Synthesis and properties of amorphous hole transport materials of triphenylamine based trihydrazones

Article abstract of DOI:10.1246/cl.2004.50

A novel class of amorphous state triphenylamine-based trihydrazones were synthesized by condensation reaction of tris(p-formylphenyl)amine with N-methyl-N-phenyl hydrazine, N,N-diphenylhydrazine, N-1-naphthyl-N-phenyl- hydrazine, and N-2-naphthyl-N-phenyl-hydrazine, respectively, and characterized using different scanning calorimetry (DSC) and cyclic voltammetry (CV).

Full text of DOI:10.1246/cl.2004.50

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Chemistry Letters Vol.33, No.1 (2004)
Synthesis and Properties of Amorphous Hole Transport Materials
of Triphenylamine Based Trihydrazones
ꢀ
Ke Jian Jiang, Ya Li Sun, Ke Feng Shao, and Lian Ming Yang
Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, P. R. China
(Received October 9, 2003; CL-030955)
A novel class of amorphous state triphenylamine-based
ning calorimetry (DSC) up to decomposition temperature. Here,
we report, for the first time, the syntheses and properties of novel
trihydrazones were synthesized by condensation reaction of
tris( p-formylphenyl)amine with N-methyl-N-phenyl hydrazine,
N,N-diphenylhydrazine, N-1-naphthyl-N-phenyl-hydrazine, and
N-2-naphthyl-N-phenyl-hydrazine, respectively, and character-
ized using different scanning calorimetry (DSC) and cyclic vol-
tammetry (CV).
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triphenylamine-based trihydrazones, triphenylamine-4,4 ,4 -tri-
carbalaldehyde tris(N-methyl-N-phenylhrdrazone) (3a), triphe-
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nylamine-4,4 ,4 -tricarbalaldehyde tris(N,N-diphenylhrdrazone)
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0
(3b), triphenylamine-4,4 ,4 -tricarbalaldehyde tris(N-1-naph-
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thyl-N-phenyl-drazone) (3c), and triphenylamine-4,4 ,4 -tricar-
balaldehyde tris(N-2-naphthyl-N-phenyl-hrdrazone) (3d), re-
spectively. As shown in Scheme 1, these compounds were
synthesized by condensation reaction of tris( p-formylphenyl)-
amine with N-methyl-N-phenyl hydrazine, N,N-diphenylhydra-
zine, N-1-naphthyl-N-phenyl-hydrazine, and N-2-naphthyl-N-
Recently, amorphous molecular materials, having stable
amorphous phase above room temperature, have been thorough-
ly investigated because of their excellent processability, trans-
parency, inexistence of grain boundaries and isotropic proper-
ties. They were extensively used in organic photoconductors
17
phenyl-hydrazine, respectively. Tris( p-formyl-phenyl)amine,
synthesized through Vilsmeier formylation of triphenylamine,
reacted with these different hydrazines with yield of 80, 77,
60, and 58%, respectively. The hydrazines were obtained by ni-
trosation and reduction reaction of corresponding diamines. The
compounds (3a–3d) were purified by chromatography, and con-
(
OPCs), light-emitting diodes (OLEDs), field effect transistors
1–4
and solid solar cells. Among these materials, the most widely
studied are hole transporting materials (HTMs). Polymers with
5
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hole transport moieties on their side or main chains, and mo-
lecularly doped systems, where small molecular hole transport
compounds are dispersed in polymer binders, can easily form
amorphous state. Most of them often show lower charge transfer
mobility compared to small molecular HTMs owing to lower
concentration of active moieties. However, small molecular
HTMs often suffer from poor thermal stability, and easy recrys-
tallization when they are allowed to stand at ambient tempera-
ture or heated above the glass state, which limit their use in pho-
toelectric devices. It is much important to design and synthesize
small molecular HTMs with high glass transition temperatures
1
firmed by H NMR.
The thermal properties of these compounds were studied us-
CHO
POCl3/DMF
N
N
OHC
CHO
1
a, R=methyl
NaNO2/HCl
Zn/HOAc
1b, R=Ph
HNRPh
a–1d
H2NNRPh
2a–2d
1c, R=1-naphthyl
1d, R=2-naphthyl
(
Tgs) and stable amorphous state. It is well known that many
1
compounds can be transformed into amorphous state by fast
cooling from the melt. Generally, the amorphous materials
should contain non-planar chemical structures, which can pre-
vent easy packing of molecules and hence hinder crystallization.
In addition, increasing number of conformer and incorporation
of bulky substitutes in the molecule also improve the formation
of amorphous state. In order to increase the glass transition tem-
perature, there are some effective measures such as incorpora-
tion of bulky substitutes in the molecules, increasing molecular
size and enhancing intermolecular interaction, which can hinder
molecular motions. Shirota and co-workers synthesized a series
of ‘burst star’ hole transport materials containing aromatic amine
CHO
N
CH=NNPhR
2
N
OHC
CHO
RPhNN=HC
CH=NNPhR
3
a–3d
Scheme 1. Synthetic routes of 3a–3d.
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groups, which form amorphous film on cooling from the melt.
Josef et al. designed spiro-linked molecular hole-transporting
materials, which show organic glass state with high thermal sta-
bility due to their high Tgs. Some other HTMs were also report-
9
10–16
ed recently.
For the most of these materials reported by now,
however, the amorphous states can be achieved only by cooling
from the melt, showing relatively low stability. Only a limited
number of compounds can completely form amorphous state,
and don’t show melting or crystallization in the differential scan-
Temperature/°C
Figure 1. DSC curves of 3a and 3c.
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.1 (2004)
51
Table 1. DSC and CV data of compounds 3a–3d
with the scan rate and no changed positions of anodic and catho-
dic peaks. The first oxidation potential increases with the molec-
ular size in the order 3a (0.17 V) < 3b (0.22 V) < 3c (0.23 V) =
Compound
Tg
Tm
ꢀ
max
/nmb
Eox
ꢁ
Ca /ꢁC
a
/Vc
/
3
d (0.23 V). Their low oxidation potentials can function as good
3
3
3
3
a
b
c
d
74
81
86
87
247
252.3, 332.0, 359.5,
0.17
241 286.1,332.7, 362.5, 382.7 0.22
hole transport materials for use in the photoelectric devices.
A series of hole transport materials were designed and syn-
thesized by consideration reaction of tris( p-formylphenyl)
amine with different hydrazines. Among these compounds, 3c
and 3d, with large substituent groups of N-1-naphthyl-N-phenyl
hydrazine and N-2-naphthyl-N-phenyl hydrazine respectively in
their molecules, show excellent stable amorphous states.
no
no
283.8, 331.4, 358.9, 381.0 0.23
270.6, 335.1, 360.7, 384.1 0.23
a
Obtained from differential scanning calorimetry (DSC) meas-
urement; no: Tg or Tm not detected. Measurement in CH2Cl–
CH2Cl solution. All data were reported relative to ferrocene
b
c
which has an oxidation potential at 0.58 V relative to Ag/
AgNO3. The sweeping voltage is from ꢂ100 to 1000 mV,
and the scan rate is 100 mV/s.
This work was supported by National Natural Science
Foundation of China (No. B59883004). We are grateful to Prof.
Yanagida and Dr. Kitamura at University of Osaka for DSC and
CV measurements.
ing differential scanning calorimetry (DSC). The glass transition
temperature (Tg), melting temperature (Tm) and recrystallization
temperature (Tc) are summarized in Table 1. 3a and 3b are crys-
talline and show a clear endothermic melting peak on first heat-
References
ꢁ
ing at 247 and 241 C, respectively. They form amorphous state
after cooling from the melt. Upon second heating, they show
1
2
D. M. Pai and J. Yanus, Photogr. Sci. Eng., 27, 14 (1983).
H. Tanaka, S. Tokito, Y. Taga, and A. Okada, Chem.
Commun., 1996, 2175.
ꢁ
glass transition at 74 and 81 C, respectively, no crystallization
ꢁ
was observed even they were heated above Tg over 300 C. It
is very interesting that, 3c and 3d didn’t show any endothermic
3
4
H. J. Bolink, C. Arts, V. V. Krasnikov, G. G. Maliaras, and
G. Hadziioannou, Chem. Mater., 9, 1407 (1997).
J. Ostrauskaite, V. Voska, J. Antulis, V. Gaidelis, V.
Jankauskas, and J. V. Grazulevicius, J. Mater. Chem., 12,
3469 (2002).
ꢁ
melting peaks on first and also on second heating up to 300 C
where they began to decompose. Although we attempted to crys-
tallize them from solution, they showed only amorphous state
ꢁ
with Tg at 86 and 87 C, respectively, which means that 3c and
3d possess higher stability compared with 3a and 3b. As report-
5
6
E. S. Kolb, R. A. Gaudiana, and P. G. Mehta, Macromole-
cules, 29, 2359 (1996).
M. Redecker, D. C. Bradley, M. Inbasekaran, W. W. Wu,
and E. P. Woo, Adv. Mater., 11, 241 (1999).
K. Katsuma and Y. Shirota, Adv. Mater., 10, 223 (1998).
Y. Shirota, J. Mater. Chem., 10, 1 (2000).
ed in the literature, triphenylamine possess non-planar chemical
structure with N atom deviating from the plane of the bonded C
ꢁ
atoms with a dihedral angle of about 75 between any phenyl
rings of the triphenylamine molecules. Their glass transition
7
8
9
1
8
ꢁ
temperatures increase from 74 to 86 C with their different hy-
drazine substitutes changing from N-methyl-N-phenylhydrazine
to N-1-naphthyl-N-phenylhydrazine in the triphenylamine mole-
S. Josef, W. Frank, and B. Jacqueline, Macromol. Symp.,
125, 121 (1997).
10 M. Thelakkat and H. W. Schmidt, Adv. Mater., 10, 219
(1998).
ꢁ
cules, whereas 3c and 3d almost possess same Tgs around 86 C
with N-1-naphthyl-N-phenylhydrazine and N-2-naphthyl-N-
phenylhydrazine substitutes, respectively. Because of large
substitutes in 3c and 3d, which can hinder regular arrangement
and motion of their molecules and lead to formalization of amor-
phous state, they show only glass state. We assume that 3c and
11 U. Bach, D. Lupo, P. Comte, J. E. Moser, F. Weiss o¨ rte, J.
Salbeck, H. Spreitzer, and M. Gr a¨ tzel, Nature, 395, 583
(1998).
12 S. Grigalevi cˇ ius, V. Getautis, J. V. Grazulevi cˇ ius, V.
Gaidelis, V. Jankauskas, and E. Montrimas, Mater. Chem.
Phys., 72, 395 (2001).
13 K. Okumota, K. Wayaku, T. Noda, H. Kageyama, and Y.
Shirota, Synth. Met., 111, 473 (2000).
14 N. X. Hu, S. Xie, Z. D. Popovic, B. Ong, and A. M. Hor,
Synth. Met., 111, 421 (2000).
15 D. E. Loy, B. E. Koene, and M. E. Thompson, Adv. Funct.
Mater., 12, 245 (2002).
16 I. Y. Wu, J. T. Lin, Y. T. Tao, and E. Balasubramaniam, Adv.
Mater., 12, 668 (2000).
17 H. Nam, D. H. Kang, J. K. Kim, and S. Y. Park, Chem. Lett.,
2002, 897.
18 M. H. Huang, F. Kartona, B. Dunn, and J. I. Zink, Chem.
Mater., 14, 5153 (2002).
3d are suitable materials to form stable amorphous glassy film,
which can be formed directly by solution coating.
The redox behavior of the hole-transporting materials were
characterised using cyclic voltammetry. The measurements were
conducted at a Pt disc electrode in dichloromethane solution
containing millimolar sample and 0.1 M tetra-n-butylammonium
hexafluorophosphate (TBAPF6) as supporting electrolyte. The
potentials were recorded against Ag/AgNO3 as reference elec-
trode and each measurement was calibrated with ferrocene/fer-
rocenium (Fc) redox system as an internal standard. Their first
oxidation potentials were listed in Table 1. The first oxidation
step for each of 3a–3d is reversible over the entire scan rate
range of 50 to 200 mV/s with the peak height varying linearly
Published on the web (Advance View) December 15, 2003; DOI 10.1246/cl.2004.50