Photoluminescence Enhancement through Anion Sublattice Engineering in (dien)In(Cl _{1– <i>x</i>} Br _<i>x</i> ) ₆ Hybrid Halides

Indium-based hybrid halides are emerging as promising lead-free highly efficient luminescent materials, yet the relationship among their composition, structure, and emission mechanism remains poorly understood. Here, we synthesized and characterized a complete (dien)In(Cl1–xBrx)6 (0 ≤ x ≤ 1) (dien─diethylenetriamine) solid-solution series to elucidate the origin of their photoluminescence (PL) behavior. Pristine (dien)InCl6 exhibits organic impurity-related emission at 430 nm and lower-energy self-trapped exciton (STE) emission, while even minor Br incorporation (as low as 3.7%) induces a sharp red shift of emission to 580 nm. Structural analysis reveals a sequence of phase transformations accompanied by halide site preference redistribution and anionic substructure ordering. Combined spectroscopic and DFT analyses show that PL originates from ligand-to-metal charge transfer (LMCT) transitions followed by radiative recombination between self-trapped excitons and halide nonbonding states. The systematic evolution of PL and PLE spectra, decreasing Stokes shift, and nonmonotonic PLQY trend, which reaches a maximum of 57% at x = 0.744, are rationalized by suppression of the impurity-associated emission and increasing orbital overlap upon bromide substitution. These results establish the emission mechanism in (dien)In(Cl1–xBrx)6 and demonstrate how subtle structural and electronic changes govern radiative processes in indium halide hybrids, offering design principles for efficient lead-free luminescent materials.