An Unusually Small Singlet-Triplet Gap in a Quinoidal 1,6-Methano[10]annulene Resulting from Baird's 4n [pi]-Electron Triplet Stabilization

• Published in: Angewandte Chemie International Edition Vol. 54; no. 20; p. 5888
• Main Authors: Streifel, Benjamin C, Zafra, José L, Espejo, Guzmán L, Gomez-Garcia, Carlos J, Casado, Juan, Tovar, John D
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 11.05.2015
• Edition: International ed. in English
• Subjects: Shells
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201500879

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 Huckel aromatic-like T1. This stability was attributed to Baird's Rule which dictates the aromaticity of 4n π-electron triplet excited states.