Cucurbituril-Induced Supramolecular pK a Shift in Fluorescent Dyes and Its Prospective Applications

Abstract

This review article provides an account on the supramolecular pK a shifts of cucurbit[7]uril-based assemblies of three chromophoric dyes, coumarin 6, Hoechst 33258 and neutral red. Since cucurbituril macrocycles act as cation-receptors due to the presence of two identical negatively polarized carbonyl portals, they preferentially encapsulate the protonated dye/drug over their unprotonated forms, resulting in pronounced pK a shifts of the guest molecules, as large as ~5.2 units. Since the stability and activity of dyes/drugs are often linked to their protonation state, the supramolecular encapsulation by CB7 and the resulting pK a shifts can be rationally exploited to activate drug molecules through thermal/metal ion-responsive pK a tuning, or photo-triggered pH-jump etc. This could act as a simple stimulus for the controlled dissociation of the complex and can make the active protonated/deprotonated form of the guest available for the desired effects. The detailed experimentation on the studied supramolecular host–guest assemblies involving the selected dyes, the associated pK a shifts and the stimuli-responsive pK a tuning, demonstrates their applications in the controlled uptake and release of drugs and photofunctional devices such as logic gates.