Structure/property relationships for molecular second-order nonlinear optics

Using a two-state model, it was recently shown that there is an optimal combination of donor and acceptor strengths for a given bridge that will maximize the first molecular electronic hyperpolarizability ((beta) ). To date, molecules with sufficiently strong donors and acceptors have not been synthesized to reach this ideal point. Here we outline synthetic strategies that lead to molecules where (beta) is optimized by tuning the degree of bond alternation in the (pi) -electron system between the donor and acceptor. Implementation of this strategy relies on the realization that molecules with strongly aromatic end groups will not have the correct balance of two limiting charge transfer resonance forms in the ground state to achieve the degree of bond alternation required to optimize the molecular hyperpolarizability. Electric field induced second harmonic generation studies on organic molecules with various bridge structures support our hypothesis.