Molecular–Supramolecular Light Harvesting for Photochemical

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Molecular/Supramolecular Light-Harvesting for Photochemical Energy Conversion – Making Every Photon Count Yi Zeng, Jinping Chen, Tianjun Yu, Guoqiang Yang, and Yi Li ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00652 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 9, 2017

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Molecular/Supramolecular Light-Harvesting for Photochemical Energy Conversion – Making Every Photon Count Yi Zeng,*,†,§ Jinping Chen,† Tianjun Yu,† 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, China. ‡

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

Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China §

University of Chinese Academy of Sciences, Beijing 100049, China

Corresponding Author *E-mail: [email protected], [email protected]

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ABSTRACT:

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Photochemical conversion and storage of the solar energy is a sustainable

way to partially cover human energy needs for the future, in which the energy conversion process is initiated by the capture of photons and the subsequent energy delivery. Diverse artificial light-harvesting techniques have been realized and applied in solar energy conversion with decades’ efforts. In this perspective, challenges and developing trends of molecular/supramolecular light-harvesting system for photochemical solar energy conversion are discussed. Advances and potential strategy on construction of exceptional light-harvesting systems by merging molecular/supramolecular tailoring, singlet fission, and triplet-triplet annihilation, are presented.

TOC GRAPHICS

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Developing clean and sustainable energy is an active research field where scientists pay great effort to meet increasing economic and environmental demands. Solar energy has been considered as one of the most promising renewable energy resource because there is clean and enormous amount of sunlight reaching the earth every day. Conversion and storage of solar energy become a major area of endeavor in the energy revolution. The most delicate fashion for solar energy conversion is photosynthesis evolved by nature, which powers living systems on the earth. The photosynthetic system comprises the reaction center surrounded by multiple light-harvesting complexes, where the energy conversion is initiated with light absorption by antenna pigments, followed by multistep electronic energy migration/transfer, and subsequent charge separation in the reaction center.1 The well evolved function and high efficiency of photosynthetic systems inspires chemists to create artificial ways to harvest and convert solar energy.2-5 A major effort of artificial photosynthesis at early stage focused on the mimic of light-harvesting complexes. Through decades of substantial efforts, a large number of artificial light-harvesting systems based on synthetic molecules and supramolecular systems, organic-inorganic hybrids, and nanomaterials have been developed, in which photon capture and subsequent directional energy funneling via Förster/Dexter mechanism, or electron transfer and resulting charge separation have been accomplished.6-9 Among the artificial designs, molecular and supramolecular systems have attracted much attention due to their easy tunability by molecular tailoring and functional assembling, potentially providing more opportunity to gain insight into the photophysical processes. In this perspective, challenges on reducing energy dissipation and utilizing the excitation energy in artificial light-harvesting 3 ACS Paragon Plus Environment

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complexes with densely packed chromophores are presented, particularly using dendritic systems

as

representative

examples.

Opportunities

for

creating

supramolecular

light-harvesting systems surpassing natural ones through the emerging singlet fission and triplet-triplet annihilation upconversion are discussed.

Preserve and utilize the harvested energy. Dendrimer and dendritic structures have been used as ideal mimics of light-harvesting complexes by utilizing their hierarchical branching structure with numerous chain ends.7 Usually, chromophores are modified at the periphery or the branching point of dendrimers as light-harvesting antenna, and an energy acceptor is anchored within the dendritic framework. Energy harnessing starts with light absorption by antenna chromophores and the harvested energy subsequently is delivered to the core through diverse ways, such as singlet/triplet energy transfer, electron transfer, resulting in population of the excited state of the acceptor or charge separation. However, compared with the subtle and ideal arrangement of antenna pigments in natural light-harvesting complexes, the artificial complexes, including dendritic and other molecule-based systems, exhibit more conformational flexibility and packing disorder. High local concentration of antenna chromophores and large space separation between the antenna and the acceptor exist in large artificial light-harvesting complexes, which lead to self-quenching of excited chromophores and descending of energy conversion efficiency, whereas natural light-harvesting complexes with dense antenna pigments have overcome such circumstance.1 Various strategies have been employed in molecular and supramolecular light-harvesting systems to alleviate the energy dissipation. Intentional selection of the constituted components can partially solve this problem, for example, by adopting a rigid dendritic 4 ACS Paragon Plus Environment

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backbone, or choosing proper antenna chromophores and introducing an energy cascade.10-12 In addition to the covalent design, supramolecular strategy provides a more convenient and versatile solution because of selectivity, reversibility, and tunability. Our group proposed a noncovalent way via host-guest assembly to enhance the energy utilization of light-harvesting dendrimers.13 The peripheral naphthyl antenna is encapsulated by the host molecule, cucurbit[7]uril, forming a pseudorotaxane-terminated structure in aqueous solution, which suppresses the interactions among the periphery naphthyl chromophores and the quenching by solvent molecules because of the protection effect of the host. Much higher fluorescence quantum yield of the peripheral naphthyl and energy transfer efficiency from the antenna naphthyl to the acceptor anthracene are obtained. Another application of supramolecular strategy on construction effective light-harvesting complexes is by adopting a unimolecular micelle antenna, which consists of a porphyrin modified with four polymer arms.14 The micelle antenna stably assembles as high as 179 units around one phycocyanin acceptor and the micelle shell inhibits self-quenching of porphyrin chromophores, leading to a high energy transfer efficiency of about 80%. Moreover, hierarchical dye aggregates with strong electronic coupling and delocalized excited states are also applied to circumventing the detrimental self-quenching issue, which is learned from the case in natural light-harvesting systems consisting of high concentration pigments involving quantum mechanical energy transfer.15-17 Artificial light-harvesting complexes derived from aggregation can associate with up to thousands of antenna dyes which are much more than natural light-harvesting complexes usually consisting of hundreds of pigments. Although fast and efficient energy migration proceeds within well-organized assemblies, quenching by the defect sites is 5 ACS Paragon Plus Environment

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frequently encountered and needs to be dealt with.18,19 Recent developments of chromophores with aggregation-induced emission (AIE) provides an alternate way to construct efficient light-harvesting complexes.20 AIE chromophores are highly emissive in aggregate or solid state mainly due to the restriction of nonradiative decay, which is different from traditional dyes with concentration quenching property, making it possible to avoid the self-quenching drawback in multiple dye complexes and improve the utilization of harvested energy.21

Figure 1. Schematic illustration of the two ways of artificial light-harvesting system working in solar energy photochemical conversion. (a) One is funneling the energy to photosensitizers which act as redox active components and participate in electron transfer process with photocatalyst, (b) and the other is directly being involved in electron transfer process with catalyst units.

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Beyond the aspect in basic research, artificial light-harvesting should be furthered into more practical regime, such as photovoltaic and photochemical conversion of solar energy. While compared with natural light-harvesting systems, artificial ones integrating abilities of capturing photon, energy transfer, and even charge separation, are still in its infancy. Natural light-harvesting system employs most of antenna pigments to capture photons and deliver excitation energy, and only a few of pigments within the reaction center to participate in electron transfer process.1 For artificial light-harvesting systems working in solar energy photochemical conversion, it is a key step to deliver the harvested energy to an active photochemical catalytic material as natural light-harvesting does, or to be involved in electron transfer process as light-absorbing and redox active components via a shortcut during mimicking photosynthesis (Figure 1). In the example of light-harvesting dendrimers capable of unidirectional triplet energy transfer from peripheral chromophores to the core, norbornadiene is used as the core acceptor, which traps the energy harvested by peripheral benzophenone and isomerizes to quadricyclane, providing a proof-of-concept harvesting and photochemical storage of photon energy.22 Photochemical hydrogen production is at the forefront of artificial photosynthesis research because it is tantalizing closer to the goal of water splitting and then alternating fossil fuels. Our group has reported a hydrogen photochemical production system consisted of a dendritic [FeFe]-hydrogenase mimic and [Ir(ppy)2(bpy)]PF6 (ppy = 2-phenylpyridine, bpy = 2,2’-bipyridine).23 The iridium complex acts as the light-absorber and the redox relay. The photocatalytic systems are highly active showing maximum quantum yield of hydrogen photoproduction up to 0.28 upon 404 nm irradiation. Bergamini and co-workers reported a photoactive PAMAM dendrimer decorated 7 ACS Paragon Plus Environment

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with [Ru(bpy)3]2+ at the periphery and platinum nanoparticles encapsulated inside the cavities of the dendrimer, giving the first example of photochemical hydrogen production in a dendrimer with the antenna sensitizer and the catalyst anchored together.24 Recently, artificial photosynthesis dendrimer integrating light-harvesting, electron delivery and hydrogen production was developed in our group.25 The peripheral antennae not only harvest photons but also act as electronic energy reservoirs by exchanging/migrating electrons among the antennas, which facilitate the photochemical catalysis. This circumstance of encapsulating the catalytic center within the electron pool of dendritic antennas, to some extent, is similar to the model of embedding reaction centers in the matrix of antenna pigments in natural photosynthetic units. Besides supramolecular assemblies constructed with polymers, gels, organic-protein hybrids, metal-organic frameworks (MOFs) is also a promising architecture for mimicking the photosynthetic antenna matrix.26 MOFs are crystalline material with well-defined topological structures, which are made of various kinds of multidentate molecular unit and metal or metal cluster connecting nodes. The porous and well-defined architecture and the versatile functionality of MOFs provide a potential way to achieve light-harvesting and photochemical conversion through catalytic component encapsulating, building block tailoring, as well as post-modifying.

Light-harvesting merging singlet fission. Although natural light-harvesting systems can effectively absorb photons in visible region, not all the energy of captured photons is used for photosynthesis. The energy threshold that initiates water splitting and biomass production is equivalent to the red region of about 1.8 eV.2 When the antenna pigment is excited by a photon with higher energy, for example, blue photon, the excess energy above the threshold 8 ACS Paragon Plus Environment

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is released thermally by internal conversion within the excited pigment. Conventional artificial light-harvesting systems also release the excess energy from higher energy photons by heat. If the portion of excess energy can be harnessed, the efficiency of solar energy conversion will be evidently improved and the exciton fission is considered to be an effective strategy. Multiple exciton generation (MEG) is a process of producing multiple (two or more) excitons upon one photon absorption, which occurs in semiconductor nanocrystals.27 MEG has received intense research because it provides a potential way to break the Shockley-Queisser limit for single junction solar cell and significant advances in enhancing light-harvesting efficiency have been achieved in photovoltaic devices involving MEG, in which the external quantum efficiency exceeding 100% has been reported.28 An analogue process of MEG occurring in organic materials is singlet fission, by which one singlet exciton can convert into two triplet excitons in an assembly of coupled organic chromophores. A single junction cell adopting singlet fission can theoretically reach the maximum efficiency over 44%, which revives the research in singlet fission since its first observation in the 1960s.29,30 Singlet fission can be descripted by equation (1) in a simplified way: 





-2

   S + S   TT   T1 + T1

(1)

where the excited singlet chromophore (S1) and the ground state chromophore (S0) form a correlated triplet pair 1(TT), a postulated intermediate with an overall multiplicity of singlet, which is also referred to multiexciton state. And then the correlated triplet pair converts into two triplets under appropriate energy. Generally, a primary requirement for chromophores capable of singlet fission is that the relaxed singlet energy E(S1) is as least twice the triplet 9 ACS Paragon Plus Environment

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energy E(T1), to make the fission process exoergic or isoergic. Furthermore, exciton coupling between chromophores is apparently needed, and the fission rate should be faster than those of competing deactivation processes of the excited chromophore, such as radiative decay, intersystem crossing, and electron transfer to other object. Although singlet fission is attractive due to the potential in organic semiconductors, a major obstacle is that only a handful of molecules have been found to effectively undergo singlet fission in their crystalline or aggregate states. As the progress made in revealing the fundamental of singlet fission, various forms of compounds capable of singlet fission have been developed, such as dimers, oligomers, and polymers.31 In addition, the advance of singlet fission in solution has raised the hope to construct singlet fission materials by supramolecular means, which may control molecular interactions and the consequent exciton coupling.32 For application of singlet fission in artificial photosynthesis, how to use the generated triplet energy is also a crucial issue to be handled. Initiating the photochemical conversion through energy transfer or electron transfer is potentially feasible by matching appropriate organic/inorganic catalysts, as the observation of effective energy or electron transfer from the triplet molecules formed in singlet fission to organic/inorganic materials like copper phthalocyanine, C60, inorganic nanocrystals, and TiO2.33-36 Combining updated underlying principles and supramolecular tools, practical light-harvesting systems taking advantage of singlet fission will serve to raise the efficiency of solar energy conversion by ‘splitting’ one exciton into two. (Figure 2)

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Figure 2. Schematic illustration of light-harvesting complexes adopting singlet fission to initiate photochemical energy conversion. Triplet-triplet annihilation upconversion for sub-bandgap photon harvesting. Besides squeezing every dime out of blue or high energy photons by singlet fission, harvesting long wavelength photons, including sub-bandgap or sub-absorption threshold photons, is also of great importance to raise the efficiency of solar energy conversion, maximizing the utilization of the solar energy that reaches the planet. One straightforward way is simply extending the absorption of light-harvesting materials into longer wavelength by lowering the energy gap. However, this may lead to more thermal decay and reduce the possibility of energy transfer and the driving force of electron transfer. A promising approach is employing photon upconversion techniques, by which low-energy photons convert to higher-energy photons, allowing to be absorbed by conventional light-harvesting materials. Compared with photon upconversion adopting second harmonic

generation, multiphoton absorption, and

lanthanide-doped materials, triplet-triplet annihilation (TTA) upconversion has prevailed in

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the research field of solar energy conversion, because TTA upconversion can effectively take place upon excitation with noncoherent, low-power light sources closing to the solar irradiance and the response wavelength can be tuned by molecular tailoring.37,38 TTA upconversion is a two component system consisting of a sensitizer and an acceptor which is also termed annihilator or emitter. A typical TTA upconversion process starts with excitation of sensitizer molecules to their singlet excited state by absorbing low-energy photons. The excited singlet sensitizers undergo fast intersystem crossing (ISC) to their triplet state and then sensitize acceptor molecules through triplet-triplet energy transfer (TTET), giving triplet acceptors. Effective encounter of two triplet acceptors produces one ground-state singlet and one excited singlet of higher energy via the TTA process. The radiative transition of the excited singlet acceptor to its ground state gives a delayed fluorescence with energy higher than the absorbed photon, achieving photon upconversion (Figure 3). The upconversion efficiency (ΦUC) can be obtained by the product of the sensitizer intersystem crossing efficiency (ΦISC), the triplet–triplet energy transfer efficiency (ΦTTET), and the triplet–triplet annihilation efficiency (ΦTTA), as well as the fluorescence quantum yield (ΦF) of DPA, expressed as ΦUC = ΦISCΦTTETΦTTAΦF.

Figure 3. Scheme for the mechanism of TTA upconversion.

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TTA upconversion is gaining renewed interest since exploration of efficient and visible upconversion systems by Castellano, Baluschev and co-workers.39,40 During the last decade, significant progress has been made in developing efficient sensitizers and acceptors to improve the TTA upconversion efficiency. Requirements of a promising sensitizer include high molar extinction efficient in the long wavelength region, especially red and near IR region, high ΦISC, and long triplet lifetime, in order to generate as many triplet acceptors as possible under certain conditions. Ru(II), Pd(II), Pt(II), Ir(III), and Re(I) complexes including cyclometalated complexes, porphyrin derivatives, as well as other chromophores with heavy atom substituents like iodide and bromide, are commonly used due to the strong spin-orbit coupling and the consequent high ISC efficiency of near unity.41,42 The acceptor must have lower triplet energy E(T1) than that of sensitizer, ensuring the occurrence of effective TTET from the triplet sensitizer to the acceptor. Furthermore, the excited singlet energy of acceptor needs to be lower than the double of the triplet energy of acceptor (2E(T1)>E(S1)) and higher than the excited singlet energy of sensitizer. The energy of excited singlet and triplet of sensitizer should fall in between those of excited singlet and triplet of acceptor. After continuous efforts in the last decade, the efficiency of TTA upconversion reaches over 30% in deaerated solution, and significant improvements in solar cell performance involving TTA upconversion were demonstrated by Schmidt’s and other’s groups.43-45 However, avoidance of oxygen quenching and incorporation of TTA upconversion into practical devices are still challenging.38 Oxygen shielding is of great importance in performing TTA upconversion because oxygen is an active quencher for triplet species. Flexible strategies have been proposed to protect against oxygen, such as sealing TTA 13 ACS Paragon Plus Environment

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upconversion systems in various types of polymeric matrix,46 gels,47 and capsules,48 which are inert during the TTA upconversion precess. Further addition of oxygen scavenger also strengthens the protection effect by depleting dissolved oxygen, and thus favoring TTA upconversion.49,50 In addition to oxygen-proof, those blending and encapsulation approaches have made a step forward into solid or quasi solid state, thus propelling the application of TTA upconversion in practical devices. Progress towards improving applications of TTA upconversion is hampered by a dilemma again. Directly increasing the concentration of sensitizers and acceptors may enhance the upconversion efficiency due to more active species produced under the same excitation, but it may be accompanied by a series of adverse effects due to the proximity of these molecules, including Förster energy transfer from the excited singlet acceptor to the sensitizer, quenching of the excited triplet acceptor by encountering a sensitizer through the external heavy-atom effect, and the TTA process between sensitizers. Although polymeric solidification or immobilization onto nanomaterials can alleviate quenching effects from diffusive collision at high density of photoactive molecules, severe aggregation or phase separation occur frequently, and thus reducing the valid species during TTA upconversion. In addition, spatially random segregation of the chromophores hinders effective sensitization of the acceptor, triplet exciton migration/diffusion and eventual annihilation, because triplet energy transfer commonly proceeds through Dexter-type mechanism involving orbital overlap that requires space proximity of chromophores. Diversified strategies such as dendritic architecture, inorganic-organic hybrid, and supramolecular assembly, provide potentials to solve the dilemma by arranging active species 14 ACS Paragon Plus Environment

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hierarchically and precisely, manipulating electronic interactions, incorporating energy relay or cascade et al.51-56 An interesting advance is that the triplet exciton migration and encounter play the key role in the supramolecular TTA upconversion systems in addition to mere molecule or chromophore diffusion. Recently, inspiring approaches based on supramolecular tactics have been reported toward efficient triplet exciton migration, TTA upconversion under condensed situation, and upconverted singlet exciton collection. A lipophilic acceptor derived from 9,10-diphenylanthracene was designed by Kimizuka et al, and used to co-assemble with porphyrin sensitizer in organic solution to form monolayer membranes. Effective sensitization of the acceptor and triplet exciton migration can occur in such assembly and up to 30% upconversion quantum yield was achieved under low excitation intensity.56 They also reported solvent-free TTA upconversion systems by employing acceptors with branched alkyl chain or ionic liquid substituent as liquid medium, which guarantee homogeneous mix with sensitizers and effective TTA upconversion process57,58 More recently, Monguzzi and co-workers dissected a TTA upconversion system in hyper-cross-linked polymeric nanoparticles and proposed a rigid platform to improve solid-state upconversion through extension of triplet exciton lifetime and diffusion length.59

Achieving peak efficiency uopn

excitation equivalent to solar irradiance is extremely challenging because of the limitation of the triplet energy diffusion rate and distance in common solvent and solid systems. Kimizuka’s group realized maximization of upconversion efficiency with excitation below the solar irradiance by incorporating MOFs as a TTA upconversion platform. Tailored acceptors and sensitizers were precisely arranged in MOFs, in which fast and long-range triplet exciton diffusion can occur, and thus promoting the TTA upconversion. Furthermore, 15 ACS Paragon Plus Environment

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the incorporation of a highly emissive singlet acceptor into the MOFs matrix dramatically increases the upconversion quantum yield by efficient collecting the upconverted singlet energy.60,61 Baldo and co-workers employed PbS nanocrystals as the sensitizer and fabricated upconversion thin film devices with a layer structure. A singlet energy cascade was built in the acceptor rubrene layer by doping with a singlet emitter dibenzotetraphenylperiflanthene which has a lower singlet excited state than the acceptor and can drain out the singlet exciton formed during TTA upconversion, achieving a peak efficiency over 1% of IR-to-Vis upconversion in such solid-state device.62 It can be envisaged that a great stride in photochemical conversion of sub-bandgap photons will be made by integrating the emerging TTA upconversion system and catalysts of water-splitting, CO2 reduction, and photoelectrochemical devices as well (Figure 4).63

Figure 4. Cartoon expression of harvesting low energy photons through TTA upconversion. Outlook. Various artificial light-harvesting complexes have thus far been prepared toward solar energy utilization through mimicking natural light-harvesting system in simplified and 16 ACS Paragon Plus Environment

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altered

ways.

In

moving

from

proof-of-concept

artificial

light-harvesting

to

application-related light-harvesting, a challenge lies in preserving the energy harvested and passing it onto conversion centers via either energy transfer or electron transfer. Moreover, endowing artificial light-harvesting systems with capability of maximizing the utilization of solar spectrum, which transcends natural systems, is especially appealing. By merging molecular tailoring, supramolecular architecture, and singlet fission or TTA upconversion, exceptional light-harvesting systems will be created, and thus reforming conventional techniques for solar energy conversion.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest. Biographies Yi Zeng is an associate professor at the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences and School of Future Technology, University of Chinese Academy of Sciences. His research is focused on energy conversion in molecular and supramolecular materials, chemo- and biosensors. Jinping Chen serves at the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences as an associate professor. He is engaged in research on preparation of functional materials for solar energy conversion and photoresist. 17 ACS Paragon Plus Environment

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Tianjun Yu is now an associate professor at the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, where he mainly conducts research on photocatalysis for energy conversion. Guoqiang Yang is a professor at the Institute of Chemistry, Chinese Academy of Sciences and the Vice-President of University of Chinese Academy of Sciences (UCAS). His research interests include photo-functional materials, novel fluorescent probes, environment and high pressure effects on luminescent properties of materials and high resolution photoresist. Yi Li is working as a full professor at the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences and School of Future Technology, University of Chinese Academy of Sciences. Her research interests include solar photochemical conversion, photophysics and photochemistry of molecular and supramolecular systems, and synthetic photochemistry.

ACKNOWLEDGMENT Financial support from the 973 program (2013CB834505 and 2013CB834703), the National Natural Science Foundation of China (Nos. 21233011, 21573266, 21673264, 21472201 and 21672226), and the Strategic Priority Research Program of Chinese Academy of Sciences (XDB17030300) is gratefully acknowledged.

REFERENCES (1) Mirkovic, T.; Ostroumov, E. E.; Anna, J. M.; van Grondelle, R.; Govindjee; Scholes, G. D. Light Absorption and Energy Transfer in the Antenna Complexes of Photosynthetic Organisms. Chem. Rev. 2016, DOI: 10.1021/acs.chemrev.6b00002. (2) Barber, J. Photosynthetic Energy Conversion: Natural and Artificial. Chem. Soc. Rev. 2009, 38, 185-196. 18 ACS Paragon Plus Environment

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(36) Schrauben, J. N.; Zhao, Y. X.; Mercado, C.; Dron, P. I.; Ryerson, J. L.; Michl, J.; Zhu, K.; Johnson, J. C. Photocurrent Enhanced by Singlet Fission in a Dye-Sensitized Solar Cell. ACS Appl. Mater. Inter. 2015, 7, 2286-2293. (37) Schmidt, T. W.; Castellano, F. N. Photochemical Upconversion: The Primacy of Kinetics. J. Phys. Chem. Lett. 2014, 5, 4062-4072. (38) Schulze, T. F.; Schmidt, T. W. Photochemical Upconversion: Present Status and Prospects for Its Application to Solar Energy Conversion. Energy Environ. Sci. 2015, 8, 103-125. (39) Keivanidis, P. E.; Baluschev, S.; Miteva, T.; Nelles, G.; Scherf, U.; Yasuda, A.; Wegner, G. Up-Conversion Photoluminescence in Polyfluorene Doped with Metal(II)-Octaethyl Porphyrins. Adv. Mater. 2003, 15, 2095-2098. (40) Islangulov, R. R.; Kozlov, D. V.; Castellano, F. N. Low Power Upconversion Using MLCT Sensitizers. Chem. Commun. 2005, 3776-3778. (41) Zhao, J. Z.; Wu, W. H.; Sun, J. F.; Guo, S. Triplet Photosensitizers: From Molecular Design to Applications. Chem. Soc. Rev. 2013, 42, 5323-5351. (42) Xun, Z. Q.; Zeng, Y.; Chen, J. P.; Yu, T. J.; Zhang, X. H.; Yang, G. Q.; Li, Y. Pd– Porphyrin Oligomers Sensitized for Green-to-Blue Photon Upconversion: The More the Better? Chem. Eur. J. 2016, 22, 8654-8662. (43) Cheng, Y. Y.; Fuckel, B.; MacQueen, R. W.; Khoury, T.; Clady, R. G. C. R.; Schulze, T. F.; Ekins-Daukes, N. J.; Crossley, M. J.; Stannowski, B.; Lips, K.; Schmidt, T. W. Improving the Light-Harvesting of Amorphous Silicon Solar Cells with Photochemical Upconversion. Energy Environ. Sci. 2012, 5, 6953-6959. (44) Monguzzi, A.; Borisov, S. M.; Pedrini, J.; Klimant, I.; Salvalaggio, M.; Biagini, P.; Melchiorre, F.; Lelii, C.; Meinardi, F. Efficient Broadband Triplet-Triplet Annihilation-Assisted Photon Upconversion at Subsolar Irradiance in Fully Organic Systems. Adv. Funct. Mater. 2015, 25, 5617-5624. (45) Li, C. H.; Koenigsmann, C.; Deng, F.; Hagstrom, A.; Schmuttenmaer, C. A.; Kim, J. H. Photocurrent Enhancement from Solid-State Triplet-Triplet Annihilation Upconversion of Low-Intensity, Low-Energy Photons. ACS Photonics 2016, 3, 784-790. (46) Simon, Y. C.; Weder, C. Low-Power Photon Upconversion through Triplet-Triplet Annihilation in Polymers. J. Mater. Chem. 2012, 22, 20817-20830. (47) Duan, P. F.; Yanai, N.; Nagatomi, H.; Kimizuka, N. Photon Upconversion in Supramolecular Gel Matrixes: Spontaneous Accumulation of Light-Harvesting Donor-Acceptor Arrays in Nanofibers and Acquired Air Stability. J. Am. Chem. Soc. 2015, 137, 1887-1894. (48) Kim, J. H.; Kim, J. H. Encapsulated Triplet-Triplet Annihilation-Based Upconversion in the Aqueous Phase for Sub-Band-Gap Semiconductor Photocatalysis. J. Am. Chem. Soc. 2012, 134, 17478-17481. (49) Marsico, F.; Turshatov, A.; Pekoz, R.; Avlasevich, Y.; Wagner, M.; Weber, K.; Donadio, D.; Landfester, K.; Baluschev, S.; Wurm, F. R. Hyperbranched Unsaturated Polyphosphates as a Protective Matrix for Long-Term Photon Upconversion in Air. J. Am. Chem. Soc. 2014, 136, 11057-11064.

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QUOTES - For artificial light-harvesting systems working in solar energy photochemical conversion, it is a key step to deliver the harvested energy to an active photochemical catalytic material as natural light-harvesting does, or to be involved in electron transfer process as light-absorbing and redox active components via a shortcut during mimicking photosynthesis.

- Combining updated underlying principles and supramolecular tools, practical light-harvesting systems taking advantage of singlet fission will serve to raise the efficiency of solar energy conversion by ‘splitting’ one exciton into two.

- An interesting advance is that the triplet exciton migration and encounter play the key role in the supramolecular TTA upconversion systems in addition to mere molecule or chromophore diffusion.

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