Photocatalytic CO2 reduction with aminoanthraquinone organic dyes - 2026.09.04 - 3

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• /opt/downloadPDF/chemwiki_pubstore/7e9875d2bdb7974d9ffecb4b0a29f8f5.pdf/Lei et al. - 2023 - Photocatalytic CO2 reduction with aminoanthraquinone organic dyes_6a9ab6d079463.pdf

Abstract Summary

This article describes a molecular photocatalytic system for the reduction of CO₂ to CO using an iron porphyrin catalyst together with aminoanthraquinone organic dyes as visible-light photosensitizers. The system operates in CO₂-saturated DMF with BIH as a sacrificial electron donor under visible irradiation. A series of six aminoanthraquinone dyes was compared, and the most active dye gave high activity together with very high CO selectivity. The work also presents mechanistic evidence that reductive quenching of the excited dye is the dominant photochemical entry step, and that reduced hydroxyanthrone-type dye species are important intermediates in delivering electrons to the iron catalyst.

Advances and Special Progress

A central advance is the use of simple aminoanthraquinone organic dyes as effective visible-light photosensitizers in a noble-metal-free molecular CO₂ reduction system. The article emphasizes that both the catalyst and the photosensitizer can reach high turnover numbers, which it states had not been achieved in current light-driven systems of this type.

The study also shows a clear structure–function relationship within a small, chemically related dye family. Changing substituents on the anthraquinone framework strongly changes photocatalytic activity. The article interprets this trend in terms of built-in donor–acceptor character within the dye, which promotes faster reductive quenching and better photocatalytic performance.

Another advance is the mechanistic analysis. The article combines fluorescence quenching, lifetime measurements, UV/Vis monitoring during photolysis, electrochemical measurements, isotopic labeling, DLS, and mercury poisoning tests. These data support a homogeneous molecular system and provide evidence for reduced dye-derived intermediates that participate in catalyst reduction.

The reported product selectivity is also notable. CO is the major product in all tested systems, and for most dyes the selectivity for CO is above 99%. The most active dye–catalyst combination gives especially high CO formation while strongly suppressing H₂ evolution.

Additional Remarks

The chemistry is relevant to solar-fuel research because CO₂-to-CO conversion stores reducing equivalents in a useful carbon-containing product. CO is an important two-electron reduction product and can serve as a feedstock for further chemical synthesis.

At the same time, this is a sacrificial photochemical system rather than a fully closed artificial photosynthetic cycle. BIH is consumed as the electron donor, so the system does not perform complete sunlight-driven fuel formation from only CO₂ and water. The solvent is DMF, not water, and the article does not report water compatibility as a feature of this system.

The work shows strong catalytic activity, but durability remains limited by catalyst decomposition. The article reports that the iron porphyrin catalyst decomposes faster than the anthraquinone dye under the tested conditions. Thus, although the system is chemically informative and highly active, long-term operational stability is still a limitation.

The mechanism is chemically elegant because the photosensitizer is not treated as a simple one-step electron shuttle. Instead, the article proposes sequential reduction and protonation chemistry of the anthraquinone scaffold, forming hydroxyanthrone-type intermediates that are then involved in catalyst reduction. This gives the study significance beyond performance alone, because it links dye structure to the detailed photochemical sequence.

Content of the Published Article in Detail

The molecular photocatalytic system contains three essential components: an iron porphyrin CO₂ reduction catalyst, an aminoanthraquinone dye photosensitizer, and BIH as the sacrificial electron donor. The reactions are carried out in CO₂-saturated DMF under white LED irradiation with wavelengths above 400 nm. Gas products in the headspace are analyzed by gas chromatography. CO is the major product, while H₂ formation is strongly suppressed.

Six aminoanthraquinone dyes, labeled 1–6 in the article, were compared. They absorb in the visible region, with maximum absorption bands from 478 to 592 nm. They also emit red fluorescence in the 600–700 nm region, with lifetimes in the nanosecond range. Electrochemical measurements show two reduction events for each dye. These photophysical and redox properties support their use as photosensitizers in CO₂ reduction.

Under a standard catalyst-focused condition of 20 μM photosensitizer, 0.6 μM FeTDHPP, and 60 mM BIH, all six dyes support CO formation, but with strongly different activities. The article reports that dyes containing amino and hydroxy substituents are substantially more active than others, and that dye 5 is the best performer in the series. Under photosensitizer-focused conditions of 5 μM photosensitizer, 20 μM FeTDHPP, and 60 mM BIH, dye 5 again gives the highest photosensitizer turnover number. The article also reports that when dye 5 and FeTDHPP are used at the same concentration, the system can achieve high turnover numbers for both components simultaneously, together with a CO quantum efficiency of 11.1% at 450 nm based on two photons per CO.

The article distinguishes observed data from mechanistic interpretation. Experimentally, fluorescence quenching and fluorescence lifetime measurements show rapid quenching of the excited photosensitizers by BIH, with quenching rate constants near the diffusion-controlled limit. Because there is strong overlap between dye absorption/emission and catalyst absorption, oxidative quenching by the catalyst could not be quantified accurately. However, the article reports no evidence for ground-state reaction between dye and BIH or between dye and FeTDHPP from UV/Vis and ¹H NMR data. Together with the much higher concentration of BIH than FeTDHPP, these results support a proposed reductive quenching pathway as the dominant mechanism.

In this proposed sequence, the dye first absorbs visible light to form an excited state. BIH then reduces the excited dye. The article further proposes that the anthraquinone scaffold undergoes sequential electron-transfer and proton-transfer steps, ultimately forming hydroxyanthrone-type species denoted PSH₂ and then PSH₂⁻. The article treats these as important intermediates in catalyst reduction. This interpretation is supported by UV/Vis studies of anthraquinone photolysis with BIH, where species assigned to protonated reduced anthraquinone intermediates appear over time, and by analogous spectral changes seen for dyes 1–6 during photocatalysis.

More specifically, the article assigns an early photoproduct of anthraquinone near 560 nm to AQH• rather than AQ•⁻ or AQ²⁻, based on comparison with known spectra. A later intermediate near 520 nm is assigned to AQH⁻. Continued reduction and protonation are proposed to generate 10-hydroxyanthrone-type species, described as AQH₂. A further photoproduct near 400–407 nm is tentatively assigned to a more reduced hydroxyanthrone species analogous to PSH₂⁻. The article supports the importance of this species by showing that when it is generated photochemically from dye 5 and BIH, subsequent addition of FeTDHPP rapidly gives CO and an Fe(I) spectral signature. In a control experiment performed before generating this species, no CO is detected after catalyst addition.

Electrochemical data are used to argue that the ordinary one- or two-electron reduced dye states are not reducing enough to generate the catalytically relevant Fe(0) state. The article states that CO₂ reduction by FeTDHPP occurs at an Fe(0) oxidation state at −1.55 V vs SCE. In contrast, the measured reduction potentials of the dyes are much more positive. Therefore, the article proposes that the stronger reductant is the hydroxyanthrone-derived PSH₂⁻ photoproduct. In situ square-wave voltammetry after irradiation shows new reduction waves at more negative than −1.90 V vs SCE for systems containing dyes 1–6 with BIH, which supports the presence of more strongly reducing photoproducts.

The catalyst redox sequence was followed by UV/Vis spectroscopy. Under white-light irradiation, the Fe(III) starting compound is converted rapidly to Fe(II), then to Fe(I), and the Fe(I) feature decreases during CO₂ reduction. The article states that this observation is consistent with a previously reported mechanism for iron porphyrin CO₂ reduction. When irradiation is restricted to wavelengths above 550 nm, the hydroxyanthrone-derived PSH₂⁻ pathway is shut down according to the article, and CO production becomes much slower. Under these conditions Fe(II) is the major observed catalyst species and Fe(I) accumulates to a lesser extent. Similar behavior is reported under 450 nm and 525 nm LED experiments. Irradiation of BIH and FeTDHPP without photosensitizer gives almost no Fe(I), only Fe(II). These observations support the article’s conclusion that BI• alone can reduce Fe(II) and Fe(I), but more slowly than the pathway involving PSH₂⁻.

The article also discusses the role of proton transfer. Proton transfer is built into the proposed dye reduction chemistry, where reduced anthraquinone states are protonated to form hydroxyanthrone-type intermediates. The catalyst cycle also requires protons in the reduction of CO₂ to CO. However, the detailed proton-transfer steps at the catalyst are not worked out beyond the proposed scheme. Competition with H₂ evolution is low for most dyes. Dye 6, which contains a strongly acidic sulfonyl group, gives somewhat lower CO selectivity and the highest H₂ selectivity in the series. The article proposes that this acidic group may promote proton transfer to the catalyst and thereby enhance H₂ formation.

Several control experiments support the molecular and homogeneous nature of the system. DLS detects no nanoparticles before or after catalysis. Mercury poisoning does not significantly change activity, arguing against amalgam-forming metal impurities as the source of catalysis. Isotopic labeling with ¹³CO₂ gives exclusively ¹³CO, confirming CO formation from CO₂. Stability tests in which fresh components are added after activity loss show that catalyst decomposition is more important than photosensitizer decomposition under the tested conditions.

Finally, the article correlates catalytic rate with reductive quenching rate across dyes 1–6. A generally linear trend is reported between the initial TOF for CO formation and the first reductive quenching rate constant. The article interprets this as evidence that faster reductive quenching promotes faster formation of the key reduced hydroxyanthrone-type intermediates and therefore higher catalytic activity.

Catalyst

The catalyst is FeTDHPP, an iron porphyrin complex. It is a molecular, porphyrinic CO₂ reduction catalyst used in homogeneous solution. The article identifies it as the CO₂ reduction catalyst throughout the photocatalytic system.

Its redox chemistry is central to function. UV/Vis experiments show conversion of Fe(III) to Fe(II) and then Fe(I) under irradiation in the catalytic mixture. The article states that catalysis proceeds through an Fe(0) oxidation state for CO₂ reduction. Electron transfer from reduced dye-derived intermediates is proposed to generate the required low-valent catalyst states.

The catalyst is highly selective for CO formation in this system, especially when paired with the better-performing aminoanthraquinone dyes. However, the article reports that the catalyst decomposes faster than the photosensitizer during extended irradiation, and this is identified as a major cause of activity loss.

Photosensitizer

The photosensitizers are aminoanthraquinone organic dyes, specifically six dyes labeled 1–6. They are molecular organic chromophores that absorb visible light and initiate the photochemical electron-transfer sequence. Dye 5 is the main and best-performing photosensitizer in the study.

These dyes show visible absorption maxima from 478 to 592 nm and emit in the 600–700 nm region. Their excited-state lifetimes are in the nanosecond range. Electrochemical measurements show two reduction events for each dye. The article uses these data to analyze how substituents alter light absorption, reduction potential, and catalytic behavior.

Mechanistically, the excited photosensitizer is proposed to undergo predominantly reductive quenching by BIH rather than oxidative quenching by the iron catalyst. The article supports this conclusion using fluorescence lifetime quenching, lack of ground-state reaction by UV/Vis and NMR, and the large excess of BIH relative to catalyst. The dye is then proposed to pass through reduced and protonated anthraquinone-derived intermediates, including PSH₂ and PSH₂⁻, before transferring electrons to the catalyst.

The substituent pattern is chemically important. The article proposes that internal donor–acceptor character within the anthraquinone framework improves activity. Dyes with amino and hydroxy substitution, especially dye 5, give the best results. The parent anthraquinone itself does not produce CO under the tested conditions, even though an AQH₂⁻-type species is observed. This shows that substituent effects are essential for productive catalysis in this dye family.

Investigation

	cat	cat conc [µM]	PS	PS conc [mM]	e-D	e-D conc [M]	.	solvent A	.	.	.	λexc [nm]	.	TON CO	.	.	TON CH4	TON H2	.	TON HCOOH	.	.
1.	FeTDHPP	0.6	1	0.02	BIH	0.06		DMF				>400		2395			not reported	not reported		not reported			"not reported" is not a number.
2.	FeTDHPP	20	1	0.005	BIH	0.06		DMF				>400		2011			not reported	not reported		not reported			"not reported" is not a number.
3.	FeTDHPP	20	1	0.02	BIH	0.06		DMF				450		not reported			not reported	not reported		not reported			"not reported" is not a number.
4.	FeTDHPP	0.6	2	0.02	BIH	0.06		DMF				>400		2738			not reported	not reported		not reported			"not reported" is not a number.
5.	FeTDHPP	20	2	0.005	BIH	0.06		DMF				>400		482			not reported	not reported		not reported			"not reported" is not a number.
6.	FeTDHPP	20	2	0.02	BIH	0.06		DMF				450		not reported			not reported	not reported		not reported			"not reported" is not a number.
7.	FeTDHPP	0.6	3	0.02	BIH	0.06		DMF				>400		3551			not reported	not reported		not reported			"not reported" is not a number.
8.	FeTDHPP	20	3	0.005	BIH	0.06		DMF				>400		1523			not reported	not reported		not reported			"not reported" is not a number.
9.	FeTDHPP	20	3	0.02	BIH	0.06		DMF				450		not reported			not reported	not reported		not reported			"not reported" is not a number.
10.	FeTDHPP	0.6	4	0.02	BIH	0.06		DMF				>400		8360			not reported	not reported		not reported			"not reported" is not a number.
11.	FeTDHPP	20	4	0.005	BIH	0.06		DMF				>400		2849			not reported	not reported		not reported			"not reported" is not a number.
12.	FeTDHPP	20	4	0.02	BIH	0.06		DMF				450		not reported			not reported	not reported		not reported			"not reported" is not a number.
13.	FeTDHPP	0.5	5	0.0005	BIH	0.06		DMF				>400		3174			not reported	not reported		not reported			"not reported" is not a number.
14.	FeTDHPP	0.5	5	0.02	BIH	0.06		DMF				>400		19158			not reported	not reported		not reported			"not reported" is not a number.
15.	FeTDHPP	0.6	5	0.02	BIH	0.06		DMF				>400		21616			not reported	not reported		not reported			"not reported" is not a number.
16.	FeTDHPP	10	5	0.005	BIH	0.06		DMF				>400		2576			not reported	not reported		not reported			"not reported" is not a number.
17.	FeTDHPP	10	5	0.01	BIH	0.06		DMF				>400		4028			not reported	not reported		not reported			"not reported" is not a number.
18.	FeTDHPP	10	5	0.02	BIH	0.06		DMF				>400		3942			not reported	not reported		not reported			"not reported" is not a number.
19.	FeTDHPP	1	5	0.001	BIH	0.06		DMF				>400		3587			not reported	not reported		not reported			"not reported" is not a number.
20.	FeTDHPP	1	5	0.02	BIH	0.06		DMF				>400		17020			not reported	not reported		not reported			"not reported" is not a number.
21.	FeTDHPP	20	5	0.005	BIH	0.06		DMF				>400		6012			not reported	not reported		not reported			"not reported" is not a number.
22.	FeTDHPP	20	5	0.01	BIH	0.06		DMF				>400		2325			not reported	not reported		not reported			"not reported" is not a number.
23.	FeTDHPP	20	5	0.02	BIH	0.06		DMF				450		not reported			not reported	not reported		not reported			"not reported" is not a number.
24.	FeTDHPP	20	5	0.02	BIH	0.06		DMF				>400		2134			not reported	not reported		not reported			"not reported" is not a number.
25.	FeTDHPP	2	5	0.002	BIH	0.06		DMF				>400		3817			not reported	not reported		not reported			"not reported" is not a number.
26.	FeTDHPP	2	5	0.005	BIH	0.06		DMF				>400		8780			not reported	not reported		not reported			"not reported" is not a number.
27.	FeTDHPP	2	5	0.015	BIH	0.06		DMF				>400		8772			not reported	not reported		not reported			"not reported" is not a number.
28.	FeTDHPP	2	5	0.01	BIH	0.06		DMF				>400		5593			not reported	not reported		not reported			"not reported" is not a number.
29.	FeTDHPP	2	5	0.02	BIH	0.06		DMF				>400		11250			not reported	not reported		not reported			"not reported" is not a number.
30.	FeTDHPP	50	5	0.02	BIH	0.06		DMF				>400		891			not reported	not reported		not reported			"not reported" is not a number.
31.	FeTDHPP	5	5	0.005	BIH	0.06		DMF				>400		4978			not reported	not reported		not reported			"not reported" is not a number.
32.	FeTDHPP	5	5	0.01	BIH	0.06		DMF				>400		4128			not reported	not reported		not reported			"not reported" is not a number.
33.	FeTDHPP	0.6	6	0.02	BIH	0.06		DMF				>400		907			not reported	not reported		not reported			"not reported" is not a number.
34.	FeTDHPP	20	6	0.005	BIH	0.06		DMF				>400		1183			not reported	not reported		not reported			"not reported" is not a number.
35.	FeTDHPP	20	6	0.02	BIH	0.06		DMF				450		not reported			not reported	not reported		not reported			"not reported" is not a number.

Investigation-Name: inv0ExportRefresh