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

AbstractThe 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 4n+2 Hückel aromaticity and open‐shell 4n Baird aromaticity.