Excited states in bis-substituted polyenes: configuration interaction description of the vertical excitation energies and nonlinear optical properties

On the basis of semi-empirical configuration interaction calculations, we analyse the influence of grafting electroactive groups on the geometric and electronic structures of polyenes. The sum-over-states formalism is applied to calculate the linear and nonlinear polarizabilities in various push-push, push-pull and pull-pull compounds. We compare the effects induced by the end groups attached to the conjugated segment on the first-, second- and third-order nonlinearities, focusing mainly on the description of the cubic response. While acceptor groups lead to a stronger enhancement of the third-order polarizabilities, γ, in short chains, donor moieties appear to be more promising candidates in order to achieve large nonlinearities in longer chains. The grafting of both strong electron-withdrawing and electron-donating groups on the conjugated chain breaks the molecular symmetry and raises considerably the γ values.