Impact of metal cation complexation and protonation on tautomeric and resonance forms of the oxaalkyl Schiff bases derived from 5-substituted salicylaldehyde and 2-hydroxy-1-naphthlaldehyde

Structures of the main resonance and tautomeric forms of three Schiff bases of 2-hydroxy-1-naphthaldehyde (1OXA), 5-bromosalicylaldehyde (2OXA) and 5-nitrosalicylaldehyde (3OXA) with 1,8-diamine-3,6-dioxaoctane, before and after protonation and complexation of monovalent metal cations, have been studied by FTIR, ¹H, ¹³C and ¹⁵N NMR methods. The spectroscopic investigations provided clear evidence that the Schiff bases exist in acetonitrile solution as three different tautomers: 1OXA in enamine-oxo, 2OXA in imine-hydroxy and 3OXA in a low-barrier O···H···N imine-oxo forms. It was found on the basis of multinuclear NMR studies, that in solid state, the enamine-oxo and imine-hydroxy tautomers are formed exclusively, but not the untypical imine-oxo tautomer, which requires strong stabilisation by solvent molecules in solution. MOG-PM6 calculations of the different tautomers allowed visualisation of their energetically the most favourable structures. Protonation of 1OXA–3OXA Schiff bases leads to formation of common forms, i.e. protonated imine-hydroxy structure, irrespectively of the structure of tautomer before protonation. In turn, coordination of monovalent metal cations implies common formation of zwitterionic forms within all studied ligands in solution. Application of FT-IR and NMR titrations in combination with ESI MS spectrometry revealed the nature and the structure of OXA complexes, whose formation is impeded by the intra- and intermolecular interactions. MOG-PM6 calculations allowed visualisation of Li⁺ and Na⁺ metal coordination sphere geometries within structure of all investigated complexes, stabilised by intermolecular interactions with solvent molecules.     

Application of Polymer‐Supported Triphenylphosphine and Microwave Irradiation to the Palladium‐Catalyzed Cyanation of Aryl Triflates

A variety of aryl nitriles were prepared in excellent yield from the palladium(II) acetate–catalyzed cross‐coupling of aryl triflates and zinc cyanide under microwave irradiation conditions. To facilitate purification, polymer‐supported triphenylphosphine was used as the palladium ligand. Comparison to the corresponding thermal conditions revealed much shorter reaction times with comparable yields.     

Continua of interactions between pairs of atoms in molecular crystals

The electron density distributions in crystals of five previously studied DMAN complexes and five Schiff bases (two new ones) have been analysed in terms of various properties of bond critical points (BCPs) found in the pair-wise interactions in their lattices. We analysed the continua of interactions including covalent/ionic bonds as well as hydrogen bonds and all other types of weak interactions for all pairs of interacting atoms. The charge density at BCPs and local kinetic and potential energy densities vary exponentially with internuclear distance (or other measures of separation). The parameters of the dependences appear to be characteristics of particular pairs of atom types. The Laplacian and the total (sum of kinetic and potential) energy density at BCPs show similar behaviour with the dependence being of the Morse type. The components lambda1, lambda2, lambda3 of the Laplacian at BCPs vary systematically with internuclear distance according to the type of atom pair. For lambda1 and lambda2 the distribution is of the exponential type, whereas lambda3 does not seem to follow any simple functional form, consistent with previous theoretical findings. Analytical nonlinear dependences of Laplacian on charge density have been found. They agree reasonably well with those obtained by least square fit of the Laplacian to charge density data. There are four distinct regions of the [symbol: see text]2rho(BCP)/rho(BCP) space, generated by E(BCP) = 0 and G(BCP)/rho(BCP) = 1 conditions. Two regions clearly correspond to the shared-shell and closed-shell interactions and the other two to some intermediate situation.     

A perfect matching algorithm for sparse bipartite graphs

This paper considers an algorithm for finding a perfect matching, if there is one, in a bipartite graph G. It is shown that the search for a perfect matching in G may be carried on separately in the strongly connected components of appropriate directed graphs. The algorithm may be particularly useful for block triangularization of very large, sparse, nonsingular matrices.     

Changes in the Luminescent Properties of Humic Acids Induced by UV Radiation

Humic acids (HA) are complex, dark, paramagnetic biopolymers, ubiquitus in the soil and aquatic ecosystems. Due to their peculiar properties (multifluorophore system capable of excitation energy transfer, continuous featureless absorption over a wide spectral range, and paramagnetism), HA play an important role as an efficient target for UV solar radiation, O₂, and O₃—detrimental environmental factors which affect the Earth's biosphere. Photooxidation of HA causes changes in the absorption and luminescence properties of HA which may be of significant importance for environmental photophysics and photochemistry. We have studied effects of UV irradiation on the degradation of several commercial HA (Fluka, Merck, and Serva). Aqueous, aerated alkaline solutions of HA (0.1–0.4 g/L in 0.006–0.1 M Na₂CO₃) were irradiated with an electrodeless Hg (254-nm) lamp in a flow system during several hours. After different times of irradiation, solutions were assayed by means of fluorescence (Fl), absorption (UV–Vis), and chemiluminescence (CL) spectroscopy. The data obtained indicate that the free radical-mediated degradative photooxidation of HA macromolecules is accompanied by a very weak, long-lived chemiluminescence (340–800 nm), a gradual decrease in absorbancy with characteristic changes in color coefficients Q _4/6, Q _2.7/6.0, and Q _2.7/4.0, and an increase in the intensity of Fl emission (340- to 560-nm) and excitation (250- to 400-nm) spectra. Processes undergoing these changes are intrinsically associated with the generation of excited states and reactive oxygen species . These processes are expected to play a vital role in the natural environment, e.g., HA-photosensitized decomposition of xenobiotics and solar energy transfer in symbiotic hydrobionts.     

(8‐Dimethylamino‐1‐naphthyl)dimethylammonium 3‐carboxy‐1,4,5,‐6,7,7‐hexachlorobicyclo[2.2.1]hept‐5‐ene‐2‐carboxylate hydrate

The structure of the title compound, C₁₄H₁₉N₂⁺·C₉H₃Cl₆O₄^?·H₂O, consists of singly ionized 1,4,5,6,7,7‐hexachlorobicyclo[2.2.1]hept‐5‐ene‐2,3‐dicarboxylic acid anions and protonated 1,8‐bis(dimethylamino)naphthalene cations. In the (8‐dimethylamino‐1‐napthyl)dimethylammonium cat­ion, a strong disordered intramolecular hydrogen bond is formed with N?N = 2.589 (3) Å. The geometry and occupancy obtained in the final restrained refinement suggest that the disordered hydrogen bond may be asymmetric. Water mol­ecules link the anion dimers into infinite chains via hydrogen bonding.     

The Impact of a 1,2,3-Triazole Motif on the Photophysical Behavior of Non-K Tetrasubstituted Pyrene with a Substitution Pattern Providing the Long Axial Symmetry

1,3,6,8-Tetrasubstituted pyrene derivatives with two types of substituents (4-(2,2-dimethylpropyloxy)pyridine, 1-decyl-1,2,3-triazole, 1-benzyl-1,2,3-triazole, and pyrazole), substituted in such a way that provides the long axial symmetry, are prepared and characterized in the present study. To the best of our knowledge, the pyrene derivative containing the same heteroaryl motif (triazole) but substituted by two various alkyls, straight decyl and benzyl-based side chains (C), is reported for the first time. For comparison, compounds with one kind of triazole motif and substituted pyridine or pyrazole groups were prepared (A and B). The photophysical properties of all molecules were evaluated by thermogravimetric analysis (TGA) and UV-Vis spectroscopy (absorption and emission spectra, quantum yields, and fluorescence lifetimes). The obtained results were compared to analogues substituted at the 1,3,6,8 positions by one kind of substituent and also with all the 1,3,6,8-tetrasubstituted pyrenes reported in the literature substituted by two kinds of substituents with a substitution pattern that provides long axial symmetry. In addition, theoretical studies based on DFT and TD-DFT were performed that supported the interpretation of the experimental results. The photophysical properties of tetrasubstituted pyrene derivatives having triazole units at the 1,8-positions, respectively, and other identical substituents at the 3,6 positions show the dominance of triazole units in the pyrene framework; the dominance is even higher in the case of the substitution of 1,3,6,8 positions by triazoles, but containing two various alkyls.     

On the different coordination of Ni, Zn and Cd cations in their model Schiff base complexes - Single crystal X-ray and solid state NMR studies

Three model metal complexes: Ni(NCS)L, Zn₂(NCS)₂L₂ and Cd₂(NCS)₂L₂, consisting of the SCN⁻ anion(s) and L = 2-[(2-dimethylaminoethylimino)-methyl]-phenolate, have been studied by X-ray diffraction and solid state NMR spectroscopy. The metal cations in these complexes have different coordination modes: Ni²⁺ is almost square-planar, the Zn²⁺ cation in Zn₂(NCS)₂L₂ is pentacoordinated, whereas Cd²⁺ is penta- and hexacoordinated in [Cd₂(NCS)₂L₂]. The different coordination of the metal cations influences the chemical shifts of the metal cations and also the nitrogen atoms. These chemical shifts can be correlated with the M-N and M-O bond lengths (M = Ni²⁺, Zn²⁺, Cd²⁺).     

Studying the luminescence efficiency of Lu<Subscript>2</Subscript>O<Subscript>3</Subscript>:Eu nanophosphor material for digital X-ray imaging applications

Scintillator materials are widely used in X-ray medical imaging detector applications, coupled with available photoreceptors like radiographic film or photoreceptors suitable for digital imaging like a-Si, charge-coupled devises (CCD), complementary metal-oxide-semiconductors (CMOS) and GaAs). In addition, scintillators can be utilized in non-medical imaging detectors such as industrial detectors for non-destructive testing (NDT) and detectors used for security purposes (i.e. airport luggage control). Image quality and dose burden in the above applications is associated with the amount of optical photons escaping the scintillator as well as the amount of optical photons captured by the photoreceptor. The former is characterized by the scintillator efficiency and the latter by the spectral matching between the emission spectrum of the scintillator and the spectral response of the photoreceptor. Recently, a scintillator material, europium-activated lutetium oxide (Lu₂O₃:Eu), has shown improved scintillating properties. Lu₂O₃:Eu samples of compact nanocrystalline non-agglomerated powder were developed in our laboratory using homogeneous precipitation from a water-toluene solution in the presence of polyvinyl alcohol as a surfactant. In order to test their light-emission properties, experimental measurements under the excitation of X-ray spectra with X-ray tube voltages between 50 kVp and 140 kVp were performed. This range of applied voltages is appropriate for X-ray radiology, NDT and security applications. Lu₂O₃:Eu was evaluated with respect to output yield and spectral compatibility of digital imaging photoreceptors (CCD-based, CMOS-based, amorphous silicon a:Si flat panels, ES20 and GaAs). High light yield and spectral compatibility increase the performance of the medical detector and reduce the dose burden to the personnel involved. In addition a theoretical model was used to determine the values for the Lu₂O₃:Eu optical photon light propagation parameters. The inverse diffusion length was found to be equal to 33 cm²/g. In addition Lu₂O₃:Eu was found to match well with several photoreceptors capable of digital imaging (i.e. GaAs).     

Effect of Na+ co-dopant and activator concentration on luminescent properties of CaGa4O7:Eu3+

Two series of calcium gallate phosphors: Ca_1?xEu_xGa₄O₇ and Ca_1?2xEu_xNa_xGa₄O₇ (x=0, 0.002, 0.01, 0.02, 0.03, 0.05) were synthesized by a modified Pechini method and their optical properties at 298 and 77 K were investigated. In undoped CaGa₄O₇ upon 255 nm excitation a bluish white emission (λ_max=500 nm) followed by an afterglow of the same color lasting for 10–20 s was observed. Eu³⁺-doping quenched the host-related luminescence and the characteristic red emission of the dopant with maximum at 613 nm appeared. Its excitation spectrum consisted of a broad band assigned to ligand to metal, O^2?→Eu³⁺, charge transfer absorption and narrow lines arising from intraconfigurational transitions within the 4f⁶ states of Eu³⁺ ion. The effects of Eu³⁺ concentration and Na⁺ co-doping on the luminescence properties and decay kinetics were studied. Low temperature emission spectra showed that Eu³⁺ ions are positioned in environments of different symmetries. Their relative populations changed with the activator content. Co-doping with Na⁺ ions led to a remarkable reduction of the number of Eu³⁺ sites as well as to noticeable improvement of the luminescence brightness though it did not affect the decay time of the emission. The quantum efficiencies of singly doped CaGa₄O₇:Eu³⁺ were very low (in the range of 1–3.7%). Na⁺ co-doping improved this parameter leading to the highest efficiency of 11% for CaGa₄O₇:3%Eu³⁺,3%Na⁺.