Theoretical Investigation of Charge Transfer in Metal Organic Frameworks for Electrochemical Device Applications

Abstract

For electrochemical device applications metal organic frameworks (MOFs) must exhibit suitable conduction properties. To this end, we have performed computational studies of intermolecular charge transfer in MOFs consisting of hexa-Zr^IV nodes and tetratopic carboxylate linkers. This includes an examination of the electronic structure of linkers that are derived from tetraphenyl benzene <b>1</b>, tetraphenyl pyrene <b>2</b>, and tetraphenyl porphyrin <b>3</b> molecules. These results are used to determine charge transfer propensities in MOFs, within the framework of Marcus theory, including an analysis of the key parameters (charge transfer integral <i>t</i>, reorganization energy λ, and free energy change Δ<i>G</i>⁰) and evaluation of figures of merit for charge transfer based on the chemical structures of the linkers. This qualitative analysis indicates that delocalization of the HOMO/LUMO on terminal substituents increases <i>t</i> and decreases λ, while weaker binding to counterions decreases Δ<i>G</i>⁰, leading to better charge transfer propensity. Subsequently, we study hole transfer in the linker <b>2</b> containing MOFs, <b>NU-901</b> and <b>NU-1000</b>, in detail and describe mechanisms (hopping and superexchange) that may be operative under different electrochemical conditions. Comparisons with experiment are provided where available. On the basis of the redox and catalytic activity of nodes and linkers, we propose three possible schemes for constructing electrochemical devices for catalysis. We believe that the results of this study will lay the foundation for future experimental work on this topic