Triplet Photosensitizers and Their Applications in Triplet–Triplet Annihilation Upconversion

Triplet–triplet annihilation upconversion (TTA-UC), which converts low-energy photons into high-energy photons, is based on linear photon absorption and shows high upconversion efficiency of luminescence with large anti-Stokes shifts; thus, it has vast applications in solar cells, photocatalysis, artificial photosynthesis, etc. Due to cascade photophysical processes of intersystem crossing (ISC) and triplet–triplet energy transfer (TTET) involved in TTA-UC, the properties of triplet photosensitizers (PSs) are critical for designing TTA-UC materials. For instance, strong light absorption and long triplet excited-state lifetimes will make PSs abundant at the triplet state, which makes the consequent intermolecular TTET process more efficient, and the upconversion efficiency will be improved as well. In this review, we include recent developments of triplet PSs for TTA-UC and focus on the photochemistry of these triplet PSs, including molecular design rationales, ISC mechanisms and electron-spin selectivity of ISC. The methods of increasing anti-Stokes shifts for TTA-UC and the applications of heavy-atom-free triplet PSs are discussed briefly. This information is helpful for the future design of heavy-atom-free TTA-UC PSs.