The relationship between the structures of semi-rigid homo- and copolyesters based on 5,5′-diphenyl-2,2′-bis(1,3,4-thiadiazole)s and their liquid crystalline and optical properties

Abstract

New semi-rigid homo- and copolyesters composed of the quaterphenyl analogue of 2,2′-bis(1,3,4-thiadiazole) (BTD), 5,5′-diphenyl-2,2′-bis(1,3,4-thiadiazole) (DBTD), were prepared by high temprature solution polycondensation of monomers, 3,3′- and 4,4′-dioxydiundecanol derivatives of DBTD with four dimethyl esters, and their liquid crystalline (LC) and optical properties were investigated. DSC measurements, texture observations using a polarizing microscope equipped with a hot stage, and powder X-ray diffraction showed that the homopolymers containing the 4,4′-DBTD unit form more stable LC phases than those having the 3,3′-DBTD unit. The 4,4′-DBTD-containing polymers and the 3,3′-DBTD unit-poor copolymers, except for the 3,3′-DBTD unit-rich copolymers and the 3,3′-DBTD-containing homopolymers (which formed highly ordered smectic or crystal mesophases), displayed a thermotropic LC smectic C phase. Solution and solid state UV-vis and photoluminescent (PL) spectra showed that the polyesters display miximum absorbances and bluish-green or blue emission based on the DBTD unit, where the Stokes shifts were observed. The peak positions in the UV-vis and the PL spectra of homopolymers composed of the 4,4′-DBTD unit were at higher wavelengths than those in the corresponding 3,3′-DBTD-containing homopolymers, due to its more conjugated structure. In the copolymers the peak maxima were shifted to shorter wavelengths with the increase of 3,3′-DBTD content.