Temperature dependence of the inclusion–dissociation behavior of the inclusion complexes between cationic substituted 3H-indoles and β-cyclodextrin: Design of a novel type of semi-rotaxane

Abstract

We report herein the syntheses of cationic surface-active molecular probes having long oligo(ethylene glycol) chains as spacers between the cationic nitrogen and the amino nitrogen, named [2-( p-oligoethyloxyethylene amino phenyl)-3,3-dimethyl-5-ethoxycarbonyl-3H-indole] methyl dihexadecyl ammonium iodide (molecules 1a, 1b and 1c), and their interactions with β-cyclodextrin (β-CD) investigated by spectral and photophysical characterizations. It was found that the inclusion–dissociation behavior occurs for the inclusion complexes of molecules 1a and 1b with β-CD, that is, the 1:3 (guest:host) inclusion complex can be formed at lower temperatures and with increasing temperature, the 1:2 inclusion complex can be found. Molecule 1c can only form the 1:1 inclusion complex in the range of temperatures studied, even though it possesses a longer spacer than 1a and 1b. This phenomenon can be interpreted in terms of that the solvation energy of 1c is much larger than that of 1a and 1b, which is unfavorable to form a complex of higher stoichiometry. The association constants of the three types of inclusion complexes at different temperatures were estimated, from which the thermodynamic parameters were also calculated. The negative entropies and enthalpies show that the inclusion complexation is an entropically unfavorable and enthalpy-driven process.