Bi-directional inclusion complexation of methylalkyl viologens with β-cyclodextrin and naphthyl group-tethered β-cyclodextrins

Inclusion complexation of methylalkyl viologens (C1CnV2+; n=7-10, 12) with mono-6-O-(2-sulfonato-6-naphthyl)-beta-CD (1) and mono-6-O-(2-naphthyl)-beta-CD (2) were studied by steady-state and time-resolved fluorescence spectroscopies and compared with the binding of the viologens with native beta-CD investigated by induced circular dichroism. The viologens form bimodal complexes with 1 and 2, in which the bipyridinium group of the viologens is placed on the primary side (type I complex) and secondary side (type II complex) of beta-CD cavity, while the group is predominantly on the secondary side in complexes with native beta-CD. The microscopic binding constants K-I and K-II were calculated from the analysis of fluorescence data. The formation of the type I complexes with 1 and 2 appears to be largely due to the charge-transfer interaction between the bipyridinium and naphthyl groups in the complexes. This work shows that the location of the bipyridinium group in beta-CD complexes and in the type II complexes of the viologens with 1 and 2 depends little on the length of alkyl chain of the viologens.