Localised Charge Transfer in Metal-Organic Frameworks for Catalysis

Abstract

In natural photosynthetic systems localised charge transfer (CT) interactions are employed to prolong photoexcited charge separated states, facilitating conversion to chemical energy. This thesis explores localised CT in redox-active Metal-Organic Frameworks (MOFs) for applications in electrocatalysis and photoelectrocatalysis. Two design strategies are adopted, the first of which incorporates cofacial dimeric units of the photo- and redox-active N,Nʹ-di(4-pyridyl)-1,4,5,8-naphthalenediimide (DPNDI) ligand into a Cd(II) MOF. Crystallographic characterisation of the structural flexibility in this MOF was achieved and linked to an enhanced capability for the MOF to stabilise photoexcited and radical states through localised Intervalence CT interactions. The ability of the material to stabilise the NDI photoexcited radical monoanion state (redox potential -2.1 V vs. SCE) was capitalised on to reduce a discrete organometallic Re(I) based CO2 electrocatalyst to its catalytically active form. Photoelectrocatalytic conversion of CO2 to CO was confirmed at modest reduction potentials of -1.2 V vs. Ag/Ag+. The second approach adopted in this project was the incorporation of the Ni bisdithiolene redox-active unit into a Zn(II) MOF, in the form of the [Ni(pedt)2]- metalloligand (where pedt represents 1-(pyridine-4-yl)ethylene-1,2-dithiolate). The combination of Zn(II) nodes and carboxylate co-ligand coordination resulted in effective electronic isolation of each [Ni(pedt)2]- ligand. The Ni bisdithiolene MOF was successfully applied as a heterogeneous proton reduction electrocatalyst under acidic conditions of 90 mM CH3COOH in 0.1 M [n-Bu4N]PF6/MeCN electrolyte. Tafel analysis of the electrocatalytic behaviour of both the ligand in solution and the MOF demonstrated similarities in catalytic mechanism, evidencing the conservation of molecular electrocatalytic behaviour.