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.
X- and Q-band electron paramagnetic resonance (EPR) investigation of different crystalline
Alq3
(tris(8-hydroxyquinoline)aluminium (III)) fractions formed by a train sublimation
method are reported. Several paramagnetic defect centres corresponding to
1/2, 1,
and 3/2
spin are observed at room temperature. Their intensity is dependent on the temperature,
nature of the crystalline phase, and preparation conditions. Spectra simulation
and analysis based on the spin Hamiltonian appropriate to a high spin system
(S≥1)
suggest the existence of randomly oriented triplets and quartets in annealed
Alq3 fractions. The
crystalline Alq3
phases responsible for the EPR powder spectra have been identified by transmission
electron microscopy measurements performed on these sample fractions.
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.
EPR and TEM investigations on sublimated solid Alq3, [tris(8- hydroxyquinoline)aluminum(III)] are reported. Different paramagnetic centers in microcrystalline Alq3 fractions as synthesized and annealed are observed. Their nature, thermal treatment dependence, as well as correlation with the crystal structure are analyzed. The existence of randomly oriented triplets and quartets in annealed Alq3 fractions is evidenced from the temperature dependence of X- and Q-band EPR spectra. The spin Hamiltonian parameters are given and proposals for the model structures are made.
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