Theory of Temperature-Dependent g-Tensor Splittings in p-Phenylenediamine–Chloranil

Abstract

A theory is developed to interpret the unusual temperature dependence of the g‐tensor splittings—the electron paramagnetic resonance lines narrow slightly as they merge into a single line—observed in the paramagnetic organic crystal p‐phenylenediamine–chloranil. Dipolar interactions are shown to delocalize the magnetic excitations over magnetically inequivalent chains of radicals. The $g$ tensor for a delocalized excitation is calculated in terms of the $g$ tensors of the radicals over which it is delocalized. The splittings decrease, and finally vanish, as the interchain dipolar field becomes sufficiently strong to delocalize the excitation entirely. The dipolar field strength depends on temperature through the strong magnetic dilution arising from large, antiferromagnetic, intrachain exchange. The theory, which accounts for the entire temperature range of the spectrum, is contrasted with the qualitatively different line merging predicted by random modulation among fixed frequencies (i.e., localized excitations), which necessarily leads to an initial line broadening greater than the line shift.