Synthesis and optical properties of water-soluble poly(p-phenylenevinylene)s

Article abstract of DOI:10.1039/a904822g

A novel conjugated polymer containing carboxylic acid and 2-[2-(2-methoxyethoxy)ethoxy]ethoxy substituents which is soluble in both organic solvents and aqueous bases is synthesized.

Full text of DOI:10.1039/a904822g

Synthesis and optical properties of water-soluble poly(p-phenylenevinylene)s
Zhonghua Peng,* Bubin Xu, Jianheng Zhang and Yongchun Pan
Department of Chemistry, University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, MO 64110, USA.
E-mail: pengz@umkc.edu
Received (in Corvallis, OR, USA) 16th June 1999, Accepted 12th Augst 1999
A novel conjugated polymer containing carboxylic acid and
2-[2-(2-methoxyethoxy)ethoxy]ethoxy substituents which is
soluble in both organic solvents and aqueous bases is
synthesized.
NaOH solutions with limited NaOH concentration (between 2
and 0.01 M). It is insoluble in solutions with NaOH concentra-
tions that are too high ( > 3 M) or too low ( < 0.001 M). The
decreased solubility of Polymer II in concentrated NaOH
solutions may be related to the crown ether–metal ion type of
bonding between ethylene oxide side chains of different
polymer chains and metal ions. Such interaction results in a less
soluble polymer network.
The most common driving force for the self-assembly of
conjugated polymers is aromatic p–p interaction.1,2 The
resulting p–p stacking of conjugated polymers has a profound
effect on the electronic and optical properties of the bulk
polymers.3,4 Controlling the self-assembly of conjugated poly-
mers is thus of great importance for optimizing materials
properties and producing novel new materials.3–6 Conjugated
polymers with polar functional groups such as hydroxy or
carboxylic acid moieties may serve this purpose.7,8 The
controllable hydrogen bonding among hydroxy or carboxylic
acid group allows modification of interchain interactions, thus
altering the properties of the polymer. One such example is the
regioregular polythiophenes with pendent carboxylic acid side
chains, which have been demonstrated to be polymer chemo-
selective sensors.8
We are interested in designing hydrogen-bonded shape-
persistent polymer networks. Our ultimate goal is to achieve
stable, nanoporous polymer networks with controllable and
uniform pore sizes. For this purpose, the polar functional groups
have to be anchored in geometrically-fixed positions, rather
than on flexible side chains.7,8 Here we report the synthesis and
optical properties of a water-soluble poly(phenylenevinylene)
(PPV)-based polymer, shown in Scheme 1 as Polymer II. PPV,
the most extensively studied electroluminescent polymer, was
chosen as the polymer backbone.9 The hydrogen-bonding
anchors, isophthalic acids,10 are covalently bonded to the PPV
backbone through oxadiazole rings arranged orthogonally to the
polymer backbone, a structure which assures shape persistence
during self-assembly. The rigid arm and the polymer backbone
are electronically cross-conjugated to enable the electronic and
optical properties of the polymer backbone to change in
response to the self-assembly process.
Polymer II was synthesized by the approach outlined in
Scheme 1. Starting from triethyl benzene-1,3,5-tricarboxylate
1, oxadiazole derivative 4 was synthesized in three steps. The
two alkoxy groups in 4 were converted to vinyl groups (7) in an
approach previously reported.11 The precursor polymer (Poly-
mer I) was synthesized by the Pd-catalyzed Heck coupling
reaction of 7 with dihalide 8,12 which was synthesized by the
reaction of 2,5-diiodohydroquinone with 3,6,9-trioxadecyl
toluene-p-sulfonate.13
Polymer I is soluble in THF, CHCl3 and CH2Cl2, and
insoluble in DMSO, water and aqueous bases. Its weight-
average molecular weight, measured by gel-permeation chro-
matography (GPC) at 30 °C with polystyrene as the standard, is
91 kDa with a polydispersity of 3.2. The hydrolysis of Polymer
I was performed in THF with KOBut as the base.14 After
hydrolysis, Polymer II shows opposite solubility characteristics
to that of Polymer I. For example, Polymer II is insoluble in
CHCl3, slightly soluble in THF, and soluble in DMSO and
dilute aqueous bases such as NaOH, NH4OH etc. The structures
of both polymers were confirmed by spectroscopic studies and
elemental analysis.
Scheme 1 Reagents and conditions: i, NH2NH2, EtOH, 52%; ii, 3, Et3N,
CHCl3, 87%; iii, POCl3, 78%; iv, BBr3, CH2Cl2, 93%; v, Tf2O, pyridine,
86%; vi, tributylvinyltin, Pd(PPh3)4, 45%; vii, 8, Pd(OAc)2, P(o-Tol)3,
DMF, 90%; viii, KOBut. THF, quant.
Polymer II shows unusual solubility behavior in aqueous
bases. For example, Polymer II is only soluble in aqueous
Chem. Commun., 1999, 1855–1856
1855

Fig. 1 1H NMR spectra of polymers (a) I and (b) II.
Fig. 2 Fluorescence and UV/Vis spectra of polymers I and II in solution. (a)
I in THF, (b) II in 1% NaOH, (c) II in DMSO.
Fig. 1 shows the 1H NMR spectra of both polymers. Before
hydrolysis, Polymer I shows chemical shifts due to ester
moieties at d 4.40 and 1.5. The chemical shifts due to the
ethylene oxide moieties appear between d 3.0 and 3.7 as broad
multiple peaks. The aromatic and vinyl protons are totally
overlapped; only one broad peak at d 8.6 is observed. After
hydrolysis, a new peak at d 13.5, corresponding to the
carboxylic acid protons, appeared in the spectrum of Polymer II.
The chemical shifts due to the ester moieties in Polymer I
disappeared, indicating complete hydrolysis. Attempts to carry
out the hydrolysis in acidic media such as TFA were
unsuccessful.15 It was found that, under acidic conditions, the
conjugated polymer backbone was destroyed, resulting in a
yellow polymer. Apparently, the acidic protons attacked the
vinyl bond in the PPV backbone. The vulnerability of the vinyl
bond to proton attack may be due to the partial polarization
induced by the alternate electron-withdrawing and electron-
donating substitution pattern of the backbones.
Polymer II also possesses good thermal stability. Differential
scanning calorimetry (DSC) measurement and thermal gravi-
metric analysis (TGA) show that Polymer II has a decomposi-
tion temperature above 325 °C. No crosslinking process,
common to the PPV backbone, is observed in the temperature
range of 50–350 °C, indicating that the vinyl bonds on different
polymer chains are well separated.
Fig. 2 shows the UV/Vis and fluorescence spectra of both
polymers. Polymer I has two absorption peaks: ca. 506 nm is the
absorption peak of the PPV backbone, ca. 310 nm can be
assigned to the conjugated oxadiazole unit. After hydrolysis, the
absorption of the PPV backbone is slightly blue-shifted,
especially in aqueous base solutions. The maximum absorption
wavelength in 1% NaOH is 484 nm.
In summary, we have synthesized a novel PPV polymer
containing carboxylic acid functional groups. This polymer
possesses solubility in both organic solvents (DMSO) and
aqueous bases (such as NaOH, NH4OH etc.). The unique
structure of this polymer makes it an attractive system for self-
assembly studies. Its high electron affinity11 and photo-
luminescence properties also warrant further study of this
polymer for light-emitting diode applications, particularly as an
electron-transporting layer in multilayer devices.
This work is supported by the University of Missouri-Kansas
City and the University of Missouri Research Board. We thank
Mr Qing Wang at the University of Chicago for the GPC
measurements.
Notes and references
1 Conjugated Polymers, ed. J. L. Bredas and R. Silbey, Kluwer, The
Netherlands, 1991.
2 T. A. Skotheim, Ed. Handbook of Conducting Polymers I & II, ed. T. A.
Skotheim, Marcel Dekker, New York, 1986.
3 N. C. Greenham and R. H. Friend, Solid State Phys., 1995, 49, 1.
4 T.-A. Chen, X. Wu and R. D. Rieke, J. Am. Chem. Soc., 1995, 117,
233.
5 T. Bjørnholm, D. R. Greve, N. Reitzel, T. Hassenkam, K. Kjaer, P. B.
Howes, N. B. Larsen, J. Bøgelund, M. Jayaraman, P. C. Ewbank and
R. D. McCullough, J. Am. Chem. Soc., 1998, 120, 7643.
6 V. V. Tsukruk, Prog. Polym. Sci., 1997, 22, 247.
7 R. D. McCullough, P. C. Ewbank and R. S. Loewe, J. Am. Chem. Soc.,
1997, 119, 633.
8 I. Benjamin, H. Hong, Y. Avny, D. Davidov and R. Neumann, J. Mater.
Chem., 1998, 8, 919.
9 A. Kraft, A. C. Grimsdale and A. B. Holmes, Angew. Chem., Int. Ed.,
1998, 37, 402.
10 K. Eichhorst-Gerner, A. Stabel, G. Moessner, D. Declerq, S. Valiya-
veettil, V. Enkelmann, K. Müllen and J. P. Rabe, Angew. Chem., Int. Ed.
Engl., 1996, 35, 1492.
11 Z. Peng and J. Zhang, Chem. Mater., 1999, 11, 1138.
12 R. F. Heck, Org. React., 1982, 27, 345.
13 B. Winkler, L. Dai and A. W. H. Mau, Chem. Mater., 1999, 11, 704.
14 F. C. Chang and N. F. Wood, Tetrahedron. Lett., 1964, 2969.
15 D. J. Pesak, J. S. Moore and T. E. Wheat, Macromolecules, 1997, 30,
6467.
Polymer I shows red fluorescence with an emission max-
imum at 615 nm. The photoluminescence quantum efficiency of
Polymer I in dilute THF solutions is 3.7%. The fluorescence of
Polymer II is significantly blue-shifted compared to that of
Polymer I and is dependent on the choice of solvents. When a
dilute DMSO solution of Polymer II was excited at 510 nm, a
maximum emission at 555 nm with a shoulder at around 595 nm
was observed. Polymer II in dilute NaOH solution, however,
has an emission maximum at 593 nm, nearly 40 nm red-shifted
compared to that of its DMSO solutions, and has a photo-
luminescence quantum efficiency of 4.2%.
Communication 9/04822G
1856
Chem. Commun., 1999, 1855–1856