Synthesis of a novel cationic water-soluble efficient blue photoluminescent conjugated polymer

Article abstract of DOI:10.1039/b000740o

A novel cationic conjugated polymer, poly[(9,9-dihexyl-2,7-fluorene)- alt-co-(2,5-bis{3-[(N,N-dimethyl)-N-ethylammonium]-1-oxapropyl}-1,4- phenylene)] dibromide, which is water-soluble and emits bright blue fluorescence both in solutions and as films, is synthesized through Suzuki coupling reaction and a post-polymerization treatment.

Full text of DOI:10.1039/b000740o

Synthesis of a novel cationic water-soluble efficient blue photoluminescent
conjugated polymer
Bin Liu,a Wang-Lin Yu,b Yee-Hing Lai*a and Wei Huang*b
a Department of Chemistry, National University of Singapore, Singapore 119260, Republic of Singapore
b Institute of Materials Research and Engineering (IMRE), National University of Singapore, 3 Research Link,
Singapore 117602, Republic of Singapore. E-mail: wei-huang@imre.org.sg
Received (in Cambridge, UK) 24th January 2000, Accepted 23rd February 2000
Published on the Web 17th March 2000
A novel cationic conjugated polymer, poly[(9,9-dihexyl-
2,7-fluorene)-alt-co-(2,5-bis{3-[(N,N-dimethyl)-N-ethylam-
monium]-1-oxapropyl}-1,4-phenylene)] dibromide, which is
water-soluble and emits bright blue fluorescence both in
solutions and as films, is synthesized through Suzuki
coupling reaction and a post-polymerization treatment.
Water-solubility of conjugated polymers may offer many new
application opportunities. Potential applications of water-
soluble conjugated polymers include the construction of active
layers in organic light-emitting diodes through a layer-by-layer
self-assembly approach,1 as buffer layer and emissive layer
materials in inkjet printing fabricated organic LEDs,2 and as
highly sensitive fluorescent sensory materials in living bodies.3
Such applications generally favor high molecular weights and
high photoluminescence (PL) efficiencies and require different
ionic types. Water-solubility of semiconducting conjugated
polymers was first demonstrated in 3-substituted polythio-
phenes4,5 and was then extended to poly(para-phenylene
vinylene) (PPV)-based6 and poly(para-phenylene) (PPP)-based
polymers.7,8 The water-solubility in such polymers was
Scheme 1 Chemical structures and synthetic route towards the polymers.
achieved by functionalizing the substituted side chains with
terminal carboxylate or sulfonate groups. These polymers are,
therefore, anionic polyelectrolytes. Until the most recent report
on the synthesis of ammonium-functionalized PPPs from
Reynolds’ group,9 there are no cationic water-soluble conju-
gated polymers available. Fluorene-based conjugated polymers
have received considerable attention in the past few years for
the high efficiencies both in PL and in electroluminescence
(EL).10 Moreover, we recently demonstrated that conjugated
polymers based on alternating fluorene and phenylene back-
bones are promising efficient and stable blue luminescent
materials.11 This work presents the successful effort in
developing a cationic water-soluble conjugated polymer based
on the alternating fluorene and phenylene backbone, which
represents the first example of fluorene-based water-soluble
conjugated polymer and exhibits efficient blue light emission.
The chemical structure of the new water-soluble conjugated
Reagents and conditions: i, 2-chlorotrimethylamine hydrochloride, anhy-
drous potassium carbonate, acetone, reflux, 3 days; ii, toluene/aqueous
potassium carbonate solution (2 M), Pd(PPh3)4, 85–90 °C, 3 days; iii,
bromoethane, DMSO–THF (1+4), 50 °C, 3 days.
The structures of both the neutral and the final water-soluble
polymers were confirmed by NMR and elemental analysis.†
The characterization of molecular weight is often a problem
for water-soluble conjugated polymers. The post-polymeriza-
tion approach for the realization of water-solubility allows us to
characterize the molecular weight at the stage of the neutral
polymer. The neutral polymer, polymer I, can be readily
dissolved in CHCl3, THF, toluene and aqueous acid, but is
insoluble in DMSO, methanol and water. Gel-permeation
chromatography (GPC) measurement using THF as eluent and
polystyrenes as the standards indicated the weight average
molecular weight to be 47 000, with a polydispersity of 1.61.
Another advantage of the post-polymerization approach is that
the quaternization degree can be controlled and thus the water-
solubility of the resultant polymer is tunable. The tunable
solubility is useful for the application of such materials as buffer
layers in inkjet printing fabrication of LEDs.2 The degree of
polymer,
poly[(9,9-dihexyl-2,7-fluorene)-alt-co-(2,5-bis{3-
[(N,N-dimethyl)-N-ethylammonium]-1-oxapropyl}-1,4-
phenylene)] dibromide (PFP-NMe2EtBr) and the synthetic
route are depicted in Scheme 1. Monomer I, 9,9-dihexyl-
fluorene-2,7-bis(trimethylene boronate), was synthesized from
2,7-dibromofluorene as the starting material.11 Monomer II,
2,5-bis[3-(N,N-dimethylamino)-1-oxapropyl)-1,4-dibromoben-
zene], was prepared from 2,5-dibromohydroquinone by reac-
tion with 2-chloroethyldimethylamine in refluxing acetone in
the presence of an excess of anhydrous potassium carbonate.9
The polymerization was carried out in a mixture of toluene and
aqueous potassium carbonate solution (2 M) containing 1 mol%
Pd(PPh3)4 under vigorous stirring at 85–90 °C for three days.
The neutral polymer, polymer I, was obtained as a fibrous white
solid with a yield of ca. 70% after purification and drying.
Conversion of the neutral polymer to the final water-soluble
polymer was achieved by treatment with bromoethane in
dimethyl sulfoxide (DMSO) and tetrahydrofuran (THF) (1+4).
1
quaternization could be determined by H NMR spectra. As
shown in Fig. 1, the neutral polymer exhibits three peaks in the
region d 7.8–7.6 arising from the aromatic protons in fluorene
and one peak at d 7.15 due to the protons in the phenylene ring.
The well resolved peaks at d 4.09, 2.67 and 2.30 correspond to
methylene groups adjacent to the oxygen (–OCH2–) and
nitrogen (–CH2N–) atoms and the methylamino groups
(–NCH3), respectively. After the treatment with bromoethane,
the peaks in the aromatic region remain almost unchanged,
whereas all the signals corresponding to –OCH2–, –CH2N–, and
–NCH3 split into two peaks, which arise from the quaternarized
(lower field) and un-quaternized components, respectively. The
DOI: 10.1039/b000740o
Chem. Commun., 2000, 551–552
This journal is © The Royal Society of Chemistry 2000
551

Fig. 2. The absorption and emission peaks appear at 359 and 416
nm, respectively.
From the application point of view, one of the most attractive
properties of the polymers is the relatively high PL quantum
yield (Fpl). Both the neutral polymer and the quaternized
polymers display strong blue fluorescence either in solutions or
as films upon exposure to UV light. The Ffl of the neutral
polymer (polymer I) is as high as 97% as measured from its
dilute solution in chloroform.‡ For polymer II (with a degree of
quaternization of 80%), the Fpl was measured to be 86% from
its dilute solution in methanol. When the measurement was
conducted in aqueous solution, the corresponding value of Fpl
is 25%. The decrease of PL efficiency may be attributed to the
aggregation of the polymer in aqueous solution. This was
supported by a further reduced PL efficiency measured in the
solid state (films on quartz plate cast from methanol solution),
which is 4% compared with 9,10-diphenylanthracene as
standard (dispersed in PMMA films with a concentration lower
than 1 3 1023 M, assuming a PL efficiency of 83%).12
In summary, we have synthesized a new cationic water-
soluble conjugated polymer based on the alternating fluorene
and phenylene backbone structure through a facile post-
polymerization approach, which permits a full structural
characterization of the polymer and control of the degree of
cation formation. The polymer emits intense blue fluorescence
both in solutions and in film states. The good water-solubility
and high fluorescence quantum yield make it attractive for
applications in fabricating organic LED devices and as
fluorescent bio-sensory materials.
Fig. 1 1H NMR spectra of polymers I (a) and II (b).
relative integrals of each pair of the split peaks can thus be used
to estimate the degree of quaternization. The highest degree of
quaternization obtained in our experiments is ca. 80%. With
this degree of quaternization, the resulting polymer shows
solubility characteristics opposite to that of polymer I, being
completely soluble in DMSO, methanol, and water but
insoluble in CHCl3 and THF.
Polymer II also possesses good thermal stability. The onset
degradation temperature of this polymer is 300 °C in nitrogen,
whereas it starts to decompose above 230 °C in air, with a small
amount of water loss at lower temperatures. In air, no residue
remained after heating to 800 °C.
The work was partially supported by the National University
of Singapore through a research grant (RP970610).
The UV–VIS absorption spectra of polymer I in chloroform
solution and as a film (on quartz plate, spin-cast from
chloroform solution) are almost identical with the same
maximum absorption at 366 nm. The PL spectrum of the
polymer solution peaks at 414 nm, whereas the polymer film
exhibits an emission maximum at 424 nm with a vibronic
shoulder around 444 nm. The emission spectral feature of the
polymer in the film state is very similar to that of the polymer
having the same backbone structure and substitution on fluorene
unit but without the terminal amino group in the phenylene side
chains.11 This implies that the terminal amino groups are
unlikely to affect the conformation of the backbone in the film
state. For the quaternized sample with the highest degree of
quaternization (ca. 80%), the electronic spectra are remarkably
dependent on the solvent, showing a bathochromic shift with a
decrease in solvent polarity. As displayed in Fig. 2, the polymer
shows absorption maxima at 343, 354 and 367 nm in water,
methanol and DMSO, respectively. The corresponding PL
maxima appear at 409, 409 and 419 nm, respectively. Uniform
and transparent films of the polymer on quartz plates were
prepared by spin-casting its aqueous solution. The UV–VIS
absorption and PL spectra of the polymer film are also shown in
Notes and references
† NMR and elemental analyses data for polymers I and II: polymer I: d (300
MHz, CDCl3) 7.79 (br, 2H, Ar-H), 7.66–7.60 (br, 4H, Ar-H), 7.15 (s, 2H,
Ar-H), 4.09 (br, 4H, –OCH2), 2.67 (br, 4H, –CH2N), 2.27 (s, 12H, NCH3),
2.05 (br, 4H, fluorene 9-H), 1.12–0.78 (br, 22H, –CH2, –CH3). Calc. for
C
39H54O2N2: C, 80.41; H, 9.27; N, 4.81; Br, 0 (terminal group). Found: C,
79.60; H, 8.99; N, 4.90; Br, 0%.
Polymer II: d (300 MHz, CD3OD) 7.93 (br, 2H, Ar-H), 7.70–7.64 (br, 4H,
Ar-H), 7.24 (br, 2H, Ar-H), 4.56 (br, 3.2H, –OCH2), 4.38 (br, 0.8H,
–OCH2), 3.77 (br, 3.2H, –CH2N), 3.52 (br, 0.8H, –CH2N), 3.42 (br, 3.2H,
NCH2CH3), 3.06 (br, 12H, NCH3), 2.87 (br, 4.8H, NCH2CH3), 2.16 (br, 4H,
fluorene 9-H), 1.28–0.80 (br, 22H, –CH2, –CH3). Calc. for C39H54O2
N2·4H2O·1.6C2H5Br (the amounts of H2O and C2H5Br were based on TGA
analysis and 1H NMR): C, 61.10; H, 8.45; Br, 15.46; N, 3.38. Found: C,
60.63; H, 8.29; Br, 16.04; N, 3.52%.
‡ The quantum yields were measured using a Perkin Elmer LS 50B
luminescence spectrometer with dilute solutions (A
< 0.2) at room
temperature.13 Quinine sulfate solution (ca. 1.0 3 1025 M) in 0.10 M
H2SO4 (quantum yield, 55%) was used as a standard.
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Fig. 2 UV–VIS absorption and photoluminescence spectra of polymer II in
solutions and as films. (a) UV in aqueous solution, (b) UV in MeOH, (c) UV
in film, (d) UV in DMSO, (e) PL in MeOH, (f) PL in aqueous solution,
(g) PL as film and (h) PL in DMSO.
Communication b000740o
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Chem. Commun., 2000, 551–552