Exciton Magnetic Resonance in Wurster's Blue Perchlorate

The paramagnetic resonance of a single crystal of Wurster's blue perchlorate (a solid ionic free radical) in the 20°—77°K temperature range provides evidence that the elementary electronic excitations are triplet excitons with excitation energy J=246 cm⁻¹. Exciton motion is indicated by the absence of nuclear hyper‐fine structure at low exciton concentrations where exciton—exciton spin exchange interaction is negligible. Exciton motion is thought to be primarily along linear free‐radical chains; the rate of chain jumping is estimated to be greater than 50 Mc/sec but probably less than 10³−10⁴ Mc/sec. At certain crystal orientations in a strong magnetic field the rate of exciton chain jumping is large compared to the difference in the fine‐structure splitting of excitons on the two linear chains in the low‐temperature monoclinic crystal; in this case the (three‐dimensional) exciton spin Hamiltonian, $\mathcal{H}{\mathrm{=E(S}}_x^2{\mathrm{-S}}_y^2)$ , with | E | = 212 Mc/sec, accounts for the observed splittings to within 2%. At other crystal orientations the fine‐structure lines are lifetime broadened (due to chain jumping) so much that they are not observable. The well‐known sudden decrease in magnetic susceptibility of Wurster's blue perchlorate at 186°K is apparently due to an orthorhombic→monoclinic phase transition in which the Wurster's blue cations pair up to form dimers with ground singlet states and excited triplet states. The observed value of E can, however, be estimated theoretically with good accuracy in terms of the known intermolecular distances for the room‐temperature orthorhombic lattice and a molecular orbital approximation for the spin distribution in the positive ions. This suggests that the dimerization involves only small molecular displacements in the crystal lattice.