Fritz Dietz

Energy Spectra and Electric and Magnetic Properties of One-Dimensional Stacks of Conjugated π-Electron Systems with Defect Surface States. II. π-Systems with Schottky and Chemisorption Surface States

A rational new method for the synthesis of variable-sized Fe3O4 nanoparticles by aerial oxidation of Fe(OH)2 under visible light irradiation in the presence of trace EDTA has been investigated. The size of the Fe3O4 nanocrystals was controlled at about 11.5−30.0 nm by varying the reaction conditions in aqueous solution at room temperature. The oxidation reaction was significantly accelerated by visible light in the presence of EDTA. A rapid oxidation rate was favorable for decreasing the size of the Fe3O4 nanocrystals. The as-obtained Fe3O4 particles showed uniform spherical shape and a narrow size distribution. The variable-sized Fe3O4 nanocrystals exhibited different magnetic properties.

Organic polymers with indirect magnetic interaction caused by the molecular topology of the elementary units

Abstract The magnetic properties of two classes of open-shell π-systems, namely diradicals and one-dimensional polymers, having frontier non-overlapping non-bonding MOs (NBMOs) localized within the monoradical units have been studied theoretically. The non-overlap and localization of the NBMOs is determined by the topology of the π-systems, and can be explained by means of the extended Coulson–Rushbrooke–Longuet-Higgins theorem. The localization and non-overlapping of the NBMOs results in vanishing of the direct (Hund) and the kinetic exchange interaction between the unpaired electrons. The indirect exchange of the unpaired electrons (spin polarization exchange) is the only contribution to the effective exchange integral and occurs via the delocalized π-electrons in the filled energy bands. A comparison of our models is accomplished with the indirect exchange in rare earth compounds (f–s model) and with the superexchange of the Kramers–Anderson model.

ENERGY-SPECTRA AND MAGNETIC-PROPERTIES OF RADIALENES, DENDRALENES AND THEIR POLY-RADICAL IONS - A NEW CLASS OF POLARONIC HIGH-SPIN PI-SYSTEMS

Abstract The energy spectra of infinite radialenes (RA) and dendralenes (DE) — the simplest one-dimensional π-systems possessing non-degenerate ground states — are theoretically investigated. Three factors, a topological, a geometrical, and a correlational, contribute to the energy gap of 3.6 eV. The charge density distribution of the Jahn-Teller active mono-cation of the [3]RA and the charged [6]RA, and the charged [6]DE is studied. The polymers derived from the poly-radical ions of infinite RAs possess a narrow half-filled band (HFB) within the energy gap. The different contributions to the effective spin exchange interaction between the unpaired electrons in the HFB: the potential exchange, the kinetic exchange, and the indirect exchange (due to the spin polarization) were calculated. In all cases the effective spin exchange is positive, i.e. the ground state of the polymers is a high-spin one. The ferromagnetic interaction within the HFB is due to the predominant role of the direct Coulomb exchange. A juxtaposition between the structural principle of the polaronic high-spin systems considered here and Fukutomes polaronic high-spin systems is discussed.

Model mechanisms for the thermal cis—trans isomerization of cyanines

Abstract Quantum-chemical calculations have been performed for rotations around the different CC bonds of streptocyanine cations and of various ion pairs of TMC + and PMC + with Cl − as the gegenion (counterion) in order to explain the experimentally well known temperature dependence of the activation energy for the thermal isomerization.

Nature of the magnetic interaction of Wurster's radicals in the solid state

Abstract The nature of the spin exchange interaction within the half-filled band in one-dimensional stacks of Wursters radicals in Wursters red bromide has been investigated theoretically. The various contributions to the Heisenberg effective exchange integral: direct, kinetic, and indirect spin exchange are evaluated quantitatively. It has been shown that the direct (Coulomb) exchange which causes the ferromagnetic coupling is small. The negative value of the effective exchange integral between Wannier functions, localized within the elementary units, has been determined by the kinetic and indirect (antiferromagnetic) exchange interaction.

Energy gap equation of 1D polyconjugated π-systems in the Hubbard approximation

Abstract It is shown that the energy gap of an arbitrary homonuclear or heteronuclear alternant or nonalternant π-conjugated system, having a singlet ground state, is given by the formula ΔEσ={Δ21(σ)corr+[Δ2(σ)corr+Δtop+Δgeom] 2} 1 2 , where Δtop and Δgeom are factors depending on the molecular topology and geometry, respectively. In the general case, the correlation corrections Δ1 (σ) and Δ2(σ) differ for MOs with different spin (σ=α,β) and ΔEβ≠ΔEβ. In some particular cases ΔEα= ΔEβ. The correlation correction Δ2(σ)corr corresponds to a non-uniform charge distribution in the π-system. In the case of homo-nuclear alternant systems Δ2(σ)corr=0. The investigations are carried out using the method of alternant molecular orbitals (AMO) in the Hubbard approximation. The band structure of the fully oxidized form of 1D poly(para-aniline)-poly(para-phenyleneimine), has been studied.

Photoswitching of the magnetic properties of one-dimensional π-electron systems

Abstract The change of the magnetic properties of the isomeric forms of photoresponsive model polymers with photochromic diarylethene elementary units substituted with radical centers are investigated theoretically. The band theory is used to calculate the effective exchange integral of the photoisomeric one-dimensional many-electron π-systems. It is shown that the magnetic properties can be reversibly switched from ferromagnetic to non-magnetic in some cases. In other cases the magnitude of the ferromagnetic exchange interaction is either increased or decreased in the photoisomers. The practicability of the photocyclization is estimated using reactivity indices in the excited state calculated in π-electron approximation with the HMO method. In most cases a reversible photocyclization (ring opening) should be possible.

Structure and energy spectra of a two-dimensional dielectric carbon allotrope

Abstract The energy spectra of two classes of infinite polymers consisting of an alternating fusion of cyclobutadiene and benzene rings are investigated theoretically. The dependence of the band gap on the topology, geometry and electron correlation is examined. By increasing the number of the cyclobutadiene and benzene rings in the elementary units the polymers tend towards a two dimensional carbon allotrope C . In contrast to graphite, the two-dimensional net C is a dielectric with a band gap ΔE ≈ 2 eV.

ENERGY-SPECTRA OF INFINITE PHENYLENES

Abstract The energy spectra of two topomers of infinite phenylenes: T (linear) and S (bent) are investigated theoretically. The band gap of topomer T (ΔE(T) = 2.12 eV) is due predominantly to electron correlation (Δcorr = 2.11 eV). This differs from the band gap of topomer S (ΔE(S) = 2.53 eV) which is determined by the topology (Δtop = 1.43 eV), geometry (Δgeom = 0.24 eV) and electron correlation (Δcorr = 1.91 eV). The topological features of both polymers are discussed.

Photoswitching of the magnetic properties of one-dimensional π-electron systems. Part II. Conjugated polymers with di-hetarylethene and benzylidene-anthrone fragments in the elementary units

The change in the magnetic properties of the isomeric forms of photoresponsive model polymers with photochromic di-hetarylethene, 10-benzylidene-anthrone, and 9-benzylidene-9,10-dihydroanthracene fragments in the elementary units and substituted with radical centers is investigated theoretically. Based on the band theory, the effective exchange integral is calculated for the photoisomeric one-dimensional conjugated π-electron systems. In the case of the polymers with di-hetarylethene fragments, an increase or decrease in the antiferromagnetic exchange is connected with the photoisomerization. A photoswitching between non-magnetic and ferromagnetic intramolecular exchange interaction (or ice ersa) of open-ring and closed-ring photoisomers should be possible with polymers containing photochromic 10-benzylidene-anthrone and 9-benzylidene-9,10-dihydroanthracene fragments.