Abstract
The incorporation of large pi-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 pi-pi interaction and the low solubility of the arylated ligands. Herein, we report the synthesis of a porous zirconium-based MOF, Zr-6(mu(3)-O)(4)(mu(3)-OH)(4)(OH)(6)(H2O)(6)(HCHC) (PCN-136, HCHC = hexakis(4-carboxyphenyl)hexabenzocoronene), which is composed of a hexacarboxylate linker with a pi-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 pi-pi 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 pi-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.