A metal complex based fluorescent chemodosimeter for selective detection of 2,4-dinitrophenol and picric acid in aqueous media

Abstract

This work describes the syntheses, characterization, crystal structures, absorption and emission spectra and DFT calculations of three dizinc(II) compounds of the composition [Zn^II₂L(μ_1,1-N₃)(N₃)₂] (1) [Zn₂L′(2,4-dinitrophenolate)₂] (2) and [Zn₂L′(picrate)₂] (3), respectively, (where HL is the 1 : 2 condensation product of 4-ethyl-2,6-diformylphenol and N-methylethylenediamine and H₂L′ (a diamino–diimino–diphenol system) is a new type of macrocyclic ligand). Compound 1 is water soluble and its aqueous solution exhibits intense fluorescence properties. 2,4-Dinitrophenol (DNP) and picric acid (PA) selectively quench the fluorescence intensity of 1 to a significant extent, revealing that 1 is a fluorescence sensor of DNP and PA. Compounds 2 and 3 were prepared by mixing 1 with DNP and PA. As a huge change in the system (acyclic to macrocyclic) occurs while exhibiting sensing behaviour, it is evident that 1 senses DNP and PA through a chemodosimetric approach in aqueous media. For sensing nitroaromatic compounds, compound 1 acts as (i) a rare chemodosimeter, (ii) a rare metal containing chemodosimeter and (iii) a rare fluorescent chemosensor in aqueous media. Based on the ESI-MS and single crystal X-ray structures, it has been possible to establish a mechanism for the conversion of the acyclic system in 1 to a new type of macrocyclic system in 2 and 3. It has been established from DFT calculations that ground state complexation via charge transfer and IFE can be considered as the major reason for the DNP/PA stimulated strong fluorescence quenching.