Synthesis, crystal structure and optical properties of 2,4-diaminotoluene based Schiff Base: experimental and theoretical approaches

A Schiff base crystal [2,2′-((1E,1′E)-((4-methyl-1,3-phenylene) bis(azanylylidene))bis(methanylylidene))diphenol] based on 2,4-diaminotoluene has been synthesized and characterized by various analytical and spectroscopic methods. The synthesized Schiff base has been used as an excellent fluorescent sensor for Al³⁺ ions at the nanomolar level (5.68 × 10⁻⁹ M). The selective and potent metal-ligand interaction between Al³⁺ ions and the synthesized non-fluorescent Schiff base leads to a 1:2 adduct, which induces fluorescence-on (fluorescence quantum yield 22.4 %) through the chelation-enhanced fluorescence (CHEF) mechanism. Job's plot, ESI-MS, fluorescence titration, FTIR, ¹H NMR titration, and XPS analysis confirm the complex formation. The metal-ligand interaction has been interpreted theoretically by density functional theory (DFT) using Gaussian 09 software. The fluorescence-off stage of the metal-Schiff base adduct occurs through quenching by EDTA (both static and dynamic). A molecular logic gate has been constructed based on the observed off-on-off photoluminescence property of the Schiff base sensor in the presence of Al³⁺ ion and EDTA chelating ligand.