THEORETICAL INVESTIGATION OF THE NONLINEAR-OPTICAL PROPERTIES OF OLIGOMERS OF POLYTHIENYLENEMETHYLIDENE, A LOW-BAND-GAP MATERIAL

We present a theoretical investigation by the sum-over-states formalis m of the static first- and third-order polarizabilities of thienylenem ethylidene oligomers; following the incorporation of a single conjugat ed carbon between the rings, these compounds present a partially quino id geometry. We use a multireference determinant single-double configu ration interaction approach to calculate excited-state energies, state dipole moments, and transition dipole moments which are introduced in the sum-over-states expressions. All the results are discussed in com parison to those obtained in thiophene oligomers of similar sizes; the y indicate that the third-order polarizabilities are smaller for the m ethine-bridged compounds than for the corresponding thiophene oligomer s. This feature is rationalized by analyzing the main optical (virtual excitation) channels that contribute to the nonlinear response.