Theoretical Study of the Effect of Alkyl and Alkoxy Lateral Chains on the Structural and Electronic Properties of π-Conjugated Polymers Consisting of Phenylethynyl-1,3,4-thiadiazole

The effects of alkyl/alkoxy pendant chains on the electronic structure and properties of a typical poly(aryl-ethynylene) (PAE) system formed by phenylethynylene units and 1,3,4-thiadiazole rings are analyzed. Optical band gaps and electronic properties (electron affinity, LUMO energy and intramolecular reorganization energy) were computed for different oligomers and then estimated for a limit polymer. The electronic properties become more influenced by the nature of the lateral chain rather than by its length. The most significant changes in the electronic properties are obtained with alkoxy side chains. The smallest band gaps were calculated for polymers containing alkoxy pendant chains. A less favored electron injection is expected for polymers with higher energy values predicted for the LUMO levels and decrease of the electron affinity. Intramolecular reorganization energies computed for all studied polymers were rather low (≤0.07 eV), which allows them to be considered as candidates for n-type semiconductors.