Escayola Sílvia, Tonnelé Claire, Matito Eduard, Poater Albert, Ottosson Henrik, Solà Miquel, Casanova David
Institute of Computational Chemistry and Catalysis and Department of Chemistry, University of Girona, C/ M. Aurèlia Capmany, 69, 17003, Girona, Catalonia, Spain.
Donostia International Physics Center (DIPC), Donostia, Euskadi, Spain.
Angew Chem Int Ed Engl. 2021 Apr 26;60(18):10255-10265. doi: 10.1002/anie.202100261. Epub 2021 Mar 24.
Pro-aromatic molecules have higher-energy diradicaloid states that are significantly influenced by resonance structures in which conjugated rings take on Hückel-aromatic character. Recently, it has been argued that there are also pro-aromatic molecules that adopt central units with 4nπ-electron Baird-aromatic character in the T state, although detailed analysis suggests that these compounds are better labelled as T Hückel-Baird hybrid molecules where Hückel-aromaticity dominates. Herein, we consider a series of symmetrically substituted conjugated rings with potential Baird aromaticity in the lowest excited triplet and singlet states. Our computational results allow us to establish general guidelines for the rational design of molecules with excited state Hückel/Baird aromaticity in pro-aromatic quinoidal compounds. We found two main strategies to promote high Baird aromatic character: 1) anionic and small conjugated rings with electron donating groups as substituents and small exocyclic groups with electron withdrawing substituents, or 2) electron deficient conjugated rings with exocyclic electron-donor substitution.
前芳香性分子具有能量较高的双自由基态，这种状态受共振结构的显著影响，在共振结构中，共轭环具有休克尔芳香性特征。最近，有人认为也存在一些前芳香性分子，它们在T态采用具有4nπ电子的拜尔德芳香性中心单元，不过详细分析表明，这些化合物更适合标记为T休克尔-拜尔德杂化分子，其中休克尔芳香性占主导。在此，我们考虑了一系列在最低激发三重态和单重态具有潜在拜尔德芳香性的对称取代共轭环。我们的计算结果使我们能够为合理设计前芳香性醌型化合物中具有激发态休克尔/拜尔德芳香性的分子建立通用指导原则。我们发现了两种促进高拜尔德芳香性特征的主要策略：1）带有供电子基团作为取代基的阴离子型小共轭环以及带有吸电子取代基的小外环基团，或者2）带有外环供电子取代基的缺电子共轭环。
