Stimuli-responsive magnetic materials: impact of spin and electronic modulation

Addressing molecular bistability as a function of external stimuli, especially in spin-crossover (SCO) and metal-to-metal electron transfer (MMET) systems, has seen a surge of interest in the field of molecule-based magnetic materials due to their enormous potential in various technological applications such as molecular spintronics, memory and electronic devices, switches, sensors, and many more. The fine-tuning of molecular components allow the design and synthesis of materials with tailored properties for these vast applications. In this Feature Article, we discuss a part of our research work into this broad topic, pertaining to the recent discoveries in the field of switchable molecular magnetic materials based on SCO and MMET systems, along with some historical background of the area and related accomplishments made in recent years.

Spin-State Modulation in FeII -Based Hofmann-Type Coordination Polymers: From Molecules to Materials

Spin crossover complexes that reversibly interconvert between two stable states imitate a binary state of 0 and 1, delivering a promising possibility to address the data processing concept in smart materials. Thus, a comprehensive understanding of the modulation of magnetic transition between high spin and low spin and the factors responsible for stabilizing the spin states is an essential theme in modern materials design. In this context, the present review attempts to provide a concise outline of the design strategy employed at the molecular level for fine-tuning the spin-state switching in Fe^II -based Hofmann-type coordination polymers and their effects on the optical and magnetic response. In addition, development towards the nanoscale architectures of HCPs, i. e., in terms of nanoparticles and thin films, are emphasized to bridge the gap between the laboratory and reality.

Impact of counter anions on spin-state switching of manganese(III) complexes containing an azobenzene ligand

Four mononuclear manganese(III) complexes coordinated with photo-active hexadentate azobenzene ligands, [Mn(5azo-sal₂-323)](X) (X = Cl, 1; X = BF₄, 2; X = ClO₄, 3; X = PF₆, 4), were prepared. The impact of various counter anions on the stabilization and switching of the spin state of the manganese(III) center was explored through detailed magneto-structural investigation using variable temperature single-crystal X-ray diffraction, magnetic, spectroscopic, and spectroelectrochemical studies, along with theoretical calculations. All four complexes consisted of an isostructural monocationic distorted octahedral MnN₄O₂ coordination environment offered by the hexadentate ligand and Cl^-, BF₄^-, ClO₄^-, and PF₆^- as counter anions respectively. Complex 1 with a spherical Cl^- counter anion showed a reversible and gradual spin-state switching between low-spin (LS) (S = 1) and high-spin (HS) (S = 2) states above 400 K, where non-covalent cation-anion interactions played a significant role in stabilizing the LS state. While, irrespective of the shape of the counter anion, complexes 2-4 remained in the HS state throughout the measured temperature range of 300-2 K, where strong π-π interaction between the azobenzene motifs among cationic units played a substantial role in stabilizing the HS state. Furthermore, magnetic data analyses revealed significantly large zero-field splitting in the S = 1 state for 1 (D = 19.4 cm^-1, E/D = 0.008) in comparison with that in the S = 2 state for 2-4 (D = 3.99-4.97 cm^-1, E/D = 0.002-0.195). Spectroelectrochemical investigations revealed the quasi-reversible reduction and oxidation of the manganese(III) center to manganese(II) and manganese(IV), respectively. A detailed theoretical calculation at the DFT and CASSCF level of theory was carried out to better understand the magneto-structural correlation.

Light-induced hidden multistability in a spin crossover metal-organic framework

The pursuit of spin crossover (SCO) materials with photo-switchable multistability is driven by the fascinating perspectives toward light-response switches and opto-magnetic memory devices. Herein, we report a 3D Hofmann-type metal...

Guest-Driven Light-Induced Spin Change in an Azobenzene Loaded Metal-Organic Framework

Stimuli-responsive materials that can be reversibly switched by light are of immense interest. Among them, photo-responsive spin crossover (SCO) complexes have great promises to combine the photoactive inputs with multifaceted outputs into switchable materials and devices. However, the reversible control the spin-state change by photochromic guests is still challenging. Herein, we report an unprecedented guest-driven light-induced spin change (GD-LISC) in a Hofmann-type metal-organic framework (MOF), [Fe(bpn){Ag(CN)₂ }₂ ]⋅azobenzene. (1, bpn=1,4-bis(4-pyridyl)naphthalene). The reversible trans-cis photoisomerization of azobenzene guest upon UV/Vis irradiation in the solid-state results in the remarkable magnetic changes in a wide temperature range of 10-180 K. This finding not only establishes a new switching mechanism for SCO complexes, but also paves the way toward the development of new generation of photo-responsive magnetic materials.

Ultrafast Photodynamics of an Azopyridine-Functionalized Iron(II) Complex: Implications for the Concept of Ligand-Driven Light-Induced Spin Change

We report on the ultrafast photodynamics of an iron(II) complex with a photoisomerizable pentadentate azo-tetrapyridylamino ligand after irradiation with ultraviolet light. The results of femtosecond transient electronic absorption spectroscopy performed on the low-spin (LS) form of the title complex show that initial excitation of the ππ* state of the azopyridine unit in the ligand at λ_pump = 312 nm is followed by an ultrafast intersystem crossing (ISC) that leads to the formation of a metal-centered (MC) ⁵T state, in competition with the intended photoswitching of the azopyridine unit. Additional measurements carried out upon excitation of the singlet metal-to-ligand charge-transfer (¹MLCT) transition at λ_pump = 455 nm suggest that this energy transfer occurs via an MLCT state. The resulting high-spin (HS) ⁵T state of the complex is metastable and recovers to the LS ground state with a time constant of ∼3 ns. The implications of these observations on the ligand-driven light-induced spin change concept are discussed.

Carboxylic Acid Functionalized Spin-Crossover Iron(II) Grids for Tunable Switching and Hybrid Electrode Fabrication

Two carboxyl-substituted iron(II) grids, one protonated, [Fe₄(HL)₄](BF₄)₄·4MeCN·AcOEt (1), and the other deprotonated, [Fe₄(L)₄]·DMSO·EtOH (2), where H₂L = 4-{4,5-bis[6-(3,5-dimethylpyrazol-1-yl)pyrid-2-yl]-1 H-imidazol-2-yl}benzoic acid, were synthesized. Single-crystal X-ray structure analyses reveal that both complexes have a tetranuclear [2 × 2] grid structure. 1 formed one-dimensional chains through intermolecular hydrogen bonds between the carboxylic acid units of neighboring grids, while 2 formed two-dimensional layers stabilized by π-π-stacking interactions. 1 showed spin transition between the 3HS-1LS and 1.5HS-2.5LS states around 200 K, while 2 showed spin-crossover between the 4LS and 2LS-2HS states above 300 K. A modified indium-tin oxide (ITO) electrode was fabricated by soaking the ITO in a solution of 1. The resultant electrode showed reversible redox waves attributed to the original redox processes of iron(II)/iron(III).

Switching Magnetic Properties by a Mechanical Motion

Switching magnetic properties have attracted a wide interest from inorganic chemist for the objectives of information storage and quantum computing at the molecular level. This review is focused on magnetic switches based on a mechanical motion, which is an innovative approach. Three main strategies to control magnetic properties by a mechanical motion have been developed in the literature and will be described. The first one (ligand-induced spin change) consists in modulating the ligand field strength by a configuration change of the ligand in spin-crossover complexes. The second one (coordination-induced spin-state switching) is based on a change in the coordination number of a metallic center that is triggered by the motion of one ligand. The third one uses the modulation of the exchange interaction between two spin-centers by a mechanical motion.

Azoimidazole functionalized Ni-porphyrins for molecular spin switching and light responsive MRI contrast agents

Molecular spin switching with light under aqueous conditions has been performed with azoimidazole functionalized Ni-porphyrins. The change in contrast in MRI images is clearly visible.

Thermal Spin Crossover Behaviour of Two-Dimensional Hofmann-Type Coordination Polymers Incorporating Photoactive Ligands

Two spin crossover (SCO)-active 2D Hofmann-type framework materials, [Fe(3-PAP)2Pd(CN)4] (A) and [Fe(4-PAP)2Pd(CN)4] (B) containing the photoactive azo-benzene-type ligands 3-phenylazo-pyridine (3-PAP) and 4-phenylazo-pyridine (4-PAP) were prepared. These materials form non-porous Hofmann-type structures whereby 2D [FeIIPd(CN)4] grids are separated by 3- or 4-PAP ligands. The iron(ii) sites of both materials (A and B) undergo abrupt and hysteretic spin transitions with characteristic transition temperatures T1/2↓,↑: 178, 190 K (ΔT: 12 K) and T1/2↓,↑: 233, 250 K (ΔT: 17 K), respectively. Photo-magnetic characterisations reveal light-induced excited spin state trapping (LIESST) activity in both A and B with characteristic T(LIESST) values of 45 and 40 K. Although both free ligands show trans- to-cis isomerisation in solution under UV-irradiation, as evidenced via absorption spectroscopy, such photo-activity was not observed in the ligands or complexes A and B in the solid state. Structural analysis of a further non-SCO active isomer to B, [Fe(4-PAP)2Pd(CN)4]·1/2(4-PAP) (B·(4-PAP)), which contains free ligand in the pore space is reported.

Light-driven coordination-induced spin-state switching: rational design of photodissociable ligands

The bistability of spin states (e.g., spin crossover) in bulk materials is well investigated and understood. We recently extended spin-state switching to isolated molecules at room temperature (light-driven coordination-induced spin-state switching, or LD-CISSS). Whereas bistability and hysteresis in conventional spin-crossover materials are caused by cooperative effects in the crystal lattice, spin switching in LD-CISSS is achieved by reversibly changing the coordination number of a metal complex by means of a photochromic ligand that binds in one configuration but dissociates in the other form. We present mathematical proof that the maximum efficiency in property switching by such a photodissociable ligand (PDL) is only dependent on the ratio of the association constants of both configurations. Rational design by using DFT calculations was applied to develop a photoswitchable ligand with a high switching efficiency. The starting point was a nickel-porphyrin as the transition-metal complex and 3-phenylazopyridine as the photodissociable ligand. Calculations and experiments were performed in two iterative steps to find a substitution pattern at the phenylazopyridine ligand that provided optimum performance. Following this strategy, we synthesized an improved photodissociable ligand that binds to the Ni-porphyrin with an association constant that is 5.36 times higher in its trans form than in the cis form. The switching efficiency between the diamagnetic and paramagnetic state is efficient as well (72% paramagnetic Ni-porphyrin after irradiation at 365 nm, 32% paramagnetic species after irradiation at 440 nm). Potential applications arise from the fact that the LD-CISSS approach for the first time allows reversible switching of the magnetic susceptibility of a homogeneous solution. Photoswitchable contrast agents for magnetic resonance imaging and light-controlled magnetic levitation are conceivable applications.

Solid-state ligand-driven light-induced spin change at ambient temperatures in bis(dipyrazolylstyrylpyridine)iron(II) complexes

We previously reported that an Fe(II) complex ligated by two (Z)-2,6-di(1H-pyrazol-1-yl)-4-styrylpyridine ligands (Z-H) presented a solid state ligand-driven light-induced spin change (LD-LISC) upon one-way Z-to-E photoisomerization, although modulation of the magnetism was trivial at ambient temperatures (Chem. Commun.2011, 47, 6846). Here, we report the synthesis of new derivatives of Z-H, Z-CN and Z-NO(2), in which electron-withdrawing cyano and nitro substituents are introduced at the 4-position of the styryl group to attain a more profound photomagnetism at ambient temperatures. Z-CN and Z-NO(2) undergo quantitative one-way Z-to-E photochromism upon excitation of the charge transfer band both in acetonitrile and in the solid state, similar to the behavior observed for Z-H. In solution, these substituents stabilized the low-spin (LS) states of Z-CN and Z-NO(2), and the behavior was quantitatively analyzed according to the Evans equation. The photomagnetic properties in the solid state, on the other hand, cannot be explained in terms of the substituent effect alone. Z-CN displayed photomagnetic properties almost identical to those of Z-H. Z-CN preferred the high-spin (HS) state at all temperatures tested, whereas photoirradiated Z-CN yielded a lower χ(M)T at ambient temperatures. The behavior of Z-NO(2) was counterintuitive, and the material displayed surprising photomagnetic properties in the solid state. Z-NO(2) occupied the LS state at low temperatures and underwent thermal spin crossover (SCO) with a T(1/2) of about 270 K. The photoirradiated Z-NO(2) displayed a higher value of χ(M)T and the modulation of χ(M)T exceeded that of Z-H or Z-CN. Z-NO(2)·acetone, in which acetone molecules were incorporated into the crystal lattice, further stabilized the LS state (T(1/2) > 300 K), thereby promoting large modulations of the χ(M)T values (87% at 273 K and 64% at 300 K) upon Z-to-E photoisomerization. Single crystal X-ray structure analysis revealed that structural factors played a vital role in the photomagnetic properties in the solid state. Z-H and Z-CN favored intermolecular π-π stacking among the ligand molecules. The coordination sphere around the Fe(II) nucleus was distorted, which stabilized the HS state. In contrast, Z-NO(2)·acetone was liberated from such intermolecular π-π stacking and coordination distortion, resulting in the stabilization of the LS state.

Light-induced spin change by photodissociable external ligands: a new principle for magnetic switching of molecules

Magnetic bistability in spin-crossover materials generally is a collective phenomenon that arises from the cooperative interaction of a large number of microscopic magnetic moments within the crystal lattice in the solid state. We now report on individual molecules in homogeneous solution that are switched between the diamagnetic and paramagnetic states at room temperature by light-driven coordination-induced spin-state switching (LD-CISSS). Switching of the coordination number (and concurrently of the spin state) was achieved by using Ni-porphyrin as a square-planar platform and azopyridines as photodissociable axial ligands. The square-planar Ni-porphyrin is diamagnetic (low-spin, S = 0), and all complexes with axial ligands are paramagnetic (high-spin, S = 1). Association constants were determined for all conceivable 1:1 and 1:2 porphyrin/azopyridine complexes. The binding constants of the trans azopyridines are larger than those of the corresponding cis isomers. Thus, upon irradiation with UV light (365 nm, trans → cis) and visible light (455 nm, cis → trans), switching of the magnetic properties was achieved. Upon substitution of the azopyridines at the 4- and 4'-positions with larger substituents, the difference in trans and cis association constants, and thus the switching efficiency, was increased. A photoinduced, reversible switching between 20 and 68% paramagnetic Ni species in solution was achieved with isopropyl substituents at room temperature.

Photomagnetism in clusters and extended molecule-based magnets

Photomagnetism in molecular systems is a new development in molecular magnetism. It traces back to 1982 and 1984 when a transient effect and then the light-induced excited-spin-state-trapping effect was discovered in spin-crossover complexes. The present contribution gives a definition of the phenomenon, a process that changes the magnetism of a (molecular) system after absorption of a photon. It is limited to the discussion of photomagnetism based on metal-metal electron transfer in clusters and extended molecule-based magnets. The paper is organized around the main pairs of spin bearers, which allowed us to evidence and to study the phenomenon: Cu-Mo, Co-Fe, and Co-W. For each metallic pair, we report and discuss the conditions of appearance of the effect and its characteristics, both in extended structures and in molecular units: structure, spectroscopy, magnetism, thermodynamics and kinetics, and applications. We conclude with some brief prospects. The field is in rapid expansion. We are convinced that the interaction of photons with magnetized matter, to provide original magnetic properties, will meet more and more interest in the future.

Control of Magnetic Properties through External Stimuli

The magnetic properties of many magnetic materials can be controlled by external stimuli. The principal focus here is on the thermal, photochemical, electrochemical, and chemical control of phase transitions that involve changes in magnetization. The molecular compounds described herein range from metal complexes, through pure organic compounds to composite materials. Most of the Review is devoted to the properties of valence-tautomeric compounds, molecular magnets, and spin-crossover complexes, which could find future application in memory devices or optical switches.