Impact of isomerism on the photoproduction of carbon monoxide and singlet oxygen by visible-light absorbing rhenium(I) photoCORMs

Abstract

Photoactive rhenium(I) complexes have high therapeutic potential, particularly in the fight against cancer and antibiotic-resistant bacteria. They have the particularity of combining several modes of action, including photogeneration of carbon monoxide (CO) and singlet oxygen. However, their design is still to be improved. One limitation is that most of them absorb in the UV range, as is the case for Re-Pyta-TPP, a very attractive photochemically CO-releasing molecule (PhotoCORM). To shift absorption to the visible, an additional nitrogen atom was introduced into the pyridyltriazole bidentate ligand of this complex. The four resulting isomers Re-Ln-TPP (n = 1–4) were thus synthesized and studied from both an experimental and a theoretical point of view. Remarkably, their spectroscopic and photochemical properties strongly depend on the substitution pattern. Three out of four complexes can indeed be excited in the visible. The photodecarbonylation yield of the new complexes was lower than that of Re-Pyta-TPP. Interesting links were found between absorption and photoluminescence data, and photoreactivity. DFT and TD-DFT calculations provided rational bases to these observations, showing that the quantum yield for CO photorelease can be correlated to the potential energy profile for the complexes in their lowest triplet state. The Re-Ln-TPP complexes that absorb and emit at the longest wavelengths have the best singlet oxygen photoproduction. This shows that it is not easy to optimize all the spectroscopic and photochemical properties at once, and that a fair balance must be found between them. This study is another step towards rationally-designed photoCORMs.