Real-Time Monitoring the Dynamics of Coordination-Driven Self-Assembly by Fluorescence-Resonance Energy Transfer

Article abstract of DOI:10.1021/jacs.7b04659

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 energ

Full text of DOI:10.1021/jacs.7b04659

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Communication
Real-Time Monitoring the Dynamics of Coordination-Driven
Self-Assembly by Fluorescence-Resonance Energy Transfer
Chang-Bo Huang, Lin Xu, Jun-Long Zhu, Yu-Xuan Wang, Bin Sun, Xiaopeng Li, and Hai-Bo Yang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b04659 • Publication Date (Web): 29 Jun 2017
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Real-Time Monitoring the Dynamics of Coordination-Driven Self-
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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 Engi-
neering, East China Normal University, 3663 N. Zhongshan Road, Shanghai 200062, P. R. China
‡Department of Chemistry, University of South Florida, Tampa 33620, United States
Supporting Information Placeholder
triangles and rectangles.6a Moreover, Hiraoka et al. studied
ABSTRACT: It is quite challenging to investigate the dy-
the self-assembly process of a Pt(II)-linked hexagonal metal-
lacycle through combination of H NMR and ESI-TOF-MS
namics of coordination-driven self-assembly due to the exist-
ence of multiple intermediates and many possible processes.
By taking advantages of high sensitivity and efficiency of
fluorescence-resonance energy transfer (FRET), for the first
time, FRET was successfully employed to real-time monitor
the dynamic behaviour of coordination-driven self-assembly.
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 re-assembly, and
stability in different solvents, have been investigated by
using FRET.
1
techniques.6b However, the analysis of such dynamic ex-
change by mass spectrometry was carried out in gas phase,
which requires high stability of metallosupramolecular archi-
tectures to avoid any structural disruption in gas phase. Con-
sidering the fact that almost all coordination-driven self-
assemblies occur in solution, it is very difficult to employ
mass spectrometry technique to real-time monitor the self-
assembly process and dynamics of metallosupramolecular
structures. In addition, although NMR technique has been
the primary tool for the solution-state study of assemblies at
millimolar concentration, the self-assembly process and dy-
namics of supramolecular architectures are best followed at
micro- to nanomolar concentrations. Therefore, the devel-
opment of new approach to real-time monitor the process
and dynamics of coordination-driven self-assembly with high
sensitivity and efficiency is highly desirable.
Coordination-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 Numer-
ous complicated and delicate supramolecular metallacycles
and metallacages with well-defined shapes, sizes, and geome-
tries have been successfully constructed by employing such
strategy, some of which have presented wide applications in
the fields of molecular recognition, supramolecular catalysis,
and drug delivery, etc.3-5 It is noteworthy that the detailed
exploration of such self-assembly process and dynamics are
not only of fundamental importance 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 dy-
namics of such self-assembly as witnessed by very few re-
ports in literatures up to date.
Fluorescence-resonance energy transfer (FRET) has been
widely used to explore dynamic process in biological systems,
including self-assembly and conformational changes.7 The
high sensitivity of 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
advantages of high sensitivity and efficiency of FRET, herein,
we present the first successful example on real-
Great efforts have been devoted to the investigation on the
dynamics of coordination-driven self-assembly.6 For exam-
ple, Stang and co-workers applied electrospray ionization
mass spectrometry (ESI-MS) technique to investigate the
constitutional exchange of Pt-N bonds of supramolecular
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Figure 1. The structure of building blocks 1-7.
time monitoring the dynamics of coordination-driven self-
assembly through FRET technique.
Self-assembly process of metallacycles driven by
coordination interactions was firstly monitored in real time
by using FRET. The dilute solutions of ligands 1 (30 μM) and
2 (30 μM) in acetone were mixed at room temperature, 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 with a decrease in donor
(coumarin) emission was observed until the equilibrium was
reached, which was consistent with a typical FRET progress
(Figure 2a and Fig. S28). The increase in the ratio of 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 nm and 467 nm was nearly linear with
regard to the self-assembly time in the range of 0–50 minutes
(Fig. S28b). The results indicated that self-assembly process
of metallacycle 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 (Fig. S28c).
These preliminary results indicated that energy transfer
might occur from coumarin to rhodamine along with the
formation of metallosupramolecular architectures.
In this study, coumarin and rhodamine moieties were
selected as the FRET donor and acceptor fluorophores,
respectively, based on the following three factors. Firstly, the
emission spectrum of coumarin and the excitation spectrum
of rhodamine have substantial overlap, which is favorable for
the FRET process. Secondly, both coumarin and rhodamine
display two well-separated emission bands with the
comparable intensities, thus ensuring the 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
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 (Fig. 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 (Table 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 (Scheme 1, Scheme S11-S13,
and Scheme 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 (Fig. S10-S25).
Scheme 1. (a) Cartoon presentation of coordination-driven
self-assembly of the donor ligand and the acceptor ligand. (b)
The formation of metallacycles M1-M8 through self-assembly
of different donor ligands and acceptor ligands.
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.
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In order to ensure that the observed FRET signal was from
By employing FRET, the solvent effect on coordination-
driven self-assembly was revealed. The real-time monitoring
of the self-assembly 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 Fig. S36, the self-assembly
of ligands 1 and 2 in acetone/water (v/v, 5:1) reached the
equilibrium faster than that in acetone, which suggested that
the presence of water facilitated the assembly of 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, M2, and 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
and Fig. 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.
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 Fig. 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 Fig. S30, almost no change in the
emission spectra of ligand 1 and complex 7 was observed,
thus ruling out the possibility of intermolecular FRET
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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 the weaker
emission at 470 nm than that of the model metallacycle M4
(10 μM) (Fig. 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 ligands 3 or 4 with
diplatinum(II) unit 2, respectively (Fig. S32-S33), again
demonstrating that the self-assembly process of
metallosupramolecular architectures driven by coordination
interactions can be monitored by FRET.
With the aim to explore the dynamic ligand exchange
between metallosupramolecular architectures by using FRET,
coumarin-functionalized 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 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 (Fig. 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 (Fig.
S39). This dynamic ligand exchange process was also
confirmed by ESI-MS (Table S3 and Fig. S40) and multiple
nuclear NMR {1H and 31P} spectroscopy (Fig. S41-S44).
Unexpectedly, both the 1H NMR and the 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 metallacycle due to a relatively large distance
between the outer functional groups and the inner
framework (Fig. S45). Moreover, ligand exchange between
metallacycle M4 and acceptor unit 2 as well as ligand
exchange between metallacycle M5 and donor ligand 1 were
further explored by employing of FRET, ESI-MS, and NMR
techniques (Fig. S46-S53), which again demonstrated that
FRET technique is a powerful technique to explore dynamic
The Förster 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 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 (Fig. 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.
Table 1. Calculation Data of Energy Transfer and
Dynamics of Metallacycles
Metallacycles
Parameter
M1
M2
M3
15.7
R[a]/Å
18.5
15.9
[b]
ΦET
49.18%
73.52%
56.74%
1.09×10-3[d]
1.60×10-3[e]
k[c]/s-1
5.67×10-4
7.80×10-4
1
[a] R is the distance between donor and acceptor (estimated using
standard bond lengths). [b] ΦET is the Förster energy transfer efficien-
cy. [c] k is rate constant for the kinetic of coordination-driven self-
assembly process. [d] in acetone; [e] in acetone/water (v/v, 5:1). De-
tailed calculation is shown in SI.
ligand exchange with the higher sensitivity than H NMR
spectroscopy in this study.
In addition, the dynamic nature of the resultant
metallacycles was further explored by adding and removing
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competitive ligands such as halide ions (F-, Cl-, Br-, and I-) to
ability to platinum atoms than pyridine, which was further
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induce reversible disassembly and re-assembly through FRET
approach (Scheme 2). As shown in Figure 3, the gradual
addition of 6.0 eq. of Br- into the solution of M1 induced a
significant decrease in emission corresponding to rhodamine
accompanied by an increase in emission related to coumarin,
indicating disassembly of metallacycle M1 with the addition
of Br-. Re-assembly of M1 was achieved by adding 6.0 eq. Ag+
into the halogenated solution of M1 as evidenced by the
reoccurrence of FRET process. The reversible assembly
process of M1 was further proved by NMR {1H and 31P}
spectroscopy (Fig. S58-S59). However, under the same
condition, the additionof 6.0 eq. F- produced a negligible
change in the fluorescence spectra of metallacycle M1 (Fig.
S55), which might be caused by the weaker nucleophilicity of
F- to platinum atom compared to Cl-, Br-, or I-. All above
results
confirmed by NMR titration experiments as well as the varia-
ble-concentration NMR {1H and 31P} studies (Fig. 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 the further investigation on solvent
effects in coordination-driven self-assembly, the dynamic
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ligand
exchange
between
metallosupramolecular
architectures, the anion-induced disassembly and re-
assembly of metallacycles, and the stability of
metallosupramolecular structures in different solvents.
Compared to NMR or ESI-MS 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.
Scheme 2. Cartoon presentation of reversible disassembly
and re-assembly of M1 induced by addition and removal of
Br-.
ASSOCIATED CONTENT
Supporting Information
Synthesis, characterization, and other experimental details
are available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*hbyang@chem.ecnu.edu.cn, lxu@chem.ecnu.edu
Notes
Figure 3. (a) Emission spectra of metallacycle M1 (0.5╳
10-5
The authors declare no competing financial interest.
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 nm and 602 nm upon titration of
TBAB in acetone-d6:D2O =5:1 (v/v); excitation at 426 nm.
ACKNOWLEDGMENT
This work was financially supported by the NSFC/China
(Nos. 21625202, 21672070, and 91427304), 973 Program (No.
2015CB856600), and STCSM (No. 16XD1401000).
indicated that the selective reversible assembly of
metallacycle could be achieved by adding or removing halide
ions and monitored by FRET in this study.
REFERENCES
Moreover, real-time monitoring of the FRET signals was
further employed to probe the stability of metallacycle in
different solvents such as dichloromethane, acetone, and
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change in the emission spectra was observed, suggesting
good stability of M1 in dichloromethane and acetone (Fig.
S60). However, the dissolution of M1 in acetonitrile caused
the destruction of the FRET process, i.e. a significant de-
crease in emission corresponding to rhodamine accompanied
by a significant increase in emission of coumarin (Fig. S61),
indicating disassembly of M1 in acetonitrile. We rationalized
that the presence of acetonitrile disrupted the Pt-N coordi-
nation bond since acetonitrile featured the stronger binding
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