Department of Chemistry , Duke University , Durham , North Carolina 27708 , United States.
Key Laboratory of Theoretical Chemistry of Environment School of Chemistry and Environment , South China Normal University , Guangzhou 510631 , China.
J Chem Theory Comput. 2018 Jun 12;14(6):3196-3204. doi: 10.1021/acs.jctc.8b00153. Epub 2018 May 30.
Thermally activated delayed florescence (TADF) is a mechanism that increases the electroluminescence efficiency in organic light-emitting diodes by harnessing both singlet and triplet excitons. TADF is facilitated by a small energy difference between the first singlet (S) and triplet (T) excited states (Δ E(ST)), which is minimized by spatial separation of the donor and acceptor moieties. The resultant charge-transfer (CT) excited states are difficult to model using time-dependent density functional theory (TDDFT) because of the delocalization error present in standard density functional approximations to the exchange-correlation energy. In this work we explore the application of the particle-particle random phase approximation (pp-RPA) for the determination of both S and T excitation energies. We demonstrate that the accuracy of the pp-RPA is functional dependent and that, when combined with the hybrid functional B3LYP, the pp-RPA computed Δ E(ST) have a mean absolute deviation (MAD) of 0.12 eV for the set of examined molecules. A key advantage of the pp-RPA approach is that the S and T states are characterized as CT states for all of experimentally reported TADF molecules examined here, which allows for an estimate of the singlet-triplet CT excited state energy gap (Δ E(ST) = CT - CT). For experimentally known TADF molecules with a small (<0.2 eV) Δ E(ST) in this data set, a high accuracy is demonstrated for the prediction of both the S (MAD = 0.18 eV) and T (MAD = 0.20 eV) excitation energies as well as Δ E(ST) (MAD = 0.05 eV). This result is attributed to the consideration of correct antisymmetry in the particle-particle interaction leading to the use of full exchange kernel in addition to the Coulomb contribution, as well as a consistent treatment of both singlet and triplet excited states. The computational efficiency of this approach is similar to that of TDDFT, and the cost can be reduced significantly by using the active-space approach.
热致延迟荧光（TADF）是一种通过利用单重态和三重态激子来提高有机发光二极管电致发光效率的机制。TADF 是通过第一单重态（S）和三重态（T）激发态之间的小能量差（ΔE(ST)）来实现的，这可以通过施主和受主部分的空间分离来最小化。由于标准密度泛函近似对交换相关能量的离域误差，所得的电荷转移（CT）激发态很难使用含时密度泛函理论（TDDFT）进行建模。在这项工作中，我们探索了粒子-粒子随机相位近似（pp-RPA）在确定 S 和 T 激发能中的应用。我们证明了 pp-RPA 的准确性与函数有关，并且当与混合泛函 B3LYP 结合使用时，pp-RPA 计算的ΔE(ST)对于所检查的分子集具有 0.12 eV 的平均绝对偏差（MAD）。pp-RPA 方法的一个关键优势是，对于所有在这里检查的实验报告的 TADF 分子，S 和 T 态都被表征为 CT 态，这允许估计单重态-三重态 CT 激发态能隙（ΔE(ST) = CT - CT）。对于实验上已知的 TADF 分子，对于这个数据集，ΔE(ST)较小（<0.2 eV），可以很好地预测 S（MAD = 0.18 eV）和 T（MAD = 0.20 eV）激发能以及ΔE(ST)（MAD = 0.05 eV）。这一结果归因于在粒子-粒子相互作用中考虑正确的反称性，导致除了库仑贡献之外还使用全交换核，以及对单重态和三重态激发态的一致处理。这种方法的计算效率与 TDDFT 相似，并且可以通过使用活性空间方法显著降低成本。
