ELECTRONIC-PROPERTIES OF POLYMERS BASED ON THIENOTHIADIAZOLE AND THIOPHENE

Ab initio crystal orbital (CO) studies on the geometric and the electr onic structures of poly(thienothiadiazole) (Poly-1a) and periodic copo lymers of thienothiadiazole and thiophene with the ratio of 1:1 (Poly- 2b) and 1:2 (Poly-3b) are presented. In Poly-1a and Poly-2b, considera ble change in the geometries is found to occur as one moves from their oligomers to the polymers. Although thienothiadiazole oligomers have a very small highest occupied molecular orbital (HOMO)-lowest unoccupi ed molecular orbital (LUMO) separation, the corresponding polymer (Pol y-1a) has a considerably large band gap. On the contrary, the geometri c and electronic structures of Poly-3b are almost identical to those o f the oligomer, and Poly-3b possesses a direct band gap of 1.3 eV esti mated by simple scaling. The reasons for these differences are discuss ed in terms of orbital interactions and nonbonding molecular orbitals in the oligomers, and the reduced pi-conjugation and enhanced intercel l interaction in the polymers. (C) 1996 American Institute of Physics.