An Electrically Conducting, Redox-Complementary Dual-Ligand 2D Graphitic MOF with Orthogonal Charge Transport Pathways

Owing to their diverse potential applications in modern electronics and energy technologies, electrically conducting metal–organic frameworks (MOFs) have emerged as one of the most coveted functional materials of the twenty-first century. The electrical conductivity is a product of frameworks’ charge carrier density and mobility, which depend on their structures and compositions. Although the hexagonal 2D MOFs composed of square-planar metal ions and trigonal planar aromatic ligands often display impressive electrical conductivities because of their unique abilities to support in-plane (through bonds) and out-of-plane (through space) charge transport, large disparities between the two orthogonal conduction pathways also render their conductivity highly anisotropic and dampen bulk conductivity. To address this issue, herein, we have developed a novel redox-complementary dual-ligand 2D MOF (CDL-MOF1), Cu3(HHTP)(HHTQ), in which the Cu-coordinated π-donor hexahydroxytriphenylene (HHTP) and a π-acceptor hexahydroxy-tricycloquinazoline (HHTQ) ligands are located at alternate corners of the heteroleptic hexagonal framework. The CDL-MOF1 layers, which support through-bond charge transport along ab-planes, are stacked in eclipsed AA fashion along the c-axis, forming parallel homomeric HHTP and HHTQ π-stacks at each corner of the hexagons, which can facilitate out-of-plane charge transport by serving as dual electron and hole transporting channels. Consequently, Cu3(HHTP)(HHTQ) MOF displayed a distinctly higher bulk conductivity (~0.12 S/m, 295 K) than the parent Cu3(HHTP)2 (7.3 x 10–2 S/m) and Cu3(HHTQ)2 (5.9 x 10–4 S/m) MOFs. This work presents a novel design strategy that can help reduce the disparity between orthogonal charge transport pathways in 2D MOFs and thereby help enhance their bulk conductivity.