Supramolecular Assemblies of Thioflavin T with Cucurbiturils: Prospects of Cooperative and Competitive Metal Ion Binding

Of late, molecular-recognition-guided supramolecular assembly and its relevance in the development of highly stable and active photofunctional materials have drawn much attention. In this review, the non-covalent interaction of a biologically important dye, thioflavin T (ThT), with cucurbituril macrocycles, especially, cucurbit[7]uril (CB7) and cucurbit[8] uril (CB8), and the response of the resulting molecular assemblies towards the metal ions have been discussed. The interaction of ThT with CB7 leads to significant enhancement in the fluorescence yield, lifetime and modifications in the spectral features of ThT. These changes are assigned to the formation of 1 : 1 and 2 : 1 (CB7.ThT) complexes. However, with CB8 a distinct evolution of a hitherto unexplored strong excimer emission band having maximum at 570 nm is observed. The strong ion-dipole interactions provided by the carbonyl portals of the CB8 adequately support the stabilization of two pi-stacked ThTs, both in 1 : 2 (CB8.ThT) and 2 : 2 stoichiometric ratio. In case of the 1 : 1 (CB7.ThT) system, the metal ion addition leads to the usual competitive binding interaction and decreases the fluorescence intensity. However, the addition of the same metal ion to the 2 : 1 (CB7.ThT) complex results in a novel cooperative metal ion binding to the complex, leading to the formation of a highly fluorescent supramolecular capsule. Further addition of amantadine hydrochloride induces rupture of the capsular complex, projecting potential application in targeted drug delivery. The 2 : 2 (CB8.ThT) complex responds to the metal ion presence in a competitive way, which allows demonstration of a fluorescence on-off mechanism.