Facile synthesis of novel monodisperse linear 3,9-linked oligocarbazoles

Article abstract of DOI:10.1016/j.tetlet.2005.08.012

Novel monodisperse linear 3,9-linked oligocarbazoles (OCAs) were prepared stepwise using Ullmann coupling reaction in seal-tubes. The resulting OCAs were soluble in common organic solvents. The UV-vis spectra of OCAs exhibited small red shift and their intensities increased linearly with the increase of the carbazole moieties, suggesting that no intramolecular π-interactions appeared at ≤8-mer. All the OCAs gave strong fluorescence and it was found that the conjugated degree of linear OCAs would be saturated when the number of carbazole units reaches four.

Full text of DOI:10.1016/j.tetlet.2005.08.012

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 6883–6886
Facile synthesis of novel monodisperse linear 3,9-linked
oligocarbazoles
Tinghua Xu, Ran Lu,* Ming Jin, Xianping Qiu, Pengchong Xue,
Chunyan Bao and Yingying Zhao
College of Chemistry, Jilin University, 2519 Jiefang Road, Changchun 130023, PR China
Received 16 June 2005; revised 1 August 2005; accepted 2 August 2005
Available online 18 August 2005
Abstract—Novel monodisperse linear 3,9-linked oligocarbazoles (OCAs) were prepared stepwise using Ullmann coupling reaction in
seal-tubes. The resulting OCAs were soluble in common organic solvents. The UV–vis spectra of OCAs exhibited small red shift and
their intensities increased linearly with the increase of the carbazole moieties, suggesting that no intramolecular p-interactions
appeared at 68-mer. All the OCAs gave strong fluorescence and it was found that the conjugated degree of linear OCAs would
be saturated when the number of carbazole units reaches four.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Monodisperse, well-defined p-conjugated oligomers
have recently attracted much attention due to their po-
tential applications in electronic or photonic devices,1
such as organic light-emitting diodes (OLEDs),2 solar
cells,3 field-effect transistors (FETs),4 etc. They are
especially, ideal models for the evaluation of structure–
functionality relationship of macromolecules.1b,2c,5–10
Nowadays, many oligomers with special properties have
been synthesized such as oligo(benzyl ether)s,5 oligofluo-
renes,6 oligo(p-phenylene)s,7 oligothiophenes,8 oligo-
naphthalenes,9 oligo(m-aniline)s,10 etc.
but it is difficult to handle and the catalyst as well as
the ligand are expensive.12 Herein, we report that the
Ullmann coupling reactions take place in seal-tubes
in which the solvents with lower boiling point can be
selected as reaction medium and the airtight system
saturated with N2 can avoid the oxidative degradation,
thereby facilitating the operations. In this letter, we pres-
ent a repetitive synthetic strategy for preparation of a
series of linear OCAs linked through 3,9-positions, in
which the formation of C–N bond will be a benefit for
the electronic energy transition between carbazole units.
It was found that these OCAs showed strong fluores-
cence emission.
The design and synthesis of carbazole-based oligomers is
also attractive because of their applications in photo-
conductors, charge-transporting and emitting materials
in OLEDs. Among them, most carbazole units are cou-
pled via 2,7-positions or 3,6-positions.11 It was well
known that the oligocarbazoles (OCAs) may show dif-
ferent conformations if the carbazole units are linked
at different positions, leading to their unique photoelec-
tron properties.11a,g Up to now, no monodisperse linear
OCAs linked via 3,9-positions have been reported. As it
is known that N-arylation of carbazole can also be real-
ized by Pd-catalyzed reactions under milder conditions,
The synthetic routes for the desired OCAs are illustrated
in Scheme 1. They were prepared stepwise employing
Ullmann coupling reactions. First, 3-iodo-9-tosylcar-
bazole was prepared according to the literature,11d,13
which was coupled with carbazole to yield compound
2 via Ullmann coupling. Dicarbazole 4 was obtained
from 3, which was the deprotected product of 2. Com-
pound 5 is key for us to extend the carbazole units of
the linear OCAs, which was synthesized from compound
2 through iodination. In the beginning, we would like to
couple 3-bromocarbazole14 with 3-iodo-9-tosylcarbazole
using 1,10-phenanthroline, CuCl as catalyst to synthe-
size compound 5.15 But the results showed that the selec-
tivity for the coupling reaction between C–Br and C–I
bond to carbazole was poor, no pure compound 5
was obtained. Cuprous-catalyzed Ullmann coupling
Keywords: Carbazoles; Oligomers; Ullmann coupling; Iodination; Seal-
tube; Fluorescence.
*
Corresponding author. Tel.: +86 431 8499179; fax: +86 431
8923907; e-mail: luran@mail.jlu.edu.cn
0040-4039/$ - see front matter ꢀ 2005Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.08.012

6884
T. Xu et al. / Tetrahedron Letters 46 (2005) 6883–6886
d
NH
N Bn
92%
1
3-iodo-9-tosylcarbazole
52%
a
N
d
NH
N
b
Bn
Ts
N
N
N
43%
67%
3
4
2
76%
c
c
N
I
N
I
b
d
Ts
Bn
N
N
54%
5
a
6
3
38%
b
N
N
NH
N
d
Bn
N
N
N
N
46%
7
a
8
5
35%
b
N
N
N
d
N
NH
N
Bn
N
N
N
N
N
N
N
35%
10
9
a 22%
6
N
N
N
N
Bn
N
N
N
11
Scheme 1. Synthesis of OCAs. Reagents and conditions: (a) Cu2O, DMAc, 170 ꢁC, 24 h; (b) KOH, DMSO, THF, H2O, reflux, 4 h; (c) KI, KIO3,
AcOH, 80 ꢁC, 4 h; (d) (i) NaH (60%), DMF, 0 ꢁC, 20 min, (ii) benzyl chloride, rt, 1 h.
reactions were then employed to achieve the growth of
OCAs due to its convenience to use and high yields.
Compound 7 was readily prepared via cross-coupling
of 3 and 5, followed by a cleavage with DMSO, THF,
and KOH/H2O.11c,d Repetitive cross-coupling and
deprotection reactions finally gave octamer 11, a mono-
disperse well-defined linear OCAs. It was found that the
reactivities of N–H in carbazole moieties would decrease
upon the increase of the number of carbazoles in
OCAs.11 For example, the Ullmann coupling reaction
between carbazole and 3-iodo-9-tosylcarbazole could
be carried out by employing Cu2O, DMAc at 160 ꢁC
for 24 h, but the coupling reaction of compounds 6
and 9 did not occur under this condition even after
2 days. When the temperature increased to 200 ꢁC, the
reaction did take place within 24 h as we hoped. All
compounds (from compounds 2 to 11) were character-
1
ized strictly with H NMR spectroscopy and MALDI/
TOF-MS measurements. They have good solubility in
common solvents, including THF, dichloromethane,
chloroform, toluene, ethyl acetate, etc.
The UV–vis spectra of monomer 1, dimer 4, tetramer 8,
hexamer 10, and octamer 11 in THF are shown in Fig-
ure 1. We observed that the absorption bands of OCAs
exhibited a small red shift and their intensities increased
linearly with the increase of the carbazole moieties com-
pared with that of 1. We deduced this phenomenon to
the large torsion angle between the cabazoles, just as
the structure of binaphthalene derivatives. The molar

T. Xu et al. / Tetrahedron Letters 46 (2005) 6883–6886
6885
2.5
2.0
1.5
1.0
0.5
0.0
600
Monomer
Dimer
Tetramer
Hexamer
Octamer
Monomer
500
Dimer
Tetramer
Hexamer
Octamer
400
300
200
100
0
250
300
350
400
350
400
450
500
Wavelength (nm)
Wavelength (nm)
Figure 1. UV–vis absorption spectra for OCAs in THF solution.
Figure 3. Emission spectra of OCAs in THF solution (excited at
290 nm).
extinction coefficient at the absorption maximum
(243 nm; 1 · 10À5 M) increased linearly depending on
the oligomer length from monomer to octamer, while
no deviations from the slope were observed at 68-mer
(Fig. 2). In this concentration, the intermolecular associ-
ation was negligible because each oligomer obeyed
BeerÕs law, so that if there were any deviations from
the slope, we can deduce the presence of intramolecular
p-interactions. Therefore, no intramolecular p-interac-
tions occurred for the OCAs at 68-mer. Figure 3 shows
the photoluminescence (PL) spectra of 9-benzylcarb-
azole and the linear OCAs in dilute solution
(1 · 10À5 M). We found that all the compounds gave
strong fluorescent peaks, and the emission bands for
OCAs red-shifted significantly from that of monomer
1. For example, 9-benzylcarbazole showed two isolated
emission bands at 347 and 363 nm, while OCAs gave
asymmetric single peaks around 380 nm except for
dicarbazole 4, whose emission was at ca. 375nm with
a shoulder, suggesting that the conjugated degree of
OCAs would be enlarged with the increase of carbazole
moieties until the number of carbazoles reached four.16
The fluorescent quantum yields are listed in Table 1.17
It shows that the fluorescent efficiency decreases rapidly
from monomer, to dimer, and up to tetramer OCAs,
where the fluorescent efficiency appears to reach a limit.
Table 1. Fluorescence quantum yields (UF) of OCAs in THFa
Samples Monomer Dimer Tetramer Hexamer Octamer
1
4
8
10
11
UF
0.49
0.250.24
0.14
0.24
a The fluorescence quantum yields were determined against quinine
sulfate in 0.1 N H2SO4 (UF = 0.546) as the standard.
synthesis of oligocarbazoles was the use of a repeating
unit 5, which was prepared from 2 with iodination. In
the seal-tube, the Ullmann coupling reaction took place
using Cu2O as a catalyst in DMAc, which can be easily
removed afterwards. Besides, seal-tube can also avoid
the byproducts from oxidation and make the operations
easy to handle. All the OCAs gave strong fluorescence
and we found that the conjugated degree would be satu-
rated when the number of carbazole units reaches four
and no intramolecular p-interactions occurred for the
OCAs at 68-mer. They were soluble in common organic
solvents.
Acknowledgements
The authors acknowledge the National Natural Science
Foundation of China (NNSFC) for financial support of
this work.
We have established a facile synthetic strategy for a
series of novel monodisperse well-defined linear oligo-
carbazoles linked through 3,9-positions. The key to the
Supplementary data
2.5x105
2.0x105
1.5x105
1.0x105
5.0x104
Experimental procedures, characterizations of the key
compounds, and computer generated 3D model of com-
pound 3 are available. Supplementary data associated
with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2005.08.012.
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Figure 2. Plotting between the molar extinction coefficient (at 243 nm)
and the degree of oligomerization for OCAs.

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T. Xu et al. / Tetrahedron Letters 46 (2005) 6883–6886
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