Stable Free Radicals. IV. Intramolecular Association in Pyridinyl Diradicals

A series of pyridinyl diradicals, Py-(CH²)ⁿPy- (in which Py- = 4-CH³OOCC⁵H⁴N- and n = 3, 4, or 5), has been prepared by reduction of the corresponding diiodides. Examination of epr spectra over a range of concentrations and temperatures reveals that spin concentration (measured by comparison with standard) divided by the titrimetric radical concentration varies from 2% (n = 3) through 20% (n = 4) to 100% (n = 5). Low relative spin concentrations are accompanied by strong visible absorption bands. Relatively minor changes in the ultraviolet absorption spectra [judged by comparison to PyR- (R = alkyl)] are found, indicating that the chemical nature of the pyridinyl radical moiety has not been changed. These observations are best explained by intramolecular association of the groups to yield singlet species analogous to the singlet pyridinyl radical dimers found previously.