Evidently, 4a 2+ and 4b 2+ are potent catalysts for oxidative C-N coupling reactions with challenging substrates. Initial mechanistic irradiation experiments with different output power settings of the LEDs and shorter reaction time (5 h, to avoid saturation effects, Table ) did not show a pronounced power-dependence, indicating a one-photon mechanism. However, the results presented so far do not provide clear mechanistic insights, in particular as (i) the radical cations 4a •+ /4b •+ could not be characterized (spectro)electro-chemically and (ii) the role of O2 in the mechanism has to be clarified. First, we had a closer look at the properties of the emissive excited singlet states of 4a 2+ /4b 2+ . The excited-state energies E00 are on the order of 2.5 eV and very similar for both compounds (Figure ). The excited states decay monoexponentially with a typical lifetime of a few nanoseconds. That lifetime is shorter in MeCN compared to CH2Cl2 by a factor of ~3 and we observed a dependence on the counteranion in the latter solvent (see Table for details and the discussion below). Importantly, there is essentially no quenching of excited 4a 2+