Chelate-free "turn-on"-type fluorescence detection of trivalent metal ions.

Trivalent metal ions are essential elements in materials and life sciences. To date, fluorescence is the gold standard for the detection of trivalent metal ions, which form a complex with fluorescent chelate ligands (Lewis base type). In this study, we report pH-responsive fluorescent probes for the selective detection of trivalent metal ions that utilize a new chelate-free approach using Brønsted base-type probes. Moreover, an X-ray structure of the outer-sphere aluminum(III) aqua complex coordinated by the fluorescent probes through hydrogen bonds was discovered. The outer-sphere complex consisting of an Al(III) aqua complex and protonated cationic probe showed cation-cation coordination.