Supramolecular Liquid-Crystalline Networks Built by Self-Assembly of

For example, the H-bonded complex 1/5 shows a smectic A phase from 176 to 156 .... Supramolecular Polymers Generated from Heterocomplementary Monomers...
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Chem. Mater. 1996, 8, 961-968

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Supramolecular Liquid-Crystalline Networks Built by Self-Assembly of Multifunctional Hydrogen-Bonding Molecules Hideyuki Kihara,† Takashi Kato,*,† Toshiyuki Uryu,† and Jean M. J. Fre´chet‡ Institute of Industrial Science, The University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan, and Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301 Received November 16, 1995. Revised Manuscript Received January 22, 1996X

Supramolecular liquid-crystalline networks have been prepared by self-assembly of multifunctional H-bond donor and acceptor molecules through the formation of intermolecular hydrogen bonds. Two tricarboxylic acids, 1,3,5-tris(2-(2-(4-carboxyphenoxy)ethoxy)ethoxy)benzene (1) and 3,4-bis(2-(2-(4-carboxyphenoxy)ethoxy)ethoxy)benzoic acid (2), have been synthesized for the use as trifunctional H-bond donors. These trifunctional H-bond donors have been complexed with bifunctional H-bond acceptors, such as 4,4′-bipyridine (3), 1,2bis(4-pyridyl)ethane (4), trans-1,2-bis(4-pyridyl)ethylene (5), and bis(2-(2-(4-(2-(4-pyridyl)ethenyl)phenoxy)ethoxy)ethyl) ether (6), maintaining a 1:1 donor/acceptor group stoichiometry. All individual components are nonmesogenic. Self-assembly of these multifunctional compounds results in the formation of liquid-crystalline network structures. For example, the H-bonded complex 1/5 shows a smectic A phase from 176 to 156 °C, while 2/5 exhibits a nematic phase between 200 and 87 °C on cooling. This behavior is attributed to the dynamics of the hydrogen bonds. These results suggest that the trifunctional H-bond donors adopt linear conformations that induce calamitic mesomorphic behavior with bifunctional H-bond acceptors.

Introduction A wide variety of self-organized molecular systems, such as liquid crystal, have attracted much attention because it has great potential as highly functional materials.1,2 Recently, new types of liquid-crystalline materials have been obtained by self-assembly through specific molecular interactions.3-12 The molecular association of two or more molecular species by intermolecular interactions results in the formation of mesomorphic molecular complexes. Such material design for liquid crystals based on noncovalent interaction is related to supramolecular chemistry, an area of great current interest.13-15 Supramolecular hydrogen-bonded liquid-crystalline materials have been prepared from †

Institute of Industrial Science, The University of Tokyo. Department of Chemistry, Cornell University. Abstract published in Advance ACS Abstracts, March 1, 1996. (1) (a) Gray, G. W., Ed. Thermotropic Liquid Crystals; Wiley: Chichestor, 1987. (b) Gray, G. W.; Goodby, J. W. Smectic Liquid Crystals; Leonard Hill: Glasgow, 1984. (2) Goodby, J. W.; Blinc, R; Clark, N. A.; Lagerwall, S. T.; Osipov, M. A.; Pinkin, S. A.; Sakurai, T.; Yoshino, K.; Zeks, B. Ferroelectric Liquid Crystals, Principals, Properties, and Applications; Gordon and Breach: Philadelphia, 1991. (3) Kato, T.; Fre´chet, J. M. J. Macromol. Symp. 1995, 98, 311. (4) Kato,T.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1989, 111, 8533. (5) Kato, T.; Fre´chet, J. M. J. Macromolecules 1989, 22, 3818. (6) Lehn, J.-M. Makromol. Chem., Macromol. Symp. 1993, 69,1. (7) Brienne, M.-J.; Gabard, J.; Lehn, J.-M.; Stibor, I. J. Chem. Soc., Chem. Commun. 1989, 1868. (8) Kato, T.; Nakano, M.; Moteki, T.; Uryu, T.; Ujiie, S. Macromolecules 1995, 28, 8875. (9) Kato, T.; Kubota, Y.; Nakano, M.; Uryu, T. Chem. Lett. 1995, 1127. (10) Ringsdorf, H.; Schlarb, B.; Venzuer, J. Angew. Chem., Int. Ed. Engl. 1988, 27, 113. (11) Ujiie, S.; Iimura, K. Macromolecules 1992, 25, 3174. (12) Percec, V.; Johansson, G.; Rodenhouse, R. Macromolecules 1992, 25, 2563. ‡

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complementary components.3-9 A great variety of Hbonded liquid crystals with unique structural features have been reported.3-9,16-35 One advantage of a molecular self-assembly process through hydrogen bonding (13) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304. (14) Whitesides, G. M.; Mathias J. P.; Seto, C. T. Science 1991, 254, 1312. (15) Philp, D.; Stoddart, J. F. Synlett 1991, 445. (16) Kato, T.; Fre´chet, J. M. J.; Wilson, P. G.; Saito, T.; Uryu, T.; Fujishima, A.; Jin, C.; Kaneuchi, F. Chem. Mater. 1993, 5, 1094. (17) Kato, T.; Wilson, P. G.; Fujishima, A.; Fre´chet, J. M. J. Chem. Lett. 1990, 2003. (18) Kato, T.; Fukumasa, M.; Fre´chet, J. M. J. Chem. Mater. 1995, 7, 368. (19) Fukumasa, M.; Kato, T.; Uryu, T.; Fre´chet, J. M. J. Chem. Lett. 1993, 65. (20) Kato, T.; Fujishima, A.; Fre´chet, J. M. J. Chem. Lett. 1990, 919. (21) Kato, T.; Adachi, H.; Fujishima, A.; Fre´chet, J. M. J. Chem. Lett. 1992, 265. (22) Kato, T.; Uryu, T.; Kaneuchi, F.; Jin, C.; Fre´chet, J. M. J. Liq. Cryst. 1993, 14, 1311. (23) Kato, T.; Kihara, H.; Uryu, T.; Ujiie, S.; Iimura, K.; Fre´chet, J. M. J.; Kumar, U. Ferroelectrics 1993, 148, 161. (24) Kumar, U.; Fre´chet, J. M. J.; Kato, T.; Ujiie, S.; Iimura, K. Angew. Chem., Int. Ed. Engl. 1992, 31, 1531. (25) Kumar, U.; Kato, T.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1992, 114, 6630. (26) Kato, T.; Kihara, H.; Uryu, T.; Fujishima, A.; Fre´chet, J. M. J. Macromolecules 1992, 25, 6836. (27) Kato, T.; Kihara, H.; Kumar, U.; Uryu, T.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. Engl. 1994, 33, 1644. (28) Fouquey, C.; Lehn, J.-M.; Levelut, A.-M. Adv. Mater. 1990, 2, 254. (29) Bladon, P.; Griffin, A. C. Macromolecules 1993, 26, 6604. (30) Alexander, C.; Jariwala, C. P.; Lee, C.-M.; Griffin, A. C. Macromol. Symp. 1994, 77, 283. (31) Wilson, L. M. Macromolecules 1994, 27, 6683. (32) Bruce, D. W.; Price, D. J. Adv. Mater. Opt. Electron. 1994, 4, 273. (33) Willis, K.; Price, D. J.; Adams, H.; Ungar, G.; Bruce, D. W. J. Mater. Chem. 1995, 5, 2195. (34) Koga, T.; Ohba, H.; Takase, A.; Sakagami, S. Chem. Lett. 1994, 2071.

© 1996 American Chemical Society

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Chem. Mater., Vol. 8, No. 4, 1996 Chart 1

is that complex structures such as three- or twodimensional networks and one-dimensional polymers can be obtained simply and spontaneously.27-31 A liquid-crystalline polymer network has been prepared by molecular self-assembly of a polyacrylate containing a side-chain benzoic acid moiety and bipyridine.27 Despite its cross-linked nature, this self-assembled network shows smectic phases and reversible phase transitions due to the dynamic nature of its hydrogen bonds. Main-chain mesogenic polymers obtained by the H bonds between chiral bifunctional molecules form helical structures.28 Ferroelectric molecular arrangement has also been achieved for side-chain polymeric complexes by the formation of hydrogen bonding between functionalized polysiloxanes and a nonmesogenic chiral stilbazole.24 In this paper, we report the induction of liquid crystallinity for H-bonded networks obtained by a selfassembly process involving multifunctional H-bonding donor and acceptor molecules shown in Chart 1. The dynamic nature of mesogenic networks and the control of mesogenic structures are also discussed.

Kihara et al. Scheme 1

Scheme 2

Experimental Section Trifunctional Hydrogen-Bond Donors 1 and 2. The syntheses of trifunctional hydrogen bond donors 1 and 2 are presented in Schemes 1 and 2, respectively. Ethyl 4-(2-(2-Hydroxyethoxy)ethoxy)benzoate (7). A mixture of 2-(2-chloroethoxy)ethanol (0.18 mol), ethyl 4-hydroxybenzoate (0.12 mol), and K2CO3 (0.24 mol) in DMF (50 mL) was stirred at 120 °C under a N2 atmosphere for 10 h. The reaction mixture was poured into water and extracted with chloroform. The organic layer was washed with water and dried over MgSO4. The crude product was purified by column chromatography (silica gel, eluent hexane/EtOAc ) 1/1) to obtain a colorless oil of ethyl 4-(2-(2-hydroxyethoxy)ethoxy)benzoate (7), yield 88%. 1H NMR for 7 (CDCl , 27 °C, ppm) δ 7.99 (d, 2 ArH ortho to 3 -COOC2H5, J ) 9.0 Hz), 6.93 (d, 2 ArH meta to -COOC2H5, J ) 9.0 Hz), 4.34 (q, 2H, -COOCH2CH3, J ) 7.1 Hz), 4.18 (t, 2H, -CH2CH2OPh-, J ) 4.6 Hz), 3.88 (t, 2H, HOCH2CH2-, (35) Malik, S.; Dhal, P. K.; Mashelkar, R. A. Macromolecules 1995, 28, 2159.

J ) 4.6 Hz), 3.76-3.66 (m, 4H, HOCH2CH2OCH2CH2-), 2.56 (s, 1H, HO-), 1.38 (t, 3H, -COOCH2CH3, J ) 7.1 Hz). Ethyl 4-(2-(2-(Tosyloxy)ethoxy)ethoxy)benzoate (8). To a solution of CH2Cl2 (100 mL) containing 7 (96.3 mmol) and pyridine (15.4 mL) was added dropwise 4-toluenesulfonyl chloride (116 mmol) dissolved in CH2Cl2 under a N2 atmosphere at 0 °C. The reaction mixture was stirred at 25 °C for 24 h. The mixture was then washed with dilute HCl and brine successively. The organic layer was dried over MgSO4 and filtered. The crude product was purified by column chromatography (silica gel, eluent hexane/EtOAc ) 2/1) to obtain a white solid of ethyl 4-(2-(2-(tosyloxy)ethoxy)ethoxy)benzoate

Supramolecular Liquid-Crystalline Networks (8), yield 64%. 1 H NMR for 8 (CDCl3, 27 °C, ppm) δ 7.99 (d, 2 ArH ortho to -COOC2H5, J ) 9.0 Hz), 7.79 (d, 2 ArH ortho to -SO2-, J ) 8.2 Hz), 7.30 (d, 2 ArH meta to -SO2-, J ) 8.2 Hz), 6.90 (d, 2 ArH meta to -COOC2H5, J ) 9.0 Hz), 4.35 (q, 2H, -COOCH2CH3, J ) 7.2 Hz), 4.20 (t, 2H, -CH2CH2OPh-, J ) 4.6 Hz), 4.10 (t, 2H, -CH2CH2OSO2-, J ) 4.6 Hz), 3.83-3.75 (m, 4H, -CH2CH2OCH2CH2-), 2.41 (s, 3H, CH3-), 1.38 (t, 3H, -COOCH2CH3, J ) 7.2 Hz). 1,3,5-Tris(2-(2-(4-(ethoxycarbonyl)phenoxy)ethoxy)ethoxy)benzene (9). A mixture of tosylate 8 (24.5 mmol), phloroglucinol (8.2 mmol), Cs2CO3 (49 mmol), and DMF (50 mL) was stirred at 25 °C under a N2 atmosphere for 2 days. After reaction, DMF was removed and the mixture was extracted with chloroform. The organic layer was washed with brine and dried over MgSO4. The crude product was purified by chromatography on a silica gel column using hexane/EtOAc (1/1) as eluent, and dried in vacuo to obtain a white solid of 1,3,5-tris(2-(2-(4-(ethoxycarbonyl)phenoxy)ethoxy)ethoxy)benzene (9), yield 28%. 1H NMR for 9 (CDCl , 27 °C, ppm) δ 7.98 (d, 6 ArH ortho to 3 -COOC2H5, J ) 8.9 Hz), 6.93 (d, 6 ArH meta to -COOC2H5, J ) 8.9 Hz), 6.11 (s, 3 ArH, 1,3,5-substituted benzene), 4.34 (q, 6H, -COOCH2CH3, J ) 7.1 Hz), 4.20 (t, 6H, -CH2OPhCOOC2H5, J ) 4.8 Hz), 4.08 (t, 6H, -CH2CH2OCH2CH2OPhCOOC2H5, J ) 4.6 Hz), 3.95-3.88 (m, 12H, -CH2CH2OCH2CH2-), 1.37 (t, 9H, -COOCH2CH3, J ) 7.1 Hz). Trifunctional Hydrogen-Bond Donor 1. To a stirred solution of 9 (2.2 mmol) in 40 mL of DMSO, maintained under N2, was added dropwise 20 mL of 20% aqueous NaOH. The solution was heated at 120 °C for 4 h. After cooling, the mixture was acidified with dilute HCl. The precipitate was filtered and recrystallized from EtOH to obtain a white crystal of 1,3,5-tris(2-(2-(4-carboxyphenoxy)ethoxy)ethoxy)benzene (1), yield 70%. 1H NMR for 1 (DMSO-d , 27 °C, ppm) δ 7.86 (d, 6 ArH ortho 6 to -COOH, J ) 8.1 Hz), 7.01 (d, 6 ArH meta to -COOH, J ) 8.1 Hz), 6.08 (s, 3 ArH, 1,3,5-substituted benzene), 4.17 (m, 6H, -CH2OPhCOOH), 4.03 (m, 6H, -CH2CH2OCH2CH2OPhCOOH), 3.79 (m, 12H, -CH2CH2OCH2CH2-). 13C NMR (DMSO-d6, 27 °C, ppm) δ 167.0 (-COOH), 162.1, 131.3, 123.0, 114.3 (ArC, -C6H4COOH), 160.3, 93.9 (ArC, 1,3,5-substituted benzene), 69.0, 68.9 (-OCH2CH2OCH2CH2O-), 67.4 (-CH2OPhCOOH), 67.2 (-CH2CH2OCH2CH2OPhCOOH). Ethyl 3,4-Bis(2-(2-(4-(ethoxycarbonyl)phenoxy)ethoxy)ethoxy)benzoate (10). Ethyl 3,4-bis(2-(2-(4-(ethoxycarbonyl)phenoxy)ethoxy)ethoxy)benzoate (10) was synthesized from tosylate 8 (36.7 mmol), ethyl 3,4-dihydroxybenzoate (18.4 mmol), and Cs2CO3 (55.1 mmol) by the same method described for the preparation of 9, yield 87%. 1H NMR for 10 (CDCl , 27 °C, ppm) δ 7.97 (d, 4 ArH, H-4, 3 J ) 8.8 Hz), 7.67 (d, 1 ArH, H-1, J ) 8.5 Hz), 7.59 (s, 1 ArH, H-3), 6.91 (d, 4 ArH, H-5, J ) 8.8 Hz), 6.90 (d, 1 ArH, H-2, J ) 8.5 Hz), 4.34 (q, 6H, -COOCH2CH3, J ) 7.1 Hz), 4.26-4.14 (m, 8H, -CH2CH2OAr), 3.95 (m, 8H, -CH2CH2OCH2CH2-) 1.38 (t, 9H, -COOCH2CH3, J ) 7.1 Hz). Trifunctional Hydrogen-Bond Donor 2. Ethyl ester 10 was hydrolyzed by the same method described for the synthesis of 1 to obtain a white crystal of 3,4-bis(2-(2-(4-carboxyphenoxy)ethoxy)ethoxy)benzoic acid (2), yield 73%. 1H NMR for 2 (DMSO-d , 27 °C, ppm) δ 7.85 (d, 4 ArH, H-4, 6 J ) 8.4 Hz), 7.53 (d, 1 ArH, H-1, J ) 8.5 Hz), 7.47 (s, 1 ArH, H-3), 7.06 (d, 1 ArH, H-2, J ) 8.5 Hz), 6.97 (d, 4 ArH, H-5, J ) 8.4 Hz), 4.14 (m, 8H, -CH2CH2OAr), 3.82 (m, 8H, -CH2CH2OCH2CH2-). 13C NMR (DMSO-d6, 27 °C, ppm) δ 167.2 (-COOH), 162.1, 131.4, 123.1, 114.3 (ArC, -C6H4COOH), 152.3, 147.7, 123.6, 123.3, 114.4, 112.8 (ArC, 3,4-substituted benzoic acid), 69.0 (-OCH2CH2OCH2CH2O-), 68.5, 68.3, 67.5 (-OCH2CH2OAr). Bifunctional Hydrogen-Bond Acceptor 6. The synthetic route of bifunctional H-bond acceptor 6 is shown in Scheme 3. Tetraethylene glycol dichloride was prepared by refluxing equimolar proportions of tetraethylene glycol and SOCl2 in benzene in the presence of pyridine. The product was purified by distillation at 137 °C under 4 mmHg. trans-4-Hydroxy-4′stilbazole was prepared by the procedure described in a

Chem. Mater., Vol. 8, No. 4, 1996 963 Scheme 3

Table 1. Thermal Properties of Multifunctional H-Bonding Molecules phase transitionsa

H-bond molecules 1 2 3 4 5 6 a

K K K K K K

175 237 112 111 152 117

I I I I I I

Transition temperatures (°C). K, crystalline; I, isotropic.

previous paper.4,25 A mixture of trans-4-hydroxy-4′-stilbazole (10.1 mmol), tetraethylene glycol dichloride (5.07 mmol), and K2CO3 (20.3 mmol) in DMF was stirred at 120 °C under N2 for 4 h. After cooling, DMF was removed and the mixture was extracted with chloroform. The organic layer was washed with brine and then dried over MgSO4. The crude product was purified by column chromatography (silica gel, CHCl3:MeOH ) 20:1) and recrystallized from CHCl3/hexane to obtain pale yellow crystals of bis(2-(2-(4-(2-(4-pyridyl)ethenyl)phenoxy)ethoxy)ethyl) ether (6), yield 22%. 1H NMR for 6 (CDCl , 27 °C, ppm) δ 8.54 (d, 4 ArH ortho to 3 N, J ) 5.9 Hz), 7.45 (d, 4 ArH meta to -OCH2CH2O-, J ) 8.6 Hz), 7.31 (d, 4 ArH meta to N, J ) 5.9 Hz), 7.23 (d, 2H, -CH)CH-, J ) 16.4 Hz), 6.92 (d, 4 ArH ortho to -OCH2CH2O-, J ) 8.6 Hz), 6.85 (d, 2H, -CH)CH-, J ) 16.4 Hz), 4.15 (t, 4H, -CH2CH2OPh-, J ) 4.8 Hz), 3.87 (t, 4H, -CH2CH2OPh-, J ) 4.8 Hz), 3.75-3.65 (m, 8H, -PhOCH2CH2OCH2CH2O-). 13C NMR (CDCl3, 27 °C, ppm) δ 159.4, 129.0, 128.4, 114.9 (phenyl), 150.1, 144.9, 120.6 (pyridyl), 132.6, 123.7 (-CH)CH-), 70.8, 70.7 (-PhOCH2CH2OCH2CH2O-), 67.5 (-CH2CH2OPh-), 69.6 (-CH2CH2OPh-). Preparation of Hydrogen-Bonded Complexes. Hydrogen-bonded complexes were prepared by the evaporation technique described in our previous papers.3-5 The pyridine solution containing stoichiometric amount of H-bond donor and acceptor moieties was evaporated under reduced pressure. The resulting solid was dried in vacuo for 24 h. Characterization. 1H and 13C NMR spectra were obtained with a JEOL GX JNM270 FT NMR spectrometer. DSC measurements were performed with Mettler DSC 30. Heating and cooling rates were 10 °C/min. Transition temperatures were taken at the maximum of transition peaks. A polarizing microscope Olympus BH2 equipped with a Mettler FP82HT hot stage was used for visual observation. Infrared measurements were conducted with a Perkin-Elmer 1600 series FTIR spectrometer. The molecular modeling was performed with Chem3D software produced by Cambridge Scientific Computing, Inc.

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Table 2. Thermal Behavior of H-Bonded Complexes Built from Trifunctional H-Bond Donor and Bifunctional H-Bond Acceptor phase transition behaviora H-bonded complexes 1/3 1/4 1/5 1/6 2/3 2/4 2/5 2/6

heating

G K

132 153 (23.3)

G or Kb G or Kb K SA

181 174 184 (49.0) 166 (3.0)

cooling K K K K N N N N

203 (78.5) 168 (93.1) 194 (77.8) 164 (77.8) 197 (27.0) 183 (21.0) 202 (12.3) 179 (9.4)

I I I I I I I I

I I I I I I I I

143 (22.4) 152 (22.4) 176 (19.5) 152 (13.5) 181 (14.4) 168 (21.6) 200 (16.2) 177 (10.4)

K SA SA SA N N N N

141 (31.0) 156 (52.3) 141 (5.2) 153 (1.4) 54 87 165 (4.2)

K K SX SA G G SA

90 (6.9) 72

K G

120 (26.7)

K

a

Transition temperatures (°C) and enthalpies of transitions (J/g, in parentheses). G, glassy; K, crystalline; S, smectic; N, nematic; I, isotropic. b Crystallization behavior of the glassy phase on heating is dependent on heating condititions.

Figure 1. DSC thermograms of H-bonded complex 1/5 on (A) heating scan and (B) cooling scan.

Results and Discussion The structures of the multifunctional H-bonding molecules used in the present study are shown in Chart 1. Compound 1 is a 1,3,5-substituted benzene containing three benzoic acid moieties. Compound 2 is a benzoic acid with two benzoic acid moieties as substituents in its 3 and 4 positions. Bipyridyl molecules of 3-6 were selected as the bifunctional H-bonding acceptor molecules. These bipyridines have previously been reported to form low molecular weight supramolecular 1:2 mesogenic complexes with 4-alkoxybenzoic acids.16,17 In this case, the bipyridine functions as a part of the central core unit of the H-bonded mesogen. All of the compounds used in this study are nonmesogenic and their melting temperatures are given in Table 1. Hydrogen-bonded complexes were prepared from the trifunctional H-bonding donors and the bifunctional H-bonding acceptors, maintaining the 1:1 stoichiometry of carboxylic acid and pyridine moieties. It was expected that intermolecular mesogenic structures were formed by complexation of these molecules. The transition temperatures of these complexes are summarized in Table 2. With the exception of complex 1/3, all of the complexes exhibit liquid crystallinity. These mesomorphic properties can be attributed to the formation of the mesogenic network structure. In all cases, calamitic mesophases are observed for the complexes. Complexes 1/4, 1/5, and 1/6 exhibit monotropic smectic phases. The DSC thermogram of complex 1/5 is shown in Figure 1. Slow crystallization is observed on cooling, while the crystal melts sharply into an isotropic liquid on heating. Complexes based on 2 exhibit enantiotropic behavior. Figure 2 shows the DSC curves of heating and cooling runs for the complex 2/5. Upon the first heating,

Figure 2. DSC thermograms of H-bonded complex 2/5 on repeated heating and cooling scans.

crystal-nematic and nematic-isotropic transitions are observed at 180 and 202 °C, respectively. The isotropic liquid of 2/5 changes to a nematic phase at 200 °C and the glassy phase is subsequently formed at 87 °C on the first cooling. Upon the second heating, the crystallization behavior of the glassy samples of 2/5 is seen. This crystallization temperature is dependent on the heating conditions. The crystalline-nematic, nematic-isotropic, and isotropic-nematic transitions on the repeated heating and cooling runs are reversibly observed for these complexes. This observation suggests the reversible formation of the H-bonding in the transition between ordered and disordered states.27 Polarizing photomicrographs of complexes 1/5 and 2/5 are shown in Figure 3. A focal conic fan texture characteristic of a smectic A phase is observed for complex 1/5 at 170 °C on cooling (Figure 3A). In contrast, a typical schlieren texture characteristic of a nematic phase is seen for complex 2/5 at 180 °C on cooling (Figure 3B). As can be seen in Figure 3C, the schlieren texture is maintained for a glassy sample of complex 2/5 at 50 °C on cooling, however, fluidity is no longer observed and wrinkles appear on the texture at this temperature. These results suggest that the nematic structure of complex 2/5 become frozen below the glass transition temperature on cooling.

Supramolecular Liquid-Crystalline Networks

Chem. Mater., Vol. 8, No. 4, 1996 965

Figure 4. Infrared spectra of 1 (A) and H-bonded complex 1/5 (B).

Chart 2

Figure 3. Photomicrographs of the liquid-crystalline state of H-bonded complexes: 1/5 at 170 °C on cooling (A), 2/5 at 180 °C on cooling (B), and nematic glassy state of 2/5 at 50 °C on cooling (C).

Infrared spectra of 1 and complex 1/5 are given in Figure 4A,B, respectively. For the single component of 1, the stretching band of the hydroxyl group involving the H-bonded dimer, and the Fermi resonances are observed at 3000, 2667, and 2554 cm-1, respectively (Figure 4A).22,36 Once the complex between 1 and 5 is formed, the O-H stretching band and its Fermi resonance appear at 2482 and 1924 cm-1, respectively, and no peak due to the dimerization of benzoic acid moieties is seen (Figure 4B). Moreover, the carbonyl band of 1 observed at 1684 cm-1 shifts to 1691 cm-1 after complexation due to the formation of the hydrogen bond between carboxylic acid and pyridine. These results support that the H-bonded network structures have been obtained by the dominant formation of the thermodynamically favored intermolecular hydrogen bonds between the individual components.16,22 (36) Kato, T.; Jin, C.; Kaneuchi, F.; Uryu, T. Bull. Chem. Soc. Jpn. 1993, 66, 3581.

It is interesting that stable liquid-crystalline behavior is induced for H-bonded networks built from multifunctional components. This is in sharp contrast to network structures built only by covalent bonds, which are known to become insoluble and infusible as is a case for a melamine resin. In our self-assembled mesogenic networks, dynamics of the hydrogen bond allow the formation of fluid liquid-crystalline phases as interchange reactions between H-bonded sites remain possible and therefore contribute to the flexibility of the system. To examine the effect of the bifunctional moieties that function as cross-linkers on thermal properties for H-bonded network complexes, we have prepared complexes consisting of a trifunctional H-bond donor and a mixture of bifunctional and monofunctional H-bond acceptors. trans-4-Methoxy-4′-stilbazole (11)26,37 was used as the monofunctional H-bond acceptor in this study (Chart 2). For a ternary complex, a mixture of x mol of bifunctional and y mol of monofunctional H-bond acceptors was complexed with (2x + y)/3 mol of a trifunctional H-bond donor to maintain stoichiometry of donor/acceptor moieties. The thermal properties of complexes consisting of 1, 5, and 11 are shown in Table 3, and the ternary phase diagram of the complexes measured on cooling is illustrated in Figure 5, in which the letter z equals 2x/ (2x + y). With increasing z, the proportion of 5 increases in the mixture of H-bond acceptor molecules. As shown in Figure 5, the complex with z ) 0 exhibits both (37) Bruce, D. W.; Dunmur, D. A.; Lalinde, E.; Maitlis, P. M.; Styring, P. Liq. Cryst. 1988, 3, 385.

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Table 3. Thermal Properties of H-Bonded Complexes Prepared from 1, 5, and 11 phase transitionsa heating

z 0 0.2 0.33 0.5 0.67 0.8 1.0

K

91 (24.0)

K K K

124 (21.5) 88 (17.9) 87 (16.6)

SA SA SA G G G

cooling

148 (11.3) 153 (2.9) 160 (5.8) 89 94 162

N N N SA SA K K

159 (2.2) 164 (6.1) 166 (7.7) 169 (15.9) 173 (14.1) 187 (39.5) 194 (77.8)

I I I I I I I

I I I I I I I

157 (4.0) 163 (5.3) 165 (6.5) 166 (15.4) 171 (17.2) 174 (18.3) 176 (19.5)

N N N SA SA SA SA

133 (0.6) 152 (2.4) 159 (4.8) 88 92 92 156 (52.3)

SA SA SA G G G K

75 (21.9) 78 (14.9) 82 (15.5)

K K K

a Transition temperatures (°C) and enthalpies of transitions (J/g, in parentheses). I, isotropic; N, nematic; S , smectic A; K, crystalline; A G, glassy.

Table 4. Thermal Properties of H-Bonded Complexes Prepared from 2, 5, and 11 phase transitionsa heating

z 0 0.2 0.33 0.5 0.67 0.8 1.0 b

G

56

G G G G

67 77 81 132

K G G SA SA SA K

142 (56.3) 55 52 90 (-)b 123 (0.5) 126 (0.8) 184 (49.0)

cooling N N N N N N N

181 (6.5) 186 (10.1) 186 (8.1) 189 (10.6) 192 (11.5) 195 (13.4) 202 (12.3)

I I I I I I I

I I I I I I I

179 (7.6) 185 (9.9) 184 (8.6) 187 (9.8) 191 (11.5) 192 (12.5) 200 (16.2)

N N N N N N N

53 53 49 80 (-)b 122 (0.5) 122 (1.2) 87

G G G SA SA SA G

64 74 73

G G G

a Transition temperatures (°C) and enthalpies of transitions (J/g, in parentheses). I, isotropic; N, nematic; S , smectic A; G, glassy. A Not detected by DSC.

Figure 5. Phase diagram of H-bonded complexes of 1 and mixtures of 5 and 11.

nematic and smectic A phases. The temperature range of the nematic phase decreases as z increases, and only a smectic A phase is observed when z exceeds 0.5. The enthalpy of transition from isotropic to mesophase increases with increasing z as shown in Table 3. These results suggest that the bifunctional H-bond acceptor molecule, which acts as cross-linker, stabilizes the smectic layer and the mesophase of the complexes. The complexes with z ) 0.2 and 0.33 show a crystalline phase below a smectic A phase though they are a ternary system. Melting endothermic transitions to liquid-crystalline phases are clearly seen in the DSC thermograms for these complexes. Wide-angle X-ray diffraction patterns have been obtained for the complexes with z ) 0.2 and z ) 0.33 at 25 °C. Sharp reflections are observed at wide angles, which suggests that crystallization occurs for this ternary complex. For

Figure 6. Phase diagram of H-bonded complexes of 2 and mixtures of 5 and 11.

example, the complex with z ) 0.33 shows sharp peaks at 5.16, 4.54, 4.33, 4.19, 3.92, and 3.09 Å. Table 4 gives the thermal properties of the complex consisting of 2, 5, and 11, and Figure 6 shows the ternary phase diagram of the complexes measured on cooling. A nematic phase is dominant in the phase diagram of the complexes containing 2. The enthalpy of transition from isotropic to mesophase also increases with z. The clearing temperatures do not significantly increase through the cross-linking density, that is z increases. Similar tendency was also observed for another H-bonded network system based on a side-chain polymer.27 Dynamic nature such as fast exchange of the hydrogen bonds may reduce the effect of the noncovalent cross-linking on the transition temperatures. It is noteworthy that the H-bonded networks based on 1 show only smectic phases while a nematic phase

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Figure 7. Molecular modeling of 1. Figure 9. Molecular modeling of 2.

Figure 8. Schematic illustration of smectic network formed by supramolecular self-assembly of 1 and a bifunctional H-bond acceptor.

is dominant for the networks derived from 2 as can be seen in Table 1. The trifunctional H-bond donor compounds 1 and 2 must adopt a linear geometry38 when complexed with the H-bond acceptor because the Hbonded complexes exhibit calamitic behavior. Figure 7 shows a molecular model for a possible conformation of 1. One of the spacer of 1 can lie parallel to the other as 1 is allowed to lower its free volume. This molecular model agrees with the result that the H-bonded complexes consisting of 1 and bifunctional H-bond acceptors have a tendency to exhibit smectic phases, as shown in (38) Attard, G. S.; Douglass, A. G.; Imrie, C. T.; Taylor, L. Liq. Cryst. 1992, 11, 779.

Figure 10. Schematic illustration of nematic network formed by supramolecular self-assembly of 2 and a bifunctional H-bond acceptor.

Figure 8. However, the illustration for the network, which is perfectly fixed and spread to a large extent, is an ideal structure and a few defects might exist. A molecular model for a possible conformation for 2 is illustrated in Figure 9. If the two spacers of 2 stretch in opposite directions, the three benzoic acid moieties are able to align, thereby lowering the free volume. The induction of nematic phases for H-bonded complexes consisting of 2 and bifunctional H-bond acceptor compounds can be explained when 2 forms such a plausible conformation as shown in Figure 10. The phase behav-

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ior can be controlled by the conformation of the individual multifunctional components for liquid-crystalline networks because H bonds fix the position of the mesogenic cores even in the mesomorphic state. Mesogenic molecular networks, which form a new structure for liquid crystals, have been obtained through a self-assembly process involving small molecular components. Moreover, the dynamic nature of hydrogen bonding contributes to the induction, stabilization, and reversibility of the mesomorphic states for the networks,

Kihara et al.

which is not observed for fully covalently cross-linked system. These results suggest the versatility of the use of noncovalent interactions for the design of selforganized functional materials. Acknowledgment. Financial support of the Asahi Glass Foundation is gratefully acknowledged. CM9505456