Design, synthesis and characterization of polyamine-based fluorescent chemosensors for selective recognition of metal cations in solution

This project describes the design, synthesis, photochemical characterization, coordination properties and X- ray crystal structures of some fluorescent polyamino-ligands, four of which (L1-L4) were found to have interesting photochemical and coordination properties. Each structure contains one or two fluorophores linked to a polyamine building block with open-chain or cyclic topology and is able to signal the presence of a specific metal ion in solution by changing its emission properties. L1 contains two hydroxychromone units linked as side arms to an ethylenediamine moiety, this ligand can be pre-organized by the coordination of a Pd(II) ion giving rise to the [Pd(H -2 L1)] species in which the four oxygen atoms bearing to the two hydroxychromone units form a negatively charged pocket able to interact with hard metal cations, such as rare earth ions. This system can be used as a selective fluorescent probe to detect the presence of Gadolinium in environmental water, due to a chelation enhancement of the fluorescence effect (CHEF) upon the coordination of Gd(III). L2 is an open-chain ligand containing two HNBO fluorophores (hydroxynaphthylbenzoxazol) linked to a dimethylethylenediamine fragment. In DMSO solution this ligand absorbs at 450 nm and is weakly fluorescent. Studies performed with Alkali and Alkaline Earth ions showed that the emission at 530 nm dramatically increases only in the presence of Mg(II) ions. Competition experiments demonstrate that the presence of Li(I), Na(I), K(I), Ca(II), Sr(II) and Ba(II) do not prevent the ability of the sensor to signal the presence of Mg(II), thus it can be used to sense this ion also in real samples such as commercial drinkable water. The presence of Mg(II) can be qualitatively appreciated with the naked eye as a yellow emission by lighting the sample with a 360 nm UV lamp. L3 is a polyamine cyclophane macrocycle in which 2,6-bis(5-(2- methylphenyl)-1,3,4-oxadiazol-2-yl)pyridine (POXAPy) acts as a fluorescent sensor and the polyamine as a metal ion binding unit. L3 performs as a PET-mediated chemosensor, with a maximum emission wavelength close to 360 nm. This gives rise to a signal that is visible to the naked eye in the blue visible range. L3 is able to detect Zn(II) and Cd(II) metal ions in an aqueous solution at pH = 7, with the coordination of the ions switching ON the emission through a CHEF effect. In contrast, paramagnetic metal ions like Cu(II) and Ni(II) completely quench the already low emission of L3 at this pH value. L4 contains the 2,6-bis(benzoxazol-2-yl)phenol fluorophore (BPhB) and shows peculiar double emission properties. The ligand is able to coordinate a Zn(II) ion in aqueous environment with concomitant keto-enol tautomerism giving rise to large Stokes shifts. Upon excitation at 342 nm the protonated form of the ligand shows a double emission at 380 nm and 507 nm, due to Excited State Intramolecular Proton Transfer (ESIPT), while in the deprotonated form only an emission band at 475 nm is observed. At pH = 7 both protonated and deprotonated forms are present in solution, thus by exciting at 342 nm an intense emission at 507 nm can be observed, while by exciting at 405 nm the emission band is centered at 476 nm. At pH = 7, the addition of selected transition metal ions causes the quenching of the keto band at 507 nm and the formation of a new band relative to the deprotonated form at 458 nm for Zn(II), Cd(II) and Pb(II). Zn(II) and Cd(II) ions give rise to highly fluorescent complexes while the other tested metal ions (Ni((II), Cu(II), Hg(II) and Pb(II)) form weakly fluorescent species. Under a UV lamp at 360 nm a solution of L4 at pH = 7 appears green. Due to the ratiometric nature of the signal the presence of Zn(II) and Cd(II) ions can be detected through a clear change of color from green to blue.