To shed light on the origin of the observed reactivities, we turned our focus to study the different reactions mechanisms by spectrofluorimetry, chemical actinometry (using F NMR), EPR spectroscopy and isotopic labelling. Fluorimetric titrations were first performed to bring evidence for the occurrence of an electron transfer (ET) process between PC1 and Langlois reagent in MeCN (the weak luminescence of the photocatalyst PC2 in DMSO turned out to be incompatible with such measurements). Stern Volmer experiments were in all cases performed in thoroughly deaerated solution of the photocatalyst, as the presence of oxygen was seen to markedly quench the fluorescence of PC1, as a consequence of a reported reverse Intersystem Crossing (rISC) mechanism. 17 Unfortunately, to our surprise, no sign of luminescence quenching was observed even upon addition of large excess (up to 10-1M) of Langlois reagent. Similar experiments carried out with styrene as a potential electron donor/acceptor, or with a mixture of styrene and CF3SO2Na led to similar observations. This suggest that if ET takes place, it occurs with very low efficiency, several orders of magnitude below classical diffusion control processes. We thus turned our attention to evaluating the photochemical quantum yields of the studied reactions, namely i/ the hydrotrifluoromethylation of styrene in DMSO (conditions I) and ii/ the formation of trifluoromethylated styrene dimer in acetonitrile (conditions II). Both studies were performed by F NMR following of the reaction ensuing irradiation of a NMR tube containing the reaction mixture (reagent 1e, PC1 or PC2 and Langlois reagent) in the corresponding deuterated solvent (PC1: MeCN-d3 or PC2: DMSO-d6), using our recently reported NMR actinometric set-up , that we recently adapted to the study of photoinduced ET mediated chemical transformations (please see SI for details about the procedure). In both cases, transformations operated very cleanly upon prolonged irradiation of the NMR sample to provide either compound 2e (in MeCN-d3, figure , SI) or compound 3ea (in DMSO-d6, figure , SI), in a kinetic that could be in both cases satisfactorily fitted to a first-order trend. Very similar quantum yield was obtained in both cases: conditions I, Φp = 0.7x10 -2 ; 19 conditions II, Φp= 1.3x10 -2 ; Interestingly this extremely low value agrees well with the very inefficient ET process suggested throughout the spectrofluorometric experiments. It is particularly remarkable, considering the inefficiency of the process, that the reaction yields in both cases one single product that can be isolated in a synthetically relevant yield, which underlines the exquisite selectivity of the associated chemical transformation. With these results, several questions raised to be able to propose plausible mechanisms in both reactions, SET oxidation of the Langlois reagent is thermodynamically favored. Thus, generating trifluoromethyl radical that could collapse to the styrene to form a new C-centred-benzylic radical. This C-centred intermediate ) should not be involved anymore in SET reactions with the catalyst since dimerization allow the formation of the desired fluorinated hexestrol derivatives. Given the nature of the stepwise mechanism the involvement of another species is required to regenerate the organophotocatalyst. Herein, the solvent or SO2 generated after oxidation of the Langlois reagent could be involved. With respect to the hydrotrifluoromethylation of styrene, the involvement of a HAT process with DMSO or reduction of the C-centred radical A generated from addition of the trifluoromethyl radical to styrene are plausible mechanisms. To do so, investigation was devoted to characterizing the radical intermediates involved in our reaction by EPR spectroscopy, using radical trapping agents. As mentioned earlier, SET from the Langlois reagent derivatives to the catalyst is thermodynamically favored. As a matter of fact, a single trifluoromethyl adduct B (Figure , a) was obtained with DMPO when the Langlois reagent was irradiated with blue LED in the presence of PC1 in MeCN (g=2.0065, aN=13.6G aH=15.85 G, a3F=0.93G), with features compatible with those reported for CF3 radicals, constituting a first direct evidence for the occurrence of a SET step between the excited photocatalyst and Langlois reagent. Complementary experiments were carried out using CF2HSO2Na as sulfinate source, which also demonstrated the formation of a single major adduct either with DMPO (adduct C, g=2.0064, aN=14.1G, aH=18.6G aF=0.6G) or PBN (adduct D, g=2.006, aN=14.62G aH=2.8G, aF=0.98G) confirming again the generation of the perfluoroalkyl radical (Figure Similarly we were also able to demonstrate that using PC2 in DMSO allows the formation of a trifluoromethyl radical adduct with PBN E (figure ) (g=2.0062, aN=14.23G aH=2.15G, a3F=1.55 G). Surprisingly, performing the same reaction with DMPO as a spin-trapping agent allowed us to observe the formation of a different species with features compatible with SO2 .-(g=2.0061) (figure 2, e), while no clear signature could be observed for the trifluoromethyl radical. This latter, may result from the reduction of SO2 generated after reduction of the Langlois reagent. The reduction of SO2 is favored either with PC1 or PC2 since both radical anions of these two photocatalysts could reduce SO2 (Ered =-0.8 V). Furthermore, SO2 .-is prone to evolve by dimerization, yielding dithionithe S2O4 2-as a white solid which was a distinctive feature that we observed at the end of the reaction. In order to get more insight especially on the missing signature of the trifluoromethyl radical under this condition, we decided to investigate the same SET reaction, this time in the presence of styrene (Figure This alternative reactivity allows difunctionalization of styrene through three component reactions. Indeed, the desired compound 4e was obtained in 50% yield in one step procedure in the presence of K3PO4 as the base (Scheme 4). 26 Scheme 4. Reaction was performed with styrene 1 (0.2 mmol, 1 equiv.), CF3SO2Na (0.4 mmol, 2 equiv.), PC1 (2 mol%) 4-cyanopyridine (0.4 mmol, 2 equiv.), K3PO4 (0.4 mmol, 2 equiv.) and ACN (2 mL). The reaction mixture was stirred at room temperature for 48 hours under blue LED irradiation and inert condition. Yield shown is of isolated product. Yield determined by F NMR spectroscopy with PhOCF3 as internal standard is schown in parenthes.