Challenges and breakthroughs in purely organic TTA-UC: the central role of heavy-atom-enhanced thermally activated delayed fluorescence photosensitizers.

The triplet-triplet annihilation upconversion (TTA-UC) mechanism, a process that converts low-energy photons into higher-energy emissions, has attracted considerable interest in photophysics and photochemistry due to its potential applications. Thermally activated delayed fluorescence (TADF) molecules, featuring a small energy gap (ΔEST) between the lowest singlet (S1) and triplet (T1) states, are promising candidates for TTA-UC systems, enabling large anti-Stokes shifts. However, their rapid reverse intersystem crossing (RISC), driven by minimal ΔEST, often compromises TTA-UC efficiency. To mitigate this limitation, the heavy atom effect (HAE) had been employed to strengthen spin-orbit coupling (SOC), thereby enhancing intersystem crossing (ISC) rates. This review highlights recent progress in optimizing TTA-UC performance through internal HAE (iHAE) in TADF molecular design, while also examining the role of external HAE (eHAE) from solvent environments. Finally, we discuss the remaining challenges and future opportunities in the development of TADF-based TTA-UC systems.