New Photon Upconversion Routes based on Coordination Compounds

Abstract

Photon upconversion (UC) is the process that converts longer wavelength (lower energy) light to shorter wavelength (higher energy) light. Among different UC mechanisms, the UC based on triplet-triplet annihilation (TTA-UC) has attracted much attention due to its advantage of high efficiency at low excitation intensity. In the typical mechanism of TTA-UC, the triplet excited state of the donor (sensitizer), generated via intersystem crossing (ISC) from its photo-excited singlet state, transfers the triplet energy to the acceptor (emitter), and the sensitized acceptor triplets collide to undergo TTA, producing the emissive acceptor excited singlet state. The efficient TTA-UC has been achieved for donor-acceptor pairs molecularly dissolved in organic solvents, however, it has several problems such as the difficulty to show efficient TTA-UC at low chromophore concentration. To solve these fundamental issues, we introduced the concept of triplet energy migration in dense chromophore assemblies and achieved highly efficient photon upconversion at low chromophore concentration by constructing coordination copolymers of donor- and acceptor-containing ligands. Another challenge of TTA-UC is to upconvert near-infrared (NIR) light to visible light. By employing Os complexes with singlet-to-triplet (S–T) direct transition and interface complexes between semiconductor nanocrystals and chromophore ligands, we have achieved an efficient NIR-to-visible TTA-UC in the solid state.