Showing papers by "Jean-François Létard published in 2016"

Surface-induced spin state locking of the [Fe(H2B(pz)2)2(bipy)] spin crossover complex.

Abstract: Temperature- and coverage-dependent studies of the Au(1 1 1)-supported spin crossover Fe(II) complex (SCO) of the type [Fe(H2B(pz)2)2(bipy)] with a suite of surface-sensitive spectroscopy and microscopy tools show that the substrate inhibits thermally induced transitions of the molecular spin state, so that both high-spin and low-spin states are preserved far beyond the spin transition temperature of free molecules. Scanning tunneling microscopy confirms that [Fe(H2B(pz)2)2(bipy)] grows as ordered, molecular bilayer islands at sub-monolayer coverage and as disordered film at higher coverage. The temperature dependence of the electronic structure suggest that the SCO films exhibit a mixture of spin states at room temperature, but upon cooling below the spin crossover transition the film spin state is best described as a mix of high-spin and low-spin state molecules of a ratio that is constant. This locking of the spin state is most likely the result of a substrate-induced conformational change of the interfacial molecules, but it is estimated that also the intra-atomic electron–electron Coulomb correlation energy, or Hubbard correlation energy U, could be an additional contributing factor.

Ordering phenomena of high-spin/low-spin states in stepwise spin-crossover materials described by the ANNNI model

Abstract: We study the complex spin-state switching mechanism in a bistable molecular crystal undergoing a stepwise conversion from high-spin to low-spin states at thermal equilibrium as well as during the relaxation process from the photoinduced metastable state. We experimentally evidence that such steps are associated with complex types of long-range and short-range ordering phenomena, resulting from the occupational modulation of bistable molecular magnetic states. The conversion is then described by using two order parameters: the totally symmetric average high spin fraction and the symmetry breaking ordering parameter. The use of the anisotropic next-nearest-neighbor Ising (ANNNI) model allows us to describe the microscopic origin of the ordering, and Monte Carlo simulations reproduce the observed stepwise thermal phase transition as well as the stepwise relaxation from the photoinduced high-spin state to the low-spin state.

Rational Control of Spin-Crossover Particle Sizes: From Nano- to Micro-Rods of [Fe(Htrz)2(trz)](BF4)

Abstract: The spin-crossover (SCO) materials based on iron (II) and triazole ligands can change their spin state under an external perturbation such as temperature, pressure or light irradiation, exhibiting notably large hysteresis in their physical properties’ transitions. If these aspects are investigated for decades, it is only in the recent years that the design of SCO particles has attracted the attention of the scientific community with increasing interest focusing on the possibility of getting wide ranges of sizes and shapes of nanoparticles. In this context, we rationalized the reverse-micellar synthesis, thanks to the scrutiny of the experimental parameters, to produce SCO particles with controlled size and shape. This approach has been performed for the reference one-dimensional (1D) polymeric spin-crossover compound of formula [Fe(Htrz)2(trz)](BF4). A synergetic effect of both time and temperature is revealed as being of paramount importance to control the final particle size. Consequently, under well-defined experimental conditions, we can now offer rod-shaped SCO particles with lengths ranging from 75 to 1000 nm.

Activation of coherent lattice phonon following ultrafast molecular spin-state photo-switching: A molecule-to-lattice energy transfer

Abstract: We combine ultrafast optical spectroscopy with femtosecond X-ray absorption to study the photo-switching dynamics of the [Fe(PM-AzA)2(NCS)2] spin-crossover molecular solid. The light-induced excited spin-state trapping process switches the molecules from low spin to high spin (HS) states on the sub-picosecond timescale. The change of the electronic state (<50 fs) induces a structural reorganization of the molecule within 160 fs. This transformation is accompanied by coherent molecular vibrations in the HS potential and especially a rapidly damped Fe-ligand breathing mode. The time-resolved studies evidence a delayed activation of coherent optical phonons of the lattice surrounding the photoexcited molecules.

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

Abstract: Spin-crossover crystals show multi-step responses to femtosecond light excitation. The local molecular photo-switching from low to high spin states occurs on the 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.

Rubidium Manganese Hexacyanoferrate Solid Solutions: Towards Hidden Phases

Abstract: The search for hidden phase using photo-induced phase transition is an important challenge in multistable materials. In this work we report the study of Prussian blue analogs of formula RbxMn[Fe(CN)6](x+2)/3. zH2O. For composition of Rb+ above 65 %, a thermal hysteresis loop is observed as a reflect of the electron-transfer phase transition from MnIII (S=5/2)-NC-FeIII (S=1/2) at high temperature (HT phase) to MnIII (S=2)-NC-FeII (S=0) at low temperature (LT phase). At low temperature the HT phase can be thermally quenched up to the T(TIESST) temperature (TIESST = Thermally-Induced Excited Spin-State Trapping) above which it relaxes back to the LT phase. Upon the decrease of Rb+ amount, the thermal hysteresis and the T(TIESST) are getting closer and overlap for values of x below 65 %. For the latter compounds, the phase transition is inhibited and the hysteresis loop is hidden. By this simple consideration we explain the observation of Light-Induced Phase Collapse already reported giving also some trends to obtain new hidden phases.