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

Copper(II) complexes formed by coordination of the Cu-parallel to ion with liquid-crystalline poly(propylene imine) dendrimer ligand Q of the first (complex 7) and second (complex 2) generations with various Cu-parallel to contents (x=Cu/L) have been studied by electron paramagnetic resonance (EPR) spectroscopy. The existence of a redox-octive blue complex I (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 I (x = 1.9) is adequately described as a mixed-valence dimer containing d(9)- and diamagnetic d(10)-configuroted copper ions, and an anti-ferromagnetically coupled NO3 center dot 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 I (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= I (completely aligned system). A combination of magnetic and orientational parameters indicates an NO, environment of the Cu-parallel to ion in green complex 1 (x = 1.9), and confirms the chain structure with intermolecular Cu-parallel to-NO3-Cu-parallel to bridges between Cu-parallel to centres in columns.