Are Heptazine-Based Organic Light-Emitting Diode Chromophores Thermally Activated Delayed Fluorescence or Inverted Singlet-Triplet Systems?
Are Heptazine-Based Organic Light-Emitting Diode Chromophores Thermally Activated Delayed Fluorescence or Inverted Singlet-Triplet Systems?
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DOI：
10.1021/acs.jpclett.1c01926
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发表时间：
2021-07
期刊：
影响因子：
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通讯作者：
A. Sobolewski;W. Domcke
中科院分区：
文献类型：
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作者：
A. Sobolewski;W. Domcke
由阿达奇及其同事合成了两种来自七嗪的发色团HAP-3 MF和HAP-3 TPA，并将其作为发光二极管（OLED）的发光体进行了测试。这两种发光体显示出量子效率超过传统荧光OLED的理论最大值。利用热激活延迟荧光（TADF）机制解释了发光效率的提高。本文采用基于波函数的从头算方法（ADC（2））研究了HAP-3 MF和HAP-3 TPA的电子激发能和激发态势能面的基本形貌特征。发现HAP-3 MF是一个反转的单重态-三重态（IST）系统，即S1和T1态的能量是鲁棒反转的，违反了Hund的多重性规则。值得注意的是，HAP-3 MF可能是第一个在OLED器件中实现的IST发射器。另一方面，在HAP-3 TPA中，S1和T1态的垂直激发能基本上是简并的。激发态沿沿着不同的弛豫坐标表现出相似量级的振动稳定能，导致绝热激发能也接近简并。发现HAP-3 TPA是TADF和IST系统的边界处的发色团。根据这些计算结果分析了阿达奇及其同事报道的HAP-3 MF和HAP-3 TPA的光谱数据。
Two chromophores derived from heptazine, HAP-3MF and HAP-3TPA, were synthesized and tested as emitters in light-emitting diodes (OLEDs) by Adachi and co-workers. Both emitters were shown to exhibit quantum efficiencies which exceed the theoretical maximum of conventional fluorescent OLEDs. The enhanced emission efficiency was explained by the mechanism of thermally activated delayed fluorescence (TADF). In the present work, the electronic excitation energies and essential features of the topography of the excited-state potential-energy surfaces of HAP-3MF and HAP-3TPA have been investigated with a wave function-based ab initio method (ADC(2)). It is found that HAP-3MF is an inverted singlet-triplet (IST) system; that is, the energies of the S1 and T1 states are robustly inverted in violation of Hund's multiplicity rule. Notably, HAP-3MF presumably is the first IST emitter which was implemented in an OLED device. In HAP-3TPA, on the other hand, the vertical excitation energies of the S1 and T1 states are essentially degenerate. The excited states exhibit vibrational stabilization energies of similar magnitude along different relaxation coordinates, resulting in adiabatic excitation energies which also are nearly degenerate. HAP-3TPA is found to be a chromophore at the borderline of TADF and IST systems. The spectroscopic data reported by Adachi and co-workers for HAP-3MF and HAP-3TPA are analyzed in light of these computational results.