Second-Order Optical Nonlinearities in Coordination Chemistry: The Case of Bis(salicylaldiminato)metal Schiff Base Complexes

Since the mid-1980s, there has been a growth of interest in the search and development of molecular second-order nonlinear optical (NLO) materials that possess various device applications. Beside the most traditional donor-acceptor (“push-pull”) organic chromophores (e.g. the stilbene family), chemists have increasingly extended their field of investigations to newer generations of organic molecules of greater complexity. In recent years, coordination complexes through their unique characteristics such as various redox and magnetic properties, in addition to their great diversity of geometries, have introduced a new dimension to the area. This review attempts to summarize the NLO capabilities of bis(salicylaldiminato)metal Schiff base complexes: (i) chromophores with enhanced hyperpolarizabilities (β) can be obtained by virtue of the coordinated metal center; (ii) the use of chiral substituents allows the engineering of the chromophores into various non-centrosymmetric environments and efficiencies of about 10 times that of urea in second-harmonic generation; (iii) the general trend observed for the high temperature stability in this family of chromophores, while combined with the large β×μ products, makes these systems interesting candidates for their incorporation into poled polymers with high glass transition temperatures (Tg); (iv) the possibility for β modulation induced by magnetic transition is also seen with respect to the concept of molecular switching.