Triplet-triplet annihilation upconversion (TTA-UC) has made significant progress in recent years in several key applications, including solar energy harvesting, photocatalysis, stereoscopic 3D printing, and disease therapeutics. In TTA-UC research, photosensitizers serve the vital function of harvesting low-energy photons. The photophysical characteristics of photosensitizers, including absorbance, triplet state quantum yield, triplet state energy level, triplet state lifetime, etc., determine the performance of TTA-UC. Thus, the study of photosensitizers has been a key aspect of TTA-UC. In recent years, multi-resonance thermally activated delayed fluorescence (MR-TADF) molecules have received extensive attention due to their excellent photophysical properties and electroluminescent device performance. MR-TADF molecules not only present a narrow energy gap between the singlet and triplet excited states, but also have stronger absorption and better wavelength regulation than conventional TADF molecules. Nowadays, the preliminary attempts in TTA-UC using MR-TADF molecules as photosensitizers have resulted in the development of green to ultraviolet, blue to ultraviolet, and even near-infrared to blue emission. This concept will summarize the research progress of MR-TADF molecules as photosensitizers in TTA-UC, analyzing the challenges and giving possible solutions. Finally, we prospect the future development of MR-TADF molecules as photosensitizers, including the molecular design as well as the possible application areas.

A series of C-8 substituted indeno[1,2-g]coumarin-based photoremovable protecting groups (PPGs) were synthesized. para-Substituted benzoic acids were employed as leaving groups to evaluate their photolytic efficiency. Substitution of phenyl groups was proved to have negative impacts on photochemical properties of the PPGs, including but not limited to: retarded photolysis course, decreased uncaging quantum yield, and unsatisfactory cargo release yield. Electron-donating diethylamino substituted PPG 3d, a structural analogue of the widely used 7-diethylaminocoumarin PPG (DEACM), exhibited red-shifted absorption maximum and improved optical properties. Photochemical characterization revealed that PPG 3d not only showed comparable photolytic efficiency to DEACM at 365 nm and 405 nm, but also demonstrated superior sensitivity towards 465 nm wavelength, to which DEACM is unable to absorb and therefore, non-responsive. The >450 nm photosensitivity makes 3d a complement to DEACM for long wavelength excitation and a promising PPG for biological applications.

The influence of activation as key parameter for oxygen sensing by luminescent metal-organic frameworks has been investigated and quantified for the archetype MOF-76(Eu). Activation at different conditions (regarding temperature and solvent-exchange for distinct vacuum pressure and heating time), shows an influence on the overall quenching, response time and cyclability due to different pore accessibility and surface area and therefore on the overall performance of the sensor. The optical sensing process is based on luminescence quenching, analyzed from high vacuum (10−7 bar) to dosing oxygen from 0.01 bar to 1 bar. Strong influence of the different activation parameters is observed, as MOF-76(Eu) activated at 50 °C shows some quenching of the luminescence intensity within 30 min, while methanol-exchange and subsequent activation at 250 °C leads to a quenching rate of 98.6%. In addition, the sensor response occurs more than 1000 times faster within 0.2 s. These results correlate well with physisorption data, which reveal a significant change in porosity and surface area according to the degree of activation. For a better understanding of the involved processes, adsorption isotherms were recorded, surface areas determined and correlated to the photophysical parameters, including Stern-Volmer kinetics and cycling experiments for the differently activated MOF sensors