Structural Correlations between Luminescent Properties and Excited State Internal Proton Transfer in Some Zinc(II) N,N′-Bis(salicylidenes)

Abstract

In this study two salicylidene ligands, N,N'-bis(salicylidene)-1,2-phenylenediamine and N,N'-bis(salicylidene)-4,5-diaminopyrimidine, and their respective aquo-zinc(II) coordination compounds were synthesized. Their characterization was performed by FTIR, proton and carbon NMR, elemental analysis,mass spectroscopy and cyclic voltammetry. Crystal structures of the ligands were determined by monocrystal X-ray diffraction. The photoluminescent properties under photostationary conditions indicate that the ligand emission predominates in both the pristine materials and their zinc(II)complexes. For both ligands, the coordination of a metal atom leads to a redshift of their emission bands in both solvent and solid state. Molecular structures and excitation energies of ligands and complexes were evaluated at DFT level using PBE0/aug-cc-pVDZ. Their ligand and complex electronic transitions can be assigned mainly to the intraligand π→π * type, mainly involving frontier molecular orbitals, with a small participation of the metal. According to our calculations, there is an increasing in the planarity of the ligand structure in the complex, which could explain the redshifting observed in the absorption and emission spectra. The dynamic photoluminescence suggests the occurrence of excited state intramolecular proton transfer from the oxygen to the nitrogen atoms in the coordination site of the sal-4,5-pym.Moreover, they are able to predict the occurrence of the excited state internal proton transfer for the sal-4,5-pym. The dynamic of this proton transfer is demonstrated by both, time resolved emission spectra (TRES) and studies in protic solvent (ethanol).

Electronic and magnetic properties of the [Ni(salophen)]: An experimental and DFT study

“…The TG/DTA curves were obtained in a Shimadzu apparatus (Kyoto, Japan) with a heating rate of 10 °C cm −1 using a dynamic atmosphere of synthetic air at a flow rate of 100 mL min −1 until 800 °C. The crystal structure of the salophen ligand has been reported [6] . The electronic absorption spectra of salophen and [Ni(salophen)] in DMSO solutions (1 × 10 −5 mol L −1 ) were acquired using a Hewlett-Packard 8452A diode array UV−vis spectrophotometer (Santa Clara, California, United States).…”

“…The procedure to obtain the ligand (salophen) has been described in detail [6] . [Ni(salophen)] was obtained by dissolving the salophen ligand (158 mg; 0.5 mmol) in ethanol (20 mL) after stirring until total solubility on a round flask balloon.…”

“…The calculations were performed within the density functional theory and its time-dependent counterpart (DFT and TD-DFT). This level of calculation has yielded reliable results in predicting the electronic spectra of chromospheres at a relatively low computational cost, and it is one of the most popular methods used for the evaluation of excitation energies [6] , [12] . The ground and first active singlet state structures of [Ni(salophen)] were optimized at the PBE0/6-311++G(d,p) [19] , [20] , [21] level of the theory using Gaussian 09 [22] .…”

“…Recently, there has been an increased interest in the chemistry of transition metal complexes containing N 2 O 2 coordination sites, such as salicylidenes [1] , due to their broad range of applications in areas such as catalysis [2] , functional materials, non-linear optics [3] , molecular magnetism [4] , and organic electronics [5] , such as light emitting diodes (OLEDs) [6] and display magnetic properties [7] . Among them, technologies based on light emission or charge transport abilities are currently receiving particular interest, leading to their use in electronic devices, such as solar cells and active components for image and data treatment storage [8] .…”

“…Their molecular structure and capable coordinate sites make salicylidenes flexible coordination compounds with several metal ions, such as Ni(II), Cu(II), Zn(II), Ru(II), Os(II), Pt(II), and Ir(III) [3] . A variety of metal ions (diamagnetic and paramagnetic) can be introduced in the coordination site, in most cases forming square planar array frameworks [6] . Schiff base nickel(II) salen-type complexes have been extensively used in catalysis [7] , [9] and for biological purposes [11] .…”