Fluorescent Cross-Linked Supramolecular Polymer Constructed by Orthogonal Self-Assembly of Metal-Ligand Coordination and Host-Guest Interaction

Article abstract of DOI:10.1002/chem.201600561

A new host molecule consists of four terpyridine groups as the binding sites with zinc(II) ion and a copillar[5]arene incorporated in the center as a spacer to interact with guest molecule was designed and synthesized. Due to the 120 ° angle of the rigid aromatic segment, a cross-linked dimeric hexagonal supramolecular polymer was therefore generated as the result of the orthogonal self-assembly of metal-ligand coordination and host-guest interaction. UV/Vis spectroscopy, ¹H NMR spectroscopy, viscosity and dynamic light-scattering techniques were employed to characterize and understand the cross-linking process with the introduction of zinc(II) ion and guest molecule. More importantly, well-defined morphology of the self-assembled supramolecular structure can be tuned by altering the adding sequence of the two components, that is, the zinc(II) ion and the guest molecule. In addition, introduction of a competitive ligand suggested the dynamic nature of the supramolecular structure. Orthogonal self-assembly: A fluorescent cross-linked supramolecular polymer was constructed by the orthogonal self-assembly of metal-ligand coordination and host-guest interaction. The well-defined morphology of the self-assembled supramolecular structure can be tuned by altering the adding sequence of the two components, that is, zinc(II) ion and the guest molecule (see figure).

Full text of DOI:10.1002/chem.201600561

DOI: 10.1002/chem.201600561
Full Paper
&
Host–Guest Systems
Fluorescent Cross-Linked Supramolecular Polymer Constructed by
Orthogonal Self-Assembly of Metal–Ligand Coordination and
Host–Guest Interaction
Xiaomin Qian,[a] Weitao Gong,*[a] Xiaopeng Li,[b] Le Fang,[a] Xiaojun Kuang,[a] and
Guiling Ning*[a]
Abstract: A new host molecule consists of four terpyridine
groups as the binding sites with zinc(II) ion and a copillar[5]-
arene incorporated in the center as a spacer to interact with
guest molecule was designed and synthesized. Due to the
1208 angle of the rigid aromatic segment, a cross-linked di-
meric hexagonal supramolecular polymer was therefore gen-
erated as the result of the orthogonal self-assembly of
metal–ligand coordination and host–guest interaction. UV/
Vis spectroscopy, 1H NMR spectroscopy, viscosity and dynam-
ic light-scattering techniques were employed to characterize
and understand the cross-linking process with the introduc-
tion of zinc(II) ion and guest molecule. More importantly,
well-defined morphology of the self-assembled supramolec-
ular structure can be tuned by altering the adding sequence
of the two components, that is, the zinc(II) ion and the
guest molecule. In addition, introduction of a competitive
ligand suggested the dynamic nature of the supramolecular
structure.
Introduction
with potential wide-ranging applications in fluorescent sens-
ing, photonics, electronics, and so on.[3d,h,k,4]
Supramolecular polymers, constructed from reduplicative mon-
omeric units held together by noncovalent interactions, have
gained increasing attention during the past few decades.[1] Be-
cause of the dynamic nature of noncovalent bonds, supra-
molecular polymers exhibit a variety of new properties, such as
stimuli-responsiveness, self-healing, reversibility, degradability,
and adaptivity,[2] which can be complementary to conventional
covalently bonded polymers. Inspired by nature, like double
helix DNA and the protein secondary structure, considerable
effort has been devoted to constructing cross-linked supra-
molecular polymers since they possess intriguing functions in
comparison with other topological structures. However, com-
pared with the abundant number of linear main chain supra-
molecular polymers, supramolecular polymers with cross-
linked topology are relatively fewer in number.[3] Moreover, in-
troduction of fluorescent properties into supramolecular poly-
mers has attracted remarkable attention, and endows them
In comparison with supramolecular polymers mainly based
on one type of noncovalent interaction, orthogonal self-assem-
bly,[5] the introduction of different types of highly specific, non-
interfering noncovalent interactions into one supramolecular
system is attracting more interest because it combines the dif-
ferent features of each interaction, which can lead to superior
architectures and specific properties. Up to now, various non-
covalent interactions have been utilized orthogonally in the
construction of new supramolecular polymers. Among them,
metal–ligand coordination and host–guest complexation are
two mainstream interactions that have been studied extensive-
ly. Metal–ligand coordination, a highly directional noncovalent
interaction, has been proven to be a powerful approach for
building supramolecular polymers since it allows the construc-
tion of well-defined supramolecular architectures and imparts
the polymers with an increased stability compared to other
noncovalent interactions. In the meantime, coordination-driven
polymers have attracted great interest due to the combination
of the properties of organic polymers as well as the magnetic,
electronic, optical, and catalytic potential of metals.[6] For this
purpose, a variety of new ligands, such as pyridine,[3i,j,7] 2,2’-bi-
pyridine,[8] 2,6-bis(1’-methylbenzimidazolyl)pyridine,[9] porphy-
rin,[10] 2,2’:6’2’’-terpyridine,[11] and so forth have been extensive-
ly explored by scientists. Among them, 2,2’:6’,2“-terpyridine
(TPY), especially for easily accessible 4’-functionalized TPY, is
a versatile building block and has been studied extensively in
recent years because of its generally high binding affinity to-
wards a variety of transition as well as rare earth metal ions,
due to dp–pp* back-bonding of the metal to the pyridine
[a] X. Qian, Dr. W. Gong, L. Fang, X. Kuang, Prof. G. Ning
Sate Key Laboratory of Fine Chemicals, School of Chemical Engineering
Dalian University of Technology
No. 2, Linggong Road, High Tech Zone, Dalian (P.R. China)
E-mail: wtgong@dlut.edu.cn
ninggl@dlut.edu.cn
[b] Dr. X. Li
Department of Chemistry and Biochemistry
Texas State University, San Marcos, Texas, 78666 (USA)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201600561.
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rings and the chelate effect.[11b] Well-designed supramolecular
architectures have been fulfilled based on metal–TPY building
blocks, paving the ways to smart materials with the capability
of being responsive to certain external changing parameters
such as pH value, anion, temperature, a competitive ligand,
and so on.[12]
Host–guest complexation, which usually refers to the forma-
tion of supramolecular inclusion complexes between macrocy-
clic host molecules and guest molecules connected together
in a highly-controlled manner, has emerged as one of the most
attractive fields in supramolecular chemistry. Pillararenes, as
a new class of macrocyclic hosts discovered after crown ether,
calixarene, cyclodextrin, cucurbituril, and so on, have facilitated
the development of supramolecular chemistry since the pio-
neering work reported by Ogoshi and co-workers in 2008.[13]
Host–guest chemistry based on pillararenes has also been
well-established and widely explored in recent years, especially
for pillar[5]arene due to its easy accessibility and versatile func-
tionalization.[14] Compared with the conventional hosts men-
tioned above, one of the most peculiar properties of pillar[5]ar-
ene is the strong binding affinities towards neutral guests in
organic solvents,[3f,i,k,4e,g,15] facilitating a new route for supra-
molecular self-assembly of neutral host–guest species in organ-
ic solvents with special physical, chemical, and optical proper-
ties.
Scheme 1. Schematic illustration of the metal–ligand coordination (host and
metal ion Zn2+), the host–guest interaction (host and guest molecule G2),
and the orthogonal self-assembly of the three components.
Supramolecular systems built on the orthogonal self-assem-
bly of metal–ligand coordination and host–guest interaction
have been studied widely in the past decades.[2e,3h,j,16] However,
to the best of our knowledge, only two examples concerning
the orthogonal self-assembly of TPY-based coordination and
pillararene-based host–guest interaction have been reported
so far. For example, Huang and co-workers prepared a solvent-
driven muscle-like metallo-supramolecular polymer from an
amino-modified pillar[5]arene.[17] The introduction of TPY moi-
eties and the rigid structure of pillar[5]arene greatly promotes
the formation of the metallo-supramolecular polymer. Yao and
co-workers presented the construction of a linear supramolec-
ular polymer with responsiveness to base stimulus and con-
centration-dependent fluorescence emission driven by pillarar-
ene-based host–guest molecular recognition and TPY-based
metal–ligand coordination.[4e] These two examples showed the
excellent cooperation motif of TPY-based metal–ligand coordi-
nation and pillararene-based host–guest interaction under deli-
cate design, but both with linear topology. Cross-linked supra-
molecular structures based on TPY and pillar[5]arene were not
found as far as we know. Hence, we envisioned that it is of
rather interest and importance to develop new cross-linked
supramolecular polymers with TPY and pillar[5]arene moieties.
Herein, we present the design and synthesis of a new host
molecule as the backbone to construct a cross-linked supra-
molecular polymer by the orthogonal self-assembly of metal–
ligand coordination and host–guest interaction. As shown in
Scheme 1, the host molecule mainly consists of two sectors,
that is, four TPY groups decorated at the rims and a copillar[5]-
arene moiety incorporated in the center. First, due to the 1208
internal angle of rigid bent-shaped aromatic segment and the
fact that two TPY groups form a pseudo octahedral complex
with one metal ion (TPY-MII-TPY),[11a] once the complexation
occurs between TPY and metal ion, a polymeric hexagonal
structure was therefore formed by intermolecular coordination.
Second, the copillar[5]arene works not only as a linking spacer
that connects the two aromatic segments covalently, but also
acts a “dynamic glue” that binds two host molecules together
noncovalently through host–guest interaction. It is noteworthy
that an appropriate alkyl chain of the copillar[5]arene is also
quite important to keep the rigidity and avoid potential forma-
tion of monocylces.[12a] Hence, a cross-linked dimeric supra-
molecular structure was anticipated after metal ion and an ap-
propriate ditopic guest molecule was introduced into the
system. Moreover, due to the rigid structure of the molecule
and the introduction of metal ion and guest molecule, tunable
fluorescence properties were also expected. These could serve
as the rationale of our design idea.
Results and Discussion
The synthetic route of the host molecule is depicted in
Scheme 2. Starting from the synthesis of substituted 2, 4, 6-tri-
phenyl pyrylium salt by an effortless condensation reaction,
substituted 2, 4, 6-triphenylbenzene was obtained in an ac-
ceptable yield by a nucleophilic reaction with sodium acetate
in acetic anhydride. Similar reactions had been widely utilized
by Hçger and co-workers in the synthesis of polycyclic aromat-
ic hydrocarbons (PAHs).[18] The 4’-(4-boronatophenyl)-2,2’:6’,2“-
terpyridine was synthesized from unprotected 4-formylphenyl-
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Scheme 2. Synthetic protocol of the host. (a) BF3·OEt2; (b) CH3COONa, acetic anhydride; (c) 4’-(4-boronatophenyl)-2,2’:6’,2“-terpyridine, [Pd(PPh3)4], K2CO3, diox-
ane/H2O; (d) BBr3, dry CH2Cl2; (e) K2CO3, KI, CH3CN.
boronic acid by a reported method.[19] Followed by a two-fold
Pd-catalyzed Suzuki coupling reaction and a subsequent de-
methylation reaction, the intermediate compound 4 with two
TPY units and a phenolic hydroxyl group was successfully ob-
tained. After a Williamson-type reaction with alkyl bromide-
decorated copillar[5]arene, the host molecule with four TPY
groups at the peripheries and a copillar[5]arene moiety as the
joint bridge was conveniently achieved in a satisfactory yield.
The guest molecule G2 was synthesized similarly to a reported
literature.[3k] All the known compounds were characterized by
1H NMR, and shown to be in full agreement with data reported
in the literatures. 1H NMR, 13C NMR spectroscopy and HRMS
were applied to characterize all the new compounds, and the
data are consistent with the structures presented.
Zinc(II) ion, as one of the most favorable transition metals
for TPY, was selected as the model to study the coordination
process. Initially, with the aim of investigating the capacity of
host to form host·Zn2+ complex in the presence of Zn2+, UV/
Vis and fluorescence titrations of host with Zn(OTf)2 in a mixed
solvent CHCl3/CH3CN (v/v, 4/1) were carried out. The monomer
host showed a single broad absorption peak at 300 nm (Fig-
ure 1a), which is attributed to the p–p* transition of the TPY
group. With incremental equivalents of Zn2+, two isosbestic
points showed up at about 294 nm and 328 nm, respectively.
The two isosbestic points implied the existence of a complicat-
ed associated system. Concomitantly, the absorption peaks at
300 nm disappeared and two new absorption bands appeared
at 286 nm and 342 nm, corresponding to the ligand-centered
(LC) p–p* transition.[20] No absorption peak was observed with
Figure 1. (a) UV/Vis and (b) emission spectra (lex =328 nm) of host
wavelength longer than 350 nm, demonstrating the absence
(0.01 mm) upon titration with Zn2+ (0 equiv, black; 2.3 equiv, green) in
of metal-to-ligand charge transfer (MLCT) process. The absorp-
CHCl3/CH3CN (4/1, v/v). Inset of (a) shows normalized absorption changes at
tion curve almost remained constant after two equivalents of
Zn2+, indicating the maximum conversion from monomer host
~
*
342 nm ( ) and 314 nm ( ); Inset of (b) shows the fluorescence color
change after addition of 2.3 equivalents of Zn2+
.
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to host·Zn2+ complex was reached. The UV/Vis titration plot
curve monitored at 314 and 342 nm (Figure 1a, inset) reveals
a 1:2 ligand-to-metal stoichiometry, which is in accordance
with our assumption as illustrated in Scheme 1. We reason that
it is not feasible for two TPY units on the same molecule to co-
ordinate with one single metal ion from a conformational con-
sideration. Therefore, it is reasonable to deduce that every two
TPY groups are linked together through one Zn2+ ion through
an intermolecular coordination rather than intramolecular co-
ordination, giving rise to a cross-linked hexagonal architecture.
The stepwise binding constants K1 and K2 by UV/Vis titration
are calculated to be 8152mÀ1 and 2036mÀ1 (K1 ꢀ4K2), which
proves to be a non-cooperative process, indicating that the
four TPY binding sites are truly identical and independent of
each other.[21]
support the perspective that only very weak interaction existed
between host and G1 (the Supporting Information, Figures S3
and S4). With this idea in mind, the second ditopic triazole
guest molecule with much longer alkyl chain G2 was intro-
duced. As shown in Figure 2, for the resulting host–guest
paired complex, alkyl chain protons H1, H2, and H3 on G2
In terms of fluorescence, when excited at 328 nm, monomer
host showed an almost identical emission as compound 3
with a strong emission band peaking at 395 nm (the Support-
ing Information, Figure S1), suggesting that the incorporation
of copillar[5]arene had little influence on the excited state of
TPY-like molecule. After Zn2+ was added, the emission intensity
of the monomer gradually quenched and a much broader
weak band finally appeared at 458 nm after 2 equivalents of
Zn2+ (Figure 1b), suggesting the formation of aggregates. The
fact that no TPY emission had been detected after fully coordi-
nated with Zn2+ indicates that the lowest excited state of TPY
Figure 2. 1H NMR spectra (400 MHz, CDCl3, room temperature) of host upon
complexation with excess G2. (a) host (1 mm); (b) host + G2 (3 mm); (c) G2
(3 mm). Inset shows amplifying chemical shifts of H4 and H5.
units is completely quenched by coordinating with Zn2+
,
which is in agreement with the result reported previously.[22]
This new weak band can be attributed to an excited state of
Zn2+-TPY complex. As a consequence, from a visual observa-
tion, the fluorescence color of the solution altered from violet
to cyan (Figure 1b, inset). Attempts to characterize the in situ
generated host·Zn2+ complex by 1H NMR spectroscopy have
failed due to the poor solubility of the complex in any solvents
we tried. To sum up, the spectroscopic results obtained verified
the idea that the coordination occurred between host and
Zn2+ in a molar ratio of 1:2, leading to the polymerization in
which the monomers are glued together by the metal ions as
presented in Scheme 1.
showed a significant chemical shift upfield as much as d=
3.357 ppm compared with the free G2 (d=3.029 ppm for H1,
3.357 ppm for H2, and 2.97 ppm for H3, respectively), support-
ing the formation of a complex based on host–guest interac-
tion. Meanwhile, proton H4 and the proton on triazole H5 also
showed a relatively small-scale upfield shift (d=0.015 ppm for
H4 and 0.064 ppm for H5, respectively), which further conclud-
ed the complexation motif. Protons on the TPY moieties did
not show apparent shifts, indicating the host–guest interaction
without the involvement of the coordination sites.
As mentioned before, pillararenes exhibit excellent molecu-
lar recognition towards neutral guest molecules. To link two
host molecules together by host–guest complexation, a ditopic
guest molecule is structurally required. Here, two ditopic neu-
tral compounds bearing two terminal cyano groups G1[3i] and
G2,[3k] which had been reported previously to have high bind-
ing affinity with pillar[5]arene, were chosen as the models to
study the slow-exchange host–guest complexation. The com-
plexation was investigated by 1H NMR spectroscopy first. As
can be seen in Figure S2 (the Supporting Information), chemi-
cal shifts of protons on the CN-attached alkyl chain of G1 did
not show any distinct movement after being added into host
in deuterated chloroform. Instead, only a broadening effect of
the peaks with no split was observed, which might imply that
the only a small part of the protons were included into the
cavity of pillar[5]arene. We speculate the length of G1 is too
short to thread into the two cavities of two pillararenes. The
negligible changes in the emission and absorption spectra also
After confirming the existence of host–guest interaction be-
1
tween host and G2 by H NMR spectroscopy, fluorescence and
UV/Vis titrations were further conducted to inspect the influ-
ence of G2 on the photophysical properties of the host, and
to provide helpful information about the self-assembly mode
in dilute solution. As we can see from the emission spectra in
Figure 3a, with increasing equivalents of G2 added into host,
the emissive peak of the host monomer centered at 395 nm
showed a progressive decrease and the intensity in the longer
wavelength increased slightly. G2 alone is non-emissive in solu-
tion. The inclusion of G2 apparently somehow affected ar-
rangements of host molecules, resulting in the fluorescence
change. The fluorescence color transformation is easily discern-
able by naked-eye under a UV light (Figure 3a, inset). From the
UV/Vis titration (Figure 3b), it is clear to see that the broad ab-
sorption band showed an apparent blue shift upon addition of
2.3 equivalents of G2 as well as an enhancement of the ab-
sorbance. The obvious absorbance change might indicate the
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to conduct due to the poor solubility of the Zn2+-coordinated
complex, fluorescence and UV/Vis tests were run to investigate
the influence of the two interactions on each other. As we can
see, when Zn2+ was added into the host·G2 complex continu-
ously, the fluorescence spectra showed a similar variation
trend as the Zn2+ added into host: the peak at 395 nm gradu-
ally decreased and a new weak peak occurred at 458 nm (the
Supporting Information, Figure S5). As for UV/Vis spectra, two
peaks at 286 nm and 342 nm appeared after addition of
2.3 equivalents of Zn2+ (the Supporting Information, Fig-
ure S6). As we modulated the addition order of the two com-
ponents, the introduction of G2 also showed a significant influ-
ence on the spectroscopic spectra of the host·Zn2+ complex
(the Supporting Information, Figures S7 and S8), which suggest
that the two interactions do not interfere with each other. That
is to say, these two interactions showed an orthogonal mode.
Aggregation behavior of the supramolecular polymer with
varying concentrations was investigated using Ubbelohde
semi-microdilution viscometer, with host for comparison. Due
to the poor solubility of the host and the complexes in organic
solvents, we began the viscosity test at a maximum concentra-
tion of 0.25 mm. For the host alone, the specific viscosity
almost remained invariant during the whole concentration
range (Figure 4), indicating no obvious aggregates were
formed. On the other hand, for the host·Zn2+ complex, the vis-
cosity exhibited a distinct uptrend with increasing concentra-
tion, suggesting the formation of high-molecular-weight ag-
gregates that resulted in a higher specific viscosity. It is note-
worthy that as for the host·Zn2+·G2 complex, the specific vis-
cosity is always higher than that of the host·Zn2+ complex.
This result proves the linking function of guest molecule. The
occurrence of the obvious aggregates at such a low concentra-
tion is probably due to the peculiarly cross-linked structure as
well as the relatively high binding affinity of metal–ligand co-
ordination and host–guest interaction. In general, the viscosity
studies clearly support that both metal–ligand coordination
and host–guest interactions are indispensable for the construc-
tion of such a supramolecular structure.
Figure 3. (a) Emission and (b) UV/Vis spectra (lex =328 nm) of host
(0.01 mm) upon titration with G2 (0 equiv, black; 2.3 equiv, green) in CHCl3/
CH3CN (4/1, v/v). Inset of (a) shows the fluorescence color change after addi-
tion of 2.3 equivalents of G2. Inset of (b) shows the Job plot of absorption
changes varying the molar ratio of host and G2 while keeping the total con-
centration at a constant (0.01 mm), revealing an approximate 2:1 stoichiom-
etry.
expansion of the conjugated structure. The Job plot (Fig-
ure 3b, inset) revealed an approximately 2:1 stoichiometry of
host to G2, suggesting that every two host molecules interact
with one G2 molecule (Scheme 1). To have an integrative con-
sideration, it is possible to deduce that H-aggregates were
generated after G2 was added into host, which involves a par-
allel interaction mode of the chromophores and are usually ac-
companied with a blueshifted absorption band as well as a rel-
atively lower fluorescence quantum yield as reported by the
literatures.[23]
Unlike most of the reported pillararene-based supramolec-
ular structures where pillar[5]arenes are usually decorated at
the linear end or at the periphery of the molecule (in which
case, the interposition of another interaction would not likely
affect the host–guest interaction from a conformational con-
sideration), here, the pillar[5]arene unit is incorporated at the
center of the backbone as a spacer. We were interested to
know whether these two interactions would affect each other
due to the structural factor since there is no much meaning
developing a supramolecular system with two main forces
2+
&
*
Figure 4. Specific viscosity (room temperature) of host ( ), host Zn com-
1
2+
~
*
*
*
work in an antagonistic way. Since H NMR studies were failed
plex ( ), and host Zn
G2 complex ( ).
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To further investigate the aggregation behavior during the
stepwise addition of Zn2+ and G2, we have performed dynam-
ic light scattering (DLS) experiments to monitor the dimension-
al changing trend during the introduction of the two interac-
tions. As we can see from Figure 5, the DLS data of the host
showed a hydrodynamic diameter of about 40 nm, indicative
of the formation of small aggregates as confirmed by SEM and
TEM in the later section. In the presence of 2 equivalents of
Zn2+, the dimension of the nanoparticles exhibited a prominent
increase up to about 600 nm, which again evidenced the effi-
cient cross-linking by metal–ligand coordination. The dimen-
sion continued to increase to about 1100 nm as G2 was
added, indicating the successful thread of G2 into the cavity of
pillar[5]arene which, as a consequence, led to a larger average
diameter distribution. However, when G2 was first added, the
DLS result indicates the formation of an aggregate with an
average hydrodynamic diameter of 145 nm. Further addition of
2 equivalents of Zn2+ induced the dimension growth up to
about 1200 nm, which is close to the result of adverse adding
order. Overall, the DLS results clearly showed a growing ten-
dency upon addition of Zn2+ and G2, suggesting that Zn2+
and guest molecule G2 worked efficiently as cross-linkers that
assisted the formation of aggregates with larger diameters.
Nanostructures with different morphologies obtained by
supramolecular self-assembly could be of interest and promi-
nent importance because of their potential applications in vari-
ous areas.[24] As for our system, to gain a further insight into
the self-assembly behavior induced by coordination and host–
guest interaction, scanning electron microscope (SEM), and
transmission electron microscope (TEM) studies were conduct-
ed (Figure 6). As we can see, host molecules self-assembled
into discrete uneven-sized spheres with an average diameter
of about (50Æ10) nm (Figure 6a). The spheres are shown to
be solid, as revealed by TEM (Figure 6a, inset). The sizes of
such spheres are much bigger than that of a single molecule
as attained by DFT (the Supporting Information, Figure S9), in-
2+
*
Figure 5. Size distribution graph of solutions of host, host Zn
,
2+
2+
*
*
*
*
*
host Zn
G2, host G2 and host G2 Zn at 258C measured by DLS.
The concentration of all the solutions is 0.1 mm.
2+
2+
*
*
*
*
Figure 6. Representative SEM and TEM images showing the morphology of (a) host; (b) host Zn complex; (d) host Zn
G2 complex; (e) host G2 com-
*
2+
2+
*
*
plex; (f) host G2 Zn complex. (c) Fluorescence microscopic image and optical microscopic (bright field, inset) image of host Zn complex, both show-
ing spherical morphologies.
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dicating that numerous molecules were involved to self-assem-
ble such a sphere. After 2 equivalents of Zn2+ were added,
clusters of solid spheres with diameters ranging from 400 nm
to 700 nm were observed (Figure 6b), which are much larger
than that of the spheres obtained by host molecules. Optical
microscopy also confirmed the existence of numerous solid
spheres (Figure 6c). Apparently the metal–ligand coordination-
induced assembly is responsible for the growth of the nano-
particles, which originates from small spheres that then coa-
lesce and anneal into the observed spherical clusters. The dif-
ferent sizes of the large particles might depend on the num-
bers of small spheres involved in the aggregation process.
After G2 was introduced, it is surprising to find out that enor-
mous porous flower-like structure was formed (Figure 6d),
which is the consequence of a host–guest interaction induced
self-assembly. As most of the reported orthogonal supramolec-
ular polymers formed rodlike or fibrous networks, it is rather
interesting to observe these well-organized morphological
changes formed simply by adding different components, espe-
cially for the last porous flower-like structure, which might find
some useful applications in catalytic area.
ligand, for example, 1,4,7,10-tetrazacyclododecane (cyclen).[25]
As we can see from Figure S10 (the Supporting Information),
the addition of excess cyclen (4 equivalents) resulted in the
fluorescence intensity enhancement as well as an emission
blueshift of the system back to the original level before the ad-
dition of Zn2+. This result suggested that the intervention of
cyclen effectively broke the cross-linked TPY-Zn2+ coordination
bond, leading to the recovery of the fluorescence. On the con-
trary, the fluorescence can also be quenched and redshifted
again by the further addition of 2 equivalents of Zn2+, indicat-
ing the re-formation of the supramolecular structure. UV/Vis
studies also affirmed this invertible process. Continuous titra-
tion with cyclen led to the disappearance of the Zn2+-TPY
peak at 342 nm, which can be restored by further addition of
Zn2+ (the Supporting Information, Figure S11). The cleavage
and formation of the competitive coordination process can be
repeated several times without obvious de-amplification.
Therefore, it is evident that the supramolecular polymer is ca-
pable of undergoing a reversible transition under external
stimulus.
To inspect how the order in which Zn2+ and G2 are added
affects the self-assembly of the system, G2 was first introduced
this time. Surprisingly, spherical structures shown to be fully
collapsed and inerratic laminar sheets with a regular width of
about 650 nm were observed, which is totally different from
the spherical clusters formed as Zn2+ was added first (Fig-
ure 6e). Apparently, host–guest interaction is the major driving
force for this morphological change. An amorphous porous
structure was generated upon further addition of Zn2+ (Fig-
ure 6 f). Though the final complexes formed by the three com-
ponents were possibly the same as concluded from the spec-
troscopic tests, the morphologies varied depending on the in-
troducing order of different interactions, as illustrated in
Scheme 3. This might indicate that the first step plays a crucial
Conclusion
We have designed and synthesized a new host molecule com-
posed of four terpyridine moieties at the four corners and a co-
pillar[5]arene unite as the linking spacer. Owing to the well-
known coordination of terpyridine towards zinc ion and the
host–guest recognition of pillar[5]arene towards neutral guest
molecule G2, a cross-linked dimeric hexagonal supramolecular
structure was successfully constructed. The two complexation
processes were both well studied and confirmed by UV/Vis
1
spectra, emission spectra and H NMR spectroscopy and they
work in an orthogonal way. Both the host and the complexes
showed good but different fluorescence properties. Viscosity
as well as DLS studies revealed that the metal–ligand coordina-
tion and the host–guest interaction are both indispensable for
the efficient construction of such a supramolecular structure.
More interestingly, changing the order in which Zn2+ and G2
are added led to totally different self-assembled morphologies
of the complex, as observed by SEM and TEM. Finally, the stim-
uli responsive property of this supramolecular complex was in-
vestigated by introducing cyclen, a stronger Zn2+ chelating
ligand, which showed good reversibility. However, due to the
inherent rigid structure, the host molecule and the therefore
formed supramolecular complex showed a poor solubility in
any solvents that we applied, which restricted the characteriza-
tions and further applications. Further studies are being under-
taken in our laboratory to design new host molecules with
better solubility and, more importantly, new properties.
Scheme 3. Schematic representation of the morphological transformation
during the introduction of different components.
role in the generation of the nanostructure and the morpholo-
gy control. Therefore, this growth mechanism study might
guide us in the fabrication of new nano-sized materials.
Experimental Section
As aforementioned in the introduction part, compared with
traditional covalently bonded polymers, one of the most fasci-
nating properties of supramolecular polymers is their stimuli-
responsiveness. In our case, the metal–ligand interaction can
be manipulated by the addition of a stronger Zn2+-chelating
Materials and Instruments
All the chemicals were purchased from commercial suppliers and
used without further purification unless mentioned. Acetonitrile
was pre-dried by shaking with type 4 Š molecular sieves, and then
Chem. Eur. J. 2016, 22, 6881 – 6890
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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distilled over calcium hydride prior to use. Dichloromethane (DCM)
was refluxed with calcium hydride and distilled prior to use. Other
solvents were used as received. Reactions were monitored by thin-
layer chromatography (TLC) plates. Visualization of the developed
plates was performed under UV light (254 nm and 365 nm).
Column chromatography was performed with silica gel (100–200
mesh) using glass columns. 1H NMR spectra were recorded on
a Bruker Av400 NMR spectrometer at 400 MHz. 13C NMR spectra
were recorded on a Bruker Av500 NMR spectrometer at 126 MHz.
Chemical shifts are given in parts per million (ppm) relative to tet-
ramethylsilane (TMS). High-resolution mass spectra (HRMS) were
recorded on a Waters GC-TOF and LC/Q-TOF mass spectrometer.
All the new compounds were characterized by 1H NMR, 13C NMR
spectroscopy and HRMS. For known compounds, we have cited
the reported characterization data that we used to compare to our
synthesized compounds and we have provided a 1H NMR spectrum
to confirm the purity of the isolated material. Fluorescence spectra
were obtained from a Jasco FP-8300 spectrofluorometer. UV/Vis
absorption spectra were all taken on a Hitachi UV-4100 spectro-
photometer. Quartz cells with 1 cm path length were used. Meas-
urements of the specific viscosities as a function of concentration
were conducted using an Ubbelohde micro-dilution viscometer at
258C. To ensure the accuracy, each sample was measured for 5
times to calculate the average flowing time. Dynamic light-scatter-
ing (DLS) tests were carried out on a Malvern Zetasizer Nano ZSP
and the solutions were filtered through a Teflon filter (pore size:
0.2 mm) before tests. SEM studies were carried out on a FEI
Quanta 450 electron microscopy operated at 20 kV. The sample for
SEM was prepared by dropcasting the solution to a freshly pre-
pared silicon wafer and then dried in the ambient environment
naturally. After that, the sample was sputtered with gold and sub-
mitted to the microscopy. TEM studies were run on a FEI Tecnai 20
electron microscopy at 200 kv. The sample for TEM was prepared
by dropcasting the solution to a carbon-coated copper grid and
dried in the ambient environment. Then the sample was submitted
to the microscopy.
127.52, 127.47, 124.77, 124.32, 123.85, 121.38, 118.65, 114.31,
55.38 ppm; TOF-LD-MS calcd for C67H47N6O: 951.3811 [M+H]+;
found: 951.3788.
Synthesis of compound 4: Under N2 atmosphere, compound 3
(0.475 g, 0.5 mmol) was dissolved in dry DCM (30 mL). Then the so-
lution was cooled to À788C and BBr3 (0.46 mL, 3 mmol) was added
in one portion. The solution turned brown immediately and was al-
lowed to obtain room temperature overnight, then quenched
slowly with a few drops of water until no gas was observed. The
precipitate was filtered and washed with DCM and CH3OH repeat-
edly, which gave 4 (0.42 g, 89.7%) as a yellow powder. 1H NMR
(400 MHz, [D6]DMSO): d=8.98 (d, J=8.1 Hz, 4H), 8.94–8.91 (m,
8H), 8.36 (t, J=7.8 Hz, 4H), 8.19 (d, J=8.3 Hz, 4H), 8.08–8.02 (m,
9H), 7.97–7.92 (m, 6H), 7.82–7.78 (m, 4H), 7.75 (d, J=8.5 Hz, 2H),
6.94 ppm (d, J=8.7 Hz, 2H); 13C NMR (126 MHz, [D6]DMSO): d=
152.10, 152.05, 152.03, 150.85, 150.84, 150.82, 150.78, 150.15,
150.13, 146.40, 146.39, 146.35, 146.31, 142.28, 142.27, 142.25,
142.23, 142.22, 142.21, 142.19, 141.22, 139.86, 138.27, 135.22,
135.20, 128.27, 127.97, 127.87, 127.84, 127.80, 127.57, 127.56,
127.54, 127.30, 126.32, 126.31, 123.43, 120.16, 115.76 ppm; TOF-LD-
MS calcd for C66H45N6O: 937.3655 [M+H]+; found: 937.3638.
Synthesis of 1,4-bis(4-bromobutoxy)benzene: 1,4-Bis(4-bromobu-
toxy)benzene was synthesized similarly according to a previously
published procedure.[27] 1H NMR (400 MHz, CDCl3): d=6.83 (s, 4H),
3.96 (t, J=6.1 Hz, 4H), 3.51 (t, J=6.7 Hz, 4H), 2.13–2.04 (m, 4H),
1
1.94 ppm (dt, J=12.6, 6.2 Hz, 4H). The H NMR data are in agree-
ment with those found in the literature.
Synthesis of copillar[5]arene: To a 250 mL three-neck round
bottom
flask,
1,4-bis(4-bromobutoxy)benzene
(1.564 g,
4.116 mmol), 1,4-dimethoxybenzene (5.687 g, 41.16 mmol) and par-
aformaldehyde (3.391 g, 113.045 mmol) were added. Then, 1,2-di-
chloroethane (100 mL) was added and the solution was degassed
with N2 for 10 min. Boron trifluoride diethyl etherate (5.7 mL,
45.22 mmol) was injected into the flask under N2 atmosphere and
the mixture was stirred at room temperature. After 30 min, metha-
nol (60 mL) was added to quench the reaction. The solvent was
evaporated under vacuum and the residue was dissolved in DCM
(50 mL) and washed with saturated NaHCO3 aqueous solution for
three times. After that, the organic phase was dried over anhy-
drous MgSO4 and crude product was obtained as a white powder.
Recrystallization from a mixture of ethanol and DCM (3:1, v: v)
Compound 1 and 2 were prepared according to our previously re-
ported method.[26]
Synthesis of 4’-(4-boronatophenyl)-2,2’:6’,2“-terpyridine: 4’-(4-
Boronatophenyl)-2,2’:6’,2”-terpyridine was synthesized according to
a previously reported method.[19] 1H NMR (400 MHz, CD3OD): d=
8.71–8.58 (m, 6H), 8.00 (td, J=7.8, 1.6 Hz, 2H), 7.83–7.68 (m, 4H),
7.50–7.45 ppm (m, 2H).The 1H NMR data are in agreement with
those found in the literature.
1
gave pure copillar[5]arene (1.3 g, 32.3%) as white crystals. H NMR
(400 MHz, CDCl3): d=6.81 (s, 2H), 6.79 (d, J=3.5 Hz, 4H), 6.76 (s,
2H), 6.72 (s, 2H), 3.84–3.75 (m, 14H), 3.70–3.63 (m, 24H), 3.21 (s,
4H), 1.78 ppm (d, J=18.6 Hz, 8H); 13C NMR (126 MHz, CDCl3): d=
150.67, 150.61, 150.55, 150.43, 149.71, 128.42, 128.26, 128.24,
128.13, 128.05, 114.63, 113.97, 113.96, 113.70, 113.49, 67.23, 55.91,
55.70, 55.64, 33.38, 29.65, 29.35, 29.14, 28.32 ppm; TOF-LD-MS
calcd for C51H60O10Br2: 990.2553 [M]+; found: 990.2544.
Synthesis of compound 3: To a 250 mL round bottom flask, 2
(0.492 g,
1 mmol),
4’-(4-boronatophenyl)-2,2’:6’,2“-terpyridine
(0.847 g, 2.4 mmol) and K2CO3 (10 mmol, 1.382 g) were suspended
in a mixture of dioxane (60 mL) and water (20 mL). After bubbling
with N2 for 10 min, [Pd(PPh3)4] (0.04 mmol, 46.24 mg) was added
immediately. The mixture was heated at 888C under N2 atmos-
phere for 36 h. After cooling to room temperature, the solvent was
evaporated and DCM (100 mL) was added. The red solution was
washed three times with water and the organic phase was collect-
ed and dried over anhydrous MgSO4. After the solvent was evapo-
rated in vacuum, compound 3 (0.55 g, 58%) was obtained as
a white powder after purification by column chromatography with
Synthesis of host molecule: Copillar[5]arene (59.4 mg, 0.06 mmol),
compound 4 (56.2 mg, 0.06 mmol), K2CO3 (82.9 mg, 0.6 mmol) and
a catalytic amount of KI (5 mg) were placed in a 100 mL round
bottom flask. Dry CH3CN (50 mL) were added and the suspension
was heated at reflux at 828C under N2 atmosphere for 3 days. After
cooling to room temperature, the pale precipitate was filtered off
and washed with deionized water and CH3CN repeatedly. After
drying in vacuum, host was obtained as a white powder which
was used without further purification. 1H NMR (400 MHz, CDCl3):
d=8.81 (s, 8H), 8.76 (d, J=3.9 Hz, 8H), 8.70 (d, J=8.1 Hz, 8H), 8.02
(d, J=8.2 Hz, 8H), 7.88 (dd, J=23.4, 15.9 Hz, 39H), 7.65 (d, J=
7.3 Hz, 4H), 7.40–7.35 (m, 8H), 7.01 (d, J=9.1 Hz, 4H), 6.88–6.67
(m, 10H), 4.26–3.52 (m, 42H), 2.04 ppm (s, 8H); 13C NMR (126 MHz,
[D8]THF): d=155.89, 154.27, 154.20, 148.56, 147.52, 147.21, 139.80,
a
mixture eluent of DCM and CH3OH (100:1, v/v). 1H NMR
(400 MHz, CDCl3): d=8.87 (s, 4H), 8.78 (d, J=4.0 Hz, 4H), 8.73 (d,
J=7.9 Hz, 4H), 8.08 (d, J=8.3 Hz, 4H), 7.94 (t, J=7.0 Hz, 4H), 7.88–
7.81 (m, 15H), 7.72–7.66 (m, 2H), 7.41 (dd, J=6.8, 5.2 Hz, 4H), 7.06
(d, J=8.8 Hz, 2H), 3.90 ppm (s, 3H); 13C NMR (126 MHz, CDCl3): d=
159.38, 156.17, 155.89, 149.64, 149.09, 142.06, 141.76, 141.18,
140.38, 139.49, 137.29, 136.90, 133.53, 128.40, 127.78, 127.76,
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139.54, 138.78, 137.45, 135.63, 134.66, 126.22, 125.73, 125.59,
125.52, 125.37, 122.32, 121.86, 118.85, 116.27, 113.67, 53.06, 52.91,
52.88, 26.40, 25.11, 24.70 ppm; TOF-LD-MS calcd for C183H147N12O12:
2704.1261 [M+H]+; found: 2704.1147.
Keywords: fluorescence · host–guest systems · morphology ·
self-assembly · supramolecular chemistry
Synthesis of guest molecule G1: To a 100 mL round bottom flask,
hydroquinone (1.1 g, 0.01 mol), 5-bromovaleronitrile (4.86 g,
0.03 mol) and K2CO3 (13.82 g, 0.1 mol) were added in one portion.
Then, 50 mL dry acetonitrile was added and the suspension was
heated at reflux at 828C for 24 h under N2 atmosphere. After cool-
ing to room temperature, acetonitrile was evaporated and 100 mL
DCM was added. The solution was washed 3 times with water and
the organic phase was dried over anhydrous MgSO4. The guest
molecule was obtained as a white powder (1.5 g, 55.1%) after pu-
rification by column chromatography with DCM as the eluent.
1H NMR (400 MHz, CDCl3) d=6.81 (s, 4H), 3.96 (t, J=5.5 Hz, 4H),
2.45 (t, J=6.8 Hz, 4H), 1.96–1.84 ppm (m, 8H); 13C NMR (126 MHz,
CDCl3): d= 152.96, 119.59, 115.34, 67.21, 28.24, 17.00 ppm; TOF-EI-
MS calcd for C16H20N2O2: 272.1525 [M]+; found 272.1532.
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Synthesis of 1,1’-[1,4-butanediylbis(oxy)]bis[4-(2-propyn-1-yl-
oxy)]benzene: 1,1’-[1,4-Butanediylbis(oxy)]bis[4-(2-propyn-1-yloxy)]-
benzene was synthesized according to a reported literature with
some modifications.[4e] 4-(Propargyloxy)phenol (4.44 g, 0.03 mol),
1,4-dibromo butane (6.48 g, 0.03 mol) and K2CO3 (12.44 g, 0.12
mol) were added to a 250 mL round-bottom flask. CH3CN (100 mL)
was added and the suspension was heated at reflux at 828C for 8–
12 h (monitored by TLC). The mixture was cooled to room temper-
ature and CH3CN was evaporated and the residue was dissolved in
DCM. After being washed three times with water, the organic
phrase was collected and dried over anhydrous MgSO4. 1,1’-[1,4-
butanediylbis(oxy)]bis[4-(2-propyn-1-yloxy)]benzene (2.44 g, 46.5%)
was obtained as a yellowish powder after purifying by column
chromatography with a mixture of hexane/ethyl acetate (10: 1, v/v)
1
as the eluent. H NMR (400 MHz, CDCl3): d=6.92 (d, J=9.2 Hz, 4H),
6.84 (d, J=9.1 Hz, 4H), 4.64 (d, J=2.4 Hz, 4H), 3.98 (t, J=5.5 Hz,
4H), 2.50 (t, J=2.4 Hz, 2H), 1.95 ppm (dt, J=5.6, 2.9 Hz, 4H). The
1H NMR data are in agreement with those found in the literature.
Synthesis of guest molecule G2: Compound G2 was synthesized
according to a reported literature with some modifications.[4e]
Sodium ascorbate (0.99 g, 5 mmol) and CuSO4·5H2O (0.62 g,
2.5 mmol) were added to a solution of 1,1’-[1,4-butanediylbis(oxy)]-
bis[4-(2-propyn-1-yloxy)]benzene (1.75 g, 5 mmol) and 5-azidopen-
tanenitrile (1.86 g, 15 mmol) in DMF(4 mL), and the mixture was
heated at 908C for 12 h. After cooling to room temperature, the
brown solution was poured into 100 mL brine and a light-yellow
precipitate was formed. The precipitate was collected and dis-
solved in DCM. The solution was washed three times with water
and the organic phase was dried over anhydrous Na2SO4. G2
(2.48 g, 82.9%) was obtained as a white solid after DCM was re-
moved by a rotary evaporator and used without further purifica-
1
tion. H NMR (400 MHz, CDCl3): d=7.62 (s, 2H), 6.92 (d, J=8.7 Hz,
4H), 6.83 (d, J=8.7 Hz, 4H), 5.17 (s, 4H), 4.44 (s, 4H), 3.98 (s, 4H),
2.41 (t, J=6.9 Hz, 4H), 2.16–2.05 (m, 4H), 1.94 (s, 4H), 1.66 ppm (s,
4H). The 1H NMR data are in agreement with those found in the lit-
erature.
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Fundamental Research Funds for the Central Universities
(DUT14LK08).
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