Energy and electron transfer reactions on silica gel and titania–silica mixed oxide surfaces

Research on Chemical Intermediates - The energy and electron transfer reactions of anthracene co-adsorbed with an electron donor on silica gel and titania–silica mixed oxides have been...     

Magnetic properties of poly(propylene imine)-copper dendromesogenic complexes: An EPR study.

Copper(II) complexes formed by coordination of the Cu(II) ion with liquid-crystalline poly(propylene imine) dendrimer ligand (L) of the first (complex 1) and second (complex 2) generations with various Cu(II) contents (x = Cu/L) have been studied by electron paramagnetic resonance (EPR) spectroscopy. The existence of a redox-active blue complex 1 (x = 1.9) and the copper(II) nitrate electron transfer associated with the valence tautomerism are revealed for the first time in copper-based dendrimers. It has been shown that the electronic structure of the blue complex 1 (x = 1.9) is adequately described as a mixed-valence dimer containing d9- and diamagnetic d10-configurated copper ions, and an antiferromagnetically coupled NO3* radical arising on the nitrate-bridged counter ligand. The activation energy value found for the electron transfer is about 0.35 meV, which indicates a low-energy charge dynamic. The ability of the blue and green complexes 1 (x = 1.9) dissolved in isotropic solvents to orient themselves in the magnetic field was revealed by EPR spectroscopy. The degree of orientation of the molecular z axis (S(z)) of these complexes in the magnetic field differs, depending on the type of copper(II)-complexing site in the dendrimer ligand, and can reach 0.76, which is close to S(z) = 1 (completely aligned system). A combination of magnetic and orientational parameters indicates an NO4 environment of the Cu(II) ion in green complex 1 (x = 1.9), and confirms the chain structure with intermolecular Cu(II)-NO3-Cu(II) bridges between Cu(II) centres in columns.     

Infrared analysis of phase transitions in liquid crystal acrylate monomers bearing urethanic groups and cholesteryl as mesogen

An infrared analysis of phase transitions in three enantiotropic liquid crystalline acrylate monomers with different spacer lengths bearing urethane groups and cholesteryl as mesogen was carried out. Through heating and cooling, spectral modifications especially at the level of carbonyl, –NH– and urethane alkoxyl oxygen groups were found. These findings reveal the influence of hydrogen bonding on mesomorphic properties of the studied compounds mentioning that both liquid crystal transitions are evidenced by the spectral changes. For all the compounds studied, the values of the transition temperatures deduced from the spectral modifications are in good agreement with those obtained from DSC measurements.     

EPR characterisation of Cu(II) complexes of poly(propylene imine) dendromesogens: using the orienting effect of a magnetic field.

Liquid-crystalline derivatives of poly(propylene imine)dendrimers of the 0th, 1st and 2nd generations, complexed with copper(II) ions, were studied by EPR spectroscopy. The structures of copper (II) complexes with different Cu(II) loadings x per dendrimer ligand L (x = Cu/L) were determined. At the lowest concentration, the Cu(II) ions form monomeric complexes with approximately square-planar N2O2 coordination of both carbonyl oxygen and amido nitrogen atoms. At higher copper content, two kinds of Cu(II) complex sites with different geometries exist. The orienting effect of a high magnetic field was used to investigate the structure and magnetic properties of the copper(II) complexes. This effect, for the first time in dendrimers, allowed the resolution of five nitrogen super-hyperfine lines on g(z) components with the unusual coupling constant of a(Nz)= 35.9 x 10(-4) cm(-1). The combination of the magnetic parameters and the orienting effect indicates the presence of a monomeric complex with pseudotetrahedral N2O2 coordination of the Cu(II) ion, as well as a "dimer" structure with fivefold coordination, presumably due to an N3O2 environment. Higher copper loadings lead to increased exchange coupling between the complex sites.