Next, we conducted a series of experiments to shed light on the mechanism of the photocatalytic transformations. Radical pathways were convincingly demonstrated through radical clock experiments (Figure ). In fact, the isolation of compounds 68 and 69 starting from NHPI esters 1al and 1am provided strong evidences for the formation of cyclopentyl methyl and cyclopropyl methyl radicals via favored 5-exo-trig and retro-3-exotrig cyclization pathways, respectively. Quantum yield measurements for both Ritter-type carboamidation and Hecktype reaction revealed that the transformations are photocatalytic in nature and that a radical chain is either absent or inefficient (Φ = 4.3 x 10 -3 for Ritter-type and Φ = 3.6 x 10 -3 for Hecktype, see Supporting Information, section 8.4). On the basis of these results and those of previous studies, a proposed mechanism is presented in Figure . Upon absorption of light, the excited state of 4-CzIPN (*E1/2 = -1.18 V vs SCE) is oxidatively quenched by the redox active ester 1. This first step is promoted by LUMO lowering activation of 1 using TFA as Brønsted acid (or zinc as Lewis acid) under carboamidation conditions. Supportive of this proposal, cyclic voltammetry revealed a positive shift in the reduction potential of 1a from -1.25 V to -1.15 V vs SCE in the presence of 10 equivalents of TFA (see Supporting Information, section 8.2). For the acid-free Heck reaction, the SET event is proposed to be facilitated by a π-π stacking 'Lewis acid-type' activation, supported by detection of non-covalent interactions between 1 and 3a via NMR studies (see Supporting Information, section 8.3). The close interaction between the two reactants was further demonstrated by preliminary studies aimed at developing a catalyst-free process. We observed that certain combinations of RAEs containing a phenyl group separated by an alkyl chain length of a specific size, together with 3a, yielded Hecktype products even in the absence of 4-CzIPN (for further details, see Supporting Information, section 5.3). This implied an electron donor-acceptor (EDA) complex pathway may arise between 1,1-diarylolefin and RAEs that contain phenyl groups tethered at a certain length. However, given the crucial role of the photocatalyst in delivering higher yields and the limited generality of the donor-acceptor-type complex formation, the method was not further explored for synthetic applicability.