Sum Frequency Generation Spectroscopic Study on Photoinduced

This may explain the disappearance of the broad band due to the water ..... of the trans-6Az10-PVA monolayer at 5 mN/m takes the orientation angels of...
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J. Phys. Chem. C 2008, 112, 172-181

Sum Frequency Generation Spectroscopic Study on Photoinduced Isomerization of Poly(vinyl alcohol) Containing Azobenzene Side Chain at the Air-Water Interface C. Ohe,† H. Kamijo,† M. Arai,† 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: May 11, 2007; In Final Form: August 20, 2007

Sum frequency generation (SFG) spectra were measured for the monolayers of a poly(vinyl alcohol) derivative (6Az10-PVA, see Figure 1A) containing azobenzene side chains at the air-water interface. The surface pressure dependence of the spectra was analyzed to elucidate structural changes of the monolayers associated with the trans-cis isomerization of the azobenzene group. The SFG spectra in the 3600-3150 cm-1 region observed for the trans-6Az10-PVA monolayer, where the OH stretching bands of water molecules associated with the PVA backbone are observed, suggest that a dissolution process of the backbone from the water surface into a bulk water takes place upon increasing the surface pressure from 20 to 45 mN/m. The spectra in the CH stretching vibration region of the alkyl group side chain indicates that the group in the cis-6Az10-PVA monolayer exists in a more irregular state containing gauche defects compared to those of the trans monolayer, although the content of the gauche defects is appreciably reduced upon compression from 3 to 7 mN/m. The simulation of the SFG spectra due to backbone stretching and bending vibrations of the azobenzene side chain and its peripheral was performed by using the density functional calculation at the B3LYP/6-311+G** level. Comparison between the calculated spectra and the surface pressure dependence of the SFG spectra of the trans and cis-6Az10-PVA monolayers in the 1650-1100 cm-1 region clarified the orientation changes of the azobenzene groups induced by compression. The long axis of the azobenzene group in the trans-6Az10PVA monolayer takes the tilt angle of ca. 20° (relative to the surface normal) in the surface pressure range of 5-20 mN/m and slightly reduces the tilt angle to ca. 15° upon compression to 45 mN/m. On the other hand, the cis-6Az10-PVA monolayer greatly changes the orientation of the azobenzene group during the compression; that is, at 3 mN/m the plane formed by a pseudo C2 axis and the NdN bond of the azobenzene group is nearly parallel to the water surface, whereas at 7 mN/m the plane rotates by ca. 90°, keeping the C2 axis parallel to the water surface.

Introduction The photoinduced trans-cis isomerization of an azobenzene group has been one of the most fascinating topics because of its function applicable to optical storage elements, light switching devices, nonlinear optical devices and liquid crystalline alignment surfaces.1-9 Understanding detailed orientation and structure changes of the azobenzene and its peripheral groups associated with the isomerization is of crucial importance to optimize the functions. To date, a wide variety of polymers containing an azobenzene group have been synthesized and various photofunctional properties have been explored in solution, films, and membranes.7-16 One of the typical and most interesting examples of azobenzene containing polymers is a poly(vinyl alcohol) derivative having an azobenzene side chain, 6Az10-PVA (see Figure 1A) which has been studied by Seki et al.13,14 According to the authors, the polymer undergoes a reversible trans-cis isomerization of the azobenzene group upon UV (365 nm) and visible light (436 nm) irradiation in solutions and monolayer states at the air-water, as schematically shown in Figure 1B. The photochromic monolayer transferred onto a solid surface can be applied to the so-called command surface, * Corresponding author. E-mail: [email protected]. † Waseda University. ‡ Nagoya University.

Figure 1. Chemical structure of trans-6Az10-PVA (A), schematic representation of UV and visible light induced structural and orientation changes of the azobenzene moiety of 6Az10-PVA monolayers (B).

which photochemically switches the alignments of liquid crystalline (LC) molecules in bulk states.7,9 Ubukata et al.15,16 found that the mixtures of the trans-6Az10-PVA (the PVA derivative with the trans azobenzene group) and a nematic LC molecule, 4′-pentyl-4-cyanobiphenyl (5CB), with the mole ratio of 5CB to the azobenzene unit less or equal to two are

10.1021/jp073602e CCC: $40.75 © 2008 American Chemical Society Published on Web 12/13/2007

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Figure 2. Optimized structures of (A) trans-1AzOMe and (B) cis1AzOMe and Eulerian angles defining the orientations. See text.

homogeneously mixed without lateral phase separation at the air-water interface. The hybrid monolayer serves as an ideal model system in order to elucidate how the surface trans-cis isomerization of the azobenzene unit regulates the homeotropic and homogeneous alignments of the LC molecule in the bulk state. Surface pressure-area (π-A) isotherm, UV-visible absorption, surface-potential, and Brewster angle microscopic measurements have been applied to the monolayers of 6Az10-PVA at the air-water interface in order to elucidate an inherent photochemical response and its cooperativity associated with conformation and orientation changes of the monolayer during the photoisomerization.13,14 The structural aspects of the monolayers thus clarified can be summarized as follows: (i) The trans-6Az10-PVA (the PVA derivative with the trans-azobenzene side chain) monolayer before compression exists in a H-type aggregate, forming iceberg like domains. (ii) Upon conversion to the cis-6Az10-PVA monolayer the molecular area is expanded by ca. 3 times compared to that of the trans monolayer. (iii) A large dipole moment associated with the cisazobenzene moiety causes a direct contact of the moiety with the water surface, resulting in the expansion of the monolayer, as schematically shown in Figure 1B. In spite of these results, however, orientation and conformation changes of the alkyl, azobenzene and PVA parts of the 6Az10-PVA monolayer due to the trans-cis isomerization have not been clarified yet. In the present paper, we applied a sum frequency generation (SFG) spectroscopy to clarify the orientation and conformation changes at the molecular level. The SFG spectroscopy is a nonlinear optical technique, in which two laser beams, one in the visible region and of fixed frequency and the other in the infrared region and of tunable frequency, are spatially and temporally overlapped at an interface to produce a vibrational spectrum specific to interfacial species. Since the SFG spectroscopy was experimentally demonstrated by the Shen laboratory,17,18 it has been established as one of the most efficient methods to extract information on surface structures from small molecules such as water to large molecular systems such as self-assembled monolayers of lipids and proteins at various interfaces.17-28 The present paper consists of the following parts; first, the experimental section; second, the simulation method of SFG spectra in the backbone stretching vibration region; third, the results of the SFG measurements in the OH stretching, CH stretching, and backbone stretching vibration regions observed for the trans- and cis-6Az10-PVA monolayers at the air-water

Figure 3. Monolayer compression (π-A) isotherms of trans-6Az10PVA (solid line) and cis-6Az10-PVA (dashed line) monolayers measured at 21 °C and the surface pressures where the SFG spectra of the monolayers were measured. See text.

interface; and fourth, the simulation of the spectra to elucidate the orientation changes of the azobenzene moiety due to the trans-cis isomerization. The results of the SFG measurements on the 5CB/6Az10PVA mixed monolayers with the trans- and cis-azobenzene side groups will be published in a separate paper. Experimental Section Materials. trans-6Az10-PVA was synthesized by the procedure already reported.13,14 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. trans-Azobenzene was purchased from Kanto Chemicals Co., Inc. and used without prurification. Preparation of Monolayers. trans-6Az10-PVA was dissolved into chloroform to prepare a stock solution with the concentration of 0.64 mg/mL. The solution of cis-6Az10-PVA was prepared by irradiating the solution of trans-6Az10-PVA for 14 min with a 360 nm light (15 mW/cm2) from a 500 W high-pressure mercury lamp (Oriel Cooporation, model 68810) equipped with a band-pass filter (KENKO Optical Co., Ltd., U-360). According to Seki et al.,13,14 the UV irradiation results in almost 90% conversion from the trans to cis form of 6Az10PVA in the solution. The monolayers at the air-water interface were prepared by spreading the stock solutions on pure water in a trough (a MiniMicro trough of KSV Instruments Ltd.), which allows us to measure monolayer compression (π-A) isotherms and to keep the pressure at a constant value by using a PC control system. A spread sample on the pure water was compressed to a certain surface pressure, kept at least 15 min to stabilize the monolayer, and served for SFG spectral measurements. The sample preparation of the monolayer of the cis-6Az10-PVA sample was performed in the dark to avoid the conversion to the trans monolayer. The absence of the conversion during the SFG measurement was checked by observing the π-A isotherm after the measurement to confirm that the isotherm gives a feature characteristic of the cis monolayer. (vide infra.) SFG Spectral Measurements. SFG spectra were measured by using the apparatus, which is virtually identical with that

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Figure 4. Surface pressure dependence of the SFG spectra in the 3600-2750 cm-1 region measured under the ssp polarization for the trans-6Az10-PVA (A) and cis-6Az10-PVA (B) monolayers (at 22 °C). The spectra, except for the bottom ones, are offset by certain values to exhibit the spectra clearly.

used in the previous studies.26-28 The spectra were recorded by combining 3 ps pulses of an 800 nm light (1 kHz, ca. 20µJ/ 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 co-propagating geometry with the angle of incidence of 43.1° from the surface normal for the visible light and the angle of 53.2° for the IR beam. All spectra were measured at 22 ( 1°C and taken with the ssp polarization combination, which refers to the sum frequency and visible and infrared beams, respectively. FT Raman Spectral Measurement. The Fourier transform (FT) Raman spectrum of trans-azobenzene in a powder state was performed by using a FT Raman spectrometer (JASCO, model FT/IR-800, λex ) 1064 nm). Simulation Method of SFG Spectra. The theory of the SFG spectroscopy has been described in detail.19,29 The intensity of the SFG signal is proportional to the square of the surface nonlinear susceptibility (2) 2 2 (2) ISFG ∝ (S/σ)2 |χ(2) NR + χR | ) (S/σ) |χNR +

∑q ω

Aq IR

- ωq - iΓq

| (1)

with

Aq,ijk )

1 0

〈(ıˆ‚lˆ)(jˆ‚m ˆ )(kˆ ‚nˆ )〉βq,lmn ∑ lmn

βq,lmn ∝ 〈g|Rlm|q〉 〈q|µn|g〉

(2) (3)

(2) where χ(2) NR and χR in eq 1 are nonresonant and resonant contribution to the susceptibility, respectively, Aq, ωq, and Γq are the line amplitude, resonant frequency and damping constant of the qth normal mode, respectively, and ωIR is the frequency of the IR beam. S is an overlapping area of the laser beams, σ is a mean molecular area of molecules forming interfaces, and the term S/σ corresponds to the number of molecules monitored by the SFG spectroscopy; in this study σ was estimated from π-A isotherms. The angle brackets in eq 2 express an average over the orientations of all the molecules at the interface, and

0, (ijk) and (lmn) denote the dielectric permittivity of vacuum, the surface fixed Cartesian coordinates ()X,Y,Z) and the molecule-fixed Cartesian coordinates ()a,b,c), respectively. Equation 3 expresses the molecular hyperpolarizability, βq,lmn, associated with the qth normal mode, where |g> and |q> are the vibrational ground and excited states, respectively, Rlm and µn are the polarizability and dipole moment operators, respectively. Equation 3 presupposes the so-called “non-resonance condition”; that is, the wavelengths corresponding to the sum and visible beams are far from those of electronic transitions of samples; the trans and cis-6Az10-PVA monolayers exhibit λmax below ca. 450 nm,13 which means that the condition can be applied to the present case. In order to simulate the SFG spectra mainly due to backbone stretching vibrations of the azobenzene moieties observed for the trans- and cis-6Az10PVA monolayers, the density functional calculation at the B3LYP/6-311+G** level was performed by following the procedures; that is, (i) the transition dipole moments, , and the Raman scattering tensor components, < g|Rlm|q>, were calculated by using the DFT method for the optimized structures of trans- and cis-4-methyl-4′-methoxy-azobenzene (1AzOMe, see Figure 2); (ii) the molecular hyperpolarizability tensor elements, βq,lmn, of normal modes mainly associated with azobenezene moiety of the trans- and cis-6Az10-PVA were evaluated from the calculated transition dipole and Raman scattering tensor components by using eq 3; (iii) the molecular hyperpolarizability elements were transformed to the surface fixed coordinate system to obtain β′q,ijk as a function of the Euler (2) angles30 defined in Figure 2; (iv) χq,YYZ of each normal mode was calculated by averaging β′q,ijk(θ,φ,χ) over φ (0-2π) at fixed θ and χ values under the assumption that the monolayers of the trans- and cis-6Az10-PVA at the air-water interface assume a uniaxial symmetry with the symmetry axis parallel to the surface normal. (The X and Z axes of the surface-fixed coordinates were taken parallel to the propagation direction of the laser beams and that of the surface normal, respectively.); (v) under the assumptions that the nonresonant term (χ(2) NR) is negligible and that each SFG band has a Lorentzian type with (2) the peak intensity proportional to the square of χq,YYZ , the spectra were calculated as a function of the molecular orientations expressed by the angles, θ and χ. In the simulation it was assumed also that there exist no coherent interference interactions between normal modes and the each band has a fixed bandwidth of 5 cm-1. The structural optimization and the calculation of normal frequencies and the transition moments of the model compounds were performed by using the Gaussian 03 program.31 The simulation procedure explained above is similar to that employed by Hore et al.,32 who determined the orientation of the sulfate group in a dodecylsulfate monolayer at the air-water interface based on the ab initio calculation of the SO3 symmetric stretching band intensity measured under various polarization combination conditions. The authors evaluated the transition dipole moments and Raman scattering tensor components by using an interpolation method, whereas we calculated the transition dipole moments and Raman scattering tensor components33 and confirmed their validity by checking the agreement between the observed IR and Raman spectra with those simulated by using the calculated transition dipole moments and scattering tensor components. In addition, we compared the calculated and observed intensities of SFG bands associated with the azobenzene group in the region of 1650-1100 cm-1 measured under the ssp polarization to determine the orientations of the group.

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Figure 5. Surface pressure dependence of the SFG spectra enlarged for the 3100-2750 cm-1 region measured under the ssp polarization for the trans-6Az10-PVA (A) and cis-6Az10-PVA (B) monolayers (at 22 °C). The spectra, except for the bottom ones, are offset by certain values to exhibit the spectra clearly. The dashed red curves under the observed plots indicate the contributions of the component bands obtained by a curve resolution procedure with an assumption of a Lorentzin curve for each component and the solid curve the sum of the contributions. See text.

Results and Discussion π-A Isotherms of the trans- and cis-6Az10-PVA Monolayers at the Air-Water Interface. Figure 3 shows the compression surface pressure-molecular area (π-A) isotherms of the trans- and cis-6Az10-PVA monolayers measured at 21 ( 1°C. The isotherms are almost identical with those already reported.13 The isotherm of the trans monolayer indicates a transformation from an expanded to a condensed state with the limiting area of ca. 35 Å.2 Upon conversion to the cis form, the molecular area is expanded by a factor of ca. 3 at a low surface pressure. A plateau region around 7 mN/m for the cis monolayer suggests a transformation from an expanded state to a less expanded one. It is also noted that the collapse pressure of the cis monolayer is much smaller than that of the trans monolayer. The SFG spectra of the trans form were measured at 5, 20 and 45 mN/m and those of the cis form at 3 and 7 mN/m. SFG Spectra in the OH Stretching Vibration Region of the Monolayers at the Air-Water Interface. Figure 4, parts A and B, exhibits the surface pressure dependence of the SFG spectra in the 3600-2750 cm-1 region observed for the transand cis-6Az10-PVA monolayers, respectively, spread on pure water. The SFG spectra of water surface and surfactants at the air/water interface measured under the ssp polarization (corresponding to SFG, visible, and IR lights) generally give rise to broad bands centered around 3200 and 3400 cm-1.22,34 The 3200 cm-1 band has been assigned to a continuum of OH symmetric stretching vibrations of tetrahedrally coordinated (or hydrogenbonded) water molecules near the surface and has been referred as an “icelike” band because of its closeness in frequency to the OH stretching of bulk ice. The 3400 cm-1 band has been assigned to more weakly correlated hydrogen-bonded OH stretching modes of water molecules in an asymmetric hydrogenbonded environment and usually labeled as a “liquidlike” band in analogy to the position of the strongest band observed in the bulk Raman spectra of liquid water.35 In addition to the abovementioned bands, the air/water interface gives a rather sharp SFG band located near 3700 cm-1, which has been attributed to the free OH bond of surface water molecules.22,34 The SFG spectra of the trans- and cis-6Az10-PVA monolayers at the air-

water interface do not show any band ascribable to the free OH bond. As can be seen from Figure 4, the SFG spectra in the 3400-3100 cm-1 region give only weak bands, contrasting to strong bands observed below 3100 cm-1, which are due to CH stretching vibrations (vide infra). However, it is noteworthy that broad bands appear around 3400 and 3200 cm-1 in the spectra measured at 5 and 25 mN/m for the trans-6Az10-PVA monolayer and that these bands almost disappear at 45 mN/m. The poly(vinyl alcohol) moiety comprises 78% of the backbone of the 6Az10-PVA sample. As the π-A isotherm shows in Figure 3, the trans monolayer has a relatively large molecular area at the lower pressures, suggesting that the PVA moiety of the trans monolayer spreads near the water surface. Presumably, the OH groups of the PVA moiety induce an ordered array of water molecules at the air-water interface, giving the broad band in the 3400-3200 cm-1 region. Upon compression of the monolayer to 45 mN/m, the monolayer is strongly compressed, resulting in submersion of the PVA moiety into the water subphase. This may explain the disappearance of the broad band due to the water molecules associated with the OH groups of the PVA moiety. On the other hand, the SFG spectrum measured for the cis-6Az10-PVA monolayer at 3 mN/m does not show any band in the frequency region above 3100 cm-1, and upon compression to 7 mN/m, there appears a broad band envelope in the 3450-3100 cm-1 region. As explained below, upon compression of the cis monolayer from 3 to 7 mN/m the group undergoes a large orientation change; that is, at 3 mN/m, the NdN bond of the group is almost parallel to the water surface, and at 7 mN/m, the bond is almost perpendicular to the surface. The cis-azobenzene group at 3 mN/m occupied a larger area than at 7 mN/m, explaining the reduction of the average molecular area induced by the compression. In addition, the cis-azobenzene group has a large dipole moment, which causes the association of water molecules around the group. So, although it is impossible to give a clear explanation to the appearance of the broad band envelop, the large orientation change may cause an ordered structure of water molecules at the air-water interface, resulting in the appearance of the envelope in the 3450-3100 cm-1 region.

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Figure 6. Surface pressure dependence of the SFG spectra in the 1750-1050 cm-1 region measured under the ssp polarization for the trans6Az10-PVA (A) and cis-6Az10-PVA (B) monolayers (22 °C). The spectra, except for the bottom ones, are offset by certain values to exhibit the spectra clearly. See text.

SFG Spectra in the CH Stretching Vibration Region of the Monolayers at the Air-Water Interface. Figure 5, parts A and B, exhibits the enlarged versions of the SFG spectra in the 3100-2750 cm-1 region observed for the trans- and cis6Az10-PVA monolayers, respectively; the dashed lines in each spectrum indicate the contribution of component bands, which are calculated by a nonlinear regression procedure explained in previous papers,26,27 and the solid line the result of the simulation or the sum of the contribution. The trans monolayer gives the SFG bands due 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 around 2850, 2870, 2925, 2940, and 2970 cm-1, respecively.26,34,36 In addition, the trans monolayer gives SFG bands observed near 3010 and 3070 cm-1, which are due to CH stretching vibrations of the benzene rings.37 The appearance of these bands indicates that the azobenzene moiety takes a more or less fixed orientation relative to the air-water interface. The relative intensities of the νs(CH3) to the νs(CH2) band in the SFG spectra of the trans monolayer remains almost constant at the surface pressures of 5 and 20 mN/m, whereas upon increasing the pressure from 20 to 45 mN/m, the relative intensity increases, suggesting that the pressure increase cause a conversion of the alkyl chains of the monolayer from an irregular structure containing gauche structures to a regular one consisting mostly of the all-trans structure.21 The π-A isotherm of the trans-6Az10-PVA monolayer indicates that the transition from the expanded to condensed state is completed during the surface pressure increase from 20 to 45 mN/m. Presumably, the transition causes the increase in the lateral packing density of the side chain of the trans-6Az10-PVA monolayer, resulting in the conversion to the more regular state of the alkyl group. This process is accompanied also by the desorption of the PVA moiety of the monolayer from the water surface, as explained in the previous section. The SFG spectra of the cis-6Az10-PVA monolayer in Figure 5B give much weaker bands than those in Figure 5A because of the smaller surface densities of the molecule. The spectra in Figure 5, however, give rise to the νs(CH2) and νs(CH3) bands at 2850 and 2870 cm-1, respectively. The dominance of the

νs(CH2) band in the spectra in Figure 5B indicates that the alkyl group in the cis monolayer exists in a more irregular state than the group of the trans monolayer,21 although on increasing the surface pressure from 3 to 7 mN/m, the intensity ratio of the νs(CH3) band to the νs(CH2) band appreciably increases, suggesting that the chain is converted to a partially ordered state. The irregular alkyl groups in the cis-6Az10-PVA monolayer indicate that the isomerization to the cis form of the azobenzene moiety causes the distortion of the alkyl side chains, which conforms to the larger molecular area of the cis monolayer compared to that of the trans monolayer. SFG Spectra in the Backbone Stretching Vibration Region of the Monolayers at the Air-Water Interface. Figure 6, parts A and B, compares the surface pressure dependence of the SFG spectra of the trans- and cis-6Az10-PVA monolayers in the 1750-1050 cm-1 region. The spectra of the trans monolayer give rise to weak band around 1720, 1590, and 1410 cm-1, which are due to CdO stretching, benzene ring stretching and CH2 bending vibrations, respectively, and strong bands near 1250 and 1140 cm-1. Comparison of the spectra with the IR spectrum of trans-6Az10-PVA dispersed in a KBr pellet shown in Figure 7 (solid line, vide infra) indicates that the SFG spectra exhibit much simpler features compared to those of the IR spectrum. The IR spectrum gives strong bands at 1738, 1603, 1583, and 1500 cm-1, corresponding bands of which are only weakly observed in the SFG spectra. The very weak feature for the CdO stretching band in the SFG spectra may be interpreted in terms of a random orientation of the CdO group and/or a more or less parallel orientation of the group relative to the water surface. A broad band envelop in the 1470-1050 cm-1 region except for the 1254, 1153, and 1142 cm-1 bands observed for the IR spectrum is absent in the SFG spectra. The envelope can be ascribed to bending, wagging, and twisting vibrations of the alkyl chains and the PVA moiety of trans-6Az10-PVA. The weakness of the SFG bands associated with the alkyl groups should be interpreted in terms of the orientation of the groups relative to the water surface by performing the spectral measurements under polarization combinations other than ssp and theoretical simulations. The DFT calculation of the model compound explained in the following section indicated that the

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Figure 7. Comparison between the IR spectrum (solid line) of trans6Az10-PVA in the KBr disk state and the spectrum (dashed line) calculated for trans-1AzOMe (see Figure 2A) at the B3LYP/6311+G** level. See text.

TABLE 1: Comparison of the Calculated IR Frequencies (cm-1) of trans-1AzOMe with Those of IR Bands Observed for trans-6Az10-PVA in the KBr State calcd. freq.a

obsd. freq.

assignmentsb

1612 1578 1508 1264 1161 1147 1047 855 560 539

1603 1583 1500 1254 1153 1142

benzene ring (ν8a)c benzene ring (ν8b)c benzene ring (ν19a)c C(benzene)-O str. C(benzene)-N str. (out-of-phase)d C(benzene)-N str. (in-phase)d C(alkyl chain)-O str. CH out-of-plane bend. out-of-plane def. of benzene ring C-NdN def.

841 565 550

a The frequencies were calculated at the B3LYP/6-311+G** and corrected by multiplying the factor proposed by Matuura and Yoshida.38 b Str. and bend. mean stretching and bending vibrations, respectively. c See ref 37 for benzene ring modes. d See text.

IR band at 1254 cm-1 band is due to a C(benzene ring)-O stretching (ν(φ-O)) and those at 1153 and 1142 cm-1 due to the out-of-phase and in-phase stretching vibrations of the C(benzene ring))-N(azo group) bonds, respectively; in the following, the out-of-phase and in-phase stretching vibrations are abbreviated to νas(φ-N) and νs(φ-N), respectively. The SFG bands at 1250 and 1140 cm-1 in Figure 6A are the counterparts of the IR bands at 1254 and 1153 cm-1 and assigned to ν(φO) and νas(φ-N) vibrations, respectively. As can be seen from Figure 6A, upon increasing the surface pressure from 5 to 20 mN/m the intensity ratio of the 1250 cm-1 band relative to the 1140 cm-1 band increases appreciably, and on further increase of the surface pressure to 45 mN/m, the ratio remains almost constant. (The overall intensities of the SFG bands measured at 20 mN/m are smaller than those measured at 5 mN/m, which does not correspond to the increase in the surface density of the 6Az10-PVA molecules deduced from the π-A isotherm in Figure 3. This may be ascribable to the following reasons, i.e., first, due to the intensity changes associated with the orientation change and, second, due to an experimental error. Although the intensities are calibrated with respect to the intensities of the visible and IR beams, a slight fluctuation of the overall intensities measured at each surface pressure cannot be ruled out in our experimental system.) The surface pressure dependence of the SFG spectra observed for the cis-6Az10-PVA monolayer in Figure 6B is quite different from that of the trans monolayer in Figure 6A. Although the spectrum measured at 7 mN/m gives SFG bands with ap-

Figure 8. Comparison of the Raman spectrum of trans azobenzene (A, solid powder, λex ) 1064 nm) and the spectrum (B) calculated for trans azobenzene at the B3LYP/6-311+G** level. See text.

preciable intensity at ca. 1250 and 1140 cm-1, the spectrum at 3 mN/m does not show any corresponding bands. This cannot be explained by a decrease in the surface density of the molecule, since the π-A isotherm of the cis-6Az10-PVA monolayer in Figure 3 predicts that the intensity ratio of each band measured at 3 mN/m relative to that at 7 mN/m should be ca. 1/4, if the intensities of the SFG bands were independent of the surface pressure. Thus, the appearance of the 1250 and 1140 cm-1 bands at 7 mN/m can be explained only in terms of an orientation change of the cis-azobenzene moiety induced by the compression. The DFT calculation for the model compound indicated that the SFG bands at 1250 and 1140 cm-1 are ascribable mainly to ν(φ-O) and νs(φ-N) vibrations, respectively. Simulation of SFG Spectra in the 1650-1100 cm-1 Region of the Monolayers at the Air-Water Interface. In order to elucidate the orientation changes of the trans- and cis-azobenzene moieties based on the observation explained in the previous section, the DFT calculation was performed, as explained in the simulation method section. The IR spectrum calculated for the trans-1AzOMe (see Figure 2A) optimized by the DFT calculation at the B3LYP/6-311+G** level is shown by the dashed line in Figure 7 and compared with the IR spectrum (solid line) of trans-6Az10-PVA in the KBr disk state. Table 1 lists the calculated and observed frequencies of main bands together with an assignment to each band. The calculated spectrum indicates that most of the sharp bands except for the 1733 cm-1 due to a CdO stretching in the IR spectrum can be ascribable to skeletal stretching and bending vibrations of the azobenzene moiety and its peripheral, and reasonable agreement in the spectral features and frequencies between the calculated and observed bands predicts that the calculation at the B3LYP/ 6-311+G** level reproduces well the transition dipole moments,

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Figure 9. Calculated SFG spectra of the trans 1AzOMe with a fixed θ angle and various χ angles. See text. (A) θ ) 10°, (B) 15°, (C) 20°, and (D) 25°.

< g|µn|q>, for the IR bands associated with the azobenzene moiety and its peripheral of trans-6Az10-PVA. Since we could not have measured the Raman spectrum of trans-6Az10-PVA, we calculated the Raman spectrum of transazobenzene and compared it with the observed spectrum in order to confirm the validity of the calculated Raman scattering tensor components, , at the B3LYP/6-311+G** level. Figure 8, parts A and B, exhibits the calculated and observed Raman spectra, respectively. Although the calculated Raman band at 1523 cm-1 differs in frequency from the corresponding observed peak at 1439 cm-1, the calculated relative intensities and frequencies of the other bands especially in the region below 1400 cm-1 correspond reasonably well with those of the observed bands. On the basis of these results, we considered that the Raman scattering tensor components calculated for the optimized structure of trans- and cis-1AzOMe can be used to estimate the molecular hyperporarizability tensor components of the azobenzene moiety and its peripheral of trans- and cis6Az10-PVA. As Figure 2 shows, the initial orientation of the optimized trans form of 1AzOMe is taken so that the long molecular axis (c axis) is parallel to the Z axis and the molecular plane (ac plane) parallel to the YZ plane. In the optimized cis form of 1AzOMe, the dihedral angle around the NN bond is 4.4°, and each benzene ring rotates relative to the average CNNC plane by about 47°, which are not so different from the corresponding values observed for the cis-azobenzene (8.0° and 53.3°).39 The cis form has a pseudo C2 axis connecting the midpoint of the

NN bond and the midpoint of the line connecting the centers of the benzene rings. The initial orientation of the cis form is taken to be the pseudo C2 axis (a axis) parallel to the X axis and the NN bond (c axis) parallel to the Z aixs. Figure 9 illustrates the calculated SFG spectra in the 16501100 cm-1 for the trans-1AzOMe; the results are given only for the tilt angle θ smaller than 30°, since the results for θ g 30° give a strong band around 1520 cm-1 ascribable to an Nd N stretching (ν(NdN)) vibration, which is very weak and discernible only in the observed spectrum at 5 mN/m (see the insert in the bottom spectrum in Figure 6A). The calculated spectra for θ ) 25° (Figure 9D) still give the 1520 cm-1 too strong intensity to be compared with the observed spectra in Figure 6A. The calculated spectra for θ ) 20° and χ ) 6090° give weak bands at 1614 and 1520 cm-1, which correspond to the SFG bands weakly observed near 1600 and 1500 cm-1 in Figure 6A, and strong bands at 1264 and 1161 cm-1, the relative intensity and frequencies of which are in reasonable agreement with the ν(φ-O) and νas(φ-N) bands, which are observed at 1250 and 1140 cm-1, respectively, in the bottom spectrum in Figure 6A. There still exists disagreement between the calculated relative intensities of the SFG bands at 1612 and 1520 cm-1 and those of the corresponding observed ones. This discrepancy is ascribable to several reasons; first, eq 3 is too simplified to calculate molecular hyperpolarizability tensors precisely, and, second, the DFT calculation at the B3LYP/6311+G** level is not accurate enough to reproduce the intensities of weak SFG bands. In spite of these facts, the

Photoinduced Isomerization of PVA

J. Phys. Chem. C, Vol. 112, No. 1, 2008 179

Figure 10. Calculated SFG spectra of the cis 1AzOMe with various θ and χ angles. See text.

agreement of the calculated and observed intensities of the ν(φ-O) and νas(φ-N) bands strongly suggests that the orientation of the azobenzene moiety of the trans-6Az10-PVA monolayer at 5 mN/m takes the orientation angels of θ ) ca. 20° and χ ) 60-90°. The large range of the χ value, which corresponds to the rotation of the molecular plane around the long axis of the trans-azobenzene group, means that the angle cannot be specified explicitly because the relative intensity is almost constant in the angle range. From Figure 9, parts A and B, it is clear that for θ e 15° the intensity of the 1261 cm-1 band is larger than that of the 1161 cm-1 band, in contrast to the case for θ ) ca. 20° and χ ) 60-90°. The result of calculation for θ e 15° corresponds to the observed spectral feature at 20 and 45 mN/m in Figure 6A; a detailed comparison between the calculated and observed relative intensity suggests that the monolayers at 20 and 45 mN/m take the orientation with θ ) ca. 15° and χ ) 60-90°. Figure 11A shows schematically how the orientation of the trans-azobenzene moiety changes its orientation upon increasing the surface pressure from 5 mN/m to a value larger than or equal to 20 mN/m. As explained in the previous sections, the conversion of the alkyl groups from the irregular to regular structure and the desorption of the PVA moiety in the trans monolayer proceed in the surface pressure range larger than 20 mN/m. Thus, the orientation change of the long molecular axis of the azobenzene group to a less tilted state relative to the surface normal proceeds in an earlier stage of the transformation from the expanded to condensed state of the trans monolayer. The simulation for the SFG spectra of the cis-6Az10-PVA monolayer was also performed for the (θ, χ) angles in the 0-90° region. Figure 10 exhibits a part of the results. The bands calculated at 1610, 1563, 1504, 1265, and 1132 cm-1 are ascribable mainly to benzene ring stretching, ν(NdN), CH bending (benzene rings), ν(φ-O) and νs(φ-N) vibrations, respectively, and the 1265 and 1132 cm-1 bands correspond to the observed SFG bands at 1250 and 1140 cm-1, respectively, in Figure 6B. As shown in the top spectrum of Figure 10, the calculated spectra only for θ and χ angles close to 90° reproduce the SFG spectrum measured at 3 mN/m (the bottom spectrum in Figure 6B), where all SFG bands are very weak and the 1250 and 1140 cm-1 are not discernible; minor changes from 90° for the (θ, χ) angles result in the appearance of a strong band at 1560 cm-1. (See, for example, the result for θ ) 75° and χ ) 90° in Figure 10.) In addition, the calculated spectra for θ g

Figure 11. Schematic representation of the orientation changes of the azobenzene moiety due to the compression of the trans-6Az10-PVA (A) and cis-6Az10-PVA (B) monolayers at the air-water interface. See text.

20° except for the case of θ ≈ 90° and χ ≈ 90° gave a strong ν(NdN) band at 1563 cm-1, which does not conform to the observed spectra in Figure 6B. In the case of θ < 20°, the calculated spectra for χ e 45° still gives a strong band at 1563 cm-1 (see for example, the calculated spectrum for θ ) 15° and χ ) 30° in Figure 10), and the calculated spectra for χ in the region of 60-90° seem to agree with the top spectrum in Figure 6B. In this way, a detailed comparison between the calculated and observed spectra was performed to conclude that the results of simulation only for θ ) 0-15° and χ ) 60-90° are in a reasonable agreement with the SFG spectrum measured for the cis-6Az10-PVA monolayer at 7 mN/m is reproduced, as can be seen from the bottom three spectra in Figure 10; the simulated spectra for θ ) 0° do not depend on the χ angle. The calculated spectra, however, give the bands at 1610 and 1563 cm-1 with appreciable intensity, which does not correspond to the observation. This is due to the error associated with the method of calculation, as in the case of the simulation of the SFG spectra of the trans monolayer. Figure 11B exhibits schematically how the orientation of the azobenzene moiety in the cis-6Az10-PVA monolayer changes its orientation upon compression. At 3 mN/m the plane formed by the pseudo C2 axis and the NdN bond is almost parallel to the water surface, and on increasing the surface pressure to 7 mN/m, the plane rotates about 90° around the C2 axis, keeping the axis parallel to the water surface. The orientations of the azobenzene moiety at both 3 and 7 mN/m explain the larger molecular area per azobenzene unit of the cis monolayer than the area of the trans monolayer. In addition, the rotation of the cis-azobenzene group causes a large orientation change for both alkyl and PVA moieties, allowing them to take a more or less

180 J. Phys. Chem. C, Vol. 112, No. 1, 2008 closely packed state. These facts account for the large molecular area reduction observed for the compression of the cis monolayer from 3 to 7 mN/m. According to Seki et al.,13 the surface potential (∆V) of the cis-6Az10-PVA monolayer exhibits a substantial decrease during the compression process. The potential is correlated to the effective dipole moment (µ) and the surface density of molecules (N) through equation ∆V ) 4πµZN/D, where µZ is the perpendicular component of the effective dipole moment of the molecule and D is the dielectric constant (usually assumed as 1). On the assumption that the effective dipole of the molecule can be assigned mainly to the cis-azobenzene group, the authors concluded that the compression accompanies the orientation change of the dipole moment from a laid state to a more or less perpendicular one relative to the water surface. Since the dipole moment is parallel to the pseudo C2 axis, the conclusion does not conform to the result of the SFG measurement, which indicated that the C2 axis stays nearly parallel to the water surface before and after compression. This discrepancy may be explained by considering that water molecules associated with the cis-azobenzene group also contribute to the effective dipole moment monitored by the surface potential measurement. This explanation still corroborates the conclusion of Seki et al.13 that the cis-6Az10-PVA monolayer is stabilized through interactions between the dipole moment of the cis-azobenzene group and water molecules at the air-water interface. Conclusion The SFG spectroscopy applied to the trans- and cis-6Az10PVA monolayers and the spectral simulation based on the DFT method gave detailed information at the molecular level about how the structures of alkyl, azobenzene, and PVA moieties of each monolayer change upon compression at the air-water interface. The results for the trans-6Az10-PVA monolayer are summarized as follows: (i) Upon compression from 20 to 45 mN/ m, the PVA moiety desorbs from the air-water interface into the water subphase: (ii) The alkyl group contains gauche defects at 5 and 20 mN/m, and on compression to 45 mN/m, the group is converted to a regular structure consisting mainly of the alltrans bonds: (iii) The long axis of the azobenzene group slightly changes the tilt angle (θ) from ca. 20 to 15° upon compression from 5 mN/m to a pressure larger than 20 mN/m. Comparison of these results with the π-A isotherm indicates that the transformation from the expanded to condensed state of the trans monolayer begins with the orientation change of the azobenzene group to the less tilted state, followed by the conversion of the alkyl group to the ordered state and the desorption of the PVA moiety from the air-water interface into the water subphase. As for the structure of the cis monolayer, the results are summarized as follows: (i) The alkyl group of the cis-6Az10PVA monolayer at the air-water interface exists in a more irregular state compared to that of the trans monolayer, although the gauche defect content is substantially reduced on compression from 3 to 7 mN/m: (ii) The cis-azobenzene group largely changes its orientation relative the water surface; that is, at 3 mN/m the plane formed by the pseudo C2 axis and the NdN bond is almost parallel to the water surface, and on compression to 7 mN/m the NdN bond rotates about 90° around the C2 axis, keeping the axis parallel to the water surface. The orientation of the azebenzene group explains the large molecular area per azobenzene group occupied by the cis monolayer at 3 mN/m. The rotation of the cis azobenzene group causes a large orientation change for both alkyl and PVA moieties, allowing

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