Open for Bismuth: Main Group Metal-to-Ligand Charge Transfer

The synthesis, characterization, and photophysical properties of 4- and 6-coordinate Bi³⁺ coordination complexes are reported. Bi(bzq)₃ (1) and [Bi(bzq)₂]Br (2) (bzq = benzo[h]quinoline) are synthesized by reaction of 9-Li-bzq with BiCl₃ and BiBr₃, respectively. Absorption spectroscopy, electrochemistry, and DFT studies suggest that 1 has 42% Bi 6s character in its highest-occupied molecular orbital (HOMO) as a result of six σ* interactions with the bzq ligands. Excitation of 1 at 450 nm results in a broad emission feature at 520 nm, which is rationalized as a metal-to-ligand charge transfer (MLCT) and phosphorescent emission resulting from bismuth-mediated intersystem crossing (ISC) to a triplet excited state. This excited state revealed a 35 μs lifetime and was quenched in the presence of oxygen. These results demonstrate that useful optoelectronic properties of Bi³⁺ can be accessed through hypercoordination with covalent organobismuth interactions that mimic the electronic structure of lead perovskites.