Ratiometric fluorescent receptors for monosaccharides based on the intramolecular charge transfer in p-dialkylaminobenzanilides

Two dual fluorescent receptors (1 and 2) for monosaccharides based on 4-dialkylaminobenzanilides (alkyl= methyl and n-butyl) containing boronic acid group at the amido aniline were synthesized and their spectral properties were investigated. These receptors exhibited dual fluorescence with the long-wavelength band displaying strong solvent-polarity dependence, indicating the occurrence of the excited-state intramolecular charge transfer (ICT). With increasing pH value in aqueous solutions, the hybridization of the boron atom changed from sp(2) to spa(3) inducing a decrease in the total fluorescence quantum yield. The experimental results indicated that the anionic form of the boronate group acted as an electron donor and the benzanilide-like charge transfer was promoted upon hybridization change. In the presence of monosaccharides, the boronic acid in 1 and 2 changed from neutral to anionic form. The intensity of the locally excited (LE) state emission decreased in the presence of sugars while a slight increase in the intensity at the charge transfer (CT) emission occurred. Based on the change in the CT to LE intensity ratios of 1 and 2 due to sugar binding, ratiometric fluorescent assays for monosaccharide sensing were established.