Polymeric iron(II) complexes of the composition Fe(atrz)3A2, where atrz = 4-amino-1,2,4-triazole, A = anions, have been synthesized and characterized. The compounds exhibit an abrupt thermally driven reversible spin transition (S=0, S=2) at temperatures varying from 175 K to 400 K depend on anion substitution. XAFS measurements were performed at the VEPP-3 storage ring at the Budker Institute of Nuclear Physics in Novosibirsk. The parameters of Fe local spatial structure were determined from EXAFS data using EXCURV92 program. In all studied compounds the Fe-N and Fe-C interatomic distances increase upon the thermally induced transition to the high spin (HS) state by amounts from 0.12 Å to 0.24 Å .
Moreover FeK EXAFS and XANES spectra of the polymeric spin-transition compounds were measured at 300 K and at 4.2 K without an optical treatment of the samples and after their irradiation with a Hg green line of 546 nm with different exposure times in order for light induced spin transition to detect and study. The spectra were obtained using fluorescence detection as well as transmission mode.
Since the rate constant of the low-temperature relaxation correlates with spin transition temperature (Tc), we have chosen for the experiments the polymeric complexes: Fe(atrz)3(NO3)2 Fe(atrz)3SiF6 and Fe(atrz)3(ClO4)2,with substantially different temperatures of the equilibrium spin transition of 342K, 255 K and 175 K, respectively.
The analysis of the data obtained by matching experimental Fourier transform (FT) magnitudes for the Fe(atrz)3(ClO4)2 and their model calculation using EXCURV92 computation for two Fe-N shells revealed that the state of complexes after irradiation can not be treated as a mixture of LS and HS states characterized by equilibrium Fe-N intetratomic distances 1.98 Å and 2.16 Å, respectively. It was found that the optical irradiation of this compound causes transition of about a half of the all Fe atoms to the metastable HS state which is accompanied by the Fe-N distance increase up to 2.16 Å. At the same time it was found that Fe atoms remaining at the LS state are characterized by strong local surrounding geometry distortions and the average Fe-N distance for them decreases to 1.93 Å.
The long-life metastable HS state of the Fe(atrz)3(NO3)2 and Fe(atrz)3SiF6 has not been found in any detectable amount after optical irradiation at 4.2 K because of their short time of life which are exponentially decreasing while the spin transition temperature increases.
The structural changes upon the light-induced spin transition was detected by the direct method for the first time showing that EXAFS spectroscopy is powerful tool to study the LIEEST peculiarities in various compounds.