Том 47, № 7 (2016)

The influence of magnetic fields on ultrasonic attenuation caused by relaxation in Jahn–Teller centers is studied using chromium ions as Zn substitute impurities in the ZnSe crystal as an example. The changes of the position and shape of the relaxation peak with the applied magnetic field induction B were observed in the temperature dependence of the ultrasonic attenuation of the shear waves at the frequency of 29.5 MHz propagating along the [110] axis with the polarization vector parallel to the [1\(\overline{1}\)0] axis. To rationalize the results, a simulation procedure was employed assuming that there are two thermal activation mechanism of relaxation with different relaxation times. One of them emerged with activation energy \(V_0=75\) K that remains unchanged in the magnetic fields, the other occurs with increasing \(V_0\) up to \(V_0=106\) K at \(B=14\) T.

A detailed study of the Signal Amplification By Reversible Exchange (SABRE) effect at high magnetic fields is performed. SABRE is formed by spin order transfer from parahydrogen to a substrate in a transient organometallic complex. Typically, such a transfer is efficient at low magnetic fields; at high fields it requires radio-frequency (RF) excitation of spins in the SABRE complex. However, recently it has been shown (Barskiy et al. in J. Am. Chem. Soc. 136:3322–3325, 2014) that high-field SABRE is also feasible due to “spontaneous” spin order transfer (i.e., transfer in the absence of RF excitation) although the transfer efficiency is low. Here, we studied the SABRE field dependence for protons in the field range 1.0–16.4 T and found an increase of polarization with the field; further optimization of proton polarization can be achieved by varying the viscosity of the solvent. As previously, polarization transfer is attributed to cross-relaxation; this conclusion is supported by additional experiments. For spin-½ hetero-nuclei, such as ¹⁵N and ³¹P, spontaneous spin order transfer is also feasible; however, in contrast to protons, it is based on a coherent mechanism. Consequently, higher transfer efficiency is achieved; moreover the ¹⁵N and ³¹P spectral patterns are remarkably different from that for protons: multiplet (anti-phase) polarization is seen for hetero-nuclei. Our study is of importance for enhancing weak nuclear magnetic resonance (NMR) signals by exploiting non-thermally polarized spins. Although the efficiency of high-field SABRE is lower than that of low-field SABRE; the high-field SABRE experiment is easy to implement for improving the sensitivity of NMR methods.

We report an experimental study of the low-temperature dynamics of electron spin fluctuations in the magnetically frustrated spinel CoAl₂O₄ as revealed by ²⁷Al nuclear magnetic relaxation measurements in a magnetic field of 7.7 T in the temperature range 4 < T < 240 K. With this local probe technique, we show that the dynamics of the correlated Co spins strongly depends on the frustration of spin interactions and on Co/Al site disorder. The anisotropy of the temperature dependences of the spin–lattice (T₁⁻¹) and spin–spin (T₂⁻¹) ²⁷Al nuclear relaxation rates reveals a coexistence of the short-range Néel order below a characteristic temperature T^* = 8 K and slow non-commensurate magnetic correlations below and above T^*, in agreement with the results of neutron diffraction experiments and our previous NMR spectroscopy data.

The one-sided Möbius band topology with its characteristic 180° twist has fascinated and inspired philosophers, artists and scientists since a long time. On the molecular level, only in the last 13 years a few chemistry groups succeeded to artificially create novel compounds with Möbius symmetry by theory-based molecular design and elaborate chemical synthesis. The interest in molecules with Möbius band symmetry was greatly stimulated in 1964 by a theoretical paper by Edgar Heilbronner from the ETH Zurich. He predicted that sufficiently large [n]annulenes with a closed-shell electron configuration of 4n π-electrons should allow for sufficient π-overlap stabilization to be synthesizable by twisting them into the Möbius topology of their hydrocarbon skeleton. In 2003, the first synthesis of an aromatic Möbius annulene was accomplished by Rainer Herges and co-workers in Kiel. In 2007, Lechosław Latos-Grażyński and co-workers in Wroclaw succeeded in synthesizing free-base di-p-benzi (Yoneda et al., Angew Chem Int Ed 53: 13169–13173, 2014) hexaphyrin (1.1.1.1.1.1), compound 1, an expanded porphyrin which can dynamically switch between Hückel and Möbius conjugation upon changes of solvent and temperature. Shortly thereafter, in 2008, Atsuhiro Osuka and his co-workers from Kyoto, Seoul and Hyogo published the synthesis of an expanded porphyrin in which metalation triggered the production of molecular twisting and Möbius aromaticity. In this minireview, among other studies also our recent EPR, ENDOR and DFT studies on open-shell states of 1, i.e., on the ground-state radical cation doublet state (total electron spin S = 1/2) and the first excited triplet state (S = 1) are summarized. The review is largely based on a previous joint publication of the current authors with the Latos-Grażyński group on radical cations of 1 (Möbius et al., Phys Chem Chem Phys 17:6644–6652, 2015). The radical cation study was the first one of an open-shell π-system with Möbius topology. In the doublet state, the hyperfine interactions of the unpaired electron spin with specific magnetic nuclei in the molecule was used as a sensitive probe for the electronic structure of the molecule and its symmetry properties. This work has now been extended to state-of-the-art DFT theory studies on photo-excited triplet states of 1. In the open-shell triplet state, besides hyperfine couplings, a change of the zero-field splitting interaction between the two unpaired electron spins is predicted to be a viable sensor for electronic structure changes upon Möbius-to-Hückel topology switching.

X-band light-induced continuous wave EPR (LEPR) spectroscopy was employed to study triplet fullerene C₇₀, ³C₇₀, in glassy decalin and o-terphenyl. The EPR lineshapes observed in the temperature ranges between 50 and 130 K (for decalin) as well as between 77 and 200 K (for OTP) were successfully simulated. The results show that the narrow LEPR line does not originate from a radical S = 1/2, but arises from jumps of the EPR lines within the LEPR spectrum of ³C₇₀ between symmetrical LEPR positions provided by paramagnetic relaxation process of ³C₇₀. This process can be described by a relaxation-induced coherence transfer between two resonance transitions having close transition frequencies.

Highly sensitive methods of the detection of the electron paramagnetic resonance (EPR) spectra based on the spin-dependent microwave photoconductivity were applied for investigation of the point defects in silicon. The specific features and properties of the excited triplet (spin S = 1) states of defects responsible for spin-dependent recombination of photo excited carriers are considered. The main attention is given to study such defects as oxygen + vacancy complexes and carbon related centers dominantly produced by irradiation.