A_0.5H₂C₆N₇O₃?4H2O (A = Ca²⁺, Sr²⁺) iso-cyamelurates with ultra-large -conjugated group and excellent nonlinear optical properties

Nonlinear optical (NLO) crystals are the core components of high-performance coherent sources. Their properties can be modulated and improved by molecular design of constituent functional motifs. In this study, inspired by a classical p-conjugated borate group (B3O6)(3-), we discovered a p-conjugated isocyamelurate [H2C6N7O3](-) anion with colossal p-conjugated orbitals, thus giving ultra-strong anisotropic first-order polarizability and ultralarge second-order susceptibility. Two new alkaline-earth metal isocyamelurates, namely, Ca0.5H2C6N7O3 center dot 4H(2)O (I) and Sr0.5H2C6N7O3 center dot 4H(2)O (II), were successfully synthesized by a facile aqueous solution method. Both I and II show excellent nonlinear optical properties, including wide band gap (>4.05 eV), large birefringence (Delta n similar to 0.24), and a strong second order harmonic generation response (>5 x KDP). Moreover, I and II exhibit a broadband ultraviolet photoluminescence around 400 nm, which indicate that they are potential multifunctional optical materials with photon-emission and photon-conversion properties. The first-principles calculations reveal that the (H2C6N7O3)(-) anionic group plays a dominant role in optimizing and enhancing the optical performance of the crystals. This study confirms that there are plenty of opportunities for (iso)-cyamelurates in designing functional crystals with superior properties.