
pmid: 20648493
AbstractNovel donor‐ and/or acceptor‐substituted cross‐conjugated carbocycles based on quinoids or expanded quinoids, with radiaannulene perimeters, were prepared and investigated to validate proaromaticity as a concept for reducing HOMO–LUMO gaps in push–pull chromophores. Analyses of IR, 1H NMR, and UV/Vis/NIR spectra in conjunction with molecular structures determined by X‐ray diffraction show that these push–pull quinoids have significant charge‐separated ground states. This feature results in small optical gaps (near IR region) and diatropic magnetic environments inside the carbocycles, as suggested by nucleus‐independent chemical shift (NICS) calculations. The NICS results, together with the bond‐length analysis of the quinoid spacers, provide strong support that proaromaticity, that is, aromatized zwitterionic mesomeric contributions in the ground state, is effective. A push–pull tetrakis(ethynediyl)‐expanded quinoid chromophore represents the first proaromatic radiaannulene.
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