Analysis of a Compound class with Triplet States Stabilized by Potentially Baird-Aromatic [10] Annulenyl Dicationic Rings

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The low-lying triplet state of a recently published compound (TMTQ, Angew. Chem. Int. Ed. 2015, 20, 5888), was analyzed quantum chemically in light of suggestions that it is influenced by Baird-aromaticity. Two mesomeric structures describe this state; a zwitterionic Baird-aromatic structure with a triplet biradical 8pi-electron methano[10]annulene (M10A) dicationic ring, and a Hückel-aromatic with a neutral closed-shell 10pi-electron ring. According to charge and spin density distributions, the Hückel-aromatic structure dominates the triplet state (the Baird-aromatic contributes at most 12%), and separation of the FLU aromaticity index into alpha and beta electron contributions emphasizes this finding. The small singlet-triplet energy gap is due to Hückel-aromaticity of the M10A ring, clarified by comparison to the smaller analogues of TMTQ. Yet, TMTQ and its analogues are Hückel-Baird hybrids allowing for tuning between closed-shell 4n+2 Hückel-aromaticity and open-shell 4n Baird-aromaticity ​
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