Interaction between β-cyclodextrin and ionic liquids in aqueous solutions investigated by a competitive method using a substituted 3H-indole probe

We report herein the interaction of three kinds of ionic liquids, i.e., 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6), 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) and 1-butyl-3-methylimidazolium chloride (bmimCl) with β-cyclodextrin (β-CD) using 2-(p-aminophenyl)-3,3-dimethyl-5-carboethyoxy-3H-indole (1) as a fluorescent probe through a competitive method. The formation of the 1:1 (guest:host) inclusion complex was suggested and the association constants at different temperatures were estimated, from which the thermodynamic parameters ΔGΘ, ΔHΘ and ΔSΘ were also obtained. The negative entropy and enthalpy changes indicated that the formation of above inclusion complexes was entropically unfavorable and enthalpy-driven. Conductivity measurements were also employed to study above systems. The results showed agreement with those based on the competitive method. Furthermore, through NMR, the location of anions in the system of ILs and β-CD was investigated. We suggested that the anion was accommodated to a great extent by the cavity forming ion pair with imidazolium cation which was totally in the cavity of β-CD. The above work demonstrated that molecule 1 is a very sensitive fluorescence probe for studying the interaction of a non-fluorescent guest with cyclodextrins.