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.