Electropolymerization of Oligothienylferrocene Complexes: Spectroscopic and Electrochemical Characterization

The new complexes 1,1‘-bis(5-(2,2‘-bithienyl))ferrocene ( 2 ) and 1,1‘-bis(5-(2,2‘:5‘,2‘ ‘-terthienyl))ferrocene ( 3 ) have been synthesized by coupling 1,1‘-bis(2-thienyl)ferrocene ( 1 ) with 2-bromothiophene and 5-bromo-2,2‘-bithiophene, respectively. The cyclic voltammograms of 1 − 3 contain a reversible Fe II/III wave with E 1/2 between 0.37 and 0.46 V vs SCE and irreversible thiophene-based oxidation waves at higher potentials. These compounds can be electrochemically oxidized to yield solutions of the monocations 1 + − 3 + . The visible−near-IR spectra of these monocations all contain low-energy bands due to an oligothienyl group to Fe III charge-transfer transition. The absorption maxima and intensities of these bands correlate to the length of the conjugation in the oligothienyl group. Above the thiophene oxidation potential and by careful exclusion of water, 2 and 3 electropolymerize resulting in the deposition of electrochromic films on the electrode surface. The films are golden-red and stable when neutral and become black upon oxidation. The cyclic voltammetry of the film growth process and the formation of electroactive films indicate that the films are conductive. Spectroelectrochemical characterization of the films demonstrates that broad, low-energy absorptions appear upon oxidation of the ferrocenyl centers and that stronger and much broader bands appear upon full oxidation of the films.