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Cooperative spin-state switching and anisotropic charge transport in two-dimensional Fe(II) Hofmann-type
R Mojica1, J Lozada1, Y Avila1
1Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada, Unidad Legaría, Instituto Politécnico Nacional, Col. Irrigación, Del. Miguel Hidalgo, Ciudad de México, CP. 11500, Mexico. rodrigo.mojica@protonmail.com.
Abstract:
Thermally induced spin crossover in Fe(II) Hofmann-type nitroprusside frameworks provides an attractive route toward multifunctional materials in which magnetic, optical, and electronic properties can be switched by external physical stimuli. Upon temperature changes, these types of layered coordination polymers undergo a reversible spin transition on sample cooling from high-spin to low-spin, accompanied by structural distortions, charge redistribution, and modifications of their transport properties. In this contribution, periodic density functional theory with hybrid functionals (HSE06) calculations, complemented by electronic UV-Vis, infrared, and Mössbauer spectroscopies, are employed to elucidate structure-property relationships in the archetypal two-dimensional Hofmann-type nitroprussides Fe(Pyridine)2 [Fe(CN)5NO] and Fe(4-methyl-isonicotinate)2 [Fe(CN)5NO]. Calculations identify the thermodynamic ground state and transient states that exhibit nearly conductive behavior. The spin crossover (SCO) process is accompanied by Fe-ligand bond shortening, lattice contraction, and band gap modulation, evidencing the strong coupling between spin state and crystal structure. Electronic structure analysis reveals that the nitroprusside moiety [Fe(CN)5NO] dominates the valence to conduction electronic transitions through metal to ligand charge transfer processes, whereas the external ligands modulate charge redistribution and transport pathways. Conductivity calculations further reveal pronounced anisotropic charge transport and enhanced electronic delocalization in NP-Py, which exhibits stronger low-energy optical features, larger spin-dependent hybridization, and a more pronounced band gap reduction than NP-4MIso. These results provide new insight into the cooperative SCO behavior of Fe(II) nitroprusside-based polymers and highlight their potential as electronically active materials with tunable magnetic and transport responses.
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