Synthesis, Structures, and Properties of Cyclopenta‐Fused Anthra[1,2‐ a ]anthracene Derivatives as New Polycyclic Aromatic Compounds

Three new cyclopenta-fused (CP) polycyclic aromatic compounds were synthesized from anthra[1,2-a]anthracenes by direct C─C bond formation in the internal region by Scholl reaction or by benzo fusion at the external corner through Pd-catalyzed cyclization. X-ray analysis and DFT calculations revealed that the aromatic frameworks are slightly twisted or curved around the internal region. Internal fusion results in significant red-shifts in the UV-vis and fluorescence spectra and ready electrochemical reduction, due to the lowered LUMO level relative to that of the parent compound. NICS and ACID analyses were used to evaluate the aromaticity of these compounds, indicating that the five-membered rings are nearly nonaromatic in all compounds.