Supramolecular strategies in artificial photosynthesis

Abstract

Artificial photosynthesis is a major scientific endeavor aimed at converting solar power into a chemical fuel as a viable approach to sustainable energy production and storage. Photosynthesis requires three fundamental actions performed in order; light harvesting, charge-separation and redox catalysis. These actions span different timescales and require the integration of functional architectures developed in different fields of study. The development of artificial photosynthetic devices is therefore inherently complex and requires an interdisciplinary approach. Supramolecular chemistry has evolved to a mature scientific field in which programmed molecular components form larger functional structures by self-assembly processes. Supramolecular chemistry could provide important tools in preparing, integrating and optimizing artificial photosynthetic devices as it allows precise control over molecular components within such a device. This is illustrated in this perspective by discussing state-of-the-art devices and the current limiting factors - such as recombination and low stability of reactive intermediates - and providing exemplary supramolecular approaches to alleviate some of those problems. Inspiring supramolecular solutions such as those discussed herein will incite expansion of the supramolecular toolbox, which eventually may be needed for the development of applied artificial photosynthesis.

Fig. 1. The fundamental actions of photosynthesis. (1) Light harvesting. (2) Charge-separation. (3) Redox catalysis. Notice that energy transfer occurs between chromophores (green arrows) after excitation to funnel the energy to the special pair where it leads to charge separation. Some energy losses are required to drive the charge separation forward. Charges are moved to catalysts that drive uphill chemical reactions. Adapted with permission from ref. 8.

Fig. 2. Operational mode of the DSSC (top) and schematic diagram of a DS-PEC (bottom). (A) Dyes anchored on a semiconductor deposited on conducting FTO glass are excited by light. (B) The dye performs charge-separation and (C) oxidizes or reduces the redox couple (RC) in the electrolyte that delivers the charge to the counter electrode (CE) closing the circuit. For n-type the dye is excited (process 1) followed by fast electron injection into the conduction band (CB) (process 2). The dark colour represents the dyes in the depicted electron configuration. The dye is oxidatively quenched by the redox couple (RC) (process 3). The RC is then regenerated at the counter electrode (CE) (process 4). In p-type the electrons move the other way around, but the processes are equivalent to n-type. The dye is excited (process 1) after which it is reductively quenched by an electron in the valence band (VB) of the NiO. The electron of the reduced dye is transferred to the RC (process 3) after which the redox mediator is regenerated at the CE. The possible recombination pathways are depicted with red arrows. A DS-PEC performs processes (A) & (B) in a similar fashion. Process (C) is altered by substituting the RC for a redox catalyst performing reactions such as water splitting. The catalyst may be anchored to the semiconductor surface or dye as diffusion is not required. Water oxidation catalysts (WOC) oxidize water to oxygen and protons. Four oxidations are required to generate a single oxygen molecule. The hydrogen evolution catalyst (HEC) combines the electrons and protons liberated at the anode to form hydrogen gas at the cathode. Every hydrogen molecule requires two electrons from the HEC. The half reactions are often separated by a proton exchange membrane (PEM).

Fig. 3. As the porphyrin dimer becomes excited, energy transfer occurs to the ring with the same rate (∼1.3 ps) no matter its size (N = 6, 8, 10, 12 or 30). A representation of the line-dipole model on the right shows the effect of strain induced by coordination. The transferred excitation is delocalized over 6 porphyrins. Adapted with permission from ref. 50.

Fig. 4. A porphyrin acceptor functionalized with ditopic pyrazine units (top) that act as guests to the donor Zn-porphyrin cage assembled with hydrogen bonds. A 8 : 1 donor : acceptor ratio is achieved here. Some atoms in the antenna system are omitted for clarity.

Fig. 5. Dendrimer with 16 porphyrin units act as a light-harvesting antenna, and achieves long lasting charge separation when electron accepting fullerenes bind the porphyrin metal atom. Used with permission from ref. 62.

Fig. 6. Atomistic model of an integrated antenna/charge-separation assembly. Adapted with permission from ref. 64.

Fig. 7. Water oxidation catalysis follows either of two mechanisms. A mononuclear WNA mechanism or the binuclear ROC mechanism. Used with permission from ref. 69.

Fig. 8. WOC encapsulated in mesoporous silica. Used with permission from ref. 73.

Fig. 9. Alkyl chain substituted WOC self-assemble into micellar vesicles in water, preorganizing the WOC for the ROC mechanism. Used with permission from ref. 74.

Fig. 10. Sulfonated WOC entrapped in a M₁₂L₂₄ supramolecular sphere. Used with permission from ref. 75.

Fig. 11. A supramolecular macrocyclic WOC trimer performs better with increased ring size. Used with permission from ref. 76.

Fig. 12. The hydrogenase enzyme with the active site depicted in red and other FeS-clusters in yellow/orange. The highlighted area shows a close-up of the H-cluster and the tight embedding in the surrounding protein environment. Adapted with permission from ref. 10.

Fig. 13. Possible mechanisms for the hydrogen evolution reaction. Used with permission from ref. 2.

Fig. 14. A sulfonated [FeFe]-hydrogenase mimic is stabilized in a cyclodextrin, but catalytically inactive. Used with permission from ref. 88.

Fig. 15. Comparison between catalyst in homogeneous solution and the catalyst controlled by a tetrahedral supramolecular cage. Used with permission from ref. 90.

Fig. 16. Comparison of catalytic parameters of a natural [FeFe]-hydrogenase enzyme and a metallopolymer [FeFe]-hydrogenase mimic. Used with permission from ref. 93.

Fig. 17. Supramolecular dye-WOC assemblies are formed by cyclodextrin host-guest chemistry. This results in enhanced stability in photocatalytic water oxidation in presence of a sacrificial oxidant. Cl⁻ ions are omitted.

Fig. 18. The components (left) are the catalytic POM (top) light-harvesting porphyrin strut (middle) and structural Zr-cluster (bottom) that lead to a supramolecular artificial photosynthetic assembly (right). Used with permission from ref. 99.

Fig. 19. A supramolecular dye-WOC assembly is formed in a 5 : 1 ratio when Ru-POM is added to π-stacked PBI in water. The Ru-oxo core of the POM is depicted besides the assembly. Used with permission from ref. 101.

Fig. 20. The supramolecular Ni-TFT cage (left) has ample space to encapsulate fluorescein (red ball). A scheme of the overall H₂S splitting at pH 11–13 is depicted (right). The cage encapsulates fluorescein (FL) and expels the dye as it is oxidized. Used with permission from ref. 102.

Fig. 21. This dye-HEC assembly sparked interest into supramolecular assemblies for proton reduction.

Fig. 22. The photo active species of an [FeFe]-hydrogenase mimic bears two different chromophores. Used with permission from ref. 109.

Fig. 23. The [FeFe]-hydrogenase mimic is incorporated into the MOF structure by PSE. Used with permission from ref. 110.

Fig. 24. Supramolecular cages self-assemble when a ligand and Co²⁺ ions are combined in a 1 : 1 stoichiometry. The cage encapsulates an organic fluorescein photosensitizer in a following step. The assembly is an efficient light drive proton reduction catalyst. Used with permission from ref. 112.

Fig. 25. Schematic representation of the influence of cone-angle on self-assembled lipid topology.

Fig. 26. A schematic representation of Re sensitizers co-embedded with [FeFe]-hydrogenase mimics in SDS micelles using ascorbic acid (H2A in the figure) as a sacrificial proton and electron donor. Used with permission from ref. 117.

Fig. 27. Schematic representation of the photosensitizer embedded and the cobaloxime catalyst in the liposome membrane. The sacrificial electron donor is triethanolamine.

Fig. 28. A self-assembled bilayer preorganizes a Ru-photosensitizer and [FeFe]-hydrogenase mimic for light driven proton reduction. Used with permission from ref. 119.

Fig. 29. Covalent artificial reaction center (1) is embedded in a lipid bilayer, where it reduces quinones (Qs) under illumination. The quinone imports protons into the vesical formed by the bilayer. The arising proton gradient is used to drive the CF_OF₁-ATP synthase enzyme for the production of ATP. Used with permission from ref. 120.

Fig. 30. The redox mediator binds the dye after electron injection due to halogen-bonding substituents on the dye. It is then quickly regenerated and as a result back electron transfer is inhibited. Used with permission from ref. 123.

Fig. 31. Supramolecular antenna complexes are assembled onto n-type semi-conductors (TiO₂) using hydrogen-bonding interactions. H₂-porphyrin acceptor (blue discs) act as the seed for the growth of Zn-porphyrin antennae acting as energy donor. Used with permission from ref. 126.

Fig. 32. Supramolecular interactions are implemented in p-type DSSC using the “blue box” as redox shuttle. The P_N dye is heterogenized onto NiO, and binds the ring prior to excitation. As soon as the ring is reduced, it loses affinity for the chromophore, dethreads and moves away while carrying the charge. Used with permission from ref. 129.

Fig. 33. Molecular p/n-junctions are accessible by supramolecular Zr^IV-phosphonate chemistry. Used with permission from ref. 131.

Fig. 34. The dye-HEC assembly is anchored to NiO to produce a functional DS-PEC producing molecular hydrogen under illumination. The catalyst is preorganized to the dye by a dative, stable during operation. Used with permission from ref. 134.

Fig. 35. Heterogenized dyes bear aliphatic tails, allowing for supramolecular preorganization of Ru-WOC in DS-PEC devices. Used with permission from ref. 137.

Fig. 36. The supramolecular layer-by-layer deposition using Zr-phosphonate chemistry preorganizes chromophores reminding of the PS II. The top system is fully artificial, but the components serve the same purpose those in the PS II Reaction Centre and transfer electrons over a large distance. Used with permission from ref. 141.