Nonlinear optical properties of flavylium salts: a quantum chemical study.

Ab initio calculations have been carried out to unravel the relationships between the structure and the first hyperpolarizability in flavylium derivatives, with the aim to design efficient second-order nonlinear optical (NLO) switching compounds. Large contrasts of the first hyperpolarizability have been obtained along the pH-controlled and photoinduced transformations for specific combinations of chemical substituents in the 4'- and 7-positions, which demonstrates that these multistate systems should behave as highly efficient molecular NLO switches.