Exploitation of the Flexidentate Nature of a Ligand To Synthesize Zn(II) Complexes of Diverse Nuclearity and Their Use in Solid-State Naked Eye Detection and Aqueous Phase Sensing of 2,4,6-Trinitrophenol.

Three Zn(II) complexes, [Zn2(HL)2(NO3)2]·H2O (1), [(Zn4L2)(μ3-OH)2](NO3)2·0.5H2O (2), and [(Zn6L2)( o-van)2(μ3-OCH3)2(μ3-OH)2](NO3)2 (3), have been synthesized by exploiting the flexidentate nature of a multidentate Schiff base ligand, H2L ( N, N'-bis(3-methoxysalicylidene)diethylenetriamine), by changing the reaction conditions and stoichiometry of the reactants. All three complexes are highly fluorescent in solution as well as in solid and have been used as luminescence sensors toward nitrophenol explosives in both the media. In aqueous/methanol medium, these complexes show very high selectivity and sensitivity with detection limit in ppb (2.03) or nM level (8.89 nM) for picric acid. The yellow color of all three Zn(II) complexes changes to red on mixing with small amount (∼5%) of picric acid in solid state, revealing the potential of these complexes for practical use in naked eye detection of 2,4,6-trinitrophenol (TNP) or picric acid in ambient light. In order to identify the host-guest interactions between Zn(II) complex and TNP, single crystals of the adduct of TNP with Zn(II) complex, [Zn2(HL)2(H2O)2][C6H2N3O7]2 (4), were grown. Its X-ray crystal structure reveals that two picrate ions are attached to a dinuclear host with the help of H-bonding and π···π interactions, throwing light into the quenching mechanism and selectivity of detection.