Multi-frequency single-crystal and powder electron paramagnetic resonance spectroscopy of [Cu2(chp)4](chp = 6-chloro-2-pyridonate)

Abstract

Powder and single-crystal X- and Q-band EPR spectra show evidence for both intra- and inter-dimer exchange in [Cu₂(chp)₄](chp = 6-chloro-2-pyridonate). The antiferromagnetic intradimer exchange gives rise to a singlet ground state and a thermally accessible S= 1 excited state which is described by the spin-Hamiltonian parameters (from single-crystal measurements at 120 K)g_zz= 2.30, g_xx= 2.07, g_yy= 2.05, |D|= 0.275 cm^–1 and |λ|(=|E/D|)= 0.01. D_zz and g_zz are colinear with the Cu—Cu vector. Interdimer exchange between neighbouring triplet state molecules is apparent by both a highly temperature dependent linewidth and an ‘extra’ feature at g_eff≈ 2.1, the intensity of which rapidly falls with decrease in temperature. Analysis of the linewidth variation with temperature from single-crystal measurements at X-band leads to an interdimer exchange parameter of J′≈ 0.15 cm^–1. The extra feature arises from averaging of the S= 1 signals of individual dimers under the action of interdimer exchange and occurs over only a narrow range of orientations in the single crystal.