Synthesis and characterization of electron-accepting nonsubstituted tetraazaacene derivatives

Article abstract of DOI:10.1246/cl.2012.937

Oligoacenes are of interest as organic p-type semiconductors for use in electronic devices, but their use as n-type semiconductors is limited. N-Heteroacenes have been investigated as oligoacene-based n-type semiconductors due to their enhanced electron affinity. Herein, we report the synthesis, X-ray crystal structures, electrochemical, and field-effect transistor properties of TANC and BTANC.

Full text of DOI:10.1246/cl.2012.937

doi:10.1246/cl.2012.937
Published on the web September 1, 2012
937
Synthesis and Characterization of Electron-accepting Nonsubstituted Tetraazaacene Derivatives
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Kyosuke Isoda,* Masaharu Nakamura, Toshinori Tatenuma, Hironori Ogata, Tomoaki Sugaya, and Makoto Tadokoro*
1
Department of Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601
Department of Chemistry, Graduate School of Science, Osaka City University, Sugimoto-cho, Sumiyoshi-ku, Osaka 558-8585
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Department of Materials Chemistry, College of Engineering, Hosei University, Koganei, Tokyo 184-8584
(
Received May 24, 2012; CL-120448; E-mail: k-isoda@rs.tus.ac.jp)
Oligoacenes are of interest as organic p-type semiconductors
for use in electronic devices, but their use as n-type semi-
conductors is limited. N-Heteroacenes have been investigated as
oligoacene-based n-type semiconductors due to their enhanced
electron affinity. Herein, we report the synthesis, X-ray crystal
structures, electrochemical, and field-effect transistor properties
of TANC and BTANC.
Scheme 1. Molecular structures of TANC and BTANC.
5,6,13,14-tetraazapentacene) are tetraazaacene analogs of tetra-
(
cene and pentacene, respectively (Scheme 1). These oligoacenes
can be purified by sublimation at reduced pressure and they
electrochemically generate stable anionic radicals by accepting
one electron. TANC and BTANC are expected to function as
good semiconductors since their structures are similar to those of
tetracene and pentacene, which have remarkable semiconducting
properties. In this report, the syntheses, structures, and physical
properties of TANC and BTANC, and their FET activities as
noble n-type semiconductors are discussed.
Oligoacenes, a class of organic molecules consisting of
linearly fused aromatic ring systems, have attracted attention
because of their potential as effective carrier-transporting
materials in electronic devices.1 Many p-type organic semi-
2
conductors based on oligoacene frameworks such as tetracene,
3
4
pentacene, and their analogs have been prepared and studied
for their conductive properties. For example, p-type oligoacene-
based semiconductors have been investigated as organic hole-
5
transporting layers in organic field-effect transistors (FETs), as
BTANC was prepared for this study according to a synthetic
6
11
spin-casted films in photovoltaic cells, and as vacuum-depos-
procedure similar to that previously reported for TANC.
ited films in a light-emitting diodes.4b In contrast, the number of
n-type organic semiconductors based on oligoacene frameworks
BTANC was synthesized by a thermal condensation between
2,3-diaminonaphthalene and 2,3-dichloroquinoxaline in ethyl-
ene glycol, followed by oxidation with PbO2 in CHCl3. After
7
is still limited, with the exception of perfluorinated oligoacene.
N-Heteroacenes are a type of hetero-oligoacene in which
some C atoms have been replaced by N atoms. The introduction
of N-imino groups to the oligoacene framework makes N-
heteroacenes good candidates for electron-transporting materials
as electron acceptors because N-imino groups have a high
electron affinity. Molecules possessing the N-heteroacene frame-
work are more susceptive to reduction than heteroacenes
containing O or S atoms.8 Winkler and co-workers have
investigated the theoretical properties of N-heteroacenes to
elucidate their ability to act as n-channel semiconducting or
electron-accepting materials.9 However, the solubility of N-
heteroacenes decreases in organic solvent as the number of fused
benzene rings in the molecule increases. Therefore, Bunz and
co-workers have synthesized highly soluble N-heteroacene
derivatives by attaching bulky triisopropylsilyl (TIPS) groups
to N-heteroacene frameworks.1 The N-heteroacene derivatives
showed high FET behavior owing to rapid electron transport
achieved by tuning the molecular arrangements in a thin film,
recrystallization from CHCl /hexane, BTANC was obtained as a
3
blackish-green solid in 90% yield. TANC and BTANC were
1
13
identified by H NMR, C NMR, and IR spectroscopy, fast atom
bombardment (FAB) mass spectrometry, X-ray crystallography,
and elemental analysis.
Single crystals of TANC and BTANC were grown from
1
2
CH3CN and CHCl3/hexane (1:2, v/v), respectively. Their
molecular structures were revealed by the crystallographic
analysis to be approximately planar (Figures 1a and 1c). The
1
3
crystal structure of TANC is similar to that of tetracene, with
a type of herringbone array of molecules with face-to-edge
1
9
packing (Figure S1 ). The shortest stacking distance between
TANC molecules is C(6)£C(7)* (3.4310(19) ¡) along the a axis
(*: ¹x ¹ 1, ¹y + 1, ¹z + 2), in which the average distance
of face-to-face stacking is 3.3771(7) ¡. The crystal array is
reinforced along the c axis by complementary, weak, and
dual CH£N hydrogen bonds (Figure 1b). The distance of
C(2)£N(1)* contact is 3.4824(19) ¡ (*: ¹x, ¹y + 1, ¹z + 3).
The BTANC crystal is constructed from one-dimensional
columnar structures with slipped stacking along the b axis.
The closest N£N distance between BTANC molecules is
N(1)£N(2)* = 3.592(3) ¡ (*: x, y + 1, z), BTANC molecules
in neighboring columns connect to each other through four
C­H£N hydrogen bonds, C(15)£N(1)* = 3.519(4) ¡ and
C(17)£N(4)* = 3.517(4) ¡ (*: ¹x + 1, ¹y ¹ 1, ¹z + 1)
(Figure 1d). The closest C£C distance for an intermolec-
ular ³-stacking interaction between BTANC molecules is
C(2)£C(14)* = 3.258(4) ¡ (*: ¹x + 1, ¹y, ¹z + 1), in which
0a
1
0
which was achieved despite the presence of bulky substituents.
However, unsubstituted N-heteroacenes, which have low sol-
ubility, are expected to have higher FET activity because high-
density stacking between molecules is possible. Moreover, the
electron-accepting properties of nonsubstituted N-heteroacenes
can be tuned by introducing appropriate groups into the N-
heteroacene frameworks.
Here, we focus on two nonsubstituted, electrochemically
stable N-heteroacenes with fundamental frameworks. The N-
heteroacenes TANC (5,6,11,12-tetraazatetracene) and BTANC
Chem. Lett. 2012, 41, 937­939
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

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Figure 2. Cyclic voltammograms of TANC (dotted line)
and BTANC (solid line) at 1.0 mM in CH3CN solutions of
¹
1
n-Bu4NPF6 (0.10 M). Sweep rate is 100 mV s
.
Figure 2 shows the results of the cyclic voltammetry (CV)
analyses of TANC and BTANC in CH3CN solutions of n-
Bu NPF . BTANC exhibits a reversible two-step and two-
4
6
electron-transfer peak. The redox potentials of BTANC observed
1
2
+
at E 1/2 = ¹0.51 V and E 1/2 = ¹1.09 V (vs. Ag/Ag ) are
attributed to the formation of an anionic radical and a dianion,
1
respectively. The E 1/2 of BTANC is less negative than that of
TANC (E 1/2 = ¹0.66 V vs. Ag/Ag ) due to the stabilization of
1
+
ELUMO. The ELUMO and EHOMO values of TANC and BTANC
1
were estimated from the redox potential E
in the cyclic
1/2
voltammograms and from the absorption edges of the UV­vis
spectra. Since ELUMO of TANC (¹4.00 eV) and BTANC
Figure 1. ORTEP drawings and single crystal structures of
TANC (a) and (b) and BTANC (c) and (d).
(
¹4.14 eV) are lower than those of typical organic electron
1
5
16
Table 1. Spectroscopic and electrochemical data of TANC and
BTANC
acceptors such as C60 fullerene and perylene diimides, TANC
and BTANC should be functional as electron-transporting
materials. Furthermore, substituted TANC derivatives have been
already reported to be higher electron acceptors rather than
UV­vis absorption Redox potentiala ELUMOb Egc
E1
E2
­max/nm
/eV /eV
1/2
1/2
1
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nonsubstituted TANC.
BTANC 274, 306, 452
TANC 260, 414
¹0.51 ¹1.09 ¹4.14 2.68
¹0.66 ¹1.20 ¹4.00 2.90
The organic FET activity of TANC and BTANC was
observed using their vapor-deposited thin films on p-doped Si/
SiO2 substrates prepared by high-vacuum evaporation using top-
contact geometry at room temperature. It is important to note
that the FET activities of TANC and BTANC had to be measured
within 30 min of forming the vapor-deposited thin films. Beyond
this period of time, electric current could not flow in the thin
films because of the formation of noncontacted crystal domains.
Analysis of the thin films revealed that both TANC and BTANC
functioned as n-type semiconductors as shown in Figure 3. The
channel current, I , increases according to an increase in V ,
aMeasured by cyclic voltammetry in CH3CN solution of
n-Bu4NPF6 (0.10 M). The redox potential of ferrocene as an
1
b
internal reference is observed at E 1/2 = 0.14 V. ELUMO is
1
calculated from first half-wave potential (E 1/2) with the
assumption that the energy level of ferrocene is ¹4.8 eV below
c
vacuum level: ELUMO = ¹ Eonset(red) ¹ 4.8 eV. Estimated from
the onset position of the UV­vis absorption spectrum in
opt
CH2Cl2: Eg = 1240/­onset.
D
G
the average distance of the face-to-face stacking is 3.298(3) ¡,
which is less than the sum of the van der Waals radii (3.40 ¡) of
two C atoms.
indicating that the thin films behave as n-type semiconduc-
1
a,14
tors.
The field-effect mobility of TANC was calculated as
¹
5
2
¹1 ¹1
8.9 © 10 cm V
s
with a threshold voltage of 23 V, and an
The spectroscopic and electrochemical data of TANC and
BTANC are summarized in Table 1. The UV­vis spectrum of
BTANC in CH2Cl2 has an absorption peak at 452 nm, which is
bathochromically shifted compared to that of TANC at 414 nm
resulting from an expanded aromatic ring system. By expansion
of its ³-conjugated system, BTANC also had a slight increase in
its HOMO energy level (EHOMO) and a decrease in its LUMO
energy level (ELUMO) relative to TANC.14 BTANC also showed a
broad absorption band at 602 nm, which is assigned as a weak
intramolecular-charge-transfer (CT) interaction between the
on/off ratio of 90. The field-effect mobility of BTANC was
¹
4
2
¹1 ¹1
calculated as 3.8 © 10 cm V
27 V, and an on/off ratio of 8 © 10 .
s
with a threshold voltage of
2
In conclusion, we have investigated the syntheses and
crystal structures of TANC and BTANC as two nonsubstituted
N-heteroacenes that are structurally similar to tetracene and
pentacene, respectively. The electrochemical properties of
TANC and BTANC were analyzed using CV and their
electron-transporting properties were studied in FET analyses.
The two N-heteroacenes behaved as electron acceptors function-
ing as novel n-type semiconductors with weaker FET activity
1
4
naphthalene and pyrazinopyradine portions of the molecule.
Chem. Lett. 2012, 41, 937­939
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

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Figure 3. FET characteristics for the thin-films of (a) BTANC
and (b) TANC. Gate voltages V were applied from 0 to 60 V in
G
12 Crystal data for TANC: C14H8N4, Mr = 232.24, Monoclinic,
space group P21/n (No. 14) with a = 4.710(3) ¡, b =
increments of 10 V.
1
4.910(10) ¡, c = 7.653(5) ¡, ¢ = 94.701(14)°, V =
3
= 1.440 g cm¹3, 99 parameter,
compared to n-type semiconductors which have been previously
535.6(6) ¡ , Z = 2, D
calcd
1
8
investigated. Although the FET activities of TANC and
BTANC are lower than those of tetracene and pentacene, the
improvement of their activity is underway by preparing thin
films containing less grain boundary in the crystal domains. In
the future, the semiconducting activity of TANC, BTANC, and
other tetraazaacenes can be improved by introducing electron-
donating or electron-accepting substituents, and changing the
number of aromatic rings and N atoms in the structures.
R1 = 0.0626, wR2 = 0.1434, and GOF = 1.223 for all
1142 data (I > 4.00·(I)); 4360 reflections collected; the
minimum and maximum residual electron densities are 0.92
¹
3
and ¹0.76 e ¡ . Crystal data for BTANC: C19H11Cl3N4,
Mr = 401.68, Monoclinic, space group P21/c (No. 14) with
a = 16.2751(3) ¡, b = 5.77490(10) ¡, c = 18.6968(3) ¡,
3
¢ = 101.7166(8)°, V = 1720.64(5) ¡ , Z = 4, Dcalcd =
¹3
1.550 g cm , 236 parameter, R1 = 0.0501, wR2 = 0.0565,
and GOF = 1.017 for all 3097 data (I > 2.00·(I)); 14964
reflections collected; the minimum and maximum residual
This work was supported partially by a Grant-in-Aid for
Young Scientists (B, No. 20568620, 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 and
Engineering (K.I.). We also thank Mitsubishi Chemical Group
Science and Technology for help with FET measurements.
¹
3
electron densities are 1.13 and ¹1.49 e ¡
.
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© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/