High-field EPR reveals the strongly temperature-dependent exchange interaction in "breathing" crystals Cu(hfac)2L(R)

Low temperature insights into the crystal and magnetic structure of a neutral radical ferromagnet

The crystal structure of the radical ferromagnet 1a at 2 K reveals a contraction in the unit cell c constant which, at the molecular level, translates into a decrease in slippage of the radical π-stacks and an increase in ferromagnetic exchange interactions along the stacking axis. The results of BS-DFT calculations using long-range corrected functionals are consistent with an overall ferromagnetic topology.

Exchange interactions in photoinduced magnetostructural states of copper(ii)–nitroxide spin dyads

We investigate intra- and intercluster exchange couplings in photoinduced states of copper(ii)–nitroxide based molecular magnets (“breathing crystals”) using variable-temperature EPR spectroscopy.

Magnetic Cationic Copper(II) Chains and a Mononuclear Cobalt(II) Complex Containing [Ln(hfac)4]- Blocks as Counterions

Four new heterospin compounds with molecular formula {[Cu₂(hfac)₃(TlTrzNIT)₂][Ln(hfac)₄]} _n·C₇H₁₆ (Ln^III = Gd (1), Tb (2), or Dy (3)) and [Co(hfac)(TlTrzNIT)₂][Dy(hfac)₄] (4), where hfac is hexafluoroacetylacetonato and TlTrzNIT is the nitronylnitroxide radical 1-( m-tolyl)-1 H-1,2,3-triazole-4-(4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide), were obtained. All structures were determined by single-crystal X-ray diffraction. In compounds 1-3, the TlTrzNIT radical is bridge-coordinated to copper(II) ions, leading to positively charged copper(II)-radical chains containing [Ln(hfac)₄]^- as counterions. In compound 4, the cobalt(II) ion is coordinated to two TlTrzNIT radicals and one hfac ligand in bidentate mode leading to a mononuclear cationic complex that contains [Dy(hfac)₄]^- as counterion. Magnetic measurements of all complexes were performed. Magnetic data were fit considering the contributions of the copper(II)-radical chain and a paramagnetic gadolinium(III) ion for 1. The sign and magnitude of the magnetic coupling constants extracted from the fit were confirmed by density functional theory calculations. The obtained spin topology shows an alternated ferro-antiferromagnetic chain. Field-induced single molecule magnet behavior was observed for the Dy derivatives 3 and 4, in agreement with CASSCF calculations performed for the latter system.

CoII Complexes with a Tripyridine Ligand, Containing a 2,6-Di-tert-butylphenolic Fragment: Synthesis, Structure, and Formation of Stable Radicals

Interaction of a tripyridine ligand bearing a 2,6-di-tert-butylphenolic fragment (L, 2,6-di-tert-butyl-4-(3,5-bis(4-pyridyl)pyridyl)phenol) with Co^II pivalate or chloride led to the formation of one-dimensional coordination polymers [Co(L)Cl₂] _n ·nEtOH (1) and [Co₃(L)₂(OH)(Piv)₅] _n (2) or a trinuclear complex Co₃(H₂O)₄(L)₂Cl₆ (3) (Piv^- = pivalate). Chemical oxidation of L and 1-3 by PbO₂ or K₃[Fe(CN)₆], as well as exposure of L (in solution or solid state) and 2 (in solid state) to UV irradiation, led to the formation of free radicals with g = 2.0024, which probably originated because of oxidation of 2,6-di-tert-butylphenolic groups. These radicals were stable for several days in solutions and more than 1 month in solid samples. Irradiation and oxidation of the solid samples probably caused formation of the phenoxyl radical only on their surface. It was shown by density functional theory calculations that exchange coupling between the unpaired electron of the phenoxyl radical and Co^II ions was negligibly weak and could not affect the electron paramagnetic resonance signal of the radical, as well as exchange coupling of Co^II ions could not be transmitted by L. The latter conclusion was confirmed by the analysis of magnetic properties of 1: temperature dependency of magnetic susceptibility (χ_M) of 1 could be simulated by a simple model for isolated Co^II ions.

Copper–nitroxide based breathing crystals: a unified mechanism of gradual magnetostructural transition supported by quantum chemistry calculations

A unified mechanism, based on the thermal average of two limit phases, governs the gradual magnetic transitions of Cu(hfac)₂L^R complexes.

Improving the accuracy of Cu(ii)–nitroxide RIDME in the presence of orientation correlation in water-soluble Cu(ii)–nitroxide rulers

Detailed analysis of artefacts in the Cu(ii)–nitroxide RIDME experiments, related to orientation averaging, echo-crossing, ESEEM and background-correction is presented.

Light-Induced Spin Transitions in Copper-Nitroxide-Based Switchable Molecular Magnets: Insights from Periodic DFT+U Calculations

The electronic structure and magnetic interactions of three members of the breathing crystal Cu(hfac)₂ L^R family (hfac=hexafluoroacetylacetonato, L^R =pyrazole-substituted nitronyl nitroxides with R=Me, Et, Pr, iPr, Bu ), mainly Cu(hfac)₂ L^Pr (1), Cu(hfac)₂ L^Bu ⋅0.5 C₈ H₁₈ (2) and Cu(hfac)₂ L^Bu ⋅0.5 C₈ H₁₀ (3), have been analyzed by means of periodic plane-wave based DFT+U calculations. These Cu^II -nitroxide based molecular magnets display thermally and optically induced switchable behavior and light-induced excited spin state trapping phenomena. The calculations confirm the presence of temperature-dependent exchange interaction within the spin triads formed by the nitroxide-copper(II)-nitroxide units, in line with the changes observed in the effective magnetic moment. Moreover, they quantify the interchain interaction mediated by the terminal nitroxide group of two spin triads in neighboring polymer chains. This interaction competes with the exchange interaction within the spin triads at high temperature, and introduces 1D exchange channels that do not coincide with the polymeric chains. The density of states reveal that the low-lying conduction states potentially involved in the UV/Vis transitions are located on the nitroxide radicals, the hfac groups and the Cu atoms. Then, the density of states is almost independent of the solvent and the R group. This suggests the possibility of light-induced spin switching for other members of this family. The 500 nm band of the low-temperature phase can be ascribed to a ligand-to-metal charge transfer transition between the nitroxide and Cu bands.

A loop chain and a three-dimensional network assembled from a multi-dentate nitronyl nitroxide radical and M(hfac)2 (M = CoII, CuII)

A multi-dentate nitronyl nitroxide radical Nit-Ph-3,5-bIm (Nit-Ph-3,5-bIm = 2-[3,5-bis(1-imidazole)-phenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) reacted with Co(hfac)2·2H2O or Cu(hfac)2 to yield two novel 3d-radical complexes {[Co(hfac)2]2(Nit-Ph-3,5-bIm)}n1 and {[Cu(hfac)2]7(Nit-Ph-3,5-bIm)3}n2 (hfac- = hexafluoroacetylacetonate). In both compounds, the Nit-Ph-3,5-bIm radical ligand behaves as a tetradentate ligand to link four M(hfac)2 units in a μ4-η1:η1:η1:η1-coordination mode via its two NO groups and two N atoms of imidazole, generating a unique loop chain for 1 and a three-dimensional framework for 2. The magnetic studies revealed a strong antiferromagnetic Co-ON exchange interaction in 1 while a ferromagnetic Cu-ON interaction is observed in 2. The dc magnetic behaviors of two complexes are analyzed by means of appropriate magnetic models. Furthermore, ac magnetic susceptibilities of the Co complex reveal slow relaxation of magnetization.

Magnetic Exchange Interaction in Nitronyl Nitroxide Radical-Based Single Crystals of 3d Metal Complexes: A Combined Experimental and Theoretical Study

Two stable nitronyl nitroxide free radicals {R ¹ = 4'-methoxy-phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide (NNPhOMe) and R ² = 2-(2'-thienyl)-4,4,5,5-tetramethylimidazoline 3-oxide 1-oxyl (NNT)} are successfully synthesized using Ullmann condensation. The reactions of these two radicals with 3d transition metal ions, in the form of M(hfac)₂ (where M = Co or Mn, hfac: hexafluoroacetylacetone), result in four metal-organic complexes Co(hfac)₂(NNPhOMe)₂, 1; Co(hfac)₂(NNT)₂·(H₂O), 2; Mn(hfac)₂(NNPhOMe)·x(C₇H₁₆), 3; and Mn(hfac)₂(NNT)₂, 4. The crystal structure and magnetic properties of these complexes are investigated by single-crystal X-ray diffraction, dc magnetization, infrared, and electron paramagnetic resonance spectroscopies. The compounds 1 and 4 crystallize in the triclinic, P1̅, space group, whereas complex 3 crystallizes in the monoclinic structure with the C2/c space group and forms chain-like structure along the c direction. The complex 2 crystallizes in the monoclinic symmetry with the P2₁/c space group in which the N-O unit of the radical coordinates with the Co ion through hydrogen bonding of a water molecule. All compounds exhibit antiferromagnetic interactions between the transition metal ions and nitronyl nitroxide radicals. The magnetic exchange interactions (J/K _B) are derived using isotropic spin Hamiltonian H = -2J∑(S _metal S _radical) for the model fitting to the magnetic susceptibility data for 1, 2, 3, and 4. The exchange interaction strengths are found to be -328, -1.25, -248, and -256 K, for the 1, 2, 3, and 4 metal-organic complexes, respectively. Quantum chemical density functional theory (DFT) computations are carried out on several models of the metal-radical complexes to elucidate the magnetic interactions at the molecular level. The calculations show that a small part of the inorganic spins are delocalized over the oxygens from hfac {∼0.03 for Co(II) and ∼0.015 for Mn(II)}, whereas a more significant fraction {∼0.24 for Mn(II) and ∼0.13 for Co(II)} of delocalized spins from the metal ion is transferred to the coordinated oxygen atom(s) of nitronyl nitroxide.

Luminescent Dinuclear Copper(I) Complexes as Potential Thermally Activated Delayed Fluorescence (TADF) Emitters: A Theoretical Study

The excited state properties of a series of binuclear NHetPHOS-Cu(I) complexes (NHetPHOS) have been investigated by means of density functional theory (DFT) and time-dependent DFT (TD-DFT). It is shown that experimental trends observed in powder, generally explored via S₁ and T₁ excited state energetics and S₁ ⇔ T₁ intersystem crossing (ISC) efficiency, are hardly analyzed on the basis of excited state properties calculated in solution. Indeed, several local minima corresponding to various structural deformations are evident on the lowest excited state potential energy surfaces (PES) when solvent correction is applied, leading to a four-state thermally activated delayed fluorescence (TADF) mechanism. In contrast, preliminary simulations performed in the solid point to the reduction of nuclear flexibility and consequently to a rather simple two-state model.

Temperature-Induced Increase of Spin Spiral Periods

Spin-polarized scanning tunneling microscopy investigations reveal a significant increase of the magnetic period of spin spirals in three-atomic-layer-thick Fe films on Ir(111), from about 4 nm at 8 K to about 65 nm at room temperature. We attribute this considerable influence of temperature on the magnetic length scale of noncollinear spin states to different exchange interaction coefficients in the different Fe layers. We thus propose a classical spin model that reproduces the experimental observations and in which the crucial feature is the presence of magnetically coupled atomic layers with different interaction strengths. This model might also apply for many other systems, especially magnetic multilayers.

Elastically driven cooperative response of a molecular material impacted by a laser pulse

Photoinduced phase transformations occur when a laser pulse impacts a material, thereby transforming its electronic and/or structural orders, consequently affecting the functionalities. The transient nature of photoinduced states has thus far severely limited the scope of applications. It is of paramount importance to explore whether structural feedback during the solid deformation has the capacity to amplify and stabilize photoinduced transformations. Contrary to coherent optical phonons, which have long been under scrutiny, coherently propagating cell deformations over acoustic timescales have not been explored to a similar degree, particularly with respect to cooperative elastic interactions. Herein we demonstrate, experimentally and theoretically, a self-amplified responsiveness in a spin-crossover material during its delayed volume expansion. The cooperative response at the material scale prevails above a threshold excitation, significantly extending the lifetime of photoinduced states. Such elastically driven cooperativity triggered by a light pulse offers an efficient route towards the generation and stabilization of photoinduced phases in many volume-changing materials.

Mechanism of Magnetostructural Transitions in Copper-Nitroxide-Based Switchable Molecular Magnets: Insights from ab Initio Quantum Chemistry Calculations

The gradual magnetostructural transition in breathing crystals based on copper(II) and pyrazolyl-substituted nitronyl nitroxides has been analyzed by means of DDCI quantum chemistry calculations. The magnetic coupling constants (J) within the spin triads of Cu(hfac)2L(Bu)·0.5C8H18 have been evaluated for the X-ray structures reported at different temperatures. The coupling is strongly antiferromagnetic at low temperature and becomes ferromagnetic when the temperature increases. The intercluster magnetic coupling (J') is antiferromagnetic and shows a marked dependence on temperature. The magnetostructural transition can be reproduced using the calculated J values for each structure in the simulation of the magnetic susceptibility. However, the μ(T) curve can be improved nicely by considering the coexistence of two phases in the transition region, whose ratio varies with temperature corresponding to both the weakly and strongly coupled spin states. These results complement a recent VT-FTIR study on the parent Cu(hfac)2L(Pr) compound with a gradual magnetostructural transition.

Dynamical effects on the magnetic properties of dithiazolyl bistable materials

The magnetic properties of molecule-based magnets are commonly rationalized by considering only a single nuclear configuration of the system under study (usually an X-ray crystal structure). Here, by means of a computational study, we compare the results obtained using such a static approach with those obtained by explicitly accounting for thermal fluctuations, and uncover the serious limitations of the static perspective when dealing with magnetic crystals whose radicals undergo wide-amplitude motions. As a proof of concept, these limitations are illustrated for the magnetically bistable 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) material. For its high-temperature phase at 300 K, we show that nuclear dynamics induce large fluctuations in the magnetic exchange interactions (JAB) between spins (up to 1000% of the average value). These deviations result in a ∼20% difference between the 300 K magnetic susceptibility computed by explicitly considering the nuclear dynamics and that computed using the X-ray structure, the former being in better agreement with the experimental data. The unveiled strong coupling between JAB interactions and intermolecular vibrations reveals that considering JAB as a constant value at a given temperature (as always done in molecular magnetism) leads to a flawed description of the magnetism of TTTA. Instead, the physically relevant concept in this case is the statistical distribution of JAB values. The discovery that a single X-ray structure is not adequate enough to interpret the magnetic properties of TTTA is also expected to be decisive in other organic magnets with dominant exchange interactions propagating through labile π-π networks.

Spatial distribution of phases during gradual magnetostructural transitions in copper(ii)–nitroxide based molecular magnets

A thorough analysis of EPR and XRD data led to conclusions on the spatial distribution of phases in copper–nitroxide molecular magnets.

A New Quinoxalinyl-Substituted Nitronyl Nitroxide Radical and its Five-Spin CuII and Four-Spin MnII Complexes: Syntheses, Crystal Structures, and Magnetic Properties

A new radical of QNXL-2NIT (1) (QNXL-2NIT = 2-(2-quinoxalinyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide) and its complexes [Cu(hfac)2]3(QNXL-2NIT)2·2CHCl3 (2) and [Mn(hfac)2]2(QNXL-2NIT)2 (3) (hfac = 1,1,1,5,5,5-hexfluoroacetylacetone) were first reasonably designed and synthesised in this paper. The X-ray structure determination revealed that by introducing the quinoxalinyl ring group to the nitronyl nitroxide radical, QNXL-2NIT affords more coordination sites for metal ions. We successfully obtained its copper complex with linear tri-nuclear five-spin structure. However, when we substituted MnII as the central metal, complex 3 exhibits a rectangle-like four-spin framework containing two MnII ions and two radicals in one molecule. The magnetic study of the two complexes shows that there are strong antiferromagnetic interactions between the radical and the terminal CuII ions with g = 2.25, J = –291.67 cm–1 and zJ′ = –0.25 cm–1 and the dominating interactions between the MnII ion and the radical in complex 3 are also antiferromagnetic with the parameters of g = 1.99, J 1 = –77.00 cm–1 and J 2 = 0.34 cm–1.