Photoluminescent chain-of-dimer anion in a novel hybrid halometallate (C₅H₁₄N₂)₃{Sb₂Br₉}₂: atypical behavior in one-dimensional anionic systems

In pursuit of identifying less toxic hybrid compounds suitable for optoelectronic applications, we synthesized a novel homopiperazinium bromoantimonate(III), (C₅H₁₄N₂)₃{Sb₂Br₉}₂. It readily crystallized from an aqueous hydrobromic acid solution and was found to be stable both in air and upon heating up to 175 °C. The crystal structure of the new bromoantimonate(III) consisted of {Sb₂Br₉}^3- zigzag chains, which were composed of strongly trigonally distorted SbBr₆^3- octahedral anions and C₅H₁₄N₂²⁺ dications. Weak interactions, including (N)H⋯Br hydrogen bonds and Br⋯Br van der Waals contacts, facilitated the assembly of the structural moieties into a three-dimensional (3D) supramolecular framework. The photoluminescence (PL) spectrum of (C₅H₁₄N₂)₃{Sb₂Br₉}₂ displayed two broad bands in visible and near-infrared regions, which were scarcely detectable at room temperature but gained intensity upon cooling. Red emission at 645 nm, typically observed in zero-dimensional halometallates, was attributed to antimony 5s² lone-pair-driven luminescence that corresponded to the emission from the self-trapped exciton (STE) state. The title compound also exhibited an atypical PL band at 880 nm, which has been discussed along with the optical properties and Raman spectrum of (C₅H₁₄N₂)₃{Sb₂Br₉}₂.