•-] = -0.59 V vs SCE) is substantially easier to reduce than DCA (E 1/2 [DCA/DCA •-] = -0.98 V vs SCE), rationalized by the electron-withdrawing nature of sulfonamide groups at its 2,6-positions (Figure ). The photocatalyst excited state oxidation potential can be approximated by a derivative of the Rehm-Weller equation. In both cases (Figure ) taking the longest wavelength absorption peaks ( max = 422 nm) and shortest wavelength emission peaks ( max = 435 nm), E 0,0 for the singlet excited state can be approximated (≈ 2.90 eV). Thus, 1 DCAS* (E 1/2 [ 1 DCAS*/DCAS •-] = +2.31 V vs SCE) is a notably more potent photooxidant than 1 DCA* (E 1/2 [ 1 DCAS*/DCA •-] = +1.93 V vs SCE). Our initial hypothesis was thus based on the SET mechanism proposed by Santamaria (Figure ). In this premise, 1 DCAS* underwent reductive quenching by trialkylamine 1 and DCAS was regenerated from oxidation of DCAS •-via O 2 (initial SET reactions of trialkylamines were also reported as the main pathway for photocatalytic oxidations to N-oxides and demethylations using thiazine and fluoresceine dyes). Trialkylamine radical cation 1' would undergo transformation to iminium ion 10, either via hydrogen atom transfer (HAT) with superoxide or by deprotonation to -amino radical 1" followed by oxidation of 1" by peroxyl radical. In the presence of H 2 O, 10 forms 11a which undergoes either hydrolysis to secondary amine or further oxidation to product 2. Alternatively, peroxide anion is more nucleophilic than H 2 O and reacts with 10 to form product 2 via the collapse of 11b. Intermediate 11b could also form via radical combination of 1" and peroxyl radical. In support of this initial hypothesis, control batch experiments employing 2.0 equiv. of DCA and DCAS under strict N 2 protection in MeCN afforded clean conversion of 1a in both cases to a 1 : 1 mixture of 1a : 3a (Figure , top). Upon exposure to light, the reaction mixtures darkened from pale yellow to purple (Figure , bottom). Upon removal of light and exposure to air, the reaction mixtures lightened to yellow again. These observations are hallmarks of cyanoanthracene radical anions and the Ndemethylation reaction confirms the SET oxidation of 1 to 1' by the organophotocatalyst. Here, PhCN was needed instead of MeCN as solvent to improve solubility in the case of the stoichiometric quantities of cyanoanthracenes employed. In the absence of additional base, 1a deprotonates 1a •+ to afford 1" (10 after a second SET oxidation), meaning the reaction fundamentally could never exceed 50% conversion. Although both reactions were slow (days), the reaction employing DCAS was markedly faster, presumably either due to its higher excited state oxidation potential or, more likely, its higher solubility (DCA was not fully partially soluble in this batch reaction). In light of the preceding discussion in support of the mechanism in Figure , one would expect 1 DCAS* to undergo more rapid fluorescence quenching than 1 DCA* by amines. Very surprisingly, the opposite was true. The Stern-Volmer quenching rate constant for 1 DCAS* by 1a (k q = 1.44 × 10 9 M -1 s -1 ) was two orders of magnitude smaller than for 1 DCA* (k q = 1.69 × 10 11 M -1 s -1 ). 100,101 Presumably, either i) the 2-methoxyethyl groups of DCAS inhibit bimolecular quenching events by sterically obstructing approach of the trialkylamine, or ii) aggregation of DCA accelerates its reductive quenching by trialkylamines which is broken up in the case of DCAS.