Geometrical and electronic structures of π-conjugated heterocycle-nonheterocycle copolymers

Theoretical work was performed to investigate the geometrical and electronic structures of new π-conjugated heterocycle-nonheterocycle copolymers. Heterocycles include thiophene, pyrrole, and furan whose homopolymers are of the aromatic forms, and nonheterocycles include cyclopentadiene, silole, oxocyclopentadiene, and thiocyclopentadiene whose homopolymers are quinoid in the ground states. AM1 band calculations show that the copolymers containing cyclopentadiene or silole rings are of the aromatic forms whereas those containing oxocyclopentadiene or thiocyclopentadiene rings are quinoid in the ground states. Modified extended Hückel band calculations predict that band gaps of the copolymers in the ground states are smaller than that of polythiophene. The π-gp* transition peaks are estimated to appear at 1.55 eV for poly(thienylene silolylene), 1.68 eV for poly(pyrrolylene silolylene), and 1.67 eV for poly(furylene silolylene). Band gaps of the copolymers are analyzed in terms of the contributions from bond-length alternations, C1–C4 interactions, and electronic effects of the bridging groups. It is revealed that the small band gaps of the copolymers come from the reduction of electronic effects of the heteroatoms compared to the effects found in heterocyclic polymers.