Molecular Nonlinear Optics: From Para-Nitroaniline to Electrochemical Switching of the Hyperpolarizability

Molecular nonlinear optics describes the nonlinear optical properties of materials at the molecular level. At this most fundamental level, the nonlinear response can be described in terms of molecular parameters, such as number of polarizable electrons, non-centrosymmetric substitution by electron-donor or electron-accepting groups, molecular conformation, position of charge, or degree of charge transfer. We recount the shifts in molecular paradigms for second-order nonlinear optics, largely based on the instrumental possibilities offered to experimentally determine the nonlinear response. We start at the shift from the dipolar paradigm to the octopolar one, initiated by the possibilities of hyper-Rayleigh scattering, and end with the perspectives recently offered by electrochemical switching of the second-order nonlinear optical response at the molecular level.