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

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.