Synthesis and functional properties of end-dendronized oligo(9,9-diphenyl) fluorenes

Article abstract of DOI:10.1021/ol0604062

A facile approach for the synthesis of a family of dendrimers OFn-EG with hole-transporting ability moieties by copper-catalyzed Buchwald's double-amination as a key reaction has been developed. These novel dendrimers exhibit good energy transfer efficiencies and very high thermal and electrochemical stabilities and have potential applications as hole transfer and emitting layers in the field of organic emitting diodes (OLEDs) or as host materials for electrophosphorescent applications.

Full text of DOI:10.1021/ol0604062

ORGANIC
LETTERS
2006
Vol. 8, No. 7
1499-1502
Synthesis and Functional Properties of
End-Dendronized
Oligo(9,9-diphenyl)fluorenes
Zhong Hui Li†,‡ and Man Shing Wong*,†
Department of Chemistry and Centre for AdVanced Luminescence Materials, Hong
Kong Baptist UniVersity, Hong Kong SAR, China
mswong@hkbu.edu.hk
Received February 16, 2006
ABSTRACT
A facile approach for the synthesis of a family of dendrimers OFn-EG with hole-transporting ability moieties by copper-catalyzed Buchwald’s
double-amination as a key reaction has been developed. These novel dendrimers exhibit good energy transfer efficiencies and very high
thermal and electrochemical stabilities and have potential applications as hole transfer and emitting layers in the field of organic emitting
diodes (OLEDs) or as host materials for electrophosphorescent applications.
Triarylamine-based dendrimers have been intensively used
as promising molecular materials for molecular magnetism
and information storage,1 energy transfer,2 and organic light-
emitting diodes.3 Monodisperse oligofluorenes have recently
attracted much attention because of their chemical and
thermal stabilities and the possibility to vary the properties
of the resulting compounds via the substitution pattern of
the 9-position of the fluorenyl unit, as well as potential
applications for optoelectronic molecular materials.4 Remark-
ably, end-capping polyfluorenes with hole-transporting moi-
eties have also been shown to improve the performance of
polyfluorene-based light-emitting diodes (PLEDs).5 Conju-
gated triarylamine-based dendrimers with redox gradients
have been investigated;1a however, oligofluorenes end-capped
with dendrons comprising triarylamine and carbazole moi-
† Hong Kong Baptist University.
‡ Present address: Department of Chemistry, Sichuan College of
Education, Chengdu, 610041 Sichuan Province, China.
(1) (a) Selby, T. D.; Blackstock, S. C. J. Am. Chem. Soc. 1998, 120,
12155. (b) Selby, T. D.; Blackstock, S. C. Chem. Mater. 2002, 14, 1685.
(c) Louie, J.; Hartwig, J. F.; Fry, A. J. J. Am. Chem. Soc. 1997, 119, 1165.
(2) Ranasinghe, M. I.; Varnavski, O. L.; Pawlas, J.; Hauck, S. I.; Louie,
J.; Hartwig, J. F.; Goodson, T. J. Am. Chem. Soc. 2002, 124, 6520.
(3) (a) Wang, P. W.; Liu, W.; Yu, J.; Devadoss, C.; Bharathi, P.; Moore,
J. S. AdV. Mater. 1996, 8, 237. (b) Freeman, A. W.; Koene, S. C.; Malenfant,
P. R. L.; Thompson, M. E.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2000, 122,
12385. (c) Satoh, N.; Cho, J.-S.; Higuchi, M.; Yamamoto, K. J. Am. Chem.
Soc. 2003, 125, 8104.
(4) (a) Grell, M.; Knoll, W.; Lupo, D.; Meisel, A.; Miteva, T.; Neher,
D.; Nothofer, H. G.; Scherf, U.; Yasuda, A. AdV. Mater. 1999, 11, 671.
(b) Li, Z. H.; Wong, M. S.; Tao, Y.; Lu, J. P. Chem. Eur. J. 2005, 11,
3285. (c) Li, Z. H.; Wong, M. S.; Fukutani, H.; Tao, Y. Chem. Mater.
2005, 17, 5032. (d) Pei, Q.; Yang, Y. J. Am. Chem. Soc. 1996, 118, 7416.
(e) Marsitzky, D.; Klapper, M.; Mu¨llen, K. Macromolecules 1999, 32,
8685.
(5) Miteva, T.; Meisel, A.; Knoll, W.; Nothofer, H. G.; Scherf, U.; Mu¨ller,
D. C.; Meerholz, K.; Yasuda, A.; Neher, D. AdV. Mater. 2001, 13, 565.
10.1021/ol0604062 CCC: $33.50
© 2006 American Chemical Society
Published on Web 03/09/2006

eties may be expected to improve the physical properties of
oligofluorenes; this still remains to be explored.
Scheme 2. Synthesis of End-Dendronized
Oligo(9,9-diphenyl)fluorenes OFn-EG
In this paper, we describe the synthesis and functional
properties of end-dendronized oligofluorenes with a redox
gradient, OFn-EG, n ) 1-3, in which the dendron is
constructed by triarylamine and carbazole moieties and the
core is constructed by oligo(9,9-diphenyl)fluorene. Since the
peripheral triarylamine-based moieties exhibit lower oxida-
tion potentials than those of carbazole units in the interior,
this configuration constructs a redox gradient and facilitates
the migration of electrons from the core to the periphery of
the dendron.
The strategy for preparation of triarylamine- and carbazole-
based dendritic oligofluorenes OFn-EG, n ) 1-3, is outlined
in Schemes 1 and 2. Transformation of the dibromide 16 into
Scheme 1. Synthesis of Diiodo-oligo(9,9-diphenyl)fluorenes
OFn-I
the synthesis of OF2-I. Importantly, the dendron EG-H was
gained by the palladium-catalyzed direct Suzuki cross-
coupling of boronic acid 7 with N-unprotected 3,6-dibro-
mocarbazole in 99% high yield. It should be noted that the
catalytic activity of Pd(PPh3)4 is remarkably higher than that
of Pd(OAc)2/2P(o-tolyl)3 in this coupling reaction. Then,
copper-catalyzed Buchwald double-amination8 of the corre-
sponding diiodide OFn-I, n ) 1-3, with the dendron EG-H
using 1:10 of CuI/1,2-trans-cyclohexadiamine as a catalyst
afforded the target products in a moderate yield.
The decomposition temperatures (Tdec) of these dendrimers
OFn-EGs are about 600 °C, indicating that all of the new
dendrimers have very high thermal stability. This good
stability may be induced by the dendritic wedges EG-H,
which have Tdec up to 565 °C (Table 1). The glass transition
temperatures (Tg) of the newly synthesized dendrimers are
192, 225, and 254 °C, respectively, and gradually increase
as the core chain length increases. The dendron EG-H also
exhibits high Tg up to 124 °C, indicating that an incorporation
of the dendrons EG onto the backbone of oligofluorene is
very important for inducing a high Tg of resulting molecules.
On the other hand, the DSC analyses further confirmed that
all of the OFn-EGs form an extremely stable amorphous
glass because neither melting transitions nor crystallization
transitions were detected after their glass transition temper-
atures.
the corresponding diiodide OF1-I was carried out in 92%
high yield by lithium-bromine exchange at low temperature,
followed by reaction with iodine at room temperature.
Transformation of the dibromide 1 into the corresponding
monobromide 2 was carried out in 81% yield by lithium-
bromine exchange, followed by reaction with trimethylsilyl
chloride. Then, transformation of the monobromide 2 into
the corresponding boronic acid 3 was easily carried out in a
yield of 72% by lithium-bromide exchange, followed by
reaction with trimethyl borate and subsequently acid hy-
drolysis. Suzuki cross-coupling between the boronic acid 3
and monobromide 2 using Pd(OAc)2/2P(o-tolyl)3 as a catalyst
afforded the desired double-trimethylsilyl end-capped bif-
luorene 4 in an excellent yield. Transformation of 4 into
diiodide OF2-I was achieved in a good yield by iodo-
desilylation at 80 °C in the presence of silver trifluoroacetate.7
Diiodide OF3-I was also prepared by a method similar to
In view of electronic absorption spectra, the absorption
bands/maxima of these new dendrimers peaked at 347, 348,
(6) Wong, K.-T.; Wang, Z.-J.; Chien, Y.-Y.; Wang, C.-L. Org. Lett. 2001,
3, 2285.
(7) Wilson, S. R.; Jacob, L. A. J. Org. Chem. 1986, 51, 4833.
(8) (a) Antilla, J. C.; Klapars, A.; Buchwald, S. L. J. Am. Chem. Soc.
2002, 124, 11684. (b) Antilla, J. C.; Baskin, J. M.; Barder, T. E.; Buchwald,
S. L. J. Org. Chem. 2004, 69, 5578.
1500
Org. Lett., Vol. 8, No. 7, 2006

Table 1. Summary of Physical Measurements of OFn-EG, n ) 1-3, and EG-H
f
λabs a/nm (ꢀ‚105/M-1cm-1
)
λem a,b/nm
Φa,c
ETEa,d/%
HOMOe/eV
E1/2e/V
Eg /eV
Tgg/°C
Tdech/°C
max
max
OF1-EG
OF2-EG
OF3-EG
EG-H
347 (1.31)
348 (1.49)
356 (1.66)
324 (0.65)
404
436
436
396
<0.01
0.31
0.38
78
68
69
5.18
5.19
5.19
5.17
0.38, 0.71
0.39, 0.69
0.39, 0.68
0.40, 0.75
2.08
2.17
2.20
1.87
192
225
254
124
596
601
601
565
<0.01
a Measured in CHCl3. b Excited at the absorption maxima. c Using quinine sulfate monohydrate (Φ313 ) 0.48) as a standard. d Energy-transfer efficiency
e
is determined by comparing the absorption maximium of the dendritic wedges of the absorption and fluorescence excitation spectra. E1/2 vs Fc+/Fc estimated
by CV method using platinum disk electrode as a working electrode, platinum wire as a counter electrode, and SCE as a reference electrode with an agar
salt bridge connecting to the oligomer solution. All of the potentials were calibrated with ferrocene, E1/2 (Fc/Fc+) ) 0.45 V vs SCE. f Estimated from the
edge of electronic absorption spectra. g Determined by differential scanning calorimeter from remelt after cooling with a heating rate of 10 °C/min under N2.
h Determined by thermal gravimetric analyzer with a heating rate of 10 °C/min under N2.
and 356 nm, respectively, and were generally structureless
and blue shifted (∼30 nm) relative to the corresponding
diphenylamino end-capped oligofluorenes4b (Figure 1a and
triarylamine to the fluorene core. The energy transfer
efficiency (ETE) of this dendritic wedge is approximately
70% as estimated by comparing the difference in intensity
at the absorption maximum of the dendritic wedges of the
absorption spectrum and the fluorescence excitation spec-
trum. The fluorescence quantum yields (ΦPL) measured in
chloroform using quinine sulfate monohydrate as a standard
are less than 0.01, 0.31, and 0.38, respectively, and increase
as the core chain length increases, indicating the longer
homologue has better coplanarity in the excited state than
the shorter one; however, these ΦPL values are quite smaller
than those (∼0.90) of the corresponding analogues only end-
capped with diphenylamino groups,4b suggesting that intro-
duction of dendrons onto the oligofluorene skeleton perturbs
the planarity and alters the fluorescent nature of oligofluorene
core.
Figure 1. (a) Absorption and fluorescence spectra of OFn-EG
and EG-H, measured in CHCl3. (b) Cyclic voltammograms of OFn-
EG and EG-H, measured in CH2Cl2.
The electrochemical behaviors of these newly synthesized
dendrimers are tabulated in Table 1 and Figure 1b. In contrast
to the triaryldiamine,4b,c the new dendrimers OFn-EG exhibit
an electrochemical behavior similar to that of the triary-
lamine- and carbazole-based dendron EG-H, in which one
reversible four-electron anodic redox couples (E1/2 ∼ 0.38
eV), corresponding to removal of electrons from the periph-
eral arylamino group of the dendrons, and another reversible
two-electron anodic redox couple (E1/2 ∼ 0.70 eV), corre-
sponding to removal of electrons from the interior carbazole
moieties forming radical tetracations and hexylcations,
respectively (Figure 1b). Both first oxidation potentials and
second oxidation potentials are essentially unaffected by the
length and structure of the oligofluorene core as the oxidation
occurs only at the wedges; however, both first oxidation
potential values and second oxidation potential values of
OFn-EGs are a bit smaller than those of EG-H (E1/2: 0.40
and 0.75 eV, respectively) (Table 1), indicating that the
incorporation of dendron EG makes the resulting dendrimers
more susceptible for electrochemical oxidation. In general,
with an incorporation of triarylamine-based dendritic end-
caps, the HOMO energy level of oligofluorenes moves up
to ∼5.20 eV (relative to the vacuum level) as estimated by
the electrochemical method. Such a high HOMO energy level
greatly reduces the energy barrier for the hole injection from
ITO (φ ) 5.0 eV) to the emissive oligofluorenes. As a result,
OFn-EGs can also be used as hole transport/injection
materials.
Table 1). There is a substantial red-shift of the absorption
maxima (λmax) of oligofluorenes (∆ 25-54 nm) upon
incorporation of strongly electron-donating diphenylamino
group(s) which is attributed to the asymmetric destabilization
of the HOMO and the LUMO levels leading to the decrease
in the energy gap;9 however, the introduction of dendrons
onto the oligofluorene backbone causes the resulting mol-
ecule less planar than the analogues end-capped with
diphenylamino groups in their electronic ground state leading
to a blue shift in λabs
(∆ 30-54 nm).10 The absorption
max
maxima (λabsmax) and molar absorptivities (ꢀmax) of OFn-EGs
increase sequentially as the core chain length increases
(Figure 1a and Table 1). Surprisingly, the emission maxima
(λemmax: 404, 436, and 436 nm, respectively) of these
dendrimers first red-shifted (∆ 5 nm) and then remained
fairly constant at 436 nm (Figure 1a and Table 1). Upon
excitation either at 324 nm attributed to the nfπ* transition
of triarylamine- and carbazole-based dendron or at 350 nm
corresponding to the πfπ* transition of oligofluorene core,
the emission spectra obtained are identical, suggesting that
energy or exciton can efficiently transfer from the peripheral
(9) Wong, M. S.; Li, Z. H. Pure Appl.Chem. 2004, 76, 1409.
(10) (a) Berlman, I. B.; J. Phys. Chem. 1970, 74, 3085. (b) Nijegorodov,
N. I.; Downey, W. S. J. Phys. Chem. 1994, 98, 5639.
Org. Lett., Vol. 8, No. 7, 2006
1501

In summary, we have presented a facile approach for the
synthesis of a family of dendrimers with hole-transporting
ability moieties by copper-catalyzed Buchwald’s double-
amination as a key reaction. We believe that this approach
will be applicable for the construction of other structurally
uniform and well-defined π-conjugated functional den-
dronized compounds. All of the end-dendronized oligofluo-
rene compounds were characterized by a combination of 1H
NMR, 13C NMR, and matrix-assisted laser desorption ioniza-
tion time-of-flight (MALDI-TOF) mass spectroscopy.11
These dendrimers exhibit very high thermal and electro-
chemical stabilities. The details on the OLEDs’ performance
using these materials as hole transfer and emitting layers or
as host materials for electrophosphorescent applications will
be reported in the future.
Acknowledgment. We are grateful to the Hong Kong
Research Grants Council (HKBU 2018/05P) for financial
support of this work.
Supporting Information Available: The synthetic pro-
cedures and physical data of all intermediates and OFn-EG,
n ) 1-3. This material is available free of charge via the
Internet at http://pubs.acs.org.
(11) Yang, J.-X.; Tao, X.-T.; Yuan, C. X.; Yan, Y. X.; Wang, L.; Liu,
Z.; Ren, Y.; Jiang, M. H. J. Am. Chem. Soc. 2005, 127, 3278.
OL0604062
1502
Org. Lett., Vol. 8, No. 7, 2006