Advances in Photofunctional Dendrimers for Solar Energy Conversion

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Advances in Photofunctional Dendrimers for Solar Energy Conversion Xiaohui Zhang, Yi Zeng, Tianjun Yu, Jinping Chen, Guoqiang Yang, and Yi Li J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 12 Jun 2014 Downloaded from http://pubs.acs.org on June 16, 2014

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The Journal of Physical Chemistry Letters

Advances in Photofunctional Dendrimers for Solar Energy Conversion Xiaohui Zhang,† Yi Zeng,∗,† Tianjun Yu,† Jinping Chen,† Guoqiang Yang,∗,‡ and Yi Li∗,†

†

Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute

of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡

Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of

Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China *Corresponding Author: [email protected]; [email protected]; [email protected] Abstract Dendrimers are regularly and hierarchically branched synthetic macromolecules with numerous chain ends all emanating from a single core, which makes them attractive candidates for energy conversion applications. During photosynthesis and photocatalysis, photoinduced electron transfer and energy transfer are the main processes involved. Studies on these processes in dendritic systems are critical for the future applications of dendrimers in photochemical energy conversion and other optoelectronic devices. In this perspective, the recent advances of photofunctional dendrimers in energy conversion based on light-harvesting systems, solar cells, and photochemical production of hydrogen will be discussed. The electron transfer and energy transfer characteristics in light-harvesting photofunctional dendrimers, and the regulation of the electron transfer process and the stabilization of charge separation state in hydrogen photoproduction are emphasized. TOC graphic

Keywords: dendrimers, light-harvesting, energy transfer, electron transfer, solar cells, hydrogen production 1

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The rapidly economic growth is exhausting the traditional fossil fuel supply around the world, which causes the air pollution and global warming from an increase of greenhouse gas emissions. In order to meet the increasing energy demands in future and solve the environmental issues, clean and sustainable sources of energy are desperately required. The solar energy has been considered to be the most viable choice for future energy supply because the sun provides enormous amount of solar energy to earth on an annual basis at a rate of 100000 TW1 and the resource is perfectly clean. However, the solar energy remains minority of the total energy consumption because of the lack of effective methods to convert, which has stimulated approaches for the conversion and storage of solar energy.2−6 Nature has created the unique machinery in solar energy conversion, which inspires and stimulates researchers to seek artificial designs. Dendrimers are regularly and hierarchically branched synthetic macromolecules with numerous chain ends all emanating from a single core. The chromophores can be accurately located at the core, focal point, periphery, or even at each branching point of the dendritic structure. The specific structure of the dendrimers makes them mimics of light-harvesting systems, where the antenna chromophores surround the central reaction center.7,8. Moreover, the nature of dendrimers with suitable cavities like natural enzymes endows them the site isolation character to stabilize the encapsulated guest including the charge separation species, which advances efficiencies of solar energy conversion in photovoltaic cells and photochemical production of hydrogen. In this perspective, recent progresses and expectations of photofunctional dendrimers in solar energy conversion are discussed. Light-harvesting systems. Studies on natural photosynthetic systems have revealed that the structure of the photosynthetic unit is a central reaction center surrounded by light-harvesting 2


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complexes.9 The remarkable character of the photosynthetic system is that the energy of any photon absorbed by antenna complexes is transferred to the reaction center and converted into a chemical potential with unit efficiency.10 In the photosynthetic process, photoinduced electron/energy transfer process plays an important role and has inspired intense studies. Tremendous efforts have been made to investigate the solar energy capture and transfer in natural and artificial systems.11−13 Dendrimer has been used as an ideal mimic of light-harvesting system by attaching abundant energy-capturing chromophores to the periphery and an energy acceptor to the core. The initial energy of radiation is captured by peripheral chromophores and results in the population of their electronic excited states, the energy of which subsequently delivers to the core through a variety of ways and mechanisms. The photoinduced electron/energy transfer processes within dendritic light-harvesting systems have been studied by several groups including ours.7,8,14−16 Andrew and coworkers have presented mechanistic insights of light-harvesting dendritic and hyperbranched macromolecules.17,18 However, compared with the evolved and efficient functions of natural systems, the artificial light-harvesting complexes are still in their infancy. The energy transfer observed in synthetic light-harvesting dendrimers mostly can be explained by Förster19 and Dexter20,21 mechanisms because the synthetic light-harvesting dendrimers usually have very weak or negligible electronic coupling between donor and acceptor except some conjugated dendrimers.22,23 Beyond the well-known Förster energy transfer theory, recent experimental and theoretical advances of energy transfer within natural light-harvesting complexes and conjugated macromolecules reveal that quantum coherent dynamics can be involved.24−26 The pioneer work of light-harvesting dendrimers with RuII and OsII polypyridine complexes 3



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as building blocks was reported by Balzani and co-workers in the early of 1990s.27 Although these dendrimers absorb visible light and undergo intramolecular energy transfer, the energy can only transfer from the core to the periphery or from the intermediate to either the core or the periphery depending on the choice of metal cores and ligands. The initial dendritic framework fulfilling unidirectional energy transfer from the periphery to the core was reported by Moore et al.28,29 They fabricated a series of conjugated phenylacetylene dendrimers functionalized with a perylene core, and the peripheral phenylacetylene units harvest light and transfer energy to the perylene core. Furthermore, they found that the a directional energy gradient from the periphery to the core can increase the rate constant by at least two orders of magnitude.30 In these dendritic light-harvesting systems the entire dendrimer framework serves both as the light-capturing antenna and as the energy transport medium. Another type of phenylacetylene dendrimer is based on unsymmetrical branching which was studied by Peng and co-workers.31 The unsymmetrical dendrimers showed broader absorption and can transfer the harvested energy to the core acceptor without significant decrease of efficiency in comparison with the symmetrical phenylacetylene dendrimers. Two different pathways of energy transfer were observed in the unsymmetrical dendrimers. One involves multistep energy migration toward acceptor incoherently, and the other appears to involve direct exciton migration from donor to acceptor which can preserve coherence, making this type of dendrimers a compelling target for quantum control setup.23,32 The Förster mechanism does not require the donor–acceptor orbital overlap, which allows the donor and acceptor chromophores to be separated by a relatively large distance (10–100 Å). The efficiency is determined by the distance between donor and acceptor, the spectral overlap between the donor emission and the acceptor absorption spectra, and the relative orientation of the donor 4


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and acceptor dipole moments. A more versatile dendritic light-harvesting system based on the Förster mechanism was developed by Fréchet and co-workers,33 in which nonconjugated poly(aryl ether) dendrimer frameworks functioned only as linkages between the peripheral light-harvesting chromophores and the central energy acceptor group, and did not play the antenna role in the energy transfer process. A pair of laser dyes, Coumarin 2 and Coumarin 343, which have large overlap integral, was carefully selected as the donor and the acceptor attached to the periphery and the core, respectively (Figure 1). In this type of light-harvesting dendrimers, the dendritic backbone is “photophysically silent” during the light-harvesting process and the efficiency of energy transfer from the peripheral donor to the core acceptor is greatly influenced by the separation of the donor and the acceptor (RDA) with a certain donor–acceptor pair. When the generation of dendrimer increases, the peripheral chromophores doubles with each generation but RDA increases as well, which allows the dendritic molecule harvesting more photons but the efficiency of energy transfer decreases at a certain generation. For the Förster mechanism, the energy transfer can occur efficiently between the peripheral donor and the core acceptor when RDA is in the effective range and a pair of donor and acceptor is selected properly. On the basis of this methodology, several kinds of light-harvesting dendrimers consisting of various dendrimer backbones decorating all sorts of energy donor and acceptor groups concerning the Förster mechanism have been developed.7,8

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Figure 1. Schematic representation of energy transfer within a Fréchet-type dendrimer The triplet–triplet energy transfer is the most common and most important type of energy transfer involved in chemical and biochemical processes, of which the mechanism is usually described by Dexter electron exchange interaction. The rate constant of energy transfer via the Dexter mechanism decreases exponentially with increasing the donor–acceptor distance and become negligibly small when the donor and the acceptor are separated more than 10 Å except for the energy transfer via a “through-bond mechanism” in a rigid or a conjugated system. To clarify the possibility and the reality of the triplet-triplet energy transfer between the peripheral donor and the core acceptor attaching to the “photophysically silent” dendritic backbones via the Dexter mechanism, a series of work on the triplet–triplet energy transfer between the periphery and the core-groups within poly(aryl ether) dendrimers has been carried out. Ceroni et al34 developed two poly(aryl ether) dendrimers (generations 2 and 3) with the naphthalene and benzophenone groups attached to the periphery and the core, and substantiated the occurrence of triplet-triplet energy transfer from the benzophenone core to the peripheral naphthalene units. When the benzophenone core was replaced by a dimethoxybenzil group, the triplet-triplet energy transfer from the dimethoxybenzil core to the peripheral naphthalene was only observed in a rigid matrix at 77 K.35

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Figure 2. Photoisomerization within benzophenone- and norbornadiene-labeled monodendron via intramolecular triplet-triplet energy transfer The first series of light-harvesting dendrimers capable of unidirectional triplet-triplet energy transfer from the peripheral chromophores to the core was reported by our group.36 These dendrimers were constructed by attaching the benzophenone (BP) chromophores and the norbornadiene (NBD) group to the outer surface and the focal point of poly(aryl ether) dendritic backbone. The results of photophysical and photochemical studies demonstrated that the triplet-triplet energy transfer occurred from the peripheral BP chromophore to the core NBD group (Figure 2). The energy transfer efficiencies and the rate constant of the triplet–triplet energy transfer did not significantly decrease as the generation increased. The intramolecular triplet–triplet energy transfer was proposed to proceed via a through–space mechanism involving the closest donor and acceptor groups by folding of the dendritic scaffold. Further studies on the triplet–triplet energy transfer within bis(dendron) BP- and NBD-labeled dendriemers suggested that the bis(dendron) systems took more congested conformation than the corresponding generation monodendron one.37 By replacing the triplet energy acceptor NBD with naphthalene (NA), the triplet-triplet energy transfer from the peripheral chromophores to the core group proceed efficiently.38 The transient absorption spectra of these dendrimers showed clearly the formation of the triplet NA along with the decay of the triplet BP with an isosbestic point at 475 7



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nm, giving a direct evidence of the triplet-triplet energy transfer from the peripheral BP chromophores to the NA core group. Electron transfer plays a central role in most photosynthetic processes of natural systems, therefore, design and fabrication of light-harvesting dendrimers based on electron transfer is also very important. The photoinduced electron transfer occurs only by electron exchange interactions and requires a donor–acceptor orbital overlap. The thermodynamic possibility of electron transfer from donor to acceptor can be estimated by the free energy changes with the Rehm-Weller equation.39 The rate constant of electron transfer depends greatly on the separation of donor and acceptor, the solvent polarity, the structure and redox potentials of donor and acceptor, and the electronic excitation energy possessed by the electron donor or acceptor. Initially, a series of Fréchet-type dendrimers with aryl groups (naphthyl or pyrenyl) and tertiary amines attached to the periphery of the skeleton and the core, respectively, was developed by Fox’s group.40 It was postulated that the fluorescence quenching of the peripheral chromophores was due to the intramolecular electron transfer from the core to the peripheral aryl groups. By attaching naphthalene groups and viologen to the periphery and the core of the poly(aryl ether) dendrimer, the photoinduced electron transfer was studied in detail.41 The photoinduced electron transfer process can occur either from the peripheral naphthyl groups or from the aryl ether dendritic backbone to the viologen core, and the production of the doubly reduced viologen requires two sites of excitation within the same dendritic molecule as a route to a two-electron reduction, thus giving a mimic of natural light-harvesting antenna systems. Photoinduced electron transfer from the poly(aryl ether) dendritic backbone to the core dication viologen was also reported by Balzani’s group.42 8


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Figure 3. Energy and electron transfer in a bifunctional dendrimer involving diarylaminopyrene peripheries and a benzthiadiazole core. Bifunctional dendritic systems capable of undergoing energy transfer and electron transfer are of great interests. A dendritic system by using diarylaminopyrene and benzthiadiazole as the energy and electron donor and acceptor attaching to the periphery and the core of poly(aryl ether) dendritic backbone was prepared (Figure 3).43 The studies revealed that the energy transfer from the excited peripheral diarylaminopyrene chromophores to the core benzthiadiazole unit took place efficiently on a picosecond time scale, and then the efficient electron transfer from the diarylaminopyrene to the sensitized benzthiadiazole occurred on a nanosecond time scale. Studies on the bifunctional dendritic systems bearing carbazole (CZ) terminal groups and a NBD core developed by our group demonstrated that the singlet electron transfer and the triplet-triplet energy transfer occurred from excited CZ units to the NBD group at ambient temperature and 77 K, respectively, which further activated the isomerization reaction of NBD to QC.44 Both the singlet electron transfer and the triplet-triplet energy transfer activated the isomerization reaction of NBD to QC.45 Furthermore, the intramolecular singlet electron transfer and energy transfer can also proceed from the poly(benzyl ether) dendron to the NBD core in the absence of other antenna 9



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chromophores, initializing the NBD isomerization reaction.46 The weaker dependence of the energy and electron-transfer efficiencies on dendrimer size was ascribed to the backfolding conformation of the flexible poly(aryl ether) backbone, which has been validated by the intramolecular exciplex formation between the peripheral chromophore or dendritic back bone and the core group.47,48 Estimation for the extent of folding back conformation indicated that the peripheral chromophores can fold inward and reach to the core vicinity for all four generation poly(aryl ether) dendrimers, which endows these light-harvesting macromolecules with extraordinary characteristics to benefit the energy/electron transfer. Systematic comparisons between linear structure and dendritic architecture using this donor–acceptor system have also been made.49−51 The energy transfer rates and efficiencies in the linear and dendritic systems have the same general scaling as increasing molecular generation, which is mainly attributed to the flexible nature of the molecular frameworks. However, the increased absorption cross-section of the donor component with increasing generation of dendritic systems demonstrates that dendritic systems show better light-harvesting abilities. In the case of charge transfer, the abundant outer donors in the dendrimers can compensate the effect of increasing average distance of donor–acceptor, favoring the charge transfer process. Interestingly, dendron-rod-coil based donor-sensitizer-acceptor triads exhibit better efficiencies than the dendron-rod and the rod-coil diads, and provide possible conduit for charge separation compared to the bisdendron structure.51 Such delicate design of molecular architecture provides unique advantages for photovoltaics. Porphyrin dendrimer is also an attractive kind of light-harvesting dendrimers inspired by the versatile functions of porphyrin derivatives in biological systems.52 Intramolecular energy 10


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migration has been investigated in flexible dendrimers with up to 64 porphyrin units.53 The energy migration and annihilation process can be affected by changing the length of connection between outer porphyrin and dendrimer backbones.54 By introducing a fullerene acceptor to each outer porphyrin noncovalently, artificial photosynthetic reaction centers were built and an extremely long charge-separation state was attained.55 Special pair-dendrons of meso-arylporphyrin were constructed upon a bisporphyrin core. The dendrons act as singlet and triplet energy acceptors or donors depending on the metal coordinating in the bisporphyrin core which alters the core energy level. This study also concluded that the closest segment of the dendritic antenna contributes the most to the energy transfer.56 As a comparison with the flexible and foldable dendrimer, porphyrin dendrimers with a cofacial free-base bisporphyrin were prepared with short linkers which substantially reduces the number of conformers. The S1-S1 and S1-Sn (n > 1) energy transfer processes from antennae to core were investigated and the conformers consisting of one or at most two antennae folded and lain near the core acceptor can facilitate the energy transfer.57 The regular and hierarchically branched structure of dendrimers makes them mimics of light-harvesting systems, where the antenna chromophores surround the central reaction center. However, in comparison with natural systems, the close-packed periphery and the scaffold conformational freedom in artificial light-harvesting dendrimers result in some disadvantageous situations, such as excimer formation and energy annihilation due to the interactions between the antenna chromophores, especially at higher generations, which affect the energy transfer efficiency within dendrimers.58 A few examples have been reported to deal with those drawbacks by using the covalent method. Müllen and co-workers utilized a rigid dendrimer scaffold to overcome those difficulties.59 Fréchet’s group chose proper antenna chromophores and introduced 11



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an energy cascade in the conformationally flexible dendrimers to favor the energy transfer process by avoiding self-quenching.60 Although the covalent method is an effective way to solve the problem, more convenient versatile means are still needed. The noncovalent modification of the periphery of dendrimers displays advantages such as reversibility, selectivity, and tunability, in comparison with the covalent one. Therefore, we proposed to increase abilities of light-harvesting dendrimers by protecting light-harvesting antenna via host-guest interaction. A host molecule, cucurbit[7]uril (CB[7]), was used to assembly with the peripheral naphthyl antenna forming a pseudorotaxane terminated structure in the aqueous solution. The interactions among the periphery naphthyl chromophores and the quenching by solvent molecules are avoided because of the protection effect of CB[7], and much higher fluorescence quantum yields are obtained. Furthermore, 9-anthracenecarboxylic acid (AN) was introduced into the dendritic system as the energy acceptor (Figure 4).61 The energy transfer efficiencies from the periphery naphthyl chromophores to AN were remarkably enhanced by the peripheral pseudorotaxane formation. Upconversion design also helps to alleviate the quenching or annihilation effect in multiphoton absorption situation,62−64 but how to avoid the quenching effect in the multichromophore complex and funnel the energy is still a significant matter needed to be dealt with.

Figure 4. Visual expression of the energy transfer process in different self-assembled dendritic systems. Although various light-harvesting dendrimers were prepared and energy conversion 12


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processes within them were studied, the development of light-harvesting dendrimers is rather empirical and proof-of-concept compared with natural complexes. Recent developments on energy transfer mechanisms demonstrate a detailed modelling of the intramolecular flow of electronic energy and deliver new physical insights into the intramolecular electrodynamics of light-harvesting dendrimer. As the advances of energy transfer theory and understanding of biological energy conversion process, it is still quite challenging and more lessons should be learned from nature to develop more efficient and practical light-harvesting dendrimers. Applications in Solar Cells. Conversion and storage of solar energy into electricity is one of the most promising approaches to the demand of future energy sources. Organic photovoltaic cells such as dye-sensitized solar cells (DSSCs) and bulk-heterojunction solar cells (BHJ solar cells) are greatly attractive to academic and industrial researchers owing to the potential of manufacturing low cost, the light weight and the flexible devices for solar conversion. Since the dye-sensitized solar cell (DSSC) was initially reported by O’Regan and Grätzel in 1991,65 great efforts have been made and the current record power conversion efficiency (PCE) of 12.3% has been obtained.5,66 The PCE of DSSCs is still insufficient to meet the demand of commercialization. One of the major problems in DSSCs is the back electron transfer from the TiO2 conduction band to the electrolyte I3‾, which restricts the open-circuit voltage and the conversion efficiency. N

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hole-transporting

phenylazomethine dendrimers with a triarylamine core (TPA-DPA) (Figure 5) and applied them in DSSC interface engineering.67 The successive layers of phenylazomethines of the TPA-DPA dendrimers generate basicity gradients of imines and thus stepwise coordination with the metal or I3‾ ions. By casting the dendrimers onto a dye-absorbed TiO2 film of DSSC, the back electron transfer at the TiO2/dye/electrolyte interface was significantly suppressed due to the association of I3‾ and imines on the TPA-DPA, resulting in higher open-circuit voltage than the bare film. The resistance of TPA-DPA for hole transfer through the dendritic shells can be reduced by complexation with SnCl2, thereby improving the fill factor. Following this design, a carbazole dendrimer containing a cyclic phenylazomethine of third generation (CPA-Cz G3) and a half-dendritic phenylazomethine of fifth generation (Half-DPA G5) were synthesized to remove I3‾ and producing I‾ on the TiO2 electrode, which suppressed the back electron transfer without retardation of the desired electron transfer.68 Several other dendrimers with N-containing heterocycles were prepared and used for improving DSSC behaviors by altering the electrochemical process.69,70 PAMAM dendron pendants were attached to the 2,2’-bipyridine ligand to synthesize a new ruthenium complex sensitizer.71 The large size of the PAMAM dendrons increased the space between electrons in TiO2 and acceptors in the electrolyte, and the amide groups in PAMAM prevented cations accessing the TiO2 surface thus suppressed electron recombination between the conduction of TiO2 and the oxidized species in electrolyte. However, the research on ruthenium (II) complex dyes with biphenyl based dendritic ligands revealed that the application of dendron did not help the DSSC performance, which indicated that molecular volume and molar extinction coefficient are both the significant parameters to be concerned in 14


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achieving high conversion efficiencies.72 In the field of DSSCs, it is also a main task to develop dyes with broad absorption in the visible and near-IR range as well as large molar extinction coefficients. The supramolecular complexes of a series of porphyrin dendrimers (Figure 6) and C60 were applied in organic photovoltaic system.73 The well-defined nanoclusters had an efficient photoresponse in the visible and near-IR regions. Under visible light irradiation, photoinduced charge separation from the porphyrin singlet excited state to C60 in the supramolecular complexes was initiated, successively, electrons from the reduced C60 were injected into the SnO2 electrode, by which photocurrent was generated. Higher photoenergy conversion efficiency was observed in the dendritic system in comarision with that without dendrimer, which was mainly attributed to the effective electron transfer from the excited porphyrin to fullerene within the dendritic matrix instead of direct electron injection from the excited porphyrin into the conduction band of SnO2. This work demonstrated that the dendrimer scaffold played a key role to stabilize the charge separation state. Later, a polypyridyl ruthenium (II) complexe was attached to the periphery of first and second generation poly(aryl ether) dendrimers, which were used as sensitizers in DSSCs.74 The higher energy conversion efficiency was observed by using the first generation dendrimer containing three Ru(II) complexes than the reference Ru(II) moieties, which was attributed to higher light-harvesting properties in the visible region and the slower charge recombination process due to the dendritic architecture. While the second generation dendrimer with six Ru(II) moieties-based DSSC showed a similar efficiency to the reference Ru(II) complex, which was ascribed to the inefficient anchoring of all of the Ru(II) moieties to the TiO2.

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Figure 6. Structure of the third generation of porphyrin dendrimer Beside applications in DSSCs, dendritic structures have also been merged in the bulk heterojunction organic cells.75,76 Photofunctional dendrimers can be designed and used as electron donor in active layers for bulk heterojunction solar cell. The oligothiophene conjugated dendrimers have showed suitable HOMO-LUMO gaps typical for organic semiconductors, long-living charge carrier ability, and broad absorption bands over the sun light region (Figure 7).77,78 Introduction of dendrimers in bulk heterojunction solar cells as p-type materials incorporated with PCBM as electron acceptor gave moderate power conversion efficiencies. Modification of the oligothiophene dendrimer with an electron-accepting core to tune the optical and electrical properties has also been reported,79 however, the efficiencies of current works are not optimal, further studies are required to improve the optoelectronic performance.

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Figure 7. Fourth generation of oligothiophene-backbone dendrimer. Photofunctional dendrimers, possessing high light-harvesting ability, well-defined structures and capability to stable the charge separation state, can be used as additive or active materials in organic solar cells and pioneer work has been paid in the field. However, due to relatively tedious synthesis and topological structures of dendrimers compared to traditional solar cell materials, it still demands much effort for design and improvements, such as combing the dendritic and the linear structures, channeling out the charge produced at the interface of donor-acceptor. Photochemical production of hydrogen. The requirement to develop clean and sustainable sources of energy has stimulated new approaches for the conversion and storage of solar energy other than by photovoltaic cells. Of these approaches, the photochemical production of hydrogen is at the forefront because hydrogen, with high specific enthalpy of combustion and a benign combustion product (water), is considered to be a potential alternative to fossil fuels. The photochemical production of hydrogen by mimicking natural photosynthesis is considered to be one of the most attractive and potentially useful approaches. In nature, the interconversion of protons and hydrogen is efficiently catalyzed by metalloenzymes known as hydrogenases, which 17



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exist in many microorganisms and rely on metals abundant on earth.80 Since the biological structures and the functionalities of the active sites of hydrogenases were proposed, chemists have made tremendous efforts to model hydrogenases for energy conversion and most of studies focused on developing novel structures of catalytic unit to increase photocatalytic activity and stability.81,82 From another point of view, the active site of hydrogenase in most natural enzymes is buried within a protein matrix which modulates the reactivity and protects the active center so as to facilitate the hydrogen production. Hence, both the catalytic units and the protein matrix are critical for efficient hydrogen production. Dendrimers have widespread applications in enzyme mimics, catalysts, and biomedical materials. Given well-defined structures of dendrimers with suitable microenvironment, the distinct dendritic architecture can be inspired to apply in hydrogenase mimics.

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Multi-Zn-MP-4

n=4

N H NH

O N

NH 2

N H H N

N

N N H

N

NH2

O O

O NH

O

NH2

N

O O

N H NH

NH2

NH 2 O

HN NH

NH 2

NH 2

Multi-Zn-MP-64

Figure 8. Structure of peptide dendrimer coordinated with Zn (II)-mesoprophyrin IX The dendritic architecture involved effectively in hydrogen production was first reported by Mihara and co-workers.83 These dendrimers were constructed by G4-PAMAM (out terminal number n = 64) backbone with 20-residual peptide parts appended at the each terminal, with which Zn (II)-mesoprophyrin IX, the photosensitizer, was coordinated between per two 18


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amphiphilic α-helices of the dendrimer (Figure 8). By using natural hydrogenase, methyl viologen

(MV2+) and triethanolamine (TEOA) as the catalyst, electron relay and sacrificial agent, respectively, photochemical production of hydrogen was achieved. Photoreduction of MV2+ was accomplished more effectively by using the photosensitizers coordinated peptide dendrimers than 4-α-helix bundle structure multi-Zn-MP-4 and Zn (II)-mesoprophyrin IX alone. By considering the photoreduction of MV2+ as the rate determining step of the catalytic system, they interpreted that the less effective performance of the negatively charged peptide dendrimer than the cationic one was attributed to the strong binding between the anionic dendrimer and the electron relay MV2+, which led to more easier back electron transfer from reduced MV•+ radical to the peptide dendrimer. A novel dendritic decorated photosensitizer was constructed by using conjugated poly(phenyleneethynylene) backbone bearing negatively charged poly(aryl ether) dendrimers (Figure 9a).84 By virtue of the three dimensional conjugated photosensitizer wrapping with the surface-charged dendrimer shell, self-quenching of the excited state photosensitizer was suppressed. Moreover, the dendritic shell enriched the positively charged electron carrier, MV2+, on the negatively charged surface to form a spatially separated donor–acceptor supramolecular complex. The spatial isolation origin from the bulk of dendritic envelope lowered the relative rate of charge recombination and the photochemical production of hydrogen took place with an overall quantum efficiency of 13% in the present of triethanolamine as the sacrificial reagent and the colloidal PVA-Pt as the catalyst (Figure 9b).

19



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Figure 9. (a) Schematic representation of dendritic poly(phenyleneethynylene); (b) Schematic diagram for photoinduced H2 evolution from water with dendritic poly(phenyleneethynylene) as sensitizer. Recently, our group reported two series of dendrimer encapsulated Pt nanoparticles which were created by using G6-PAMAM dendrimers terminated with different numbers of hydroxyl groups to mimic hydrogenases.85 The artificial hydrogenases were successfully applied to a hydrogen production system with Pt-tppa+, ethyl viologen, and TEOA as the photosensitizer, electron relay, and sacrificial reagent, respectively (Figure 10). At the optimal conditions, the maximum turnover numbers (TONs) 300 ± 30 and 270 ± 22 per Pt atom were achieved for three hydroxyl groups and one hydroxyl group-terminated dendrimers, respectively. The dendrimers provided cavities to maintain the integrity of small Pt nanoparticles and prevent agglomeration, and no passivation effect is caused by the periphery hydroxyl groups of dendrimers. Most recently, monodisperse Pt nanoparticles inside the cavities of G4-PAMAM dendrimer decorated with [Ru(bpy)3]2+ at the periphery was synthesized and introduced to photocatalytic production of H2 by M. Natali et al (Figure 11).86 Photocatalytic hydrogen evolution was performed with ascorbic acid as the electron donor, giving the TON and turnover frequency (TOF) values 63 and 44.5 h-1 per Pt nanoparticle (the average number of platinum atoms per nanoparticles is 76 Pt/Np), 20


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respectively. Control experiments suggest that the dendrimer scaffold seems to act as a shield for the Pt nanoparticles to limiting bimolecular electron transfer process and facilitate intramolecular electron transfer from the photogenerated reduced sensitizer to the Pt nanoparticles.

Figure 10. Schematic diagram of light driven hydrogen production catalyzed by Pt nanoparticles encapsulated by PAMAM dendrimer.

Figure 11. Photoactive dendrimer combined sensitizer and catalyst for hydrogen production in water. Reprinted from Reference 86. A much more inspiring result of photochemical production of hydrogen based on dendritic [FeFe]-hydrogenase mimics was reported recently by our group.87 The hydrogenase mimics (Hy-Gn) were constructed by attaching two Fréchet-type dendrons (Gn, n = 1–4) to a [2Fe2S] cluster. The photocatalytic evolution of hydrogen by Hy-Gn (n = 1–4) was examined by adopting [Ir(ppy)2(bpy)]PF6 (ppy = 2-phenylpyridine, bpy = 2,2’-bipyridine) as the photosensitizer to absorb light and triethylamine (TEA) as the sacrificial electron donor (Figure 12). The dendritic 21



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hydrogenase mimics exhibit exceptional activity for photocatalytic hydrogen production, giving the corresponding TONs and the initial TOFs per artificial hydrogenase molecule up to 18100, 19000, 21500, 22200, and 6190, 6360, 7000, 7240 h−1 for the hydrogenase mimics of generations 1–4, respectively. It is worth to emphasize that the quantum yield of hydrogen evolution is up to 0.28 for Hy-G4 upon irradiation with 404 nm laser, which is the best quantum yields for [2Fe2S]-based catalysts so far. In this work, the dendritic frameworks provide a distinct microenvironment to regulate the electron-transfer process and protect the active site, similar to natural proteins, thus consequently advancing the photocatalysis.

Figure 12. The homogeneous photocatalytic system using Hy-G4 as the catalyst. The three-dimensional dendrimer structure benefits the mimics of photochemical hydrogen production

by

providing

protein-like

matrices

for

active

center

protection

and

hydrophobic/hydrophilic tunnel for substrate transportation, as well as multichromophores at the periphery for light capture. This nanosize scaffold provides an unsophisticated way to construct artificial photosynthesis system and promising photocatalysts as well, which has been demonstrated by the recent experiment designs. Conclusions and outlook Unique architecture, specific characters and functionalization in predetermined position of dendritic structures with varied chromophores make dendrimers artificial light-harvesting systems. 22


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The artificial light-harvesting systems can harvest photons from a broad absorption regions and the harvested energy can be efficiently channeled to an appointed focal site through energy transfer or electron transfer. The specific structure of dendrimers also provides a distinct microenvironment, which makes dendrimers ideal candidates of mimics of natural enzymes. The artificial hydrogenases can be constructed by encapsulating a catalytic unit within dendritic architectures. The site isolation effect can stabilize the encapsulated catalytic unit and regulate the energy-transfer and electron-transfer processes, similar to natural proteins, advancing the photocatalysis. Taking advantages of light-harvesting ability, well-defined structures and capability to stable the charge separation state, photofunctional dendrimers have been used as additive or active materials in organic solar cells. The photophysical and photochemical features can be easily tuned by varying the components of the dendrimer molecules, resulting in different photofunctionality. Although there are many scientific designs for constructing photofunctional dendrimers and benefits are attained by merging the dendritic structure, challenges are still remain to enhance the utilization of harvested energy, to expend ways of energy utilization, to broaden applications, and also to deal with their tedious synthesis and undesired energy dissipation. Recent developments on special chromophores possessing character of aggregation-induced emission (AIE) demonstrated that the non-irradiated decay of the AIE chromophores can be suppressed by conformation restriction.88

Considering

the

crowded

conformation

of

dendrimers,

novel

efficient

light-harvesting systems may be built by using the AIE chromophores as antenna groups to compensate concentration quenching of multichromophore complex and further advancing the utilization of harvested energy. The performance of dendritic structure on hydrogenase mimics 23



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demonstrates that the dendritic structures can modulate the electron transfer process and stabilize the charge separation state. Because electron transfer is the fundamental process for energy conversion in photosynthetic systems, therefore, we can speculate that application of dendritic structure in other photocatalytic systems may benefit the catalytic efficiencies. With inspiring biomimetic approaches, the progress of photofunctional dendrimers will be well continued. It is expected that the future generation of photofunctional dendrimers will be exploited in novel and advanced ways leading to applications such as solar energy conversion devices, photocatalysts, fluorescent sensors, and so on, as complementary substances to traditional macromolecular materials. Biographies Xiaohui Zhang is a Ph.D. student in Prof. Li’s Lab at Technical Institute of Physics and Chemistry, CAS. His research focuses on photoresponsive dendrimer and dendritic assembly. Yi Zeng received his B.S. from University of Science and Technology of China and Ph.D. from Technical Institute of Physics and Chemistry, CAS. His research interests are in photofunctional supramolecular systems. Tianjun Yu obtained his B.S. at Beijing Normal University and his Ph.D. from Technical Institute of Physics and Chemistry, CAS. His research aims to build artificial complexes for photochemical hydrogen production. Jinping Chen received his B.S. at Shandong University and his Ph.D. from Technical Institute of Physics and Chemistry, CAS. His current research interests are synthesis, characterization and application of conjugated molecules. Guoqiang Yang is a full professor at the Institute of Chemistry, CAS. His scientific interests are 24


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focused on optical functional materials, the effects on the luminescent properties of materials, and chemo/bio-probes. Yi Li has been working as a full professor at the Technical Institute of Physics and Chemistry, CAS, since 1999. Her research interests include photophysics and photochemistry of supramolecular systems and synthetic photochemistry. Acknowledgements We are grateful for funding from the National Natural Science Foundation of China (grant nos. 21173245, 21233011, 21073215, 21004072, and 21273258), the 973 program (nos. 2013CB834703, 2013CB834505, 2010CB934500), and the Solar Energy Initiative of the Chinese Academy of Sciences. References (1) Barber, J. Photosynthetic Energy Conversion: Natural and Artificial. Chem. Soc. Rev. 2009, 38, 185–196. (2) Kamat, P. V. Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion. J. Phy. Chem. C 2007, 111, 2834–2860. (3) Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera, D. G. Solar Energy Supply and Storage for the Legacy and NonLegacy Worlds. Chem. Rev. 2010, 110, 6474–6502. (4) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446–6473. (5) Hagfeldt, A.; Boschloo, G.; Sun, L. C.; Kloo, L.; Pettersson, H. Dye-Sensitized Solar Cells. Chem. Rev. 2010, 110, 6595–6663. (6) Kamat, P. V. Graphene-Based Nanoassemblies for Energy Conversion. J. Phys. Chem. Lett. 2011, 2, 242–251. (7) Astruc, D.; Boisselier, E.; Ornelas, C. Dendrimers Designed for Functions: From Physical, Photophysical, and Supramolecular Properties to Applications in Sensing, Catalysis, Molecular Electronics, Photonics, and Nanomedicine. Chem. Rev. 2010, 110, 1857–1959. (8) Zeng, Y.; Li, Y. Y.; Chen, J. P.; Yang, G. Q.; Li, Y. Dendrimers: A Mimic Natural Light-Harvesting System. Chem. Asian J. 2010, 5, 992–1005. (9) Mcdermott, G.; Prince, S. M.; Freer, A. A.; Hawthornthwaitelawless, A. M.; Papiz, M. Z.; Cogdell, R. J.; Isaacs, N. W. Crystal-Structure of an Integral Membrane Light-Harvesting Complex from Photosynthetic Bacteria. Nature 1995, 374, 517–521. (10) Hu, X. C.; Damjanovic, A.; Ritz, T.; Schulten, K. Architecture and Mechanism of the Light-Harvesting Apparatus of Purple Bacteria. P. Natl. Acad. Sci. USA 1998, 95, 5935–5941. 25



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Quotes: Although various light-harvesting dendrimers were prepared and energy conversion processes within them were studied, the development of light-harvesting dendrimers is rather empirical and proof-of-concept compared with natural complexes. Dendrimers have widespread applications in enzyme mimics, catalysts, and biomedical materials. Given well-defined structures of dendrimers with suitable microenvironment, the distinct dendritic architecture can be inspired to apply in hydrogenase mimics. Although there are many scientific designs for constructing photofunctional dendrimers and benefits are attained by merging the dendritic structure, challenges are still remain to enhance the utilization of harvested energy, to expend ways of energy utilization, to broaden applications, and also to deal with their tedious synthesis and undesired energy dissipation.

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