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.

Direct observation of nuclear reorganization driven by ultrafast spin transitions

“…As depicted in Fig. 5, we observe markedly different structural dynamics for BPY than in our previous UED study of another Fe (II)-based SCO compound, AZA 38 . In the latter case, the observed evolution of Bragg peak intensity followed a monoexponential decay behavior with a 2.3-ps time constant; this is fully described by structural dynamics wherein all motions-the Fe-N bond elongations, ligand butterfly motions, and local unit cell expansions-are coupled.…”

“…In most previous UED studies 14,38,47 , the phase transitions of interest could be activated both thermally and optically. Since the thermal and photoinduced states are known to be nearly identical with respect to their spectral features and other properties 14,38,47 , the thermally induced structure is used as the reference for determining the time-dependent evolution of atomic coordinates from the ground to photoinduced structural changes. In the case of SCO, this prior knowledge of the excited state structure can confirm that the observed evolution of the Bragg peak intensities is due to photoinduced SCO.…”

“…In the presented analysis, four key structural modes (Fe-N bond elongation (ξ 1 ), Fe-ligand elongation (ξ 2 ), Fe-N rotation (ξ 3 ), and Fe-ligand rotation (ξ 4 )) are selected to model the structural dynamics of SCO. Their selection was inferred from previous studies to avoid chemically unreasonable structures 28,37,38 . A model is thus constructed to map the known structural changes between the LS and HS states in solution to those in the single crystal, allowing for a parameterized reconstruction of the evolution of the BPY crystal structure under photoexcitation (see Supplementary Note 3 and Supplementary Table 1).…”

“…The potential nuclear motions leading to an expanded HS molecule were first discussed in an earlier timeresolved X-ray diffraction (TR-XRD) experiment 36 and a number of theoretical studies 7,11,13,37 . This nuclear reorganization has since been directly observed by ultrafast electron diffraction (UED) in the related SCO system, [Fe II (PM-AzA) 2 ](NCS) 2 (PM-AzA = N-2'-pyridyl-methylene-4-(phenyl-azo)) (AZA) 38 . We note that BPY has been previously studied by TR-XRD 39 but under extremely high excitation fluence and the results therein are inconsistent with those from other works 26,[40][41][42][43] .…”

“…In this work, we exploit the simultaneous femtosecond time resolution and high structural sensitivity of UED 14,38 to directly observe the structural dynamics coupled to the spin transitions in photoinduced SCO in single-crystal BPY, to provide the full extent of the nuclear reorganization process. Electron diffraction resolves the atomic positions of molecules by scattering electrons off the Coulomb potentials of the nuclei to give full structural details 14 .…”