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

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 Co^II 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., Rh^III(OEP)Cl (OEP = 2,3,7,8,12,13,17,18-octaethylporphyrin) and Co^II chlorin through coordination with the molecular wires with pyridine moieties at the end. The electrocatalytic O₂ reduction was performed with Co^II chlorin immobilized on an electron-conductive metal-oxide substrate by molecular wires. The turnover frequency for H₂O₂ production using the Co^II chlorin coordinated by molecular wires with rotaxane encapsulation was 331 ± 75, which is significantly higher than that of 82 ± 8 obtained for Co^II chlorin immobilized by that without rotaxane encapsulation. These results clearly indicate that the molecular wires with rotaxane encapsulation are beneficial for Co^II chlorin complexes to exhibit high electrocatalytic activity and selectivity to H₂O₂.