Near-Infrared-to-Visible Photon Upconversion Sensitized by a Metal Complex with Spin-Forbidden yet Strong S0-T1 Absorption

Near-infrared (NIR)-to-visible (vis) photon upconversion (UC) is useful for various applications; however, it remains challenging in triplet-triplet annihilation-based UC, mainly due to the energy loss during the S-1-to-T-1 intersystem crossing (ISC) of molecular sensitizers. In this work, we circumvent this energy loss by employing a sensitizer with direct S-0-to-T-1 absorption in the NIR region. A mixed solution of an osmium complex having a strong S-0-T-1 absorption and rubrene emitter upconverts NIR light (lambda = 938 nm) to visible light (lambda = 570 nm). Sensitizer-doped emitter nanoparticles are prepared by re-precipitation and dispersed into an oxygen-barrier polymer. The obtained composite film shows a stable NIR-to-vis UC emission based on triplet energy migration (TEM), even in air. A high UC quantum yield of 3.1% is observed for this TEM-UC system, expanding the scope of molecular sensitizers for NIR-to-vis UC.