Color Change Indicators for Molecules Using Methyl Red-Modified Cyclodextrins

Abstract

β-Cyclodextrin derivatives, MRCD and p-MRCD, which have a 4-(dimethylamino)azobenzene moiety with a carbonyl substituent at 2‘ and 4‘ positions, respectively, have been prepared as color change indicators for detecting organic compounds. In a 10% ethylene glycol solution, MRCD and p-MRCD form intramolecular self-complexes in which the pendant dye moiety is included in the cyclodextrin cavity with an orientation parallel and perpendicular to the cyclodextrin axisis, respectively. When guest molecules are added to the acidic solutions of MRCD (pH 1.60) and p-MRCD (pH 2.40), they exhibit color changes from yellow to red for MRCD and from orange to red for p-MRCD. These color changes, which arise from the structural change of the dye moieties from the azo form to the azonium one, are caused when MRCD and p-MRCD undergo a conformational change in which the dye moieties inserted in the cyclodextrin cavities are excluded to outside of the cavities upon guest accommodation. The extent of the guest-induced color changes of MRCD and p-MRCD depend on the shape, size, number, and position of the functional group of guest molecules. Selectivities between MRCD and p-MRCD in guest detection are roughly parallel and reflected in the host−guest binding constants. Among guest molecules examined, ursodeoxycholic acid and chenodeoxycholic acid were detected by MRCD and p-MRCD with high sensitivities. 1-Adamanetanecarboxylic acid and (−)-borneol were also detected with high sensitivities. In neutral conditions, however, the selectivity in guest detection of p-MRCD is different from that in acidic conditions as shown by the fact that, for example, 1-adamantanol and 2-adamantanol were detected by p-MRCD with larger sensitivities than 1-adamanetanecarboxylic acid. The result indicates that the ionic nature of the guest molecules is an important factor for detection of the guest molecules. All these results demonstrate that MRCD and p-MRCD can be used as color change indicators for detecting various organic compounds in aqueous solution.