Multiple Resonance TADF Sensitizers Enable Green-to-Ultraviolet Photon Upconversion: Application in Photochemical Transformations

Efficient visible-to-ultraviolet (UV) triplet-triplet annihilation upconversion (TTA-UC) with large anti-Stokes shift is highly promising in solar-powered and indoor applications. Nonetheless, the excitation wavelength is confined to the blue region (< 450 nm) mainly due to large energy loss during triplet sensitization. Herein, a series of multiple resonance thermal activated delayed fluorescence (MR-TADF) compounds are constructed as pure organic sensitizers for the purpose of energy-loss reduction, which also feature intense absorbance at visible region, high intersystem crossing efficiencies, and long triplet lifetimes. By pairing the MR-TADF sensitizers with appropriate UV-emissive acceptors, green-to-UV TTA-UC systems were realized with anti-Stokes shift up to 1.05 eV, upconversion quantum yield up to 7.6% and threshold excitation intensity as low as 9.2 mW cm-2 in solution. Proof-of-concept demonstrations suggest the TTA-UC pairs could be applied as internal or external source of UV photons to trigger energy-demanding photopolymerization and photoligation reactions even under excitation of low-power-density green LED light, suggesting the broad utility of these molecular upconverters.