Global Edge Institute, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, Japan.
J Phys Chem B. 2013 May 2;117(17):5180-7. doi: 10.1021/jp401474n. Epub 2013 Apr 12.
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.
基于三重态-三重态湮灭（TTA）的光子上转换（UC）是一种新兴的波长转换技术，适用于阳光。之前我们发现，在离子液体（ILs）中进行的 TTA-UC（ΦUC）的量子效率取决于所使用的 IL 的类型。在本文中，我们研究了实施 TTA 的三重态发射体分子（苝）的动力学，以确定 ΦUC 的 IL 依赖性的起源。我们测量了由五种基于咪唑的 IL 制成的样品的时间分辨延迟 UC 荧光强度，并用分析模型分析其强度衰减曲线。结果，阐明了关于一阶和二阶衰减的几个重要方面。结果表明，ΦUC 的 IL 依赖性主要源自于两个三重态发射体分子相遇时 TTA 的分支比的 IL 依赖性。此外，还发现 IL 的粘度与 TTA 的分支比之间存在很强的相关性。这一发现得到了温度依赖性测量的支持，结果表明 ΦUC 显著受到 IL 粘度的影响。本研究的结果应该为进一步提高ΦUC 提供线索。
