Cyclodextrin-grafted poly(phenylene ethynylene) with chemically-responsive properties

Article abstract of DOI:10.1039/b605804c

Water-soluble poly(phenylene ethynylene) carrying β-cyclodextrin was prepared; the polymer exhibited a fluorescence color change or quenching, depending on the kind of guest. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b605804c

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Cyclodextrin-grafted poly(phenylene ethynylene) with chemically-
responsive properties{
Tomoki Ogoshi, Yoshinori Takashima, Hiroyasu Yamaguchi and Akira Harada*
Received (in Cambridge, UK) 25th April 2006, Accepted 10th July 2006
First published as an Advance Article on the web 2nd August 2006
DOI: 10.1039/b605804c
dimer unit; Polym-1 is taken as a polymer of CD dimer, poly(CD
dimer). To the best our knowledge, the polymerization of CD
dimer has not been reported. Polym-1 is structurally interesting as
each poly(phenylene ethylene) is flanked by two CD groups.
Polym-1 is soluble in water, while Polym-2 is insoluble in water.
This is because the bulky hydrophilic CD groups of Polym-1
increase its solubility.
Water-soluble poly(phenylene ethynylene) carrying b-cyclodex-
trin was prepared; the polymer exhibited a fluorescence color
change or quenching, depending on the kind of guest.
Cyclodextrins (CDs) have received a great deal of interest because
they selectively recognize guests such as aliphatic, aromatic and
chiral compounds, and form complexes with these guests in
aqueous solution.1 Thus, CDs have been used for sensors,
components of rotaxanes, polyrotaxanes and supramolecular
polymers.2
Fig. 1(a) shows the absorption spectra of Polym-1, Polym-2 and
the Monomer Model in DMF; that of Polym-1 being in aqueous
solution. The absorption maximum of Polym-1 in DMF was
observed at 443 nm, while the spectrum of the Monomer Model
exhibited peaks around 360 nm, indicating the expansion of
p-conjugation along the polymer backbone. The absorption of
Polym-2 in DMF was almost same as that of Polym-1. In H2O, the
UV-vis absorption band of Polym-1 red-shifted by about 30 nm
and broadened. This peak shift indicates an increase in the
conjugation length in aqueous solution by intermolecular p-stack-
ing between p-conjugated polymer backbones.
p-Conjugated polymers are attracting significant interest
because they are used in electrical conductivity, electrolumines-
cence, light-emitting diodes and chemosensors.3 Among them, one
of the current interests in the field of conjugated polymers focuses
on the tuning of their optical and electrical properties by stimuli
such as oxidation–reduction, pH and metal cations.4
Herein, we describe a water-soluble chemically-responsive
fluorescent polymer employing CD; fluorescence color changes
and quenching of the conjugated polymer are induced by host–
guest complex formation in aqueous solution. We designed and
synthesized b-CD modified poly(phenylene ethynylene) (Polym-1)
via the Sonogashira coupling reaction (Scheme 1). As a reference
for Polym-1, poly(phenylene ethynylene) without b-CD moieties
(Polym-2) and a Monomer Model of Polym-1 were also prepared
(see ESI for experimental details{). Polym-1 consists of a CD
Additional information regarding the effect of solvents on
Polym-1 comes from the emission spectra (Fig. 1(b)). In DMF,
Polym-1 exhibited a strong fluorescence (quantum yield: Wfl
=
0.24), with emission at 481 nm. On the other hand, the fluorescence
intensity decreased and the fluorescence peak was red-shifted with
a fluorescence maximum at 493 nm (Wfl = 0.07) in H2O, indicating
the intermolecular p-stacking of polymer backbones in aqueous
solution. By diluting a Polym-1 aqueous solution, the fluorescence
at 493 nm from aggregates decreased while fluorescence at 450 nm,
derived from completely soluble Polym-1, appeared at the same
time (ESI{). Polym-1 is not completely soluble in its monomeric
form but forms partially quenched aggregates in water. 1H NMR
studies in solution reveal further evidence of the intermolecular
Scheme 1 Chemical structure of Polym-1, Polym-2 and the Monomer
Model.
Department of Macromolecular Science, Graduate School of Science,
Osaka University, Toyonaka, Osaka 560-0043, Japan.
E-mail: harada@chem.sci.osaka-u.ac.jp; Fax: +81 6-6850-5445;
Tel: +81 6-6850-5445
{ Electronic supplementary information (ESI) available: Experimental
section, optical properties of polymers, concentration dependence of the
fluorescence of Polym-1 in aqueous solution, 1H NMR spectra of Polym-1
in D2O and DMSO-d6, molecular model of Polym-1, 1H NMR spectra of
Polym-1 with AdCA and changes in fluorescence of Monomer Model upon
addition of viologen derivatives. See DOI: 10.1039/b605804c
Fig. 1 (a) UV-vis and (b) emission spectra (excited at 400 nm) of Polym-
1, Polym-2 and Monomer Model (0.020 mM) in DMF and aqueous
solutions.
3702 | Chem. Commun., 2006, 3702–3704
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p-stacking in aqueous solution (ESI{). The aromatic polymer
backbone of Polym-1 was observed in DMSO-d6 but was not
found in D2O, indicating the low rotational freedom of the
aromatic polymer backbone segment in aqueous solution
compared to in DMSO-d6. From molecular modelling studies of
Polym-1, p–p stacking of the benzene part-modified b-CD was
difficult due to steric hindrance by the b-CD moieties. On the other
hand, the space around benzene group-modified triethylene oxide
moieties was enough to form p–p stacking (see ESI on molecular
modelling{).
Changes to optical properties by adding guest compounds to
aqueous Polym-1 solutions were investigated. With aliphatic
1-adamantanecarboxylic acid (AdCA) as a guest compound, the
UV-vis absorption peak at 470 nm, derived from the stacking of
p-conjugated polymer backbones, largely decreased and comple-
tely disappeared at 0.20 mM of AdCA (Fig. 2(a)). The emission
with the peak at 490 nm, resulting from the intermolecular
p-stacking between polymer chains, also decreased and a new
emission peak around 460 nm appeared with increasing AdCA
concentration (Fig. 2(b)). The emission color changed from light
green (Wfl = 0.07) to deep blue (Wfl = 0.11) by addition of AdCA
(Scheme 2, path (i)). In contrast, little change in UV-vis and
emission spectra were found in the Monomer Model with AdCA
(Fig. 2(c) and 2(d)). These observations indicate dissociation of the
intermolecular p-stacking of polymer backbones by complexation
of AdCA to the b-CD moieties of Polym-1.5 Repulsive forces
between the complexed anions might prevent polymer chains from
aggregating. The pH of the solution was neutral in the experiment,
thus the proton of the carboxylic acid of AdCA should be
Scheme 2 Changes in fluorescence induced by host–guest complexes.
dissociated. It is generally well known that carboxylic acid groups
become carboxylate anions around pH 4–5.6
Fig. 3 shows the changes in fluorescence induced by adding
viologen derivatives. It has been demonstrated that the fluores-
cence of poly(phenylene ethynylene)s is quenched by electron-
accepting viologen groups.7 By adding an adamantane-modified
viologen group (AdBpyMe), large scale fluorescence quenching
was observed (Scheme 2, path (ii)). On the other hand, by using
N,N9-dimethyl-4,49-bipyridinium (MeBpyMe), low fluorescence
quenching occurred because the cationic viologen group barely
formed a host–guest complex with CD.8 These data indicate that
the fluorescence quenching by adding AdBpyMe is due to inclusion
complex formation between the adamantane moiety of AdBpyMe
and the b-CD of Polym-1. Holding viologen groups on the side
chain of Polym-1 via host–guest formation results in efficient
electron transfer from the poly(phenylene ethynylene) backbone to
the viologen moiety of AdBpyMe. By using the Monomer Model
instead of Polym-1, weak fluorescence quenching was observed
upon addition of AdBpyMe (ESI{). These data indicate that the
fluorescence quenching of Polym-1 with AdBpyMe is amplified
compared to that of the Monomer Model. Upon addition of an
adamantane-bearing pyridinium moiety (AdPy) instead of
Fig. 2 (a) UV-vis and (b) emission spectral changes (excited at 400 nm)
of aqueous Polym-1 solutions (0.020 mM) by adding AdCA. (c) UV-vis
and (d) emission spectral changes (excited at 360 nm) of aqueous Monomer
Model solutions (0.020 mM) by adding AdCA.
Fig. 3 Emission spectral changes (excited at 400 nm) of aqueous Polym-
1 solutions (0.020 mM) by adding viologen derivatives (0.20 mM).
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AdBpyMe, the fluorescence quenching was not observed because
the pyridinium group did not act as an electron-acceptor from
poly(phenylene ethynelene)s.
In conclusion, the fluorescence of Polym-1 was changed by the
addition of guest compounds; the fluorescence color change and
quenching of Polym-1 occurred in aliphatic and acceptor guests,
respectively (Scheme 2). The chemically-responsive fluorescence
color change and quenching system in aqueous solution are
interesting and little known. As the fluorescence changes take place
in aqueous solution and the conjugated polymer backbone of
Polym-1 is surrounded by biocompatible CD moieties, there is
promise that they will see application in molecular sensing drug-
delivery systems (DDS).
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5 Formation of a host–guest complex between AdCA and b-CD of Polym-
We thank Dr. A. Hashidzume of Osaka University for many
useful discussions. This work was supported by a Grant-in-Aid for
Science Research from the Ministry of Education, Science, Sports,
and Culture of Japan. T. O. is a research fellow of the Japan
Society for the Promotion of Science, 2005–2007.
1
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3704 | Chem. Commun., 2006, 3702–3704
This journal is ß The Royal Society of Chemistry 2006