A new photo-switchable "on-off" host-guest system

Article abstract of DOI:10.1039/c1pp05055a

A new photo-switchable "on-off" host-guest system comprising cucurbit[7]uril (CB[7]) and a photoresponsive cinnamamide derivative (trans-(3-phenyl-acryloylamino)-acetic acid, E-1) is studied. The cinnamamide derivative and CB[7] forms a stable 1:1 host-gu

Full text of DOI:10.1039/c1pp05055a

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Cite this: Photochem. Photobiol. Sci., 2011, 10, 1415
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PAPER
A new photo-switchable “on-off” host–guest system†‡
a
a,b
a
a
a,b
Youngkook Kim, Young Ho Ko, Minseon Jung, Narayanan Selvapalam and Kimoon Kim*
Received 1st February 2011, Accepted 8th March 2011
DOI: 10.1039/c1pp05055a
A new photo-switchable “on-off” host–guest system comprising cucurbit[7]uril (CB[7]) and a
photoresponsive cinnamamide derivative (trans-(3-phenyl-acryloylamino)-acetic acid, E-1) is studied.
The cinnamamide derivative and CB[7] forms a stable 1 : 1 host–guest complex (CB[7]·E-1) with a high
4
-1
binding constant (K = 2.1 ¥ 10 M ). Irradiation of UV light (300 nm) to an aqueous solution of
CB[7]·E-1 induces the E- to Z-conformational change of the cinnamamide derivative, which then leads
1
to the dissociation of the complex as evidenced by UV-visible and H-NMR spectroscopy. The reverse
process, photo-induced host–guest complex formation between CB[7] and Z-1 is achieved by irradiation
of UV light at 254 nm. The photo-switchable “on-off” host–guest system shows high reversibility and
switching efficiency, which makes it potentially useful in designing photoresponsive gating systems.
1
6
Introduction
and alkali metal ions across a lipid membrane. This success
encouraged us to extend this work to CB[7]-based, stimuli-
responsive gated ion channels, especially photo-responsive gated
ion channels. As a part of this effort, we first decided to investigate
the light-controlled host–guest complexation of CB[7]. Although
The control of host–guest recognition behavior by external
1,2
stimuli has received much attention because of its potential
3
applications in many areas, including molecular logic gates,
information storage and gating systems. Especially, stimuli-
responsive gating systems are useful for the construction of drug
releasing systems or stimulus-gated artificial ion channels to
mimic natural ion channels. The development of the latter is
important not only for the fundamental understanding of natural
4
5
17
18
the redox-controlled and pH-controlled guest complexation
of CB[7] has been investigated, the guest complexation and
decomplexation of CB[7] triggered by light has not been studied.
Cinnamamide (Scheme 1), whose photoisomerization proper-
ties such as transform efficiency, wavelength-dependence, and
thermal stability have been well studied, is an attractive pho-
6
7
ion channels but also for applications such as in drug discovery
8
and sensors. Despite extensive studies, however, there are only a
19
toresponsive component for molecular switch systems, because
of the high conformational thermal stability of the Z isomer
9–11
few examples
of stimuli-responsive gated ion channels partly
because stimuli-responsive “on-off” host–guest systems for gating
are rare. Thus, the design and synthesis of an efficient host–guest
system that forms a complex and dissociates in response to external
stimuli still pose a challenge in chemistry for the development of
artificial gated ion channels.
12,13
Cucurbit[n]uril (CB[n], n = 5–10),
a family of host molecules
comprising n glycoluril units, have a hydrophobic cavity and
two identical carbonyl-laced portals. Their unique ability to
form stable host–guest complexes with high binding affinity and
14
15
specificity make them useful in many applications. Some years
ago, we reported artificial ion channels incorporating long alkyl
chain attached CB[n] (n = 5, 6), which can transport proton
a
National Creative Research Initiative Center for Smart Supramolecules
(
CSS) and Department of Chemistry, Pohang University of Science
and Technology (POSTECH), Pohang, 790-784, Korea. E-mail: kkim@
postech.ac.kr
b
Division of Advanced Materials Science, Pohang University of Science and
Technology (POSTECH), Pohang, 790-784, Korea
†
This article is published as part of a themed issue in honour of Yoshihisa
Inoue’s research accomplishments on the occasion of his 60th birthday.
Electronic supplementary information (ESI) available: 2D COSY and
‡
ROESY NMR spectra, ITC profiles, calibration curves of E-1 and
CB[7]·E-1. See DOI: 10.1039/c1pp05055a
Scheme 1 Photo-switchable “on-off” host–guest system.
This journal is © The Royal Society of Chemistry and Owner Societies 2011 Photochem. Photobiol. Sci., 2011, 10, 1415–1419 | 1415

20
after E to Z photoisomerization. The Z isomer conserves its
conformation at ambient conditions until irradiation with light
of specific wavelength (254 nm) for the reverse reaction. This
fascinating physical property provides a suitable operating stability
to molecular switches or stimuli-responsive systems. Very recently,
we discovered that CB[7] and (E)-cinnamamide form a stable 1 : 1
5
-1
host–guest complex with a binding affinity of 2.24 ¥ 10 M (Fig.
S1 of ESI‡). This observation prompted us to design a photo-
switchable host–guest complexation system based on CB[7] and
cinnamamide. Herein we report a new switchable “on-off” host–
guest system incorporating CB[7] and a cinnamamide derivative
(
trans-(3-phenyl-acryloylamino)-acetic acid, E-1) triggered by UV
light irradiation as illustrated in Scheme 1.
Results and discussion
As mentioned above, (E)-cinnamamide has a high binding affinity
to CB[7]. We also tried to measure the binding affinity of (Z)-
cinnamamide with CB[7] to check the effect of the isomerization
on the binding affinity. However, the maximum E to Z pho-
toisomerization of cinnamamide by UV irradiation (300 nm) is
approximately 85% (Z to E isomer ratio = 6 : 1), which hampered
the accurate measurement of the binding affinity of the pure Z
isomer to CB[7]. Nevertheless, the binding affinity of a 6 : 1 mixture
of Z isomer and E isomer toward CB[7] was measured to be 9.2
1
Fig. 1 H NMR spectra of (a) E-1, (b) after irradiation of UV light
300 nm) to E-1 for 3 min (Z-1:E-1 = 6 : 1), (c) CB[7]·E-1, (d) after addition
of 1 equiv. of CB[7] to (b), (e) after irradiation of 300 nm light to (c) for
(
3
2
min, and (f) after irradiation of 254 nm light to (e) for 30 min in D
C. The signals labelled with * correspond to the CB[7]. The Z-1:E-1
ratio was calculated by integration of H NMR signals.
2
O at
4
-1
¥
10 M (see Fig. S2 of the ESI‡), suggesting that the binding
◦
5
4
-1
affinity of the Z isomer is approximately 7 ¥ 10 M assuming a
linearity in the binding affinity. The difference in binding affinity
between (E)-cinnamamide and (Z)-cinnamamide to CB[7] is too
small to construct a photo-switchable “on-off” host–guest system.
We thus decided to design a cinnamamide derivative (E-1)
1
(
Scheme 1) containing a negatively-charged carboxylate group
attached to the amide terminal of cinnamamide. We anticipated
that the phenyl ring of E-1 would be included in the CB[7] cavity
while the negatively-charged carboxylate terminal group would be
located at the outside of CB[7] cavity, sufficiently far away from the
electron-rich carbonyl groups of the CB[7] portal. In case of the
Z isomer (Z-1), however, the inclusion of the phenyl ring of the
guest would generate a significant repulsive interaction between
the portal of CB[7] and the carboxylate group of Z-1 (Scheme 1),
which might prevent the binding of the Z isomer to the host.
With this idea in mind, we first investigated the photoisomer-
1
ization properties of E-1 to Z-1 by H NMR and UV-visible
1
spectroscopy. The H NMR spectrum of E-1 showed noticeable
changes in the chemical shifts upon irradiation of UV light
(
300 nm), indicating that E to Z photoisomerization occurred.
Fig. 2 Absorption spectra of E-1 ( ), after irradiation with 300 nm light
to E-1 for 3 min (---), CB[7]·E-1 ( ◊ ◊ ◊ ) and after irradiation with 300 nm
The ratio of Z to E isomers increased with increasing irradiation
time and reached a maximum (Z-1:E-1 = 6 : 1) at 3 min (Fig. 1a and
1
light to CB[7]·E-1 for 3 min (-·-) in H
2
O.
b). Similar to cinnamamide, further irradiation of UV light did
not increase the Z to E ratio. The E to Z photoisomerization
was accompanied by a blue shift (~17 nm) of the absorption
maximum in the UV-visible spectrum (Fig. 2), similar to that
mixture of a ~1 : 2 mixture of Z-1 and E-1 as characterized by UV
spectroscopy (see Fig. S5b of the ESI‡).
1
of cinnamamide. No appreciable changes in UV-visible and H
The host–guest complex formation between CB[7] and E-1 was
studied by NMR spectroscopy, which confirmed the formation
of a stable 1 : 1 host–guest complex (CB[7]·E-1) in neutral water.
NMR spectra (Fig. 1b) were observed even after 1 week at room
temperature, indicating the high stability of the Z isomer. The
reverse Z to E isomerization was achieved by irradiation of UV
light at 254 nm. For example, irradiation of a 6 : 1 mixture of
Z-1 and E-1 with UV light at 254 nm for 30 min produced a
1
The H NMR spectrum of E-1 revealed noticeable changes in the
chemical shifts upon complexation with CB[7] (Fig. 1c), the peak
assignments of which have been aided by 2D NMR spectroscopy
1
416 | Photochem. Photobiol. Sci., 2011, 10, 1415–1419 This journal is © The Royal Society of Chemistry and Owner Societies 2011

(
see Fig. S3 of the ESI‡). The signals for two methine protons, 4
with CB[7]. The weak interaction between Z-1 and CB[7] was also
reflected in the ITC experiment; the association of Z-1 with CB[7]
is too weak to be measured by ITC (see Fig. S4 of the ESI‡).
Presumably, as we expected, the repulsive interaction between
the negatively charged carboxylate group of Z-1 and the polar
carbonyl groups on the CB[7] portal hindered the formation of a
stable host–guest complex between CB[7] and Z-1.
and 5, as well as phenyl protons, 1–3, in the cinnamamide moiety
of E-1 shifted upfield remarkably upon complexation with CB[7].
In particular, the phenyl protons of E-1 are subjected to large
upfield-shifts up to ~0.70 ppm relative to those of the free E-1,
indicating that the cinnamamide moiety is encapsulated inside the
CB[7] cavity as illustrated in Scheme 1. However, the signal for
the methylene protons, 6, adjacent to the carboxylate group of
E-1 is down-field shifted, which means that the carboxylate group
is located outside of the CB[7] cavity (Scheme 1). The binding
The photo-induced dissociation of the host–guest complex
CB[7]·E-1 was then investigated. Irradiation of UV light (300 nm)
to a solution of CB[7]·E-1 for 3 min resulted in large changes in
4
-1
1
constant of E-1 to CB[7] was measured to be 2.1 ¥ 10 M
the H NMR spectrum of the complex, especially in the chemical
in pure water at 295 K by ITC (Fig. 3). The presence of the
terminal carboxylate group makes the binding affinity of E-1 to
CB[7] smaller compared to that of cinnamamide, but it is still
high enough to form a stable host–guest complex. The absorption
spectrum of CB[7]·E-1 is almost the same as that of free E-1 except
a small decrease in the optical density at lmax (277 nm) with relative
to that of free E-1.
shifts of the cinnamamide moiety (Fig. 1e). A close inspection
of the spectrum revealed that this spectrum was almost the same
as Fig. 1d described above, indicating that the UV irradiation of
CB[7]·E-1 resulted in dissociation of most of the guest 1 from
CB[7], which was triggered by photoisomerization of E-1 to Z-1.
The photo-induced conformational change of the guest complexed
with the host was also confirmed by UV-visible spectroscopy.
The irradiation of UV light (300 nm) to the aqueous solution of
CB[7]·E-1 caused the same blue-shift of the absorption maximum
as for the isomerization of free E-1 to Z-1 (Fig. 2). Although the
detailed mechanism remains to be elucidated, the dissociation of
the host–guest complex CB[7]·E-1 was achieved by UV irradiation
which induced the photoisomerization of the guest.
The reverse process, photo-induced host–guest complex forma-
1
tion between CB[7] and Z-1, has also been studied by H NMR
spectroscopy. Upon irradiation of UV light at 254 nm to the
previously generated mixture of Z-1/E-1 (6 : 1) and CB[7] for
3
0 min, the signals for Z-1 decreased while those for CB[7]·E-1
increased (Fig. 1f), indicating that the reverse reaction occurred
to produce E-1 which then formed the host–guest complex with
CB[7]. As the NMR peaks were too broad to calculate the
conversion, the ratio of Z-1 to E-1 in the photosteady state was
determined by UV-visible spectroscopy (Z-1:E-1 = 1 : 4) (see Fig.
S5, Table S1 and Table S2 of the ESI‡).
Finally, we investigated the reversibility of the photocontrolled
“
on-off” switching host–guest system by UV-visible spectroscopy.
Upon alternating irradiation of 300 nm (3 min) and 254 nm
30 min), the host–guest system was reversibly switched on and
off as illustrated in Fig. 4. It should be noted that the CB[7]·E-
system shows a significantly higher Z to E photoisomerization
efficiency compared with free E-1 presumably due to the formation
(
1
21
of the stable 1 : 1 host–guest complex between E-1 and CB[7].
Fig. 3 ITC profile of CB[7]·E-1 at 298.15 K.
We also studied the host–guest interaction between CB[7] and
Z-1. After generating a 6 : 1 mixture of Z-1 and E-1 by irradiation
of UV light to a solution of E-1 for 3 min as described above, we
added 1 equiv. of CB[7] to the mixture. The NMR spectrum of
the resulting mixture (Fig. 1d) showed that although significantly
broadened, most of the signals for the cinnamamide moiety were
found almost at the same positions as those of free Z-1 (Fig. 1b),
suggesting that the major component Z-1 remained as a free guest,
while the minor component E-1 formed a host–guest complex
Fig. 4 Changes in the percentage of E-1 or CB[7]·E-1 upon alternating
irradiation of 300 nm (3 min) and 254 nm (30 min) light.
This journal is © The Royal Society of Chemistry and Owner Societies 2011 Photochem. Photobiol. Sci., 2011, 10, 1415–1419 | 1417

Conclusions
Isothermal titration calorimetry (ITC)
The association constants and thermodynamic parameters for the
host–guest complexation of (E)-cinnamamide, the mixture of (Z)-
cinnamamide and (E)-cinnamamide (Z isomer : E isomer = 6 : 1),
E-1 and the mixture of Z-1 and E-1 (Z-1 : E-1 = 6 : 1) with CB[7]
were determined by isothermal titration calorimetry with a VP-
ITC instrument (MicroCal, Inc.). The solutions were prepared
in purified water (Milli-Q, Millipore). All solutions were degassed
prior to titration experiment according to the procedures provided
by MicroCal, Inc. An aqueous solution (0.18 mM) of CB[7] was
placed in the sample cell (1.398 mL). As a 2.5 mM solution of
guest (8 mL) was added in a series of 22–24 injections, the heat
evolved was recorded at T = 298.15 K. The heat of dilution was
corrected by injecting the guest solutions into deionized water and
subtracting these data from those of the host–guest titration. The
data were analyzed and fitted by the Origin software adapted for
ITC data analysis (MicroCal, Inc).
An “on-off” photo-switchable host–guest system comprising
CB[7] and a cinnamamide derivative (1) has been demonstrated.
The cinnamamide derivative in E-conformation and CB[7] forms a
stable 1 : 1 host–guest complex (“on” state). Upon irradiation with
3
00 nm light, the complex was dissociated (“off” state) due to the
conformational change of the guest from E- to Z-conformation,
1
as confirmed by UV-visible and H-NMR spectroscopy. The
dissociation of the guest from CB[7] is presumably due to the
repulsive interaction between the carboxylate terminal of the guest
and the polar carbonyl groups on the CB[7] portal upon E to Z
photoisomerization. Irradiation of 254 nm light to the “off” state
reversed the process to turn the system “on”. The high reversibility
and efficiency of the photoswitchable “on-off” host–guest system
was demonstrated by several cycles of alternating irradiation of
3
00 and 254 nm light. This system may thus be useful for designing
photoresponsive gating systems. Currently, we are working on an
artificial gated ion channels based on this photo-switchable “on-
off” host–guest system.
Calibration curve
UV light (300 nm) was irradiated to an aqueous solution of E-1
Experimental
(1 mM, D
2
O) with different exposure times (0, 50, 70, 120 and
180 s). The remaining percentage of E-1 in each solution was
General
1
calculated by integration of H NMR signals (Table S1, ESI‡).
The absorbance at 277 nm of 20-times diluted solution of each
was measured by UV-visible spectroscopy. Finally, a calibration
curve correlating the UV absorbance at 277 nm with percentage
of E-1 was plotted. After addition of equimolar amount of CB[7]
was added into each solution, the UV absorbance at 277 nm of
each solution was also measured. A calibration curve for CB[7]·E-
13b
CB[7] was prepared according to our previous report. All the
NMR data were recorded on a Bruker DRX500 spectrometer
operating at the proton Larmor frequency of 500.23 MHz.
D O was used for field-frequency lock. UV-visible absorption
2
spectra were recorded on a Hewlett-Packard 8453 diode array
spectrophotometer. Photo irradiation experiments were carried
on an illuminator (Xe DC arc lamp, 300 W, Newport Co. USA)
with suitable filters (Newport Co. USA). The distance between
the optical fiber and sample cell was 5 cm. All reagents were
commercial products and used without further purification.
1
was also constructed from the UV absorbance. See Table S1 and
Fig. S5 in the ESI‡.
Acknowledgements
We gratefully acknowledge the CRI, BK 21, and WCU (Project
No. R31-2008-000-10059-0) programs of the Korean Ministry of
Education, Science and Technology for support of this work.
Synthesis of trans-(3-phenyl-acryloylamino)acetic acid (E-1).
Glycine (410 mg, 5.45 mmol) was dissolved in 10 mL of 10% NaOH
aqueous solution. Cinnamoyl chloride (780 mg, 4.68 mmol) was
slowly added to the solution with vigorous stirring at room
temperature and the resulting solution was stirred for 3 h at rt.
HCl (37%) was slowly added to the mixture until pH 2. A white
solid product was collected, washed with water and ether and then
Notes and references
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◦
dried in vacuo (921 mg, 96%). Mp = 180–181 C; Anal. calcd for
1
052–1058.
C
11
H
11NO
3
1
: C, 64.38; H, 5.40; N, 6.83%, found: C, 64.10; H, 5.45;
◦
2 M. Liu, X. Yan, M. Hu, X. Chen, M. Zhang, B. Zheng, X. Hu, S. Shao
and F. Huang, Photoresponsive host–guest systems based on a new
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Eur. J., 1997, 3, 1992–1996.
N, 6.86%. H NMR (500 MHz, D
2
O, 25 C) d = 7.68 (m, 2 H),
7
4
1
.60 (d, J = 16 Hz, 1 H), 7.49 (m, 3H), 6.74 (d, J = 16 Hz, 1 H),
1
3
◦
.07 (s, 2H), C NMR (125 MHz, D
2
O, 25 C) d = 174.8, 169.5,
42.2, 134.7, 130.7, 129.5, 128.4, 120.0, 42.3; MS (ESI): m/z 206.09
+
[M + H] .
CB[7]·E-1. To a solution of E-1 (0.41 mg, 2.0 mmol) in D
2
O
4
M. Cavallini, F. Biscarini, S. Le o` n, F. Zerbetto, G. Bottari and D. A.
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(
2 mL), CB[7] (2.3 mg, 2.0 mmol) was added and the resulting
solution was sonicated until all solid materials were dissolved. The
5
1
formation of the inclusion complex was confirmed by H NMR
1
and MALDI/TOF mass spectrometry. H NMR (D
2
2
O, 500 MHz,
◦
5 C) d = 7.06 (br s, 1H), 6.79 (br s, 1H), 6.68 (br s, 2H), 6.54 (br s,
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H), 6.38 (br s, 1H), 5.74 (d, J = 15.5 Hz, 14 H), 5.45 (s, 14 H), 4.15
(
d, J = 15.5 Hz, 14 H), 4.15–4.10 (br s, 2 H); MS (MALDI/TOF):
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However, we can speculate two possibilities, one involving isomer-
ization followed by dissociation of the guest and the other involving
dissociation followed by isomerization. In the latter case, the efficiency
of the E to Z photoisomerization would not affected by the presence of
CB[7] as the product Z-1 has little interaction with CB[7], whereas that
of the reverse process is expected to increase as the product E-1 forms a
stable host–guest complex with CB[7], which is what we observed here.
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This journal is © The Royal Society of Chemistry and Owner Societies 2011 Photochem. Photobiol. Sci., 2011, 10, 1415–1419 | 1419