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Cite this: DOI: 10.1039/c4dt02996h

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Recent advances in the construction of fluorescent metallocycles and metallocages via coordination-driven self-assembly Lin Xu, Yu-Xuan Wang and Hai-Bo Yang* During the last few years, the construction of fluorescent metallocycles and metallocages has attracted considerable attention because of their wide applications in fluorescence detection of metal ions, anions,

Received 28th September 2014, Accepted 13th November 2014

or small molecules, mimicking complicated natural photo-processes, and preparing photoelectric devices, etc. This perspective focuses on the recent advances in the construction of a variety of fluo-

DOI: 10.1039/c4dt02996h

rescent metallocycles and metallocages via coordination-driven self-assembly. In addition, the fluo-

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rescence properties and the applications of these organometallic architectures have also been discussed.

Introduction The extensive studies on macrocycles, such as crown ethers, cyclodextrins, calixarenes, cyclophanes, cavitands, pillararenes, have greatly contributed to the development of the field of supramolecular chemistry.1 In particular, the design and preparation of novel supramolecular macrocycles or cages have inspired the intensive research interests because of their wide applications within supramolecular chemistry and materials science.2 Coordination-driven self-assembly, which is based on the metal–ligand coordination interaction, has proven to be a

Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, 3663 N. Zhongshan Road, Shanghai, 200062, People’s Republic of China. E-mail: [email protected]; Fax: (+86) 21-62235137; Tel: (+86) 21-62235137

Lin Xu

Lin Xu received his PhD degree in chemistry from the East China University of Science and Technology (ECUST, Shanghai) in 2012 under the supervision of Professor Xuhong Qian. Subsequently, he joined the East China Normal University (ECNU, Shanghai) as an assistant professor and was promoted to associate professor in 2014. His research interests focus mainly on supramolecular selfassembly and fluorescence sensing.

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successful strategy for the construction of discrete two-dimensional (2-D) metallocycles and three-dimensional (3-D) metallocages with well-defined shapes, sizes, and geometries.3 Since the metal–ligand bonds are highly directional and relatively strong, the coordination-driven self-assembly methodology offers considerable synthetic advantages such as fewer steps, fast and facile construction of the final products, and inherently self-correcting and defect-free assembly. During the last three decades, the groups of Lehn,4 Stang,5 Fujita,6 Mirkin,7 Raymond,8 Newkome,9 and others10 have successfully developed the coordination-driven self-assembly protocol to build numerous metallosupramolecular architectures. In fact, coordination-driven self-assembly of discrete organometallic polygons and polyhedra has evolved to be one of the most prevalent areas within modern supramolecular chemistry.

Yu-Xuan Wang

Yu-Xuan Wang received his BS degree in chemistry from the Beijing Normal University in 2013. He is currently pursuing his MS degree in the laboratory of Prof. Hai-Bo Yang at the East China Normal University. His current research interests are focused on the preparation of chiral metallacycles via coordination-driven self-assembly and their applications in catalysis, sensing, etc.

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Notably, there has been an increasing interest in the construction of functionalized supramolecular metallocycles and metallocages through the incorporation of functional groups into the building blocks.11,12 For example, Stang et al. have successfully introduced various functional groups, such as crown ether,13 ferrocene,14 and Fréchet-type dendrons,15 at the vertex of building blocks, which enabled the construction of a series of novel functionalized metallocycles. It is worth noting that the coordination-driven self-assembly strategy allows for precise control over the shape and size of the final construction as well as the distribution and total number of incorporated functional moieties. Thus coordination-driven selfassembly has provided a highly efficient approach for preparation of functional organometallic architectures. Recently, the design and construction of fluorescent metallocycles and metallocages have attracted considerable attention because of their wide applications in fluorescence detection of metal ions, anions, or small molecules, mimicking complicated natural photo-processes, and preparing photoelectric devices, etc.16 Although it has become one of the most active and interesting areas in modern supramolecular chemistry, surprisingly, there have been few reviews in the literature on the description of the construction of fluorescent supramolecular metallocycles and metallocages via coordination-driven self-assembly.17 Considering the fact that significant progress has been made during the last few years in this field, it is time to summarize the recent development in the construction of such fluorescent metallosupramolecular architectures. In this review we will highlight the recent advances in the construction of fluorescent metallocycles and metallocages via coordination-driven self-assembly. In addition, the fluorescence properties and the applications of these architectures will also be discussed.

Fluorescent metallocycles Since the construction of metal-cornered molecular squares by Fujita and Stang in the early 1990s,18 a wide variety of metallo-

Hai-Bo Yang

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Hai-Bo Yang obtained his PhD degree at the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) in Beijing in 2004. Then he joined Professor Peter J. Stang’s group at the University of Utah as a Postdoctoral Fellow. Since the end of 2008, he has started his independent research as a PI at the East China Normal University in Shanghai. Prof. Yang’s research interests span the areas of organic, organometallic, and supramolecular chemistry.

cycles consisting of varied 2-D geometries and sizes ranging from triangles and rhomboids to hexagons have been prepared.3 According to the “directional bonding” model and the “symmetry interaction” model, the shape of an individual 2-D metallocycle is usually determined by the value of the turning angle within its angular components.19 For instance, a discrete hexagonal metallocycle could be prepared from 120° building blocks with the complementary 120° subunits or 180° linear units.20 From the topological point of view, theoretically, there are four methods for incorporating fluorophores into the discrete metallocycles. Firstly, through the introduction of a fluorophore moiety onto the exterior surface of a directional building block, an exo-functionalized fluorescent metallocycle can be prepared. Secondly, a fluorophore moiety could be attached into the “inside” of a directional building block with a turning angle less than 180°, which allows for the formation of an endo-functionalized supramolecular fluorescent metallocycle. In addition, a fluorophore moiety can be incorporated into or as the edge or corner of a building block to construct an edge or corner-functionalized fluorescent metallocycle. Anthracene is a structurally simple fluorophore that contains three benzene rings fused in a linear manner with strong rigidity. Because of their strong electron donor ability and strong fluorescence characteristics, anthracene and its derivatives have been widely used as fluorescence sensors.21 Recently, Mukherjee et al. reported the synthesis of a clip-type donor 1 (1,8-bis(4-pyridyl)ethynylanthracene) and its [2 + 2] self-assembly with 180° acceptor 2 (4,4′-bis[trans-Pt(PEt3)2(O3SCF3)(ethynyl)]biphenyl) to generate the rectangle 3 (Fig. 1).22 Due to the presence of the fluorophore anthracene, the rectangle 3 showed typical fluorescence characteristics in DMF solution. For example, the rectangle 3 featured the maximum emission peaks at 458 and 491 nm. Its fluorescence quantum yield was determined to be 0.12 in DMF solution. It was worth noting that the linking of anthracene and ethynyl moieties in 3 could extend the π-conjugation and enhance the electron donating ability of the rectangle 3. A further investigation revealed that the anthracene-containing rectangle 3 was suitable for sensing the secondary chemical explosive picric acid (PA) with high selectivity among various other electron deficient aromatic compounds. PA is a common chemical frequently used in several organic transformations and dye industries as a pigment.23 It has been proven that a long time exposure to the vapor of picric acid can cause headaches, anemia, and liver injury.24 Thus, the detection of PA is an important and attractive topic.25 As shown in Fig. 2a, with addition of picric acid into a solution of the rectangle 3, the fluorescence intensity of 3 was quenched significantly without any change in the wavelength of the emission spectra. The reason for the observed quenching of the initial fluorescence intensity of the rectangle 3 was caused by the formation of an efficient ground state charge-transfer (CT) complex between the π-electron-rich 3 and the electron-poor PA, which was further verified by the 1H NMR titration experiment. The PA-titration experiment indicated that rectangle 3 could sense the presence of picric acid even at the ppb level of

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Fig. 1 Cartoon representation of the formation of multi-anthracene rectangles 3, 5, and 8.

Fig. 2 (a) Fluorescence quenching of 3 (8.0 × 10−7 M) with picric acid (1.0 × 10−3 M) in chloroform–methanol solution; (b) relative fluorescence quenching of the rectangle 3 observed upon addition of various analytes: BA = benzoic acid, BQ = benzoquinone, 4-MeBA = 4-methoxybenzoic acid, NB = nitrobenzene, NT = nitrotoluene, NP = nitrophenol, PA = picric acid. Copyright 2011 American Chemical Society.

concentration. More importantly, as shown in Fig. 2b, rectangle 3 displayed good selectivity for PA over many other electron deficient aromatic compounds including benzoic acid, benzoquinone, 4-methoxybenzoic acid, nitrobenzene, nitrotoluene, and nitrophenol. The selectivity of 3 for PA was probably due to the stronger dipolar or electrostatic interactions of the electron-poor PA with the π-electron-rich metallocycle. Based on a similar strategy, another anthracene-containing rectangle 5 with a smaller cavity was prepared through the self-

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Perspective

assembly of 1 with a short linear acceptor 4 (1,4-bis[trans(ethynyl)Pd(PEt3)2(NO3)]benzene) by Mukherjee and coworkers (Fig. 1).26 Although anthracene-containing rectangles 3 and 5 have similar recognition units, rectangle 5 showed a poor selectivity for sensing PA compared to rectangle 3. This finding suggested that the cavity size of the anthracene-containing metallocycle played an important role in the selective sensing of PA. In recent years, the development of fluorescent sensors for trace metal ions like Mn2+, Fe3+, Ni2+, Cu2+, Zn2+, and Cd2+ is of great interest because they have great impact on human health.27 In particular, some of them are closely related to the severe damage to human health and environment. For instance, zinc is the second most abundant of the essential nutrients required for normal cell growth and development. However, zinc overload can result in a variety of diseases, such as Parkinson’s, epileptic seizures and renal and liver damage.28 Recently, Mukherjee et al. prepared a rectangle 8 by stirring the mixture of anthracene-containing clip acceptor 6 with 7 (N,N′-bis(4-pyridylidene)ethylenediamine) in a 1 : 1 molar ratio in 94% yield (Fig. 1).29 For rectangle 8, the presence of four imine nitrogen atoms made it a suitable pocket for interaction with moderately hard 3d metal ions like Cu2+, Ni2+, Mn2+, and so forth. The fluorescence studies showed that the fluorescence intensity of 8 was quenched efficiently upon titration with transition metal ions like Mn2+, Fe3+, Ni2+, and Cu2+. However, such quenching was not observed upon the addition of the soft metal ions (Zn2+ or Cd2+) containing a d10 electronic configuration. The fluorescence quenching of 8 by transition metal ions could be explained by the photo-induced electron transfer (PET) mechanism. The design and synthesis of probes for the detection of anions has attracted considerable attention because of the important roles of anions in biology, chemistry, medicine, and environmental science.30 It is well known that anions are ubiquitous in biological systems in the form of their aqueous solutions, thus it is necessary yet challenging to design and synthesize water-soluble probes for anions. Yu et al. prepared a water-soluble fluorescent metallocycle 11 through the selfassembly of anthracene-containing diimidazole ligand 9 with 10 ((TMEDA)Pd(NO3)2) (TMEDA = tetramethylethylenediamine) in a 1 : 1 molar ratio in water at room temperature (Fig. 3).31 The aqueous solution of metallocycle 11 exhibited a characteristic anthracene emission. For example, the fluorescence maxima were found to be at 401, 422, and 447 nm. Interestingly, the addition of NO3− to the solution of 11a (the PF6− salt of 11, which was obtained through an anion exchange with excess NH4PF6 in a methanol solution) in H2O–CH3CN (2 : 1, v/v; 1.0 × 10−5 M) resulted in a linear increase of the fluorescence intensity of 11a until the NO3− reached 1 equiv. concentration (1.0 × 10−5 M) (Fig. 4). This observation implied that 11a was sensitive to NO3− and could be potentially used to quantitatively detect NO3− with a concentration ranging from 2.0 × 10−6 to 1.0 × 10−5 M. Recently, the construction of carbazole-containing metallocycles was of considerable interest because of the intrinsic

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Fig. 3 Cartoon representation of the formation of multi-anthracene metallocycle 11 from donor 9 and acceptor 10.

Fig. 4 Fluorescence emission of 11a in H2O–CH3CN (2 : 1, v/v; 1.0 × 10−5 M) upon the addition of NO3− (0–2.0 equiv.). The inset shows the emission titration curve of 11a upon the addition of NO3− at 422 nm. λex = 370 nm. Copyright 2011 American Chemical Society.

photoelectric properties of carbazoles as well as the easy preparation of carbazole-based building blocks with a 90° geometry.32 For example, Mukherjee et al. constructed a square-type metallocycle 14 by stirring the mixture of carbazole-containing di-Pt(II) acceptor 12 (3,6-bis[trans-platinum(triethylphosphine)2(nitrate)(ethynyl)] carbazole) and an equimolar amount of the flexible ditopic donor 13 (1,3-bis(4-pyridyl)isophthalamide) (Fig. 5).33 It was worth noting that organometallic square 14 was a fluorophore-receptor system, which consisted of the carbazole group as the fluorophore unit and the amide functional group as the receptor site for binding anions through hydrogen-bonding interactions. The anion binding studies revealed that the addition of F−, ClO4−, and H2PO4− into the solution of 14 did not induce any obvious changes in its emission spectrum (Fig. 6). However, metallocycle 14 exhibited a dramatic fluorescence enhancement upon the addition of P2O74−. It should be noted that P2O74− plays an important role in various biological processes such as energy transduction in organisms. Moreover, P2O74− was found to control the metabolic processes by participating in various enzymatic reactions. The enhancement of

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Fig. 5 Cartoon representation of the formation of multi-carbazole square 14 from acceptor 12 and donor 13.

Fig. 6 (a) Changes in the fluorescence spectra of metallocycle 14 in DMF (red line) upon addition of increasing amounts of P2O74− in H2O; (b) change in fluorescence intensity of the metallocycle 14, in relation to the free ligand, after addition of the anions. Copyright 2010 American Chemical Society.

fluorescence emission of 14 was attributed to the inhibition of the PET process from the amide receptor to the excited state of the carbazole-based fluorophore subunit. Interestingly, although several Zn2+-based fluorescent sensors for P2O74− have been reported, metallocycle 14 represented a new class of PtII4 fluorescent sensors for P2O74− prepared through coordination-driven self-assembly. Since the discovery of buckminsterfullerene C60 by Kroto, Smalley, and Curl, the inclusion of C60 into artificial organic hosts has been well investigated with the aim to modify fullerene.34 It has been reported that a bowl-shaped host was an ideal host to embrace the curved-surface of C60 through concave–convex, “ball-and-socket” π–π interactions or C–H–π interactions. Recently, Mukherjee, Stang, and Chi et al. found that the metallocycle 17 could form a stable inclusion complex with C60 (Fig. 7).35 Metallocycle 17 was prepared through the self-assembly of the carbazole-based 90° dipyridyl donor 15 (3,6-di(4-pyridylethynyl)carbazole) and Pd(II)-based 90° ditopic acceptor 16 (cis-(dppf )Pd(OTf )2) (dppf = 1,1′-Bis(diphenylphosphino)ferrocene). An investigation of the molecular structure of 17 using a single crystal X-ray diffraction revealed a bowl-shaped confor-

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Fig. 7 Cartoon representation of the formation of multi-carbazole metallocycle 17 from donor 15 and acceptor 16; the crystal structure of 17.

Fig. 8 Reduction in the emission intensity of macrocycle 17 (a) upon the gradual addition of C60 and its corresponding Stern–Volmer plot (b). Inset: stoichiometry plot. Copyright 2012 American Chemical Society.

mation with a large π-conjugated concave aromatic surface. In addition, the internal diameter of metallocycle 17 was determined to be 14.71 Å, which was larger than the van der Waals radius (∼10 Å) of C60. Thus, the metallocycle 17 would be an ideal host to form a stable host–guest complex with C60 through π–π interactions. The fluorescence titration experiment revealed that the emission intensity of 17 depleted rapidly upon gradually adding C60 into the solution of 17 in the mixed solvents of acetonitrile–tetrachloroethane (Fig. 8a), which might be caused by the formation of the charge-transfer inclusion complex between the bowl-shaped host 17 and the convex guest C60. Moreover, the Stern–Volmer binding constant of 17 with C60 was estimated to be 1.0 × 105 M−1 (Fig. 8b). The high value of the binding constant indicated that the metallocycle 17 featured a propensity to form a stable complex with C60. During the last few years, pyrene and its derivatives have been extensively explored, and they exhibit interesting optical properties such as a relatively long excited-state lifetime, high fluorescence quantum yield, and especially a considerable distinction between the fluorescence bands for monomers and excimers.36 Therefore, the incorporation of the pyrene subunit

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Fig. 9 Cartoon representation of the formation of multi-pyrene hexagons 21, 22 and 26, 27.

into the scaffold to construct pyrene-containing architectures has evolved into one attractive subject.37 However, the introduction of multiple pyrene groups into a well-defined supramolecular structure in a controlled manner is still a challenge. Recently, as shown in Fig. 9, through the coordination-driven self-assembly strategy, Yang and Xu et al. constructed a series of tris- and hexakis( pyrene) hexagonal metallocycles 21 and 22 with well-defined shapes and sizes.38 These metallocycles were easily prepared by stirring the mixture of the pyrene-modified 120° acceptor 18 and the complementary 120° donor 19 or 180° donor 20, respectively. Furthermore, by employing a diverse library of donor and acceptor building blocks, another series of “isomeric” multipyrene hexagons 26 and 27 were

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Fig. 10 Emission spectra of 18 (10−5 M), 23 (10−5 M), 21 (0.33 × 10−5 M), 22 (0.17 × 10−5 M), 26 (0.33 × 10−5 M), and 27 (0.17 × 10−5 M) in CH2Cl2. Copyright 2013 American Chemical Society.

obtained through the self-assembly of the 120° pyrene-modified donor 23 and complementary 120° acceptor 24 or 180° acceptor 25, respectively. Although both hexagons 21, 22 and 26, 27 possessed a similar hexagonal geometry, the fluorescence studies revealed that these novel multipyrene hexagons displayed dramatically different emission behaviours (Fig. 10). For example, hexagons 26 (0.33 × 10−5 M) and 27 (0.17 × 10−5 M) in CH2Cl2 exhibited a longer fluorescence emission band at λmax 550 nm, which corresponded to the excimer emission of the pyrene chromophore. However, at the same concentration, hexagons 21 (0.33 × 10−5 M) and 22 (0.17 × 10−5 M) were not able to form excimers. The results indicated that the combination mode of building blocks could influence the excimer formation of the resulting pyrene-containing self-assemblies. Recently, the post-self-assembly modification strategy has been successfully employed to build functional architectures owing to the advantage of tailoring the properties of supramolecular assemblies after the initial self-assembly.39 The post-self-assembly modification strategy allows for the further tuning of supramolecular species to achieve new structures with high complexity and additional functionality. Generally, the post-self-assembly modification of discrete supramolecular structures can be achieved via supramolecular transformations39a or covalent modifications.39b Post-self-assembly covalent modification usually employs certain organic reactions to efficiently introduce a new moiety onto a supramolecular scaffold bearing functional sites. It has been reported that the cyclooctyne group allowed for the efficient copper-free strain-promoted azide–alkyne cycloaddition (SPAAC) reactions with other functionalized azide-containing derivatives to incorporate a new functionality under mild conditions.40 Stang and Chakrabarty prepared the rhomboid 30 decorated with cyclooctyne groups through self-assembly of 120° cyclooctyne-tethered donor 28 and 60° acceptor 29 (3,6-bis[trans-Pt-(PEt3)2(NO3)2]phenanthrene) in a 1 : 1 ratio in CD2Cl2 (Fig. 11).41 Subsequently, the covalent post-synthetic modification of rhomboid 30 yielded bispyrene-functionalized fluorescent rhomboid 32 by treating 30 with 31 (1-(azidomethyl)pyrene) in a ratio of 1 : 5 in CD2Cl2 at room temperature via the SPAAC reaction. Additionally, through the reaction of 30 with azidomethylbenzene or 2-(azidoethyl)biotinamide, two different kinds of functionalized rhomboids were obtained as well.41

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Fig. 11 Cartoon representation of the formation of rhomboid 30 from 120° donor 28 and 60° acceptor 29 and post-assembly modification of discrete rhomboid 30 with pyrene-containing azide 31 via copper-free click chemistry.

This result indicated that the post-self-assembly modification strategy was a facile way to incorporate a wide range of chemical functionalities on appropriate supramolecular assemblies with relatively synthetic ease. In addition to being a successful methodology to construct homo-functional supramolecular metallocycles, the coordination-driven self-assembly strategy has proven to be a readily and efficient approach to construct hetero-functional metallocycles through the combination of complementary precursors substituted with different functional moieties.42 The incorporation of different functional moieties into metallocycles generally endows them with versatile properties. Dendrimers are hyperbranched and well defined macromolecules composed of several dendritic wedges that extend outward from an internal core.43 The incorporation of dendritic groups as functional moieties into the fluorescent metallocycles to construct dendritic fluorescent metallocycles has been attracting much attention because of not only their aesthetically pleasing structures but also their interesting photophysical properties of fluorophores induced by the introduction of dendritic groups.44 Yang and Xu et al. prepared a variety of [G-1]–[G-3] pyrene-containing rhomboidal metallodendrimers 38–43 by

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Fig. 13 Cartoon representation of the formation of rectangle 46 from acceptor 44 and donor 45.

Fig. 12 Cartoon representation of the formation of hetero-functional rhomboids 38–40 and 41, 42 from 120° [G0]–[G2] dendritic donors 33–35 and 120° acceptors 36 or 37, respectively.

stirring the mixture of 60° dendritic dipyridyl donors 33–35 and pyrene-containing 120° di-Pt(II) acceptors 36 or 37 under mild conditions (Fig. 12).44 The fluorescence studies revealed that 38–40 exhibited moderate fluorescence quantum yields (0.14–0.17) in CH2Cl2, while 41–43 exhibited high fluorescence quantum yields (0.47–0.61). More importantly, it was worth noting that the fluorescence quantum yields of 38–40 and 41–43 were higher than those of their pyrene-containing precursors 36 (0.03) and 37 (0.09), respectively. The enhancement of quantum yields might be due to the introduction of dendritic groups which prevented the aggregation of pyrene. This work provided a highly efficient approach to construct several fluorescent dendrimers with different fluorescence quantum yields through the appropriate choice of subunits. It is known that the oxalate anion, an important and essential class of oxyanion found in nature, plays an important role in the functioning of the human body since quite a number of diseases are associated with the disruption of the oxalate exchange.45 In addition, citrate and tartrate are key metabolites in cellular intermediary metabolism and essential sources of fatty acids and cholesterol.46 Thus, the detection of multi-carb-

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oxylates has gained increasing attention over the last few years.47 Although considerable efforts have been made in this area, very few examples of effective fluorescent sensors for citrate and tartrate have been reported to date. Chi, Stang, and Lee et al. constructed a rectangle 46 through self-assembly of naphthacenedione–ruthenium acceptor 44 and dipyridyl amide ligand 45 in nitromethane–methanol (1 : 1) solution (Fig. 13).48 For rectangle 46, the amide linkage provided the propensity for binding with anions through hydrogen bonding interactions. At the same time, the naphthacenedione–Ru moiety could serve as a new fluorescent sensing motif. Interestingly, the emission studies showed that the fluorescence intensity of 46 was significantly lower than that of the acceptor 44 at the same molar concentration of the naphthacenedione–Ru moiety, which suggested the occurrence of an intramolecular photoinduced electron transfer (PET) in 46 from the naphthacenedione–Ru fluorophore to the amide donor. More importantly, as shown in Fig. 14, the anionsensing investigation revealed that the addition of 2.5 equiv. of anions like F−, Br−, Cl−, or CH3COO− to the solution of 46 did not lead to significant changes in the emission spectrum while the addition of oxalate, tartrate or citrate anion resulted in an immediate large increase in emission intensity. For instance, the gradual addition of an oxalate, tartrate, or citrate anion led to a 2.5-, 3-, or 4.5-fold fluorescence enhancement compared with the emission intensity of free 46, respectively. The enhancement in emission of 46 with the addition of multi-carboxylates might be attributed to the blocking of the PET process from the arene–Ru fluorophore to the amide receptor upon binding the carboxylate to the amide moiety through hydrogen bonding interactions. These results indicated that the rectangle 46 could be employed in the selective sensing of multi-carboxylate anions such as oxalate, tartrate, and citrate anions.

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Fig. 14 (a) Changes in the emission spectra of 46 in methanol upon addition of incremental amounts of aqueous solutions of various anions (Cl = tetrabutylammonium chloride, Br = tetrabutylammonium bromide, F = tetrabutylammonium fluoride, AcO = sodium acetate, mal = disodium malonate, suc = disodium succinate, and ox = disodium oxalate). (b) Changes in the emission spectra (the inset shows the Stern–Volmer plot) of 46 in methanol upon addition of incremental amounts of an aqueous solution of oxalate anion. Copyright 2011 Wiley-VCH.

Fig. 15 Cartoon representation of the formation of multi-naphthalimide hexagon 49 from 120° donor 47 and 120° acceptor 48.

In addition to anions, the detection of proton (H+) has also gained increasing attention since H+ plays an important role in many chemical and biological processes.49 Recently, Xu et al. prepared a naphthalimide-containing hexagonal metallocycle 49 by stirring the mixture of 120° 1,8-naphthalimide-containing donor 47 and the complementary 120° diplatinum acceptor 48 in dichloromethane at ambient temperature in excellent yield (97%) (Fig. 15).50 It should be noted that 1,8naphthalimide was selected as the fluorophore due to its high quantum yield, good photostability, and favourable compatibility.51 Moreover, the naphthalimide functional moiety was introduced into the classical 120° di-pyridine donor by the non-conjugate incorporation method, which could avoid the quenching of fluorescence, thus maintaining the fluorescence detection ability. Fluorescence detection studies revealed that hexagonal metallocycle 49 exhibited a good performance on fluorescence detection of protons. For example, when the pH values change from 7.5 to 3.5, the fluorescence intensity of the metallocycle

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Fig. 16 Fluorescence spectra of 49 (a, 20 mM) upon addition of proton in aqueous solution (acetone–water, 4/1, v/v); inset (a) and (b): curve of fluorescence intensity at 514 nm of 49 (20 mM) versus increasing concentrations of CF3COOH. Copyright 2014 Royal Society of Chemistry.

49 at 514 nm gradually increased up to 75-fold (Fig. 16a). This enhancement was attributed to the protonation of the N atom in the N-methyl piperazine moiety under acidic conditions, which blocked the PET channel synchronously. Moreover, the enhancement of fluorescence intensity of 49 at 514 nm was found to be corresponding to the concentration of H+ (0–60 mM) in a linear manner (linearly dependent coefficient: R2 = 0.9906), which indicated that the metallocycle 49 could be used to quantitatively detect H+ concentration below 60 μM (Fig. 16b). Porphyrin has been extensively explored to prepare functional materials since its rich photochemistry may impart some novel functionality to the final functionalized structures.52 For example, an investigation of porphyrin-containing artificial complexes provided insights into the mechanisms of biological processes such as photosynthesis.53 Although the synthesis of multiporphyrin systems based on covalent bonds have been realized, coordination-driven self-assembly has proven to be a more efficient method to construct such complicated multiporphyrin structures.54 Lehn et al. prepared two tetra-porphyrin squares 54 and 55 through the reaction of 5,10-bipyridyl porphyrin 50 with cis-Pt(NCPh)2Cl2 51 (90° corner) or 5,15-bipyridyl porphyrin 52 with trans-Pd(NCPh)2Cl2 53 (180° linear unit), respectively (Fig. 17).55 The corresponding tetra-Zn-porphyrin squares 58 and 59 were prepared from 5,10-bipyridyl Zn-porphyrin 56 or 5,15-bipyridyl Zn-porphyrin 57, respectively. Similarly, Stang et al. constructed two tetra-porphyrin squares 60 and 61 through the reaction of the angular porphyrin precursors 50 or 56 with 180° acceptor 25 (Fig. 17).56 Furthermore, a series of tetra-porphyrin squares 64–67 were prepared through the reaction of bistriflates of dppp-Pd(II) 62 or dppp-Pt(II) 63 (dppp = 1,3-bis(diphenylphosphino)propane) with 5,15-di(4′-pyridyl)-10,20-diphenylporphyrin (trans-DPyDPP) (52) and their zinc-containing analogues 57 (Fig. 18). More importantly, two chiral tetra-porphyrin squares 70 and 71 were formed in good yields through the reaction of trans-DPyDPP 52 with pure R (+)- or S (−)-BINAP-Pd(II) bistriflates 68 and 69 (Fig. 18). Furthermore, the reaction of transZnDPyDPP (57) with 68 yielded tetra-porphyrin square 72. The optical property investigation of the above-mentioned tetra-porphyrin squares revealed that the maxima of the Soret and Q-bands, the characteristic of porphyrin chromophores,

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Fig. 18 Cartoon representation of the formation of multi-porphyrin squares 64–67 and 70, 71.

Fig. 17 Cartoon representation of the formation of multi-porphyrin squares 54, 55 and 58–61.

were red-shifted by 8–12 nm in the squares 60, 61, 64–67, and 70–72 as compared to those of the starting porphyrin precursors. Moreover, the Soret bands of the squares 61, 62, 64–67, and 70–72 were found to be broader, which might be attributed to an intramolecular exciton coupling between the porphyrin chromophores in the squares. However, the initial fluorescence of porphyrins was quenched drastically upon formation of the squares. For instance, an overall 30–60% drop in fluorescence quantum yield compared to that of free ligands was observed. Moreover, the addition of dppp-Pt(II) rather than dppp-Pd(II) triflate induced a more pronounced decrease in fluorescence intensity, which suggested that the heavy atom effect was the major contributor to the fluorescence quenching. Interestingly, CD spectra of the chiral macrocycles dis-

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Fig. 19 Cartoon representation of the formation of squares 74 and 75 from 180° donor 73 and 90° acceptor 62 or 63, respectively.

played a strong exciton coupling between the porphyrin fluorophores in the squares. Such undesired fluorescence quenching was overcome in perylene bisimide-based squares 74 and 75, of which the fluorescence quantum yields were almost unchanged in chloroform.57 The squares 74 and 75 were prepared by Würthner et al. by stirring the mixture of ditopic perylene ligand 73 with 62 or 63 in CH2Cl2 in high yield (>90%), respectively (Fig. 19).

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Fig. 21 Cartoon representation of the formation of multi-BODIPY rhomboid 79 from donor 78 and acceptor 63.

Fig. 20 Cartoon representation of the formation of square 77 from 180° donor 76 and 90° acceptor 63.

Since the pyridyl moieties were located at the imide positions of the perylene core, which had only a small electronic coupling to the HOMO and LUMO of the perylene chromophore, the fluorescence properties of these ligands remained intact in the metal-assembled squares. The fluorescence quantum yields of 74 and 75 were calculated to be 86% and 88% in chloroform, respectively. The construction of light harvesting systems to mimic the conversion of sunlight into chemical energy in natural photosynthesis is one of the most fascinating scientific topics in supramolecular chemistry.58 In continuation of the development of perylene bisimide-based metallocycles, Würthner et al. prepared a bichromophoric square 77, which contained sixteen pyrene dye units and four perylene bisimide units through the equimolar reaction of 76 with [Pt(dppp)][(OTf )2] 63 in CH2Cl2 at room temperature (Fig. 20).59 Interestingly, an energy transfer was observed from the outer pyrenes to the inner perylene dyes upon photo excitation. For example, upon excitation of the pyrenes at 344 nm the observed emission arose mainly from the S0–S1 transition of the perylene bisimides and only a very weak emission of the pyrenes was found upon magnification. This result suggested an almost quantitative energy transfer process from the pyrene to the perylene fluorophores. BODIPY are attractive fluorophores because of their excellent photophysical properties such as bright photoluminescence with high quantum yields, strong photo- and chemostability, high extinction coefficients, sharp absorptionfluorescence spectra, extraordinary color tunability through appropriate exocyclic substitution, and/or extension of the π-conjugation.60 In addition, the sp3 hybridized fluorine atoms

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of bodipy could be synthetically replaced by appropriate functional rigid donors such as 4-ethynylpyridine, leading to highly symmetric and rigid 3D molecular platforms.61 Recently, Pistolis et al. synthesised a novel 109.5° bipyridinesubstituted BODIPY donor 78, from which the brightly fluorescent rhomboid 79 was prepared by mixing the donor 78 with 90° acceptor [Pt(dppp)][(OTf )2] 63 in a 1 : 1 stoichiometric ratio in CHCl3 or CH2Cl2 (Fig. 21).62 It was worth noting that such self-assembly was not predicted if strictly considering the angular “combinatorial” capability of the “Molecular Library” for preprogrammed assemblies. The investigation of photophysical properties revealed that the rhomboid 79 conserved the excellent photophysical properities of the parent BODIPY dye including high quantum yield (Φ = 0.86) and high fluorescence anisotropy (r ≈ 0.37). More recently, based on a similar strategy, Pistolis et al. further reported the synthesis of another brightly fluorescent rhomboid 82 composed of two different BODIPYs through the self-assembly of the BODIPY-containing donor 80 and the BODIPY-containing acceptor 81 in a 1 : 1 ratio in CHCl3 (Fig. 22).63 Similarly, no apparent quenching of the fluorescence signals was found with the formation of rhomboid 82. Furthermore, the highly efficient and unidirectional intrahost and guest-to-host energy transfer between the rhomboid 82 and 1,3,6,8-tetrasulfopyrene was demonstrated as well. Bisthienylethenes have been extensively explored in photochemistry and materials science owing to their ability to undergo a reversible transformation between ring-open and ring-closed conformations.64 Moreover, these two conformations feature markedly different optical and electronic properties. Recently, Zhu and Yang et al. prepared a family of photochromic hexagons 84 and 85 containing different numbers of bisthienylethene units through self-assembly of bisthienylethene-containing donor 83 (2,3-bis(2-methyl5-( pyridin-4-yl)thiophen-3-yl)benzo[b]thiophene-1,1-dioxide) with 120° acceptor 48 or 180° acceptor 25 at room temperature in a 1 : 1 molar ratio, respectively (Fig. 23).65

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Fig. 22 Cartoon representation of the formation of multi-BODIPY rhomboid 82 from donor 80 and acceptor 81.

Perspective

Fig. 24 (a) Absorption spectral changes of hexagon 84 (6.7 × 10−6 M in CH2Cl2) upon UV irradiation at 365 nm; (b) emission spectral changes of hexagon 84 (6.7 × 10−6 M in CH2Cl2; λex = 315 nm) upon UV irradiation at 365 nm; (c) fatigue resistance of hexagon 84 (1.7 × 10−5 M in CH2Cl2) upon alternating UV (365 nm) and visible-light (>510 nm) irradiation. The inset photographic images in (a) and (b) show the absorption and fluorescence behavior of hexagon 84 (6.7 × 10−6 M in CH2Cl2) upon alternating UV and visible-light irradiation. Copyright 2012 American Chemical Society.

multi-bisthienylethene hexagon was totally reversible (Fig. 24c). Notably, the conversions from the ring-open form to the ring-closed form were achieved almost quantitatively (yields as high as 99%), which were much better than that of free 83 (88%). Similar results for 85 were observed. This work provided the first examples of well-controlled reversible structural transformations of discrete self-assembled metallocycles triggered by light irradiation.

Fluorescent metallocages Fig. 23 Cartoon representation of the formation of multi-bisthienylethene hexagons 84 and 85 and their structural transformations.

The investigation of photophysical properties of hexagons 84 and 85 displayed that they were highly sensitive and responsive to photostimuli, especially allowing for quantitative reversible supramolecular transformations triggered by light irradiation. For instance, as shown in Fig. 24a, irradiation of hexagon 84 at 365 nm resulted in the ring-closed conformation of 84, accompanied by a color change from colorless to cyan along with the formation of two new absorption bands at ca. 420 and 622 nm. Upon irradiation at 315 nm, the strong fluorescence of hexagon 84 was completely quenched (Fig. 24b). Moreover, the photocyclization process for the

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In addition to the preparation of discrete 2-D metallocycles, coordination-driven self-assembly has proven to be an efficient strategy for the construction of 3-D metallocages with welldefined shapes and sizes.19 In general, the supramolecular metallocages feature the promising applications in the fields of selective guest encapsulation and recognition, cavity induced catalysis, and gas storage, etc.66 During the last few years, there has been a growing interest in the construction of well-defined supramolecular metallocages including fluorescent metallocages. The development of the selective detection of nitric oxide (NO) has attracted much attention of chemists and biochemists since NO has been identified as an important signaling molecule in the immune, cardiovascular, and nervous systems.67 NO plays critical roles in many diverse physiological

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Fig. 26 Families of the luminescence spectra of (a) 87 (15 μM) upon addition of the free radical PTIO up to 0.45 mM; (b) 87 (15 μM) and PTIO (0.30 mM) upon addition of NO up to 0.45 mM; (c) luminescence selectivity of 87 (15 μM) and PTIO (0.3 mM) treated with various ROS and RNS. NO: 0.5 mM; H2O2: 1.0 mM; OCl−: 1.0 mM NaOCl; 1O2: 1.0 mM H2O2 + 1.0 mM NaOCl; NO2−: 1.0 mM NaNO2; NO3−: 1.0 mM NaNO3; ONOO−: 1.0 mM NaONOO. Intensities were recorded at 470 nm with the excitation at 350 nm. Copyright 2011 American Chemical Society. Fig. 25 Cartoon representation of the formation of tetrahedron 87 and the detection of NO.

and biological processes such as vasodilation, carcinogenesis, neurodegenerative disorders, and neurotransmission.68 Recently, Duan and He et al. prepared a fluorescent tetrahedron 87 through the reaction of triphenylamine-containing tridentate ligand 86 and Ce(NO3)3·6H2O in CH3OH (Fig. 25).69 When excited at 350 nm, tetrahedron 87 (15 μM) exhibited an emission band centered at 470 nm in DMF with a quantum yield of 0.01. Interestingly, tetrahedron 87 was able to encapsulate 2-phenyl-4,4,5,5-tetra-methylimidazolineyloxyl-3-oxide (PTIO, 88), a specific spin-labeling NO trapper used for detecting NO in biological systems. As shown in Fig. 26a, the addition of PTIO into the solution of 87 induced an almost complete fluorescence emission quenching of 87. The Hill-plot of the fluorescence titration curves demonstrated a 1 : 1 stoichiometric host–guest complex between 87 and PTIO. More interestingly, introducing NO (0.45 mM) into the mixture containing 15 μM 87 and 0.3 mM PTIO in DMF–H2O at room temperature immediately resulted in the recovery of the fluorescence of 87 with a 12-fold increase of intensity (Fig. 26b). Under identical conditions, however, nearly no fluorescence intensity changes were observed in emission spectra with the addition of other reactive species including H2O2, 1O2, ClO−, NO2−, NO3−, and ONOO− (Fig. 26c). The high selectivity toward NO over other species was likely due to the special environment provided by the cavities of the polyhedron. The kinetic studies revealed that the tetrahedron 87 could function as an enzyme-like

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Fig. 27 Fluorescence imaging of compound 87 (15 μM, suspended in DMF) and PTIO (20 μM, suspended in DMF) induced by sodium nitroprusside (2.0 mM). (Bottom row) Bright-field images of MCF-7 cells shown in top panel. Fluorescence image of MCF-7 cells incubated with compound 87 (top left); incubated with PTIO in top row (middle); further incubated with sodium nitroprusside (top right). Excited at 405 nm. Copyright 2011 American Chemical Society.

pocket to encapsulate the spin-trapping agent molecule and prompt a spin-trapping reaction with NO. Furthermore, fluorescence microscopy experiments proved that tetrahedron 87 could be used for monitoring the intracellular NO (Fig. 27). It is worth noting that the organometallic tetrahedron 87 was the first example of metal–organic polyhedron that was successfully employed for biological imaging

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Fig. 28 Cartoon representation of the formation of cage 90 from 89 and Ce(NO3)3·6H2O.

Fig. 30 Cartoon representation of the formation of multi-anthracene prism 92 from donor 1 and acceptor 91.

Fig. 29 Fluorescence spectra of 90 (10 μM) in a DMF–CH3CN (1 : 9, v/v) solution upon the addition of increasing concentrations of Mg2+. The inset shows the fluorescence responses of 90 towards Mg2+ over various cations with the emission intensity at 480 nm (excitation at 365 nm). Copyright 2014 Royal Society of Chemistry.

in living cells. Based on a similar strategy, Duan and He et al. constructed a series of fluorescent tetrahedra for the selective detection of natural carbohydrates and cyclotrimethylenetrinitramine (RDX).70 Mg2+ is the most abundant divalent cation in cells, which is involved in a variety of cellular processes.71 Although Mg2+ has many important cellular functions, few Mg2+ fluorescent probes have been reported in the literature.72 Duan and He et al. successfully synthesized a triple-strand cage 90 by stirring the mixture of amide-containing ligand 89 and Ce(NO3)3·6H2O in a 1 : 1 ratio at room temperature (Fig. 28).73 It was worth noting that the cage 90 had a lantern-like cavity with six oxygen atoms of the ether bond inside, which could work as an efficient receptor for the metal ion. Due to the presence of the PET process, the solution of free 90 (10 μM) in DMF–CH3CN exhibited a weak fluorescence band at 480 nm when excited at 365 nm. Interestingly, the addition of 10 equiv. of Mg2+ caused a ca. 4.5 fold luminescence enhancement (Fig. 29). The significant emission enhancement of 90 was attributed to the coordination of Mg2+ with the oxygen atoms of the ether bond group. The coordination with Mg2+ could favor electronic delocalization of the electronic distribution, which would lead to the further blocking of the PET processes. A further investigation revealed that, under identical conditions, the addition of other different-

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Fig. 31 Quenching of fluorescence intensity of 92 on gradual addition of TNT (a) and Stern–Volmer plot (b). Copyright 2009 American Chemical Society.

sized metal ions such as Al3+, Li+, Ca2+, Na+, Ba2+ and K+ did not cause any significant emission spectra variation. Although the addition of d-block metal ions such as Cu2+, Zn2+, and Fe2+ caused fluorescence quenching of the solution of 90, the lower concentrations in living systems of these ions compared to that of the Mg2+ ion could make their influence negligible in the biochemical imaging of Mg2+. Furthermore, compared with cage 90, ligand 89 exhibited lower selectivity for Mg2+. This result suggested that the geometry constraints of the cavity and the robustness of the coordination polyhedron of the cerium ions possibly provided the additional size selectivity of the cage. Recently, polyacetylenes and other conjugated organic polymers have been proven as efficient fluorescent sensing materials for nitroaromatic explosives such as 2,4,6-trinitrotoluene (TNT).74 Mukherjee et al. prepared a [2 + 3] prism 92 through the self-assembly of the tripodal Pt3-organometallic acceptor 91 (4,4′,4″-tris[ethynyl-trans-Pt(PEt3)2(NO3)]triphenylethane) and the donor 1 (1,8-bis(4-pyridyl)ethynylanthracene) (Fig. 30).75 In the UV-vis spectrum of prism 92, two main peaks at 421 and 398 nm corresponding to the anthracene part were observed. When excited at 400 nm, prism 92 displayed a strong emission between 400 and 500 nm. As shown in Fig. 31, the addition of TNT resulted in a gradual decrease in the fluorescence intensity of prism 92 in DMF. The fluorescence

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Fig. 32 Cartoon representation of the formation of prism 95 from acceptor 93 and donor 94.

Fig. 33 Fluorescence spectra of 95 (1.0 × 10−6 M in methanol) in the presence of TNT (a, from 0 to 175 μM) and PA (b). λex = 280 nm, the fluorescence intensity was monitored at 350 nm. Copyright 2011 American Chemical Society.

quenching might be caused by the formation of a charge transfer complex between the excited state of π-electron rich prism 92 and the electron-deficient oxidizing TNT. Interestingly, no quenching of the fluorescence intensity of 91 and very weak quenching of the fluorescence intensity of ligand 1 were noticed when they were titrated with TNT solution. These results suggested that the incorporation of the electrondeficient TNT inside the cavity of electron-rich prism 92 along with the host–guest interactions between them were the possible reasons for this quenching. During the last few years, the construction of discrete metallosupramolecules from the transition-metal complexes with an octahedral geometry is a growing research field.76 Recently, Stang, Chi, and Mukherjee et al. reported the selfassembly of octahedral ruthenium(II) metal-containing bidentate acceptor 93 with tridentate pyridyl donor 94 (1,3,5-tris-(4pyridylethynyl)benzene) in a 3 : 2 ratio at room temperature to afford trigonal prism 95 (Fig. 32).77 As shown in Fig. 33, the incorporation of electron-rich ethynyl moieties endowed the cage 95 with strong fluorescence with a quantum yield of 0.12. When a variety of aromatic compounds such as benzoic acid, 4-methoxybenzoic acid, 1,4-benzoquinone, 4-nitrotoluene, nitrobenzene, 4-nitrophenol, TNT,

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and PA were added to the solution of cage 95, the nitroaromatics efficiently quenched the fluorescence emission. For instance, the addition of TNT caused 70% quenching in the fluorescence intensity and the addition of PA induced nearly complete quenching. The nitroaromatic detection of cage 95 was consistent with the expected mode of π–π interactions, in which electron-deficient nitroaromatics acted as fluorescence quenchers via either excited-state electron transfer from the electron-rich prism 95 or the charge-transfer complex formation. Due to its excellent photophysical properties and tolerable coordination with a variety of metal ions, the bis-anthracene ligand 96 has been explored in the construction of functional supramolecular architectures.78 For example, Yoshizawa et al. prepared a series of molecular capsules 97–103 through the reaction of ligand 96 with Zn2+, Cu2+, Pd2+, Pt2+, Co2+, Ni2+, and Mn2+ in a 2 : 1 ratio in quantitative yield, respectively (Fig. 34).79 Interestingly, the photophysical properties of these supramolecular capsules 97–103 revealed a striking metal-ion dependence. For example, upon irradiation at 396 nm, the solution of Zn2+-capsule 97 emitted a strong blue emission with an absolute quantum yield of Φ = 0.81, which closely matched that of the ligand 96 (Φ = 0.79). However, the Cu2+, Pd2+, Pt2+, and Co2+ capsules 98–101 were non-emissive (Φ = 0.00) and the Ni2+ and Mn2+ capsules 102 and 103 were weakly emissive (Fig. 35a). Switching from Zn2+ to Ni2+ and Mn2+ resulted in a significant quenching of the emission due to the presence of the paramagnetic metal ions. More interestingly, the Cu2+ capsule 98 displayed a striking solvatochromism and solvent-dependent emission behavior. As shown in Fig. 35c, an impressive range of colored solutions was obtained for capsule 98 by simply changing the solvent: DMF (green-yellow), DMSO (yellow), THF (green), and MeCN (brown) solutions. Furthermore, as shown in Fig. 35b, with the slow addition of DMSO into a non-emissive solution of complex 98 in MeCN, the solution became visibly emissive under irradiation with UV light. The quantum yields of 98 were measured to be 0.76, 0.33, and 0.22 in DMSO, DMF, and THF, respectively. The unusual solvatochromism of Cu2+ capsule 98 most likely arose from the coordination of the solvent at the apical position of the octahedral d 9 Cu2+ center. Additionally, it was worth noting that each of the self-assembled capsules 97–103 possessed a cavity with a diameter of ∼1 nm that could encapsulate mediumsized spherical and planar molecules as well as a very large molecule such as C60.80 It is well-known that the porphyrin containing empty H2N4 pockets have the ability to bind with Zn2+.81 In addition, the Zn2+-embedded porphyrin derivatives feature the characteristic photophysical properties. Thus, porphyrin derivatives are of great interest to chemists for the construction of fluorescent probes for Zn2+.82 Although various porphyrin-functionalized molecular architectures have been developed by using both covalent and supramolecular synthetic strategies, porphyrinfunctionalized supramolecular cages were less studied.83 Recently, Mukherjee et al. constructed a series of prisms

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Fig. 34 Cartoon representation of the formation of multi-anthracene cages 97–103 from 96 and Zn2+, Cu2+, Pd2+, Pt2+, Co2+, Ni2+, or Mn2+, respectively.

Fig. 36 Cartoon representation of the formation of tetrahedron 108–110 and their corresponding Zn2+-embedded porphyrin derivatives 111–113.

Fig. 35 (a) Fluorescence spectra (degassed MeCN, λex = 396 nm, 1.5 mm cages 97–103, RT) of cages 97–103 (M = Zn, Pd, Pt, Ni, Co, Cu, and Mn). Insets: photographs of the solutions of cages 97–103 in MeCN with UV-light irradiation (λex = 365 nm); (b) changes in the emission on the slow addition of DMSO to a non-emissive solution of cage 98 in MeCN (0.7 mm) at RT. The DMSO layers (bottom) of 98 showed strong blue emission upon UV-light irradiation (λex = 365 nm). (c) Solventdependent UV/Vis (RT) and (d) fluorescence spectra (λex = 396 nm, 1.5 mm, RT) of cage 98. Insets: photographs of the solutions of cage 98 in DMF, DMSO, THF, and MeCN with and without UV-light irradiation (λex = 365 nm). Copyright 2012 Wiley-VCH.

108–110 through the self-assembly of the tetratopic ligand 104 ((5,10,15,20-tetrakis(4-pyridyl)porphyrin)) with various 90° acceptors 105 (cis-(tmen)Pd(NO3)2), 106 (cis-(Meen)Pd(NO3)2), or 107 (cis-(2,2′-bipy)Pd(NO3)2) in MeCN in a 2 : 1 molar ratio (tmen = N,N,N′,N′-tetramethylethylene diamine, Meen = 1,2diaminopropane, and 2,2′-bipy = 2,2′-bipyridine) (Fig. 36).84 Upon treatment of complexes 108–110 with Zn(OAc)2·2H2O at room temperature, their corresponding Zn2+-embedded porphyrin derivatives 111–113 were obtained. The investigation of photophysical properties of these prisms 108–113 revealed that all of them showed two emission

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bands at 707 and 771 nm. Compared to their corresponding free analogues 108–110, in the emission spectra of the zincembedded complexes 111–113, the intensity of the higherenergy emission maxima was decreased and the intensity of the lower-energy emission maxima was enhanced. For example, the I110/I113 of the higher-energy emission maxima was 1.06 and the I113/I110 of the lower-energy emission maxima was 4.86. The results indicated that the prisms 108–110 could be used as a sensitive fluorescence probe for Zn2+. Multicomponent self-assembly is a widespread phenomenon in biological systems.85 For example, viral capsids such as tomato bushy stunt virus and rhinovirus are assembled by three and four different subunits, respectivley.86 In order to broaden the diversity of coordination-driven self-assembly and the resulting supramolecules, the self-assembly of discrete supramolecular structures from more than two distinct tectons has attracted much attention.87 Recent research revealed that three-component systems composed of a square planar Pt(II) center, pyridine, and anionic carboxylate donors could selfassemble into multicomponent supramolecular rectangles or prisms with high efficiency.88 According to this strategy, Stang and Zheng et al. prepared a multicomponent porphyrin cage 117 through the reaction of 114 (cis-Pt(PMe3)(OTf )2) with 115 (tetrakis(4-pyridyl) porphyrin) and 116 (sodium terephthalate) in a 4 : 1 : 2 ratio in an aqueous acetone solution (v/v 1 : 1)

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Fig. 37 Cartoon representation of the formation of prism 117 from 114, 115, and 116.

Fig. 39 Changes in the (a) fluorescence spectra (λex = 590 nm) of 121 (67 nM) with the addition of tetrabutylammonium acetate (0–5.77 µM) in acetonitrile. Inset shows photographs of 121 (67 nM) in acetonitrile in the absence and presence of acetate (5.77 µM); (b) relative changes in the fluorescence intensity of 121 (67 nM) at 653 nm in the presence of selected anions; (c) fluorescence spectrum with a photograph for a mixture of 122 (110 nM) and perylene (330 nM) in acetonitrile. λex = 365 nm. Copyright 2014 Royal Society of Chemistry.

Fig. 38 Cartoon representation of 121–123.

the formation

of tetrahedra

(Fig. 37).89 A further investigation revealed that the nanocavity of 117 was able to encapsulate aromatic guest like triphenylene (TP) in aqueous acetone solution. Recently, Nitschke et al. reported the other example of multicomponent self-assembly involving the simultaneous formation of both dynamic covalent (CvN) and coordinative bonds to yield fluorescent tetrahedra.90 As shown in Fig. 38, the reaction between bis-(aminophenyl)Bodipy 118 and 2-formylpyridine 119 together with Fe(OTf )2 or Zn(NTf2)2 in acetonitrile yielded the tetrahedron 121 or 122. Similarly, the reaction of 118 with pyrene-appended formylpyridine (120) and Fe(OTf )2 afforded the corresponding tetrahedron 123. Interestingly, all these tetrahedra 121–123 underwent welldefined changes upon cage formation. For instance, as compared to precursor 118, the absorption maxima of tetrahedra 121–123 were shifted into the shorter wavelength region (Δλ = 70 nm to 100 nm). Moreover, the fluorescence of the Bodipy moiety was “turned-on” during self-assembly and the quantum yields were found to be 0.012, 0.015 and 0.055 for 121–123, respectively. The changes in the optical properties of the precursors upon tetrahedron formation might be attributed to the disruption of ICT/exciplex states during imine formation. Furthermore, the investigations of the anion binding

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properties of these tetrahedra indicated that both 121 and 122 were capable of encapsulating anions (Fig. 39a). For example, the addition of acetate and azide induced significant enhancements in the fluorescence of 121, whereas moderate responses were obtained with fluoride and chloride. However, only negligible changes were observed upon the addition of bromide, iodide, nitrate, hexafluorophosphate, triflate, perchlorate and tetrafluoroborate (Fig. 39b). The fluorescence responses of pyrene-containing tetrahedron 122 towards anions were similar to those of 121. The addition of acetate to a solution of 123 in acetonitrile resulted in a significant bathochromic shift in the absorption maximum from 559 to 606 nm, which was comparable to the changes observed with 121 and 122. However, a negligibly small enhancement in the fluorescence intensity of 123 was observed upon the addition of acetate, which indicated that the fluorescence response of 123 differed significantly from those of 121 and 122. In contrast to the observations with cages 121–123, addition of acetate to 118 resulted in only minor changes in its absorbance. The results revealed that the positive charges as well as the three-dimensional cavities of tetrahedra were important for anion recognition of 121–123. Additionally, the tetrahedron 122 was found to encapsulate perylene driven by π–π stacking and thereby formed a white light emitting ensemble. As shown in Fig. 39c, the fluorescence from a mixture of 122 and perylene in a 1 : 3 ratio had equivalent intensities in the blue, green, and red regions of the electromagnetic spectrum. This emission spectrum was broad and covered the entire visible region, leading to white light emission. Moreover, the tetrahedron 122 could function

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Fig. 41 Fluorescence switching between capsule 125 and cage 126; (a) emission of capsule 125 (left) and cage 126 (right) under UV irradiation ([124] = 1 mM), (b) fluorescence spectra of capsule (red) and cage (blue) complexes ([124] = 10 μM, CH3CN, 293 K, λex = 284 nm), and (c) reversible switching cycles of fluorescence intensity (λem = 360 nm) by alternate addition of Hg2+ ions and the cryptand. Copyright 2007 American Chemical Society. Fig. 40 Cartoon representation of the formation of 125 and 126 and their structural transformations.

as a reaction-based indicator for the visual recognition of amino acids. The reversible arrangement of molecular building blocks in self-assembled architectures has been investigated in the construction of the structural and functional molecular switching systems.91 Generally, in metal-assembled systems, a reversible change in the coordination number and geometry of metal centers can be triggered by external stimuli so the dynamic structural switching of the architectures would take place in association with a notable change in their chemical and physical properties.92 Recently, Shionoya et al. prepared a capsuleshaped complex 125 and a cage-shaped complex 126 through the self-assembly of tris-monodentate ligands 124 and 0.75 equivalent Hg2+ ions or 1.5 equivalents Hg2+ ions in CD3CN, respectively (Fig. 40).93 Interestingly, the capsule 125 and the cage 126 could be quantitatively and structurally switched through the addition of Hg2+ and the Hg2+-chelating reagent [2.2.2]-cryptand. For instance, upon addition of 0.75 equiv. of Hg(OTf )2 to the solution of 125, the capsule quickly and quantitatively switched to the cage 126. In contrast, when 0.75 equiv. of [2.2.2]-cryptand was added to the solution of the cage 126, the capsule 125 was simultaneously and quantitatively regenerated. More interestingly, such a structural interconversion system was found to trigger a remarkable change in fluorescence properties between the two states. Upon excitation at 284 nm, the capsule 125 exhibited violet-light emission at 360 nm in CH3CN at 293 K. However, the cage 126 showed almost no emission. In a cycle, when 0.75 equiv. of Hg2+ ions and [2.2.2]-

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cryptand were added in this order to the initial solution of 125, the violet-fluorescence disappeared and appeared again (Fig. 41a and b). This on–off switching process could be repeated at least ten times without any loss of fluorescence efficiency (Fig. 41c). Switchable coordination cages have presented promising applications in separation and purification tasks, the capturing of hazardous chemicals, the stabilization of reactive intermediates, and the realization of sensing systems.94 As a wastefree external stimulus, light is one of the most ideal stimuli for switchable coordination cages. However, few examples of lightswitchable coordination cages have been reported.95 Recently, Clever et al. constructed two cages 129 and 130 through the reaction of the dithienylethene-based pyridyl ligands 127 and 128 with [Pd(CH3CN)4](BF4)2 in a 2 : 1 ratio in CD3CN, respectively (Fig. 42).95 Due to the presence of dithienylethene, the ring-open cage 129 and ring-closed cage 130 could be smoothly interconverted by irradiation with UV or white light. The studies of the anion encapsulation showed that two cages 129 and 130 had the ability to control the encapsulation and release of the guest [B12F12]2−. The controllable of cages 129 and 130 for encapsulation and release of anionic guests might find applications in fields such as supramolecular catalysis and drug delivery.

Conclusions Recently, the preparation of various kinds of fluorescent materials including fluorescent metallocycles and metallocages, supramolecular fluorescent polymers,96 fluorescent

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Perspective Table 1

The summary of metallocycles or metallocages for the detection of metal ions, anions, or small molecules

Recognition guest

Reference

1

Picric acid

Inorg. Chem., 2011, 50, 11736

2

NO3–

Inorg. Chem., 2011, 50, 6055

3

P2O74−

Organometallics, 2010, 29, 2971

4

C60

Inorg. Chem., 2012, 51, 4817

5

Oxalate anion

Chem. Eur. J., 2011, 17, 7837

No.

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Donor

Acceptor

Metallocycle or metallocage

6

Ce(NO3)3·6H2O

NO

J. Am. Chem. Soc., 2011, 133, 12402

7

Ce(NO3)3·6H2O

Mg2+

Dalton Trans., 2014, 43, 335

8

TNT

Organometallics, 2009, 28, 4288

9

TNT and PA

Inorg. Chem., 2011, 50, 1506

Dalton Trans.

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tectures, there have been relatively less reports on endo-functionalized fluorescent metallocycles and metallocages, which could be constructed through the self-assembly of the endofunctionalized fluorophore ligands. It is worth noting that the existence of cavities may be beneficial to improving the selectivity or sensitivity of these supramolecular architectures for sensing guests. Particularly, the self-assembly of fluorophorecored metallocycles and metallocages may generate novel photoelectric materials with special photophysical properties such as aggregation-induced emission. Secondly, with the aim to diversify biological applications, water soluble and especially near-infrared fluorescent metallocycles and metallocages should be designed and prepared. Thirdly, the selfassembly of fluorescence resonance energy transfer (FRET) metallocycles and metallocages should also be considered, which may be helpful to understand and mimic some photosynthesis and energy transfer in nature. In a word, considering the aesthetically pleasing structures and potential applications of fluorescent metallocycles and metallocages, there is no doubt that such supramolecular architectures will attract more and more attention and play an important role in supramolecular chemistry as well as in materials science in the coming decades.

Fig. 42 Cartoon representation of the formation of 129 and 130 and their structural transformations.

MOF,97 and fluorescent small-molecule probes98 have attracted intensive research interests. In this perspective, we summarized the recent advances in the construction of fluorescent metallocycles and metallocages via coordination-driven selfassembly. By employing such a strategy, a variety of fluorescent 2-D and 3-D metallosupramolecular architectures with predetermined shapes and sizes as well as the distribution and total number of fluorophores were successfully prepared. The above examples provided strong evidence that coordinationdriven self-assembly was a simple and highly efficient approach with considerable synthetic advantages, such as few steps, fast and facile construction of the final products, and to construct fluorescent metallocycles and metallocages. Moreover, the coordination-driven self-assembly strategy has proven to be an easy and efficient methodology to construct heterofunctional fluorescent metallocycles through the combination of complementary precursors substituted with different functional moieties. In addition, their photophysical properties and applications such as sensing cations, anions, and small molecules, fluorescence imaging in living cells, and the encapsulation and release of guests were also discussed in this perspective (Table 1). Although a lot of achievements have been gained in this area, in our opinion, three important aspects should be considered in the future development of supramolecular fluorescent metallocycles and metallocages. Firstly, compared to the many examples of exo-functionalized organometallic archi-

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Acknowledgements Thanks to all excellent authors whose names appear in the references. The work was financially supported by the National Natural Science Foundation of China (no. 21322206, 21302058, and 21132005), the Key Basic Research Project of Shanghai Science and Technology Commission (no. 13JC1402200), the Fok Ying Tung Education Foundation (no. 131014), the Research Fund for the Doctoral Program of Higher Education of China (no. 20130076120006), and the Opening Projects of Shanghai Key Laboratory of Chemical Biology and the Chongqing Key Laboratory of Environmental Materials and Remediation Technology (no. CEK1402).

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Recent advances in the construction of fluorescent metallocycles and metallocages via coordination-driven self-assembly.

During the last few years, the construction of fluorescent metallocycles and metallocages has attracted considerable attention because of their wide a...
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