Highly sensitive and selective colorimetric and off-on fluorescent reversible chemosensors for Al³⁺ based on the rhodamine fluorophore.

A series of rhodamine derivatives L1-L3 have been prepared and characterized by IR, 1H-NMR, 13C-NMR and ESI-MS. These compounds exhibited selective and sensitive "turn-on" fluorescent and colorimetric responses to Al3+ in methanol. Upon the addition of Al(III), the spiro ring was opened and a metal-probe complex was formed in a 1:1 stoichiometry, as was further confirmed by ESI-MS spectroscopy. The chemo-dosimeters L1-L3 exhibited good binding constants and low detection limits towards Al(III). We also successfully demonstrate the reversibility of the metal to ligand complexation (opened ring to spirolactam ring).