Hydrostatic Pressure-DrivenBand Gap Tuning and Self-TrappedExciton Formation in (4FPEA)2SnBr4 Halide Perovskite

Two-dimensional tin-halide perovskites provide a highly tunable platform for exciton–phonon coupling and local lattice distortions, enabled by their intrinsically soft lattice. We report a combined temperature- and pressure-dependent photoluminescence study of the layered perovskite (4FPEA)2SnBr4. At room temperature, its optical response is dominated by near-band-edge (NBE) excitons, which red-shift linearly under hydrostatic pressure up to ∼3 GPa, indicating a rigid band-edge behavior without phase transitions. Cooling reveals a broad, strong Stokes-shifted self-trapped exciton (STE) emission, evidencing a crossover from delocalized to self-localized excitonic states. Strikingly, while NBE emission red-shifts under pressure, STE emission exhibits an anomalous blue shift, reflecting pressure-induced modification of the exciton–phonon energy landscape. In contrast, the iodide analogue (4FPEA)2SnI4 shows no STE emission under identical conditions, highlighting the critical role of lattice rigidity and dielectric screening in stabilizing self-trapped excitons.