Guest-induced colour changes and molecule-sensing abilities of p-nitrophenol-modified cyclodextrins

Abstract

Two p-nitrophenol modified β-CDs (1, 2) were synthesized in order to assess colour changes induced by host–guest complexation as well as to investigate their conformations and binding properties as colour changeable indicators for the presence of organic molecules in pH neutral aqueous solutions. Host 3, which has an α-CD in place of the β-CD of 1, was also synthesized for comparison. The induced circular dichroism spectra of 1–3 suggest that the p-nitrophenol unit of 1 and 3 is not completely included in their CD cavities and that of 2 is fully included in its β-CD cavity. The spectra of 1–3 in the presence of guest supported these results, slightly enhancing the absolute intensities for 1 and 3 and decreasing the intensity for 2. The guest-induced conformation changes of 1–3 caused the colour to change from yellow to colourless, which is associated with the conversion of the appending moiety from phenolate anion to neutral phenol species at pH 5.10 for 1 and 3 and at pH 6.50 for 2. The binding constants of 1 were markedly larger than those for 2, reflecting that the p-nitrophenol moiety of 1 acts as an effective hydrophobic cap while that of 2 acts as an intramolecular inhibitor upon guest accommodation. These results demonstrate that various colour changeable indicators for molecules which work under neutral conditions may be constructed on the basis of the appropriate design of the appending part in dye-modified cyclodextrins.