Donor Position Research Articles

Synthesis, Photophysical Behavior, and Electronic Structure of Push−Pull Purines

"Push-pull" purines have been synthesized by the introduction of electron-accepting functional groups (A = CN, CO(2)Me, and CONHR) to the heterocyclic C(8) position to complement typical electron-donating substituents at C(2) (D(1)) and C(6) (D(2)). The donor-acceptor purines show significantly altered, and overall improved photophysical properties relative to their acceptor-free precursors (A = H); these include red-shifted (20-50 nm) absorption maxima, highly solvatochromic emission profiles (em lambda(max) from 355-466 nm depending on substitution pattern and solvent) with excellent linear correlations between emission energy and solvent polarity (E(T)(N)), improved photochemical stability upon continuous irradiation, and enhanced (up to 2500%) fluorescence quantum yields. Comprehensive structure-property studies show how the absorption/emission maxima and quantum yields depend on donor and acceptor structure, relative donor position (C(2) or C(6)), and solvent (1,4-dioxane, dichloromethane, acetonitrile, methanol, and in some cases water). Further insight regarding electronic structure comes from a quantitative treatment of the solvent-dependent emission data (that provides Delta mu(ge) values ranging from 1.9 to 3.4 D) and DFT (B3LYP/6-311++G**) electronic structure calculations. X-ray crystal structures of several derivatives showcase the molecular recognition capabilities of the donor-acceptor chromophores that overall have photophysical and structural properties suitable for applications in biosensing and materials.

Whose aid? Whose influence? China, emerging donors and the silent revolution in development assistance

Rising economies including China, the United Arab Emirates, Brazil, Korea, India, Kuwait and Saudi Arabia are subtly changing the rules of foreign aid with profound consequences for the role of multilateral institutions and conditionality. Fears abound that this new aid is bolstering rogue states, fuelling corruption, and increasing the debt burdens of poor countries. This article critically assesses these arguments before dissecting the attractions of emerging donors' aid against a background of established donors' failure to deliver on promises to increase aid, reduce conditionality, better coordinate and align aid efforts, and reform the aid architecture. It argues that a silent revolution is taking place whereby the emerging donors are not overtly attempting to overturn the rules of multilateral development assistance, nor to replace them. Rather, by quietly offering alternatives to aid-receiving countries, they are weakening the bargaining position of western donors. The resulting tensions underscore the urgency of reforming the multilateral aid system.

Interplay between Hydrogen-Bond Formation and Multicenter π-Electron Delocalization: Intramolecular Hydrogen Bonds

The specific case of intramolecular hydrogen bonds assisted by pi-electron delocalization is thoroughly investigated using multicenter delocalization analysis. The effect of the pi-electron delocalization on the intramolecular hydrogen-bond strength is determined by means of the relative molecular energies of "open" and "closed" structures, calculated at the B3LYP/6-311++G(d,p) level of theory. These relative energies are compared to variations in the multicenter electron delocalization indices and covalent hydrogen-bond indices, which are shown to correlate very well with the relative strength of the intramolecular hydrogen bonds studied. The multicenter electron delocalization indices and covalent bond indices have been computed using the quantum theory of atoms in molecules approach. The hydrogen bonds are formed with oxygen, nitrogen, or sulfur as acceptor atom, which are also the atoms considered to be bonded to the donor hydrogen. Malonaldehyde is taken as reference; the substitution of oxygen by other atoms at the acceptor and donor positions and the effect of the aromaticity have been studied. The results shown here match perfectly with the qualitative expectations derived from the resonance models. In addition, they provide a quantitative picture of the role played by the pi-electron delocalization on the relative strength of intramolecular hydrogen bonds.

Energy migration as related to the mutual position and orientation of donor and acceptor molecules in LH1 and LH2 antenna complexes of purple bacteria

Many approaches to discovering the interaction energy of molecular transition dipoles use the well-known coefficient xi(phi, psi (1) psi (2)) = (cos phi - 3 cos psi (1) cos psi (2))(2), where phi, Psi (1), and Psi (2) are inter-dipole angles. Unfortunately, this formula often yields rather approximate results, in particular, when it is applied to closely positioned molecules. This problem is of great importance when dealing with energy migration in photosynthetic organisms, because the major part of excitation transfers in their chlorophyllous antenna proceed between closely positioned molecules. In this paper, the authors introduce corrected values of the orientation factor for several types of mutual orientation of molecules exchanging with electronic excitations for realistic ratios of dipole lengths and spacing. The corrected magnitudes of interaction energies of neighboring bacteriochlorophyll molecules in LH2 and LH1 light-absorbing complexes are calculated for the class of photosynthetic purple bacteria. Some advantageous factors are revealed in their mutual positions and orientations in vivo.

A study of the second coordination sphere in 8-azaxanthinato salts of divalent metal aquacomplexes

The interaction of the 4,6-dimethyl and 4-monomethyl derivatives of 1,2,3-triazolo[4,5- d]pyrimidin-5,7-dione (which may be named also as purine derivatives, 1,3-dimethyl-8-azaxanthine, Hdmax and 3-methyl-8-azaxanthine, H3max) with the divalent cations of Mn, Co, Ni, Zn and Cd in aqueous media generate solids with formulation ML 2 · 6H 2O. The crystal structure of the Mn and Cd compounds with dmax and the Cd compound with 3max have been determined by single-crystal X-ray diffraction revealing that the compounds are salts with [M(H 2O) 6] 2+ as cations and dmax − or 3dmax − as anions. The second-sphere interactions in these compounds have been analysed, consisting in a network of very well-defined hydrogen bonds, with all available potential donor and acceptor positions involved. The topology of the motifs generated by these hydrogen bonds has been characterised, adapting to the second coordination sphere concepts usually applied to the first (chelate, bridge, monodentate, …). Monodimensional (tapes) superstructures with the building blocks rather tightly bounded appear in the compounds with dmax − as anion, whereas the corresponding superstructure in the Cd compound with 3max − is bidimensional. These superstructures further interact among them in a less tight fashion to generate the three dimensional crystal structures. Powder X-ray diffraction strongly suggests that Mn, Co, Zn and Cd compounds of each ligand are isostructural, so the results of the samples determined by single-crystal X-ray diffraction may be extended to all these compounds. On the other hand, powder X-ray diffraction indicates that the nickel compounds have a different structure and the spectroscopic data for these compounds suggest that the ligand is directly attached to the metal for them.

Binding energy of an off-center hydrogenic donor in a spherical quantum dot with strong parabolic confinement

In this paper, off-center impurity effects in a spherical quantum dot are theoretically studied by degenerate perturbation method in strong confinement. The binding energies are computed for the typical GaAs material as a function of the donor position. The numerical results show the quantum size effect. We note that crosses will not come forth any more, but gaps will appear between degenerate states in comparison with the results gained by using the method as in Bose [Physica E 4 (1999) 180].

Single-Layered Films of Diblock Copolymer Micelles Containing Quantum Dots and Fluorescent Dyes and Their Fluorescence Resonance Energy Transfer

The nanoscale positioning of QD donors and dye acceptors enables an effective control of the fluorescence resonance energy transfer from QDs to dyes in a single-layered film of diblock copolymer micelles.

Experimental and theoretical investigation of first hyperpolarizability in aminophenols

This work reports theoretical and experimental studies on the first hyperpolarizability ( β) of aminophenols, evaluating the influence of the NH 2 group position relative to the OH group on the hyperpolarizability. A new extension of hyper-Rayleigh scattering technique using picosecond pulse trains was employed to obtain the experimental absolute values of ( β). The theoretical static β(0) values were calculated using AM1 method implemented in the AMPAC program. The theoretical and experimental data show a clear dependence between β and the relative position of the electron donor ( D) and acceptor ( A) groups, presenting the 2-aminophenol the higher values. Moreover, calculations show excellent qualitative agreement between theoretical and experimental data, which are improved when the simulations considering the solvated molecule in a combination of discrete solvent molecules interacting with the solute and the application of continuous dielectric model. Besides, the study indicates that the experimental hyperpolarizabilities seem to be a property of the solute-solvation shell system. These facts have affirmed that the theoretical approach employed can be successfully used to foresee the variation in β due to modifications in the D/ A position. Moreover, a theoretical study of the ground state absorption is performed and compared with experimental data.

Binding energy of an off-center hydrogenic donor in a spherical Gaussian quantum dot

The energy levels of an off-center hydrogenic donor confined by a spherical Gaussian potential have been calculated as a function of the potential radius for different donor position by exact diagonalization method. The results have clearly demonstrated the so-called quantum size effect. The binding energy is dependent on the dot radius R, the impurity ion distance D, and the confining potential depth V 0 .

Fine Splitting of Electron States in Silicon Nanocrystal with a Hydrogen-like Shallow Donor

Electron structure of a silicon quantum dot doped with a shallow hydrogen-like donor has been calculated for the electron states above the optical gap. Within the framework of the envelope-function approach we have calculated the fine splitting of the ground sixfold degenerate electron state as a function of the donor position inside the quantum dot. Also, dependence of the wave functions and energies on the dot size was obtained.

Polaron effects on the energy of a hydrogenic donor impurity in GaAs–(Ga,Al)As quantum-well wires

In this work using the effective-mass approximation within a variational approach we study the effect of the 3D longitudinal optical phonons on the energy of a shallow donor impurity in a GaAs–(Ga,Al)As quantum-well wire with a magnetic field applied in the wire-axis direction. We obtain the energy of the lower impurity states as a function of the wire radius, the donor position inside the wire, and under the effect of the applied magnetic field. The polaron effects on the impurity energy are studied using second order perturbation theory. We include non-parabolicity effects to explain the optical experiments under applied high magnetic fields.

Valley-orbit splitting in doped nanocrystalline silicon:k∙pcalculations

The valley-orbit splitting in silicon quantum dots with shallow donors has been theoretically studied. In particular, the chemical-shift calculation was carried out within the frames of $\mathbf{k}∙\mathbf{p}$ approximation for single- and many-donor cases. For both cases, the great value of the chemical shift has been obtained compared to its bulk value. Such increase of the chemical shift becomes possible due to the quantum confinement effect in a dot. It is shown for the single-donor case that the level splitting and chemical shift strongly depend on the dot radius and donor position inside the nanocrystal. In the many-donor case, the chemical shift is almost proportional to the number of donors.

Ground state energies of neutral and negatively charged donors in nanowire superlattice

Using the Bastard-type trial function for the neutral donor and the Hylleraas-type trial function for the negatively charged donor, we investigate the effect of donor positions inside the cylindrical GaAs/Ga(Al)As nanowire superlattice on their ground state energies. Results of calculation, presented in the form of contour plots of the energies corresponding to different donor positions along a cross section through symmetry axis, show a close relationship between the energies of the neutral and negatively charged donors and the charge distributions in one- and two-electron nanowire superlattices, respectively, at the point of their locations. The higher the charge density resulting from the unbound electron at a point in the heterostructure, the lower are the ground state energies of the neutral and negatively charged donors located at this point.

Specificity of L,D-Transpeptidases from Gram-positive Bacteria Producing Different Peptidoglycan Chemotypes

We report here the first direct assessment of the specificity of a class of peptidoglycan cross-linking enzymes, the L,D-transpeptidases, for the highly diverse structure of peptidoglycan precursors of Gram-positive bacteria. The lone functionally characterized member of this new family of active site cysteine peptidases, Ldt(fm) from Enterococcus faecium, was previously shown to bypass the D,D-transpeptidase activity of the classical penicillin-binding proteins leading to high level cross-resistance to glycopeptide and beta-lactam antibiotics. Ldt(fm) homologues from Bacillus subtilis (Ldt(Bs)) and E. faecalis (Ldt(fs)) were found here to cross-link their cognate disaccharide-peptide subunits containing meso-diaminopimelic acid (mesoDAP(3)) and L-Lys(3)-L-Ala-L-Ala at the third position of the stem peptide, respectively, instead of L-Lys(3)-d-iAsn in E. faecium. Ldt(fs) differed from Ldt(fm) and Ldt(Bs) by its capacity to hydrolyze the L-Lys(3)-D-Ala(4) bond of tetrapeptide (L,D-carboxypeptidase activity) and pentapeptide (L,D-endopeptidase activity) stems, in addition to the common cross-linking activity. The three enzymes were specific for their cognate acyl acceptors in the cross-linking reaction. In contrast to Ldt(fs), which was also specific for its cognate acyl donor, Ldt(fm) tolerated substitution of L-Lys(3)-D-iAsn by L-Lys(3)-L-Ala-L-Ala. Likewise, Ldt(Bs) tolerated substitution of mesoDAP(3) by L-Lys(3)-D-iAsn and L-Lys(3)-L-Ala-L-Ala in the acyl donor. Thus, diversification of the structure of peptidoglycan precursors associated with speciation has led to a parallel evolution of the substrate specificity of the L,D-transpeptidases affecting mainly the recognition of the acyl acceptor. Blocking the assembly of the side chain could therefore be used to combat antibiotic resistance involving L,D-transpeptidases.

The use of 2- O-propagyloxycarbonyl protecting group in the selective formation of 1,2- trans-glycosidic linkage

The effect of N-phenylcarbamoyl (Car) and propagyloxycarbonyl (Poc) protecting groups at the O-2 position of donors was examined. The usefulness of Poc group in the selective formation of 1,2- trans-glycosidic linkage is shown by comparing the reactivity of donors having Car or acyl (Bz) groups.

The thermotropic phase behaviour and phase structure of a homologous series of racemic β- d-galactosyl dialkylglycerols studied by differential scanning calorimetry and X-ray diffraction

The thermotropic phase behaviour of aqueous dispersions of some synthetic 1,2-di- O-alkyl-3- O-(β- d-galactosyl)- rac-glycerols ( rac-β- d-GalDAGs) with both odd and even hydrocarbon chain lengths was studied by differential scanning calorimetry (DSC), small-angle (SAXS) and wide-angle (WAXS) X-ray diffraction. DSC heating curves show a complex pattern of lamellar (L) and nonlamellar (NL) phase polymorphism dependent on the sample's thermal history. On cooling from 95 °C and immediate reheating, rac-β- d-GalDAGs typically show a single, strongly energetic phase transition, corresponding to either a lamellar gel/liquid-crystalline (L β/L α) phase transition ( N ≤ 15 carbon atoms) or a lamellar gel/inverted hexagonal (L β/H II) phase transition ( N ≥ 16). At higher temperatures, some shorter chain compounds ( N = 10–13) exhibit additional endothermic phase transitions, identified as L/NL phase transitions using SAXS/WAXS. The NL morphology and the number of associated intermediate transitions vary with hydrocarbon chain length. Typically, at temperatures just above the L α phase boundary, a region of phase coexistence consisting of two inverted cubic (Q II) phases are observed. The space group of the cubic phase seen on initial heating has not been determined; however, on further heating, this Q II phase disappears, enabling the identification of the second Q II phase as P n 3 ¯ m (space group Q 224). Only the P n 3 ¯ m phase is seen on cooling. Under suitable annealing conditions, rac-β- d-GalDAGs rapidly form highly ordered lamellar-crystalline (L c) phases at temperatures above ( N ≤ 15) or below ( N = 16–18) the L β/L α phase transition temperature ( T m). In the N ≤ 15 chain length lipids, DSC heating curves show two overlapping, highly energetic, endothermic peaks on heating above T m; corresponding changes in the first-order spacings are observed by SAXS, accompanied by two different, complex patterns of reflections in the WAXS region. The WAXS data show that there is a difference in hydrocarbon chain packing, but no difference in bilayer dimensions or hydrocarbon chain tilt for these two L c phases (termed L c1 and L c2, respectively). Continued heating of suitably annealed, shorter chain rac-β- d-GalDAGs from the L c2 phase results in a phase transition to an L α phase and, on further heating, to the same Q II or H II phases observed on first heating. On reheating annealed samples with longer chain lengths, a subgel phase is formed. This is characterized by a single, poorly energetic endotherm visible below the T m. SAXS/WAXS identifies this event as an L c/L β phase transition. However, the WAXS reflections in the di-16:0 lipid do not entirely correspond to the reflections seen for either the L c1 or L c2 phases present in the shorter chain rac-β- d-GalDAGs; rather these consist of a combination of L c1, L c2 and L β reflections, consistent with DSC data where all three phase transitions occur within a span of 5 °C. At very long chain lengths ( N ≥ 19), the L β/L c conversion process is so slow that no L c phases are formed over the time scale of our experiments. The L β/L c phase conversion process is significantly faster than that seen in the corresponding rac-β- d-GlcDAGs, but is slower than in the 1,2- sn-β- d-GalDAGs already studied. The L α/NL phase transition temperatures are also higher in the rac-β- d-GalDAGs than in the corresponding rac-β- d-GlcDAGs, suggesting that the orientation of the hydroxyl at position 4 and the chirality of the glycerol molecule in the lipid/water interface influence both the L c and NL phase properties of these lipids, probably by controlling the relative positions of hydrogen bond donors and acceptors in the polar region of the membrane.

New low band gap alternating polyfluorene copolymer-based photovoltaic cells

New low band gap alternating polyfluorene copolymers were synthesized for use in plastic solar cells and their optical, electrochemical, and photovoltaic characteristics were determined. These polymers incorporated fluorene units alternating with groups including electron-withdrawing (A) and electron-donating (D) groups in donor–acceptor–donor (DAD) sequence to achieve the lowering of band gaps. The HOMO–LUMO values were estimated from electrochemical studies. By varying the donor and acceptor strength and position of the solubilizing substituents, similar HOMO values were obtained. These values were also found to correlate well with the open circuit voltage ( V OC) values determined from photovoltaic data of the polymers blended with the acceptor PCBM. Despite similar HOMO values, the absorption spectra of the polymers differ significantly. This prompted the preparation of photovoltaic devices consisting of blends of two polymers with complementary absorptions in combination with PCBM to harvest more photons in the polymer solar cells.

Chemical-shift enhancement for strongly confined electrons in silicon nanocrystals

Theoretical study of the chemical shift and ground level splitting in spherical silicon quantum dot with a shallow donor has been carried out within the framework of the k–p method. We have found the great value of the chemical shift compared to the one in bulk. It is shown that the level splitting strongly depends on the dot radius and the donor position inside the nanocrystal.

Tuning the silver(I) complexes of 3-(2-pyridyl)pyrazole-based ligands: Syntheses and crystal structures of the complexes, as well as theoretical investigations on the coordination abilities of the ligands

In our efforts to investigate the influences of different pendant aromatic groups and the spatial position of N donors in 3-(2-pyridyl)pyrazole-based ligands on the structures of their metal complexes, five structurally related ligands: 1-[3-(2-pyridyl)pyrazol-1-ylmethyl]benzene ( L 1 ), 1-[3-(2-pyridyl)pyrazol-1-ylmethyl]naphthalene ( L 2), 8-[3-(2-pyridyl)pyrazol-1-ylmethyl]quinoline ( L 3 ), 3-[3-(2-pyridyl)pyrazol-1-ylmethyl]pyridine ( L 4 ) and 4-[3-(2- pyridyl)pyrazol-1-ylmethyl]pyridine ( L 5 ), have been used to react with AgClO 4 to form five Ag(I) complexes, [Ag( L 1 ) 2](ClO 4) ( 1), [Ag( L 2 ) 2](ClO 4) ( 2), [Ag( L 3 )(H L 3 )](ClO 4) 2(CH 3CN) ( 3), {[Ag( L 4 )](ClO 4)} 2 ( 4), and {[Ag( L 5 )](ClO 4)} ∞ ( 5). The structural differences of these complexes may be attributed to the coordination geometries or N donor position of the pendant aromatic groups in ligands L 1 – L 5 . Also, the result reveals that various intra- and/or inter-molecular weak interactions, such as π⋯π stacking, C–H⋯π and C–H⋯O H-bonding interactions, play important roles in the formation of 1– 5, especially in the aspect of linking the multi-nuclear discrete subunits or low-dimensional entities into high-dimensional frameworks. Moreover, the coordination behaviors of ligands L 1 – L 5 have been briefly evaluated by DFT calculations.

Privation et mesure du bien-être social

Nous introduisons un nouveau critere pour effectuer des comparaisons de bien-etre, qui repose sur la notion d’absence de privation. Ce quasi-ordre constitue une alternative naturelle a l’approche standard en economie normative, qui consiste a comparer les courbes de Lorenz generalise des distributions de revenu. Le quasi-ordre de Lorenz generalise satisfait aux principes des transferts selon lequel le bien-etre social augmente suite a un transfert progressif. Le critere que nous proposons verifie une condition moins exigeante que le principe des transferts : le bien-etre social doit augmenter uniquement pour des combinaisons de transferts progressifs particulieres, ou les positions des individus impliques dans ces transformations jouent un role crucial.