Identifying Key Physical Properties of Simple Organic Drop Cast Films that give Visible to Ultraviolet Light Up-Conversion

We demonstrate UV-emitting films of 2,5-diphenyloxazole (PPO) sensitised by 3,3′-carbonylbis(7-diethylaminocoumarin) (CBDAC), prepared by simple drop casting with rapid solvent evaporation, giving up-converted emission even at low excitation intensities. The mechanisms of up-conversion and triplet quenching in these films has been studied through time-resolved and steady-state spectroscopy. The CBDAC sensitizer aggregates strongly even at low concentrations, with CBDAC phosphorescence being observed in all films, indicating that many triplet excitons do not transfer to the surrounding PPO annihilator. Nonetheless, at very low sensitiser concentrations (1 : 40000 CBDAC : PPO), up-converted PPO UV emission is observed at room temperature which is critically dependent on the film formation conditions. Only in films cast onto substrates held at 60 °C is dominant TTA-UC observed. Comparing to similar sensitised films of TIPS-naphthalene, which are TTA-inactive, we deduce that π-stacking of the PPO, prevented by the TIPS side groups in the TIPS-naphthalene films is crucial for efficient triplet diffusion and up-conversion.