Lowering Electrocatalytic CO Reduction Overpotential Using N-Annulated Perylene Diimide Rhenium Bipyridine Dyads with Variable Tether Length.

We report the design, synthesis, and characterization of four N-annulated perylene diimide (NPDI) functionalized rhenium bipyridine [Re(bpy)] supramolecular dyads. The Re(bpy) scaffold was connected to the NPDI chromophore either directly [] or via an ethyl [], butyl [], or hexyl [] alkyl-chain spacer. Upon electrochemical reduction in the presence of CO and a proton source, all exhibited significant current enhancement effects, while did not. During controlled potential electrolysis (CPE) experiments at = -1.8 V vs Fc, all achieved comparable activity (TON ∼ 25) and Faradaic efficiency (FE ∼ 94%). Under identical CPE conditions, the standard catalyst Re(dmbpy) was inactive for electrocatalytic CO reduction; only at = -2.1 V vs Fc could Re(dmbpy) achieve the same catalytic performance, representing a 300 mV lowering in overpotential for . At higher overpotentials, both outperformed , indicating the possibility of coinciding electrocatalytic CO reduction mechanisms that are dictated by tether-length and overpotential. Using UV-vis-nearIR spectroelectrochemistry (SEC), FTIR SEC, and chemical reduction experiments, it was shown that the NPDI-moiety served as an electron-reservoir for Re(bpy), thereby allowing catalytic activity at lower overpotentials. Density functional theory studies probing the optimized geometries and frontier molecular orbitals of various catalytic intermediates revealed that the geometric configuration of NPDI relative to the Re(bpy)-moiety plays a critical role in accessing electrons from the electron-reservoir. The improved performance of dyads at lower overpotentials, relative to Re(dmbpy), highlights the utility of chromophore electron-reservoirs as a method for lowering the overpotential for CO conversion.