We report the direct observation by x-ray diffraction of a photoinduced paraelectric-to-ferroelectric structural phase transition using monochromatic 100-picosecond synchrotron pulses. It occurs in tetrathiafulvalene-p-chloranil, a charge-transfer molecular material in which electronic and structural changes are strongly coupled. An optical 300-femtosecond laser pulse switches the material from a neutral to an ionic state on a 500-picosecond time scale and, by virtue of intrinsic cooperativity, generates self-organized long-range structural order. The x-ray data indicate a macroscopic ferroelectric reorganization after the laser irradiation. Refinement of the structures before and after laser irradiation indicates structural changes at the molecular level.

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We study the basic mechanisms allowing light to photoswitch at the molecular scale a spin-crossover material from a low- to a high-spin state. Combined femtosecond x-ray absorption performed at LCLS X-FEL and optical spectroscopy reveal that the structural stabilization of the photoinduced high-spin state results from a two step structural trapping. Molecular breathing vibrations are first activated and rapidly damped as part of the energy is sequentially transferred to molecular bending vibrations. During the photoswitching, the system follows a curved trajectory on the potential energy surface.

International audiencePhotoinduced phase transformations [1,2] occur when a laser pulse impacts a material, thereby transforming its electronic and/or structural orders, consequently directing the functionalities [3,4,5,6,7]. The transient nature of photoinduced states has thus far severely limited the application scope. It is of paramount importance to explore whether structural feedback during the solid deformation has capacity to amplify and stabilize photoinduced transformations. Contrary to coherent optical phonons long under scrutiny [8,9,10] , coherently propagating cell deformations over acoustic timescale [11,12,13,14] have not been explored to similar degree, particularly in light of cooperative elastic interactions. Herein we demonstrate experimentally and theoretically a self-amplified responsiveness in a spin-crossover material [15] during its delayed volume expansion. The cooperative response at material scale prevails above a threshold excitation, significantly extending the lifetime of photoinduced states. Such elastically-driven cooperativity triggered by a light pulse offers a new efficient route to the generation and stabilization of photoinduced phases in many volume-changing materials

We investigate the out-of-equilibrium switching dynamics of a molecular Fe(III) spin-crossover solid triggered by a femtosecond laser flash. The time-resolved x-ray diffraction and optical results show that the dynamics span from subpicosecond local photoswitching followed by volume expansion (nanosecond) and thermal switching (microsecond). We present a physical picture of the consecutive steps in the photoswitching of molecular materials.

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New neutron-diffraction data collected at 200 K with deuterated p-terphenyl, and published X-ray data collected at room temperature with protonated p-terphenyl, have been used to determine the characteristics of the double-well potential for the librations of the central ring about the long molecular axis, in the disordered phase. A picture of the double well is obtained simply by halving the atoms outside the molecular axis and imposing constraints on their geometries and thermal parameters. The R values compared with that obtained with the simple-well model, are significantly improved. The deuterated benzene rings, compared to the protonated ones, show a systematic and hard-to-explain distortion: all the internuclear C-C bond lengths parallel to the long axis are shortened and the others lengthened, the mean bond lengths being the same in the two compounds. The double-well potential-barrier height is found to be about 0.6 kcal mo1-1 at 200 K and at room temperature. This value is in good agreement with that given by potential-energy calculations. The rotation angle between the two wells is about 26 °. Only for the internal g mode is the thermal energy kT sufficient to bring the rings near the top of the barrier. For the central-ring librations the doubly peaked distribution function is an argument for an order-disorder regime. However, the smallness of the double-well barrier height suggests that the p-terphenyl structural phase transition is near the boundary between the order-disorder and displacive regimes.

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 allows 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