Synergic on/off Photoswitching Spin State and Magnetic Coupling between Spin Crossover Centers

A mononuclear iron(II) complex constructed using a complementary ligand pair exhibits intrinsic luminescence-spin-crossover coupling

Molecular materials that exhibit synergistic coupling between luminescence and spin-crossover (SCO) behaviors hold significant promise for applications in molecular sensors and memory devices. However, the rational design and underlying coupling mechanisms remain substantial challenges in this field. In this study, we utilized a luminescent complementary ligand pair as an intramolecular luminophore to construct a new Fe-based SCO complex, namely [FeL1L2](BF4)2·H2O (1-Fe, L1 is a 2,2':6',2''-terpyridine (TPY) derivative ligand and L2 is 2,6-di-1H-pyrazol-1-yl-4-pyridinecarboxylic acid), and two isomorphic analogs (2-Co, [CoL1L2](BF4)2·H2O and 3-Zn, [ZnL1L2](BF4)2·H2O). Magnetic studies reveal that 1-Fe exhibits thermally induced SCO within the temperature range of 150-350 K. Variable-temperature fluorescence emission spectral analysis of the three complexes confirmed the occurrence of SCO-luminescence coupling in 1-Fe. Furthermore, variable-temperature UV-vis absorption spectra and time-dependent density functional theory (TD-DFT) calculations elucidate the intramolecular luminescence emission behavior, highlighting the critical role of charge transfer processes between the L1 ligand and FeII ions with different spin states. Our research presents a novel construction strategy for synthesizing synergistic SCO-luminescent materials and contributes to the understanding of the mechanisms underlying SCO-luminescence coupling.

Effects of mono- or di-fluoro-substitution on spin crossover behavior in a pair of Schiff base-like FeII-coordination polymers

Spin crossover (SCO) has long been a hot topic in the field of molecular magnetism owing to its unique bistability character. Rational control of thermal hysteresis and transition temperature (T1/2) is crucial for their practical applications, which rely on precise manipulation of the substituents of SCO coordinating ligands and molecular packing interactions. In this study, we designed two different bridging ligands (2-FDPB: 4,4'-(2-fluoro-1,4-phenylene)dipyridine; 2,3-FDPB: 4,4'-(2,3-difluoro-1,4-phenylene)dipyridine) featuring one and two fluoro substitution on the central benzene ring and applied a Schiff base-like equatorial tetradentate ligand {diethyl(E,E)-2,2'-[4,5-difluoro-1,2-phenyl-bis(iminomethylidyne)]bis(3-oxobutanoate)-(2-)-N,N',O3,O3'} (H2L) to coordinate with the FeII ion. Two FeII-coordination chain polymers [FeII(L)(2,3-FDPB)]·0.25CH2Cl2 (1) and [FeII(L)(2-FDPB)]·0.5CH3OH (2) were obtained. 1 crystallizes in the monoclinic P21/n space group with only one FeII center, while 2 crystallizes in the triclinic P1̄ space group with two independent FeII centers. Unlike the identical 2D layer stacking in 1, 2 exhibited alternating stacking of the extending 2D layers and meshed chains. Magnetic measurements revealed the typical thermally induced spin crossover behavior (SCO): 1 exhibited a 41 K-wide thermal hysteresis with transition temperatures of T1/2↑ = 245 K and T1/2↓ = 204 K, while 2 showed a higher transition temperature (T1/2 = 330 K) with no thermal hysteresis. Magneto-structural correlation studies suggest that the electron-withdrawing effect present in the fluoro substituents does not have a significant impact on the SCO behaviors. Despite the fluoro substituents having a similar atomic radius of hydrogen atoms, variations in the number of these substituents can alter the crystallization behavior of these complexes, which in turn affects the solvents, molecular stacking patterns, and intermolecular interactions, ultimately influencing the SCO behaviors.

Reversible light-induced spin state switching in a dinuclear Fe(II) spin crossover complex

A dinuclear Fe(II) spin crossover (SCO) complex with the formula [Fe2L5(NCS)4]·2DMF·2H2O (1) was synthesised from 1-naphthylimino-1,2,4-triazole (L). Complex 1 exhibits an incomplete thermally induced spin transition with a transition temperature T1/2 of 95 K and a thermally trapped metastable high-spin state at low temperatures. Furthermore, it undergoes a reversible light-induced spin crossover by alternate irradiation with 532 and 808 nm lasers.

N-N-Bridged Polynuclear POM-Based Coordination Polymers Based on a V-Type Ligand: Proton Conduction and Magnetism

The construction of polyoxometalate (POM)-based coordination polymers, in the presence of a nitrogen heterocyclic ligand, is intriguing due to the potential for obtaining diverse structures. These structures exhibit extensive application possibilities in the fields of proton conductivity and magnetism. Herein, four new POM-based polynuclear coordination polymers with the formulas of {[Fe2(btb)3(H2O)2(SiW12O40)]·3H2O}n (1), {[Cd2(btb)2(H2O)6(HPMoVI10MoV2O40)]·2H2O}n (2), {[Co3(OH)2(btb)2(H2O)5(HPMoVI10MoV2O40)]·7H2O}n (3), and {[Cu3(OH)(btb)2(H2O)(HP2Mo5O23)]·6H2O}n (4) have been prepared using the V-type 1,3-bis(4H-1,2,4-triazole-4-yl)benzene (btb) ligand. Compounds 1 and 2 feature similar two-dimensional (2D) structures, derived from the binuclear Fe2N6 and Cd2N4 subunits connected by tridentate btb ligands. Meanwhile, in compound 3, hexanuclear Co6(OH)4 units are bound by quadridentate btb ligands forming a 2D layer with the same 4-c sql topology simplification as compounds 1 and 2. In compound 1, Keggin-type polyoxoanions are monodentate-coordinated to metal ions and suspended on the 2D structure, while, in compounds 2 and 3, they act as discrete counterions residing in the interstitial spaces between two adjacent layers, thereby extending the 2D structures into 3D structures through hydrogen bonding interactions. In compound 4, trinuclear Cu3(OH) subunits are further constructed into a 3D framework through cooperation with four tridentate and quadridentate btb ligands as well as Strandberg-type anions. Furthermore, the proton conduction of the four compounds has been investigated. They display high proton conductivities at 358 K and 98% RH with powdered samples, which are 1.26 × 10-3, 1.24 × 10-3, 3.24 × 10-4, and 2.57 × 10-4 S cm-1, respectively. Interestingly, by mixing with Nafion, the composite membranes of compounds 2 and 4 exhibit enhanced proton conductivities, measuring at 4.87 × 10-2 and 1.28 × 10-2 S cm-1, respectively, at 358 K and 98% RH, which suggests excellent potential for applications. In addition, compounds 1, 3, and 4 display antiferromagnetic behaviors due to similar magnetic interactions. This work can provide research insights into the assembly of 2D POM-based coordination polymers with nitrogen heterocyclic ligands and Keggin-type POMs and further promote their research progress in proton conduction.

Magnetic couplings and applied electric field regulation in diradical SiC defect diamond-like nanoclusters

We computationally characterize the diradical characters and explore the magnetic spin coupling characteristics of SiC defect diamond-like nanoclusters and their regulation by applied electric field.

Antagonist elastic interactions tuning spin crossover and LIESST behaviours in FeII trinuclear-based one-dimensional chains

A new 1-D spin SCO coordination polymer based on FeII trinuclear units covalently linked by a flexible coligand has been reported as an unusual platform and model system for experimental study on the origin of the step-like feature in 1-D systems.

Enhanced dielectricity coupled to spin-crossover in a one-dimensional polymer iron(ii) incorporating tetrathiafulvalene

A concerted bending–flattening motion of the redox-active TTF within constructed one-dimensional Fe^II–TTF–Schiff-base chain with bridging 4,4′-bpy enhances the dielectric constant coupled to its spin-crossover transition above room temperature.

In designing multifunctional materials for potential switches that can be used as memory devices, the high-spin (HS) to low-spin (LS) crossover (SCO) one-dimensional polymer, [Fe^II(L)(4,4′-bpy)]_n, was constructed from a designed redox-active tetrathiafulvalene (TTF) functionalized Schiff-base and the ditopic linker 4,4′-bipyridine (bpy). It exhibits an 8 K hysteretic SCO centred at T_1/2 = 325 K which is coupled to changes in its dielectric constant. The crystal structures above and below the transition temperature reveal similar parallel linear ···Fe–bpy–Fe–bpy··· chains displaying expansion of the Fe^II octahedron in the HS state. Density functional theory (DFT) calculations reveal a concerted electronic charge and spin change represented by the Mülliken charge of the Fe and the magnitude and direction of the dipole moment which substantiate the experimental observations.

Tunable microwave absorption of switchable complexes operating near room temperature

Materials that are able to switch microwave radiation are strongly desired for their potential applications in electronic devices. In this paper, we show the spin-dependant interaction of spin-crossover materials with microwave radiation, namely, the ability of coordination compounds [Fe(NH₂trz)₃]Br₂ and [Fe(NH₂trz)₃](NO₃)₂ that undergo a cooperative spin transition between low-spin and high-spin states to operate as thermoswitchable microwave absorbers. The characteristics of the microwave reflection and transmission of these spin-crossover complexes were investigated at variable temperatures. The evolution of both the transmission and reflection spectra in the 26–37 GHz frequency band within the temperature range of spin crossover showed significant differences in the interaction of microwave radiation with the high-spin and low-spin forms of the compounds. The microwave transmission coefficient shows a notable decrease upon transition to the high-spin state, while the reflection coefficient can be both increased or decreased on the characteristic frequencies during the spin transition. The different microwave absorbing properties of the low-spin and high-spin forms are found to be associated with a notable microwave permittivity change upon spin crossover. The switchable microwave reflection/transmission correlates well with the transition characteristics found in the optical and differential scanning calorimetry measurements. These results widen the spectroscopic range in which spin-crossover materials can be applied and contribute to the creation of a preliminary database of the microwave absorbing properties of spin-crossover complexes.

Iron(ii) spin-crossover complexes are shown to be effective microwave switches operating near room temprature.

Improving the Light-Induced Spin Transition Efficiency in Ni(II)-Based Macrocyclic-Ligand Complexes

The structural stability and photoabsorption properties of Ni(II)-based metal-organic complexes with octahedral coordination having different planar ligand ring structures were investigated employing density functional theory (DFT) and its time-dependent extension (TD-DFT) considering the M06 exchange-correlation functional and the Def2-TZVP basis set. The results showed that the molecular composition of different planar cyclic ligand structures had significant influences on the structural stability and photoabsorption properties of metal-organic complexes. Only those planar ligands that contained aromatic rings met the basic criteria (thermal stability, structural reversibility, and appropriate excitation frequency domain) for light-induced excited spin state trapping, but their spin transition efficiencies were very different. While, in all three aromatic cases, the singlet electronic excitations induced charge distribution that could help in the singlet-to-triplet spin transition, and triplet excitations, which could assist in the backward (triplet-to-singlet) spin transition, was found only for one complex.

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.

Photo-switched magnetic coupling in spin-crossover complexes

This feature article summarizes the recent progress in the magnetically coupled spin-crossover (SCO) complexes. The photo-switched molecular nanomagnet property, long range magnetic ordering, and the perspectives of SCO complexes are also presented.

Spin crossover and photomagnetic behaviors in one-dimensional looped coordination polymers

Thermal- and light-induced SCO behaviors have been studied on two one-dimensional looped coordination polymers.

Abrupt Spin Crossover Behavior in a Linear N1,N2-Triazole Bridged Trinuclear Fe(II) Complex

The synthesis, structures and magnetic properties of a new trinuclear spin crossover complex [FeII3(pyrtrz)6(TsO)6]·10H2O·2CH3OH (C2) and its analogue binuclear [FeII2(pyrtrz)5(SCN)4]·7H2O (C1), are reported here. These two compounds are synthesized based on the pyrrolyl functionalized Schiff base 1,2,4-triazole ligand 4-((1H-pyrrol-2-yl)methylene-amino)-4H-1,2,4-triazole (pyrtrz), which represent rare discrete multi-nuclear species, with µ2-N1,N2-triazole bridges linking the FeII centers. DC magnetic susceptibility measurements revealed an abrupt single-step spin crossover (SCO) behavior for compound 2 on the central FeII site and single-crystal X-ray diffraction (173 K) showed that this compound crystallizes in the monoclinic space group (P21/c), and multiple intramolecular interactions were found responsible for the abrupt transition. Compound 1 is a binuclear complex with thiocyanate as terminal ligands. This compound stays in high spin state over the whole temperature range and displays weak antiferromagnetic exchange coupling.

Fluorescence modulation via photoinduced spin crossover switched energy transfer from fluorophores to FeII ions

Molecular materials possessing phototunable fluorescence properties have attracted great interest owing to their potential applications in optical switches and storage. However, most fluorescence modulation is realized through light-responsive structural isomerization in solution. It is a formidable challenge to achieve phototunable fluorescence emission with high fatigue resistance and a fast response rate in the solid state for the development of devices. Here, a mononuclear compound was constructed via the coordination of fluorophores with Fe^II ions, whose electronic configuration changed from low spin to high spin upon light irradiation. The photoinduced spin crossover of Fe^II ions was accompanied by a 20% increase in the fluorescence emission intensity. A temperature-dependent spectroscopic study together with time-dependent density functional theory calculations revealed that the effective spectral overlap between the emission of the fluorophores and the absorption band of the Fe^II ions differed between the low spin and high spin states. The photoinduced spin crossover switched the energy transfer from the fluorophore to the Fe^II ion, resulting in fluorescence modulation. The presented results provide a novel approach for developing optical memory and sensors via electron rearrangement of photoinduced spin crossover.

Spin crossover in discrete polynuclear iron(ii) complexes

A comprehensive review of 127 dinuclear to octanuclear complexes, mostly 2012-present, reveals key design features and future directions for spin crossover active supramolecular assemblies.