Effect of methoxy substituents on fluorescent Zn 2+ /Cd 2+ selectivity of bisquinoline derivatives with a N , N '-dimethylalkanediamine skeleton.

BQDMEN ( N , N '-bis(2-quinolylmethyl)- N , N '-dimethylethylenediamine) and its 6-methoxyquinoline derivative (6-MeOBQDMEN) are fluorescent Zn 2+ sensors with minor response to Cd 2+ ( I Zn / I Cd = 3.9 for BQDMEN and I Zn / I Cd = 2.2 for 6-MeOBQDMEN in the presence of 1 equiv. of metal ion). Nevertheless, the introduction of three methoxy substituents at the 5,6,7-position of both quinoline rings of BQDMEN reversed the fluorescent metal ion selectivity to favor Cd 2+ ( I Zn / I Cd = 0.22 for TriMeOBQDMEN in the presence of 1 equiv. of metal ion). This reversal of Zn 2+ /Cd 2+ preference in fluorescence enhancement by trimethoxy substitution was also valid for 1,3-propanediamine derivatives. The X-ray crystallography, ESI-MS, fluorescence lifetime and pH profile of the fluorescence intensity suggest that the dinuclear cadmium complex is a key component of the fluorescent Cd 2+ selectivity in TriMeOBQDMEN.