Unlocking Nonlinear Optics and Photoluminescence with Simultaneous Enhancement of Bandgap and Birefringence in Hybrid Sb-Based Halides

Organic–inorganic hybrid metal halides have emerged as highly promising nonlinear optical (NLO) candidates due to their structural diversity and excellent growth habits. Herein, two novel hybrid Sb-based halides, (C 4 H 8 N 5 ) 2 SbBr 5 ( MGSB ) and (C 4 H 8 N 5 ) 2 SbCl 5 ·H 2 O ( MGSC ), were successfully synthesized by halogen substitution and hydrogen bond engineering. MGSB crystallizes in the centrosymmetric (CS) space group P 1̅, whereas MGSC adopts the noncentrosymmetric (NCS) space group Pca 2 1 . The NCS structure of MGSC originates from synergistic effects among asymmetric hydrogen bonding interactions, highly distorted inorganic polyhedra, and the ordered zigzag arrangements of organic molecules in the lattice. MGSC shows a promising second-harmonic generation (SHG) response (1.8 × KDP), a wide bandgap (3.78 eV), and appropriate birefringence (0.16@546 nm), making it a competitive NLO candidate. Moreover, MGSC exhibits bright blue fluorescence under ultraviolet light, indicating that MGSC achieves an unprecedented quadruple enhancement in optical properties relative to MGSB . This work provides an effective strategy for developing multioptical-functional hybrid metal halides.