Improving Triplet-Triple 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 ul-traviolet (UV) region. Our study introduces a protocol employing a benzothiophene-based compound, TIPS-BT, as an energy transfer mediator between the sensitizer 4CzBN and the annihilator TIPS-Naphthalene. Through detailed kinetic modeling, we elucidate the underlying mechanism, highlighting the role of hetero-TTA (triplet–triplet annihilation between the media-tor and annihilator). Notably, we report the first estimation of a hetero-TTA rate constant, which exceeds the homo-TTA rate by a factor of two. This work broadens the design space for TTA-UC systems by enabling the use.