Synergistic Design of UV Nonlinear Optical Crystals: Harnessing Flexible π-Conjugated Anions and Guanidinium Cations for Enhanced Performance

Nonlinear optical (NLO) materials are vital for advanced optical applications, particularly in the laser industry. While borate-based NLO crystals have been extensively studied, recent research has expanded to novel materials featuring flexible organic π-conjugated systems. In this work, we report the synthesis of a new ultraviolet NLO crystal [C(NH2)3][NH2(CH2COO)2], which crystallizes in the noncentrosymmetric monoclinic space group Pc (No. 7). The structure is composed of a three-dimensional network formed by the [C(NH2)3]+ cation and the flexible π-conjugated [NH2(CH2COO)2]- anion, linked through hydrogen bonding and Coulombic interactions. Optical measurements reveal a short ultraviolet cutoff edge at 216 nm, a significant birefringence (0.194 at 546.1 nm), and a notable second-harmonic generation response (1.5 × KDP). Theoretical calculations suggest that synergistic interactions between the cation and anion group enhance the material's anisotropic polarizability and hyperpolarizability, positioning it as an excellent candidate for high-performance ultraviolet NLO applications. This study demonstrates the potential of combining flexible organic anions with strategically selected cations to design advanced NLO materials for next-generation optical devices.