Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St Andrews, KY16 9ST St Andrews, U.K.
Laboratory for Chemistry of Novel Materials, University of Mons, 7000 Mons, Belgium.
J Phys Chem A. 2023 Jun 1;127(21):4743-4757. doi: 10.1021/acs.jpca.2c08201. Epub 2023 May 17.
The importance of intermediate triplet states and the nature of excited states has gained interest in recent years for the thermally activated delayed fluorescence (TADF) mechanism. It is widely accepted that simple conversion between charge transfer (CT) triplet and singlet excited states is too crude, and a more complex route involving higher-lying locally excited triplet excited states has to be invoked to witness the magnitude of the rate of reverse inter-system crossing (RISC) rates. The increased complexity has challenged the reliability of computational methods to accurately predict the relative energy between excited states as well as their nature. Here, we compare the results of widely used density functional theory (DFT) functionals, CAM-B3LYP, LC-ωPBE, LC-ωPBE, LC-ωHPBE, B3LYP, PBE0, and M06-2X, against a wavefunction-based reference method, Spin-Component Scaling second-order approximate Coupled Cluster (SCS-CC2), in 14 known TADF emitters possessing a diversity of chemical structures. Overall, the use of the Tamm-Dancoff Approximation (TDA) together with CAM-B3LYP, M06-2X, and the two ω-tuned range-separated functionals LC-ωPBE and LC-ωHPBE demonstrated the best agreement with SCS-CC2 calculations in predicting the absolute energy of the singlet S, and triplet T and T excited states and their energy differences. However, consistently across the series and irrespective of the functional or the use of TDA, the nature of T and T is not as accurately captured as compared to S. We also investigated the impact of the optimization of S and T excited states on Δ and the nature of these states for three different functionals (PBE0, CAM-B3LYP, and M06-2X). We observed large changes in Δ using CAM-B3LYP and PBE0 functionals associated with a large stabilization of T with CAM-B3LYP and a large stabilization of S with PBE0, while Δ is much less affected considering the M06-2X functional. The nature of the S state barely evolves after geometry optimization essentially because this state is CT by nature for the three functionals tested. However, the prediction of the T nature is more problematic since these functionals for some compounds interpret the nature of T very differently. SCS-CC2 calculations on top of the TDA-DFT optimized geometries lead to a large variation in terms of Δ and the excited-state nature depending on the chosen functionals, further stressing the large dependence of the excited-state features on the excited-state geometries. The presented work highlights that despite good agreement of energies, the description of the exact nature of the triplet states should be undertaken with caution.
近年来，中间三重态态和激发态的性质在热激活延迟荧光（TADF）机制中引起了关注。人们普遍认为，电荷转移（CT）三重态和单重激发态之间的简单转换过于粗糙，必须采用更复杂的途径来涉及更高的局域激发三重态激发态，以见证反向系间窜越（RISC）速率的速率。这种复杂性增加了对计算方法准确性预测激发态之间相对能量及其性质的可靠性的挑战。在这里，我们比较了广泛使用的密度泛函理论（DFT）泛函，CAM-B3LYP、LC-ωPBE、LC-ωPBE、LC-ωHPBE、B3LYP、PBE0 和 M06-2X 与基于波函数的参考方法，自旋分量缩放二阶近似耦合簇（SCS-CC2）在 14 种已知的 TADF 发射体中，这些发射体具有多种化学结构。总体而言，使用 Tamm-Dancoff 近似（TDA）与 CAM-B3LYP、M06-2X 以及两种 ω 调谐范围分离函数 LC-ωPBE 和 LC-ωHPBE 相结合，在预测单重态 S 的绝对能量以及三重态 T 和 T 激发态及其能量差异方面，与 SCS-CC2 计算结果的吻合度最好。然而，在整个系列中，无论功能如何，也无论是否使用 TDA，与 S 相比，T 和 T 的性质都不能被准确捕获。我们还研究了三种不同功能（PBE0、CAM-B3LYP 和 M06-2X）优化 S 和 T 激发态对Δ和这些状态性质的影响。我们观察到，使用 CAM-B3LYP 和 PBE0 功能时，Δ会发生很大变化，这与 CAM-B3LYP 中 T 的稳定化以及 PBE0 中 S 的稳定化有关，而在考虑 M06-2X 功能时，Δ的影响要小得多。S 态的性质在几何优化后几乎没有变化，这主要是因为这三种测试的功能都使 S 态具有 CT 性质。然而，T 性质的预测则更为成问题，因为对于某些化合物，这些功能会非常不同地解释 T 的性质。基于 TDA-DFT 优化几何形状的 SCS-CC2 计算导致 Δ和激发态性质的变化很大，这取决于所选的功能，这进一步强调了激发态特征对激发态几何形状的强烈依赖性。所提出的工作强调，尽管能量吻合度很好，但应谨慎进行三重态的确切性质描述。
