Promoting photocatalytic CO 2 reduction through facile electronic modification of N-annulated perylene diimide rhenium bipyridine dyads.

The development of CO 2 conversion catalysts has become paramount in the effort to close the carbon loop. Herein, we report the synthesis, characterization, and photocatalytic CO 2 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, -NO 2 , -OPh, -NH 2 , or pyrrolidine (-NR 2 ). 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 CO 2 reduction performance. Although some R-groups ( i.e. -Br and -OPh) did not influence the performance of CO 2 photocatalysis (relative to -H; TON co ∼60), the use of an electron-withdrawing -CN was found to completely deactivate the catalyst (TON co < 1) while the use of an electron-donating -NH 2 improved CO 2 photocatalysis four-fold (TON co = 234). Despite being the strongest EWG, the -NO 2 derivative exhibited good photocatalytic CO 2 reduction abilities (TON co = 137). Using a combination of CV and UV-vis-nIR SEC, it was elucidated that the -NO 2 derivative undergoes an in situ transformation to -NH 2 under reducing conditions, thereby generating a more active catalyst that would account for the unexpected activity. A photocatalytic CO 2 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 NPDI 2- to Re(bpy) electron-transfer both significantly influence photocatalytic CO 2 reduction. These results help provide rational design principles for the future development of related supramolecular dyads.