Phenolphthalein-modified β-cyclodextrin as a molecule-responsive colorless-to-color change indicator

Phenolphthalein-modified β-CD, 1, was synthesized for the purpose of developing a new type of guest-responsive color change indicator. The pH titration curve of 1 depends on its concentration, suggesting that 1 exists not only as an self-inclusion form but also as an association form at a concentration of 10^-4 M in neutral aqueous solution. At pH 11.0, the association species dissociates into the monomer one, taking a dianion form in the phenolphthalein part. Upon the guest addition at pH 9.70, 1 exhibits the color change from colorless to purple at its concentration of 5.0 x 10^{- 6} M due to the 1:1 host-guest complex formation. The guest-induced absorption changes were used for molecule sensing. The sensing abilities of 1 for various guests are roughly parallel to the binding constants.