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 pKa of Re(CO)3(CH 3bbpyCOOH)Cl obtained from the pH titration curve of the complex absorption at 409 nm was 2.5.
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