
==== Front
iScience
iScience
iScience
2589-0042
Elsevier

S2589-0042(24)01919-9
10.1016/j.isci.2024.110694
110694
Perspective
Functional molecular models of photosynthesis
Fukuzumi Shunichi fukuzumi@chem.eng.osaka-u.ac.jp
12∗
Lee Yong-Min yomlee@ewha.ac.kr
13∗∗
Nam Wonwoo wwnam@ewha.ac.kr
14∗∗∗
1 Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea
2 Department of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
3 Research Institute for Basic Sciences, Ewha Womans University, Seoul 03760, Korea
4 Henan Key Laboratory of Function-Oriented Porous Materials, College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471934, China
∗ Corresponding author fukuzumi@chem.eng.osaka-u.ac.jp
∗∗ Corresponding author yomlee@ewha.ac.kr
∗∗∗ Corresponding author wwnam@ewha.ac.kr
08 8 2024
20 9 2024
08 8 2024
27 9 110694© 2024 The Author(s)
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
Summary

This perspective focuses on functional models of photosynthesis to achieve molecular photocatalytic systems that mimic photosystems I and II (PSI and PSII). A long-lived and high-energy electron-transfer state of 9-mesityl-10-methylacridinium ion (Acr+-Mes) has been attained as a simple and useful model of the photosynthetic reaction center. Acr+-Mes has been used as an effective photoredox catalyst for photocatalytic hydrogen evolution and regioselective reduction of NAD(P)+ from plastoquinone analogs as a molecular functional model of PSI. A functional molecular model system to mimic the function of PSII has also been developed to oxidize water by plastoquinone analogs to produce O2 and plastoquinol analogs. The PSI molecular models have finally been integrated with the PSII molecular models to achieve production of a solar fuel (hydrogen) and NAD(P)H and its analogs from water by use of solar energy as a molecular artificial photosynthesis.

Graphical abstract

Chemical engineering; Chemistry; Natural sciences

Subject areas

Chemical engineering
Chemistry
Natural sciences
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pmcIntroduction

Increasing extraction and consumption of fossil fuels, such as coal, natural gas, and oil, to meet our current human energy demand have led to a significant increase in CO2 emission, which has already caused serious climate change.1,2 Since fossil fuels are the products of photosynthesis, sustainable solar fuels should be produced by developing artificial photosynthesis using solar energy.3,4,5 Artificial photosynthesis, in broad terms, is the process of converting solar energy into useful fuels for storage and mobile use, as photosynthesis in plants and algae does. Solar-driven water splitting to produce molecular hydrogen (H2) as a solar fuel has been achieved in relatively simple systems by employing particulate semiconductor materials as photocatalysts.6,7,8,9 However, the energy conversion efficiency needs to be significantly improved for practical use as an alternative to fossil fuels. In such heterogeneous catalytic systems, regioselective nicotinamide adenine dinucleotide phosphate (NADP+) reduction by H2O to form NAD(P)H like natural photosynthesis would be extremely difficult to achieve. In contrast to heterogeneous systems, molecular replication of the natural photosynthetic processes in homogeneous systems would mark a significant achievement in the production of clean energy while also reducing CO2 emission to the atmosphere, providing more mechanistic insights by detecting reactive intermediates. Photosynthesis is composed of five units: (1) the light–harvesting (LH) component, (2) the charge–separation (CS) component, (3) the catalytic component for the reduction of NADP+, (4) the catalytic component for water oxidation, and (5) the catalytic component for CO2 fixation. Molecular-based functional models of these five units occurring in photosynthesis have attracted much attention.

Two photosystems, photosystems I and II (PSI and PSII), both of which contain the photosynthetic reaction centers (PRC), are required for oxygenic photosynthesis, where multi-step CS processes proceed.10,11,12 In PSII, multi-step CS at the PRC results in the oxidation of water (H2O) to produce dioxygen (O2) at the oxygen evolving complex (OEC), accompanied by the two-electron/two-proton reduction of PQ to PQH2.10,11 The multi-step CS at the PRC in PSI results in two-electron/two-proton oxidation of plastoquinol (PQH2) to produce plastoquinone (PQ), accompanied by the two-electron/one-proton reduction of NADP+ to produce the dihydro form (NADPH), which acts as an electron and proton source for CO2 fixation to produce carbohydrates in the Calvin cycle.13 The accumulated products of photosynthesis were converted to fossil fuels over billions of years. This perspective focuses on molecular artificial photosynthesis to achieve molecular photocatalytic systems that mimic catalytic functions of PSI and PSII. First, photosynthetic reaction center models are discussed and then molecular models of photosystems I and II are followed to discuss the catalytic mechanisms. Finally, functional models of PSI and PSII are combined to achieve production of H2 and NAD(P)H from water as molecular-based artificial photosynthesis, which provides promising perspective for low cost production of sustainable solar fuels by combining water oxidation and CO2 reduction using solar energy.

Photosynthetic reaction center models

Extensive efforts have been made to generate molecular PRCs composed of electron donors, electron acceptors, and photosensitizers over decades.14,15,16,17,18,19,20,21 Among many PRC model compounds, a ferrocene-(zinc porphyrin)3-fullerene pentad [Fc-(ZnP)3-C60] exhibits the best charge separation performance to mimic multi-step electron-transfer processes at the PRC as shown in Figure 1, where fullerene (C60) as the terminal electron acceptor and ferrocene (Fc) as the terminal electron donor are tethered at the both ends of the zinc porphyrin trimer [(ZnP)3] with the edge-to-edge distance (Ree) of 46.9 Å.22 The final CS lifetime was determined to be 0.53 s by monitoring the decay of the CS state with use of electron paramagnetic resonance (EPR) spectroscopy in frozen benzonitrile (PhCN), which was similar to that at the native PRC for the first time, with a high quantum yield of the CS state (Φ = 0.83).22 The 1st-order kinetics for the decay of the CS state through the intramolecular back electron transfer in frozen PhCN was changed to the 2nd-order kinetics because of the bimolecular back electron transfer between two molecules of CS state in PhCN at 298 K.22 The 1st-order decay of the CS state was much slower than the 2nd-order bimolecular decay of the CS state because of the much longer distance between the C60·– and the Fc+ moiety in the same pentad molecule than that between two CS molecules that are close to each other.22,23 The 2nd-order bimolecular decay rate constant of two CS molecules in PhCN (2.7 × 109 M−1 s−1) was close to the diffusion-limited value.22,23Figure 1 A pentad molecule, Fc-(ZnP)3-C60, to attain the long-lived CS state via multi-step photoinduced electron transfer; Ar = 3,5-tBu2C6H3

Reprinted with permission from ref.22 Copyright 2004, John Wiley and Sons.

As mentioned previously, multi-step electron-transfer processes resulted in the generation of a long-lifetime CS state because of an increase in the CS distance as the case of the native PRC. Each electron-transfer step should be exergonic (downhill) and therefore multi-step electron-transfer processes result in a large energy loss, which compensates for a long lifetime of the CS state.22,23,24 Thus, the energy of the final CS state of Fc+-(ZnP)3-C60·– (1.03 eV) is lower than the initial excited state energy of ZnP (1.50 eV). In such a case, the reduction potential of Fc+-(ZnP)3-C60·– (Ered vs. SCE = 0.37 V for Fc+ moiety) is not high enough to oxidize water, although the reducing ability of Fc+-(ZnP)3-C60·– (Eox vs. SCE = −0.67 V for C60·– moiety) is sufficient to reduce H2O.22

PSI and PSII are used in natural photosynthesis to obtain the high oxidizing ability to oxidize water to evolve O2 as well as the high reducing ability to reduce NADP+ to NADPH using two PRCs.10 It seems almost impossible to design and synthesize a unified molecular system to combine PSI and PSII. Thus, it is highly desired to design and synthesize simple molecular electron donor-acceptor dyads, which undergo rapid photoinduced charge separation to obtain long-lived CS states that are capable of oxidizing and reducing water at the same time.

In 2004, 9-mesityl-10-methylacridinium ion (Acr+-Mes), which is an organic electron donor-acceptor dyad, was reported to exhibit an extremely long lifetime with strong oxidizing power as well as strong reducing power.25 In the X-ray structure of Acr+-Mes (Figure 2A), the mesityl moiety (electron donor) is orthogonally connected at the 9-position of the acridinium ion (electron acceptor) and the dihedral angle between the planes of two aromatic rings in Acr+ and Mes moieties is nearly 90°.25 This indicates that there is no orbital interaction between the mesityl (electron donor) and acridinium (electron acceptor) moieties, while the absorption spectrum of Acr+–Mes is composed of the superposition of those of mesitylene and 10-methylacridinium ion.25 The excitation energy of Acr+-Mes was determined from the absorption maximum (430 nm) and fluorescence maximum (501 nm) to be 2.68 eV.25 HOMO and LUMO orbitals of Acr+-Mes (Figures 2B and 2C) are located on the mesitylene and acridinium moieties, respectively.25 The high energy of the electron-transfer state (Acr·-Mes·+; 2.37 eV) was determined by the one-electron oxidation and reduction potentials of Acr+-Mes (Eox vs. SCE = 1.88 V and Ered vs. SCE = −0.49 V, respectively) in PhCN.25 the ET state. The long-lived ET state of Acr+-Mes was questioned by Benniston et al. who reported that the triplet energy (1.96 eV) of the Acr+ moiety of Acr+-Mes is lower than the ET state.26 However, such a locally excited triplet state of the Acr+ moiety of Acr+–Mes would never be able to oxidize electron donors whose one-electron oxidation potentials are higher than the one-electron reduction potential of the locally excited triplet state of the Acr+ moiety of Acr+-Mes (Ered vs. SCE = 1.47 V).27 By the same token, the locally excited triplet state of the Acr+ moiety of Acr+-Mes would never be able to reduce electron acceptors whose one-electron reduction potentials are lower than the one-electron oxidation potential of the triplet locally excited state of Acr+-Mes (Eox vs. SCE 0.08 V vs. SCE).27 However, the photoexcitation of Acr+-Mes with electron donors and/or acceptors resulted in electron-transfer oxidation of a variety of electron donors that have much higher Eox values (e.g., 2.0 V) than 1.53 V, and also ET reduction of electron acceptors that have much lower one-electron reduction potentials than 0.08 V (e.g., −0.4 V) as clearly observed by transient absorption measurements.27 The triplet energy of 1.96 eV (vide supra) was reported to result from 9-mesitylacridine that contained as an impurity in 9-meistyl-10-methylacridinium ion.28Figure 2 X-ray structure, and photoinduced electron transfer and back electron transfer of Acr+-Mes

(A) Crystal structure, (B) HOMO (highest occupied molecular orbital) and (C) LUMO (lowest unoccupied molecular orbital) of Acr+-Mes.

(D) Eyring plot of ln(kBET/T) vs. T−1 for intramolecular back electron transfer from the Acr· moiety to the Mes·+ moiety of Acr·-Mes·+. Red and blue parts in (A) correspond to the mesityl and acridinium ion moieties, respectively. Reprinted with permission from ref.25 Copyright 2004, American Chemical Society.

The formation of the electron-transfer state of Acr+-Mes was observed by using laser-induced transient absorption measurements via electron transfer from the Mes moiety to the singlet excited state of the Acr+ moiety upon laser excitation. The singlet electron-transfer state [1(Acr·-Mes·+)] produced first was converted to the triplet state [3(Acr·-Mes·+)] by the intersystem crossing with nearly 100% quantum yield.27 The lifetime of 1(Acr·-Mes·+) (τ = 7.7 ns) is too short for intermolecular ET reactions with concentrations less than 0.1 mM when the maximum quenching of 1(Acr·-Mes·+) with the diffusion-limited rate constant (1010 M−1 s−1) is less than 1%.27 The decay rate of 3(Acr·-Mes·+) obeyed 2nd-order kinetics, because the intramolecular back electron transfer from the Acr· component to the Mes·+ component in the triplet state [3(Acr·-Mes·+)] is much slower than the bimolecular back electron transfer between two molecules of 3(Acr·-Mes·+) species. The 2nd-order decay rate constant of 3(Acr·-Mes·+) species was determined to be nearly diffusion-limited,25,27 as observed for the decay of the CS state of a tetrad, Fc+-ZnP-H2P-C60·–.22,23 At higher temperatures in PhCN, however, the 3(Acr·-Mes·+) decay kinetics was switched over from the 2nd-order to the 1st-order kinetics (Figure 2D) because of the much larger activation enthalpy for the 1st-order decay by the intramolecular back electron transfer than the 2nd-order decay via the diffusion.25

The long lifetime of 3(Acr·-Mes·+) has enabled the observation of structural changes from the ground state of Acr+-Mes(ClO4−) to triplet ET state [3(Acr·-Mes·+)] in the crystal upon photoexcitation using laser pump, which was analyzed by X-ray probe crystallography.29 Upon photoexcitation of Acr+-Mes, the N-methyl moiety of Acr+ was bent to that of Acr· with the concomitant rotation and movement of the ClO4− anion by the electrostatic interaction with the Mes·+ part.29 Such bending of the N-methyl moiety and the movement of anion result from the formation of the ET state of Acr+-Mes upon photoillumination.29

Functional models of PSI

Since Acr+-Mes affords a long-lived triplet electron-transfer state [3(Acr·-Mes·+)], which has the strong reducing ability as well as the oxidizing ability (vide supra), with nearly 100% yield upon photoexcitation, Acr+-Mes has been quite often employed as an effective photoredox catalyst to enable various chemical transformations.30,31,32,33,34,35 At the PRC of PSI, the charge separation enables the two-electron/one-proton reduction of NADP+ to NADPH regioselectively, whereas plastoquinol (PQH2) is oxidized to plastoquinone (PQ).10 Various electron and proton donors have been used for the photocatalytic proton reduction to produce molecular hydrogen (H2).36,37 H2 is feasible to reduce NAD(P)+ by catalysis of the iridium(III) complex to regioselectively generate NAD(P)H.37,38,39,40 Thus, the combination of photocatalytic reduction of proton to H2 and the catalytic reduction of NAD(P)+ by H2 provides PSI functional models. CO2 can also be reduced by H2 in the presence of transition-metal catalysts to yield formic acid, methanol, and methane as alternative H2 carrier.41,42

Acr+-Mes was used as an efficient photocatalyst for H2 evolution in the presence of a hydrogen evolution catalyst (HEC), such as polyvinylpyrrolidone-protected Pt nanoclusters (Pt-PVP), and an electron/proton donor, such as NADH, without using an electron mediator (e.g., methyl viologen MV2+]).43 Surprisingly, the H2 evolution rate without MV2+ is 300-fold greater than that with MV2+ in the photocatalytic system.43 The faster hydrogen evolution results from the faster electron transfer from Acr·-Mes to Pt-PVP than the electron transfer from MV·+ to Pt-PVP.43 The quantum yield of H2 evolution was determined to be 52% with a high hydrogen yield (95%), accompanied by the oxidation of NADH to NAD+ using Acr+-Mes as a photoredox catalyst, Pt-PVP as a HEC, and NADH as a reductant in phthalic acid buffer/MeCN (1:1) at 298 K43

The photocatalytic production of H2 by the two-electron reduction of protons with one photon is made possible by photoinduced NADH oxidation by Acr+-Mes with one photon as shown in Scheme 1, where the triplet electron transfer state [3(Acr·-Mes·+)] is formed upon photoexcitation of Acr+-Mes and the intersystem crossing, followed by the electron transfer from NADH to the Mes·+ moiety of 3(Acr·-Mes·+) to form NADH·+ and Acr·-Mes.43,44 NADH·+ undergoes deprotonation to produce NAD·, which is a much stronger reductant than NADH, thereby, NAD· is readily oxidized by the Acr+ moiety to produce NAD+ and Acr·-Mes.43,44 Therefore, in the presence of one equivalent of NADH, the one photon absorption by Acr+-Mes forms two equivalents of Acr·-Mes, which could produce one equivalent of H2 with two protons by injecting two electrons into Pt-PVP.43,44 The overall stoichiometry is given by Equation 1. Besides acridinium ion derivatives, a quinolinium ion derivative (2-phenyl-4-(1-naphthyl)quinolinium ion) has also been shown to act as an efficient photoredox catalyst for photocatalytic H2 evolution with NADH.45,46(Equation 1) NADH +2Acr+−Mes →hν NAD+ +2Acr·-Mes+H+

Scheme 1 Photoinduced two-electron oxidation of NADH to NAD+ by the Acr+ moiety with one photon under visible light irradiation (λ > 390 nm)

Reprinted with permission from ref.43 Copyright 2007, Royal Society of Chemistry.

NADH and Pt-PVP in the photocatalytic hydrogen evolution could be replaced by hydroquinone derivatives (X-QH2; e.g., hydroquinone [QH2], tetrachlorohydroquinone [Cl4QH2], and tetramethylhydroquinone [Me4QH2]) as a plastoquinol analog, and CoIII(dmgH)2pyCl (py = pyridine; dmgH– = dimethylglyoximate) as a HEC to achieve a molecular functional model of PSI (Scheme 2).47 Visible light photoillumination (λ > 420 nm) of a solution containing CoIII(dmgH)2pyCl and Acr+-Mes in the presence of QH2 and H2O under an Ar atmosphere in MeCN resulted in the production of hydrogen (100% yield) with the concomitant formation of p-benzoquinone (Q) by the oxidation of QH2.47 The photocatalytic mechanism was elucidated by using laser transient absorption measurements (Scheme 3).47 Laser excitation of a deaerated MeCN solution of Acr+-Mes resulted in the formation of the triplet electron-transfer state [3(Acr·-Mes·+)] via intramolecular electron transfer from the Mes moiety to the singlet excited state of the Acr+ moiety, followed by intersystem crossing. Electron transfer from Cl4QH2 to the Mes·+ moiety of 3(Acr·-Mes·+) with the rate constant of ket = 7.2 × 107 M−1 s−1 resulted in the formation of Cl4QH2·+ and Acr·-Mes at 298 K.47 The electron transfer reaction is exergonic because the one-electron reduction potential (Ered vs. SCE) of the Mes·+ moiety of 3(Acr·-Mes·+) and one-electron oxidation peak potential (Epa vs. SCE) of Cl4QH2 are 2.06 V and 1.05 V, respectively, indicating that the Ered value of the Mes·+ moiety of 3(Acr·-Mes·+) is higher than the Epa value of Cl4QH2.47 Since Ered value of CoIII(dmgH)2pyCl and Eox value of the Acr· moiety of Acr·-Mes are −0.16 V and −0.57 V vs. SCE, respectively, the exergonic electron-transfer reaction (ket = 4.2 × 107 M−1 s−1) of Acr·-Mes and CoIII(dmgH)2pyCl also takes place to form [CoII(dmgH)2pyCl]– with the concomitant reproduction of Acr+-Mes at 298 K.47 Cl4QH2·+ deprotonates to produce a semiquinone radical species (Cl4QH·). Then, [CoII(dmgH)2pyCl]– abstracts a hydrogen atom from Cl4QH· to form [CoIII(dmgH)2pyCl(H)]–, which is a CoIII-hydride complex. The reaction of [CoIII(dmgH)2pyCl(H)]– with H+ produces H2 molecule with the concomitant reproduction of CoIII(dmgH)2pyCl species (Scheme 3).47Scheme 2 Photocatalytic generation of H2 by plastoquinol analogs

(A) Chemical structures of plastoquinol analogs (X-QH2).

(B) Photocatalytic generation of H2 molecule by plastoquinol analogs with CoIII(dmgH)2pyCl and Acr+-Mes upon photoillumination (λ > 420 nm). Reprinted with permission from ref.47 Copyright 2020, American Chemical Society.

Scheme 3 Proposed mechanism of photocatalytic H2 evolution by hydroquinone (plastoquinol model) with CoIII(dmgH)2pyCl and Acr+-Mes upon photoillumination (λ > 420 nm)

Reprinted with permission from ref.47 Copyright 2020, American Chemical Society.

The photocatalytic system of H2 evolution from X-QH2 (Scheme 2) can be applied for the photocatalytic reduction of NAD+ instead of H+ to regioselectively form NADH by tetrachlorohydroquinone (Cl4QH2), which is a plastoquinol model, as the stoichiometry of the reduction of NADP+ by plastoquinol in PSI.48 As a result of light irradiation of a toluene/TFE/borate buffer (20:1:19, pH 7.0) solution of Cl4QH2, NAD+, CoIII(dmgH)2pyCl and Acr+-Mes for 4 h, NADH was produced with a yield of approximately 90% based on the initially used concentration of Cl4QH2.48 The turnover number (TON) of NADH formation in the reduction of NAD+ by Cl4QH2 after 5 h of light irradiation was 52 and 26 based on concentrations of CoIII(dmgH)2pyCl and Acr+-Mes, respectively.48 Cl4QH2 can be replaced by hydroquinone (QH2) and durohydroquinone (Me4QH2) for the photocatalytic NAD+ reduction to NADH.48 As shown in Scheme 4, NADH is produced regioselectively in the photocatalytic reduction of NAD+ by a hydroquinone derivative (X-QH2) as a PSI model reaction.48 Photoexcitation of the visible absorption band of Acr+-Mes results in formation of the triplet electron-transfer state (Acr·-Mes·+), which has both strong reducing and oxidizing capabilities with an extremely long lifetime.25,27,33,34 Then, exergonic electron transfer from X-QH2 (Epa [anodic peak potential] vs. SCE = 0.85–1.05 V) to the Mes·+ moiety of Acr·–Mes·+ (Ered vs. SCE = 2.06 V) occurs rapidly to form X-QH2·+ and Acr·-Mes (Eox vs. SCE = −0.57 V), followed by electron transfer from Acr·-Me to CoIII(dmgH)2pyCl (Ered vs. SCE = −0.16 V) to generate [CoII(dmgH)2pyCl]–, accompanied by regeneration of Acr+-Mes.48 X-QH2·+ undergoes deprotonation to afford the semiquinone radical (X-QH·) from which H atom is transferred to [CoII(dmgH)2pyCl]– to form the Co(III)-hydride complex ([CoIII(H)(dmgH)2pyCl]–) and X-Q.48 Then, hydride transfer occurs predominantly from [CoIII(H)(dmgH)2pyCl]– to NAD+ rather than H+, producing NADH regioselectively.48 The regioselectivity arises due to a kinetically favorable transition state, which has the six-membered ring in which the hydride ligand interacts with the C4-position of NAD+ regioselectively.49Scheme 4 Proposed mechanism for the photocatalytic NAD+ reduction by X-QH2 with CoIII(dmgH)2pyCl and Acr+-Mes to form NADH

Reprinted with permission from ref.48 Copyright 2024, American Chemical Society.

Molecular models of PSII

Plastoquinol (PQH2) that is used as a reductant in PSI is produced by the two-electron/two-proton reduction of plastoquinone (PQ), accompanied by the four-electron/four-proton oxidation of H2O in PSII, as given by Equation 2.10(Equation 2) 2H2O+2PQ→O2+2PQH2

The first functional model of PSII was reported for the photocatalytic reduction of plastoquinone analogs (i.e., p-benzoquinones [X-Q]) with the concomitant four-electron/four-proton oxidation of water to evolve O2 (Equation 3) in the presence of [FeII(N4Py)]2+ (N4Py = N,N-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)- methanamine) as a water oxidation catalyst (WOC).50(Equation 3) 2H2O+2X−Q→hνO2+2X−QH2

Upon white light photoirradiation of an MeCN solution of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) in the presence of [FeII(N4Py)]2+ and water, the oxidation of H2O evolved O2 with virtually 100% yield, whereas H2O reduced DDQ to generate DDQH2 (Figure 3).50 Longer photoillumination time decreased the yield of O2 product because DDQH2 was re-oxidized by O2 to yield H2O2 (Equation 4).50 DDQ can be replaced by p-benzoquinone (BQ), tetrachloro-1,4-benzoquinone (Cl4Q), 2,5-dimethyl-1,4-benzoquinone (PXQ), and duroquinone (DQ) for the photocatalytic four-electron H2O oxidation to produce O2 with more than 90% yield (Figure 3).50(Equation 4) DDQH2+O2→hνDDQ+H2O2

Figure 3 Time profiles of the formation of O2 by p-benzoquinone derivatives (0.50 mM; DDQ [black circles], p-benzoquinone [BQ: blue circles], tetrachloro-1,4-benzoquinone [Cl4Q: green circles], 2,5-dimethyl-1,4-benzoquinone [PXQ: red circles] and duroquinone [DQ: orange circles]) with [FeII(N4Py)]2+ (0.20 mM) and H2O (500 mM) upon white light photoirradiation under an Ar atmosphere in MeCN at 298 K

The yield of O2 in the case of DDQ reached 100% and then decreased at prolonged photoirradiation time because the reduced DDQ (DDQH2) reduce O2 to H2O2. Reprinted with permission from ref.50 Copyright 2019, American Chemical Society.

The catalytic mechanism of the photocatalytic four-electron/four-proton oxidation of H2O by DDQ as an oxidant and [FeII(N4Py)]2+ as a WOC is proposed in Scheme 5.50 Upon photoexcitation, DDQ was converted to the singlet excited state, which is further converted to triplet excited state (3DDQ∗) by intersystem crossing.51 Photoinduced electron-transfer reaction of 3DDQ∗ and [FeII(N4Py)]2+ formed DDQ·– and [FeIII(N4Py)]3+, and the electron-transfer rate constant was determined to be 1.0 × 1010 M−1 s−1 at 298 K, which is close to the diffusion-limited value.50 The iron(III) complex produced reacted with H2O to form [FeIII(OH)(N4Py)]2+ (pathway a in Scheme 5), whereas the protonation of DDQ·– by water generated DDQH·.50 The electron-transfer reaction of [FeIII(OH)(N4Py)]2+ and 3DDQ∗ took place, followed by proton transfer to form an iron(IV)-oxo species, [FeIV(O)(N4Py)]2+, and DDQH· (pathway b in Scheme 5).50 Moreover, the electron-transfer reaction of [FeIV(O)(N4Py)]2+ and 3DDQ∗ further generated a putative iron(V)-oxo species, [FeV(O)(N4Py)]3+, and DDQ·– (pathway c in Scheme 5).50 The formation of [FeV(O)(N4Py)]3+ was detected by laser-induced transient absorption measurements.52 The nucleophilic attack of H2O to [FeV(O)(N4Py)]3+ produced an iron(III)-OOH species, [FeIII(OOH)(N4Py)]2+, and H+ (pathway d in Scheme 5).50,52 H+ reacted with DDQ·– to produce DDQH·. The disproportionation of DDQH· produced DDQ and DDQH2.50 It should be noted that [FeIII(OOH)(N4Py)]2+, which was independently prepared by the reaction of [FeII(N4Py)]2+ and H2O2,53 was thermally oxidized by DDQ to evolve O2 and simultaneously regenerated [FeII(N4Py)]2+ (pathways e and f in Scheme 5).50Scheme 5 Proposed mechanism of the photocatalytic four-electron/four-proton oxidation of H2O by DDQ as an oxidant and [FeII(N4Py)]2+ as a WOC to release O2, accompanied by reduction of DDQ to DDQH2

Reprinted with permission from ref.50 Copyright 2019, American Chemical Society.

Combination of PSI and PSII models

The stoichiometry of photosynthesis obtained by combining PSI and PSII is given by Equation 5, where H2O is used as a reductant (an electron and proton source) to reduce(Equation 5) 2H2O+2NAD(P)+→hνO2+2NAD(P)H+2H+

NAD(P)+ to generate NAD(P)H regioselectively, accompanied by O2 evolution under solar irradiation. Since NAD(P)H can reduce water with an NAD(P)+ reduction catalyst to evolve H2,40 the stoichiometry of the overall artificial photosynthesis for solar fuel production is given by Equation 6 that is water splitting. Water splitting has also been investigated using heterogeneous semiconductor photocatalysts.6,54(Equation 6) 2H2O→hν2H2+O2

The water splitting with molecular catalysts has been realized by combining a PSI molecular model system (Scheme 3) and a PSII molecular model system (Scheme 5) by using two liquid membranes, as shown in Figure 4.55 Toluene phases as well as two solvent mixture phases of H2O/TFE (v/v 3:1) are separated by use of two glass membranes in the two compartment cell for the combination of PSI and PSII models (Figure 4).55 The PSII molecular model contains an X-Q used as a PQ analog in the toluene phase, whereas an iron(II) complex ([FeII(N4Py)]2+) employed as a homogeneous WOC in the H2O/TFE (v/v 3:1) phase. It was confirmed that in this PSII model, a stoichiometric amount of O2 was produced simultaneously with the generation of X-QH2 as PQH2 analogs.55 In the part of PSI model, nothing is contained in the toluene phase, whereas X-QH2, Acr+-Mes, and CoIII(dmgH)2pyCl are used in the H2O/TFE (v/v 3:1) phase as a PQH2 analog, a photoredox catalyst (photosynthetic reaction center model), and an H2 evolution catalyst, respectively.55 It was also confirmed that a stoichiometric amount of H2 was produced simultaneously with the regeneration of X-Q in this part of PSI model.55 The combination of functional molecular models of photosystems I and II separated by two glass membranes (Figure 4) have made it possible to split water to H2 and O2, achieving over 100 TON for H2 evolution by Q/QH2 cycle, where Q is reduced to QH2 in a PSII model, which is oxidized back to Q in a PSI model (Scheme 6).55Figure 4 A molecular photocatalytic system made up of two compartment cells employed for functional models of PSI (H2 evolution) and PSII (water oxidation)

Reprinted with permission from ref.55 Copyright 2022, American Chemical Society.

Scheme 6 Molecular photocatalytic cycle for water oxidation (O2 evolution) combined with H2 production by a combination of PSI and PSII models

Reprinted with permission from ref.55 Copyright 2022, American Chemical Society.

The same photocatalytic system for water splitting (Figure 4) has been employed to achieve the regioselective NAD(P)+ reduction by H2O to form NAD(P)H, combined with water oxidation to evolve O2 (Equation 5), as shown in Figure 5.48 The photoillumination of a two-phase mixed solution of toluene, TFE and 0.10 M borate aqueous buffer (pH = 7.0; 50:1:49) of Cl4Q and [(N4Py)FeII]2+ in the left side cell and also a two-phase mixed solution of toluene, TFE and 0.10 M borate aqueous buffer (pH = 7.0; 50:1:49) of NAD(P)+ or its analog (i.e., 1-benzyl-3-carbamoylpyridinium cation [BNA+] as an NAD+ model compound), Acr+-Mes and CoIII(dmgH)2pyCl in the right side cell resulted in the production of NAD(P)H or its analog (BNAH) with nearly 100% yield based on the initial amount of NAD(P)+ or BNA+ together with evolution of H2 in the right side cell and the evolution of O2 in the left side cell (Figure 5).48 The TON of NADH formation reached 24 based on Cl4Q (initial amount), 16 based on CoIII(dmgH)2pyCl, and 12 based on Acr+-Mes.48 Thus, Cl4Q, CoIII(dmgH)2pyCl, and Acr+-Mes act as the combination catalysts for the overall photocatalytic reduction of NAD(P)+ by H2O. In particular, X-Q plays the same role as plastoquinone acting as a redox mediator catalyst in the photosynthesis (Scheme 7),48 where H2O is oxidized by plastoquinone (PQ) in PSII to produce O2 and plastoquinol (PQH2), whereas NADP+ is reduced by PQH2 to produce NADPH, accompanied by regeneration of PQ in PSI.10 In the photosynthetic model system, X-Q is reduced by H2O with [(N4Py)FeII]2+ used as a WOC in the PSII model to QH2 with Acr+-Mes (photosynthetic reaction center model) and CoIII(dmgH)2pyCl (reduction catalyst) to form NADPH and regenerate X-Q in the PSI model (right side cell in Figure 5).48Figure 5 A photochemical O-type tube used to produce NADH by photocatalytic reduction of NAD+ by H2O by combining functional models of photosystems I and II using two glass membranes

Reprinted with permission from ref.48 Copyright 2024, American Chemical Society.

Scheme 7 Molecular photocatalytic cycle for production of NADH by photocatalytic reduction of NAD+ by H2O with O2 evolution in combination of PSI and PSII models

Reprinted with permission from ref.48 Copyright 2024, American Chemical Society.

Conclusion and perspective

Simple organic dyads have been shown to undergo short range charge separation but tremendously slow charge recombination with minimized energy loss, while long-range charge separation in the multi-step electron-transfer processes requires a significant amount of energy loss in the natural photosynthetic reaction center as well as in the model compounds. These simple donor-acceptor pairs have been successfully applied to build the efficient photocatalytic systems as functional models of PSI, where plastoquinol analogs are oxidized to the corresponding plastoquinone analogs, accompanied by hydrogen evolution and regioselective reduction of NAD(P)+ to NAD(P)H. On the other hand, plastoquinone analogs are reduced to plastoquinol analogs by water to evolve O2 with [FeII(N4Py)]2+ via successive electron transfer from [FeII(N4Py)]2+ to the triplet excited state of plastoquinone analogs to produce [FeV(O)(N4Py)]3+. The iron(V)-oxo complex ([FeV(O)(N4Py)]3+) oxidizes water to evolve O2. The functional molecular models of PSI were then successfully combined with a functional molecular model of PSII, to achieve production of H2 and NAD(P)H by photocatalytic reduction of water and NAD(P)+ using a homogeneous molecular photocatalyst and a reduction catalyst. Such a combination of functional molecular models of photosystems I and II provides valuable mechanistic insights enabling detection of reaction intermediates, which would otherwise be impossible to achieve as long as heterogeneous photocatalysts are employed.38 The molecular photocatalytic H2 production from H2O can be applied for the production of liquid solar fuels from H2O and CO2 by replacing H2 evolution catalysts by CO2 hydrogenation catalysts (Scheme 8). The photocatalytic regioselective reduction of NAD(P)+ by water to produce NAD(P)H can also be combined with NAD(P)H dependent enzymatic reactions using water as an electron and proton source.Scheme 8 Molecular photocatalytic cycle for CO2 reduction by H2O by combining PSI and PSII models with a hydrogenation catalyst

Acknowledgments

The authors gratefully acknowledge the contributions of his collaborators and coworkers mentioned in the references. This work was supported by the 10.13039/501100003725 National Research Foundation of Korea through 10.13039/100018501 CRI (NRF-2021R1A3B1076539 to W.N.), Basic Science Research Program (NRF-2023R1A2C1007668 to Y.-M.L.), and Brain Pool Program (NRF-2022H1D3A2A01045098 to W.N. and S.F.), Korea, Grants-in-Aid from MEXT, Japan (no. 16H02268 and 23K04686 to S.F.), and Henan Center for Outstanding Overseas Scientists, China (GZS2024020 to W.N.).

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Declaration of interests

The authors declare no competing interests.
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