N,N-Diethylamine appended binuclear Zn(ii) complexes: highly selective and sensitive fluorescent chemosensors for picric acid†
Novel binuclear Zn(II) complexes (1–2) derived from bis-chelating salen type ligands (H2L1 and H2L2) possessing N,N-diethylamine moieties on the periphery of the molecules have been synthesized and thoroughly characterized by satisfactory elemental analyses and spectral (FT-IR, 1H, 13C NMR, UV-vis, fluorescence and ESI-MS) studies. The structures of H2L1 and 1 have been authenticated by single crystal X-ray diffraction analyses. Complexes 1 and 2 strongly fluoresce and act as highly selective and sensitive chemosensors for picric acid in different organic as well as aqueous media. Both 1 and 2 showed strong potential to detect traces of PA in vapour/solid phase through contact mode analysis. Spectral and theoretical (DFT) studies suggested that the observed fluorescence quenching may be associated with ground state (GS) charge transfer as well as electrostatic interactions between 1/2 and PA. The fluorescence lifetime for the representative complex 1 displayed a double exponential curve and unaltered lifetime (τav, 0.63 nm) in the absence and presence of PA and strongly suggested that quenching follows a static mechanism. Further, DFT calculations on 1 and 2 strongly supported the static mechanism through GS charge transfer between complexes and PA. In addition, 1H NMR spectral studies on 1–2 in the presence of PA firmly advocated strong hydrogen bonding and π–π stacking between the phenolic rings of 1–2 and the aromatic ring of PA. These complexes are capable of detecting PA either individually or in a competitive environment of other nitro- explosives. Florescence spectral studies on the model complex M lacking N,N-diethylamine groups revealed moderate selectivity and sensitivity towards PA and supported the key role of N,N-diethylamine moieties in the selectivity and sensitivity of complexes.