Synthesis of a new solvent-responsive pillar[5]arene-based [1]rotaxane molecular machine

Article abstract of DOI:10.1039/d0nj01859g

In this work, we designed a new pillar[5]arene-based molecular machine responsive to the polarity of different solvents, which can exist in an interlocked structure in CDCl3 and CD3OD, and can exist in an extended form in DMSO and was studied by 1H and 2D NMR spectroscopy, HR(MS) and fluorescence spectroscopy.

Full text of DOI:10.1039/d0nj01859g

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DOI: 10.1039/D0NJ01859G
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Synthesis of A New Solvent-driven Pillar[5]arene-Based
[1]Rotaxane Molecular Machine
Received 00th January 20xx,
Accepted 00th January 20xx
Huasheng Tian, Runan Li, Po-Han Lin, Kamel Meguellati*
DOI: 10.1039/x0xx00000x
pillar[5]arene is usually used to construct more complicated
nanomaterials such as fluorescent probes38, ion recognition39,
In this work, we designed a new pillar[5]arene-based molecular
machine responsive to the polarity of different solvents which can
exist in an interlocked structure in CDCl3 and CD3OD, and can exist
in an extended form in DMSO and was studied by 1H and 2D NMR
spectroscopy, HR(MS) and fluorescence spectroscopy.
gels40
and
so
on.
However,
pillar[5]arene-based
(pseudo)[1]rotaxane or [1]rotaxane made up of one macrocycle
and one axle have been poorly reported mainly due to their
lower synthetic yield and the relative instability of the self-
assembled structures. Ogoshi et al. reported the first mono-
substituted pillar[5]arene-based pseudo[1]rotaxane, which
indicated that mono-guest-functionalized pillar[5]arene can be
self-included in CDCl3 and in an open form in acetone41. Xue et
al. reported a new strategy for the design and the synthesis of
[1]rotaxane via the condensation of a carboxylic acid containing
pillar[5]arene and with an amine in one step42. Yang et al.
reported a light-driven molecular machine, which can display
different conformations by light triggered Z/E isomerization of
stiff stilbene43. The aforementioned approaches represent very
convenient strategies for the design of multi-functionalized
(pseudo)[1]rotaxane or [1]rotaxane.
With the development of supramolecular chemistry1 and the
emergence of macrocycles such as crown ethers2,
cyclodextrins3, calixarenes4 and pillar[5]arenes5-10
,
etc.
mechanically interlocked molecules (MIMs)11-14 such as
rotaxanes15-17, catenanes18, 19 and molecular knots20, 21, have
attracted attention due to their broad applications in self-
assembly12, 22, drug delivery23, light harvesting24, etc. Rotaxane,
composed of one or more macrocycles threading one or more
25
axles with two stoppers at the ends
and sensitive to the
tunability of the host and guest interactions, has potential
applications in the design of molecular shuttles26, molecular
machines27 and ion recognitions28. Depending on the number of
building units, it can be defined as [1]rotaxane29, [n]rotaxane9
and poly[n]rotaxane30. [1]rotaxane is a key member in the
rotaxane family, and has attracted more chemists to find special
architectures and applications.
Solvent-responsive smart molecular machines have been poorly
reported. Wang et al. reported a self-locked solvent responsive
pseudo[1]catenane containing one pillar[5]arene motif linked
to an alkyl chain with two urea groups. However, the structure
of synthetic self-locked pseudo[1]rotaxane cannot be changed
by the modification of solvent14. Jiang et al. reported a kind of
pillar[5]arene based [2]rotaxane constructed with 1,4-
diethoxypillar[5]arene as a wheel that can slide depending on
the solvent along a long alkyl axle where a bodipy chromophore
was used as a stopper44. There are a very few reported mono-
substituted pillar[5]arene based molecular machine that can
exist in the form of self-included [1]rotaxane in CDCl3 and
dissociated in DMSO. Herein, A new strategy to construct
stimuli-responsive [1]rotaxane is described (Scheme 1). Firstly,
we prepared a mono-guest functionalized pillar[5]arene P3 via
click reaction from a monohydroxy-pillar[5]arene and 1-
Pillar[5]arene31 was first reported by Ogoshi in 2008. They are
composed of five hydroquinone units linked by methylene
bridges at the 2- and 5- positions. Pillar[5]arene has an electron-
rich cavity which makes it as an excellent container to host
various electron-poor guests32. The synthesis and
functionalization on both rims spotlight them as a macrocycle
of choice for the design of a series of hydrophobic, hydrophilic
and amphiphilic derivatives which could be applied in molecular
shuttles33,
nano-containers34,
ion
recognitions35,
supramolecular gels36, drug delivery37, etc. Mono-substituted
a. Huasheng Tian, Runan Li, Po-Han Lin, Prof. Kamel Meguellati
International Joint Research Laboratory of Nano-Micro Architecture Chemistry
(NMAC), College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun
130012, (PR China).
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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DOI: 10.1039/D0NJ01859G
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O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Br
N
Br
O
OH
O
4)
1)
2)
3)
N
N
N
N
N
O
O
O
N
N
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
P1
P2
P3
P4
N
N
Br
N
Br
N
OH
7)
8)
9)
O
5)
6)
N
N
O
N
N
O
N
O
N
N
N
O
M2
M3
M1
O
M4
O
M5
o
Scheme 1 Synthetic route of [1]rotaxane P4 and the related monomer M5. 1) BBr3, CHCl3, 0 C, 1 h, 22%; 2) propargyl bromide,
NaH, DMF, 80 oC, 24 h, 56%; 3) 1-aza-8-bromooctane, sodium ascorbate, CuSO4⸱5H2O, THF, H2O, 24 h, 87%; 4) M2, CHCl3, 90 oC,
24 h, 18%. 5) NBS, DMF, 0 oC, 4 h, 60%; 6) pyridine-4-boronic acid, K2CO3, Pd(PPh3)4, DMF, 120 oC, 24 h, 67%; 7) propargyl bromide,
o
K2CO3, CH3CN, 85 C,24 h, 56%; 8) 1-aza-8-bromooctane, sodium ascorbate, CuSO4⸱5H2O, THF, H2O, 24 h, 75 %; 9) M2, CHCl3,
90oC, 24h, 84%.
compound P3 was synthesized from the monohydroxy-
substituted pillar[5]arene P1, propargyl bromide and 1-aza-8-
aza-8-bromooctane, which is self-included in CDCl3. The final
bromooctane. Then, the mechanically interlocked molecule P4
mechanically interlocked molecules were prepared from P3 and
was prepared directly through a reaction of substitution
a triphenylamine derivative. The equilibrium between the self-
between P3 and M2. For a better understanding of the
included structure and its extended form depends on the
switchability of compound P4, the related monomer M5 was
environment. By introducing two different electron deficient
synthesized according to a similar method than compound P4.
positions, that act as stations, solvent responsive mechanically
interlocked molecules can change conformation and move
upon addition of a stimuli. Furthermore, by introducing a
triphenylamine moiety as a fluorescent group in the molecule,
the solvent-dependent transformation of the structure can be
monitored via NMR analysis and fluorescence analysis.
As compound P3 is a precursor of the mechanically interlocked
molecule P4, so the preliminary dynamic study of P3 was very
important to prepare P4. When it was dissolved in CDCl3, we
1
noticed that some peaks were below 0 ppm from the H NMR
studies (Fig. 1), which indicated that these protons were
shielded by pillar[5]arene. This phenomenon was certified by
It is well known that the host-guest interactions can be affected
2D H-1H NOESY NMR studies. From the 2D H-1H NOESY NMR
spectrum of P3 in CDCl3 (Fig. 2), we can see correlation peaks
between aromatic protons of pillar[5]arene and aliphatic
protons of the axle which confirmed the interlocked structure
of P3. Furthermore, to determine the intermolecular assembly
or the process of dynamic interlocked assembly of compound
1
1
by changing the polarities of solvents45, by increasing the
temperature8, by adding guests that have stronger interactions
with the host14. In this study, a solvent responsive mechanically
interlocked molecule P4 was prepared according to the
procedure described in Scheme 1. Firstly, the self-included
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P3, the concentration related NMR titration experiments were 2D 1H-1H NOESY NMR spectrum of P4 in CDCl3 (FigV.ieSw2A6r)ti,csletOronlninge
conducted in CDCl3 (Fig. S25). From the concentration related correlation peaks between aromatic protons of pillar[5]arene
DOI: 10.1039/D0NJ01859G
NMR spectra of compound P3, we can notice similar chemical and aliphatic protons of axle were seen which led us to conclude
shifts of P3 by increasing the concentration of P3 from 4 mM to to a fully interlocked structure for compound P4. 1H NMR
64 mM, which indicated that the structure of compound P3 in (Fig.S13), 2D 1H-1H COSY NMR (Fig. 4) and 2D 1H-1H NOESY NMR
1
CDCl3 is attributed to an intramolecular assembly.
(Fig. 5) 2D H-13C HSQC NMR studies were also conducted in
CD3OD. From the 2D 1H-1H NOESY NMR spectrum of compound
P4 in CD3OD, a correlation of the corresponding signals between
aromatic protons of pillar[5]arene and aliphatic protons of axle
can be seen clearly which confirmed a self-interlocked structure
for P4 in CD3OD.
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O
O
O
1
O
N
Br
O
N
N
8 7 6 5 4 3 2
9
O
9.0 8.5 8.0 7.5 7.0 6.5
O
δ (ppm)
O
O
O
a )
8
1
7
2
5,6 3,4
8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0
δ (ppm)
b )
Fig. 1 1H NMR spectroscopy of compound P3 (300 MHz, CDCl3,
298 K).
9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5
δ (ppm)
H2
H3, H4, H5, H6
Fig. 3 1H NMR spectra comparison between M5 and P4 in CDCl3
(300 MHz, 298 K).
CH
& ArOCH
Ar 2Ar
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
3
O
O
8
7
6&1 5&2
4
3
O
8
O
N
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Br
1 2 3O 4 5 6 7
N
N
9
O
O
O
Br
N
O
O
8
O
O
N
N
1 2 3O 4 5 6 7
O
N
O
9
O
N
O
O
O
O
H
Ar
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8
δ (ppm)
Fig. 2 2D 1H-1H NOESY NMR spectroscopy of compound P3 (600
MHz, CDCl3, 298 K).
6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0
δ (ppm)
P3 can be defined as an important precursor of self-included
[1]rotaxanes by adding a stopper at the end of the axle. In this
work, based on the self-included structure of compound P3, we
constructed a [1]rotaxane P4 directly via a nucleophilic
substitution from P3 and triphenylamine derivate. To study the
self-included structure of compound P4, 1H NMR of compounds
P4 (Fig. 3b) and M5 (Fig. 3a)were compared in CDCl3, and we
can see that some aliphatic protons of the axle and some
aromatic protons of triphenylamine shifted to higher magnetic
field proving that some protons of the axle were shielded by the
cavity of pillar[5]arene, hence the compound P4 can be
described as an interlocked molecule in CDCl3 (Fig. 3). From the
1
Fig. 4 2D H-1H COSY NMR spectroscopy of compound P4 (600
MHz, CD3OD-d4, 298 K).
Generally, the increase of the polarity of solvent reduces the
host-guest interactions. To further explore the stability of the
self-interlocked structure of P4, P4 was dissolved
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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and shifted to an extended form (Fig. 6 and Fig. S27). Then
CDCl , CD OD and DMSO (respective increased pVoielawrAirttyic)lewOnelirnee
DOI: 10.1039/D0NJ01859G
3
3
used to study the shift effect of P4. The compound P4 was
dissolved in these three solvents to see the modification of the
chemical shifts by 1H NMR studies (Fig. 7). From the spectra of
compound P4, we can see that the aromatic protons close to
the pyridinium moiety shifted to lower magnetic field by
increasing the polarity of solvent. Furthermore, proton H9 and
some protons of the axle shifted to higher magnetic field in
CD3OD and then shifted to lower magnetic field in DMSO. All of
these phenomena indicated that P4 shifted along the axle with
the increase of polarity of solvent until reaching an extended
state in DMSO, which led us to conclude to the successful
preparation of a new dynamic molecular machine. The self-
inclusion is entropically more favorable but the disruption of
the cation-π interactions extended the structure. In fact, the
solvent responsive feature is dependent on the polarity of the
solvent, which can modulate the cation-π interactions between
the cavity of the macrocycle and the pyridinium moiety. The
lower polarity of CDCl3 promotes the cation-π interactions and
the localization of the pyridinium moiety inside the cavity. The
increase of polarity disrupts this interaction leading to a
progressive dethreading of the macrocycle from the axle into an
extended form once we used DMSO.
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
5
6
7
8
ArCH2Ar
& ArOCH3
O
O
Br
N
O
8
O
N
N
1 2 3O 4 5 6 7
N
N
9
O
9
O
O
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O
O
ArH
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4
δ (ppm)
Fig. 5 2D 1H-1H NOESY NMR spectroscopy of compound P4 (600
MHz, CD3OD-d4, 298 K).
4)
3)
DMSO
2)
1)
8
7
6&1 5&2
4
3
9
CD3OD
9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5
δ (ppm)
CDCl
3
Fig. 6 1H NMR spectra comparison of compound P4 by
increasing the polarity of solvents: 1) CDCl3; 2)
CDCl3/DMSO=3/1; 3) CDCl3/DMSO=1/1; 4) DMSO.
9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5
δ (ppm)
O
O
O
O
O
O
in different solvents with different polarities (CDCl3,
CDCl3/DMSO=3/1, CDCl3/DMSO=1/1, DMSO), and from the
spectra of compound P4 in four different solvents (Fig. 6), it can
be seen that with the increase of polarity of the mixed solvents,
the aromatic protons close to the pyridinium cation shifted to
lower magnetic field, and proton H9 was at higher magnetic
field in VCDCl3/VDMSO = 3/1 and then moved to lower magnetic
field in VCDCl3/VDMSO = 1/1. Meanwhile, part of aliphatic protons
including H3 and H4 belonging to the axle were at high magnetic
field firstly and then moved to low magnetic field by increasing
the ratio of DMSO which indicated that the shield effect of the
pyridinium moiety by aromatic protons decreased by increasing
the polarity of solvent, and that pillar[5]arene shifted along the
axle with the increase of polarity. Furthermore, when P4 was
dissolved in pure DMSO, no proton was found below 0 ppm, as
well as the comparison between the 1H NMR spectra of P4 and
M5, where the aliphatic protons have almost the same chemical
shifts, proved that the self-interlocked structure was destroyed
O
O
O
O
N
N
N
N
N
N
N
O
O
N
O
O
O
O
N
O
O
O
O
N
State I
State II
O
O
O
O
O
O
N
N
N
N
N
O
O
O
O
State III
Fig. 7 1H NMR spectra comparison of compound P4 by
increasing the polarity of solvents: these NMR comparisons
helped us to propose different possible conformations for
4 | J. Name., 2012, 00, 1-3
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compound P4: CDCl3 (State I), CD3OD (State II) and DMSO (State 5.
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0
CH2Cl2
9
THF
CHCl3
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CH3CN
DMF
CH3OH
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DMSO
500
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Wavelength (nm)
Fig. 8 Fluorescence spectroscopy of compound P4 in different
solvents.
As triphenylamine is a fluorescent molecule with aggregation
induced enhanced emission effect, we performed fluorescent
spectroscopy studies of compound P4 by dissolving it in
different solvents (CH2Cl2, THF, CHCl3, CH3CN, DMF, CH3OH,
DMSO). From Fig. 8, we can see that the emission peak of
triphenylamine derivate is around 550 nm and the fluorescent
intensity decreased with the increase of polarity of solvent; it
can be noticed that the host-guest interactions were enhanced
in CDCl3 coupled with a reduction of intramolecular vibrations
17.
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leading to
a fluorescent enhancement. The host-guest
interactions which were weakened by the increase of the
polarity of solvent and intramolecular vibrations led to the
decrease of the fluorescence intensity. These results suggested
that this molecular machine is responsive to the polarity of
solvents.
Conclusions
In summary, we synthesized a new pillar[5]arene-based
molecular machine. This molecular machine can undergo into
different self-interlocked structures in CDCl3 and CD3OD, and in
an extended form in DMSO. This new [1]rotaxane solvent-
responsive molecular machine was made with a new synthetic
route. This machine will be useful for the construction of new
stimuli responsive smart materials with applications in biology
and material science.
27.
28.
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30.
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