Triplet–triplet annihilation upconversion sensitized with nanocrystals for a new generation of photocatalytic systems

Photon upconversion (UC) is a quantum mechanical process that converts two (or more) lower-energy photons (typically in the NIR or visible range) into a higher-energy photon (in the visible or UV range, respectively). Triplet–Triplet Annihilation (TTA) is one of the most promising UC processes as it can occur directly under non-coherent sources, such as sunlight. The TTA mechanism requires a sensitizer and an annihilator, both of which are generally organic or organo-metallic dyes. Recently, novel TTA-UC systems sensitized with nanocrystals have been developed, offering significant advantages compared to molecular systems, such as the possibility of easily tuning their absorption and emission wavelengths across the solar spectrum and enhanced photostability. These TTA-UC systems are excellent candidates for a wide range of applications, including 3D printing, bioimaging and, especially, photovoltaics and photocatalysis. This review provides a comprehensive and up-to-date overview of the recent advances in the field, addressing the key challenges and current goals, such as maximizing the UC quantum yield. After outlining the principles and mechanisms of TTA, we focus on the main TTA components. Special emphasis is placed on TTA-UC systems sensitized with nanocrystals and their emerging applications, with particular attention to photo-driven reactions. Our aim is to inspire interest in future studies in this exciting yet still emerging subject.