Benzenehexol-Based 2D Conjugated Metal–Organic Frameworks with Kagome Lattice Exhibiting a Metallic State

Abstract

2D conjugated metal–organic frameworks (2D c-MOFs) are emerging as unique electroactive materials for electronics and spintronics. The structural design and discovery of Kagome-type 2D c-MOFs exhibiting a metallic state are of paramount significance, yet remain rarely explored. Here, the solution synthesis of benzenehexol-based 2D c-MOFs based is presented on the tetrahydroxy-1,4-quinone (THQ) ligand. This study shows that controlling the pH of the reaction system to ≈7.5 yields an energetically favorable nonporous Cu3(C6O6) with a Kagome lattice, while at a pH of ≈10, the known porous Cu3(C6O6)2 with a honeycomb lattice is obtained. The crystal structures of both Cu3(C6O6)2 and Cu3(C6O6) are resolved with near-atomic precision (resolution, 1.8 Å) using an imaging technique. Unlike the p-type semiconducting behavior of Cu3(C6O6)2, theoretical studies identify Cu3(C6O6) as a metal due to its unique structural topology. The metallic state of Cu3(C6O6) is experimentally validated by terahertz time-domain spectroscopy (THz-TDS), which shows an increase in conductivity upon cooling. Scattering-type scanning near-field optical microscopy (s-SNOM) measurements further support these findings by revealing an increase in normalized reflectivity with decreasing temperature. This work provides a new avenue for tailoring the structural topology of 2D c-MOFs to attain the Kagome lattice and metallic state.