Photocatalytic CO2 reduction with aminoanthraquinone organic dyes - 2026.09.04 - 4

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Abstract Summary

This study examines a molecular photocatalytic system for the light-driven reduction of CO₂ to CO using aminoanthraquinone organic dyes as photosensitizers and an iron porphyrin as the CO₂-reduction catalyst. The system operates in CO₂-saturated DMF under visible light with BIH as a sacrificial electron donor. Among six related dyes, the brominated hydroxy-aminoanthraquinone photosensitizer labeled 5 gave the highest activity. The article reports high turnover numbers for both the catalyst and the photosensitizer, while maintaining very high selectivity for CO over H₂. Mechanistic experiments support a reductive quenching pathway and suggest that hydroxyanthrone-type reduced dye intermediates participate in electron transfer to the iron catalyst.

Advances and Special Progress

A central advance is the use of simple aminoanthraquinone organic dyes as effective visible-light absorbers in a noble-metal-free molecular CO₂ reduction system. The article emphasizes that this system achieves high turnover numbers for both the photosensitizer and the catalyst, which it identifies as an unmet challenge in current light-driven systems.

The structure-function comparison across dyes 1-6 is another important advance. By varying substituents on the anthraquinone framework, the study links photocatalytic performance to built-in donor-acceptor character within the dye. The best-performing dye, 5, combines substituents with distinct electronic effects and gives the highest reported activity in the series.

The work also provides mechanistic progress. Spectroscopic, electrochemical, quenching, isotopic labeling, DLS, and mercury-poisoning experiments are used to support a homogeneous molecular system and a proposed reductive quenching pathway. The data further support the formation of reduced and protonated dye-derived intermediates that can reduce the iron catalyst to catalytically relevant states.

The reported CO selectivity is also notable. Most systems based on dyes 1-5 gave CO selectivities above 99%, showing efficient suppression of H₂ evolution under the reported conditions.

Additional Remarks

The chemistry is significant because photochemical conversion of CO₂ into CO stores solar energy in a chemically useful reduced carbon product. CO is also a valuable intermediate for further chemical synthesis. This makes selective CO₂-to-CO photoreduction a relevant model transformation in artificial photosynthesis.

At the same time, the system remains a sacrificial photochemical system rather than a complete solar-fuel cycle. BIH is consumed as the electron donor, so the setup does not perform the oxidative half-reaction of water splitting. The solvent is DMF, and water compatibility was not reported for this photocatalytic system.

The article also identifies catalyst stability as a limitation. Component-addition experiments indicate that the iron porphyrin catalyst decomposes faster than the anthraquinone dye during catalysis. Thus, strong activity does not remove the durability challenge.

Competition from H₂ formation is generally suppressed, but not absent. The sulfonyl-containing dye 6 gave lower CO selectivity than the other dyes in the series, which the article associates with enhanced proton transfer to the catalyst and increased H₂ generation.

Content of the Published Article in Detail

The reported photocatalytic system contains three essential molecular components: an aminoanthraquinone dye photosensitizer, the iron porphyrin catalyst FeTDHPP, and BIH as the sacrificial electron donor. Experiments were carried out in CO₂-saturated DMF under irradiation with white LED light of λ > 400 nm. Product gases were analyzed by gas chromatography. CO was the major product, while H₂ formation was strongly suppressed in most cases.

The article compares six dyes, labeled 1-6. These dyes absorb strongly in the visible region, with maximum absorption bands ranging from 478 to 592 nm. They also emit in the red region, with fluorescence lifetimes in the nanosecond range. Electrochemical studies show two reduction events for each dye. These photophysical and redox properties are presented as the basis for using the dyes as molecular photosensitizers in CO₂ reduction.

For catalytic testing, two types of performance evaluation were highlighted. In one, catalyst turnover numbers were measured using 20 μM photosensitizer, 0.6 μM FeTDHPP, and 60 mM BIH. Under these conditions, dye 5 gave the best catalytic performance, with TON_Fe = 21,616 ± 2351 for CO and CO selectivity greater than 99.9%. In another set of conditions designed to evaluate photosensitizer performance, 5 μM photosensitizer, 20 μM FeTDHPP, and 60 mM BIH were used. Under those conditions, dye 5 gave TON_PS = 6012 ± 606. The article further reports that when dye 5 and FeTDHPP were used at the same concentration, the system achieved TON = 4978 ± 326 and a CO quantum efficiency of 11.1% ± 0.9% at 450 nm.

Mechanistically, the article argues that two broad quenching pathways are possible in such systems: reductive quenching of the excited photosensitizer by BIH, and oxidative quenching of the excited photosensitizer by the catalyst. The data support reductive quenching as the dominant pathway. Fluorescence lifetime quenching by BIH gave quenching rate constants near the diffusion-controlled limit, above 10⁹ M⁻¹ s⁻¹. By contrast, oxidative quenching constants with the catalyst could not be measured accurately because the dye and catalyst strongly overlap spectrally. The article also notes that the BIH concentration in catalysis is more than 3000 times greater than the FeTDHPP concentration in the main catalytic conditions, which further supports reductive quenching.

Ground-state interactions were examined by UV/Vis and ¹H NMR studies. These experiments indicated no reaction between the dyes and BIH or FeTDHPP before irradiation, ruling out a static quenching pathway according to the article.

To understand the photochemical sequence, the article studied anthraquinone itself as a model. Upon irradiation of AQ in the presence of BIH, UV/Vis spectroscopy showed rapid formation of a species near 560 nm, assigned to a proton-coupled one-electron reduction product AQH•. Continued irradiation produced a species near 520 nm, assigned to AQH⁻. The article then proposes further protonation to form a hydroxyanthrone species AQH₂. Additional spectroscopic evidence suggested formation of another photoproduct near 400-407 nm during irradiation. By analogy with the dye systems, and based on follow-up experiments, this ~400 nm species was tentatively assigned as a reduced hydroxyanthrone-type intermediate, PSH₂⁻.

The mechanistic proposal is that after the dye absorbs light, BIH reductively quenches the excited dye. Through sequential electron-transfer and proton-transfer steps, the dye is converted into reduced and protonated intermediates, culminating in PSH₂ and then photoexcited PSH₂*; reductive quenching of PSH₂* gives PSH₂⁻. The article proposes that this PSH₂⁻ intermediate is a key strong reductant that transfers electrons to the iron catalyst.

Electrochemical reasoning is central here. The article states that CO₂ reduction by FeTDHPP occurs at an Fe(0) oxidation state at −1.55 V vs SCE. The directly reduced dye species PS⁻, PS²⁻, and their protonated forms were considered insufficiently reducing for this step. In situ square-wave voltammetry after photolysis showed new reduction waves at potentials more negative than −1.90 V vs SCE, supporting the proposal that the photogenerated PSH₂⁻ species is sufficiently reducing to generate the required Fe(0) intermediate.

The reduction sequence of the catalyst was monitored by UV/Vis spectroscopy. Under white LED irradiation, the Fe(III) catalyst was reported to convert rapidly to Fe(II), then to Fe(I), and the Fe(I) species decreased during ongoing CO₂ reduction. The article interprets this as consistent with catalytically relevant stepwise reduction of the iron porphyrin.

The role of BIH extends beyond the initial reductive quenching event. A peak attributed to BI• was observed electrochemically after irradiation. The article concludes that BIH donates two electrons in the overall CO₂ reduction process under low-BIH conditions. However, the article also argues that direct reduction of the catalyst by BI• is slower than the pathway involving PSH₂⁻. This conclusion is supported by irradiation experiments at λ > 550 nm, which suppress formation of PSH₂⁻. Under these conditions, CO production became much slower and Fe(II) accumulated as the major catalyst-derived species, with less Fe(I) than under broadband visible irradiation.

Several control experiments support the mechanistic interpretation and the homogeneous nature of the system. DLS measurements detected no nanoparticles before or after catalysis. Mercury poisoning had little effect on activity, arguing against catalysis by amalgam-forming metal impurities. Isotopic labeling under ¹³CO₂ gave exclusive ¹³CO, showing that CO originates from CO₂.

The article also compares dye structures. Dyes containing amino and hydroxyl substituents generally outperformed less favorably substituted analogues. The article interprets this in terms of internal donor-acceptor design on the anthraquinone framework, which promotes faster reductive quenching and higher catalytic rates. A generally linear relationship was reported between the initial TOF for CO production and the first reductive quenching rate constant across dyes 1-6.

Stability tests showed that adding more BIH alone after rate decay did not restore activity, implying decomposition of the photosensitizer or catalyst rather than mere donor depletion. Adding catalyst and BIH restored significantly more activity than adding photosensitizer and BIH. The article therefore concludes that the iron porphyrin catalyst decomposes faster than the anthraquinone dye during photocatalysis.

Catalyst

The catalyst is FeTDHPP, an iron porphyrin complex used as the molecular CO₂-reduction catalyst. It is a homogeneous iron porphyrin system and serves as the site at which CO₂ is reduced to CO.

The article discusses catalytically relevant Fe(III), Fe(II), Fe(I), and Fe(0) states. UV/Vis monitoring under irradiation showed conversion of Fe(III) to Fe(II) and then Fe(I). The article states that CO₂ reduction occurs at an Fe(0) oxidation state. Thus, the catalyst functions through sequential reduction of the iron center.

The catalyst gives high CO selectivity in this system, especially with the better-performing dyes. However, stability experiments indicate that FeTDHPP decomposes faster than the anthraquinone photosensitizer during catalysis, making catalyst durability a major limitation under the reported conditions.

Photosensitizer

The photosensitizers are a series of six aminoanthraquinone organic dyes labeled 1-6. They are molecular organic dyes that absorb visible light and initiate the photochemical electron-transfer sequence. The main photosensitizer in the highest-performing system is 5.

These dyes show visible absorption maxima from 478 to 592 nm and fluorescence in the 600-700 nm region. Their lifetimes are reported as 0.66-1.02 ns. Electrochemical studies showed two reduction events for each dye, supporting their ability to participate in sequential photoredox chemistry.

The article proposes that the dyes operate mainly through reductive quenching. After excitation, BIH quenches the excited dye. The dye then undergoes further electron-transfer and proton-transfer chemistry to form reduced hydroxyanthrone-type intermediates such as PSH₂ and the proposed strongly reducing PSH₂⁻. This latter species is presented as a key reductant for the iron catalyst.

Among the six dyes, 5 gave the highest activity. The article attributes the superior performance of dyes such as 4 and 5 to substituent-controlled donor-acceptor character on the anthraquinone scaffold. The dye series was also used to correlate reductive quenching kinetics with catalytic CO formation rates.

The photosensitizer is more durable than the catalyst under the tested conditions, but light competition from decomposed species was suggested to limit complete recovery of activity after component re-addition experiments.

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		5258			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.

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