Near-Infrared Light-Induced Spin-State Switching Based on Fe(II)-Hg(II) Spin-Crossover Network

The development of molecular switches with tunable properties has garnered considerable interest over several decades. A novel spin-crossover (SCO) material based on iron(II) complexes incorporating 4-acetylpyridine (4-acpy) and [Hg(SCN)₄]^2- anions was synthesized and formulated as [Fe(4-acpy)₂][Hg(μ-SCN)₄] (1). Compound 1 is crystallized in a three-dimensional network in the non-centrosymmetric orthorhombic space group Pna2₁ with two octahedral [Fe(4-acpy)₂(NCS)₄] entities featuring two distinct Fe centers (Fe1 and Fe2). Crystallographic, magnetic, and Mössbauer measurements reveal an incomplete SCO exclusively at Fe2, with transition temperature T_1/2≈102 K. Photomagnetic studies conducted at 10 K with lasers ranging from 405 to 1310 nm evidence light-induced excited spin-state trapping (LIESST) and reverse-LIESST effects, with a unique near-infrared-responsive LIESST phenomenon at 1064 and 1310 nm. Advanced photocrystallographic studies at 40 K provide precise structural evidence for these metastable states. The optical and vibrational properties consistently corroborate with magnetic and photomagnetic studies. Additionally, temperature- and light-dependent terahertz (THz) absorptions are associated with phonon vibrations around Fe2 centers, through SCO behavior, as supported by ab initio calculation. The Fe(II)-Hg(II) systems can be promising benchmarks for exploring synergistic switching effects in structural, magnetic, and spectroscopic properties.