71 research outputs found
Charge migration in organic materials: Can propagating charges affect the key physical quantities controlling their motion?
Charge migration is a ubiquitous phenomenon with profound implications
throughout many areas of chemistry, physics, biology and materials science. The
long-term vision of designing functional materials with tailored molecular
scale properties has triggered an increasing quest to identify prototypical
systems where truly molecular conduction pathways play a fundamental role. Such
pathways can be formed due to the molecular organization of various organic
materials and are widely used to discuss electronic properties at the nanometer
scale. Here, we present a computational methodology to study charge propagation
in organic molecular stacks at nano and sub-nanoscales and exploit this
methodology to demonstrate that moving charge carriers strongly affect the
values of the physical quantities controlling their motion. The approach is
also expected to find broad application in the field of charge migration in
soft matter systems.Comment: 18 pages, 6 figures, accepted for publication in the Israel Journal
of Chemistr
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Single-Electron Lanthanide-Lanthanide Bonds Inside Fullerenes toward Robust Redox-Active Molecular Magnets
A characteristic phenomenon of lanthanide-fullerene interactions is the transfer of metal valence electrons to the carbon cage. With early lanthanides such as La, a complete transfer of six valence electrons takes place for the metal dimers encapsulated in the fullerene cage. However, the low energy of the σ-type Ln-Ln bonding orbital in the second half of the lanthanide row limits the Ln2 → fullerene transfer to only five electrons. One electron remains in the Ln-Ln bonding orbital, whereas the fullerene cage with a formal charge of -5 is left electron-deficient. Such Ln2@C80 molecules are unstable in the neutral form but can be stabilized by substitution of one carbon atom by nitrogen to give azafullerenes Ln2@C79N or by quenching the unpaired electron on the fullerene cage by reacting it with a chemical such as benzyl bromide, transforming one sp2 carbon into an sp3 carbon and yielding the monoadduct Ln2@C80(CH2Ph). Because of the presence of the Ln-Ln bonding molecular orbital with one electron, the Ln2@C79N and Ln2@C80(R) molecules feature a unique single-electron Ln-Ln bond and an unconventional +2.5 oxidation state of the lanthanides.In this Account, which brings together metallofullerenes, molecular magnets, and lanthanides in unconventional valence states, we review the progress in the studies of dimetallofullerenes with single-electron Ln-Ln bonds and highlight the consequences of the unpaired electron residing in the Ln-Ln bonding orbital for the magnetic interactions between Ln ions. Usually, Ln···Ln exchange coupling in polynuclear lanthanide compounds is weak because of the core nature of 4f electrons. However, when interactions between Ln centers are mediated by a radical bridge, stronger coupling may be achieved because of the diffuse nature of radical-based orbitals. Ultimately, when the role of a radical bridge is played by a single unpaired electron in the Ln-Ln bonding orbital, the strength of the exchange coupling is increased dramatically. Giant exchange coupling in endohedral Ln2 dimers is combined with a rather strong axial ligand field exerted on the lanthanide ions by the fullerene cage and the excess electron density localized between two Ln ions. As a result, Ln2@C79N and Ln2@C80(CH2Ph) compounds exhibit slow relaxation of magnetization and exceptionally high blocking temperatures for Ln = Dy and Tb. At low temperatures, the [Ln3+-e-Ln3+] fragment behaves as a single giant spin. Furthermore, the Ln-Ln bonding orbital in dimetallofullerenes is redox-active, which allows its population to be changed by electrochemical reactions, thus changing the magnetic properties because the change in the number of electrons residing in the Ln-Ln orbital affects the magnetic structure of the molecule. © 2019 American Chemical Society
Local origin of the strong field-space anisotropy in the magnetic phase diagrams of CeLaB measured in a rotating magnetic field
Cubic f-electron compounds commonly exhibit highly anisotropic magnetic phase
diagrams consisting of multiple long-range ordered phases. Field-driven
metamagnetic transitions between them may depend not only on the magnitude, but
also on the direction of the applied magnetic field. Examples of such behavior
are plentiful among rare-earth borides, such as RB or RB ( = rare
earth). In this work, for example, we use torque magnetometry to measure
anisotropic field-angular phase diagrams of La-doped cerium hexaborides,
CeLaB ( = 0, 0.18, 0.28, 0.5). One expects that
field-directional anisotropy of phase transitions must be impossible to
understand without knowing the magnetic structures of the corresponding
competing phases and being able to evaluate their precise thermodynamic energy
balance. However, this task is usually beyond the reach of available
theoretical approaches, because the ordered phases can be noncollinear, possess
large magnetic unit cells, involve higher-order multipoles of 4f ions rather
than simple dipoles, or just lack sufficient microscopic characterization. Here
we demonstrate that the anisotropy under field rotation can be qualitatively
understood on a much more basic level of theory, just by considering the
crystal-electric-field scheme of a pair of rare-earth ions in the lattice,
coupled by a single nearest-neighbor exchange interaction. Transitions between
different crystal-field ground states, calculated using this minimal model for
the parent compound CeB6, possess field-directional anisotropy that strikingly
resembles the experimental phase diagrams. This implies that the anisotropy of
phase transitions is of local origin and is easier to describe than the ordered
phases themselves.Comment: To be published in Phys. Rev. B; v2: minor typographic correction
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High Blocking Temperature of Magnetization and Giant Coercivity in the Azafullerene Tb 2 @C 79 N with a Single-Electron Terbium–Terbium Bond
The azafullerene Tb 2 @C 79 N is found to be a single-molecule magnet with a high 100-s blocking temperature of magnetization of 24 K and large coercivity. Tb magnetic moments with an easy-axis single-ion magnetic anisotropy are strongly coupled by the unpaired spin of the single-electron Tb−Tb bond. Relaxation of magnetization in Tb 2 @C 79 N below 15 K proceeds via quantum tunneling of magnetization with the characteristic time τ QTM =16 462±1230 s. At higher temperature, relaxation follows the Orbach mechanism with a barrier of 757±4 K, corresponding to the excited states, in which one of the Tb spins is flipped. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA
Transverse Electronic Transport through DNA Nucleotides with Functionalized Graphene Electrodes
Graphene nanogaps and nanopores show potential for the purpose of electrical
DNA sequencing, in particular because single-base resolution appears to be
readily achievable. Here, we evaluated from first principles the advantages of
a nanogap setup with functionalized graphene edges. To this end, we employed
density functional theory and the non-equilibrium Green's function method to
investigate the transverse conductance properties of the four nucleotides
occurring in DNA when located between the opposing functionalized graphene
electrodes. In particular, we determined the electrical tunneling current
variation as a function of the applied bias and the associated differential
conductance at a voltage which appears suitable to distinguish between the four
nucleotides. Intriguingly, we observe for one of the nucleotides a negative
differential resistance effect.Comment: 19 pages, 7 figure
Record-high thermal barrier of the relaxation of magnetization in the nitride clusterfullerene Dy 2
- …