Thermally Activated Delayed Photoluminescence Based on Functionalized Quantum Dots

Colloidal quantum dots (QDs) are promising light-harvesting materials that offer advantages of high extinction coefficients and tunable absorption/emission wavelengths. However, the exciton lifetime, another key property of these materials, is relatively short (nanoseconds) and is challenging to manipulate. In recent years, the hybrid structures of QD surface-functionalized with organic molecules have emerged a versatile platform to regulate the exciton lifetime. Through rapid triplet energy transfer (TET) and thermally activated reverse TET (rTET) between the QD exciton and the molecular triplet, the exciton lifetime can be prolonged to sub-milliseconds with thermally activated delayed photoluminescence (TADPL) that resembles thermally activated delayed fluorescence (TADF) in molecules. Reported TADPL systems thus far have successfully covered the spectral range from violet to near-infrared. This review introduces the principles for constructing efficient TADPL systems and identifies three strategies that allow for facile regulation of TADPL lifetimes. The advantages of TADPL systems over TADF molecules are also highlighted, which include a smaller Stokes shift and a narrower emission linewidth. Applications of TADPL systems in triplet-triplet annihilation up-conversion and photochemical reactions are also reviewed, which are enabled by the substantial increase in exciton lifetime that facilitates efficient charge/energy transfer to substrate molecules via diffusion in solution. Finally, we summarize the current limitations of these materials and provide an outlook for future directions to be explored.