Photochemistry of P,N-bidentate rhenium(I) tricarbonyl complexes: reactive species generation and potential application for antibacterial photodynamic therapy

In this work, we describe the photoisomerization of facial rhenium(I) tricarbonyl complexes bearing P,N-bidentate pyridyl/phosphine ligands with different chelating rings and anions: RePNBr, RePNTfO, and RePNNBr, which are triggered under irradiation at 365 nm in solutions. The apparent photodegradation rate constants (k(app)) depend on the coordinating ability of the solvent, being lowest in acetonitrile. The k(app) value increases as the temperature rises, suggesting a reactive IL excited state thermally populated from the MLCT excited state involved. Using the Eyring equation, positive activation enthalpies (Delta H-not equal) accompanied by high negative values for the activation entropy (Delta S-not equal) were obtained. These results suggest whatever the P,N-ligand or anion, the reaction proceeds through a strongly solvated or a compact transition state, which is compatible with an associative mechanism for the photoisomerization. A 100-fold decrease in the log(10) CFU value is observed for E. coli and S. aureus in irradiated solutions of the compounds, which follows the same tendency as their singlet oxygen generation quantum yield: RePNBr > RePNTfO > RePNNBr, while no antibacterial activity is observed in the darkness. This result indicates that the generation of singlet oxygen plays a key role in the antibacterial capacity of these complexes.

Automated summary

This study investigates the photoisomerization of facial rhenium(I) tricarbonyl complexes in solutions with different P,N-bidentate ligands and anions under irradiation at 365 nm. The reaction involves a reactive IL excited state thermally populated from the MLCT excited state, with positive activation enthalpies and high negative values for the activation entropy. The complexes exhibit antibacterial activity correlated with their singlet oxygen generation quantum yield, suggesting a key role of singlet oxygen in their antibacterial capacity.