Tuning the Redox Potential of Conducring Polymeric Films

Abstract

Abstract Hybrid transition metal-containing conducting polymers represent a new class of advanced material that combine the electron transfer properties of transition metal ions embedded within a conducting polymer matrix. Attaching a thiophene residue (a precursor to the conducting polymer polythiophene) to a transition metal via a covalent bond to a ligand within the complex provides a route to novel hybrid polythiophene conducting polymers. In this project, a range of new tetraaza-macrocyclic ligands bearing appended thiophenyl groups have been prepared with a view to subsequent polymerization via an electrochemical synthesis (electropolymerisation) Two strategies were chosen to synthesize these new types of ligands. Copper-directed Mannich reactions have been used to produce ligands with pendent thiophene groups introduced via a thiophene containing nitro-substituted ‘carbon acid’. A number of new nitro, thiophene-substituted macrocyclic ligands were isolated and characterized as their Cu(II) complexes. Reduction of the nitro groups led to isolation of the amino analogues as their free ligands. Alternatively, thiophene substituents were introduced by a reductive alkylation process involving condensation between a suitable thiophene (or terpthiophene) aldehyde and the exocyclic primary amine of an existing tetraaza macrocycle. Redox active Co(III), Cu(II), Ni(II) and Mn(III) complexes of these ligands were prepared and fully characterized structurally, spectroscopically and electrochemically in their monomeric forms. Electropolymerisation of these complexes was attempted as well as with the metal free ligands. However, none of these thiophene-appended ligands or complexes proved suitable despite several variables being investigated including solvent, co-polymer, catalysts or types of electrode. However, terthiophene-appended macrocyclic ligands and complexes obtained from a similar reductive alkylation procedure were very effective precursors to polythiophene hybrid polymers. Electropolymerisation of the Co(III) complexes or free ligands yielded conducting polymeric films of good stability and morphology. Electrochemistry of the solid film adhered to an electrode showed that the Co centres within the polymer were redox active and could be cycled between their tri- and divalent oxidation states reversibly. Characterization of the films with X-ray photoelectron spectroscopy confirmed the presence of Co within the film. The elemental ratio of Co:S (1:3) within the polythiophene:Co polymer was the same as that within the monomer complex itself