Dependence of intervalence-transfer bands for mixed-valence oligo(1,1 '-dihexylferrocenylene)s on the oxidation number and the number of ferrocene units from two to six

Intervalence-transfer (IT) bands at all of the mixed-valence states of olig o(1,1'-dihexylferrocenylene)s from the dimer to the hexamer, 2-6, were dete rmined by analysis of the absorption spectra of the oligomers oxidized quan titatively with 1,1'-dichloroferrocenium hexafluorophosphate, 1, in CH2Cl2- acetone. Characteristic features of the IT bands are a higher energy shift in nu(max) as the oxidation number for each oligomer increases and a lower energy shift in nu(max) for the monocationic forms of the oligomers as the number of ferrocene units increases. A simple analysis of the shift in nu(m ax) of IT bands was made based on a model assuming that the mixed-valence s tates are a linear combination of reduced (Red) and oxidized (Ox) nuclei. I t is qualitatively rationalized by taking into account the change in neighb oring-site combination, especially between Red-Ox and Ox-Ox, formed by phot oelectron transfer, which requires extra energy due to the strain in intern uclear distance.