Novel optical chemosensors for anions and cations based on an amino acid core functionalised with benzimidazoles

A series of novel unnatural phenylalanine derivatives bearing a benzimidazole as coordinating/reporting unit, substituted with electron donor (methoxy) or acceptor (cyano and nitro) groups were synthesised, and their evaluation as colorimetric/fluorimetric chemosensors was carried out in acetonitrile solution. They were tested for the recognition of organic and inorganic anions, (such as Br−, F−, CN−, AcO−, BzO−, NO3−, ClO4−, HSO4−, H2PO4−, and OH−) and of alkaline, alkaline-earth, and transition metal cations, (such as Cu2+, Cd2+, Co2+, Ca2+, Na+, Cr3+, Zn2+, Hg2+, Fe2+, Fe3+, Ni2+, and Pd2+). The nature of the substituent at the benzimidazole allowed the fine tuning of the sensory properties and signaling mode, as seen by spectrophotometric and spectrofluorimetric titrations, which showed that derivatives 3a-c are fluorimetric chemosensors, with 3a being selective and highly sensitive for Pd2+ whereas 3b was also very sensitive to this cation but not selective, while derivative 3d bearing a nitro group at the benzimidazole behaves as a colorimetric chemosensor for OH−.