Application of time-resolved electron paramagnetic resonance spectroscopy in the mechanistic study of thermally activated delayed fluorescence (TADF) materials
Application of time-resolved electron paramagnetic resonance spectroscopy in the mechanistic study of thermally activated delayed fluorescence (TADF) materials
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DOI：
10.1039/d1tc04888k
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发表时间：
2021-12-14
影响因子：
6.4
通讯作者：
Zhao, Jianzhang
中科院分区：
文献类型：
--
作者：
Chen, Xi;Xiao, Xiao;Zhao, Jianzhang
在有机发光二极管(OLED)中，三重态激子的捕获是至关重要的，因为由电子和空穴复合产生的三重态占激子总数的75%，而单重态则占25%。然而，有机分子的三重态通常要么在室温下不发光，要么具有很长的磷光寿命，这两种情况都对OLED不利。热激活延迟荧光(TADF)发射器是应对这一挑战的一种方法。典型的TADF发射体是基于施主-受主结构基元的，低能态包括电荷转移单态((CT)-C-1)、三态((CT)-C-3)和紧密局域三态((LE)-L-3)。尽管已经开发了许多高效的有机发光二极管TADF发射体，但TADF的光物理过程，如电荷分离、正向系间交叉(ISC)和反向系间交叉(RISC)，以及这些发射体激发态之间的耦合，还远不清楚。在此，我们介绍了用时间分辨电子顺磁共振(TREPR)方法研究TADF发射体的光物理过程的最新进展。这一光谱工具提供了参与TADF过程的瞬时顺磁物种动力学的独特信息，例如(CT)-C-3和(LE)-L-3态，以及ISC机制。用TREPR谱研究了TADF机制，如同时观测(CT)-C-3和(LE)-L-3态，振动和自旋-振动耦合介导的ISC，以及(CT)-C-3和(LE)-L-3态波函数的电子组态和空间离域。文中还简要介绍了通过理论计算得到的有利于TADF的因素。
Triplet exciton harvesting is crucial in organic light emitting diodes (OLEDs) because the triplet states produced by electron and hole recombination account for up to 75% of the total excitons, whereas the singlet states account for 25%. However, the triplet state of organic molecules is usually either non-emissive at room temperature or gives very long phosphorescence lifetimes, both of which are detrimental to OLEDs. Thermally activated delayed fluorescence (TADF) emitters are one answer to this challenge. Typical TADF emitters are based on an electron donor-acceptor structure motif, and the low-lying states include the charge transfer singlet ((CT)-C-1) state and triplet ((CT)-C-3) state, and a closely-lying localized triplet ((LE)-L-3) state. Although many efficient TADF emitters have been developed for OLEDs, the underpinning photophysical processes of TADF, for instance, the charge separation, the forward intersystem crossing (ISC) and the reverse ISC (rISC), and the coupling of the excited states of these emitters, are far from clear. Herein, we introduce recent developments in the study of the photophysical processes of TADF emitters using the time-resolved electron paramagnetic resonance (TREPR) spectroscopy method. This spectral tool supplies unique information on the dynamics of the transient paramagnetic species involved in the TADF processes, for instance, the (CT)-C-3 and the (LE)-L-3 states, as well as the ISC mechanisms. Physical insights have been obtained with TREPR spectra on the TADF mechanism, such as simultaneous observation of the (CT)-C-3 and (LE)-L-3 states, vibrational and spin-vibronic coupling mediated ISC, and the electronic configuration and spatial delocalization of the (CT)-C-3 and (LE)-L-3 states' wave functions. Factors beneficial to TADF obtained via theoretical computations are also briefly introduced.