State Key Laboratory of Physical Chemistry of Solid Surfaces and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, P. R. China.
Chem Asian J. 2020 Nov 2;15(21):3444-3450. doi: 10.1002/asia.202000900. Epub 2020 Sep 21.
Based on Hückel's and Baird's rules, species are aromatic either in the lowest singlet state (S ) or the lowest triplet state (T ) only. Thus, species with adaptive aromaticity (with aromaticity in both the S and T states) is particularly rare. On the other hand, σ-aromaticity in the T state has been underdeveloped, let alone adaptive σ-aromaticity. Herein, via various aromaticity indices including NICS, ACID and EDDB, we demonstrate adaptive σ-aromaticity in an unsaturated three-membered ring, which is a traditional area dominated by π-aromaticity. The origin of adaptive σ-aromaticity could be attributed to the excitation mode of the T state formed from out-of-plane π molecular orbital to the π* orbitals. Thus the σ-aromaticity of the three-membered ring in the S state could hold in the T state. Our findings extend the concept of adaptive σ-aromaticity into three-membered rings and could be useful to further develop the concept of both σ-aromaticity and adaptive aromaticity.
基于休克尔（Hückel）和贝尔德（Baird）规则，物种仅在最低单重态（S）或最低三重态（T）中具有芳香性。因此，具有适应性芳香性（在 S 和 T 态中都具有芳香性）的物种特别罕见。另一方面，T 态中的σ-芳香性尚未得到充分发展，更不用说适应性σ-芳香性了。在此，通过包括 NICS、ACID 和 EDDB 在内的各种芳香性指数，我们证明了不饱和三员环中存在适应性σ-芳香性，这是一个传统上由π-芳香性主导的领域。适应性σ-芳香性的起源可以归因于 T 态的激发模式，即从平面外π分子轨道到π*轨道的激发。因此，S 态中环的σ-芳香性可以在 T 态中保持。我们的发现将适应性σ-芳香性的概念扩展到了三员环中，并可能有助于进一步发展σ-芳香性和适应性芳香性的概念。
