ELECTRONIC-STRUCTURE AND NONLINEAR OPTICAL-PROPERTIES OF PUSH-PULL POLYENES - THEORETICAL INVESTIGATION OF BENZODITHIA POLYENALS AND DITHIOLENE POLYENALS

We report the results of ab initio Hartree-Fock calculations on the geometric and electronic structure and first-order (alpha) and second-order (beta) molecular polarizabilities of a series of push-pull polyene molecules. The acceptor is in all cases an aldehyde group while the donor is either a benzodithia, a dithiolene, or a dithiolane group. The benzodithia polyenal compounds have recently been shown to present among the largest mu.beta-values ever measured. Theoretical results are found to be in excellent agreement with experimental data. They provide a detailed microscopic understanding of the evolution of the electronic and optical properties as a function of the length of the polyenic segment. While dipole moments and polarizabilites are only weakly sensitive to the conjugation path length, beta-values evolve over 2 orders of magnitude, thus strikingly pointing out the relevance of higher order nonlinear susceptibilities to account in more detail for electronic polarization related features of conjugated molecules. A detailed analysis of the angular deviation between the dipole moment mu and the vector component-beta of the rust-order hyperpolarizability clearly illustrates the necessity to complement EFISHG measurements by theoretical computations in order to reach the modulus of tensor-beta. Large values of beta, otherwise hidden by solution measurements, may then eventually be unraveled.