High-field EPR of copper(II)-nitroxide compound exhibiting three-step phase transition: structural insights from the field-induced sample orientation.

Copper(II)-nitroxide based Cu(hfac)2LR compounds exhibit unusual magnetic behavior that can be induced by various stimuli. In many aspects, the magnetic phenomena observed in Cu(hfac)2LR are similar to classical spin-crossover behavior. However, these phenomena originate from polynuclear exchange-coupled spin clusters Cu2+-O˙-N< or >N-˙O-Cu2+-O˙-N<. Such peculiarities may result in additional multifunctionality of Cu(hfac)2LR compounds, making them promising materials for spintronic applications. Herein, we investigate the Cu(hfac)2LMeMe material, which demonstrates a three-step temperature-induced magnetostructural transition between high-temperature, low-temperature, and intermediate states, as revealed by magnetometry. Two main steps were resolved using variable-temperature Fourier-transform infrared and Q-band electron paramagnetic resonance (EPR) spectroscopies. The intermediate-temperature states (∼40-90 K) are characterized by the coexistence of two types of copper(II)-nitroxide clusters, corresponding to the low-temperature and high-temperature phases. High-field EPR experiments revealed the effect of partial alignment of Cu(hfac)2LMeMe microcrystals in a strong (>20 T) magnetic field. This effect was used to unveil the structural features of the low-temperature phase of Cu(hfac)2LMeMe, which were inaccessible using single-crystal X-ray diffraction (XRD) technique. In particular, high-field EPR allowed us to determine the relative direction of the Jahn-Teller axes in CuO6 and CuO4N2 units.

Selective Ion Exchange in Supramolecular Channels in the Crystalline State.

Artificial ion channels are of increasing interest because of potential applications in biomimetics, for example, for realizing selective ion permeability through the transport and/or exchange of selected ions. However, selective ion transport and/or exchange in the crystalline state is rare, and to the best of our knowledge, such a process has not been successfully combined with changes in the physical properties of a material. Herein, by soaking single crystals of Li2 ([18]crown-6)3 [Ni(dmit)2 ]2 (H2 O)4 (1) in an aqueous solution containing K+ , we succeeded in complete ion exchange of the Li+ ions in 1 with K+ ions in the solution, while maintaining the crystalline state of the material. This ion exchange with K+ was selectively conducted even in mixed solutions containing K+ as well as Na+ /Li+ . Furthermore, remarkable changes in the physical properties of 1 resulted from the ion exchange. Our finding enables not only the realization of selective ion permeability but also the development of highly sensitive biosensors and futuristic ion exchange agents, for example.

Spin-state-correlated optical properties of copper(ii)-nitroxide based molecular magnets.

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.

A magnetically isolated cuprate spin-ladder system: synthesis, structures, and magnetic properties.

We synthesized and characterized two magnetically isolated spin ladders, Cu2(CO3)(ClO4)2(NH3)6 (1) and Cu2(CO3)(ClO4)2(H2O)(NH3)5 (2), which are the first examples of carbonate bridging molecular spin ladders. Compounds 1 and 2 form a ladder configuration by stacking a structural unit composed of two Cu(2+) ions and one CO3(2-), where the Cu-O-Cu interactions form the rungs and legs of each ladder and the counter anions (ClO4(-)) occupy the space between the ladders and ensure their magnetic isolation. A S = 1/2 magnetically isolated spin-ladder model with a ladder-rung magnetic interaction J1/k(B) = 364 K (where J is defined as positive for antiferromagnetic interactions) and a ladder-leg magnetic interaction J2/k(B) = 27.4 K accurately predicts the temperature dependence of the molar magnetic susceptibility for 1. The ladder configuration of 2 is similar to that of 1 except that the CO3(2-) is alternately skewed in different directions in the stacked structural unit. Interestingly, this minor structural variation in 2 results in its remarkably different magnetic behavior; the magnetic susceptibility curve of 2 is accurately described by an alternating chain model with J3/k(B) = 7.26 K and J4/k(B) = 4.42 K.

Photoswitching of a thermally unswitchable molecular magnet Cu(hfac)2L(i-Pr) evidenced by steady-state and time-resolved electron paramagnetic resonance.

Most photoswitchable molecular magnets exhibit thermally induced switching, as is typical of spin crossover (SCO), valence tautomerism and SCO-like phenomena. We report a rare case of a copper-nitroxide based molecular magnet Cu(hfac)2L(i-Pr) that does not exhibit quantitative SCO-like behavior in the temperature range of its chemical stability (2-350 K); however, it can be switched to a metastable thermally inaccessible spin state via visible/near-IR light at cryogenic temperatures. By means of photogeneration, unique information on this otherwise unobservable spin state has been obtained using steady-state Q-band (34 GHz) and time-resolved W-band (94 GHz) electron paramagnetic resonance (EPR) spectroscopy. In particular, we have found that the electronic structure and relaxation properties of the photoinduced state in Cu(hfac)2L(i-Pr) are very similar to those in its sister compound Cu(hfac)2L(n-Pr) that is thermally switchable and has been exhaustively characterized by many analytical methods, previously. The first observation of photoswitchable behavior in a thermally unswitchable copper-nitroxide based molecular magnet Cu(hfac)2L(i-Pr) paves the way for photoswitching applications of this and similar compounds in the remarkably broad temperature range of 2-350 K.