Efficient Fluorescent Detection of Mercuric Ions Based on 2-Thienylbenzimidazole/Cucurbit[7]uril Complexes

2-Thienylbenzimidazole (TBI)/cucurbit[7]uril (CB7) host/guest complex was used to significantly improve the detection of Hg2+ in water samples. The unique mechanism of the sensing process is based on the binding of CB7 host to TBI guest. The host−guest complexation was confirmed by UV−visible absorption, time-resolved fluorescence, and proton NMR spectroscopy. The complexations of TBI and TBI/CB7 with Hg2+ ions in solid-state were confirmed by FTIR, mass, and TGA. The most stable structures were unfolded by DFT calculations. In aqueous solution, a five-fold higher stability constant of the protonated TBIH+/CB7 complex was observed when compared to the neutral TBI/CB7 complex as manifested in an increase in pKa values by ~ 0.5 unit in an excited state. The static fluorescence quenching of TBI on binding to Hg2+ ions was significantly enhanced in the presence of CB7. Macrocycles are useful additives for the highly sensitive recovery of mercury ions in water samples.