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

Abstract

Intervalence-transfer (IT) bands at all of the mixed-valence states of oligo(1,1′-dihexylferrocenylene)s from the dimer to the hexamer, 2–6, were determined by analysis of the absorption spectra of the oligomers oxidized quantitatively with 1,1′-dichloroferrocenium hexafluorophosphate, 1, in CH₂Cl₂–acetone. Characteristic features of the IT bands are a higher energy shift in ν_max as the oxidation number for each oligomer increases and a lower energy shift in ν_max for the monocationic forms of the oligomers as the number of ferrocene units increases. A simple analysis of the shift in ν_max of IT bands was made based on a model assuming that the mixed-valence states are a linear combination of reduced (Red) and oxidized (Ox) nuclei. It is qualitatively rationalized by taking into account the change in neighboring-site combination, especially between Red–Ox and Ox–Ox, formed by photoelectron transfer, which requires extra energy due to the strain in internuclear distance.