# Phenoxazine-Sensitized CO2-to-CO Reduction with an Iron Porphyrin Catalyst: A Redox Properties-Catalytic Performance Study

Martin Kientz, Grace Lowe, Blaine G Mccarthy, Garret M Miyake, Julien Bonin, Marc Robert

► To cite this version:

Martin Kientz, Grace Lowe, Blaine G Mccarthy, Garret M Miyake, Julien Bonin, et al., Phenoxazine-Sensitized CO2-to-CO Reduction with an Iron Porphyrin Catalyst: A Redox Properties-Catalytic Performance Study. ChemPhotoChem, 2022, 6 (7), pp.e202200009. 10.1002/cptc.202200009. hal-04703893

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# Phenoxazine-Sensitized CO₂-to-CO Reduction with an Iron Porphyrin Catalyst: A Redox Properties-Catalytic Performance Study

Martin Kientz,[a] Grace Lowe,[a] Blaine G. McCarthy,[b] Garret M. Miyake,[b] Julien Bonin,[*a] and Marc Robert*[a, c]

We have evaluated six phenoxazine derivatives as visible light photosensitizers for the photochemical reduction of CO₂ to CO with an iron porphyrin catalyst in organic media. The phenoxazine core was functionalized with electron-donating or -withdrawing groups to modify the photophysical properties. Both singlet and triplet excited state potentials of the sensitizers spanned several hundred mV range and the ground state oxidation potentials spanned 250 mV. We observed that no correlation can be established between the production of CO

and the excited state potential of the phenoxazine, which determines the driving force for electron transfer to the catalyst. On the contrary, a clear correlation can be made between the oxidation potentials of the phenoxazine and the production of CO. This observation indicates that the process was limited by photosensitizer regeneration and highlights the fact that electron transfers not directly related to the catalyst activation could play a key role in homogeneous photocatalytic systems.

## Introduction

The conversion of CO₂ into useful carbon-neutral C1 products for industrial chemical feedstocks and fuels is a major research challenge.[1] The chemical stability of CO₂ requires the use of catalysts able to drive its reduction in a selective and efficient way, and the conversion also requires a large amount of energy. For the past few years we have been developing photocatalytic systems incorporating iron and cobalt-based homogeneous catalysts that address both aspects of photo-catalytic CO₂ reduction.[2-9] One challenge using this approach is to harvest enough energy from the visible domain of the solar spectrum to drive the process. To achieve this, the catalyst is associated with a visible light photosensitizer which, once photo-excited, can transfer an electron to the catalyst. For decades, processes requiring such visible light photosensitizers have largely employed ruthenium and iridium based inorganic complexes. These inorganic photosensitizers absorb visible light with high quantum yields to generate long-lived excited states that are both highly reducing and oxidizing.[10,11] These photophysical properties make iridium and ruthenium photosensitizers excellent candidates for reactions like photochemical CO₂ reduction. However, they incorporate expensive and rare metals which is a problem for scaling-up these systems. As a result, photosensitizers made from Earth-abundant materials that exhibit similar photophysical characteristics to those of iridium and ruthenium photosensitizers are highly sort after.

Many photosensitizers have been synthesized using Earth-abundant elements, including inorganic complexes incorporating metals such as chromium, iron, copper or zinc,[12,13] and purely organic dyes.[14-18] Organic photosensitizers include commonly used dyes such as coumarins[19] or eosin Y,[20] as well as bodipy compounds.[21-24] Earth-abundant photosensitizers and dyes have been employed in applications such as solar cells.[25,26] However, most of these compounds have not been successfully used for photocatalytic CO₂ reduction to date. This is because they generally do not combine the three required properties: a strong absorption in the visible range, a long-lived charge transfer excited state, and a strong reduction potential. Purpurin is an exception and has been reported to efficiently sensitize an iron porphyrin to form CO from CO₂ with high selectivity.[5]

Phenoxazines (Phen) are a class of organic compounds that have been reported as visible light photoredox catalysts.[27-30] They exhibit reversible electron transfer behavior, strong visible-light absorption, and they have long-lived (up to several hundreds of µs), highly reducing charge transfer triplet excited states. These are all essential characteristics of a good photosensitizer.[30] Previously, we reported our initial study of an iron porphyrin catalyst combined with a phenoxazine sensitizer (Phen 1, Scheme 1) in a DMF, triethylamine solution. We showed that this photosensitizer can facilitate CO₂ reduction to CO and even methane.[31] We also observed that the catalytic process sensitized with this phenoxazine was both more productive (by a factor of 2 in TON) and more robust

<figure><img src="image_3.png" /></figure>

Supporting information for this article is available on the WWW under https://doi.org/10.1002/cptc.202200009

<footnote>[a] M. Kientz, Dr. G. Lowe, Dr. J. Bonin, Prof. Dr. M. Robert Laboratoire d'Electrochimie Moléculaire (LEM) Université de Paris, CNRS 75006, Paris (France)
E-mail: julien.bonin@u-paris.fr
robert@u-paris.fr</footnote>

<footnote>[b] Dr. B. G. McCarthy, Prof. Dr. G. M. Miyake Department of Chemistry Colorado State University Fort Collins, Colorado 80523 (United States)</footnote>

<footnote>[c] Prof. Dr. M. Robert Institut Universitaire de France (IUF) 75005, Paris (France)</footnote>
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>Scheme 1. Chemical structures of the phenoxazine sensitizers.</figcaption></figure>

(production over 100 h) than the iridium sensitizer that we had previously used.[31] Hence, we decided to explore this class of photosensitizer in more detail, specifically by investigating the influence of structural modifications on the redox and photo-physical properties of these compounds. Several Phen derivatives (Scheme 1) were used as sensitizers for the photochemical CO₂ reduction to CO and the overall performance of each catalytic system was compared to the properties of the photosensitizer.

## Results and Discussion

The six derivatives studied here are based on the N-aryl phenoxazine core (in blue, Scheme 1) and were modified with electron-donating groups (Phen 11 and 14), electron-withdrawing groups (Phen 12 and 13), or by extending the conjugated aromatic framework (Phen 1 and 16). Phenoxazines bearing electron-donating core substituents are more easily oxidized in the ground state and exhibit a more negative triplet excited state potential. The opposite is observed with those bearing electron-withdrawing groups which have a more positive triplet excited state potential and being less easily oxidized. Extension of the conjugation leads to a wider absorption in the visible range with a red shift of the absorption maximum (Table 1). Triplet excited state potentials (E°*T) of these photosensitizers span 490 mV, from Phen 12 (–1.42 V vs. SCE) to Phen 11 (–1.91 V vs. SCE) whereas the singlet excited state potentials (E°*S) span a range of 390 mV, from Phen 16 (–1.54 V vs. SCE) to Phen 13 (–1.93 V vs. SCE). For all Phen, both excited states are reductive enough to generate the active state of the catalyst. The oxidation potentials (E°ox) of the Phen sensitizers span a range of 250 mV, from Phen 11 (+0.37 V vs. SCE) to Phen 12 (+0.62 V vs. SCE). The E°*T and the E°ox of the reference compound fac-tris(2-phenylpyridine) iridium (Ir(ppy)₃) sensitizer

<table><caption>**Table 1.** Photophysical and redox properties of the phenoxazine sensitizers studied in this report, together with Ir(ppy)₃ as typical strongly reducing sensitizer as reference compound.</caption><thead><tr><th>Phen<sup>[a]</sup></th><th>λ<sub>max</sub><br>abs<br>[nm]</th><th>λ<sub>max</sub><br>em<br>[nm]<sup>[b]</sup></th><th>E<sup>O*</sup><sub>T</sub><br>(Phen*<sup>+</sup>/<br>Phen*)</th><th>E<sup>O*</sup><sub>S</sub><br>(Phen*<sup>+</sup>/<br>Phen*)</th><th>E<sup>O</sup><sub>ox</sub><br>(Phen*<sup>+</sup>/<br>Phen)</th></tr></thead><tbody><tr><td>Phen 1</td><td>388</td><td>506</td><td>-1.70</td><td>-1.80</td><td>+0.42</td></tr><tr><td>Phen 11</td><td>363</td><td>528</td><td>-1.91</td><td>-1.81</td><td>+0.37</td></tr><tr><td>Phen 12</td><td>411</td><td>506</td><td>-1.42</td><td>-1.75</td><td>+0.62</td></tr><tr><td>Phen 13</td><td>388</td><td>469</td><td>-1.58</td><td>-1.93</td><td>+0.58</td></tr><tr><td>Phen 14</td><td>382</td><td>527</td><td>-1.88</td><td>-1.83</td><td>+0.30</td></tr><tr><td>Phen 16</td><td>379</td><td>548</td><td>-1.85</td><td>-1.54</td><td>+0.45</td></tr><tr><td>Ir(ppy)<sub>3</sub><sup>[d]</sup></td><td>375</td><td>518</td><td>-1.73</td><td>-</td><td>+0.77</td></tr></tbody></table>

[a] Phenoxazine numbers were kept identical to Ref. [30] for simplicity.

[b] Measured at room temperature in acetonitrile, with λ<sub>wxc</sub> = 400 nm. [c] In V vs. SCE, computationally predicted for E°*T or experimentally measured for E°*S and E°ox as described in Ref. [30]. [d] Data from Ref. [32].

also lies in these ranges (–1.73 V and +0.77 vs. SCE, respectively).

Emission quenching measurements, with the excitation wavelength set at 400 nm were conducted to determine if each Phen excited state can be photo-oxidized by the catalyst (Figure S2). Note that this experiment likely probes the singlet excited state of the organic Phen derivatives because they are typically not phosphorescent in solution at ambient temperature. However, reactivity from both the singlet and triplet excited states of phenoxazines are relevant in catalysis and provide insight into the reactivity.[33] All six Phen derivatives possess an absorption in the visible domain (Figure S1) with extinction coefficients at the 400 nm lower threshold ranging from 8.4×10³ to 3.1×10⁴ (Table S1). For all six Phen derivatives, a clear emission decay was observed in the presence of increasing amount of FepTMA catalyst. The Stern-Volmer analysis of these data shows a linear relationship between the emission intensity ratio and the catalyst concentration, as expected from this model. The Stern-Volmer constant K<sub>SV</sub>, representing the quenching efficiency, ranges from 3×10⁴ to 1×10⁵ M⁻¹ (Table S2), which corresponds to a quenching rate constant with the singlet state k<sub>q</sub>⁵ between 7.3×10¹¹ and 2.4×10¹² M⁻¹s⁻¹, assuming a singlet-excited state lifetime of ca. 45 ns for the Phen (the singlet excited-state lifetime of Phen 1).[34] Such large rate constants were indeed reported to correspond to diffusion-limited bimolecular photoinduced electron transfer with organic chromophores in acetonitrile.[35,36] Electron transfers of interest for photoredox catalysis can occur from either the singlet or the triplet state of the Phen,[30] but the longer-lived triplet state is very likely responsible for most ET in such bimolecular processes. So, considering a triplet-excited state lifetime of ca. 450 µs for the Phen (the triplet excited-state lifetime of Phen 1),[29,31] the rate constant k<sub>q</sub>⁷ thus ranges between 7.3×10⁷ and 2.4×10⁸ M⁻¹s⁻¹. A series of control experiments was conducted by adding increasing amount of sacrificial electron donor (BIH) instead of FepTMA catalyst, and no emission quenching occurred (Figure S3). The electron transfer thus proceeds efficiently from the Phen excited state to the catalyst to activate it (ET<sub>act</sub> step, Scheme 2), through a so-called oxidative quenching process. As a result,
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>**Scheme 2.** Proposed mechanism for the phenoxazine sensitized photochemical CO₂ reduction to CO with an iron porphyrin catalyst.</figcaption></figure>

the oxidized form of the Phen is formed after each photo-induced ET to the catalyst. The catalyst must undergo three one-electron reduction reactions to reach the formal Fe⁽⁰⁾ active state from the initial Fe⁽³³⁾ state. Previous work from our laboratory indeed showed that only the Fe⁽⁰⁾ active state is able to bind and reduce CO₂ in DMF.[³⁷] The same is true in acetonitrile with FepTMA, as observed by cyclic voltammetry under CO₂ atmosphere (Figure S5) in which a catalytic wave is observed at the potential corresponding to the formation of Fe⁽⁰⁾.

The Phen derivatives were all evaluated as visible light sensitizers using the following conditions: the reaction media consisted of a homogeneous acetonitrile (ACN) solution, saturated with CO₂ and containing 2 µM FepTMA catalyst, 200 µM Phen photosensitizer, 5 mM 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) sacrificial electron donor, and 0.1 M trifluoroethanol (TFE) external weak acid. Note that FepTMA catalyst also absorbs at 400 nm, with an extinction coefficient of 45500 M⁻¹cm⁻¹. However, the molar ratio of Phen to catalyst was 100 (200 vs. 2 µM, respectively), and taking into account the extinction coefficient of the Phen (Table S1), the catalyst absorption only accounts for a very minor part of the total absorption, from ca. 1% (with the higher absorbing Phen 14) to ca. 5% (with the lower absorbing Phen 11).

Under visible light illumination (> 400 nm) CO and H₂ formation was observed through gas chromatography analysis (Figure S4). For all the systems containing Phen photosensitizers, except for Phen 14, CO was formed as the main gaseous product (see Table S3 for complete catalytic performance data) and H₂ as the minor product. No liquid product was detected by ionic chromatography or NMR, and no CO or H₂ was formed when the same reaction mixture was saturated with argon instead of CO₂. All derivatives are in the same range of catalytic performances, except for Phen 14 which produced a lower TON and selectivity for CO (see below). TON(CO) ranges in between 88 for Phen 16 to 115 for Phen 12, and CO selectivity ranges from 80% with Phen 1 to 100% with Phen 12 and 16 (Table 2). Thus, TON(CO) and CO selectivity seem to be insensitive to photosensitizer absorption coefficient and absorptivity. Evolution of TON(CO) with time also presents a very similar trend for

<table><caption>**Table 2.** Turnover number (TON) and catalytic selectivity (CS) after 21 h of visible light (> 400 nm) illumination of a CO₂-saturated ACN solution containing 2 µM FepTMA catalyst, 200 µM Phen photosensitizer, 5 mM BIH sacrificial electron donor and 0.1 M TFE weak acid.</caption><thead><tr><th>Phen<br>PS</th><th>TON(CO)</th><th>TON(H<sub>2</sub>)</th><th>CS(CO)<br>[%]</th><th>CS(H<sub>2</sub>)<br>[%]</th></tr></thead><tbody><tr><td>Phen 1</td><td>103</td><td>25</td><td>80</td><td>20</td></tr><tr><td>Phen 11</td><td>88</td><td>12</td><td>94</td><td>6</td></tr><tr><td>Phen 12</td><td>115</td><td>0</td><td>100</td><td>0</td></tr><tr><td>Phen 13</td><td>112</td><td>12</td><td>90</td><td>10</td></tr><tr><td>Phen 14</td><td>32</td><td>57</td><td>36</td><td>64</td></tr><tr><td>Phen 16</td><td>89</td><td>0</td><td>100</td><td>0</td></tr></tbody></table>

all Phen compounds, once again except for Phen 14 (Figure S6).

The functionalization of the Phen core varied the energy levels of the photosensitizers and their intermediates formed during photocatalysis. To investigate the effects of this, TON(CO) was plotted as a function of the E°<sub>ox</sub>(Phen⁺/Phen), E°*<sub>s</sub>(Phen⁺/Phen*) and E°*<sub>T</sub>(Phen⁺/Phen*) (Figure 1). In both cases, the data point corresponding to Phen 14 (into brackets) is apart from the general trend, due to its low CO₂ selectivity (larger H₂ production as compared to CO) in contrast with other derivatives. This observation is quite surprising since we have previously shown that FepTMA catalyst, without sensitizer and with BIH electron donor, is remarkably selective towards CO formation, with no H₂ formed even in the presence of a

<figure><img src="image_8.png" /><figcaption>**Figure 1.** TON for CO as a function of (A) the oxidation potential E°<sub>ox</sub> and (B) the singlet E°*<sub>s</sub> and triplet E°*<sub>T</sub> excited state reduction potential of the phenoxazine derivatives.</figcaption></figure>
<figure><img src="image_9.png" /></figure>

relatively strong proton donor such as phenol.[4] The generation of H₂ with iron porphyrins was indeed proposed to be related to the formation of a hydride from the active Fe(⁰) state,[38] but FepTMA catalyst is less prone to follow such pathway thanks to the strong electron withdrawing effect of the trimethylamino groups. Remarkably, Phen 14 is the only one of the six derivatives bearing groups with acid-base properties. Triphenylamine is however known to be a poor base due to the delocalization of the nitrogen lone pair so its protonation is rather unlikely.[39] The exact reasons why Phen 14 leads to H₂ formation are thus still unclear but the triphenylamino groups play for sure a non-innocent role.

For the plot of TON(CO) vs. E°<sub>ox</sub>(Phen•⁺/Phen) (Figure 1(A)), a clear trend (red dotted line) can be observed. TON(CO) increases as E°<sub>ox</sub>(Phen•⁺/Phen) shifts to more positive values. The driving force of the ET<sub>regen</sub> step (Scheme 2) and the rate of Phen regeneration is determined by the difference in potential between the oxidized Phen and the sacrificial electron donor (BIH). The more positive E°<sub>ox</sub>(Phen•⁺/Phen) and the larger that driving force, the faster the regeneration of the photosensitizer. Since BIH has an oxidation potential of +0.33 V vs. SCE,[40] the ET<sub>regen</sub> step thus proceeds at 40 mV (with Phen 11) to 290 mV driving force (with Phen 12), which corresponds to the lower and the higher TON(CO) observed, respectively. This behavior indicates that photosensitizer regeneration is likely the rate-limiting step in our system.

The driving force and rate of the ET<sub>act</sub> step (Scheme 2) is determined by the difference in potential between the iron porphyrin Fe<sup>(n-1)</sup>/Fe<sup>(n)</sup> redox couple and the sensitizer excited state E°*(Phen•⁺/Phen•). It is generally thought that the performances of a photocatalytic system are very sensitive to the catalyst activation step, and consequently related to the driving force for the ET<sub>act</sub> step. The iron porphyrin catalyst used here possesses three redox couples, Fe<sup>(III)</sup>/Fe<sup>(II)</sup>, Fe<sup>(III)</sup>/Fe<sup>(I)</sup> and Fe<sup>(II)</sup>/Fe<sup>(I)</sup>, at potentials of -0.10, -0.95 and -1.47 V vs. SCE, respectively.[41] When the energy of the excited state is varied by functionalizing the Phen this should also vary the driving force for ET<sub>act</sub> because the porphyrin potentials stay constant. As presented in Figure 1(B), no clear trend can be observed between either the singlet E°*<sub>S</sub> (in green) or the triplet E°*<sub>T</sub> (in blue) excited state potential and the TON(CO). The higher TON(CO) (115, Table 2) is obtained with Phen 12, i.e. the derivative having the less reducing triplet excited state potential and a medium singlet excited state potential. On the contrary, the lowest TON(CO) (88-89) is obtained with the two most reducing triplet excited state potentials, with Phen 11 and Phen 16, whereas the latter possesses the less negative singlet excited state potential. If we focus on the triplet excited state, the results of Figure 1(B) actually mirror those of Figure 1(A) since the derivative having the most negative excited state potential (Phen 11) is also the one that is the most easily oxidized, and inversely. This strongly supports our hypothesis that the rate-determining step is the photosensitizer regeneration.

From our results, we can propose the following mechanistic scenario (Scheme 2). On one hand, the excited state potential of the Phen sensitizer does not play a significant role in the

catalytic performance of the system as long as the potential is negative enough to activate the catalyst, i.e. to achieve the ET<sub>act</sub> step. We indeed observe that an increase in the electron transfer driving force, induced by a more negative triplet excited state potential, has a negative effect on the production of CO. Although further work is needed to consider the triplet quantum yield and excited-state lifetimes, our initial observations suggest that the oxidation potential of the radical cation is involved in the rate determining step of the reaction. The oxidation potential of Phen appears to have a significant effect on the catalytic cycle which is indicated by a correlation between the increase in the TON(CO) and the positive shift of the Phen oxidation potential. In other words, if the driving force and hence the rate of the ET<sub>regen</sub> step increases, more CO is formed. This indirect effect of the redox properties of the sensitizer on the catalysis could probably be explained by the fact that a more efficient/rapid regeneration of the Phen sensitizer, due to the electron transfer from the sacrificial electron donor, would allow to maintain a higher steady-state concentration of available, possibly excited, sensitizer, the consequence being that more catalysts molecules are activated.

## Conclusions

In summary, our evaluation of six phenoxazine derivatives as visible light photosensitizers for the catalytic reduction of CO₂ with an iron porphyrin catalyst reveals that no correlation can be established between the production of CO and their excited state potential. This suggests that the direct ET<sub>act</sub> step involving the sensitizer excited state is either not sensitive to the driving force of the reaction or not the rate determining step of the process. On the contrary, the production of CO is strongly related to the oxidation potential of the Phen sensitizer, thus implying that the redox properties of the Phen is an important parameter for its regeneration through the ET<sub>regen</sub> step with the sacrificial electron donor. This indirect effect emphasizes the importance of ET steps not directly involved in the catalyst activation step of a photocatalytic cycle when the sensitizer excited state is oxidatively quenched by the catalyst. Note that in the case of a reductive quenching between the sensitizer excited state and the sacrificial donor, that leads to the formation of the reduced state PS⁻ of the sensitizer, the ET step activating the catalyst could then be the limiting factor.

Many advances have been made in homogeneous photochemical reduction of CO₂ by focusing on optimizing the ET between photosensitizer and catalyst. However, this work shows that optimizing the interaction between photosensitizer and sacrificial electron donor is crucial for developing functional photocatalytic systems, especially those composed of Earth-abundant elements.
<figure><img src="image_10.png" /></figure>

# Experimental Section

## Chemicals and Materials

The synthesis of chloro iron(III) 5,10,15,20-tetra(4'-N,N,N-trimeth-
ylanilinium)porphyrin (Fe-p-TMA) hexafluorophosphate,[42] and of
Phen 1, Phen 11, Phen 12, Phen 13, Phen 14 and Phen 16[27,28,30]
have been described previously. 1,3-dimethyl-2-phenyl-2,3-dihy-
dro-1H-benzo[d]imidazole (BIH) was prepared according to liter-
ature procedure.[43] 2,2,2-trifluoroethanol (TFE, Romil Ltd, > 99.5 %),
and acetonitrile (ACN, Acros Organics, 99.9%) were used as
received. Argon (>99.998%) and CO₂ (> 99.7%) were from Air
Liquide.

Photochemical Setup

Argon or CO₂-saturated ACN solutions typically contained the catalyst (FepTMA, 2 µM), the photosensitizer (Phen, 200 µM), the sacrificial electron donor (BIH, 5 mM) and the external weak acid (TFE, 0.1 M). Solutions were saturated with gas for 20 minutes prior to irradiation. Light illumination was conducted at right angle in a closed 1×1 cm, 3.5 mL quartz suprasil cuvette (Hellma 117.100F-QS) equipped with a home-designed 20 mL headspace glassware. The light source was a Newport LCS-100 solar simulator, delivering 1 Sun irradiance thanks to an AM 1.5G standard filter, associated with a Schott GG400 longpass filter to only select the visible domain. Phen concentration was chosen to ensure an efficient quenching with the catalyst (bimolecular reaction) and to maximize light absorption.

Optical Measurements

UV-Visible absorption spectra were measured with a Cary 60 (Agilent Technologies) UV/Vis spectrophotometer. Emission quenching measurements on argon saturated solutions were conducted with a Cary Eclipse fluorescence spectrophotometer (Agilent Technologies), with the excitation wavelength set at 400 nm corresponding to the lower limit of the visible domain used for illumination in the catalytic process. Emission intensities used for the Stern-Volmer analysis were taken at the emission maximum of each Phen.

Reaction Products Analysis

Gaseous products in the photochemical cell headspace were
analyzed with an Agilent Technologies 7820 A gas chromatography
(GC) system equipped with a thermal conductivity detector and a
CP-CarboPlot P7 capillary column (27.46 m in length and 25 µm
internal diameter), with argon as carrier gas (9.5 mL min⁻¹ constant
flow, 0.5 bar constant pressure). Temperature was set to 150 °C for
the detector and to 34 °C for the oven. Injection was performed
with a 250 µL Hamilton gas-tight sample-lock syringe. CO and H₂
molar amounts were determined through the conversion of GC
peaks area integration into mole using individual calibration
curves.

Turnover Number Calculation

TON is defined as the number of catalytic cycles after a given
illumination time. Data points are the results of at least two
individual experiments and relative error on TON is ca. 5%,
corresponding to the size of the data points.

Acknowledgements

M.K., G.L., J.B. and M.R. acknowledge the MARS consortium (GRDF, GRTgaz and Teréga) for financial support. Partial financial support to M.R. from the Institut Universitaire de France (IUF) is gratefully acknowledged. B.G.M and G.M.M acknowledge support from the National Institute of General Medical Sciences (Award R35GM119702) of the National Institutes of Health.

Conflict of Interest

The authors declare no conflict of interest.

Data Availability Statement

The data that support the findings of this study are available
from the corresponding author upon reasonable request.

Keywords: carbon dioxide reduction · carbon monoxide · molecular catalysis · phenoxazine · photosensitizers

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Manuscript received: January 13, 2022

Revised manuscript received: March 8, 2022

Accepted manuscript online: March 11, 2022

Version of record online:

<figure><img src="image_12.png" /></figure>
<figure><img src="image_13.png" /></figure>

**Pumping iron:** The amount of carbon
monoxide produced from the photo-
chemical catalytic reduction of carbon
dioxide by an iron porphyrin catalyst
is limited by the electron transfer re-

generating the phenoxazine photo-
sensitizer and not by the driving force
of the electron transfer between the
excited sensitizer and the catalyst.

M. Kientz, Dr. G. Lowe, Dr. B. G.
McCarthy, Prof. Dr. G. M. Miyake,
Dr. J. Bonin*, Prof. Dr. M. Robert*

1 – 7

Phenoxazine-Sensitized CO₂-to-CO
Reduction with an Iron Porphyrin
Catalyst: A Redox Properties-
Catalytic Performance Study

<figure><img src="image_14.png" /></figure>