Low-spin Ground State Research Articles

Structure and spin of the low- and high-spin states of Fe2+(phen)3 studied by x-ray scattering and emission spectroscopy.

The structure and spin of photoexcited Fe2+(phen)3 in water are examined by x-ray scattering and x-ray emission spectroscopy with 100 ps time resolution. Excitation of the low-spin (LS) ground state (GS) to the charge transfer state 1MLCT* leads to the formation of a high-spin (HS) state that returns to the GS in 725 ps. Density functional theory (DFT) predicts a Fe-N bond elongation in HS by 0.19 Å in agreement with the scattering data. The angle between the ligands increases by 5.4° in HS, which allows the solvent to get 0.33 Å closer to Fe in spite of the expansion of the molecule. The rise in solvent temperature from the return of photoproducts to the GS is dominated by the formation dynamics of HS, 1MLCT* → HS, which is followed by a smaller rise from the HS → GS transition. The latter agrees with the 0.61 eV energy gap E(HS)-E(LS) calculated by DFT. However, the temperature rise from the 1MLCT → HS transition is greater than expected, by a factor of 2.1, which is explained by the re-excitation of nascent HS* by the 1.2 ps pump pulse. This hypothesis is supported by optical spectroscopy measurements showing that the 1.2 ps long pump pulse activates the HS* → 5MLCT* channel, which is followed by the ultrafast return to HS* via intersystem crossing. Finally, the spins of the photoproducts are monitored by the Kβ emission and the spectra confirm that the spins of LS and HS states are 0 and 2, respectively.

Unveiling the Low-Lying Spin States of [Fe3S4] Clusters via the Extended Broken-Symmetry Method.

Photosynthetic water splitting, when synergized with hydrogen production catalyzed by hydrogenases, emerges as a promising avenue for clean and renewable energy. However, theoretical calculations have faced challenges in elucidating the low-lying spin states of iron-sulfur clusters, which are integral components of hydrogenases. To address this challenge, we employ the Extended Broken-Symmetry method for the computation of the cubane-[Fe3S4] cluster within the [FeNi] hydrogenase enzyme. This approach rectifies the error caused by spin contamination, allowing us to obtain the magnetic exchange coupling constant and the energy level of the low-lying state. We find that the Extended Broken-Symmetry method provides more accurate results for differences in bond length and the magnetic coupling constant. This accuracy assists in reconstructing the low-spin ground state force and determining the geometric structure of the ground state. By utilizing the Extended Broken-Symmetry method, we further highlight the significance of the geometric arrangement of metal centers in the cluster's properties and gain deeper insights into the magnetic properties of transition metal iron-sulfur clusters at the reaction centers of hydrogenases. This research illuminates the untapped potential of hydrogenases and their promising role in the future of photosynthesis and sustainable energy production.

How Do Differences in Electronic Structure Affect the Use of Vanadium Intermediates as Mimics in Nonheme Iron Hydroxylases?

We study active-site models of nonheme iron hydroxylases and their vanadium-based mimics using density functional theory to determine if vanadyl is a faithful structural mimic. We identify crucial structural and energetic differences between ferryl and vanadyl isomers owing to the differences in their ground electronic states, i.e., high spin (HS) for Fe and low spin (LS) for V. For the succinate cofactor bound to the ferryl intermediate, we predict facile interconversion between monodentate and bidentate coordination isomers for ferryl species but difficult rearrangement for vanadyl mimics. We study isomerization of the oxo intermediate between axial and equatorial positions and find the ferryl potential energy surface to be characterized by a large barrier of ca. 10 kcal/mol that is completely absent for the vanadyl mimic. This analysis reveals even starker contrasts between Fe and V in hydroxylases than those observed for this metal substitution in nonheme halogenases. Analysis of the relative bond strengths of coordinating carboxylate ligands for Fe and V reveals that all of the ligands show stronger binding to V than Fe owing to the LS ground state of V in contrast to the HS ground state of Fe, highlighting the limitations of vanadyl mimics of native nonheme iron hydroxylases.

Nagaoka ferromagnetism in an array of phosphorene quantum dots.

We consider an array of four quantum dots defined in phosphorene containing three excess electrons, i.e., in the conditions of near half filling when itinerant Nagaoka ferromagnetism is expected to appear in a square array with isotropic interdot hopping. The interdot hopping in the array arranged in a square inherits the anisotropy from the form of the phosphorene conduction band. We apply the configuration interaction method for discussion of the appearance and stability of the spin-polarized ground state and discuss the compensation of the effective mass anisotropy by the geometry of the quantum dot array. Our study shows strong stability of Nagaoka ferromagnetism for optimized geometry of the array, with the Nagaoka gap as large as ∼230µeV. A phase diagram for the ground-state spin ordering versus the geometric parameters of the array is presented. We study the suppression of the ferromagnetism in a transition of the [Formula: see text] array to a quasi-1D chain and indicate that the shift of one of the quantum dots away from the array center is enough to transform the system to a quantum dot chain. A shift in the zigzag crystal direction induces the low-spin ground state more effectively than a shift along the armchair direction. We also discuss the robustness of the spin ordering against detuning one of the dots. The ferromagnetic ground-state survives as long as the detuning is not large enough to trap one of the electrons within a single quantum dot (for positive detuning) or remove one of the quantum dots of the accessible energy range (for negative detuning).

Understanding Antiferromagnetic and Ligand Field Effects on Spin Crossover in a Triple-Decker Dimeric Cr(II) Complex.

Two possible explanations for the temperature dependence of spin-crossover (SCO) behavior in the dimeric triple-decker Cr(II) complex ([(η5-C5Me5)Cr(μ2:η5-P5)Cr(η5-C5Me5)]+) have been offered. One invokes variations in antiferromagnetic interactions between the two Cr(II) ions, whereas the other posits the development of a strong ligand-field effect favoring the low-spin ground state. We perform multireference electronic structure calculations based on the multiconfiguration pair-density functional theory to resolve these effects. We find quintet, triplet, and singlet electronic ground states, respectively, for the experimental geometries at high, intermediate, and low temperatures. The ground-state transition from quintet to triplet at an intermediate temperature derives from increased antiferromagnetic interactions between the two Cr(II) ions. By contrast, the ground-state transition from triplet to singlet at low temperature can be attributed to increased ligand-field effects, which dominate with continued variations in antiferromagnetic coupling. This study provides quantitative detail for the degree to which these two effects can act in concert for the observed SCO behavior in this complex and others subject to temperature-dependent variations in geometry.

Chemical Manipulation of the Spin-Crossover Dynamics through Judicious Metal-Ion Dilution

In 2019, our groups described a unique FeII complex, [Fe(2MeL)(NCBH3)2] (2MeL = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-1,2-ethanediamine) possessing a low-spin ground state that is not easily accessible due to the extremely slow dynamics of the high-spin to low-spin phase transition. Herein, we report the successful chemical manipulation of this spin-crossover (SCO) process through controlled metal-ion dilutions. The emergence or suppression of the thermally induced SCO behavior was observed depending on the radius of the metal ion used for the dilution (NiII or ZnII). Reversible photo-switching has been confirmed in all mixed-metal complexes whether the low-spin state is thermally accessible. Remarkably, the dilution with ZnII metal ions stabilizes HS FeII complexes with complete suppression of the thermally induced SCO process without destroying the reversible photoswitchability of the material.

Theoretical Understanding of Reactions of Rhenium and Ruthenium Tris(thiolate) Complexes with Unsaturated Hydrocarbons: Noninnocent Nature of the Ligand, Mechanism, and Origin of Differential Reactivity

In retrospect to the complexity induced by the noninnocent ligands in identifying the transition metal's oxidation state and correlating the ligand's noninnocence with reactivity, the reactions of alkene/alkyne addition to rhenium/ruthenium tris(thiolate) complexes are particularly good cases for shedding light on the chemistry of the dppbt ligand, including its noninnocent nature, ligand-centered mechanism, and origin of differential reactivity. Density functional theory (DFT) combined with the high-level ab initio calculations performed herein demonstrates that, upon alkene/alkyne addition, the orbital symmetry properly regulates the reaction to form ligand-centered cis-interligand dithioethers as the most favorable pathway. The neutral and cationic Re and Ru dithioethers are revealed via DFT calculations to be in a low-spin ground state; on the contrary, high-level ab initio methods confirm that the dicationic Re-dithioethers exhibit obvious multireference character with antiferromagnetic coupling between Re-dyz and S1-py. The metal-stabilized thiyl radicals play a pivotal role in delivering the reactivity of [RuL3]+ and [ReL3]+/2+ toward alkene/alkyne rather than [ReL3], where [RuL3]+ and [ReL3]+/2+ present significant radical characters on ligand S2, yet neutral [ReL3] has little such feature, from which differential reactivity arises. Faster styrene addition to Ru tris(thiolate) in contrast to Re tris(thiolate) has been properly interpreted using DFT calculations with major products assigned. The deeper understanding gained in this work would illuminate further experimental exploration in adding alkene/alkyne to other metal-stabilized thiyl radicals.

Magnetic behavior and ground spin states for coordination {L·[MII(Hal)2]3}3- assemblies (Hal = Cl or I) of radical trianion hexacyanohexaazatriphenylenes (L) with three coordinated high-spin FeII (S = 2) or CoII (S = 3/2) centers.

A series of trianion assemblies of hexaazatriphenylenehexacarbonitrile {HAT(CN)6} and hexaazatrinaphthylenehexacarbonitrile {HATNA(CN)6} with three Fe(II) or Co(II) ions: {cryptand(K+)}3·{HATNA(CN)6·(FeIII2)3}3-·2C6H4Cl2 (1), {cryptand(K+)}3·{HATNA(CN)6·(CoIII2)3}3-·2C6H4Cl2 (2), and (CV+)3·{HAT(CN)6·(CoIICl2)3}3-·0.5(CVCl)·2.5C6H4Cl2 (3) are synthesized (CVCl = crystal violet). Salt 1 has a χMT value of 9.80 emu K mol-1 at 300 K, indicating a contribution of three high-spin FeII (S = 2) and one S = 1/2 of HATNA(CN)6˙3-. The χMT value increases with cooling up to 12.92 emu K mol-1 at 28 K, providing a positive Weiss temperature of +20 K. Such behavior is described using a strong antiferromagnetic coupling between S = 2 and S = 1/2 with J1 = -82.1 cm-1 and a weaker FeII-FeII antiferromagnetic coupling with J2 = -7.0 cm-1. As a result, the spins of three Fe(II) ions (S = 2) align parallel to each other forming a high-spin S = 11/2 system. Density functional theory (DFT) calculations support a high-spin state of CoII (S = 3/2) for 2 and 3. However, the χMT value of 2 and 3 is 2.25 emu K mol-1 at 300 K, which is smaller than 6 emu K mol-1 calculated for the system with three independent S = 3/2 and one S = 1/2 spins. In contrast to 1, the χMT values decrease with cooling to 0.13-0.36 emu K mol-1 at 1.9 K, indicating that spins of cobalt atoms align antiparallel to each other. Data fitting using PHI software for the model consisting of three high-spin Co(II) ions and an S = 1/2 radical ligand shows very large CoII-L˙3- coupling for 2 and 3 with J1 values of -442 and -349 cm-1. The CoII-CoII coupling via the ligand (J2) is also large, being -100 and -84 cm-1, respectively, which is more than 10 times larger than that of 1. One of the reasons for the J2 increase may be the shortening of the Co-N(L) bonds in 3 and 2 to 2.02(2) and 1.993(12) Å. DFT calculations support the population of the quartet state for the Co3 system, whereas the high-spin decet (S = 9/2) state is positioned higher by 680 cm-1 and is not populated at 300 K. This is explained by the large CoII-CoII coupling. Thus, a balance between J1 and J2 couplings provides parallel or antiparallel alignment of the FeII and CoII spins, leading to high- or low-spin ground states of {L·[MII(Hal)2]3}3-.

A heterometallic [Mn9Ni2] cluster consisting of the [M4(μ3-O)3(μ3-Cl)]n+ cubane and [MnIII3(μ3-O)4]+ “V-shaped” sub-units appearing in the giant [Mn84] and [Mn70] compounds and its [Mn9CoIII2] analogue

Two new heterometallic clusters exhibiting related cores, [Mn9Ni2(μ3-Ο)10(μ3-Cl)2(O2CMe)11(py)4(H2O)4]·2H2O (1·2H2O) (py = pyridine) and [Mn9Co2(μ3-Ο)10(μ3-ΟΗ)2(O2CEt)10(EtOH)3(H2O)(py)5](ClO4)2Cl (2) are reported. Compounds 1·2H2O/2 were prepared from reactions of [Mn3O(O2CMe)6(py)3]·py/[Mn3O(O2CEt)6(py)3](ClO4) with MCl2·6H2O (M = Ni2+, Co2+) and (Bun4N)MnO4 in a 1:3:0.1/1:1:0.1 molar ratio in MeCN (15 ml)/MeOH-EtOH (15–0.5 ml) in ∼ 28 %/< 2 % yields, respectively. Their cores consist of two [MnIVMnIII2Mn+(μ3-Ο)3(μ3-X)]m+ cubanes (Mn+ = Ni2+, X = Cl−, m = 5+, 1·2H2O; Mn+ = Co3+, X = OH−, m = 6+, 2) at the terminal positions linked through a central [MnIII3(μ3-Ο)4]+ slightly V-shaped sub-unit. Interestingly, this core has appeared as a fragment in the giant [Mn84] and [Mn70] torus–like single–molecule magnets (SMMs). Magnetism studies of 1·2H2O revealed the presence of competing ferromagnetic and antiferromagnetic exchange interactions leading to a fairly low spin ground state value of ST ≈ 4 or 3 and fully visible out-of-phase ac signals (above 1.8 K) indicative of slow relaxation of the magnetization and SMM behavior.

Unexpected Light-Induced Thermal Hysteresis in Matrix Embedded Low Cooperative Spin Crossover Microparticles

The embedding of spin-crossover micro- or nanocrystals in various surroundings dramatically changes their functionalities based on first-order spin transitions. The dampening of their internal cooperativity, together with introducing a new kind of interactions occurring at interfaces between spin-crossover particles and their environment, results in spectacular effects, as an enhanced hysteresis with non-cooperative transitions. In this work, we deal with the influence of the embedding matrix on the light-induced thermal hysteresis (LITH) in the case of spin-crossover microparticles of Fe(phen)2(NCS)2. Despite the low cooperativity of this compound, the competition between the continuous photoexcitation towards the metastable high spin state and the relaxation down to low spin ground state leads to a light-induced thermal hysteresis, with a quasi-static width of around 10 K. This unexpected hysteresis is explained by considering a switch-on/cutoff mechanism of the particle–matrix interactions in the framework of a mean-field approach based on negative external pressures, with Gaussian distributed variations and of an Ising-like model with various interactions with the environment. Additional first-order reversal curves measurements and corresponding calculated distributions are in line with relaxations under light and confirm the existence of a non-kinetic LITH.

Influence of the heme distal pocket on nitrite binding orientation and reactivity in Sperm Whale myoglobin.

Nitrite binding to recombinant wild-type Sperm Whale myoglobin (SWMb) was studied using a combination of spectroscopic methods including room-temperature magnetic circular dichroism. These revealed that the reactive species is free nitrous acid and the product of the reaction contains a nitrite ion bound to the ferric heme iron in the nitrito- (O-bound) orientation. This exists in a thermal equilibrium with a low-spin ground state and a high-spin excited state and is spectroscopically distinct from the purely low-spin nitro- (N-bound) species observed in the H64V SWMb variant. Substitution of the proximal heme ligand, histidine-93, with lysine yields a novel form of myoglobin (H93K) with enhanced reactivity towards nitrite. The nitrito-mode of binding to the ferric heme iron is retained in the H93K variant again as a thermal equilibrium of spin-states. This proximal substitution influences the heme distal pocket causing the pKa of the alkaline transition to be lowered relative to wild-type SWMb. This change in the environment of the distal pocket coupled with nitrito-binding is the most likely explanation for the 8-fold increase in the rate of nitrite reduction by H93K relative to WT SWMb.

Non-heme perferryl intermediates: Effect of spin state on the epoxidation enantioselectivity

• Chiral non-heme iron complexes catalyze epoxidation of olefins with H 2 O 2 in up to 96 % ee . • The active oxoiron(V) intermediates have been detected by EPR spectroscopy. • The Fe V =O intermediates adopt either low-spin ( S = 1/2) or high-spin ( S = 3/2) ground state. • The spin state is governed by the electron-donating ability of p -substituents at the pyridine rings. • The high-spin Fe V =O intermediates are more enantioselective than their low-spin counterparts. The catalyst systems Fe 2 (PDP NR1R2 ) 2 /H 2 O 2 /RCOOH (PDP = N , N′ -bis(pyridyl-2-methyl)-( S , S )-2,2′-bipyrrolidine, NR 1 R 2 = NMe 2 , NEt 2 , NMe i Pr or N(CH 2 ) 4 at the para -position in the pyridine rings, RCOOH = acetic or 2-ethylhexanoic acid) generate the high-spin ( S = 3/2) oxoiron(V) oxidizing intermediates with proposed structure [(PDP NR1R2 )Fe V =O(OC(O)R)] 2+ and the EPR spectrum g 1 , = 4.30, g 2 = 3.69, g 3 = 1.96. In contrast, the catalyst systems Fe 2 (PDP Me2OMe ) 2 /H 2 O 2 /RCOOH and Fe 2 (PDP MeOCH2CF3 ) 2 /H 2 O 2 /RCOOH, based on similar iron complexes, containing 3,5-Me 2 -4-OMe and 3-Me-4-OCH 2 CF 3 substituents at the pyridine rings, generate the low-spin ( S = 1/2) oxoiron(V) intermediates with characteristic EPR spectrum g 1 = 2.07, g 2 = 2.01, g 3 = 1.96. Both high and low-spin intermediates directly epoxidize cyclohexene, with the low-spin species being much more reactive than the high-spin congeners; the corresponding second-order rate constants have been evaluated. Crucially, the catalyst systems exhibiting the high-spin oxidizing intermediates demonstrate higher enantioselectivity in the epoxidation of prochiral olefinic substrates, compared with the catalyst systems featuring the low-spin intermediates. This enhanced enantioselectivity is explained in terms of attenuated reactivity of the high-spin intermediates, leading to more product-like transition state with tighter substrate-catalyst interactions.

A diamagnetic iron complex and its twisted sister - structural evidence on partial spin state change in a crystalline iron complex.

We report here the syntheses of a diamagnetic Fe complex [Fe(HL)2] (1), prepared by reacting a redox non-innocent ligand precursor N,N'-bis(3,5-di-tert-butyl-2-hydroxy-phenyl)-1,2-phenylenediamine (H4L) with FeCl3, and its phenoxazine derivative [Fe(L')2] (2), which was obtained via intra-ligand cyclisation of the parent complex. Magnetic measurements, accompanied by spectroscopic, structural and computational analyses show that 1 can be viewed as a rather unusual Fe(III) complex with a diamagnetic ground state in the studied temperature range due to a strong antiferromagnetic coupling between the low-spin Fe(III) ion and a radical ligand. For a paramagnetic high-spin Fe(II) complex 2 it was found that, when crystalline, it undergoes a thermally induced process where 25% of the molecules in the material change to a diamagnetic low-spin ground state below 100 K. Single crystal X-ray studies conducted at 95 K afforded detailed structural evidence for this partial change of spin state of 2 showing the existence of crystallographically distinct molecules in a 3 : 1 ratio which exist in high- and low-spin states, respectively. Also, the magnetic behaviour of 2 was found to be related with the crystallinity of the material as demonstrated by near-IR radiation to unpaired electrons conversion ability of amorphous sample of 2.

Unusual mixed spin-state of Co3+ in the ground state of LaSrCoO4: Combined high-pressure and high-temperature study

The nature of the non-magnetic to paramagnetic transition of Co3+ oxide LaCoO3 was strongly disputed in the literature for many decades from a low-spin (LS) state below 20 K and to a mixed LS state and high-spin (HS) state or an intermediate-spin (IS) state above 100 K. In this context, the layered perovskite LaSrCoO4 is more favorable for a Jahn-Teller-active IS state because of an elongated distortion, but has been scarcely studied with experimental X-ray spectroscopies as a function of temperature or external pressure. Here, our Co-L2,3 X-ray absorption spectroscopic study indicates a mixture of 40% HS-Co3+ and 60% LS-Co3+ for LaSrCoO4 against 25% HS-Co3+ and 75% LS-Co3+ for LaCoO3 at 300 K and ambient pressure (AP). At 10 K, we observed a sizable magnetic-circular-dichroism signal and a clear HS state of the magnetic Co3+ ion from the Co-L2,3 edge of LaSrCoO4. This result demonstrates that the HS state is already populated in the ground state versus a pure LS ground state in LaCoO3. A quantitative change of quantum number of the spin of the Co3+ ion of LaSrCoO4 as a function of pressure and temperature investigated systematically with Co-Kβ X-ray emission experiments firmly demonstrates not only a mixed state of LS/HS at 300 K and AP but also a presence of the pure LS-Co3+ and HS-Co3+ states only under high pressure and high temperature, respectively.

Laser-assisted decay spectroscopy and mass spectrometry of Au178

A comprehensive study of the isotope 178 Au has been made at the CERN-ISOLDE facility, using resonance laser ionization. Two long-lived states in 178 Au were identified—a low-spin ground state and a high-spin isomer—each of which were produced as pure beams. Using the ISOLTRAP precision Penning trap, the excitation energy of the isomeric state in 178 Au was determined to be E ∗ = 189 ( 14 ) keV . The α -decay fine structure patterns of the two states were studied using the Windmill decay station, providing information on the low-lying states in the daughter nucleus 174 Ir . Nuclear spin assignments of I ( 178 Au g ) = ( 2 , 3 ) and I ( 178 Au m ) = ( 7 , 8 ) are made based on the observed β -decay feeding and hyperfine structure intensity patterns. These spin assignments are used for fitting the hyperfine structures of the two states from which values for the magnetic dipole moments are extracted. The extracted moments are compared with calculations using additivity relations to establish the most probable configurations for 178 Au g , m .

Surface effects on temperature-driven spin crossover in Fe(phen)2(NCS)2.

Despite their importance in molecular spintronics, the surface effects on spin crossover (SCO) behaviors are still poorly understood. Here, we report the impact of substrates on thermal SCO in Fe(phen)2(NCS)2 (phen = 1,10-phenanthroline) deposited on metallic surfaces and monolayer two-dimensional materials. By first-principles calculations, we show that temperature-driven SCO is preserved on both hexagonal boron nitride and molybdenum disulfide (MoS2), while low-spin ground states are locked on metal surfaces, including Cu(111), Ag(111), and Au(111). On the contrary, the molecule in contact with graphene exhibits a high-spin ground state. We demonstrate that the spin transition temperature Tc depends critically on surface environments, and we correlate this effect with the modification of electronic structures and molecular vibrations upon adsorption. In particular, a sulfur vacancy in MoS2 considerably increases Tc. These findings open a way to nanoscale applications related to spin state bistability.

Contribution of the Multiplicity Fluctuation in the Temperature Dependence of Phonon Spectra of Rare-Earth Cobaltites.

We have studied, both experimentally and theoretically, the unusual temperature dependence of the phonon spectra in NdCoO3, SmCoO3 and GdCoO3, where the Co3+ ion is in the low-spin (LS) ground state, and at the finite temperature, the high-spin (HS) term has a nonzero concentration due to multiplicity fluctuations. We measured the absorption spectra in polycrystalline and nanostructured samples in the temperature range 3–550 K and found a quite strong breathing mode softening that cannot be explained by standard lattice anharmonicity. We showed that the anharmonicity in the electron–phonon interaction is responsible for this red shift proportional to the concentration.

Interplay of Biradicaloid Character and Singlet/Triplet Energy Splitting for cis-/trans-Diindenoacenes and Related Benzothiophene-Capped Oligomers as Revealed by Extended Multireference Calculations.

The biradicaloid character of different types of polycyclic aromatic hydrocarbons is an important quality that guides the development of new materials with interesting magneto-optical properties. Diindenoacene-based systems represent such a class of promising compounds. In this work, the three types trans-diindenoacenes, cis-diindenoacenes, and trans-dicyclopentaacenobis(benzothiophenes) have been studied by means of advanced ab initio methods. The descriptors singlet/triplet splitting energy (ΔES-T), effective number of unpaired electrons (NU), unpaired electron density, and the harmonic oscillator measure of aromaticity have been used to characterize the biradicaloid properties of these species. For all trans structures, low-spin singlet ground states were found, in agreement with previous investigations. Increasing the length of the inner acene chain decreased the ΔES-T and strongly increased the NU values of the singlet state. The cis-diindenoacenes displayed a greatly increased biradicaloid character, indicating enhanced chemical instability. The thiophene rings in the trans-dicyclopentaacenobis(benzothiophenes) structures were found to simultaneously restrict the unpaired electron density from extending into the terminal six-membered rings and confine the unpaired electron density found in the core benzene rings.

The effect of Hubbard-like interaction on molecular magnetism of TM-coronene complex (TM = Fe and Co)

The adsorption of two transition metal adatoms, Fe and Co added to a coronene molecule were studied by means of a full potential local orbital method in the framework of relativistic density functional theory. A sequence of fixed spin moment calculations based on the Hubbard-like interaction (U) were carried out on the basis of the generalized gradient approximation (GGA) of the exchange–correlation functional. We found a transition from low-spin to high-spin state upon increasing the transition metal-coronene distance. Furthermore, the magnetism of both Fe-coronene and Co-coronene complexes revealed sensitivity to the magnitude of the U values. In our GGA+U (U = 2 eV) calculations, a low-spin ground state was found with in-plane magnetic anisotropy for both Fe-coronene and Co-coronene, whereas the GGA+U (U = 4 eV) calculations resulted in high-spin state with out-of-plane and in-plane magnetic anisotropy for Fe-coronene and Co-coronene, respectively.

Slow Dynamics of the Spin-Crossover Process in an Apparent High-Spin Mononuclear FeII Complex.

A mononuclear FeII complex that shows a high-spin (S=2) paramagnetic behavior at all temperatures (with standard temperature-scan rates, ≈1 K min-1 ) has, in fact, a low-spin (S=0) ground state below 100 K. This low-spin state is not easily accessible due to the extremely slow dynamics of the spin-crossover process-a full relaxation from the metastable high-spin state to the low-spin ground state takes more than 5 h below 80 K. Bidirectional photo-switching of the FeII state is achieved reproducibly by two selective irradiations (at 530-590 and 830-850 nm). The slow dynamics of the spin-crossover and the strong structural cooperativity result in a remarkably wide 95-K hysteresis loop induced by both temperature and selected light stimuli.