Charge Transfer and Delocalization in Conjugated (Ferrocenylethynyl)oligothiophene Complexes

Abstract

A series of conjugated mono(ferrocenylethynyl)oligothiophene and bis(ferrocenylethynyl)oligothiophene complexes have been prepared. The cyclic voltammograms of the complexes all contain a reversible ferrocene oxidation wave and an irreversible oligothiophene-based wave. The potential difference between the two waves (ΔE) varies from 0.38 to 1.12 V, depending on the length and substitution of the oligothiophene group. Several of the mono(ferrocenylethynyl)oligothiophene complexes couple when oxidized, resulting in the deposition of a redox-active film on the electrode surface. In solution, electrochemical oxidation of the FeII centers yields the corresponding monocations and dications, which exhibit oligothiophene-to-FeIII charge-transfer transitions in the near-IR region. The band maxima of these low-energy transitions correlate linearly with ΔE, while the oscillator strengths show a linear correlation with negative slope with ΔE. The complexes with similar charge-transfer transition dipole lengths show an increase in the extent of charge delocalization with smaller ΔE. Comparisons between complexes with different length oligothiophene ligands show that a reduction in ΔE results either in greater delocalization of charge or in charge being delocalized further along the rigid oligothiophene ligand. These results have important implications in understanding charge delocalization in metal-containing polymers.