Theoretical study of geometrical and electronic structures of various thiophene-based tricyclic polymers

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Abstract
A theoretical study of a variety of tricyclic polymers [–( C 8 H 2 X 2 Y ) n –] with two different types of bridging groups was performed, X=S and Y=CH 2 , SiH 2 , C=O , C=S , or C=CH 2 for the fused bithiophene system and vice versa for the thieno-bicyclic system. These two types of the bridging groups are different from each other in that S favors the aromatic form of a cyclic polymer and the other groups prefer the quinonoid form. Geometrical structures of the polymers were obtained from semiempirical self-consistent-field (SCF) band calculations and the electronic properties from the modified extended Hückel band calculations. It is found that the ground-state geometrical structures of the tricyclic polymers are determined by the bridging groups in the outer rings. That is, the fused bithiophene system is aromatic in the ground state and the thieno-bicyclic system is quinonoid. The ground-stateband gaps (which correspond to the absorption peaks of π–π * band transition) of the polymers were estimated to be in the range of 0.7–2.0 eV. The band gaps were analyzed in terms of the bond-length alternation along the conjugated carbon backbone, the C1–C4 interactions, and the electronic effect of the bridging groups. We also investigated the geometrical and electronic structures of polydicyanomethylene-cyclopenta-dicyclopentadiene (PDICNCY). Unlike the theoretical predictions of Toussaint and Bredas [Synth. Met. 69, 637 (1995)], PDICNCY in the ground state was estimated to be of the quinonoid form and to possess a large band gap (2.55 eV) comparable with the gap of polythiophene.