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.