Blue‐to‐UVB Upconversion, Solvent Sensitization and Challenging Bond Activation Enabled by a Benzene‐Based Annihilator

Several energy‐demanding photoreactions require harsh UV light from inefficient light sources. The conversion of low‐energy visible light to high‐energy singlet states via triplet‐triplet annihilation upconversion (TTA‐UC) could offer a solution for driving such reactions under mild conditions. We present the first annihilator with an emission maximum in the UVB region that, combined with an organic sensitizer, is suitable for blue‐to‐UVB upconversion. The annihilator singlet was successfully employed as an energy donor in subsequent FRET activations of aliphatic carbonyls. This hitherto unreported UC‐FRET reaction sequence was directly monitored using laser spectroscopy and applied to mechanistic irradiation experiments demonstrating the feasibility of Norrish chemistry. Our results provide clear evidence for a novel blue light‐driven substrate or solvent activation strategy, which is important in the context of developing more sustainable light‐to‐chemical energy conversion systems. Blue‐to‐UVB photon upconversion (UC) enabled by a benzene‐based annihilator is presented for the first time. The annihilator's unprecedented singlet‐excited energy of 4.15 eV was successfully employed to activate inert carbonyls via a subsequent FRET energy transfer. This UC‐FRET reaction scheme was directly monitored by laser techniques and exploited in lab‐scale NMR experiments demonstrating its potential in light‐to‐chemical energy conversion.