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Role of Hydrogen Bonding in Azobenzene-Urea Assemblies. The Packing State and Photoresponse Behavior in Langmuir Monolayers Takahiro Seki* and Takashi Fukuchi Photofunctional Chemistry Division, Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Magatsuta, Midori-ku, Yokohama 226-8503, Japan
Kunihiro Ichimura Research Laboratory for Science and Technology, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 248-8510, Japan Received December 6, 2001. In Final Form: April 11, 2002 Three homologous azobenzene (Az)-containing urea amphiphiles having two, one, and no hydrogen(s) at the terminal nitrogen of urea were synthesized by stepwise substitutions with methyl group(s). The spreading behavior, molecular packing, and photomechanical response of the Langmuir monolayers on water were investigated together with a homologous carboxylic acid derivative. The film stability, packing state, and photoreactivity that leads to photomechanical responses were clearly categorized into two groups: (1) the urea amphiphiles having two and one hydrogen(s) at the molecular end and (2) the urea amphiphile with no hydrogen at this position. The properties of the latter monolayer resembled that of the carboxylic acid homologue. Most likely, such distinct changes are attributable to the difference in the mode of the intermolecular hydrogen bond, namely, whether the bifurcated hydrogen bonds that are typically found in urea derivative assemblies can be formed. This work presents a typical example that the minimum chemical modification definitely alters the molecular functions in the assembled state.
1. Introduction The utilization of hydrogen bonds instead of covalent ones has become a prevalent tool in liquid crystal1-3 and macromolecular4,5 chemistry. Hydrogen bonds are moderately strong (4-25 kJ mol-1) and directional.6,7 One of the fascinating features of hydrogen bonding from the viewpoint of materials science is that the bond formation can be reversibly switched by heating or other physical stimuli under relatively mild conditions.8 The urea moiety is simple in structure but can be used as a very useful hydrogen bond building block for supramolecular organization.7,9,10 The study of Langmuir monolayers of urea derivatives at the air-water interface started very early. Adam11 showed in 1922 that long-chain urea derivatives exhibit a unique polymorphism having two forms of packing states, which is altered by temperature changes. According to Glazer and Alexander,12,13 in * To whom correspondence should be addressed. Fax: +81-45924-5247. E-mail:
[email protected]. (1) Kato, T.; Fre´chet, J. M. J. Macromolecules 1989, 22, 3818. (2) Brienne, M.-J.; Gabaard, J.; Lehn, J.-M.; Stibor, I. J. Chem. Soc., Chem. Commun. 1989, 1868. (3) Peleos, C. M.; Tsiourvas, D. Angew. Chem., Int. Ed. Engl. 1995, 34, 1696 (review). (4) Lange, R. F. M.; van Gurp, M.; Meijer, E. W. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 3657. (5) Bladon, P.; Griffin, A. C. Macromolecules 1993, 26, 6604. (6) Jeffey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: New York, 1997; pp 253-256. (7) MacDonald, J. C.; Whitesides, G. M. Chem. Rev. 1994, 94, 2383. (8) Kato, T.; Kihara, H.; Kumar, U.; Uryu, Y.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. Engl. 1994, 33, 1644. (9) Hanabusa, K.; Shimura, K.; Hirose, K.; Kimura, M.; Shirai, H. Chem. Lett. 1996, 885. (10) Van Esch, J.; De Feyter, S.; Kellog, R. M.; De Schryver, F.; Feringa, B. L. Chem.sEur. J. 1997, 3, 1238. (11) Adam, N. K. Proc. R. Soc. London, Ser. A 1922, 101, 452. (12) Alexander, A. E. Proc. R. Soc. London, Ser. A 1942, 179, 470. (13) Glazer, J.; Alecander, A. E. Trans. Faraday Soc. 1951, 47, 401.
the R form (high-temperature form, 0.20-0.21 nm2 per molecule) the urea headgroups align vertically with no hydrogen bonding between them, and the alkyl chains are fully extended with vertical orientations. On the other hand, in the β form (low-temperature form, 0.25-0.27 nm2) the urea groups are more tilted with two hydrogen bonds between the oxygen atom of one molecule and the two nitrogen atoms of a neighbor. This is termed bifurcated hydrogen bonding. This peculiar polymorphism is also a subject of recent active research.14-18 Huo et al.19 very recently reported a remarkable observation with a urea compound having two symmetrically introduced diacetylene long chains that the formation of the bifurcated hydrogen bond strand in the center of the assembly leads to the formation of a nontraditional form of Langmuir monolayer. Our recent efforts have been directed to the photofunctionalization of urea amphiphiles by introduction of the photochromic azobenzene (Az).20-22 The monolayers of the Az-containing urea have the following unusual features. (i) The Langmuir monolayer of 6Az10-urea (Chart 1) forms a tilted aggregate of Az in which the trans-to-cis (14) Kato, T.; Akiyama, H.; Yoshida, M. Chem. Lett. 1992, 565. (15) Shimizu, M.; Yoshida, K.; Iimura, K.; Suzuki, N.; Kato, T. Colloids Surf., A 1995, 102, 69. (16) Hunter, D. S.; Barnes, G. T.; Godfrey, J. S.; Grieser, F. J. Colloid Interface Sci. 1990, 138, 307. (17) Hayami, Y.; Kawano, M.; Motomura, K. Colloid Polym. Sci. 1991, 269, 167. (18) Urai, Y.; Ohe, C.; Itoh, K.; Yoshida, M.; Iimura, K.; Kato, T. Langmuir 2000, 16, 3920. (19) Huo, Q.; Russev, S.; Hasegawa, T.; Nishijo, J.; Umemura, J.; Puccetti, S.; Russell, K. C.; Leblanc, R. M. J. Am. Chem. Soc. 2000, 122, 7890. (20) Seki, T.; Fukuchi, T.; Ichimura, K. Bull. Chem. Soc. Jpn. 1998, 71, 2807. (21) Seki, T.; Fukuchi, T.; Ichimura, K. Langmuir 2000, 16, 3564. (22) Kobayashi, T.; Seki, T.; Ichimura, K. Chem. Commun. 2000, 1193.
10.1021/la011762q CCC: $22.00 © 2002 American Chemical Society Published on Web 06/06/2002
Hydrogen Bonding in Azobenzene-Urea Assemblies Chart 1
photoisomerization is completely impeded despite the fact that this monolayer is more expanded than that of a corresponding carboxylic acid homologue (6Az10-COOH in Chart 1) in which the photoisomerization moderately proceeds.20 (ii) The packing state of the transferred 6Az10urea on a hydrophilic surface is highly sensitive to the atmospheric humidity.21 (iii) The systematic change of the spacer length leads to clear odd-even effects in the molecular packing state.22 In the previous work, marked features of the Az-urea amphiphile assembly were extracted in comparison with the data of 6Az10COOH;20,21 however, precise understanding of the role of hydrogen bonding would not be obtained by such an approach using different polar groups. In this context, we newly synthesized a series of Azurea derivatives in which mono- or dimethyl substitutions at the terminal nitrogen were made (6Az10-UM and 6Az10-UM2 in Chart 1). It was anticipated that the substitution with methyl group(s) would be able to change the mode of hydrogen bonding in the monolayer assemblies. The monosubstituted urea still has one hydrogen at the terminal nitrogen atom, and thus it is still able to form the intermolecular bifurcated hydrogen bonds. On the other hand, when the hydrogen atoms are fully substituted by two methyl groups, such a hydrogenbonding bridge will not be formed. This paper focuses on the spreading behavior and some photoresponsive characteristics of Langmuir monolayers at the air-water interface. The detailed structural characterizations of the form of multilayered assemblies deposited on solid substrates will be reported elsewhere.
Langmuir, Vol. 18, No. 14, 2002 5463 was uncorrected. 1H NMR spectra were recorded on a Brucker AC-200 spectrometer using Me4Si as an internal standard. IR spectra were taken on a JASCO FT/IR-300. Elemental analysis was performed on a Yanaco MT-5 CHN CORDER. N-(11-{4-[(4-Hexylphenyl)azo]phenoxy})decyl-N′-methylurea (6Az10-UM). To a solution of 6Az10-COOH (1.50 g, 3.22 mmol) in dry benzene (40 mL) were added diphenylphosphoryl azide (DPPA) (1.60 g, 4.8 mmol) and triethylamine (730 mg, 4.8 mmol). After stirring at refluxing temperature for 4 h, the solvent was evaporated. To the residue was added ether (40 mL), and dried methylamine gas was passed into the solution for 1 h at room temperature. After the pale yellow solid was precipitated, the mixture was vigorously stirred overnight. The solid was filtered off and recrystallized from chloroform. Yield: 840 mg (53%); mp: 133-134 °C. IR (KBr): 3345 (νNH), 1623 (νCdO), 1246 (νC-O-C), and 842 (νCH, p-Ph). 1H NMR (δ, CDCl3): 0.88 (3H, t, J ) 7 Hz, CH3-), 1.31-1.89 (24H, m, -CH2-), 2.67 (2H, t, J ) 8 Hz, -CH2Ph), 2.77 (3H, d, J ) 4 Hz, -NH-CH3), 3.15 (2H, q, J ) 6 Hz, -CH2-NH-), 4.03 (2H, t, J ) 6 Hz, -O-CH2-), 4.20 (2H, m, -NH-), 6.99 (2H, d, J ) 9 Hz, Ph-H), 7.29 (2H, d, J ) 8 Hz, Ph-H), and 7.75-7.90 ppm (4H, m, Ph-H). Found: C, 72.70%; H, 9.25%; N, 11.38%. Calcd for C30H46N4O2: C, 72.87%; H, 9.31%; N, 11.34%. N-(11-{4-[(4-Hexylphenyl)azo]phenoxy})decyl-N′,N′-dimethylurea (6Az10-UM2). The synthesis was achieved similarly to the procedure of 6Az10-UMe starting from 1.50 g of 6Az10-COOH. At the final stage, dry dimethylamine gas was passed into the dry ether solution. The crude product was recrystallized from ethyl acetate. Yield: 250 mg (15%); mp: 81-83 °C. IR (KBr): 1630 (νCdO), 1246 (νC-O-C), and 842 (νCH, p-Ph). 1H NMR (δ, CDCl3): 0.89 (3H, t, J ) 6 Hz, CH3-), 1.31-1.88 (24H, m, -CH2), 2.67 (2H, t, J ) 7 Hz, -CH2-Ph), 2.89 (6H, s, -NH-(CH3)2), 3.21 (2H, q, J ) 8 Hz, -CH2-NH-), 4.03 (2H, t, J ) 7 Hz, -OCH2-), 4.31 (1H, bs, -NH-), 6.99 (2H, d, J ) 9 Hz, Ph-H), 7.29 (2H, d, J ) 9 Hz, Ph-H), and 7.77-7.90 ppm (4H, m, Ph-H). Found: C, 72.97%; H, 9.56%; N, 10.97%. Calcd for C31H48N4O2: C, 73.23%; H, 9.45%; N, 11.02%. 2.2. Methods. The spreading behavior of Az-containing monolayers was evaluated with a Lauda FW1 film balance in subdued red light. Unless stated otherwise, pure water (Milli-Q grade, 18 MΩ cm-1, pH ) 5.8) filled the trough. After evaporation of the solvent, the monolayer was compressed at a speed of 20 cm2 min-1, and the surface pressure was recorded versus the molecular area. The temperature of the subphase was controlled by water circulation using a Yamato-Komatsu CTE-22W at an accuracy of (0.5 °C. UV-visible absorption spectra for the floating monolayer were taken on a spectrometric system composed of a photodiode array detector (MCPD-2000, Ohtsuka Electronics) assembled with a deuterium/halogen lamp (MC-2530, Ohtsuka Electronics) and a processing computer. Light irradiation was performed with a 150 W Hg-Xe lamp (San-ei UV Supercure-230S). The 365 nm line was selected with a combination of Toshiba optical filters (UV-35/UV-D36A). The light intensity was estimated with an optical power meter (Advantest TQ-8210).
3. Results and Discussion 2. Experimental Section 2.1. Materials. Solvents used for Langmuir film spreading and spectroscopic measurements were of spectroscopic grade (Uvasol, Ciba-Merck). The synthetic procedure for 6Az10-urea was already reported.20 The Az derivatives having a monomethyl (6Az10-UM) or dimethyl substituted (6Az10-UM2) urea headgroup were synthesized in a similar manner to that for 6Az10-urea, starting from a carboxylic acid derivative [11-{4-[(4-hexylphenyl)azo]phenoxy}undecanoic acid (6Az10-COOH)]20 through a conversion to isocyanate23 followed by reaction with methylamine or dimethylamine.24 Identification of synthesized materials was performed using the following apparatus. Melting point was measured with a Yanaco MP-S3 melting point apparatus and (23) Shioiri, T.; Ninomiya, K.; Yamada, S. J. Am. Chem. Soc. 1972, 94, 6203. (24) Jones, L. W.; Mason, J. P. J. Am. Chem. Soc. 1927, 49, 2528.
3.1. Spreading Behavior at the Air-Water Interface. Figure 1 shows surface pressure-area (π-A) isotherms of 6Az10-UM (a) and 6Az10-UM2 (b) monolayers in the trans-Az form on pure water at 10-35 °C. The π-A curves for 6Az10-urea and 6Az10-COOH were indicated in the previous paper.20 The shape of the curves of the 6Az10-UM monolayer (a) was almost identical at all temperatures examined except for slight decreases in the collapse pressure at 35 °C. The limiting areas per Az unit, which are estimated by extrapolating the steepest slope to zero pressure, of this monolayer coincided well with each other within the range of 0.30-0.33 nm2. These data agree well with those obtained with 6Az10-urea,20 indicating that one methyl group substitution at the terminal nitrogen atom did not alter the packing state and its high thermal stability.
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Figure 1. π-A isotherms of 6Az10-UM (a) and 6Az10-UM2 (b) monolayers on pure water at 10 °C (-.-.-), 20 °C (s), and 35 °C (- - -).
In contrast, the substitution with two methyl groups at the terminal nitrogen (6Az10-UM2) caused drastic changes. The monolayer of 6Az10-UM2 indicated clear temperature dependence. The shape of the π-A curve at 10 °C resembled that of 6Az10-UM or 6Az10-urea, giving the limiting molecular area at 0.31 nm2. At 20 °C, the monolayer indicated a phase transition at ca. 15 mN m-1 indicated by a small plateau region at this pressure. This phase transition was associated with the change in the packing state of Az as will be mentioned in section 3.2. At 35 °C, a further expansion in the π-A curve was observed at low pressures in which the lift-off area shifted to ca. 0.40 nm2. Thus, 6Az10-UM2 forms a temperaturesensitive flexible monolayer assembly, which is not the case for the non-methylated (6Az10-urea) or monomethylated (6Az10-UM) urea derivatives. In Figure 2, the π-A curves of the monolayers of three Az ureas (6Az10-urea, 6Az10-UM, and 6Az10-UM2) at 20 °C were collected. In the trans-Az form (a), the π-A curves of the three derivatives were similar. The molecular occupying area (0.30-0.25 nm2) and the collapse pressure at ca. 55 mN m-1 did not change significantly. Only a small difference is admitted in the 6Az10-UM2 monolayer showing a two-step isotherm as stated above. In the cis-Az form (b), in contrast, a large difference was observed in the collapse pressure around 0.35 nm2 per molecule. The 6Az10-UM2 monolayer showed a collapse at 23 mN m-1, showing a marked reduction in comparison with the values of the other homologues (ca. 50 mN m-1). The shape of the plateau region around 0.4-
Seki et al.
Figure 2. π-A isotherms of 6Az10-urea (- - -), 6Az10-UM (-.-.-), and 6Az10-UM2 (s) on pure water at 20 °C in the trans state (a) and cis-rich (ca. 90% content) state (b) of azobenzene.
0.8 nm2 also indicated a different feature. Here, it is stressed that the collapse pressure of the 6Az10-urea or 6Az10-UM monolayer is exceptionally high (mechanically stable) for the cis-Az state. For other similar Az derivatives with a C10 alkylene spacer including both low-molecularmass compounds and polymeric materials, the collapse pressures were commonly observed below 25 mN m-1, that is, 18 and 25 mN m-1 for 6Az10-COOH and a poly(vinyl alcohol) derivative with a 6Az10 side chain (6Az10PVA), respectively.25 Therefore, 6Az10-UM2 has the common feature of hitherto investigated Az amphiphiles, and the monolayers of 6Az10-urea and 6Az10-UM, on the other hand, indicate a exceptional mechanical stability. A small pressure decrease was observed in the π-A curve in the cis form around 0.85 nm2 per molecule for 6Az10urea and 6Az10-UM. This does not indicate a collapse of the monolayer state because highly homogeneous images were obtained in Brewster angle microscopic observations. The pressure reduction seems to reflect a contraction of the monolayer due to a rapid hydrogen bond formation in the lateral directions. 3.2. UV-Visible Absorption Spectroscopy. The packing state of the trans-Az unit in the monolayer was evaluated by UV-visible absorption spectroscopy on the water surface. Figure 3 compares the absorption spectra obtained for 6Az10-MU (a) and 6Az10-UM2 (b) monolayers at 20 °C. Upon compression, the spectrum of the (25) Seki, T.; Fukuda, R.; Yokoi, M.; Tamaki, T.; Ichimura, K. Bull. Chem. Soc. Jpn. 1996, 69, 2375.
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Figure 4. The position of the absorption maximum (λmax) of the π-π* long axis transition of the 6Az10-UM2 monolayer on water as a function of surface pressure at 10 °C (O), 20 °C (9), and 35 °C (]).
Figure 3. UV-visible absorption spectra of 6Az10-UM (a) and 6Az10-UM2 (b) monolayers on water at 20 °C upon compression.
6Az10-UM monolayer showed regular enhancements in proportion to the area decrease without changing the spectral shape giving λmax (peak wavelength of the long axis π-π* band) at 355 nm,26,27 indicating that the molecular packing state is fixed from the initial stage of spreading and maintained in the compression process. In the case of 6Az10-UM2, in contrast, the lateral compression induced marked spectral changes. A sudden and discontinuous hypsochromic shift of λmax from 323 to 314 nm was observed between 10 and 15 mN m-1. This shift was accompanied by a decrease of the absorption intensity in the normal incidence, which should indicate that the orientation of the molecule changed to a more perpendicular one with respect to the water surface plane. The absorption spectrum of the 6Az10-UM monolayer was considerably broader than that of 6Az10-MU2, which should be attributed to the existence of different aggregation states of the Az unit. In Figure 4, λmax is plotted against surface pressure at three temperatures. As stated, a sudden spectral jump was observed in the region of 10-15 mN m-1 at 20 °C. At 35 °C, λmax shifted rather continuously from 331 to 317 nm and stayed almost constant at 316-317 nm above 20 mN m-1. At these two temperatures, the pressure region (26) The position of λmax ()355 nm) nearly agreed with that in chloroform (352 nm), but this agreement is incidental. The Az unit in 6Az10-urea and 6Az10-UM forms firmly fixed tilt aggregation (ref 27). (27) Shimomura, M.; Ando, R.; Kunitake, T. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 1134.
of the spectral shift corresponded to the region from the expanded to the condensed state of the monolayer (Figure 1b). λmax at 10 °C showed minor shifts from 316 to 312 nm upon compression. The packing state of the film should be still flexible as compared to those of 6Az10-urea or 6Az10-MU at 20 °C because the latter indicated no spectral shift upon compression.20 3.3. Photochemical and Mechanical Response. Figure 5 displays the change in the absorption spectrum of the monolayers of 6Az10-urea (a), 6Az10-UM (b), 6Az10-UM2 (c), and 6Az10-COOH (d) upon irradiation with 365 nm (UV) light at 20 °C. In these measurements, the surface pressure was kept constant at 2 mN m-1. Figure 6 shows the area changes of the monolayers observed under the above conditions. The light exposure energy was ca. 0.5 mW cm-2. As seen from Figure 5, no photoreaction proceeded in the 6Az10-urea and 6Az10-UM monolayers even at a large exposure dose of 5 J cm-2 that corresponds to a 50100-fold excess dose to reach the photostationary state in solution (below 100 mJ cm-2). A minor gradual decrease in the absorbance was seen, but this originated from an experimental artifact of baseline shift. Consequently, no area changes were induced by UV irradiation for these monolayers (Figure 6a,b). In sharp contrast, the spectrum of the 6Az10-UM2 monolayer exhibited rapid changes under the identical irradiation conditions (Figure 5c). The intensity of the π-π* band (λmax ) 325 nm) decreased, and instead that of the n-π* band (around 440 nm) was enhanced. Thus, the spectral changes are indicative of the proceeding of simple trans-to-cis photoisomerization. The film showed a continuous photoinduced expansion under the pressure condition (2 mN m-1, see below) upon irradiation which changes the number of chromophores in the observation beam spot, and therefore no isosbestic points were observed. The spectral change almost ceased at 150 mJ cm-2, indicating that the efficiency of the photoreaction is comparable to that in solutions. The photoisomerization induced an obvious expansion of the monolayer (Figure 6c) upon UV light illumination. The films show an approximately 3-fold expansion. The film area reverted to the original level upon irradiation with visible (436 nm) light. This photomechanical response closely resembled that of the 6Az10-PVA monolayer as previously reported.25 The time constant of the expansion process did not coincide with the course of the photoreaction. The
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Figure 6. Photoinduced area changes of 6Az10-urea (a), 6Az10-UM (b), 6Az10-UM2 (c), and 6Az10-COOH (d) monolayers on water at 2 mN m-1 and 20 °C. “UV” and “Vis” in the figures denote the starting points of light irradiation with 365 and 436 nm light, respectively.
The 6Az10-COOH monolayer was also photoresponsive. In this case, more prolonged irradiation was required for the trans-to-cis photoreaction. This can be attributed to the stronger aggregation of Az in the film as seen from the further hypsochromic spectral shift (Figure 5d).25 Except for the light energy required, the photoresponse behavior was essentially the same between the 6Az10UM2 (Figure 6c) and 6Az10-COOH (Figure 6d) monolayers. In summary of this section, the photoreaction and the resulting area responses were observed for the 6Az10UM2 and 6Az10-COOH monolayers, and they were completely inhibited in the 6Az10-urea and 6Az10-UM ones. 4. Discussion
Figure 5. Changes in the UV-visible absorption spectrum of the monolayers of 6Az10-urea (a), 6Az10-UM (b), 6Az10UM2 (c), and 6Az10-COOH (d) at 2 mN m-1 upon irradiation with 365 nm light.
photostationary state was attained in 300 s; however, a slow expansion still continued after 3600 s. It is already demonstrated in the previous paper using 6Az10-PVA that the actual film expansion is considerably delayed from the photoreaction.28
The properties of the four Langmuir monolayers are summarized in Table 1. The mechanical stability can be evaluated from the collapse pressure in the cis-Az form (the second column in Table 1). In the trans-Az form, the monolayers had almost equal stability as indicated in Figure 2a due to the ordered lateral packing through van der Waals interaction and π-π stacking. The influence from the hydrophobic head part became prominent when such ordered chain packing was lost in the cis-Az form. As shown in Figure 2b, the monolayers of 6Az10-urea and 6Az10-UM are (28) Seki, T.; Sekizawa, H.; Morino, S.; Ichimura, K. J. Phys. Chem. B 1998, 102, 5313.
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Table 1. Various Properties of the Monolayers on a Water Surface at 20 °C
compound
collapse pressurea/ mN m-1
λmaxb/nm
6Az10-urea 6Az10-UM 6AZ10-UM2 6Az10-COOH
50 50 23 16
355 355 324 f 311 305 f 295
photoresponsec no no yes yes
supposed hydrogen bond bifurcated bifurcated single single
a In the cis-rich state. b Peak position of the π-π* band in the trans form of Az. c Both the proceeding of the trans-to-cis isomerization and the resulting photomechanical effect upon irradiation with 365 nm light.
exceptionally stable in the cis-Az state giving the collapse pressure at 50 mN m-1. This pressure is almost comparable to that of the trans-Az monolayers. The collapse pressures for 6Az10-UM2 and 6Az10-COOH are below 25 mN m-1 as observed for other homologues.25,28 It was confirmed by UV-visible absorption spectroscopy that the compression of the monolayers of 6Az10-urea and 6Az10-UM in the cis form does not cause a back isomerization reaction to the trans form during the π-A isotherm measurement (data not shown). The UV-visible spectral data provide information on the packing state of Az (the third column). The packing state of Az in the 6Az10-urea and 6Az10-UM monolayers is firmly fixed immediately after the spreading and solvent evaporation. The lateral compression does not alter the packing state of these nonelastic solid (crystalline) monolayers. On the other hand, the flexibility of the packing state is retained in the 6Az10-UM2 and 6Az10-COOH monolayers at low surface pressures. Stronger H-type aggregates are formed in the course of lateral compression. With respect to the trans-to-cis photoisomerization of Az and the resulting photomechanical response, the difference was most prominent (the fourth column). The photoresponse behavior is observed in the all-or-none fashion. The 6Az10-urea and 6Az10-UM monolayers showed no response, and the other two, 6Az10-UM2 and 6Az10-COOH, exhibited clear photomechanical responses. Altogether, the monolayers can be classified into the two groups, (i) 6Az10-urea and 6Az10-UM and (ii) 6Az10-UM2 and 6Az10-COOH. The monolayer of monomethyl-substituted 6Az10-UM exactly maintains the features of the mother compound (6Az10-urea). On the other hand, the full substitution with two methyl groups completely changes such features, and the monolayer properties resemble those of 6Az10-COOH. Such effects are reasonably interpreted by assuming the difference in the mode of intermolecular hydrogen bonding as illustrated in Figure 7. The former class (6Az10-urea and 6Az10-UM) can form the bifurcated N-H‚‚‚OdC hydrogen bonds that are characteristic in the assemblies of many urea compounds.9-19 For the latter class (6Az10-UM2 and 6Az10-COOH), only the single hydrogen bond (N-H‚‚‚OdC or OH‚‚‚OdC29) is available among the headgroups. The discrepant characters between the two classes should be the consequence of the formation of bifurcated or single intermolecular hydrogen bonding in the two dimensions. We are not yet able to obtain direct information on the hydrogen bonding of monolayers on water due to experimental difficulties; nevertheless, the above interpretation seems highly plausible.30 (29) Ozaki, Y.; Fujimoto, Y.; Terashita, S.; Katayama, N.; Iriyama, K. Spectroscopy 1993, 8, 36.
Figure 7. Schematic illustrations of plausible hydrogenbonding formation between the headgroups. Most probably, the bridge via bifurcated hydrogen bonds is formed in the monolayers of 6Az10-urea and 6Az10-UM, whereas only a single hydrogen bond is formed for the rest.
In conclusion, it is demonstrated that the systematic synthetic approach for the urea amphiphiles is highly informative to gain knowledge on the intermolecular hydrogen bonding. The results obtained here provide a typical example that the minimum modification of the molecular structure can drastically change the functions of the molecular assemblies. The experimental elucidation of the packing state is quite difficult in the monolayer state on water. We are now proceeding with structural characterizations of deposited multilayers on solid substrates. The data will be reported in due course. Acknowledgment. We thank Drs. S. Morino and M. Nakagawa for helpful discussions. This work was in part supported by the Nissan Science Foundation. LA011762Q (30) For deposited Langmuir-Scheafer multilayered films of 6Az10UM, only a sharp single N-H stretching band was observed at 3343 cm-1 despite the fact that this compound has two N-H bonds at different nitrogen atoms. This fact indicates that the two N-H‚‚‚OdC bonds are equivalent in the film and provides strong evidence for the formation of bifurcated hydrogen bonds participating with an equivalent contribution.
