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

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• 作者：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 ; and Jerome A. Berson
• 刊名：Journal of the American Chemical Society
• 出版年：1997
• 出版时间：February 12, 1997
• 年：1997
• 卷：119
• 期：6
• 页码：1406 - 1415
• 全文大小：323K
• 年卷期：v.119,no.6(February 12, 1997)
• ISSN：1520-5126

文摘

Semiempirical quantum chemical calculations (AM1/CI and PM3/CI)confirm the qualitative perturbationalprediction that electron-withdrawing groups on the ring nitrogen of a3,4-dimethylenepyrrole should diminish theenergy separation of the singlet and triplet states to near zero.Syntheses of a series of precursors of suchbiradicalshave been developed. Study of the chemistry and spectroscopy ofthe biradicals has revealed persistent singletstates for the cases where the substituent is methyl, isobutyryl, andpivaloyl. In the cases ofN-arenesulfonyl-3,4-dimethylenepyrroles, both a singlet and a triplet form can be observedas persistent species. In this paper, the propertiesof the singlets in this series are described. Although energytransfer from the excited triplet state of the sensitizerxanthone to the diazene precursor ofN-p-toluenesulfonyl-3,4-dimethylenepyrrole isobserved by nanosecond time-resolved spectroscopy, the chemical behavior of the biradicalintermediate is the same as that observed in the directphotolysis or thermolysis of the diazene. The reactive form of thebiradical under these conditions appears to be thesinglet.