MetalâOrganic Frameworks: Versatile Materials for Heterogeneous

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Metal-Organic Frameworks: Versatile Materials for Heterogeneous Photocatalysis Le Zeng, Xiangyang Guo, Cheng He, and Chunying Duan ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02228 • Publication Date (Web): 13 Oct 2016 Downloaded from http://pubs.acs.org on October 14, 2016

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Metal-Organic Frameworks: Versatile Materials for Heterogeneous Photocatalysis Le Zeng, Xiangyang Guo, Cheng He, and Chunying Duan* State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian, 116024, Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300071, China. ABSTRACT: Photocatalysis is one of the most important chemical methods to face up to the energy and environmental crisis via converting inexhaustible solar energy into clean chemical potential. The general history of the development of photocatalysis based on porous metalorganic frameworks (MOFs) is simply divided into three branches with focus placed on the distinct structural role of the photocatalytic center: the inorganic cluster nodes, the organic linkers and the guests in the pores of MOFs. In each branch, these photocatalytic centers are considered to be mono-dispersed within the crystal lattices with the other two structure roles regularly distributed to isolate the active centers and sometimes, to provide more functions other than photoactivity. This distinctive nature has rendered MOFs as promising candidates for photocatalysis not only because they combine the benefits of heterogeneous catalysis and homogeneous catalysis, but also for the possibility to merging multifunctional catalytic sites for concerted or cascade photocatalysis. The design strategy and improvement approaches for MOFbased photocatalysts are also introduced with emphasis on structure. Our intention is for this comprehensive view of MOFs-involved photocatalysts to inspire new ideas for designing heterogeneous photocatalysts toward the better utilization of solar energy.

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Keywords: metal-organic framework; heterogeneous photocatalysis; structure-function relationship; porosity; mono-disperse; multifunctional; cascade or concerted photocatalysis.


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Introduction In nature, green plants use solar radiation to convert water and carbon dioxide into energy and biomolecules, which are essential for their survival.1 With the ever-increasing energy and environment demand to provide high quality of life, humans are urged to learn from how nature obtains energy and materials through a sustainable approach called artificial photosynthesis or simply photocatalysis.2-6 The classical photocatalytic process consists of three steps: first, a photosensitizer absorbs light to reach its excited state; second, the excited state transforms into a charge separation state, producing a mobile electron and a hole; finally, the transportable electron or hole reacts with substrate. Photocatalyst that absorbs the energy of light and most importantly uses the energy to produce a new redox active center is the heart of photocatalysis. Ever since Fujishima and Honda discovered in 1972 that semiconducting TiO2 can drive water splitting under UV irradiation, extensive efforts have been devoted to investigating semiconductor-based photocatalysts.7-11 TiO2 is the flagship of semiconductor photocatalysts and has been extensively applied to photo-driven water splitting and organic contaminant degradation.9-11 However, the insensitivity of TiO2 to a large portion of the solar energy spectrum (44%)—the visible light portion—has forced researchers to develop photocatalysts capable of absorbing visible light.12 Transition-metal complexes such as Ru(bpy)32+ are thus attracting increasing attention for their enormous potential to catalyze useful and unique organic transformations upon visible-light illumination.13-16 Generally, an excellent photocatalyst should own the following features: strong absorption of visible light, long lifetime of excited state, high yielding of charge separation states and good


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charge mobility. Heterogeneous photocatalysts are in great demand for their recyclability and ease of separation from workup.17 Well known as a hybrid porous crystalline material linked by coordination bonds between organic linkers and metal or metal cluster nodes, metal-organic frameworks (MOFs) are ideal candidates for photocatalyst design.18-27 First, the three distinct components of MOFs—metal nodes, organic linkers and pores—can all be easily tailored for photocatalysis thanks to the modular nature of MOFs, combining the merits of organic and inorganic chemistry. Second, the uniform channels or pores are beneficial for substrate-catalyst interactions and the well-defined crystalline structure of MOFs is useful for elucidating structurefunction relationships and good for charge mobility. Third, as a solid material, MOFs are easy to separate from reaction mixtures and can then be reused for the next run, possibly extending the lifetime of the catalysts and reducing waste and contamination.

Figure 1 Scheme presentation of the three types of MOF photocatalysts showing the distinct locations of photo-initiated redox centers: type I, The inorganic cluster nods as mono-dispersed nano-semiconductors photocatalysts; type II, the photocatalytic organic or metal-organic dyes as


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linkers to be heterogenized; type III, the photocatalytic units as guests encapsulated in the pores of MOFs. The exploiting of MOFs as photocatalysts has followed the progress of photocatalysis and has been accompanied by the growth of MOF structural and functional complexity. To date, numerous reports demonstrating the unique merits of MOFs-based photocatalysis over purely organic or inorganic systems have been reported, and excellent reviews are published almost yearly.28-34 In this perspective, MOF-based photocatalysts are classified into three groups according to the intrinsic structural roles of the photocatalytic centers (Figure 1). In type I MOFs, the 0D inorganic cluster nodes act as semiconductor dots photocatalysts that well-isolated by the organic linkers and sometimes also by the pores of the MOFs. The entire MOFs are described as isolated semiconductor dots that regularly and uniformly mono-dispersed in the lattice of the crystals. In type II MOFs, the linkers are functional organic and metal-organic dye-based photocatalysts that consolidated and separated by the metal nods. The entire MOFs are considered to be heterogenized dye-based photocatalyst that well positioned into isolated fashion with ordered arrays. In type III MOFs, the photocatalytic species that have suitable sizes and structures are encapsulated within the pores of the MOFs. The entire MOFs are considered as regular distributed supramolecular systems that were isolated by the MOF scaffold. Outstanding examples of MOF photocatalysis are presented to reveal the design routes in terms of these three-types of MOFs. Type I MOFs-based photocatalysts with 0D inorganic clusters as metal nods. Semiconductors, especially inorganic semiconductors, have been the leading players in photocatalysis for decades.7-11 Scheme 1 shows the elementary steps that occur in a photocatalysis event mediated by semiconductors. Upon the absorption of photons with the energy


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greater than the band gap between the conduction band (CB) and the valence band (VB), charge separation occurs in the semiconductor, producing an electron in the CB and a hole in the VB. The generated charges (holes or electrons) then migrate to the surface of the semiconductor to react with reductive or oxidizing substrates. The long journey of MOF-based photocatalysis began with the semiconducting property because of the inherent connection between MOFs and semiconductors.35-37 In case of the metal-oxygen clusters as the nods of the MOFs, these clusters can be simplified as semiconductor dots that are isolated and ordered distributed within the networks of MOFs (Scheme 1).19 Compared to traditional semiconductors, semiconductor dots are more efficient as photocatalysts because the detrimental charge recombination is no longer a problem.

Scheme 1 (Left) Elementary steps of semiconductor photocatalysis: i) light absorption, ii) electron promotion from the VB to the CB, iii) charge migration to the surface of the particle, oxidation of the substrate (S) by the positive hole (h+), and reduction of the S by an electron (e−). Reprinted from ref 32. Copyright 2016, Wiley-VCH. (Right) Scheme presentation of type I MOF as mono-dispersed semiconductor dots distributed on the networks of the lattice.


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MOF scaffold can exploit the full potential of semiconductor dots as photocatalysts because 1) the high porosity allows the substrate to be close to semiconductor dots so that the produced active electrons or holes do not need to travel to the surface of the whole material; 2) the density of semiconductor dots can be higher than the homogeneous systems, while no quenching will occur thanks to the isolation of inorganic clusters by organic linkers; 3) the organic linkers can serve as the photon antennae to increase the visible light absorption of semiconductors, besides the isolators for the semiconductor dots. This rational deduction is confirmed by lots of examples. In 2007, Garcia and co-workers were the first to provide experimental evidence supporting the behavior of MOF-5 as a semiconductor (Scheme 2).35 A charge-separated state of MOF-5 was observed to decay at the microsecond time scale after irradiation, and the band gap was estimated to be 3.4 eV. Thereafter, many MOFs, including the UiO-66 and MIL series, have been reported to behave as semiconductors.36 These semiconductor-like MOFs have been successfully applied to photocatalysis systems involving the photodegradation of organic pollutants and solar energy conversion, the home ground of semiconductor photocatalysis.37-40 Moreover, the tunability and diversity of MOF structure provide numerous methods for MOFs to function even better than real semiconductors.36, 44-45 For example, the organic linker can be adjusted to be a better photon antenna via the facile introduction of –NH2 group. At the same time, noble-metal nanoparticles (NPs) can be encapsulated into the MOF pores as electron reservoirs, which can increase the charge mobility by orders of magnitude. Recently, Grzybowski and coworkers demonstrated that adding Ag nano-clusters to a Rb-CD(cyclodextrin)-MOF not only imparts moderate electrical conductivity to an otherwise insulating material but also renders it photoconductive, with conductivity increasing by up to 4 orders of magnitude upon light irradiation.46 Lanthanide (Ln) ions are a unique type of photoactive metal node in MOFs and have been widely used to


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construct Ln-MOFs that function as luminescent sensors and light-emitting materials.41-43 However, the Laporte-forbidden f-f transition inherent to Ln ions limits the application of LnMOFs as photocatalysts.

Scheme 2 Photophysical processes that occur after the irradiation of the MOF-5 solid material, suggesting semiconductor behavior. Reprinted from ref 35. Copyright 2007, Wiley-VCH. Type II MOFs with dye molecules as organic linkers. Dye molecules particularly metallo-organic dye Ru(bpy)32+ have been extensively studied as photocatalysts in homogeneous systems for their beneficial photophysical properties, which include strong visible-light absorption and long-lived excited states.12-16,

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seminal results obtained using photoinduced electron transfer (PET), researchers have demonstrated various dye molecules to be very efficient in photoredox reactions through excitation.12-16 Incorporation of these photoredox-capable dye molecules into MOFs broadens and deepens the MOF photocatalysis to achieve more sophisticated and meaningful photoconversions. Meanwhile, MOFs enable the high-density and orderly distribution of isolated dyephotocatalysts along the heterogeneous support; by contrast, in homogeneous systems, the same


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concentration of dyes often induces detrimental aggregation that suppresses activity or causes self-quenching.49-52 Most importantly, the crystalline nature of MOF structure provides other possibilities for the transport of excited states or photoinduced electrons and holes.53-56 For example, Lin and coworkers recently reported that the “through space” energy jumping of singlet excited states beyond the nearest neighbor can account for up to 67% of the energy transfer rate in truxene-based materials (Figure 2).57 This phenomenon can be observed only in highly ordered networks and may provide an additional pathway for energy transfer and exciton migration, which are in favor of photocatalysis.

Figure 2 Cartoon showing the step-by-step nearest neighbor hopping (NNH) and long-distance jumping pathways of excitation migration in a network of chromophores. Reprinted from ref 57. Copyright 2016, American Chemical Society. Type III MOFs with photoredox species encapsulated in pores. In addition to the metal nodes and the organic or metal-organic linkers, pores can be another origin of photoredox activity for MOFs, which enrich MOFs as photocatalysts while also providing additional possibilities for photocatalysis. Photoredox species of appropriate size can be introduced into the pores of MOFs as guests with sustained and often improved photoactivity


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owning to the isolation from each other and the mutual effect with the MOF scaffold.58-59 Interestingly, the microenvironment of MOFs' pores can be finely adjusted for energy and electron transfer, thus making the MOF-based photocatalysis more similar to enzymatic catalysis.60-62 Importantly, the high porosity provides not only sufficient space for the catalystsubstrate interaction but also size-selectivity for photocatalysis.65, 68 Polyoxometalates (POMs), because of their outstanding performance as acid catalysts and limited surface area as a solid, are continually introduced into MOF pores to obtain stable porous catalysts with POMs dispersed at the molecular level.61-67 Su and coworkers were the first to report well-defined crystalline POM/MOF composites and the first to use them as true heterogeneous acid catalysts (Figure 3).63 Keggin-type POMs were incorporated into HKUST-1 scaffolds through a one-step hydrothermal reaction, resulting in a POM/MOF series. One of these POM-MOF composites exhibited high activity and reusability in the hydrolysis of esters in excess water. Furthermore, selectivity based on substrate size and accessibility to the pore surface was observed, which highlighted the heterogeneity of these POM-MOF catalysts. Inspired by this work, researchers have devoted much effort to the research of catalytic POMMOF assemblies, and photoactive POM-MOF systems have recently become a topic of great interest.61-62,64-67 The three types of MOF-based photocatalysts offer plenty of ideas for researchers to explore and have been successfully applied in photocatalysis.28-34 One more particular thing to be noted regarding the superiority of MOFs as photocatalysts is that multiple functional components can be organized and integrated into a single material in an ordered and hierarchical manner for facile energy and electron transfer, thus providing infinite possibilities for synergistic or cooperative catalysis.69-70 In the following section, we provide an overall profile of MOF


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photocatalysis, with a special focus on the design aspect. Meaningful and successful application examples of the three types of MOFs will be introduced.

Figure 3 View of a (001) sheet with two types of pores, A and B, in NENU-n (n=1-6). The CuBTC framework and Keggin polyanions are represented by wireframe and polyhedral models, respectively. Reprinted from ref 63. Copyright 2009, American Chemical Society. Photocatalysis with Type I MOFs Inorganic semiconductors were the first and remain the most widely studied heterogeneous photocatalysts.7-9 When integrated into an MOF scaffold, the metal clusters maintain their semiconducting properties, and the porosity of the MOF structure helps make full use of the metal clusters as photocatalysts. As a traditional photocatalyst that has received lasting attention, semiconductors are of particular interest in the photodegradation of organic pollutants and in solar energy conversion processes such as water splitting.8-11 Naturally, the application of type I MOFs as photocatalysts is primarily focused on these fields. Nowadays, there already are reviews on the photoinduced decomposition of organic pollutants, especially organic dyes, the photochemical reduction of protons and carbon dioxide and the photooxidation of alcohols using


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MOFs as photocatalysts.37-40 Almost all the photoactive MOFs in these aforementioned reviews are consist of nano-semiconductors, the metal-oxygen clusters. The development of MOFs for the photoreduction of protons and carbon dioxide will be discussed below whereas the photodegradation of organic pollutants and the photooxidation of alcohols using MOFs will not be discussed in this perspective because these applications share the same design and improvement strategies.

Scheme 3 Mechanistic proposal to rationalize the photophysics of Zr-MOF used to photo-drive the hydrogen evolution reaction (HER). Reproduced from ref 79. Copyright 2010, Wiley-VCH. The reduction of protons to hydrogen and the reduction of carbon dioxide to carbon monoxide or simple carbon-containing organics are regarded as two powerful methods to solve the energy and environment dilemma.3-6 Various inorganic semiconductors and noble metal complexes have been studied to achieve these two valuable reduction reactions with the assistance of photo-energy.3-11 Many of the MOFs capable of photocatalytic hydrogen evolution or carbon dioxide reduction contain metal-oxygen clusters, taking advantage of their semiconducting properties.37,39-40 Only a few examples involve the incorporation of proton/carbon dioxide reduction catalysts into MOF scaffolds or pores.71-78 However, although rare, these materials can also exhibit favorable catalytic efficiency.


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After observing the semiconductor behavior of MOF-5, Garcia and coworkers continued to explore the potential of MOFs as photocatalysts, leading to the first example of an MOF-based photocatalyst for the hydrogen evolution reaction (HER).79 The photocatalytic HER was driven by highly water-stable Zr-MOFs UiO-66 and NH2-UiO-66 in a water/methanol mixture under UV irradiation. The introduction of an amino group into the organic linker added another absorption band beyond 300 nm to NH2-UiO-66 and enabled it to perform better in the photocatalytic HER than UiO-66. The presence of Pt nanoparticles as co-catalysts can promote the photoconversion to nearly five times the yield. The long-lived charge separation states recorded by the laser flash photolysis spectra of UiO-66 and NH2-UiO-66 confirmed the semiconductor role of MOFs for this photodriven HER (Scheme 3).

Scheme 4 Proposed mechanism for the photocatalytic carbon dioxide reduction over NH2-MIL125(Ti) under visible-light irradiation. Reprinted from ref 81. Copyright 2012, Wiley-VCH. Ti-MOFs attract considerable interest as photocatalysts because they contain titanium-oxo species. MIL-125(Ti) has been intensively studied as a photocatalyst since Sanchez, Serre and coworkers synthesized this intriguing MOF and observed its photochromic behavior, revealing


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the photoinduced conversion from Ti(IV) to Ti(III).80 Inspired by this work, Li and co-workers added an -NH2 group into MIL-125(Ti), generating NH2-MIL-125(Ti), which was active in the photocatalytic reduction of CO2 to HCOO− under visible light.81 The multinuclear Ti-centers in NH2-MIL-125(Ti) are both photo- and redox-active. The amino functionality not only transfers the absorption band for MIL-125(Ti) from the UV region to the visible region but also results in a higher absorption capability toward carbon dioxide. The photodriven carbon dioxide reduction of NH2-MIL-125(Ti) was performed in acetonitrile (MeCN) with triethanolamine (TEOA) as the electron donor (Scheme 4). The generation of Ti3+ upon visible-light illumination was observed as a color change from bright-yellow to green when NH2-MIL-125(Ti) was irradiated under nitrogen in the absence of carbon dioxide. When carbon dioxide was pumped in, the color turned back to yellow and the carbon dioxide was reduced by Ti3+ to yield HCOO−. TEOA offered an electron to regenerate the pristine NH2-MIL-125(Ti). Later on, Li and coworkers observed that NH2-UiO-66(Zr) catalyzes the photocatalytic reduction of carbon dioxide under visible light with TEOA as a sacrificial agent and the photogenerated ZrIII was firstly confirmed and revealed.82 The partial substitution of the organic linker improved the efficiency of the photocatalytic reduction of carbon dioxide. More further, the metal Zr can also be partially replaced by Ti through a post-synthetic exchange method to obtain a more capable MOF-photocatalyst for both the carbon dioxide reduction and hydrogen evolution under visible light.83 These works definitely show the enormous potential of MOFs, which benefit from the facile adjustment of structure and functionality, for developing better catalysts for photocatalytic carbon dioxide reduction and hydrogen evolution.84-85 Very recently, another useful strategy to improve the performance of MOFs as a semiconductor-photocatalyst was reported by Gascon, Vlugt, Reek and coworkers.77 The pores


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of NH2-MIL-125(Ti) were decorated with the Co-based electrocatalyst Co-dioxime-diimine via a “ship-in-a-bottle” strategy (Figure 4), and the resulting Co@MOF composite outperformed all previously reported MOF-based analogues for photochemical hydrogen production. Though the redox catalytic center for substrate transformation is inside the MOF pores, this Co@MOF composite is still belong to the type I MOF-catalyst because the photo-induced redox center is still the Ti-oxygen clusters. This Co@MOF composite is a recyclable catalyst free of noble metals producing hydrogen from water under visible light. It displays a stable TOF of 0.8 h-1 even after 65 h of operation. The introduction of the cobalt molecular catalyst into the cavities of the framework resulted in a 20-fold enhancement of photocatalytic activity compared to the pristine NH2-MIL-125(Ti), whereas the cobalt molecular catalyst could not produce hydrogen under the same conditions because of the lack of a photosensitizer. This example reveals the potential of MOFs as an ideal modular functional material for photocatalysis via the elaborate design and integration of well-defined molecular building blocks. The higher density of the semiconductor nodes within the crystal lattices features a new fashion to enhance the catalytic efficiency.

Figure 4 Concept of photocatalytic hydrogen production using a Co@MOF composite with Codioximediimine encapsulated into the pores of NH2-MIL-125(Ti). Reprinted from ref 77. Copyright 2015, Royal Society of Chemistry.


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The assembly of a functional unit within MOF scaffold can trigger new properties beyond the simple merging of two parts. Jiang and co-workers found that a deep electron trap state occurred after the integration of porphyrin into Zr-MOF, PCN-222 and the photocatalytic conversion of carbon dioxide into formate anion with PCN-222 was significantly improved than the corresponding porphyrin ligand.86 The semiconductor character of PCN-222 was illustrated by a Mott−Schottky plot that showed the possibility for photoreduction of CO2 by PCN-222. As expected, the HCOO− anion was continuously produced with PCN-222 as the photocatalyst and TEOA as the electron donor in CH3CN under visible-light irradiation. ESR studies revealed the generation of ZrIII ions in this photoreduction, indicating the electron transfer from the excited porphyrin ligand to the Zr6 metal nodes. In this case, the pristine homogenous photocatalyst porphyrin served as antenna to sensitize Zr-oxo clusters, which then transfer electron to substrate CO2, making PCN-222 a type I MOF. When H2TCPP was employed as the photocatalyst under similar conditions, clearly weaker performance was observed: only 2.4 μmol of HCOO− was produced after 10 h (30 μmol for PCN-222). This result shows that the photocatalytic activity of the porphyrin was greatly enhanced by the formation of the MOF structure, inspiring a series of experiments to unveil the reason. Steady-state photoluminescence (PL) measurement of PCN-222 indicated a pronounced PL emission quenching from H2TCPP, suggesting greatly suppressed radiative electron−hole recombination in PCN-222 relative to that in H2TCPP. Moreover, ultrafast transient absorption (TA) spectroscopy of PCN-222 revealed that the ΔA recovery converges to an asymptote (dashed line in Figure 5b) with a non-zero value of approximately −1.2 mOD within the probedelay limit of the pump/probe spectrometer (~3 ns). Notably, the nearly perfect parallelism between the asymptote and the ΔA = 0 line suggests that the eventual recovery to ΔA = 0


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features an extremely long lifetime (τ3), possibly indicating a third trap state. The detrapping of electrons from this deep trap state was so slow that its corresponding radiative electron−hole recombination was dramatically suppressed, which is in good line with the steady-state PL spectra. In summary, as a direct result of framework formation, a deep electron trap state emerged in PCN-222 and this trap state could boost electron−hole separation and then supply long-lifetime electrons for the photoreduction of carbon dioxide via effective suppression of the detrimental electron−hole recombination. This work reinforces the advantage of MOF structures for the integration of functional units and supply surprise than common mixing.

Figure 5 (a) Transient TA spectra of PCN-222 registered at different probe delays (pump at 500 nm). (b) Representative TA kinetics of PCN-222 taken at the probing wavelength of 430 nm. In (a) and (b), the TA signal is given as absorbance change (ΔA) with unit of mOD. Reprinted from ref 86. Copyright 2015, American Chemical Society. Although type I MOFs are typical MOF-based catalysts for photocatalytic HER and carbon dioxide reduction, other types of MOF-photocatalysts for these two key transformations also exhibit satisfactory activity.71-78 Homogeneous photocatalysts such as Re or Ir complexes, porphyrin derivatives and POMs have been built into MOF structures as linkers to obtain type II


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MOF photocatalysts for these two reactions.71-75 Excellent examples will be introduced in detail in the following section. Photocatalysis with Type II MOFs Organic photocatalysts are gaining increasing attention and renewed enthusiasm in regard to photosynthesis.12-16 The poor absorption of light prevents type I MOFs from being powerful photocatalysts for more sophisticated and useful organic transformations. The incorporation of rigid photocatalyst as linkers into MOF scaffolds opens a new avenue for photocatalytic organic conversions. In fact, the immobilization of active photocatalysts onto a suitable matrix is a longpursued goal for researchers to avoid self-quenching between photocatalysts and the contamination of the product, especially now that the most widely commercialized photocatalysts are all heterogeneous in nature for easy workup and relatively low cost.17,49 In this context, MOFs are the perfect heterogeneous platform for photoactive linkers because they are orderly dispersed as single-site catalysts in MOF scaffold and the distance between them can be finely adjusted for better performance; the porous nature of MOFs provides sufficient space for substrate activation and product departure.18-27 At the meantime, the strong coordination bonds ensure the stability and recyclability of MOFs. More and more researchers has demonstrated that the pristine homogeneous photocatalyst can drive photocatalysis more efficiently or completely after being incorporated or embedded into MOFs.28-34 The homogeneous star-photocatalysts such as porphyrin and [Ru(bpy)3]2+ were the first batch of photofunctional organic linkers used to construct efficient MOF photocatalysts for meaningful organic syntheses.71,90-97 In early 2011, Wu and coworkers reported a SnIV–porphyrin-based MOF (Zn-SnIV–TPyP) for the efficient photooxygenation of phenol and sulfides (Figure 6).91 The metalloporphyrin building block SnIV–TPyP presents distinct photocatalytic activity in the


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homogeneous phase. Upon excitation, the SnIV–TPyP reached to its triplet excited state, which transfer energy with oxygen to produce 1O2, one of the famous reactive oxygen species (ROS). 1

O2 then oxidizes substrate to corresponding product. However, this photoactivity was heavily

influenced by reaction with singlet oxygen. After immobilization of SnIV–TPyP into Zn-SnIV– TPyP, the quenching process was overcome efficiently thanks to the enough space between photocatalytic units SnIV–TPyP and the easy interaction between 1O2 and substrate. Thus, ZnSnIV–TPyP provided remarkable activity for the photooxygenation of phenol and sulfides.

Figure 6 Perspective view of the 3D porous framework of Zn-SnIV–TPyP down the c axis (left) and a schematic representation of the photooxidation of phenol and sulfides (right). Reprinted from ref 50. Copyright 2014, American Chemical Society. Almost simultaneously, Lin and coworkers demonstrated the doping of catalytically competent Ir and Ru complexes into a highly stable and porous UiO-67 and applied the resultant phosphorescent [Ru(bpy)3]2+-derived MOF 5 and [Ir(ppy)2(bpy)]+-derived MOF 6 to three photocatalytic organic transformations (aza-Henry reaction, aerobic amine coupling, and aerobic oxidation of thioanisole), with good results (Figure 7).71 [Ru(bpy)3]2+and [Ir(ppy)2(bpy)]+ have been extensively investigated as photoredox catalysts in various photocatalytic organic reactions.16,47 The incorporation of these precious-metal-containing photocatalysts into porous MOFs is highly desirable for a prolonged catalytic lifetime because of the stabilization effect of


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MOFs and their recyclability. The importance of MOF permanent porosity was demonstrated by the poor catalytic performance of amorphous nanoparticles in the photodriven aza-Henry reaction (18% conversion, corresponding to the background reaction). The same strategy was also applied for the immobilization of the CO2-reduction photocatalyst, [ReI(dcbpy)(CO)3Cl], obtaining MOF 4. When irradiated in CO2-saturated MeCN with TEA as sacrificial agent, MOF 4 turned from orange to green and CO and H2 were detected by gas chromatography (GC). During the first six hours, The CO-TONs reached 5.0 and the molar ratio of the CO and H2 production was around 10. Unfortunately, MOF 4 became inactive in CO generation after two six-hour reaction runs, due to the detachment of Re-carbonyl moieties from the dcbpy group in the MOF 4 framework. Even so, the total CO-TON of the MOF 4 are still two times higher than the homogenous system for 20 h reaction, presumably owing to the catalyst stabilization by the MOF framework.

Figure 7 Schematic presentation of the Ir and Ru complexes-doped UiO-67 MOFs for the photocatalysis of three organic transformations. Reproduced from ref 71. Copyright 2011, American Chemical Society.


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Following this work, Lin and coworkers developed a new approach for the photocatalytic HER with Ru/Ir complex-derived MOFs.87,61-62 Pt NPs and various POMs were introduced to improve electron transfer during the HER; these Ru/Ir complex-derived MOFs are among the most efficient MOF-based photocatalysts for HER. For example, Lin and co-workers synthesized two Pt@MOF composites by loading Pt nanoparticles into the pores of [Ir(ppy)2(bpy)]+-based MOFs 1 and 2 (Figure 8).87 The photochemical hydrogen evolution with Pt@MOF composites was operated with TEA as the electron donor and with the Ir-phosphor absorbing visible light and transferring electrons to Pt nanoparticles for the final reduction step. A total Ir-TON of 7000 was obtained for MOF 2 via 48 h hydrogen evolution experiments, which is approximately five times the TON value afforded by the homogeneous control. The better cooperation between photoexcitation of the MOF frameworks and electron injection into the entrapped Pt NPs was responsible for the markedly improved efficiency. Moreover, these Pt@MOF composites could be readily recycled and reused.

Figure 8 Phosphorescent Zr-carboxylate MOFs (1 and 2) and subsequent loading of Pt NPs inside MOF cavities to form the Pt@1 and Pt@2 assemblies for the synergistic photogeneration of hydrogen. Reprinted from ref 87. Copyright 2012, American Chemical Society.


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The modular nature of MOF scaffold endows MOFs with infinite potential for the construction of multifunctional materials, which is also the case for the photoactive speciesinvolved MOF. Various functionalities can be added into photoactive MOFs, being the metal nods or the second linkers. Therefore, distinct capable units in a photoactive MOF can operate simultaneously and even synergistically toward substantial utilization of solar energy. Farha, Hupp and co-workers described a dual-function MOF that simultaneously detoxified two chemical warfare agent (CWA) stimulants at room temperature (Figure 9).92 Efficient deactivation of CWAs requires multiple detoxification pathways, such as hydrolysis and oxidation, to occur simultaneously because predicting which CWAs will need to be deactivated is difficult. The multifunctional nature of MOFs makes them a perfect material to incorporate distinct catalytic moieties to generate broad-spectrum detoxification materials. The dual-function MOF was constructed from a Zr6-containing node and a porphyrin organic linker. Upon visiblelight (LED) irradiation, a simulant of mustard gas, 2-chloroethyl ethyl sulfide, was oxidized to a nontoxic product by singlet oxygen, which was generated from the photosensitized porphyrin moieties. Meanwhile, the Zr6 nodes containing Zr-OH-Zr in the same MOF, mimicking the ZnOH-Zn active site in PTE, hydrolyzed another simulant of nerve agents (such as GD or VX), dimethyl 4-nitrophenyl phosphate, to a nontoxic complex. The structure of this dual-function MOF remained intact after catalysis, and the degradation efficiency for fresh CWAs was as the same as for the first run. The versatility and tunability of MOFs suggests that suitably engineered successors may eventually prove useful for air-filtration equipment and for the destruction of stockpiles or spills of chemical warfare agents. In addition, reducing the size of MOF crystals to the nano-regime leads to acceleration of the catalysis.


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Figure 9 A dual-function MOF constructed from a phosphortriesterase-like Zr6-containing node and a photoactive porphyrin linker for the simultaneous degradation of simulants of two CWAs. Reproduced from ref 92. Copyright 2015, American Chemical Society. The combination of chiral catalysis and photocatalysis is a hot topic in chemistry because chiral complexes are highly desirable as medical materials but their traditional chemical synthesis is often very tedious. Through careful selection and design, Duan and coworkers realized the first integration of a triphenylamine photoactive unit and the chiral component L- or D-pyrrolidin-2-ylimidazole

(PYI) in an MOF and successfully applied these chiral photoactive

MOFs (Zn−PYIs) in the light-driven asymmetric α-alkylation of aldehydes (Figure 10).60 The targeted Zn−PYIs could not be obtained through direct solvothermal synthesis due to the instability of PYI. A tert-butoxycarbonyl (Boc) group was thus introduced to protect the catalytically active N−H site of pyrrolidine and the resulting precursor MOF (Zn−BCIP) was converted into the active Zn−PYI simply by heating in DMF solution. The asymmetric α-alkylation of aldehydes with Zn−PYI1 started with phenylpropylaldehyde and diethyl 2bromomalonate as the coupling partners and a common fluorescent lamp (26 W) as the light source. A high reaction efficiency (74% yield) and excellent enantioselectivity (92% ee) were achieved, demonstrating the successful execution of photoactive chiral MOF design. Absorption


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and luminescence experiments suggested that the quenching process was typically attributed to the photoinduced electron (PET) process from Zn−PYI1* to diethyl 2-bromomalonate. The chiral PYI moieties acted as cooperative organocatalytic active sites to further induce the PETgenerated active intermediate in an MOF channel with remarkable stereoselectivity. Therefore, Zn−PYI1 represents the first example of an MOF-based heterogeneous asymmetric photocatalyst for this important reaction. Because of the restricted movement of the substrates within the MOF’s interior and multiple chiral inductions as well, the integration of both the photocatalyst and asymmetric organocatalyst into a single MOF makes the enantioselection superior to that achieved through simple mixing of the corresponding MOFs with the chiral adduct.

Figure 10 The integration of the stereoselective organocatalyst

L-

or

D-PYI

(blue) and a

triphenylamine photoredox group (red) into one single MOF to promote the asymmetric αalkylation of aliphatic aldehydes in a heterogeneous manner. Reprinted from ref 60. Copyright 2012, American Chemical Society. For typical photocatalysis with a PET process, an excited state of photosensitizer is obtained after the absorption of a photon. Specially, some photosensitizers may produce active radicals right after the absorption. Xing and co-workers utilized anthracene, one photosensitizer of this particular type to construct a visible light responsive MOF, NNU-35, for the promising


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photocatalytic atom-transfer radical polymerization (ATRP) reaction of methacrylate monomers.94 The incorporation of the anthracene-based ligand L1 into NNU-35 resulted in absorption and emission bands that were broader and redshifted because of energy transfer and/or charge transfer interactions in the MOF structure. The resonance EPR signal after exposure to visible light was assignable to the anthracene-based ligand and it can sustained for about several minutes even without light irradiation, clearly suggesting its free-radical nature. The broad visible-light absorption and desirable photoinduced charge separation in NNU-35 are intriguing for light-switchable ATRP. ATRP is an efficient method for controlled radical polymerization, which allows non-experts facile access to functionalized polymer materials with well-defined structures and architectures.

Figure 11 Proposed mechanism for NNU-35 mediated ATRP under visible light. Reprinted from ref 94. Copyright 2016, Royal Society of Chemistry. More recently, photoinduced ATRP has received intensive attention because of its unique features of temporal and spatial control of the chain extension process. A typical ATRP reaction system of MMA with a copper complex as the catalyst and EBiB as the initiator was chosen to test the photoactivity of NNU-35. 48% monomers were polymerized after 8 h of irradiation with


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520 nm light, and a relatively narrow molecular weight distribution (Mw/Mn) of 1.12 was observed. No polymerization occurred in the absence of NNU-35. More importantly, the reaction could be triggered easily through light switching. When the visible light was periodically turned on and off, the polymerization also occurred periodically, exhibiting the characteristics of living radical polymerization. Figure 11 clearly illustrates the successful use of MOF NNU-35 for the light-switchable ATRP reaction via continuous regeneration of the copper catalyst. The heterogeneous photocatalyst NNU-35 first transfers an electron to an oxidized Cu(II)/PMDETA complex after the visible-light-induced formation of a radical. The generated Cu(I) complex then reacts with the alkyl halide (R–X) initiator, forming radicals (R•) to initiate the ATRP. The implementation of NNU-35 for the ATRP reaction successfully expanded the application range of photoactive MOFs to radical chemistry. A breakthrough in photocatalysis involving consecutive photoinduced electron transfer (conPET) processes was reported by König in 2014.48 Before that, the energy conferred by visible-light excitation for subsequent redox chemistry was limited to the one single absorbed photon. Therefore useful reactions requiring high energy are either performed under UV irradiation or with highly active substrate precursors under strict conditions, both of which are obstacles to wide application. Through the integration of perylene diimide (PDI) into MOF ZnPDI, the conPET process for the efficient visible-light-driven reduction of aryl halides was introduced into the heterogeneous catalysis field, as accomplished by Duan and co-workers (Figure 12).95 Perylene diimide-derivative H2PDI was chosen as the organic ligand because the conPET process was discovered using a PDI derivative. However, the poor solubility of PDIs and their strong tendency to aggregation, resulting from their large planar conjugated structure, severely restrict the development of this conPET process for other catalytic transformations in


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homogeneous systems. Strong π···π interaction between PDIs still existed in the Zn-PDI sheets; however, the coordination effect worked synergistically, making the aggregation controllable. Each of the three PDI molecules formed a close J-aggregate group. The formation of Jaggregates in Zn-PDI is beneficial for photocatalysis because of the excellent ability of Jaggregates to delocalize and migrate excitations. Moreover, the appropriate space between Jaggregates and the rigid framework of Zn-PDI helped reduce self-quenching, as concluded on the basis of the obviously weaker fluorescence intensity of H2PDI solutions with the same PDI concentration.

Figure 12 Diagram illustrating the strategy of assembling insoluble PDI into organized arrays in porous solid Zn-PDI to obtain an efficient photocatalyst for the visible-light-driven reduction of aryl halides and the oxidation of alcohols and amines. Reprinted from ref 95. Copyright 2016, American Chemical Society.


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The conPET process with Zn-PDI was illustrated through the heterogeneous visible-lightdriven reduction of aryl halides, starting from 4′-bromoacetophenone. The 87% yield of acetophenone, which was three times higher than the yield afforded by the homogeneous counterpart, was detected after 1 h of irradiation with blue LEDs and the use of 72 equivalents of the electron donor Et3N. The reason for the high performance is explained on the basis of the aforementioned studies: the rigid Zn-PDI framework isolated the active sites of PDI but allowed them to be dense at the same time, which may be optimal for electron or energy transfer. Moreover, the most inert aryl chlorides were also effectively reduced using higher loadings of Zn-PDI. Normally, the photoreduction of inert aryl chlorides cannot proceed without highly sensitive and active donor molecules, UV-A irradiation and strictly inert reaction conditions. The integration of conPET-active PDI bricks into the Zn-PDI architecture thus represents a critical step toward more efficient solar energy utilization via a conPET process. In addition to direct incorporation into MOF scaffold, photocatalysts can be introduced to MOFs through post-synthetic modification (PSM), benefiting from the porosity and the welldefined structure.76, 87-89 Xu described the synthesis of photoactive MOF-253-Pt for the reduction of water to form hydrogen under visible-light irradiation through PSM of a 2,2′-bipyridine-based MOF with platinum ions.76 The visible-light induced hydrogen evolution of the MOF-253-Pt was examined in the presence of 15 vol% TEOA as a sacrificial electron donor in water at pH 8.5; the amount of hydrogen produced from MOF-253-Pt was nearly five times greater than that produced from the Pt(bpydc)Cl2 complex. For many capable molecular photocatalyst, the direct solvothermal synthesis often failed to produce the wanted MOFs while PSM are more feasible. This PSM strategy combines the advantages of molecular catalysts with a highly ordered and stable inorganic support and it is sure to be widely applicable for MOFs as photocatalysts.89


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Photocatalysis with Type III MOFs Beyond the semiconducting metal-oxo clusters or photoactive linkers, the inherent pores of MOFs can also be exploited to render MOF photoactivity. Suitable photocatalysts can be dispersed into MOF pores through one-pot synthesis during synthesis or through in situ deposition after the formation of the MOF scaffold.65,98 Though a lot of photoactive species, including perylene and DMASM (4-[p-(dimethylamino)styryl]-1-methylpyridinium), have been introduced into MOF cavities as guests and have exhibited favorable properties resulting from the host-guest interaction.99-101 Thus far, this type of MOF-photocatalysts has remained in its infancy; metal NPs and POMs are the rare examples of photocatalysts encapsulated into MOF pores and applied to photoconversion.65-68,102-103 Metal NPs were initially introduced into photoactive MOFs as co-catalysts to facilitate energy transfer. Duan, Chen and co-workers utilize the localized surface plasmon resonance (LSPR) of MOF-encapsulated NPs for photocatalysis.98,102-103 Au@ZIF-8 single- or multi-core– shell structures were obtained by epitaxial growth or coalescence of nuclei depending on the density of ZIF-8 nuclei on Au NPs.102 The photodriven oxidation of benzyl alcohol by these Au@ZIF-8 composites was performed in acetonitrile under visible light given the LSPR-related absorption of Au NPs (Figure 13). The conversion was 25.8% for single-core Au@ZIF-8 and 51.6% for multi-core Au@ZIF-8 after 24 h of irradiation. This difference in conversion is ascribed to plasmonic coupling between Au NPs in the multi-core structures. The size-selectivity of ZIF-8 that restricts benzyl alcohol from entering pore cavities too close to Au, according to the authors, is responsible for the conversion lower than that afforded by Au–SiO2 (56.9%). Later, the same authors reported that the introduction of Au NPs into NH2-UiO-66 favored the electron transfer from Au NPs to NH2-UiO-66 with a localized electronic state characterized by


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C-AFM, leading to nearly six-fold greater conversion than that provided by NH2-UiO-66 for the photooxidation of benzyl alcohol.103

Figure 13 Schematic of the photooxidation of benzyl alcohol by Au@ZIF-8 single- or multicore–shell structures under visible light. Reprinted from ref 102. Copyright 2014, Royal Society of Chemistry. MOFs, as excellent porous materials, have long been intensively studied for gas storage and separation. This intriguing feature can be exploited in photocatalysis with gas-phase compounds. Jiang and co-workers recently reported the synthesis of a Pd nanocubes@ZIF-8 composite material for the efficient and selective catalytic hydrogenation of olefins at room temperature under 1 atm H2 and light irradiation (Figure 14).68 The encapsulation of Pd nanocubes in ZIF-8 solved several problems impeding catalysis, such as the propensity for aggregation and the random dispersion of Pd NCs under homogenous conditions. In addition, the plasmonic photothermal effects of the Pd nanocube cores endowed the composite Pd NCs@ZIF-8 with photoactivity for the hydrogenation of olefins.


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Figure 14 Self-assembly of Pd NCs@ZIF-8 and plasmon-driven selective catalysis of the hydrogenation of olefins. Reprinted from ref 68. Copyright 2016, Wiley-VCH. The uniform pores of the ZIF-8 shell played important roles in the efficient hydrogenation: they accelerated the reaction by H2 enrichment and acted as a “molecular sieve” for olefins with specific sizes. Upon light irradiation, Pd NCs@ZIF-8 drove the hydrogenation of 1-hexene to almost complete conversion in 90 min, whereas only 58% conversion was achieved with Pd NCs. Moreover, the yields with Pd NCs@ZIF-8 remained approximately 100% after three consecutive runs, whereas a low yield of 21% with the Pd NCs was observed in the third run. Remarkably, the catalytic efficiency of hydrogenation with Pd NCs@ZIF-8 under 60 mW cm-2 of full spectrum or 100 mW cm-2 of visible-light irradiation at room temperature was comparable to that of a process driven by heating at 50 °C. Thus, hydrogenation with H2 may utilize solar energy instead of high-temperature heating, which is dangerous, energy-consuming and environmentally damaging. The strategy of combining the photothermal effects of metal nanocrystals with the favorable properties of MOFs for efficient and selective catalysis generates synergistic advantages and opens up an avenue to the prospects for MOF photocatalysis.


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POMs, in addition to NPs, are also frequently embedded into MOF cavities due to their excellent redox properties.61-67 However, most investigations have focused on thermal reactions or multi-electron processes, with little attention devoted to the photoactivity of POMs. However, in 2015, Duan and co-workers reported a POM-MOF, CR–BPY1, that is active toward the photocatalytic oxidative coupling of low-reactive sp3 C–H bonds with environmentally benign and inexpensive oxygen as the oxidant (Figure 15).65 CR–BPY1 was constructed from the incorporation of a ruthenium-substituted POM, [SiW11O39Ru(H2O)]5-, which exhibited excellent photoactivity in various catalytic oxidation processes of organic substrates, into copper-BPY networks. Copper atoms were first connected by the BPY ligands to form a two-dimensional square grid; adjacent sheets were then connected together via the coordination of Cu to a terminal oxygen atom of the deprotonated [SiW11O39Ru(H2O)]5- to generate a 3D framework. The catalytic potential of CR–BPY1 was examined via a model reaction using N-phenyltetrahydroisoquinoline and nitromethane as coupling partners under irradiation by an 18 W fluorescent lamp. A yield of 90% was observed after 24 h of irradiation, whereas less than half the conversions were obtained using the same equiv of copper(II) salts or/and K5[SiW11O39Ru(H2O)] as catalysts. These results suggested that the direct connection of copper(II) ions to [SiW11O39Ru(H2O)]5- anions was the key factor for improving the catalytic performance; this speculation was confirmed by a series of spectroscopic analyses and an elaborate control experiment that is discussed below. The significantly weaker (46% conversion) catalytic activity of CR–BPY2 than CR–BPY1 in the model C–C coupling reactions demonstrated that the direct CuII–O–W(Ru) bridges were the key factor in achieving the synergistic catalysis between the photocatalyst and the metal catalyst. Inspired by this work, the same author recently demonstrated the encapsulation of photosensitizing [W10O32]4– into MOF


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pores brought about a series of POM-MOFs as capable heterogeneous photocatalysts for the light-driven acceleration of the - or -site C–H alkylation of aliphatic nitriles; these results suggest that the POM-MOF is a promising photocatalyst.66-67

Figure 15 Illustration of CR–BPY1 for the oxidative coupling of low-reactive sp3 C–H bonds through the synergistic catalysis of the photoredox polyoxometalate [SiW11O39Ru(H2O)]5- and the Cu2+ metal node. Reprinted from ref 65. Copyright 2015, Royal Society of Chemistry. Conclusions and outlook After nearly a decade of development, photocatalysis with MOFs has reached its booming stage. Designing MOFs for photocatalysis is a feasible approach to obtain highly active, recyclable and environmentally benign photocatalysts with simple workup, largely thanks to the tunable structure and high porosity of these crystalline materials. The photoactive species can be integrated into MOF scaffolds as inorganic cluster nodes (type I) or organic linkers (type II) or as an inclusion (type III). For type I MOF-based photocatalysts, the metal-oxo clusters can be regarded as semiconductor dots that are mono-dispersed and isolated in the frameworks to enhance the density of the catalytic sites meanwhile avoiding the aggregations of these nanosize


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dots, resulting in the high photoactivity toward proton or CO2 reduction as well as the degradation of organic contaminates in water and air. Introducing well-designed functional groups into the organic linkers to shift the absorption bands or using metal NPs as co-catalysts are the primary methods used to improve the performance of type I MOF photocatalysts. In the case of type II MOF-based photocatalysts, the structure of MOFs serves as an ideal platform for the immobilization of homogeneous photoactive organic and metal-organic dyes. In addition to the separation and stabilization of photoactive sites, MOFs also provide an opportunity for the photoactive unit to work with other functionalities simultaneously or synergistically, giving rise to broad-spectrum catalysts or the integration between photocatalysis and metal- or organocatalysis. The photocatalysis of traditional organic synthesis, radical chemistry and novel conPET processes have all been accomplished using this type of MOFs as photocatalysts. The photocatalysis with respect to type III MOF photocatalysts is indeed underdeveloped. Only metal NPs and POMs have been embedded into the pores of MOFs for photocatalysis, although the integration of MOF porosity and photocatalyst has led to the formation of high-performance photocatalysts. The future development of MOF-based photocatalysts requires deeper understanding of the advantages related to MOF-based materials over other systems. First, the excellent crystallinity of MOF-based materials can be further utilized to construct photocatalysts with inherent semiconducting property or even special conducting property. Second, the post-synthetic modification approach can be widely used to obtain photoactive MOFs through metal- or ligandexchange or photoactive species encapsulation, which will surely enlarge the pool of MOF photocatalysts and widen the application range of MOF-mediated photocatalysis. Third, distinct functionality could be introduced into a single MOF for the construction of an MOF-based


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photocatalyst with cooperative and tandem catalysis. Fourth, the environment of pores within MOFs should be finely adjusted to promote enzymatic-type behavior of the encapsulated photocatalysts. Finally, MOFs could be scaled to nanosize for better interaction with the substrate and improved absorption of light. We are confident that the tunable and diverse MOFs are an excellent platform for the development of an ideal photocatalyst toward a greener world. Corresponding Author Email: [email protected] Acknowledgements This work was supported by the NNSF (Nos. 21421005, 21231003 and 21501041) and the MOST "973 Project" (NO3. 2013CB733700). REFERENCES (1) Barber, J. Chem. Soc. Rev. 2009, 38, 185-196. (2) Wasielewski, M. R. Chem. Rev. 1992, 92, 435-461. (3) Bard, A. J.; Fox, M. A. Acc. Chem. Res. 1995, 28, 141-145. (4) Ravelli, D.; Dondi, D.; Fagnoni, M.; Albini, A. Chem. Soc. Rev. 2009, 38, 1999-2011. (5) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2009, 42, 1890-1898. (6) Berardi, S.; Drouet, S.; Francas, L.; Gimbert-Surinach, C.; Guttentag, M.; Richmond, C.; Stoll, T.; Llobet, A. Chem. Soc. Rev. 2014, 43, 7501-7519. (7) Fujishima, A.; Honda, K. Nature 1972, 238, 37-38. (8) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. Rev. 1995, 95, 735-758. (9) Wang, H.; Zhang, L.; Chen, Z.; Hu, J.; Li, S.; Wang, Z.; Liu, J.; Wang, X. Chem. Soc. Rev. 2014, 43, 5234-5244. (10) Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Chem. Rev. 2010, 110, 6503-6570. (11) Habisreutinger, S. N.; Schmidt-Mende, L.; Stolarczyk, J. K. Angew. Chem. Int. Ed. 2013, 52, 7372-7408.


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