The combination of metal catalysis and photochemistry has received considerable attention over the last decade and holds great promise for milder and more sustainable organic syntheses. This rising field, termed metallaphotoredox catalysis and pioneered by the MacMillan group, enables diverse transformations utilizing oxidative or reductive excited state quenching mechanisms to form transient organic radicals from readily available starting materials. Additionally, employed photosensitizers can undergo energy transfer (EnT) processes and thus enable an alternative activation mechanism in light-mediated catalysis holding great promises in future synthetic applications. Naturally occurring carboxylic acids are an ideal and abundant substrate to undergo reductive or oxidative quenching during metallaphotoredox catalysis. When activated, carboxy radicals are formed which rapidly liberate carbon dioxide in a highly exergonic reaction (-23.2 kcal mol -1 ), to form a carbon-centered radical. This approach, when combined with metal catalysis, has enabled various new kinds of C-C and C-N bond formation reactions. A suitable activation method for carboxylic acids are hypervalent iodine(III) reagents, such as phenyliodo diacetate (PIDA), which are well known sources for aliphatic radicals and have been activated with photochemistry in chain reactions. On the one hand, single-electron transfer (SET) or reduction of PIDA can occur from highly reducing excited state photocatalysts (PC, Figure ). On the other hand, access to triplet states of PIDA is feasible due to the heavy-atom effect of iodine and can be reached via direct Dexter-type energy transfer of long-lived triplet excited states of the photocatalyst. However, the operating mechanism is debated in the literature: In a seminal report by Liang et al. on the copper-catalyzed N-alkylation, oxidative quenching of the photocatalyst and subsequent reduction of PIDA was described as the initial step in the reaction mechanism to form a carboxy radical, which rapidly decarboxylates to form the active alkyl radical. For most substrates, light activation was not necessary and thermal copper-catalyzed coupling between N-nucleophiles and alkyl fragments was broadly achieved. The proposed photocatalytic mechanism was recently investigated by Mane et al. with DFT calculations shining light on the sequence of intermediate reaction steps. In a more complex scenario, the MacMillan group developed a radical sorting mechanism of unsymmetrical hypervalent iodine-derived radicals with nickel catalysis which operated under an energy transfer mechanism with thioxanthone (TXO) and a SET mechanism with 2,4,5,6-tetrakis(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) as photocatalysts respectively. To this end, it is not clear how distinct this differentiation can be made. In 2023, the group of Murakami reported a detailed study where they found switching behavior of mechanisms with hypervalent iodine reagents. Following SET reactivity, an ionic pathway led to decarboxylative esterification, whereas EnT resulted in radical-radical homo-dimerization. Merging the mechanistic diversity of hypervalent iodine reagents with copper catalysis is scarce and will be applied to the radical methylation reaction. In particular methyl radicals are significantly destabilized compared to commonly employed primary or secondary radicals (relative radical stabilization energy, ΔRSE(1°) ~ -3.2 kcal mol -1 , ΔRSE(2°) ~ -5.6 kcal mol -1 ) rendering methyl radicals a more difficult target. The combination of metal catalysis and photochemistry is envisioned to extend methylation reactions beyond nucleophilic substitution reactions. Methylation of organic molecules is a general strategy to influence their properties and found tremendous applications in drug design coined the "magic methyl-effect". In particular, N-heterocycles are common targets for methylation reactions and commonly methyl iodide or dimethyl sulfate are used under basic conditions. However, both reagents are known carcinogens. Catalytic, non-toxic, and mild alternatives thus remain of considerable interest and have transformative potential for pharmaceutical syntheses. In the study by Liang et al. only two examples were described to undergo successful methylation reaction. Inspired by this study, we were wondering whether the differentiation between SET and EnT mechanisms can be applied to copper-catalyzed methylation reactions. We were aiming to develop a purely light-controlled process which yields methylated Nheterocycles. Followed by a detailed mechanistic understanding the underlying copper-catalyzed mechanism should enable us to derive general reaction conditions, which can be expanded to additional weaker nucleophiles.