Optimizing TTA-UC performance by chemically tuning sensitizers and orderly organizing sensitizers and annihilators

Triplet-triplet annihilation upconversion (TTA-UC), which can efficiently convert low-energy light into high-energy light, has received extensive attention in cutting-edge fields such as photovoltaics, bioimaging, and photopolymerization. Currently, optimizing the performance of upconversion materials is a key issue in expanding the application scope of TTA-UC and an important driving force for promoting the progress of this field. This review summarizes recent advancements in enhancing TTA-UC performance through two primary strategies: (1) molecular design: chemical modification of sensitizers to optimize their photophysical properties. By optimizing their molecular structures, the energy difference between the singlet and triplet states of molecules was precisely adjusted, thus the energy losses in intersystem crossing (ISC) and triplet-triplet energy transfer (TTET) processes were effectively reduced, and the anti-Stokes shift was significantly expanded; (2) molecular organization: implementing strategies to control the spatial arrangement of sensitizers and annihilators at the molecular level. This includes the utilization of supramolecular host-guest systems. For example, by taking advantage of the special cavities of host molecules such as cyclodextrins and pillararenes, sensitizers or annihilators can be accurately encapsulated. Through supramolecular host-guest interactions, the intermolecular distance is reduced, thereby facilitating energy transfer between the sensitizers and annihilators. In addition, through co-crystallization, forming frameworks, introducing multiple hydrogen bonds, or leveraging organic/inorganic hybrid materials, the long-range ordered arrangement of annihilators has been realized, opening up new avenues for improving the efficiency of TTA-UC. Furthermore, this review also explores the existing challenges and future development directions. It aims to provide comprehensive guidance and new research ideas for researchers entering this field.