Light-induced Excited Spin State Trapping Research Articles

Understanding spin-dependent vibrational frequencies in Fe(II) metal organic coordination complexes

We investigated the compositional and temperature (T) dependences of vibrational frequencies in Hofmann-type Fe(L)2[M(CN)4] spin-crossover (SCO) coordination polymers in which {M = Ni, Pd or Pt with L = pyridine (py)}, or {L = 3-Cl-py or 3-methylpy with M = Ni}, using Raman spectroscopy. The SCO-driven peak shifts (in wavenumber) ranged up to ∼170 cm−1, manifesting significant spin-dependent structural evolutions. Furthermore, there appear clear HS signatures even at T ≪ TSCO for L = 3-Cl-py or 3-methylpy implying the steric effects of the organic ligands on the HS trapping. Meanwhile, for L = py, such HS trapping at the low temperature was not significant although some spectra taken under high laser fluence exhibit light-induced excited spin state trapping (LIESST) effect. The mechanism of the LIESST is discussed in detail in terms of the M d – C 2sp hybridization effects.

Observation of TLIESST above Liquid Nitrogen Temperature and Disclosure of Hidden Hysteresis in Multiresponsive Hofmann-type Coordination Polymers.

Photoresponsive spin-crossover (SCO) molecules are an important class of bistable magnetic molecules with intriguing potential in device applications. The light-induced excited spin state trapping (LIESST) and the combined application of light and temperature can provide access to the metastable region of the SCO profile. The primary obstacle in utilizing light stimuli is the manifestation of light-induced trappings at extremely low temperatures. Herein, we report two novel multiresponsive 2D Hofmann-type coordination polymers exhibiting light-induced excited spin state trapping above liquid nitrogen temperature (TLIESST = 82 and 81 K). Stimulating the samples in conjugation with light and temperature successfully unveils hysteresis, which is otherwise concealed. Apart from light and temperature, we found that the SCO phenomenon is also responsive to external hydrostatic pressure and exhibits modulation of the hysteresis width and transition temperature shifts with changes in pressure.

Control of Iron(II)-Tris(2,2'-Bipyridine) Light-Induced Excited-State Trapping via External Electromagnetic Fields.

Light-induced excited spin-state trapping reactions in iron pyridinic complexes allow the iron's low-to-high spin transition in a sub-picosecond timescale. Employing a recently developed model for [Fe(2,2'-bipyridine)3]2+ photochemical spin-crossover reaction in conjunction with quantum wavepacket dynamics, we explore the possibility of controlling the reaction through external electromagnetic fields, aiming at stabilizing the initial metal-to-ligand charge transfer states. We show that simple Gaussian-shaped electromagnetic fields have a minor effect on the population kinetics. However, introducing vibrationally excited initial wavepacket representations allows for maintaining the population trapped in the metal-to-ligand charge transfer states. Using optimal control theory, we propose an electromagnetic field shape that increases the lifetime of metal-to-ligand charge transfer states. These results open the route for controlling the iron photochemistry through the action of external electric fields.

A cyanide-bridged FeII/FeIII mixed-valence chain exhibiting spin transition in the Fe(II) sites

A cyanide-bridged {FeIII2FeII}n one-dimensional chain {[Fe(Tp)(CN)3]2Fe(btb)}n (1) (Tp = hydrotris(pyrazolyl)borate, btb = 1,4-(1H-1,2,4-triazole)butane) has been synthesized via the hydrothermal reaction of Li[Fe(Tp)(CN)3], btb, and FeII(ClO4)2·6H2O at 120 °C. Single-crystal X-ray analysis reveals that compound 1 is an isolated neutral mixed-valence {FeIII2FeII}n double-zigzag chain with each [FeIII(Tp)(CN)3]− unit connecting with two Fe(II) centres via two of its three CN− groups in the cis-positions, and two triazole groups from btb coordinated with Fe(II) ion in its axial positions. Single-crystal data obtained at different temperature together with magnetic data indicate that complex 1 exhibits thermal induced spin transition in the Fe(II) sites with T1/2 ≈ 165 K. Additionally, light-induced excited spin-state trapping (LIESST) of 1 was also observed at low temperature.

Correlating Valence and 2p3d RIXS Spectroscopies: A Ligand-Field Study of Spin-Crossover Iron(II).

The molecular spin-crossover phenomenon between high-spin (HS) and low-spin (LS) states is a promising route to next-generation information storage, sensing applications, and molecular spintronics. Spin-crossover complexes also provide a unique opportunity to study the ligand field (LF) properties of a system in both HS and LS states while maintaining the same ligand environment. Presently, we employ complementing valence and core-level spectroscopic methods to probe the electronic excited-state manifolds of the spin-crossover complex [FeII(H2B(pz)2)2phen]0. Light-induced excited spin-state trapping (LIESST) at liquid He temperatures is exploited to characterize magnetic and spectroscopic properties of the photoinduced HS state using SQUID magnetometry and magnetic circular dichroism spectroscopy. In parallel, Fe 2p3d RIXS spectroscopy is employed to examine the ΔS = 0, 1 excited LF states. These experimental studies are combined with state-of-the-art CASSCF/NEVPT2 and CASCI/NEVPT2 calculations characterizing the ground and LF excited states. Analysis of the acquired LF information further supports the notion that the spin-crossover of [FeII(H2B(pz)2)2phen]0 is asymmetric, evidenced by a decrease in eπ in the LS state. The results demonstrate the power of cross-correlating spectroscopic techniques with high and low LF information content to make accurate excited-state assignments, as well as the current capabilities of ab initio theory in interpreting these electronic properties.

Simultaneous manipulation of iron(II) spin crossover and LIESST behaviour using pressure, temperature and light.

Spin crossover (SCO) and light-induced excited spin state trapping (LIESST) effects were studied using high pressure X-ray diffraction at cryogenic temperatures on a single crystal of the {[FeII(pyrazole)4]2[NbIV(CN)8]·4H2O}n (FeNb) coordination polymer. The studied compound does not show SCO or LIESST at ambient pressure, but these effects can be enforced by a mechanical stimulus. The obtained results demonstrate the manipulation of the spin state via the appropriate combination of multiple stimuli simultaneously.

Rotational order-disorder and spin crossover behaviour in a neutral iron(II) complex based on asymmetrically substituted large planar ionogenic ligand.

Octahedrally coordinated spin crossover (SCO) FeII complexes represent an important class of switchable molecular materials. This study presents the synthesis and characterisation of a novel complex, [FeII(ppt-2Fph)2]0·2MeOH, where ppt-2Fph is a new asymmetric ionogenic tridentate planar ligand 2-(5-(2-fluorophenyl)-4H-1,2,4-triazol-3-yl)-6-(1H-pyrazol-1-yl)pyridine. The complex exhibits a hysteretic thermally induced SCO transition at 285 K on cooling and at 293 K on heating, as well as light induced excited spin state trapping (LIESST) at lower temperatures with a relaxation T(LIESST) temperature of 73 K. Single crystal analysis in both spin states shows that the compound undergoes an unusual partial (25%) reversible order-disorder of the asymmetrically substituted phenyl group coupled to the thermal SCO. The highly cooperative SCO transition, analysed by structural energy framework analysis at the B3LYP/6-31G(d,p) theory level, revealed the co-existence of stabilising and destabilising energy variations in the lattice. The observed antagonism of intermolecular interactions and synchronous rotational disorder, which contributes to the overall entropy change, is suggested to be at the origin of the cooperative SCO transition.

Bidirectional photomagnetism, exciplex fluorescence and dielectric anomalies in a spin crossover Hofmann-type coordination polymer.

Stepped spin crossover (SCO) complexes with three or more spin states have promising applications in high-order data storage, multi-switches and multi-sensors. Further synergy with other functionalities, such as luminescence and dielectric properties, will provide a good chance to develop novel multifunctional SCO materials. Here, a bent pillar ligand and luminescent pyrene guest are integrated into a three-dimensional (3D) Hofmann-type metal-organic framework (MOF) [Fe(dpoda){Au(CN)2}2]·pyrene (dpoda = 2,5-di-(pyridyl)-1,3,4-oxadiazole). The magnetic data show an incomplete and two-step SCO behavior with the sequence of 1 ↔ 1/2 ↔ 1/4. The rare bi-directional light-induced excited spin-state trapping (LIESST) effect and light-induced stepped thermal relaxation after LIESST are observed. The pyrene guests interact with dpoda ligands via offset face-to-face π⋯π interactions to form intermolecular exciplex emissions. The competition between thermal quenching and stepped SCO properties results in a complicated and stepped exciplex fluorescence. Moreover, the stepped dielectric property with higher dielectric permittivity at lower temperature may be related to the more frustrated octahedral distortion parameters in the intermediate spin states. Hence, a 3D Hofmann-type MOF with bent pillar ligands and fluorescent guests illustrates an effective way for the development of multifunctional switching materials.

Light-induced spin-state switching in Fe(II) spin-crossover complexes with thiazole-based chelating ligands.

Homoleptic complexes [Fe(4bt)3](ClO4)2 (1), [Fe(2bt)3](ClO4)2 (2), and [Fe(3tpH)3](ClO4)2 (3) were obtained by a reaction between the Fe(II) precursor salt and the corresponding thiazole-based bidentate ligand (L = 4bt = 4,4'-bithiazole, 2bt = 2,2'-bithiazole, 3tpH = 3-(thiazol-2-yl)pyrazole). X-ray crystal structure determination revealed crystallization of solvent-free complex 1, a solvate 2·MeOH, and a co-crystal 3·2(3tpH). The crystal packing of all these complexes is dominated by one-dimensional interactions between the [Fe(L)3]2+ cations. These interactions are stronger in 2·MeOH and 3·2(3tpH), leading to cooperative and slightly hysteretic transitions between the high-spin and low-spin electronic configurations at ∼235 K and 159 K, respectively. In contrast, weaker intermolecular interactions in 1 result in a gradual spin crossover above 300 K, with the maximum fraction of the HS state ∼25% achieved at 400 K. Complexes 2 and 3·2(3tpH) exhibit light-induced excited spin state trapping (LIESST) under irradiation with white light or a 532 nm laser at 5 K. After the photoexcitation, the trapped metastable HS state relaxes to the ground LS state with the average relaxation temperature of 81 K and 68 K, respectively. Examination of the relaxation dynamics by optical absorption spectroscopy on a single crystal of 3·2(3tpH) revealed the sigmoidal shape of the relaxation curves at lower temperatures, attributed to cooperative effects, as well as a plateau at ∼10% of the HS fraction at intermediate temperatures, hinting at a more complex mechanism for the relaxation of the LIESST phase in this material.

Lattice solvent- and substituent-dependent spin-crossover in isomeric iron(II) complexes.

Spin-state switching in iron(II) complexes composed of ligands featuring moderate ligand-field strength-for example, 2,6-bi(1H-pyrazol-1-yl)pyridine (BPP)-is dependent on many factors. Herein, we show that spin-state switching in isomeric iron(II) complexes composed of BPP-based ligands-ethyl 2,6-bis(1H-pyrazol-1-yl)isonicotinate (BPP-COOEt, L1) and (2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)methylacetate (BPP-CH2OCOMe, L2)-is dependent on the nature of the substituent at the BPP skeleton. Bi-stable spin-state switching-with a thermal hysteresis width (ΔT1/2) of 44 K and switching temperature (T1/2) = 298 K in the first cycle-is observed for complex 1·CH3CN composed of L1 and BF4- counter anions. Conversely, the solvent-free isomeric counterpart of 1·CH3CN-complex 2a, composed of L2 and BF4- counter anions-was trapped in the high-spin (HS) state. For one of the polymorphs of complex 2b·CH3CN-2b·CH3CN-Y, Y denotes yellow colour of the crystals-composed of L2 and ClO4- counter anions, a gradual and non-hysteretic SCO is observed with T1/2 = 234 K. Complexes 1·CH3CN and 2b·CH3CN-Y also underwent light-induced spin-state switching at 5 K due to the light-induced excited spin-state trapping (LIESST) effect. Structures of the low-spin (LS) and HS forms of complex 1·CH3CN revealed that spin-state switching goes hand-in-hand with pronounced distortion of the trans-N{pyridyl}-Fe-N{pyridyl} angle (ϕ), whereas such distortion is not observed for 2b·CH3CN-Y. This observation points that distortion is one of the factors making the spin-state switching of 1·CH3CN hysteretic in the solid state. The observation of bi-stable spin-state switching with T1/2 centred at room temperature for 1·CH3CN indicates that technologically relevant spin-state switching profiles based on mononuclear iron(II) complexes can be obtained.

Substituent group tuned structural dimensionalities and magnetic properties of V-shaped bis-pyridyl ligands based Fe(II) coordination polymers

A series of V-shaped thioether-based bis-pyridyl ligands containing different substituent groups have been successfully employed to assemble Fe(II)-based complexes by using H-tube diffusion method, resulting in six new coordination polymers with different structure types, [Fe(L1)(SCN)2]n·1.5nCH3OH (1), [Fe(L1)(SeCN)2]n·nCH3OH·nH2O (2), [Fe(L2)(SCN)2]n (3), [Fe(L2)(SeCN)2]n (4), [Fe(L3)(SCN)2]n (5), [Fe(L3)(SeCN)2]n (6) (L1 = 1,3-bis((pyridin-4-ylthio)methyl)benzene, L2 = 4,4′-(((2,4,6-trimethyl-1,3-phenylene)bis(methylene))bis(sulfanediyl))dipyridine, L3 = 4,4′-(((perchloro-1,3-phenylene)bis(methylene))bis(sulfanediyl))dipyridine). All the complexes have been structurally characterized by elemental analysis, FT-IR spectroscopy, single crystal and powder X-ray diffraction. The single crystal structure analysis revealed the one-dimensional (1D) chain for complexes 1–4, in which the different conformation the ligand adopted was found, and two-dimensional (2D) grid-like network for 5 and 6, respectively. Investigation of the temperature dependent magnetic susceptibilities discovered the always high spin state of the Fe(II) ion in all the compounds within the whole temperature range of 10-300 K, except complex 2 with the obvious temperature dependent color change from yellow (293 K) to violet (100 K). The thermo- and photomagnetic property study for complex 2 showed the incomplete spin transition behavior with interesting light-induced excited spin state trapping effect (LIESST). The current results indicated that the steric effect and/or the electronegativity of the substitute groups on the ligand and the conformation of the ligand could play an important role on the topologic structure of the target products, therefore putting obvious influence on spin state of the central Fe(II) ion.

Solution phase high repetition rate laser pump x-ray probe picosecond hard x-ray spectroscopy at the Stanford Synchrotron Radiation Lightsource.

We present a dedicated end-station for solution phase high repetition rate (MHz) picosecond hard x-ray spectroscopy at beamline 15-2 of the Stanford Synchrotron Radiation Lightsource. A high-power ultrafast ytterbium-doped fiber laser is used to photoexcite the samples at a repetition rate of 640 kHz, while the data acquisition operates at the 1.28 MHz repetition rate of the storage ring recording data in an alternating on-off mode. The time-resolved x-ray measurements are enabled via gating the x-ray detectors with the 20 mA/70 ps camshaft bunch of SPEAR3, a mode available during the routine operations of the Stanford Synchrotron Radiation Lightsource. As a benchmark study, aiming to demonstrate the advantageous capabilities of this end-station, we have conducted picosecond Fe K-edge x-ray absorption spectroscopy on aqueous [FeII(phen)3]2+, a prototypical spin crossover complex that undergoes light-induced excited spin state trapping forming an electronic excited state with a 0.6-0.7 ns lifetime. In addition, we report transient Fe Kβ main line and valence-to-core x-ray emission spectra, showing a unique detection sensitivity and an excellent agreement with model spectra and density functional theory calculations, respectively. Notably, the achieved signal-to-noise ratio, the overall performance, and the routine availability of the developed end-station have enabled a systematic time-resolved science program using the monochromatic beam at the Stanford Synchrotron Radiation Lightsource.

Iron(II) Complexes of 2,6-Di[4-(ethylcarboxy)pyrazol-1-yl]pyridine with Reversible Guest-Modulated Spin-Crossover Behavior.

Three solvatomorphs of the iron(II) complex of 2,6-di[4-(ethylcarboxy)pyrazol-1-yl]pyridine (bpCOOEt2p) of formulas [Fe(bpCOOEt2p)2](ClO4)2·1.5MeNO2 (1), [Fe(bpCOOEt2p)2](ClO4)2·MeNO2 (2), and [Fe(bpCOOEt2p)2](ClO4)2·2MeNO2 (3) have been prepared and characterized. They show interesting spin-crossover (SCO) properties ranging from partial to complete thermal spin transitions and a light-induced excited spin-state trapping (LIESST) effect. In solvatomorph 2, a robust structure is formed with channels that enable the entrance or removal of solvent molecules by vapor diffusion without losing the crystallinity. Thus, solvent-exchanged samples [Fe(bpCOOEt2p)2](ClO4)2·MeNO2 (2·MeNO 2 ), [Fe(bpCOOEt2p)2](ClO4)2·MeCN (2·MeCN), [Fe(bpCOOEt2p)2](ClO4)2·0.5Me2CO (2·Me 2 CO), and [Fe(bpCOOEt2p)2](ClO4)2·MeCOOH (2·MeCOOH) were prepared by vapor diffusion of the solvents in a crystal of the compound previously heated to 400 K in a single-crystal to single-crystal (SCSC) fashion. Interestingly, this causes a change of spin state with a stabilization of the low-spin state in 2·Me 2 CO and the high-spin state in 2·MeCN. Therefore, the SCO properties of 2 can be tuned in a reversible way by exposure to different solvents.

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.

Defying the inverse energy gap law: a vacuum-evaporable Fe(ii) low-spin complex with a long-lived LIESST state.

The novel vacuum-evaporable complex [Fe(pypypyr)2] (pypypyr = bipyridyl pyrrolide) was synthesised and analysed as bulk material and as a thin film. In both cases, the compound is in its low-spin state up to temperatures of at least 510 K. Thus, it is conventionally considered a pure low-spin compound. According to the inverse energy gap law, the half time of the light-induced excited high-spin state of such compounds at temperatures approaching 0 K is expected to be in the regime of micro- or nanoseconds. In contrast to these expectations, the light-induced high-spin state of the title compound has a half time of several hours. We attribute this behaviour to a large structural difference between the two spin states along with four distinct distortion coordinates associated with the spin transition. This leads to a breakdown of single-mode behaviour and thus drastically decreases the relaxation rate of the metastable high-spin state. These unprecedented properties open up new strategies for the development of compounds showing light-induced excited spin state trapping (LIESST) at high temperatures, potentially around room temperature, which is relevant for applications in molecular spintronics, sensors, displays and the like.

Thermal- and light-induced SCO effect in Fe(II) complexes and coordination polymers

Abstract The review presents several families of spin crossover (SCO) active Fe(II) coordination compounds with photoactive N-donor heterocyclic ligands, in which the photoinduced structural changes can activate reversible change of spin state and thus control magnetic properties under isothermal conditions. Detailed description of structural, spectral, and magnetic behavior for selected examples of photoisomerizable coordination compounds are provided. From the application point of view, light is an excellent tool to control SCO properties. The first and best known approach called Light Induced Excited Spin State Trapping (LIESST) has a significant technological limitation due to low temperatures (< 120 K) required for the trapping and existence of photoexcited metastable states. The second and more elegant approach known as Ligand-Driven Light-Induced Spin Crossover (LD-LISC) seems to be a very suitable strategy utilizing light-induced structural changes to control the spin. Isomerization of photoswitchable groups, such as azobenzenes or stilbenes, can cause reversible transformation between two isomeric forms after exposition to selective wavelengths at ambient temperature. A very recent third approach, the Guest-Driven Light-Induced Spin Crossover (GD-LISC) effect employing the photoisomerizable guest molecules to control the spin state has also been introduced.

A series of new Fe(II) coordination polymers based-on [Hg(XCN)3]− or [Hg(XCN)4]2− (X = S, Se) building blocks: Synthesis, crystal structure and magnetic property

Self-assembly of the tetrahedral metalloligand building block [(HgII(XCN)4]2-, where X ​= ​S or Se, a series of bis-monodentate pyridine-like ligands and iron(II) compounds has afforded seven new heterobimetallic HgII–FeII coordination polymers. Single crystal X-ray diffraction structural analysis revealed the infinite two-dimensional (2D) sheet with different topologies for all the complexes except 3, which belonged to inherent 3D-structure. The results of the temperature dependent magnetic susceptibility investigation over all the complexes discovered the paramagnetic behavior for the high spin iron(II) ion in all the complexes within the temperature range of 2–300K, except complex 6. The thermo- and photomagnetic property study for complex 6 showed incomplete spin crossover (SCO) behaviors with interesting thermomagnetic properties and light-induced excited spin state trapping effect (LIESST). The results show that the ligand field, configurations, rigidity, and polarity of the bridging ligands, as well as the coordination mode with the Fe(II) ion of the building units, are important for the design and synthesis of the targeted coordination polymers.

Abrupt Spin Transition in a Heteroleptic Fe(II) Complex with Pendant Naphthalene Functionality.

A heteroleptic spin-crossover (SCO) complex, [Fe(tpma)(xnap-bim)](ClO4)2 (1; tpma = tris(2-pyridylmethyl)amine, xnap-bim = 8,15-dihydrodiimidazo[1,2-a:2',1'-c]naphtho[2,3-f][1,4]diazocine), has been obtained by reacting a Fe(II) precursor salt with tetradentate tpma and bidentate xnap-bim ligands. Depending on crystallization conditions, two different solvates have been obtained, 1·2.25py·0.5H2O and 1·py. The former readily loses the interstitial solvent to produce either a powder sample of 1 upon filtration or crystals of 1 if the solvent loss is slowed by placing the crystals of 1·2.25py·0.5H2O in diethyl ether. In contrast, 1·py exhibits higher stability toward solvent loss. The crystal packing of both solvates and of the solvent-free structure features double columns of [Fe(tpma)(xnap-bim)]2+ cations formed by efficient π-π interactions between the pyridyl groups of tpma ligands, as well as by stacks supported by π-π interactions between interdigitated naphthalene fragments of xnap-bim ligands. While both solvates show a gradual SCO between the high-spin (HS) and low-spin (LS) states of the Fe(II) ion, solvent-free complex 1 exhibits an abrupt spin transition centered at 127 K, with a narrow 2 K thermal hysteresis. Complex 1 also shows a light-induced excited spin state trapping effect, manifested as LS → HS conversion upon irradiation with white light at 5 K. The metastable HS state relaxes to the ground LS state when heated above 65 K.

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.

Regulation of Multistep Spin Crossover Across Multiple Stimuli in a 2-D Framework Material.

We investigate the effects of a broad array of external stimuli on the structural, spin-crossover (SCO) properties and nature of the elastic interaction within the two-dimensional Hofmann framework material [Fe(cintrz)2Pd(CN)4]·guest (cintrz = N-cinnamalidene 4-amino-1,2,4-triazole; A·guest; guest = 3H2O, 2H2O, and Ø). This framework exhibits a delicate balance between ferro- and antiferro-elastic interaction characters; we show that manipulation of the pore contents across guests = 3H2O, 2H2O, and Ø can be exploited to regulate this balance. In A·3H2O, the dominant antiferroelastic interaction character between neighboring FeII sites sees the low-temperature persistence of the mixed spin-state species {HS-LS} for {Fe1-Fe2} (HS = high spin, LS = low spin). Elastic interaction strain is responsible for stabilizing the {HS-LS} state and can be overcome by three mechanisms: (1) partial (2H2O) or complete (Ø) guest removal, (2) irradiation via the reverse light-induced excited spin-state trapping (LIESST) effect (λ = 830 nm), and (3) the application of external hydrostatic pressure. Combining experimental data with elastic models presents a clear interpretation that while guest molecules cause a negative chemical pressure, they also have consequences for the elastic interactions between metals beyond the simple chemical pressure picture typically proposed.