Synthesis and characterization of starburst 9-phenylcarbazole/triazatruxene hybrids

Article abstract of DOI:10.1246/cl.2008.986

Novel starburst triazatruxenes functionalized with six N-phenylated carbazole units have been synthesized and characterized. Good yields and high purity were readily obtained via an optimized microwave-enhanced six-fold Suzuki coupling methodology. High thermal stability and good amorphous properties as well as deep blue emissions (423-434 nm) were demonstrated. The introduction of six-substituted triazatruxene scaffolds could provide a new guideline for exploring novel solution-processable amorphous materials with dual functions for light-emitting and/or hole-transporting applications. Copyright

Full text of DOI:10.1246/cl.2008.986

986
Chemistry Letters Vol.37, No.9 (2008)
Synthesis and Characterization of Starburst 9-Phenylcarbazole/Triazatruxene Hybrids
Wen-Yong Lai,1;2 Qi-Yuan He,1 Dao-Yong Chen,1 and Wei HuangÃ2
1Institute of Advanced Materials, Fudan University, 220 Handan Road, Shanghai 200433, P. R. China
2Institute of Advanced Materials, Nanjing University of Posts and Telecommunication,
66 XinMoFan Road, Nanjing 21000, P. R. China
(Received May 7, 2008; CL-080465; E-mail: iamwhuang@njupt.edu.cn)
Novel starburst triazatruxenes functionalized with six N-
reaction was used to attach the phenyl bridge onto the nitrogen
atom of carbazole to give the N-phenylcarbazole derivatives 1
and 3 in good yields (Scheme 1). CuI/L-proline proved to be
an effective catalyst system for this reaction. Subsequent treat-
ment of the above bromides with n-BuLi and 2-isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane gave the corresponding
boronic esters 4 or 5 efficiently.
Scheme 2 depicts the synthesis towards target molecules.
An optimized microwave-enhanced multiple Suzuki coupling
methodology6b was used to afford the two designed materials
C1 and C2 with ideal yields (75.8 and 77% respectively) and
high purity. It should be noted that such good yields and high
purity were generally difficult to achieve under traditional
conditions where partially substituted by-products are generally
involved for such complex six-fold couplings and this presented
serious challenges to separate the desired targets from those
partially substituted counterparts.6b
phenylated carbazole units have been synthesized and character-
ized. Good yields and high purity were readily obtained via
an optimized microwave-enhanced six-fold Suzuki coupling
methodology. High thermal stability and good amorphous
properties as well as deep blue emissions (423–434 nm) were
demonstrated. The introduction of six-substituted triazatruxene
scaffolds could provide a new guideline for exploring novel
solution-processable amorphous materials with dual functions
for light-emitting and/or hole-transporting applications.
Carbazole derivatives1 are playing an active role as key
building blocks in the field of organic electronics, i.e. organic
light-emitting diodes (OLEDs),1–3 due to their intrinsic photo-
physical properties, good thermal stability, and hole-transport
properties. They have also been recongnized as potential host
materials for triplet emitters in OLEDs because of their tunable
triplet energies and good carrier transporting characteristics.4
Star-shaped compounds consisting of a central core with ra-
dial linear conjugated arms have shown numerous promising ad-
vantages for thin-film device applications, including truly amor-
phous properties, well-defined size and structure, and high purity
as well as the ease to fine-tune their physical properties via prop-
er choice of the central core and conjugated arms.5,6 Various
kinds of star-shaped materials mainly based on three-substituted
scaffolds with either a benzene, a 1,3,5-triphenylbenzene, a tri-
phenylamino, or a triazine moiety as the core structure have been
developed for light-emitting and/or charge-transporting applica-
tions.5 In particular, carbazole-based star-shaped materials
where carbazole units are used either as rigid core or radial arms
exhibited interesting optoelectronic and redox properties.3,5
Despite this progress, for improved amorphous properties
and morphology stability, it would be interesting to explore more
bulky and complex materials based on carbazole building
blocks, yet it remains a challenge because of the synthetic
difficulties.
C1 shows moderate solubility in THF and CHCl3. With
t-butyl chains on the 3,6-positions of carbazole units, C2 is
readily soluble in common solvents, such as CH2Cl2, CHCl3,
THF, toluene, etc. The structure and purity were confirmed by
spectroscopic methods and elemental analysis. In MALDI-
TOF mass spectrometry, the molecular peaks of C1 and C2 were
ii
i
HN
Br
N
HN
Cl
+
85%
87%
80%
O
iii
3
ii
2
iii
O
Br
N
B
N
B
N
80%
O
O
1
4
5
Scheme 1. Reagents and conditions: (i) ZnCl2, CH3NO2, 5 h
at room temperature. (ii) 1,4-Dibromo benzene, K2CO3, CuI,
L-proline, DMSO, 140 ꢀC. (iii) a) n-BuLi, À78 ꢀC; b) 2-isopro-
poxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, À78 ꢀC to room
temperature.
We have recently developed a series of starburst triazatrux-
enes functionalized with oligofluorenes and have found them
rather attractive for efficient light-emitting applications.6 Here,
we present the synthesis and characterization of a series of
starburst 9-phenylcarbazole–triazatruxene hybrids in which six
carbazole units are introduced to surround the rigid triazatruxene
core via a phenyl bridge. The combination of carbazole periph-
ery and the overlapped three-carbazole-fragment triazatruxene
structure is expected to offer improved thermal and morpholog-
ical stabilities and the dual functions for light-emitting and
charge-transporting applications.
X
X
X
N
Br
X
N
Br
Br
X
R
N
Br
Br
R
N
4 or 5
R
N
N
N
N
R
X
X
N
N
i
N
R
6
R
X
Br
R: n-C6H13
N
X
C1: X = H
C2: X = t-C4H9
X
X
X
Triazatruxene core 6 was synthesized according to the pro-
cedures described in our previous work.6 A modified Ullmann
Scheme 2. Synthesis towards C1 and C2: (i) Microwave heat-
ing, pressurized-vessel, Pd(PPh3)4, THF, 150 ꢀC, 15 min.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.9 (2008)
987
Table 1. Physical properties of C1, C2, and T16a
ꢁabs/nm
(THF)
ꢁem/nm
(THF)
ꢂf
(THF)
ꢁabs/nm
(film)
ꢁPL/nm
(film)
ꢂf
(film)
EHOMO/eV
ELOMO/eV
EgCV/eV
Tg/ꢀC
Td/ꢀC
C1
C2
T1
292.5, 344
297, 345.5
282, 348.5
423
423
429
0.65
0.68
0.72
295.5, 348.5
298.5, 349.5
289.5, 354.5
428
434
438
0.35
0.40
0.38
À5:35
À5:34
À5:16
À2:09
À2:17
À2:08
3.26
3.18
3.08
NA
NA
442.9
438.1
410.3
81.3
stabilities. No distinct thermal processes were observed
during the DSC scanning cycles from 20 to 250 ꢀC for C1 and
C2, while a glass transition (Tg ꢁ 81:3 ꢀC) was determined for
T1 (Figure S6, Supporting Information). TGA study also showed
much higher decomposition temperatures (Td) for C1 and C2
(442.9 and 438.1 ꢀC, respectively), compared to that of T1
(410.3 ꢀC).
Due to the rich carbazole structure along with the electron-
donating properties of triazatruxene,6c C1 and C2 exhibit rela-
tively high HOMO energy levels (À5:35 and À5:34 eV respec-
tively), which match well with the work function of ITO/
PEDOT:PSS (À5:2 eV). These HOMO levels are even higher
than general hole-transporting materials (TPD: À5:5 eV; NPB:
À5:4 eV). Thus, good hole injection and transport ability can
be expected when such materials are used as active materials
in OLEDs.
In conclusion, novel starburst 9-phenylcarbazole/triazatrux-
ene hybrids C1 and C2 were successfully synthesized and
characterized. Deep blue emission (ꢁmax, 423–434 nm) was
obtained both in solution and as films. Good thermal stability
and amorphous properties as well as proper HOMO energy
levels and large band gaps (3.18–3.26 eV) suggest the potential
of these starburst 9-phenylcarbazole/triazatruxene materials
that are featured by rich carbazole units and unique 3D structures
as promising light-emitting and/or hole-transporting materials
as well as host materials for triplet emission.7
UV-solution
PL-solution
UV-film
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
PL-film
C1
C2
UV-solution
PL-solution
UV-film
PL-film
300 350 400 450 500 550 600
Wavelength /nm
Figure 1. Absorption and PL spectra of C1 and C2 in THF and as
films.
found at m=z 2044.7 (calcd MW: 2043.94) and at m=z 2718.4
(calcd MW: 2716.69), respectively, which agreed very well with
the calculated values.
The UV–vis absorption and PL spectra of C1 and C2 in
dilute tetrahydrofuran (THF) solution and in thin films are shown
in Figure 1 and the photophysical data are summarized in
Table 1. The absorption spectra of C1 and C2 were structureless
with absorption peaks at 344 nm for C1 and 345.5 nm for C2 in
THF solution. The absorption maxima at around 345 nm were at-
tributed to a strong ꢀ–ꢀÃ transition. In film state, the absorption
spectra of C1 and C2 remain almost identical with only 3–
4.5 nm red shifts for the absorption maxima compared to those
in dilute THF solution, suggesting suppressed molecular interac-
tions in the ground state owing to the bulky starburst architec-
tures. Although typical absorption peaks of carbazole units at
around 290 nm can be clearly identified, no distinct characteris-
tic absorption peaks of the triazatruxene core (peaks at 315.8,
256.2 nm in THF and peaks at 321 nm in solid state)6c were ob-
served in the absorption spectra of C1 and C2 both in solution
and in the solid state, implying efficient ꢀ-delocalization
through the central triazatruxene core across the 9-phenyl-
carbazole arms. Both in solution and as films, C1 and C2 exhibit
deep-blue fluorescence with an intense single emission peak
(ꢁmax) ranging at 423–434 nm and comparable quantum yields
(ꢂf) relative to those of monofluorene-functionalized triazatrux-
ene T16a (Table 1).
This work was financially supported by the National Natural
Science Foundation of China (Grants Nos. 60325412, 90406021,
and 50428303), the NUPT Foundation (Grant No. NY207040).
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Thermal and morphological properties of C1 and C2 were
investigated by DSC and TGA studies (Table 1). The results
are compared with those of T1 to demonstrate the significance
of incorporating carbazole functionality into the starburst molec-
ular systems on improving the thermal and morphological
7
Supporting Information is available electronically on the CSJ-Journal
Web site, http://www.csj.jp/journals/chem-lett/index.html.
Published on the web (Advance View) August 23, 2008; doi:10.1246/cl.2008.986