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 (α) and second-order (β) 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 μ·β 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, β 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 μ and the vector component β of the first-order hyperpolarizability clearly illustrates the necessity to complement EFISHG measurements by theoretical computations in order to reach the modulus of tensor β. Large values of β, otherwise hidden by solution measurements, may then eventually be unraveled.