Synthesis, spectral studies, solvent effect and on-off fluorescence of 1-benzyl-4-fluoro-2-phenyl-1 <i>H</i> -benzo[ <i>d</i> ]imidazole: Fe ³⁺ -induced quenching and reversal by H ₃ PO ₄

1-benzyl-4-fluoro-2-phenyl-1H-benzo[d]imidazole (BFPBI) was prepared and its identity was confirmed by 1H and 13C NMR spectroscopy, Mass spectrometry and elemental analysis. A shift in the absorption bands of BFPBI was observed in different solvents. PL spectral study of BFPBI was conducted in various solvents; peak maxima shift to larger wavelengths when polarity is raised. BFPBI has the ability to function as a selective fluorescence sensor for the identification of the Fe3+ ion. Other tested ions, including Pb2+, Ni2+, Co2+, Cd2+, Zn2+, Mg2+and Ca2+ have not considerable influence on the fluorescence. Interestingly, fluorescence intensity was restored upon the addition of H3PO4, indicating a reversible fluorescence behavior. The fluorescence quenching is attributed to PET mechanism. The limit of detection for Fe3+ was determined to 5 × 10−5 M, confirming the high sensitivity. Other tested ions exhibited negligible interference, demonstrating high selectivity. A 1:1 stoichiometry was assumed, and the binding constant was calculated to 4.2 × 103 M−1 using the Benesi–Hildebrand model, indicating appreciable affinity for Fe3+. Investigation of the electric dipole moment and hyperpolarisability indicates that the imidazole derivative synthesized exhibits behavior of non-linear optics. This chromophore has a more suitable off-diagonal vs. diagonal tensorial component ratio that represents the anisotropy of in-plane non-linearity.