We also assessed the performance of these PCs in the photocatalytic atom transfer radical addition (ATRA) reaction of styrene and p-toluenesulfonyl chloride (TsCl). The generally accepted mechanism involves an oxidative quenching of the PC* by TsCl, generating a tosyl radical (Ts • ), which engages in a radical addition onto the olefin. The benzylic radical is then oxidized by PC +• , regenerating the ground-state neutral catalyst and forming a cationic intermediate that is subsequently trapped by the liberated chloride anion (Figure ). The reduction potential of TsCl (Ered = -0.94 V vs SCE) is comfortably less negative than the Eox* of these PCs (Table ), meaning that all four PCs are capable of reducing TsCl. Separately, the PC •+ must be capable of oxidizing the carbon-centred radical following addition to the olefin; therefore, a moderate ground-state oxidation potential is also needed to turn over this reaction.