Solvent Dependent Luminescence Sensing of Nitro-Explosives by a Terbium-Based Metal-Organic Complex

Powder sample of a terbium based metal-organic complex (MOC) [Tb-2(pydcH)(6)]3H(2)O {H(2)pydc= 2,6-Pyridinedicarboxylic acid}, 1, were synthesized by simple solvent evaporation method in water medium. Powder X-ray diffraction data of complex 1 confirmed a molecular complex of tricapped trigonal prismatic Tb3+ ion with a free carboxylic acid site of each H(2)pydc ligands by comparing with the single crystal X-ray diffraction data of isomorphous europium based compound, [Eu-2(pydcH)(6)]3H(2)O. Compound 1 showed very strong visible green luminescence which was sensitive towards nitro explosives; and interestingly, the sensitivity was dependent on the solvent used. The luminescence was emitted by ligand sensitized Tb metal centre of compound 1 and was quenched so quickly and efficiently that an instant colour change from visible green to colourless was observed by naked eye. The experiments were done for the detection of nitro-explosive materials such as 2,4,6-trinitrophenol (TNP), 2,4-dinitrophenol (2,4-DNP), 2,4-dinitro toluene (DNT), 1,3-dinitro benzene (DNB), 4-nitrotoluene (NT) and nitro benzene (NB) in water (H2O), methanol (MeOH), acetonitrile (CH3CN), and tetrahydrofuran (THF) solvents. Mechanism of the luminescence quenching behaviour in different solvents was successfully explained by combined effect of resonance energy transfer, absorption of excitation light and the possibility of electron transfer. Experimental results were also supported by DFT calculations.