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Studies on the Molecular Environment and Reaction Kinetics of Photo-Oligomerization in Langmuir-Blodgett Films of 4-(4-(2-(Octadecyloxycarbonyl)vinyl)cinnamoylamino)benzoic Acid Jiang Zhao,†,‡ Haruhisa Akiyama,† Koji Abe,† Zhongfan Liu,‡ and Fusae Nakanishi*,† National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, and Center for Intelligent Materials Research, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China Received June 24, 1999. In Final Form: November 3, 1999 The amphiphilic diolefin compound 4-(4-(2-(octadecyloxycarbonyl)vinyl)cinnamoylamino)benzoic acid has been found to form a stable monolayer on a water surface and to be successful in fabricating photoreactive Langmuir-Blodgett (LB) films. The photo-oligomerization in LB films have been studied by UV-visible and infrared spectroscopy. The infrared spectroscopy reveals a breaking of hydrogen bonding between the amide groups and a distortion of alkyl chains during the reaction. Furthermore, a correlative two-step kinetics of olefinic groups and the changes in orientation and packing state of the alkyl chains is found. This correlation strongly suggests the dependence of the reaction kinetics on the structural change in the LB films induced by the reaction process itself.
Introduction Photopolymerization in ordered ultrathin organic films has been attracting considerable research interest because of its multiple prospective applications, such as surface modifications, ultrahigh-density information storage, the fabrication of nanoscale molecular devices for electronics and photonics.1-5 Its importance has also been recognized in relation to gaining a fundamental understanding of the reaction process itself in low-dimensional molecular systems, which are believed to have unique characteristics compared with those processes in bulk crystallites.6-26 * Corresponding author. Tel: +81-298-54-4671. Fax: +81-29854-4673. E-mail:
[email protected]. † National Institute of Materials and Chemical Research. ‡ Peking University. (1) Ulman, A. An Introduction to Ultrathin Organic Films: from Langmuir-Blodgett to Self-assembly; Academic Press: Boston, 1991; pp 176-191. (2) Lando, J. B.; Fort, T., Jr. In Polymerization of Organized Systems; Elias, H.-G., Ed.; Gordon and Breach: New York, 1977; pp 63-78. (3) Naegele, D.; Ringsdorf, H. In Polymerization of Organized Systems; Elias, H.-G., Ed.; Gordon and Breach: New York, 1977; pp 79-88. (4) An Introduction to Molecular Electronics, Petty, M. C., Bryce, M. R., Bloor, D., Eds.; Oxford University Press: New York, 1995. (5) Molecular Electronics: A ‘Chemistry for the 21st Century’ Monograph; Jortner, J., Ratner, Eds.; Blackwell Science: London, 1997. (6) Laschewky, A.; Ringsdorf, H.; Schmidt, G. Thin Solid Films 1985, 134, 153. (7) Laschewky, A.; Ringsdorf, H. Macromolecules 1988, 21, 1936. (8) Naegele, D.; Lando, J. B.; Ringsdorf, H. Macromolecules 1977, 10, 1339. (9) Cemel, A.; Fort, T., Jr.; Lando, J. B. J. Polym. Sci.: A-1 1972, 10, 2061. (10) Fukuda, K.; Shibasaki, Y.; Nakahara, H. J. Macromol. Sci.Chem. 1981, A15, 999. (11) Tanaka, Y.; Nakayama, K.; Iijima, S.; Shimizu, T.; Maitani, Y. Thin Solid Films 1985, 133, 165. (12) Kuriyama, K.; Kikuchi, H.; Kajiyama, T. Langmuir 1996, 12, 6468. (13) Saito, A.; Urai, Y.; Itoh, K. Langmuir 1996, 12, 3938. (14) Yamamoto, M.; Wajima, T.; Kameyama, A.; Itoh, K. J. Phys. Chem. 1992, 96, 10365. (15) Saito, A.; Wajima, T.; Yamamoto, M.; Itoh, K. Langmuir 1995, 11, 1277. (16) Ohe, C.; Ando, H.; Sato, N.; Urai, Y.; Yamamoto, M.; Itoh, K. J. Phys. Chem. B 1999, 103, 435. (17) Kim, T.; Ye, Q.; Sun, L.; Chan, K. C.; Crooks, R. M. Langmuir 1996, 12, 6065.
Because polymerization in the solid state depends largely on the crystalline structure,27,28 it is reasonable to assume that polymerization in thin organized films will also rely on the two-dimensional crystalline structures. In this aspect, a number of studies using spectroscopy, microscopy, and X-ray techniques have been made.13-16,20-25 Recently, a grazing-angle X-ray study on a Langmuir monolayer revealed the topochemical rule valid for the two-dimensional systems.21 On the other hand, the twodimensional structure is also subject to change during the reactions because of the drastic differences in the electronic structures of reaction center atoms and, consequently, in the molecular conformations induced by the reactions.3,29,30 This kind of structural change may be further propagated to the unreacted molecules through intermolecular forces, modifying the molecular microenvironment, in terms of molecular packing, molecular orientation, etc., thereby further affecting the reaction itself. Therefore, studies of these phenomena can provide information on the relationship between the molecular environment and reactivity13-16 and also help to build up suitable low-dimensional reaction systems in which the reaction process can be controlled to some extent. (18) Rabe, J. P.; Rabolt, J. F.; Brown, C. A.; Swalen, J. D. Thin Solid Films 1985, 133, 153. (19) Beredjick, N.; Burlant, W. J. J. Polym. Sci.: A-1 1970, 8, 2807. (20) Wang, S.; Vidon, S.; Leblanc, R. M. J. Colloid Interface Sci. 1998, 207, 303. (21) Weissbuch, I.; Bouwman, W.; Kjaer, K.; Als-Nielsen, J.; Lahav, M.; Leiserowitz, L. Chirality 1998, 10, 60. (22) Mao, G.; Tsao, Y.; Tirrell, M.; Davis, H. T.; Hessel, V.; Ringsdorf, H. Langmuir 1993, 9, 3461. (23) Werkman, P. J.; Wieringa, R. H.; Vorenkamp, E. J.; Schouten, A. J. Langmuir 1998, 14, 2119. (24) Cai, M.; Mowery, M. D.; Menzel, H.; Evans, C. E. Langmuir 1999, 15, 1215. (25) Grim, P. C. M.; Feyter, S. D.; Gesquie`re, A.; Vanoppen, P.; Ru¨cker, M.; Valiyaveettil, S.; Moessner, G.; Mu¨llen, K.; De Schryver, F. C. Angew. Chem., Int. Ed. Engl. 1997, 36, 2601. (26) Letts, S. A.; Fort, T. J. Colloid Interface Sci. 1998, 202, 341. (27) Savion, Z.; Wernick, D. L. J. Org. Chem. 1993, 58, 2424, and references therein. (28) Schmidt, G. M. J. Pure Appl. Chem. 1971, 27, 647. (29) Nakanishi, F. J. Polym. Sci.: C: Polym. Lett. 1988, 26, 159. (30) Nakanishi, F.; Okada, S.; Nakanishi, H. Polymer 1989, 30, 1959.
10.1021/la9908236 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/13/2000
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A Langmuir-Blodgett (LB) film is a typical species of organized molecular film systems that is physisorbed on solid substrates.1-3 The molecular interactions are especially important for the assemblies in these molecular systems because of the relatively weak anchoring. In addition, due to this weak adsorption, the monolayer structures are subject to change under certain driving forces, such as external heat or mechanical forces as well as the chemical reactions. In photopolymerization, the reaction occurs in response to the absorption of external photoenergy, and the original changes in electronic structures and molecular conformations will affect the monolayer structures. We have previously evaluated the reaction-induced structural changes and the dependence of kinetics on the monolayer structure in the dimerization of 4-(4-(2-(decyloxycarbonyl)vinyl)cinnamoylamino)benzoic acid (DCAB) in its Langmuir monolayer with the aid of stearic acid on a water surface.31 However, in that case, the monolayer on the water surface was difficult to transfer onto solid substrates because of the weak hydrophobicity of the DCAB molecule. This difficulty hindered the spectroscopic monitoring of the reaction, especially for infrared spectroscopy, which is powerful in providing detailed molecular information. One approach to solving this problem might be to increase the hydrophobicity of the molecule’s end group, which could result in a stable Langmuir monolayer on the water surface and satisfactory LB deposition onto solid substrates. In this paper, we describe our study of the photooligomerization of an amphiphilic p-phenylenediacrylic acid derivative with longer alkyl chain, 4-(4-(2-(octadecyloxycarbonyl)vinyl)cinnamoylamino)benzoic acid (abbreviated as OCAB), in its LB monolayers. The capacity of this molecule to form a good monolayer for LB deposition was evaluated. Photo-oligomerization was induced in its LB films, and the relationship between the reaction kinetics and the monolayer structure was studied by UVvisible and infrared spectroscopy. Experimental Section Preparation of Materials. OCAB was synthesized by the reaction of the acid chloride of p-phenylenediacrylic acid (p-PDA) monooctadecyl ester with p-aminobenzoic acid and was purified by its reprecipitation from dimethyl sulfoxide (DMSO) solution with water. For LB film fabrication, the OCAB was dissolved in a mixed solvent of tetrahydrofuran (THF) and benzene (1:3) at a concentration of 2.4 × 10-4 M. π-A Isotherm and LB Deposition. The surface pressuremolecular area (π-A) isotherm measurement and the fabrication of the LB films were conducted in a commercial trough (Lauda Film Balance, Germany). The subphase was first deionized and then distilled water, with its temperature kept at ∼20 °C. Ten minutes after the sample solution had been spread on the water surface, compression of the surface began at a compression rate of 14 cm2/min. After the monolayer had become stable at a fixed surface pressure, the monolayer deposition was conducted by means of a vertical method. For the surface morphologic study, freshly cleaved mica plates were adopted as substrates, while CaF2 plates were used as substrates for the spectroscopic studies. The surfaces of the CaF2 substrates were made to be hydrophilic by successive ultrasonication in ethanol, acetone, chloroform, and distilled water for 10 min each. The mica substrates were put below the water surface before solution spreading, and deposition was conducted later. The CaF2 substrates were allowed to rest in air before the start of the deposition process. Morphologic Observation of the Langmuir and Langmuir-Blodgett Monolayer. The surface morphology of the Langmuir monolayer of OCAB on the water surface was observed by a Brewster angle microscope (BAM; see ref 31 for the (31) Zhao, J.; Abe, K.; Akiyama, H.; Liu, Z.; Nakanishi, F. Langmuir 1999, 15, 2543.
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Figure 1. π-A (surface pressure-molecular area) isotherm of OCAB on water surface at 20 °C. Inset: BAM images of monolayer of OCAB on water surface at 20 °C at different surface pressure of 0.0 mN/m (A and B), 10.0 mN/m (C), and 35.0 mN/m (D). experimental details). Other experimental conditions such as the subphase temperature and surface compression rate were kept identical to those in the monolayer fabrication. Observations of the morphology of the OCAB LB monolayer on a mica surface were carried out with an atomic force microscope (AFM; Pico SPM, Molecular Images) in an ambient environment. The force constant of the cantilever of the probe tip was 0.38 N/m. In its operation, the force between the tip and the monolayer’s surface was adjusted to be very small. Spectroscopic Studies of Photo-Oligomerization. UVvisible spectra were measured by a Shimadzu 2500 PC spectrophotometer. The IR spectral measurement was carried out with a Perkin-Elmer System 2000 FTIR spectrometer equipped with a narrow-band MCT detector cooled by liquid nitrogen. The resolution of the system was set at 4 cm-1. The sample chamber was purged with dry nitrogen to eliminate the interference of water vapor in air. To avoid the interference effect due to reflections from the two surfaces of the CaF2 plate at normal incidence, an incident angle of 15° was chosen in the IR measurement. To induce oligomerization in the LB films, a 500 W extrahigh-pressure mercury lamp (Ushio UI-501C, Japan) with a combination of optical filters (Kenko U-330 and 0.265 g/L K2Cr2O7 + 0.5 wt % Na2CO3 aqueous solution of 1.0 cm path length) to obtain monochromic light centered at 313 nm was used as an irradiation source. The light source was equipped with a timing shutter system to conduct irradiation at different time intervals, between which the UV-visible spectra and IR spectra were measured. To determine the molecular weight of the products, the irradiated LB films on the CaF2 substrate were put in THF in a small bottle for several hours, and the THF solution was completely dried. Afterward, a very small amount of THF (∼60 µL) was added to the bottle to dissolve the products. This THF solution was subjected to measurement by gel permeation chromatography (GPC) (Shimadzu C-R2AX, Japan). The columns used were Shodex GPC KF-805 and KF 803, and the standard samples used for calibration were polystyrene.
Results and Discussion Langmuir Monolayer and Langmuir-Blodgett Film Fabrication. Figure 1 shows the π-A isotherm of OCAB on a water surface at 20 °C. It can be seen that the onset of the increase in surface pressure occurs at a value
Langmuir-Blodgett Films of OCAB
of the molecular area of 0.40 nm2, and further compression of the surface very quickly leads to a solid-phase region with a limiting molecular area of ∼0.31 nm2. No kink or plateau in the π-A curve occurs before the monolayer collapses at approximately 60 mN/m. To obtain a clear picture of the morphology of the Langmuir monolayer, a BAM observation was conducted, and the resulting images are presented in the inset of Figure 1. (Note that in each image only the center part is in good focus and the dimensions of the images are 0.65 × 0.90 mm2.) At 0.0 mN/m (A), the image shows a discontinuous feature: separated islands are observed that can be assigned to the domains formed by the OCAB molecules. In the current case of OCAB, the intermolecular interaction is believed to be strong, consisting of hydrophobic interactions between the alkyl chains, the π-π interactions between the conjugated part of the molecule, i.e., the two olefinic and two phenylene groups, and the hydrogen bonding between the amide and carboxyl groups. The existence of these attractive interactions can have a great effect on the organization of the molecules, as is evidenced by the formation of large-scale domains (>0.4 × 0.65 mm2), even in the resting state, i.e., at 0.0 mN/m (B). As the compression continued and the surface pressure increased, a decrease in the average distance between the domains was observed, though the discontinuous feature of the surface monolayer persisted (C for 10.0 mN/m). As the compression proceeded with the increase of the surface pressure, the monolayer morphology began to exhibit some continuous and homogeneous features. Figure 1D shows an example of the morphology of the monolayer at 35.0 mN/m, in which a homogeneous domain of a scale larger than the field of view of the microscope (0.65 × 0.9 mm2) is observed. Recalling the absence of any plateau or kink in the π-A isotherm, it can be said that no phase transition occurs during the compression, and the formation of the homogeneous monolayer is achieved by the coalescence of formerly separated small domains as they are pushed together. When the surface film was kept under 35 mN/m for a long time (>2 h), its area remained constant, being only slightly smaller than that when 35.0 mN/m had just been reached, indicating that a stable monolayer had formed, and consequently, the monolayer deposition was conducted at this surface pressure. By comparing these data with those for the previously studied DCAB, which has difficulty forming a stable monolayer on a water surface at room temperature,31 it is obvious that an increase in the length of the alkyl chain greatly improves the molecule’s ability to form the stable monolayer. The origin of this improvement should be the stronger hydrophobicity of the alkyl groups and the stronger attractive van der Waals interactions between them. At the surface pressure of 35.0 mN/m, the monolayer was successfully transferred onto either mica substrates or hydrophilic CaF2 substrates. In experiment, a monolayers was deposited on mica, and a Y-type five-layer film was fabricated on CaF2 substrates. The transferring ratios were higher on the upstrokes than the downstrokes, and an average value of 0.9 was obtained. An AFM study of the LB monolayer on mica has demonstrated the domain structure of the monolayers, along with defects inside the domains.32 The height differences at the edges of the domains demonstrated (32) Hansma, H. G.; Gould, S. A. C.; Hansma, P. K.; Gaub, H. E.; Longo, M. L.; Zasadzinski, J. A. N. Langmuir 1991, 7, 1051, and references therein.
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Figure 2. An AFM image of the surface of a typical domain of LB monolayer of OCAB deposited on mica surface.
variations in the film thickness of 2.1-2.9 nm. These values are smaller than the estimated molecular length of OCAB (∼3.6 nm), indicating a tilting orientation of the molecules in the monolayer. Inside a continuous domain, the surface was found to be corrugated, as shown in Figure 2. The average depth and width of the holes and voids in this figure are approximately 0.6 and 30 nm, respectively. Some of the deep voids have a depth of nearly 1.0 nm. This kind of corrugation could be a result of the strong molecular interactions, as evidenced by the BAM observation described before. The strong attractive interaction and the formation of the H-aggregation is also evidenced in the UV-visible spectra of the monolayer, in which the position of the π-π* absorption exhibits a 37 nm blue shift with respect to that in THF solution.33 The holes and voids observed by the AFM could be a trace of this formation process. (Note that in the LB monolayer deposition, the dipping speed was controlled as low as 2 mm/min to avoid possible effects of the dipping.) Spectroscopic Studies of the Photo-Oligomerization of OCAB in LB Multilayers UV-Visible Spectra. The changes in the UV-visible spectra of a five-layer LB film on CaF2 substrate upon irradiation are presented in Figure 3. Before irradiation, the spectrum exhibits an absorption peak at 296 nm, assigned to the absorption of the conjugated chromophor of the OCAB molecule, similar to other p-phenylenediacrylic acid derivatives.31 Upon irradiation, this absorption band decreases in intensity and also shifts toward a short wavelength (λmax ) 286 nm at 90 min). Meanwhile, an increase in absorption near 200 nm is observed. This kind of spectral behavior can be attributed to the reduction in conjugation length due to the cycloaddition: cyclobutane rings are formed by the olefinic groups.34,35 The GPC analysis of the products after 90 min irradiation showed the existence of dimer, and no products with (33) Kasha, M. Radiat. Res. 1963, 20, 55. (34) Rao, C. N. R. Ultra-violet and Visible Spectroscopy: Chemical Applications; Butter Worths: London, 1967. (35) From UV-visible spectra, it is difficult to distinguish the transcis isomerization. However, we intend to exclude it in the present case for two reasons. First, the closely packed structure of the film provides little free space for the molecules to undergo isomerization. Second, the IR absorbance of ν(CdC) is believed to increase when this isomerization takes place and will result in a kinetics with opposite feature, i.e., first slow because the isomerization should be much faster than oligomerization. The lack of a good isosbestic point in the spectra is considered to be attributed to the experimental error caused by the shift in sample film position in the spectrophotometer because it had to be measured alternatively by UV-visible and IR spectroscopy.
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Figure 3. UV-visible spectra of a five-layer LB film of OCAB on CaF2 substrate upon UV irradiation. Inset: a sketch of the reaction scheme. Scheme 1
higher molecular weight were observed. However, after the irradiated LB film had been sufficiently washed with THF, its UV-visible spectral measurement still showed a remaining absorption of considerable intensity. The shape of this spectrum is similar to that of the film right after the reaction, but the absorbance reduced to about one-third to one-half. The remaining substances are very stable and difficult to remove by ordinary organic solvents. An effective way of removing them is by ozone treatment, after which no absorption could be observed in the spectrum. It is reasonable to consider the remaining insoluble substances to be oligomers having higher molecular weights than the dimer (Scheme 1). It should be noted in the present case that because the amount of sample in the LB films was small, it is difficult to make a precise determination of the molecular structures of the products. Making this determination will require nuclear magnetic resonance spectroscopy and X-ray diffraction analysis. We suppose the product to be cyclobutane dimers and oligomers, based on the possible reaction scheme for OCAB molecules (see the inset in Figure 3), as well as previous work with p-phenylenediacrylic acid derivatives, either in films or in crystallites,
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where dimerization and polymerization through cycloaddition reactions have been clearly verified.31,36,37 Infrared (IR) Spectra. Figure 4 displays the IR spectra of the same five-layer LB film under irradiation in the frequency range 1800-1400 cm-1, as summarized in the following. Photoreactivity. In Figure 4, continuous changes in the absorbance of a number of bands are shown, indicating the occurrence of the cycloaddition reaction. The most obvious indication is with the doublet peaks located at 1636 and 1629 cm-1, assigned to the stretching vibration of the olefinic groups on the ester side (ν(CdC)I) and amide side (ν(CdC)II), respectively. (The assignment of the bands of these two olefinic groups was discussed previously.31) The decrease in the absorbance of these two bands is due to the consumption of olefinic groups by cycloaddition. The absorption of the carbonyl group in ester and amide groups is also a clear indicator of the reaction. As we know, the vibration frequency of the CdO stretching mode is strongly affected by its local chemical environment. When a carbonyl group is conjugated with other π electron systems, its stretching frequency is lowered.38,39 In the current case, either of the carbonyl groups in the ester or amide are conjugated with an olefinic group on each side of the phenylene ring before the reaction occurs. Upon irradiation, the olefinic groups of the two neighbor molecules form a cyclobutane ring, and the conjugation of the former CdC groups is lost. This loss of conjugation will surely shift the frequencies of the CdO group in either the ester or amide groups to higher values. As for the IR spectra, before the reaction occurs, the absorption band of the stretching vibration mode of the carbonyl group in the ester, ν(CdO)ester, is found at 1704 cm-1 and that in the amide group (amide I) at 1665 cm-1. (The absorption band of the shoulder feature located at 1679 cm-1 is assigned to the ν(CdO) mode in the carboxyl group.) As the irradiation proceeds, the band intensity of ν(CdO)ester and amide I decreases significantly, and new absorption bands are observed around 1760-1718 and 1694-1681 cm-1. To resolve these overlapping peaks, we have utilized curve fitting. (In the curve fitting, a mixed-band profile of Gauss and Lorentzian types is suggested initially.) The fitting results show the emergence of two new absorption bands at 1729 and 1692 cm-1, which can be assigned to the stretching mode of the carbonyl group in the ester and amide of the products, denoted as ν(CdO)′ester and amide I′, respectively. Reaction Kinetics. The absorbance of the two olefinic groups versus the time of irradiation is shown in the inset of Figure 4. No remarkable difference can be observed between the kinetics of the two olefinic groups. Interestingly, a two-step characteristic of this kinetics is clearly seen: a faster process before an irradiation time of 8 min and a slower one thereafter. This two-step feature appears to be the effect of structural change in the monolayer during the reaction, based on its correlation with the kinetics of the changes in the molecular environment, as described by the orientation and packing of alkyl chains later. This kind of two-step feature of the reaction kinetics has also been observed in the polymerization reactions in (36) Hasegawa, M.; Suzuki, Y.; Nakanishi, H.; Nakanishi, F. In Progress in Polymer Science Japan; Imahori, K.; Murahashi, S.; Eds.; Kodansha Ltd.: Tokyo, John Wiley & Sons: New York, 1973; Vol. 5, p 143. (37) Nakanishi, F.; Hasegawa, M.; Tasai, T. Polymer 1975, 16, 218. (38) Lin-Vien, D.; Colthup, N. B.; Fatelay, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: Boston, 1991. (39) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Methuen: London, 1958.
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Figure 4. Infrared (IR) spectra of a five-layer LB multilayer of OCAB on CaF2 substrate upon UV irradiation. (A ) absorbance). The time of irradiation for each spectrum is 0.0, 1.0, 2.0, 4.0, 8.0, 12.0, 20.0, 30.0, 40.0, 50.0, 70.0, 90.0 min, respectively. Inset: Absorbance of stretching vibration mode of olefinic groups at the ester side (0) and amide side (O) versus the time of UV irradiation.
other LB films.14,15 In those cases, the analysis of the reaction kinetics along with the temporal behavior of the orientations of the alkyl chains has revealed an accumulation of the repulsive force among the molecules in the monolayers. Molecular Orientation and Molecular Interaction. The vibration modes of amide groups and alkyl groups can be very informative in showing the hydrogen bonding behavior as well as the molecular orientation and packing in the monolayer and are important indicators of the molecular environment during the reaction. Before irradiation, the ν(N-H) exhibits a broad absorption centered at 3318 cm-1, which lies in the frequency region of the hydrogen-bonded secondary amide group with trans configuration.38,39 As the irradiation proceeds, a very sharp peak begins to emerge at 3393 cm-1. This frequency is much higher than those usually considered to be indicative of hydrogen bonding and is near the region to be assigned as “free” amide.38,39 The sharpness of this new absorption (fwhm ) 12 cm-1) also strongly suggests that this band belongs to a weakly hydrogen-bonded amide group. Correspondingly, an obvious change in the amide II band is observed, as illustrated by the band near 1520 cm-1 in Figure 4, in which a continuous decrease at 1531 cm-1 and an increase at 1519 cm-1 are observed. The amide II band is a combination of the deformation vibration of the N-H bond and the stretching vibration of the C-N bond, and it has an opposite tendency of frequency shifting in response to the strength of the hydrogen bonding, compared with stretching modes such as ν(N-H) and amide I band; i.e., its frequency shifts to a lower value as the hydrogen bonding becomes weaker.40 All of these data clearly show that some of the hydrogen bonding between the amide groups begins to break as the reaction proceeds. This phenomenon can be attributed to the changes in the molecular distance, conformation, or orientation that occur with the oligomerization reaction. When the distance between the amide groups is not short enough for hydrogen bonds to form, or if the adjacent amide groups are no longer in suitable mutual positions or orientation, the hydrogen bonding will surely be weakened. Two important features can be seen in the spectra of the methylene. One is the decrease in the absorbance of
Figure 5. Temporal profiles of frequencies of symmetric and antisymmetric stretching vibration of methylene groups, νs(CH2) and νa(CH2), versus the time of UV irradiation. Inset: Absorbance of νs(CH2) versus the time of irradiation.
the νa(CH2) and νs(CH2) modes, and the other is the shift in their frequencies toward higher values. The absorbance of the νs(CH2) mode versus the time of irradiation is shown in the inset of Figure 5, in which a monotonic decrease with a two-step feature is found, similar to the kinetics of the olefinic groups. The transition dipole moment of νs(CH2) lies in the methylene plane and is parallel to the bisector of the H-C-H bond angle and orthogonal to the backbone axis of the alkyl chain. In this case, the alkyl chains are considered to take on an all-trans conformation, (40) Zhang, J.; Zhao, J.; Zhang, H. L.; Li, H. L.; Liu, Z. F. Chem. Phys. Lett. 1997, 271, 90.
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as evidenced by the low frequencies of the methylene absorption, 2918 cm-1 for νa(CH2) and 2850 cm-1 for νs(CH2).41-43 Because the electric vector of the IR probe beam forms a 15° angle with respect to the plane of the LB film’s surface, this decrease in the absorbance of νs(CH2) indicates a tilting of the alkyl chain away from the beam direction. As revealed by the AFM observation, which shows a monolayer thickness of 2.1-2.9 nm, the molecules tilt with respect to the surface normal at an average tilting angle much larger than 15°. Therefore, the decrease in the νs(CH2) absorption indicates a further tilting of the alkyl chain toward the surface plane. Regarding the kinetic behavior, a changing point from a faster decay to a slower one can be found around 8 min of irradiation. In addition to the orientation of the alkyl chains, the frequencies of the C-H stretching vibration of the methylene group can provide information regarding the packing status of the alkyl chains. Upon irradiation, shifts in the frequencies of νa(CH2) and νs(CH2) toward high values were observed. In addition, monotonic increases in these frequencies versus the time of irradiation can be clearly seen (Figure 5). The increases in the frequencies of these two methylene modes indicate a change in the alkyl chain conformation away from the former all-trans one, reflecting a change in the packing status toward a loosely packed and less ordered one. Another interesting characteristic of the frequency shift lies in the kinetics, which shows a two-step feature: a faster one before 8-10 min of irradiation and a slower one thereafter. Photoreaction Behavior. On the basis of the data described above, photo-oligomerization (including dimerization) proceeds most probably through cycloaddition. This reaction brings about considerable changes in the LB film structure, including the breaking of hydrogen bonds and a distortion of the alkyl chains. Most importantly, the data reveal a correlation between the dynamic features of the spectral behaviors of different groups as well as a correlation between the dynamics in their different spectral characteristics: the two-step kinetics of the absorbance of ν(CdC)I&II and the absorbance and frequencies of methylene vibrations. These correlations suggest a very close relationship between the reaction kinetics and the kinetics of the changes in the molecular microenvironment. The above phenomena can be attributed to the big difference in the molecular conformation between the monomer and the dimer or oligomer, as sketched in Figure 3. The OCAB molecule has a planar conjugation. When the former sp2 hybridization of the carbon atoms in the CdC group is changed to sp3 by cycloaddition, this planar structure is changed to an out-stretching structure of the cyclobutane products. Also, at the same time, the two neighboring monomer molecules which were formerly separated at about 4 Å are pulled together by the cyclobutane ring at a covalent distance of ∼1.5 Å. In other words, both molecular conformational changes and lateral (41) Snyder R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta 1978, 34A, 395. (42) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (43) Naselli, C.; Rabolt, J. F.; Swalen, J. D. J. Chem. Phys. 1985, 82, 2136. (44) The effect of the heat from the light source could be neglected because the distance between the sample film and the output window of the light source was long (∼85 cm), at which point the irradiation intensity was measured to be low (∼2 mW/cm2 at 313 nm). (Description of the method: Heller, H. G.; Langan, J. R. J. Chem. Soc., Perkin 2 1981, 341.)
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movements occur during the reaction, causing a modification of the intermolecular relationship. As is known, the formation of hydrogen bonds is highly directional, depending especially on the molecular distance and mutual orientations. If the molecules are moved away from their former optimum position or orientation, the hydrogen bonding will be weakened or broken. In the current case, it can be seen in the IR spectra that the amide groups being very weakly hydrogen bonded emerge as the reaction proceeds. Regarding the packing state of the alkyl chains, although the molecules are pulled together by the cyclobutane rings, a distortion in the packing of the alkyl chains results because they are forced to tilt away from each other by the conformation change from olefinic groups to a cyclobutane ring. In this case, the alkyl chains cannot be packed in an ordered state as before, and they move away from each other, taking on a more tilting orientation toward the surface plane. This kind of mutual movement and orientational changes will surely destroy the formerly ordered states. The two-step feature of the reaction kinetics can also be attributed to the molecular environment change induced by the reaction. Referring to the kinetics data of olefinic groups shown in Figure 4, the slowing down of the reaction kinetics after 8 min implies that the changes in the monolayer structure induced by the reaction are toward a state that makes conditions less suitable for the reaction to proceed. This is because the changes in the reacted molecules affect the unreacted ones, in changing either their mutual distance, orientation, or position and influencing the reaction kinetics.44 Conclusion 4-(4-(2-(Octadecyloxycarbonyl)vinyl)cinnamoylamino)benzoic acid is capable of forming a stable monolayer on a water surface at room temperature, and its photoreactive LB films can be successfully fabricated on substrates such as fresh-cleaved mica and hydrophilic CaF2 plates. By UV irradiation, oligomerization (including dimerization) through cycloaddition of the olefinic group can be successfully carried out. The breaking of hydrogen bonds and the distortion of alkyl chains were observed during the reaction by IR spectroscopy. More importantly, correlative two-step temporal profiles were found in the absorbance of olefinic groups as well as in the absorbance and frequency shifts of the stretching vibration of the methylene group. These results reflect the strong dependence of the reaction kinetics on the monolayer structure, or more specifically, the molecular environment, which includes molecular mutual packing, orientation, and interactions. The drastic change in the molecular conformation appears to be the cause of these phenomena, resulting in both a change in the shape of the molecules and in the lateral translational movement and, further, causing a shift in the optimum conditions for the oligomerization. Acknowledgment. Financial support from STA (Science and Technology Agency) Postdoctoral Fellowship is appreciated. The authors would like to express their thanks to M. Higuchi, M. Matsumoto, J. Nagasawa, K. Tamada, S. Yokokawa, and M. Yoshida of National Institute of Materials and Chemical Research for their kind help in carrying out experiments. J.Z. would like to thank Prof. J. Umemura of Kyoto University for his helpful discussion. LA9908236
