A Chiral Low-Molecular-Weight Gelator Based on Binaphthalene with

Fluorous ?ponytails? lead to strong gelators showing thermally induced structure evolution. Harshita Kumari , Sarah E. Armitage , Steven R. Kline , Kr...
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Langmuir 2007, 23, 1478-1482

A Chiral Low-Molecular-Weight Gelator Based on Binaphthalene with Two Urea Moieties: Modulation of the CD Spectrum after Gel Formation Cheng Wang,†,‡ Deqing Zhang,*,† and Daoben Zhu*,† Beijing National Laboratory for Molecular Sciences, Organic Solids Laboratory, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, and Graduate School of Chinese Academy of Sciences, Beijing 100080, China ReceiVed September 7, 2006. In Final Form: October 14, 2006 The synthesis and characterization of a new chiral LMWG 1 based on the axially chiral binaphthalene with two urea moieties were described. A transparent gel in cyclohexane with LMWG 1 was obtained and characterized by SEM, XRD and CD techniques. The results of 1H NMR measurement indicated that the intermolecular H-bonds and π-π interaction may be responsible for the gel formation. It was demonstrated that the gel phase could be destroyed by addition of F- due to the disruption of intermolecular H-bonds. After gel formation, modulation of the CD spectrum of 1 was observed.

Introduction Molecular gels have been the subject of increasing attention in recent years due to their unique structures and wide range of potential applications.1 Molecular gels are entangled threedimensional networks with solvent molecules entrapped inside. They are formed through aggregation of the fibrous architectures that result from assembly of low-molecular-weight gelators (LMWGs).2 Intermolecular interactions, such as π-π stacking, H-bonding, and charge transfer are the driving forces for the gelation of solvents in the presence of gelators. In general, modification of the chemical structures of the LMWGs would affect the corresponding intermolecular interactions, and therefore, the structures (e.g., morphology) and physical properties of the generated gels.3 Therefore, development of new gelators is crucial for obtaining molecular gels that show fascinating properties useful for a wide range of advanced applications. A number of LMWGs such as those containing urea groups4 have been reported so far, thanks to the interdisciplinary efforts from many research groups.5 Manipulation of chirality is one of the important issues of modern chemistry and has been addressed in different ways. Chirality transcription from the chiral gelators into the supramolecular structures and gels seems to be a very promising approach.6 Chiral gelators containing cholesterol and amino acid moieties have been described.7,8 Feringa et al. recently reported the * E-mail: [email protected]. † Institute of Chemistry, Chinese Academy of Sciences. ‡ Graduate School of Chinese Academy of Sciences. (1) (a) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133-3159. (b) Sangeetha, N. M.; Maitra, U. Chem. Soc. ReV. 2005, 34, 821-836. (2) (a) Wang, R.; Geiger, C.; Chen, L. H.; Swanson, B.; Whitten, D. G. J. Am. Chem. Soc. 2000, 122, 2399-2400. (b) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263-2266. (c) van Esch, J.; Schoonbeek, F.; de Loos, M.; Kooijman, H.; Spek, A. L.; Kellogg, R. M.; Feringa, B. L. Chem.sEur. J. 1999, 5, 937-950. (3) (a) Wang, S.; Shen, W.; Feng, Y. L.; Tian, H. Chem. Commun. 2006, 1497-1499. (b) Paulusse, J. M. J.; Sijbesma, R. P. Angew. Chem., Int. Ed. 2006, 45, 2334-2337. (c) Wang, C.; Zhang, D. Q.; Zhu, D. B. J. Am. Chem. Soc. 2005, 127, 16372-16373. (d) Li, Y. T.; Tang, Y. Q.; Narain, R.; Lewis, A. L.; Armes, S. P. Langmuir 2005, 21, 9946-9954. (e) Kuroiwa, K.; Shibata, T.; Takada, A.; Nemoto, N.; Kimizuka, N. J. Am. Chem. Soc. 2004, 126, 2016-2021. (f) Kawano, S.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2004, 126, 8592-8593. (g) An, B. K.; Lee, D. S.; Lee, J. S.; Park, Y. S.; Song, H. S.; Park, S. Y. J. Am. Chem. Soc. 2004, 126, 10232-10233.

photoswitching of the chirality of the gel phases based on a chiral gelator derived from a dithienylethene derivative.9 Binaphthalene molecules are axially chiral species and show strong CD signals which are dependent on the dihedral angles between the two naphthalene rings.10 However, to the best of our knowledge, no chiral gelators based on the binaphthalene framework have been known. In this paper, we report a new LMWG (compound 1 in Scheme 1) based on S-binaphthalene with two urea moieties. The motivation of this work includes the following: (1) development of new chiral LMWGs based on the framework of binaphthalene, which can assemble through intermolecular interactions such as H-bonding and π-π stacking in solution to induce the gelation of organic solvents; (2) possibility to modulate the CD spectrum due to the intermolecular interactions, with the result that a thermally driven chiral switch may be realized in association with the gel-solution transition. The 1H NMR measurements of 1 at different concentrations (in CDCl3) indicate that the intermolecular H-bonds due to urea moieties may be responsible for the gelation. It is interesting to note that (1) the gel phase of 1 in cyclohexane can be destroyed by addition of F; (2) the CD spectrum of 1 can be modulated after gel formation. (4) (a) Wu¨rthner, F.; Hanke, B.; Lysetska, M.; Lambright, G.; Harms, G. S. Org. Lett. 2005, 7, 967-970. (b) van der Laan, S.; Feringa, B. L.; Kellogg, R. M.; van Esch, J. Langmuir 2002, 18, 7136-7140. (c) Estroff, L. A.; Hamilton, A. D. Angew. Chem., Int. Ed. 2000, 39, 3447-3450. (d) de Loos, M.; van Esch, J.; Kellogg, R. M.; Feringa, B. L. Angew. Chem., Int. Ed. 2001, 40, 613-616. (e) Schoonbeek, F. S.; van Esch, J. H.; Wegewijs, B.; Rep, D. B. A.; de Haas, M. P.; Klapwijk, T. M.; Kellogg, R. M.; Feringa, B. L. Angew. Chem., Int. Ed. 1999, 38, 1393-1397. (5) (a) Balakrishnan, K.; Datar, A.; Zhang, W.; Yang, X. M.; Naddo, T.; Huang, J. L.; Zuo, J. M.; Yen, M.; Moore, J. S.; Zang, L. J. Am. Chem. Soc. 2006, 128, 6576-6577. (b) Li, H. F.; Homan, E. A.; Lampkins, A. J.; Ghiviriga, I.; Castellano, R. K. Org. Lett. 2005, 7, 443-446. (c) George, S. J.; Ajayaghosh, A. Chem.sEur. J. 2005, 11, 3217-3227. (d) Ajayaghosh, A.; George, S. J. J. Am. Chem. Soc. 2001, 123, 5148-5149. (e) Shirakawa, M.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 4164-4165. (f) Kishimura, A.; Yamashita, T.; Aida, T. J. Am. Chem. Soc. 2005, 127, 179-183. (g) Shirakawa, M.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2003, 125, 9902-9903. (h) Duncan, D. C.; Whitten, D. G. Langmuir 2000, 16, 6445-6452. (6) (a) George, S. J.; Ajayaghosh, A.; Jonkheijm, P.; Schenning, A.; Meijer, E. W. Angew. Chem., Int. Ed. 2004, 43, 3422-3425. (b) Brizard, A.; Oda, R.; Huc, I. Top. Curr. Chem. 2005, 256, 167-218. (c) Hirst, A. R.; Smith, D. K.; Feiters, M. C.; Geurts, H. P. M. Chem.sEur. J. 2004, 10, 5901-5910. (d) Yagai, S.; Nakajima, T.; Kishikawa, K.; Kohmoto, S.; Karatsu, T.; Kitamura, A. J. Am. Chem. Soc. 2005, 127, 11134-11139. (e) Ajayaghosh, A.; Varghese, R.; George, S. J.; Vijayakumar, C. Angew. Chem., Int. Ed. 2006, 45, 1141-1144.

10.1021/la062621x CCC: $37.00 © 2007 American Chemical Society Published on Web 11/18/2006

Chiral LMW Gelator Based on Binaphthalene

Langmuir, Vol. 23, No. 3, 2007 1479

Scheme 1 Chemical Structure of LMWG 1 and the Synthetic Approacha

a (a) 2-Bromoethanol, DEAD, PPh , THF, reflux; (b) NaN , DMF, 80 °C; (c) zinc powder, NH Cl, ethanol/H O, reflux; (d) dodecylisocyanate, 3 3 4 2 THF, ambient temperature.

Experimental Section Materials. Dodecylisocyanate was purchased from Acros (Belgium). THF was dried over sodium/benzophenone before use. DMF and acetonitrile was dried over CaH2 before use. All other reagents and solvents (standard grade) were used as received unless otherwise stated. CAUTION: NaN3 is potentially explosive, and it should be handled in small quantities. Characterization Techniques. Melting points were measured with an XT4-100X apparatus and uncorrected. 1H NMR and 13C NMR spectra were recorded with Bruker 300 and 400 MHz spectrometers. Infrared spectra were obtained on a Perkin-Elmer System 2000 FT-IR spectrometer. MS spectra were determined with BEFLEX III for TOF-MS and AEI-MS 50 for EI-MS. HRMS was determined with FTICR-APEX. Elemental analyses were performed on a Carlo-Erba-1106 instrument. XRD data were collected on a Rigaku D/max-2500 X-ray diffractometer with Cu KR radiation (λ ) 1.5406 Å). CD spectra were recorded in a Jasco J-810 spectrophotometer; the scan rate was 500 nm/min, and all of the spectra were accumulated two times. For scanning electron microscopy (SEM) experiments, a JEOL JSM 6700F field emission scanning electron microscope was used, and the sample was sputtered with platinum. Synthesis of Compound 2. It was synthesized according to the literature.10j Mp 96-98 °C; [R]20D -53.2 (c 3.00, CH2Cl2); 1H NMR (400 MHz, CDCl3, ppm): δ ) 7.97 (2H, d, J ) 8.7 Hz), 7.88 (2H, d, J ) 7.8 Hz), 7.42 (2H, d, J ) 9.0 Hz), 7.36 (2H, t, J ) 7.2 Hz), 7.23 (2H, d, J ) 6.8 Hz), 7.13 (2H, d, J ) 8.5 Hz), 4.26-4.17 (4H, m), 3.21 (4H, m). 13C NMR (75 MHz, CDCl3, ppm): δ ) 153.5, 134.0, 129.8, 129.7, 127.9, 126.5, 125.4, 124.1, 121.0, 116.3, 69.9, 29.1. HRMS calcd for C24H20Br2O2: 497.9830 (M+), 499.9810 (M + 2)+, 501.9879 (M + 4)+; found 497.9828 (M+), 499.9814 (M + 2)+, 501.9803 (M + 4)+. Synthesis of Compound 3. A solution of 2 (2.2 g, 4.4 mmol) in anhydrous DMF (40 mL) was treated with NaN3 (5.7 g, 88.0 mmol) at 25 °C under N2. The resulting reaction mixture was warmed at (7) (a) Zinic, M.; Vo¨gtle, F.; Fages, F. Top. Curr. Chem. 2005, 256, 39-76. (b) Sugiyasu, K.; Fujita, N.; Shinkai, S. Angew. Chem., Int. Ed. 2004, 43, 12291233. (c) Ishi-i, T.; Iguchi, R.; Snip, E.; Ikeda, M.; Shinkai, S. Langmuir 2001, 17, 5825-5833. (d) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664-6676. (e) Ajayaghosh, A.; Vijayakumar, C.; Varghese, R.; George, S. J. Angew. Chem., Int. Ed. 2006, 45, 456-460. (8) (a) Zhan, C. L.; Gao, P.; Liu, M. H. Chem. Commun. 2005, 462-464. (b) Gao, P.; Zhan, C. L.; Liu, L. Z.; Zhou, Y. B.; Liu, M. H. Chem. Commun. 2004, 1174-1175. (c) Gronwald, O.; Shinkai, S. Chem.sEur. J. 2001, 7, 4328-4334. (d) Yoza, K.; Amanokura, N.; Ono, Y.; Akao, T.; Shinmori, H.; Takeuchi, M.; Shinkai, S.; Reinhoudt, D. N. Chem.sEur. J. 1999, 5, 2722-2729.

80 °C for 8 h before 20 mL of H2O was added. The aqueous solution was extracted with dichloromethane (3 × 40 mL), and the combined extracts were washed with saturated aqueous NaCl (2 × 40 mL), dried (Na2SO4), and concentrated in a vacuum. After column chromatography on silica gel with CH2Cl2/petroleum (60-90 °C) (1:2, v/v) as eluant, compound 3 was obtained as a white solid (1.6 g) in 85% yield. Mp 60-61 °C; [R]20D -25.6 (c 2.50, CH2Cl2); 1H NMR (300 MHz, CDCl3, ppm): δ ) 7.96 (2H, d, J ) 9.0 Hz), 7.88 (2H, d, J ) 8.1 Hz), 7.41 (2H, d, J ) 9.0 Hz), 7.35 (2H, t, J ) 7.2 Hz), 7.22 (2H, d, J ) 6.7 Hz), 7.13 (2H, d, J ) 8.4 Hz), 4.18-4.00 (4H, m), 3.17 (4H, m). 13C NMR (75 MHz, CDCl3, ppm): δ ) 153.8, 134.0, 129.7, 129.6, 127.9, 126.4, 125.3, 123.9, 120.6, 115.8, 68.8, 50.5. MS (EI): 424 (M+). IR (KBr, cm-1): 2109 (-N3). Anal. Calcd for C24H20N6O2: C, 67.91; H, 4.75; N, 19.80. Found: C, 67.74; H, 4.73; N, 19.78. Synthesis of Compound 4. A solution of 3 (0.5 g, 1.2 mmol) in ethanol/H2O (9:1, v/v, 80 mL) was treated with zinc powder (0.9 g, 14.0. mmol) and NH4Cl (1.9 g, 36.0 mmol) under N2. The resulting reaction mixture was refluxed for 6 h before 10 mL of aqueous NaOH (1 M) was added. The mixture was refluxed for another 0.5 h. The aqueous solution was extracted with dichloromethane (3 × 50 mL), dried (Na2SO4), and concentrated in a vacuum. After column chromatography on silica gel with CH2Cl2/CH3OH (20:1, v/v) as eluant, compound 3 was obtained as a white solid (0.3 g) in 68% yield. Mp 81-83 °C; [R]20D -82.8 (c 2.30, CH2Cl2). 1H NMR (300 MHz, CDCl3, ppm): δ ) 7.96 (2H, d, J ) 9.0 Hz), 7.88 (2H, d, J ) 8.2 Hz), 7.43 (2H, d, J ) 9.0 Hz), 7.34 (2H, t, J ) 7.0 Hz), 7.24 (2H, d, J ) 7.7 Hz), 7.18 (2H, d, J ) 8.4 Hz), 4.09-3.89 (4H, m), 2.68 (4H, m). 13C NMR (75 MHz, CDCl3, ppm): δ ) 153.9, 133.9, 129.5, 129.4, 128.0, 126.5, 125.3, 123.9, 120.4, 115.7, 71.6, 41.3. MS (EI): 372 (M+). HRMS: Calcd for C24H24N2O2 372.1838. Found 372.1835. Synthesis of Compound 1. A solution of dodecylisocyanate (0.12 (9) de Jong, J. J. D.; Lucas, L. N.; Kellogg, R. M.; van Esch, J. H.; Feringa, B. L. Science 2004, 304, 278-281. (10) (a) Mason, S. F.; Seal, R. H.; Robert, D. R. Tetrahedron 1974, 30, 1671. (b) Ferrarini, A.; Moro, G. J.; Mordio, P. L. Liq. Cryst. 1995, 189, 397-399. (c) Di Bari, L.; Piscitelli, G.; Salvadori, P. J. Am. Chem. Soc. 1999, 121, 7998-8004. (d) Di Bari, L.; Piscitelli, G.; Marchetti, F,; Salvadori, P. J. Am. Chem. Soc. 2000, 122, 6395-6398. (e) Circular Dichroism: Principles and Applications; Berova, N., Nakanishi, K., Woody, R. W., Eds.; Wiley-VCH: New York, 2000. (f) Beer, G.; Niederalt, C.; Grimme, S.; Daub, J. Angew. Chem., Int. Ed. 2000, 39, 32523255. (g) van Delden, R. A.; Hurenkamp, J. H.; Feringa, B. L. Chem.sEur. J. 2003, 9, 2845-2853. (h) Pieraccini, S.; Masiero, S.; Spada, G. P.; Gottarelli, G. Chem. Commun. 2003, 598-599. (i) Zhou, Y. C.; Zhang, D. Q.; Zhang, Y. Z.; Tang, Y. L.; Zhu, D. B. J. Org. Chem. 2005, 70, 6164-6170. (j) Zhou, Y. C.; Zhang, D. Q.; Zhu, L. Y.; Shuai, Z. G.; Zhu, D. B. J. Org. Chem. 2006, 71, 2123-2130.

1480 Langmuir, Vol. 23, No. 3, 2007 g, 0.56 mmol, 0.14 mL) in dry THF (10 mL) was slowly added to a solution of compound 4 (0.10 g, 0.27 mmol) in dry THF (30 mL) under N2. The reaction mixture was stirred overnight at room temperature. The solvents were removed in a vacuum, and the residue was purified by column chromatography on silica gel with CH2Cl2/CH3OH (100:1, v/v) as eluant to afford compound 1 as a white powder (0.10 g) in 48% yield. Mp 87-88 °C; [R]20D -54.2 (c 2.20, CH2Cl2). 1H NMR (400 MHz, CDCl3, ppm): δ ) 7.98 (2H, d, J ) 9.0 Hz), 7.91 (2H, d, J ) 7.8 Hz), 7.44 (2H, d, J ) 9.0 Hz), 7.38 (2H, t, J ) 7.0 Hz), 7.28 (2H, d, J ) 7.8 Hz), 7.17 (2H, d, J ) 8.4 Hz), 4.17-3.93 (4H, m), 4.10 (2H, s), 3.80 (2H, s), 3.23 (4H, m), 2.95 (4H, m), 1.39-1.27 (40H, m), 0.88 (6H, t, J ) 6.5 Hz). 13C NMR (75 MHz, CDCl3, ppm): δ ) 158.1, 154.1, 133.8, 129.7, 129.5, 128.2, 126.6, 125.3, 123.9, 120.5, 116.3, 69.8, 40.3, 39.8, 31.9, 30.3, 29.7, 29.5, 29.4, 27.0, 22.7, 14.1. MS (MALDI-TOF): 795.9 (M + 1)+. Anal. Calcd for C50H74N4O4: C, 75.53; H, 9.38; N, 7.05. Found: C, 75.79; H, 9.37; N, 6.95. Gel Formation. In a typical gelation experiment, a weighed amount of the gelator 1 and 1.0 mL of the solvent were placed in a test tube, which was sealed and then heated until the compound dissolved. The solution was allowed to cool to 15 °C.

Wang et al.

Figure 1. The CD spectra of LMWG 1 (5.0 × 10-6 M, acetonitrile) in the presence of different amounts of (C4H9)4F recorded in a 1-cm cell. Table 1. Gelation Experimental Results with Compound 1

Results and Discussion Synthesis and Characterization. The synthesis of LMWG 1 was carried out in four steps, starting from (S)-(-)2,2′dihydroxy-1,1′-binaphthyl, as shown in Scheme 1. Compound 2 was prepared from (S)-(-)2,2′-dihydroxy-1,1′-binaphthyl by using the Mitsunobu reaction.11 Reaction of compound 2 with NaN3 led to compound 3, which was reduced to yield compound 4. Reaction of compound 4 with dodecylisocyanate afforded LMWG 1 in a total yield of 17%. As expected, LMWG 1 (5 × 10-6 M in acetonitrile) showed strong bisignated CD bands in the region 210-250 nm with λθ)0 ) 229 nm. The CD spectrum of 1 was similar to that of (S)-(-)2,2′-dihydroxy-1,1′-binaphthyl, indicating that 1 should retain the S configuration.12 It is known that the urea moieties are able to bind with different anions.13 Thus, the two urea moieties of 1 may be able to bind with anions, and as a result, the dihedral angle between the two naphthalene rings would be modulated. As shown in Figure 1, where the CD spectra of 1 in the presence of F- was displayed, the intensities of CD bands at about 223 and 236 nm increased after addition of F- to the solution of 1 (5.0 × 10-6 M in acetonitrile). Such a CD spectral change observed for the diluted solution of 1 in the presence of F- was likely due to the binding of F- with two urea moieties of 1,14 leading to the modulation of the dihedral angle between two naphthalene rings and thus the change of CD spectrum, according to previous studies.10f-j Interestingly, the CD spectrum of 1 remained almost unchanged (11) Mitsunobu, O. Synthesis 1981, 1-28. (12) It was reported that the optically active 2,2′-substituted binaphthalene enantiomers was very stable and could not be racemized at high temperatures (>150 °C); see: Hall, D. M.; Turner, E. E. J. Chem. Soc. 1955, 1242-1251, and Pu, L. Chem. ReV. 1998, 98, 2405-2494. Thus, compound 1 should retain the S configuration under the present experimental conditions. (13) (a) Gunnlaugsson, T.; Davis, A. P.; O’Brien, J. E.; Glynn, M. Org. Biomol. Chem. 2005, 3, 48-56. (b) Jose, D. A.; Kumar, D. K.; Ganguly, B.; Das, A. Org. Lett. 2004, 6, 3445-3448. (c) Lee, D. H.; Im, J. H.; Lee, J. H.; Hong, J. I. Tetrahedron Lett. 2002, 43, 9637-9640. (d) Kwon, J. Y.; Jang, Y. J.; Kim, S. K.; Lee, K. H.; Kim, J. S.; Yoon, J. Y. J. Org. Chem. 2004, 69, 5155-5157. (e) Bondy, C. R.; Gale, P. A.; Loeb, S. J. J. Am. Chem. Soc. 2004, 126, 5030-5031. (f) Cho, E. J.; Moon, J. W.; Ko, S. W.; Lee, J. Y.; Kim, S. K.; Yoon, J.; Nam, K. C. J. Am. Chem. Soc. 2003, 125, 12376-12377. (g) Varghese, R.; George, S. J.; Ajayaghosh, A. Chem. Commun. 2005, 593-595. (h) Ren, J.; Wang, Q. C.; Qu, D. H.; Zhao, X. L.; Tian, H. Chem. Lett. 2004, 33, 974-975. (i) Liu, B.; Tian, H. Chem. Lett. 2005, 34, 686-687. (14) It was reported earlier that urea moieties could form H-bonds with F-. But, N-H deprotonation occurred when excess F- was present. Large CD spectral variation was observed when 1.0 equiv of F- was added, but almost no more change was detected after more than 1.0 equiv of F- was added: (a) EstebanGomez, D.; Fabbrizzi, L.; Liechelli, M. J. Org. Chem. 2005, 70, 5717-5720. (b) Boiocchi, M.; Del Boca, L.; Esteban-Gomez, D.; Fabbrizzi, L.; Licchelli, M.; Monzani, E. Chem.sEur. J. 2005, 11, 3097-3104.

a

solvent

gelationa

n-hexane cyclohexane p-xylene chloroform 1,2-dichloroethane acetonitrile 2-propanol

P G (6 mg/mL) S S S P P

Gel ) G; solution ) S; precipitation ) P.

in the presence of Cl-, Br-, I-, H2PO4-, and HPO42-. This may be because the binding of these anions with the urea groups of 1 was rather weak. Gel Formation and CD Spectrum Modulation. The gelation ability of 1 was tested in several solvents at 15 °C (see Table 1). Among the solvents tested, a transparent gel was formed in cyclohexane with LMWG 1 (6 mg/mL) (see Figure 2). The gel was found to be stable in the temperature range 15-20 °C. When the temperature was over 20 °C, the transparent gel was transformed into the corresponding solution. This sol-gel interconversion was fully thermoreversible by several cycles of heating and cooling. Figure 3 shows the SEM images of xerogel of 1 formed in cyclohexane. As expected, an entangled network of thin solid fibers with lengths up to tens of microns was formed. The gel of 1 showed an XRD pattern exhibiting a peak around 3.9° corresponding to d ) 2.4 nm, which was about the same length as 1 (ca. 1.9 nm, see Figure 4).15 1H NMR spectrum of 1 (ca. 30 mg/mL in CDCl ) was recorded 3 separately at 300, 310, and 320 K (see Figure S1 of Supporting Information). At 300 K, the proton signals of the urea groups of 1 were almost overlapped with those of -CH2 groups. At 320 K, a broad signal due to the protons of urea groups was detected. These results implied that the intermolecular H-bonds were formed between neighboring urea groups at 300 K, and consequently, the corresponding proton signals of urea groups were downfield-shifted. The intermolecular H-bonds would become weak by increasing temperature, and as a result, the proton signals of urea groups would be upfield-shifted at 320 K. The 1H NMR spectra of 1 were also recorded in CDCl3 at different concentrations. When the concentration of 1 increased, the corresponding signals at 3.77 and 4.11 ppm assigned to urea moieties were gradually downfield-shifted, as shown in Figure 5. For instance, the signals at 3.77 and 4.11 ppm were shifted to 3.86 and 4.16 ppm by changing the concentration of 1 from 2 mg/mL to 20 mg/mL. These results indicated that the (15) The theoretical calculation was carried out with AMPAC, version 5.0; Semichem: Shawnee, KS, 1994.

Chiral LMW Gelator Based on Binaphthalene

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Figure 2. Illustration of the gel formation in cyclohexane containing LMWG 1 (8 mg/mL) and destroying the gel phase by addition of N(C4H9)4F (3.2 mg/mL).

Figure 3. SEM image of the gel of 1 in cyclohexane.

Figure 5. The 1H NMR spectra of 1 in CDCl3 at different concentrations recorded at 298 K: (A) 2 mg/mL; (B) 10 mg/mL; (C) 20 mg/mL; (D) 30 mg/mL.

Figure 4. XRD patterns of the gels of 1 in cyclohexane and the energy-minimized structure of LMWG 1.

intermolecular H-bonds were formed between the urea moieties of neighboring molecules of 1. These intermolecular H-bonds may be responsible for the gel formation of 1 in cyclohexane. However, the chemical shifts due to the formation of intermolecular H-bonds between urea moieties were relatively small compared to those observed for other LMWGs with urea moieties. This result implied that weak intermolecular H-bonds were formed between urea moieties, which was likely because the two urea moieties were not well oriented to form H-bonds. This may be in agreement with the fact that a gel was only produced in cyclohexane with 1 among the several solvents tested. Apart from the intermolecular H-bonds, intermolecular π-π interaction may also contribute to the gelation. As discussed above, the urea moieties are able to bind with

F-;13 thus, the intermolecular H-bonding between the urea moieties would be disrupted in the presence of F-. It was reported earlier that the binding of F- with urea-functionalized oligo(p-phenylenevinylene)s induced fluorescence enhancement.13g The 1H NMR spectrum of 1 (in CDCl3) was also recorded in the presence of 1.0 equiv of F- (see Figure S2 of Supporting Information). After the addition of F-, the proton signals due to urea groups of 1 became weak, which may be due to the deprotonation of urea groups by F- according to previous reports.13h-i As shown in Figure 2, the solution of 1 in cyclohexane containing (C4H9)4NF could not be transformed into the gel under the same condition. This result also provided support for the assumption that the intermolecular H-bonds between the urea moieties should be responsible for the gel formation. However, no absorption spectral changes were observed for 1 after addition of F- (see Figure S3 of Supporting Information). This may be understandable, since the urea groups of 1 are linked to the binaphthalene rings by σ-bonding spacers. (16) The linear CD may also contribute to the CD spectrum variation observed for the gel of 1 in cyclohexane. (17) Xiao, D. B.; Yang, W. S.; Yao, J. N.; Lu, X.; Xia, Y.; Shuai, Z. G. J. Am. Chem. Soc. 2004, 126, 15439-15444. (18) The recording of the CD spectrum was performed by rotating the quartz substrate following the procedures described in the literature (see ref 8b).

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the chirality inversion observed for the nanoparticles of the chiral binaphthalene molecule.17 The CD spectrum of the xerogel of 1 was also measured (see Figure S4 in Supporting Information).18 The two CD bands are further bathochromically shifted by ca. 5 nm, which may also result from the intermolecular exciton coupling of two naphthyl moieties.

Conclusion

Figure 6. The CD spectra of compound 1 (6 mg/mL) in cyclohexane in a 0.05 mm quartz cell: (a) solution recorded at 28 °C; (b) gel recorded at 20 °C; (c) solution after heating the gel recorded at 28 °C.

The CD spectra of 1 before and after gel formation were measured (see Figure 6). Obviously, after gel formation, the intensities of CD bands around 222 and 238 nm were largely enhanced, and concomitantly, the CD bands were slightly shifted. Note that such CD spectrum variation can be repeated for several cycles by alternately heating and cooling the solution of 1. The intensity enhancement for the CD bands after gelation may be due to the fact that gelation could lead to the intermolecular exciton coupling of two naphthyl moieties of neighboring molecules.16 Previously, it was reported that such intermolecular exciton coupling of two naphthyl moieties was responsible for

In summary, we reported a new chiral LMWG 1 based on the axially chiral binaphthalene with two urea moieties. Gelation of cyclohexane occurred in the presence of 1, and a transparent gel was obtained. The results of 1H NMR measurement indicated that the intermolecular H-bonds and π-π interaction may be responsible for the gel formation. Moreover, it was demonstrated that the gel phase could be destroyed by the addition of F- due to the disruption of intermolecular H-bonds. After gel formation, modulation of the CD spectrum of 1 was observed, and the modulation could be reversibly realized by alternate heating and cooling. Acknowledgment. The present research was financially supported by NSFC, Chinese Academy of Sciences, and National Basic Research Program (2006CB806200). D.-Q. Zhang thanks National Science Fund for Distinguished Young Scholars. Supporting Information Available: NMR, absorption, and CD spectra. This material is available free of charge via the Internet at http://pubs.acs.org. LA062621X