Sébastien Pillet

Excited-state structure by time-resolved X-ray diffraction

X-ray crystallography has traditionally been limited to the study of the ground-state structure of molecules and solids. Recent technical advances are removing this limitation as demonstrated here by a time-resolved stroboscopic study of the photo-induced 50 micros lifetime excited triplet state of the [Pt(2)(pop)(4)](4-)ion [pop = pyrophosphate, (H(2)P(2)O(5))(2-)], performed at helium temperatures with synchrotron radiation. The shortening of the Pt-Pt bond by 0.28(9)A upon excitation is compatible with the proposed mechanism involving promotion of a Pt-Pt antibonding dsigma* electron to a weakly bonding p orbital. The contraction is accompanied by a 3 degree molecular rotation. The time-resolved diffraction technique described here is applicable to reversible light-driven processes in the crystalline solid state.

Giant Ising-Type Magnetic Anisotropy in Trigonal Bipyramidal Ni(II) Complexes: Experiment and Theory

This paper reports the experimental and theoretical investigations of two trigonal bipyramidal Ni(II) complexes, [Ni(Me(6)tren)Cl](ClO(4)) (1) and [Ni(Me(6)tren)Br](Br) (2). High-field, high-frequency electron paramagnetic resonance spectroscopy performed on a single crystal of 1 shows a giant uniaxial magnetic anisotropy with an experimental D(expt) value (energy difference between the M(s) = ± 1 and M(s) = 0 components of the ground spin state S = 1) estimated to be between -120 and -180 cm(-1). The theoretical study shows that, for an ideally trigonal Ni(II) complex, the orbital degeneracy leads to a first-order spin-orbit coupling that results in a splitting of the M(s) = ± 1 and M(s) = 0 components of approximately -600 cm(-1). Despite the Jahn-Teller distortion that removes the ground term degeneracy and reduces the effects of the first-order spin-orbit interaction, the D value remains very large. A good agreement between theoretical and experimental results (theoretical D(theor) between -100 and -200 cm(-1)) is obtained.

Magnetostructural relationship in the spin-crossover complex t-{Fe(abpt)2[N(CN)2]2}: polymorphism and disorder phenomenon.

t-{Fe(abpt)(2)[N(CN)(2)](2)} [abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole] is an intriguing spin-crossover system that crystallizes in two polymorphs. Polymorph A is paramagnetic; its crystal structure consists of a single molecule located at the center of inversion symmetry. Polymorph B, on the other hand, exhibits a rather complicated two-step-like spin transition; its crystal structure consists of two symmetry-independent molecules. The crystal structure of polymorph B has been derived in the different spin states: above the high-temperature step (300 K), between the two steps (90 K), below the incomplete low-temperature step (50 K), in the light-induced metastable state (15 K), in the thermally quenched metastable state (15 K), and after relaxation from the quenched state (15 K). The correlation between the structure and magnetic properties is precisely established, allowing the complicated magnetic behavior of polymorph B to be well understood. A unique order-disorder phase transition, resulting in a modulation of the metastable state structures, is detected for the first time on such spin-transition compounds. The modulation of the structure originates from a particular ordering of the dicyanamide ligand at one of the two Fe sites.

Ligand-Driven Light-Induced Spin Change Activity and Bidirectional Photomagnetism of Styrylpyridine Iron(II) Complexes in Polymeric Media

Two pseudo-octahedral iron(II) complexes, Fe(stpy)(4)(NCSe)(2), containing photoresponsive ligands (cis <--> trans isomerization of -CHCH-) were prepared with trans- or cis-styrylpyridine (stpy) isomers. The magnetic behavior of the polycrystalline solids was previously shown to depend on the configuration of the stpy ligand. The crystal X-ray structures were determined at 293 and 104 K for both isomers. The all-trans and all-cis compounds crystallize in the orthorhombic (Pna2(1)) and the monoclinic space groups (C2/c), respectively. No symmetry change occurs upon cooling to 104 K. The Fe(II) centers lie in axially compressed octahedra with NCSe anions in the apical position and the four pyridinic nitrogens in the meridional plane. The variation of metal-ligand bond lengths as a function of temperature reflects the thermal S = 0 <--> S = 2 crossover of all-trans complexes and the S = 2 ground state of all-cis complexes. The unit-cell volumes per metal ion also change accordingly, and the relative variation due to the spin-crossover compares those associated with the formal change of configuration of the four stpy isomers. The photomagnetic responses were investigated at 130 K with doped polymer thin films containing all-cis (high-spin) or all-trans species (partly low-spin). The 130 K illumination of these doped poly(methyl methacrylate) (PMMA) films leads to the UV-vis absorption features typical for the cis <--> trans photoisomerization of the stilbenoid moiety. The direct magnetic measurements have unambiguously established the photomagnetic effect named ligand-driven light-induced spin change (LD-LISC). The 355 nm excitation of doped thin films produces very long lifetime states that are manifested by high-spin to low-spin (all-cis complex) and low-spin to high-spin (all-trans complex) changes of the Fe(II) magnetic behavior; the process is bidirectional. A structural analysis based on the single-crystal X-ray diffraction data has been proposed to rationalize the LD-LISC activity detected here for doped PMMA thin films.

Photoinduced HS state in the first spin-crossover chain containing a cyanocarbanion as bridging ligand

A new polymeric approach, based on cyanocarbanion ligands, for the design of spin crossover (SCO) compounds led us to the compound [Fe(abpt)(2)(tcpd)] () (tcpd(2-) = (C[C(CN)(2)](3))(2-), abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) which has been characterised as the first SCO molecular chain involving a cyanocarbanion as bridging ligand.

Recovering experimental and theoretical electron densities in corundum using the multipolar model: IUCr Multipole Refinement Project

This electron-density study on corundum (alpha-Al2O3) is part of the Multipole Refinement Project supported by the IUCr Commission on Charge, Spin and Momentum Densities. For this purpose, eight different data sets (two experimental and six theoretical) were chosen from which the electron density was derived by multipolar refinement (using the MOLLY program). The two experimental data sets were collected on a conventional CAD4 and at ESRF, ID11 with a CCD detector, respectively. The theoretical data sets consist of static, dynamic, static noisy and dynamic noisy moduli of structure factors calculated at the Hartree-Fock (HF) and density functional theory (DFT) levels. Comparisons of deformation and residual densities show that the multipolar analysis works satisfactorily but also indicate some drawbacks in the refinement. Some solutions and improvements during the refinements are proposed like contraction or expansion of the inner atomic shells or increasing the order of the spherical harmonic expansion.

Cyanocarbanion-based spin-crossover materials: photocrystallographic and photomagnetic studies of a new iron(II) neutral chain.

A new iron(II) chain of formula [Fe(abpt)(2)(tcpd)] [1; (tcpd)(2-) = [C(10)N(6)](2-) = (C[C(CN)(2)](3))(2-) = 2-dicyanomethylene-1,1,3,3-tetracyanopropanediide anion, abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole] has been synthesized and characterized by IR spectroscopy, detailed variable-temperature single-crystal X-ray diffraction, magnetic and photomagnetic measurements. The crystal structure determination of 1 reveals a one-dimensional structural architecture in which the (tcpd)(2-) cyanocarbanion acts as a μ(2)-bridging ligand and the two abpt molecules act as chelating ligands. Detailed X-ray diffraction studies as a function of the temperature (293-10 K) showed a strong modification of the iron coordination sphere, whose characteristics are in agreement with the presence of a spin-crossover transition from high spin (HS) to low spin (LS) in 1. The average Fe-N distances at room temperature, at 10 K following a flash cooling, and at 10 K after subsequent HS-to-LS relaxation are in the range expected for 100%, 50%, and 25% fractions of HS Fe(II), respectively. These observations are consistent with the presence of ca. 25% residual HS species at low temperatures, as derived from the magnetic data. The signature of a photoinduced metastable HS state in 1 has been detected by performing coupled photomagnetic and photocrystallographic analyses. The limiting T(LIESST) value associated with the light-induced excited-spin-state trapping effect was derived as 35 K, in good agreement with the thermal dependence of the unit cell volume upon irradiation. Kinetic studies governing the photoinduced HS/LS process have been recorded at different temperatures; a reverse-LIESST effect has been evidenced at 10 K as a reduction of the residual HS fraction by irradiating the sample at 830 nm.

Magnetic bistability and thermochromism in a molecular Cu(II) chain.

An original magnetic bistability and a thermochromic transition are observed in a new Cu(II) molecular chain. Thermal structural studies reveal changes in the Cu(II) coordination sphere, driven by a more pronounced Jahn-Teller effect at low temperature. These distortions provoke a gradual color change. The structural study at 10 K shows a dimerization of the molecular chain, in agreement with the abrupt magnetic transition observed at 30 K.

On the precision and accuracy of structural analysis of light-induced metastable states

Bragg diffraction data were collected on single crystals of the spin-crossover complex [Fe(phen)2(NCS)2] in its low-spin and light-induced metastable high-spin states. Experimental variables included the temperature (32 and 15 K), the X-ray source (sealed tube and synchrotron), and the time interval between laser light excitation of the sample (λ = 647 nm). From a comparison of the structural parameters refined, it is shown that photo-crystallographic measurements suffer significantly and systematically from bias if the probed sample contains residual ground-state species, resulting from an incomplete photo-conversion or a significant metastable- to ground-state relaxation. It follows that a 4% population of species in a different spin state affects the Fe—N bond lengths by more than three standard deviations, and the FeN6 polyhedron volume by as much as seven standard deviations, while the mean atomic position misfit exceeds 0.005 A.

Out-of-equilibrium charge density distribution of spin crossover complexes from steady-state photocrystallographic measurements : experimental methodology and results

Abstract The electron density distribution of a light-induced molecular excited state, i.e. the high spin metastable state of [Fe(phen)2(NCS)2], was determined from steady-state photocrystallographic measurements. We defined the experimental conditions under which the accuracy of the measured diffraction data is compatible with an electron density analysis. These include: (i) a large structural and electronic contrast between high spin (HS) and low spin (LS) states, (ii) an efficient photoconversion under light irradiation and (iii) slow relaxation of the HS metastable state. Multipolar modeling of the electron density yielded a deformation density and 3d-orbital populations for Fe(II) characteristic of a high spin (t2g4eg2) electron configuration and support the assumption of significant σ-donation and π-backbonding of the Fe—N interactions. The electron density distribution in the intermolecular regions confirms anisotropic intermolecular interactions with possibly a layer topology parallel to the orthorhombic (ab) plane, related to the system cooperativity.