Photophysics and acid-base chemistry of Re(CH₃bpyCOOH)(CO) ₃X (X=CI^-, and imidazole) complexes

Rhenium tricarbonyl (4′-methyl-2,2′-bipyridine-4-carboxylic acid) X (where X is Cl^- and imidazole) complexes have been prepared. These two complexes exhibit similar spectroscopic properties. The metal-to-ligand charge-transfer (MLCT) absorption and the corresponding emission are observed. This charge transfer band is highly solvent dependent. Due to the stronger electron-withdrawing ability of the -COOH (in comparison to the -COO^- group), the MLCT band has red-shifted during protonation. Emission quantum yields are dramatically reduced while lifetime remains similar upon protonation. These behaviors are typical in static quenching mechanism by protons. The ground state pK_a of Re(CO)₃(CH ₃bbpyCOOH)Cl obtained from the pH titration curve of the complex absorption at 409 nm was 2.5.