Synthesis of quinoline dicarboxylic esters as biocompatible fluorescent tags

A series of dicarboxylic quinoline derivatives bearing electron-releasing or -withdrawing substituents have been synthesized using mono- or/and biphasic methodologies. By controlling the regioselectivity of addition into our electrophilic intermediate, we also characterized by which mechanism the Doebner-Miller cyclization step occurred. As anticipated, electron-releasing substituents induce a red shift of the low-energy absorption allowing excitation in the visible region. In addition, by playing on the strength and position of the electron-releasing substituents, chromophore having interesting fluorescent properties such as large Stoke shifts, good fluorescent quantum yields, emission in the visible green-yellow region and reasonable two-photon absorption in the NIR region have been obtained. These small-size fluorophores, which can be made water-soluble and have been shown to be non-toxic, can be hetero- and/or polyfunctionalized and thus represent promising key units for fluorescence-based physiological experiments with low background interactions.