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Real-Time Monitoring the Dynamics of Coordination-Driven Self-Assembly by ... and kinetic aspects of a series of discrete, well-defined metallacycles ...
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Real-Time Monitoring the Dynamics of Coordination-Driven SelfAssembly by Fluorescence-Resonance Energy Transfer Chang-Bo Huang,† Lin Xu,*,† Jun-Long Zhu,† Yu-Xuan Wang,† Bin Sun,† Xiaopeng Li,‡ and Hai-Bo Yang*,† †

Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, 3663 North Zhongshan Road, Shanghai 200062, P. R. China ‡ Department of Chemistry, University of South Florida, Tampa 33620, United States S Supporting Information *

high stability of metallosupramolecular architectures to avoid any structural disruption in gas phase. Considering the fact that almost all coordination-driven self-assemblies occur in solution, it is very difficult to employ a mass spectrometry technique to real-time monitor the self-assembly process and dynamics of metallosupramolecular structures. In addition, although the NMR technique has been the primary tool for the solution-state study of assemblies at millimolar concentration, the self-assembly process and dynamics of supramolecular architectures are best followed at micro- to nanomolar concentrations. Therefore, the development of a new approach to real-time monitor the process and dynamics of coordination-driven self-assembly with high sensitivity and efficiency is highly desirable. Fluorescence-resonance energy transfer (FRET) has been widely used to explore the dynamic process in biological systems, including self-assembly and conformational changes.7 The high sensitivity of the FRET technique allows for the detection of interacting species at nanomolar concentrations. More importantly, the change of the individual components as well as the assemblies can be observed directly through FRET due to its simplicity and easy detection of sample in solution. Therefore, FRET is an ideal technique to monitor the process and dynamics of supramolecular self-assembly in real time with high sensitivity and efficiency. For instance, Rebek and co-workers successfully developed an innovative method to characterize the dynamic features of supramolecular capsules based on hydrogen bonds by using FRET.8 By taking advantage of the high sensitivity and efficiency of FRET, herein, we present the first successful example on real-time monitoring the dynamics of coordinationdriven self-assembly through the FRET technique. In this study, coumarin and rhodamine moieties were selected as the FRET donor and acceptor fluorophores, respectively, based on the following three factors. First, the emission spectrum of coumarin and the excitation spectrum of rhodamine have substantial overlap, which is favorable for the FRET process. Second, both coumarin and rhodamine display two wellseparated emission bands with comparable intensities, thus ensuring accuracy when determining their intensities and ratios. Finally, both coumarin and rhodamine exhibit good photostability. Therefore, the dipyridyl ligand 1 and the diplatinum(II) unit 2 labeled with 7-(diethylamino)-coumarin and rhodamine as

ABSTRACT: It is quite challenging to investigate the dynamics of coordination-driven self-assembly due to the existence of multiple intermediates and many possible processes. By taking advantage of the high sensitivity and efficiency of fluorescence-resonance energy transfer (FRET), FRET was successfully employed to real-time monitor the dynamic behavior of coordination-driven selfassembly. The Förster energy transfer efficiencies and kinetic aspects of a series of discrete, well-defined metallacycles have been determined. Moreover, the dynamic characteristics of these supramolecular assemblies, such as the dynamic ligand exchange, anion-induced disassembly and reassembly, and stability in different solvents, have been investigated by using FRET.

C

oordination-driven self-assembly, which is based on the formation of metal−ligand bonds, has evolved to be one of the most attractive topics within supramolecular chemistry and materials science during the past few decades.1,2 Numerous complicated and delicate supramolecular metallacycles and metallacages with well-defined shapes, sizes, and geometries have been successfully constructed by employing such a strategy, some of which have presented wide applications in the fields of molecular recognition, supramolecular catalysis, drug delivery, etc.3−5 It is noteworthy that the detailed exploration of such a self-assembly process and dynamics is not only fundamentally important for understanding the nature of self-assembly but also crucial for the design and preparation of new metallosupramolecular materials with desirable functionality. However, because of the existence of multiple intermediates and many possible processes within self-assembly, it is extremely challenging to study the dynamics of such self-assembly as witnessed by very few reports in literature to date. Great efforts have been devoted to the investigation into the dynamics of coordination-driven self-assembly.6 For example, Stang and co-workers applied an electrospray ionization mass spectrometry (ESI-MS) technique to investigate the constitutional exchange of Pt−N bonds of supramolecular triangles and rectangles.6a Moreover, Hiraoka et al. studied the self-assembly process of a Pt(II)-linked hexagonal metallacycle through combination of 1H NMR and ESI-TOF-MS techniques.6b However, the analysis of such dynamic exchange by mass spectrometry was carried out in gas phase, which requires the © 2017 American Chemical Society

Received: May 6, 2017 Published: June 29, 2017 9459

DOI: 10.1021/jacs.7b04659 J. Am. Chem. Soc. 2017, 139, 9459−9462

Communication

Journal of the American Chemical Society

using FRET. The dilute solutions of ligands 1 (30 μM) and 2 (30 μM) in acetone were mixed at rt, and the changes in the fluorescence emission spectra were recorded with time. Initially, the strong emission from donor ligand 1 was found under the excitation wavelength of coumarin (λex = 426 nm) since the donor ligand 1 and acceptor unit 2 were isolated from each other in solution and no FRET behavior occurred. Over time an obvious increase in acceptor (rhodamine) emission accompanied by a decrease in donor (coumarin) emission was observed until equilibrium was reached, which was consistent with typical FRET progress (Figures 2a and S28). The increase in the ratio of

Figure 1. Structure of building blocks 1−7.

donor and acceptor fluorophores were designed and synthesized (Figure 1). Moreover, in order to investigate the structural effects on FRET, e.g., the relative position and distance of donor and acceptor fluorophores, two different ligands, short ligand 3, and coumarin endo-functionalized ligand 4 were designed and prepared. The normalized emission and absorption spectra of ligands 1−4 showed that there was obvious overlap between the emission spectrum of coumarin-modified ligands 1, 3, or 4 and the excitation spectrum of rhodamine-modified unit 2 (Figure S26). This finding indicated that the introduction of coumarin and rhodamine into the dipyridyl or diplatinum(II) building blocks did not obviously change the optical properties of the fluorophores (Tables S1−S2). From the ligands 1−4, three different metallacycles M1−M3 modified with both coumarin and rhodamine at alternative vertices were prepared through coordination-driven self-assembly (Schemes 1, S11−S13, and

Figure 2. (a) Time-dependent changes in the emission spectra of the mixture of ligand 1 (30 μM) and unit 2 (30 μM) in acetone; (b) 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 1 (30 μM) and unit 2 (30 μM) in acetone.

the fluorescence intensities of rhodamine at 602 nm to the fluorescence intensities of coumarin at 467 nm was observed until the equilibrium was reached (Figure 2b). More importantly, the enhancement of the ratio of fluorescence intensities at 602 and 467 nm was nearly linear with regard to the self-assembly time in the range of 0−50 min (Figure S28b). The results indicated that the self-assembly of metallacycles could be monitored in real time. Moreover, the diagram produced by the Commission Internationale de L’Eclairage (CIE) software directly showed that the fluorescent color of the mixture solution of 1 and 2 changed from blue to red as the assembly proceeded (Figure S28c). These preliminary results indicated that energy transfer might occur from coumarin to rhodamine along with the formation of metallosupramolecular architectures. In order to ensure that the observed FRET signal was from the self-assembly, the control experiment was conducted by mixing coumarin-pyridine ligand 1 and rhodamin−platinum−iodine complex 7 for 22 h (Scheme S22 and Figure S30). The platinum atoms in complex 7 were protected by iodine moieties, which cannot coordinate with ligand 1 to form a metallacycle. As shown in Figure S30, almost no change in the emission spectra of ligand 1 and complex 7 was observed, thus ruling out the possibility of intermolecular FRET between coumarin-modified ligand 1 and rhodamine-modified unit 2. Moreover, under the excitation wavelength of coumarin (λex = 426 nm), the metallacycle M1 (10 μM in acetone) exhibited the stronger emission at 602 nm than that of the model metallacycle M5 (10 μM) while exhibiting a weaker emission at 470 nm than that of the model metallacycle M4 (10 μM) (Figure S31). This observation further demonstrated that energy transfer occurred from coumarin to rhodamine within the metallacycle M1. Similar FRET processes were also observed upon combination of dipyridyl ligand 3 or 4 with diplatinum(II) unit 2, respectively (Figures S32−S33), again demonstrating that the self-assembly process of metallosupramolecular architectures driven by coordination interactions can be monitored by FRET.

Scheme 1. (a) Cartoon Presentation of Coordination-Driven Self-Assembly of the Donor Ligand and the Acceptor Ligand; (b) Formation of Metallacycles M1−M8 through SelfAssembly of Different Donor Ligands and Acceptor Ligands

S19). Additionally, the model metallacycles M4, M5, M7, and M8 functionalized with either coumarin or rhodamine were prepared as well (Schemes S14−S18). The model metallacycle M6 without any fluorophore was also constructed. All metallacycles M1−M8 were well characterized by using multinuclear NMR {1H, 13C, and 31P} spectroscopy and ESI-MS spectrometry (Figures S10−S25). The self-assembly process of metallacycles driven by coordination interactions was first monitored in real time by 9460

DOI: 10.1021/jacs.7b04659 J. Am. Chem. Soc. 2017, 139, 9459−9462

Communication

Journal of the American Chemical Society

emission corresponding to rhodamine was observed, which was consistent with the occurrence of FRET caused by the formation of new metallacycles containing both coumarin and rhodamine, thus demonstrating the dynamic ligand exchange between the metallacycles M4 and M5. After 5 h, the dynamic ligand exchange process reached equilibrium as indicated by the unchanged emission signals corresponding to both coumarin and rhodamine (Figure S38). The rate constant k for the ligand exchange between metallacycles M4 and M5 in acetone/water (v/v, 5:1) was calculated to be 1.58 × 10−4 S−1 (Figure S39). This dynamic ligand exchange process was also confirmed by ESI-MS (Table S3 and Figure S40) and multiple nuclear NMR {1H and 31P} spectroscopy (Figures S41−S44). Unexpectedly, both the 1H and 31P NMR spectra for the final equilibrium system did not show obvious distinction from the initial ones for the mixture of metallacycles M4 and M5, which might be caused by the fact that the ligand exchange had little effect on the inner framework of the metallacycle due to a relatively large distance between the outer functional groups and the inner framework (Figure S45). Moreover, ligand exchange between metallacycle M4 and acceptor unit 2 and ligand exchange between metallacycle M5 and donor ligand 1 were further explored by employing FRET, ESI-MS, and NMR techniques (Figures S46−S53), which again demonstrated that the FRET technique is powerful to explore dynamic ligand exchange with higher sensitivity than 1H NMR spectroscopy in this study. In addition, the dynamic nature of the resultant metallacycles was further explored by adding and removing competitive ligands such as halide ions (F−, Cl−, Br−, and I−) to induce reversible disassembly and reassembly through FRET approach (Scheme 2). As shown in Figure 3, the gradual addition of 6.0 equiv of Br−

The Fö rster energy transfer efficiency (ΦET) between coumarin and rhodamine in metallacycles M1−M3 was calculated based on fluorescence spectroscopy to learn the structural effect on FRET (Table 1 and section 6 in SI). ΦET was Table 1. Calculation Data of Energy Transfer and Dynamics of Metallacycles Metallacycles Parameter

M1

M2

M3

Ra/Å ΦETb kc/s−1

18.5 49.18% 1.09 × 10−3d 1.60 × 10−3e

15.9 73.52% 5.67 × 10−4

15.7 56.74% 7.80 × 10−4

a

R is the distance between donor and acceptor (estimated using standard bond lengths). bΦET is the Förster energy transfer efficiency. c k is rate constant for the kinetic of coordination-driven self-assembly process. dIn acetone. eIn acetone/water (v/v, 5:1). Detailed calculation is shown in SI.

measured as the ratio of fluorescence intensities of the donor (coumarin) in the absence (for the model metallacycles M4, M7, and M8) and presence (for metallacycles M1, M2, and M3) of the acceptor (rhodamine). ΦET between coumarin and rhodamine in metallacycles M1−M3 was calculated to be 49.18%, 73.52%, and 56.74%, respectively, which indicated the occurrence of an efficient FRET process in every system. The metallacycle M2 displayed a higher ΦET than metallacycle M1, which might be attributed to the shorter distance between the coumarin and rhodamine moieties in M2 (Figure S35). Furthermore, the metallacycle M3 exhibited a lower ΦET than M2 although the distance between coumarin and rhodamine moieties in M2 and M3 were similar, which indicated that the donor (coumarin) being positioned inside the metallacycle was not conducive to the energy transfer. By employing FRET, the solvent effect on coordination-driven self-assembly was revealed. The real-time monitoring of the selfassembly process of ligands 1 and 2 in either acetone or the mixed solvents of acetone/water (v/v, 5:1) by FRET was carried out. As shown in Figure S36, the self-assembly of ligands 1 and 2 in acetone/water (v/v, 5:1) reached equilibrium faster than that in acetone, which suggested that the presence of water facilitated the assembly of the metallacycle in this case. Moreover, the quantitative emission spectra acquired over time allowed the kinetics of coordination-driven self-assembly processes to be determined. The rate constants for self-assembly of M1−M3 were obtained by monitoring the change in emission intensities of the acceptors. Analysis of the data indicated that self-assembly of M1−M3 in either acetone or acetone/water (v/v, 5:1) followed the first-order kinetics with different rate constants (Table 1; Figure S37).9 For example, the rate constant for the self-assembly of M1 in acetone/water (1.60 × 10−3 s−1) was higher than that in acetone (1.09 × 10−3 s−1), which was consistent with the previous result that the presence of water was able to facilitate the assembly of metallacycles in this study. With the aim to explore the dynamic ligand exchange between metallosupramolecular architectures by using FRET, coumarinfunctionalized metallacycle M4 and rhodamine-functionalized metallacycle M5 were mixed in a 1:1 ratio in acetone/water (v/v, 5:1) (Scheme S26). Initially, only those emission signals corresponding to coumarin and rhodamine in each metallacycle were observed. As time went on, a decrease in emission corresponding to coumarin accompanied by an increase in

Scheme 2. Cartoon Presentation of Reversible Disassembly and Reassembly of M1 Induced by Addition and Removal of Br−

into the solution of M1 induced a significant decrease in emission corresponding to rhodamine accompanied by an increase in

Figure 3. (a) Emission spectra of metallacycle M1 (0.5 × 10−5 M) upon titration of TBAB in acetone-d6/D2O = 5:1 (v/v); (b) fluorescence intensity changes of the fluorescence intensity of metallacycle M1 at 470 and 602 nm upon titration of TBAB in acetone-d6/D2O = 5:1 (v/v); excitation at 426 nm. 9461

DOI: 10.1021/jacs.7b04659 J. Am. Chem. Soc. 2017, 139, 9459−9462

Communication

Journal of the American Chemical Society ORCID

emission related to coumarin, indicating disassembly of metallacycle M1 with the addition of Br−. Reassembly of M1 was achieved by adding 6.0 equiv of Ag+ into the halogenated solution of M1 as evidenced by the reoccurrence of the FRET process. The reversible assembly process of M1 was further proved by NMR {1H and 31P} spectroscopy (Figures S58−S59). However, under the same conditions, the addition of 6.0 equiv of F− produced a negligible change in the fluorescence spectra of metallacycle M1 (Figure S55), which might be caused by the weaker nucleophilicity of F− to a platinum atom compared to Cl−, Br−, or I−. All the above results indicated that the selective reversible assembly of metallacycle could be achieved by adding or removing halide ions and monitored by FRET in this study. Moreover, real-time monitoring of the FRET signals was further employed to probe the stability of metallacycles in different solvents such as dichloromethane, acetone, and acetonitrile. M1 was dissolved in dichloromethane or acetone (10−5 M) and allowed to stand for several hours. A very slight change in the emission spectra was observed, suggesting good stability of M1 in dichloromethane and acetone (Figure S60). However, the dissolution of M1 in acetonitrile caused the destruction of the FRET process, i.e. a significant decrease in emission corresponding to rhodamine accompanied by a significant increase in emission of coumarin (Figure S61), indicating disassembly of M1 in acetonitrile. We rationalized that the presence of acetonitrile disrupted the Pt−N coordination bond since acetonitrile featured the stronger binding ability to platinum atoms than pyridine, which was further confirmed by NMR titration experiments as well as the variable-concentration NMR {1H and 31P} studies (Figures S62−S64). In conclusion, this study presents the first successful example on real-time monitoring the process and dynamics of coordination-driven self-assembly by employing FRET. The high sensitivity and efficiency of FRET allowed for monitoring the self-assembly process and dynamics of the metallosupramolecular architectures in real time. Both the Förster energy transfer efficiency and the kinetic aspects of these metallacycles were determined on the basis of quantitative emission spectra results. Moreover, application of FRET allowed for further investigation of solvent effects in coordination-driven self-assembly, the dynamic ligand exchange between metallosupramolecular architectures, the anion-induced disassembly and reassembly of metallacycles, and the stability of metallosupramolecular structures in different solvents. Compared to the NMR or ESIMS technique, FRET is capable of providing much more information about the process and dynamics of the supramolecular metallacycles, which undoubtedly deepens the understanding of the coordination-driven self-assembly process, thus allowing for the design of new functional metallosupramolecular materials in the future.

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Xiaopeng Li: 0000-0001-9655-9551 Hai-Bo Yang: 0000-0003-4926-1618 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NSFC/China (Nos. 21625202, 21672070, and 91427304).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b04659. Synthesis, characterization, and other experimental details (PDF)

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DOI: 10.1021/jacs.7b04659 J. Am. Chem. Soc. 2017, 139, 9459−9462