Synthesis and properties of 9,9-diarylfluorene-based triaryldiamines

Article abstract of DOI:10.1021/ol016051y

(matrix presented) 9,9-Diaryl-2,7-dibromofluorene was synthesized by a triflic acid promoted Friedel-Crafts reaction. Introduction of diarylamino groups at its C2 and C7 positions by a Pd-catalyzed amination results in the formation of a novel class of triaryldiamines. The 9,9-diaryl substituents at the central linkage play a less important role in the photophyscial properties but affect the oxidation potential and improve the morphological stability of these new triarylamines.

Full text of DOI:10.1021/ol016051y

ORGANIC
LETTERS
2001
Vol. 3, No. 15
2285-2288
Synthesis and Properties of
9,9-Diarylfluorene-Based Triaryldiamines
Ken-Tsung Wong,* Zi-Jien Wang, Yuh-Yih Chien, and Chien-Lung Wang
Department of Chemistry, National Taiwan UniVersity, Taipei 106, Taiwan
kenwong@ccms.ntu.edu.tw
Received May 1, 2001
ABSTRACT
9,9-Diaryl-2,7-dibromofluorene was synthesized by a triflic acid promoted Friedel−Crafts reaction. Introduction of diarylamino groups at its C2
and C7 positions by a Pd-catalyzed amination results in the formation of a novel class of triaryldiamines. The 9,9-diaryl substituents at the
central linkage play a less important role in the photophyscial properties but affect the oxidation potential and improve the morphological
stability of these new triarylamines.
Amorphous triarylamines with high glassy state stability are
suitable as hole-transporting materials in organic light-
emitting devices (OLED). Many endeavors have been made
to develop new amorphous triarylamines with high morpho-
logical stability. For example, dendritic molecules with a
triarylamine core have shown to form thin films of good
quality with high glass transition temperatures (Tg)1. Intro-
duction of a spiro linkage has also generated triarylamines
with higher Tg.2 Another approach involves replacing the
phenyl group of triarylamines by a bulkier aryl group.
Asymmetric substituted triarylamines normally yield thin
films of high thermal stability.3 The primary structural feature
for an amorphous triarylamine to exhibit high morphological
stability is a stable non-coplanar conformation, which can
inhibit the crystallization efficiently. However, a chro-
mophore without a rigid planar skeleton usually emits
fluorescence with a low quantum yield.4 To have an efficient
OLED device, it is important to improve the quantum yield
of photoluminescence and also to retain the morphological
stability of a triarylamine that is used as a hole transporter
as well as a light emitter.
interchain interaction but also improved the thermal stability
of polyfluorene.5 Inspired by their results, we report here a
new synthetic route for the preparation of 9,9-diarylfluorene-
based triaryldiamines. The rigidity of the fluorene core could
be beneficial for improving the quantum efficiency. Sys-
tematic tuning of the structure of the central linkage by
introducing different aryl substituents at the C9 position of
fluorene provides new possibilities for probing the properties
of resulting triaryldiamines. According to Mu¨llen’s addition-
cyclization strategy, only symmetric aryl groups can be
introduced at C9 of fluorene as substituents. We developed
another synthetic strategy for the synthesis of 2,7-dibromo-
9,9-diarylfluorene, in which the two aryl groups can be
different. The synthetic route to 9,9-diarylfluorene-based
triaryldiamines is shown in Scheme 1.
Three different aryl Grignard reagents, i.e., C6H5MgBr,
p-TolMgBr, and 1-NpMgBr (where Np is naphthyl) were
selected for the reaction with 2,7-dibromofluorenone 16 in
diethyl ether at reflux temperature. The corresponding alcohol
derivatives 2a,7 2b, and 2c were isloated in 90%, 82%, and
97% yields, respectively. The Friedel-Crafts reaction8 of
2a with benzene was carried out in the presence of an excess
By introducing two bulky groups at the C9 position of
polyfluorene, Mu¨llen and Leising not only blocked the
(5) Setayesh, S.; Grimsdale, A. C.; Weil, T.; Enkelmann, V.; Mu¨llen,
K.; Meghdadi, F.; List, E. J. W.; Leising, G. J. Am. Chem. Soc. 2001, 123,
(1) Shirota, Y. J. Mater. Chem. 2000, 10, 1.
(2) Salbeck, J.; Yu, N.; Bauer, J.; Weisso¨rtel, F.; Bestgen, H. Synth. Met.
1997, 91, 209.
946.
(6) Hauser, A.; Thurner, J.-U.; Hinzmann, B. J. Prakt. Chem. 1988, 330,
367.
(3) Koene, B. E.; Loy, D. E.; Thompson, M. E. Chem. Mater. 1998, 10,
2235.
(4) Nizhegorodov, N. I.; Downey, W. S. J. Phys. Chem. 1994, 98, 5639.
(7) Weber, E.; Dorpinghaus, N.; Csoregh I. J. Chem. Soc.; Perkin Trans.
2 1990. 2167.
(8) Ohta, T.; Shudo, K.; Okamoto, T. Tetrahedron Lett. 1983, 24, 71.
10.1021/ol016051y CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/30/2001

yields (entries 2, 3, and 5, Table 1).9 The reactions of the
9,9-diaryl-2,7-dibromofluorene derivatives 3a-3e with dif-
ferent diarylamines were carried out in the presence of a
catalytic amount of Pd(OAc)2 and PtBu3 in toluene at 100
°C with NaOtBu as the base.9,10 After column chromatog-
raphy purification, the isolated yields for the products 4a-
4g were good to excellent (Table 2). This synthetic strategy
Scheme 1
Table 2. Isolated Yields for the Pd-Catalyzed Reactions of
9,9-Diaryl-2,7-dibromofluorene 3 with Diarylamines
a Et2O, Ar1MgBr (2 equiv), reflux, 2-6 h. b CF3SO3H (2 equiv),
Ar2H, 50 °C, 10-30 min; see text. c Pd(OAc)2 (5 mmol %), PtBu3
(20 mmol %), NaOtBu (2 equiv), toluene, 100 °C, 4-8 h.
amount of CF3SO3H for 6 h at 80 °C to give 2,7-dibromo-
9,9-diphenylfluorene (3a) with a yield of 88% (entry 1, Table
1). Under the same condition, the reaction of 2c with benzene
Table 1. Isolated Yields for the CF3SO3H-Promoted
Friedel-Crafts Reaction of the Alcohol Derivatives 2 with
Benzene or Toluene
a R-Np is 1-naphthyl.
allows us to tailor the desired properties of the products by
the use of different combinations of aryl substituents at the
C9 position and the diarylamino substituents at the C2 and
the C7 positions of the central fluorene linkage.
The electronic absorption maxima (λmax), the emission
maxima (λem), and the quantum yields for the 9,9-diarylfluo-
rene-based triaryldiamine derivatives 4 are summarized in
Table 3. Triaryldiamines 4 with N-R-naphthyl-N-pheny-
lamine as diarylamino substituents (4a-4e) have demon-
failed and led to the decomposition of the starting material.
Only a low yield of 3b (less than 10%) was obtained when
the same reaction mixture was stirred at room temperature
overnight. However, 3b could be isolated in 56% yield after
a reaction time of 30 min at 50 °C (entry 4, Table 1). As a
result of its electron richness, toluene reacted very efficiently
with 2a, 2b, and 2c in a period of 10 min at 50 °C. This led
to the 9,9-diarylfluorene derivatives 3c, 3d, and 3e in good
2286
Org. Lett., Vol. 3, No. 15, 2001

Table 3. Physical Properties of 9,9-Diarylfluorene Bearing Triaryldiamines 4
λmax
λem
ΦF
E1pa, E1
E2pa, E2
∆E
Tg
(°C)d
pc
pc
compound
(nm, log ꢀ)a
(nm)a
(%)b
[mV, (onset)]
740, 640 (625)
(mV)
(mV)c
4a
352, 4.55
379, 4.55
378, 4.55
356, 4.53
353, 4.46
379, 4.46
274, 4.40
381, 4.54
350, 4.53
379, 4.56
308, 4.17
380, 4.38
337, 4.55
387, 4.62
335
457
15
15
16
18
15
58
69
16
1080, 1000
1060, 980
1140, 1060
1100, 1000
1080, 980
1040, 940
1100, 1000
1100, 1020
340
340
360
350
360
340
330
280
129
125
127
109
134
97
4b
461
720, 620 (600)
780, 700 (670)
740, 660 (600)
720, 640 (600)
700, 620 (600)
780, 700 (675)
840, 740 (730)
4c
455
4d
450
4e
457
4f
401, 420
410, 427
449
4g
125
100e
R-NPD
a In EtOAc, photoluminesence maximum was recorded upon irradiation at the absorption maximum. b Quantum yield of PL, compared to coumarin I in
EtOAc. c ∆E ) E2 - E1 d Determined by differential scanning calorimetry (DSC) analysis of the liquid nitrogen quenched samples. e From ref 3.
.
pa
pa
strated a similar photophysical behavior, irrespective of the
different aryl substituents at the C9 position of the fluorene
linkage. The second absorption bands of 4a-4g are signifi-
cantly red-shifted with respect to that of R-NPD (N,N′-di-
R-naphthyl-N,N′-diphenyl-4,4′-diphenyl). This indicates that
the rigid and planar fluorenyl core plays an important role
in reducing the HOMO-LUMO energy gap.
However, the triaryldiamine derivatives 4a-4e with N-R-
naphthyl-N-phenylamine substituents, as well as R-NPD,
exhibited similar emission maxima and quantum yields. The
introduction of different diarylamino substituents into the 9,9-
diaryl-substituted fluorenyl core (e.g., 4f, 4g) results in a
substantial blue shift of the first absorption band and the
properties of triaryldiamines 4 are dominated by the stuctural
feature of diamino substituents at C2 and C7 of the central
fluorene linkage.
Cyclic voltammetry was performed in CH2Cl2 (with 0.1
M tetra-n-butylammonium hexafluorophosphate,TBAPF6, as
supporting electolyte). Similarly to the conventional triaryl-
diamines, the 9,9-diarylfluorene-based triarydiamines 4 have
two quasireversible anodic redox couples, corresponding to
the removal of an electron from each triarylamine group.
The results are summarized in Table 3, and a comparison of
cyclic voltammogrammes of 4c, 4e, 4f, and R-NPD is shown
in Figure 2. As a result of the rigidity and planarity of the
λ
em with respect to that of 4a and R-NPD. A comparison of
the UV-vis spectra and the photoluminescent spectra of 4a,
4f, and R-NPD is shown in Figure 1. The photopysical
Figure 2. A comparison of normalized cyclic voltammograms of
4c, 4e, 4f, and R-NPD (0.1 M TBAPF6 in CH2Cl2)
Figure 1. A comparison of normalized absorption and photolu-
minescence specta of 4a, 4f, and R-NPD.
central fluorene linkage, the onset of the first oxidation step
of triarydiamines 4 is evidently lower compared to that of
Org. Lett., Vol. 3, No. 15, 2001
2287

R-NPD. These observations could be attributed to the better
π-conjugation along the molecular axis in 4 compared to
that in R-NPD. Significant potential differences between the
first oxidation step and the second oxidation step also reveal
that the radical cation could efficiently delocalize in the
molecule once it is formed. For the case that the central
linkage carries electron-rich aryl substituents (e.g., 4c), the
first oxidation step is obviously more difficult. By introducing
the electron-rich diarylamino substituents (e.g., 4f) both
oxidation potentials were reduced. However, for 4e with the
same diarylamino substituents as R-NPD, the second oxida-
tion potential exhibited no significant changes.
The morphological stability of 4 was investigated by
differential scanning calorimetry (DSC) analysis. The tri-
aryldiamines derivatives 4 exhibited good glass forming
properties with relative high glass transition temperatures
(Table 3). The triaryldiamines 4 with the same diarylamino
substituents as R-NPD exhibited a higher Tg compared to
that of R-NPD. The increase of the morphological stability
could be explained by an increase of the molecular weight
of the triaryldiamines 4, due to the introduction of the aryl
substituents at the central linkage. However, the two aryl
substituents at the C9 position of fluorene may hinder the
further crystallization process, which could also be ascribed
to the increase in Tg of triaryldiamines 4.
In summary, we have successfully established a new
method for the synthesis of sterically hindered 9,9-diarylfluo-
rene-based triaryldiamines. This new class of compounds has
revealed that the planarity of the fluorenyl core has a
significant effect on reducing the HOMO-LUMO energy
gap, which is observed from the red shifts in the absorption
spectra and the lower anodic oxidation onset. The diary-
lamino substituents at C2 and C7 of 9,9-diarylfluorene,
introduced by Pd-catalyzed amination, play a predominant
role in the photophysical properties. The introduction of
diaryl substituents at the C9 position of fluorene significantly
enhances the morphological stability of these fluorene-based
triaryldiamines. Further modifications of the central fluorene
linkage by introducing various aryl substituents with a higher
molecular weight and the application of these triaryldiamines
in OLED are currently under investigation.
Acknowledgment. We thank Mr. Chung-Shen Kao for
the DSC analysis and Dr. Jo¨rn Wirsching for his help on
the preparation of the manuscript. This work was supported
by the National Science Council of Taiwan (NSC-89-2113-
M002-053).
Supporting Information Available: Experimental pro-
cedures and characterization data of all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(9) Please see Supporting Information for the experimental procedures.
(10) Yamamoto, T.; Nishiyama, M.; Koie, Y. Tetrahedron Lett. 1998,
39, 2367.
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Org. Lett., Vol. 3, No. 15, 2001