Etude par R.P.E. des excitons triplets dans le dimethyl-1,5 nortropinone nitroxyde

Dimerization of 1,5 dimethyl-nortropinone nitroxide in the solid state is investigated by E.S.R. spectroscopy. In this compound, the arrangement of the two neighbouring NO fragments is not rectangular. The E.S.R. spectra of the single crystal are characteristic of a thermally accessible triplet state. The absence of resolved hyperfine structure, the observation of only one pair of E.S.R. lines despite the existence of the two translationally inequivalent dimers A and B and the anisotropy of linewidth show that the excitations move rapidly from one dimer to the other. This motion occurs at a frequency sufficient to average the dipolar tensors D A and D B of the two dimers A and B. Thus, the spectra are well described by the spin hamiltonian in which D is the average dipolar tensor. The zero-field splittings parameters are: The hopping rate between translationally inequivalent dimers is obtained from the study of the anisotropic linewidth; this rate is not activated and is about 3 × 10 8 Hz.