Amphiphilic molecular design as a rational strategy for tailoring bicontinuous electron donor and acceptor arrays: Photoconductive liquid crystalline oligothiophene-C60 dyads

Article abstract of DOI:10.1021/ja802757w

For tailoring solution-processable optoelectronic thin films, a rational strategy with amphiphilic molecular design is proposed. A donor-acceptor dyad consisting of an oligothiophene and C₆₀, when modified with a hydrophilic wedge on one side and a paraffinic wedge on the other (1_Amphi), forms over a wide temperature range a photoconducting smectic A liquid crystal having bicontinuous arrays of densely packed donor and acceptor units. In contrast, when modified with only paraffinic wedges (1_Lipo), the dyad forms a smectic A liquid crystalline mesophase, which however is poorly conductive. As indicated by an absorption spectral feature along with a synchrotron radiation small-angle X-ray scattering profile, 1_Lipo in the lamellar structure does not adopt a uniform head/tail orientation. Such defective donor and acceptor arrays likely contain a large number of trapping sites, leading to short-lived charge carriers, as observed by a flash photolysis time-resolved microwave conductivity study. Copyright

Full text of DOI:10.1021/ja802757w

Published on Web 06/18/2008
Amphiphilic Molecular Design as a Rational Strategy for Tailoring
Bicontinuous Electron Donor and Acceptor Arrays: Photoconductive Liquid
Crystalline Oligothiophene-C60 Dyads
Wei-Shi Li,*,† Yohei Yamamoto,† Takanori Fukushima,*,†,‡ Akinori Saeki,§ Shu Seki,§
Seiichi Tagawa,§ Hiroyasu Masunaga,# Sono Sasaki,# Masaki Takata,#,| and Takuzo Aida*,†,‡
ERATO-SORST Nanospace Project, Japan Science and Technology Agency (JST), National Museum of Emerging
Science and InnoVation, 2-41 Aomi, Koto-ku, Tokyo 135-0064, Japan, Department of Chemistry and Biotechnology,
School of Engineering and Center for NanoBio Integration, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan, Institute of Scientific and Industrial Research (ISIR), Osaka UniVersity, 8-1, Mihogaoka,
Ibaraki, Osaka 567-0047, Japan, Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Kouto, Sayo-cho,
Sayo-gun, Hyogo 679-5198, Japan, and RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo-cho, Sayo-gun,
Hyogo 679-5148, Japan
Received April 15, 2008; E-mail: li@nanospace.miraikan.jst.go.jp; fukushima@nanospace.miraikan.jst.go.jp;
aida@macro.t.u-tokyo.ac.jp
Chart 1. Structures of 1Amphi, 1Lipo, and References 2 and 3
Tailoring bicontinuous electron donor (D) and acceptor (A) arrays
in solution-processable materials is an essential step for the
realization of thin-film organic optoelectronics.1 Casting of a D/A
mixture may give rise to photoconductive bulk materials.2 However,
without proper choice of the components, many defects are usually
formed, leading to trapping and rapid annihilation of charge carriers.
In view of photovoltaics, the presence of bicontinuous D and A
arrays is essential, but cast films from D/A mixtures likely contain
their charge-transfer complexes. So far, nanofibers and tubes with
bicontinuous D and A arrays have been obtained by self-assembly
of covalently linked D-A dyads.3,4 Nevertheless, solution-pro-
cessable bulk materials with a D/A heterojunction5 might be more
realistic for device applications. Here we report a photoconductive
liquid crystal (LC) with bicontinuous arrays of densely packed D
and A components, tailored from amphiphilic oligothiophene-C60
dyad 1Amphi (Chart 1).6 Together with contrasting results for a
nonamphiphilic reference (1Lipo), we highlight a crucial role of the
amphiphilic design both in structuring and in photoconductivity.
Figure 2, I). Thus, each repeating layer consists of laterally coupled
tail-to-tail pairs of 1Amphi, and such layers are connected at the
fullerene-appended hydrophilic head part of 1Amphi to form a 2D
lamellar structure. In this configuration, the hydrophilic and
paraffinic wedges are separated by 5.3 nm.
The molecular structures of 1Amphi and 1Lipo are identical to each
other except for the terminal wedges. 1Amphi bears a hydrophilic
wedge with triethylene glycol chains and, on the other side, a
hydrophobic wedge with paraffinic chains. In contrast, 1Lipo
possesses only the paraffinic wedges. We also prepared compounds
2 and 3, which are equivalent to the donor and acceptor components
of 1Amphi, respectively. They are all soluble in CH2Cl2, where the
absorption spectral profiles (Figure S3) and redox properties (Figure
S4) of 1Amphi and 1Lipo were essentially identical to one another.6
Similar to 1Amphi, nonamphiphilic 1Lipo formed a smectic A LC
mesophase in a temperature range from 111.4 to 12.5 °C, as
Dyad 1Amphi formed a LC smectic A mesophase over a wide
temperature range from 136.1 to 18.3 °C (Figure S5).6 Polarized
optical microscopy (POM) of LC-1Amphi displayed a typical focal-
conic texture (Figure 1a). Synchrotron radiation small-angle X-ray
scattering (SAXS) analysis showed sharp peaks with d spacings of
10.6, 5.3, 3.5, and 2.6 nm (Figure 1c), which can be indexed as
(100), (200), (300), and (400) reflections of a lamellar structure
with a layer width of 10.6 nm. On the basis of a CPK model of
1
Amphi, the observed layer width is almost twice as large as the
distance from the paraffinic end to the fullerene unit (∼5.5 nm;
† ERATO-SORST Nanospace Project, JST.
‡ The University of Tokyo.
§ Osaka University.
# JASRI.
Figure 1. POMs at 33 °C and SAXS patterns at 30 °C (inset: magnified
| RIKEN.
at q ) 1.5-2.5 nm-1) of (a,c) LC-1Amphi and (b,d) LC-1Lipo
.
9
8886 J. AM. CHEM. SOC. 2008, 130, 8886–8887
10.1021/ja802757w CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
mixture of 2 and 3 formed neither regular structures nor photo-
conductivity (black). Noteworthy, LC-1Lipo (red) was much less
conductive than LC-1Amphi, where the photocurrent was only 1/10
that of 1Amphi observed under identical conditions. This result is
interesting since the electron transfer efficiencies in LC-1Amphi and
LC-1Lipo, judging from their fluorescence quenching profiles (Figure
S6),6 are considered equally high. Having these features in mind,
we conducted flash photolysis time-resolved microwave conductiv-
ity (FR-TRMC) measurements, which are informative of the
behaviors of mobile charge carriers in a short distance (∼10 nm).
With a 355 nm laser pulse at 25 °C (Figure 3d), the maximum
conductivity (φΣµmax in cm2/V·s) of LC-1Amphi (4.1 × 10-4) was
markedly greater than that of the 2/3 mixture (1.2 × 10-4) but
almost comparable to that of LC-1Lipo (3.7 × 10-4). However, the
charge carrier, generated in LC-1Lipo, was shorter-lived than that
in LC-1Amphi, suggesting the presence of a larger number of trapping
sites in LC-1Lipo. This is in accord with the aforementioned low
structural integrity of LC-1Lipo (Figure 2, II) and its poor photo-
conducting nature in a macroscopic scale (Figure 3b).
Figure 2. Schematic representations of molecular orientations in LC-1Amphi
(I) and LC-1Lipo (II) with a smectic A mesophase.
In summary, we have demonstrated that amphiphilic molecular
design7 can be a rational strategy for the spontaneous formation of
bicontinuous donor and acceptor arrays in liquid crystalline
materials. Site-specific modification of donor-acceptor dyads with
hydrophilic and paraffinic wedges not only prohibits donor-acceptor
interactions leading to the trapping of charge carriers but also
ensures a long-range conducting pathway in the materials. As liquid
crystals are solution-processable and self-healable, our design
strategy may contribute to the development of high-performance
organic optoelectronics.
Acknowledgment. The synchrotron radiation experiments were
carried out on the BL40B2 of SPring-8 under the Priority Nano-
technology Support Program administrated by JASRI (Proposal No.
2008A1650).
Supporting Information Available: Details of synthesis and
characterization of 1Amphi, 1Lipo, 2, and 3, and supporting Figures S1-S6.
This material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 3. (a) Absorption spectra at 25 °C of 1Amphi (blue) and 1Lipo (red)
in the LC state (solid curves) and in CH2Cl2 (broken curves), normalized
at 450 nm. Photoconducting properties at 25 °C of LC-1Amphi (blue), LC-
1Lipo (red), and a 1:1 mixture of 2 and 3 (black); (b) I-V profiles without
(broken curves) and with photoirradiation (solid curves) and (c) changes in
photogenerated electric current under an applied voltage of +1 V. (d) FP-
TRMC profiles (λex ) 355 nm) of LC-1Amphi (blue) and LC-1Lipo (red).
References
(1) (a) Segura, J. L.; Mart´ın, N.; Guldi, D. M. Chem. Soc. ReV. 2005, 34, 31–
47. (b) Gu¨nes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107,
1324–1338. (c) Thompson, B. C.; Fre´chet, J. M. J. Angew. Chem., Int. Ed.
2008, 47, 58–77.
(2) (a) Percec, V.; Glodde, M.; Bera, T. K.; Miura, Y.; Shiyanovskaya, I.; Singer,
K. D.; Balagurusamy, V. S. K.; Heiney, P. A.; Schnell, I.; Rapp, A.; Spiess,
H.-W.; Hudsonk, S. D.; Duank, H. Nature 2002, 419, 384–387. (b) Schmidt-
Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.; Friend, R. H.;
MacKenzie, J. D. Science 2006, 293, 1119–1122. (c) Liscio, A.; De Luca,
G.; Nolde, F.; Palermo, V.; Mu¨llen, K.; Samor`ı, P. J. Am. Chem. Soc. 2008,
130, 780–781.
(3) (a) Yamamoto, Y.; Fukushima, T.; Suna, Y.; Ishii, N.; Saeki, A.; Seki, S.;
Tagawa, S.; Taniguchi, M.; Kawai, T.; Aida, T. Science 2006, 314, 1761–
1764. (b) Yamamoto, Y.; Fukushima, T.; Saeki, A.; Seki, S.; Tagawa, S.;
Ishii, N.; Aida, T. J. Am. Chem. Soc. 2007, 129, 9276–9277.
(4) (a) Wu¨rthner, F.; Chen, Z.; Hoeben, F; J, M.; Osswald, P.; You, C.-C.;
Jonkheijm, P.; Herrikhuyzen, J. v.; Schenning, A. P. H. J.; van der Schoot,
P. P. A. M.; Meijer, E. W.; Beckers, E. H. A.; Meskers, S. C. J.; Janssen,
R. A. J. J. Am. Chem. Soc. 2004, 126, 10611–10618. (b) Ro¨ger, C.; Mu¨ller,
M. G.; Lysetska, M.; Miloslavina, Y.; Holzwarth, A. R.; Wu¨rthner, F. J. Am.
Chem. Soc. 2006, 128, 6542–6543.
observed by DSC and POM profiles (Figures 1b and S5).6 SAXS
pattern (Figure 1d) displayed intense and weak diffractions with d
spacings of 5.7 and 2.9 nm, respectively, which were indexed as
(100) and (200) reflections of a lamellar structure with a layer width
of 5.7 nm (Figure 2, II). The observed width is close to the
molecular length of 1Lipo. Considering that both ends of 1Lipo are
paraffinic, the head/tail orientation of 1Lipo in the lamellar structure
is most likely nonuniform (Figure 2, II). In relation to these
structural aspects, the absorption spectrum of LC-1Lipo was different
from that of LC-1Amphi, while both were obviously broader than
those in CH2Cl2 (Figure 3a). In particular, a broad band observed
for LC-1Lipo at 600-800 nm suggests a charge-transfer interaction
between the oligothiophene and fullerene units, which can occur
unless 1Lipo adopts a uniform head/tail orientation.
(5) (a) Samor`ı, P.; Yin, X.; Tchebotareva, N.; Wang, Z.; Pakula, T.; Jackel, F.;
Watson, M. D.; Venturini, A.; Mu¨llen, K.; Rabe, J. P. J. Am. Chem. Soc.
2004, 126, 3567–3575. (b) Peeters, E.; van Hal, P. A.; Meskers, S. C. J.;
Janssen, R. A. J.; Meijer, E. W. Chem.—Eur. J. 2002, 8, 4470–4474.
(6) See Supporting Information.
As shown in Figure 3b,c, LC-1Amphi (blue) exhibited a distinct
photoconductive character. At an applied voltage of, for example,
+2 V, the current at 25 °C increased abruptly from 0.09 to 26 pA
(on/off ratio ) 290) upon irradiation and quickly retrieved the
original value when the light was turned off. In contrast, a 1:1
(7) For review, see: Tschierske, C. J. Mater. Chem. 2001, 11, 2647–2671.
JA802757W
9
J. AM. CHEM. SOC. VOL. 130, NO. 28, 2008 8887