Kinetics of Photon Upconversion in Ionic Liquids: Energy Transfer between Sensitizer and Emitter Molecules

Abstract

The efficiency of triplet-sensitized photon upconversion in ionic liquids was previously found to be dependent on the type of ionic liquid employed. The properties of the intermolecular energy transfer need to be understood in order to improve the upconversion efficiency. Here, we investigate the kinetics of the triplet energy transfer from the triplet sensitizing molecule to the emitter molecule where the latter is responsible for delayed upconversion fluorescence emission. The collision kinetics between the sensitizer and emitter molecules in imidazolium ionic liquids are investigated by systematically changing the alkyl chain length of the ionic liquid cation. Stern-Volmer analysis reveals unique diffusion behavior of the solute molecules in ionic liquids, and this observation is attributed to the microheterogeneity of the ionic liquids. Through time-resolved transient absorption measurements and determination of the triplet-triplet absorption coefficient of the sensitizer molecule used, we find that the quantum efficiency of the triplet energy transfer in the present system is sufficiently high (ca. 0.75) and independent of the type of ionic liquid. These findings show that the ionic liquid dependence of the upconversion efficiency arises from the later processes pertaining to the emitter molecule rather than the triplet energy transfer process.