Vapochromic effect in switchable molecular-based spin crossover compounds.

Coordination complexes based on transition metal ions displaying [Ar]3d4-3d7 electronic configurations can undergo the likely most spectacular switchable phenomena found in molecular coordination chemistry, the well-known Spin Crossover (SCO). SCO phenomena is a detectable, reproducible and reversible switch that occurs between the high spin (HS) and low spin (LS) electronic states of the transition metal actuated by different stimuli (i.e. light, temperature, pressure, the presence of an analyte). Moreover, the occurrence of SCO phenomena causes different outputs, one of them being a colour change. Altogether, an analyte in gas form could be detected by naked eye once it has triggered the corresponding HS ↔ LS transition. This vapochromic effect could be used to detect volatile molecules using a low-cost technology, including harmful chemical substances, gases and/or volatile organic compounds (VOCs) that are present in our environment, in our home or at our workplace. The present review condenses all reported iron coordination compounds where the colour change induced by a given molecule in its gas form is coupled to a HS ↔ LS spin transition. Special emphasis has been made on describing the nature of the post-synthetic modification (PSM) taking place in the material upon the analyte uptake. In this case, three types of PSM can be distinguished: based on supramolecular contacts and/or leading to a coordinative or covalent bond. In the latter, a colour change not only indicates the switch of the spin state in the material but also the formation of a new compound with different properties. It is important to indicate that some of the SCO coordination compounds discussed in the current report have been part of other spin crossover reviews, that have gathered thermally induced SCO compounds and the influence of guest molecules on the SCO behaviour. However, in the majority of examples in these reviews, the change of colour upon the uptake of analytes is not associated with a spin transition at room temperature. In addition, the observed colour variations have been mainly discussed in terms of host-guest interactions, when they can also be induced by a PSM taking place in different sites of the molecule, like the Fe(II) coordination sphere or by chemically altering its inorganic and/or organic linkers. Therefore, we present here for the first time an exhaustive compilation of all systems in which the interaction between the coordination compounds and the vapour analytes leads to a colour change due to a spin transition in the metal centre at room temperature.

Concomitant Thermochromic and Phase-Change Effect in a Switchable Spin Crossover Material for Efficient Passive Control of Day and Night Temperature Fluctuations.

The increasing environmental protection demand has prompted the development of passive thermal regulation systems that reduce temperature fluctuations in buildings. Here, it is demonstrated that the heat generated by the sun can trigger a spin crossover (SCO) in a molecule-base material, resulting in a concomitant color variation (from pink to white) and a phase transition. This leads to a cooling effect with respect to other thermochromic materials. In addition, when the material is cooled, a dampening of the temperature decrease is produced. Therefore, these materials can potentially be implemented for passive temperature control in buildings. Furthermore, SCO materials are remarkably stable upon cycling and highly versatile, which allows for the design of compounds with properties tailored for the desired climatic conditions and comfortable temperature.

Tunable Proton Conductivity and Color in a Nonporous Coordination Polymer via Lattice Accommodation to Small Molecules.

Nonporous coordination polymers (npCPs) able to accommodate molecules through internal lattice reorganization are uncommon materials with applications in sensing and selective gas adsorption. Proton conduction, extensively studied in the analogue metal-organic frameworks under high-humidity conditions, is however largely unexplored in spite of the opportunities provided by the particular sensitivity of npCPs to lattice perturbations. Here, AC admittance spectroscopy is used to unveil the mechanism behind charge transport in the nonporous 1·2CH3 CN. The conductance in the crystals is found to be of protonic origin. A vehicle mechanism is triggered by the dynamics of the weakly coupled acetonitrile molecules in the lattice that can be maintained by a combination of thermal cycles, even at low humidity levels. An analogue 1·pyrrole npCP is formed by in situ exchange of these weakly bound acetonitrile molecules by pyrrole. The color and conduction properties are determined by the molecules weakly bonded in the lattice. This is the first example of acetonitrile-mediated proton transport in an npCP showing distinct optical response to different molecules. These findings open the door to the design of switchable protonic conductors and capacitive sensors working at low humidity levels and with selectivity to different molecules.

Spin-state-dependent electrical conductivity in single-walled carbon nanotubes encapsulating spin-crossover molecules.

Spin crossover (SCO) molecules are promising nanoscale magnetic switches due to their ability to modify their spin state under several stimuli. However, SCO systems face several bottlenecks when downscaling into nanoscale spintronic devices: their instability at the nanoscale, their insulating character and the lack of control when positioning nanocrystals in nanodevices. Here we show the encapsulation of robust Fe-based SCO molecules within the 1D cavities of single-walled carbon nanotubes (SWCNT). We find that the SCO mechanism endures encapsulation and positioning of individual heterostructures in nanoscale transistors. The SCO switch in the guest molecules triggers a large conductance bistability through the host SWCNT. Moreover, the SCO transition shifts to higher temperatures and displays hysteresis cycles, and thus memory effect, not present in crystalline samples. Our results demonstrate how encapsulation in SWCNTs provides the backbone for the readout and positioning of SCO molecules into nanodevices, and can also help to tune their magnetic properties at the nanoscale.

Water Soluble Iron-Based Coordination Trimers as Synergistic Adjuvants for Pancreatic Cancer.

Pancreatic cancer is a usually fatal disease that needs innovative therapeutic approaches since the current treatments are poorly effective. In this study, based on cell lines, triazole-based coordination trimers made with soluble Fe(II) in an aqueous media were explored for the first time as adjuvant agents for the treatment of this condition. These coordination complexes were effective at relatively high concentrations and led to an increase in reactive oxygen species (ROS) in two pancreatic cancer cell lines, PANC-1 and BXPC-3, and this effect was accompanied by a significant reduction in cell viability in the presence of gemcitabine (GEM). Importantly, the tested compounds enhanced the effect of GEM, an approved drug for pancreatic cancer, through apoptosis induction and downregulation of the mTOR pathway. Although further evaluation in animal-based models of pancreatic cancer is needed, these results open novel avenues for exploring these iron-based materials in biomedicine in general and in pancreatic cancer treatment.

Tracking the Light-Induced Excited-State Dynamics and Structural Configurations of an Extraordinarily Long-Lived Metastable State at Room Temperature.

Time-resolved X-ray (Tr-XAS) and optical transient absorption (OTA) spectroscopy on the pico-microsecond timescale coupled with density functional theory calculations are applied to study the light-induced spin crossover processes of a Fe-based macrocyclic complex in solution. Tr-XAS analysis after light illumination shows the formation of a seven-coordinated high-spin quintet metastable state, which relaxes to a six-coordinated high-spin configuration before decaying to the ground state. Kinetic analysis of the macrocyclic complex reveals an unprecedented long-lived decay lifetime of approximately 42.6 µs. Comparative studies with a non-macrocyclic counterpart illustrate a significantly shortened approximately 568-fold decay lifetime of about 75 ns, and highlight the importance of the ligand arrangement in stabilizing the reactivity of the excited state. Lastly, OTA analysis shows the seven-coordinated high-spin state to be formed within approximately 6.2 ps. These findings provide a complete understanding of the spin crossover reaction and relaxation pathways of the macrocyclic complex, and reveal the importance of a flexible coordination environment for their rational design.

A switchable iron-based coordination polymer toward reversible acetonitrile electro-optical readout.

Efficient and low cost detection of harmful volatile organic compounds (VOCs) is a major health and environmental need in industrialized societies. For this, tailor-made porous coordination polymers are emerging as promising molecular sensing materials thanks to their responsivity to a wide variety of external stimuli and could be used to complement conventional sensors. Here, a non-porous crystalline 1D Fe(ii) coordination polymer acting as a porous acetonitrile host is presented. The desorption of interstitial acetonitrile is accompanied by magneto-structural transitions easily detectable in the optical and electronic properties of the material. This structural switch and therefore its (opto)electronic readout are reversible under exposure of the crystal to acetonitrile vapor. This simple and robust iron-based coordination polymer could be ideally suited for the construction of multifunctional sensor devices for volatile acetonitrile and potentially for other organic compounds.

A Three-Dimensional Dynamic Supramolecular "Sticky Fingers" Organic Framework.

Engineering high-recognition host-guest materials is a burgeoning area in basic and applied research. The challenge of exploring novel porous materials with advanced functionalities prompted us to develop dynamic crystalline structures promoted by soft interactions. The first example of a pure molecular dynamic crystalline framework is demonstrated, which is held together by means of weak "sticky fingers" van der Waals interactions. The presented organic-fullerene-based material exhibits a non-porous dynamic crystalline structure capable of undergoing single-crystal-to-single-crystal reactions. Exposure to hydrazine vapors induces structural and chemical changes that manifest as toposelective hydrogenation of alternating rings on the surface of the [60]fullerene. Control experiments confirm that the same reaction does not occur when performed in solution. Easy-to-detect changes in the macroscopic properties of the sample suggest utility as molecular sensors or energy-storage materials.

Current Switching Coupled to Molecular Spin-States in Large-Area Junctions.

The fabrication of large-area vertical junctions with a molecular spin-crossover complex displaying concerted changes of spin degrees of freedom and charge-transport properties is reported. Fabricated devices allow spin-state switching in the spin-crossover layer to be triggered and probed by optical means, while detecting associated changes in electrical resistance in the junctions.

Counter anion dependent gradual spin transition in a 1D cobalt(ii) coordination polymer.

One Co(ii) SCO coordination polymer [Co(enbzpy)(µ1,5-dca)]n(PF6)n (·PF6) has been isolated and characterised structurally and magnetostructurally. The properties of ·PF6 are compared with the reported perchlorate analogue [Co(enbzpy)(µ1,5-dca)]n(ClO4)n (·ClO4). The gradual spin transition in ·PF6 in contrast to an abrupt spin transition with a hysteresis loop in ·ClO4 has been analysed in terms of structural factors.

New topologies in pentanuclear nickel/oximato clusters: structural and magnetic characterization.

In the present work, five new Ni5 clusters employing the versatile 2-pyridylcyanoxime ligand have been synthesized and chemically, structurally, and magnetically characterized. The crystallographic examination of these Ni5 clusters together with those already published in the literature, giving a total number of 14 complexes, exhibiting up to 8 different topologies for which the relationship between topology, reaction conditions and magnetic response has been analyzed. DC magnetic measurements were carried in the 300-2 K range for the new complexes and the analysis of the experimental data revealed an antiferromagnetic response for the oximato mediated interactions with a variety of ground states (S = 0, 1, 3) as function of the cluster topology.

Three-way crystal-to-crystal reversible transformation and controlled spin switching by a nonporous molecular material.

Porous materials capable of hosting external molecules are paramount in basic and applied research. Nonporous materials able to incorporate molecules via internal lattice reorganization are however extremely rare since their structural integrity usually does not resist the guest exchange processes. The novel heteroleptic low-spin Fe(II) complex [Fe(bpp)(H2L)](ClO4)2·1.5C3H6O (1; bpp = 2,6-bis(pyrazol-3-yl)pyridine, H2L = 2,6-bis(5-(2-methoxyphenyl)pyrazol-3-yl)pyridine) crystallizes as a compact discrete, nonporous material hosting solvate molecules of acetone. The system is able to extrude one-third of these molecules to lead to [Fe(bpp)(H2L)](ClO4)2·C3H6O (2), switching to the high-spin state while experiencing a profound crystallographic change. Compound 2 can be reversed to the original material upon reabsorption of acetone. Single crystal X-ray diffraction experiments on the latter system (1') and on 2 show that these are reversible single-crystal-to-single-crystal (SCSC) transformations. Likewise, complex 2 can replace acetone by MeOH and H2O to form [Fe(bpp)(H2L)](ClO4)2·1.25MeOH·0.5H2O (3) through a SCSC process that also implies a switch to the spin state. The 3→1 transformation through acetone reabsorption is also demonstrated. Besides the spin switching at room temperature, this series of SCSC transformations causes macroscopic changes in color that can be followed by the naked eye. The reversible exchanges of chemicals are therefore easily sensed at the temperature at which these occur, contrary to what is the case for most of the few existing nonporous spin-based sensors, which feature a large temperature gap between the process monitored and the mechanism of detection.

High-temperature photo-induced switching and pressure-induced transition in a cooperative molecular spin-crossover material.

The thermal and photo-induced switching properties of the recently published molecular spin crossover complex [Fe(H4L)2](ClO4)2·H2O·2(CH3)2CO () have been investigated in detail through Raman spectroscopy. Magnetometric and single crystal X-ray diffraction kinetic studies within the hysteresis loop have proven its metastable character, which allowed establishing the shape of the true, quasi-static hysteresis loop. This is related to the proximity of the temperatures of the thermal high to low spin SCO (T1/2↓) to those of the relaxation of the thermally or photo-generated metastable states (T(TIESST) and T(LIESST), respectively). Green light irradiation within the hysteresis loop results in a complete low to high spin photo-switch. In addition, the SCO of at room temperature can be induced by applying pressure, as followed by Raman spectroscopy.

Multimetastability in a spin-crossover compound leading to different high-spin-to-low-spin relaxation dynamics.

The relaxation kinetics of both the thermally trapped and photoinduced high-spin (HS) states of the spin-crossover compound [Fe(H4L)2](ClO4)2·H2O·2(CH3)2CO (1) were measured and found to differ significantly. Calorimetry measurements then demonstrated that relaxation of the thermally trapped phase was concurrent with two separate processes, not previously detected as such. Determination of the photogenerated HS structure revealed a new metastable HS state of the system, much closer structurally to the low-spin phase than the thermally trapped one. This difference is proposed as the root of the disparate kinetic behavior, which is proposed to require two processes in the case of the structurally more complex thermally trapped state. Therefore, light irradiation is shown as a mechanism to decouple effectively the structural and magnetic phase transitions that occur in 1 during the course of its spin crossover.

Local coordination geometry and spin state in novel Fe(II) complexes with 2,6-bis(pyrazol-3-yl)pyridine-type ligands as controlled by packing forces: structural correlations.

A substituted 2,6-bis(pyrazol-3-yl)pyridine (3-bpp) ligand, H(4)L, created to facilitate intermolecular interactions in the solid, has been used to obtain four novel Fe(II) complexes: [Fe(H(4)L)(2)](ClO(4))(2)⋅2 CH(3)NO(2)⋅2 H(2)O, [Fe(H(4)L)(H(2)LBF(2))](BF(4))⋅5 C(3)H(6)O (H(2)LBF(2) is an in situ modified version of H(4)L), [Fe(H(4)L)(2)](ClO(4))(2)⋅2 C(3)H(7)OH and [Fe(H(4)L)(2)](ClO(4))(2)⋅4 C(2)H(5)OH. Changing of spin-inactive components (solvents, anions or distant ligand substituents) causes differences to the coordination geometry of the metal that are key to the magnetic properties. Magnetic measurements show that, contrary to the previously published complex [Fe(H(4)L)(2)](ClO(4))(2)⋅H(2)O⋅2 CH(3)COCH(3), the newly synthesised compounds remain in the high-spin (HS) state at all temperatures (5-300 K). A member of the known family of Fe(II)/3-bpp complexes, [Fe(3-bpp)(2)](ClO(4))(2)⋅1.75 CH(3)COCH(3)⋅1.5 Et(2)O, has also been prepared and characterised structurally. In the bulk, this compound exhibits a gradual and incomplete spin transition near 205 K. The single-crystal structure is consistent with it being HS at 250 K and partially low spin at 90 K. Structural analysis of all these compounds reveals that the exact configuration of intermolecular interactions affects dramatically the local geometry at the metal, which ultimately has a strong influence on the magnetic properties. Along this line, the geometry of Fe(II) in all published 3-bpp compounds of known structure has been examined, both by calculating various distortion indices (Σ, Θ, θ and Φ) and by continuous shape measures (CShMs). The results reveal correlations between some of these parameters and indicate that the distortions from octahedral geometry observed on HS systems are mainly due to strains arising from intermolecular interactions. As previously suggested with other related compounds, we observe here that strongly HS-distorted systems have a larger tendency to remain in that state.

A molecular [Mn14] coordination cluster featuring two slowly relaxing nanomagnets.

An extended polypyrazolyl ligand has been used to assemble Mn ions into high spin entities, in the form of one stable molecule composed of two well defined clusters. The slow relaxation of the magnetisation observed is demonstrated to arise from each "half-SMM" composing this molecular cluster pair.

Coexistence of intramolecular ligand-mediated and through hydrogen-bond magnetic interactions in a chain of dicopper(II) units.

The reaction of 2,8-dimethyl-5,11-bis(pyridin-2-ylmethyl)-1,4,5,6,7,10,11,12-octahydroimidazo[4,5-h]imidazo[4,5-c][1,6]-diazecine (dimp) with copper(II) nitrate in water produces the compound [Cu(2)(dimp)(H(2)O)(2)(NO(3))(2)](NO(3))(2). The single-crystal X-ray structure shows the formation of hydrogen-bonded chains in the lattice that are formed by dicopper(II) units doubly connected by nitrate/water bridges. Within the one-dimensional chains, the Cu ions are separated by either intramolecular or intermolecular distances of 7.309(2) Å or 6.255(2) Å, respectively. The magnetic susceptibility data revealing weak antiferromagnetic exchange interactions between the copper(II) ions were interpreted by considering two possible models, namely, an isolated dinuclear and a 1-D chain picture. The latter leads to an alternation J(1) = -11.6 and J(2) = -3.0 cm(-1) along the chain. In order to clarify the relative strengths of the exchange couplings through hydrogen bonds and via the bridging dimp ligand, solution EPR studies and quantum chemical calculations were carried out. EPR studies unambiguously conclude on the existence of an exchange interaction J(a) mediated by the dinucleating dimp ligand, while the through-H coupling J(b) is physically absent in solution. On the basis of dinuclear units extracted from the X-ray data, J(a) was estimated around -5.0 cm(-1) from DFT-based calculations (M06 functional), whereas J(b) is negligible. In contrast, wave function configuration interaction calculations (DDCI) support a description where both inter- and intramolecular pathways coexist with a preeminent role of H bonds with J(a) = -2.8 and J(b) = -10.4 cm(-1). Not only are these values very consistent with the extracted set of parameters (J(1), J(2) = -11.6, -3.0 cm(-1)) but the possibility to generate leading exchange coupling through weak bonds is evidenced by means of wave function-based calculations.

Coupled crystallographic order-disorder and spin state in a bistable molecule: multiple transition dynamics.

A novel bispyrazolylpyridine ligand incorporating lateral phenol groups, H(4)L, has led to an Fe(II) spin-crossover (SCO) complex, [Fe(H(4)L)(2)][ClO(4)](2)⋅H(2)O⋅2 (CH(3))(2)CO (1), with an intricate network of intermolecular interactions. It exhibits a 40 K wide hysteresis of magnetization as a result of the spin transition (with T(0.5) of 133 and 173 K) and features an unsymmetrical and very rich structure. The latter is a consequence of the coupling between the SCO and the crystallographic transformations. The high-spin state may also be thermally trapped, exhibiting a very large T(TIESST) (≈104 K). The structure of 1 has been determined at various temperatures after submitting the crystal to different processes to recreate the key points of the hysteresis cycle and thermal trapping; 200 K, cooled to 150 K and trapped at 100 K (high spin, HS), slowly cooled to 100 K and warmed to 150 K (low spin, LS). In the HS state, the system always exhibits disorder for some components (one ClO(4)(-) and two acetone molecules) whereas the LS phases show a relative ≈9 % reduction in the Fe-N bond lengths and anisotropic contraction of the unit cell. Most importantly, in the LS state all the species are always found to be ordered. Therefore, the bistability of crystallographic order-disorder coupled to SCO is demonstrated here experimentally for the first time. The variation in the cell parameters in 1 also exhibits hysteresis. The structural and magnetic thermal variations in this compound are paralleled by changes in the heat capacity as measured by differential scanning calorimetry. Attempts to simulate the asymmetric SCO behaviour of 1 by using an Ising-like model underscore the paramount role of dynamics in the coupling between the SCO and the crystallographic transitions.

Reliability and storage capacity: a compromise illustrated in the two-step spin-crossover system [Fe(bapbpy)(NCS)(2)].

The design of bistable magnetic systems should enable the storage of information by manipulation of the spin degrees of freedom. However, such a strategy relies on the preparation of target objects, whose environment must be controlled to favor a hysteretic behavior. Here, we report the successful modeling of a highly cooperative two-step spin-crossover iron(II) compound, [Fe(bapbpy)(NCS)(2)]. The magnetic susceptibility measurements and low- and high-temperature hysteretic cycles reflect the presence of an intermediate phase, which controls the memory-storage capacity of this material. It is shown that the hysteresis loop widths can be traced theoretically by evaluating the electrostatic contributions between the transiting units. Despite the apparent reduction of intermolecular interactions upon cooling, it is suggested that the enhanced fluctuations of the Madelung field are responsible for the observed hysteresis width changes. This counterintuitive scenario makes the preparation of information storage devices an even more challenging task, where theoretical inspections are very insightful.