Marina Servol

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.

Structural dynamics of photoinduced molecular switching in the solid state.

Fast and ultra-fast time-resolved diffraction is a fantastic tool for directly observing the structural dynamics of a material rearrangement during the transformation induced by an ultra-short laser pulse. The paper illustrates this ability using the dynamics of photoinduced molecular switching in the solid state probed by 100 ps X-ray diffraction. This structural information is crucial for establishing the physical foundations of how to direct macroscopic photoswitching in materials. A key feature is that dynamics follow a complex pathway from molecular to material scales through a sequence of processes. Not only is the pathway indirect, the nature of the dynamical processes along the pathway depends on the timescale. This dictates which types of degrees of freedom are involved in the subsequent dynamics or kinetics and which are frozen or statistically averaged. We present a recent investigation of the structural dynamics in multifunctional spin-crossover materials, which are prototypes of molecular bistability in the solid state. The time-resolved X-ray diffraction results show that the dynamics span from subpicosecond molecular photoswitching followed by volume expansion (on a nanosecond timescale) and additional thermoswitching (on a microsecond timescale).

100 Picosecond Diffraction Catches Structural Transients of Laser‐Pulse Triggered Switching in a Spin‐Crossover Crystal

We study by 100 picosecond X-ray diffraction the photo-switching dynamics of single crystal of the orthorhombic polymorph of the spin-crossover complex [(TPA)Fe(TCC)]PF(6), in which TPA = tris(2-pyridyl methyl)amine, TCC(2-) = 3,4,5,6-Cl(4)-Catecholate(2-). In the frame of the emerging field of dynamical structural science, this is made possible by using optical pump/X-ray probe techniques, which allow following in real time structural reorganization at intra- and intermolecular levels associated with the change of spin state in the crystal. We use here the time structure of the synchrotron radiation generating 100 picosecond X-ray pulses, coupled to 100 fs laser excitation. This study has revealed a rich variety of structural reorganizations, associated with the different steps of the dynamical process. Three consecutive regimes are evidenced in the time domain: 1) local molecular photo-switching with structural reorganization at constant volume, 2) volume relaxation with inhomogeneous distribution of local temperatures, 3) homogenization of the crystal in the transient state 100 µs after laser excitation. These findings are fundamentally different from those of conventional diffraction studies of long-lived photoinduced high spin states. The time-resolution used here with picosecond X-ray diffraction probes different physical quantities on their intrinsic time-scale, shedding new light on the successive processes driving macroscopic switching in a functionalized material. These results pave the way for structural studies away from equilibrium and represent a first step toward femtosecond crystallography.

The Role of Ligand‐Field States in the Ultrafast Photophysical Cycle of the Prototypical Iron(II) Spin‐Crossover Compound [Fe(ptz)6](BF4)2

Light-induced excited spin-state trapping (LIESST) in iron(II) spin-crossover compounds, that is, the light-induced population of the high-spin (S=2) state below the thermal transition temperature, was discovered thirty years ago. For irradiation into metal-ligand charge transfer (MLCT) bands of the low-spin (S=0) species the acknowledged sequence takes the system from the initially excited (1) MLCT to the high-spin state via the (3) MLCT state within ca. 150 fs, thereby bypassing low-lying ligand-field (LF) states. Nevertheless, these play a role, as borne out by the observation of LIESST and reverse-LIESST on irradiation directly into the LF bands for systems with only high-energy MLCT states. Herein we elucidate the ultrafast reverse-LIESST pathway by identifying the lowest energy S=1 LF state as an intermediate state with a lifetime of 39 ps for the light-induced high-spin to low-spin conversion on irradiation into the spin-allowed LF transition of the high-spin species in the NIR.

Polymorphism in the spin‐crossover ferric complexes [(TPA)FeIII(TCC)]PF6

We have identified two polymorphs of the molecular complex [(TPA)Fe((III))(TCC)]PF(6) [TPA = tris(2-pyridylmethyl)amine and TCC = 3,4,5,6-tetrachlorocatecholate dianion]: one is monoclinic and the other is orthorhombic. By lowering the temperature both undergo a thermal spin-crossover between a high-spin (S = 5/2) and a low-spin (S = 1/2) state, which we detected by magnetic, optical and X-ray diffraction measurements. The thermal crossover is only slightly shifted between the polymorphs. Their crystalline structures consist of similar cation layers alternating with PF(6) anion layers, packed differently in the two polymorphs. The magnetic and optical properties of the polymorphs are presented.

Multi-phonon dynamics of the ultra-fast photoinduced transition of (EDO-TTF)2SbF6

We report here the first observation of the photoinduced insulating-to-metal phase transition in the (EDO-TTF)2SbF6 salt, which occurs on the picosecond time-scale. The time-resolved optical experiments performed with 80 fs time-resolution demonstrate that the dynamical process involves several low-frequency phonons, as the crystalline structure is destabilized upon laser excitation.

Ultrafast non-thermal laser excitation of gigahertz longitudinal and shear acoustic waves in spin-crossover molecular crystals [ Fe ( PM – AzA ) 2 ( NCS ) 2 ]

We report GHz longitudinal as well as shear acoustic phonon photoexcitation and photodetection using femtosecond laser pulses in a spin-crossover molecular crystal. From our experimental observation of time domain Brillouin scattering triggered by the photoexcitation of acoustic waves across the low-spin (LS) to high-spin (HS) thermal crossover, we reveal a link between the molecular spin state and photoexcitation of coherent GHz acoustic phonons. In particular, we experimentally evidence a non-thermal pathway for the laser excitation of GHz phonons. We also provide experimental insights into the optical and mechanical parameters evolving across the LS/HS spin crossover temperature range.

Symmetry, Aperiodicity and Ultrafast Photo-Switching in Spin-Crossover Compounds

The optical control of materials and related physical properties (electronic state, magnetic, optical...) by laser irradiation has gained tremendous interest within the emerging field of photoinduced phase transitions. Light-induced changes of molecular systems involve subtle coupling between the electronic and structural degrees of freedom, which are essential to stabilize the photo-excited state, different in nature from the stable state. Therefore the new experimental field of photocrystallography plays a key role. Its outreach goes far beyond simple structural analysis under laser excitation. By playing on different physical parameters and developing the techniques and analysis, one can investigate new out of equilibrium physics through light-driven cooperative dynamics and transformations in materials, or follow a chemical reaction in real time [1]. The use of photo-crystallography allows to investigate the nature, the mechanisms and the dynamics of photoinduced phase transitions. Here we will present photocrystallography studies of the photo-switching process in spin-crossover materials. On the one hand, ultrafast diffraction allows to follow the structural dynamics [2] and to probe the different processes following femtosecond laser excitation. Recent studies performed on the LCLS XFEL have shown that the structural molecular changes, with the characteristic Fe-N bond elongation, occur within 160 fs. On the other hand, we will present investigations on the effect of light excitation on spin-state concentration waves, which may be of aperiodic nature (Figure, [3]). Time-resolved x-ray diffraction studies reveal that the high symmetry phase is reached only after milliseconds.