A Two-Dimensional Organic Salt With a Deep-Ultraviolet Cutoff Edge and Strong Second-Harmonic Generation

Targeted synthesis of ultraviolet (UV) nonlinear optical (NLO) materials, especially organic salts, has experienced considerable difficulties due to the stringent requirement that these materials simultaneously possess ultra-wide UV transparency, strong second-harmonic generation (SHG), and short SHG phase-matching cutoff wavelength. We report herein the designed synthesis of the first deep-UV-transparent NLO organic guanidinium salt [C(NH₂)₃][CH₃(CH₂)₃BF₃], featuring a honeycomb-layered structure with uniformly aligned π-conjugated [C(NH₂)₃]⁺ cations. The compound possesses a deep-UV cutoff edge (197 nm), strong SHG responses at both 1064 nm (4.5 × KH₂PO₄) and 532 nm (0.7 × β-BaB₂O₄), and sufficient birefringence (0.12 @ 546 nm). The calculated phase-matching wavelength fully covers its optical transparency range (extending down to 197 nm), confirming that [C(NH₂)₃][CH₃(CH₂)₃BF₃] is a very promising UV NLO material. Theoretical calculations and crystal structure analysis show that the rare coexistence of optimal performance in the key optical properties can be attributed to an ideal two-dimensional layered structure, combined with the π-conjugated [C(NH₂)₃]⁺ cations and the flexible [CH₃(CH₂)₃BF₃]^- anions. This research affords a new paradigm for the designed synthesis of high-performance UV NLO organic salts.