Hydration-driven Structural Reorganization and Switchable Thermal-Photoinduced Spin-State Dynamics in Iron(II) Spin-crossover Crystalline Solids

Abstract

We present a comprehensive structural and spectroscopic investigation of the Fe(II) spin-crossover (SCO) crystalline solid, [Fe(3-bpp)2]2[Cr(ox)3](ClO4)·5H2O, highlighting the pivotal role of hydration in governing spin-state switching dynamics. Single-crystal XRD at 100 and 296 K reveals an orthorhombic Pca2₁ structure with two crystallographically non-equivalent Fe(II) centers, sustained by a robust 3D supramolecular network of hydrogen bonding, π–π, and CH–π interactions involving water molecules, oxalate, and perchlorate anions. Variable-temperature PXRD shows a reversible first-order dehydration-driven transition to a higher-symmetry tetragonal structure, directly correlated with the thermally induced low-spin (LS) → high-spin (HS) conversion. Optical absorption spectroscopy reveals pronounced phase-dependent differences. The hydrated phase exhibits partial low-temperature HS retention due to vacuum-induced kinetic stabilization and lattice water rearrangement, whereas the dehydrated phase undergoes a gradual and incomplete SCO arising from microstructural disorder, KBr-induced inhomogeneity, tensile strain, and electrostatic perturbations, along with additional pelletization-induced mechanical effects that stabilize the HS state by broadening and lowering the zero-point energy difference (ΔE0HL). Light-induced excited spin-state trapping (LIESST) further underscores hydration effects: the hydrated phase shows a low T(LIESST) of 16 K with complex two-step relaxation involving domain dynamics and lattice flexibility, whereas the dehydrated phase displays a higher T(LIESST) of 64 K, slower relaxation, and enhanced trapping of the photoinduced HS state, consistent with increased lattice rigidity. Time-resolved spectroscopy confirms that HS → LS relaxation in both phases proceeds predominantly via temperature-independent quantum tunneling, with kinetics influenced by hydration-dehydration-induced lattice rearrangements. Overall, this study establishes direct correlations between hydration and spin-state dynamics- thermal and photoinduced- demonstrating how non-covalent interactions and local structural environments dictate SCO energetics and kinetics. These insights provide guiding principles for designing environmentally responsive molecular materials with tunable spin-switching behavior for advanced electronic, sensing, and photonic applications.