Physical Gelation of Organic Fluids by Anthraquinone−Steroid-Based

Mar 20, 1996 - Physical Gelation of Organic Fluids by Anthraquinone−Steroid-Based Molecules. ... Understanding the Role of H-Bonding in Self-Aggrega...
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Physical Gelation of Organic Fluids by Anthraquinone-Steroid-Based Molecules. Structural Features Influencing the Properties of Gels Ravindranath Mukkamala and Richard G. Weiss* Department of Chemistry, Georgetown University, Washington, D.C. 20057 Received August 7, 1995. In Final Form: November 30, 1995X Ten new molecules containing 2-anthraquinonyl, steroidal groups, and various linking groups have been synthesized. Several structural properties of the new molecules have been found to influence gelation abilities: (1) gelation is facilitated by β stereochemistry at C-3 of the steroidal moiety; (2) minor modifications of the C-17 alkyl chain of the steroidal moiety do not destroy gelling ability; and (3) the length and functionality of the linker group can affect gelation ability significantly, although the number of examples studied is not sufficient to discern a clear correlation. Gelation temperatures (Tg) were measured for several gels by the “inverse-flow” and “ball-drop” methods. Since gels composed of cholesteryl 4-(2-anthraquinonyloxy)butanoate (1) and D-, L-, or DL-2-octanol have virtually the same Tgs over a range of gelator concentrations, specific liquid-gelator interactions do not appear to be a sensitive variable, at least when the liquid molecules being compared are similar in shape. Liquid polarity, on the other hand, has a significant effect on Tg. Gels of 1/alcohol exhibit Tg values that are much lower than those of 1/alkane gels; the opposite trend was observed for cholesteryl 4-(2-anthryloxy)butanoate gels. Circular dichroism spectral studies give rise to extrinsic circular dichroism that is not observable in isotropic phases. Again, liquid chirality has no apparent effect on the chirality of the gelator aggregates, indicating that specific diastereomeric liquid-gelator interactions do not dictate the mode of molecular packing of the gelators.

Introduction Although physical gelation of organic liquids by low molecular weight molecules (gelators) has been known for many years,1 there has been recent interest not only in discovering new classes of gelators but also in probing the process of gelation, and in the structure and properties of the gels once formed.2-24 To date, most studies have been anecdotal, and comparisons among gelators of similar structure in one liquid are rare. X Abstract published in Advance ACS Abstracts, February 15, 1996.

(1) Tachibana, T.; Kayama, K.; Takeno, H. Bull. Chem. Soc. Jpn. 1972, 45, 415. (2) Twieg, R. J.; Russell, T. P.; Siemens, R.; Rabolt, J. R. Macromolecules 1985, 18, 361. (3) Terech, P.; Chachaty, C.; Gaillard, J.; Giroud-Godquin, A. M. J. Phys. (Paris) 1987, 48, 663. (4) Terech, P.; Schaffhauser, V.; Maldivi, P.; Guenet, J. M. Langmuir 1992, 8, 2104. (5) Wade, R. H.; Terech, P.; Hewat, E. A.; Ramasseul, R.; Volino, F. J. Colloid Interface Sci. 1986, 114, 442. (6) Lin, Y.-C.; Weiss, R. G. Macromolecules 1987, 20, 414. (7) (a) Lin, Y.-C.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542. (b) Lin, Y.-C. Ph.D. Thesis, Georgetown University, Washington, D.C., 1987. (8) Furman, I.; Weiss, R. G. Langmuir 1993, 9, 2084. (9) Mukkamala, R.; Weiss, R. G. J. Chem. Soc., Chem. Commun. 1995, 375. (10) Itoh, T.; Katsoulis, D. E.; Mita, I. J. Mater. Chem. 1993, 3, 1303. (11) Bujanowski, V. J.; Katsoulis, D. E.; Zeimelis, M. J. J. Mater. Chem. 1994, 4, 1181. (12) (a) Terech, P.; Furman, I.; Weiss, R. G. J. Phys. Chem. 1995, 99, 9558. (b) Terech, P.; Furman, I.; Bouas-Laurent, H.; Desvergne, J.-P.; Ramasseul, R.; Weiss, R. G. Faraday Discuss., in press. (13) (a) Ostuni, E.; Weiss, R. G. Unpublished results. (b) Ostuni, E.; Weiss, R. G. 29th Middle Atlantic Regional Meeting for the American Chemical Society, May 1995; American Chemical Society: Washington, DC, 1995; Abstract 238. (14) Brotin, T.; Utermohlen, R.; Fages, F.; Bouas-Laurent, H.; Desvergne, J.-P. J. Chem. Soc., Chem. Commun. 1991, 416. (15) Thierry, A.; Straupe, C.; Lotz, B.; Wittmann, J. C. Polym. Commun. 1990, 31, 299. (16) Hanabusa, K.; Okui, K.; Karaki, K.; Koyama, T.; Shirai, H. J. Chem. Soc., Chem. Commun. 1992, 1371. (17) Hanabusa, K.; Tange, J.; Taguchi, Y.; Koyama, T.; Shirai, H. J. Chem. Soc., Chem. Commun. 1993, 390. (18) Hanabusa, K.; Matsumoto, Y.; Miki, T.; Koyama, T.; Shirai, H. J. Chem. Soc., Chem. Commun. 1994, 1401. (19) de Vries, E. J.; Kellogg, R. M. J. Chem. Soc., Chem. Commun. 1993, 238.

We have studied thermally reversible gels comprised of a variety of organic liquids and a family of molecules (ALS), whose 2-anthryl or 2-anthraquinonyl groups (A) are connected to C-3 of steroids (S) via a linker group (L).6-8 The gelator molecules aggregate in interacting, long strands that immobilize the liquid component by surface tension. These gels have superstructures similar to those of polymeric gels, like those formed in cross-linked polystyrene, but they differ in several important ways. Among these are the thermal reversibility of the low molecular weight gels and the fact that the colloidal networks of fibers form in a nucleation phenomenon that involves essentially one-dimensional crystal growth. Although a wide variety of liquids can be gelled, minor structural modifications to an ALS are capable of causing major changes in gel stability.7 Here, report the syntheses and gelation properties of 10 new ALS molecules9 (Chart 1). Three specific aspects of gelation are discussed: the relationship between ALS structure and gelation ability, gel stability (as indicated by gelation temperatures), and the aggregation of gelator molecules. The behavior of neat phases of the ALS molecules is also reported and correlated with gelation ability. Experimental Section Methods and Materials. THF (Fisher) was distilled over sodium benzophenone ketyl radical prior to use.25 Benzene (Fisher) was dried overnight over anhydrous calcium chloride, (20) Aoki, M.; Nakashima, K.; Kawabata, H.; Tsutsui, S.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2 1993, 347. (21) van Nostrum, C. F.; Picken, S. J.; Nolte, R. J. M. Angew. Chem., Int. Ed. Engl. 1994, 33, 2173. (22) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 666. (23) Tata, M.; John, V. T.; Waguespack, Y. Y.; McPherson, G. L. J. Am. Chem. Soc. 1994, 116, 9464. (24) Hanabusa, K.; Miki, T.; Taguchi, Y.; Koyama, T.; Shirai, H. J. Chem. Soc., Chem. Commun. 1993, 1382. (25) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals; Pergamon Press: New York, 1988; p 367.

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Langmuir, Vol. 12, No. 6, 1996 1475 Chart 1

followed by azeotropic distillation; spectro-grade benzene (J. T. Baker) was used as received. 2-Hydroxy-9,10-anthraquinone26 (mp 300-302 °C; lit.26 mp 302-303 °C) and 3β-(2-hydroxyethoxy)5-cholestene27 (mp 102.5-103.5 °C; lit.27 mp 102-104 °C) were prepared according to literature procedures. Oxalyl chloride (98%), triphenylphosphine (99%), diethyl azodicarboxylate (>98%), lithocholic acid (98%), 4-bromobutanoyl chloride (95%), ptoluenesulfonyl chloride (99+%), 3β-thiocholesterol, 3β-cholestanol (98%), 4-(dimethylamino)pyridine (99%), D-2-octanol (99%), L-2octanol (99%), and DL-2-octanol (98%) were purchased from Aldrich. Anthraquinone-2-carboxylic acid (99+%, TCI), 3βCholestanyl chloroformate (mp 99-101 °C, Eastman), cholesterol (99%, Eastman), and stigmastanol (98%, Sigma) were used as received. Stigmasterol (mp 169 °C; lit.28 mp 168 °C) was recrystallized twice from chloroform/methanol before use. Benzyl alcohol (Fisher) was purified by vacuum distillation. Silicone oil (Dow Corning 704 fluid) was obtained from Dow Corning Corporation. Melting points (corrected) were recorded on a Bausch & Lomb microscope with a Thomas hot stage or a Leitz Wetzlar (SMLUX-POL) microscope with a Leitz 585 thermostatting stage; photographs were taken with a Pentax K1000 camera mounted on the latter. Differential scanning calorimetry (DSC) was done on DuPont Instruments 910 differential scanning calorimeter, controlled by either DuPont 1090B Thermal Analyzer or TA Instruments Thermal Analyst 2000; the rate of heating and cooling of samples was 2 or 3 deg per minute. UV/vis absorption spectra were recorded on a Perkin-Elmer Lambda 6 UV-vis spectrophotometer and were analyzed using Perkin-Elmer PECSS software. Circular dichroism (CD) spectra (26) Mihai, G. G.; Tarassoff, P. G.; Filipescu, N. J. Chem. Soc., Perkin Trans. 1 1975, 1376. (27) Ahmed, M. S.; S. C. Logani, S. C. Aust. J. Chem. 1971, 24, 143. (28) Barton, D. H. R.; Cox, J. D. J. Chem. Soc. 1948, 783.

were recorded on a JASCO J-710 spectropolarimeter using a Hellma jacketed quartz cell of 0.1 mm path length. No difference in the CD spectra was observed in two perpendicular sample orientations. Proton NMR spectra were recorded on a Bruker HFX/WH-90, a Bruker AM-300 WB, or a Bruker 270 MHz nuclear magnetic resonance spectrometer with a Tecmag operating system. IR spectra were recorded on a MIDAC FT-IR spectrometer, using either KBr pellets or chloroform solutions. Highperformance liquid chromatography (HPLC) was performed on a Waters Associates liquid chromatograph with a Model 440 constant-wavelength (254 nm) absorbance detector and an Alltech 250 mm × 4.4 mm 5 µm silica gel column. Elemental analysis on 1 was performed by Guelph Chemical Laboratories, Ltd., Ontario, Canada. Mass spectra were obtained on Finnigan 4500 (for EI) and Finnigan 4600 (for CI) mass spectrometers at the National Institutes of Health, Bethesda, MD, by Dr. Quanlong Pu. Gelation temperatures by the inverse-flow22 or ball-drop method29 were measured typically on gels formed from 50 or 100 µL of a solvent and a gelator of known weight, in flame-sealed or septum-capped glass tubes (ca. 5 cm length and 0.5 cm diameter) while being heated in a water bath at a rate of ca. 4 °C/min. In the ball-drop method, a steel ball (ca. 200 mg and ca. 2.5 mm diameter) was placed on a gel and the temperature (range) at which the ball fell to the bottom was taken as the Tg. In the inverse-flow method, a gel sample in a sealed glass tube was inverted, immersed, and heated in a water bath at a rate of ca. 4 °C/min. Tg was taken to be when a gel fell to the bottom as a mass (for samples that melted over a narrow range) or when there was a continuous flow (for gels with broad melting ranges). The Tg values ((3 °C for the gels of 1 and (5 °C for the gels of 8-10) are the average of at least three measurements on one sample. (29) Takahashi, A.; Sakai, M.; Kato, T. Polym. J. 1980, 12, 335.

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Cholesteryl 4-Bromobutanoate. Cholesterol (2.0 g, 5.17 mmol) and then a few drops of dry pyridine were added to a stirred solution of 4-bromobutanoyl chloride (0.81 mL, 1.3 g, 7.0 mmol) in dry benzene (50 mL). The reaction mixture was warmed at 65 °C for 6 h in a dry atm, cooled to room temperature, diluted with benzene (50 mL), washed sequentially with water, 10% aqueous sodium bicarbonate, and water, and dried (Na2SO4). The residue from evaporation was chromatographed on silica gel (1/20 ethyl acetate/hexane) to give cholesteryl 4-bromobutanoate as a white crystalline solid (2.4 g, 96%): mp 85-87 °C; IR (KBr) 2933, 2862, 1724 cm-1; 1H NMR (90 MHz, CDCl3) δ 5.34 (bm, 1H, olefinic), 4.58 (m, 1H, COOCH), 3.46 (bt, J ) 6.3 Hz, 2H, BrCH2), 2.4-0.67 (m, 47H, CH2CH2COO and steroidal). Cholesteryl 4-(2-Anthraquinonyloxy)butanoate (1). A mixture of 2-hydroxyanthraquinone (0.8 g, 3.8 mmol), cholesteryl 4-bromobutanoate (1.94 g, 4.0 mmol), and potassium carbonate (2.7 g, 20.0 mmol) in 40 mL of dry acetone was refluxed for 25 h under a nitrogen atmosphere. The residue after removal of solvent was extracted into chloroform (150 mL). The chloroform was washed with water (5 × 100 mL), dried (anhydrous Na2SO4), and evaporated to residue. After column chromatography on silica gel (1/20 ethyl acetate/hexane) and two recrystallizations from hot ethyl acetate, 0.37 g of 1 was obtained (15%, >99.6% pure by HPLC): mp 137-138 °C (solid to liquid crystalline) and 174-175 °C (liquid crystalline to isotropic); IR (chloroform) 1724, 1672, 1595 cm-1; 1H NMR (270 MHz, CDCl3/TMS) δ 8.35-8.22 (m, 3H, aromatic), 7.84-7.67 (m, 3H, aromatic), 7.30-7.22 (m, 1H, ArH), 5.37 (bm, 1H, olefinic), 4.63 (bm, 1H, COOCH), 4.254.17 (t, J ) 6.3 Hz, 2H, OCH2), 2.58-0.65 (m, 47H, CH2CH2COO and steroidal); MS m/z 678 (M+, 2.5), 368 (50). Anal. Calcd for C45H58O5: C, 79.60; H, 8.61. Found: C, 79.60; H, 8.65. 2-Anthraquinonyl 3β-cholestanyl carbonate (2). Soon after a few drops of dry pyridine were added to a stirred mixture of 2-hydroxyanthraquinone (0.1 g, 0.5 mmol) and cholestanyl chloroformate (0.22 g, 0.5 mmol) in dry benzene (5 mL) at room temperature, the color turned from red to pale yellow. The mixture was then heated at 50 °C for 5 h in a dry atm. A pale yellow solid was filtered, washed with 20 mL of benzene, and dissolved in 50 mL of chloroform. The chloroform solution was washed with water (2 × 50 mL), dried (anhydrous Na2SO4), and evaporated to residue. The residue was washed with hot ethyl acetate and filtered under suction to yield 0.14 g (62%, 98.2% pure by HPLC) of 2: mp 229.5-232.8 °C; IR (chloroform) 1759, 1676, 1595 cm-1; 1H NMR (300 MHz, CDCl3/TMS) δ 8.34-8.37 (m, 2H, ArH), 8.11 (bs, 1H, ArH), 7.81-7.76 (m, 2H, ArH), 7.627.58 (m, 2H, ArH), 4.71 (m, 1H, COOCH), 1.99-0.65 (m, 46H, steroidal); MS m/z 656 (M+ + 18, CI with NH4+, 100), 640 (M+ + 2, 3). Methyl lithocholate was prepared by the standard esterification procedure30 of heating a mixture of lithocholic acid (1.0 g, 2.64 mmol), methanol (20 mL), and a few drops of concentrated sulfuric acid under reflux for 8 h in a dry atm. The crude ester was recrystallized from a hexane/ethyl acetate mixture to yield 0.75 g (73%) of white crystals: mp 126.8-127.8 °C (lit.31 mp 130 °C). Methyl 3r-((Anthraquinon-2-ylcarbonyl)oxy)lithocholate (3). After being heated at 50 °C with stirring under a dry atm for 1 h, a mixture of anthraquinone-2-carboxylic acid (0.28 g, 1.2 mmol) and oxalyl chloride (0.9 mL, 10.1 mmol) in 25 mL of dry benzene afforded a transparent yellow solution. Excess oxalyl chloride and benzene were distilled at atmospheric pressure, and additional dry benzene (2 × 10 mL) was added and distilled to remove traces of oxalyl chloride. Dry benzene (25 mL), methyl lithocholate (0.47 g, 1.2 mmol), and a few drops of pyridine were added to the solid residue, and the reaction mixture was stirred at room temperature for 16 h in a dry atm. Removal of the solvent under reduced pressure gave a yellow residue which was purified by column chromatography on silica gel (1/20 ethyl acetate/hexane) and two recrystallizations from hot ethyl acetate: 0.34 g (46%, 98.5% pure by HPLC); mp 197.4-200.4 °C; IR (chloroform) 2941, 2860, 1734, 1678 cm-1; 1H NMR (300 MHz, CDCl3/TMS) δ 8.95 (bs, 1H, aromatic), 8.52-8.32 (m, 4H, aromatic), 7.85-7.82 (m, 2H, aromatic), 5.1 (m, 1H, COOCH), (30) Furniss, B. S.; Hannaford, A. J.; Smith, P. W. G.; Tatchell, A. R. Vogel’s Textbook of Practical Organic Chemistry; Wiley: New York, 1989; p 699. (31) Osawa, R.; Yamasaki, K. Bull. Chem. Soc. Jpn. 1959, 32, 1302.

Mukkamala and Weiss 3.66 (s, 3H, COOCH3), 2.36-0.67 (m, 37H, steroidal); MS m/z 642 (M+ + 18, CI with NH4+, 100), 627 (M+ + 3, 2). 2-(3β-Cholesteryloxy)ethyl Anthraquinone-2-carboxylate (4). The procedure for the preparation of 3 was followed using 0.5 g (1.16 mmol) of 3β-(2-hydroxyethoxy)-5-cholestene as the steroid. 4 was obtained as a yellow crystalline solid in 43% yield (99.4% pure by HPLC): mp 177.2-179.2 °C (after recrystallization from ethyl acetate); IR (chloroform)) 2943, 2866, 1724 and 1678 cm-1; 1H NMR (90 MHz, CDCl3/TMS) δ 8.9 (bs, 1H, aromatic), 8.6-8.2 (m, 4H, aromatic), 7.8 (m, 2H, aromatic), 5.3 (m, 1H, olefinic), 4.5 (bt, J ) 5.0 Hz, 2H, COOCH2), 3.8 (bt, J ) 5.0 Hz, 2H, CH2O), 3.6 (m, 1H, OCH), 2.4-0.5 (m, 43 H, steroidal); MS m/z 682 (M+ + 18, CI with NH4+, 100), 666 (M+ + 2, 2). 3β-Cholestanyl Tosylate. Tosyl chloride (0.99 g, 5.2 mmol) and then 4-(dimethylamino)pyridine (ca. 20 mg) were added to an ice-cooled solution of 3β-cholestanol (1.0 g, 2.57 mmol) in 7 mL of dry pyridine, and the reaction mixture was stirred for 15 h at room temperature under a dry atm. It was poured into ca. 20 mL of cold 2 N sulfuric acid, stirred for 0.5 h, and extracted with chloroform. The organic layer was washed successively with water, aqueous NaHCO3, and water, and dried (anhydrous Na2SO4). Solvent removal afforded a pale yellow solid which was recrystallized from an ethyl acetate/hexane mixture to obtain 0.8 g (58%) of white solid: mp 142-143 °C; IR (chloroform) 2945, 2834, 1601, 1466 cm-1; 1H NMR (90 MHz, CDCl3/TMS) δ 7.8 (d, J ) 8.0 Hz, 2H, aromatic), 7.3 (d, J ) 8.0 Hz, 2H, aromatic), 4.4 (m, 1H, SO3CH), 2.4 (s, 3H, ArCH3), 2.1-0.6 (m, 46H, steroidal). 3r-Cholestanol. Using a literature procedure,32 3R-cholestanol (mp 185.3-187.3 °C; lit.33 mp 185 °C) was prepared from 3β-cholestanyl tosylate in 42% yield. 3r-Cholestanyl Anthraquinone-2-carboxylate (5). The procedure for preparation of 3 was followed using 3R-cholestanol as the steroid. 5 was obtained as yellow crystals in 37% yield (98.4% pure by HPLC): mp 212.8-215 °C (after recrystallization from ethyl acetate); IR (chloroform) 2932, 2856, 1734, 1678 cm-1; 1H NMR (90 MHz, CDCl /TMS) δ 8.9 (bs, 1H, aromatic), 8.6-8.2 3 (m, 4H, aromatic), 7.8 (m, 2H, aromatic), 5.3 (m, 1H, COOCH), 2.2-0.5 (m, 46H, steroidal); MS m/z 622 (M+, 80), 482 (20), 370 (55), 254 (100). 1-(2-Anthraquinonyloxy)-2-(3β-cholesteryloxy)ethane (6). 3β-(2-Hydroxyethoxy)-5-cholestene (0.2 g, 0.48 mmol) was added to a mixture of 2-hydroxyanthraquinone (0.1 g, 0.48 mmol), triphenylphosphine (0.13 g, 0.5 mmol), and diethyl azodicarboxylate (0.08 mL, 0.5 mmol) in 10 mL of dry THF.34 The mixture was stirred in the dark for 3 days at room temperature under a dry atm. After removal of solvent (rotary evaporator), the residue was chromatographed (silica gel, 1/20 ethyl acetate/ hexane) and recrystallized twice from hot ethyl acetate to yield 0.104 g (35%, one peak by HPLC) of yellow crystals: mp 141.3143.0 °C (solid to liquid crystalline) and 156.8-157.4 °C (liquid crystalline to isotropic); IR (chloroform) 3043, 2941, 1670, 1599 cm-1; 1H NMR (300 MHz, CDCl3/TMS) δ 8.32-8.25 (m, 3H, aromatic), 7.80-7.75 (m, 3H, aromatic), 7.33-7.29 (m, 1H, aromatic), 5.37 (m, 1H, olefinic), 4.31 (t, J ) 5.8 Hz, 2H, OCH2), 3.91 (t, J ) 5.8 Hz, 2H, CH2O), 3.28 (m, 1H, OCH), 2.05-0.68 (m, 43H, steroidal); MS m/z 654 (M+ + 18, 100, CI with NH4+), 637 (M+ + 1, 50). 3β-(2-Anthraquinonyloxy)cholestane (7). 3R-Cholestanol (0.1 g, 0.25 mmol) was added to a mixture of 2-hydroxyanthraquinone (0.054 g, 0.26 mmol), triphenylphosphine (0.068 g, 0.26 mmol), and diethyl azodicarboxylate (0.040 mL, 0.26 mmol) in 6 mL of dry THF, and the mixture was stirred in the dark for 4 days at room temperature under a dry atm.34 After removal of solvent (rotary evaporator), the residue was chromatographed on silica gel (1/20 ethyl acetate/hexane) and recrystallized twice from hot ethyl acetate to yield 0.03 g (20%, 97.5% pure by HPLC) of 7 as a pale yellow crystalline solid: mp 170.7-171.7 °C; IR (chloroform) 2943, 2854, 1672, 1591, 1327 cm-1; 1H NMR (300 MHz, CDCl3/TMS) δ 8.32-8.23 (m, 3H, aromatic), 7.80-7.70 (m, 3H, aromatic), 7.27-7.22 (m, 1H, aromatic), 4.47 (m, 1H, OCH), 2.02-0.67 (m, 46H, steroidal); MS m/z 612 (M+ + 18, 50, CI with NH4+), 595 (M+ + 1, 100). (32) Raduchel, B. Synthesis 1980, 292. (33) Marker, R. E.; Whitmore, F. C.; Kamm, O. J. Am. Chem. Soc. 1935, 57, 2358. (34) Manhas, M. S.; Hoffman, W. H.; Lal, B.; Bose, A. K. J. Chem. Soc., Perkin Trans. 1 1976, 461.

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Langmuir, Vol. 12, No. 6, 1996 1477 Chart 2

Stigmasteryl Anthraquinone-2-carboxylate (8). Using the procedure for 3, anthraquinone-2-carboxylic acid chloride was prepared from anthraquinone-2-carboxylic acid (0.28 g, 1.2 mmol) and oxalyl chloride (0.9 mL, 10.07 mmol). The acid chloride was added to stigmasterol (0.45 g, 1.1 mmol) and a few drops of pyridine in 20 mL of dry pyridine, and the reaction mixture was heated under reflux for 24 h in a dry atm. After removal of solvent (rotary evaporator), the residue was dissolved in chloroform, washed with aqueous NaHCO3 and water, and dried (anhydrous Na2SO4). After removal of solvent, the solid residue was eluted on a silica gel column (1/20 ethyl acetate/hexane) and recrystallized twice from hot ethyl acetate to yield 0.05 g (7%, one peak by HPLC) of yellow solid: mp 196.3-198.4 °C (solid to liquid crystalline) and 236.3-246.6 °C (liquid crystalline to isotropic; decomp); IR (chloroform) 2932, 2856, 1718, 1676, 1602 cm-1; 1H NMR (300 MHz, CDCl3/TMS) δ 8.94 (m, 1H, aromatic), 8.43-8.35 (m, 4H, aromatic), 7.65 (m, 2H, aromatic), 5.45 (m, 1H, olefinic), 5.35-4.85 (m, 4H, olefinic and COOCH), 2.510.72 (m, 43H, steroidal); MS m/z 664 (M+ + 18, CI with NH4+, 100), 648 (M+ + 2, 30). 3β-Thiocholesteryl Anthraquinone-2-carboxylate (9). Using the procedure for 3, anthraquinone-2-carboxylic acid chloride was prepared from anthraquinone-2-carboxylic acid (0.28 g, 1.2 mmol) and oxalyl chloride (0.9 mL, 10.07 mmol). The acid chloride was added to 3β-thiocholesterol (0.403 g, 1.0 mmol), magnesium turnings (0.03 g, 1.2 mmol), and iodine (two small crystals) in 25 mL of dry benzene,35 and the reaction mixture was refluxed for 24 h with stirring in a dry atm. The reaction mixture was poured into water and extracted with chloroform (3 × 50 mL). The combined organic layers were washed with aqueous NaHCO3 and water and dried (anhyd Na2SO4). After removal of solvent, the solid residue was eluted on a silica gel column (1/20 ethyl acetate/hexane) and recrystallized twice from hot ethyl acetate/chloroform mixture to yield 0.305 g (49%, one peak by HPLC) of 9 as a pale yellow, fluffy solid: mp 216.4219.5 °C (solid to liquid crystalline) and 288.1 °C (liquid crystalline to isotropic; decomp); IR (KBr) 2959, 2864, 1676, 1638 (-COS-), 1593 cm-1; 1H NMR (270 MHz, CDCl3/TMS) δ 8.95 (m, 1H, aromatic), 8.48 (m, 4H, aromatic), 7.82 (m, 2H, aromatic), 5.41 (m, 1H, olefinic), 3.42 (m, 1H, COSCH), 2.5-0.7 (m, 43H, steroidal); MS m/z 636 (M+, 10), 608 (50), 521 (20), 368 (100). Stigmastanyl Anthraquinone-2-carboxylate (10). The procedure for the preparation of 9 was followed35 using stigmastanol (0.416 g, 1.0 mmol) as the steroid. 10 was obtained as a pale yellow crystalline solid (0.311 g, 49%, one peak by HPLC): mp 202.5-203.5 °C (solid to liquid crystalline) and 247.1-248.1 °C (liquid crystalline to isotropic); IR (KBr) 2957, 2866, 1726, 1678, 1593 cm-1; 1H NMR (270 MHz, CDCl3/TMS) δ 8.95 (m, 1H, aromatic), 8.49 (m, 4H, aromatic), 7.92 (m, 2H, m, aromatic), 5.0 (m, 1H, COOCH), 2.05-0.65 (50H, m, steroidal); MS m/z 668 (M+ + 18, CI with NH4+, 100), 652 (M+ + 2, 35).

Results and Discussion Aggregation of ALS molecules into strands with exceedingly large aspect ratios is responsible for their (35) Kreisky, S. Acta Chem. Scandi. 1957, 11, 913.

physical gelation of liquids. Strands form due to attractive intermolecular interactions based primarily on van der Waals forces. However, when the liquid component contains hydroxy or other functional groups capable of hydrogen bonding, stronger gelator-liquid interactions can also contribute to the type and stability of the gel formed. In other (non-ALS) systems, strongly attractive electrostatic and/or hydrogen-bonding interactions have been shown to lead not only to self-aggregation of singlecomponent gelators1,3-5,15-21 but also to specific recognition between molecules of some two-component gelator assemblies (such as surfactant-phenol23 and pyrimidinebarbituric acid24). Recently, we have found that charge transfer interactions can produce intercalated twocomponent ALS gels with interesting properties.13 The gelation ability of ALS molecules is known to be highly sensitive to their molecular shape and electronic properties.7 For example, several ALS containing 2-anthryl,7b 2-anthraquinonyl,7b or azobenzene22 A groups gelled various liquids while those with phenyl, 2-naphthyl, 9-anthryl, or biphenyl groups did not.7b Additionally, ALS molecules without a long alkyl chain on C-17 of the steroid moiety did not form gels with any of the liquids tested.7 To complicate matters further, several ALS with three linearly fused aromatic rings in the A part and long alkyl chains at C-17 of the S part are still ineffective as gelators. This suggests that additional structural features (either independent of or in combination with those mentioned above) contribute to the gelation ability of an ALS molecule. Herein, the relationship between ALS structure and gelation abilities of compounds 1-10 (Chart 1) has been examined. These gelators allow more detailed structural comparisons of features, such as the R/β stereochemistry at C-3 of the S group, the nature of the alkyl chain on C-17 of the S group, and the length and functionality of the L group, to be made; the aromatic group, 2-anthraquinonyl, is not varied. The stereochemistry of C-3 of the steroidal moiety is β for all of the molecules in the chart except 3 and 5 for which it is R. The C-17 alkyl chain is different from the C8H17 group of cholesterol in 3, 8, and 10. Previously, we found that both ester (as in 3, 5, 8, and 10) and etherester (as in 1) functionalities for L support gelation. New linker groups, such as thioester (in 9), ether (in 7), diether (in 6), ester-ether (in 4), and carbonate (in 2) functional groups, have been employed here; the single oxygen atom of 7 is the shortest L group used in an ALS thus far. Cholesteryl 4-(2-anthraquinonyloxy)butanoate (1) is the anthraquinone analogue of the extensively studied cholesteryl 4-(2-anthryloxy)butanoate7,8 (CAB, Chart 2). We

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Table 1. Liquids Tested for Gelation by 2 wt% ALS Gelator liquid methanol ethanol 1-propanol 2-propanol 1-butanol tert-butyl alcohol 1-pentanol 1-hexanol 1-heptanol 4-heptanol 1-octanol 2-octanol 1-nonanol 1-decanol isoamyl alcohol tert-amyl alcohol 2,4,4-trimethylpentanol 2-tridecanol ethylene glycol 1,2-propanediol 2-ethanolamine hexanoic acid 2-decanone n-hexane n-heptane isooctane n-dodecane n-hexadecane cyclohexane methylcyclohexane phenylcyclohexane benzene styrene toluene pyridine methyl acrylate methyl methacrylate acetonitrile benzyl alcohol 1,2-dimethoxyethane diethyl ether ethyl acetate carbon tetrachloride dichloromethane chloroform Dow Corning 704 silicone oile a

1 I I G P G G G G G G G G G G G

I I T S S I I G G G GCa GCa

2

3

4

5

I I I

I I SS

I I SS

I I E

I I I

P I P

P SS P

E

I I

P P P

P P P

E

P

6

7

8

I I B

I I

I I

G G

SS P

I G

E E E

G GCa

P

P

P

G

SS

SS P S P

P P I G G GCa GCa P S P S T P P SS GCd

S S S

S S S

E P

9

10

I I

I I

G G

G

G G G

G G G

G G G

E

G

G G I G G

G

P I

P I

G I

P I

P P

G G G

G G B

G G B

S S S

S S S

S S P

S S S

P

P B I G

I GCa SS GCa

B GCc SS G

SS

SS

SS

P S

P S

S S

I I

S S S S GCb GCb SS G P I P S S S

I

P

I

I

P P I

I I

SS P SS

P P P

P P P

S S P P

S S S S

S S S S

S S S

P

P

I P P S

P S S S

P P S S S

S S S

GCd

GCd

P b

c

d

e

Cooled under tap water at ca. 10 °C. Cooled in a freezer (ca. -10 °C). Cooled in ice. Cooled in a refrigerator (ca. 4 °C). Gelator was dissolved using methylene chloride as a cosolvent.10,11 B ) broken gel, E ) emulsion, G ) gel formed at room temperature, GC ) gel formed upon cooling below room temperature, I ) gelator insoluble at boiling temperature of liquid, P ) gelator soluble but precipitated upon cooling, S ) gelator soluble at room temperature or upon slight heating and remaining in solution even upon cooling, SS ) sparingly soluble at boiling temperature of liquid and gel not formed upon cooling, T ) thickening upon cooling in a freezer (ca. -10 °C).

have concentrated on 1 to discern its aggregation behavior and to compare the stabilities of its gels with those of CAB. Gelation Properties of 1-10. Typically, an ALS and an organic liquid were heated in a sealed tube until the solid dissolved. The hot solution was cooled either in the air, under cold tap water (ca. 10 °C), in a refrigerator (ca. 4 °C), in ice, or in a freezer (ca. -10 °C); the maximum length of the cooling period was ca. 2 h. Gelation was considered successful if the solution became a semitransparent solidlike phase with no apparent flow upon inversion of the tube. In 2% wt/vol concentrations, compounds 1, 6, and 8-10 were able to gel a wide variety of linear and branched alcohols and alkanes, and a few other solvents. Compounds 2-5 and 7 did not, however, gel any of these liquids. Various organic fluids were tested with 1-10, and the results are summarized in Table 1. Concentrations from ca. 0.5-5 wt% of 1 and 9 were tested in several liquids; for the other ALS, concentrations were kept near 2 wt%.

All of the compounds 1-10 could be dissolved readily in chloroform, dichloromethane, benzene, or toluene without gels being formed. Interestingly, chloroform is gelled by a crown ether-based phthalocyanine,21 and aromatic solvents are gelled by dibenzylidene sorbitol;15 unlike the ALS, both of these gelators are able to make strong hydrogen bonds. Relatively low molecular weight alcohols (methanol and ethanol) and alkanes (hexane and heptane) were not gelled due, at least in part, to their inability to dissolve adequate quantities of ALS at the temperatures where the liquid boils. All the molecules in Chart 1 (except 2 in hexadecane and dodecane) could be dissolved in higher homologues of alkanes or alcohols by heating the ALS-fluid mixtures to temperatures close to the fluid’s boiling point. Some of the ALS reported here and CAB7 form weak gels with cyclic alkanes, like cyclohexane, that persist for very short periods. This behavior is in marked contrast to the extreme stability of gels from the same liquids and the D-azahomosteroid 11 (Chart 2) investigated by Wade et al.5

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Although none of the ALS in Chart 1 formed gels with ethyl acetate, the more polar methyl acrylate and methyl methacrylate could be gelled by 1, 9, and/or 10. Gelation of hexanoic acid, ethylene glycol, 1,2-propylene glycol, and several other liquids was tested with only a few of the ALS, and no gels were obtained. 2-Decanone was gelled by 1-2 wt% of 9, but not by any of the other ALS. Gelation of a silicone oil (Dow Corning 704 Fluid) was tested by 2 wt% 1, 8, and 10 (using methylene chloride as a cosolvent10,11) also; 1 and 10 gave weak gels at subambient temperatures while 8 merely precipitated. These results suggest that ALS gelators are most effective for gelation of long chain alkanes and alcohols in which the necessary balance between solubility and insolubility is attained. The ALS molecules must disperse in solution (at elevated temperatures) and then selfassemble into three-dimensional networks of intertwined fibers,7 which encompass and immobilize the fluid (at lower temperatures). Some previously prepared ALS molecules like CHAQ and CAQ (Chart 2) have also been highly effective in the gelation of several silicone fluids.10,11 In spite of their being prepared by a different method, the microstructures of these gels are very similar to those of gels containing other organic fluids. ALS Structure-Gelation Correlations. From the gelating abilities of 1-10 and other ALS studied previously, it appears that gelation is facilitated when the stereochemistry at C-3 of the steroid moiety is β. Although the β isomer of 5 (CHAQ, Chart 2) is known to be an effective gelator of several alcohols and alkanes,7 neither 5 nor 3, another ALS molecule with R stereochemistry at C-3 of the S unit, gels the same liquids. The importance of stereochemistry at C-3 of the S unit can be rationalized on the basis of geometric considerations. An ALS with β stereochemistry is much more rodlike in shape than its R diastereomer when both are in extended conformations. The more bent shape of the R isomer may lead to packing arrangements less conducive to stable strand formation. However, both the R and β epimers of cholesteryl azobenzenecarboxylates22 and some quaternary ammonium salts of 3-aminocholestane36 do act as gelators. In these cases, the greater importance of interactions among A-type groups may be the determining factor in aggregation. Since both 8 and 10 are able to gel several alcohols and alkanes, minor modifications of the cholesterol-type chain at C-17 of CAQ, such as introduction of unsaturation and/ or an ethyl substituent, modify but do not destroy gelling ability. However, several ALS molecules with shortened and otherwise grossly modified chains at C-17 are unable to gel alkanes and alcohols.7 The long alkyl chain may help to moderate the solubility of an ALS in a fluid, allowing aggregates of strands, rather than bulk crystals, to form on cooling. There may be specific gelator-liquid interactions involving these alkyl chains during the initial period of aggregation. 11 with gem-di-n-propyl groups at C-17 is a better gelator of cyclic alkanes than of n-alkanes;5 the shape of the gem-dipropyl group mimics that of a cycloalkane. Although the length and/or functionality of the L group is important (and the length is often changed with the functionality), we can discern no clear relationship between either one and gelation ability from the structures in Chart 1. The number of examples does not permit specific conclusions to be drawn. However, some comparisons can be made. Although both 6 and cholesteryl 2-(2-anthryloxy)ethanoate7 have L groups with similar lengths, the former, a diether, is a successful gelator while the latter, an ether-ester, is not. Similarly, 8, 10, CAQ,7 (36) Lu, L.; Weiss, R. G. Langmuir 1995, 11, 3630.

Langmuir, Vol. 12, No. 6, 1996 1479 Table 2. Phase Transition Temperatures (Optical Microscopy) and Heats of Transitions (DSC) of ALS Molecules ∆H (kcal/mol) ALS

phase transition

1

solid-cholesteric cholesteric-isotropic solid-isotropic solid-isotropic solid-isotropic solid-isotropic solid-cholesteric cholesteric-isotropic solid-isotropic solid-cholesteric cholesteric-isotropicc solid-cholesteric cholesteric-isotropicc solid-cholesteric cholesteric-isotropic

2 3 4 5 6 7 8 9 10

temp

(°C)a

137 174 235 197 177 213 141 156 171 196 236 216 288 203 247

Ia

IIb

9.5 0.3

7.0 0.5

9.6 0.3

9.3 0.3

8.4 1.4 9.6 d 7.5 0.09

6.7 0.07

a First heating. b Second heating. c With decomposition. measured.

d

Not

cholesteryl anthracene-2-carboxylate7 (with carboxy groups), and 9 (a thioester) gel a variety of liquids. ALS with L groups like carbonate (2) or a simple ether (7) did not gel a representative selection of alcohols or n-alkanes. Thermal Behavior of ALS. Of the molecules in Chart 1, only those that were found to gel alkanes and alcohols (i.e., 1, 6, and 8-10) form enantiotropic cholesteric liquidcrystalline phases. The mesophases were identified by optical microscopy and differential scanning calorimetry. The other ALS, 2, 3-5, and 7, melted directly from solid to isotropic phases upon heating. However, there are examples of other ALS that are mesomorphic but not gelators (when A is naphthyl or biphenyl7b) and vice versa, and many non-ALS gelators are not mesomorphic. We suggest that mesomorphism aids gel formation, perhaps at the stage of gelator nucleation, but that it is not a necessary property. Table 2 collects the phase information for 1-10. Gel Stability. We take gelation temperature Tg (the temperature at which a gel phase becomes isotropic and/ or phase separates into its solid and liquid constituents upon heating) and gel lifetime (the period which a gel persists at room temperature) as measures of gel stability. Thus, a very stable gel has a high Tg and a long gel lifetime; however, a high Tg need not be accompanied by a long gel lifetime, since different thermodynamic and kinetic factors are involved. Gels persisted for longer periods at higher gelator concentrations (up to a limit) and at lower incubation temperatures. Also, lifetimes at ambient temperatures seem to depend on the polarity of both the gelator and the liquid component. For instance, the approximate lifetimes of gels of 2 wt% 1 in benzyl alcohol and in n-dodecane are 4-6 days and 10 h, respectively. For gels of 2 wt% 6, the longer lifetime (4 days) is found with n-dodecane, and the lifetime in benzyl alcohol is only 5 min! Gels comprised of 1 or 6 and alkanols persisted for varied time periods (1-12 h). No relationship was discernable between gel lifetime and either the length or the branching of alkanol chains. Gelation Temperatures. In order to gain a more quantitative understanding of the stabilities of ALS gels, gelation temperatures of various concentrations of 1 in a variety of organic fluids were measured by the inverseflow22 and/or ball-drop29 methods (Figures 1 and 2). Upon being heated, many of these gels melted over narrow ranges, ca. 3 °C, indicating a rather homogeneous microstructure. As previously observed,7 initially Tg in-

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Mukkamala and Weiss Table 3. Gelation Temperatures (Tg) for 1 and 1-Alkanols Measured by the Inverse-Flow Method Tg (°C)a

1-alkanol no. of carbon atoms

1.5 wt%b

2.0 wt%b

3 4 5 6 7 8 9 10

52 42 37 37 39 38 36 36

45 52 47 (50)c 41 43 (43)d 43 (42)e 43 45

a (3 °C. b Concentration of 1. c From isoamyl alcohol. 4-heptanol. e From DL-2-octanol.

Figure 1. Gelator concentration versus gelation temperature for 1 and various liquids.

Figure 2. Gelation temperatures for 1 wt% (9) or 1.5 wt% (0) of 1 in 1-alkanols versus ∆f (solvent polarity) and n (number of carbon atoms in the alcohol molecules).

creased rapidly with increasing gelator concentration before approaching a “saturation” value (Figure 1). It is important to distinguish between the influence of the liquid component on gel formation (cooling) and gel destruction (heating) since the two may be very different. Upon being cooled, a solution of gelator and liquid may experience interactions that preclude or promote nucleation leading to stands (and a gel). On heating, the ancillary influence of the liquid component on molecular packing may be lost since many gels appear to exclude molecules of the liquid component from the strands (vide infra). In this case, Tg values from heating the gel phase will reflect mostly the solubility of the gelator in the liquid and/or the melting temperature of the strands as zero gels. Also, the rheologically-based Tg values represent the temperatures at which the junctions between intersecting strands are ruptured (i.e., where cross-linking is destroyed). These are not necessarily the temperatures at which extensive gelator aggregation is lost.8 In spite of its chiral nature, 1 formed gels with DL-, D-, or L-2-octanol whose Tg values (heating cycles) were indistinguishable within experimental error. The diastereomeric gelator-liquid interactions must be too small

d

From

in magnitude to be detected by our methods. On the other hand, the enantiomeric gelator-gelator interactions in some amino acid-based gelators appear to be quite important for gelation since racemic mixtures could not gel nonchiral fluids, but the individual enantiomers could.16,17 Smaller diastereomeric gelator-gelator interactions allow the copper(II) salt of racemic 2-ethylhexanoic acid to gel some nonpolar liquids.4 These results suggest that specific molecular interactions between gelator and fluid need not play a significant role in gel formation. Fluorescence studies on gels of CAB in mixed fluid systems comprised of molecules with similar shapes indeed show that Tg even from cooling cycles depends more on a fluid’s bulk polarity than on its functionality.8 By contrast, the Tg values (heating cycles) of gels from 1 and 1-alkanols with 5-10 carbon atoms are virtually invariant (Figure 2). The aberrant behavior of the gel from 1.5 wt% 1 and 1-propanol is reproducible but not understood at this time. It may result from smaller molecules being incorporated at lower temperatures into the gelator strands at junction zones.12a Since the Lippert polarity function, ∆f ) [{( - 1)/(2 + 1)} - {(n2 - 1)/(2n2 + 1)}] (where  is the dielectric constant and n is the refractive index),37 varies in a continuous fashion throughout the series of alcohols, Tg must not be not responding to the small changes in bulk polarity. Interestingly, CAB gels have the same (spectroscopically measured) Tg value (cooling cycle) throughout a series of n-alkanes (hexane to hexadecane) at one gelator concentration.7 In fact, the similarity among the Tg values in Table 3 for 1 in isomeric alcohols exemplifies the greater influence of bulk polarity than of molecular structure on gel stability. The results suggest that specific gelator-fluid molecular interactions do not contribute significantly to the melting of these gels. Larger polarity changes than those found in Figure 2 can have a measurable influence on Tg. Tg values of gels from 1 and dodecane (∆f ) -0.0013) are considerably higher than those from 1 and alcohols (Tables 3 and 4 and Figures 1 and 2) when the gelator concentration is 2 wt% or higher. By contrast, gels from CAB, the anthryl analogue of 1, exhibit much lower Tg values with alkanes than with alcohols.8 The source of the differing trends is probably related to 1 being more soluble in alcohols than is CAB. Recent investigations on CAB gels using small angle neutron and X-ray scattering techniques confirm that the gelator strands are interconnected by “junction zones” that, like the cross-linking points of polymeric gelators, form a three-dimensional network.12a The scattering data also show that the molecular assembly of the CAB at the junction zones in alcohol gels is very similar to that of the solid state; molecules of the liquid component (37) Rohatgi-Mukherjee, K. K. Fundamentals of Photochemistry; Wiley Eastern Limited: New Delhi, India, 1978; p 105.

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Table 4. Gelation Temperatures (Tg) for 1 in Benzyl Alcohol and Dodecane Measured by the Ball-Drop Method (A), Optical Microscopy (B), and Inverse-Flow Method (C) Tg (°C)a benzyl alcohol gelator concn (wt%)

(A)

(B)

1 2 3 4 5

26 38 43 46 49

28 34 40 47

a

dodecane (C) 26 59 66 73 69

(3 °C. Table 5. Gelation Temperatures (Tg) for 8-10 in Measured by the Inverse-Flow Method

DL-2-Octanol

gelator

concn (wt%)

Tg (°C)a

8

1.0 2.0 1.0 2.0 1.0 2.0

47 68 79 84 28 35

9 10 a

Figure 3. Optical micrograph (×357, crossed polars) of the gel phase for 2 wt% 1 and benzyl alcohol at 24 °C.

(5 °C.

are excluded from these aggregates. In decane gels, however, the junction zones are swollen due to solvent penetration and they assume lyotropic organizations.12a Thus, the CAB/alcohol gels must be heated to higher temperatures than the CAB/alkane gels to dissolve the junction zones and, thereby, liquify the phase. As mentioned, gel stabilities based on Tgs and gel lifetimes need not be similar. At room temperature, the gels of 1/n-dodecane with higher Tg values have shorter lifetimes than those of 1/benzyl alcohol with lower Tg values. For gels of 1/n-alkanols, the Tg values are lower than those of 1/n-dodecane gels but the lifetimes at room temperature are similar. Since the lifetimes at room temperature seem to depend upon the resistance of strands to undergo transitions from less stable to more stable packing arrangements, to a first approximation they should not depend upon the liquid component at all. However, there must be a slow exchange of gelator molecules in the gel phase between those in the liquid component and those in the strands,23 and some strands incorporate molecules of the liquid.12a Either of these exchanges can, in principle, catalyze strand phase transitions and lead to destruction of the gels. Note that the Tg and melting or isotropization temperatures of the neat gelators (Table 2) do not correlate. Gelation temperatures were also measured for the gels of 8, 9, and 10 in DL-2-octanol by the inverse-flow (heating cycles) method. Although the gel-melting transitions in all these cases were much broader (>10 °C) than those of the corresponding 1 gels, some interesting trends in stability can be gleaned (Table 5). The gels of 8 melt at considerably higher temperatures than those of 10. The presence of two double bonds in the former may induce stronger aggregation between the gelator molecules due to increased van der Waals attractions and stiffer chains at C-17. The greater polarizability of the thioester group may have a similar influence and be responsible for the gels of 9 having higher Tg values than those of 8 or 10. Gel Microstructure by Optical Microscopy. ALS gels are known to contain intricate networks of strands of gelator molecules which hold the liquid component in place via surface tension.7 Optical micrographs of gels of CAB/dodecane and CAB/1-octanol show colloids several micrometers in diameter consisting of intricate networks

Figure 4. Circular dichroism spectra (A) and absorption spectra (B) of 2 wt% 1 in DL-2-octanol (gel phase, s, 22 °C and isotropic phase, - - -, 55 °C).

of branched gelator fibers, each of ∼10-20 nm in width.7,12 The shapes of the fibers in the two gels are rather different.7 When gels from various concentrations of 1 in benzyl alcohol were examined between cover slips by optical microscopy using crossed polars, networks of gelator strands could be seen (Figure 3). The density of strands increased with gelator concentration. Over a range of gelator concentrations (Table 4), the temperatures

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There is a known weak transition above 400 nm in anthraquinones,39 and it is prominent in the circular dichroism spectra of 1 (Figures 4 and 5). The CD spectra of 2 wt% 1 in D-, L-, and DL-2-octanol were studied in both gel and isotropic phases. (Figure 4 shows only the spectra in DL-2-octanol, and almost identical spectra were observed in both D- and L-2-octanol.) The gel phase spectra have induced CD bands between 300-500 nm which are absent in the corresponding isotropic phases. As mentioned above, the weak transition above 400 nm in the absorption spectra of 1 (characteristic of anthraquinone and its derivatives10,39) is pronounced with its typical vibronic structure in the gel phase CD spectra. The signs, positions, and approximate intensities of the bands for the three gels are virtually identical, indicating that the chirality of the solvent has no discernable influence on the extrinsic chirality of the gelator aggregates. Consistent with this, the gel phase CD spectrum of 1 in 1-decanol (Figure 5) is similar to those of 1 in the chiral alcohols. From comparisons of CD spectra in Figures 4 and 5, it can be concluded also that polarity of the liquid component has no apparent influence on the extrinsic chirality of these gelator aggregates. Solvent polarity does affect the CD of gels of CAB.40

Figure 5. Circular dichroism spectra (A) and absorption spectra (B) of 2 wt% 1 in dodecane (gel phase, s, 22 °C and isotropic phase, ‚‚‚, 77 °C) and 1.5 wt% 1 in 1-decanol (gel phase, - - -, 22 °C and isotropic phase, -‚-, 55 °C).

at which strands disappeared upon heating closely corresponded to the Tg from the ball-drop method. Circular Dichroism. Samples of CAB in dodecane and alcohols, and cholesteryl azobenzenecarboxylates in several alcohols and alkanes, show extrinsic circular dichroism (CD) in the gel phase from the chiral array of the aggregated gelator molecules.7,22 This phenomenon is similar to liquid crystal-induced circular dichroism.38 The absorption spectrum of 1 in benzene has maxima at 330 ( 4069 L mol-1 cm-1) and 364 nm ( 3295 L mol-1 cm-1) and a shoulder at ca. 410 nm ( ca. 150 L mol-1 cm-1). The energies of the absorption bands do not change appreciably in solvents of different polarities or when gelation occurs. However, gelation of CAB/dodecane samples is marked by new absorption bands from excitation dipole coupling between stacked anthracenyl groups.7 (38) Saeva, F. D.; Wysocki, J. J. J. Am. Chem. Soc. 1971, 93, 5928.

Conclusions We have investigated the gelation properties of 10 new molecules belonging to the ALS family of gelators. These studies allow the influence of some key structural features of the ALS (such as the stereochemistry at C-3, the alkyl chain at C-17 of the S group, and the length and the functionality of the L group) on gelation ability to be understood. From the gelation temperatures and circular dichroism spectra of several samples of 1, we conclude that specific solvent-gelator molecular interactions need not be present to ensure gel stability. Solvent polarity, on the other hand, can have a profound influence on both the formation and stability of a gel by controlling the solubility of an ALS. A comparison of Tg values of 1 in benzyl alcohol from rheological measurements indicates that aggregation into strands and physical gelation occur almost synchronously. Efforts to further understand the molecular packing of 1 in its neat solid state and in gel strands are in progress.41 Acknowledgment. We thank Mr. W. G. Smith and Dr. I. Furman for preparing 3β-(2-hydroxyethoxy)-5cholestene used in the syntheses of 4 and 6. We also thank Dr. Quanlong Pu and Dr. Xue-Feng Pei of the National Institutes of Health for mass spectral analyses and TA Instruments, Inc. for the use of a Thermal Analyst 2000 controller. The National Science Foundation (Grants CHE-9213622 and CHE-9422560) is gratefully acknowledged for its support of this work. LA950666K (39) Itoh, T. Spectrochim. Acta, Part A 1986, 42, 1083. (40) Gottarelli, G.; Spada, G. P.; Weiss, R. G. Unpublished results. (41) Terech, P.; Mukkamala, R.; Weiss, R. G. Manuscript in preparation.