title: Kinetics of photon upconversion in ionic liquids: time-resolved analysis of delayed fluorescence.
abstract: Photon upconversion (UC) based on triplet-triplet annihilation (TTA) is an emerging wavelength shifting technology, which is applicable to sunlight. Previously we found that the quantum efficiency of TTA-UC (ΦUC) carried out in ionic liquids (ILs) is dependent on the type of IL employed. In this article we investigate the kinetics of the triplet emitter molecules (perylene) that implement TTA to determine the origin of the IL dependence of ΦUC. We measure the time-resolved delayed UC fluorescence intensities from samples made with five imidazolium-based ILs, and their intensity decay curves are analyzed with an analytical model. Consequently, several important aspects regarding both the first-order and second-order decays are elucidated. It is revealed that the IL dependence of ΦUC primarily originates from the IL dependence of the branching ratio toward TTA upon an encounter of two triplet emitter molecules. Additionally, a strong correlation between the viscosity of the ILs and the branching ratios toward TTA is found. This finding is supported by temperature-dependent measurements, from which ΦUC is found to be significantly affected by the viscosity of the IL. The results of this study should provide a clue for further improving ΦUC.
authors.affiliation: Global Edge Institute, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, Japan. murakami.y.af@m.titech.ac.jp
authors.forename: Yoichi
authors.initials: Y
authors.lastname: Murakami
authors.forename: Hitomi
authors.initials: H
authors.lastname: Kikuchi
authors.forename: Akio
authors.initials: A
authors.lastname: Kawai