Blue light emitting poly(n-arylcarbazol-2,7-ylene)s

Article abstract of DOI:10.1246/cl.2005.900

(Chemical Equation Presented) N-(4-alkoxyphenyl) substituted poly(carbazol-2,7-ylene)s were synthesized by Ni-catalyzed polycondensations of 2,7-dihalogenocarbazoles. The polymers intensely fluoresced blue in solution and in the solid-film state by irradi

Full text of DOI:10.1246/cl.2005.900

900
Chemistry Letters Vol.34, No.7 (2005)
Blue Light Emitting Poly(N-arylcarbazol-2,7-ylene)s
Masashi Kijima,ꢀ Ryohei Koguchi, and Shota Abe
Institute of Materials Science, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573
(Received March 31, 2005; CL-050430)
N-(4-alkoxyphenyl) substituted poly(carbazol-2,7-ylene)s
X
were synthesized by Ni-catalyzed polycondensations of 2,7-
dihalogenocarbazoles. The polymers intensely fluoresced blue in
solution and in the solid-film state by irradiation of UV light. The
introduction of the aryl group at the N-position of poly(carbazol-
2,7-ylene) enhanced fluorescent quantum efficiency of the poly-
mer, which showed higher performance than N-alkyl substituted
polymers.
X
X
X
+
CuI, K3PO4, CHDA
N
N
H
dodecane, 1,4-dioxane
110 °C
X'
OR
OR
X = Cl, Br
X' = Br, I
1a: X = Cl, R = dodecyl
1b: X = Cl, R = EH
1c: X = Br, R = EH
(EH = 2-ethylhexyl)
A: Ni(cod)2, bpy
THF−DMF, 70 °C
Development of blue light emitting polymers with high per-
formance such as good stability, pure blue color emitting, and
high quantum efficiency is inevitable to be applied in light emit-
ting diodes (PLED) for full color displays.1 Conjugated poly-
mers have a merit to efficiently emit even in the solid thin-film
state, which can simplify the process for fabrication of organic
LEDs by an appropriate manner such as spin coating and ink-
jet printing. Recently, poly(N-alkylcarbazol-2,7-ylene)s have
been considered as a candidate for blue light emitters instead
of fluorene-based polymers.2 Poly(carbazol-2,7-ylene) is regard-
ed as a poly(4,40-biphenylylene) consisting of the strained plan-
ner unit of imino-bridged biphenylylene, which must show a
similar properties to polyfluorene.3 Furthermore, a remarkable
feature of pyrrole-based conjugated polymers is a relatively high
HOMO level around ꢁ5:5 eV,3,4 which might be more advanta-
geous than polyfluorene for hole injection from the indium tin
oxide (ITO) anode in the PLED device. In this paper, we inves-
tigate potential of poly(carbazol-2,7-ylene)s as the blue light
emitter by modifying the polymer appropriately.
n
N
B: Zn, PPh3, NiCl2, bpy
DMAc, 80 °C
2a: R = dodecyl
2b: R = EH
OR
Scheme 1. Synthesis of poly(N-arylcarbazol-2,7-ylene)s 2.
Table 1. Polymerization results of 1
Monomer
(Method)
Yield
/%
d.p.b
a
Polymer
Mn
Mw=Mn
1a (A)
1b (A)
1b (B)
1c (B)
2a
2b
2b
2b
68
77
89
75
6000
6600
7700
4700
1.5
1.2
1.9
1.4
14
18
21
13
aThe number-average and the weight-average molecular
weights (Mn and Mw) were determined by gel permeation
chromatography vs polystyrene standards (THF). bThe de-
gree of polymerization was estimated from Mn.
In our previous investigations of the pyrrole-based poly-
mers,4 photoluminescent (PL) property of polypyrroles could
be largely improved from almost non-emissive to good lumines-
cent by introducing aromatic moieties in the emissive core unit,
which suggested that increase of rigidity and extended ꢀ-conju-
gation at the emissive core enhanced the PL efficiency. By con-
trast, the increase of rigidity on the conjugated polymer back-
bone decreased solubility of the polymer in organic solvent. In
addition, excimeric PL emission sometimes seriously influences
the efficiency and purity of the emitting color. Considering the
above mentioned, phenyl groups with long alkoxy side chains
are introduced into the N-position of poly(carbazol-2,7-ylene).
The synthetic route of poly(N-arylcarbazol-2,7-ylene)s is
summarized in Scheme 1. N-Arylation of 2,7-dichlorocarbazole2
with 1 equiv. p-alkoxybromobenzene was carried out by the
CuI-trans-1,2-cyclohexanediamine (CHDA)-catalyzed method,5
affording monomers in good yields (1a: 95%, 1b: 89%), respec-
tively. In the case of N-arylation of 2,7-dibromocarbazole,6 5
equiv. p-alkoxyiodobenzene was reacted under the same condi-
tions (1c: 50%). The monomers were polymerized by Yamamoto
coupling reactions either with 2.4 equiv. Ni(cod)2 and 2.4 equiv.
2,20-bipyridine (bpy) in THF–DMF at 70 ꢂC for 1.5 days (meth-
od A) or with 2.5 equiv. Zn, 0.6 equiv. PPh3, 0.05 equiv. bpy,
and 0.05 equiv. NiCl2 in N,N-dimethylacetamide at 80 ꢂC for 3
days (method B).2 The results of the polymerizations of mono-
mers 1 are summarized in Table 1.
In all cases, polymers 2 were obtained as a pale-yellow
powdery solid in good yields, respectively. The polymers 2
obtained after purification by reprecipitations were soluble in
usual organic solvents such as THF and CHCl3. They showed
a good thermal stability with a high glass transition temperature
(Tg > 150 ꢂC) and a high softening temperature (Ts > 210 ꢂC).
The chemical structure of 2 was confirmed by NMR.7 All poly-
mers have a good processability to make a thin cast film.
The polymers 2 showed an intense blue fluorescence in
solution and in the solid thin-film state. Optical properties of 2
are summarized in Table 2, and the typical spectra of absorption
and PL both in solution and in the thin solid-film state are shown
in Figure 1.
The polymers 2 showed a UV–vis absorption band due to ꢀ–
ꢀꢀ transition around 300–400 nm. In the case of 2a having the
long dodecyl side chain, the absorption spectrum broadened in
the solid state, and there was a considerable difference of absorp-
tion maximum (ꢁmax) in CHCl3 solution and in the solid state.
The difference might be attributed to interchain interactions.
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.7 (2005)
901
Table 2. Optical properties of the polymers 2
emitting core in poly(carbazol-2,7-ylene) by introducing the aryl
group enhanced quantum efficiency in comparison with poly-
(N-alkylcarbazol-2,7-ylene)s (ꢂ ꢃ 0:8).2 The branched 2-ethyl-
hexyl side chain effectively worked disturbing formation of
excimers. The polymerization of 1 by Zn with the Ni catalyst
(method B) was enough to obtain the polymer with good optical
properties, which had a merit without using the hazardous
Ni(cod)2. Furthermore, preliminary manufacturing of PLED
device with 2 (method A) composing of ITO/PEDOT(PSS)/2/
Ca–Al realized the blue emission the same to the PL spectra in
the solid state at a low turn on voltage (ꢃ3 V) with a maximum
luminance (about 2000 cd/m2) and an efficiency ðꢃcÞ > 0:2 cd/
A at 10 V, which was superior to those of poly(N-alkylcarbazol-
2,7-ylene)s. The bridged triphenylamine structure might affect a
hole transporting ability of 2, which resulted in an appropriate
bipolar valance of the device.
Solution ꢁmax (nm) Thin solid ꢁmax (nm)
Polymer
(method)
ꢂfd
abs.
Emc
abs.
em.
2a (A)a
2b (A)a
2b (B)a
2b (B)b
380
379
384
372
418
420
417
415
418
381
389
373
(441) 467
432
433
0.87
ꢃ1
ꢃ1
428
0.80
aPrepared from 2,7-dichlorocarbazole monomers (1a and 1b).
bPrepared from 2,7-dibromocarbazole 1c. cThe polymer concen-
tration was about 10ꢁ6 mol/dm3. dFluorescence quantum yields
(ꢂf) were determined in CHCl3 against 9,10-diphenylanthracene
in cyclohexane (ꢂf ¼ 0:9) as the standard.
This work was partially supported from Promotion of
Creative Interdisciplinary Materials Science for Novel Functions
by The 21st Century COE Program. We thank to Fujitsu Lab.
Ltd. for making and evaluation of the PLED devices, and the
Chemical Analysis Center, University of Tsukuba, for NMR
spectroscopic and elemental analysis data.
References and Notes
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Figure 1. UV–vis absorption and fluorescence spectra of 2b in
CHCl3 (solid line) and in the solid state (broken line).
¨
2
J.-F. Morin and M. Leclerc, Macromolecules, 34, 4680 (2001);
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By contrast, 2b having the branched 2-ethylhexyl (EH) side
chain showed the absorption ꢁmax, both in the solution and in
the solid state, was around 380 nm (Figure 1). This suggests that
2b has very similar conformations in both states. From the ab-
sorption edge observed in the thin-film samples, HOMO–LUMO
energy gap (Eg) is estimated to be 2.80 eV for 2a and 2.90 eV for
2b, respectively. The HOMO level is also determined by electro-
chemical method,8 being about ꢁ5:8 eV.
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Macromolecules, 35, 8413 (2002); J.-F. Morin, S. Beaupre,
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PL spectra of 2b, which was symmetrical with a vibronic
fine structure to the absorption spectra, showed a maximum
emission (ꢁmax) around 420 nm in CHCl3 and 430 nm in the
solid state (Figure 1). Almost the same and narrow shape of
the emission spectra with slight difference of fluorescence
ꢁmax suggests that the emission is essentially the same both in
solution and in the solid state without formation of excimer. The
degree of polymerization and the terminal residue influence the
optical properties. On the other hand, 2a showed wider spectrum
than 2b in the solid state, and the emission ꢁmax shifted longer in
wavelength (Table 1). The symmetrical Gaussian fitting of the
fluorescence spectrum for 2a suggests that the spectrum is com-
posed of three peaks at 434, 461, 493 (shoulder) nm. The peak
around 430 nm may be formed by the radiative singlet exciton
decay with the Stokes’ shift of 50 nm and the most intense peak
around 465 nm may be attributed to the radiative decay of exci-
mer. The Stokes’ shifts are small within about 40 nm in solution
and 50 nm in the solid state. The fluorescence quantum efficien-
cy of 2 in CHCl3 is almost quantitative.
5
6
7
A. Klapars, J. C. Antilla, X. Huang, and S. L. Buchwald, J. Am.
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¨
2470.
NMR data for 2a:1H NMR in CDCl3; ꢄ (ppm) 0.88 (3H, t,
CH3), 1.27–1.85 (20H, CH2), 4.05 (2H, t, OCH2–), 6.93–
8.13 (10H, carbazole, Ph); 13C NMR in CDCl3, 14.6 (CH3),
23.1, 26.5, 29.8, 29.87, 29.93, 30.11, 30.14, 32.4 (CH2), 68.8
(OCH2–), 109.1, 120.9, 122.5, 128.9, 140.7, 142.9, 158.7
(carbazole), 115.2, 116.1, 130.3, 159.0 (Ph), and for 2b:
1H NMR in CDCl3; ꢄ (ppm) 0.92, 0.99 (6H, CH3), 1.34–1.80
(8H, CH2), 3.95 (2H, OCH2–), 6.95, 7.58 (4H, phenyl), 7.10,
7.49, 8.15 (6H, carbazole); 13C NMR in CDCl3, 11.6, 14.6
(CH3), 23.5, 24.4, 29.6, 31.0, (CH2), 39.9 (CH), 71.3
(OCH2–), 109.1, 116.1, 122.5, 129.1, 130.3, 142.9 (carbazole),
115.2, 120.8, 140.7, 159.2 (Ph).
8
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In conclusions, increase of rigidity at the absorption and
Published on the web (Advance View) May 28, 2005; DOI 10.1246/cl.2005.900