Stimuli‐Responsive Low‐Frequency Terahertz Absorption ON‐OFF Switchability in Spin‐Crossover Material

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Thermal and optical‐induced ON‐OFF switchable materials show vast potential in various fields like sensors, spintronics, and electronic devices, but remain underexplored in the essential terahertz (THz) region. In this context, a unique 1D spin‐crossover (SCO) network, {[Fe II (4‐cyanopyridine) 2 ][Hg II ( µ ‐SCN) 2 (SCN)(4‐cyanopyridine)] 2 } n ( 1 ), has been designed. Temperature‐dependent crystallographic, magnetic, and THz absorption spectroscopic studies indicate an abrupt SCO phenomenon from a high‐spin (HS) state to a complete or partial low‐spin (LS) state, depending on the cooling rate. At low temperatures, the LS state can be converted into the metastable HS state via the light‐induced excited spin‐state trapping (LIESST) effect using visible or near‐infrared lights. Both temperature and light reversibly modulate the THz absorbance (e.g., 0.82 and 1.37 THz) associated with phonons around Fe(II) centers, confirmed by first‐principles calculations and photocrystallographic analysis. This work advances comprehension of the intersection between structures, THz properties, and external‐stimuli switching effects and is pivotal for future THz device applications.