Maximizing the linear and nonlinear optical responses of alkaline tricyanomelaminate

High-performance bi-functional materials are in urgent demand for the next-generation integrated optical devices.In this work,we successfully synthesized the first tricyanomelaminate with bi-functional optical responses,namely Cs_(3)C_(6)N_(9)•H_(2)O(I),from its analogue Na_(3)C_(6)N_(9)•3H_(2)O by a facile ion exchange method.In contrast to Na_(3)C_(6)N_(9)•3H_(2)O,I realizes an optimal arrangement of𝜋π-conjugated(C_(6)N_(9))3−anion groups in its crystal structure.As a result,the second-order nonlinear optical(NLO)response is greatly enhanced from nearly zero of Na_(3)C_(6)N_(9)•3H_(2)O to∼9.8×KH_(2)PO_(4)of I.Furthermore,I exhibits a giant linear optical anisotropic response(i.e.birefringence)of 0.52 at the wavelength of 550 nm.Both responses are almost the largest among the inorganic compounds ofπ-conjugated rings,which indicates that I has great potential as a bi-functional optical crystal.Structural and theoretical analyses reveal the microscopic origin of excellent optical properties.This work would attract a lot of interest to the persistently neglected potential of tricyanomelaminates as linear optical and NLO crystals.