Triplet-Triplet Annihilation Upconversion in Molecular Aggregation Systems

Triplet-triplet annihilation (TTA) upconversion, a unique technique that converting low-energy photons into higher-energy photons, has attracted much attention owing to its potential applications in various fields, such as solar cells, bioimaging, photocatalysis and photoelectric device. TTA upconversion has several advantages over other upconversion methods, such as allowing for the use of low excitation power density, readily tunable excitation/emission wavelength and high upconversion quantum yield. Both triplet-triplet energy transfer (TTET) and TTA processes in TTA upconversion follow the Dexter energy transfer mechanism. The components involved have to diffuse in the media and collide within the lifetime of their excited states to complete the energy transfer. Thus, most efficient TTA-based upconversion has been achieved with donor-acceptor pairs that are molecularly dissolved in deaerated organic solvents, which however significantly limited their practical applications. In recent years, more and more efforts have beendevoted to explore high-efficient TTA upconversion under aerated conditions by employing specific solid materials or viscous liquids as matrices to block oxygen. The recent advance of research of TTA upconversion in aggregated systems is summarized, including rubbery polymers, gels, molecular crystals, nanoparticles and supramolecular self-assemblies.