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.