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Poly (squaramides): Synthesis, Anion Sensing, and
Self-Assembly
by
Ali Rostami
A thesis submitted in conformity with the requirements for the
degree of Doctor of Philosophy
Department of Chemistry University of Toronto
© Copyright by Ali Rostami 2012
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Poly (squaramides): Synthesis, Anion Sensing, and Self-
Assembly
Ali Rostami
Doctor of Philosophy
Department of Chemistry University of Toronto
2012
Abstract
The focus of the research presented in this thesis is the
design, synthesis, and anion recognition
properties of a structurally novel class of poly (amides) that
incorporates the
diaminocyclobutenedione (squaramide) group into the polymer
backbone.
In Chapter 1, a brief overview of different anion-responsive
synthetic macromolecules is
presented. Emphasis is placed on the wide structural diversity
of the polymers, the mechanisms
of their anion-induced responses, and features such as signal
amplification, multivalency, and
cooperative behavior that can be exploited productively in the
context of anion recognition and
sensing.
Chapter 2 describes a new method for the regioselective
preparation of squaramides, using Lewis
acid-catalyzed condensations of diethyl squarate and different
anilines. Zinc
trifluoromethanesulfonate promotes efficient condensations of
anilines with squarate esters,
providing access to symmetrical and unsymmetrical squaramides in
high yields from readily
available starting materials. Colorimetric anion-sensing
behavior and computational studies
illustrating the enhanced hydrogen bond donor ability and
acidity of squaramides in comparison
to ureas are presented.
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In Chapter 3, the application of the synthetic method described
above to the selective preparation
of polysquaramides composed of 1,2-isomeric repeat units is
described. The optical, thermal and
aggregation properties of these materials are also
discussed.
Finally, Chapter 4 describes self-assembly properties as well as
applications of these materials in
the area of anion recognition and sensing. 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 fluorescence sensor
for dihydrogenphosphate ions. This unusual behavior is the
result of a cooperative, anion-
triggered aggregation process that was further probed by dynamic
light scattering (DLS),
transmission electron microscopy (TEM) and laser confocal
microscopy.
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Dedicated to my lovely parents, Parvaneh and Abbas
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Acknowledgments
First and foremost I offer my sincerest gratitude to my
supervisor, Dr. M. S. Taylor, who has
supported me throughout my thesis with his patience and
knowledge whilst allowing me the
room to work in my own way. I attribute the level of my Ph.D.
degree to his encouragement and
effort and without him this thesis, too, would not have been
completed or written. One simply
could not wish for a better or friendlier supervisor.
My appreciation goes to all the members of Taylor’s group for
their continuous help and
friendship. In particular, many thanks go to Sunny Lai, Mike
Chudzinski, Corey McClary, Doris
Lee, Elena Dimitrijevic and Gholam Sarwar for our great
discussions and shared moments in the
Lab. I would also like to thank members of Winnik’s group
especially Mohsen Soliemani and
Dr. Gerald Guerin fantastic scientists and polymer chemists, I
learned not only about polymer
chemistry but also about enjoying life. I gratefully thank Chu
Jun Wei for her assistance in the
synthesis and sensory properties of squaramide-based model
compound required for our second
publication. Also, many thanks go to Xiao Yu Li and Alexis
Collin, undergraduate students who
helped me with catalyst screening and UV/vis titration
experiments required for our first
publication. I would also like to extend my acknowledgment to
all spectral services staff for their
fantastic work and patience.
I gratefully acknowledge the funding sources NSERC and
University of Toronto that made my
Ph.D. work possible.
My most sincere gratitude goes to my lovely parents, for
constantly supporting my education,
both financially and emotionally. Last but not least; I would
like to thank all my friends,
especially Ali Alavi, for his presence, encouragement and
motivations throughout this period.
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Table of Contents
ABSTRACT………………………………………………………………………………………ii
ACKNOWLEDGEMENTS………………………………………………………………………iv
LIST OF TABLES………………………………………………………………………………..ix
LIST OF SCHEMES………………………………………………………………………………x
LIST OF FIGURES……………………………………………………………………….……..xii
LIST OF APPENDICES ………………………………………………………………...……...xvi
LIST OF ABBREVIATIONS…………………………………………………………………..xvii
1 CHAPTER1……………………………………………………………………………………..1
1 Polymers for anion recognition and
sensing…………………………………………………….1
1.1 Introduction……………………………………………………………………………………1
1.2 Polymer-based chemical sensors: general
principles………………………………………….2
1.3 Anion responsive polymers……………………………………………………………………4
1.3.1 Anion responsive
chemodosimeters………………………………………………………....4
1.3.2 Lewis acid–base interactions: organoboron, organosilicon,
and metal cation-containing
anion-responsive polymers…………………………………………………………………..…....5
1.3.3 Ion-pairing interactions: anion recognition by cationic
polyelectrolytes…………………15
1.3.4 Polymers bearing hydrogen bond-donor
groups……………………………………….......19
1.4 Conclusions…………………………………………………………………………………..26
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1.5 References……………………………………………………………………………………28
2 CHAPTER 2…………………………………………………………………………………...31
2 N,N!-Diarylsquaramides: Synthesis and applications in
colorimetric anion sensing………...31
2.1 Introduction…………………………………………………………………………………..31
2.2 Squaramide preparation……………………………………………………………………...34
2.3 Development of a Lewis-acid-promoted condensation of
anilines with squarate Esters…....36
2.4 Hydrogen bonding of N,N!-diarylsquaramides with neutral
acceptors: solid-state and
computational studies………………………………………………………………………..40
2.5 Colorimetric anion sensing by nitro-substituted N,N!-
diarylsquaramides…………………..42
2.6 Computational investigation of the acidity of
squaramides………………………………….51
2.7 Conclusions…………………………………………………………………………………..53
2.8 References……………………………………………………………………………………54
2.9 Experimental procedures……………………………………………………………………..57
3 CHAPTER 3…………………………………………………………………………………...64
3 Polysquaramides: synthesis, thermal and optical
properties……………………………….......64
3.2 Aromatic polyamide
synthesis…………………………………………………………….....65
3.3 Polysquaramides: synthetic challenges and
limitations……………………………………...68
3.4 Monomer synthesis…………………………………………………………………………..70
3.5 Polysquaramide synthesis by Lewis acid catalyzed
condensation polymerization………….73
3.6 Thermal properties of
polysquaramides……………………………………………………...77
3.7 Aggregation properties of polysquaramides in
solution……………………………………..78
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3.8 Computational modeling of
oligo(fluorenesquaramides)…………………………………....79
3.8.1 Conformation of
oligo(fluorenesquaramides)……………………………………………..79
3.8.2 Bandgap calculations on
oligo(fluorenesquaramides)……………………………..………81
3.9 Optical properties of
polysquaramides………………………………………………………82
3.10 Conclusions…………………………………………………………………………………85
3.11 References…………………………………………………………………………………..85
3.12 Experimental procedures…………………………………………………………………....87
4 CHAPTER 4………………………………………………………………………………….104
4 Polysquaramides: anion sensing and
self-assembly…………………………………………..104
4.1 Introduction……………………………………………………………………………........104
4.2 Synthetic receptors with rigidity and
preorganization……………………………………...106
4.3 Self-assembled molecular
capsules…………………………………………………………108
4.4 Self-assembled polymers…………………………………………………………………...111
4.4.1 Anion detection by a fluorescent
polysquaramide……………………………………......111
4.4.2 Nature and magnitude of emission responses: molecular
weight effect………………….114
4.4.3 Selectivity towards different
anions…………………………………………………........116
4.4.4 Aggregation properties: microscopy and light scattering
studies………………………...118
4.4.5 Anion-induced self-assembly of polymeric
chains…………………………………...…..121
4.4.6 Nature and magnitude of emission response: structural
effects…………………………..124
4.4.7 Selectivity of structurally different polymers towards
different anions……………….…127
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4.4.8 Anion induced self-assembly of different
polysquaramides……………………………...128
4.5 Conclusions ………………………………………………………………………………...130
4.6 References…………………………………………………………………………………..132
4.7 Experimental procedures…………………………………………………………………....135
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List of Tables
Table 2.01 Evaluation of Lewis acid catalysts for the
condensation of 3,5-bis(trifluoromethyl)-
aniline and diethyl squarate………………………………………………………………………37
Table 2.02 Preparation of symmetrically substituted
N,N!-diarylsquaramides by Zn(OTf)2-
catalyzed condensation…………………………………………………………………………..38
Table 2.03 Preparation of unsymmetrically substituted
N,N"-diaryl squaramides by Zn(OTf)2-
catalyzed condensation…………………………………………………………………………..39
Table 3.01 Effect of solvent ratio on polymer molecular weight
and polydispersity……………74
Table 3.02 Molecular weight and polydispersities for different
Polysquaramides determined by
GPC………………………………………………………………………………………………75
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List of Schemes
Scheme 2.01 Applications of squaramide in chemistry, biology and
material sciences…………31
Scheme 2.02 Recognition of carboxylate anion with
squaramide-based receptor……………....32
Scheme 2.03 A squaramide fluorescent ensemble for monitoring
sulfate in water……………...33
Scheme 2.04 Squaramide based chloride ion
receptor…………………………………………..33
Scheme 2.05 Traditional methods for the preparation of
mono-squaramide and bis-
squaramides………………………………………………………………………………………34
Scheme 2.06 Mechanism for the synthesis of squaramides and
squaraines……………………..35
Scheme 2.07 Preparation of bis-squaramides by (a) condensation
reaction and (b) copper
catalyzed cross-coupling
reaction………………………………………………………………..36
Scheme 2.08 Sequential deprotonations of bis(4-nitrophenyl)urea
as the basis for colorimetric
anion sensing……………………………………………………………………………………..43
Scheme 2.09 DFT-Calculated gas-phase acidities for
N,N!-diphenylurea and N,N!-diphenyl-
squaramide……………………………………………………………………………………….52
Scheme 3.01 Commercial aromatic
polyamides…………………………………………………64
Scheme 3.02 Aromatic polyamide synthesis utilizing diacid and
diisocyanates………………...67
Scheme 3.03 Aromatic polyamide synthesis utilizing diamines and
diacids…………………....67
Scheme 3.04 Aromatic polyamide synthesis utilizing
palladium-catalyzed condensation……...67
Scheme 3.05 Polysquaramide copolymer syntheses by condensation
of squaric acid and
diamine…………………………………………………………………………………………...68
Scheme 3.06 Formation of 1,3-diamide isomer in basic
media………………………………….69
Scheme 3.07 Polysquaramide synthesis by solid-state thermal
condensation protocol…………69
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Scheme 3.08 Synthesis of diaminofluorenes with varied side
chains……………………………70
Scheme 3.09 Synthesis of diaminotrifluorene
monomer………………………………………...71
Scheme 3.10 Synthesis of aminofluorene monomer with a
hydrophilic, flexible spacer………..72
Scheme 3.11 Other aromatic diamine monomers utilized for
poly(squaramide) synthesis……...72
Scheme 3.12 Scandium (III) triflate-catalyzed polycondensation
of diaminofluorene and diethyl
squarate…………………………………………………………………………………………..73
Scheme 3.13 Preparation of polysquaramides with well-defined end
groups…………………...74
Scheme 3.14 Preparation of model compound 3.28 and
poly(squaramides) Poly1#Poly5 by
Lewis acid catalyzed condensation………………………………………………………………76
Scheme 3.15 Three calculated structures (B3LYP/6-31G(d)) of
difluorenesquaramide………...80
Scheme 3.16 DFT calculations on three different conformers of
trifluorenesquaramides………81
Scheme 3.17 Bandgap calculations on
oligo(fluorenesquaramides)…………………………….82
Scheme 3.18 Squaramide based receptors with chiral side
chains………………………………84
Scheme 4.01 A shape-persistent triazole based
macrocycle……………………………………106
Scheme 4.02 Chloride ion templation of a cyclic hexaurea
macrocycle……………………….107
Scheme 4.03 Cavitand-based coordination cage via self-assembly
approach………………….108
Scheme 4.04 Encapsulation of an anionic guest by a neutral
cage……………………………..109
Scheme 4.05 Encapsulation of sulfate anion by crystallization of
the bipyridine-urea ligand with
nickel (II) sulfate……………………………………………………………………………..…110
Scheme 4.06 Analyte-induced
self-assembly…………………………………………………..122
Scheme 4.07 Sulfate-induced micellization of amphiphilic block
copolymer…………………123
Scheme 4.08 Sulfate-induced dimerization of
squaramide……………………………………..123
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List of Figures
Figure 1.01 Signal ‘gain’ in fluorescence quenching of
conjugated polymers by a paraquat
derivative.………………………………………………………………………………………….3
Figure 1.02 (a) Anion-induced desilylation/cyclization as the
basis for fluoride detection. (b) A
polymeric chemodosimeter for
fluoride…………………………………………………………...5
Figure 1.03 Representative anion-responsive organoboron
polymers…………………………….6
Figure 1.04 Structure of an anion-responsive, fluorescent
poly(silane)…………………………10
Figure 1.05 Turn-on fluorescence response of an anionic PPE–Cu2+
adduct to pyrophosphate...11
Figure 1.06 Turn-on fluorescence response of
imidazole-functionalized polyacetylene#Cu22+
adduct to cyanide ion…………………………………………………………………………….12
Figure 1.07 Turn-on fluorescence response of
imidazole-functionalized polyfluorene#Cu22+
adduct to cyanide ion…………………………………………………………………………….12
Figure 1.08 Benzimidazole-functionalized polyfluorene
metal-binding polymer for phosphate
and pyrophosphate detection…………………………………………………………………….13
Figure 1.09 Turn-on fluorescence response of
DCPDP#functionalized poly hydroxyethyl
methacrylate#Cu22+ adduct to pyrophosphate
ion………………………………………………14
Figure 1.10 Conjugated polypyrrole zwitterionic polymer
responsive to alkali metal iodide
salts………………………………………………………………………………………………16
Figure 1.11 Anion responsive cationic
polymers.…………………………………………..........17
Figure 1.12 Fluorescent, polymeric anion sensors based on acidic
OH groups…………………20
Figure 1.13 Polymeric anion sensors based on acidic NH
groups……………………………….24
Figure 2.01 Solid-state structures (ORTEP) of the hydrogen-
bonded complexes of 2.20 and
DMSO (top); 2.20b and DMSO (middle) and 2.35 and DMSO
(bottom)……………………….41
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Figure 2.02 Structures and DFT-calculated energies of
interaction (gas phase, B3LYP/6-
311++G**) of squaramide- DMSO and urea-DMSO hydrogen-bonded
complexes……………42
Figure 2.03 (a) Absorption spectrum of squaramide 2.34 as a
function of solvent composition. (b)
UV-vis spectra of 2.34 in DMSO as a function of
concentration………………………………..44
Figure 2.04 Changes in absorption spectrum of squaramide 2.34
upon addition of tetra-n-
butylammonium fluoride………………………………………………………………………...45
Figure 2.05 Changes in the absorption spectrum of 2.34 upon
addition of tetrabutylammonium
fluoride…………………………………………………………………………………………...46
Figure 2.06 (a) Changes in the absorption spectrum of 2.34 upon
addition of tetrabutyl-
ammonium acetate. (b) Absorption intensity of 2.34 at 459 nm as
a function of added acetate in
acetonitrile……………………………………………………………………………………….47
Figure 2.07 (a) Changes in the absorption spectrum of 2.34 upon
addition of tetrabutyl-
ammonium p-toluenesulfonate. (b) Absorption intensity of 2.34 at
395 nm as a function of added
Bu4NOTs in DMSO…………………………………………………………………………….49
Figure 2.08 Changes in the 1H NMR spectrum of 2.34 upon addition
of tetrabutylammonium p-
toluenesulfonate………………………………………………………………………………….50
Figure 2.09 Photographs of 2.34 in the presence of (left to
right): Bu4NOTs; no analyte;
Bu4NOAc; Bu4NF………………………………………………………………………………..51
Figure 2.10 DFT-calculated highest occupied molecular orbitals
for the conjugate bases of (top)
N,N!-diphenylurea and (bottom) N,N!
-diphenylsquaramide…………………………………….53
Figure 3.01 GPC profiles for structurally different
Polysquaramides…………………………...75
Figure 3.02 1H NMR spectrum of Poly1a in
d6-DMF…………………………………………...77
Figure 3.03 (a) TGA data for different Polysquaramides. (b) TGA
curve of Poly4 under
nitrogen……………………………………………………………………..................................78
Figure 3.04 GPC profiles of Poly1a in pure NMP and NMP/LiCl (0.2
wt%)…………………..79
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Figure 3.05 Variable temperature UV/Vis spectra in
DMF……………………………………79
Figure 3.06 Absorption spectra of different polysquaramides and
the model compound in (a)
pure NMP and (b) NMP/H2O (9:1) at a
concentration………………………………………….83
Figure 3.07 (a) Effect of molecular weight on the absorption
spectra of squaramide based
receptors. (b) Wavelength absorption maxima of different
polysquaramides. (c) CD spectrum of
Poly2 in DMF. (d) CD spectrum of model compound 3.29 in
DMF…………………………....84
Figure 4.01 (a) Emission and (b) absorption responses of Poly1a
to Bu4N+H2PO4# (10% water in
NMP solvent; $ex = 415 nm)……………………………………………………………………112
Figure 4.02 (a) Plots of 1H NMR spectra of polymer Poly1a in
d7-DMF after addition of various
quantities of Bu4N+H2PO4# to demonstrate the reversible nature
of the interaction between the
receptor and phosphate: (i) polymer Poly1a; (ii, iii, iv)
polymer Poly1a/Bu4N+H2PO4# mixture.
(v) Addition of TFA to the mixture of Poly1a/Bu4N+H2PO4#. (b) A
multiple and reversible
emission response of Poly1a to
H2PO4#………………….………………………………………………………………………113
Figure 4.03 (a) Fluorescence response of polymer Poly1a to
Bu4N+H2PO4#. (b) Linear Hill plot
associated with the interactions between polymer Poly1a and
Bu4N+H2PO4#…………………114
Figure 4.04 Concentration dependence of the fluorescence
response and degree of cooperativity
of squaramide-based receptors with different number of repeat
units to Bu4N+H2PO4#……….115
Figure 4.05 (a) Normalized fluorescence response (I/I0) of
polymer Poly1a (black bars) and model compound (grey bars) to
anions X# (b) Normalized fluorescence response (I/I0) of polymer
Poly1a to divalent and multivalent anions Xn#………………………………………..117
Figure 4.06 (a) Fluorescence response (I/I0) of polymer Poly1a to
Bu4N+H2PO4# in 20% H2O/NMP mixture. (b) Linear Hill plot associated
with the interactions between Poly1a and Bu4N+H2PO4# in 20 %
H2O/NMP mixture……………………………………………………...118 Figure 4.07 (a)
Autocorrelation functions and (b) normalized CONTIN plots from DLS
measurements at 90° of polysquramide Poly1a (2.4 % 10#5 M): in the
absence (red ) and the presence (blue ) of Bu4N+H2PO4# (2.4 % 10#3
M) in DMF…………………………119 Figure 4.08 (a) Autocorrelation functions on
Poly1a at different times. (b) Effect of F# and HSO4# on the
self-assembly process…………………………………………………………....120 Figure 4.09 (a)
TEM image of dried films of different polysquaramide Poly1a and
Bu4N+H2PO4#
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(sample was cast from a DMF solution of Poly1a (2.4 % 10#5 M)
and Bu4N+H2PO4# (2.4 % 10#3 M). Spherical polymer aggregates are
clearly visible. The lacy pattern is the copper framework of the
carbon-coated copper TEM gird. (b) A laser confocal microscopy
image of a solution of Poly1a in DMF (2.4 % 10#5 M) in the presence
of Bu4N+H2PO4# (2.4 % 10#3 M)………...……121 Figure 4.10 calculated
structures (B3LYP/6-31G(d)) of bis(fluorenyl)squaramides in the (a)
absence and (b) presence of dihydrogenphosphate
anion………………………………………124 Figure 4.11 Absorption and emission spectra
of different polysquaramides and the model compound 3.28 in the
presence of Bu4N+H2PO4# in NMP/H2O mixture………………………125 Figure 4.12
(a) Solvent dependent emission spectra of Poly2 upon addition of
water in pure NMP. (b) Emission spectra of structurally different
polysquaramides in pure NMP……….….126 Figure 4.13 a) Magnitude of
fluorescence response (b) Nature of fluorescence response of
structurally different squaramide-based receptors to
Bu4N+H2PO4#………………………..….126 Figure 4.14 Normalized fluorescence
response (I/I0) of structurally different polymers to anions
X#……………………………………………………………………………………………….127 Figure 4.15 (a)
Hydrodynamic radii and polydispersity indexes of different
polysquaramide aggregates in the presence of H2PO4#. (b) Normalized
CONTIN plots from DLS measurements at 90 ° on different
polysquaramides (2.4 % 10 #5 ) in the presence of Bu4N+H2PO4# (2.4
% 10#3 M) in DMF. Autocorrelation functions of different
polysqsuaramides (2.4 % 10#5 M): (c) Poly3, (d) Poly4 in the
absence (red ) and the presence (blue ) of Bu4N+H2PO4#. (e) Effect
of Bu4N+H2PO4# on the self-assembly process…………………………………………129
Figure 4.16 TEM image of dried films of different polysquaramides
((a) Poly2, (b) Poly3, and (c) Poly4) and Bu4N+H2PO4# (sample was
cast from a DMF solution of different polysquaramides (2.4 % 10#5
M) and Bu4N+H2PO4# (2.4 % 10#3 M). Spherical polymer aggregates are
clearly visible. The lacy pattern is the copper framework of the
carbon-coated copper TEM gird.
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List of Appendices
Appendix I A. Gas-phase acidity
caculations………………………………………………..…136
Appendix I B. Calculations of the urea-DMSO and squaramide-DMSO
hydrogen-bonded
complexes………………………………………………………………………………………140
Appendix II X-Ray diffraction data of compound
2.37…...........................................................143
Appendix II X-Ray diffraction data of compound
2.20b….........................................................155
Appendix II X-Ray diffraction data of compound
2.20…..........................................................167
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List of Abbreviations
ADP Adenosine diphosphate
AMP Adenosine monophosphate
Anal. Elemental analysis
app Apparent
aq Aqueous
Ar Aryl
ATP Adenosine triphosphate
ATR Attenuated total reflectance
Bn Benzyl
brs Broad singlet
Bu Butyl
Calcd Calculated
CD Circular dichroism
13C NMR Carbon nuclear magnetic resonance spectroscopy
Conc Concentrated
DCPDP Dicyanomethylene-4H-pyran diamino pyridine
dd Doublet of doublets
ddd Doublet of doublets of doublets
DFT Density functional theory
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DLS Dynamic light scattering
DMA N,N-Dimethylacetamide
DME 1,2-Dimethoxyethane
DMF N,N-Dimethylformamide
DMSO Dimethyl sulfoxide
DNA Deoxyribonucleic acid
DPQ dipyrrolyl quinoxaline
EI Electron impact
ESI Electrospray ionization
ESIPT excited-state intramolecular proton transfer
Et Ethyl
!ex Excitation wavelength
FTIR Fourier-transform infrared
GPC Gel permeation chromatography
HAADF High angle annular dark field
HBO 2,5-bis(benzoxazol-2’-yl)benzene-1,4-diol
HEMA 2-hydroxyethyl methacrylate
HMPA Hexamethyl phosphoramide
HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid)
1H NMR Proton nuclear magnetic resonance spectroscopy
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HOMO Highest Occupied Molecular Orbital
HPLC High performance liquid chromatography
hr Hour/hours
HRMS High-resolution mass spectrometry
ICT Intramolecular charge transfer
IPC Isophthaloyl dichloride
ICP-MS Inductively coupled plasma mass spectrometry
IR Infrared spectroscopy
KSV Stern-Volmer constant
LA Lewis acid
LUMO Lowest Unoccupied Molecular Orbital
m Multiplet
Me Methyl
mg Milligrams
min Minute/minutes
mL Milliliters
mmol Millimoles
m.p. Metlting point
MPD m-Phenylenediamine
nm Nanometer
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NMP N-Methyl-2-pyrrolidone
ODA 3,4’-Diaminodiphenyl ether
ORTEP Oak Ridge Thermal-Ellipsoid Plot
PDI Polydispersity index
Ph Phenyl
Pi Inorganic phosphate
PMPI Poly(m-phenylene isophthalamide)
PPBA Poly(p-benzamide)
PPD p-Phenylenediamine
PPi Pyrophosphate
ppm Parts per million
PPPT Poly(p-phenylene terephthalamide)
Py Pyridine
q Quartet
R Generic alkyl groups
Rf Retention factor (in chromatography)
Rh Hydrodynamic radii
RAFT Reversible addition fragmentation chain transfer
polymerization
rt Room temperature
s Singlet
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SDBS Sodium dodecyl benzene sulfonic acid
SDP Sodium dodecyl phosphate
SDS Sodium dodecyl sulfate
SDX Sodium dodecyl carboxylate
STEM Scanning transmission electron microscopy
t Triplet
Td Decomposition temperature
TEM Transmission electron microscopy
TFA Trifluoroacetic acid
TGA Thermal gravimetric analysis
THF Tetrahydrofuran
TLC Thin layer chromatography
TPC Terephthaloyl dichloride
TPP Triphenylphosphite
UV/vis Ultraviolet–visible spectroscopy
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Chapter 1 Polymers for anion recognition and sensing*
1.1 Introduction Molecules capable of the selective recognition
and sensing of anions represent an intriguing class
of targets, with implications in medical diagnostics,
environmental monitoring and remediation,
and the discovery of biological probes or potential therapeutic
agents. It is assumed that anions
are engaged in 70 % of all enzymatic processes. They are
ubiquitous in the environment from
farming to heavy industries and are often hazardous pollutants:
phosphates found in agricultural
fertilizers and nerve agents, fluoride is the byproduct of sarin
gas, sulfate is directly related to
nuclear-waste remediation and acid rain, and cyanide that is
extremely toxic to mammals is
common in gold mining and metallurgy.1, 2
Anions present unique challenges for molecular recognition: they
are characterized by diverse
geometries, charge distributions and sizes, and their high
enthalpies of hydration present a
significant obstacle to developing receptors able to function in
aqueous medium. The range of
potential applications and the challenging nature of the problem
have motivated intensive
research efforts in the area of anion recognition in recent
years.3
The exceptional affinities and selectivities displayed by
naturally occurring anion-binding
proteins serve as a source of inspiration for the development of
synthetic systems. For example,
the crystal structure of a sulfate-binding protein indicates
that anion binding is achieved without
the direct participation of positively charged moieties, through
precisely positioned hydrogen-
bonding interactions between the sulfate anion and the NH groups
of the protein backbone,
serine OH, or tryptophan NH groups.4 The principles of
complementarity and preorganization
underlie the design of synthetic anion receptors and sensors
able to interact with analytes through
hydrogen bonding, Lewis acid–base coordination, Coulombic
interactions, and other noncovalent
contacts (anion–arene interactions and halogen bonding, for
example).
* Part of this chapter has recently been published: Rostami, A.;
Taylor, M. S. Macromol. Rapid Commun 2012, 33,
21!34
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Research reported over the past few decades has identified a
wide variety of functional groups
and molecular architectures for this purpose, and thermodynamic
studies have provided insight
into the roles played by such factors as conformational rigidity
and solvation. In several
instances, these discoveries have been exploited in the
development of sensors for ‘real-life’
monitoring of analyte concentration in complex environments.5
Challenges remaining for the
field include the development of receptors capable of selective
and high-affinity anion
recognition in water: in this regard, the performance of
Nature’s anion-binding proteins remains
unparalleled by synthetic systems.
The development of anion-responsive polymeric systems has
recently emerged as an exciting
direction for research in anion recognition. Macromolecules
offer unique opportunities for
achieving sensitivity and selectivity by mechanisms that take
advantage of their distinct
properties and behaviors;6 however, anion sensory materials have
emerged slowly in comparison
to those for cation sensing.
In this chapter recent advances in polymer-based
anion-responsive materials will be discussed,
with an emphasis on the wide structural diversity of polymers
that can be employed for this
purpose, and the mechanisms of their anion-induced responses.
Following an introduction to the
key concepts that form the basis of polymer-based chemical
sensing schemes, the application of
these concepts to anion-responsive systems will be covered; the
polymers are grouped according
to the nature of their interaction with anionic analytes (Lewis
acid–base, Coulombic, hydrogen
bonding, etc.).
1.2 Polymer!based chemical sensors: general principles Polymers
possess unique properties that can offer special advantages for the
development of
analyte-responsive systems. Several classes of conjugated
polymers display optoelectronic
properties that can be harnessed for signal transduction by
electrochemical, colorimetric, or
fluorescence techniques; mechanisms for signal transduction
include anion-induced changes in
polymer conformation that result in modulation of the effective
conjugation length, or the
introduction or destruction of local traps, giving rise to
fluorescence quenching or unquenching
events. The ability to process such materials into thin films or
membranes facilitates their
integration into functional devices, and the repeating
structures of polymers may give rise to
effects such as multivalency or cooperativity in the context of
supramolecular interactions.
2
-
However, the most striking distinctions between small-molecule
and polymeric chemical sensors
often arise from the ability of the latter to achieve signal
amplification, giving rise to high levels
of sensitivity for the analyte of interest. The ability of
fluorescent conjugated polymers to
function as amplifying chemical sensors was elucidated more than
15 years ago and the concept
forms the basis of several high-performance sensory materials
for use in trace detection and
biosensing.6 Studies by Swager and Zhou of the fluorescence
quenching-based response of
cyclophane-based poly(phenyleneethynylenes) to bis(pyridinium)
ions demonstrated that the
incorporation of analyte-binding motifs gave rise to signal gain
due to the ‘molecular wire effect’
(Figure 1.01).7 Control experiments using non-polymeric
cyclophane receptor 1.01 indicated that
the polymeric system 1.02 exhibited a 65-fold enhancement of
sensitivity (as measured by the
Stern–Volmer constants KSV for static quenching), resulting from
the ability of excitons to
diffuse along the polymer chain and thus to ‘sample’ the
occupancy of multiple analyte-binding
sites. Later studies (particularly in the context of the
detection of ultra-trace concentrations of
trinitrotoluene and related nitroaromatics) revealed that
harnessing inter- in addition to
intrapolymer exciton diffusion (for example, in thin films)
provides a mechanism for increased
levels of signal amplification.8 The concepts emerging from this
pioneering work have proved to
be remarkably general, and underlie the application of
conjugated polymers for the detection of
diverse analytes in a wide variety of media.
Figure 1.01 Signal ‘gain’ in fluorescence quenching of
conjugated polymers by a paraquat derivative.
C12H25O
O
O
O O O O
O O O O
OC12H25
ON
O NC8H17
C8H17
C8H17
C8H17
O
O
O O O O
O O O On
KSV = 1.63 ! 103 M-1
N N
Analyte = paraquat (PQ2+)
1.01
1.02 (Mn = 1.05 ! 105 g/mol)KSV = 1.05 ! 10 5 M"1
Sensitivity enhancement = KSV(1.02)/ KSV(1.01) = 65
3
-
1.3 Anion responsive polymers Several of the pioneering studies
illustrating the ability of analytes to effect changes in the
conformation, bandgap, or aggregation behavior of polymers, and
the results of these changes on
photophysical properties or conductivity, were conducted in the
context of cation detection.9
Although many of the initially reported polymer-based sensors
were designed for the detection
of cationic species, an expanding body of research carried out
by several groups illustrates that
anionic analytes also represent interesting targets for these
approaches. The wide structural
diversity of the polymers that have been discovered for anion
recognition is described in the
following sections: anion-responsive polymers are grouped by
structure and according to the
nature of the functional group that is proposed to interact with
the anionic species. The principle
of signal amplification outlined in the previous section has
played a key role in these efforts: a
number of interesting polymer backbones that enable the
transduction of anion-binding events
into measurable signals have been devised. In addition,
polymer-based phenomena other than the
‘molecular wire’ effect – including cooperativity and
multivalency – have been brought to bear
on the problem of anion sensing. The increased sensitivity or
affinity resulting from these effects
enables the application of polymers to solve challenging
problems in anion recognition,
including developing sensors that function in aqueous
environments.
1.3.1 Anion!responsive chemodosimeters
Although the majority of the anion-responsive polymers developed
to date have been designed to
interact reversibly with their analytes (see below),
anion-induced, irreversible covalent bond
formation or cleavage represents another viable detection
method. An implementation of this
concept in the context of a polymeric anion sensor was reported
by Kim and Swager, who
exploited the reactivity of fluoride toward Si–O bonds as a
basis for the selective detection of
this anion (Figure 1.02).10 Reaction of polymer 1.04 with
fluoride (Bu4N+F–, 1.6 x 10–7 M in
THF) triggered cleavage of the silyl ether group and cyclization
to the corresponding coumarin
(Figure 1.02b), accompanied by a significant red-shift of the
polymer emission spectrum.
Control experiments with an alkylidenemalonate not appended to a
poly(phenyleneethynylene)
indicated that the degree of amplification resulting from
exciton migration to the newly
generated fluorophore was approximately 100-fold.
4
-
Figure 1.02 (a) Anion-induced desilylation/cyclization as the
basis for fluoride detection; (b) A polymeric chemodosimeter for
fluoride.
1.3.2 Lewis acid–base interactions: organoboron, organosilicon,
and metal cation-containing anion-responsive polymers
Diverse strategies for incorporating Lewis acidic main group or
metal-based functional groups
into stimulus-responsive polymers have been developed. These
include applications of boron-
and silicon-based polymers, as well as materials that
incorporate metal cations.
Organoboron-based polymers display interesting optoelectronic
properties when the empty
boron-based p orbitals contribute to the conjugated ! system of
the polymer backbone.11 The
occupancy of the boron-based p orbital may have a significant
influence on the effective
conjugation length, providing a mechanism for transducing
anion-binding events into measurable
signals. A pioneering application of anion sensing by
organoboron-based conjugated polymers
was reported by Chujo and co-workers:12 A hydroboration
polymerization methodology was
used for the synthesis of the polymer containing a tricoordinate
aryl borane functionality.
Addition of fluoride ion to a solution of
poly(vinylenephenylenevinylene borane) polymer 1.05
in chloroform led to a hypsochromic shift of the UV-vis
absorption maximum, resulting in
change in color of the solution from yellow to colorless. This
behavior was interpreted as
O
CO2Et
CO2Et
Si(tBu)(CH3)2NH3CH2C
CH2CH3NH3CH2C
CH2CH3O
CO2Et
O
F-
(a)
(b)
C12H25O OC12H25
H3C
S
CO2Et
CO2Et
n
O
F-C12H25O OC12H25
H3C
S
CO2Et
n
O
O
Si(iPr)3
1.04
1.03
Responsive to F! in THF
5
-
involving coordination of fluoride to the Lewis acidic boron
moiety. This polymer displayed a
fluorescence quenching response to fluoride anion in chloroform
solvent. The response was
selective for fluoride over the other halide anions such as Cl!,
Br!, and I!, and the observation of
complete quenching upon addition of 0.5 equivalents of n-Bu4N+F–
(~ 10–6 M in CHCl3)
indicated some degree of signal amplification relative to a
non-polymeric borane species.
Figure 1.03 Representative anion-responsive organoboron
polymers.
B
n
B
CN
x y
x / y = 9
B
i-Pr
i-Pri-PrHex Hex
n
i-Pri-Pr
i-Pr
1.05Responsive to n-Bu4+F! in CHCl3 1.06
Responsive to n-Bu4+F! / CN! / I! in THF
1.07Responsive to n-Bu4+F! / CN! in THF
B B
Tip
Hex Hex
Tip
n
Hex Hex
n-1
BHB
Tip
Hex Hex
n
Hex Hex
n-1 Tip
[nBu4N]+CN!
B B
Hex Hex
n
Hex Hex
n-1 TipTip
CN CN
CN
Titration of 1.07 with n-Bu4+CN! in THF
[nBu4N]+CN!
(c)
(b)(a)
(d)
6
-
Figure 1.03 Representative anion-responsive organoboron
polymers.
In recent years, several novel anion-responsive polymers bearing
Lewis acidic borane functional
groups have been developed.13 A random conjugated
dialkylfluorene/dibenzoborole copolymer
was reported by Bonifácio and coworkers.13b In this conjugated
polymer the dibenzoborole
moiety was electronically stabilized by incorporation of a cyano
group in the para-position of the
9-phenyl substituent, which increased the environmental
stability of the polymer. The polymer
was prepared utilizing Yamamoto-type aryl-aryl coupling with a
9:1 molar ratio of the co-
monomers, and displayed a fluorescence quenching-based response
to F–, CN– and I– (n-Bu4N+
counterion, THF solvent), both in solution and in thin films:
fluoride concentrations as low as 0.1
mM could be detected using a spin-coated film of the polymer.
Upon addition of fluoride and
cyanide to a solution of the polymer 1.06, defined anionic
complexes were formed with the
boron, the formation of which were manifested by characteristic
isosbestic points at 490 nm (for
F!) and 510 nm (for CN!) and a blue shift in absorption maximum.
Incomplete fluorescence
quenching upon fluoride and cyanide addition was reasoned in
terms of static quenching
mechanism, whereas complete quenching with iodide was attributed
to the collisional-quenching
mechanism. The Stern-Volmer quenching constant (KSV) was much
higher for iodide (Kiodide =
23.2 " 106) than fluoride (Kfluoride= 0.49 " 106) and cyanide
(Kcyanide= 4.78 " 106).
B
SS
Hex
n
BO
O S S
OO
OOSi
Si
O
OB
SS
OO
OO
n
1.08Responsive to n-Bu4+F! / CN! in THF
1.09Responsive to amines and Bu4+F! in THF
(e) (f)
N B B
Hex Hex
n
i-Pr
i-Pr
i-Pr
i-Pri-Pr i-Pr
1.10Responsive to n-Bu4+F! / CN! in THF
(g)
7
-
Jäkle and co-workers developed efficient transmetallation-based
protocols for the synthesis of
fluorene-based organoborane polymers, in which bulky mesityl or
tris(isopropyl)phenyl groups
provide steric protection from air oxidation.13c Polymer 1.07
showed selective colorimetric and
fluorescence responses to fluoride and cyanide anions in
tetrahydrofuran solvent, but were non-
responsive to larger, less basic anions (Cl–, Br–). A two-step
binding process was inferred from
the nature of the spectral changes: initial binding (up to ~ 0.5
equivalents of anion) was proposed
to occur at alternating boron sites, resulting in a red-shifted
emission maximum consistent with
charge-transfer between adjacent tetracoordinate (electron-rich)
and tricoordinate (electron-
deficient) moieties (Figure 1.03d).
Another report from the same group described an emissive
donor-#-acceptor polymer with
alternating electron deficient bifunctional triarylborane and
electron rich triphenylamine
moieties.11e Addition of an excess amount of fluoride or cyanide
to a solution of polymer 1.10 in
THF led to a red shift in the absorption maximum, concomitant
with a quench in the emission
band at 459 nm and formation of a weak blue-shifted emission
band. The polymer was not
responsive to other anions such as chloride, bromide, and
nitrate, suggesting the absence of
coordination of these larger anions. A two-step binding
mechanism, similar to that described for
polymer 1.07 was proposed for this polymer (Figure 1.03g)
Although incorporation of the boron-based p orbital into the
main chain of a !-conjugated
system has been identified as a useful design principle, a
number of anion-responsive boron-
containing materials that do not benefit from this type of
conjugation have been developed.14 For
example, functionalization of polystyrene with
thiophene-substituted arylboranes was achieved
through a series of organometallic transformations of
poly(4-trimethylsilylstyrene).13a The
presence of the tricoordinate boron functionality in the pendent
group provided an opportunity
for the recognition of anions. Changes in the absorbance and
emission spectra of the resulting
materials signaled the presence of tetrabutylammonium fluoride
and cyanide in THF solvent.
Upon addition of fluoride to a solution of polymer 1.08, a
gradual decrease in absorption
intensity at 378 nm followed by the formation of a new band at
338 nm was evident by UV-vis
spectroscopy. These changes were assigned to the formation of
bithiophene groups with a
tetracoordinated boron moiety. Based on the absorption and
emission spectra, it was clear that
fluoride and cyanide bind tightly to the boron polymers where as
other anions revealed no major
response. Stern–Volmer quenching constants indicated roughly
8-fold signal amplification for
8
-
polymer 1.08 relative to a non-polymeric model compound:
intrapolymer exciton migration was
proposed as a mechanism for this effect (Figure 1.03e).
The borasiloxane-based polythiophenes synthesized via
electropolymerization by Lee and co-
workers are another structurally distinct class of organoboron
polymers that have been employed
for chemical sensing. The axial open coordination sites of the
boron serves as the recognition
sites for anion binding.13d Red shifts of the absorption and
emission spectra of polymer 1.10
relative to model compounds were consistent with some degree of
through-space electronic
communication through the borasiloxane cages. Although detection
of amine vapors was pursued
as the major sensory application of 1.10, a film of the polymer
on an indium tin oxide-coated
glass electrode displayed a green-to-orange colorimetric
response to Bu4N+F– in THF solvent
(Figure 1.03f).
Polysilanes display interesting optoelectronic properties that
result from the delocalization of "-
conjugated electrons along the polymer backbone. This feature,
combined with the high fluoride
affinity of many organosilicon compounds, has been exploited in
anion recognition by Fujiki and
co-workers.15 Fluoroalkylated polysilane 1.11, synthesized by
the sodium-mediated coupling
reaction of 3,3,3-trifluoropropylmethyldichlorosilane in octane
(Figure 1.04), adapted a rodlike
conformation stabilized by weak Si...F-C interactions between
the trifluoropropyl side-chains and
the silicon moieties at the main chain. This polymer displayed a
sensitive fluorescence
quenching-based response to nanomolar concentrations of fluoride
anion (n-Bu4N+ counterion,
THF solvent), with a Stern–Volmer quenching constant of 1.35 "
107 M–1 indicating significant
levels of amplification due to exciton migration in this system.
The polymer displayed a high
level of selectivity for fluoride, and was unresponsive to other
anions such as bromide and
chloride. The structure of 1.11 was tuned to enhance the
fluoride affinity of the polysilane
backbone: replacement of the electron-withdrawing fluoroalkyl
groups by nonfluorinated
moieties resulted in a loss of sensitivity of more than three
orders of magnitude, while replacing
the methyl substituents with more sterically encumbered alkyl
groups led to a loss of the
fluorescence response.
9
-
Figure 1.04 Structure of an anion-responsive, fluorescent
poly(silane).
As alluded to above, metal cations have been pursued for more
than a decade as targets of
polymer-based chemical sensors. The polymers obtained upon
cation binding may be viewed as
potential anion-responsive hybrid materials, in which
interactions between the analyte and
polymer-bound metal ion result in alterations of the
optoelectronic properties of the polymer. A
sensory scheme of this type was reduced to practice by Schanze
and co-workers, who employed
carboxylate-functionalized poly(phenyleneethynylene) 1.12 as an
amplifying fluorescent
indicator (Figure 1.05).16 The conjugated polyelectrolyte was
shown to interact strongly with
Cu2+ ions, resulting in a fluorescence quenching response with a
KSV of approximately 106 M–1 in
aqueous HEPES buffer. Titration of the polymer–Cu2+ adduct with
pyrophosphate (PPi) anion
resulted in a recovery of fluorescence (30-fold enhancement in
the presence of 20 mM PPi): an
analytical detection limit of 80 nM was estimated. The system
was shown to be selective for
pyrophosphate over other monovalent and divalent anions (F–,
Cl–, Br–, I–, HSO4–, NO3–, HCO3–,
H2PO4–, CH3CO2–, SO42–, CO32–, HPO42–), and a control experiment
with the metal chelator
EDTA provided support for a mechanistic hypothesis involving
sequestration of Cu2+ by the PPi
anion. The system was employed in a real-time assay of the
activity of an alkaline phosphatase
enzyme. Bunz, Rotello and co-workers described another efficient
sensor for pyrophosphate
based on a carboxylate-functionalized PPE: in their study,
interaction of the anion PPE with 10
nm cobalt–iron spinel nanoparticles resulted in quenching of the
polymer fluorescence.17
Addition of pyrophosphate to the polymer–nanoparticle adduct
effected a fluorescence recovery
10
-
that enabled the detection of PPi at high nanomolar
concentrations, even in the presence of
relatively high concentrations (0.1 mM) of phosphate anion.
Figure 1.05 Turn-on fluorescence response of an anionic PPE–Cu2+
adduct to pyrophosphate
A number of related systems involving anion-induced
‘unquenching’ of metal-bound fluorescent
polymers have been developed: Li and coworkers described the
synthesis of light emitting
polyacetylene containing imidazole moieties through a post
functionalization strategy.18 The blue
fluorescence of polyacetylene 1.13 was quenched in the presence
of Cu 2+ ions, while in the
absence of the imidazole moieties, no change was observed in the
emission behavior of the
polymer. Addition of a solution of CN! (7.0 " 10!5 M) led to
emission recovery of the
completely quenched polymer: cyanide ion has more affinity
towards Cu2+ than imidazole, as
reflected by the high stability constant of the complex of CN!
and Cu2+. The alteration of the
interaction between Cu2+ and imidazole by cyanide ion leads to
the recovery of the emission.
Other anions were not able to recover the quenched emission
except for H2PO4- and PO43-, which
caused a slight increase in fluorescence intensity. Polymer 1.13
was able to detect cyanide in the
solid state by first dipping the film into an aqueous solution
of Cu2+ and subsequently to a
solution of CN!.
O
On
Cu2+
PPi
OO
OO
O
On
OO
O
On
OO
OO
Cu2+
OO
P
P
O
O
O
O
O
Cu2+
Responsive to P2O72! in HEPES buffer
1.12
emissive (ON) non-emissive (OFF)
O
O
11
-
Figure 1.06 Turn-on fluorescence response of
imidazole-functionalized polyacetylene!Cu22+ adduct to cyanide
ion
In a separate report, Li described a blue emissive
imidazole-functionalized polyfluorene
derivative as a fluorescence probe for cyanide ion utilizing
Lewis acid-anion coordination
chemistry through turn-off-turn-on cycle (figure 1.07).19
Addition of aliquots of aqueous solution
of Cu2+ ions (3.0 " 10!8 M) to a dilute solution of polymer 1.14
led to a rapid quenching of the
emission with a Stern-Volmer quenching constant (KSV) of 2.1 "
106 M!1 and recovery of the
initial emission intensity in the presence of CN! (1.2 " 10!5
M!1). The present polyfluorene metal
ion chemosensor showed an improved sensitivity (12 µM) towards
CN! compared to the
previous work by the same group on the imidazole-functionalized
polyacetylene (70 µM).
Figure 1.07 Turn-on fluorescence response of
imidazole-functionalized polyfluorene!Cu22+ adduct to cyanide
ion
Recently Iyer reported a neutral copolymer
polyfluorene-alt-1,4-phenylene derivative containing
the benzimidazole moiety as the pendant group and Fe3+ for the
detection of phosphates
(H2PO4!, HPO42!).20 Polymer 1.15 was synthesized by Suzuki cross
coupling of dibrominated
fluorene and 4-phenylbronic acid in 80% yield. Addition of
aqueous solution of Fe3+ metal salts
(chloride and perchlorate) induced fluorescence quenching of the
copolymer with a 97%
emissive (ON)
C C C(CH2)3
C(CH2)3NCl
N
x yx = 0.36y = 0.64
N
N
N
N
Cu2+N
N
N
N
N
N
N
N
CN-
Cu2+[Cu(CN)x]n-
emissive (ON) non-emissive (OFF)
1.13
N
C6H13 C6H13
n
N
N
N
C6H13 C6H13
n
N
N
N
C6H13 C6H13
n
N
N
Cu2+ CN-
[Cu(CN)x]n-
emissive (ON) non-emissive (OFF) emissive (ON)
Cu2+
1.14
12
-
decrease in fluorescence intensity, reflecting the strong
affinity of the benzimidazole moiety
towrds Fe3+. The polymer showed a Stern-Volmer quenching
constant (KSV) of 7.8 " 105 M!1 and
a detection limit of 3.38 " 10!6 M. A recovery of both emission
and absorption spectra were
accomplished after addition of phosphates (H2PO4!, HPO42!) to a
quenched solution of polymer
suggesting that phosphate anions were able to displace Fe3+ from
the benzimidazole moiety. The
polymer-Fe3 assay was further applied for the detection of
phosphate (1.44 mmol/L) in saliva.
Figure 1.08 Benzimidazole-functionalized polyfluorene
metal-binding polymer for phosphate and pyrophosphate detection
Recently, the groups of Tian and Zhu reported that a hydrophilic
copolymer containing
dicyanomethylene-4H-pyran moiety display a turn-on fluorescence
response towards
pyrophosphate anion (P2O74-) both in solution and in thin
film.21 The copolymer (HEMA-co-
DCPDP) 1.16 was prepared by free-radical copolymerization of
DCPDP functionalized 2-
hydroxyethyl methacrylate. Treatment of the polymer with a
solution of Cu(ClO4)2 in methanol
provided a polymer-metal complex as the anion sensory material.
An intramolecular charge
transfer (ICT) process was expected due to its donor-#-acceptor
(D-#-A) structure. In the
DCPDP moiety, the diaminopyridine groups act as
electron-donating substituents whose donor
ability depends on cation coordination. Addition of Cu2+ to a
solution of the polymer led to a
decrease in fluorescence intensity (KSV = 1.42 " 105 M-1), and a
hypochromic shift in the
absorption spectra was consistent with the coordination of
diaminopyridine groups to the Cu2+,
thus reducing the electron donor ability of the amino groups in
the ICT process. Addition of a
solution of pyrophosphate anion to the ensemble of polymer-Cu2+
complex recovered the
fluorescence enenhancement (I/I0 = 4.8). The selectivity of the
system towards different anions
was evaluated and it was found other anions such as F!, Cl!,
Br!, I!, H2PO4!, HSO4!, HCO3!,
NN
NN
n1.15
Responsive to phosphates in THF/water mixture
13
-
NO3!, SO42!, CO32!, CH3COO!, ADP, ATP and AMP showed almost no
interference except for
the little influence of PO43!. The association constant of the
polymer-Cu2+ complex was found to
be 3.1 " 104 M!1 by fluorometric titration curve. The emission
enhancement was related to the
electrostatic interaction between pyrophosphate and Cu2+ in
polymer-copper complex, in which
pyrophosphate two oxygen atoms coordinate to the Cu2+ reducing
its electron withdrawing by
partial charge neutralization. A spin-coated thin film of
copolymer-Cu2+ showed a turn-on
response in the presence of a solution of pyrophosphate at 590
nm with a slight bathochromic
shift.
Figure 1.09 Turn-on fluorescence response of
DCPDP!functionalized poly hydroxyethyl methacrylate!Cu22+ adduct to
pyrophosphate ion
In each of the systems described above, the reversibility of the
anion-binding process has not
been demonstrated, and it may be that the polymer–metal anion
adducts act as dosimeters rather
than sensors. Nonetheless, the versatility of this general
approach, in which both the functional
group interacting with the metal ion and the identity of the
metal itself may be tuned, are
apparent from the range of anionic analytes that have been
successfully targeted in the
applications reported to date.
OON
Ox y
OH N
O
NN
N
NC
CN
OON
Ox y
OH N
O
NN
N
NC
CN
PPi
OON
Ox y
OH N
O
NN
N
NC
CN
Cu2+
Cu2+
emissive (ON) non-emissive (OFF) emissive (ON)
1.16
Cu2 +
P
PO
O
OO
OOO
14
-
1.3.3 Ion-pairing interactions: anion recognition by cationic
polyelectrolytes
Taking advantage of ion-pairing interactions through the use of
cationic receptors has been a key
design principle for achieving anion recognition in competitive
(aqueous) medium. Indeed, the
majority of ‘small-molecule’ anion receptors that function in
water are positively charged
species.22 Likewise, polymers bearing cationic substituents
represent interesting candidates for
use as anion-responsive macromolecules. Although sensory
applications of cationic
polyelectrolyes have generally been targeted toward DNA, a
number of examples illustrate the
utility of such macromolecules in high-affinity detection of
mono- or divalent anions in aqueous
medium.
One of the earliest reports of an anion-responsive organic
polymer is the work of Tour and co-
workers, who observed that planar conjugated polypyrrole
zwitterionic polymer 1.17 displayed a
near-IR colorimetric response to alkali metal iodide salts in
methanol solvent.23 The polymer (Mn
= 4980, PDI = 1.54) was synthesized by copper-bronze-promoted
polymerization process
coupling reaction of a diiodo pyrrole derivative in DME. A low
iodine-doped conductivity of 4.2
" 10!4 $!1 cm!1 was observed. The presence of the carbonyl
functionality in the polymer
backbone was proposed to inhibit polaronic/bipolaronic
migrations. Dropwise addition of
aqueous NaOH (0.05 M) to a solution of polymer in THF showed a
reversible pH-dependent
shift in the absorption spectra with a decrease in absorption
maximum (#max = 512 nm) and
formation of a new band at higher wavelength (#max = 881 nm)
with a color change from red to
pale orange. This large change in the absorption maximum to the
near-IR region was assigned to
the Brønsted base-induced planarization of the conjugated
system, and increased delocalization
of the #-electrons. Control experiments indicated that cation
binding to the triethylene glycol
moieties was required for the iodochromic response, and that the
nucleophilicity of the halide
anion also played an important role. The results were
interpreted in terms of a cation-assisted
nucleophilic attack of iodide on the carbonyl ylide functional
groups.
More recently, the group of Leclerc has developed a cationic,
water-soluble imidazolium-bearing
polythiophene derivative 1.18a that displays highly selective
colorimetric and fluorescence
responses to iodide anion in deionized water.24 This polymer was
prepared by oxidative
polymerization of an imidazolium-substituted thiophene in
chloroform. The red shift of the
15
-
maximum absorption from its original wavelength towards lower
energy values became more
significant upon addition of NaI to the cationic polymer
solution and a color change from yellow
to red-violet was observed. This red shift of the absorption
spectrum was accompanied by a
significant decrease in emission intensity upon addition of
iodide (concentrations as low as 2 x
10–6 M). The absorption spectrum of the polymer in the presence
of aqueous NaI was similar to
that of the polymer alone in the solid state, suggesting an
anion-induced aggregation mechanism,
whereby iodide induces changes in polymer conformation from a
random-coil (yellow emissive)
to planar and aggregated form (quenched).
Figure 1.10 Conjugated polypyrrole zwitterionic polymer
responsive to alkali metal iodide salts
The polymer was not responsive towards other anions such as
SO42!, CO32!, HCO3!, H2PO4!,
and CH3COO!. Noteworthy aspects of this system include its high
selectivity for iodide, its
ability to function in pure aqueous medium, and the dependence
of the sensory properties on
subtle structural features of the polymer (for example,
polythiophene 1.18b was considerably
less sensitive toward iodide than 1.18a).
N
N
N
NOO O
OO
O
OO
O
O
O
O
O
O
O O
O
O
O
O
n/4 N
N
N
NOO O
OO
O
OO
O
O
O
O
O
O
O O
O
O
O
O
n/4
I!
visible absorption near-IR absorption
Responsive to KI in methanol
1.17
I
K+
K+
K+
K+
16
-
Figure 1.11 Anion-responsive cationic polymers.
Another example of anion detection by an
imidazolium-functionalized conjugated polymer was
reported recently by Lee, Yoon and co-workers, who described
applications of polydiacetylene
1.19 for the detection of anionic surfactants in aqueous
medium.25 The polymer was synthesized
by UV irradiation of suspensions of highly ordered
self-assembled imidazolium functionalized
diacetylene and showed a good film forming property due to
double hydrogen bonding and
aromatic interactions involved between imidazolium rings.
Surfactants are applied extensively
in a variety of industrial settings and are of concern as
environmental pollutants. The
imidazolium-bearing poly(diacetylene) displayed a colorimetric
response, accompanied by an
increase in emission intensity, upon addition of anionic
surfactants such as sodium dodecyl
sulfate (SDS, at concentrations as low as 2 x 10–7 M), sodium
dodecyl carboxylate (SDX),
sodium dodceyl phosphate (SDP) and sodium dodecylbenzenesulfonic
acid (SDBS) in aqueous
HEPES buffer. The system was selective for anionic surfactants,
showing no response to other
anionic species, neutral or cationic surfactants; furthermore,
the four anionic surfactants elicited
three different color changes that enabled the differentiation
of SDS, SDC/SDP and SDBS: SDS
induced a blue-to-yellow transition, SDC and SDP induced a
blue-to-orange transition, and
SDBS produced a blue-to-red transition. Addition of SDS to 10 µM
of polymer induced a
S
OH3C (CH2)3 N
NCH3
H3C Cl!
n
1.18a (m = 3), 1.18b (m = 2)
Responsive to NaI in water
C12H21 C12H21 C12H21 C12H21
N
N
N
N
N
N
N
N
n
O O O O
N N N N
H H HH
Responsive to anionic surfactantssuch as SDS in aqueous
buffer
1.19
O
O
HNO
NHO
NH3Cl!
NH3Cl!H3NCl!
NH3Cl!
H3N
H3NCl!
Cl!
n
Responsive to pyrophosphatein aqueous MES buffer
1.20
17
-
decrease in absorption at 620 nm with a concomitant increase at
490 nm and 523 nm. The
fluorescence spectrum of polymer 1.19 showed a 34-fold increase
in emission intensity at 565
nm when 4 µM SDS was added to a 10 µM solution of the polymer.
The detection limit of the
polymer for SDS was estimated to be 2 " 10!7 M (56 ppb) and
similar changes were observed for
the other anionic surfactants. The polymer–surfactant
complexation was formulated as
involving ion-pairing interactions between the surfactant head
groups and the imidazolium
moieties, as well as contacts between the hydrocarbon regions of
the surfactant and polymer,
driven by the hydrophobic effect. Computational studies
indicated that binding of this type
could result in distortion of the polydiacetylene !-system in a
manner consistent with the
observed spectroscopic changes.
Ammonium-functionalized, conjugated polyelectrolyte 1.20 was
employed as a selective,
ratiometric fluorescence-based sensor for pyrophosphate anion in
2-(N-morpolino)ethanesulfonic
acid (MES)-buffered aqueous solution by Schanze and
co-workers.26 The polymer was prepared
by Sonogashira polymerization of a diiodobenzene derivative with
branched polyamine side
chains and 1,4-diethynylbenzene. The water-soluble polymer was
obtained after deprotection
under acidic conditions (4 M HCl). Addition of PPi at pH 6.5
triggered a red-shift in the
absorbance spectrum of the polymer from its original place
related to the conversion between the
“ free chain state” and the “aggregate state” of the polymer
chains. This spectral change was
parallel with the decrease of a fluorescence emission band at
435 nm and simultaneous increase
of emission intensity at 530 nm; both effects were suitable for
determination of PPi concentration
(with a detection limit of approximately 340 nM) using a
convenient and robust ratiometric
format. The spectral changes were consistent with backbone
planarization and enhanced
interpolymer exciton coupling arising from anion-induced polymer
aggregation. The polymer
was insensitive towards halide ions, carbonate, and sulfate,
while biologically important anions
such as adenosine monophosphate (AMP), adenosine diphosphate
(ADP), and adenosine
triphosphate (ATP) did somewhat interfere with the emission
response.
The observed selectivity toward PPi over other inorganic anions
such as phosphate was
interpreted as arising from the unique ability of pyrophosphate
to cross-link polymer chains,
giving rise to polymer aggregation. In this regard, similarities
may be drawn between this system
and the behavior of pyridinium-based polymers, which have been
reported to undergo selective,
18
-
anion-induced self-assembly into micellar aggregates (without an
associated fluorescence
response).27
1.3.4 Polymers bearing hydrogen bond-donor groups
The strength and directionality of hydrogen bonding
interactions, combined with the ability to
incorporate hydrogen bond donor groups into a range of
preorganized scaffolds, have contributed
to the extensive development of anion receptors based on these
interactions. Likewise, diverse
approaches for incorporating hydrogen bond donor groups into
polymeric scaffolds have been
investigated for the purpose of anion recognition. The polymeric
nature of these receptors gives
rise to unique effects, including signal amplification,
cooperative binding, and enhanced
selectivity.
Several fluorescent polymers functionalized with acidic OH
groups have been found to act as
sensitive materials for anion detection (Figure 1.12). For
example, polyquinoline 1.21 was
studied by Wang and co-workers as a fluorescent chemosensor for
Bu4N+F– in DMSO solvent.28
The polymer was synthesized by a nickel(0)-catalyzed coupling
reaction / deprotection sequence
(Mn = 6000, Mw/Mn = 1.15). A fluoride-induced red-shifted
absorption peak at 500 nm and a
color change from colorless to red were accompanied by formation
of a new feature in the
emission spectrum at 620 nm. The increased acidity of the
phenolic proton due to the strong
electron withdrawing ability of the quinoline moiety resulted in
deprotonation of the OH acidic
proton by strong bases such as fluoride, and subsequent
development of an intramolecular charge
transfer (ICT) between the phenolate anion and the quinoline
moiety. The response was selective
for F– over less basic anions (Cl–, Br–), and only a minimal
response toward
dihydrogenphosphate anion was observed. Comparison with a
non-polymeric hydroxyquinoline
revealed that the polymer displayed enhanced sensitivity to 100
equivalents of fluoride (147-fold
enhancement, compared to 17-fold for the model compound in
DMSO), as well as an improved
F–/H2PO4– selectivity in comparison to its non-polymeric analog.
The higher selectivity of the
polymer compared to the small molecule model compound was
attributed to the coil structure of
the polymer in which penetration of smaller anions such as
fluoride is more facile.
19
-
Figure 1.12 Fluorescent, polymeric anion sensors based on acidic
OH groups.
Another report from the same group demonstrated a series of
polyphenylenes composed of
phenol-substituted oxadiazole moieties as fluorescent
chemosensors for fluoride ion.29 The
copolymer 1.22 was synthesized by Suzuki copolymerization of
phenol-substituted oxadiazole
monomers with 2,5-dibutoxy-1,4-phenylenediboronic acid and
2,7-dibromo-9,9-dioctylfluorene.
Addition of anions such as F! and H2PO4! (10 !3 M) to a solution
of polymer (5 µM in
chloroform) led to fluorescence quenching with Stern-Volmer
constants of KSV (F!) = 7.1 " 105
and KSV (H2PO4!) = 9.5 "104 and the formation of a
longer-wavelength absorption band at 462
nm with a reduction of the main absorption band at 375 nm.
Polymers containing two phenol
groups in each oxadiazole repeat unit exhibited higher
sensitivities towards fluoride anion over
dihydrogen phosphate anion in comparison to those having only
one phenolic group per repeat
unit. The polymer showed quenching up to 380 fold while a small
molecule model compound
displayed only a 3-fold reduction of emission intensity,
consistent with an amplification effect.
The polymer fluorescence lifetime was independent of fluoride
anion concentration, suggesting a
static quenching process rather than a dynamic process. The
polymer was unresponsive towards
other anions such as Cl!, Br!, I!, BF4!, PF6!, while H2PO4! only
induced a 6-fold decrease in
emission intensity and a color change from colorless to yellow.
The selectivity of the polymer
towards fluoride ion over dihydrogen phosphate ion was
attributed to the coil conformation of
the polymer in solution, providing steric hindrance for larger
anions such as H2PO4! to access the
hydroxyl group.
OC4H9
C4H9O
OC4H9
C4H9O C8H17 C8H17
x y
1.22Responsive to F! and H2PO4! in CHCl3
N
HO
Nn
1.21Responsive to F! in DMSO
OHN
N
O
OH
O
NHO
OHC6H13 C6H13
O
N n
1.23Responsive to F!, OH!, and AcO! in THF
20
-
Pang reported a fluorene #-conjugated polymer based chemosensor
composed of 2,5-
bis(benzoxazol-2$-yl)benzene-1,4-diol (HBO) units synthesized by
Suzuki-Miyaura cross
coupling reaction.30 The absorption spectrum of polymer 1.23
showed bands at 332, 400, and
421 nm while the emission spectrum showed a peak at 616 nm with
a large Stokes shift (~200
nm), assigned to excited-state intramolecular proton transfer
(ESIPT) in the bis(HBO)
chromophore. Upon addition of anions such as OH!, F!, and AcO!,
a decrease in absorption
bands at 400 and 421 nm, and evolution of a new absorption band
between 510-540 nm (color
change from to red) was observed. Addition of fluoride or
acetate anions also induced a
fluorescence enhancement by a factor of ~20. The corresponding
excitation spectrum resembled
to the absorption spectrum, indicating that the new emission
band originated from their new
absorption band. This observation was assigned to anion-induced
perturbation of the hydrogen-
bonding structure in the bis(HBO) unit in the ground state.
In addition to OH groups, acidic NH groups have been
incorporated into conjugated polymers to
give rise to anion-sensory materials; among the functional
groups that have been employed are
pyrrolyl (1.24a, 1.24b, 1.25), amido (1.26) and ureido (1.27,
1.28, 1.29, 1.30) groups (Figure
1.13 and Figure 1.14). Important precedent for these efforts is
the application of polypyrrole
films in anion-selective electrodes for detection of chloride,
bromide, nitrate and perchlorate: a
combination of hydrogen bonding interactions involving the
pyrrole NH groups, along with ion-
pairing interactions accompanying oxidation of the polymer, are
likely responsible for the
polypyrrole–analyte interactions.31 Pioneering work in the
development of new classes of
hydrogen-bonding polymers for anion recognition was carried out
by Aldakov and Anzenbacher,
who incorporated anion-binding dipyrrolylquinoxaline (DPQ)
moieties into thiophene-based
polymers by electrochemical polymerization.32 In analogy to the
polypyrrole sensors mentioned
above, a key feature of this design is the prospect for
modulating the level of positive charge in
the polythiophene – and thus the extent of Coulombic interaction
with anionic analytes, or,
perhaps, the pKa of acidic NH groups – by applying an electric
potential. Polymers 1.24a and
1.24b displayed colorimetric responses to tetrabutylammonium
fluoride, pyrophosphate and
phosphate in DMSO/water mixed solvent at a constant potential of
!0.12 V (potentials are
referenced to the ferrocene/ferrocenium redox couple), with
tetrabutylammonium perchlorate as
the supporting electrolyte. Apparent association constants for
1.24a with these anions ranged
from 1.1 " 104 M–1 for pyrophosphate to 9.0 " 10–4 M–1 for
dihydrogenphosphate.
21
-
Electrochemical oxidation of polymer 1.24a resulted in an
increase of the apparent affinity
constant for pyrophosphate from 1.1 x 104 M–1 at !0.12 V to
>106 M–1 at +0.58 V, and in situ
conductivity measurements using interdigitated microelectrodes
revealed a concentration-
dependent decrease in the conductivity of polymer 1.24b upon
addition of tetrabutylammonium
pyrophosphate. The ability to use a single material for
colorimetric and/or conductivity-based
anion sensing is a noteworthy aspect of this system from the
standpoint of developing robust and
versatile sensory devices.
Sun and coworkers reported a fluorescent poly(phenylene
ethynylene) polymer containing
dipyrrolylquinoxaline (DPQ) groups that display changes in color
and fluorescence intensity
upon addition of fluoride ion.33 Copolymer 1.25 was synthesized
by palladium-catalyzed
Sonogoshira cross-coupling reaction of a
dibromodipyrrolylquinoxaline monomer and 1,4-
diethynylbenzene. Addition of fluoride or pyrophosphate anions
to a solution of polymer in
CH2Cl2, resulted in a color change, from yellow to red. The
quenching of the emission band
at #max = 550 nm was assigned to the anion-induced deprotonation
of the pyrrole NH protons in
the DPQ moiety. The sensitivity of the anion-induced quenching
was considerably increased (34-
fold) compare to a small molecule model compound due to rapid
exciton migration. The
oxidation-induced affinity enhancement was assigned to the
higher acidity of the DPQ unit upon
p-doping in the polymeric system. The polymer exhibited a linear
Stern-Volmer response with a
Stern-Volmer constant of (KSV) = 5.20 " 105, and the
fluorescence lifetimes of the polymer
solution did not vary with the concentrations of the anion,
consistent with static quenching.
Recently Tian reported a variety of poly(methylmethacrylate)s
composed of naphthalimide
signaling moieties and either thiourea34a or imide34b
recognition groups by reversible addition-
fragmentation chain transfer polymerization (RAFT). Upon
addition of AcO! and H2PO4! to a
solution of polymer 1.27 containing the thiourea moiety in DMSO
(10!5 M), only a slight
modulation of the absorption spectra was observed with a color
change from light yellow to
orange, while addition of excess F! led to a decrease of the
absorption band intensity at 450 nm
and evolution of a new band at 560 nm. Similar results were seen
for polymeric films. The
spectral changes were attributed to the acidity of the thiourea
protons and an intramolecular
charge transfer process. Monitoring the two thiourea N!H protons
by 1H NMR spectroscopy
upon addition of F! to a solution of polymer in DMSO-d6 revealed
the disappearance of these
22
-
protons and formation of a new triplet signal at 16.1 ppm which
was assigned to the bifluoride
[FHF]!anion, suggesting the deprotonation of thiourea N!H.
Another report by the same group demonstrated a naphthaimide
polyphenylacetylene which was
synthesized by rhodium [Rh(nbd)Cl]2 catalyzed polymerization of
the corresponding acetylenic
monomer and further utilized in fluorescence sensing of fluoride
ion.34c The polymer 1.26
showed an absorption peak between 310-400 nm assigned to #!#*
transition of the
polyphenylacetylene conjugated backbone and the fluorescence
spectra of the polymer showed
an emission band at 460 nm (#ex = 340 nm) with a characteristic
shape of the naphthalimide unit,
yet lower quantum yield (%PL = 69%). This lower emission of the
polymer was attributed to
photoinduced electron/energy transfer from the
polyphenylacetylene backbone to the
naphthalimide moiety. Upon addition of fluoride ion to a
solution of polymer in acetonitrile, the
absorption band at 360 nm decreased while a new band at 490 nm
developed with concomitant
color changes from yellow to pale red. A significant decrease in
emission intensity of the
polymer, with a red shift towards higher energy at 580 nm was
also observed. A linear
correlation between intensity ratio of absorbance at 490 nm and
360 nm (A490/A360) and fluoride
concentration in acetonitrile was observed which could be
applied for ratiomeric sensing. The
changes in absorption and emission spectra was attributed to the
electron donating abilities of the
amide group in the presence and absence of fluoride ion and its
influence on the intramolecular
charge transfer process from the amide group to the
electron-withdrawing imide moiety. The
selectivity of the polymer towards other anions (Cl!, Br!, and
I!) was also investigated and no
substantial responses with respect to these anions were
observed. The linear emission intensity
ratio at 580 nm and 460 nm (I580/I460) was used to evaluate the
polymer efficiency in detecting
fluoride ions.
Yang reported fluorescence fluorene-based conjugated polymer
1.28 that carries a pendant urea
group for anion detection.35 The copolymer (Mn = 5000 g/mol )
was synthesized by Suzuki
cross-coupling reaction of a dibromo fluorene containing two
phenyl urea moieties, 2,7-dibromo-
9,9-dihexyl fluorene and 1,4-phenylenediboronic acid with Mn =
5000 g/mol. The UV-vis
spectrum of the polymer in THF displayed absorption maxima at
367 nm, 236 and 267 nm, of
which the former was ascribed to the #!#* transition and the
latter to the diphenyl urea side
chain. Addition of fluoride anion to the solution of polymer in
THF resulted in a hyperchromic
shift of the band at 237 nm and a hypochromic shift of that at
269 nm. These spectroscopic
23
-
changes were attributed to the hydrogen bonding interaction
between F! with the urea NH
protons. Addition of F! and AcO! resulted in quenching of the
polymer fluorescence, while
dihydrogenphosphate anion (Ka = 5.46 " 104 M!1) led to only a
medium quenching and other
anions such as Cl!, Br!, and I! were ineffective. The quenching
behavior was assigned to
photoinduced electron transfer triggered upon interaction of the
urea protons with the anion.
A conceptually distinct and versatile method for transducing
hydrogen-bonding events into
measurable signals using a polymeric architecture is described
in a series of publications by
Kakuchi and co-workers: incorporation of acidic NH groups into
the side chains of a
poly(phenylacetylene) results in a system in which hydrogen
bonding interactions with anions
influence the helical conformation of the polymer.36
24
-
Figure 1.13 Polymeric anion sensors based on acidic NH
groups.
CH
C
O NH
N
O
O
(CH2)7CH3
n
1.26Responsive to F! in CH3CN
CH3
CH3C C
H2
CH3 S
S
O
N OO
NH
NHS
O
O n
1.27Responsive to F!, AcO!, and H2PO4! in DMSO
NNSS
O O O O
HNNH
R R
n
1.24a (R = Cl), 1.24b (R = H)Responsive to H2PO42!, P2O72!, and
F!
(Bu4N+ salts, DMSO)
OC12H25
C12H25O N N
HNNH
n
1.25Responsive to n-Bu4+F! (Bu4N+ salts, CH2Cl2)
25
-
Figure 1.14 Polymeric anion sensors based on acidic NH
groups.
Urea-containing poly(phenylacetylenes) were synthesized using
rhodium catalyzed
polymerization of a functionalized phenyl acetylene. Polymer
1.29 bearing urea-linked L-leucine
substituents did not show evidence of a defined helical
conformation in THF solvent, but
addition of tetrabutylammonium chloride, bromide or acetate
resulted in dramatic induction of
Cotton effects in the circular dichroism (CD) spectrum, along
with a red-shift of the UV-vis
absorption spectrum and color changes from pale yellow to red.
These spectral changes implied
that anion binding was accompanied by an alteration in the
polymer conformation, leading to a
biased helical conformation along with a change in the
conjugation length of the ! system. The
polymer response was dependent upon the ionic radius (and not,
apparently, on the basicity) of
the anions, indicating that the spacing of the urea groups along
the polymer backbone gives rise
to a significant level of selectivi