Magnetic Excitations in Charge-Transfer Complexes. I. p-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 $\mathrm{(S\hspace{0.17em}=\hspace{0.17em}}\frac{1}{2})$ are described by a single, almost axially symmetric $g$ factor, with $\text{g‖\hspace{0.17em}=\hspace{0.17em}2.0024\hspace{0.17em}±\hspace{0.17em}0.0002}$ , $\text{g⊥\hspace{0.17em}=\hspace{0.17em}2.0054\hspace{0.17em}±\hspace{0.17em}0.0002}$ , and ${\mathrm{|\hspace{0.17em}g}}_x{\mathrm{\hspace{0.17em}-\hspace{0.17em}g}}_y\text{\hspace{0.17em}|\hspace{0.17em}∼\hspace{0.17em}0.0001}$ . 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.