Abstract: To tailor organic p/n hetero-
junctions with molecular-level preci-
sion, a rational design strategy using
side-chain incompatibility of a cova-
lently connected donor–acceptor (D–
A) dyad has been successfully carried
out. An oligothiophene–perylenedi-
imide dyad, when modified with tri-
ethylene glycol side chains at one ter-
minus and dodecyl side chains at the
other (2Amphi), self-assembles into nano-
fibers with a long-range D/A hetero-
junction. In contrast, when the dyad is
modified with dodecyl side chains at
both termini (2Lipo), ill-defined micro-
fibers result. In steady-state measure-
ments using microgap electrodes, a cast
film of the nanofiber of 2Amphi displays
far better photoconducting properties
than that of the microfiber of 2Lipo
.
Flash-photolysis time-resolved micro-
wave conductivity measurements, in
conjunction with transient absorption
spectroscopy, clearly indicate that the
nanofiber of 2Amphi intrinsically allows
for better carrier generation and trans-
port properties than the microfibrous
Keywords: amphiphiles
· hetero-
junctions · nanostructures · photo-
conduction · self-assembly
assembly of 2Lipo
.
Introduction
D and A components tend to assemble together by means
of a charge-transfer (CT) interaction,[8] unfavorable for pho-
toelectric conversion. To overcome this essential problem,
we proposed a molecular design strategy using ꢁside-chain
incompatibilityꢂ (Figure 1).[9] The basic motif of this strategy
is a covalently connected D–A dyad (1D–A), site-specifically
functionalized with two incompatible side chains. Dyad 1D–A
likely forms a p-stack architecture with molecules on top of
each other with an alternating geometry due to an intermo-
lecular CT interaction. However, this geometry requires two
incompatible side chains to mix, thereby causing a thermo-
dynamic instability in the assembly. Consequently, 1D–A pre-
fers to stack homotropically in terms of its D and A units.
Overall, the p- and n-type semiconducting molecular layers
could form and be conjugated together at a very wide inter-
face. This strategy seems to work for the formation of a D/
A heterojunction in bulk materials. In fact, we demonstrated
that a covalent D–A dyad consisting of oligothiophene and
C60 units, when properly functionalized at its termini with
long alkyl and oxyethylene side chains, self-assembles into a
photoconductive smectic liquid crystal (LC) with a layered
p/n heterojunction.[9]
Remarkable advances in synthetic organic chemistry have
made it possible to design a wide variety of p-conjugated
molecules with attractive electronic functions for organic de-
vices.[1–4] However, without proper molecular design that
allows for long-range ordering of p-conjugated molecules,
the resulting devices rarely show expected performances.
This issue is particularly important for the development of
thin-film organic photovoltaic devices, since they require
electron-donor (D) and -acceptor (A) molecules to assemble
homotropically to form a heterojunction.[1,5–7] Furthermore,
for achieving a highly efficient photoinduced charge separa-
tion, the resultant p- and n-type semiconducting domains
must be connected at a wide interface. However, in general,
[a] Prof. Dr. W.-S. Li
Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences
345 Ling-Ling Road, Shanghai 200032 (China)
Fax : (+86)21-5492-5381
In the present study, we synthesized, from oligothiophene
(OT) and perylenediimide (PDI), covalently linked D–A
dyads 2Amphi and 2Lipo (Figure 2),[10] which bear at their termi-
ni incompatible and compatible side chains, respectively.
Whereas C60 is spherical and tends to aggregate homotropi-
cally, PDI is planar and known to form CT complexes with
various electron donors.[8c,11] Hence, we thought that dyads 2
[b] Dr. A. Saeki, Prof. Dr. S. Seki
Graduate School of Engineering, Osaka University
and PRESTO, Japan Science and Technology Agency (JST)
2-1 Yamadaoka, Suita, Osaka 565-0871
ACHTUNGTRNE(NUNG Japan)
[c] Dr. Y. Yamamoto, Prof. Dr. T. Aida
ERATO-SORST Nanospace Project, Japan Science and Technology
Agency (JST)
National Museum of Emerging Science and Innovation
2-3-6 Aomi, Koto-ku, Tokyo 135-0064 (Japan)
Fax : (+81)3-5841-7310
[9]
could be more appropriate than those with C60 for discus-
sing the general versatility of our design strategy for making
a heterojunction. We found that both dyads 2 self-assemble
into fibrous structures rather than LC, in which the assembly
of 2Amphi that has two incompatible side chains exhibits a
much greater photoconducting output than that of 2Lipo with
the compatible side chains.
[d] Dr. T. Fukushima
Advanced Science Institute, RIKEN
2-1 Hirosawa, Wako, Saitama 351-0198 (Japan)
Fax : (+81)48-467-8214
[e] Dr. N. Ishii
National Institute of Advanced Industrial Science and Technology
(AIST)
Tsukuba Central-6, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8566 (Japan)
Results and Discussion
[f] Dr. K. Kato, Prof. Dr. M. Takata
RIKEN SPring-8 Center
Compounds 2Amphi and 2Lipo were synthesized by Sonogashira
coupling of an ethynyl-group-terminated OT derivative with
the corresponding iodophenyl-group-appended PDI deriva-
tives, respectively (see the Supporting Information). These
1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148 (Japan)
Supporting information for this article is available on the WWW
Chem. Asian J. 2010, 5, 1566 – 1572
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1567