Resonant Second-Harmonic Generation in a Hybrid Manganese Bromide: [3-Quinuclidinone]₂MnBr₄

Second-harmonic generation (SHG) is an upconversion process occurring in noncentrosymmetric crystals and can be used to transform near-infrared light to the visible region, hence fundamentally and technologically important for the development of lasing, sensing, and imaging. Herein, we report ketone-substituted [3-quinuclidinone]₂MnBr₄ (QO-MnBr₄), derived from [quinuclidine]₂MnBr₄ (space group P2₁/c), which crystallizes in the noncentrosymmetric space group Pmn2₁. This compound exhibits green emission at about 520 nm upon UV excitation, attributed to the ⁴T₁ → ⁶A₁ radiative recombination of [MnBr₄]^2- tetrahedra. More importantly, QO-MnBr₄ shows a remarkable SHG response under 1064 nm excitation, which is 1 order of magnitude larger than that of y-cut quartz. Further wavelength-dependent SHG measurements reveal that the giant SHG response is ascribed to the excited state resonance. This work demonstrates symmetry-breaking engineering as a strategy to construct resonant channels for SHG enhancement in manganese-based bromides, offering a new design platform for high-performance SHG materials.