Photochemical Properties of Host–Guest Supramolecular Systems

Dec 26, 2018 - The confined interactions between the host and the guest have shown fascinating effects of promoting and controlling light-induced chem...
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Article Cite This: Acc. Chem. Res. 2019, 52, 100−109

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Photochemical Properties of Host−Guest Supramolecular Systems with Structurally Confined Metal−Organic Capsules Published as part of the Accounts of Chemical Research special issue “Supramolecular Chemistry in Confined Space and Organized Assemblies”. Xu Jing, Cheng He, Liang Zhao, and Chunying Duan*

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State Key Laboratory of Fine Chemicals, Zhang Dayu College of Chemistry, Dalian University of Technology, Dalian, 116024, P. R. China ABSTRACT: Inspired by natural photosynthesis, researchers have designed symmetric metal−organic hosts with large inner pockets that are spontaneously generated through preorganized ligands and functionalized metallocorners to construct dye-containing host−guest systems. The abundant noncovalent interaction sites in the pockets of the hosts facilitated substrate−catalyst interactions for possible enrichment, fixation, and activation of substrates/reagents, providing special electron transfer pathways for regio- or stereoselectively photocatalytic chemical transformations. In this Account, we focus our attention on metal−organic hosts that contain photoactive or redox-active units to evaluate electron transfer and charge separation between host and guest units in these supramolecular systems and elucidate the related photoinduced chemical reactions controlled by these electron transfer processes within the structurally confined pockets of these interesting metal−organic hosts. We have been engaged in developing methods to isolate a series of chromophores for charge separation in supramolecular systems, incorporating organic dyes as photosensitizers in metal−organic hosts with electron acceptor/donor guests is a promising way to enable typical enzyme-like photocatalytic transformations within a confined microenvironment. Related to these inter- and intramolecular photoinduced electron transfer (PET) processes, the formation of host−guest supramolecular systems to fix and isolate the donor−acceptor pair with a short through-space distance provided a new PET pathway to stabilize the charge-separated ion pair. Highly efficient photosynthetic systems can be obtained when charge transfer to electron donors/ acceptors occurs faster than the charge recombination. This Account starts with a brief summary of the potential approaches for constructing photoactive metal−organic hosts through the incorporation of dye molecules within ligand backbones or as a part of the metal nodes of the architecture. Following the methodological summary is a discussion on the mechanisms governing the photoinduced charge separation and electron transfer pathways within the dye-incorporated metal−organic hosts. We also searched for strategies for constructing photoactive supramolecular systems through encapsulating dye molecules within the inner space of redox-active hosts. The photochemistry of these systems demonstrated the following advantages due to the structural confinement: avoiding excited state quenching caused by other chemical species, including aggregated dyes, stabilizing the radical intermediate and tuning the absorption or emission of the guest through electron/energy transfer pathways. The photoinduced dye to redox-active host electron transfer is a new and efficient pathway that is meaningful for chemists to realize and understand many important enzymatic processes and to reveal the secrets of a substance and energy metabolism in biological systems. The confined interactions between the host and the guest have shown fascinating effects of promoting and controlling light-induced chemical transformations.

1. INTRODUCTION

with a catalytically active site, they often bind substrates via tailored, hydrophobic cavities, activating them via the cumulative influence of many noncovalent interactions. Chemists have constructed “molecular containers” with defined cavities that emulate the environments of enzyme pockets to catalyze unique chemical transformations, echoing the remark-

Catalytic synthetic methods inspired by natural prototypes (enzymes) that react under ambient conditions using benign solvents and green energy sources are a major research area in synthetic chemistry. The initial ideas in this area have focused on catalysis in which a substrate is bound next to the catalytically active site learning from natural systems, resulting in numerous interesting examples of biomimetic models.1 As enzymes are much more than just a combination of a substrate binding site © 2018 American Chemical Society

Received: September 13, 2018 Published: December 26, 2018 100

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Accounts of Chemical Research able properties of enzymes.1 Following the pioneering studies to create different classes of covalent artificial hosts, the application of coordination bonds as an organizational and structural element for the preparation of chemically novel metal−organic capsules with a wide range of physicochemical properties and functions has been a crucial driving force in advancing the field of supramolecular catalysis.2,3 In contrast to typical covalently connected organic supramolecular hosts, metal−organic capsules are usually spontaneously generated by simply mixing modular building units, that is, preorganized ligands and functionalized metallocorners, to produce symmetrically predesigned structures.4 Investigations have demonstrated that the highly directional and predictable nature of the metal−ligand coordination bonds with a robust pocket restricted the orientation and rotation of internally bound substrates, resulting in high selectivity with appropriate conformational host−guest matching. The special conformational chiralities for the coordination geometries enable the generation of chirality from substitutionally inert metal ions and the selective recognition of an enantiomer substrate within homochiral environments of the pocket.5 The possibility of merging functional catalytic sites from the ligands and the metal ions makes metal−organic capsules promising candidates for supramolecular catalysis with concerted and tandem properties.6,7 Although many photosensitizers have been devised to induce single-electron transfer processes via light adsorption,8 incorporating organic dyes as photosensitizers within the ligand backbones to create metal−organic hosts as photocatalysts is quite significant. Typical enzyme-like dynamic behavior is expected for the photocatalytic transformation in a structurally confined microenvironment. Supramolecular photoactive systems are also constructed through encapsulating dye guests within the inner space of redox-active metal−organic hosts. The formation of a host−dye complex results in efficient photoinduced electron transfer from the excited state of the dyes to the structurally confined hosts, enhancing the stability of the charge separation intermediate formed by the photoinduced electron transfer. Determining the type of electron transfer that occurs in structurally confined hosts is meaningful for chemists to realize and understand many important enzymatic processes and to reveal the secrets of substance and energy metabolism in biological systems. Clearly, dye-contained host−guest supramolecular systems are commonly located at the middle point between intermolecular and intramolecular photocatalytic systems. Considering that the free energy of a photogenerated charge separation intermediate reinforces the electron transfer processes, supramolecular systems within which donors and acceptors are fixed and isolated in close proximity could be advantageous for artificial photosynthesis.9 In this Account, the construction and photochemical properties of two varieties of photoactive supramolecular systems (Scheme 1) with metal− organic hosts dye-incorporated photoactive supramolecules and dye-encapsulated photoactive supramolecules are respectively discussed.

Scheme 1. Strategies for the Construction of Two Kinds of Photoactive Supramolecular Systems and the Potential Pseudo-intramolecular PET Pathways in the Dye-Contained Host−Guest Systems

transformations within a confined microenvironment. Generally, the paramagnetic or redox active transition metal ions as the essential components of metal−organic hosts always lead to quenching of the excited state of organic or organometallic dyes in the ligand backbones. To retain the original photocatalytic properties of dyes, closed-shell metal ions (i.e., Zn2+, Cd2+, or lanthanide ions) without quenching groups were chosen as suitable structural components. However, these closed-shell metal ions always exhibited substitutionally active coordination behavior;10 therefore, ligands containing potential emission quenchers such as strong electron donors or multidentate ligands are necessary to stabilize the host structures within the solution. Accordingly, the careful modification of both ligands and metal ions is necessary to design the photoactive metal− organic hosts. Metal−Organic Hosts with Organometallic Dyes as a Part of the Metal Nodes

As the most efficient and well investigated photosensitizers, metalloorganic photoactive functional groups, such as Ir(III), Ru(II), and Pt(II) complexes, represent important building blocks for the construction of photoactive or multifunctional metal−organic capsules,11 despite the difficulty in derivation of the special coordination donors within the greatly stabilized aforementioned Ir(III), Ru(II), and Pt(II) complexes. The most simple and direct preparative approach is based on the use of linear bridging ligands to connect the photoactive Ir(III), Ru(II) and Pt(II) complexes with metal ions to form homo- and heteronuclear metal−organic capsules, respectively. Despite the fact that the direct connection of the photoactive groups always caused self-quenching of the excited state, several interesting architectures have been created by Stang, Würthner, and several other research groups, which were used to selectively recognize and detect (luminescent) small molecules (Figure 1a−c).12−14 Still the low quantum yields of the excited states and the lack of efficient catalytic active sites always precluded them from serving as efficient photocatalysts for light-driven chemical transformation. Also noted is the fact that the macrocycle, which gathers three ruthenium centers, accelerates the rate of water oxidation via a water nucleophilic attack mechanism with remarkable catalytic turnover frequencies. However, the

2. PHOTOCHEMICAL PROPERTIES OF DYE-INCORPORATED METAL−ORGANIC HOSTS In nature, a series of chromophores are used to separate charges in supramolecular systems, and incorporating organic or organometallic dyes as photosensitizers into metal−organic hosts with electron acceptor/donor guests is a promising way to mimic natural enzyme systems in terms of photocatalytic 101

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Figure 1. Metal−organic hosts in which the metallic photoactive functional groups are linked by bridging ligands: (a) Pt-based metallocycle H1; (b) Ru-based metallacycle H2; (c) Ru-Ln metallacycle H3.

Figure 2. (a) Ru-metalloligand based Ru−Pd cage H4 for photocatalytic hydrogen production and the asymmetric C−C bond formation. (b) The binding of CO32− pathways in Ir-metalloligand-based Ir−Zn (H5) and Ir−Co (H6) polyhedrons.

aerobic and anaerobic conditions although through distinct pathways that nevertheless involve the same radical intermediates. This unusual dimerization constitutes an exceedingly rare example of asymmetric induction in biaryl coupling by making use of coordination cages with dual functionality photoredox reactivity and stereoselectivity. Of course, the use of zinc ions to connect together the photoactive fragments can enable conservation of the photoactive properties, that is, the original emission of the building block. As shown in Figure 1, fac-tris(4-(2-pyridinyl)phenylamine)iridium containing a Zn-based trigonal bipyramidal H5 metal−organic polyhedron exhibited a strong emission center at 508 nm, which is assigned to the Ir(ppy)3 emission.18 When the host molecule captured CO2 to form a carbonate anion via the mimicking of natural carbonic anhydrases, the emission was quenched through photoinduced electron transfer from the photosensitizer to the anion. As a result, the cobalt(II)based capsule H6 did not exhibit any obvious emission, due to the intramolecular photoinduced electron transfer from the excited state of Ir(III) centers to Co(II) sites.19 The capsule exhibited efficient catalytic ability of the visible-light activated trichloromethylation via the synergistic combination of both photocatalysis and transition metal in one host. With the encapsulation of the carbonate anion, the photocatalytic reaction was completely quenched. It is postulated that beside the capsule−capsule conversion by the carbonate binding, the efficient photoinduced electron transfer from the Ir(III) centers to the anion, such as that occurring in the zinc-based capsule, was also an important factor that influenced the catalytic performance (Figure 2b).

capability of [Ru(bda)bpb]3 for water oxidation requires additional photosensitizer, that is, Ru(bpy)3Cl2, under photocatalytic reaction conditions.13 Another approach to construct photoactive polyhedrons is based on the modification of efficient coordinate donors in the ligand backbone of these promising photosensitizers. Ignoring the difficulty of the derivation process for the stable coordination compounds, such an approach enabled the maintenance of all photochemical properties relevant to the building blocks and provided the possibility of incorporating two or more different functions within one molecular host for photocatalytic transformation. The most promising example reported by Su and coauthors is a nanosized Pd−Ru heteronuclear metal−organic capsule (H4) that was stepwise synthesized from a predesigned redox- and photoactive Ru(II)-metalloligand and naked Pd(II) ion (Figure 2a).15−17 The presence of multiple photoactive and catalytic metal centers endows the promising polyhedron with a potential hydrogen-evolving photochemical molecular device. Efficient hydrogen production may derive from directional electron transfers through multiple channels due to proper organization of the photoactive and catalytic units within the octahedral cage, which may open up a novel pathway for the design of photochemical molecular devices with well-organized metallosupramolecules for homogeneous photocatalytic applications.16 The photoinduced regio- and enantioselective coupling of naphthols and derivatives thereof is achieved in the confined chiral coordination space of this metalloligandbased cage. The cages encapsulate naphthol guests, which then undergo a regiospecific 1,4-coupling,17 rather than the normal 1,1-coupling, to form 4-(2-hydroxy-1-naphthyl)-1,2-napthoquinones. The photocatalytic transformation proceed under both 102

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Figure 3. Structures of lanthanide-based metal−organic capsules constructed from multidentate chelating ligands (from left to right: H7, H8, and H9).

Figure 4. Organic dye-incorporated metal−organic capsules. (a) BODIPY-Fe tetrahedron H10. (b) Interlocking donor−acceptor double capsules based on mixed tetracyanobenzene and naphthyl ligands H11s. Reproduced with permission from ref 26. Copyright 2016 American Chemical Society. (c) quinolone-Zn octahedron H12.

Lanthanide ions are also interesting building blocks for the construction of luminescent active metal−organic capsules; several groups have synthesized many types of structures for luminescence-based detection of important small molecules.20,21 Interesting examples involve the use of unusual Ce3+ ions as robust building blocks for the construction of luminescent active molecular capsules. Considering the relationship between the environmentally sensitive character of these parity-allowed electric-dipole 4f−5d transitions and the electronic conformation of the ligands, the formation of hydrogen bonds with the amide groups affected the electronic transitions associated with the Ce3+ ions, leading to significant changes in their optical properties (Figure 3).22−24 However, because of the always lower quantum yield of the lanthanide emission, these capsules are seldom used for photocatalytic chemical transformation.

substitutionally active coordination character of the full-shell redox inactive metal ions, dynamically inert, low spin-state Fe(II) ion was used to construct FeII4L6 tetrahedral cages H10 containing one or two distinct BODIPY moieties, as well as mixed cages that contain both BODIPY chromophores (Figure 4a).25 Upon the cage formation, strong excitonic interactions were observed between at least two BODIPY chromophores along the edges, arising from the electronic delocalization through the metal centers. The cages exhibited the same progression from an initial bright singlet state to a delocalized dark state, driven by interactions between the transition dipoles of the ligands, and subsequently into geometrically relaxed species. In the case of cages loaded with C60 or C70 fullerenes, ultrafast host-to-guest electron transfer was observed to compete with the excitonic interactions, with basic geometrical considerations sufficient to explain the observed host−guest charge-transfer behavior. In fact, the formation of host−guest supramolecular systems to fix and isolate the donor−acceptor pair could provide a new PET pathway to stabilize the charge-separated ion pairs. In a topologically interlocking donor−acceptor system, the dense packing of the coordination cages ensured that the unique host− guest system contained strong host−guest interactions.26 The mixed-ligand capsules H11 enabled close communication between donors and acceptors; the transient absorption spectrum for this sample contained an absorption pattern that was similar to the sum of the absorption spectra for the radicals of the homogeneous donor and acceptor cages, showing the formation of a relatively stable charge-separated state upon excitation consisting of a radical cation and a radical anion.

Metal−Organic Hosts With Organic Dyes Incorporated within the Ligand Backbones

Generally, organic dyes are promising candidates for the construction of photoactive metal−organic capsules, since the derivation of desirable coordination groups can be followed by the normal ligand synthesis procedure, with the assembly processes controlled by the synthetic strategies used for the coordination architectures reported previously. The most important issue concerns the maintenance of the original photoactive properties of the dyes during the formation of the metal−organic capsules, as the paramagnetic or redox-active transition metal ions are efficient quenchers for the excited states of organic dyes in the ligand backbones. Considering the typical 103

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Figure 5. Organic dye-incorporated metal−organic capsules: (a) triazine ligand-based cage, H13; (b)carbazole ligand-based cage, H14; (c)naphethyltriazine ligand-based cage H15. The photoinduced electron transfer and charge separation processes in the photoresponsive host−guest systems: (d) photooxidation of alkanes in H13; (e) light-induced hydrogen evolution in H14; (f) light-induced chemical transformation in H15.

ligand backbone to the reducing active sites in the guest, considering that the hydrogen bonds which encapsulated the guests could enhance the emission intensity directly. As the proton reduction by the active model compound is faster than that of the back electron transfer, the oxidized host molecules reacted with the electron donor that was added to facilitate relaxation to the initial state. Hydrogen gas could be produced efficiently and directly (Figure 5d). The Fujita group reported unusual photooxidation of alkanes within the cavity of their electron-poor triazine ligand-based cage, H14 (Figure 5b).30 The proposed reaction mechanism involves the generation of a host anion radical and a guest cation radical via guest-to-host photoinduced electron transfer. The postulated host anion radical was elucidated by in situ IR spectroscopy. The triazine core of the panel ligands is very electron-poor because of the three coordinated pyridine moieties and is thus a good electron acceptor. In the meantime, the guest molecules are tightly bound and packed within the cage cavity, facilitating the unusual photoinduced electron transfer (Figure 5e). Notably, the formation of host−guest supramolecular systems leads to new photochemical behavior for catalytic conversion: the promising pseudointramolecular photoinduced electron transfer combined with the special structural constraints could lead to several new and unexplored photocatalytic chemical transformations. Raymond’s tetrahedron is an interesting example that includes special substrates for the catalytic transformation, as the formation of host−guest supramolecular systems that fixed and isolated the donor−acceptor pair provides

To obtain enhanced photoactive properties compared to the original organic dyes, a powerful methodology to create such metal−organic capsules based on the use of tridentate chelating units as efficient building blocks to fix the orientation of ligands coordinated to a transition metal center has been reported.27 For example, with the presence of molecules with large aromatic rings such as quinolone and the use of chelation-enhanced fluorescence to enhance the stability of the excited state, octahedral host H12 exhibited significant luminescence (Figure 4c).28 Most importantly, the luminescence enhancement occurred upon the addition of glucosamine, suggesting that the formation of hydrogen bonds between the amide groups enhanced the stability of the excited state. Such hydrogen bonding interaction-enhanced emission demonstrated that this type of metal−organic capsule is promising for supramolecular catalytic transformation, as the hydrogen bonding interactions relative to the host can not only strongly recognize the guest molecules but at the same time also enable the enhancement of the lifetime and quantum yield of the excited state of the dyes that were incorporated into the host. An interesting example is a photoactive basket-like metal−organic tetragon containing carbazole fragments as photosensitizers, H13 (Figure 5a).29 The encapsulation of a [FeFe]-H2 ase model compound into a host enforced close proximity between carbazole photosensitizers and the enzyme active sites, ensuring enhancement of host−guest photoinduced electron transfer. The formation of a host−guest supramolecular system caused the luminescence quenching of the host molecule, which is reasonably assigned to the electron transfer from the excited state of the dyes in the 104

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Figure 6. Dye-incorporated metal−organic architectures using the structurally robust properties of the metal−organic hosts: (a) TPE-based cage H16. Reproduced with permission from ref 32. Copyright 2017 American Chemical Society. (b) Triphenylamine-based tetrahedron H17. (c) Triphenylamine-based metal−organic framework M1.

Figure 7. Chromophores encapsulated in the metal−organic hosts: (a) coronene in trigonal prismatic H18 cage, (b) acenaphthenequinone in H19, and (c) tetraazaporphinein H20.

6b) was developed, which functioned as an enzyme-like pocket.33 When the nitroxide spin-trapping agent PTIO was encapsulated, the bright blue emission of the triphenylaminebased tetrahedron H17 was quenched significantly. Back electron transfer from the charge separated state was stopped through the formation of a new species during the spin-trapping processes. When NO was introduced into a mixture containing the tetrahedron and PTIO, the spin-trapping reaction between PTIO and NO took place before PTIO trapped the electron from the excited state of the dyes. Strong emission was recovered and used for the selective detection and bioimaging of NO in solution and living cells. Our investigation suggested that the incorporation of triphenylamine and its derivatives as the core of C3-symmetric facial ligands with carboxylate donors is a useful platform for new photoactive metal−organic frameworks (Figure 6c).34−36 Of all the prepared photoactive systems containing dye-incorporated ligands, the architectures containing triphenylamine fragments exhibited advantages due to chemical stability and excellent photochemical properties. Host−guest photoactive systems show promise for applications in photocatalysis; however, photosensitization by metal− organic hosts via the absorption of light energy and subsequent electron transfer to an encapsulated guest acceptor to elicit a chemical transformation has not been thoroughly explored. The challenge lies in the demonstration of the construction. To be employed as a building block for an efficient photoactive metal− organic host, besides the essential photochemical properties (high quantum yield, long excited state lifetime), the dye moieties should be chemically stable, show a low structure relaxation and be easily derived with coordination sites.

a new PET pathway to stabilize the charge-separated ion pairs. In the assembled tetrahedron host H15(Figure 5c),31 the naphethyl bridging ligands, as photosensitizers, donate an excited-state electron to the 1-cinnamylalkylammonium ion, facilitating a 1,3-rearrangement. The donation of excited-state electrons of the host to the closely encapsulated cation results in heterolytic C−N cleavage, forming a tertiary amine and a geminate radical ion pair. Back electron transfer from either the allyl radical or the tertiary amine to the ligand-based radical cation forms a stabilized allylcation or tertiary amine radical cation and re-establishes the original charge on the ligand. The encapsulated tertiary amine recombines with the allylcation within the cavity to form the 3-substituted allyl product (Figure 5f). Another interesting approach to construct dye-incorporated metal−organic hosts is based on the use of the structurally robust properties of the metal−organic hosts, as the rigid coordinated bonds and the robust properties of the metal− organic hosts can prohibit rotation or other structural relaxation of the fluorophores upon excitation. Stang and co-workers reported the fabrication and construction of two tetragonal prismatic platinum(II) metallacages (H16),which contain two TPE-based ligands held in a cofacial arrangement (Figure 6a).32 The cage not only fluoresces in dilute solutions but also exhibits tunable visible-light emission with molecular aggregation. The unprecedented photophysical properties originate from the metal-to-ligand charge-transfer process and twisted ligand conformations within the rigid hosts as well as special aggregated behaviors in different solvents. The emission wavelength is dictated by solubility/aggregate formation, with the higher polarity environment leading to a lower light-emitting efficiency under conditions of similar solubility. With the presence of triphenylamine as the core of the C3symmetric facial ligand containing three multidentate-coordinating sites, a robust cerium-based tetrahedron (H17, Figure

3. PHOTOCHEMICAL PROPERTIES OF DYE-ENCAPSULATED METAL−ORGANIC HOSTS Besides the incorporation of dyes in the ligand backbone to assemble the dye incorporated metal−organic hosts, another 105

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Figure 8. Redox active metal−organic capsules encapsulating dyes for light-driven hydrogen production: (a) H21 with fluorescein guest, (b) H22 with anionic Ru(dcbpy)3 guest, and (c) H24 with fluorescein guest.

Figure 9. Fluorescein-encapsulated H23 for light-driven hydrogen evolution and the possible electron communication pathways.

A reversible charge-transfer process for a fluorophore within a triazine ligand-containing metallacage, H20, was described by the Fujita’s group. When an electron-deficient fluorophore, tetraazaporphine, was selected as the guest for the columnar host, with triazine backbones (Figure 7c),39 the red-fluorescent dye tetraazaporphine within coordination cage H20 endowed high water solubility and prevented dye aggregation in the solution and solid state. Unlike typical aromatic hydrocarbon guests, tetraazaporphine did not form a charge transfer complex with cage H20 and remained emissive. This dimeric sandwich motif deserves further investigations on unique photochemical, redox, and catalytic properties. For the case of redox-active metal−organic capsules, it is postulated that photoinduced electron transfer between the dye encapsulated in the host and the redox active metal centers in the host is efficient. The confined space protects the excited state of the dye, avoiding the energy and electron transfer to other species except the host molecule. Interaction of an integrated tetraphenylethylene moiety with C4-symmetric tetrakis-bidentate connectors and a C3-symmetric metal center with three 2,2′bipyridineimine chelating ligands resulted in O-symmetric cubic structures H21, which have a general formula of M8L6 (Figure 8a).40 Cyclic voltammograms for the molecular cages indicated a coupled reduction process at approximately −0.80 V (vs. Ag/ AgCl). For the case of dye guests being incorporated in the inner space of these redox-active hosts to construct photoactive systems, the ground states and the excited states of the dyes could be directly communicated with the redox active hosts via the charge transfer interactions. This communication can be conducted in such a way that the supramolecular systems are used to produce hydrogen from the solution in the presence of electron donors.

strategy for constructing supramolecular photoactive systems is the encapsulation of dye molecules in the inner space of redoxactive hosts. Such systems could avoid self-quenching caused by aggregation, stabilize the radical intermediate, and tune the absorption or emission of the guest through an energy transfer pathway. The confined interaction between the host and the guest exhibited the fascinating effects of promoting and controlling light-induced chemical transformations, which are meaningful for chemists to realize and understand a diversity of important enzymatic processes. Stang and co-workers built a supramolecular host−guest complex via encapsulation of a coronene molecule in a trigonal prismatic cage, H18, containing electron deficient triazine ligands (Figure 7a).37 The encapsulation of coronene in the capsule H18 induced a guest-to-cage charge transfer which broaden the spectral response in the visible region. The microenvironment inside the metallacage inhibits nonradiative decay processes, resulted in the prolonged triplet lifetime that are particularly effective for photoinduced applications. Clearly, the formation of dye contained host guest supramolecular systems provided a route to optimize the photophysical properties of photosensitizers by tuning their electronic structures. Shionoya and co-workers used a π-electron-rich Znporphyrin-based ligand to coordinate Ag ions, forming a cofacial porphyrin dimer [Ag4L2]4+ cage H19.38 The distance between the two porphyrins was ideal for intercalating a series of aromatic molecules. H19 exhibited a remarkably high intercalation ability toward 3 π-electron-deficient, size-suitable aromatic guests, and the Zn-porphyrin emission was efficiently quenched, owing to the efficiently photoinduced electron transfer from the host cage to the guest. 106

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between the encapsulated substrate and the structurally confined photoactive hosts. The regiospecific and stereospecific PET processes within the host are in an early stage of development but have already revealed their importance through the forging of organic reactions with tandem steps or intrinsic selectivity. The encapsulation of photosensitizers into the hydrophobic pocket of a host after PET to the redox center entails a new strategy to control redox event positions in the catalytic region of the hosts. In view of the interest regarding developing host−guest systems mimicking natural photosynthesis systems, the localization of bioinspired cofactors into the pocket and of the photosensitizer outside the pocket was postulated as an alternatively way to construct artificial systems.44,45

We have also synthesized a nickel(II) contained hexanuclear metal−organic cylinder, H22. to encapsulate an anionic ruthenium polypyridyl photosensitizer Ru(dcbpy)3 (dcbpy = 2,2′-bipyridine-4,4′-dicarboxylic acid).41 This resulted a pseudointramolecular photoinduced electron transfer process between the [Ru(dcbpy)3] unit and the host H22, leading to an efficient light-driven hydrogen production based on this system. The new, well-elucidated reaction pathways and the increased molarity of the reaction within the confined space render these supramolecular systems superior to other relevant systems. The unique communication between the dye guest and host would be the direct PET from the excited state of the dyes to the hosts, possibly meaningful for chemists to reveal the secrets of substance and energy metabolism in biological systems. By incorporating thiosemicarbazone bidentate chelators with potential guest-accessible sites into a tripod ligand backbone, we assembled a metal−organic polyhedron, H23, which acts as a host for encapsulated organic dye molecules and redox catalyst for the photocatalytic generation of hydrogen from water (Figure 9).42 The cobalt ions in this polyhedron are coordinated by three thiosemicarbazone (nitrogen and sulfur) chelating ligands and exhibit a redox potential suitable for electrochemical proton reduction. The close proximity between the redox site and the photosensitizer encapsulated in the pocket enables PET from the excited state of the photosensitizer to the cobalt-based catalytic sites via a distinctive PET pathway. The fact that the luminescence of the fluorescein was quenched during encapsulation in the host while the luminescence lifetime(4.50 ns) was maintained suggested that a pseudointramolecular PET from the excited state of the photosensitizer to the redox cobalt sites occurred, avoiding unwanted electron transfer processes. The well-separated charges enable the direct reduction of protons within the pocket of the cage, whereas the oxidized dye leaves the pocket through a guest exchange reaction to recover the initial host−guest dye-encapsulated system. The modified supramolecular system exhibits TONs comparable to the highest values reported for related cobalt/fluorescein systems. The new, well-elucidated reaction pathways and the increased effective concentration of the reactants within the confined space render these supramolecular systems superior to other relevant systems. The redox-active vessel H24, which contains an octahedral pocket, encapsulated an organic dye and performed photocatalytic proton reduction in the inner space of the pocket to obtain molecular hydrogen and oxidized dye (Figure 8c).43 The oxidized dye leaves the pocket via equilibrium-controlled host− guest interactions, which results in sulfide oxidation outside the host to yield elemental sulfur. The overall loop constitutes hydrogen sulfide splitting to form molecular hydrogen and elemental sulfur, which is analogous to the water-splitting reaction. The high efficiency of this reaction, simple separation of hydrogen gas and sulfur solid from solution, and easy handling, and recyclable procedure enable potential applications for this system in the chemical industry.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Liang Zhao: 0000-0001-8197-6686 Chunying Duan: 0000-0003-1638-6633 Notes

The authors declare no competing financial interest. Biographies Xu Jing obtained his Ph.D. from Dalian University of Technology in 2015. He joined the Zhang Dayu Chemical School of Dalian University of Technology as an Associate Professor. His research interests are primarily in the area of host−guest supramolecular systems. Cheng He earned his Ph.D. degree in 2000 from Nanjing University. He has worked at the Dalian University of Technology since 2006 as a Professor. His research interest is in supramolecular coordination chemistry. Liang Zhao obtained his PhD at Dalian University of Technology in 2014 and then began his faculty appointment as an associate professor in the State Key Laboratory of Fine Chemicals at Dalian University of Technology. His research interests include coordination chemistry and supramolecular chemistry. Chunying Duan is a Professor in Dalian University of Technology. He completed his Ph.D. in 1992 at Nanjing University. His research interest is in functional coordination chemistry and supramolecular chemistry.

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation of China (21531001, 21501041, and 21501019).

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REFERENCES

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4. CONCLUSION AND OUTLOOK The host−guest chemistry within dye-containing metal− organic hosts permitted additional thermodynamic activation and modification of the electron transfer route for the occurrence of chemical reactions in these supramolecular systems. The enzyme-like close proximity between the guest molecule and the pockets promotes efficient through-space PET 107

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DOI: 10.1021/acs.accounts.8b00463 Acc. Chem. Res. 2019, 52, 100−109

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DOI: 10.1021/acs.accounts.8b00463 Acc. Chem. Res. 2019, 52, 100−109