Triplet–Triplet Annihilation Upconversion with Large Anti-Stokes Shift

Triplet-triplet annihilation (TTA) upconversion is a phenomenon that converts low-energy photons into high-energy ones. TTA upconversion with a large anti-Stokes shift is highly desired but less easily obtained because intersystem crossing (ISC), triplet energy transfer (TET), and the TTA process in upconversion are all accompanied by significant energy losses. This review, on the basis of the three above-mentioned energy loss channels, systematically summarized three roads to large anti-Stokes shift: (i) Reduce the energy loss of ISC process by utilizing photosensitizers based on small S₁/T₁ energy gap due to charge transfer, small S₁/T₁ energy gap due to multiresonance, direct excitation into S₀ → T₁, minimal exchange splitting (S₁/T₁) of inorganic materials, etc.; (ii) Reduce the energy loss of TET process by selecting energy matched photosensitizer and annihilator; and (iii) Reduce the energy loss of TTA process by regulating the singlet and triplet energy levels of annihilator. In addition, the representative applications of TTA upconversion with large anti-Stokes shift in life science, photocatalysis, and 3D printing are exhibited. This study summarizes cutting-edge strategies and research advances in large anti-Stokes upconversion, offering a critical analysis of both progress and persistent challenges in the field, which further provides a deep understanding of the mechanism of TTA upconversion with large anti-Stokes shifts and the construction of innovative and high-performance upconversion systems.