Magnetic Excitations in Charge-Transfer Complexes. I. <i>p</i>-Phenylenediamine–Chloranil

Abstract

The electron magnetic resonance of single crystals of p-phenylenediamine–chloranil (PDC) at 160 Mc/sec, 9.5, and 35 Gc/sec is reported. The thermally accessible (activation energy 0.13 ± 0.01 eV) magnetic excitations for the linear chains of exchange-coupled, alternately PD cation and chloranil anion radicals (S = 12) are described by a single, almost axially symmetric g factor, with g‖ = 2.0024 ± 0.0002, g⊥ = 2.0054 ± 0.0002, and | gx − gy | ∼ 0.0001. Temperature-dependent g-factor splittings are observed below 315°K, while a single, strongly exchange-narrowed line (width ∼ 250 mG) is observed above 315° for any orientation of the crystal. The angular dependence of the splittings below 270°K corresponds to three magnetically inequivalent, independent free-radical chains related to each other by a threefold axis parallel to the chain axis, with a 6° angle between the chain axis and the normal to the molecular planes of the radicals. The collapse of the splittings between 270° and 315° is explained qualitatively in terms of electron–electron dipolar interactions between magnetic excitations on inequivalent chains. Evidence is presented that the magnetic excitations in organic charge-transfer crystals based, like PDC, on planar, strong electron donors and acceptors are Wannier spin excitons above a diamagnetic, completely ionic ground state. Delocalized magnetic excitations are indicated by the single, averaged g factor for each chain, by the absence of nuclear hyperfine splittings, and by the strongly exchange-narrowed, Lorentzian lines observed. Evidence that the pairs of spins forming a Wannier spin exciton are independent is obtained from the temperature and pressure dependence of the resonance linewidth and from the absence of fine-structure splittings. The observed near independence of the magnetically inequivalent, exchange-coupled free-radical chains vindicates the usual description of spin excitations in organic free radicals by one-dimensional models.