THEORETICAL-STUDY OF THE CONFORMATIONS AND ELECTRONIC-STRUCTURES OF PHENYLENE PYRROLE AND PHENYLENE FURAN COPOLYMERS

A theoretical study of the conformations and electronic structures of phenylene-pyrrole and phenylene-furan copolymers with methyl and methoxy substituents on the phenylene ring was performed. Torsional potential surfaces for the corresponding monomers were calculated using the partial retention of diatomic differential overlap method. The preferred structures of the copolymers were predicted to be coplanar except for the alkyl-substituted pyrrole system, where strong steric repulsion between the alkyl and HN groups was found. Hydrogen bonding between the oxygen of the alkoxy group and the hydrogen bound to the pyrrole nitrogen was found to play a key role in keeping the dialkoxy-substituted pyrrole analog coplanar. The electronic structures of the copolymers were calculated with the modified extended Huckel band method. The calculated electronic properties of the copolymers, except for the bandwidths, are similar to those of the homopolymers and are in excellent agreement with experimental observations. The three highest dispersive valence bands of the copolymers are separated from each other by 0.6-0.7 eV. The highest valence bandwidths of the copolymers are about one-third of those of the corresponding homopolymers and are probably responsible, in part, for the much lower conductivities of these systems.