COMPARATIVE-STUDY OF OPTICAL NONLINEARITIES IN SUBSTITUTED POLYYNES VERSUS THE CORRESPONDING POLYENES AND POLYPHENYLS

The frequency-dependent polarizability, hyperpolarizability, and secon d hyperpolarizability of a series of donor-acceptor polyynes (III) hav e been calculated by the correction vector approach, using both singly and doubly excited configurations. The results are compared with thos e obtained for the corresponding polyenes (I) and polyphenyls (II). Bo th the SHG and THG coefficients of the polyynes (III) are predicted to be significantly larger than those of the polyphenyls (II) but inferi or to those of the polyenes (I) at this level of theory. The inclusion of doubly excited configurations in the evaluation of the hyperpolari zability tensor for the polyynes increases the absolute value relative to calculations which include singly excited configurations alone, bu t the reverse is true for the polyenes where the absolute value decrea ses with the inclusion of doubly excited configurations.