Crystallography and nanocrystallography of switchable materials

  • Design of functional switchable molecular materials and nanomaterials.

Our group has a specific expertise in the study of switchable molecular materials such as photomagnetic bistable materials (e.g. spin crossover), photochromic linkage isomers (e.g. ML5Z ; Z = NO, NO2, SO2, N2), and piezoelectric materials. We focus our attention on the structure-properties relationship of such materials in various forms : single crystals, nanoparticles, and nanocomposites that we design and elaborate. Nancomposites are based on photoactive molecules or nanocrystals confined in porous silica matrices and nanostructured ordered porous thin films, which could be attractive for the development of new molecule based sensors.

Durand2013Room temperature bistability with wide thermal hysteresis in a spin crossover silica nanocomposite. Durand et al., J. Mater. Chem. C, 1 (2013), 1901-2070.

Nanocomposites have been processed by confined growth of nanoparticles of the spin crossover complex [Fe(Htrz)2(trz)](BF4) within pre-formed transparent mesoporous silica monoliths of well controlled porosity. Monodisperse spherical nanoparticles of mean diameter 3.2(5) nm, homogeneously distributed within the pores of the silica monolith were achieved, whose spin transition behaviour exhibits a room temperature bistability and 65 K wide hysteresis.


  • Development of crystallographic approaches for space- and time-resolved structural analysis.

We have developed a laboratory diffraction instrument for millisecond time-resolved experiments under in situ optical excitation in a pump-probe approach. The instrument is based on a 4-circle X-ray diffractometer equipped with a microfocus X-ray source and a hybrid pixel X-ray detector (XPAD). The detector is synchronized to the output of a Nd-YAG ns pump pulsed laser equipped with an optical parametric oscillator (400nm<λ<700nm). The experiment is further coupled to a transient optical absorption measurement, so that the x-ray diffraction and optical absorption measurements can be run in parallel on the very same single crystal sample. Henthforce, the optical and structural signature of photo-excited metastable states can be obtained in the ms time scale. photocristallo
(left) schematic of the experiment. (right) experimental setup.


  • Structure / property relationship of switchable materials.

The structure-property relationship of switchable materials involves several important aspects that we are investigating using various approaches. Nanomaterials are investigated in order to probe the structural response to size reduction, to understand the physics of thermally induced and photo-induced phase transitions in nanoscale systems. Specific attention are attached on several crucial structural aspects, such as changes of lattice symmetry, lattice expansion, strain and lattice distortion, as well as surface relaxation. Detailed information on the size dependence of the order parameters, critical behaviour, structural fluctuations, coherence length, are specifically studied. We focus our attention on nistrosyl coordination complexes, and spin transition materials.

pillet2012Multimetastability, phototrapping, and thermal trapping of a metastable commensurate superstructure in a FeIIspin-crossover compound. Pillet et al., Phys. Rev. B, 86 (2012), 064106.

The photoinduced switching and relaxation regime of the two-step spin-crossover compound [Fe(bapbpy)(NCS)2] has been investigated by time-dependent photo-crystallography. The relaxation from the metastable HS state proceeds in two steps associated with two symmetry breaking processes associated to an intermediate superstructure, characterized by a long-range-ordered [HS-LS-LS] motif coupled to a commensurate displacive modulation.

Chakraborty2013Light induced bistability in the 2D coordination network {[Fe(bbtr)3](BF4)2}: wavelength-selective addressing of molecular spin states. Chakraborty et al., Chem. Eur. J., 19 (2013), 11418-11428.

In the covalently linked 2D coordination network of {[Fe(bbtr)3](BF4)2}¥, FeII stays in the high-spin state down to 10 K. However, strong cooperative effects of elastic origin result in reversible, persistent, and wavelength-selective photoswitching between the low-spin and high-spin manifolds. This compound thus shows true light-induced bistability below 100 K.

schaniel2013Unraveling the mechanism of NO ligand photoisomerism by time-resolved infrared spectroscopy. Gallé et al. Chem. Phys. Lett., 552 (2013), 64-68.

Femtosecond time-resolved infrared spectroscopy demonstrate that the photoisomerization of the NO ligand in a Na2[Fe(CN)5NO].2H2O single crystal occurs in about 350 fs. The optical excitation leads to an excited state from which the rapid transition into MS2 and GS occurs within 300 fs due to vibronic coupling. The rotation of the NO ligand either to 90° or back to 0° proceeds with this time constant. After these fast (~300 fs) transitions the vibrational–rotational thermalization to the GS and MS2 potential minima occurs within a few picoseconds.

dinitrosyl1Photocrystallography and IR-spectroscopy of light-induced linkage NO isomers in [RuBr(NO)2(PCyp3)2]BF4. Casaretto et al. Acta Cryst.., B71 (2015), 788-797.

One single photo-induced linkage NO isomer (PLI) is detected and characterized in the dinitrosyl pentacoordinated compound [RuBr(NO)2(PCyp3)2]BF4 by a combination of photocrystallographic and infrared (IR) analysis. In the ground state, the molecule adopts a trigonal-bipyramidal structure with the two NO ligands almost linear. After light irradiation of 405 nm at T = 10 K, a metastable state is formed, characterized by the rotation of one single nitrosyl ligand towards the Br atom, forming a bent κN bonded configuration. A clear signature of the formed metastable state is provided by infrared spectroscopy under irradiation. The experimental results are further supported by DFT calculations.

dinitrosyl2Multiple light-induced NO linkage isomers in the dinitrosyl complex [RuCl(NO)2(PPh3)2]BF4 unraveled by photocrystallographic and infrared analysis. Casaretto et al. IUCr J, 2 (2015), 35-44.

Three light-induced reversible metastable NO linkage isomers are detected in the dinitrosyl compound [RuCl(NO)2(PPh3)2]BF4 by a combination of photocrystallographic and infrared analysis. The IR signature for each of the three PLI states has been clearly identified. The photocrystallographic analysis indicates a selectivity of the photo-isomerisation process, only one of the two NO ligands—the bent NO—changes its position, while the linear NO is unaffected. After irradiation with 405 nm, the orientation is changed by rotation towards the Cl ligand in opposite to the linear NO, with an isonitrosyl Ru-O-N binding mode.


  • Nanocristallographie

The structural determination and analysis of nanoscale objects and nanocomposites is a very delicate task. For that purpose, we use a combination of total X-ray scattering methods with pair distribution function (PDF) analysis, and solid state NMR, which provide the structural organisation with atomic resolution and dynamics informations.

PDF-RMN2Structure determination of molecular
nanocomposites by combining pair distribution function analysis and solid-state NMR.
Hsieh et al. RSC Adv., 3 (2013), 26132.

High energy X-ray total scattering with atomic pair distribution function analysis (PDF), together coupled to Solid State NMR techniques, allowed to investigate the structural properties of isolated Sodium nitroprusside (SNP) molecules incorporated in the 1nm nanopores of mesoporous silica matrices. The atomic PDF shows that the SNP complexes are embedded in the form of isolated molecules inside the matrix pores. 29Si MAS NMR measurements show that the gel matrix structure is not modified by the SNP embedding procedure. 13C and 23Na MAS NMR measurements show that inside the matrix pores the SNP neither takes a crystalline like structure nor does it adhere to the pore surface. The confined complex adopts solution like structure and dynamics with isotropic reorientation below the μs timescale. Subsequent NMR experiments at different relative humidity levels show the presence of weakly interacting water molecules inside the pores.

PDF-RMN1Structure determination of molecular nanocomposites by combining pair distribution function analysis and solid-state NMR. Bendeif et al. RSC Adv., 5 (2015), 8895.

Sodium nitroprusside (SNP) nanocomposites were prepared by embedding SNP molecules in transparent mesoporous silica monoliths of well controlled porosity and narrow pore size distribution around 6 nm. The nanocomposites were caracterized by X-ray total scattering coupled to atomic pair distribution function analysis (PDF) and solid-state NMR spectroscopy. The PDF analysis allows the identification of various existing phases: SNP isolated molecules and SNP crystalline nanoparticles with the same molecular structure as the bulk crystalline material. The multi-nuclei Solid State NMR investigation confirms the structural model proposed by the PDF analysis and assigns the isolated molecules to dynamic disorder of a solvated phase.