Mechanistic insights into intramolecular energy transfer dynamics in photosensitizers for triplet-triplet annihilation upconversion

Photosensitizers constitute a crucial element in the process of triplet-triplet annihilation upconversion, necessitating robust absorption of visible or near-infrared light, high intersystem crossing efficiency, prolonged triplet state lifetime, and minimal energy dissipation during intersystem crossing and vibrational relaxation. Nonetheless, conventional monomeric photosensitizers frequently fail to simultaneously meet these requirements. In recent years, researchers, including our group, have fabricated photosensitizers that incorporate multiple covalent linkages, such as dyads and triads, which are regarded more likely to achieve comprehensive performance optimization. This review article explores the design and characteristics of recently synthesized dyads and triads photosensitizers that operate on the principles of intramolecular singlet energy transfer and intramolecular triplet energy transfer, demonstrating their outstanding efficacy in high-efficiency triplet-triplet annihilation upconversion. We provide an exhaustive explanation of the design rationales, photophysical, and photochemical properties of these photosensitizers, along with suggestions for the creation of photosensitizers with enhanced performance. Moreover, we discuss potential avenues and opportunities for the future development of triplet-triplet annihilation upconversion technology.