Host-guest assembly of squaraine dye in cucurbit[8]uril: Its implication in fluorescent probe for mercury ions

Article abstract of DOI:10.1039/c002219p

The binding interactions between SQ2 squaraine dye and cucurbit[8]uril (CB8) effectively removes the aggregation of SQ2 in aqueous solution by forming a 1:1 inclusion complex. The resulting SQ2·CB8 complex exhibits highly selective fluorescence quenching by Hg²⁺ ions, as a result of the synergetic binding between CB8, SQ2 and metal cation.

Full text of DOI:10.1039/c002219p

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Host–guest assembly of squaraine dye in cucurbit[8]uril: its implication
in fluorescent probe for mercury ionsw
Yongqian Xu, Matthew J. Panzner, Xiaopeng Li, Wiley J. Youngs and Yi Pang*
Received 1st February 2010, Accepted 23rd March 2010
First published as an Advance Article on the web 20th April 2010
DOI: 10.1039/c002219p
The binding interactions between SQ2 squaraine dye and
cucurbit[8]uril (CB8) effectively removes the aggregation of
SQ2 in aqueous solution by forming a 1 : 1 inclusion complex.
The resulting SQ2ÁCB8 complex exhibits highly selective
2+
fluorescence quenching by Hg ions, as a result of the synergetic
binding between CB8, SQ2 and metal cation.
Mercury ion is one of the most toxic environment pollutants
and can be accumulated in the organs of human or animal
bodies through the food chain. Even low concentrations can
1
give rise to a wide variety of disease and health problems. In
recent years, many studies have been directed to develop
2
+
2
Scheme 1 Schematic representation of metal ion binding in the
inclusion complex with a benzothiazolium derivative (a) and structure
of SQ2 (b).
various probes specifically for Hg ion detection. The most
attractive approach in this field is fluorescence-based sensors
3
owing to their intrinsic sensitivity and selectivity. However,
2
while a few near-infrared (NIR) fluorescent Hg probes are
+
Cucurbit[8]uril (CB8), a synthetic pumpkin-shaped cation
2
c,d
available
it remains a greater challenge to design NIR
receptor, is a member of the host family of cucurbit[n]urils,
2
+
Hg -specific fluorescent probes.
which show a strong affinity for cationic species such as
12,13
As a class of well-studied organic dyes, squaraines exhibit
intense absorption and fluorescence in the red to near-infrared
4
region. These dyes exhibit high tendency to form aggregates
protonated amines.
The binding between a cationic
organic molecule and CB is thought to be driven by an inter-
play of hydrophobic interactions with the inner cavity as well
13
in solution, which significantly affects their optical properties.
Depending on surrounding media, squaraine dyes can form
J-aggregates that give red-shifted absorption bands (as
compared to monomer absorption), or H-aggregates that give
blue shifted absorption bands. Sensitivity of aggregate formation
to the surrounding media has made squaraines suitable candidates
as polar interaction with the carbonyl groups at the portals.
The inclusion complex with the cationic benzothiazolium as
guest could be very interesting (Scheme 1), as the polar
interaction between the positively charged nitrogen atom
and carbonyl will place the sulfur atom near the portals of
CB8. The unique geometry of CB8 portals, in conjunction
with appropriate location of the sulfur atom, could enhance
the selective interaction of metal ion with benzothiazolium
moiety while restricting the access of other interfering
chemicals. Herein we demonstrate such concept by using the
squaraine dye (SQ2) which forms a guest–host complex with
CB8. The resulting complex exhibits improved selectivity in
5
for cation sensors. The different aggregates show different
emission properties with H-aggregates usually being poor
6
emitters while J-aggregates show efficient luminescence.
Formation of a supramolecular guest–host assembly represents
an effective strategy to control the molecular aggregation.
Successful examples include encapsulating squaraine dyes to
7
form rotaxanes, formation of an inclusion complex between
2+
recognizing Hg cation.
8
thioflavin and cucurbit[8]uril (CB8), and using cationic and
9
SQ2 was synthesized by using a modified literature
14
procedure. The crystal structure of SQ2 reveals that both
1
nonionic surfactants. The H- and J-aggregates of a cyanine
0
1
dye can be controlled by using cucurbit[7]uril. Utilization of
1
sulfur atoms are pointing to the same side. The squaraine
skeleton is nearly planar (Fig. 1), while two benzothiazole
rings are twisted away slightly by B101 from the coplanarity.
surfactant has been shown to improve the fluorescence
9
response of semisquaraine dye and phenylene-ethynylene
1
0
2+
dendrimer to achieve the selective detection of Hg cation.
To our best knowledge, no study has been reported to use CB8
2+
to aid the Hg chelation and selective detection.
Department of Chemistry & Maurice Morton Institute of Polymer
Science, The University of Akron, Akron, OH 44325, USA.
E-mail: yp5@uakron.edu
w Electronic supplementary information (ESI) available: Synthetic
procedures, characterization of SQ2, and additional spectroscopic
data. CCDC 764291. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c002219p
Fig. 1 ORTEP view of the molecular structure of SQ2 from the front
(left) and side (right). All hydrogen atoms are omitted for clarity.
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Chem. Commun., 2010, 46, 4073–4075 | 4073

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Fig. 3 Optimized geometry of the SQ2ÁCB8 complex in side (a) and
front view (b), where the C, N, S and O atoms are in cyan, blue, yellow
and red, respectively. The pyridinyl groups in view (a) are indicated by
double arrows, and the CB[8] in view (b) is shown in the stick model.
Fig. 2 Absorption spectra of SQ2 (5 mM) with different equivalent of
CB8 in water. The molar extinction coefficients (e) of the SQ2ÁCB8
complex at 655 nm appears to be lower than that of non-complexed
form.
2
The amino nitrogen (N8) adopts the sp hybridization,
indicating a strong interaction present between the nitrogen
atom and the p-conjugated backbone of SQ2. The crystal
structure also shows that the two pyridinyl rings in the
di(2-pyridinylmethyl)amine (DPA) group are located on the
opposite sides of the squaraine main plane.
In solution, SQ2 exhibits absorption peaks at 344 and
655 nm at low concentration (1 mM), which can be attributed
to the DPA segment and SQ2 backbone, respectively. At 5 mM
concentration, a new absorption peak appears at B600 nm
(Fig. 2), which is assigned to H-aggregates on the basis of the
Fig. 4 Fluorescence spectra of SQ2 (5 uM) in water in the presence of
2+
different equivalents of CB8 and Hg
observed spectral blue-shift (B55 nm) in comparison with the
absorption of the monomer. The assignment is consistent with
the crystal packing of SQ2, as the X-ray crystal structure reveals
significant parallel overlap of two conjugated backbones.
The binding interaction between SQ2 and CB6 and CB8
were investigated in aqueous solution. Addition of CB6 (by as
much as 8 equiv.) shows nearly no visible influence on the
aggregation absorption band of SQ2. In sharp contrast, the
presence of about 3 equiv. of CB8 causes the complete
disappearance of the H-aggregate absorption band at B600 nm
.
metal ions in aqueous solution. The fluorescence of SQ2 at
B673 nm is nearly unaffected upon addition of 1 equiv. of
2
Hg
+
cation (Fig. 4). Introduction of CB8 to the solution
2
+
containing SQ2 and Hg , however, leads to gradual
fluorescence quenching, which is accompanied with disappearance
of the H-aggregate absorption band at 600 nm (Fig. S4, ESIw).
The results reveal that the CB8 has a profound effect on the
2
+
(
Fig. 2), suggesting the formation of the guest–host complex
SQ2–Hg interaction.
SQ2ÁCB8. Mass spectrometry detected SQ2ÁCB8 (m/z =
The response of the host–guest supramolecular assembly
2+
À
1
942.5974) without the CF
3
SO
3
counterion (Fig. S1, ESIw).
The complexation is observable in the H NMR spectrum
SQ2ÁCB8 to Hg was further evaluated by addition of three
1
equiv. of CB8 to SQ2 aqueous solution, which ensures all SQ2
molecules are incorporated into the CB host. The fluorescence
signal of SQ2ÁCB8 is completely quenched after addition of 3
(
Fig. S2, ESIw). The corresponding equilibrium association
5
À1
constant (K) is determined to be 2.4 Â 10 M on the basis of
: 1 stoichiometry of the complexes (revealed from Job plot,
2
+
1
equiv. of Hg ion (Fig. 4). In the absence of CB8, however,
2
+
Fig. S3, ESIw).
50 equiv. of Hg
are required to completely quench the
A molecular modeling study (AM1) further shows that the
SQ2 can fit into CB[8] (Fig. 3). The two pyridinyl groups,
which point to the opposite sides of the conjugated plane (as
seen from the crystal structure), appears to still allow the guest
molecule SQ2 to thread through the cavity of CB8. The result
also explains why CB6 of smaller cavity is ineffective to
remove aggregation of SQ2, since an adequate cavity size
from the cucurbit[n]uril is essential to fit the guest molecule.
The bulky DPA group also prevents the inclusion of a second
SQ2 in the CB8 cavity for dimer formation, which can occur
fluorescence of SQ2. Dramatic difference in the optical
+
2
response of SQ2 to Hg
cation (Fig. 5) strongly indicates
the involvement of the guest molecule CB8. The absorption
and fluorescence titration data indicate a 1 : 2 binding model
2
+
between SQ2 and Hg cation (Fig. S5, ESIw). The effective
2
+
Hg -binding constants are determined to be (4.3 Æ 0.6) Â
1
0
À2
7
À2
10
M
and (7.3 Æ 0.1) Â 10 M for SQ2ÁCB8 complex
and SQ2, respectively (by using fluorescence titration data). In
other words, formation of the CB8 complex increases the
2
Hg -binding efficiency by nearly three orders of magnitude.
+
8
2+
when the guest is less hindered.
The 1 : 2 binding ratio suggests the possibility that two Hg
The photophysical behaviors of SQ2 in the absence and
cations are bonding to each end of CB8 to quench the
presence of CB8 were examined upon addition of various
fluorescence of SQ2ÁCB8.
4
074 | Chem. Commun., 2010, 46, 4073–4075
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2
+
(
Fig. S7, ESIw), showing that the binding of Hg to SQ2ÁCB8
is a reversible process.
In conclusion, we have demonstrated that H-aggregation of
the NIR-emitting dye SQ2 in aqueous solution can be effectively
controlled by forming the inclusion complex SQ2ÁCB8. The
array of the carbonyl groups at the CB8 portals provides strong
ion–dipole interaction to stabilize the complex with the positively
charged guest molecule SQ2 (in 1 : 1 ratio). Utilization of the
oxygen array from CB8, in connection with the fluorophore SQ2
2+
in the complex, leads to a highly selective and reversible Hg
sensor. On forming the inclusion complex with CB8, the binding
2
+
Fig. 5 Fluorescence response of SQ2 (5 mM) in water to Hg ion in the
presence (black squares) and absence (red circles) of 3 equiv. of CB8.
2+
strength between SQ2 and Hg can be enhanced by up to three
orders of magnitude. The example illustrates a new approach to
design and optimize the performance of fluorescent sensors by
using a supramolecular strategy.
This work was supported by The University of Akron and
Coleman endowment. We also wish to thank The National
Science Foundation (CHE-9977144) for funds used to purchase
the NMR instrument used in this work.
Notes and references
1
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Fig. 6 Fluorescence quenching data for SQ2 (5 mM) in water with (a)
or without (b) 3 equiv. of CB8 upon addition of various metal ions
(2 equiv.).
2
(a) E. M. Nolan and S. J. Lippard, Chem. Rev., 2008, 108,
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cations are added to the solution of SQ2 under the same
(
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conditions (in the presence of 3 equiv. of CB8). Addition of
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2+
(
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2+
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3+
Mn , Zn , Cd , Cu , Fe , Ag , Pb , Cr , Co
(
,
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3
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Ni ) does not induce any apparent fluorescence quenching
Fig. 6). The ratio of F/F at 675 nm (here, F indicates the
(
0
0
fluorescence intensity of free SQ2 and F675 indicates the fluores-
cence intensity upon addition of 2 molar equiv. of respective
metal ions) is examined in the presence of various other metal
4
5
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2+
ions. For Hg , the F675/F value is almost 0.2, while the values
0
3+
for the other metal ions are approximately 1 (except for Fe , the
value is 0.4). Therefore, the host–guest supramolecular assembly
6 S. Alex, M. C. Basheer, K. T. Arun, D. Ramaiah and S. Das,
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2+
is a highly selective chemosensor for Hg
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.
2+
In the absence of CB8, the F675/F value for Zn cation is
0
about 0.83, indicating moderate interaction. The observation
1
is consistent with the H NMR result, which detects no
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2
+
Zn
in DMSO-d (Fig. S6, ESIw). In the presence of CB8,
6
9
2+
however, the F675/F value for Zn cation is 1, indicating no
0
1
1
1
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fluorescence response, despite the fact that DPA is a well
2
+
15
known chelator for Zn cation. It appears that the middle
segment of SQ2 is shielded by the host CB8, making the DPA
2
+
group less accessible. The trend observed from Zn
2+
true for most of the other metal ions (except Hg , Cr
is also
3+
,
3+
Fe ), exhibiting a larger F675/F
0
value in the presence of CB8
1
as the chromophore is shielded from interaction with other
cations to some extent.
2
+
The selective fluorescence quenching of SQ2ÁCB8 by Hg
1
is likely to occur via binding to the sulfur atom, which
remains accessible in the complex. High affinity of sulfur to
2
+
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1
Hg
ion is also expected to play an important role in
1
the observed selectivity. Addition of EDTA to an aqueous
6
6 X. Chen, S. Nam, M. J. Jou, Y. Kim, S. Kim, S. Park and J. Yoon,
Org. Lett., 2008, 10, 5235–5238.
2
+
solution of SQ2ÁCB8–Hg recovers the emission of SQ2ÁCB8
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Chem. Commun., 2010, 46, 4073–4075 | 4075