Solvent effects on the second-order nonlinear optical responses in the keto-enol equilibrium of a 2-hydroxy-1-naphthaldehyde derivative

Hyper-Rayleigh scattering experiments and ab initio calculations are combined to investigate the solvent effects on the second-order nonlinear optical responses of a 2-hydroxy-1-naphthaldehyde derivative that commutes between an enol and a keto form. Different binary mixtures of cyclohexane and ethanol are used to displace the tautomeric equilibrium. We show that increasing the solvent polarity increases the population of the keto form, shifts the lowest energy band of the absorption spectra to lower energy, and gives rise to a large enhancement of the first hyperpolarizability. Using theoretical calculations, the global solvent effect on the latter is shown to originate from both the displacement of the tautomeric equilibrium and the modification of the second-order nonlinear optical response of the individual tautomeric forms.