A bis-benzimidazole-derived N, S macrocycle as sensor for transition metal ions in aqueous solution

A fluorescent molecule (φ=0.32) comprising two benzimidazole fluorophores condensed with a N, S macrocycle with sufficient solubility in water (10−4 M) has been studied as sensor for a series of metal ion aqueous solutions (including Cr3+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Hg2+ and Pb2+). The complex formation constant for dipositive cations in water is low (∼0.2 M−1), but it is remarkably larger for tripositive cations, particularly for Cr3+ (100.2 M−1). This indicates that the main factor controlling complexation is the Lewis acidity of the cation and its interaction with hard N atoms. The emission is quenched by the presence of the above metal ions at a diffusion control quenching rate.