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

The fluorescent behaviour in aqueous solution of two types of amino-beta -c yclodextrin (amino-beta -CDx) isomers 1 and 2 bearing the naphthoamide grou p at the 1- and 2-position as a fluorescent-sensing unit was investigated u sing mainly fluorescence spectroscopy. The isomers 1 and 2 show both strong temperature- and pH-responsive photophysical properties in aqueous solutio n. The protonation of the amino groups of 1 and 2 in acidic media affects e fficiently their fluorescence intensities. The equilibrium and thermodynami c quantities of the two-state equilibrium model for 1 and 2 have been deter mined spectrophotometrically. Solution NMR behaviour in a DMSO-D2O mixed so lvent confirms the stereospecific relationship between the appended naphtha lene fluorophore and the hydrophobic cavity of CDx. Restricted and/or flexi ble 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 e vident from the NMR studies. The potential application of 1 to anion sensin g is also discussed.