Construction of type iii-c rotaxane-branched dendrimers and their anion-induced dimension modulation feature

Article abstract of DOI:10.1021/jacs.9b06739

Starting from a novel rotaxane building block with dendrimer growth sites being located at both the wheel and axle component, we realized the successful construction of a new family of rotaxane-branched dendrimers, i.e., Type III-C rotaxane-branched dendr

Full text of DOI:10.1021/jacs.9b06739

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Article
Construction of Type III-C Rotaxane-Branched Dendrimers
and Their Anion-Induced Dimension Modulation Feature
Xu-Qing Wang, Wei-Jian Li, Wei Wang, Jin Wen, Ying Zhang, Hongwei Tan, and Hai-Bo Yang
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 14 Aug 2019
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Construction of Type III-C Rotaxane-Branched Dendrimers and Their
Anion-Induced Dimension Modulation Feature
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Xu-Qing Wang†, Wei-Jian Li†, Wei Wang†*, Jin Wen§, Ying Zhang‡, Hongwei Tan‡, Hai-Bo Yang†*
†Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular
Engineering, Chang-Kung Chuang Institute, East China Normal University, 3663 N. Zhongshan Road, Shanghai
200062, P. R. China
§ Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 16610 Prague 6, Czech
Republic
‡ Department of Chemistry, Beijing Normal University, Beijing, 100050, P.R. China
ABSTRACT: Starting from a novel rotaxane building block with dendrimer growth sites being located at both the wheel and
axle component, we realized the successful construction of a new family of rotaxane-branched dendrimers, i.e. Type III-C
rotaxane-branched dendrimers up to fourth-generation as a highly branched [46]rotaxane through a controllable divergent
approach. In the resultant rotaxane-branched dendrimers, the wheel components of the rotaxane units are located on the
branches as well as at the branching points, making them excellent candidates to mimic the amplified collective molecular
motions. Thus, taking advantage of the urea moiety inserted into the axle components of the rotaxane units as the binding
sites, the addition or removal of acetate anion as stimulus endows the individual rotaxane unit switchable feature that lead
to a collective expansion-contraction motion of the integrated rotaxane-branched dendrimers, thus allowing for the
remarkable and reversible size modulation. Such three-dimensional size switching feature makes Type III-C rotaxane-
branched dendrimers a very promising platform towards the fabrication of novel dynamic smart materials.
INTRODUCTION
Inspired by biomolecular machines in living systems that
perform vital biological functions such as synthesis,
replication, motion, and transport etc.,1 a great number of
artificial molecular machines2 such as molecular shuttle,3
molecular muscle,4 molecular motor,5 molecular assembler6
etc., have been constructed by chemists. As an intriguing
feature of biomolecular machines, the remarkable
amplification of collective molecular motions of functional
nanomechanical moieties with specific arrangements leads
to the essential biological functions as outputs.7 Taking the
units were investigated in the most cases, while the
construction of novel integrated systems with two-
dimensional (2-D) or even three-dimensional (3-D)
arrangements of artificial molecular machines units have
been rarely explored, which might be caused by the lack of
facile and efficient synthetic approaches that enable the
precise arrangements of multiple mechanically interlocked
moieties.
macroscopic motion of muscles as
a representative
example, such process originates from the coordinative
movements of sarcomeres in a cooperative manner.8 In
order to mimic such biological process, the integration of
multiple artificial molecular machine units with a delicate
arrangement has attracted more and more attentions
towards the construction of novel dynamic bio-inspired
materials. For instance, by using the rotaxane-based
molecular muscles9 (especially [c2]daisy chain rotaxanes10)
Type III-A
Type III-B
Type III-C
Fig. 1 Cartoon presentation of Type III rotaxane-branched
dendrimers: Type III-A, III-B, and III-C, respectively.
as key building blocks,
a
series of muscle-like
Considering the highly branched and star-shaped
structure feature as well as the nanometer-scale
dimensions of dendrimers,12 the introduction of rotaxane
units, especially ones with the controllable motion
behaviors,
supramolecular polymers have been constructed, whose
macroscopic properties could be modulated by the
controllable motions of the molecular muscle units at
nanoscale.11 However, in these reports, only one-
dimensional (1-D) linear arrangements of nanomechanical
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O
O
O
O
H3CO
H3CO
O
O
H3CO
H3CO
O
O
O
CH2
PEt3
Pt
CH2
PEt3
Pt
O
O
N
H
N
H
N
N
H
H
PEt3
PEt3
R
R
O
O
5
R
R
O
CH3
6
7
R =
O
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+ stimulus
- stimulus
Contracted Type III-C rotaxane-
branched dendrimer
Stretched Type III-C rotaxane-
branched dendrimer
Fig. 2 Cartoon representation of the dimensional modulation of Type III-C rotaxane-branched dendrimer upon the addition or
removal of external stimuli.
into the branches of dendrimers give rise to rotaxane-
branched dendrimers with 3-D monodispersed
via a controllable divergent approach. Notably, with the
addition of acetate anion as external stimulus, the
translational motion of the macrocycle components along
the branch was realized. More importantly, due to the
existence of rotaxane moieties both on the branches and at
the branching points, the anion-triggered switchable
feature of the individual rotaxane branches endows the
arrangements of rotaxane units within the dendritic
skeleton.13 The unique collective amplification of molecular
motions of each branch might make the resultant rotaxane-
branched dendrimers a very promising platform towards
the exploration of the amplified collective molecular
motions and their applications in the construction of
artificial molecular machines and smart materials. It should
be noted that, in the original definition by Kim et al.,14a only
two types of rotaxane-branched dendrimers, i.e. Type III-A
and III-B ones, were proposed depending on the position of
wheel components either on the branches or at the
branching points, respectively. As the last piece of the
‘rotaxane-branched dendrimer puzzle’, Type III-C rotaxane-
branched dendrimers with the combined structural
features of both III-A and III-B ones, i.e. the wheel
components are located on the branches as well as at the
branching points, have been only proposed by Stoddart15 in
2016 and the construction of high-generation Type III-C
rotaxane-branched dendrimers has not been realized yet
(Fig. 1). Notably, compared with Type III-A or III-B ones, the
relative movement of the wheel and axle components in
Type III-C rotaxane-branched dendrimers might lead to the
remarkable changes of the length of each branch in a direct
way. Furthermore, such collected contraction-extension
motion of each branch under the external stimuli will
probably result in the remarkable size and
microenvironment modulation of the integrated rotaxane-
branched dendrimers.
integrated rotaxane-branched dendrimers
a
three-
dimensional switching feature via a collective extension-
contraction motion, thus allowing for the remarkable and
reversible size modulation (Fig. 2).
RESULTS AND DISCUSSION
Synthesis of the switchable [2]rotaxane 3 as key
precursor and its anion-switching behavior. To construct
Type III-C rotaxane-branched dendrimers, according to the
retrosynthetic analysis, a new [2]rotaxane building block
with dendrimer growth sites being located at both the
wheel and axle component is needed. Following such design
strategy, two protected alkynes that can be gently exposed
to the alkyne group for the subsequent dendrimer growth
were attached at the periphery of the pillar[5]arene (P[5])
macrocycle,17 thus leading to the new wheel component 1
(Scheme S1). In addition,
a semi-blocked rod-like
compound 2 was further prepared as the axle precursor
through a multistep synthetic route (Scheme S2), where the
urea moiety was inserted into the axle component as a
binding site. By employing threading-followed-by-
stoppering strategy, the reaction of 1, 2, and Pt(PEt3)2I2 in a
ratio of 6:1:4 in CHCl3/i-Pr2NH (v/v, 2:1) in the presence of
CuI as
Based on our on-going interests on rotaxane-branched
dendrimers,16 herein, we report the first successful
synthesis of dynamic Type III-C rotaxane-branched
dendrimers up to fourth-generation with 45 switchable
[2]rotaxane units dispersing within the dendrimer skeleton
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catalyst resulted in the successful preparation of
TIPS
O
O
H3CO
1
2
3
4
5
6
7
8
organometallic [2]rotaxane 3 as the basic building block in
a good yield (70%) on gram scale (Fig. 3). Notably, in
[2]rotaxane 3, the mono-substituted platinum-acetylide
moiety not only served as the stopper to prevent
dethreading, but also acted as the reactive sites for the
subsequent dendrimer growth. The structure of
[2]rotaxane 3 was fully characterized by 1-D multinuclear
(1H, 13C, and 31P), 2-D NMR (1H-1H COSY, ROESY)
spectroscopy, and MALDI-TOF-MS analysis (Figs. S1-S6).
O
3
3
PEt3
Pt
PEt3
O
O
O
O
+
I
I
H2
C
+
N
N
H
H
H3CO
TIPS
4
O
1
2
CuI, CHCl3, i-Pr2NH
r.t., 8 h, 77%
O
H3CO
H3CO
O
O
9
11
5
7
9
CH2
PEt3
Pt
PEt3
O
In organometallic [2]rotaxane 3, the P[5] ring preferred
being located at the urea and adjacent methylene moieties
due to the stronger hydrogen bonding interactions between
methoxy group of P[5] macrocycle and the urea moiety.
Considering the ability of urea moiety to serve as hydrogen
bonding donor, the addition of hydrogen bonding acceptor,
which could compete with P[5] macrocycle to complex with
the urea moiety, into the rotaxane system would induce the
translational motion of P[5] along the axle from urea to the
neutral alkyl chain. To test such hypothesis, acetate anion
was selected as the stimulus (Fig. 3). To our delight, by
sequentially adding tetrabutylammonium acetate (TBAA)
from 1.0 equiv to 5.0 equiv into the solution of [2]rotaxane
N
N
I
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13
10
R
12
4
6
8
H
H
O
14 15
R
O
[2]rotaxane 3
NaPF6
Acetate Anion
O
O
H3CO
H3CO
O
O
CH2
PEt3
Pt
PEt3
O
N
H
N
H
I
R
O
O
R
O
CH3
R =
O
TIPS
3
in tetrahydrofuran-d8 (THF-d8), 1H NMR titration
experiments revealed that the signals of the protons H14 and
H15 on the urea moiety as well as the protons nearby (H10-
13) displayed the obvious downfield shifts. While the
Fig. 3 Synthesis of [2]rotaxane 3 starting from compounds 1, 2
and Pt(PEt3)2I2, and the anion-induced switching motion of
[2]rotaxane 3.
H3CO
OCH3
(a)
O
R
O
O
4
H2
O
C
HN
HN
O
R
O
O
H3CO
H3CO
O
O
CH2
PEt3
Pt
PEt3
O
N
N
I
H
H
O
R
Et3
P Pt PEt3
R
O
CuI, Et2NH
r.t., 70%
[2]rotaxane 3
+
Et3
P
PEt3
Pt
Pt
PEt3
Et3
P
R
O
O
O
O
N
NH
4
CH2
H
NH
NH
O
R
O
O
R
O
H2
C
4
O
R
H3CO
O
O
O
OCH3
OCH3
[4]rotaxane G1
OCH3
(b)
G1
I
II
III
[10]rotaxane G2
[22]rotaxane G3
[46]rotaxane G4
Fig. 4 (a) Synthesis of Type III-C rotaxane-branched dendrimer G1 by a CuI-catalyzed coupling reaction of 3 and 1,3,5-
triethynylbenzene; (b) Schematic representation of a controllable divergent strategy for the synthesis of Type III-C rotaxane-
branched dendrimers G2-G4. Reaction conditions: (I) (a) Bu4NF·3H2O, THF, r.t., 2h; (b) 3, CuI, Et2NH, r.t., 8h. 52%; (II) (a)
Bu4NF·3H2O, THF, r.t., 2h; (b) 3, CuI, Et2NH, r.t., 8h. 42%; (III) (a) Bu4NF·3H2O, THF, r.t., 2h; (b) 3, CuI, Et2NH, r.t., 8h, 29%.
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peaks of protons (H4-9) attributed to the neutral alkyl chain
Page 4 of 9
The identity of these Type III-C rotaxane-branched
dendrimers Gn (n = 1-4) was firstly determined by 1-D NMR
measurements. In the 1H NMR spectra of G1-G4, no proton
signal ascribed to the free terminal acetylenes was
observed, indicating the full conversion of the terminal
acetylenes towards the formation of platinum–acetylide
bonds18 during the dendrimer growth process. In addition,
the peaks below 0.0 ppm that are attributed to the protons
on the axle of the rotaxane units remained, suggesting that
the rotaxane units were intact during the synthetic
processes. In the case of 31P NMR analysis, compared with
the building block [2]rotaxane 3, G1 displayed a remarkable
downfield shift from 8.91 to 11.64 ppm, which directly
supported the formation of platinum-acetylide bonds as the
dendrimer growth step. Similarly, the higher-generation
G2-G4 also featured a signal peak at 11.68 ppm (G2), 11.86
ppm (G3), and 11.66 ppm (G4), respectively, revealing the
high symmetry feature of the resultant Type III-C rotaxane-
branched dendrimers (Figs. S14-S27). The matrix-assisted
1
2
3
4
5
6
7
8
exhibited the remarkable upfield shifts. These observations
suggested the acetate anions replaced the P[5] ring to bind
with urea moiety, which thus allowed the macrocycle
component moving from the urea moiety to neutral alkyl
chain. The binding constant (K) of [2]rotaxane 3 for TBAA
was sequentially calculated to be (6.41 ± 0.08) ×103 M-1.
Subsequently, upon adding 10.0 equiv. of NaPF6 into the
mixture of [2]rotaxane 3 and TBAA, which could completely
remove acetate anion as NaOAc precipitate, the resultant 1H
NMR spectrum was almost the same as the original
spectrum of the[2]rotaxane 3, thus suggesting the
movement of the macrocycle back to the urea moiety (Figs.
S7-S12). Therefore, the anion-triggered translational
motion of P[5] ring along the axle in [2]rotaxane 3 was
achieved in a reversible manner through the addition and
removal of acetate anion, thus allowing for a switchable
rotaxane system. Moreover, the theoretical calculation also
clearly indicated that the wheel component underwent the
translational motion from urea moiety to the neutral alkyl
chain upon the binding between urea moiety and acetate
anion (Fig. S13). All these results confirmed the anion-
induced switchable molecular motion of [2]rotaxane 3, thus
laying the foundation of the construction of stimuli-
responsive Type III-C rotaxane-branched dendrimers by
employing [2]rotaxane 3 as the key building block.
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laser
desorption/ionization-time
of
flight
mass
spectrometry (MALDI-TOF-MS) and gel permeation
chromatography (GPC) experiments further confirmed the
successful synthesis of the targeted Type III-C rotaxane-
branched dendrimers. In the MALDI-TOF-MS spectrum of
G1 (Fig. 5a), peaks of m/z = 6,952.7 and m/z = 3,476.5 were
found, which agreed with the theoretical values of [G1 +
Na]+ ion (m/z = 6,953.5) and [G1 + H + Na]2+ ion (m/z =
3,477.2), respectively. In the case of G2, the peaks of m/z =
19,592.5 and m/z = 9,809.0 were observed, which fitted
well with the theoretical values of [G2 + Na]+
Synthesis and characterization of Type III-C
rotaxane-branched dendrimers. Starting from the
switchable [2]rotaxane 3, a controllable divergent strategy
was employed to construct the targeted Type III-C
rotaxane-branched dendrimers. Fig. 4 outlines the detailed
synthetic route. Firstly, by utilizing 1,3,5-triethynylbenzene
as the scaffold core and CuI as catalyst, the coupling reaction
of [2]rotaxane
3
with 1,3,5-triethynylbenzene in
diethylamine (Et2NH) resulted in the formation of first-
generation Type III-C rotaxane-branched dendrimer (G1) in
70% yield, which contained three rotaxane units in the
dendrimer skeleton. Subsequently, the deprotection of G1
with tetrabutylammonium fluoride trihydrate (Bu4NF ·
3H2O) allowed for the generation of the intermediate G1-
YNE with six alkyne groups at the periphery of the P[5] ring.
The sequential employment of coupling reaction of G1-YNE
with [2]rotaxane 3 in the presence of CuI as catalyst in
Et2NH generated the second-generation Type III-C
rotaxane-branched dendrimer (G2) with nine rotaxane
units dispersed in the dendrimer skeleton in a yield of 52%.
By repeating such iterative deprotection-coupling
reactions, the construction of the third-generation and
fourth-generation
Type
III-C
rotaxane-branched
dendrimers (G3 and G4) were achieved through a divergent
approach (Scheme S3-S6). All the obtained Type III-C
rotaxane-branched dendrimers were neutral and soluble in
common organic solvents, which made the purification
process facile and efficient through the combination of flash
column chromatography and preparative gel permeation
chromatography (GPC). Notably, for the resultant G4 with a
molecular weight as high as 95,371 Da., forty-five individual
switchable rotaxane units were distributed in the
dendrimer skeleton in a monodispersed manner to afford a
highly branched [46]rotaxane system.
Fig. 5 MALDI-TOF-MS spectra of (a) G1 and (b) G2. GPC (c) and
DLS (d) spectra of Type III-C rotaxane-branched dendrimers
G1-G4.
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ion (m/z = 19,592.7) and [G2 + 2Na]2+ ion (m/z = 9,807.9)
(a)
(b)
1
(Fig. 5b), respectively. For the higher-generation G3 and G4,
because of their high molecular weight and low
ionizationefficiency, we didn’t obtain the satisfied data even
after continuous attempts. Thus GPC experiments were
performed to confirm the formation of G3 and G4. In the
GPC spectra (Figs. 5c and S28-S31), all rotaxane-branched
dendrimers G1-G4 displayed a single peak and narrow
distributions for the number-averaged molecular weight
(Mn) and the polydispersity index (PDI) (for G1, PDI = 1.02;
for G2, PDI = 1.04; for G3, PDI = 1.03; for G4, PDI = 1.04),
indicating the monodispersity of G1-G4. In addition, with
the increase of the generation, the Mn values significantly
increased (for G1, Mn = 6,614; for G2, Mn = 14,465; for G3,
Mn = 29,997; for G4, Mn = 46,548). Such changes are in
accord with the expected trend, which indirectly proved the
successful synthesis of G3 and G4. Notably, the absolute
molecular weight of G4 was determined as 105,800 Da. (Fig.
S31), which was quite close to the Mw value (95,371 Da.),
thereby again supporting the successful synthesis of the
[46]rotaxane-branched dendrimer G4.
2
3
4
5
6
7
8
9
(d)
(c)
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(e)
(f)
Moreover, the 2-D diffusion-ordered spectroscopy
(DOSY) spectra of all Type III-C rotaxane-branched
dendrimers, especially for G3 and G4, revealed one set of
signals, respectively, indicating the monodispersity of G1-
G4. In addition, when compared with the precursor
[2]rotaxane 3, the diffusion coefficient (D) decreased
significantly from (14.13± 0.14) × 10-10 m2s-1 (3) to (10.96±
0.24)× 10-10 m2s-1 (G1), (5.13±0.12) × 10-10 m2s-1 (G2), (2.75
±0.16) × 10-10 m2s-1 (G3), and (1.66±0.15) × 10-10 m2s-1 (G4),
respectively. Considering the fact that diffusion coefficient
is inversely proportional to the hydrodynamic size, such
progressive increase of the hydrodynamic size along with
the increase of the dendrimer generation provided the
additional support for the existence of Type III-C rotaxane-
branched dendrimers G1-G4 (Figs. S32-S36). Dynamic light
scattering (DLS) is also a useful technique to determine the
dimension of rotaxane-branched dendrimers in solution.
The hydrodynamic size (Dh) of G1 was determined as 2.11
± 0.10 nm, and a size progression was clearly observed with
the increase of dendrimer generation (3.67 ± 0.14 nm for
G2; 5.36 ± 0.08 nm for G3; 7.12 ± 0.10 nm for G4) (Figs. 5d
and S37-S40).
(g)
(h)
Fig.
6
AFM images of Type III-C rotaxane-branched
dendrimers. (a) G1; (b) the height range of G1 is 1.75 ± 0.15
nm; (c) G2; (d) the height range of G2 is 2.45 ± 0.20 nm; (e) G3;
(f) the height range of G3 is 3.30 ± 0.20 nm; (g) G4; (h) the
height range of G4 is 4.80 ± 0.25 nm.
successful synthesis of the targeted Type III-C rotaxane-
branched dendrimers, their stimuli-responsive properties
were then evaluated (Scheme S7-S10). Same as the
switchable building block [2]rotaxane 3, acetate anion was
1
selected as the stimulus. H NMR titration experiments of
G1-G4 were recorded in THF-d8 at 298K. It was found that,
upon the addition of TBAA, the obvious chemical shifts of
the protons (H14, H15) on the urea moieties as well as the
protons (H4-13) on the axle were observed, which displayed
the similar anion-induced switching behaviors with the
individual [2]rotaxane 3, thus supporting the feasibility of
anion-induced collective switching motion of the individual
Transmission electron microscopy (TEM) and atomic
force microscopy (AFM) analysis were then employed to
visualize the morphologies of the resultant Type III-C
rotaxane-branched dendrimers G1-G4. From the TEM
analysis (Figs. S41-S44), the discrete and uniform near-
spherical morphologies were observed for G1-G4,
respectively. And along with the increase of the generation,
the size of G1-G4 were determined to be 1.70 ± 0.20 nm,
2.65 ± 0.22 nm, 3.60 ± 0.30 nm, and 4.65 ± 0.30,
respectively. Moreover, spherical particles were also
observed in the AFM images (Fig. 6). With the increase of
the generation, the average height gradually increased from
1.75 ± 0.15 nm (G1) to 2.45 ± 0.20 nm (G2), 3.30 ± 0.20 nm
(G3) and 4.80 ± 0.25 nm (G4), which is similar with the TEM
analysis.
1
rotaxane branches. As indicated by H NMR spectra (Figs.
S45-S48), for each urea moiety, 5.0 equiv. of TBAA was
needed to induce the translational motion of P[5]
macrocycle from urea moiety to the alkyl chain. Similarly,
NaPF6 was introduced to promote the macrocycle return
back to the original urea station, suggesting the reversible
translational motion of macrocycle along the axle on each
branch.
As above-mentioned, the addition or removal of acetate
anion could reversibly change the location of macrocycle
component on the branches. It should be noted that the
macrocycles are located both on the branches and at the
branching points, the movement of the wheel in the
Anion-induced dimension modulation of Type III-C
rotaxane-branched dendrimers. After confirming the
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upon the addition of TBAA, the increase of the averaged
height for each generation rotaxane-branched dendrimer
was observed (Figs. S65-S68), which demonstrated the
anion-induced swelling behaviors with the swelling ratios
of 20.0% for G1, 24.4% for G2, 30.0% for G3, and 34.8% for
G4, respectively. Through the combination of 2-D DOSY,
DLS and AFM analysis, two trends were clearly revealed: (1)
the addition or removal of acetate anion could reversibly
regulate the expansion and contraction of the integrated
Type III-C rotaxane-branched dendrimers; (2) along with
the increase of dendrimer generation, the swelling ratio
increased simultaneously, which might be attributed to the
amplification of the responsiveness through the integration
of multiple switchable rotaxane moieties within the
dendrimer skeleton.
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contraction of the integrated rotaxane-branched
dendrimers. In order to confirm this hypothesis, 2-D DOSY
experiments were carried out in THF-d8 at 298K, which
could provide the direct evidence for the size change of
Type III-C rotaxane-branched dendrimers. For instance, it
was found that the diffusion coefficient (D) remarkably
decreased from (10.96± 0.24) × 10-10 m2 s-1 to (8.91± 0.20)
× 10-10 m2 s-1 with the addition of TBAA (Fig. 7), suggesting
the increase of the hydrodynamic size of G1. Sequentially,
the introduction of NaPF6 into the resultant mixture
resulted in the increase of diffusion coefficient (D = (10.23±
0.25) × 10-10 m2 s-1), which almost returned back to the
original value, thus demonstrating the reversibility of such
size modulation processes. For the higher-generation
rotaxane-branched dendrimers G2, G3 and G4, the change
of D value showed the same tendency with G1. On the basis
of the Stokes-Einstein equation, the swelling ratio of
different Type III-C rotaxane-branched dendrimers was
determined to be 22.9% for G1, 28.8% for G2, 34.7% for G3,
and 38.3% for G4, respectively (Figs. S49-S56).
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In order to further confirm such amplified anion-induced
dimension modulation of Type III-C rotaxane-branched
dendrimers, a semi-empirical method PM6-D3H4 with the
dispersion corrections was used in the geometry
optimizations for G2 before and after the addition of TBAA
using the program MOPAC2016.19 To predict the surface
areas and volumes of different complexes, a series of probe
radii were carried out by 3V: Voss Volume Voxelator
program.20 For ideal spheres, the ratio of their lengths could
be presented as the ratios of the cube roots of their volumes.
When we assume the thicknesses of these rotaxane-
branched dendrimers are similar, their size difference is
calculated as the ratio of the square roots of their surface
areas or volumes instead of the ratio of the cube roots of
their volumes. If the probe radius is large enough, the ratios
of square roots of the surface areas should be close to those
of the volumes. As shown in Table S1-S2, the square root
of converges to 1.28 when the probe radius is 80 Å. We
found that the ratio of the square roots of the volumes of G2
is close to that of surface areas when the probe radius is
larger than 70 Å. This means that 70 Å as a probe radius is
large enough to compare the sizes of different rotaxane-
branched dendrimers. As a prediction by our computational
modeling, the sizes of G2 is increased by 28% after the
anion is included in the dendrimer, which is in accord with
the swelling ratio determined by 2-D DOSY and DLS
analysis, thus confirming the amplified anion-switched
dimension modulation of these novel Type III-C rotaxane-
branched dendrimers (Fig. 8).
Furthermore, DLS analysis was also employed to evaluate
the stimuli-induced size modulation of rotaxane-branched
dendrimers. As shown in Fig. 7, with the addition of TBAA,
the size of G1 increased from 2.11 ± 0.10 to 2.60 ± 0.15 nm
with a swelling ratio of 23.2%. Upon adding NaPF6 to
remove the acetate anions, the size almost went back to the
original value, which was in accord with the 2-D DOSY
analysis. Similarly, the swelling ratio of G2, G3 and G4 were
calculated to be 26.7%, 31.3%, and 36.7%, respectively
(Figs. S57-S64). In addition, according to the AFM analysis,
12
Gn
Gn+TBAA
Gn+TBAA+NaPF6
10
8
6
4
D
2
0
G1
G2
G3
G4
11
10
9
To gain more insight into such size modulation
phenomenon, the model dendrimer G1-a without urea
moieties was prepared from the corresponding model
[2]rotaxane 3-a via the same divergent approach (Scheme
S11). For both 3-a and G1-a, due to the absence of binding
Gn
Gn+TBAA
Gn+TBAA+NaPF6
8
7
1
site, the H NMR spectra showed no obvious changes with
6
the addition of TBAA (Figs. S70-S71), suggesting that
neither 3-a nor G1-a showed anion-responsiveness. More
importantly, the diffusion coefficient (D) from 2-D DOSY
experiments and hydrodynamic size from DLS analysis,
respectively, were almost the same with the addition of
TBAA (for G1-a, D = (1.55±0.14) × 10-9 m2 s−1, and the size
was 1.54 ± 0.10 nm; for the mixture of G1-a and TBAA, D =
(1.48±0.12) × 10-9 m2 s−1, and the size was 1.52 ± 0.17 nm)
(Figs. S72-S75). The control experiment indicated that the
size modulation of rotaxane-branched dendrimers was
5
4
3
2
1
0
G1
G2
G3
G4
Fig. 7 2-D DOSY (THF-d8, 298 K, 500 MHz) (top) and DLS
(bottom) results of anion-induced size switching of Type III-C
rotaxane-branched dendrimers G1-G4.
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attributed to the collective translational motion of
1
2
macrocycle along the axle on each branch.
Notes
The authors declare no competing interest.
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4
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ACKNOWLEDGMENT
This work was financially supported by the NSFC/China (Nos.
21625202 and 21572066), Innovation Program of Shanghai
Municipal Education Commission (No. 2019-01-07-00-05-
E00012), and Program for Changjiang Scholars and Innovative
Research Team in University for financial support. X. W. is
grateful to the Guanghua Excellent Postdoctoral Scholarship
for support. J. W. is grateful to the Institute of Organic
Chemistry and Biochemistry, Academy of Sciences of the Czech
Republic (RVO 61388963).
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Fig. 8 Optimized structures of Type III-C rotaxane-branched
dendrimer G2 before (left) and after (right) the addition of
TBAA as stimulus by using the program MOPAC2016.
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CONCLUSION
In conclusion, by employing a controllable divergent
approach, we have realized the successful synthesis of a
brand-new Type III-C rotaxane-branched dendrimers as the
last member of the proposed rotaxane-branched dendrimer
family15 up to the fourth-generation with 45 individual
switchable rotaxane units dispersing within the dendrimer
skeleton. In particular, with the addition or removal of the
acetate anion, the collective expansion-contraction motion
on each branch led to the reversible size switching of the
integrated rotaxane-branched dendrimers. Herein we
provide another prototype of stimuli-responsive rotaxane-
branched dendrimers with the controllable switching
features that may mimic the amplified collective molecular
motions in biomolecular machines. Such unique dynamic
supramolecular systems may find promising applications in
the controllable cargo release/capture and switchable
supramolecular catalysis in the future.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website.
Additional
information
concerning
the
synthesis,
characterization, and other experimental details.
AUTHOR INFORMATION
Corresponding Author
*hbyang@chem.ecnu.edu.cn (H.-B. Yang);
*weiwang@stu.ecnu.edu.cn (W. Wang)
ORCID
Jin Wen: 0000-0001-6136-8771
Hai-Bo Yang: 0000-0003-4926-1618
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14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
TOC
O
O
O
H3CO
H3CO
O
O
H3CO
O
O
O
CH2
PEt3
Pt
CH2
PEt3
Pt
O
O
N
H
N
H
N
N
H
H
PEt3
O
PEt3
R
H3CO
R
O
O
R
R
O
CH3
R
=
O
+ stimulus
- stimulus
Contracted Type III-C rotaxane-
branched dendrimer
Stretched Type III-C rotaxane-
branched dendrimer
ACS Paragon Plus Environment