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>i.e.</i> -Br and -OPh) did not influence the performance of CO₂ 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 -NH₂ improved CO₂ photocatalysis four-fold (TON_co = 234). Despite being the strongest EWG, the -NO₂ derivative exhibited good photocatalytic CO₂ reduction abilities (TON_co = 137). Using a combination of CV and UV-vis-nIR SEC, it was elucidated that the -NO₂ derivative undergoes an <i>in situ</i> 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 NPDI^2- 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.