Vibronic Theory of Ultrafast Intersystem Crossing Dynamics in a Single Spin-Crossover Molecule at Finite Temperature beyond the Born–Oppenheimer Approximation

Quantum density matrix theory is carried out to study the ultrafast dynamics of the photoinduced state in a spin-crossover (SC) molecule interacting with a heat bath. The investigations are realized at finite temperature and beyond the usual Born–Oppenheimer (BO) approach. We found that the SC molecule experiences in the photoexcited state (PES) a huge internal pressure, estimated at several gigapascals, partly released in an “explosive” way within ∼100 fs, causing large bond length oscillations, which dampen in the picosecond time scale because of internal conversion processes. During this regime, the BO approximation is not valid. Depending on the tunneling strength, the ultrafast relaxation may proceed through the thermodynamic metastable high-spin state or prevent it. Interestingly, we demonstrate that final relaxation toward the low-spin state always follows a local equilibrium pathway, where the BO approach is valid. Our formulation reconciles the nonequilibrium and the equilibrium properties of this fascinating phenomenon and opens the way to quantum studies on cluster molecules

Elastic Frustration in 1D Spin-Crossover Chains: Evidence of Multi-Step Transitions and Self-Organizations of the Spin States

We consider a 1D elastic spin-crossover (SCO) chain in which each site may be in the low-spin or in the high-spin (HS) state. The sites interact elastically through a harmonic coupling, and the local equilibrium distances depend on the spin states of the interacting sites. The Hamiltonian of the system is solved by the Monte Carlo method running on the spin states and the atomic displacements. By considering the existence of an elastic frustration between the equilibrium distances of the nearest-neighbors and the next-nearest-neighbors, we succeeded to highlight a number of original behaviors of the thermal dependence of the high-spin fraction, like multistep transitions, incomplete spin transitions, emergence of self-organized structures, and re-entrant spin transitions, by adjusting only one control parameter. The obtained results allow understanding several experimental data of 1D spin-crossover materials which seem to be model systems for elastic frustration

Cooperative 1D Triazole-Based Spin Crossover Fe^II Material With Exceptional Mechanical Resilience

Cooperative 1D Triazole-Based Spin Crossover Fe^II Material With Exceptional Mechanical Resilienc

Thermal-Driven Guest-Induced Spin Crossover Behavior in 3D Fe(II)-Based Porous Coordination Polymers

3D porous coordination polymers [Fe(bpb)2dca]ClO4·0.5CH3OH·Guest (bpb = 1,4-bis(4-pyridyl)benzene; Guest = chloroform or dichloromethane) (1·3CHCl3 and 1·2.5CH2Cl2, 1 = [Fe(bpb)2dca]ClO4·0.5CH3OH) were synthesized and characterized by single-crystal X-ray diffraction, thermogravimetric, magnetic, optical, and calorimetric measurements. The 3D polymer 1·3CHCl3 displays upon cooling an incomplete one-step transition centered at T1/2 ∼ 85 K according to SQUID measurements. The 3D polymer 1·2.5CH2Cl2, however, displays a two-step incomplete spin crossover behavior located between T1/2 ∼ 218 K for the highest transition and T1/2 ∼ 124 K for the lower one. In addition, these materials show different colors due to accommodating chloroform (yellow color) or dichloromethane (orange color) molecules in their cavities. Cryogenic optical microscopy was used to image the spatiotemporal properties of the thermal transition at the scale of a single crystal of both compounds. A gradual transition with a homogeneous color change was detected in 1·3CHCl3 around 90 K, in fair agreement with magnetic data. In contrast, a cooperative hysteretic thermal transition accompanied by delamination and the appearance of well spatially organized microcracks was evidenced in 1·2.5CH2Cl2 around 140 K. The complex two-step spatiotemporal front transformations observed in this system are attributed to the interplay between the SCO and the structural transition. In light of all the above elements, it is thus inferred that host–guest interactions in the crystal cavity can modulate these polymers’ magnetic and optical properties. Such materials can realize the interconversion of high-spin and low-spin states under the stimulation of guest molecules, thus having potential applications as reusable storage for chemical and gas sensors

Thermal-Driven Guest-Induced Spin Crossover Behavior in 3D Fe(II)-Based Porous Coordination Polymers

3D porous coordination polymers [Fe(bpb)2dca]ClO4·0.5CH3OH·Guest (bpb = 1,4-bis(4-pyridyl)benzene; Guest = chloroform or dichloromethane) (1·3CHCl3 and 1·2.5CH2Cl2, 1 = [Fe(bpb)2dca]ClO4·0.5CH3OH) were synthesized and characterized by single-crystal X-ray diffraction, thermogravimetric, magnetic, optical, and calorimetric measurements. The 3D polymer 1·3CHCl3 displays upon cooling an incomplete one-step transition centered at T1/2 ∼ 85 K according to SQUID measurements. The 3D polymer 1·2.5CH2Cl2, however, displays a two-step incomplete spin crossover behavior located between T1/2 ∼ 218 K for the highest transition and T1/2 ∼ 124 K for the lower one. In addition, these materials show different colors due to accommodating chloroform (yellow color) or dichloromethane (orange color) molecules in their cavities. Cryogenic optical microscopy was used to image the spatiotemporal properties of the thermal transition at the scale of a single crystal of both compounds. A gradual transition with a homogeneous color change was detected in 1·3CHCl3 around 90 K, in fair agreement with magnetic data. In contrast, a cooperative hysteretic thermal transition accompanied by delamination and the appearance of well spatially organized microcracks was evidenced in 1·2.5CH2Cl2 around 140 K. The complex two-step spatiotemporal front transformations observed in this system are attributed to the interplay between the SCO and the structural transition. In light of all the above elements, it is thus inferred that host–guest interactions in the crystal cavity can modulate these polymers’ magnetic and optical properties. Such materials can realize the interconversion of high-spin and low-spin states under the stimulation of guest molecules, thus having potential applications as reusable storage for chemical and gas sensors

Thermal-Driven Guest-Induced Spin Crossover Behavior in 3D Fe(II)-Based Porous Coordination Polymers

3D porous coordination polymers [Fe(bpb)2dca]ClO4·0.5CH3OH·Guest (bpb = 1,4-bis(4-pyridyl)benzene; Guest = chloroform or dichloromethane) (1·3CHCl3 and 1·2.5CH2Cl2, 1 = [Fe(bpb)2dca]ClO4·0.5CH3OH) were synthesized and characterized by single-crystal X-ray diffraction, thermogravimetric, magnetic, optical, and calorimetric measurements. The 3D polymer 1·3CHCl3 displays upon cooling an incomplete one-step transition centered at T1/2 ∼ 85 K according to SQUID measurements. The 3D polymer 1·2.5CH2Cl2, however, displays a two-step incomplete spin crossover behavior located between T1/2 ∼ 218 K for the highest transition and T1/2 ∼ 124 K for the lower one. In addition, these materials show different colors due to accommodating chloroform (yellow color) or dichloromethane (orange color) molecules in their cavities. Cryogenic optical microscopy was used to image the spatiotemporal properties of the thermal transition at the scale of a single crystal of both compounds. A gradual transition with a homogeneous color change was detected in 1·3CHCl3 around 90 K, in fair agreement with magnetic data. In contrast, a cooperative hysteretic thermal transition accompanied by delamination and the appearance of well spatially organized microcracks was evidenced in 1·2.5CH2Cl2 around 140 K. The complex two-step spatiotemporal front transformations observed in this system are attributed to the interplay between the SCO and the structural transition. In light of all the above elements, it is thus inferred that host–guest interactions in the crystal cavity can modulate these polymers’ magnetic and optical properties. Such materials can realize the interconversion of high-spin and low-spin states under the stimulation of guest molecules, thus having potential applications as reusable storage for chemical and gas sensors

Thermal-Driven Guest-Induced Spin Crossover Behavior in 3D Fe(II)-Based Porous Coordination Polymers

3D porous coordination polymers [Fe(bpb)2dca]ClO4·0.5CH3OH·Guest (bpb = 1,4-bis(4-pyridyl)benzene; Guest = chloroform or dichloromethane) (1·3CHCl3 and 1·2.5CH2Cl2, 1 = [Fe(bpb)2dca]ClO4·0.5CH3OH) were synthesized and characterized by single-crystal X-ray diffraction, thermogravimetric, magnetic, optical, and calorimetric measurements. The 3D polymer 1·3CHCl3 displays upon cooling an incomplete one-step transition centered at T1/2 ∼ 85 K according to SQUID measurements. The 3D polymer 1·2.5CH2Cl2, however, displays a two-step incomplete spin crossover behavior located between T1/2 ∼ 218 K for the highest transition and T1/2 ∼ 124 K for the lower one. In addition, these materials show different colors due to accommodating chloroform (yellow color) or dichloromethane (orange color) molecules in their cavities. Cryogenic optical microscopy was used to image the spatiotemporal properties of the thermal transition at the scale of a single crystal of both compounds. A gradual transition with a homogeneous color change was detected in 1·3CHCl3 around 90 K, in fair agreement with magnetic data. In contrast, a cooperative hysteretic thermal transition accompanied by delamination and the appearance of well spatially organized microcracks was evidenced in 1·2.5CH2Cl2 around 140 K. The complex two-step spatiotemporal front transformations observed in this system are attributed to the interplay between the SCO and the structural transition. In light of all the above elements, it is thus inferred that host–guest interactions in the crystal cavity can modulate these polymers’ magnetic and optical properties. Such materials can realize the interconversion of high-spin and low-spin states under the stimulation of guest molecules, thus having potential applications as reusable storage for chemical and gas sensors

Quantitative Contact Pressure Sensor Based on Spin Crossover Mechanism for Civil Security Applications

Detailed studies of impacts on the new 1D spin transition polymer [Fe­(hyetrz)₃]­I₂·0.5EtOH have been performed under several controlled contact pressures, showing for high energy values a color change of the compound and allowing a visual detection of the spin transition from high-spin to low-spin states. By performing detailed investigations on freshly impacted samples, using spectroscopic diffuse optical reflectivity, we could follow the variation of the optical spectra as a function of the energy of the impact. The meticulous analysis of the obtained spectra allowed us to establish an absorption peak at 550 nm whose intensity and position well correlate to the energy of the impact. This concept provides a reliable method of measuring the energy of a chock even if the sample does not change its color so much in the visible range. This might be of high importance in several civil security applications, like transportation of artwork or other fragile valuable objects or even in the evaluation of the degree of alteration of a material after a collision

Control of the Speed of a Light-Induced Spin Transition through Mesoscale Core–Shell Architecture

The rate of the light-induced spin transition in a coordination polymer network solid dramatically increases when included as the core in mesoscale core–shell particles. A series of photomagnetic coordination polymer core–shell heterostructures, based on the light-switchable Rb_<i>a</i>Co_<i>b</i>[Fe­(CN)₆]_<i>c</i>·<i>m</i>H₂O (RbCoFe-PBA) as core with the isostructural K_<i>j</i>Ni_<i>k</i>[Cr­(CN)₆]_<i>l</i>·<i>n</i>H₂O (KNiCr-PBA) as shell, are studied using temperature-dependent powder X-ray diffraction and SQUID magnetometry. The core RbCoFe-PBA exhibits a charge transfer-induced spin transition (CTIST), which can be thermally and optically induced. When coupled to the shell, the rate of the optically induced transition from low spin to high spin increases. Isothermal relaxation from the optically induced high spin state of the core back to the low spin state and activation energies associated with the transition between these states were measured. The presence of a shell decreases the activation energy, which is associated with the elastic properties of the core. Numerical simulations using an electro-elastic model for the spin transition in core–shell particles supports the findings, demonstrating how coupling of the core to the shell changes the elastic properties of the system. The ability to tune the rate of optically induced magnetic and structural phase transitions through control of mesoscale architecture presents a new approach to the development of photoswitchable materials with tailored properties