EPR Studies of Spin Correlation in Some Ion Radical Salts

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.