A series of hydrogen-bonding associated main-chain complexes based on dipyridyl and dicarboxylic acid derivatives have been prepared by slow solvent evaporation, and a comparative investigation has been conducted on their thermotropic phase transitions and liquid crystalline (LC) behavior by FTIR, DSC and polarized optical microscopy. All the complex series based on 4',4''-dicarboxy-1,6-diphenoxyhex- ane (C6-2COOH) and 4',4''-dicarboxy-1,10-diphenoxydecane (C10-2COOH) possessed higher melting and clearing points, usually showed soft crystal or crystalline smectic phases and crystal polymorphs except C10-2COOH/p-PhBPy complex displaying also an enantiotropic nematic phase, while the complexes based on 4,4'-dicarboxy-α,ω-diphenoxy tetraethyleneglycol ether (C8O4-2COOH) with a longer flexible spacer exhibited monotropic fluidic smectic SmA and/or nematic phases. On the other hand, for those complexes based on such variant dipyridyls as 4,4'-bipyridine (4,4'-BPy), trans-1,2-bis-(4-pyridyl)ethene (4,4'-BPyE) and bis-isonicotinic ester of hydroquinone (p-PhBPy) with the same dicarboxylic acid, the LC order decreased and the fluidity increased accordingly. This kind of phase transition behavior could be mainly ascribed to the decrease in rigidity and increase in deformability and polarity from 4,4'-BPy to 4,4'-BPyE then to p-PhBPy. Furthermore, some discrepancies in earlier literature have been pointed out and our results may provide guidance for hydrogen-bonding association, in particular for the design and assembly of supramolecular polymers based on carboxylic acid/pyridyl pairs.

Key words: liquid crystal, supramolecular polymer, carboxy-pyridyl hydrogen bonding association, fluidic smectic phase, nematic phase