The prediction of accurate singlet triplet (ST) gaps of polyaromatic hydrocarbons has been challenging due to the differential multireference character of the two states. The ST gaps of linear polyacenes have shown an exponential decay with system size due to the decreasing stability and increasing multireference nature of the singlet state. These low ST gaps can ideally be leveraged towards energy applications but is hindered by the decreasing stability of the system. While non-linear or kinked polyacenes are characterised by higher stability, multireference calculations on these systems are limited. In our work, we show that while the singlet states of kinked polyacenes are markedly less multireference, the triplet states are highly multireference in these systems and therefore, the correct trend of ST gap in the kinked polyacenes requires high-level multireference calculations. We show that unlike linear polyacenes, in the non-linear systems the ST gaps increase marginally with system size. The ST gaps also show absolutely no correlation with HOMO-LUMO gaps. These surprising trends are a combined effect of the non-linear connections (topology) and the geometrical factors. These results are in stark contrast to the observations in linear polyacenes.

中文翻译：

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非线性多环芳烃中低位态的奇妙案例

由于两种状态的差异多参考特征，多环芳烃的准确单重态三重态 (ST) 间隙的预测一直具有挑战性。由于单线态的稳定性降低和多参考性质增加，线性多并苯的 ST 间隙随系统尺寸呈指数衰减。这些低 ST 差距可以理想地用于能源应用，但会受到系统稳定性下降的阻碍。虽然非线性或扭结多并苯的特点是稳定性更高，但对这些系统的多参考计算是有限的。在我们的工作中，我们表明，虽然扭结多并苯的单重态的多指性明显较低，但三重态在这些系统中具有高度的多指性，因此，扭结多并苯中 ST 间隙的正确趋势需要高水平的多参考计算。我们表明，与线性多并苯不同，在非线性系统中，ST 间隙随着系统尺寸的增加而略有增加。ST 间隙也显示出与 HOMO-LUMO 间隙绝对没有相关性。这些令人惊讶的趋势是非线性连接（拓扑）和几何因素的综合影响。这些结果与线性多并苯的观察结果形成鲜明对比。