Stereochemical Activity and Luminescence Mechanism in (C₄H₁₂N)₂SnCl₆ Doped with ns² Metal Ions

Metal halide luminescent materials, particularly those doped with ns² ions, exhibit exceptional optical properties. However, the luminescent mechanisms associated with the stereochemical activity of lone-pair electrons remain insufficiently explored. In this study, zero-dimensional (0D) tin-based halide (C₄H₁₂N)₂SnCl₆ is utilized as a model system to investigate the effects of lattice distortion and lone-pair electron expression on fluorescence emission characteristics by doping with 6s² and 5s² ions. The findings reveal that compared to 6s² ions, 5s² ion doping results in stronger electron-phonon coupling, causing significant octahedral lattice distortion and producing unique dual emissions along with a large Stokes shift of 317 nm. Remarkably, even at temperatures as high as 360 K, (C₄H₁₂N)₂SnCl₆:Sb maintains its dual emission due to insufficient thermal activation energy to promote electron transition from excited state 1 to excited state 2. In contrast, 6s² doping exhibits a single blue emission with a small Stokes shift of 101 nm. Additionally, calculations of the electron localization function reveal that 5s² lone pairs exhibit static stereochemical expression with specific spatial orientation, while 6s² lone pairs display dynamic behavior. These findings highlight the critical role of lone-pair stereochemistry in governing the photophysical properties of ns²-doped metal halides, offering new insights into their luminescent mechanisms.