New Bichromophoric Triplet Photosensitizer Designs and Their Application in Triplet-Triplet Annihilation Upconversion

As indispensable molecular components, photosensitizers play a crucial role in determining the quantum efficiency of triplet-triplet annihilation upconversion (TTA UC). This emergent technology has attracted great attention in recent years for realizing large anti-Stokes shifts with noncoherent excitation sources. In a typical TTA UC, low-energy photons are first harvested by the photosensitizers, which upon intersystem crossing (ISC) undergo triplet-triplet energy transfer (TTET) to emitters (i.e., annihilators). Following the bimolecular TTA among the emitters, high-energy photons are given off by the singlet excited state of the emitters. Apparently, the efficiencies of photon absorption, ISC, and TTET are all dependent on the sensitizers. With a Dexter-type ET mechanism requiring collisional interactions, a long triplet lifetime of the energy donor (photosensitizer) is evidently favorable for enhancing the efficiency of TTET. This progress report summarizes the recent developments of photosensitizers used for TTA UC, many of which feature a bichromophoric molecular scaffold. Among the various consequences and functions entailed by such bichromophoric designs, the extended triplet lifetime is a particularly advantageous property for TTA UC. Additionally, these new potent photosensitizers with long triplet lifetimes are also useful for other applications such as singlet oxygen sensitization and oxygen sensing.