[Interaction of Cucurbit[8]uril with β-Indoleacetic Acid and Methylviologen]

The interaction between Q[8] with β-indoleacetic acid and the methylviologen was studied in aqueous solution with electronic absorption spectroscopy (UV-Vis), fluorescence spectroscopy, 1H NMR spectroscopy and isothermal titration calorimetry (ITC) in details. The authors explored the mode of action, action site and thermodynamic properties of the host-guest system. The electronic absorption and fluorescence spectroscopy data showed that the Q[8]/IAA system and Q[8]/MV²⁺ system informed 1:1 inclusion complexes in aqueous solution. ITC results showed that the changes of Gibbs free energy and enthalpy are all negative, it suggested that complex formation was spontaneous and exothermic reaction. Moreover, ITC results for the Q [8] and IAA with MV²⁺ indicate that the association constants of the Q[8]-IAA and Q[8]-MV²⁺ complexes were (3.22 ± 0.96) x 10⁵ L · mol⁻¹ and (3.90 ± 0.91) x 10⁶ L · mol⁻¹, respectively. Therefore, the interaction between Q[8] and IAA with the MV²⁺ was a competitive process. This likely occurs because the MV²⁺ and IAA molecules attempt to occupy the Q[8] cavity, which reduces the fluorescence and absorption spectra intensity of Q[8]-IAA because of the formation of a new inclusion complex between Q[8] and IAA with MV²⁺. In addition, with the addition of MV²⁺ to a Q[8]/IAA complex, 1H NMR results showed that the indole moieties of β-indoleacetic acid and bipyridyl group of methylviologen can be incorporated into Q[8] cavities because of electronic transfer MV²⁺ with PQ in a Q[8] cavity with ternary complexes. These results provides the potential applications for the supramolecular self-assembly in cucurbit[n]urils field.