1-Aminoanthracene-9,10-dione based chromogenic molecular sensors: effect of nature and number of nitrogen atoms on metal ion sensing behavior

To examine the consequences of nature and number of nitrogen atoms on metal ion sensing properties, four new molecular receptors based on 1-aminoanthracene-9,10-dione as chromogenic moiety and different types of nitrogen atoms viz. arylamine, alkylamine, and pyridyl nitrogen as appendages have been synthesized. These receptors in CH₃OH/H₂O (1:1) (v/v) at pH 7.0, on addition of heavy metal ions show selective and/or semi-selective interactions. These binding interactions are visible to naked eye due to remarkable color change and are associated with λ_max shift by 85-125 nm. Molecular receptor 2, with two sp² hybridized nitrogen atoms and one arylamine nitrogen, selectively binds with Cu²⁺ but 2-Cu²⁺ complex is stable only between pH 7.0 and 8.75. However, the conversion of imine nitrogen to alkylamine in molecular receptor 6, increases the binding ability toward Cu²⁺ along with significant binding affinities toward Ni²⁺ and Co²⁺. Receptor 6 shows the stability of its complexes in the order Cu²⁺>Ni²⁺>Co²⁺ in a broader pH range 6-12. Dipicolylamine based receptor 8, possessing two pyridyl nitrogen atoms, one tertiaryamine and one arylamine nitrogen atoms as ligating sites, also binds semiselectively in the order Cu²⁺>Co²⁺>Ni²⁺. Receptor 10, possessing anilide group in the place of arylamine in receptor 8, on addition of Cu²⁺, Ni²⁺ or Co²⁺ shows bathochromic shift of λ_max associated with color change from yellow to russet (brown) and on addition of Zn²⁺ shows hypsochromic shift of its λ_max associated with disappearance of yellow color. Additionally, all the four chemosensors show ratiometric response toward all these metal ions and thus increase the usability and the dynamic range of estimation.