Oligothiophenes and conducting metallopolymers : fundamental studies and development of functional materials
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Diimine rhenium(I) tricarbonyl complexes are known as phosphorescent emitters and electro- and photocatalysts for the reduction of CO₂ to CO. Conducting metallopolymers containing this rhenium(I) moiety should not only retain the photoluminescent and catalytic properties of the complex but also gain the conductivity, processability, and mechanical flexibility typical of [pi]-conjugated polymers. A series of tricarbonyl rhenium(I) diimine-type monomers and metallopolymers have been prepared. Appended to the ligands are thiophene and 3,4-ethylenedioxythiophene (EDOT) groups for electropolymerization of the metal complexes. UV-Vis absorption and emission spectroscopy studies of the monomers indicate that light emission originates from triplet ligand-centered (³LC) [pi] [right arrow] [pi]* and triplet metal-to-ligand charge transfer (³MLCT) excited states. Additionally, both the monomers and metallopolymers show electrocatalytic activity towards the reduction of CO₂ to CO. Furthermore, the EDOT-functionalized diimine-type ligand (EDOT₂-BPP) also serves as a good sensitizing ligand for luminescent lanthanide emission. A series of lanthanide complexes that utilize tris([beta]-diketonates) and EDOT₂-BPP ligands have been synthesized and studied using X-ray crystallography and photophysical techniques. Large quantum yields and microsecond lifetimes were found for the EuIII and SmIII complexes. Complexes of TbIII were found to have weak luminescent emission due to the less-than-optimal energy gap between the sensitizing ligands and the excited state of the TbIII ion. Oligothiophenes are models for polymeric systems because of solution processability, controlled chain length, and a well-defined structure. We have synthesized a library of alkyl and polyfluoroalkyl-substituted oligothiophenes to study how molecular structure, long-range order, spatial orientation, and varying degree of electronic coupling between molecules influences charge separation in photovoltaics. These oligoalkylthiophenes have been characterized by X-ray diffraction, photophysical methods, electrochemistry, and UV-Vis and EPR spectroscopies. Although the electronic properties of these oligoalkylthiophenes do not vary with alkyl group, aggregates of oligooctylthiophene, made through solution processing, have distinct morphologies with varying amounts of electronic disorder. The extent of electronic disorder within the aggregate is determined by comparing the suppression of the 0-0 vibronic band in the fluorescence spectra to that of the non-aggregated parent molecule. This extent of electronic disorder was correlated with the local contact potential of individual aggregates through Kelvin probe force microscopy (KPFM) measurements.