Design and synthesis of stable triarylamines for hole-transport applications

Article abstract of DOI:10.1016/S0040-4039(01)00702-X

Three new tetrakis(triarylamino)methanes have been designed and synthesized. These triarylamines have been shown to exhibit high glass-transition temperatures and optimal oxidation potential for achieving efficient OLEDs. The para-position of the aryl rings is blocked with electron-donating t-butyl or methoxy groups, which enhances the radical cation stability of these molecules - a highly desirable quality for hole transporters.

Full text of DOI:10.1016/S0040-4039(01)00702-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 4421–4424
Design and synthesis of stable triarylamines for hole-transport
†
applications
Hongda Zhao, Christine Tanjutco and S. Thayumanavan*
Department of Chemistry, Tulane University, New Orleans, LA 70118, USA
Received 22 March 2001; accepted 2 April 2001
Abstract—Three new tetrakis(triarylamino)methanes have been designed and synthesized. These triarylamines have been shown to
exhibit high glass-transition temperatures and optimal oxidation potential for achieving efficient OLEDs. The para-position of the
aryl rings is blocked with electron-donating t-butyl or methoxy groups, which enhances the radical cation stability of these
molecules—a highly desirable quality for hole transporters. © 2001 Elsevier Science Ltd. All rights reserved.
Following the seminal report of 4,4%-bis(m-tolyphenyl-
amino)biphenyl (TPD, 1a) as an efficient hole trans-
1
R
porter, several triarylamine-based small molecules and
N
N
polymers have been investigated for applications such
as xerography and organic light-emitting diodes
R
2
,3
(OLEDs).
However, the design of triarylamine
1
molecules that possess high stability, while maintaining
high efficiency, has been minimal. For example, in a
recent study, the ionization potential of TPD was
1a; TPD; R = H
1b; MeO-TPD; R = p-OMe
1c; F-TPD; R = m-F
4,5
modified by varying the R-group in structure 1. Upon
studying the device property of the polymer versions of
these molecules, it was recognized that the F-TPD (1c)
exhibited poor stabilities, although its external quantum
efficiency was the highest among the TPD derivatives
is decreased, when compared to 1a. Similarly, the hole-
injection barrier is decreased and the hole–electron
recombination energy is increased for 1b.
In organic electroluminescent (EL) materials, the holes
and electrons that are generated can be considered to
be the radical cations and anions of the charge-trans-
porting molecular species, respectively. Therefore, the
difference in the stability of the devices was attributed
to the difference in radical cation stability. The mecha-
nism of decomposition of triarylamine radical cations
has been well established and is illustrated in Scheme
1. The difference in stability among 1a–c was ascribed
to the ease of proton removal in compounds that bear
an electron-withdrawing group (step 2 of Scheme 1).
Note that although the product of the decomposition is
also a triarylamine, this transformation impacts the
hole-transporting ability of the material for two rea-
6
studied. It was also noted in this study that the TPD
derivative that had an electron-donating substituent
(
MeO-TPD (1b)) exhibited good stability, albeit low
efficiency.
The difference in efficiency of these molecules is
ascribed to the difference in the oxidation potential of
these compounds and the resulting effect on the charge-
8
6,7
injection barrier and charge-recombination energy.
5
,9
As would be expected, F-TPD (1c) and MeO-TPD (1b)
exhibited the highest and lowest oxidation potentials,
respectively. In the case of 1c, the hole-injection barrier
is increased and the hole–electron recombination energy
R
R
-2 H+
H
R
R
R
N
R
R
R
R
R
H
N
N
N
N
N
R
R
H
Scheme 1.
*
Corresponding author.
This paper is dedicated to Professor Peter Beak on the occasion of his 65th birthday.
†
0
040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00702-X

4
422
H. Zhao et al. / Tetrahedron Letters 42 (2001) 4421–4424
R1
R2
sons: (i) the oxidation potential of the product triaryl-
amine is different from that of the starting material.
Therefore, the small concentration of the product could
act as traps for the charges and reduce the hole-trans-
porting ability of the material. (ii) The formation of
protons as by-products could deteriorate the efficiency
of the electroluminescent devices through undesirable
side reactions.
N
R1
N
R2
R2
R1
With these factors in mind, we set out to design triaryl-
amines that exhibit oxidation potentials similar to that
of F-TPD (1c), but exhibits much higher radical cation
stability. It has been well established that incorporation
of electron-donating substituents at the para-position
of triarylamines affords stable, often isolable, radical
N
N
R2
R1
2
3
4
(R1 = R2 = -OMe)
(R1 = OMe; R2 = -t-Bu)
(R1 = R2 = -t-Bu)
1
0
cations. Therefore, we designed triarylamines 2–4,
where the para-positions are blocked with electron-
donating groups. We particularly targeted electron-
donating para-substituents that lack benzylic
hydrogens. Thus, this design obviates not only the
dimerization pathways through the para-position in
Scheme 1, but also the possible dimerization through a
Ullmann reaction conditions. The product 4 was
obtained in 24% yield.
5
benzylic position. It is conceivable that all of the above
Cyclic voltammetry studies were carried out on the
triarylamines 2–4 to determine their oxidation poten-
tial. The electrochemical studies were performed in 0.1
M solution of (Bu N)PF in THF with a glassy carbon
structural features can be incorporated into a trisubsti-
tuted triphenylamine. However, these simpler structures
would lack the amorphous nature, a necessary feature
4
6
1
1
for hole-transporting materials. Therefore, we envi-
sioned that the tetrahedral architecture in structures
electrode as the working electrode. All potentials are
reported with respect to ferrocene/ferrocenium, which
was used as the internal standard. The E1/2 of 272, 351,
and 440 mV were observed for the tetra-
kis(triarylamines) 2, 3, and 4, respectively. The oxida-
tion waves of the compounds 2–4 were fully reversible,
compared to the irreversible oxidation waves observed
with triphenylamine itself (see Fig. 1). These results
were taken to indicate that the stability of the radical
cations of the triarylamines increases with the incorpo-
ration of electron-donating groups. Note that the first
oxidation potentials of TPD (1a) and F-TPD (1c) are at
269 and 371 mV, respectively. The oxidation potential
of the tetrakis(triarylamine) 2 at 272 mV is essentially
the same potential observed for the commonly used
TPD (1a). The oxidation potential of the compound 3
compares very well with that of the more efficient, but
less stable hole-transport material, F-TPD (1c). The
presence of electron-donating groups in 3 to stabilize
the radical cation is promising to afford stable and
efficient electroluminescent devices.
2
–4 would also provide the targeted materials with high
12
glass-transition temperatures (Tg).
The triarylamines were synthesized using the palladium-
catalyzed carbonꢀnitrogen bond-forming methodo-
1
3,14
15
logy
or by Ullmann coupling. The aniline 5 or 6
was treated with the aryl bromide 7 or 8 in the presence
of sodium tert-butoxide and catalytic amounts of
Pd (dba) and DPPF, to afford the diarylamines 9–11
2
3
in 72–98% yields, as shown in Scheme 2. Treatment of
1
6
excess 9 and 10 with tetrakis(4-iodophenyl)methane
(
12) under the above reaction conditions afforded prod-
ucts 2 and 3 in 9% and 48% yield, respectively. How-
ever, reaction of the diarylamine 11 with 12 did not
afford the product 4 under the same conditions. The
reason for the difference in yields with varying sub-
stituents in the diarylamines is not apparent to us at
this time. To achieve the targeted tetrakis(triarylamine)
product 4, we treated the diarylamine 11 with 12 under
I
C
Pd (dba) , DPPF
2
3
R2
R1
4
NaOt-Bu, toluene
12
R1
NH2
Products
2,3, or 4
N
H
R2 Br
"Pd" (or) Ullmann
conditions
5
, R = -OMe
1
7
2-97% yield
7
, R = -OMe
2
6
, R = -t-Bu
1
9
(R1 = R2 = -OMe)
8
, R = -t-Bu
2
1
0 (R1 = OMe; R2 = -t-Bu)
1 (R1 = R2 = -t-Bu)
1
Scheme 2.

H. Zhao et al. / Tetrahedron Letters 42 (2001) 4421–4424
4423
Since the oxidation potential of the benzidine derivative
is much lower than that of triphenylamine, it could be
oxidized by the radical cation of triphenylamine. This
transformation could also be due to the reaction of any
unreacted SbCl with the benzidine product. In the case
5
of 3, a slight increase in the radical cation peak at 720
nm was observed over time. These results suggest that
the radical cation of 3 is significantly more stable than
5
those of TPD or Ph N. The small increase in absorp-
3
tion is ascribed to the slow reaction between the triaryl-
amine and SbCl , since the reaction was carried out
5
with extremely dilute solutions to facilitate observation
by absorption spectroscopy.
The tetrahedral molecular architecture was also chosen
in the system to provide amorphous materials that have
high glass-transition (T ) and high recrystallization tem-
g
peratures to prevent failures associated with joule heat-
1
1
ing at device interfaces. The T of 2, 3, and 4 were
g
measured to be 169, 145, and 185°C, respectively, as
measured using differential-scanning calorimetry
(
DSC). These glass-transition temperatures are superior
Figure 1. Electrochemical comparison of triphenylamine and
compound 2.
to those observed for the TPD derivatives 1, which
were around 50°C.
4
In order to further confirm that blocking the para-posi-
tion enhanced the chemical stability of the radical
cations of the designed triarylamines, radical cations of
In summary, we have designed and synthesized a new
series of triarylamine derivatives that exhibit high radi-
cal cation and thermal stabilities. In addition, the pre-
liminary electrochemical investigations suggest that the
redox potentials of these compounds are also suitable
to provide high efficiency in electroluminescent devices.
Efforts are underway to collaboratively study the hole-
transporting ability of these triarylamines in electro-
luminescent devices.
3
and triphenylamine were also chemically generated by
reaction with SbCl . The stabilities of these radical ions
5
were compared by monitoring the absorption spectra of
these radical cations (Fig. 2). In the case of triphenyl-
amine, the peak that belongs to the triphenylamine
radical cation (658 nm) decayed rapidly with time,
while the peak at 484 nm increased with time. The
latter peak corresponds to the absorption spectra of the
5
monocation of N,N,N%,N%-tetraphenylbenzidine, which
is the decomposition product of triphenylamine
Acknowledgements
(
Scheme 1). These results suggest that the benzidine
product is immediately formed even in dilute solutions.
The authors would like to thank the 3M Company for
funding through a Non-Tenured Faculty Award to S.T.
They are grateful to Dr. K. Mohanalingam and Mr.
Josh Yukich for their help with electrochemistry. They
would also like to thank Mr. Amir Sabahi for synthe-
sizing compound 9.
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