Light-driven open-close motion of chiral molecular scissors

Article abstract of DOI:10.1021/ja034994f

The first example of light-driven chiral molecular scissors (1), which consists of 1,1-,3,3-tetraarylferrocene as a pivot part and azobenzene as a driving part, was synthesized. Absorption, circular dichroism (CD), and 1H NMR spectral studies on the photoinduced isomerization process of an enantiomer of 1 agreed well with a prediction by a DFT calculation, where a motion of the handles via light-driven contraction/expansion of the connecting azobenzene strap was transformed, through a pivotal motion of the ferrocene unit, into an open-close motion of the blade parts. Copyright

Full text of DOI:10.1021/ja034994f

Published on Web 04/18/2003
Light-Driven Open-Close Motion of Chiral Molecular Scissors
Takahiro Muraoka, Kazushi Kinbara,* Yuka Kobayashi,† and Takuzo Aida*
Department of Chemistry and Biotechnology, School of Engineering, The UniVersity of Tokyo,7-3-1, Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan
Received March 5, 2003; E-mail: aida@macro.t.u-tokyo.ac.jp; kinbara@macro.t.u-tokyo.ac.jp
Scheme 1. Synthesis of 1a
Control of mechanical motions of single molecules by external
stimuli has attracted great attention, not only in relation to biological
supramolecular machines but also for the development of molecular
machines and devices. Until now, a variety of molecular machiner-
ies including motors, shuttles, switches, and tweezers has been
reported.1 Herein we report on “light-driven chiral molecular
scissors,” where a motion of a photoisomerizable part is transformed
into an open-close motion of the blade moieties. Examples of such
an interlocking mechanical motion are very rare2 in the precedent
molecular machineries, which are mostly operative by individual
motions of driving parts.
A pair of scissors consists of three different essential components,
i.e., handles, pivot, and blades. A change in a distance between
the two handles should be transformed, through an angular motion
of the pivot, into an open-close motion of the blades. The pivot
should be comprised of two parallel planes, which can rotate freely.
a (a) 3-Ethynylaniline, Pd(PPh3)2Cl2, CuI, Et3N, rt, 98%. (b) PtO2, H2,
EtOH, THF, rt, 80%. (c) CuCl, air, pyridine, rt, 60%. See Supporting
Information for details of the synthesis of 1-4.
We chose ferrocene as the candidate for the pivot part, since the
two cyclopentadienyl (Cp) rings are parallel to each other and rotate
freely even at low temperature.3 As the driving group to operate
the handles azobenzene was chosen because it expands and contracts
reversibly upon irradiation with visible and ultraviolet lights,
respectively. Thus, we designed molecular scissors 1, consisting
of two phenyl groups (A) as the blade moieties, 1,1′,3,3′-
tetrasubstituted ferrocene as the pivot part, and two phenylene
groups (B) as the handle parts, which are strapped by an azobenzene
unit through ethylene linkages (Scheme 1). Compound 1 is
asymmetric due to the planar chirality of the 1,1′,3,3′-tetrasubstituted
ferrocene unit, and allows us to evaluate the angular motion of the
pivotal ferrocene unit by means of circular dichroism (CD)
spectroscopy.
A DFT calculation of 1 predicted that the blade parts are closed
when the connecting azobenzene unit adopts a trans configuration,
while they are open in cis-1 (Figure 1). In trans-1, the expanded
conformation of the azobenzene unit places the two-phenylene
groups in the handle parts apart from one another, while the phenyl
Figure 1. Molecular structures of trans-1 (left) and cis-1 (right), optimized
groups in the blade parts are in close proximity to each other with
with DFT calculation (B3LYP/3-21G*) (top), and schematic representation
of its open-close motion induced by photoisomerization of the azobenzene
unit (bottom).
a bite angle of 9.2°. On the other hand, the blade phenyl groups in
cis-1 are separated from each other with a bite angle of 58.2°. Thus,
the photoisomerization of the azobenzene unit can be expected to
induce the angular motion of the ferrocene unit, resulting in the
open-close motion of the blade parts.
Scheme 1 shows the synthetic route to molecular scissors 1 (for
details, see Supporting Information). Tetrasubstituted ferrocene 2,4
obtained as a mixture of meso and racemic isomers from 1-aryl-
4-phenylcyclopenta-1,3-diene, was reacted with 3-ethynylaniline to
afford 3 in 98% yield.5 Compound 3, thus obtained, was subjected
to preparative HPLC to isolate the racemic isomer in 45% yield.
The configuration of 3 in the racemic form was successfully
confirmed by the X-ray crystallographic analysis of 4, obtained by
hydrogenation of 3 with Adam’s reagent (80% yield).6 Finally,
oxidative coupling of the aniline amino groups in 4, in the presence
of CuCl, gave desired product 1 in 60% yield as a mixture of trans
and cis isomers with respect to the azobenzene unit.7 Then, 1 was
subjected to preparative HPLC to isolate trans-1 and cis-1 at a molar
ratio of 67:33. The optically active forms of trans-1 ([CD(-)280]-
trans-1 and [CD(+)280]-trans-1) were successfully resolved by
chiral HPLC with Daicel Chiralpak OT (+).
When trans-1 in tetrahydrofuran (THF) was irradiated with a
UV light (150-W xenon arc lamp, λ ) 350 nm) for 180 s at 20 °C,
the intensity of the absorption band centered at 420 nm was
increased, while that of the absorption band centered at 330 nm
was decreased (Figure 2a). 1H NMR spectroscopy after irradiation
showed an upfield shift of the signals due to the aromatic protons
† Responsible for theoretical calculation of 1.
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C O M M U N I C A T I O N S
reversible angular motion of the Cp rings of the ferrocene unit
induced by the photoisomerization of the azobenzene unit.
1H NMR spectroscopy was conducted to investigate a possible
interlocking motion of 1, associated with the pivotal motion around
the ferrocene unit on the isomerization of the azobenzene part. After
1
the trans-to-cis isomerization of 1, the H NMR signals due to
aromatic protons Ha,b (δ 6.99 ppm) and Hc (δ 7.12 ppm) in the
blade phenyl groups (Figure 1) showed downfield shifts by 0.09
and 0.13 ppm, respectively. On the other hand, aromatic signals
due to Hd and He in the handle phenylene groups, observed
respectively at 7.16 and 7.13 ppm for trans-1, showed upfield shifts
by 0.30 and 0.58 ppm, after the isomerization to the cis form. These
spectral change profiles are quite reasonable for the molecular
models of the trans and cis isomers of 1 (Figure 1). Namely, the
two blade phenyl groups in trans-1, which are in close proximity
to each other, become apart from one another on the isomerization
to the cis form. This motion will lead to the lowering of a magnetic
shielding effect from one phenyl group to the other (downfield
shift). On the other hand, the two handle parts in 1 come closer to
each other on the trans-to-cis isomerization of the azobenzene part,
so that each phenylene unit can receive a magnetic effect more
efficiently from the counterpart. Hence, it can now be concluded
that compound 1 undergoes a scissors-like open-close motion upon
light stimuli.
In conclusion, we have succeeded in the molecular design of
the first “light-driven molecular scissors”, where the open-close
motion of the blade parts is interlocked by that of the handle parts
strapped by a photoisomerizable azobenzene unit, due to the pivotal
motion of the connecting ferrocene unit. This work provides a basis
for the development of the next-generation molecular machineries,
which realize further complex motions interlocked by many joint
parts.
Figure 2. (a) Absorption and (b) circular dichroism (CD) spectral changes
of trans-1 (3.5 × 10-5 mol dm-3) in THF at 20 °C upon irradiation with
a UV light (λ ) 350 nm). (c) Light-induced CD intensity change at 285
nm, upon sequential irradiation with a UV light (λ ) 350 nm) for 180 s
(blue areas) and a visible light (λ > 400 nm) for 15 s (yellow areas).
Supporting Information Available: Synthesis of 1-4, details of
crystal structure determination of 4, 1H NMR spectra of 1, and details
of DFT calculation of trans-1 and cis-1 (PDF). X-ray crystallographic
file in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
of the azobenzene unit. These spectral changes are typical of those
for the trans-to-cis isomerization of azobenzene derivatives.8 The
molar ratio of trans-1 to cis-1, as determined by HPLC, was 11:89
after photoirradiation. On the other hand, upon irradiation with a
visible light (λ > 400 nm) for 15 s at 20 °C, a backward
isomerization took place to furnish the cis/trans isomer ratio of
54:46. Thus, the azobenzene unit in compound 1 can reversibly
isomerize in response to UV and visible lights.
The enantiomers of trans-1 displayed characteristic CD spectra.
For example, [CD(-)280]-trans-1 showed a negative Cotton effect
at 272.5 nm and positive Cotton effects at 256.0 and 315.5 nm
(Figure 2b). The trans-to-cis isomerization of [CD(-)280]-trans-
1, upon UV-irradiation (λ ) 350 nm) for 180 s, was accompanied
by a CD spectral change at 240-300 nm due to the major
absorption of the tetraarylferrocene unit. On the other hand, upon
irradiation with visible light, (λ > 400 nm), a reverse CD spectral
change occurred, where the system quickly reached a photo-
stationary state in 15 s. On sequential irradiation with UV and
visible lights in an alternating manner, the sign of the Cotton effect,
e.g., at 280-290 nm was inversed from negative to positive and
then from positive to negative (Figure 2c). These results suggest a
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