Light-induced Thermal Hysteresis Research Articles

Interface versus Bulk Light-Induced Switching in Spin-Crossover Molecular Ultrathin Films Adsorbed on a Metallic Surface.

Spin-crossover molecules present the unique property of having two spin states that can be controlled by light excitation at low temperature. Here, we report on the photoexcitation of [FeII((3, 5-(CH3)2Pz)3BH)2] (Pz = pyrazolyl) ultrathin films, with thicknesses ranging from 0.9 to 5.3 monolayers, adsorbed on Cu(111) substrate. Using X-ray absorption spectroscopy measurements, we confirm the anomalous light-induced spin-state switching observed for sub-monolayer coverage and demonstrate that it is confined to the first molecular layer in contact with the metallic substrate. For higher coverages, the well-known light-induced excited spin-state trapping effect is recovered. Combining continuous light excitation with thermal cycling, we demonstrate that at low temperature light-induced thermal hysteresis is measured for the thicker films, while for sub-monolayer coverage, the light enables extension of the thermal conversion over a large temperature range. Mechanoelastic simulations underline that, due to the intermolecular interactions, opposite behaviors are observed in the different layers composing the films.

Light-induced thermal hysteresis and high-spin low-spin domain formation evidenced by optical microscopy in a spin-crossover single crystal

The low-temperature photoinduced effects of the spin-crossover $[{\mathrm{Fe}{(2\text{\ensuremath{-}}\mathrm{pytrz})}_{2}[\mathrm{Pd}{(\mathrm{CN})}_{4}]}].3{\mathrm{H}}_{2}\mathrm{O}$ single crystal have been investigated by means of a cryogenic optical microscopy technique down to 10 K from which the imaging and quantitative analysis of the spatiotemporal transformation are derived. The magnetic investigations revealed that this compound exhibits an incomplete spin transition between a full high-spin (HS) state at high temperature and an intermediate HS and low-spin (LS) state, where HS and LS species coexist, as a result of the existence of an elastic frustration at the molecular scale, most likely caused by the rigidity of the interconnected $[\mathrm{Pd}{(\mathrm{CN})}_{4}$] [Fe(II)/Pd(II)] two-dimensional network. At low temperature (10 K), thanks to reverse light-induced excited spin-state trapping effect, we could switch the system from the intermediate HS-LS state to the fully photoinduced LS state by irradiating the sample in near-infrared region, revealed by photomagnetic and optical microscopy studies. Optical microscopy images showed monotonous and homogeneous transformation of the crystal color along this process, corresponding to a gradual change of the spin state under light. In contrast, the thermal relaxation in the dark of this photoinduced LS-LS state shows a transition to the intermediate HS-LS state at $\ensuremath{\sim}90$ K with domain formation, which is characteristic of a first-order transition at equilibrium. Interestingly, the same behavior is also obtained during the heating process of the reverse light-induced thermal hysteresis cycle with a heating branch almost unchanged, confirming that light does not act on this transition. It is then concluded that the transition from the full LS to the intermediate HS-LS transition is of first order and therefore the LS state reached by light is a hidden stable state of the system until $\ensuremath{\sim}90$ K. These experimental results are modeled using an adapted version of the electroelastic model including photoexcitation effects and solved by Monte Carlo method. Both thermal equilibrium and light-induced thermal hysteresis are reproduced, in fair qualitative agreement with the experimental data of photomagnetism and optical microscopy.

Unexpected Light-Induced Thermal Hysteresis in Matrix Embedded Low Cooperative Spin Crossover Microparticles

The embedding of spin-crossover micro- or nanocrystals in various surroundings dramatically changes their functionalities based on first-order spin transitions. The dampening of their internal cooperativity, together with introducing a new kind of interactions occurring at interfaces between spin-crossover particles and their environment, results in spectacular effects, as an enhanced hysteresis with non-cooperative transitions. In this work, we deal with the influence of the embedding matrix on the light-induced thermal hysteresis (LITH) in the case of spin-crossover microparticles of Fe(phen)2(NCS)2. Despite the low cooperativity of this compound, the competition between the continuous photoexcitation towards the metastable high spin state and the relaxation down to low spin ground state leads to a light-induced thermal hysteresis, with a quasi-static width of around 10 K. This unexpected hysteresis is explained by considering a switch-on/cutoff mechanism of the particle–matrix interactions in the framework of a mean-field approach based on negative external pressures, with Gaussian distributed variations and of an Ising-like model with various interactions with the environment. Additional first-order reversal curves measurements and corresponding calculated distributions are in line with relaxations under light and confirm the existence of a non-kinetic LITH.

Thermo- and photoinduced spin state switching in an iron(II) 2D coordination network associated with large light-induced thermal hysteresis and tuning of dimensionality via ligand modulation.

Three iron(ii) complexes, [Fe(L1)2(NCS)2(MeOH)2] (1), [Fe(L1)2(NCSe)2(MeOH)2] (2), and [Fe(L2)2(NCS)2]n (3) (L1 = 2,5-dipyridyl-3,4-ethylenedioxythiophene and L2 = 2,5-diethynylpyridinyl-3,4-ethylenedioxythiophene), have been synthesized using redox-active luminescent ethylenedioxythiophene (EDOT)-based ligands, and characterized by variable temperature single-crystal X-ray diffraction, (photo)magnetic, optical reflectivity, and spectroscopy studies. Magneto-structural investigations revealed that 1 and 2 are mononuclear with a FeN4O2 octahedral coordination geometry and remain in a high-spin (HS) (S = 2) state in a temperature range of 2-280 K. Interestingly, a 2D coordination network structure with FeN6 surrounding each iron center was observed for 3, which exhibits reversible thermo-induced spin-state switching between the paramagnetic high-spin (HS) (S = 2) and diamagnetic low-spin (LS) (S = 0) states at around 105 K (T1/2). Furthermore, optical reflectivity and photomagnetic measurements at low temperature confirmed that 3 shows reversible ON/OFF switching between the photoinduced excited paramagnetic HS metastable state and diamagnetic LS state under light irradiation (ON mode using red light and OFF mode using green light). Finally, the photoinduced excited HS state can be reversibly relaxed back to the diamagnetic ground LS state by heating the system at ca. 88 K (TLIESST = 88 K) (light-induced excited spin state trapping (LIESST) effect). Furthermore, 3 also showed an exciting and unique 18 K wide light-induced thermal hysteresis (LITH) effect above liquid nitrogen temperature (100 K). DFT and CASSCF level theoretical calculations were utilized to better understand the magneto-structural correlations of these complexes.

Two-Step Thermoinduced Metal-to-Metal Electron Transfer and ON/OFF Photoswitching in a Molecular [Fe2Co2] Square Complex.

A cyanide-bridged [Fe2Co2] molecular square complex, {[Fe(Tp)(CN)3]2[Co(L)2]2}(BF4)2·2CH3CN·6H2O [1; Tp = hydrotris(pyrazol-1-yl)borate and L = bis(1-ethyl-1H-imidazol-2-yl)ketone], has been synthesized and characterized fully by single-crystal X-ray diffraction, (photo)magnetic measurements, optical reflectivity, and other physical measurements. 1 exhibits a two-step metal-to-metal electron-transfer (MMET)-induced spin transition accompanied by thermal hysteresis (T1/2↑ = 332 and 407 K and T1/2↓ = 320 and 405 K, respectively), converting the low-temperature diamagnetic {FeIILS-CN-CoIIILS} ground state into the high-temperature paramagnetic {FeIIILS-CN-CoIIHS} state. Additionally, 1 displays reversible photoinduced MMET under light irradiation (ON mode using 808 nm laser light and OFF mode using 532 nm laser light), as confirmed by optical reflectivity and (photo)magnetic measurements. The photoinduced paramagnetic metastable state relaxes back to the diamagnetic ground state at 91 K (TLIESST = 91 K). Astonishingly, 1 also exhibits a 27 K wide light-induced thermal hysteresis below 100 K. The overall results show that 1 is a multistimuli-responsive bistable material that exhibits reversible switching between the diamagnetic state, {FeIILS-CN-CoIIILS}, and the paramagnetic state, {FeIIILS-CN-CoIIHS}, under the application of temperature and light.

Kinetic effects on double hysteresis in spin crossover molecular magnets analyzed with first order reversal curve diagram technique

In this paper, we analyze two types of hysteresis in spin crossover molecular magnets compounds in the framework of the First Order Reversal Curve (FORC) method. The switching between the two stable states in these compounds is accompanied by hysteresis phenomena if the intermolecular interactions are higher than a threshold. We have measured the static thermal hysteresis (TH) and the kinetic light induced thermal hysteresis (LITH) major loops and FORCs for the polycrystalline Fe(II) spin crossover compound [Fe1−xZnx(bbtr)3](ClO4)2 (bbtr = 1,4-di(1,2,3-triazol-1-yl)butane), either in a pure state (x = 0) or doped with Zn ions (x = 0.33) considering different sweeping rates. Here, we use this method not only to infer the domains distribution but also to disentangle between kinetic and static components of the LITH and to estimate the changes in the intermolecular interactions introduced by dopants. We also determined the qualitative relationship between FORC distributions measured for TH and LITH.

Thermal‐, Pressure‐ and Light‐Induced Spin‐Crossover Behaviour in the Two‐Dimensional Hofmann‐Like Coordination Polymer [Fe(3‐Clpy)2Pd(CN)4

AbstractThe thermal spin‐crossover behaviour, photoexcitation and subsequent relaxation, as well as the pressure‐induced spin‐crossover behaviour at 298 K are discussed for the non‐porous two‐dimensional Hofmann‐like coordination polymer [Fe(3‐Clpy)2Pd(CN)4] (1). The title compound undergoes a two‐step, cooperative thermal‐induced SCO with critical temperatures Tc1↓ = 159.6 K and Tc1↑ = 164.5 K for the first step and Tc2↓ = 141.4 K and Tc2↑ = 148.4 K for the second step. Irradiation of the low‐spin state with green light (514 nm) at 10 K induced the photoexcitation of around 60 % of the iron(II) centres to the high‐spin state (LIESST effect). The subsequent cooperative relaxation processes were recorded at several temperatures and analysed. The effect of cooperativity on the photoexcitation was investigated by the LITH (light‐induced thermal hysteresis) procedure. At 300 K, the coordination polymer 1 underwent a cooperative spin‐crossover transition at around 0.6 GPa. The thermodynamic analysis of the pressure‐induced SCO transition afforded the enthalpy and cooperativity parameters ΔHHL(p) = 22.01 kJ mol–1 and Γ(p) = 7.47 kJ mol–1, consistent with previous results.

Goldilocks Effect in Magnetic Bistability: Remote Substituent Modulation and Lattice Control of Photoinduced Valence Tautomerism and Light-Induced Thermal Hysteresis

The thermal-induced and photoinduced valence tautomerism of a series of Co(dioxolene)(2)(4-X-py)(2) complexes (dioxolene = 3,5-di-tert-butylcatecholate or 3,5-di-tert-butylsemiquinonate; 4-X-py = 4-(X)pyridine, X = H (1), OMe (2), Me (3), CN (4), Br (5), NO(2) (6)) is described. The thermal valence tautomerism (ls-Co(III)(SQ)(Cat)(4-X-py)(2) <--> hs-Co(II)(SQ)(SQ)(4-X-py)(2)) is only observed for complexes 4, 5, and 6 where each is accompanied by a hysteresis loop of ca. 5 K. When a crystalline sample of 4-6 is held at 10 K in a SQUID magnetometer and irradiated with white light (lambda = 400-850 nm), the hs-Co(II) tautomer is formed. When the light source is removed, and the sample is slowly heated, the hs-Co(II) tautomer persists until ca. 90 K, approximately 40 K higher than the thermal stability of previously reported complexes. Heating and cooling the sample while maintaining irradiation results in the appearance of a new light-induced thermal hysteresis loop below 90 K (DeltaT = ca. 25 K). Below 50 K, the hs-Co(II) tautomer displays temperature-independent relaxation to the ls-Co(III) form, and above 50 K, the relaxation is thermally activated with an activation energy E(a) > ca. 1500 cm(-1). The coordination geometry (trans-pyridines), pyridine substitution, and crystal packing forces conspire to create the comparatively thermally stable photogenerated hs-Co(II) tautomer, thus providing an excellent handle for molecular and crystal engineering studies.

Abrupt spin transition around room temperature and light induced properties in FeII complexes with N4O2 coordination sphere

Mononuclear spin-crossover complex Fe(II)(L)(2) x CH(3)OH (1 x CH(3)OH) and its desolvated product 1 exhibit abrupt spin transitions around room temperature. 1 also shows a light induced excited spin state trapping (LIESST) effect and a light induced thermal hysteresis (LITH) phenomenon at low temperature.

Opposite effects of interactions and disorder on the switching properties of the spin transition compound [Fe II(L) 2][ClO 4] 2 · C 7H 8

We report on a detailed analysis of the thermal transition hysteresis loop and the relaxation curves of the recently investigated compound [Fe II(L) 2][ClO 4] 2 · C 7H 8 [V. Mishra et al., Inorg. Chem. (2008)]. We developed a variant of a previous kinetic cooperative model, modified for including short-range and long-range Gaussian distributions of energy parameters. The resulting set of coupled master equations was solved numerically for the dynamic and the quasi-static case. We have elucidated the various correlations between the distribution widths and the long-range interaction parameter, which basically have opposite effects. It is shown that presence of a steep hysteresis loop may hide a sizable short-range distribution, while the stretched character of the relaxation curves may also hide the presence of strong interactions. We also show that the coherent analysis of the set of relaxation curves at different temperatures does not allow to disentangle properly all the relevant parameters. We discuss these data with respect to the other photomagnetic experiments, T(LIESST) curve and light-induced thermal hysteresis (LITH) loop, and finally we try to address the question of the real nature of the cooperative relaxation process.

Light-induced phase separation (LIPS) into like-spin phases observed by Laue neutron diffraction on a single crystal of [Fe(ptz)6](BF4)2

Abstract We have followed the photo-transformation, the light-induced thermal hysteresis, and the light-induced phase separation (LIPS) into like-spin phases on a [Fe(ptz)6](BF4)2 single crystal, in the quenched rhombohedral phase, through the analysis of neutron Laue diffraction patterns. In agreement with the analysis of previous magnetic measurements, phase separation was observed in the close vicinity of the static light-induced instability, but not otherwise. The positions of the Laue spots were analysed in terms of variation in the a/c lattice-parameter ratio which is well correlated with the variation of the high-spin fraction predicted by a simple homogeneous mean-field model. Using a mapped version of the same model, we analyzed the line shapes of the Laue spots during photo-excitation and relaxation experiments so as to determine the width of the distribution of irradiation intensities, a crucial parameter in phase separation. The predictions of the mapped model failed to reproduce the rapid initial kinetics of the phase separation process, but were in agreement with the further evolution. These features are attributed to a nucleation and growth process, for which the crossover from microscopic to macroscopic regimes has been observed.

An optical microscope study of photo-switching and relaxation in single crystals of the spin transition solid [Fe(ptz) 6](BF 4) 2, with image processing

We report on the use of optical microscopy for investigating quantitatively the thermal and optical switching of spin states. The sample was [Fe(ptz) 6](BF 4) 2 single crystal in the thermally quenched structure (rhombohedral). We followed the photo-excitation from the stable LS state to the metastable HS state, the subsequent thermally activated relaxation and the light-induced thermal hysteresis. We evidenced the presence of temperature gradients, with larger differences between the crystal support and the cold finger of the cryostat, and observed the development of cracks in the crystal at crucial stages of the spin state transformations, associated with an irreversible increase in diffusion of light. We used the green and red optical densities for monitoring, respectively, the variations of the spin fractions and of diffusion of light. We processed the images so as to blur out the digital character of the data and make possible establishing a temperature map of the crystal in the photo-stationary state. We also discussed the conditions required for visualising the recently observed light-induced phase separation into like-spin domains, which are far form reached at the present state of art.

Rate-dependent light-induced thermal hysteresis of [Fe(PM-BiA)2(NCS)2] spin transition complex

The light-induced bistability under permanent irradiation (LITH: light-induced thermal hysteresis), due to the competition between light irradiation and self-accelerated relaxation, shows crucially rate-dependent hysteresis. Major and minor hysteresis loops for [Fe(PM-BiA)2(NCS)2] spin crossover compound have been measured. Using appropriate master equations, distribution data from the first-order reversal curves diagrams of the purely thermal hysteresis, and parameter values previously derived from relaxation curves, we could simulate the LITH experimental data. The tunneling contribution to the relaxation rate revealed to be a key factor for the final agreement.

Photoexcitation and Relaxation Properties of a Spin-Crossover Solid in the Case of a Stable High-Spin State

The molecular solid [Fe(II)L(2)](ClO(4))(2).CH(3)CN where L is 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine provides a stable high-spin (HS) state at low temperature. Photoexcitation and subsequent relaxation have been studied using light-induced excited state spin trapping [LIESST(H --> L)] in the 700-850 nm range, determination of T(LIESST), relaxation curves at different temperatures, and temperature dependence of the light-induced spin equilibrium under constant irradiation. The measured photoinduced population of the metastable low-spin (LS) state (<30%) was drastically limited by the concomitant L --> H photoprocess. The absence of static light-induced thermal hysteresis and the stretched exponential shape of the relaxation curves respectively revealed the absence of sizable interactions and a large spreading of the activation energies attributed to the ligand flexibility. The whole data set has been simulated using a linear rate equation, with a simplified correction for the bulk extinction of light in the powder sample.

Nature and mechanism of the photoinduced spin transition in[Fe(PM−BiA)2(NCS)2]

We report the investigation of a prototype cooperative photoinduced spin transition in $[\mathrm{Fe}(\mathrm{PM}\text{\ensuremath{-}}\mathrm{BiA}{)}_{2}(\mathrm{NCS}{)}_{2}]$ by x-ray diffraction. The structural rearrangement accompanying the change of an electronic state between the low spin and the photoinduced out-of-equilibrium high spin phases is similar to the one occurring around the phase transition at thermal equilibrium. The cooperative behavior of the light-induced optical hysteresis and light-induced thermal hysteresis phenomena investigated here show a strong similarity with thermal hysteresis around first-order phase transitions, since they are driven by a domain nucleation process.

First-order reversal curves analysis of rate-dependent hysteresis: The example of light-induced thermal hysteresis in a spin-crossover solid

The first-order reversal curves (FORC) diagram method is applied here to the light-induced thermal hysteresis of spin-crossover compounds. Due to the kinetic nature of the photoinduced process, the FORC analysis has been mainly focused on the dependence of the FORC diagram with respect to the temperature sweep rate. Most of the features experimentally observed are explained by adequate simulations using the mean-field macroscopic master equation of a two-level system.

Light-induced thermal hysteresis and intensity thresholds in molecular switcheable solids, by mean-field macroscopic master equation approach: Discussion of the experimental data obtained for Co–Fe Prussian Blue Analogues

Abstract The mean-field macroscopic master equation (MFMME) provides a convenient first approach to the static and dynamic properties of photo-switcheable molecular solids under irradiation. The MFMME static approach is formally equivalent to those of the Ising-like or regular solution models in mean-field (or Bragg) approximation. In addition to the previously described light-induced instability of the low-temperature photo-stationary states [A. Desaix, O. Roubeau, J. Jeftic, J.G. Haasnoot, K. Boukheddaden, E. Codjovi, J. Linares, M. Nogues, F. Varret, Eur. Phys. J. B6 (1998) 183], the MFMME approach also provides an easy access to the intensity threshold values involved in photo-switching experiments performed in the thermal hysteresis loop [N. Shimamoto, S. Ohkoshi, O. Sato, K. Hashimoto, Chem. Lett. 4 (2002) 486–487; H.W. Liu, K. Matsuda, Z.Z. Gu, K. Takahashi, A.L. Cui, R. Nakajima, A. Fujishima, O. Sato, Phys. Rev. Lett. 90 (2203) 167403; E. Freysz, S. Montant, S. Letard, and JF Letard, Chem. Phys. Lett. 394 (2004) 318] and to the reversible mechanism suggested in [H.W. Liu, K. Matsuda, Z.Z. Gu, K. Takahashi, A.L. Cui, R. Nakajima, A. Fujishima, O. Sato, Phys. Rev. Lett. 90 (2203) 167403]. The kinetic aspects of photo-switching are also accounted for. The light-induced thermal hysteresis loop of a photomagnetic Prussian Blue Analogue is reported for the first time, and the resulting value for the threshold intensity is discussed.

Photomagnetic properties of the [Fe(L222(N3O2))(CN)2]·H2O complex: a fascinating example of multi-metastability

Today, it is known that the [Fe(L222(N3O2))(CN)2]·H2O complex exhibits an atypical thermal spin-crossover behaviour probably connected to an unusual change of the iron(II) environment from a six to a seven coordination sphere. Here, we show that the photomagnetic properties of this derivative are also atypical. When the sample is initially prepared in the LS form, by a cooling process of 3 K/min, the light irradiation gives access to a long-lived metastable HS state with a T(LIESST) value of 132 K. This photoinduced HS state is moreover characterised by a strong cooperative lattice, as reflected by the kinetic relaxations which follow a sigmoidal model and the existence of a Light-Induced Thermal Hysteresis (LITH) loop. At the opposite, when the sample is carefully placed into the LS/HS mixture state, by using a slow cooling process of less than 1 K/min, the light irradiation generates a metastable state with a T(LIESST) value of only 73 K and the influence of the cooperativity is weak. The relaxation curves are indeed satisfactory fitted by using a stretched exponential model and the LITH phenomenon is not present. All these data will be discussed in term of multimetastability.

A photomagnetic study of three iron(II) compounds containing ligands from the 2,6-di(pyrazol-1-yl)pyridine series

A comprehensive study of the photomagnetic behaviour of [Fe(L1) 2](BF 4) 2, [Fe(L2) 2](BF 4) 2 and [Fe(L2) 2](ClO 4) 2 where L1=2,6-di(pyrazol-1-yl)pyridine and L2=2,6-di(pyrazol-1-yl)pyrazine has been carried out. The critical Light Induced Excited Spin State Trapping (LIESST) temperatures ( T(LIESST)) of all three compounds are presented. The kinetics of the transition from the metastable high spin state to the low spin state have been investigated and examination of the photomagnetic behaviour under constant irradiation has shown the Light Induced Thermal Hysteresis (LITH) behaviour of the compounds. This work completes the crystallographic data previously published and proves the quantitative photoconversion at low temperature of all these derivatives.

FORC method applied to the thermal hysteresis of spin transition solids: first approach of static and kinetic properties

We have analyzed by the FORC method the thermal hysteresis properties of spin crossover compounds [Fe x Zn 1− x (btr) 2(NCS) 2]H 2O], detected by optical reflectivity. The corresponding FORC distributions are discussed in terms of bias and coercivity effects. The origin of these distributions is attributed to internal stresses and domain size distribution. Simulated properties of the light-induced thermal hysteresis provide a preliminary investigation of kinetic effects.