Scavenging of Organic C‑Centered Radicals by Nitroxides

Scavenging of Organic C‑Centered Radicals by Nitroxide

Self-Decelerating Relaxation of the Light-Induced Spin States in Molecular Magnets Cu(hfac)₂L^R Studied by Electron Paramagnetic Resonance

Molecular magnets Cu­(hfac)₂L^R (hfac = hexafluoroacetylacetonate) called “breathing crystals” exhibit thermally and light-induced magnetic anomalies very similar to iron­(II) spin-crossover compounds. They are physically different systems, because the spin-state switching occurs in exchange-coupled nitroxide–copper­(II)–nitroxide clusters, in contrast to classical spin crossover in d⁴–d⁷ transition ions. Despite this difference, numerous similarities in physical behavior of these two types of compounds have been observed, including light-induced excited spin-state trapping (LIESST) phenomenon recently found in the Cu­(hfac)₂L^R family. Similar to iron­(II) spin-crossover compounds, the excited spin state in breathing crystals relaxes to the ground state on the time scale of hours at cryogenic temperatures. In this work, we investigate this slow relaxation in a series of breathing crystals using electron paramagnetic resonance (EPR). Three selected compounds represent the cases of relatively strong or weak cooperativity and different temperature of thermal spin transition. They all were studied in a neat magnetically concentrated form; however, sigmoidal self-accelerating relaxation was not observed. On the contrary, the relaxation shows pronounced self-decelerating character for all studied compounds. Relaxation curves and their temperature dependence could be fitted assuming a tunneling process and broad distribution of effective activation energies in these 1D materials. A number of additional experimental and theoretical arguments support the distribution-based model. Because self-decelerating relaxation behavior was also found in 1D polymeric iron­(II) spin-crossover compounds previously, we compared general relaxation trends and mechanisms in these two types of systems. Both similarities and differences of copper–nitroxide-based breathing crystals as compared to iron­(II) spin-crossover compounds make future research of light-induced phenomena in these new types of spin-crossover-like systems topical in the field of molecule-based magnetic switches

Structural Equilibrium in New Nitroxide-Capped Cyclodextrins: CW and Pulse EPR Study

Design of the new spin-labeled cyclodextrins can significantly extend the functionality of nitroxides. A series of new complexes based on fully methylated cyclodextrin (TRIMEB) covalently bound to the piperidine, pyrroline, pyrrolidine, and pH-sensitive imidazoline type nitroxides has been synthesized and studied using pulse and continuous wave electron paramagnetic resonance (EPR). The influence of the radical and linker properties on the structure of complexes formed has been investigated. Using the electron spin echo envelope modulation technique, we have analyzed quantitatively the accessibility of radicals to solvent molecules in studied complexes depending on the structure and length of the linkers. In all studied systems we observed different types of equilibria between conformations with radical fragment being outside the TRIMEB cavity and radical fragment capping the cavity of TRIMEB. The observed guest-induced shift of equilibrium toward the complex with radical capping TRIMEB cavity was explained by a change of macrocyclic configuration of TRIMEB. Complex with the −NH–CO– linker has been found most perspective for the applications requiring close location of nitroxide to the inclusion complex of TRIMEB. Using continuous wave EPR, we have shown that the pH-sensitive radical covalently bound to TRIMEB maintains its pH-sensitivity, but this complexation does not reduce radical reduction rate in the reaction with ascorbic acid

Study of a DNA Duplex by Nuclear Magnetic Resonance and Molecular Dynamics Simulations. Validation of Pulsed Dipolar Electron Paramagnetic Resonance Distance Measurements Using Triarylmethyl-Based Spin Labels

Pulse dipole–dipole electron paramagnetic resonance (EPR) spectroscopy (double electron–electron resonance [DEER] or pulse electron–electron double resonance [PELDOR] and double quantum coherence [DQC]) allows for measurement of distances in biomolecules and can be used at low temperatures in a frozen solution. Recently, the possibility of distance measurement in a nucleic acid at a physiological temperature using pulse EPR was demonstrated. In these experiments, triarylmethyl (TAM) radicals with long memory time of the electron spin served as a spin label. In addition, the duplex was immobilized on modified silica gel particles (Nucleosil DMA); this approach enables measurement of interspin distances close to 4.5 nm. Nevertheless, the possible influence of TAM on the structure of a biopolymer under study and validity of the data obtained by DQC are debated. In this paper, a combination of molecular dynamics (MD) and nuclear magnetic resonance (NMR) methods was used for verification of interspin distances measured by the X-band DQC method. NMR is widely used for structural analysis of biomolecules under natural conditions (room temperature and an aqueous solution). The ultraviolet (UV) melting method and thermal series ¹H NMR in the range 5–95 °C revealed the presence of only the DNA duplex in solution at oligonucleotide concentrations 1 μM to 1.1 mM at temperatures below 40 °C. The duplex structures and conformation flexibility of native and TAM-labeled DNA complexes obtained by MD simulation were the same as the structure obtained by NMR refinement. Thus, we showed that distance measurements at physiological temperatures by the X-band DQC method allow researchers to obtain valid structural information on an unperturbed DNA duplex using terminal TAM spin labels

Saccharides as Prospective Immobilizers of Nucleic Acids for Room-Temperature Structural EPR Studies

Pulsed dipolar electron paramagnetic resonance (EPR) spectroscopy is a powerful tool for structural studies of biomolecules and their complexes. This method, whose applicability has been recently extended to room temperatures, requires immobilization of the studied biosystem to prevent averaging of dipolar couplings; at the same time, the modification of native conformations by immobilization must be avoided. In this work, we provide first demonstration of room-temperature EPR distance measurements in nucleic acids using saccharides trehalose, sucrose, and glucose as immobilizing media. We propose an approach that keeps structural conformation and unity of immobilized double-stranded DNA. Remarkably, room-temperature electron spin dephasing time of triarylmethyl-labeled DNA in trehalose is noticeably longer compared to previously used immobilizers, thus providing a broader range of available distances. Therefore, saccharides, and especially trehalose, can be efficiently used as immobilizers of nucleic acids, mimicking native conditions and allowing wide range of structural EPR studies at room temperatures

Synthesis of 2,5-Bis(spirocyclohexane)-Substituted Nitroxides of Pyrroline and Pyrrolidine Series, Including Thiol-Specific Spin Label: An Analogue of MTSSL with Long Relaxation Time

The nitroxides of 7-azadispiro[5.1.5.2]­pentadecane and 7-azadispiro[5.1.5.2]­pentadeca-14-ene series have been prepared, including thiol-specific methane thiosulfonate spin label for site-directed spin labeling. The effect of spirocyclohexane moieties on chemical and spectral properties has been studied. The obtained temperature dependencies of electron spin relaxation parameters demonstrate that new nitroxides may be suitable for PELDOR distance measurements at 80–120 K. Moreover, the new nitroxides demonstrated much higher stability toward reduction by ascorbate than spirocyclohexane-substituted nitroxides of piperidine series and showed 1.3–3.14 times lower reduction rates compared to corresponding 2,2,5,5-tetramethyl nitroxides

Light-Induced Magnetostructural Anomalies in a Polymer Chain Complex of Cu(hfac)₂ with <i>tert</i>-Butylpyrazolylnitroxides

We report the study of light-induced magnetostructural anomalies in a polymer chain complex of Cu­(hfac)₂ (hfac = hexafluoroacetylacetonate) with an unusual acyclic <i>tert</i>-butylpyrazolylnitroxide radical (L_tert^Me) using EPR. This complex ([Cu­(hfac)₂L_tert^Me]_<i>n</i>) belongs to the family of thermo- and photoswitchable molecular magnets “breathing crystals”. Compared to previously studied breathing crystals with nitronyl nitroxides, [Cu­(hfac)₂L_tert^Me]_<i>n</i> shows much weaker absorption bands in the visible spectral region and therefore is superior for optical manipulation of the spin states. Illumination with light (λ ≈ 540 nm) at cryogenic temperatures leads to formation of a metastable weakly coupled spin state, which relaxes to the ground strongly coupled spin state on a time scale of hours. These phenomena are in many aspects similar to the light-induced excited spin state trapping (LIESST) well-known for spin-crossover compounds. Remarkably, the photoinduced spin state in [Cu­(hfac)₂L_tert^Me]_<i>n</i> is metastable at temperatures up to <i>T</i>_LIESST ≈ 60 K, which is a significant improvement compared to that of previously studied breathing crystals with nitronyl nitroxides (<i>T</i>_LIESST ≈ 20 K). We describe LIESST-like behavior observed in [Cu­(hfac)₂L_tert^Me]_<i>n</i> and discuss possible reasons for the increased stability of the photoinduced spin state

W‑Band Time-Resolved Electron Paramagnetic Resonance Study of Light-Induced Spin Dynamics in Copper–Nitroxide-Based Switchable Molecular Magnets

Molecular magnets Cu­(hfac)₂L^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)₂L^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)₂L^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)₂L^R at low (liquid helium) temperatures and approaches developed for photoswitching/relaxation studies. Then we analyze the kinetics of the excited spin state at <i>T</i> = 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

Structural Dynamics in a “Breathing” Metal–Organic Framework Studied by Electron Paramagnetic Resonance of Nitroxide Spin Probes

Reversible structural rearrangements (“breathing”) of metal–organic frameworks (MOFs) are interesting and complex phenomena with many potential applications. They are often triggered by small amounts of adsorbed guest molecules; therefore, the guest–host interactions in breathing MOFs are intensively investigated. Due to the sensitivity limitations, most analytical methods require relatively high concentrations of guests in these studies. However, because guest molecules are not “innocent”, breathing behavior may become suppressed and unperturbed structural states inaccessible. We propose here the use of guest nitroxide molecules in tiny concentrations (such as 1 molecule per 1000 unit cells), which serve as spin probes for electron paramagnetic resonance (EPR), for effective study of breathing phenomena in MOFs. Using a perspective MIL-53­(Al) framework as an example, we demonstrate the great advantage of this general approach, which avoids perturbation of the framework structure and allows in-depth investigation of guest–host interactions in the breathing mode

FTIR Study of Thermally Induced Magnetostructural Transitions in Breathing Crystals

“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