Cyclodextrin-Based [c2]Daisy Chain Rotaxane Insulating Two Diarylacetylene Cores

Article abstract of DOI:10.1002/chem.202004505

A [c2]daisy chain rotaxane with two diarylacetylene cores was efficiently synthesized in 53 % yield by capping a C₂-symmetric pseudo[2]rotaxane composed of two diarylacetylene-substituted permethylated α-cyclodextrins (PM α-CDs) with aniline stoppers. The maximum absorption wavelength of the [c2]daisy chain rotaxane remained almost unchanged in various solvents, unlike that of the stoppered monomer, indicating that the two independent diarylacetylene cores were insulated from the external environment by the PM α-CDs. Furthermore, the [c2]daisy chain rotaxane exhibited fluorescence emission derived from both diarylacetylene monomers and the excimer, which implies that the [c2]daisy chain structure can undergo contraction and extension. This is the first demonstration of a system in which excimer formation between two π-conjugated molecules within an isolated space can be controlled by the unique motion of a [c2]daisy chain rotaxane.

Full text of DOI:10.1002/chem.202004505

Accepted Article
Accepted Article
Title: Cyclodextrin-Based [c2]Daisy Chain Rotaxane Insulating Two
Diarylacetylene Cores
Authors: Susumu Tsuda, Yoshitsugu Komai, Shin-ichi Fujiwara, and
Yutaka Nishiyama
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.202004505
Link to VoR: https://doi.org/10.1002/chem.202004505
01/2020

10.1002/chem.202004505
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Cyclodextrin-Based [c2]Daisy Chain Rotaxane Insulating Two
Diarylacetylene Cores
Susumu Tsuda,*[a] Yoshitsugu Komai,[b] Shin-ichi Fujiwara,[a] and Yutaka Nishiyama*[b]
[a]
Dr. S. Tsuda, Prof. Dr. S. Fujiwara
Department of Chemistry
Osaka Dental University
Hirakata, Osaka 573-1121 (Japan)
E-mail: tsuda-s@cc.osaka-dent.ac.jp
Y. Komai, Prof. Dr. Y. Nishiyama
Faculty of Chemistry, Materials and Bioengineering
Kansai University
[b]
Suita, Osaka 564-8680 (Japan)
E-mail: nishiya@kansai-u.ac.jp
Supporting information for this article is given via a link at the end of the document.
Abstract: A [c2]daisy chain rotaxane with two diarylacetylene cores
was efficiently synthesized in 53% yield by capping a C2-symmetric
pseudo[2]rotaxane composed of two diarylacetylene-substituted
permethylated -cyclodextrins (PM -CDs) with aniline stoppers. The
maximum absorption wavelength of the [c2]daisy chain rotaxane
remained almost unchanged in various solvents, unlike that of the
stoppered monomer, indicating that the two independent
diarylacetylene cores were insulated from the external environment
by the PM -CDs. Furthermore, the [c2]daisy chain rotaxane exhibited
fluorescence emission derived from both diarylacetylene monomers
and the excimer, which implies that the [c2]daisy chain structure can
undergo contraction and extension. This is the first demonstration of
a system in which excimer formation between two -conjugated
molecules within an isolated space can be controlled by the unique
motion of a [c2]daisy chain rotaxane.
threading each axle through the other ring to form a
pseudo[2]rotaxane that is mechanically interlocked by capping
with bulky terminal groups (Figure 1b). This strategy is efficient for
symmetric pseudo[2]rotaxane formation because the host–guest
interactions between the two subunits are doubled, as each unit
includes both host (ring) and guest (axle) moieties. Kaneda’s
group synthesized a C2-symmetric [2]rotaxane (which they called
a Janus [2]rotaxane) in high yield using lipophilic azobenzene-
substituted permethylated -cyclodextrin (PM -CD) derivatives
that were also soluble in aqueous solutions.[6a] Therefore, this
strategy is suitable for creating a supramolecular architecture that
can insulate two identical -conjugated hydrocarbon cores.[9]
Herein, we prepared a [c2]daisy chain rotaxane insulating two
diarylacetylene cores using a PM -CD bearing a diarylacetylene
moiety. Furthermore, the physical properties of the obtained
rotaxane were investigated using UV-Vis absorption and
fluorescence measurements.
-Conjugated hydrocarbons have attractive optical and electronic
properties that can be exploited for the development of light-
emitting and electronic devices.[1] Several -conjugated
hydrocarbons (e.g., pyrene, perylene, and diarylacetylene
derivatives) form excimers in solution and in the solid state.[2–4]
The emission of such excimers is often shifted to a longer
wavelength than the intrinsic monomer emission owing to the H-
and J-aggregation of multiple -conjugated hydrocarbon
molecules. Insulating the two identical molecules involved in
excimer formation from other molecules and/or the external
environment allows the essential properties of the excimer to be
investigated in detail, which will lead to the development of new
functional molecules.
Figure 1. Synthetic strategies for rotaxanes insulating two -conjugated
In pioneering studies by Inouye and colleagues, two perylene
(or pyrene) cores were successfully insulated in the cavities of two
-cyclodextrins (-CDs), cyclic oligosaccharides consisting of
molecular cores.
eight -D-glucopyranoses, using a simple rotaxane strategy
(Figure 1a). The resulting spatially restricted excimers were found
to emit strong circularly polarized luminescence.[5] However, the
desired [4]rotaxane was obtained in very low yield. Therefore, a
new strategy is required for the efficient production of functional
molecules that can generate spatially restricted excimers.
Several research groups have previously developed synthetic
methods for [c2]daisy chain rotaxanes,[6–8] wherein two subunits
bearing both axle and ring components are associated by
Scheme 1. Synthesis of monomer 1.
1
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Monomer 1 with a PM -CD moiety as a ring component and a
diarylacetylene moiety as an axle component was prepared from
PM -CD monotosylate[10] in 75% yield (3 steps), as shown in
Scheme 1. Using monomer 1, the formation of pseudorotaxanes
chromatography. The symmetrical and dimeric structure of
rotaxane 2 was confirmed based on its 1H NMR spectrum
(Scheme 2e) and its ESI-HRMS spectrum, in which the peaks at
m/z 1182.8757 and 1762.8195 corresponded to sodium
adducts[12] (calculated m/z of [Mꞏ3Na]3+: 1182.8755; calculated
m/z of [Mꞏ2Na]2+: 1762.8186) (Figure S1). The peaks in the
aromatic region of the 1H NMR spectrum of rotaxane 2 were
assigned to protons A′′–E′′ and the N–H proton by 1H NMR and
2D NMR measurements. Notably, these assignments correspond
reasonably well with those estimated for the protons of the
pseudo[2]rotaxane.
Stoppered monomer 3 was also prepared via the condensation
reaction in dichloromethane. The assignments of the peaks in the
aromatic region of the 1H NMR spectrum of stoppered monomer
3 are shown in Scheme 2a. Protons C′′ and D′′ in rotaxane 2 are
shifted downfield (C′′–C′′′ +0.57 ppm and D′′–D′′′ +0.17 ppm)
relative to protons C′′′ and D′′′ in stoppered monomer 3 owing to
the deshielding effect of the glycosidic oxygens located inside the
CD cavity. In contrast, protons A′′ and B′′ in rotaxane 2 are shifted
upfield relative to protons A′′′ and B′′′ in stoppered monomer 3
(A′′–A′′′ −0.29 ppm and B′′–B′′′ −0.13 ppm) by the
paracyclophane-like proximity of the benzene rings bound directly
to the PM -CDs. These shifts indicate that the two
diarylacetylene cores in rotaxane 2 face each other in a symmetric
manner, as depicted in Scheme 2. The benzene rings connected
directly to the PM -CDs partially overlap, whereas the other
benzene rings are located within the PM -CD cavities.
was
investigated
using
solvent-dependent
1H
NMR
measurements. As shown in Scheme 2b, four doublets derived
from the diarylacetylene moiety of monomer 1 were observed in
the aromatic region of the 1H NMR spectrum in CDCl3. Host–guest
interactions between monomer units are generally weakened in
chloroform. Therefore, the doublets were attributed to
nonassociated monomer 1,[11] which is assumed to be the only
species in chloroform. The doublets were assigned to aromatic
protons A–D of nonassociated monomer
1 by 2D NMR
measurements. In CD3OD, in addition to the peaks of the
nonassociated species, small broad peaks corresponding to an
associated species appeared at 8.00, 7.45, 7.02, and 6.69 ppm
(Scheme 2c). Furthermore, in CD3OD/D2O (2/1), the peaks of
nonassociated monomer 1 almost disappeared while four new
broad doublets were clearly observed (Scheme 2d). These broad
doublets were attributed to protons A′–D′ of diarylacetylene
insulated in a pseudo[2]rotaxane by comparison with the NMR
data for previously reported [c2]daisy chain rotaxanes.[6a,b]
The structure of the pseudo[2]rotaxane was then fixed by
capping with a water-soluble dimethylaniline derivative via a
condensation
reaction
with
1-ethyl-3-(3-
dimethylaminopropyl)carbodiimide hydrochloride (EDCꞏHCl) in
CH3OH/H2O (2/1). Desired [c2]daisy chain rotaxane 2 was
obtained in 53% yield after purification by silica gel column
OR
A''' B'''
C''' D'''
E'''
OR
O
H2N
O
O
N
S
B'''C'''
PM -CD
R = (CH2CH2O)3H
EDCꞏHCl, DMAP
a)
H
E'''
3 in CDCl3
A''' D'''
A D
N-H
CH2Cl2
below 2 °C, 15 min
and then rt, 19 h
stoppered monomer 3
S
(OMe)17
b)
46%
B C
1 in CDCl3
C'
D'
A
B
C D
O
O
COOH
A' B'
S
c)
O
O
COOH
1 in CD3OD
PM -CD
PM -CD
MeOH/H2O (2/1)
PM -CD
HOOC
O
O
(OMe)17
d)
(OMe)17
1
in CD3OD/D2O (2/1)
(OMe)17
S
D'
C'
B'
A'
monomer 1
pseudo[2]rotaxane
C'' D''
OR
S
B''
E''
A''
e)
EDCꞏHCl
N-H
D''
H2N
OR
E''
2 in CDCl3
O
A''
B''
O
O
MeOH/H2O (2/1)
C''
N
PM -CD
below 2 °C, 2 h
and then rt, 4 h
H
N
H
PM -CD
O
O
53%
(OMe)17
O
RO
R = (CH2CH2O)3H
(OMe)17
 (ppm)
[c2]daisy chain rotaxane 2
Scheme 2. Partial 1H NMR spectra[a] of a) stoppered monomer 3 in CDCl3 at rt, b) monomer 1 in CDCl3 at rt, c) monomer 1 in CD3OD at rt, d) monomer 1 in
CD3OD/D2O (2/1) at rt, and e) rotaxane 2 in CDCl3 at rt, and syntheses of [c2]daisy chain rotaxane 2 and stoppered monomer 3. [a] The chloroform peak is denoted
by S in the 1H NMR spectra.
2
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As shown in Figure 2, stoppered monomer 3 exhibited only
monomer emission in various solvents at a concentration of 1.9 ×
10−6 M. In contrast, rotaxane 2 showed both monomer and
excimer emission at a much lower concentration (2.6 × 10−7 M).
As there is an alkyl spacer between the diarylacetylene unit and
the aniline stopper, the distance between the two diarylacetylene
cores in rotaxane 2 can be varied by contraction or extension of
the molecular length along the long axis (Figure 2a). This is a
characteristic behavior of [c2]daisy chain rotaxane, also known as
the molecular muscle.[6–8,13] When the two diarylacetylene cores
are in close proximity, an excimer can form, resulting in excimer
emission; otherwise, monomer emission occurs. These results
imply that contraction and extension of the [c2]daisy chain
structure of rotaxane 2 occurs in solution by shuttling the PM -
CDs along the axle components. Unfortunately, multiple chemical
Table 1. Maximum absorption peaks (max) of rotaxane 2 and stoppered
monomer 3.[a]
Stoppered monomer 3
Solvent
Rotaxane 2 [nm]
[nm]
Chloroform
Tetrahydrofuran
Methanol
297, 316
296, 315
296, 315
296, 315
297, 316
296, 315
295, 314
293, 311
294, 313
297, 316
Acetonitrile
Dimethyl sulfoxide
1
species with different conformations were not observed by H
[a] Conditions: [2] = 1.7 × 10−5 M, [3] = 3.4 × 10−5 M.
NMR spectroscopy owing to the fast shuttling of the subunits in
rotaxane 2 on the NMR time scale.
In summary, we efficiently synthesized a [c2]daisy chain
rotaxane insulating two diarylacetylene cores using the Janus
[2]rotaxane strategy. The obtained rotaxane showed both excimer
and monomer emission in various solvents, which implied that fast
shuttling of the rotaxane subunits occurred in solution. This work
demonstrates for the first time that excimer generation can be
controlled within an isolated space by the unique structural motion
of the [c2]daisy chain rotaxane. Thus, the [c2]daisy chain
rotaxane strategy is a powerful tool for developing optical and
electronic materials based on spatially restricted excimers of -
conjugated hydrocarbons. It is expected that switchable optical
and electronic materials will be developed based on [c2]daisy
chain rotaxane units. In our future study, we will investigate the
motion of rotaxane and its effect on the optical properties.
Acknowledgements
This work was supported by the Iketani Science and Technology
Foundation (No. 0321005-A). The electrospray ionization mass
spectrometry measurements were performed at the Analytical
Instrument Facility, Graduate School of Science, Osaka
University. We thank Prof. Dr. Rui Umeda (Faculty of Chemistry,
Materials and Bioengineering, Kansai University) for discussions
of the absorption and fluorescence data.
Figure 2. Fluorescence spectra of (a) rotaxane 2 and (b) stoppered monomer
3. Conditions: [2] = 2.6 × 10−7 M, [3] = 1.9 × 10−6 M, excitation at the maximum
absorption wavelength in the range of 311–316 nm.
Conflict of interest
The absorption peaks (max) of rotaxane 2 and stoppered
monomer 3 in various solvents are listed in Table 1. For rotaxane
2, the absorption spectra in various solvents overlapped well
(Figure S2a), and the absorption maximum did not shift
significantly. In contrast, a shift of up to 5 nm was observed for
the absorption maximum of stoppered monomer 3 (Figure S2b).
Similar trends were also observed for the monomer emission of 2
and 3 (Figure 2). The shapes of the bands in the absorption
spectra of rotaxane 2 and stoppered monomer 3 were similar,
which suggests that the ground-state interaction between the two
diarylacetylenes in rotaxane 2 is weak. These results confirmed
that the two independent diarylacetylene cores of rotaxane 2 were
insulated from the external environment by the PM -CDs.
The authors declare no conflict of interest.
Keywords: [c2]daisy chain • cyclodextrins • diarylacetylene •
excimer • rotaxanes
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4
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Entry for the Table of Contents
A [c2]daisy chain rotaxane, in which two independent diarylacetylene cores are insulated by two permethylated -cyclodextrins (PM -
CDs), was efficiently synthesized using the Janus [2]rotaxane strategy. The observation of both monomer and excimer emission in
various solvents revealed that frequent shuttling of the rotaxane subunits occurs in solution.
5
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