Novel thermotropic liquid-crystalline (LC) copolyesters were prepared with three disubstituted (4,4 '-, 3,4 '-, and 3,3 '-) dioxydiundecanol derivatives of terphenyl analogues of 1,3,4-thiadiazole [2,5-diphenyl-1,3,4-thiadiazole (DPTD)], and their optical and electrochemical properties were examined. Their structures were characterized with Fourier transform infrared, H-1 NMR spectroscopy, and elemental analyses. The thermal and mesomorphic properties of the copolyesters were investigated with differential scanning calorimetry measurements, polarized microscopy observations, and X-ray analyses; the data suggested that these copolymers formed LC smectic or nematic phases. The mesomorphic tendency decreased in the following order: 4,4 '-DPTD and 3,4 '-DPTD copolyesters > 4,4 '-DPTD and 3,3 '-DPTD copolyesters > 3,4 '-DPTD and 3,3 '-DPTD copolyesters. Solution and solid-state ultraviolet-visible (UV-vis) and photoluminescence spectra indicated that the copolyesters displayed maximum absorbances and blue emissions according to the DPTD unit; the peak maxima of absorption and emission spectra of the copolyesters shifted to lower wavelengths in the aforementioned order for the LC properties. Cyclic voltammetry measurements indicated that the electrochemical band gaps of the polyesters estimated from the onset of reduction and oxidation processes were almost the same as the optical band gaps determined from the solid-state UV-vis spectral data. The DPTD unit enhanced the hole-injection barrier and improved the charge-injection balance in these polyesters. (c) 2005 Wiley Periodicals, Inc.