Ligand-Driven Light-Induced Spin Change Activity and Bidirectional Photomagnetism of Styrylpyridine Iron(II) Complexes in Polymeric Media

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.

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.

A Mononuclear Iron(II) Spin-Crossover Molecule Decorated by Photochromic Azobenzene Group

Aiming at constructing photoresponsive spin crossover (SCO) behavior, herein we designed a new ligand Abtz (Abtz = (E)-N-(4-((E)-phenyldiazenyl)phenyl)-1-(thiazol-4-yl)methanimine) which was decorated by a photochromic azobenzene group. Based on this photochromic ligand, a mononuclear Fe(II) SCO molecule [Fe(Abtz)₃](BF₄)₂·(EAC)₂ (1, EAC = ethyl acetate) was successfully synthesized and showed a complete one-step SCO behavior. Under continuous UV light and blue-light exposure, the cis–trans photoisomerization of both ligand Abtz and compound 1 in the liquid phase was confirmed through UV–Vis spectra. Moreover, the ¹H-NMR spectra of Abtz reveal a trans–cis conversion ratio of 37%. Although the UV–Vis spectra reveal the photochromic behavior for 1 in the solution phase, the SCO behavior in the liquid state is absent according to the variable-temperature Evans method, suggesting the possible decomposition. Moreover, in the solid state, the cis–trans photoisomerization of both Abtz and 1 was not observed, due to the steric hindrance.

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...

Photoisomerization of Linear and Stacked Isomers of a Charged Styryl Dye: A Tandem Ion Mobility Study

The photoisomerization behavior of styryl 9M, a common dye used in material sciences, is investigated using tandem ion mobility spectrometry (IMS) coupled with laser spectroscopy. Styryl 9M has two alkene linkages, potentially allowing for four geometric isomers. IMS measurements demonstrate that at least three geometric isomers are generated using electrospray ionization with the most abundant forms assigned to a combination of EE (major) and ZE (minor) geometric isomers, which are difficult to distinguish using IMS as they have similar collision cross sections. Two additional but minor isomers are generated by collisional excitation of the electrosprayed styryl 9M ions and are assigned to the EZ and ZZ geometric isomers, with the latter predicted to have a π-stacked configuration. The isomer assignments are supported through calculations of equilibrium structures, collision cross sections, and statistical isomerization rates. Photoexcitation of selected isomers using an IMS-photo-IMS strategy shows that each geometric isomer photoisomerizes following absorption of near-infrared and visible light, with the EE isomer possessing a S₁ ← S₀ electronic transition with a band maximum near 680 nm and shorter wavelength S₂ ← S₀ electronic transition with a band maximum near 430 nm. The study demonstrates the utility of the IMS-photo-IMS strategy for providing fundamental gas-phase photochemical information on molecular systems with multiple isomerizable bonds.

Multiscale Approach of Spin Crossover Materials: A Concept Mixing Russian Dolls and Domino Effects

The spin crossover (SCO) phenomenon corresponds to a modification that originates at the atomic scale. However, the simple consideration of the transformations that occur following the SCO at this scale or in its close vicinity does not allow anyone to truly understand, anticipate and thus take advantage of what happens at the scale of the material, and even less at the device one. As the fruit of years of work and experience on this phenomenon, we formalize here the concept of the multiscale understanding of SCO. Clearly, the deflagration generated by the initial impressive atomic modification on all the physical scales of the solid must be understood in terms of structure-properties relationships that fit together, like Russian dolls, and propagate according to a kind of domino effect. Each scale can both give different and independent consequences from those of the other scales but at the same time can influence those of a larger or smaller scale, the whole being imperatively to take into account. The concept appears well illustrated by the volume modification, always the same at the atomic level but drastically different and adaptable, in amplitude and sense, at any other physical scale. This approach results in a much wider range of potential applications than the atomic level alone initially suggests, including one serious path to shape memory materials.

Spin crossover crystalline materials engineered via single-crystal-to-single-crystal transformations

This highlight illustrates the latest crystalline materials engineered via SCSC transformations, with emphasis on the onset and progress of spin crossover in a crystal control.

Spin-crossover modulation via single-crystal to single-crystal photochemical [2 + 2] reaction in Hofmann-type frameworks

Spin-crossover (SCO) system integrated with photochemical [2 + 2] reaction is firstly reported. The dimerization of azastilbene ligands dramatically modulated SCO properties, providing a new strategy for exploiting photo-responsive SCO materials.

This study reports the first modulation of spin-crossover (SCO) behavior via a photochemical [2 + 2] cycloaddition reaction. Here we construct two no-solvent Fe(ii)–Ag(i) bimetallic Hofmann-type frameworks, [Fe(4-spy)₂{Ag(CN)₂}₂] (1) and [Fe(2,4-bpe)₂{Ag(CN)₂}₂] (2) (4-spy = 4-styrylpyridine, 2,4-bpe = trans-1-(2-pyridyl)-2-(4-pyridyl)ethylene). For 1, the dimerization of 4-spy results in a single-crystal to single-crystal (SCSC) transformation from 2D interdigitated layers to a 3D interpenetrated structure. Additionally, a 3D → 3D structural transformation accompanied with Ag(i)–N bond breaking is achieved via the photochemical cycloaddition reaction of 2,4-bpe in 2. More importantly, both the spin transition temperatures and the SCO character are effectively modulated; thus, this approach provides a new strategy for constructing photo-responsive SCO magnetic materials.

Transport property of ligand-driven light-induced spin-change Fe-based spin crossover complexes

The Fe-based spin-crossover (SCO) complexes, especially the ligand-driven light-induced spin-change (LD-LISC) systems with high spin-transition temperature, are considered as the most promising building blocks for designing molecular spintronic devices due to their bistability between the high-spin (HS) and low-spin (LS) states. Here, we explore the transport properties of Fe(stpy)₄(NCS)₂ LD-LISC SCO complexes with the trans and cis configurations sandwiched between Au electrodes by performing extensive density functional theory calculations combined with the non-equilibrium Green's function method. As for the trans configuration, the current through the molecular junction with the HS state is significantly larger than that of the LS state, which indicates that this Fe-based LD-LISC SCO complex with the trans configuration could act as a molecular switch when the spin transition is triggered by external stimuli. Remarkably, we observe the nearly perfect spin-filtering effect and obvious negative differential resistance feature in the Fe(stpy)₄(NCX)₂ junctions with the trans and cis configurations, which is attributed by the dramatically different electronic structures of two spin channels and the bias-dependent transmission spectra, respectively. These obtained theoretical findings suggest that the examined Fe-based LD-LISC SCO complexes hold great potential in molecular spintronics.

The transport properties of LD-LISC Fe-based SCO complexes with the trans and cis configuration exhibit robust spin-filtering and NDR features, which highlight them for promising applications in molecular devices.

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.

The influence of NCE− (E = S, Se, BH3) ligands on the temperature of spin crossover in a family of iron(ii) mononuclear complexes

The temperature of spin crossover was systematically tuned by replacing the NCE⁻ (E = S, Se, BH₃) co-ligands in a family of mononuclear complexes.

A flexible polymer–nanoparticle hybrid material containing triazole-based Fe(ii) with spin crossover properties for magneto-optical applications

Centimeter-scale materials exhibiting solid state-like spin-crossover properties and easy handling under ambient conditions are obtained that preserve their thermochromic behavior over months.

Molecular isomerism induced Fe(ii) spin state difference based on the tautomerization of the 4(5)-methylimidazole group

Two iron(ii) structural isomers with only one methyl position difference in their ligands exhibit different spin states due to the competition of the electronic effect and steric crowding.

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

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

Bidirectional photoswitching of magnetic properties at room temperature: ligand-driven light-induced valence tautomerism

Valence tautomeric (VT) metal complexes are highly promising bistable molecular compounds for applications as molecular switches in molecular electronics and spintronics. Although VT species can be switched with light, the photoswitching in all reported systems requires very low temperatures (usually below 20 K) because photoinduced states are highly unstable at room temperature. The thermal instability hinders any practical application of these complexes in genuine devices. In this report, for the first time we demonstrate photoswitching of VT species and associated magnetic properties at room temperature. The bidirectional photoswitching in solution is due to cis-trans photoisomerizable 4-styrylpyridine ligands deliberately integrated into cobalt dioxolene molecular complexes. The novel type of photoswitching has been coined Ligand-Driven Light-Induced Valence Tautomerism (LD-LIVT). The photoconversion of VT states of 28% has been achieved in solution at room temperature. The photoinduced states show extraordinary thermal stability for hours at room temperature, as compared to common nanoseconds reported previously. The switching proceeds at molecular level with the effective photoswitching rate of 3 × 1013 molecules per s under our conditions. Consequently, this work may open new horizons in applications of molecular switches based on VT metal complexes in molecular devices functioning at room temperature.

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.

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.

Molecular spin crossover phenomenon: recent achievements and prospects

Recently we assisted a strong renewed interest in the fascinating field of molecular spin crossover complexes by (1) the emergence of nanosized spin crossover materials through direct synthesis of coordination nanoparticles and nanopatterned thin films as well as by (2) the use of novel sophisticated high spatial and temporal resolution experimental techniques and theoretical approaches for the study of spatiotemporal phenomena in cooperative spin crossover systems. Besides generating new fundamental knowledge on size-reduction effects and the dynamics of the spin crossover phenomenon, this research aims also at the development of practical applications such as sensor, display, information storage and nanophotonic devices. In this critical review, we discuss recent work in the field of molecule-based spin crossover materials with a special focus on these emerging issues, including chemical synthesis, physical properties and theoretical aspects as well (223 references).

Magnetic bistability of molecules in homogeneous solution at room temperature

Magnetic bistability, as manifested in the magnetization of ferromagnetic materials or spin crossover in transition metal complexes, has essentially been restricted to either bulk materials or to very low temperatures. We now present a molecular spin switch that is bistable at room temperature in homogeneous solution. Irradiation of a carefully designed nickel complex with blue-green light (500 nanometers) induces coordination of a tethered pyridine ligand and concomitant electronic rearrangement from a diamagnetic to a paramagnetic state in up to 75% of the ensemble. The process is fully reversible on irradiation with violet-blue light (435 nanometers). No fatigue or degradation is observed after several thousand cycles at room temperature under air. Preliminary data show promise for applications in magnetic resonance imaging.