Strongly Polar [H₂PO₃]^- Induced π-Conjugated Group Arrangement for Balanced UV Nonlinear Optical Performance

Achieving a delicate balance among the short UV cutoff edge (λ_UV), strong second harmonic generation (SHG) response, appropriate birefringence (Δn), and favorable growth habits has always been a major challenge in the development of ultraviolet (UV) nonlinear optical (NLO) materials. Herein, the highly polar [H₂PO₃]^- group was utilized to induce an ordered arrangement of the planar triangle π-conjugate [C(NH₂)₃]⁺, [C(NH₂)₃]H₂PO₃ has been successfully designed and synthesized, which exhibits a short UV cutoff edge of 200 nm, a large SHG response of 1.7 × KH₂PO₄ (KDP), and an appropriate birefringence of 0.117@1064 nm. Furthermore, by replacing [C(NH₂)₃]⁺ with a double triangular coupled [C₂N₄H₇O]⁺ group, another guanylurea phosphite [C₂N₄H₇O]H₂PO₃ was screened out, which possesses a larger SHG response of 3.5 × KDP, a larger birefringence of 0.155@1064 nm, and a short λ_UV of 215 nm. Theoretical calculations revealed that their optical performance originates from the synergistic effect of the π-conjugated groups and [H₂PO₃]^-. More importantly, centimeter-sized crystals of both compounds were successfully obtained from the aqueous solution. These results highlight their potential as short-wave UV NLO crystals, offering new insights for the design of novel NLO materials.