Heavy-Atom-Free Triplet-Triplet Annihilation Upconversion in Photo-cross-linked Polymer Poly(ethylene glycol)...

Heavy-atom-free triplet-triplet annihilation (TTA) upconversion sensitized by a thermally activated delayed fluorescence (TADF) molecule is investigated in a dried gel made of a photo-cross-linked polymer as the solid-state matrix. The upconversion fluorescence quantum yields, Φ_UC, of the solid-gel TTA system at different penetration depths are measured accurately based on a developed internal-reference method. It is found that Φ_UC is greatest at the surface and then decreases exponentially with increasing depth, influenced by the substrate absorption. The same process is also performed in a TTA solution at different depths, but a completely different result is obtained; there is little difference for Φ_UC. To the best of our knowledge, this is the first time the quantum yields at different transmission depths have been mentioned and calculated experimentally. These results illustrate the importance of accurately measuring the quantum yield of solid-phase TTA upconversion and provide a novel way to improve the solid-phase TTA quantum yield by reducing the thickness of the substrate.