Spirooxazine molecular switches with nonlinear optical responses as selective cation sensors

Abstract

Spirooxazine, a photochromic material, can transform into metallic open-form merocyanine by molecular switching, giving rise to large contrasts in its second-order nonlinear optical (NLO) properties. The switching properties are particularly large when various metal ions (Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe²⁺, Zn²⁺, and Ag⁺) are introduced, as evidenced by density functional theory calculations, which show that the spirooxazine undergoes a pronounced change in geometry accompanied by formation of a larger π-conjugated system. The resultant merocyanine derivatives have 10–21-fold higher static second-order NLO responses. Spirooxazine can therefore be used as a powerful and multi-use detection tool. The large first hyperpolarizability (β_tot) is shown to rely on the alkaline earth metal, causing β_tot values to increase nearly 21-fold, as evidenced by the larger charge distribution, lower transition energy, and separate distribution of first hyperpolarizability density. In contrast, variation of β_tot in the Fe²⁺ derivative is not obvious, owing to stronger complexation, a larger amount of charge transferred from the napthoxazine moiety to the metal, and the reduction in N⋯O distance between the ligand heads. Therefore, spiropyran-to-merocyanine molecular switching can be used to distinguish alkaline earth metals and determine the efficiency of cation detection.