Victor I. Ovcharenko

High-Field EPR Reveals the Strongly Temperature-Dependent Exchange Interaction in “Breathing” Crystals Cu(hfac)2LR

In the overwhelming majority of the exchange-coupled clusters investigated in field of molecular magnetism, the exchange interaction is constant on temperature. “Breathing” crystals of composition Cu(hfac)2LR undergo temperature-induced reversible structural rearrangements accompanied by significant changes of the effective magnetic moment. Using high-field (W-band) EPR, we provide a solid proof of drastic temperature dependence of exchange interaction J(T) in these compounds that originates from temperature dependence of inter-spin distances. Strong dependence J(T) revealed by EPR makes Cu(hfac)2LR breathing crystals interesting and promising systems in the research toward creation of molecular-magnetic switches and related spin devices.

Light‐Induced Excited Spin State Trapping in an Exchange‐Coupled Nitroxide‐Copper(II)‐Nitroxide Cluster

The discovery of light-induced excited spin state trapping (LIESST) on a spin-crossover complex of iron(II) in 1984 attracted much attention in the field of photomagnetism and triggered a large number of subsequent studies. Apart from being an interesting photophysical phenomenon, LIESST is one of the most promising effects for potential applications in molecular-level light-operated magnetic devices, because the metastable multiplet state excited by light may be trapped for hours to days at sufficiently low temperatures (typically below Tc 50 K). Recent research on LIESST has been aimed at increasing the blocking temperature Tc and expanding the range of compounds exhibiting this effect. To date, LIESST has been observed on compounds containing spin-crossover complexes of iron(II) and, more recently, iron(III). In all of these cases, including exchange-coupled dinuclear complexes, LIESST is based on the spin-crossover nature of iron. Herein, we report the first example of a LIESST-like effect observed in a system of principally different type, that is, an exchange cluster of copper(II) with two nitronyl nitroxide ligands. This compound does not contain any metals displaying spin crossover, and an efficient trapping of the light-induced state of a cluster on a timescale of hours originates from highly cooperative structural rearrangements in a one-dimensional system. Photoswitching of the exchange interaction has been studied in other one-dimensional systems aiming at ultrafast switching times. In contrast, the present system shows a behavior more similar to the LIESST effect in iron compounds and is characterized by a long lifetime of the photoinduced state. Our finding introduces a new class of photoswitchable compounds and thus broadens the scope and range of potential applications of such systems. The polymeric complex [Cu(hfac)2L ] (Figure 1) belongs to the family of so-called “breathing crystals” that we have studied extensively over the last few years. Breathing

Nitronyl and imino nitroxides: improvement of Ullman's procedure and report on a new efficient synthetic route.

The synthesis of nitronyl and imino nitroxides has been reexamined with the aim of both increasing yields and of offering opportunities for new structures. The conditions for the formation of 2,3-bis(hydroxyamino)-2,3-dimethylbutane, the key intermediate of Ullmans route, have been carefully studied, and a new procedure is proposed, which affords the free base in a very pure form and up to 60% yield. Full characterization of this intermediate including an X-ray crystal structure is presented. An alternative synthetic route through 2,3-diamino-2,3-dimethylbutane and the corresponding imidazolidines which bypasses the delicate synthesis of the bis(hydroxyamino) compound is described. It is shown that 3-chloroperbenzoic acid is an effective oxidant for the transformation of adequately substituted imidazolidines into nitronyl nitroxides, which are obtained in high yield. An illustration of the potentialities of this new route, a new nitronyl nitroxide with two ethyl substituents in positions 4 and 5 of the imidazoline ring, is reported. The scope and limitations of the two routes are discussed.

Unusual spin transitions

New heterospin complexes of Cu(hfac)2 (hfac, hexafluoroacetylacetonate) with pyrazole-substituted nitronyl nitroxides have been found that in the solid state exhibit thermally induced spin transitions analogous to spin crossover. For the first complex, [Cu(hfac)2Li-Pr], at room temperature, the Cu—OL distances, where OL is the oxygen atom of the nitroxyl group, are very short (2.143 Å). This leads to a strong antiferromagnetic exchange (∼-120cm−1) in the > N—·O—Cu2+—O·—N < exchange clusters. The CuO6 coordination units formed by four O atoms of the two hfac anions and by the nitroxyl O atoms of the two bridging nitroxides have a rare form of flattened octahedra, transformed at low temperatures into elongated octahedra with shorter Cu—OL distances (2.143 Å→2.002Å) and two longer Cu—Ohfac distances (2.130 Å→2.293 Å). For the second complex, [Cu(hfac)2LBu·0.5C6H14], unusual low temperature structural dynamics of heterospin systems have been found. It is characterized by the formation of two types of CuO6 unit. The axial Cu—OL distances are lengthened in one unit (2.250 Å→2.347 Å) and shortened in the other (2.250 Å → 2.006 Å). This leads to a sophisticated μeff(T) dependence with μeff drastically decreased at 163 K as a result of full coupling of two spins in half of all >N—·O—Cu2+—O·—N < exchange clusters and to a shift from antiferromagnetic to ferromagnetic exchange in the other half.

Intercluster Exchange Pathways in Polymer-Chain Molecular Magnets Cu(hfac)2LR Unveiled by Electron Paramagnetic Resonance

Polymer-chain complexes Cu(hfac)(2)L(R) represent an interesting type of molecular magnets exhibiting thermally induced and light-induced magnetic switching, in many respects similar to a spin crossover. In the majority of these compounds the polymer chain consists of alternating one- and three-spin units composed of copper(II) ions and nitronyl nitroxides. The principal one-dimensional structure of the complexes has previously been assumed to play a key role in the observed magnetic anomalies. Using Q-band electron paramagnetic resonance (EPR) spectroscopy, we have reliably demonstrated that these complexes are indeed one-dimensional in the sense of the topology of their exchange channels; however, the magnetic chains spread across the structural polymer chains and consist solely of spin triads of nitroxide-copper(II)-nitroxide. Using four selected examples of complexes Cu(hfac)(2)L(R), we have found the exchange coupling values between spin triads of neighboring polymer chains to range from <1 to ca. 10 cm(-1). This conclusion could only be reached due to the selective probing of one- and three-spin units by EPR and correlates perfectly with both previous magnetic susceptibility data and quantum chemical calculations performed in this work. These findings give new insights into the cooperativity effects and mechanisms of magnetic anomalies in the Cu(hfac)(2)L(R) family of molecular magnets.

A thorough investigation of the synthetic problems of vic-bis-hydroxylamine - the precursor of Ullman's nitroxides

Abstract The products formed in the course of the reduction of 2,3-dimethyl-2,3-dinitrobutane to 2,3-dihydroxylamino-2,3-dimethylbutane in a Zn/NH4Cl/EtOH-H2O system have been investigated. An improved synthesis of 2,3-dihydroxylamino-2,3-dimethylbutane (and its monosulphate salt) - the key precursor of Ullmans nitroxides - in a Zn/NH4Cl/THF-H2O system is proposed.

Ultrafast Photoswitching in a Copper‐Nitroxide‐Based Molecular Magnet

Molecular compounds with photoswitchable magnetic properties have been intensively investigated over the last decades due to their prospective applications in nanoelectronics, sensing and magnetic data storage. The family of copper-nitroxide-based molecular magnets represents a new promising type of photoswitchable compounds. We report the first study of these appealing systems using femtosecond optical spectroscopy. We unveil the mechanism of ultrafast (<50 fs) spin state photoswitching and establish its principal differences compared to other photoswitchable magnets. On this basis, we propose potential advantages of copper-nitroxide-based molecular magnets for the future design of ultrafast magnetic materials.

W-band time-resolved electron paramagnetic resonance study of light-induced spin dynamics in copper-nitroxide-based switchable molecular magnets.

Molecular magnets Cu(hfac)(2)L(R) represent a new type of photoswitchable materials based on exchange-coupled clusters of copper(II) with stable nitroxide radicals. It was found recently that the photoinduced spin state of these compounds is metastable on the time scale of hours at cryogenic temperatures, similar to the light-induced excited spin state trapping phenomenon well-known for many spin-crossover compounds. Our previous studies have shown that electron paramagnetic resonance (EPR) in continuous wave (CW) mode allows for studying the light-induced spin state conversion and relaxation in the Cu(hfac)(2)L(R) family. However, light-induced spin dynamics in these compounds has not been studied on the sub-second time scale so far. In this work we report the first time-resolved (TR) EPR study of light-induced spin state switching and relaxation in Cu(hfac)(2)L(R) with nanosecond temporal resolution. To enhance spectral resolution we used high-frequency TR EPR at W-band (94 GHz). We first discuss the peculiarities of applying TR EPR to the solid-phase compounds Cu(hfac)(2)L(R) at low (liquid helium) temperatures and approaches developed for photoswitching/relaxation studies. Then we analyze the kinetics of the excited spin state at T = 5-21 K. It has been found that the photoinduced spin state is formed at time delays shorter than 100 ns. It has also been found that the observed relaxation of the excited state is exponential on the nanosecond time scale, with the decay rate depending linearly on temperature. We propose and discuss possible mechanisms of these processes and correlate them with previously obtained CW EPR data.

Temperature-Dependent Exchange Interaction in Molecular Magnets Cu(hfac)2LR Studied by EPR: Methodology and Interpretations

Exchange-coupled spin triads nitroxide-copper(II)-nitroxide are the key building blocks of molecular magnets Cu(hfac)(2)L(R). These compounds exhibit thermally induced structural rearrangements and spin transitions, where the exchange interaction between spins of copper(II) ion and nitroxide radicals changes typically by 1 order of magnitude. We have shown previously that electron paramagnetic resonance (EPR) spectroscopy is sensitive to the observed magnetic anomalies and provides information on both inter- and intracluster exchange interactions. The value of intracluster exchange interaction is temperature-dependent (J(T)), that can be accessed by monitoring the effective g-factor of the spin triad as a function of temperature (g(eff)(T)). This paper describes approaches for studying the g(eff)(T) and J(T) dependences and establishes correlations between them. The experimentally obtained g(eff)(T) dependences are interpreted using three different models for the mechanism of structural rearrangements on the molecular level leading to different meanings of the J(T) function. The contributions from these mechanisms and their manifestations in X-ray, magnetic susceptibility and EPR data are discussed.

Theory of spin-peierls transitions in chains of exchange clusters.

A theoretical approach to the analysis of magnetostructural phase transitions of chain polymeric heterospin complexes is suggested. The approach is based on a model of the spin-Peierls transition in chains of exchange clusters. The chain elasticity parameter is found to be a main factor determining the order of phase transition.