Large quadratic nonlinear optical efficiencies in pseudosymmetric streptocyanine dyes

The quadratic (proportional to E-2) nonlinear optical (NLO) properties of pseudosymmetric organic cations are investigated. These chromophores belong to a family of highly polarizable streptocyanine dyes built in two steps by amination of their related carboxonium salts. They exhibit a nearly perfect optical transparency in the 200-1100 nm range, except around 700 nm, where a single but narrow band is present. Some of these dyes crystallize in noncentrosymmetric space groups, which, despite a symmetrical pi-electronic structure, leads to symmetry lowering and restores a push-pull character to the optical transition and hence a sizable molecular hyperpolarizability. One of these molecules is optimized for a solid-state NLO effect and exhibits an efficiency 120 times that of urea in second harmonic generation at 1.907 mu m.