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.
Highlights
Synthesis of N3-C2-N-annulated perylene diimide (NPDI)-R precursors that are converted into Re(bpy-C2-NPDI-R) dyads
N3-C2-NPDI-CN had to be synthesized starting from HNPDI
We focus on developing a molecular orbital (MO) description of these Re(bpyC2-NPDI-R) dyads that helps account for the observed differences in photocatalytic CO2 reduction performance (Figure 5)
Summary
The adverse effects on climate change related to increased anthropogenic CO2 emissions has inspired the utilization of excess CO2 as a sustainable feedstock for value-added chemicals and fuels.[1,2] While the activation of CO2 is kinetically unfavorable, it can be readily accomplished electro-/photocatalytically via protoncouple multielectron chemical reductions.[3,4] the development of capable molecular electro-/photocatalysts has mainly focused on improving the efficiency and selectivity of the CO2 conversion process.[5,6,7,8,9] Among the many comprehensively studied molecular catalyst systems, Re(2,2’-bipyridine)(CO)3Cl [Re(bpy)] is notable for its highly selective CO2-to-CO conversion.[10]. A mechanism based on molecular orbital (MO) energy levels is proposed to explain the observed differences in photocatalytic CO2 reduction performance for these dyads caused by the installation of EWGs and EDGs on NPDI.
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