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

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.

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

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.

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

Crystallographic elucidation of purely structural, thermal and light-induced spin transitions in an iron(II) binuclear complex.

The intricate phase diagram of the binuclear iron(II) spin-crossover complex [{Fe(3-bpp)(NCS)(2)}(2)(4,4'-bypiridine)].2CH(3)OH where 3-bpp is 2,6-bis(pyrazol-3-yl)pyridine has been investigated by variable temperature single crystal X-ray diffraction including a study into the effect of photo-irradiation. This sample is known to exhibit an incomplete spin transition at low temperature. At room temperature, in phase I, iron ions are all crystallographically equivalent, adopting the high spin state (HS). X-Ray structural investigation has revealed two phase transitions in the range (300-30 K). The first transition (T approximately 161 K) leading to phase II is of a purely structural nature and corresponds to a break in symmetry as a result of a twist of the two rings of 4,4'-bipyridine; the two iron sites of the binuclear unit becoming crystallographically independent but remaining all HS. The second structural transition corresponds to the spin crossover, one of the two Fe(II) ions of the binuclear complex being in the low spin state (LS) in phase III. The crystal structure shows an ordered HS-LS crystal packing where HS and LS sites are clearly identified and not randomly distributed in the metal ion sites as often observed. Moreover, light irradiation of single crystals in phase III at 30 K, leading to phase III*, induces a light-induced spin-state trapping (LIESST) effect corresponding to the full conversion of all the iron sites to HS. The crystal packing in phase III* is closer to that of phase III than to those observed in the other HS phases, I and II. This reveals an unusual differentiation between the thermal and light-induced HS states. A deeper analysis of the structural properties first demonstrates the key role of the bipyridine bridge in the peculiar preliminary pure structural transition shown by the title compound. Elsewhere, it also shows that the molecular packing is strongly dependent on the nature of the external perturbation contrary to the iron coordination sphere geometry that appears to be only dependent on the spin state. Moreover, in the HS phase II, the distortion of the iron sites that will subsequently undergo a spin crossover demonstrates some differences with the distortion of the iron sites that remain HS. The predominant role of the iron environment distortion in the spin crossover phenomenon is thus clearly evidenced.

Adaptable thermochromism in the CuMo1-xWxO4 series (0

The members of the CuMo1-xWxO4 series (0alpha transition can occur between 260 and 360 K, and the alpha-->gamma transition between 175 and 275 K as a function of x. The control of the alpha/gamma transition temperatures with x is related to the larger propensity of tungsten compared to molybdenum, to adopt a tetrahedral environment.