Charge transfer and delocalization in conjugated (ferrocenylethynyl)oligothiophene complexes

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 (DeltaE) 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 Fe-II centers yields the corresponding monocations and dications, which exhibit oligothiophene-to-Fe-III charge-transfer transitions in the near-IR region. The band maxima of these tow-energy transitions correlate linearly with DeltaE, while the oscillator strengths show a linear correlation with negative slope with DeltaE. The complexes with similar charge-transfer transition dipole lengths show an increase in the extent of charge delocalization with smaller DeltaE. Comparisons between complexes with different length oligothiophene ligands show that a reduction in DeltaE 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.