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Tuning the Singlet−Triplet Energy Gap in a Non-Kekulé Series by Designed Structural Variation. The Singlet States of N-Substituted-3,4-dimethylenepyrrole Biradicals
journal contribution
posted on 1997-02-12, 00:00 authored by Linda C. Bush, Richard B. Heath, Xu Wu Feng, Patricia A. Wang, Ljiljana Maksimovic, Anne In Song, Wen-Sheng Chung, Alain B. Berinstain, J. C. Scaiano, Jerome A. BersonSemiempirical quantum chemical calculations (AM1/CI and PM3/CI)
confirm the qualitative perturbational
prediction that electron-withdrawing groups on the ring nitrogen of a
3,4-dimethylenepyrrole should diminish the
energy separation of the singlet and triplet states to near zero.
Syntheses of a series of precursors of such
biradicals
have been developed. Study of the chemistry and spectroscopy of
the biradicals has revealed persistent singlet
states for the cases where the substituent is methyl, isobutyryl, and
pivaloyl. In the cases of
N-arenesulfonyl-3,4-dimethylenepyrroles, both a singlet and a triplet form can be observed
as persistent species. In this paper, the properties
of the singlets in this series are described. Although energy
transfer from the excited triplet state of the sensitizer
xanthone to the diazene precursor of
N-p-toluenesulfonyl-3,4-dimethylenepyrrole is
observed by nanosecond time-resolved spectroscopy, the chemical behavior of the biradical
intermediate is the same as that observed in the direct
photolysis or thermolysis of the diazene. The reactive form of the
biradical under these conditions appears to be the
singlet.