Directional molecular transportation based on a catalytic stopper-leaving rotaxane system

Article abstract of DOI:10.1021/jacs.6b01852

Ratchet mechanism has proved to be a key principle in designing molecular motors and machines that exploit random thermal fluctuations for directional motion with energy input. To integrate ratchet mechanism into artificial systems, precise molecular design is a prerequisite to control the pathway of relative motion between their subcomponents, which is still a formidable challenge. Herein, we report a straightforward method to control the transportation barrier of a macrocycle by selectively detaching one of the two stoppers using a novel DBU-catalyzed stopperleaving reaction in a rotaxane system. The macrocycle was first allowed to thread onto a semidumbbell axle from the open end and subsequently thermodynamically captured into a nonsymmetrical rotaxane. Then, it was driven energetically uphill until it reached a kinetically trapped state by destroying its interaction with ammonium site, and was finally quantitatively released from the other end when the corresponding stopper barrier was removed. Although the directional transportation at the present system was achieved by discrete chemical reactions for the sake of higher transportation efficiency, it represents a new molecular transportation model by the strategy of using stopper-leavable rotaxane.

Full text of DOI:10.1021/jacs.6b01852

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Article
Directional Molecular Transportation Based on
a Catalytic Stopper-Leaving Rotaxane System
Zheng Meng, Jun-Feng Xiang, and Chuan-Feng Chen
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 14 Apr 2016
Downloaded from http://pubs.acs.org on April 14, 2016
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Directional Molecular Transportation Based on a Catalytic Stopper-
Leaving Rotaxane System
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Zheng Meng,†,‡ JunꢀFeng Xiang,† and ChuanꢀFeng Chen*,†
†Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute
of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
‡University of Chinese Academy of Sciences, Beijing 100049, China.
ABSTRACT: Ratchet mechanism has proved to be a key principle in designing molecular motors and machines that exploit ranꢀ
dom thermal fluctuations for directional motion with energy input. To integrate ratchet mechanism into artificial systems, precise
molecular design is prerequisite to control the pathway of relative motion between their subcomponents, which is still formidable
challenge. Herein, we report a straightforward method to control the transportation barrier of a macrocycle by selectively detaching
one of the two stoppers using a novel DBUꢀcatalyzed stopperꢀleaving reaction in a rotaxane system. The macrocycle was firstly
allowed to thread onto a semiꢀdumbbell axle from the open end and subsequently thermodynamically captured into a nonsymmetꢀ
rical rotaxane. Then it was driven energetically uphill and got to a kinetically trapped state by destroying its interaction with ammoꢀ
nium site, and was finally quantitatively released from the other end when the corresponding stopper barrier was removed. Altꢀ
hough the directional transportation at the present system was achieved by discrete chemical reactions for the sake of higher transꢀ
portation efficiency, it represents a new molecular transportation model by the strategy of using stopperꢀleavable rotaxane.
macrocycle transportation system where the dibenzoꢀ24ꢀ
INTRODUCTION
crownꢀ8 (DB24C8) was firstly allowed to thread onto a semiꢀ
Biomolecular machines and motors1 are ubiquitously used in
dumbbell axle from its open end and subsequently covalently
captured16 by a nonsymmetrical rotaxane, then motivated to a
metastable, kinetically trapped state by destroying its interacꢀ
tion with ammonium station, and finally quantitatively reꢀ
leased from the other end by selectively detaching the correꢀ
sponding phenol stopper. The present system provides a new
directional molecular transportation model based on the enerꢀ
gy ratchet mechanism.
cells to implement cargo delivery,2 organelles movement,3
active transmembrane transport4 and proteins synthesis5 which
are crucial to the metabolism process. A distinguish feature of
these tiny and exquisite machines is their ability to employ
stochastic ratchet mechanisms6 to rectify random thermal
noise into unidirectional transportation and other useful work
with defined energy input. Mechanically interlocked moleꢀ
cules (MIMs),7 especially rotaxanes8 and catenanes,9 have
proved to be promising candidates to design molecular switchꢀ
es and molecular machines10 because of the controllability of
nonꢀcovalent interactions between their subcomponents, which
results large amplitude relative motions. Not merely mimickꢀ
ing their programmed controlled motion, to further evolve
MIMsꢀbased artificial molecular machines comparable to the
natural analogues in functions, great efforts have been devoted
by chemists to utilize ratchet mechanism in MIM system.10b,11
Pioneering contributions made by Leigh, Stoddart and Credi,
respectively, have rendered serials of scrumptious performancꢀ
es realized in MIMsꢀbased molecular machines, such as direcꢀ
tional molecular motion,12 sequenceꢀspecific peptide syntheꢀ
sis,13 and active transport.14 The ratchet mechanism, especially
energy ratchet, which raises stringent need on precise molecuꢀ
lar design to control over kinetic barriers,11b,14c,15 however, is
still formidable challenge to integrate into artificial systems.
In this study, we have discovered a DBUꢀcatalyzed stopperꢀ
leaving reaction within a rotaxane series, which executed
highly regioꢀ and chemoꢀ selective cleavage of the stopper
when it was linked at the Cꢀ4 position of the electron deficient
N–methyltriazolium (MTA) group through two carbon atoms,
following a basedꢀcatalyzed βꢀelimination mechanism. This
clean and efficient reaction was then harnessed to design a
RESULTS AND DISCUSSION
Discovery of the Stopper-Leaving Reaction. The MTA
group, known for its relatively weak interaction with dibenzoꢀ
24ꢀcrownꢀ8 host compared with ammonium,17 can serve as a
very useful recognition site in building various MIMs based
molecular switches.17a,18 Because the Nꢀ3 position of the 1,2,3ꢀ
triazole was methylated, the whole positively charged aroꢀ
matic ring is electron deficient. It is reasonable to recognize
the MTA group as an electronꢀwithdrawing group. Typical
electronꢀwithdrawing groups, such as ester, amide and cyano
group, usually activate the CꢀH bond of their α position, makꢀ
ing it vulnerable to base. Although we may envision its possiꢀ
ble electronic effect, it is still not clear how and to what extent
would the MTA group affect the reactivity of its two neighborꢀ
ing (Nꢀ1 and Cꢀ4 substituted) carbon atoms.
Our discovery of the stopperꢀleaving reaction was originatꢀ
ed from the study of the shuttling dynamic of a pair of stereoꢀ
isomeric rotaxanes, R1ꢀH•3PF6 and R2ꢀH•3PF6 shown in Figꢀ
ure 1 (see Supporting Information for the details of synthetic
procedures). The two MTA groups in each rotaxane were
linked by the same spacers (butylene and ethylene chain, reꢀ
spectively) between the dibenzylammonium recognition site
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Figure 1. Comparison of the different outcomes of a pair of stereoꢀisomeric rotaxanes with the addition of DBU. (a) Adding base DBU to
R1ꢀH•3PF6 generate a degenerate [2]rotaxane R1•2PF6. (b) The addition of DBU to R2ꢀH•3PF6 not only produces the degenerate
[2]rotaxane R2•2PF6, but also subsequently detaches its stoppers causing the DB24C8 ring to dethread from the axle molecule. Conditions:
(i) 1.50 equiv of DBU, 298 K; (ii) heating at 348 K for 2.5 hours.
adding 1.50 equiv of DBU to (a) and then heating at 348 K for 2.5
hours; (c) R2ꢀH•3PF6; (d) the solution obtained immediately after
adding 1.50 equiv of DBU to (c); (e) the solution in (d) after heatꢀ
ing at 348 K for 2.5 hours. The proton assignments correspond to
those shown in Figure 1. (f) HRꢀESIꢀMS of the solution in (e).
The corresponding calculated values, also shown in the spectra,
were found to be consistent with the experimental values.
and the 3,5ꢀdimethylphenol stopper. The only difference of the
two rotaxanes is the connection mode of the MTA group. In
R1ꢀH•3PF6, the MTA group is linked between the ammonium
site and the stopper via Cꢀ4 and Nꢀ1 position, while the cirꢀ
cumstance is reversed in R2ꢀH•3PF6. When 1.50 equiv of
DBU was added to the acetonitrile solution of R1ꢀH•3PF6 in
which the DB24C8 ring was located at the dibenzylammonium
site (Figure 2a, S3), a degenerate [2]rotaxane19 R1•2PF6 was
formed with the DB24C8 ring rapidly oscillating between the
two MTA groups (Figure 2b, Figure S6, S7). With the increasꢀ
ing of the temperature, the oscillating frequency accelerated.
Unexpectedly, when the same amount of DBU was added to
R2ꢀH•3PF6 (Figure 2c) under the same condition, the deꢀ
threading of DB24C8 ring slowly took place in the degenerate
[2]rotaxane R2•2PF6 (Figure 2d), giving a new set of DB24C8
signals that exactly consistent with its free form in the H
1NMR spectra (Figure S16, S20). When the solution was kept
under 348 K for 2.5 hours, total dethreading of DB24C8 was
1
accomplished. H NMR spectrum of resulted solution showed
a new quartet at 6.77 to 6.68 ppm, and two double peak at 6.17,
5.97 ppm, exhibiting the characteristics of terminal alkene
signals, however with considerable downfield shifts (Figure
Figure 2. Partial 1H NMR spectra (500 MHz, 298 K, c = 5.00 mM,
CD3CN, 298 K) of (a) R1ꢀH⋅3PF6; (b) the solution obtained after
2e). Meanwhile, high resolution electrospray ionization mass
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spectrometry also gave a strong m/z peak at 278.6734 of a A detailed study on mechanism of the stopperꢀleaving reꢀ
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doubly positively charged species (Figure 2f), besides the m/z
peaks corresponding to DB24C8. With these clues in hand, the
structure of terminal alkene E2 as drawn in Figure 1b was
figured out, implying that the slippage of DB24C8 ring was
caused by the leaving of the 3,5ꢀdimethylphenol stopper
through a βꢀelimination.
action was then performed using L2 as a substrate (Figure 3a),
in which the amount of DBU used and the temperature were
explored. When the concentration of the product 3,5ꢀ
dimethylphenol was plotted versus time and the data points
were fitted according to a model of a firstꢀorder kinetic, good
agreement was obtained (Figure 3b, Figure S30). As equimolar
amount of DBU was quickly (far less than 1 min at 298 K)
neutralized by the ammonium group, the actual content of
DBU in solution was only the excess part. Kinetic NMR exꢀ
periments showed that only a 10% excess of DBU was able to
lead a clear and quantitative transformation of L2 to the termiꢀ
nal alkene E2 and 3,5ꢀdimethylphenol within 10 hours at 348
K. However, when it was performed under insufficient DBU
(such as 0.90 equiv), no corresponding product was detected
even after a heating at that temperature for one day (Figure
S32). With the increasing of concentrations of DBU, the reacꢀ
tion rate constants k′ showed a perfect linear growth (Figure
3c). Further considering the stoichiometric ratio of L2 and net
DBU, it was deduced that the base DBU was a catalyst and the
stopperꢀleaving reaction, in fact, should be characterized as a
DBUꢀcatalyzed pseudo firstꢀorder reaction (Figure S31).20
Based on the rate constants k′ calculated at different temperaꢀ
tures (Figure 3d), kinetic parameters of the stopperꢀleaving
reaction were derived using the Eyring equation (Figure 3e,
Figure S34), in which the activation enthalpy (ꢁH≠) and the
activation entropy (ꢁS≠) are 14.0 kcal molꢀ1 and −32.9 cal Kꢀ1
molꢀ1, respectively.
Mechanism Study. The distinct reactivity of isomeric rotaxꢀ
anes highlighted the regioꢀselectivity of the stopperꢀleaving
reaction which preferred to happen at the Cꢀ4 end of the MTA
group. We further found that when the free axle component L2
of R2•2PF6 was treated with 1.50 equiv of DBU, the correꢀ
sponding elimination reaction still happened, as proved by the
1H NMR and HRꢀESIꢀMS spectrum (Figure S28, S29). This
result indicated that the stopperꢀleaving reaction was a general
phenomenon for MTA group, regardless of whether the subꢀ
strate is an interlocked architecture or not.
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N
N
N
N
N
(a)
N
+
+
DBU
O
+
O
O
O
N
-
3PF6
H2
L2
N
N
N
N
N
N
+
+
O
O
2HO
+
N
H
-
2PF6
E2
2
+
(c)
(b)
0.08
10
Based on these investigations on L2, a possible mechaꢀ
nism of stopperꢀleaving reaction was outlined in Figure 4. Due
to electron withdrawing effect of the MTA group, the αꢀ
hydrogen of its Cꢀ4 position is easily attacked by the strong
base DBU with its sp2 N atom.21 The generated deprotonated
species II can stabilize the negative charge on the αꢀcarbon
through pꢀπ conjugation with electron deficient MTA group.
DFT calculations (see Supporting Information for details) reꢀ
vealed that in active intermediate II, the CꢀO bond linking the
phenol stopper and MTA group was notably lengthened to
1.5613 Å compared with 1.4598 Å in I. The intermediate then
underdoes a unimolecular dissociation to give the terminal
alkene III and 3,5ꢀdimethylphenol anion. As the methylꢀ
substituted phenol anion (pKa> 27.5, Figure S35) is more basic
than DBU (pKa = 24.1) in acetonitrile,22 the protonated DBU
would be neutralized by the former, making the catalyst DBU
recovered. Considering that the activation entropy (ꢁS≠) deꢀ
termined by experiments was reasonably negative, it is deꢀ
duced that the attack of substrate I by DBU is the rate deterꢀ
mining step of this reaction.23 The energy raise of the calculatꢀ
ed transition state of this nucleophilic attack step21a,24 correꢀ
sponding to the substrates is 10.8 kcal molꢀ1, much higher than
that of the unimolecular splitting step (1.9 kcal molꢀ1, see also
the Supporting Information). It was also in accord with the
fact that the rate constant of stopperꢀleaving reaction in rotaxꢀ
ane R2•2PF6 (0.0259 minꢀ1) was smaller than that in free axle
molecule L2 (0.0425 minꢀ1), which may result from the hinꢀ
drance effect of DB24C8.
8
6
4
2
0
y = 0.0773x + 0.002
0.06
0.04
0.02
0.00
R2 = 0.997
ꢀ
2.00 E.Q.
1.75 E.Q.
1.50 E.Q.
1.25 E.Q.
1.10 E.Q.
0
50
100
150
t/min
200
250
300
0.0
0.2
0.4
0.6
0.8
1.0
Concentration of net DBU/5 mM
(e)
(d)
10
8
-13.0
-13.2
-13.4
-13.6
-13.8
-14.0
-14.2
-14.4
y = −7040x + 7.17
Intercept error: ± 1.2
Slope error: ± 405
6
ꢀ
328 K
333 K
338 K
343 K
348 K
4
2
0.00285
0.00290
0.00295
0.00300
0.00305
0
50
100 150 200 250 300 350 400
1/T/K-1
t/min
Figure 3. (a) The stopperꢀleaving reaction of ammonium L2 in
the presence of excess DBU and the corresponding graphical repꢀ
resentation. (b) Concentrations of the detached stopper change
with the time after the addition of 1.10 to 2.00 equiv of DBU to
L2 (c = 5.00 mM in CD3CN) at 348K. The xꢀaxis denotes the
concentration of DBU that was the excess part. Solid lines repreꢀ
sent the data fitting according to a firstꢀorder kinetic model. The
rate constants k′ were obtained with deviations < 2.5% for all
circumstances. (c) The rate constants change with the concentraꢀ
tion of net DBU. (d) Concentrations of the detached stopper
change with the time after the addition of 1.50 equiv of DBU to
L2 (c = 5.00 mM in CD3CN) at 328 to 348K. Solid lines repreꢀ
sents the data fitting according to a firstꢀorder kinetic model, givꢀ
ing the rate constants k′ with deviations < 3.5%. (e) The rate conꢀ
stants k′ and temperature T were fitted according to the Eyring
equation.
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N
N
N
(Figures S23ꢀS24). When TBD, a slightly stronger base than
R
TS1
O
1
2
3
4
DBU was used, faster rate of the stopperꢀleaving reaction was
observed (Figures S25ꢀS27). This was probably because a
stronger base would promote the deprotonation of the αꢀ
hydrogen of MTA group which we recognized as the rate deꢀ
termining step of the stopperꢀleaving reaction.
slow
H
N
N
N
I
R
O
O
N
HN
II
N
N
(DBU)
N
N
N
R
5
fast
6
O
Unidirectional Molecular Transportation. The DBU has
played a dual role in releasing the macrocycle of R2ꢀH•3PF6
and R3ꢀH•3PF6. It not only neutralized the ammonium, and
thus raised the energy of DB24C8 by destroying its strong
hydrogen bonding interaction with the ammonium site, but
also catalyzed a βꢀelimination at the Cꢀ4 end of the MTA
group, leading the macrocycle to release into solution after the
stopper was detached. In an energy ratchet mechanism, the
directed motion is driven by modulation both of the depths of
energy wells and the heights of energy barriers.15a Undoubtedꢀ
ly, the catalytic stopperꢀleaving reaction provides us a method
to control the direction of the macrocycle release by selectiveꢀ
ly detaching one of two stoppers in a rotaxane. We anticipated
that unidirectional macrocycle transportation could be realized
based on an energy ratchet mechanism if we could further
control the threading path of the macrocycle in the same sysꢀ
tem.
TS2
7
8
9
OH
N
N
N
R
III
Figure 4. Mechanism of DBUꢀcatalyzed stopperꢀleaving reaction.
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To this end, the ammonium guest A was synthesized with
one of its two terminals preꢀcapped with a 3,5ꢀdimethylphenol
stopper, which was linked by two carbon atoms at the Cꢀ4
position of the 1,2,3ꢀtriazole group (see Supporting Inforꢀ
mation for the synthetic details). Due to the semiꢀdumbbellꢀ
shaped structure of axle A, the macrocycle was only allowed
to thread into the axle through the open terminal (the left side,
1
see Figure 6aꢀb). The H NMR spectra (Figure 7aꢀc) showed
that in a 1:1 solution (c= 5.00 mM) of acetonitrile, the comꢀ
plexation between A and DB24C8 was a slow exchange proꢀ
cess on NMR time scale. From the integral values, it was calꢀ
culated that 45% of macrocycle was transferred to A, correꢀ
sponding to an association constant of 298 Mꢀ1. In a less polar
solvent of dichloromethane that facilitates the noncovalent
interactions, the ratio of macrocycle threaded is higher than 85%
(Ka = 8.44 × 103, Figure S39). The complex A⊂DB24C8 was
then covalently captured16 by blocking the open end with anꢀ
other 3,5ꢀdimethylphenol stopper using Huisgen alkyne–azide
1,3ꢀdipolar cycloaddition in the AꢀDB24C8 (molar ratio
1:1.20) solution in CH2Cl2. Subsequent methylation of triazole
group gave the rotaxane RꢀH•3PF6 in an overall yield of 78%
(The yield is based on the axle A, and see Supporting Inforꢀ
mation for synthetic details). RꢀH•3PF6 has a nonsymmetrical
structure with the two stoppers linked at the Nꢀ1 and Cꢀ4 posiꢀ
tion of the MTA group, respectively, and macrocycle DB24C8
is thermodynamically trapped in the ammonium energy well
(Figure 7d).
The addition of 1.50 equiv DBU to RꢀH•3PF6 quickly genꢀ
erated a quasiꢀdegenerate rotaxane R•2PF6 (Figure 7e), in
which the macrocycle was in a metastable state25 and oscillatꢀ
ing between the two MTA groups. The MTA moiety was so
weak a molecular station for DB24C8 that there is in fact no
complexation between their separated form in solution, beꢀ
cause the enthalpic energy (directly related to the weak nonꢀ
covalent interactions between DB24C8 and MTA) cannot
compensate the energy loss caused by entropic
increase.17b,18a,26 The macrocycle in R•2PF6 was thereby not in
the stable state as in the bulk solution where the system had
lower free energy, but kinetically trapped in the deprotonated
Figure 5. (a) The addition of excess DBU to R3ꢀH•3PF6 detaches
its stoppers causing the DB24C8 ring to dethread from the axle. (b)
Adding excess DBU to R4ꢀH•3PF6 only generated a degenerate
[2]rotaxane R4•2PF6. The conditions used are as demonstrated in
Figure 1.
As implied from the calculated results, the leaving of the
phenol group and the subsequent generating of the conjugated
olefin were likely the main driving force for this reaction.
Consequently, another two rotaxanes, R3ꢀH•3PF6 and R3ꢀ
H•3PF6 as shown in Figure 5, which shared the same structure
with R2ꢀH•3PF6 except that the former with 3,5ꢀdiꢀtertꢀ
butylphenol stoppers and the latter with a C3 spacer between
the stopper and the MTA group, were synthesized (see Supꢀ
porting Information for their synthesis) for reactivity test. It
was found that the stopperꢀleaving reaction happened for the
former in the presence of DBU after heating (Figure 5a, Figure
S21). However, under the same conditions, the latter only gave
a degenerate rotaxane (Figure 5b, Figure S22).
We also performed another five groups of control reactions
on the axle L2 and the rotaxane R2ꢀH•3PF6 respectively with
commonlyꢀused
bases,
including
(DMAP),
pyridine,
4ꢀ
1,4ꢀ
dimethylaminopyridine
diazabicyclo[2.2.2]octane (DABCO), triethylamine, and 1,5,7ꢀ
triazabicyclo[4.4.0]decꢀ5ꢀene (TBD), whose pKa values cover
from 12.3 to 26.0 in acetonitrile. The stopperꢀleaving phenomꢀ
enon was not detected for pyridine, DMAP, DABCO and triꢀ
methylamine which are weaker bases compared with DBU
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rate of the macrocycle was much more rapid (k = 3.58 sꢀ1 deꢀ
axle due to the insurmountable stopper barriers (Figure 6c).
1
2
3
4
The excess amount of DBU further catalyzed the stopperꢀ
leaving reaction at the Cꢀ4 side of the MTA group. Based on
the selectivity of the stopperꢀleaving reaction demonstrated
above, only the stopper on the right side would be detached,
termined by 2D EXSY spectrum, see Figure S40) than the
leaving of stopper. In terms of the power stroke, the catalytic
stopperꢀleaving reaction is the decisive factor for the kinetic
properties of the energy release process; while the friction
from subcomponents of the MIM itself is negligible.27 After
the solution was kept at 348 K for 5 hours, quantitate macroꢀ
cycle releasing was achieved (Figure 7g). Taking the whole
process into account, 65% ((78%/1.2)×100%) of the macrocyꢀ
cles were transported from the initial DB24C8ꢀA solution.
1
which was further proved by the H NMR spectra (Figure 7f).
5
Once the kinetic barrier on the right side was removed, the
macrocycle spontaneously diffused to the solution from the
same side. The rate constant of macrocycle release was 0.0134
minꢀ1 at 348 K, slightly smaller (ꢀk′=1.3×10ꢀ5 sꢀ1) than that of
the stopper leaving (0.0142 minꢀ1), a sign that the oscillation
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Figure 6. Unidirectional transportation of the DB24C8 macrocycle based on an energy ratchet mechanism driven by chemical energy. (a)
Chemical drawings and (b) cartoon representations of the transportation, and (c) energy profiles representing the free energy of the system
during the ring’s moving from the left to the right side of the axle.
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Figure 7. 1H NMR spectra (500 MHz, c = 5.00 mM, CD3CN) for characterizing the unidirectional transportation of the macrocycle. Partial
1H NMR spectra of (a) DB24C8, (b) semiꢀdumbbell axle A, (c) 1:1 solution of DB24C8 and A, (d) rotaxane RꢀH•3PF6, (e) rotaxane Rꢀ
H•3PF6 immediately after adding 1.50 equiv of DBU, (f) the solution in (e) after heating at 348 K for 5.0 hours, and (g) rotaxane RꢀH•3PF6
after adding 1.50 equiv of DBU and then kept at 348 K for 10 to 280 min. The proton assignments correspond to those shown in Figure 6a.
The red and blue dotted lines marked part of the signals that grew or vanished with time, respectively. Spectra (a)−(f) were recorded at 298
K, and (g) was at 348 K.
details of theoretical calculations. This material is available free
of charge via the Internet at http://pubs.acs.org.
CONCLUSIONS
In summary, we have discovered a DBUꢀcatalyzed stopperꢀ
leaving reaction within a rotaxane series. The reaction showed
high regioꢀ and chemoꢀselectivity, and happened at the Cꢀ4
AUTHOR INFORMATION
end of electron deficient MTA group only when the phenol
stopper was linked at the Cꢀ4 position through two carbon
atoms. Detailed kinetic NMR experiments and theoretical
Corresponding Author
cchen@iccas.ac.cn
studies showed that the reaction was followed a baseꢀcatalyzed
βꢀelimination mechanism. Based on an energy ratchet mechaꢀ
nism, a unidirectional molecular transportation system was
designed and constructed exploiting the stopperꢀleaving reacꢀ
tion, in which the macrocycle DB24C8 was firstly allowed to
thread into a semiꢀdumbbell axle molecule from the open end
and subsequently covalently captured by a nonsymmetrical
rotaxane to form a thermodynamically trapped product. The
macrocycle was then driven to a metastable, kinetically
trapped state by destroying its interaction with ammonium,
and finally directionally released from the other end by selecꢀ
tively detaching the stopper barrier.
The present system provides a new directional molecular
transportation model by the strategy of using stopperꢀleavable
rotaxane to implement the catalytic energy releasing process,
circumventing the contingents in controlling local kinetic barꢀ
riers. Besides its high efficiency, the rate of the stopper leaving
can be also expediently tuned by varying the catalyst loading
or temperature under relatively benign and succinct conditions,
signifying the facility in controlling molecular release. It is
anticipated that this new method would find its wide applicaꢀ
tions in designing other artificial molecular transportation
system, and in constructing controlled release devices28 by
grafting MIMs onto nanoparticles or surfaces.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by National Natural Science Foundation
of China (21332008, 91527301, and 21521002), and the Strategic
Priority Research Program of the Chinese Academy of Sciences
(XDB12010400).
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