Thermally-activated Upconversion Based on Triplet Fusion for Deep Red Photoactivation of Ru(ii) Complexes

Expanding the anti-Stokes shift in triplet-triplet annihilation upconversion (TTA-UC) systems and harvesting light within the biological transparency window (700-900 nm) are of great importance for their biomedical applications. In this study, we have developed a TTA-UC system that converts deep red-light into blue photons using a highly endothermic sensitization pair, wherein the triplet energy level of the sensitizer is 0.19 eV below that of the acceptor. The thermally activated triplet energy transfer compensates the energy loss during intersystem crossing (ISC) of the sensitizer, achieving a 0.82 eV anti-Stokes shift upconversion with over 6% quantum yield. This TTA-UC system activates a Ru(ii)-based photoactivated chemotherapeutic agent, causing DNA to bleach under 700 nm LED irradiation. These results demonstrate a feasible way to develop upconversion systems that operate within the biological transparency window, with potential applications in biological phototherapy.