Multi-stimuli-engendered radical-anionic MOFs: Visualization of structural transformation upon radical formation

Understanding the dynamic structural changes accompanying radical formation provides detailed insights into the design of stimuli-responsive redox-active materials. In this study, we develop a metal-organic framework (MOF) comprising a π-acidic 1,4,5,8-naphthalenediimide (NDI)-based ligand and Nd3+, viz. X-DGIST-4 (X = synthesis temperature in °C, DGIST = Daegu Gyeongbuk Institute of Science and Technology). The reduction of the NDI moieties to the radical-anionic state (NDI·−) in 75-DGIST-4 can be triggered by various stimuli (heat, X-rays, and ultraviolet, visible, and infrared light), indicating the presence of structure-guided multiple electron transfer pathways. The single-crystal-to-single-crystal transformation of 75-DGIST-4 upon X-ray-induced radical formation reveals sequential structural changes. The solvothermally synthesized radical-anionic MOF, viz. 150-DGIST-4, offers a more complete structural understanding of the radical-anionic state. Importantly, the stabilization of radical-anionic NDI·− by adjacent benzoates allows this state to be visualized by single-crystal X-ray diffractometry; the pendant benzoate is twisted to optimize the electrostatic interaction between NDI·− and the benzoate phenyl group.