Synthesis and characterization of N-carbazole end-capped oligofluorenes

Article abstract of DOI:10.1016/j.tetlet.2006.11.007

A series of N-carbazole end-capped oligofluorenes (CF_n, n = 1-3) were synthesized. The 9-position of the carbazole moiety was attached to the terminal ends of the oligofluorene cores using an Ullmann coupling reaction. These molecules exhibit red shifts in absorption and photoluminescence spectra with respect to the number of fluorene units and excellent electrochemical reversibility. They were found to be potential blue light-emitting or hole-transporting materials for organic light-emitting diodes (OLEDs).

Full text of DOI:10.1016/j.tetlet.2006.11.007

Tetrahedron Letters 48 (2007) 89–93
Synthesis and characterization of N-carbazole
end-capped oligofluorenes
*
Vinich Promarak, Sayant Saengsuwan, Siriporn Jungsuttiwong,
Taweesak Sudyoadsuk and Tinnagon Keawin
Department of Chemistry, Faculty of Science, Ubon Ratchathani University, Warinchumrap, Ubon Ratchathani 34190, Thailand
Received 21 August 2006; revised 23 October 2006; accepted 1 November 2006
Available online 21 November 2006
n
Abstract—A series of N-carbazole end-capped oligofluorenes (CF , n = 1–3) were synthesized. The 9-position of the carbazole moiety
was attached to the terminal ends of the oligofluorene cores using an Ullmann coupling reaction. These molecules exhibit red shifts in
absorption and photoluminescence spectra with respect to the number of fluorene units and excellent electrochemical reversibility.
They were found to be potential blue light-emitting or hole-transporting materials for organic light-emitting diodes (OLEDs).
Ó 2006 Elsevier Ltd. All rights reserved.
A series of well-defined monodispersed oligofluorenes
have recently attracted much attention as non-defect
structural materials for electronic devices and as impor-
cations.13 Moreover, the thermal stability and glassy
state durability of these organic molecules were found
to be significantly improved upon incorporation of a
carbazole moiety in the structure. To the best of our
knowledge, there is no report on N-carbazole-end
capped oligofluorenes.
1
1
4
tant models to understand the fundamental properties
2
of polyfluorenes (PF), which are promising blue light-
emitting materials for organic light-emitting diodes
3
(
OLEDs). The optical, thermal, and electrochemical
properties of oligofluorenes have known to be changed
by varying the substitution patterns at the 9-positions
of the fluorenyl ring, by introduction of specific func-
tional groups on the terminal ends of molecules or by
incorporating oligofluorenes into different molecular
In this letter, we report the synthesis of oligofluorenes
(n = 1–3) bearing carbazole units at both ends and
investigate their basic physical properties.
The synthetic strategy to the target N-carbazole end-
4
5
architectures. Introduction of aryl pendant groups,
capped oligofluorenes (CF , n = 1–3) is outlined in
n
6
15
chiral alkyl substituents or a spiro-configured struc-
Scheme 1. 2,7-Dibromofluorene 1 was alkylated at
7
ture to the 9-positions significantly improved the mor-
the C-9 position with n-hexylbromide followed by cou-
pling of the resultant bromide 2 with an excess of carb-
azole under Ullmann coupling conditions in the presence
of Cu-bronze as catalyst and K CO as a base to give
phology and thermal stability of the oligofluorenes,
while the electronic properties were retained. More
recently, oligofluorenes end-capped with triarylamine-
carbazole dendrons with very high thermal and
electrochemical stabilities were reported.8 Due to its
unique optical, electrical, and chemical properties, car-
bazole has been widely employed as a functional build-
ing block or substituent in the construction of organic
2
3
the desired compound CF as a white solid in quantita-
1
,9
tive yield. Under similar conditions but with only
1 equiv of carbazole, the intermediate 7-bromo-2-carb-
azole-9,9-bis-n-hexylfluorene 3 was isolated in 45% yield
1
as a colorless solid. The H NMR spectrum of mono-
1
0
11
photoconductors, non-linear optic (NLO) materials,
hole-transport and light-emitting materials for OLED
adduct 3 showed a characteristic doublet signal for 4-H
and 5-H of the carbazole at d 8.19 ppm (J = 7.8 Hz)
1
2
0
0
devices and host materials for phosphorescence appli-
and the non-equivalent 4 -H and 5 -H of fluorene were
observed as two doublets at d 7.89 ppm (J = 7.8 Hz)
and d 7.65 ppm (J = 8.4 Hz). Coupling of the resulting
Keywords: Oligofluorenes; Carbazole; Ullmann coupling; Organic
intermediate
3
with 9,9-bis-n-hexylfluorene-2,7-di-
light-emitting diode.
*
Corresponding author. Tel.: +66 1 5930005; fax: +66 45 288379;
e-mail: pvinich@sci.ubu.ac.th
boronic acid 4 under Suzuki cross-coupling reaction
conditions using Pd(PPh ) as a catalyst and Na CO
3
4
2
3
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.11.007

90
V. Promarak et al. / Tetrahedron Letters 48 (2007) 89–93
carbazole, Cu, K CO
2
3
Br
Br
N
N
PhNO , reflux; 98%
2
X
X
R
R
1
2
X = H
C H13Br, 50%NaOH(aq)
PTC, DMSO, 93%
CF1
6
X = R
carbazole (1 equiv.), Cu, K CO
2
3
PhNO , reflux; 45%
2
Pd(PPh ) , Na CO , H O,
3
4
2
3
2
THF, reflux; 60%
N
Br
N
N
HO
OH
R
R
R
R
CF3
3
B
B
3
HO
OH
R
R
4
NiCl H O, Zn
Br2, I2, CH2Cl2
81%
2
. 2
Br
H
H
Br
Br
PPh , DMF; 23%
3
X
X
R
R
2
R
R
2
5
6
X = H
7
8
C H13Br, 50%NaOH(aq)
PTC, DMSO, 89%
6
X = R
carbazole, Cu
K CO , PhNO
2
N
N
R = n-hexyl
2
3
R
R
2
reflux; 67%
CF2
n
Scheme 1. Synthetic route to the target molecules CF (n = 1–3).
as a base in THF afforded the target CF as a light yellow
3
solid in 60% yield. 2-Bromofluorene was bis-alkylated at
the C-9 position with n-hexylbromide and subsequently
converted to a dimer via a Ni(0)-catalyzed homo-
coupling reaction with NiCl ÆH O, Zn powder, and
2
2
PPh as catalysts in DMF to give the fluorene dimer 7
3
in low yield. Selective bromination of the 2- and 7-posi-
tions using Br followed by coupling with excess carb-
2
azole under similar Ullmann coupling conditions give
the target CF2 as a white solid in reasonable yield.
Compounds CF (n = 1–3) were fully characterized by
n
1
6
standard spectroscopic methods.
In order to understand the geometries and electronic
properties of compounds CF (n = 1–3), quantum chem-
n
ical calculations were performed using the BLYP/
3
Figure 1. HOMO (bottom) and LUMO (top) of model CF .
*
*
6
-31G method. As illustrated in Figure 1, the carbazole
moieties attached to both ends of the molecule are
nearly perpendicular to the plane of the oligofluorene
moiety. Therefore, p-electron delocalization between
those units will be negligible. However, in their HOMO
and LUMO orbitals the p-electrons are able to delocal-
ize over the whole oligofluorene backbone and
end-capped carbazole moieties through the lone electron
pair of the nitrogen of the carbazole.
of CF (n = 1–3) normalized at 294 nm are dominated
n
by two major absorption bands, the less intense band
*
around 294 nm being assigned to the p–p local excita-
tion of carbazole moieties at the terminal ends and the
strong absorption band at longer wavelength corre-
*
sponding to the p–p electron transfer of the entire con-
jugated backbone. The intensities and absorption
maxima of the latter band progressively increased as
the number of the fluorene units per molecule increased.
This result can be explained in terms of the decreasing
HOMO–LUMO energy gap as delocalization of the
The optical properties of the N-carbazole-capped oligo-
fluorenes CFn (n = 1–3) in chloroform are shown in
Figure 2 and Table 1. The UV–vis absorption spectra

V. Promarak et al. / Tetrahedron Letters 48 (2007) 89–93
91
The electrochemical properties of these newly synthe-
sized end-capped oligomers are shown in Figure 3 and
CF3
CF2
summarized in Table 1. Compounds CF (n = 1–3) exhi-
n
CF1
bit two, three, and four chemically reversible oxidation
processes, respectively. The first oxidation process can
be attributed to the removal of electrons from the
peripheral carbazole moieties and other reversible oxi-
dation processes corresponding to the removal of elec-
trons from the interior oligofluorene backbone. During
the successive oxidation cycles of all compounds a slight
shift of the CV curves was observed indicating a weak
oxidative coupling at the 3,6-positions of the peripheral
carbazole moieties (Fig. 4). This is usually detected in
most carbazole derivatives with open 3,6-positions.
250
300
350
400
450
500
The first oxidation potential of CF (n = 1–3) is not sig-
n
Wavelength (nm)
nificantly affected by the number of fluorene units within
the core as the oxidation occurs only at the end-capped
carbazole units. Moreover, these oxidation potential
values are slightly smaller than that of carbazole
Figure 2. UV–vis absorption and photoluminescence spectra of CF
n = 1–3) measured in dilute CHCl solution.
n
(
3
p-electron system along the backbone is extended. In
comparison to the absorption maxima of their corre-
sponding oligofluorenes, the absorption maxima of
CFn (n = 1–3) were about 8–20 nm red-shifted. This
indicates that there is p-conjugation through the lone
electron pair at the nitrogen atom of the carbazole units
and that the p-electrons are delocalized over the entire
conjugated backbone as indicated by the quantum
chemical calculation results. The photoluminescence
-
-
-
6
4
2
0
2
4
6
CF3
CF2
CF1
(
PL) spectra of CF (n = 1–3) exhibited strong emission
n
in the blue region with the emission maxima being grad-
ually red-shifted with respect to an increase of the p-con-
jugation length of the fluorescent cores as observed in
the absorption spectra. Upon excitation either at
3
50 nm corresponding to the absorption of the oligoflu-
orene cores or at 294 nm due to the carbazole units, the
resulting emission spectra obtained were identical. This
suggests that energy or exciton can efficiently transfer
from the peripheral carbazole via the lone electron pair
of the nitrogen atom to the oligofluorene backbone.
800
1200
1600
2000
+
Potential vs Ag/Ag (mV)
Figure 3. CV curves of CF (n = 1–3) measured in CH Cl at a scan
rate of 100 mV/s.
n
2
2
n
Table 1. Summary of the physical measurements of CF (n = 1–3)
a
b
c
d
e
d
f
g
Compound
k
abs (nm)
k
lum (nm)
E
g
(eV)
E
1/2/DE (V)
T
g
/T
m
/T
(°C)
HOMO (eV)
LUMO (eV)
CF
1
2
340
361
3.45
0.51/0.31
.85/0.33
60/118/350
5.28
1.83
0
CF
349
391
3.20
3.09
0.46/0.25
69/196/425
5.28
2.08
2.23
0
1
.81/0.33
.21/0.27
CF
3
360
414
0.45/0.22
71/NA/427
5.32
0
0
1
.79/0.28
.93/0.23
.17/0.26
a
b
c
Measured in dilute chloroform solution.
Excited at the absorption maxima.
Estimated from the onset of absorption (E = 1240/konset).
g
d
Measured using a glassy carbon electrode as a working electrode, a platinum rod as a counter electrode, and SCE as a reference electrode in CH
2 2
Cl
containing 0.1 M tetrabutylammonium hexafluorophosphate as a supporting electrolyte at a scan rate of 100 mV/s. All the potentials were
+
calibrated with ferrocence, E1/2(F
c
/F
c
) = 0.47 V (DE = 0.16 V) versus SCE.
e
f
Obtained from DSC measurements with a heating rate of 10 °C/min under N
2
.
onset 17
Calculated using the empirical equation: HOMO ¼ ð4:44 þ E
Þ.
ox
g
Calculated from LUMO = HOMO ꢀ E
g
.

92
V. Promarak et al. / Tetrahedron Letters 48 (2007) 89–93
Acknowledgements
6
4
2
0
2
4
6
8
This research was supported by the National Metal and
Materials Technology Centre (MTEC) of Thailand
Grant MT-S-46-POL-24-249-G). We also thank Chu-
labhorn Research Institute (CRI) of Thailand for
HRMS measurements.
(
-
-
-
-
st
References and notes
1
2
3
cycle
nd
rd
cycle
cycle
1. Yasuda, T.; Fujita, K.; Tsutsui, T.; Geng, Y.; Culligan, S.
W.; Chen, S. H. Chem. Mater. 2005, 17, 264–268.
. Lee, S. H.; Tsutsui, T. Thin Solid Films 2000, 363, 76–
0.
. Kreyenschmidt, M.; Klaerner, G.; Fuhrer, T.; Ashenhurst,
J.; Karg, S.; Chen, W. D.; Lee, V. Y.; Scott, J. C.; Miller,
R. D. Macromolecules 1998, 31, 1099–1103.
2
3
0.8
1.0
1.2
1.4
1.6
1.8
8
+
Potential vs Ag/Ag (V)
1 2 2
Figure 4. The successive CV curves of CF measured in CH Cl at a
scan rate of 100 mV/s.
4
5
. Zhou, X. H.; Yan, J.-C.; Pei, J. Org. Lett. 2003, 5, 3543–
3
546.
. Wong, K. T.; Chien, Y. Y.; Chen, R. T.; Wang, C. F.; Lin,
Y. T.; Chiang, H. H.; Hsieh, P. Y.; Wu, C. C.; Chou, C.
H.; Su, Y. O.; Lee, G. H.; Peng, S. M. J. Am. Chem. Soc.
2002, 124, 11576–11577.
(
E1/2 = 1.09 V), indicating that the incorporation of car-
bazole at the terminal ends makes the resulting mole-
cules more susceptible to electrochemical oxidation.
The HOMO and LUMO energy levels were estimated
from the absorption onset and the onset oxidation
potential and are listed in Table 1. The results reveal that
with the incorporation of dicarbazole end-caps, the
HOMO energy level of the oligofluorenes moves up to
5.32 eV (relative to the vacuum level). Such a high-
lying HOMO energy level greatly reduces the energy
barrier for hole injection from indium thin oxide (ITO)
6. Geng, Y.; Trajkovska, A.; Katsis, D.; Ou, J. J.; Culligan,
S. W.; Chen, S. H. J. Am. Chem. Soc. 2002, 124, 8337–
8
347.
7
. Katsis, D.; Geng, Y. H.; Ou, J. J.; Culligan, S. W.;
Trajkovska, A.; Chen, S. H.; Rothberg, L. J. Chem.
Mater. 2002, 14, 1332–1339.
. Li, Z. H.; Wong, M. S. Org. Lett. 2006, 8, 1499–1502.
. Zhang, Q.; Chen, J. S.; Cheng, Y. X.; Geng, Y. H.; Wang,
L. X.; Ma, D. G.; Jing, X. B.; Wang, F. S. Synth. Met.
2005, 152, 229–232.
ꢁ
8
9
(
/ = 4.80 eV) to the emissive Alq3 (/ = 5.80 eV).
Accordingly, compounds CF (n = 1–3) can also be used
as hole transport and injection materials for double-
layer OLEDs.
10. Grazulevicius, J. V.; Strohriegl, P.; Pielichowski, J.;
n
Pielichowski, K. Prog. Polym. Sci. 2003, 28, 1297–1353.
1
1
1
1. Kotler, Z.; Segal, J.; Sigalov, M.; Ben-Asuly, A.; Khodor-
kovsky, V. Synth. Met. 2000, 115, 269–273.
2. Kundu, P.; Thomas, K. R. J.; Lin, J. T.; Tao, Y.-T.;
Chien, C.-H. Adv. Funct. Mater. 2003, 13, 445–451.
3. Van Dijken, A.; Bastiaansen, J. J. A. M.; Kiggen, N. M.
M.; Langeveld, B. M. W.; Rothe, C.; Monkman, A.; Bach,
I.; Stossel, P.; Brunner, K. J. Am. Chem. Soc. 2004, 126,
7718–7727.
Thermal analysis reveals that the oligomers CF (n =
n
1
–3) are thermally stable with the onset of decomposi-
tion temperatures in the range of 350–427 °C under
nitrogen. Differential scanning calorimetry (DSC) mea-
surements show that both compounds CF and CF
1
2
are semi-crystalline. Their glass transition temperatures
14. Koene, B. E.; Loy, D. E.; Thompson, M. E. Chem. Mater.
1998, 10, 2235–2250.
15. Lee, J. K.; Klaerner, G.; Miller, R. D. Chem. Mater. 1997,
(
T ) range from 60 to 69 °C (Table 1). Compound CF
g
3
behaves totally differently. Only the glass transition at
about 71 °C can be seen on repeated DSC heating cycles
with no crystallization or melting peaks observed. The
1
1, 1083–1088.
1
6. Characterization data for compounds, CF ; IR (KBr)
1 1
1
ꢀ
045, 2926, 1597, 1450, 1230, and 748 cm ; H NMR
3
ability of CF to form a molecular glass and the possibil-
3
(
(
(
300 MHz, CDCl ) d 0.83 (6H, t, J = 6.9 Hz), 0.90–0.94
4H, m), 1.18–1.28 (12H, m), 2.04–2.10 (4H, m), 7.30–7.37
3
ity to prepare thin films from CF both by evaporation
3
and by solution casting are highly desirable for applica-
tions in OLEDs.
4H, m), 7.44–7.52 (8H, m), 7.61–7.65 (4H, m), 8.00 (2H,
13
d, J = 8.7 Hz), and 8.20 (4H, d, J = 7.7 Hz); C NMR
(
75 MHz, CDCl ) d 14.0, 22.5, 24.1, 29.6, 31.6, 40.2, 55.7,
3
In conclusion, we have demonstrated an efficient syn-
thetic method for the preparation of a series of N-carb-
azole end-capped oligofluorenes. Carbazole was attached
at the C-9 position to both ends of the oligofluorenes via
Ullmann coupling. The absorption and emission spectra
were gradually red shifted when the number of fluorene
units was increased. The oligomers exhibit excellent elec-
trochemical reversibility with the first oxidation poten-
tial being essentially unaffected by the chain length
extension. They are suitable blue light-emitting or
hole-transporting materials for electroluminescent
devices.
109.8, 119.9, 120.4, 121.0, 121.9, 123.4, 125.9, 126.0, 136.8,
139.6, 141.0, and 152.9; HRMS-ESI m/z: [MH ] calcd for
C
Compound CF ; IR (KBr) 3057, 2926, 1597, 1463, 1230,
8
(
m), 2.07–2.18 (8H, m), 7.32–7.37 (4H, m), 7.44–7.51 (8H,
m), 7.59–7.61 (4H, m), 7.72 (2H, s), 7.75 (2H, d,
J = 8.1 Hz), 7.90 (2H, d, J = 7.8 Hz), 7.97 (2H, d,
J = 7.4 Hz), and 8.20 (4H, d, J = 8.4 Hz); C NMR
(
+
49
H
49
N
2
665.3890; found, 665.3888.
2
ꢀ
1 1
18, and 749 cm ; H NMR (300 MHz, CDCl
3
) d 0.83
12H, t, J = 7.2 Hz), 0.87–0.92 (8H, m), 1.17–1.22 (24H,
13
75 MHz, CDCl ) d 14.0, 22.6, 23.9, 29.6, 31.5, 40.3, 55.6,
3
109.8, 119.9, 120.2, 120.4, 120.9, 121.5, 121.9, 123.4, 125.9,
125.9, 126.4, 136.4, 139.6, 140.1, 140.7, 151.9, and 152.9;

V. Promarak et al. / Tetrahedron Letters 48 (2007) 89–93
93
+
HRMS-ESI m/z: [MH ] calcd for C74
H
81
N
2
997.6394;
d 14.0, 23.8, 31.5, 40.2, 55.6, 109.8, 119.8, 120.0, 120.1,
found, 997.6409.
120.4, 120.9, 121.5, 121.9, 123.4, 125.8, 125.9, 126.2, 126.3,
Compound CF ; IR (KBr) 3056, 2928, 1597, 1461, 1233,
136.3, 139.5, 140.1, 140.4, 140.9, 141.1, 151.8, 151.9, and
3
ꢀ
1
1
+
8
0
7
7
17, and 746 cm ; H NMR (300 MHz, CDCl
3
) d 0.81–
152.9; HRMS-ESI m/z: [MH ] calcd for C99
H
113
N
2
.89 (30H, m), 1.17 (36H, br), 2.11–2.20 (12H, m), 7.32–
.37 (4H, m), 7.45–7.52 (8H, m), 7.59–7.60 (4H, m), 7.72–
.78 (8H, m), 7.87–1.92 (4H, m), 7.97 (2H, d, J = 8.7 Hz),
1329.8898; found, 1329.8912.
17. Meng, H.; Zheng, J.; Lovinger, A. J.; Wang, B.-C.; Van
Patten, P. G.; Bao, Z. Chem. Mater. 2003, 15, 1778–
1787.
1
3
3
and 8.21 (4H, d, J = 7.6 Hz); C NMR (75 MHz, CDCl )