Attachment of different donor groups to a cryptand for modulation of two-photon absorption cross-section

Two-photon absorption (TPA) properties of a laterally nonsymmetric aza cryptand with attached side arms have been investigated. This series of Schiff base derivatives supports the mechanistic approach for enhancing the TPA process, which is usually dictated by molecular geometry, pi-bridging, delocalization length, and corresponding charge-transfer possibilities. The results described here suggest that on increasing the branching units, the TPA cross-section, sigma((2)), can be tuned to a larger value. The TPA activity is "switched on" when a metal atom enters the cavity and serves as a conduit of electronic delocalization. The sigma((2)) value increases as the donor strength increases. The maximum value is obtained on moving from the single-branched system to the nearly threefold symmetry. This serves as a useful synthetic strategy for designing novel octupolar molecules with high sigma((2)) values. Theoretical calculations at the B3LYP functional with the 6-31G* basis set under DFT formalism provide supporting evidence that the communication between the side arms through the metal d orbital and more ordered geometry of chromophores leads to a smaller HOMO-LUMO gap, which has a great influence upon the electronic properties of the molecules.     

One Novel Cu(II)–Amino Acid Schiff Base Complex Derived from Salicylaldehyde and L-Serine: Identification of Unusual Monodentate 4,4′-Bipyridine

One novel copper(II) complex [Cu(L)(4,4′-bipy)](ClO₄) (1), (where L: tridentate Schiff base derived from salicylaldehyde and L-serine) has been synthesised and characterised by spectroscopic and electrochemical studies. The single-crystal structure of the complex was determined. The crystal structure features the presence of [Cu(L)(4,4′-bipy)]⁺ cations and ClO₄⁻ anions aggregated by hydrogen bonding. Here, 4,4′-bipyridine functions as a monodentate ligand, which appears to be an unusual phenomenon.     

Equilibrium glassy phase in a polydisperse hard-sphere system

The phase diagram of a polydisperse hard-sphere system is examined by numerical minimization of a discretized form of the Ramakrishnan-Yussouff free-energy functional. Crystalline and glassy local minima of the free energy are located and the phase diagram in the density-polydispersity plane is mapped out by comparing the free energies of different local minima. The crystalline phase disappears and the glass becomes the equilibrium phase beyond a "terminal" value of the polydispersity. A crystal-to-glass transition is also observed as the density is increased at high polydispersity. The phase diagram obtained in our study is qualitatively similar to that of hard spheres in a quenched random potential.     

Metal binding characteristics of a laterally nonsymmetric aza cryptand upon functionalization with a pi-acceptor group

The laterally nonsymmetric aza cryptand synthesized by condensing tris(2-aminoethyl)amine (tren) with tris[[2-(3-(oxomethyl)phenyl)oxy]ethyl]amine readily forms mononuclear inclusion complexes with both transition and main-group metal ions. In these complexes, the metal ion occupies the tren-end of the cavity making bonds with the three secondary amino and the bridgehead N atoms. When a strong pi-acceptor group such as 2,4-dinitrobenzene is attached to one of the secondary amines, the binding property of the cryptand changes drastically. When perchlorate or tetrafluoroborate salts of Ni(II), Cu(II), Zn(II), or Cd(II) are used, the metal ion enters the cavity which can be monitored by the hypsochromic shift of the intramolecular charge-transfer transition from the donor amino N atom to the acceptor dinitrobenzene. However, in the presence of coordinating ions such as Cl(-), N(3)(-), and SCN(-), the metal ion comes out of the cavity and binds the cryptand outside the cavity at a site away from the dinitrobenzene moiety. Four such complexes are characterized by X-ray crystallography. Thus, a metal ion can translocate between inside and outside of the cryptand cavity depending upon the nature of the counter anion.     

A Versatile CuII Metal–Organic Framework Exhibiting High Gas Storage Capacity with Selectivity for CO2: Conversion of CO2 to Cyclic Carbonate and Other Catalytic Abilities

A linear tetracarboxylic acid ligand, H₄L, with a pendent amine moiety solvothermally forms two isostructural metal–organic frameworks (MOFs) L_M (M=Zn^II, Cu^II). Framework L_Cu can also be obtained from L_Zn by post‐ synthetic metathesis without losing crystallinity. Compared with L_Zn , the L_Cu framework exhibits high thermal stability and allows removal of guest solvent and metal‐bound water molecules to afford the highly porous, L_Cu′ . At 77 K, L_Cu′ absorbs 2.57 wt % of H₂ at 1 bar, which increases significantly to 4.67 wt % at 36 bar. The framework absorbs substantially high amounts of methane (238.38 cm³ g^?1, 17.03 wt %) at 303 K and 60 bar. The CH₄ absorption at 303 K gives a total volumetric capacity of 166 cm³ (STP) cm^?3 at 35 bar (223.25 cm³ g^?1, 15.95 wt %). Interestingly, the NH₂ groups in the linker, which decorate the channel surface, allow a remarkable 39.0 wt % of CO₂ to be absorbed at 1 bar and 273 K, which comes within the dominion of the most famous MOFs for CO₂ absorption. Also, L_Cu′ shows pronounced selectivity for CO₂ absorption over CH₄, N₂, and H₂ at 273 K. The absorbed CO₂ can be converted to value‐added cyclic carbonates under relatively mild reaction conditions (20 bar, 120 °C). Finally, L_Cu′ is found to be an excellent heterogeneous catalyst in regioselective 1,3‐dipolar cycloaddition reactions (“click” reactions) and provides an efficient, economic route for the one‐pot synthesis of structurally divergent propargylamines through three‐component coupling of alkynes, amines, and aldehydes.     

Kinetics of Forward Extraction of V(IV) in V(IV)‐SO42−(H+, Na+)‐Cyanex 301–Kerosene System Using the Lewis Cell Technique

Kinetics of the extraction of V(IV) from acidic sulfate medium by Cyanex 301 dissolved in kerosene has been investigated using a constant interfacial area stirred cell with laminar flow (Lewis cell). Reaction orders with respect to different concentration terms, rate constant, activation energy, entropy change in activation, and the enthalpy change in activation have been measured at a phase circulation speed of 3 Hz. The rate equation at 293 K is F (kmol/m²s) = 10^?7.02 (1 + 0.025[V(IV)]^?1)^?1 (1 + 398[H⁺])^?1 (1 + 0.10 [HA]_o^?1)^?1 (1 +  3.16 [SO₄^2?]). Based on the experimental results, the mechanism of extraction has been proposed.     

DSC,density and refractive index studies on two nematogenic p-alkylphenyl-2-chloro-4-(p-alkylbenzoyloxy)-benzoates and their mixtures

The compounds p-pentylphenyl-2-chloro-4-(p-pentylbenzoyloxy)-benzoate [PCPB] and p-octylphenyl-2-chloro-4-(p-heptylbenzoyloxy)-benzoate [OCHB], and mixtures of different mole fractions of them, show a nematic phase with a supercooling effect. From DSC studies both ΔH and ΔS at T _NI are found to have minimum values for the equimolar mixture, which happens to be an eutectic mixture (Mix-II). However the density of this mixture is found to be less than that of the pure compounds. Refractive indices (n _o, n_e) are measured by thin prism method at different temperatures for pure components and the eutectic mixture. The birefringence values of the eutectic mixture are less than those of the individual components at higher temperature region, and are in-between at lower temperatures. The principal polarizibilities (α _o, α _e) of all the compounds have been calculated using both Vuks’ and Neugebauer's methods. These values have been compared with those calculated from additivity rules of bond polarizability. The average polarizability α increases with chain length. The orientational order parameter (P ₂) at different temperatures has been calculated using Haller's extrapolation procedure. (P ₂) values of OCHB are found to be greater than those of PCPB and Mix-II at higher temperatures, and are consistent with those determined from our X-ray studies. The temperature dependence of (P ₂) of all the compounds has been compared with Maier-Saupe theoretical values.     

Influence of Nonequilibrium LO Phonons on High-frequency Performance of One-dimensional Hot Electrons in Quantum Wires of Polar Semiconductors

Small-signal ac transport of degenerate one-dimensional hot electrons in quantum wires of GaAs and In_0.53Ga_0.47As is studied for lattice temperatures of 77 K and 300 K. The carrier energy loss via polar optic phonons and momentum losses via polar optic phonons, acoustic phonons and ionized impurities are included in the calculations. Alloy disorder scattering in momentum loss is additionally incorporated for (In,Ga)As. The consideration of nonequilibrium optical phonons or hot phonons is found to enhance the 3dB cut-off frequency (f_3dB) considerably, where the ac mobility falls to 0.707 of its low frequency value. f_3dB is generally higher for (In,Ga)As quantum wire than for GaAs.