Charge transfer and delocalization in conjugated (ferrocenylethynyl)oligothiophene complexes

A series of conjugated mono(ferrocenylethynyl)oligothiophene and bis(ferroc enylethynyl)oligothiophene complexes have been prepared. The cyclic voltamm ograms of the complexes all contain a reversible ferrocene oxidation wave a nd an irreversible oligothiophene-based wave. The potential difference betw een the two waves (DeltaE) varies from 0.38 to 1.12 V, depending on the len gth and substitution of the oligothiophene group. Several of the mono(ferro cenylethynyl)oligothiophene complexes couple when oxidized, resulting in th e deposition of a redox-active film on the electrode surface. In solution, electrochemical oxidation of the Fe-II centers yields the corresponding mon ocations and dications, which exhibit oligothiophene-to-Fe-III charge-trans fer 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 wi th similar charge-transfer transition dipole lengths show an increase in th e extent of charge delocalization with smaller DeltaE. Comparisons between complexes with different length oligothiophene ligands show that a reductio n in DeltaE results either in greater delocalization of charge or in charge being delocalized further along the rigid oligothiophene ligand. These res ults have important implications in understanding charge delocalization in metal-containing polymers.