V. Likodimos

Electronic structure of polycrystalline polyamine copper dinitrate complexes investigated by photoacoustic and electron paramagnetic resonance spectroscopy

Photoacoustic and electron paramagnetic resonance (EPR) spectroscopies have been applied to resolve the electronic structure in powder polycrystalline samples of three biogenic polyamine copper complexes, spermine copper dinitrate, aqua norspermine copper dinitrate, and homospermine copper dinitrate. The fine structure of the intense absorption band in the photoacoustic spectra is assigned to the d–d transitions between the crystal field split levels of copper ions, that cannot be discriminated in the UV/vis solution absorption spectra. Combination with the EPR results allows one to probe the variation of the electronic properties and bonding interaction at the copper site, consistent with the structural data for the crystalline complexes and further supports the reliability of the photoacoustic method to resolve the d–d transition band. A dominant contribution of the in-plane ligand field due to equatorial nitrogen atoms is deduced for the complexes of polyamines with copper salts.

Magnetism in pristine and chemically reduced graphene oxide

The evolution of magnetism for graphene oxide (GO) before and after chemical reduction was investigated by means of static magnetization and electron spin resonance (ESR) spectroscopy. Strong paramagnetism with a saturation magnetization of ∼1.2u2009emu/g and weak antiferromagnetic interactions were identified in pristine GO. Apart from spin-half defect centers, ESR spectroscopy indicated the excitation of high spin states, consistently with the high spin (Su2009=u20092) magnetic moments derived from the magnetization analysis, corroborating the formation of spatially “isolated” magnetic clusters in GO. A marked reduction of GOs magnetization (∼0.17u2009emu/g) along with an appreciable rise of diamagnetism (−2.4u2009×u200910−6u2009emu/g Oe) was detected after chemical reduction by sodium borohydride, reflecting the drastic removal of paramagnetic defects and the concomitant growth of sp2 domains in reduced graphene oxide (rGO). ESR revealed a large drop of the spin susceptibility for rGO, which, in addition to the main paramagnetic...

Magnetic properties of carbon nanotube poly(ether-ester) nanocomposites

The magnetic and electronic properties of composites consisting of oxidized multiwall carbon nantubes (MWNTs) dispersed in elastomeric poly(ether-ester) segmented block copolymer have been studied by means of electron spin resonance (ESR) and dc magnetization measurements. A marked reduction in the MWNT diamagnetic response is identified, indicative of substantial hole doping related to the oxygen functional groups on the oxidized carbon nanotube’s surface. Both ESR and the static magnetization reveal considerable enhancement of the spin susceptibility due to an excessive increase in the density of paramagnetic defects, which are sensitive to the dynamical polymer relaxation and thus to the MWNT-polymer interfacial coupling.

Influence of water molecule coordination on the magnetic properties of polyamine copper dinitrate complexes

The magnetic properties of two biogenic polyamine copper complexes were investigated by dc magnetization and electron paramagnetic resonance (EPR) measurements. The variation in temperature of the low field magnetization reveals that the absence or presence of water molecules in the copper coordination sphere results, respectively, in the enhancement or suppression of antiferromagnetic interactions between Cu2+ ions. Analysis of the EPR spectra shows considerable temperature dependence of both the g values and EPR linewidths that persist in the paramagnetic regime for both complexes. Such variation of EPR parameters is attributed to the interplay of demagnetizing field effects, pertinent to the purely paramagnetic compound, and to the presence of short-range magnetic order that applies to the antiferromagnetic polyamine complex.

Low temperature magnetic phase transition and interlayer coupling in double-wall carbon nanotubes

The magnetic properties of double wall carbon nanotubes (DWCNTs) were investigated using electron spin resonance (ESR) spectroscopy. An asymmetric resonance line of low intensity was identified and analyzed by the superimposition of a narrow and a broad metallic lineshape, attributed to the distinct contributions of defect spins located on the inner and outer DWCNTs shells. The spin susceptibilities of both ESR components revealed a ferromagnetic phase transition at low temperatures (Tu2009<u200910u2009K) with small variation in the corresponding Curie-Weiss temperatures, approaching closely that of metallic single wall carbon nanotubes. Interlayer coupling between the DWCNT layers is suggested to effectively reduce the difference between the transition temperatures for the inner and outer shells and enhance spin-spin interactions between defect spins via the RKKY-type interaction of localized spins with conduction electrons.

Copper Defects and Conductivity in Bi—Pb—Sr—Ca—Cu—O Glasses

Glass ceramics of the composition (Bi0.8Pb0.2)4Sr3Ca3Cu4O8 prepared by the melt quenching technique and the crystalline phases produced by the rapid thermal annealing have been studied by electrical resistivity and electron paramagnetic resonance (EPR) measurements in the temperature range from liquid helium up to room temperature. The concentration of the EPR active Cu2☎ paramagnetic centers decreases as conductivity increases for the glass ceramics and disappears after crystallization and the growth of superconducting phases, similar to bulk high-Tc superconductors. The KPR spectra of both glass and crystallized ceramics after short-time annealing indicate the coexistence of Cu2☎ paramagnetic ions and the exchange coupled clusters.

EPR Study of Polyamine Copper Complexes

Organometallic complexes of naturally occurring polyamines (biogenic amines) attract particular interest due to their implication in a number of biological processes. In particular, the centers of polyamine interactions with other bioligands constitute potential coordination sites of metal ions present in living cells, while polyamines may be treated as the interfering factor in the formation of complexes between metal ions and nucleic acids [1,2]. Considerable research effort has been accordingly devoted to crystallographic and spectroscopic studies of crystalline polyamine complexes with various inorganic cations and anions, in order to determine the structural conformation of polyamine cations and explore the relation between protonation and metalation of biogenic polyamines [3–5].