Elena G. Bagryanskaya

Physiological-Temperature Distance Measurement in Nucleic Acid using Triarylmethyl-Based Spin Labels and Pulsed Dipolar EPR Spectroscopy

Resolving the nanometer-scale structure of biomolecules in natural conditions still remains a challenging task. We report the first distance measurement in nucleic acid at physiological temperature using electron paramagnetic resonance (EPR). The model 10-mer DNA duplex has been labeled with reactive forms of triarylmethyl radicals and then immobilized on a sorbent in water solution and investigated by double quantum coherence EPR. We succeeded in development of optimal triarylmethyl-based labels, approach for site-directed spin labeling and efficient immobilization procedure that, working together, allowed us to measure as long distances as ~4.6 nm with high accuracy at 310 K (37 °C).

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

Novosibirsk Free Electron Laser—Facility Description and Recent Experiments

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.

Scavenging of organic C-centered radicals by nitroxides.

Elena G. Bagryanskaya*,†,‡,⊥ and Sylvain R. A. Marque* †N.N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences, Pr. Lavrentjeva 9, Novosibirsk 630090, Russia ‡International Tomography Center, Siberian Branch of the Russian Academy of Sciences, Institutskaya 3A, Novosibirsk 630090, Russia Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russia Aix-Marseille Universite,́ CNRS, ICR UMR 7273, case 551, Avenue Escadrille Normandie-Niemen, 13397 Marseille Cedex 20, France

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.

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.

pH-Sensitive C―ON Bond Homolysis of Alkoxyamines of Imidazoline Series with Multiple Ionizable Groups As an Approach for Control of Nitroxide Mediated Polymerization

Recently, a new concept of pH-switchable agents for reversible addition-fragmentation chain transfer (RAFT) polymerization has been introduced by Benaglia et al. (J. Am. Chem. Soc.2009, 131, 6914-6915). In this paper we extended the concept of pH-switchable mediators to nitroxide mediated polymerization (NMP) by employing nitroxides with basic or acidic groups as controlling agents. Four alkoxyamines, the derivatives of 2-(4-(dimethylamino)-2-ethyl-5,5-dimethyl-2-(pyridin-4-yl)-2,5-dihydro-1H-imidazol-1-oxyl and 2-(2-carboxyethyl)-5,5-diethyl-2,4-dimethyl-2,5-dihydro-1H-imidazol-1-oxyl, have been prepared. The influence of pH on alkoxyamine homolysis rate constants (k(d)) and on the nitroxide-alkyl radical recombination rate constants (k(c)) was studied. All alkoxyamines under study as well as the parent nitroxides have several basic groups, which under pH variation can undergo consecutive protonation. It was shown that the k(d) value under basic conditions are significantly (up to 15-fold) higher than in acidic solution at the same temperature, whereas the k(c) value in basic solutions decrease by a factor of 2 only. The efficiency of NMP is known to be dependent on k(d) and k(c), both constants being dependent on the monomer structure; therefore the performance of NMP of different monomers in the controlled mode requires different conditions. It is shown that the pH value crucially affects the polymerization regime, changing it from the controlled to the uncontrolled mode. The controlled regime of NMP of different hydrophilic monomers (sodium 4-styrenesulphonate and acrylamide) in aqueous solution under mild conditions (90 °C) can be achieved using the same alkoxyamine by the variation of the pH value. The chain length of polymers depends on pH value during the polymerization.

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.

Room-Temperature Electron Spin Relaxation of Triarylmethyl Radicals at the X- and Q-Bands.

Triarylmethyl radicals (trityls, TAMs) represent a relatively new class of spin labels. The long relaxation of trityls at room temperature in liquid solutions makes them a promising alternative for traditional nitroxides. In this work we have synthesized a series of TAMs including perdeuterated Finland trityl (D36 form), mono-, di-, and triester derivatives of Finland-D36 trityl, the deuterated form of OX63, the dodeca-n-butyl homologue of Finland trityl, and triamide derivatives of Finland trityl with primary and secondary amines attached. We have studied room-temperature relaxation properties of these TAMs in liquids using pulsed electron paramagnetic resonance (EPR) at two microwave frequency bands. We have found the clear dependence of phase memory time (Tm ∼ T2) on the magnetic field: room-temperature Tm values are ∼1.5-2.5 times smaller at the Q-band (34 GHz, 1.2 T) than at the X-band (9 GHz, 0.3 T). This trend is ascribed to the contribution from g-anisotropy that is negligible at lower magnetic fields but comes into play at the Q-band. In agreement with this, the difference between T1 and Tm becomes more pronounced at the Q-band than at the X-band due to increased contributions from incomplete motional averaging of g-anisotropy. Linear dependence of (1/Tm - 1/T1) on viscosity implies that g-anisotropy is modulated by rotational motion of the trityl radical. On the basis of the analysis of previous data and results of the present work, we conclude that, in the general situation where the spin label is at least partly mobile, the X-band is most suitable for application of trityls for room-temperature pulsed EPR distance measurements.