Quantum chemical study on the electronic structure and second-order nonlinear optical properties of salen-type Schiff bases

Semi-empirical (ZINDO-SOS), time-dependent density functional theory and ab initio quadratic response function (DDRPA) calculations on a series of donor–acceptor substituted salen-type Schiff bases are used to aid in the design of dyes with useful optical nonlinearities (molecular quadratic hyperpolarizabilities βvec). 4-Phenylazo-2-phenyliminomethylphenols 1 are calculated to be less suitable as nonlinear optical materials than the isomeric 5-phenylazo-2-phenyliminomethylphenols 2. The largest hyperpolarizabilities are predicted for compounds with an acceptor-containing phenylazo and a donor-substituted phenyliminomethyl moiety. The calculations also clearly indicate the intramolecular charge transfer nature of the first ππ∗-transition in the investigated Schiff bases.