Langmuir 1998, 14, 4495-4502
4495
Macrocyclic Amphiphiles. 3.† Monolayers of O-Octacarboxymethoxylated Calix[4]resorcinarenes with Azobenzene Residues Exhibiting Efficient Photoisomerizability Masanori Fujimaki, Sumie Kawahara, Yoko Matsuzawa, Eiichi Kurita, Yuko Hayashi, and Kunihiro Ichimura* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan Received November 17, 1997. In Final Form: April 13, 1998 O-Octacarboxymethoxylated calix[4]resorcinarenes having four azobenzene residues at the lower rim were prepared to study photoisomerizability in monolayers. UV irradiation of the amphiphilic macrocycles at 365 nm on a water surface resulted in the formation of about 60% of the Z-isomer even in densely packed monolayers while 99% of the Z-isomer was formed in solution photochemistry. Langmuir-Blodgett monolayered films deposited on a quartz substrate displayed about 72% of E-to-Z conversion at a photostationary state. The amphiphilic macrocycles adsorbed firmly on a quartz plate to give relatively densely packed monolayers. About 86% of Z-isomer was formed when these monolayers were exposed to 365 nm light. The efficient E-to-Z photoisomerizability in monolayers is ascribable to large difference in cross-sectional areas between the cylindrical framework of calix[4]resorcinarene (1.7 nm2) and azobenzenes (0.25 × 4 ) 1.0 nm2) to ensure a two-dimensional free volume. The E-to-Z photoisomerization in adsorbed monolayers deviated from the first-order plots, indicating that steric constraint exists more or less.
Introduction Molecular layers incorporating photochromic moieties have been attracting ever increasing interest in conjunction with photofunctionalization of material surfaces including the photoregulation of versatile material properties1 including ionic permeability, membrane potential, electroconductivity, wettability, dispersibility of colloidal silica,2 and alignment of liquid crystals.3 Azobenzenes have been employed extensively for these purposes owing to their good availability and reasonable photofatigue resistance. However, problems arise occasionally from the fact that the E/Z photoisomerization of the chromophores is accompanied by a relatively large sweep volume so that the geometrical E-to-Z photoisomerization is suppressed in densely packed molecular films.4 The photoisomerizability in densely packed molecular films has been so far ensured by using polymeric chains as hydrophilic headgroups,6 bulky hydrophilic headgroups,7 and cyclodextrin inclusion complexes8 and by mixing of amphiphilic azobenzenes with a shorter alkyl surfactant.9 During the course of our systematic studies on the photoregulation of liquid crystal alignment by photochromic molecular as well as polymeric films, well-defined †
Macrocyclic amphiphiles. 2.: Ref. 12.
(1) Anzai, J.; Osa, T. Tetrahedron 1994, 14, 4039. (2) (a) Ueda, M.; Kim, H.-B.; Ichimura, K. J. Mater. Chem. 1994, 4, 883. (b) Ueda, M.; Fukushima, N.; Kudo, K.; Ichimura, K. J. Mater. Chem. 1997, 7, 641. (3) Ichimura, K. In Electrooptical and Photooptical Polymers as Active Media; Shibaev, V., Ed.; Elsevier: Amsterdam; 1996; p 138. (4) Nakahara, N.; Fukuda, K.; Shimomura, M.; Kunitake, T. Nippon Kagaku Kaishi 1988, 1001. (5) Nishiyama, K.; Fujihira, M. Chem. Lett. 1988, 1257. (6) Seki, T.; Ichimura, K. Polym. Commun. 1989, 30, 108. (7) Tachibana, H.; Nakamura, T.; Matsumoto, M.; Komizu, H.; Manda, E.; Niino, H.; Yabe, A.; Kawabata, Y. J. Am. Chem. Soc. 1989, 111, 380. (8) Yabe, A.; Kawabata, Y.; Niino, H.; Matsumoto, M.; Ouchi, A.; Takahashi, H.; Tamura, S.; Takagi, W.; Nakahara, H.; Fukuda, K. Thin Solid Films 1988, 160, 33. (9) Maack, J.; Ahuja, R. C.; Tachibana, H. J. Phys. Chem. 1995, 99, 9221.
molecular layers such as Langmuir-Blodgett (LB) films with embedded azobenzene units have been highly required for the elucidation of interactions at interfaces between surface azobenzenes and liquid crystal molecules. Amphiphilic polymers with azobenzene side chains have been extensively studied for this purpose.10 Taking notice of the unique chemical structure of a crown conformer of calix[4]resorcinarenes (CRAs) with amphiphilic characteristics, we prepared novel CRA derivatives with four azobenzene substituents tethered to the upper rim of the cylindrical molecular framework and examined the photoisomerizability in densely packed monomolecular layers.11 As discussed in our previous paper, a cross-sectional area of CRA is estimated to be ∼1.3 nm2 while that of four azobenzenes is ∼1.0 nm2 so that the E-to-Z photoisomerization takes place to some extent in a densely packed monolayer on a water surface.11 It was revealed on the other hand that cross-sectional areas of CRA on a water surface are enlarged by replacing the phenolic OH groups by the other hydrophilic units such as carboxymethoxyl and hydroxyethyl units.12 Furthermore, it was found that CRAs modified by carboxymethoxy groups (CRA-CMs) adsorb on polar surfaces through hydrogen bonds relatively firmly from nonpolar solvents to give monolayers.12 These facts led us to examine the photoisomerizability of CRAs with eight carboxymethoxy residues at the upper rim and four azobenzene chromophores at the lower rim under anticipation that the E-to-Z photoisomerizability of monolayers is enhanced because of a larger cross-sectional area of the modified CRA. We have mentioned in our preliminary (10) Seki, T.; Sakuragi, M.; Kawanishi, Y.; Tamaki, T.; Fukuda, R.; Ichimura, K. Langmuir 1993, 9, 857. (11) Ichimura, K.; Fukushima, N.; Fujimaki, M.; Kawahara, S.; Matsuzawa, Y.; Hayashi, Y.; Kudo, K. Langmuir 1997, 25, 6780. (12) Kurita, E.; Fukushima, N.; Fujimaki, M.; Matsuzawa, Y.; Kudo, K.; Ichimura, K. J. Mater. Chem. 1998, 8, 397.
S0743-7463(97)01257-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/15/1998
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report that this is the case.13 This paper deals with the detailed description of the behavior of CRA-CMs possessing azobenzene residues in floating monolayers, Langmuir-Blodgett films and chemisorbed monolayers to stress the significant effect of O-carboxymethylation to ensure the two-dimensional free volume required for E-to-Z photoisomerization. Experimental Section Materials. 2,8,14,20-Tetrakis{3-[4-(4-butylphenylazo)phenoxy]propyl}-4,6,10,12,16,18,22,24-octahydroxycalix[4]arene (5b) was prepared by the reaction of resorcinol and 4-[(4-butylphenylazo)phenoxy]propanal diethylacetal and purified according to our previous work.2b 2,8,14,20-Tetrakis(3-chloropropyl)-4,6,10,12,16,18,22,24octahydroxycalix[4]arene (1). A solution of 18.3 g of resorcinol and 30.0 g of 4-chlorobutanal diethylacetal in 100 mL of 2-methoxyethanol containing 4 mL of 12 N hydrochloric acid was stirred at 80 °C for 15 h under a nitrogen atmosphere. After the solvent was evaporated, diethyl ether was added and the solution was washed with water. An organic layer was dried over magnesium sulfate and removed the solvent to give a residual solid. The solid of a crude desired product was treated with hot diethyl ether under reflux, and an insoluble solid was collected by filtration. This process was repeated five times to obtain 10.8 g (32.7% yield) of pure crown conformer of 2,8,14,20-tetrakis(3-chloropropyl)-4,6,10,12,16,18,22,24-octahydroxycalix[4]arene, which decomposed above 260 °C without melting. Anal. Calcd for C40H44O8Cl4 + H2O: C, 59.00; H, 5.69; Cl, 16.77. Found: C, 59.12; H, 5.71; Cl, 17.45. 1H NMR (DMSO): δ 1.412.31 (br, 16H, ClCH2CH2CH2CH-), 3.63 (t, 8H, ClCH2-, J ) 6 Hz), 4.20-4.58 (br, 4H, ClCH2CH2CH2CH-), 6.23 (s, 4H, ArH), 7.02 (s, 4H, Ar-H), 8.81 (s, 8H, OH). 2,8,14,20-Tetrakis(3-chloropropyl)-4,6,10,12,16,18,22,24octabenzoyloxycalix[4]arene (2). To a solution of 1.50 g of 1 and 1.84 g of triethylamine in 10 mL of dichloromethane was added 2.9 g of benzoyl chloride to perform octabenzoylation to give 2.70 g (87% yield) of colorless solid of 2. Anal. Calcd for C96H76O16Cl4 + H2O: C, 70.07; H, 4.78; Cl, 8.62. Found: C, 70.17; H, 4.32; Cl, 9.00. 1H NMR (CDCl3) δ 1.39-2.65 (br, 16H, ClCH2CH2CH2CH-), 3.22-3.71 (br, 8H, ClCH2CH2CH2CH-), 4.33-4.71 (br, 4H, ClCH2CH2CH2CH-), 6.30-6.78 (br, 4H, Ar-H), 6.80-7.61 (m, 28H, Ar-H of calixarene and Ar-H of benzoyl), 7.60-8.10 (br, 16H, Ar-H of benzoyl). 2,8,14,20-Tetrakis(3-iodopropyl)-4,6,10,12,16,18,22,24octabenzoyloxycalix[4]arene (3). A solution of 1.90 g of 2 and 1.50 g of sodium iodide in acetone was refluxed for 10 h, followed by a conventional workup to afford 1.31 g of colorless crystals of 3 of mp 273-274 °C. Anal. Calcd for C96H76O16I4: C, 57.85; H, 3.84; I, 25.47. Found: C, 57.78; H, 3.41; Cl, 25.15. 1H NMR (CDCl3): δ 1.55-2.65 (br, 16H, ICH2CH2CH2CH-), 3.00-3.38 (br, 8H, ICH2CH2CH2CH-), 4.40-4.73 (br, 4H, ICH2CH2CH2CH-), 6.30-6.78 (br, 4H, Ar-H of calixarene), 6.80-7.61 (m, 28H, Ar-H of calixarene and Ar-H of benzoyl), 7.61-8.10 (br, 16H, Ar-H of benzoyl). 2,8,14,20-Tetrakis[3-(4-phenylazophenoxy)propyl]-4,6,10,12,16,18,22,24-octahydroxycalix[4]arene (5a). To a 0.50 g of 3 in 7 mL of DMF was added 0.42 g of p-hydroxyazobenzene and 0.61 g of potassium carbonate. The solution was stirred for 20 h at room temperature, followed by a usual workup to give 0.35 g of an orange solid of 2,8,14,20-tetrakis[3-(4-phenylazophenoxy)propyl]-4,6,10,12,16,18,22,24-octabenzoyloxycalix[4]arene (4a), which was purified by silica gel column chromatography using a 1:7 mixture of ethyl acetate and hexane. Anal. Calcd for C88H80N8O12: C, 76.04; H, 4.96; N, 4.93. Found: C, 76.33; H, 4.74; N, 4.67. 1H NMR (CDCl3): δ 1.47-2.61 (br, 16H, -OCH2CH2CH2CH-), 3.52-4.07 (br, 8H, ICH2CH2CH2CH-), 4.40-4.73 (br, 4H, ICH2CH2CH2CH-), 6.47-6.88 (d, 12H, Ar-H of calixarene and Ar-H of azobenzene), 7.00-8.18 (m, 72H, Ar-H of calixarene and Ar-H of azobenzene). A solution of 0.34 g of 4a in 10 mL of methanol was mixed with 5 mL of water dissolving 2.10 g of potassium hydroxide and stirred (13) Preliminary report: Fujimaki, M.; Matsuzawa, Y.; Kurita, E.; Hayashi, Y.; Ichimura, K. Chem. Lett. 1998, 165.
Fujimaki et al. at room temperature. The alkaline solution was acidified with hydrochloric acid to form an orange precipitate that was dissolved in a 2:1 mixture of THF and methanol to remove an insoluble substance. After drying of the solution over magnesium sulfate and removal of the solvent, a residual solid was recrystallized from a mixture of THF and methanol to give 0.09 g of the desired compound. Mp > 260 °C (dec). Anal. Calcd for C88H80N8O12 + 3H2O: C, 70.66; H, 5.80; N, 7.49. Found: C, 69.94; H, 5.40; N, 6.80. 1H NMR (DMSO): δ 1.67-1.74 (br, 8H, -OCH2CH2CH2CH-), 2.24-2.28 (br, 8H, -OCH2CH2CH2CH-), 4.05 (t, 8H, -OCH2CH2CH2CH-, J ) 6 Hz), 4.42 (t, 4H, -OCH2CH2CH2CH-, J ) 6 Hz), 6.25 (s, 4H, Ar-H of calixarene), 6.89-7.05 (d, 8H, Ar-H of azobenzene, J ) 9 Hz), 7.28 (s, 4H, Ar-H of calixarene), 7.35-7.58 (m, 12H, Ar-H of azobenzene), 7.607.87 (m, 12H, Ar-H of azobenzene), 8.77 (s, 8H, Ar-OH). 2,8,14,20-Tetrakis{3-[4-(4-cyclohexylphenylazo)phenoxy]propyl}4,6,10,12,16,18,22,24-octahydroxycalix[4]arene (5c). This was prepared in a way similar to that for 5a in a 20% yield. 1H NMR (DMSO): δ 1.24-1.50 (br, 28H, H of cyclohexyl and -OCH2CH2CH2CH-), 1.68-1.90 (br, 28H, H of cyclohexyl and -OCH2CH2CH2CH-), 2.47-2.54 (br, 4H, H of cyclohexyl), 3.96-4.05 (br, 8H, -OCH2CH2CH2CH-), 4.36 (t, 4H, -OCH2CH2CH2CH-, J ) 6 Hz), 6.18 (s, 4H, Ar-H of calixarene), 6.86 (d, 8H, Ar-H of azobenzene, J ) 9 Hz), 7.26 (s, 4H, Ar-H of calixarene), 7.36 (d, 8H, Ar-H of azobenzene, J ) 8 Hz), 7.70 (d, 8H, Ar-H of azobenzene, J ) 8 Hz), 7.72 (d, 8H, Ar-H of azobenzene, J ) 9 Hz), 8.21 (br, 8H, Ar-OH). 2,8,14,20-Tetrakis(3-chloropropyl)-4,6,10,12,16,18,22,24octaethoxycarbonylmethoxymethylcalix[4]arene (6). A solution of 8.8 g of 1, 19.3 g of ethyl bromoacetate and 16.1 g of potassium carbonate in 200 mL of dried acetone was refluxed under nitrogen for 24 h. The reaction mixture was filtered, followed by evaporating the solvent to dryness. A residue was treated with methanol to form a colorless precipitate that was recrystallized from methanol to give 12.94 g of colorless crystals of mp 110.0110.5 °C. Anal. Calcd for C72H92O24Cl4: C, 58.30; H, 6.25; Cl, 9.56. Found: C, 58.10; H, 6.17; Cl, 10.08. 1H NMR (CDCl3): δ 1.27 (t, 24H, -OCH2CH3, J ) 7 Hz), 1.63-2.23 (br, 16H, ClCH2CH2CH2CH-), 3.59 (t, 8H, Cl-CH2-, J ) 6 Hz), 4.19 (q, 16H, OCH2CH3, J ) 7 Hz), 4.27 (s, 16H, -OCH2CO-), 4.61 (t, 4H, Cl-CH2CH2CH2CH-, J ) 6 Hz), 6.18 (s, 4H, Ar-H), 6.62 (s, 4H, Ar-H). 2,8,14,20-Tetrakis(3-iodopropyl)-4,6,10,12,16,18,22,24octaethoxycarbonylmethoxymethylcalix[4]arene (7). A solution of 9.0 g of 6 and 7.3 g of sodium iodide in 200 mL of 2-butanone was refluxed for 12 h. The reaction mixture was treated with a mixture of ethyl acetate and water to separate an organic layer. After washing with a saturated NaCl aqueous solution and drying over magnesium sulfate, the solvent was removed. A residual mass was subjected to silica gel column chromatography using ethyl acetate and recrystallization from 2-propanol to give 10.1 g of colorless crystals of mp 128-129 °C. Anal. Calcd for C72H92O24I4 + H2O: C, 46.32; H, 5.07; I, 27.19. Found: C, 46.70; H, 5.26; I, 26.97. 1H NMR (CDCl3): δ 1.27 (t, 24H, CH3, J ) 7 Hz), 1.67-2.19 (br, 16H, ICH2CH2CH2CH-), 3.12-3.39 (br, 8H, ICH2-), 4.21 (q, 16H, -OCH2CH3, J ) 7 Hz), 4.27 (s, 16H, -OCH2COO-), 4.50-4.70 (br, 4H, I-CH2CH2CH2CH-), 6.17 (s, 4H, Ar-H), 6,61 (s, 4H, Ar-H). 2,8,14,20-Tetrakis[3-(4-phenylazophenoxy)propyl]-4,6,10,12,16,18,22, 24-octaethoxycarbonylmethoxymethycalix[4]arene (8a). To a solution of 3.0 g of 7 in 90 mL of DMF was added 1.6 g of p-hydroxyazobenzene and 1.1 g of potassium carbonate. The mixture was stirred for 12 h at room temperature to be subjected to a conventional workup. The product was purified by silica gel column chromatography using a 2:1 mixture of ethyl acetate and hexane and recrystallized from 2-propanol to give 2.9 g of orange crystals (8a) of mp 75-76 °C. Anal. Calcd for C120H128N8O28: C, 67.66; H, 6.06; N, 5.26. Found: C, 67.32; H, 5.88; N, 5.15. 1H NMR (CDCl3): δ 1.27 (t, 24H, CH3, J ) 7 Hz), 1.81-2.01 (br, 8H, -OCH2CH2CH2CH-), 2.04-2.24 (br, 8H, -OCH2CH2CH2CH-), 4.05 (t, 8H, -OCH2CH2CH2CH-, J ) 6 Hz), 4.21 (q, 16H, -OCH2CH3, J ) 7 Hz), 4.29 (s, 16H, -OCH2COO-), 4.76 (t, 4H, -OCH2CH2CH2CH-, J ) 6 Hz), 6.24 (s, 4H, Ar-H of calixarene), 6.77 (s, 4H, Ar-H of calixarene), 6.88 (d, 8H, Ar-H of azobenzene,
Macrocyclic Amphiphiles J ) 9 Hz), 7.34-7.51 (m, 12H, Ar-H of azobenzene), 7.78 (d, 8H, Ar-H of azobenzene, J ) 9 Hz), 7.81 (d, 8H, Ar-H of azobenzene, J ) 8 Hz). 2,8,14,20-Tetrakis{3-[4-(4-cyclohexylphenylazo)phenoxy]propyl}4,6,10,12,16,18,22,24-octaethoxycarbonylmethoxymethylcalix[4]arene (8b). This was prepared in a similar manner described above in a 88% yield. Orange crystals had mp 76.5-78 °C. Anal. Calcd for C144H168N8O28: C, 70.33; H, 6.89; N, 4.56. Found: C, 68.47; H, 6.61; N, 4.09. 1H NMR (CDCl3): δ 1.27 (t, 24H, CH3, J ) 7 Hz), 1.32-1.52 (br, 20H, H of cyclohexyl), 1.70-2.01 (br, 28H, H of cyclohexyl and -O-CH2CH2CH2CH-), 2.05-2.21 (br, 8H, -O-CH2CH2CH2CH-), 2.48-2.64 (br, 4H, H of cyclohexyl), 4.05 (t, 8H, -O-CH2CH2CH2CH-, J ) 6 Hz), 4.21 (q, 16H, -OCH2CH3, J ) 7 Hz), 4.29 (s, 16H, -OCH2COO-), 4.75 (t, 4H, -O-CH2CH2CH2CH-, J ) 6 Hz), 6.25 (s, 4H, Ar-H), 6.77 (s, 4H, Ar-H), 6.88 (d, 8H, Ar-H of azobenzene, J ) 9 Hz), 7.25 (d, 8H, Ar-H of azobenzene, J ) 8 Hz), 7.74 (d, 8H, Ar-H of azobenzene, J ) 8 Hz), 7.78 (d, 8H, Ar-H of azobenzene, J ) 9 Hz). 2,8,14,20-Tetrakis[3-(4-phenylazophenoxy)propyl]-4,6,10,12,16,18,22,24-octacarboxymethoxycalix[4]arene (9a). A solution of 1.0 g of 8a in 15 mL of THF was mixed with 15 mL of water dissolving 0.80 g of potassium hydroxide and stirred for 1.5 h at room temperature. After removal of THF, an alkaline solution was acidified with hydrochloric acid to precipitate an orange substance that was collected by centrifugation and washed with water repeatedly, followed by drying under reduced pressure to give 0.73 g of orange crystals of mp 220-222 °C in a 82% yield. Anal. Calcd for C104H96N8O28 + 3H2O: C, 63.73; H, 5.25; N, 5.71. Found: C, 63.61; H, 5.09; N, 5.88. 1H NMR (DMSO): δ 1.66-1.90 (br, 8H, -OCH2CH2CH2CH-), 1.91-2.20 (br, 8H, -OCH2CH2CH2CH-), 3.91-4.13 (br, 8H, -OCH2CH2CH2CH-), 4.39 (q, 16H, -OCH2COOH, J ) 18 Hz), 4.68 (t, 4H, -OCH2CH2CH2CH-, J ) 6 Hz), 6.48 (s, 4H, Ar-H), 6.73 (s, 4H, Ar-H), 6.91 (d, 8H, Ar-H of azobenzene, J ) 9 Hz), 7.39-7.56 (m, 12H, Ar-H of azobenzene), 7.64-7.83 (m, 16H, Ar-H of azobenzene). 2,8,14,20-Tetrakis{3-[4-(4-cyclohexylphenylazo)phenoxy]propyl}4,6,10,12,16,18,22,24-octacarboxymethoxycalix[4]arene (9b). This was prepared in a manner similar to that described above in a 77% yield. Orange crystals of mp 228.5-230 °C. Anal. Calcd for C128H136N8O28 + 5H2O: C, 66.13; H, 6.33; N, 4.82. Found: C, 66.33; H, 6.02; N, 4.52. 1H NMR (DMSO-d6): δ 1.08-1.47 (br, 20H, H of cyclohexyl), 1.55-1.90 (br, 28H, H of cyclohexyl and -O-CH2CH2CH2CH-), 1.92-2.20 (br, 8H, -O-CH2CH2CH2CH-), 2.32-2.52 (br, 4H, H of cyclohexyl), 3.90-4.10 (br, 8H, -O-CH2CH2CH2CH-), 4.40 (q, 16H, -OCH2COOH, J ) 18 Hz), 4.68 (t, 4H, -O-CH2CH2CH2CH-, J ) 6 Hz), 6.48 (s, 4H, ArH), 6.79 (s, 4H, Ar-H), 6.89 (d, 8H, Ar-H of azobenzene, J ) 9 Hz), 7.12 (d, 8H, Ar-H of azobenzene, J ) 8 Hz), 7.55 (d, 8H, Ar-H of azobenzene, J ) 8 Hz), 7.63 (d, 8H, Ar-H of azobenzene, J ) 9 Hz). Measurements. UV-visible absorption spectra of solutions were taken on a Hitachi UV-320 spectrometer. Absorption spectra of monolayers on quart plates were recorded on a JASCO MAC-1 spectrometer. NMR spectra were taken on FT-NMR spectrometers of JEOL FX-90Q and Bruker AC200. Photoirradiation. A Hg-Xe lamp (SAN-EI SUPERCURE-203S; San-ei Electronics) was used for photoisomerization experiments using combination of glass filters of UV-D36A and UV-35 (Toshiba) and of Y43 and V44 (Toshiba), respectively, to obtain 365 nm light and blue light centered at 440 nm. UV light intensity was 1.5 mW cm-2, while that of blue light was 1.5 mW cm-2 for solution photoisomerization. Azobenzene monolayers on quartz plates were irradiated with UV light intensity of 0.5 mW cm-2 and with blue light intensity of 0.5 mJ cm-2. π-A Isotherm Measurements. Surface pressure-area isotherms were obtained by using a Lauda film balance at 20 ( 1 °C. A CRA derivative was spread on pure water, which was purified by Milli-Q, from a 1:1 mixture of chloroform and THF solution of a concentration of ∼5 × 10-5 mol dm-3. After evaporation of the solvents for 15 min, a monolayer was compressed at a speed of 30 cm2 min-1 to record a surface pressure-area isotherm. Absorption spectra of monolayers on a water surface were recorded on a multichannel photodiode MCPD-2000 (Ohtsuka Electronics) equipped with a quartz optical
Langmuir, Vol. 14, No. 16, 1998 4497
Figure 1. Synthesis pathways to CRA derivatives. fiber. Brewster angle microscope images were taken with an NL-LB/EMM-001 (Nippon Laser Electronics). Surface Adsorption Experiments. Quartz plates were immersed in 2-propanol solution dissolving potassium hydroxide and washed in water. They were treated ultrasonically with acetone, distilled water, concentrated nitric acid, a sodium bicarbonate aqueous solution, and finally distilled water and used immediately. Surface adsorption of CRA derivatives was carried out by immersing a clean quartz plate in their 2-butanone solutions of a concentration of 10-4 mol dm-3 or less under gentle shaking at 60 °C. The plate was rinsed in pure 2-butanone shortly and dried at 120 °C in an oven. LB Film Preparation. LB film deposition was carried out using a Lauda film balance at 20 ( 1 °C. A CRA derivative was spread on pure water from a 1:1 mixture of chloroform and THF solution. After evaporation of the solvent for 15 min, a monolayer was compressed with a moving barrier at a speed of 30 cm2 min-1. It took approximately 15 min to reach a constant area at 20 mN m-1. Monolayers were transferred onto cleaned quartz plates by a vertical dipping method. The dipping rate was 5 mm min-1.
Results and Discussion Synthesis and Solution Photochemistry. Figure 1 shows reaction pathways to azo-modified CRA derivatives. In our previous work, a crown conformer of CRA having 4-butylazobenzene was prepared by the cyclotetramerization of resorcinol with the corresponding azobenzene having an acetal unit.11 For convenience to synthesize a family of CRAs with substituted azobenzene chro-
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Fujimaki et al.
Figure 2. Surface pressure/area isotherms of (a) 9a and (b) 9b on pure water at 20 °C. Table 1. UV Absorption Spectral Data of Azobenzene-Modified CRA Derivatives compound
λmax (nm)
(dm3 mol-1 cm-1)
half-value width (nm)
5a 5c 9a 9b
346.0 349.5 347.5 351.5
9.20 × 104 1.13 × 105 8.56 × 104 9.23 × 104
65.0 61.5 60.6 61.4
mophores, CRA substituted with four iodopropyl residues (3) was prepared in this work as a common intermediate through the corresponding chloropropylated CRA (1). The crown conformer (1) was obtained by the reaction of resorcinol with 4-chlorobutanal diethylacetal under an acidic condition and purified by treatment with diethyl ether. OH groups of 1 were protected by benzoylation to give an O-octabenzoylated CRA (2), followed by the conversion into 3 to activate the subsequent substitution with a 4-hydroxyazobenzene. Octabenzoylated CRAs modified with four azobenzene residues (4) were subjected to alkaline hydrolysis to give the desired CRA derivatives having four azobenzenes (5a) and four p-cyclohexylazobenzenes (5b), respectively. To prepare O-octacarboxymethoxylated CRA derivative (CRA-CM), 1 was transformed into O-octaethoxycarbonyloxymethoxylated CRA with four chloropropyl residues (6). 6 was converted into the corresponding iodopropyl derivative (7), which was reacted with a 4-hydroxyazobenzene to give the corresponding octaester of CRA having four azobenzene residues (8) which was hydrolyzed in an alkaline condition to yield CRA-CMs with four azobenzene residues (9). They were soluble in THF, but hardly soluble in toluene. UV absorption data of 5 and 9 having azobenzenes are summarized in Table 1. The changes in absorption spectra of 9a in THF upon 365 nm light irradiation indicated the normal E/Z photoisomerization to give 99% of Z-isomer at a photostationary state. The appearance of isosbestic points implies that there is no molecular interaction among the azobenzene chromophores to form H-aggregates intramolecularly though they stretch out from the lower rim of the cyclic skeleton in the same direction, just as in the case of CRA having four p-butylazobenzene units (5b).12 The same situation was observed for 9b. Photochemistry in Monolayers on a Water Surface. Figure 2 shows π-A isotherms of 9a and 9b. Both of them agree with each other except for collapse pressures. A higher collapse pressure for 9b is evidently due to the introduction of a cyclohexyl substituent, which shows a cross-sectional area similar to that of azobenzene. The occupied areas of both macrocyclic amphiphiles are estimated to be 1.7 nm2 and prominently in line with those of crown conformers of CRA-CM derivatives substituted with long-alkyl chains instead of azobenzenes.12 The area
Figure 3. Spectral changes of (a) monolayers of 9a and (b) of 9b on a water surface at various surface pressures. Each inset shows absorbances at λmax at various surface pressures.
agrees well with that obtained by a space-filling model. These facts indicate that the occupied area of the CRACMs on a water surface is essentially determined by the base area of the CRA framework irrespective of the nature of substituents tethered from the methylene bridges. Figure 3 shows absorption spectra of the CRA-CMs (9a and 9b) on a water surface at various surface pressures. It is worthy to stress here that both the absorbance and wavelength of the π,π*-absorption band are not altered at all during surface compression for both 9a and 9b. This fact indicates strongly that the compression of monolayers induce neither molecular reorientation nor molecular interactions among azobenzenes. This makes a sharp contrast to our previous observation that CRA substituted with p-butylazobenzene residues (4b) with an occupied area of 1.3 nm2 displays a considerable blue shift of π,π*absorption band due to the formation of a H-aggregate in a monolayer during compression. Figure 3 tells us also that λmax of azobenzene chromophores in monolayers of the CRA-CMs (9) is not far from that in solution, informing that essentially no aggregation is formed even in densely packed monolayers. The increase in a two-dimensional area of 0.4 nm2 plays a significant role to suppress thoroughly the molecular aggregation of azobenzene moieties. Observation with Brewster angle microscopy
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Langmuir, Vol. 14, No. 16, 1998 4499
Figure 4. Spectral changes of monolayers of 9a at surface pressures of (a) 0.0 and (b) 20.0 mN m-1 and 9b at surface pressures of (c) 0.0 and (d) 20.0 mN m-1 on a water: (1) before UV irradiation; (2) after UV irradiation; (3) after subsequent visible light irradiation.
revealed that monolayers of both 9a and 9b on a water subphase exhibit a homogeneous appearance during compression. All of these results give us a conclusion that the CRA-CMs (9) form quite stable monolayers on water while their occupied areas are strictly determined by the base of the cyclic skeleton so that molecular interactions among the chromophores are effectively suppressed. Figure 4 shows the spectral alteration of monolayers of both 9a and 9b upon UV and subsequent visible light irradiation. Whereas the extent of E-to-Z photoisomerization accompanied by the increment of a cross-sectional area is partially reduced even at surface pressure ) 0 mN m-1, more than half of the E-isomer is transformed into the Z-counterpart under application of a surface pressure. Evidently, the expansion of an occupied area owing to the CRA-CM framework ensures a two-dimensional free volume required for the E-to-Z photoisomerization. The level of Z-isomer at a photostationary state (Y) was estimated by absorbances due to π,π*-absorption of UVirradiated samples according to the following expression (eq 1) for further discussion
Y ) [Z]/[E]0 ) (1 - A/A0)/(1 - Z/E)
(1)
where [Z] and [E0] denote the concentrations of Z-isomer
in a photostationary state and of E-isomer before UV irradiation, respectively, whereas A0 and A are absorbances at absorption maximum and Z and E are absorption coefficients of Z- and E-isomers at the absorption maximum. The content of Z-isomer was about 60% for both 9a and 9b. It was observed that the level of E-to-Z photoisomerizability in monolayers is not influenced by the introduction of a bulky cyclohexyl substituent. It is very likely that the cyclohexyl substituent of photoisomerized azobenzene chromophore is directed not to the outside of a space determined by the cylindrical CRA moiety but rather to the inside of the CRA cross section to minimize its occupied area. Monolayers on Substrate Plates. Monolayers of azomodified CRA-CM were obtained by the following two methods: LB deposition and an adsorption technique from solutions. LB films of 9a and 9b were prepared by transferring the corresponding monolayers on a water subphase onto quartz plates at a surface pressure of 20 mN min-1, respectively. Figure 5 shows spectral changes of LB films of both CRA-CM upon UV irradiation. It is evident that both cyclic compounds display efficient E-to-Z photoisomerizability even in LB films. The content of Z-isomer at a photostationary state in LB films was about 72% for both.
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Figure 6. Adsorption isotherms of (a) 9a and (b) 9b on a quartz surface from a 2-butanone solution at 60 °C. Figure 5. Spectral changes of LB films of (a) 9a and (b) 9b on quartz substrates during irradiation with 365 nm light.
As described in our previous article,12 crown conformers of CRAs and their derivatives adsorb on surfaces of polar solids such as a silica plate and a poly(vinyl alcohol) film from their solutions in nonpolar solvents to form densely packed, monomolecular layers. This provides us a convenient way to modify polar surfaces with functional monolayers using CRA derivatives. The adsorption behavior of CRA-CM with azobenzene residues was examined to approach to our goal to cover a surface of silica plates with azobenzene monolayers. Figure 6 shows the adsorption isotherms of 9a and 9b in butanone at 60 °C. The results were consequently analyzed according to the Langmuir adsorption equation (eq 2)
C/W ) (1/a Ws ) + (1/Ws ) C
(2)
where W and Ws stand for a coverage with CRA-CM on a quartz plate in molecules per nm2 from a solution of a concentration C in mol dm-3 and a saturated coverage with the adsorbate, respectively, while a denotes an equilibrium constant in dm3 mol-1. The extent of coverage with CRA-CM was estimated UV spectroscopically under the assumption that essentially no modification of absorption coefficient of azobenzene is induced by surface adsorption. The validity of this assumption was confirmed by the thorough desorption of 9a from a quartz plate in an ethanol solution dissolving 30 vol % ethanolamine to be subjected to UV absorption spectral measurement of the solution. The amount of adsorbed CRA molecules measured in the latter procedure was in good agreement with that estimated by taking a UV absorption spectrum of a plate.
The Langmuir plots are given in Figure 7. In both cases, a good linear relationship was obtained to allow us to estimate the values of a and Ws, which are 8.0 × 105 mol dm-3 and 0.53 molecules nm-2 for 9a and 1.2 × 106 mol dm-3 and 0.51 molecules nm-2 for 9b, respectively. These Ws values are approximately equal to those for CRA-CMs having alkyl substituents,12 indicating that the adsorption kinetics is governed specifically by the hydrophilic upper rim, being irrespective of the nature of substituents at the methylene bridges. It was confirmed thus that the CRAs with eight carboxymethoxy residues as cyclic azobenzene amphiphiles adsorb effectively on a silica surface. The occupied areas expressed as a reciprocal of Ws were calculated to be 1.9 nm2 and 2.0 nm2 for 9a and 9b, respectively. As stated above, the base area of CRA-CM amounts to 1.7 nm2 according to their π-A isotherm measurements and is not far from occupied areas of adsorbed molecules, implying that the surface adsorption technique leads to a simple procedure to cover a substrate surface with azobenzene monolayers in a densely packed state. Since CRAs having four normal alkyl chains adsorb on a silica plate surface in a range of Ws ) 0.72-0.77, as revealed in our previous report,12 the adsorption experiments for some CRAs substituted with azobenzenes at the methylene bridges from chloroform solutions were achieved to evaluate the effect of the nature of a hydrophilic rim on the properties of adsorbed layers. The results are compiled in Table 2. Contrary to our expectation that CRAs with azobenzene chromophores with a smaller base area of 1.3 nm2 would give occupied areas smaller than that of the corresponding CRA-CMs, no marked difference in the occupied areas was observed between CRAs and
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Figure 7. Langmuir plots for (a) 9a and (b) 9b in 2-butanone at 60 °C. Table 2. UV Absorption Spectral Data of Monolayers of Azobenzene-Modified CRA Derivatives Adsorbed on Quartz Plates
compound
isomer for surface adsorption
λmax (nm)
occupied area (nm2/molecule)
content of Z-isomer (%)
5a 5c 9a 9a 9b 9b
E E E Z E Z
342.0 340.0 342.5 344.0 345.5 346.0
1.7 1.9 1.9 2.0 2.0 2.6
n.d. 80 87 85 85 91
CRA-CMs. This may reflect both the bulkiness and the mobility of azobenzene chains, when compared with longchain alkyl chains, to increase an apparant cross-sectional area of CRAs. In other words, cross-sectional areas of both CRAs and CRA-CMs may be determined by the nature of the azobenzene residues. Table 2 presents also the influence of isomeric structures of azobenzene chromophores on adsorption behavior. Before adsorption experiments, a solution of azo-modified 9a or 9b was exposed to UV light to form a Z-isomer in photostationary states. After immersion of a quartz plate in the photoirradiated solution, the plate was heated at 80 °C for the recovery of E-isomer to be subjected to absorption spectral measurement. It was found that occupied areas of the CRA-CM are enlarged when the azobenzenes are in the Z-form, showing that the conversion into Z-isomer gives an alternate method to control two-dimensional density of the CRA-CM. Isomerization in Adsorbed Monolayers. Figure 8 shows absorption spectra of 9a and 9b adsorbed on quartz plates before and after UV irradiation at 365 nm, respectively. It is clearly seen that E-to-Z photoisomer-
Figure 8. Absorption spectra of (a) 9a and (b) 9b adsorbed on quartz plates before (spectrum 1) and after (spectrum 2) UV irradiation.
ization takes place more effectively in adsorbed monolayers than that in tightly compressed monolayers on a water surface as a result of larger occupied areas of the former. The results are summarized in Table 2. It is shown here that the level of Z-isomer after UV irradiation is enhanced when the CRA-CM is adsorbed from a solution of Z-isomer. In particular, a plate adsorbing Z-isomer of 9b, followed by thermal reversion, displays a higher E-to-Z photoisomerizability in line with a larger occupied area of 2.6 nm2. Photoisomerization behavior of adsorbed azobenzenes was followed by recording absorption spectra during UV irradiation and analyzed according to the following expression
ln([E]0 - [E]∞)/([E] - [E]∞) ) [E]0/[E]∞(B + K)t (3) where [E]0, [E]∞, and [E] are concentrations of E-isomer before, after, and during UV irradiation at time t whereas A, B, and K are constants. The results are summarized in Figure 9. All first-order plots deviate from linearity, indicating that the E-to-Z photoisomerization in adsorbed monolayers suffers from steric constraint more or less because of existence of a two-dimensional free volume. The deviation from the linearity is markedly observed for 9b when the cyclic compound is adsorbed in E-form. Evidently, this is due to the bulky cyclohexyl substituent exhibiting a larger sweep volume for the E/Z photoisomerization.
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Figure 9. First-order plots for E f Z photoisomerization in monolayers prepared by surface adsorption of (O) E-form 9a, (b) Z-form of 9a, (4) E-form of 9b and (2) Z-form of 9b. Monolayers were annealed before 365 nm light irradiation for Z-to-E thermal reversion.
Conclusion The CRA-CM moiety possesses the following features in assembling monomolecular layers exhibiting E/Z photoisomerizability which is accompanied by a relatively large sweep volume. The first is based on the ability to form stable and homogeneous monolayers on a water surface due to the cyclic framework with amphiphilic characteristics. An occupied area of 1.7 nm2 is determined specifically by the base of the cylindrical macrocycles irrespective of the nature of four substituents tethered to the methylene bridges, leading to the ease in the molecular design of functional monolayers.
Fujimaki et al.
The second feature arises from multisite adsorptivity on a silica surface through hydrogen bonds due to eight polar substituents attached to the lower rim of the cyclic structure. Equilibrium constants are in a high range of 106 dm3 mol-1 so that absorbed monolayers with photoreactivity are readily available simply by immersing a silica plate in a dilute solution of CRA-CM substituted with azobenzenes. The occupied area of the CRA-CMs substituted with azobenzenes is ∼2.0 nm2 or less so that relatively densely packed monolayers with photofunctionality become readily available even though four residues with complicated azo-chromophores are tethered to the cylindrical CRA structure. The third point of the macrocyclic amphiphiles is that the E-to-Z photoisomerizability of azobenzene chromophores is secured to a reasonable extent in contrast to our previous report that azobenzene-modified CRA displays quite a low extent of E-to-Z photoisomerizability on a water surface. Approximately half of the azobenzene units are transformed into Z-isomer of CRA-CM under UV irradiation in closely packed monolayers on a water surface while about 90% of the E-isomer is converted into Z-isomer in surface-adsorbed monolayers. The E-to-Z photoisomerizability is enhanced to some extent by UV irradiation of a solution of CRA-CM with azobenzene residues, followed by heat treatment to lead to the thermal reversion of the isomer. These facts indicate that CRA-CM derivatives provide a promising way to assemble monolayers with photoreactive moieties both by the LB deposition technique and by surface-adsorption from dilute solutions. Acknowledgment. This work was supported by the Grant-in-Aid for Priority-Area-Research on “Supramolecular Architectures” (No. 07141222 for K. I.) from the Ministry of Education, Sports, Science and Culture. LA971257M
