Efficient colorimetric anion detection based on positive allosteric system of urea-functionalized poly(phenylacetylene) receptor

Article abstract of DOI:10.1021/ma1016852

Poly(phenylacetylene) with rationally designed urea groups was demonstrated to be a superior anion receptor possessing both a colorimetric response ability and a positive homotropic allosteric binding mode. The target polymer, poly(phenylacetylene) with [

Full text of DOI:10.1021/ma1016852

7406 Macromolecules 2010, 43, 7406–7411
DOI: 10.1021/ma1016852
Efficient Colorimetric Anion Detection Based on Positive Allosteric
System of Urea-Functionalized Poly(phenylacetylene) Receptor
Ryosuke Sakai,†,§ Shota Okade,† Eric B. Barasa,† Ryohei Kakuchi,† Magdalena Ziabka,†,^
Satoshi Umeda,‡ Katsuyuki Tsuda,‡ Toshifumi Satoh,† and Toyoji Kakuchi*,†
†Division of Biotechnology and Macromolecular Chemistry, Graduate School of Engineering, Hokkaido
University, Sapporo 060-8628, Japan, and ‡Department of Materials Chemistry, Asahikawa National College
§
of Technology, Asahikawa 071-8142, Japan. Present address: Department of Materials Chemistry,
Asahikawa National College of Technology, Asahikawa 071-8142, Japan. ^ Present address:
Department of Biomaterials, Faculty of Materials Science and Ceramics, AGH University of Science and
Technology, 30-059 Krakow, Poland.
Received January 20, 2010; Revised Manuscript Received August 16, 2010
ABSTRACT: Poly(phenylacetylene) with rationally designed urea groups was demonstrated to be a superior
anion receptor possessing both a colorimetric response ability and a positive homotropic allosteric binding
mode. The target polymer, poly(phenylacetylene) with [bis(trifluoromethyl)phenyl]urea pendants (poly-1),
was synthesized by the stereoselective polymerization with Rh(nbd)BPh4 (nbd = norbornadiene) in high
yield. The anion sensing ability of poly-1 was evaluated using the tetra-n-butylammonium (TBA) salts of a
series of anions in THF. Upon the addition of anions such as CH3CO2-, C6H5CO2-, F-, Cl-, Br-, NO3
-
,
N3-, and HSO4-, the yellow color of the THF solution of poly-1 immediately turned to a different color
1
depending on the types of anions, indicating the anion recognition capability of poly-1. The H NMR
titrations of poly-1 by using increasing amounts of CH3CO2- revealed that the colorimetric response of poly-
1 was considered to be the direct consequence of the hydrogen-bonding complex formation between the urea
functionality and the anions. Interestingly, during the course of the determination of the anion-binding
affinity, we encountered highly cooperative binding ability of poly-1. The Hill plot analysis provided an
exceptional positive cooperativity, e.g., 8.4 of Hill coefficient in the complexation between poly-1 and
C6H5CO2-. This result clearly indicated that this binding mode was based on a positive homotropic allos-
terism, in which the partially formed urea/anion complex units in the polymer chain should produce a change
in the whole main chain conformation, facilitating further anion binding. Additionally, by comparing the
anion binding of poly-1 with that of previously prepared urea-functionalized poly(phenylacetylene) (poly-2),
an enhanced anion binding ability based on two electron-withdrawing -CF3 groups was found to be essential
to realize such cooperativity.
Introduction
highly difficult to artificially prepare the appropriate receptor mole-
cule with the positive homotropic allosterism. Therefore, only
Anions have been recognized as a significant analyte in diverse
fields including environmental, industrial, biological, and medi-
cal ones.1 Therefore, a reliable, robust, and highly sensitive anion
sensor has beenincreasingly in demand.2-7 Inparticular, a colori-
metric anion sensor has been recently appreciated because of
its capability that allows a simple and quick analysis and even
naked-eye detection.2,3,5 For example, Sessler et al. achieved the
fabrication of a number of colorimetric anion sensors by simply
using commercially available chemicals.8 Anzenbacher et al.
established a reliable method for multianion sensing based on the
pattern recognition of colorimetric sensor arrays.9 Although such
sophisticated colorimetric anion sensors have already been deve-
loped, further improvement in the detection sensitivity is still desired.
For the realization of a highly efficient anion detection, one of
the rational approaches should be the construction of a positive
homotropic allosteric system.10,11 This system rests on the co-
operative binding process, in which the initial molecular binding
facilitates the subsequent interactions, thus providing an effici-
ent sensing ability. Although the system is ubiquitously seen in
nature as an efficient information transduction process, it is
limited examples have been known as the anion recognition with
such a cooperative binding mode, in which small molecular
receptor bearing several binding sites is employed.12-15 Conside-
ring that the positive allosteric system essentially needs to possess
dynamic multiple binding sites, a polymeric receptor would be a
rational candidate because (1) the introduction of a great number
of binding sites (more than 100) into one molecule is synthetically
easy and (2) the initial guest-binding event influences the sub-
sequent complexation through the conformational change of the
whole polymer chain. In particular, by adopting a π-conjugated
polymer as such receptor polymer, the optical output ability is
highly expected as well as the cooperative binding.16-23
On the basis of this concept, we have investigated the anion
recognition ability of poly(phenylacetylene) receptors.24-27
Among them, poly(phenylacetylene)s with amino acid-derived
urea pendants were found to exhibit a drastic anion-induced colori-
metric response, in which the resulting color was varied depend-
ing on the employed anion.24,26 However, the desired positive
allosteric binding was not achieved probably due to the low
binding ability of the urea receptor. In order to enhance the
binding affinity, a rational molecular design must be required in
receptor units.
*Corresponding author: Tel þ81 11 706 6602; Fax þ81 11 706 6602;
e-mail kakuchi@poly-bm.eng.hokudai.ac.jp.
pubs.acs.org/Macromolecules
Published on Web 08/30/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 18, 2010 7407
Figure 1. 1H NMR spectrum of poly-1 in CD3OD at ambient tempe-
rature.
Scheme 1. Synthesis of Urea-Functionalized Poly(phenylacetylene)
(Poly-1)
Figure 2. (a) Photograph of THF solutions and (b) absorption spectra
of poly-1 in the presence of TBA salts of a series of anions ([monomeric
units of poly-1] = 1.3 mM in photograph and 130 μM in absorption
spectra, [anion]/[monomeric units of poly-1] = 5).
We have now designed poly(phenylacetylene) with [bis(trifluoro-
methyl)phenyl]urea pendants (poly-1), which emerges as an ideal
candidate because of the expectation that the electron-withdrawing
-CF3 substituents will enhance the hydrogen-bonding ability of
1
urea functionalities.28,29 The UV-vis and H NMR measure-
ments in the presence of a series of anions have been carried out
to elucidate the colorimetric anion sensing of poly-1, providing
insight into positive allosteric binding based on the rationally
designed polymeric receptor.
Results and Discussion
The polymerization of 1 was carried out using Rh(nbd)BPh4 in
dry THF at ambient temperature to produce the desired poly-
(phenylacetylene) with [bis(trifluoromethyl)phenyl]urea pendants
(poly-1) in 89% yield (Scheme 1). The molecular weight and
polydispersity of poly-1 were determined to be 1.7ꢀ 105 and 1.8
Figure 3. 1H NMR spectra of poly-1 in the presence of (a) 0, (b) 0.05,
(c) 0.1, (d) 0.2, (e) 0.3, (f) 0.5, (g) 0.7, and (h) 1.0 equiv of tetra-
n-butylammonium acetate in THF-d8. Closed circles show the peaks
due to the urea protons.
1
based on the SEC measurement, respectively. In the H NMR
spectrum of poly-1, the signal corresponding to the main chain
proton was observed at 5.94 ppm along with those due to aro-
matic protons at 7.76, 7.25, 7.11, and 6.78 ppm, indicating that
poly-1 possessed the cis configuration as the main chain stereo-
regularity (Figure 1).
for the color variation. Although poly-1 seems to bind to almost
all the above anions without articulate selectivity under this con-
dition, the multicolor response capability allows a rough dis-
crimination of the anions by the naked eye.
In anion reception chemistry, a color change of a urea receptor
molecule has been known to be based on a complex formation
with an anion or anion-induced deprotonation.30-33 To provide
an insight into the colorimetric response of poly-1, we performed
1H NMR titrations of poly-1 in the presence of increasing
amounts of CH3CO2 (Figure 3). In the H NMR spectrum,
a remarkable broadening was observed in the signals due to the
urea, main chain, and aromatic protons of poly-1. Additionally,
The anion sensing ability of poly-1 was evaluated using the
tetra-n-butylammonium (TBA) salts of a series of anions in THF.
Poly-1 exhibits a yellow color due to the π-conjugated system in
the main chain structure. Upon the addition of almost all anions,
the yellow polymer solution immediately turned to a different
color depending on the types of anions, as shown in Figure 2a.
For more detailed discussions, UV-vis measurements of poly-1
-
1
were performed in the presence of these anions. As shown in
-
a downfield shift in the urea protons reflects the establishment of
Figure 2b, a large bathochromic shift was observed for CH3CO2
,
- 34
-
C6H5CO2-, F-, Cl-, Br-, NO3-, N3-, and HSO4-, though ClO4
a hydrogen bond interaction with CH3CO2
.
These results
indicated that the colorimetric response of poly-1 was considered
to be the direct consequence of the supramolecular complex
formation between the urea functionality and various anions.
apparently produced no spectral change. Interestingly, the anion
recognition of poly-1 provides a different absorption spectrum
basically depending on the kinds of anions, which is the reason

7408 Macromolecules, Vol. 43, No. 18, 2010
Sakai et al.
Figure 4. Job’s plots upon mixing of THF solutions of poly-1 and
CH3CO2-, obtained by means of UV-vis spectroscopy. The total con-
centration of monomeric units of poly-1 and CH3CO2- was kept con-
stant at 260 μM.
In order to further elucidate this complex formation, the stoi-
-
chiometry of the poly-1/CH3CO2 system was determined by
means of continuous-variation plots (Job’s plots), in which the
variation of the absorbance at 470 nm was plotted against the
molar fraction of the monomeic units of poly-1 (χ) (Figure 4).
Although the shape of plots was not symmetrical because of the
cooperativity that is described later, the maximum value was
clearly observed at 0.5 of χ, which indicated that the urea receptor
in poly-1 and CH3CO2- formed the 1:1 complex.
To understand the binding ability of poly-1, an absorption
titration experiment was conducted using CH3CO2-. By adding
CH3CO2-, the absorption corresponding to the polymer back-
bone gradually red-shifted and the λmax reached 466 nm when
Figure 5. (a) UV-vis absorption titration and (b) the resulting titration
curve of poly-1 with CH3CO2- in THF ([monomeric units of poly-1]=
130 μM, [CH3CO2-]/[monomeric units ofpoly-1]=0-5.0). Inset shows
the Hill plot analysis of this complexation.
-
1.0 equiv of CH3CO2 was added to the system (Figure 5a).
Figure 5b shows the plots of the absorbance at 470 nm as a
function of the CH3CO2- amount. The saturation in the absorp-
tion change was produced by the addition of almost an equimolar
amount of CH3CO2- relative to the urea units of poly-1, implying
that the efficient anion reception event was accomplished. More
importantly, the resulting titration curve displayed a sigmoidal
curvature, indicative of a cooperative binding mode.10,11 Thus,
this binding process was analyzed using the Hill equation, which
is a suitable equation for determining both the binding constant
and cooperativity in the case of a cooperative binding system.35
The monomer unit-based apparent association constant (Ka) and
Hill coefficient (n) of this binding system was determined to be
11500 M-1 and 8.0, respectively (the inset of Figure 5b). The
conformational change for the polymeric receptor, poly(phenyl-
acetylene) backbone should be important for the observed
allosteric anion-binding capability of poly-1. The inherent rigi-
dity and stiffness of the poly(phenylacetylene) backbone may
contribute to the exceptional cooperativity.
We further examined the effect of the molecular design of poly-
1 on the anion binding affinity by comparison to that of poly-
(phenylacetylene) featuring urea receptors derived from L-glutamic
acid (poly-2), which is one of the polymers that we previously
reported.40 The change in the absorption spectrum with the
addition of CH3CO2- and the resulting titration curve are shown
in Figure S1 of the Supporting Information. Poly-2 exhibited
a bathochromic shift similar to poly-1 upon the addition of
CH3CO2-. However, the spectral change was gradual, and thus
resulting Ka value obviously indicates that poly-1 possesses a
- 36
superior binding ability to CH3CO2
.
More importantly, an
exceptional n value was observed in this binding system, implying
a positive homotropic allosterism. The n value describes the
cooperativity in the binding process and the values greater than
one are obtained in the case of positive allosteric binding mode, in
which the initial guest-binding facilitates further complex forma-
tion. Given the observed large n value, it is obvious that the
cooperative and positive allosteric binding occurs between poly-1
and CH3CO2-, thus providing the increasing anion binding
ability. For this binding system, the partially formed urea/
-
more than 15 equiv of CH3CO2 was necessary for completing
the change. On the basis of the Hill analysis, the Ka and n values
of this binding system were found to be 2800 M-1 and 1.3,
respectively, which were both significantly lower than those for
poly-1. In particular, the resulting n value indicates little co-
-
operativity in the binding process between poly-2 and CH3CO2
.
These results suggest that the both binding ability and coopera-
tivity were remarkably enhanced for the newly designed poly-1.
The most reasonable explanation in the improvement of the
binding ability of poly-1 is the rational molecular design of the
urea moiety. Two electron-withdrawing -CF3 groups probably
endow the urea receptor with a more robust and efficient hydrogen-
bonding ability, thus enabling the improved anion binding
affinity. For realization of the highly cooperative binding, the
rational molecular design of the receptor units was therefore
clarified to be indispensable as well as the inherent nature of the
poly(phenylacetylene) backbone.
-
CH3CO2 complex units in the polymer chain should produce
a change in the whole main chain conformation, which is more
-
suitable for further CH3CO2 binding. Therefore, the binding
ability of poly-1 nonlinearly increases with the increasing amount
-
of CH3CO2 bound to poly-1. The observed positive allosteric
binding mode is definitely attributable to the molecular feature
that cooperative conformation change of the polymer backbone
is actualized. A similar cooperative conformation change has
been particularly observed for helical poly(phenylacetylene)s,37
which is the so-called chiral amplification via the sergeants and
soldiers principle.38,39 Given the scarcity of such highly cooperative
Finally, we carried out the absorption titration experiment and
Hill analysis for all the anions in order to elucidate the binding

Article
Macromolecules, Vol. 43, No. 18, 2010 7409
Chart 1. Structure of Poly-2
-CF3 groups realized a significant enhancement in the binding
ability without disturbing the capability of showing the color
variation depending on the types of anions. A detailed investiga-
tion clarified that both the poly(phenylacetylene) scaffold and the
enhanced hydrogen bonding ability of the urea receptor were
essential for the exceptional cooperativity. The concept of this
cooperative anion binding is applicable to other polymer and
supramolecular receptor that are possessing dynamic multipoint
binding sites. Therefore, we believe that the demonstrated posi-
tive allosterism based on the poly(phenylacetylene) receptor will
contribute to the further design of reliable and highly sensitive
anion sensor materials.
Table 1. Apparent Association Constant (Ka) and Hill Coefficient (n)
Experimental Section
of the Complexation of Poly-1 with Various Anionsa
Materials. 3,5-Bis(trifluoromethyl)phenyl isocyanate, tetra-
n-butylammonium azide, nitrate, fluoride, bromide, benzoate,
and acetate were purchased from Aldrich Chemical Co., Inc.,
and used as received. Tetra-n-butylammonium chloride and
perchlorate were available from Tokyo Kasei Kogyo Co., Ltd.
(TCI, Tokyo, Japan). 4-Ethynylaniline and tetra-n-butylammonium
hydrogen sulfate were purchased from Wako Pure Chemical
Industries, Ltd., and used without further purification. Dry
THF was available from Kanto Chemicals Co., Ltd., and
used without further purification. Rhþ(2,5-norbornadiene)[(η6-
C6H5)B-(C6H5)3] (Rh(nbd)BPh4) was prepared in accordance
with a previous report.42 The synthesis of poly-2 has already
been reported.26 The number-average molecular weight and
polydispersity of poly-2 are 1.7ꢀ 105 and 2.2, respectively.
anion
Ka (M-1
)
n
-
CH3CO2
C6H5CO2
F-
11500
11700
6500
9100
5300
4900
12300
11500
8.0
8.4
2.6
3.6
1.6
1.4
1.4
1.4
-
Cl-
Br-
NO3
N3
HSO4
-
-
-
a Determined from absorption titrations by using the Hill equation.
ability of poly-1 to other anions (see Supporting Information;
Figures S3-S5). In the resulting titration curve, the amount of
anion for reaching to the saturation and the curve profile highly
depended on anions. The Ka and n values could be determined for
1
Instruments. The H and 13C NMR spectra were recorded
using a JEOL JNM-A400II instrument. The size exclusion
chromatography (SEC) was performed at 40 ꢀC using a Jasco
high-performance liquid chromatography (HPLC) system (PU-
980 Intelligent HPLC pump, CO-965 Column oven, RI-930
Intelligent RI detector, and Shodex DEGAS KT-16) equipped
with a Shodex Asahipak GF-310 HQ column (linear, 7.6 mmꢀ
300 mm; pore size, 20 nm; bead size, 5 μm; exclusion limit, 4 ꢀ
104) and a Shodex Asahipak GF-7 M HQ column (linear,
7.6 mm ꢀ 300 mm; pore size, 20 nm; bead size, 9 μm; exclusion
limit, 4ꢀ107) in DMF containing lithium chloride (0.01 M) at a
flow rate of 0.4 mL min-1. The number-average molecular
weight (Mn) and polydispersity (Mw/Mn) of the polymer were
calculated on the basis of a polystyrene calibration. The ultra-
violet-visible (UV-vis) spectrum was measured using a Jasco
V-550 spectrophotometer equipped with a Jasco ETC-505T
temperature controller. The melting points of the compounds
were determined by the differential scanning calorimetry (DSC)
analysis using a Bruker AXS DSC 3100 SA under a nitrogen
atmosphere.
Synthesis of 1-(4-Ethynylphenyl)-3-[3,5-bis(trifluoromethyl)-
phenyl]urea (1). To a solution of 3,5-bis(trifluoromethyl)phenyl
isocyanate (4.40 g, 17.3 mmol) in dry THF (45 mL) was added
a solution of 4-ethynylaniline (2.12 g, 18.1 mmol) in dry THF
(25 mL) at room temperature under a N2 atmosphere. The
reaction mixture was stirred overnight. After the removal of the
solvent, the residue was dissolved in ethyl acetate and washed
with 1 N HCl, followed by water. The organic layer was dried
over anhydrous MgSO4 and evaporated to dryness. The residue
was purified by recrystallization from acetonitrile to give 1 as a
solid. Yield=6.26 g (97.5%); mp 156 ꢀC. 1H NMR (400 MHz,
DMSO-d6, δ): 9.44 (s, 1H, -NH-CO-), 9.19 (s, 1H, -NH-
CO-), 8.14 (s, 2H, aromatic), 7.64 (s, 1H, aromatic), 7.52 (d, J=
8.8 Hz, 2H, aromatic), 7.42 (d, J=8.7 Hz, 2H, aromatic), 4.06 (s,
1H, HCtC-). 13C NMR (100 MHz, DMSO-d6, δ): 153.14,
142.58, 140.64, 133.34, 131.65 (q, J=33 Hz), 124.21 (q, J=273
Hz), 119.5, 119.10, 116.26, 115.45, 84.50, 80.49. Anal. Calcd for
C17H10F6N2O: C, 54.85; H, 2.71; N, 7.53. Found: C, 54.51; H,
2.94; N, 7.42.
-
anions except for ClO4 by using the Hill analysis, and the
resulting values are summarized in Table 1. Poly-1 was found to
possess good binding affinities to these anions, of which the Ka
values ranged from 4900 to 12300, suggesting that poly-1 was a
suitable receptor for all these anions. Moreover, a positive allosteric
binding similar to the case of CH3CO2- was observed for C6H5-
CO2-, F-, and Cl-, though almost no cooperativity was found for
Br-, NO3-, N3-, and HSO4-. In particular, the poly-1/C6H5CO2
-
complex exhibited the highest n value of 8.4. The observed n value
increased in the order of NO3 ≈ N3 ≈ HSO4- <Br- <F- <
Cl- , CH3CO2- < C6H5CO2-. The important point to note is
that the order is not well explained by only the binding constant.
For the anion recognition chemistry, the binding affinity is
generally dictated by the basicity of anions, which increases in
-
-
the order of ClO4- < HSO4 ≈ Br- ≈ NO3- < Cl- < N3
<
-
-
C6H5CO2-< CH3CO2- < F-.41 However, neither the Ka nor
n values well correlated with the basicity of the anions. Considering
that good relationship was also not observed between the colori-
metricresponse of poly-1 andthe basicity of the employedanions,
other factor should be concerned in this anion recognition event.
In the both processes of the colorimetric response and the co-
operative anion binding, the polymer conformation change plays
a crucial role as well as the anion binding at the urea function-
alities. Therefore, the anion size, which has a significant effect in
the polymer conformation change, could be considered as one of
the prime factors in this anion recognition. Although the exact
mechanism has yet to be revealed, the anion specificity in the
colorimetric response and the cooperative binding were clearly
demonstrated in this examination. Further investigation is cur-
rently being conducted to clarify this mechanism, which includes
the evaluation of anion recognition of poly(phenylacetylene) bear-
ing bulkier pendants.
Conclusions
We demonstrated the efficient colorimetric anion sensing based
on the highly cooperative binding ability of urea-functionalized
poly(phenylacetylene). The introduction of electron-withdrawing
Polymerization. The polymerization of 1 was carried out in
a dry Shlenk flask under an argon atmosphere. Under an Ar

7410 Macromolecules, Vol. 43, No. 18, 2010
Sakai et al.
atmosphere, 1 (1.00 g, 2.69 mmol) was weighed into a flask and
dissolved in dry THF (85.5 mL). To the solution was added a
solution of Rh(nbd)BPh4 (27.6 mg, 53.7 μmol) in dry THF (4.0
mL). After stirring at room temperature for 24 h, to the reaction
mixture was added triphenylphosphine (84.6 mg, 322 μmol).
The solution was concentrated and then poured into a large
amount of CH2Cl2. The precipitate was purified by reprecipita-
tion with CH2Cl2 and then dried under reduced pressure to give
poly-1 as a black powder. Yield: 887 mg (88.7%). Mn = 1.7 ꢀ
105, Mw/Mn =1.8.
(3) Gunnlaugsson, T.; Glynn, M.; Tocci, G. M.; Kruger, P. E.; Pfeffer,
F. M. Coord. Chem. Rev. 2006, 250 (23-24), 3094–3117.
(4) Gale, P. A.; Quesada, R. Coord. Chem. Rev. 2006, 250 (23-24),
3219–3244.
(5) Anzenbacher, P., Jr.; Nishiyabu, R.; Palacios, M. A. Coord. Chem.
Rev. 2006, 250 (23-24), 2929–2938.
(6) Davis, A. P. Coord. Chem. Rev. 2006, 250 (23-24), 2939–2951.
(7) Katayev, E. A.; Ustynyuk, Y. A.; Sessler, J. L. Coord. Chem. Rev.
2006, 250 (23-24), 3004–3037.
(8) Miyaji, H.; Sessler, J. L. Angew. Chem., Int. Ed. 2001, 40 (1),
154–157.
(9) Palacios, M. A.; Nishiyabu, R.; Marquez, M.; Anzenbacher, P.
Typical Experimental Procedure for UV-vis Absorption Mea-
surements. All UV-vis absorption measurements were perfor-
med in dry THF. The concentrations of poly-1 and poly-2,
which were calculated on the basis of the monomeric units, were
130 μM for all the measurements. A typical experimental
procedure is described as follows: Stock solutions of poly-1
(537 μM) and tetra-n-butylammonium acetate (TBAA) (1.34 mM)
in THF were prepared in flasks equipped with stopcocks,
respectively. The poly-1 solution (1 mL) and the TBAA solution
(0.4 mL) were transferred to a vial, and then the resulting
mixture was diluted with dry THF (2.6 mL) to give the sample
solution, of which [monomeric units of poly-1] and [TBAA]/
[monomeric units of poly-1] were adjusted to be 130 μM and 1.0,
respectively. The UV-vis absorption spectrum of the resulting
sample solution was measured in a glass cell with a 10 mm path
length at 25 ꢀC.
Absorption Titrations and Hill Plots. Sample solution with
varying anion amount was prepared in accordance with the above
procedure, and the absorption spectra were taken to determine
the change. The titration curve was obtained by plotting the
absorbance at the proper wavelength versus the [anion]/[urea]
ratio. The binding data were further analyzed using the Hill
equation: log(Y/(1 - Y))=n log[anion] þ n log Ka, where Y, Ka,
and n represent the fractional saturation of the host, the
apparent microscopic association constant, and the Hill coeffi-
cient, respectively. The Y value was calculated by the following
equation: Y=ΔAobs/ΔAmax =(Aobs - A0)/(Amax - A0), where
A0, Aobs, and Amax represent the inherent absorbance of polymer
sample, the absorbance in the presence of anion guests, and the
maximum absorbance obtained when the absorption change
was completed at the selected wavelength. The Ka and n values
were determined from the slope and Y-intercept in the resulting
Hill plots. The Ka value estimated in this calculation means the
apparent binding constant of one urea receptor unit, that is, the
monomeric unit in the cases of poly-1 and poly-2.
J. Am. Chem. Soc. 2007, 129 (24), 7538–7544.
(10) Takeuchi, M.; Ikeda, M.; Sugasaki, A.; Shinkai, S. Acc. Chem. Res.
2001, 34 (11), 865–873.
(11) Shinkai, S.; Ikeda, M.; Sugasaki, A.; Takeuchi, M. Acc. Chem. Res.
2001, 34 (6), 494–503.
(12) Takeuchi, M.; Shioya, T.; Swager, T. M. Angew. Chem., Int. Ed.
2001, 40 (18), 3372–3376.
(13) Hirata, O.; Takeuchi, M.; Shinkai, S. Chem. Commun. 2005, 30,
3805–3807.
(14) dos Santos, C. M. G.; McCabe, T.; Watson, G. W.; Kruger, P. E.;
Gunnlaugsson, T. J. Org. Chem. 2008, 73 (23), 9235–9244.
(15) Willans, C. E.; Anderson, K. M.; Potts, L. C.; Steed, J. W. Org.
Biomol. Chem. 2009, 7 (13), 2756–2760.
(16) Okamoto, Y.; Nakano, T. Chem. Rev. 1994, 94 (2), 349–372.
(17) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000,
100 (7), 2537–2574.
(18) Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.; Moore, J. S.
Chem. Rev. 2001, 101 (12), 3893–4012.
(19) Nakano, T.; Okamoto, Y. Chem. Rev. 2001, 101 (12), 4013–4038.
(20) Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M.; Sommerdijk,
N. A. J. M. Chem. Rev. 2001, 101 (12), 4039–4070.
(21) Thomas, S. W.; Joly, G. D.; Swager, T. M. Chem. Rev. 2007,
107 (4), 1339–1386.
(22) Yashima, E.; Maeda, K.; Nishimura, T. Chem. Eur. J. 2004,
10 (1), 42–51.
(23) Yashima, E.; Maeda, K. Macromolecules 2008, 41 (1), 3–12.
(24) Kakuchi, R.; Nagata, S.; Sakai, R.; Otsuka, I.; Nakade, H.; Satoh,
T.; Kakuchi, T. Chem. Eur. J. 2008, 14 (33), 10259–10266.
(25) Kakuchi, R.; Nagata, S.; Tago, Y.; Sakai, R.; Otsuka, I.; Satoh, T.;
Kakuchi, T. Macromolecules 2009, 42 (5), 1476–1481.
(26) Kakuchi, R.; Tago, Y.; Sakai, R.; Satoh, T.; Kakuchi, T. Macro-
molecules 2009, 42 (13), 4430–4435.
(27) Kakuchi, R.; Kodama, T.; Shimada, R.; Tago, Y.; Sakai, R.;
Satoh, T.; Kakuchi, T. Macromolecules 2009, 42 (12), 3892–3897.
(28) Amendola, V.; Boiocchi, M.; Colasson, B.; Fabbrizzi, L. Inorg.
Chem. 2006, 45 (16), 6138–6147.
(29) Nishiyabu, R.; Anzenbacher, P. Org. Lett. 2006, 8 (3), 359–362.
(30) Gomez, D. E.; Fabbrizzi, L.; Licchelli, M.; Monzani, E. Org.
Biomol. Chem. 2005, 3 (8), 1495–1500.
(31) Jose, D. A.; Kumar, D. K.; Ganguly, B.; Das, A. Tetrahedron Lett.
2005, 46 (32), 5343–5346.
Job’s Plots. THF solutions of poly-1 and TBAA were mixed
to prepare sample solution with varying molar fractions of the
monomeic units of poly-1 (χ) from 0 to 1. The total concentra-
-
(32) Amendola, V.; Esteban-Gomez, D.; Fabbrizzi, L.; Licchelli, M.
Acc. Chem. Res. 2006, 39 (5), 343–353.
(33) Gale, P. A. Acc. Chem. Res. 2006, 39 (7), 465–475.
(34) Boiocchi, M.; Del Boca, L.; Gomez, D. E.; Fabbrizzi, L.;
Licchelli, M.; Monzani, E. J. Am. Chem. Soc. 2004, 126 (50),
16507–16514.
tion of monomeric units of poly-1 and CH3CO2 was kept
constant at 260 μM. UV-vis measurement of each sample was
performed. Absorbance at 470 nm were normalized to the
variation of absorbance (ΔA470) with the following equation:
ΔA470 = Aobs - χA0, where A0 is absorbance of poly-1 itself in
THF at 260 μM (χ = 1). Job’s plots were obtained by plotting
ΔA470 values against χ.
(35) The binding data were analyzed using the Hill equation: log(Y/(1 -
Y))=n log[anion] þ n log Ka, where Y is the fractional saturation of
the host, Ka is the apparent microscopic association constant, and n
is the Hill coefficient. See: Connors, K. A. Binding Constants: the
Measurement of Molecular Complex Stability; Wiley: New York,
1987; pp 59-65.
(36) One reviewer of this paper pointed out the possibility that the newly
developed absorbance at 470 nm corresponds not to the quantity of
anion bound to polymer but to the extent of conformational
change. This is definitely sensible. In fact, Yashima et al. have
reported amplification of circular dichroism (CD) signal based on
cooperative conformational change of polymer chain.37 However,
we think such possibility is excluded by the following reasons:
(1) Poly-1 shows the colorimetric response by the addition of anions,
in which conformational change of poly-1 actually plays crucial
roles as well as the complex formation. However, the colorimetric
response is unexplainable by only the conformation change of poly-1.
In contrast to CD signals, abosorption change should be ascribable
to the complex formation event. (2) In anion recognition chemistry,
Acknowledgment. We gratefully acknowledge the Global
COE Program (Catalysis as the Basis for Innovation in Materials
Science) of the Ministry of Education, Culture, Sports, Science
and Technology, Japan, for the financial support.
Supporting Information Available: UV-vis absorption
titration of poly-1 and poly-2 in the presence of TBA salts of
anions and the resulting Hill plots. This material is available free
of charge via the Internet at http://pubs.acs.org.
References and Notes
(1) Caltagirone, C.; Gale, P. A. Chem. Soc. Rev. 2009, 38 (2), 520–563.
(2) Martinez-Manez, R.; Sancenon, F. Chem. Rev. 2003, 103 (11),
4419–4476.

Article
a number of small molecular receptors with chromophores show
Macromolecules, Vol. 43, No. 18, 2010 7411
(37) Nonokawa, R.; Yashima, E. J. Am. Chem. Soc. 2003, 125 (5), 1278–
1283.
(38) Green, M. M.; Peterson, N. C.; Sato, T.; Teramoto, A.; Cook, R.;
the colorimetric response as a direct consequence of anion recogni-
tion event, in which the color change is considered to be based on
the complex formation. Therefore, the colorimetric response of
poly-1 would correlate closely with the anion recognition. (3) The
Job’s plots as shown in Figure 4 provided additional important
information on the colorimetric response of poly-1. The Job’s plots
showed the maximum value at 0.5 of χ. If the signal amplification
based on the cooperative conformation change occurs, the ΔA470
values in the range from 0.5 to 1.0 should be much higher due to the
signal amplification effect. Therefore, we believe that the absor-
bance at 470 nm is almost identical with the quantity of anion
bound to the polymer. At least, the determination of the apparent
binding constant and cooperativity using this absorbance is con-
sidered to be appropriate.
Lifson, S. Science 1995, 268 (5219), 1860–1866.
(39) Green, M. M.; Park, J. W.; Sato, T.; Teramoto, A.; Lifson, S.;
Selinger, R. L. B.; Selinger, J. V. Angew. Chem., Int. Ed. 1999,
38 (21), 3139–3154.
(40) Due to the good solubility in THF, poly-2 was employed to
compare the anion binding ability. See ref 26.
(41) The acid-base equilibrium constants in DMSO were used as an
alternative. For F-: (a) Frederick, G. B. Acc. Chem. Res. 1988, 21,
456–463. For other anions: (b) Izutsu, K. Acid-Base Dissociation
Constants in Dipolar Aprotic Solvents; Blackwell: London, 1990.
(42) Kishimoto, Y.; Itou, M.; Miyatake, T.; Ikariya, T.; Noyori, R.
Macromolecules 2002, 28 (19), 6662–6666.