Composition, stability and fluorescence properties of metal complexes of an aza-flavonol analog 1-methyl-2-phenyl-3-hydroxy-4(1H)-quinolone in aqueous solution

Aza-flavonols are compounds containing the 3-hydroxy-4(1H)-quinolone moiety, a unique scaffold with applications expanding from fluorescent probes and bioimaging to potential metal-related pharmaceutical drugs. Compositions and stability constants of metal complexes of an aza-flavonol 1-methy1-2-pheny1-3-hydroxy-4(1H)quinolone (LH) in aqueous micellar medium of hexadecyltrimethylammonium bromide were established by fluorescence and spectrophotometric titration of LH in the presence of metal ions of different type at variable pH. Strong binding with quenching of ligand fluorescence was observed for Cu2+, Fe2+/(3+) and Pb2+ cations. The complexation with Zn2+ induced strong fluorescence enhancement in ZnL+ complex and smaller enhancement in ZnL2 complex. Al3+, Ga3+ and Me2Sn2+ cations induced strong fluorescence enhancement in 1:1 ML complexes, but quenching in ML2 complexes. Together with the relevant literature data these results indicate that the quenching effect in 1:2 complexes along with strong fluorescence enhancement in respective 1:1 complexes is a general phenomenon for 3-hydroxypyranone and 3-hydroxypyridinone ligands. The quantum mechanical Natural Transition Orbital analysis of ground and excited states of LH, AlL2+ and AlL2+ species reveals that the fluorescence of AlL2+ complex is quenched by an internal electron transfer process, which is absent in the 1:1 AlL2 complex. The results emphasize strong dependence of the coordination induced fluorescence changes on the complex composition.