When the formation of 2 was plotted against the ground and excited state redox potentials, no correlation was observed, which suggests that a mechanism via single electron transfer is unlikely (Figure ). For the expected oxidative quenching event (Figure and S6C) even sensitizers which are lower in excited state oxidation potential than the necessary Ered(PIDA) = -1.12 V vs. SCE performed well under the optimized conditions. However, plotting the triplet state energies of the photosensitizers against reaction progress shows a clear cut-off between 55 and 60 kcal mol -1 (Figure ). a limited number of acridinium and ruthenium sensitizers were available below this threshold (Table ). Based on the behavior displayed in Figure we estimate the triplet state energy of PIDA to be around 56-59 kcal mol -1 , which is in good agreement with 58.6 kcal mol -1 obtained from DFT calculations (B3LYP D3/def 2 level of theory, Table ). With these results in hand a triplet-triplet energy transfer mechanism (TTEnT) is reasonable and likely the main operating mechanism for quenching of hypervalent iodine reagents in photocatalysis (Figure , right). Therefore, we propose a catalytic cycle which starts with excitation of a photosensitizer (PS) with (visible) light (Figure ). After intersystem crossing, the long-lived triplet state (τ(TXO) = 0.76 µs, τ(4CzIPN) = 5.1 µs, τ(Ir(ppy)3) = 1.9 µs) undergoes triplet energy transfer to hypervalent iodine(III) to populate the T1 excited state. Fragmentation and loss of CO2 leads to the formation of active methyl radical (I), acetate (vide infra), and either neutral phenyl iodide or the iodanyl radical cation (PhI •+ ). The formation of methyl radical (I) was confirmed by addition of 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO, 2.0 equiv) as the respective radical adduct II formed in 165% 1 H NMR yield, whereas the coupling reaction was completely suppressed (Figure ). Radical addition to Cu(II) intermediates occurs fast (k ~ 10 -9 s), and forms Cu(III) species which readily undergo reductive elimination, forming the desired product 2. The resulting Cu(I) intermediate (Ered (Cu(acac)2) = -1.65 V vs. SCE) can be oxidized by PIDA or iodanyl radical cation and in the presence of Nnucleophile regenerates Cu(II) closing the catalytic cycle. This oxidation is further supported by the need for two equivalents of PIDA for high conversions (Figure ). The role of DBU as non-nucleophilic base was investigated by direct aprotic injection experiments in a quadrupole MS instrument (Figure ). In the negative mode many di-and trinuclear copper species were identified, which are likely off-cycle intermediates as known from conventional Chan-Lam coupling reactions (Figure ). Thereof, DBU seems to be crucial in returning these intermediates back onto the catalytic cycle. The active nature of copper(acac)x-DBU complexes in the catalytic cycle are further supported by the recent theoretical study of Mane et al.. In line with the base dependency of the reaction described above, we conducted a UV/Vis study to follow changes of the copper catalyst with each component as well as over time and irradiation (see SI). A broad peak around 620 nm from Cu(acac)2 is observed under normal reaction conditions (Figure ). Upon irradiation of the reaction with 440 nm LEDs a bathochromic shift to 670 nm occurs (Figure ), which builds up over the initial 10 minutes of reaction time and can be assigned to the formation of copper(II) acetate in the presence of PIDA. This equilibrium is in line with the literature for the fast exchange between acac -and -OAc ligands (Figure ). Additionally, energy transfer from the sensitizer can further induce the dissociation of acac ligands after photoexcitation. After initial photoinduced acac dissociation, stronger O-nucleophiles, such as acetate, can ligate copper. Combined with the MS results (vide supra), acetate seems to assist as base to fulfil the overall reaction stoichiometry. Without addition of photocatalyst, no change in absorption behavior is observed (Figure , left) confirming the nature of the photochemical process. Based on literature precedence, large excess of N-nucleophiles is required to induce formation of adducts with Cu(acac)2, which may explain the small change in the UV/Vis (Figure , blue graphs). Alongside the formation of the desired N-methylated product, methyl acetate formed at similar rates with a minor initial lag phase (Figure ). The addition of methyl radicals to copper(II) acetate was described to form a Cu(III) species (Figure ), which upon fast reductive elimination liberates methyl acetate showing a distinct shift in the 1 H NMR at around 3.65 ppm (Figure ). Additionally, indazoles show the formation of N-acetylated products. We probed the origin of the acetyl group and concluded it must arise from direct reaction of N-nucleophiles with PIDA, since neither methyl acetate nor Norrish-type fragmentation of the ligand on copper led to the Nacetyl indazole (Scheme S6). Acyl transfer from PIDA to nucleophiles is unusual but plausible, and may enable intriguing redox active ester-type reactivity in the future. Likewise radical addition or coupling with the substrate was anticipated, since a highly stabilized α,α-amine radical intermediate would form (Scheme S7). Therefore, 2-cyclopropylbenzimidazole (S1) was synthesized and subjected to the standard conditions. No radical clock ring opening product S2' was obtained and only the desired product S2 was obtained in 29% yield, indicating that no intermediate tertiary radical within the heterocycle was formed (Scheme S8). With a solid mechanistic understanding provided by the combination of NMR, GC, luminescence quenching, UV/Vis spectroscopy, and aprotic MS experiments we were wondering whether weak nucleophiles, which are not readily methylated with common reagents like methyl iodide, are amenable to copper-catalyzed energy transfer catalysis. The direct photoexcited fragmentation of 1-butoxy 1-λ 3 -benzo[d][1,2]iodaoxol-3(1H)-one (IBB) in the presence of aliphatic acids was reported by Itami and Murakami and resulted in intramolecular O-benzylated products with 2-iodobenzoic acid via a polar SET mechanism. Thermal decomposition of PIDA combined with copper catalysis was used by the group of Zhang in the methylation of mostly mono-substituted benzoic acids. After quick solvent optimization we were pleased to see efficient reactivity of various benzoic acids upon combination of copper and energy transfer catalysis. In particular aprotic polar solvents performed well, and acetone was chosen due to the highest yields across diverse electronic substrates (Figure ). Many benzoic acids were compatible with our protocol and even showed competitive yields across the board (see experimental section in the SI). Mono-, di-, and trisubstituted benzoates were applicable and the respective products 11 to 17 were obtained in isolated yields of up to 79% (Scheme 2). All reaction components are required for high performance rendering a purely lightdriven process (Scheme S11) and shows high robustness toward the addition of various additives (Scheme S12). Secondary alcohols, imides, anhydrides, amines, olefins, alkynes, aryl bromides, and even carbonyl and nitro groups are well tolerated with only minor decrease in yield (Scheme S12, green). Only limited functionalities show inhibition of the reaction towards 11 such as N-hydroxy phthalimide (see SI). Multiple electron-rich and electron-poor heterocycles like pyridine, pyrazine, thiophene, furan, indazole, or pyrazolo-pyrimidine gave good conversion and up to 91% yield was obtained for products 18 to 24. Competition experiments show that electron-withdrawing groups increase the yield and N-heterocycles are generally detrimental (Scheme S13-14). Last we subjected smaller and larger active pharmaceutical intermediates (APIs) as substrates to show the broad application and mild nature of this coppercatalyzed energy transfer mechanism. Aspirin, Candesartan, and Apixaban performed all well and methylated products 25-27 were obtained in 47%, 66%, and 54% isolated yield, respectively, displaying the outstanding potential of this method.