L’apport de la spectroscopie d’absorption des rayons-x pour l’étude des composes moléculaires photocommutables.

Abstract

This work focuses on the contribution of X-ray absorption spectroscopy (XAS) on photoswitchable molecular compounds such as sodium nitroprusside (Na2[F e(CN)5NO]2H2O SNP). The study is based on the electronic and structural properties of the local l environment of Iron (Fe) atom in the SNP sample by XAS spectroscopy technique. XAS technique is a non-destructive and sensitive method to a specific element in a given material. It provides information on the electronic and structural properties around the absorbing atom. The XAS spectrum is obtained from the measurement of the absorption coefficient which has three regions namely : the pre-edge, the XANES (X-ray Absorption Near Edge Spectroscopy) and the EXAFS (Extended X-ray Fine Structure Absorption). XANES provides information on electronic properties, chemical composition and oxidation state in a material. As for the EXAFS, it gives access to the local environment of an absorbing atom, making it possible to determine the number and nature of neighboring atoms, the interatomic distance and the thermal disorder. The experimental measurements are carried out on the XAFS beamline of ELETTRA at the iron K edge on the SNP single crystal cut along the crystallographic axes a, b and c on the one hand ; and on the SNP single crystal in the ground state irradiated by LASER light on the other hand. The theoretical results of the ab initio calculation simulated by the FEFF9 software made it possible to carry out an in-depth study on thermal agitation, polarization, and the Debye Waller factor. The results obtained showed a change in the XAS spectra when these different parameters were varied. An in-depth study on the ground state GS and the metastable states MSI and MSII has been conducted through XAS data acquisition obtained after LASER irradiation of GS state of SNP. The results showed that after 15 H of blue LASER irradiation with the wavelength of 476 nm, the metastable MSI can be transferred from the GS. The transfer of MSII metastable is obtained after infrared irradiation of the wavelength 1064 nm.