Dimerization of radical cations of mono-6-(1-alkyl-4,4’-bipyridino)-beta-cyclodextrins (beta-CD-CnV.+, n = 6, 7, 8) formed via Ru(bpy)(3)(2+)-sensitized photochemical reduction of beta-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 x 10(4) M(-1) for beta-CD-C6V.+, 8.9 x 10(5) M(-1) for beta-CD-C7V.+, and 6.8 x 10(6) M(-1) for beta-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 beta-CD or amphiphilic molecules that are included into beta-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 beta-CD-CnV.+ molecule into the beta-CD cavity of the counter molecule. Association constants of beta-CD-CnV.+ with beta-CD and amphiphilic molecules have been determined from the dependence of dimerization constants on the concentrations of beta-CD Or amphiphiles. The results suggested that the terminal methyl and ethyl groups of the alkyl chains of beta-CD-C7V.+ and beta-CD-C8V.+, respectively, are included in the beta-CD cavities of the same molecules, and this intramolecular inclusion affects the association of beta-CD-CnV.+ with beta-CD and amphiphiles.