Electronic structure of mixed quinoid/aromatic thiophene copolymers

We report results of geometry optimization and electronic structure calculations for a class of recently synthesized thiophene copolymers with mixed aromatic/quinoid character which exhibit unusually large third-order optical nonlinearities. The orbital charges on their aromatic/quinoid portions are calculated and a relation is suggested between them and the energy dispersion of the orbital. On the basis of these results, good localization properties are predicted for the HOMO and even for the LUMO, especially if somewhat larger quinoid sequences could be synthesized.