title: Structure and electronic configuration of an iron(II) complex in a LIESST state: a pump and probe method.
abstract: Two polymorphs of mononuclear six-coordinate iron(II) spin-crossover complex trans-[Fe(tzpy)(2)(NCS)(2)] (tzpy = 3-(2-pyridyl)[1,2,3]triazolo[1,5-a]pyridine) (1) were isolated and structurally characterized. According to the thermally dependent magnetic measurements, polymorph A undergoes a gradual spin transition from a paramagnetic high-spin state ((5)T(2), S = 2, HS-1) above 200 K to a diamagnetic low-spin state ((1)A(1), S = 0, LS-1) below 120 K, whereas polymorph B shows an abrupt spin transition with T(1/2) at 102 K. Molecular and crystal structures of polymorph A in the HS-1 and LS-1 states were studied at 300 and 40 K, respectively. Significant differences in Fe-N distances and coordination geometries of Fe were found between the two spin states, as expected. Light-induced excited spin state trapping (LIESST) was observed upon irradiating the crystal with 532 nm laser light at 40 K, whereupon a metastable high-spin state (HS-2) was formed; the molecular and crystal structure of this metastable state were investigated by a pump and probe method because of its relatively fast relaxation. The electronic configuration of the Fe center in the HS-1, LS-1, and LIESST (HS-2) states were further confirmed by Fe K- and L-edge absorption spectroscopy. In addition, the C[triple bond]N stretching frequency on the ligand can also be followed through the spin transition. The excitation and relaxation process concerning such metastable state were followed by the C[triple bond]N stretching frequency and magnetic susceptibility measurements in the temperature ranges 15-55 K and 5-80 K, respectively. The structure and electronic configuration of the LIESST state of polymorph A were firmly established by X-ray diffraction, X-ray absorption, infrared absorption, and magnetic measurements. A single-crystal-to-single-crystal transition through irradiation was demonstrated. The changes in structure and electronic configuration as a result of the spin transition are believed to occur concurrently.
authors.affiliation: Department of Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei, Taiwan 106.
authors.forename: Chou-Fu
authors.initials: CF
authors.lastname: Sheu
authors.forename: Kowa
authors.initials: K
authors.lastname: Chen
authors.forename: Szu-Miao
authors.initials: SM
authors.lastname: Chen
authors.forename: Yuh-Sheng
authors.initials: YS
authors.lastname: Wen
authors.forename: Gene-Hsiang
authors.initials: GH
authors.lastname: Lee
authors.forename: Jin-Ming
authors.initials: JM
authors.lastname: Chen
authors.forename: Jyh-Fu
authors.initials: JF
authors.lastname: Lee
authors.forename: Bing-Ming
authors.initials: BM
authors.lastname: Cheng
authors.forename: Hwo-Shuenn
authors.initials: HS
authors.lastname: Sheu
authors.forename: Nobuhiro
authors.initials: N
authors.lastname: Yasuda
authors.forename: Yoshiki
authors.initials: Y
authors.lastname: Ozawa
authors.forename: Koshiro
authors.initials: K
authors.lastname: Toriumi
authors.forename: Yu
authors.initials: Y
authors.lastname: Wang