EPR Studies of Spin Correlation in Some Ion Radical Salts

Electron spin resonance studies have been carried out on a number of ion‐radical salts based on the strong π‐acceptor tetracyanoquinodimethane (TCNQ). These studies have established that spin correlation exists between the magnetic electrons in these salts, giving rise to a ground singlet state and a thermally accessible triplet state. Typical singlet‐triplet separations (J) derived from observed temperature dependences of EPR signal intensities are 0.034 ev for the [(C₂H₅)₃NH]⁺(TCNQ)₂⁻ salt, 0.062 ev for the (_φ3PCH₃)⁺(TCNQ)₂⁻ and (_φ3AsCH₃)⁺(TCNQ)₂⁻ salts, and 0.41 ev for the (morpholinium)⁺(TCNQ)⁻ salt. The triplet states of the (_φ3PCH₃)⁺(TCNQ)₂⁻ and (_φ3AsCH)⁺(TCNQ)₂⁻ salts are further manifested by an anisotropic splitting of the resonance into a doublet (zero‐field splitting) and the appearance of half‐field Δm=±2 transitions. Detailed analysis reveals that both salts may be represented by the spin Hamiltonian $$\mathcal{H}{\mathrm{=\beta H·g·S+DS}}_z^2{\mathrm{+E(S}}_x^2{\mathrm{-S}}_y^2\mathrm{),}$$ with | D/hc |=0.0062 cm⁻¹, | E/hc |=0.00098 cm⁻¹, and g_x, g_y, and g_z equal to 2.004₀, 2.003₁, and 2.002₇, respectively. Temperature‐dependent exchange interactions observed for the (_φ3PCH₃)⁺(TCNQ)₂⁻ and (_φ3AsCH₃)⁺(TCNQ)₂⁻ salts cause the doublet zero‐field components to broaden, move together, and collapse into a single, progressively sharper line as the temperature is increased. The exchange frequency follows an exponential temperature dependence and is governed by the concentration of triplet entities in the solid (J=0.062 ev). The importance of paramagnetic impurities in studies of the magnetic properties of organic solids is emphasized. Examples are given which illustrate how a percent or so of doublet‐state impurity species can dominate resonances near g=2.002 in molecular solids obeying singlet‐triplet statistics.