Coordinated Water Molecules Regulate Exciton Localization and Bright Luminescence in Sb3+-Doped Indium Bromide Hybrid

Abstract Inner-sphere coordination in metal halide hybrids (MHHs) represents a powerful yet largely unexplored handle for regulating excitonic emission. Here, we demonstrate that inner-sphere coordinated water molecules govern the photophysics of Sb3+-doped indium bromide hybrid (Sb:DAMPInBr), synthesized using 1-(2-dimethylaminoethyl)-4-methylpiperazine (DAMP). The zero-dimensional hybrid comprises two inequivalent halometallate units, (In/Sb)Br6 and water-coordinated (In/Sb)Br4(H2O)2, enabling direct interrogation of coordination-induced structural and electronic effects on excitonic emission. The hydrated phase exhibits intense yellow emission (∼570 nm, PLQY 47%), whereas dehydration produces a red-shifted, weak orange emission (∼600 nm, PLQY ∼4%). Density functional theory reveals that coordinated water actively participates in the excited-state electronic structure, enhancing exciton localization, thereby stabilizing self-trapped excitons (STE) within the [(Sb/In)Br4(H2O)2] unit and promoting radiative recombination. In contrast, dehydration weakens exciton localization, leading to increased nonradiative decay. The reversible restoration of emission upon rehydration establishes a hydration-driven luminescence switching mechanism and identifies coordinated water as an active structural motif rather than a passive source of lattice distortion. More broadly, this work extends the current understanding of luminescence in MHHs by demonstrating that structural distortion-induced STE formation alone does not fully account for the observed emission efficiency. Instead, our findings establish exciton localization, directly regulated by inner-sphere coordinated water molecules, as a key factor governing radiative recombination and luminescence efficiency in hydrated MHHs, providing a new design principle for engineering stimuli-responsive luminescent materials.