Singlet Oxygen Mediated Photochemical Upconversion of NIR Light

We report photochemical upconversion (UC) of near-infrared (NIR) light to the visible spectrum mediated by molecular oxygen. Thereby, we address two of the main challenges in the field of photochemical UC, UC of photons above 800 nm and (ii) utilization of molecular oxygen, which is necessarily excluded in conventional photochemical UC systems. In the employed system, singlet oxygen is generated upon photoexcitation of the sensitizer molecules and then acts as an energy transmitter for the UC process. The excitation energy of two singlet oxygen molecules is subsequently harvested by emitter molecules, which in turn gives rise to delayed fluorescence of the emitter species. We discuss strategies for improvement of the currently achieved efficiencies of <= 0.01% to produce excited singlet states in the emitter molecules.