Anion detection by a fluorescent poly(squaramide): Self-assembly of anion-binding sites by polymer aggregation

Article abstract of DOI:10.1002/anie.201006884

Winner by aggregate: Incorporating an anion-binding squaramide group into a polymeric architecture results in drastic alterations in the selectivity and magnitude of its anion-induced response, resulting in a sensitive and discriminating turn-on fluoresce

Full text of DOI:10.1002/anie.201006884

DOI: 10.1002/anie.201006884
Anion Recognition
Anion Detection by a Fluorescent Poly(squaramide): Self-Assembly of
Anion-Binding Sites by Polymer Aggregation**
Ali Rostami, Chu Jun Wei, Gꢀrald Guꢀrin, and Mark S. Taylor*
The selective recognition and sensing of anions has been the
subject of intensive research effort, motivated by applications
in medical diagnostics, environmental and industrial monitor-
ing, and nuclear waste cleanup.[1] Selectivity and affinity may
be achieved by preorganization of binding groups in an
orientation complementary to the geometry and charge
distribution of the anionic guest: sulfate- and phosphate-
binding proteins represent impressive illustrations of this
principle.[2] While there has been some progress toward
synthetic hosts capable of high-affinity, selective anion
recognition in competitive (aqueous) environment,[3] the
synthesis of preorganized targets having the ideal number
and orientation of binding groups for a given analyte is often
difficult,[4] and rates of guest binding and release may become
problematic. An attractive solution to this problem employs
reversible processes (noncovalent or dynamic covalent inter-
actions) as the basis for receptor self-assembly. Self-assem-
bled capsules,[5] dynamic covalent libraries,[6] and coordina-
tion complexes[7–9] capable of anion binding are successful
implementations of this concept.
unprecedented alteration of anion selectivity as well as an
enhancement in anion sensitivity.
Our investigations began with the synthesis of poly(squar-
amide) 1a (Scheme 1). This material is unlikely to be capable
of long-range exciton transport, a property that has been
exploited to generate signal “gain” in conjugated polymer-
Here, we describe experiments demonstrating that aggre-
gation of an organic polymer composed of repeating hydro-
gen-bond donor groups may be exploited to achieve remark-
able enhancements in anion affinity and selectivity. In
particular, a polymer based on the 3,4-diaminocyclobutene-
1,2-dione (squaramide) functional group shows a selective
“turn-on” fluorescence response to dihydrogenphosphate
Scheme 1. Preparation of poly(squaramides) by Lewis acid catalyzed
condensation.
À
(H2PO4 ) ions in the competitive medium 10% water in N-
methylpyrrolidinone (NMP). The dual role of the squaramide
groups in controlling both the aggregation of the polymer and
its anion-responsive properties results in complex behavior,
including cooperativity in analyte binding. Comparison with a
non-polymeric reference compound indicates that incorpo-
ration of the squaramide group into a polymer results in an
based detection schemes.[10,11] However, we sought to test the
hypothesis that 1a could display anion-responsive properties
resulting from the precedented aggregation behavior of
polyamides in general,[12] and of materials containing squar-
amide[13] and urea[14] groups in particular, coupled with the
known anion-binding properties of these functional groups.[15]
Squaramides bind tightly to anions even in competitive
media, a property attributed to the high acidity of squaramide
NH groups relative to those of amides or (thio)ureas.[16]
Polymer 1a was prepared by scandium(III) triflate-
catalyzed condensation of 9,9-dioctylfluorene-2,7-diamine
and diethyl squarate employing a modification of our recently
developed procedure for the preparation of aniline-based
squaramides.[16b] Under Lewis acid catalyzed conditions, the
poly(squaramide) was obtained with good molecular weight
and polydispersity (Mn = 1.8 ꢀ 104 gmolÀ1, Mw/Mn = 1.7) as
judged by gel permeation chromatography (GPC) in 0.2 wt%
LiCl/NMP with poly(methyl methacrylate) standards. Poly-
merization in the presence of 4-tert-butylaniline yielded
polymer 1b carrying defined end groups. Values of Mn for
[*] A. Rostami, C. J. Wei, Dr. G. Guꢀrin, Prof. M. S. Taylor
Department of Chemistry, Lash Miller Laboratories
University of Toronto
80 St. George St., Toronto, ON M5S 3H6 (Canada)
Fax: (+1)416-978-8775
E-mail: mtaylor@chem.utoronto.ca
Homepage: http://www.chem.utoronto.ca/staff/MST/taylorgroup
[**] This work was funded by NSERC (Discovery Grants Program), the
Canadian Foundation for Innovation, the Province of Ontario, the
Connaught Foundation, and the University of Toronto. We are
grateful to Prof. Mitch Winnik for advice and for access to
instrumentation for GPC, DLS, and fluorescence experiments, to
Sandeep Sagoo and Prof. Rebecca Jockusch for ESI-MS experi-
ments, and to Mike Chudzinski for DFT calculations.
1
1b determined by H NMR spectroscopy (5.0 ꢀ 103 gmolÀ1)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006884.
and GPC (7.0 ꢀ 103 gmolÀ1) were in good agreement. These
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Communications
materials, which are soluble in amide solvents such as NMP
To evaluate the importance of the polymeric architecture
of squaramide 1a, compound 2 was prepared as a control
receptor. Like polymer 1a, squaramide 2 underwent an
increase in fluorescence quantum yield upon addition of
and N,N-dimethylformamide (DMF), represent the first
examples of soluble, well-defined squaramide-based poly-
mers, a new class of poly(amides).[17] The unusual anion
sensory properties of these materials are discussed in the
following paragraphs; experiments probing their mechanical
and electronic properties are underway.
The interactions of polymer 1a with anionic analytes were
studied by fluorescence spectroscopy in 10% water/NMP, a
competitive medium in which anion recognition by neutral
hosts represents a challenge. Addition of tetra-n-butylammo-
nium dihydrogenphosphate (Bu4N+H2PO4À) to solutions of
1a resulted in an increase in emission intensity (Figure 1a).
À
Bu4N+H2PO4 in 10% H2O/NMP, but the cooperative
response characteristic of 1a was not observed. Instead, 2
showed behavior consistent with 1:1 binding (Ka = 2.8 ꢀ
103 mÀ1, determined by fitting changes in fluorescence inten-
sity to a 1:1 binding curve).[18,22] Polymer 1a is approximately
four times more sensitive to phosphate ions than is 2: the half-
maximal fluorescence responses of 1a and 2 were observed at
À
H2PO4 concentrations of 50 mm and 200 mm, respectively.
The magnitude of the fluorescence response of 1a is also
greater (maximum values of I/I0 for 1a and 2 were 25 and 5,
respectively); this is largely a consequence of the lower
fluorescence of 1a as compared to 2 in the absence of analyte,
likely due to aggregation of the former (see below). We
attribute the enhanced emission of 1a and 2 in the presence of
H2PO4À ions to an intramolecular charge-transfer mechanism.
Incorporating the squaramide group into a poly(amide)
backbone has a profound effect on the selectivity of system
(Figure 2). The fluorescence response of polymer 1a is
Figure 2. Normalized fluorescence response I/I0 of polymer 1a (black
bars) and model compound 2 (gray bars) to anions XÀ (nBu4N+XÀ,
10% H2O/NMP solvent; anion concentration 140 mm for 1a and
1200 mm for 2, respectively).
Figure 1. a) Fluorescence response of polymer 1a to Bu4N+H2PO4
À
(10% water in NMP solvent; lex =415 nm); b) Concentration depend-
*
ence of the fluorescence response (lem =À453 nm) of 1a (blue ) 1b
selective for H2PO4À over other monovalent anions, including
basic analytes such as fluoride and hydroxide. In contrast,
model compound 2 shows the largest fluorescence response
+
~
^
(green ), and 2 (red ) to Bu4N H2PO4
.
À
This effect was evident to the naked eye upon irradiation of
solutions of 1a in the presence or absence of H2PO4À ions with
a hand-held UV lamp (Figure 1a, inset).
towards fluoride, followed by H2PO4 ions. The difficulty in
developing receptors capable of the selective binding of
H2PO4 ions in the presence of FÀ has been documented
À
previously.[23] Selective detection of inorganic phosphate ions
in aqueous medium is of interest, both from the perspective of
its relevance in biological systems and human health, and its
role as a pollutant causing eutrophication of fresh water.[10d,24]
It is noteworthy that the affinity and selectivity of polymer 1a,
À
A graph of fluorescence intensity as a function of H2PO4
concentration shows the sigmoidal shape characteristic of a
cooperative anion response, in which polymer–phosphate
interactions trigger additional binding events (Figure 1b). A
À
Hill plot indicates positive cooperativity for the 1a–H2PO4
interaction (nH = 2.5),[18] although the complex nature of the
polymer–anion interaction (see below) likely precludes a
straightforward application of this type of analysis.[19] While
cooperativity is a hallmark of binding and recognition
processes in biological systems,[20] its exploitation in synthetic
receptors represents an emerging area of research.[21]
which has no evident preorganized “binding site” for H2PO4
À
ions, compare favorably with those of state-of-the-art,
uncharged receptors capable of phosphate binding in com-
petitive medium.[25]
The unusual enhancements in affinity and selectivity
observed upon incorporation of the squaramide group into
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Angew. Chem. Int. Ed. 2011, 50, 2059 –2062

polymer 1a appear to be results of the interplay between
polymer aggregation and analyte binding. As is typical of
aromatic amide-based polymers, 1a is prone to aggregation:
GPC analysis with NMP as eluent resulted in an apparent Mn
of 2.0 ꢀ 106 gmolÀ1; upon addition of LiCl, which promotes
the deaggregation of polyamides, the reported molecular
weight of 1.8 ꢀ 104 gmolÀ1 was determined. Temperature-
dependent UV/Vis spectra in DMF and transmission electron
microscopy (TEM) also indicated aggregation of 1a.[18]
Dynamic light scattering (DLS) measurements in DMF
solution demonstrated that addition of phosphate ions
triggers the formation of well-defined aggregates (mean
hydrodynamic radius 120 nm, PDI = 0.1; Figure 3a,b) from
an initially heterogeneous distribution in its absence (low
scattering intensity: hydrodynamic radius 50 nm, PDI = 0.3).
The results of TEM experiments were in qualitative agree-
ment with the solution-phase behavior: in the absence of
Bu4N+H2PO4À, aggregates of 1a varying in size and shape
were observed,[18] but images of 1a in the presence of
Bu4N+H2PO4 indicated spherical aggregates of 200–800 nm
À
diameter (Figure 3c).[26] The larger of these aggregates were
visible by laser confocal microscopy in DMF.[18] This behavior
stands in contrast to previously reported deaggregation-based
detection schemes employing conjugated polymers[10a,27] and
urea-based oligomers.[14c]
It thus appears that anion-triggered reorganization of
polymer aggregates gives rise to the cooperative response and
altered analyte selectivity that characterize the poly(squar-
À
amide) system. We propose that H2PO4 ions are able to
engage in hydrogen-bonding interactions with multiple squar-
amide groups, bringing polymer chains together in a manner
that results in a cooperative anion response.[28] Consistent
with this hypothesis, the magnitude and degree of coopera-
tivity of the anion-induced fluorescence response increased as
a function of polymer molecular weight (see Figure 1b and
the Supporting Information). Precedent for this proposal
includes the existence of urea-based receptors that bind to
H2PO4 with a 2:1 urea:anion stoichiometry,[25a] and the
À
observation of a squaramide host that undergoes sulfate-
induced dimerization.[29] Calculations (B3LYP/6-311 + G**)
suggest a geometry for such a 2:1 squaramide:phosphate
complex (Figure 3d). Recent studies suggest that secondary,
attractive intra-receptor interactions are a powerful strategy
for achieving high-affinity binding in competitive medium;[30]
the present example appears to represent a new illustration of
this principle.
The superior mechanical properties of poly(amides) that
underlie their commercial utility are largely the result of
interpolymer hydrogen-bonding interactions. This work dem-
onstrates for the first time that polyamides may be employed
for selective anion detection in competitive medium, and
indicates that the aggregation behavior of this class of
materials may offer significant benefits in the context of
chemical sensing. Viewed in another light, this represents a
system in which the supramolecular aggregation of a polymer
is under the control of a specific small-molecule stimulus.
Efforts to develop a more detailed mechanistic understanding
of the anion-induced response, and to modify the polymer
structure to achieve selective anion binding in still more
competitive media, are ongoing in our laboratories.
Received: November 3, 2010
Published online: January 24, 2011
Figure 3. a) Autocorrelation functions and b) normalized CONTIN
plots from DLS measurements at 908 on polymer 1a (2.4ꢁ10À5 m) in
Keywords: aggregation · anions · molecular recognition ·
polymers · self-assembly
.
*
*
the absence (red , c) and the presence (blue , c) of
Bu4N+H2PO4 (2.4ꢁ10À3 m) in DMF. c) TEM image of a dried film of
À
1a and Bu4N+H2PO4À (sample was cast from a DMF solution of 1a
(2.4ꢁ10À5 m) and Bu4N+H2PO4À (2.4ꢁ10À3 m)). Spherical polymer
aggregates are clearly visible. The lacy pattern is the copper framework
of the carbon-coated copper TEM grid. d) Calculated (B3LYP/6-
311+G**)Àgeometry of the 2:1 complex of N,N’-dimethylsquaramide
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