Conducting Polymers Containing In-Chain Metal Centres : Electropolymerisation and Charge Transport

Abstract: Conjugated polymers that exhibit high electronic conductivities play key roles in the emerging field of molecular electronics. In particular, linking metal centres with useful electrochemical, photophysical, or catalytic properties to the backbone, or within the polymer chain itself, is a topic which has attracted a significant amount of interest lately. Structurally rigid monomers that can be electropolymerised to form highly conducting molecular wires may provide new insights into conduction mechanisms, e.g., exploiting resonant superexchange (electron-hopping) by tuning the energies of redox centre and bridge states. The focus of this thesis lies on the electrochemical investigation of preparation, growth dynamics, and charge transport dynamics of oligothiophene/transition metal hybrid materials. The incorporation of ruthenium(II) and osmium(II) terpyridine complexes into such polymeric assemblies was accomplished by an electropolymerisation procedure, to produce rod-like oligothienyl-bridged metallopolymers. The properties of the monomers used were characterised by optical spectroscopy and electrochemical techniques. Charge transport was studied in detail for some of the materials created, and it was found that the electron transport rate and dc conductivity was enhanced by up to two orders of magnitude compared to relevant non-conjugated polymers, demonstrating the usefulness of this approach for optimization of charge transport in metallopolymers. The charge transport diffusion coefficent was determined to (2.6 ± 0.5) x 10-6 cm2 s-1 for a quaterthienyl-bridged {Os(tpy)2} polymer by use of an electrochemical steady-state method carried out using a transistor-like experimental geometry. It was found that charge transport in these materials is concentration-gradient driven. The rate limiting step of the charge transport process was investigated using electrochemical impedance spectroscopy. The electropolymerisation dynamics of one of the monomers was studied using microelectrodes, and the results obtained shows that electropolymerisation is highly efficient, and indicate that mass transport controls this process. Through a combination of controlled potential deposition and SEM imaging it was demonstrated that it is possible to exploit the edge effect of microelectrodes to promote film growth in a direction co-planar with the electrode surface.