Abstract
The effects of cyclodextrins (CDs) on the monomer-dimer equilibria of 1,1′-dialkylviologen radical cations (Cn Cn V.+ :n = 1–5) and one-electron reduced 1,1′-bis(3-sulfopropyl)viologen (SPV) were investigated spectroelectrochemically. The dimerization is enhanced upon addition ofγ-CD, while the addition ofα-CD andβ-CD suppresses the dimerization. This is attributed to the stabilization of dimers by the inclusion of the dimer in the cavity ofγ-CD From the dependence of the apparent dimerization constants (KD′) on the concentration ofγ-CD the association constants of the monomer (KC1) and dimer (KC2) withγ-γ-CD were determined. TheKC1 value increases slightly, from 5 M−1 forn = 1 to 35 M−1 forn = 5, as the length of the alkyl substituents of viologen increases. TheKC2 value depends strongly on the alkyl substituents giving the smallest value of 85 M−1 for (SPV.−)2 and the largest one of 3500 M−1 for (C4C4V.+)2. These results, together with the difference in UV-vis absorption spectra obtained in the absence and in the presence ofγ-CD, suggest that the alkyl substituents and part of the bipyridine ring of viologen radicals are included in theγ-CD cavity forming dimers in which the bipyridine rings are stacked at an oblique angle.
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