Creating Well-Defined Hexabenzocoronene in Zirconium Metal–Organic Framework by Postsynthetic Annulation

The incorporation
of large π-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 π–π interaction and the low solubility
of the arylated ligands. Herein, we report the synthesis of a porous
zirconium-based MOF, Zr6(μ3-O)4(μ3-OH)4(OH)6(H2O)6(HCHC) (PCN-136, HCHC = hexakis­(4-carboxyphenyl)­hexabenzocoronene),
which is composed of a hexacarboxylate linker with a π-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 π–π 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 π-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.