Complex formation of phenolphthalein and some related compounds with β-cyclodextrin

Abstract

The complex formation of phenolphthalein and β-cyclodextrin (cyclohepta-amylose) has been investigated by spectrophotometric, c.d., and potentiometric methods. It has been shown that complexes are formed with the protonated and dissociated forms of phenolphthalein as well; the dianion is complexed in alkaline solution in a colourless form. The complexes formed have the largest stability constants among cyclodextrin complexes known up to now. The unusual stabilities and colour change can be explained by the combined effect of the accessibility of the phenolic-phenolate moiety and presence of the carboxy–carboxylate group simultaneously, providing optimal space filling and the possibility for the cyclodextrin molecule to interact with three functional groups of phenolphthalein at the same time.