A redesign of light-driven rotary molecular motors

Article abstract of DOI:10.1039/b715652a

Structural modification of unidirectional light-driven rotary molecular motors in which the naphthalene moieties are exchanged for substituted phenyl moieties are reported. This redesign provides an additional tool to control the speed of the motors, and should enable the design and synthesis of more complex systems. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b715652a

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
A redesign of light-driven rotary molecular motors†‡
Michael M. Pollard, Auke Meetsma and Ben L. Feringa*
Received 11th October 2007, Accepted 23rd November 2007
First published as an Advance Article on the web 13th December 2007
DOI: 10.1039/b715652a
Structural modification of unidirectional light-driven rotary molecular motors in which the
naphthalene moieties are exchanged for substituted phenyl moieties are reported. This redesign
provides an additional tool to control the speed of the motors, and should enable the design and
synthesis of more complex systems.
Introduction
Rotary molecular motors are ubiquitous in biological systems
where they provide the essential mechanochemical driving force
for cellular functions including ion and proton pumping, cellular
1
translocation, and ATP synthesis. Synthetic molecular motors
would allow chemists the possibility to mimic their biological
functions as well as to invent new applications in, for instance,
smart materials and nanodevices. Successful designs for synthetic
2
Fig. 1 Design modifications of the original molecular motor 1, acceler-
ated motor 2, and new motor 3.
rotary molecular motors have been reported, including those
3–5
6,7
powered by light energy, and chemical energy.
steric interactions that lead to the formation of the metastable
intermediates upon photoirradiation of both cis and trans isomers.
In addition to cis → trans photoisomerization, a number of
rigidified stilbenes undergo photocyclization as the predominant
One of the most promising designs is based on chiral over-
crowded alkenes, comprising a naphthalene chromophore linked
through a central alkene to an identical or different aromatic
chromophore to give the ‘first-generation’ (e.g. 1 and 2, Fig. 1) and
16
side reaction. However, this photocyclization does not occur
in stilbene derivatives in which the geminal substituents of the
3,4
‘
second-generation’ molecular motors, respectively. Two major
challenges to overcome to be able to harness work from these
systems include accelerating the speed of rotation, and function-
alization to allow incorporation into more complex molecular
systems. Addressing these challenges has been deceptively difficult.
Several recent investigations on the modification of these motors
have revealed that minor alterations in their structure can lead to
17
central alkene are fused through 5-membered rings. Thus, it was
anticipated that a motor based on this core would not suffer from
photochemical cyclization side reactions. In 3, the naphthalene
function of 2 was exchanged with a dimethyl-substituted phenyl
ring to preserve the steric interactions in the fjord region which
are necessary to generate conformational bistability.
8–10
11,12
a dramatic enhancement,
as well as sometimes undesired
effects on the motor’s speed, in some cases destroying the
11,13
function as a motor altogether.
Moreover, the synthesis of
Results and discussion
functionalized versions of these systems is mired by the necessity to
incorporate functionalized naphthalene moieties into the system.
Here we report the redesign of both our first- and second-
generation motors, wherein the naphthalene moiety is exchanged
for a substituted phenyl ring (structure 3 illustrates this modi-
fication to first-generation motor 2, Fig. 1). This simplification
is anticipated to both provide a convenient handle for tuning
Overcrowded alkene 3 was conveniently prepared in two steps by
a tandem Friedel–Crafts/Nazarov cyclization of methacrylic acid
onto p-xylene in polyphosphoric acid to give 2,4,7-trimethylindan-
18
1
-one, followed by a McMurry coupling to give a 3.5 : 1 mixture
of cis-3 and trans-3 in 32% overall yield (Fig. 2). Key data support-
1
ing the structural assignment include the upfield shift of the H
14
NMR absorptions (CDCl ) of the aryl methyl groups located in the
3
the speed of the motor, as well as facilitating the synthesis
of functionalized motors by avoiding the need to derivatize the
fjord region of the cis isomer (1.51 ppm) relative to the trans isomer
15
naphthalene moiety through multistep syntheses. Key challenges
to achieving this goal include tuning of the structure to avoid
11
photocyclization of the cis isomer, and to maintain the necessary
Stratingh Institute for Chemistry, University of Groningen, Groningen, The
Netherlands. E-mail: b.l.feringa@rug.nl; Fax: +31 50 363 4278; Tel: +31 05
3
63 4296
1
13
†
Electronic supplementary information (ESI) available: H NMR and
C
1
NMR spectra of cis-3, trans-3 and 6, in addition to H NMR spectra of
the PSS mixtures. See DOI: 10.1039/b715652a
‡
CCDC reference numbers 666199 and 666200. For crystallographic data
Fig. 2 Tandem Friedel–Crafts/Nazarov cyclization, followed by a Mc-
Murry coupling in the synthesis of cis-3 and trans-3.
in CIF or other electronic format see DOI: 10.1039/b715652a
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(
2.04 ppm), which are consistent with the expected shielding
caused by their close proximity to the adjacent aromatic rings.
X-Ray crystallography‡ showed that in both cis and trans
isomers, the stereogenic methyl group adopts a pseudo-axial
conformation to minimize steric strain imposed by the adjacent
substituents on the overcrowded central alkene (Fig. 3). Compar-
ison of the X-ray structures of the stable isomers of 3 with the
analogous isomers of 2 show how the increased steric demands
3
of the sp centers in the fjord region (due to the presence of the
2
methyl substituents) relative to sp centers (as in 2) influences the
geometry around the central olefin moiety of both isomers.
Fig. 4 Rotary cycle of molecular motor 3.
unstable trans-3 in which the stereogenic methyl substituents are
forced into a conformationally strained pseudo-equatorial con-
formation (step 1). This strain is released during the thermal helix
inversion step (step 2) whereby the molecule isomerizes to give
stable trans-3, in which the stereogenic methyl groups resume an
energetically favoured axial orientation. A second photochemical
double bond isomerization (step 3) generates unstable cis-3, which
is followed by a second thermal helix inversion (step 4) which
ultimately regenerates stable cis-3. The two photochemical and
Fig. 3 (a) Pluto drawing (top) and Newman projection (bottom) of stable
trans-3. (b) Pluto drawing (top) and Newman projection (bottom) of stable
cis-3.
The X-ray structure of stable trans-3 (Fig. 3) reveals that it
adopts a twisted, syn-folded conformation, similar to the trans
isomer of 2. However, in trans-3, the fold angle of the double
◦
bond (199.7 ) is slightly larger than that observed in the trans
◦
isomer of 2 (197.8 ), hinting at an increase in strain generated
3
◦
by the bulkier sp hybridized methyl substituent. This might in
two thermal steps add up to a full 360 rotation of the upper
turn cause a greater destabilization of the unstable form of trans-3
compared with 2 (vide infra).
part relative to the lower part. The unidirectionality of the rotary
cycle is dictated by the helicity of the overcrowded alkene and the
absolute configuration of the stereogenic centers.
In contrast to the cis isomer of 2, which adopts an anti-folded
conformation, cis-3 adopts an anti-twisted conformation (Fig. 3).
This is attributed to the additional steric demands generated by
Irradiation of a solution of stable cis-3 in hexane with 313 nm
◦
light at −80 C showed a red-shift of the major long wavelength
3
the two interfering sp centers of the methyl groups present in the
band in the UV spectrum from 310 nm to an absorption centered
at 325 nm (Fig. 5). This red-shift is consistent with increased strain
on the central double bond, and hence the generation of a higher
fjord region of the cis isomer.
By analogy with 1 and 2, it was anticipated that 3 should
function as a light-powered molecular rotary motor which pro-
ceeds through 4 distinct steps in a unidirectional fashion (Fig. 4).
Starting with stable cis-3, a photochemical isomerization generates
12
energy isomer.
◦
After irradiation (kmax = 313 nm, −80 C) of stable cis-3 in
1
CDCl
3
, its H NMR spectrum showed that the photostationary
Fig. 5 Left: UV spectra of hexane solutions of stable cis-3 at −50 C (black, solid), the PSS313nm containing unstable trans-3 at −50 ◦C (grey, solid).
◦
Right: stable trans-3 (black, dashed) at rt, and a PSS365nm containing unstable cis-3 at rt (grey, dashed).
5
08 | Org. Biomol. Chem., 2008, 6, 507–512
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state (PSS) contained a mixture of stable cis-3 and unstable trans-3
in a 8 : 1 ratio. The identity of unstable trans-3 was evident from
the appearance of a new set of absorptions which were consistent
with the trans structure, including the benzylic methyl protons
in the fjord region which no longer experience shielding by the
adjacent aromatic rings. This change in their environment leads
to a downfield shift from 1.51 ppm in stable cis-3 to 2.11 ppm
in unstable trans-3. Informative changes also include the upfield
shift of the doublet generated by the stereogenic methyl group
from 1.05 ppm§ in stable cis-3 to 0.82 ppm in unstable trans-3.
The kinetic parameters of the thermal conversion of unstable
cis-3 to stable cis-3 in heptane solution were determined by mon-
itoring the change in absorption at 350 nm at four temperatures
◦
(T = 70, 80, 90, 100 C). Using the Eyring equation, the Gibbs
−
1
free energy of activation was determined to be 101.2 kJ mol ,
◦
indicating that isomerization is slow (at rt: t1/2 >1.5 d, at 60 C:
t
1/2 = 20 min) compared with the thermal isomerization in step 2
(unstable trans-3 → stable trans-3). This isomerization step of 3 is
also much slower than the isomerization of the analogous unstable
trans-2 to stable trans-2, which has a half-life of 74 min at rt.
We then investigated the effect of exchanging the naphthalene
for a substituted phenyl moiety on second-generation motors
1
This shift is consistent with analogous changes in the H NMR
12
spectrum of 2.
Upon warming the sample at −20 ◦C for 20 min, the
1
H
(Fig. 6). The original second-generation motor 4 has a half-life
4
NMR spectrum revealed that unstable trans-3 had undergone a
quantitative thermal helix inversion to generate stable trans-3.
Each of the absorptions from the unstable isomer were replaced
by signals from the stable isomer, including the doublet from
the stereogenic methyl at 0.82 ppm shifting to 1.08 ppm upon
going from an equatorial to an axial conformation. These data
demonstrate that this structurally modified motor undergoes the
crucial thermal conversion from its unstable isomer to the stable
isomer in an irreversible, unidirectional fashion.
of 233 h at rt, and has been the subject of intense efforts to reduce
14
this time. It was found that when the naphthalene moiety in
the upper half of 4 was exchanged with a benzene, irradiation
11
lead to photocyclization. The alkenes containing a lower half
derived from fluorenone have proven to be attractive for study
because they exhibit a large change in their UV–vis spectrum
upon isomerization between the stable and unstable isomers, and
generally have good photoequilibria. We believed that the same
redesign could be used as a means to adjust the speed of second
generation motors (i.e. 5 → 6) as well as to expand the range of
structures that function as unidirectional molecular motors.
The thermal isomerization was followed by monitoring the
change in absorption at 335 nm at four different temperatures
◦
(
−45, −40, −35 and −30 C). Using the Eyring equation, it
was determined that the isomerization has a Gibbs free energy
‡
◦
−1
of activation (D G ) of 71 kJ mol . This value corresponds to a
half-life at rt of 1.2 s, which shows that the helix inversion is faster
than the analogous helix inversion of the related motor 2, of which
the unstable trans to stable trans isomerization occurs with a half-
life of 18 s under identical conditions. It is possible that a slight
increase in steric crowding between the stereogenic methyl groups
and the benzylic methyl groups in the fjord region (compared with
the aromatic ring present in 2) results in subsequent destabilization
of unstable trans-3 relative to unstable trans-2.
Fig. 6 Design modifications of the original second-generation molecular
motor 4, accelerated motor 5, and redesigned 6.
Alkene 6 was prepared in 3 steps using a Staudinger-type diazo-
The UV spectrum of stable trans-3 and unstable cis-3, generated
by irradiation (kmax = 313 nm) of pure stable trans-3 at rt, also
shows a dramatic red-shift of the major absorption band. This
includes a red-shift of the absorption at 290 nm to a broad
absorption with a maximum at 340 nm, which is consistent with
the generation of unstable cis-3, reflecting the increased torsional
strain on the central double bond.
20
18
thioketone synthesis. 2,4,7-Trimethylindan-1-one was treated
with Lawesson’s reagent to give the thioketone 7 (Fig. 7) that
21
is unstable and degrades on silica gel. Therefore, most of the
Lawesson’s reagent was removed by precipitation with pentane,
and the crude filtrate was treated with diazofluorenone to give
alkene 6. Our previous experience with related derivatives that
contain the naphthalene function suggests that electron donating
substituents on the aromatic moiety should improve the stability
of the intermediate thioketone.
Irradiation of a benzene-d
6
solution of stable trans-3 at 313 nm
generated a PSS comprising a mixture of stable trans-3 : unstable
19
cis-3 in a 1.1 : 1 ratio. Photoconversion to the unstable cis isomer
was apparent from the upfield shift of the aryl methyl groups in
the fjord region from 2.04 ppm in stable trans-3 to 1.61 ppm in
unstable cis-3. Additionally, the doublet from the methyl group at
the stereogenic center shifted upfield from 1.12 ppm to 1.35 ppm,
1
consistent with the analogous shifts observed in 2. The H NMR
◦
spectrum of the sample after heating (60 C for 2 h) revealed
that all of the absorptions of unstable cis-3 were replaced by
the absorptions expected from stable cis-3, exemplified by the
shift of the doublet from the stereogenic methyl group from 1.35
to 1.10 ppm. These H NMR data indicated that the thermal
isomerization quantitatively gave stable cis-3.
Fig. 7 Synthesis of 6.
1
Like 3, it was anticipated that 6 would avoid photocyclization
side reactions due to insufficient p-orbital overlap because it also
contains 5-membered rings fused to the central olefin (Fig. 8).
The UV–vis spectrum of stable 6 in hexane contains a major
band centered at 360 nm (Fig. 9). Irradiation (kmax = 365 nm)
§
This value was obtained at low temperature, and is slightly shifted from
the corresponding absorption measured at rt.
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Org. Biomol. Chem., 2008, 6, 507–512 | 509

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facilitating the design and synthesis of systems functionalized in
ways which were previously impractical.
Experimental
General remarks
Chemicals were purchased from Acros, Aldrich, Fluka or Merck.
Solvents for extraction and chromatography were technical grade.
All solvents used in reactions were freshly distilled from appropri-
ate drying agents before use. All other reagents were recrystallized
or distilled as necessary Analytical TLC was performed with
Merck silica gel 60 F254 plates and visualization was accomplished
by UV light. Flash chromatography was carried out using Merck
silica gel 60 (230–400 mesh ASTM). NMR spectra were obtained
using a Varian Mercury Plus and a Varian Unity Plus Varian-
Fig. 8 Photochemical and thermal isomerization of 6.
5
00, operating at 399.93 and 499.86 MHz, respectively, for the
1
H nucleus or at 100.57 and 125.70 MHz, respectively, for
the C nucleus. Chemical shifts are reported in d units (ppm)
relative to the residual deuterated solvent signals of CHCl
NMR: d 7.26 ppm; C NMR: d 77.0 ppm). MS (EI) spectra
were obtained with a Jeol JMS-600 spectrometer, C
NMR: d 2.09 ppm) and C D H ( H NMR: d 7.15; C NMR:
1
3
1
3
( H
Fig. 9 UV spectra of stable 6 (black) and unstable 6 (grey) in hexane at
13
◦
−
40 C.
1
7
13
D
7
H ( H
◦
1
of this sample at −40 C resulted in a thermally reversible red-
6
5
shift of the major band to a broad absorption centered at 385 nm,
128.0 ppm). UV measurements were performed on a Hewlett-
Packard HP 8543 FT spectrophotometer in conjunction with an
Oxford Optistat using Uvasol grade solvents (Merck). Solvents
were distilled and dried before use by standard methodology.
Irradiation experiments were performed with a Spectroline model
ENB-280C/FE lamp. Photostationary states (PSS) were ensured
by monitoring composition changes in time by taking UV spectra
at distinct intervals until no changes were observed. Thermal
helix inversions were monitored by UV–vis spectroscopy using
the apparatus described above with a cut-off filter (313 nm for cis-
3 and trans-3, 350 nm for 6) to minimize irradiation by the analysis
lamp, as well as 20 s interval times of data point collection. 2,4,7-
22
analogous to related motor 5. The reversion of the PSS spectrum
grey) back to its original spectrum (black) was monitored by the
change in absorption at 410 nm at four different temperatures
(
◦
23
(
−15, −10, 0 and 10 C); using the Eyring equation we could
‡
◦
−
1
establish that the thermal step of 6 had a D G of 79.1 kJ mol ,
and a t1/2 at rt of 15 s, which is over 12 times faster than that
reported for 5.
◦
Irradiation (kmax = 365 nm) of stable 6 in toluene-d
8
at −40 C
1
to its PSS allowed H NMR spectroscopic characterization of
unstable 6. A 3 : 1 ratio of unstable 6 and stable 6 was present at
the PSS. The identity of unstable 6 is evident from the downfield
shift of the absorption from the stereogenic methyl group from
18
25
Trimethylindan-1-one and diazofluorenone were synthesized
by following literature procedures.
1
.13§ to 1.39 ppm upon changing from a pseudo-axial to pseudo-
equatorial conformation. This change in conformation is also
supported by the upfield shift of the methine proton from 3.94§ to
Synthesis
3
.54 ppm which is consistent with Hax → Heq. Warming this sample
to rt resulted in the rapid regeneration of the original spectrum of
cis-3 and trans-3. TiCl4 (1.10 mL, 10.0 mmol) was added
stable 6.
dropwise to a vigorously stirred suspension of Zn (20.0 mmol,
1
.30 g) in THF (15 mL). This mixture was heated to reflux for 2 h,
and cooled to rt. 2,4,7-Trimethylindan-1-one (871 mg, 5.00 mmol)
was added (2 × 0.5 mL THF to rinse the flask), and the mixture
was heated to reflux for 2 d. This mixture was then cooled, diluted
with EtOAc (15 mL) and washed with aq. HCl (0.1 N, 3 × 5 ml).
The mixture was then washed with water (2 × 5 mL), brine,
Conclusion
The naphthalene group in first- and second-generation light-
driven rotary molecular motors can be exchanged for a dimethyl-
substituted phenyl group, while preserving the photochemical and
thermal steps crucial to their 4-step rotary cycles. The change in
structure from 2 → 3 leads to a modest acceleration of the thermal
isomerisation step from the unstable to the stable trans isomer, but
a significant deceleration of the corresponding unstable → stable
cis isomer. When the same structural alteration was applied to
the second-generation motors possessing a fluorene-derived lower
half (5 → 6), the motor function was preserved with a modest
dried (Na
2
SO ) and concentrated in vacuo. The crude residue was
4
purified by flash chromatography (SiO
2
, Rf -trans = 0.6, Rf -cis = 0.68,
heptane, trans isomer is less soluble) to give the cis (197 mg, 25%)
and trans (56 mg, 7%) isomers, both as white solids. X-Ray quality
crystals were obtained by slow evaporation from MeOH (cis) or
◦
heptane (trans). Stable cis-3: mp 181–182 C; kmax(hex)/nm 213
3
−1
−1
1
sh (e/dm mol cm 7150), 308 sh (3850); H NMR (500 MHz,
CDCl
24
acceleration of the thermal helix inversion. It is anticipated that
this simplification of the original structure without compromising
the motor function will guide future efforts of motor design, by
providing a new handle to control the motors’ speed as well as
3
) d 1.07 (d, J = 7.0 Hz, 6H), 1.51 (s, 6H), 2.26 (s, 6H),
2.44 (d, J = 15.0 Hz, 2H), 3.09 (dd, J = 15.0, 6.5 Hz, 2H), 3.35
(quin., J = 6.5 Hz, 2H), 6.85 (d, J = 7.5 Hz, 2H), 6.92 (d, J =
1
3
8.0 Hz, 2H); C NMR (APT, 100 MHz, C
6
6
D ) d 18.5, 20.6, 21.0,
5
10 | Org. Biomol. Chem., 2008, 6, 507–512
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39.0, 42.2, 128.6, 128.7, 130.8, 133.6, 141.1, 141.2, 144.3; HRMS
Photochemical generation of PSS sample of unstable cis-3.
A
(
EI) calcd for C24 28; 316.2191, found 316.2201. Stable trans-3:
H
solution of 3 mg of stable trans-3 in C
at 313 nm for 2 h at 0 C, with intermittent analysis of its H
6
D
6
(0.7 mL) was irradiated
◦
3
−1
−1
◦
1
dec. ∼135 C; kmax(hex)/nm 295 (e/dm mol cm 2800), 308 sh
1
(
2570); H NMR (400 MHz, C
6
D
6
) d 1.12 (d, J = 6.4 Hz, 6H),
NMR spectrum throughout. No further changes were observed
1
2
6
2
1
1
.04 (s, 6H), 2.13 (d, J = 14.8 Hz, 2H), 2.46 (s, 6H), 2.64 (dd, J =
.0, 14.8 Hz, 2H), 2.98 (quin, J = 6.0 Hz, 2H), 6.95 (d, J = 8.0 Hz,
H), 7.04 (d, J = 8.0 Hz, 2H); C NMR (100 MHz, C ) d
7.5, 18.8, 21.4, 38.4, 41.6, 127.9, 128.4, 130.6, 130.9, 140.7, 141.3,
41.9; HRMS (EI) calcd for C24 28; 316.2191, found 316.2199.
after 1.5 h. Unstable cis-3: H NMR (400 MHz, C
6
D ) d 1.35 (d,
6
J = 6.0 Hz, 6H), 1.61 (s, 6H), 2.14 (d, J = 14.0 Hz, 2H), 2.13 (s,
6H), 2.53 (dd, J = 5.6, 14.4 Hz, 2H), 2.80–3.00 (unresolved peak
due to overlap with an absorption from remaining stable trans-3,
2H), 6.81 (d, J = 8.0 Hz, 2H), 6.87 (d, J = 7.6 Hz, 2H).
1
3
6
D
6
H
Photoirradiation of stable 6 to give a PSS mixture containing
unstable 6 and stable 6. A solution of stable 6 (4 mg) in toluene-
9
-(2,4,7-Trimethyl-2,3-dihydro-1H-inden-1-ylidene)-9H-fluo-
rene 6. ,4,7-Trimethylindan-1-one (944 mg, 4.00 mmol) was
heated with Lawesson’s reagent (1.78 g, 4.00 mmol) in CHCl
sealed tube to 95 C for 4 h. The thioketone appears as a dark pur-
ple spot on TLC (heptane–toluene; 10 : 1) and runs faster than the
starting material ketone (this product degrades to some extent on
TLC). Other byproducts are evident as slower eluting spots on the
d
−
8
(0.7 mL) was irradiated (kmax = 365 nm) in an NMR tube at
3
in a
◦
40 C for 3 h. The photochemical conversion was followed by
◦
1
◦
intermittent analysis by H NMR at −40 C. When no additional
1
changes were apparent in the H NMR spectrum, the mixture
contained a 3 : 1 mixture of unstable 6 : stable 6. Unstable 6: H
1
◦
NMR (500 MHz, toluene d
8
at −60 C) d 1.33 (d, J = 6.4 Hz, 3H),
1
TLC (possibly the vinyl sulfide). The crude H NMR spectrum sug-
2
6
6
.05 (s, hidden by solvent peak, 3H), 2.10 (s, 3H), 2.35 (dd, J =
gested a mixture of compounds were present, including the starting
material ketone and the thioketone in a ratio of 13 : 1. The mixture
was cooled, concentrated to a volume of ∼5 mL, diluted with
heptane (10 mL), filtered, concentrated in vacuo, diluted again with
heptane (with trituration of the solid material), decanted, then
concentrated in vacuo, diluted with toluene (10 mL) and treated
.0, 14.0 Hz, 2H), 2.79 (dd, J = 16.0, 8.5 Hz, 2H), 3.48 (sext., J =
.7 Hz, 2H), 6.83 (d, J = 7.5 Hz, 2H), 6.86–7.11 (2H, overlapped
with solvent), 7.18 (quinapparent, J = 7.3 Hz, 2H), 7.44 (d, J = 8.0 Hz,
2
H), 7.50–7.62 (m, 3H).
◦
◦
directly with the diazofluorenone at 50 C for 1 h, then 80 C for
h. (Efforts to purify the thioketone by flash chromatography
Acknowledgements
5
gave an impure product in <10% yield due to the instability of the
thioketone.) This material was then filtered over silica gel, concen-
Wesley R. Browne, Edzard Geertsema, and Dirk Pijper are
gratefully acknowledged for their helpful suggestions. Financial
support from the Netherlands Organization for Scientific Research
(NWO-CW) through a NWO-CW Top-grant, Spinoza grant from
NWO and a Nanoned grant is gratefully acknowledged.
trated, and purified by flash chromatography (SiO
0% AgNO , 10% toluene in heptane, though the eluent required
is sensitive to the amount of AgNO employed). This yields the de-
sired motor 6 (240 mg, 18%) as a yellow solid as well as bifluorene
∼500 mg). Recrystallization from EtOH gave fine yellow needles:
mp 147–148 C; kmax(hex)/nm 213 (e/dm mol cm 62 100), 241
2
doped with
1
3
3
(
◦
3
−1
−1
References and notes
(
56 800), 268 sh (29 300), 291 (13 100), 304 (11 500), 359 (16 900);
1 Molecular Motors, ed. M. Schliwa, Wiley-VCH, Weinheim, 2003.
1
2
For a seminal review see: E. R. Kay, D. A. Leigh and F. Zerbetto, Angew.
Chem., Int. Ed., 2007, 46, 72. For progress on molecular motors at work
see: W. R. Browne and B. L. Feringa, Nature Nanotechnol., 2006, 1, 25.
N. Koumura, R. W. J. Zijlstra, R. A. van Delden, N. Harada and B. L.
Feringa, Nature, 1999, 401, 152.
H NMR (400 MHz, toluene-d
8
) d 1.15 (d, J = 6.4 Hz, 3H), 2.10
(s, 3H, hidden by solvent peak, can be observed in CDCl
(
3
), 2.27
s, 3H), 2.29 (d, J = 14.0 Hz, 1H), 2.89 (dd, J = 6.0, 14.4 Hz, 1H),
3
4
3
2
8
.97 (quin., J = 6.8 Hz, 1H), 6.87 (d, J = 8.0 Hz, 2H), 6.93 (s,
H), 7.14 (t, J = 7.5 Hz, 1H), 7.20–7.28 (m, 2H), 7.59 (dapparent, J =
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1
3
.0 Hz, 2H), 7.65 (d, J = 7.2 Hz, 1H), 7.88 (d, J = 7.6 Hz, 1H); C
NMR (125 MHz, CDCl
3
3
) d 18.2 (CH
3
), 19.0 (CH
3
), 21.1 (CH ),
3
9.9 (CH ), 43.9 (CH), 119.1 (CH), 119.6 (CH), 120.3 (CH), 123.6
2
(
(
(
(
CH), 123.9 (CH), 126.5 (CH), 126.76 (CH), 128.81 (CH), 127.0
CH), 128.7 (CH), 130.08 (CH), 130.11 (CH), 131.5 (CH), 134.5
CH), 137.8 (C), 139.4 (C), 139.6 (C), 139.7 (C), 139.8 (C), 145.2
5
(a) D. A. Leigh, J. K. Y. Wong, F. Dehez and F. Zerbetto, Nature, 2003,
4
24, 174; (b) J. V. Hernandez, E. R. Kay and D. A. Leigh, Science, 2004,
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6
7
T. R. Kelly, H. De Silva and R. A. Silva, Nature, 1999, 401, 150.
S. P. Fletcher, F. Dumur, M. M. Pollard and B. L. Feringa, Science,
C), 152.3 (C). HRMS (EI) calcd C25H22 322.1712, found 322.1731.
2
005, 310, 80.
8
9
J. Vicario, A. Meetsma and B. L. Feringa, J. Am. Chem. Soc., 2006,
128, 5127.
M. K. J. ter Wiel, R. A. van Delden, A. Meetsma and B. L. Feringa,
J. Am. Chem. Soc., 2003, 125, 15076.
1
H NMR characterization of unstable intermediates
Photochemical generation of PSS of unstable trans-3. Unstable
1
1
0 S. Kuwahara, T. Fujita and N. Harada, Eur. J. Org. Chem., 2005, 4544.
1 E. M. Geertsema, N. Koumura, M. K. J. ter Wiel, A. Meetsma and
B. L. Feringa, Chem. Commun., 2002, 2962.
trans-3: A solution of 2.4 mg of stable cis-3 in C
7
8
D (0.7 mL) was
◦
irradiated at 313 nm for 8 h at −80 C, with periodic analysis
1
of its H NMR spectrum throughout. No further changes were
12 M. K. J. ter Wiel, R. A. van Delden, A. Meetsma and B. L. Feringa,
1
◦
observed after 6 h. H NMR (500 MHz, −80 C, CDCl
3
) d 0.82
d, J = 6.5 Hz, 6H), 2.11 (s, 6H), 2.33 (d, J = 14.0 Hz, 2H), 2.43
s, 6H), 2.62 (dd, J = 5.3, 14.8 Hz, 2H), 2.83 (quin., J = 6.0 Hz,
H), 7.00 (d, J = 7.5 Hz, 2H), 7.04 (d, J = 7.5 Hz, 2H).
J. Am. Chem. Soc., 2005, 127, 14208.
1
1
3 M. K. J. ter Wiel, M. G. Kwit, A. Meetsma and B. L. Feringa, Org.
Biomol. Chem., 2007, 5, 87.
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Mater., 2007, 17, 718.
(
(
2
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1
5 We recently reported a synthetic route to functionalized first generation
motors which required 9 steps, see: M. K. J. ter Wiel and B. L. Feringa,
Synthesis, 2005, 11, 1789.
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21 This thioketone likely tautomerizes to the thioenol, which is deleterious
in the diazo-thioketone coupling.
22 J. Vicario, A. Meetsma and B. L. Feringa, Chem. Commun., 2005, 5910.
23 K. J. Laidler and M. C. King, J. Phys. Chem., 1983, 87, 2657.
24 While the ‘stable → unstable’ photoequilibria of 3 and 6 are less
favourable than for their naphthalene-containing counterparts, their
4-step cycle remains unidirectional.
1
1
8 W. Kaminsky, O. Rabe, A.-M. Schauwienold, G. U. Schupfner, J. Hanss
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9 This step also proceeds with similar efficiency in chloroform, however
the use of benzene allows superior resolution of the absorptions in the
mixture.
25 A. Sch o¨ nberg, W. Awad and N. Latif, J. Chem. Soc., 1951, 1368.
5
12 | Org. Biomol. Chem., 2008, 6, 507–512
This journal is © The Royal Society of Chemistry 2008