GEOMETRICAL AND ELECTRONIC-STRUCTURES OF PI-CONJUGATED HETEROCYCLE-NONHETEROCYCLE COPOLYMERS

Theoretical work was performed to investigate the geometrical and elec tronic structures of new pi-conjugated heterocycle-nonheterocycle copo lymers. Heterocycles include thiophene, pyrrole, and furan whose homop olymers are of the aromatic forms, and nonheterocycles include cyclope ntadiene, silole, oxocyclopentadiene, and thiocyclopentadiene whose ho mopolymers are quinoid in the ground states. AMI band calculations sho w that the copolymers containing cyclopentadiene or silole rings are o f the aromatic forms whereas those containing oxocyclopentadiene or th iocyclopentadiene rings are quinoid in the ground states. Modified ext ended Huckel band calculations predict that band gaps of the copolymer s in the ground states are smaller than that of polythiophene. The pi- pi transition peaks are estimated to appear at 1.55 eV for poly(thien ylene silolylene), 1.68 eV for poly(pyrrolylene silolylene), and 1.67 eV for poly(furylene silolylene). Band gaps of the copolymers are anal yzed in terms of the contributions from bond-length alternations, C1-C 4 interactions, and electronic effects of the bridging groups. It is r evealed that the small band gaps of the copolymers come from the reduc tion of electronic effects of the heteroatoms compared to the effects found in heterocyclic polymers.