Practical synthesis of an amphiphilic, non-ionic poly(paraphenyleneethynylene) derivative with a remarkable quantum yield in water

Article abstract of DOI:10.1039/b413616k

Aggregation of poly(para-phenyleneethynylene)s is efficiently suppressed by introduction of branched oligoethyleneglycol side chains rendering the polymer backbone, which is readily obtained using an A₂ + BB′ polycondensation protocol, soluble and highly emissive in aqueous environments. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b413616k

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Practical synthesis of an amphiphilic, non-ionic poly(para-
phenyleneethynylene) derivative with a remarkable quantum yield in
water{
Anzar Khan,ab Stephan Mu¨llera and Stefan Hecht*ab
Received (in Cambridge, UK) 6th September 2004, Accepted 23rd November 2004
First published as an Advance Article on the web 10th January 2005
DOI: 10.1039/b413616k
10 equiv. of water in acetonitrile at room temperature for 3 d. The
commercial availability of TMSA and the rapid synthetic access to
monomer 3 render this synthesis of polymer 5 highly practical.
Polymer 5 was obtained in 83% isolated yield after single
precipitation into diethyl ether and extraction to remove residual
inorganic impurities. GPC analysis13 of polymer 5 indicated
reasonable degrees of polymerization (DP # 30) and polydisper-
sities (PDI # 2.0) typical for polycondensation processes.
1H-NMR showed successful incorporation of the monomer repeat
units into polymer 5 with no discernible end-groups, while in
13C-NMR no diacetylene defects could be detected.7 Polymer 5
displayed good solubility in a wide range of different solvents,
most importantly in water (0.7 mg mL21).
Aggregation of poly(para-phenyleneethynylene)s is efficiently
suppressed by introduction of branched oligoethyleneglycol
side chains rendering the polymer backbone, which is readily
obtained using an A2 + BB9 polycondensation protocol, soluble
and highly emissive in aqueous environments.
The detection of biologically relevant analytes can be achieved in
high sensitivity using sensory materials based on conjugated
organic polymers.1 In situ sensing methods operating under
ambient conditions necessitate the synthesis of water-soluble,
emissive or (semi)conducting polymers that has proven to be
challenging due to the high tendency of conjugated hydrophobic
backbones to aggregate in aqueous environments, thereby
rendering the materials insoluble and leading to detrimental
emission quenching. Several strategies have been developed to
introduce water-solubility and overcome aggregation, for instance
in poly(para-phenyleneethynylene)s (PpPE)2 including the use of
ionic3 and polyhydroxylated4 substituents as well as site isolation
utilizing branched5 and linear6 polymer side chains. Here, we
present a practical synthetic route to a non-ionic, non-protic,
amphiphilic PpPE derivative, which displays both respectable
solubility and an exceptionally high fluorescence quantum yield in
water.
UV–vis spectra of polymer 5 in various organic solvents exhibit
typical absorption maxima lmax 5 437–441 nm (Fig. 1, Table 1)
suggesting that the full effective conjugation length has been
reached.8 In water however, a considerable hypsochromic shift
(lmax 5 427 nm) was observed, similar to the reported case of an
ionic PpPE.3d Since the data suggest a negligible polarity effect of
the medium, water seems to cause certain structural reorganization
and/or induce some degree of aggregation, both potentially
distinguishable by emission spectroscopy. Corrected and normal-
ized fluorescence spectra of polymer 5 in the same solvents were
qualitatively similar yet showed distinctly different photolumines-
cence efficiencies (Fig. 1). Calibration using quinine sulfate as
reference standard14,{ yielded the corresponding quantum yields
(Table 1) that do not scale with solvent polarity. Interestingly,
chloroform being a good solvent for both polar side chains and
non-polar core leads to less intense emission than acetonitrile
which preferentially solvates the polar corona. Increasing solvent
polarity from acetonitrile via methanol to water caused a decrease
in photoluminescence efficiency.
Recently, we have reported on an in situ deprotection–coupling
Sonogashira–Hagihara polycondensation protocol for the synth-
esis of lengthy and defect-free amphiphilic poly(meta-phenyl-
eneethynylene)s.7,8 The aim of the present work was to extend our
newly developed A2 + BB9 polycondensation method9 to the
synthesis of amphiphilic, site isolated PpPEs from readily available
monomer building blocks. In order to efficiently suppress
aggregation arising from ionic substituents and their associated
counter-ions or from hydrogen-bonding protic residues, two
branched oligoethyleneglycol (OEG) side chains10,11 were intro-
duced at every repeat unit. The polar, non-ionic and non-protic
side chains should render the backbone soluble in a variety of
media including water and provide a chemically inert scaffold.
Monomer synthesis was accomplished by alkylation of 2,5-
diiodohydroquinone 112 with branched OEG tosylate 210
(Scheme 1).{ Applying our A2 + BB9 polycondensation protocol,7
involved stoichiometric reaction of diiodide 3 (A2 monomer) with
trimethylsilylacetylene 4 (TMSA, BB9 monomer) in the presence of
5 mol% freshly prepared Pd(PPh3)4 catalyst, 10 mol% CuI
cocatalyst, 6 equiv. of diazabicyclo[5.4.0]undec-7-ene (DBU), and
In order to span the entire emission intensity range, the solvent
was systematically varied from acetonitrile to water (Fig. 2).
Fluorescence emission was found to decrease with increasing
amount of water in a non-linear fashion (inset Fig. 2). It is
important to note that the shape of the fluorescence band did not
change considerably,{ indicating almost negligible aggregation that
would have causes a broad, red-shifted, excimer-like emission. This
finding is furthermore supported by UV–vis absorption measure-
ments on annealed films that did not show any sign of aggregation,
such as narrower and bathochromically shifted absorption
bands.8,{ Therefore, the observed solvent effects are most likely
due to different solvation of the amphiphilic polymer, causing
structural reorganization of the backbone chromophores in polar/
aqueous environments.
{ Electronic supplementary information (ESI) available: synthesis and
characterization details. See http://www.rsc.org/suppdata/cc/b4/b413616k/
*hecht@mpi-muelheim.mpg.de
584 | Chem. Commun., 2005, 584–586
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Scheme 1 Synthesis of water-soluble PpPE derivative 5 from readily available monomers 3 and 4 via an A2 + BB9 polycondensation route.
Fig. 1 UV–vis absorption and fluorescence emission spectra of polymer
5 in various solvents (25 uC). Absorption spectra are scaled to the same
optical density, while emission spectra are corrected and normalized.
Fig. 2 Fluorescence emission spectra of polymer 5 in acetonitrile–water
solvent mixtures (25 uC). The inset shows the calculated quantum yields as
a function of solvent composition (the line obtained from the non-linear
least squares fit of the data is included as a guide to the eye).
Table 1 Absorption and emission characteristics of polymer 5
Solvent
labsa/nm
lema/nm
Wfb
molecular mechanics calculations.{ In related poly(para-
phenylene)s, these branched OEG side chains were found to
efficiently prevent aggregation in the solid state.10 In addition to
the chosen substituents, the observed high quantum yields can be
explained by the absence of diacetylene defects due to our method
of polymer preparation.7,3a
Chloroform
Acetonitrile
Methanol
Water
437
441
438
427
465
468
466
464
0.53
0.63
0.57
0.43
b
a
lmax of absorption and emission bands, respectively. Using
quinine sulfate as reference standard.14,
{
The utility of our A2 + BB9 polycondensation approach7 arising
from its practicality due to rapid monomer availability as well as
the quality of the obtained materials has successfully been
demonstrated by the synthesis of a chemically inert, water-soluble,
and highly emissive PpPE derivative, which could potentially be
used in ultrasensitive biosensing.1,15 In this context it is important
to note that in aqueous solutions of polymer 5, variation of several
other parameters, such as addition of metal cations (Li+, Mg2+,
Ca2+, Ba2+, Zn2+) and changing pH (4–11) did not lead to a
noticeable change in the qualitative and quantitative absorption
and emission characteristics. The approach described should be
generally applicable to the design of conjugated polymers both
soluble and emissive in aqueous environments.
In general, the quantum yields in solution are high and even in
water, which imposes an enormous hydrophobic effect to drive
aggregation, remarkably efficient photoluminescence has been
observed. In fact, to the best of our knowledge polymer 5 exhibits
by far the highest quantum yield in water (Wf 5 0.43) of any
known PpPE derivative (ionic PpPEs:3 Wf 5 0.03–0.10; poly-
hydroxylated PpPE:4 Wf 5 0.07). This exceptional behavior is
attributed to the significant steric demand of the employed side
chains that are attached twice per repeat unit and place their
branch points in close proximity to the backbone. Thereby, an
efficient shielding of the PpPE core is achieved as indicated by
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P. Samori, J. P. Rabe, H. Ha¨ger and W. Heitz, J. Colloid. Interface Sci.,
1999, 212, 24; (c) D. T. McQuade, A. H. Hegedus and T. M. Swager,
J. Am. Chem. Soc., 2000, 122, 12389; (d) C. Tan, M. R. Pinto and
K. S. Schanze, Chem. Commun., 2002, 446; (e) M. R. Pinto, B. M. Kristal
and K. S. Schanze, Langmuir, 2003, 19, 6523; (f) review on water-
soluble, conjugated polyelectrolytes including PPEs: M. R. Pinto and
K. S. Schanze, Synthesis, 2002, 1293.
4 K. Kuroda and T. M. Swager, Chem. Commun., 2003, 26.
5 Dendritic PpPE (like) backbones via macromonomer routes: (a) T. Sato,
D.-L. Jiang and T. Aida, J. Am. Chem. Soc., 1999, 121, 10658; (b)
W.-S. Li, D.-L- Jiang and T. Aida, Angew. Chem. Int. Ed., 2004, 43,
2943.
Ongoing studies in our laboratories include detailed character-
ization in the solid state to obtain structural insight into the
remarkable ability of the branched OEG side chains to efficiently
encapsulate and solubilize the hydrophobic PpPE backbone in
water as well as time-resolved fluorescence studies to deduce
conformational and aggregation behavior in aqueous solution.
Furthermore, we are currently exploring the alignment of such
polymers on templating substrate surfaces.16
Note added in proof: after submission of this manuscript, Jiang,
Aida, and coworkers reported on a dendronized, polyanionic
PpPE derivative with an extraordinarily high fluorescence
quantum yield of 57% in water.17
6 PpPE brushes via graft-from approaches: (a) Y. Wang, B. Erdogan,
J. N. Wilson and U. H. F. Bunz, Chem. Commun., 2003, 1624; (b)
C. A. Breen, T. Deng, T. Breiner, E. L. Thomas and T. M. Swager,
J. Am. Chem. Soc., 2003, 125, 9942.
Generous support by the Sofja Kovalevskaja Award of the
Alexander von Humboldt Foundation, endowed by the Federal
Ministry of Education and Research (BMBF) within the Program
for Investment in the Future (ZIP) of the German Government, as
well as the German Research Foundation (DFG Sfb448) is
gratefully acknowledged. We also thank Holger Frauenrath
(FUB) for some GPC measurements.
7 A. Khan and S. Hecht, Chem. Commun., 2004, 300.
8 For a comprehensive review on the synthesis and properties of PPE
derivatives, consult: U. H. F. Bunz, Chem. Rev., 2000, 100, 1605.
9 The reactive sites have been labeled using polycondensation terminol-
ogy: A refers to aryl iodide, B to free terminal acetylene, and B9 to TMS-
protected terminal acetylene moieties.
10 U. Lauter, W. H. Meyer, V. Enkelmann and G. Wegner, Macromol.
Chem. Phys., 1998, 199, 2129.
11 For related non-ionic polysilanes, see for example: T. J. Cleij,
S. K. Y. Tsang and L. W. Jenneskens, Chem. Commun., 1997, 329.
12 Q. Zhou and T. M. Swager, J. Am. Chem. Soc., 1995, 117, 7017.
13 Due to the rigid rod character of PpPEs, their molecular weight is most
likely overestimated by a factor of 1.5–2 using GPC calibrated with
flexible polystyrene standards: (a) V. Francke, T. Mangel and K. Mu¨llen,
Macromolecules, 1998, 31, 2447; (b) L. Kloppenburg, D. Jones and
U. H. F. Bunz, Macromolecules, 1999, 32, 4194; (c) L. Kloppenburg,
D. Jones, J. B. Claridge, H.-C. zur Loye and U. H. F. Bunz,
Macromolecules, 1999, 32, 4460; (d) ref. 8.
Anzar Khan,ab Stephan Mu¨llera and Stefan Hecht*ab
aInstitut fu¨r Chemie/Organische Chemie, Freie Universita¨t Berlin,
Takustr. 3, 14195, Berlin, Germany
bMax-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1,
45470, Mu¨lheim and der Ruhr, Germany.
E-mail: hecht@mpi-muelheim.mpg.de; Fax: +49-208-306-2980;
Tel: +49-208-306-2230
Notes and references
14 W. H. Melhuish, J. Phys. Chem., 1961, 65, 229.
15 L. Chen, D. W. McBranch, H.-L. Wang, R. Helgeson, F. Wudl and
D. G. Whitten, Proc. Natl. Acad. Sci. USA, 1999, 96, 12287;
D. G. Whitten, R. M. Jones, S. A. Kushon, K. D. Ley, W. Xia,
S. Kumaraswamy and D. W. McBranch, QTL Biosystems, Llc., USA,
WO 2004046687, 2004.
16 N. Severin, A. Khan, S. Hecht and J. P. Rabe, unpublished results.
17 D.-L. Jiang, C.-K. Choi, K. Honda, W.-S. Li, T. Yuzawa and T. Aida,
J. Am. Chem. Soc., 2004, 126, 12084.
1 A comprehensive review is given by: D. T. McQuade, A. E. Pullen and
T. M. Swager, Chem. Rev., 2000, 100, 2537.
2 PpPE backbones exhibit intrinsically high photoluminescence efficiencies
and enable efficient signal amplification by the conjugated wire
approach pioneered by Swager and coworkers, see: T. M. Swager,
Acc. Chem. Res., 1998, 31, 201.
3 (a) H. Ha¨ger and W. Heitz, Macromol. Chem. Phys., 1998, 199, 1821; (b)
H. Schnablegger, M. Antonietti, C. Go¨ltner, J. Hartmann, H. Co¨lfen,
586 | Chem. Commun., 2005, 584–586
This journal is ß The Royal Society of Chemistry 2005