Photo Induced Structure in Spin Crossover Complex

The spin crossover complex, Fe(phen)$_{2}$(NCS)$_{2}$, is widely recognized for the temperature and photo-induced spin-state transition of the Fe$^{2+}$ ions. With the increase of temperature, the Fe$^{2+}$ ions show the transition from the low-spin (LS: S=0) to the high-spin (HS: S=2) state at Tc=180K. In addition, a photo-excitation by a green light induces a similar spin-state transition at low temperature and the HS state is trapped after the irradiation. Our purpose is the structural analysis on the charge density level of the photo-induced phase using synchrotron radiation powder diffraction. The powder diffraction data of the complex were measured at 92K under 532nm laser irradiation by using Large Debye-Scherrer Camera installed at BL02B2, SPring-8. The imaging plate was used as a detector to collect whole powder patterns simultaneously. The N$_{2}$ gas flow type system was used for the low-temperature measurements. The wavelength of the incident X-ray was 1.0Å. The photo-induced diffraction data with a high-counting statistics were successfully obtained. We have analyzed the powder diffraction data of photo-induced state by the MEM/Rietveld method. The difference of bonding nature between the photo-exited HS state and temperature-induced HS state has been revealed.