Triple-decker cadmium phthalocyanine sandwich complexes: self-assembled EPR active complexes

Abstract

The discovery and characterization of the first example of a bis-cadmiumtris-phthalocyanine triple-decker sandwich complex is reviewed. The compound, with each Pc ring substituted at the eight non-peripheral positions with hexyl chains, was obtained unexpectedly during recrystallization of the corresponding monomeric 1,4,8,11,15,18,22,25-octa-hexyl cadmium phthalocyanine from CH 2 Cl 2 and methanol. The scope for obtaining further examples of this new class of complex bearing different ring substituents is also described. A feature of the compounds is that they give an EPR signal. An electrochemical study has shown that the rest state of the core of the sandwich structure is a dianion arising from the imbalance of the charges on the two cadmium atoms and the three Pc 2- ligands. This is a spin 0 species. It is proposed that the free radical character arises because the potential for the first one-electron oxidation is unusually low for a phthalocyanine, allowing for the partial presence of the spin ½ species for non-peripherally substituted complexes and a more extensive presence of the EPR active redox state for peripherally substituted analogs. Results of chemical oxidation using iodine are also discussed. A spectroscopic and electrochemical study on the monomeric precursors revealed the mode of formation of the triple-decker complexes. It has been established that octa-alkyl cadmium phthalocyanines unexpectedly exist very predominantly as dimeric structures in CH 2 Cl 2 and these are proposed to be intermediates in the formation of the triple-decker complexes. Results from a series of cross experiments using differently substituted Pcs indicate that cadmium can be scrambled between a CdPc and a metal-free Pc . Furthermore, when either species is added to a solution of a triple-decker complex, the ligand from the added species becomes incorporated into new "mixed" triple-decker complexes. From these results it is proposed that the triple-decker structures are formed by self-assembly processes and that they can disassemble and reassemble in the solution phase. Preliminary measurements have identified ring substitution patterns that lead to higher oligomers.