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, H=E(Sx2−Sy2), 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.