Promoting photocatalytic CO2 reduction through facile electronic modification of N-annulated perylene diimide rhenium bipyridine dyads.

Abstract

The development of CO2 conversion catalysts has become paramount in the effort to close the carbon loop. Herein, we report the synthesis, characterization, and photocatalytic CO2 reduction performance for a series of N-annulated perylene diimide (NPDI) tethered Re(bpy) supramolecular dyads [Re(bpy-C2-NPDI-R)], where R = –H, –Br, –CN, –NO2, –OPh, –NH2, or pyrrolidine (–NR2). The optoelectronic properties of these Re(bpy-C2-NPDI-R) dyads were heavily influenced by the nature of the R-group, resulting in significant differences in photocatalytic CO2 reduction performance. Although some R-groups (i.e. –Br and –OPh) did not influence the performance of CO2 photocatalysis (relative to –H; TONco ∼60), the use of an electron-withdrawing –CN was found to completely deactivate the catalyst (TONco < 1) while the use of an electron-donating –NH2 improved CO2 photocatalysis four-fold (TONco = 234). Despite being the strongest EWG, the –NO2 derivative exhibited good photocatalytic CO2 reduction abilities (TONco = 137). Using a combination of CV and UV-vis-nIR SEC, it was elucidated that the –NO2 derivative undergoes an in situ transformation to –NH2 under reducing conditions, thereby generating a more active catalyst that would account for the unexpected activity. A photocatalytic CO2 mechanism was proposed for these Re(bpy-C2-NPDI-R) dyads (based on molecular orbital descriptions), where it is rationalized that the photoexcitation pathway, as well as the electronic driving-force for NPDI2− to Re(bpy) electron-transfer both significantly influence photocatalytic CO2 reduction. These results help provide rational design principles for the future development of related supramolecular dyads.