Synthesis, characterisation and charge transport properties of a series of osmium containing polymers

Abstract

A series of metallopolymers, based on poly(4-vinylpyridine) (PVP) and poly(N-vinylimidazole) (PVI), has been prepared containing both osmium- and ruthenium-bis (2,2’-bipyridyl) centres. The polymers have been character ised using uv/vis and emission spectroscopy as well as, thermal and electrochemical methods. The results obtained are compared wi th those obtained for the metallopolymers containing only osmium or ruthenium centres as well as, model monomeric complexes. The results for mixed metal polymers suggest that there is little interact ion between the metal centres when in solution, either in the ground or excited state. The rate of homogeneous charge t ranspor t through [Os(bipy)2 (PVP)nCl]Cl films (n = 5, 10, 15, 20 and 25), has been examined using potentia l step methods and cyclic voltammetry . The effect of redox site loading, electrolyte type and concentra tion and temperature on the charge transport diffusion coefficient for homogeneous charge t rans fer wi thin the immobilised film for the Os(II/III) oxidation, has been investigated. The relevant activation parameters, enthalpy, ent ropy and free energy changes and activation energy for diffusional charge transport are presented. The results obtained suggest that at short times mass and charge transport can be decoupled, while at longer times the mass transport required to maintain electroneutrality occurs. The standard rate constant k° and transfer coefficient character is ing the heterogeneous electron transfer reaction from the under lying electrode into the modifying film have also been evaluated. The effect of variations in osmium content within the film, electrolyte type and concentration and temperature on these parameters is examined. The relevant enthalpies for the heterogeneous electron transfer reaction have been evaluated. The reaction entropy is similar for all redox site loading electrolyte concentration combinations examined suggesting that the local micro environment of the redox centre remains largely unal tered by changes in the nature of the supporting electrolyte or the active site loading. The charge t ransport properties of [Os(bipy)2 (PVI )nCl]Cl where PVI is poly(n-vinylimidazole) and n = 5, 10, 15, 20 and 25 has been explored in a range of electrolytes based on chloride, sulphate, tosylate and perchlorate anions. The effect of electrolyte and redox site loading and temperature on the homogeneous charge transport process reveals that for low redox site loading/high electrolyte concent ration segmental polymer chain motion limits Dq-j-. In contra st, for high active site/low electrolyte concent ration combinations the rate of charge transport is controlled by ion transport within the film. The effect of electrolyte and active site loading on k° is also explored. Homogeneous and heterogeneous charge t rans fer through poly(N-vinylimidazole) conta ining [Os(N)g] ' moieties has been examined as a function of the nature of the contacting electrolyte solution and of temperature . The charge transport parameters are sensitive to the nature of the electrolyte anion. In sulphur ic acid charge transport is rapid and is consistent with a swollen porous film. In perchloric acid the films appear compact. The abil ity of the [Os(bipy)2 (PVP)jgCl]Cl polymer to catalyse the reduction of Fe(III) to Fe(II) in both 0.1 M H2 SO4 and 1.0 M HCIO4 electrolytes is demonst rated. Catalysis occurs at a large por tion of the active sites wi thin the polymer in sulphur ic acid leading to three dimensional catalysis. In perchloric acid catalysis only occurs wi thin a region of molecular dimensions at the film/electroly te interface