Ketoprofen Recognition and Sensing by Zn(II) Complexes with Fluorogenic Triamine Receptors

Ligands L1 and L2 are composed, respectively, by a diethylenetriamine or a dipropylenetriamine moiety linked at their extremities to anthracene units through methylene bridges and form stable 1:1 complexes with Zn(II), in which the metal is coordinated by all three nitrogens of the ligands. Zn(II) binding by L1 leads to a marked enhancement of the fluorescence emission, thanks to the inhibition of the photoinduced electron transfer (PET) process from the benzylic amine groups of the triamine sub-unit to the excited fluorophore, which normally quenches the emission of fluorescent polyamine receptors. Conversely, the emission of L2 is somewhat quenched by Zn(II) binding likely due-as also indicated by ab initio calculations and molecular dynamics simulations-to the formation of cation π quenching contacts between the metal and the anthracene moieties that overcome the effects of PET inhibition. The Zn(II) complexes of both ligands are able to bind ketoprofen (KP) in its anionic form, thanks to the formation of COO^--Zn(II) coordinative bonds, to form [KPZnL]⁺ and [(KP)₂ZnL] (L = L1 or L2) ternary adducts. While KP binding to [ZnL2]²⁺ enhances the fluorophore emission, coordination of KP to [ZnL1]²⁺ slightly reduces the anthracene emission, due, once again, to the formation in the L1 ternary complexes of marked cation π contacts.