Complexation of naphthalene-viologen linked compounds with α- cyclodextrin: Effect of linkage length on thermodynamic parameters and complex structures

Inclusion complexation between α-CD and naphthalene-viologen compounds linked with a flexible polymethylene bridges [-(CH₂)(n)-; n = 3, 5, 7, 9, 12] is examined by NMR spectroscopy. Thermodynamic parameters such as equilibrium constants, enthalpy changes, and entropy changes of complexation were obtained. The equilibrium constants increase dramatically as the chain becomes longer. It is shown that the chain-length dependence of the equilibrium constant is dominated mainly by the entropy factor due to the hydrophobic interaction. Effects of intramolecular complexation on the apparent complexation constants are discussed. Structural features of the complex are also examined from chemical shift change and ROESY experiment. The main inclusion site seems to be the aromatic moiety for short chain molecules, and it moves toward the alkyl chain part as the chain becomes longer.