Theoretical Investigations of Structural Changes in Bimetallic MIL-53(Al/Cr)MOF Under Different States and Liquids.

MIL-53(Al/Cr) can exhibit pore expansion or contraction in response to different external stimuli and liquid environments. These macroscopic structural transitions are governed by the local rhombic distortion of the Cr3+ coordination octahedron. Based on the experimentally measured EPR spectra, the structural distortion characteristics of MIL-53(Al/Cr) are quantitatively studied in this work by using the perturbation formulas of the spin Hamiltonian parameters (SHPs) for 3d3 ions in rhombically distorted octahedra. The microstructure features under different states or liquids (specifically the bond-length distortion ΔR (0.0529-0.0671 Å) and bond-angle distortion δ (7.99°-10.14°) of the Cr3+ site, which exhibit a cooperative variation pattern characterized by an increase in ΔR accompanied by a decrease in δ during the narrow-pore to large-pore transition and the reverse trend upon the opposite transition) are studied. Based on this structural information, the relationship between the large-pore and narrow-pore breathing states and the microstructure is further clarified, thus elucidating the impact of the state and liquid on the macroscopic structure of the material.