On-top π-stacking of quasiplanar molecules in hole-transporting materials: Inducing anisotropic carrier mobility in amorphous films

Article abstract of DOI:10.1002/anie.201400068

Dimers of partially oxygen-bridged triarylamines were designed and synthesized as hole-transporting materials. X-ray structural analyses revealed that these compounds form on-top π-stacking aggregates in the crystalline state. TRMC measurements showed tha

Full text of DOI:10.1002/anie.201400068

Angewandte
Chemie
DOI: 10.1002/anie.201400068
Heterocycles
On-Top p-Stacking of Quasiplanar Molecules in Hole-Transporting
Materials: Inducing Anisotropic Carrier Mobility in Amorphous
Films**
Atsushi Wakamiya,* Hidetaka Nishimura, Tatsuya Fukushima, Furitsu Suzuki, Akinori Saeki,
Shu Seki, Itaru Osaka, Takahiro Sasamori, Michihisa Murata, Yasujiro Murata,* and
Hironori Kaji*
Abstract: Dimers of partially oxygen-bridged triarylamines
were designed and synthesized as hole-transporting materials.
X-ray structural analyses revealed that these compounds form
on-top p-stacking aggregates in the crystalline state. TRMC
measurements showed that high levels of anisotropic charge
transport were induced in the direction of the p-stacking.
Surprisingly, even in vacuum-deposited amorphous films, these
compounds retained some of the face-on p-stacking, thus
facilitating an out-of-plane carrier mobility.
acenes or increasing the C/H ratio in the ring-fused aromatic
compounds can be effective strategies for a molecular design
in which CH-p interactions are prevented and a better
packing motif with facilitated charge mobility is obtained.[2,6]
However, molecular design approaches that allow effective
control over the molecular orientation in the solid state
remain limited. Representative examples for hole-transport-
ing materials used in OLEDs are triarylamines with propeller-
like structures, such as TPD[7a] and a-NPD.[7b] These materials
are used as amorphous films, and the conformations of
individual molecules as well as of their aggregates have not
been determined unambiguously, thus rendering precise
conclusions regarding molecular design strategies difficult.[8]
Accordingly, the elucidation of the exact relationship
between the molecular orientation in the crystalline and
amorphous state remains a key challenge to be addressed in
the design of advanced charge-transporting materials.
In this study, we focused on partially bridged triarylamines
as the p-conjugated skeletons for the charge-transporting
materials. In these amines, three phenyl groups are con-
strained in a quasiplanar fashion by two oxygen bridges
(Figure 1). We envisioned that the use of quasiplanar
scaffolds should be beneficial to provide: a) delocalized p-
conjugation by enhanced planarity, and b) dense p-stacking in
the solid state, arising from slightly twisted molecular
structures. In addition, the dispersion in the direction of the
T
he development and characterization of improved charge-
transporting materials still remains a crucial issue for the
achievement of high-performance organic devices, for exam-
ple, for organic light-emitting diodes (OLEDs),[1] organic
field-effect transistors (OFETs),[2] and organic photovoltaics
(OPVs).[3] Efficient charge transport in molecular solids
requires charges to move easily from molecule to molecule.[4]
In the molecular design of these materials, it is therefore
important not only to control the p-electronic structure for
tuning the HOMO and LUMO levels and lowering of the
reorganization energy (l), but also to control the packing
structure in the solid state to facilitate large charge transfer
integrals (J). Acenes, such as pentacene, are widely used as
charge-transporting materials in OFETs, because of their
rigid p-conjugated skeletons with high planarity. In the solid
state, pentacene adopts a herringbone structure with a tilted
molecular arrangement, which is the result of p-p interactions
and the presence of CH-p interactions (electrostatic inter-
action).[5] Introducing substituents in the peri-position of
À
C H bonds in the lateral position of the quasiplanar
molecules should decrease the contribution of the CH-p
interactions between neighboring molecules, so that on-top p-
[*] Prof. Dr. A. Wakamiya, H. Nishimura, Dr. T. Fukushima, F. Suzuki,
Prof. Dr. T. Sasamori, Dr. M. Murata, Prof. Dr. Y. Murata,
Prof. Dr. H. Kaji
Prof. Dr. A. Wakamiya
Precursory Research for Embryonic Science and Technology
(PRESTO) (Japan) Science and Technology Agency
4-1-8 Honcho, Kawaguchi, Saitama 332-0012 (Japan)
Institute for Chemical Research, Kyoto University
Uji, Kyoto 611-0011 (Japan)
E-mail: wakamiya@scl.kyoto-u.ac.jp
yasujiro@scl.kyoto-u.ac.jp
[**] This work was partially supported by JST and the Collaborative
Research Program at the ICR, Kyoto Univ. Support was also received
from the Funding Program for World Leading Innovative R&D on
Science and Technology (FIRST Program). The NMR measurements
were supported by the Joint Usage/Research Center (JURC) at the
ICR, Kyoto Univ. Synchrotron single-crystal X-ray analysis was
carried out with the SPring-8 beam line BL38B1 with the approval of
JASRI (2012A1448, 2012B1319, and 2013A1489). 2D GIXD experi-
ments were performed at BL19B2 of SPring-8 with the approval of
JASRI (2013A1634). We thank Dr. T. Koganezawa (JASRI) for advice
on the 2D GIXD measurements.
kaji@scl.kyoto-u.ac.jp
Dr. A. Saeki, Prof. Dr. S. Seki
Department of Applied Chemistry
Graduate School of Engineering, Osaka University
2-1 Yamadaoka, Suita, Osaka 565-0871 (Japan)
Dr. I. Osaka
Emergent Molecular Function Research Group, RIKEN CEMS
Wako, Saitama 351-0198 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201400068.
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13 (65%). Homocoupling of brominated 8b, 8c, 10, and 13
yielded the corresponding dimers 1–4 (54–98%) as pale
yellow solids (Scheme 1b).
The electrochemical and photophysical data as well as
thermal analyses for 1–4 are summarized in Table 1. A
detailed discussion of these properties can be found in the
Supporting Information.
The optimized structures of 1–4, calculated at the B3LYP/
6-31G(d) level of theory, supported the expected quasiplanar
structures for the partially oxygen-bridged triarylamine
moieties (see Figure S17 in the Supporting Information).
The HOMOs of 1–4 are highly delocalized over the whole p-
skeleton (see Figure S18 in the Supporting Information),
which allows effective tuning of the HOMO levels through
the peripheral substituents on the outer phenyl rings. Fully
delocalized HOMOs should also lower the reorganization
energies of the hole transfer. For 1–4, these were calculated at
the B3LYP/6-31G(d) level to be lower (1: l+ = 0.15 eV; 2:
0.15 eV; 3: 0.18 eV; 4: 0.17 eV) than those for TPD (l+ =
0.26 eV) or a-NPD (l+ = 0.28 eV).
Figure 1. Molecular design aspects for charge-transporting materials
with a quasiplanar structure as the key feature.
stacking could be facilitated in the solid state. As model
compounds, we designed and synthesized a series of partially
oxygen-bridged triarylamine dimers 1–4 (Figure 1). Herein,
we report on the characterization of 1–4 in
solution and the solid state, as well as on the
relationship between the packing structures
and the anisotropic charge-carrier-transport
properties, which were observed for 1 and 2
both in the crystalline and amorphous
states.
The key reaction step to establish qua-
siplanar structures in 8a–c is a twofold
intramolecular nucleophilic aromatic sub-
stitution (Scheme 1a). Compounds 7a–c
were prepared by Ullmann reactions of 5a
or 5b and 6a or 6b (61–68%). After
removal of the protecting methyl groups in
7a–c with BBr3, a subsequent treatment
with K2CO3 in DMF at 1008C resulted in
the formation of cyclized products 8a–c
(67–95%). This synthetic route is straight-
forward, based on commercially available
starting materials, and can be easily scaled
up. In contrast to a previously reported
synthetic route for 8a via the linear inter-
mediate 2,6-bis(2-bromophenoxy)aniline,[9]
our route allows the preparation of deriva-
tives containing substituents in the desired
positions. For example, the bromination of
triarylamine 7a with 1 equiv of NBS
resulted in the selective formation of 9
(75%). The subsequent twofold cyclization
of 9 gave monobrominated 10, an isomer of
8b, in almost quantitative yield. The unsym-
metric triarylamine 13 was obtained from
two successive coupling reactions of 6b with
different aryl components. Pd0-catalyzed
Scheme 1. Reagents/conditions: i) Cu, K2CO3, o-dichlorobenzene, 1808C; ii) 1. BBr3, CH2Cl2,
2. K2CO3, DMF, 1008C, (then CH3I, 608C for 8c); iii) NBS (1 equiv), CHCl3/AcOH, iv) [Pd2-
(dba)3]·CHCl3, P(tBu)3, NaOtBu, toluene, 1008C; v) [Ni(cod)2], bipyridine, cod, THF, 608C.
NBS=N-bromosuccinimide, dba=trans,trans-dibenzylideneacetone, cod=cycloocta-1,5-
arylation of 5a with 6b resulted in the
selective formation of 11 (68%). A subse-
quent second Ullmann arylation afforded
12 (75%), which was doubly cyclized to give diene.
2
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Table 1: Thermal analysis, electrochemical, and photophysical data for
1–4.
Interestingly,
2 exhibited crystalline polymorphism.
Recrystallization of 2 from toluene (110–1208C) resulted in
the deposition of needlelike crystals (P1), containing one-
dimensional on-top p-stacks of 2 along the short a-axis
(Figure 3a,c), similar to 1, 3, and 4. On the other hand,
recrystallization from o-dichlorobenzene at higher temper-
ature (140–1608C) or sublimation (2658C, 0.1mmHg) pro-
ꢀ
Td5 Tg[b] E1/2, ox
IP[d] A[e]
[eV] [nm]
(loge)
FLliq
FLsol
[a]
[c]
[e]
[f]
[8C] [8C] [V]
[nm]
[nm]
(F)[g]
(F)[g]
1
2
3
4
449 64 +0.25 +0.44
441 123 +0.23 +0.42
408 56 +0.41 +0.58
5.05 387
(4.22)
5.03 397
(4.48)
5.21 385
(4.16)
4.82 405
(4.10)
451
(0.51)
449
(0.69)
439
(0.45)
489
491
(0.40)
508
(0.51)
485
(0.14)
533
443 34 +0.02 +0.17
+0.86[h]
(0.77)
(0.19)
[a] 5% weight loss temperature measured by thermogravimetric analysis
(TGA). [b] Glass transition temperature measured by differential scan-
ning calorimetry (DSC). [c] Half-wave oxidation potential (versus Fc/Fc+)
measured by cyclic voltammetry in CH2Cl2 (0.3 mm) with [(nBu)4N][PF6]
(0.1m) as the supporting electrolyte. [d] Estimated ionization potentials
(IPs) from the first oxidation potential (+4.80 eV).[10] [e] UV absorption/
fluorescence measured in CH2Cl2. [f] Fluorescence measured in the solid
state (ground crystals). [g] Absolute quantum yields determined by
a calibrated sphere system. [h] Corresponding to a two-electron
oxidation.
Single crystals of 1–4 suitable for X-ray diffraction
analysis were obtained by sublimation or recrystallization.
The structural analysis confirmed quasiplanar structures for
dimers 1–4 in the crystalline state. For example, the dihedral
angles between the outer and inner phenyl rings in 1 (18.68/
23.08; see Figure 2a) are in good agreement with the
Figure 3. X-ray crystal structures of 2: ORTEP drawings (50% proba-
bility for thermal ellipsoids) and packing structures for crystals
obtained from toluene (a,c) and from o-dichlorobenzene (b,d).
Figure 2. X-ray crystal structure of 1: a) ORTEP drawing (50% proba-
bility for thermal ellipsoids) and b) packing structure.
vided platelike crystals (P21/a), in which two-dimensional p-
stacks of 2 with a slipped arrangement were observed
(Figure 3b,d). A determination of the face index confirmed
that the longest axis in both types of crystals corresponds to
the a-axis in the p-stacking direction (see Figure S29 in the
Supporting Information). Since neither a phase transition
between the two crystals, nor a difference in the melting point
could be observed, we assume that the platelike crystals form
by entropy-driven processes at higher temperature.
To gain insight into the relationship between packing
structures and carrier-transport properties, we conducted
theoretical calculations at the PW91/DZP level and evaluated
the charge-transfer integrals (J) for crystals of 2 with different
packing modes. For the one-dimensional p-stacking structure
shown in Figure 3c, a large J value (198 meV) was obtained
for the hole-transfer integral in the direction of the on-top p-
stacking (a-axis), whereas small values were observed in the
other directions (0.72–12.8 meV). For the two-dimensional p-
stacking structure shown in Figure 3d, only moderate J values
(4.8–27.0 meV) were obtained, with less anisotropy in all
calculated optimized structure (22.88). However, the dihedral
angles between the inner phenyl rings obtained from the
optimized structures of 1–4 (33.8–35.18) are not consistent
with those observed in the crystal structures for 1 (0.08) and 4
(49.78). This discrepancy can probably be attributed to
packing forces. The packing structures of dimers 1–4 in the
crystals are all one-dimensional on-top p-stacks, with inter-
molecular distances of 3.72–3.79 ꢀ (Figures 2b and 3c, see
also Figures S24 and S27 in the Supporting Information), thus
suggesting that compounds with this quasiplanar structural
motif share a general tendency for the observed packing
mode. DFT calculations at the B3LYP/6-31G(d) level sug-
gested an inversion barrier for the flipping of the two phenyl
rings in monomer 8a of 9.1 kcalmolÀ1 (see Figure S19 in the
Supporting Information), which indicates that it should
proceed easily in solution or under sublimation conditions.
This flexibility should facilitate dense p-stacking in the on-top
mode.
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possible directions (see Tables S2 and S3 in the Supporting
Information).
direction perpendicular (Smper) to the quartz substrate (1:
0.98 cm2 VÀ1 sÀ1; 2: 0.57 cm2 VÀ1 sÀ1) was approximately three
times higher than that in the direction parallel (Smpar) to the
substrate (1: 0.38 cm2 VÀ1 sÀ1; 2: 0.19 cm2 VÀ1 sÀ1). To validate
that the quasiplanar structure of 1 and 2 is responsible for this
anisotropy, we conducted TRMC control measurements on
vacuum-deposited films of TPD, which has a corresponding
nonplanar structure (see Figure S32 in the Supporting Infor-
mation). However, a minimal anisotropy of the carrier
To examine the relationship between packing structures
and carrier-transport properties experimentally, time-
resolved microwave conductivity (TRMC) measurements[11]
were carried out on several dozens of crystals aligned on
a quartz substrate (Figure 4a,b). TRMC measures the local
mobility of charge carriers (fSm) under an oscillating micro-
mobility was observed for this TPD film (Smper
=
0.53 cm2 VÀ1 sÀ1; Smpar = 0.37 cm2 VÀ1 sÀ1; Smper/Smpar = 1.4); it
was significantly less prominent than those in films of 1 (Smper
/
Smpar = 2.6) and 2 (Smper/Smpar = 3.0).
To confirm this anisotropy, the ordering structure of 2 in
thin film was investigated by X-ray diffraction studies.
Vacuum-deposited films of 2 did not exhibit any distinct
diffraction peaks in measurements using a laboratory X-ray
diffractometer with a sealed-tube X-ray generator (RIGAKU
Ultima IV; see Figure S33 in the Supporting Information),
which suggests that these films are essentially amorphous.
However, in the two-dimensional grazing-incidence X-ray
diffraction (2D GIXD), and using a synchrotron radiation
source,[3e] we observed a diffraction halo corresponding to the
p-stacking along the qz-axis (out-of-plane; q = 1.69–1.74 ꢀÀ1,
Figure 4d), whereas no distinct diffraction was observed in
the direction of the qxy-axis (in-plane). The determined p-
stacking distance (dp) of 3.6–3.7 ꢀ is in good agreement with
the results obtained from the crystal structures. This finding
suggests a horizontal molecular orientation (face-on), in
which the p-stacking structure was retained to some extent,
even in the amorphous state.[13] These results accordingly
support the anisotropy of the carrier mobility observed by
TRMC. In contrast, on the basis of variable angle spectro-
scopic ellipsometry measurements, nonplanar TPD was
reported to be randomly oriented in vacuum-deposited
films under similar conditions.[13b] It is therefore feasible to
conclude that the anisotropy of the carrier mobility is most
likely enhanced by the quasiplanar structural motif of oxygen-
bridged triarylamines 1 and 2.
In summary, we demonstrated that quasiplanar structures
can be used as the key feature in the molecular design of
triarylamines to obtain delocalized HOMOs and on-top p-
stacks in the crystalline state, which leads to the observed high
and anisotropic carrier mobilities in the p-stacking direction.
We also found that these compounds retain some degree of
the face-on p-stacking in amorphous films, thus facilitating
out-of-plane charge transport. The properties exhibited by
these quasiplanar triarylamine model compounds are
expected to be extraordinarily beneficial for charge-trans-
porting materials in OLEDs, OPVs, and perovskite-sensitized
solar cells,[14] all of which require high levels of out-of-plane
charge transport. Investigations concerning the applications
of these types of charge-transporting materials are currently
in progress in our laboratory and the results will be reported
in due course.
Figure 4. Anisotropy of the carrier mobility measured by TRMC for
a) needle-like crystals, b) platelike crystals and c) vacuum-deposited
films of 2 under excitation at 355 nm. d) 2D GIXD image of a vacuum-
deposited film of 2.
wave electric field in the absence of contact between the
semiconductors and the metal of electrodes, where f is
defined as the charge carrier generation efficiency. It should
be noted that the local mobility (Sm) obtained by TRMC is
different from the bulk mobility in time-of-flight (TOF),
space-charge-limited current (SCLC), and organic field-effect
transistor (OFET) measurements, as the absence of the
contact between the semiconductors and the electrode in
TRMC measurements allows the evaluation of the intrinsic
charge carrier mobility and its anisotropy.[11] Significant
anisotropy of the intrinsic charge carrier mobility was
observed in the crystals with one-dimensional p-stacking.
It was substantially higher in the direction of the on-
top
p-stacking
(fSm = 6.1 ꢁ 10À5 cm2 VÀ1 sÀ1
;
Sm =
0.34 cm2 VÀ1 sÀ1 [12]) compared to the other directions (fSm =
1.6–1.8 ꢁ 10À5 cm2 VÀ1 sÀ1; Sm = 0.09–0.10 cm2 VÀ1 sÀ1 [12]). No
such anisotropy was observed in the crystals with two-
dimensional p-stacks (fSm = 1.5–1.8 ꢁ 10À5 cm2 VÀ1 sÀ1; Sm =
0.09–0.10 cm2 VÀ1 sÀ1 [12]). These results are consistent with
the hole-transfer integrals.
In many of the organic electronic devices such as OLEDs
and OPVs, the charge-transporting materials are used in the
form of films. Therefore, we also conducted TRMC measure-
ments on vacuum-deposited films (100 nm thickness) of 1 (see
Figure S31 in the Supporting Information) and 2 on a quartz
substrate (Figure 4c). Interestingly, we also observed a sig-
nificant anisotropy of the carrier mobility. The mobility in the
Received: January 4, 2014
Revised: March 8, 2014
Published online: && &&, &&&&
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Communications
Heterocycles
A. Wakamiya,* H. Nishimura,
T. Fukushima, F. Suzuki, A. Saeki, S. Seki,
I. Osaka, T. Sasamori, M. Murata,
Y. Murata,* H. Kaji*
&&&& — &&&&
On-Top p-Stacking of Quasiplanar
Molecules in Hole-Transporting
Materials: Inducing Anisotropic Carrier
Mobility in Amorphous Films
Taking charge: Dimers of partially oxygen-
bridged triarylamines that form on-top p-
stacking aggregates in the crystalline
state were shown to induce high levels of
anisotropic charge transport in the
direction of the p-stacking. Surprisingly,
these compounds retained some of the
face-on p-stacking even in vacuum-
deposited amorphous films, thus facili-
tating an out-of-plane carrier mobility.
6
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