Highly efficient and stable deep blue light emitting poly(9,9- dialkoxyphenyl- 2,7-silafluorene): Synthesis and electroluminescent properties

Article abstract of DOI:10.1021/ma102446b

The synthesis of a soluble poly(9,9-dialkoxyphenyl-2,7-silafluorene) (PSF) by the Yamamoto reaction was reported. In order to get high yield of 9,9-dichloro-2,7-dibromosilafluorene, excess SiCl₄ was added to suppress the formation of spirosilafluorene and was stripped eventually before the addition of alkoxyphenyllithium. After the solution of 9,9-di(4-(30,70-dimethyloctyloxy))-2,7-dibromosilafluorene and nickel catalyst was stirred at 85°C for 2 days, bromobenzene was added to diminish the bromo end groups which may cause formation of green emission aggregates. It can be seen that the absorption spectrum of PSF in solution are almost identical to that in film, with the absorption maximum at 391-392 nm, implying that the conjugation length does not change upon the film was formed after spin-coating. The absorption onset of PSF is around 438 nm; thus, the optical bandgap of PF is estimated at ca. 2.83 eV. PSF exhibits excellent thermal stability from the TGA and DSC measurements.

Full text of DOI:10.1021/ma102446b

Macromolecules 2011, 44, 17–19 17
DOI: 10.1021/ma102446b
In this Communication, we report the synthesis a soluble
poly(9,9-dialkoxyphenyl-2,7-silafluorene) (PSF) by the Yama-
moto reaction. The thermal, photophysical, and EL properties
of the obtained polymer were investigated.
Highly Efficient and Stable Deep Blue Light Emitting
Poly(9,9-dialkoxyphenyl- 2,7-silafluorene): Synthesis
and Electroluminescent Properties
Jun Wang,† Chang-qing Zhang,† Cheng-mei Zhong,‡
Su-jun Hu,‡ Xue-yi Chang,§ Yue-qi Mo,*,†
Xiwen Chen,*,^ and Hong-bin Wu*,‡
Experimental Section. The synthesis of PSF is shown in
Scheme 1. 9,9-Dichloro-2,7-dibromosilafluorene17 is the key
intermediate for the synthesis of 9,9-di(4-(30,70-dimethyl-
octyloxy))-2,7-dibromosilafluorene (1). We tried another
intermediate;9-phenyl-9-chloro-2,7-dibromosilafluorene;but
it is difficult to get the objective product (1). In order to get high
yield of 9,9-dichloro-2,7-dibromosilafluorene, excess SiCl4 was
added to suppress the formation of spirosilafluorene18 and was
stripped eventually before the addition of alkoxyphenyl-
lithium. PSF was obtained via the Yamamoto reaction. After
the solution of 1 and nickel catalyst was stirred at 85 ꢀC for
2 days, bromobenzene was added to diminish the bromo end
groups which may cause formation of green emission aggre-
gates.19After purification, PSF with Mn of 23 000 and Mw of
92 000 was obtained. PSF was readily soluble in common
organic solvents, including toluene, chlorobenzene, chloro-
form, and THF.
Results and Discussion. The UV-vis absorption spectra
and photoluminescent spectra (PL) of PSF in CH2Cl2 and in
solid state are shown in Figure 1. It can be seen that the absorp-
tion spectrum of PSF in solution are almost identical to that in
film, with the absorption maximum at 391-392 nm, implying
that the conjugation length does not change upon the film was
formed after spin-coating. The absorption onset of PSF is
around 438 nm; thus, the optical bandgap (Eg) of PF is esti-
mated at ca. 2.83 eV, slightly smaller than that of poly(9,
9-dialkysilafluorene) (2.93 eV).5 As the onset potential of the
oxidation process (p-doping) of PSF occurs at about 1.38 V
against Hg/Hg2Cl2 (as shown in Figure S5), the HOMO level of
PSF was calculated to be -5.78 eV according to the empirical
formula EHOMO = -(Eox þ 4.4) (eV),20 which is quite close to
that of poly(9,9-dialkysilafluorene) (-5.77 eV).5 The LUMO
level was calculated to be about -2.95 eV from the HOMO level
and the optical bandgap (Eg).
As can be seen from the PL spectra of PSF in CH2Cl2 (ca.
5 ppm) and in the solid state, which were obtained by a
Fluorolog JY luminescence spectrometer under excitation of
340 nm, the PL spectrum of PSF in film shows a 14 nm of red
shift as compared with that in CH2Cl2 while both PL spectra
show strong vibronic structure. The PL maximum of PSF in
film is located at 437 nm, which shows about 12 nm of red
shift compared with poly(9,9-dialkysilafluorene) film,5 with
two vibronic sidebands at 461 and 495 nm, respectively. The
PL quantum yields of PSF in dilute 1,2-dichloroethane and
in film were estimated to be ca. 95% and 75% as measured in
an integrated sphere, respectively.
To investigate the electroluminescence properties of the
PSF, single-active-layer devices with the configuration ITO/
PEDOT:PSS/PSF/Ba/Al and ITO/PEDOT:PSS/PVK /PSF/
Ba/Al have been fabricated, where poly(N-vinylcarbazole)
(PVK) was used as hole transporting layer. The EL spectra of
the devices and the PL spectra of the PSF are shown in Figure 2.
We note that the EL spectra of the devices in both configura-
tions are nearly identical to that of the PL spectrum, with an
emission peak at ca. 435 nm and a shoulder at 458 nm, implying
that EL and PL have the same origin. Unlike poly(dioctylfluo-
rene), in which pronounced emission in the longer wavelength
†Key Laboratory of Special Functional Materials, South China
University of Technology, Guangzhou 510640, P. R. China,
‡Institute of Polymer Optoelectronic Materials and Devices,
South China University of Technology, Guangzhou 510640,
P. R. China, §Canton Oledking Optoelectronic Materials Co., ltd,
Guangzhou 510640, P. R. China, and ^CSIRO Materials Science
and Engineering, Bayview Avenue, Clayton, VIC 3168, Australia
Received October 27, 2010
Revised Manuscript Received December 5, 2010
Introduction. Highly efficient blue emitting polymer with
good color purity is one of the key issues of the commercial-
ization of the polymer light emitting diodes (PLEDs)1 for
the next-generation flat-panel displays. Although many
efforts have been focused on poly( p-phenylene)s, polycarba-
zoles, polythiophenes, etc., 2,7-fluorene-based polymers2 are
regarded as the most successful blue light emitters. Very
recently, Yang et al.3 and Chen et al.4 reported highly
efficient blue emitting PLEDs based on alkoxyphenyl-sub-
stituted poly(2,7-fluorene) derivatives. On the other hand,
poly(9,9-dialkylsilafluorene)s are another promising candi-
date for blue light emitting PLEDs, which was first reported
by Holmes’ group5 and us nearly the same time in 2005.6 In
general, polysilafluorenes exhibit even more stable blue
emission due to the higher oxidative stability of Si at the
9-position of polysilafluorenes as compared to that of
the C-9 carbon of polyfluorenes, which usually result in the
formation of the keto defects.7 Moreover, Si-containing
polymers are expected to have higher electron affinity owing
to the σ*-π* conjugation.8 As a result, photovoltaic cells
based on 9,9-dialkylsilafluorene-based copolymers have
been demonstrated to have higher open voltage circuit as
compared to that based on its polyfluorene analogue,9 and
the related PLEDs exhibit very good EL performance,10,11
which received intense attention.12 However, we note that
the device based on the homopolymer of 9,9-dialkylsilafluor-
ene shows a moderate external quantum efficiency (EQE) of
0.66% and a maximal luminous efficiency of 0.53 cd/A.10
Given the fact that polymers with aryl side chains usually
have higher thermal stability due to higher glass transition
temperatures than those with alkyl side chains,13,14 bulky
aryl substitutes were thought to result in more effective
suppression of aggregation than alkyl substitutes. As it can
lead to the suppression of excimer formation and long-
wavelength emission,15 it is very interesting to develop
homopolymers of 9,9-diarylsilafluorene for blue light emit-
ting devices. Although some 9,9-diarylsilafluorene-contain-
ing compounds had been reported previously,16 to the best of
our knowledge, processable homopolymers of 9,9-diarylsila-
fluorene have not appeared in the literature.
*Corresponding authors. E-mail: moyueqi@263.net (Y.M.); hbwu@
scut.edu.cn (H.W.); Xiwen.Chen@csiro.au (X.C.).
r 2010 American Chemical Society
Published on Web 12/15/2010
pubs.acs.org/Macromolecules

18 Macromolecules, Vol. 44, No. 1, 2011
Communication
Scheme 1. Synthesis of the PSF
Table 1. Device Performance of the PLEDs Based on PSFa
LEmax
entry treatment (cd/A)
EQEmax
(%)
Von
(V)
Lmax
(cd/m2)
CIE
1
2
0.32
0.53
0.31
0.51
3.2
2.8
1540
2362
(0.16, 0.12)
(0.16, 0.12)
120 ꢀC,
30 min
3
4
2.1
2.3
2.02
2.21
3.2
3.4
6278
6107
(0.17, 0.13)
(0.17, 0.13)
120 ꢀC,
30 min
a Configurations of 1 and 2: ITO/PEDOT:PSS/PSF/Ba/Al. Config-
urations of 3 and 4: ITO/PEDOT:PSS/PVK/PSF/Ba/Al. LE: luminous
efficiency; EQE: external quantum efficiency; L: luminance; Von: turn-on
voltage.
polyfluorene analogue;poly[9,9-bis(4-(2-ethylhexyloxy)phe-
nyl)fluorene-2,7-diyl];was reported to a luminous efficiency
of 1.9 cd/A (corresponding to an EQE of 1.8%) with CIE
coordinates of (0.16, 0.12).3 The current density-luminance-
voltage (J-L-V) characteristics of device A and the LE-
current density (LE-J) characteristics of both devices are
plotted in the Figures S6 and S7.
Figure 1. Absorption and PL spectra of PSF in CH2Cl2 and in solid state.
The thermal properties of PSF were investigated using ther-
mogravimetric analysis (TGA) and differential scanning calo-
rimetry (DSC) under a nitrogen atmosphere. PSF exhibits
excellent thermal stability from the TGA and DSC measure-
ments (shown in Figures S8 and S9). In the DSC trace of heating
(10 ꢀC/min), PSF exhibited a glass transition at about 120 ꢀC,
which is slightly higher than that of poly(9,9-dialkoxyphenyl-
2,7-fluorene) (108 ꢀC)14 and much higher than that of poly(9,
9-dihexyl-3,6-fluorene) (83 ꢀC).6 TGA curve shows that decom-
position temperature of PSF starts at 426 ꢀC (5% weight loss),
which is close to that of poly(9,9-dialkysilafluorene).5 We attri-
bute the excellent thermal stabilities of PSF to the 9,9-diphenyl
substitutes.
Conclusion. In conclusion, the soluble poly(9,9-dialkoxy-
phenyl-2,7-silafluorene) was prepared via Yamamoto reac-
tions. This polymer with high glass transition temperature of
120 ꢀC exhibits a high external quantum efficiency of 75%
and a luminous efficiency of 2.1-2.3 cd/A.
Figure 2. PL and EL spectra of the devices fabricated from PSF in
device configuration of ITO/PEDOT:PSS/PSF/Ba/Al and ITO/PED-
OT:PSS/PVK/PSF/Ba/Al.
region typically exists and unstable blue emission upon increase
of temperature and operation stress,15 the EL spectra of the PSF
devices remain nearly unchanged even the devices were an-
nealed at 120 ꢀC for 30 min. The excellent spectral stabilities of
PSF can be attributed to the bulky 9,9-diphenyl substitutes,
which could result in the suppression of aggregation and/or
excimer formation.
Acknowledgment. The authors are grateful for financial sup-
port from Fundamental Research Funds for the Central Univer-
sities of China (No. 2009ZM0307), Ministry of Science and
Technology Project (No. 2009CB623601), and the National Nature
Science Foundation of China (No. U0634003). We thank Prof.
Hongzhi Wang for help with the PL measurements.
The CIE coordinates of the blue emission were found to
be around from (0.17, 0.12) to (0.17,0.13), representing deep
blue emission in the 1931 CIE diagram. Table 1 summarizes the
performance of the PSF devices fabricated from two device
configurations. With a device configuration of ITO/PEDOT:
PSS/PSF/Ba/Al, the maximal LE of 0.53 cd/A (corresponding
to an EQE of 0.51%.) was obtained after annealing at 120 ꢀCfor
0.5 h. Upon the incorporation of a thin layer of PVK (∼30 nm),
despite the turn-on voltage (defined as the voltage at which a
luminance of 1 cd/m2 was measured) slightly increased, the LE
dramatically increased to 2.3 cd/A, while the EQE of the device
reached 2.2%. We noticed that the best performance of a
Supporting Information Available: NMR spectra of mono-
mers and PSF, cyclic voltammograms of PSF, DSC heating curves
of PSF, TGA of PSF, the current density-luminance-voltage
(J-L-V) characteristics and the LE-current density (LE-J)
characteristics of the devices from PSF. This material is available
free of charge via the Internet at http://pubs.acs.org.
References and Notes
(1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.;
Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990,
347, 539–541.

Communication
Macromolecules, Vol. 44, No. 1, 2011 19
(2) (a) Ohmori, Y.; Uchida, M.; Muro, K.; Yoshino, K. Jpn. J. Appl.
Phys. 1991, 30, L1941–L1943. (b) Janietz, S.; Bradley, D. D. C.; Grell,
M.; Giebeler, C.; Inbasekaran, M.; Woo, E. P. Appl. Phys. Lett. 1998,
73, 2453–2455.
(3) Liu, J.; Zou, J. H.; Yang, W.; Wu, H. B.; Li, C.; Zhang, B.; Peng,
J. B.; Cao, Y. Chem. Mater. 2008, 20, 4499–4506.
(4) Huang, C. W.; Peng, K. Y.; Liu, C. Y.; Jen, T. H.; Yang, N. J.;
Chen, S. A. Adv. Mater. 2008, 20, 3709–3716.
(5) Chan, K. L.; McKiernan, M. J.; Towns, C. R.; Holmes, A. B.
(12) (a) Beaupre, S.; Boudreault, P. L. T.; Leclerc, M. Adv. Mater. 2010,
22, E6–E27. (b) Cheng, Y. J.; Yang, S. H.; Hsu, C. S. Chem. Rev. 2009,
109, 5868–5923. (c) Chen, J. W.; Cao, Y. Macromol. Rapid Commun.
2007, 28, 1714–1742. (d) Zhao, Q.; Liu, S. J.; Huang, W. Macromol.
Chem. Phys. 2009, 210, 1580–1590. (e) Grimsdale, A. C.; Chan, K. L.;
Martin, R. E.; Jokisz, P. G.; Holmes, A. B. Chem. Rev. 2009, 109,
897–1091. (f ) Wong, W. W. H.; Holmes, A. B. Adv. Polym. Sci. 2008,
212, 85–98.
(13) (a) Yu, W. L.; Pei, J.; Huang, W.; Heeger, A. J. Adv. Mater. 2000,
€
J. Am. Chem. Soc. 2005, 127, 7662–7663.
12, 828–831. (b) Klarner, G.; Davey, M. H.; Chen, W. D.; Scott, J. C.;
(6) (a) Mo, Y. Q.; Tian, R. Y.; Shi, W.; Cao, Y. Chem. Commun. 2005,
4925–4926. (b) Chan, K. L.; Watkins, S. E.; Mak, C. S. K.; McKiernan,
M. J.; Towns, C. R.; Pascu, S. I.; Holmes, A. B. Chem. Commun. 2005,
5766–5768.
(7) (a) List, E. J. W.; Guentner, R.; Freitas, P. S.; Scherf, U. Adv.
Mater. 2002, 14, 374–378. (b) Kulkarni, A. P.; Kong, X.; Jenekhe, S. A.
J. Phys. Chem. B 2004, 108, 8689–8701. (c) Liu, L. L.; Tang, S.; Liu,
M. R.; Xie, Z. Q.; Zhang, W.; Lu, P.; Hanif, M.; Ma, Y. G. J. Phys.
Chem. B 2006, 110, 13734–13740. (d) Abbel, R.; Wolffs, M.; Bovee,
R.; van Dongen, J.; Lou, X.; Henze, O.; Feast, W.; Meijer, E.;
Schenning, A. Adv. Mater. 2009, 21, 597–602.
Miller, R. D. Adv. Mater. 1998, 10, 993–997.
(14) Lee, J. H.; Hwang, D. H. Chem. Commun. 2005, 2836–2837.
(15) (a) Hung, M. C.; Liao, J. L.; Chen, S. A.; Chen, S. H.; Su, A. C.
J. Am. Chem. Soc. 2005, 127, 14576–14577. (b) Sims, M.; Bradley,
D. D. C.; Ariu, M.; Koeberg, M.; Asimakis, A.; Grell, M.; Lidzey, D. G.
Adv. Funct. Mater. 2004, 14, 765–781. (c) Zhou, X. H.; Zhang, Y.;
Xie, Y. Q.; Cao, Y.; Pei, J. Macromolecules 2006, 39, 3830–3840.
(d) Zeng, G.; Yu, W. L.; Chua, S. J.; Huang, W. Macromolecules 2002,
35, 6907–6914.
(16) (a) Chen, R. F.; Fan, Q. L.; Liu, S. J.; Zhu, R.; Pu, K. Y.; Huang,
W. Synth. Met. 2006, 156, 1161–1167. (b) Couzijn, E. P. A.; van den
Engel, D. W. F.; Slootweg, J. C.; de Kanter, F. J. J.; Ehlers, A. W.;
Schakel, M.; Lammertsma, K. J. Am. Chem. Soc. 2009, 131, 3741–
3751. (c) Yan, D. C.; Mohsseni-Ala, J.; Auner, N.; Bolte, M.; Bats, J. W.
Chem.;Eur. J. 2007, 13, 7204–7214. (d) Shimizu, M.; Mochida,
K.; Hiyama, T. Angew. Chem., Int. Ed. 2008, 47, 9760–9764. (e) Yabusaki,
Y.; Ohshima, N.; Kondo, H.; Kusamoto, T.; Yamanoi, Y.; Nishihara, H.
Chem.;Eur. J. 2010, 16, 5581. (f ) Li, L.; Xu, C.; Li, S. Tetrahedron Lett.
2010, 51, 622–624.
€
(8) (a) Palilis, L. C.; Makinen, A. J.; Uchida, M.; Kafafi, Z. H. Appl. Phys.
Lett. 2003, 82, 2209. (b) Yamaguchi, S.; Endo, T.; Uchida, M.; Izumizawa,
T.; Furukawa, K.; Tamao, K. Chem.;Eur. J. 2000, 6, 1683–1692.
(9) (a) Duan, C. H.; Cai, W. Z.; Huang, F.; Zhang, J.; Wang, M.; Yang,
T. B.; Zhong, C. M.; Gong, X.; Cao, Y. Macromolecules 2010, 43,
5262–5268. (b) Wang, E. G.; Wang, L.; Lan, L. F.; Luo, C.; Zhuang,
W. L.; Peng, J. B.; Cao, Y. Appl. Phys. Lett. 2008, 92, 033307. (c)
Boudreault, P.-L. T.; Michaud, A.; Leclerc, M. Macromol. Rapid
Commun. 2007, 28, 2176–2179.
(10) Wang, E. G.; Li, C.; Peng, J. B.; Cao, Y. J. Polym. Sci., Part A 2007,
45, 4941–4949.
(11) (a) Mo, Y. Q.; Deng, X. Y.; Jiang, X.; Cui, Q. H. J. Polym. Sci.,
Part A 2009, 47, 3286–3296. (b) Wang, E. G.; Li, C.; Mo, Y. Q.; Zhang,
Y.; Ma, G.; Shi, W.; Peng, J. B.; Yang, W.; Cao, Y. J. Mater. Chem.
2006, 16, 4133–4140.
(17) Liu, Y. X.; Stringfellow, T. C.; Ballweg, D.; Guzei, I. A.; West, R.
J. Am. Chem. Soc. 2002, 124, 49–57.
(18) Xiao, H. B.; Leng, B.; Tian, H. Polymer 2005, 46, 5707–5713.
(19) Chen, X. W.; Tseng, H. E.; Liao, J. L.; Chen., S. A. J. Phys. Chem.
B 2005, 109, 17496–17502.
(20) Bredas, J. L.; Silbey, R.; Boudreaux, D. S.; Chance, R. R. J. Am.
Chem. Soc. 1983, 105, 6555–6559.