Femtosecond X-ray and optical studies of ultrafast photoswitching dynamics in molecular-based magnetic materials – FEMTOMAT

The goal of this project is to lay the foundations for physical properties light-control (magnetic, optical, …) of molecular-based materials using the cooperative and coherent response at ultrashort time-scale. The Institute of Physics in Rennes (IPR, Partner 1) is a leading actor of this emerging field of ultrafast structural dynamics of photoinduced phase transition, which represents a next leap in materials science. This project at the physics-chemistry interface may have direct consequences for the development of new light-driven based technologies if we can understand how to control materials. This requires advanced experimental analysis and especially ultrafast structural science, as molecular reorganization play a key role in the stabilization of the photoinduced state. We will install a betatron femtosecond X-ray absorption spectroscopy station at Laboratoire d'Optique Appliqué (LOA, Partner 2), well adapted to our scientific cases.
This scientific project focuses on bistable or multi-stable molecular-based materials, made of metallic centres which may have different spin states. Spin-crossover materials and Prussian blue analogues, developed at ICMMO (Partner 3) and ICMCB (Partner 4) are prototypes photomagnetic systems. The rich literature deals with slow response to continuous laser irradiation, where the basic mechanisms are hidden in a statistical average precluding access to the elementary processes at the molecular level. Few ultrafast studies were reported on such molecules in solution, where the general inter-system crossing mechanism was revealed. However, the coupling of electronic and structural degrees of freedom is neither well understood nor controlled. In addition, because we are interested in materials, the crystal medium, itself made of switchable molecules is active. We want to take advantage of interactions between molecules for realizing highly efficient response. We recently observed in one system, by combining femtosecond XAS at X-FEL and optical spectroscopy, that the structural stabilization of the photo-excited state involves several intra-molecular phonons and that cooperative response can be observed on ultra-fast time scale, as a single photon transforms several molecules during the structural relaxation.
For understanding universal and specific aspects of these processes, a rich variety of systems will be investigated, showing different origin or behaviour associated with photo-magnetism:
-Spin-crossover materials: we observed quite different inter-system crossing time scales for Fe(II) and Fe(III) compounds (50-250fs). This more or less efficient relaxation is probably due to different electron-phonon or phonon-phonon coupling as several modes are involved.
-Ligand-Driven light-induced spin change: the goal is to control the magnetic state on ultra-fast time-scale through a structural modification of the ligand. Nothing is known on the time-scales of the process, where the energy is not deposited on the magnetic metal centre but on the ligand.
-Prussian blue analogues, where the change of magnetic state is driven by charge-transfer between two metal centres. This is also accompanied by structural relaxation around each metal centre.

These scientific cases need developing state of the art ultra-fast techniques, sensitive to the change of electronic state and/or to the change of molecular structure:
- Femtosecond X-ray diffraction and XAS by using X-FEL (LCLS) and home-based pulsed X-ray sources (Betatron radiation at LOA)
- 10 fs optical spectroscopy and IR spectroscopy at IPR

The feedback from the experiments will help chemists designing more efficient materials as control parameters will be revealed. It is clear that this project is an important key to support research activities in France around the competitive field of photoinduced phase transition and ultrafast structural science at the world-class level.