Narrow Results

The magnons transport properties of molecular wires connecting two Heisenberg ferromagnets are studied within the framework of the matching method and with use of a realistic atomic structure. The model system consists of two nanostructured ferromagnetic films on either side of the junction and the atomic wire consists of a linear molecule connecting two ultrathin solid ferromagnetic films. A theoretical model is presented for the study of the transmission and the reflection of spin waves at the atomic wire junction. The calculation was made at the atomic scale for two identical waveguides with ordered spins and coupled by Heisenberg exchange interaction between first neighbors. Our analysis yields a detailed understanding of the spin-wave coherent scattering at the linear molecular junction. We calculate, in particular, the coherent reflection R and transmission T coefficients, which constitute the elements of the scattering matrix in accordance with the Landauer–Büttiker scattering formalism, as well as the magnon transmittance of the atomic wire for spin-waves incident from the interior of the film on the junction. The most representative numerical results obtained for the system of two slabs made up of three Fe ferromagnetic atomic layers connected with an Fe or Gd atomic wire are presented as function of the dimensionless frequency Ω in the magnons energy band. The coherent reflection and transmission scattering cross sections show characteristic spectral features, depending on the length of the wire, on the cut-off frequencies for the propagating magnons, as well as on the magnons incidence angle. The results illustrate the occurrence of Fano resonances in the transmitted spectra due to the interaction of localized spin states on the atomic wire with the propagating spin waves of the waveguide. An interesting physical effect is observed for this magnetic atomic junction, namely the frequency selective conductance of the spin waves via Fano resonances, by an appropriate choice of the spin-wave incident angle.

In the aim to prepare thick porphyrin molecular wires, which is visible by atomic force microscopy (AFM), even on the rough surfaces of nanogap electrodes fabricated by electron beam lithography, dendrimer protected porphyrins whose two meso-positions are substituted with ethynyl groups. The porphyrin monomer was reacted with palladium catalyst to make oligomers. Analyses of them with time-of-flight mass spectroscopy (TOF-MS), gel-permeation-chromatography (GPC) revealed that the oligomers were distributed up to 16 mer, whose molecular weight was about 38 000 Daltons.

The conductance properties of the thiophene bithiol molecular wire, a nano-wire connecting two metallic electrodes, were investigated using quantum-mechanical based methods such as Density Functional Theory, in conjunction with non-equilibrium Green's function formalism. Using the quantum mechanics methods, the Hamiltonians of the three main parts of system, i.e. the right lead, the device, the left lead and conductance properties of this molecular wire such as I-V curve, were calculated.

In this work, a detailed theoretical investigation of the substituent groups effect on the geometric and electronic properties of the newly proposed molecular wire has been performed with the DFT-B3LYP/6-31G* level of theory by considering the influence from the external electric field (EF). The results show that the performance of molecular wire is very sensitive to the electron donating (–NH₂ and –OH), electron withdrawing (–NO₂ and –F) groups and external EF intensities. The energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), E_LUMO–E_HOMO (HLG) in all cases decreased, furthermore, the electron withdrawing –NO₂ group had a much lower HLG than the other substituted groups. Results of this study indicate that the substitutions and external electric field can be used to tune the properties of a molecular scale device effectively.

Theoretical investigations on Au and S substituted poly (2,3-di-(4′-hydroxyphenyl)-1,4-phenylene ethynyl) as a molecular wire have been carried out by incorporating the external electric field (EF). The results demonstrate that both geometric and electronic structures of conjugated molecular wire are sensitive to EF. The Z component of the dipole moment and total dipole moment increases continuously since EF polarizes the molecule. Static EF modifies the frontline molecular orbitals and HLG. The I-V curve indicates the symmetrical trend respect to the original point.

The synthesis and structural characterization and the study of the electronic properties of two novel porphyrin-bridge-fullerene molecules, where a free-based porphyrin and [60]fullerene are connected through one and two units of ethylenedioxythienylenevinylene π-conjugated bridges, is reported. The absorption studies, voltamperometric measurements and theoretical calculations at DFT level are presented. A HOMO–LUMO gap as low as 1.41 eV has been found for compound 6.

A diruthenium complex with a free-base porphyrin linker 1 is synthesized and characterized by ¹H and ³¹P NMR, IR, and ESI-TOF-MS spectroscopy. A cyclic voltammogram of 1 shows two reversible waves attributed to the redox processes at the ruthenium centers, and a compropotionation constant (K_C) has been determined to be 1.8 × 10⁵, indicating that a mixed valence state of 1⁺ is thermodynamically stable. The monocationic complex 1⁺ obtained by chemical oxidation of 1 by [Cp₂Fe]PF₆ shows an intervalence charge transfer (IVCT) band in the NIR region. On the basis of the electronic coupling (V_ab) of 2644 cm^-1 obtained by analysis of the IVCT band, complex 1^+• is assigned as a Class III compound according to the Robin–Day classification. DFT calculation and IR study suggest that the strength of π-back donation is one of key determinants for a strong electronic coupling between the two metal centers.

meso–meso Directly-linked trimeric and pentameric porphyrin–hexaphyrin hybrid arrays 5 and 6 comprising of electron-deficient porphyrin units were prepared by cross-condensation of monomeric and dimeric electron-deficient meso-formyl porphyrins with a tripyrrane. The solid-state structures of 5 and 6 have been determined by single crystal X-ray diffraction analysis. The latter is the largest crystal structure of meso–meso linked multiporphyrinic array analogues reported to date.

In the aim to prepare thick porphyrin molecular wires, which is visible by atomic force microscopy (AFM), even on the rough surfaces of nanogap electrodes fabricated by electron beam lithography, dendrimer protected porphyrins whose two meso-positions are substituted with ethynyl groups. The porphyrin monomer was reacted with palladium catalyst to make oligomers. Analyses of them with time-of-flight mass spectroscopy (TOF-MS), gel-permeation-chromatography (GPC) revealed that the oligomers were distributed up to 16 mer, whose molecular weight was about 38 000 Daltons.