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'-bithi ophene, 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 t hiophene-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 b ands due to an oligothienyl group to Fe-III charge-transfer transition. The absorption maxima and intensities of these bands correlate to the length o f the conjugation in the oligothienyl group. Above the thiophene oxidation potential and by careful exclusion of water, 2 and 3 electropolymerize resu lting in the deposition of electrochromic films on the electrode surface. T he films are golden-red and stable when neutral and become black upon oxida tion. The cyclic voltammetry of the film growth process and the formation o f electroactive films indicate that the films are conductive. Spectroelectr ochemical characterization of the films demonstrates that broad, low-energy absorptions appear upon oxidation of the ferrocenyl centers and that stron ger and much broader bands appear upon full oxidation of the films.