H. Cailleau

Inelastic neutron scattering study of structural phase transitions in polyphenyls

Abstract Inelastic neutron scattering was used to study structural phase transitions in polyphenyls. These transitions result from the stabilization of the non-planar molecular configuration with respect to a torsional angle between the planes of phenyl rings. For biphenyl the zone boundary mode shows a pronounced softening in the temperature range 60–200 K and becomes overdamped close to the transition temperature. For p-terphenyl the spectrum contains a quasi-elastic anisotropic central component diverging on lowering the temperature.

Developing 100 ps-resolved X-ray structural analysis capabilities on beamline NW14A at the Photon Factory Advanced Ring

NW14A is a newly constructed undulator beamline for 100 ps time-resolved X-ray experiments at the Photon Factory Advanced Ring. This beamline was designed to conduct a wide variety of time-resolved X-ray measurements, such as time-resolved diffraction, scattering and X-ray absorption fine structure. Its versatility is allowed by various instruments, including two undulators, three diffractometers, two pulse laser systems and an X-ray chopper. The potential for the detection of structural changes on the 100 ps time scale at NW14A is demonstrated by two examples of photo-induced structural changes in an organic crystal and photodissociation in solution.

Incommensurate phases in biphenyl

Abstract Successive phase transitions in deuterated biphenyl are re-examined with higher resolution elastic neutron scattering. The two low temperature phases were found to be incommensurate. The wave vector characterizing the reflections are: q s = δ a a ∗ + 1 2 (1 - δ b ) b ∗ in phase II existing between TII = 21 K and TI = 38 K, and q s = 1 2 (1 - δ b ) b ∗ in phase III below TII. The variations of δa and δb with temperature small. The deviations of satellite reflections exhibit clearly a jump at TII.

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.

Observation of excitations in the incommensurable phases of biphenyl by inelastic neutron scattering

Abstract We have used a very high resolution triple-axis spectrometer installed on a neutron cold source to observe a new excitation superimposed on an overdamped mode in the incommensurable phases of biphenyl. The possibility of spurious effects has been examined and discarded. The measured temperature dependence of the excitation is discussed. Its dispersion is found to follow a linear law originating at the satellite reflection with a slope very much lower than that of the lowest acoustic mode.

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).

Structural investigation of the photoinduced spin conversion in the dinuclear compound {[Fe(bt)(NCS)2]2(bpym)}: toward controlled multi‐stepped molecular switches

The photocrystallographic investigation of the light-induced excited spin-state trapping effect in the dinuclear spin-crossover compound {[Fe(bt)(NCS)2]2(bpym)} is reported. In this system, each of the two Fe sites may be either in the high-spin (HS) or in the low-spin (LS) state, so that the molecule corresponds to a three-state system (LS–LS, HS–LS and HS–HS). At low temperature, the laser excitation wavelength controls the photoswitching from the stable LS–LS state to one of the metastable excited states (HS–LS or HS–HS), and also between these two excited states. Significant changes in the crystalline structure associated with the photoinduced change of spin state are detailed here. The low-temperature photoinduced states look similar to the corresponding states observed at thermal equilibrium within the unit-cell thermal contraction.

Raman Scattering Study of the order—disorder phase transition in para-terphenyl

Abstract We have investigated the effect of the order—disorder phase transition on the Raman spectrum of para-terphenyl in the spectral range 3–170 cm−1. The Raman spectrum does not present a sudden change at the transition temperature but a progressive evolution in the low temperature phase. The low temperature spectra (15 K) exhibit external modes and torsional internal modes.

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.

Calorimetric Study of the Phase Transition of Para-Terphenyl

Abstract At 300 K crystalline para-terphenyl (PTP) may be considered to be planar, but the librational amplitude of the central ring is unusually high (16°). This behaviour has been explained by the occurrence of a double well potential for the central cycle which occupies at random one of the two possible positions and jumps continuously from one minimum to the other one.1 When the temperature is lowered, a superstructure appears: an antiferro-reorientational ordering results from the stabilization of the central ring in one of the two possible wells.2 A recent treatment of the diffraction data (neutron as well as x-ray) collected in the high temperature phase has confirmed this hypothesis: a double-peaked distribution function was obtained for the central ring librations.3