Novel functionalised imidazo-benzocrown ethers bearing a thiophene spacer as fluorimetric chemosensors for metal ion detection

Abstract

Novel phenylalanine derivatives bearing benzimidazole and crown ethers as coordinating/reporting units and thiophene as a spacer unit were synthesized for the first time, and their evaluation as fluorimetric chemosensors was carried out in acetonitrile and acetonitrile/water solutions. 15-Crown-5 benzimidazolyl phenylalanine methyl ester, 15-crown-5 thienylbenzimidazolyl phenylalanine methyl ester and 18-crown-6 thienylbenzimidazolyl phenylalanine methyl ester were tested for alkaline, alkaline-earth and transition metal ions (such as Na+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Fe3+, Hg2+, Ni2+, Pd2+ and Zn2+). The different crown ether binding moieties as well as the electronic nature and the length of the π-bridge linked to the benzimidazole heterocycle allowed the fine tuning of the sensory properties as seen by spectrofluorimetric titrations. Therefore, 15-crown-5 benzimidazolyl phenylalanine methyl ester is a fluorimetric chemosensor, being selective and sensitive for Cu2+ and Pd2+ in aqueous solutions (ACN/H2O; 80:20). On the other hand, the metal cation sensing properties displayed by 15-crown-5 thienylbenzimidazolyl phenylalanine methyl ester bearing an arylthienyl spacer showed that this is a promising candidate as fluorimetric chemosensor for Fe3+, Pb2+ and Pd2+ in acetonitrile solution.