Molecular Triplet Sensitization and Photon Upconversion Using Colloidal Semiconductor Nanocrystals

Colloidal semiconductor nanocrystals have recently emerged as a novel class of photosensitizers for molecular spin-triplet states. This sensitization strategy combines the advantages of strong and readily tunable light absorption of nanocrystals with the exceptionally long lifetime of molecular triplet states, which can find a broad range of photochemical and energy applications. A unique advantage for the application in triplet–triplet annihilation photon upconversion is that the negligible bright-to-dark exciton relaxation energy loss of nanocrystals allows for a large photon energy gain (i.e., anti-Stokes shift). This Focus Review highlights recent advances in this emerging field, mainly including a summary of the principles and mechanisms of triplet sensitization along with the factors controlling the sensitization efficiencies and an overview of the nanocrystal materials that have been applied to triplet sensitization and photon upconversion. We end the review with our perspectives on the remaining challenges in this field and the potential applications of this technology in the future.