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Photoinduced Dimerization of a p-Phenylenediacrylic Acid Derivative in a Langmuir Monolayer Mixed with Stearic-d35 Acid on a Water Surface Jiang Zhao,†,‡ Koji Abe,† Haruhisa Akiyama,† 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 October 16, 1998. In Final Form: January 26, 1999 Photoinduced cycloaddition of a p-phenylenediacrylic acid derivative, 4-(4-(2-(decyloxycarbonyl)vinyl)cinnamoylamino)benzoic acid, was conducted successfully in a mixed Langmuir monolayer with stearicd35 acid on a water surface at room temperature. The reaction process was investigated by UV-visible and infrared spectroscopy, gel permeation chromatography (GPC), and Brewster-angle microscopy (BAM). The product was identified as the cyclobutane dimer. The formation of the stable monolayer and the dimerization reaction on the water surface were evaluated at the molecular level by observation of the monolayer’s behavior (surface pressure, monolayer area, and change of surface morphology). The BAM observation and GPC analysis of the photoproducts obtained under different states of the monolayer reveal that the molecular arrangement with higher mobility gives a much higher reaction speed.
Introduction Photoinduced polymerization in ultrathin organized films has been attracting continuous research interest because it is believed to have prospective applications in fabrication of molecular electronic and photonic devices, surface engineering, and ultrahigh-density information storage, etc.1-14 In these processes, well organized molecular systems, such as Langmuir monolayers,3,8,17,18 Langmuir-Blodgett (LB) films,4-13 and self-assembled * Corresponding author. Phone: +81-298-54-4671. Fax: +81298-54-4673. 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; p 176-191. (2) Lando, J. B.; Fort, T., Jr. In Polymerization of Organized Systems; Elias, H.-G., Ed.; Gordon and Breach: New York, 1977; p 63-78. (3) Neagele, D.; Ringsdorf, H. In Polymerization of Organized Systems; Elias, H.-G., Ed.; Gordon and Breach: New York, 1977; p 79-88. (4) Laschewky, A.; Ringsdorf, H.; Schmidt, G. Thin Solid Films 1985, 134, 153. (5) Laschewky, A.; Ringsdorf, H. Macromolecules 1988, 21, 1936. (6) Neagele, D.; Lando, J. B.; Ringsdorf, H. Macromolecules 1977, 10, 1339. (7) Cemel, A.; Fort, T., Jr.; Lando, J. B. J. Polym. Sci., Part A-1 1972, 10, 2061. (8) Fukuda, K.; Shibasaki, Y.; Nakahara, H. J. Macromol. Sci., Chem. 1981, A15, 999. (9) Tanaka, Y.; Nakayama, K.; Iijima, S.; Shimizu, T.; Maitani, Y. Thin Solid Films 1985, 133, 165. (10) Kuriyama, K.; Kikuchi, H.; Kajiyama, T. Langmuir 1996, 12, 6468. (11) Saito, A.; Urai, Y.; Itoh, K. Langmuir 1996, 12, 3938. (12) Yamamoto, M.; Wajima, T.; Kameyama, A.; Itoh, K. J. Phys. Chem. 1992, 96, 10365. (13) Saito, A.; Wajima, T.; Yamamoto, M.; Itoh, K. Langmuir 1995, 11, 1277. (14) Kim, T.; Ye, Q.; Sun, L.; Chan, K. C.; Crooks, R. M. Langmuir 1996, 12, 6065. (15) Savion, Z.; Wernick, D. L. J. Org. Chem. 1993, 58, 2424 and references therein. (16) Schmidt, G. M. J. Pure Appl. Chem. 1971, 27, 647. (17) Rabe, J. P.; Rabolt, J. F.; Brown, C. A.; Swalen, J. D. Thin Solid Films 1985, 133, 153. (18) Beredjick, N.; Burlant, W. J. J. Polym. Sci., Part A-1 1970, 8, 2807.
monolayers (SAM)14 made of monomer molecules are first fabricated. Afterward, polymerization proceeds upon irradiation. The polymerization process can help stabilize the monolayer structure since it makes the individual molecules connect each other together through chemical bonds. On the other hand, the molecular conformation also changes largely during the reaction because of the drastic changes in the electron orbital hybridization of the reaction center atoms. In the case of dimerization of olefinic groups, a hybridization change of sp2 to sp3 happens. This change in molecular conformation can alter the local molecular environment and will also exert some effect on the monolayer structure. Since the dimerization in the solid state depends largely on crystallite lattice structure,15,16 the monolayer structure change induced by reaction will also have its effects on the reaction itself. Studies on this question can provide useful information in building suitable molecular monolayer reaction systems. The Langmuir monolayer at the air-water interface is a good model system to make an investigation on this aspect, because the macroscopic parameters, such as surface pressure (π) and monolayer area (A), which provide important information on the microscopic status of the molecules at the interface, can be easily measured. Previously, it has been reported that both expansion and shrinkage of the Langmuir monolayers were observed during the polymerization of several systems.3,17,18 In our previous work on polymerization of p-phenylenediacrylic acid (p-PDA) monododecyl ester on a water surface, a large increase in either surface pressure or monolayer area was observed.19 In this paper, we report our study on the photoinduced cycloaddition of a p-phenylenediacrylic acid derivative, 4-(4-(2-(decyloxycarbonyl)vinyl)cinnamoylamino)benzoic acid in the Langmuir monolayer on a water surface (abbreviated as DCAB, one of its possible structures is shown by Chart 1). In experiments, a mixed system of DCAB and stearic-d35 acid is chosen to prepare a stable monolayer, and the effects of the reaction on the surface pressure and monolayer area were studied. A Brewster(19) Nakanishi, F. J. Polym. Sci., Part C: Polym. Lett. 1988, 26, 159.
10.1021/la9814539 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/05/1999
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Chart 1
angle microscope (BAM) was adopted to study the morphological changes of the monolayer, and the photoproducts obtained under fixed surface pressure and at fixed monolayer area were analyzed by UV-visible spectroscopy, infrared spectroscopy, and gel permeation chromatography (GPC). A relationship between molecular environment and reaction kinetics is revealed both by BAM observation and GPC analysis. Experimental Section 4-(4-(2-(decyloxycarbonyl)vinyl)cinnamoylamino)benzoic acid (DCAB) was synthesized by reaction of the acid chloride of p-PDA monodecyl ester with p-amino benzoic acid and purified by reprecipitation of it from dimethyl sulfoxide (DMSO) solution with water. Stearic-d35 acid, CD3(CD2)16COOH, was purchased from Aldrich. DCAB was dissolved in a mixed solvent of tetrahydrofunan (THF) and benzene (1:4 by volume) at a concentration of 1.0 mM, and stearic-d35 acid in chloroform (1.0 mM). The Langmuir monolayer experiment was conducted with a commercial trough (Lauda Film Balance, Germany). In this experiment, first deionized and then distilled water was adopted as the subphase and its temperature was 22 ( 0.3 °C. When the sample solution was spread on the water surface, the compression of the monolayer started 10-30 min later, with a compression ratio of ∼14 cm2/min. After a stable monolayer had been formed on the water surface, UV irradiation was conducted with a highpressure 400 W Hg lamp (Toshiba, Japan), equipped with a glass filter to cut off radiation below 300 nm. The whole area of the water surface could be irradiated by the lamp. After irradiation, the product was collected from the water surface with a clean glass slide. The products were dried and then dissolved in THF. Their molecular weight was determined by GPC (Shimadzu C-R2AX, Japan) using the calibration curve of standard polystyrene, and UV-visible spectra were measured by a Shimadzu UV-2500PC spectrophotometer. The product THF solution was also deposited and dried on CaF2 substrate in order to have its infrared (IR) spectrum measured. The IR measurement was done with a Perkin-Elmer System 2000 FTIR spectrometer equipped with a MCT detector. The surface morphology observation of the Langmuir monolayer on water was done by a Brewster-angle microscope (BAM) (NL-EMM633-KS, Nippon Laser & Electronics Lab., Japan) on a trough system (NL-LB240S-MWC, Nippon Laser & Electronics Lab., Japan). The BAM was equipped with a Helium-Neon laser (633 nm), and the images were recorded by a CCD camera and an image recording and processing system. In each experiment, the polarization of the analyzer and the incident laser beam were adjusted to be p-polarization. To irradiate the spot under observation on the water surface, a 200 W Mercury and Xenon lamp with a fiber light guide was used (SUNCURE 202, Asahi Glass Company, Japan).
Results and Discussion Langmuir Monolayer Behavior. To evaluate the behavior of a DCAB molecule on the water surface, its π-A (surface pressure-molecular area) isotherm was measured, as shown by the dashed line in Figure 1. Although the surface pressure begins to rise at large molecular area (∼0.40 nm2), it is difficult for this molecule to form a stable monolayer on the water surface. At both relatively low and high pressures (4.6 and 13.8 mN/m, respectively), the film’s area could not be retained and the barrier moved all the way to the position which corresponds to a molecular area of less than 0.2 nm2, where
Figure 1. π-A (surface pressure-molecular area) isotherm at 20 °C on water surface: DCAB (dashed line); DCAB + stearicd35 acid (solid line). 1 and 2 denote the fixed pressure adopted in the experiment, 7.4 and 13.8 mN/m, respectively.
the film on the water surface is no longer a monolayer but a multilayer. To verify this, one layer of the surface film at this stage was transferred onto freshly cleaved mica by the vertical deposition method and an atomic force microscope (AFM) observation was done. The film’s thickness is found to vary from 6 to 12 nm, which is much larger than the molecular length of DCAB (∼3 nm), indicating a multilayer structure of the film transferred and also the film on the water surface. To improve the stability of the monolayer, it might be possible to introduce into this system another kind of molecule which has the capacity to form a stable monolayer, and stabilization can be achieved through the molecular interaction between the two kinds of molecules. In this effort, aliphatic acid should be a good choice and the hydrophobic interaction of the alkyl groups may play an important role in the monolayer stabilization. In this study, stearic-d35 acid is adopted, which also will not interfere with DCAB in both infrared and UV-visible spectra. In experiment, the two kinds of solutions were mixed with a volume ratio of 3:1 and, consequently, a mole ratio of 3:1 (stearic-d35 acid/DCAB). The π-A isotherm of the mixed monolayer is also shown in Figure 1. It consists of three stages, stage I (A > 0.25 nm2), stage II (0.20 nm2 < A < 0.25 nm2), and stage III (A < 0.20 nm2). Judging from the molecular area, it is possible that monolayer structure exists both in stage I and II.20 However, in these two stages, the molecular area value on the π-A curve could not be retained at fixed values of surface pressure. In this experiment, the trough was set to compress to a fixed surface pressure of 7.4 mN/m (in stage I, denoted as 1 in Figure 1) and 13.8 mN/m (in stage II, denoted as 2), respectively. In both cases, the moving barrier could not stop at the point on the π-A curve. When the pressure was set at 13.8 mN/m, the barrier stopped at a point corresponding to a mean molecular area of 0.16 nm2, which is almost equal to that of stearic-d35 acid.21 (20) Upon dynamic compression and expansion of the surface film (no stabilization is conducted), the isotherm below the surface pressure of 1 is reversible while that below 2 is not. However, the hysteresis below 2 is reversible (not shown in Figure 1). This means that, during this dynamic process, a monolayer on water surface persists. (21) Kawai, T.; Umemura, J.; Takenaka, T. Chem. Phys. Lett. 1989, 162, 243.
Dimerization of p-Phenylenediacrylic Acid Derivative
Therefore, at this point, the film on the water surface is not a monolayer. Taking into account the fact that stearic acid is much more stable than the DCAB molecule on the water surface, it is reasonable to judge that, on the water surface, a stable monolayer of stearic-d35 acid is formed and the DCAB molecule has been squeezed out of the monolayer.22 On the other hand, when the surface pressure was set at 7.4 mN/m, the barrier stopped finally (about 2 h later) at the point corresponding to a mean molecular area of 0.23 nm2, in which case the corresponding partial molecular area of DCAB is estimated as 0.26 nm2, adopting the molecular area of stearic-d35 acid as 0.22 nm2. This indicates that a stable monolayer of stearic-d35 acid and DCAB molecules is formed successfully at a suitable surface pressure. When the surface film (either at 7.4 mN/m or 13.8 mN/ m) had become stable, UV irradiation was carried out. (In these cases, the trough was kept under constant surface pressure.) The behaviors of the surface films on UV irradiation at these two surface pressures were quite different, reflecting the difference in structure of these surface films. At 13.8 mN/m, no change in either surface pressure or monolayer area was observed, while at 7.4 mN/m, upon 10 min of irradiation, first an increase in surface pressure (∼0.4 mN/m) was seen and, afterward, an expansion of the monolayer happened. Ideally, the monolayer area should increase right after an increase in surface pressure is detected. However, a delay in the feedback of the trough system results in the observation of considerable increase in surface pressure before the expansion of the monolayer. The increase in monolayer area is around 4%. Since the stearic-d35 acid monolayer is stable under irradiation, this expansion is attributed only to the reacted DCAB molecules, and therefore, about 16% increase in partial molecular area of DCAB is estimated. The olefinic groups in the DCAB molecule will undergo a dimerization reaction when irradiated by light with a wavelength longer than 300 nm (verified by the GPC and spectral data described later). The fact that the dimerization happens indicates that these two kinds of molecules do not mix with each other homogeneously and, instead, form separated domains consisting of DCAB and stearicd35 acid themselves in the monolayer, similar to the case reported on other mixed monolayers of conjugated and simple alkyl amphiphilic molecules.23,24 As we know, the carbon atom in the olefinic group bears sp2 hybridization and the two olefinic groups form, with the phenylene groups, a planar conjugation part of the DCAB molecule. When the dimerization reaction occurs, two olefinic groups of two adjacent molecules form a cyclobutane ring and the hybridization of the carbon atom changes to sp3. In this case, the former planar structure no longer exists. When the cycloaddition reaction occurs, two neighbor molecules are connected together by the cyclobutane ring and a new dimer molecule is formed. This dimer molecule has an extended structure, with a cyclobutane ring in the center and the conjugated phenylene group, amide group, as well as the alkyl chains extending outward, as illustrated schematically in Figure 2. It should be noted that, before reaction, the intermolecular distance is relatively large (∼4 Å). When the cyclobutane ring is formed, the distance is reduced to ∼1.5 (22) A similar example: Kawabata, Y.; Sekiguchi, T.; Tanaka, M.; Nakamura, T.; Komizu, H.; Honda, K.; Manda, E. J. Am. Chem. Soc. 1985, 107, 5270. (23) Gaines, G. L., Jr.; Mellamy, W. D.; Tweet, A. G. J. Chem. Phys. 1964, 41, 538. (24) Era, M.; Tsutsui, T.; Saito, S. Langmuir 1989, 5, 1410.
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Figure 2. Schematic illustration of the photodimerization of DCAB in its monolayer.
Å.15,16 Since DCAB molecules have a large conjugation part, although the dimerization draws two molecules closely together, the extending dimer structure results in a larger molecular area of the dimer molecule than that of the two formerly individual monomer molecules. Since, in the monolayer, the molecules are closely packed, this extension in molecular structure will exert a force to the surrounding molecules. In the case of the Langmuir monolayer, this extension brings about the increase in surface pressure and thereafter an increase in film area when a constant surface pressure is maintained. This can be the reason an expansion in monolayer area was observed under irradiation.25 As for the case of 13.8 mN/m, since the photoactive DCAB molecules are not inside the monolayer, the reaction and its resulting molecular conformation change cannot exert any effect on the monolayer, and therefore, no changes in surface pressure and monolayer area can be observed. A different behavior of the monolayer was observed under irradiation when a surface pressure of 7.4 mN/m had just been reached (before the monolayer is stabilized, or namely “as-formed” monolayer), in which case, a shrinkage of the monolayer area and a decrease in surface pressure were found upon irradiation. According to the π-A isotherm, the partial molecular area for DCAB is about 0.38 nm2 for the monolayer at this stage. In this case, the average surface molecular density is lower than the stabilized monolayer. Regarding the monolayer structure, either a larger intermolecular distance or a more tiling molecular orientation can be applied to the “asformed” monolayer. Therefore, a shrinkage of the monolayer upon irradiation is found in this case, indicating a smaller molecular area occupied by the dimer molecule than by the two formerly unreacted molecules. This may result from either a decrease in molecular distance or a standup of the molecules due to the reaction. Brewster-Angle Microscope Observation. The typical Brewster-angle microscope (BAM) images of the mixed monolayer are shown in Figure 3a-d. The dimensions of (25) An increase of less than 0.3 °C in temperature was recorded during the irradiation, which cannot contribute much to the area expansion of the monolayer. This can also be verified by the fact that the increased area still held when the temperature fell down.
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Figure 4. Brewster-angle microscope (BAM) images of the Langmuir monolayer of stearic-d35 acid + DCAB (3:1) on the water surface at ∼22 °C right after 7.4 mN/m is reached: (a) before irradiation; (b) after irradiation.
Figure 3. Brewster-angle microscope (BAM) images of the Langmuir monolayer of stearic-d35 acid + DCAB (3:1) on the water surface at ∼22 °C: (a) right after; (b) 1 h after; (c) 2 h after 7.4 mN/m is reached; (d) image of the mixed film stabilized at 13.8 mN/m. The dimension of the images are 0.65 × 0.9 mm2.
these images are approximately 0.65 × 0.90 mm, and in each image, the central part of the field of view is in good focus. In Figure 3a, when surface pressure of 7.4 mN/m has just been reached, the feature of the monolayer is not very clear due to the low reflectivity. As time goes on, the reflectivity of the surface becomes higher and the feature of the surface is clearly shown; it consists of a dark background and many small bright spots randomly distributed. An increase of brightness of the separated spots is observed as the film is stabilized (parts b and c of Figure 3). Comparing with the BAM image of the stearicd35 acid monolayer, which has a much lower reflectivity (not shown here), the bright spots can be assigned to the domains formed by DCAB molecules. From the appearance, two possibilities exist regarding to the structure of the spots, condensed domains or three-dimensional crystallites of DCAB. The former shows a monolayer formed by the two kinds of molecules, stearic-d35 acid and DCAB, while the later indicates a collapse of the surface film. According to the behavior of the monolayer in its π and A parameters described above, a monolayer is possibly formed under surface pressure of 7.4 mN/m, while a collapse of DCAB molecules is surely in the case of the pressure of 13.8 mN/m. An image of the stable surface film at 13.8 mN/m is shown in Figure 3d, in which very tiny spots with medium brightness are found. These tiny spots can be assigned to the structure formed by DCAB molecules, which can be either the three-dimensional crystallites of DCAB or the multilayer structure of DCAB, which have been squeezed out of the monolayer formed by stearic-d35 acid only. Comparing parts c and d of Figure 3, it is seen that the brightness of the spots of the stable
film under 7.4 mN/m is much higher than that of the collapsed one under 13.8 mN/m, indicating a much higher molecular density of the spots in the former case than the three-dimensional crystallite in the later one. This can be understood as that the assemblies of DCAB molecules are under the surface pressure at 7.4 mN/m while those at 13.8 mN/m are free from pressure because they are squeezed out of the monolayer. Also, a difference in their size can clearly be seen. Taking into account the differences in these two cases, the three-dimensional crystallite origin of the bright spots under 7.4 mN/m can be ruled out, and they are assigned to be condensed domains formed by DCAB molecules. Therefore, the BAM observations show that a monolayer is successfully formed by DCAB and stearic-d35 acid at surface pressure of 7.4 mN/m, and these two kinds of molecules form separated domains inside this monolayer. When the monolayer has been stabilized at 7.4 mN/m, UV irradiation is introduced. Although the monolayer is found to expand under irradiation, no remarkable change in monolayer morphology can be observed through BAM. On the other hand, in the case of monolayer formed right after 7.4 mN/m has been reached (the “as-formed” monolayer), a big change in surface morphology is found upon irradiation. As shown in Figure 4a, right after 7.4 mN/m is reached, the monolayer morphology is similar to that shown in Figure 3a, in which small spots with medium brightness are observed. Upon irradiation, these mediumbright spots disappear and the whole image turns very dark, with only several small bright spots remaining (Figure 4b). Referring to the principle of the Brewsterangle microscope,26-28 this decrease in reflectivity is due to the decrease in refractive index in the monolayer, which is originated from the occurrence of the cycloaddition reaction. As we know, when two olefinic groups form a cyclobutane ring, the conjugation of the former individual molecules will be lost partially. This change in status of conjugation influences the macroscopic refractive index of the molecular assembly largely. According to the previous intensive research on the relationship between the refractive index and the molecular structure of a conjugated molecular system, the linear molecular polarizability (R), which is directly related to the refractive index (n), strongly depend on the length of the conjugation (L), and a relation of R ∝ L3 is revealed for linear conjugated (26) Born, M.; Wolf, E. Principles of Optics, 3rd ed.; Pergamon Press: Oxford, 1965; Chapters 1 and 2. (27) He´non, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936. (28) Ho¨nig, D.; Mo¨bius, D. Thin Solid Films 1992, 210/211, 64.
Dimerization of p-Phenylenediacrylic Acid Derivative
molecules.29,30 Obviously, a decrease in the refractive index will result from the decrease in conjugation length of the molecules. In the present case, partial loss of conjugation happens when the cycloaddition reaction takes place, and consequently, the refractive index value of the DCAB molecular assemblies is reduced. During the morphological observation, the reflectivity of the DCAB domains to the incident laser beam of BAM is reduced at the occurrence of the reaction. Depending on the morphological observation, information on monolayer structure and reaction kinetics is obtained. The fact that the change in surface morphology in the case of the “as-formed monolayer” is much more remarkable than that of the stabilized monolayer indicates a much faster reaction speed in the former case. This feature suggests a strong dependence of cycloaddition reaction kinetics on the monolayer structure. Since molecular density is lower in the case of the “as-formed” monolayer, the above feature shows that a higher reaction speed is associated with the more loosely packed structure. This is also in agreement with the results of the GPC analysis described later. UV-Visible Spectroscopy. The UV-visible spectra of the products after 2.5 min and 10 min irradiation at constant monolayer area are shown in parts b and c of Figure 5. Comparing with the spectra of DCAB solution (Figure 5a), the absorption around 330 nm is attributed to the unreacted DCAB molecules. After irradiation, a new absorption peak appears at 270 nm. This spectroscopic feature can be understood as a reduction in conjugation length by cycloaddition of olefinic groups at either the ester side or amide side, which results in a blue shift of the chromophor absorption.31 Therefore, this new absorption is assigned to the absorption of the cyclobutane dimer produced by the reaction. The conversion of dimer estimated from the absorption intensities at 270 and 330 nm is about 60-70% at 10.0 min of irradiation. In this determination, the value of the extinction coefficient () of the cyclobutane dimer is estimated from our results on photodimerization of another p-phenylenediacrylic acid derivative compound, 4-(4-(2-ethyloxycarbonyl)vinyl)cinnamoylamino)benzoic acid, in the crystalline state, in which case the separation of the dimer and monomer and thereafter its spectral measurement is successfully carried out. The value of the dimer is 7 × 104 at 270 nm, close to that of the monomer (6 × 104 at 330 nm). The polymerization, including dimerization, of p-phenylenediacrylic derivatives through cycloaddition reactions has been studied intensively in the crystalline state previously.32,33 In those works, the cycloaddition process was examined sufficiently by spectroscopic and X-ray techniques. In the present case of the monolayer reaction system, it should be important to have an exact determination of the molecular structure and conformation of (29) Nicoud, J. F.; Twieg, R. J. In Nonlinear Optical Properties of Organic Molecules and Crystals; Chemla, D. S., Zyss, J., Eds.; Academic Press: Orlando, 1987; Vol. 1, p 227 and references therein. (30) As examples of some cyclic molecules, the refractive index of cyclohexane, cyclohexene, and benzene are found to be 1.4264, 1.4428, and 1.50108 at 589 nm, respectively. The Merck Index, 9th ed.; Windholz, M., Budavari, S., Stroumtsos, L. Y., Fertig, M. N., Eds.; Merck & CO.: Rahway, 1976; p 138, 356. The values of cyclohexane and benzene are 1.427 and 1.501 at 633 nm, respectively. Garito, A. F.; Kuzyk, M. G. In CRC Handbook of Laser Science and Technology, Supplement 2: Optical Materials; Weber, M. J., Ed.; CRC Press: Boca Raton, 1995; p 289. (31) Rao, C. N. R. Ultra-violet and Visible Spectroscopy: Chemical Applications; Butter Worths: London, 1967. (32) 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. (33) Nakanishi, F.; Hasegawa, M. Polymer 1975, 16, 218.
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Figure 5. UV-visible spectra of THF solution of DCAB (a) and products of irradiation at constant monolayer area for 2.5 and 10.0 min ((b) and (c)).
the photoproduct by methods of X-ray diffraction and nuclear magnetic resonance (NMR) spectroscopy. However, the fact that the amount of sample in the Langmuir monolayer is very small (∼28 µg for a 100% collection of product from the water surface) makes these characterizations very difficult. Besides the spectroscopic characterizations used (UV-visible and infrared spectroscopy, gel permeation chromatography (GPC), as described later in this paper), another experiment was adopted to prove the formation of the cyclobutane ring. That is the photocleavage of cyclobutane rings in solution. For example, diethyl R-truxillate acid, which is a typical dimer with a cyclobutane ring, is cleaved into ethyl cinnamate on irradiation.33 In the present experiment, after UVvisible measurement, the THF solution of product after 10 min irradiation (the case of Figure 5c) was irradiated by UV light centered at around 280 nm from a monochromator. During this process, a monotonic decrease in absorbance of the dimer (∼270 nm) and an increase of the monomer (∼330 nm) were observed. Further investigation on the product’s distribution by GPC clearly showed a drastic increase in the relative ratio between monomer and dimer (monomer/dimer); that is, by irradiation around 280 nm, the dimer in the products was converted back to monomer, showing the occurrence of the dedimerization reaction.32,33 This typical property of cyclobutane dimer also verifies the cycloaddition reaction in the Langmuir monolayer. Infrared (IR) Spectroscopy. Figure 6a is the IR spectra of the DCAB molecule in a KBr pellet, and Figure 6b is the spectra of products of the mixed monolayer on the water surface under irradiation of 10 min at constant monolayer area and under constant surface pressure,
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Figure 6. Infrared spectra of DCAB in a KBr pellet before irradiation (a) and products of mixed stearic-d35 acid + DCAB (3:1) monolayer irradiated at 7.4 mN/m for 10 min (b) ((A) at constant monolayer area, (B) under constant surface pressure). Inset in (a), the magnified spectra for ν(CdC)I (1636 cm-1) and ν(CdC)II (1627 cm-1).
respectively. The band assignments are summarized in Table 1. The assignment of the spectra, especially to distinguish the two olefinic groups, is done by comparing the spectra of DCAB with those of similar but different compounds synthesized in the laboratory, such as didecyl p-phenylene diacrylate (ν(CdC) at 1636 cm-1), monodecyl p-phenylene diacrylate (ν(CdC) at both 1637 and 1627 cm-1), and p-phenylene diacrylic acid (ν(CdC) at 1627 cm-1). It can be seen that, upon irradiation, the absorption of the stretching vibration mode of CdC double bonds at both the ester side and amide side (ν(CdC)I (1636 cm-1) and ν(CdC)II (1627 cm-1), respectively) decreases drastically and a strong absorption appears at 1305 cm-1, corresponding to the absorption of the bending vibration
of the CH group in the cyclobutane ring, γ(CH).34-37 Therefore, these IR spectra provide some evidence of the cyclobutane dimer formation in the reaction and also that both of the two olefinic groups undergo the cycloaddition reaction. Gel Permeation Chromatography (GPC). Figure 7 shows the results of the GPC analysis of the products at (34) 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. (35) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Methuen: London, 1958. (36) Annamalai, A.; Keiderling, T. A. J. Mol. Spectrosc. 1985, 109, 46.
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Table 1. Assignment of IR Absorption Main Bandsa DCAB wavenumber 3319 2955 2921 2851 1702 1679 1663 1636 1627 1607 1598 1529 1467 1426 1407 1323 1304 1249 1200 1171
(cm-1)
product assignment ν(N-H) νa(CH3) νa(CH2) νs(CH2) ν(CdO)ester ν(CdO)carboxyl amide I ν(CdC)I ν(CdC)II benzene ring stretching benzene ring stretching amide II δ(CH2) δ(OH, ip) benzene ring stretching γs(CHdCH) γa(CHdCH) amide III ν(C-O)ester ν(C-O)ester
wavenumber 3334 2955 2920 2850 2191 2086 1692 1602 1567 1529 1462 1433 1405 1366 1353 1305 1250 1166 1091 1086
(cm-1)
assignment ν(N-H) νa(CH3) νa(CH2) νs(CH2) νa(CD2) νs(CD2) ν(CdO) combinationb benzene ring stretching benzene ring stretching amide II δ(CH2) δ(OH, ip) benzene ring stretching δs(CH3) ν(phen-N) δ(CH)cyclobutane ring amide III ν(C-O)ester δ(CD2) δ(CD2)
a ν, stretching vibration; δ, bending; γ, rocking; a, asymmetric; s, symmetric. ν(CdC) and ν(CdC) , stretching vibration of olefinic group I II at ester side and amide side, respectively. b When the cycloaddition reaction occurs, the conjugation loss causes the vibration frequency of the carbonyl group in ester and amide to shift to higher frequencies. The combination of stretching vibration modes of these carbonyl groups results in a broad absorption in the region of 1790-1640 cm-1.
Figure 7. GPC results of products of mixed stearic-d35 acid + DCAB (3:1) monolayer irradiated under constant surface pressure for 2.5 (a), 5.0 (b), 10.0 min (c) and at constant monolayer area for 2.5 (d), 5.0 (e), and 10.0 min (f).
irradiation times of 2.5, 5.0, and 10.0 min, at constant monolayer area and under constant surface pressure, respectively. In experiment, the GPC probe wavelength (37) After irradiation, the absorbances of γs(CHdCH) at 1319-1323 and γa(CHdCH) at 1301-1304 cm-1 decrease, making the emergence of the new band of δ(CH) of the cyclobutane ring be very predominant in this vicinity. The relative absorption intensity of δ(CH)cyclobutane ring increases further and becomes quite unique around 1305-1307 cm-1 in the case of higher dimer yield. These features makes the assignment of δ(CH) quite confident even though the difference between frequencies of δ(CH) and γa(CHdCH) modes is around the limitation of resolution of the IR instrument.
was set to be 270 nm. In these data, both dimer and monomer can be observed, which are represented by the peaks appearing at retention time of 20.5 and 21.0 min, respectively. In both cases (constant π and constant A), the relative ratio between the dimer and monomer increases with the irradiation time, showing the progress of the dimerization reaction. This ratio appears to be similar in the two cases of constant surface area and constant surface pressure. The similarity of GPC data between these two cases suggests that the monolayer structures and the molecular
2550 Langmuir, Vol. 15, No. 7, 1999
environment during reaction process in these two cases do not differ very much; otherwise, a big difference in the dimer/monomer ratio should be observed, as deduced from the fact that the dimerization process in the solid state depends largely on the crystallite structure.16 Under constant surface pressure, the monolayer experiences an expansion when the reaction goes on. Actually, this expansion is a process for the molecules to make some rearrangement, either in their own orientation or in their mutual position. When the reaction happens, the extending of the dimer molecules exerts a force in the monolayer, and consequently, results in an increase in surface pressure. However, any increase in surface pressure is compensated by the expansion of the monolayer. Therefore, the structure of the two-dimensional assembly formed by the unreacted molecules is expected to be the same as or very similar to that before irradiation, and the reaction kinetics will not change much at different stages of irradiation. In the case of constant monolayer area, an increase in surface pressure (∼2.0 mN/m) is observed upon 10 min of irradiation. Since the monolayer area is fixed, there is less free space for the molecules to make rearrangement, compared with the case of constant surface pressure. However, the GPC data which show a similar kinetics of reaction suggest that, in this case, the monolayer structure still retains under the increased surface pressure and does not differ very much from the case of constant surface pressure. It is very likely that the stearicd35 acid molecules stabilize the monolayer, and a 2 mN/m increase in surface pressure is not high enough to change the monolayer structure largely. UV irradiation was also conducted right after 7.4 mN/m is reached, i.e., “as-formed” monolayer. In the GPC data of products of 5.0 min irradiation both under constant surface pressure and at constant surface area, the amount of dimer predominates and monomer can hardly be observed. These results show a much higher reaction speed with the monolayer under this condition, compared to the stabilized monolayer case. As described before, at this state, the molecular density is lower and the molecules are more loosely packed, having much larger free space and higher mobility for the molecules. Since the cycloaddition of DCAB is accompanied by the movement of molecules, the microenvironment of the molecules is a very important factor in the reaction kinetics. From these data, it is reasonable to conclude that the molecule in the environment of larger mobility has a faster reaction speed. This result is in agreement with the observation by BAM.
Zhao et al.
d35 acid on a water surface at room temperature under suitable surface pressure. In this monolayer, photoinduced dimerization takes place successfully upon UV irradiation above 300 nm, whose product is considered to be cyclobutane dimer by UV-visible and IR spectroscopy and GPC. An expansion in monolayer area and an increase in surface pressure are found upon irradiation, showing an increase in molecular area and a movement of the molecule during the reaction. The Brewster-angle microscope observation reveals the separated domain structure of the monolayer, and the reaction speed is found to be higher in the less condensed monolayers. In GPC analysis, the photoreaction conducted at the state before the formation of the stable monolayer has a much higher yield of dimer than that of the stable monolayer, indicating a much higher reaction speed with the molecules having higher mobility. As for the understanding at a molecular scale, since a sp2 to sp3 hybridization change occurs in the cycloaddition reaction, the molecular conformation experiences a big change when the cyclobutane dimer is formed (Figure 2). This change in molecular conformation brings about a movement of the molecules, which requires free space in the reaction system, and therefore, the molecules that have higher mobility undergo the cycloaddition reaction at a much higher speed. It should be noted that the Langmuir monolayer on the water surface brings about some difficulties in real-time spectroscopic studies, especially for the infrared spectral measurement. It would be better to monitor the reaction in LB films.11-13 However, it is difficult for the present mixed monolayer to be transferred onto solid substrates (transfer ratio is about 0.4). As an effort in this direction, a work using both spectroscopy and microscopy on polymerization in an LB film of p-phenylenediacrylic acid derivatives with stronger hydrophobic interaction, 4-(4(2-(octadecyloxycarbonyl)vinyl)cinnamoylamino)benzoic acid, has been done.38 Acknowledgment. Financial support from STA (Science Technology Agency) Fellowship is appreciated. The authors express their thanks to Y. K. Gong, 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 experiments. LA9814539
Conclusion 4-(4-(2-decyloxycarbonyl)vinyl)cinnamo)benzoic acid (DCAB) is found to form a stable monolayer with stearic-
(38) Zhao, J.; Akiyama, H.; Abe, K.; Liu, Z.; Nakanishi, F. Manuscript in preparation.
