Derivatization of 4-(Dimethylamino)benzamide to Dual Fluorescent Ionophores: Divergent Spectroscopic Effects Dependent on N or O Amide Chelation

• Published in: Helvetica chimica acta 2001-09, Vol.84 (9), p.2439-2451
• Main Authors: Malval, Jean-Pierre, Lapouyade, René
• Format: Article
• Language: English
• ISSN: 0018-019X; 1522-2675
• DOI: 10.1002/1522-2675(20010919)84:9<2439::AID-HLCA2439>3.0.CO;2-S

Starting from the pentafluorophenyl ester of 4‐(dimethylamino)benzoic acid, two dual fluorescent amide ligands with aza‐15‐crown‐5 and 2‐(aminomethyl)pyridine were obtained for sensing, respectively, alkali (alkaline‐earth) and transition (heavy) metal cations. The crystal structure of the copper(II) complex is reported. The Cu2+ is coordinated through the pyridine N‐ and amide O‐atoms of two symmetry‐related ligands. The azacrown‐directed Ca‐chelation to the N‐atom of the amide leads to a slight quenching of the two fluorescence bands. In contrast, the pyridine directed CuII‐chelation to the O‐atom of the amide enhances the short‐wavelength emission 17‐fold over the locally excited state (LE), quenching the twisted intramolecular charge‐transfer (TICT) emission, and, as a result, the intensity ratio I(LE)/I(TICT) provides an accurate and sensitive measurement of the CuII concentration. These different cation effects are dependent on which atom (N vs. O) of the amide function participates in cation coordination: while the Ca2+ interaction with the N‐atom electron pair leads to the deconjugation of the amide N‐atom from the fluorophore, Cu2+ interaction with the lone pair of the O‐atom of the carbonyl group increases the energy of the n‐π* but also of the 1La transition and therefore close the channel to the TICT state.