cis-6Az10-PVA monolayer in Figure 3B indicates that the alkyl moiety of the cis monolayer takes an irregular state containing an appreciable amount of gauche defects.16,17 The overlap of the bands due to the alkyl group of 5CB and those due to the alkyl group of 6Az10-PVA in the SFG spectra of the mixed monolayers, however, hampers a detailed spectral analysis to elucidate the orientation and conformation of the alkyl group of 6Az10-PVA. Therefore, spectral measurements were performed on mixed monolayers of 6Az10-PVA and 5CBd11. SFG Spectra in the CH Stretching Vibration Region of 6Az10-PVA/5CB-d11 Mixed Monolayers at the Air-Water Interface. Parts A and B of Figure 4 exhibit the SFG spectra in the 3100-2750 cm-1 region observed for 5CB-d11/trans6Az10-PVA and 5CB-d11/cis-6Az10-PVA mixed monolayers at the air-water interface, respectively. For comparison purposes, the SFG spectra of the monolayers of pure trans- and cis-6Az10-PVA are also added (the bottom spectra in Figure 4). To estimate the line amplitude of each SFG band in Figure 4, the spectra were simulated by using the following equation:19,20
ISFG ∝ (S/σ)2|χ(2)|2 )
|
2
(S/σ) χNR
(2)
+
∑q ω
Aqeiθq IR
- ωq + iΓq
|
2
(1)
where S is the overlapping area of the laser beams, σ the mean molecular area estimated from the π-A isotherms in Figure 2, and the term S/σ the number of side chains at the air-water interface monitored by SFG spectroscopy. χ(2) is the average molecular surface nonlinear susceptibility, and χNR(2) is its nonresonant contribution. Aq in eq 1 is the mean molecular line amplitude of the qth vibrational mode with phase angle θq, resonant frequency ωq, and damping constant Γq. The mean molecular line amplitude represents an average contribution in a mixed monolayer to the observed SFG band intensity for a certain mole fraction of either 5CB or 6Az10-PVA, and its relative value was calculated by multiplying the line amplitude determined from the SFG spectra (vide infra) by the corresponding mean molecular surface area, σ, under the assumption that the area of the cross section, T, is constant. Vibrational modes of nearby or overlapping bands can constructively and/ or destructively interfere with each other through eq 1 because of the coherent nature of SFG spectroscopy. The parameters in eq 1 were determined to get the best fits between the calculated and observed SFG spectra in Figure 4 by using a nonlinear regression procedure (built-in algorithms in the software package IGOR (version 4.0.5.1) were used for the calculation) under the assumption that (i) χNR(2) is negligibly small and (ii) the difference in the phase angles, θq, among the SFG bands is zero. Strictly speaking assumption ii is not correct, since the phase factors are affected by the orientation of the vibrational modes.21,22 However, we adopted the assumption on the basis of the fact that it gave reasonable fits between the calculated spectra and the observed ones, as in the cases of other studies.19,20 Parts A and B of Figure 5 exhibit the mean molecular line amplitudes of the νs(CH2) and νs(CH3) bands as a function of the mole fraction of 5CB estimated for mixed monolayers containing trans- and cis-6Az10-PVA, respectively. As Figure 5A shows, the line amplitude of the νs(CH3) band in the SFG spectra of 5CB-d11/trans-6Az10-PVA mixed monolayers decreases almost linearly with the mole fraction of 5CB-
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Figure 5. Mean molecular amplitudes of the νs(CH2) and νs(CH2) bands as a function of the mole fraction of 5CB-d11 observed for 5CB-d11/ trans-6Az10-PVA (A) and 5CB-d11/cis-6Az10-PVA/5CB (B) mixed monolayers. The vertical bar on each point indicates the error of the amplitude.
Figure 6. SFG spectra in the ν(CN) region observed at 22 ( 1 °C for 5CB/trans-6Az10-PVA (A) and 5CB/cis-6Az10-PVA (B) mixed monolayers with various mixing ratios. The spectra, except for the bottom ones, are offset by certain values to exhibit the spectra clearly.
d11 (x(5CB-d11)), the plots being virtually on the straight dashed line connecting the plot at x(5CB-d11) ) 0 and the point corresponding to the point at x(5CB-d11) ) 1.0. On the other hand, the plots for the line amplitudes of the νs(CH2) band appreciably deviate from the dashed line in the negative direction. Since the amplitude of the νs(CH2) band has been known to be a measure of the gauche defect of the alkyl chain,11,12 the deviation indicates that the introduction of 5CBd11 into the monolayers causes a conversion of the alkyl side chain of the trans-6Az10-PVA molecule from an irregular structure containing gauche defects to a regular structure consisting mainly of trans bonds. (The gauche to trans conversion should cause a line amplitude change of the νs(CH3) band. The change, however, may be too small to to be detected by our SFG measurement system.) The line amplitude of the νs(CH2) band in the spectra of 5CB-d11/cis-6Az10-PVA mixed monolayers decreases almost in proportion to x(5CB), as can
be seen form the plots in Figure 5B. (The plots for the νs(CH3) band, however, deviate from the dashed line. This is due to an experimental error; i.e., as can be seen from Figure 4B, the intensities of the νs(CH3) band are too weak to measure accurately.) Thus, the introduction of 5CB-d11 does not affect the conformation and orientation of the alkyl group of cis6Az10-PVA. This is reasonable, since the π-A isotherms indicated that mixed monolayers of 5CB and cis-6Az10-PVA exist in a phase-separated state consisting of a pure monolayer of each constituent. SFG Spectra in the CN Stretching Vibration Region of 6Az10-PVA/5CB Mixed Monolayers at the Air-Water Interface. Parts A and B of Figure 6 exhibit the SFG spectra in the CN stretching vibration (ν(CN)) associated with 5CB observed for 5CB/trans-6Az10-PVA and 5CB/cis-6Az10-PVA mixed monolayers, respectively, at the air-water interface. The corresponding spectrum of a pure 5CB monolayer is also given
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Figure 7. Mean molecular amplitudes of the ν(CN) band as a function of the mole fraction of 5CB observed for 5CB/trans-6Az10-PVA and 5CB/cis-6Az10-PVA/5CB mixed monolayers. The vertical bar on each point indicates the error of the amplitude.
at the top of Figure 6A. It should be noted that the spectra of the 1:1 and 2:1 mixed monolayers containing trans 6Az10-PVA (the bottom two spectra in Figure 6A) give a very weak ν(CN) band, in contrast to the case of the 3:1 and 6:1 mixed monolayers and also to the case of the spectra of mixed monolayers containing cis-6Az10-PVA. These results are more clearly recognized by plotting the line amplitudes of the ν(CN) band as a function of x(5CB). The plots are shown in Figure 7. The line amplitude in the SFG spectra of mixed monolayers containing cis-6Az10-PVA increases in proportion to x(5CB), which is again consistent with the fact that the monolayers exist in the phase-separated state consisting of monolayers of cis6Az10-PVA and 5CB, where the 5CB molecules are arranged so that the CN groups are in direct contact with the water surface. On the other hand, in the case of mixed monolayers containing trans-6Az10-PVA the amplitudes for the mixed monolayers with x(5CB) ) 0.5 and 0.67 (the amplitudes for the 1:1 and 2:1 mixed monlayers) are very small compared to the amplitudes at x(5CB) ) 0.75 and 0.86 (the amplitudes of the 3:1 and 6:1 mixed monolayers). As already explained, the
Ohe et al 1:1 and 2:1 mixed monolayers form a homogeneous state composed of 5CB and trans-6Az10-PVA, while the 3:1 and 6:1 monolayers are composed of a homogeneous monolayer of the 2:1 mixed monolayer and a pure 5CB monolayer. Thus, the plots indicate that the 5CB molecules in the homogeneous monolayers give abnormally weak ν(CN) bands for the mole fractions. Since the normal vibrations of the chemical groups assuming an inversion symmetry should not give any SFG band,23 most of the 5CB molecules in the homogeneous state are arranged to form a dimer with an inversion symmetry with respect to the two CN bonds. (Since the SFG spectra preferentially probe molecular transition dipole moments perpendicular to the surface normal under the ssp polarization condition, the result may be explained by considering that the moment of the CN bond in the homogeneous state is parallel to the interface. This explanation is rejected by the following two facts. First, if it were the case, the mean molecular amplitudes of the 1:1 and 2:1 mixed monolayers would be much larger than those observed in the π-A isotherms in Figure 2. Second, according to the X-ray reflectometry study on the 1:1 mixed monolayer at the air-water interface,10 the 5CB molecules are almost perpendicular to the surface, being interdigitated with the transazobenzene groups.) Each trans-azobenzene unit accommodates at most two 5CB molecules, and when the mole fraction of 5CB is increased to the level where the 5CB/azobenze unit ratio is larger than 2, the excess 5CB molecules are assembled to form monolayers with the hydrophilic CN group attached to the water surface, as in the case of 5CB/cis-6Az10-PVA mixed monolayers. Presumably, the trans-azobenzene unit forms a hydrophobic pocket to accommodate the dimeric structure, which is favorable for an electrostatic dipole-dipole interaction. A schematic model for the 1:1 hybrid monolayer is given in Figure 8. The alkyl groups of 6Az10-PVA in 5CB/cis-6Az10-PVA mixed monolayers contain gauche defects, as shown by the predominance of the νs(CH2) SFG band in Figure 3B, which prevents the formation of the hydrophobic pocket. In addition, as already indicated,1,11,12 a large electric dipole moment of cisazobenzene favors a direct interaction of the unit with the water surface. These factors explain the formation of the phaseseparated state of 5CB/cis-6Az10-PVA mixed monolayers,
Figure 8. A schematic representation of the hybrid monolayer consisting of 5CB and trans-6Az10-PVA with a mole ratio (r) of 1. See the text.
Interaction between 5CB and a PVA Derivative where the components assemble separately to form pure monolayers at the air-water interface. Conclusion SFG spectroscopy gave detailed information at the molecular level about how the trans-azobenzene unit in the 6Az10-PVA monolayer at the air-water interface interacts with the 5CB molecule to form homogeneous hybrid monolayers. The weak feature of the SFG band due to the CN band of 5CB in the hybrid monolayers and the stoichiometry of their formation showed that each azobenzene unit accommodates at most two 5CB molecules, which form a dimer with an inversion symmetry. The result could be explained by considering that the side chains containing the trans-azobenzene unit form a hydrophobic pocket, into which the dimer is geometrically fixed. A hydrophobic environment of the pocket enhances the electrostatic dipole-dipole interaction of the CN bonds to cause the formation of the dimer. In the case of 5CB/cis-6Az10-PVA mixed monolayers the cis-azobenzene units do not afford such a hydrophobic environment and the 5CB and cis-6Az10-PVA molecules form monolayers separately at the air-water interface. Acknowledgment. We sincerely thank Professor Emeritus Kunihiro Ichimura of the Tokyo Institute of Technology for valuable discussions and encouragement. This work has been partly supported by the 21COE “Practical Nano-Chemistry” from MEXT. References and Notes (1) Ohe, C.; Arai, M.; Kamijo, H.; Adachi; M.; Miyazawa, H.; Itoh, K.; Seki, T. J. Phys. Chem. C 2008, 112, 172.
J. Phys. Chem. C, Vol. 112, No. 16, 2008 6365 (2) Rau, H. In Photochemistry and Photophysics, Rabech, J. F., Ed.; CRC Press: Boca Raton, FL, 1990; Vol. 2, Chapter 4, pp 119-141. (3) Swandry, M.; Schmidt, A.; Stamm, M.; Knoll, W.; Urban, C.; Ringsdorf, H. Polym. AdV. Technol. 1991, 2, 127. (4) Delaire, J. A.; Nakatani, K. Chem. ReV. 2000, 100, 1817. (5) Ichimura, K.; Chem. ReV. 2000, 100, 1847. (6) Natasohn, A.; Rochon, P. Chem. ReV. 2002, 102, 4139. (7) Ubukata, T.; Seki, T.; Ichimura, K. J. Phys. Chem. B 2000, 104, 4141. (8) Ubukata, T.; Seki, T.; Ichimura, K. J. Phys. Chem. B 2003, 107, 13831. (9) Ubukata, T.; Seki, T.; Morino, S.; Ichimura, K. J. Phys. Chem. B 2000, 104, 4148. (10) Kago, K.; Seki, T.; Schu¨cke, R. R.; Mouri, E.; Matsuoka, H.; Yamaoka, H. Langmuir 2002, 18, 3875. (11) Seki, T.; Fukuda, R.; Yokoi, M.; Tamaki, T.; Ichimura, K. Bull. Chem. Soc. Jpn. 1996, 69, 2375. (12) Seki, T.; Sekizawa, H.; Morino, S.; Ichimura, K. J. Phys. Chem. B 1998, 102, 5313. (13) Ohe, C.; Ida, Y.; Matsumoto, S.; Sasaki, T.; Goto, Y.; Noi, M.; Tsurumaru, T.; Itoh, K. J. Phys. Chem. B 2004, 108, 18081. (14) Ohe, C.; Goto, Y.; Noi, M.; Arai, M.; Kamijo, H.; Itoh, K. J. Phys. Chem. B 2007, 111, 1693. (15) Ohe, C.; Sasaki, T.; Noi, M.; Goto, Y.; Itoh, K. Anal. Bioanal. Chem. 2007, 388, 73. (16) Guyot-Sionnest, P.; Hunt, J. H.; Shen, Y. R. Phys. ReV. Lett. 1987, 59, 1597. (17) Bain, C. D. J. Chem. Soc., Faraday Trans. 1995, 91, 1281. (18) Varsanyi, G. Vibrational Spectra of Benzene DeriVatiVes; Academic Press: New York, 1969. (19) Kim, G.; Gurau, M. C.; Lim, S. M.; Cremer, P. S. J. Phys. Chem. B 2003, 107, 1403. (20) Walker, R. A.; Gruetzmacher, J. A.; Richmond, G. L. J. Am. Chem. Soc. 1998, 120, 6991. (21) Himmelhaus, M.; Eisert, F.; Buck, M.; Grunze, M. J. Phys. Chem. B 2000, 104, 576. (22) Humbert, C.; Busson, B.; Abid, J.-P.; Six, C.; Girault, H. H.; Tadjeddine, A. Electrochim. Acta 2005, 50, 3101. (23) Shen, Y. R. Nature 1987, 337, 519.
