Electron-Transporting Liquid-Crystalline N-Heteroacene Derivatives
Kyosuke ISODA,* Tomonori ABE, and Makoto TADOKORO
Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka,
Shinjuku-ku, Tokyo 162-8601 Japan
Designable molecules programmed by non-covalent intermolecular interactions such as a hydrogen and
coordination bonds, electrostatic and a van der Waals interactions, can fabricate the highly ordered superstructures
in the bulk material. Liquid-crystalline (LC) molecules can form various self-organized nanostructures in thin films
unlike an amorphous material and an organic polymer, because a molecular arrangement in a LC state is already
adapted to an ordered superstructure by an intermolecular weak interaction such as a hydrophobic attraction of long
alkyl chains. LC molecules composed of the designable molecules, which have the -conjugated framework of
5,6,11,12-tetraazanaphthacene (TANC) with long alkyl chains, are created to form the 1- and 2-dimensional ordered
nanostructures by self-organization.1 As a result, they are expected to transport charge carriers as electrons through
a overlap of -orbitals between neighbouring -conjugated molecules.
TANC as N-heteroacene, in which N atoms partially substitute C atoms in a -conjugated polycyclic
oligoacene, behaves as an electron acceptor owing to the presence of electron deficient imino-N atoms. We have
previously reported on an electron-accepting TANC as N-heteroacene, which have reversible two one-electron
reduction peaks (E11/2 = –0.66 V and E21/2 = –1.20 V vs. Ag/Ag+ in MeCN).2–4 TANC thin films fabricated by a
vapour deposition also show a n-type FET activity with the electron mobility of 8.9 x 10–5 cm2 V–1s–1.2 The
application of a TANC module with the n-type activity into a LC molecule should fabricate into the
electron-transporting thin film based on the TANC framework.
Herein, we have reported on LC TANC derivatives
1–3 having racemic long alkoxy groups (Figure 1). They
self-assemble into the formation of the columnar LC for 1 and
the smectic A LC phases for 2 and 3 in a wide temperature
range including room temperature. In a cyclic voltammetry, 1–
3 work as an electron acceptor due to having two one-electron
transfer reductions as the same as non-substituted TANC.
Moreover, it is revealed that the electron mobilities of 2 and 3 Figure 1. Molecular structures of LC TANC derivatives.
in the SmA LC phases are measured by a time-of-flight (TOF)
technique and are the order of 10–4 cm2 V–1s–1 under ambient atmosphere, respectively. To our knowledge, there is
the first report on the carrier-transporting function of LC N-heteroacene semiconductors.
Differential scanning calorimetry (DSC) revealed that 1–3 show the LC phase in a wide temperature range
including room temperature, of which phase transition temperatures are 73.6, 91.3, and 113.8 ˚C, respectively, Their
transition temperatures tend to gradually decrease with the increase in the volume of substituted alkyl chains. The
X-ray diffraction patterns have indicated that 1 having two alkoxy chains form the hexagonal columnar (Col h) LC
phase, whereas 2 and 3 bearing a alkoxy chain show the smectic A (SmA) LC phase, respectively. The polarized
optical microscopic (POM) observation of 2 shows the fan-like texture characteristic of the SmA LC phase. For 1,
the uniaxial alignment of Colh LC structure can be achieved by shearing the sample between sandwiched glass
plates. The birefringence of the aligned sample of 1 alternately changes between light and dark upon by 45˚ rotation
under the cross Nicols condition.
The electric properties of 1–3 have revealed by the cyclic voltammetry. Compound 1–3 show the
reversible two one-electron reduction waves in the negative region, which are ascribed to the formation of the
radical anion and dianion. Furthermore, the electron-transporting properties of 2 and 3 are investigated by the TOF
measurements. The electron mobilities of 2 and 3 are on the order of 10–4 cm2 V–1 s–1, which should suggest that 1–3
are expected to function as an electron-transporting material.
In conclusion, we have prepared LC TANC derivatives 1–3 self-assembling to form the Colh and the SmA
LC structures, respectively. The thin films fabricated by electron-accepting LC TANC 1–3 are expected to function
as the n-type organic semiconductors.
References
1. Isoda and Tadokoro et al., Chem. Asian J., 2013, 8, 2951;
2. Isoda and Tadokoro et al., Chem. Lett., 2012, 41, 937;
3. Tadokoro et al., Angew. Chem. Int. Ed., 2006, 45, 5144;
4. Tadokoro and Isoda et al., ChemPhysChem, 2011, 12, 2561.
5. Isoda and Tadokoro et al., to be submitted.
Acknowledgement: This work was supported partially by a Grant-in-Aid for Scientific Research on Innovative
Areas of “New Polymeric Materials Based on Element-Blocks (No. 2401)” (no. 25102540, K.I.) from the Ministry
of Education, Culture, Sports, Science, and Technology of Japan; Hitachi Metals•Material Science Foundation
(K.I.); The Ogasawara Foundation for the Promotion of Science & Engineering (K.I.); and The Hattori Hokokai
Foundation (K.I.).
We specially thank Prof. M. Funahashi at Kagawa university for cooperation with the TOF measurement and the
fruitful discussion.
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* presenting author; E-mail: k-isoda@rs.tus.ac.jp