Synthesis and properties of novel bis(triarylamines) based on a 3,3′-diphenyl-2,2′-bithiophene core

Article abstract of DOI:10.1039/b103194p

An efficient synthesis of 3,3′-diphenyl-2,2′-bithiophene based bis(triarylamines) and their physical properties are reported.

Full text of DOI:10.1039/b103194p

Synthesis and properties of novel bis(triarylamines) based on a
3,3A-diphenyl-2,2A-bithiophene core
Ken-Tsung Wong,*a Tsung Hsi Hung,a Shen C. Kao,b Chung Hsien Choua and Yuhlong Oliver Sua
a Department of Chemistry, National Taiwan University, Taipei 106, Taiwan.
E-mail: kenwong@ccms.ntu.edu.tw; Fax: +886 2 23636359; Tel: +886 2 23630231 ext. 3315
b Advance Instrumentation Center, National Taiwan University, Institute of Organic and Materials,
National Taipei University of Technology, Taiwan
Received (in Cambridge, UK) 10th April 2001, Accepted 28th June 2001
First published as an Advance Article on the web 3rd August 2001
An efficient synthesis of 3,3A-diphenyl-2,2A-bithiophene
based bis(triarylamines) and their physical properties are
reported.
Accordingly, the resulting new class of bis(triarylamines) based
on 3,3A-diphenyl-2,2A-bithiophene could enhance the thermal
and morphological stability, which are crucial for their
application in optoelectronic devices.
Amorphous triarylamines with high glass transition tem-
peratures (Tg) are widely used as hole-transporting materials in
organic light-emitting devices (OLED).1 Thiophene and oligo-
thiophene-linked triarylamines and bis(triarylamines), with
phenyl rings separating the thiophene system from the terminal
diarylamino groups were first introduced by Shirota et al.2
These amorphous materials form thin films with high morpho-
logical stability and also show interesting hole-transporting
properties, resulting in their successful use as colour-tunable
emitting materials for OLED devices. Triarylamines with
diarylamino group(s) directly attached to thiophene are one of
the least exploited class of hole-transporting materials.3,4
Recently, Watanabe and co-workers5 have accomplished a
systematical study on Pd-catalysed amination for the synthesis
of 2-diarylaminothiophenes and 2,5-bis(diarylamino)thio-
phenes. In addition to this, a new synthetic strategy involving
transition-metal free cyclization followed by thermal decarbox-
ylation has also been introduced for the synthesis of triar-
ylamines bearing a thiophene moiety.6 However, the physical
properties of thiophene based triarylamines are still limited.5,6
In this communication, we report the synthesis and physical
properties of a new class of bis(triarylamines) based on 3,3A-
diphenyl-2,2A-bithiophene as a central linkage. It is well
documented that the physical properties of oligothiophenes and
polythiophenes strongly depend on the nature of the sub-
stituents. However, the similar oxidation potential and absorp-
tion lmax of bithiophene and 3,3A-diphenyl-2,2A-bithiophene
have been attributed to counterbalancing the resonance effect of
phenyl substituents by the steric effect.7 Interestingly, poly-
thiophenes with phenyl substituent(s) at its 3- and 4-position
exhibit higher thermal stability and lower oxidation potentials.8
Cu-promoted Ullman reaction for the synthesis of 3,3A-
diphenyl-2,2A-bithiophene was first reported by Johnson in
1976.9 Further synthetic efforts based on the Ni-catalysed
Kumada coupling reaction of 2-halo-3-phenylthiophene with
2-magnesium-3-phenylthiophene gave 3,3A-diphenyl-2,2A-bi-
thiophene in poor yields.7,8 We report herein a more efficient
synthetic pathway (Scheme 1). In the presence of a catalytic
amount of Pd(PPh3)4, 3,3A-dibromo-2,2A-bithiophene 110 was
treated with phenylboronic acid in DME to afford 3,3A-
diphenyl-2,2A-bithiophene (2) in 86% yield. Regioselective
bromination11 of 2 was accomplished by treating 2 with
bromine in AcOH–CHCl3 (1+2) solution, which afforded 5,5A-
dibromo-3,3A-diphenyl-2,2A-bithiophene 3 in 95% yield. Six
different diarylamines were screened to react with 3 re-
spectively in the presence of a catalytic amount of Pd(OA-
c)2 and PBut3 in toluene12 at reflux temperature, resulting in
compounds 4a–4f as bright yellow solids in moderate yields
(Table 1). For a comparative study of the influences of phenyl
† Electronic supplementary information (ESI) available: experimental
details and NMR spectra. See http://www.rsc.org/suppdata/cc/b1/
b103194p/
Scheme 1 Reagents and conditions: a, PhB(OH)2, Pd(PPh3)4, Na2CO3,
DME, reflux 2 d, 86%; b, Br2, AcOH–CHCl3 (1+2), 0 °C to rt 98%; c,
diarylamine, Pd(OAc)2, PBut3, NaOBut, toluene, reflux overnight.
Table 1 Chemical yields and physical properties of 5,5A-bis(diarylamino)-3,3A-diphenyl-2,2A-bithiophenes 4 and model compound 5
Compound
Yield (%)a
lmax/nm, log eb
lem/nmc
Epa, Epc /mVd
Tg/°Ce
4a
4b
4c
4d
4e
4f
5
62
56
60
52
51
53
62
263 (4.59), 367 (4.12)
262 (4.69), 365 (4.26)
260 (4.96), 317 (4.87)
254 (4.90), 363 (4.09)
256 (4.99), 376 (4.54)
265 (4.94), 381 (4.48)
273 (4.38), 392 (4.42)
535
535
538
539
494
458, 503
480
550, 430
595, 440
570, 465
605, 470
(545, 455), (650, 570)
600, 500
(460, 385), (695, 610)
55
85
83
114
119
124
67
a Isolated yield by column chromatography on SiO2 (hexane–CH2Cl2 = 4+1) with satisfactory spectral analyses (1H, 13C, mass, HRMS, and IR). b Recorded
in EtOAc; units of e mol21 dm3 cm21 c Recorded in EtOAc and excited at lmax d Performed in 0.1 M solution of n-Bu4NPF6 in CH2Cl2, carbon electrode
. .
was used as the working electrode, scan rate 100 mV s21, Ag/AgCl as reference electrode. e Analyzed by differential scanning calorimetry (DSC), Tg was
recorded from heating (10 °C min21) a liquid nitrogen quenched melt-sample.
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Chem. Commun., 2001, 1628–1629
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b103194p

Fig. 1 Absorption and photoluminescence spectra of 4b (solid), 4e (dot),
and 5 (dash-dot) in EtOAc (1.0 3 1025 M)
Fig. 2 Cyclic voltammogram of 4b (solid), 4e (dot), and 5 (dash-dot) in
CH2Cl2.
substituents on the physical properties, model compound 5 was
also synthesized by the Pd-catalysed amination of 5,5A-
dibromo-2,2A-bithiophene.
that of 4e. The model compound 5 exhibited two well-resolved
redox couples (Epa 460 mV, Epc 385 mV and Epa 695 mV, Epc
610 mV vs. Ag/AgCl). The lower oxidation onset and larger
potential difference (230 mV) reveal that the the central
bithiophene linkage is in a coplanar conformation.
The absorption spectra of 4a–4f in ethyl acetate showed
similar behaviour, displaying two absorption maxima (Table 1).
The second bands of 4e and 4f were slightly red-shifted
compared to that of 4a–4d. Compound 4a–4d were highly
fluorescent. These results indicate that the absorption lmax is
relatively insensitive to the nature of the diarylamino sub-
stitutents. However, the photoluminescence efficiency and
emission maxima are strongly dependent on the structural
feature of the terminal diarylamino groups. Thus, bis(triar-
ylamines) (4a–4d) with less conjugated diarylamino sub-
stituents showed more efficient photoluminescence than when
compared with those bearing more conjugated terminal diaryla-
mino groups (4e–4f). The significant Stokes shift (ca. 170 nm
for 4a–4d) reveals that the 3,3A-diphenyl-2,2A-bithiophene
linkage may be highly twisted in the ground state. The emission
maxima with relatively long wavelength could be attributed to
the relaxation from an excited state with a more coplanar
conformation. The influence of phenyl substituents of the
bithiophene linkage on the photophysical properties are demon-
strated by a comparison of the UV-Vis and photoluminescent
spectra of 4b, 4e, and 5 (Fig. 1). Bis(triarylamine) 5 exhibited a
sharper, red-shifted absorption and a blue-shifted emission
compared to that of 4b. The longer emission wavelength of 4b
further confirms a more conjugated excited state.
All the compounds in this study exhibited an amorphous
nature evidenced by the presence of the glass transition
temperature (Table 1). The asymmetric diarylamino substituent
and the diphenyl substituted central bithiophene linkage
significantly contribute to the high Tgs. Compounds (4d, 4e, 4f)
with higher molecular weight aryl groups showed higher Tgs
compared to that of 4a, 4b and 4c bearing a lower molecular
weight aryl group in the terminal diarylamino substituents.
In summary, we have successfully established an efficient
method for the synthesis of a new class of bis(triarylamines)
bearing 3,3A-diphenyl-2,2A-bithiophene as a central linkage. The
introduction of phenyl substituents on the central linkage apart
from improving the morphological stability, twists the con-
formation of the central bithiophene linkage in the ground state,
which results in interesting photophysical and electrochemical
properties. Further studies on the modification of the central
bithiophene linkage by introducing bulkier aryl groups and their
applications in OLED are under way and will be reported in due
course.
We thank the National Science Council of Taiwan for
providing financial support (NSC-89-2113-M002-053).
Compounds 4a–4f exhibited only quasi-reversible anodic
oxidation (Table 1). The onset of oxidation and Epa (V vs. Ag/
AgCl) varied with the nature of the terminal diarylamino
groups. Differing from the conventional bis(triarylamines),13
only one redox couple (Epa 550 mV, Epc 430 mV vs. Ag/AgCl)
was detected for 4a and 4c. Coulometry in a thin layer cell
showed the redox process of 4a to be a two-electron oxidation.
The lack of co-planarity of the central 3,3A-diphenyl-2,2A-
bithiophene linkage prevents the extension of p-conjugation
along the molecular axis. The rate of second oxidation in 4a
may be faster than the conformational change reaching a more
coplanar conformation. Therefore, the first radical cation can
not efficiently delocalise in the whole molecule, the two
triarylamine systems behave independently but are oxidized
simultaneously without any communication. Fig. 2 shows a
comparison of the cyclic voltammogram of 4b, 4e, and 5. Two
partially resolved redox processes (Epa 545 mV, Epc 455 mV
and Epa 650 mV, Epc 570 mV vs. Ag/AgCl) of 4e were detected,
which were assigned to be a two-step one-electron redox couple
corresponding to removal of an electron from each triarylamine
system. The redox potential difference (110 mV) of 4e indicates
that the second oxidation could occur via a radical cation with
more coplanar conformation. 4b, 4d, and 4f showed similar
redox behaviour but were less resolved when compared with
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