Creating Well-Defined Hexabenzocoronene in Zirconium Metal-Organic Framework by Postsynthetic Annulation.
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abstract
The incorporation of large -conjugated ligands into metal-organic frameworks (MOFs) can introduce intriguing photophysical and electrochemical properties into the framework. However, these effects are often hindered by the strong - interaction and the low solubility of the arylated ligands. Herein, we report the synthesis of a porous zirconium-based MOF, Zr6(3-O)4(3-OH)4(OH)6(H2O)6(HCHC) (PCN-136, HCHC = hexakis(4-carboxyphenyl)hexabenzocoronene), which is composed of a hexacarboxylate linker with a -conjugated hexabenzocoronene moiety. Direct assembly of the Zr4+ metal centers and the HCHC ligands was unsuccessful due to the low solubility and the unfavorable conformation of the arylated HCHC ligand. Therefore, PCN-136 was obtained from aromatization-driven postsynthetic annulation of the hexaphenylbenzene fragment in a preformed framework (pbz-MOF-1) to avoid - stacking. This postsynthetic modification was done through a single-crystal-to-single-crystal transformation and was clearly observable utilizing single -crystal X-ray crystallography. The formation of large -conjugated systems on the organic linker dramatically enhanced the photoresponsive properties of PCN-136. With isolated hexabenzocoronene moieties as photosensitizers and Zr-oxo clusters as catalytic sites, PCN-136 was employed as an inherent photocatalytic system for CO2 reduction under visible-light irradiation, which showed increased activity compared with pbz-MOF-1.
