Ligand π-Conjugation Area Modulates Hydrogen Evolution Performance in Metal-Organic Frameworks

Metal-organic frameworks (MOFs) combine permanent porosity, tunable compositions, ordered structures, and well-defined coordination environments, making them versatile platforms for electrocatalyst design. Although ligand engineering has been widely explored for the hydrogen evolution reaction (HER), few studies have treated ligand π-conjugation area as a defined molecular variable and systematically examined its relationship with HER performance. Here, we use a conjugated-ligand engineering strategy to systematically vary ligand π-conjugation area while maintaining comparable metal coordination environments. Pyromellitic dianhydride (PMDA), naphthalene-1,4,5,8-tetracarboxylic dianhydride (NTDA), and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), featuring benzene-, naphthalene-, and perylene-based aromatic cores with 6, 10, and 20 π electrons, respectively, are used as precursors to the corresponding tetracarboxylate ligands to construct related Ni- and Co-based MOFs with six-coordinate metal centers. Across both metal series, increasing ligand π-conjugation area consistently lowers overpotentials and Tafel slopes, decreases charge-transfer resistance and mass-transport-related impedance, and increases electrochemically active surface area. This systematic ligand variation is accompanied by metal-center binding-energy shifts and bandgap narrowing. These results establish a clear structure-performance relationship between ligand π-conjugation area and HER performance across the investigated MOF series, highlighting conjugated-ligand engineering as an effective strategy for MOF electrocatalyst design.