Rational design of novel benzimidazole-based sensor molecules that display positive and negative fluorescence responses to anions.

Two novel and neutral benzimidazole derivatives-based anion receptors bearing a 1,10-phenanthroline fluorophore, N,N'-di-(2'-benzimidazolyl-methylene)-1, 10-phenanthroline-2,9-diamide (1) and N,N'-di-[2'-(benzimidazolyl-2'-) ethyl-]-1,10-phenanthroline-2,9-diamide (2), which exhibited turn-on and turn-off fluorescence responses to various anions, were rationally designed and synthesized and their fluorescent response toward anions was investigated in DMSO solution. In the process of anions binding, there were two different fluorescent responses in presence of anions: a quenching of the fluorescence emission for F- and AcO- and an enhancement of the fluorescence emission for Cl-, Br- and I-. Two different luminescent mechanisms of the receptors 1 and 2 resulting from various anions were exploited to rationalize quenching and enhancement of the fluorescence emission: a photo-induced electronic transfer mechanism (PET) and the increase of the rigidity of the host molecules, respectively. In particular, chloride could be recognized selectively from the anions tested according to changes of fluorescence spectrum.