Electrochemical and EPR studies of two substituted bis-cadmium tris-phthalocyanine complexes: elucidation of unexpectedly different free-radical character

The electronic and electron transfer behaviour of two examples of a recently discovered class of triple-decker sandwich complex based on three phthalocyanine ligands linked by two chelated cadmium ions has been investigated by EPR spectroscopy and cyclic voltammetry, square wave voltammetry and linear sweep voltammetry experiments. The two compounds, 1 and 2, differ in the location of the eight alkyl groups attached to each of the phthalocyanine rings; at the non-peripheral sites in 1 and the peripheral sites in 2. Quantitative comparison of the free radical character of 1 and 2 in solutions was undertaken by EPR spectroscopy and revealed that 1 exists as a mixture of s = 0 and s = ½ species, whereas compound 2 exists essentially as a spin ½ species alone. The electrochemical study of 1 and 2 was undertaken in both dichloromethane (CH₂Cl₂) and tetrahydrofuran (THF). The two compounds show comparable but subtly different redox behaviour which can only be attributed to the different locations of the substituents. Seventeen of the possible eighteen one-electron transfer processes could be identified for 1. The first oxidation wave for 2, both in THF and in CH₂Cl₂ solutions, was encountered at ca. 160 mV lower potential than for 1 implying that 2 is much easier to initially oxidise than 1. This finding provides a rationale for the EPR results described above. In separate experiments, oxidation of 1 and 2 as solutions and spin-coated film formulations was achieved using iodine and was characterised by significant changes in the visible region absorption spectra of the compounds.