Third-Order Nonlinear Optical Responses of Bis(15-crown-5)-stilbenes Binding to One- or Two-Alkali Metal Cation (Li⁺, Na⁺ and K⁺)

Bis(15-crown-5)-stilbenes containing crown ether parts have been widely used in a variety of chemical applications, such as cation detectors, because of their ability to selectively bind to alkali metal cations, Bis(15-crown-5)-stilbenes and its derivatives with complexation of one- or two-alkali metal cation (Li⁺, Na⁺ and K⁺) have been theoretically investigated by quantum chemistry methods. The coordination of alkali cations results in partial shrinkage of crown ethers, which directly affected natural distribution analysis charges and molecular orbital energy levels. The number of alkali metal ions has significant effects on absorption spectra and mean second hyperpolarizability. When one alkali metal ion was added to the anticonformer of bis(15-crown-5)-stilbene, the absorption spectra were obviously redshifted and the mean second hyperpolarizability values were slightly increased; while two alkali metal ions were added to bis(15-crown-5)-stilbene, the absorption spectra were obviously blue shifted and the mean second hyperpolarizability values decreased. On the other hand, as the radius of the alkali ions increaseed, the mean second hyperpolarizability values of the compounds increased gradually. It indicated that the mean second hyperpolarizability value was sensitive to the number and radius of the alkali metal cations, thus the third order nonlinear optical response can be used as a signal to detect the number and type of alkali metal ions.