A lockable light-driven molecular shuttle with a fluorescent signal

Article abstract of DOI:10.1002/anie.200453708

A turn on! A lockable photodriven molecular shuttle is based on a [2]rotaxane that consists of an α-cyclodextrin threaded by a stilbene derivative. The fluorescent signals originate from the naphthalimide stopper and indicate the position of the shuttle. Once unlocked, the device can be turned on by irradiation at 335 nm and turned off by irradiation at 280 nm. The shuttle can be locked or unlocked by the addition acid or alkali, respectively (see picture).

Full text of DOI:10.1002/anie.200453708

Angewandte
Chemie
Molecular Devices
A Lockable Light-Driven Molecular Shuttle with
a Fluorescent Signal**
Qiao-Chun Wang, Da-Hui Qu, Jun Ren,
Kongchang Chen, and He Tian*
Rotaxanes have received considerable interest in recent years
because of their potential applications in nanostructured
functional materials and molecular devices.[1–5] Deeper insight
into biological molecular motors have stimulated a surge of
activity towards the construction of artificial molecular
machines, especially molecular shuttles, which offer possibil-
ity of long-range translational motion of a threaded macro-
cyclic “shuttle” along a “rail track”. Such a supramolecular
system could be also used to store information if the shuttling
motion (output) is controlled by proper external stimulus
(input).[6] The use of photons as input (light-driven shuttling)
is now under prominent consideration because excitation with
lasers can lead to a fast response and can function in a small
space without producing any by-products.[7] Temperature
change (entropy-driven shuttling) is another clean input
because no chemical reaction and no covalent structural
changes are involved.[8] However, recording of the changes by
1H NMR, absorption, and circular dichroism spectroscopies
are the usual methods used to monitor the coconformational
[*] Dr. Q.-C. Wang, D.-H. Qu, J. Ren, Prof. K. Chen, Prof. Dr. H. Tian
Lab for Advanced Materials and
Institute of Fine Chemicals
East China University of Science & Technology
Shanghai 200237 (P. R. China)
Fax: (+86)21-6425-2288
E-mail: tianhe@ecust.edu.cn
[**] H.T. acknowledges the support by NSFC/China (20273020) and
Education Committee of Shanghai. We thank Professor Yu Liu
(NanKai University/China) for his relevant advice on the synthesis
and Dr. H. L. Anderson (Oxford University/UK) for his helpful
discussion.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200453708
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change of rotaxanes, and this might not be convenient in
CD:Pd of 1:1.2:1.5:0.08, and we isolated the rotaxane a-CD-
NPSI in 7.6% yield by chromatography (silica gel, 1:2:5 acetic
acid/n-butanol/water as the eluent). The “dumbbell” NPSI
was also synthesized by using the same conditions as those for
the synthesis of a-CD-NPSI but in the absence of a-CD (yield
37%). The chemical structure of the rotaxane was confirmed
by 1H NMR and 1H ROESY NMR spectroscopies, and TOF-
MS. The NOEs spectra shown in Figure 2 and Figure 3
future applications because the difficulty in transforming
these spectral signal changes into the easily detected output
signals. In contrast, the use of changes in fluorescence as an
output signal is preferable because the signal enables easy
remote reading and is inexpensive. However, reports based
on rotaxanes that can be induced to switch between different
fluorescent states (output) in response to clean inputs are
rare.
Herein, we report an example of [2]rotaxane a-CD-NPSI
(Figure 1; CD is cyclodextrin), in which the hydrogen bonding
Figure 2. 1H ROESY NMR (500 MHz) spectrum of a-CD-NPSI in D2O
recorded at 298 K.
Figure 1. The chemical structures of B2, A7, NPSI and [2]rotaxane
a-CD-NPSI.
observed from the aromatic protons He, Hf, Hg, and Hh (no Hi)
of the dumbbell to the internal proton 5-H of the a-CD, from
Hf, Hg, Hh, and Hi (no He) to 3-H, and from the He, Hd to 6-H,
indicate that the 2, 3-OH rim of the a-CD is near to the
isophthalic acid stopper, and that the a-CD ring sits over the
stilbene unit. It is to our surprised that only one isomer was
obtained, in which the orientation of a-CD was confirmed as
shown in Figure 1.
between the a-CD macrocycle and the isophthalic acid unit
stops the shuttling motion of the a-CD macrocycle between
two binding sites. The disruption of the hydrogen bonding by
adding a base enables shuttling, which is exclusively con-
trolled by light energy without the formation of by-products,
fully reversible, and accompanied by an obvious change in the
intensity of fluorescence. This molecular machine can be a
switch, and features the convenient use of optical input and
easy reading of the optical output.
a-CD-NPSI consists of an a-CD macrocycle locked into a
thread, which has two binding sites—a biphenyl (P) site and a
stilbene (S) site—linked covalently, and trapped mechanically
by two stoppers: 4-amino-3,6-disulfonic-1,8-naphthalimide
disodium salt (N) unit and a isophthalic acid (I) unit as
shown in Figure 1. It was reported that [2]rotaxanes can be
prepared by the Pd-catalyzed Suzuki coupling in the presence
of CD.[4a,b] We treated A7 with B2 in an alkaline aqueous a-CD
solution, with a Pd(OAc)2 catalyst, in the mole ratio A7:B2:a-
The E/Z photoisomerism of the [2]rotaxane a-CD-NPSI
in aqueous solution was investigated by monitoring the
changes in 1H NMR spectra as shown in Figure 4 and
Figure 5. Efficient photoisomerization was found in a stil-
bene-based rotaxane set up by Anderson and co-workers, in
which the E/Z isomerization was accompanied by the
shuttling motion of the a-CD ring from the stilbene unit to
the biphenyl unit.[9] But to our surprise, after the irradiation of
a-CD-NPSI (5.0 mm in D2O) at 335 nm for 65 min, no change
1
in H NMR signals was found,[10] which indicates that no E
isomer was converted to Z isomer. This fact is probably due to
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Figure 3. 1H ROESY NMR (500 MHz) spectrum of [a-CD-NPSI]2ꢀ in
D2O recorded at 298 K.
Figure 5. 1H ROESY NMR (500 MHz) spectrum of [a-CD-NPSI]2ꢀ in
D2O recorded at 298 K afte irradiation at 335 nm for 65 min.
still found over the stilbene unit. Irradiation of the product at
335 nm for 65 min leads to new sets of NOEs from Hb, Hc to 5-
H and 6-H, from Hd, He to 3-H, and from Hi to 2-H, on the
external surface of the a-CD, as shown in Figure 5. These
results indicate that [a-CD-NPSI]2ꢀ undergoes photoisome-
rization and the a-CD ring shifts from the stilbene unit to the
biphenyl unit, as shown in Figure 6. Significant signal broad-
ening and a new signal at d = 5.19 ppm (Ha of the Z form) and
a decrease in d = 5.07 ppm (Ha of the E form) was found in
1H NMR spectra after irradiation, as illustrated in Figure 4.
Integrals of the two signals at d = 5.07 ppm and d = 5.19 ppm
appear with a ration of 3:5, which suggests that at the
photostationary about 63% of [a-CD-NPSI]2ꢀ was trans-
formed from the E form to the Z form.
Figure 4. 1H NMR spectra (500 MHz in D2O at 298 K) of [a-CD-
NPSI]2ꢀ a) before irradiation and b) after irradiation at 335 nm for
65 min (b).
The absorption changes for the E/Z photoisomerism of
systems in aqueous solution are shown in Figure 7. After
irradiation at 335 nm of a-CD-NPSI (7.8 ” 10ꢀ6 m) for 48 h,
little change in absorption was found,[10] which is in accor-
dance with the lack of photoisomerization found, as men-
tioned above. The conversion of a-CD-NPSI into its disodium
salt [a-CD-NPSI]2ꢀ by adding 10 moles of Na2CO3 (pH 9.6)
resulted in photoisomerization to give a E/Z mixture after
irradiation at 335 nm for 48 min, characterized by an increase
in the absorption at around 280 nm (DA = 0.040), a decrease
in the absorption at 335 nm (DA = 0.026), and by the presence
of two isobestic points at 317 nm and 370 nm as shown in
Figure 7. Similarly, the irradiation on the disodium salt of
the strong hydrogen bonding between the OH groups in a-
CD and the two carboxyl groups in the isophthalic acid unit,
which consequently prevent the a-CD ring from shuttling
away to the biphenyl unit, as illustrated in Figure 6. Con-
version of the carboxyl groups into the disodium salt by
adding 10 moles of solid Na2CO3 to the solution (pH 10.5),
which destroys the strong hydrogen bonds, enables the
photoisomerization of [a-CD-NPSI]2ꢀ. Before irradiation,
the same NOEs between the aromatic protons and 3-H, 5-H
and 6-H protons on the interior of the a-CD annulus were
found, as shown in Figure 3, which indicates that the ring is
dumbbell NPSI (Na+ [NPSI]2ꢀ) for 15 min caused an increase
2
in the absorption at around 280 nm (DA = 0.049) and a
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Figure 6. The strong hydrogen bonding between the OH groups in a-CD and the two carboxyl groups in the isophthalic acid unit locks the photo-
driven motion of a-CD. The addition of Na2CO3 destroys the strong hydrogen bonds, thereby unlocking the shuttling motion.
Figure 8, while there is only a small increase by about 4%
in the intensity of the same fluorescence band in [NPSI]2ꢀ.[10]
This behavior is the result of the rigidity of the a-CD ring
shifting away from the stilbene unit to the biphenyl unit,
which hinders the vibration and the rotation of the bonds in
the methylene and biphenyl units. This is confirmed by the
finding of the weaker fluorescent intensity of dumbbell
[NPSI]2ꢀ,[10] in which the vibration and rotation modes more
facile without the macrocycle.
When the photoisomerization of [a-CD-NPSI]2ꢀ from the
E form to the Z form and the consequent a-CD shuttling have
Figure 7. The absorption spectra of an aqueous solution of [a-CD-
NPSI]2ꢀ a) before and b) after irradiation at 335 nm for 4 min,
c) 16 min, d) 48 min. e) These changes in the absorption spectra can
be reversed by irradiating the solution at 280 nm (25 min). The absorp-
tion changes of an aqueous solution of [NPSI]2ꢀ (7.8”10ꢀ6 m) at 258C
after irradiation (335 nm) for 15 min were very similar.
decrease at 335 nm (DA = 0.032). The fact that the spectral
changes for [NPSI]2ꢀ are greater than for [a-CD-NPSI]2ꢀ
indicates that [NPSI]2ꢀ undergoes E!Z photoisomerization
more easily.[11] This is not surprising as the E!Z isomer-
ization becomes more difficult in the presence of the a-CD
ring.[9] The maximum absorption of 4-amino-3,6-disulfonic-
1,8-naphthalimide disodium salt at around 422 nm changes
little, because this unit is separated from the residues by the
methylene group.
Figure 8. The increasing intensity of fluorescence (emission at
422 nm) of an aqueous solution of [a-CD-NPSI]2ꢀ (7.8”10ꢀ6 m) after
irradiation at 335 nm (0 to 48 min). These changes in the absorption
spectra can be reversed by irradiating the solution at 280 nm (25 min).
Insert: Changes in fluorescence intensity at 530 nm (excited at
422 nm) for aqueous solution of [a-CD-NPSI]2ꢀ on alternating irradia-
tion at 335 nm and 280 nm.
The absorption changes in [a-CD-NPSI]2ꢀ are accompa-
nied by a parallel increase by about 46% in the intensity of
the fluorescence band at lmax = 530 nm, as illustrated in
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Chemie
Commun. 2001, 1860 – 1861; e) A. M. Brouwer, C. Frochot, F. G.
been performed, the equilibrium can be shifted back to the E
form by irradiation at 280 nm. This process is accompanied by
a decrease in the absorption at around 280 nm and a rise at
335 nm (Figure 7), as well as a decrease in the intensity of
fluorescence at lmax = 530 nm (Figure 8). Because of the full
reversibility of the photoisomerization process, the photo-
induced shuttling motion of the a-CD can be repeated
(Figure 6) with reversible fluorescent output signals (Figure 8
insert). Thus, the photoinduced shuttling motion in a-CD-
NPSI, which is a lockable molecular switch, can be easily
detected by means of a fluorescent output signal at 530 nm.
In summary, a new [2]rotaxane, [a-CD-NPSI]2ꢀ, is a light-
driven molecular shuttle, in which the a-CD macrocycle can
shuttle back and forth between the stilbene unit and biphenyl
unit by alternating the irradiation frequency. This shuttling is
accompanied by obvious changes in the intensity of fluores-
cence at 530 nm. The shuttle can be locked by simple
acidification. It also should be noted that the inputs (switching
on at 335 nm and off at 280 nm) are photochemical.
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Received: January 9, 2004
Revised: March 3, 2004 [Z53708]
Keywords: cyclodextrins · molecular devices · photochemistry ·
.
rotaxanes · supramolecular chemistry
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