Tetracyanoanthraquinodimethanes having biaryl substituents: Synthesis, crystal structures, and physical properties

Article abstract of DOI:10.1246/cl.2012.482

11,11,12,12-Tetracyano-9,10-anthraquinodimethane (TCNAQ) derivatives having four types of aryl substituents, phenyl, biphenyl-4-yl, thien-2-yl, and 2,2′-bithiophen-5-yl groups at their 2-position, were synthesized and characterized. Their crystal structures were determined by single-crystal X-ray structure analysis, revealing that the TCNAQ moiety and electron-donating substituents form donoracceptor-segregated columnar structures. The combination of nonplanar electron acceptor TCNAQ and planar donor units provides a good way to afford segregated columnar structures.

Full text of DOI:10.1246/cl.2012.482

doi:10.1246/cl.2012.482
482
Published on the web April 21, 2012
Tetracyanoanthraquinodimethanes Having Biaryl Substituents:
Synthesis, Crystal Structures, and Physical Properties
Hiroshi Chiba, Jun-ichi Nishida, and Yoshiro Yamashita*
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering,
Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8502
(Received January 28, 2012; CL-120070; E-mail: yoshiro@echem.titech.ac.jp)
11,11,12,12-Tetracyano-9,10-anthraquinodimethane (TCNAQ)
NC CN
O
O
derivatives having four types of aryl substituents, phenyl,
biphenyl-4-yl, thien-2-yl, and 2,2¤-bithiophen-5-yl groups at
their 2-position, were synthesized and characterized. Their
crystal structures were determined by single-crystal X-ray
structure analysis, revealing that the TCNAQ moiety and
electron-donating substituents form donor-acceptor-segregated
columnar structures. The combination of nonplanar electron
acceptor TCNAQ and planar donor units provides a good way to
afford segregated columnar structures.
Br
R
R
(ii)
(i)
O
O
NC CN
2
3a-d
1a-d
R =
S
S
S
a
b
c
d
Scheme 1. Synthetic route to compounds 1a-1d. (i) 3a and
3b: R-B(OH)2, [Pd(PPh3)4], K2CO3(aq), toluene, reflux; 3c and
3d: R-SnBu3, [Pd(PPh3)4], toluene, reflux; (ii) H2C(CN)2, TiCl4,
pyridine, CH2Cl2, rt.
Donor-acceptor-segregated columnar structures have at-
tracted much attention since they afford good carrier trans-
portation, and are advantageous for organic photovoltaics.1
Oligothiophene-C60 dyads2 and ³-extended sumanenes,3 which
consist of nonplanar acceptor units linked with planar aryl donor
units, form donor-acceptor-segregated columnar stacks, sug-
gesting that nonplanar electron acceptors with planar electron-
donating substituents are promising compounds to afford such
structures.
Table 1. Optical and electrochemical data of 1a-1d9
b
c
Epc
Epa
/V
Compound -abs/nm (log ¾)a
/V
1a
1b
1c
1d
298 (4.44), 324 (4.47)
¹0.35
282 (4.54), 335 (4.58), 425 (3.98) ¹0.36
288 (4.36), 343 (4.50), 442 (3.87) ¹0.38
11,11,12,12-Tetracyano-9,10-anthraquinodimethane (TCNAQ)4
is a dibenzo analogue of tetracyanoquinodimethane (TCNQ) and
has a butterfly-shaped structure due to the steric interaction
between the dicyanomethylene parts and the neighboring hydro-
gens.5 The TCNAQ derivatives are expected to form donor-
acceptor-segregated columnar structures by introducing planar
electron donor units to the nonplanar electron acceptor moiety.
We have now synthesized TCNAQs with four types of aryl
substituents, phenyl (1a), biphenyl-4-yl (1b), thien-2-yl (1c), and
2,2¤-bithiophen-5-yl (1d) groups at their 2-position, and inves-
tigated their crystal structures by single-crystal X-ray structure
analysis. We report here the donor-acceptor-segregated colum-
nar structures in the TCNAQs with biaryl substituents.
286 (4.35), 351 (4.58), 501 (4.02) ¹0.38 +1.35
b
aIn CH2Cl2. 0.1 M n-Bu4NPF6 in CH2Cl2, Pt electrode, V vs.
c
SCE. CV, irreversible wave, peak potential.
ents and TCNAQ moiety upon reduction.14 TCNAQs 1a-1c
showed no oxidation waves, whereas 1d showed an irreversible
oxidation wave originating from the bithienyl units.
Their optical properties were investigated by the absorption
spectra. Figure 1 shows the UV-vis absorptions of 1a-1d in
CH2Cl2. The absorption maxima of 1b, 1c, and 1d in the visible
region were observed at 425, 442, and 501 nm, respectively,
which are attributed to charge transfer from the aryl donor
substituents to the TCNAQ moiety.6 Increasing the electron-
donating properties of aryl substituents brings about more
bathochromic shifts. The reflection spectra of 1a-1d in the solid
state were collected by using an integrated sphere and are
depicted in Figure 2. Compared to the spectra in solution, the
absorption edges reach longer wavelengths. This is due to the
stronger intra- and intermolecular charge transfer in the solid
state.
Their crystal structures were investigated by single-crystal
X-ray structure analysis. Single crystals of 1a-1d suitable for
X-ray analysis were obtained by slow evaporation of the solvent
from their CH2Cl2/hexane solution. Figure S117 shows the
crystal structures of 1a and 1c. These two molecules form
dimeric structures, and no interaction was observed between the
aryl substituents. In the case of biphenyl-substituted derivative
The synthesis of TCNAQs is outlined in Scheme 1.
2-Bromoanthraquinone (2) reacted with aryl boronic acids by
the palladium-catalyzed Suzuki coupling reaction, or reacted
with tributylstannyl reagents by the palladium-catalyzed Stille
coupling to give the aryl-substituted derivatives.6,7 TCNAQs
1a-1d were obtained by a TiCl4-catalyzed Knoevenagel reaction
with malononitrile.6,8 They were characterized by mass spec-
1
trometry (MS) and H NMR along with elemental analysis.
Cyclic voltammograms of TCNAQs 1a-1d shown in
Figure S517 were measured in CH2Cl2 to investigate their
electrochemical properties. The redox potentials are summarized
in Table 1. All these compounds 1a-1d showed a reversible
reduction wave ascribed to two-electron reduction of the
TCNAQ moiety.13 The reduction potentials appeared at ca.
¹0.37 V vs. SCE and are almost the same among 1a-1d. This
suggests weak electronic interactions between the aryl substitu-
Chem. Lett. 2012, 41, 482-484
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

483
Figure 4. Crystal packing of 1d.
Figure 1. UV-vis absorption spectra of 1a-1d in CH2Cl2.
Figure 2. Reflection spectra of 1a-1d in the solid state after
K-M transformation.
Figure 5. Ambipolar characteristic of 1d.
Similar segregated stacking was observed in the crystal
structure of 2,2¤-bithiophen-5-yl-substituted derivative 1d shown
in Figure 4. In this case, effective ³-stacking interaction can be
seen between the bithienyl units, where the distance between
these ³-stacked units is 3.44 ¡. This intermolecular interaction
between the planar donor units seems to result in formation of a
donor-acceptor-segregated columnar structure.
To investigate the relationship between the donor-acceptor-
segregated structures and carrier-transport properties, field-effect
transistor (FET) characteristics based on 1a-1d were measured.
The FET devices were fabricated with bottom-contact config-
uration. Thin films (500 ¡) of compounds 1a-1d were deposited
on the channel regions kept at rt by vacuum evaporation. The
measurements were carried out in situ under high vacuum
(10¹5 Pa). The films of 1b and 1d exhibited n-type semi-
conducting behavior. The electron mobilities (on/off ratio) were
¹1
calculated to be 6.6 © 10¹7 cm2 V¹1 s
(800) for 1b and
3.4 © 10¹6 cm2 V¹1 s (1000) for 1d.16 In addition, an ambi-
polar characteristic of 1d was observed due to the electron-
donating bithienyl units (Figure 5). The hole mobility (on/off
ratio) of 1d was calculated to be 3.6 © 10¹7 cm2 V¹1 s¹1 (1000).
In contrast, 1c showed no semiconducting behavior. This
indicates that the donor-acceptor-segregated structure of 1d is
advantageous for carrier transport. Unfortunately, X-ray diffrac-
tograms of the films of 1a-1d showed that the films were not
completely crystalline.
¹1
Figure 3. Crystal structure of 1b viewed along the b axis.
1b, the solvent CH2Cl2 molecules are included and interactions
between the biphenyl units are observed (Figure S317). The
single crystals without the solvent molecules were obtained by
slow sublimation. In the crystal of 1b, the TCNAQ moieties
assemble themselves, and a donor-acceptor-segregated colum-
nar structure is constructed (Figure 3). In this case, CH-³
interactions are observed, and they seem to contribute to the
formation of herringbone stacking of the biphenyl units. This
remarkable change at the structure in 1b compared to 1a can be
attributed to the extended ³ conjugation of the biphenyl group to
give favorable intermolecular interactions.15
In summary, TCNAQs bearing four types of aryl substitu-
ents 1a-1d were synthesized, and their crystal structures were
investigated by X-ray analysis. The derivatives with electron-
donating biaryl substituents, 1b and 1d form donor-acceptor-
segregated columnar structures. This is attributed to the
Chem. Lett. 2012, 41, 482-484
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

484
intermolecular interactions between donor units. The combina-
tion of nonplanar acceptor TCNAQ and extended ³-conjugated
planar donor units provides a good way to afford such
segregated structures. The greater elongation of the ³ conjuga-
tion of donor units is thought to promote better carrier
transportation, and study along this line is underway in our
laboratory.
1874.6(3) ¡3, Z = 4, Dcalcd = 1.348 g cm¹3, 17841 reflec-
tions collected, 4290 independent (Rint = 0.0654),
GOF = 0.977, R1 = 0.0505 (I > 2.00·(I)), wR2 = 0.1548
for all reflections. Crystal Data for 1b: C32H16N4, Mr =
456.51, red block, crystal dimensions 0.78 © 0.60 ©
0.15 mm3, monoclinic, space group P21/c, a =
25.424(8) ¡, b = 10.236(4) ¡, c = 26.623(9) ¡, ¢ =
¹3
98.363(4)°, V = 6855(4) ¡3, Z = 12, Dcalcd = 1.327 g cm
,
References and Notes
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X-ray measurement of single crystals of 1a-1d was carried
out using a RAXIS-RAPID imaging plate diffractometer
with Mo K¡ radiation (- = 0.71075 ¡) at ¹180.0 °C. The
structures were solved by the direct method (SIR200410) and
refined by the full-matrix least-squares method on F2. The
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Crystal Data for 1a: C26H12N4, Mr = 380.41, orange
platelet, crystal dimensions 0.79 © 0.35 © 0.08 mm3,
monoclinic, space group P21/c, a = 10.5382(8) ¡, b =
8.8850(6) ¡, c = 20.5763(14) ¡, ¢ = 103.342(3)°, V =
Chem. Lett. 2012, 41, 482-484
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/