Triplet species from dihydropyrrolo[3,4-d]pyridazines, the diazene precursors of N-arenesulfonyl-3,4-dimethylenepyrroles

In contrast to the photolyses of N-methyl- or N-pivaloylpyrrolo[3,4]dihydropyridazines in low-temperature matrices, which give the blue, ESR-inactive singlet 3,4-dimethylenepyrrole biradical at all wavelengths, the photolyses of the N-arenesulfonyl derivatives in this series are strongly wavelength-dependent. In three separate instances, both singlet and triplet N-arenesulfonyl-3,4-dimethylenepyrrole biradicals have been observed. In each case, one or two other triplets are seen by ESR spectroscopy after irradiation under special conditions, These are tentatively assigned conformationally isomeric diazenyl biradical structures. The possible origins of the slow intersystem crossing rates for singlet-triplet interconversions in N-arenesulfonyl-3,4-dimethylenepyrrole biradicals are examined. Near-zero ionic character of the singlet wave function is considered to be a less likely cause than conformational control of the spin state. The latter circumstance would require that intersystem crossing be coupled to conformational change, which can be very slow at low temperatures. The zero-field splittings determined from the ESR spectra of N-tosyl-3,4-dimethylenepyrrole biradical are found to be temperature-dependent, which could be caused by librations and/or internal rotational (conformational) motions of the biradical in the matrix.