Photon Upconversion Systems Based on Triplet–Triplet Annihilation as Photosensitizers for Chemical Transformations

Photon upconversion (UC) based on triplet–triplet annihilation (TTA) is considered one of the most attractive methodologies for switching wavelengths from lower to higher energy. This two-photon process, which requires the involvement of a bimolecular system, has been widely used in numerous fields such as bioimaging, solar cells, displays, drug delivery, and so on. In the last years, we have witnessed the harnessing of this concept by the organic community who have developed new strategies for synthetic purposes. Interestingly, the generation of high-energetic species by this phenomenon has provided the opportunity not only to photoredox activate compounds with high-energy demanding bonds, expanding the reactivity window that lies outside the energy window of the initial irradiation wavelength, but also to sensitized conventional photocatalysts through energy transfer processes even employing infrared irradiation. Herein, an overview of the principal examples found in literature is described where TTA–UC systems are found to be suitable photosensitizers for several chemical transformations.