Halogen Contacts-Induced Unusual Coloring in BiIII Bromide Complex: Anion-to-Cation Charge Transfer via Br⋅⋅⋅Br Interactions.

A yellow bromobismuthate {(2-BrPy)2 H}[BiBr4 ] (1, 2-BrPy=2-bromopyridinium) transforms into the unusually deeply colored cherry-red (2-BrPyH)2 [BiBr5 ] (2). A combination of structural studies and theoretical calculations confirms that the appearance of short non-covalent Br⋅⋅⋅Br interactions (≈3.3 Å) in 2 is responsible for the anion-to-cation charge transfer (LP(Brligand )→σ*(Br-C)), yielding dramatic changes in optical behavior. This effect opens the way towards novel halogen bonding-templated halometalate-based hybrid materials with enhanced optical properties.