Antiferromagnetic Spin Coupling Research Articles

Single-hole spectra of Kitaev spin liquids: from dynamical Nagaoka ferromagnetism to spin-hole fractionalization

The dynamical response of a quantum spin liquid upon injecting a hole is a pertinent open question. In experiments, the hole spectral function, measured momentum-resolved in angle-resolved photoemission spectroscopy (ARPES) or locally in scanning tunneling microscopy (STM), can be used to identify spin liquid materials. In this study, we employ tensor network methods to simulate the time evolution of a single hole doped into the Kitaev spin-liquid ground state. Focusing on the gapped spin liquid phase, we reveal two fundamentally different scenarios. For ferromagnetic spin couplings, the spin liquid is highly susceptible to hole doping: a Nagaoka ferromagnet forms dynamically around the doped hole, even at weak coupling. By contrast, in the case of antiferromagnetic spin couplings, the hole spectrum demonstrates an intricate interplay between charge, spin, and flux degrees of freedom, best described by a parton mean-field ansatz of fractionalized holons and spinons. Moreover, we find a good agreement of our numerical results to the analytically solvable case of slow holes. Our results demonstrate that dynamical hole spectral functions provide rich information on the structure of fractionalized quantum spin liquids.

Bis(ethylenedithio)tetrathiafulvalene and dibenzotetrathiafulvalene salts with dicyanoiron(III) phthalocyanine anions

Electrooxidation of bis(ethylenedithio)tetrathiafulvelene (BEDT-TTF) and dibenzotetrathiafulvelene (DB-TTF) in the presence of dicyanoiron(II) phthalocyanine (Pc) dianions in form of the {Cryptand(K+)}2{FeII(CN)2Pc}2− salt has been studied in o-dichlorobenzene and acetonitrile solutions (where Cryptand is cryptand[2.2.2]). Initially iron(II) is oxidized to iron(III) and after that tetrathiafulvelenes are oxidized forming crystalline salts (BEDT-TTF+){FeIII(CN)2Pc}−·C6H4Cl2 (1) and (DB-TTF+){FeIII(CN)2Pc}−·C6H4Cl2 (2). Both salts contain closely packed 2D layers composed of the {FeIII(CN)2Pc}− anions and cationic layers containing solvent molecules. Since cations cannot partially oxidize Pc in these salts, two S = 1/2 spins are positioned on FeIII and the cations. The χMT values are 0.90 and 0.87 emu·K/mol per formula unit at 300 K. Antiferromagnetic coupling of spins is observed in 1 and 2 due to both Fe-Cation coupling of −0.4 and −3.5 cm−1 and Fe-Fe couplings of −3.5 and −7.9 cm−1, respectively. Due to its smaller size of DB-TTF+, complex 2 has denser packing of the components and shows stronger exchange couplings. An unusual compound forms when DB-TTF and {FeII(CN)2Pc}2− are oxidized together in acetonitrile: (DB-TTF0)2{Cryptand(K+)}{FeIII(CN)2Pc}− (3) where charges on DB-TTF and Fe are closer to zero and +3, respectively. The {FeIII(CN)2Pc}− anions form layers in 3 among which chains with short C,N…C,N contacts between Pc can be outlined. Only weak intermolecular antiferromagnetic coupling is manifested in 3 with estimated zJ value of −1.6 cm−1.

Spin model for the honeycomb NiPS3

In the Van der Waals material NiPS3, Ni atoms have spin S = 1 and realize a honeycomb lattice. Six sulfur atoms surround each Ni and split their d manifold into three filled and two unfilled bands. Aimed to determine the spin Hamiltonian of NiPS3, we study its exchange mechanisms using a two-band half-filled Hubbard model. Hopping between d-orbitals is mediated by p orbitals of sulfur and gives rise to bilinear and biquadratic spin couplings in the limit of strong electronic correlations. The microscopic model exposed a ferromagnetic biquadratic spin interaction K1, allowing the completion of a minimal J1−J3−K1 spin Hamiltonian for NiPS3. In bulk, a ferromagnetic first nearest neighbor J1 and a more significant antiferromagnetic third nearest neighbor spin coupling J3 agreed with the literature, while in monolayer, J1 is positive and very small in comparison. Using a variational scheme, we found that a zig–zag antiferromagnetic order is the ground state of bulk samples. The zig–zag pattern is adjacent to commensurate and incommensurate spin spirals, which could hint at the puzzling results reported in NiPS3 monolayers.

Quantum geometric spin frustration of antiferromagnetic CuFeO2 enables photocatalytic applications

Photocatalysis, containing a potential solution to the global crises of energy shortage and the greenhouse effect, has been studied extensively in recent years. The CuFeO2 (abbreviated as CFO) is a material with a Delafossite structure, and has also been widely studied because of its triangular lattice antiferromagnets. The triangular lattice composed of Fe3+ ions, along with strong antiferromagnetic spin coupling between the Fe3+ ions, leads to a geometric spin frustration. Furthermore, CFO is also a potential candidate for the photocatalytic material due to its narrow band gap of 1.1–1.6 eV, electrochemical stability in an aqueous environment, high charge carrier mobility, flat-band potential positioned at ∼ 1 eV, and contains an abundance of raw materials in the earth. In this study, we successfully grew the size and highly-quality CFO single crystals using the optical floating-zone technique. Powder x-ray diffraction (XRD) and Rietveld refinement indicate that the CFO is a rhombohedral structure with a space group of R3̅m, consisting of edge-shared FeO6 octahedron to form the FeO2 layer, and the Cu cations occupied between the two FeO2 layers. The core-level electron energy loss (EEL) spectra found the valence of Fe and Cu in CFO is +3 and +1, respectively. The low-loss EEL spectrum observed two volume plasmons at ∼5 eV and ∼23 eV contributed from the collective oscillation of Cu-O antibonding states σCu−O* and mixed nonbonding Cu 3dxy, σCu−O bonding states, and Fe-O, respectively, based on the density function theory calculation. The optical absorption spectrum measured the energy bandgap for CFO is about 1.5 eV. The Raman spectrum shows significant Eg and A1 g vibration modes corresponding to the in-plane Fe-O and out-of-plane Cu-O vibrations, respectively. Both magnetic susceptibility, χ(T), with the applied magnetic field along the c-axis and specific heat capacity reveal the two magnetic phase transition temperatures at low temperatures: TN1 = 13.5 K when transition from paramagnetic (PM) phase to partially disordered antiferromagnetic, and TN2 = 10.5 K when complete transition from partially disordered antiferromagnetic to antiferromagnetic phase. The additional calculation of magnetic entropy change is 0.15, which is roughly close to 0.2, meaning that only 1/5 of the Fe3+ spins are disordered when transforming from PM states to partially disordered at T < TN1 in this study. The presence of instantaneous ferromagnetic spin arrangement (↑↑↓) extends the residence time of H2O on the surface of CFO, facilitating electron exchange with CFO and promoting photocatalytic water splitting reactions.

The Fully Oxidized State of the Glutamate Coordinated O2-Tolerant [NiFe]-Hydrogenase Shows a Ni(III)/Fe(III) Open-Shell Singlet Ground State.

The oxygen tolerance of the [NiFe]-hydrogenase from H. thermoluteolus was recently assigned to originate from an unusual coordination sphere of the active site nickel atom (Shomura et al. Science 2017, 357, 928-932, 10.1126/science.aan4497). In the oxidized state, a terminal cysteine residue is displaced by a bidentate coordinating nearby Glu32 and thus moves to occupy a third μ-cysteine bridging position. Spectral features of the oxidized state were assigned to originate from a closed-shell Ni(IV)/Fe(II) state (Kulka-Peschke et al. J. Am. Chem. Soc. 2022, 144, 17022-17032, 10.1021/jacs.2c06400). Such a high-valent nickel oxidation state is unprecedented in biological systems. The spectral properties and the coordination sphere of that [NiFe]-hydrogenase can, however, also be rationalized by an energetically lower broken-symmetry Ni(III)/Fe(III) state of the active site which was not considered. In this open-shell singlet, the ligand-mediated antiferromagnetic spin-coupling leads to an overall S = 0 spin state with evenly distributed spin densities over the metal atoms. Experiments are suggested that may clarify the final assignment of redox states.

Weak ferromagnetism at 48.5 K in rubidium-intercalated biphenyl

Aromatic hydrocarbon molecules with π-conjugated structure have great potential for the search of exotic states such as high temperature superconductivity and novel quantum magnetism. In this work, we found that high temperature weak ferromagnetism can be achieved by intercalating rubidium into the shortest poly(para-phenylene) molecule—biphenyl. Both dc and ac magnetic susceptibility measurements showed that the synthesized material exhibits a typical antiferromagnetic transition at 48.5 K, accompanying a ferromagnetic character in the magnetically ordered region. Combining theoretical calculations and Raman spectroscopic studies, it was found that the mole ratio between rubidium and biphenyl is 1:1, and the molecular magnetic moments are formed by transferring the Rb-5s electrons to the C-2p orbitals. The weak ferromagnetism is produced by intralayer spin-canted antiferromagnetic and interlayer ferromagnetic spin couplings. Our finding enriches the physicochemical functionality of biphenyl and opens a window for the search of π-electron organic magnets in poly(para-phenylene) molecules.

Electron-Coupled Proton Transfer Governed Magnetic Spin Couplings and Switching in Defect Nano Silicon Carbide

The formation mechanism, defect characterization, and potential applications of the Si vacancy (VSi) with internal H atoms in the silicon carbide (SiC) materials have been widely investigated. However, the inherent electronic properties and related phenomena induced by internal H dynamics in VSi, especially the C-radical spin interaction dynamics, are still poorly understood in such nano SiC. In this work, using the density functional calculations, we explore the magnetic coupling characteristics and dynamic behaviors induced by H-motion in nano SiC with four different types of H-doped VSi defect centers, (VSi + H)−, (VSi + H)+, (VSi + 2H), and NVH [i.e., (NCVSi + H)0 in which one of C-radical sites in VSi is replaced by N]. Our results reveal their unique ferromagnetic (FM) or antiferromagnetic (AFM) spin coupling characteristics and H-motion-induced interconversion. In general, the internal H-motion goes through the electron-coupled proton transfer (ECPT) in a C–H···C unit which has great impact on molecular orbitals, spin densities, and thus spin couplings among the C-radicals, allowing the VSi center to undergo magnetic switching between FM and AFM coupling. Interestingly, as in the (VSi + H)− center, such ECPT can further drive the excess electron delocalized on three C-radicals to gather at the ECPT unit, forming a three-center-four-electron (3c–4e) [C···H···C]− covalent H-bond with the switching of magnetic coupling. This work provides the dynamics insights into the spin coupling characteristics and its regulation by internal H-motion and multiple radical characters of such defect materials and the formation of a 3c–4e H-bond assisted by excess electron, and also provides inspired information for the design of inorganic magnetic materials and logic devices which can be controlled by interior doping and dopant dynamics.

Layered Organic Conductors Based on BEDT-TTF and Ho, Dy, Tb Chlorides

Molecular semiconductors with lanthanide ions have been synthesized based on BEDT-TTF and lanthanide chlorides: (BEDT-TTF)2[HoCl2(H2O)6]Cl2(H2O)2 (1, which contains a 4f holmium cation), and (BEDT-TTF)2LnCl4(H2O)n (Ln = Dy, Tb, Ho (2–4), which contain 4f anions of lanthanides). Conductivity and EPR measurements have been carried out along with the SQUID magnetometry, and the crystal structure has been established for 1. The structure of 1 is characterized by an alternation of organic radical cation layers composed of BEDT-TTF chains and inorganic layers consisting of chains of the [HoCl2(H2O)6]+ cations interlinked by chlorine anions and crystallization water molecules. The magnetic susceptibility of 1–3 determined mainly by lanthanide ions follows the Curie–Weiss law with the Weiss temperatures of −3, −3, −2 K for 1–3, respectively, indicating weak antiferromagnetic coupling between paramagnetic lanthanide ions. The signals attributed to the BEDT-TTF+· radical cations only are observed in the EPR spectra of 1–3, which makes it possible to study their magnetic behavior. There are two types of chains in the organic layers of 1: the chains composed of neutral molecules and those formed by BEDT-TTF+· radical cations. As a result, uniform 1D antiferromagnetic coupling of spins is observed in the BEDT-TTF+· chains with estimated exchange interaction J = −10 K. The study of dynamic magnetic properties of 1–3 shows that these compounds are not SMMs.

Endohedral σ-Diradical Nitrogen-Vacancy Diamond Nanoclusters with a Confined Magnetic Space and Strong Electronic Spin Couplings.

The electronic properties and their modulations for the nitrogen-vacancy (NV) centers in various nanoscale diamonds are of profound current interest because of their potential applications. However, although the NV centers as chromophores in diamond are the most widely studied, surprisingly, little is known about their magnetic spin coupling properties up to now. Here, we for the first time show, using the spin-polarized DFT calculations, that the NV centers can act as unique endohedral σ-diradical magnets in diamond nanoclusters and exhibit quite strong ferromagnetic (FM) or antiferromagnetic (AFM) spin coupling characteristics due to their unique endotetrahedral structures with favorable radical-radical contacts. Although the neutral NV center (NV0) in its doublet ground state exhibits quite strong AFM spin coupling among three radical C-sites (i.e., an AFM triradical center), interestingly, excess electron injection can convert it to a FM diradical magnet (i.e., the triplet ground state NV-) with almost unchanged J-coupling magnitude, and the J-coupling of the nanocluster can be noticeably enhanced by F-termination of the surface due to triradical spin delocalization mediated by excess electron. However, interior modification (one C in the endotetrahedron core is substituted by N or B or is hydrogenated) can assign the nanocluster perfect AFM diradical character. The spin coupling strength presents a quasilinear correlation with the distance between the two C radicals in the NV core for the same size of the clusters and a high linear correlation with the energy difference between two singly occupied molecular orbitals. Clearly, the FM and AFM couplings as well as their switching behavior in such NV defect diamond nanoclusters featuring the endohedral σ-diradicals are a novel type of promising magnetic material motifs. These findings open up promising spintronic application prospects of the NV diamonds and provide helpful information for the design of inorganic magnetic materials and logic devices.

Different types of interactions between fullerene C60 and C70 anions and metal tetraphenylporphyrins in the (PMDAE+)(MIITPP)(Fullerene−)⋅Solvent complexes (M = Co, Mn, Zn) containing coordinating N, N, N, N‘, N‘-pentamethyldiaminoethane cations

Series of anionic salts of fullerenes C60 and C70 with different metal tetraphenylporphyrins, MIITPP, where M = Co, Mn and Zn, and N, N, N, N‘, N‘-pentamethyldiaminoethane cations (PMDAE+) has been obtained in a crystalline form. Composition of the complexes is (PMDAE+)⋅(MIITPP)(C60−)⋅XC6H4Cl2 (M = Co, X = 1.25 (1), M = Mn, X = 0.75 (2) and M = Zn, X = 0.75 (3)) and {(PMDAE+)⋅ZnIITPP}2⋅(C70−)2⋅4C6H4Cl2 (4). Both fullerene anion and nitrogen-containing PMDAE+ cation can coordinate to metal porphyrin competing with each other. Weak Co-C(C60−) coordination bond of 2.426(7) Å length (120 K) is formed in 1 whereas PMDAE+ cation only weakly coordinates to CoIITPP. Dissociation of Co-C(C60−) bond observed at 90–310 K is accompanied by a transition of 1 from singlet to triplet state with an energy gap of 574 ± 3 K. According to the calculations negative charge on C60− is only −0.548 due to electron transfer to the macrocyclic ligand through the cobalt atom. Charge transfer bands are observed in {CoIITPP(C60)}− at 700 and 1240 nm. On the contrary, PMDAE+ cations coordinate to MnIITPP and ZnIITPP in 2 and 3, respectively, with short M−N(PMDAE+) bonds of 2.27–2.33 Å length. In these cases fullerenes are preserved in the non-coordinated radical anion state. Fullerenes are arranged in closely packed π-stacking {C60•−}2 dimers with short interfullerene center-to-center distances of 9.92–9.95 Å. Nevertheless, the C60•− radical anions do not form intercage CC bonds down to 1.9 K. Strong antiferromagnetic coupling of spins is observed in the {C60•−}2 dimers of 3 with exchange interaction of J/kB = -21 K. Both {(PMDAE+)⋅MnIITPP} and C60•− species are paramagnetic in 2 having S = 5/2 and S = 1/2 spin states, respectively. These spins are coupled antiferromagnetically with Weiss temperature of −16 K. It is obvious that magnetic coupling occurs not only in the {C60•−}2 dimers but MnIITPP is also involved in this coupling. The C70•− radical anions are dimerized in 4 forming diamagnetic singly-bonded (C70−)2 dimers which manifest intense NIR absorption. As a result, complex 4 is EPR silent.

Complexes of transition metal carbonyl clusters with tin(II) phthalocyanine in neutral and radical anion states: methods of synthesis, structures and properties.

Coordination of tin(II) phthalocyanine to transition metal carbonyl clusters in neutral {SnII(Pc2-)}0 or radical anion {SnII(Pc˙3-)}- states is reported. Direct interaction of Co4(CO)12 with {SnII(Pc2-)}0 yields a crystalline complex {Co4(CO)11·SnII(Pc2-)} (1). There is no charge transfer from the cluster to phthalocyanine in 1, which preserves the diamagnetic Pc2- macrocycle. The Ru3(CO)12 cluster forms complexes with one or two equivalents of {SnII(Pc˙3-)}- to yield crystalline {Cryptand[2.2.2](Na+)}{Ru3(CO)11·SnII(Pc˙3-)}- (2) or {Cryptand[2.2.2](M+)}2{Ru3(CO)10·[SnII(Pc˙3-)]2}2-·4C6H4Cl2 (3) (M+ is K or Cs). Paramagnetic {SnII(Pc˙3-)}- species in 2 are packed in π-stacking [{SnII(Pc˙3-)}-]2 dimers, providing strong antiferromagnetic coupling of spins with exchange interaction J/kB = -19 K. Reduction of Ru3(CO)12, Os3(CO)12 and Ir4(CO)12 clusters by decamethylchromocene (Cp*2Cr) and subsequent oxidation of the reduced species by {SnIVCl2(Pc2-)}0 yield a series of complexes with high-spin Cp*2Cr+ counter cations (S = 3/2): (Cp*2Cr+){Ru3(CO)11·SnII(Pc˙3-)}-·C6H4Cl2 (4), (Cp*2Cr+){Os3(CO)10Cl·SnII(Pc˙3-)}-·C6H4Cl2 (5) and (Cp*2Cr+){Ir4(CO)11·SnII(Pc˙3-)}2- (6). It is seen that reduced clusters are oxidized by SnIV, which is transferred to SnII, whereas the Pc2- macrocycle is reduced to Pc˙3-. In the case of Os3(CO)12, oxidation of the metal atom in the cluster is observed to be accompanied by the formation of Os3(CO)10Cl with one OsI center. Rather weak magnetic coupling is observed between paramagnetic Cp*2Cr+ and {SnII(Pc˙3-)}- species in 4, but this exchange interaction is enhanced in 5 owing to Os3(CO)10Cl clusters with paramagnetic OsI (S = 1/2) also being involved in antiferromagnetic coupling of spins. The formation of {SnII(Pc˙3-)}- with radical trianion Pc˙3- macrocycles in 2-5 is supported by the appearance of new absorption bands in the NIR spectra and essential Nmeso-C bond alternation in Pc (for 3-5). On the whole, this work shows that both diamagnetic {SnII(Pc2-)}0 and paramagnetic {SnII(Pc˙3-)}- ligands substitute carbonyl ligands in the transition metal carbonyl clusters, forming well-soluble paramagnetic solids absorbing light in the visible and NIR ranges.

Copper(II)-Nickel(II) Induced Cyanide Bridged 2D Heterometallic Coordination Polymer: Synthesis, X-ray Structure and Magnetic Study

One new two-dimensional heterometallic coordination polymer [Cu3Ni(L)2(CN)6]n (1) [L = 2-methylethylenediamine, CN– = cyanide ion] was synthesized and characterized by X-ray crystallography. Compound 1 crystallizes in the monoclinic C2/c space group. The single crystal X-ray structure of compound 1 indicated that it is a 2D coordination polymer with three copper(II) ion embedded with [Ni(CN)4]2- molecular ion in architecture through μ-CN/μ-NC bridges results heterometallic polymer. In compound 1, each Cu1 center with a CuN5 chromophore is encompassed by four N atoms of the two different bidentate amine (L) and one N atom of μ-NC ion, while both Cu2 and Cu3 centres are connected by three N atoms of μ-NC ions having CuN3 chromosphere and Ni1 are coordinated by three μ-CN and one terminal CN ions with NiC4 chromophore. Three copper centers and one nickel center are interlinked through μ-CN/μ-NC forming {-Cu1(L)2(μ-NC)Ni1(CN)(μ-CN)2Cu2(μ- NC)Cu3(μ-NC)-} unit to fabricate a 2D hetero metallic polymer. In the crystalline state of compound 1, each 2D polymer is further propagated through intermolecular N-H···N hydrogen bonds corroborant a supramolecular 2D network structure. Variable temperature magnetic susceptibility study displayed a weak antiferromagnetic spin coupling presumably due to the diamagnetic tetracyanonickellate bridging among the CuII ions.

Band gap reduction and quenching of p-d exchange interaction in sol-gel derived Zn(Al,Cu)O nanostructures

The objective of the present study is to address the gap in the fundamental knowledge on the effect of doping and co-doping in ZnO nanostructures. In this regard, we explore the significant impact of Al- and Cu co-doping on band-gap and associated changes in the ZnO system. Sol-gel co-precipitation was used to synthesize ZnO-based nanostructures to accomplish the objective. Crystallite size determined by XRD was in the range of 6.44-37.58 nm and the lattice constant, c, initially decreased with co-doping, accompanied by an increase. Strong co-doping with Cu altered the nature of microstrain from tensile to compressive. FTIR studies predicted that Al and Cu were incorporated at Zn-O site through the formation of Al-O and Cu-O bonds, while UV-vis studies suggested the reduction of the band-gap when Al and Cu were incorporated in ZnO. The underlying reason was Cu-3d and O-2p exchange interaction in Zn(Al,Cu)O system. Quenching of this exchange interaction occurred in the presence of specific combination of dopant and co-dopant, along with blocking of the low energy transitions, eventually leading to a band-gap slightly greater than undoped ZnO. Furthermore, the emission peak observed in the photoluminescence spectra implied redshift induced by Al-doping, which was not influenced on Cu co-doping. Another important observation was the presence of ferromagnetic character in all samples, where saturation magnetization decreased with the increase in Al and Cu content in ZnO matrix, a behaviour attributed to anti-ferromagnetic coupling of spins of Cu ions at the substitutional sites of Zn-O. Lastly, electron microscopy revealed that the morphology of undoped ZnO transformed from granular to sand-rose on doping with Al, while Cu co-doping led to the formation of heavy clusters. The new insights on the band-gap reduction and associated structural changes in doped ZnO-based nanostructured materials have the potential for next generation of spintronic devices.

A novel family of hepta-coordinated Cr(III) complexes with a planar pentadentate N3O2 Schiff base ligand: synthesis, structure and magnetism

A series of seven-coordinate pentagonal-bipyramidal (PBP) Cr(III) complexes with pentadentate pyridine-based ligands, 2,6-diacetylpyridine bis(4-methoxybenzoylhydrazone), H2DAPMBH (H2LOCH3) or 2,6-diacetylpyridine bis(benzoylhydrazone), H2DAPBH (H2L) and different axial ligands have been prepared. The reaction of the H2LOCH3 with CrCl2·4H2O in methanol or CrCl3·6H2O in CH3CN led to a novel seven-coordinate pentagonal-bipyramidal (PBP) complex [Cr(HLOCH3)Cl2] (1) with the mono-deprotonated chelating ligand in the equatorial plane and two apical Cl atoms. Then, taking advantage of lability of the apical Cl ligands in 1, a number of PBP CrIII complexes with charged (viz. CH3O−, N3−, CN−, NCS−) and neutral (viz. CH3OH, H2O) apical ligands was obtained and characterized: [Cr(HLOCH3)Cl2]·4CHCl3 (1), [Cr(HLOCH3)Cl2]·CH3OH (1a), [Cr(HLOCH3)(H2O)Cl]PF6·CH3OH (2), [Cr(HL)(H2O)Cl]ClO4·0.25H2O (3), [Cr(HLOCH3)(H2O)2](NO3)2·H2O·C2H5OH (4), [Cr(LOCH3)(CH3OH)(OCH3)]·CH3OH (5), [Cr(HLOCH3)(NCS)2]·1.5H2O (6), [Cr(HLOCH3)(N3)2]·xH2O (7, x = 0.2, 8, x = 0, 9, x = 0, three phases in the same synthesis), [Cr(LOCH3)(N3)2][Na(CH3OH)2]·2CH3OH (10), [Cr(LOCH3)(CN)2][Na(H2O)(C2H5OH)] (11). Single crystal X-ray analysis reveals that all the complexes 1–11 have the PBP geometry with a pentadentate ligand in a form of [HLOCH3]− or [LOCH3]2− in the equatorial plane. The PBP complexes are prone to aggregate into dimers or polymers, either due to strong hydrogen bonds or due to the transformation of terminal ligands into bridging between different metallic centers. All complexes 1–11 exhibit considerable in-plane distortion of the CrN3O2 pentagon due to the shift of the CrIII ion from the central position, which is caused by the strong first-order Jahn-Teller (JT) effect for the high-spin 3d3 configuration in the PBP ligand field. The mechanism of JT distortions is rationalized in terms of DFT calculations. DC magnetic measurements indicate a high-spin (S = 3/2) ground state of complex [Cr(HLOCH3)Cl2]·4CHCl3 (1); theoretical analysis of its magnetic properties reveals negative zero-field splitting energy with the anisotropy parameter D = –1.8 cm−1 and weak dimer-like antiferromagnetic spin coupling J = –0.23 cm−1 between neighboring PBP units [CrIII(HLOCH3)Cl2] mediated by π-stacking of planar H2LOCH3 ligands.

Molecular Dynamics Characterization of Dielectron Hydration in Liquid Water with Unique Double Proton Transfers.

Radiation chemistry of water and aqueous solutions has always been an interesting scientific issue owing to involving electronic excitations, ionization of solvated species, and formation of radiolytic species and many elementary reactions, but the underlying mechanisms are still poorly understood. Here, we for the first time molecular dynamics characterize the hydration dynamics of two correlated electrons and their triggered unique phenomena in liquid water associated with radiolysis of water using the combined hybrid functional and nonlocal dispersion functional. Hydration of two electrons may experience two distinctly different mechanisms, one forming a spin-paired closed-shell unicaged dielectron hydrate (e22-aq) and the other forming a spin-paired metastable open-shell bicaged hydrated electron pair (e-aq···e-aq) which exhibits intriguing antiferromagnetic spin coupling dynamics (in a range of -40 cm-1 to -500 cm-1). e-aq···e-aq can recombine to e22-aq through a unique solvent fluctuation-controlled gradual-flowing mechanism, and enlarging fluctuation can promote the conversion. Interestingly, we directly observe that e22-aq as the precursor can trigger hydrogen evolution via unique continuous spontaneous double proton transfer to the dielectron with a short-lived H-aq intermediate, but e-aq···e-aq does not directly. This is the first direct observation for the connection between e22-aq and spontaneous hydrogen evolution including participation of H-aq in aqueous solution, bridging relevant experimental phenomena. This work also evidences an unnoticed process, the double proton transfer mediated charge separation, and presents the first detailed analysis regarding the evolution dynamics of e22-aq for the understanding of the radiolysis reactions in aqueous solutions.

Cyclic Heterometallic Interactions formed from a Flexible Tripeptide Complex Showing Effective Antiferromagnetic Spin Coupling

Developing tunable motifs for heterometallic interactions should be beneficial for fabricating functional materials based on cooperative electronic communications between metal centers. Reported here is the efficient formation of cyclic heterometallic interactions from a complex containing an artificial tripeptide with metal binding sites on its main chain and side chains. X-ray structural analysis and X-ray absorption spectroscopy revealed that the cyclic metal-metal arrangements arise from the amide groups connecting four square-planar CuII centers and four octahedral NiII centers in a cyclic manner. UV/Vis spectral studies suggested that this efficient formation was achieved by the selective formation of the square-planar CuII centers and a crystallization process. Magnetic measurements using SQUID clarified that the cyclic complex represented the S=2 spin state at low temperatures due to effective antiferromagnetic interactions between the NiII and CuII centers.

Cyclic Heterometallic Interactions formed from a Flexible Tripeptide Complex Showing Effective Antiferromagnetic Spin Coupling

AbstractDeveloping tunable motifs for heterometallic interactions should be beneficial for fabricating functional materials based on cooperative electronic communications between metal centers. Reported here is the efficient formation of cyclic heterometallic interactions from a complex containing an artificial tripeptide with metal binding sites on its main chain and side chains. X‐ray structural analysis and X‐ray absorption spectroscopy revealed that the cyclic metal–metal arrangements arise from the amide groups connecting four square‐planar CuII centers and four octahedral NiII centers in a cyclic manner. UV/Vis spectral studies suggested that this efficient formation was achieved by the selective formation of the square‐planar CuII centers and a crystallization process. Magnetic measurements using SQUID clarified that the cyclic complex represented the S=2 spin state at low temperatures due to effective antiferromagnetic interactions between the NiII and CuII centers.

Solid-State Properties of Hexaazatriphenylenehexacarbonitrile HAT(CN)6 .- Radical Anions in Crystalline Salts Containing Cryptand(M+ ) and Crystal Violet Cations.

Crystalline {Cryptand[2.2.2](Na+ )}{HAT(CN)6 .- }⋅0.5C6 H4 Cl2 (1), {Cryptand[2.2.2](K+ )}{HAT(CN)6 .- } (2), (CV+ ){HAT(CN)6 .- } (3), and (CV+ ){HAT(CN)6 .- }⋅2C6 H4 Cl2 (4) salts (where CV+ is the crystal violet cation) containing hexaazatriphenylenehexacarbonitrile radical anions have been obtained. The solid-state molecular structure as well as the optical and magnetic properties of HAT(CN)6 .- are studied. The formation of HAT(CN)6 .- in 1-4 leads to the appearance of new bands in the visible range, at 694 and 740 nm. The HAT(CN)6 .- radical anions have spin state S=1/2 and are packed in one-dimensional stacks containing the {HAT(CN)6 .- }2 dimers alternated with weaker interacting pairs of HAT(CN)6 .- in 1 and nearly isolated {HAT(CN)6 .- }2 dimers in 2. The {HAT(CN)6 .- }2 dimers are diamagnetic in 1 but they effectively mediate one-dimensional antiferromagnetic coupling of spins within the stacks with moderate exchange interaction of J/kB = -80 K. The behaviour of salt 2 is described by a singlet-triplet model for the {HAT(CN)6 .- }2 dimers with an energy gap of 434(±7) K. Magnetic behaviour of both salts agree well with the data of extended Hückel calculations. Salts 3 and 4 contain isolated stacks of alternated HAT(CN)6 .- and CV+ ions, and in this case, nearly paramagnetic behaviour is observed with Weiss temperatures of -1 and -7 K, respectively. Narrow Lorentzian EPR signals with g = 2.0033-2.0039 were found for the HAT(CN)6 .- radical anions in 1 and 4 but in solution g-factor shifts to 1.9964. The electronic structure of HAT(CN)6 .- is analysed based on X-ray diffraction data for 2, showing a Jahn-Teller distortion of the radical anion that reduces the symmetry from D3h to Cs and splits the initially degenerated LUMOs.

π-electron weak ferromagnetism in potassium-intercalated 9-phenylanthracene

Intercalation of alkali metal is an effective way to tune the physicochemical property of polyaromatic hydrocarbons. Herein, we show for the first time that the weak ferromagnetism with a Curie temperature of 5.2 K can be realized in an all-carbon π-electron compound – potassium-intercalated 9-phenylanthracene. The magnetic measurements reveal that there exists an antiferromagnetic transition around 35 K before transition to the weak ferromagnetic phase. An in-depth theoretical investigation indicates that the weak ferromagnetism is produced by the intramolecular ferromagnetic and intermolecular antiferromagnetic spin couplings in the crystal unit cell. Our finding enriches the physical functionality of anthracene derivatives and adds a new member of organic magnetic materials.

Polariton condensate trapping by parametric pair scattering

Spatially confined, trapped polariton condensates have been shown to exhibit strong stochastic on-site spin polarization and in longer polariton condensate chains, distance controlled ferromagnetic and antiferromagnetic spin couplings. Until now, little is known, on how such polariton condensates spatially separated from their exciton reservoirs are trapped and formed. Here, we investigate the properties and formation dynamics of two main families of polariton condensates, those overlapping with the pump reservoir and those in confined geometries, under pulsed nonresonant excitation. The observed reduction in polariton condensation threshold and energy blueshift in trapped case is attributed to exciton reservoir-condensate spatial separation, whereas time-resolved photoluminescence measurements, reveal distinct relaxation and condensate formation dynamics with pair parametric scattering process being the dominant relaxation mechanism in trapped geometry.