Synthesis of novel dansyl appended cyclodextrins. Self-inclusion and sensor properties

Abstract

The synthesis of three dansyl appended cyclodextrin derivatives, differing in the spacer length between cyclodextrin and the dansyl moiety, is described. In compound 4 the fluorophore is directly attached to the cyclodextrin. Compound 5 contains an ethyl spacer and compound 6 a triethylene glycol spacer. These compounds are designed to detect neutral organic guest molecules like cyclohexanol and adamantanecarboxylic acid in water by fluorescence spectroscopy. At neutral pH none of the compounds is sensitive towards guest molecules. For compound 4 this is due to the fact that the dansyl group is located outside the cyclodextrin cavity. For compunds 5 and 6 the low sensitivity is the result of a strong self-inclusion of the dansyl group. Lowering the pH results in protonation of the dimethylamino group of the dansyl moiety, which makes the self-inclusion less favourable leading to a strongly increased response towards guests. This phenomenon allows the sensors to be switched on and off by lowering or increasing the pH of the solution. Compound 6 is able to detect adamantanecarboxylic acid at 5 × 10^–7 mol^–1 dm³ concentration at pH 1.