Controlling molecular rotary motion with a self-complexing lock

Article abstract of DOI:10.1002/anie.200906064

(Figure Presented) The key to motion: A second-generation molecular rotary motor, which contains a DB24C8 macrocycle ring incorporated into the lower stator half and a dialkyl ammonium ion attached to the upper rotor half, forms a [1]pseudorotaxane in less polar solvents such as CH₂Cl₂. In this self-complexing system, acid-base-controlled threading-dethreading movements can be utilized to unlock or lock the molecular motor (see picture).

Full text of DOI:10.1002/anie.200906064

Angewandte
Chemie
DOI: 10.1002/anie.200906064
Molecular Devices
Controlling Molecular Rotary Motion with a Self-Complexing Lock**
Da-Hui Qu and Ben L. Feringa*
Biological motors, such as the kinesin or myosin linear and
ATPase rotary motor systems,[1] have been a source of
inspiration for the development of a variety of artificial
molecular mechanical devices[2] (including
switches, shuttles, muscles, and rotors) and
of elegant molecular motor systems.[3] Self-
complexing and self-assembling systems[4]
represent important dynamic compounds
that play a prominent role in the field of
molecular recognition and molecular devi-
ces. The uses of such supramolecular
systems add attractive features to the
construction of advanced nanoscale molec-
ular machinery because of their potential
to undergo controllable intramolecular
complexation in response to a particular
stimulus.
of 1) a second-generation light-driven molecular motor[8–10]
based on an overcrowded alkene, in which the molecular
rotor (2,6-dioxonaphthalene, upper half) rotates 3608 relative
Scheme 1. The chemical structure of a lockable light-driven molecular motor featuring a self-
complexing [1]pseudorotaxane system and the acid–base-controlled threading–-dethreading
(locking–unlocking) movements.
Of these self-assembling systems, sec-
+
ondary dialkylammonium ions (R2NH2 )
are well-known for their ability to thread
through, for example, a dibenzo[24]crown-
8 (DB24C8) ring to give a [2]pseudorotaxane,[5] owing to a
to the stator (xanthene, lower half) upon repetitive photo-
chemical trans–cis isomerizations and subsequent thermal
irreversible helix inversion steps, 2) a DB24C8 macrocyclic
ring incorporated into the xanthene lower stator half, which
can act as a socket for the dialkylammonium moiety, and 3) a
À
combination of strong [ + N H···O] hydrogen bonding and
+
À
[C H···O] interactions. Upon deprotonation of the R2NH2
À
moiety, the two [ + N H···O] hydrogen bonds are eliminated,
and the [2]pseudorotaxanes are dethreaded. Although several
groups[6] have combined both the dibenzo[24]crown-8 and
dialkylammonium ion structural motifs into a single system,
most of these feature versatile intermolecular complexation
rather than unique intramolecular self-complexation.
+
R2NH2 moiety attached to the upper rotor half by a short
spacer, which can insert itself as a plug into the DB24C8
macrocycle socket. The free OH group at the end of the arm
can be easily functionalized to construct interlocked rotax-
anes. cis-1-H·PF6 was prepared in 20 steps (Schemes S1–S4 in
the Supporting Information) and characterized by 1H and
13C NMR spectroscopy and high-resolution mass spectrome-
try.[11]
Herein we report the design, characterization, and
operation of a lockable[7] light-driven molecular rotary
motor featuring
a
self-complexing [1]pseudorotaxane
system. By taking advantage of the complexation between
+
1
the R2NH2 and the DB24C8 units in the system, acid–base-
The H NMR spectrum (500 MHz, 298 K) of the hexa-
controlled threading–dethreading movements can be utilized
to unlock or lock the molecular rotary motor. The design of
the molecular system with a self-complexing lock is illustrated
in Scheme 1. The molecular system cis-1-H·PF6 is composed
fluorophosphate salt cis-1-H·PF6 (Figure 1a), recorded in
[D6]DMSO, has similar splitting patterns as the spectrum of
the unprotonated cis-1 in CD2Cl2 (Figure 1b),[6a] which can be
rationalized by attributing it to the uncomplexed species, that
is, the dialkylammonium ion does not reside inside the
DB24C8 cavity.[6c] A more complicated 1H NMR spectrum of
cis-1-H·PF6 was obtained in CD2Cl2 (Figure 1c), as is evident
in the region d = 3.5–4.4 ppm corresponding to the resonances
of the protons in the DB24C8 ring, that is, formation of a
complex. The signals of HA (HB or HC) were split owing to the
unsymmetric structure of the DB24C8 ring in the self-
complexing system. The NOEs (Figures S2 and S3 in the
Supporting Information) observed between the protons in the
dialkylammonium arm and protons of the crown ether ring
(from HA to Ha, HB,C to Hb,g), as well as the NOEs observed
between HA and Hm in the aromatic part of the macrocycle
ring provide good evidence for the complexation of the
[*] Dr. D.-H. Qu, Prof. Dr. B. L. Feringa
Stratingh Institute for Chemistry and Center for Systems Chemistry
Zernike Institute for Advanced Materials, University of Groningen
Nijenborgh 4, 9747 AG, Groningen (The Netherlands)
Fax: (+31)50-363-4296
E-mail: b.l.feringa@rug.nl
Homepage: http://feringa.fmns.rug.nl/
[**] Financial support from The Netherlands Organization for Scientific
Research (NWO-CW), Zernike Institute for Advanced Materials, and
the University of Groningen is gratefully acknowledged. We thank
Dr. T. Sasaki and Mr. G. London for helpful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906064.
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Figure 1. 1H NMR spectra (500 MHz, 298 K) of a) cis-1-H·PF6 in
[D6]DMSO, b) cis-1 in CD2Cl2, c) cis-1-H·PF6 in CD2Cl2. The protons of
each compound were assigned using 1H–1H COSY NMR spectrosco-
py.[11] The resonance of Ha’ (or Hb’) was shifted upfield from that of Ha
(or Hb), owing to the shielding effect of the upper half in the cis
isomer. The assignments correspond to the structure shown in
Scheme 1.
Figure 2. Partial 1H NMR spectra of a) cis-1-H·PF6, b) after addition of
1.1 equiv DBU, c) irradiation of sample (b) at 365 nm at room temper-
ature for 2 h, and d) reprotonation with addition of 1.3 equiv TFA to
sample (b) in CD2Cl2. The assignments correspond to Scheme 1. dT, hT,
iT donate Hd, Hh, Hi of the trans-stable isomer.
dialkylammonium moiety and the DB24C8 ring. Electrospray
ionization mass spectrometry (ESI-MS) showed a single peak
at m/z 906.44, which corresponds to the single positively
charged ion [MÀPF6]+. No peak corresponding to the dimer,
trimer, or polymer was found in the mass spectrum. From
1H NMR and NOESY spectroscopy and ESI-MS spectro-
metry, we conclude that cis-1-H·PF6 can self-complex to form
the [1]pseudorotaxane in a less polar solvent such as CD2Cl2.
To study the chemically driven threading and dethreading
movements in this self-complexing system, 1,8-diazabicyclo-
[5.4.0]undec-7-ene (DBU) was used to deprotonate the
dialkylammonium ion moiety.[12] Addition of 1.1 equiv DBU
in CD2Cl2 resulted in the elimination of the hydrogen bonding
between the dialkylammonium ion and the DB24C8 ring and
the dethreading of the pseudorotaxane. As a result, the
structure of the deprotonated species became less crowded,
and the downfield shifts of the resonances for Hf (from d = 5.7
to 6.2 ppm) and Hd (from d = 6.5 to 6.8 ppm) were detected
(Figure 2b). When 1.3 equiv trifluoroacetic acid (TFA) was
added to this solution, the original spectrum was regenerated
completely, thus indicating a return to the self-complexing
[1]pseudorotaxane structure (Figure 2d).
To verify that this molecule can function as a rotary
molecular motor after deprotonation, the photochemical and
thermal isomerization of stable cis-1 and stable trans-1 was
1
also investigated with low-temperature H NMR spectrosco-
py (Figures S7 and S8 in the Supporting Information). Upon
irradiation of cis-1 (365 nm, 3 h, À258C) in CD2Cl2, new
signals corresponding to the unstable trans isomer appeared.
In the photostationary state (PSS) of the first photoequili-
brium (step 1 in Scheme 2), around 75% unstable trans-1 is
present.[14] Upon standing for 30 min at 208C in the dark,
conversion of the unstable trans-1 to the expected stable
trans-1 isomer is observed. Careful analysis revealed that not
all unstable trans-1 was converted to stable trans-1: in a
competing process 30% of the unstable trans-1 was thermally
converted back to stable cis-1. After the first thermal step
(step 2 in Scheme 2), the ratio between trans-1 and cis-1 was
therefore around 47:53.
Similar experiments starting with trans-1 indicated a
highly selective trans-to-cis photoisomerization. Starting
with stable trans-1, the ratio of unstable cis-1 and stable
trans-1 was determined to be 95:5 in the PSS of the second
photoequilibrium (step 3 in Scheme 2). In the second thermal
step (step 4 in Scheme 2), competing thermal processes also
operate, that is, a forward helix inversion (minor pathway,
30%) and a backward cis-to-trans isomerization (major
pathway, 70%) take place simultaneously. After the second
thermal step, the ratio between cis-1 and trans-1 was around
30:70. Compared with the first thermal step, the backward cis-
to-trans isomerization significantly increased in the second
thermal step, which is attributed to the drastically enhanced
steric interactions between the two parts of the molecules that
slip past each other when the thermal helix inversion happens
from unstable cis-1 (step 4). This competing thermal isomer-
Irradiation of cis-1-H·PF6 in CD2Cl2 at 365 nm did not
result in any change in the 1H NMR spectrum. After addition
of 1.1 equiv DBU, irradiation at 365 nm at room temperature
resulted in the formation of a mixture of trans- and cis-stable
isomers with a ratio of 55:45 (Figure 2c).[13] We attribute this
change in photochemical behavior to the fact that in the self-
assembling [1]pseudorotaxane system, the hydrogen-bonding
+
interactions between the R2NH2 unit and the DB24C8
macrocycle are strong enough to prevent the conformational
changes needed for the rotational motion of the rotor part,
that is, the motor is in a “locked” state. After deprotonation,
the motor is in an “unlocked” state, and photoisomerization is
allowed.
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that this backward thermal pathway
leaves the overall unidirectionality
intact; steps 2 and 4 are still strictly
unidirectional, as evident from
1H NMR spectroscopic analysis of
the thermal and photochemical steps
during the overall forward process.
The locking–unlocking proper-
ties of the molecular system were
also demonstrated using low-tem-
perature
UV/Vis
spectroscopy.
Upon irradiation (365 nm, À208C)
of cis-1-H·PF6 in CH2Cl2, no change
was observed in the UV/Vis absorp-
tion spectrum (Figure 3a). This find-
ing means that even at very low
concentration (10À5 molLÀ1), the
system is kept in the “locked” state.
Addition of excess DBU led to a
small red shift of the spectrum
(Figure 3b). Subsequent irradiation
resulted in the appearance of a long-
wavelength band around 423 nm in
the UV/Vis spectrum (Figure 3c),
which is due to the enhanced twist
of the central olefinic bond and is
characteristic for the unstable trans-
1.[9] A clear isosbestic point was
observed around 387 nm, indicating
a unimolecular process. Subsequent
warming of the sample to room
Scheme 2. Photochemical and thermal isomerization processes involved with the rotary cycle of
motor 1. Competing thermal processes are the forward helix inversion and the backward cis–trans
isomerization. Racemic motors were used for the NMR spectroscopy studies; single enantiomers
are shown for clarity.
ization pathway has not been observed with the previously
described light-driven molecular motors based on the over-
crowded alkenes, except for a push–pull system.[15] In the
latter case, the unusual behavior is attributed to a large
polarizing effect on the central olefinic bond evident from a
resonance structure featuring a single bond connecting the
rotor and the stator as the central axis. In the case of cis-1, the
enhanced steric hindrance for helix inversion opens a com-
petitive thermal cis-to-trans isomerization. It should be noted
temperature resulted in the disappearance of the long-wave-
length band, thus indicating thermal isomerization and
formation of the stable isomers (Figure 3d). Similar experi-
ments starting with trans-1 indicated a corresponding trans-to-
cis isomerization in the unlocked state, as shown in the
Supporting Information.[11,16,17]
In conclusion, we have demonstrated the operation of a
lockable light-driven molecular motor based on a self-
complexing [1]pseudorotaxane system. By taking advantage
+
of the complexation between the R2NH2 moiety and the
DB24C8 macrocycle ring present in the system, as well as its
acid–base switching properties in solution, the locking and
unlocking of the molecular motor can be achieved. These
results represent an important step towards fabricating more
advanced molecular devices and achieving control of inte-
grated nanomechanical motion at the single-molecule level.
Received: October 28, 2009
Published online: December 28, 2009
Keywords: molecular devices · molecular motors ·
.
photochemistry · protonation · self-complexation
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Figure 3. UV/Vis spectra (CH2Cl2, À208C) of a) cis-1-H·PF6, b) after
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Communications
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[11] The Supporting Information includes full experimental proce-
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1
H·PF6 and its intermediates and UV/Vis and H NMR spectral
data of stable and unstable isomers of motor 1.
[12] Tributylamine, triethylamine, and diisopropylethylamine were
also used to deprotonate the dialkylammonium ion, but the
deprotonation was incomplete in each case, even if a large excess
of base was added.
[13] The Gibbs free energy of activation of the thermal isomerization
steps in the motor without any substituent is rather low
(19.9 kcalmolÀ1), corresponding to a half-life of 77 s at room
temperature. In this case, unstable isomers could not be
observed at room temperature upon irradiation, owing to fast
thermal helix inversion.
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[14] Under conditions in which no thermal isomerization occurred, a
secondary, slower photochemical process was also observed in
which stable cis-1 isomerized directly to stable trans-1. Con-
tinued irradiation ultimately gave
a photostationary state
containing all three isomers of 1 in a 100:20:7 ratio of unstable
trans/stable cis/unstable cis. Only trace amounts of stable trans-1
were observed, as it was converted into unstable cis-1 quanti-
tatively upon irradiation; for an extensive analysis of competing
processes in molecular motors, see, for example, E. M. Geert-
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[16] Compared with the case without DBU, the ratios between the
unstable form and the stable form were enhanced in the
presence of DBU upon irradiation of both cis-1 and trans-1
isomers at low temperature in the UV/Vis measurements. This
result is attributed to some photodegradation of CH2Cl2, which
can produce trace amounts of HCl that effect the isomerization
at very low concentration. Addition of base can neutralize the
traces of acid and promote the photochemical conversion.
[17] The presence of an ammonium group does not interfere with the
photochemistry.
[8] For examples of first-generation molecular motors, see: a) N.
Koumura, R. W. J. Zijlstra, R. A. van Delden, N. Harada, B. L.
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