(C₆H₇N₂O)NO₃: π-Conjugation-Driven Synergistic Enhancement of Second-Harmonic Generation and Birefringence

Nonlinear optical (NLO) crystal materials, endowed with unique photoelectric conversion properties, have garnered widespread utilization in laser science and technology. Herein, an ultraviolet (UV) NLO crystal (C₆H₇N₂O)NO₃, constructed from the π-conjugated organic cation [C₆H₇N₂O]⁺ and the inorganic anion [NO₃]^-, was synthesized via a π-conjugation-driven synergistic strategy. The crystal exhibits a large second-harmonic generation (SHG) response (2.75 × KH₂PO₄ (KDP)), a large birefringence (0.271@546 nm), and excellent physicochemical stability. Combined structural characterization and computational studies demonstrate that the exceptional performance stems from synergistic interactions between the π-conjugated structural units [C₆H₇N₂O]⁺ and [NO₃]^-. This work establishes a promising strategy for developing excellent organic-inorganic hybrid NLO materials.