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Tetrahedron 76 (2020) 131024

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Tetrahedron

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Tetrahedron report 1201

Bioinspired artificial photosynthesis systems

Shunichi Fukuzumi a, b, *, Yong-Min Lee a, c, *, Wonwoo Nam a, d, *

a Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, South Koreab Graduate School of Science and Engineering, Meijo University, Nagoya, Aichi 468-8502, Japanc Research Institute for Basic Sciences, Ewha Womans University, Seoul 03760, South Koread State Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), ChineseAcademy of Sciences, Lanzhou 730000, China

a r t i c l e i n f o

Article history:Received 5 December 2019Received in revised form10 January 2020Accepted 4 February 2020Available online 7 February 2020

Keywords:Artificial photosynthesisPhotoredox catalysisPhotosynthetic reaction centerPhotocatalytic mechanism

* Corresponding authors. Department of ChemistWomans University, Seoul 03760, South Korea.

E-mail addresses: fukuzumi@chem.eng.osaka-u.acewha.ac.kr (Y.-M. Lee), wwnam@ewha.ac.kr (W. Nam

https://doi.org/10.1016/j.tet.2020.1310240040-4020/© 2020 Elsevier Ltd. All rights reserved.

a b s t r a c t

In photosynthesis, the oxidizing equivalents generated by photoinduced charge separation in thephotosynthetic reaction center in photosystem II (PSII) are used for four-electron oxidation of water todioxygen, whereas the reducing equivalents are used to reduce two equivalents of plastoquinone inquinone pool to produce two equivalents of plastoquinol. Plastoquinol is used as a two-electron/two-proton reductant to reduce NADPþ regioselectively to NADPH in photosystem I (PSI). This review fo-cuses on bioinspired artificial photosynthetic systems to mimic charge-separation processes in thephotosynthetic reaction centers and also the functions of PSII to oxidize water, accompanied by reductionof plastoquinone to plastoquinol as well as PSI to reduce protons by plastoquinol to produce hydrogen.Simple photosynthetic reaction center model compounds, which undergo fast charge separation butslow charge recombination, are applied as effective photoredox catalysts to catalyze a variety of chemicaltransformations under photoirradiation including mimicry of the function of PSI.

© 2020 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1. Photosynthetic reaction center models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2. Long-lived ET states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.3. Photosystem I (PSI) models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.4. Photosystem II (PSII) mimic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2. Conclusion and perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1. Introduction

In natural photosynthesis, there are two photosystems withinthylakoid membranes, designated as photosystem I (PSI) andphotosystem II (PSII), both of which contain the photosyntheticreaction centers for charge-separation processes [1e9]. In PSII,

ry and Nano Science, Ewha

.jp (S. Fukuzumi), yomlee@).

water is oxidized in the oxygen evolving complex (OEC) to reduceplastoquinone (PQ) to plastoquinol (PQH2) [1e6], whereas PQH2 isoxidized back to PQ to reduce nicotinamide adenine dinucleotidephosphate (NADPþ) to NADPH in PSI [6e9]. NADPH reduces CO2into carbohydrates in the Calvin cycle [10,11]. Fossil fuels wereproduced from fossilized carbohydrate compounds, which are theaccumulated products of photosynthesis over billions years. TheEarth’s environment has been threatened by an increase in CO2 inthe atmosphere that can be attributed to the rapid consumption offossil fuels caused by human development. Thus, development ofartificial photosynthesis to produce sustainable solar fuels instead

S. Fukuzumi et al. / Tetrahedron 76 (2020) 1310242

of fossil fuels has attracted increasing attention [12e18].There have been extensive studies on chemical models of the

photosynthetic reaction center by designing and synthesizingelectron donor-acceptor ensembles linked with electronmediators,the photoexcitation of which results in multi-step photoinducedelectron transfer (ET) to produce charge-separated states with longlifetimes [19e33]. This review focuses on development of bio-inspired artificial photosynthesis systems to mimic charge-separation processes in the photosynthetic reaction center, whichis the primary step in photosynthesis. Then, photosynthetic reac-tion center model compounds are used as effective photocatalystsin the PSI model systems for photocatalytic hydrogen evolution,accompanied by oxidation of various substrates. A recent PSIImodel is also discussed together with the possible combination ofPSI and PSII models to produce a solar fuel.

1.1. Photosynthetic reaction center models

Photoinduced electron-transfer (ET) processes in the photo-synthetic reaction center (PRC) with the reported structure frompurple bacteria are illustrated in Fig. 1 (left side) [34e38]. EfficientET from the light harvesting unit surrounding the PRC to the specialpair (P) results in formation of the singlet excited state (P*), fol-lowed by ET from P* to the primary electron acceptor, bacter-iopheophytin (HL), to produce the initial charge-separated (CS)state (P�þHL�e) within 4e7 ps [34]. The subsequent ET fromHL�e tothe next electron acceptor, quinone (QA), along the L branch affordsthe charge-separated state (P�þQA

�e) within the time range of200e250 ps, which is much faster than the primary charge

Fig. 1. Structure of PRC from Rhodopseudomonas viridis with the lifetimes of CS statesand the distances between the components, such as ‘special pair’ (P), auxiliarybacteriochlorophyll (BL), bacteriopheophytin (HL), and menaquinone (QA) (left side).Structure of (MP)2Q-Q2HP-C60 with the lifetimes of CS states and the inter-chromophoric distances, when M in MP is Zn(II) or 2H and Ar is 3,5-di-tert-butyl-phenyl (right side). Reprinted with permission from Ref. [56]. Copyright 2016, RoyalSociety of Chemistry.

recombination (0.6e10 ns), at room temperature (Fig. 1, left side)[34e38]. The final charge separation occurs from QA

�e to the nextquinone called QB at the time range of 100 ms [34e38]. When PRCslack a quinone at the QB site the electron on QA

�e recombines withthe hole on P�þ with a 100 ms lifetime at room temperature[34e38].

Porphyrins and fullerenes have frequently been used instead ofbacteriochlorophylls and quinones to synthesize PRC model com-pounds that undergo multi-step photoinduced charge-separationprocesses (Fig. 1, right side) [39e55]. The porphyrins, (MP)2 in(MP)2Q-Q2HP-C60 (M ¼ Zn and 2H), are linked directly using aTr€oger’s base bridge on the porphyrin b-pyrrolic position [56]. The(MP)2 unit is closely mimicked by using the Tr€oger’s base linkedporphyrin dimer (ZnP)2 [56]. It has been confirmed by the X-raycrystal structure that there is the pseudo C2-symmetry of theporphyrin dimer, which is the mimetic model of the ‘special-pair(P)’ of natural PRCs [57]. The distance of intra-dimer porphyrin-center to porphyrin-center is 6.2 Å for this Tr€oger’s base dimer,which is close to the interchromophore separation of 7.0 Å foundwithin P in the natural PRC [58]. In addition, (MP)2Q-Q2HP-C60 hasthe additional functionality through the bonding of the fullereneelectron acceptor, which permits the occurrence of the secondarycharge-separation processes, mimicking the electron acceptorfunction of the quinones in the natural system [57]. The distances of18.7 and 15.0 Å in (MP)2Q-Q2HP-C60 for ET reactions are also similarto those of 18.0 and 14.3 Å found in the PRC (Fig. 1) [56,59]. Theoccurrence times for the (MP)2Q-Q2HP-C60 model compound are20e40 ps for the first charge separation, 250e500 ps for the sec-ondary charge separation, 0.3e2 ns for the primary chargerecombination, and 5e90 ms for the secondary charge recombina-tion, which are similar to those of the natural system except for thesecondary charge recombination time that is shorter by 3e4 ordersof magnitude than that of the natural system [56].

The best model compound mimicking multi-step ET processesin the PRC reported up to now is a ferrocene-meso,meso-linkedporphyrin trimer-fullerene pentad (Fc-(ZnP)3eC60; see Fig. 2),where the C60 and ferrocene (Fc) moieties are tethered at the bothends of (ZnP)3 (Ree¼ 46.9 Å) [60]. The lifetime of the final CS state is0.53 s in frozen PhCN at 163 K, which is comparable to that in thenatural PRC, with a high quantum yield (F¼ 0.83) [60]. It should benoted, however, that the decay rate of the CS state in solutionobeyed second-order kinetics due to the bimolecular back ET be-tween two CSmolecules with the rate constant of 2.7� 109M�1 s�1,which is close to the diffusion rate constant in PhCN [60]. Such along-lifetime of the CS state is attained through multi-step ETprocesses, which result in a significant increase in the charge sep-aration distance by mimicking the natural PRC (vide supra). How-ever, a significant amount of energy is lost during the multi-step ETprocesses, because each ET step is exergonic (downhill). The CSstate energy of Fcþ-(ZnP)3eC60

�e is thereby quite low (1.04 eV), whenthe one-electron reduction potential of Fcþ and the one-electronoxidation potential of C60�e are 0.37 and �0.67 V vs. SCE, respec-tively [60]. Thus, the oxidizing ability of Fcþ-(ZnP)3eC60

�e is ratherlow. In the case of natural photosynthesis, two photosystems (I andII) are required to recover the energy loss via the multi-step ETprocesses in PRCs and to obtain the strong oxidizing power to beable to oxidize water in PSII and the strong reducing power to beable to reduce NADþ coenzyme in PSI [2,3,61]. The design andsynthesis of molecular systems to mimic two photosystems in na-ture seem to be extremely difficult or almost impossible. Thus, it ishighly desired to design simple molecular electron donor-acceptordyads, which are capable of fast charge separation but retain slowcharge recombination of the high-energy CS state, which are able tooxidize and reduce water at the same time.

Fig. 2. Multi-step photoinduced ET in Fc-(ZnP)3eC60 where Ar ¼ 3,5-tBu2C6H3. Reprinted with permission from Ref. [60]. Copyright 2004, John Wiley and Sons.

S. Fukuzumi et al. / Tetrahedron 76 (2020) 131024 3

Various donor-acceptor linked molecules have been designedand synthesized to attain long-lived CS states, where the donor andacceptormolecules are connected using a short linkage tominimizethe reorganization energy of solvent, which prevents back ET[62e72]. The longest lifetime of the CS state in solution was ob-tained for a simple electron donor-acceptor dyad, (10-[4’-(N,N-dimethylamino)phenyl]isoalloxazine: DMAeFl in Fig. 3), in whichan electron donor (DMA) is linked with a flavin analog (Fl) [71].Photoexcitation of DMAeFl results in ET from the DMA moiety tothe singlet excited state of the Fl moiety to produce the CS state(DMA�þeFl�e) that exhibits an extremely long lifetime (2.1 ms) in aPhCN solution at 298 K. The quantum yield of CS state was deter-mined to beF¼ 31% [71]. The one-electron oxidation and reductionpotentials (Eox and Ered) of DMAeFl are determined to be 0.94and �0.83 V vs. SCE, respectively. Since there is no significantorbital interaction between DMA�þ and Fl�e in the CS state(DMA�þeFl�e), the Eox and Ered values of DMAeFl correspond to theEred and Eox values of DMA�þeFl�e, respectively. In such a case, theDMA�þ moiety in DMA�þeFl�e can oxidize bis(ethylenedithiol)tetrathiafulvalene (BEDT-TTF) (Eox ¼ 0.40 V vs. SCE) to produceBEDT-TTF�þ (lmax ¼ 1000 nm) and the Fl�e moiety in DMA�þeFl�e

can reduce p-dinitrobenzene (Ered ¼ �0.71 V vs. SCE) to produce p-dinitrobenzene radical anion (lmax ¼ 920 nm) with the rate con-stant of 4.9 � 109 M�1 s�1 in PhCN at 298 K, which is close to thediffusion rate constant [71]. Thus, the CS state, DMA�þeFl�e, acts asa strong one-electron reductant as well as a one-electron oxidant atthe same time.

The stronger reducing ability of the CS state than that ofDMAeFl was obtained for aniline derivative-coumarin dyads (D-CM: D ¼ 4-(N,N-diphenyl)aniline (DPA), 4-(N,N-diethyl)aniline(DEA), 4-(N,N-dimethyl)aniline (DMA), and 2-methyl-4-(N,N-dimethyl)aniline (MeDMA) in Fig. 4) [72]. The Eox values ofDPA�þeCM�e, DEA�þeCM�e, DMA�þeCM�e, and MeDMA�þeCM�e

are �1.29, �1.33, �1.33, and �1.35 V vs. SCE, respectively. Femto-second laser excitation of DPAeCM resulted in ET from DPA to thesinglet excited state of CM to produce the singlet CS state,1(DPA�þeCM�e), with the rate constant of 1.5 � 1012 s�1, followed

Fig. 3. Chemical structures of DMAeFl and the reference compound (MeFl) [71].

by the intersystem crossing to produce the triplet CS state,3(DPA�þeCM�e), with the rate constant of 6.9 � 1010 s�1. The decayof 3(DPA�þeCM�e) obeyed the second-order kinetics with the rateconstant of 5.6 � 109 M�1 s�1 due to the bimolecular back electrontransfer (BET) between two 3(DPA�þeCM�e) molecules rather thanintramolecular BET to the ground state [72]. In the cases of3(DEA�þeCM�e), 3(DMA�þeCM�e), and 3(MeDMA�þeCM�e), thedecay of the CS states in MeCN at 298 K obeyed first-order kineticsto afford the CS lifetimes of 34, 31, and 100 ms, respectively [72]. Thetriplet CS state, 3(D�þeCM�e), is capable of reducing various elec-tron acceptors [72]. ET from the CM�emoiety of 3(DMA�þeCM�e) tonitrobenzene (Ered ¼ �1.15 V vs. SCE) [73], tetramethyl-p-benzo-quinone (Ered ¼ �0.85 V vs. SCE) [74], and p-benzoquinone(Ered ¼ �0.50 V vs. SCE) [74] occurs to produce the correspondingradical anions [72]. However, no ET from the CM�e moiety inDMA�þeCM�e to 1,4-dicyanobenzene (Ered ¼ �1.72 V vs. SCE) [73]occurs, because the ET is highly endergonic (DGet ¼ 0.39 eV).

1.2. Long-lived ET states

A simple electron donor-acceptor dyad, 9-mesityl-10-methylacridinium ion (AcrþeMes), which has the strongestoxidizing power upon photoexcitation, was developed by con-necting an electron donor moiety (mesityl group) at the 9-positionof the acridinium ion [75]. X-ray single crystal structure of Acrþ-Mes is shown in Fig. 5a, in which the dihedral angle between thetwo aromatic ring planes is close to 90�, indicating no orbitalinteraction between the donor and acceptor moieties [75]. Thus,UVevisible spectrum of AcrþeMes is the spectral superposition ofeach component (i.e., mesitylene and 10-methylacridinium ion).DFT calculations with Gaussian 98 (B3LYP/6-31G* basis set) affordHOMO and LUMO orbitals of AcrþeMes, which are localized onmesitylene (donor) and acridinium (acceptor) moieties (Fig. 5b andc), respectively [75]. The energy of the ET state (Acr�eMes�þ) inPhCN was determined by the redox potentials of each componentof AcrþeMes using cyclic voltammetry to be 2.37 eV [75]. The ETstate (Acr�eMes�þ) corresponds to the charge-shifted state ofAcrþeMes (not charge-separated state) [75].

Nanosecond laser photoexcitation of a deaerated PhCN solutionof AcrþeMes at 430 nm results in generation of the triplet ET state[3(Acr�eMes�þ)] with a very high quantum yield (F ¼ 98%) viaphotoinduced ET from theMesmoiety to the singlet excited state ofthe Acrþ moiety (1Acrþ*-Mes), followed by intersystem crossingfrom the singlet ET state [1(Acr�eMes�þ)] [75]. The intramolecularback ET from the Acr�moiety to theMes�þmoiety in 3(Acr�eMes�þ)was too slow to compete with the bimolecular back ET betweentwo 3(Acr�eMes�þ) molecules. In such a case, the decay rate of3(Acr�eMes�þ) obeyed second-order kinetics with the rate

Fig. 4. Chemical structures of aniline derivativeelinked coumarin dyads [72].

Fig. 5. (a) X-ray structure of AcrþeMes. (b) Highest occupied molecular orbital (HOMO) and (c) lowest unoccupied molecular orbital (LUMO) calculated by DFT. (d) Eyring plot ofln(kBET/T) vs. T�1. Reprinted with permission from Ref. [75]. Copyright 2004, American Chemical Society.

S. Fukuzumi et al. / Tetrahedron 76 (2020) 1310244

constant, which is near to the diffusion rate constant [75,76]. Thelifetime of the triplet ET state [3(Acr�eMes�þ)] becomes longer withdecreasing temperature to approach an almost infinite lifetime at77 K [75]. The observation of the second-order decay kinetics of theET state is the same as the observed for the case of Fcþ-ZnP-H2PeC60

�e, in which bimolecular back ET predominates due to theslow intramolecular back ET (vide supra) [39]. As temperatureincreased, the decay kinetics of Acr�eMes�þ is changed from thesecond-order kinetics to the first-order kinetics in PhCN [75]. Thisindicates that the rate of the intramolecular back ET from the Acr�

moiety to the Mes�þ moiety in 3(Acr�eMes�þ) becomes much fasterthan the rate of the intermolecular back ET between two3(Acr�eMes�þ) molecules at higher temperatures because of thelarger activation energy of the intramolecular ET than the inter-molecular ET [75].

Benniston et al. reported that the local triplet excited state of theAcrþ moiety (3Acrþ*eMes) could be formed rather than the tripletET state [3(Acr�eMes�þ)] and that the energy of 3Acrþ*eMes islower than that of 3(Acr�eMes�þ) [77,78]. The energy of the localtriplet excited state of AcrþeMes (3Acrþ*eMes) was reported to be1.96 eV from the phosphorescence maximum [77]. If this value wascorrect, the one-electron oxidation potential (Eox) of 3Acrþ*eMeswould be only 0.10 V vs. SCE, which is estimated from the Eox valueof the Mes moiety (2.06 V vs. SCE) [79] and the energy of tripletexcitation (1.96 V vs. SCE) [77]. In such a case, ET from 3Acrþ*eMes(Eox ¼ 0.10 V vs. SCE) to hexyl viologen (HV2þ: Ered ¼ �0.42 V vs.SCE) would be energetically impossible (highly endergonic)judging from the largely positive free energy change of ET (0.52 eV).However, the nanosecond laser photoexcitation of a deaeratedMeCN solution of AcrþeMes in the presence of HV2þ (5.0� 10�4 M)

S. Fukuzumi et al. / Tetrahedron 76 (2020) 131024 5

results in generation of HV�þ, which was detected by the well-known transient absorption band at 605 nm [80], accompaniedby the decay of transient absorption band at 510 nm due to the Acr�

moiety of the triplet ET state (Fig. 6a) [76]. Thus, ET from the Acr�

moiety in 3(Acr�eMes�þ) (Eox ¼ �0.57 V vs. SCE) to HV2þ

(Ered ¼ �0.42 V vs. SCE), which is exergonic with the free energychange of ET of �0.15 eV (exergonic), occurs to produceAcrþeMes�þ and HV�þ.

The rate of formation of HV�þ increases with increasing con-centration of HV2þ (Fig. 6b), and the pseudo-first-order rate con-stant increases linearly with an increase in concentration of HV2þ

(Fig. 6c) [76]. The second-order rate constant was determined fromthe slope of kobs vs. [HV2þ] in Fig. 6c to be 4.1� 108M�1 s�1 inMeCNat 298 K [76]. Similarly ET from the Acr� moiety of 3(Acr�eMes�þ)(Eox ¼ �0.57 V vs. SCE) to p-benzoquinone (Q: Ered ¼ �0.51 V vs.SCE) occurs with the free energy change of ET of �0.06 eV (exer-gonic) to produce AcrþeMes�þ and Q�e [76].

Furthermore, there would be almost no formation of3Acrþ*eMes from the beginning, because 1Acrþ*-Mes undergoes

Fig. 6. (a) Transient absorption spectra of Acrþ-Mes (1.0 � 10�4 M) in the presence of HV2þ (circle) upon nanosecond laser excitation at 355 nm. (b) Time profiles of the absorbance rise aHV�þ against concentrations of HV2þ. Reprinted with permission from Ref. [76]. Copyright

fast ET from the Mes moiety to the 1Acrþ* moiety to produce thesinglet ET state [1(Acr�eMes�þ)] with the lifetime of only 4.2 ps inMeCN, which is much faster than the intersystem crossing to pro-duce 3Acrþ*eMes [76]. The singlet ET state [1(Acr�eMes�þ)] un-dergoes the intersystem crossing to produce the triplet ET state[3(Acr�eMes�þ)] with the lifetime of 7.7 ns. Thus, 3(Acr�eMes�þ)instead of 1(Acr�eMes�þ) as an effective photoredox catalyst to beable to oxidize electron donors (D) to D�þ and reduce electron ac-ceptors (A) to A�e at the same time in contrast to the case of thelocal triplet excited state of AcrþeMes (3Acrþ*eMes), which haslow capability to reduce electron acceptors.

The reported triplet energy of 3Acrþ*eMes (1.96 eV) may resultfrom an acridine impurity involved in the preparation of ArþeMesby Benniston et al., who synthesized the compound viamethylationof the corresponding acridine [77], because acridine may remain asan impurity even after purification of acridinium ion by recrystal-lization. When AcrþeMes was prepared by the Grignard reaction of10-methyl-9(10H)-acridone with 2-mesitylmagnesium bromide,there was no acridine left as an impurity [75,76]. Thus, AcrþeMes

5.0 � 10�4 M) in deaerated MeCN at 298 K taken at 2 ms (closed circle) and 20 ms (opent 600 nm due to HV�þ. (c) Plot of the first-order rate constants (kobs) of the formation of2016, John Wiley and Sons.

S. Fukuzumi et al. / Tetrahedron 76 (2020) 1310246

without acridine afforded no phosphorescence spectrum in bothdeaerated glassy 2-MeTHF and ethanol at 77 K [75e78]. However,acridine derivatives are known to exhibit strong phosphorescenceat 15,650e15850 cm�1 [81]. Indeed, the phosphorescencemaximum of 9-phenylacridine in glassy 2-MeTHF at 77 K agreeswell with that reported by Benniston et al. [81]. Thus, the reportedlow lying local triplet energy of AcrþeMes, which contradicts to thelong-lived triplet ET state, results from the acridine impurity [82].

The long lifetime of the triplet ET state of AcrþeMes has made itpossible to observe the crystal structural change directly in theAcrþeMes(ClO4

�) crystal upon intramolecular photoinduced ET byusing laser pump and X-ray probe crystallographic analysis (Fig. 7)[83]. Laser photoexcitation of the single crystal of AcrþeMes(ClO4

�)resulted in the structural change in the N-methyl group of the Acrþ

moiety, which became bent with its bending angle of 10.3(16)�, andalso the movement of the N-methyl carbon, which became0.27(4) Å away from the mean plane of the aromatic ring (Fig. 7)[83]. Such bending of the N-methyl group results from the photo-induced ET from the Mes moiety to the Acrþ moiety toproduce 3(Acr�eMes�þ), because the sp2 carbon of the N-methylgroup of the Acrþ moiety at the ground state is changed to thesp3 carbon (Acr�) in the triplet ET state [83]. The bending of the N-methyl group by photoexcitation is accompanied by the movementand rotation of the ClO4

� due to the electrostatic interactionwith theMes�þ moiety in the triplet ET state (Fig. 7) [83]. The observedbending of the N-methyl group together with the movement ofClO4

� has provided strong evidence for the formation of the long-lived triplet ET state of AcrþeMes upon photoexcitation of thesingle crystal. In contrast to the case of AcrþeMes, no structuraldifference was observed upon photoexcitation of single crystals of

Fig. 7. Single crystal X-ray structural change upon photoexcitation of AcrþeMes: (a)Structural change in the N-methyl group with numbers, indicating the volumes of thedivided cavity formed by the yellow dotted line (left side) with structure change (greento red) surrounding ClO4

� (right side). (b) Photoinduced geometrical changes in frag-ments of the ground and triplet ET state of AcrþeMes molecules (green and red,respectively). The red dashed line shows the electrostatic interaction between Mes�þ

moiety and ClO4�. Reprinted with permission from Ref. [83]. Copyright 2012, American

Chemical Society.

AcrþePh, which results in no formation of the triplet ET state [83].AcrþeMes cation molecules were incorporated and immobi-

lized in nanosized mesoporous silica-alumina (AlMCM-41), whichcontain cation exchange sites to afford a nanocomposite(AcrþeMes@AlMCM-41) [84]. The shape and size of nanosizedAlMCM-41 were controlled by changing the preparation conditionsas shown in Fig. 8, where TEM images exhibit a tubular or rod-like(tAlMCM-41) morphology with the diameter of 50e100 nm and thelength of 0.2e2 mm array (A) and also a sphere morphology(sAlMCM-41, B) [84]. Because the molecular size of AcrþeMes issmall enough to be incorporated inside the pore of mesoporoussilica with the diameter of more than 3 nm, the cation exchange ofNaþ with AcrþeMes occurs easily upon mixing NaþeexchangedAlMCM-41 with AcrþeMes in MeCN [84]. The percentages of thecation exchange of tAlMCM-41 and sAlMCM-41 by AcrþeMes weredetermined to be 16% and 18%, respectively [84]. The incorporatedAcrþeMes into AlMCM-41 was stable without leaching out inMeCN at room temperature [84].

Photoexcitation of the nanocomposite of AcrþeMes@tAlMCM-41 suspended in MeCN results in ET form the Mes moiety to thesinglet excited state of the Acrþ moiety within 10 ps to produce thetriplet ET state via intersystem crossing as detected by laser flashphotolysis and EPR measurements [84]. The distance between twoelectron spins in the triplet ET state was estimated from the zero-field splitting parameters of the EPR signal at 77 K to be 7.7 Å,which agrees with the expected distance of 7.2 Å between an sp2

carbon atom of the 4 position of the Mes moiety and sp2 carbonatoms of the 3 and 6 positions of the Acr moiety in 3(Acr�eMes�þ)[84]. The triplet ET state of Acrþ-Mes-incorporated into tAlMCM-41has an extremely long lifetime with more than 1 s at room tem-perature, decaying via intramolecular back ET from the Acr� moietyto the Mes�þ moiety of the triplet ET state in contrast to fastintermolecular back ET between two 3(Acr�eMes�þ) moleculesobserved at the diffusion rate in solution (vide supra) [84]. Thus,AcrþeMes@AlMCM-41 acts as an excellent photoredox catalyst,which is quite robust under photoirradiation [84]. Moreover,AcrþeMes@AlMCM-41 can be easily recovered by filtration afterphotocatalytic reactions [84].

1.3. Photosystem I (PSI) models

As described above, AcrþeMes is a simple but an excellentmodel of the PRC, affording a long-lived ET state upon excitationwith a high quantum yield to exhibit not only the strong oxidizingcapability but also the reducing capability at the same time. Thus,AcrþeMes has been frequently used as an excellent photoredoxcatalyst, catalyzing efficiently a variety of chemical transformations[85e95]. In PSI, the charge separation in the PRC results in theregioselective reduction of nicotinamide adenine dinucleotidephosphate (NADPþ) to NADPH, accompanied by oxidation of plas-toquinol (PQH2) to plastoquinone (PQ). In PSI model systems,various electron and proton donors are used to reduce protons toH2. Once H2 is produced, NAD(P)þ can be reduced by H2 to produceNAD(P)H in the presence of an Ir(III) complex [96e99]. H2 can alsoreduce CO2 using appropriate catalysts to produce formic acid,methanol and methane, which can be used alternative H2 carrier[100e104]. The catalytic hydrogenation of abundantly availablecarboxylic acids by H2 yield the corresponding alcohols, which canalso be used as alternative organic energy carriers instead of H2 oras platform chemicals [105].

AcrþeMes was used as a simple PRC model compound for effi-cient photocatalytic H2 evolution without an electron mediatorsuch asmethyl viologen (MV2þ) using poly(N-vinyl-2-pyrrolidone)-protected platinum nanoclusters (Pt-PVP) and NADH, as an H2-evolution redox catalyst and an electron/proton donor, respectively

Fig. 8. High resolution TEM images of (A) tAlMCM-41 and (B) sAlMCM-41. Reprinted with permission from Ref. [84]. Copyright 2012, National Academy of Sciences.

S. Fukuzumi et al. / Tetrahedron 76 (2020) 131024 7

[106]. The rate of H2 evolution in the photocatalytic system in theabsence of MV2þ is 300 times faster than that in the presence MV2þ

as shown in Fig. 9, because direct electron transfer (ET) fromAcr�eMes to Pt-PVP for H2 evolution from NADH is much fasterthan ET from MV�þ to Pt-PVP [106]. A very high quantum yield(F ¼ 52%) and a high H2 yield (95%) based on the initial amount ofNADH were obtained under photoirradiation of a deaerated pH 4.5phthalic acid buffer/MeCN (v/v 1:1) solution containing AcrþeMes(0.10 mM), NADH (1.0 mM), and Pt-PVP (0.20 mg) at 298 K [106].

Photoexcitation of AcrþeMes results in intramolecular ET fromthe Mes moiety to the singlet excited state of the Acrþ moiety ofAcrþeMes to produce the triplet ET state [3(Acr�eMes�þ)] (videsupra), followed by subsequent intermolecular ET from NADH tothe Mes�þ moiety of 3(Acr�-Mes�þ) to produce NADH�þ and Acr�-Mes (Scheme 1) [106]. Then, NADH�þ deprotonates to produceNAD�, which is oxidized by ET to AcrþeMes to yield NADþ and Acr�-

Fig. 9. Time course of H2 evolution under photoirradiation (l > 390 nm) of a deaerated pH 4(1.0 mM), and Pt-PVP (0.20 mg) in the absence (red circles) and presence (blue circles) of MVSociety of Chemistry.

Mes as revealed by laser-induced transient absorption measure-ments [106]. Thus, the absorption of one photon by AcrþeMes inthe presence of NADH resulted in formation of two equivalents ofAcr�eMes to achieve the one-photon two-electron processes(Scheme 1) [106].

ET from Acr�eMes to PtNPs in the presence of protons resultedin H2 production [106]. As the H2 evolution redox catalyst, PtNPswith cubic shape and a diameter size of 6.3 ± 0.6 nm exhibited themaximum activity for the photocatalytic H2 production [107]. Theobserved rate of H2 production in the photocatalytic reaction wasvirtually the same as the rate of ET from Acr�eMes to PtNPs [106].The rate constant of the ET from Acr�eMes to PtNPs (ket) increasedlinearly with increasing Hþ concentration [107]. When Hþ (H2O)was replaced by Dþ (D2O), the inverse kinetic isotope effect(KIE¼ ket(H)/ket(D)¼ 0.47) was observed for the rate constant of ETfrom Acr�eMes to PtNPs [107]. Such an inverse KIE together with

.5 phthalic acid buffer/MeCN (v/v 1:1) solution containing AcrþeMes (0.10 mM), NADH2þ (5.0 mM) at 298 K. Reprinted with permission from Ref. [106]. Copyright 2007, Royal

Scheme 1. One-photon two-electron processes in photodriven oxidation of NADH by AcrþeMes. Reprinted with permission from Ref. [106]. Copyright 2007, Royal Society ofChemistry.

Scheme 2. H2 evolution via PCET from Acr�eMes to PtNPs. Reprinted with permissionfrom Ref. [107]. Copyright 2011, John Wiley and Sons.

S. Fukuzumi et al. / Tetrahedron 76 (2020) 1310248

the first-order dependence of ket with respect to proton concen-tration indicates that the PteH bond is formed by proton-coupledelectron transfer (PCET) from Acr�eMes to PtNPs, which is therate-determining step (r.d.s.) for the photocatalytic production ofhydrogen (Scheme 2) [107].

When 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPhþeNA)[108] was used as a PRC model compound instead of AcrþeMes,photocatalytic H2 production occurred efficiently even under basicconditions (pH 10) using NADH as an electron/proton donor andPtNPs as a redox catalyst [109]. PtNPs can be replaced by metalnanoparticles using more earth-abundant metals such as RuNPs asan H2 evolution catalyst, which exhibits almost the same catalyticactivity obtained with PtNPs for the photocatalytic H2 evolutionfrom NADH [109]. Photoexcitation of QuPhþeNA results in intra-molecular ET from the NA moiety to the singlet excited state ofQuPhþ moiety to produce the triplet ET state of QuPhþeNA[3(QuPh�eNA�þ)] via intersystem crossing from 1(QuPh�eNA�þ),followed by subsequent intermolecular ET from NADH to the NA�þ

moiety of QuPh�eNA�þ to produce NADH�þ and QuPh�eNA [109].Then, two equivalents of QuPh�eNA were produced via deproto-nation of NADH�þ and the subsequent intermolecular ET fromNAD�

to QuPhþeNA to produce NADþ and QuPh�eNA (Scheme 3) [109].Finally, ET from QuPh�eNA to RuNPs occurs to produce H2 evenunder basic conditions (e.g., pH 10) because of the strong reducingability of QuPh�eNA (vide infra) [109]. In this case, the rate of ET

Scheme 3. Chemical structure of QuPhþeNA (left side) and the scheme of the pho-tocatalytic hydrogen evolution (right side) using QuPhþeNA as a photocatalyst andmetal nanoparticles as a redox catalyst (MNPs, M ¼ Ru, Pt). Reprinted with permissionfrom Ref. [109]. Copyright 2011, American Chemical Society.

from QuPh�eNA to RuNPs is much faster than that of H2 evolution.RuNPs with the average diameter of 4.1 nm showed the highest rateof hydrogen-evolution normalized by the weight of RuNPs [109].

In the case of AcrþeMes, ET from Acr�eMes to PtNPs requiresthe assistance of Hþ and the rate of PCET increases with increasingproton concentration [107]. In contrast to the case of theAcrþeMes/PtNPs system, ET from QuPh�eNA toMNPs occurs in theabsence of Hþ due to the much stronger reducing ability ofQuPh�eNA than that of Acr�eMes, resulting from the significantlymore negative Eox value of QuPh�eNA (Eox ¼ �0.90 V vs. SCE) thanthat of Acr�eMes (Eox ¼ �0.57 V vs. SCE) [109]. Thus, the rate of ETfrom QuPh�eNA to MNPs remains about the same even under basicconditions (pH 10). RuNPs can be replaced by more earth abundantNiNPs, although the catalytic reactivity of NiNPs remains 40% basedon the that of commercially available PtNPs using the same weight[110].

Oxalic acid, which is air-stable at room temperature and pro-duced in the body of a plant, can be used as an electron/protondonor instead of NADH for the photocatalytic hydrogen productionwith QuPhþeNA in a deaerated pH 6.0 aqueous buffer/MeCN (v/v1:1) [111]. Oxalate and its conjugate acid act as a two-electrondonor to produce one equivalent of H2 and two equivalents ofCO2 (Equation (1)). In this case as well, the catalytic reactivity ofNiNPs in the photocatalytic evolution of H2 in an MeCN/H2O mixedsolution remains 32% of PtNPs [111].

(COOH)2 / 2CO2 þ H2 (1)

QuPhþeNAwas immobilized into nanosized mesoporous silica-alumina (sAlMCM-41), which has a spherical shape, by cation ex-change to afford the composite photocatalyst(QuPhþeNA@sAlMCM-41), which can be used in an aqueous so-lution without the use of an aprotic solvent such as MeCN [112].Immobilization of cationic electron donor-acceptor dyads intonanosized mesoporous silica-alumina and the laser photoexcita-tion resulted in formation of the triplet ET states, which have muchlonger lifetimes as compared with those in solution, because ofinhibition of intermolecular back ET between 3(QuPh�eNA�þ)molecules incorporated inside the composite photocatalysts[113,114]. TEMmeasurements of sAlMCM-41 revealed the sphericalshape nanoparticles with the diameter size that ranged from 200 to700 nm [112]. K2PtCl6 was used as a catalyst precursor for PtNPsproduced in situ in sAlMCM-41 during the photocatalytic reaction.Due to the larger size of PtNPs than the pore size of sAlMCM-41,PtNPs could not be incorporated into sAlMCM-41 directly [112].Thus, combination of QuPhþeNA@sAlMCM-41 and PtNPs resultedin no H2 evolution under photoirradiation (l > 340 nm) of a pH 4.5phthalate buffer solution [112]. When K2PtCl6 instead of PtNPs wasadded into the reaction solution as a precursor of an H2 evolutioncatalyst, however, efficient H2 evolution occurred, because PtNPs

S. Fukuzumi et al. / Tetrahedron 76 (2020) 131024 9

were produced inside QuPhþeNA@sAlMCM-41 during the photo-catalytic reaction [112]. A copper(II) nitrate complex, CuII(NO3)2,could also be used as a precursor catalyst for the photocatalyticproduction of H2 from oxalate in an aqueous solution at pH 4.5[112]. Thus, hybrid composites of an organic photocatalyst andmetal nanoparticles, which derived from earth-abundant metalsalts (QuPhþeNA@sAlMCM-41), act as efficient H2 evolution pho-tocatalysts working in an aqueous solution (Fig. 10) [112].

AcrþeMes (3: see Scheme 4) can also be used as an efficientphotoredox catalyst for oxidation of alkenes such as styrene withwater, accompanied by proton reduction using a Co(III) complex(CoIII(dmgH)2pyCl) as a redox catalyst to evolve H2 under visiblelight irradiation (Equation (2)) [115]. The photocatalyticmechanismis shown in Scheme 4 [115]. Photoexcitation of AcrþeMes results inormation of the long-lived triplet ET state [3(Acr�eMes�þ)] (videsupra), followed by ET from an alkene to the Mes�þ moiety of thetriplet ET state to produce an alkene radical cation and Acr�eMes.Then, the anti-Markovnikov addition of water and deprotonationaffords a carbon centered-radical intermediate III, which may befurther oxidized by CoIII(dmgH)2pyCl (4) to produce a cation in-termediate IV and CoII(dmgH)2py, respectively (Scheme 4)

(2)

.The Mes�þ moiety of the ET state is also suggested to oxidize III

to IV, accompanied by formation of Acr�eMes (5) (Scheme 4) [115].5 reacts with 4 to regenerate catalyst 3 and CoII(dmgH)2py (6).Although the Eox value of 5 (Eox ¼ �0.56 V vs. SCE) is less negativethan the Ered value of 4 (Ered ¼ �0.67 V vs. SCE), ET from 5 to 4mayoccur because of the irreversible reduction of 4 [116]. Afterdeprotonation of IV, the enoleketo tautomerism produces thecorresponding carbonyl compounds 2. Acr�eMes (5) is proposed toreduce CoII(dmgH)2py (6) to produce [CoI(dmgH)2py]e (7). Theprotonation of 7 affords the CoIII-hydride complex([CoIII(H)(dmgH)2py]) (9) via 8 (Scheme 4) [115]. However, the Eredvalue of CoII(dmgH)2py (6) (�1.12 V vs. SCE) is much lower than theEox value of the Acr�eMes (�0.56 V vs. SCE) [117]. In such a case. noET from Acr�eMes to 6would occur. Alternatively PCET from III to 6may occur to yield 2 and 9. As a result, the protonation of the CoIII-

Fig. 10. Photocatalytic H2 evolution from oxalate using QuPhþeNA, which is immo-bilized inside sAlMCM-41 as a photoredox catalyst with Pt or Cu salts as a precursorredox catalyst for metal nanoparticles as an H2 evolution catalyst, which are producedin situ inside sAlMCM-41. Reprinted with permission from Ref. [112]. Copyright 2013,Royal Society of Chemistry.

hydride complex (9) released H2 and completes the catalytic cycle[114]. However, the alternative homolytic mechanism involvingtwo Co(III)eH hydrides to produce H2 cannot be ruled out [115].

In the absence of water, photocatalytic dimerization of styreneoccurs, accompanied by H2 evolution in the presence of Acrþ-Mesand CoIII(dmgH)2pyCl as shown in Scheme 5 [118]. The photo-catalytic dimerization of styrene is also initiated by ET from styreneto the Mes�þ moiety of the triplet ET state of AcrþeMes to producestyrene radical cation and Acr�eMes [118]. Styrene radical cationreacts with styrene to produce the dimer radical cation, followed bydeprotonation to produce the dimer neutral radical [118]. On theother hand, ET from Acr�eMes to CoIII(dmgH)2pyCl occurs to pro-duce CoII(dmgH)2py, accompanied by regeneration of AcrþeMes[118]. Then, hydrogen atom transfer from the dimer neutral radicalto CoII(dmgH)2py occurs to yield the dimerized product, accom-panied by formation of the Co(III)-hydride complex(CoIII(H)(dmgH)2py). CoIII(H)(dmgH)2py reacts with Hþ to evolveH2, accompanied by regeneration of CoIII(dmgH)2pyCl to completethe catalytic cycle [118].

AcrþeMes can also catalyze the oxidative [4 þ 2] annulationreaction between aromatic NH ketimines and alkenes to form 3,4-dihydroisoquinoline derivatives with high regioselectivity andtrans diastereoselectivity, accompanied by H2 evolution in thepresence of cocatalysts such as [CoIII(dmgH)2py2]PF6 (Scheme 6)[119]. In this case as well, the catalytic reaction is started by ET fromb-methylstyrene to the Mes�þ moiety of 3(Acr�eMes�þ) occurs toproduce b-methylstyrene radical cation and Acr�eMes, followed bythe nucleophilic attack of benzophenone imine to b-methylstyreneradical cation to produce the adduct radical species after thedeprotonation (Scheme 7) [119]. The addition process proceedswith b-regioselectivity because of the stability of the generatedbenzyl radical. Then, the radical cyclization of B in Scheme 7 occursto afford the intermediate C [119]. On the other hand, ET fromAcr�eMes to [CoIII(dmgH)2py2]PF6 occurs to produceCoII(dmgH)2py2, accompanied by regeneration of AcrþeMes [119].Then, hydrogen atom transfer from the cyclized radical C toCoII(dmgH)2py2 occurs to the desired 3,4-dihydroisoquinolineproduct and the Co(III)-hydride complex (CoIII(H)(dmgH)2py)[119]. CoIII(H)(dmgH)2py reacts with Hþ to evolve H2, accompaniedby regeneration of [CoIII(dmgH)2py2]PF6 [119].

A selective C(sp2)-H amination of arenes (alkyl-substitutedbenzenes, anisole derivatives and biphenyl) accompanied by H2evolution can also be achieved by using AcrþeMes as a photoredoxcatalyst and [CoIII(dmgH)2Cl2]e as a thermal redox catalyst (Scheme8) [120]. The photocatalytic reaction is again started by ET fromarenes such as p-xylene to the triplet ET state of AcrþeMes occursto produce p-xylene radical cation and Acr�eMes, followed by thenucleophilic attack of pyrazole to produce the adduct radical afterthe deprotonation (Scheme 8) [120]. On the other hand, ET fromAcr�eMes to [CoIII(dmgH)2Cl2]e occurs to produce[CoII(dmgH)2Cl]e, accompanied by regeneration of AcrþeMes[120]. Then, hydrogen atom transfer from the adduct radical speciesto [CoII(dmgH)2Cl]e occurs to produce the amination product and[CoIII(H)(dmgH)2Cl]e that reacts with Hþ to evolve H2, accompaniedby regeneration of [CoIII(dmgH)2Cl2]e [120]. CeH/CeH cross-coupling between electron-rich arenes and styrene derivativeshas also been made possible by using a dual catalytic system con-taining AcrþeMes as a photoredox catalyst and a cobaloxime redoxcatalyst in 1,2-dichloroethane (DCE) [121]. The photocatalyticmechanism is shown in Scheme 9, where firstly ET from electrondonor-substituted benzenes such as 1,3,5-trimethoxybenzene(TMB; 1 in Scheme 9) to the Mes�þ moiety of the triplet excitedstate of AcrþeMes occurs to produce TMB�þ and Acr�eMes [121].TMB�þ reacts with styrene to produce the coupled radical species(14 in Scheme 9) after the deprotonation. On the other hand,

Scheme 4. Photocatalytic cycle of oxygenation of alkenes with H2O, accompanied by H2 evolution in the presence of Acrþ-Mes and CoIII(dmgH)2pyCl (4). Reprinted with permissionfrom Ref. [115]. Copyright 2016, American Chemical Society.

Scheme 5. Photocatalytic cycle of dimerization of styrene, accompanied by H2 evolution in the presence of Acrþ-Mes and CoIII(dmgH)2pyCl. Reprinted with permission fromRef. [118]. Copyright 2018, Elsevier.

S. Fukuzumi et al. / Tetrahedron 76 (2020) 13102410

Acr�eMes reduces a Co(III) complex to a Co(II) complex, accom-panied by regeneration of AcrþeMes. Because Acr�eMes cannotreduce the Co(II) complex to Co(I) complex (vide supra), a hydrogenatom transfer from the coupled radical species to the Co(II) complex

may occur to produce the desired product (3 in Scheme 9) and theCo(III)-hydride complex, which reacts with Hþ to evolve H2 [121].

The combination of AcrþeMes and its derivatives with a coba-loxime catalyst has also made it possible to achieve decarboxylative

Scheme 6. Photocatalytic [4 þ 2] annulation for the synthesis of dihydroisoquinolines together with hydrogen evolution. Reprinted with permission from Ref. [119]. Copyright 2018,John Wiley and Sons.

Scheme 7. Photocatalytic cycles of [4 þ 2] annulation for the synthesis of dihydroisoquinolines together with hydrogen evolution. Reprinted with permission from Ref. [119].Copyright 2018, John Wiley and Sons.

S. Fukuzumi et al. / Tetrahedron 76 (2020) 131024 11

elimination with H2 evolution (Scheme 10) [122]. The Co(I) com-plex is first produced by reducing the starting Co(III) complex withone equivalent of zinc before being subjected to the reactionmixture. Protonation of Co(I) by an a-amino carboxylic acid pro-vides an a-amino carboxylate. ET from a-amino carboxylate to theMes�þ moiety of the triplet ET state of AcrþeMes occurs to producean a-amino radical species after decarboxylation and Acr�eMes.The Co(III) complex is reduced by Acr�eMes to the Co(II) complex,accompanied by regeneration of AcrþeMes [122]. Then, a hydrogenatom transfer from a-amino radical species to the Co(II) complexoccurs to yield the desired olefin product and the Co(III)-hydridecomplex, which reacts with a-amino carboxylic acid to evolve H2and provide a-amino carboxylate [122]. Ultimately the trans-formation would produce CO2 and H2 as the only stoichiometricbyproducts [122].

The dual catalytic system containing AcrþeMes photoredoxcatalyst and a cobaloxime proton-reducing catalyst [Co(dmgH)2py]Cl at ambient temperature has also been used for photocatalyticphosphinyloxy radical addition/cyclization cascade of arylphos-phinic acids or arylphosphonic acid monoesters with alkynes,providing an efficient and practical access to various phosphaiso-coumarins [123]. Thus, the dual catalytic system containingAcrþeMes and a cobaloxime provides a powerful synthetic tool forapplication to the synthesis of important natural products, phar-maceuticals, and functional materials. Because development ofefficient catalysts for hydrogen evolution has been acceleratedrecently [124e129], a cobaloxime may be replaced by more effi-cient catalysts combined with AcrþeMes for improvement of theoverall catalytic activity.

Scheme 8. Photocatalytic cycles of selective C(sp2)-H amination of arenes together with hydrogen evolution. Reprinted with permission from Ref. [120]. Copyright 2017, SpringerNature.

Scheme 9. Photocatalytic cycles of CeH/CeH cross-coupling between electron-rich arenes and styrene derivatives using AcrþeMes and a cobaloxime catalysts. Reprinted withpermission from Ref. [121]. Copyright 2018, Royal Society of Chemistry.

S. Fukuzumi et al. / Tetrahedron 76 (2020) 13102412

1.4. Photosystem II (PSII) mimic

The oxidizing equivalents produced at the electron donor side ofPSII are used to oxidize water, whereas the reducing equivalents

accumulated at the electron acceptor side of PSII are used to reducetwo quinone molecules, QA and QB, which act as one- and two-electron gates, respectively [1e3,130]. Electrons and protons arefinally transferred to plastoquinone (PQ) in the quinone pool to

Scheme 10. Photocatalytic elimination of N-Boc-phenylalanine using several organicphotoredox catalysts combined with commercially available cobaloxime, Co(dmgH)2-pyCl. Reprinted with permission from Ref. [122]. Copyright 2018, American ChemicalSociety.

S. Fukuzumi et al. / Tetrahedron 76 (2020) 131024 13

form plastoquinol (PQH2) [1e3,130]. The overall photodriven wateroxidation, accompanied by reduction of PQ, is given by Equation(3), where PQ is reduced by water to generate dioxygen and PQH2[1e3,130].

2H2O þ 2PQ / O2 þ 2PQH2 (3)

Photodriven four-electron oxidation of H2O by p-benzoquinonederivatives (X-Q), which is mimic of PSII [Equation (3)], has beenmade possible by using a nonheme iron(II) complex, [(L)FeII]2þ

(L ¼ N4Py ¼ N,N-bis(2-pyridylmethyl)-N-bis-(2-pyridyl)methyl-amine) as a water oxidation catalyst (Equation (4)) [131].

2H2O þ 2X-Q / O2 þ 2PQH2 (4)

As shown in Fig. 11, photoirradiation of an MeCN solution con-taining 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), [(L)FeII]2þ and H2O (0.50 M) with a Xe-lamp resulted in dioxygen

Fig. 11. Time profiles of O2-evolution by photocatalytic oxidation of H2O by X-Q[0.50 mM; DDQ (black), BQ (blue), DQ (orange), CA (green), and PXQ (red)] in thepresence of [(L)FeII]2þ (0.20 mM) and H2O (0.50 M) under white light irradiation in adeaerated MeCN at 298 K. Reprinted with permission from Ref. [131]. Copyright 2019,American Chemical Society.

evolution, where the yield of O2 evolution based on the initialamount of DDQ used reaches nearly 100%. DDQwas reduced byH2Oto produce DDQH2 [131]. At prolonged photoirradiation time, theyield of O2 evolution decreased because of the oxidation of DDQH2by O2 to produce H2O2, accompanied by regeneration of DDQ[Equation (5)] [131]. When DDQ was replaced by p-benzoquinone(Q), p-chloranil (CA) and 2,5-dimethyl-p-benzoquinone (PXQ), theO2 yields were 90% [130]. In the case of tetramethyl-p-benzoqui-none (DQ), however, the O2 yield became smaller (ca. 50%) [131]. Inany case, H2O is oxidized by various p-benzoquinone derivativewith [(L)FeII]2þ as a redox catalyst under photoirradiation to evolveO2.

DDQH2 þ O2 / DDQ þ H2O2 (5)

The catalytic mechanism of the photodriven H2O oxidation by X-Q in the presence of [(L)FeII]2þ is shown in Scheme 11, where DDQ isemployed as an oxidant [131]. Photoexcitation of DDQ results in theformation of 3DDQ* via intersystem crossing from the singletexcited state [132e134]. ET from [(L)FeII]2þ to 3DDQ* occurs toproduce [(L)FeIII]3þ and DDQ�e with the rate constant of1.0 � 1010 M�1 s�1, which is close to the diffusion limited value at298 K. [(L)FeIII]3þ reacts with water to produce the Fe(III)-hydroxocomplex, [(L)FeIII(OH)]2þ (Scheme 11, reaction pathway a),whereas DDQ�e is protonated to afford DDQH�, which dispropor-tionates to produce DDQ and DDQH2. Then, ET from [(L)FeIII(OH)]2þ

to 3DDQ* occurs to produce the FeIV(O) complex ([(L)FeIV(O)]2þ)and DDQH� (Scheme 11, reaction pathway b) [131]. The formation of[(L)FeIV(O)]2þ was confirmed by the absorption spectrumwith lmaxof 690 nm. [(L)FeIV(O)]2þ is further oxidized by ET from [(L)FeIV(O)]2þ to 3DDQ* to produce the FeV(O) complex ([(L)FeV(O)]3þ)and DDQ�ewith the rate constant of 9.4� 109M�1 s�1, which is alsoclose to the diffusion-limited value at 298 K (Scheme 11, reactionpathway c) [131]. [(L)FeV(O)]3þ may react with H2O rapidly toproduce the Fe(III)-hydroperoxo complex, [(L)FeIII(OOH)]2þ, and Hþ

(Scheme 11, reaction pathway d) [131]. It should be noted that [(L)FeV(O)]3þ could not be detected by transient absorption measure-ments probably due to the high reactivity towards H2O. [(L)FeIII(OOH)]2þ is further oxidized by DDQ via H-atom transfer toafford DDQH� and [(L)FeIII(O2

�e)]2þ (Scheme 11, reaction pathway e),followed by a rapid release of O2 from [(L)FeIII(O2

�e)]2þ to generateagain [(L)FeII]2þ (Scheme 11, reaction pathway f) [131]. It was

Scheme 11. Photocatalytic mechanism of the photodriven H2O oxidation by DDQ inthe presence of [(L)FeII]2þ. Reprinted with permission from Ref. [131]. Copyright 2019,American Chemical Society.

S. Fukuzumi et al. / Tetrahedron 76 (2020) 13102414

confirmed that [(L)FeIII(OOH)]2þ was oxidized by DDQ thermally toyield O2 with 100% yield based on initial amount of DDQ, when [(L)FeIII(OOH)]2þ was independently prepared by the reaction of [(L)FeII]2þ and H2O2 [135].

2. Conclusion and perspective

As demonstrated above, simple molecular dyads such asAcrþeMes have been shown to be capable of fast charge separationbut extremely slow charge recombination with minimized energyloss, whereas a loss of significant amount of energy is required forthe long-range charge separation in the multi-step ET processes innatural photosynthetic reaction center. Such readily synthesizeddonor-acceptor dyads can be successfully applied to constructefficient photocatalytic systems with thermal redox catalysts forhydrogen evolution, which is regarded as PSI models, becausehydrogen (H2) can reduce NAD(P)þ coenzymes to NAD(P)H that isthe product of PSI. Such dual catalytic system containing a photo-redox catalyst such as AcrþeMes and a thermal redox catalyst suchas cobaloxime provides a powerful synthetic tool for application tothe synthesis of important natural products, pharmaceuticals, andfunctional materials, accompanied by H2 evolution. Many organiccompounds are used as electron and proton sources for H2 evolu-tion. Unfortunately, photocatalytic H2 evolution using plastoquinolderivatives as a more-bioinspired PSI model has yet to be achieved.Combination of PSI and PSII models would expand the scope ofartificial photosynthesis for not only solar fuel production but alsofor the synthesis of important natural products, pharmaceuticals,and functional materials.

Acknowledgements

The authors gratefully acknowledge the contributions of theircollaborators and coworkers mentioned in the cited references, andfinancial supports by a JST SENTAN project and JSPS KAKENHI(Grant No. 16H02268 to S.F.) from MEXT, Japan and by the CRI(2012R1A3A2048842 toW.N.), GRL (2010-00353 toW.N.), and BasicScience Research Program (2017R1D1A1B03029982 to Y.M.L. and2017R1D1A1B03032615 to S.F.) through NRF of Korea.

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Shunichi Fukuzumi earned a bachelor’s degree and Ph.D.degree in applied chemistry at Tokyo Institute of Tech-nology in 1973 and 1978, respectively. After working as apostdoctoral fellow from 1978 to 1981 at Indiana Univer-sity in USA, he became an Assistant Professor in 1981 atOsaka University where he was promoted to a Full Pro-fessor in 1994. His research has focused on electrontransfer chemistry, in particular artificial photosynthesis.He is currently a Distinguished Professor of Ewha WomansUniversity, Korea, a Designated Professor of Meijo Uni-versity, and a Professor Emeritus of Osaka University,Japan.

Yong-Min Lee received BS, Master and Ph.D. degrees inChemistry under the supervision of Professor Sung-NakChoi at Pusan National University, Republic of Korea in1990, 1995 and 1999, respectively. After he worked inthe Centro di Ricerca di Risonanze Magnetiche (CERM)at Universit�a degli Studi di Firenze, Italia, as a Postdoctoralfellow and Researcher under the direction of ProfessorsIvano Bertini and Claudio Luchinat from 1999 to 2005,he joined the Center for Biomimetic Systems at EwhaWomans University, as a Research Professor in 2006. Heis currently a Special Appointment Professor at EwhaWomans University since 2009.

Wonwoo Nam earned his B.S. (Honors) degree in Chem-istry from California State University, Los Angeles, and hisPh.D. degree in Inorganic Chemistry from University ofCalifornia, Los Angeles (UCLA) under the supervision ofProfessor Joan S. Valentine in 1990. After working as apostdoctoral fellow at UCLA for one year, he became anAssistant Professor at Hong Ik University in 1991. In 1994,he moved to Ewha Womans University, where he iscurrently a Distinguished Professor. His current researchhas been focusing on the dioxygen activation, wateroxidation, and important roles of metal ions in bio-inorganic chemistry.