For the proposed photocatalytic transformation, we suggest the following mechanism (Scheme 10). A complex comprising CeF4, benzoic acid and a base is excited by 395 nm light, leading to the single electron oxidation of the deprotonated form of benzoic acid (3.1) to yield the respective radical species 3.2. This radical compound can then abstract a hydrogen atom from the starting material 1, wherein the resulting carbon-centred radical (1.1) is stabilised in the tertiary position within the carbon chain of ester 1. The nucleophilic radical is subsequently trapped by the phthalimide derivative 2, and upon transfer of a trifluoromethyl thio-radical, product 4 is formed. The remaining nitrogen-centred phthalimide radical can re-oxidise Ce 3+ to the Ce 4+ species, completing the cycle as it converts into the phthalimide anion 2.2. Alternatively, instead of benzoic acid, BINOL phosphate, hydroxamic acids or bromide ligands can be considered as an active hydrogen atom transfer (HAT) catalyst in combination with Ce 4+ . Scheme 10. Proposed mechanism for the C-H activation of tertiary carbon atoms via cerium-benzoic acid radical. CONCLUSION In this project, we explored the range of LMCTactive ligands capable of forming complexes with CeF4 and other cerium compounds, such as CeO2. Benzoic acid, phosphates, hydroxamic acids and halides such as bromides were successfully employed as ligands for the selective photocatalytic C-H functionalisation, resulting in tertiary carbon-centred radicals. Other ligands did not promote the photoreaction. Complex formation attempts between CeF4 and tetrabutylammonium bromide yielded Br3 -quantitively. Mixing CeF4, a base and KSCN, gave a coloured complex, possibly resulting from a ground-state redox reaction between cerium and the ligand, which was not active in C-H photoactivation. In general, the formation of coloured coordination compounds upon ligand addition to CeF4 does not correlate with productive LMCT C-H activation. Many parameters define LMCT-active cerium complexes, including the mostly unknown structure and dy-namic of the coordination compounds, the specific photophysical processes induced upon irradiation and the lifetimes of generated reactive intermediates in the context of the kinetic parameters of the C-H activation reaction.