The energy gap between the lowest singlet and triplet excited states (ΔE_ST) is a key property of thermally activated delayed fluorescence (TADF) emitters, where these states are dominated by charge-transfer (CT) character. Despite its well-known shortcomings concerning CT states, time-dependent density functional theory (TD-DFT) is widely used to predict this gap and study TADF. Moreover, polar CT states exhibit a strong interaction with their molecular environment, which further complicates their computational description. Addressing these two major challenges, this work studies the performance of Tamm–Dancoff-approximated TD-DFT (TDA-DFT) on the recent STGABS27 benchmark set,¹ exploring different strategies to include orbital and structural relaxation, as well as dielectric embedding. The results show that the best-performing strategy is to calculate ΔE_ST at the ground-state structure using functionals with a surprisingly small amount of Fock exchange of ≈10% and without a (complete) solvent model. However, as this approach heavily relies on error cancellation to mimic dielectric relaxation, it is not robust and exhibits large systematic deviations in excited state energies, state characters, and structures. More rigorous approaches, including state-specific solvation, do not share these systematic deviations, but their predicted ΔE_ST values exhibit larger statistical errors. We thus conclude that for the description of CT states in dielectric environments, none of the tested TDA-DFT methods is competitive with the recently presented ROKS/PCM approach regarding robustness, accuracy, and computational efficiency.

中文翻译：

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准确的单重态-三重态间隙或激发态结构：测试和了解 TADF 发射器的 TD-DFT 性能

最低单线态和三线态激发态 (Δ E _ST )之间的能隙是热激活延迟荧光 (TADF) 发射器的一个关键特性，其中这些状态由电荷转移 (CT) 特性决定。尽管其在 CT 状态方面存在众所周知的缺点，但时间相关密度泛函理论 (TD-DFT) 被广泛用于预测这种差距并研究 TADF。此外，极性 CT 态表现出与其分子环境的强烈相互作用，这进一步使其计算描述复杂化。为了解决这两个主要挑战，这项工作研究了 Tamm–Dancoff 近似 TD-DFT (TDA-DFT) 在最近的 STGABS27 基准集上的性能，¹探索不同的策略，包括轨道和结构弛豫，以及电介质嵌入。结果表明，性能最佳的策略是使用具有 ≈10% 的惊人少量 Fock 交换且没有（完整）溶剂模型的泛函来计算基态结构的Δ E _{ST 。}然而，由于这种方法严重依赖误差消除来模拟介电弛豫，因此它不稳健并且在激发态能量、状态特征和结构方面表现出较大的系统偏差。更严格的方法，包括状态特定的溶剂化，不共享这些系统偏差，但它们预测的 Δ E _ST值表现出较大的统计误差。因此，我们得出结论，对于介电环境中 CT 状态的描述，经测试的 TDA-DFT 方法在鲁棒性、准确性和计算效率方面均无法与最近提出的 ROKS/PCM 方法相媲美。