Comparative study of optical nonlinearities in substituted polyynes versus the corresponding polyenes and polyphenyls

The frequency-dependent polarizability, hyperpolarizability, and second hyperpolarizability of a series of donor-acceptor polyynes (III) have been calculated by the correction vector approach, using both singly and doubly excited configurations. The results are compared with those obtained for the corresponding polyenes (I) and polyphenyls (II). Both the SHG and THG coefficients of the polyynes (III) are predicted to be significantly larger than those of the polyphenyls (II) but inferior to those of the polyenes (I) at this level of theory. The inclusion of doubly excited configurations in the evaluation of the hyperpolarizability tensor for the polyynes increases the absolute value relative to calculations which include singly excited configurations alone, but the reverse is true for the polyenes where the absolute value decreases with the inclusion of doubly excited configurations.