Organic Triplet Photosensitizers for Triplet-Triplet Annihilation Upconversion

Triplet-triplet annihilation (TTA) upconversion, a photophysical phenomenon of conversion of low-energy photon into high-energy photon, is a promising approach due to its low excitation power requirement and high upconversion quantum yields. Most of the triplet photosensitizers (PSs) used for TTA upconversion are based on transition metal complexes and organic chromophores containing heavy atom. Recently TTA upconversion with heavy atom-free PSs has gained much attention due to its minimal detrimental effects. In this chapter we summarize organic triplet PSs with different intersystem crossing (ISC) mechanism used in TTA upconversion. We briefly describe different types of triplet PSs for TTA upconversion such as heavy atom-containing organic chromophores, BODIPY dimers involving exciton coupling, and electron spin convertor-bearing dyes. Recently new triplet PSs involving charge recombination (CR)-induced ISC and thermally activated delayed fluorescence (TADF) have been used in the TTA upconversion. We discuss the benefits of these PSs for TTA upconversion. This book chapter provides current progress in the field of TTA upconversion and designing new efficient triplet PSs.