Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014 Helsinki, Finland.
Phys Chem Chem Phys. 2018 Jul 4;20(26):17705-17713. doi: 10.1039/c8cp03112f.
Aromatic properties of two recently synthesized dithienothiophene-bridged (DTT) [34]octaphyrins have been investigated by calculating magnetically induced current densities and vertical excitation energies. These intriguing molecules have been proposed to be the first synthesized neutral bicycloaromatic compounds. The triplet state of their dications was even suggested to be Baird-type bicycloaromatic rendering them very interesting as a new prototype of molecules possessing simultaneously the two rare types of aromaticity. Here, we investigate computationally the aromatic properties of the neutral as well as the singly and doubly charged DTT-bridged [34]octaphyrins. Our study provides unambiguous information about changes in the aromatic properties of the DTT-bridged [34]octaphyrins upon oxidation. The calculations identify two independent diatropic ring currents in the neutral DTT-bridged [34]octaphyrins, showing that they are indeed bicycloaromatic. The current-density flow of the two independent ring currents of the bicycloaromatic compounds are visualized and individual aromatic pathways are quantified by performing numerical integration. The calculations show that two independent diatropic ring currents can indeed be sustained by molecules consisting of two aromatic rings that share a common set of π electrons. The current density calculations on the singly charged DTT-bridged [34]octaphyrins show that they are weakly antiaromatic, which does not agree with the suggested aromatic character deduced from spectroscopical studies. The triplet state of the two DTT-bridged [34]octaphyrin cations with very similar molecular structures have unexpectedly different aromatic character. One of them is Baird-type bicycloaromatic, whereas the triplet state of the other dication has one aromatic and one nonaromatic ring, which could not be resolved from available spectroscopical data. Calculations of excitation energies reveal that a simple model cannot be employed for interpreting the electronic excitation spectra of the present molecules, because more than 20 excited states contribute to the spectra above 2.5 eV (500 nm) showing the importance of computations. The present work illustrates how detailed information about molecular aromaticity can nowadays be obtained by scrutinizing calculated current densities.
两种最近合成的二噻吩并噻吩桥联（DTT）[34]卟啉的芳香性质已经通过计算磁诱导电流密度和垂直激发能进行了研究。这些引人入胜的分子被提议为第一个合成的中性双环芳烃化合物。它们的二阳离子的三重态甚至被提议为贝尔德型双环芳烃，这使得它们作为同时具有两种罕见芳香性类型的新型分子原型非常有趣。在这里，我们通过计算研究了中性以及单和双电荷 DTT 桥联[34]卟啉的芳香性质。我们的研究提供了关于 DTT 桥联[34]卟啉氧化时芳香性质变化的明确信息。计算确定了中性 DTT 桥联[34]卟啉中两个独立的反磁环电流，表明它们确实是双环芳烃。通过执行数值积分，可视化了双环芳烃化合物的两个独立环电流的电流密度流，并量化了各个芳香途径。计算表明，由共享一组π电子的两个芳香环组成的分子确实可以维持两个独立的反磁环电流。DTT 桥联[34]卟啉单电荷的电流密度计算表明，它们是弱反芳香性的，这与从光谱研究中推断出的芳香特征不一致。具有非常相似分子结构的两个 DTT 桥联[34]卟啉阳离子的三重态具有出乎意料的不同的芳香特征。其中一个是贝尔德型双环芳烃，而另一个二阳离子的三重态有一个芳香环和一个非芳香环，这无法从现有的光谱数据中分辨出来。激发能的计算表明，简单的模型不能用于解释目前分子的电子激发光谱，因为超过 20 个激发态对 2.5 eV（500nm）以上的光谱有贡献，这表明计算的重要性。本工作说明了如何通过仔细研究计算电流密度来获得有关分子芳香性的详细信息。
