Inclusion complex formation of ionic liquids with cucurbit [7]uril studied by competitive binding of berberine fluorescent probe

Abstract

The encapsulation of two monocationic ionic liquids and one dicationic ionic liquid in cucurbit [7]uril (CB7) was studied by absorption, fluorescence, and NMR spectroscopic methods in strong acidic aqueous solution. Berberine (BER) was used as a fluorescent probe to study the encapsulation of the ionic liquid in CB7. Addition of the ionic liquid to the aqueous solution of BER–CB7 complex causes considerable fluorescence intensity diminution. It is due to the extrusion of berberine from the macrocycle into the aqueous phase by the competitive inclusion of the ionic liquid. NMR and PM6 semi-empirical calculations also support the 1:1 strong complexation and reveal multiple C–O=C hydrogen bonding contacted between the imidazolium C(2)-proton(s) and portal carbonyl oxygen of CB7. Energy-minimized structures demonstrate that the ethyl, butyl or part of alkyl ether chain of imidazolium moiety in ionic liquid is responsible for the complexation with CB7. CB7 has a preference for ethyl and butyl over methyl for C2MIm+ and C4MIm+, with trends of C4MIm+BF4−>polyalkyl ether bridged dicationic ionic liquid >C2MIm+BF4− in binding constants with CB7.