Face selectivity of inclusion complexation of viologens with β-cyclodextrin and 6-O-(2-sulfonato-6-naphthyl)-β-cyclodextrin

Abstract

Face selectivity in binding of methyloctyl viologen (C 1C 8V 2+: 1) and adamantylmethyl viologen (AdaC 1V 2+: 2) with β-CD and 6-O-(2-sulfonato-6-naphthyl)-β-CD (β-CD-NS: 4) has been investigated. Circular dichroic titration of 1 and 2 with β-CD gave the binding constants and the molar ellipticities [θ] 254nm of the β-CD complexes as 890 M -1 and -2000 deg cm 2 dmole -1 for 1, and as 7300 M -1 and -3600 deg cm 2 dmole -1 for 2. The [θ] values of the complexes are much less negative than -8500 deg cm 2 dmole -1 of β-CD-viologen compound 3 in which the viologen is covalently bonded to the primary face of β-CD. Fluorescence quenching of the naphthyl group of 4 by 1 and 2 is much more efficient than that of 6-methoxy-2-naphthalenesulfonate (MNSS), but residual fluorescence was observed even at high viologen concentration. The binding constants of the viologens with 4 and the fractions of the residual fluorescence with respect to fully monomeric 4 were determined as 4700 M -1 and 0.09 for 1 and 10 400 M -1 and 0.15 for 2. Comparing the steady-state fluorescence quenching and time-resolved fluorescence studies, it appeared that the bipyridinium moiety of 2 is predominantly on the secondary side of β-CD of 4, whereas that of 1 favors the primary side, mainly due to the direct charge transfer interaction between the naphthyl and bipyridinium groups: without the interaction, it would prefer the secondary face 14 times more favorably to the primary face. The rate constants of the photoinduced electron-transfer reactions between the bipyridinium group on the secondary face and the naphthyl group are 1.9 × 10 8 s -1 for the 1/4 complex and 7.9 × 10 8 s -1 for the 2/4 complex.