Understanding triplet exciton diffusion between organic thermally activated delayed fluorescence (TADF) molecules is a challenge due to the unique cycling between singlet and triplet states in these molecules. Although prompt emission quenching allows the singlet exciton diffusion properties to be determined, analogous analysis of the delayed emission quenching does not yield accurate estimations of the triplet diffusion length (because the diffusion of singlet excitons regenerated after reverse-intersystem crossing needs to be accounted for). Herein, we demonstrate how singlet and triplet diffusion lengths can be accurately determined from accessible experimental data, namely the integral prompt and delayed fluorescence. In the benchmark materials 4CzIPN and 4TCzBN, we show that the singlet diffusion lengths are (9.1 ± 0.2) and (12.8 ± 0.3) nm, whereas the triplet diffusion lengths are negligible, and certainly less than 1.0 and 1.2 nm, respectively. Theory confirms that the lack of overlap between the shielded lowest unoccupied molecular orbitals (LUMOs) hinders triplet motion between TADF chromophores in such molecular architectures. Although this cause for the suppression of triplet motion does not occur in molecular architectures that rely on electron resonance effects (e.g. DiKTa), we find that triplet diffusion is still negligible when such molecules are dispersed in a matrix material at a concentration sufficiently low to suppress aggregation. The novel and accurate method of understanding triplet diffusion in TADF molecules will allow accurate physical modeling of OLED emitter layers (especially those based on TADF donors and fluorescent acceptors).

中文翻译：

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精确实验确定热活化延迟荧光分子之间单重态和三重态激子扩散的方法

由于有机分子在单重态和三重态之间的独特循环，因此了解有机热活化延迟荧光（TADF）分子之间的三重态激子扩散是一项挑战。尽管快速发射猝灭可以确定单重态激子的扩散性质，但延迟发射猝灭的类似分析不能准确估计三重态的扩散长度（因为需要考虑反向系统间交叉后再生的单重态激子的扩散） 。在本文中，我们演示了如何从可访问的实验数据（即完整的即时荧光和延迟荧光）中准确确定单线态和三线态扩散长度。在基准材料4CzIPN和4TCzBN中，我们表明单重态扩散长度为（9.1±0.2）和（12.8±0.3）nm，而三重态扩散长度可以忽略不计，并且分别小于1.0 nm和1.2 nm。理论证实，在这种分子结构中，屏蔽的最低未占据分子轨道（LUMO）之间缺乏重叠会阻碍TADF生色团之间的三重态运动。尽管在依赖电子共振效应的分子结构中不会出现抑制三重态运动的原因（例如DiKTa），我们发现当这类分子以足够低的浓度分散在基质材料中以抑制聚集时，三重态扩散仍然可以忽略不计。理解TADF分子中三重态扩散的新颖，准确的方法将允许对OLED发射器层（尤其是基于TADF供体和荧光受体的那些）进行精确的物理建模。