Donor-Acceptor Diphenylacetylenes: Geometric Structure, Electronic Structure, and Second-Order Nonlinear Optical Properties

We present a theoretical study of the geometry and electronic structure in a series of donor-acceptor diphenylacetylene molecules containing one-four triple bonds in the conjugated segment. We investigate the second-order molecular polarizabilities β at the intermediate neglect of differential overlap/configuration interaction (INDO/ CI) sum-over-states level; the theoretical results are compared to detailed experimental data which have been recently reported. We test the convergence of the sum-over-states approach and the validity of the two-state model. In compounds with one or two triple bonds in the conjugated segment, after examining a wide range of donors and acceptors, a linear dependence is obtained between β and, on the one hand, the inverse of the squared lowest transition energy (1/hω_eg)²) and, on the other hand, the dipole moment change (Δμ) in going from the ground state to the low-lying charge-transfer excited state. In these shorter compounds, we find the two-state model to provide excellent trends. However, the results indicate that increasing the conjugation length of the molecules beyond two triple bonds leads to a total breakdown of the two-state approximation; this is because of the appearance of several nearly isoenergetic excited states which significantly contribute to the molecular second-order polarizability β. This feature is fully consistent with the experimental data.