Synthesis of novel star-shaped carbazole-functionalized triazatruxenes

Article abstract of DOI:10.1016/j.tetlet.2006.07.086

The syntheses of novel well-defined star-shaped carbazole-functionalized triazatruxenes, which consist of six carbazole arms linked with a central triazatruxene core consisting of three carbazoles with one phenyl, are presented. Both 9-hexylcarbazole and 9-phenylcarbazole as arms gave high yields. Their chemical formulas were confirmed by means of ¹H NMR, MALDI-TOF mass spectroscopy, and elementary analysis. The fluorescent spectra showed that these compounds exhibit good blue fluorescence. The maximum fluorescence emission wavelengths were between 420 and 430 nm.

Full text of DOI:10.1016/j.tetlet.2006.07.086

Tetrahedron Letters 47 (2006) 7089–7092
Synthesis of novel star-shaped carbazole-functionalized
triazatruxenes
Guo-Liang Feng,a Wen-Yong Lai,b Shun-Jun Jia,* and Wei Huangb,c,d,
*
aKey Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry and Chemical Engineering,
Soochow University, Hengyi Road, Suzhou Industrial Park, Suzhou 215123, People’s Republic of China
bInstitute of Advanced Materials (IAM), Fudan University, 220 Handan Road, Shanghai 200433, People’s Republic of China
cInstitute of Advanced Materials (IAM), Nanjing University, 22 Hankou Road, Nanjing 210093, People’s Republic of China
dFaculty of Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Republic of Singapore
Received 17 May 2006; revised 17 July 2006; accepted 18 July 2006
Available online 10 August 2006
Abstract—The syntheses of novel well-defined star-shaped carbazole-functionalized triazatruxenes, which consist of six carbazole
arms linked with a central triazatruxene core consisting of three carbazoles with one phenyl, are presented. Both 9-hexylcarbazole
1
and 9-phenylcarbazole as arms gave high yields. Their chemical formulas were confirmed by means of H NMR, MALDI-TOF
mass spectroscopy, and elementary analysis. The fluorescent spectra showed that these compounds exhibit good blue fluorescence.
The maximum fluorescence emission wavelengths were between 420 and 430 nm.
ꢀ 2006 Elsevier Ltd. All rights reserved.
1. Introduction
new electrical, optical, and morphological properties.
Presently star-shaped conjugated architectures have at-
tracted increasing attention as possible alternatives to
linear conjugated oligomers. Fluorene or thiophene
based truxene derivatives have been reported,5 with only
three-arms linked with the central truxene core. We
recently described methods for the preparation of
oligo-fluorene triazatruxenes.6 In this letter, we describe
the synthesis and characterization of novel star-shaped
materials in which six short carbazole chains are
connected to a central rigid triazatruxene core. In this
structure, the triazatruxene core and chains are all
During the past few years, the synthesis and application
of carbazole derivatives has been of great interest for
chemists and materials scientists due to their intrinsic
photophysical and redox properties, i.e., they exhibit
relatively intense luminescence and undergo reversible
oxidation processes making them suitable as hole carri-
ers.1 Carbazole derivatives are also widely used as build-
ing blocks for potential organic semiconductors,2
especially for organic light-emitting diodes.3 Despite
the huge amount of work already invested in these var-
ious applications, it is clear that further progress in these
fields require intense research efforts focused on the
design and synthesis of new compounds with electro-
chemical, optical, and electronic properties specifically
tailored for each type of application.
Br
N
Br
N
Br
Br
H
N
C6H13
N
H
a
C6H13
Star-shaped oligomers with a central core have proven
to be very useful in field-effect transistors, light-emitting
diodes, and photovoltaic cells.4 Star molecules consist-
ing of linear molecular arms with a central core provide
Br
Br
Br
Br
N
N
H
Br
C6H13
Br
Br
Br
T0
*
Corresponding authors. Tel.: +86 512 6588 0086; fax: +86 512 6588
0089 (S.-J.J.); tel.: +86 21 5566 4188; fax: +86 21 6565 5123
(W.H.); e-mail addresses: shunjun@suda.edu.cn; wei-huang@fudan.
edu.cn; chehw@nus.edu.sg
Scheme 1. Synthesis of hexabromo-trihexyl-triazatruxene (T0).
Reagents and conditions: (a) hexabromo-triazatruxene:1-bromohex-
ane = 1:3.3, KOH, tetrabutyl ammonium bromide, DMSO, 100 ꢁC.
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.07.086

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G.-L. Feng et al. / Tetrahedron Letters 47 (2006) 7089–7092
carbazoles. We consider such structures will have highly
efficient hole transport ability, together with high
thermal and electrochemical stability.
Carbazole arms were directly attached to the central tri-
azatruxene core. For this purpose, at first, we synthe-
sized triazatruxene according to the literature6 and the
C6H13
N
C6H13
N
C6H13
N
c
d
H
N
b
OH
Br
B
HO
1
C1
N
e
N
N
f
g
OH
Br
B
HO
2
C2
Scheme 2. Synthesis of carbazole-3-boronic acids C1–C2. Reagents and conditions: (b) carbazole:1-bromohexane = 1:1.1, KOH, tetrabutyl
ammonium bromide, DMSO, 100 ꢁC; (c) 9-hexyl-9H-carbazole:NBS = 1:1, DMF, dark, 0 ꢁC; (d) n-BuLi (1.2 equiv), THF, À78 ꢁC, (i-PrO)3B
(1.5 equiv), H2O/HCl; (e) carbazole:1-iodobenzene = 1:1, K2CO3, CuI, 1,10-phenanthroline (0.1 equiv), p-xylene, 180 ꢁC; (f) 9-phenyl-9H-
carbazole:NBS = 1:1, DMF, 0 ꢁC; (g) n-BuLi (1.2 equiv), THF, À78 ꢁC, (i-PrO)3B (1.5 equiv), H2O/HCl.
N
C6H13
N
C6H13
cat. Pd(PPh3)4
2M K2CO3-H2O/toluene
70°C
, 72 h
C6H13
N
N
C6H13
C1
C6H13
N
C6H13
N
N
C6H13
Br
Br
N
N
C6H13
C6H13
T1
C6H13
N
N
C6H13
Br
Br
Br
N
N
N
C6H13
Br
cat. Pd(PPh3)4
C6H13
N
2M K2CO3-H2O/toluene
N
C6H13
70°C , 72 h
N
C2
N
N
C6H13
N
N
T2
Scheme 3. Suzuki cross-coupling route to T1 and T2 triazatruxene-carbazole oligomers.

G.-L. Feng et al. / Tetrahedron Letters 47 (2006) 7089–7092
7091
Table 1. Optical absorption and emission in CH2Cl2 solution and in
the solid state
hexabromo-triazatruxene core with hexyl terminal
substituents, exploiting procedures similar to those
described for alkylation of carbazole in the synthesis
of 9-hexylcarbazole (Scheme 1).
Compound
kabs,max (nm)
kem,max
kem,max
(nm, solution)
(nm, solid)
T1
T2
240, 295, 350
248, 299, 350
422
422
428
428
In general, we used a repetitive procedure for the con-
version of bromo derivatives 1–2 into the corresponding
boronic acids C1–C2 via lithiathion with n-BuLi,
followed by quenching with triisopropyl borate and
hydrolysis under acidic conditions (Scheme 2).
For products T1–T2,7 the carbazole moiety was intro-
duced by C–C double-bond connection at its 3-position
through a Suzuki cross-coupling of its boronate with
hexabromo-triazatruxene derivative T0 using Pd(PPh3)4
as the catalyst (Scheme 3). We also found that 9-hexyl-
carbazole and 9-phenylcarbazole gave similar high
yields in syntheses of T1 (53.2% yield) and T2 (48.2%
yield), however, the dissolution of T1 was better than
that of T2.
It is known that arylboronic acids form dimers through
intermolecular hydrogen bonding or infinite networks in
the case of polyfunctional boronic acids. As a result, OH
1
protons are often ‘invisible’ in their H NMR spectra.
Moreover, they easily lose water, yielding the corre-
sponding tricyclic anhydrides boroxines. Because the
presence of water in the Suzuki cross-coupling reaction
results in the hydrolysis of boronates back to the acids,
the reactivity of both boronic acids and boronates are
similar. In our case, arylboronic acids are protected with
pinacol and then purified by flash chromatography on
silica gel to give the pure products.
2. Optical properties
The optical properties of (T1–T2) were investigated by
UV–vis absorption spectroscopy and photolumines-
cence spectroscopy in dichloromethane solution and in
the solid state, respectively (Fig. 1). The spectral data
are listed in Table 1.
2.5
The UV–vis spectra of T1 and T2 are shown in Figure 1.
The absorption spectra of these carbazole derivatives
exhibit the characteristic absorption peaks of carbazole
unit between 240 and 350 nm. The absorption maximum
of T2 shows a small red-shift compared with that of T1.
As shown in Figure 1, the emission spectra of T1 and T2
in dichloromethane solution (1 · 10À5 M) or in the solid
state are nearly identical. T1 or T2 has a red-shift of
6 nm in the solid state compared with that in dichloro-
methane solution. This implies that interaction between
T1 or T2 molecules exists in the ground state.
T1
T2
2.0
1.5
1.0
0.5
0.0
In summary, we have synthesized two novel star-shaped
molecules with a triazatruxene core. By using N-alkyl-
ated and N-phenylated carbazole units as side arms of
the novel molecules, we obtained materials with high
solubility in common organic solvents. Moreover, their
absorption and photoluminescence in CH2Cl2 solution
were studied. They have different emitting maximum
fluorescence wavelengths ranging from 420 to 430 nm
which may be served as good blue light-emitting materi-
als. The application of two novel star-shaped molecules
with a triazatruxene core is under way in our laboratory.
200
300
400
500
Wavelength (nm)
1.0
T1 (solution)
T2 (solution)
T1 (solid)
0.8
0.6
0.4
0.2
0.0
T2 (solid)
Acknowledgments
This work was partially supported by the National Nat-
ural Science Foundation of China (Grants 60325412,
90406021, 50428303, 20172039, and 20472062), the
Shanghai Commission of Science and Technology
(Grants 03DZ11016 and 04XD14002), and the Shanghai
Commission of Education under Grant 03SG03. The
work was also supported by a research grant from the
Innovation Project for graduate students of Jiangsu
Province, the Key Laboratory of Organic Synthesis of
400
450
500
550
Wavelength (nm)
Figure 1. The UV–vis absorption and photoluminescence emission
spectra of T1–T2 in CH2Cl2 (ca. 10À5 mol/L) and in the solid state.

7092
G.-L. Feng et al. / Tetrahedron Letters 47 (2006) 7089–7092
Kwong, H. L.; Lee, S. T. Chem. Mater. 2005, 17, 1208–
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7. General procedure: To a degassed (N2) solution of hexa-
bromo-trihexyltriazatruxene T0 (1.00 g, 0.93 mmol) and
Pd(PPh3)4 catalyst (0.45 g, 0.39 mmol) in toluene (30 mL), a
solution of carbazole-boronic acid (8.37 mmol) in toluene
(20 mL) and 2 M aqueous K2CO3 solution (10 mL) were
added via syringe. The reaction mixture was stirred at 70 ꢁC
for 72 h. After cooling, the product was extracted with
DCM, washed with water, and dried over MgSO4. The
solvent was evaporated, affording the crude mixture.
Following column chromatography on silica gel eluting
with PE–DCM (5:1), pure compound T1–2 was obtained.
Tris(9-hexylcarbazol-3-yl)triazatruxene (T1): yield (53.2%).
Mp: 153–155 ꢁC. 1H NMR (400 MHz, CDCl3): d (ppm)
8.55 (s, 3H); 8.27 (s, 6H); 8.05 (t, J = 7.8 Hz, 6H); 7.82 (s,
3H); 7.46–7.33 (m, 18H); 7.21–7.14 (m, 12H); 5.12 (t,
J = 7.6 Hz, 6H); 4.26–4.20 (m, 12H); 2.28–2.20 (m, 6H);
1.85–1.80 (m, 12H); 1.35–1.22 (m, 54H); 0.84 (t, J = 7.4 Hz,
18H); 0.73–0.69 (m, 9H). MALDI-TOF MS (m/z): calcd for
C
150H165N9: 2092; found: 2092. (M+), 2008 ([MÀC6H12]+).
Anal. calcd for C150H165N9: C, 86.04; H, 7.94; N, 6.02.
Found: C, 85.98; H, 7.77; N, 6.14. Tris(9-phenylcarbazol-3-
1
yl)triazatruxene (T2): yield (48.2%). Mp: 219.5–221 ꢁC. H
NMR (400 MHz, CDCl3): d (ppm) 8.54 (s, 6H); 8.27 (s,
6H); 8.08 (t, J = 8.0 Hz, 6H); 7.80 (s, 3H); 7.63–7.50 (m,
21H); 7.42–7.32 (m, 21H); 7.26–7.21 (m, 15H); 5.3–5.07 (m,
6H); 2.34–2.23 (m, 6H); 1.56–1.03 (m, 18H); 0.88–0.78 (m,
9H). MALDI-TOF mass (m/z): calcd for C150H117N9:
2043.94; found: 2044 (M+), 1960 ([MÀC6H12]+). Anal.
calcd for C150H117N9: C, 88.07; H, 5.77; N, 6.16; found: C,
87.98; H, 5.69; N, 6.29.
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