The construction of H-shaped fluorescent materials based on building blocks consisting of triphenylamine and fluorene

Article abstract of DOI:10.1246/cl.2010.520

A class of complicated 9,9-diarylfluorenes as nonplanar building blocks were synthesized and utilized to prepare a novel H-shaped oligofluorene. High glass transition temperatures and excellent solubility of the obtained oligomers indicated the 9,9diarylfluorenes would be promising building blocks for the construction of H-shaped optoelectronic materials.

Full text of DOI:10.1246/cl.2010.520

doi:10.1246/cl.2010.520
520
Published on the web April 17, 2010
The Construction of H-shaped Fluorescent Materials Based on Building Blocks Consisting
of Triphenylamine and Fluorene
Cheng Zhang,* Yu-jian Zhang, Wen-qin Xiang, Bin Hu, Mi Ouyang, Yi Xu, and Chun-an Ma
Department of Applied Chemistry, Zhejiang University of Technology, Hangzhou 310014, P. R. China
(Received February 1, 2010; CL-100093; E-mail: czhang@zjut.edu.cn)
A class of complicated 9,9-diarylfluorenes as nonplanar
MTPA
CF3SO3H
1,4-dioxane
O
Ar1
Ar1
Ar1
building blocks were synthesized and utilized to prepare a novel
H-shaped oligofluorene. High glass transition temperatures and
excellent solubility of the obtained oligomers indicated the 9,9-
diarylfluorenes would be promising building blocks for the
construction of H-shaped optoelectronic materials.
OH
Ar1-MgBr
N
1a−1f
2a−2f
3a−3f
CH3
H3C CH3
F
F
F
CH3
F
F
1a−1f
F
F
Fluorene-based materials, such as terfluorenes, oligofluorene,
and polyfluorenes (PFs), have emerged as promising classes
of organic semiconductors in the field of organic electronics,
especially in OLEDs, because of their excellent thermal and
chemical stability as well as high photoluminescence (PL)
quantum efficiencies.1 However, an obvious deficiency of fluorene
derivatives concerns their tendency to form long-wavelength
aggregates/excimers or keto defects in the solid state upon thermal
annealing or device operation. Another intractable issue is the low
HOMO energy level, which leads to inefficient hole injection.2
Highly electron-rich triphenylamines known for their high hole
mobility have been extensively used to increase the HOMO energy
levels and reduce the energy barrier for hole injection from the
indium tin oxide (ITO) to the emissive layer. Additionally, various
strategies have also been developed to reduce the formation of
aggregation, which include the introduction of sterically hindered
side chains and spiro-links.3
9,9-Diarylfluorenes (DAF) have been proven to be an
efficient building blocks to suppress the formation of aggregates
or excimers of PFs main chains because of the three-dimensional
cardo structure of substituted fluorene derivatives.2b,4 Various 9,9-
bis(triarylamines)-substituted fluorenes which help to improve
hole-injection ability are reported recently.5 Inspired by their
results, in this study, a series of novel DAF building blocks
consisting of triphenylamine core and fluorene/fluorinated arene
peripheries is designed and prepared. Then, several novel H-
shaped oligomers based on the above building blocks are
synthesized. The desired oligomers consist of two parts: ter(bi-
phenyls or phenyls) as the arms of the H-shape and 4-
methyltriphenylamine (MTPA) as the rung. The fluorene and
MTPA moieties are connected through the sp3-hybridized C9
carbon atom, which may not only hinder close packing and
crystallization but also lead to pronounced morphological stability
of amorphous materials. Meanwhile, the hole injection ability of
the resulting oligomers are effectively enhanced by the introduc-
tion of the MTPA group. More importantly, the introduction of
MTPA groups to the 9-position of fluorine is confirmed to be
easily achieved by the Friedel-Crafts reaction.
Ar1=
2a−2f
3a−3f
a
b
c
d
e
f
Scheme 1. Synthetic route of DAF building blocks.
Br
Br
N
TB-MTPA
N
MTPA
CF3SO3H
1,4-dioxane
OH
Br
Br
Br
R
Br
BBrF
R
Pd(PPh3)4, toluene
K2CO3, 90 °C, 48 h
Phenylboronic acid or
4-Biphenylboronic acid
R
R
DP-MTPA
DDP-MTPA
R=phenyl
R= biphenyl
Scheme 2. Synthesis of DP-MTPA and DDP-MTPA.
with the yields of more than 83%. The results indicated that
CF3SO3H as a Lewis acid is probably an effective catalyst for the
Friedel-Crafts reaction of tertiary alcohol 2a-2f with MTPA.
Additionally, a novel approach for the synthesis of bipolar
oligofluorenes containing triphenylamine and fluorinated arene
using the Friedel-Crafts reaction and Grignard reagents is
presented.
The efficiency of CF3SO3H could be further confirmed by the
following reaction of tertiary alcohol BBrF with MTPA
(Scheme 2), which indeed exhibited excellent reactivity with
yield of 90%. In addition, this reaction revealed that the dibromo
substituents had little influence on the reactivity of tertiary
alcohol. Then, two stable blue-light emitters (DP-MTPA and
DDP-MTPA) with nonplanar conformation were constructed with
4.4 equiv of corresponding boronic acid and the tetrabrominated
TB-MTPA by the Suzuki coupling reaction. The yields were 72%
and 65%, respectively.
The different aryl Grignard reagents, 1a-1d were synthesized
for the reaction with fluorenone in tetrahydrofuran at room
temperature (Scheme 1). The corresponding alcohols 2a-2d were
isolated in more than 80% yields. Under the same condition, only a
low yield 2e and 2f (less than 35%) were obtained due to the
coupling reaction of Grignard reagents. The Friedel-Crafts
reaction of 2a-2f with MTPA was carried out in the presence of
an excess amount of CF3SO3H for 6 h at 80 °C, to obtain 3a-3f
The resulting oligofluorenes presented good solubility in
common organic solvents and had much higher solubility than the
Chem. Lett. 2010, 39, 520-521
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

521
Table 1. Physical Properties of DP-MTPA and DPP-MTPA
Abs -max/nm
(sol/film)
PL -max/nm
(sol/film)
d
Compounds
Tg/°C
Td/°C
E
ox/V
Eg/eV
HOMOb/eV
¾c/105
¯
F
DP-MTPA
DDP-MTPA
316/322
337/342
421/431
445/450
178
201
450
®
0.77
0.76
3.23
3.06
¹5.15
¹5.14
1.13
1.08
0.18
0.24
a
aUp to 500 °C without obvious decomposition. bEstimated from the onset oxidation potentials by using EHOMO = ¹Eox ¹ 4.38 eV.7 cMolar extinction
d
coefficient (¾, M¹1 cm¹1). Using quinine sulfate (ΦF = 0.55) in 0.05 M H2SO4 as a standard.
Figure 1. Absorption and fluorescence spectra of DP-MTPA and DDP-
MTPA in CHCl3 solution (hole) and solid state (solid).
Figure 2. Cyclic voltammograms of DP-MTPA and DPP-MTPA (10
dispiro analog.6 The UV-vis absorption and emission spectra of
successive scans) in CH2Cl2 with 0.10 M of TBAP as a supporting
¹1
DP-MTPA and DDP-MTPA were measured in dilute chloroform
solutions, as shown in Figure 1. A strong ³-³* absorption band
was observed in the solution spectrum with an absorption
maximum at 316 nm for DP-MTPA and 337 nm for DDP-MTPA,
showing a red-shift of the absorption band with the increase in the
conjugation length of the oligofluorene arms. The PL emission
wavelength of two H-shaped oligofluorenes from solution to solid
film did not change remarkably, which suggested that the unique
conformation significantly reduced aggregation. In addition, no
obvious change in the emission film of DDP-MTPA was observed
even after annealing at 120 °C for 12 h in air, which indicated that
the luminescent stability of oligofluorenes was successfully
improved.9
Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) were carried out to investigate the thermal
properties of DP-MTPA and DDP-MTPA, as shown in Table 1.
Compared with dispiro compounds8 2,2¤,7,7¤-tetraphenyl-9,9¤-
spirobifluorene and 2,2¤,7,7¤-tetra(biphenyl-4-yl)-9,9¤-spirobi-
fluorene, the two H-shaped oligofluorenes exhibited high decom-
position temperature, more than 450 °C with 5% weight loss and a
high Tg over 178 °C.9 However, the fluorescence quantum yields
(ΦF) of the two dispiro compounds, with the better rigidity, are
higher values than that of DP-MTPA (0.18) and DDP-MTPA
(0.24).
Cyclic voltammetry (CV) was performed to probe eletro-
chemical proerties of obtained oligomers (Figure 2). The anode
scans had similar values of onset oxidation potential (EOX) for
both oligofluorenes (0.77 V for DP-MTPA and 0.76 V for DDP-
MTPA). The estimated EHOMO were ¹5.15 eV for DP-MTPA and
¹5.14 eV for DDP-MTPA, respectively (Table 1). The results
demonstrated that the energy barrier for hole-injection from the
ITO anode (WITO) ¹4.8 eV) to the oligofluorenes were signifi-
cantly lower than that of fluorene homopolymer (5.8 eV), which
could be attributed to the existence of the MTPA-based rung in
H-shaped oligofluorenes. Figure 2 also shown that p-doping
processes of DP-MTPA and DDP-MTPA were reversible, indicat-
ing a high stability for hole injection. Under successive multiple
potential scans, DDP-MTPA demonstrated extraordinarily high
electrochemical stability, with the CV curves remaining un-
changed in 10 scans.
electrolyte, scan rate 50 mV s
.
In conclusion, based on synthesis of the novel building block,
H-shaped oligofluorenes with high thermal stability, solubility,
and blue emission were prepared. The presence of the MTPA-
based rung raised the HOMO energy levels of DP-MTPA and
DDP-MTPA. Moreover, the conjugation length of the oligofluo-
rene arms had less influence on the ability of hole-injection. The
results shown that DAFs 3a-3f are potential building blocks for
constructing organic luminescent materials without the need for a
hole-transporting layer.
Natural Science Foundation of Zhejiang Province (No.
Y40902 60), Major Science and Technology, Special and Priority
Themes of Zhejiang Province, China (No. 2009C14004) and
Project for “XinMiao” Talents Training Plan of Zhejiang Province.
References and Notes
1
X. W. Chen, H.-E. Tseng, J.-L. Liao, S.-A. Chen, J. Phys. Chem. B 2005,
109, 17496; K. Becker, J. M. Lupton, J. Feldmann, B. S. Nehls, F.
Galbrecht, D. Q. Gao, U. Scherf, Adv. Funct. Mater. 2006, 16, 364; R.
Grisorio, G. P. Suranna, P. Mastrorilli, C. F. Nobile, Adv. Funct. Mater.
2007, 17, 538.
2
a) S. Y. Cho, A. C. Grimsdale, D. J. Jones, S. E. Watkins, A. B. Holmes, J.
Am. Chem. Soc. 2007, 129, 11910. b) K. L. Chan, M. J. McKiernan, C. R.
Towns, A. B. Holmes, J. Am. Chem. Soc. 2005, 127, 7662.
H. J. Jiang, J. H. Wan, W. Huang, Sci. China, Ser. B Chem. 2008, 51, 497.
B.-M. Zhao, N.-N. Fu, L.-H. Xie, L.-H. Wang, W. Huang, Chem. Lett.
2008, 37, 622; C. Tang, F. Liu, Y.-J. Xia, L.-H. Xie, A. Wei, S.-B. Li, Q.-L.
Fan, W. Huang, J. Mater. Chem. 2006, 16, 4074.
3
4
5
6
C. Ego, A. C. Grimsdale, F. Uckert, G. Yu, G. Srdanov, K. Müllen, Adv.
Mater. 2002, 14, 809; S. B. Jiao, Y. Liao, X. J. Xu, L. P. Wang, G. Yu, L. M.
Wang, Z. M. Su, S. Ye, Y. Q. Liu, Adv. Funct. Mater. 2008, 18, 2335; Z.
Zhao, J.-H. Li, P. Lu, Y. Yang, Adv. Funct. Mater. 2007, 17, 2203.
a) L.-H. Xie, X.-Y. Hou, C. Tang, Y.-R. Hua, R.-J. Wang, R.-F. Chen, Q.-L.
Fan, L.-H. Wang, W. Wei, B. Peng, W. Huang, Org. Lett. 2006, 8, 1363. b)
L.-H. Xie, X.-Y. Hou, Y.-R. Hua, C. Tang, F. Liu, Q.-L. Fan, W. Huang,
Org. Lett. 2006, 8, 3701.
7
8
Q. G. Kong, D. Zhu, Y. W. Quan, Q. M. Chen, J. F. Ding, J. P. Lu, Y. Tao,
Chem. Mater. 2007, 19, 3309.
J. Salbeck, M. Schörner, T. Fuhrmann, Thin Solid Films 2002, 417, 20; T. P.
I. Saragi, T. Spehr, A. Siebert, T. Fuhrmann-Lieker, J. Salbeck, Chem. Rev.
2007, 107, 1011.
9
Supporting Information is available electronically on the CSJ-Journal Web
site, http://www.csj.jp/journals/chem-lett/index.html.
Chem. Lett. 2010, 39, 520-521
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/