title: Proaromaticity: organic charge-transfer chromophores with small HOMO-LUMO gaps.
abstract: Novel 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, (1)H 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.
authors.affiliation: Laboratorium für Organische Chemie, ETH-Zürich, Hönggerberg, HCI, 8093 Zürich, Switzerland.
authors.forename: Yi-Lin
authors.initials: YL
authors.lastname: Wu
authors.forename: Filip
authors.initials: F
authors.lastname: Bures
authors.forename: Peter D
authors.initials: PD
authors.lastname: Jarowski
authors.forename: W Bernd
authors.initials: WB
authors.lastname: Schweizer
authors.forename: Corinne
authors.initials: C
authors.lastname: Boudon
authors.forename: Jean-Paul
authors.initials: JP
authors.lastname: Gisselbrecht
authors.forename: François
authors.initials: F
authors.lastname: Diederich