Integrating Fluorinated [BF ₄ ] Anion With π‐Conjugated Six‐Membered Rings for Short‐Wavelength UV Frequency Conversion

Abstract

Abstract Nonlinear optical (NLO) crystals for the short‐wavelength ultraviolet (UV) region are vital for all‐solid‐state lasers, particularly for generating 266 nm light via the direct fourth harmonic generation. While organic planar π‐conjugated six‐membered rings are key functional units, their typically narrow band gaps hinder short‐wavelength UV application. In this study, we targeted melamine [C 3 H 8 N 6 ] for its strong hyperpolarizability and combined it with the wide‐energy gap fluorinated [BF 4 ] tetrahedra to synthesize three new compounds. Among them, the non‐centrosymmetric α ‐phase, α ‐C 3 H 8 N 6 (BF 4 ) 2 ·(H 2 O), exhibits balanced properties: a short UV cutoff edge and a high powder second‐harmonic generation efficiency of 6.4 × KDP, surpassing most known short‐wavelength UV transparent melamine salts and hybrid crystals with π‐conjugated six‐membered rings. More importantly, it achieves phase‐matched 266 nm generation with an efficiency ∼1.4 × that of classic BBO crystal. Theoretical calculations confirm that the [C 3 H 8 N 6 ] cation is the primary source of the NLO response, while the fluorinated [BF 4 ] anion is responsible for the wide band gap. This work not only presents a high‐performance melamine crystal for 266 nm output but also demonstrates a synergistic assembly strategy using π‐conjugated cations and wide energy gap fluoride anions to balance the critical trade‐off between band gap and nonlinearity in NLO materials.