Interaction of lanthanide β-diketonate complexes with polyvinylpyrrolidone: proton-controlled switching of Tb3+...

In the present study the interaction of lanthanide β-diketonate complexes with polyvinylpyrrolidone (PVP) has been systematically investigated using fluorescence spectroscopy. The emission of terbium complexes with hexafluoroacetylacetone (hfa) ligands is almost completely quenched by PVP in the absence of H(+) ions, while it is enhanced by more than 30 times in the presence of H(+) ions. Conversely, the H(+) ions increase the quenching efficiency of terbium complex with acetylacetone ligands. The strikingly different behavior of complexes with fluorinated and nonfluorinated β-diketonates is accounted for by the interaction mechanism, in which the important role of hydrogen bonds is indicated. The proposed mechanism includes ion-dipol Coulombic interactions between negative charge localized on ligands O-C-CH2-C-O group and positive charge induced in N atom of PVP pyrrolidone group, as well as interactions through hydrogen bonds formed between ligand C-O groups and PVP, occurring directly or through solvent molecules. Additional stabilizing effect, significantly influencing the binding strength of ligands with PVP, results from hydrogen bonds formed by terminal (CF3 or CH3) substituents of ligands with C-O group of PVP. Importantly, the quenching and enhancement of luminescence of terbium complexes with hfa ligands in PVP solution is a reversible process. This enables one to obtain the emission switch-OFF-ON system triggered externally by H(+) ions, which can find possible application in the development of molecule-based devices.