To reveal the presence of the ligated boryl radical, we recorded an EPR spectrum of a deoxygenated benzene solution of PC1 (0.05 M) and B1 (0.05 M) containing phenyl N-tertbutylnitrone (PBN, 0.0125 M) as a spin trap. Prior to irradiation no signal was observed, however, after continuous irradiation for 15 minutes ( = 460 nm) we clearly saw the appearance of two distinct features: one can be attributed to the reduced photocatalyst (acridine radical), while the second feature is derived from the trapping of the ligated boryl radical with PBN (Figure , for further details see Section 6.1 in the Supporting Information). The assignment is further supported by the observation of the PBN*NHCBH2 • adduct in HRMS (Figure ). These experiments unequivocally show the formation of the desired LBR under photoredox conditions. Direct observation of the ligated boryl radical via EPR was unsuccessful due to its short lifetime. As further evidence, we also tracked the interaction between PC1 and B1 via UV-Vis spectroscopy. In particular, when a deoxygenated CH3CN solution (10 -5 M) of PC1 and B1 was irradiated at 456 nm, we observed that the former was directly converted into a species absorbing in the 450-700 nm range (two isosbestic points were discerned, see Figure ). We propose this species to be the acridine radical generated upon single-electron reduction by B1, based on a comparison with the literature for a similar compound. In order to get some insights into the radical chain mechanism, we conducted experiments with deuterium-labeled substrates, and the results are collected in Figure . The deuterium incorporation was calculated via 1 H-NMR on purified products. Overall, these experiments showed that deuterium incorporation (product 3-d1) was only observed when deuterated borane B1-d3 was exploited as XAT agent, while the use of B1 resulted in the formation of product 3 exclusively. These results show that the hydroalkylated product is obtained upon HAT from another molecule of B1 rather than the solvent, thus suggesting a radical chain mechanism. We next wondered if the latter could be the rate-determining step of the transformation; therefore, we set off to evaluate the kinetic isotope effect (KIE) of the reaction (Figure ). First, we measured the KIE through a competition experiment. Hereto, we performed our benchmark reaction in the presence of an equimolar mixture of B1 and B1-d3 (1.2 equiv. each) and a KIE value of 3 was found. However, when we double-checked this value with the parallel reactions method by performing independent reactions under optimized conditions, one containing B1 and one containing B1-d3, we found a KIE of only 1.3. Taken together, these experiments suggest that the HAT step might not be involved in the rate-determining step of the reaction. Finally, we determined a quantum yield of 2% for this visible-light promoted transformation. Such a modest value is in accordance with a process being supported by either short-lived radical chain propagations, an inefficient initiation process or decomposition of the photocatalyst (further mechanistic insights are reported in Section 6 of the Supporting Information). To gain further insights into the reaction mechanism, we initiated a computational investigation intended to model the entire reaction profile through the simulation of all the key steps, including some possible parasitic pathways. Thus, we adopted DFT at the B97xD/def2TZVP level of theory to optimize the relevant stationary points, also including the effect of the solvent through an implicit model (Figure ; see also Section 10 of the Supporting Information for further details).