To gain mechanistic insights we envisioned different scenarios and investigated unsensitized as well as sensitized conditions using TXO, homoleptic tris(2-phenylpyridine)iridium(III) (Ir(ppy)3), and 4CzIPN as photocatalysts, where energy-transfer and electron-transfer processes are plausible respectively. We probed the background reaction under UV (365 nm) and visible light excitation (450 nm) and observed that product formation was moderate with the high-energy UV light, whereas no reaction took place with blue light (Figure ). Recently, the Nakalima and Nemoto groups investigated the direct excitation of hypervalent iodine species and found evidence for the spin-forbidden transition from the ground state (S0) to a high-lying triplet state (Tn, Figure ). The groups could show the experimental utility of this approach in various group transfer reactions and is in line with our observation. To the best of our knowledge, this strategy has not been applied to copper catalysis to date. When we added photosensitizers to the reaction, a significant boost in conversion was observed and high product formation was found for TXO, 4CzIPN, and Ir(ppy)3 within a few minutes (Figure ). The kinetic profiles for each case reveal significant rate enhancement for the sensitized cases (Figure ). TXO enhances the UV light background reaction, and the quantitative formation of 2 is achieved after 5 minutes. For the other two sensitizers the kinetic difference likely originates from lower quenching efficiency of 4CzIPN compared to Ir(III) as determined by Stern-Volmer experiments (Figure ). The triplet state quantum yields of all three photocatalysts are available from the literature (Φ(TXO) = 0.12 in MeOH at 298 K, Φ(Ir(ppy)3) = 0.97 in 2-Me-THF at 298 K, and Φ(4CzIPN) = 0.18 in MeCN at 300 K). The respective quantum yields or efficiencies of the quenching process were determined to be 0.021 for TXO, 0.016 for Ir(ppy)3 and 0.005 for 4CzIPN (see SI). The quenching efficiency is largest for TXO and Ir(ppy)3, which is in line with the observations from reaction kinetics (Figure , see SI). Next, we probed for the likeliness of excited state single electron transfer (SET), and we screened a variety of photocatalysts and -sensitizers (Table ).