Crystal Structure and Optical Properties Characterization in Quasi-0D Lead-Free Bromide Crystals (C₆H₁₄N)₃Bi₂Br₉·H₂O and (C₆H₁₄N)₃Sb₃Br₁₂

Low dimensional organic inorganic metal halide materials have shown broadband emission and large Stokes shift, making them widely used in various fields and a promising candidate material. Here, the zero-dimensional lead-free bromide single crystals (C₆H₁₄N)₃Bi₂Br₉·H₂O (1) and (C₆H₁₄N)₃Sb₃Br₁₂ (2) were synthesized. They crystallized in the monoclinic crystal system with the space group of P2₁ and P2₁/n, respectively. Through ultraviolet–visible–near-infrared (UV–vis–NIR) absorption analysis, the band gaps of (C₆H₁₄N)₃Bi₂Br₉·H₂O and (C₆H₁₄N)₃Sb₃Br₁₂ are found to be 2.75 and 2.83 eV, respectively. Upon photoexcitation, (C₆H₁₄N)₃Bi₂Br₉·H₂O exhibit broad-band red emission peaking at 640 nm with a large Stokes shift of 180 nm and a lifetime of 2.94 ns, and the emission spectrum of (C₆H₁₄N)₃Sb₃Br₁₂ are similar to those of (C₆H₁₄N)₃Bi₂Br₉·H₂O. This exclusive red emission is ascribed to the self-trapping exciton transition caused by lattice distortion, which is confirmed through both experiments and first-principles calculations. In addition, due to the polar space group structure and the large spin–orbit coupling (SOC) associated with the heavy elements of Bi and Br of crystal 1, an obvious Rashba effect was observed. The discovery of organic inorganic metal bromide material provides a critical foundation for uncovering the connection between 0D metal halide materials’ structures and properties.