Modulation in the charge transfer characteristics of flexible bis-benzimidazole probes: independent sensing mechanisms for Hg²⁺ 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 Hg²⁺ (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 Hg²⁺ ion coordinates with the probe molecule <i>via</i> the pyridyl nitrogen end (acceptor side), while the F^- ion forms a hydrogen-bonded complex involving benzimidazole -NH groups (donor side). Though interaction with Hg²⁺ 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 CH₃CN medium when compound 2 with indole as the terminal residue was considered. The latter compound demonstrates sensitive yet not so specific interaction with Hg²⁺ 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.