Photon upconversion, which is the process of converting low-energy photons into high-energy photons, has shown unique advantages in the fields of solar energy harvesting, photocatalysis, and bio-imaging, etc. Triplet-triplet annihilation upconversion (TTA-UC) based on organic materials is emerging as the next-generation upconversion luminescence that surpasses the limitations of their inorganic counterparts due to its remarkable merits, such as low excitation power density, high upconversion quantum yield (ΦUC, upper limit: 50 %), and finely tunable excitation and emission wavelengths. Although remarkable progress has been made in visible-light-activatable TTA-UC, the near-infrared (NIR) light-sensitized TTA-UC has more promising prospects due to the superiority of NIR light (700–1,700 nm) in terms of its deep penetration and anti-scattering characters and the development of NIR-to-blue TTA-UC with a large anti-Stokes shift is extremely challenging because of the energy loss during the intersystem crossing (ISC).
