The synthesis, characterization, thermal stability, optical spectroscopic, electronic structure, and second-order nonlinear optical (NLO) response of a series of donor-acceptor bis(salicylaldiminato)nickel(II) Schiff base complexes and the free ligand precursors are reported. The effect of the metal center in such complexes is manifold: it templates the formation of acentric molecular structures, imparts high thermal stability to the chelate ring, and both 'switches on' and enhances NLO response. Metal complexation imparts new linear optical spectroscopic features, having metal-to-ligand charge transfer character, which are responsible for the second-order nonlinearity. Moreover, the present synthetic strategy represents a novel route to inorganic NLO chromophores. Solution-phase hyperpolarizability values, deduced by electric field-induced second-harmonic-generation experiments are as high as -79 x 10-30 cm5 esu-1 (hω = 0.92 eV). Experimental linear and nonlinear optical features are fully consistent with INDO/SCI-SOS theoretical calculations. They provide a rationale for the NLO response of these materials and are attractive for designing new, highly efficient second-order nonlinear optical inorganic chromophores.
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