A light-driven rotaxane molecular shuttle with dual fluorescence addresses

Article abstract of DOI:10.1021/ol049605g

A molecular shuttle containing an α-CD macrocycle, an azobenzene unit, and two different fluorescent naphthalimide units was synthesized. The cis-trans photoisomerization of the azobenzene unit resulted in the motion of the CD macrocycle on the track. Because of the easy regulation and full reversibility of the fluorescence change of the two stopper units, the molecular shuttle could be used as a molecular storage medium or switch with all-optical inputs and outputs.

Full text of DOI:10.1021/ol049605g

ORGANIC
LETTERS
2004
Vol. 6, No. 13
2085-2088
A Light-Driven Rotaxane Molecular
Shuttle with Dual Fluorescence
Addresses
Da-Hui Qu, Qiao-Chun Wang, Jun Ren, and He Tian*
Lab for AdVanced Materials and Institute of Fine Chemicals, East China UniVersity of
Science & Technology, Shanghai 200237, P.R. China
tianhe@ecust.edu.cn
Received March 2, 2004
ABSTRACT
A molecular shuttle containing an r-CD macrocycle, an azobenzene unit, and two different fluorescent naphthalimide units was synthesized.
The cis−trans photoisomerization of the azobenzene unit resulted in the motion of the CD macrocycle on the track. Because of the easy
regulation and full reversibility of the fluorescence change of the two stopper units, the molecular shuttle could be used as a molecular
storage medium or switch with all-optical inputs and outputs.
A rotaxane1 is described as a molecular system in which a
macrocycle threads a linear subunit with two bulky stoppers.
Rotaxanes have attracted more and more attention because
of their challenging constructions and potential applications
in areas such as molecular switches,2 molecular logic gates,3
and molecular wires.4 To realize their full potential, rotaxanes
must have two basic properties. On one hand, the binary
states, namely, the “0” state and “1” state, must be reversible.
On the other hand, as output signals, they must be easily
recognizable. In previous studies, the binary states of most
molecular machines were distinguished by NMR spectra,5,6
cyclic voltammetry,7 complexation ability differences of
certain ions of the two states,8 or circular dichroism. It is
rather inconvenient to transform these spectral signals into
easily detected output codes, however. Using fluorescence
change as an output is attractive because the signal can be
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Chem. Soc. 1994, 116, 9399. (b) Livoreil, A.; Sauvage, J.-P.; Armarolim
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(1) (a) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew.
Chem., Int. Ed. 2000, 39, 3348. (b) Feringa, B. L. Molecular Switches;
Wiley-VCH: Weinheim, 2001.
(2) (a) Asakawa, M.; Higuchi, M.; Mattersteig, G.; Nakamura, T.; Pease,
A. R.; Raymo, F. M.; Shimizu, T.; Stoddart, J. F. AdV. Mater. 2000, 12,
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10.1021/ol049605g CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/03/2004

Figure 1. A4, B5, dumbbell NN, and [2]rotaxane NNCD studied in this work.
easily and remotely detected with low cost. However, reports
on induction of a rotaxane to switch between different
fluorescent states (output) as a response to a clean input are
rare.9
also prepared using the same conditions, in the absence of
R-CD, with a yield of 30%.
Both rotaxane NNCD and dumbbell NN can be character-
1
ized by H NMR spectra. Because of the low solubility of
In this report, we designed and synthesized an unsym-
metrical light-driven molecular shuttle based on a [2]rotaxane
NNCD (Figure 1) with an R-CD as the ring, azobenzene
and biphenyl chain as the threading molecular chain, and
two different fluorescent naphthalimide moieties: 4-amino-
1,8-naphthalimide-3,6-disulfonic disodium salt (the “N”
stopper) and 1,8-naphthalimide-5-sulfonic sodium salt (the
“S” stopper) as the two bulky terminal stoppers. In this
[2]rotaxane NNCD, reversible motion of the R-CD macro-
cycle between two stations (azobenzene station and biphenyl
station) takes place after UV irradiation at 360 and 430 nm,
respectively. This motion affects the fluorescence spectra of
the two naphthalimide stoppers. When the azobenzene unit
is in the trans form, the R-CD ring is located here. After a
2 min of irradiation with 360 nm UV light, the trans form
((E)-NNCD) changes to the cis form ((Z)-NNCD) and the
R-CD moves to the biphenyl station. Irradiation with 430
nm light converts the (Z)-NNCD back to (E)-NNCD (Figure
1). The [2]rotaxane NNCD was synthesized by Pd-catalyzed
Suzuki coupling where phenyl boronic acid B5 reacted with
azobenzene phenyl halide A4 in the presence of R-CD in an
Ar-saturated Na2CO3 aqueous solution according to Ander-
son’s method.10 As shown in Figure 1, treating equal
equivalents of A4 and B5 with 2.5 equiv of R-CD for 20 h
at 80 °C in an Ar-saturated 0.2 M Na2CO3 aqueous solution
with 0.05 equiv of Pd(OAc)2 catalyst led to the formation
of [2]rotaxane NNCD. Chromatography (silica gel, the upper
layer was 1.3:2:5 acetic acid/n-butanol/water) gave pure
NNCD with a yield of 15%. The non-CD dumbbell NN was
dumbbell NN in D2O, the measurements were conducted in
DMSO-d6 (298 K). Figure 2 shows the H NMR spectra of
1
Figure 2. 1H NMR spectra (500 MHz in DMSO-d6 at 298 K) of
(a) dumbbell NN, (b) rotaxane (E)-NNCD, and (c) the same b
solution after irradiation at 360 nm for 30 min ((Z)-NNCD) and
(d) 1H NMR spectra (500 MHz in D2O at 298 K) of rotaxane (E)-
NNCD.
NN and NNCD in DMSO-d6 (298 K). Because of the
influence of R-CD, the chemical shifts of the protons b-e
of the thread shown in Figure 1 have different changes: Hb
(∆δ ) 0.06 ppm), Hc (∆δ ) -0.09 ppm), Hd (∆δ ) 0.24
ppm), He (∆δ ) -0.02 ppm). Compared to the methylene
(9) Wang, Q.-C.; Qu, D.-H.; Ren, J.; Chen K.-C.; Tian, H. Angew. Chem.,
Int. Ed. 2004, 43, 2661.
(10) (a) Stanier, C. A.; Alderman, S. J.; Claridge, T. D. W.; Anderson,
H. L. Angew. Chem., Int. Ed. 2002, 41, 1769. (b) Stanier, C. A.; O’Connell,
M. J.; Clegg, W.; Anderson, H. L. Chem. Commun. 2001, 493.
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Org. Lett., Vol. 6, No. 13, 2004

resonance of NN for Ha,h (a singlet at δ 5.31), the methylene
resonance of (E)-NNCD is split into a doublet at δ 5.48 (Ha,
∆δ ) 0.17 ppm) and a singlet at δ 5.37 (Hh, ∆δ ) 0.06
ppm) due to the influence of R-CD that is near the methylene
of Ha and far from the methylene of Hh. The measurement
of NNCD was also conducted in D2O at 298 K (Figure 2d).
Compared to neat R-CD, an obvious upfield shift of H3 (∆δ
) 0.15 ppm) and H5 (∆δ ) 0.11 ppm) of the R-CD
macrocycle in NNCD was observed. This is a result of the
aromatic shielding effect of the threaded benzene rings in
the cavity of the R-CD, suggesting that the R-CD ring is
threaded by dumbbell NN.
At the same time, the resonance peaks of the two
methylenes are split completely. The resonance of Ha splits
into multiplets, while the resonance of Hh is a singlet. The
two-dimensional ROESY NMR of NNCD in DMSO-d6
reveals the relativity of the protons of the aromatic regions
and R-CD. As shown in Figure 3, strong NOEs are observed
Figure 4. High-field portions of the two-dimensional ROESY
NMR spectrum of NNCD under the same conditions as Figure 3.
This is attributed to the deshielding effect of R-CD due to
the shifting of the ring away from the azobenzene unit to
the biphenyl unit. The two signals at δ 5.57 (Hh of the cis
form) and δ 5.48 (Ha of the trans form) appear at a 10:7
ratio. This suggests that, at the photostationary state, about
59% of (E)-NNCD was transformed to its cis form (Z)-
NNCD.
Similar to Nakashima’s rotaxane,5c both compounds
NNCD and NN in this work showed photoisomerization. UV
light (360 nm) irradiation of the DMF solution of NNCD
(1.0 × 10-5 M) caused photoisomerization from the trans to
cis configuration of the azobenzene unit in NNCD, which
returned to the trans configuration following irradiation at
430 nm for 2 min. The change is characterized by an increase
in absorption at around 275 nm (∆A ) 0.08) and a decrease
in absorption at 350 nm (∆A ) 0.07), characteristic of the
photoisomerization of the azobenzene units (shown in
Supporting Information). Prolonging irradiation beyond 2
min under this experimental condition does not result in any
change in the absorption and the fluorescence spectra.
A phenomenon was found in the fluorescence spectra of
compound NNCD. UV light (360 nm) irradiation of NNCD
in DMF (1.0 × 10-5 M) makes the fluorescence at around
520 nm (due to the “N” stopper) weaker and the fluorescence
at around 395 nm (due to the “S” stopper) stronger, as
illustrated in Figure 5a. As the R-CD ring is shifting away
from the azobenzene unit toward the biphenyl unit, the
biphenyl unit becomes more rigid. The vibration and the
rotation of the bonds in the methylene and azobenzene units
are enhanced, and the vibration and the rotation of the bonds
in the methylene and biphenyl units are hindered. This is
confirmed by the fluorescent intensity of NN. The fluores-
cence of NN at around 395 nm is weaker than that of (Z)-
NNCD after irradiation, and the fluorescence at around 520
nm is weaker than that of (E)-NNCD. The vibration and
rotation of the bonds in NN is facilitated by the absence of
the macrocycle. Additionally, there is little change in the
fluorescence of NN before and after UV irradiation, as is
Figure 3. Two-dimensional ROESY NMR spectrum of NNCD
(500 MHz in DMSO-d6 at 298 K) after a mixing time of 300 ms.
from the Hb and Hc to positions on the 2,3-rim of the R-CD
(mostly H-3, point B) and from the Hd,He to positions on
the 6-rim of the R-CD (mostly H-5, point A). Simultaneously,
relatively strong NOEs are observed from Hf to the H-6 and
OH-6 of the R-CD (point C in Figure 3) and from Ha to the
OH-2 of R-CD (Figure 4, point A). No NOE was found
between Ha and the OH-6 of R-CD, demonstrating that the
rotaxane NNCD exists as a single isomer. The orientation
of the R-CD on the thread was confirmed as that shown in
Figure 1.
After UV irradiation by 360 nm for 30 min, there was
very little change in the resonance of the aromatic positions
of NNCD in DMSO-d6, while change in the resonance of
the methylene protons was more profound. The latter was
split into two doublets and one singlet as compared to one
doublet and one singlet before the irradiation. A new signal
was found at δ 5.57 (Hh of the cis form shown in Figure 2).
Org. Lett., Vol. 6, No. 13, 2004
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Figure 6. Changes in the fluorescence intensity of NNCD in DMF
(the upper line is the emission at λ ) 395 nm, and the other line
is the emission at λ ) 520 nm) along with changes in irradiation
time and light source. Light sources of 360 and 430 nm UV light
were alternated every 2 min.
changes (blue at 395 nm and green at 520 nm) act as the
“0” state and the “1” state, respectively. Alternation of the
“0” state and the “1” state is accomplished completely by
optical stimuli, UV light at 360 and 430 nm. Furthermore,
the fluorescence signals indicating the molecular shuttle
movement are very sensitive.
In summary, the [2]rotaxane NNCD is a light-driven
molecular shuttle in which the R-CD macrocycle can be
stimulated to shuttle back and forth between the azobenzene
unit and biphenyl unit by irradiation. This is accompanied
by reversible changes in fluorescence intensity of the two
fluorescent stoppers, which make it possible for the rotaxane
to be a molecular storage or a molecular logic gate. It should
be noted that the output is a fluorescence signal, which can
be easily detected remotely, in contrast to other spectral
signals. The inputs to this system are also optical, which are
superior to chemical or electrochemical inputs because of
their cleanness and convenience. Finally, the complete
reversibility of the state exhibited by the molecular system
allows for repetitive processing of the input signals.
Figure 5. Fluorescence spectra of [2]rotaxane (a) NNCD and (b)
NN in DMF (1 × 10-5 M) at 25 °C after irradiation with UV light
360 nm for different times.
shown in Figure 5b. Figure 5 also demonstrates that the
motion of the CD in NNCD greatly affects the fluorescence
of the two stoppers. Similar to the absorption spectra of
NNCD, the fluorescence spectra of NNCD could be repeated
over many cycles with UV irradiation alternating between
360 and 430 nm in 2 min intervals. The relative fluorescence
quantum yields of compound NNCD and NN are listed in
Table S1 of the Supporting Information. The fluorescent
quantum yield ratio between (Z)-NNCD and (E)-NNCD is
close to 2 at 395 nm and 0.65 at 520 nm. The fluorescence
of the dumbbell NN did not change after UV irradiation,
further proving that the R-CD strongly affects the fluores-
cence of the stoppers.
Acknowledgment. This work was supported by NSFC/
China (20273020) and Education Committee of Shanghai.
We thank Prof. Yu Liu (Nan Kai University/China) for his
relevant advice on the synthesis.
Figure 6 shows changes in the fluorescence intensity of
NNCD following irradiation. In Figure 6, the upper line is
the fluorescence intensity change of the “S” stopper (λex
)
336 nm) and the lower line is for the fluorescence intensity
change of the “N” stopper (λex ) 438 nm). As shown in
Figure 6, the rotaxane has excellent recovery properties,
which is useful for studying information storage and mo-
lecular logic gates. The two kinds of fluorescence signal
Supporting Information Available: Absorption spectra
and synthetic procedures of NNCD and NN. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL049605G
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Org. Lett., Vol. 6, No. 13, 2004