ELECTRONIC-STRUCTURE OF MIXED QUINOID AROMATIC THIOPHENE COPOLYMERS/

We report results of geometry optimization and electronic structure ca lculations for a class of recently synthesized thiophene copolymers wi th mixed aromatic/quinoid character which exhibit unusually large thir d-order optical nonlinearities. The orbital charges on their aromatic/ quinoid portions are calculated and a relation is suggested between th em and the energy dispersion of the orbital. On the basis of these res ults, good localization properties are predicted for the HOMO and even for the LUMO, especially if somewhat larger quinoid sequences could b e synthesized.