Intramolecular charge transfer dual fluorescent sensors from 4-(dialkylamino)benzanilides with metal binding site within electron acceptor

Abstract

Three fluoroionophores ( 2a– c) were designed as the intramolecular charge transfer (CT) dual fluorescent sensors for metal cations with metal binding site within the electron acceptor. These sensors were derived from 4-dialkylaminobenzanilides (alkyl=methyl, ethyl, and n-butyl) with the amido phenyl ring being an arm of 15-crown-5 thus bearing binding site for alkaline and alkaline earth metal cations. Compounds 2a– c were expected to have two possible CT channels of opposite direction. The absorption and fluorescence spectra of 2a– c and their crown-ether free model molecules 3a– c in a variety of solvents were recorded. Dual fluorescence was observed with 2a– c and was assigned to the LE and the CT states, respectively. In nonpolar or less polar solvents the CT occurring with 2a– c was identified as that occurred with benzanilides (BA) with the amido anilines being the electron donor (the BA-like CT), while in polar solvents such as acetonitrile (ACN), the CT was still mainly the BA-like. In the presence of alkali and alkaline earth metal cations in ACN, the CT dual fluorescence underwent substantial changes so as increased total quantum yield, red-shifted LE band and enhanced CT to LE intensity ratio. Binding of the metal cations at the 15-crown-5 moiety of 2a– c was shown to turn the CT direction that the dialkylamino group in the binding complexes being the electron donor while the benzo-15-crown-5 moiety now being within the electron acceptor. The occurrence of this CT enhances metal cation binding to 15-crown-5 ether in 2a– c, which was confirmed by the observed higher metal binding constants. Compounds 2a– c as the CT dual fluorescent sensors were shown to operate under the mechanism of the metal cation binding induced switching of the CT character from the BA-like to that occurred with 4-(dimethylamino)benzamides (the DMABA-like). Compounds 2a– c therefore represent successful examples for the CT dual fluorescent sensors for cations with the metal binding site within the electron acceptor and can be employed as sensitive ratiometric fluorescent sensors for metal cations of improved sensing performance.