Self-assembled supramolecular artificial light-harvesting nanosystems: construction, modulation, and applications

Artificial light-harvesting systems, an elegant way to capture, transfer and utilize solar energy, have attracted great attention in recent years. As the primary step of natural photosynthesis, the principle of light-harvesting systems has been intensively investigated, which is further employed for artificial construction of such systems. Supramolecular self-assembly is one of the feasible methods for building artificial light-harvesting systems, which also offers an advantageous pathway for improving light-harvesting efficiency. Many artificial light-harvesting systems based on supramolecular self-assembly have been successfully constructed at the nanoscale with extremely high donor/acceptor ratios, energy transfer efficiency and the antenna effect, which manifests that self-assembled supramolecular nanosystems are indeed a viable way for constructing efficient light-harvesting systems. Non-covalent interactions of supramolecular self-assembly provide diverse approaches to improve the efficiency of artificial light-harvesting systems. In this review, we summarize the recent advances in artificial light-harvesting systems based on self-assembled supramolecular nanosystems. The construction, modulation, and applications of self-assembled supramolecular light-harvesting systems are presented, and the corresponding mechanisms, research prospects and challenges are also briefly highlighted and discussed.


Introduction
Sunlight is the original source that supplies energy for all the biological activities in the world. [1][2][3][4] A huge amount of sunlight is received by Earth every day; however, only a small part of it is utilized by human beings. [5][6][7][8][9][10][11][12] Nature always takes advantage of photosynthesis to capture solar energy and construct higher carbon products, which offers great inspiration to fabricate articial ones according to the principle of photosynthesis. In higher plants and photosynthetic bacteria, light-harvesting antenna supramolecular complexes absorb the solar energy and transfer it through long-range energy funnels to reaction centers, which can further drive the reactions to convert the transferred light energy into chemical energy. 4,[13][14][15][16][17][18][19] Nevertheless, from capturing solar energy to nally forming higher carbon products, photosynthesis is a multistep complex process, which has been hard to completely reproduce articially so far. Thus, at the present stage, mimicking some of the steps of photosynthesis can help us gradually recognize the whole photosynthesis as well as improve the utilization efficiency of solar energy. [20][21][22] Among all the steps, light-harvesting is the primary step of photosynthesis, which plays an important role in determining the utilization efficiency of solar energy. Therefore, more and more attention has been focused on developing articial light-harvesting systems (ALHSs).
Antenna chromophores are employed rst to absorb and transfer light energy in common ALHSs. Then, to avoid the Chem. and mechanical interlocked molecules.

Quan Li is Distinguished Chair
Professor and Director of Institute of Advanced Materials at Southeast University. He held appointments in USA, Germany, and France. Li received his PhD in Organic Chemistry from the Chinese Academy of Sciences in Shanghai, where he was promoted to the youngest Full Professor of Organic Chemistry and Medicinal Chemistry in February 1998. He is a Fellow of the Royal Society of Chemistry. He has been elected as a member of the European Academy of Sciences and the European Academy of Sciences and Arts. He was also honored as a Professor and Chair Professor at several universities. His current research interests span from stimuli-responsive smart so matter, advanced photonics, and optoelectronic materials for energy harvesting and energy saving to functional biocompatible materials and nanoparticles to nanoengineering and device fabrication.
dissipation of light energy that is captured by antenna chromophores, Förster resonance energy transfer (FRET) is one feasible process for achieving the energy transfer. 16,[23][24][25] Thus, the antenna chromophores are referred to as FRET donors, which transfer the absorbed light energy to FRET acceptors to realize the articial light-harvesting process. [26][27][28][29][30][31][32] According to the mechanism of FRET and the requirements of ALHSs, there are four key considerations: (1) a high donor/acceptor molecular ratio; (2) a large overlap between the emission band of the donor and the absorption band of the acceptor; (3) a distance in the range of 0.1 to 10 nm between donors and acceptors; (4) avoiding emission quenching of donors even though a number of donor chromophores are stacked for energy transfer to one acceptor. Therefore, to construct ALHSs satisfying the above listed points, systems at the nanoscale, i.e., articial lightharvesting nanosystems are an available and viable choice.
In the past decade, many types of ALHSs have been developed by employing dendrimers, 33-35 molecular arrays, [36][37][38][39] polymers, 40-47 anisotropic uid 48 and amphiphilic selfassembled structures. [49][50][51][52][53][54][55][56][57] Both covalent and non-covalent interactions are used to bind light-harvesting donors and acceptors. Covalent binding between donors and acceptors is a stable linkage for light-harvesting, but the donor/acceptor ratio is hard to reach at the highest level of the natural lightharvesting system (∼200 : 1) because of the too complicated synthesis challenge to covalently link large number of donors to one acceptor. 58,59 In contrast, non-covalent binding endows ALHSs with more possibilities in a relatively easier way to achieve a high donor/acceptor ratio as well as controllable properties, but a rational design based on strong driving forces is required to achieve high efficiency of FRET. 60,61 Therefore, different supramolecular construction strategies have been developed to pursue both high donor/acceptor ratios and improved light-harvesting efficiency. Two main factors are chosen to evaluate the light-harvesting efficiency, energy transfer efficiency (F ET ) and the antenna effect (AE), which reect the percentage of energy transferred from the donor and the emission enhancement of the acceptor caused by donor antennas, respectively. [62][63][64] In consideration to construct highly efficient ALHSs, supramolecular nanosystems receive extensive attention because donor and acceptor chromophores in such nanoscale supramolecular systems can easily keep a suitable distance in the nanometer range for achieving high transfer efficiency.
Recently, Hu and coworkers have reviewed ALHSs based on macrocycle-assisted supramolecular assembly in aqueous media. 14 Xiao, Elmes and coworkers have reported ALHSs fabricated via supramolecular host-guest interactions. 65 Moreover, Yang, Xu and coworkers have presented supramolecular ALHSs with a special emission property, aggregation-induced emission (AIE). 60 Considering recent research progress and previously presented reviews on ALHSs, we think that it is a very important topic to summarize the advances in supramolecular self-assembled light-harvesting nanosystems (nano-ALHSs). In this review, we introduce the recent advances in supramolecular nano-ALHSs from the viewpoints of construction, controllable supramolecular synthesis methods, and applications. Host-guest complexes, biomaterials and metal-organic complexes are common construction structures for nano-ALHSs, which are summarized in detail in the rst section. Subsequently, stimuliresponsive and other controlled methods are introduced. Furthermore, some potential applications such as photocatalysis, cell imaging, encryption, and anti-counterfeiting are summarized in the third section. Therefore, in this review article, we aim to provide inspiration for the construction of highly efficient and applicable supramolecular nano-ALHSs. Furthermore, mechanisms, research prospects, and challenges are also briey discussed.

Self-assembled supramolecular nano-ALHSs based on host-guest complexation
Host-guest complexation via pillararenes is an effective pathway for constructing nano-ALHSs. As shown in Fig. 1A, Wang, Hu and coworkers synthesized a salicylaldehyde azine derivative (G1) as a light-harvesting donor, which can coassemble with a pillar [6]arene carboxylate (WP6) to construct an articial light-harvesting platform. 80 Such a light-harvesting platform exhibited spherical nanoparticles (average diameter: 109.2 nm) and enhanced the emission of the donor G1 by 28 times. The G1-WP6 co-assembly is a versatile articial lightharvesting platform that can transfer its emission energy to two different acceptor uorophores, Nile red (NiR) and Eosin Y (ESY). When loading trace amounts of NiR as well as ESY into G1-WP6 nanoparticles, both the two ALHSs work at high efficiency. The AEs of the two ALHSs are calculated to be 25.4 for the NiR loaded ALHS at a 150 : 1 donor/acceptor ratio and 28.0 for the ESY loaded ALHS at a 200 : 1 donor/acceptor ratio. Furthermore, they developed a highly efficient nano-ALHS based on a two-step sequential FRET supramolecular selfassembly system based on self-assembly between a bola-type bis(4-phenyl)acrylonitrile derivative (G2) and pillar [5]arene carboxylate (WP5). (4,7-Bis(thien-2-yl))-2,1,3-benzothiadiazole (DBT) and NiR were selected as two sequential FRET acceptors which can be loaded in G2-WP5 self-assemblies to realize an efficient ALHS (Fig. 1B). 81 In this ALHS system, the emission energy of G2 effectively transfers to DBT at a 350 : 1 molar ratio, which is referred to as a light-harvesting process, while the second stage of FRET is realized at Beyond nano-ALHSs based on the host-guest complexation between anionic pillararene and cationic ammonium in aqueous solution, the molecular recognition between pillararene and alkyl nitriles in non-aqueous solution can also be used for constructing nano-ALHSs. 82 As shown in Fig. 1C, a rhomboidal metallacycle bearing four pillar [5]arene macrocycles WP5-Pt was synthesized as a host building block to construct a light-harvesting platform with 9,10-distyrylanthracene-containing dinitrile guest G3 via host-guest complexation in THF/water. A hydrophobic uorophore diketopyrrolopyrrole DPP was selected as a light-harvesting acceptor to construct a (WP5-PtIG3)IDPP dual-donor nano-ALHS. Such a nano-ALHS has taken advantage of metal-ligand coordination, host-guest complexation and hydrophobic interactions, and it exhibits an excellent capacity for harvesting both UV and blue light. The F ET and AE reached 53.1% and 12.8, respectively, which indicates that such a strategy for constructing nano-ALHSs has great potential for applications.
The host-guest complexation between pillararenes and alkyl chains has been also utilized for constructing metal-organic nano-ALHSs. 83 As shown in Fig. 1D, CsPbBr 3 quantum dots were synthesized as light-harvesting donors, whose emission band showed a good overlap with that of a common uorescent acceptor, ESY. Thus, a two bromo-butyl modied ESY derivative (EYB) was synthesized as a guest uorophore to bind with a thymine functionalized-pillar [5]arene (PTY) for preparing a supramolecular polymer. Such an EYB-PTY based supramolecular polymer further assembled with CsPbBr 3 quantum dots to form an efficient light-harvesting system. The obtained lightharvesting system showed a high F ET of up to 96.5% and displayed excellent photocatalytic activity in cross-coupling hydrogen evolution reactions, which led to more than 2.5 times the product yield than using only EYB itself for the same catalytic reaction. Therefore, such a two-stage self-assembled nano-ALHS based on host-guest complexation and metalorganic complexation provides a new pathway for photocatalysis with solar energy.
Pillararenes can be further tethered to synthetic polymers as a medium to construct nano-ALHSs. Yang, Tang and coworkers reported a nano-ALHS based on a newly synthesized linear copolymer with a number of pillar [5]arene dangling side chains acting as hosts (Fig. 2). 43 Based on the host-guest interactions between pillar [5]arene and cyano groups and the AIE behavior of tetraphenylethene (TPE), the authors synthesized a TPEbased guest molecule and complexed such a guest molecule with pillararene modied polymers to form spherical supramolecular nanoparticles through host-guest complexation. The thus obtained supramolecular nanoparticles exhibited an extremely high uorescence quantum yield of 98.22% in THF. Then, another luminophore, 9,10-distyrylanthracene (DSA), was introduced into the supramolecular nanoparticles as an acceptor to achieve an efficient nano-ALHS. The quantum yield of the ALHS was measured to be 51.69% aer introducing a preferable amount of the acceptor. As the molar ratio between the donor and acceptor reached 1 : 0.25, the F ET reached as high as 88.34% and the AE value was calculated to be 3.63. Furthermore, F ET , AE, emission and quantum yields of such ALHSs can all be tuned by controlling the solvent as well as the donor/acceptor ratio.
Cyclodextrin is also a promising macrocycle for fabricating efficient nano-ALHSs. Liu and coworkers constructed an efficient nano-ALHS by utilizing a cyclic polysaccharide, sulfonatob-cyclodextrin (SCD), an oligo(phenylenevinylene) derivative (OPV) and NiR. OPV was employed as the donor of the ALHS and transferred the light energy to NiR (Fig. 3A). 55 OPV and SCD coassembled into ∼100 nm spherical nanoparticles through electrostatic and amphiphilic interactions. The constructed supramolecular nanoparticles enhanced the uorescence of OPV by several times as well as provide loading sites for NiR, ensuring an efficient FRET process between the two uorophores. The FRET process started to happen at an extremely high donor/acceptor molar ratio (125 : 1), which was considered an efficient ALHS system. Such an OPV-SCD-NiR system showed both a high F ET (72%) and AE (32.5).
The host-guest complexation based on cyclodextrin for further self-assembly is also an effective pathway for fabricating nano-ALHSs. As shown in Fig. 3B, an amphiphilic hydroxy-avone derivative (HFD) with a hydrophilic side chain modication was synthesized as the light-harvesting donor, which can co-assemble with both band g-cyclodextrin (b-CD and g-CD) to form two different light-harvesting platforms. 84 Aer this, a typical uorophore, Cy5, was employed as the light-harvesting acceptor to be loaded into the two types of donor assemblies. Although a-cyclodextrin (a-CD) cannot include the HFD because its cavity volume is too small for a HFD molecule, the HFD can still co-assemble with a-CD and the HFD@a-CD co-assembly can also be constructed into HFD@a-CD-Cy5 ALHSs. The three kinds of ALHSs (HFD@a-CD-Cy5, HFD@b-CD-Cy5, and HFD@g-CD-Cy5) showed both high F ET (34.8%, 54.6% and 24.5%) and AE (15.1, 16.6 and 12.8), respectively, which provided a promising design for highly efficient nano-ALHSs through controlling uorescence intensity and changing the size of self-assemblies.
Moreover, as a kind of brand-new synthetic macrocycle, a water-soluble phosphate-based cyclotrixylohydroquinoylene (WPCTX) was reported for investigation of light-harvesting ability. 85 As shown in Fig. 3C, WPCTX was successfully synthesized through three-step reactions. The new anionic macrocycle exhibits obvious host-guest complexation ability for ammonium as well as pyridinium groups. The association constant was calculated to be up to (284 ± 11) M −1 , which indicated the rather good capacity of host-guest complexation between WPCTX and these cationic groups in aqueous solution. Thus, an amphiphilic tetraphenylethene ammonium derivative (G4) has been synthesized to assemble with WPCTX to form AIE coassemblies through host-guest complexation and amphiphilic interactions. Such co-assemblies can act as a light-harvesting platform, which exhibits highly efficient energy transfer behavior to both ESY and NiR, two typical light-harvesting acceptors, at a high donor/acceptor ratio (150 : 1 for ESY and 100 : 1 for NiR). Both high F ET (50.1% and 58.9%) and AE (9.1 and 11.0) are obtained from G4-WPCTX-ESY and G4-WPCTX-NiR light-harvesting systems, respectively.

Self-assembled supramolecular nano-ALHSs based on biomacromolecules
With regard to construction of nano-ALHSs, the rst step is to build up the supramolecular nanoparticles as a platform based on non-covalent interactions for the energy transfer process to take place. Applying non-toxic biomacromolecules directly to assemble into supramolecular nanoparticles is a fascinating method, and makes the variation of luminescence more important for the great potential to be applied in living cells, whether in the elds of biosensing, cell imaging, drug delivery, gene delivery, etc.
Recently, researchers all around the world have constructed various supramolecular assemblies based on DNA, proteins, peptides and some other biomacromolecule such as synthesized polymers.
Light-harvesting complexes capture light energy and deliver it, and nally transform it into chemical energy. Construction of light-harvesting systems with DNA and various luminophores (different luminophores acting as donors and acceptors respectively) has the following advantages: (1) DNA is a natural biomacromolecule with high biocompatibility; (2) the double stranded structure of DNA as a supramolecular scaffold can separate the luminophores and reduce the possibilities of the quenching behavior, making it easier to achieve high energy transfer efficiency and antenna effect; (3) the non-covalent interactions between DNA enables the supramolecular assemblies with stimuli responsive properties, making the supramolecular light-harvesting system more precisely controlled by different stimuli; (4) the helical structure and DNA-templated assembly behavior sometimes can induce the chirality of assemblies, together with variation in circularly polarized luminescent signals.
Liu and coworkers employed DNA as the assembly scaffold, where three distinct chromophores were introduced into a nano-ALHS, with ethynylpyrene (Py) acting as a primary donor array and a cyanine-derived dye (Cy3) acting as a secondary or intermediate donor, together with an Alexa Fluor 647 (AF) acting as the acceptor (Fig. 4A). 86 Beneting from the well-dened DNA-templated assembly and the three distinct chromophore arrays, seven triad congurations with precise interchromophore distance and a well-dened donor-acceptor ratio were established. As the ratio among Py : Cy3 : AF in the ALHS was 6 : 6 : 1, energy transfer efficiency reached 90%, which is a signicant high light-harvesting efficiency. This work successfully demonstrated that DNA is an excellent platform to organize arrays of distinct chromophores to further construct high-efficiency light-harvesting supramolecular assemblies with stimuli responsive properties. Such a feasible control was realized only with a convenient variation in the ratio between distinct dyes, which could greatly modulate the light-harvesting efficiency. Different from the way of simply complexing luminophores with DNA to construct articial light-harvesting systems, Häner and coworkers reported a series of lightharvesting supramolecular phenanthrene-containing polymer hybrids with DNA as a scaffold to assemble and various luminophores are covalently linked to a series of DNA acting as photonic wires (Fig. 4B). 87 In this work, four different uorophores, phenanthrene, Cy3, Cy5 and Cy5.5 are employed. A phenanthrene containing oligomer acts as a primary donor, while a series of different dye labelled oligonucleotides, that is covalently substituted DNA scaffolds, acted both as assembly blocks and block containing acceptors. The authors complexed different dye-labeled DNA with the primary donor and successfully constructed a high efficiency light-harvesting system. Owing to the amphiphilic properties of DNA, the noncovalent interactions and the backbone provided by DNA chains, the complex successfully assembled with these separated dyes, which restrained the aggregation and caused quenching to achieve remarkable energy transfer efficiency. Such an ALHS from a simple complexation of a phenanthrene oligomer and Cy3-labeled DNA achieved an AE of 22.8. Among the obtained complexes, a signicant light-harvesting assembly formed by the phenanthrene oligomer, Cy3-labeled DNA, Cy5labeled DNA and Cy5.5-labeled DNA achieved an F ET as high as 59%. The high F ET was not only relevant to the wellorganized state of assembly beneting from DNAs, but also owing to the stepwise energy transfer process from the primary energy donor, phenanthrene, to the intermediate acceptors, Cy3 and Cy5, then nally to a Cy5.5 acceptor through a FRET mechanism.
The related studies demonstrated that DNA is an excellent building block to fabricate nano-ALHSs and has great potential to be further used in various applications due to its stimuliresponsive properties. Besides DNA, other biomacromolecules such as proteins and peptides have also been developed in supramolecular nano-ALHSs. Perrier and coworkers reported an ALHS mediated by a cyclic peptide polymer (Fig. 4C). 88 Based on a careful investigation of the absorption and emission spectra of uorophores, the authors selected three types of dyes, and then synthesized three assembly blocks, pyrene-cyclic peptidepoly(ethylene glycol), cyanine3-cyclic peptide-poly(ethylene glycol) and naphthalene monoamide-cyclic peptide-poly(ethylene glycol), by covalently modifying the dyes to the peptides. Due to the relative strong interactions between cyclic peptides, especially the existence of strong hydrogen bonding between the at rings of the peptides and the exceptional amphiphilic properties that peptides exhibit, the dye-modied cyclic peptides formed stable co-assemblies in aqueous media. Moreover, owing to the covalent connection of the dyes to the peptides, the stacking of uorophores was greatly reduced; thus signicantly avoiding the aggregation induced quenching effect. Such stable co-assemblies in aqueous solution maintained an appropriate distance between the three dyes for the two-step FRET process and realized high efficiency of the ALHS. By simply adjusting the molar ratio between the three types of dye-modied peptides carefully, the light-harvesting system achieved a quantum yield of over 30% in aqueous media. Interestingly, the emission color of such an ALHS was successfully tuned continuously from blue to green and nally orange including pure white emission with a quantum yield of up to 29.9%. Remarkably, such an ALHS can work stably whether at an extremely low concentration (<1 mM) or at relatively high temperature (>80°C). This work revealed that peptides with strong interactions and amphiphilic characters are exceptional assembly blocks to construct a highly emissive light-harvesting system and mimic the photosynthesis process in nature. Amphiphilic polysaccharides can also be employed to construct articial light-harvesting systems beyond DNA and peptides. Liu and coworkers reported a supramolecular nano-ALHS by complexation of the following components: 4-(4bromophenyl)pyridine-1-ium bromide modied hyaluronic acid, cucurbit [8]uril and LAPONITE® clay (Fig. 5). 89 The successfully constructed multivalent supramolecular assemblies exhibited phosphorescence emissive properties enabling the assemblies to act as donors themselves, while two distinct dyes, rhodamine B (RhB) or sulfonated rhodamine 101 (SR101) were loaded into the luminescent assemblies as acceptors. Due to the existence of 4-(4-bromophenyl)pyridine-1-ium bromide, the supramolecular assemblies could exhibit wholly organic room-temperature phosphorescence with a phosphorescence lifetime of up to 4.79 ms in aqueous solution. It was revealed that there existed a large band overlap between the spectra of the emission of the co-assemblies and the absorption of RhB and SR101. Thus, the luminescent supramolecular assembly showed universal potential as an energy donor. Upon loading RhB into the assembly as an acceptor, when the ratio between the donor and acceptor reached 25 : 1, the F ET of the system remarkably reached 80% and the AE value reached 361.6. When SR101 was loaded into the system and the ratio between the donor and acceptor reached 75 : 1, the F ET reached 73.4% and the AE value reached 307.5. Furthermore, by adjusting the molar ratio between the donor and acceptor, a wide range of emission color from blue to orange was achieved when loading RhB, and the emission color was changed from blue, to white, and all the way nally to pink when SR101 acted as the acceptor. Finally, the multicolor emissive nano-ALHS was applied as smart luminescent inks for potential encryption materials.

Self-assembled supramolecular nano-ALHSs based on metal-ligand interactions
To fabricate a supramolecular nano-ALHS, the rst step is to construct a platform, for example, supramolecular assemblies at the nanoscale, for the FRET process to take place. As for the driving force to form supramolecular assemblies, apart from host-guest interactions mediated by various macrocycles and the non-covalent interactions among intra/intermacromolecules, coordination interactions between various metal ions and ligands are also important and commonly used. Coordination-induced self-assembly is an important strategy in the construction of supramolecular assemblies, not only because of their strong, directional, stable and specic properties, but also for the usually well-dened shapes and nanoscale sizes of the generated assemblies. More importantly, with the metal ions involved in the self-assembly system it enables such assemblies to have the capabilities of magnetic, optic, and electronic properties, and catalytic potential. Furthermore, the introduction of the coordination interactions endows the supramolecular system with stimuli-responsive properties. The solvent, ligands, or introduction of other metals can all act as variables to tune the properties of the nanosystem. The rigid, orderly packing of ligands and metals, together with complexed luminophores in the system makes the fabrication of efficient ALHSs extremely easy.
Stang, Acharyya and coworkers reported uorescent hexagonal Pt(II) metallacycles with planar structures as a new platform and fabricated a nano-ALHS by combination with a typical light-harvesting acceptor, ESY. 66 The metallacycles were constructed from a triphenylamine-based ditopic ligand and two diplatinum metallic centers as illustrated in Fig. 6A. The fabricated hexagonal metallacycles possess strong uorescent emission properties in both solution and solid states. Remarkably, the luminescent metallacycles showed strong AIE properties in a mixture of DMSO and water, and could well selfassemble into spherical nanoparticles in H 2 O-DMSO (4 : 1; v/v) with an average diameter of ∼250 nm, as validated by TEM and SEM investigations. These properties made the metallacycles serve as an exceptional light-harvesting platform and at the same time as the donor. Considering the large overlap between the emission spectra of metallacycles and the absorption spectra of luminophore dyes, a commercial uorophore ESY was applied as the acceptor. When the molar ratio between the donor and acceptor was 10 : 1 in H 2 O-DMSO (4 : 1; v/v), the energy transfer efficiency reached as high as 65%.
Apart from metallacycles that are driven by the coordination interactions between metal centers and ligands, metallacages, three-dimensional coordination structures, are newly synthesized and further applied in the construction of nano-ALHSs (Fig. 6B) as reported by Stang, Deria and Li et al. 90 In this work, the authors have designed and synthesized two Pt(II) metallacages based on triphenylamine and anthracene, the assembly of which was driven by coordination interactions. The two Pt(II) metallacages were constructed by two anthracenetriphenylamine-based tripyridyl ligands as two faces, three dicarboxylates with different length rigid spacers as pillars and six 90°Pt(II) acceptors as linkers. The uorescent emission properties of metallacages can be easily tuned by polarity of solvents, temperature, and concentration. The emission color of metallacages can be continuously tuned from deep blue (in toluene) to bright yellow (in DMSO) under a UV lamp. Considering that the emission maxima of the metallacage is right at 554 nm, which is exactly close to the absorption of a commonly used commercial dye, NiR, NiR was chosen as the acceptor for further constructing the light-harvesting system. When NiR was loaded into the assemblies of the metallacages in DMSO, the energy transfer efficiency reached as high as 93% from the donor to the acceptor. This work has inspired researchers to consider a new aspect to construct highly efficient ALHSs and to nd a way to offer a rigid scaffold for the energy transfer process to take place.

Modulation of self-assembled supramolecular nano-ALHSs
The development direction of materials science in recent years has been to nd smart materials or intelligent materials, which can change their behavior by responding to external stimuli such as light, pH, temperature, force, electric elds, etc. 3,91-94 These smart materials have played an important role in many elds such as biosensing, automobiles, building and construction, the aviation industry, and electronic and medical applications. 95-100 Therefore, the modulation of light-harvesting systems obtains researchers' intensive attention and will be the next popular topic in this eld.

Light-controlled supramolecular nano-ALHSs
As for self-assembled supramolecular nanosystems, light is a simple but efficient tool to achieve regulation of lightharvesting efficiency. Liu and coworkers have reported a lightcontrolled supramolecular nano-ALHS constructed with four building blocks including polyanionic g-cyclodextrin (COONag-CD), a pyrene derivative (PYC12), NiR, and a diarylethene (DAE) derivative in aqueous solution (Fig. 7A). 101 PYC12 exhibited uorescence emission at 375 nm and 395 nm, while its emission in the excimer state was red-shied to around 490 nm aer co-assembling with COONa-g-CD. Thus, PYC12 has become a remarkable energy donor in the PYC12/COONa-g-CD co-assembled light-harvesting platform. Subsequently, NiR was selected as an energy acceptor and loaded into the PYC12/ COONa-g-CD supramolecular assembly. A highly efficient energy transfer process occurred in this system, and the optimal donor/acceptor ratio is 160 : 1 with an energy transfer efficiency of up to 83%. Beneting from this ALHS, multicolor containing pure white light-emitting hydrogels were successfully achieved by tuning the molar ratios of the donor and acceptor. With the addition of NiR, the luminescent color changed from cyan to red, and white-light emission with the CIE 1931 chromaticity coordinates of (0.32, 0.30) was also achieved. In order to achieve the photoswitchable uorescence behavior of this system, a secondary acceptor DAE was loaded into the PYC12/COONa-g-CD/NiR light harvesting system. DAE possesses an excellent photoisomerization ability between open and closed forms, which can endow the DAE-loaded PYC12/COONa-g-CD/NiR supramolecular light-harvesting system with controllability by UV light and >600 nm visible light. Under UV light irradiation, the emission of NiR at 640 nm decreased and then recovered upon >600 nm visible light irradiation due to the energy transfer between CF-DAE and PYC12/COONa-g-CD/NiR.
Circularly polarized luminescence (CPL), one important feature of chiral molecular systems, has been used widely in display, sensing, and information storage. Tunable CPL systems have been well developed via supramolecular assembly strategies, and supramolecular nano-ALHSs provided a special pathway for tunable CPL systems (Fig. 7B). 102 Xing, Hao and coworkers reported a photoresponsive supramolecular nano-ALHS matrix with tunable chiroptical properties. Cholesterolappended cyanostilbene (CSC) was selected as a luminophore with a chiral center, which can transform from Z-into E-form under UV light irradiation. The CSCs self-assembled into supramolecular vesicles with exciton Cotton effects and CPL which exhibited a high dissymmetry g-factor at a 10 −2 order of magnitude. Aer UV light irradiation, Cotton effects of CSC further enhanced by about 3-fold, while the CPL dissymmetry factor was almost unchanged aer light irradiation. Subsequently, NiR was used as an acceptor to allow for energy and chirality transfer with luminescence as well as CPL color change from orange to red while retaining a high g lum value. Furthermore, the photo-responsiveness of the NiR loaded CSC ALHS was investigated. Under 380 nm UV irradiation, Cotton effects show a merely slight variation, but uorescence and CPL exhibit a remarkable decrease when 5% NiR was loaded in CSC supramolecular assemblies, which provides a new direction for designing smart chiroptical materials in water.

Thermo-responsive supramolecular nano-ALHSs
Thermally responsive luminescent materials have great potential for temperature sensing, smart display, and anticounterfeiting because of their high sensitivity to temperature changes. 3,93,94,[103][104][105] In some ALHSs, the molecular order of lightharvesting donors shows a signicant effect on the lightharvesting efficiency, which can be easily modulated by temperature. Uvdal and coworkers reported an efficient nano-ALHS based on the self-assembly of nanoscale coordination polymers (NCPs), which showed temperature responsiveness for guest capturing and releasing to achieve emission enhancement (Fig. 8A). 106 The NCPs were prepared by the co-assembly between a linear p-conjugated dicarboxylate (L1) with lanthanide metal ions Gd(III), Eu(III), and Yb(III) in DMF. The guest molecule trans-4-styryl-1-methylpyridiniumiodide (D1) or methylene blue (D2) was encapsulated into NCPs. It was found that the D1-loaded NCPs exhibited an efficient lightharvesting process with energy transfer from NCPs to D1. The light-harvesting properties of D1-loaded NCPs were investigated at both 20°C and 140°C. The emission color changed from bright blue-violet of L1 to orange aer the addition of D1 and Gd(OAc) 3, which showed an efficient light-harvesting effect, while the orange emission intensity further increased upon raising the temperature to 140°C.
Li, Yang and Chen et al. have reported a thermochromic ALHS based on anisotropic uids. In this ALHS, the energy donor is a saddle-shaped discotic liquid crystal compound LC6 bearing a rigid COTh uorophore and multiple dodecyl chains linked to the periphery, which exhibits excellent temperaturedependent uorescence chromism (Fig. 8B). 48 NiR was selected as the energy acceptor that was loaded into this thermochromic liquid crystal platform, and then the light- harvesting efficiency could be tuned at different temperatures by regulating the molecular order of the compound LC6. Meanwhile, not only temperature but also the molar ratio of the donor/acceptor could lead to the variation of the emitting color of such an ALHS. A high AE of up to 39.1 was achieved when the ratio between LC and NiR reached 100 : 1. Furthermore, during the cooling process of the 10 : 1 LC6-NiR light-harvesting system from 80°C to −30°C, the uorescence spectra underwent a red shi and the uorescence color changed from yellow to red.
Besides, Hayashi, Uchihashi, and Oohora et al. developed another method to construct thermo-responsive nano-ALHSs (Fig. 8C). 107 A mutant of a thermostable hemoprotein, hexameric tyrosine-coordinated heme protein (HTHP) was employed to generate a protein assembly under heating by reacting with maleimide-tethering thermoresponsive poly(Nisopropylacrylamide), PNIPAAm. It was found that the PNI-PAAm-modied HTHP (PNIPAAm-HTHP) can self-assemble into a 43 nm spherical structure at 60°C while disassembling at 20°C. The temperature controlled self-assembly can occur reversibly at least 5 times. A photosensitizer Zn protoporphyrin IX, ZnPP, was loaded into a solution of apoHTHP V44C (a type of HTHP), and further modied with maleimide-terminated PNI-PAAm (PNIPAAm-MI) to obtain PNIPAAm-ZnPP (n/6) HTHP V44C . The PNIPAAm-ZnPP (n/6) HTHP V44C was also found to show thermoresponsive assembling behavior, which could provide a micellar assembly (ZnPP (n/6) micelle) aer heating. To evaluate the energy migration, the uorescence quenching, uorescence lifetime and uorescence anisotropy decay were characterized. It was revealed that energy migration within the ZnPPsubstituted micelle occurred within several tens of picoseconds.

pH-Responsive supramolecular nano-ALHSs
pH sensors are oen employed to detect pH variation, and are widely used for evaluating water quality, sensing blood, etc. Due to the strong emission and good dispersity at a trace amount of luminescent acceptor of ALHSs in water, ALHSs have greater potential applications with higher sensitivities than traditional pH sensors. Therefore, not only in pH sensors but also in pH controlled photocatalysis, pH-responsive ALHSs have attracted much attention. Liu, He and coworkers reported a highly emissive microgel showing both pH and temperature responsiveness (Fig. 9). 108 The microgel was synthesized by polymerizing TPE based comonomers, acrylic acid, N-isopropylacrylamide (NIPAM), and permanent crosslinker ethylenebisacrylamide (BIS), which acted as the donor with energy transfer to the acceptor RhB. The pH value showed an obvious effect on the energy transfer efficiency for this light-harvesting system, increasing from 21.6% (pH = 2.0) to 52.3% (pH = 8.0) accompanying the diverse luminescent colors at a molar ratio of TPE/RhB of 5 : 1.

Nano-ALHSs controlled by donor/acceptor ratios
Undoubtedly, the molar ratio of the donor and acceptor is a key factor for ALHSs to determine light-harvesting efficiency as well as the emergent luminescence because it is an entry point for the construction of each ALHS. The F ET can be regulated by the molar ratio of the donor and acceptor, and the emission intensity of the acceptor will also change accordingly. Therefore, upon well matching between the donor and acceptor, some ALHSs can facilely change their luminescent colors by varying the molar ratio of the donor and acceptor. Wang, Xiao and coworkers constructed a nano-ALHS through the coassembly of a typical cationic amphiphile, CTAB, and a polymerized UPy-functionalized TPE derivative (1) as a donor and NiR as an acceptor (Fig. 10A). 109 The tunable emission behavior of the light-harvesting 1-NiR nanoparticles has been achieved by changing the added amount of NiR. Initially, the nanoparticles of 1 in water showed light blue emission color without NiR. When the addition of NiR was increased as the molar ratio of the donor and acceptor decreased from 1500 : 1 to 100 : 1, the emission color changed from light blue to bright red, meanwhile a strong white emission emerged when the molar ratio reached 250 : 1. Based on this research, they further developed another nano-ALHS to achieve controllable luminescence (Fig. 10B). 110 In this system, a bifunctional monomer (CSU) with AIE properties was synthesized by linking cyanostilbene (CS) with ureidopyrimidinone (UPy) units, which can self-assemble into a supramolecular polymer. The CSU acts as the donor and DBT as the energy acceptor. By changing the addition amount of DBT, the luminescence color showed a variation from blue to yellow. A white light emission was also realized when the molar ratio between donor and acceptor reached 1000 : 1.
More broad luminescence changing ranges are achieved when we employ luminophores with more luminescent colors as the donors and acceptors. Except for the luminescence color changing from blue to red or yellow, the green to orange color is also obtained by regulating the donor/acceptor ratio of a green emitting donor and an orange emitting acceptor. Yang, Tang and coworkers fabricated a nano-ALHS by employing luminescent supramolecular polymer nanoparticles with cyanovinylene (CV)-based chromophores (CV-1-CN and CV-2-CN) as donors and NiR as an acceptor, whereby efficient FRET processes occur and broaden the initial emission spectra (Fig. 11). 111 CV-2-CN was included in the cavity of pillar [5]arene from a pillararene-based polymer (PH-2) through host-guest complexation. When the molar ratio between CV-2-CN and NiR ranged from 40 : 0 to 40 : 40, the emission color of the lightharvesting system CV-2-CN&NiR3PH-2 supramolecular polymer nanoparticles changed from green to orange.
The light-harvesting effect of ALHSs can also be controlled by ionic strength. Xing and coworkers presented a co-assembled multi-component system with switchable uorescence as well as CPL that also showed response to SO 2 derivatives (Fig. 12). 112 Pentylamine-substituted cholesteryl naphthalimide (PNC) and a cholesteryl coumarin (CC) derivative were used to co-assemble into vesicles and nanohelices and acted as the energy transfer donor and acceptor, respectively. With different molar ratios between PNC and CC, the emission color changed from green to yellow, orange, and nally to red; meanwhile, the emission wavelength red shied from 542 nm to 624 nm. The F ET reached 44.8% (544 nm) at a donor/acceptor ratio of 100 : 5. The chiroptical properties of this system were also investigated. The results showed that energy transfer between PNC and CC allowed for CPL color evolution from green to red depending on the fraction of CC. And aer adding SO 3 2− , the CPL exhibited a blue shi to 540 nm, which demonstrated that an anionresponsive CPL-active ALHS was achieved.

Anti-counterfeiting
Anti-counterfeiting is well-known in modern human society, which avoids nancial forgeries and the harm of fake and shoddy products. [113][114][115][116][117] Beneting from controllable luminescence, ALHSs can be used as luminescent ink to encrypt information for anti-counterfeiting, namely, information that is not observed under natural light but appears under special light, which is superior to typical uorescent inks due to the tunable luminescent color and high sensitivity resulting from the trace amount of the acceptor. Liu and coworkers have developed an anti-counterfeiting-oriented highly efficient nano-ALHS based on a supramolecular assembly constructed pillar [5]arene (WP5) with a pyridinium modied TPE derivative (Py-TPE) (Fig. 13). 118 In this articial light-harvesting system, Py-TPE/WP5 acted as a donor because of the AIE effect of Py-TPE induced by WP5, which nally resulted in aggregation induced emission enhancement. Then, SR101 was selected as the rst acceptor, which could be packaged into the hydrophobic layer of the Py-TPE/WP5 supramolecular co-assembly with a donor/acceptor ratio of 150 : 1 to achieve highly efficient FRET. Finally, the second acceptor, AlPcS 4 , was employed to form a two-step sequential energy transfer accompanied by uorescence variations from blue to near-infrared (NIR) emission. Moreover, this articial light-harvesting system with NIR emission was used as uorescent security ink. The word 'N' was written using SR101 on a paper with negligible uorescence under UV light, while a red uorescent 'N' appeared when Py-TPE/WP5 supramolecular assembly solution was written on the same position of this special paper. Thus, information encryption is realized for use  in the anti-counterfeiting eld. Among all the secret information, a ngerprint is one of the most important pieces of individual information, which can be used as personal security measures and even for personal identication in criminal cases under some circumstances. 119

Cell imaging
Fluorescent nanomaterials are developed as one of the research frontiers of imaging applications, which can provide entrapment space for drug loading and delivery to realize integration of diagnosis and treatment. To enhance luminescence instead of uorescence quenching in an aggregation state from nanosystems is a key issue for cell imaging oriented uorescent nanomaterials. 74,76,120,121 Liu and coworkers developed a twostage mediated near-infrared emissive supramolecular assembly for lysosome-targeted cell imaging. 4,4 ′ -Anthracene-9,10-diylbis(ethene-2,1-diyl)bis(1-ethylpyridin-1-ium)bromide (ENDT) with weak uorescence emission at 625 nm, was selected as a guest molecule for cucurbit [8]uril (CB [8]) to form a supramolecular assembly (Fig. 14B). 122 The uorescence emission of this supramolecular assembly was enhanced and red shied to 655 nm because of J-aggregation of ENDT induced by CB [8]. Then, the ENDT&CB [8] assemblies were used to assemble with lower-rim dodecyl-modied sulfonatocalix [4] arene (SC4AD), resulting in a second-stage enhancement of NIR emission. Such co-assemblies were demonstrated to image the lysosome by highly consistent co-staining with Lysotracker blue in human lung adenocarcinoma cells (A549 cells). Because of their many excellent properties such as good light stability, strong and multicolor luminescence, and aggregation without quenching, supramolecular ALHSs possess huge potential for cell-imaging, which has inspired a hot research direction. Li, Yang and coworkers have reported a highly efficient nearinfrared emissive supramolecular nano-ALHS for imaging in the Golgi apparatus (Fig. 14C). 123 A naphthyl-1,8-diphenyl pyridinium derivative (NPS) has been synthesized and used as a uorescent donor with a weak uorescence emission at 593 nm. Then, NPS co-assembled with SC4AD to form nanoparticles, exhibiting a sharp aggregation induced emission enhancement with an obvious blue shi to 550 nm. In order to realize near-infrared emission, NiB was loaded into the NPS-SC4AD supramolecular assembly as an energy acceptor due to the sufficient overlap between the absorption band of NiB and the emission band of NPS-SC4AD nanoparticles, which led to an efficient ALHS with 675 nm emission. Furthermore, it has been found that when the donor/acceptor ratio of the NPS-SC4AD-NiB system is 250 : 1 in PBS buffer, the energy transfer efficiency (F ET ) and antenna effect (AE) reach up to 60.8% and 33.1, respectively, which manifests that the NPS-SC4AD-NiB system is a highly efficient ALHS. Finally, NPS-SC4AD-NiB nano-ALHS was used in cell imaging and was found to stain the Golgi apparatus in human prostate cancer cells (PC-3 cells). Furthermore, such NPS-SC4AD supramolecular co-assembly light-harvesting platforms are further demonstrated for efficient energy transfer to NiB and NiR, and these ALHSs show synchronous imaging capability in the Golgi apparatus and lysosome. 124 Taking advantage of its long lifetime, organic roomtemperature-phosphorescence (RTP) can greatly avoid the inuence of biological uorescence and background interference, promoting great progress in bio-imaging. 125 However, there is a limitation to RTP materials; the weak emission caused by amorphous aggregates needs to be solved urgently. 126 Thus, designing and constructing an ALHS with RTP is promising for solving this problem. Liu and coworkers developed a highly efficient RTP energy transfer nano-ALHS in water (Fig. 14D). 127 This light-harvesting system was constructed via a free bromonaphthalene-connected methoxyphenyl pyridinium salt (G), CB [8] and an amphiphilic calixarene (SC4AH) by a two-stage assembly strategy, exhibiting remarkably enhanced RTP emission at 530 nm. Moreover, two organic dyes, NiB and NiR, were loaded into the hydrophobic layer of G3CB [8]@SC4AH as acceptors, respectively, which endowed the light-harvesting system with delayed near-infrared emission at 635 nm and 675 nm. The F ET and AE of the two RTP light-harvesting systems were further investigated. The F ET was calculated to be 64.1% and phosphorescent AE was 352.9 at a donor/acceptor ratio of 150 : 1 for the G3CB[8]@SC4AH/NiR ALHS. Meanwhile, the F ET and AE of G3CB [8]@SC4AH/NiB were calculated to be 49.6% and 123.5 at a donor/acceptor ratio of 300 : 1. Thus, compared to G3CB [8]@SC4AH/NiR, the excellent phosphorescencecapturing capability of the G3CB[8]@SC4AH/NiB system will play an important role in cell imaging, which was found to mark lysosomes with relatively good stability in living cells (A549). Except for nano-ALHSs based on supramolecular nanoparticles, light-harvesting systems based on 2D nanosheets can also be successfully constructed for cell imaging.

Photocatalysis
In natural light-harvesting systems, the processes that realize the transformation of solar energy into chemical energy in a series of steps are collectively called photosynthesis. Therefore, mimicking natural photosynthesis is one of the nal aims for the research on ALHSs. 67,128-132 Light-harvesting systems can be used as photocatalysts like other traditional catalysts for chemosynthesis. Yang, Wang and coworkers have reported a nano-ALHS for the photooxidation reaction and aerobic cross dehydrogenative coupling reaction (Fig. 15A). 133 The ALHS is constructed by a TPE-branched rotaxane dendrimer TGn (n = 1, 2, 3) self-assembling with the uorescent dye ESY acting as the uorescence energy acceptor (TGn-ESY, n = 1, 2, 3). When the molar ratio of TPE/ESY was set as 3 : 1, the F ET were calculated to be 42.5% for TG1-ESY, 68.2% for TG2-ESY, and 71.6% for TG3-ESY, respectively. As for photooxidation, the TGn-ESY ALHSs were supposed to be capable of producing singlet oxygen (O 2 ), which guarantees the photocatalyst system for both the photooxidation reaction and aerobic crossdehydrogenative coupling (CDC) reaction. 2-(Ethylsulfanyl)ethanol was selected as a model compound for photooxidation, and the full conversion from sulde to sulfoxide was achieved aer 10.0 h, 8.0 h and 4.0 h of reaction using TG1-ESY, TG2-ESY and TG3-ESY as photocatalysts, respectively. When keeping the same amount of the TPE units, the higher generation TG was proven to have a boosted photocatalytic performance. Furthermore, N-phenyl-1,2,3,4tetrahydroisoquinoline with an indole group was chosen as another model compound for a typical aerobic cross-coupling reaction. The full conversion of the target product was achieved aer 11.0 h, 8.0 h, and 4.0 h for TG1-ESY, for TG2-ESY for TG3-ESY as photocatalysts with isolated yields of 95%, 90%, and 89%, respectively, which is consistent with the photooxidation results. Zhang and coworkers have developed a nano-ALHS by employing an emissive poly(ethylene glycol)decorated tetragonal prismatic platinum(II) metallacage as the donor and ESY as the energy acceptor in aqueous solution because of the good spectral overlap between the emission band of the metallacage and the absorption band of ESY (Fig. 15B). 134 A TPE-based metallacage was constructed through the coassembly of TPE-based sodium benzoate ligands, the dipyridyl ligand with eight poly(ethylene glycol) chains and cis-Pt(PEt 3 ) 2 (OTf) 2, as the donor for the FRET process in aqueous light-harvesting systems. This aqueous ALHS was further used to catalyze the cross-coupling hydrogen evolution reaction for benzothiazole with phosphine oxide. Compared with those obtained when using only ESY, both the conversion of benzothiazole and the yield of benzo[d]thiazol-2-yldiphenylphosphine oxide increased dramatically when using the platinum(II)-cagebased nano-ALHS as a catalyst. Aer reaction for 12 h, the conversion of benzothiazole reached 88% and the yield of benzo [d]thiazol-2-yldiphenylphosphine oxide reached 65%; however, only 40% conversion and 33% yield were obtained with only ESY as the catalyst.
Nano-ALHSs with a two step FRET effect have also been developed for photocatalysis. Yi, Zhang and coworkers have presented a metallacycle-based ALHS with a sequential energy transfer process for the photocatalyst (Fig. 16). 135 A quadrilateral platinum(II) metallacycle containing a TPE-based ligand (M1) can self-assemble in H 2 O/MeOH (19/1,v/v) which is used as a donor to build a light-harvesting platform owing to its AIE effect. Two uorescent dyes, ESY and SR101, were selected as the energy transfer acceptors for the rst and second steps, respectively. Then, the two acceptors were loaded into M1 selfassemblies to form the M1-ESY-SR101 ALHS with high efficiency. Subsequently, the alkylation of C-H bonds for phenyl vinyl sulfone and tetrahydrofuran was catalyzed by the M1-ESY-SR101 system in an aqueous medium under irradiation of a Xe lamp at 50°C. The result of the reaction showed that the conversion of phenyl vinyl sulfone reached 99% and the yield of 2-(2-(phenylsulfonyl)ethyl)tetrahydrofuran is 48% aer 1 h of irradiation in H 2 O-THF (19 : 1, v/v) containing 1 mol% of M1-ESY-SR101. In contrast, without SR101, the remaining M1-ESY catalyst only exhibited 21% yield of 2-(2-(phenylsulfonyl)ethyl) tetrahydrofuran, which proved that the two sequential energy transfers can improve catalytic efficiency signicantly.
Wang, Hu and coworkers have also reported a nano-ALHS based on the supramolecular assembly between water-soluble WP5 and a bola-type TPE-functionalized dialkyl ammonium derivative (TPEDA) to achieve a two-step sequential energytransfer process in aqueous solution (Fig. 17). 136 The AIE properties of the TPE group makes TPEDA to become a light-harvesting donor, whose emission can be enhanced by coassembling with WP5 to form supramolecular nanoparticles. The formed emissive nanoparticles can act as a light-harvesting donor platform for energy transfer to an energy acceptor ESY as the rst step. Furthermore, NiR was added to realize the second sequential energy-transfer process at a 200 : 1 : 1 molar ratio of TPEDA/ESY/NiR. Furthermore, the WP5ITPEDA-ESY-NiR nanoreactor was used to catalyze the dehalogenation reaction of a-bromoacetophenone with Hantzsch ester in aqueous solution. The yield of product acetophenone reached 96% with WP5ITPEDA-ESY-NiR aer 8 h of irradiation, which increased signicantly in comparison to that obtained with other control catalysts. The photobleaching effect and uorescence quenching of the WP5ITPEDA-ESY-NiR nanoreactor were restrained, which led to the high catalytic efficiency of photosynthesis.

Conclusion and outlook
In this review, we systematically summarize the recent advances in supramolecular nano-ALHSs with a focus on three aspects: construction of supramolecular nano-ALHSs based on different substrates, the modulation strategy for supramolecular nano-ALHSs, and applications of the obtained supramolecular nano-ALHSs. Compared with the covalent bonding between light-harvesting donors and acceptors to achieve ALHSs, supramolecular nano-ALHSs based on non-covalent interactions can simplify the complex synthesis and provide higher light-harvesting efficiency and more exible options for energy matching between donor and acceptor chromophores. Although non-covalent interactions from supramolecular nano-ALHSs may lead to less stability of the created structures as well as the effect of ALHSs, remarkable efforts and inspiring progress have been made to strengthen supramolecular nano-ALHSs toward meeting the standard of practical applications. Some potential methods are suggested here toward increasing their stability: (1) active reaction precursors can be modied on building blocks, so that in situ reactions can be carried out to stabilize the assembled structure aer the formation of supramolecular nano-ALHSs; (2) inspired by natural light-harvesting systems that are protected in chloroplasts, we can design a protective enclosure to avoid the degradation of supramolecular nano-ALHSs; (3) because most of the supramolecular nano-ALHSs were constructed in a solution state that can be easily inuenced by the environment of the solvents, the design of solvent-free supramolecular nano-ALHSs may serve as a preferable way to construct ALHSs with higher stability.
Natural light-harvesting systems belong to nanosystems, so constructing nano-ALHSs is probably the most desirable and available pathway for mimicking the natural light-harvesting process. In addition, according to the principle of FRET during the light-harvesting process, nano-ALHSs provide a suitable spatial distance (0.1-10 nm) for higher energy transfer efficiency. Moreover, like the natural light-harvesting system that serves as the rst step of photosynthesis, one of the most important and rational applications of nano-ALHSs is to realize highly efficient photocatalysis, where not only a large specic surface area but also special bonding and electronic states on the surface of nano-ALHSs can provide for improving the catalytic efficiency and selectivity. Besides photocatalytic applications that are closest to photosynthesis, the captured light energy can be directly utilized for smart display, information encryption and anti-counterfeiting. Although nano-ALHSs do not seem to show obvious advantages in this application area, the molecular order and alignment of supramolecular assemblies in ALHSs possess great potential for advanced information encryption, which depends on the nanostructures of supramolecular ALHSs. Furthermore, from a dimension perspective, only nano-ALHSs could work in living cells. According to this, cell imaging as well as further biological applications offer a promising and interesting research eld, not only because of the bright emission obtained from acceptors of nanoscale ALHSs, but also because it endows living beings with the potential to harvest light energy directly from the sun. As is well-known, natural photosynthesis can only occur in higher plants and some photosynthetic bacteria, while most higher animals do not have such an ability. Thus, we can imagine, if animals have the ability to obtain energy through ALHSs in their cells, the efficiency of solar energy will be greatly improved. Overall, in consideration of all the potential applications that are discussed, we expect that supramolecular nano-ALHSs will be very promising to be further developed into elds such as articial photosynthesis, advanced encryption, integration of diagnosis and treatment, and beyond.

Conflicts of interest
The authors declare no conict of interest.