Chemodosimetric mechanistic insight through trapping intermediate for selective detection of picric acid in aqueous medium by a dinuclear Cd(II) complex

A fluorescent dinuclear cadmium(II) based discrete metal complex of composition [CdII2L(µ-Cl)Cl2] (1) is used {HL = 2,6-bis[2-(methylamino)ethyliminomethyl]-4-Ethylphenol} for the specific recognition of 2,4,6-trinitrophenol (picric acid; PA) via fluorescence quenching phenomenon among various nitroaromatic compounds through a chemodosimetric approach. It has been established that 1 is a chemodosimeter in pure water. We have successfully able to isolate three compounds: chemodosimeter 1; an intermediate complex 2 of composition [CdII(LH2)Cl2](Picrate) and final association complex 3 of composition [NH3(CH2CH2)NH2CH3](Picrate)2. Compounds have been characterised by CHN elemental analyses, single crystal X-ray crystallography, PXRD, NMR and FTIR. Selective interaction of 1 with PA was evaluated by fluorescence, UV-Vis and life time measurements. Fluorescence quenching of 1 occurs definitely due to the formation of compound 3 via intermediate 2 involving partial decomplexation, hydrolysis and proton transfer phenomena in solution during the course of sensing. The quenching constant (Ksv), association constant (Ka) and detection limit (LOD) of the complex 1 for picric acid are ~1.55×105 M‒1, ~1.8× 1010 M−2 and ~0.47 µM (0.108 ppm), respectively. Mechanism of sensing is proposed and the very rare case of isolation and characterization of intermediate in picric acid sensing is discussed.