Recent developments in light-emitting carbon nanodots and molecular organic semiconductors have seen renewed interest in the properties of polycyclic aromatic hydrocarbons (PAHs) as a family. The networks of delocalized π electrons in sp²-hybridized carbon grant PAHs light-emissive properties right across the visible spectrum. However, the mechanistic understanding of their emission energy has been limited due to the ground state-focused methods of determination. This computational chemistry work, therefore, seeks to validate existing rules and elucidate new features and characteristics of PAHs that influence their emissions. Predictions based on (time-dependent) density functional theory account for the full 3-dimensional electronic structure of ground and excited states and reveal that twisting and near-degeneracies strongly influence emission spectra and may therefore be used to tune the color of PAHs and, hence, carbon nanodots. We particularly note that the influence of twisting goes beyond torsional destabilization of the ground-state and geometric relaxation of the excited state, with a third contribution associated with the electric transition dipole. Symmetries and peri-condensation may also have an effect, but this could not be statistically confirmed. In pursuing this goal, we demonstrate that with minimal changes to molecular size, the entire visible spectrum may be spanned by geometric modification alone; we have also provided a first estimate of emission energy for 35 molecules currently lacking published emission spectra as well as clear guidelines for when more sophisticated computational techniques are required to predict the properties of PAHs accurately.

中文翻译：

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尺寸并不代表一切：多环芳烃的几何调节及其对碳纳米点的影响

发光碳纳米点和分子有机半导体的最新发展引起了人们对多环芳烃（PAH）家族特性的新兴趣。 sp ²杂化碳中的离域 π 电子网络赋予 PAH 跨越可见光谱的发光特性。然而，由于以基态为重点的测定方法，对其发射能量的机械理解受到限制。因此，这项计算化学工作旨在验证现有规则并阐明影响其排放的多环芳烃的新特征和特性。基于（时间相关）密度泛函理论的预测解释了基态和激发态的完整 3 维电子结构，并揭示了扭曲和近简并强烈影响发射光谱，因此可用于调整 PAH 的颜色，因此，碳纳米点。我们特别注意到，扭曲的影响超出了基态的扭转不稳定和激发态的几何弛豫，第三个贡献与电跃迁偶极子相关。对称性和围凝结也可能有影响，但这无法得到统计证实。在实现这一目标的过程中，我们证明，只需对分子大小进行最小的改变，仅通过几何修饰就可以跨越整个可见光谱；我们还提供了目前缺乏公布的发射光谱的 35 种分子的发射能量的初步估计，以及何时需要更复杂的计算技术来准确预测 PAH 的特性的明确指南。