Tuning the singlet-triplet energy gap in a non-Kekule series by designed structural variation. The singlet states of N-substituted-3,4-dimethylenepyrrole biradicals

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. Berson*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

35 Scopus citations

Abstract

Semiempirical 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.

Original languageEnglish
Pages (from-to)1406-1415
Number of pages10
JournalJournal of the American Chemical Society
Volume119
Issue number6
DOIs
StatePublished - 12 Feb 1997

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