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 formalism of the static first- and third-order polarizabilities of thienylenemethylidene oligomers; following the incorporation of a single conjugated carbon between the rings, these compounds present a partially quinoid geometry. We use a multireference determinant singles-double configuration 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 comparison to those obtained in thiophene oligomers of similar sizes; they indicate that the third-order polarizabilities are smaller for the methine-bridged compounds than for the corresponding thiophene oligomers. This feature is rationalized by analyzing the main optical (virtual excitation) channels that contribute to the nonlinear response.