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

Inclusion complexation between alpha-CD and naphthalene-viologen compounds linked with a flexible polymethylene bridges [-(CH2)(n)-; n = 3, 5, 7, 9, 1 2] is examined by NMR spectroscopy. Thermodynamic parameters such as equili brium constants, enthalpy changes, and entropy changes of complexation were obtained. The equilibrium constants increase dramatically as the chain bec omes longer. It is shown that the chain-length dependence of the equilibriu m constant is dominated mainly by the entropy factor due to the hydrophobic interaction. Effects of intramolecular complexation on the apparent comple xation constants are discussed. Structural features of the complex are also examined from chemical shift change and ROESY experiment. The main inclusi on site seems to be the aromatic moiety for short chain molecules, and it m oves toward the alkyl chain part as the chain becomes longer.