For our initial substrate, benzene, several Stern-Volmer quenching studies were carried out (Table ) using different solvents (CH2Cl2 and MeCN), 4a 2+ as well as 4b 2+ and both counter anions (SbF6 -or BF4 -). All quenching rate constants kq so obtained are in the range from 1.5 to 5.0 × 10 8 M -1 s -1 , i.e. slightly below the diffusion limit. Under all conditions investigated, the excited N-Ph-derivative 4b 2+ has a shorter lifetime than the N-Me-derivative 4a 2+ , but this is usually compensated by a faster kq for 4b 2+ such that the overall quenching efficiencies are comparable for both catalysts. An extrapolation of these results to the actual irradiation conditions (5.6 M of benzene) gives quenching efficiencies on the order of 90%, which implies high quantum yields for the overall reaction and short irradiation times. Nanosecond laser flash photolysis (LFP) studies upon 355 nm excitation of 4a 2+ in the presence of benzene concentrations allowing up to 90% *4a 2+ quenching were carried out to detect both quenching products. However, we only observed baseline-like spectra without any evidence for the benzene radical cation and 4a •+ , despite the high sensitivity of our LFP setup. Unproductive and ultrafast back-electron transfer in the socalled solvent cage is spin-allowed for this reaction, which can be regarded as a general problem for photoinduced electron transfer reactions with photoexcited singlet states. In this particular case, the quantum yield for productive cage escape to yield radical ions that can be detected and intercepted seems to be very low (< 5%).