Bisquinoline-based fluorescent zinc sensors

Abstract

Several bisquinoline derivatives, N,N'-bis(2-quinolylmethyl)-N,N'-dialkylethylnediamines (alkyl=methyl, ethyl, isopropyl and t-butyl), have been synthesized and their fluorescent responses toward zinc ion were investigated. These compounds exhibit zinc ion-induced fluorescence and their intensities decrease as the alkyl groups become larger. The t-butyl derivative (BQDtBEN) exhibited negligible fluorescence even in the presence of zinc ion. The fluorescence intensity of the zinc complex of the bisquinoline derivative (BQDMEN) is higher than that of TQEN (N,N,N',N'-tetrakis(2-quinolylmethyl)ethylenediamine), indicating that the TQEN-Zn complex has an intramolecular quenching mechanism due to the energy transfer among four quinoline rings and the remaining photoinduced electron transfer (PET) mechanism. Introduction of methoxy substituents into the quinoline ring shifted the excitation and emission wavelengths towards a lower-energy direction and increased the fluorescence intensity, which allows N,N'-bis(6-methoxy-2-quinolylmethyl)-N,N'-dimethylethylenediamine (6-MeOBQDMEN) to be used for cellular fluorescent microscopic analysis (lambdaex=331 nm, lambdaem=406 nm and phi=0.28 for 6-MeOBQDMEN-Zn complex).