Functional molecular models of photosynthesis

Abstract

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.

Keywords: Chemical engineering; Chemistry; Natural sciences.

Graphical abstract

Figure 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. Copyright 2004, John Wiley and Sons.

Figure 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(k_BET/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. Copyright 2004, American Chemical Society.

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. Copyright 2007, Royal Society of Chemistry.

Scheme 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 Co^III(dmgH)₂pyCl and Acr⁺-Mes upon photoillumination (λ > 420 nm). Reprinted with permission from ref. Copyright 2020, American Chemical Society.

Scheme 3

Proposed mechanism of photocatalytic H₂ evolution by hydroquinone (plastoquinol model) with Co^III(dmgH)₂pyCl and Acr⁺-Mes upon photoillumination (λ > 420 nm) Reprinted with permission from ref. Copyright 2020, American Chemical Society.

Scheme 4

Proposed mechanism for the photocatalytic NAD⁺ reduction by X-QH₂ with Co^III(dmgH)₂pyCl and Acr⁺-Mes to form NADH Reprinted with permission from ref. Copyright 2024, American Chemical Society.

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 [Fe^II(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. Copyright 2019, American Chemical Society.

Scheme 5

Proposed mechanism of the photocatalytic four-electron/four-proton oxidation of H₂O by DDQ as an oxidant and [Fe^II(N4Py)]²⁺ as a WOC to release O₂, accompanied by reduction of DDQ to DDQH₂ Reprinted with permission from ref. Copyright 2019, American Chemical Society.

Figure 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. 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. Copyright 2022, American Chemical Society.

Figure 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. 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. Copyright 2024, American Chemical Society.

Scheme 8

Molecular photocatalytic cycle for CO₂ reduction by H₂O by combining PSI and PSII models with a hydrogenation catalyst