Modulation in the charge transfer characteristics of flexible bis-benzimidazole probes: independent sensing mechanisms for Hg and F.

Flexible bis-benzimidazole-based V-shaped amphiphilic probes (1 and 2) that form a fluorescent nanoscopic assembly in aqueous media have been designed. The ion-binding properties of compound 1 are investigated in both polar protic (water) and aprotic (acetonitrile) solvents. In acetonitrile, the compound shows a distinct chromogenic response towards Hg (LOD: 8.7 ppb) and F ions (LOD: 13.2 ppb) owing to bathochromic shifts of the charge transfer band to different extents. The mechanistic investigation indicated that the Hg ion coordinates with the probe molecule the pyridyl nitrogen end (acceptor side), while the F ion forms a hydrogen-bonded complex involving benzimidazole -NH groups (donor side). Though interaction with Hg has been perceptible even in aqueous medium, no response is witnessed with the F ion. Also, a significant change in the ion-binding properties was observed in the CHCN medium when compound 2 with indole as the terminal residue was considered. The latter compound demonstrates sensitive yet not so specific interaction with Hg ions and also no response towards anions despite having imidazole units. Furthermore, probe 1 has been utilized for rapid, on-site screening of real-life water samples using pre-coated, chemically modified paper strips.