EPR Studies of Spin Correlation in Some Ion Radical Salts

TLDR

EPR studies were performed on ion‑radical salts derived from the π‑acceptor TCNQ. The study emphasizes the role of paramagnetic impurities in the magnetic behavior of organic solids. The experiments revealed spin‑correlated singlet–triplet ground states in TCNQ‑based salts, with measured singlet–triplet separations ranging from 0.034 to 0.41 eV, anisotropic zero‑field splitting and half‑field transitions in certain salts, and temperature‑dependent exchange interactions that broaden and collapse the zero‑field components, all described by a detailed spin Hamiltonian.

Abstract

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 H=βH·g·S+DSz2+E(Sx2−Sy2),with | D/hc |=0.0062 cm−1, | E/hc |=0.00098 cm−1, and gx, gy, and gz 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.