Discriminative interaction of a 4-methyl-2,6-diformylphenol based molecule with various metal ions

We report here discriminative interaction of 3-(((6-chlorobenzo[d]thiazol-2-yl)imino)methyl)-2-hydroxy-5-methylbenzaldehyde (HL) with Zn²⁺, Cu²⁺ and Ni²⁺. HL was synthesized by a reaction between 4-methyl-2,6-diformyl phenol and 6-chlorobenzo[d]thiazol-2-amine in 1:1 ratio. It showed moderate fluorescence intensity at 530 nm in 10 mM HEPES buffer in water/DMF = 4:1 (v/v) (pH 6.0) upon excitation at 420 nm. On addition of Zn²⁺, its fluorescence maximum shifted to 480 nm and then its intensity gradually enhanced at 480 nm signifying ratiometric detection of the metal ion. In the presence of Cu²⁺ and Ni²⁺, its fluorescence intensity at 530 nm quenched severely. Other metal ions could not alter the fluorescence of HL. Quantum yield of HL changed from 0.166 to 0.299, 0.059 and 0.010 in the presence of Zn²⁺, Cu²⁺ and Ni²⁺, respectively. It formed 1:1 complexes with each of these cations with an association constant of 3.30 × 10³, 9.00 × 10⁴ and 9.70 × 10⁴ M⁻¹ with Zn²⁺, Cu²⁺ and Ni²⁺, respectively. The values of limit of detection were determined to be 3.44 × 10⁻⁸, 3.81 × 10⁻⁸ and 3.30 × 10⁻⁸ M towards Zn²⁺, Cu²⁺ and Ni²⁺, respectively indicating impressive sensitivity. Binding of Zn²⁺ to HL results in the restriction of CN isomerization and CHEF mechanism leading to the high fluorescence. Due to the paramagnetic nature of the metal center, fluorescence intensity of HL decreases with Cu²⁺ and Ni²⁺. HL was applied in imaging these metal ions in living HeLa cells.