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An Unusually Small Singlet–Triplet Gap in a Quinoidal 1,6‐Methano[10]annulene Resulting from Baird’s 4 n π‐Electron Triplet Stabilization
Author(s) -
Streifel Benjamin C.,
Zafra José L.,
Espejo Guzmán L.,
GómezGarcía Carlos J.,
Casado Juan,
Tovar John D.
Publication year - 2015
Publication title -
angewandte chemie international edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.831
H-Index - 550
eISSN - 1521-3773
pISSN - 1433-7851
DOI - 10.1002/anie.201500879
Subject(s) - diradical , annulene , aromaticity , intersystem crossing , open shell , singlet state , antiaromaticity , chemistry , singlet fission , molecule , chemical physics , triplet state , photochemistry , excited state , computational chemistry , molecular physics , atomic physics , physics , organic chemistry
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 S 0 to a more planar shape in the Hückel aromatic‐like T 1 . This stability was attributed to Baird’s Rule which dictates the aromaticity of 4 n π‐electron triplet excited states.