MAGNETISM AND SUPERCONDUCTIVITY 50 PHOTOINDUCED MOLECULAR SWITCHING STUDIED BY POLARISED NEUTRON DIFFRACTION A. Goujon 1 , B. Gillon 1 , A. Gukasov 1 , J. Jeftic 2 , Q. Nau 1,2 , E. Codjovi 3 , F. Varret 3 1 Laboratoire Léon Brillouin (CEA-CNRS), CEA-Saclay, 91191 Gif-sur-Yvette cedex, France 2 Laboratoire Ecole Nationale de Chimie de Rennes (ENSCR), UMR CNRS 6052, Av. du Général Leclerc, Campus de Beaulieu,35700 Rennes. 3 Laboratoire d‘Optique et de Magnétisme de Versailles (LMOV), CNRS-Université de Versailles UMR 8634, 45 Avenue des Etats Unis,78035 Versailles. The design of molecules that could be utilised for information storage is one of the main challenge in molecular material science and optical switching is one of the most intense aeras of interest in memory molecules. Spin crossover solids represent a promising example of photo-switchable materials, studied for future applications as optical memories or numerical displays [1]. They contain an octahedrally coordinated transition metal ion with the 3d n electronic configuration and can cross over between a low spin (LS) and a high spin (HS) state. The flip between the two states usually occurs with a temperature change, under pressure or under light illumination. Spin crossover compounds containing the Fe 2+ ion have a LS and a HS spin states characterized by spins of S=0 (diamagnetic) and S=2 (paramagnetic). Photo- excitation at low temperature, with a suitable light wavelength, can provide a switching of the system to a photoindcued metastable state having an extremely long lifetime at low temperatures. Therefore the effect is called Light Induced Excited Spin State Trapping (LIESST).[2] The photo-process involves, either a metal to ligand charge transfer, or d-d transitions. For typical Fe 2+ compounds, absorption bands for the LSﾆHS process are located around ~500 (Metal Ligand Charge Transfer), 550 (d-d) respectively. The reverse process (HSﾆLS) occurs by irradiation at ~750 nm, with a lower efficency, due to branching ratio 4:1 for the direct and reverse processes, respectively. Polarised neutron diffraction is a powerful tool to study the magnetisation densities in crystals and has never been applied so far to the study of photo- induced magnetic states. Figure 1 shows the new experimental setup designed to carry out “in situ” photo-excitation experiments, we have developed for this purpose. The photo-switching process of [Fe(ptz) 6 ](BF 4 ) 2 compound was observed by polarised neutron diffraction measurements (PND) as shown on Figure 2. The moment of 4.05(7) B on the iron site (with 2 = 5.11) obtained by refinement is very close to the theoretical value of the Fe 2+ moment at saturation (S=2). This evidences a complete photo-transformation of the crystal. Laser H uur Sample holder rod Lifting-counter Quartz plate crystal Optical fibre 0 P uur Neutron beam Figure 1 Schematic experimental setup of the polarized neutron diffractometer. In the inset, the sample holder allowing light irradiation is sketched. P 0 corresponds to the neutron polarisation direction . 0 20 40 60 80 100 120 0.0 0.1 0.2 0.3 0.4 LIGHT ON 473 nm H=5 Tesla T= 2K reflection (0 1 2) 1-R Time(min) Figure 2 Kinetics of the photo-excitation of [Fe(ptz) 6 ](BF 4 ) 2 . at 473 nm, 2 K, 5 T. Flipping ratio of (1 0 2) reflection with the magnetic field parallel to the [001] direction as a function of time. The first point is measured before illumination and corresponds to a reference point. The solid line is a guide for the eyes.