Electron spin relaxation by spin-rotation interaction in benzoyl and other acyl type radicals

In various studies of the spin dynamics in radical pairs, benzoyl-type radicals have been one of the two paramagnetic pair species. Their electron spin relaxation has been assumed to be slow enough to be neglected in the data analysis. This assumption is checked by measuring the electron spin relaxation in a sequence of three acyl radicals (benzoyl, 2,4,6-trimethylbenzoyl and hexahydrobenzoyl) by time-resolved electron paramagnetic resonance spectroscopy. In contrast to the assumed slow relaxation, rather short spin-lattice relaxation times (100-400 ns) are found for benzoyl and 2,4,6-trimethylbenzoyl radicals from the decay of the integral initial electron polarization to thermal equilibrium at different temperatures and viscosities. The relaxation is induced by a spin-rotation coupling arising from two different types of radical movements: overall rotation of the whole radical and hindered internal rotation of the CO group. The predominant second contribution depends on the barrier of the internal rotation. The obtained results are well explained in the frame of Bull's theory when using a modified rotational correlation time τ J . The size of the spin-rotation coupling due to the internal CO group rotation in benzoyl radicals is estimated to be |C α|=1510 MH

TRITYL RADICALS AS SPIN LABELS

This work was supported by Russian Science Foundation project № 21-14-00219

Photoswitching of a thermally unswitchable molecular magnet Cu(hfac) 2Li-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)2Li-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)2Li-Pr are very similar to those in its sister compound Cu(hfac)2Ln-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)2Li-Pr paves the way for photoswitching applications of this and similar compounds in the remarkably broad temperature range of 2-350 K. © 2014 American Chemical Society

FTIR study of thermally induced magnetostructural transitions in breathing crystals

© 2015 American Chemical Society. "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

HIGLY STABLE NITROXIDE SPIN LABELS FOR BIOPHYSICAL RESEARCH

This work was supported by the RSF, project # 19-13-000235 and Ministry of Science and Higher Education of the Russian Federation (grant No. 14. W03.31.0034)