EPR study of the x,α and β structures of lithium phthalocyanine

Abstract

The magnetic properties of the lithium phthalocyanine radical are investigated for the x,α and β crystal structures via electron paramagnetic resonance (EPR). Different magnetic behaviours (susceptibility, line shape) are evidenced in PcLi powders and interpreted in terms of the molecular packing. A Curie–Weiss susceptibility is observed for the α-phase with a mean-field temperature, T_CW of ca. 4.5±1 K, whereas a two-component susceptibility must be considered for the x-phase (powders grown in acetonitrile) involving a Curie–Weiss contribution (T_CWca. –3±1 K) and a thermally activated contribution with an activation energy E_a of ca. 0.04 eV. For the PcLi β structure two temperature domains can be distinguished: for T>50 K a Curie–Weiss susceptibility with a mean-field temperatureT_CW of ca. –60±5 K is observed while for T<20 K a Curie–Weiss-like susceptibility withT_CWca. –7±1 K dominates. The overall magnetic properties of the PcLi polymorphs are discussed in terms of McConnell’s mechanism for ferromagnetism and antiferromagnetism in solid free radicals. The magnetic properties of β-PcLi needles (magnetic susceptibility, anisotropy of spin diffusion) have been investigated. Adsorbed oxygen is shown to alter strongly the line shape, the linewidth and the g-factor anisotropy in β-PcLi crystallites and powders at temperatures below 55 K.