Carboxylic Ester Amidation under Mild Conditions and Its Application in Spectral Sensing

In this work, a series of carboxylic esters bearing strong electron-withdrawing groups (e.g., -NO₂) on the aromatic ring were designed. These activated esters undergo amidation with organic amines in aprotic solvents (e.g., acetonitrile) under mild conditions (room temperature, ambient pressure, and without any exogenous catalysts), a combination rarely achieved in conventional ester amidation protocols. By incorporating chromophores (e.g., nitrophenol) or fluorophores (e.g., 2-naphthol) onto the ester structures, the amidation processes can be conveniently monitored by UV-vis absorption or fluorescence spectroscopy. Notably, these esters, such as ester 1 and ester 6, exhibited remarkably high selectivity in their spectral responses toward tris-(2-aminoethyl) amine (TAEA), which is attributed to the unique molecular structure of TAEA (possessing three -NH₂ groups), enabling synergistic interactions of these -NH₂ groups with the carbonyl group of the esters. Therefore, using UV-vis absorption spectra-based ester 1 and fluorescence spectra-based ester 6, the limits of detection (LOD) for TAEA were determined to be 1.10 and 0.19 μM, respectively. This work provides a proof-of-concept molecular spectral sensing system based on ester amidation, operating under mild, exogenous catalyst-free, homogeneous solution conditions with high selectivity and sensitivity.