Synthesis And Characterization Of Novel Supramolecular Liquid Crystals Through Intermolecular Hydrogen-bonding

Hydrogen bonding is one of the key interactions in the process of molecular aggregation and recognition in nature and it can be used to design and assemble supramolecular architectures. Supramolecular liquid crystalline materials through intermolecular hydrogen–bonding have attracted much attention because they are good candidates for the next generation of materials, for which dynamic function, environmental benignity, and low energy processing are required.The present paper deals with the synthesis and the thermotropic liquid crystalline behaviour of supramolecular liquid crystalline materials containing the p-methoxyazobenzene mesogenic group through intermolecular hydrogen–bonding. The main results are as follows:1.1 Side chain liquid crystalline pesudopolymers via intermolecular double hydrogen–bonding (open type).Novel hydrazide modified p-methoxyazobenzene derivatives, namely 4-{n-[4-(4-methoxy-phenylazo)-phenoxy]- alkoxy }- benzoic acid hydrazide (Dn, n=3,6,10), were designed and synthesized. The FTIR spectra of Dn were examined in the range of 3500-1000cm-1 at different temperatures. The results showed that the stretching vibration frequencies of the -NH and -C=O groups shifted to higher frequency with the increase of temperature, while the -NH2 bending, combination of -C-N-H shifted slightly to lower frequency. Intermolecular hydrogen bonding between -NH, -NH2 and -C=O groups is proposed and the schematic supramolecular structure of Dn is given. This may explain the higher Dn transitional temperatures compared to that of 4-{n-[4-(4-methoxy-phenylazo)-phenoxy]- alkoxy }-benzoic ethyl ester (Cn, n=3,6,10).1.2 Liquid crystalline dimers through intermolecular hydrogen-bonding between –C=O and –OH groups (closed type).A novel type of liquid crystals, namely 4-{n-[4-(4-methoxy-phenylazo)-phenoxy]- alkoxy }-benzoic acid (En, n=3,6,10), was synthesized and characterized. The FTIR spectra of En in the range of 2800 cm-1-900cm-1 at different temperatures were examined and the results showed that the dimers were formed through double hydrogen bonding between -C=O and –OH groups. The thermal behavior of En was studied by DSC, WAXD and POM and the results showed that E6 and E10 were enantiotropic liquid crystals and E3 was monotropic nematic one. Higher En transitional temperatures and high order smectic (E10) or nematic (E3 and E6) phase behavior compared with that of Cn were attributed to the hydrogen-bonded dimerization.1.3 Supramolecular liquid crystalline complexes based on single hydrogen-bonding between benzoic acid derivative and 4,4'-bipyridine (closed type).The hydrogen-bonded liquid crystalline complexes (Fn, n=3,6,10) between 4-{n-[4-(4-methoxy-phenylazo)-phenoxy]- alkoxy }-benzoic acid (En, n=3,6,10) and 4,4'-bipyridine have been prepared and characterized. The IR spectra of Fn were examined and the results showed that trimers were formed through single hydrogen bonding between carboxyl and pyridyl groups. The existence of smectic A(F10) and nematic(F3 and F6) mesophases of these complexes was confirmed. Broader mesophase of Fn and lower liquid crystalline transitional temperatures compared to that of En were attributed to the formation of intermolecular hydrogen-bonding between carboxyl and pyridyl groups.