Efficient electrocatalytic H2O2 evolution utilizing electron-conducting molecular wires spatially separated by rotaxane encapsulation

Abstract

Immobilization of a catalytically active metal complex onto an electrode surface in highly dispersed form is crucial to assure high catalytic activity observed in solution. In this report, mononuclear CoII chlorin complexes were successfully immobilized on a conductive metal-oxide electrode by using electron conducting molecular wires comprising phenylene–ethynylene-based π-conjugation covered by a linked permethylated α-cyclodextrin to enhance electrocatalysis for selective two electron reduction of molecular oxygen to hydrogen peroxide. First, the rotaxane-encapsulation effect of the molecular wire on electron transfer was examined by the comparison of electrochemical behaviors of ferrocene (Fc) molecules immobilized on an ITO substrate using the molecular wires with or without rotaxane encapsulation. Then, electrodes were modified with metalloporphyrinoids, i.e., RhIII(OEP)Cl (OEP = 2,3,7,8,12,13,17,18-octaethylporphyrin) and CoII chlorin through coordination with the molecular wires with pyridine moieties at the end. The electrocatalytic O2 reduction was performed with CoII chlorin immobilized on an electron-conductive metal-oxide substrate by molecular wires. The turnover frequency for H2O2 production using the CoII chlorin coordinated by molecular wires with rotaxane encapsulation was 331 ± 75, which is significantly higher than that of 82 ± 8 obtained for CoII chlorin immobilized by that without rotaxane encapsulation. These results clearly indicate that the molecular wires with rotaxane encapsulation are beneficial for CoII chlorin complexes to exhibit high electrocatalytic activity and selectivity to H2O2.