Synthesis, characterization and nonlinear optical (NLO) properties of a push-pull bisboronate chromophore with a potential electric field induced NLO switch

A new bisboronate with five six-membered ring heterobicycles was prepared by reaction of [2-(2-hydroxy-4-diethylaminobenzylidene)amino]-5-nitroaminophenol and 1,4-phenyldiboronic acid, and its quadratic nonlinear optical (NLO) response was compared to that of a related monoboronate species. A computational investigation conducted within the framework of the DFT theory, at the B3PW91/6-31G* level, indicates that, while the diboronate derivative exhibits a centrosymmetric conformation in the ground state, and hence a vanishing quadratic hyperpolarizability (beta), the application of an external electric field provides a gradual intramolecular rotation of the two "push-pull'' sub-units which reaches 128 degrees for a field intensity of 10(-3) a.u. and leads to a beta value equal to 49.2 x 10(-30) cm(5) esu(-1). This result suggests a possibility for a molecular NLO switch induced by an electric field in such systems. Experimentally, the NLO response measured by the electric field induced second harmonic (EFISH) technique indicates an NLO response (expressed as the mu x beta product) 1.95 times larger for the bisboronate than for the monoboronate analogue, thus suggesting that, although the ground state conformation is centrosymmetric, the "push-pull'' sub-units are in quasi-free rotation at room temperature.