GUEST-INDUCED COLOR CHANGES AND MOLECULE-SENSING ABILITIES OF P-NITROPHENOL-MODIFIED CYCLODEXTRINS

Two p-nitrophenol modified beta-CDs (1, 2) were synthesized in order t o assess colour changes induced by host-guest complexation as well as to investigate their conformations and binding properties-as colour ch angeable indicators for the presence of organic molecules in pH neutra l aqueous solutions, Host 3, which has an alpha-CD in place of the bet a-CD of 1, was also synthesized for comparison, The induced circular d ichroism 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 full y included in its beta-CD cavity, The spectra of 1-3 in the presence o f guest supported these results, slightly enhancing the absolute inten sities for 1 and 3 and decreasing the intensity for 2, The guest-induc ed conformation changes of 1-3 caused the colour to change from yellow to colourless, which is associated with the conversion of the appendi ng moiety from phenolate anion to neutral phenol species at pH 5.10 fo r 1 and 3 and at pH 6.50 for 2, The binding constants of 1 were marked ly larger than those for 2, reflecting that the p-nitrophenol moiety o f 1 acts as an effective hydrophobic cap while that of 2 acts as an in tramolecular inhibitor upon guest accommodation. These results demonst rate that various colour changeable indicators for molecules which wor k under neutral conditions may de constructed on the basis of the appr opriate design of the appending part in dye-modified cyclodextrins.