A pyrazole-based orthogonal ferromagnetically coupled [2 × 2] homoleptic square Cu4 grid: Magnetostructural correlations

The synthesis and structure of a pyrazole-based orthogonal ferromagnetically coupled tetracopper(II) 2 × 2 homoleptic grid complex [Cu₄(PzOAPyz)₄(ClO₄)₂](ClO₄)₂ · 6H₂O (1), formed by the reaction between the ditopic ligand PzOAPyz and Cu(ClO₄)₂ · 6H₂O, are described. The ligand contains terminal pyrazole and pyrazine residues bound to a central flexible diazine subunit (N–N) as well as one potentially bridging alkoxo group. The two adjacent metal centers are linked by an alkoxo oxygen forming essentially a square Cu₄(μ-O₄) cluster. In the Cu₄(μ-O₄) core, out of the four copper centers, two copper centers are penta-coordinated and the remaining two are hexa-coordinated. In each case of hexa-coordination, the sixth position is occupied by one of the oxygen atoms of a coordinated perchlorate ion. Complex 1 has been characterized structurally and magnetically. Although the large Cu–O–Cu bridge angles (137–138°) and short Cu–Cu distances (3.964–3.970 Å) are suitable for the transmission of the expected antiferromagnetic coupling, the square-based Cu₄(μ-O₄) cluster exhibits an intramolecular ferromagnetic exchange (J = 7.47 cm⁻¹) between the metal centers with an S = 2 magnetic ground state associated with the quasi orthogonal arrangement of the magnetic orbitals (d_x2-y2${\text{d}}_{x^2-y^2}$). The exchange pathway parameters have been evaluated from density functional calculations.     

Effect of electrostatic interaction on the location of molecular probe in polymer-surfactant supramolecular assembly: a solvent relaxation study

Effect of electrostatic interaction on the location of a solubilized molecular probe with ionic character in a supramolecular assembly composed of a triblock copolymer, P123 ((ethylene oxide) 20-(propylene oxide) 70-(ethylene oxide) 20) and a cosurfactant cetyltrimethylammonium chloride (CTAC) in aqueous medium has been studied using steady-state and time-resolved fluorescence measurements. Coumarin-343 dye in its anionic form has been used as the molecular probe. In the absence of the surfactant, CTAC, the probe C343 prefers to reside at the surface region of the P123 micelle, showing a relatively less dynamic Stokes' shift, as a large part of the Stokes' shift is missed in the present measurements due to faster solvent relaxation at micellar surface region. As the concentration of CTAC is increased in the solution, the percentage of the total dynamic Stokes' shift observed from time-resolved measurements gradually increases until it reaches a saturation value. Observed results have been rationalized on the basis of the mixed micellar structure of the supramolecular assembly, where the hydrocarbon chain of the CTAC surfactant dissolves into the nonpolar poly(propylene oxide) (PPO) core of the P123 micelle and the positively charged headgroup of CTAC resides at the interfacial region between the central PPO core and the surrounding hydrated poly(ethylene oxide) (PEO) shell or the corona region. The electrostatic attraction between the anionic probe molecule and the positively charged surface of the PPO core developed by the presence of CTAC results in a gradual shift of the probe in the deeper region of the micellar corona region with an increase in the CTAC concentration, as clearly manifested from the solvation dynamics results.     

Ultrafast bimolecular electron transfer dynamics in micellar media

Ultrafast photoinduced bimolecular electron transfer (ET) dynamics between 7-aminocoumarin derivatives and N,N-dimethylaniline (DMAN) has been studied in neutral (TX100), cationic (DTAB) and anionic (SDS) micellar media. A very fast decay time constant (tau(fast)) shorter than approximately 10 ps has been observed for the coumarins in the presence of DMAN in all of the three micellar media. In this time scale, reactants in the micellar phase undergo ET interactions without involving diffusion or reorientation of the reactants and thus can be envisaged as equivalent to nondiffusive bimolecular ET reaction. The fastest ET rates estimated as the inverse of the shortest lifetime components of the fluorescence decay (k(et) congruent with tau(fast)(-1)) nicely follow the predicted Marcus inversion behavior with reaction exergonicity (-DeltaG degrees), irrespective of the nature of micelles considered. Onset of inversion in ET rates occur at approximately 0.61 eV lower exergonicity in SDS and TX100 micelles compared with that in DTAB micelle and are rationalized following two-dimensional ET (2DET) theory. These differences suggest the possibility of tuning Marcus inversion by proper selection of micelles. Interestingly, ET rates (k'(et)) obtained from the conventional Stern-Volmer analysis of the relatively longer time constants of the fluorescence decays also exhibit similar Marcus correlation with DeltaG degrees, showing clear inversion behavior. Fitting of Marcus correlation curves for k(et) and k'(et) indicate two largely different values for the electronic coupling parameters. In micellar media, as the interacting donor-acceptor molecules are on an average expected to be separated by an intervening surfactant chain and the reorientation rate of the reactants is quite slow, it is predicted that the ultrafast ET (k(et)) component arises because of the surfactant separated donor-acceptor pairs that are orientated perfectly to give the maximum electronic coupling. The slower ET (k'(et)) is predicted to arise because of those pairs where the donor-acceptor orientations are not very suitable but good enough to give a sizable electronic coupling.     

Structural,magnetic, and physical properties of La(1–x)MnO3±δ nano-manganite

A detailed structural, magnetic and physics properties of La_1?xMnO_3±δ (LMO) nanomanganites were investigated to find out the role of cationic vacancies (La vacancy with Mn³⁺/Mn⁴⁺) in grain size modulation. Crystal structure and phase analysis of all samples were carried out by Rietveld refinement of high-resolution XRD and neutron diffraction data. We report here, the oxygen content in studied LMO compound decreases with increase in La vacancies in parent site and a parasitic Mn₃O₄ phase has been evolved in the range of 0.9 ≥ La/Mn ≥ 0.7. Para to ferro magnetic transition temperature (T_C) of all nanometric samples (La/Mn < 0.9) was found at high temperature side (≥260 K) whereas, the same for bulk one (La/Mn ≥ 0.9) was around 160 K. The enhancement of T_C (~70 K) with size reduction is attributed to broadening of bandwidth due to compaction of MnO₆ octahedra in system unit cell. In bulk sample, a secondary cluster/spin glassy phase is found below 50 K, whereas the glassy phase has been suppressed in nanoscale. Field-dependent magneto-resistance measurements are also carried out for all samples at different temperatures to get a profound insight of magneto-transport dynamics of the present system.     

Surface patch binding induced intermolecular complexation and phase separation in aqueous solutions of similarly charged gelatin-chitosan molecules

The formation of selective surface patch binding induced complex coacervates between polyions, chitosan (cationic polyelectrolyte), and alkali-processed gelatin (polyampholyte), both carrying similar net charge, was investigated for two volumetric mixing ratios: r = [chitosan]/[gelatin] = 1:5 and 1:10. Formation of soluble intermolecular complexes between gelatin and chitosan molecules was observed in a narrow range of pH, though these biopolymers had the same kind of net charge, which was evidenced from electrophoretic measurement. This clearly established the role played by selective surface patch binding driven interactions. The temperature sweep measurements conducted on these coacervate samples through rheology and differential scanning calorimetry (DSC) studies yielded two characteristic melting temperatures located at approximately 68 +/- 3 degrees C and 82 +/- 3 degrees C. In the flow mode, the shear viscosity (eta) of the coacervate samples was found to scale with (power-law model) applied shear rate (gamma*) as eta(gamma*) approximately (gamma*)(-k); this yielded k = 0.76 +/- 0.2 (1 s(-1) < gamma* < 100 s(-1)), indicating non-Newtonian behavior. The static structure factor (I(q)) deduced from small angle neutron scattering (SANS) data in the low q (q is the scattering wavevector) (0.018 A(-1) < q < 0.072 A(-1)) region was fitted to the Debye-Bueche regime, I(q) approximately 1/(1 + zeta(2)q(2))2 that yielded a size of zeta approximately 215 +/- 20 A (for r = 1:10) and zeta approximately 260 +/- 20 A (for r = 1:5) samples, implying change in the size of inhomogeneities present with mixing ratio. In the intermediate q region, called the Ornstein-Zernike regime, I(q) approximately 1/(1 + xi(2)q(2)) gave a correlation length of xi approximately 10.0 +/- 2.0 A independent of the mixing ratio. The results taken together imply the existence of a weakly interconnected and heterogeneous network structure inside the coacervate phase separated by domains of polymer-poor regions.     

Some recent developments in SUSY dark matter analyses

A brief summary of some of the recent developments in the analyses of supersymmetric dark matter is given.