title: An Unusually Small Singlet-Triplet Gap in a Quinoidal 1,6-Methano[10]annulene Resulting from Baird's 4n π-Electron Triplet Stabilization.
abstract: Within the continuum of π-extended quinoidal electronic structures exist molecules that by design can support open-shell diradical structures. The prevailing molecular design criteria for such structures involve proaromatic nature that evolves aromaticity in open-shell diradical resonance structures. A new diradical species built upon a quinoidal methano[10]annulene unit is synthesized and spectroscopically evaluated. The requisite intersystem crossing in the open-shell structure is accompanied by structural reorganization from a contorted Möbius aromatic-like shape in S0 to a more planar shape in the Hückel aromatic-like T1. This stability was attributed to Baird's Rule which dictates the aromaticity of 4n π-electron triplet excited states.
authors.affiliation: Department of Chemistry, Johns Hopkins University, Baltimore MD, 21218 (USA).
authors.forename: Benjamin C
authors.initials: BC
authors.lastname: Streifel
authors.forename: José L
authors.initials: JL
authors.lastname: Zafra
authors.forename: Guzmán L
authors.initials: GL
authors.lastname: Espejo
authors.forename: Carlos J
authors.initials: CJ
authors.lastname: Gómez-García
authors.forename: Juan
authors.initials: J
authors.lastname: Casado
authors.forename: John D
authors.initials: JD
authors.lastname: Tovar
keywords: Baird’s rule; annulenes; aromaticity; diradical species; quinoidal structures