Macrocyclic Amphiphiles. 1. Properties of Calix[4]resorcinarene

Dec 10, 1997 - Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan, and Instit...
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Macrocyclic Amphiphiles. 1. Properties of Calix[4]resorcinarene Derivatives Substituted with Azobenzenes in Solutions and Monolayers Kunihiro Ichimura,*,† Noriaki Fukushima,† Masanori Fujimaki,† Sumie Kawahara,† Yoko Matsuzawa,† Yuko Hayashi,† and Kazuaki Kudo‡ Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan, and Institute of Industrial Science, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106, Japan Received July 2, 1997. In Final Form: October 1, 1997X Photochemical behavior of calix[4]resorcinarenes (CRAs) and O-octaacetylated derivatives having four azobenzene residues at their lower rim in solutions and monolayers is described. UV irradiation of an O-octaacetylated CRA in homogeneous solution gave rise to a photostationary state consisting of a possible six isomers as a result of normal photoisomerization. The compound exhibited reversible precipitation/ dissolution cycles in a diluted methanol solution upon alternate irradiation with UV and visible light owing to its scarce solubility of all-trans isomer. Wherease azobenzene chromophores tethered from the cyclic skeleton displayed marked intermolecular interactions in monolayers on a water surface, trans-to-cis photoisomerization took place even in compressed monolayers. The level of the photoisomerizability was dependent on the chemical structure and surface pressures. Monolayers of azobenzene CRAs exhibited the reversible alteration of their areas under a constant surface pressure upon UV and visible light irradiation because of the drastic shape change of the chromophores.

Introduction The acid-catalyzed condensation of resorcinol with an equimolar amount of an aldehyde gives cyclic isomers termed calix[4]resorcinolarenes (CRA) consisting of four resorcinol moieties linked by alkylidene bridges.1 Owing to the intramolecular hydrogen bond formation of the phenolic hydroxyl groups, a crown conformer of the cone structure, in which four residues derived from the aldehyde are tethered from the lower rim of the cylindrical structure in the same direction, is thermodynamically most stable among the others having partial cone, 1,2-alternate and 1,3-alternate structures and forms preferentially after prolonged reaction.2 Because of its unique molecular structure, the crown isomers have been attracting intensive interest connected with assembling supramolecular systems exhibiting molecular recognition functions.3 The crown structure possesses an amphiphilic character in nature as far as hydrophobic aldehydes are used for the cyclomerization. In fact, the crown isomer of a CRA derived from long-chain aliphatic aldehyde forms a stable monolayer film at a water/air interface.4 Thus, the isomers with crown structure can be regarded as macrocyclic amphiphiles with fixed molecular frameworks having hydrophilic residues at the upper rim. A typical structure of amphiphilic compounds consists usually of a hydrophobic long alkyl chain substituted with a hydrophilic residue at the end of the molecule to stabilize * Author to whom correspondence should be addressed. Tel.: +81-45-924-5266. Fax: +81-45-924-5276. E-mail: ichimur@ res.titech.ac.jp. † Tokyo Institute of Technology. ‡ University of Tokyo. X Abstract published in Advance ACS Abstracts, November 15, 1997. (1) Vincens, J.; Bo¨hmer, V. CalixarenessA Versatile Class of Macrocyclic Compounds: Klumwer Academic Publishers: Dordrecht, 1991. (2) Ho¨gberg, A. G. S. J. Am. Chem. Soc. 1980, 102, 6046. (3) (a) Aoyama, Y.; Tanaka, Y.; Sugahara, S. J. Am. Chem. Soc. 1989, 111, 5397. (b) Tanaka, Y.; Kato, Y.; Aoyama, Y. J. Am. Chem. Soc. 1990, 112, 2807. (c) Kikuchi, Y.; Kato, Y.; Tanaka, T.; Toi, H.; Aoyama, Y. J. Am. Chem. Soc. 1991, 113, 1349. (d) Kikuchi, Y.; Kobayashi, K.; Aoyama, Y. J. Am. Chem. Soc. 1992, 114, 1451. (4) Kurihara, K.;Ohta, K.; Tanaka, Y.; Aoyama, Y.; Kunitake, T. Thin Solid Films 1989, 179, 21.

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interfaces between two different phases. Molecular organization of amphiphilic compounds formed at interfaces is based on the hydrophobic interaction between long alkyl chains and polar residues with surfaces having a higher surface energy.5 This situation led us to regard the crown isomer as a fragment of these assembled monomolecular layers and to develop organic thin films with various functionalities by synthesizing CRA derivatives having functional residues. Azobenzenes have been employed most extensively as representative photoactive molecules triggering photoresponsiveness of materials on account of their good availability and acceptable photofatigue resistance. Particular interest has been concentrated on the exploitation of photofunctionality of ultrathin films embedding azobenzene chromophores to lead to the photoregulation of versatile physical properties6 including ionic permeability, membrane potential, and electroconductivity and of alignment of liquid crystals.7 In this context, crown conformers substituted with four azobenzene groups at the same rim play a significant role as building blocks for photofunctional thin films owing to their unique structure in which the photoactive chromophores stretch out from the rim of the cyclic skeleton in the same direction. In fact, we have recently observed that a CRA having four azobenzene residues at the upper rim adsorbs on colloidal silica, leading to the photocontrolled dispersibility of the particles in a nonpolar solvent upon alternate exposure to UV and blue light for the reversible photoisomerization.8 The photofunctionalization of monomolecular films incorporating azobenzene units is realized by guaranteeing a free volume in films for trans-to-cis photoisomerization since the photochemical process requires a sweep volume. The photoisomerizability of azobenzenes in densely packed molecular films has been so far ensured by employing (5) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (6) Anzai, J.; Osa, T. Tetrahedron 1994, 14, 4039. (7) Ichimura, K. In Electrooptical and Photooptical Polymers as Active Media; Shibaev, V., Ed.; Elsevier: New York, 1996; p 138. (8) Ueda, M.; Fukushima, N.; Kudo, K.; Ichimura, K. J. Mater. Chem. 1997, 7, 641.

© 1997 American Chemical Society

Properties of Calix[4]resorcinarene Derivatives

polymeric chains as hydrophilic head groups,9,10 bulky hydrophilic head groups,11 or cyclodextrin inclusion complexes12 and by mixing of amphiphilic azobenzenes with a shorter alkyl surfactant.13 It should be stressed here that the area of the base of the cyclic skeleton of CRA amounts to ca. 1.3 nm2 4,14 whereas the cross-sectional area of four azobenzenes is approximately 1.0 nm2 since each azobenzene has a cross-sectional area of 0.25 nm2. This rough estimation indicates that even a closely packed monolayered film of a kind of CRA substituted with azobenzene chromophores possesses a two-dimensional free volume of ca. 0.3 nm2, leading to photoisomerizability. During our extensive studies on this type of photoactive CRA derivatives, closely related work has been recently reported on the behavior of monolayers of calixarenes substituted with azobenzenes on a water surface.15 The major concern of this paper is to reveal the photochemical behavior of CRAs with azobenzenes in solution and in monolayers on a water surface and to present a novel procedure to give densely packed monolayers exhibiting reasonable photoisomerizability. Experimental Sections Azobenzene Acetals (1). 4-[4-(4-Butylphenylazo)phenoxy]butanal diethyl acetal (1a) was prepared according to our previous paper.8 The other derivatives were prepared in a similar way. 4-[4-(4-Octylphenylazo)phenoxy]butanal diethyl acetal (1b) of mp 37.0-39.0 °C in a 81.7% yield. Anal. Found: C, 73.57; H, 9.02; N, 6.28%. Calcd. for C28H42N2O3: C, 73.97; H, 9.31; N, 6.16%. 1H-NMR (CDCl3): δ 0.89 (t, 3H, CH3CH2(CH2)5CH2-Ar), 1.2-1.4 (t + br, 13H, CH3CH2O + CH3CH2(CH2) 5CH2-Ar), 1.6 (br, 2H, CH3CH2(CH2)6-Ar), 1.90 (br, 4H, -OCH2CH2CH2CH-), 2.69 (t, 2H, CH3CH2(CH2)5CH2-Ar), 3.60 (m, 4H, -OCH2CH3), 4.06 (t, 2H, -OCH2CH2-), 4.55 (t, 1H, CH(OCH2CH3)2), 6.9-8.0 (m, 8H, Ar). 4-[4-(4-Cyclohexylphenylazo)phenoxy]butanal diethyl acetal (1c) of mp 112.5-113.5 °C in a 36.9% yield. Anal. Found: C, 73.71; H, 8.26; N, 6.53%. Calcd. for C26H36N2O3: C, 73.55; H, 8.55; N, 6.60%. 1H-NMR (CDCl3): δ 1.22 (t, 6H, CH3CH2O), 1.40 (br, 10H, (CH2)5CH), 1.80 (br, 4H, -OCH2CH2CH2CH2-), 2.56 (m, 1H, (CH2)5CH-Ar), 3.60 (m, 4H, -OCH2CH3), 4.06 (t, 2H, -OCH2CH2CH2CH2-), 4.55 (t, 1H, CH(OCH2CH3)2), 6.9-8.0 (m, 8H, Ar). 4-(4-Phenylazo)phenoxybutanal diethyl acetal (1d) of mp 37.0-39.0 °C in a 72.9% yield. Anal. Found: C, 70.14; H, 7.88; N, 8.16%. Calcd. for C20H26N3O2: C, 70.15; H, 7.65; N, 8.18%. 1H-NMR (CDCl3): δ 1.23 (t, 6H, CH3CH2O), 1.90 (br, 4H, -OCH2CH2CH2CH2-), 3.60 (m, 4H, -OCH2CH3), 4.05 (t, 2H, -OCH2CH2 CH2CH2-), 4.55 (t, 1H, CH(OCH2CH3)2), 6.98.0 (m, 8H, Ar). Azobenzene Calix[4]resorcinarenes (2). 2,8,14,20-Tetrakis{3-[4-(4-butylphenylazo)phenoxy]propyl}-4,6,10,12,16,18, 22,24-octahydroxycalix[4]arene (4AzCRA; 2a) was prepared and purified according to our previous work.8 The other calix[4]arenes were prepared in a similar manner, but their purification was not attained because no crystalline product was obtained and chromatographic separation failed due to strong adsorptivity of CRAs. Octaacetoxylated Azobenzene Calixarenes (3). 2,8,14, 20-Tetrakis{3-[4-(4-butylphenylazo)phenoxy]propyl}-4,6,10, 12,16,18,22,24-octaacetoxycalix[4]arene (4AzCRA-Ac; 3a) as a typical example: To a suspension of 0.30 g of 4AzCRA in 9 mL of acetic anhydride was added 0.3 mL of pyridine at 50 °C to give (9) Nishiyama, K.; Fujihira, M. Chem. Lett. 1988, 1257. (10) Seki, T.; Ichimura, K. Polym. Commun. 1989, 30, 108. (11) Tachibana, H.; Nakamura, T.; Matsumoto, M.; Komizu, H.; Manda, E.; Niino, H.; Yabe, A.; Kawabata, T. J. Am. Chem. Soc. 1989, 111, 3080. (12) 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. (13) Maack, J.; Ahuja, R. C.; Tachibana, H. J. Phys. Chem. 1995, 99, 9221. (14) Kurita, E.; Fukushima, N.; Fujimaki, M.; Matsuzawa, Y.; Kudo, K.; Ichimura, K. J. Mater. Chem. in press. (15) Tyson, J. C.; Moore, J. L.; Hughes, K. D.; Collard, D. M. Langmuir 1997, 13, 2068.

Langmuir, Vol. 13, No. 25, 1997 6781 a homogeneous solution which was stirred for 4 h at 80 °C. The reaction mixture was added to 1 L of water, and an orange precipitate was collected by filtration and recrystallized from a mixture of chloroform and methanol to give 0.28 g of orange crystals melting at 189.5-191.0 °C. Anal. Found: C, 71.62; H, 6.63; N, 5.87%. Calcd. for C120H128N8O20: C, 71.97; H, 6.64; N, 5.60%. 1H-NMR (CDCl3): δ 0.94 (t, 12H, CH3), 1.37 (m, 8H, CH3CH2), 1.54-1.66 (m, 24H, CH2), 2.14 (m, 24H, CH3CO), 2.65 (t, 8H, ArCH2), 3.95 (t, 8H, OCH2), 4.29 (t, 4H, CH), 6.84 (d, 8H, Ar-H of azobenzene, J ) 9 Hz), 6.96 (broad s, 4H, Hc), 7.23 (d, 8H, Ar-H of azobenzene, J ) 9 Hz), 7.75 (double d, 16H, Ar-H of azobenzene, J ) 9 Hz). The proton signal of Hb was not detected at room temperature. The following octaacetylated compounds were synthesized by acetylation of the corresponding crude calixresorcinarene in a similar way. 2,8,14,20-Tetrakis{3-[4(4-octylphenylazo)phenoxy]propyl}-4,6,10,12,16,18,22,24octaacetoxycalix[4]arene (8AzCRA-Ac; 3b) as orange crystals in a 33% yield starting from 1b. Anal. Found: C, 73.21; H, 7.23; N, 5.19%. Calcd. for C136H160N8O20: C, 73.36; H, 7.24; N, 5.03%. 1H-NMR (CDCl ): δ 0.91 (t, 12H, CH ), 1.2-1.4 (m, 64H, CH ), 3 3 2 1.5-1.7 (m, 64H, CH2), 2.15 (broad s, 24H, CH3CO), 2.64 (t, 8H, ArCH2), 3.96 (t, 8H, OCH2), 4.29 (t, 4H, CH), 6.84 (d, 8H, Ar-H of azobenzene, J ) 9 Hz), 6.97 (broad s, 4H, Ar-Hc), 7.23 (d, 8H, Ar-H of azobenzene, J ) 9 Hz), 7.75 (double d, 16H, Ar-H of azobenzene, J ) 9 Hz). 2,8,14,20-Tetrakis{3-[4-(4-cyclohexylphenylazo)phenoxy]propyl}-4,6,10,12,16,18,22,24-octaacetoxycalix[4]arene (C6AzCRA-Ac; 3c) in a 30% yield starting from the corresponding acetal (1c). Anal. Found: C, 72.98; H, 6.47; N, 5.42%. Calcd. for C128H136N8O20: C, 72.70; H, 6.48; N, 5.68%. 1H-NMR (CDCl ): δ 1.2-1.9 (m, 56H, CH ), 2.15 (broad, s, 24H, 3 2 CH3CO), 2.56 (m, 4H, (CH2)5CH-Ar), 3.94 (broad, t, 8H, OCH2), 4.29 (t, 4H, CH), 6.84 (d, 8H, Ar-H of azobenzene, J ) 9 Hz), 6.97 (broad, s, 4H, Ar-Hc), 7.26 (d, 8H, Ar-H of azobenzene, J ) 9 Hz), 7.75 (double d, 16H, Ar-H of azobenzene, J ) 9 Hz). 2,8,14,20Tetrakis[3-(4-phenylazophenoxy)propyl]-4,6,10,12,16,18,22,24octaacetoxycalix[4]arene (0AzCRA-Ac; 3d) in a 26% yield starting from 1d. Anal. Found: C, 70.22; H, 5.52; N, 6.35%. Calcd. for C104H96N8O20: C, 70.26; H, 5.44; N, 6.30%. 1H-NMR (CDCl3): δ 1.59 (broad, 16H, -OCH2CH2CH2CH2-), 2.16 (broad, s, 24H, CH3CO), 3.96 (t, 8H, OCH2 CH2CH2CH2), 4.28 (t, 4H, CH), 6.85 (d, 8H, Ar-H of azobenzene, J ) 9 Hz), 6.97 (broad s, 4H, Ar-H), 7.38-7.44 (m, 12H, Ar-H of azobenzene), 7.78-7.82 (double d, 16H, Ar-H of azobenzene, J ) 9 Hz). Physical Measurements. UV-visible absorption spectra were taken on a diode array spectrometer (Hewlett-Packard HP8452A) while NMR spectra were recorded on an FT-NMR spectrometer (JEOL FX-90Q), respectively. Brewster angle microscope images were taken with an NL-EMM633 (Nippon Laser Electronics). Photoirradiation of Solutions. A solution of 4AzCRA-Ac in a 1:2 (v/v) mixture of ethyl acetate and chloroform was irradiated with 365 nm light from a 500 W super-high-pressure mercury arc through a band-pass filter (UV-D36B; Toshiba). Samples of 20 µL were taken out at intervals during photoirradiation to be subjected to high-performance liquid chromatography (HPLC) analysis using a multichannel photodiode array spectrometer (MCPD-350, Ohtsuka Electronics). Product distribution was estimated by the peak area of each fraction which was monitored by following absorbances at an isosbestic point ()305 nm) during photoirradiation. The validity of this method is supported by the appearance of the isosbestic point in solution photochemistry. The trans-to-cis ratio of each isomer was simply estimated by the absorbance of π,π*-absorption band of a transisomer relative to that at the isosbestic point. Observation of Solubility Change. A methanol solution of 4AzCRA filled in a quartz cuvett was placed in a cell holder of a photodiode array spectrometer (Hewlett-Packard HP8452A) and irradiated with light from a 500 W super-high-pressure mercury arc passed through a suitable glass filter to select 365 nm or 436 m light. The turbidity alteration was monitored by measuring the transmittance at 630 nm. π-A Isotherm Measurements. Measurements of π-A isotherms were performed on pure water (Milli-Q) filled in a USI FSD-110 film balance at 20 °C. A spreading solution was prepared by dissolving CRA derivatives in chloroform (4.5-6.6

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Ichimura et al. Table 1. UV Absorption Properties of 1, 2, and 3 in THF compound

λmax (nm)

 (dm3 mol-1 cm-1)

half-value width (nm)

4Az (1a) 4AzCRA-Ac (3a) 8Az (1b) 8AzCRA-Ac (3b) CHAz (1c) CHAzCRA-Ac (3c) 0Az (1d) 0AzCRA-Ac (3d)

351.5 349.5 352.0 349.0 352.0 349.0 348.5 344.5

3.07 × 104 1.13 × 105 2.92 × 104 1.17 × 105 3.00 × 104 1.18 × 105 2.50 × 104 9.81 × 104

58.5 61.0 57.5 60.0 60.5 58.5 57.0 60.0

Figure 1. Azobenzene acetals and CRA derivatives. × 10-4 mol dm-3). After the completion of solvent evaporation for 15 min, a monolayered film was compressed at a speed of 10.0 mm min-1. Absorption Spectral Measurements and Photoirradiation of Monolayers on Water. A chloroform solution of a CRA was spread on water in the dark in a manner similar to that for π-A isotherm measurements. UV-visible absorption spectra of monolayers on water were recorded on a multichannel photodiode spectrometer MCPD-1000 (Ohtsuka Electronics) equipped with a quartz optical fiber. The whole area of a monolayer on water was irradiated with expanded light from a 500 W super-highpressure mercury arc passed through a band-pass filter (UVD36B) or a combination of a cut-off filter (Y-43; Toshiba) and a neutral density filter (ND50%; Toshiba) for the selection of 365 nm light or visible light. The photoreaction was followed by taking absorption spectra on the same spectrometer.

Results and Discussion Synthesis and Structural Elucidation. Resorcinol reacts with an aldehyde in boiling ethanol in the presence of hydrochloric acid to give a calix[4]resorcinarene.2,3 Because the precipitation of condensation product(s) took place at the early stage of heating of an ethanol solution of an azobenzene acetal (1) and resorcin, the solvent was replaced by 2-methoxyethanol in order to prevent from the heating of the reaction mixture in a heterogeneous state. Among the acetals employed, the butylated azobenzene (1a: R ) C4H9) gave a crystalline cyclotetramer (4AzCRA; 2a) and was purified by recrystallization (Figure 1). Purification of the other CRAs (2b-d) were unsuccessful so that purified materials were obtained after acetylation to give 3, followed by column chromatography. The crown conformation of 4AzCRA was elucidated by NMR spectra. The appearance of two signals due to phenolic OH groups indicates that four OH groups are intramolecularly hydrogen-bonded while the others are free from the hydrogen bond.3 Both aromatic protons (Hb and Hc) of phenyl rings derived from resorcinol appear as a singlet in a temperature range from -60 to 50 °C, supporting the C4v symmetry of the molecule. These observations indicate that the cyclotetramer has a crown structure. The acetylation breaks down the intramolecular hydrogen bond of AzCRA so that the molecular flexibility of the ring system is enhanced. This situation was determined by the NMR spectra at various temperatures. Both Hb and Hc protons exhibit temperature-dependence. The

Figure 2. Distribution of photoisomers of 4AzCRA-Ac as a function of UV irradiation time.

signal of the Hb protons are split into two peaks at -50 °C while the signals become so broad that no absorption is detected at room temperature and at 50 °C. The protons of acetyl groups are also temperature-dependent; two peaks observed at -50 °C are combined at higher temperatures. These all give evidence that the ring system of 4AzCRA-Ac (3a) is fixed to form a boat conformation at a lower temperature and that at elevated temperatures four phenyl rings of the CRA moiety display a rapid motion to form a crown conformation. Quite similar molecular motion was observed for the other three CRA-Ac derivatives (3b-d), judged from their temperature-dependence of the NMR spectra. Photoisomerization in Solutions. The π,π*-absorption band of 4AzCRA is essentially identical with that of the corresponding octaacetate (4AzCRA-Ac) and displays a slight blue shift of 2 nm when compared with the monomeric compound (1a). As summarized in Table 1, the absorption maxima of the cyclotetramers are all blueshifted slightly, and the half-value width of the π,π* band increases slightly for the cyclotetramers except the cyclohexyl-substituted derivative (CHCRA-Ac). These suggest that there is an intramolecular interaction in the multichromphoric systems in solution. In a striking contrast to this, λmax of cis-isomers are not influenced by whether the chromophores are attached to the cyclic skeleton, indicating that there is essentially no intramolecular interaction for the cis-isomers in solution. The photoisomerization of 4AzCRA-Ac in a solution of a 1:2 v/v mixture of ethyl acetate and chloroform under 365 nm irradiation was followed by monitoring absorbances at an isosbestic point of 305 nm by means of HPLC. Whereas six isomers consisting of cyclic tttt, tttc, tctc, ttcc, tccc, and cccc should exist, five fractions were detected during the photoreaction. Here t and c stand for transand cis-isomers, respectively. Spectral analysis of each fraction indicated that two isomers of ttcc and tctc are not separated under the present condition. Figure 2 shows the changes in the distribution of each isomer as a function of irradiation time. The predominant isomer is cccc at a photostationary state whereas the level of the other isomers except for tccc is quite small. Assuming that the

Properties of Calix[4]resorcinarene Derivatives

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Figure 4. π-A isotherms of 4AzCRA and 4AzCRA-Ac at an air/water interface.

Figure 3. Transmittance changes of a 1.5 × 10-4 mol dm-3 solution of 8AzCRA-Ac in methanol upon alternate irradiation with 365 and 436 nm light.

cis content (x) at a photostationary state is not influenced at all by isomeric structures, the probability of the formation of each isomer should be (x/100)4, 4(x/100)3[(100 - x)/100], 6(x/100)2[(100 - x)/100]2, 4(x/100)[(100 - x)/ 100]3, and [(100 - x/100)]4 for cccc, ccct, cctt, and ctct, cttt, and tttt, respectively. On the basis of the percentage of each isomer shown in Figure 2, the conversion was calculated to be 95% as a mean value. The complete thermal reversion into tttt occurred upon storage of a photoirradiated solution in the dark at room temperature for a month. Photoinduced Solubility Change. During the course of the photoisomerization experiment of 8AzCRA-Ac in solutions, it was found that photoirradiation of a methanol solution of the compound results in the reversible solubility change even though a concentration is very low in a range of 10-4 mol dm-3. Figure 3 shows an example of the solubility alteration of 8AzCRA-Ac upon alternate irradiation with UV and visible light at 436 nm for the trans-cis photoisomerization. The solubility changes were followed by the monitoring of the transmittance of a solution at 630 nm light. Before UV irradiation, a solution was turbid because of scarce solubility in methanol. UV irradiation at 365 nm caused the formation of a transparent solution which became again turbid upon irradiation with visible light. The transmittance increased gradually after repeated UV irradiation as seen in Figure 3, because the CRA derivative was deposited on a UVirradiated cell wall and not completely dissolved upon visible light irradiation. Although it was reported that photoisomerization of photochromic side chains attached to polymer backbones leads to the reversible solubility modification of the polymers,16 such a marked solubility photocontrol of a low-mass photochromic compound has been hardly described. A representative compound exhibiting a remarkable solubility change triggered by photoirradiation is bilirubin which becomes water-soluble upon visible light irradiation, providing the working principle for the phototherapy for neonatal jaundice.17 Such a noticeable solubility modification of low-mass photochromic molecules results evidently from the cooperative work of multichromophoric systems. Photochemical Properties of Monolayers on a Water Surface. It was described that a stable monolayer (16) (a) Irie, M.; Tanaka, J. Macromolecules 1983, 16, 2418. (b) Irie, M.; Iwayamagi, T.; Taniguchi, Y. Macromolecules 1985, 18, 2418. (17) McDonagh, A. F.; Palma, L. A.; Lightner, D. A. J. Am. Chem. Soc. 1982, 104, 6865.

Figure 5. λmax and absorbances of the π-π* transition of the azobenzene chromophore of monolayered 4AzCRA as a function of surface pressure.

is formed when CRA derived from resorcinol and dodecanal is spread on a water surface, exhibiting an occupied area of 1.3 nm2 which is in line with that estimated by a spacefilling molecular model.4,14 This suggests that the density of a closely packed CRA monolayer is determined specifically by the base area of the cyclic framework, being independent of a cross-sectional area of substituents tethered from the lower rim of the tetramer. As suggested in the Introduction, this situation allows us to ensure a free volume for trans-to-cis photoisomerization in a densely packed monolayer whereas the photoisomerization accompanied by a sweep volume is strictly suppressed for monolayers of long alkyl amphiphilic molecules incorporating an azobenzene chromophore. Figure 4 shows π-A isotherms of 4AzCRA and 4AzCRAAc. The other acetylated CRAs exhibited π-A isotherms similar to that of 4AzCRA-Ac. The results are summarized as follows. First, all compounds display high collapse pressures, supporting the considerable stability of monolayers. Second, an occupied area of 4AzCRA is estimated to be 1.3 nm2 and equal to that of CRA having four long alkyl chains. Thirdly, the areas of octaacetylated azobenzene CRAs are not influenced by the nature of a substituent at the azobenzene chromophore at all. These facts indicate that the occupied area is determined in fact solely by the base area of CRA. Absorption spectra of a 4AzCRA monolayer on a water surface was influenced by surface pressures. Figure 5 shows λmax and absorbance of π-π* transition as a function of surface pressure. When compared with λmax of the CRA in solution (Table 1), λmax of a monolayer suffered from a marked blue shift up to 340 nm though no surface pressure was applied. Observation with Brewster angle microscopy of a monolayer on water was in accordance with this spectral characteristic; a monolayer on a water surface forms iceberg-like domain structures with sharp edges even without a surface pressure, suggesting that 4AzCRA molecules aggregate readily on water. The increase in a surface pressure results in the increase in the π-π* absorbance at the early stage of compression, reflecting

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Figure 6. Spectral changes of a monolayer of 4AzCRA on a water surface at surface pressures of (a) 0.0 mN/m and (b) 16.5 mN/m before (spectra-1) and after irradiation with UV (spectra2) and visible light (spectra-3).

the increment of a two-dimensional concentration of the CRA. Further compression causes a considerable blue shift, accompanied by the reduction of absorbance. This situation indicates that azobenzene chromophores form an H-aggregate. In a sharp contrast to the marked blue shift for 4AzCRA, λmax of the O-octaacetylated 4AzCRAAc on water showed only a slight blue shift. The difference in spectral alteration between 4AzCRA and 4AzCRA-Ac is ascribed evidently to that in their occupied areas. Photoinduced spectral changes of monolayers of 4AzCRA on a water surface were achieved to evaluate the photoisomerizablity and shown in Figure 6. Under no surface pressure application, UV irradiation of a monolayer at 365 nm resulted in the decrease in the absorption band due to trans-to-cis isomerization whereas reverse photoisomerization was induced by exposure of the monolayer to 436 m light. An absorption spectrum at a photostationary state had λmax at ca. 330 nm which was much longer than that of cis-isomer in solution. This implied that the geometrical photoisomerization is suppressed more or less even without a surface pressure, probably due to intermolecular interactions including intermolecular hydrogen-bond formation of phenolic OH groups at the lower rim. The surface pressure application gives rise to considerable suppression of trans-to-cis photoisomerization (Figure 6b). It is noteworthy to mention again that λmax of a monolayer at a photostationary state under 365 nm light irradiation is centered at around 330 nm while the absorption band decreases only slightly. This spectral behavior exhibits the sufficient suppression of photoisomerization because of a restricted free volume in a monolayer to result in the partial generation of the cisisomer leading to the dissociation of an H-aggregate of trans-isomer. On the contrary, trans-to-cis photoisomerization takes place sufficiently for an octaacetylated

Ichimura et al.

Figure 7. Spectral changes of a monolayer of 4AzCRA-Ac on a water surface at surface pressures of (a) 0.6 mN/m and (b) 10.0 mN/m before (spectra-1) and after irradiation with UV (spectra-2) and visible light (spectra-3). The spectra in the lower panel are shown after smoothing.

compound (4AzCRA-Ac), as shown in Figure 7. This is due to its larger occupied area. In other words, the extent of trans-to-cis photoisomerization is controlled by an appropriate choice of an occupied area which is determined by the nature of hydrophilic residues attached to the lower rim of CRA. Photocontrol of Areas of Monolayers. The suppression of trans-to-cis photoisomerization in monolayers on a water surface caused at a constant area may be reduced to result in area expansion of monolayers at a constant surface pressure. In fact, it has been observed that prominent area changes of monolayers of amphiphilic polymers with azobenzene side chains on a water surface are induced by their photoisomerization when a surface pressure is kept at a constant value. This phenomenon was influenced by the chemical structure of CRA derivatives and a surface pressure at which photoirradiation was made. The results are shown in Figure 8. It was found that monolayers of CRA derivatives with azobenzenes exhibit the reversible alteration of their area upon alternate photoirradiation with UV and visible light except for 0AzCRA-Ac (Figure 8d), possessing no substituent at the azobenzene chromophore. While distinct reversibility in the areal alteration arose from alternate photoirradiation for 8AzCRA-Ac (Figure 8b) and 4AzCRA (Figure 8e) even at a relatively higher surface pressure at 30 mN m-1, no reversibility was observed for monolayers of 4AzCRA-Ac (Figure 8a) and CHAzCRA-Ac (Figure 8c) at the same surface pressure probably because of the collapse of monolayers induced by UV irradiation. As seen in Figure 8e, the first UV irradiation of a monolayer of 4AzCRA resulted in a rapid reduction of its area at the early stage, followed by an area increase upon further irradiation. The same behavior was observed for 0AzCRA-

Properties of Calix[4]resorcinarene Derivatives

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Figure 8. Reversible changes in areas of monolayers of (a) 4AzCRA-Ac, (b) 8AzCRA-Ac, (c) C6AzCRA-Ac, (d) 0AzCRA-Ac, and (e) 4AzCRA at various surface pressures on a water surface upon alternate irradiation with UV and visible light.

Ac, 8AzCRA-Ac, and CHAzCRA-Ac. Such anomalous behavior was not observed upon subsequent UV irradiation cycles any longer. This may be interpreted in terms of a photoinduced orientational change of azobenzene chromophores in a fresh monolayer leading to the modification of a packing mode of two-dimensional molecular sheets, though further studies are required. Figure 8 shows that the level of area increment (∆A) upon UV irradiation decreases with an increase in surface pressures. This arises from the suppression of trans-tocis photoisomerization at higher surface pressures to reduce a content of cis-azobenzene at photostationary states, as revealed by spectral observation of monolayers exposed to UV light (Figures 6 and 7). The results summarized in Figure 8 tell us the following: First, ∆A induced by UV irradiation is much smaller than those observed for monolayered films of polymers with azobenzene side chains. In the case of amphiphilic poly(vinyl alcohols) substituted by p-alkylazobenzenes, an area of a monolayer becomes doubled or more upon UV irradiation at a constant surface pressure.18 Such a considerable lateral expansion of UV-irradiated polymeric monolayers has been interpreted as the direct interaction of polar cis-azobenzene chromophores with a water surface leading to the drastic transformation of polymer chain structures. The relatively smaller ∆A of the present system suggests that the flat-laid orientation of a CRA skeleton on a water surface is not changed before and after UV irradiation at all so that no direct interaction of cis-isomer units with a water surface is induced. Second, ∆A is not dependent on the bulkiness of a p-substituent at every surface pressure in spite of our anticipation that a bulkier substitutent at the p-position should bring about the more areal expansion upon UV irradiation. It is thus very likely that p-alkyl substituents of photoisomerized cis-azobenzene chromophores are directed not to the outside of the cylindrical structure of the CRA moiety to enhance the steric repulsion between CRA molecules but rather to the inside of the CRA cross-section to minimize its occupied (18) Seki, T.; Fukuda, R.; Yokoi, M.; Tamaki, T.; Ichimura, K. Bull. Chem. Soc. Jpn. 1996, 69, 2375.

area. Third, ∆A of 4AzCRA is rather smaller than that of octaacetylated CRAs, in particular, at a surface pressure of 0.5 mN m-1. This observation was against our expectation that 4AzCRA with a smaller cross-sectional area would demonstrate a larger expansion of a monolayer film as a result of more efficient steric repulsion of cischromophores. The level of trans-to-cis photoisomerization is not so high, though no surface pressure is applied, as judged from an absorption spectra (Figure 6a). As discussed above, this arises from effective intermolecular interactions of the CRA on a water surface. Thus, the smaller ∆A of 4AzCRA is ascribed to the efficient suppression of the photoisomerization. Conclusion CRAs having azobenzenes tethered from the lower rim of the cylindrical moiety of CRA are a multichromophoric system with amphiphilic characters. Slight molecular interaction leading to the modification of absorption spectra was observed in solution due to the crown conformation favorable for intermolecular interactions of azobenzenes. The trans-to-cis photoisomerization in solution proceeds normally without being affected by multichromophoric interactions to give a photostationary state consisting of a possible six isomers. The solubility of 4AzCRA is drastically changed in methanol before and after UV irradiation so that reversible precipitation/ dissolution cycles are performed photochemically even in a dilute methanol solution. Spectral characteristics and photoisomerization behavior of monolayers on a water surface were distinctly different from those in solution, reflecting efficient interactions of azobenzene chromophores. In particular, 4AzCRA with an occupied area of 1.3 nm2 displays a considerable blue shift of π,π*-absorption band due to the formation of an H-aggregate. The trans-to-cis photoisomerization takes place in a monolayer of 4AzCRA even under surface compression, although the photoisomerizability is more or less restricted. This situation arises from the fact that an occupied area of CRA molecules is

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determined by the base area of the cyclic framework so that a two-dimensional free volume is ensured in a densely packed monolayer. The trans-to-cis photoisomerizability is enhanced effectively by O-octaacetylation of the CRA as a result of the enlargement of an occupied area by the introduction of acetoxy residues. This means consequently that the photoisomerizability requiring a sweep volume is controlled by an appropriate choice of hydrophilic residues attached to the upper rim of the CRA skeleton even in densely packed molecular films.

Ichimura et al.

The cross-sectional area of azobenzene CRA derivatives is regulated reversibly by the photoisomerization. The level of area changes of monolayers on a water surface depends on the chemical structure of cyclic molecules and surface pressures. It was assumed that the flat-laid conformation of the CRA derivatives on a water surface is not altered by UV irradiation despite a polar nature of the cis isomer owing to the rigid macrocyclic structure with amphiphilic characteristics. LA970701I