Metal-Ion-Mediated Assemblies of Thiazole Orange with Cucurbit[7]uril: A Photophysical Study

The formation of molecular superstructures by metal-ion-mediated noncovalent self-assembly has been demonstrated using the macrocycle, cucurbit[7]uril (CB7), and the dye, thiazole orange (TO), as building blocks. Interestingly, the association of these molecular building blocks can be tuned by the chemical environment, leading to self-assembled structures of different stoichiometries, which is supported by absorption, fluorescence, 1H NMR, and AFM measurements. Most importantly, the self-assembly process of the CB7/TO/metal ion system is observed to be remarkably different for alkali (Na+) and alkaline earth (Ca2+) metal ions. Fluorescence enhancement is observed in the presence of Ca2+ ions, which is attributed to the formation of short dimeric structures composed of two 1:1 CB7–TO complexes. Solution turbidity is detected in the presence of Na+ ions, which is proposed to be due to the formation of extended structures by the assembly of many 1:1 CB7–TO complexes.