Molecular stirrers in action

Article abstract of DOI:10.1021/ja507711h

A series of first-generation light-driven molecular motors with rigid substituents of varying length was synthesized to act as "molecular stirrers". Their rotary motion was studied by ¹H NMR and UV - vis absorption spectroscopy in a variety of

Full text of DOI:10.1021/ja507711h

Article
pubs.acs.org/JACS
Molecular Stirrers in Action
Jiawen Chen, Jos C. M. Kistemaker, Jort Robertus, and Ben L. Feringa*
Centre for Systems Chemistry, Stratingh Institute for Chemistry and Zernike Institute for Advanced Materials, University of
Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
*
W Web-Enhanced Feature *S Supporting Information
ABSTRACT: A series of first-generation light-driven molec-
ular motors with rigid substituents of varying length was
synthesized to act as “molecular stirrers”. Their rotary motion
1
was studied by H NMR and UV−vis absorption spectroscopy
in a variety of solvents with different polarity and viscosity.
Quantitative analyses of kinetic and thermodynamic parameters show that the rotary speed is affected by the rigidity of the
substituents and the length of the rigid substituents and that the differences in speed are governed by entropy effects. Most
pronounced is the effect of solvent viscosity on the rotary motion when long, rigid substituents are present. The α values
obtained by the free volume model, supported by DFT calculations, demonstrate that during the rotary process of the motor, as
the rigid substituent becomes longer, an increased rearranging volume is needed, which leads to enhanced solvent displacement
and retardation of the motor.
1
. INTRODUCTION
central carbon−carbon double bond acts as the axle for
rotation, and the rotary motion is induced by photochemical
stilbene-type photoisomerization. Repetitive rotation around
the double bond is achieved by two energetically uphill
photochemical steps, each followed by a thermally activated,
energetically downhill step. The chirality of the molecular
motor allows the rotation to proceed in a unidirectional sense.
The combination of being able to rotate repetitively with
Molecular motors are abundant in nature, driving a variety of
essential biological processes. These ingenious systems are a
great source of inspiration and set the stage for a long-standing
goal in current nanotechnology toward the design and
exploration of molecular systems that govern controlled rotary
and translational motion and ultimately make it possible to
operate more complex machine-like functions. In studying the
dynamics of motors in solution at the nanoscale, under
conditions of low Reynolds numbers and where Brownian
motion rules, it should be remembered that non-covalent
interactions are key, in contrast to the functioning of
macroscopic motors, where inertia is a dominant factor. A
recurrent question is to what extent molecular size and rigidity
and the effects of the nanoscale environmenti.e., solvent
polarity and viscosity and surface frictionplay roles in
governing rotary motion.
1
2
13
controlled directionality, powered by light energy, qualifies
3
the system as a rotary molecular motor and distinguishes the
14
motor from many systems based on molecular switches. In
addition, the large geometrical change of the two halves of the
first-generation motor with respect to each other upon
isomerization can be used for stereodynamic control in
4
15
mesoscopic and macromolecular materials and for precise
positioning of functional groups in a dynamic cycle with
sequence control. This resulted in a variety of applications,
including dynamic control of intramolecular H-stacking of
Control over rotary and translational motion with molecular
16
perylenebisimide, controlling intramolecular through-space
systems has been achieved in the past decade with a variety of
rotors, motors, shuttles, propulsion systems, and walkers,
both in solution and on surfaces, and these systems are
powered by light, heat, redox processes, or chemical
conversions. Our program on molecular motors focuses on
light-driven unidirectional rotary motors based on chiral,
1
7
18
5
6
7
8
9
magnetic interactions, functioning as molecular “gear box”,
19
10
and photoswitchable chiral organocatalysts. The geometrical
change that the motor undergoes during its rotary process
might be able to displace the surrounding solvent molecules,
20
and the large-amplitude motion is expected to be affected by
solvent friction. It has indeed been demonstrated that
isomerization processes that involve large-amplitude rotational
motion are controlled by the viscosity of the medium, although
1
1
overcrowded alkenes.
1
2
The first-generation motor consists of two identical
chromophores linked by an olefinic bond (Figure 1). The
21f,g
the effects are in some cases remarkably small.
External
frictional effects have been observed in viscous solvents and
compressed liquids for photochemical and thermal isomer-
2
2
23
izations of stilbenes and azobenzenes, rearrangements and
24
25
fragmentations, local rotations in peptides and proteins, E−
Figure 1. Molecular stirrer based on a first-generation light-driven
molecular motor.
Received: July 29, 2014
©
XXXX American Chemical Society
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Scheme 1. First-Generation Light-Driven Molecular Motors 1−5 with Different Rods at the 6- and 6′-Positions
2
1g
Z isomerization in photochromic dendrimers,
and rotary
2. RESULTS AND DISCUSSION
.1. Design. The molecular stirrers in the present study are
based on first-generation molecular motors with two pending
rods (Scheme 1). In the design of light-driven motors with rods
of various lengths and flexibility, it is essential that the pending
rods do not interfere with the rotary behavior (Scheme 1).
DFT calculations suggest that substituents placed at the 6- and
26
motion of surface-bound rotors. A pertinent example of an
azobenzene derivative acting as a “molecular stirrer” was
reported by Tanaka, resulting in the controlled release of a
guest from a zeolite pore. The influence of substituents on the
photoisomerization of dendrimer-based azobenzenes has been
2
27
28
discussed by Mu
rigidity and the size of the rigid dendritic wedges have a modest
factor of 3) but significant effect on the rate constant of the
̈
llen and De Schryver, showing that the
6
′-positions of the naphthalene ring of first-generation motor 1
(
(
Scheme 1) do not exert a direct effect on the steric crowding
thermal cis−trans isomerization in higher generation den-
drimers with an azobenzene core. Other examples include
isomerization of 1,4-diphenyl-1,3-butadiene, merocyanine,
bisoxonols, carbocyanine, and zinc dithizonate. Adam and
Trofimov emphasized that solvent viscosity studies and analyses
in terms of a free-volume model provide important insight into
in the fjord region, as shown by the insignificant differences
found for the enthalpies of activation (Supporting Information
29
30
35
(SI), Table S3). In addition, as reported previously, electronic
3
1
32
33
effects caused by substitution at this position do not have a
significant influence on the rotary speed of the motor.
Therefore, a series of molecular motors was designed with
different substituents, including long, flexible hexadecyl chains
and rigid phenylethynylene oligomers with varying lengths
which have been introduced on both sides of the motor core
structure (Scheme 1). Phenylethynylene oligomers were
chosen because they are known to be shape persistent and
34
frictional effects in molecular transformations. We considered
it of great interest to explore whether an extension of the rotary
motor with longer “arms” would experience greater frictional
effects and enable it to displace more solvent, with the ultimate
goal of applying these systems as light-powered molecular
stirrers (Figure 1). Therefore, exploring the properties of
molecular motors with substituents of distinct size, length, and
rigidity in an environment of increasing viscosity could help us
gain more insight in the rotary process of molecular motors.
In the present study, two main questions were investigated.
First, what is the effect of covalently bound large and/or rigid
groups on the rotary motion of the light-driven motor? Second,
what is the effect on the speed of a motor when it rotates in a
viscous solvent compared to a non-viscous solvent? In the
nanoscale world, the possibility exists that a molecular motor
would rotate inside its solvent shell and therefore not be
affected by the size of substituents and the viscosity of the
surrounding solvent. Increasing the length of the motor’s
substituents might result in a protrusion of the solvent shell
which would allow the viscosity of the surrounding solvent to
affect the rotary motion. We anticipated that a rod-like
substituent will require a different degree of order of the
surroundings than a flexible chain, and we expected that it
might display a higher entropic barrier.
36
their synthesis allows for well-defined sizes of the rods. With
this design, we envisioned good control over the length of the
rigid oligomers attached to the motors. In the molecules 1−5
(
Scheme 1), the length of the rods varied from H (1.09 Å) to
tetramer (32.0 Å). The rotary motion of these motors was
1
studied by H NMR and UV−vis spectroscopy using solvents
with different polarities and viscosities.
2
.2. Synthesis. The synthesis of molecular motors 1−5
involves dibromo-substituted, overcrowded alkene 9 as a key
intermediate for subsequent coupling reactions of various rods.
For the regioselective synthesis of the 6-bromo ketone 8
37
(
Scheme 2), we started with ketone 6, which was treated with
bromine under Lewis acid conditions at 40 °C, resulting first in
the α-brominated intermediate which was not isolated. Upon
addition of an excess of bromine, a second bromine substituent
was introduced by electrophilic aromatic substitution of the
naphthalene moiety with high regioselectivity in favor of the 6-
position. After recrystallization from EtOH, dibrominated
compound 7 was obtained in moderated yield. Removal of
the bromine substituent at the α-position of the ketone was
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3
9
Scheme 2. Synthesis of Bis-acetylene-Substituted First-
Generation Molecular Motor 11
applied. Dibromo motor 9 was reacted with hexadecyllithium
t
solution at room temperature in toluene using Pd[(P Bu) ] as
a catalyst (Scheme 4). The reaction was completed within 3 h,
3
2
Scheme 4. Coupling of the Motors with Rigid Rods of
Various Lengths
achieved by treating 7 with chlorotrimethylsilane and sodium
iodide, giving ketone 8 in nearly quantitative yield . Reductive
McMurry coupling of 8 using titanium tetrachloride and zinc
provided the overcrowded alkene 9 as a 1:1 mixture of trans
and cis isomers. Further modification was achieved by coupling
9
1
with trimethylsilylacetylene via Sonagashira coupling, yielding
0 in 71% yield. Deprotection of the trimethylsilyl groups of 10
using tetrabutylammonium fluoride (TBAF) gave rise to motor
1, which bears two terminal acetylenes that were suitable to be
1
used in a later stage.
For the synthesis of the rigid substituents of varying lengths,
38
building block 12, which contains two hexyl groups to
improve the solubility of the oligomers, was employed.
Phenylethynylene oligomers of defined sizes were prepared
via a step-by-step synthesis, using Sonogashira cross-coupling
methodology (Scheme 3).
giving 80% yield of the disubstituted product. This coupling
method offered in our hands milder conditions, shorter reaction
time, higher selectivity, and better yields to install two alkyl
groups as in trans-2 than the corresponding Suzuki coupling.
The pure trans isomer of 2 was obtained by chromatography,
and the isomerization processes was studied in accordance with
analogues trans-3−5 (vide infra).
36
Scheme 3. Synthesis of the Rigid Rods of Various Lengths
For the synthesis of 3, after several reaction conditions were
screened, motor 9 was successfully functionalized with
phenylacetylene at both the upper and lower halves by using
Pd(PPh ) , Cs CO , and Ag O as catalysts (Scheme 4). This
3
4
2
3
2
40
method prevents homo-coupling of the acetylene compound.
After column chromatography, the first-generation molecular
motor 3 was isolated as a mixture of trans and cis isomers in a
1
0:1 ratio. In contrast, when the same conditions were applied
to couple 9 with 17, no conversion was observed (Scheme 4).
Therefore, a reverse coupling approach was used, starting with
diacetylene compound 11 (Scheme 4). The cross-coupling
between 11 and 16 was performed by using Pd(Ph ) Cl , CuI,
3
2
2
and (i-Pr) NH, resulting in trans-4. The low isolated yield
2
A portion of bromide 12 was converted into the
corresponding iodo compound 13. It is essential to keep the
temperature of the reaction mixture strictly below −90 °C to
avoid any side reaction. Subsequently, another portion of
bromide 12 was treated with TBAF to generate acetylene 14.
Taking advantage of the iodo−bromo selectivity in the
Sonagashira cross-coupling, dimer 15 was obtained in 65%
yield. No symmetrically coupled oligomers of 14 were
observed. This synthetic approach was repeated in an iterative
fashion, with the cross-coupling of 16 and 17, providing
tetramer 18 which was further converted into the more reactive
aryl iodide 19.
Subsequently, a variety of cross-coupling methods was
employed to connect the light-driven molecular motors with
different substituents. For the preparation of 2, attempts to
react dibromo motor 9 with hexadecylboronic acid via Suzuki
cross-coupling resulted in a mixture of mono- and disubstituted
products which were difficult to separate due to the low polarity
of both compounds. Instead, a novel organolithium-based
cross-coupling method, recently developed in our group, was
(26%) of pure trans-4 is due to poor separation of 4 from
various side products by chromatography, while also several
recrystallizations were required. Only the monosubstituted cis
isomer was found; the absence of the disubstituted cis isomer of
4 might be due to steric hindrance. Employing the same
coupling reaction of 11 with 19, the pure trans isomer of 5 was
obtained as the sole disubstituted product in 18% yield. The
structures of the isomers were assigned by comparing spectral
data with those of various other first-generation motors (for
1
2
characterization and structural assignment, see the SI).
1
2.3. H NMR Studies. The photoisomerization and thermal
helix inversion steps from stable trans to stable cis isomer of
1
motors 1−5 (Scheme 5) were studied by H NMR spectros-
copy. For first-generation motors, it is expected that, when a
stable trans isomer with the methyl substituents at the
stereogenic center in a pseudoaxial orientation is irradiated
with UV light, a photochemical trans−cis isomerization takes
12
place. This photochemical step results in an unstable cis
isomer, in which the methyl substituents at the stereogenic
centers are forced to adopt a less favored pseudoequatorial
C
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Scheme 5. Photochemical and Thermal Helix Inversion
H in the stable trans and unstable cis isomers. Leaving the
c
Steps of 1−5
sample overnight at room temperature with exclusion of light
led to the thermal helix inversion from unstable cis-4 to stable
cis-4, which is indicated by a shift of all absorptions (Figure 2c).
The doublet at 2.7 ppm was assigned as H in the stable cis
c
isomer since it has a similar coupling pattern as that in stable
trans isomer. The absorption at 3.5 ppm shifted downfield and
the multiplet at 3.9 ppm shifted upfield, so these absorptions
are observed together in the region 3.5−3.7 ppm. Notably, the
ratio of stable cis-4/stable trans-4 (3:2) is equivalent to the ratio
of unstable cis-4/stable trans-4. It confirms the unidirectionality
of the thermal isomerization of unstable cis-4, indicating the
absence of the thermal E−Z isomerization in accordance with
our previous observation with related first-generation mo-
orientation. To release the steric strain, the two naphthalene
rings need to slip past each other, resulting in an irreversible
thermal isomerization with inverted helical conformation of the
molecules. This thermal helix inversion step generates the
corresponding stable cis isomer, in which the methyl
substituents at the stereogenic center adopt a more favored
pseudoaxial orientation.
1
2,18
tors.
Motors 2, 3 and 5 also show similar changes in spectra
and PSS ratios (unstable cis/stable trans, 3:2) (SI, Figures S2−
S4).
1
Figure 2a shows the partial H NMR spectrum of a solution
of stable trans-4 in dichloromethane-d2 (CD Cl ). (These
2.4. UV−Vis Spectroscopy Studies. The photochemical
isomerization and thermal helix inversion processes of motors
2
2
2
−5 were also studied by UV−vis spectroscopy. The UV−vis
spectrum of stable trans-2 in tetrahydrofuran (THF) is shown
in Figure 3a (λmax = 355 nm, solid line). The absorption
1
Figure 2. Partial H NMR of 4 (CD Cl , −20 °C): (a) stable trans-4,
2
2
before irradiation (λ ≥ 365 nm), (b) after irradiation, and (c) after
standing at room temperature in the dark overnight. (Single
enantiomer shown.)
Figure 3. UV−vis spectra in THF of molecular motors, trans isomer
(solid line), unstable cis isomer at PSS (dashed line), and stable cis
isomer (dotted line) at room temperature: (a) motor 2, (b) motor 3,
spectra are representative also for the other motors studied; see
the SI.) Distinctive features are the signals of the aliphatic
1
2,18
(c) motor 4, and (d) motor 5.
protons H , H , and H .
There is only negligible coupling
a
b
c
between H and H due to their relative orientations as a result
of the conformation of the five-membered ring, and therefore
a
c
maxima at 340 and 355 nm are characteristic absorptions of the
the doublet at 2.4 ppm can be assigned to H . As H can couple
stable trans isomer of the five-membered first-generation
c
b
12
not only to its geminal proton H but also to vicinal proton H ,
molecular motors. Upon irradiation (λ ≥ 365 nm), a
bathochromic shift was observed in the absorption maximum
from 355 to 407 nm. The presence of an isosbestic point at 385
nm indicates that no secondary reactions occur during the
experiments, and the bathochromic shift can be attributed to
the formation of an unstable cis isomer (λmax = 407 nm, dashed
line, Figure 3a). Subsequently, the sample was left in the dark
to allow the thermal helix inversion to occur. A blue shift was
observed from 407 to 362 nm, suggesting the generation of
stable cis-2 (λmax = 362 nm, dotted line, Figure 3a). Motor 3
was analyzed under identical condition as described above; a
clear isosbestic point was found at 412 nm and similar changes
(stable trans-3 → unstable cis-3 → stable cis-3) were observed
(Figure 3b), indicating the photochemical and thermal helix
inversion processes. Stable trans-4 displayed similar changes in
the spectra (Figure 3c) when it was converted to unstable cis-4
c
a
the double doublet at 2.9 ppm can be assigned to H . The
b
absorption at 3.1 ppm is assigned to proton H , which shows a
c
multiplet signal due to the coupling with the methyl group and
proton H . A solution of stable trans-4 in CD Cl was irradiated
b
2
2
(λ ≥ 365 nm) at −20 °C, and it was found that all absorptions
shifted downfield, indicating the formation of a new isomer
which was identified as unstable cis-4 (Figure 2b). Notably, Hc
shifts from 2.4 ppm (doublet) to 3.2 ppm (double doublet).
Unstable cis-4 adopts a different conformation than that of
stable trans-4, which allows the coupling between H and H .
a
c
The new absorption at 3.45 ppm can be identified as H , and
b
the multiplet at 3.9 ppm can be assigned as H in the unstable
a
cis isomer. Extended irradiation resulted in a photostationary
state (PSS) with a ratio of 3:2 (unstable cis-4/stable trans-4),
which was determined by integration of the signals for proton
D
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⧧
by irradiation (dashed line) and subsequent to stable cis-4 after
warming (dotted line). Besides the featured absorptions of
motor at 458 and 472 nm, stable trans-5 shows an additional
strong absorption at 420 nm (λmax = 420 nm, solid line, Figure
of Δ G° of 1−5 is predominately attributed to the change in
⧧
⧧
entropy, Δ S°. Increasing Δ S° values of 1−5 indicate an
increasing demand of order during thermal helix inversion upon
an increase in size of the motor. It is postulated that this
increase in order is caused by a need to displace solvent
molecules during the movement of the motor rods in the
thermal helix inversion process from the unstable cis isomer to
the stable cis isomer. This might explain why the half-life of
motor 2 remains similar to that of the parent motor, because
the flexible hexadecyl alkyl chains can reorientate upon helix
inversion to minimize their interactions with solvents, while the
rigid substituents cannot. Going from motor 3 to 5, as the rigid
substituents become longer, an increased interaction with the
surrounding solvent molecules might be generated, and as a
consequence a retardation of the rotary speed is observed. To
further exclude the possibility that this increased half-life is due
to aggregation of the motors or the rigid substituents,
concentration-dependent experiments were performed (Figure
4). The linear relationship between concentration and UV−vis
3d), which can be attributed to the rigid tetramer rod in 5. After
irradiation, the formation of unstable cis-5 generated a
bathochromic shift of the long-wavelength absorption band
20
(λmax = 514 nm, dashed line, Figure 3d) and an isosbestic point
at 487 nm which is notably red-shifted, compared to those of 3
and 4. This shift originates from the structure of 5, as it
possesses a more extended conjugated system than 3 and 4.
Leaving the sample in the dark overnight resulted in the
formation of stable cis-5 (λmax = 421 nm, dotted line, Figure
3
d).
Since the photoisomerization step of molecular motors takes
4
1
place on the picosecond time scale, the rate of the thermal
helix inversion step is rate-limiting in these systems and
determines the speed of the motor.
4
1c,42
The kinetics of the
thermal helix inversion step can be studied by UV−vis
1
8
spectroscopy. The helix inversion, from unstable cis to stable
cis isomer for molecular motors 1−5, was followed by
monitoring the UV−vis absorption at a certain wavelength
with respect to time at different temperatures (20, 25, 30, and
3
5 °C; for details, see SI, Figure S5). From these data, the half-
⧧
life (t1/2) and Gibbs free energy of activation (Δ G°) at room
temperature (20 °C) could be obtained by means of an Eyring
analysis (SI, Figure S6), as well as the enthalpy of activation
⧧
⧧
(
Δ H°) and entropy of activation (Δ S°). The kinetic
parameters of the thermal helix inversion steps of motors 2−
at room temperature in THF are listed in Table 1.
5
Table 1. Kinetic Parameters of the Helix Inversion Step of
−5 in THF at 20 °C
1
calcd
⧧
⧧
⧧
Δ G°
Δ H°
Δ H°
⧧
Figure 4. Concentration dependence of the UV−vis absorption of
trans and cis isomers (at PSS) for motors 4 and 5 in THF at room
temperature.
motor
t1/2 (min)
(kJ/mol) Δ S° (J/mol·K) (kJ/mol) (kJ/mol)
1
2
3
4
5
74.0 ± 0.1
78.5 ± 0.1
97.8 ± 0.1
123.7 ± 0.1
214.6 ± 0.1
93.2
93.3
93.8
94.4
95.7
−31.7
−31.6
−33.1
−35.1
−39.3
84.0
84.0
84.1
84.2
84.2
85.9
84.5
85.2
85.0
85.6
absorption for 4 and 5 confirms the absence of aggregation and
⧧
therefore ensures that the change in Δ S° is attributed to the
different interactions between motor and solvent molecules.
2
.5. Solvent Effects on Thermal Helix Inversion. To
Motor 2, which has two flexible hexadecyl chains instead of
rigid rods like in 3−5, was found to have almost the same half-
further confirm that different rotary speeds are due to different
interactions between motors and solvents, motors 1−5 were
studied in solvent mixtures of increasing viscosity. Solvent−
solvent interactions are higher for a viscous solvent compared
to a non-viscous solvent; it is therefore expected that
rearrangements of a motor in which it has to displace solvent
molecules will be more difficult in solvents of higher
life (t1 = 78.5 min) as parent motor 1 (t1/2 = 74.0 min). The
/2
t1/2 of 3 (97.8 min) is slightly increased compared to that of
parent motor 1 by extending the motor core structure with
phenylacetylenes on both sides. The t1/2 of 4, which comprises
two rigid diphenylacetylene rods attached to the motor,
increases to 123.7 min, which indicates that the thermal helix
inversion step of motor 4 is almost 2 times slower than that of
motor 1. Motor 5 has the longest rigid rods and exhibits the
lowest isomerization rate (t1/2 = 214.6 min), which is about 3
times slower than for motor 1. It suggests that the rate of the
thermal helix inversion from unstable cis to stable cis isomer is
affected by the rigidity of the substituent and the length of the
rigid substituent. Similar activation enthalpies for 1−5 imply
that the substituents at the 6- and 6′-positions do not generate
any significant steric effects in the fjord region, nor do possible
electronic effects influence the thermal helix inversion. These
findings are in good agreement with the computationally
2
0,26
viscosity.
Choosing different solvents of varying viscosities
might cause undesired side effects due to nonlinearity in
polarity or solubility. In order to keep the changes in motor−
solvent interactions linear, a mixed solvent system was chosen
in which glycerol was added to THF to a maximum of 80%
(volume %). This solvent mixture (η = 1060 cP) is about
2000 times more viscous than THF (η = 0.45 cP). This change
in viscosity might affect the rate of thermal helix inversion, but
the rate might also be affected by an increase in polarity.
Therefore, the rates of thermal helix inversion of motors 1−5
were first analyzed in hexane and MeOH, which both possess a
viscosity similar to THF but are of lower and higher polarity,
25
43
⧧
44
predicted Δ H° values (SI, Table S3). Therefore, the variation
respectively. The half-lives of motors 1−5 in hexane and
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MeOH are similar to those measured in THF, indicating that
the polarity and hydrogen-bonding of a solvent have no
significant effect on the thermal helix inversion (SI, Table S1).
This suggests that solvent−motor interactions are not
dominant a factor, pointing to solvent−solvent interactions,
i.e, viscosity, as the origin of retardation (vide infra).
Concentration dependence was also investigated in THF/
glycerol (1/4, v/v) to ensure the absence of aggregation under
the experimental conditions (SI, Figure S1). When studied in
glycerol/THF (4/1), motors 1−5 displayed an increase in half-
In contrast to translation diffusion, molecular rearrangement
(such as the thermal helix inversion under investigation)
involves only a portion of the molecule. Thus, only a fraction
(α) of the critical volume V is required to execute the internal
0
molecular motion. Since α is expressed as a fraction, α is less
than 1 because no more than the whole molecule could be
subjected to frictional effects during a molecular change; α
50
values higher than 0.3 have been observed before but are
47−49
rather exceptional.
The rate constant k of the molecular
rearrangement is given by eq 2, and substitution of eq 1 into eq
45
life of the thermal helix inversion (Figure 5).
2 affords eq 3, an expression that was first applied to rationalize
47a
the viscosity dependence for the isomerization of stilbene.
0
k = k exp(−αV /V )
(2)
0
f
0
α
k = k (A/η)
(3)
The double-logarithmic form of eq 3 predicts a linear
dependence of ln k versus ln η (eq 4). The α value is readily
ln k = const − α ln η
(4)
accessible from the slope of the plotted data. For this purpose,
motors 1−5 were studied in mixed solvent systems of glycerol
and THF in different ratios, providing a range of different
viscosities (SI).
As is evident from Figure 6, the rate constant k strongly
depends on the viscosity for all motors 1−5 under
Figure 5. Half-lives of motors 1−5 in THF (red) and glycerol/THF
(
4/1, v/v) (green) at room temperature.
Parent motor 1 (t1/2 = 167.3 min) was found to be
approximately 2 times slower in glycerol/THF than when
measured in THF (t1/2 = 74.0 min), whereas motor 5, which
bears two rigid tetramers, showed a prominent retardation of
the rotary speed (t1/2 = 2974.2 min), being >10 times slower
compared to THF (t1/2 = 214.6 min). These results confirm
that the rotary speed of a motor is affected by the size of its
substituents. Notably, the increase in half-life becomes
increasingly larger for larger substituents. In THF, motors 3,
4
, and 5 are 1.3, 1.7, and 2.9 times slower than parent motor 1,
respectively, while motor 2 has almost the same half-life.
However, in the viscous solvent mixture, the thermal
isomerization steps in the rotary process of motors 2, 3, 4,
and 5 are 2.0, 2.3, 5.3, and 17.8 times slower than for motor 1,
22−25,29−33
Figure 6. Double-logarithmic plot of solvent viscosity and rate
constant of thermal helix inversion of motors 1−5.
respectively. This suggests that the “frictional effect”
of the solvent molecules on the motor during the thermal helix
inversion becomes more pronounced in viscous solvents.
investigation, suggesting that the thermal helix inversion of
these motors obeys the free-volume model over a wide viscosity
range. The slope of the double-logarithmic plot affords the α
value for each motor (Table 2). The obtained α values are
comparable with or exceed the values reported previously for a
2.6. Free Volume Model Analysis. To further understand
how solvent viscosity and the shape and size of the substituents
affect the rotary speed of motors, the free-volume model is
4
6
applied. This model has been used successfully to rationalize
the viscosity dependence of processes that involve large-
47,48
4
7−49
number of viscosity-controlled isomerizations
rangements.
and rear-
amplitude rotation of molecules.
According to Doolit-
49
4
6a
tle, molecular motion in a liquid medium is only possible
As expected, parent motor 1, which has no substituents (R =
H), exhibits the lowest α value, 0.16. The α value for motor 2
with hexadecyl chains was found to be slightly higher, at 0.20.
This value is similar to that of motor 3 (α = 0.19), which has
phenylacetylene functionalities but is significantly smaller in
mass and calculated volume (∼2/3) (Table 2). This indicates
that the rigidity of the substituents is an important factor
affecting the rotary speed of motors. When comparing 2 and 3
to parent motor 1, it is evident that motor 2 shows a stronger
when the free volume (V ) per molecule is larger than its
f
−1
“
critical volume” (V ). The fluidity (η ) is proportional to the
0
probability factor [exp(−V /V )] for the translation motion.
0
f
Therefore, the free-volume dependence of the viscosity can be
expressed by eq 1, in which A is a proportionality factor.
η = A exp(V /V )
(1)
0
f
F
dx.doi.org/10.1021/ja507711h | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Table 2. α Values for Motors 1−5
An increase in atomic displacement when substituents are
added is evident from the IRCs (Figure 7, 179.3 vs 295.6
amu ·bohr for 1 and 3, respectively), but the change in ratio
between the displacement of the central alkene carbons and the
calcd mol
volume
α × mol
1/2
volume
distance from alkene to
center of mass (Å)
3
3
motor
α
(Å /mol)
(Å /mol)
peripheral 6- and 6′-carbons is remarkable (Table 3). This
1
2
3
4
5
0.16
0.20
0.19
0.28
0.40
441.2
1137
765.3
2837
4179
70.60
227.4
145.4
794.4
1671
1.58
4.78
3.71
9.32
14.1
Table 3. Summed Atomic Displacement of 1 and 3
viscosity dependence than motor 1 or 3. As mentioned before,
motor 2 is almost as fast as its parent analogue 1 in pure THF
(
Figure 5) since the hexadecyl chains of motor 2 can re-
orientate to minimize the disruption of the surrounding solvent
molecules. However, in viscous solvents these interactions,
originating from van der Waals interactions of the long aliphatic
chains with glycerol, are not negligible. Therefore, in the most
viscous mixture, motor 2 slows down by such a large degree
that its speed becomes comparable to that of motor 3 (Figure
motor
alkene atoms (Å)
atoms 6+6′ (Å)
ratio
1
2
2.9
4.0
8.2
9.2
2.8
2.3
indicates that, even though the internal behavior remains the
same (meaning internal coordinates show similar changes
during thermal helix inversion), its external behavior (meaning
that with respect to the surroundings) changes. The central
displacement increases compared to the peripheral movement
from motor 1 to 3. To illustrate this effect, the rotation of the
alkene was followed over the course of the IRC (specifically,
the angle the double bond makes during the thermal helix
inversion with respect to the double bond of the unstable cis
state, Figure 8).
5). Going from motor 3 to motors 4 and 5, the α value
increases gradually from 0.19 to 0.28 and 0.40, respectively. It
seems to correspond to the hypothesis that substitutions at the
6
- and 6′-positions would directly contribute to the rearranging
volume (α × molecular volume, Table 2), since they are on the
periphery of the rotating moieties. If the relationship between
molecular volume and rearranging volume is linear, an α value
of 0.91 would be expected for motor 5 [(volume 5 − volume 1
+
rearranging volume 1)/volume 5 = (4179 − 441.2 + 70.60)/
4179)]. However, experimentally a significantly lower value of
α = 0.40 is found for motor 5, which indicates that not all of the
added volume also adds to the rearranging volume.
To investigate this phenomenon, intrinsic reaction coor-
dinates (IRCs) of motors 1 and 3 were compared, which
showed very similar behaviors of the two motors (Figure 7).
Figure 8. Rotational deviation of the double bond from the unstable
cis isomer to stable cis isomer during thermal helix inversion of 1 and 3,
in degrees.
Figure 8 shows a clear distinction between the rotational
behavior of the overcrowded alkene of motors 1 and 3. The
double bond of motor 3 shows not only a larger deviation from
its starting point (the unstable cis state) but also a change in
behavior characterized by the difference in overall shape. This
change in behavior is best shown by an overlay of the
animations (see Movie 1) of the IRCs of motors 1 (in red) and
3 (in blue). Motors 4 and 5 are expected to follow this
behavior, which predicts them to undergo the thermal helix
inversion using the same pathway with comparable internal
behavior. It is expected that the change in external behavior for
motors 4 and 5 will be larger to accommodate for a minimal
Figure 7. Intrinsic reaction coordinates of thermal helix inversion and
6
- and 6′-substituent dihedral angle during thermal helix inversion of
motors 1 and 3.
Motor 3 has to displace a large amount of mass, which is equal
to volume on account of a consistent degree of hybrization
throughout the molecule (Figure 7). Comparing any internal
coordinates during the rotation, for example, the 6- and 6′-
substituent dihedral angles plotted in Figure 7, shows the same
overall behavior. Since no distinct differences were observed
when comparing internal relationships, absolute displacements
were analyzed.
51
mass displacement, while still increasing the total amount of
G
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Journal of the American Chemical Society
Article
displacement of mass and, as a consequence, solvent displace-
ment compared to motors 1 and 3 (Table 2). These results
explain why the volume added in the 6- and 6′-positions does
not add completely to the rearranging volume but only partially
because of changes in spatial reorganization. Indeed, motors 4
and 5 possess an increasingly larger volume, which displaces
solvent during rotation. The rigid substituents present in motor
AUTHOR INFORMATION
■
Corresponding Author
b.l.feringa@rug.nl
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
5
increase the volume which displaces solvent during rotation
■
3
3
from 70.60 Å for parent 1 to 1671 Å , thereby using a 24 times
larger volume to “stir its surroundings”.
Financial support from NanoNed, The Netherlands Organ-
ization for Scientific Research (NWO-CW), the European
Research Council (Advanced Investigator Grant, No. 227897 to
B.L.F), the Ministry of Education, Culture and Science (Gravity
program 024.001.035), and the “Top Research School”
program of the Zernike Institute for Advanced Materials
under the Bonus Incentive Scheme (BIS) is gratefully
acknowledged. The synthesis of compound 2 via organo-
lithium-based cross-coupling method was done with the help of
3. CONCLUSIONS
A series of first-generation motors with substituents of varying
rigidity and length has been synthesized. Their rotation from
stable trans to stable cis isomer upon irradiation and subsequent
1
warming has been confirmed by H NMR and UV−vis
spectroscopy. The rate of their thermal helix inversion, which
is the rate-determining step of the rotary motion of these
molecular motors, has been studied in a variety of solvents and
solvent mixtures of different polarity and viscosity. The data
shows that the size and rigidity of the substituents have a strong
effect on the entropic barrier for rotation, decreasing the rate
for larger and more rigid substituents. Whereas changes in
polarity exhibited no significant influence on the rotation rate, a
strong dependency was found for the viscosity of the solvent
system. For larger and more rigid substituents, a more
pronounced viscosity dependency was found. The free-volume
model showed that, with increasing substituent size, an
increased rearranging volume of the motor participates in the
rotation, indicating the requirement for a larger solvent
displacement. The rearranging volume of the molecular stirrer
has been increased 24-fold from non-substituted motor 1 to
tetramer-substituted motor 5. Computed IRCs show a change
in the external rotational behavior upon an increase in
substituent size, while the internal relationships remains
constant during the thermal helix inversion, explaining the
trend found in α values. This work helps us to understand the
substituent effect on the rotation of molecular motors and can
therefore provide important guidelines for designing more
advanced molecules. The results presented here on the thermal
isomerization step, being the rate-determining step in the entire
rotary process, are in line with medium effects observed very
́
Dr. Martin Fan
̃
́
anas-Mastral.
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W Web-Enhanced Feature
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