QUANTUM-CHEMICAL STUDY OF GEOMETRICAL AND ELECTRONIC-STRUCTURES OF THIOPHENE-BASED BICYCLIC POLYMERS

A quantum-chemical study was performed to investigate the geometrical and electronic structures of a variety of thiophene-based bicyclic pol ymers [-(C6H2SX)(n)(-)], where X=CH2, SiH2, C=O, C=S, or C=CH2. These two (S and X) types of the bridging groups are different from each oth er in that S favors the aromatic form of a cyclic polymer and X the qu inonoid form. Geometrical structures of the polymers were obtained fro m AML band calculations and the electronic properties from the modifie d extended Huckel band calculations. It is predicted that the bicyclic polymers with weak electron-donating groups (CH2 and SiH2 groups) are of the aromatic forms in the ground state and that the polymers with electron-withdrawing groups (C=O, CIS and C=CH2 groups) are of the qui nonoid forms as observed in the thiophene copolymers, -[(C4H2S)- (C4H2 X)](n)(-). The band gaps (which correspond to the absorption peaks of pi-pi band transition) of the bicyclic polymers in the ground state a re estimated to be in the range of 1.4-1.9 eV. The band gaps were anal yzed in terms of the bond-length alternation along the conjugated carb on backbone, the C1-C4 interactions, and the electronic effect of the bridging groups. In comparison with the contributions found in the thi ophene copolymers, the contribution from the bond-length alternation t o the band gaps decreases, and the contributions from the C1-C4 intera ctions and the electronic perturbation of S increase. As a result, the band gaps of the bicyclic polymers are about 0.2 eV smaller than thos e of the corresponding thiophene copolymers.