Single-molecule magnets (SMMs) with total electron spin S = 3/2 are promising platforms for optical magnetization control. Their study in such a context is relevant in both magnetically concentrated and diluted samples and assumes the excitation of magnetic dipole transitions by resonant radiation. Such experiments require precise determination of the energy splitting between Kramers doublets (EZFS), since this value determines the resonant radiation frequency. The direct spectroscopic method available for such measurements is frequency-domain Fourier transform terahertz electron paramagnetic resonance (FD-FT THz-EPR) spectroscopy. It usually requires a setup that includes an FTIR-spectrometer and a superconducting magnet to create a magnetic field in the sample area. In this work, we propose a method for the rapid determination of EZFS of SMMs without using a superconducting magnet. The method is based on circular dichroism of an SMM placed in a static external magnetic field. It was implemented using a standard FTIR spectrometer with a sample holder modified by inserting a permanent magnet. Circular polarization was created using a grid polarizer and a quarter-wave plate. Model Co(II)-based SMMs with a varying degree of magnetic dilution were investigated by the proposed method. The results were confirmed by FD-FT THz-EPR. The obtained EZFS values of 43.4-43.6 cm-1 demonstrate a slight downward trend with a decrease in the paramagnetic center content. Such preliminary measurements of EZFS in magnetically diluted SMMs pave the way for the resonant excitation of magnetic dipole transitions using narrowband sources and even for coherent control of magnetic quantum states.

Molecular spin triangles have been theoretically predicted to exhibit spin electric coupling (SEC) holding significant promising in quantum information technology. The possibility of nanoscale manipulation of spin states by the application of electric field offers several advantages over the standard magnetic field-based control of spin states. Experimentally, the antiferromagnetically coupled spins systems arranged in a triangular cluster (M3) where M = Cu2+, Co2+, Fe3+ have been explored to reveal spin electric coupling by utilizing voltage pulses for modulating the phase accumulation within spin-echo sequences in pulsed electron paramagnetic resonance (EPR) experiments. Despite the rapid experimental progress in this field in last one decade, the role of spin anisotropy, antisymmetric exchange interactions and structural distortions in driving the strength of magnetoelectric coupling effects remains underexplored. Here we report an isostructural series of molecular spin triangles based on triaminoguanidine ligand scaffold with metal centres as Cu2+ (S = 1/2, complex 1), Co2+ (S = 3/2, complex 2), and Mn2+ (S = 5/2, complex 3) ions laying groundwork for chemically tunable magnetoelectric spin qubits. Herein, for the first time, we report the antiferromagnetically coupled Mn3 triangle by incorporating the triaminoguanidine ligand motif. DFT calculations reveal distinct magnetic exchange coupling strengths among the Mn3, Co3, and Cu3 spin triangles. Continuous-wave (cw) EPR spectroscopy reveals a paramagnetic ground state for all M3 triangle studied for the antiferromagnetically coupled clusters with half integer spin on each metal ion. Pulsed EPR measurements establish microsecond phase memory times (T2 ~ 0.8–2.3 μs) and long spin-lattice relaxation times for all three complexes at cryogenic temperatures, confirming the quantum coherence prerequisites for spin-electric coupling (SEC) detection. This series spanning S = 1/2, 3/2, and 5/2 within a single structural scaffold provides a chemically controlled platform for systematically disentangling the microscopic contributions to spin electric coupling across the 3d transition metal series.

This study presents a theoretical analysis of the interaction of high-spin systems with inhomogeneous alternating electric field and homogeneous alternating magnetic field that induce electric quadrupole (QT, E2) and magnetic dipole transitions (MT, M1), respectively. In order to distinguish QTs from MTs, an analytical expression for the intensities and selection rules for a model system with a half-integer total spin S = 3 / 2 was derived using the spin Hamiltonian and operator-equivalent approaches. The direct comparison of the absorption patterns for the QT and MT of a model high-spin Co(II) system was performed in a frequency domain corresponding to Frequency Domain Fourier Transform Terahertz Electron Paramagnetic Resonance spectroscopy. This type of systems often exhibits the properties of a single molecular magnet at helium temperatures and is characterized by a large zero field splitting. Despite more flexible selection rules for electric quadrupole transitions, the powder spectra of QT and MT were shown to be similar, emphasizing the need for precise spectral measurements to determine the dominant transition type in high-spin systems. The approach developed in the paper not only solves a rather complex quantum mechanical problem that includes the estimation of the quadrupole moment of unpaired electrons, but also demonstrates a possible way for advanced manipulation of spin states, a capability crucial for the development of quantum computing and information storage technologies. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-Non Commercial 4.0International (CC BY-NC) license

Transient or time-resolved electron paramagnetic resonance spectroscopy (TR EPR) is a powerful method for studying various photogenerated paramagnetic species. The use of low-energy quanta, such as terahertz (THz) radiation, as an external stimulus in TR EPR allows the initiation of spin dynamics without generating new paramagnetic species other than those already present in the system. This spin dynamic reflects the return of the system to thermodynamic equilibrium, governed by a spin-lattice relaxation time, T1. The latter, together with a phase memory time, is of paramount importance for the practical implementation of single-molecule magnets and molecular spin qubits. In this work, we present TR EPR spectroscopy with pulsed heating by THz pulses as a versatile spectroscopic method for determining T1 in a wide range of paramagnetic systems. To define the scope of the method, we developed a numerical model based on the Liouville-von Neumann equation, with the equilibrium density matrix defined by the temperature profile of the lattice. Using experimental data obtained for [CoTp2] (cobalt(II) bis[tris(pyrazolyl)borate]) with S = 3/2, we compared the proposed method with two other commonly used techniques: alternating current (AC) magnetometry and pulsed EPR. All three methods were found to be in qualitative agreement and provided complementary information about the relaxation properties. TR EPR spectroscopy showed the orientation dependence of T1. AC magnetometry revealed the dependence of T1 on the value of the external magnetic field, which was attributed in the literature to a field-induced Raman process. Finally, pulsed EPR spectroscopy was found to be biased by strong spectral diffusion.

Six aryl- and pyridine-substituted nitronyl-nitroxide radicals were synthesized and characterized to investigate their optical anisotropic properties. Single-crystal X-ray diffraction analysis revealed molecular packing organized by either halogen and hydrogen bonding or hydrogen bonding alone. Single-crystal electronic absorption spectra in the visible region of three studied radicals exhibit pronounced linear dichroism, while single crystals of other radicals do not demonstrate this property. Time-dependent DFT and ab initio calculations were employed to determine the transition dipole moment (TDM) vectors corresponding to the long-wavelength absorption bands. For all radicals, these vectors are found to be practically parallel to the O & ctdot;O direction of the nitronyl-nitroxide chromophore. Correlation between the dichroic properties and crystal structure was established through comprehensive analysis of TDM vector orientations relative to the crystal surface. The strongest dichroic effect was observed in crystals where all projections of the TDM vectors onto the illuminated face are parallel to each other, while weaker or absent effects correspond to non-parallel arrangements. This study constitutes the first systematic investigation of linear dichroism in paramagnetic organic crystals, thereby establishing new avenues for developing multifunctional materials that respond to both optical and magnetic stimuli.

Single-ion magnets (SIMs) are the most promising candidates for magnetic storage and processing of information at the molecular level. Apart from the properties of each isolated SIM, their performance crucially depends on intermolecular exchange interactions in bulk materials or those occurring when individual SIMs are assembled into arrays for practical applications. However, compared with their magnetic anisotropy, the correlation of in-bulk intermolecular exchange interactions and magnetic relaxation parameters has been established to a much lesser extent. In this work, using an extended protocol involving XRD, multifrequency and frequency-domain EPR, SQUID magnetometry, DFT and ab initio calculations, we investigated intermolecular interactions in a new series of four bis-(phenoxy Schiff base) Co(II) complexes representing field-induced SIMs. Combined multifrequency EPR ranging from 9 GHz to >1 THz allowed unique measurement of weak intermolecular exchange couplings of the order of 0.01 cm(-1). Consolidation of EPR data with ac/dc SQUID magnetometry and detailed structural and quantum-chemical analyses allowed us to reliably analyze the key performance characteristics of these SIMs, the degree of polyhedral distortion, and the values of intermolecular exchange couplings in an attempt to achieve a firm interplay among them. Such analysis can become prototypical for future design and evaluation of crystals, assemblies and arrays of SIMs.

The interaction of photochromic 8-methoxy-1',3',3'-trimethyl-6-nitro-spiro[chromene-2,2'-indole] (MNSP) spiropyran with copper(II) hexafluoroacetylacetonate yields a deep red-violet solution due to a coordination-induced transition of MNSP from its closed form to the colored, open merocyanine (MC) form. The crystal structure of {CuII(hfac)2·MNSP} (1) shows coordination of two oxygen atoms in MNSP to CuII, forming a distorted octahedral geometry with four short (1.94-1.97 Å) and two longer (2.22-2.44 Å) Cu-O bonds. The χMT value of 0.43 emu K mol-1 at 300 K corresponds to a g-factor of 2.16 for CuII (S = 1/2). The electron paramagnetic resonance (EPR) signal fits well with g∥ = 2.071, g⊥ = 2.347 (giso = 2.167), and A⊥ = 420 MHz. Slow magnetic relaxation is observed under a static magnetic field of 2000 Oe with χ''(ν) curves showing well-defined maxima in the 2.0-6.0 K range. The dependence of ln(τ) vs. 1/T is well described using a linear combination of direct and Raman relaxation mechanisms. A magnetic hysteresis loop is observed at 0.5 K, classifying complex 1 as a single-ion magnet. This loop closes at zero field due to quantum tunneling of the magnetization but reopens at fields exceeding ±100 Oe. Pulsed EPR spectroscopy reveals quantum coherence with Tm ∼ 0.3 μs at 10 K. The spin-lattice relaxation time, T1 = 0.7 ms, correlates well with values obtained from AC susceptibility data. A blue shift of 40-60 nm in the main absorption band of the open MC form of MNSP is observed upon complex formation. The complex dissociates in dilute solutions but reforms reversibly under ultraviolet (UV) and green light excitation. It remains stable in concentrated solutions or films. In solution, green and UV light reversibly switch the ligand between the open and closed forms within the complex, whereas only partial switching is observed in films.

A completely original open-shell scaffold, based on the furazano[3,4-b]pyrazine moiety, has been recently discovered and has already shown certain advantages over mainstay radical blocks. Here, we present a way to synthetically extend this class by changing an oxadiazole moiety into a triazole, yielding triazolo[4,5-e]furazano[3,4-b]pyrazines, thereby unlocking a site for substituents in the scaffold. We also propose new transformation conditions for the key cyclization step of bis(arylhydrazones) to the corresponding 6-aryltriazolo[4,5-e]furazano[3,4-b]pyrazines. The anion radicals, resulting from their easy one-electron oxidation, exhibit good stability and unexpected NIR luminescence.

The removal of nitrogen dioxide (NO2) is becoming increasingly critical due to its rising concentration in the atmosphere, driven by growing exhaust emissions and industrial production. However, many types of sorbents degrade due to the high reactivity of NO2, rendering them unsuitable for cyclic use. In this study, a low-cost and scalable approach is proposed to apply and regenerate zirconium-based MOF-801 framework for NO2 removal from gas mixtures, allowing its reuse multiple times. Through in situ electron paramagnetic resonance (EPR) spectroscopy, ex situ ATR-FTIR spectroscopy and breakthrough measurements, the mechanism of NO2 sorption in MOF-801 is elucidated. The majority of the sorption capacity is attributed to the formation of nitrates and nitrites, which can be conveniently eliminated from the sorbent. To facilitate this process, a novel regeneration method using low-cost chemicals is developed. The effectiveness of MOF-801 exposed to low concentrations of NO2 (400 ppm, typical for the flue gases) is demonstrated in breakthrough experiments. The whole cycle of NO2 adsorption, product removal, and sorbent regeneration can be employed repeatedly for capturing NO2 from dilute mixtures, paving the way for practical environmental applications.

Copper(II)-nitroxide based molecular magnets exhibit spin-crossover-like spin state switching, which is topical in field of molecular magnetism. However, establishing reliable structure-property relationships in these systems is still challenging, especially regarding the light-induced switching of spin states. In this paper, we report the investigation of photoswitching and relaxation in a series of heterospin Cu(hfac)2LR complexes with pyridine-based nitroxide ligands (LR), which belong to this family of materials. Using electron paramagnetic resonance (EPR) for detection, we demonstrate very long lifetimes of photoinduced spin states at liquid helium temperatures (<15 K), where relaxation to the ground state does not exceed 15% within two hours. At the same time, the efficiency of photoswitching strongly depends on the structure of the radical ligand in this series: the bulkier the ligand, the smaller the fraction of heterospin clusters that undergo photoswitching. These findings expand the understanding of mechanisms and factors behind photoswitching and relaxation in copper(II)-nitroxide molecular magnets and aid in further research aiming to optimize their functional properties.

The application of terahertz (THz) science in industrial technology and scientific research requires efficient THz detectors. Such detectors should be able to operate under various external conditions and conform to existing geometric constraints in the required application. Pyroelectric THz detectors are among the best candidates. This is due to their versatility, outstanding performance, ease of fabrication, and robustness. In this paper, we propose a compact pyroelectric detector based on a bioriented poled polyvinylidene difluoride film coated with sputtered metal electrodes for in situ absorption measurement at cryogenic temperature. The detector design was optimized for the registration system of the electron paramagnetic resonance (EPR) endstation of the Novosibirsk Free Electron Laser facility. Measurements of the detector response to pulsed THz radiation at different temperatures and electrode materials showed that the response varies with both the temperature and the type of electrode material used. The maximum signal level corresponds to the temperature range of 10–40 K, in which the pyroelectric coefficient of the PVDF film also has a maximum value. Among the three coatings studied, namely indium tin oxide (ITO), Au, and Cu/Ni, the latter has the highest increase in sensitivity at low temperature. The possibility of using the detectors for in situ absorption measurement was exemplified using two typical molecular spin systems, which exhibited a transparency of 20–30% at 76.9 cm−1 and 5 K. Such measurements, carried out directly in the cryostat with the main recording system and sample fully configured, allow precise control of the THz radiation parameters at the EPR endstation.

The present letter demonstrates a simple method to characterize the molecular electron spin qubits, for which a large number of chemical compounds with non-zero spin have been proposed. The method is based on calculating the value of π using a one-qubit protocol based on obtaining and processing Rabi oscillations. It was implemented using a model system of a Finland trityl radical with an electron spin S = 1/2 and a pulsed electron paramagnetic resonance spectrometer. As a result, the value of π was obtained with an accuracy of two decimal places, and an analysis of statistical and systematic errors was carried out.

The development and technological applications of molecular spin systems require versatile experimental techniques to characterize and control their static and dynamic magnetic properties. In the latter case, bulk spectroscopic and magnetometric techniques, such as AC magnetometry and pulsed electron paramagnetic resonance, are usually employed, showing high sensitivity, wide dynamic range, and flexibility. They are based on creating a nonequilibrium state either by changing the magnetic field or by applying resonant microwave radiation. Another possible source of perturbation is a laser pulse that rapidly heats the sample. This approach has proven to be one of the most useful techniques for studying the kinetics and mechanism of chemical and biochemical reactions. Inspired by these works, we propose an inductive detection of temperature-induced magnetization dynamics as applied to the study of molecular spin systems and describe the general design and construction of a particular induction probehead, taking into account the constraints imposed by the cryostat and electromagnet. To evaluate the performance, several coordination compounds of VO2+, Co2+, and Dy3+ were investigated using low-energy pulses of a terahertz free electron laser of the Novosibirsk free electron laser facility as a heat source. All measured magnetization dynamics were qualitatively or quantitatively described using a proposed basic theoretical model and compared with the data obtained by alternating current magnetometry. Based on the results of the research, the possible scope of applications of inductive detection and its advantages and disadvantages in comparison with standard methods are discussed.

The crystallization of paramagnetic species in a magnetic field gradient under microgravity-like conditions is an area of interest for both fundamental and applied science. In this paper, a setup for the crystallization of paramagnetic species in the magnetic field up to 7 T generated by a superconducting magnet is described. The research includes calculations of the conditions necessary to compensate for the gravitational force for several types of paramagnetic substances using the magnetic field of superconducting magnets (4.7 T, 7 T, 9.4 T, and 16.4 T). Additionally, for the first time, the crystallization of copper sulfate and cobalt sulfate, as well as a mixture of copper sulfate and cobalt sulfate under gravitational force compensation in a superconducting magnet, was performed. This paper experimentally demonstrates the feasibility of growing paramagnetic crystals within the volume of a test tube on the example of copper and cobalt sulfate crystals. A comparison of crystals grown from the solution of a mixture of copper and cobalt sulfates under the same conditions, with and without the presence of a magnetic field, showed changes in both the number and size of crystals.

A new binuclear "paddle-wheel" complex, [Co2(bhbz)4(EtOH)2]·4EtOH (1, Hbhbz-3,5-di(tert-butyl)-4-hydroxybenzoic acid); an isostructural zinc complex (2); a and magnetically diluted sample of [Zn1.93Co0.07(bhbz)4(EtOH)2]·4EtOH (3) were obtained. Molecular structures of 1 and 2 were determined by single-crystal X-ray diffraction. DFT calculations for 1 indicate strong Co-Co antiferromagnetic exchange interactions in the binuclear fragment. It was shown that when one paramagnetic ion in the binuclear molecule is replaced by a diamagnetic zinc(II) ion, the remaining cobalt(II) ion can be considered as an isolated center with magnetic anisotropy, the parameters of which are determined by ab initio calculations. Magnetic properties for samples 1 and 3 were investigated and analyzed in detail.

A one-stage synthesis of paramagnetic Re(I) compounds based on the reaction of [Re(CO)(5)Br] with spin-labeled N-donor heterocycles was developed. Nitronyl nitroxide derivatives of 1H-imidazole (L-1), 1H-pyrazole (L-2), di (1H-pyrazol-1-yl)methane (L-3) and 4-methyl-2-(1H-pyrazol-1-yl)quinoline (L-4) can easily substitute two coordinated CO molecules in [Re(CO)(5)Br], resulting in a series of paramagnetic fac-[Re(CO)(3)(L-i)(n)Br]center dot xSolv complexes. Single crystal XRD studies showed that the paramagnetic L-1-L-4 ligands are coordinated by Re(I) through the imine atoms N of the heterocycles. According to SQUID measurements and EPR spectroscopy data, magnetic behavior of [Re(CO)(3)(L-1)(2)Br] (1), [Re(CO)(3)(L-2)(2)Br] (2), and [Re(CO)(3)(L-3)Br] (3) complexes are typical for biradicals, while [Re(CO)(3)(L-4)Br] (4) is typical for monoradical. The combination of Re(I) therapeutic properties and contrasting characteristics of nitroxide radicals in diagnostic magnetic resonance imaging, stability and simplicity of synthesis make [Re(CO)(3)(L-i)(n)Br] valuable model objects for material design for theranostics purposes.

Photo-excited triplet states represent a new class of spin labels in pulse electron paramagnetic resonance (EPR), attracting increasing attention because of their unique spectroscopic properties. Despite certain advantages, the use of photo-labels has also some challenges, e.g. low repetition rates due to technical laser-related limitations and intrinsic properties of the labels. The application of additional pulse trains for multiple refocusing of the electron spin echo and integration of all observed echoes can significantly enhance sensitivity at a given repetition rate. In this work, we demonstrate that the use of Carr-Parcel-Meiboom-Gill (CPMG) blocks followed by multiple echo integration is a promising route for sensitivity gain in pulsed EPR utilizing photo-excited triplet states, including light-induced pulsed dipolar spectroscopy (LiPDS). The reduction of accumulation time by a factor of 5.3 has been achieved using a commercial pulsed EPR spectrometer with the implementation of a CPMG block and an external digitizer. The methodology of using CPMG refocusing with multiple echo integration in light-induced pulsed EPR experiments is discussed, aiding future applications of this approach in LiPDS experiments.