Gelation of Organic Liquids by Some 5Î±-Cholestan-3Î²-yl N-(2-Aryl

20

Langmuir 2000, 16, 20-34

Gelation of Organic Liquids by Some 5r-Cholestan-3β-yl N-(2-Aryl)carbamates and 3β-Cholesteryl 4-(2-Anthrylamino)butanoates. How Important Are H-Bonding Interactions in the Gel and Neat Assemblies of Aza Aromatic-Linker-Steroid Gelators?† Liangde Lu, T. Matthew Cocker, Robert E. Bachman, and Richard G. Weiss* Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227 Received November 24, 1998. In Final Form: February 17, 1999 Thermally reversible organogels, comprised of a variety of organic liquids and low concentrations of relatively low molecular mass aza analogues of previously investigated ALS (i.e., aromatic linker steroid) molecules have been investigated. The aza-ALS gelators are derivatives of 3β-cholesteryl 4-(2-anthrylamino)butanoate (CAAB) and 5R-cholestan-3β-yl aryl- or alkylcarbamates. The relationship between gelator structure and efficiency of gelation is explored. The molecular packing arrangements of the gelators in the gel (strands) and other phases are investigated by several physical methods. The results provide insights into the factors that do and do not lead to efficient gelators of this type. For instance, UV-vis absorption and fluorescence spectra of the gel and crystalline phases of 5R-cholestan-3β-yl N-(2-anthryl)N-methylcarbamate (CAMC) exhibit red-shifted bands that can be associated with J-type aggregates. From comparative infrared spectral investigations, it is concluded that there are no hydrogen-bonding interactions of 5R-cholestan-3β-yl N-(2-anthryl)carbamate (CAC) or 5R-cholestan-3β-yl N-(2-naphthyl)carbamate (CNC) molecules in the gels; the larger interactions of CAC molecules than of CAMC molecules in the gels appear to be a consequence of the smaller size of the N-substituent of CAC that allows closer molecular packing in strands. The small transition dipoles of naphthyl rings of CNC molecules are apparently too weak to promote detectable exciton coupling of aggregates (if present) in their gel and solution phases. Strong extrinsic circular dichroism of the gel phases of CAC indicate that its aggregates are in macrochiral arrangements. More than one strand morph can be formed by changing the cooling protocol associated with sol w gel transitions of CAC/1-pentanol samples. Dichroism of the gel phases of CAMC and CNC is large but significantly weaker and less well defined than that of CAC. Although CAAB, the amino analogue of the excellent “oxa” gelator, 3β-cholesteryl 4-(2-anthryloxy)butanoate, gelled none of several liquids examined, its amide derivatives did. As with CAC, IR spectra of CAAB and single-crystal X-ray diffraction and IR data for 5R-cholestan-3β-yl N-(4-n-butylphenyl)carbamate (CBPC, a nongelator) provide no clear evidence for H-bonding to oxygen atoms in their crystalline phases. Thus, hydrogen bonding, a critical factor in many other aggregate geometries, including those of various gelator strands, is subordinated to other van der Waals forces.

Introduction Gelation of liquids by small quantities of a second component, usually a polymer, has been known for several centuries.1,2 Although the most important factors necessary for macroscopic stabilization of these gels have been identified,3 the models are somewhat qualitative or apply to only a select set of systems.4-6 More recently, thermoreversible gelation of organic liquids by relatively low molecular mass organic molecules (hereafter referred to as “gelators”) has been investigated in diverse systems.7 An understanding of the microscopic factors responsible for gelation in these systems, as well as the nature of the † Part of the Special Issue “Clifford A. Bunton: From Reaction Mechanisms to Association Colloids; Crucial Contributions to Physical Organic Chemistry”.

(1) Garlaschelli, G.; Ramaccini, F.; Della Sala, S. Nature 1991, 353, 507. (2) (a) Graham, T. Trans. R. Soc. 1861, 151, 183. (b) Graham, T. J. Chem. Soc. 1864, 17, 318. (3) Flory, P. J. Discuss. Faraday Soc. 1974, 57, 7. (4) Tanaka, T. Sci. Am. 1981, 244, 14. (5) Hermans, P. H. Colloid Science; Kruyt, H. R., Ed.; Elsevier: Amsterdam, 1949; Vol. II, p 483. (6) Jordon-Lloyd, D. In Colloid Chemistry; Alexander, J., Ed.; The Chemical Catalog Co.: New York, 1926; Vol. 1, p 767. (7) (a) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (b) Terech, P. In Specialist Surfactants; Robb, I. D., Ed.; Chapman & Hall: Cambridge, 1997; Chapter 8.

gelator aggregates, has resulted from detailed spectroscopic and diffraction studies. Some attempts have been made to correlate the properties of the gels and the structures of the gelators.8-12 In almost all of the known examples, gelator molecules aggregate into fibers or strands that form intricate interlocking networks. The fibers or strands have cross sections typically in the 10-100 nm range. Cross-linking (8) (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, DC, 1987. (c) Lin, Y.-C.; Weiss, R. G. Macromolecules 1987, 20, 414. (9) (a) 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. (b) Murata, K.; Aoti, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. DIC Tech. Rev. 1996 (2), 39, and references cited therein. (c) Jeong, S. W.; Murata, K.; Shinkai, S. Supramol. Sci. 1996, 3, 83. (10) Mukkamala, R.; Weiss, R. G. Langmuir 1996, 12, 1474. (11) (a) Brotin, T.; Utermohlen, R.; Fages, F.; Bouas-Laurent, H.; Desvergne, J.-P. J. Chem. Soc., Chem. Commun. 1991, 416. (b) Placin, F.; Colomes, M. Desvergne, J.-P. Tetrahedron Lett. 1997, 38, 2665. (12) (a) Tata, M.; John, V. T.; Waguespack, Y. Y.; McPherson, G. L. J. Am. Chem. Soc. 1994, 116, 9464. (b) Tata, M.; John, V. T.; Waguespack, Y. Y.; McPherson, G. L. J. Phys. Chem. 1994, 98, 3809. (c) Xu, X.; Ayyagari, M.; Tata, M.; John, V. T.; McPherson, G. L. J. Phys. Chem. 1993, 97, 11350. (d) Terech, P.; Allegraud, J. J.; Garner, C. M. Langmuir 1998, 14, 3991. (e) Garner, C. M.; Terech, P.; Allegraud, J.-J.; Mistrot, B.; Nguyen, P.; de Geyer, A.; Rivera, D. J. Chem. Soc., Faraday Trans. 1998, 94, 2173.

10.1021/la981637n CCC: $19.00 © 2000 American Chemical Society Published on Web 07/27/1999

Gelation of Organic Liquids

points, called “junction zones”,13 allow the fibers to immobilize the liquid component primarily by surface tension.14 Previously, we have shown that many (but not all) members of the ALS family (i.e., molecules with aromatic (A), linking (L), and steroidal (S) groups, as typified by 3β-cholesteryl 4-(2-anthryloxy)butanoate (CAB) and 3βcholesteryl anthracene-2-carboxylate (CA-2)) are effective gelators of a wide variety of organic liquids.8,10,15 Their strand networks form colloidal units which intersect to immobilize the liquid component. The packing of ALS molecules within a strand can depend in a very sensitive way on the liquid component as well as the history of gel formation.16 Although several variations of the L part have been made,10 none allows the possibility of both intermolecular H-bond donation and acceptance among gelator molecules in a strand. Stabilization and regulation of biological structures by H-bonding interactions are very prevalent in aqueous environments. Hydrogen bonding can be also be an important factor in nonaqueous media, leading to stable, noncovalently linked molecular arrays,17 including those responsible for several gelator aggregates.4,7,9,12,18 However, the importance of H-bonding in strands of nonaqueous gels relative to other attractive (and repulsive) packing interactions,19 such as London dispersion forces among alkyl chains and steroidal groups and π-π stacking among aromatic groups,20 has not been explored. The energies of O-H‚‚‚O and N-H‚‚‚N type hydrogen bonds are typically in the range of 2-6 kcal/ mol.21 Those of O-H‚‚‚aromatic and N-H‚‚‚aromatic (13) Terech, P.; Furman, I. Weiss, R. G. J. Phys. Chem. 1995, 99, 9558. (14) Heller, W. In Polymer Colloids II; Fitch, R. M., Ed.; Plenum: New York, 1980; p 153. (15) Ostuni, E. M.S. Thesis, Georgetown University, Washington, DC, 1995. (16) Furman, I.; Weiss, R. G. Langmuir 1993, 9, 2084. (17) (a) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312. (b) Appella, D. H.; Christianson, L. A.; Karle, I. L.; Powell, D. R.; Gellman, S. H. J. Am. Chem. Soc. 1996, 118, 13071. (c) Fuhrhop, J.-H.; Schneider, P.; Rosenberg, J.; Boekema, E. J. Am. Chem. Soc. 1987, 109, 3387. (d) Hidaka, S.; Murata, M.; Onai, T. J. Chem. Soc., Chem. Commun. 1984, 562. (e) Haneissian, S.; Simard, M.; Roelens, S. J. Am. Chem. Soc. 1995, 117, 7630. (f) Menger, F. M.; Lee, S. J. J. Am. Chem. Soc. 1994, 116, 5987. (g) He, C.; Donald, A. M.; Griffin, A. C.; Waigh, T.; Windle, A. H. J. Polym. Sci., Part B: Polym. Phys. 1998, 36, 1617. (h) De, S.; Aswal, V. K.; Goyal, P. S.; Bhattacharya, S. J. Phys. Chem. 1998, 102, 6152. (18) (a) Sommerdijk, N. A. J. M.; Lambermon, M. H. L.; Feiters, M. C.; Nolte, R. J. M.; Zwanenburg, B. Chem. Commun. 1997, 1423. (b) Bhattacharya, S.; Acharya, S. N. G.; Raju, A. R. Chem. Commun. 1996, 2101. (c) de Rango, C.; et al. J. Am. Chem. Soc. 1992, 114, 5475. (d) Terech, P.; Ramasseul, R.; Volino, F. J. Colloid Interface Sci. 1983, 91, 280. (e) Terech, P. J. Colloid Interface Sci. 1985, 107, 244. (f) Tachibana, T., Mori, T.; Hori, K. Bull. Chem. Soc. Jpn. 1980, 53, 1714. (f) Haering, G.; Luisi, L. P. J. Phys. Chem. 1986, 90, 5892. (g) Hanabusa, K.; Miki, T.; Taguchi, Y.; Koyama, T.; Shirai, H. J. Chem. Soc., Chem. Commun. 1993, 1382. (h) Yasuda, Y.; Iishi, E.; Inada, H.; Shirota, Y. Chem. Lett. 1996, 575. (i) Gulik-Krzywicki, T.; Fouquey, C.; Lehn, J.-M. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 163. (j) Jeong, S. W.; Murata, K.; Shinkai, S. Supramol. Sci. 1996, 3, 83. (k) Aoki, M.; Nakashima, K.; Kawabata, H.; Tsutsui, S.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2 1993, 347. (l) Newkome, G. R.; et al. J. Am. Chem. Soc. 1990, 112, 8458. (m) Yamasaki, S.; Tsutsumi, H. Bull. Chem. Soc. Jpn. 1995, 68, 123. (n) Menger, F. M.; Yamasaki, Y.; Catlin, K. K.; Nishimi, T. Angew. Chem., Int. Ed. Engl. 1995, 34, 585. (o) de Loos, M.; van Esch, J.; Stokroos, I.; Kellogg, R. M.; Feringa, B. L. J. Am. Chem. Soc. 1997, 119, 12675. (o) Kato, T.; Kutsuna, T.; Hanabusa, K.; Ukon, M. Adv. Mater. 1998, 10, 606. (p) Yoza, K.; Ono, Y.; Yoshihara, K.; Akao, T.; Shinmori, H.; Takeuchi, M.; Shinkai, S.; Reinhoudt, D. N. Chem. Commun. 1998, 907. (q) Ragunathan, K. G.; Bhattacharya, S. Chem. Phys. Lipids 1995, 77, 13. (19) See for instance: Ramdas, S.; Thomas, N. W. In Organic Solid State Chemistry; Desiraju, G. R., Ed.; Elsevier: Amsterdam, 1987; Chapter 12. (20) (a) Hunter, C. A. Chem. Soc. Rev. 1994, 101. (b)Burley, S. K.; Petsko, G. A. Science 1985, 229, 23. (c) Cornil, J.; dos Santos, D. A.; Crispin, X.; Silbey, R.; Bredas, J. L. J. Am. Chem. Soc. 1998, 120, 1289.

Langmuir, Vol. 16, No. 1, 2000 21

bonds in the gas phase have been estimated to be 1.822a and 2.4 kcal/mol,22b respectively. These are not inconsequential, but are they sufficiently large to control the packing23 of gelator molecules in their strands? Here, we use a combination of spectroscopic and physical measurements to construct a rather comprehensive picture of how molecular organization in ALS gelator strands is affected when an N-H group, capable of H-bond donation, is either added to gelators such as CA-2 or exchanged for an ether linkage of gelators such as CAB in the L parts (Scheme 1). Information concerning π-π interactions of gelator molecules has been obtained from UV-vis absorption, emission, and excitation spectra. Circular dichroism has provided insights into local and long-range chiral packing of gelators. The importance of H-bonding interactions and other aspects of molecular packing have been gleaned from infrared spectroscopic and single-crystal X-ray diffraction measurements. Finally, fluorescence and rheological techniques have been employed to measure gelation temperatures. Structural modifications of the ALS have been made to compare the relative contributions of π-π interactions among rigid aromatic groups and dispersive interactions among the rigid S parts (or flexible alkyl chains) to gel stability. The liquids chosen for detailed investigation are 1-alkanols and n-alkanes. Experimental Section Materials. All starting materials and liquids were reagent grade or better and were used as received except where indicated otherwise. Gelators CNC, CPC, CBPC, CHC, and COC were synthesized by methods similar to that for CAC.24 CAB was available from previous studies.8 5r-Cholestan-3β-yl N-(2-Anthryl)carbamate (CAC). A solution of 5-R-cholestan-3β-yl chloroformate (1.4 g, 3.1 mmol) in distilled benzene (20 mL) and dried pyridine (1 mL) was added to a stirred solution of 2-aminoanthracene (0.6 g, 3.1 mmol) in distilled benzene (azeotropically dried) (15 mL) at 0 °C under a dry atmosphere. The reaction was continued for 1 h at 35 °C. The reaction mixture was partially dissolved in CHCl3 (50 mL) and filtered. The filtrate was reduced to a solid residue (rotary evaporation) which was recrystallized twice from 30 mL of 2/1 benzene/methanol and chromatographed on silica gel (1/9 ethyl acetate/hexane) to yield a yellowish solid (0.55 g, 37%; one peak by HPLC with 1/9 ethyl acetate/hexane): mp (dec) 270-272 °C; IR (KBr) 3412 (NH, sharp), 3059 (C-H, w), 2951 and 2866 (CH, vs), 1736 (CdO, s), 1211 (O-C-O, s) cm-1; 1H NMR (300 MHz, CDCl3) δ 8.4-7.4 (m, 9H, anthryl), 6.7 (s, 1H, N-H), 4.75 (m, 1H, CH), 2.0-0.6 (m, 49H, cholestanyl); MS (CI with NH3) m/e 625 (M+ + 17, 100), 608 (M+, 40). 5r-Cholestan-3β-yl N-(2-Naphthyl)carbamate (CNC). A white solid: mp 178-180 °C, in 83% yield (one peak by HPLC with 1/9 ethyl acetate/hexane; Rf ) 0.45 on silica gel (1/9 ethyl acetate/hexane)); IR (KBr) 3407 (NH, sharp), 3060 (aromatic C-H, w), 1730 (CdO, s) cm-1; 1H NMR (270 MHz, CDCl3) δ 8.00, 7.80-7.72, 7.46-7.28 (m, 7H, aromatic), 6.72 (s, 1H, N-H), 4.804.65 (m, 1H, CH), 2.20-0.60 (m, 47H, cholestanyl); MS (CI with NH3) m/e 575 (M+ + 17, 48), 558 (M+, 11). 5r-Cholestan-3β-yl N-Phenylcarbamate (CPC). A white solid: mp 150-151 °C, in 64% yield (one peak by HPLC with 1/9 ethyl acetate/hexane; Rf ) 0.77 on silica gel (15/85 ethyl acetate/ hexane)); IR (KBr) 3435 (NH, one sharp peak), 2957 and 2860 (21) (a) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; Freeman: San Francisco, 1960; pp 82ff. (b) Joesten, M. D.; Schaad, L. J. Hydrogen Bonding; Marcel Dekker: New York, 1974; pp 162ff. (22) (a) Suzuki, S.; Green, P. G.; Bumgarner, R. E.; Dasgupta, S.; Goddard, W. A., III; Blake, G. A. Nature 1992, 257, 942. (b) Rodham, D. A.; Suzuki, S.; Suenram, R. D.; Lovas, F. J.; Dasgupta, S.; Goddard, W. A., III; Blake, G. A. Nature 1993, 362, 735. (23) Desiraju, G. R. Crystal Engineering: The design of Organic Solids; Elsevier: Amsterdam, 1989. (24) Lu, L. Ph.D. Thesis, Georgetown University, Washington, DC, 1997.

22

Langmuir, Vol. 16, No. 1, 2000

Lu et al. Scheme 1. ALS Gelators

(C-H, vs), 1741 (CdO, s) cm-1; 1H NMR (270 MHz, CDCl3) δ 7.46-7.28 (m, 5H, aromatic), 7.08-7.02 (t, J ) 7.1 Hz, 1H), 6.51 (s, 1H, N-H), 4.68 (m, 1H, CH), 2.0-0.60 (m, 47H, cholestanyl); MS (CI with NH3) m/e 525 (M+ + 17, 100), 508 (M+, 14). 5r-Cholestan-3β-ylN-(4-n-Butylphenyl)carbamate(CBPC). A white solid: mp 142-144 °C, in 44% yield (one peak by HPLC with 1/9 ethyl acetate/hexane; Rf ) 0.56 on silica gel (1/9 ethyl acetate/hexane)); IR (KBr) 3400 (NH, one sharp peak), 2917 and 2833 (C-H, vs), 1738 (CdO, s) cm-1; 1H NMR (270 MHz, CDCl3) δ 7.28-7.24 (d, J ) 8.55 Hz, 2H), 7.11-7.08 (d, J ) 8.55 Hz, 2H), 6.44 (s, 1H, N-H), 4.65-4.75 (m, 1H, CH), 2.58-2.53(t, J ) 7.57 Hz, 2H), 2.20-0.60 (m, 54H, cholestanyl); MS (CI with NH3) m/e 581 (M+ + 17, 100), 564 (M+, 17). 5r-Cholestan-3β-yl N-Octadecylcarbamate (COC). A white solid: mp 70-72 °C, in 80% yield (96% pure by HPLC with a differential refractometer detector and 1/9 ethyl acetate/hexane as eluant; Rf ) 0.87 on silica gel (1/4 ethyl acetate/hexane)); IR (KBr) 3384 (NH, sharp), 2940 and 2864 (vs), 1708 (CdO, s) cm-1;

1H NMR (270 MHz, CDCl ) δ 4.55 (m, 2H, CH), 3.18-3.10 (m, 3 2H, CH2), 2.00-0.64 (m, 81H, cholestanyl and heptadecyl groups); MS (CI with NH3) m/e 701 (M+ + 17, 100), 684 (M+, 72). 5r-Cholestan-3β-yl N-Heptylcarbamate (CHC). A white solid: mp 75-78 °C, in 67% yield (96% pure by HPLC with a differential refractometer detector and 1/9 ethyl acetate/hexane as eluant; Rf ) 0.70 on silica gel (1/9 ethyl acetate/hexane)); IR (KBr) 3351 (NH, sharp), 2936 and 2860 (vs), 1694 (CdO, s) cm-1; 1H NMR (270 MHz, CDCl ) δ 4.62-4.50 (m, 2H, CH), 3.20-3.10 3 (m, 2H, CH2), 2.00-0.64 (m, 59H, cholestanyl and hexyl); MS (CI with NH3) m/e 547 (M+ + 17, 100), 530 (M+, 68). 3β-Cholesteryl 4-(2-Anthrylamino)butanoate (CAAB). A mixture of 2-aminoanthracene (0.108 g, 0.56 mmol) and NaH (0.016 g, 0.67 mmol) in dried benzene (10 mL) was stirred at room temperature for 1.5 h under a dry atmosphere. After removal of the benzene, a solution of 3β-cholesteryl 4-bromobutanoate (0.2 g, 0.37 mmol) in anhydrous DMF (10 mL) was added and the mixture was stirred for 16 h at 70 °C. A yellow solid (30 mg,

Gelation of Organic Liquids (3% yield), >99% pure by HPLC with 2/3 ethyl acetate/hexane as eluant), mp 189-192 °C, was obtained after filtering the reaction mixture and washing the solid with water, eluting it on a silica column (15/85 ethyl acetate/hexane), and recrystallizing it from the condensed eluant (a mixture of ethyl acetate/hexane) at 0 °C: IR (KBr) 3384 (NH, sharp), 3050 (aromatic C-H, w), 2932, 2840 (C-H, vvs), 1717(CdO, vs), 1637 (aromatic, vs) cm-1; 1H NMR (90 MHz, CDCl ) δ 8.2-7.4 (m, 9H, anthryl), 5.40 (m, 3 1H, CHd), 4.65 (m, 1H, CH), 3.4 (t, J ) 6.4 Hz, 2H, CH2), 2.5-0.6 (m, 57H, cholesteryl). 5r-Cholestan-3β-yl N-(2-Anthryl)-N-methylcarbamate (CAMC). Iodomethane (0.2 mL, 2.2 mmol) was added to a stirred mixture of 5R-cholestan-3β-yl N-(2-anthryl)carbamate (0.3 g, 0.48 mmol), tetraheptylammonium bromide (0.024 g, 0.032 mmol), powdered NaOH (0.5 g), and Na2CO3 (1 g) in benzene (10 mL), and the mixture was refluxed for 10 h at 60 °C in the air.25 Benzene (40 mL) and water (30 mL) were added, and the organic layer was separated, washed with water (3 × 30 mL), dried over sodium sulfate (anhydrous), and evaporated under vacuum. The solid residue was chromatographed on silica gel (3/2 CH2Cl2/hexane) to yield 0.25 g (84%) of a white solid: mp 213-216 °C (one peak by HPLC with 3/2 CH2Cl2/hexane; Rf ) 0.82 on silica gel (3/2 CH2Cl2/hexane)); IR (KBr) 3050 (C-H, w), 2968 and 2847 (C-H, vs), 1706 (CdO, s), 1165 (C-O-C, s) cm-1; 1H NMR (270 MHz, CDCl3) δ 8.4-7.4 (m, 9H, anthryl), 4.7 (m, 1H, CH), 3.45 (s, 3H, CH3), 1.8-0.6 (m, 56H, cholestanyl); MS (CI with NH3) m/e 639 (M+ + 17, 50), 622 (M+, 100). Instrumentation and Methods. 1H NMR spectra were recorded on Nicolet 270 (270 MHz), Bruker AM-300 WB (300 MHz), and Bruker HFX/WH-90 (90 MHz) spectrometers using tetramethylsilane (TMS) as standard. Mass spectra were obtained on Finnigan 4600 (CI for chemical ionization) mass spectrometer at the National Institutes of Health, Bethesda, MD. HPLC was performed on a Waters Associates liquid chromatograph with a model 440 constant-wavelength (254 nm) absorbance detector (unless indicated otherwise) and an Alltech 250 mm × 4.4 mm 5 µm silica gel column. Melting points ((0.5°, uncorrected) were observed on a Leitz 585 SM-LUX-POL optical microscope equipped with a Leitz 350 heating stage connected to an Omega HH21 microprocessor thermometer. Infrared spectra were obtained on a MIDAC FT-IR spectrometer (2 cm-1 resolution) using KBr pellets for solid samples, except as noted. Nujol mulls were sandwiched between NaCl plates, and solutions were placed in NaCl cells. Gel samples were either sandwiched between NaCl plates or dabbed on one plate; the frequencies of the absorbances were the same in the two cases, but the relative intensities of peaks attributable to the gelator and liquid differed due to flow of the liquid between the plates in the sandwich after pressure was applied. Fluorescence emission and excitation spectra were recorded on a Spex 111 Fluorolog fluorimeter equipped with an Osram 150 W high-pressure xenon lamp and an IBM-compatible computer. Excitation spectra are corrected for different intensities of the excitation lamp. Solid-state spectra of CAMC were obtained as a fine dispersion in a KBr pellet. Gel samples were prepared by dissolving an amount of gelator in methylene chloride or chloroform in a Kimax flattened glass capillary (8 mm × 0.4 mm i.d.) with one end sealed. The solvent was evaporated by heating the capillary in an oil bath while a stream of N2 passed over the cell mouth. Then a volume of liquid was added, and the capillary was flame-sealed under N2. The sealed capillary was placed in a dodecane-filled quartz cuvette (1 cm path length) that was previously thermostated. Fluorescence was detected at a rightangle geometry from the back of the capillary, placed at a ca. 45° angle with respect to the incident radiation beam. UV-vis absorption spectra were measured on a Perkin-Elmer Lambda 6 spectrophotometer and analyzed using Perkin-Elmer PECSS software. Typically, quartz cuvettes (1 or 0.3 cm path length) were used to measure solution spectra. For gel samples, a hot solution of gelator in a liquid was transferred to a Hellma jacketed quartz cell (0.1 mm path length). Circular dichroism (CD) spectra were recorded on a JASCO J-710 spectropolarimeter using the Hellma cell. (25) (a) Gajda, T.; Zwierzak, A. Synthesis 1981, 1005. (b) Koziara, A.; Zawadaki, S.; Zwierzak, A. Synthesis 1979, 527. (c) Burke, P. O.; Spillane, W. J. Synthesis 1985, 935.

Langmuir, Vol. 16, No. 1, 2000 23 Tests for Gelation. A known weight of potential gelator and a measured aliquot of liquid were placed into a half-sealed Pasteur pipet. The pipet was flame-sealed and heated until the solid dissolved. The sol was cooled to room temperature in air (fastcooling) or at ca. 2 deg/min in a water bath (slow-cooling), to ca. 10 °C under cold tap water or to 0 °C in an ice-bath. Inversion of the tube without sample flow constituted a positive test. In some cases, the liquid was not completely immobilized; solution and solidlike gel phases, referred to as “partial gels”, coexisted. Temperatures of sol w gel or gel w sol transitions (Tg) were measured using three different methods. In method 1 (an inversion-flow technique26), a sealed tube containing a gel was inverted and immersed in a water or oil bath. The temperature was raised at ca. 2 deg/min until the gelled material began to fall and was continued until the last of the material fell or phaseseparated. The range of temperatures over which the “melting” occurred and its midpoint or the onset gelation temperature is reported as Tg. In method 2, the fluorescence intensity of a gelled sample was followed while the temperature of the sample was raised gradually; in method 3, the fluorescence intensity of a hot solution was recorded while its temperature was lowered gradually. The gelation temperatures, determined from analyses of the data by Quattro Pro,27 were taken as the points of largest slope in plots of fluorescence intensity versus temperature. X-ray Crystallographic Studies. Crystals were mounted on a glass fiber using epoxy cement. All aspects of data collection were performed on a Siemens P4/RA or SMART system diffractometer using Mo KR radiation (λ ) 0.71073 Å) at 173((2) K. The structures were solved using direct methods and refined against F2 using the SHELXTL/PC v5.0 software suite.28 Data were corrected for Lorentz and polarization effects, but not for absorption. All non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were included in calculated positions using a standard riding model and thermal parameters proportional to the non-hydrogen atom to which they were attached. Methyl groups were allowed to rotate freely around the carboncarbon bond to position the hydrogen atoms at electron density maxima. Due to the lack of anomalous scatters, absolute structures were not determined reliably. Full details, including atomic positions and anisotropic displacement parameters (see Supporting Information), have been deposited with the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K. Quote reference number 101174. CAMC (Table 1). A colorless plate (from benzene by slow evaporation) of approximately 0.4 × 0.4 × 0.2 mm3. The unit cell parameters were determined by a least-squares analysis of 36 random, carefully centered reflections which were well distributed in reciprocal space (9.0° e 2θ e 25.0°). Instrument and crystal stability were verified by monitoring three check reflections every 100 data points. No measurable decay was detected. The lack of any noticeable change in appearance of the crystal, even under a microscope, after data cooling and data collection, indicates no phase changes between room temperature and 173 K. CBPC (Table 1). A colorless plate (from 1-butanol by slow evaporation) of approximately 0.12 × 0.30 × 0.48 mm3. The initial unit cell was determined using a least-squares analysis of a random set of reflections collected from three series of 0.3° wide scans (20 frames/series) which were well distributed in reciprocal space. The intensity data was then collected using ω-scans (0.3° wide) with a crystal-to-detector distance of 5.0 cm to provide a complete sphere of data to a maximum resolution of 0.75 Å (2θmax ) 58.72°). The final unit cell parameters were determined by a least-squares analysis of 8192 reflections from the full data set. Due to the overall poor quality of the data (particularly at high angle), the data set was truncated at a resolution of 0.84 Å (2θ ) 49.99°) during the later stages of refinement.

Results and Discussion Gelation and Gelator/Liquid Structures. Results from experiments with 2 wt % of gelator and a selection of organic liquids are listed in Table 2. The more (26) Takahashi, A.; Sakai, M.; Kato, T. Polym. J. 1980, 12, 335. (27) Quattro Pro for Windows, version 5.0, Borland International, Inc., 1994. (28) Siemens Analytical X-ray Instruments, Madison, WI.

24


Lu et al.

Table 1. Crystallographic Data and Structure Refinement for CAMC and CBPC

crystal system space group cell constant volume/Z crystal size/color θ range index ranges reflections collected independent reflections refinement method data/restraints/parameters goodness-of-fit (F2) final R indices absolute structure parameter

CAMC C43H59NO2/621.91

CBPC C38H61NO2/563.88

triclinic P1 a ) 6.5584(15) Å, R ) 87.084(12)° b ) 14.548(3) Å, β ) 86.414(15)° c ) 19.076(3) Å, γ ) 79.85(2)° 1786.6(6) Å3/2 0.2 × 0.4 × 0.4 mm3/colorless 1.07 to 20.00° -1 e h e 6, -13 e k e 13, -18 e l e 18 4502 4502 [R(int) ) 0.0000] full-matrix least-squares on F2 4498/3/841 1.078 R1 ) 0.0545 [I > 2σ(I)] WR2 ) 0.1473 [all data] -1.29 (177)

monoclinic P2(1) a ) 13.9344(2) Å b ) 7.65150(10) Å, β ) 92.0590(10)° c ) 15.8237(2) Å 1686.02(4) Å3/2 0.12 × 0.3 × 0.48 mm3/colorless 1.29 to 29.36° -18 e h e 18, -10 e k e 10, -21 e l e 20 21169 7865 [R(int) ) 0.1524] full-matrix least-squares on F2 5876/1/371 1.172 R1 ) 0.0635 [I > 2σ(I)] WR2 ) 0.1638 [all data] -1.01 (181)

Table 2. Liquids Tested for Gelation by 2 wt % ALS Gelatorsa liquid

CAC

CAMC

CNC

CPC

CBPC

CHC

COC

n-heptane n-dodecane n-hexadecane benzene styrene tetramethyltetraphenyltrisiloxanec benzyl alcohol ethanol 1-propanol 2-propanol 1-butanol 1-pentanol 1-octanol

I G G P P G

I P P S S P P I I I G G P

G G G S S G

P P P

S S S

S S S

S

S

S

S

S S S

S S

S S S

S S S S S P Gb Gb P P P P P

I G G G G

S I Gb Gb Gb Gb P

P S S

CAcAB

CTsAB

CBzAB

P P P S S

G G S S

P P S S Gb P Gb Gb Gb

P G G

S Gd Gd Gd Gd G G

a Key: I ) gelator insoluble at boiling temperature of liquid; G ) gel formed at room temperature (RT); P ) solubilized gelator precipitated upon cooling to RT; S ) solution at RT. b Gel formed by cooling at 1 year >1 year >2 months

>1 year >1 year >1 year >1 year >1 year

a N ) no evidence of gel at room temperature (RT): sol, ppt ) solution upon heating, precipitate at RT; insol ) gelator insoluble at boiling temperature of liquid; partial gel ) solution and solidlike gel phases coexist; Y ) gel at RT. b After evaporation of cosolvent (CHCl3).

polar protic liquids such as 1-pentanol (Tables 3 and 4 and Figure 1A). CAAB, the aza analogue of the excellent


Langmuir, Vol. 16, No. 1, 2000 25

Table 4. CAMC Gelation Tests in Sealed Glass Vials liquid n-heptane n-dodecane n-hexadecane tetramethyltetraphenyltrisiloxane ethanol 1-propanol 1-butanol 1-pentanol 4-heptanol 1-octanol 2-octanol 1-decanol styrene benzene

wt % of gelator

phasea

2.0 1.0, 2.0 1.0, 2.0 2.0

N (insol) N (sol, ppt) N (sol, ppt) Nb (sol, ppt)

2.0 2.0 2.0 1.0 1.0, 2.0 2.0 1.0, 2.0 2.0 2.0 2.0 2.0

N (insol) N (insol) Y N (sol, ppt) Y N (sol, ppt) N (sol, ppt) N (sol, ppt) N (sol, ppt) N (sol) N (sol)

a N ) no evidence of gel at room temperature (RT); insol ) gelator insoluble at boiling temperature of liquid; sol, ppt ) solution upon heating, precipitate at RT. b N ) precipitate from a solution phase after evaporation of cosolvent (CHCl3). Y ) gel stable for ca. 2 h at RT.

ether-containing gelator, CAB,8 gelled none of the several liquids examined.29 Gelation Temperatures and Gelator/Liquid Structures (Figure 1). Tg, as determined by inversion-flow26 and fluorescence methods,8 depends on many factors, including the age of a gel,5 sample cooling rate from the sol, thermal history, degree of hysteresis, and whether the transition is approached from higher or lower temperature. The range of temperatures over which Tg is observed can depend on the method of observation.24 Despite this, several previous investigations have shown that the Tg values from methods 1-3 are usually very similar. Unless specified otherwise, the Tg values of CAC, CAMC, and CNC have been measured by the inverse flow method (method 1) on samples whose histories are similar.10 Typical data for CNC are included in Figure 2. In the monomeric emission region, below 420 nm, the ca. 30% increase in fluorescence intensity that ac(29) By contrast, gelators in which the amino hydrogen atom of CAAB has been replaced by selected bulky groups do gel some liquids!24 For example, 3β-cholesteryl 4-(N-2-anthryl-N-benzyloxyformylamino)butanoate (CBzAB) forms very stable gels with alkanes and alkanols. Both 3β-cholesteryl 4-(N-acetyl-N-2-anthrylamino)butanoate (CAcAB) and 3β-cholesteryl 4-(N-2-anthryl-N-p-toluenesulfonylamino)butanoate (CTsAB), although similar in structure to CBzAB, but containing (respectively) smaller and more polar substituents, formed less stable gels and only with alkanols. 3β-Cholesteryl 4-(N-acetyl-N-2-anthrylamino)butanoate (CAcAB). A white solid: mp 142-145 °C, in 23% yield (99.2% pure by HPLC with 2/3 ethyl acetate/hexane; Rf ) 0.30 on silica gel (3/7 ethyl acetate/hexane)); IR (KBr) 3050 (C-H, w), 2949 and 2869 (C-H, vs), 1726 (CdO near cholesteryl, s), 1676 (CdO on acetyl, s) cm-1; 1H NMR (300 MHz, CDCl3) δ 8.49-7.20 (m, 9H), 5.30-5.20 (d, J ) 4.69 Hz, 1H, CHd), 4.55-4.45 (m, 1H, CH), 3.90-3.75 (m, 2H, CH2), 2.40-2.30 (t, J ) 7.6 Hz, 2H, CH2), 2.17 (s, 3H, CH3), 2.00-0.60 (m, 85H, cholesteryl group); MS (CI with NH3) m/e 690 (M+, 100). 3βCholesteryl 4-(N-benzyloxyformyl-N-2-anthrylamino)butanoate (CBzAB). A white solid: mp 155-158 °C, in 30% yield (one peak by HPLC with 1/10 ethyl acetate/hexane; Rf ) 0.24 on silica gel (3/1 CH2Cl2/hexane)); IR (KBr) 3050 (aromatic C-H, w), 2951 and 2866 (C-H, vs), 1730 (CdO near cholesteryl, s), 1703 (CdO near benzyl, s) cm-1; 1H NMR (300 MHz, CDCl3) δ 8.5-7.3 (m, 14H, aromatic), 5.3 (m, 1H, CHd), 5.2 (s, 2H, CH2), 4.5 (m, 1H, CH), 3.9 (t, J ) 7.0 Hz, 2H, CH2), 2.3 (t, J ) 7.0 Hz, 2H, CH2), 2.2-0.6 (m, 40H, cholesteryl group); MS (CI with NH3) m/e 800 (M+ + 17, 98), 783 (M+, 100). 3β-Cholesteryl 4-(N-p-toluenesulfonyl-N-2-anthrylamino)butanoate (CTsAB). A white solid: mp 188192 °C, in 35% yield (99% pure by HPLC with 1/10 ethyl acetate/hexane; Rf ) 0.73 on silica gel (9/1 CHCl3/hexane)); IR (KBr) 3444 (H2O, broad), 3050 (aromatic C-H, w), 2947 and 2868 (C-H, vs), 1726 (CdO, s), 1360 (SdO, s), 1167 (SdO, vs) cm-1; 1H NMR (300 MHz, CDCl3) δ 8.56-7.69 (m, 13H, aromatic), 5.3 (s, 1H, CHd), 4.5 (m, 1H, CH), 3.75 (t, J ) 6.77 Hz, 2H, CH2), 2.50-2.35 (m, 5H, CH3, CH2), 2.25-2.10 (m, 2H, CH2), 2.0-0.7 (m, 56H, cholesteryl group); MS (CI with NH3) m/e 819 (M+ + 17, 100), 802 (M+, 13).

Figure 1. (A) Onset gelation temperatures versus gelator concentration for (9) CAC/n-dodecane and (b) CAC/1-pentanol, and median gelation temperature with range (method 1) versus gelator concentration for (+) CAMC/1-pentanol. (B) Range and median gelation temperatures of 1.0 wt % CNC/n-alkane gels vs the number of carbon atoms in the alkane molecules.

companies gelation of CAC/1-pentanol sols is too small to allow precise measurement of Tg by method 2 or 3. Fortunately, gelation is accompanied by a new, red-shifted excitation band at 431 nm whose intensity increases by 20-fold in the small temperature range that includes Tg (Figure 3). Gelation temperaturs based upon this band (method 3) in CAC/1-pentanol samples are ca. 40° higher than those from method 1. The Tg values of CAC in dodecane (Figure 1A) are 4050 °C higher than those in 1-pentanol at comparable concentrations. A similar trend is found for gels employing 3β-cholesteryl 4-(2-anthraquinonyloxy)butanoate, the anthraquinonyl analogue of CAB,10 but gels of CAB exhibit much lower Tg values with alkanes than with alkanols.8 The trends depend on solubility: gelator strands dissolve at lower temperatures in liquids in which they are more compatible; strand formation occurs at higher temperatures in poorer solvents.7,10 In n-dodecane at 1 wt % of gelator, the Tg of CAC is much higher than that of CNC. The relative strengths of π-π intermolecular interactions among molecules of the resulting aggregates are indicated to some extent by features in the spectra (vide infra). However, the much smaller transition dipoles of a naphthyl group make excitonic coupling bands from its aggregates intrinsically weaker (and, therefore, more difficult to observe) than those from analogous anthryl aggregates.30,31 (30) (a) Yanagidate, M.; Takayama, K.; Takeuchi, M.; Nishimura, J.; Shizuka, H. J. Phys. Chem. 1993, 97, 8881. (b) Lewis, F. D.; Yang, J.-S. J. Phys. Chem. B 1997, 101, 1775. (31) (a) Hochstrasser, R. M.; Kasha, M. Photochem. Photobiol. 1964, 68, 441. (b) Kasha, M. Rev. Mod. Phys. 1959, 31, 162. (c) Czikkely, H.; Foersterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 11. (d) Czikkely, H.; Foersterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 207.

26


Figure 2. (A) Fluorescence intensity of a 0.9 wt % CNC gel in n-octane (λem ) 400 nm, λex ) 340 nm) vs temperature during (s) slow-heating and slow-cooling (- - -). (B) Gelation temperatures (indicated by arrows in (A)) vs gelator concentration of CNC/n-octane gels during heating (b, method 2) and cooling (9, method 3). The cooling rate (in the range investigated) did not affect the gelation temperatures.

For 1.0 wt % CNC, Tg is ∼56 °C in a range of n-alkanes from heptane to hexadecane (Figure 1B), but methylcyclohexane is gelled only partially at room temperature. Similar differences between unbranched and cyclic alkane liquids have been found when CAB is the gelator.8 By contrast, very stable gels with cycloalkanes are formed by a D-17-azahomosteroid.32 Phase-Dependent Changes in Spectra of CAC, CAMC, and CNC. UV-vis Absorption and Fluorescence Excitation Spectra. A solution of 1.5 × 10-3 wt % CAC in methylene chloride (a nongelled liquid) shows absorption bands characteristic of a weak, perturbed 1La anthryl (short-axis polarization) transition33 at 394 nm ( ) 3300 L mol-1 cm-1) and a strong, perturbed 1Bb transition (longaxis polarization) at 263 nm ( ) 91 000 L mol-1 cm-1). Virtually the same spectrum is obtained from a solution of 1.4 × 10-2 wt % CAC in 1-pentanol or even 0.3 wt % in (the nongelled liquid) benzene (Figure 3 A(a)). Absorption spectra of partially gelled 0.3 wt % CAC samples in 1-pentanol or n-dodecane (Figure 3A(c)) contain a new red-shifted band at ca. 420 nm, characteristic of excitoncoupled chromophores31 in J-aggregates,34 and another excitonic band overlapping the 0-0 band of the monomer near 395 nm. The shapes of the complex CAC excitation spectra with 1-pentanol (Figure 4B) or n-dodecane are the same and independent of both cooling rate and λem. The absorption (32) Wade, R. H.; Terech, P.; Hewat, E. A.; Ramassuel, R.; Volino, F. J. Colloid Interface Sci. 1986, 114, 442. (33) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules, 2nd ed.; Academic: New York, 1971; pp 72ff. (34) Whitten, D. G. Acc. Chem. Res. 1993, 26, 502.

Lu et al.

Figure 3. (A) Absorption spectra at room temperature of ca. 0.3 wt % CAC in (‚‚‚, a) benzene solution, (- - -, b) 1-pentanol partial gel by slow or fast cooling, and (s, c) n-dodecane partial gel by slow or fast cooling at room temperature (0.1 mm cell). (B) Absorption spectra of CAC in 1-pentanol: (‚‚‚, a) 0.014 wt % solution at room temperature (0.3 cm cell); (s, b) 1.0 wt % fresh gel phase (same spectrum upon slow or fast cooling) at room temperature (0.1 mm cell); and (- - -, c) 1.0 wt % solution phase at 95 °C (0.1 mm cell).

and normalized excitation spectrum of the 0.014 wt % CAC/1-pentanol solution at room temperature are similar (Figure 3B(a)). However, at least a small component of the excitonic band at 395 nm is discernible in excitation. The ratios of intensities between the absorption band near 395 nm and any of the “monomeric” bands at lower wavelengths from the 0.014 wt % sample (that lacks completely the ca. 420 nm J-band) and a free-flowing 1.0 wt % sample in 1-pentanol at 95 °C (Figure 3B(c)) are almost the same; the anomolous intensity at 395 nm and the band at 420 nm must be due to different (coexisting) aggregate types. The excitation spectrum of 1.0 wt % CAC at 95 °C (Figure 4B) does not correspond in shape or position to its absorption spectrum. The most prominent band in the excitation spectrum at ca. 410 nm has no equivalent in any of the other CAC excitation (or absorption) spectra, and its “monomeric” vibrational progression is offset in wavelength from the other spectra.8 In the free-flowing state, large aggregates can still exist,35 but junction zones between them either do not exist or their population is severely depleted. Thus, we believe the anomolous intensity at 395 nm and the band at ca. 420 nm are due to strands and junction zones, respectively. At ca. 1.0 wt % CAC below Tg, the gel phases in 1-pentanol and n-dodecane are translucent and their absorption spectra resemble each other (Figure 3B(b)). The monomeric vibronic peaks in the 300-380 nm region are diminished in intensity and are nearly smoothed by another progression from aggregates. The appearance of the excitation spectrum is similar in this wavelength region. An additional new band at 431 nm (red-shifted by 21 nm from the 410 nm peak detected above Tg) and a plateau region appear in the excitation spectrum (Figure (35) Schurr, O.; Ostuni, E.; Glinka, C.; Weiss, R. G. Manuscript in preparation.


Langmuir, Vol. 16, No. 1, 2000 27

Figure 5. Absorption spectra of 1.0 wt % CAMC in 1-pentanol (0.1 mm cell): (‚‚‚) solution at 72 °C; (s) air-cooled gel at room temperature; (- - -) ppt/sol at room temperature.

Figure 4. Emission (A, λex ) 375 nm) and excitation (B, λem ) 500 nm) spectra of CAC in N2-saturated 1-pentanol: (s) 1.0 wt % fast-cooled fresh gel phase at room temperature; (- - -) 1.0 wt % slow-cooled fresh gel phase at room temperature; (-‚‚-) 1.0 wt % slow-cooled gel phase after aging 1 week at room temperature; (‚‚‚) 1.0 wt % solution at 95 °C; and (-‚-) 0.014 wt % solution at room temperature. The small feature near 420 nm in the excitation spectrum of the 1.0 wt % fast-cooled fresh gel and in Figure 6B are instrumental artifacts.

4B). The 431 nm feature lends support to the conclusion reached from experiments with other gelators13,35,36 that intermolecular interactions among CAC molecules in junction zones and strands are different. The features at 395 and 420 nm are about twice as intense in 1-pentanol as those in n-dodecane due to a combination of factors that includes aggregate sizes,8 aggregation modes,16 and solubility (i.e., the mole fractions of gelator molecules incorporated in the strands and dissolved in the liquid component). The spectral shapes and intensities do not depend on cooling rates but do change slightly upon aging. The part of CAC aggregated in various samples can be estimated from the spectroscopic data above (Appendix I). Accordingly, g12% at 95 °C (liquid state) and g48% at room temperature (gelled state) of the CAC in a 1.0 wt %/1-pentanol sample are aggregated. An even greater fraction of CAC molecules is incorporated into n-dodecane gel strands. From analogous spectral comparisons, the amount of J-aggregates in (nongelled) 0.35 wt % CAC/ benzene at room temperature must be negligible. If different morphs of the same gelator form in different liquids, the influence of the medium will be manifested indirectly. Since the shape and peak position of the CAC aggregate-derived bands are very similar in nonpolar (ndodecane) and more polar, H-bonding (1-pentanol) liquids, (36) Terech, P.; Furman, I.; Bouas-Laurent, H.; Desvergne, J.-P.; Ramassuel, R.; Weiss, R. G. Faraday Discuss. 1995, 101, 345.

the strand assemblies that constitute the gelator networks must contain little, if any, liquid molecules. However, the detection of prominent bands in some excitation spectra that are not seen in the corresponding absorption spectra provides evidence for rapid energy migration to “trap” sites,31,34 perhaps at junction zones. The absorption spectrum of a solution of 1.6 × 10-3 wt % CAMC in methylene chloride exhibits λmax at 380 nm ( ) 5000 L mol-1 cm-1) as well as other blue-shifted vibronic bands from a weak 1La-type transition,33 and at 258 nm ( ) 97 000 L mol-1 cm-1) from a 1Bb-type transition. A very similar spectrum is obtained from a solution of 10-2 wt % CAMC in 1-pentanol at room temperature or 1.0 wt % at 72 °C (above Tg) (Figure 5). However, a very strong band at 396 nm appears in the excitation spectrum of 1.0 wt % CAMC/1-pentanol at 70 °C. The vibronic progressions in the “monomeric” region are offset from those of the very dilute (unaggregated CAMC) solution at room temperature (Figure 6B). Although the absorption spectra do not provide evidence for significant aggregation above Tg, the excitation spectra clearly do. When the hot 1.0 wt % 1-pentanol solution was cooled in air, the absorbances decreased and the peaks merged into a broad plateau with an extra shoulder (i.e., an exciton coupling band31) at ca. 410 nm. Finally, as the gel separated macroscopically into solid and liquid parts, the band intensity decreased further due to much of the precipitated CAMC no longer being in the UV beam. As a result, the ppt/sol spectra in Figures 5 and 6B are deceptively simple: they resemble the spectrum of dissolved (unaggregated) CAMC more than that of the gel; the weakness of the broad band near 415 nm is not indicative of the true concentration of solid. The shapes of the absorption spectra of solid (KBr pellet) and gelled CAMC are very similar. The excitation spectra of both samples contain a 411 nm peak (like the 415 nm peak found in absorption) and the same progressions of vibronic bands. The excitation spectrum of the precipitate/ liquid also contains the 411 nm peak, as well as a redshfted shoulder that is found in the gel phase. Solution spectra of 1.0 wt % CNC in n-octane at 71 °C and 0.01 wt % in benzene at room temperature are nearly

28


Figure 6. (A) Emission (A, λex ) 355 nm) and excitation (B, λem ) 451 nm) spectra of CAMC in N2-saturated 1-pentanol: (-‚-) 0.01 wt % solution at room temperature; (‚‚‚) 1.0 wt % solution at 70 °C; (s) 1.0 wt % air-cooled gel phase at room temperature; (- - -) 1.0 wt % ppt/sol phase at room temperature.

Figure 7. Absorption spectra of 1.0 wt % CNC in n-octane (0.1 mm cell): (‚‚‚) solution at 71 °C; (s) air-cooled gel at room temperature.

identical. The lowest energy 1Lb-type (long-axis polarized) absorption bands from CNC33 in the gel phase from the cooled n-octane solution are red-shifted by 1 nm, broadened, and slightly more intense (Figure 7), but there are no new bands. The excitation spectra resemble closely the absorption spectra. They change only slightly from gel (λem 332-341 nm) to solution (λem ca. 340 nm) phases or from nonpolar to polar liquids.

Lu et al.

Fluorescence Spectra. (See the Supporting Information for more details.) Very large differences in position, shape, and intensity are found between CAC solution and gel emission spectra. In addition, the excitation spectra of some CAC samples and emission spectra of some CAMC samples are dependent upon λem and λex, respectively; no new peaks appear, but the relative intensities of the existing ones change. This behavior is consistent with emission emanating from excited molecules in more than one state of aggregation. Although some of the differences among the emission spectra for various temperatures and concentrations of one gelator may be ascribable to diffusional processes (e.g., dynamic excimer formation37), the absorption and excitation spectra indicate otherwise. The gel spectra of one gelator frequently differ from each other when protocols for sample preparation or concentrations are changed. They also differ from spectra of solutions in which the gelators are well dispersed. It is likely that the gelators exist in different morphs and/or there is rapid energy transfer to trap sites. CAC in n-dodecane provides a clear example. The emission spectra of a 0.9 wt % CAC/n-dodecane gel at room temperature and at 95 °C are superimposable in shape, but not in intensity. They do not resemble the emission from a 0.3 wt % CAC/n-dodecane sol at 95 °C. The emission maximum of the fast-cooled gel16 from 1.0 wt % CAC in 1-pentanol (Figure 4A) is 23 nm to the red of the solution phase (like the shift in the corresponding excitation spectra (Figure 4B)). The emission spectrum of the corresponding slow-cooled gel has λmax 441 nm and a shoulder that is initially about half as intense at ca. 415 nm. After 1 week at room temperature, the emission spectrum is very similar to that of a fast-cooled gel. Since the shoulder at 415 nm is in the region of emission from dilute CAC solutions, initially unaggregated molecules from the slow-cooled gel may subsequently become a part of the colloidal gel framework. By contrast, emission spectra from 0.65 to 0.90 wt % CAC/n-dodecane gels are not dependent upon cooling rate. This may be due to a different nucleation process stemming from less aggregation above Tg. The positions of the CAMC emission bands of the 1-pentanol solution and gel phases in 1-pentanol and in the neat solid phase were independent of excitation wavelength. One broad peak at 419 nm constitutes the emission spectra of the solutions containing 0.01 and 1.0 wt % CAMC (Figure 6A). Reversion of the metastable gel to a precipitate and a liquid led to a red-shifted and transformed spectrum that was nearly the same as that of the neat solid (KBr pellet). Like the absorption and excitation spectra of CNC, its (excitation wavelength independent) fluorescence spectra (λem ca. 355 nm (gel) and 351-355 nm (solution)) contain no bands attributable to new aggregates.30 Excitonic interactions are too weak to be detected. Circular Dichroism (CD). Circular dichroism has been induced in optically inactive molecules by several methods, including dissolving them in cholesteric liquid crystals38 and forming complexes with polypeptides39 or aggregating dyes40 in helical arrangements. Very large increases in circular dichroism can also be induced upon gelation of some ALS gelators which are optically active but whose centers of chirality (S parts) are too remote from the (37) Morita, M.; Kishi, T.; Tanaka, M.; Tanaka, J.; Ferguson, J.; Sakata, Y.; Misumi, S.; Hayashi, T.; Mataga, N. Bull. Chem. Soc. Jpn. 1978, 51, 3449 (38) Saeva, F. D.; Wysocki, J. J. J. Am. Chem. Soc. 1971, 3, 5928. (39) Mason, S. F.; McCaffery, A. J. Nature 1964, 204, 468. (40) Berg, R. A.; Haxby, B. A. Mol. Cryst. Liq. Cryst. 1970, 12, 93.


chromophores (A parts) to provide measurable ellipticities in “normal” solutions;8,9 CD reflects the overall chiral effect from aggregation. The aza-ALS gelators investigated here behave in a similar fashion. Contributions from linear dichroism are negligible since only slight changes in CD intensity were detected when samples were rotated by 90°. The very large increase in CD ellipticities of CAC and smaller increases of CAMC and CNC as their solutions become gelled indicate that the aggregates must be chiral on a macroscale. The gel phase of CAC/1-pentanol has a very high ellipticity (up to 1°), due to a very twisted packing arrangement of molecules in gelator strands. Since the spectrum of the corresponding solution phase at 95 °C shows a much smaller ellipticity, larger chiral arrays are transformed into smaller ones and monomers at the higher temperature. The CD intensities of CAC/n-dodecane gels are considerably smaller than those of 1-pentanol gels, especially between 200 and 300 nm. Thus, the more helically arranged aggregates form in the more polar, protic liquid. The origin of this difference must be related to selective gelator-liquid interactions during the initial nucleation steps leading to strand formation (vide infra).16 The shapes and intensities of the CD spectra of CAC gels, especially, also depend on nonstructural/electronic factors such as cooling rate,8,16 thermal history,8 and aging.8 The CD spectra of slow-cooled 1.0 wt % CAC/1-pentanol gels have a positive Cotton effect (Figure 8). The absence of bisignate exciton-splitting bands may be due to parallel orientation of short-axis transition dipoles of neighboring molecules;41 weaker negative signals, if present, may be buried under the stronger positive CD absorption. The corresponding fast-cooled (fresh) CAC/1-pentanol gels give an apparently conservative exciton-splitting band at λ[θ])0 ) 420 nm, the absorption maximum of the redshifted band. The sign of the exciton-splitting band is related to the helical sense of the aggregates9,38 or, more specifically, to the orientations of the coupling 1La-type transition dipoles of the anthryl part. The negative Cotton effect and the enormous ellipticity in the gel phase are consistent with the presence of right-handed helical (Phelicity) fibrous superstructures from counterclockwiseoriented coupling transition dipoles.41,42 On the basis of the CD spectra of the gels, the macrochiral configurations of the aggregates are the same sense in protic polar and aprotic nonpolar liquids. Since the 395 nm CD peak derives from a mixture of monomer and exciton transitions, its analysis in terms of chromophore orientations is not straightforward. Effects of aging on fast-cooled CAC/1-pentanol gels were followed by their CD spectra. The magnitude of the ellipticity increased dramatically between the first and second cooling and after incubation for 1 day (Figure 9). During this process, the band at ca. 420 nm lost its conservative nature and became the negative image of the slow-cooled gel band! Spectral “equilibrium” was established within the first 24 h. The spectra indicate that aging leads to an increased specificity of molecular packing or slow incorporation of initially dissolved gelator molecules into the gel strands. The temporal dependence of CD spectra from slow-cooled CAC/1-pentanol gels was not investigated. Although the fluorescence spectra in Figure 4A indicate that significant changes should be (41) Tachibana, T.; Mori, T.; Hori, K. Bull. Chem. Soc. Jpn. 1980, 53, 1714. (42) Fuhrhop, J.-H.; Svenson, S.; Boettcher, C.; Roessler, E.; Vieth, H.-M. J. Am. Chem. Soc. 1990, 112, 4307.

Langmuir, Vol. 16, No. 1, 2000 29

Figure 8. (A) Absorption spectrum of a 1.0 wt % CAC fastcooled fresh gel in 1-pentanol at room temperature. The shape of the absorption spectrum of the n-dodecane gel is virtually the same. (B) CD spectra of fresh gels with 1.0 wt % CAC: fast-cooled in 1-pentanol (s) or n-dodecane (‚‚‚, 4×), and slowcooled fresh gel in 1-pentanol (- - -). (C) CD spectra of fastcooled fresh gels at room temperature: (s) 1.0 wt % CAMC in 1-pentanol and (‚‚‚) 1.0 wt % CNC in n-octane. Cell thickness ) 0.1 mm in all cases.

expected in CD spectra from aging slow-cooled gels also, these temporal changes were not investigated. Slow-cooled CAC/1-pentanol gels have a strong, positive CD band at 420 nm and an overall positive set of 1La and 1 Bb bands. Fast-cooling leads to CD spectra with a roughly inverted 1Bb region and makes the 420 nm band weaker and negative (Figure 8B). In addition, the 1La vibronic progressions of the slow- and fast-cooled gels are offset. Similar changes in the spectra of CAB gels have been observed and attributed to different morphs of the gelator strands.8,16 The data in Figure 8B indicate that CAC gelator strands can be polymorphous also. Sol w organogel transformations of CAB in both protic and nonpolar aprotic liquids led to a new prominent absorption (and positive CD) band in the 1La region8 that is analogous to the 420 nm feature of slow-cooled CAC/ 1-pentanol gels. It was attributed to aggregation of CAB

30


Figure 9. CD spectra of a 1.0 wt % CAC gel in 1-pentanol at room temperature: (s, a) first fast-cooling; (- - -, b) second fastcooling after aging 1 day at room temperature and reheating; and (‚‚‚, c) 1 day after (b).

molecules in which the anthryl groups are overlapping and stacked in a helical arrangement. Nakanishi has shown that chiral excitonic interactions between anthryl groups do not provide a new band in the 1La region when the chromophores are not facially stacked.43 In a roughly coplanar chiral arrangement, the major CD feature is a strong, conserved split-exciton band in the 1Bb region near 250 nm. The presence of a new band in the 1La region of CAC is a clear indication of extensive facial overlap of anthryl groups in the strands of the organogels. The CD intensities of gels of CAMC in 1-pentanol are ca. 10% those of the corresponding CAC gels between 300 and 450 nm and only ca. 2% near 250 nm (Figure 8); no bisignate exciton-splitting band is observed for CAMC. The ellipticities of CNC/hydrocarbon gels are even smaller, ca. 1% of the analogous CAC gels near 250 nm (Figure 8). Anthryl groups in strands of CAMC gels must be stacked less cofacially based upon the much lower ellipticities in Figure 8C and the virtual absence of a band near 420 nm (Figures 5 and 8C). Only in the excitation spectrum of the CAMC gel is the 420 nm feature apparent (Figure 6B). Overall, the CD results demonstrate that the aza-ALS gelators adopt helical arrangements in their gel strands, like other ALS gelators.8,9 The degree of helicity is dependent upon the nature of the liquid being gelled (i.e., how the liquid influences nucleation of the gelator molecules) and, of course, the specific structure of the gelator. Infrared Spectroscopy. The infrared spectra indicate that H-bonding is suppressed effectively within the gelator strands by other packing forces. While this conclusion is clearly not general, it demonstrates that the possibility of H-bonding in gelator assemblies should not presume its actuality. The infrared spectrum of crystalline CAAB (KBr pellet) contains a strong, sharp carbonyl stretching band at 1717 cm-1, the same frequency as in CAB,8 and a sharp N-H doublet centered at 3384 cm-1. The CdO stretching frequency of N-H‚‚‚OdC bonded ester groups are expected in the region 1695-1715 cm-1; free aliphatic ester (43) Harada, N.; Nakanishi, K. Circular Dichroism Spectroscopy; University Science Books: Mill Valley, CA, 1983. (b) Harada, N.; Takuma, Y.; Uda, H. J. Am. Chem. Soc. 1976, 98, 5408.

Lu et al.

stretches occur above 1730 cm-1.44,45 Despite this, the equivalency of the carbonyl stretches leads us to believe that the amino group of CAAB does not participate in intra- or intermolecular H-bonds with ester groups. Thus, the failure of CAAB as a gelator cannot be attributed to inter- or intramolecular H-bonding, at least in the solid morph obtained upon precipitation from solution; others and we have determined that the IR absorption spectra of gelators can be phase dependent.9c,46 Representative spectra for CAC in each phase, for CNC in the gel phase, and for CAMC in the solid phase are presented in Figure 10. The stretching frequencies of N-H bonds occur at 3455 (free), 3385 (H-bonded to an ester group), and 3305 cm-1 (H-bonded to an amide group).45 Sharp peaks from (non-H-bonded) N-H stretches are at 3395 cm-1 for CAC and 3426 cm-1 for CNC. In the neat solid phase of CAC (Figure 10d), each sharp peak is superimposed on a much larger (in area) broad peak (from H-bonded N-H stretches). In very dilute (ca. 0.15 wt %) alcohol-free chloroform solutions of CAC or CNC, only a sharp N-H stretching band at 3437 cm-1 was observed. The spectrum of CAMC is included for completeness; as expected, it lacks bands above 3100 cm-1. In 1.0 wt % n-dodecane gels, there are two sharp, closely spaced peaks at 3395 and 3385 cm-1 (CAC) and at 3396 and 3449 (minor) cm-1 (CNC). Two N-H stretching bands are consistent with the prior indications of gelator strands with molecules held in more than one conformation or environment. The CdO stretching of the neat solids appeared at 1729 (CAC), 1738 (CNC), and 1708 (CAMC) cm-1. All were relatively sharp and strong. In chloroform, the bands remained sharp but were at 1728 (CAC), 1726 (CNC), and 1688 (CAMC) cm-1, indicating a probable change in conformation between the solid and liquid states for the latter two molecules. Were hydrogen bonding the cause of the shifts, the liquid-state frequencies should have been higher than those in the solid. In the gelled state, the CdO stretches remained single, sharp peaks: 1730 cm-1 for CAC and 1733 cm-1 for CAMC. Another band near 1500 cm-1 is associated with secondary amides and carbamates in transoid conformations (amide-II).47 Sharp bands were present for CAC at 1515 cm-1 in the gel and solid phases and at 1521 cm-1 in solution. For CAMC, two bands were detected in the spectrum from each phase: 1503 and 1528 cm-1 (solid); 1503 and 1536 cm-1 (solution); 1503 and 1538 cm-1 (gel). We believe that the data provide no clear evidence for H-bonding from N-H to other amino, carbamate, or carboxylate groups in CAC and CAMC gels. There is no evidence for H-bonding in solid CBPC; infrared spectra show sharp, strong bands at 3418 (N-H stretch) and 1737 (CdO stretch) cm-1. Molecular Packing of Crystalline CBPC and the Lack of H-Bonding Interactions (Table 1). Singlecrystal X-ray diffraction data indicate that molecular packing of CBPC molecules is dominated by interactions between steroidal groups. There is no evidence of either π-π stacking (aligned aromatic rings are separated by 7.65 Å, the b-axis length of a unit cell) or an aromatic herringbone motif like that found in crystals of benzene, naphthalene, and anthracene; the amide linkages are sterically isolated. The closest intermolecular contact (44) Iriondo, P.; Iruin, J. J.; Fernandez-Berridi, M. J. Polymer 1995, 36, 3235. (45) Kaczmarczyk, B. Polymer 1998, 39, 5853. (46) Ostuni, E.; Kamaras, P.; Weiss, R. G. Angew. Chem., Int. Ed. Engl. 1996, 35, 1324. (47) Nakanishi, K.; Solomon, P. H. Infrared Absorption Spectroscopy, 2nd ed.; Holden-Day: San Francisco, 1977.


Langmuir, Vol. 16, No. 1, 2000 31

Figure 10. FT-IR spectra of CNC (a), CAC (b-d), and CAMC (e): (a, b) 1.0 wt % gel in n-dodecane; (c) 0.12 wt % chloroform solution; (d, e) solid phase (Nujol mull >3060 cm-1; KBr pellet 20 nm shifts observed in some bands upon CAC or CAMC aggregation.31 The specific molecular interactions responsible for these large shifts are not clear to us; they may involve chargetransfer states between carbamate groups and aromatic π-clouds.54 Whatever their origins, the bands indicate that (52) Lu, L.; Weiss, R. G. Chem. Commun. (Cambridge) 1996, 2029. (53) Rohatgi-Mukerjee, K. K. Fundamentals of Photochemistry; Halsted Press: New Delhi, 1978; pp 61-210.

Langmuir, Vol. 16, No. 1, 2000 33

the anthryl groups in the gels interact very strongly. Further insights into how these interactions occur are not possible at this time since we have been unable to prepare single crystals of CAC suitable for structural analyses. Even with the CAMC crystal structures, it will be necessary to relate their molecular packing to that in the gelator strands.38 However, the packing arrangements within the crystal do indicate the type of π‚‚‚π interactions that support excitonic interactions and macrohelicity. Interestingly, they also indicate that CdO‚‚‚H3C-N intermolecular interactions may help to stabilize the overall molecular organization. Additionally, one of the new bands in the UV-vis absorption spectra of the partial gel and gel (or concentrated solution) phases of CAC overlaps the longest wavelength 1La-type monomer band at 395 nm (shortaxis polarization) and the other is further (bathochromically) shifted to 420 nm. On this basis, the new bands are ascribed to interacting short-axis transition dipoles in their J-type aggregates.34 Conclusions In general, the aza-ALS molecules are less efficient gelators than their oxa analogues and, in some cases, do not gel any of the liquids that were examined! This conclusion is contrary to initial expectations since many of the aza gelators are capable of both H-bond donation and acceptance; the oxa gelators can act as H-bond acceptors only. Spectroscopic measurements provide clear evidence for stacking of A groups, but there is none for H-bonding within the gel strands! The properties of some of the gelators are sensitive to the rate of sol cooling (i.e., how rapidly the sol w organogel transition occurs) and to aging. Dispersive attractive forces (especially among steroidal groups) and steric repulsive interactions (which are more difficult to characterize) appear to mandate the mode of molecular packing. Although H-bonding interactions among gelator molecules have been shown to be present and important in strands of other organogels, they are not here. The nature of the liquid is more germane to Tg (via solubility considerations) and the type of morph formed by the gelator molecules as they aggregate (via nucleation as the sol w organogel transition occurs) than on the structure of the gel itself. A relatively wide variety of polar protic and nonpolar aprotic liquids can be gelled by some of the aza-ALS gelators. Although this research addresses some fundamental questions concerning the role of different packing interactions on the viability and stability of gel formation, it does not resolve a very important outstanding question: Why does the one-dimensional crystal growth needed for gel formation occur in any case? Work to address this question is in progress. Appendix I We take as a limiting case that no aggregation exists in 0.014 wt % (2.3 × 10-4 M) CAC in 1-pentanol at room temperature. In fact, the excitation spectrum in Figure 7B shows a very small amount of aggregation for this solution. Then, from Beer’s law, 356 ) A/bc ) 0.29/(0.3 × 2.3 × 10-4) ) 4.2 × 103 L mol-1 cm-1 and 395 ) 3.1 × 103 L mol-1 cm-1. These values are almost the same as the (54) (a) We thank Professor Jerry Perlstein for this suggestion. (b) For a higher energy charge-transfer transition from a carboxyl to an amido group, see: Pajcini, V.; Chen, X. G.; Bormett, R. W.; Geib, S. J.; Li, P.; Asher, S. A.; Lidiak, E. G. J. Am. Chem. Soc. 1996, 118, 9716.

34


molar extinction coefficients calculated for ca. 10-5 M CAC in methylene chloride. If the molar extinction coefficients are temperature independent in the range 25-100 °C and there is no aggregation in 1.0 wt % (1.6 × 10-2 M) CAC/1-pentanol at 95 °C, A is calculated to be 0.59 when b ) 0.01 cm. From Figures 3 and 4, 356 of aggregated CAC is certainly less than that of monomeric CAC but must be >0. An upper limit for Cmono, the concentration of unaggregated CAC, can be calculated when 356 is taken to be zero for unaggregated CAC. For example, Cmono < A356/(356b) ) 0.59/(4.2 × 103 × 0.01) ) 1.4 × 10-2 M; thus,

Gelation of Organic Liquids by Some 5Î±-Cholestan-3Î²-yl N-(2-Aryl

Recommend Documents