Triplet Species from Dihydropyrrolo[3,4-d]pyridazines, the Diazene Precursors of N-Arenesulfonyl-3,4-dimethylenepyrroles

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• 作者：Linda C. Bush ; Ljiljana Maksimovic ; Xu Wu Feng ; Helen S. M. Lu ; 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：1416 - 1427
• 全文大小：381K
• 年卷期：v.119,no.6(February 12, 1997)
• ISSN：1520-5126

文摘

In contrast to the photolyses of N-methyl-or N-pivaloylpyrrolo[3,4]dihydropyridazines inlow-temperaturematrices, which give the blue, ESR-inactive singlet3,4-dimethylenepyrrole biradical at all wavelengths, thephotolysesof the N-arenesulfonyl derivatives in this series arestrongly wavelength-dependent. In three separateinstances,both singlet and tripletN-arenesulfonyl-3,4-dimethylenepyrrole biradicals have beenobserved. In each case, one ortwo other triplets are seen by ESR spectroscopy after irradiation underspecial conditions. These are tentativelyassigned conformationally isomeric diazenyl biradical structures.The possible origins of the slow intersystem crossingrates for singlet-triplet interconversions inN-arenesulfonyl-3,4-dimethylenepyrrole biradicals areexamined. Near-zero ionic character of the singlet wave function is considered to be aless likely cause than conformational controlof the spin state. The latter circumstance would require thatintersystem crossing be coupled to conformationalchange, which can be very slow at low temperatures. The zero-fieldsplittings determined from the ESR spectra ofN-tosyl-3,4-dimethylenepyrrole biradical are found to betemperature-dependent, which could be caused by librationsand/or internal rotational (conformational) motions of the biradical inthe matrix.