Theoretical assessment of the photosensitization mechanisms of porphyrin–ruthenium(II) complexes for the formation of reactive oxygen species

• We propose photosensitization mechanisms for the formation of reactive oxygen species.
• Energy-transfer and electron-transfer processes were assessed.
• Generation of 1O2 is favored for Ru(II) complexes in their triplet excited state.
• Anionic radical of porphyrin–Ru(II) complex favors the formation of the superoxide anion radical.

Following a thermodynamics point of view, we analyzed a set of photosensitization mechanisms (energy transfer, electron transfer) given by seven reactions applied to porphyrin–ruthenium(II) complexes (1–6) and their fragments (ruthenium complexes: 7–12; porphyrin: 13) at the B3LYP/6–31G(d,p)/LANL2DZ level of theory in condensed phase (aqueous solution). We found that whereas for the fragments (7–13) an energy-transfer process between their first triplet excited state (T1) and molecular oxygen (3O2) to yield singlet oxygen (1O2) is favored, the privileged channels for the photosensitizers (1–6) correspond to production of a superoxide anion radical (O2−) by electron transfer between 3O2 and the photosensitizers (P) either in their P(T1) excited state or as a radical anion P−. The reaction channels involving P− are more exergonic by 13–17 kcal/mol with respect to the process involving P(T1). These results suggest that the anionic photosensitizer would likely acquire more importance for the photodamage in the cell.

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