Multi‐Resonance Thermally Activated Delayed Fluorescence Molecules for Triplet‐Triplet Annihilation Upconversion

Triplet-triplet annihilation upconversion (TTA-UC) has made significant progress in recent years in several key applications, including solar energy harvesting, photocatalysis, stereoscopic 3D printing, and disease therapeutics. In TTA-UC research, photosensitizers serve the vital function of harvesting low-energy photons. The photophysical characteristics of photosensitizers, including absorbance, triplet state quantum yield, triplet state energy level, triplet state lifetime, etc., determine the performance of TTA-UC. Thus, the study of photosensitizers has been a key aspect of TTA-UC. In recent years, multi-resonance thermally activated delayed fluorescence (MR-TADF) molecules have received extensive attention due to their excellent photophysical properties and electroluminescent device performance. MR-TADF molecules not only present a narrow energy gap between the singlet and triplet excited states, but also have stronger absorption and better wavelength regulation than conventional TADF molecules. Nowadays, the preliminary attempts in TTA-UC using MR-TADF molecules as photosensitizers have resulted in the development of green to ultraviolet, blue to ultraviolet, and even near-infrared to blue emission. This concept will summarize the research progress of MR-TADF molecules as photosensitizers in TTA-UC, analyzing the challenges and giving possible solutions. Finally, we prospect the future development of MR-TADF molecules as photosensitizers, including the molecular design as well as the possible application areas.