ESR studies on a new phenyl t-butyl nitroxide biradical based on calix[4]arene

A new paramagnetic calix[4]arene (III)bearing two nitroxide radicals in one molecule was synthesized. At room temperature III in toluene gives a typical spectrum of a nitroxide biradical with g = 2.0067 and a nitrogen coupling constant of 6.6G, which is ascribed to a strong spin-spin exchange interaction. At 77K a characteristic triplet ESR fine structure was observed. DeltaM(S) = +/-2 half-field transitions were clearly detected at 1674G as multiple lines due to the electron-nitrogen coupling. Zero-field splitting parameters are estimated: D/hc = 0.0175cm(-1), E/hc = 0.0008cm(-1). These results are discussed in terms of conformational effects.