Bisquinoline-based fluorescent cadmium sensors.

Rational molecular design afforded fluorescent Cd ²⁺ sensors based on bisquinoline derivatives. Introduction of three methoxy groups at the 5,6,7-positions of the quinoline rings of BQDMEN ( N , N '-bis(2-quinolylmethyl)- N , N '-dimethylethylenediamine) resulted in the reversal of metal ion selectivity in fluorescence enhancement from zinc to cadmium. Introduction of bulky alkyl groups and an N , N -bis(2-quinolylmethyl)amine structure, as well as replacement of one of the two tertiary amine binding sites with an oxygen atom and the use of a 1,2-phenylene backbone significantly improved the Cd ²⁺ specificity. The fluorescent cadmium ion selectivity could be explained by the differential binding with Cd ²⁺ and Zn ²⁺ , and the formation of a bis(μ-chloro) dinuclear cadmium complex in contrast to the mononuclear zinc complex.