Enhancing photostability of cyanine dye by cucurbituril encapsulation

Article abstract of DOI:10.1016/j.dyepig.2012.01.022

A linear cyanine dye (LDP) guest comprising two binding sites, one 4-[2-[4-(dimethylamino)phenyl]ethenyl]pyridinium group and one p-aminophenoxy ethylene group, was synthesized and proved could form inclusion complexes with cucurbit[7]uril (CB[7]) in aqueous solution. A partially encapsulated ([2]pseudorotaxane) state in which one CB[7] ring moved fast between the two binding sites was originally detected and was then transferred into a full encapsulated ([3]pseudorotaxane) state where two CB[7] macrocycles resided on both binding sites with the continuously adding of CB[7] to LDP. The two binding constants K₁ and K₂ were determined as (6.29 ± 0.88) × 10⁴ M^-1 and (1.58 ± 0.17) × 10⁴ M^-1, respectively. Compared with the free LDP, the photostability of the [3]pseudorotaxane is found been distinctively promoted, while that of the [2]pseudorotaxane state is hardly improved.

Full text of DOI:10.1016/j.dyepig.2012.01.022

Dyes and Pigments 94 (2012) 266e270
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Enhancing photostability of cyanine dye by cucurbituril encapsulation
*
Hongyuan Zhang, Lele Liu, Chao Gao, Ruyi Sun, Qiaochun Wang
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, No. 130 Meilong Road, Shanghai 200237, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A linear cyanine dye (LDP) guest comprising two binding sites, one 4-[2-[4-(dimethylamino)phenyl]
ethenyl]pyridinium group and one p-aminophenoxy ethylene group, was synthesized and proved could
form inclusion complexes with cucurbit[7]uril (CB[7]) in aqueous solution. A partially encapsulated
([2]pseudorotaxane) state in which one CB[7] ring moved fast between the two binding sites was
originally detected and was then transferred into a full encapsulated ([3]pseudorotaxane) state where
two CB[7] macrocycles resided on both binding sites with the continuously adding of CB[7] to LDP. The
Received 15 August 2011
Received in revised form
8 October 2011
Accepted 21 January 2012
Available online 28 January 2012
two binding constants K1 and K2 were determined as (6.29 ꢀ 0.88) ꢁ 104
M
ꢂ1 and (1.58 ꢀ 0.17) ꢁ 104 Mꢂ1
,
Keywords:
Pseudorotaxane
Cucurbituril
respectively. Compared with the free LDP, the photostability of the [3]pseudorotaxane is found been
distinctively promoted, while that of the [2]pseudorotaxane state is hardly improved.
Ó 2012 Elsevier Ltd. All rights reserved.
Cyanine dye
Photostability
Inclusion complex
Supramolecular chemistry
1. Introduction
a cyanine dye by simply mixing it with Cucurbit[7]uril (CB[7]) in
aqueous solution to form a [3]pseudorotaxane.
The cyanine dyes are undoubtedly one of the most important
classes of dyes for their extensive applications, for example, in
the area of imaging sensitizer, DNA indicator, ion probe, as well as in
data storage and dye-sensitized solar cell [1e7]. It is obvious that
photobleaching of cyanine dyes is an undesirable characteristic in
most cases. Efforts have been made to improve the stability in
regard to the photochemical decomposition by structural modifi-
cation or adding additives [8e10]. Yet, these methods are somewhat
complex or less efficient. Anderson group has also reported
a strategy to enhance the photostability of dyes by threading them
into the cavity of cyclodextrin [11], but this effective method should
be faced with challenging rotaxane construction. Cucurbiturils
(CBs) are pumpkin-shaped macrocylic host molecules synthesized
from the acid-catalyzed cyclization of glycoluril and formaldehyde.
Like cyclodextrins, they are commercially available with good
stability, and can form inclusion complexes with a variety of organic
guests. Moreover, CBs have higher affinity and better selectivity
than cyclodextrins especially for those guests with positive charge,
thus could boost stable inclusion complex formation [12]. To the
best of our knowledge, reports on slowing down the photo-
bleaching of a dye with cucurbituril are extremely rare [13]. Herein
we describe a facile method to reduce the photobleaching of
2. Experimental
2.1. Chemicals and instruments
Cucurbit[7]uril was synthesized according to reported method
[14]. Other reagents were commercially available and used without
further purification.
1H NMR spectra were recorded on a Brüker AM 400 spectrom-
eter with tetramethyl silane (TMS) as internal reference. ESI-MS
spectra were conducted on Waters LCT Premier XE mass spec-
troscopy. Absorption spectra and Fluorescence spectra were
measured on a Varian Cary100 UVeVis spectrophotometer and
a Varian Cary Eclipse Fluorescence spectrophotometer, respec-
tively. The photobleaching experiments were carried out by using
a 500 W xenon lamp emitting visible light with an intensity of
14,000 wcmꢂ2. The photographs were taken by Pentax ker digital
single lens reflex camera.
2.2. Synthesis
2.2.1. Preparation of 1-[2-(4-Nitrophenoxy)ethyl]-4-methyl
pyridinium bromide(1)
A
mixture of 22.0 g (89.8 mmol) 1-(2-bromoethoxy)-4-
* Corresponding author. Tel. /fax: þ86 2164252758.
E-mail address: qcwang@ecust.edu.cn (Q. Wang).
nitrobenzene [15] and 12.0 g (129 mmol) 4-methylpyridine in
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.01.022

H. Zhang et al. / Dyes and Pigments 94 (2012) 266e270
267
50 ml DMF was stirred at 100 ꢃC for 10 h, then 60 ml ethyl acetate
was added and the resulting mixture was stirred and cooled to
room temperature, the precipitate was collected by filtration,
washed with 50 ml ethyl acetate and dried to give 21.5 g (71%) 2 as
a pale yellow solid. m.p. 240e242 ꢃC; 1H NMR (400 MHz, DMSO-
6.49(d, 2H, J ¼ 8.7 Hz); 4.77(t, 2H, J ¼ 4.7 Hz); 4.31(t, 2H, J ¼ 4.7 Hz);
3.03(s, 6H); MS (m/z): [M-Br]þ calcd. for [C23H26N3O]þ 360.21;
found: 360.2.
3. Results and discussion
d6):
d
¼ 8.84(d, 2H, J ¼ 6.4 Hz); 8.15(d, 2H, J ¼ 9.3 Hz); 7.33(d, 2H,
J
¼6.4 Hz); 8.25(d, 2H, J ¼ 9.3 Hz); 4.78(t, 2H, J1 ¼4.6 Hz,
3.1. The formation of inclusion complex
J2 ¼ 4.8 Hz); 4.59(t, 2H, J1 ¼4.8 Hz, J2 ¼ 4.6 Hz); 2.76(s, 3H)。
A dimethylamino phenyl moiety (DMA) and a p-aminophenoxy
ethylene group (OA) were arranged at the two ends of LDP, as
shown in Scheme 1. They were both proved as binding sites for
CB[7] in our previous study [16], and as a result, a [3]pseudor-
otaxane might finally be obtained when one LDP molecule binds
two CB[7] rings. To confirm the binding ratio, measurements of the
fluorescence of a series of LDP solutions with different CB[7] molar
fraction were carried out with a total concentration remaining
constant. The resulting job plot curve is shown in Fig. 1. The
maximum value of the vertical axle corresponds to 0.33 at hori-
zontal axle, and the stoichiometry of LDP and CB[7] is verified to be
1:2.
Although a final [3]pseudorotaxane (abbreviated as LDP$CB2)
would form, LDP may also display as other different architectures
when binding to CB[7] at low molar ratio. These are the [2]pseu-
dorotaxanes (abbreviated as LDP$CB) in which one CB[7] macro-
cycle encircles either of the two binding sites. To investigate the
inclusion behaviors of LDP and CB[7], the 1H NMR spectra of LDP
aqueous solution titrating with CB[7] were monitored. The 1H
NMR technique is a very efficient method to identify the binding
site of CBs-based complex. It is a common rule that the protons
inside the hydrophobic cucurbituril cavity undergo shielding effect
while the outside ones conduct deshielding effects, and those near
the carbonyl rim are hardly affected [17,18]. To prevent the pro-
tonization of the OA group, the 1H NMR experiments were
recorded at a weak base environment (pD ¼ 9.6, and the pKa of OA
2.2.2. Preparation of 4-[2-[4-(dimethylamino)phenyl]ethenyl]-
1-[2-(4-nitrophenoxy)ethyl] pyridinium bromide (2)
A mixture of 11.3 g (33.4 mmol) 1, 6.0 g (40.3 mmol) 4-(dime-
thylamino)benzaldehyde and 90 ml ethanol was heated to 60 ꢃC
with stirring, then 1.0 ml piperidine was added and the solution
was stirred under reflux for 6 h. After cooled to room temperature,
the precipitate was collected by filtration, and recrystallized from
ethanol to give 10.2 g (65%) 3 as a red solid. m.p. 269e271 ꢃC; 1H
NMR(400 MHz, DMSO-d6):
d
¼ 8.82(d, 2H, J ¼ 6.7 Hz); 8.23(d, 2H,
J ¼ 9.2 Hz); 8.08(d, 2H, J ¼ 6.9 Hz); 7.94(d, 1H, J ¼ 16.0 Hz); 7.60
(d, 2H, J ¼ 8.9 Hz); 7.19(d, 1H, J ¼ 16.0 Hz); 7.16(d, 2H, J ¼ 9.3 Hz);
6.79(d, 2H, J ¼ 9.0 Hz); 4.89(t, 2H, J ¼ 4.6 Hz); 4.65(t, 2H, J ¼ 4.6 Hz);
3.04(s, 6H).
2.2.3. Preparation of 4-[2-[4-(dimethylamino)phenyl]ethenyl]-
1-[2-(4-aminophenoxy)ethyl] pyridinium bromide (LDP)
To a solution of 10.0 g (21.3 mmol) 2 in 80 ml hot ethanol, was
added 8.0 g (143 mmol) iron powder, 30 ml water and 1.0 ml 45%
HBr with stirring. The resulting mixture was heated under refluxed
for 4 h and filtered hot, the filtrate was concentrated by rotary
evaporation and the resulting solid was recrystallized from ethanol
twice to give 4.6 g (51%) LDP as a red powder. m.p. 251e253 ꢃC; 1H
NMR (400 MHz, DMSO-d6):
d
¼ 8.78(d, 2H, J ¼ 6.9 Hz); 8.07(d, 2H,
J ¼ 6.9 Hz); 7.94(d, 1H, J ¼ 16.1 Hz); 7.60(d, 2H, J ¼ 8.9 Hz); 7.19
(d, 1H, J ¼ 16.1 Hz); 6.79(d, 2H, J ¼ 9.0 Hz); 6.63(d, 2H, J ¼ 8.8 Hz);
Scheme 1. Synthetic route of cyanine dyes LDP and its complexation behavior with CB[7].

268
H. Zhang et al. / Dyes and Pigments 94 (2012) 266e270
also a CB[7] ring staying at the DMA moiety. The above two
deductions seem inconsistent when the molar ratio of CB[7]/LDP is
lower than 2.0 equiv. However, when taking into account the fact
that there are only increasing movements but not splitting of the
proton signals observed with the continuously increasing of the
molar ratio of CB[7]/LDP until the full [3]pseudorotaxane (for
example CB[7]/LDP ¼ 4.0 in Fig. 2d) is realized, we can come to the
conclusion that the CB[7] ring undergoes fast exchanging between
the two binding sites till they are both encapsulated. Moreover, in
view of the obvious deshielding effect of the pyridinium vinyl
moiety, the exchanging route should be OAdsolventdDMA
(Scheme 1), but not OAdpyridiniumdDMA (in this manner, the
pyridinium group should be in shielding region).
At the condition that pH<7.31, the OA group would be proton-
ized and according to the common rule, CB[7] should stay at the
dicationic side. But unfortunately, though repeated efforts had been
made, we did not get the clear NMR signals (the best NMR spec-
trum among these is shown in Fig. 2f) to confirm the exact archi-
tecture of the complex. Taking into account that the protonated LDP
molecule can be regarded as another cyanine dye, we did not carry
out further investigation.
Fig. 1. Job plot for the complexation of LDP and CB[7] (the total concentration of CB[7]
and LDP is 50 mM at pH ¼ 9.6).
is 7.31 after encapsulated by CB[7] [16]), as illustrated in Fig. 2.
Compared with LDP alone (Fig. 2a), most protons of LDP with the
increasing addition of CB[7] (Fig. 2c) were found shift distinctly.
On one hand, the pyridine proton H2 experiences a downfield-shift
3.2. Binding constants
The above investigations show that, with the increasing addi-
tion of CB[7] to the LDP aqueous solution, LDP$CB comes into being
where CB[7] moves fast between the two binding sites and then is
transferred into the 1:2 complex LDP$CB2. The association equi-
librium can be written as the following equation (Eq. (1)):
(
DdH2 increases from þ0.11 ppm in 0.5 equiv to þ0.25 ppm in
4.0 equiv) and the OA protons H8, H9, H10, H11 upfield-shift
DdH8 is ꢂ0.19 ppm in 0.5 equiv and ꢂ0.40 in 4.0 equiv;
(
DdH9 is ꢂ0.20 ppm in 0.5 equiv and ꢂ0.48 ppm in 4.0 equiv;
DdH10 is ꢂ0.21 ppm in 0.5 equiv and ꢂ0.47 ppm in 4.0 equiv; DdH11
is ꢂ0.22 in 0.5 equiv and ꢂ0.48 ppm in 4.0 equiv). These facts
mean that pyridinium group undergoes deshielding effect and OA
site conducts shielding effect, and a CB[7] ring residing over
the OA moiety can thus be deduced. On the other hand,
shielding effects on the DMA group (DdH5 is ꢂ0.03 ppm in
0.5 equiv and ꢂ0.31 ppm in 4.0 equiv; dH6 is ꢂ0.03 ppm in
0.5 equiv and ꢂ0.21 ppm in 4.0 equiv; DdH7 is ꢂ0.03 ppm
in 0.5 equiv and ꢂ0.49 ppm in 4.0 equiv) and the deshielding
effects on the vinyl unit (DdH3 is þ0.03 in 0.5 equiv and þ0.06 ppm
in 4.0 equiv; and DdH4 is þ0.05 ppm in 0.5 equiv and þ0.09 ppm in
4.0 equiv) can also be found. These phenomena show that there is
K1
LDP þ CB½7ꢄ # LDP$CB
(1)
K2
LDP$CB þ CB½7ꢄ # LDP$CB2
The gradual addition of CB[7] to LDP up to about 50 equiv leads
to a remarkable increase in the fluorescence intensity by 14 folds, as
can be seen from Fig. 3. This exciting observation should result from
the increasing rigidity [19] and the reduced environment polarity
[20] when LDP enters into the cavity of CB[7]. The binding
constants K1 and K2 are calculated by analyzing the gradual changes
in the intensity of the fluorescence with the increasing concen-
tration of the host. The detected fluorescent intensity If at any stage
Fig. 2. 1H NMR spectra (400 MHz, D2O) of LDP (3 mM) at pD ¼ 9.6 in different envi-
ronment: (a), LDP alone, (b) LDP and 0.5 equiv CB[7], (c)LDP and 1.1 equiv CB[7], (d)
LDP and 4 equiv CB[7], (e) LDP and 5 equiv CB[7]. (f) At pD ¼ 6.5 with 1.1 equiv CB[7].
Fig. 3. The emission spectrum of LDP (1 ꢁ10ꢂ5 M in H2O) in the presence of increasing
concentration of CB[7] (0e50 equiv) at pH ¼9 .6. Inset: the experimental data shows
the best fit to the 1:2 binding model at 599 nm.

H. Zhang et al. / Dyes and Pigments 94 (2012) 266e270
269
3.3. Photostability
The absorption of LDP aqueous solution undergoes an intense
bathochromic shift when the chromophore is encapsulated into the
cavity of the CB[7], as illustrated in Fig. 4. The maximum absorption
peak of LDP is found at 461 nm, and the color of the solution
is yellow. The [3]pseudorotaxane aqueous solution shows
a
maximum absorption peak at 493 nm, which is a 32 nm bath-
ochromic shift compared with the free LDP, and represents an
obvious color change to orange red. The large red-shift is the result
of the enhancement of intramolecular charge-transfer caused by
the deshielding effect to the pyridinium unit and the shielding
effect to the DMA moiety when the two binding sites are encap-
sulated by CB[7].
It is generally considered that CB[7] can prevent the dye from
photo-oxidation and ozonolysis when a chromophore is encapsu-
lated inside the CB[7] ring [23]. So it is expected that the photo-
stability of LDP could be improved by adding CB[7]. The
photostability investigation of LDP (25
mM) and its [3]pseudor-
Fig. 4. UVeVis absorption spectral changes of LDP (dash line) and the LDP$CB2 (solid
line) complex (both LDP concentrations are 25 mM, and the latter 50 equiv of CB[7]
otaxane LDP$CB2 (25
m
M LDP þ 50 equiv of CB[7]) were then
added) in H2O at pH ¼ 9.6 upon irradiation (the irradiation intensity is 14,000 wcmꢂ2).
carried out. The two solutions were put into two quartz cuvettes
respectively, and then exposed to the xenon lamp irradiation. Their
fading processes were photographed, as shown in Fig. 5. The
aqueous solution of LDP was thoroughly bleached after 70 min,
whereas LDP$CB2 still remained an evident color. Their relative
absorption spectra are shown in Fig. 4. It shows that the absorption
of the free LDP aqueous solution displays a sharp decrease with the
continuous irradiation, while the LDP$CB2 solution under the same
situation is affected rather limitedly. The photobleaching rate
constants were then calculated [24,25] according to the fading data,
as can be seen in Fig. 6. The light-fading processes fit to the first-
order kinetic decay curves, which are in accordance with the
mechanism of the photobleaching of cyanine dyes [26,27], and the
is contributed by LDP, LDP$CB and LDP$CB2 according to the
concentrations of their presence in the solution. So the calculation
equation can be written as [21,22]:
I0 þ ILDP$CBK1½CBꢄ0 þ ILDP$ðCBÞ2K1K2½CBꢄ20
f
If
¼
(2)
1 þ K1½CBꢄ0 þ K1K2½CBꢄ02
where K1 and K2 are the binding constants for the formation of the
respective pseudorotaxanes LDP$CB and LDP$CB2; If is the detected
fluorescent intensity at any stage; I0f is the fluorescent intensity of
LDP in the absence of CB[7]; ILDP$CB is the fluorescent intensity of
the 1:1 inclusion complex; and ILDP$(CB)2 is the fluorescent intensity
of the [3]pseudorotaxane. [CB]0 is the total concentrations of the CB
[7] used. The K1 and K2 were then estimated to be (6.29 ꢀ 0.88) ꢁ
104 Mꢂ1 and (1.58 ꢀ 0.17) ꢁ 104 Mꢂ1 after applying Eq. (2) to fit the
fluorescent intensities at 599 nm at different concentration of CB
[7], as shown in Fig. 3 inset.
constants
are
determined
as
kL ¼ 0.042 minꢂ1
and
kLC2 ¼ 0.012 minꢂ1 (kL and kLC2 are the photobleaching rate of LDP
and LDP$CB2 respectively). The promotion of the photostability
through encapsulation is accordingly proved.
It is noteworthy that the 1:1 complex LDP$CB exhibits little
photostability improvement relative to LDP alone. The light-fading
Fig. 5. The fading process of LDP (25
m
M) and LDP$CB2 (25
m
M LDP þ 1.25 mM CB[7]) at pH ¼ 9.6 were recorded by camera with increasing irradiation time. The intensity of
irradiation was constant with 14,000 wcmꢂ2. All the photos were shot at the same environment and parameters: Shutter speed: 1/20s; focal length: 35 mm; aperture: F/5; ISO: 200;
exposure compensation: 0 EV.

270
H. Zhang et al. / Dyes and Pigments 94 (2012) 266e270
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In summary, a stable [3]pseudorotaxane complex is constructed
by simply mixing the linear cyanine dye LDP guest and CB[7] host
in aqueous solution, the threading of one LDP molecule into two
CB[7] mcrocycles gives rise to a 32 nm bathochromic shift in the
absorption and a 13 folds of fluorescent enhancement. It is worthy
of noting that the photostability of the cyanine dye is distinctively
improved after the encapsulation by CB[7], which may offer a new
route to enhance the light-resistance of dyes.
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This work is financially supported by the NSFC/China (21072058),
National Basic Research 973 Program (2011CB808400), and the
Foundation for the Author of National Excellent Doctoral Disserta-
tion of PR China (200758).
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