Department of Chemistry, National Taiwan University^* National Synchrotron Radiation Research Center, Hsinchu, Taiwan^**
○Yu-Chun Chuang^* Jin-Ming Chen^** Yu Wang^*

Spin crossover (SCO) complexes are considered to be one of the potential molecular switch materials. The spin state of the metal ion in such complexes could be switched between high spin (HS) and LS (low spin) state by changing in temperature; pressure or by light irradiation. The Light induced excited spin state trapping (LIESST) phenomenon is associated with pumping electron from LS to metastable HS state with laser irradiation of proper wavelength. Such phenomenon is our special concern due to its potential applications in the future. Evolutions of electronic configuration accompanied with spin transitions were studied for several Fe SCO systems by x-ray absorption spectroscopy at both Fe K- and L-edge. The L_2,3-edge XAS is a powerful fingerprint of the electronic structure for 3d-transition-metal center. An unexpected excitation to HS state was recorded at ca. 8K with soft x-ray irradiation. Similar finding was reported before, where they propose a so called “SOXIESST” (soft X-ray induced excited spin state trapping) process. Nevertheless, the energy is too high to be fitted in the same scheme for the LIESST scheme, where the excitation is mainly through the metal ligand charge transfer band. The photo emission spectrum out of the soft x-ray excitation on the system is therefore designed in order to unravel the mechanism of the LS to HS process. An emission peak at about 430nm is observed at 8K for three different Fe SCO complexes. In addition, the emission spectrum seems highly dependent on the intensity of the incident beam. We therefore propose that the emission line at 430 nm is the one which is responsible for the LIESST process. Detail analysis for the emission spectra will be discussed.