Origin of inverse emission behaviour in strain-engineered zero-dimensional tin halides: all-inorganic vs. hybrid

Zero-dimensional (0D) tin halides exhibit strong self-trapped exciton (STE) mediated emission driven by the stereochemically active ns² lone pair, making them promising candidates for next-generation light-emitting technologies. Strain engineering provides a powerful tool to tune STE behavior, offering a platform for pressure-tunable photoluminescence (PL). However, emission parameters like PL intensity, brightness, emission wavelength and PL broadness do not follow a universal pressure-dependent response. In a comparable external pressure range, all-inorganic Cs₄SnBr₆ undergoes a transition from weak narrowband to intense broadband emission, whereas the hybrid (C₉NH₂₀)₂SnBr₄ shifts from bright red broadband to weak narrowband emission. However, the origin of such inverse behaviour has not been established. Using state-of-the-art density functional theory (DFT) calculations, we have unveiled the atomistic understanding behind such an inverse trend. Our results demonstrate that at a certain critical pressure, both systems undergo a transition in emission, however, with a completely inverse nature, which arises from the different fate of STEs. In Cs₄SnBr₆, compression induces octahedral twisting and inter-octahedral Br-Br interactions already in the ground state, providing a favourable geometry for electron-hole localization, thereby stabilizing STEs resulting in brighter emission. In contrast, the hybrid system exhibits no significant ground-state structural rearrangement of the inorganic units as the bulky soft organic cations effectively screen the applied pressure by altering its packing nature, thereby limiting the pressure effect to the inorganic units. However, the structural response emerges exclusively in the excited state, where asymmetric distortion of the SnBr₄^2- unit forces the STE centre to switch from a disphenoidal to a trigonal pyramid geometry, reducing electron-hole confinement and destabilizing the STE, leading to a dark state. Such inverse responses demonstrate that, unlike previous popular understanding, pressure-dependent luminescence in 0D halides cannot be rationalized purely from ground-state ns² lone-pair activity. Instead, in the excited state, the reorganization of the STE hosting unit governs the emission behaviour, mediated by the interplay among coordination symmetry, packing constraints, and local lattice flexibility. This study reveals the mechanistic origin of experimentally observed strain-modulated optoelectronic behaviour in 0D tin halides and lays a critical foundation for the rational design of strain engineered luminescent materials, with implications for advancing light emitting technologies.