Ab initio investigation on the second-order nonlinear optical responses in keto-enol equilibria of salicylideneanilines

The keto-enol (K-E) tautomerization equilibrium, more precisely, the keto-amine/enol-imine equilibrium, has been investigated for a series of substituted salicylideneanilines in view of designing compounds with a contrast of second-order nonlinear optical properties. Substituting the salicylidene ring by an acceptor group or the other ring by a donor prevents the K form from being stable, whereas in the other cases, the K form can easily be converted to the E form due to the small activation barrier, figuring out in most cases that the K form is metastable. For a representative set of donor/acceptor substituents, the E and K forms present a sufficiently large contrast of β to allow its detection by using electric-field-induced second harmonic generation or hyper-Rayleigh scattering. The largest β values are mainly associated with species bearing a donor in the para position of the salicylidene ring and an acceptor on the other ring whereas the largest β values are generally found for the E form.