In subsequent experiments, we investigated the mechanism of Sc-based photooxidative catalysis using toluene oxidation as the representative example. Irradiation of an oxygen-saturated acetonitrile solution containing Sc(OTf)3 (0.1 M) and toluene (2 M) with a 400 nm LED at room temperature resulted a distinct isotropic EPR signal (Fig. ). The pronounced eight-line isotropic spectrum was attributed to the formation of a complex of superoxide radical anion with scandium ion, Sc 3+ -O2 •-. This resulted in superhyperfine splitting (aSc = 4.28 G) due to the 7/2 scandium nuclear spin and featured an isotropic g value, giso = 2.0160. This observation is consistent with reported EPR spectra in the literature. Interestingly, while previous studies required a low temperature to observe Sc 3+ -O2 •-by EPR, we were able to obtain well-defined spectra at room temperature. The kinetic recordings showed stability of the Sc 3+ -O2 •-complex for about 2 min after the cessation of irradiation (Fig. , insight). Importantly, no significant EPR signal appeared upon irradiation of a blank solution containing no toluene. Irradiation of the acetonitrile solution of Sc(OTf)3 and toluene under anaerobic conditions also did not reveal any EPR signals. Replacing Sc(OTf)3 with La(OTf)3 (which showed no catalytic activity in photooxidative procedures) led to a very faint EPR signal, even after an extended period of irradiation (see Supplementary Section S8). These observations provide evidence that all species -toluene as a substrate and electron donor, oxygen as a terminal electron acceptor and scandium as mediator -are necessary for efficient photoinduced electron transfer and stabilised Sc 3+ -O2 •-formation (see Fig. for a proposed mechanism).