Supramolecular Coordination Cages for Artificial Photosynthesis and Synthetic Photocatalysis

Abstract

Because sunlight is the most abundant energy source on earth, it has huge potential for practical applications ranging from sustainable energy supply to light driven chemistry. From a chemical perspective, excited states generated by light make thermodynamically uphill reactions possible, which forms the basis for energy storage into fuels. In addition, with light, open-shell species can be generated which open up new reaction pathways in organic synthesis. Crucial are photosensitizers, which absorb light and transfer energy to substrates by various mechanisms, processes that highly depend on the distance between the molecules involved. Supramolecular coordination cages are well studied and synthetically accessible reaction vessels with single cavities for guest binding, ensuring close proximity of different components. Due to high modularity of their size, shape, and the nature of metal centers and ligands, cages are ideal platforms to exploit preorganization in photocatalysis. Herein we focus on the application of supramolecular cages for photocatalysis in artificial photosynthesis and in organic photo(redox) catalysis. Finally, a brief overview of immobilization strategies for supramolecular cages provides tools for implementing cages into devices. This review provides inspiration for future design of photocatalytic supramolecular host-guest systems and their application in producing solar fuels and complex organic molecules.

Figure 1

Schematic representation of a supramolecular coordination cage with different positions that are available for the introduction of (photo)catalytic functions.

Figure 2

(a) Schematic representation of photosystem II, which catalyzes the oxidation of water to oxygen while charge separating protons and electrons. Flow of electrons is shown by the black arrows. (b) Schematic picture of an artificial photosynthetic device with an anode for light-driven water oxidation (PS = chromophore, WOC = catalyst) and a cathode for light-driven proton reduction catalysis (PS = chromophore, PRC = proton reduction catalyst). (c) Schematic representation of the relevant energy levels for light-driven water oxidation (left) and proton reduction (right) pathways unproductive charge recombination indicated in red (CB = conduction band, VB = valence band).

Figure 3

(a) General mechanisms of photo(redox)catalysis: energy transfer (EnT) and photoinduced electron transfer (PET) involving a photosensitizer (PS), single electron oxidants [Ox] and reductants [Red], and energy acceptor [A] that accepts excited-state energy. (b) Schematic representation of the energy levels for the Dexter (red) and Förster (blue) EnT processes from an excited-state PS.

Figure 4

Schematic representation of light-absorbing supramolecular coordination cages with the photosensitizer (PS) at different positions (a) in bulk solution, (b) encapsulated inside the cage, (c) as part of the cage linker, and (d) as part of the cage metal node.

Figure 5

(a) Comparison of the two mechanisms WNA (left) and I2M (right) on the example of (b) Sun’s catalysts [Ru(pda)(pic)₂] (1) and [Ru(bpa)(pic)₂] (2).

Figure 6

Structures of supramolecular triangles (C1a–c) and typical Ru(bpy)₃ derivates (3a–c) used by the group of Würthner. Each triangle contains three Ru-based WOC catalysts.

Figure 7

Structures of supramolecular ring structures based on Ru nodes that are active in water oxidation catalysis containing oligo-ethylene glycol chains for improved solubility in aqueous media.

Figure 8

Different imidazolate ligands 4a–d and XRD structures of their respective cobalt cages C4a–d for water oxidation developed by the group of Li. The magnified coordination environment of the Co center plays an important role in catalysis. Atoms: C = gray, N = blue, O = red, Co = purple, Br = light brown.⁻

Figure 9

Proton reduction with [(Zr₃O(OH)₃Cp₃)₄(5)₆)]Cl₄C5 as catalyst and rhodamine B (6) as photosensitizer. TEOA acts a sacrificial reductant.

Figure 10

(a) Photochemical proton reduction catalyzed with [M₆7₈] (M = Co(II) C6a or Ni(II) C6b) cages, Ru(bpy)₃Cl₂3a as PS, and AA as sacrificial donor in ascorbate buffer solution (pH = 4). (b) Crystal structure of C6a, atoms: C = gray, Co = green, Cl = purple, N = blue, Si = pale yellow.

Figure 11

Cubic porphyrin 8 based cage [Fe₈8₆] C7 that may encapsulate Pt^II(dmb)Cl₂ catalyst 9 for the production of hydrogen with Ru(bpy)₃Cl₂3a as photosensitizer and methyl viologen 10 as redox mediator.

Figure 12

(a) Photocatalytic system for CO₂ reduction to formate based on supramolecular Rh(II)-paddlewheel (Rh₂)₁₂L₂₄ spheres C8a and C8b with Ru(bpy)₃Cl₂3a as the photosensitizer and TEOA as the sacrificial electron donor. (b) Polymers are obtained by mixing spheres C8a and C8b with ditopic ligand 12. Adapted with permission from ref (122). Copyright 2022 American Chemical Society.

Figure 13

(a) Host–guest binding properties of [Co₄13₄] (C11) and [Co₄14₄] (C12). C11 readily encapsulates one molecule of fluorescein 15, while C12 is too small to bind 15. (b) [Ni₄16₆] C13 binds 15. Addition of ATP leads to expulsion of 15 due to stronger binding. [Ni17₃] C14 is too small to bind either of these guests.

Figure 14

Schematic structure of box-like cages C15 and C16 based on tetraphenylethylene ligands 18 and 19 that is able to encapsulate fluorescein 15 and use the Fe centers for H₂ production.

Figure 15

Schematic structure of DHPA moieties containing ligand 20 for the self-assembly of [Co₃20₃] bowl C17 for fluorescein 15 encapsulation.

Figure 16

Schematic structure of the [Ni₆21₆]¹²⁺C18 cage, which can bind anionic Ru(dcbpy)₃^4–3c through electrostatic interactions and uses the NiII nodes as catalysts for hydrogen production.

Figure 17

Schematic structure of the supramolecular triangles C19a and C19b, based on ligands 23a and 23b, respectively, that can encapsulate fluorescein 15. Mononuclear reference complex C20 based on ligand 24 is also shown which cannot encapsulate 15.

Figure 18

(a) Structure and crystal structure of C21 and ligand 25. (b) Structure of C22 and ligand 26. (c) Structure and crystal structure of C23 and 27. Atoms: C = gray, Co = purple, O = red, N = lilac, S = bright yellow.

Figure 19

Schematic structure of triarylamine based ligand 28 in [Ni₆28₄] cage C24 and mononuclear analogue C25 based on ligand 29, used for photochemical H₂S splitting.

Figure 20

Crystal structures of [Fe₂L₃] C26 and C27, and chemical structures of their linkers 30 and 31. Atoms: C = gray, Fe = orange, O = red, N = blue.

Figure 21

Crystal structures of C28 and mononuclear complex C29, along with the chemical structures of their ligands 32 and 33, respectively. C28 is able to encapsulate 15 and perform photocatalyzed proton and CO₂ reduction. Atoms: C = gray, Ni = green, N = blue, S = bright yellow.

Figure 22

Schematic representation of [FeFe]-hydrogenase mimic 35 encapsulated into porphyrin 34-based [Fe₄34₆] cage C30, which performs photochemical proton reduction.

Figure 23

Schematic structure of cage C31 based on carbazole-containing ligand 37 and nonacoordinate Ce^IV atoms that can bind FeFe-hydrogenase mimic 36 for proton reduction.

Figure 24

Schematic structure of tetrahedral cage C32, based on ligand 38, used for photochemical proton reduction. The amino groups can be functionalized with Pt nanoparticles to further enhance hydrogen evolution.

Figure 25

(a) Schematic structure of Pd₆(RuL₃)₈C33, which catalyzes light-driven proton reduction and the electron transfer pathway to the catalytically active Pd node as indicated by ultrafast TA spectroscopy. (b) Structure of tetrathiafulvalene 40, which can be encapsulated in the cage and acts as electron relay.

Figure 26

Schematic representation of photochemical CO₂ reduction by catalyst 41 incorporated into Zr(IV)-tetrahedron C34.

Figure 27

Schematic representation of tetrahedral cage C35 based on ligands 43 and 44, which performs photochemical CO₂ reduction to CO in MeCN/H₂O (4/1) with TEA as a sacrificial reductant.

Figure 28

Schematic representation of an encapsulated photoactive guest in a photochemically inert host, and the reaction by excitation of the substrate (S) to intermediate (I), yielding product (P).

Figure 29

(a) Schematic structures of triazole 45 based Fujita cages with achiral (C36a) and chiral (C36b) Pd capped nodes. (b) C36a induced photochemical cyclization of α-diketones. (c) C36a induced photochemical radical coupling of quinones with benzylic carbons.^,

Figure 30

(a) Structures of cubic Zn₈52₆ C37 and anthraquinone 53 guest. (b) photochemical dechlorination of various chlorobenzene derivatives with C37 and 53.

Figure 31

Highly selective photochemical homo- and hetero [2+2] cycloadditions in the cavity of cage C36a or bowl C37.

Figure 32

Photochemical hetero [2+2] cycloadditions of maleimides with (a) acenaphthylene 57a in C36a, showing an induced fit, and (b) fluoranthene derivates 65a and 65b in C36b, providing chiral products with moderate ee’s.^,

Figure 33

Oxidation of benzylic carbons via host–guest charge transfer complexes with C36a and electron rich aromatic rings (a) 67, (b) 68, and (c) 69.

Figure 34

Proposed mechanism of the photoinduced oxidation of toluene to benzaldehyde by host–guest charge transfer complexes developed by Dasgupta and co-workers.

Figure 35

Condition-dependent reactions of spiroepoxy naphthalenone 78 in C39. Conditions: (a) photochemically induced epoxide ring opening to form 79, (b) thermally induced aerobic Wacker-type oxidation to form 80.

Figure 36

Self-assembly of guest-adaptive capsule C40 (Pd₆81₃) and bowl C41 (Pd₄81₂) that forms upon addition of guest to C40.

Figure 37

Photocatalytic oxidation of thioanisoles to corresponding sulfoxides by guest-adaptive capsule C40/bowl C41 hosts.

Figure 38

Proposed mechanism of the photoinduced oxidation of sulfides to sulfoxides catalyzed by guest-adaptive capsule C40/bowl C41 hosts. Circles indicate capsule C40 and squares indicate bowl C41.

Figure 39

(a) Guest-assisted self-assembly of 84 to form [Pd₂84₂] cage C42 in quantitative fashion. (b) Photoredox active guest W₁₀O₃₂85, which may be used as templating guest for the quantitative formation of [85⊂C42]. (c) Photocatalyzed aerobic oxidation of toluene derivatives with [85⊂C42] under blue LED irradiation. (a) reaction time = 5 h. (b) reaction time = 9 h.

Figure 40

Schematic representation of a supramolecular cage containing a photoactive linker. The excitation of the photosensitizer (PS) starts the reaction by transforming substrate (S) to intermediate (I), yielding product (P).

Figure 41

Photochemical reactions in cage C36a upon irradiation of the corresponding host–guest complex with UV light (a) oxidation of adamantane, (b) hydration of aryl alkynes, (c) oxidation of triquinacene, and (d) demethylenation of steroid 95.⁻

Figure 42

Photoinduced 1,3-rearragement of phenyl allylic quaternary amines inside the cavity of K₁₂ Ga₄96₆ (C43) cage.

Figure 43

Proposed mechanism for the photoinduced 1,3-rearrangement of cinnamyl ammonium ions in the cavity of C43. Diffusion out of the cavity is indicated by gray arrows.

Figure 44

Comparison between preorganized and nonpreorganized photocatalyzed reductive dehalogenations of alkyl bromides with phenothiazines.

Figure 45

(a) Oxidative dimerization of 2-naphthol derivatives promoted by C33 in regio- and stereoselective fashion. (b) Reaction conditions and outcomes of the oxidative dimerization of 2-naphthol derivatives.

Figure 46

Enantioselective [2+2] cycloaddition of acenaphthylenes 56a and 56c, induced by the cavity of Δ-C33 or Λ-C33 to give the optically active head-to-head anti-dimers.

Figure 47

Regioselective photocatalyzed [2+2] cycloadditions by racemic C33 for (a) homocoupling and (b) heterocoupling.

Figure 48

Schematic representation of a supramolecular cage containing a photoactive metal node. The excitation of the photosensitizer (PS) starts the reaction by transforming substrate (S) to intermediate (I), yielding product (P).

Figure 49

(a) Cage to cage conversion of cage C45 induced by carbonate anions in solution. (b) Photocatalytic activity of C45 for the trichloromethylation on the α-carbonyl position of 2-acylpyridines.

Figure 50

Enantiopure C46 cage (a) self-assembly, (b) substrate selectivity of the photocatalytic E–Z isomerization between 111 and 113 by Λ-C46. (c) Crystal structure of Λ-C46 atoms: C = gray, F = lime green, N = dark blue, P = orange, Ir = navy blue.

Figure 51

Self-assembled coordination cages C47–C49 containing 115 units investigated for aggregation induced emission.

Figure 52

Proposed reaction mechanism for the light-harvesting C48:RhB complex in the photoinduced oxidative cyclization between N,N-methyl aniline 118 and maleimides 119a–f.

Figure 53

Different strategies for cage immobilization on solid support: (a) drop-casting, (b) electrostatic interactions (e.g., on charged polymers or alumina), (c) hydrophobic interactions with SAM on gold, (d) stepwise assembly on gold surface by first installing an SAM of cavitand then followed by metal-mediated assembly of the cage.

Figure 54

Coordination cage immobilized on polymer via electrostatic interactions used in continuous flow setup for catalysis. Adapted with permission from ref (221). Copyright 2020 American Chemical Society.