Dinuclear rhenium complexes as redox-active pendants in a novel electrodeposited polycyclopentadithiophene material

The novel [Re2(μ-H)(μ-OOC-CPDT)(CO)6(μ-3-Me-pydz)] complex (1; OOC-CPDT = 4H-cyclopenta[2,1-b:3,4-b′]dithiophene-4-carboxylate, 3-Me-pydz = 3-methylpyridazine) has been prepared and characterized by single-crystal X-ray diffraction, density functional theory (DFT), and time-dependent DFT computations, UV-vis absorption and emission spectroscopy, and cyclic voltammetry (CV). The measured properties indicate the lack of electronic communication in the ground state between the CPDT and the rhenium diazine moieties. Oxidative electropolymerization of 1, achieved by repeated potential cycling (-0.4 to +1.0 V vs Fc+/Fc, in acetonitrile) with different supporting electrolytes, on different electrodes, afforded an electroactive and stable metallopolymer (poly-1). In situ measurements of the mass of the growing film (on a gold electrode, with an electrochemical quartz microbalance) confirmed the regularity of the polymerization process. The polymer exhibits two reversible oxidation peaks of the thiophene chain and a broad irreversible reduction peak (-1.4 V, quite close to that observed for the reduction of monomer 1), associated with a remarkably delayed sharp return peak, of comparable associated charge, appearing in close proximity (+0.3 V) to the first oxidation peak of the neutral polythiophene chain. This charge-trapping effect can be observed upon repeated cycles of p and n doping, and the negative charge is maintained even if the charged electrode is removed from the solution for many hours. Electrochemical impedance spectroscopy showed that the main CV oxidation peak corresponds to facile charge transfer, combined with very fast diffusion of both electrons and ions within the polymer. In summary, poly-1 provides a new example of a metallopolymer, in which the conductive properties of the π-conjugated system are added to the redox behavior of the pendant-isolated complexes.