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 Cs₃C₆N₉•H₂O (I), from its analogue Na₃C₆N₉•3H₂O by a facile ion exchange method. In contrast to Na₃C₆N₉•3H₂O, I realizes an optimal arrangement of π-conjugated (C₆N₉)^3- anion groups in its crystal structure. As a result, the second-order nonlinear optical (NLO) response is greatly enhanced from nearly zero of Na₃C₆N₉•3H₂O to ∼9.8 × KH₂PO₄ 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 of π-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.