Amorphous molecular materials with high carrier mobilities: Thiophene- and selenophene-containing tris(oligoarylenyl)amines

Article abstract of DOI:10.1246/cl.2004.1266

A novel class of amorphous molecular materials, thiophene-and selenophene-containing tris(oligoarylenyl)amines, were developed. They were found to exhibit high hole drift mobilities greater than 10^-2 cm ²V^-1s^-1 at an electric field of 1.0 × 10⁵ Vcm^-1 at 293 K in their amorphous glassy states. These values are of the highest level among those reported for organic disordered systems.

Full text of DOI:10.1246/cl.2004.1266

1266
Chemistry Letters Vol.33, No.10 (2004)
Amorphous Molecular Materials with High Carrier Mobilities:
Thiophene- and Selenophene-Containing Tris(oligoarylenyl)amines
Hitoshi Ohishi, Masatake Tanaka, Hiroshi Kageyama, and Yasuhiko Shirotaꢀ
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka, Suita, Osaka 565-0871
(Received June 14, 2004; CL-040683)
A novel class of amorphous molecular materials, thiophene-
TTPA and TPTPA were synthesized by the coupling reac-
tions of tris(4-iodophenyl)amine with the corresponding oligo-
arylenylmagnisium bromide in tetrahydrofuran (THF) in the
presence of 1,3-bis(diphenylphosphino)propane nickel(II) chlo-
ride as a catalyst. TSePA and TPSePA were synthesized by
the coupling reactions of tris(4-iodophenyl)amine with the cor-
responding oligoarylenylzinc chloride in THF in the presence
of tetrakis(triphenylphosphine)palladium as a catalyst. They
and selenophene-containing tris(oligoarylenyl)amines, were de-
veloped. They were found to exhibit high hole drift mobilities
greater than 10ꢁ2 cm2Vꢁ1sꢁ1 at an electric field of 1:0 ꢂ 105
Vcmꢁ1 at 293 K in their amorphous glassy states. These values
are of the highest level among those reported for organic disor-
dered systems.
1
were identified by mass spectrometry, H and 13C NMR spec-
troscopies, and elemental analysis.8
Charge transport in organic disordered systems plays an im-
portant role in the operation of organic electronic and optoelec-
tronic devices, e.g., photoreceptors in electrophotography, elec-
troluminescent devices, and photovoltaic devices.
These tris(oligoarylenyl)amines were found to form amor-
phous glasses above room temperature when the melt samples
were cooled, as evidenced by differential scanning calorimetry
(DSC), X-ray diffraction, and polarized light microscopy. The
glass transition temperatures of TTPA, TPTPA, TSePA and
TPSePA were 70, 83, 80, and 105 ꢃC, respectively, as deter-
mined by DSC. To our knowledge, TSePA and TPSePA are
the first examples of selenium-containing amorphous molecular
materials.
Hole drift mobility was measured by a time-of-flight meth-
od. An appropriate amount of material was heated to melt on an
indium–tin–oxide (ITO)-coated glass, which was pressed by an-
other ITO glass and then cooled with ice water. The cell was
mounted in a cryostat and irradiated with pulsed N2 laser light
(wavelength: 337 nm, pulse duration: 3 ns).
Organic disordered systems such as polymers and molecu-
larly doped polymers, where low molecular weight organic com-
pounds are dispersed into polymer binders, have been studied
extensively. We have performed a series of studies on the crea-
tion of low molecular-weight organic compounds that readily
form stable amorphous glasses above room temperature, namely,
‘‘amorphous molecular materials.’’1 The creation of amorphous
molecular materials has enabled studies of charge transport in
their amorphous glassy states without polymer binders.1–3 Re-
cently, there have been numerous studies on charge transport
in amorphous molecular materials.4–7 It has been revealed that
amorphous molecular materials exhibit drift mobilities in the
range from 10ꢁ5 to 10ꢁ3 cm2Vꢁ1sꢁ1, greatly depending upon
the structures of materials.3 Materials that exhibit drift mobilities
of the order of 10ꢁ3 cm2Vꢁ1sꢁ1 have been limited in number.4,6,7
It is a challenging subject to develop amorphous molecular
materials with one order greater drift mobilities, 10ꢁ2
cm2Vꢁ1cmꢁ1, in the amorphous glassy state. In the light of
the consideration that oligoarylenes exhibit high hole mobilities
of ca. 1 ꢂ 10ꢁ3 cm2Vꢁ1sꢁ1,3 and that the incorporation of the
thiophene or selenophene ring may result in a higher degree of
planarity of the oligoarylene unit, leading to better intermolecu-
lar overlap of ꢀ electrons, we have designed and synthesized a
new class of amorphous molecular materials, thiophene- and se-
lenophene-containing tris(oligoarylenyl)amines, tris[4-(2-thie-
nyl)phenyl]amine (TTPA), tris{4-(5-phenylthiophen-2-yl)phen-
yl}amine (TPTPA), tris[4-(2-selenyl)phenyl]amine (TSePA),
and tris{4-(5-phenylselenophen-2-yl)phenyl}amine (TPSePA).
All the molecular glasses of TTPA, TPTPA, TSePA, and
TPSePA exhibited nondispersive transient photocurrents in the
wide range of temperatures and electric fields. The transit time
(t ) was determined from the cusp of the plot of log iph vs
ꢁ
log t, where iph and t represent the transient photocurrent and
time, respectively, according to the Scher–Montroll theory.9
The hole drift mobility (ꢂ) was calculated from the formula
ꢂ ¼ L2=t V, where L is the thickness of the sample and V is
ꢁ
the applied voltage. The hole drift mobilities of these molecular
glasses were found to exceed 10ꢁ2 cm2Vꢁ1sꢁ1 as shown in
Table 1. These values are of the highest level among those re-
ported for organic disordered systems.
The electric-field and temperature dependencies of the
charge carrier drift mobilities of these molecular glasses were
analyzed in terms of the disorder formalism (Eq 1), which as-
sumes that charge transport in disordered systems occurs by hop-
ping through a manifold of states, subject to fluctuations in both
the hopping site energies (energetic disorder) and the intermo-
lecular wavefunction overlap (positional disorder),10
X
X
"
#
( "
#
)
ꢀ
ꢁ
ꢀ
ꢁ
2
2ꢁꢄ2 E1=2 ð1Þ
2ꢃ
3kT
ꢃ
kT
ꢂ ¼ ꢂ0 exp ꢁ
exp C
N
N
X
X
X
X
where ꢃ and ꢄ are the parameters that characterize the energetic
and positional disorder, respectively, ꢂ0 represents a hypotheti-
cal mobility in the energetic disorder free system, E is the elecric
X = S : TTPA
X = Se : TSePA
X = S : TPTPA
X = Se : TPSePA
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.10 (2004)
1267
field, k is the Boltzmann constant, T is the temperature, and C is
an empirical constant.
cule and the degree of intermolecular wavefunction overlap.
The energetic disorder ꢃ, namely, the fluctuation of the hopping
site energy, is understood as determined both by the variation of
molecular geometry caused by bond rotation and by the fluctua-
tion of polarization energy resulting from van der Waals and
charge-dipole interactions. It is thought that the incorporation
of the thiophene- and selenophene-containing oligoarylenylene
unit enhances the intrinsic charge transporting ability and/or
the degree of intermolecular wavefunction overlap and that the
synthesized molecules with symmetrical structures reduce
energetic disorder because of a small degree of variation of
the molecular geometry.
Figure 1 shows the electric field dependencies of the hole
drift mobilities at various temperatures observed for the TSePA
molecular glass. The hole drift mobilities followed the electric
field dependence of expðSE1=2Þ, where S represents a coefficient.
Hole drift mobilities at the zero electric field, ꢂðE ¼ 0Þ, were
obtained by the extrapolation of the electric field dependence
of the hole drift mobilities to the zero electric field. The values
of ꢂ0 and ꢃ were obtained from the intersect and the slope, re-
spectively, of the linear plot of the logarithm of ꢂðE ¼ 0Þ vs
T
ꢁ2. The ꢄ value was determined from the intersection at
2
2
S ¼ 0 in the linear plot of S vs ðꢃ=kTÞ , where ðꢃ=kTÞ ¼ ꢄ2
holds. The hole transport parameters based on the disorder for-
malism for these molecular glasses are summarized in Table 1.
The results show that while ꢄ and C values for TTPA,
TPTPA, TSePA, and TPSePA are more or less similar to those
for many other amorphous molecular materials, the pre-expo-
nential factor ꢂ0 for these molecular glasses are approximately
one order of magnitude greater than those for a number of amor-
phous molecular materials. In addition, the ꢃ values for these
molecular glasses are relatively small as compared with other
amorphous molecular materials. It is therefore indicated that
the high hole drift mobilities observed for these molecular
glasses are attributed both to the large pre-exponential factor
ꢂ0 and to the relatively small energetic disorder ꢃ.
In summary, novel thiopehen- and selenophene-containing
tris(oligoarylenyl)amines were developed. They were found to
exhibit high hole drift mobilities exceeding 10ꢁ2 cm2Vꢁ1sꢁ1
,
which are of the highest level among those reported for organic
disordered systems. It is shown that the high hole drift mobilities
are attributable to a large pre-exponential factor and to a relative-
ly small energetic disorder in terms of the disorder formalism.
The present study provides a new molecular design concept
for amorphous molecular materials with high mobilities, paving
the way for future development of amorphous molecular materi-
als with high-performance charge transport.
References and Notes
1
2
3
4
5
Y. Shirota, J. Mater. Chem., 10, 1 (2000) and references cited
therein.
K. Nishimura, T. Kobata, H. Inada, and Y. Shirota, J. Mater.
Chem., 1, 897 (1991).
Y. Shirota, S. Nomura, and H. Kageyama, Proc. SPIE-Int. Soc.
Opt. Eng., 3476, 132 (1998) and references cited therein.
P. M. Borsenberger, L. Pautmeier, R. Richert, and H. Bassler,
¨
J. Chem. Phys., 94, 8276 (1991).
The pre-exponential factor, namely, the hypothetical mobi-
lity in the absence of energetic disorder, is thought to be deter-
mined by the intrinsic charge-transporting ability of the mole-
G. G. Malliaras, Y. Shen, D. H. Dunlap, H. Murata, and Z. H.
Kafafi, Appl. Phys. Lett., 79, 2582 (2001).
K. Okumoto and Y. Shirota, Chem. Lett., 2000, 1034.
6
7
8
K. Okumoto and Y. Shirota, Mater. Sci. Eng., B, 85, 135 (2001).
TTPA. Yield: 34%. Mass (TOF): m=z ¼ 491 (Mþ). 1H NMR
(750 MHz, THF-d8): ꢅ 7.03 (dd, 3H), 7.12 (d, 6H), 7.29 (d,
3H), 7.30 (d, 3H), 7.55 (d, 6H). 13C NMR (188 MHz, THF-d8):
ꢅ 123.2, 124.9, 125.2, 127.4, 128.6, 130.4, 144.7, 147.4. Anal.
Calcd. for C30H21NS3: C, 73.28; H, 4.30; N, 2.85; S, 19.56.
Found: C, 73.31; H, 4.26; N, 2.82; S, 19.16%. TPTPA. Yield:
7%. Mass (EI): m=z ¼ 719 (Mþ). 1H NMR (600 MHz, THF-
d8): ꢅ 7.16 (d, 6H), 7.24 (t, 3H), 7.33 (d, 3H), 7.36 (dd, 6H),
7.37 (d, 3H), 7.61 (d, 6H), 7.65 (d, 6H). 13C NMR (150 MHz,
THF-d8): ꢅ 124.3, 124.9, 125.2, 160.0, 127.2, 128.0, 129.6,
130.2, 135.2, 143.6, 143.9, 147.5. Anal. Calcd for C48H33NS3:
C, 80.07; H, 4.62; N, 1.95; S, 13.36. Found: C, 79.96; H, 4.61;
N, 1.91; S, 13.35%. TSePA. Yield: 21%. Mass (TOF): m=z ¼
635 (Mþ). 1H NMR (400 MHz, THF-d8): ꢅ 7.10 (d, 6H), 7.25
(dd, 3H), 7.44 (dd, 3H), 7.51 (d, 6H), 7.94 (dd, 3H). 13C NMR
(100 MHz, THF-d8): ꢅ 125.1, 125.3, 127.8, 130.1, 131.1,
132.4, 147.5, 150.9. Anal. Calcd for C30H21NSe3: C, 56.98; H,
3.35; N, 2.21; Se, 37.46. Found: C, 56.78; H, 3.38; N, 2.31%.
TPSePA. Yield: 15%. Mass (EI): m=z ¼ 863 (Mþ). 1H NMR
(600 MHz, THF-d8): ꢅ 7.14 (d, 6H), 7.24 (t, 3H), 7.34 (dd,
6H), 7.48 (d, 3H), 7.53 (d, 3H), 7.55 (d, 6H), 7.59 (d, 6H).
13C NMR (150 MHz, THF-d8): ꢅ 125.2, 126.4, 126.4, 127.1,
127.6, 128.1, 129.6, 132.3, 137.3, 147.5, 149.7, 149.9. Anal.
Calcd for C48H33NSe3: C, 66.98; H, 3.86; N, 1.63; Se, 27.52.
Found: C, 66.77; H, 3.86; N, 1.63%.
Figure 1. Electric field dependencies of hole drift mobilities
for the molecular glass of TSePA.
Table 1. Hole drift mobilities and charge transport parameters
determined for the molecular glasses of TTPA, TPTPA, TSePA,
and TPSePA in terms of the disorder formalism
ꢂ
ꢂ0
ꢃ
/eV
C
Material
ꢄ
/cm2Vꢁ1sꢁ1a /cm2Vꢁ1sꢁ1
/(cmVꢁ1
)
1=2
TTPA
1:1 ꢂ 10ꢁ2
1:0 ꢂ 10ꢁ2
1:5 ꢂ 10ꢁ2
1:1 ꢂ 10ꢁ2
1:1 ꢂ 10ꢁ1
1:8 ꢂ 10ꢁ1
1:6 ꢂ 10ꢁ1
1:1 ꢂ 10ꢁ1
0.064 1.1
0.075 1.3
0.063 1.8
0.067 1.0
3:3 ꢂ 10ꢁ4
4:1 ꢂ 10ꢁ4
3:6 ꢂ 10ꢁ4
3:7 ꢂ 10ꢁ4
TPTPA
TSePA
TPSePA
9
10 H. Bassler, Phys. Status Solidi B, 107, 9 (1981); H. Bassler,
H. Scher and E. W. Montroll, Phys. Rev. B, 12, 2455 (1975).
¨
¨
Phys. Status Solidi B, 175, 15 (1993).
aMeasured at an electric field of 1:0 ꢂ 105 Vcmꢁ1 at 293 K.
Published on the web (Advance View) September 4, 2004; DOI 10.1246/cl.2004.1266