Lanthanide(III) SMMs with cationic and anionic complex fragments formed by a Schiff base: structure, luminescence, magnetic properties and ab initio calculations

The syntheses, structures, luminescence and magnetic properties of a new series of Ln(III) complexes of the formula [Ln(L)(H2O)2(DMF)2][Ln(L)2] (in which H2L is N,N'-ethylaminebis[1-phenyl-3-methyl-4-formylimino-2-pyrazoline-5-one]; Ln(III) - Gd (1), Tb (2), or Dy (3) ions). The crystal structures were determined by single-crystal X-ray diffraction measurements for all the above-mentioned complexes. The crystals of these compounds consist of cationic [Ln(L)(H2O)2(DMF)2]+ and anionic [Ln(L)2] moieties which form a 3D supramolecular architecture by the H-bonds and electrostatic forces. Luminescence emission in the visible range was observed for Tb(III) and Dy(III) compounds upon ligand sensitization, with moderate quantum yields of 3.2% for the Dy complex and 24.2% for the Tb analogue. Moreover the Tb(III) complex demonstrates triboluminescence activity. The dynamic magnetization studies revealed that 1 and 2 demonstrate field-induced magnetic relaxation with effective energy barriers, ΔE|kB = 24 K (for 1) and 85 K (for 2), while the Dy complex 3 exhibits slow relaxation of magnetization in zero field with an activation energy of 256 K.

Bidirectional photomagnetism, exciplex fluorescence and dielectric anomalies in a spin crossover Hofmann-type coordination polymer

Stepped spin crossover (SCO) complexes with three or more spin states have promising applications in high-order data storage, multi-switches and multi-sensors. Further synergy with other functionalities, such as luminescence and dielectric properties, will provide a good chance to develop novel multifunctional SCO materials. Here, a bent pillar ligand and luminescent pyrene guest are integrated into a three-dimensional (3D) Hofmann-type metal-organic framework (MOF) [Fe(dpoda){Au(CN)2}2]·pyrene (dpoda = 2,5-di-(pyridyl)-1,3,4-oxadiazole). The magnetic data show an incomplete and two-step SCO behavior with the sequence of 1 ↔ 1/2 ↔ 1/4. The rare bi-directional light-induced excited spin-state trapping (LIESST) effect and light-induced stepped thermal relaxation after LIESST are observed. The pyrene guests interact with dpoda ligands via offset face-to-face π⋯π interactions to form intermolecular exciplex emissions. The competition between thermal quenching and stepped SCO properties results in a complicated and stepped exciplex fluorescence. Moreover, the stepped dielectric property with higher dielectric permittivity at lower temperature may be related to the more frustrated octahedral distortion parameters in the intermediate spin states. Hence, a 3D Hofmann-type MOF with bent pillar ligands and fluorescent guests illustrates an effective way for the development of multifunctional switching materials.

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.

Synchronously tuning the spin-crossover and fluorescence properties of a two-dimensional Fe(ii) coordination polymer by solvent guests

Synchronously tuning the spin-crossover and fluorescence properties of a two-dimensional Fe(ii) coordination polymer by solvent guests.

Synergic Bistability between Spin Transition and Fluorescence in Polyfluorene Composites with Spin Crossover Polymers

In recent years, several examples of materials combining the molecular bistability of spin crossover (SC) and fluorescent moieties have flourished in the literature. Fluorescence is a sensitive probe, and SC may provide modulation of the signal, thus affording systems in which physicochemical changes in the environment of the SC centers could be effectively detected. On the contrary, organic semiconductor polymers are of great interest and, furthermore, have been successfully applied in different optoelectronic devices, such as transistors, solar cells, and light-emitting devices. Herein, we report on the fabrication of composites comprising a fluorescent, organic semiconductor polymer (polyfluorene) and a spin crossover compound, an Fe(II)-triazole coordination polymer. A strong synergy was observed between the spin transition of the Fe(II) compound and variations in the fluorescence of the organic polymer. The fluorescence modulation was shown to be reversible, with an increase of ≤20% with respect to the original value.

Ferroelectric and Spin Crossover Behavior in a Cobalt(II) Compound Induced by Polar-Ligand-Substituent Motion

Ferroelectric spin crossover (SCO) behavior is demonstrated to occur in the cobalt(II) complex, [Co(FPh-terpy)₂ ](BPh₄ )₂ ⋅3ac (1⋅3 ac; FPh-terpy=4'-((3-fluorophenyl)ethynyl)-2,2':6',2''-terpyridine) and is dependent on the degree of 180° flip-flop motion of the ligand's polar fluorophenyl ring. Single crystal X-ray structures at several temperatures confirmed the flip-flop motion of fluorobenzene ring and also gave evidence for the SCO behavior with the latter behavior also confirmed by magnetic susceptibility measurements. The molecular motion of the fluorobenzene ring was also revealed using solid-state ¹⁹ F NMR spectroscopy. Thus the SCO behavior is accompanied by the flip-flop motion of the fluorobenzene ring, leading to destabilization of the low spin cobalt(II) state; with the magnitude of rotation able to be controlled by an electric field. This first example of spin-state conversion being dependent on the molecular motion of a ligand-appended fluorobenzene ring in a SCO cobalt(II) compound provides new insight for the design of a new category of molecule-based magnetoelectric materials.

Controlling dynamic magnetic properties of coordination clusters via switchable electronic configuration

Large-sized coordination clusters have emerged as a new class of molecular materials in which many metal atoms and organic ligands are integrated to synergize their properties. As dynamic magnetic materials, such a combination of multiple components functioning as responsive units has many advantages over monometallic systems due to the synergy between constituent components. Understanding the nature of dynamic magnetism at an atomic level is crucial for realizing the desired properties, designing responsive molecular nanomagnets, and ultimately unlocking the full potential of these nanomagnets for practical applications. Therefore, this review article highlights the recent development of large-sized coordination clusters with dynamic magnetic properties. These dynamic properties can be associated with spin transition, electron transfer, and valence fluctuation through their switchable electronic configurations. Subsequently, the article also highlights specialized characterization techniques with different timescales for supporting switching mechanisms, chemistry, and properties. Afterward, we present an overview of coordination clusters (such as cyanide-bridged and non-cyanide assemblies) with dynamic magnetic properties, namely, spin transition and electron transfer in magnetically bistable systems and mixed-valence complexes. In particular, the response mechanisms of coordination clusters are highlighted using representative examples with similar transition principles to gain insights into spin state and mixed-valence chemistry. In conclusion, we present possible solutions to challenges related to dynamic magnetic clusters and potential opportunities for a wide range of intelligent next-generation devices.

Interplay between spin crossover and proton migration along short strong hydrogen bonds

The iron(ii) salt [Fe(bpp)2](isonicNO)2·HisonicNO·5H2O (1) (bpp = 2,6-bis(pyrazol-3-yl)pyridine; isonicNO = isonicotinate N-oxide anion) undergoes a partial spin crossover (SCO) with symmetry breaking at T 1 = 167 K to a mixed-spin phase (50% high-spin (HS), 50% low-spin (LS)) that is metastable below T 2 = 116 K. Annealing the compound at lower temperatures results in a 100% LS phase that differs from the initial HS phase in the formation of a hydrogen bond (HB) between two water molecules (O4W and O5W) of crystallisation. Neutron crystallography experiments have also evidenced a proton displacement inside a short strong hydrogen bond (SSHB) between two isonicNO anions. Both phenomena can also be detected in the mixed-spin phase. 1 undergoes a light-induced excited-state spin trapping (LIESST) of the 100% HS phase, with breaking of the O4W⋯O5W HB and the onset of proton static disorder in the SSHB, indicating the presence of a light-induced activation energy barrier for proton motion. This excited state shows a stepped relaxation at T 1(LIESST) = 68 K and T 2(LIESST) = 76 K. Photocrystallography measurements after the first relaxation step reveal a single Fe site with an intermediate geometry, resulting from the random distribution of the HS and LS sites throughout the lattice.

Homochiral versus racemic polymorphs of spin-crossover iron(ii) complexes with reversible LIESST effect

Homochiral and racemic polymorphs show different spin-crossover behaviours due to different intermolecular interactions, and reversible LIESST effects can be realized on homochiral complexes.

Interplay between spin-crossover and luminescence in a multifunctional single crystal iron(ii) complex: towards a new generation of molecular sensors

We present a new example of a mononuclear iron(ii) complex exhibiting a correlated spin-crossover (SCO) transition and strong fluorescence, whose coordination sphere is saturated, for the first time, by six phosphorescent ligands. The interplay between SCO and light emission properties in the thermal region of the spin transition was investigated by means of magnetic, fluorescence, optical absorption and optical microscopy measurements on a single crystal. Overall, the results show an excellent correlation between fluorescence and magnetic data of the present gradual transition, indicating an extreme sensitivity of the optical activity of the ligand to the spin state of the active iron(ii) ions. These results open the way for conceiving new prototypes of pressure and temperature sensors based on this synergy between SCO and luminescence properties. In particular, the fact that cooperative SCO material is not a prerequisite for obtaining such synergetic effects, is useful for the design of thin films or nanoparticles, in which the cooperativity is reduced, for appropriate implementation in nanosized devices to enhance the sensing properties at the nanoscale.

Manipulating Spin Transition To Achieve Switchable Multifunctions

The spin transition of metal ions involves interconversion between electron configurations exhibiting considerably different functions and plays a substantial role in the chemical, physical, and biological fields. The photoinduced spin transition offers a promising approach to tune various physical properties with high spatial and temporal resolutions for producing smart multifunctional materials not only to explore their basic science but also to satisfy the demands of the next-generation photoswitchable-molecule-based devices. Therefore, it is attracting considerable interest to utilize photoinduced spin transition to simultaneously tune multifunctions. However, two issues are challenging in obtaining reversible and swift manipulation of functions: (1) the interconversion between different electron configurations of photoresponsive units should be reversibly switched via photoinduced spin transition; (2) effective coupling should be built between the photoresponsive and functional units to produce photoswitchable functions utilizing photoinduced spin transition. In this Account, we will review our recent advances in the usage of spin transition of metal ions as actuators for tuning the magnetic, dielectric, fluorescence, and mechanical properties, wherein the role of a photoswitchable spin transition is highlighted. We mainly focus on the study of two spin-transition categories, including spin-crossover (SCO) of one metal ion and metal-to-metal charge transfer (MMCT). Initially, we will describe a strategy for developing photoinduced reversible SCO and MMCT. The role of flexible intermolecular interactions, in particular, π···π interactions, is discussed with respect to a photoinduced reversible MMCT. Then, the SCO and MMCT units were assembled using metallocyanate building blocks to form a chain, wherein the spin states, anisotropy, and magnetic coupling interactions can be photoswitched to tune the single-chain magnet behavior. Besides magnetic properties, the photoinduced spin transition that is associated with the concomitant changing of charge distribution, bond lengths, and absorption spectra can be utilized to tune the multifunctions. Therefore, the transfer of an electron from a central cobalt site to one of the two iron sites in linear trinuclear Fe₂Co compounds resulted in the transformation of a centrosymmetric nonpolar molecule into an asymmetric polar molecule, and the molecular electric dipole and dielectric properties can be reversibly switched. Moreover, the spin transition usually involved significant expansion or contraction of the coordination sphere of metal ions because of the population/depopulation of the antibonding e_g orbitals. Therefore, colossal positive and negative thermal expansion behaviors were achieved in a layered compound by manipulating the spin-transition process and the rotation of the functional units, thereby providing a strategy for synthesizing phototunable nanomotors. Photoinduced spin transition can also be used to modulate the fluorescence properties by controlling the energy transfer between the fluorescent ligands and the metal sites showing SCO. Finally, we will provide a perspective and detail the remaining challenges that are associated with this research area. We believe that an increasing number of fascinating photoswitchable SCO and MMCT systems will emerge in the near future and that the materials exhibiting various properties and functions that can be manipulated using photoinduced spin transition will provide novel opportunities for the development of smart multifunctional materials and devices.

A Known Iron(II) Complex in Different Nanosized Particles: Variable-Temperature Raman Study of Its Spin-Crossover Behavior

The spin-crossover (SCO) polymorph B (complex 1) of the known compound [Fe^II{N(CN)₂}₂(abpt)₂], where abpt is 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole, has been prepared in three different particle sizes averaging ∼300 (sample 1a), ∼80 (sample 1b), and ∼20 nm (sample 1c). Two independent octahedral molecules possessing Fe1 and Fe2 were found to be present in the crystal of B. Magnetostructural relationships had established that at room temperature both Fe^II sites are in the high-spin state (HS-HS), whereas a decrease in the temperature to 90 K induces the complete high-spin to low-spin conversion of the Fe1 site, with Fe2 remaining in the high-spin state (LS-HS). The three samples have been characterized by elemental analyses, ATR spectra, solution UV/vis spectra (to exclude resonance Raman effects) and powder X-ray diffraction patterns, while their morphological characteristics have been examined by scanning electron microscopy (SEM). The SCO behavior of the originally prepared sample 1a has been monitored in detail by variable-temperature Raman studies in the 300-80 K range using mainly low-frequency ν(Fe-N) and δ(NFeN) modes and the ν(C≡N) mode of the axial dicyanamido groups as spin-sensitive vibrations. The new peaks that appear in the low-temperature Raman spectra of the LS-HS form of the complex are reproduced in the calculated spectrum of the LS state of [Fe^II{N(CN)₂}₂(abpt)₂]. The influence of the average particle size on the SCO properties of 1 has also been studied by variable-temperature Raman spectra. The studies indicate that, during the HS-HS → LS-HS transition, the latter form of the complex appears at higher temperatures for the smaller particles; the T_1/2 shift accomplished by manipulating the particle size within a range of roughly 1 order of magnitude (300-20 nm) may be as high as ∼30 K. The SCO features of 1, as deduced from the Raman study, are in excellent agreement with those derived from a traditional variable-temperature magnetic susceptibility study, indicating the utility of the former.

Deciphering the Influence of Meridional versus Facial Isomers in Spin Crossover Complexes

Chelate coordination of non-symmetrical didentate pyrazine-benzimidazole (L1) or pyridine-benzimidazole (L2) N-donor ligands around divalent iron in acetonitrile produces stable homoleptic triple-helical spin crossover [Fe(Lk)₃ ]²⁺ complexes existing as mixtures of meridional (C₁ -symmetry) and facial (C₃ -symmetry) isomers in slow exchange on the NMR timescale. The speciation deviates from the expected statistical ratio mer/fac=3:1, a trend assigned to the thermodynamic trans-influence, combined with solvation effects. Consequently, the observed spin state Fe^II _low-spin ↔Fe^II _high-spin equilibria occurring in [Fe(Lk)₃ ]²⁺ refer to mixtures of complexes in solution, an issue usually not considered in this field, but which limits rational structure-properties correlations. Taking advantage of the selective and quantitative formation of isostructural facial isomers in non-constrained related spin crossover d-f helicates (HHH)-[LnFe(Lk)₃ ]⁵⁺ (Ln is a trivalent lanthanide, Lk=L5, L6), we propose a novel strategy for assigning pertinent thermodynamic driving forces to each spin crossover triple-helical isomer. The different enthalpic contributions to the spin state equilibrium found in mer-[Fe(Lk)₃ ]²⁺ and fac-[Fe(Lk)₃ ]²⁺ reflect the Fe-N bond strengths dictated by the trans-influence, whereas a concomitant solvent-based entropic contribution reinforces the latter effect and results in systematic shifts of the spin crossover transitions toward higher temperature in the facial isomers.

Spin States of Homochiral and Heterochiral Isomers of [Fe(PyBox)2 ]2+ Derivatives

The following iron(II) complexes of 2,6-bis(oxazolinyl)pyridine (PyBox; L^H ) derivatives are reported: [Fe(L^H )₂ ][ClO₄ ]₂ (1); [Fe((R)-L^Me )₂ ][ClO₄ ]₂ ((R)-2; L^Me =2,6-bis{4-methyloxazolinyl}pyridine); [Fe((R)-L^Ph )₂ ][ClO₄ ]₂ ((R)-3) and [Fe((R)-L^Ph )((S)-L^Ph )][ClO₄ ]₂ ((RS)-3; L^Ph =2,6-bis{4-phenyloxazolinyl}pyridine); and [Fe((R)-L^iPr )₂ ][ClO₄ ]₂ ((R)-4) and [Fe((R)-L^iPr )((S)-L^iPr )][ClO₄ ]₂ ((RS)-4; L^iPr =2,6-bis{4-isopropyloxazolinyl}pyridine). Solid (R)-3⋅MeNO₂ exhibits an unusual very gradual, but discontinuous thermal spin-crossover with an approximate T1/2 of 350 K. The discontinuity around 240 K lies well below T1/2 , and is unconnected to a crystallographic phase change occurring at 170 K. Rather, it can be correlated with a gradual ordering of the ligand conformation as the temperature is raised. The other solid compounds either exhibit spin-crossover above room temperature (1 and (RS)-3), or remain high-spin between 5-300 K [(R)-2, (R)-4 and (RS)-4]. Homochiral (R)-3 and (R)-4 exhibit more twisted ligand conformations and coordination geometries than their heterochiral isomers, which can be attributed to steric clashes between ligand substituents [(R)-3]; or, between the isopropyl substituents of one ligand and the backbone of the other ((R)-4). In solution, (RS)-3 retains its structural integrity but (RS)-4 undergoes significant racemization through ligand redistribution by ¹ H NMR. (R)-4 and (RS)-4 remain high-spin in solution, whereas the other compounds all undergo spin-crossover equilibria. Importantly, T1/2 for (R)-3 (244 K) is 34 K lower than for (RS)-3 (278 K) in CD₃ CN, which is the first demonstration of chiral discrimination between metal ion spin states in a molecular complex.

Polymorphism of a chiral iron(ii) complex: spin-crossover and ferroelectric properties

A chiral iron(ii) complex with two polymorphs, one that undergoes a gradual spin crossover, whilst the other remains in a high-spin state but shows a typical ferroelectric feature, is reported.

Iron(iii) complexes of 2-methyl-6-(pyrimidin-2-yl-hydrazonomethyl)-phenol as spin-crossover molecular materials

Four new mononuclear Fe(iii) complexes are obtained by using 2-methyl-6-(pyrimidin-2-yl-hydrazonomethyl)-phenol. The perchlorate complex displays spin crossover with a hysteresis of 32 K, while the neutral complex exhibits a gradual incomplete spin transition.