Photoisomerization of spiropyran for driving a molecular shuttle

Article abstract of DOI:10.1021/ol7015862

A novel light-powered molecular shuttle was synthesized which can switch the movement of a macrocycle between two distinct stations-dipeptide and zwitterionic ME-by exploiting the photoisomerization of spiropyran. The macrocycle resides selectively in the

Full text of DOI:10.1021/ol7015862

ORGANIC
LETTERS
2007
Vol. 9, No. 20
3929-3932
Photoisomerization of Spiropyran for
Driving a Molecular Shuttle
Weidong Zhou, Dugang Chen, Junbo Li, Jialiang Xu, Jing Lv, Huibiao Liu, and
Yuliang Li*
National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids,
Institute of Chemistry, Chinese Academy of Sciences, Beiijng 100080, P. R. China
ylli@iccas.ac.cn
Received July 5, 2007
ABSTRACT
A novel light-powered molecular shuttle was synthesized which can switch the movement of a macrocycle between two distinct stations
s
dipeptide and zwitterionic ME by exploiting the photoisomerization of spiropyran. The macrocycle resides selectively in the dipeptide station
s
in the SP form and moves to the ME station under the irradiation of UV light. This movement process of the macrocycle is accompanied by
reversible absorptive output signals which can be detected by the naked eye.
With the great development of biotechnology and biochem-
istry in the past decades, scientists have been encouraged to
construct molecular motors for simulating natural motors.1
Considering the extreme complexity of the natural motors,
what can be done, at present, is to construct simple prototypes
of artificial molecular motors, consisting of a few compo-
nents capable of moving in a controllable way, and to
investigate the associated problems. Mechanically interlocked
molecules, such as catenanes and rotaxanes, have become
typical candidates in the design of artificial molecular
machines because they consist of a few noncovalently
interacting components with the ability to move reversibly
between two or more stations on application of external
stimuli.2-5 Various stimuli have been employed to induce
such switching, including metal binding,2 configurational
changes,3 and alteration of the oxidation state4 or protonation
level5 of the molecule. Among them, the chemical fuel-
powered artificial motors described so far are not autonomous
because, after the mechanical movement induced by a
chemical input, they need another, opposite, chemical input
to reset, which also implies generation of waste products.
Light and electricity powered artificial motors are now under
prominent consideration in the construction of molecular
devices due to the convenience of energy input and the
absence of waste products.6
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10.1021/ol7015862 CCC: $37.00
© 2007 American Chemical Society
Published on Web 09/06/2007

Spiropyran (SP), a typical photoisomerizable compound,
has been widely exploited for the construction of various
logic gates and optical switches due to the remarkable
difference of the two isomers and their high reversibility
under the irradiation of different light. As shown in Figure
1, the colorless SP does not show absorption bands at
Here, we describe a [2]rotaxane in which the light powered
isomerization of the SP in the thread induces the movement
of the macrocycle along the thread. The molecular shuttle
SP-1/ME-1 consists of an isophthalamide-based macrocycle
mechanically locked onto a thread, SP-2/ME-2, featuring
two potential H-bonding stations - short peptide unit and an
open form ME unit. The short peptide unit has previously
been shown10 to be an excellent geometrical and electronic
fit for isophthalamide macrocycles. The second station is
related to the SP group which is a weakly hydrogen bonding
acceptor as the ester group in the SP form (gray) but a
powerful hydrogen-bond acceptor as the phenolate anion in
the ME form (purple) (Scheme 1). So, the macrocycle resides
Scheme 1. Synthesis Route of the Molecular Shuttle
Figure 1. Photoisomerization process of spiropyran (SP).
wavelengths greater than 400 nm. When it is irradiated by
ultraviolet light, it switches to the purple merocycanine (ME),
accompanied by the appearance of an absorption band at 568
nm and an emission band at 640 nm.7 These bands disappear
when the solution is irradiated with visible light, as ME
switches completely to SP. Both transformations are highly
reversible and can be detected by the naked eye, even without
the help of spectral equipment. As indicated in the structural
drawing, the open form exhibits a zwitterionic structure with
a nitrogen cation and a phenolic anion. This suggests that
ME has strong electronic donor ability and can function as
a favorable metal ion ligand and a hydrogen bond acceptor.8
Amide-amide hydrogen bonding of short peptide units
with isophthalamide macrocycles has been well studied by
the Leigh group for template-induced synthesis of rotax-
anes.9,10 Recently, Leigh has successfully demonstrated that
strong anion hydrogen bonding translocates the isophthal-
amide macrocycle from a neutral hydrogen-bonding station
in a molecular shuttle, though there is limited data or theory
with which to compare the hydrogen-bonding ability of
anions with neutral functional groups.9
(6) (a) Eelkema, R.; Pollard, M. M.; Vicario, J.; Katsonis, N.; Ramon,
B. S.; Bastiaansen, C. W. M.; Broer, D. J.; Feringa, B. L. Nature 2006,
440, 163-163. (b) Bern, J.; Leigh, D. A.; Lubomska, M.; Mendoza, S. M.;
Pe´rez, E. M.; Rudolf, P.; Teobaldi, G.; Zerbetto, F. Nature Mater 2005, 4,
704-710; c) Balzani, V.; Clemente-Leon, M.; Credi, A.; Ferrer, B.; Venturi,
M.; Flood, A. H.; Stoddart, J. F. Proc. Natl. Acad. Sci. U.S.A. 2006, 103,
1178-1183.
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Chem. Soc. 2003, 125, 2361-2364. (c) Raymo, F. M.; Giordani, S. Org.
Lett., 2001, 3, 3475-3478. (d) Raymo, F. M.; Giordani, S. Org. Lett. 2001,
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(11) Altieri, A.; Bottari, G.; Dehez, F.; Leigh, D. A.; Wong, J. K. Y.;
Zerbetto, F. Angew. Chem., Int. Ed. 2003, 42, 2296.
selectively in the dipeptide station in the SP form, and moves
to the ME station under the irradiation of UV light. The
macrocycle will return to the dipeptide station with the
isomerization of ME to SP upon the irradiation of visible
light.
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Org. Lett., Vol. 9, No. 20, 2007

The shuttle was synthesized according to Scheme 1. What
surprised us is that the hydroxyl group in SP-OH did not
react with normal acid under the catalysis of EDCI and
DMAP or even the benzoyl chloride, which might be due to
the passivation effect from the SP unit. In fact, the yield is
less than 40% even when it reacts with one equiv of super
active chloroacetyl chloride under the base of Et3N. The
target rotaxane SP-1, incorporating glycylglycine and SP unit
in the thread and an isophthalamide macrocycle, was synthe-
sized through a template induced clipping strategy. Treatment
of the thread SP-2 with 10 equiv of both p-xylylene diamine
and isophthaloyl dichloride (CH2Cl2, Et3N, 4 h, high dilution)
provided [2]rotaxanes SP-1 in 20% yields.
Since the xylylene parts of the macrocycle shields encap-
sulated regions of the thread, the position of the macrocycle
could be determined by comparing the chemical shift of the
protons in the rotaxane with those of the corresponding
thread.9 The 1H NMR spectra (Figure 2) of thread SP-2 and
rotaxane SP-1 confirm the interlocked nature of SP-1 and
show that, in the form of SP-1, the macrocycle is largely
localized on the dipeptide region of the thread (Scheme 1).
The upfield shifts of the methylene resonances of the
dipeptide station (He 1.078, Hc 0.402) in SP-1 are charac-
teristics9 of aromatic shielding by the encapsulating macro-
cycle. The ester carbonyl is a weaker hydrogen bond acceptor
than the amides and does not appear to contribute signifi-
cantly to the intercomponent binding.
The thread amide resonances, Hd and Hf, are shifted
downfield by 0.505 and 0.513 ppm, respectively, in the
rotaxane as a result of the collective influences of (i) aromatic
shielding (upfield shift) and (ii) inductive effects of hydrogen
bonding to the thread amide groups from macrocycle
(downfield shift). There is no evidence of any interaction
between the macrocycle and the SP group.
UV light irradiation (365 nm, high press mercury lamp,
100W) of SP-1 for 3 min results in significant changes to
1
the H NMR spectrum in CD3CN (Figure 2c). The opened
structure ME-1 is approximately 26% of the total isomers
according to the analysis of the 1H NMR spectrum7 (see the
Supporting Information). When the thread SP-2 is irradiated
under the same conditions, the opened form ME-2 is so
active that it returns to SP-2 during the process of 1H NMR
measurement, though the change of color can be detected.
Kinetic studies indicate that the rate constant of ME-2 to
SP-2 is about 6 times as large as that of ME-1 to SP-1 (see
the Supporting Information). This increased stability of ME-1
indicates an interaction between the macrocycle and the ME
unit in the [2]rotaxane, which stabilizes the zwitterionic
ME-1. The upfield shifts of the methylene resonances
adjacent to the ME station (Hii′ 0.89, Figure 2c) in ME-1
are characteristics of aromatic shielding by the encapsulating
macrocycle, because the signals of Hii′ would downfield shift
with the isomerization in the free SP unit.7e Twenty-six
percent of the integral area of the signals assigned to He
downfield shifts to 4.00 ppm, which is strong evidence for
the movement of the macrocycle away from the dipeptide
during isomerization of the SP unit. That is, about 26% of
the cyclic tetraamide moves from the peptide to the ME
1
Figure 2. Partial H NMR spectra (600 MHz, 298 K, CD3CN, 1
× 10-3 M) of the SP-2 (a), SP-1 (b), ME-1 (gained from irradiation
of SP-1 with 365 nm UV light for 3 min) (c), and SP-1(gained
from irradiation of ME-1 with sunlight for 3 min) (d).
station on irradiation. In addition, 26% of two signals
belonging to Hll’ shifted downfield to a single signal due to
the equalization in the isomerization process, which indicates
that 26% of the spiropyran opens up on irradiation. Both
features indicate that the cyclic tetraamide is highly selective
around the ME station in ME-1 (near 100%) (Scheme 1).
As shown in Figure 2d, all 1H NMR signals of ME-1 return
to those of SP-1 upon irradiation with sunlight, which
indicates the cyclic tetraamide moves back to the dipeptide
end when ME is isomerized to SP and supports the
reversibility of this shuttle.
The absorption spectra of colorless solutions SP-2 and
SP-1 (Figure 3) do not show bands at wavelengths greater
Org. Lett., Vol. 9, No. 20, 2007
3931

Figure 4. Absorption spectrum of SP-1 (5 × 10-5 M, CH3CN, 25
°C) at 568 nm under irradiation of alternate light (UV light at 365
nm and sun light) for six cycles.
Figure 3. Absorption spectra of SP-1, SP-2, ME-1, and ME-2 (5
× 10-5 M, CH3CN, 25 °C). Insets are the emission spectra of ME-1
and ME-2 (5 × 10-5 M, CH3CN, 25 °C). ME-1 and ME-2 were
gained from the irradiation of SP-1 and SP-2 with 365 nm UV
light for 3 min.
(Figure 4). This movement of the macrocycle is accompanied
by reversible absorptive output signals which can be detected
even by the naked eye.
In conclusion, we have synthesized a novel photopowered
molecular shuttle that can switch the movement of a
macrocycle between two distinct stationssdipeptide and
zwitterionic MEsby exploiting the photoisomerization of
spiropyran. The highly reversible shuttle of the macrocycle
was accompanied by an obvious absorption output signal,
which can be detected by the naked eye. The incorporation
of one switchable unit in the molecular shuttle as a bulk
stopper and an interaction site will introduce more under-
standing on the construction of future molecular shuttles.
than 400 nm. Upon irradiation of both solutions with
ultraviolet light, an absorption band at 568 nm and an
emission band at 640 nm appeared, which revealed the
colorless SP-2 and SP-1 switched to purple ME-2 and
ME-1, respectively.7 As shown in Figure 3, the absorption
and emission wavelength of ME-1 is similar to that of
ME-2, but the intensity of absorption at 568 nm and emission
at 640 nm from ME-1 is larger than that from ME-2. Though
the change of absorption spectra does not reflect the
movement process of the macrocycle directly, the stronger
absorption of SP-1 than SP-2 at 568 nm after the same
irradiation of UV light verifies the fact that the macrocycle
promotes the isomerization of SP-1 to ME-1.
Different photostationary states exist for SP-1 and ME-1
(relative percentage of SP-1/ME-1 was alternated from 100:0
to 74:26), and evident absorptive and fluorescent changes
could be achieved by irradiating with light of different wave-
lengths. Because of the good reversibility of the photo-
isomerization process, the photoinduced shuttling motion of
the macrocycle could be repeated remarkably well many
times without obvious degradation at room temperature
Acknowledgment. This work was supported by the
National Natural Science Foundation of China (20531060,
20473102, 20571078) and the Major State Basic Research
development Program (2006CB806200, 2007CB936401).
Supporting Information Available: Full experimental
details pertaining to the preparation and characterization of
SP-1/ME-1. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL7015862
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Org. Lett., Vol. 9, No. 20, 2007