Triplet-triplet annihilation upconversion (TTA-UC) is a process in which two photons are converted into one of higher energy. Therefore, it is attractive for multiple applications such as drug activation, sensing, photovoltaics and others.[1] TTA-UC relies on the long lifetime of triplet excitons for efficient energy transfer from sensitizer to annihilator and frequent intermolecular interaction between triplet excitons of annihilator. In the solution state, under ambient conditions, dissolved molecular oxygen caused a major measurement problem because it strongly quenches the triplet states, limiting the evaluation of intrinsic photophysical parameters. Several techniques could be used to prevent oxygen quenching, including solution bubbling with inert gases, such as nitrogen, which physically replaces dissolved oxygen in the solvent, or the freeze-pump-thaw method, where the solution is frozen with liquid nitrogen, trapping dissolved gasses inside. After vacuuming and heating a frozen solution, gaseous oxygen is removed.
Although these methods effectively remove oxygen, they require relatively sophisticated technological equipment and are time-consuming. Another way to prevent oxygen quenching is by using oxygen scavengers. After quenching the triplet state, molecular oxygen becomes excited and forms reactive singlet oxygen, which, in the presence of oxygen scavenges, effectively reacts, resulting in the stable form of oxidized scavengers. In this way, oxygen concentration is reduced in the solution.
In this study, various oxygen scavengers were tested, including common organic solvents such as THF, 2-methyltetrahydrofuran, DMSO or sulphur-based scavengers[2] such as dimethylsulfide, trithiane and others. It has been observed that DMSO removes oxygen the most efficiently, however, DMSO, due to its large dipole moment, is particularly suitable solvent for polar molecules. Therefore, alternative scavenging materials, even less efficient, are very attractive.