Establishing the anion recognition correlation of the 2-(2-methoxyphenyl)-1H-imidazo [4, 5-f][1,10] phenanthroline and its Ru(bipy)₂²⁺ complex via fluorimetry

2-(2-methoxyphenyl)-1H-imidazo [4, 5-f][1, 10] phenanthroline (1) and its Ru(bipy)₂²⁺ complex (2) were synthesized and the anion sensing properties of the two were well evaluated and compared by fluorescence titration experiments. Both 1 and 2 were fluorescent in DMSO solution and possessed an anion binding imidazole N–H site to interact with the anions. 1 did not exhibit any fluorescent changes with even most basic anions in DMSO solution but on introduction of Ru metal center via synthesis of its Ru(bipy)₂²⁺ complex (2), the N–H acidity enhanced appreciably and it could detect acetate even at very low concentrations (LOD=5.25×10⁻⁷). The fluorescence intensity of 2 was almost completely quenched and its fluorescent red orange color disappeared with the addition of acetate ions. The spatial arrangement of receptor 2 fits well for the triangular shape of AcO⁻ ion, hence making it the most preferred anion for binding. Both ¹H NMR titrations and DFT computational calculations supported the deprotonation mechanism.