Phase behaviour of hydrogen-bonded liquid crystalline complexes of alkoxycinnamic acids with 4,4 '-bipyridine

The hydrogen-bonded liquid crystalline complexes of 4-n-alkoxycinnamic acid s (nCNA: n = 4-8, 10, 12, 16, 18, where n is the number of carbons in the a lkoxy chain) with 4,4'-bipyridine (BPy) have been prepared and characterize d. The existence of smectic C, smectic A, and nematic mesophases of these c omplexes was demonstrated by a combination of polarized optical microscopy and X-ray diffraction (XRD). In this H-bonded mesogenic structure, nonmesog enic BPy functions as the core unit of the mesogen through the H-bond, as c onfirmed by infrared spectroscopy and XRD. A general comparison of nCNABPy with the benzoic acid analogues (nOBA)(2)-BPy, showed that the elongated nC NABPy mesogen behaves as a rod unit and increases the transition temperatur e. Smectic phases in nCNAs (5 less than or equal to n less than or equal to 12) were induced on complexation with BPy. The favouring of smectic phases in these complexes is believed to originate from the increment of polarity of the mesogen by intermolecular H-bonding. With increasing chain length o f the nCNABPy complexes the smectic C phase becomes stabilized, like conven tional rod-coil molecules.