Microgravity-like Crystallization of Paramagnetic Species in Strong Magnetic Fields.

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.

Exchange interactions in photoinduced magnetostructural states of copper(ii)-nitroxide spin dyads.

Copper(ii) complexes with stable nitroxide radicals are capable of magnetostructural spin-crossover like anomalies induced by external stimuli. Photoswitching in such systems is particularly important; however, retrieving the properties of photoinduced states is challenging and requires development of novel approaches. In this work, we investigate the exchange interactions in metastable photoinduced states of two compounds containing copper(ii)-nitroxide dyads. Using Electron Paramagnetic Resonance (EPR) with photoexcitation we obtain temperature dependence of magnetic susceptibility in the photoinduced state and estimates for the corresponding values of exchange coupling in the studied complexes. The interplay between intra- and inter-cluster exchange couplings is considered and analyzed. The proposed methodology is applicable also to other photoswitchable exchange-coupled systems.

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.

Light-Induced Spin State Switching and Relaxation in Spin Pairs of Copper(II)-Nitroxide Based Molecular Magnets.

Similar to spin-crossover (SCO) compounds, spin states of copper(II)-nitroxide based molecular magnets can be switched by various external stimuli including temperature and light. Although photoswitching and reverse relaxation of nitroxide-copper(II)-nitroxide triads were investigated in some detail, similar study for copper(II)-nitroxide spin pairs was still missing. In this work we address photoswitching and relaxation phenomena in exchange-coupled spin pairs of this family of molecular magnets. Using electron paramagnetic resonance (EPR) spectroscopy with photoexcitation, we demonstrate that compared to triad-containing compounds the photoinduced weakly coupled spin (WS) states of copper(II)-nitroxide pairs are remarkably more stable at cryogenic temperatures and relax to the ground strongly coupled spin (SS) states on the scale of days. The structural changes between SS and WS states, e.g., differences in Cu-Onitroxide distances, are much more pronounced for spin pairs than for spin triads in most of the studied copper(II)-nitroxide based molecular magnets. This results in higher energy barrier between WS and SS states of spin pairs and governs higher stability of their photoinduced WS states. Therefore, the longer-lived photoinduced states in copper(II)-nitroxide molecular magnets should be searched within the compounds experiencing largest structural changes upon thermal spin transition. This advancement in understanding of LIESST-like phenomena in copper(II)-nitroxide molecular magnets allows us to propose them as interesting playgrounds for benchmarking the basic factors governing the stability of photoinduced states in other SCO and SCO-like photoswitchable systems.