Interplay between host structure, oxygen quenching and triplet-triplet annihilation upconversion in hybrid polymer hosts

The ability to convert low-energy photons to those of higher energy through triplet-triplet annihilation upconversion (TTA-UC) is of significant interest for diverse applications including solar energy harvesting, sensing and anti-counterfeiting. However, the efficiency of TTA-UC under ambient conditions is often severely limited due to quenching of excited triplet states by molecular oxygen. Therefore, when designing TTA-UC systems, it is crucial to effectively characterize the extent of oxygen quenching, and to use these insights to drive material design that effectively prevent the permeation of oxygen. In this work, we investigate the oxygen barrier properties of three organic-inorganic hybrid polymers known as ureasils, previously determined to be effective TTA-UC hosts under ambient conditions. Through both direct oxygen permeation measurements, and kinetic analysis of the phosphorescence quenching of palladium (II) octaethylporphyrin (PdOEP), we investigate how the ureasil structure, particularly with respect to silica content and molecular weight and branching of the polymer chains, affects the bulk and local oxygen permeabilities. We also demonstrate the interplay of oxygen quenching with the wider TTA-UC process, confirming that variation in oxygen permeability is the primary factor affecting ambient TTA-UC efficiency between different ureasil structures. This emphasizes the importance considering oxygen barrier properties as a key metric in the design of future host materials for more efficient ambient TTA-UC systems.