Optical Phenomena in Molecule-Based Magnetic Materials

Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

A metal-radical hetero-tri-spin SCM with methyl-pyrazole-nitronyl nitroxide bridges

The preparation, crystal structures, and magnetic properties of a family of hetero-tri-spin 1-D coordination polymers with the formula [Ln(hfac)₃Cu(hfac)₂(4-NIT-MePyz)₂] (Ln = Gd, 1, Tb, 2, Dy, 3; hfac = hexafluoroacetylacetonate; 4-NIT-MePyz = 2-{4-(1-methyl)-pyrazolyl}-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) are reported. In these complexes, the 4-NIT-MePyz radical acts as a linker to bridge the Cu^II and Ln^III ions through its pyrazole and aminoxyl groups to form a chain structure. Magnetic properties typical of spin-chains are observed for Dy and Tb derivatives but single-chain magnet (SCM) behavior was evidenced only for the Tb compound which is characterized by an energy gap for demagnetization Δ_τ/k_B of 31 K.

Synthesis, crystal structures, and magnetic properties of nitroxide radical-coordinated manganese(II) and cobalt(II) complexes

Coordination complexes constructed from the nitronyl nitroxide radical NIT-Pyra-3-Isobu and MII(hfac)2(H2O)2 as building blocks (NIT-Pyra-3-Isobu = 2-(3-isobutyl-pyrazole)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, hfac = hexafluoroacetylacetonato, and M = Mn or Co) are successfully synthesized. The crystal structures of the coordination compounds reveal the usefulness of the functionalized radical in providing discrete or extended architectures. Single-crystal X-ray diffraction analyses indicated that the ligand is coordinated through the oxygen atom of NO• and the N atom of the pyrazole linkage to the metal, and the metals adopt an octahedral geometry leading to one-dimensional zig-zag chain systems. The magnetic behavior of the manganese and cobalt shows overall antiferromagnetic interactions between the metal center and the organic radical.

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

We investigate intra- and intercluster exchange couplings in photoinduced states of copper(ii)–nitroxide based molecular magnets (“breathing crystals”) using variable-temperature EPR spectroscopy.

Aromatic SNF-Approach to Fluorinated Phenyl tert-Butyl Nitroxides

The interaction of octafluorotoluene (1a), as well as pentafluorobenzonitrile (1b) with tert-butylamine, followed by the oxidation of thus formed tert-butylanilines (2a,b) with meta-chloroperoxybenzoic acid led to functionalized perfluorinated phenyl tert-butyl nitroxides [namely, 4-(N-tert-butyl(oxyl)amino)heptafluorotoluene (3a) and 4-(N-tert-butyl(oxyl)amino)tetrafluorobenzonitrile (3b)] with nearly quantitative total yields. The molecular and crystal structures of nitroxide 3a were proved by single crystal X-ray diffraction analysis. The radical nature of both nitroxides was confirmed by ESR data. The interaction of Cu(hfac)₂ with the obtained nitroxides 3a,b gave corresponding trans-bis(1,1,1,5,5,5-hexafluoropentane-2,4-dionato-κ²O,O′)bis{4-(N-tert-butyl(oxyl)amino)perfluoroarene-κO}copper (II) complexes ([Cu(hfac)₂(3a)₂] and [Cu(hfac)₂(3b)₂]). X-ray crystal structure analysis showed square bipyramid coordination of a centrally symmetric Cu polyhedron with the axial positions occupied by oxygen atoms of the nitroxide groups. Magnetic measurements revealed intramolecular ferromagnetic exchange interactions between unpaired electrons of Cu(II) ions and paramagnetic ligands, with exchange interaction parameters J_Cu–R reaching 53 cm⁻¹.

Copper–nitroxide based breathing crystals: a unified mechanism of gradual magnetostructural transition supported by quantum chemistry calculations

A unified mechanism, based on the thermal average of two limit phases, governs the gradual magnetic transitions of Cu(hfac)₂L^R complexes.

The structurally variable network of spin couplings and migrating paramagnetic centers in binuclear o-quinone CoII complexes with biradical acene linkers: a computational DFT study

A series of new magnetically-active coordination compounds comprising binuclear mixed-ligand complexes of cobalt bis-diketonates with acene linkers functionalized by two redox-active o-quinone moieties has been designed by means of density functional theory (DFT UB3LYP*/6-311++G(d,p)) calculations of their electronic structure, energy characteristics and magnetic properties. Two types of redox-active ligands include those with an acene linker bridging two o-benzoquinone fragments and the ligands containing an integrated π-conjugated system formed by annulation of o-quinone rings to the polycyclic core. The calculations reveal the dependence of spin density distribution in the compounds under study on the type of ligand. The considered binuclear CoII diketonate adducts manifest the capability of undergoing one- and two-step spin transitions induced by intramolecular electron transfers between the metal ions and the ligand system. An increase in the number of condensed rings of the acene linker promotes the stabilization of the biradicaloid state of the linker and enhances exchange interactions between all paramagnetic centers of the complexes giving rise to the formation of a flexible spin coupling network. The predicted unusual magnetic properties make the binuclear cobalt complexes with di-o-quinone ligand containing acene linker groups the promising building blocks for molecular electronic and spintronic devices.

Light-Induced Spin State Switching in Copper(II)-Nitroxide-Based Molecular Magnet at Room Temperature

Molecular magnets Cu(hfac)₂L^R exhibit an unusual type of photoinduced magnetostructural switching in exchange-coupled copper(II)-nitroxide clusters. Such photoswitching from strongly coupled to weakly coupled spin state (SS → WS) was recently found to be ultrafast, thus enhancing the interest in these systems and the scope of their potential applications. However, to date such SS → WS photoswitching was demonstrated only at cryogenic temperatures, being limited by the absence of suitable SS states and short relaxation times at T > 100 K. In this work we selected model compound Cu(hfac)₂L^iso-Pr residing in the mixed SS/WS state at room temperature and investigated it using femtosecond optical spectroscopy. Photoinduced spin dynamics was detected, and an ultrafast SS → WS photoswitching was for the first time demonstrated at room temperature, constituting an important milestone in the development of copper(II)-nitroxide molecular magnets for practical purposes.

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

Pronounced thermochromism of copper(ii)–nitroxide based molecular magnets is explained.