Controlling the Photochromism of Zirconium Pyromellitic Diimide-Based Metal-Organic Frameworks through Coordinating Solvents

Metal–organic frameworks (MOFs) are promising platforms for designing photoresponsive materials due to their structural versatility and tunable properties. However, challenges remain in fine-tuning the photoresponsive behavior while maintaining the high stability of MOFs. In this study, we synthesized a MOF containing redox-active pyromellitic diimide (PMDI) groups and unsaturated Zr6 clusters (named Zr-PMDI-DMF) and fine-tuned its photochromic properties by exchanging the coordination solvent molecules on the Zr sites. Unlike traditional Zr6 clusters with bidentate carboxylate coordination, Zr-PMDI-DMF features monodentate carboxylate coordination with the exposed Zr sites occupied by solvent molecules. We post-synthetically exchanged the coordinated N, N-dimethylformamide (DMF) solvent molecules with 2-(dimethylamino)ethanol (DMAE), N-methyltetrahydropyrrole (NMP), and dimethyl sulfoxide (DMSO), and determined the structures of the coordinated solvent molecules using single-crystal X-ray diffraction. Through photochromic and bleaching cycle experiments, electron paramagnetic resonance spectroscopy, and density functional theory calculations, we found that the coordinated solvents act as electron donors. In contrast, the PMDI ligands act as electron acceptors, causing the intra-framework electron transfer and the photochromism. The rate of the photochromic response correlated with the electron-donating ability of the solvents, following the trend of DMAE > NMP > DMSO > DMF.