Proton-Coupled Electron Transport in Anthraquinone-Based Zirconium Metal Organic Frameworks

The ditopic ligands 2,6-dicarboxy-9,10-anthraquinone and 1,4dicarboxy-9,10-anthraquinone were used to synthesize two new UiO-type metal organic frameworks (MOFs; namely, 2,6-Zr-AQMOF and 1,4-Zr-AQMOF, respectively). The Pourbaix diagrams (E vs pH) of the MOFs and their ligands were constructed using cyclic voltammetry in aqueous buffered media. The MOFs exhibit chemical stability and undergo diverse electrochemical processes, where the number of electrons and protons transferred was tailored in a Nernstian manner by the pH of the media. Both the 2,6-Zr-AQMOF and its ligand reveal a similar electrochemical pK(a) value (7.56 and 7.35, respectively) for the transition between a two-electron, two-proton transfer (at pH < pK(a)) and a two-electron, one-proton transfer (at pH > pKa). In contrast, the position of the quinone moiety with respect to the zirconium node, the effect of hydrogen bonding, and the arnount,of defects in I,4-Zr-AQMOF lead to the transition from a two-electron, thre-proton transfer to a two-electron, on-proton transfer. The pKa of this framework (5.18) is analogous to one of the three electrochemical plc values displayed by its ligand (3.91, 5.46, and 8.80), which also showed intramolecular hydrogen bonding. The ability of the MOFs to tailor discrete numbers of protons and electrons suggests their application as charge carriers in electronic devices.