Molecular Aggregation of Disklike Benzenetricarboxamides

Ashleigh R. Borges , Marilise Hyacinth , Michelle Lum , Clayton M. Dingle , Paris L. Hamilton , Maksymilian Chruszcz , Lin Pu , Michal Sabat and Kevin...
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Langmuir 2004, 20, 9513-9519

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Molecular Aggregation of Disklike Benzenetricarboxamides Containing Diacetylenic Groups in Bulk and Organic Solvents Seung Ju Lee, Chong Rae Park, and Ji Young Chang* School of Materials Science and Engineering, and Hyperstructured Organic Materials Research Center, College of Engineering ENG445, Seoul National University, Seoul 151-744, Korea Received March 14, 2004. In Final Form: August 5, 2004 We prepared disklike tris(4-alkylbutadiynylphenyl)-1,3,5-benzenetricarboxamide and tris[4-(4-alkyloxyphenyl)butadiynylphenyl]-1,3,5-benzenetricarboxamide, where three phenyl diacetylenic and diphenyl diacetylenic groups are connected to a benzene ring through amide linkages, respectively. The structures of self-assembled substances were investigated by using transmission electron microscopy, scanning electron microscopy, and X-ray diffraction techniques. All the compounds were highly viscous in melt states, and only compound 9 having three phenyl diacetylenic groups and hexyl tails showed a thermotropic mesophase on cooling. The compound with three diphenyl diacetylenic groups and dodecyloxy tails (13) formed a stable gel in THF/cyclohexane. The IR and X-ray analyses showed that in the gel state molecules were assembled into a rectangular columnar lattice and held each other by hydrogen bondings between amide groups. The compound with tetradecyloxy tails (14) formed stable colloidal particles in cyclohexane. The UV irradiation of 13 in a gel and 14 in a colloidal particle did not result in a long conjugated polymer because of the inappropriate alignment of diacetylenic groups for the topochemical polymerization.

Introduction Self-assembly of discotic molecules consisting of a diskshaped aromatic core and peripheral flexible side chains occurs through microsegregation between the aromatic core and the peripheral chains in bulk1-12 or in organic solvents.13-19 Noncovalent interactions such as π-π interactions, hydrogen bondings, and solvophobic effects are the main driving forces for the microsegregation. In previous work, we prepared polymerizable disklike compounds that had a benzene ring core and three rigid diacetylenic units. The diacetylenic units were attached to the benzene ring via ester linkages, thereby constructing (1) Chandrasekhar, S. Mol. Cryst. Liq. Cryst. 1981, 63, 171. (2) Pugh, C.; Percec, V. J. Mater. Chem. 1991, 1, 765. (3) Giroud-Godquin, A. M.; Billard, J. Mol. Cryst. Liq. Cryst. 1981, 66, 147. (4) Piechocky, C.; Simon, J.; Skoulios, A.; Guillon, D.; Weber, P. J. Am. Chem. Soc. 1982, 104, 5245. (5) Barbera, J.; Cativiela, C.; Serrano, J. L.; Zurbano, M. M. Adv. Mater. 1991, 3, 602. (6) Ebert, E.; Wendorff, J.; Lattermann, G. Liq. Cryst. 1990, 7, 553. (7) Lattermann, G.; Straufer, G.; Brezesinski, G. Liq. Cryst. 1991, 10, 169. (8) Ciuchi, F.; Nicola, G. D.; Franz, H.; Gottarelli, G.; Mariani, P.; Bossi, M. G. P.; Spada, G. P. J. Am. Chem. Soc. 1994, 116, 7064. (9) Zhang, H.; Lai, C. K.; Swager, T. Chem. Mater. 1995, 7, 2067. (10) Lai, C. K.; Tsai, C.-H.; Pang, Y.-S. J. Mater. Chem. 1998, 8, 1355. (11) Zhang, Y.-D.; Jespersen, K. G.; Kempe, M.; Kornfield, J. A.; Barlow, S.; Kippelen, B.; Marder, S. R. Langmuir 2003, 19, 6534. (12) Wu, J.; Watson, M. D.; Zhang, L.; Wang, Z.; Mullen, K. J. Am. Chem. Soc. 2004, 126, 177. (13) van Gorp, J. J.; Vekemans, J. A. J. A.; Meijer, E. W. J. Am. Chem. Soc. 2002, 124, 14759. (14) Camerel, F.; Faul, C. F. J. Chem. Commun. 2003, 1958. (15) Yasuda, Y.; Iishi, E.; Inada, H.; Shirota, Y. Chem. Lett. 1996, 575. (16) Palmans, A. R. A.; Vekemans, J. A. J. A.; Fischer, H.; Hikmet, R. A.; Meijer, E. W. Chem.sEur. J. 1997, 3, 300. (17) Palmans, A. R. A.; Vekemans, J. A. J. A.; Hikmet, R. A.; Fischer, H.; Meijer, E. W. Adv. Mater. 1998, 10, 873. (18) Brunsveld, L.; Zhang, H.; Glasbeek, M.; Vekemans, J. A. J. A.; Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 6175. (19) Ryu, S. Y.; Kim, S.; Seo, J.; Kim, Y.-W.; Kwon, O.-H.; Jang, D.-J.; Park, S. Y. Chem. Commun. 2004, 70.

a part of the rigid disks. The compounds self-assembled into a columnar structure mainly through π-π interaction in their thermotropic liquid crystalline (LC) states.20,21 Mesogenic diacetylenes are of great interest as their polymerization in LC states proceeds by a 1,4-polymerization pathway.22-27 The resulting polymers could maintain the aligned structures over a wide range of temperatures. This paper describes self-aggregation and photoreaction of benzenetricarboxamides, in which three phenyl diacetylenic or diphenyl diacetylenic groups were connected to a benzene ring through amide linkages. Since the amide groups can function as both a hydrogen bond donor and an acceptor, we expected that hydrogen bonding should play an important role in the self-assembly of these discotic molecules. In general, columns formed from the molecules having a relatively small discotic core through π-π interaction alone have irregular or “loose” stacking. Intermolecular hydrogen bonding can aid the formation of well-ordered supramolecules. 1,3,5-Benzenetricarboxamide derivatives,15,19,28-30 cis-1,3,5-cyclohexanetricarboxamide derivatives,31 benzenehexamine derivatives,32 and tris(stearoylamino)triphenyl-amine33 have been inten(20) Chang, J. Y.; Baik, J. H.; Lee, C. B.; Han, M. J.; Hong, S.-K. J. Am. Chem. Soc. 1997, 119, 3197. (21) Chang, J. Y.; Yeon, J. R.; Shin, Y. S.; Han, M. J.; Hong, S.-K. Chem. Mater. 2000, 12, 1076. (22) Tsibouklis, J. Adv. Mater. 1995, 7, 407. (23) Garito, A. F.; Teng, C. C.; Wong, K. Y.; Khamiri, O. Z. Mol. Cryst. Liq. Cryst. 1984, 106, 219. (24) Tsibouklis, J.; Campbell, C.; Werninck, A. R.; Shand, A. J.; Milburn, G. H. W. Polym. Bull. 1992, 29, 661. (25) Schen, M. A.; Kotowski, K.; Cline, J. Polymer 1991, 32, 1843. (26) Izuoka, A.; Okuno, T.; Ito, T.; Sugawara, T.; Sato, N.; Kamei, S.; Tohyama, K. Mol. Cryst. Liq. Cryst. 1993, 226, 201. (27) Hammond, P. T.; Rubner, M. F. Macromolecules 1995, 28, 795. (28) Hanabusa, K.; Koto, C.; Kimura, M.; Shirai, H.; Kakehi, A. Chem. Lett. 1997, 429. (29) Lightfoot, M. P.; Mair, F. S.; Pritchard, R. G.; Warren, J. E. Chem. Commun. 1999, 1945. (30) Brunsveld, L.; Schenning, A. P. H. J.; Broeren, M. A. C.; Janssen, H. M.; Vekemans, J. A. J. M.; Meijer, E. W. Chem. Lett. 2000, 292.

10.1021/la0493417 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/29/2004

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sively investigated in terms of assembly into columns through intermolecular hydrogen bonding. We investigated molecular aggregation behaviors of benzenetricarboxamides in organic solvents as well as in bulk. Experimental Section Materials and Instrumentation. 4-Iodophenol, 1-bromodecane, 1-bromododecane, 1-bromotetradecane, 1-nonyne, 1-decyne, 1-dodecyne, (trimethylsilyl)acetylene, dichlorobis (triphenylphosphine)palladium(II), copper(I) iodide, copper(II) acetate, 1,3,5-benzenetricarbonyl trichloride, and cyanuric chloride were purchased from Aldrich and used as received. 1-Undecyne was purchased from Tokyo Kasei Kogyo Co. and used as received. Reagent-grade solvents were dried and purified as follows. Triethylamine (TEA) was dried and distilled from calcium hydride. Methanol was dried over molecular sieves (4 Å) and distilled. Tetrahydrofuran (THF) and pyridine were dried and distilled from sodium. 1H and 13C NMR spectra were recorded with the use of a Bruker Avance DPX-300 (300 MHz) or a Bruker Avance 500 (500 MHz). FT-IR spectra were obtained with the use of a Perkin-Elmer Spectrum 2000 FT-IR spectrometer. Elemental analyses were performed by an EA1110 (CE instrument). Thermal analyses were performed by a TA modulated differential scanning calorimeter 2920. Polarizing optical microscopy (POM) study was performed with a Leica MPS 30 equipped with a Mettler Toledo FP82HT heating stage and a Mettler Toledo FP90 controller. Powder X-ray diffractograms were obtained with the use of a Bruker SAXS with general area detector diffraction (Cu KR radiation, λ ) 1.54 Å) or a Bruker NANOSTAR SAXS system (Cu KR radiation, λ ) 1.54 Å). UV-vis spectra were obtained with the use of a Hewlett-Packard HP8452A UV-vis spectrometer. Photoluminescence spectra were obtained with the use of a Shimadzu RF-5301PC spectrofluorophotometer. Scanning electron microscopy (SEM) images were obtained with the use of a JEOL JSM 6330F field-emission scanning electron microscope. Transmission electron microscopy (TEM) images were obtained with the use of a JEM-2000EXII transmission electron microscope. 1-Decyloxy-4-iodobenzene. This compound was prepared according to our previous reports.20,21,34,35 To a solution of 4-iodophenol (8.8 g, 40.00 mmol) in ethanol (95%, 100 mL) were added potassium hydroxide (2.24 g, 40.00 mmol) and potassium iodide (0.13 g, 2 mol %). The mixture was heated to 60 °C, and 1-bromodecane was added dropwise. The reaction mixture was refluxed for 12 h. After the reaction mixture was cooled, the precipitates were removed by filtration. After evaporation of the solvent, the product was isolated by column chromatography on silica gel using methylene chloride/n-hexane (2/1, v/v) as the eluent; yield, 12.04 g (84%). 1H NMR (CDCl , 300 MHz): δ 7.50, 6.64 (dd, 4H, aromatic 3 ring protons), 3.87 (t, 2H, OCH2), 1.78-1.25 (m, 20H, CH2), 0.87 (t, 3H, CH3). 1-Dodecyloxy-4-iodobenzene. This compound was prepared from 4-iodophenol and 1-bromododecane as described for 1-decyloxy-4-iodobenzene; yield, 66%. 1H NMR (CDCl , 300 MHz): δ 7.52, 6.65 (dd, 4H, aromatic 3 ring protons), 3.89 (t, 2H, OCH2), 1.79-1.24 (m, 20H, CH2), 0.86 (t, 3H, CH3). 1-Tetradecyloxy-4-iodobenzene. This compound was prepared from 4-iodophenol and 1-bromotetradecane as described for 1-decyloxy-4-iodobenzene; yield, 82%. 1H NMR (CDCl , 300 MHz): δ 7.51, 6.66 (dd, 4H, aromatic 3 ring protons), 3.89 (t, 2H, OCH2), 1.80-1.24 (m, 20H, CH2), 0.88 (t, 3H, CH3). (31) Hanabusa, K.; Kawakami, A.; Kimura, M.; Shirai, H. Chem. Lett. 1997, 191. (32) Kohne, B.; Praefcke, K.; Derz, T.; Hoffmann, H.; Schwandner, B. Chimia 1986, 40, 171. (33) Yasuda, Y.; Takebe, Y.; Fukumoto, M.; Inada, H.; Shirota, Y. Adv. Mater. 1996, 8, 740. (34) Lee, C. J.; Lee, S. J.; Chang, J. Y. Tetrahedron Lett. 2002, 43, 3863. (35) Lee, S. J.; Chang, J. Y. Tetrahedron Lett. 2003, 44, 7493.

Lee et al. 1-Decyloxy-4-(trimethylsilylethynyl)benzene. This compound was prepared according to our previous reports.20,21,34,35 To a solution of 1-decyloxy-4-iodobenzene (11.04 g, 30.64 mmol) and (trimethylsilyl)acetylene (5.20 mL, 36.77 mmol) in triethylamine were added dichlorobis(triphenylphosphine) palladium(II) (0.43 g, 2 mol %) and copper(I) iodide (58.4 mg, 1 mol %) at 0 °C. The reaction mixture was stirred for 20 h at room temperature under nitrogen. After evaporation of the solvent, the product was isolated by column chromatography on silica gel using ethyl acetate/n-hexane (1/9, v/v) as the eluent; yield, 9.12 g (90%). 1H NMR (CDCl , 300 MHz): δ 7.46, 6.75 (dd, 4H, aromatic 3 ring protons), 3.97 (t, 2H, OCH2), 1.78-1.23 (m, 16H, CH2), 0.87 (t, 3H, CH3), 0.25 (s, 9H, Si(CH3)3). 1-Dodecyloxy-4-(trimethylsilylethynyl)benzene. This compound was prepared from 1-dodecyloxy-4-iodobenzene and (trimethylsilyl)acetylene as described for 1-decyloxy-4-(trimethylsilylethynyl)benzene; yield, 89%. 1H NMR (CDCl , 300 MHz): δ 7.45, 6.75 (dd, 4H, aromatic 3 ring protons), 3.95 (t, 2H, OCH2), 1.80-1.24 (m, 20H, CH2), 0.86 (t, 3H, CH3), 0.25 (s, 9H, Si(CH3)3). 1-Tetradecyloxy-4-(trimethylsilylethynyl)benzene. This compound was prepared from 1-tetradecyloxy-4-iodobenzene and (trimethylsilyl)acetylene as described for 1-decyloxy-4-(trimethylsilylethynyl)benzene; yield, 85%. 1H NMR (CDCl , 300 MHz): δ 7.47, 6.76 (dd, 4H, aromatic 3 ring protons), 3.98 (t, 2H, OCH2), 1.82-1.26 (m, 24H, CH2), 0.88 (t, 3H, CH3), 0.24 (s, 9H, Si(CH3)3). 1-Decyloxy-4-ethynylbenzene. This compound was prepared according to our previous reports.20,21,34,35 To a solution of 1-decyloxy-4-(trimethylsilylethynyl)benzene (7.75 g, 23.44 mmol) in methanol/methylene chloride (50 mL/50 mL) was added potassium carbonate (0.32 g, 10 mol %). The reaction mixture was stirred for 4 h at room temperature. After evaporation of the solvent, the product was isolated by column chromatography on silica gel using methylene chloride/n-hexane (1/4, v/v) as the eluent; yield, 5.75 g (95%). 1H NMR (CDCl , 300 MHz): δ 7.40, 6.81 (dd, 4H, aromatic 3 ring protons), 3.93 (t, 2H, OCH2), 3.00 (s, 1H, -CtCH), 1.781.25 (m, 16H, CH2), 0.88 (t, 3H, CH3). 1-Dodecyloxy-4-ethynylbenzene. This compound was prepared from 1-dodecyloxy-4-(trimethylsilylethynyl)benzene as described for 1-decyloxy-4-ethynylbenzene; yield, 89%. 1H NMR (CDCl , 300 MHz): δ 7.39, 6.81 (dd, 4H, aromatic 3 ring protons), 3.93 (t, 2H, OCH2), 2.97 (s, 1H, -CtCH), 1.781.24 (m, 20H, CH2), 0.86 (t, 3H, CH3). 1-Tetradecyloxy-4-ethynylbenzene. This compound was prepared from 1-tetradecyloxy-4-(trimethylsilylethynyl)benzene as described for 1-decyloxy-4-ethynylbenzene; yield, 91%. 1H NMR (CDCl , 300 MHz): δ 7.36, 6.79 (dd, 4H, aromatic 3 ring protons), 3.95 (t, 2H, OCH2), 2.99 (s, 1H, -CtCH), 1.811.23 (m, 24H, CH2), 0.87 (t, 3H, CH3). 1-(4-Aminophenyl)-1,3-undecadiyne (1). This compound was prepared according to our previous reports.20,34,35 To a solution of 4-ethynylaniline34-37 (2.34 g, 20.00 mmol) and 1-nonyne (1.24 g, 10.00 mmol) in pyridine/methanol (100 mL/100 mL) was added copper(II) acetate (5.45 g, 30.00 mmol), and the reaction mixture was refluxed for 18 h under nitrogen. Insoluble solids were removed by filtration. After evaporation of the solvent, the product was isolated by column chromatography on silica gel using methylene chloride/n-hexane (2/1, v/v) as the eluent; yield, 31%. 1H NMR (CDCl , 300 MHz): δ 7.28, 6.58 (dd, 4H, aromatic 3 ring protons), 3.85 (s, 2H, NH2), 2.34 (t, 2H, -CtCCH2), 1.611.28 (m, 10H, CH2), 0.89 (t, 3H, CH3). 1-(4-Aminophenyl)-1,3-dodecadiyne (2). This compound was prepared from 4-ethynylaniline and 1-decyne as described for compound 1; yield, 33%. 1H NMR (CDCl , 300 MHz): δ 7.26, 6.57 (dd, 4H, aromatic 3 ring protons), 3.84 (s, 2H, NH2), 2.33 (t, 2H, -CtCCH2), 1.581.28 (m, 12H, CH2), 0.88 (t, 3H, CH3). (36) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis 1980, 627. (37) Chang, J. Y.; Rhee, S. B.; Cheong, S.; Yoon, M. Macromolecules 1992, 25, 2666.

Aggregation of Disklike Benzenetricarboxamides 1-(4-Aminophenyl)-1,3-tridecadiyne (3). This compound was prepared from 4-ethynylaniline and 1-undecyne as described for compound 1; yield, 34%. 1H NMR (CDCl , 300 MHz): δ 7.29, 6.60 (dd, 4H, aromatic 3 ring protons), 3.85 (s, 2H, NH2), 2.35 (t, 2H, -CtCCH2), 1.611.28 (m, 14H, CH2), 0.89 (t, 3H, CH3). 1-(4-Aminophenyl)-1,3-tetradecadiyne (4). This compound was prepared from 4-ethynylaniline and 1-dodecyne as described for compound 1; yield, 31%. 1H NMR (CDCl , 300 MHz): δ 7.28, 6.57 (dd, 4H, aromatic 3 ring protons), 3.85 (s, 2H, NH2), 2.34 (t, 2H, -CtCCH2), 1.601.27 (m, 10H, CH2), 0.88 (t, 3H, CH3). 1-(4-Aminophenyl)-4-(4-decyloxyphenyl)butadiyne (5). This compound was prepared from 4-ethynylaniline and 1-decyloxy-4-ethynylbenzene as described for compound 1; yield, 1.67 g (45%). 1H NMR (CDCl , 300 MHz): δ 7.43, 6.83 (dd, 4H, aromatic 3 ring protons), 7.32, 6.60 (dd, 4H, aromatic ring protons), 3.95 (t, 2H, OCH2), 3.88 (s, 2H, NH2), 1.80-1.27 (m, 16H, CH2), 0.88 (t, 3H, CH3). 1-(4-Aminophenyl)-4-(4-dodecyloxyphenyl)butadiyne (6). This compound was prepared from 4-ethynylaniline and 1-dodecyloxy-4-ethynylbenzene as described for compound 1; yield, 40%. 1H NMR (CDCl , 300 MHz): δ 7.43, 6.82 (dd, 4H, aromatic 3 ring protons), 7.32, 6.59 (dd, 4H, aromatic ring protons), 3.95 (t, 2H, OCH2), 3.87 (s, 2H, NH2), 1.80-1.26 (m, 16H, CH2), 0.88 (t, 3H, CH3). 1-(4-Aminophenyl)-4-(4-tetradecyloxyphenyl)butadiyne (7). This compound was prepared from 4-ethynylaniline and 1-tetradecyloxy-4-ethynylbenzene as described for compound 3; yield, 50%. 1H NMR (CDCl , 300 MHz): δ 7.43, 6.83 (dd, 4H, aromatic 3 ring protons), 7.32, 6.60 (dd, 4H, aromatic ring protons), 3.95 (t, 2H, OCH2), 3.88 (s, 2H, NH2), 1.80-1.27 (m, 16H, CH2), 0.88 (t, 3H, CH3). Tris(4-undeca-1,3-diynylphenyl)-1,3,5-benzenetricarboxamide (8). To a solution of compound 1 (0.51 g, 2.14 mmol) and pyridine (0.35 mL, 4.28 mmol) in THF (30 mL) was added 1,3,5-benzenetricarbonyl trichloride (0.16 g, 0.60 mmol) under nitrogen. The reaction mixture was stirred for 8 h at room temperature. The precipitates were removed by filtration. After evaporation of the solvent, the product was isolated by column chromatography on silica gel using THF/n-hexane (1/3, v/v) as the eluent; yield, 66%. 1H NMR (DMSO-d , 300 MHz): δ 10.79 (s, 3H, CONH), 8.69 6 (s, 3H, central aromatic ring protons), 7.86, 7.56 (dd, 12H, aromatic ring protons), 2.41 (t, 6H, -CtCCH2), 1.52, 1.26 (m, 30H, CH2), 0.86 (t, 9H, CH3). 13C NMR (CDCl3, 500 MHz): δ 165.7, 137.9, 135.8, 133.5, 128.2, 120.0, 119.2, 85.2, 75.2, 74.3, 65.5, 31.9, 29.2, 29.0, 28.6, 22.9, 19.9, 14.3. Anal. Calcd for C60H63N3O3: C, 82.44; H, 7.26; N, 4.81. Found: C, 82.60; H, 7.42; N, 4.97. Tris(4-dodeca-1,3-diynylphenyl)-1,3,5-benzenetricarboxamide (9). This compound was prepared from compound 2 and 1,3,5-benzenetricarbonyl trichloride as described for compound 8. The product was isolated by column chromatography on silica gel using ethyl acetate/n-hexane (1/3, v/v); yield, 63%. 1H NMR (DMSO-d , 300 MHz): δ 10.81 (s, 3H, CONH), 8.71 6 (s, 3H, central aromatic ring protons), 7.88, 7.57 (dd, 12H, aromatic ring protons), 2.42 (t, 6H, -CtCCH2), 1.51, 1.27 (m, 36H, CH2), 0.87 (t, 9H, CH3). 13C NMR (CDCl3, 500 MHz): δ 165.7, 137.9, 135.9, 133.6, 128.3, 120.1, 119.2, 85.2, 75.2, 74.3, 65.5, 32.1, 29.4, 29.3, 29.2, 28.6, 22.9, 19.9, 14.4. Anal. Calcd for C63H69N3O3: C, 82.58; H, 7.59; N, 4.59. Found: C, 82.67; H, 7.68; N, 4.59. Tris(4-trideca-1,3-diynylphenyl)-1,3,5-benzenetricarboxamide (10). This compound was prepared from compound 3 and 1,3,5-benzenetricarbonyl trichloride as described for compound 8. The product was isolated by column chromatography on silica gel using ethyl acetate/n-hexane (1/3, v/v); yield, 65%. 1H NMR (DMSO-d , 300 MHz): δ 10.79 (s, 3H, CONH), 8.70 6 (s, 3H, central aromatic ring protons), 7.87, 7.56 (dd, 12H, aromatic ring protons), 2.41 (t, 6H, -CtCCH2), 1.53, 1.26 (m, 42H, CH2), 0.86 (t, 9H, CH3). 13C NMR (CDCl3, 500 MHz): δ 165.7, 137.9, 135.9, 133.6, 128.2, 120.1, 119.2, 85.2, 75.2, 74.3, 65.5, 32.1, 29.7, 29.5, 29.4, 29.2, 28.6, 22.9, 19.9, 14.3. Anal. Calcd

Langmuir, Vol. 20, No. 22, 2004 9515 for C66H75N3O3: C, 82.72; H, 7.89; N, 4.38. Found: C, 83.03; H, 7.89; N, 4.40. Tris(4-tetradeca-1,3-diynylphenyl)-1,3,5-benzenetricarboxamide (11). This compound was prepared from compound 4 and 1,3,5-benzenetricarbonyl trichloride as described for compound 8. The product was isolated by column chromatography on silica gel using ethyl acetate/n-hexane (1/3, v/v); yield, 60%. 1H NMR (DMSO-d , 300 MHz): δ 10.76 (s, 3H, CONH), 8.71 6 (s, 3H, central aromatic ring protons), 7.87, 7.57 (dd, 12H, aromatic ring protons), 2.42 (t, 6H, -CtCCH2), 1.52, 1.27 (m, 48H, CH2), 0.86 (t, 9H, CH3). 13C NMR (CDCl3, 500 MHz): δ 165.5, 137.9, 135.8, 133.5, 128.4, 120.1, 119.1, 85.2, 75.2, 74.4, 65.5, 32.1, 29.8, 29.7, 29.5, 29.4, 29.2, 28.6, 22.9, 19.9, 14.4. Anal. Calcd for C69H81N3O3: C, 82.84; H, 8.16; N, 4.20. Found: C, 83.16; H, 8.38; N, 4.17. Tris[4-(4-decyloxyphenyl)butadiynylphenyl]-1,3,5-benzenetricarboxamide (12). This compound was prepared from compound 5 and 1,3,5-benzenetricarbonyl trichloride as described for compound 8. The product was isolated by column chromatography on silica gel using THF/n-hexane (1/2, v/v) as the eluent; yield, 0.46 g (60%). 1H NMR (CDCl , 300 MHz): δ 9.27 (s, 3H, CONH), 7.79 (s, 3H, 3 central aromatic ring protons), 7.54-6.74 (m, 24H, aromatic ring protons), 3.87 (t, 6H, OCH2), 1.86-1.27 (m, 48H, CH2), 0.89 (t, 9H, CH3). 13C NMR (CDCl3, 500 MHz): δ 165.2, 160.0, 138.0, 135.6, 134.4, 133.5, 128.8, 120.3, 118.9, 114.7, 113.8, 82.5, 80.7, 75.1, 73.2, 68.2, 32.1, 29.8, 29.7, 29.6, 29.5, 26.3, 22.9, 14.3. Anal. Calcd for C87H93N3O6: C, 81.85; H, 7.34; N, 3.29. Found: C, 81.55; H, 7.47; N, 3.15. Tris[4-(4-dodecyloxyphenyl)butadiynylphenyl]-1,3,5benzenetricarboxamide (13). This compound was prepared from compound 6 and 1,3,5-benzenetricarbonyl trichloride as described for compound 8; yield, 65%. 1H NMR (CDCl , 300 MHz): δ 9.34 (s, 3H, CONH), 7.68 (s, 3H, 3 central aromatic ring protons), 7.55-6.73 (m, 24H, aromatic ring protons), 3.88 (t, 6H, OCH2), 1.74-1.27 (m, 60H, CH2), 0.88 (t, 9H, CH3). 13C NMR (CDCl3, 500 MHz): δ 165.1, 160.0, 138.0, 135.5, 134.4, 133.5, 128.8, 120.4, 118.9, 114.7, 113.7, 82.5, 80.7, 75.1, 73.2, 68.2, 32.1, 29.9, 29.9, 29.8, 29.7, 29.6, 29.5, 26.2, 22.9, 14.3. Anal. Calcd for C93H105N3O6: C, 82.08; H, 7.78; N, 3.09. Found: C, 81.74; H, 7.92; N, 2.91. Tris[4-(4-tetradecyloxyphenyl)butadiynylphenyl]-1,3,5benzenetricarboxamide (14). This compound was prepared from compound 7 and 1,3,5-benzenetricarbonyl trichloride as described for compound 8; yield, 63%. 1H NMR (CDCl , 300 MHz): δ 9.21 (s, 3H, CONH), 7.84 (s, 3H, 3 central aromatic ring protons), 7.53-6.75 (m, 24H, aromatic ring protons), 3.87 (t, 6H, OCH2), 1.76-1.27 (m, 72H, CH2), 0.88 (t, 9H, CH3). 13C NMR (CDCl3, 500 MHz): δ 165.0, 160.0, 138.0, 135.6, 134.4, 133.5, 128.9, 120.4, 118.9, 114.7, 113.7, 82.5, 80.7, 75.1, 73.2, 68.3, 32.2, 29.9, 29.9, 29.8, 29.7, 29.6, 29.5, 26.3, 22.9, 14.3. Anal. Calcd for C99H117N3O6: C, 82.29; H, 8.16; N, 2.91. Found: C, 82.09; H, 8.40; N, 2.75. Gelation Test. A weighed amount of a 1,3,5-benzenetricarboxamide and 1 mL of the solvent were put in a vial. After the vial was tightly sealed, the compound was dissolved by heating and cooled to room temperature. When the resulting substance exhibited no gravitational flow, it was judged to be a gel.

Results and Discussion Synthesis. Linear diacetylenic side groups were synthesized according to Scheme 1. 4-Ethynylaniline was synthesized by following the procedures in the literature.34-37 Phenyl diacetylenes 1-4 were prepared by the oxidative coupling reaction of 4-ethynylaniline with 1-alkyne. Diphenyl diacetylenes 5-7 were prepared in a similar manner. Alkyloxy tails with various lengths were introduced to benzene rings by the reaction of sodium 4-iodophenoxide with alkyl bromides. After acetylation with trimethylsilylacetylene in the presence of a palladium catalyst, the resulting (4-alkoxyphenyl)acetylenes were coupled with 4-ethynylaniline to yield diphenyldiacetylenes. The linear diacetylenes 1-7 were reacted with 1,3,5benzenetricarbonyl trichloride in THF in the presence of

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Lee et al.

Scheme 1

Scheme 2 Figure 1. DSC thermograms of (a) 8, (b) 9, (c) 10, and (d) 11 obtained on the second heating and the second cooling. Heating and cooling rates were 5 and 1 °C min-1, respectively.

Figure 2. (a) POM image and (b) SEM image of compound 9, which were obtained at 126 °C and at room temperature after quenching from 126 °C, respectively.

pyridine at room temperature (Scheme 2). The products were isolated by column chromatography on silica gel and characterized by 1H and 13C NMR spectroscopy and elemental analysis. One singlet peak for the core ring protons and two peaks for the core ring carbons appeared in the 1H and 13C NMR spectra, respectively, indicating that all chloro groups were replaced. Thermal Behavior. Figure 1 shows differential scanning calorimetry (DSC) thermograms of 1,3,5-benzenetricarboxamides 8-11 with an aromatic core comprising four benzene rings. On heating of 8 at a rate of 5 °C min-1, a broad endothermic peak corresponding to melting of small crystals (premelting) was observed around 90 °C, followed by an exotherm at 98 °C for recrystallization, and a strong endothermic peak corresponding to melting appeared at 117 °C. When cooled from the isotropic state at a rate of 1 °C min-1, 8 showed two broad, small exothermic peaks (the sum of two enthalpy changes ) 7.1

kJ mol-1) around 97 and 90 °C. Compound 9 showed a small endotherm around 89 °C for premelting, followed by an exotherm at 96 °C for recrystallization. A strong endotherm at 114 °C and a small endotherm at 130 °C for melting and isotropization, respectively, were observed. The compound was shearable between 114 and 130 °C. When cooled from the isotropic state at a rate of 1 °C min-1, 9 showed two broad, small exothermic peaks (the sum of two enthalpy changes ) 7.6 kJ mol-1) around 100 and 94 °C. Compound 10 showed thermal behavior similar to that of 8 on heating. A small exotherm around 88 °C (∆H ) 5.0 kJ mol-1) was observed on cooling. Compound 11 exhibited only one small, broad endotherm at 68 °C on heating corresponding to premelting and became a liquid above 90 °C. No exothermic peak was detected on cooling. Compounds 8-11 were thermally polymerized above 190 °C. Under polarizing microscopy observation, only compound 9 exhibited a birefringent phase below 100 °C on cooling (Figure 2a). The SEM image of compound 9 quenched from 126 °C to room temperature reveals that microsegregation occurred to form a granular structure with particle sizes of 20-30 nm (Figure 2b). Figure 3 shows small-angle X-ray diffractograms of 9 quenched from 130, 120, 110, 100, and 85 °C on cooling. The samples prepared at 130, 120, and 110 °C exhibited

Aggregation of Disklike Benzenetricarboxamides

Figure 3. X-ray diffractograms of 9 quenched from (a) 130, (b) 120, (c) 110, (d) 100, and (e) 85 °C on cooling.

similar X-ray diffraction (XRD) patterns: a strong reflection with a d spacing of 24.8 Å and a weak reflection with a d spacing of 12.8 Å. The d spacings were in the ratio of 1:1/2, implying that compound 9 arranged into a loose layered structure. With further cooling below 100 °C, the compound began to crystallize. The samples obtained at 100 and 85 °C showed sharp, strong reflections with d spacings of 25.4, 21.7, and 20.8 Å and weak reflections with d spacings of 17.9, 12.6, and 12.0 Å. Compounds 12-14 with diphenyl diacetylenic groups and alkyloxy tails (decyloxy, dodecyloxy, or tetradecyloxy) showed higher clearing transition temperatures (208 °C for 12, 203 °C for 13, and 207 °C for 14) than compounds 8-11. No distinct textures showed up under polarizing microscopy observation on heating or cooling. Thermal polymerizations of the compounds gradually proceeded after clearing transitions. Compared to corresponding benzenetricarboxylates,20,21 the benzenetricarboxamides had much higher viscosities in melt states. High viscosities presumably prevented self-aggregation of the molecules into long-range ordered structures. Aggregation in Organic Solvents. Many organic solids show colloidal behaviors in liquids, and gelation or precipitation occurs when colloidal particles interact physically.38 Compounds 12-14 were soluble in polar organic solvents such as THF, DMF, chloroform, methylene chloride, ethyl acetate, and acetone but insoluble in n-alkane and slightly soluble in cyclohexane. For gelation tests, the compounds were added to a mixture of cyclohexane and THF. The mixture was heated to be isotropic and cooled to room temperature. Compound 13 (1 wt %) formed a stable gel in THF/cyclohexane (1/8-1/20, v/v). Compound 14 also formed a gel in THF/cyclohexane at g10-3 M but phase-separated macroscopically from the organic solvent. In the case of compound 12, precipitation occurred. The benzenetricarboxamides with phenyl diacetylenic groups (8-11) have relatively small aromatic cores and did not show any specific aggregation behaviors in organic solvents. Gelation of THF/cyclohexane from the compounds at high concentrations is certainly a consequence of regulation of fiber-organic liquid interfacial energy to control solubility. Crystallization was prevented by adding a small amount of THF (dielectric constant  ) 7.6) to cyclohexane ( ) 2.0) and thus tuning the polarity of organic solvents. (38) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237.

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Figure 4. FT-IR spectra of (a) a THF solution of compound 13 and (b) a xerogel of compound 13 (1 wt %) from THF/cyclohexane (1/8, v/v).

Figure 5. TEM image of a gel of 10-3 M compound 13 in THF/ cyclohexane (1/20, v/v). The bar represents 500 nm.

In FT-IR spectra, the N-H stretch and amide I bands of compound 13 in THF appeared at 3421-3327 and 1663 cm-1, respectively. When the sample was prepared as a xerogel from THF/cyclohexane (1/8, v/v), they shifted to 3245 and 1646 cm-1 (Figure 4). These spectral changes indicate that molecules held each other by hydrogen bondings between amide groups in the gel state.39 The structure of the gel of compound 13 was investigated by using TEM and XRD techniques. Figure 5 shows the TEM image of a gel of 10-3 M compound 13 in THF/ cyclohexane (1/20, v/v), where a fibrous morphology is observed. Relatively straight fibrous rods consisting of a bundle of fibers are intertwined with each other, and the thickness of the smallest rod is about 50 nm. In the smallangle X-ray diffractogram of a xerogel of compound 13 from THF/cyclohexane (1/8, v/v) (Figure 6), a set of reflection peaks corresponding to d spacings of 42.6, 28.6, 23.7, and 14.6 Å is shown. The radius of compound 13 was calculated to be about 30 Å by simple molecular modeling (MM+, HyperChem 5.0). In combination with the modeling, these reflections can be indexed in sequence as (100), (010), (110), and (020) reflections from a rectangular columnar (Colr) lattice with the lattice parameters of a ) 42.6 Å and b ) 28.6 Å. The fact that a is smaller than the calculated diameter was attributable to interdigitation of flexible alkyl tails. Compounds 12-14 were dissolved in cyclohexane (10-4 or 10-5 M) by heating. When the solutions were cooled to room temperature, the compounds were finely dispersed. (39) Socrates, G. In Infrared Characteristic Group Frequencies, 2nd ed.; John Wiley & Sons: Chichester, 1994; pp 104-107.

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Lee et al. Table 1. Molecular Aggregations of the 1,3,5-Benzenetricarboxamides in Organic Solvents Depending on Concentration and Number of Peripheral Alkyloxy Tail Carbons (m)a organic solvent THF/cyclohexane (1/8, v/v) cyclohexane n-alkane a

Figure 6. X-ray diffractogram of a xerogel of compound 13 (1 wt %) from THF/cyclohexane (1/8, v/v).

Figure 7. SEM image of compound 14 in cyclohexane (10-4 M). The bar represents 1 µm.

The dispersion state was maintained over more than 4 months for compound 14 at room temperature, whereas compounds 12 and 13 precipitated after several hours and several days, respectively. At concentrations higher than 10-4 M, precipitation occurred for all the compounds. SEM analysis of a dispersion of compound 14 in cyclohexane (10-4 M) revealed that colloidal particles with diameters of 0.3-0.5 µm formed (Figure 7). Colloids are thermodynamically unstable with respect to the bulk, which is demonstrated by the equation dG ) γdσ, where G is Gibbs free energy, σ is the surface area of the substance, and γ is the interfacial surface tension.40 Therefore, the kinetics of collapse plays an important role in the stable existence of colloidal particles. For colloidal particles whose surfaces are electrically neutral, the pairwise potential energy of interaction consists of the short-range repulsive interaction energy and the longrange attractive interaction energy. The principle of suppressing the long-range attractions between colloidal particles is to make the surfaces of the particles compatible with the surrounding medium, coated with a protective film, or covered with negative or positive charges to repel an approach from another similarly charged particle. It is presumed that in cyclohexane π-π interactions and strong intermolecular hydrogen bondings of the aromatic cores made the molecules aggregate into small particles whose surfaces were coated with lyophilic alkyloxy chains. Compound 14 with the longest alkyloxy chains solely formed stable colloidal particles at low concentrations (10-4 and 10-5 M). It is likely that long alkyl chains are more favorable for their compatibility (40) Atkins, P. W. In Physical Chemistry, 6th ed.; Oxford University Press: Oxford, 1998; pp 702-704.

concentration 12 13 14 (M) (m ) 10) (m ) 12) (m ) 14) g10-3

P

G

PG

>10-4 10-4 10-5

P P P I

P P P I

P colloid colloid I

P ) precipitation, G ) gel, PG ) partial gel, and I ) insoluble.

with cyclohexane than shorter alkyloxy chains, leading to stable colloidal particles. Table 1 summarizes molecular aggregations of compounds 12-14 in organic solvents according to concentration and number of peripheral alkyloxy tail carbons. Photoreaction. A gel of 1 wt % 13 in THF/cyclohexane (1/8, v/v) was irradiated with a UV lamp (mercury arc lamp, 100 W) for 1 h at room temperature. IR spectroscopy did not show significant intensity changes of the bands at 2213 and 2144 cm-1 from symmetric and asymmetric stretching vibrations of carbon-carbon triple bonds, indicating that photopolymerization did not proceed. For topochemical 1,4-addition reaction of diacetylenes, monomeric diacetylenes should be aligned to have appropriate geometry.41,42 Mair et al. reported the crystal structure of the triamide, N,N′,N′′-tris(2-methoxyethyl)benzene-1,3,5tricarboxamide, which is composed of infinite π-stacked rods supported by a triple helical network of hydrogen bonds.43 Considering such structural feature of the triamide, the molecular arrangement of 13 in the gel state is probably unsuitable for 1,4-addition reaction of diacetylenes. Figure 8 shows UV-vis and photoluminescence (PL) spectra of a dilute dispersion of 14 in cyclohexane (10-4) before and after photoirradiation for 1 and 2 h. On photoirradiation of the dispersion, no significant UV-vis absorption spectral changes were observed except that the absorption limits were extended up to about 550 nm. The dispersion emitted weak greenish fluorescence when illuminated with a UV lamp. After photoirradiation, the emissions disappeared. This was confirmed by PL spectroscopy. PL of a dispersion of 14 in cyclohexane (10-4 M) before and after photoirradiation was measured with an excitation wavelength of 330 nm (Figure 8). The PL intensity of the dispersion greatly reduced after photoirradiation for 2 h, and the maximum emission wavelength red-shifted from 508 to 549 nm. The UV-vis and PL spectroscopy results indicate that only partial polymerization occurred at the surface of colloidal particles to yield short conjugated oligomers. The photoirradiated dispersions also showed long-term stability without precipitation. In summary, we investigated molecular aggregations of discotic 1,3,5-benzenetricarboxamides having three phenyl diacetylenic or diphenyl diacetylenic groups in bulk and in organic solvents. Compound 9 having three hexylbutadinylphenyl groups showed a thermotropic mesophase, but others did not form long-range ordered (41) Enkelmann, V. In Polydiacetylenes; Cantow, H. J., Ed.; SpringerVerlag: Berlin, 1984; p 91. (42) Sarkar, A.; Okada, S.; Matsuzawa, H.; Matsuda, H.; Nakanishi, H. J. Mater. Chem. 2000, 10, 819. (43) Lightfoot, M. P.; Mair, F. S.; Pritchard, R. G.; Warren, J. E. Chem. Commun. 1999, 1945.

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Figure 8. (a) UV-vis spectra and (b) photoluminescence emission spectra (λex ) 330 nm) of 14 in cyclohexane (10-4 M) before photoirradiation (solid line) and after photoirradiation for 1 h (dashed line) and 2 h (dotted line).

phases. This is attributable in part to high viscosities resulting from intermolecular hydrogen bonding and π-π interaction, which would prevent molecular aggregation into ordered structures. Molecular aggregation behaviors in organic solvents were dependent on the size of the aromatic core comprising benzene rings and rigid diacetylenic groups, as well as flexible tail lengths. Compound 13 gelated a mixture of THF and cyclohexane. In cyclohexane alone, compound 13 was precipitated, while compound 14 formed stable colloidal particles. Diacetyl-

enic groups were poorly photoreactive in gels or colloidal particles due to their inappropriate alignment for the topochemical polymerization. Acknowledgment. The financial support from the Korea Science and Engineering Foundation through the Hyperstructured Organic Materials Research Center is gratefully acknowledged. LA0493417