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 (4 n + 2)π-electron perimeter. Single-crystal and 1 H 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.