SPring-8, the large synchrotron radiation facility

Date & Time: 14:00-16:00-, December 8, 2005

Speaker: Prof. Claude Lecomte
Affiliation: LCM3B Universiti Henri Poincari Nancy I

Abstract
　　Upon variation of external conditions (T, P, hν), some transition metals (3d4-3d7) in octahedral surrounding may undergo HS-LS reversible phase transition depending when the ligands force field at the metal center is of the order of magnitude of the electron pairing energy [1,2]. The talk will focuss on iron II complexes which undergo photo or thermoexcited transitions.

　　The first part will give details on the mechanism of the phase transition by a carefull study of the crystallographic lattice changes:

- isostructural spin transitions [3,4] or incommensurate modulation of the photoexcited HS state [5]
- domains formation: dynamics and orientation [3,6].

　　The second part of the talk will first compare the structural properties of phases 1 to 5 (figure below) and analyses for the first time the valence electron density of the HS and LS states of Fe(btr)₂(NCS)₂ H₂O and Fe(Phen)₂(NCS)₂.

1] P. Gütlich, A. Hauser, H. Spiering, Angew. Chem. Int. Ed. Engl. 33, 2024 (1994)
[2] P. Gütlich, Y. Garcia, T. Woike, Coord. Chem. Rev., 219-221, 839 (2001)
[3] S. Pillet, J. Hubsch, C. Lecomte, Eur. Phys. J. B. 38, 541-552 (2004)
[4] V. Legrand, C. Carbonera, S. Pillet, M. Souhassou, J.F. Létard, P. Guionneau, C. Lecomte, J. Phys.: Conference Series 21, 73-80 (2005)
[5] S. Pillet, C. Lecomte, C.F. Sheu, Y.C. Lin, I.J. Hsu, Y. Wang, J. Phys.: Conference Series 21, 221-226 (2005)
[6] V. Legrand, S. Pillet, C. Lecomte, in preparation

Date & Time: 14:00-16:00-, December 8, 2005

Speaker: Prof. Philip Coppens
Affiliation: State University of New York at Buffalo

　　Although ground-state structure determination is quite routine, the determination of transient species offers considerable challenges. To meet this challenge, time-resolved diffraction on a μsec time-scale has been implemented at the 15-ID Chem/Mat CARS beamline at the Advanced Photon Source.¹ The analysis of 3 types of complexes has been completed so far: a) binuclear Pt and Rh complexes in which the metal-metal distance shortens by as much as 0.85(4) in [Rh₂(dimen)₄](PF₆)₂. b) Cu(I) dimethylphenanthroline photosensitizer dyes in which the molecular geometry is changed by metal to ligand charge transfer. c) the trinuclear Cu(I) complex [3,5-(CF₃)₂pyrazolateCu]₃, in which reversible intermolecular Cu-Cu bond formation occurs (see fig.).² The experiments are complemented by parallel theoretical calculations.
A fast shutter of novel design has been developed and is now being applied in sub-microsecond studies. ³ The advantages and disadvantages of monochromatic vs. pink-beam Laue techniques will be discussed.

1 P. Coppens, I. I. Vorontsov, T. Graber, M. Gembicky and A. Yu. Kovalevsky, Acta Crystallogr. A 61, 162-172 (2005).
2 I. I. Vorontsov, A. Yu. Kovalevsky, Y.-S. Chen, T. Graber, M. Gembicky, I. V. Novozhilova, M. A. Omary, P. Coppens, Phys. Rev. Lett. 94, 193003/1-193003/4 (2005)
3 M. Gembicky, D. Oss, R. Fuchs, P. Coppens, J. Synchrotron Rad. 12, 665-669, (2005).