Effect of methoxy substituents on fluorescent Zn2+/Cd2+ selectivity of bisquinoline derivatives with a N,N′-dimethylalkanediamine skeleton

Abstract

BQDMEN (N,N′-bis(2-quinolylmethyl)-N,N′-dimethylethylenediamine) and its 6-methoxyquinoline derivative (6-MeOBQDMEN) are fluorescent Zn²⁺ sensors with minor response to Cd²⁺ (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 Cd²⁺ (I_Zn/I_Cd = 0.22 for TriMeOBQDMEN in the presence of 1 equiv. of metal ion). This reversal of Zn²⁺/Cd²⁺ 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 Cd²⁺ selectivity in TriMeOBQDMEN.