Anomalous Magnetic Behavior in Three Kinds of 3-(Aryl-substituted)-1,5-diphenylverdazyl Radical Crystals (<i>p</i>-FPDV,<i>p</i>-PyDV and<i>m</i>-PyDV) Induced by Frustrated Spin Interaction

Abstract

Magnetic susceptibilities (χ M ) of three kinds of 3-(aryl-substituted)-1,5-diphenyl-verdazyl radical crystals (aryl: 4-fluorophenyl ( p -FPDV), 4-pyridyl ( p -PyDV), and 3-pyridyl ( m -PyDV)) have been measured between 1.8 and 300 K, showing a broad maximum T max at 200, 272, and 157 K, respectively. The values of T max observed are 1 or 2 orders of magnitude larger than those for usual organic neutral radical crystals, suggesting strong intermolecular antiferromagnetic (AFM) exchange interactions ( J ) in the crystals. The temperature dependence of the susceptibilities cannot be explained by a simple model, such as the one-dimensional (1D) Heisenberg AFM linear- and alternating-chain model. The values of χ M observed for the p -FPDV, p -PyDV, and m -PyDV radicals are about 42, 76, and 35% larger than the corresponding values calculated by a 1D Heisenberg AFM linear-chain model at each temperature. The result of the crystal structure analysis of p -FPDV indicates the existence of butterfly-type spin frustration in the system. The origin of the anomalous magnetic behavior observed for these verdazyl radical crystals has been discussed based on the results of the crystal structure analysis and unpaired spin density calculation.