Unveiling Phase Transition Dynamics Using Cryogenic Optical Microscopy in a Charge-Transfer Fe2Co2 Switchable Molecular Material

Understanding the phase transition mechanism in switchable materials is crucial to optimizing their properties. In this study, we present the thermal electron transfer-coupled spin transition (ETCST) mechanism, revealed through the cryogenic optical microscopy (OM) measurements on a cyanide-bridged square complex, {[Fe(Tp)(CN)₃]₂[Co(vbik)₂]₂}·2ClO₄·2CH₂Cl₂ (1·ClO₄), where Tp is tris(pyrazolyl)borate and vbik is bis(1-vinylimidazolyl)ketone. A one-step thermal ETCST of 1·ClO₄ is observed using conventional techniques such as single-crystal X-ray diffraction (SC-XRD) and bulk sample magnetic measurements. The ETCST is cooperative, with quite different transition temperatures for the single crystal (T_1/2↑ = 251.5 K for heating and T_1/2↓ = 243.5 K) and bulk sample (T_1/2↑ = 273 K and T_1/2↓ = 255 K). In contrast, the direct visualization of the thermal ETCST in the single crystal of 1·ClO₄ through OM and the subsequent image analysis disclose for the first time a phase transition of unexpected complexity. The ETCST progresses in three steps along the a, b, and c axes: (i) Initially, the strip domains form and rapidly extend along the a-axis. (ii) These domains then gradually widen in the b-axis covering the entire ab layer. (iii) The final step involves a layer-by-layer extension of ETCST along the c-axis. Structural analysis of 1·ClO₄ reveals that two types of intermolecular interactions govern the two preferential propagation directions of ETCST. The first type, mediated by the ClO₄^– anion, drives the rapid propagation of ETCST along the a axis, occurring so rapidly that conventional methods like magnetometry and SC-XRD are unable to detect it. The second type, along the b axis, involves the π–π stacking of vbik ligands, which contributes to the slower ETCST propagation in this direction. This mechanistic insight into the anisotropic propagation patterns of ETCST in 1·ClO₄ underscores the role of intermolecular interactions in modulating the dynamics of molecular switching.