Synthesis and Luminescence Properties of (TMS)₄Bi_1–<i>x</i>Sb_<i>x</i>Br₇: A Series of 0D Hybrid Metal Halides Containing S-Based Organic Cations

Zero-dimensional (0D) metal halides, particularly hybrid lead-free metal halides, have recently attracted considerable research attention because of their intriguing emission properties and structural flexibility. Understanding the correlation between structural characteristics and luminescence properties is crucial for the development of new 0D metal halides with enhanced optoelectronic features for practical applications. In this work, we synthesized 0D mixed metal halides containing a sulfur-based organic cation, (TMS)4Bi1–xSbxBr7 [x = 0, 0.4, 0.8, 1]; (TMS)+ = [(CH3)3(S)+], and investigated their photoluminescence properties. All of the compounds are crystallized in the trigonal R3̅m space group and contain isolated regular [MBr6]3– (M = Bi/Sb) octahedra surrounded by (TMS)+ cations and linear TMS–Br units. The M–Br bond lengths are identical in the [MBr6]3– octahedral units and the bond angle is very close to the ideal value (∠Br–M–Br ≈ 90°). Optical analysis indicates that the compounds exhibit characteristics of the indirect band gap type, with minimal energy differences between the direct and indirect band gap values. Photoluminescence (PL) investigations show that the substitution of Sb at the Bi site in (TMS)4Bi1–xSbxBr7 leads to superior emission properties. The pristine Sb compound exhibits strong PL emission at 588 nm, with a photoluminescence quantum yield (PLQY) of 26.6% and a large Stokes shift of 165 nm, while the Bi analogue shows a very weak emission at 505 nm with a PLQY of 0.09%. Theoretical studies show greater distortion and isolation of the [SbBr6]3– polyhedra compared to the Bi analogue in the excited state. The higher PLQY of the Sb compound may be attributed to the relatively greater distortion and isolation of the [SbBr6]3–octahedra in the excited state. This work provides significant insight into the relationship between the structural, electronic, and photoluminescence properties of 0D lead-free metal halides containing sulfur-based A-site cations.