Second, we considered the mechanism operative under blue-light irradiation. Under this set of conditions, PC1 absorbs light resulting in formation of the corresponding highly oxidizing excited state (E(PC*/PCred) = + 2.06 V vs SCE). This excited state can react with B1 (E(B1 •+ /B1) = + 0.76 V vs SCE) via single-electron transfer to afford LBR (I) upon deprotonation. The latter intermediate is entrusted with the desired XAT step, which is expected to be relatively fast (k ~ 10 8 M -1 s -1 ), thus yielding the alkyl radical II. This radical can subsequently be trapped by the electron-poor olefin to give adduct III. The observed inhibition effect of O2 (Table , Entries 10-11) can be explained by taking into account that LBR I is known to react even faster with molecular oxygen (k > 10 8 M -1 s -1 ), thus confiscating this crucial intermediate for the XAT event. Alternatively, it has been reported that molecular oxygen can also quench the excited state of PC1 at diffusion controlled rates to form singlet oxygen (k = 2•10 9 M -1 s -1 ). Next, as shown by the deuterium labelling experiments, III abstracts a hydrogen-atom from B1 in a polarity-matched step to give product 3 and subsequently kicks off the chain propagation. 32