Spin transition molecular materials: intelligent contrast agents for magnetic resonance imaging

Spin Crossover Quenching by "Racemization" in a Family of trans-1,2-Di(tetrazol-1-yl)cyclopentane-Based Fe(II) 1D Coordination Polymers

Optically pure (RR)- and racemic (RR/SS)-trans-1,2-di(tetrazol-1-yl)cyclopentane were synthesized and used to prepare homo- and heterochiral Fe(II) coordination compounds. [Fe((RR/SS)-C7H10N8)2(CH3CN)2](BF4)2 (1A), [Fe((RR/SS)-C7H10N8)2(C2H5CN)2](BF4)2 (2A), [Fe((RR)-C7H10N8)2(CH3CN)2](BF4)2·2CH3CN (1B·solv), and [Fe((RR)-C7H10N8)2(C2H5CN)2](BF4)2 (2B) form a family of one-dimensional coordination polymers. Fe(II) cations in these complexes are characterized by a heteroleptic coordination environment: the neighboring metal centers are bridged by two 1,2-di(tetrazol-1-yl)cyclopentane molecules, while the nitrile molecules (acetonitrile or propionitrile, respectively) occupy the axial positions. Independently of the kind of nitrile coligands, an ability to thermally induce spin crossover (SCO) is governed by chirality. 1B·solv and 2B exhibit abrupt and complete SCO occurring at T1/2 = 144 K and T1/2 = 228 K, respectively. Desolvated form, 1B (of the same stoichiometry as 2B), also exhibits SCO (T1/2 = 215 K). In contrast, an exchange within the polymeric chain of half of the RR molecules with the SS enantiomeric form results in formation of 1A and 2A, which remain in stable high-spin (HS) form down to 10 K. It has been shown that moving from a homochiral to a heterochiral system changes the structure of the polymeric unit (while maintaining the same polymer dimensionality and bridging fashion) that leads to the deep reorganization of the further coordination spheres, including the anion network.

Spin crossover iron complexes with spin transition near room temperature based on nitrogen ligands containing aromatic rings: from molecular design to functional devices

During last decades, significant advances have been made in iron-based spin crossover (SCO) complexes, with a particular emphasis on achieving reversible and reproducible thermal hysteresis at room temperature (RT). This pursuit represents a pivotal goal within the field of molecular magnetism, aiming to create molecular devices capable of operating in ambient conditions. Here, we summarize the recent progress of iron complexes with spin transition near RT based on nitrogen ligands containing aromatic rings from molecular design to functional devices. Specifically, we discuss the various factors, including supramolecular interactions, crystal packing, guest molecules and pressure effects, that could influence its cooperativity and the spin transition temperature. Furthermore, the most recent advances in their implementation as mechanical actuators, switching/memories, sensors, and other devices, have been introduced as well. Finally, we give a perspective on current challenges and future directions in SCO community.

Spin crossover {[Fe(atrz)3](OTs)2}n monolith: a green synthesis approach for Robust switchable materials

This work presents a straightforward, room-temperature synthesis of a robust {[Fe(atrz)3](OTs)2}n monolith. This approach offers a green alternative to traditional nanoparticle synthesis for manipulating spin crossover (SCO) behaviour. The monolith exhibits a more gradual SCO transition at lower temperatures compared to the bulk material, aligning with observations in smaller particle systems. Notably, the synthesis employs a solvent- and surfactant-free approach, simplifying the process and potentially reducing environmental impact, aligning with the principles of green chemistry.

Extremely Slow Thermally-Induced Spin Crossover in the Two-Dimensional Network [Fe(bbtr)3 ](BF4 )2

Cooling [Fe(bbtr)3 ](BF4 )2 (bbtr=1,4-di(1,2,3-triazol-1-yl)butane) triggers very slow spin crossover below 80 K (T1/2 ↓ =76 K). The spin crossover (SCO) is accompanied by a hysteresis loop (T1/2 ↑ =89 K). In contrast to isostructural perchlorate analogue [Fe(bbtr)3 ](ClO4 )2 in which spin crossover during cooling is preceded by phase transition at TPT =126 K in tetrafluoroborate phase transition does not occur to the beginning of spin crossover (80 K). Studies of mixed crystals [Fe(bbtr)3 ](BF4 )2(1-x) (ClO4 )2x (0.5≤x≤0.9) showed that a phase transition precedes spin crossover, however, for x≅0.46 intersection of T1/2 (x) and TPT (x) dependencies takes place. The application of pressure of 1 GPa shifts the spin crossover in [Fe(bbtr)3 ](BF4 )2 to a temperature above 270 K. High-pressure studies of neat tetrafluoroborate and perchlorate, as well as mixed crystals [Fe(bbtr)3 ](BF4 )2(1-x) (ClO4 )2x (0.1≤x≤0.9), revealed that at 295 K P1/2 value changes linearly with x indicating similar mechanism of spin crossover under elevated pressure in all systems under investigation. Variable pressure single crystal X-ray diffraction studies confirmed that in contrast to thermally induced spin crossover undergoing differently in tetrafluoroborate and perchlorate an application of high pressure removes this differentiation leading to a similar mechanism depending at first on start spin crossover and then P-3→P-1 phase transition occurs. In this report we have shown that 2D coordination polymer [Fe(bbtr)3 ](BF4 )2 (bbtr=1,4-di(1,2,3-triazol-1-yl)butane) treated to date as spin crossover silent shows thermally induced spin crossover phenomenon. Spin crossover in tetrafluoroborate is extremely slow. Determination of the spin crossover curve required carrying measurement in the settle mode-cooling from 85 to 70 K took about 600 h (average velocity of change of temperature ca. 0.0004 K/min).

Transition Temperature for Spin-Crossover Materials with the Mean Value Ensemble Hubbard-U Correction

Calculation of transition temperatures T1/2 for thermally driven spin-crossover in condensed phases is challenging, even with sophisticated state-of-the-art density functional approximations. The first issue is the accuracy of the adiabatic crossover energy difference ΔEHL between the low- and high-spin states of the bistable metal-organic complexes. The other is the proper inclusion of entropic contributions to the Gibbs free energy from the electronic and vibrational degrees of freedom. We discuss the effects of treatments of both contributions upon the calculation of thermochemical properties for a set of 20 spin-crossover materials using a Hubbard-U correction obtained from a reference ensemble spin-state. The U values obtained from a simplest bimolecular representation may overcorrect, somewhat, the ΔEHL values, hence giving somewhat excessive reduction of the T1/2 results with respect to their U = 0 values in the crystalline phase. We discuss the origins of the discrepancies by analyzing different sources of uncertainties. By use of a first-coordination-sphere approximation and the assumption that vibrational contributions from the outermost atoms in a metal-organic complex are similar in both low- and high-spin states, we achieve T1/2 results with the low-cost, widely used PBE generalized gradient density functional approximation comparable to those from the more costly, more sophisticated r2SCAN meta-generalized gradient approximation. The procedure is promising for use in high-throughput materials screening, because it combines rather low computational effort requirements with freedom from user manipulation of parameters.

Interpenetration Phenomena via Anion Template Effects in Fe(II) and Co(II) Coordination Networks with a Bis-(1,2,4-triazole) Ligand

Seven new coordination networks, [Fe(tbbt)3](BF4)2 (1), [Co(tbbt)3](BF4)2 (2), [Fe(tbbt)3](ClO4)2 (3), [Co(tbbt)3](ClO4)2 (4), [Fe(NCS)2(tbbt)2] (5), [Co(NCS)2(tbbt)2] (6), and [Fe(H2O)2(tbbt)2]Br2·2H2O (7), were synthesized with the linker 1,1'-(trans-2-butene-1,4-diyl)bis-1,2,4-triazole (tbbt) and structurally investigated. The structure of complexes 1-4 is composed of three interpenetrating, symmetry-related 3D networks. Each individual 3D network forms a primitive, nearly cubic lattice (pcu) with BF4- or ClO4- anions present in the interstitial spaces. The structure of compounds 5 and 6 is composed of two-dimensional sql layers, which are parallel to each other in the AB stacking type. These layers are interpenetrated by one-dimensional chains, both having the same formula unit, [M(NCS)2(tbbt)2] (M = Fe, Co). The structure of compound 7 consists of parallel, two-dimensional sql layers in the ABCD stacking type. The interpenetration in 1-6 is not controlled by π-π-interactions between the triazole rings or C=C bonds, as could have been expected, but by (triazole)C-H⋯F4B, C-H⋯O4Cl, and C-H⋯SCN anion hydrogen bonds, which suggests a template effect of the respective non-coordinated or coordinated anion for the interpenetration. In 7, the (triazole)C-H⋯Br anion interactions are supplemented by O-H⋯O and O-H⋯Br hydrogen bonds involving the aqua ligand and crystal water molecules. It is evident that the coordinated and non-coordinated anions play an essential role in the formation of the networks and guide the interpenetration. All iron(II) coordination networks are colorless, off-white to yellow-orange, and have the metal ions in the high-spin state down to 77 K. Compound 5 stays in the high spin state even at temperatures down to 10 K.

Study of Spin-Crossover in Iron(II) Complexes with 2,6-Bis(4,5-Dimethyl-1H-Imidazol-2-yl)Pyridine and closo-Borate Anions

Abstract

New iron(II) coordination compounds with 2,6-bis(4,5-dimethyl-1H-imidazol-2-yl)pyridine (L) and closo-borate(2–) anions [FeL2]B10H10⋅2H2O and [FeL2]B12H12⋅H2O have been synthesized. The compounds have been identified and studied by CHN analysis, electron spectroscopy (diffuse reflection spectroscopy), IR, Mössbauer, and EXAFS spectroscopies, X-ray powder diffraction, and static magnetic susceptibility. The structures of the coordination knots of complexes [FeL2]B10H10⋅2H2O and [FeL2]B12H12⋅H2O has been obtained by modeling the EXAFS spectra. The ligand is coordinated by the iron(II) ion in a tridentate-cyclic manner by two nitrogen atoms of imidazole cycles and a nitrogen atom of pyridine to form the FeN6 coordination knot. The study of the temperature dependence of the magnetic susceptibility in the range of 80–500 K has showed that the high-temperature spin-crossover 1А1 ↔ 5Т2 manifests itself in the obtained compounds.

Toward High-Temperature Light-Induced Spin-State Trapping in Spin-Crossover Materials: The Interplay of Collective and Molecular Effects

Spin-crossover (SCO) materials display many fascinating behaviors including collective phase transitions and spin-state switching controlled by external stimuli, e.g., light and electrical currents. As single-molecule switches, they have been fêted for numerous practical applications, but these remain largely unrealized-partly because of the difficulty of switching these materials at high temperatures. We introduce a semiempirical microscopic model of SCO materials combining crystal field theory with elastic intermolecular interactions. For realistic parameters, this model reproduces the key experimental results including thermally induced phase transitions, light-induced spin-state trapping (LIESST), and reverse-LIESST. Notably, we reproduce and explain the experimentally observed relationship between the critical temperature of the thermal transition, T_1/2, and the highest temperature for which the trapped state is stable, T_LIESST, and explain why increasing the stiffness of the coordination sphere increases T_LIESST. We propose strategies to design SCO materials with higher T_LIESST: optimizing the spin-orbit coupling via heavier atoms (particularly in the inner coordination sphere) and minimizing the enthalpy difference between the high-spin (HS) and low-spin (LS) states. However, the most dramatic increases arise from increasing the cooperativity of the spin-state transition by increasing the rigidity of the crystal. Increased crystal rigidity can also stabilize the HS state to low temperatures on thermal cycling yet leave the LS state stable at high temperatures following, for example, reverse-LIESST. We show that such highly cooperative systems offer a realistic route to robust room-temperature switching, demonstrate this in silico, and discuss material design rationale to realize this.

NAST: Nonadiabatic Statistical Theory Package for Predicting Kinetics of Spin-Dependent Processes

We present a nonadiabatic statistical theory (NAST) package for predicting kinetics of spin-dependent processes, such as intersystem crossings, spin-forbidden unimolecular reactions, and spin crossovers. The NAST package can calculate the probabilities and rates of transitions between the electronic states of different spin multiplicities. Both the microcanonical (energy-dependent) and canonical (temperature-dependent) rate constants can be obtained. Quantum effects, including tunneling, zero-point vibrational energy, and reaction path interference, can be accounted for. In the limit of an adiabatic unimolecular reaction proceeding on a single electronic state, NAST reduces to the traditional transition state theory. Because NAST requires molecular properties at only a few points on potential energy surfaces, it can be applied to large molecular systems, used with accurate high-level electronic structure methods, and employed to study slow nonadiabatic processes. The essential NAST input data include the nuclear Hessian at the reactant minimum, as well as the nuclear Hessians, energy gradients, and spin-orbit coupling at the minimum energy crossing point (MECP) between two states. The additional computational tools included in the NAST package can be used to extract the required input data from the output files of electronic structure packages, calculate the effective Hessian at the MECP, and fit the reaction coordinate for more advanced NAST calculations. We describe the theory, its implementation, and three examples of application to different molecular systems.

Realizing shape and size control for the synthesis of coordination polymer nanoparticles templated by diblock copolymer micelles

The combination of polymers with nanoparticles offers the possibility to obtain customizable composite materials with additional properties such as sensing or bistability provided by a switchable spin crossover (SCO) core. For all applications, a precise control over size and shape of the nanomaterial is highly important as it will significantly influence its final properties. By confined synthesis of iron(II) SCO coordination polymers within the P4VP cores of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) micelles in THF we are able to control the size and also the shape of the resulting SCO nanocomposite particles by the composition of the PS-b-P4VP diblock copolymers (dBCPs) and the amount of complex employed. For the nanocomposite samples with the highest P4VP content, a morphological transition from spherical nanoparticles to worm-like structures was observed with increasing coordination polymer content, which can be explained with the impact of complex coordination on the self-assembly of the dBCP. Furthermore, the SCO nanocomposites showed transition temperatures of T_1/2 = 217 K, up to 27 K wide hysteresis loops and a decrease of the residual high-spin fraction down to γ_HS = 14% in the worm-like structures, as determined by magnetic susceptibility measurements and Mössbauer spectroscopy. Thus, SCO properties close or even better (hysteresis) to those of the bulk material can be obtained and furthermore tuned through size and shape control realized by tailoring the block length ratio of the PS-b-P4VP dBCPs.

Single crystal-to-single crystal transformation - from two distinct to three distinct spin crossover centers in 2D coordination polymer [Fe(bbtr)3](CF3SO3)2

1,4-Di(1,2,3-triazol-1-yl)butane (bbtr) forms a two-dimensional (2D) coordination polymer (1) in a reaction with iron(II) triflate. In the crystal lattice there are two crystallographically unique iron(II) ions surrounded octahedrally by a 1,2,3-triazole ring coordinated through nitrogen atoms N3. Single crystal X-ray diffraction studies revealed that spin crossover for each crystallographically independent iron(II) ion proceeds at a different temperature (T_1/2(Fe1) = 201 K; T_1/2(Fe2) = 216 K), while the magnetic measurements showed that there is one step, complete thermally induced spin crossover (T_1/2 = 205 K). Complex 1 undergoes, with time, single crystal-to-single crystal transformation (SCSC) to the converted system (1c) from the R3̄ to the P6₃ space group, accompanied by significant changes in the lattice parameter c (a shortening of approximately one-third) and consequently unit cell volume. Structural transformation is associated with rebuilding of the polymeric layer as well as the anion network, which is reflected in the results of Mössbauer studies. In the polymorphic system (1c) there are three crystallographically independent iron(II) ions. The temperature dependence results for magnetic susceptibility indicated complete, one-step spin crossover very similar to that of 1; however, single-crystal X-ray diffraction studies of 1c revealed that spin crossover for each crystallographically independent iron(II) ion occurs in a different manner, revealing three elementary stages (T_1/2(Fe1) = 200 K; T_1/2(Fe2) = 212 K, T_1/2(Fe3) = 214 K).

On the optimization of imaging parameters for magnetic resonance imaging thermometry using magnetic microparticles

Magnetic Resonance Imaging thermometry is an extremely useful technique which allows one to determine, noninvasively, the temperature deep in the tissue in two or three dimensions. Many methods of MR thermometry have been developed, including those that rely on the intrinsic MR properties of tissue and those which depend on the addition of contrast agents injected into the tissue to create temperature dependent MR images. One such method is to introduce magnetic particles whose magnetization's temperature dependence influences the MR properties of the surrounding tissue and obtain temperature from calibrated intensity changes of T₂* weighted MR images. One limitation of this method is the temperature resolution which is determined by the rate of change of the magnetization with temperature. One can change the MR response either through varying the particles properties or finding the MR scan parameters which maximize the image contrast due to T₂* weighting of images. In this work we calculate the MR signal strength, using known values of T₁ and T₂* relaxation times for agarose gel phantoms with embedded magnetic particles, and compared this with the temperature dependent intensity of experimental MR images. We seek to optimize the change in signal intensity with temperature by varying the selectable MR scanner parameters: echo time, repetition time, and flip angle. Based on comparison with experimental data we find that the change in signal with temperature can be significantly increased (by as much as 100%) through the appropriate choice of MR scan parameters.

Molecular Spin State Switching and Photochromism in the Red and Near Infrared with Ni(II) Chlorin and Ni(II) Bacteriochlorin

Molecules or ions are either paramagnetic (unpaired electrons) or diamagnetic (all electrons are paired). Switching between the two states under ambient conditions was considered a typical solid state phenomenon and has been termed spin crossover. The first single-molecule spin state switches operated with light in solution were developed a decade ago and offer a number of technical applications that are not accessible to solid state systems. Magnetic switching in biological environments, however, requires water solubility, and for in vivo applications, switching wavelengths within the bio-optical window (650-950 nm) are needed. We now present molecular spin state switches that are water-soluble and switchable in the far-red and near-infrared region. At the same time, they are photochromic compounds with excellent photophysical properties. trans-cis isomerization is induced with 505 nm radiation, and cis-trans conversion with 620 or 720 nm radiation. The metastable cis isomers are stable at room temperature for at least several weeks. The detailed mechanism of this surprising and unprecedented long wavelength photoisomerization of azobenzenes is still under investigation.

Magnetic resonance thermometry using a GdIII-based contrast agent

The complexes described here serve as contrast agents for magnetic resonance imaging thermometry. The complexes differentially enhance contrast between 275 and 325 K. The basis of the temperature response of the fluorinated contrast complex is the modulation of water exchange caused by trifluoromethyl groups that can be chemically controlled.

Spatiotemporal Studies of the One-Dimensional Coordination Polymer [Fe(ebtz)2 (C2 H5 CN)2 ](BF4 )2 : Tug of War between the Nitrile Reorientation Versus Crystal Lattice as a Tool for Tuning the Spin Crossover Properties*

Reaction of 1,2-di(tetrazol-2-yl)ethane (ebtz) with Fe(BF₄ )₂ ⋅6 H₂ O in different nitriles yields one-dimensional coordination polymers [Fe(ebtz)₂ (RCN)₂ ](BF₄ )₂ ⋅nRCN (n=2 for R=CH₃ (1) and n=0 for R=C₂ H₅ (2) C₃ H₇ (3), C₃ H₅ (4), CH₂ Cl (5)) exhibiting spin crossover (SCO). SCO in 1 and 3-5 is complete and occurs above 160 K. In 2, it is shifted to lower temperatures and is accompanied by wide hysteresis (T_1/2 ^↓ =78 K, T_1/2 ^↑ =123 K) and proceeds extremely slowly. Isothermal (80 K) time-resolved single-crystal X-ray diffraction studies revealed a complex nature for the HS→LS transition in 2. An initial, slow stage is associated with shrinkage of polymeric chains and with reduction of volume at 77 % (in relation to the difference between cell volumes V_HS -V_LS ) whereas only 16 % of iron(II) ions change spin state. In the second stage, an abrupt SCO occurs, associated with breathing of the crystal lattice along the direction of the Fe-nitrile bonds, while the nitriles reorient. HS→LS switching triggered by light (808 nm) reveals the coupling of spin state and nitrile orientation. The importance of this coupling was confirmed by studies of [Fe(ebtz)₂ (C₂ H₅ CN/C₃ H₇ CN)₂ ](BF₄ )₂ mixed crystals (2 a, 2 b), showing a shift of T_1/2 to higher values and narrowing of the hysteresis loop concomitant with an increase of the fraction of butyronitrile. This increase reduces the capability of nitrile molecules to reorient. Density functional theory (DFT) studies of models of 1-5 suggest a particular possibility of 2 to adopt a low (140-145°) value of its Fe-N-C(propionitrile) angle.

Spin Crossover in New Iron(II) Coordination Compounds with Tris(pyrazol-1-yl)Methane

We review here new advances in the synthesis and investigation of iron(II) coordination compounds with tris(pyrazol-1-yl)methane and its derivatives as ligands. The complexes demonstrate thermally induced spin crossover accompanied by thermochromism. Factors that influence the nature and temperature of the spin crossover are discussed.

Multifield and inverse-contrast switching of magnetocaloric high contrast ratio MRI labels

PURPOSE

Demonstrating multifield and inverse contrast switching of magnetocaloric high contrast ratio MRI labels that either have increasing or decreasing moment versus temperature slopes depending on the material at physiological temperatures and different MRI magnetic field strengths.

METHODS

Two iron-rhodium samples of different purity (99% and 99.9%) and a lanthanum-iron-silicon sample were obtained from commercial vendors. Temperature and magnetic field-dependent magnetic moment measurements of the samples were performed on a vibrating sample magnetometer. Temperature-dependent MRI of different iron-rhodium and lanthanum-iron-silicon samples were performed on 3 different MRI scanners at 1 Tesla (T), 4.7T, and 7T.

RESULTS

Sharp, first-order magnetic phase transition of each iron-rhodium sample at a physiologically relevant temperature (~37°C) but at different MRI magnetic fields (1T, 4.7T, and 7T, depending on the sample) showed clear image contrast changes in temperature-dependent MRI. Iron-rhodium and lanthanum-iron-silicon samples with sharp, first-order magnetic phase transitions at the same MRI field of 1T and physiological temperature of 37°C, but with positive and negative slope of magnetization versus temperature, respectively, showed clear inverse contrast image changes. Temperature-dependent MRI on individual microparticle samples of lanthanum-iron-silicon also showed sharp image contrast changes.

CONCLUSION

Magnetocaloric materials of different purity and composition were demonstrated to act as diverse high contrast ratio switchable MRI contrast agents. Thus, we show that a range of magnetocaloric materials can be optimized for unique image contrast response under MRI-appropriate conditions at physiological temperatures and be controllably switched in situ.

Imaging-based internal body temperature measurements: The journal Temperature toolbox

Noninvasive imaging methods of internal body temperature are in high demand in both clinical medicine and physiological research. Thermography and thermometry can be used to assess tissue temperature during thermal therapies: ablative and hyperthermia treatments to ensure adequate temperature rise in target tissues but also to avoid collateral damage by heating healthy tissues. In research use, measurement of internal body temperature enables us the production of thermal maps on muscles, internal organs, and other tissues of interest. The most used methods for noninvasive imaging of internal body temperature are based on different parameters acquired with magnetic resonance imaging, ultrasound, computed tomography, microwave radiometry, photoacoustic imaging, and near-infrared spectroscopy. In the current review, we examine the aforementioned imaging methods, their use in estimating internal body temperature in vivo with their advantages and disadvantages, and the physical phenomena the thermography or thermometry modalities are based on.

Magnetic resonance thermometry and its biological applications - Physical principles and practical considerations

Most parameters that influence the magnetic resonance imaging (MRI) signal experience a temperature dependence. The fact that MRI can be used for non-invasive measurements of temperature and temperature change deep inside the human body has been known for over 30 years. Today, MR temperature imaging is widely used to monitor and evaluate thermal therapies such as radio frequency, microwave, laser, and focused ultrasound therapy. In this paper we cover the physical principles underlying the biological applications of MR temperature imaging and discuss practical considerations and remaining challenges. For biological tissue, the MR signal of interest comes mostly from hydrogen protons of water molecules but also from protons in, e.g., adipose tissue and various metabolites. Most of the discussed methods, such as those using the proton resonance frequency (PRF) shift, T₁, T₂, and diffusion only measure temperature change, but measurements of absolute temperatures are also possible using spectroscopic imaging methods (taking advantage of various metabolite signals as internal references) or various types of contrast agents. Currently, the PRF method is the most used clinically due to good sensitivity, excellent linearity with temperature, and because it is largely independent of tissue type. Because the PRF method does not work in adipose tissues, T₁- and T₂-based methods have recently gained interest for monitoring temperature change in areas with high fat content such as the breast and abdomen. Absolute temperature measurement methods using spectroscopic imaging and contrast agents often offer too low spatial and temporal resolution for accurate monitoring of ablative thermal procedures, but have shown great promise in monitoring the slower and usually less spatially localized temperature change observed during hyperthermia procedures. Much of the current research effort for ablative procedures is aimed at providing faster measurements, larger field-of-view coverage, simultaneous monitoring in aqueous and adipose tissues, and more motion-insensitive acquisitions for better precision measurements in organs such as the heart, liver, and kidneys. For hyperthermia applications, larger coverage, motion insensitivity, and simultaneous aqueous and adipose monitoring are also important, but great effort is also aimed at solving the problem of long-term field drift which gets interpreted as temperature change when using the PRF method.

Mononuclear complexes of FeII, CoII and CoIII containing imine-based ligands of 8-aminoquinoline and 7-aminoindazole: spin state tuning of FeII complexes in solution

Careful selection of the fused ring heterocycle and aldehyde synthons resulted in the spin state tuning of iron(ii) complexes in solution.

Magnetocaloric materials as switchable high contrast ratio MRI labels

PURPOSE

To develop switchable and tunable labels with high contrast ratio for MRI using magnetocaloric materials that have sharp first-order magnetic phase transitions at physiological temperatures and typical MRI magnetic field strengths.

METHODS

A prototypical magnetocaloric material iron-rhodium (FeRh) was prepared by melt mixing, high-temperature annealing, and ice-water quenching. Temperature- and magnetic field-dependent magnetization measurements of wire-cut FeRh samples were performed on a vibrating sample magnetometer. Temperature-dependent MRI of FeRh samples was performed on a 4.7T MRI.

RESULTS

Temperature-dependent MRI clearly demonstrated image contrast changes due to the sharp magnetic state transition of the FeRh samples in the MRI magnetic field (4.7T) and at a physiologically relevant temperature (~37°C).

CONCLUSION

A magnetocaloric material, FeRh, was demonstrated to act as a high contrast ratio switchable MRI contrast agent due to its sharp first-order magnetic phase transition in the DC magnetic field of MRI and at physiologically relevant temperatures. A wide range of magnetocaloric materials are available that can be tuned by materials science techniques to optimize their response under MRI-appropriate conditions and be controllably switched in situ with temperature, magnetic field, or a combination of both.

Unravelling the spin-state of solvated [Fe(bpp)2]2+ spin-crossover complexes: structure-function relationship

This paper reports firstly the syntheses, crystal structures, and thermal and magnetic properties of spin crossover salts of formulae [Fe(bpp)₂]₃[Cr(CN)₆]₂·13H₂O (1) and [Fe(bpp)₂][N(CN)₂]₂·H₂O (2) (bpp = 2,6-bis(pyrazol-3-yl)pyridine) exhibiting hydrogen-bonded networks of low-spin [Fe(bpp)₂]²⁺ complexes and [Cr(CN)₆]^3- or [N(CN)₂]^- anions, with solvent molecules located in the voids. Desolvation of 1 is accompanied by a complete low-spin (LS) to a high-spin (HS) transformation that becomes reversible after rehydration by exposing the sample to the humidity of air. The influence of the lattice water on the magnetic properties of spin-crossover [Fe(bpp)₂]X₂ complex salts has been documented. In most cases, it stabilises the LS state over the HS one. In other cases, it is rather the contrary. The second part of this paper is devoted to unravelling the reasons why the lattice solvent stabilises one form over the other through magneto-structural correlations of [Fe(bpp)₂]²⁺ salts bearing anions with different charge/size ratios (X^n-). The [Fe(bpp)₂]²⁺ stacking explaining these two different behaviours is correlated here with the composition of the second coordination sphere of the Fe centers and the ability of these anions to form hydrogen bonds and/or π-π stacking interactions between them or the bpp ligand.

Spin-Crossover Iron(II) Coordination Polymer with Fluorescent Properties: Correlation between Emission Properties and Spin State

A spin-crossover coordination polymer [Fe(L1)(bipy)]_n (where L = a N₂O₂^2- coordinating Schiff base-like ligand bearing a phenazine fluorophore and bipy = 4,4'-bipyridine) was synthesized and exhibits a 48 K wide thermal hysteresis above room temperature (T_1/2↑ = 371 K and T_1/2↓ = 323 K) that is stable for several cycles. The spin transition was characterized using magnetic measurements, Mössbauer spectroscopy, and DSC measurements. T-dependent X-ray powder diffraction reveals a structural phase transition coupled with the spin transition phenomenon. The dimeric excerpt {(μ-bipy)[FeL1(MeOH)]₂}·2MeOH of the coordination polymer chain crystallizes in the triclinic space group P1̅ and reveals that the packing of the molecules in the crystal is dominated by hydrogen bonds. Investigation of the emission properties of the complexes with regard to temperature shows that the spin crossover can be tracked by monitoring the emission spectra, since the emission color changes from greenish to a yellow color upon the low spin-to-high spin transition.

Design, synthesis, and evaluation of nickel dipyridylmethane complexes for Coordination-Induced Spin State Switching (CISSS)

The coordination of pyridine to a nickel(ii) dipyridylmethane complex changes the spin state.

Synthesis of Coordination Polymer Nanoparticles using Self-Assembled Block Copolymers as Template

Nowadays there is a high demand in specialized functional materials, for example, for applications as sensors in biomedicine. For the realization of such applications, nanostructures and the integration in a composite matrix are indispensable. Coordination polymers and networks, for example, with spin crossover properties, are a highly promising family of switchable materials in which the switching process can be triggered by various external stimuli. An overview over different strategies for the synthesis of nanoparticles of such systems is given. A special focus is set on the use of block copolymer micelles as templates for the synthesis of nanocomposites. The block copolymer defines the final size and shape of the nanoparticle core. Additionally it allows a further functionalization of the obtained nanoparticles by variation of the polymer blocks and an easy deposition of the composite material on surfaces.

Synthesis of [Fe(Leq)(Lax)] n coordination polymer nanoparticles using blockcopolymer micelles

Spin-crossover compounds are a class of materials that can change their spin state from high spin (HS) to low spin (LS) by external stimuli such as light, pressure or temperature. Applications demand compounds with defined properties concerning the size and switchability that are maintained when the compound is integrated into composite materials. Here, we report the synthesis of [Fe(L_eq)(L_ax)]_n coordination polymer (CP) nanoparticles using self-assembled polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer (BCP) micelles as template. Variation of the solvent (THF and toluene) and the rigidity of the axial ligand L_ax (L_ax = 1,2-di(pyridin-4-yl)ethane) (bpea), trans-1,2-di(pyridin-4-yl)ethene (bpee), and 1,2-di(pyridin-4-yl)ethyne) (bpey); L_eq = 1,2-phenylenebis(iminomethylidyne)-bis(2,4-pentanedionato)(2-)) allowed the determination of the preconditions for the selective formation of nanoparticles. A low solubility of the CP in the used solvent and a high stability of the Fe-L bond with regard to ligand exchange are necessary for the formation of composite nanoparticles where the BCP micelle is filled with the CP, as in the case of the [FeL_eq(bpey)] _n @BCP. Otherwise, in the case of more flexible ligands or ligands that lead to high spin complexes, the formation of microcrystals next to the CP-BCP nanoparticles is observed above a certain concentration of [Fe(L_eq)(L_ax)] _n . The core of the nanoparticles is about 45 nm in diameter due to the templating effect of the BCP micelle, independent of the used iron complex and [Fe(L_eq)(L_ax)] _n concentration. The spin-crossover properties of the composite material are similar to those of the bulk for FeL_eq(bpea)] _n @BCP while pronounced differences are observed in the case of [FeL_eq(bpey)] _n @BCP nanoparticles.

Spin-crossover and high-spin iron(ii) complexes as chemical shift 19F magnetic resonance thermometers

The potential utility of paramagnetic transition metal complexes as chemical shift 19F magnetic resonance (MR) thermometers is demonstrated. Further, spin-crossover FeII complexes are shown to provide much higher temperature sensitivity than do the high-spin analogues, owing to the variation of spin state with temperature in the former complexes. This approach is illustrated through a series of FeII complexes supported by symmetrically and asymmetrically substituted 1,4,7-triazacyclononane ligand scaffolds bearing 3-fluoro-2-picolyl derivatives as pendent groups (L x ). Variable-temperature magnetic susceptibility measurements, in conjunction with UV-vis and NMR data, show thermally-induced spin-crossover for [Fe(L1)]2+ in H2O, with T1/2 = 52(1) °C. Conversely, [Fe(L2)]2+ remains high-spin in the temperature range 4-61 °C. Variable-temperature 19F NMR spectra reveal the chemical shifts of the complexes to exhibit a linear temperature dependence, with the two peaks of the spin-crossover complex providing temperature sensitivities of +0.52(1) and +0.45(1) ppm per °C in H2O. These values represent more than two-fold higher sensitivity than that afforded by the high-spin analogue, and ca. 40-fold higher sensitivity than diamagnetic perfluorocarbon-based thermometers. Finally, these complexes exhibit excellent stability in a physiological environment, as evidenced by 19F NMR spectra collected in fetal bovine serum.

Proton-driven coordination-induced spin state switch (PD-CISSS) of iron(ii) complexes

A pH dependent reversible spin state switch is observed for strong field iron(ii) complexes in line with a significant relaxivity gap between the high spin and the low spin state.

pH-Dependent spin state population and 19F NMR chemical shift via remote ligand protonation in an iron(ii) complex

An Fe^II complex that features a pH-dependent spin state population and ¹⁹F chemical shift, by virtue of a variable ligand protonation state, is described.

Synthesis of [Fe(L)(bipy)]n spin crossover nanoparticles using blockcopolymer micelles

Nowadays there is a high demand for specialized functional materials for specific applications in sensors or biomedicine (e.g. fMRI). For their implementation in devices, nanostructuring and integration in a composite matrix are indispensable. Spin crossover complexes are a highly promising family of switchable materials where the switching process can be triggered by various external stimuli. In this work, the synthesis of nanoparticles of the spin crossover iron(ii) coordination polymer [Fe(L)(bipy)]_n (with L = 1,2-phenylenebis(iminomethylidyne)bis(2,4-pentanedionato)(2-) and bipy = 4,4'-bipyridine) is described using polystyrene-poly-4-vinylprididine blockcopolymer micelles as the template defining the final size of the nanoparticle core. A control of the spin crossover properties can be achieved by precise tuning of the crystallinity of the coordination polymer via successive addition of the starting material Fe(L) and bipy. By this we were able to synthesize nanoparticles with a core size of 49 nm and a thermal hysteresis loop width of 8 K. This is, to the best of our knowledge, a completely new approach for the synthesis of nanoparticles of coordination polymers and should be easily transferable to other coordination polymers and networks. Furthermore, the use of blockcopolymers allows a further functionalization of the obtained nanoparticles by variation of the polymer blocks and an easy deposition of the composite material on surfaces via spin coating.