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)]PF₆, in which TPA=tris(2‐pyridyl methyl)amine, TCC^2?=3,4,5,6‐Cl₄‐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 three‐dimensional intermolecular network formed via water molecules in trans‐bis(nitrito‐κN)tetrakis(pyridine‐κN)ruthenium(II) dihydrate

The molecular geometry of the tetragonal crystal structure of the title compound, [Ru(NO₂)₂(C₅H₅N)₄]·2H₂O, differs from that previously determined by powder diffraction [Schaniel et al. (2010). Phys. Chem. Chem. Phys. 12 , 6171–6178]. In the [Ru(NO₂)(C₅H₅N)₄] molecule, the Ru atom lies at the intersection of three twofold axes (Wyckoff position 8b). It is coordinated by four N atoms of the pyridine rings, as well as by two N atoms of N‐nitrite groups. The last two N atoms are located on a twofold axis (Wyckoff position 16f). The O atoms of the water molecules are situated on a twofold axis (Wyckoff position 16e). Short intermolecular contacts are observed in the crystal structure, viz. N—O...OW and N—O...H—OW contacts between nitrite and water, and weak C—H...OW hydrogen bonds between pyridine and water. Thus, the intercalated water molecules act as bridges connecting the trans‐[Ru(NO₂)₂(py)₄] molecules into a three‐dimensional network.     

Impacting materials by light and seeing their structural dynamics

The emergence of the field of photo-induced transformation triggered by intense and ultra-short light pulse in a material has opened a new route to acting on matter. It combines many of today’s issues such as far away from equilibrium phenomena, coherence and quantum control, complex correlations and non linear responsiveness, inter-conversion of energy and information. The advent of ultrafast structural dynamics motivated developing new tools to directly watch how matter works, by probing the time- and length-scales on which photo-induced processes take place. Different points will be briefly overviewed: (i) the development of laser pump and X-ray probe for watching ultrafast structural dynamics, (ii) the physical features resulting from this way of impacting materials by light, (iii) the presentation of some examples investigated by time-resolved X-ray diffraction and diffuse scattering experiments to illustrate the complex structural dynamics of a photo-induced transformation.     

Down to the quantum limit at the neutral-to-ionic phase transition of (BEDT-TTF)-(ClMe-TCNQ): symmetry analysis and phase diagram

We report on the neutral-to-ionic (N-I) phase transition in the one-dimensional organic complex (BEDT-TTF)-(ClMeTCNQ). The X-ray studies at room temperature show that the neutral phase of (BEDT-TTF)-(ClMeTCNQ) is already characterized by a polar long-range ordering, at variance with other charge-transfer compounds comprising noncentrosymmetric molecules. From a detailed neutron diffraction study of this complex under high pressure, we present the phase diagram of the N-I transition down to the quantum limit. We discuss the symmetry breaking associated with the transition and the evolution of its first-order character under pressure.     

Capturing one-dimensional precursors of a photoinduced transformation in a material

Achieving control of photoinduced phase transitions requires understanding how materials work during transformation induced by a laser pulse. Here we investigate the precursors of a photoinduced phase transition in the highly cooperative charge-transfer molecular crystal tetrathiafulvalene-p-chloranil and provide key insights. The photogeneration of one-dimensional nanoscale clusters was detected by time-resolved diffuse x-ray scattering with 50-ps time resolution. Such clustering of structurally relaxed electronic excitations is expected to be a common process in many materials presenting photoinduced transformations.     

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.     

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

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.