Dominating Antiaromatic Character of as-Indacene Decides Overall Properties of a Formally Aromatic Dicyclopenta[c]fluorenothiophene

Dicyclopenta[c]fluorenothiophene 5 was synthesized as the isoelectronic polycyclic heteroarene analogue of an as-indacenodifluorene with a (4n + 2)π-electron perimeter. Single-crystal and 1H NMR analyses indicated a quinoidal ground state for 5, which was supported by theoretical calculations while suggesting a degree of antiaromaticity of the as-indacene subunit greater than that for s-indacenodifluorene 3. The dominant antiaromaticity for 5 was evidenced by the broad weakly intense absorption tail reaching the near-IR region, four-stage redox amphotericity, and small HOMO-LUMO energy gap.