Activation of coherent lattice phonon following ultrafast molecular spin-state photo-switching: A molecule-to-lattice energy transfer

Abstract

We combine ultrafast optical spectroscopy with femtosecond X-ray absorption to study the photo-switching dynamics of the [Fe(PM-AzA)2(NCS)2] spin-crossover molecular solid. The light-induced excited spin-state trapping process switches the molecules from low spin to high spin (HS) states on the sub-picosecond timescale. The change of the electronic state (<50 fs) induces a structural reorganization of the molecule within 160 fs. This transformation is accompanied by coherent molecular vibrations in the HS potential and especially a rapidly damped Fe-ligand breathing mode. The time-resolved studies evidence a delayed activation of coherent optical phonons of the lattice surrounding the photoexcited molecules.

Photoinduced Structural Dynamics of Molecular Systems Mapped by Time-Resolved X-ray Methods

“…113,211 Similar results were obtained in the derivative SCO material [Fe(PM-AzA)2(NCS)2]. 291 This ultrafast structural dynamic in SCO solids was also investigated by x-ray diffraction by Chergui, Elsaesser and co-workers. 292 They used a laser-plasma source X-ray source delivering (Cu Kα, λ = 0.154 nm, 100 fs pulse duration)…”

“…288,290,291 In SCO Fe II molecular solids, X-ray diffraction reveals important changes of the molecular structure, around the FeN6 octahedron, between LS and HS states at thermal equilibrium but also in the long-lived photoinduced HS state at very low temperature. In the prototype [Fe(phen)2(NCS)2] material, the main structural deformation is the increase of the average Fe-N distance from <Fe-N>LS  1.97 Å to <Fe-N>HS  2.16 Å, 290 as in solutions (see § 4.2), accompanied by a torsion of the octahedron.…”

Coherent ultrafast lattice-directed reaction dynamics of triiodide anion photodissociation

“…Our results suggest that the corresponding gas-phase reaction scheme needs to be significantly modified to take full account of the system-bath interactions in the crystalline environment (between reactant, photoproduct and lattice). The very recent surge of interest [43][44][45] in the study of the coupling of lattice dynamics to various reactive molecular systems on ultrafast timescales also demonstrate the varieties of lattice participation, which will require investigation on a case-by-case basis to eventually arrive at a generalized description of lattice-coupled reaction dynamics. Furthermore, our study motivates future work to verify the present findings using structure-resolving probes 46 that directly access the spatial degrees of freedom, which are fundamental to understanding chemistry.…”

Perspective: Opportunities for ultrafast science at SwissFEL

“…More recently, Collet and co-workers extended these studies to SCO molecular crystals 137 . At SACLA, studies were performed on an Fe-complex, [Fe III (C 2 O 4 ) 3 ] 3+ in solution, 138 delivering insight into the photoinduced electronic and structural changes in the system, while the photoinduced charge carrier dynamics was investigated in the case of WO 3 nanoparticles, 139 and of TiO 2 140 .…”