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.

Dye-Binding Metal–Organic Cage for Highly Efficient Multiphoton Activation of C(sp3)–H Bonds

“…Catalytic synthetic methods inspired by natural enzymes that react under ambient conditions using green energy sources are one of the frontier fields in synthetic chemistry. − To echo the laudable catalytic properties and underpin the reactivity and selectivity that are not observed in normal solution, chemists have employed molecular containers with defined pockets that emulate the catalytic domain of enzymes to optimize the reactivity of substrates, achieving substantial rate enhancements. − While natural enzymes, especially those associated with C–H activation, are usually able to integrate multiactive sites to form a catalytic domain for synergistically activating reactants to achieve thermodynamically very demanding reactions, , few metal – organic cages possess multifunctional sites to activate reactants simultaneously; so far, examples of the C–H functionalization systems via synergistic activation of metal – organic capsules have not been reported. − As a powerful strategy to achieve the high activity, multiphoton excitation was applied to drive thermodynamically increasingly challenging reactions with photocatalysts under irradiation. , We envisioned that decorating well-established multiphotocatalytic sites within metal – organic cages would provide an enhanced understanding of the impacts of electron transfer features on multiphoton excitation, offering an ideal platform for the design and fabrication of new metal – organic photocatalysts with enhanced photoredox abilities …”

“…15,16 We envisioned that decorating well-established multiphotocatalytic sites within metal−organic cages would provide an enhanced understanding of the impacts of electron transfer features on multiphoton excitation, offering an ideal platform for the design and fabrication of new metal−organic photocatalysts with enhanced photoredox abilities. 17 Recently, photomediated hydrogen atom transfer (HAT) has developed as a powerful strategy to activate inert C(sp 3 )−H, 18 especially when it was combined with the consecutive multiphoton excitation. 19,20 However, controlling metal− organic cages bearing both organic dyes and hydrogen-atom transfer precursors with multiple weak terminal ligands faced marked challenges.…”

“…31 We herein report an octahedral cage (H 1 ) with six Ce III −Cl nodes that allow us to peripherally bind the organic dye 2-chloro-9H-thioxanthen-9-one (TXO). It facilitates a multiphoton excitation approach for activating inert C(sp 3 )−H that involves integrating photoinduced electron transfer (PET) to generate Ce IV −Cl species, LMCT to achieve the chloride radical, and then HAT to activate C(sp 3 )−H consecutively, enabling a multiphoton excitation driven cascade reaction (Figure 1). 15 The single crystal X-ray structure of H 1 reveals that the Ce 6 L 4 Cl 12 6+ octahedron had formed by connecting six Ce III ions and four ligands in an alternating fashion (Figures 2a and S1).…”

“…Irradiating a solution containing H 1 ⊃ TXO, styrene, and di-tert-butyl azodicarboxylate (DBAD) afforded the Cl • -radical-trapped product in mass spectrometry (Figures 4a and S39). 27,48 These results suggested that H 1 ⊃ TXO would possess efficient photomediated HAT to activate C(sp 3 )−H for special functionalization.…”

“…Inspired by the given method in photomediated organic synthesis, the alkylation of C(sp 3 )−H is chosen as a benchmark for investigating the catalytic performance. 49 In a standard experiment, reactions were performed in DMSO/CH 3 CN (1:4; v/v) irradiated under argon with a 365 nm LED at room temperature.…”