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Light-induced excited spin state trapping effect on [Fe(mepy)3tren](PF6)2 solvated crystals†‡ Antoine Tissot,*a,b Eric Rivière,a Régis Guillot,a Loic Toupet,c Eric Colletc and Marie-Laure Boillot*a A spin-crossover solvated compound [Fe(mepy)3tren](PF6)2·C7H8·C2H3N has been prepared and its switching properties have been compared to those reported for the non-solvated solid. The thermal spin transition occurs at 88 K with the opening of a 3.5 K wide hysteresis loop, while a fairly steep transition at 215 K without hysteresis has been previously reported for the non-solvated analogue. This feature has been rationalized by the analysis of the high-spin (HS) and low-spin (LS) structures, evidencing a relative stabilization of the high-spin state, as well as strong intermolecular interactions in the solvated compound. The photoswitching of the solvated solid, based on the light-induced excited spin state trapping effect, leads to a quantitative transformation from the low-spin to the high-spin state at 10 K. The long lifetime of the metastable HS state at 10 K allows the measurement of the photo-induced HS structure, where the cooperative interactions are enhanced, compared to those of the thermally populated HS structure. Then,the HS-to-LS relaxations have been studied between 45 and 60 K. They are sigmoidal in shape and
Received 11th September 2013, Accepted 8th October 2013
can be well fitted in the frame of the mean-field approximation. The relative stability of the photoinduced HS state in this family of spin crossover compounds is not directly related to their thermal spin
DOI: 10.1039/c3dt52495g
transition temperature. This unexpected observation is rationalized by a careful analysis of their structural
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characteristics.
Introduction Photoswitchable molecular compounds present a wide interest, as they may be applied in future information storage or display devices.1 Spin-crossover compounds, whose spin state can be interconverted between low-spin (LS) and high-spin (HS) state under the effect of temperature, pressure, magnetic field or light, are one of the most studied classes of switchable
a Université Paris Sud, ICMMO-ECI, UMR CNRS 8182, Université Paris-Sud, 91405 Orsay, France. E-mail: [email protected] b Université de Genève, Département de Chimie Physique, 30 Quai E. Ansermet 1211 Genève 4, Switzerland. E-mail: [email protected] c Institut de Physique de Rennes, UMR 6251, Université Rennes 1-CNRS, Rennes, France † This work has been supported by the ANR (09-BLAN-0212) project. ‡ Electronic supplementary information (ESI) available: Tables containing the analysis of the short-ring interactions (Table S1), the Pi-ring interactions (Table S2) and the intermolecular contacts (Table S3) as well as the Ortep view of the HS structure at 293 K and 100 K (Fig. S1), the view of the 10 K packing showing the cations alignment (Fig. S2), the evolution of the χMT product on a large temperature range (Fig. S3), the powder X-ray diffraction pattern and magnetization measurements of desolvated compounds coming from different syntheses (Fig. S4) and the effect of grinding on the shape of the relaxation curves (Fig. S5) are given in ESI. CCDC 928909, 928910, 929981 and 929982. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt52495g
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solids.2 The spin-state switching of such compounds is displayed by drastic changes in their physical (optical, structural, electrical…) properties, which may be used for developing light-based devices.3 The conversion of such compounds may be triggered at low temperature by direct irradiation of the metallic center through the light-induced excited spin state trapping (LIESST) effect, which induces the population of the metastable HS state.4,5 Photoswitching of spin domains inside the hysteresis loop has also been reported.6 An approach called ligand-driven light-induced spin change (LD-LISC), based on photo-isomerization and structural reorganization of the ligand can also be considered in order to control the spin state and the physical properties of solids with light.7 Recent efforts have been dedicated to the study of the photoswitching mechanism involved in the LIESST effect. Ultrafast techniques have been used to probe the early stages of this transformation in solution and in crystalline solids, as new developments in time-resolved X-ray diffraction allow catching some transient structural reorganizations.8 The development of photocrystallography experiments makes it now possible to obtain the structure of the photo-excited states for compounds whose metastable HS lifetime is long enough at low temperature.9 One important issue concerns the structure of the photo-induced state and its comparison to that of
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the thermally populated HS state. Indeed, the HS phase reached by a route resulting from photo-excitation of the LS state stable at low temperature can differ from the HS phase stable at high temperature in terms of structure, order or symmetry. This is due to the fact that light allows reaching false HS ground states at low temperature.10 [Fe(mepy)3tren](PF6)2 is a well-known prototypical spincrossover compound, which undergoes a fairly steep thermal spin transition between LS (S = 0) and HS (S = 2) around 215 K.11 The photoswitching properties, based on the LIESST effect of [Zn1−xFex(mepy)3tren](PF6)2 (x = 0.005) diluted single crystals have been investigated by means of optical spectroscopy. In such crystals, the photo-induced HS state presents a 7 s lifetime at 10 K.12 The corresponding structure of the neat compound has been recently determined by X-ray diffraction measurement under continuous irradiation.13 The LS analogue [Fe( py)3tren](PF6)2 has been thoroughly studied in solution by ultrafast X-ray and optical spectroscopy for analyzing the lifetime of the photo-induced HS state and the corresponding local structure.14 The intrinsic flexibility of the hexadentate (R-py)3tren ligand through the three chelating arms linked to a seventh nitrogen atom, which can be considered as a potential coordination center for the FeII ion, arises questions on the molecular reorganization associated with the photo-induced transformation of such compound. The role of structural distortion, resulting from the molecular packing, in the stabilization of the photo-induced HS state has been recently underlined.15 This work describes the synthesis of a solvated analogue of this compound, [Fe(mepy)3tren](PF6)2·C7H8·C2H3N. The thermo- and photo-induced switching of this crystalline material have been studied by magnetization and X-ray diffraction measurements, both at variable temperature and under light irradiation, providing a better understanding of the photoswitching behavior of the [Fe(mepy)3tren]2+ cation.
Experimental section Synthesis The molecular compound [Fe(mepy)3tren](PF6)2 has been prepared as described elsewhere.11 Red crystals of the solvated compound [Fe(mepy)3tren](PF6)2·C7H8·C2H3N have been obtained by slow diffusion of a saturated acetonitrile solution of the non-solvated compound in toluene. Elemental analysis: formula C36H44F12FeN8P2 Calcd (%): C 46.27, H 4.75, N 11.99; found (%): C 46.92, H 4.76, N 10.53. Loss of weight at 140 °C compared to RT measured by thermogravimetric analysis: Calcd: 14.3% (corresponding to one molecule of acetonitrile and one molecule of toluene by complex); found: 13.9%. Grinded crystals have been used to probe the influence of the crystal quality on the properties of this compound. Thermogravimetric measurements on such grinded solids confirm that the grinding does not induce any solvent removal. For comparison purpose, the phase precipitated from an acetonitrile–toluene mixture has also been prepared, as
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previously described16 (chemical analysis, IR, UV-vis, magnetic and XRD characterizations supplied in ref. 16). Single crystal X-ray diffraction At 293 and 100 K. The X-ray diffraction data have been collected by using a Kappa X8 APPEX II Bruker diffractometer with graphite-monochromated MoKα radiation (λ = 0.71073 Å). The single-crystal has been mounted on a CryoLoop (Hampton Research) with Paratone-N (Hampton Research) as a cryoprotectant and then flashfrozen in a nitrogen-gas stream at 100 K. The temperature of the crystal has been maintained at the selected value by means of a 700 series Cryostream cooling device within an accuracy of ±1 K. The data were corrected for the Lorentz polarization and the absorption effects. The structures were solved by direct methods using SHELXS-9717 and refined against F2 by full-matrix least-squares techniques using SHELXL-9718 with anisotropic displacement parameters for all non-hydrogen atoms. Hydrogen atoms were located on a difference Fourier map and introduced into the calculations as a riding model with isotropic thermal parameters. All calculations were performed by using the Crystal Structure crystallographic software package WINGX.19 At 10 K. Structural investigations at thermal equilibrium and under continuous light irradiation (λ = 660 nm) were performed by X-ray diffraction on another single crystal. Data were collected on a four-circle Oxford Diffraction Xcalibur 3 diffractometer (MoKα radiation) with a 2D Sapphire 3 CCD detector, on a sample with a typical size around 100 × 100 × 50 μm3 in different experimental conditions. An Oxford Diffraction Helijet Helium-flow cryostat has been used for photocrystallography studies. The unit-cell parameters and the data reduction were obtained using CrysAlis software from Oxford Diffraction. The structures were solved with SIR-9720 and refined with SHELXL.18 Crystallographic data are given in Table 1. Magnetization measurements Magnetic measurements were carried out using a Quantum Design SQUID magnetometer (MPMS5S Model) calibrated against a standard palladium sample. The data were corrected from the diamagnetic contribution of the molecular compound and the sample holder. The solid in the form of singlecrystals has been freezed at 250 K prior to the magnetic measurements in order to avoid any change in the solvent content. Photo-excitation experiments were performed on a small collection of crystals deposited on Scotch tape without grinding. The magnetometer has been equipped with an optical fiber (UV grade fused silica) connected to a Nd:YAG pulsed laser Surelite–Continuum Performance (450 mJ at 1064 nm and harmonic options for 532 or 355 nm outputs). The TLIESST measurement21 has been performed with a sweeping rate of 0.3 K min−1. The HS-to-LS relaxation measurements were carried out after in situ excitation of the sample (λ = 532 nm, ∼10 mW cm−2) at a working temperature till the photostationary state has been reached. Then the decay of magnetization has been recorded in the dark. The curves were
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Crystal data and structure refinement for [Fe(mepy)3tren](PF6)2·C7H8·C2H3N at 293, 100 and 10 K before and after irradiation at 660 nm (ζ)
Crystal phase, temperature
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Empirical formula Formula weight Crystal system Space group a/Å b/Å c/Å β/° V/Å3 Z R
HS, 293 K
HS, 100 K
LS, 10 K
HS, 10 K (ζ)
13.0765(8) 16.957(1) 17.994(1) 104.200(7) 3868.0(4) 4 0.0467
13.2599(4) 16.8595(4) 18.2715(5) 103.897(3) 3965.1(2) 4 0.0387
C36H44F12FeN8P2 934.56 Monoclinic P21/n 13.4040(5) 17.4684(6) 18.3855(7) 101.934(1) 4211.9(3) 4 0.0794
scaled with respect to the values expected for a complete phototransformation.
Results and discussion Thermo-induced spin-state switching Structural properties. [Fe(mepy)3tren](PF6)2·C7H8·C2H3N crystallizes in a monoclinic P21/n space group and no symmetry change occurs upon cooling from 293 down to 10 K, nor after a 10 K irradiation at 660 nm. The relevant crystallographic parameters are collected in Table 1. The properties of the 10 K HS phase are discussed below in the section concerning the photo-induced spin-state switching. At 293 K, the salt in the form of a double solvate has a relatively expanded unit-cell, which allows large vibrational motions of fluorine (PF6−) and carbon (CH3–Ph) atoms. A low temperature (100 K) is required to minimize these effects through a large unit-cell contraction of ∼5.1% (see Fig. S1‡). This thermal variation is more pronounced along b (∼3%) (c, 1%; a, 0.7%) and is coupled to the aperture of the β angle (1.7%). As shown in Fig. 1, the coordination sphere of cations is constituted by the FeII ion chelated by 6 N atoms of the ligand in a pseudo-octahedral geometry (see Fig. S1‡ for an ORTEP view of the structure). The average Fe–N bond lengths (in Table 2) are characteristic of a HS FeII complex at 293 (2.232 Å) or 100 K (2.208 Å), while at 10 K, a value typical for a LS FeII
Fig. 1 Molecular structure of the [Fe(mepy)3tren]2+ cation at 10 K, showing 50% probability displacement ellipsoids. Hydrogen atoms, solvent molecules, and counter ions are omitted for clarity.
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13.3157(4) 16.9676(6) 18.1951(6) 103.620(1) 3995.3(2) 4 0.0376
Table 2
The geometry of the Fe site surroundings
Coordination sphere
HS, 293 K
HS, 100 K
LS, 10 K
HS, 10 K
Fe–N1/Å Fe–N2/Å Fe–N3/Å Fe–N5/Å Fe–N6/Å Fe–N7/Å 〈Fe–N〉/Åa Fe–N4/Åb ζ/Åc Σ/°d
2.286(3) 2.309(3) 2.332(3) 2.151(3) 2.141(3) 2.172(3) 2.232 3.233 0.463 107
2.247(1) 2.282(1) 2.302(1) 2.143(1) 2.123(1) 2.151(1) 2.208 3.253 0.414 104.9
2.092(3) 2.100(3) 2.092(3) 1.962(3) 1.979(3) 1.958(3) 2.031 3.552 0.385 81.7
2.286(2) 2.321(3) 2.257(3) 2.135(3) 2.168(2) 2.152(3) 2.220 3.241 0.409 107.0
6 1X dFeNi . b Distance to the nitrogen atom of the amine 6 i¼1 6 12 X X group (see Fig. 1). c ζ ¼ j90 ϕi j. jFe Li kFe Llj. d Σ ¼
a
kFe Nl ¼
i¼1
i¼1
complex (2.031 Å) indicates the occurrence of a thermallyinduced spin state change at very low temperature. As expected during the HS-to-LS transition,22 the average Fe–N distance is shortened by 0.177 Å and the asymmetry of the coordination sphere, characterized by the Σ parameter, is reduced by 23.2° (Table 2). Moreover, the geometry of the seventh nitrogen atom of the ligand, noted N4 in Fig. 1, changes from a pyramidal geometry in the HS state to a trigonal planar one in the LS state. Therefore, the Fe–N4 distance increases by 0.299 Å during the transition from HS to LS state, contrarily to the other Fe–N bonds, which decrease. The evolution of the coordination sphere driven by the spinstate switching nicely compares to those of close analogues,23,24 [Fe(mepy)3tren](ClO4)2 and [Fe(C6-mepy)3tren](ClO4)2, ((C6-mepy)3tren being the ligand substituted by aliphatic chains). As shown in Fig. 2, the crystal packing consists of chains of [Fe(mepy)3tren]2+ cations arranged along the b crystallographic axis which are held together with van der Waals contacts involving the PF6− anions and the two solvent molecules (see Fig. S2‡ for a view along the a + c axis). Along b, each cation is stabilized by direct π-stacking interactions ( parallel-displaced type) with the two adjacent complexes (distances between ring centroids in Table S1‡). The corresponding distances between atoms of pyridinic rings are 3.374 Å (interaction A) and 3.426 Å (interaction B) in the LS state (10 K), 3.455 Å (interaction A) and 3.462 Å (interaction B) in the HS state (at 100 K).
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Fig. 2 View of π-stacking contacts at 10 K along the b crystallographic axis. The two different interactions are noted A and B. Hydrogen atoms, acetonitrile molecules and PF6− anions are omitted for clarity.
Weaker π-contacts, observed for example, between the third pyridinic ring of the cation and toluene, result from a tilt angle (and larger distances) between the aromatic planes. Moreover, several X–H⋯π contacts are also observed (Table S1‡). It is worth noticing that Seredyuk et al. have described similar π-contacts (3.327 Å between atoms of pyridinic rings of [Fe(mepy)3tren]2+ cations despite distinctive features of the crystal packing in the LS state.23 In Table S3,‡ the few van der Waals contacts (interatomic distances < (sum of van der Waals radius) – 0.1 Å) ensuring the solid cohesion at 293 K are weak. At 100 K, both the unit-cell contraction and the establishment of significant direct and indirect 3D interactions (numerous π-stacking and van der Waals interactions) contribute to a phase with cooperative features. Below the transition temperature, at 10 K, these tendencies are still enhanced (Δa/a ∼ 0.018, Δb/b < 0.001, Δc/c ∼ 0.011, Δβ/β ∼ −0.006) resulting in a denser crystal packing of the LS species. Magnetic properties. The crystalline powder presents a complete spin conversion (see Fig. S3‡), with χMT values (χM being the molar magnetic susceptibility, T, the temperature) characteristic of a fully LS state at 10 K (χMT (10 K) = 0.17 cm3 K mol−1) and a HS state at 200 K (χMT (200 K) = 3.33 cm3 K mol−1). Fig. 3 presents the evolution of the HS fraction γHS, extracted from the magnetization measurement between 60 and 110 K, assuming the χMT = 3.40 cm3 K mol−1 value for the pure HS compound. A first-order phase transition
Fig. 3 Temperature dependence of the HS fraction for [Fe(mepy)3tren](PF6)2·C7H8·C2H3N extracted from a magnetization measurement at 0.5 K min−1 (in red) along with a mean-field fit of the curve (in black).
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characterized by the opening of a 3.5 K wide hysteresis loop (T1/2↓ = 86.0 K, T1/2↑ = 89.5 K) is observed. For FeII spin-crossover compounds with low transition temperature, kinetic effects leading to the trapping of residual HS species at low temperature can be observed.25 The fact that the transition is complete in this example is related to the relatively fast HS-toLS relaxation rate above 60 K (see below). This result contrasts with the behavior of the non-solvated compound (crystallized with PF6− or ClO4−), which present a fairly steep spin crossover without hysteresis centered around 220 K.11,23 The thermo-induced spin-transition curve has been fitted with eqn (1) according to the mean-field model.26 γ HS ΔH 0 TΔS0 2Γðγ HS 1=2Þ ¼ exp kB T 1 γ HS
ð1Þ
where γHS is the HS fraction, ΔH0 and ΔS0 are the standard enthalpy and entropy variation associated with the spin transition and Γ is the interaction constant. The obtained thermodynamical parameters (ΔH0 = 5.0 kJ mol−1, ΔS0 = 57 J K−1 mol−1, Γ = 1.7 kJ mol−1) fall within the range of expected values for FeII spin-crossover complexes.2 In the absence of interactions involving donor atoms, the decrease of transition temperature, compared to the non-solvated compound, can be partly accounted for by the net increase (27%) of the unit cell volume per FeII ion (in the LS states, 967 Å3/FeII ion for [Fe(mepy)3tren](PF6)2·C7H8·C2H3N and 764 Å3/FeII ion for [Fe(mepy)3tren](PF6)2)13 due to the presence of solvent molecules within the unit cell. Indeed, the expansion of the unit cell volume is expected to stabilize the HS state and thus decrease the transition temperature. Similar observations were reported by Seredyuk et al. for the compound [Fe(C6-mepy)3tren](ClO4)2 as the presence of an aliphatic chain has been associated with a decrease of the transition temperature (ca. 150 K compared to 235 K for the unsubstituted compound).24 The calculated interaction parameter Γ is slightly lower here (1.7 kJ mol−1) than that of the ClO4− salt (2.8 kJ mol−1). Nevertheless, the transition appears more cooperative for the solvated compound. This nicely illustrates the fact that the appearance of an hysteresis loop is related to the balance between Γ and 2RT1/2 (for [Fe(mepy)3tren](ClO4)2, Γ = 2.8 kJ mol−1 < 2RT1/2 = 3.6 kJ mol−1 and for [Fe(mepy)3tren](PF6)2·C7H8·C2H3N, Γ = 1.7 kJ mol−1 > 2RT1/2 = 1.5 kJ mol−1). Heating the compound at 140 °C induces a full desolvatation of the solid, accompanied by a disintegration of the single crystals, which is favored by the large difference of volume between the solvated and non-solvated solids. Powder X-ray diffraction and magnetization measurements show that the desolvatation by thermal treatment at 140 °C leads to the obtainment of a crystalline phase that is different from the one characterizing the previously reported non-solvated compound (see Fig. S4‡).11 Interestingly, this phase is similar to the one obtained by fast precipitation in an anti-solvent described by some of us in ref. 16.
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Fig. 4 Magnetic measurement of the [Fe(mepy)3tren](PF6)2·C7H8·C2H3N sample performed with a sweeping rate at 0.3 K min−1: on cooling (blue), upon irradiation at 10 K with λ = 532 nm (black) and upon heating in the dark after reaching the photostationary state at 10 K.
Photo-induced spin-state switching Photomagnetic investigation. The sample has been irradiated within the magnetometer at 10 K with a 532 nm Nd:YAG laser (see Fig. 4) until the obtainment of a photostationary state. This wavelength corresponds to the metal-toligand charge transfer (MLCT) absorption band of the LS complex. An increase of the χMT product from 0.06 to 3.06 cm3 K mol−1 is observed at 10 K, corresponding to a complete LS-to-HS photoconversion through the LIESST effect, a feature that is fully consistent with the 10 K structural analysis (below). The solid in the photo-induced HS state has then been heated up in the dark from 10 to 100 K at 0.3 K min−1 in order to evaluate at which temperature the HS-to-LS relaxation becomes faster than the characteristic magnetization measurement time (few 10 s). The corresponding temperature, called TLIESST and evaluated here at 55 K is defined as the temperature where half of the photo-excited complexes have relaxed back to the LS state.21 Structural investigation. As the lifetime of the photoinduced HS state is larger than hours at 10 K, the structure of the photo-induced HS phase has been measured by X-ray diffraction, after irradiation of a single-crystal with a 660 nm laser. This wavelength corresponds to the tail of the LS MLCT absorption band, where the laser penetration depth in the crystal is maximized.12 The photo-induced HS phase remains in the same space group than the one observed in the thermoinduced process. As shown by the metal–ligand bond lengths collected in Table 2 or the typical geometry inversion at the N4 center, the photo-induced process corresponds to a complete LS-to-HS transition. The 10 K unit-cell parameters change accordingly with a relative expansion of the volume at ∼2.5% that is smaller than the one including the thermal effect between 10 and 100 K (3.2%). Contrarily to the thermal process, the photo-induced change at 10 K originates from the crystal expansion localized in the (ac) plane (a: 1.4%; c, 1.5%). The slight reduction (−0.6%) of the b axis and the
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Fig. 5 HS-to-LS relaxation curves performed with magnetization measurements at different temperatures (○), along with their modelization in the mean-field approximation (-).
strengthening of π stacking interactions (involving either interaction A or interaction B between the pyridinic rings of the ligand, Table S1‡) suggest that the cooperativity mediated by the π-contacts is still reinforced with respect to one of the LS phases. Investigation of the HS-to-LS relaxation dynamics. The relaxation curves were registered by means of magnetization measurements at various temperatures in order to get a better insight into the relaxation mechanism (see Fig. 5). They are sigmoidal in shape, as a consequence of cooperative processes in solids.5 The present observation is fully consistent with the strong cooperativity of the thermal process and the reinforcement of the intersite interactions in the photo-induced phase. The relaxation curves can be modeled in the frame of the mean-field approximation, where the relaxation rate constant depends on the HS fraction:5 kHL ðT; γ LS Þ ¼ kHL ðT; γ LS ¼ 0ÞeαðTÞγ LS
ð2Þ
kHL(T, γLS) being the HS-to-LS relaxation rate constant for a given value of temperature T and γLS, the LS fraction. In the thermally activated regime of relaxation, the acceleration factor α(T ) can be related to the interaction parameter Γ by the equation: αðTÞ ¼
Γ kB T
ð3Þ
Therefore, a global fit has been performed by assuming that the value of Γ can be extracted from the thermal spin transition. This model well reproduces the experimental data. A small deviation for γHS close to 1 is observed at low temperature. It may be related to some inhomogeneities in the photoexcitation process or to the warming of the sample.27 For γHS lower than 0.1, a deviation from the fit consisting in a systematic slowing down of the relaxation, compared to the calculated curves is also observed. In the literature, the observation of stretched relaxation curves is accounted for by a distribution of environments due to the presence of crystalline defects in the sample.28 This feature has been confirmed here by the
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Fig. 6 Arrhenius plot of the average relaxation rate constants kav extracted from the magnetic measurements (■) along with a linear fit in the thermally-activated regime (-).
strong alteration between the relaxation curves of the compound before and after the crystals grinding (see Fig. S5‡). Moreover, more complicated phenomena, like phase separation and formation of HS and LS domains may also take place and contribute to the deviation from the mean-field approximation.15,29 Analysis of the relaxation rates. The average relaxation rate constants, kav, which were extracted from the modeling at each temperature following eqn (3), are reported in the Arrhenius plot shown in Fig. 6.5 kav ðTÞ ¼ kHL ðT; γ LS ¼ 0Þeα=2
ð4Þ
Above 50 K, a linear evolution is characterized, as expected for a HS-to-LS relaxation occurring in the thermally activated region. At 45 K, a small slowing down of the relaxation, compared to the expected value for the thermally-activated region, is indicative of a kinetic influenced by the quantum tunneling relaxation mechanism at low temperature. The HS-to-LS
relaxation timescale measured at 45 K gives an upper limit of the value of kHL(T → 0) and leads us to estimate this value at 10−5–10−6 s−1. It can be compared to the 7 s lifetime of the photo-induced HS state at 10 K measured on [Zn1−xFex(mepy)3tren](PF6)2 (x = 0.0005, T1/2 = 210 K) doped crystals by Hauser et al. Both sets of data are in agreement with the correlation between kHL(T → 0) and T1/2, previously established by Hauser.12 Analysis of the TLIESST value. The compilation of TLIESST data of a considerable set of FeII spin-crossover materials has been performed by J.-F. Létard.21 The correlations between TLIESST and T1/2 temperature values have been analyzed as a function of the apticity of the ligands. With respect to this work, the values of TLIESST = 55 K and T1/2 = 88 K observed for [Fe(mepy)3tren](PF6)2·C7H8·C2H3N compare to those reported for monodentate ligands, while (mepy)3tren is an hexadentate one. In order to discuss this observation, we have collected in Table 3 the TLIESST and T1/2 values along with some structural parameters of related tren-based complexes, which undergo a spin-crossover and, for a number of them, the LIESST effect. From Fig. 7, the first observations are: (i) for all the related compounds, the (TLIESST, T1/2) points fall in the range corresponding in ref. 21 to monodentate or bidentate ligands, (ii) there is no clear correlation between these temperature values in contrast to the literature.21 For the compounds {Fe[(Mepy)3tren]}(ClO4)2 and {Fe[(Mepy)3tren]}(PF6)2, the fact that the relaxation at 10 K is too fast for the magnetic detection precludes the determination of any TLIESST temperature. However, a TLIESST value of 30–40 K would be expected for a monodentate ligand with a thermal spin transition centered around 200–250 K and the observed relaxation timescale is therefore faster than expected. We note that the role of the conformational rearrangement of the ligands has been proposed in the analysis of the photomagnetic35,36 or the dynamical optical properties of some spin-crossover compounds. The link
Table 3 Comparison between TLIESST, T1/2 and some structural parameters of tren-based spin-crossover compounds including pyridinyl, imidazolyl or pyrazolyl heterocycles
Label
a b c d g e f h
Δ〈Fe–N〉/Å
P Δ =°
233 No 214 No LIESST structure 87.8 55 LIESST structure 97 73 113 73
0.203 0.206 0.192 0.179 0.192 0.155 0.173
122 155.5 139 146 210
0.204 0.221 0.199 0.228 0.208
Compound
T1/2/K
{Fe[(Mepy)3tren]}(ClO4)2 {Fe[(Mepy)3tren]}(PF6)2 {Fe[(Mepy)3tren] }(PF6)2·S S = C2H3N·C7H8 [Fe(L2)]Br(CF3SO3) [Fe(L3)]Cl·I3 [Fe(L1)3tren](PF6)2 (Phase 1) (Phase 2) [Fe(L4)](NO3)2·CH3NO2 [Fe(C6-trenMe)](ClO4)2 [Fe(L2)](BF4)2 3H2O
TLIESST/K
81 53 80–82 56 75
Δd(Fe–N4)/Å
ΔVSCO/Å3
Ref.
27 20.4 21.7 23.2 25.5 20.2 17.8
0.362 0.243 0.233 0.302 0.317 0.278 0.332
27.2 — 29.2 31.8 24.3 — —
23 13
50.9 44.7 25.0 34.5 49.0
0.406 0.315 0.474 0.357 0.634
— — 26.9 — —
dis
This work 34 33 30 32 24 31
HS LS P P P P Δr(Fe–N) is the average metal–ligand bond length. Δ ¼ , where is the sum of the deviation from 90° of the 12 cis N–Fe–N angles. dis
dis
dis
dis
Δd(Fe–N4) is the distance between the iron and the nitrogen atom labelled N4 in Fig. 1. ΔVSCO is the volume change associated with the spin crossover. Ligand names: (C6-trenMe) = tris[3-aza-4-(5-Cn)(6-R)(2-pyridyl)but-3-enyl]amine, L1 = (nBu-im)3tren = tris(4-{n-butylated-1H-imidazol-2yl}-3-aza-3butenyl)amine, L2 = tris{[2-((imidazol-4-yl)methylidene)amino]ethyl}amine, L3 = tris{[2-((2-methylimidazol-4-yl)methylidene)amino]ethyl}amine, L4 = (pz)3tren = tris(4-{pyrazol-3-yl}-3-aza-3-butenyl)amine.
1068 | Dalton Trans., 2014, 43, 1063–1071
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close to the FeII ion of a seventh N atom connecting the three –NvCH2-py coordinating groups leads to peculiar degrees of freedom and distortions of the FeII coordination sphere which can be used to change the TLIESST and the dynamic properties.
Conclusions
Fig. 7 Correlation diagram between TLIESST and T1/2 for several FeII spin-crossover compounds with tren-based hexadentate ligands (see labels in Table 3). The area of the points is proportional to the value of Δd(Fe–N4) reported in Table 3. The lines 1 and 2 correspond to the correlation between TLIESST and T1/2 proposed by Létard for monodentate and bidentate ligands respectively.21
between the distortion of the coordination sphere and the photo-induced HS lifetime has been recently established by Buron et al. from photocrystallography measurements of two spin-crossover polymorphs.15 Concerning the tren-based derivatives, the importance of this distortion has been pointed for discussing the ability of the metal ion to be thermally switched.32 On this basis, we can consider the structural data in Table 3. We observe in the two cases, for which the photo-induced HS structure has been determined, the similarities with the structures of the thermally populated HS state. If we assume that this fact is a general feature for this family of compounds, we can discuss their photo-induced properties by comparing first the LS and thermally-populated HS structures. For all the compounds, the variations of the metal–ligand bond length and volume during the P spin transition are very close. The Δ values that may characdis
terize the distortion of the coordination sphere, varies between 20 and 50° but no simple correlation appears between this parameter and the TLIESST value. Another marker of the distortion coupled to the spin-crossover process,23 the variation of the distance between the FeII and the seventh nitrogen atom of the ligand, noted Δd(Fe–N4), seems to be more relevant to explain the evolution in Fig. 7. The tendencies that appear are (i) for a given value of TLIESST, T1/2 increases when Δd(Fe–N4) increases (see for example b34 < c33 < d30 < e32 < h31) while (ii) for a given value of T1/2, TLIESST increases with Δd(Fe–N4). Therefore, the lifetime of the photo-induced HS state clearly depends on the ability of the ligand to undergo a large reorganization, characterized here by the change of distance between the metal center and the seventh nitrogen atom, during the spin transition. The reorganization amplitude is controlled by the intermolecular interactions in the solid state. Finally, we can suggest from this analysis that the introduction
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This work presents the synthesis of a solvated spin-transition solid [Fe(mepy)3tren](PF6)2·C7H8·C2H3N. It demonstrates the strong influence of the crystallinity on the switching properties of the [Fe(mepy)3tren]2+ cation. Indeed, while the non-solvated solid displays a fairly steep spin crossover centered at 215 K, the solvated one presents a cooperative transition with the opening of a 3.5 K wide hysteresis loop at 88 K. The relative stabilization of the HS state in the solvated compound can be accounted for by a packing of a larger unit cell volume, which favors the spin species of larger volume. As a consequence, the HS-to-LS lifetime at 10 K becomes larger than hours and a photo-induced HS structure has been determined. This HS structure shows rather similar features with the thermally populated one. The overall volume is obviously smaller, due to the absence of thermal expansion effect at 10 K. In contrast to the thermal process, the slight unit-cell compression along b axis suggests a gain of cooperativity upon photoswitching, as it is associated with strong and direct intersite interactions (π-stacking). The HS-to-LS relaxation has been studied at different temperatures in the 40–60 K range. Sigmoidal relaxation curves are observed and can be satisfactorily fitted in the frame of the mean-field theory, taking as a reference the thermodynamic parameters extracted from the thermal spin transition. The lifetime of the photo-induced HS state has then been compared with those of complexes containing tren-type hexadentate ligands. This set of experimental data discards the previously established correlations between TLIESST and T1/2. We have shown that this discrepancy may be a consequence of the coupling between different degrees of freedom involved in the spin crossover, the swelling and the distortion of the coordination sphere due to the presence of a seventh nitrogen atom in the ligand. This observation suggests that the photoinduced HS state lifetime might be modulated by using this new structural parameter.
Notes and references 1 (a) Molecular Switches, ed. B. Feringa, Wiley-VCH, Weinheim, 2nd edn, 2001; (b) O. Sato, J. Tao and Y.-Z. Zhang, Angew. Chem., Int. Ed., 2007, 46, 2152. 2 Spin Crossover in Transition Metal Compounds I – III, ed. P. Gütlich and H. A. Goodwin, Top. Curr. Chem., Springer, Heidelberg, 2004, pp. 233–235. 3 A. Bousseksou, G. Molnar, L. Salmon and W. Nicolazzi, Chem. Soc. Rev., 2011, 40, 3313. 4 (a) P. Gütlich, A. Hauser and H. Spiering, Angew. Chem., Int. Ed. Engl., 1994, 106, 2971; (b) S. Decurtins, P. Gütlich,
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PAPER
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Light-induced excited spin state trapping effect on [Fe(mepy)3tren](PF6)2 solvated crystals†‡ Antoine Tissot,*a,b Eric Rivière,a Régis Guillot,a Loic Toupet,c Eric Colletc and Marie-Laure Boillot*a A spin-crossover solvated compound [Fe(mepy)3tren](PF6)2·C7H8·C2H3N has been prepared and its switching properties have been compared to those reported for the non-solvated solid. The thermal spin transition occurs at 88 K with the opening of a 3.5 K wide hysteresis loop, while a fairly steep transition at 215 K without hysteresis has been previously reported for the non-solvated analogue. This feature has been rationalized by the analysis of the high-spin (HS) and low-spin (LS) structures, evidencing a relative stabilization of the high-spin state, as well as strong intermolecular interactions in the solvated compound. The photoswitching of the solvated solid, based on the light-induced excited spin state trapping effect, leads to a quantitative transformation from the low-spin to the high-spin state at 10 K. The long lifetime of the metastable HS state at 10 K allows the measurement of the photo-induced HS structure, where the cooperative interactions are enhanced, compared to those of the thermally populated HS structure. Then,the HS-to-LS relaxations have been studied between 45 and 60 K. They are sigmoidal in shape and
Received 11th September 2013, Accepted 8th October 2013
can be well fitted in the frame of the mean-field approximation. The relative stability of the photoinduced HS state in this family of spin crossover compounds is not directly related to their thermal spin
DOI: 10.1039/c3dt52495g
transition temperature. This unexpected observation is rationalized by a careful analysis of their structural
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characteristics.
Introduction Photoswitchable molecular compounds present a wide interest, as they may be applied in future information storage or display devices.1 Spin-crossover compounds, whose spin state can be interconverted between low-spin (LS) and high-spin (HS) state under the effect of temperature, pressure, magnetic field or light, are one of the most studied classes of switchable
a Université Paris Sud, ICMMO-ECI, UMR CNRS 8182, Université Paris-Sud, 91405 Orsay, France. E-mail: [email protected] b Université de Genève, Département de Chimie Physique, 30 Quai E. Ansermet 1211 Genève 4, Switzerland. E-mail: [email protected] c Institut de Physique de Rennes, UMR 6251, Université Rennes 1-CNRS, Rennes, France † This work has been supported by the ANR (09-BLAN-0212) project. ‡ Electronic supplementary information (ESI) available: Tables containing the analysis of the short-ring interactions (Table S1), the Pi-ring interactions (Table S2) and the intermolecular contacts (Table S3) as well as the Ortep view of the HS structure at 293 K and 100 K (Fig. S1), the view of the 10 K packing showing the cations alignment (Fig. S2), the evolution of the χMT product on a large temperature range (Fig. S3), the powder X-ray diffraction pattern and magnetization measurements of desolvated compounds coming from different syntheses (Fig. S4) and the effect of grinding on the shape of the relaxation curves (Fig. S5) are given in ESI. CCDC 928909, 928910, 929981 and 929982. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt52495g
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solids.2 The spin-state switching of such compounds is displayed by drastic changes in their physical (optical, structural, electrical…) properties, which may be used for developing light-based devices.3 The conversion of such compounds may be triggered at low temperature by direct irradiation of the metallic center through the light-induced excited spin state trapping (LIESST) effect, which induces the population of the metastable HS state.4,5 Photoswitching of spin domains inside the hysteresis loop has also been reported.6 An approach called ligand-driven light-induced spin change (LD-LISC), based on photo-isomerization and structural reorganization of the ligand can also be considered in order to control the spin state and the physical properties of solids with light.7 Recent efforts have been dedicated to the study of the photoswitching mechanism involved in the LIESST effect. Ultrafast techniques have been used to probe the early stages of this transformation in solution and in crystalline solids, as new developments in time-resolved X-ray diffraction allow catching some transient structural reorganizations.8 The development of photocrystallography experiments makes it now possible to obtain the structure of the photo-excited states for compounds whose metastable HS lifetime is long enough at low temperature.9 One important issue concerns the structure of the photo-induced state and its comparison to that of
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the thermally populated HS state. Indeed, the HS phase reached by a route resulting from photo-excitation of the LS state stable at low temperature can differ from the HS phase stable at high temperature in terms of structure, order or symmetry. This is due to the fact that light allows reaching false HS ground states at low temperature.10 [Fe(mepy)3tren](PF6)2 is a well-known prototypical spincrossover compound, which undergoes a fairly steep thermal spin transition between LS (S = 0) and HS (S = 2) around 215 K.11 The photoswitching properties, based on the LIESST effect of [Zn1−xFex(mepy)3tren](PF6)2 (x = 0.005) diluted single crystals have been investigated by means of optical spectroscopy. In such crystals, the photo-induced HS state presents a 7 s lifetime at 10 K.12 The corresponding structure of the neat compound has been recently determined by X-ray diffraction measurement under continuous irradiation.13 The LS analogue [Fe( py)3tren](PF6)2 has been thoroughly studied in solution by ultrafast X-ray and optical spectroscopy for analyzing the lifetime of the photo-induced HS state and the corresponding local structure.14 The intrinsic flexibility of the hexadentate (R-py)3tren ligand through the three chelating arms linked to a seventh nitrogen atom, which can be considered as a potential coordination center for the FeII ion, arises questions on the molecular reorganization associated with the photo-induced transformation of such compound. The role of structural distortion, resulting from the molecular packing, in the stabilization of the photo-induced HS state has been recently underlined.15 This work describes the synthesis of a solvated analogue of this compound, [Fe(mepy)3tren](PF6)2·C7H8·C2H3N. The thermo- and photo-induced switching of this crystalline material have been studied by magnetization and X-ray diffraction measurements, both at variable temperature and under light irradiation, providing a better understanding of the photoswitching behavior of the [Fe(mepy)3tren]2+ cation.
Experimental section Synthesis The molecular compound [Fe(mepy)3tren](PF6)2 has been prepared as described elsewhere.11 Red crystals of the solvated compound [Fe(mepy)3tren](PF6)2·C7H8·C2H3N have been obtained by slow diffusion of a saturated acetonitrile solution of the non-solvated compound in toluene. Elemental analysis: formula C36H44F12FeN8P2 Calcd (%): C 46.27, H 4.75, N 11.99; found (%): C 46.92, H 4.76, N 10.53. Loss of weight at 140 °C compared to RT measured by thermogravimetric analysis: Calcd: 14.3% (corresponding to one molecule of acetonitrile and one molecule of toluene by complex); found: 13.9%. Grinded crystals have been used to probe the influence of the crystal quality on the properties of this compound. Thermogravimetric measurements on such grinded solids confirm that the grinding does not induce any solvent removal. For comparison purpose, the phase precipitated from an acetonitrile–toluene mixture has also been prepared, as
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Dalton Transactions
previously described16 (chemical analysis, IR, UV-vis, magnetic and XRD characterizations supplied in ref. 16). Single crystal X-ray diffraction At 293 and 100 K. The X-ray diffraction data have been collected by using a Kappa X8 APPEX II Bruker diffractometer with graphite-monochromated MoKα radiation (λ = 0.71073 Å). The single-crystal has been mounted on a CryoLoop (Hampton Research) with Paratone-N (Hampton Research) as a cryoprotectant and then flashfrozen in a nitrogen-gas stream at 100 K. The temperature of the crystal has been maintained at the selected value by means of a 700 series Cryostream cooling device within an accuracy of ±1 K. The data were corrected for the Lorentz polarization and the absorption effects. The structures were solved by direct methods using SHELXS-9717 and refined against F2 by full-matrix least-squares techniques using SHELXL-9718 with anisotropic displacement parameters for all non-hydrogen atoms. Hydrogen atoms were located on a difference Fourier map and introduced into the calculations as a riding model with isotropic thermal parameters. All calculations were performed by using the Crystal Structure crystallographic software package WINGX.19 At 10 K. Structural investigations at thermal equilibrium and under continuous light irradiation (λ = 660 nm) were performed by X-ray diffraction on another single crystal. Data were collected on a four-circle Oxford Diffraction Xcalibur 3 diffractometer (MoKα radiation) with a 2D Sapphire 3 CCD detector, on a sample with a typical size around 100 × 100 × 50 μm3 in different experimental conditions. An Oxford Diffraction Helijet Helium-flow cryostat has been used for photocrystallography studies. The unit-cell parameters and the data reduction were obtained using CrysAlis software from Oxford Diffraction. The structures were solved with SIR-9720 and refined with SHELXL.18 Crystallographic data are given in Table 1. Magnetization measurements Magnetic measurements were carried out using a Quantum Design SQUID magnetometer (MPMS5S Model) calibrated against a standard palladium sample. The data were corrected from the diamagnetic contribution of the molecular compound and the sample holder. The solid in the form of singlecrystals has been freezed at 250 K prior to the magnetic measurements in order to avoid any change in the solvent content. Photo-excitation experiments were performed on a small collection of crystals deposited on Scotch tape without grinding. The magnetometer has been equipped with an optical fiber (UV grade fused silica) connected to a Nd:YAG pulsed laser Surelite–Continuum Performance (450 mJ at 1064 nm and harmonic options for 532 or 355 nm outputs). The TLIESST measurement21 has been performed with a sweeping rate of 0.3 K min−1. The HS-to-LS relaxation measurements were carried out after in situ excitation of the sample (λ = 532 nm, ∼10 mW cm−2) at a working temperature till the photostationary state has been reached. Then the decay of magnetization has been recorded in the dark. The curves were
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Dalton Transactions Table 1
Crystal data and structure refinement for [Fe(mepy)3tren](PF6)2·C7H8·C2H3N at 293, 100 and 10 K before and after irradiation at 660 nm (ζ)
Crystal phase, temperature
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Empirical formula Formula weight Crystal system Space group a/Å b/Å c/Å β/° V/Å3 Z R
HS, 293 K
HS, 100 K
LS, 10 K
HS, 10 K (ζ)
13.0765(8) 16.957(1) 17.994(1) 104.200(7) 3868.0(4) 4 0.0467
13.2599(4) 16.8595(4) 18.2715(5) 103.897(3) 3965.1(2) 4 0.0387
C36H44F12FeN8P2 934.56 Monoclinic P21/n 13.4040(5) 17.4684(6) 18.3855(7) 101.934(1) 4211.9(3) 4 0.0794
scaled with respect to the values expected for a complete phototransformation.
Results and discussion Thermo-induced spin-state switching Structural properties. [Fe(mepy)3tren](PF6)2·C7H8·C2H3N crystallizes in a monoclinic P21/n space group and no symmetry change occurs upon cooling from 293 down to 10 K, nor after a 10 K irradiation at 660 nm. The relevant crystallographic parameters are collected in Table 1. The properties of the 10 K HS phase are discussed below in the section concerning the photo-induced spin-state switching. At 293 K, the salt in the form of a double solvate has a relatively expanded unit-cell, which allows large vibrational motions of fluorine (PF6−) and carbon (CH3–Ph) atoms. A low temperature (100 K) is required to minimize these effects through a large unit-cell contraction of ∼5.1% (see Fig. S1‡). This thermal variation is more pronounced along b (∼3%) (c, 1%; a, 0.7%) and is coupled to the aperture of the β angle (1.7%). As shown in Fig. 1, the coordination sphere of cations is constituted by the FeII ion chelated by 6 N atoms of the ligand in a pseudo-octahedral geometry (see Fig. S1‡ for an ORTEP view of the structure). The average Fe–N bond lengths (in Table 2) are characteristic of a HS FeII complex at 293 (2.232 Å) or 100 K (2.208 Å), while at 10 K, a value typical for a LS FeII
Fig. 1 Molecular structure of the [Fe(mepy)3tren]2+ cation at 10 K, showing 50% probability displacement ellipsoids. Hydrogen atoms, solvent molecules, and counter ions are omitted for clarity.
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13.3157(4) 16.9676(6) 18.1951(6) 103.620(1) 3995.3(2) 4 0.0376
Table 2
The geometry of the Fe site surroundings
Coordination sphere
HS, 293 K
HS, 100 K
LS, 10 K
HS, 10 K
Fe–N1/Å Fe–N2/Å Fe–N3/Å Fe–N5/Å Fe–N6/Å Fe–N7/Å 〈Fe–N〉/Åa Fe–N4/Åb ζ/Åc Σ/°d
2.286(3) 2.309(3) 2.332(3) 2.151(3) 2.141(3) 2.172(3) 2.232 3.233 0.463 107
2.247(1) 2.282(1) 2.302(1) 2.143(1) 2.123(1) 2.151(1) 2.208 3.253 0.414 104.9
2.092(3) 2.100(3) 2.092(3) 1.962(3) 1.979(3) 1.958(3) 2.031 3.552 0.385 81.7
2.286(2) 2.321(3) 2.257(3) 2.135(3) 2.168(2) 2.152(3) 2.220 3.241 0.409 107.0
6 1X dFeNi . b Distance to the nitrogen atom of the amine 6 i¼1 6 12 X X group (see Fig. 1). c ζ ¼ j90 ϕi j. jFe Li kFe Llj. d Σ ¼
a
kFe Nl ¼
i¼1
i¼1
complex (2.031 Å) indicates the occurrence of a thermallyinduced spin state change at very low temperature. As expected during the HS-to-LS transition,22 the average Fe–N distance is shortened by 0.177 Å and the asymmetry of the coordination sphere, characterized by the Σ parameter, is reduced by 23.2° (Table 2). Moreover, the geometry of the seventh nitrogen atom of the ligand, noted N4 in Fig. 1, changes from a pyramidal geometry in the HS state to a trigonal planar one in the LS state. Therefore, the Fe–N4 distance increases by 0.299 Å during the transition from HS to LS state, contrarily to the other Fe–N bonds, which decrease. The evolution of the coordination sphere driven by the spinstate switching nicely compares to those of close analogues,23,24 [Fe(mepy)3tren](ClO4)2 and [Fe(C6-mepy)3tren](ClO4)2, ((C6-mepy)3tren being the ligand substituted by aliphatic chains). As shown in Fig. 2, the crystal packing consists of chains of [Fe(mepy)3tren]2+ cations arranged along the b crystallographic axis which are held together with van der Waals contacts involving the PF6− anions and the two solvent molecules (see Fig. S2‡ for a view along the a + c axis). Along b, each cation is stabilized by direct π-stacking interactions ( parallel-displaced type) with the two adjacent complexes (distances between ring centroids in Table S1‡). The corresponding distances between atoms of pyridinic rings are 3.374 Å (interaction A) and 3.426 Å (interaction B) in the LS state (10 K), 3.455 Å (interaction A) and 3.462 Å (interaction B) in the HS state (at 100 K).
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Fig. 2 View of π-stacking contacts at 10 K along the b crystallographic axis. The two different interactions are noted A and B. Hydrogen atoms, acetonitrile molecules and PF6− anions are omitted for clarity.
Weaker π-contacts, observed for example, between the third pyridinic ring of the cation and toluene, result from a tilt angle (and larger distances) between the aromatic planes. Moreover, several X–H⋯π contacts are also observed (Table S1‡). It is worth noticing that Seredyuk et al. have described similar π-contacts (3.327 Å between atoms of pyridinic rings of [Fe(mepy)3tren]2+ cations despite distinctive features of the crystal packing in the LS state.23 In Table S3,‡ the few van der Waals contacts (interatomic distances < (sum of van der Waals radius) – 0.1 Å) ensuring the solid cohesion at 293 K are weak. At 100 K, both the unit-cell contraction and the establishment of significant direct and indirect 3D interactions (numerous π-stacking and van der Waals interactions) contribute to a phase with cooperative features. Below the transition temperature, at 10 K, these tendencies are still enhanced (Δa/a ∼ 0.018, Δb/b < 0.001, Δc/c ∼ 0.011, Δβ/β ∼ −0.006) resulting in a denser crystal packing of the LS species. Magnetic properties. The crystalline powder presents a complete spin conversion (see Fig. S3‡), with χMT values (χM being the molar magnetic susceptibility, T, the temperature) characteristic of a fully LS state at 10 K (χMT (10 K) = 0.17 cm3 K mol−1) and a HS state at 200 K (χMT (200 K) = 3.33 cm3 K mol−1). Fig. 3 presents the evolution of the HS fraction γHS, extracted from the magnetization measurement between 60 and 110 K, assuming the χMT = 3.40 cm3 K mol−1 value for the pure HS compound. A first-order phase transition
Fig. 3 Temperature dependence of the HS fraction for [Fe(mepy)3tren](PF6)2·C7H8·C2H3N extracted from a magnetization measurement at 0.5 K min−1 (in red) along with a mean-field fit of the curve (in black).
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characterized by the opening of a 3.5 K wide hysteresis loop (T1/2↓ = 86.0 K, T1/2↑ = 89.5 K) is observed. For FeII spin-crossover compounds with low transition temperature, kinetic effects leading to the trapping of residual HS species at low temperature can be observed.25 The fact that the transition is complete in this example is related to the relatively fast HS-toLS relaxation rate above 60 K (see below). This result contrasts with the behavior of the non-solvated compound (crystallized with PF6− or ClO4−), which present a fairly steep spin crossover without hysteresis centered around 220 K.11,23 The thermo-induced spin-transition curve has been fitted with eqn (1) according to the mean-field model.26 γ HS ΔH 0 TΔS0 2Γðγ HS 1=2Þ ¼ exp kB T 1 γ HS
ð1Þ
where γHS is the HS fraction, ΔH0 and ΔS0 are the standard enthalpy and entropy variation associated with the spin transition and Γ is the interaction constant. The obtained thermodynamical parameters (ΔH0 = 5.0 kJ mol−1, ΔS0 = 57 J K−1 mol−1, Γ = 1.7 kJ mol−1) fall within the range of expected values for FeII spin-crossover complexes.2 In the absence of interactions involving donor atoms, the decrease of transition temperature, compared to the non-solvated compound, can be partly accounted for by the net increase (27%) of the unit cell volume per FeII ion (in the LS states, 967 Å3/FeII ion for [Fe(mepy)3tren](PF6)2·C7H8·C2H3N and 764 Å3/FeII ion for [Fe(mepy)3tren](PF6)2)13 due to the presence of solvent molecules within the unit cell. Indeed, the expansion of the unit cell volume is expected to stabilize the HS state and thus decrease the transition temperature. Similar observations were reported by Seredyuk et al. for the compound [Fe(C6-mepy)3tren](ClO4)2 as the presence of an aliphatic chain has been associated with a decrease of the transition temperature (ca. 150 K compared to 235 K for the unsubstituted compound).24 The calculated interaction parameter Γ is slightly lower here (1.7 kJ mol−1) than that of the ClO4− salt (2.8 kJ mol−1). Nevertheless, the transition appears more cooperative for the solvated compound. This nicely illustrates the fact that the appearance of an hysteresis loop is related to the balance between Γ and 2RT1/2 (for [Fe(mepy)3tren](ClO4)2, Γ = 2.8 kJ mol−1 < 2RT1/2 = 3.6 kJ mol−1 and for [Fe(mepy)3tren](PF6)2·C7H8·C2H3N, Γ = 1.7 kJ mol−1 > 2RT1/2 = 1.5 kJ mol−1). Heating the compound at 140 °C induces a full desolvatation of the solid, accompanied by a disintegration of the single crystals, which is favored by the large difference of volume between the solvated and non-solvated solids. Powder X-ray diffraction and magnetization measurements show that the desolvatation by thermal treatment at 140 °C leads to the obtainment of a crystalline phase that is different from the one characterizing the previously reported non-solvated compound (see Fig. S4‡).11 Interestingly, this phase is similar to the one obtained by fast precipitation in an anti-solvent described by some of us in ref. 16.
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Fig. 4 Magnetic measurement of the [Fe(mepy)3tren](PF6)2·C7H8·C2H3N sample performed with a sweeping rate at 0.3 K min−1: on cooling (blue), upon irradiation at 10 K with λ = 532 nm (black) and upon heating in the dark after reaching the photostationary state at 10 K.
Photo-induced spin-state switching Photomagnetic investigation. The sample has been irradiated within the magnetometer at 10 K with a 532 nm Nd:YAG laser (see Fig. 4) until the obtainment of a photostationary state. This wavelength corresponds to the metal-toligand charge transfer (MLCT) absorption band of the LS complex. An increase of the χMT product from 0.06 to 3.06 cm3 K mol−1 is observed at 10 K, corresponding to a complete LS-to-HS photoconversion through the LIESST effect, a feature that is fully consistent with the 10 K structural analysis (below). The solid in the photo-induced HS state has then been heated up in the dark from 10 to 100 K at 0.3 K min−1 in order to evaluate at which temperature the HS-to-LS relaxation becomes faster than the characteristic magnetization measurement time (few 10 s). The corresponding temperature, called TLIESST and evaluated here at 55 K is defined as the temperature where half of the photo-excited complexes have relaxed back to the LS state.21 Structural investigation. As the lifetime of the photoinduced HS state is larger than hours at 10 K, the structure of the photo-induced HS phase has been measured by X-ray diffraction, after irradiation of a single-crystal with a 660 nm laser. This wavelength corresponds to the tail of the LS MLCT absorption band, where the laser penetration depth in the crystal is maximized.12 The photo-induced HS phase remains in the same space group than the one observed in the thermoinduced process. As shown by the metal–ligand bond lengths collected in Table 2 or the typical geometry inversion at the N4 center, the photo-induced process corresponds to a complete LS-to-HS transition. The 10 K unit-cell parameters change accordingly with a relative expansion of the volume at ∼2.5% that is smaller than the one including the thermal effect between 10 and 100 K (3.2%). Contrarily to the thermal process, the photo-induced change at 10 K originates from the crystal expansion localized in the (ac) plane (a: 1.4%; c, 1.5%). The slight reduction (−0.6%) of the b axis and the
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Fig. 5 HS-to-LS relaxation curves performed with magnetization measurements at different temperatures (○), along with their modelization in the mean-field approximation (-).
strengthening of π stacking interactions (involving either interaction A or interaction B between the pyridinic rings of the ligand, Table S1‡) suggest that the cooperativity mediated by the π-contacts is still reinforced with respect to one of the LS phases. Investigation of the HS-to-LS relaxation dynamics. The relaxation curves were registered by means of magnetization measurements at various temperatures in order to get a better insight into the relaxation mechanism (see Fig. 5). They are sigmoidal in shape, as a consequence of cooperative processes in solids.5 The present observation is fully consistent with the strong cooperativity of the thermal process and the reinforcement of the intersite interactions in the photo-induced phase. The relaxation curves can be modeled in the frame of the mean-field approximation, where the relaxation rate constant depends on the HS fraction:5 kHL ðT; γ LS Þ ¼ kHL ðT; γ LS ¼ 0ÞeαðTÞγ LS
ð2Þ
kHL(T, γLS) being the HS-to-LS relaxation rate constant for a given value of temperature T and γLS, the LS fraction. In the thermally activated regime of relaxation, the acceleration factor α(T ) can be related to the interaction parameter Γ by the equation: αðTÞ ¼
Γ kB T
ð3Þ
Therefore, a global fit has been performed by assuming that the value of Γ can be extracted from the thermal spin transition. This model well reproduces the experimental data. A small deviation for γHS close to 1 is observed at low temperature. It may be related to some inhomogeneities in the photoexcitation process or to the warming of the sample.27 For γHS lower than 0.1, a deviation from the fit consisting in a systematic slowing down of the relaxation, compared to the calculated curves is also observed. In the literature, the observation of stretched relaxation curves is accounted for by a distribution of environments due to the presence of crystalline defects in the sample.28 This feature has been confirmed here by the
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Fig. 6 Arrhenius plot of the average relaxation rate constants kav extracted from the magnetic measurements (■) along with a linear fit in the thermally-activated regime (-).
strong alteration between the relaxation curves of the compound before and after the crystals grinding (see Fig. S5‡). Moreover, more complicated phenomena, like phase separation and formation of HS and LS domains may also take place and contribute to the deviation from the mean-field approximation.15,29 Analysis of the relaxation rates. The average relaxation rate constants, kav, which were extracted from the modeling at each temperature following eqn (3), are reported in the Arrhenius plot shown in Fig. 6.5 kav ðTÞ ¼ kHL ðT; γ LS ¼ 0Þeα=2
ð4Þ
Above 50 K, a linear evolution is characterized, as expected for a HS-to-LS relaxation occurring in the thermally activated region. At 45 K, a small slowing down of the relaxation, compared to the expected value for the thermally-activated region, is indicative of a kinetic influenced by the quantum tunneling relaxation mechanism at low temperature. The HS-to-LS
relaxation timescale measured at 45 K gives an upper limit of the value of kHL(T → 0) and leads us to estimate this value at 10−5–10−6 s−1. It can be compared to the 7 s lifetime of the photo-induced HS state at 10 K measured on [Zn1−xFex(mepy)3tren](PF6)2 (x = 0.0005, T1/2 = 210 K) doped crystals by Hauser et al. Both sets of data are in agreement with the correlation between kHL(T → 0) and T1/2, previously established by Hauser.12 Analysis of the TLIESST value. The compilation of TLIESST data of a considerable set of FeII spin-crossover materials has been performed by J.-F. Létard.21 The correlations between TLIESST and T1/2 temperature values have been analyzed as a function of the apticity of the ligands. With respect to this work, the values of TLIESST = 55 K and T1/2 = 88 K observed for [Fe(mepy)3tren](PF6)2·C7H8·C2H3N compare to those reported for monodentate ligands, while (mepy)3tren is an hexadentate one. In order to discuss this observation, we have collected in Table 3 the TLIESST and T1/2 values along with some structural parameters of related tren-based complexes, which undergo a spin-crossover and, for a number of them, the LIESST effect. From Fig. 7, the first observations are: (i) for all the related compounds, the (TLIESST, T1/2) points fall in the range corresponding in ref. 21 to monodentate or bidentate ligands, (ii) there is no clear correlation between these temperature values in contrast to the literature.21 For the compounds {Fe[(Mepy)3tren]}(ClO4)2 and {Fe[(Mepy)3tren]}(PF6)2, the fact that the relaxation at 10 K is too fast for the magnetic detection precludes the determination of any TLIESST temperature. However, a TLIESST value of 30–40 K would be expected for a monodentate ligand with a thermal spin transition centered around 200–250 K and the observed relaxation timescale is therefore faster than expected. We note that the role of the conformational rearrangement of the ligands has been proposed in the analysis of the photomagnetic35,36 or the dynamical optical properties of some spin-crossover compounds. The link
Table 3 Comparison between TLIESST, T1/2 and some structural parameters of tren-based spin-crossover compounds including pyridinyl, imidazolyl or pyrazolyl heterocycles
Label
a b c d g e f h
Δ〈Fe–N〉/Å
P Δ =°
233 No 214 No LIESST structure 87.8 55 LIESST structure 97 73 113 73
0.203 0.206 0.192 0.179 0.192 0.155 0.173
122 155.5 139 146 210
0.204 0.221 0.199 0.228 0.208
Compound
T1/2/K
{Fe[(Mepy)3tren]}(ClO4)2 {Fe[(Mepy)3tren]}(PF6)2 {Fe[(Mepy)3tren] }(PF6)2·S S = C2H3N·C7H8 [Fe(L2)]Br(CF3SO3) [Fe(L3)]Cl·I3 [Fe(L1)3tren](PF6)2 (Phase 1) (Phase 2) [Fe(L4)](NO3)2·CH3NO2 [Fe(C6-trenMe)](ClO4)2 [Fe(L2)](BF4)2 3H2O
TLIESST/K
81 53 80–82 56 75
Δd(Fe–N4)/Å
ΔVSCO/Å3
Ref.
27 20.4 21.7 23.2 25.5 20.2 17.8
0.362 0.243 0.233 0.302 0.317 0.278 0.332
27.2 — 29.2 31.8 24.3 — —
23 13
50.9 44.7 25.0 34.5 49.0
0.406 0.315 0.474 0.357 0.634
— — 26.9 — —
dis
This work 34 33 30 32 24 31
HS LS P P P P Δr(Fe–N) is the average metal–ligand bond length. Δ ¼ , where is the sum of the deviation from 90° of the 12 cis N–Fe–N angles. dis
dis
dis
dis
Δd(Fe–N4) is the distance between the iron and the nitrogen atom labelled N4 in Fig. 1. ΔVSCO is the volume change associated with the spin crossover. Ligand names: (C6-trenMe) = tris[3-aza-4-(5-Cn)(6-R)(2-pyridyl)but-3-enyl]amine, L1 = (nBu-im)3tren = tris(4-{n-butylated-1H-imidazol-2yl}-3-aza-3butenyl)amine, L2 = tris{[2-((imidazol-4-yl)methylidene)amino]ethyl}amine, L3 = tris{[2-((2-methylimidazol-4-yl)methylidene)amino]ethyl}amine, L4 = (pz)3tren = tris(4-{pyrazol-3-yl}-3-aza-3-butenyl)amine.
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close to the FeII ion of a seventh N atom connecting the three –NvCH2-py coordinating groups leads to peculiar degrees of freedom and distortions of the FeII coordination sphere which can be used to change the TLIESST and the dynamic properties.
Conclusions
Fig. 7 Correlation diagram between TLIESST and T1/2 for several FeII spin-crossover compounds with tren-based hexadentate ligands (see labels in Table 3). The area of the points is proportional to the value of Δd(Fe–N4) reported in Table 3. The lines 1 and 2 correspond to the correlation between TLIESST and T1/2 proposed by Létard for monodentate and bidentate ligands respectively.21
between the distortion of the coordination sphere and the photo-induced HS lifetime has been recently established by Buron et al. from photocrystallography measurements of two spin-crossover polymorphs.15 Concerning the tren-based derivatives, the importance of this distortion has been pointed for discussing the ability of the metal ion to be thermally switched.32 On this basis, we can consider the structural data in Table 3. We observe in the two cases, for which the photo-induced HS structure has been determined, the similarities with the structures of the thermally populated HS state. If we assume that this fact is a general feature for this family of compounds, we can discuss their photo-induced properties by comparing first the LS and thermally-populated HS structures. For all the compounds, the variations of the metal–ligand bond length and volume during the P spin transition are very close. The Δ values that may characdis
terize the distortion of the coordination sphere, varies between 20 and 50° but no simple correlation appears between this parameter and the TLIESST value. Another marker of the distortion coupled to the spin-crossover process,23 the variation of the distance between the FeII and the seventh nitrogen atom of the ligand, noted Δd(Fe–N4), seems to be more relevant to explain the evolution in Fig. 7. The tendencies that appear are (i) for a given value of TLIESST, T1/2 increases when Δd(Fe–N4) increases (see for example b34 < c33 < d30 < e32 < h31) while (ii) for a given value of T1/2, TLIESST increases with Δd(Fe–N4). Therefore, the lifetime of the photo-induced HS state clearly depends on the ability of the ligand to undergo a large reorganization, characterized here by the change of distance between the metal center and the seventh nitrogen atom, during the spin transition. The reorganization amplitude is controlled by the intermolecular interactions in the solid state. Finally, we can suggest from this analysis that the introduction
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This work presents the synthesis of a solvated spin-transition solid [Fe(mepy)3tren](PF6)2·C7H8·C2H3N. It demonstrates the strong influence of the crystallinity on the switching properties of the [Fe(mepy)3tren]2+ cation. Indeed, while the non-solvated solid displays a fairly steep spin crossover centered at 215 K, the solvated one presents a cooperative transition with the opening of a 3.5 K wide hysteresis loop at 88 K. The relative stabilization of the HS state in the solvated compound can be accounted for by a packing of a larger unit cell volume, which favors the spin species of larger volume. As a consequence, the HS-to-LS lifetime at 10 K becomes larger than hours and a photo-induced HS structure has been determined. This HS structure shows rather similar features with the thermally populated one. The overall volume is obviously smaller, due to the absence of thermal expansion effect at 10 K. In contrast to the thermal process, the slight unit-cell compression along b axis suggests a gain of cooperativity upon photoswitching, as it is associated with strong and direct intersite interactions (π-stacking). The HS-to-LS relaxation has been studied at different temperatures in the 40–60 K range. Sigmoidal relaxation curves are observed and can be satisfactorily fitted in the frame of the mean-field theory, taking as a reference the thermodynamic parameters extracted from the thermal spin transition. The lifetime of the photo-induced HS state has then been compared with those of complexes containing tren-type hexadentate ligands. This set of experimental data discards the previously established correlations between TLIESST and T1/2. We have shown that this discrepancy may be a consequence of the coupling between different degrees of freedom involved in the spin crossover, the swelling and the distortion of the coordination sphere due to the presence of a seventh nitrogen atom in the ligand. This observation suggests that the photoinduced HS state lifetime might be modulated by using this new structural parameter.
Notes and references 1 (a) Molecular Switches, ed. B. Feringa, Wiley-VCH, Weinheim, 2nd edn, 2001; (b) O. Sato, J. Tao and Y.-Z. Zhang, Angew. Chem., Int. Ed., 2007, 46, 2152. 2 Spin Crossover in Transition Metal Compounds I – III, ed. P. Gütlich and H. A. Goodwin, Top. Curr. Chem., Springer, Heidelberg, 2004, pp. 233–235. 3 A. Bousseksou, G. Molnar, L. Salmon and W. Nicolazzi, Chem. Soc. Rev., 2011, 40, 3313. 4 (a) P. Gütlich, A. Hauser and H. Spiering, Angew. Chem., Int. Ed. Engl., 1994, 106, 2971; (b) S. Decurtins, P. Gütlich,
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5 6
7
8
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