Second-order nonlinear optical properties of stilbene, benzylideneaniline and azobenzene derivatives. The effect of π-bridge nitrogen insertion on the first hyperpolarizability

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The second-order nonlinear optical (NLO) and electronic properties of stilbenes (CC bridge), benzylideneanilines (CN and NC bridges) and azobenzenes (NN bridge) containing either an N,N-dimethylamino donor and/or a nitro acceptor were investigated using EFISH, UV spectroscopy, cyclic voltammetry and PPP/SCF calculations. It appeared that although first hyperpolarizabilities of the ethylene and azo bridged donor–acceptor compounds are of comparable magnitude, substitution of one carbon by a nitrogen atom reduces the NLO activity. Differences in hyperpolarizabilities were rationalized with the aid of a two-level model, which revealed that they find their origin in the redox activity of the nitrogen-containing bridges.