Synthesis and photophysical properties of triphenylamine-based dendrimers with 1,3,5-triphenylbenzene cores

Article abstract of DOI:10.1016/j.tetlet.2007.06.052

Two new conjugated dendrimers bearing a triphenylamine moiety as dendrons and 1,3,5-triphenylbenzene as a core have been synthesized through a convergent synthetic strategy. These conjugated dendrimers have high fluorescence quantum yields and exhibit similar absorption and emission behaviors in solutions and in solid films, which demonstrate that these dendrimers form good amorphous states.

Full text of DOI:10.1016/j.tetlet.2007.06.052

Tetrahedron Letters 48 (2007) 5877–5881
Synthesis and photophysical properties of triphenylamine-based
dendrimers with 1,3,5-triphenylbenzene cores
Haijian Xia, Jiating He, Ping Peng, Yinhua Zhou, Yaowen Li and Wenjing Tian*
Key Laboratory for Supramolecular Structure and Materials of Ministry of Education, Jilin University, Changchun 130023, China
Received 5 April 2007; revised 9 June 2007; accepted 12 June 2007
Available online 15 June 2007
Abstract—Two new conjugated dendrimers bearing a triphenylamine moiety as dendrons and 1,3,5-triphenylbenzene as a core have
been synthesized through a convergent synthetic strategy. These conjugated dendrimers have high fluorescence quantum yields and
exhibit similar absorption and emission behaviors in solutions and in solid films, which demonstrate that these dendrimers form
good amorphous states.
ꢀ 2007 Elsevier Ltd. All rights reserved.
In the past few decades, p-conjugated polymers have
attracted considerable interest due to their potential
applications in opto-electronic devices, such as organic
light-emitting diodes (OLEDs),1 photovoltaic cells,2
optical power limiting,3 and field-effect transistors.4
However, the aggregation of p-conjugated skeletons
often results in low quantum yields of luminescence.5
Compared with the organic small molecules and
polymers, p-conjugated dendritic systems with large
branching building blocks can exhibit intrinsic two or
three-dimensional architectures, which can overcome
the quenching of luminescence. Furthermore, such large
dendritic structures also efficiently improve the film for-
mation ability. Therefore, they have become one of the
most promising prospects among p-conjugated materi-
als, and have been applied as active materials in elec-
tronic and photoelectronic devices.6
ylamine derivatives are connected by the N–C single
bond and obtained either through palladium-catalyzed
cross-coupling reactions requiring expensive and extre-
mely air-sensitive palladium catalysts or through cop-
per-catalyzed Ullmann condensations usually involving
high temperatures and prolonged reaction times,8 but
few papers about triphenylamine derivatives connected
by the C–C double bond were published.9 In this Letter,
therefore, we report the design, synthesis, characteriza-
tion, and photophysical properties of two representative
conjugated dendrimers having 1,3,5-triphenylbenzene as
the core with three triphenylamine branches connected
by phenylenevinylene units, which have the interesting
photochemical and photophysical properties through
Heck reaction and Horner–Wittig reaction. Heck reac-
tion and Horner–Wittig reaction were extensively uti-
lized to construct the trans double bond in each
phenylenevinylene conjugation unit.9a
Triphenylamine (TPA) derivatives have been widely
investigated for almost two decades because these
compounds have showed excellent thermal and electro-
chemical stability, electron donating ability, and opto-
electronic properties.7 Considerable effort in synthetic
chemistry, in particular by Shirota and co-workers,
has led to the development of many classes of TPA-
based compounds as hole-transporting or electrolumi-
nescent materials.7a,b To our knowledge, most of triphen-
The synthetic routes to the first-generation dendrimer
are shown in Scheme 1. The 1,3,5-tris-(4-methyl-
phenyl)benzene (1) was synthesized in high yield via
tetrachlorosilanemediated cyclotrimerization of 4-
methylacetophenone under solvents of ethanol.10 The
bromination of 1 by employing N-bromosuccinimide
(NBS) in benzene successfully afforded the desired 2.
Compound 3 can be easily prepared from 2 through
the reaction with the triethyl phosphate under N2.11
The first-generation dendrimer TPB-TPA3 (4) was syn-
thesized via the typical Wittig– Horner reaction between
aldehyde 5 and the core tris(phosphonate) 3 in dry tetra-
hydrofuran, using potassium tert-butoxide as base.12
Keywords:
Triphenylamine;
Phenylenevinyl;
Dendrimers;
Fluorescence.
*
Corresponding author. Tel.: +86 431 85166212; fax: +86 431
85193421; e-mail: wjtian@mail.jlu.edu.cn
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.06.052

5878
H. Xia et al. / Tetrahedron Letters 48 (2007) 5877–5881
CH2Br
NBS, BPO, C H
6
SiCl CH CH OH,
6
4
,
3
2
reflux, 3 h
RT, 24 h
O
BrH2
C
CH2Br
1
2
58%
84%
N
PO(OC2H5)2
N
CHO ,t -BuOK
THF,RT, 12 h
P(OC H ) ,N
2
2
5 3
reflux, 24 h
(C2H5O)2OP
PO(OC2H5)2
N
N
3
90%
73%
4TPB-TPA3
Scheme 1. Synthesis of the first-generation dendrimer 4.
The synthetic routes to the second-generation dendrimer
are shown in Scheme 2. Iodination of 5 with KI/KIO3 in
AcOH at 85 ꢁC provided compound 6 in a yield of
95%,13 an important intermediate for the synthesis of
larger triphenylamine dendrons. N,N-Diphenyl-4-vinyl-
benzenamine 7 was prepared with 5 by the Wittig reac-
tion with methyltriphenylphosphonium bromide in a
good yield.14 Triphenylamine dendron 8 was obtained
by the Heck reaction15 between 6 and 7. The synthetic
method of the second-generation dendrimer TPB-
TPA9 (9) was similar to that of TPB-TPA3.12
Dendrimers are soluble in common organic solvents
such as chloroform, dichloromethane, 1,2-dichloroeth-
I
N
KI (4/3 equiv), KIO3 (2/3 equiv)
AcOH, 85oC, 5 h
N
CHO
6
Pd(OAc)2, K3PO4, DMAc
140 oC, 24 h
95%
N
CHO
I
N
CHO
Ph3 PCH3Br, t-BuOK
THF,RT, 2 h
N
5
N
8
55%
7
76%
N
N
N
3, t-BuOK, THF
RT, 12 h
N
N
N
N
N
N
9 TPB-TPA9 56%
Scheme 2. Synthesis of the second-generation dendrimer 9.

H. Xia et al. / Tetrahedron Letters 48 (2007) 5877–5881
5879
ane, and THF, which are helpful for separation and
1.0
0.8
0.6
0.4
0.2
0.0
1.0
purification. All intermediates and final products were
characterized by FT-IR, 1H NMR spectroscopy, 13C
NMR spectroscopy, and MALDI-TOF mass spectro-
metry. The 1H NMR spectra (CDCl3) of TPB-TPA3
and TPB-TPA9 showed sharp and well resolved signals
at room temperature. The coupling constant
(J ꢀ 16.5 Hz) of olefinic protons in the TPB-TPA3 and
TPB-TPA9 indicates that Heck reaction and Wittig–
Horner reaction afforded the pure all-trans isomers.
The pure all-trans isomers of TPB-TPA3 and TPB-
TPA9 were further confirmed by the characterization
of vibration band of trans double bond at 962 cmÀ1
and 659 cmÀ1 in FT-IR spectra, respectively.16
TPB-TPA3
TPB-TPA9
TPB-TPA3
TPB-TPA9
0.8
0.6
0.4
0.2
0.0
300
350
400
450
500
550
600
650
700
Wavelength (nm)
In order to investigate their photophysical properties,
the absorption and the photoluminescent (PL) spectra
of TPB-TPA3 and TPB-TPA9 both in dilute chloroform
solutions and in the solid films were recorded. Figure 1
illustrates the absorption and PL spectra of TPB-TPA3
and TPB-TPA9 in dilute chloroform solutions. For the
UV–vis absorption spectrum in chloroform solutions,
TPB-TPA3 displayed two absorption peaks at 304 nm
and 384 nm, which were ascribed to the absorption of
the triphenylamine moiety and p–p* transition, respec-
tively. It was also observed that TPB-TPA9 displayed
two absorption peaks at 305 nm and 411 nm, which
exhibited behaviors similar to that of TPB-TPA3. None-
theless, the intensity of the peak at about 304 nm of
TPB-TPA9 decreased in comparison with that of TPB-
TPA3, because the relative number of triphenylamine
in TPB-TPA9 was reduced. For the emission spectra
in dilute chloroform solutions, TPB-TPA3 and TPB-
TPA9 also displayed similar behaviors. The emission
peaks of compounds TPB-TPA3 and TPB-TPA9 are
located at 456 nm and 490 nm, respectively. The emis-
sion peak of TPB-TPA9 was red shifted 34 nm from that
of TPB-TPA3, which showed that there was obvious p–
p* delocalization with the increase of the generation of
dendrimers. These results also demonstrated that the
effective conjugation length significantly improved with
the increasing generation of the dendrimers.17 The fluo-
Figure 2. Normalized absorption (left) and fluorescence spectra (right)
of novel conjugated dendrimers in the solid films. Emission spectra
were obtained upon excitation at the absorption maximum.
rescence quantum yields (uf) of TPB-TPA3 and TPB-
TPA9 in dilute chloroform solutions were measured to
be 100% by using quinine sulfate as a standard. The
fluorescence life time of TPB-TPA3 (0.38 ns) is longer
than that of TPB-TPA9 (0.16 ns) due to the stronger
core–core interactions in TPB-TPA3 because of the
absence of the second dendron.18
Figure 2 outlines the absorption and emission spectra of
TPB-TPA3 and TPB-TPA9 in the solid films. The
absorption behaviors of TPB-TPA3 and TPB-TPA9 in
the solid films were quite similar to those in solutions.
The absorption maximum of TPB-TPA3 and TPB-
TPA9 in films were red shifted in relation to those in
solutions, due to the aggregation effect, with the degree
of red shift depending on the generation number of the
dendrimers. The absorption maximum of TPB-TPA3
and TPB-TPA9 in films, for example, were red shifted
by 3 nm and 1 nm, respectively, or in other words, the
higher the generation, the smaller the red shift in the
absorption.19 These results indicated the absence of
aggregation in the dendrimers, because of large branches
and lots of the noncoplanarity of triphenylamine. There
is no significant ordering of the dendrimers either from
aggregation or crystallization, and hence the dendrimer
films are in a good amorphous state,20 which is very
important for the application of materials in optical
and electronic devices such as OLEDs. The emission
peaks of the first-generation dendrimer TPB-TPA3
and the second-generation dendrimer TPB-TPA9 in
the solid films were red shifted 10 nm and 1 nm from
those in solutions, respectively. The results indicated
that the emission peak of the higher-generation dendri-
mer had a smaller shift than that of the lower one, indic-
ative to a certain extent of a site isolation or dendron
dilution effect, reducing the extent or possibility of
aggregation of the triphenylbenzene core. The films were
fabricated by spin casting onto quartz plates from their
solutions in chloroform. The quantum efficiencies of the
films are 17.2% and 5.2% for TPB-TPA3 and TPB-
TPA9 relative to 100% in solutions, respectively. The
much-reduced fluorescence quantum yields in films
compared to solutions result from the intermolecular
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
TPB-TPA3
TPB-TPA9
TPB-TPA3
TPB-TPA9
300
400
500
600
700
Wavelength (nm)
Figure 1. Normalized absorption (left) and fluorescence spectra (right)
of novel conjugated dendrimers in dilute chloroform solution
(1 · 10À6 M). Emission spectra were obtained upon excitation at the
absorption maximum.

5880
H. Xia et al. / Tetrahedron Letters 48 (2007) 5877–5881
Table 1. Photophysical properties of TPB-TPA3 and TPB-TPA9
s/nsa sol
uf sol (%)
kabs/nm films
kem/nm films
ufl (%) films
b
c
kabs/nm sol
kem/nm sol
TPB-TPA3
TPB-TPA9
384
411
456
490
0.38
0.16
100
100
387
412
465
491
17.2
5.2
a Double-exponential decay.
b Quinine sulfate was used as a standard (kexc = 365 nm; uf = 0.546 in H2O).
c Determined by using an integrating sphere.
6. Schryver, F. C.; Vosch, T.; Cotlet, M.; Auweraer, M. V.;
Mllen, K.; Hofkens, J. Acc. Chem. Res. 2005, 38, 514.
7. (a) Shirota, Y. J. Mater. Chem. 2000, 10, 1; (b) Shirota, Y.
J. Mater. Chem. 2005, 15, 75; (c) Thelakkat, M. Macro-
mol. Mater. Eng. 2002, 287, 442.
interactions such as aggregates, which can be caused by
the relatively dense packing of dendrimer units in films.
The photophysical properties of TPB-TPA3 and TPB-
TPA9 are summarized in Table 1.
8. (a) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-
Razzaq, F.; Lee, H. E.; Adachi, C.; Burrows, P. E.;
Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2001,
123, 4304; (b) Kuwabara, Y.; Ogawa, H.; Inada, H.;
Noma, N.; Shirota, Y. Adv. Mater. 1994, 6, 677; (c)
Sonntag, M.; Kreger, K.; Hanft, D.; Strohriegl, P.;
Setayesh, S.; de Leeuw, D. Chem. Mater. 2005, 17, 3031.
9. (a) Wei, P.; Bi, X.; Wu, Z.; Xu, Z. Org. Lett. 2005, 7, 3199;
(b) Li, J.; Liu, D.; Li, Y.; Lee, C.-S.; Kwong, H.-L.; Lee,
S.-T. Chem. Mater. 2005, 17, 1208.
In conclusion, we have synthesized two new conjugated
dendrimers bearing a triphenylamine moiety as den-
drons and 1,3,5-triphenylbenzene as a core through a
convergent synthetic strategy. These conjugated dendri-
mers exhibit similar absorption and emission behaviors
in solutions and in solid films, which demonstrate that
these dendrimers form amorphous states. They also
have high fluorescence quantum yields, which indicate
that these dendrimers are candidates for the application
in OLED as light emitting materials. We are currently
investigating the electroluminescent properties of these
dendrimers.
10. (a) Elmorsy, S. S.; Pelter, A.; Smith, K. Tetrahedron Lett.
1991, 32, 4175; (b) Plater, M. J. Synth. Lett. 1993, 6, 405;
(c) Cherioux, F.; Guyard, L. Adv. Funct. Mater. 2001, 11,
305.
11. He, Q.; Huang, H.; Yang, J.; Lin, H.; Bai, F. J. Mater.
Chem. 2003, 13, 1085.
12. The general procedure used for the Wittig–Horner reac-
tion: 546 mg (2.0 mmol) of 4-(diphenylamino)benzalde-
hyde and 378 mg (0.5 mmol) of 3 were dissolved in 20 ml
of dry THF. The resulting solution was added dropwise
slowly to 280 mg (2.5 mmol) of t-BuOK in 20 ml of dry
THF at 0 ꢁC, then the reaction mixture was warmed to
room temperature and stirred under N2 overnight. The
mixture was poured into water and extracted with
dichloromethane. The organic phase was washed with
water, brine, and dried over MgSO4. After removing the
solvent, the product was purified by column chromatog-
raphy using dichloromethane/petroleum ether (1/4) gave 5
(406 mg, 73%) as a yellow solid.
Acknowledgments
This work was supported by the State Key Development
Program for Basic Research of China (Grant No.
2002CB613401), the National Natural Science Founda-
tion of China (Grant No. 20474023, 50673035), the Pro-
gram for Changjiang Scholars and Innovative Research
Team in University (Grant No. IRT0422), the 111 Pro-
ject (Grant No. B06009), the Research Project of Jilin
Province (Grant No. 20050504, 20060702) and the Re-
search Project of Changchun City (Grant No. 06GH03).
1H NMR (500 MHz CDCl3) d 7.02–7.08 (m, 15H, Ar),
7.12–7.15 (m, 15H, Ar), 7.25–7.28 (m, 12H, Ar), 7.42 (d,
J = 8.5 Hz, 6H, Ar), 7.61 (d, J = 8.0 Hz, 6H, Ar), 7.70 (d,
J = 8.5 Hz, 6H, Ar), 7.81 (s, 3H, Ar) 13C NMR (75 MHz
CDCl3) 123.06, 123.53, 124.51, 126.40, 126.79, 127.39,
127.54, 128.39, 129.28, 131.39, 137.01, 139.84, 141.93,
147.42, 147.50. MALDI/TOF MS: Calcd for C84H63N3:
1113.5, Found: 1114.3. Anal. Calcd for C84H63N3: C,
90.53; H, 5.70; N, 3.77. Found: C, 90.31; H, 5.82; N, 3.58.
The same procedure was used to prepare the dendrimer
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2007.06.052.
1
References and notes
TPB-TPA9: Compound 9: H NMR (500 MHz CDCl3) d
6.96–7.05 (m, 36H, Ar), 7.08–7.15 (m, 48H, Ar), 7.24–7.29
(m, 30H, Ar), 7.36–7.40 (m, 18H, Ar), 7.43 (d, J = 8.0 Hz,
6H, Ar), 7.60 (d, J = 8.0 Hz, 6H, Ar), 7.69 (d, J = 7.5 Hz,
6H, Ar), 7.80 (s, 3H, Ar) 13C NMR (75 MHz CDCl3)
122.95, 123.64, 124.04, 124.33, 124.41, 124.63, 126.40,
126.67, 126.84, 126.90, 127.16, 127.22, 127.48, 127.56,
128.32, 129.25, 131.73, 131.84, 132.51, 136.95, 139.86,
141.94, 146.33, 146.68, 147.10, 147.54. MALDI/TOF MS:
Calcd for C204H153N9 2728.2, Found: 2729.4. Anal. Calcd
for C204H153N9: C, 89.74; H, 5.65; N, 4.62. Found: C,
89.54; H, 5.81; N, 4.48.
1. (a) Jenekhe, S. A. Adv. Mater. 1995, 7, 309; (b) Grimsdale,
A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37, 402;
(c) Miller, J. S. Adv. Mater. 1993, 5, 671; (d) Carroll, R. L.;
Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4378.
2. Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J.
Science 1995, 270, 1789.
3. Bhawalkar, J. D.; He, G. S.; Prasad, P. N. Rep. Prog.
Phys. 1996, 59, 1041.
4. Schon, J. H.; Dodabalapur, A.; Kloc, C.; Batlogg, B.
Science 2000, 290, 963.
5. Joswick, M. D.; Cambell, I. H.; Barashkov, N. N.;
Ferraris, J. P. J. Appl. Phys. 1996, 80, 2883.
13. Shirota, Y.; Kobata, T.; Noma, N. Chem. Lett. 1989,
1145.

H. Xia et al. / Tetrahedron Letters 48 (2007) 5877–5881
5881
14. Zhang, X. H.; Choi, S. H.; Choic, D. H.; Ahn, K. H.
Tetrahedron Lett. 2005, 46, 5273.
15. Yao, Q.; Kinney, E. P.; Yang, Z. J. Org. Chem. 2003, 68,
7528.
18. Palsson, L.-O.; Beavington, R.; Frampton, M. J.; Lupton,
J. M.; Magennis, S. W.; Markham, J. P. J.; Pillow, J. N.
G.; Burn, P. L.; Samuel, I. D. W. Macromolecules 2002,
35, 7891.
16. Beavington, R.; Frampton, M. J.; Lupton, J. M.; Burn, P.
L.; Samuel, I. D. W. Adv. Funct. Mater. 2003, 13, 211.
17. Jiang, Y.; Wang, J.; Ma, Y.; Cui, Y.; Zhou, Q.; Pei, J. Org.
Lett. 2006, 8, 4287.
19. Xu, T.; Lu, R.; Qiu, X.; Liu, X.; Xue, P.; Tan, C.; Bao, C.;
Zhao, Y. Eur. J. Org. Chem. 2006, 4014.
20. Paul, G. K.; Mwaura, J.; Argun, A. A.; Taranekar, P.;
Reynolds, J. R. Macromolecules 2006, 39, 7789.