Electrochromism in the transition-metal phthalocyanines. Part 3.—Molecular organisation, reorganisation and assembly under the influence of an applied electric field. Response of [Fe(pc)] and [Fe(pc)Cl]

Abstract

The electrochromic behaviour of α-[Fe(pc)] and [Fe(pc)Cl] has been studied, using cyclic voltammetry, electronic absorption spectroscopy and Mössbauer spectroscopy, the latter on ⁵⁷Fe-enriched [Fe(pc)] films. [Fe(pc)] films have been shown to cycle reversibly ca. 10³ times between neutral (blue) and reduced (purple-red) states (negative voltages), but decompose when oxidised repeatedly (positive voltages) in aqueous KCI electrolytes. When oxidised then reduced in < ca. 1 s [Fe(pc)Cl] is formed. This indicates that the Cl^– ion adds as a ligand during the electrochromic oxidation and this takes place concomitantly with a lattice reorganisation. [Fe(pc)] films on oxidation produce red [Fe^III(pc˙)Cl₂] and contain a (pc˙) phthalocyanine radical anion. On reduction to neutral they become [Fe(pc)Cl]. [Fe(PC)Cl] films have been shown to reduce electrochromically from an initial green film to a purple reduced film without first becoming [Fe(pc)]. However, on reoxidation to neutral, a blue [Fe(pc)] film is formed; this then behaves like a normal [Fe(pc)] film. These studies show that the axial Cl^–on [Fe(pc)Cl], once removed, passes out of the lattice during the electrochromic reduction, and that under the influence of the field the lattice reorganises, assembling predominantly as the α-[Fe(pc)] phase. This is the first attempt to explain widespread lattice reorganisation that involves metallophthalocyanines with axial ligands attaching and detaching under controlled voltage.