The excitation of [Ru(bpy)3] 2+ , which is a sufficiently strong photooxidant for the desired reaction, with 532 nm in the presence of 4a gave the expected post-quenching spectra with a superposition of [Ru(bpy)3] •+ and 4a •+ (upper part of Figure ). The sensitizer-derived photoproduct (gray spectrum in Figure ) with a well-known TA spectrum showing an absorption band peaking at ~510 nm and a characteristic ground-state bleach at ~440 nm decays on a microsecond timescale. The remaining TA signals are attributed to the quenching by-product 4a •+ . This assignment is based on (i) the similarity to the UV-Vis spectrum of 3a •+ (Figure ), (ii) our successful spectral separation displayed in the upper part of Figure and (iii) additional kinetic TA measurements (Figure ). Taking the known difference molar absorption coefficient for [Ru(bpy)3] •+ at 500 nm (~12000 M -1 cm -1 ), where 4a •+ is essentially transparent, the 4a •+ spectrum can be calibrated. The calibrated absorption spectra of the hybrid 4a and its oxidation states are shown in Figure (middle). While 4a •+ is inaccessible by electrochemical oxidation/reduction, our investigations clearly demonstrated its photochemical generation from both 4a and 4a 2+ , also via a challenging arene oxidation with the singlet-excited state of the latter species. However, the difficulties in obtaining high concentrations of 4a •+ under conditions similar to those used for catalysis hamper further meaningful spectroscopic investigations. We, therefore, performed mechanistic irradiation experiments to obtain (indirect) evidence for the 4a 2+ regeneration in the catalytic cycle under study. well as a BP •+ reference spectrum (violet) and the resulting difference spectrum (brown).