Advances in functionalized annihilators to extend triplet–triplet annihilation upconversion performances

Triplet–triplet annihilation upconversion (TTA-UC) has made major advances in many emerging fields in recent years, such as solar light harvesting, photocatalysis, biological imaging, and sensing. TTA-UC consists of photosensitizers and annihilators. In addition to acting as emitters, chemical modification of annihilators has expanded their roles to include the formation of organic gel to avoid oxygen-mediated triplet quenching, amplifying the asymmetry factor of circularly polarized luminescence, constructing an upconversion sensor as recognition units, serving as photoremovable protecting groups, and photocatalysts to realize long-wavelength light-driven organic transformations. Here, we will focus on the significant applications of functionalized annihilators other than photoluminescence, which are manifested via chemical modification with other functional units. Finally, we will elaborate on the existent issues with TTA-UC, including challenges in molecular design, material development, and emerging field applications. In accordance with our research experience, we will propose potential solutions.