Charge Injection and Transport in Metal-Containing Conducting Polymers: Spectroelectrochemical Mapping of Redox Activities

Electropolymerization of tris­(dioximate) cage complexes furnished metal-containing conducting polymers (MCPs) that deposit directly onto the electrode surface as uniform films. The injection of electrons into, or removal of electrons from, these electroactive materials proceeds via different pathways with different rates, the underlying molecular mechanisms of which were investigated by a combination of electrochemical, spectroscopic, and focused-ion-beam–scanning electron microscopy (FIB-SEM) cross-section analysis studies. For cobalt-containing polymers, both the metal centers and π-conjugated organic backbone work cooperatively as hopping stations for migrating holes, whereas the reduced polymer utilizes less-efficient self-exchange between cobalt­(II) and cobalt­(I) centers for electron transport. A small molecule model of such reductively doped polymer was prepared independently, which provided compelling electrochemical and spectroelectrochemical evidence to support the structural integrity of the metal centers upon redox switching. A well-defined metal-to-ligand charge transfer (MLCT) band of the n-doped polymer was exploited further as a straightforward spectroscopic tool to quantify the number of redox-active metal centers directly and to estimate the lower distance limit of diffusional charge transport across the bulk material.