Multiaddressable Self-Assembling Organogelators Based on 2H

Jun 5, 2002 - Upon a further thermal treatment, the system is reversibly converted back to the supramolecular network. Controlled gelation could be ...
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Langmuir 2002, 18, 7096-7101

Articles Multiaddressable Self-Assembling Organogelators Based on 2H-Chromene and N-Acyl-1,ω-amino Acid Units† Saleh A. Ahmed,‡ Xavier Sallenave,‡ Fre´de´ric Fages,‡ Gudrun Mieden-Gundert,§ Walter M. Mu¨ller,§ Ute Mu¨ller,§ Fritz Vo¨gtle,§ and Jean-Luc Pozzo*,‡ Group of Supramolecular Chemistry, Biomimetism and Nanoscience, University Bordeaux 1, 33405 Talence Cedex, France, and Kekule´ -Institut fu¨ r Organische Chemie und Biochemie der Universita¨ t Bonn, 53121 Bonn, Germany Received January 16, 2002. In Final Form: March 14, 2002 Multiaddressable organogelators are 3,3-diphenyl-3H-naphtho[2,1-b]pyrans covalently linked to sodium N-acyl-11-aminoundecanoate. These molecules have been designed to respond to changes to their environment. They are shown to act as efficient gelators for polar organic fluids, and obviously they exhibit a thermosensitive answer as low molecular mass organogelators. In these fluids, the aggregative properties are totally suppressed upon conversion to neutral carboxylic species. The gels of these carboxylate sodium salts are shown to be markedly affected by light irradiation. Supramolecular gelating assemblies can be disrupted by the photoinduced ring opening of the chromene subunit, so that the macroscopic flowing property is recovered. Upon a further thermal treatment, the system is reversibly converted back to the supramolecular network. Controlled gelation could be achieved using temperature, light, or acidity as external stimuli.

Introduction Successful synthesis of organized supramolecular assemblies is the fundamental step on the way to new materials or functional supramolecular devices.1-3 Recently, thermoreversible physical gels generated from relatively low molecular mass organic molecules4 have emerged as a fascinating new class of self-assembling organic materials which opens up new prospects in elaborating nanostructured materials and devices. Although many aspects of gelation remain unclear5 and the variety of low molecular mass organogelators is still increasing,6 it is usually stated that gel formation relies on the spontaneous self-recognition of individual organic gelators into fiberlike structures, which in turn assemble into larger aggregates. Thus solvents, entrained within the interstices of a network, are immobilized by surface tension. Aggregation of low molecular mass organogelators is usually driven by specific noncovalent intermolecular * To whom correspondence should be addressed. E-mail: [email protected]. † This article is part of the special issue of Langmuir devoted to the emerging field of self-assembled fibrillar networks. ‡ Group of Supramolecular Chemistry, Biomimetism and Nanoscience, CNRS UMR 5802, 351 crs Libe´ration, F-33405 Talence Cedex, Fax: (+33) 556-84-6646. § Kekule ´ Institut fu¨r Organische Chemie und Biochemie, Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany. (1) Lehn, J.-M. Supramolecular Chemistry. Concepts and Perspectives; VCH: Weinheim, 1995. (2) Schneider, H.-J.; Yatmirsky, A. Principles and Methods in Supramolecular Chemistry; Wiley: Chichester, 2000. (3) Supramolecular Materials and Technologies; Reinhoudt, D. N., Ed.; Perspectives in Supramolecular Chemistry, Vol. 4; Wiley: New York, 1999. (4) (a) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (b) Terech, P. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 1630. (c) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263. (5) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237.

forces, and several examples have been reported based on hydrogen bond formation, hydrophobic interactions, dipole-dipole interactions, charge transfer, van der Waals interactions, or metal coordination bond formation. The control by external stimuli of the gelation process, which is a macroscopic expression of self-assembling, remains a tremendous challenging task.7 A promising approach toward such smart gels is the introduction of an addressable function into the supramolecular building blocks. Different approaches using cation complexation or pH variation have already been proven successful but were reported to slightly modulate the gelling abilities or the medium viscosity.8,9 Organic photochromes10 could (6) (a) Hanabusa, K.; Tanaka, R.; Suzuki, M.; Kimura, M.; Shirai, S. Adv. Mater. 1997, 8, 740. (b) van Esch, J.; de Feyter, S.; Kellogg, R. M.; de Schryver, F.; Feringa, B. L. Chem.sEur. J. 1997, 3, 1238. (c) Hishikawa, Y.; Sada, K.; Watanabe, R.; Miyata, M.; Hanabusa, K. Chem. Lett. 1998, 795. (d) Yoza, K.; Ono, Y.; Yoshihara, K.; Akao, T.; Shinmori, H.; Takeuchi, M.; Shinkai, S.; Reinhoudt, D. N. Chem. Commun. 1998, 907. (e) Shinkai, S.; Murata, K. J. Mater. Chem. 1998, 8, 485. (f) Maitra, U.; Kumar, P. V.; Chandra, N.; D’Souza, L. J.; Prasanna, M. D.; Raju, A. R. Chem. Commun. 1999, 595. (g) van Esch, J.; Schoonbeek, F.; De Loos, M.; Kooijman, H.; Spek, A. L.; Kellogg, R. M.; Feringa, B. L. Chem.s Eur. J. 1999, 5, 937. (h) Clavier, G.; Mistry, M.; Fages, F.; Pozzo, J. L. Tetrahedron Lett. 1999, 40, 9021. (i) Ihara, H.; Yoshitake, M.; Takafuji, M.; Yamada, T.; Sagawa, T.; Hirayama, C.; Hachisako, H. Liq. Cryst. 1999, 26, 1021. (j) Cuccia, L. A.; Lehn, J. M.; Homo, J. C.; Schmutz, M. Angew. Chem., Int. Ed. 2000, 39, 233. (k) Lu, L. D.; Cocker, T. M.; Bachman, R. E.; Weiss, R. G. Langmuir 2000, 16, 20. (l) Wang, R.; Geiger, C.; Chen, L. H.; Swanson, B.; Whitten, D. G. J. Am. Chem. Soc. 2000, 122, 2399. (m) Duncan, D. C.; Whitten, D. G. Langmuir 2000, 16, 2399. (n) Lucas, L. N.; van Esch, J.; Kellogg, R. M.; Feringa, B. L. Chem. Commun. 2001, 759. (o) Lyon, R. P.; Atkins, W. M. J. Am. Chem. Soc. 2001, 123, 4408. (p) Ajayagosh, A.; George, S. J. J. Am. Chem. Soc. 2001, 123, 5148. (q) Makarevic, J.; Jokic, M.; Peric, B.; Tomsic, V.; Kojic-Prodic, B.; Zinic, M. Chem.sEur. J. 2001, 7, 3328. (r) Ko¨lbel, M.; Menger, F. M. Langmuir 2001, 17, 4490. (s) George, M.; Weiss, R. G. J. Am. Chem. Soc. 2001, 123, 10393. (7) van Esch, J.; Schoonbeek, F.; De Loos, M.; Veen, E. M.; Kellogg, R. M.; Feringa, B. L. Supramolecular science: where it is and where it is going; Nato ASI Series C, Mathematical and physical sciences, Vol. 527; Kluwer Academic: Dordrecht, 1999; p 233.

10.1021/la025545g CCC: $22.00 © 2002 American Chemical Society Published on Web 06/05/2002

Multiaddressable Self-Assembling Organogelators

offer the possibility to modify the self-assembly process of the individual molecules and also the resulting supramolecular network by means of light. Indeed, photochromic systems display two different molecular states in terms of their respective absorption spectra. More interestingly, the light-induced transformation causes reversibly important structural and physicochemical changes. Photoswitchable hydrogen-bonded in self-organized cylindrical peptide systems have been successfully prepared, the E/Z isomerization of the azobenzene moiety allowing the reversible interconversion of inter- to intramolecular hydrogen bonding.11 Cholesterol-based organogelators that incorporate an azobenzene unit have been reported to exhibit a slightly higher gel-to-sol phase transition temperature in the alltrans state than in the photostationary state, so the system can be changed from solution to gel by light irradiation.8 A photochromic gelator based on a nitrospiroindolinopyran connected to L-glutamic acid has been used to determine the critical aggregation concentration of gelator molecules.12 Mixed gels that incorporate aromatic gelators and structurally closed photochromes have been reported by some of us to be photoswitchable gels in a short range of temperatures.13 More interestingly, azobenzene coaggregating guest has been used for chiral recognition in bis(urea)-based aggregates and organogels.14 These particular molecules when mixed could markedly affect the medium viscosity. According to our knowledge, these examples constitute the rare attempts toward photocontrolled organogelation. In this study, we have designed photochromic organogelators that are based on the covalent association of a photostimulable 2H-chromene and an acid-sensitive selfassembling unit. We report the synthesis and the gelation abilities of naphthopyran derivatives that incorporate an N-acyl-1,ω-amino acid moiety which have been recently reported to induce gelation of various organic fluids.15

Experimental Section 3,3-Diphenyl-8-hydroxymethyl-[3H]naphtho[2,1-b]pyran (4). 6-Hydroxymethyl-2-naphthol (871 mg, 5 mmol) and 1,1diphenyl-propyn-1-ol (1.04 g, 5 mmol) were dissolved in dry acetonitrile. A catalytic amount of p-toluenesulfonic acid (100 mg) was added, and the reaction mixture was stirred at 25 °C (8) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664. (9) Pozzo, J. L.; Clavier, G.; Desvergne, J. P. J. Mater. Chem. 1998, 8, 125. (10) (a) Applied Photochromic Polymer Systems; McArdle, C. B., Ed.; Blackie: Glasgow, 1992. (b) Organic Photochromic and Thermochromic Compounds; Crano, J. C., Guglielmetti, R., Eds.; Plenum: New York, 1999; Vols. 1 and 2. (11) Vollmer, M. S.; Clark, T. D.; Steinem, C.; Ghadiri, M. R. Angew. Chem., Int. Ed. 1999, 38, 1598. (12) Hachisako, H.; Ihara, H.; Kamiya, T.; Hirayama, C.; Yamada, K. Chem. Commun. 1997, 19. (13) Pozzo, J. L.; Clavier, G.; Rustmeyer, F.; Bouas-Laurent, H. Mol. Cryst. Liq. Cryst. 2000, 344, 101. (14) De Loos, M.; van Esch, J.; Kellogg, R. M.; Feringa, B. L. Angew. Chem., Int. Ed. 2001, 40, 613. (15) Mieden-Gundert, G.; Klein, L.; Fischer, M.; Vo¨gtle, F.; Heuze´, K.; Pozzo, J. L.; Vallier, M.; Fages, F. Angew. Chem., Int. Ed. 2001, 40, 3164.

Langmuir, Vol. 18, No. 19, 2002 7097 for 2 days. The reaction mixture was then concentrated and chromatographed on silica gel (dichloromethane) to yield a white solid (1.27 g, 70%): mp 147-148 °C; 1H NMR (250 MHz, CDCl3) δ 1.96 (br, 1H, OH), 4.81 (sl, 2H, CH2OH), 6.29 (d, J ) 10.0 Hz, 1H, H-2), 7.21 (d, J ) 10.0 Hz, 1H, H-1), 7.24-7.38 (m, 7H, H-5, H-4′ and H-3′), 7.42-7.52 (m, 5H, H-2′ and H-9), 7.67 (d, J ) 8.9 Hz, 1H, H-6), 7.71 (d, J ) 1.9 Hz, 1H, H-7), 8.02 (d, J ) 8.8 Hz, 1H, H-10); 13C NMR (62.5 MHz, CDCl3) δ 65.3 (CH2OH), 82.6 (C3), 114.1 (C1a), 118.6, 119.6, 121.8, 126.0, 126.3, 127.1 (C2′), 127.6 (C4′), 128.0, 128.2 (C3′), 129.2, 129.9, 136.0, 144.9 (C1′), 150.7 (C4a); MS (EI+) m/e 364 (M, 100), 333 (M - CH2OH, 24), 287 (M - C6H5, 98). Methyl 11-(N-3,3-diphenyl-[3H]-naphtho[2,1-b]pyran-8methoxycarbonyl)aminoundecanoate (5). A mixture of the chromene-8-methanol 4 (182.23 mg, 0.5 mmol) and triethylamine (0.108 mL, 0.76 mmol.) is dissolved in 10 mL of dry THF under a nitrogen atmosphere. Methyl 11-isocyanatoundecanoate (120 mg, 0.5 mmol) was then added, and the reaction mixture was heated at 73 °C for 12 h. The solvent was evaporated under reduced pressure to lead to a brown oil residue which was washed three times with hexane. The product was then chromatographed on silica gel (dichloromethane) to yield a white viscous oil (105 mg, 35%). 1H NMR (250 MHz, CDCl3) δ 1.20-1.30 (m, 12H, 6xCH2), 1.40-1.64 (m, 4H, CH2CH2COOCH3 and CH2CH2NH), 2.30 (t, J ) 7.5 Hz, 2H, CH2COOCH3), 3,17 (m, 2H, CH2NH), 3.68 (s, 3H, COOCH3), 4.71 (m, 1H, CH2NHCO), 5.19 (s, 2H, CH2O), 6.27 (d, J ) 10.0 Hz, 1H, H-2), 7.24-7.38 (m, 8H, H-1, H-5, H-4′ and H-3′), 7.42-7.52 (m, 5H, H-2′ and H-9), 7.64 (d, J ) 8.9 Hz, 1H, H-6), 7.70 (d, J ) 1.9 Hz, 1H, H-7), 7.94 (d, J ) 8.6 Hz, 1H, H-10); 13C NMR (62.5 MHz, CDCl3) δ 24.3, 26.9, 29.1, 29.2, 29.3 (xC), 29,4, 30.0 (CH2CH2NH), 34.1 (CH2COOCH3), 41.3 (CH2NH), 51.5 (COOCH3), 65.3 (CH2O), 82.6 (C3), 114.1 (C1a), 118.7, 119.5, 121.8, 126.9, 127.1 (C2′), 127.6 (C4′), 127.9, 128.0, 128.2 (C3′), 129.2, 129.9, 136.0, 144.9 (C1′), 150.7 (C4a); MS (EI+) m/e 605 (M, 20), 528 (25, M - C6H5, 25), 364 (ArCH2OH, 100). 11-(N-3,3-Diphenyl-[3H]-naphtho[2,1-b]pyran-8-methoxycarbonyl)aminoundecanoic Acid (6). A mixture of ester 5 (303 mg, 0.5 mmol) and lithium hydroxide (150 mg, 6.25 mmol) was dissolved in 10 mL of methanol and 3 mL of water and then stirred at 20 °C for 6 h. Methanol was removed, and the residue was dissolved in water and acidified to pH ) 2 with a 2 N hydrochloride solution and then stirred for 5 h. The product was extracted with dichloromethane (3 × 30 mL). The organic layer was dried over MgSO4 and concentrated. The resulting residue was chromatographed on silica gel (9/1 dichloromethane/ethyl acetate as eluent) to yield a white solid (136 mg, 46%): mp 3942 °C; 1H NMR (250 MHz, CDCl3) δ 1.20-1.30 (m, 12H, 6xCH2), 1.40-1.64 (m, 4H, CH2CH2COOH and CH2CH2NH), 2.32 (t, J ) 7.5 Hz, 2H, CH2COOH), 3.17 (m, 2H, CH2NH); 4.71 (m, 1H, CH2NHCO), 5.19 (s, 2H, CH2O), 6.27 (d, J ) 10.0 Hz, 1H, H-2), 7.24-7.38 (m, 8H, H-1, H-5, H-4′ and H-3′), 7.42-7.52 (m, 5H, H-2′ and H-9), 7.64 (d, J ) 8.9 Hz, 1H, H-6), 7.70 (d, J ) 1.9 Hz, 1H, H-7), 7.94 (d, 8.6, 1H, H-10); MS (FAB+) m/e 591 (M, 10); 514 (M - C6H5, 30), 347 (ArCH2+, 100). Sodium 11-(N-3,3-Diphenyl-[3H]-naphtho[2,1-b]pyran8-methoxycarbonyl)aminoundecanoate (6-Na). A mixture of 6 (296 mg, 0.5 mmol) and 500 mg of 1 N NaOH was stirred in methanol (10 mL) at room temperature for 3 h. The solvent was removed under vacuum, and the product was washed with diethyl ether and dried to yield a white solid (295 mg, 96%): mp (dec) 122-124 °C. 3,3-Diphenyl-[3H]naphtho[2,1-b]pyran-8-carboxylic Acid (7). A mixture of 6-hydroxynaphthalene-2-carboxylic acid (0.94 g, 5 mmol) and 1,1-diphenylpropyn-1-ol (1.04 g, 5 mmol) was dissolved in 150 mL of dry acetonitrile. A catalytic amount of p-toluenesulfonic acid (100 mg) was added, and the reaction mixture was stirred for 2 days. The reaction mixture was concentrated, and the residue was chromatographed on silica gel (8/2 dichloromethane/ethyl acetate as eluent) to yield a white solid (0.980 g, 52%): mp 285-287 °C; 1H NMR (250 MHz, DMSOd6), δ 6.64 (d, J ) 9.95 Hz, 1H, H-2), 7.23-7.40 (m, 8H, H-1, H-5, H-4′ and H-3′), 7.46-7.52 (m, 4H, H-2′), 7.94 (dd, J ) 8.9 and 1.9 Hz, 1H, H-9), 7.99 (d, J ) 8.95 Hz, 1H, H-6), 8.17 (d, J ) 8.75 Hz, 1H, H-10), 8.48 (d, J ) 1.9 Hz, 1H, H-7); 13C NMR (62,5 MHz, DMSO-d6), δ 83.3 (C3), 115.2, 120.2, 120.4, 123.1, 126.8, 127.6

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(C2′), 127.7, 128.8 (C4′), 129.3 (C3′), 129.4, 130.0, 130.1, 132.3, 138.9, 145.8, 152.9, 170.1; MS (EI+) m/e 378 (M, 60), 301 (M C6H5, 100). Methyl 11-[N-3,3-Diphenyl-[3H]-naphtho[2,1-b]pyran-8carbonyl]aminoundecanoate (8). Acid 7 (378 mg, 1 mmol) was dissolved in dry dichloromethane (60 mL) and then cooled at 0 °C. Then methyl 11-aminoundecanoate hydrochloride (284 mg, 1 mmol), DCC (300 mg, 1.5 mmol), and DMAP (50 mg, 0.4 mmol) were added. The reaction mixture was kept at 0 °C and stirred for 7 h. N,N′-Dicyclohexylurea was collected by filtration, and the filtrate was concentrated under reduced pressure. The residue was chromatographed on silica gel (dichloromethane) to yield a white solid (127 mg, 22%): mp 96-97 °C; 1H NMR (250 MHz, CDCl3) δ 1.25-1.35 (m, 12H, 6xCH2), 1.50-1.60 (m, 4H, CH2CH2COOCH3 and CH2CH2NH), 2.29 (t, J ) 7.2 Hz, 2H, CH2COOCH3), 3,23 (m, 2H, CH2NH), 3.65 (s, 3H, COOCH3), 6.24 (t, J ) 4.9 Hz, 1H, NH), 6.29 (d, J ) 10.0 Hz, 1H, H-2), 7.17-7.36 (m, 8H, H-1, H-5, H-4′ and H-3′), 7.46-7.52 (m, 4H, H-2′), 7.71 (d, J ) 8.8 Hz, 1H, H-6), 7.80 (dd, J ) 8.9 and 1.7 Hz, 1H, H-9), 7.97 (d, J ) 8.9 Hz, 1H, H-10), 8.13 (d, J ) 1.7 Hz, 1H, H-7); 13C NMR (62.5 MHz, CDCl3) δ 24.3, 26.9, 29.1, 29.2, 29.3 (xC), 29.4, 30.0 (CH2CH2NH), 34.1 (CH2COOCH3), 41.3 (CH2NH), 51.5 (COOCH3), 82.9, 114.0, 119.2, 119.9, 121.8, 124.4, 127.3 (C2′), 127.6 (C4′), 127.7, 127.9, 128.2 (C3′), 128.5, 130.8, 131.2, 144.6, 152.0, 167.4, 173.4; MS (EI+) m/e 575 (M, 100), 478 (M - C6H5, 17), 361 (ArCO+, 85). 11-[N-3,3-Diphenyl-[3H]-naphtho[2,1-b]pyran-8-carbonyl]aminoundecanoic Acid (9). Compound 8 (576 mg, 1 mmol) and lithium hydroxide (150 mg, 6.25 mmol) were dissolved in 20 mL of methanol and 5 mL of water and then stirred at 20 °C for 6 h. Methanol was removed, and the residue was dissolved in water and acidified to pH ) 2 with a 2 N hydrochloride solution and then stirred for 5 h. The product was extracted with dichloromethane (3 × 40 mL). The organic layer was dried over MgSO4, and the residue was chromatographed on silica gel (7/3 dichloromethane/ethyl acetate as eluent) to yield a white solid (164 mg, 30%): mp 147 °C; 1H NMR (250 MHz, CDCl3) δ 1.201.40 (m, 12H, 6xCH2), 1.50-1.60 (m, 4H, CH2CH2COOH and CH2CH2NH), 2.32 (t, J ) 7.2 Hz, 2H, CH2COOH), 3,44 (m, 2H, CH2NH), 4.32 (m, 1H, NH), 6.26 (m, 1H, COOH), 6.29 (d, J ) 9.95 Hz, 1H, H-2), 7.17-7.36 (m, 8H, H-1, H-5, H-4′ and H-3′), 7.46-7.52 (m, 4H, H-2′), 7.71 (d, J ) 8.8 Hz, 1H, H-6), 7.80 (dd, J ) 8.9 and 1.7 Hz, 1H, H-9), 7.97 (d, J ) 8.9 Hz, 1H, H-10), 8.11 (d, J ) 1.8 Hz, 1H, H-7); 13C NMR (62.5 MHz, CDCl3) δ 24.3, 26.9, 29.1, 29.2, 29.3 (xC), 29.4, 30.0 (CH2CH2NH), 34.1 (CH2COOCH3), 41.3 (CH2NH), 119.2, 119.9, 121.8, 124.4, 127.3 (C2′), 127.6 (C4′), 127.7, 127.9, 128.2 (C3′), 130.8, 131.1, 167.5, 177.9; MS (FAB+) m/e 562 (MH+, 40), 484 (M - C6H5, 30), 361 (ArCO+, 100). Sodium 11-[N-3,3-Diphenyl-[3H]-naphtho[2,1-b]pyran8-carbonyl]aminoundecanoate (9-Na). A mixture of 9 (280 mg, 0.5 mmol) and 500 mg of a 1 N sodium hydroxide solution was stirred in methanol (10 mL) at room temperature for 3 h. The solvent was removed under vacuum. The product was washed with diethyl ether and then dried under vacuum to yield a white solid (271 mg, 94%): mp (dec) 196 °C.

Results and Discussion Rational Design. Previously, we found that sodium salts of various N-acyl-1,ω-amino acids15 are able to gelate polar organic fluids such as dimethylformamide, dimethylsulfoxide, propylene carbonate, and dimethylacrylamide, whereas the neutral carboxylic derivatives are totally soluble. This study clearly demonstrates the versatile construction of organogelators based on the N-acyl amino acid scaffold by incorporating different segments in terms of chemical nature and shape ranging from short linear alkyl chains to heterocycles or aromatic moieties. Remarkably, the gelling ability is retained for compounds that bear a bulky substituent such as the adamantane moiety. This versatility encouraged us to prepare photoresponsive derivatives which are sterically demanding compounds. It occurred to us that incorporation of a photoresponsive unit in such a supramolecular system could lead to smart

Ahmed et al. Scheme 1. Photochromic Equilibrium for 3,3-Diphenyl-3H-naphtho[2,1-b]pyran.

Scheme 2. Synthetic Pathway for the Formation of 6 and 6-Naa

a (i) 1,1-Diphenylpropyn-1-ol; (ii) OdCdN-(CH2)10-COOMe, TEA; (iii) LiOH then HCl; (iv) NaOH 1 N.

gels sensitive to light, temperature, and acidity. 2HChromenes are known to display interesting photochromic properties such as fatigue resistance and a wide range of absorption in the visible region.16 The photochromic properties of the 2H-chromene structure and, hence, 3Hnaphtho[2,1-b]pyrans are based on a reversible color change observed upon irradiation with UV light. There are two interconvertible isomers: the colorless or closed form (CF) and the colored or open form (OF) (Scheme 1). The latter which is predominantly under its TC conformation,17 is thermally unstable and reverts back to the pyran form by an electrocyclization process. Then, under appropriate external light stimulus, changes of geometry and polarity of the photochromic entity can lead to a topologic change of a covalently connected molecular system, modifying in turn the physical properties of the material under design. It emerges from previously prepared gelator series that compounds bearing an aromatic group such as benzyloxy or naphthyl (1-Na and 2-Na, respectively) represent appropriate candidates to be transformed into naphtho[2,1-b]pyran derivatives, the photoresponsive subunit being closely introduced to the aggregating amide function. Molecular modeling experiments establish that the shape and the molecular volume of designed compounds 6-Na and 9-Na are comparable to those of adamantane gelator 3-Na (Schemes 2 and 3).18 One could expect that this peculiar structural modification would not prevent gelation. Synthesis. Naphthopyrans can be prepared, in reasonable yield, by cyclization of a suitable naphthol with (16) van Gemert, B.; Kumar, A.; Knowles, D. B. Mol. Cryst. Liq. Cryst. 1997, 297, 131. (17) Delbaere, S.; Luccioni-Houze, B.; Bochu, C.; Teral, Y.; Campredon, M.; Vermeersch, G. J. Chem. Soc., Perkin Trans. 2 1998, 1153. (18) Molecular modelling experiments were performed using the CAChe 3.1 program, Oxford Molecular.

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Scheme 3. Synthetic Pathway for the Formation of 9 and 9-Naa

a (i) 1,1-Diphenylpropyn-1-ol; (ii) HCl, NH2-(CH2)10-COOMe, DCC, DMAP; (iii) LiOH then HCl; (iv) NaOH 1 N.

alkynol in the presence of a catalytic amount of protonic acid.19 Here, 1,1-diphenylpropyn-1-ol was employed and the synthetic procedure was simply adapted by using dry acetonitrile as the solvent for solubility reasons due to the increased polarity of starting naphthols as previously described.20 Subsequently, the urethane derivative 5 was obtained through condensation with an appropriate isocyanate. The amide was synthesized according to the classic peptide coupling procedure using DCC and DMAP. The resulting esters 5 and 8 were saponified under mild conditions using lithium hydroxide to yield the neutral carboxylic forms after hydrolysis. Gelation Properties. The gelation behavior was first investigated via the inverted test tube method.8 DMF and DMSO were selected as fluids according to the mimimum gelation concentrations which were found in the millimolar range for some sodium salts of N-acyl amino acids.15 Furthermore, compounds 6-Na and 9-Na are insoluble in most of the usual organic fluids. They are sparingly soluble in DMF and DMSO, but upon heating at 70-120 °C and sonication, they gradually dissolved. Upon cooling to ambient temperature, gels are readily obtained which are then stable for months. Both chromenic derivatives were found to gelate these fluids over a wide range of concentration. For comparison, model compounds 1-Na and 2-Na have been tested in similar conditions. The results are collected in Figure 1. Although there are clear differences in minimum gelation concentration, the presence of the pyran ring disubstituted by two phenyl groups does not prevent the gelation. The gel-to-sol phase transition temperatures (hereafter denoted Tgel) of DMF gels in a concentration of 1% wt/vol21 are 59 and 69 °C for 6-Na and 9-Na, respectively, whereas 144 °C was found for Tgel for compound 2-Na which could be considered to act as a supergelator.22 The neutral carboxylic acid derivatives 6 and 9 were found to be very soluble in DMF and DMSO at 25 °C. Upon addition of sodium hydroxide, they rapidly convert to the gel state. So, for the targeted naphthopyrans, the interesting gelation ability is retained and the presence (19) Pozzo, J. L.; Samat, A.; Guglielemetti, R.; Dubest, R.; Aubard, J. Helv. Chim. Acta 1997, 80, 725. (20) Samat, A.; Lokshin, V.; Chamontin, K.; Levi, D.; Pepe, G.; Guglielemetti, R. Tetrahedron 2001, 57, 7349. (21) 1% wt/vol concentrations expressed in mol L-1: 1.52 × 10-2 for 6-Na, 1.70 × 10-2 for 9-Na, and 1.81 × 10-2 for 2-Na. (22) Gronwald, O.; Shinkai, S. Chem.sEur. J. 2001, 7, 4329.

Figure 1. (a) Tgel in DMF as a function of organogelator concentration: 1-Na (9), 2-Na (×), 6-Na (b), and 9-Na ((). (b) Tgel in DMSO as a function of organogelator concentration: 1-Na (9), 2-Na (×), 6-Na (b), and 9-Na (().

of carboxylate groups and sodium counterions is still of major importance since the addition of acid could suppress the gel formation. TEM observations give direct evidence for the microscopic organization formed in DMF. Numerous juxtaposed and intertwined fibrous aggregates of low molecular mass gelator 6-Na are formed. The morphology of gels was based on long fibers which form an entangled network. The diameter of the smallest entities which can be distinguished is 50-70 nm. This represents several gelator lengths. These gels were further studied by infrared spectroscopy. For such purpose, deuterated dimethylsulfoxide was used as the solvent in order to avoid strong peaks in the NH-stretch and in amid I and amid II regions. The maxima of these peaks are characteristics for the presence of the hydrogen-bonded amide group.23 Alternative evidence supporting the self-aggregation of naphthopyrans in DMF gel was provided by rheological measurements.24 The frequency sweep experiment shows that the elastic modulus G′ and the loss modulus G′′ are fairly independent of frequency over more than 3 decades. Typical values are here reported for 1.5% wt/vol of 6-Na in DMF. The G′ value (1 × 104 Pa) was observed to be 1 order of magnitude higher than that of G′′ (1 × 103 Pa) indicating that the system still exhibits a solidlike behavior. Photochromic Properties. The photochromic characteristics were first determined for the carboxylic acids 6 and 9 in both DMF and DMSO using a 366 nm irradiation lamp coupled to a UV-vis spectrometer. The presence of the amino acid scaffold directly linked on the (23) FTIR absorption peaks for amid-Na in DMSO-d6 gel: NH stretch, 3331 cm-1; amid I, 1626 cm-1; and amid II, 1571 cm-1. (24) Rheological measurements were performed with a TA Instrument AR 1000 stress-controlled rheometer. A measuring cell with a cone and plate geometry (a diameter of 20 mm and a cone angle of 4°) was used. The heated solution was introduced in the cell regulated at 60 °C and then cooled rapidly (environ 20 °C/min) at 20 °C. The elastic modulus G′ and loss modulus G′′ were measured as a function of angular frequency ω at 20 °C. The oscillatory frequency was varied from 10 to 0.01 Hz.

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Scheme 4. Illustration of Interconversion between the Different Molecular Species and Corresponding Macroscopic States upon External Stimuli (X ) none for 9, X ) CH2O for 6)

Figure 2. Overlay spectra illustrating the thermal decay of colored forms and superimposed spectrum of recovered closed form of 6 at 20 °C in DMF.

8-position of the 3,3-diphenyl-3H-naphtho[2,1-b]pyran subunit does not alter the photochromic behavior. The actinic wavelengths were determined to be 348 and 361 nm for the urethane derivative, and similarly 346 and 360 nm for the amide compound. Thermal fading rate constants are also closed ranging from 0.06 to 0.17 s-1. For both compounds, the light-induced open forms exhibit an absorption band around 440 nm at 20 °C. The wavelength of the absorption maxima denoted λmax does not show any dependence on solvent polarity as previously reported for the 3,3-diphenyl-3H-naphtho[2,1-b]pyran parent molecule.25 The λmax values are 426 nm for the urethane derivative and 437 nm for the amide one. Irradiation of Gels. Photochromic gels have been studied in thermostated cells with a 1 mm optical pathway, (25) Pozzo, J. L.; Samat, A.; Guglielmetti, R.; Lokshin, V.; Minkin, V. Can. J. Chem. 1996, 74, 1649.

allowing one to follow coloration and subsequent decoloration by UV-vis measurements and to keep homogeneity within the samples. Upon irradiation using a 366 nm lamp, gels become rapidly colored and then begin to flow as we have set up an inverted cell. One can observe the appearance of yellowcolored liquid areas within the sample along with aggregate zones. The shape of the absorption band displayed by the irradiated gel is totally superimposable with those arising from irradiated acid indicating that the conversion of some gelator molecules into open forms takes place. The primary photoinduced ring opening event clearly diminishes the strength of the network which is not totally suppressed. Indeed, the strongly colored open forms could act as an internal filter preventing total conversion. When irradiation ceases, the coloration progressively disappears to yield a colorless viscous solution which does not revert spontaneously into a gel. This could be simply achieved upon heating and then cooling. This complete interconversion process between gel state, viscous liquid, and isotropic solution has been cycled 10 times without any detectable degradation side products. This can be unambiguously assigned due to the Tgel values which are invariable. Conclusion We have demonstrated that incorporation of a photoresponsive unit on the sodium N-acyl-11-aminoundecanoate scaffold does not suppress the gelation ability. On the other hand, the formation of intermolecular hydrogen bonds, and thereafter the supramolecular aggregates, is strongly affected by photoinduced structural changes of the photochromic subunit. Upon addition of sodium hydroxide, the DMF and DMSO homogeneous solutions of neutral carboxylic species are reversibly converted into gels. Such multiaddressable self-assembling organogelators are excellent building blocks for the development of functional materials and devices. This approach constitutes a new route toward nanosized

Multiaddressable Self-Assembling Organogelators

architectures which could be reversibly influenced upon irradiation and protonation. These systems represent example of smart gels which display the ability to respond to various changes of their environment. Acknowledgment. The financial support from CNRS, the Deutsche Forschunggemeinschaft (DFG, Vo 145/491), and COST D-11 Supramolecular Chemistry Project

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D11/0015/99 is gratefully acknowledged. The authors thank Dr. M. Lescanne, CRPP, University Bordeaux, for rheological experiments. Supporting Information Available: TEM micrograph and rheological frequency sweep experiment. This material is available free of charge via the Internet at http://pubs.acs.org. LA025545G