Pi Complexes between Organic Free Radicals

The spectroscopic and magnetic properties of N-ethylphenazyl and Wurster's blue perchlorate show characteristic changes at low temperatures. Using Mulliken's charge-transfer theory, we have shown that the formation of π complexes can account for these changes. The intense long wavelength absorption band which appears at low temperatures in both cases, is attributed to a transition to a state which is an antisymmetrical combination of two charge-transfer configurations, in which all the electrons are paired. We have compared the properties of these π complexes with the properties of the biradicals related to 4,4′-bis-diphenylmethylenediphenyl (chichibabin hydrocarbon.)