p-Dimethylaminobenzamide as an ICT dual fluorescent neutral receptor for anions under proton coupled electron transfer sensing mechanism

The intramolecular charge transfer (ICT) dual fluorescence of p-dimethylaminobenzamide (DMABA) in acetonitrile was found to show highly sensitive response to HSO4− over several other anions such as H2PO4−,AcO− and ClO4−. In the presence of bisulfate anion the dual fluorescence intensity ratio and the total intensity of DMABA decreased while the dual emission band positions remained unchanged. Absorption titration indicated that a 1:1 hydrogen bonding complex was formed between bisulfate anion and DMABA, which gave a binding constant of 2.02×104 mol−1 l that is two orders of magnitude higher than those for other anions. The obvious isotopic effect observed in the fluorescence quenching [KSV(HSO4−)/KSV( DSO4−)=1.63] suggests that the hydrogen atom moving is an important reaction coordinate. It was assumed that the dual fluorescence response was due to proton coupled electron transfer mediated by hydrogen bonds within the 1:1 HSO4−-DMABA hydrogen-bonding complex.