Control of Symmetry and Photophysical Properties of Hybrid Antimony Chlorides via Mixed Organic Cations

Zero-dimensional organic-inorganic hybrid metal halides (0D OIHMHs) have emerged as a promising platform for exploring structure-property relationships in low-dimensional optical materials. However, achieving photoluminescence (PL) and second-harmonic generation (SHG) simultaneously in 0D OIHMHs remains challenging due to the conflicting structural and electronic requirements. Here, we investigate the PL and SHG properties of four newly synthesized 0D Sb-based chlorides via an organic-cation-mixing strategy. These four compounds, (TMA)₂(MIA)SbCl₆ (1), (DMBA)₂(MIA)SbCl₆·H₂O (2), (TMA)₂(IA)₃Sb₂Cl₁₁ (3), and (DMBA)₂(IA)SbCl₆ (4), possess 0D inorganic units and all crystallize in the noncentrosymmetric space group P2₁. Compounds 4, 2, and 1 exhibit broadband emission with PLQYs of 52%, 57%, and 95%, respectively, primarily correlated with the increased octahedral separation. Meanwhile, compounds 2 and 3 exhibit SHG responses of 0.43 and 0.78 × KDP, respectively. The stronger SHG activity of compound 3 originates from the collective noncentrosymmetry of all components, while in the other three compounds, the noncentrosymmetry is limited to imidazole units. These results suggest that mixed-organic-cation engineering dictates the symmetry, octahedral connectivity, and separation, thereby mediating both PL and SHG responses. This work elucidates the important role of mixed organic cations in the control of symmetry and photophysical properties, offering new insights into the rational design of multifunctional low-dimensional hybrid materials.