Hydrogen Bonding and Photochemical Processes of a Cinnamic Acid

(L) of 1-9 prepared by changing surface pressures (10-30 mN/m) gave four sets of ν(CdO) and ν(CdC) bands at 1730 and 1641, 1684 and 1624, 1674 and 1...
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J. Phys. Chem. 1996, 100, 18483-18490

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Hydrogen Bonding and Photochemical Processes of a Cinnamic Acid Derivative in Langmuir-Blodgett Films Masato Yamamoto, Naoki Furuyama, and Koichi Itoh* Department of Chemistry, School of Science and Engineering, Waseda UniVersity, Shinjuku-ku, Tokyo 169, Japan ReceiVed: June 4, 1996; In Final Form: August 26, 1996X

Fourier-transform infrared reflection absorption (IRA) spectroscopy was applied to study the structures and UV-light-induced photochemical processes of a cinnamic acid derivative (C22CA) in Langmuir-Blodgett (LB) films on a silver substrate. The IRA spectra of the LB films of C22CA with the number of monolayers (L) of 1-9 prepared by changing surface pressures (10-30 mN/m) gave four sets of ν(CdO) and ν(CdC) bands at 1730 and 1641, 1684 and 1624, 1674 and 1624, and 1715 and 1638 cm-1. The first set was assigned to the C22CA molecules forming a hydrogen-bonded chain of carboxyl groups or a “lateral hydrogen-bonded state”, the second and third sets to “cis hydrogen-bonded dimers” (the cis conformation is defined in terms of the position of CdO and CdC groups around the C-C bond connecting the two groups), and the fourth set to the molecules forming a hydrogen-bonding between neighboring LB monolayers or a “face-to-face hydrogen-bonded state”. The lateral hydrogen-bonded state exists only in a one-monolayer LB film (L ) 1) and in the first monolayer of multimonolayer LB films (L g 3), while the cis hydrogen-bonded dimers are formed in the multimonolayer LB films assuming a structure similar to that of a crystalline state of C22CA. When the multimonolayer LB films were annealed at 100 °C, the face-to-face hydrogen-bonded state was converted to the hydrogen-bonded dimer with the trans conformation, giving the set of the ν(CdO) and ν(CdC) bands at 1694 and 1632 cm-1, respectively. Upon UV irradiation, the C22CA molecules in the lateral hydrogen-bonded state do not show any photochemical process, while the molecules forming the cis and trans hydrogen-bonded dimers undergo photodimerization. On the other hand, the irradiation causes the conversion from the face-to-face hydrogen-bonded state to an irregular state consisting of the lateral hydrogenbonded state and a non-hydrogen-bonded state. The conversion to the trans hydrogen-bonded dimer caused by the thermal treatment and the conversion to the irregular state due to the irradiation indicate that the face-to-face hydrogen bonded state is thermodynamically in a metastable (or a kinetically trapped) state. Thus, the IRA spectroscopy characterized a variety of structures of the cinnamic acid derivative forming LB monolayer assemblies and elucidated the correlation between the structures and photochemical processes induced by UV irradiation.

Introduction Fourier-transform infrared reflection absorption (IRA) spectroscopy is a method to obtain grazing incident angle p-polarized reflection spectra of adsorbates on metal surfaces. The high sensitivity of this technique allows us to observe high-quality vibrational spectra of even submonolayer quantities of the adsorbates. The Langmuir-Blodgett (LB) technique is an efficient method for fabrication of monomolecular layers. The study of reaction processes of molecules with purposely designed arrangements obtained by the LB technique gives insight into the relationship between the molecular arrangements and reactions. Recently we have applied the IRA spectroscopy to investigate the structures and photochemical processes of LB films containing stilbazole1,2 and diaryl-1,3-butadiene derivatives;3 the results indicated that upon UV-light irradiation the LB films of the stilbazole and butadiene derivatives undergo dimerization, the kinetic features of which are closely related to geometrical arrangements of the reacting molecules in the films. In the present paper we prepared LB films of 3-phenyl-transp-docosyloxy-2-propenoic acid (trans-p-docosyloxy-2-cinnamic acid), C22CA; see Figure 1A), on a silver substrate by changing surface pressures, and applied IRA spectroscopy to study the structures and UV-light-induced processes of the LB films. X

Abstract published in AdVance ACS Abstracts, November 1, 1996.

S0022-3654(96)01621-8 CCC: $12.00

Figure 1. Structures of CA and C22CA (A) and schematic presentation of photodimerization of CA in the R crystalline state (B).

Photochemical processes of cinnamic acid (3-phenyl-propenoic acid, CA; see Figure 1B) and its derivatives in the crystalline states have been extensively studied by Schmidt and coworkers.4 With regard to the structures and photochemical processes of CA itself they found that (i) the carboxyl groups form hydrogen-bonded dimers and the crystalline states are classified into R, β, and γ forms in terms of the modes of arrangements of the molecules, (ii) upon UV irradiation, the © 1996 American Chemical Society

18484 J. Phys. Chem., Vol. 100, No. 47, 1996 CA molecules in the R and β crystalline states undergo photodimerization (see Figure 1B), and (iii) the configurations of the dimerization products are closely related to the molecular arrangements in the crystals.5 The study of Schmidt and coworkers was a starting point of so-called “topochemistry”. The solid-state photodimerization of a cinnamic acid derivative, trans-p-bromocinnamic acid, has also been studied by Wernick and Savion,6 who used infrared spectroscopy to monitor photodimerization reaction. They observed transient infrared bands associated with changes in stacking states of the reacting molecules induced by the dimerization and related these changes with kinetic features of the process. In the present investigation, we extended these studies to the LB films of C22CA and found that the mode of hydrogen bonding and the structures of the cinnamic acid derivative depend on the surface pressures (1030 mN/m) employed for the preparation of the LB films and the number of monolayers (1-9). Further, through the analyses of the IRA spectral changes induced by UV irradiation of the LB films, we could elucidate how the structures and hydrogen bonding are related to photochemical processes including dimerization reaction. These results provided new insight into topochemistry of photochemical processes of C22CA in the LB films. This paper consists of two parts. In the first part we report the transmission infrared (TIR) spectral changes accompanied by the photodimerization of CA and C22CA in the crystalline state. In the second part we analyzed the UV-light-induced IRA spectral changes of the LB films of C22CA based on the results for CA and C22CA in the crystalline state and explain the photochemical processes of the LB films. To the best of our knowledge there has been no vibrational spectroscopic study on the photochemical processes of the LB films of C22CA. Experimental Section Materials. The trans-cinnamic acid derivative, C22CA, was synthesized by the procedures already reported.7 It was purified three times by recrystallization from chloroform and the purity was checked by measuring the 1H NMR spectra of its chloroform-d solution. trans-Cinnamic acid (CA) was purchased from Kanto Chemical Co., Inc. CA in the R crystalline state was prepared by recrystallization from benzene.4 The crystalline structure of the R form was checked by measuring X-ray powder diffraction patterns. Fabrication of LB Films. A Kyoma Kaimen Kagaku, HBMAP2, Langmuir trough with a Wilhelmy balance was used. Trough water was purified by a Millipore water purification system (Milli-Q, 4-bowl) and its pH value was adjusted at 3.54.0 with aqueous HCl, NaOH, and NaHCO3 solutions, so that the carboxyl group of C22CA in the LB films takes the protonated form. Temperature of the trough water was kept at 13 °C. A silver film with a thickness of about 100 nm, which was deposited on a glass slide (26 × 60 mm) by a homemade vacuum evaporator with a liquid nitrogen trap under a pressure of 10-6 Torr, was used as a substrate for LB films. A monomolecular layer was spread on the trough water by dropwise addition of a 10-3 mol/L chloroform solution of C22CA. The monolayer was transferred to the substrate by a vertical dipping method at the surface pressure of 10-30 mN/ m, at the dipping speed of 5 mm/min. The first transfer was always performed in an upstroke mode. The transfer ratio of the odd and even number layers were 1.0 ( 0.05 and 0.7 ( 0.1, respectively. All the LB films were of a Y-type structure. Photochemical Reactions. Photochemical processes of CA in the crystalline state and C22CA in the LB films as well as in the crystalline state were induced by irradiation with a 100 W

Yamamoto et al. high-pressure Hg lamp (Oreal, Model 6283). By using a quartz lens (f ) 10 cm) the light was collected onto each LB film to get a light intensity of about 10 mW/cm2 at the surface. Usually we did not use any filter. In order to investigate the wavelength dependence of the photochemical processes, however, filters supplied by Hoya Co. Ltd., U340, B370, B460, and HA50, were used to get light sources with wavelengths in the UV-near-IR region. Measurements. Infrared spectra were measured by using JEOR JIR-5500 and Bio-Rad FTS-45A Fourier-transform infrared spectrometers with a resolution of 4 cm-1, except for the spectra in Figure 2 which were measured with a resolution of 0.5 cm-1. A liquid nitrogen cooled mercury cadmium telluride (MCT) detector was used for the measurement of the IRA spectra of LB films, which requires a high sensitivity of about 10-4 in absorbance scale. A DTGS detector was used for the measurement of the TIR spectra of crystals dispersed in a KBr disk. For the IRA measurements a JEOL IR-RSC110 reflection attachment was used at the incidence angle of 80° of the p-polarized IR beam. The temperature dependence of IR spectra was measured by using a homemade cryostat. Results and Discussion IR Spectral Change during Photodimerization of CA in the r Crystalline State. As already explained, CA in the R crystalline state undergoes photodimerization upon UV irradiation (Figure 1B). The IR spectral changes associated with the photodimerization in the crystalline state are illustrated in Figure 2. The dimerization causes the frequency shift of a broad band centered at 1684 cm-1, which is ascribable to a CdO stretching vibration (ν(CdO)) of the carboxyl group forming a hydrogenbonded dimer, to 1705 cm-1, and the intensity reduction of the bands at 1632 and 981 cm-1, which are assigned to ν(CdC) and CH out-of-plane bending (γ(CH)) vibrations of the olefinic group, respectively. In addition, there appear new bands at 1119, 1082, 1031, and 913 cm-1, which may be ascribable to the cyclobutane ring of the dimerization product. The broad feature due to ν(CdO) around 1684 cm-1 in Figure 2A suggests the existence of several component bands. In order to clarify this point, the temperature dependence of the IR spectrum of the R crystal was measured. The result illustrated in Figure 3 indicates the following facts. (i) The ν(CdO) band in the temperature range of 369-215 K consists of two component bands (near 1691 and 1678 cm-1, see below) and the relative intensities of these bands change reversibly with temperature. (ii) In the temperature range below 200 K another sharp component appears at 1684 cm-1 and increases its intensity, as the temperature is reduced further. (iii) The ν(CdC) band is observed near 1630 cm-1 in the temperature range of 369-215 K, although it slightly changes its band shape and position. Upon reducing the temperature below 200 K a shoulder band appears in the lower frequency side and gives a clear feature at 1624 cm-1 at 107K. The 1684 and 1624 cm-1 components are ascribable to an identical structure. In order to quantify the intensities of the component bands in the result (i) we performed a curve resolution procedure on the reversible spectral change in the 369-215 K region under the assumption that the ν(CdO) band is composed of two Gaussian components at 1691 and 1678 cm-1. From the area intensities of the components as a function of temperature we determined the enthalpy and entropy differences (∆H and ∆S) between the two structures by the method of Hartman et al.;8 the results indicated that the structure associated with the 1678 cm-1 component is thermodynamically slightly more stable than the structure associated with the 1691 cm-1 component, the values of ∆H

Photochemical Processes of a Cinnamic Acid Derivative

J. Phys. Chem., Vol. 100, No. 47, 1996 18485

Figure 3. Temperature dependence of the infrared transmission spectra of the R crystalline CA dispersed in a KBr disk. Number at the righthand side of each spectrum indicates absolute temperature.

Figure 2. Infrared spectral changes in the 1760-1560 cm-1 (A) and 1140-860 cm-1 (B) regions during photodimerization of CA in R crystalline state. Number at the right-hand side of each spectrum indicates irradiation time (seconds).

Figure 4. Structures of a cis hydrogen-bonded dimer (A) and a trans hydrogen-bonded dimer (B) of CA.

and ∆S being 356 cal mol-1 and 2.94 cal K-1 mol-1, respectively. According to the X-ray crystal analysis,9 the carboxyl moieties forming the hydrogen-bonded dimer in the R crystalline state at room temperature are in a disordered state interchanging the carboxyl and hydroxyl groups. This means that, as illustrated in Figure 4, both the trans and cis hydrogen-bonded dimers coexist at the room temperature and that the doublet features observed for the ν(CdO) band in the 369-215 K region (Figure 3) can be explained in terms of the trans and cis dimers. The X-ray analyses of CA in the R-form at low temperatures should be performed to determine whether the 1678 cm-1 is assigned to the cis hydrogen-bonded dimer or to the trans dimer. Since the analysis has not been done yet, we performed preliminary molecular orbital calculations on the hydrogen-bonded dimers of CA; semiempirical calculation using the PM3 method suggests that the cis dimer is energetically more stable than the trans dimer by about 1.5 kcal/mol and ab initio calculation based on the STO-3G basis set also predicts that the cis dimer is more stable than the trans one by about 1.0 kcal/mol. On the basis of these results, we tentatively concluded that the ν(CdO) band at 1678 cm-1 is ascribable to the CA molecules forming the cis hydrogen-bonded dimer and the band at 1691 cm-1 to the CA molecules forming the trans dimer. Leiserowitz10 summarized the crystal structures of a series of R- and β-unsaturated carboxylic acids forming a hydrogen-bonded dimer and concluded that the trans and cis conformations of the dimers are

determined by a balance between intramolecular and interlayer interactions. Further, he indicated that the intramolecular interactions favor the cis conformation. (Leiserowitz classified the cis and trans dimers as synplanar and antiplanar conformations of the CdCsCdO group, respectively.) This is consistent with the result of our calculation. As can be recognized from Figure 4, the interconversion between the trans and cis conformations is a simultaneous double proton transfer process. The preliminary calculations indicate that the proton transfer along the one-dimensional linear coordinate of OsH‚‚‚O should overcome an energy barrier of more than 40 kcal/mol. Such a high barrier cannot be overcome by thermal energy. Then, the transfer should be regarded as the phonon-assisted proton tunneling process, as extensively discussed for the hydrogen bonds in the crystalline lattices of carboxylic acids.9 The appearance of the sharp band at 1684 cm-1 below 200 K in Figure 3 indicates a transition from the R crystalline phase to another one. Presumably the cis hydrogen-bonded state is retained in the lower temperature phase and the 1684 cm-1 band may be ascribable to a cis hydrogen-bonded dimer. In order to get detailed insight into the dimerization process from Figure 2, the spectral changes in the CdO stretching vibration region were analyzed by applying a curve-resolution procedure under the assumption that the spectra are composed of three Gaussian components at 1705, 1691, and 1678 cm-1. Figure 5 plots the area intensities of the components against

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Figure 5. Plots of the area intensities against irradiation time observed for the infrared bands of CA in the R-crystalline state during photodimerization (see text). (4) ν(CdO) at 1705 cm-1; (0) ν(CdO) at 1691 cm-1; (O) ν(CdO) at 1678 cm-1; (2) ν(CdC) band at 1632 cm-1; (1) γ(CH) at 981 cm-1.

irradiation time; the plots for the intensities of the ν(CdC) band centered at 1632 cm-1 and the γ(CH) band of the olefinic group at 981 cm-1 are also included in the figure. The 1678 cm-1 component, which was tentatively assigned to the cis hydrogenbonded dimer, the ν(CdC) band at 1632 cm-1, and the γ(CH) band at 981 cm-1 decrease in intensity with the onset of irradiation and virtually disappear after about 10 min, while the intensity of the 1705 cm-1 band assigned to ν(CdO) of the photodimerization product increases upon irradiation and almost saturates after about 10 min. These results indicate that the CA molecules forming the cis hydrogen-bonded dimer in the R crystalline state readily dimerize from the beginning of irradiation. On the other hand, the other component of ν(CdO) at 1691 cm-1, which was assigned to the trans hydrogen-bonded dimer, shows intensity change appreciably different from that of the 1678 cm-1 component; i.e., the intensity of the 1691 cm-1 component at first increases, and after about 40 s it begins to decrease; even after 10 min the intensity is still about one-half of the original value. The initial intensity increase in the 1691 cm-1 component may be explained by considering that the dimerization of the cis hydrogen-bonded dimers causes a concomitant conversion of a part of the cis dimers to the trans hydrogen-bonded state. The intensity decrease observed for the 1691 cm-1 component after 40 s of irradiation indicates that, when the dimerization of the cis hydrogen-bonded dimer proceeds to a certain extent, the trans-hydrogen bonded dimers change their stacking states, causing the photodimerization reaction of the trans hydrogen-bonded dimers. Presumably, overlapping between the π-electron wave functions of the neighboring CdC bonds in the cis hydrogen-bonded dimers is more favorable to dimerization than in the trans hydrogenbonded dimers. Detailed molecular orbital calculation will give more direct evidence for this conclusion. Together with the theoretical calculation, we are now studying the temperature dependence of the kinetic features of the photodimerization process of CA in the crystalline state, the result of which will be published in a separate paper. IR Spectral Changes during Photodimerization of C22CA in the Crystalline State. Figure 6 illustrates the spectral changes induced by UV irradiation of C22CA in the crystalline state. The ν(CdO) and ν(CdC) bands at 1675 and 1624 cm-1 correspond to the 1678 and 1630 cm-1 bands, respectively, observed for CA forming the cis hydrogen-bonded dimer (Figure 3); this result suggests that the carboxyl groups of C22CA in

Figure 6. Infrared spectral changes in the 1770-1530 cm-1 (A) and 1140-890 cm-1 (B) regions during photodimerization of C22CA in the crystalline state dispersed in a KBr disk. Number at the right-hand side of each spectrum indicates irradiation time (minutes).

the crystalline state assumes the hydrogen-bonded dimer with the cis conformation. Upon UV irradiation, the ν(CdO) band at 1675 cm-1 shifts to 1703 cm-1 and the bands at 1624 and 982 cm-1, which are due to the ν(CdC) and γ(CH) vibrations of the olefinic group, respectively, reduced their intensity; these results provide evidence for photodimerization of C22CA in the crystalline state. Further, Figure 6B indicates that upon irradiation there appear new bands at 1113 and 912 cm-1. These bands correspond to those observed at 1119 and 913 cm-1 for the photodimerization product (R-truxillic acid) from CA in the R-crystalline state (Figure 2B). In the lower frequency region the photodimerization product from C22CA gives IR bands at 658 and 583 cm-1, which are the counterparts of the 663 and 581 cm-1 bands observed for R-truxillic acid (not shown in this paper). We concluded from these results that the dimerization product from C22CA in the crystalline state assumes a structure similar to that of R-truxillic acid (Figure 1B). Structure of One-Monolayer LB Films of C22CA. Figure 7 shows the surface pressure vs area (π-A) plot observed for C22CA at 13 °C and pH 4.0. As the surface area is compressed, the surface pressure at first increases rapidly. When the pressure becomes larger than 20 mN/m, the rate of increase is much reduced. Presumably, at surface pressures below 20 mN/m the C22CA molecules form a monolayer on the water surface. Upon further compression of the surface area, a part of the monolayer is corrupted forming bilayers or multilayers, which exist in a

Photochemical Processes of a Cinnamic Acid Derivative

Figure 7. Surface pressure-area plot of C22CA at 13 °C and pH 4.0. Arrows in the figure indicate surface pressures at which one-monolayer LB films were prepared (see text).

Figure 8. IRA spectra of one-monolayer LB films of C22CA prepared at various surface pressures. Number at the right-hand side of each spectrum indicates the surface pressure.

crystalline-like state. This is proved by measuring the IRA spectra of one-monolayer LB films of C22CA prepared on a silver substrate at surface pressures in the 10-30 mN/m region. (The one-monolayer LB film (L ) 1) does not necessarily mean that the film forms a true one-monolayer on the substrate; it means that the sample was prepared by just one vertical stroke of the substrate in the LB technique. See Experimental Section.) As can be seen from Figure 8, the one-monolayer LB films prepared at 10-15 mN/m give rise to the ν(CdO) and ν(CdC) bands at 1730 and 1641 cm-1, respectively, in addition to a weak ν(CdO) band near 1684 cm-1. At the surface pressure of 20 mN/m the film gives the 1684 cm-1 band of appreciable intensity (with a shoulder band near 1674 cm-1) and the intensities of these bands increase with the surface pressure. The intensity increase is much enhanced in the region above 20 mN/m compared to that below 20 mN/m. Further, at surface pressures above 20 mN/m there appears a ν(CdC) band at 1624 cm-1 and its intensity increases with the surface pressure. These spectral changes correspond to the discontinuous change of π-A around 20 mN/m (Figure 7). CA in the gaseous state gives rise to the ν(CdO) and ν(CdC) bands at 1755 and 1642 cm-1, respectively.10 The ν(CdO) frequency observed for the LB films prepared at lower surface pressures (1730 cm-1) is appreciably lower than that

J. Phys. Chem., Vol. 100, No. 47, 1996 18487 observed for the gaseous sample and it is still higher than that observed for the CA molecules forming the hydrogen-bonded dimers (1691 and 1678 cm-1). According to Nuzzo et al.11 and Sun et al.,12 self-assembled one-monolayer films containing carboxyl groups give rise to a ν(CdO) band in the region of 1718-1720 cm-1 ascribable to linear polymeric chains of hydrogen bonds of the groups. On the basis of these results we concluded that the ν(CdO) band at 1730 cm-1 and the ν(CdC) band at 1641 cm-1 are ascribable to a hydrogen-bonded chain of the carboxyl groups. When a monomolecular layer on the water surface is transferred to the silver substrate at a lower pressure (20 mN/m) correspond to the IR bands at 1684, 1678, and 1630(or 1624) cm-1, respectively, observed for CA in Figure 3 and the IR bands near 1675 and 1624 cm-1 for C22CA in Figure 6A. It is reasonable to consider that, when the bilayers and multilayers formed on the water surface at the surface pressures above 20 mN/m are transferred to the substrate, the C22CA molecules in the LB films take on a structure similar to that of CA and C22CA in the crystalline state, giving the IRA bands at 1684, 1674, and 1624 cm-1. These IRA bands are ascribable to C22CA forming cis hydrogen-bonded dimers. Photochemical Process of One-Monolayer LB Films of C22CA. Figure 9, parts A and B, exhibits the spectral changes induced by UV irradiation of the one-monolayer LB films of C22CA prepared at 10 and 30 mN/m, respectively. As can be seen from Figure 9A, the irradiation does not cause any appreciable change except for slight frequency lower of the 1730 cm-1 band. Thus, the C22CA molecules in the lateral hydrogenbonded state does not show any photochemical process. On the other hand, the LB film prepared at 30 mN/m exhibits appreciable changes; i.e., with the onset of irradiation the 1684, 1674, and 1624 cm-1 bands reduce their intensity and there appears a new band at 1705 cm-1 (Figure 9B). These spectral changes are similar to those observed for the photodimerization process of C22CA in the crystalline state (Figure 6A). From these results we concluded that the C22CA molecules forming the cis hydrogen-bonded dimer in the one-monolayer LB film undergo photodimerization. As in the case of the LB film prepared at 10 mN/m, the 1730 and 1641 cm-1 bands in Figure 9B do not show any change upon irradiation. Structures of Multilayer LB Films of C22CA. Figure 10 exhibits the IRA spectra of the LB films of C22CA with the number of monolayers (L), 1, 3, 5, 7, and 9, which were prepared at 10 mN/m. The films with L g 3 give ν(CdO) bands at 1715 and 1680 cm-1 and a ν(CdC) band at 1638 cm-1 in addition to the 1732 and 1641 cm-1 bands observed for the onemonolayer LB films. The intensities of the 1715, 1680, and 1638 cm-1 bands increase with L. In contrast, the intensity of the 1732 cm-1 band due to the lateral hydrogen-bonded state remains virtually constant, which indicates that the state exists only in the first monolayer in the multilayered films (L g 3). Interactions between the first monolayer and the silver substrate may stabilize the lateral hydrogen-bonded state. The LB films with L g 3 in Figure 10 are of a Y-type structure, which means that the carboxyl groups of C22CA in neighboring monolayers may interact with each other through hydrogen bonding. The appearance of the ν(CdO) bands at 1680 and 1715 cm-1 only for the LB films with L g 3 suggests

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Figure 9. IRA spectral changes during UV irradiation of onemonolayer LB films of C22CA prepared at surface pressures at 10 mN/m (A) and 30 mN/m (B). Number at the right-hand side of each spectrum indicates irradiation time (seconds).

Figure 10. IRA spectra of the LB films of C22CA with the number of monolayers from 1 to 9 (prepared at 10 mN/m). Number at the righthand side of each spectrum indicates the number of monolayers.

that these bands are ascribable to hydrogen bonding between the neighboring monolayers. The 1680 cm-1 band corresponds to the 1675 cm-1 band in crystalline states in Figure 6A and

Yamamoto et al.

Figure 11. (A) IRA spectral changes during UV irradiation of a threemonolayer LB film of C22CA (prepared at 10 mN/m). Number at the right-hand side of each spectrum indicates irradiation time (seconds). (B) Difference spectra calculated from the IRA spectra in part A (see text).

the 1684 or 1674 cm-1 bands in one-monolayer LB film prepared at the surface pressures above 25 mN/m (Figure 8). Then, the 1680 cm-1 band is assigned to the C22CA molecuels forming the cis hydrogen-bonded dimer between the neighboring monolayers. (The ν(CdC) band from the hydrogen-bonded dimer is expected to appear around 1624 cm-1. This band, however, is buried in between the 1638 and 1607 cm-1 bands in the spectra of the LB films with L ) 3-9 in Figure 10.) The 1715 and 1638 cm-1 bands in Figure 10 indicate that the LB films with L g 3 contain another hydrogen-bonded state in addition to the lateral hydrogen-bonded state and the cis hydrogen-bonded dimer. Although the structure of this state has not been clarified yet, the frequency of the ν(CdO) band at 1715 cm-1, which is appreciably lower than that of the lateral hydrogen-bonded state (1732 cm-1), suggests that the third state forms a hydrogen-bonded dimer between the carboxyl groups in neighboring monolayers. The difference in the ν(CdO) frequency compared to those of the cis and trans hydrogenbonded dimers of CA and C22CA in the crystalline state may be explained in terms of differences in intermolecular interactions and the CdCsCdO backbone structures. Hereafter the structure associated with the 1715 and 1638 cm-1 band is called a “face-to-face hydrogen-bonded state”. Photochemical Processes of Multilayer LB Films of C22CA. Figure 11A shows the IRA spectral changes induced by UV irradiation of the LB film of C22CA with L ) 3 prepared at 10 mN/m. With the onset of irradiation the 1680 cm-1 band

Photochemical Processes of a Cinnamic Acid Derivative reduces its intensity, and there appears a shoulder band near 1705 cm-1. These spectral changes are similar to those observed in Figure 9B. Thus, the C22CA molecules forming the cis hydrogen-bonded dimer in the multilayer LB film undergo photodimerization. From Figure 11A it is also clear that the irradiation causes intensity reduction of the 1715 cm-1 band. This result cannot be interpreted as due to photodimerization, because the corresponding ν(CdC) band at 1638 cm-1 does not show intensity decrease comparable to that of the 1715 cm-1 band. More detailed insight into the spectral changes can be obtained by a series of difference spectra in Figure 11B, which are calculated by subtracting a spectrum measured at an irradiation time t2 from that measured at t1 (t2 > t1). Positive and negative peaks in the figure indicate intensity increase and decrease, respectively, caused by the irradiation. The negative peaks at 1676 and 1624 cm-1 and the positive one at 1705 cm-1 observed in an initial step of irradiation (0-30 s) correspond to the photodimerization of the C22CA molecules forming the cis hydrogen-bonded dimer. A negative peak at 1717 cm-1 accompanies broad positive peaks observed in the 1730-1750 cm-1 region. The ν(CdO) band near 1730 cm-1 is ascribable to the lateral hydrogen-bonded state and that near 1750 cm-1 to a non-hydrogen-bonded state. Then, the result suggests that the C22CA molecules forming the face-to-face hydrogen-bonded state are converted to an irregular state containing the lateral hydrogen-bonded state and the non-hydrogen-bonded state. A positive peak around 1643 cm-1, which is ascribable to the ν(CdC) band of the lateral hydrogen-bonded state and/or the non-hydrogen-bonded state, is observed for t2 g 30 s; this result corroborates the conversion from the face-to-face hydrogenbonded state to the irregular state. In parallel with the irradiation experiment with the 100 W high-pressure Hg lamp without using any filter, we investigated the wavelength dependence of irradiation light on the photochemical processes of the LB film of C22CA with L ) 3 prepared at 10 mN/m by using a series of filters (see Experimental Section) to get irradiation wavelength from the UV to the near-IR region. The results confirmed that both the photodimerization process in the cis hydrogen-bonded state and the conversion from the face-to-face hydrogen-bonded state to the irregular state are induced only by irradiation at an UV wavelength region. It is impossible to consider that the latter conversion is a true photochemical process. Presumably, the conversion process is a secondary process induced by the photodimerization of the cis hydrogen-bonded dimer. Structural Change of a Three-Monolayer LB Film of C22CA Induced by Annealing. Figure 12 exhibits the IRA spectrum of the LB film of C22CA with L ) 3 prepared at 25mN/m and the spectra of the same film measured after annealing at 100 °C for 10 and 20 min. Upon annealing, the 1715 and 1639 cm-1 bands due to the face-to-face hydrogenbonded state reduce their intensity, and new bands appear at 1694 and 1632 cm-1. The latter bands correspond to those observed at 1691 and 1630 cm-1 in Figure 3, which were ascribed to a hydrogen-bonded dimer with the trans conformation. This result indicates that the face-to-face hydrogen-bonded state is thermodynamically in a metastable state (or a kinetically trapped state) and converted to a more stable state, i.e., the trans hydrogen-bonded dimer. The thermodynamic instability of the face-to-face hydrogen-bonded state conforms to the fact that, as explained in the preceding section, it is converted to the lateral hydrogen-bonded state and/or to the non-hydrogen-bonded state during the photodimerization of the cis hydrogen-bonded dimer in the LB film with L ) 3. From Figure 12 it is clear that the 1734 cm-1 band due to the lateral hydrogen-bonded state

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Figure 12. IRA spectra of a three-monolayer LB films of C22CA (prepared at 25 mN/m) annealed at 100 °C for 0, 10, and 20 min. The measurement was performed at room temperature.

Figure 13. IRA spectral changes during UV irradiation of an annealed three-monolayer LB film of C22CA (prepared at 25 mN/m). The measurement was performed at room temperature. Number at the righthand side of each spectrum indicates irradiation time (seconds).

reduces its intensity. Thus, the heat treatment induces also the conversion of the lateral hydrogen-bonded state to another one (probably the trans hydrogen-bonded state). Photochemical Process of an Annealed Three-Monolayer LB Film of C22CA. Figure 13 shows the IRA spectral changes induced by UV irradiation observed for the annealed threemonolayer LB film of C22CA prepared at 25 mN/m. With the onset of irradiation the 1694, 1681, and 1632 cm-1 bands reduce their intensity , and there appears a new band at 1705 cm-1 ascribable to the photodimerization product. Thus, the C22CA molecules forming the trans and cis hydrogen-bonded dimers in the multimonolayer LB film undergo the photodimerization. This is contrasted to the case of the photodimerization of CA in the R crystalline state, where at an initial stage of irradiation only the cis hydrogen-bonded dimer undergoes the dimerization reaction. A difference in the molecular packing between the crystal and the LB film may explain the difference in the photochemical reactivity. (The 1634 cm-1 band measured after irradiation for 10 m is ascribable to the face-to-face hydrogenbonded state, which still exists after the irradiation. The corresponding 1714 cm-1 band is buried in the higher frequency side of the 1705 cm-1 band.)

18490 J. Phys. Chem., Vol. 100, No. 47, 1996 Conclusion The IRA spectroscopy proved that the molecular assemblies of C22CA fabricated by the LB technique assumes at least four kinds of hydrogen-bonded states, that is, a lateral hydrogenbonded state, a face-to-face hydrogen-bonded state, and cis and trans hydrogen-bonded dimers. Although some of the structure models proposed to these states are still in a level of speculation, the LB technique was very effective to prove that an amphiphile such as C22CA, which has potential diversity in terms of conformations (cis-trans isomerism) and intermolecular interactions (hydrogen-bonded chain and hydrogen-bonded dimer), can assume a variety of structures, when it is assembled into twoand three-dimensional arrangements. The lateral hydrogenbonded state exists in the one-monolayer LB film and in the first monolayer of the multimonolayer LB films (L g 3); these results suggest that the state is stabilized through a direct interaction of the monolayer and the substrate surface. The cis hydrogen-bonded dimer is formed in the one-monolayer LB film prepared at surface pressures larger than 20 mN/m and in the multimonolayer LB films (L g 3). The face-to-face hydrogenbonded state was also found only in the multimonolayer LB films (L g 3). Then, the cis hydrogen-bonded dimer and the face-to-face hydrogen-bonded state are formed between neighboring monolayers. The latter state is thermodynamically in a metastable state and readily converted to the trans hydrogenbonded dimer upon annealing at an elevated temperature (e.g., at 100 °C). The IRA spectroscopy clarified also the topochemical aspects of photochemical processes of the hydrogen-bonded states of C22CA in the LB films. C22CA in the lateral hydrogen-bonded state does not undergo photodimerization, indicating that the molecule forming a hydrogen-bonded chain cannot assume a stacking state appropriate for the reaction. Photodimerization

Yamamoto et al. proceeds only for C22CA forming the cis and trans hydrogenbonded dimers. Presumably, the formation of the hydrogenbonded dimers results in stacking states appropriate for the dimerization in the LB films. Upon UV-light irradiation the C22CA molecules forming the face-to-face hydrogen-bonded state in the multimonolayer LB films (L g 3) are converted to the non-hydrogen-bonded state and/or to the lateral hydrogenbonded state. This conversion is induced by the photodimerization of the C22CA molecules in the cis hydrogen-bonded state, which coexist in the LB films. Thus, the thermodynamic instability of the face-to-face hydrogen-bonded state is reflected by its photochemical behavior. References and Notes (1) Yamamoto, M.; Wajima, T.; Kameyama, A.; Itoh, K. J. Phys. Chem. 1992, 96, 10365. (2) Yamamoto, M.; Itoh, K.; Nishigaki, A.; Ohshima, S. J. Phys. Chem. 1995, 99, 3655. (3) Saito, A.; Wajima, T.; Yamamoto, M.; Itoh, K. Langmuir 1995, 11, 1277. (4) Schmidt, G. M. J. Chem. Soc. 1964, 2014. (5) Cohen, M. D.; Schmidt, G. M. J. J. Chem. Soc. 1964, 1996. (6) Savion, Z.; Wernick, D. L. J. Org. Chem. 1993, 58, 2424. (7) Gray, G. W.; Jones, B. J. Chem. Soc. 1954, 1467. (8) Hartman, K. O.; Carlson, G. L.; Witkowski, R. E.; Fately, W. G. Spectrochim. Acta 1968, 24A, 157. (9) Bryan, R. F.; Freyberg, D. P. J. Chem. Soc., Perkin Trans. 2 1975, 1835. (10) Leiserowitz, L. Acta Crystallogr. 1976, B32, 775. (11) Skinner, J. L.; Trommsdorff, H. P. J. Chem. Phys. 1988, 89, 897. (12) Kharitonov, Yu. Ya.; Oleinik, I. I. Dokl. Akad. Nauk. SSSR 1990, 313, 2, 384. (13) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (14) Sun, Li.; Kepley, L. J.; Crooks, R. M. Langmuir 1992, 8, 2101.

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