Electron spin resonance studies of Co(tbp)·C₆₀ single crystal

ESR measurements have been made on a newly synthesized cocrystallite of C₆₀ with the Co^II complex of 5, 10, 15, 20-tetrakis[3, 5-(di-tert-butyl)-phenyl]porphyrin (tbp)-single crystal to study the strong intermolecular interactions which are suggested by x-ray structural analyses. A single anisotropic ESR absorption line is observed at room temperature with the principal g-values g_⊥ = 2.423 and g_∥= 2.005, which are typical values for the low-spin Co^II ions (S = 1/2). The antiferromagnetic superexchange interaction between Co^II ions via C₆₀ is confirmed from the observation of the finite Weiss temperature, which is further supported by the absence of the hyperfine splitting due to ⁵⁹Co (I = 7/2) nuclei. As the temperature is lowered, the g_⊥-value decreases monotonically suggesting an increase of the crystal-field splitting energy. This result is consistent with the fact that the intermolecular distance between Co^II and C₆₀ becomes shorter as the temperature decreases. The peak-to-peak ESR linewidths ΔH_pp along both principal axes increase with temperature due to the shorter spin-lattice relaxation time.