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 degrees C. The crystal structure of the new bromoantimonate(iii) consisted of {Sb2Br9}3- zigzag chains, which were composed of strongly trigonally distorted SbBr63- octahedral anions and C5H14N22+ dications. Weak interactions, including (N)H & ctdot;Br hydrogen bonds and Br & ctdot;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.