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; 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 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.

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Introduction

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 proton-couple multielectron chemical reductions.^3,4 Consequently, the development of capable molecular electro-/photocatalysts has mainly focused on improving the efficiency and selectivity of the CO₂ conversion process.^5–9 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 The versatile bpy ligand has been modified with a variety of substituents to change both the electronic properties and/or the second-sphere H-bonding character of the catalyst.^11–18 And while Re(bpy) alone can be used as an effective CO₂ reduction photocatalyst,^19–24 the photocatalytic CO₂ reduction performance is greatly enhanced via the direct functionalization of Re(bpy) with photosensitizing (PS) units.^25–31

The development of ruthenium(II) diimine photosensitized Re(bpy) supramolecular dyads has been extensively reported by the Ishitani group.^5,25–31 These Ru^II–Re^I dyads make use of a Z-scheme architecture whereby the photoexcited electrons of the Ru^II-moiety are reductively quenched and subsequently transferred to the Re^I catalyst center to enable CO₂ reduction. To facilitate efficient electron-transfer (eT) and CO₂ photocatalysis, several supra-molecular dyad design principles have been established. First, the photoexcited electron should be localized near the tethering portion between the PS and the catalyst.⁵ Second, the tether between the PS and catalyst moieties should be as short as possible (without being through-conjugated) to enable rapid intramolecular eT.^27–30 Third, increasing the molar absorptivity of the PS-moiety (i.e. by incorporating multiple PS units) can improve the quantum efficiency capabilities and the ensuing eT dynamics of the supramolecular dyad.³¹ Restricted by the first two design principles, attempts to improve the quantum efficiency of these Re(bpy) dyads has been made by using more strongly absorbing PS units, such as porphyrins,^32–37 naphthalimide,^38,39 naphthalene diimide,^40–42 and perylene diimide (PDI).^43,44 Although the photophysical dynamics of these dyads appear fundamentally well-understood, only a handful have been properly evaluated as CO₂ reduction photocatalysts (see ESI, Table S1†).^35–39

Recently, we reported on four N-annulated perylene diimide (NPDI) functionalized Re(bpy) dyads as CO₂ reduction electro-catalysts.⁴⁵ Our investigation of these Re(bpy)–NPDI dyads revealed that the PS unit (NPDI) functions as an electron-reservoir for Re(bpy), enabling efficient CO₂ reduction at an overpotential 300 mV lower than conventional Re(bpy)-type electrocatalysts. Moreover, it was also elucidated that the tether length between Re(bpy) and NPDI governs which CO₂ reduction mechanism is preferred for the supramolecular dyad(s), where the ethyl-linked Re(bpy)–NPDI dyad possessed the greatest degree of electronic communication. These promising results from our initial Re(bpy)–NPDI dyads led us to hypothesize that eT from the electron-reservoir to the Re(bpy) catalyst could be improved by electronically modifying NPDI in two different ways. It was theorized that the introduction of electron withdrawing groups (EWGs) on NPDI may inductively stabilize the entire dyad, thus enabling more efficient eT by increasing the overall electron affinity of Re(bpy)-moiety. Alternatively, the use of electron donating groups (EDGs) on NPDI could make the electron-reservoir more electron-rich and thus more willing to transfer electrons to the Re(bpy)-moiety. To determine which of these two opposing hypotheses was correct, a series of electronically modified ethyl-linked Re(bpy)–NPDI dyads [Re(bpy-C2-NPDI-R)] (where R = –H, –Br, –CN, –NO₂, –OPh, –NH₂, or –NR₂) were designed (Fig. 1, left) and, for the first time, their photocatalytic CO₂-to-CO reduction performance was evaluated. It was revealed that installing an EDG, such as –NH₂, led to a four-fold enhancement in turnover numbers of CO (TON_co), with respect to the benchmark Re(bpy-C2-NPDI-H) dyad. 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.

Fig. 1 (Left) chemical structures and (right) UV-vis absorption spectra of Re(bpy-C2-NPDI-R) dyads in CHCl₃ (∼10⁻⁵ M), where R = –H (red), R = –Br (orange), R = –CN (yellow), R = –NO₂ (green), R = –NH₂ (blue), R = –NR₂ (purple), and R = –OPh (magenta). The optical profiles of all Re(bpy-C2-NPDI-R) dyads overlap with the emission spectrum of the blue LED (λ = 470 ± 30 nm; 4 mW cm⁻²) used for photocatalytic CO₂ reduction testing.

Results & discussion

Synthesis & characterization

The synthesis of all azide-ethyl-NPDI precursors (N₃-C2-NPDI-R) starts from the HNPDI synthon (Scheme 1).^47–49 The pyrrolic nitrogen was alkylated with 1,2-dibromoethane and a terminal azide was installed using S_N2 chemistry to give N₃-C2-NPDI-H.^45,47 Treatment of N₃-C2-NPDI-H with either fuming HNO₃ (at −78 °C) or Br₂ (at 20 °C) affords the N₃-C2-NPDI-NO₂ and N₃-C2-NPDI-Br precursors, respectively. When refluxed with pyrrolidine, the N₃-C2-NPDI-Br precursor can be converted to N₃-C2-NPDI-NR₂. Alternatively, reacting N₃-C2-NPDI-Br with an excess of phenol and K₂CO₃ in N,N-dimethylformamide (DMF; at 80 °C) generates N₃-C2-NPDI-OPh. Attempts to cyano-functionalize N₃-C2-NPDI using the Rosenmund–von Braun reaction conditions resulted in the elimination of the terminal azide. Consequently, N₃-C2-NPDI-CN had to be synthesized starting from HNPDI. After selectively brominating HNPDI,⁵⁰ HNPDI-Br was reacted with excess CuCN in refluxing DMF to yield HNPDI-CN. This intermediate was then alkylated and azide-functionalized to give the N₃-C2-NPDI-CN precursor.

Scheme 1 Synthesis of N₃-C2-NPDI-R precursors that are converted into Re(bpy-C2-NPDI-R) dyads.

Next, these six N₃-C2-NPDI-R precursors were linked to Re(5-ethynyl-2,2′-bipyridine)(CO)₃Cl [Re(ethynyl-bpy)] using a standard copper catalyzed azide–alkyne cycloaddition (CuAAC) procedure (see ESI Section II for more details†).^45,47 While most Re(bpy-C2-NPDI-R) dyads (where R = –H, –Br, –CN, –NR₂, and –OPh) were obtained in excellent yields as the exclusive product, the CuAAC reaction between N₃-C2-NPDI-NO₂ and Re(ethynyl-bpy) generated both Re(bpy-C2-NPDI-NO₂) (major product) and Re(bpy-C2-NPDI-NH₂) (minor product). To obtain each catalyst selectively, two additional CuAAC protocols were developed. The selective synthesis of Re(bpy-C2-NPDI-NO₂) was accomplished by changing the catalyst from CuSO₄ (with sodium ascorbate) to CuI, suggesting that the substoichiometric sodium ascorbate used under our standard CuAAC conditions acted as a reducing agent.^51–53 With this insight, Re(bpy-C2-NPDI-NH₂) was afforded exclusively using catalytic CuSO₄ and a stoichiometric excess of sodium ascorbate. The identity of all Re(bpy-C2-NPDI-R) dyads was confirmed using ¹H and ¹³C NMR spectroscopies, as well as MALDI-TOF mass spectrometry and CHN elemental analysis (Fig. S1–S54†). Note, each specific Re(bpy-C2-NPDI-R) dyad will henceforth be referred to by their R-group only (where R = –H, –Br, –CN, –NO₂, –NH₂, –NR₂, and –OPh).

The optical properties of these Re(bpy-C2-NPDI-R) dyads were probed using both UV-visible-near-infrared (UV-vis-nIR) absorption and Fourier transform infrared (FTIR) spectroscopies. Visually, the optical properties of all Re(bpy-C2-NPDI-R) dyads were dominated by the NPDI-moiety (Fig. 1 and S55–S56†). Relative to –H (λ_max = 521), the installation of –Br, –CN, –NO₂, and –OPh groups on NPDI all caused a minor bathochromic shift of the absorption profile (Δλ_max = 1–13 nm) while maintaining most of the vibronic transitions of the parent complex. As for the –NH₂ and –NR₂ dyads, the optical spectra were significantly red-shifted (λ_max = 558 and 572 nm, respectively) and the molar absorptivity also decreased (from ∼60 000 to ∼40 000 M⁻¹ cm⁻¹). The broadened absorption of –NH₂ and –NR₂ near the λ_max is consistent with the donation of electron-density from the lone-pair of the amino nitrogen into the NPDI π-system.^54–56 Notably, all dyads exhibit an absorption peak around λ = 341–343 nm, characteristic of a Re(bpy)-based metal-to-ligand charge transfer band.⁵⁷ By FTIR, all Re(bpy-C2-NPDI-R) dyads possessed three carbonyl stretching frequencies (v_co ∼1900, 1925, and 2025 cm⁻¹), which is consistent with other Re(bpy) catalysts (Fig. S57†).^11–18,58 Together, these data confirm successful Re(bpy-C2-NPDI-R) dyad formation.

The electrochemical properties of these Re(bpy-C2-NPDI-R) dyads were next evaluated using cyclic voltammetry (CV). CV analysis was first performed in CH₂Cl₂ (Fig. 2A and S65†), with all reported redox events being referenced to the Fc^+/0 internal standard. Under an atmosphere of argon, all Re(bpy-C2-NPDI-R) dyads exhibited four reduction and two oxidation redox processes. The first two reversible reductions may be assigned to the NPDI˙^−/0 and NPDI^2−/˙⁻ redox couples while the third and fourth reductions correspond to the quasi-reversible bpy˙^−/0 and the irreversible Re^0/I redox events, respectively.^11–18,45 With respect to the oxidation events, the irreversible Re^II/I redox process remains consistently near E_p ≈ +1.0 V. The quasi-reversible NPDI-based oxidation event, on the other hand, underwent dramatic shifts in potential depending on the nature of the electronic substituent.

Based on the presented CV data (Fig. 2A, S64 and S65†), it was observed that the installation of EWGs (i.e. –Br, –CN, and –NO₂) on NPDI caused both the redox events assigned to the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) energy levels of NPDI to shift to more positive potentials. In all cases, the electronic bandgap was narrowed because the LUMO energy level was more significantly perturbed than the HOMO energy level. Conversely, the installation of EDGs (i.e. –OPh, –NH₂, and –NR₂) on NPDI caused the redox events associated with NPDI HOMO and LUMO energy levels to shift to more negative potentials. Once again, the electronic bandgap was decreased mainly due to the more substantial effects experienced by the HOMO energy level. The observed shifts in E_1/2 for first NPDI-R based reduction and oxidation events (Fig. 2B and C), relative to –H, show good Hammett parameter correlation.⁴⁶ Similar correlations were previously reported for electronically-substituted Re(4,4′-R-bpy) complexes by Kubiak et al.,¹¹ where EWGs shifted redox events more positively and EDGs shifted redox events more negatively. In principle, combining these two relationships could assist with proper energy level matching when designing future supramolecular dyads based on the Re(bpy-C2-NPDI-R) architecture.

Fig. 2 Cyclic voltammograms (A) of Re(bpy-C2-NPDI-R) dyads where R = –NR₂ (purple), –NH₂ (blue), –OPh (magenta), –H (red), –Br (orange), –CN (yellow), and –NO₂ (green). All measurements were recorded at 100 mV s⁻¹, under argon in CH₂Cl₂ with 0.1 M TBAPF₆ supporting electrolyte (WE = glassy carbon, CE = Pt-wire, RE = Ag/AgCl, and Fc^+/0 as internal reference standard). Correlation diagrams plot the observed shifts in E_1/2 (relative to R = –H) for the first reduction (B) and oxidation (C) as a function of R-group Hammett parameter.⁴⁶

Next, the electrochemistry of these Re(bpy-C2-NPDI-R) dyads were assessed in DMF, where only the reduction processes were measured due to solvent window effects.⁵⁹ Under argon, all Re(bpy-C2-NPDI-R) dyads exhibited the same four previously assigned reduction events (vide supra), where solvent effects caused the potential of most redox events to shift positively by ∼50–100 mV, relative to CH₂Cl₂ (Fig. S66†).⁶⁰ Upon measuring CVs at variable scan rates, each dyad displayed a diffusion-limited current response when fitted to the Randles–Sevcik equation (see ESI, eqn (i)†), with a calculated diffusion coefficient between D = 2.3–4.4 × 10⁻⁶ cm² s⁻¹ (Fig. S69–S74†). Upon subjecting these Re(bpy-C2-NPDI-R) dyads to an atmosphere of CO₂ (in DMF), a moderate CV current enhancement was observed underneath the fourth redox couple (Fig. 3, red traces with E_cat/2 ≈ −2.1 V). When a catalyst is supplied with the appropriate combination of substrates at a sufficient applied potential, a CV current enhancement is often observed as a result of initiating an electron-consuming catalytic process.^61,62 Moreover, these CV current enhancements may be monitored as a function of catalyst or proton-source concentration to gather preliminary mechanistic data on the catalytic process (see ESI, eqn (ii)†).^63,64

Fig. 3 CV current enhancement plots as a function of 2,2,2-trifluoroethanol (TFE) concentration for Re(bpy-C2-NPDI-R) derivatives, where R = –NR₂ (A), R = –NH₂ (B), R = –OPh (C), R = –H (D), R = –Br (E), R = –CN (F), and R = –NO₂ (G). TFE was incrementally added at 0 M (red), 0.5 M (yellow), 1.0 M (green), 1.5 M (blue), and 2 M (purple). All measurements were recorded at 100 mV s⁻¹, under an atmosphere of CO₂, in DMF with 0.1 M TBAPF₆ supporting electrolyte (WE = glassy carbon, CE = Pt-wire, RE = Ag/AgCl, and Fc^+/0 as internal reference standard). Note, the observed current enhancement at E_p = −1.35 V (vs. Fc^+/0) for R = –NO₂ (G) results from an in situ electrochemical conversion to R = –NH₂.

When the Re(bpy-C2-NPDI-R) concentration was varied, a linear CV current enhancement response was also observed for most dyads (Fig. S68†). Similarly, when proton-source 2,2,2-trifluoroethanol (TFE) was incrementally added to a CO₂-saturated Re(bpy-C2-NPDI-R) dyad solution, it induced a CV current enhancement increase under the fourth reduction event (Fig. 3). We further note that incremental addition of TFE also caused the NPDI^2−/˙⁻ redox couple to gradually shift to more positive potentials. This behavior has been modeled for similar rylene diimide materials as a 2H⁺/2e⁻ proton-coupled electron-transfer process,⁶⁵ whereby the NPDI imide oxygens are protonated by the proton-source.^66,67 While the collected CV data was not obtained under steady-state conditions (i.e. plateau current),⁶⁸ modelling these measured CV current enhancements as CO₂ reduction variables can qualitatively describe the effects of catalyst and proton-source as first-order and second-order rate-dependent variables, respectively. In other words, these data imply that one Re(bpy-C2-NPDI-R) dyad, with the assistance of two proton-source molecules, can enable electrocatalytic CO₂ conversion.⁴⁵ The obvious exception to these generalized electrochemical trends is the –NO₂ dyad (Fig. 3G). An in-depth evaluation of the electrochemical behaviour of –NO₂ under argon and CO₂ is provided in the ESI (see Section VIII, Fig. S84–S86†). Thorough analysis of this data strongly suggests that –NO₂ is converted in situ to –NH₂ under reducing conditions; as such, it is difficult to establish meaningful trends for –NO₂ (vide infra).

CO₂ electro-/photocatalysis

The electrocatalytic CO₂ reduction abilities of these Re(bpy-C2-NPDI-R) dyads were evaluated using controlled potential electrolysis (CPE). All experiments were performed in DMF (with 2 M TFE) using our previously described two-compartment H-cell.^45,69 At an applied potential (E_appl) of −1.8 V (Fig. S75†), all Re(bpy-C2-NPDI-R) dyads (except –CN) achieved comparable turnover numbers of CO (TON_co = 21–25) and faradaic efficiencies (FE_co = 87–99%) after 6 hours of electrocatalysis. The –CN derivative, on the other hand, attained about half the performance (TON_co = 13) at a slightly lower FE_co (87%). This drop in performance is consistent with decreased efficacy of the electron-reservoir effect due to the increased electron-affinity of NPDI-CN (Fig. S77†). Unlike the electronically modified Re(4,4′-R-bpy) series reported by Kubiak et al., where electrocatalytic CO₂-to-CO conversion efficiency was highly dependent on the nature of the R-group,¹¹ all Re(bpy-C2-NPDI-R) dyads achieved high FE_co at an overpotential that is ∼300 mV lower than the measured E_cat/2 ≈ −2.1 V (Fig. S67†). It should be noted, however, that when an E_appl = −1.7 V was used for CPE (Fig. S76†), electrocatalytic CO₂ reduction was essentially shut-off for all Re(bpy-C2-NPDI-R) dyads (TON_co ≤ 6). This could indicate that altering the electronic properties of the electron-reservoir may not be the most effective strategy towards further lowering the overpotentials required to enable electrocatalytic CO₂ reduction.

In our previous report, we noticed that the use of blue light during CPE experiments significantly increased the rates of CO production for –H.⁴⁵ To determine the origin of this TON_co enhancement,^28,70,71 we next turned our attention towards the photocatalytic CO₂ reduction capabilities of these Re(bpy-C2-NPDI-R) dyads. Following established literature protocols,^31,34–37 each dyad (30 μM) was dissolved in a (5 : 1) DMF: triethanolamine (TEOA) mixture containing sacrificial reducing agent 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH; 3 mM). The glass vials were sealed with rubber septa, sparged with CO₂, and then irradiated with blue light (λ = 470 ± 30 nm; 4 mW cm⁻²) for 24 hours (Table 1).

Table 1 Optimized photocatalytic CO₂ reduction of Re(bpy-C2-NPDI-R)

R =	TONCO^a	TONH2^a	CO : H2^b
a Calculated based on bulk catalyst concentration from quadruplicate trials. b No other gaseous (i.e. CH4) or liquid (i.e. HCOO–) products were detected. c Re(bpy-C2-NPDI-NO2) is in situ converted to Re(bpy-C2-NPDI-NH2).
–H	57 ± 1	1.8 ± 1	97 : 3
–CN	<1	<1	—
–NO2^c	137 ± 15	3 ± 0.6	98 : 2
–Br	61 ± 5	2.5 ± 1	96 : 4
–NR2	86 ± 8	1.4 ± 1	98 : 2
–OPh	59 ± 6	3.6 ± 2	95 : 5
–NH2	234 ± 13	2.0 ± 0.1	>99 : 1

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Over the 24 h testing period, the Re(bpy-C2-NPDI-R) dyads all showed good activity for ∼9 hours, after which CO production would level-off for the remainder of the experiment (Fig. S78†). The benchmark dyad, –H, achieved a TON_co of 57 ± 1 with a selectivity for CO of 97%. The –Br (TON_co = 61 ± 5) and –OPh (TON_co = 59 ± 6) dyads achieved the same performance with roughly the same CO selectivity (≥95%). The –CN derivative was completely inactive for CO₂ photocatalysis under these conditions. Interestingly, despite being the most EWG, the –NO₂ dyad achieved the second best TON_co (134 ± 15) with a very high CO selectivity of 98%. When EDGs were functionalized on NPDI, the CO₂-to-CO production and selectivity was improved to (TON_co = 86 ± 8) for –NR₂ and (TON_co = 234 ± 13) for –NH₂. We note that the photocatalytic performance of –NH₂ was not greatly improved over the 24 h testing period by replenishing both BIH and CO₂ in 6 h intervals (TON_co = 294; Fig. S79†), suggesting that depletion of substrate was not the limiting factor for TON_co.

To confirm the importance of each component in the photocatalytic CO₂ reduction setup, various control experiments were conducted. As expected, the omission of Re(bpy-C2-NPDI-R) dyads or CO₂ from the setup stopped the production of CO. When sacrificial reducing agent BIH was excluded (Table S2†), the TON_co was decreased by at least two-fold for all dyads. This result implies that while TEOA alone can simultaneously act as the proton-source and the sacrificial electron-donor,²⁰ BIH is more efficient at reductively quenching the photoexcited dyads. When TEOA was replaced by TFE (Table S3†), the production of CO for all dyads was decreased almost four-fold, except for –NH₂ (TON_co = 143). Overall, this result points towards the utility of TEOA as a sacrificial electron-donor, as well as the importance of forming Re(bpy)-adducts during CO₂ reduction, as seen in previous literature examples.^5,26–31 When the irradiation source was switched to a green light LED array (λ = 525 ± 32 nm; 1.9 mW cm⁻²), the measured TON_co decreased at least five-fold for all Re(bpy-C2-NPDI-R) dyads (Table S4†). This significant drop in performance could be the result of either inefficient photoexcitation pathways in the dyad^40–44 and/or the elimination of a photo-assisted CO cleavage process.^70,71 Lastly, the importance of tethering the NPDI-moiety to the Re(bpy)-moiety was demonstrated by combining N₃-C2-NPDI-R with Re(bpy), where it was observed that all samples obtained the same performance as Re(bpy) alone (Table S5†). We further showed that all N₃-C2-NPDI-R precursors were essentially inactive for CO₂ conversion under the optimized photocatalysis conditions (TON_co < 3; Table S6†).

While these control experiments clearly highlight the necessity of each component in the photocatalytic CO₂ reduction process, it does not fully account for the performance differences of each Re(bpy-C2-NPDI-R) dyad. Therefore, based on the photocatalytic CO₂ reduction results, the Re(bpy-C2-NPDI-R) dyads may be grouped together in three categories: (i) standard catalysts (weak EWGs/EDGs = –H, –Br, and –OPh), (ii) inactive catalysts (strong EWGs = –CN), and (iii) top-performing catalysts (strong EDGs = –NR₂ and –NH₂). Note, the –NO₂ dyad can also be classified as a top-performing catalyst because –NO₂ undergoes an in situ transition to –NH₂ under photocatalytic CO₂ reduction conditions (see ESI Section VIII for more details†).

Mechanistic investigation

During electro-/photocatalytic CO₂ reduction testing of these Re(bpy-C2-NPDI-R) dyads, a series of dramatic colour changes were observed. Prior to light irradiation, the absorption profile of all Re(bpy-C2-NPDI-R) dyads were unchanged by the addition of both TEOA and BIH, confirming that neither reagent reduces Re(bpy-C2-NPDI-R) immediately. After sparging with CO₂, the samples were then subjected to blue light and the progression of colour changes was monitored periodically by UV-vis-nIR spectroscopy (Fig. S80†). Each Re(bpy-C2-NPDI-R) dyad underwent a transition from their initial colour to either a green (–H, –Br, and –OPh), a dark blue (–CN), or a beige (–NO₂, –NR₂, and –NH₂) colour that was found to persist throughout the remainder of catalysis. Photoluminescence spectroscopy revealed that the photoexcited state of all Re(bpy-C2-NPDI-R) dyads can be reductively quenched by both TEOA and BIH (Fig. S81†). Thus, to gain further insight into the photoelectrochemical processes that were occurring during CO₂ reduction catalysis, UV-vis-nIR and FTIR spectroelectrochemistry (SEC) experimentation was conducted. UV-vis-nIR and FTIR SEC data was collected by monitoring an air-free Re(bpy-C2-NPDI-R) dyad solution that was held at a constant E_appl (where Red1, Red2, and Red3 correspond to the NPDI˙^−/0, NPDI^2−/˙⁻, and bpy˙^−/0 reductions, respectively).

The UV-vis-nIR SEC data of each Re(bpy-C2-NPDI-R) dyad (Fig. 4) correlates very well with what was observed when we periodically monitored our photocatalytic experiments. When using an E_appl = Red1, the λ_max of all Re(bpy-C2-NPDI-R) dyads was bathochromically shifted, the relative molar absorptivity of this λ_max was stronger, and some new vibrational fine-structure was also observed between 800–1000 nm. While the nature of installed R-group influences the position of the λ_max and the absorption fine-structure, all these absorption features are consistent with selective formation of NPDI˙⁻.^43,44 When the E_appl is switched to Red2, the spectral features of NPDI˙⁻ are rapidly depleted and replaced by shifted broad-band absorption peak(s). These spectral features are commonly associated with NPDI²⁻,⁶⁷ where the vibronic structure of the NPDI²⁻ absorption profile is once again influenced by the nature of installed R-group. With respect to –H (Fig. 4D), the incorporation of HOMO-modifying EDGs (i.e. –NR₂, –NH₂, and –OPh) resulted in minimal changes to the NPDI²⁻ absorption profile shape (Fig. 4A–C). Conversely, when LUMO-modifying EWGs (i.e. –Br and –CN) are used, the vibronic structure of NPDI²⁻ is significantly different (Fig. 4E and F). When the E_appl was changed to Red3, no other significant spectral changes were observed. This result is unsurprising given the differences in molar absorptivity of the NPDI and Re(bpy)-moieties.^57,72 Except for –NO₂ (Fig. 4G), all Re(bpy-C2-NPDI-R) dyads displayed very similar spectral transitions. In-depth analysis of the UV-vis-nIR SEC data for –NO₂ (Fig. S86†) shows that –NO₂ is in situ converted to –NH₂ under the conditions necessary for CO₂ reduction catalysis. These results help confirm why –NO₂ served as an efficient CO₂ reduction photocatalyst despite having a stronger EWG than the totally inactive –CN dyad derivative.

Fig. 4 UV-vis-nIR SEC absorbance spectra of Re(bpy-C2-NPDI-R) derivatives, where R = –NR₂ (A), R = –NH₂ (B), R = –OPh (C), R = –H (D), R = -Br (E), R = –CN (F), and R = –NO₂ (G). All experiments were performed in DMF with 0.1 M TBAPF₆ supporting electrolyte (WE = Pt-mesh, CE = Pt-wire, pseudo-RE = Ag-wire).

Moving onto the FTIR SEC data, the behavior of all Re(bpy-C2-NPDI-R) dyads were nearly identical (Fig. S58–S63†). While no spectral changes were detected at E_appl = Red1 and Red2, significant shifts in v_co were observed at E_appl = Red3. At Red3, the Re(bpy)-moiety of these dyads is formally reduced by one-electron [Re^I(bpy˙⁻-C2-NPDI²⁻-R)]. The added electron density at the Re(bpy)- moiety results in a lowering of v_co from 1895, 1915, and 2019 cm⁻¹ to roughly 1865, 1885, and 1995 cm⁻¹, respectively. Over time, an equilibration process occurs whereby electron-density is shifted from bpy˙⁻ to Re, and leads to Re–Cl dissociation [Re⁰(bpy-C2-NPDI²⁻-R)].⁵⁸ This crucial process generates a 5-coordinate Re metal-center and can be characterized by a Δv_co to 1843, 1862, and 1978 cm⁻¹. Although this Cl-dissociation process was more readily observed for dyads bearing EWGs (–CN, –NO₂, and –Br) than it was for dyads bearing EDGs (–NR₂, –NH₂, and –OPh), it was still detected to some degree for all Re(bpy-C2-NPDI-R) catalysts.

The similarities between our SEC data and the results reported for other Re(bpy) and Ru^II–Re^I catalysts suggests that our Re(bpy-C2-NPDI-R) systems likely operate via similar photocatalytic CO₂ reduction mechanisms.^5,25–31 It should be noted, however, that the photocatalytic CO₂ reduction mechanism for Re(bpy) is still heavily debated.^18–31,73 As such, the goal of our proposed mechanism was not to precisely determine the exact identity, electron-spin configuration (singlet vs. triplet), and/or the rate dynamics of eT for all photocatalytic CO₂ reduction intermediates. Instead, we focus on developing a molecular orbital (MO) description of these Re(bpy-C2-NPDI-R) dyads that helps account for the observed differences in photocatalytic CO₂ reduction performance (Fig. 5). The presented mechanism was modeled after the benchmark dyad, –H.

Fig. 5 Molecular orbital description for the proposed photocatalytic CO₂ reduction mechanism of Re(bpy-C2-NPDI-R), where Re(bpy) orbitals are shown in blue and NPDI-R orbitals are shown in red. The influence of electron withdrawing groups (EWGs, orange) and electron donating groups (EDGs, purple) on NPDI-R molecular orbitals is highlighted (right). Both BIH and TEOA are depicted as sacrificial electron donors that reductively quench the photoexcited electrons.

By CV, it was elucidated for all Re(bpy-C2-NPDI-R) dyads (except –NH₂ and –NR₂) that the HOMO is Re(bpy)-based and the LUMO is NPDI-based (Fig. 5A). Consequently, the first two photoreductions most likely result from a Re–π* intersystem crossing (ISC) process; however, a direct NPDI-based π–π* transition may also be possible.^43,44 The third photoreduction likely occurs either via direct Re(bpy) ³MLCT¹⁹ or eT from a photoexcited state of NPDI²⁻.^43,44 Following the formal three electron reduction of the Re(bpy-C2-NPDI-R) dyads (Fig. 5B), the next step is Re–Cl dissociation which generates a 5-coordinate Re metal-center whose axial position subsequently forms an adduct with TEOA.^20,74 Another consequence of the chloro-dissociation step is that the Re metal-center undergoes a rehybridization process that lowers the overall energy of the Re(bpy)-based MOs.^45,75 Following the formation of the TEOA-Re(bpy) adduct, it is possible for CO₂ insertion to occur without direct eT from the catalyst center (Fig. 5D).^20,27–30 Protonation of the resulting carbonate intermediate induces a reorganization process that releases TEOA and forms a Re-CO₂˙⁻ species (Fig. 5E).²² Due to the presence of ¹³CO and H¹³CO₃⁻ in the ¹³C{¹H} NMR spectrum after blue light irradiation (Fi. S83†), it is postulated that these Re(bpy-C2-NPDI-R) dyads operate via a BIH-mediated disproportionation reaction between Re–CO₂˙⁻ and another equivalent of CO₂ that liberates HCO₃⁻,²⁵ rather than a proton-coupled electron transfer process from NPDI²⁻ to the Re(bpy)-moiety that liberates OH⁻.¹⁸ From there, reductive quenching of a photoexcited electron restores NPDI²⁻ (Fig. 5G) and the ensuing transfer of this electron to the Re(bpy)-moiety produces CO, as well as opens a coordination site for TEOA (Fig. 5H).²¹ The photocatalytic cycle is completed by the photoexcitation and reductive quenching of the electron to regenerate NPDI²⁻.

Based on photocatalytic CO₂ reduction performance, the catalysts were loosely grouped into three categories: (i) standard catalysts (–H, –Br, and –OPh), (ii) inactive catalysts (–CN), and (iii) top-performing catalysts (–NR₂ and –NH₂). Looking at the proposed photocatalytic CO₂ reduction mechanism, it is also possible to map out the effects of EWGs and EDGs on the provided MO description of these Re(bpy-C2-NPDI-R) dyads (Fig. 5, highlighted in yellow). In the case of –Br and –OPh, the overall influence of these R-group does not appear to change the eT dynamics of the Re(bpy-C2-NPDI-R) dyad (with respect to –H). The HOMO–LUMO transition (Re–π*) is identical and the relative shift(s) of the NPDI²⁻ energy level does not significantly alter the electronic driving-force of eT between NPDI²⁻ and Re(bpy). In the case of –CN, the fixation of that EWG on NPDI served to lower the energy of the NPDI-based HOMO and LUMOs. While the net result of this transformation retains the original HOMO–LUMO transition (Re–π*), it appears to lower the energy level of NPDI²⁻ enough to effectively prevent eT from NPDI²⁻ to the Re(bpy)-moiety, thus shutting down catalysis. Conversely, the installation of strong EDGs (–NR₂ and –NH₂) causes the energy levels of the NPDI-based HOMO and LUMOs to increase. The overall result of this transformation not only changes the HOMO–LUMO transition to an exclusively NPDI-based process (π–π*), but it also increases the driving-force for eT from NPDI²⁻ to Re(bpy). The sum of these two effects together lead to improved dyad eT dynamics, thereby enhancing photocatalytic CO₂ reduction (with respect to –H).

It should be noted that another feasible explanation for the improved performance of the amino-functionalized dyads, in particular –NH₂, is the possibility of second-sphere H-bonding effects.^15,18 Previously we calculated the optimized geometries of various intermediates during CO₂ catalysis for the –H dyad.⁴⁵ It was shown in this study that, although the NPDI was initially folded over the Re(bpy)-moiety, reduction of the –H dyad caused the two moieties to extend away from one another (likely due to coulombic repulsion effects). By analogy, the amino-functionalized NPDI-R bay position would most likely also be extended away from the Re(bpy)-moiety during CO₂ photocatalysis. While in this case the distance between the catalyst center and the amino-groups of –NR₂ and –NH₂ make it unlikely that second-sphere H-bonding effects are involved in CO₂ photocatalysis, they can't be conclusively ruled out at this time. If nothing else, the synthetic versatility of the NPDI chromophore means that future iterations of the Re(bpy-C2-NPDI-R) motif could incorporate proximal second-sphere H-bonding groups as a means to further improve CO₂ conversion performance.

Conclusions

In conclusion, we present the synthesis and full characterization of six new Re(bpy-C2-NPDI-R) supramolecular dyad materials (where R = –Br, –CN, –NO₂, –OPh, –NH₂, or –NR₂). The installation of R-groups on NPDI altered the optoelectronic properties of these dyads, as well as impacted the photocatalytic CO₂ reduction performance. Relative to the benchmark Re(bpy-C2-NPDI-H) dyad (TON_co = 57), the incorporation of EDGs (i.e. –NH₂) led to an over four-fold improvement in photocatalytic CO₂ reduction performance (TON_co = 234) while strong EWGs (i.e. –CN) resulted in complete deactivation of the dyads. Despite being the most electron-withdrawing, the –NO₂ functionalized NPDI was among the top performing CO₂ reduction photocatalysts (TON_co = 137), making it an outlier to the proposed trend. Through CV and UV-vis-nIR SEC experimentation, it was elucidated that –NO₂ undergoes an in situ conversion to –NH₂, thereby forming a different dyad that is responsible for catalysis. A photocatalytic CO₂ reduction mechanism is proposed for these dyads, where EDGs served to accelerate CO₂ reduction rates by simultaneously changing the HOMO–LUMO excitation pathway and by increasing the electronic driving-force of intramolecular electron transfer from NPDI²⁻ to Re(bpy). Conversely, EWGs shifted the LUMO energy levels of NPDI to the point where photocatalysis is shut down because there is no electronic driving force for eT between NPDI²⁻ and Re(bpy). This study clearly highlights the importance of evaluating structure–property relationships to develop and optimize the future design of new supramolecular dyad photocatalysts.

Author contributions

JDBK performed all experimental work and data analysis and prepared the manuscript. GCW directed the project and provided resources. WEP co-directed the project and provided resources.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

GCW acknowledges funding from the NSERC DG program (2019-04392), the Canada Foundation for Innovation, and the University of Calgary. WEP acknowledges the Canada Research Chairs Program. JK acknowledges NSERC CGS-D scholarship program. This research was undertaken thanks in part to funding from the Canada First Research Excellence Fund (CFREF).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1sc05465a

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