EFFECTS OF SIMPLE SUBSTITUTIONS ON CONFORMATIONAL AND ELECTRONIC-PROPERTIES OF RIGID ROD POLYMER MODELS

Conformational energies, electronic band gaps dagger and second-order hyperpolarizabilities were calculated as a function of torsion angle for structures designed to model the rigid rod polymers poly(p-phenylenebenzobisoxazole), poly(p-phenylenebenzobisimidazole) and poly(p-phenylenebenzobisthiazole), including examples in which the phenylene group is mono- and dimethylated, using AM1 semi-empirical molecular orbital techniques. An increase in torsion angle decreases pi electron delocalization, which is the dominant factor in electronic band gap and second-order hyperpolarizability changes. Methylation of the phenyl group destabilizes the flat structures in all cases, and the resulting calculated barrier heights show the result of the opposing steric and conjugative effects. The methyl substituents have negligible direct effect on opto-electronic properties.