Chromogenic recognition of Cr(III) and Cu(II) using a novel Schiff base and “turn-on” fluorescent detection of CO32− by its reduced scaffold in aqueous media

Abstract

An upsurge of severe toxic inorganic ionic species in water arising due to anthropogenic activities, has caused global concern and detection of such harmful contaminants is a matter of supreme importance. In this regard, a novel trans-1,2-diaminocyclohexane containing Schiff base N,N′-di(4-acetamidophenylmethylene)-1,2-cyclohexanediamine (1) and its reduced scaffold N,N’-di(4-acetamidophenylmethyl)-1,2-cyclohexanediamine (2) have been synthesized and characterized by FT-IR and 1H & 13C NMR while the structures of both 1 and 2 have been determined by X-ray single crystal diffraction analysis. The detection response of 1 and 2 have been investigated toward same set of analytes and found to be entirely different both in absorption and emission studies. Compound 1 selectively and very sensitively detects Cr3+ and Cu2+via hydrolytic rupture of >C = N- bond as indicated by ratiometric change and emergence of an isosbestic point in UV–vis analysis. Further, the kinetic studies clearly suggested rapid hydrolysis of 1 (< 3.0 mins) in the presence of Cr3+ whereas it requires ∼1.0 h for complete hydrolysis with Cu2+. In contrast, the recognition of Cr3+ and Cu2+ not be observed through fluorescence technique. Moreover, 2 remains silent toward Cr3+/Cu2+in UV–vis analysis, however, it selectively recognizes CO32− through ‘turn-on’ fluorescence output in the background presence of several tested cations and anions. Job's plot and mass analysis suggested 1:2 (CO32−: 2) binding stoichiometry between CO32− and 2. Remarkably, the present findings demonstrate entirely different functionality of a Schiff base and its reduced form in presence of same analytes. The main objective of this report is to illustrate that how a subtle structural change of a molecule governs the analyte detection ability.