Improving Triplet–Triplet Annihilation Upconversion Output by a Triplet Mediator Approach: Mechanistic Insights on Homo and Hetero-Annihilation in Three-Component Systems

Triplet-triplet annihilation photon upconversion (TTA-UC) is a promising strategy for converting low-energy photons into higher-energy emission, with potential applications in solar energy harvesting, bioimaging, and photocatalysis. A challenge in TTA-UC systems is minimizing the reabsorption of upconverted photons by the annihilator molecules. To address this, we present a mediator-assisted TTA-UC approach utilizing a neutral mediator molecule to facilitate upconversion in the ultraviolet (UV) and visible regions. Our study introduces a general protocol, and through detailed kinetic modeling, we elucidate the underlying mechanism, highlighting the role of hetero-TTA (triplet-triplet annihilation between the mediator and annihilator). Notably, we report the first estimation of a hetero-TTA rate constant, which exceeds the homo-TTA rate by a factor of 2. This work broadens the design space for TTA-UC systems by enabling the use of neutral, noncovalently linked mediators, expanding beyond the conventional reliance on charged or covalently tethered species.