2020, Vol. 31 
b Key Laboratory of Resource Chemistry of Ministry of Education, Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Normal University, Shanghai 200234, China;
c State Key Laboratory of New Drug and Pharmaceutical Process, Shanghai Institute of Pharmaceutical Industry, China State Institute of Pharmaceutical Industry, Shanghai 201203, China;
d State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China
Over the past few decades, the fabrication of discrete two-dimensional (2-D) metallacycles with well-defined shapes, sizes, and geometries via coordination-driven self-assembly has attracted significant attention because of their widespread applications in the fields of sensing [1-3], artificial light-harvesting [4], catalysis [5-7], photodynamic therapy [8-11], and so on [12-27]. For instance, the groups of Lehn [28, 29], Stang [30-32], Fujita [33], Mirkin [34], Mukherjee [35, 36], Chi [37, 38], and others [39-56] have successfully adopted the coordination-driven self-assembly strategy to build numerous complicated and delicate metallacycles. Recently, we have also developed a series of functional metallacycles especially fluorescent metallacycles with wide applications in the fields of sensing [57], supramolecular catalysis [58], energy transfer [59], tumor treatment [60], supramolecular polymer [61], etc. [62, 63]. Therefore, the construction of metallacycles has evolved to be one of the most prevalent areas within modern supramolecular chemistry and materials science.
Fluorescence-resonance energy transfer (FRET), which relies on the distance-dependent energy transfer from a donor molecule to an acceptor molecule, has been described as an important physical phenomenon over 70 years ago [64, 65]. Generally, there are at least three criteria that should be satisfied in order for fluorescence-resonance energy transfer to occur [66]: (i) The absorption spectrum of the acceptor must overlap the fluorescence emission spectrum of the donor; (ii) The donor and acceptor molecules must be in close proximity to each other (typically 1-10 nm); (iii) The donor and acceptor transition dipole orientations must be approximately parallel. Based on the calculation formula for FRET efficiency, the rate of FRET depends on the extent of spectral overlap between the donor and acceptor molecules, the distance separating the donor and acceptor molecules, the quantum yield of the donor, and the relative orientation of the donor-acceptor transition dipole moments [67]. As a result, the strong distancedependence of the FRET efficiency has been widely utilized in fluorescence sensing, measuring the structure and the intermolecular or intramolecular interactions [68-80]. For example, recently, Qian and co-worker developed FRET-based mito-specific fluorescent probe for ratiometric detection and imaging of endogenous peroxynitrite [81]. Since well-defined metallacycles always afford the precise positions and numbers of different building blocks such as chromophores, fluorescence-resonance energy transfer could efficiently occur and be fine-tuned within the metallacycles.
Recently, the design and construction of fluorescent metallacycles featuring the fluorescence-resonance energy transfer behavior have attracted increasing attention. However, to the best of our knowledge, no review on the development of such kind of metallacycles has been summarized. Considering that significant progress has been made in this field, it is time to summarize the recent development of such fluorescent metallacycles. In this review, we will focus on recent advances on the design and preparation of fluorescent metallacycles featuring the fluorescence-resonance energy transfer behavior. Moreover, their properties, functions, and applications will be discussed as well. For clarity, we will classify these fluorescent metallacycles based on their functions and applications.
2. Fluorescent metallacycles display the interesting FRET propertyIn 2003, Würthner and co-worker constructed a metallacycle 3 containing sixteen pyrene and four perylene chromophores through the coordination-driven self-assembly of perylene diimide-based 180° dipyridyl donor 1 with [Pt(dppp)][(OTf)2] (2) in a 1:1 molar ratio in CH2Cl2 at room temperature (Fig. 1a) [82]. Moreover, in order to properly assign the absorption and emission bands, two fluorescent model compounds 4 and 5 were synthesized. As shown in Fig. 1b, the individual bands of the pyrene and the perylene chromophores can be clearly observed, which indicated that the pyrene and perylene chromophores of ligand 1 exhibit no interaction in the ground state and could be excited with high selectivity at 344 nm for pyrene chromophore and at 400–600 nm for perylene chromophore. Notably, the absorption spectrum of metallacycle 3 is similar to that of ligand 1 (Fig. 1c), revealing that the metallacycle 3 exhibited a square tetrameric scaffold of weakly coupled perylene chromophores absorbing in the visible range which is surrounded by sixteen independent pyrene chromophores absorbing in the UV range. Upon photoexcitation of metallacycle 3 at 344 nm, an intense red fluorescence emission arising from the S0–S1 transition of the perylene bisimides as well as a very weak emission of the pyrenes was observed, which suggested an almost quantitative energy transfer process from the pyrene to the perylene chromophores (Fig. 1d).
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| Fig. 1. (a) The preparation of metallacycle 3 and the fluorescent model compounds 4 and 5; UV–vis absorption spectra in CH2Cl2 for (b) the tetrapyrene–perylene ligand 1 (black line), pyrene 4 (blue line) and perylene 5 (red line) and (c) the self-assembled metallacycle 3 (black line). In addition in (c) the calculated spectrum ε = 4 × ε(5) + 16 × ε(4) is shown (red line). (d) UV-vis absorption (red line), fluorescence excitation (blue line, λem = 630 nm) and emission (black line, λex = 344 nm) spectra for metallacycle 3 in CH2Cl2. The inset shows a magnification of the residual pyrene emission. Reproduced with permission [82]. Copyright 2003, the Royal Society of Chemistry. | |
In addition, through the similar strategy, Würthner et al. constructed a series of multichromophore supramolecular square composing of sixteen pyrene antennas such as dimethylaminonaphthalimide and pyrene moieties attached to four ditopic bayfunctionalized perylene bisimide chromophores [83, 84]. It was showed that light captured by dimethylaminonaphthalimide antennas could be efficiently transferred to the perylene bisimide core with high energy-transfer efficiency.
Recently, Pistolis et al. prepared the rhombic metallacycle 9 and hexagonal metallacycle 10 by stirring the mixtures of boron dipyrromethane (BODIPY)-containing diplatinum acceptor 6 and 180° perylene bisimide (PBI)-containing donor 7 or 120° perylene diimide-containing donor 8, respectively, in ClCH2CH2Cl in a 1:1 stoichiometric ratio at ambient temperature for 1 h (Fig. 2a) [85]. Upon excitation of dilute solutions of metallacycles 9 and 10 in ClCH2CH2Cl at 500 nm (the main absorption band of the BODIPY subunit 6), the intense fluorescence of 6 almost totally disappeared and was replaced by the fluorescence spectrum of the PBI unit 7 (Figs. 2b-e). The fluorescence emission spectra of both metallacycles 9 and 10 exhibited the typical PBI emission with the maximum at 623 nm. The results revealed the presence of efficient energy transfer from the BODIPY units to the PBI units of the metallacycles 9 and 10. Moreover, the energy transfer efficiency of these metallacycles 9 and 10 were determined to be approximately 97% as a basis of the comparison of the quantum yields of the donor moiety in the metallacycles and free form.
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| Fig. 2. (a) Schematic illustration of the coordination-driven supramolecular synthesis of the fluorescent metallacycles 9 and 10. The inset shows the crystal structure of a BODIPY based on tecton 6; Absorption (black lines), fluorescenceexcitation (open circles) and fluorescence spectra (colored lines) of the free building blocks (b) 6 (λex = 495 nm), (c) 7 (λex = 550 nm) and their assemblies (d) 9 and (e) 10 (λex = 500 nm). The extinction coefficient (ε) for the assemblies 9 and 10 refers to the constituent building blocks 6 and 7. Copied with permission [85]. Copyright 2015, the Royal Society of Chemistry. | |
Although coordination-driven self-assembly is an efficient method to construct discrete supramolecular metallacycles and metallacages, it is quite challenging to explore the dynamics of coordination-driven self-assembly due to the existence of multiple intermediates and many possible processes. As mentioned above, the rate of FRET depends on the distance between the donor and acceptor chromophores, which enables FRET to be an ideal technique to monitor the process and dynamics of coordinationdriven self-assembly in real time with high sensitivity and efficiency. Yang and Xu et al. prepared the dipyridyl ligand 11 and the diplatinum(II) unit 12 labeled with 7-(diethylamino)-coumarin and rhodamine as donor and acceptor fluorophores since the emission spectrum of coumarin and the excitation spectrum of rhodamine have substantial overlap (Fig. 3) [59]. Firstly, the selfassembly process of metallacycle driven by coordination interactions was monitored in real time by FRET. As shown in Figs. 3c and d, an obvious increase in acceptor (rhodamine) emission accompanied by a decrease in donor (coumarin) emission was observed upon mixing the donor ligand 11 and acceptor ligand 12 in acetone, which was consistent with typical FRET progress and indicated that energy transfer occurred from coumarin to rhodamine along with the formation of metallacycle 16. Further demonstration of the energy transfer from coumarin to rhodamine within the metallacycle 16 was conducted by mixing coumarinpyridine ligand 11 and rhodamin-platinum-iodine complex 13 for 22 h, which displayed almost no change in the emission spectra of ligand 11 and complex 13.
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| Fig. 3. (a) Structures of building blocks 11–15. (b) Cartoon presentation of coordination-driven self-assembly of the donor ligand and the acceptor ligand. (c) Time-dependent changes in the emission spectra of the mixture of ligand 11 (30 μmol/L) and unit 12 (30 μmol/L) in acetone. (d) Change in the ratio of the fluorescence intensity with time of the acceptor intensity maximum (602 nm) and donor emission maximum (467 nm) upon combination of ligand 11 (30 μmol/L) and unit 12 (30 μmol/L) in acetone. Reproduced with permission [59]. Copyright 2017, the American Chemical Society. | |
With the aim of investigating the structural effects, such as the relative position and distance of donor and acceptor fluorophores, on FRET, the short donor ligand 14 and coumarin endo-functionalized ligand 15 were designed and prepared. Correspondingly, metallacycles 17 and 18 were constructed through selfassembly of donor ligands 14 and 15 with acceptor ligand 12, respectively. As expected, energy transfer efficiency (ΦET) between coumarin and rhodamine in metallacycles 16, 17 and 18 was calculated to be 49.18%, 73.52% and 56.74%, respectively. Moreover, the rate constant for self-assembly of metallacycles 16, 17 and 18 was determined to be 1.09 × 10–3, 5.67 × 10–4 and 7.80 × 10–4, respectively. Furthermore, the dynamic ligand exchange between metallasupramolecular architectures, the anion-induced disassembly and reassembly of metallacycles, and the stability of metallasupramolecular structures in different solvents were comprehensively investigated by FRET. For instance, the gradual addition of 6.0 equiv. of Br- into the solution of metallacycle 16 caused a remarkable decrease in emission related to rhodamine accompanied by an increase in emission corresponding to coumarin (Fig. 4), which indicated the disassembly of metallacycle 16 with the addition of Br- due to the stronger nucleophilicity of Br- to the platinum atom than that of pyridine unit. On the contrary, the addition of 6.0 equiv. of Ag+ into the halogenated solution of metallacycle 16 induced the reassembly of 16 as evidenced by the reoccurrence of the FRET process. Moreover, realtime monitoring of the FRET signals of metallacycle 16 in different solvents such as dichloromethane, acetone, and acetonitrile showed that metallacycle 16 displayed good stability in dichloromethane and acetone while instability in acetonitrile since acetonitrile featured the stronger binding ability to platinum atoms than pyridine. Therefore, this study presented the first successful example on real-time monitoring the process and dynamics of coordination-driven self-assembly through the FRET technique, which undoubtedly deepened the understanding of the coordination-driven self-assembly process and mechanism.
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| Fig. 4. (a) Cartoon presentation of reversible disassembly and reassembly of metallacycle 16 induced by addition and removal of Br-. (b) Emission spectra of metallacycle 16 (0.5 ×10–5 mol/L) upon titration of TBAB in acetone-d6/D2O = 5:1 (v/v). (c) Fluorescence intensity changes of the fluorescence intensity of metallacycle 16 at 470 and 602 nm upon titration of TBAB in acetone-d6/ D2O = 5:1 (v/v). λex = 426 nm. Reproduced with permission [59]. Copyright 2017, the American Chemical Society. | |
Photoswitchable fluorescence-resonance energy transfer within the metallacycles has attracted considerable attention because of their potential applications in fluorescence imaging, information storage, photoswitchable molecular devices, and reversible erase/rewrite optical materials. Recently, Yang and Xu et al. prepared a rhombic metallacycle 21 by stirring a mixture of the diarylethene-containing diplatinum(II) acceptor 19 and an Ir complex-containing donor 20 in a 1:1 ratio in a solution of acetone and water (Fig. 5) [86]. Notably, the large overlap between the emission spectrum of 20 and the absorption spectrum of ringclosed form of acceptor 19 (C-19) enabled the occurrence of FRET while the lack of spectral overlap between the emission spectrum of 20 and the absorption spectrum of ring-open form of acceptor 19 prevented the energy transfer from Ir complex to diarylethene unit. As depicted in Figs. 5c-e, upon irradiation the metallacycle 21 at 365 nm in a nitrogen atmosphere, a new absorption peak at 560 nm appeared and the emission peak at 630 nm decreased rapidly, which provided the direct evidence for the existence of FRET from the Ir complex to the ring-closed form diarylethene. However, after irradiation the above solution with visible light (λ > 430 nm), the absorption spectrum and the emission spectrum of metallacycle 21 could almost be fully recovered to its original state due to the reverse photoisomerization of diarylethene moieties. These results indicated that FRET can efficiently occur when the diarylethene unit was in its ring-closed form and light-controlled FRET switching was successfully achieved.
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| Fig. 5. (a) Self-assembly of the diarylethene containing diplatinum acceptor 19 and the Ir complex containing donor 20 into discrete rhombic metallacycle 21. (b) Illustration of photoswitchable FRET within the metallacycle 21. (c) Absorption spectral changes of metallacycle 21 (1.6 × 10-5 mol/L in CH2Cl2) under irradiation at 365 nm in a N2 atmosphere. (d) Fatigue resistance of metallacycle 21 (1.6 × 10-5 mol/L in CH2Cl2) upon alternating cycles of UV (365 nm) and visible light (> 430 nm) irradiation. (e) Emission spectral changes of metallacycle 21 (1.6 × 10-5 mol/L in CH2Cl2, λex = 380 nm) upon irradiation at 365 nm. (f) Changes in the emission intensity of metallacycle 21 at 630 nm over a few cycles of photoisomerization (1.6 × 10-5 mol/L in CH2Cl2, λex = 380 nm). Copied with permission [86]. Copyright 2019, the Royal Society of Chemistry. | |
By taking advantage of the obvious color and fluorescence changes in metallacycle 21 upon irradiation, metallacycle 21 was applied to prepare versatile photoswitchable fluorescent materials including the filter paper and the PVDF film. As shown in Fig. 6, the characters on the filter paper were initially light yellow under ambient conditions and bright orange luminescence under the excitation of hand-held UV lamp (365 nm). However, upon UV irradiation of this filter paper caused it turn purple and display almost no luminescence. These results revealed that the luminescence of metallacycle 21 could also be efficiently switched in the solid state.
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| Fig. 6. (a) Color and fluorescence images of characters written using metallacycle 21 before and after irradiationwith UV or visible light. (b) Emission spectral changes of a PVDF film loaded with metallacycle 21 (2 wt%) upon irradiation at 365 nm. Copied with permission [86]. Copyright 2019, the Royal Society of Chemistry. | |
During the past few years, the fabrication of activatable photosensitizers demonstrating the ability of the controlled generation of 1O2 has been of great interest since they could reduce the side effects and unselective killing of healthy cells by minimizing nonspecific photodamage from the undesirably generated 1O2 in photodynamic therapy [87, 88]. Recently, Yang and Tian et al. prepared the metallacycle 23 containing both porphyrin photosensitizer and diarylethene photochromic-switch moieties by stirring a mixture of the diarylethene-containing diplatinum(II) acceptor 19 and porphyrin-containing donor 22 in a 1:1 ratio in the mixture solution of acetone and water (Fig. 7a) [60]. Upon irradiation of metallacycle 23 at 365 nm led to the generation of the ring-closed form metallacycle 23 (C-23), thus inducing an increase in absorption between 500–680 nm (Figs. 7b and c). Moreover, alternating irradiation with UV and visible light resulted in the reversible photoswitching of metallacycle 23 between the ring-open form and the ring-closed form several times, which indicated that the metallacycle 23 had good fatigue resistance and the presence of porphyrin units did not affect the photoswitching (Figs. 7d and e). The efficient energy transfer from porphyrin moieties to the ring-closed form diarylethene units in metallacycle 23 endowed it with the controlled emission as well as the controlled generation of 1O2. For example, upon irradiating with 365 nm light, the emission and the 1O2 generation efficiency of metallacycle 23 were gradually quenched. The 1O2 generation efficiency of metallacycle 23 was about 23 times higher than that of the ring-closed form metallacycle 23 (C-23).
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| Fig. 7. (a) Self-assembly of the diplatinum acceptor 19 and porphyrin-containing donor 22 into discrete hexagonal metallacycle 23. (b) The absorption spectra of the ligands 19, C-19, 22, and metallacycles 23, C-23; Inset: photographs of the color change of metallacycle 23 solutions. (c) Absorption spectral changes of metallacycle 23 (10 μmol/L in CH2Cl2) under irradiation at 365 nm; The insets show the amplifying absorption spectral changes from 450 nm to 700 nm and the absorbance at 560 nm. (d) Fatigue resistance of metallacycle 23 (10 μmol/L in CH2Cl2) upon alternating UV (365 nm) and visible light (> 470 nm) irradiation. (e) Changes in emission intensity of metallacycle 23 (10 μmol/L in CH2Cl2) at 652 nm over a few cycles of photoisomerization. Reproduced with permission [60]. Copyright 2019, the American Chemical Society. | |
With the aim of evaluating the applicability of metallacycle 23 in biological systems, metallacycle 23 was incorporated into the nanoparticles (NPs) by an amphiphilic polymer mPEG-DSPE since NPs have been considered as a promising platform to transport medicine into biological systems (Fig. 8a). It should be noted that the encapsulation of the metallacycles 23 into NPs had a negligible effect on their photophysical properties and the 1O2 generation efficiency of O-NPs (the nanoparticles contain the metallacycle 23) was about 155 times higher than that of C-NPs (the nanoparticles contain metallacycle C-23).
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| Fig. 8. (a) Schematic representation of the controllable generation of 1O2 in metallacycle 23 and NPs. mPEG-DSPE agents are used for nanoparticles formation. Nanoparticles in ring-open form state (O-NPs) are in the on state of photosensitization and generate 1O2. Nanoparticles in ring-closed form state (C-NPs) are in the off state of photosensitization and do not generate 1O2. (b) Tumor growth inhibition curves after PDT treatment. (c) Mice body weight curves with relevant treatments. (d) Tumor weight curves with relevant treatments. (e) Photo of tumors of each group after PDT. One tumor in the O-NPs + light group was completely eradicated at the end point. (f) Histology of tumor slices of mice in various groups after PDT treatment. Copied with permission [60]. Copyright 2019, the American Chemical Society. | |
Motivated by the above results, light-controlled generation of 1O2 for cancer therapy was investigated. As shown in Figs. 8b-f, once the mice were injected with O-NPs, tumor growth was remarkably retarded and the tumor sizes became smaller under light irradiation. However, the injection of C-NPs into the mice only inhibited tumor growth to some extent under light irradiation owing to the 1O2 quenching in the ring-closed form metallacycle 23 (C-23). Additionally, an unobvious inhibition of tumor growth was observed when the mice were injected with nanoparticles including O-NPs or C-NPs yet without light irradiation. These results indicated that the energy-transfer-based control of 1O2 generation for cancer therapy could be realized in the tumor model. This study provides the first successful example of light-controlled generation of 1O2 in the discrete metallacycle.
6. ConclusionIn this review, we comprehensively summarized the development of the fluorescent metallacycles featuring the fluorescence-resonance energy transfer behavior. This review focuses on the design principles, preparation methods, optical properties, and the wide applications of these fluorescent metallacycles in the realtime monitoring the dynamics of coordination-driven selfassembly, photoswitching fluorescence-resonance energy transfer, and light-controlled generation of singlet oxygen for cancer therapy. To the best of our knowledge, this is the first review that systematically summarized the development of the fluorescence-resonance energy transfer within the fluorescent metallacycles. Since their wide applications in materials science, supramolecular chemistry, and biological science, there is no doubt that such kind of fluorescent metallacycles will gain more and more attention in the next decades.
Although there have been achievements in this area, in our opinion, at least two important aspects need to be considered for further developing the fluorescent supramolecular coordination complexes (SCCs) featuring the fluorescence-resonance energy transfer behavior. First, the fabrication of new fluorescent metallacycles with high energy transfer efficiency is particularly necessary. Second, although fluorescent metallacycles with the fluorescence-resonance energy transfer behavior have been widely explored, three-dimensional (3-D) fluorescent metallacages featuring the fluorescence-resonance energy transfer behavior have rarely been reported. It is exciting that Zhang et al. prepared two tetragonal prismatic platinum(II) cages with a reverse FRET process very recently via coordination-driven self-assembly [89]. Considering that 3-D metallacages exhibit well-defined shapes, sizes, and cavities, more attention will be paid to investigate the fluorescence-resonance energy transfer within the fluorescent metallacages in future.
Declaration of competing interestThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
AcknowledgmentsThis work was supported by the National Nature Science Foundation of China (Nos. 21922506, 21871092, 21672070 and 31702070), Shanghai Pujiang Program (No. 18PJD015), the Fundamental Research Funds for the Central Universities, Shanghai Municipal Natural Science Foundation (No. 19ZR1437900), the Opening Projects of Shanghai Key Laboratory of Green Chemistry and Chemical Processes, and State Key Laboratory of Fine Chemicals (No. KF 1801).
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