Synthesis of a nonionic water soluble semiconductive polymer

Article abstract of DOI:10.1039/b209561k

A new nonionic water-soluble fluorescent conjugated polymer is reported with hydroxyl and amide side chains surrounding an aromatic polymer backbone.

Full text of DOI:10.1039/b209561k

Synthesis of a nonionic water soluble semiconductive polymer
Kenichi Kuroda and Timothy M. Swager*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA, USA 02139
Received (in Columbia, MO, USA) 29th September 2002, Accepted 14th November 2002
First published as an Advance Article on the web 28th November 2002
A new nonionic water-soluble fluorescent conjugated poly-
mer is reported with hydroxyl and amide side chains
surrounding an aromatic polymer backbone.
Conjugated polymer biosensory materials are of great interest
because of the high sensitivity of their optical and conducting
properties in the presence of analytes.1,2 In particular, water-
soluble conjugated polymers offer powerful new methods for
the trace detection of analytes in aqueous environments.
However, applications to aqueous sensing have been limited as
a result of the challenges associated with strong interactions
between hydrophobic backbones and aromatic p–p stacking
that severely restricts their water solubility. The conventional
approach to water-soluble conjugated polymers employs ionic
(often in the side chains) sulfonic,3 carboxylic4 or ammonium
groups.5 These groups give strong enthalpic interactions with
water and electrostatic repulsions between the polymer chains,
Scheme 1 (a) Ethyl bromacetate, K2CO3, 2-butanone, reflux; (b) (i)
both of which promote solubility. Ionic conjugated polymers
diethanolamine, ethanol, reflux, (ii) triisopropylsilyl chloride, imidazole,
have been applied in biosensor schemes,5–7 however, these
THF; (c), (e) TBAF, THF; (d) (i) (trimethylsilyl)acetylene, CuI,
systems have several disadvantages: the solution’s pH and ionic
strength have to be adjusted to prevent aggregation of
conjugated polymers and non-specific electrostatic interactions
between ionic polymers and biomolecules such as proteins and
DNA will reduce target specificity. Considering the demerits of
ionic sensory polymers, we have chosen to develop nonionic
(neutral) water-soluble semiconductive polymers as a platform
for high specificity biosensory polymers.
(PPh3)2PdCl2, benzene/triethylamine, 60°C, (ii) K2CO3, MeOH/THF.
protection of the hydroxyl group in contrast to the synthesis of
6. This monomer is completely soluble in DMF, DMSO, and
water.
Monomers 6 and 9 (54 mM) were polymerized by Sonoga-
shira cross-coupling reaction in the presence of 5% (PPh3)4Pd
and 5% CuI in morpholine at 50 °C overnight. The polymer was
precipitated in ethyl acetate and dried under vacuum. The
obtained polymer A is readily soluble in water dissolving in
essentially all proportions instantaneously without heating. Gel
permeation chromatography analysis (GPC) (eluent: DMF,
PMMA standards) indicated that Mw and Mn of the obtained
polymer are 45 000 and 32 000, respectively. We also synthe-
sized a homopolymer, polymer B, by coupling 4 and 6 in the
same polymerization procedure. (Mn = 12 487, Mw = 24 122).
To achieve high water-solubility without ionic groups, we
have investigated a number poly(phenylene ethynylene)s
(PPEs) with hydrophilic groups in the side chains. The
extremely hydrophobic nature of the PPE backbone made it
necessary to place many hydroxyls proximate to the polymer
backbone and thereby shield it from water. Based upon this
design principle, we synthesized nonionic and completely
water-soluble PPE A with hydroxyl and amide groups surround-
ing the polymer chain. Polymer A was synthesized by a
Sonogashira cross-coupling reaction of diiodobenzene (9) and
diethynylene benzene derivatives (6) prepared according to
Schemes 1 and 2. As shown in Scheme 1 diiodohydroquinone
(1) was reacted with ethyl bromacetate to give ester (2) which
after reaction with diethanolamine was treated with triisopro-
pylsilyl chloride in THF to give organic soluble 3, which is
easily purified by re-crystallization from CH2Cl2/methanol.
Compound 5 is obtained from 3 by standard Sonogashira cross-
coupling procedures. The TIPS groups were removed by
treatment with tributylammonium fluoride (TBAF) in THF and
compound 6 precipitates from MeOH/CH2Cl2.† Diiodobenzene
derivative 4 was made by cleavage of TIPS groups from 3 in the
same procedure for 6.
However this material is only slightly soluble (est. 0.1 mg ml21
in water after heating at 60°C overnight.
)
The most hydrophilic monomer 9† was prepared in three
steps from 2 (Scheme 2). The ester group of 2 is hydrolyzed,
converted to the acid chloride, and reacted with diethyl
iminodiacetate to give 8. The target compound 9 was afforded
by adaptation of Newkome’s synthesis of arborols.8 Compound
8 is treated with tris(hydroxylmethyl)aminomethane (Tris) in
DMSO in the presence of K2CO3 at room temperature.8,9 After
workup the oily residue was rinsed with CH2Cl2 to give 9 as a
precipitate. In this synthetic approach, 9 was obtained without
Scheme 2 (a) NaOH/methanol, reflux; (b) (i) oxalyl chloride, reflux, (ii)
diethyl iminodiacetate, CH2Cl2, Et3N; (c) Tris, DMSO, K2CO3.
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CHEM. COMMUN., 2003, 26–27
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solution prepared by dissolving polymer A in water at room
temperature showed small peaks in the higher wavelength
region of the absorption and fluorescence. These features
disappeared after heating at 60 °C overnight. Aggregation of
conjugated polymers generally produces such new low energy
absorption and emission features. Therefore, the spectral
change indicates that polymer A is initially aggregated under
the conditions of the fluorescence experiments but dissociates
after heating. Quantum yields of polymer A were 0.07 in water
and 0.27 in DMF determined with quinine sulfate as a standard.
The origin of the reduced quantum yield in water is unclear. It
is still possible that the strong hydrogen bonding tendency of the
side chains produces an unfavorable organization about the
polymers. Although not apparent from the spectra after heating,
a very weak aggregation can not be ruled out. Newkome
reported aggregation and gelation in water of arborols, which
consist of two hydrophilic Tris-based dendron spheres con-
nected with a lipophilic bridge such as an alkyl chain or phenyl
groups.8 Further spectroscopic characterizations of the polymer
solution are in progress in our laboratory.
This work was supported by the National Science Foundation
through the Center for Materials Science and Engineering at
MIT and NASA.
UV-vis and fluorescence spectra of aqueous and DMF
solutions of polymer A are shown in Fig. 1. The aqueous
Notes and references
1
† Compound 6: H NMR (300 MHz, DMSO-d6): d 6.91 (s, 2H), 5.00 (t,
2H), 4.94 (s, 2H), 4.71 (t, 2H), 4.34 (s, 2H), 3.59-3.33 (m, 16H); 13C NMR
(75 MHz, DMSO-d6): 167.15, 152.89, 116.88, 111.84, 86.06, 79.98, 66.21,
58.61, 49.23, 47.73. HR-MS (ESI) calcd. for C22H28N2O8: 449.1918 (M+ +
H), found: 449.1903 (M+ + H), 471.1725 (M+ + Na). Compound 9: 1H NMR
(300 MHz, DMSO-d6): d 7.73 (bs, 2H), 7.42 (bs, 2H), 7.23 (bs, 2H), 4.85
(bs, 4H), 4.62 (bs, 12H), 4.06 (bs, 8H), 3.95 (bs 4H), 3.56 (bd, 24H). HR-
MS (ESI) calcd. for C34H54I2N6O20: 1143.1374 (M+ + Na), found:
1143.1377 (M+ + Na).
1 D. T. McQuade, A. E. Pullen and T. M. Swager, Chem. Rev., 2000, 100,
2537.
2 T. M. Swager, Acc. Chem. Res., 1998, 31, 201.
3 S. Shi and F. Wudl, Macromolecules, 1990, 23, 2119.
4 H. Häger and W. Heitz, Macromol. Chem. Phys., 1998, 199, 1821.
5 M. Stork, B. S. Gaylord, A. J. Heeger and G. C. Bazan, Adv. Mater., 2002,
14, 361.
6 C. Fan, K. W. Plaxco and A. J. Heeger, J. Am. Chem. Soc., 2002, 124,
5642.
7 R. M. Jones, T. S. Bergstedt, D. W. McBranch and D. G. Whitten, J. Am.
Chem. Soc., 2001, 123, 6726.
8 G. R. Newkome, X. Lin, C. Yaxiong and G. H. Escamilla, J. Org. Chem.,
1993, 58, 3123.
9 L. J. Twyman, A. E. Beezer, R. Esfand, M. J. Hardy and J. C. Mitchell,
Tetrahedron Lett., 1999, 40, 1743.
Fig. 1 UV-vis absorption spectra and fluorescence spectra of polymer A:
aqueous solution before heating (dashed line), after heating (solid line) and
DMF solution (broken-dot line).
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