The interaction of C.I. Basic Red 5 with complementary negatively charged hosts: 4-Sulfonatocalix[4]arene and carboxymethyl-β-cyclodextrin

The formation of the inclusion complexation between C.I. Basic Red 5 and two types of negatively charged host molecules, namely 4-sulfonatocalix[4]arene and carboxymethyl-β-cyclodextrin, was studied using fluorescence, UV–visible absorption and nuclear magnetic resonance spectroscopy. Different fluorescence and absorption behavior were observed upon complexation of the dye with host molecules that possess a hydrophobic cavity. 4-Sulfonatocalix[4]arene showed strong binding ability for the acidic form of the dye, which reduced the fluorescence emission of the dye. In contrast, carboxymethyl-β-cyclodextrin was more suitable for complexation of the neutral form of the dye for which an increase in fluorescence emission was seen. The difference in the complexation behavior of dye towards the two hosts is attributed to the specific effects of the dominant binding modes, namely electrostatic attraction in the case of 4-sulfonatocalix[4]arene and hydrophobic interaction in the case of carboxymethyl-β-cyclodextrin. The thermodynamic parameters for inclusion complexation, which were determined using van't Hoff analysis, supported the different binding modes of the dye with the two host molecules; 1H NMR further confirmed the conclusion.