Inverted singlet–triplet gap (INVEST) materials have promising photophysical properties for optoelectronic applications due to an inversion of their lowest singlet (S₁) and triplet (T₁) excited states. This results in an exothermic reverse intersystem crossing (rISC) process that potentially enhances triplet harvesting, compared to thermally activated delayed fluorescence (TADF) emitters with endothermic rISCs. However, the processes and phenomena that facilitate conversion between excited states for INVEST materials are underexplored. We investigate the complex potential energy surfaces (PESs) of the excited states of three heavily studied azaphenalene INVEST compounds, namely, cyclazine, pentazine, and heptazine using two state-of-the-art computational methodologies, namely, RMS-CASPT2 and SCS-ADC(2) methods. Our findings suggest that ISC and rISC processes take place directly between the S₁ and T₁ electronic states in all three compounds through a minimum-energy crossing point (MECP) with an activation energy barrier between 0.11 to 0.58 eV above the S₁ state for ISC and between 0.06 and 0.36 eV above the T₁ state for rISC. We predict that higher-lying triplet states are not populated, since the crossing point structures to these states are not energetically accessible. Furthermore, the conical intersection (CI) between the ground and S₁ states is high in energy for all compounds (between 0.4 to 2.0 eV) which makes nonradiative decay back to the ground state a relatively slow process. We demonstrate that the spin-orbit coupling (SOC) driving the S₁-T₁ conversion is enhanced by vibronic coupling with higher-lying singlet and triplet states possessing vibrational modes of proper symmetry. We also rationalize that the experimentally observed anti-Kasha emission of cyclazine is due to the energetically inaccessible CI between the bright S₂ and the dark S₁ states, hindering internal conversion. Finally, we show that SCS-ADC(2) is able to qualitatively reproduce excited state features, but consistently overpredict relative energies of excited state structural minima compared to RMS-CASPT2. The identification of these excited state features elaborates design rules for new INVEST emitters with improved emission quantum yields.

中文翻译：

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倒置单线态-三线态间隙分子中反向系间窜越详细机制的计算研究

反转单线态-三线态间隙（INVEST）材料由于其最低单线态（S ₁）和三线态（T ₁）激发态的反转而在光电应用中具有有前景的光物理性质。与具有吸热 rISC 的热激活延迟荧光 (TADF) 发射器相比，这导致放热反向系间窜越 (rISC) 过程，可能会增强三重态收获。然而，促进 INVEST 材料激发态之间转换的过程和现象尚未得到充分探索。我们使用两种最先进的计算方法（即 RMS-CASPT2 和 SCS-）研究了三种经过深入研究的氮杂菲那烯 INVEST 化合物（环嗪、五嗪和庚嗪）激发态的复杂势能表面 (PES)。 ADC(2) 方法。我们的研究结果表明，ISC 和 rISC 过程直接发生在所有三种化合物的 S ₁​​ 和 T ₁电子态之间，通过最小能量交叉点 (MECP)，其激活能垒高于 S ₁态 0.11 至 0.58 eV。 ISC 以及rISC高于 T ₁状态 0.06 至 0.36 eV 之间。我们预测较高位置的三重态不会被填充，因为这些态的交叉点结构在能量上无法到达。此外，所有化合物的基态和S ₁态之间的圆锥形交叉点(CI)能量都很高（0.4 至2.0 eV 之间），这使得非辐射衰变回到基态是一个相对缓慢的过程。我们证明，驱动 S ₁ -T ₁转换的自旋轨道耦合 (SOC)通过与具有适当对称振动模式的较高单重态和三重态的振动耦合得到增强。我们还合理地认为，实验观察到的环嗪的反 Kasha 发射是由于亮 S ₂和暗 S ₁态之间能量上难以接近的 CI 造成的，阻碍了内部转换。最后，我们表明，SCS-ADC(2) 能够定性地再现激发态特征，但与 RMS-CASPT2 相比，始终高估激发态结构最小值的相对能量。这些激发态特征的识别详细阐述了具有改进的发射量子产率的新型 INVEST 发射器的设计规则。