Photochemical Properties of Host−Guest Supramolecular Systemswith Structurally Confined Metal−Organic CapsulesPublished as part of the Accounts of Chemical Research special issue “Supramolecular Chemistry in ConfinedSpace and Organized Assemblies”.
Xu Jing, Cheng He, Liang Zhao, and Chunying Duan*
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, researchershave designed symmetric metal−organic hosts with largeinner pockets that are spontaneously generated throughpreorganized ligands and functionalized metallocorners toconstruct dye-containing host−guest systems. The abundantnoncovalent interaction sites in the pockets of the hostsfacilitated substrate−catalyst interactions for possible enrich-ment, fixation, and activation of substrates/reagents, providingspecial electron transfer pathways for regio- or stereo-selectively photocatalytic chemical transformations. In thisAccount, we focus our attention on metal−organic hosts thatcontain photoactive or redox-active units to evaluate electrontransfer and charge separation between host and guest units inthese supramolecular systems and elucidate the related photoinduced chemical reactions controlled by these electron transferprocesses 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 supramolecularsystems, incorporating organic dyes as photosensitizers in metal−organic hosts with electron acceptor/donor guests is apromising way to enable typical enzyme-like photocatalytic transformations within a confined microenvironment. Related tothese inter- and intramolecular photoinduced electron transfer (PET) processes, the formation of host−guest supramolecularsystems to fix and isolate the donor−acceptor pair with a short through-space distance provided a new PET pathway to stabilizethe 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 forconstructing photoactive metal−organic hosts through the incorporation of dye molecules within ligand backbones or as a partof the metal nodes of the architecture. Following the methodological summary is a discussion on the mechanisms governing thephotoinduced 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 moleculeswithin the inner space of redox-active hosts. The photochemistry of these systems demonstrated the following advantages dueto 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 transferpathways. The photoinduced dye to redox-active host electron transfer is a new and efficient pathway that is meaningful forchemists to realize and understand many important enzymatic processes and to reveal the secrets of a substance and energymetabolism in biological systems. The confined interactions between the host and the guest have shown fascinating effects ofpromoting and controlling light-induced chemical transformations.
1. INTRODUCTION
Catalytic synthetic methods inspired by natural prototypes(enzymes) that react under ambient conditions using benignsolvents and green energy sources are a major research area insynthetic chemistry. The initial ideas in this area have focused oncatalysis in which a substrate is bound next to the catalyticallyactive site learning from natural systems, resulting in numerousinteresting examples of biomimetic models.1 As enzymes aremuch more than just a combination of a substrate binding site
with a catalytically active site, they often bind substrates viatailored, hydrophobic cavities, activating them via thecumulative influence of many noncovalent interactions.Chemists have constructed “molecular containers” with definedcavities that emulate the environments of enzyme pockets tocatalyze unique chemical transformations, echoing the remark-
Received: September 13, 2018Published: December 26, 2018
able properties of enzymes.1 Following the pioneering studies tocreate different classes of covalent artificial hosts, the applicationof coordination bonds as an organizational and structuralelement for the preparation of chemically novel metal−organiccapsules with a wide range of physicochemical properties andfunctions has been a crucial driving force in advancing the fieldof supramolecular catalysis.2,3
In contrast to typical covalently connected organic supra-molecular hosts, metal−organic capsules are usually sponta-neously generated by simply mixing modular building units, thatis, preorganized ligands and functionalized metallocorners, toproduce symmetrically predesigned structures.4 Investigationshave demonstrated that the highly directional and predictablenature of the metal−ligand coordination bonds with a robustpocket restricted the orientation and rotation of internallybound substrates, resulting in high selectivity with appropriateconformational host−guest matching. The special conforma-tional chiralities for the coordination geometries enable thegeneration of chirality from substitutionally inert metal ions andthe selective recognition of an enantiomer substrate withinhomochiral environments of the pocket.5 The possibility ofmerging functional catalytic sites from the ligands and the metalions makes metal−organic capsules promising candidates forsupramolecular catalysis with concerted and tandem proper-ties.6,7
Although many photosensitizers have been devised to inducesingle-electron transfer processes via light adsorption,8 incor-porating organic dyes as photosensitizers within the ligandbackbones to create metal−organic hosts as photocatalysts isquite significant. Typical enzyme-like dynamic behavior isexpected for the photocatalytic transformation in a structurallyconfined microenvironment. Supramolecular photoactive sys-tems are also constructed through encapsulating dye guestswithin the inner space of redox-active metal−organic hosts. Theformation of a host−dye complex results in efficient photo-induced electron transfer from the excited state of the dyes to thestructurally confined hosts, enhancing the stability of the chargeseparation intermediate formed by the photoinduced electrontransfer. Determining the type of electron transfer that occurs instructurally confined hosts is meaningful for chemists to realizeand understand many important enzymatic processes and toreveal the secrets of substance and energy metabolism inbiological systems. Clearly, dye-contained host−guest supra-molecular systems are commonly located at the middle pointbetween intermolecular and intramolecular photocatalyticsystems. Considering that the free energy of a photogeneratedcharge separation intermediate reinforces the electron transferprocesses, supramolecular systems within which donors andacceptors are fixed and isolated in close proximity could beadvantageous for artificial photosynthesis.9 In this Account, theconstruction and photochemical properties of two varieties ofphotoactive supramolecular systems (Scheme 1) with metal−organic hosts dye-incorporated photoactive supramolecules anddye-encapsulated photoactive supramolecules are respectivelydiscussed.
In nature, a series of chromophores are used to separate chargesin supramolecular systems, and incorporating organic ororganometallic dyes as photosensitizers into metal−organichosts with electron acceptor/donor guests is a promising way tomimic natural enzyme systems in terms of photocatalytic
transformations within a confined microenvironment. Gener-ally, the paramagnetic or redox active transitionmetal ions as theessential components of metal−organic hosts always lead toquenching of the excited state of organic or organometallic dyesin the ligand backbones. To retain the original photocatalyticproperties of dyes, closed-shell metal ions (i.e., Zn2+, Cd2+, orlanthanide ions) without quenching groups were chosen assuitable structural components. However, these closed-shellmetal ions always exhibited substitutionally active coordinationbehavior;10 therefore, ligands containing potential emissionquenchers such as strong electron donors or multidentateligands are necessary to stabilize the host structures within thesolution. Accordingly, the careful modification of both ligandsand metal ions is necessary to design the photoactive metal−organic hosts.Metal−Organic Hostswith Organometallic Dyes as a Part ofthe 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 buildingblocks for the construction of photoactive or multifunctionalmetal−organic capsules,11 despite the difficulty in derivation ofthe special coordination donors within the greatly stabilizedaforementioned Ir(III), Ru(II), and Pt(II) complexes. The mostsimple and direct preparative approach is based on the use oflinear bridging ligands to connect the photoactive Ir(III), Ru(II)and Pt(II) complexes with metal ions to form homo- andheteronuclear metal−organic capsules, respectively. Despite thefact that the direct connection of the photoactive groups alwayscaused self-quenching of the excited state, several interestingarchitectures have been created by Stang, Wurthner, and severalother research groups, which were used to selectively recognizeand detect (luminescent) small molecules (Figure 1a−c).12−14Still the low quantum yields of the excited states and the lack ofefficient catalytic active sites always precluded them from servingas efficient photocatalysts for light-driven chemical trans-formation. Also noted is the fact that the macrocycle, whichgathers three ruthenium centers, accelerates the rate of wateroxidation via a water nucleophilic attack mechanism withremarkable catalytic turnover frequencies. However, the
Scheme 1. Strategies for the Construction of Two Kinds ofPhotoactive Supramolecular Systems and the PotentialPseudo-intramolecular PET Pathways in the Dye-ContainedHost−Guest Systems
capability of [Ru(bda)bpb]3 for water oxidation requiresadditional photosensitizer, that is, Ru(bpy)3Cl2, under photo-catalytic reaction conditions.13
Another approach to construct photoactive polyhedrons isbased on the modification of efficient coordinate donors in theligand backbone of these promising photosensitizers. Ignoringthe difficulty of the derivation process for the stable coordinationcompounds, such an approach enabled the maintenance of allphotochemical properties relevant to the building blocks andprovided the possibility of incorporating two or more differentfunctions within one molecular host for photocatalytic trans-formation. The most promising example reported by Su andcoauthors is a nanosized Pd−Ru heteronuclear metal−organiccapsule (H4) that was stepwise synthesized from a predesignedredox- and photoactive Ru(II)-metalloligand and naked Pd(II)ion (Figure 2a).15−17 The presence of multiple photoactive andcatalytic metal centers endows the promising polyhedron with apotential hydrogen-evolving photochemical molecular device.Efficient hydrogen production may derive from directionalelectron transfers through multiple channels due to properorganization of the photoactive and catalytic units within theoctahedral cage, which may open up a novel pathway for thedesign of photochemical molecular devices with well-organizedmetallosupramolecules for homogeneous photocatalytic appli-cations.16 The photoinduced regio- and enantioselectivecoupling of naphthols and derivatives thereof is achieved inthe confined chiral coordination space of this metalloligand-based cage. The cages encapsulate naphthol guests, which thenundergo a regiospecific 1,4-coupling,17 rather than the normal1,1-coupling, to form 4-(2-hydroxy-1-naphthyl)-1,2-napthoqui-nones. The photocatalytic transformation proceed under both
aerobic and anaerobic conditions although through distinctpathways that nevertheless involve the same radical inter-mediates. This unusual dimerization constitutes an exceedinglyrare example of asymmetric induction in biaryl coupling bymaking use of coordination cages with dual functionalityphotoredox reactivity and stereoselectivity.Of course, the use of zinc ions to connect together the
photoactive fragments can enable conservation of the photo-active properties, that is, the original emission of the buildingblock. As shown in Figure 1, fac-tris(4-(2-pyridinyl)-phenylamine)iridium containing a Zn-based trigonal bipyrami-dal H5 metal−organic polyhedron exhibited a strong emissioncenter at 508 nm, which is assigned to the Ir(ppy)3 emission.18
When the host molecule captured CO2 to form a carbonateanion via the mimicking of natural carbonic anhydrases, theemission was quenched through photoinduced electron transferfrom the photosensitizer to the anion. As a result, the cobalt(II)-based capsule H6 did not exhibit any obvious emission, due tothe intramolecular photoinduced electron transfer from theexcited state of Ir(III) centers to Co(II) sites.19 The capsuleexhibited efficient catalytic ability of the visible-light activatedtrichloromethylation via the synergistic combination of bothphotocatalysis and transition metal in one host. With theencapsulation of the carbonate anion, the photocatalyticreaction was completely quenched. It is postulated that besidethe capsule−capsule conversion by the carbonate binding, theefficient photoinduced electron transfer from the Ir(III) centersto the anion, such as that occurring in the zinc-based capsule,was also an important factor that influenced the catalyticperformance (Figure 2b).
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) Thebinding of CO3
2− pathways in Ir-metalloligand-based Ir−Zn (H5) and Ir−Co (H6) polyhedrons.
Lanthanide ions are also interesting building blocks for theconstruction of luminescent active metal−organic capsules;several groups have synthesized many types of structures forluminescence-based detection of important small mole-cules.20,21 Interesting examples involve the use of unusualCe3+ ions as robust building blocks for the construction ofluminescent active molecular capsules. Considering the relation-ship between the environmentally sensitive character of theseparity-allowed electric-dipole 4f−5d transitions and theelectronic conformation of the ligands, the formation ofhydrogen bonds with the amide groups affected the electronictransitions associated with the Ce3+ ions, leading to significantchanges in their optical properties (Figure 3).22−24 However,because of the always lower quantum yield of the lanthanideemission, these capsules are seldom used for photocatalyticchemical transformation.
Metal−Organic Hosts With Organic Dyes Incorporatedwithin the Ligand Backbones
Generally, organic dyes are promising candidates for theconstruction of photoactive metal−organic capsules, since thederivation of desirable coordination groups can be followed bythe normal ligand synthesis procedure, with the assemblyprocesses controlled by the synthetic strategies used for thecoordination architectures reported previously. The mostimportant issue concerns the maintenance of the originalphotoactive properties of the dyes during the formation of themetal−organic capsules, as the paramagnetic or redox-activetransitionmetal ions are efficient quenchers for the excited statesof organic dyes in the ligand backbones. Considering the typical
substitutionally active coordination character of the full-shellredox inactive metal ions, dynamically inert, low spin-stateFe(II) ion was used to construct FeII4L6 tetrahedral cages H10containing one or two distinct BODIPY moieties, as well asmixed cages that contain both BODIPY chromophores (Figure4a).25 Upon the cage formation, strong excitonic interactionswere observed between at least two BODIPY chromophoresalong the edges, arising from the electronic delocalizationthrough the metal centers. The cages exhibited the sameprogression from an initial bright singlet state to a delocalizeddark state, driven by interactions between the transition dipolesof the ligands, and subsequently into geometrically relaxedspecies. In the case of cages loaded with C60 or C70 fullerenes,ultrafast host-to-guest electron transfer was observed to competewith the excitonic interactions, with basic geometricalconsiderations sufficient to explain the observed host−guestcharge-transfer behavior.In fact, the formation of host−guest supramolecular systems
to fix and isolate the donor−acceptor pair could provide a newPET pathway to stabilize the charge-separated ion pairs. In atopologically interlocking donor−acceptor system, the densepacking of the coordination cages ensured that the unique host−guest system contained strong host−guest interactions.26 Themixed-ligand capsules H11 enabled close communicationbetween donors and acceptors; the transient absorptionspectrum for this sample contained an absorption pattern thatwas similar to the sum of the absorption spectra for the radicalsof the homogeneous donor and acceptor cages, showing theformation of a relatively stable charge-separated state uponexcitation consisting of a radical cation and a radical anion.
Figure 3. Structures of lanthanide-basedmetal−organic capsules constructed frommultidentate chelating ligands (from left to right:H7,H8, andH9).
Figure 4. Organic dye-incorporated metal−organic capsules. (a) BODIPY-Fe tetrahedron H10. (b) Interlocking donor−acceptor double capsulesbased onmixed tetracyanobenzene and naphthyl ligandsH11s. Reproduced with permission from ref 26. Copyright 2016 American Chemical Society.(c) quinolone-Zn octahedron H12.
To obtain enhanced photoactive properties compared to theoriginal organic dyes, a powerful methodology to create suchmetal−organic capsules based on the use of tridentate chelatingunits as efficient building blocks to fix the orientation of ligandscoordinated to a transitionmetal center has been reported.27 Forexample, with the presence of molecules with large aromaticrings such as quinolone and the use of chelation-enhancedfluorescence to enhance the stability of the excited state,octahedral hostH12 exhibited significant luminescence (Figure4c).28 Most importantly, the luminescence enhancementoccurred upon the addition of glucosamine, suggesting thatthe formation of hydrogen bonds between the amide groupsenhanced the stability of the excited state. Such hydrogenbonding interaction-enhanced emission demonstrated that thistype of metal−organic capsule is promising for supramolecularcatalytic transformation, as the hydrogen bonding interactionsrelative to the host can not only strongly recognize the guestmolecules but at the same time also enable the enhancement ofthe lifetime and quantum yield of the excited state of the dyesthat were incorporated into the host. An interesting example is aphotoactive basket-like metal−organic tetragon containingcarbazole fragments as photosensitizers, H13 (Figure 5a).29
The encapsulation of a [FeFe]-H2 ase model compound into ahost enforced close proximity between carbazole photo-sensitizers and the enzyme active sites, ensuring enhancementof host−guest photoinduced electron transfer. The formation ofa host−guest supramolecular system caused the luminescencequenching of the host molecule, which is reasonably assigned tothe electron transfer from the excited state of the dyes in the
ligand backbone to the reducing active sites in the guest,considering that the hydrogen bonds which encapsulated theguests could enhance the emission intensity directly. As theproton reduction by the active model compound is faster thanthat of the back electron transfer, the oxidized host moleculesreacted with the electron donor that was added to facilitaterelaxation to the initial state. Hydrogen gas could be producedefficiently and directly (Figure 5d).The Fujita group reported unusual photooxidation of alkanes
within the cavity of their electron-poor triazine ligand-basedcage, H14 (Figure 5b).30 The proposed reaction mechanisminvolves the generation of a host anion radical and a guest cationradical via guest-to-host photoinduced electron transfer. Thepostulated host anion radical was elucidated by in situ IRspectroscopy. The triazine core of the panel ligands is veryelectron-poor because of the three coordinated pyridinemoieties and is thus a good electron acceptor. In the meantime,the guest molecules are tightly bound and packed within thecage cavity, facilitating the unusual photoinduced electrontransfer (Figure 5e).Notably, the formation of host−guest supramolecular systems
leads to new photochemical behavior for catalytic conversion:the promising pseudointramolecular photoinduced electrontransfer combined with the special structural constraints couldlead to several new and unexplored photocatalytic chemicaltransformations. Raymond’s tetrahedron is an interestingexample that includes special substrates for the catalytictransformation, as the formation of host−guest supramolecularsystems that fixed and isolated the donor−acceptor pair provides
Figure 5. Organic dye-incorporated metal−organic capsules: (a) triazine ligand-based cage, H13; (b)carbazole ligand-based cage, H14;(c)naphethyltriazine ligand-based cageH15. The photoinduced electron transfer and charge separation processes in the photoresponsive host−guestsystems: (d) photooxidation of alkanes in H13; (e) light-induced hydrogen evolution in H14; (f) light-induced chemical transformation in H15.
a new PET pathway to stabilize the charge-separated ion pairs.In the assembled tetrahedron host H15(Figure 5c),31 thenaphethyl bridging ligands, as photosensitizers, donate anexcited-state electron to the 1-cinnamylalkylammonium ion,facilitating a 1,3-rearrangement. The donation of excited-stateelectrons of the host to the closely encapsulated cation results inheterolytic C−N cleavage, forming a tertiary amine and ageminate radical ion pair. Back electron transfer from either theallyl radical or the tertiary amine to the ligand-based radicalcation forms a stabilized allylcation or tertiary amine radicalcation and re-establishes the original charge on the ligand. Theencapsulated tertiary amine recombines with the allylcationwithin the cavity to form the 3-substituted allyl product (Figure5f).Another interesting approach to construct dye-incorporated
metal−organic hosts is based on the use of the structurallyrobust properties of the metal−organic hosts, as the rigidcoordinated bonds and the robust properties of the metal−organic hosts can prohibit rotation or other structural relaxationof the fluorophores upon excitation. Stang and co-workersreported the fabrication and construction of two tetragonalprismatic platinum(II) metallacages (H16),which contain twoTPE-based ligands held in a cofacial arrangement (Figure 6a).32
The cage not only fluoresces in dilute solutions but also exhibitstunable visible-light emission with molecular aggregation. Theunprecedented photophysical properties originate from themetal-to-ligand charge-transfer process and twisted ligandconformations within the rigid hosts as well as special aggregatedbehaviors in different solvents. The emission wavelength isdictated by solubility/aggregate formation, with the higherpolarity environment leading to a lower light-emitting efficiencyunder conditions of similar solubility.With the presence of triphenylamine as the core of the C3-
symmetric facial ligand containing three multidentate-coordi-nating sites, a robust cerium-based tetrahedron (H17, Figure
6b) was developed, which functioned as an enzyme-likepocket.33 When the nitroxide spin-trapping agent PTIO wasencapsulated, the bright blue emission of the triphenylamine-based tetrahedron H17 was quenched significantly. Backelectron transfer from the charge separated state was stoppedthrough the formation of a new species during the spin-trappingprocesses. When NO was introduced into a mixture containingthe tetrahedron and PTIO, the spin-trapping reaction betweenPTIO and NO took place before PTIO trapped the electronfrom the excited state of the dyes. Strong emission was recoveredand used for the selective detection and bioimaging of NO insolution and living cells. Our investigation suggested that theincorporation of triphenylamine and its derivatives as the core ofC3-symmetric facial ligands with carboxylate donors is a usefulplatform for new photoactive metal−organic frameworks(Figure 6c).34−36 Of all the prepared photoactive systemscontaining dye-incorporated ligands, the architectures contain-ing triphenylamine fragments exhibited advantages due tochemical stability and excellent photochemical properties.Host−guest photoactive systems show promise for applica-
tions in photocatalysis; however, photosensitization by metal−organic hosts via the absorption of light energy and subsequentelectron transfer to an encapsulated guest acceptor to elicit achemical transformation has not been thoroughly explored. Thechallenge lies in the demonstration of the construction. To beemployed as a building block for an efficient photoactive metal−organic host, besides the essential photochemical properties(high quantum yield, long excited state lifetime), the dyemoieties should be chemically stable, show a low structurerelaxation and be easily derived with coordination sites.
Besides the incorporation of dyes in the ligand backbone toassemble the dye incorporated metal−organic hosts, another
Figure 6.Dye-incorporatedmetal−organic architectures using the structurally robust properties of themetal−organic hosts: (a) TPE-based cageH16.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.
strategy for constructing supramolecular photoactive systems isthe encapsulation of dye molecules in the inner space of redox-active hosts. Such systems could avoid self-quenching caused byaggregation, stabilize the radical intermediate, and tune theabsorption or emission of the guest through an energy transferpathway. The confined interaction between the host and theguest exhibited the fascinating effects of promoting andcontrolling light-induced chemical transformations, which aremeaningful for chemists to realize and understand a diversity ofimportant enzymatic processes.Stang and co-workers built a supramolecular host−guest
complex via encapsulation of a coronene molecule in a trigonalprismatic cage, H18, containing electron deficient triazineligands (Figure 7a).37 The encapsulation of coronene in thecapsule H18 induced a guest-to-cage charge transfer whichbroaden the spectral response in the visible region. Themicroenvironment inside the metallacage inhibits nonradiativedecay processes, resulted in the prolonged triplet lifetime thatare particularly effective for photoinduced applications. Clearly,the formation of dye contained host guest supramolecularsystems provided a route to optimize the photophysicalproperties of photosensitizers by tuning their electronicstructures.Shionoya and co-workers used a π-electron-rich Zn-
porphyrin-based ligand to coordinate Ag ions, forming a cofacialporphyrin dimer [Ag4L2]
4+ cage H19.38 The distance betweenthe two porphyrins was ideal for intercalating a series of aromaticmolecules.H19 exhibited a remarkably high intercalation abilitytoward 3 π-electron-deficient, size-suitable aromatic guests, andthe Zn-porphyrin emission was efficiently quenched, owing tothe efficiently photoinduced electron transfer from the host cageto the guest.
A reversible charge-transfer process for a fluorophore within atriazine ligand-containing metallacage, H20, was described bythe Fujita’s group. When an electron-deficient fluorophore,tetraazaporphine, was selected as the guest for the columnarhost, with triazine backbones (Figure 7c),39 the red-fluorescentdye tetraazaporphine within coordination cage H20 endowedhigh water solubility and prevented dye aggregation in thesolution and solid state. Unlike typical aromatic hydrocarbonguests, tetraazaporphine did not form a charge transfer complexwith cage H20 and remained emissive. This dimeric sandwichmotif 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 dyeencapsulated in the host and the redox activemetal centers in thehost is efficient. The confined space protects the excited state ofthe dye, avoiding the energy and electron transfer to otherspecies except the host molecule. Interaction of an integratedtetraphenylethylene moiety with C4-symmetric tetrakis-biden-tate connectors and aC3-symmetric metal center with three 2,2′-bipyridineimine chelating ligands resulted inO-symmetric cubicstructures H21, which have a general formula of M8L6 (Figure8a).40 Cyclic voltammograms for themolecular cages indicated acoupled reduction process at approximately −0.80 V (vs. Ag/AgCl). For the case of dye guests being incorporated in the innerspace of these redox-active hosts to construct photoactivesystems, the ground states and the excited states of the dyescould be directly communicated with the redox active hosts viathe charge transfer interactions. This communication can beconducted in such a way that the supramolecular systems areused to produce hydrogen from the solution in the presence ofelectron donors.
Figure 8.Redox active metal−organic capsules encapsulating dyes for light-driven hydrogen production: (a)H21with fluorescein guest, (b)H22withanionic 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.
We have also synthesized a nickel(II) contained hexanuclearmetal−organic cylinder, H22. to encapsulate an anionicruthenium polypyridyl photosensitizer Ru(dcbpy)3 (dcbpy =2,2′-bipyridine-4,4′-dicarboxylic acid).41 This resulted a pseu-dointramolecular photoinduced electron transfer processbetween the [Ru(dcbpy)3] unit and the host H22, leading toan efficient light-driven hydrogen production based on thissystem. The new, well-elucidated reaction pathways and theincreased molarity of the reaction within the confined spacerender these supramolecular systems superior to other relevantsystems.The unique communication between the dye guest and host
would be the direct PET from the excited state of the dyes to thehosts, possibly meaningful for chemists to reveal the secrets ofsubstance and energy metabolism in biological systems. Byincorporating thiosemicarbazone bidentate chelators withpotential guest-accessible sites into a tripod ligand backbone,we assembled a metal−organic polyhedron,H23, which acts as ahost for encapsulated organic dye molecules and redox catalystfor the photocatalytic generation of hydrogen from water(Figure 9).42 The cobalt ions in this polyhedron are coordinatedby three thiosemicarbazone (nitrogen and sulfur) chelatingligands and exhibit a redox potential suitable for electrochemicalproton reduction. The close proximity between the redox siteand the photosensitizer encapsulated in the pocket enables PETfrom the excited state of the photosensitizer to the cobalt-basedcatalytic sites via a distinctive PET pathway. The fact that theluminescence of the fluorescein was quenched duringencapsulation in the host while the luminescence lifetime(4.50ns) was maintained suggested that a pseudointramolecular PETfrom the excited state of the photosensitizer to the redox cobaltsites occurred, avoiding unwanted electron transfer processes.The well-separated charges enable the direct reduction ofprotons within the pocket of the cage, whereas the oxidized dyeleaves the pocket through a guest exchange reaction to recoverthe initial host−guest dye-encapsulated system. The modifiedsupramolecular system exhibits TONs comparable to thehighest values reported for related cobalt/fluorescein systems.The new, well-elucidated reaction pathways and the increasedeffective concentration of the reactants within the confinedspace render these supramolecular systems superior to otherrelevant systems.The redox-active vessel H24, which contains an octahedral
pocket, encapsulated an organic dye and performed photo-catalytic proton reduction in the inner space of the pocket toobtain molecular hydrogen and oxidized dye (Figure 8c).43 Theoxidized dye leaves the pocket via equilibrium-controlled host−guest interactions, which results in sulfide oxidation outside thehost to yield elemental sulfur. The overall loop constituteshydrogen sulfide splitting to form molecular hydrogen andelemental sulfur, which is analogous to the water-splittingreaction. The high efficiency of this reaction, simple separationof hydrogen gas and sulfur solid from solution, and easyhandling, and recyclable procedure enable potential applicationsfor this system in the chemical industry.
4. CONCLUSION AND OUTLOOKThe host−guest chemistry within dye-containing metal−organic hosts permitted additional thermodynamic activationand modification of the electron transfer route for theoccurrence of chemical reactions in these supramolecularsystems. The enzyme-like close proximity between the guestmolecule and the pockets promotes efficient through-space PET
between the encapsulated substrate and the structurallyconfined photoactive hosts. The regiospecific and stereospecificPET processes within the host are in an early stage ofdevelopment but have already revealed their importancethrough the forging of organic reactions with tandem steps orintrinsic selectivity. The encapsulation of photosensitizers intothe hydrophobic pocket of a host after PET to the redox centerentails a new strategy to control redox event positions in thecatalytic region of the hosts. In view of the interest regardingdeveloping host−guest systems mimicking natural photosyn-thesis systems, the localization of bioinspired cofactors into thepocket and of the photosensitizer outside the pocket waspostulated as an alternatively way to construct artificialsystems.44,45
The authors declare no competing financial interest.
Biographies
Xu Jing obtained his Ph.D. from Dalian University of Technology in2015. He joined the Zhang Dayu Chemical School of Dalian Universityof Technology as an Associate Professor. His research interests areprimarily 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 aProfessor. His research interest is in supramolecular coordinationchemistry.
Liang Zhao obtained his PhD at Dalian University of Technology in2014 and then began his faculty appointment as an associate professorin the State Key Laboratory of Fine Chemicals at Dalian University ofTechnology. His research interests include coordination chemistry andsupramolecular chemistry.
Chunying Duan is a Professor in Dalian University of Technology. Hecompleted his Ph.D. in 1992 at Nanjing University. His researchinterest is in functional coordination chemistry and supramolecularchemistry.
■ ACKNOWLEDGMENTSWe acknowledge the financial support from the NationalNatural Science Foundation of China (21531001, 21501041,and 21501019).
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