Manipulating metal-to-metal electron transfer of cyano-bridged {W2Co3} assemblies toward photo-switchable 2D molecular magnets

Abstract

Photo-switchable molecular magnets are promising in the applications of molecular spintronics, memory and switching devices. Introducing photo-responsive metal-to-metal electron transfer units into the molecular magnets provides a feasible means of controlling the magnetic and related physical properties. Here, we assembled the redox-active octacyanotungstate building block [W^V(CN)₈]^5- with Co^II ion to obtain the metal-to-metal electron transfer (MMET) molecular magnets. Four structure-related two-dimensional (2D) compounds 1–4 derived from [W(CN)₈]₂[Co₃(L)₁₂]·solv (L = 1-phenylimidazole) were constructed by modifying phenylimidazole ligands with substituent R at the para position of the aromatic ring (R = CN (L1), NO₂ (L2), F (L3) and Cl (L4)). Structural analysis indicated that 1–4 underwent a thermal-induced single-crystal to single-crystal (SCSC) phase transition to the desolvated 1a–4a, accompanied by the MMET from (Co^III_LS)₂-W^IV₂-Co^II_HS (LS: low-spin) phase to (Co^II_HS)₂- W^V₂-Co^II_HS (HS: high-spin) phase. 1a–4a also exhibited incomplete (Co^II_HS)₂-W^V ₂-Co^II_HS (HS) phase ⇆ (Co^III_LS)₂-W_IV2-Co^II_HS (LS) phase transition with thermal hysteresis loops. Photomagnetic characterizations revealed that 1–4 and 1a–4a exhibited a reversible light-induced interconversion between the HS* (*: metastable phase generated by light irradiation) state and LS* state. Moreover, for 1, 1a and 2, their magnetic hysteresis can be reversibly produced and eliminated by alternating the 946- and 671-nm light irradiations, respectively, showing the photo-switchable molecular magnet behavior. The compared magnetostructural analysis indicated that the intermolecular π …π stacking interactions between modified phenylimidazole ligands can be modulated by the different electronic and steric hindrance effects of substituents, which regulate the distortion degree of the CoN₆ octahedron and ligand field around the cobalt ions. The anomalous increase of octahedral distortion parameter Σ_Co, which increases as the interlayer π…π interaction weakens, not only affects the transition temperature, but also plays an important role in stabilizing the photoexcited metastable state. This study demonstrates the feasibility of regulating intermolecular interactions through molecular engineering strategy in developing photo-responsive molecular magnetic materials with enhanced photo operating temperatures, and may offer new insights for advancing the construction of photo-switchable 2D magnets and related molecule-based spintronic devices.