Cholic acid-based high sensitivity fluorescent sensor for α,ω-dicarboxylate: an intramolecular excimer emission quenched by complexation

Fluorescent receptors (1 and 2) bearing two binding units on C3 and C24 and two signal display units on C7 and C12 of cholic acid, respectively, were designed and synthesized. Both 1 and 2 emit a much weaker fluorescence than that of the control compound 3 lacking of the binding units reflecting that a PET process originated from the C-3 thiourea group to the plural pyrenyl pendant groups is operative. Addition of terminal dicarboxylate anions to the CH₃CN solution of 1 or 2 enhances the PET process, which leads to a significant and highly sensitive fluorescence response, resulting in an almost complete quenching of the excimer emission of the signal units. To maximize the interaction of the host and the guest, carboxylates of more than five carbon chains could penetrate through the space between the two pyrenyl pendants of the host, triggering a considerable conformational change of the fluorophores. As a result, an enhancement of the monomer emission at the expense of the excimer emission will take place. The binding properties and mechanism of 1 and 2 to dicarboxylates in CH₃CN were manifested by the combined fluorescence, UV-vis, and ¹H NMR spectroscopic method.