Antimony Dopant-Driven Monomer–Dimer Structural Transition in a Zero-Dimensional Indium Halide Hybrid

While metal-ion doping is typically known to preserve the structural and dimensional integrity of zero-dimensional (0D) host metal halide hybrids, introducing only subtle local perturbations, this work uncovers a striking deviation from this paradigm. We demonstrate that dopant incorporation can serve as an effective overall structural modulator, inducing transformations in coordination geometry and framework connectivity/dimensionality. Specifically, Sb³⁺ ions act as dopants that drive a transition in a 1-methylpiperazine-based indium bromide hybrid ((C₅H₁₄N₂)₂InBr₇·H₂O) from a monomeric 0D structure with isolated [InBr₆]^3- octahedral units to a dimeric framework ((C₅H₁₄N₂)₂In_1.81Sb_0.19Br₁₀·(H₂O)₂) featuring edge-sharing [(In/Sb)₂Br₁₀]^4- octahedral units that exhibit strong orange emission (photoluminescence quantum yield of ≈59%). The structural modulation persists across a broad Sb³⁺ concentration range and mirrors the dimeric nature of the Sb analogue control sample ((C₅H₁₄N₂)₂Sb₂Br₁₀·(H₂O)₂). DFT and thermodynamic formation energy analyses confirm self-trapped exciton-mediated Sb³⁺ emission and the energetic preference for the dimeric phase, establishing doping as a powerful route for tuning the overall structure/dimensionality and luminescence of 0D metal halide hybrids.