In molecular spin-crossover materials, the spin-state switching is strongly coupled with changes in the structural degrees of freedom, which play a central role in the alteration of functions and ...mechanisms of transformation from the molecular scale (bond lengths) to the material scale (volume). This article reviews various techniques currently used, ranging from microscopy to diffraction, to study the structural aspects related to spin crossover at different physical scales. Some of the most important recent advances concern the resolution of complex crystal structures, the determination of microstructures, the study of phase coexistence, temporal studies ranging from second to femtosecond, and the description of multistep transition involving intricate ordering of molecules in different electronic states.
Dans les matériaux moléculaires à conversion de spin, la modification de l'état de spin est fortement couplée aux changements des degrés de liberté structuraux, qui jouent un rôle central dans l'altération des propriétés et dans les mécanismes de transformation, de l'échelle de la molécule (longueur de liaison) à l'échelle du matériau (volume). Cet article passe en revue différentes techniques utilisées actuellement, allant de la microscopie à la diffraction, pour étudier les aspects structuraux fondamentaux liés au changement d'état de spin aux différentes échelles physiques. Les avancées récentes les plus marquantes concernent la résolution de structures cristallines complexes, la détermination des microstructures, l'étude de la coexistence de phases, les études temporelles allant de la seconde à la femtoseconde et la description de transition multi-étapes impliquant des mises en ordre complexes de molécules dans différents états électroniques.
A one-dimensional coordination solid 1c is synthesized by reaction of a bispyridyl dithienylethene (DTE) photochromic unit with the highly anisotropic dysprosium-based single-molecule magnet ...Dy(Tppy)F(pyridine)2PF6. Slow magnetic relaxation characteristics are retained in the chain compound 1c , and photoisomerization of the bridging DTE ligand induces a single-crystal-to-single-crystal transformation that can be monitored using photocrystallography. Notably, the resulting chain compound 1o exhibits faster low-temperature relaxation than that of 1c , which is apparent in magnetic hysteresis data collected for both compounds as high as 4 K. Ab initio calculations suggest that this photomodulation of the magnetic relaxation behavior is due to crystal packing changes rather than changes to the crystal field splitting upon ligand isomerization.
Photoinduced phase transformations occur when a laser pulse impacts a material, thereby transforming its electronic and/or structural orders, consequently affecting the functionalities. The transient ...nature of photoinduced states has thus far severely limited the scope of applications. It is of paramount importance to explore whether structural feedback during the solid deformation has the capacity to amplify and stabilize photoinduced transformations. Contrary to coherent optical phonons, which have long been under scrutiny, coherently propagating cell deformations over acoustic timescales have not been explored to a similar degree, particularly with respect to cooperative elastic interactions. Herein we demonstrate, experimentally and theoretically, a self-amplified responsiveness in a spin-crossover material during its delayed volume expansion. The cooperative response at the material scale prevails above a threshold excitation, significantly extending the lifetime of photoinduced states. Such elastically driven cooperativity triggered by a light pulse offers an efficient route towards the generation and stabilization of photoinduced phases in many volume-changing materials.
Using ultrafast optical absorption spectroscopy, the room‐temperature spin‐state switching dynamics induced by a femtosecond laser pulse in high‐quality thin films of the molecular spin‐crossover ...(SCO) complex Fe(HB(tz)3)2 (tz = 1,2,4‐triazol‐1‐yl) are studied. These measurements reveal that the early, sub‐picosecond, low‐spin to high‐spin photoswitching event, with linear response to the laser pulse energy, can be followed under certain conditions by a second switching process occurring on a timescale of tens of nanoseconds, enabling nonlinear amplification. This out‐of‐equilibrium dynamics is discussed in light of the characteristic timescales associated with the different switching mechanisms, i.e., the electronic and structural rearrangements of photoexcited molecules, the propagation of strain waves at the material scale, and the thermal activation above the molecular energy barrier. Importantly, the additional, nonlinear switching step appears to be completely suppressed in the thinnest (50 nm) film due to the efficient heat transfer to the substrate, allowing the system to retrieve the thermal equilibrium state on the 100 ns timescale. These results provide a first milestone toward the assessment of the physical parameters that drive the photoresponse of SCO thin films, opening up appealing perspectives for their use as high‐frequency all‐optical switches working at room temperature.
Size reduction effects are evidenced on the room temperature photoswitching dynamics of spin‐crossover thin films induced by femtosecond laser excitations. A thermally activated switching is observed on the 20–40 ns timescale, governed by the intramolecular energy barrier between the two spin states. This additional switching step is suppressed in the thinnest films.
The description of ultrafast nonadiabatic chemical dynamics during molecular photo-transformations remains challenging because electronic and nuclear configurations impact each other and cannot be ...treated independently. Here we gain experimental insights, beyond the Born-Oppenheimer approximation, into the light-induced spin-state trapping dynamics of the prototypical Fe(bpy)
compound by time-resolved X-ray absorption spectroscopy at sub-30-femtosecond resolution and high signal-to-noise ratio. The electronic decay from the initial optically excited electronic state towards the high spin state is distinguished from the structural trapping dynamics, which launches a coherent oscillating wave packet (265 fs period), clearly identified as molecular breathing. Throughout the structural trapping, the dispersion of the wave packet along the reaction coordinate reveals details of intramolecular vibronic coupling before a slower vibrational energy dissipation to the solution environment. These findings illustrate how modern time-resolved X-ray absorption spectroscopy can provide key information to unravel dynamic details of photo-functional molecules.
We review the tremendous advances in ultrafast X-ray science, over the past 15 years, making the best use of new ultrashort X-ray sources including table-top or large-scale facilities. Different ...complementary X-ray-based techniques, including spectroscopy, scattering, and diffraction, are presented. The broad and expanding spectrum of these techniques in the ultrafast time domain is delivering new insight into the dynamics of molecular systems, of solutions, of solids, and of biosystems. Probing the time evolution of the electronic and structural degrees of freedom of these systems on the time scales of femtosecond to picoseconds delivers new insight into our understanding of dynamical matter.
Molecular magnetic switches are expected to form the functional components of future nanodevices. Herein we combine detailed (photo‐) crystallography and magnetic studies to reveal the unusual ...switching properties of an iron(III) complex, between low (LS) and high (HS) spin states. On cooling, it exhibits a partial thermal conversion associated with a reconstructive phase transition from a HS‐HS to a LS‐HS phase with a hysteresis of 25 K. Photoexcitation at low temperature allows access to a LS‐LS phase, never observed at thermal equilibrium. As well as reporting the first iron(III) spin crossover complex to exhibit reverse‐LIESST (light‐induced excited spin state trapping), we also reveal a hidden hysteresis of 30 K between the hidden LS‐LS and HS‐LS phases. Moreover, we demonstrate that FeIII spin‐crossover (SCO) complexes can be just as effective as FeII systems, and with the advantage of being air‐stable, they are ideally suited for use in molecular electronics.
Light revealing: Thermal and optical switching in an iron(III) spin‐crossover complex reveals a hidden hysteresis of 30 K. The results show that FeIII spin‐crossover complexes can be just as effective as FeII systems, and with the advantage of being air‐stable, they are suited for use in molecular electronics.
Light‐induced excited spin‐state trapping (LIESST) in iron(II) spin‐crossover compounds, that is, the light‐induced population of the high‐spin (S=2) state below the thermal transition temperature, ...was discovered thirty years ago. For irradiation into metal–ligand charge transfer (MLCT) bands of the low‐spin (S=0) species the acknowledged sequence takes the system from the initially excited 1MLCT to the high‐spin state via the 3MLCT state within ca. 150 fs, thereby bypassing low‐lying ligand‐field (LF) states. Nevertheless, these play a role, as borne out by the observation of LIESST and reverse‐LIESST on irradiation directly into the LF bands for systems with only high‐energy MLCT states. Herein we elucidate the ultrafast reverse‐LIESST pathway by identifying the lowest energy S=1 LF state as an intermediate state with a lifetime of 39 ps for the light‐induced high‐spin to low‐spin conversion on irradiation into the spin‐allowed LF transition of the high‐spin species in the NIR.
The path of LIESST resistance: Ultrafast transient absorption with an infrared pump and a UV probe allows for a detailed elucidation of the sequence of events on irradiation into the spin‐allowed ligand‐field transition of the high‐spin species of a prototypical iron(II) spin‐crossover complex, thereby identifying the lowest‐energy triplet state as a short‐lived intermediate state with a lifetime of 39 ps in the passage from the excited quintet to the singlet ground state.
Domain wall motion is detected for the first time during the transition to a ferroelastic and spin state ordered phase of a spin crossover complex. Single‐crystal X‐ray diffraction and resonant ...ultrasound spectroscopy (RUS) revealed two distinct symmetry‐breaking phase transitions in the mononuclear Mn3+ compound Mn(3,5‐diBr‐sal2(323))BPh4, 1. The first at 250 K, involves the space group change Cc→Pc and is thermodynamically continuous, while the second, Pc→P1 at 85 K, is discontinuous and related to spin crossover and spin state ordering. Stress‐induced domain wall mobility was interpreted on the basis of a steep increase in acoustic loss immediately below the the Pc‐P1 transition
Stresses and strains: Domain wall motion is detected for the first time in a spin crossover crystal. Mobility of ferroelastic domain walls was interpreted on the basis of a steep increase in acoustic loss below a first order transition. Spin state ordering and domain formation is associated with collapse of the Jahn–Teller distortion on switching from the spin quintet to spin triplet form of a mononuclear Mn3+ complex.
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