Figures of merit of push-pull molecules in photorefractive polymers

Push-pull molecules are the most widely used nonlinear optical chromophores incorporated in polymers to generate photorefractivity. They are at the origin of the Pockels and the Kerr electrooptic effects in high-T-g photorefractive polymers and give also rise to the optical birefringence in low-T-g photorefractive polymers. High-T-g and low-T-g figures of merit characterize the efficiency of push-pull molecules to generate phototrefractivity. We present, here, the two-level figures of merit of various push-pull molecules determined by using electrooptical absorption measurements in solution. Analysis of the results within the framework of the two-form two-state model provides useful guidelines for the design of push-pull molecules for photorefractive applications. This model enables us to evaluate the optimum structures as well as the corresponding maximum optical birefringence, Pockels and Kerr electrooptic responses. It also shows how these maxima are related. Tuning a molecular structure to reach an optimum one will not automatically lead to a very large figure of merit. An essential parameter that is usually overlooked is the magnitudes of these maxima. We show that the maxima are affected both by the conjugation length and by the choice of the donor/acceptor substituents. Some of the longest compounds investigated here exhibit very large high-T-g and low-T-g figures of merit. (C) 2000 American Institute of Physics. [S0021-9606(00)51004-6].