Facile Dimerization of Viologen Radical Cations Covalently Bonded to β-Cyclodextrin and Suppression of the Dimerization by β-Cyclodextrin and Amphiphiles

Dimerization of radical cations of mono-6-(1-alkyl-4,4‘-bipyridino)-β-cyclodextrins (β-CD-CnV•+, n = 6, 7, 8) formed via Ru(bpy)32+-sensitized photochemical reduction of β-CD-CnV2+ was studied spectroscopically, and the results were compared with those of methylalkyl viologens. Spectral characteristics of monomer and dimer of viologen radical cations were derived. The dimerization constants have been determined to be 4.0 × 104 M-1 for β-CD-C6V•+, 8.9 × 105 M-1 for β-CD-C7V•+, and 6.8 × 106 M-1 for β-CD-C8V•+. These values are 2−3 orders of magnitude greater than those for the corresponding methylalkyl viologen radical cations. The dimer formation is driven by a large enthalpy decrease and a moderate entropy increase. The dimerization is suppressed upon addition of β-CD or amphiphilic molecules that are included into β-CD. These results indicate that the dimers are stabilized by inclusion of the alkyl chain of the 1-alkyl-4,4‘-bipyridino moiety of the β-CD-CnV•+ molecule into the β-CD cavity of the counter...