Emissive or Nonemissive? Molecular Insight into Luminescence Properties of Tin(II) Metal Halides

In addition to the easy oxidation of tin(II) (Sn²⁺), the poor repeatability in synthesizing luminescent Sn²⁺-based halide perovskites can be attributed to the structural diversity among Sn compositions, with many structures failing to exhibit luminescence. Furthermore, compared to luminescent compounds, there is insufficient attention on the photophysical properties of these nonluminescent compositions, which impedes a deeper understanding of the relationship between their structures and optical properties. In this work, we report two Sn²⁺-based halide compounds, (DFPD)₆SnBr₈ and (DFPD)₂SnBr₄ (DFPD⁺ = 4,4-difluoropiperidinium). Both exhibit excellent air stability, with the former demonstrating a high luminescence efficiency of ∼92%, while the latter is essentially nonluminescent. Theoretical calculations suggest that the nonluminescence of (DFPD)₂SnBr₄ arises from charge transfer between two adjacent [SnBr₄]^2- units in the first excited state. In contrast, significant structural distortion and localization of excitons in (DFPD)₆SnBr₈ indicate that its emission originates from self-trapped excitons. As a demonstration, we prepared an X-ray scintillator based on (DFPD)₆SnBr₈ with a high light yield up to 27,600 ph/MeV and a low detection limit of 84.7 nGy/s, which is significantly better than the commercial LuAG/Ce scintillator (22,000 ph/MeV, 2.32 μGy/s).