Roman Bertoni

Femtosecond Spin-State Photoswitching of Molecular Nanocrystals Evidenced by Optical Spectroscopy†

In the field of control science, which aims at switching the physical properties of materials, photoinduced phase transitions open fascinating perspectives for driving a material towards a new state, far from thermal equilibrium. Such photoswitching will impact future technologies as it provides doorways to the light-control of various photoswitchable functions (for example, magnetic, optical, conducting, and ferroelectric). In control science, tailored laser pulses are widely regarded as the most likely source for achieving that goal. In that respect, the great challenge for molecularbased materials is directing the functionality, both at the relevant size and time scales. If we attempt simple parallels here, the goal is to achieve at the level of a material what femtochemistry has accomplished at the level of a molecule. In observing and understanding how materials work during elementary dynamical processes, several challenging basic questions are confronted. For instance, ultrafast information processing based on the control of light-driven switching of the physical properties of materials requires that such systems be directed through a complex pathway from atomic to material scales, and that the fundamental limits of transformation speed be overcome, or circumvented. Molecular magnets, and especially the spin-crossover compounds (SCO), are ideal candidates for photo-active prototypes, which show photomagnetic and photochromic properties driven by the switching of the constituent molecules between their electronic low spin (LS) and high spin (HS) states. Herein we report the ultrafast spin state photoswitching of a spin-crossover nanocrystal of an Fe complex, [Fe(3-MeOSalEen)2]PF6 (Figure 1), as studied through femtosecond optical spectroscopy (H-3-MeO-SalEen being the condensation product of 3-methoxy-substituted salicylaldehyde and N-ethyl-ethylenediamine). This result provides proof-of-principle for femtosecond switching at the nanoscale in SCO materials showing photomagnetic and photochromic responses. SCO materials are bistable systems, for which nanosecond laser excitation within the range of thermal hysteresis can generate LS to HS transition. Ultrafast investigations of similar photo-transformations have been mainly limited to single molecules in solution, and only recently also carried out on crystals. Despite formidable progress in the chemistry and engineering of spin-crossover nanoparticles, as well as their nano-patterning and nanoscale assembling 11] while preserving their switchable properties, the ultrafast switching of such materials has not yet been observed. Herein we study the ultrafast LS-to-HS spin-state photoswitching pathway of nanocrystals (Figure 1), taking advantage of growing knowledge in the field of ultrafast chemical physics. [*] R. Bertoni, Dr. M. Lorenc, Dr. M. Servol, Prof. E. Collet Institut de Physique de Rennes, UMR CNRS 6251 Universit Rennes 1, 35042 Rennes cedex (France) E-mail: maciej.lorenc@univ-rennes1.fr eric.collet@univ-rennes1.fr

Ultrafast spin-state photoswitching in a crystal and slower consecutive processes investigated by femtosecond optical spectroscopy and picosecond X-ray diffractionw

We report the spin state photo-switching dynamics in two polymorphs of a spin-crossover molecular complex triggered by a femtosecond laser flash, as determined by combining femtosecond optical pump-probe spectroscopy and picosecond X-ray diffraction techniques. The light-driven transformations in the two polymorphs are compared. Combining both techniques and tracking how the X-ray data correlate with optical signals allow understanding of how electronic and structural degrees of freedom couple and play their role when the switchable molecules interact in the active crystalline medium. The study sheds light on crossing the border between femtochemistry at the molecular scale and femtoswitching at the material scale.

Ultrafast Light-Induced Spin-State Trapping Photophysics Investigated in Fe(phen)2(NCS)2 Spin-Crossover Crystal

Few photoactive molecules undergo a complete transformation of physical properties (magnetism, optical absorption, etc.) when irradiated with light. Such phenomena can happen on the time scale of fundamental atomic motions leading to an entirely new state within less than 1 ps following light absorption. Spin crossover (SCO) molecules are prototype systems having the ability to switch between low spin (LS) and high spin (HS) molecular states both at thermal equilibrium and after light irradiation. In the case of Fe(II) (3d(6)) complexes in a nearly octahedral ligand field, the two possible electronic distributions among the 3d split orbitals are S = 0 for the LS diamagnetic state and S = 2 for the HS paramagnetic state. In crystals, such photoexcited states can be long-lived at low temperature, as is the case for the photoinduced HS state of the [Fe(phen)2(NCS)2] SCO compound investigated here. We first show how such bistability between the diamagnetic and paramagnetic states can be characterized at thermal equilibrium or after light irradiation at low temperature. Complementary techniques provide invaluable insights into relationships between changes of electronic states and structural reorganization. But the development of such light-active materials requires the understanding of the basic mechanism following light excitation of molecules, responsible for trapping them into new electronic and structural states. We therefore discuss how we can observe a photomagnetic molecule during switching and catch on the fly electronic and structural molecular changes with ultrafast X-ray and optical absorption spectroscopies. In addition, there is a long debate regarding the mechanism behind the efficiency of such a light-induced process. Recent theoretical works suggest that such speed and efficiency are possible thanks to the instantaneous coupling with the phonons of the final state. We discuss here the first experimental proof of that statement as we observe the instantaneous activation of one key phonon mode precluding any recurrence towards the initial state. Our studies show that the structural molecular reorganization trapping the photoinduced electronic state occurs in two sequential steps: the molecule elongates first (within 170 femtosecond) and bends afterwards. This dynamics is caught via the coherent vibrational energy transfer of the two main structural modes. We discuss the transformation pathway connecting the initial photoexcited state to the final state, which involves several key reaction coordinates. These results show the need to replace the classical single coordinate picture employed so far with a more complex multidimensional energy surface.

Cooperative elastic switching vs. laser heating in [Fe(phen)2(NCS)2] spin-crossover crystals excited by a laser pulse

Spin-crossover crystals show multi-step responses to femtosecond light excitation. The local molecular photo-switching from low to high spin states occurs on sub-picosecond timescale. It is followed by additional conversion due to elastic (ns) and thermal (μs) effects. In [Fe(phen)2(NCS)2] crystals discussed herein, the thermal switching can be made unobtrusive for the investigation of cooperative elastic switching. We evidence a cooperative transformation induced by lattice expansion through elastic coupling between molecules in the crystal, where up to 3 molecules are transformed per photon.

Femtosecond spin-state photo-switching dynamics in an FeIII spin crossover solid accompanied by coherent structural vibrations

We investigate light-induced excited spin-state trapping (LIESST) dynamics of an FeIII spin-crossover material from low (S = 1/2) to high (S = 5/2) spin states. Our results show that this process occurs only at the molecular level as evidenced by the linear dependence of the fraction of photo-switched molecules with the excitation density as well as with the initial fraction of low spin molecules. The inter-system crossing from photoexcited LS (S = 1/2) to HS (S = 5/2) occurs within ≈200 fs and is accompanied by coherent non-equilibrium vibrational relaxation in the photo-induced HS state. These results reveal similar dynamical features to those already reported for LIESST in FeII systems. The activation of coherent molecular vibrations is essential for rapidly reaching the HS potential on the timescale of molecular motions, whereas their fast damping allows an efficient trapping in the HS potential. The observed coherent oscillations are attributed to photoinduced molecules in the HS states, as supported by Raman spectroscopy at thermal equilibrium, and DFT analyses of molecular vibrations and TD-DFT calculations of optical absorption.