Photophysics and photochemistry of a prospective light-activated anticancer dirhodium complex

Dirhodium complexes Rh(II,II) with organic ligands are known to demonstrate light-induced cytotoxicity, combining both oxygen-dependent (like in the case of photodynamic therapy, PDT) and oxygen-independent (like in the case of photo-activated chemotherapy, PACT) anticancer activity. Photophysics and photochemistry of the previously reported light-activated anticancer dirhodium complex cis-[Rh₂(μ-O₂CCH₃)₂(dppn)₂](O₂CCH₃)₂ (dppn = benzo[i]dipyrido[3,2-a:2',3'-c]phenazine) were studied by means of several stationary (UV spectroscopy and capillary electrophoresis) and time-resolved (laser flash photolysis, ultrafast TA spectroscopy and direct singlet oxygen detection) methods. The only observed photochemical reaction is photoaquation with the release of an acetate ligand; its quantum yield is low (0.026%). The quantum yields of singlet oxygen formation in different solvents (D₂O, CH₃CN, and CD₃OD) lie in the range (2-8)%, which is consistent with the presumably oxygen-independent pathway of light-induced cell-damage reported in the literature. Complicated behavior of intermediate absorption in the time interval from 100 fs to 20 μs was interpreted by "ladder" transitions between excited states of different nature, ¹IL* → ³IL* → ³MLCT* → ³MC*. The lifetime of the longest-living excited state ³MC* is 6.2 μs. Estimations show that the oxygen-independent light-induced cytotoxic effect could be provided even without preliminary binding of the complex to the substrate.