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

The low‐lying triplet state of a recently published compound ( TMTQ ) was analyzed quantum chemically in light of suggestions that it is influenced by Baird aromaticity. Two mesomeric structures describe this state: 1) a zwitterionic Baird aromatic structure with a triplet diradical 8π‐electron methano[10]annulene (M10A) dicationic ring and 2) a Hückel aromatic with a neutral closed‐shell 10π‐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 aromatic fluctuation index (FLU) into α and β 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 4 n +2 Hückel aromaticity and open‐shell 4 n Baird aromaticity.