Integrating Fluorinated [BF₄] Anion With π-Conjugated Six-Membered Rings for Short-Wavelength UV Frequency Conversion

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 [C₃H₈N₆] for its strong hyperpolarizability and combined it with the wide-energy gap fluorinated [BF₄] tetrahedra to synthesize three new compounds. Among them, the non-centrosymmetric α-phase, α-C₃H₈N₆(BF₄)₂·(H₂O), 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 [C₃H₈N₆] cation is the primary source of the NLO response, while the fluorinated [BF₄] 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.