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Refocusing pulse trains with multiple echo integration enhances sensitivity in pulse EPR experiments with photo-labels, aiding in applications of light-induced pulsed dipolar spectroscopy.

Intriguing heterogeneities and nanostructural reorganizations of glassy ionic liquids (ILs) have recently been found using electron paramagnetic resonance (EPR) spectroscopy. Alkyl chains of IL cations play the key role in such phenomena and govern the anomalous temperature dependence of local density and molecular mobility. In this paper we evidence and study similar manifestations in a variety of common non-IL glasses, which also contain molecules with alkyl chains. A series of phthalates clearly demonstrates very similar behavior to imidazolium-based ILs with the same length of alkyl chain. Glasses of alkyl alcohols and alkyl benzenes show only some similarities to the corresponding ILs, mainly due to a lower glass transition temperature hindering the development of the anomaly. Therefore, we demonstrate the general nature and broad scope of nanoscale structural anomalies in organic glasses based on alkyl-chain compounds. The 'roadmap' for their occurrence is provided, which aids in understanding and future applications of these anomalous nanoheterogeneities.

The Cu(hfac)2 complex with a paramagnetic ligand 2-(1-ethyl-imidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L5Et) was synthesized in the form of two polymorphs: α-[Cu(hf...

Unlike synchrotrons with multiple beamlines, free-electron lasers (FELs) are single-beam facilities, which nevertheless have a number of endstations. The latter requires development of complex radiation transport line, which should be efficient enough to avoid scaling down the FEL capabilities. Keeping the beam shape and radiation power level along the beamline is a challenge because the total length of the FEL radiation transport line can exceed a hundred meters. The FELs around the world differ from each other in both radiation parameters and endstations’ layout requiring individual design of their radiation transport lines. In this work, we describe the 120-m beamline for transporting the radiation of the Novosibirsk FEL facility, consisting of three FELs of the terahertz (THz), far-infrared, and mid-infrared ranges. The radiation of these three FELs is directed to the same beam transport channel, which is able to deliver the radiation to any of 14 endstations already commissioned. To compare the expected beam parameters with the actual ones, the radiation intensity distribution was measured in a number of places of the beamline that are THz radiation outputs for some endstations. Possible causes of the parameters’ mismatching observed for distant endstations are discussed. The problem of radiation absorption by water vapor is considered in detail.

The influence of the electric field on the spin state of the single crystal of [Fe(1-Bpp)2][BF4]2 demonstrating the spin crossover phenomenon is studied. The spin state of the complex is determined by the indirect method: FT-IR microscopy. The experimental results show that the application of the external electrostatic field with an intensity of 13 kV/cm exerts no effect on the temperature of the magnetostructural transition of the compound within 0.2 K.

The interaction of Cu(acF6)2 with the stable imino nitroxide 2-(pyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl (ImN-mPy) at a ratio of 2:1 led to the formation of a new complex [...

A new 1D-coordination polymer [Co(Piv)2(NH2(CH2)6NH2)]n (1, Piv is Me3CCO2− anion) was obtained, the mononuclear fragments {Co(O2CR)2} within which are linked by μ-bridged molecules of hexamethylenediamine (NH2(CH2)6NH2). For this compound, two different monoclinic C2/c (α-1) and P2/n (β-1) phases were found at room temperature by single-crystal X-ray diffraction analysis, with a similar structure of chains and their packages in unit cells. The low-temperature phase (γ-1) of crystal 1 at 150 K corresponds to the triclinic space group P-1. As the temperature decreases, the structural phase transition (SPT) in the α-1 and β-1 crystals is accompanied by an increase in the crystal packing density caused by the rearrangements of both H-bonds and the nearest ligand environment of the cobalt atom (“octahedral CoN2O4 around the metal center at room temperature” → “pseudo-tetrahedral CoN2O2 at 150 K”). The SPT was confirmed by DSC in the temperature range 210–150 K; when heated above 220 K, anomalies in the behavior of the heat flow are observed, which may be associated with the reversibility of SPT; endo effects are observed up to 300 K. The SPT starts below 200 K. At 100 K, a mixture of phases was found in sample 1: 27% α-1 phase, 61% γ-1 phase. In addition, at 100 K, 12% of the new δ-1 phase was detected, which was identified from the diffraction pattern at 260 K upon subsequent heating: the a,b,c-parameters and unit cell volume are close to the structure parameters of γ-1, and the values of the α,β,γ-angles are significantly different. Further heating leads to a phase transition from δ-1 to α-1, which both coexist at room temperature. According to the DC magnetometry data, during cooling and heating, the χMT(T) curves for 1 form a hysteresis loop with ~110 K, in which the difference in the χMT values reaches 9%. Ab initio calculations of the electronic structure of cobalt(II) in α-1 and γ-1 have been performed. Based on the EPR data at 10 K and the ab initio calculations, the behavior of the χMT(T) curve for 1 was simulated in the temperature range of 2–150 K. It was found that 1 exhibits slow magnetic relaxation in a field of 1000 Oe.

Single-ion magnets (SIMs) with large magnetic anisotropy are promising candidates for realization of single-molecule based magnetic memory and qubits. Creation of materials with magnetically uncoupled spatially separated SIMs requires dilution in a diamagnetic matrix. Herein, we report that progressive dilution of paramagnetic Co(II) by diamagnetic Zn(II) in the SIM ([CoxZn(1−x)(piv)2(2-NH2-Py)2], x = 1 ÷ 0) beyond a threshold of 50% reveals an abrupt structural change, where the distorted tetrahedral Zn coordination structure is superimposed on the remaining Co ions, which were initially in a distorted octahedral environment. Dilution-induced structure modification switches the magnetic anisotropy from easy-plane (D = 36.7 cm−1) to easy-axis type (D = −23.9 cm−1), accompanied by a fivefold increase of the magnetic relaxation time at 2 K. Changes of the static and dynamic magnetic properties are monitored by electron paramagnetic resonance spectroscopy and AC susceptibility measurements. Complementary quantum chemical ab initio calculations quantify the influence of structural changes on the electronic structure and the magnetic anisotropy. Thus, magnetic dilution hits two goals at once, the creation of isolated magnetic centres and an improvement of their SIM properties.

Copper(ii) complexes with stable nitroxide radicals are capable of magnetostructural spin-crossover like anomalies induced by external stimuli. Photoswitching in such systems is particularly important; however, retrieving the properties of photoinduced states is challenging and requires development of novel approaches. In this work, we investigate the exchange interactions in metastable photoinduced states of two compounds containing copper(ii)-nitroxide dyads. Using Electron Paramagnetic Resonance (EPR) with photoexcitation we obtain temperature dependence of magnetic susceptibility in the photoinduced state and estimates for the corresponding values of exchange coupling in the studied complexes. The interplay between intra- and inter-cluster exchange couplings is considered and analyzed. The proposed methodology is applicable also to other photoswitchable exchange-coupled systems.

We report on the investigation of the localized surface plasmon resonance (LSPR) in periodical Au nanostructures. The arrays of Au nanoclusters and dimers were fabricated on Si and Si/SiO2 surfaces by electron beam lithography. Diameters and periods of nanoclusters with disk shape vary in the range of 30–150 and 130–200 nm, respectively. Because of the opaque nature of the substrates, optical reflection spectroscopy was chosen to probe the plasmonic properties of the metal nanostructures. From a comparison of experimental reflection spectra with those numerically simulated by the Finite Difference Time Domain (FDTD) method, we determined the model structural parameters of the plasmonic nanostructures. These parameters were further used for the calculation of absorptance spectra of the plasmonic structures for which absorptance in the substrate was subtracted. LSPR positions were determined from the maxima of the absorptance spectra. This study reveals a strong dependence of the LSPR position on nanocluster size, distance between nanoclusters, as well as on the SiO2 layer thickness in the nanometer range. In the case of dimer arrays, the plasmon anisotropy in the dimers leads to a splitting of the LSPR plasmon into two modes with orthogonal polarizations. The absorptance spectra reveal a transverse LSPR mode corresponding to the excitation of plasmons in nanoclusters induced by scattered fields from the neighboring ones. This research provides a pathway for a fast and cost-effective determination of the LSPR position from optical reflection spectra. A broad field of potential applications of metal structures with well-controlled plasmonic properties includes surface-enhanced infrared absorption, photoluminescence, and Raman scattering as well as signal transmission in silicon photonics.

A simple technique is proposed to modulate the output radiation power in a free-electron laser (FEL) oscillator that allows the average radiation power to be adjusted smoothly, and short (as brief as 35 μs) radiation macrobunches to be obtained. It is based on the periodic shift of the electron bunch phase with respect to that of an FEL radiation bunch stored in the optical cavity, resulting in lasing suppression. The phase shift frequency required to suppress lasing is relatively low and does not change the electron bunch repetition rate appreciably. A computer-based test bench is created to demonstrate the feasibility of the proposed technique. The test bench contains standard CAMAC blocks: a G0609 modulator timer and a G0601 clock generator. The electron bunch phase is shifted via programmable skipping of the reference frequency periods used to start the current modulator in the electron gun. The skipping is controlled by a computer program operating in the real-time mode. The clock generator is used to send synchronization signals to the radiation user’s equipment. The test bench is tested via EPR spectroscopy with a temporal resolution that allows the duration of the generated macrobunches to be estimated.

Free heme is a highly toxic molecule for a living organism and its detoxification is a very important process, especially for carnivorous animals. Here we report the discovery of a previously unknown process for neutralizing free heme in the digestive tract of domestic cats. The cornerstone of this process is the encapsulation of heme into carbonated hydroxyapatite nanoparticles, followed by their excretion with faeces. This way of heme neutralization resembles the formation of insoluble heme-containing particles in the digestive tracts of other hematophagous species (for example, the formation of insoluble hemozoin crystals in malaria-causing Plasmodium parasites). Our findings suggest that the encapsulation of heme molecules into a hydroxyapatite matrix occurs during the transition from the acidic gastric juice to the small intestine with neutral conditions. The formation of these particles and their efficiency to include heme depends on the bone content in a cat’s diet. In vitro experiments with heme-hydroxyapatite nanoparticles confirm the proposed scenario.

Spin-labeled cyrhetrenes [(NNCp)Re(CO)3] and [(INCp)Re(CO)3], where NNCp is nitronyl nitroxide 2-(η5-cyclopentadienyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl and INCp is the corresponding imino nitroxide, were synthesized and characterized by EPR, CV, XRD, magnetochemistry and quantum chemistry methods. The correlations between different arrangements of paramagnetic centers and the magnetic exchange interactions for three polymorphs of [(NNCp)Re(CO)3] were studied. It was concluded that high kinetic stability of nitroxide-substituted cyrhetrenes is a promising feature of compounds for the creation of multifunctional contrast agents.

Structural control over the magnetic properties of molecular nanomagnets (MNM) is key to bottom up design strategies for novel molecule-based magnetic materials, for e.g. data storage, magnetic cooling or spintronics. Herein, we switch the magnetism of materials made of the single-molecule magnet [CoxZn(1 x)(piv)2(2 NH2 Py)2], by structural changes induced by gradual replacement of complexes containing paramagnetic Co(II) with analogues containing diamagnetic Zn(II). Combining advanced structural and magnetic characterization techniques, we show that a change of the ratio of the two ions induces a change of the Co(II) coordination structure, changed magnetic relaxation times and a switch from easy-plane to easy-axis type magnetic anisotropy. Complementary quantum chemical calculations reveal magneto structural correlations, rationalizing the structural origin of the changed magnetic properties upon magnetic dilution. This approach provides an important new lever to understand, predict and tune the magnetic properties of a large range of MNM based magnetic materials.

Zero-field splitting (ZFS) of three high-spin Co(I) (S = 1) clathrochelate complexes was determined by frequency-domain Fourier-transform THz-EPR (FD-FT THz-EPR). The following axial and rhombic ZFS values (D and E, respectively) were determined: [N(n-Bu)4]CoI(GmCl2)3(BPh)2 (1, D/hc = +16.43(1) cm–1, E/hc = 0.0(1) cm–1), [P(Me2N)4]CoI(GmCl2)3(BPh)2 (2, D/hc = +16.67(4) cm–1, E/hc = 0.0(1) cm–1), and [P(C6H5)4]CoI(GmCl2)3(BPh)2 (3, D/hc = +16.72(2) cm–1, E/hc = 0.24(3) cm–1). Complementary susceptibility χT measurements and quantum chemistry calculations on 1 revealed hard-axis-type magnetic anisotropy and allowed for a correlation of ZFS and the electronic structure. Increased rhombicity of 3 as compared to 1 and 2 was assigned to symmetry changes of the ligand structure induced by the change of the counterion. 1 and 3 exhibited temperature-dependent ZFS values. Possible reasons for this phenomenon, such as structural changes and weak chain-like intermolecular antiferromagnetic interactions, are discussed.

Q-band electron paramagnetic resonance (EPR) data conclusively demonstrate that the iron and cobalt centers in the solid solution [Fe(bpp)2]0.97[Co(terpy)2]0.03[BF4]2 (bpp = 2,6-dipyrazol-1-ylpyridine) undergo allosteric spin-state switching during light-induced excited-spin-state trapping (LIESST) at 20 K and thermal relaxation around 80 K. EPR of [Cu(terpy)2]2+ and [Cu(bpp)2]2+, doped into the same host lattice, also indicates expansion of the copper coordination sphere during LIESST excitation.

Electron Paramagnetic Resonance (EPR) station at the Novosibirsk Free Electron Laser (NovoFEL) user facility is described. It is based on X-band (∼9 GHz) EPR spectrometer and operates in both Continuous Wave (CW) and Time-Resolved (TR) modes, each allowing detection of either direct or indirect influence of high-power NovoFEL light (THz and mid-IR) on the spin system under study. The optics components including two parabolic mirrors, shutters, optical chopper and multimodal waveguide allow the light of NovoFEL to be directly fed into the EPR resonator. Characteristics of the NovoFEL radiation, the transmission and polarization-retaining properties of the waveguide used in EPR experiments are presented. The types of proposed experiments accessible using this setup are sketched. In most practical cases the high-power radiation applied to the sample induces its rapid temperature increase (T-jump), which is best visible in TR mode. Although such influence is a by-product of THz radiation, this thermal effect is controllable and can deliberately be used to induce and measure transient signals of arbitrary samples. The advantage of tunable THz radiation is the absence of photo-induced processes in the sample and its high penetration ability, allowing fast heating of a large portion of virtually any sample and inducing intense transients. Such T-jump TR EPR spectroscopy with THz pulses has been previewed for the two test samples, being a useful supplement for the main goals of the created setup.

Terahertz (THz) range, which lies between the microwave and infrared regions of the electromagnetic spectrum, presents a new frontier containing an abundance of technical applications and fundamental research problems. There are several challenges limiting the progress in the THz science and technology. One of them is the limited range of the guided-wave propagation of THz radiation, owing to the high loss from the finite conductivity of metals and the high absorption coefficient of dielectric materials in the THz range. In this work, we discuss the design and fabrication of multimodal hollow THz waveguides used at the X-band Electron Paramagnetic Resonance (EPR) endstation located at the Novosibirsk Free Electron Laser facility (NovoFEL). Experiments carried out at the EPR endstation are aimed to investigate the impact of THz and far infrared radiation on the spin system of different inorganic complexes and organic radicals. The EPR probehead and cryostat impose strict constraints on the possible ways of transferring the THz radiation to the sample, requiring the development, manufacturing, and characterization of specific waveguides. The proposed waveguides for X-band EPR measurements under THz radiation resemble hollow metal/dielectric waveguides. They have the shape of a hollow cylinder, tapering towards the end, with a silver-coated inner surface, and allow THz radiation to be transmitted over a distance of about 60 cm in a wide frequency range from THz to middle infrared. The waveguides performance was characterized at different frequencies of NovoFEL radiation and compared with numerical simulations. Following the requirements of polarization-sensitive EPR experiments, the waveguides were further modified by a miniature attachment, consisting of a polarizer, a semiconductor mirror, and a quarter-wave plate. The attachment allows preserving the initial polarization properties of THz radiation, turns its propagation vector, and creates the circular-polarized light irradiating the sample – all inside the limited volume of the EPR probehead.

Advanced atomically resolved characterization methods unveil the mechanism of a promising photocatalytic Co@MOF(Ti) system for H2 production. The combination of X-ray absorption spectroscopy (XAS) and electron paramagnetic resonance (EPR) experiments allows for the characterization of atomic and electronic rearrangements in the photoinduced species. This information provides the basis for the optimization of photocatalyst design.

New series of Ln (III) adducts with triazole-based ligands has been synthesized and characterized as potential fluorescent emitters. The general formulas are Ln (C2H6O2) (H2O)2(NO3)3∙2bpt∙2H2O where Ln – Sm (1), Eu (2), Gd (3), Tb (4) and Dy (5); bpt - 3,5-bis(pyridine-4-yl)-1,2,4-triazole. The crystal structures of 2 and 4 were determined by single-crystal X-ray analysis. Luminescence studies have been carried out for the complexes in the solid-state phase. All luminescence spectra showed characteristic emission bands of lanthanide ions and broad band of ligand-centred emission that indicates about incomplete suppression of the ligand-centred fluorescence is observed. As a result, it leads to an inefficient ligand-to-metal energy transfer. Complexes of Tb(III) and Eu(III) ions demonstrate thermosensing properties based on dual emission as result of energy transfer between 3,5-bis(pyridine-4-yl)-1,2,4-triazole molecules and Ln (III) ions.

Similar to spin-crossover (SCO) compounds, spin states of copper(II)–nitroxide based molecular magnets can be switched by various external stimuli including temperature and light. Although photoswitching and reverse relaxation of nitroxide–copper(II)–nitroxide triads were investigated in some detail, similar study for copper(II)–nitroxide spin pairs was still missing. In this work we address photoswitching and relaxation phenomena in exchange-coupled spin pairs of this family of molecular magnets. Using electron paramagnetic resonance (EPR) spectroscopy with photoexcitation, we demonstrate that compared to triad-containing compounds the photoinduced weakly coupled spin (WS) states of copper(II)–nitroxide pairs are remarkably more stable at cryogenic temperatures and relax to the ground strongly coupled spin (SS) states on the scale of days. The structural changes between SS and WS states, e.g., differences in Cu–Onitroxide distances, are much more pronounced for spin pairs than for spin triads in most of ...

We synthesized 1-ethylimidazolyl-substituted nitronyl nitroxides, i.e., 2-(1-ethylimidazol-4-yl)- (L4Et) and 2-(1-ethylimidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide-1-oxyl (L5Et). The stable radical L5Et is an ethyl analog of 2-(1-methylimidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole 3-oxide-1-oxyl (L5Me) described earlier, the reaction of which with Cu(hfac)2 (hfac is 1,1,1,5,5,5-hexafluoropentane-2,4-dionate) leads to the formation of the [Cu(hfac)2(L5Me)2] jumping crystals. The reaction of Cu(hfac)2 with L5Et with reagent ratios 1: 2 and 1: 1 yields heterospin complexes [Cu(hfac)2(L5Et)2] and [Cu(hfac)2L5Et]2, respectively. X-ray diffraction study of the mononuclear complex [Cu(hfac)2(L5Et)2] determined that the compound has a packing similar to that of jumping crystals studied earlier, with the only difference being that the O...O contacts between neigh- boring nitroxide groups were found to be 0.3—0.5 A longer than in [Cu(hfac)2(L5Me)2]. As a result of the lengthening of these contacts, [Cu(hfac)2(L5Et)2] crystals lack chemomechanical activi- ty. We found that when cooling crystals of binuclear complex [Cu(hfac)2L5Et]2 below 50 K, the antiferromagnetic exchange between unpaired electrons of the >N—•O groups of neighboring molecules leads to the full spin-pairing of the nitroxides, with only the Cu2+ ions contributing to the residual paramagnetism of the compound.

The X-band Electron Paramagnetic Resonance (EPR) spectroscopy endstation at the Novosibirsk Free Electron Laser (NovoFEL) facility is capable for steady-state (CW) EPR and time-resolved (TR) continuous wave EPR experiments. Recently several upgrades in experimental hardware and software have been done, aiming to more productively use the time of experimental session and to increase the control of experimental parameters during the session. Firstly, we installed detectors with different time resolution for tracking radiation produced by NovoFEL. Secondly, on the basic of fsc2 program, we implemented a possibility of a simultaneous use of two oscilloscopes, configured at different timescale and/or sensitivity. Finally, we developed software for on-the-fly CW and TR experimental data viewing and treatment.

During the last decades, persistent tris(2,3,5,6-tetrathiaaryl)methyl radicals (TAMs) have attracted much attention due to their applications in oximetry, EPR tomography, and as spin labels in pulsed dipolar EPR spectroscopy. Recently, researchers proposed to use TAM radicals as spin labels and/or a partner for photoinduced spin labels. Thus, the questions of their photochemical stability and mechanism of degradation under UV irradiation have become relevant and important. In this study, steady-state photolysis and flash photolysis of TAM radicals were investigated. A detailed mechanism of TAM phototransformations was proposed and confirmed by NMR, gel permeation chromatography, and mass-spectrometric analyses of the products.

Molecular magnets based on copper(ii) ions and stable nitroxide radicals exhibit promising switchable behavior triggered by a number of external stimuli; however, their spin-state-correlated optical properties vital for photoinduced switching have not been profoundly investigated to date. Herein, the electronic absorption spectra of single crystals of three representatives of this unique family are studied experimentally and theoretically in the visible and near-IR regions. We established that the color of the complexes is mainly determined by optical properties of the nitroxide radicals, whereas the Cu(hfac)2 fragment contributes to the near-IR range with the intensity smaller by an order of magnitude. The thermochromism of these complexes evident upon thermal spin state switching is mainly caused by a spectral shift of the absorption bands of the nitroxides. The vibrational progression observed in the visible range for single crystals as well as for solutions of pure nitroxides is well reproduced by DFT calculations, where the C-C stretching mode governs the observed progression. The analysis of the spectra of single crystals in the near-IR region reveals changes in the energy and in the intensity of the copper(ii) d-d transitions, which are well reproduced by SOC-NEVPT2 calculations and owe to the flip of the Jahn-Teller axis in the coordination environment of copper. Further strategies for designing bidirectional magnetic photoswitches using these appealing compounds are discussed.

In this work, high pressure high temperature (HPHT) diamonds synthesized in the Mg–C system with germanium, silicon and boron additives were studied. In the photoluminescence spectra of the samples doped with Ge, an intense system 602 nm attributed to germanium-vacancy defects was detected. In the electron paramagnetic resonance (EPR) spectra of these samples, a new paramagnetic center with S = 1 was detected along with substitutional nitrogen P1 and silicon-vacancy KUL1 (SiV0) centers. The angular dependence investigation of the new spectrum allowed us to establish its spin Hamiltonian parameters: g|| = 2.0025, g⊥ = 2.0027, D = 80.3 mT, E = 0. The center was determined to have the symmetry axis parallel to 〈111〉. Hyperfine structure (HFS) of one 73Ge atom (I = 9/2) was observed for the new spectrum. The novel paramagnetic center was proposed to be the neutral germanium split-vacancy defect. EPR and luminescence studies of diamonds doped with Si and B revealed a new paramagnetic center that can be associated with the sharp luminescence system 720 nm. An analysis of the angular dependence of the EPR spectrum showed that it had electronic spin S = 1/2 and anisotropic g-factor: g1 = 2.0033, g2 = 2.0004, and g3 = 2.0024. Based on the principal values and directions of the g-tensor the detected center was suggested to have the structure of silicon and boron atoms in the nearest carbon positions.

We report on the study of localized surface plasmon resonances (LSPRs) arising in arrays of metal (Au) nanoclusters, dimers, and nanoantennas under the influence of external electromagnetic radiation. Using nanolithography, the plasmonic arrays with different morphologies including cylindrical nanoclusters and dimers, linear and H-type nanoantennas were fabricated. Their LSPR frequencies were determined from the analysis of optical reflection and IR spectra. LSPR frequency depends strongly on morphology of metal nanostructures and is varied from visible to terahertz spectral range with variation of the structures from cylindrical nanoclusters to the nanoantennas with a high aspect ratio. We established the interconnection between the structural parameters of the plasmonic arrays and their LSPR frequencies based on the results of 3D electrodynamic simulations.

Hybrid materials bearing organic and inorganic motifs have been extensively discussed as playgrounds for the implementation of atomically resolved inorganic sites within a confined environment, with an exciting similarity to enzymes. Here, we present the successful design of a site-isolated mixed-metal metal organic framework (MOF) that mimics the reactivity of soluble methane monooxygenase enzyme and demonstrates the potential of this strategy to overcome current challenges in selective methane oxidation. We describe the synthesis and characterization of an Fe-containing MOF that comprises the desired antiferromagnetically coupled high-spin species in a coordination environment closely resembling that of the enzyme. An electrochemical synthesis method is used to build the microporous MOF matrix while integrating the atomically dispersed Fe active sites in the crystalline scaffold. The model mimics the catalytic C-H activation behavior of the enzyme to produce methanol and shows that the key to this reactivity is the formation of isolated oxo-bridged Fe units.

The reaction of Cu(hfac)2 with a spirocyclic nitronyl nitroxide 2-(1-ethylpyrazol-4-yl)-4,5-bis(spirocyclopentane)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (LEt) gave a heterospin complex [Cu(hfac)2LEt]-I, whose solid state is formed by polymer chains with a head-to-tail motif (the bridging function inside the chains is performed by the paramagnetic organic ligand). At 295 K the distance from the Cu atom to the N atom of the pyrazole ring and to the O atom of the NO group is 2.333(2) and 2.451(2) A, respectively. The cooling of the compound to 110 K leads to a structural transition, as a result of which the unit cell volume decreases by more than 7%. The elongated axis of the octahedron changes from N–Cu–ONO to Ohfac–Cu–Ohfac in all coordination units as a result of a significant decrease in the Cu–ONO and Cu–N bond lengths to 2.002(4) and 2.034(5) A, respectively, and an increase in the two Cu–Ohfac distances from 1.950(2) and 1.955(2) A to 2.263(4) and 2.213(4) A, respectively. The [Cu(hfac)2LEt]-I crystals are elastic and stable during the thermally induced spin crossover. The transition from the high-temperature to low-temperature phase is accompanied by a spin transition at 125 K at 1.0 × 10−4 GPa, as a result of which [Cu(hfac)2LEt]-I transforms from a paramagnetic compound to a diamagnetic one due to the complete spin coupling in the {Cu(II)–O˙–N

The Novosibirsk free electron laser is the first multi-turn energy-recovery linear accelerator with three separate laser systems (the terahertz, far-infrared and mid-infrared ones). The facility is well equipped with optical elements and instrumentation available to Russian and foreign users. In this paper, we describe in brief the workstations of the facility and survey selected recent experiments using intense monochromatic terahertz laser radiation, which can be tuned from 90 to 240 μm.

We present the results of an investigation of surface-enhanced Raman scattering (SERS) by optical phonons in colloidal CdSe nanocrystals (NCs) homogeneously deposited on both arrays of Au nanoclusters and Au dimers using the Langmuir–Blodgett technique. The coverage of the deposited NCs was less than one monolayer, as determined by transmission and scanning electron microscopy. SERS by optical phonons in CdSe nanocrystals showed a significant enhancement that depends resonantly on the Au nanocluster and dimer size, and thus on the localized surface plasmon resonance (LSPR) energy. The deposition of CdSe nanocrystals on the Au dimer nanocluster arrays enabled us to study the polarization dependence of SERS. The maximal SERS signal was observed for light polarization parallel to the dimer axis. The polarization ratio of the SERS signal parallel and perpendicular to the dimer axis was 20. The SERS signal intensity was also investigated as a function of the distance between nanoclusters in a dimer. Here the maximal SERS enhancement was observed for the minimal distance studied (about 10 nm), confirming the formation of SERS “hot spots”.

The design and operational characteristics of the Novosibirsk free electron laser facility are described. Selected experiments in the terahertz range carried out recently at the user stations are surveyed in brief.

Copper-nitroxide-based molecular magnets of the Cu(hfac)2LR family (often called “breathing crystals”) are interesting and appealing systems exhibiting various magnetic anomalies triggered by temperature or light, which can generally be viewed as spin-crossover-like phenomena. The presence of paramagnetic copper(II) ions and stable nitroxide radicals in the polymer chains of breathing crystals makes electron paramagnetic resonance (EPR) widely applicable to these systems. In addition to the determination of typical parameters (g-tensor, hyperfine splitting constants, etc.), EPR allows the monitoring of magnetostructural switching between different states and evaluation of exchange interactions in the exchange-coupled clusters of copper(II) with nitroxides and those between the exchange-coupled clusters. This review highlights the capabilities of EPR in investigating breathing crystals and demonstrates the obtaining of crucial information on magnetic interactions inaccessible by other techniques.

"Breathing crystals" based on copper(II) hexafluoroacetylacetonates and pyrazolyl-substituted nitronyl nitroxides comprise the exchange-coupled clusters within the polymeric chains. Owing to an interplay of exchange interaction between copper(II) and nitroxide spins and Jahn-Teller nature of copper(II) complex, the breathing crystals demonstrate thermally and light-induced magnetostructural transitions in many aspects similar to the classical spin crossover. Herewith, we report the first application of variable temperature (VT) far/mid Fourier transform infrared (FTIR) spectroscopy and mid FTIR microscopy to breathing crystals. This VT-FTIR study was aimed toward clarification of the transitions mechanism previously debated on the basis of superconducting quantum interference device, X-ray diffraction, and electron paramagnetic resonance data. VT-FTIR showed the onset of new vibrational bands during phase transitions occurring at the expense of several existing ones, whose intensity was significantly reduced. The most pronounced spectral changes were assigned to corresponding vibrational modes using quantum chemical calculations. A clear-cut correlation was found between temperature-dependent effective magnetic moment of studied compounds and the observed VT-FTIR spectra. Importantly, VT-FTIR confirmed coexistence of two types of copper(II)-nitroxide clusters during gradual magnetostructural transition. Such clusters correspond to weakly coupled and strongly coupled spin states, whose relative contribution depends on temperature. The pronounced difference in the VT-FTIR spectra of two states in breathing crystals is a fingerprint of magnetostructural transition, and understanding of these characteristics achieved by us will be useful for future studies of breathing crystals as well as their diamagnetic analogues.

Diamonds synthesized in the Mg0.9–Ge0.1–C system at high pressure-high temperature (HPHT) conditions are investigated by the EPR and luminescence methods. A new GeV paramagnetic center with the spin S = 1 and D 3d symmetry is detected in the EPR spectra of the Ge-doped samples, along with the impurity nitrogen (Р1) and silicon-vacancy defect KUL1 (SiV0) centers. The investigation demonstrates that this new center has a double semi-vacancy structure with the germanium atom in the center. The studied GeV center is in a neutral charge state and is assigned to the 602 nm system in the photoluminescence spectra.

Efficient and straightforward methods for characterization of polymers with inherent microporosity are demanded in their targeted design for particular applications. Among critical parameters to be obtained are the size of the pores and polymer stability against photoirradiation. Herewith, we demonstrate the efficiency of electron paramagnetic resonance (EPR) spectroscopy applied to this task. We use stable nitroxide radicals (2,2,6,6-tetramethylpyperidine-1-oxyl) (TEMPO) as reporter spin probes for EPR and investigate a series of perspective polymers with inherent microporosity developed for pressure sensitive paints (PSP), namely, poly(1,1,1,3,3,3-hexafluoroisopropylmethacrylate-co-2,2,3,3,4,4,4-heptafluorobutylmethacrylate) (FIB), its two modifications poly(1,1,1,3,3,3-hexafluoroisopropylmethacrylate-co-2,2,3,3,4,4,4-heptafluorobutylmethacrylate-co-1-(4-(4-chloro-2,3,5,6-tetrafluorophenyl)piperazin-1-yl)prop-2-en-1-one) (NS4) and poly(1,1,1,3,3,3-hexafluoroisopropylmethacrylate-co-2,2,3,3,4,4,4-heptafluorobutylmethacrylate-co-1-(4-(4-tert-butylphenylsulfonyl)piperazin-1-yl)prop-2-en-1-one) (NS5), as well as poly(1-trimethylsilyl-1-propyne) (PTMSP) and poly(1,1,2,2-perfluorooctylmethacrylate) (PFOMA). Nitroxides were incorporated into the pores of the polymers post-synthetically via a gas-phase sorption, and the mobility of nitroxides tracked by EPR yielded information on the pore sizes and polymer degradation under ultraviolet light. The conclusions obtained by EPR have been supported by a variety of other techniques, thus demonstrating EPR to be a very convenient tool for express analysis of porous polymers.

Copper(ii)-nitroxide based molecular magnets Cu(hfac)2L(R) exhibit thermally-induced transitions between high- and low-temperature (HT/LT) magnetostructural states. In this work we report the first study on the spatial distribution of HT/LT phases during gradual transitions in these compounds. We explore the possibility of domain formation at intermediate temperatures, which has never been addressed before. For this purpose, we reexamine the available electron paramagnetic resonance (EPR) and X-ray diffraction data, and perform numerical calculations of EPR spectra for different models of exchange-coupled networks. A thorough analysis shows that during gradual transitions, molecular magnets Cu(hfac)2L(R) represent solid solutions of disordered HT and LT phases, and the formation of single-phase domains larger than a few nanometers in size is unlikely.

We present a synthetic strategy for the efficient encapsulation of a derivative of a well-defined cobaloxime proton reduction catalyst within a photoresponsive metal–organic framework (NH2-MIL-125(Ti)). The resulting hybrid system Co@MOF is demonstrated to be a robust heterogeneous composite material. Furthermore, Co@MOF is an efficient and fully recyclable noble metal-free catalyst system for light-driven hydrogen evolution from water under visible light illumination.

The inclusion of Ti(IV) in water-stable metal-organic frameworks has been proposed as a strategy for the generation of high surface area heterogeneous photocatalysts, with UiO-66 being a promising candidate. We find that the site of binding of Ti(IV) is at linker defect sites and not incorporated into the inorganic secondary building unit through metal exchange. We also demonstrate the choice of titanium source is critical in the generation of an active hydrogen evolution catalyst, and explain the observed activity using density functional theory calculations, which also enable rational design of future Ti(IV) coordination environments in MOF catalysts

The first alkyl chain-anchored heteropentacene, dithieno[2,3-d;2′,3′-d′]benzo-[1,2-b;3,4-b′]dithiophene (DTmBDT), mono- or disubstituted with a nitronyl nitroxide group has been prepared through a cross-coupling synthetic procedure of the corresponding dibromo-derivative (Br 2 -DTmBDT) with a nitronyl nitroxide-2-ide gold(I) complex. The synthesized nitroxides possess high kinetic stability, which allowed us to investigate their structure and thermal, optical, electrochemical, and magnetic properties. Single-crystal X-ray diffraction of both mono- and diradicals revealed that the nitronyl nitroxide group lies almost in the same plane as the nearest side thiophene ring. Such arrangement favors formation of edge-to-edge dimers, which then form close π-stacks surrounded by interdigitating alkyl chains. Before melting, these nitronyl nitroxide radical substituted molecules undergo at least two different phase transitions (PTs): for the monoradical, PTs are reversible, accompanied by hysteresis, and occur near 13 and 83 °C; the diradical upon heating shows a reversible PT with hysteresis in the temperature range 2–11 °C and an irreversible PT near 135 °C. PTs of this type are absent in Br 2 -DTmBDT. Therefore, the step-by-step substitution of bromine atoms by nitronyl nitroxide groups changes the structural organization of DTmBDT and induces the emergence of PTs. This knowledge may facilitate crystal engineering of π-stacked paramagnets and related molecular spin devices.

Similar to potassium bromide (KBr), which is used in mid-IR as a matrix for pellet preparation, polyethylene (PE) is widely applied in far-IR because it is an inert material without intensive absorption lines in this range. However, despite the material itself being transparent, the transmission of pellet made of PE powder strongly depends on the wavelength tending to decrease almost to zero in a short-wave part of the far-IR range. Heating of the pellet to PE melting temperature significantly improves its transparency, but this approach can only be applied to thermally stable compounds. In this work 8 types of PE powders of different mean particle size (MPS) ranging from 43 to 3 µm have been synthesized. Far-IR spectra of PE pellets made of these powders at room temperature have been measured and the inverse correlation between overall far-IR pellet transparency and MPS of its PE powder has been found. The imaginary part of the effective refractive index of the studied pellets obeys light-scattering scale invariance rule, indicating that the transmission of PE pellets is limited by light scattering by PE grains. It has been shown that PE powders with MPS of ~4 µm or less are suitable as a matrix for pellet preparation in far-IR spectroscopy without the necessity of pellet melting.

Two six-coordinate Sn(IV) complexes, bis-[2,4,6,8-tetra-(tert-butyl)-9-oxyphenoxazinyl-1-onolate]Sn(IV) 4 and bis-[2,4,6,8-tetra-(tert-butyl)-phenoxazinyl-1-onolate]Sn(IV) dichloride 5 with redox-active tridentate 2,4,6,8-tetra(tert-butyl)-9-hydroxyphenoxazin-1-one 2 and bidentate 1-H-1-oxo-2,4,6,8-tetra(tert-butyl) phenoxazine-1-one 3 ligands were synthesized and their molecular structures determined with the use of X-ray crystallography. Electronic structures of the complexes were investigated with DFT B3LYP*/6-311++G(d,p) calculations, SQUID magnetometry and EPR spectroscopy and assigned to the biradical Sn(IV)(CatNSQ)2 and Sn(IV)(ISQ)2Cl2 electromeric forms, where (CatNSQ)2− and (ISQ)− are radical-anions formed by the ligands 2 and 3, correspondingly. Weak ferromagnetic coupling of the two paramagnetic centers was established in complex Sn(IV)(CatNSQ)2 with ligands 2, whereas in the same type complex formed by ligands 3 the coupling of the paramagnetic centers bears weak antiferromagnetic character. The calculated exchange coupling constants J for 4 and 5 (5 cm−1 and −8 cm−1, respectively) are in sufficiently good agreement with the values obtained from analysis of μeff(T) dependences (3.5 cm−1 and −1 cm−1).

Vanadium(IV) complexes are actively studied as potential candidates for molecular spin qubits operating at room temperatures. They have longer electron spin decoherence times than many other transition ions, being the key property for applications in quantum information processing. In most cases reported to date, the molecular complexes were optimized through the design for this purpose. In this work, we investigate the relaxation properties of vanadium(IV) ions incorporated in complexes with lanthanides using electron paramagnetic resonance (EPR). In all cases, the VO6 moieties with no nuclear spins in the first coordination sphere are addressed. We develop and implement the approaches for facile diagnostics of relaxation characteristics in individual VO6 moieties of such compounds. Remarkably, the estimated relaxation times are found to be close to those of other vanadium-based qubits obtained previously. In the future, a synergistic combination of qubit-friendly properties of vanadium ions with single-molecule magnetism and luminescence of lanthanides can be pursued to realize new functionalities of such materials.