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
[**] 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.
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
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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