Fig. 1: Schematic structure, thermally induced phase transition, and experimental setup.
a Schematic of the structure of 1 in the high-temperature (HT) phase. Red dotted lines are the halogen bond (XB) between I and S atoms. Black balls are carbon atoms; blue balls are nitrogen atoms; red balls are oxygen atoms; brown balls are iron (III) atoms; purple balls are iodine atoms; yellow balls are sulfur atoms; white balls are nickel atoms. Hydrogen atoms are skipped to improve clarity. b Schematic of the local structure of 1 which shows the thermally induced phase transition. In the low-temperature (LT) phase (left panel), the system is in the low-spin (LS) state of [Fe(Iqsal)2]+ cations with singlet-spin state (strong dimerization (SD)) of [Ni(dmit)2]− anions. It is in the high-spin (HS) state of [Fe(Iqsal)2]+ cations with paramagnetic state (weak dimerization (WD) of [Ni(dmit)2]− anions) in the HT phase (right panel). The blue and red circles represent [Fe(Iqsal)2]+ cations. Spin configuration on t2g and eg orbitals of the Fe(III) ion are shown inside the circles. The blue circles are smaller than the red circles to emphasize the Fe-ligand expansion during SCO. The horizontal thick lines between the [Fe(Iqsal)2]+ cations represent positions of the [Ni(dmit)2]− anions. The arrows on the [Ni(dmit)2]− anions represents the localized spins and become spin-singlet due to SD in the LT phase. c Schematic of the experimental setup and the diffraction images. After initial photoexcitation with 400-nm pump laser, ultrafast dynamics of 1 was measured individually by (i) transient absorption spectroscopy, (ii) IR reflectivity spectroscopy, and (iii) ultrafast electron diffraction.
