Additional control experiments (see Figs. and (Fig. ), indicating the involvement of a radical pathway in this oxidation process. Similarly, the cyanation of diphenyl ether was almost completely halted in the presence of TEMPO (see Supplementary Section S7 for details). Stern-Volmer quenching experiments revealed that toluene, diphenyl ether and 1,3,5-trimethoxybenzene quenched the fluorescence of a Sc(OTf)3 solution in acetonitrile with Stern-Volmer constants Ks of 3, 22 and 42 L mol -1 , respectively (see example in Fig. ). This signified SET from the tested substrates to an excited scandium complex (see Supplementary Section S7 for details). As mentioned earlier, the UV-Vis absorption spectrum of Sc(OTf)3 in acetonitrile showed an elongated tail in the visible region around 400 nm and a fluorescence emission spectrum with a sharp band at 505 nm (Fig. ). ESI-MS measurements aimed at elucidating the structure of the scandium complex responsible for this absorption in a solution of Sc(OTf)3 in MeCN. It suggested the formation of a cluster of complexes, including [Sc(OTf)2(MeCN)2] + , as the primary species. However, these particles do not absorb in the visible region but below 260 nm, as indicated by the theoretical spectra (see Supplementary Section S10). By contrast, the [Sc(MeCN)3((η 2 -O2)] 3+ particle in S0 state, a complex with three MeCN molecules and coordinated oxygen molecule, seems to absorb in the visible region (about 415 nm) as documented by the theoretical spectrum, consistent with our experimental results (Fig. ). Indeed, the coordination of oxygen molecule is ascribed to the oxophilicity of Sc 3+ ions and is further supported by our EPR measurements (see below). It should be noted that the isomeric [Sc(MeCN)3(η 1 -O2)] 3+ complex (in T1 state) is more stable by 0.950 eV (91.67 kJ mol -1 ) at DLNPO-CCSD(T)/aug-cc-pVTZ level of theory (see Supplementary Section S10 for further information) compared to the S0 state [Sc(MeCN)3((η 2 -O2)] 3+ , nevertheless, [Sc(MeCN)3(η 1 -O2)] 3+ is proposed to not absorb in the visible region (absorption below 300 nm, see Supplementary section S12). We believe the small barrier can be overcome thermally.