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

The development of fluorescent Cd 2+ sensors requires strict selectivity over Zn 2+ because of the high availability of Zn 2+ 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 Cd 2+ selectivity rather than Zn 2+ . 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 Cd 2+ -selective fluorescence enhancement over Zn 2+ in DMF-HEPES buffer (1:1, 50 mM HEPES, 0.1 M KCl, pH = 7.5) ( I Zn / I Cd = 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 ( I Zn / I Cd = 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 Cd 2+ complex with TriMeO- N,O -BQMAE ( 3 ) exhibits a long fluorescence lifetime similar to that of TriMeOBQDMEN via intramolecular excimer emission, whereas the Cd 2+ complex with TriMeO- N,N -BQMAE ( 6 ) exhibits a short lifetime from monomer emission.