Reactions at thin-film electrodes: Polymer-based and semiconductor-based systems

Reactions and properties of electrodes modified with thin-film metallo-polymers and thin films of nanocrystalline semiconductors are investigated. New phenanthroline-containing complexes of ruthenium featuring widely varying bipyridyl ligand lengths are prepared via an oxidative electropolymerization scheme. The films show strong size selective permeability towards molecular reactants based on oxidation of electro-active species at the modified electrodes. This leads to a process whereby the reactivity of the underlying electrode can be controlled. The observed molecular size cut-off allows for film pore size and composition to be gauged. The mechanism for polymerization is elucidated to involve electrochemical activation of the metal complex that renders the coordinated phenanthroline capable of undergoing nucleophilic attack by a bipyridyl ligand coordinated to a second metal center.; Reactions at nanocrystalline titanium dioxide semiconductor interfaces are studied via laser-based spectroelectrochemical methods. Titanium dioxide sensitized with a light absorbing species such as functionalized ruthenium-tris-2,2{dollar}spprime{dollar}-bipyridine undergoes visible excitation in which the sensitizer rapidly injects an electron into the semiconductor. This is followed by slow return electron transfer (ET). The kinetics of the back ET process is monitored by transient absorbance spectroscopy. Significant results such as pH independent rate constants, and normal region reactivity observed from chromophore-based variations in driving force, point to a new mechanism for interfacial electron transfer. The proposed scheme involves proton-decoupled (pH independent) electron transfer in which the back ET step has a smaller driving force than the overall reaction driving force. Studies based on osmium and iron based dyes offer further insights into dye-sensitized semiconductor technology.