Experimental and theoretical exploration of sensing and magnetic properties of a triply bridged dicopper(II) complex: The first discrete metal complex to sense picric acid in pure water

Abstract

This article reports the syntheses, crystal structures, fluorescence spectra, picric acid (PA) sensing and magnetic properties of a μ-phenoxo-bis(μ-chloro) dicopper(II) compound of composition [Cu2L(μ-Cl)2]Cl·4.5H2O (1), where HL is the [1 + 2] condensation product of 2,6-diformyl-4-ethylphenol and 2-(2-aminoethyl)pyridine. Compound 1 is water soluble and exhibits fluorescence behaviour in pure water. Interestingly among the various aromatic, non aromatic nitro compounds as well as non nitro aromatic compounds screened, PA specifically quenches the fluorescence intencity of 1 significantly in water, revealing that 1 is a turn-off fluorosensor for PA. The sensing pathways have been investigated by spectral titration, time resolved fluorescence decay and DFT studies. Spectral and theoretical (DFT) studies suggest that the observed fluorescence quenching is associated with ground state (GS) charge transfer as well as electrostatic interactions between 1 and picrate ion. Unaltered fluorescence life time of 1 in the absence and presence of PA strongly suggests that quenching follows a static mechanism. Nature of binding modes or interactions between the complex 1 and PA was disclosed by the single crystal structure analysis of the corresponding association complex of composition [CuII2L(μ-Cl)(H2O)2]2Picrate·DMF (2). The quenching constant (Ksv), association constant (Ka) and detection limit (LOD) of the complex 1 for picric acid was found to be 1.1 × 105 M‒1, 4.64 × 1010 M‒2 and 4.8 μM respectively. Variable-temperature (2.5‒300 K) magnetic susceptibility measurement of the complex 1 reveals antiferromagnetic exchange interaction with J = ‒ 132.40 cm‒1. The magnetic property has been nicely rationalized in terms of spin density and magnetic orbitals within broken-symmetry (BS) framework. Compound 1 is the first example of a discrete metal compound to sense picric acid in pure water. Some other interesting aspects have been discussed.