In contrast to what has been observed for some acridinium and structurally related isoacridone As the two-electron oxidation/reduction potential (Figure ) seems inappropriate for the determination of the required potential for the 4a 2+ /4a •+ couple and the excited-state reduction potential E(*4a 2+ /4a •+ ) accordingly, we carried out a detailed Rehm-Weller analysis to obtain that quantity. For that, we carried out time-resolved quenching experiments with nine different benzene and biphenyl derivatives and analyzed the resulting quenching rate constants as a function of the oxidation potential of the quencher (Figure and Figure ). These studies were carried out in MeCN because reliable quencher oxidation potentials, which are crucial for this analysis, are available in this solvent. In line with the expected quenching through the transfer of a single electron from the quencher to the excited catalyst, the bimolecular rate constants decrease with increasing oxidation potentials of the former. The E(*4a 2+ /4a •+ ) resulting from a fit based on the Rehm-Weller model is as high as +2.52 V vs. SCE (corresponding to +2.14 V vs.