To further probe the importance of the rate of mesolytic cleavage, an intermolecular competition was conducted between 9-bromoanthracene ( ) and 1-iodonaphthalene (13) (Figure ), both of which are known to go through stepwise dissociative electron transfer pathways. While the former has a more positive reduction onset potential (-2.07 V for 12 vs -2.07 V for 13), the radical anion of the latter is known to undergo a 2000-fold faster rate of mesolytic cleavage. The reaction afforded exclusive formation of 14b, arising from oxidative addition into 1-iodonaphthalene. Together with the experiment in Figure , this suggests that mesolytic cleavage is at least product-determining in the mechanism due to the competition with unproductive back-electron transfer. However, the combined data from the Hammett plot and the competition reaction in Figure suggest that the rate of mesolytic cleavage (vs backelectron transfer) and the rate of electron transfer may both contribute to the observed rate of the reaction. Indeed, this combined influence of the electron transfer (favored for more electron-deficient arenes) and the rate of mesolytic cleavage (favored for more electron-rich arenes) may also help rationalize the low ρ value observed in the Hammett plot obtained for a mechanism proceeding via a SET mechanism (Figure ). It is worth pointing out that the Hammett series obtained points against mesolytic cleavage being the sole rate-determining step, since a correlation with a negative ρ value would be expected. The mechanistically grounded importance of the mesolytic cleavage step can rationalize the exquisite selectivity of this system for oxidative addition into aryl thianthrenium species and aryl iodides, which have significantly faster rates of radical anion fragmentation relative to other aryl (pseudo)halides. Based on the mechanistic picture established for the oxidative addition process into aryl iodides, we entertained the possibility of expanding the scope of electrophiles that can be reductively activated by the combination of 1a and red light. We found that several classes of alkyl electrophiles that are unreactive in the presence of 1a in the ground state (dark), can be cleanly activated in the presence of red light. The red light-active 1a could readily undergo SET-based oxidative additions into non-chlorinated redox active esters, as well as secondary alkyl iodides and bromides (Figure ).