Endothermic and Exothermic Energy Transfer Made Equally Efficient for Triplet-Triplet Annihilation Upconversion.

Expanding the anti-Stokes shift for triplet-triplet annihilation upconversion (TTA-UC) systems with high quantum yields without compromising power density thresholds ( I _th ) remains a critical challenge in photonics. Our studies reveal that such expansion is possible by using a highly endothermic TTA-UC pair with an enthalpy difference of +80 meV even in a polymer matrix 1000 times more viscous than toluene. Carrying out efficient endothermic triplet-triplet energy transfer (TET) requires suppression of the reverse annihilator-to-sensitizer TET, which was achieved by using sensitizers with high molar extinction coefficients and long triplet state lifetimes as well as optimized annihilator concentrations. Under these conditions, the sensitizer-to-annihilator forward TET becomes effectively entropy driven, yielding upconversion quantum yields comparable to those achieved with the exothermic TTA-UC pair but with larger anti-Stokes shifts and even lower I _th , a previously unattained achievement.