Synthesis of Bisimidazole Derivatives for Selective Sensing of Fluoride Ion.

Rapid and efficient analysis of fluoride ion is crucial to providing key information for fluoride ion hazard assessment and pollution management. In this study, we synthesized one symmetrical structure called 1,4-bis(4,5-diphenyl-1-imidazol-2-yl)benzene () and two asymmetrical structures, namely 2-(4-(4,5-diphenyl-1-imidazol-2-yl)phenyl)-1-phenanthro(9,10-)imidazole () and 2-(4-(4,5-diphenyl-1-imidazol-2-yl)phenyl)-1-imidazo(4,5-)(1,10)phenanthroline (), which served as an efficient anion sensor for fluoride ion over a wide range of other anions (Cl, Br, I, NO₃, ClO₄, HSO₄, BF₄, and PF₆) owing to imidazole group in the main backbone. The absorption intensity of compound at λ 358 nm slightly decreased; however, a new band at λ 414 nm appeared upon the addition of fluoride ion, while no evident change occurred upon the addition of eight other anions. The photoluminescence intensity of compound at λ 426 nm was nearly quenched and fluorescence emission spectra were broadened when fluoride ion was added into dimethyl sulfoxide (DMSO) solution of compound . Compared with the optical behaviors of the DMSO solution of compound in the presence of Bu₄N⁺F, compounds and exhibited considerable sensitivity to fluoride ion due to the increase in coplanarity. Furthermore, compared with the fluorescence emission behaviors of the DMSO solutions of compounds and in the presence of Bu₄N⁺F, compound exhibited the most significant sensitivity to fluoride ion due to the charge transfer enhancement. Consequently, the detection limits of compounds - increased from 5.47 × 10 M to 4.21 × 10 M to 9.12 × 10 M. Furthermore, the largest red shift (75 nm) of the DMSO solution compound in the presence of fluoride ion can be observed. Our results suggest that the increase in coplanarity and the introduction of electron-withdrawing groups to the imidazole backbone can improve the performance in detecting fluoride ion.