Effect of Substitution at Amine Functionality of 2,6-Diaminopyridine-Coupled Rhodamine on Metal-Ion Interaction and Self-Assembly

2,6-Diaminopyridine-coupled rhodamines 1 and 2 have been synthesized, and the effect of substitution on amine functionality toward metal-ion interactions and self-assembly is thoroughly investigated. Both the compounds effectively recognize different metal ions of biological significance fluorimetrically and colorimetrically with a high degree of selectivity and sensitivities. While compound 1 is sensitive to Fe³⁺ ions, compound 2 is responsive to both Fe³⁺ and Al³⁺ ions in aqueous CH₃CN (4/1, v/v; 10 mM tris HCl buffer, pH 6.8). The sensing mechanism involves the metal-ion chelation-induced spirolactam ring opening of the rhodamine scaffold that results in both color and fluorescence changes, while the extent of interactions with the metal ions is truly governed by the chemical structure of the compounds. Both 1 and 2 are proficient in detecting Fe³⁺ and Al³⁺ ions in human lung cancer cells (A549). As new findings, unlike 1, compound 2 formed a faint pink gel in the toluene-hexane mixture solvent (1:1, v/v), and the gel state of 2 selectively recognizes Ag⁺ ions by exhibiting a phase change from gel to purple sol. Experimental findings establish the role of the formamide moiety in forming the self-assembly.