Heterocyclic benzoxazole-based liquid crystals

The synthesis, characterization, and mesomorphic properties of a new type of heteronuclear compounds 1a- c and a Pd complex 1d derived from benzoxazole as the core group are reported. These compounds were prepared by the ring closure reaction of 4-alkoxybenzoic acid 4-[(4-alkoxy-2-hydroxyphenylimino) methyl] phenyl esters 6 in the presence of lead(IV) acetate. All the compounds were characterized by H-1 and C-13 NMR spectroscopies and elemental analysis. The phase behaviour of these mesogenic compounds was characterized and studied by differential scanning calorimetry and polarizing optical microscopy. All the compounds 1a exhibited nematic (N) and/or smectic C (SmC) phases, as expected for rod-like molecules; however, the compounds 1b and 1c exhibited crystal phases. For those compounds 1a having shorter carbon chains (n=1, 3, 4) nematic phases were observed, whereas for compounds having longer carbon chains ( n=6, 7, 8, 10, 12, 14) smectic C behaviour was also observed at lower temperatures. The greater aspect ratio (l/d) of compounds 1a compared with 1b and 1c was found to be required for the observation of liquid crystallinity. The fluorescent properties of these compounds were also examined. All lambda(max) peaks of the absorption and photoluminescence spectra of compounds 1a - 1c occurred at c. 316 - 322 nm and 371 - 382 nm, respectively. The quantum yields of some compounds were relatively low, and also slightly solvent-dependent.