Oxidation state dependent conjugation controls electrocatalytic activity in a two-dimensional di-copper metal-organic framework

Molecularly defined two dimensional conjugated metal-organic frameworks combine properties of molecular and material based electrocatalysts, enabling tunable active site and bandgap design. The rational optimization of these systems requires an understanding of the complex interplay between the various metal sites and their influence on catalysis. For this purpose, copper-phthalocyanine-based two-dimensional conjugated metal-organic framework (CuPc-CuO4 2D c-MOF) films with an edge-on layer orientation were transferred to (Ni-NTA) functionalised graphite electrodes and analyzed via electrochemical resonance Raman spectroscopy. With the help of Density Functional Theory (DFT) calculations for the first time a detailed assignment of the vibrational bands for different Cu oxidation states could be achieved and correlated to their electrocatalytic activity in respect to the oxygen reduction reaction (ORR). Potential dependent Raman spectroscopy made it possible to determine the redox potentials of the Cu in the CuPc moieties and the Cu-catecholate nodes individually with ECuPc= -0.04V and ECuO4= +0.33 V vs. Ag|AgCl. Albeit the Cu-catecholates are generally seen as the active site in these systems, electrocatalytic ORR was only observed below -0.1 V were both Cu units were present in their respective CuI state. DFT calculations of bandgaps and density of states (DOS) showed a significant decrease in bandgap and increase in π-conjugation upon transition from the inactive mixed CuII/CuI state to the active CuI/CuI state suggesting that slow electron supply in the mixed state limits catalysis at the Cu-catecholates. Our results indicate that the coupling between metal oxidation changes and π-conjugation of the 2D c-MOF is a key parameter towards achieving good electrocatalytic activity.