Self-inclusion behavior and circular dichroism of aliphatic chain-linked β-cyclodextrin-viologen compounds and their reduced forms depending on the side of modification

Abstract

The self-inclusion behavior and induced circular dichroism (ICD) characteristics of two β-cyclodextrin (β-CD) derivatives, in which a 1-methyl-4,4′-bipyridinium (viologen) group is connected by an octamethylene chain to either the primary (22+) or secondary (3 2+) side of β-CD, and of their reduced forms, are investigated. 1H NMR studies showed that 22+ forms an intramolecular self-inclusion complex with Kin = 3.1 ± 0.4, whereas 3 2+ forms a head-to-head type of dimer with KD = 65 ± 10 M-1 at 25 °C. 22+ and 32+ form [2]pseudorotaxanes with α-CD, with the secondary side of the α-CD facing the viologen moiety. The ICD characteristics of mono-6-[4-(1-methyl-4- pyridinio)-1-pyridinio]-β-CD (12+), 22+, 3 2+, and methyloctyl viologen-β-CD complexes were obtained for the oxidized and reduced states of the viologen units. The results indicated dimer formation for 1°, and intramolecular complexation for 2.+ and 2° in which the reduced viologen units are outside the β-CD cavity. The results also indicated intramolecular complexation for 3.+ and 3°, but with reduced viologen units inside the cavity. This work provides unequivocal evidence of the preference of the secondary side of cyclodextrins for viologen groups, regardless of their oxidation states, and the dependence of ICD of the viologen chromophores on their location with respect to the CD cavity. © 2005 American Chemical Society.