Drawing from the outcomes of control experiments, mechanistic studies using EPR and transient absorption measurements (TA), and considering previous reports, a plausible mechanism for the Sc(OTf)3-catalysed photooxidation reactions is postulated (Fig. ). Initially, the Sc 3+ complex can be excited upon coordination with molecular oxygen, as demonstrated by the quantum chemical calculations (see Fig. and Supplementary Section S10). This type of complex is proposed to involve acetonitrile ligands and the participation of triflates either as counterions or ligands. Upon excitation, the scandium complex forms a singlet excited state, which can subsequently undergo an intersystem crossing process, as elucidated by TA, to generate a triplet excited state of the scandium complex.