New efficient blue light emitting polymer for light emitting diodes

Article abstract of DOI:10.1039/a905482k

The synthesis, by the Suzuki coupling reaction, of a novel soluble blue light emitting polymer, poly{(9,9-dihexyl-2,7-fluorene-alt-co-[2,5-bis(decyloxy)-1,4-phenylene]} (PDHFDDOP) is reported. PDHFDDOP exhibits photoluminescence (PL quantum efficiency of

Full text of DOI:10.1039/a905482k

New efficient blue light emitting polymer for light emitting diodes
Wang-Lin Yu,ab Jian Pei,ab Yong Cao,bc Wei Huang* and Alan J. Heeger*
a
bc
a
Institute of Materials Research and Engineering, National University of Singapore, Singapore 119260, Republic of
Singapore
Institute for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, CA
b
c
9
3106-5090, USA. E-mail: ajh@physics.ucsb.edu
UNIAX Corporation, 6780 Cortona Drive, Santa Barbara, CA 93117, USA
Received (in Columbia, MO, USA) 6th July 1999, Accepted 9th August 1999
The synthesis, by the Suzuki coupling reaction, of a novel
soluble blue light emitting polymer, poly{(9,9-dihexyl-
2
,7-fluorene–alt-co-[2,5-bis(decyloxy)-1,4-phenylene]}
(
PDHFDDOP) is reported. PDHFDDOP exhibits photo-
luminescence (PL quantum efficiency of 40% in neat films)
and electroluminescence (EL) with deep blue emission.
The design and synthesis of light emitting polymers have
received attention in the last decade following the discovery of
EL in conjugated polymers and the development of polymer
1–4
light emitting diodes (PLEDs) as a display technology.
Although the three primary colors have been demonstrated in
PLEDs, only red (orange) and green PLEDs have sufficient
efficiencies and lifetimes to be of commercial value.3 Thus, to
achieve full color polymer emissive displays, there is a need for
stable blue-emitting conjugated polymers with high photo-
luminescence efficiency.
,4
Scheme 1 Reagents and conditions: i, 1-bromohexane, 50% NaOH aqueous
3 3
solution, TBACl, 80 °C; ii, (1) Mg, THF, (2) B(OMe ) , THF, 278 °C to
room temp., 48 h, (3) H SO4 (5% in ice); iii, Propane-1,3-diol, toluene,
2
The first blue PLED was fabricated from poly(para-
reflux, 3 h; iv, Toluene/aqueous potassium carbonate solution (2 M),
Pd(PPh ) , reflux, 48 h.
3 4
5
phenylene) (PPP), which is not soluble in its conjugated form.
Subsequently, soluble substituted PPPs were synthesized and
high external EL quantum efficiencies were demonstrated.6
However, low molecular weight and poor stability have limited
the application of blue-emitting conjugated polymers. Blue EL
has also been demonstrated in poly(para-phenylene vinylene)
derivatives,7 3,4-disubstituted polythiophenes, copolymers
absorption and PL spectra obtained from the films on quartz are
displayed in Fig. 1. The PL spectrum of poly(9,9-dihexyl-
2
,7-fluorene) (PDHF) is also given in Fig. 1 for comparison.
The PL spectrum of PDHFDDOP has a maximum at 420 nm
with a well defined vibronic feature at 448 nm. The emission
peaks in the blue are very similar to those of PDHF. However,
PDHF also emits at longer wavelengths (486, 534 nm). This
longer wavelength emission is common in 9,9-disubstituted
8
9
,10
with and without non-conjugated segments,
Recently, the 9,9-disubstituted polyfluorenes
have drawn attention as efficient blue-emitting polymers.
and other
structures.1
1,12
13–16
polyfluorenes and has been attributed to interchain excimer
We report here, the synthesis of a new blue light emitting
conjugated polymer and preliminary data which characterize
the emission from this polymer. As depicted in Scheme 1,
poly{(9,9-dihexyl-2,7-fluorene)–alt-co-[2,5-bis(decyloxy)-
formation.1
3,16
By contrast, the PL spectrum of PDHFDDOP
does not include the longer wavelength components. The
absolute PL efficiency of PDHFDDOP in neat films, as
measured in an integrating sphere at room temperature, was
found to be ca. 40%.
1,4-phenylene]} (PDHFDDOP) comprises alternating fluorene
and phenylene units. The long hydrocarbon chains at the
fluorene bridge (‘9’) methylene and on the phenylene ring
ensure the solubility of the polymer. PDHFDDOP was
synthesized through the Suzuki coupling reaction.17 The
monomer 9,9-dihexylfluorene-2,7-bis(trimethylene boronate) I,
was synthesized from 2,7-dibromofluorene a as starting mate-
rial.† The monomer 1,4-dibromo-2,5-bis(decyloxy)benzene II,
was prepared from hydroquinone by reaction with 1-bromo-
decane in a solution of NaOEt in ethanol, followed by
bromination. Polymerization was performed in a refluxing
toluene/aqueous potassium carbonate solution (2 M) containing
Cyclic voltammetry (CV) was performed with a polymer film
on a glassy carbon disc working electrode (ca. 0.2 cm ) using a
platinum wire as counter electrode and a silver wire as a quasi-
reference electrode (electrochemical potential ca. 0.01 V vs.
2
3 4
1.0–1.5 mol% of Pd(PPh ) under vigorous stirring for 48 h.
The polymer was isolated by pouring the reaction mixture into
methanol. After being purified and dried under dynamic
vacuum at room temperature, the polymer was obtained as a
white solid with the yield of ca. 85%. The structure was
confirmed by NMR and elemental analysis.‡
PDHFDDOP readily dissolves in solvents, such as THF,
chloroform, toluene and xylene. Uniform colorless films were
prepared with thicknesses of ca. 750 Å on quartz and ITO
coated glass substrates by spin coating from solution in toluene
Fig. 1 Absorption (2), photoluminescence (-) and electroluminescence
(5) spectra of PDHFDDOP. The photoluminescence spectrum of PDHF
(8) is also given for comparison.
(
3%) at a spin rate of 1500 rpm. Upon exposure to UV radiation,
the films emitted intense blue fluorescence. The UV–VIS
Chem. Commun., 1999, 1837–1838
1837

Fig. 2 Cyclic voltammograms of PDHFDDOP recorded from the first scan
2
1
(
––––) and the tenth scan (– – – –) in a solution of TBAPF
6
(0.10 mol L
)
Fig.
3 Light output and I–V characteristics of a device in the
21
in MeCN at room temp. Scan rate = 40 mV s
.
ITO|PVK|PDHFDDOP|Ca configuration.
SCE). The data are shown in Fig. 2. Both the oxidative and the
reductive reactions are reversible. The onset of reduction occurs
at ca. 21.78 V above which the cathodic current quickly
increases, and a cathodic peak appears at 22.38 V. A
corresponding re-oxidation peak appears at 22.33 V. The n-
afforded 2,7-dibromo-9,9-dihexylfuorene b as white crystals after purifica-
tion by recrystallization from ethanol. 2,7-Dibromo-9,9-dihexylfuorene b
was treated with magnesium (turnings) in THF to form the Grignard
reagent. The Grignard reagent solution was slowly dropped into a sitrred
solution of trimethyl borate in THF at 278 °C. The mixture was stirred at
1/2
doping potential, ERed , is 22.36 V. For oxidation, the onset
potential is ca. 1.22 V and an anodic peak occurs at 1.42 V with
the corresponding re-reduction peak at 1.37 V. The oxidation
2
78 °C for 2 h and then at room temperature for two days. The reaction
mixture was then poured into crushed ice containing sulfuric acid (5%)
while stirring. The mixture was extracted with diethyl ether and the
combined extracts were evaporated to give a white solid. Recrystallization
of the crude product from hexane–acetone (80+20) afforded pure 9,9-dihex-
ylfuorene-2,7-diboronic acid c as a white solid. The diboronic acid c was
then reacted with propane-1,3-diol in toluene under reflux to produce
1/2
potential, EOx , is 1.39 V. The difference between the p- and
n-doping onset potentials is 3.00 V, implying that the p–p*
band gap of the polymer is 3.00 eV; the same as the value
determined from the onset of optical absorption. The HOMO
and LUMO energy levels of the polymer were estimated from
the p- and n-doping onset potentials18 to be 5.66 and 2.62 eV,
respectively. As shown in Fig. 2, after ten scans for the same
polymer film, both the potentials and the current intensities of
the redox peaks remain almost unchanged.
monomer I.
1
‡
7
PDHFDDOP, H NMR (CDCl
.58 (d, 2H), 7.14 (s, 2H), 3.99 (t, 4H), 2.05 (br, 4H), 1.72 (t, 4H), 1.50–0.97
3
, 200 MHz) d 7.80 (d, 2H), 7.74 (s, 2H),
(m, 44H), 0.82 (t, 12H). Anal. Found: C, 84.50; H, 10.59. Calc. for
51 76 2
C H O
: C, 84.94; H, 10.62%.
EL devices with the configuration of ITO|PVK
900 Å)|PDHFDDOP (750 Å)|Ca were fabricated, where PVK
1
J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K.
Mackay, R. H. Friend, P. L. Burn and A. B. Holmes, Nature (London),
(
is poly(N-vinylcarbazole). Because of the low HOMO energy of
PDHFDDOP, 5.66 eV below vacuum there is a large energy
barrier for hole injection from ITO into the polymer layer. Here
we use PVK as hole transporting layer. A typical device emits
visible blue light at ca. 10 V forward bias (ITO wired positive)
1
990, 347, 539.
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1
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2
2
and reaches the brightness of 115 cd m for a bias of 23.8 V.
2
1
This gives an efficiency of 0.34 cd A and a luminosity of
2
1
5 G. Grem, G. Leditzky, B. Ullrich and G. Leising, Adv. Mater., 1992, 4,
0
0
.045 lm W . The maximum external quantum efficiency was
.60%. The current and light output characteristics are given in
3
6.
6
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Fig. 3. The EL spectrum is displayed in Fig. 1. As noted for
other polymer LED devices,11 the HOMO level of PL (6.1 eV
below vacuum) causes a large barrier for hole injection.
Nevertheless, the external quantum efficiency is three times
higher than that of the blue polymer LED device of 9,9-dio-
ctylpolyfluorene, in which a polymeric triphenyldiamine is used
as hole transporting layer and the thickness of the emissive
8
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14
polymer layer has been optimized.
1
1
1
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In summary, a new light emitting polymer, poly{(9,9-
dihexyl-2,7-fluorene)–alt-co-[2,5-bis(decyloxy)-1,4-phenyl-
ene]}, has been synthesized. The polymer emits deep blue light
without longer wavelength components in the emission spec-
trum. Relatively high PL and EL efficiences have been
demonstrated. Improved EL performance, lower operating
voltage and higher efficiency can be expected by further
improving the hole-injection. The results demonstrate that this
new polymer is a promising candidate for blue-emitting
polymer LEDs.
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Notes and references
†
The reaction of 2,7-dibromofluorene with 1-bromohexane in 50% NaOH
aqueous solution in the presence of tetrabutylammonium chloride at 80 °C
Communication 9/05482K
1838
Chem. Commun., 1999, 1837–1838