Using fluorescence to detect biologically relevant metals has been studied extensively due to its rapid and low detection limit ability. Sodium and potassium differentiation is significant in diagnosis of many medical conditions. For this, we designed coumarin dimers as flexible fluorescent probes using ethylene glycol units for differentiation of sodium and potassium. To our best knowledge, use of these easy-to-synthesize coumarin dimers linked through ethylene glycol units are first in the literature. In fluorescence titration experiments, diethylene glycol linked coumarin-3-carboxylate dimer is responsive for sodium ions but not for potassium ions. The driving force for the complexation of metal cation and fluorescence probes is thought to be size-matching. To further explain the phenomenon, we synthesized coumarin dimer using 1,8- octanediol as the linker, and methyl ester of coumarin-3-carboxylic acid to investigate the effect of structural changes on the fluorescence intensity. These two compounds could not differentiate the sodium and potassium. Flexible coumarin dimers as fluorophores are shown to be useful for sensing sodium cation in the presence of potassium cation.