Tuning the limits of pH interference of a rhodamine ion sensor by introducing catechol and 3-hydroxy-4-pyridinone chelating units

A previously reported fluorescein-based dye containing a catechol unit showed high sensitivity to Fe(III) at physiological pH, but also undesirable pH sensitivity, mainly due to the lower pKa values of the phenolic hydroxyl groups of the fluorescein core. Aiming to synthesize compounds that interact with metal ions at physiological pH, exhibiting no pH sensitivity, we designed two novel fluorescent compounds by assembling, through amide linkage, a rhodamine fluorophore with two selected chelating moieties, catechol and 3-hydroxy-4-pyridinone, both possessing high affinity for Fe(III) but distinct pKa values. The fluorescent compounds were prepared using straightforward synthetic protocols and characterized by NMR, mass spectrometry and electronic spectroscopy (UV-Vis and fluorescence) and one of the compounds was also characterized by single crystal X-ray diffraction. The results reveal that, while the fluorescent 3-hydroxy-4-pyridinone derivative exhibits a significant dependence of fluorescence emission with increasing pH, due to the pKa values arising from the 3-hydroxy-4-pyridinone residue, the fluorescent catechol ligand responds mainly to interactions with metal ions, preferentially to Fe(III), showing less sensitivity to pH than the related ligands over the pH range 3-8, fact that is relevant concerning the application of this compound as an ion sensor in biological media.