ESR studies of the triplet state of [n.n] paracyclophanes

Abstract

ESR experiments were performed on the triplet state of randomly oriented paracyclophanes in a variety of rigid glasses at temperatures ranging from 103 to 15°K. Spectra were recorded for [2.2]; [3.3]; (4,7,12,15)-tetramethyl [2.2]-paracyclophane and stagger-ring paracyclophane. For all the samples except stagger ring, a four-ringed paracyclophane, only the Hmin feature was observed from which D*, the root-mean-square zero-field splitting, was calculated. For stagger ring the triplet spectrum has two features in the Δ ms= 1 region in addition to the Hmin feature. From these, the zero-field splittings, D and E, were calculated. The triplet spectra for the paracyclophanes show that there is strong transannular interaction with electron delocalization over all benzene rings. There is evidence for strongly coupled intramolecular exciton effects. The effect of increasing the inter-ring separation from [2.2] paracyclophane to [3.3] paracyclophane is to decrease the transannular interaction. The effect of methyl substitution is to increase transannular effects relative to the parent compound. Transannular interactions in stagger ring are greater than in [2.2] paracyclophane despite the increased electron delocalization possible through the introduction of more than two rings. The large value of E for stagger ring represents a significant deviation from axial symmetry for the zero-field-splitting tensor and indicates that the methylene bridges, the methyl substituents, or the ring distortion, may make important contributions to the electronic distribution of the triplet state.