Franco Olga, Jakoby Marius, Schneider Rebekka V, Hundemer Fabian, Hahn Daniel, Richards Bryce S, Bräse Stefan, Meier Michael A R, Lemmer Uli, Howard Ian A
Department of Electrical Engineering and Information Technology, Light Technology Institute, Karlsruhe Institute of Technology, Karlsruhe, Germany.
Department of Electrical Engineering and Information Technology, Institute of Microstructure Technology, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen, Germany.
Front Chem. 2020 Feb 26;8:126. doi: 10.3389/fchem.2020.00126. eCollection 2020.
Beyond their applications in organic light-emitting diodes (OLEDs), thermally activated delayed fluorescence (TADF) materials can also make good photonic markers. Time-gated measurement of their delayed emission enables "background-free" imaging in, for example, biological systems, because no naturally-occurring compounds exhibit such long-lived emission. Attaching a strongly-absorbing antenna, such as a phenylene ethynylene oligomer, to the TADF core would be of interest to increase their brightness as photonic markers. With this motivation, we study a sequence of TADF-oligomer conjugates with oligomers of varying length and show that, even when the absorption of the oligomer is almost resonant with the charge-transfer absorption of the TADF core, the antenna transfers energy to the TADF core. We study this series of compounds with time resolved emission and transient absorption spectroscopy and find that the delayed fluorescence is essentially turned-off for the longer antennae. Interestingly, we find that the turn-off of the delayed fluorescence is not caused by quenching of the TADF charge-transfer triplet state due to triplet energy transfer of the lower-lying triplet state to the antenna, but must be associated with a decrease in the reverse intersystem crossing rate. These results are of relevance for the further development of TADF "dyes" and also, in the broader context, for understanding the dynamics of TADF molecules in the vicinity of energy donors/acceptors (i.e., in fluorescent OLEDs wherein TADF molecules are used as an assistant dopant).
除了在有机发光二极管（OLED）中的应用外，热激活延迟荧光（TADF）材料还可以成为良好的光子标记物。对其延迟发射进行时间分辨测量能够在例如生物系统中实现“无背景”成像，因为没有天然存在的化合物会表现出如此长寿命的发射。将强吸收天线，如亚苯基乙炔低聚物，连接到TADF核上，对于提高它们作为光子标记物的亮度将是有意义且令人感兴趣的。出于这个动机，我们研究了一系列TADF - 低聚物共轭物，其中低聚物长度不同，并表明，即使低聚物的吸收几乎与TADF核的电荷转移吸收发生共振，天线仍会将能量转移到TADF核上。我们通过时间分辨发射光谱和瞬态吸收光谱研究了这一系列化合物，发现对于较长的天线，延迟荧光基本上被猝灭。有趣的是，我们发现延迟荧光的猝灭不是由于较低能级三重态向天线的三重态能量转移导致TADF电荷转移三重态的猝灭，而是一定与反向系间窜越速率的降低有关。这些结果对于TADF“染料”的进一步发展具有重要意义，并且在更广泛的背景下，对于理解能量供体/受体附近（即，在使用TADF分子作为辅助掺杂剂的荧光OLED中）TADF分子的动力学也具有重要意义。
