Shi Baimei, Nachtigallová Dana, Aquino Adélia J A, Machado Francisco B C, Lischka Hans
School of Pharmaceutical Science and Technology, Tianjin University, Tianjin 300072, P. R. China.
Phys Chem Chem Phys. 2019 May 8;21(18):9077-9088. doi: 10.1039/c9cp00635d.
The study of electronically excited states of stacked polycyclic aromatic hydrocarbons (PAHs) is of great interest due to promising applications of these compounds as luminescent carbon nanomaterials such as graphene quantum dots (GQDs) and carbon dots (CDs). In this study, the excited states and excitonic interactions are described in detail based on four CD model dimer systems of pyrene, coronene, circum-1-pyrene and circum-1-coronene. Two multi-reference methods, DFT/MRCI and SC-NEVPT2, and two single-reference methods, ADC(2) and CAM-B3LYP, have been used for excited state calculations. The DFT/MRCI method has been used as a benchmark method to evaluate the performance of the other ones. All methods produce useful lists of excited states. However, an overestimation of excitation energies and an inverted ordering of states, especially concerning the bright HOMO-LUMO excitation, are observed. In the pyrene-based systems, the first bright state appears among the first four states, whereas the number of dark states is significantly larger for the coronene-based systems. Fluorescence emission properties are addressed by means of geometry optimization in the S1 state. The inter sheet distances for the S1 state decrease in comparison to the corresponding ground-state values. These reductions are largest for the pyrene dimer and decrease significantly for the larger dimers. Several minima have been determined on the S1 energy surface for most of the dimers. The largest variability in emission energies is found for the pyrene dimer, whereas in the other cases a more regular behavior of the emission spectra is observed.
由于堆叠多环芳烃（PAHs）作为发光碳纳米材料（如石墨烯量子点（GQDs）和碳点（CDs））具有很有前景的应用，对其电子激发态的研究备受关注。在本研究中，基于芘、蒄、环 - 1 - 芘和环 - 1 - 蒄的四个碳点模型二聚体系统，详细描述了激发态和激子相互作用。两种多参考方法，DFT/MRCI和SC - NEVPT2，以及两种单参考方法，ADC(2)和CAM - B3LYP，已用于激发态计算。DFT/MRCI方法已被用作基准方法来评估其他方法的性能。所有方法都生成了有用的激发态列表。然而，观察到激发能的高估和态的顺序反转，特别是关于明亮的HOMO - LUMO激发。在基于芘的系统中，第一个明亮态出现在前四个态之中，而基于蒄的系统中暗态的数量明显更多。通过在S1态进行几何优化来研究荧光发射特性。与相应的基态值相比，S1态的层间距离减小。芘二聚体的这些减小最大，对于较大的二聚体则显著减小。大多数二聚体在S1能量表面上已确定了几个极小值。芘二聚体的发射能量变化最大，而在其他情况下观察到发射光谱的行为更规则。
