Cd2+-Selective Fluorescence Enhancement of Bisquinoline Derivatives with 2-Aminoethanol Skeleton

Abstract

The development of fluorescent Cd2+ sensors requires strict selectivity over Zn2+ because of the high availability of Zn2+ in the natural environment. In this paper, bisquinoline-based fluorescent sensors with a 2-aminoethanol backbone were investigated. The weak coordination ability of quinoline compared to well-studied pyridine is suitable for Cd2+ selectivity rather than Zn2+. In the presence of 3 equiv. of metal ions, TriMeO-N,O-BQMAE (N,O-bis(5,6,7-trimethoxy-2-quinolylmethyl)-2-methylaminoethanol (3)), as well as its N,N-isomer TriMeO-N,N-BQMAE (N,N-bis(5,6,7-trimethoxy-2-quinolylmethyl)-2-methoxyethylamine (6)), exhibits Cd2+-selective fluorescence enhancement over Zn2+ in DMF-HEPES buffer (1:1, 50 mM HEPES, 0.1 M KCl, pH = 7.5) (IZn/ICd = 26–34%), which has similar selectivity in comparison to the corresponding ethylenediamine derivative TriMeOBQDMEN (N,N’-bis(5,6,7-trimethoxy-2-quinolylmethyl)-N,N’-dimethylethylenediamine) under the same experimental condition (IZn/ICd = 24%). The fluorescence mechanisms of N,O- and N,N-isomers of BQMAE are quite different, judging from the fluorescence lifetimes of their metal complexes. The Cd2+ complex with TriMeO-N,O-BQMAE (3) exhibits a long fluorescence lifetime similar to that of TriMeOBQDMEN via intramolecular excimer emission, whereas the Cd2+ complex with TriMeO-N,N-BQMAE (6) exhibits a short lifetime from monomer emission.