Structural and electronic effects of 1,3,4-thiadiazole units incorporated into polythiophene chains

A series of conjugated triaryl compounds have been synthesized, consisting of thiophene or selenophene peripheral units and a central 1,3,4-thiadiazole (TDA) heterocycle. X-ray crystallographic studies on four of the materials reveal that the molecules are planar in the solid state and feature an array of intramolecular (heteroatomic) and intermolecular (heteroatomic and π-π) noncovalent close contacts. Electrochemical oxidative polymerization affords insoluble polymers for EDOT-TDA-EDOT and EDTT-TDA-EDTT. The band gaps of the polymers have been deduced by cyclic voltammetry and electronic absorption spectroscopy and were found to be 1.8-1.9 eV. Both polymers show good stability toward n-doping and the EDTT analogue is more readily reduced than the EDOT-containing system. The enhanced stability toward n-doping, compared with the homopolymers PEDTT and PEDOT, is attributed to the presence of the electron deficient thiadiazole unit. The LUMO of poly(EDTT-TDA-EDTT) is 0.3 eV lower than that of poly(EDOT-TDA-EDOT), demonstrating that the substituent effect of the chalcogen atom is an important contributor to the electronic properties of the polymers.