Light-controlled reversible self-assembly of nanorod suprastructures

Article abstract of DOI:10.1039/c7cc03280c

Nanorod suprastructures constructed by the coordination of zinc ions with the inclusion complex of 4,4′-dipyridine in β-cyclodextrin can dissociate and rebuild repeatedly via alternate visible light irradiation in the presence of photoacid merocyanine in

Full text of DOI:10.1039/c7cc03280c

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
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Light-controlled reversible self-assembly of nanorod
suprastructures
Received 00th January 20xx,
a
a,b
a
a,b
Jie Guo, Heng-Yi Zhang,* Yan Zhou and Yu Liu*
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
7
Nanorod suprastructures constructed by the coordination of coordination and supramolecular chemistry. Its neutral and
zinc ions with the inclusion complex of 4,4’-dipyridine in
β
-
rigid N-donor site
cyclodextrin can dissociate and rebuilt repeatedly via the
alternate visible light irradiation in the presence of photoacid
merocyanine in aqueous solution.
Supramolecular smart materials are currently intensively
studied for their potential applications in drug delivery, self-
healing materials, controllable catalysis, tissue engineering. In
designing and fabricating new supramolecular smart materials,
one of the crucial features is their reversibility. There are lots
of reports on smart materials showing reversible responses to
1
2
3
4
pH, competitive binding, redox stimuli/inputs, light, and
5
enzyme. Of which the pH adjustment is intensively
investigated for existing plentiful easy protonation groups in
supramolecular assembly units. However, the operation
induced by acid and base would produce waste products,
Scheme 1 Schematic representation of light-controlled reversible self-assembly of
which would stop the operation. In sharp contrast, the light nanorod suprastructures besed on polypseudorotaxane 1.
response is particularly attractive, because it not only can carry
out the remote control, but also there are no additional
make DPD as an excellent candidate which can be used as a
chemical species to be introduced into systems. If those smart
materials with the pH-responsive groups could be operated by
light instead of acid and base, it will facilitate reversibility of
supramolecular smart materials. Photoacids have an excited
bridging linker for the two- and three-dimensional
8
supramolecular architectures. We previously investigated the
inclusion complexation of DPD with
β-cyclodextrin (β-CD) and
the following coordination of the inclusion complex DPD⊂β-CD
with Ni , and prepared a series of polypseudorotaxanes with
2
+ 9
a
electronic state pK much lower than that in their ground
2
+
2+
state, so have proven useful in operating molecular switch and
6
logic gates by the pH change. On the other hand, 4,4’-
different morphologic nanoarchitectures by Co or Zn ions
coordinating with N-donor of DPD in the inclusion complex
1
0
bipyridine (DPD) has been extensively investigated in
DPD⊂β-CD. Herein, we report the light-controlled reversible
self-assembly of nanorod suprastructures formed by the
polypseudorotaxane (1) of the inclusion complex DPD⊂β-CD
a.
2+
Department of Chemistry, State Key Laboratory of Elemento-Organic Chemistry,
Nankai University, Tianjin 300071, P. R. China.
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin 300071, P. R. China.
with Zn in aqueous solution containing the protonated
merocyanine (MEH). Using NMR, UV, FT-IR, Dynamic light
scattering (DLS), and the transmission electron microscopy
b.
E-mail: hyzhang@nankai.edu.cn, yuliu@nankai.edu.cn
Electronic supplementary information (ESI) available: The synthesis of
polypseudorotaxane 1, H NMR, FT-IR, TEM images, DLS, and UV/vis. See DOI:
(
TEM), we will show that both the monomer DPD and the
†
1
included DPD in DPD⊂β-CD can be protonated and
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1
deprotonated induced by MEH with light, and the following released from the ring-closing reaction of MEH to SP, leading to a
1
2
+
nanorod superstructures formed by Zn ions with DPD⊂β-CD small change of the pH value.
can dissociate and re-form via the reversible transition of
photoacid. Our particular interest is to regulate the nano-
superstructures containing the only pH-responsive groups by
light instead of acid and base. The illustration of the
formation/dissociation of nanorod superstructures is shown in
Scheme 1.
Fig. 2 The pH values of (a) MEH before irradiation (black “■”), after irradiation at 420
nm for 5 min (red “●”), and kept in dark for 15 min (blue “ ”) at different MEH
concentration, and (b) 1 (2.0 × 10− M) + MEH before irradiation (black “■”), after
▲
4
irradiation at 420 nm for 5 min (orange “●”), and kept in dark for 15 min (green “▲”) at
different MEH concentration.
The detail information about the morphology of assemblies
formed from polypeseudorotaxane 1 was obtained from their TEM
1
0
experiments. As expected,
nanorod suprastructures were
observed in the absence of MEH (Fig. 3a). When the pH value of the
solution of 1 was adjusted to 3 with HCl, a lot of smaller nanorods
were observed with a length of about 30 nm in the TEM image (Fig.
S8b, ESI†). This observation should be attributed to the protonation
of DPD in 1 at low pH leading to the dissociation of the coordination
1
-
Fig. 1 Partial H NMR spectra (400 MHz, D
2
O, 25 °C) of polypseudorotaxane 1 (2.0 × 10
4
2+
M) at different pH upon the sequential addition of HCl and NaOH.
bond between Zn and the N-atom in DPD. We further adjusted
the pH value to 7 with NaOH, the original rod-like structures were
It is well demonstrated that the host-guest interaction between observed again (Fig. S8c, ESI†). To visualize the impact of pH on the
β-CD and DPD are contributed to hydrophobic interactions, and dissociation and reforming process of polypseudorotaxane 1, a
DPD of mono/double-protonated form interacts with β-CD more series of TEM measurements were performed at three different pH
weakly than its initial unprotonated form, which occupy a large values (from 4 to 6). As can be seen from Fig. S9 (ESI†), the pH value
1
2
proportion below pH 5. Therefore, we firstly investigated the of solution has a significant impact on its dissociation and reforming
1
1
influence of pH on the protonation of DPD in polypseudorotaxane 1. process, in which there is a similar phenomenon for the solution
As can be seen from Fig. 1b, both meta and ortho protons of DPD of pH 4 with the above pH 3, but the TEM images at the pH value of
exhibit slightly downfield shifts at pH 4-6, and an obvious shift is 5 and 6 still remain the nanorod suprastructures.
observed at the pH value of 3. This observation indicates that DPD
should be the double-protonated form when the pH value reaches
ca. 3. And then, we measured the pH values of MEH in the different
concentration with irradiation. When the aqueous solution of MEH
was irradiated by visible light of 420 nm, MEH underwent a light-
induced ring-closing reaction to the SP with the release of a proton
13
to the environment. After the light was turned off, the SP reverted
back to the MEH in ca. 5 minutes. As expected, the pH value of MEH
exhibits no significant change when the concentration of MEH
increased gradually from 1.0 × 10− M to 6.0 × 10 M (Fig. 2a).
However, after irradiation with visible light at 420 nm for 5 min, the
extent of pH variation of MEH shows clear concentration
dependency. The pH value decreases about 1.2 units to pH 4.9 at
4
−4
.0 × 10− M, but 2.6 units to pH 3.4 at 6.0 × 10 M. When the light
4
−4
1
was removed, the pH value of MEH gradually comes back to its
initial value. However, upon irradiation of the mixed solution
containing 1 (2.0 × 10− M) and MEH with 420 nm for 5 min, the
corresponding changes of the pH values are smaller than those of
free MEH (Fig. 2b). One reasonable explanation is that the N-atom
of DPD in 1 may undergo a protonation process with the protons
4
−
5
−5
−4
Fig. 3 TEM Images of (a) 1 (1.0 × 10 M), (b) 1 (1.0 × 10 M) + MEH (6 × 10 M), (c) 1
−5
−4
−5
(
1.0 × 10 M) + MEH (6 × 10 M) after irradiation at 420 nm for 5 min, (c) 1 (1.0 × 10
−4
M) + MEH (6 × 10 M) was irradiated at 420 nm for 5 min and kept in the dark.
2
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−
5
When the aqueous solution of 1 (1.0 × 10 M) was mixed with isomerization of MEH into SP. When keeping the solution in the
-4
MEH (6 × 10 M), no evidence for dissociation was obtained by dark, the representative absorption band of MEH at 423 nm quickly
TEM observations (Fig. 3b). However, after the mixture was appears, suggesting SP to MEH.
irradiated at 420 nm for 5 min, the light-induced transition from
aggregation to disperse state was observed (Fig. 3c). Keeping the
solution in the dark leads to the re-formation of the nanorod
suprastructures (Fig. 3d).
-
4
-6
Fig. 5 UV-vis absorption of MEH (2 × 10 M) + 1 (2 × 10 M) before (orange) and 2 min
-
4
after irradiation (yellow) (inset figures show colour changes of 1 (2.0 × 10 M) + MEH
before and after irradiation at 420 nm for 5 min, respectively).
Once the first cycle has completed, the dissociation–reforming
sequence can be repeated. To evaluate the reversibility of the
system, we repeatedly switched the system. Fig. 6 shows five cycles,
in which the pH value of the mixture of 1 and MEH can be reversibly
switched between ca. 6.5 and ca. 5.7 just by turning the visible light
on and off. We observed no significant changes in the samples,
indicating that the switching process is fully reversible. Because of
the good reproducibility of the protonation and deprotonation
process of DPD, the photoinduced dissociation and reforming
process of polypeseudorotaxane 1 could be repeated many times
without appreciable degradation at room temperature.
Fig. 4 DLS of (a) 1 (2.0 × 10 M) + MEH (6 × 10−4 M), (b) 1 (2.0 × 10−4 M) + MEH (6 ×
1
−
4
−
4
−4
−4
0 M) after irradiation at 420 nm for 5 min, (c) 1 (2.0 × 10 M) + MEH (6 × 10 M)
kept in the dark for 15 min after irradiation at 420 nm for 5 min.
We further used DLS to confirm the reversible dissociation and
re-formation
polypeseudorotaxane 1 in aqueous solution. For the mixed solution
containing and MEH, the DLS results show that
polypeseudorotaxane forms the assemblies with size
process
of
assemblies
formed
by
1
1
a
distribution of 2.76 ꢀm (Fig. 4a). After irradiation at 420 nm for 5
min, the size distribution of the assemblies falls into two categories:
one is from 0.84 to 0.62 ꢀm, and the other is from 3.71 to 2.49 ꢀm.
The former should belong to the smaller nanorods due to the
dissociation of the original assemblies, and the latter still belongs to
the undissociated assemblies (Fig. 4b). The sample was placed in
the dark for 15 min, the size distribution returns to 4.29 ꢀm (Fig. 4c).
The similar observations were also found in the control experiments
with HCl/NaOH instead of MEH (Fig. S10, ESI†).
The UV-vis spectroscopy was further performed to investigate
the reversibility of the assembly process of nanorod
suprastructures. As can be seen from Fig. 5, the colour of the
aqueous solution containing 1 and MEH is yellow with an
absorption band in λmax = 423 nm, which can be attributed to the
Fig. 6 Cycles of pH change under irradiation and in the dark. For each cycle, the
−4
−4
−4
solution of 1 (2.0 × 10 M) + MEH (6.0 × 10 M) (red) and 1 (2.0 × 10 M) (blue) were
protonated MEH as illustrated in Scheme 1. After irradiated by irradiated with 420 nm for 5 min and kept in the dark for 15 min.
visible light at 420 nm for 5 min, the colour of the solution becomes
a pale yellow accompanying the decrease of the absorption
In summary, we have demonstrated a method to regulate the
intensity in λmax = 423 nm. This observation is due to the self-assembly of the nanometer materials containing the only pH-
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responsive groups by light instead of acid and base. We firstly
constructed the nanorod suprastructures by the coordination of the
9
(a) Y. Liu, Y.-L. Zhao, H.-Y. Zhang, E.-C. Yang, X.-D. Guan, J.
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photoacid MEH, and found that the nanorod superstructures can
dissociate and re-form repeatedly via the alternate visible light
irradiation. From the NMR and UV-vis studies, we elucidated that
there is a perfect match of the pH changes between the
protonation of DPD and the ring-closing/opening reaction of MEH.
The present results may open the way toward the effective
development of new strategies in light-controlled soft
nanotechnology.
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