EPR STUDY OF THE X-STRUCTURE, ALPHA-STRUCTURE AND BETA-STRUCTURE OF LITHIUM PHTHALOCYANINE

The magnetic properties of the lithium phthalocyanine radical are inve stigated for the x, alpha and beta crystal structures via electron par amagnetic resonance (EPR). Different magnetic behaviours (susceptibili ty, line shape) are evidenced in PcLi powders and interpreted in terms of the molecular packing. A Curie-Weiss susceptibility is observed fo r the alpha-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 contri bution (T-CW ca. -3 +/- 1 K) and a thermally activated contribution wi th an activation energy E-a of ca. 0.04 eV. For the PcLi beta structur e two temperature domains can be distinguished: for T > 50 K a Curie-W eiss susceptibility with a mean-held temperature T-CW of ca. -60 +/- - 5 K is observed while for T < 20 K a Curie-Weiss-like susceptibility w ith T-CW ca. -7 +/- 1 K dominates. The overall magnetic properties of the PcLi polymorphs are discussed in terms of McConnell's mechanism fo r ferromagnetism and antiferromagnetism in solid free radicals. The ma gnetic properties of beta-PcLi needles (magnetic susceptibility, aniso tropy of spin diffusion) have been investigated. Adsorbed oxygen is sh own to alter strongly the line shape, the linewidth and the g-factor a nisotropy in beta-PcLi crystallites and powders at temperatures below 55 K.