1094 Chem. Mater. 1993,5, 1094-1100 Hydrogen-Bonded Liquid Crystals. Novel Mesogens Incorporating Nonmesogenic Bipyridyl Compounds through Complexation between H-Bond Donor and Acceptor Moieties Takashi Kato,**t Jean M. J. Fr&het,'P* Paul G. Wilson,$Takeshi Saito,+ Toshiyuki Uryu,t Akira Fujishima,Q Chihiro Jin,l and Fumiko KaneuchiJ- Institute of Industrial Science, The University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan, Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, Department of Synthetic Chemistry, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan, and Japan Spectroscopic Co., Ltd., Hachioji, Tokyo 192, Japan Received February 11, 1993. Revised Manuscript Received May 18,1993 Mesogenic structures have been built from 2:l (molar ratio) mixtures of 4-alkoxy- or 4-alkylbenzoic acid (nOBA or nBA; n is the carbon number of the alkyl chain) and 4,4'-bipyridine (BPy) or trans-l,2-bis(4-pyridyl)ethylene (BPyE). In these complexes, the benzoicacid derivative functions as an H-bond donor and the bipyridyl compound operates as a bifunctional H-bond acceptor. Well-defined structures of the mesogenic complexes are formed from independent and different molecules. These complexes exhibit stable mesophases that are not observed for each of the single components. For example, a 2:l (molar ratio) complex prepared from 4-ethoxybenzoic acid (20BA) and 4,4'-bipyridine (BPy) shows a nematic phase from 150 to 169 "C while both individual compounds are nonmesogenic. The liquid-crystalline phase is induced by the hydrogen bonding. The 2:l complex of 4-butoxybenzoic acid (40BA) and trans-l,2- bis(4-pyridy1)ethylene (BPyE) exhibits a smectic A phase (146-168 "C) and a nematic phase (168-177 "C). The smectic phase displayed by the complex is not observed for 40BA and BPyE. The isotropization temperature is increased by the complexation through the H bonds. The effect of the terminal alkyl chain length on thermal properties has been examined for the 2:l H-bonded complexes of a series of the benzoic acids and the bipyridyl compound. The type of mesophases obtained is affected by the alkyl chain length. Infrared study suggests that the hydrogen bond is an un-ionic type with a double minimum potential energy and ita stability is greatly dependent on the order of the molecular complexes. Phase diagrams have been obtained for the binary mixture of H-bond donor and acceptor moieties. The isotropization temperature curves show significant positive deviations because of the intermolecular H-bond interaction. Introduction Hydrogen bonding is one of the key interactions for chemical and biological processes in nature. For molecular aggregates, hydrogen bonding plays an important role in the association of molecules. In liquid crystals, meso- morphism results from the proper combination of mol- ecular interactions and the shape of molecules. Dipole- dipole interactions have long been taken into consideration in the design of liquid-crystalline molecules.'P2 In contrast, and with few exceptions, intermolecular hydrogen bonds, that are stronger than dipole-dipole interactions had been considered to be deleterious for thermotropic liquid crystallinity in the past because in many cases they cause molecular associations that raise melting temperatures or destroy molecular order of the mesophase. Aromatic acid derivatives with alkoxy or alkyl terminal groups are known to show mesomorphism due to the t Institute of Industrial Science, The University of Tokyo. * Department of Chemistry, Cornell University. 1 Department of Synthetic Chemistry, The University of Tokyo. * Japan Spectroecopic Co. Ltd. (1) Gray, G. W., Ed. Thermotropic Liquid Crystals; Wiley: Chich- ester: 1987. Hik Glasgow, 1984. (2) Gray, G. W.; Goodby, J. W. Smectic Liquid Crystals; Leonard dimerization of their carboxvlic acid ~OUDS through hydrogen b~nding.~ A few mbnosacch&ide;- bearing long alkyl chains and some aliphatic compound^^-^ that contain several hydroxyl groups exhibit thermotropic liquid crystallinity. Diisobutylsilanediol exhibits a co- lumnar mesophase.'O In these cases, the intermolecular hydrogen bonds between hydroxyl groups induce meso- morphism. A series of aromatic-aliphatic polyesteramides shows thermotropicliquid crystallinitythrough the lateral H-bonded interaction between amide linkages.'l The lateral H-bond interaction is also effectivefor the molecular arrangementof a smectic C phase for tropone compounds.12 ~ (3) (a)Bradfield, A. E.; Jones, B. J. Chem. SOC. 1929,2660. (b) Jones, B. J. Chem. SOC. 1935,1874. (c) Gray, G. W.; Jones, B. J. Chem. SOC. 1963,4179. (d) Weygand, C.;Gabler, R. Z. Phys. Chem. 1940, B46,270. (4) Noller, C. R.; Rockwell, W. C. J. Am. Chem. SOC. 1988,60,2076. (5) (a)Jeffrey, G. A. Acc. 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