Lai Chung Liu

Mapping molecular motions leading to charge delocalization with ultrabright electrons.

Ultrafast processes can now be studied with the combined atomic spatial resolution of diffraction methods and the temporal resolution of femtosecond optical spectroscopy by using femtosecond pulses of electrons or hard X-rays as structural probes. However, it is challenging to apply these methods to organic materials, which have weak scattering centres, thermal lability, and poor heat conduction. These characteristics mean that the source needs to be extremely bright to enable us to obtain high-quality diffraction data before cumulative heating effects from the laser excitation either degrade the sample or mask the structural dynamics. Here we show that a recently developed, ultrabright femtosecond electron source makes it possible to monitor the molecular motions in the organic salt (EDO-TTF)2PF6 as it undergoes its photo-induced insulator-to-metal phase transition. After the ultrafast laser excitation, we record time-delayed diffraction patterns that allow us to identify hundreds of Bragg reflections with which to map the structural evolution of the system. The data and supporting model calculations indicate the formation of a transient intermediate structure in the early stage of charge delocalization (less than five picoseconds), and reveal that the molecular motions driving its formation are distinct from those that, assisted by thermal relaxation, convert the system into a metallic state on the hundred-picosecond timescale. These findings establish the potential of ultrabright femtosecond electron sources for probing the primary processes governing structural dynamics with atomic resolution in labile systems relevant to chemistry and biology.

Ring-Closing Reaction in Diarylethene Captured by Femtosecond Electron Crystallography

The photoinduced ring-closing reaction in diarylethene, which serves as a model system for understanding reactive crossings through conical intersections, was directly observed with atomic resolution using femtosecond electron diffraction. Complementary ab initio calculations were also performed. Immediately following photoexcitation, subpicosecond structural changes associated with the formation of an open-ring excited-state intermediate were resolved. The key motion is the rotation of the thiophene rings, which significantly decreases the distance between the reactive carbon atoms prior to ring closing. Subsequently, on the few picosecond time scale, localized torsional motions of the carbon atoms lead to the formation of the closed-ring photoproduct. These direct observations of the molecular motions driving an organic chemical reaction were only made possible through the development of an ultrabright electron source to capture the atomic motions within the limited number of sampling frames and the low data acquisition rate dictated by the intrinsically poor thermal conductivity and limited photoreversibility of organic materials.

Spectral Signatures of Ultrafast Spin Crossover in Single Crystal [FeII(bpy)3](PF6)2

Solvated iron(II)-tris(bipyridine) ([Fe(II)(bpy)3](2+)) has been extensively studied with regard to the spin crossover (SCO) phenomenon. Herein, the ultrafast spin transition dynamics of single crystal [Fe(II)(bpy)3](PF6)2 was characterized for the first time using femtosecond transient absorption (TA) spectroscopy. The single crystal environment is of interest for experiments that probe the nuclear motions involved in the SCO transition, such as femtosecond X-ray and electron diffraction. We found that the TA at early times is very similar to what has been reported in solvated [Fe(II)(bpy)3](2+), whereas the later dynamics are perturbed in the crystal environment. The lifetime of the high-spin state is found to be much shorter (100 ps) than in solution due to chemical pressure exerted by the lattice. Oscillatory behavior was observed on both time scales. Our results show that single crystal [Fe(II)(bpy)3](PF6)2 serves as an excellent model system for localized molecular spin transitions.

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

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.

Femtosecond Electron Diffraction Study of the Spin Crossover Dynamics of Single Crystal [Fe(PM-AzA)2](NCS)2

The atomic motions involved in spin crossover photo-switching dynamics of single crystal [Fe(PM-AzA)2](NCS)2 are investigated by femtosecond electron diffraction (FED). The experiment was performed with an ultrabright femtosecond electron source using 8.5 × 104 electrons per pulse with 400 fs temporal instrument response function.

Ultrabright Femtosecond Electron Sources: Ultrafast Structural Dynamics in Labile Organic Crystals

1. Max Planck Institute for the Structure and Dynamics of Matter and, Hamburg Centre for Ultrafast Imaging, Luruper Chaussee 149, 22761 Hamburg, Germany. 2. Departments of Chemistry and Physics, 80 St. George Street, University of Toronto, Toronto, Ontario, M5S 3H6, Canada. 3. Interactive Research Center of Science, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8502, Japan. 4. PRESTO, Japan Science and Technology Agency, Honcho, Kawaguchi 332-0012, Japan. 5. Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ōokayama, Meguroku, Tokyo 152-8551, Japan. 6. CREST, Japan Science and Technology Agency (JST), 5-3, Yonbancho, Chiyoda-ku, Tokyo 1028666, Japan. 7. Research Center for Low Temperature and Materials Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan. 8. Faculty of Agriculture, Meijo University, Shiogamaguchi 1-501 Tempaku-ku, Nagoya 468-8502, Japan. *. Present address: Department of Chemistry, 200 University Ave. W, University of Waterloo, Ontario, N2L 3G1, Canada.

Ultrafast Dynamics Related to Spin Crossover Processes in Single Crystal [FeII(bpy)3](PF6)2

Transient absorption spectroscopy is used to characterize the ultrafast spin crossover process in iron(II)-tris(bipyridine)-bis(hexafluorophosphate) single crystals. Preliminary data shows evidence of instrument response limited excited state absorption, long-lived (>1 ns) ground state bleach, and oscillatory signals on multiple time scales.

Ab Initio Solution of Structural Dynamics with Ultrafast Electron Diffraction and Charge Flipping

Ultrafast electron diffraction is used to probe the photoinduced structural dynamics of single crystal (EDO-TTF)2PF6 with femtosecond time resolution. Structure factor phases at key time points are solved ab initio using the charge-flipping method.