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

Abstract

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 T1/2 ≈ 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.