Synthesis and properties of acene dimers linked by a 1,3,4-oxadiazole spacer

Article abstract of DOI:10.3987/COM-06-S(K)4

Acene dimers linked by a 1,3,4-oxadiazole spacer were synthesized and their physical, optical, and electrochemical properties were investigated. The melting points and electron affinities of the dimers increased in the following order: benzene, naphthalene, and anthracene. The longest absorption maximum was red-shifted with an increase in the number of benzene rings. The conjugation between the two acene moieties through the oxadiazole spacer was not strong. Although no OFET properties were observed in a bottom contact device using the anthracene dimer as an active layer, the molecules were perpendicularly arranged on the substrate to form π-stacking films as observed in the X-Ray diffraction analysis.

Full text of DOI:10.3987/COM-06-S(K)4

HETEROCYCLES, Vol. 72, 2007
85
HETEROCYCLES, Vol. 72, 2007, pp. 85 - 90. © The Japan Institute of Heterocyclic Chemistry
Received, 21st August, 2006, Accepted, 28th September, 2006, Published online, 3rd October, 2006. COM-06-S(K)4
SYNTHESIS AND PROPERTIES OF ACENE DIMERS LINKED BY A
1,3,4-OXADIAZOLE SPACER
Katsuhiko Ono,* Mayuko Wakida, Ryohei Hosokawa, Katsuhiro Saito,
Jun-ichi Nishida,† and Yoshiro Yamashita†
Department of Materials Science and Engineering, Nagoya Institute of
Technology, Gokiso, Showa-ku, Nagoya 466-8555, Japan; e-mail:
ono.katsuhiko@nitech.ac.jp
†Department of Electronic Chemistry, Interdisciplinary Graduate School of
Science and Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku,
Yokohama 226-8502, Japan
Abstract – Acene dimers linked by a 1,3,4-oxadiazole spacer were synthesized
and their physical, optical, and electrochemical properties were investigated.
The melting points and electron affinities of the dimers increased in the following
order: benzene, naphthalene, and anthracene. The longest absorption maximum
was red-shifted with an increase in the number of benzene rings. The
π-conjugation between the two acene moieties through the oxadiazole spacer was
not strong. Although no OFET properties were observed in a bottom contact
device using the anthracene dimer as an active layer, the molecules were
perpendicularly arranged on the substrate to form π-stacking films as observed in
the X-Ray diffraction analysis.
The synthetic research of acene compounds has attracted considerable attention since they are promising
materials for the active layers of organic field-effect transistors (OFETs).1 Pentacene is one of the most
useful materials, and it exhibits good p-type semiconducting properties with a mobility of 3 cm2 V−1 s−1
and an on/off current ratio of 105.2 Recently, modified acene compounds were also synthesized and
their properties were investigated.3–8 For example, a trimer and dimer of anthracene exhibited good
performances as p-type semiconductors with mobilities of 0.07 and 0.01 cm2 V−1 s−1, respectively.4 The
anthracene and tetracene dimers linked by a bithiophene spacer are also p-type semiconductors with
mobilities of 0.1 and 0.5 cm2 V−1 s−1, respectively.5 With regard to this, we synthesized anthracene and

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HETEROCYCLES, Vol. 72, 2007
naphthalene dimers (1) and (2) linked by a 1,3,4-oxadiazole spacer as a new type of modified acene
compounds. Both the acene moieties are expected to be arranged in the form of linear molecular shapes.
Further, these compounds are of interest as an approach to n-type semiconductors due to the inherent
electron affinity of the oxadiazole moiety, and they were prepared by the condensation of corresponding
hydrazides in polyphosphoric acid (PPA).9 In this paper, we report the synthesis and properties of these
acene dimers together with compound (3).
O
O
O
N N
N N
N N
1
2
3
The synthesis of the anthracene dimer (1) is shown in Scheme 1. Methyl 2-anthracenecarboxylate (6)
was obtained with a yield of 96% by the esterification of carboxylic acid (5). Carbohydrazide (7) was
prepared with a yield of 88% by the reaction of 6 with hydrazine monohydrate. The anthracene dimer
(1) was prepared with a yield of 15% by the condensation of 7 in PPA at 180 °C for 4 h.10
A
naphthalene dimer (2) was obtained by a procedure similar to 1.11 The yields of the condensation,
melting points, and color of compounds (1)–(3) are summarized in Table 1. The yields of the
condensation decreased with an increase in the number of benzene rings in the acene moieties due to the
low solubility of carbohydrazide. The melting point increases with an increase in the number of benzene
rings. The differential scanning calorimetry (DSC) of 1 revealed a melting point of 370.3 ºC (Tm) and a
crystallization temperature of 332.1 ºC (Tc) (Figure 1),12 suggesting that the anthracene moieties caused a
molecular packing with strong intermolecular interactions. Compound (1) was obtained in the form of
yellow crystals, which were very slightly soluble in chloroform and dichloromethane and were insoluble
in the other common solvents.
CO2H
CO2Me
H2SO4
MeOH
5
6
NH2NH2·H2O
MeOH
PPA
CONHNH2
1
180 °C
7
Scheme 1. Synthesis of anthracene dimer (1).

HETEROCYCLES, Vol. 72, 2007
87
Table 1. Condensation yields, melting points, and color of 1–3
Compound Yield / %
mp / ºC
370.3a
color
1
2
3
15
yellow
colorless
colorless
50
194–195
138–139
86–92b
aDSC measurement. bReference 9a.
Figure 1. DSC profiles of compound (1).
The absorption spectra of 1–3 in dichloromethane are shown in Figure 2. The absorption bands are
red-shifted with an increase in the number of benzene rings of the acene moieties. The longest
absorption maxima were observed as 281 nm (log ε 4.42) for 3, 317 nm (log ε 4.51) for 2, and 402 nm for
1. Since the absorption maxima are not much shifted to longer wavelengths as compared to those of
benzene (λmax = 254 nm), naphthalene (λmax = 273 nm), and anthracene (λmax = 376 nm), the
π-conjugation between the two acene moieties through the oxadiazole spacer appears to be weak. The
HOMO-LUMO energy gaps were evaluated to be 3.77 eV (3), 3.35 eV (2), and 2.95 V (1) according to
the absorption edges. The cyclic voltammetry (CV) of 2 and 3 in DMF revealed reversible reduction
waves.13 However, the voltammogram of 1 was not recorded because of its insolubility. The
half-wave reduction potentials of 2 and 3 were observed to be −2.28 V and −2.50 V vs. Fc/Fc+,
respectively. The MNDO-PM3 calculations14 also indicated that electron affinity increased with an
increase in the number of benzene rings in the acene moieties.15
A bottom contact OFET was fabricated using compound (1) as an active layer with a thickness of 60 nm
on a SiO2 (300 nm) dielectric substrate by high vaccum (10−5 Pa) deposition. However, the OFET
properties were not observed.

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HETEROCYCLES, Vol. 72, 2007
100
80
60
40
20
0
1
2
3
250
300
350
400
450
Wavelength / nm
Figure 2. UV-vis spectra of compounds (1)–(3) in dichloromethane.
Kwon et al. recently reported that molecules of compound (3) were arranged horizontally on a Cu(111)
surface to form a π-stacking film. This was observed using scanning tunneling microscopy (STM) and
density function theory (DFT).16 In order to investigate the thin films of compound (1) formed on the
SiO2 substrate, X-Ray diffraction (XRD) analysis was performed (Figure 3). The XRD profile exhibits
sharp diffraction peaks, indicating that these molecules were regularly arranged to form the π-stacking
film. The distance between the molecular layers was calculated to be 2.25 nm from the diffraction peak
of 2θ = 3.92°. This distance is comparable to the molecular length calculated by the PM3 method as
shown in Figure 4. Therefore, these molecules were considered to be arranged perpendicularly on the
SiO2 substrate. This result suggests that no OFET behavior of 1 is not due to the molecular arrangement,
but to the insufficient electron affinity.
Figure 3. XRD of a thin film of 1 formed on a SiO2 substrate.

HETEROCYCLES, Vol. 72, 2007
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2.18 nm
Figure 4. Molecular structure of 1 with the molecular length calculated
by the NMDO-PM3 method.
In conclusion, the preparation of crosslinking acenes by using a 1,3,4-oxadiazole spacer is a useful
method for producing modified large acene molecules. Their physical and electronic properties such as
melting point, electron affinity, and HOMO-LUMO energy gap can be turned by changing the acene units.
The π-conjugation between the acenes through the oxadiazole spacer was not strong. According to the
XRD measurement and the PM3 calculations, the two acene moieties were arranged to form a linear
molecular structure in the thin films. Although the oxadiazole spacer does not possess sufficient
electron affinity, other heterocycle spacers with higher electron affinities may afford new n-type
semiconductors.
ACKNOWLEDGEMENTS
This work was supported by a Grant-in-Aid (No. 17750037, 17550033) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan. We wish to thank Ichihara International Scholarship
Foundation and Yazaki Memorial Foundation for Science and Technology for financial support.
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1
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