Fluorescent behaviour in host–guest interactions. Part 2. Thermal and pH-dependent sensing properties of two geometric isomers of fluorescent amino-β-cyclodextrin derivatives

Abstract

The fluorescent behaviour in aqueous solution of two types of amino-β-cyclodextrin (amino-β-CDx) isomers 1 and 2 bearing the naphthoamide group at the 1- and 2-position as a fluorescent-sensing unit was investigated using mainly fluorescence spectroscopy. The isomers 1 and 2 show both strong temperature- and pH-responsive photophysical properties in aqueous solution. The protonation of the amino groups of 1 and 2 in acidic media affects efficiently their fluorescence intensities. The equilibrium and thermodynamic quantities of the two-state equilibrium model for 1 and 2 have been determined spectrophotometrically. Solution NMR behaviour in a DMSO–D₂O mixed solvent confirms the stereospecific relationship between the appended naphthalene fluorophore and the hydrophobic cavity of CDx. Restricted and/or flexible motion of the naphthalene moiety of 1 and 2 about the bond between the amide group and the amino group at the C-6 carbon atom in the CDx ring is evident from the NMR studies. The potential application of 1 to anion sensing is also discussed.