Structural Dynamics upon Photoexcitation in a Spin Crossover Crystal Probed with Femtosecond Electron Diffraction

Abstract

Photoexcitation of spin crossover (SCO) complexes can trigger extensive electronic spin transitions and transformation of molecular structure. However, the precise nature of the associated ultrafast structural dynamics remains elusive, especially in the solid state. Here, we studied a single‐crystal SCO material with femtosecond electron diffraction (FED). The unique capability of FED allows us to directly probe atomic motions and to track ultrafast structural changes within a crystal lattice. By monitoring the time‐dependent changes of the Bragg reflections, we observed the formation of a photoinduced structure similar to the thermally induced high‐spin state. The data and refinement calculations indicate the global structural reorganization within 2.3 ps, as the metal–ligand bond distribution narrows during intramolecular vibrational energy redistribution (IVR) driving the intermolecular rearrangement. Three independent dynamical group are identified to model the structural dynamics upon photoinduced SCO.

Ultrafast non-thermal laser excitation of gigahertz longitudinal and shear acoustic waves in spin-crossover molecular crystals [Fe(PM–AzA)2(NCS)2]

“…The possibility to trigger their properties by light [3], especially in ultrafast fashion offers further prospects in future applications as optically controlled switching devices [2,4]. While such compounds have already been widely studied [5][6][7][8], there is a shortage of information about the interplay between the change of molecular spin state, the change of the unit cell volume (5% change) and the covalent bonding (10% change). From this standpoint the study of crystal deformations that trigger excitation of coherent acoustic phonons clearly deserves further experimental investigations.…”

Sublimable Spin‐Crossover Complexes: From Spin‐State Switching to Molecular Devices

“…Light irradiation of the SCO solids with femtosecond laser pulses leads to an out‐of‐equilibrium dynamical spin‐state switching process, which involves a sequence of three consecutive steps, namely, photo‐, elastic, and thermal switching at distinct ps, ns, and μs timescales, respectively [88, 127, 128] . The photo‐induced step involves ultrafast electronic and structural reorganization, that is, LS→HS switching and the associated volume change, at the molecular scale.…”

Sublimierbare Spin‐Crossover‐Komplexe: Vom Schalten des Spinzustands zu molekularen Bauelementen

“…Bei Bestrahlung von SCO‐Festkörpern mit Femtosekunden‐Laserpulsen setzt sich eine dynamische, dreistufige Reaktion jenseits des Gleichgewichtszustandes in Gang, bei der nacheinander eine lichtinduzierte, eine elastische und eine thermische Umschaltung im Piko‐, Nano‐, und Mikrosekundenbereich durchlaufen werden [88, 127, 128] . Der lichtinduzierte Schritt beinhaltet die ultraschnelle elektronische und strukturelle Reorganisation auf molekularer Ebene, also ein LS→HS‐Schalten mitsamt der damit verbundenen Volumenänderung.…”