Synthesis, physical properties, and field-effect mobility of isomerically pure syn -/ anti -anthradithiophene derivatives

Article abstract of DOI:10.1021/ol301626u

Isomerically pure syn-/anti-isomers of 2,8-dimethylanthradithiophene (DMADT) were synthesized in five steps and characterized using thermogravimetry, X-ray single crystal analysis, UV-vis absorption, and electrochemical measurements. The physical properties in solution were slightly different for each isomer, whereby the more obvious differences were observed in the solid state. A field-effect transistor using the anti-isomer showed a much higher performance than that using the syn-isomer.

Full text of DOI:10.1021/ol301626u

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Synthesis, Physical Properties, and
Field-Effect Mobility of Isomerically Pure
syn-/anti-Anthradithiophene Derivatives
Masashi Mamada,* Tsukuru Minamiki, Hiroshi Katagiri, and Shizuo Tokito*
Graduate School of Science and Engineering, Yamagata University, Yonezawa,
Yamagata, 992-8510 Japan
mamada@yz.yamagata-u.ac.jp; tokito@yz.yamagata-u.ac.jp
Received June 13, 2012
ABSTRACT
Isomerically pure syn-/anti-isomers of 2,8-dimethylanthradithiophene (DMADT) were synthesized in five steps and characterized using
thermogravimetry, X-ray single crystal analysis, UVꢀvis absorption, and electrochemical measurements. The physical properties in solution
were slightly different for each isomer, whereby the more obvious differences were observed in the solid state. A field-effect transistor using the
anti-isomer showed a much higher performance than that using the syn-isomer.
Fused heterocyclicaromatichydrocarbons areknown as
semiconductors with high charge carrier mobilities and
have been a subject of considerable interest for organic
electronics applications such as organic field-effect transis-
tors (OFETs).1 Most hydrocarbons in OFETs contain thio-
phene units due to their similarity in chemical structure to
benzene rings and good stability against photo-oxidation.2 In
addition, the incorporation of heteroatoms enhances inter-
molecular interactions, resulting in excellent OFET device
performance.3
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Figure 1. Structures of BDT, NDT, and ADT derivatives.
Benzodithiophene (BDT) and naphthodithiophene (NDT),
which are heteroacenes corresponding to anthracene and tetra-
cene, are useful as core structures of organic semiconductors or
r
10.1021/ol301626u
XXXX American Chemical Society

Scheme 1. Synthesis of 1 (syn-DMADT) and 2 (anti-DMADT)
a building block for extended π-conjugation systems.4,5
Anthradithiophene (ADT) is analogous to pentacene,
which is the most common organic semiconductor.6
ADT showed better stability than pentacene and good
hole transporting properties.7 However, ADT is generally
synthesized and used as a mixture of syn-/anti-isomers.
Recently, several groups have obtained isomerically pure
ADT derivatives, such as those shown in Figure 1. The
pure anti-ADT derivatives which are substituted by alkyl
groups were synthesized through a new synthetic route
presented by Geerts et al.8 The synthetic method allows for
a more detailed investigation of the characteristics of ADT
derivatives, but it is limited to anti-derivatives. However,
the pure syn-ADT derivatives (syn-ABBT) were developed
and used as semiconductors by Tykwinski et al., although
extended π-conjugation systems were only synthesized.9
Anthony et al. synthesized both syn-/anti-ADT derivatives
(ADTAs), and their performance in solar cells was inves-
tigated and compared for each isomer.10 However, those
derivatives are substituted by large functional groups in
order to separate them by recrystallization because ADT
derivatives are basically insoluble. Thus, a synthetic ap-
proach to obtain simple and pure syn-/anti-ADTs and the
comparison of their physical properties and FET charac-
teristics for corresponding isomers are of significant inter-
est. Herein, we report on a newly established synthesis
method of pure syn-/anti-ADT derivatives and the com-
parison of their crystal structures and UVꢀvis absorption
and redox potential measurements in solution as well as in
the solid state. Also, FET device performance for each
isomer is described.
(4) (a) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J.; Dodabalapur,
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Niihara, N.; Otsubo, T. J. Am. Chem. Soc. 2004, 126, 5084. (c) Pan, H.; Li,
Y.;Wu, Y.;Liu, P.;Ong, B. S.;Zhu, S.;Xu, G.Chem. Mater. 2006, 18, 3237.
(d) Pan, H.; Wu, Y.; Li, Y.; Liu, P.; Ong, B. S.; Zhu, S.; Xu, G. Adv. Funct.
Mater. 2007, 17, 3574. (e) Ebata, H.; Miyazaki, E.; Yamamoto, T.;
Takimiya, K. Org. Lett. 2007, 9, 4499. (f) Wang, Y.; Parkin, S. R.; Watson,
M. D. Org. Lett. 2008, 10, 4421. (g)Zhou, Y.;Lei, T.;Wang, L.;Pei, J.;Cao,
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Nakano, M.; Shinamura, S.; Houchin, Y.; Osaka, I.; Miyazaki, E.;
Takimiya, K. Chem. Commun. 2012, 48, 5671. (c) Takimiya, K.;
Shinamura, S.; Osaka, I.; Miyazaki, E. Adv. Mater. 2011, 23, 4347.
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T. D. Mater. Res. Soc. Symp. Proc. 2003, 2, 678.
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Soc. 1998, 120, 664. (b) Chen, M. C.; Kim, C.; Chen, S. Y.; Chiang, Y. J.;
Chung, M. C.; Facchetti, A.; Marks, T. J. J. Mater. Chem. 2008, 18,
1029. (c) Jurchescu, O. D.; Hamadani, B. H.; Xiong, H. D.; Park, S. K.;
Subramanian, S.; Zimmerman, N. M.; Anthony, J. E.; Jackson, T. N.;
Gundlach, D. J. Appl. Phys. Lett. 2008, 92, 132103. (d) Jurchescu, O. D.;
Subramanian, S.; Kline, R. J.; Hudson, S. D.; Anthony, J. E.; Jackson,
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The synthetic formulation strategy is based on the
synthesis of a naphthothiophene ring, whereby the starting
materials are phthalic anhydride and thiophene (Scheme 1).11
The naphthothiophene ring can be considered a part of
the ADT ring. Thus, pyromellitic dianhydride, which has
two reaction sites, should provide the ADT ring as shown
in Scheme 1. In this scheme, 2-methylthiophene was used
because dimethyl-substituted ADT (DMADT) showed
better semiconducting properties than ADT.7b The
first reaction step gave a mixture of a meta-isomer (1d)
and para-isomer (2d), where the geometry of thiophene
for ADT syntheses is fixed on the benzene ring. There-
fore, key to this strategy is the separation of these
compounds. Fortunately, similar isomers from pyromel-
litic dianhydride with benzene have been separated by
(9) Lehnherr, D.; Hallani, R.; McDonald, R.; Anthony, J. E.; Tykwinski,
R. R. Org. Lett. 2012, 14, 62.
(10) Li, Z.; Lim, Y.-F.; Kim, J. B.; Parkin, S. R.; Loo, Y.-L.;
Malliaras, G. G.; Anthony, J. E. Chem. Commun. 2011, 47, 7617.
(11) (a) Tedjamulia, M. L.; Tominaga, Y.; Castle, R. N. J. Hetero-
cycl. Chem. 1983, 20, 1143. (b) Carruthers, W.; Douglas, A. G.; Hill, J.
J. Chem. Soc. 1962, 704.
(8) Tylleman, B.; Velde, C. M. L. V.; Balandier, J.-Y.; Stas, S.;
Sergeyev, S.; Geerts, Y. H. Org. Lett. 2011, 13, 5208.
B
Org. Lett., Vol. XX, No. XX, XXXX

recrystallization from acetic acid,12 and the same method
could be adopted for the separation of isomers 1d and 2d.
The ratio of isomers 1d and 2d was 2:1, which is different
from benzene derivatives. In the third step, the Rf values
of isomers 1b and 2b on the silica gel plate were different
when eluted with ethyl acetate due to a difference in polarity
derived from their dipole moment. Thus, isomeric purity is
high, even when a small amount of opposite isomers remain
in the recrystallization for isomers 1d and 2d. The separated
isomers 1d and 2d could be converted to DMADTs 1 and 2,
respectively, with good yields under the same reaction
conditions used for the synthesis of naphthothiophene.
Finally, the synthesis of DMADTs was successfully con-
ducted through regiochemical control and the products
were purified by sublimation. The solubility of these pro-
ducts in common organic solvents is relatively low.
different. The molecules in isomer 2 are arranged with
slipped herringbone packing, which slightly reduces the
overlap between rings.
TG-DTA for isomers 1 and 2 revealed decomposition
above 400 °C, suggesting high thermal stabilities. The 5%
weight losses were at temperatures of 424 and 440 °C for
isomer 1 (syn-isomer) and isomer 2 (anti-isomer), respec-
tively (see Supporting Information (SI)). The higher de-
composition temperature for isomer 2 might be related to
its different molecular structure as well as differences in
intermolecular interactions.
Figure 3. Normalized absorption spectra of isomers 1 and 2 in
CH2Cl2 (solid line) and in film (dashed line).
Table 1. Optical and Electrochemical Properties
λabs
(nm)a
Eox1 Eox2
(V)b (V)b
Ered
(V)b
HOMO LUMO
compd
(eV)c
(eV)c
1
2
solution
calcdd
film
479
488
530
480
490
519
0.23 0.78 ꢀ2.35
ꢀ5.03
ꢀ4.70
ꢀ5.18
ꢀ5.03
ꢀ4.69
ꢀ5.16
ꢀ2.45
ꢀ1.85
ꢀ3.02
ꢀ2.46
ꢀ1.86
ꢀ3.10
ꢀ
ꢀ
ꢀ
0.38 0.83 ꢀ1.78
0.23 0.77 ꢀ2.34
solution
calcdd
film
ꢀ
ꢀ
ꢀ
0.36 0.80 ꢀ1.70
a The lowest energy maxima. b Determined by differential pulse
voltammetric measurement in 0.1 M solution of Bu4NPF6 in o-dichlor-
obenzene at 100 °C or in film with 0.1 M solution of Bu4NPF6 in CH2Cl2
at rt (vs Fc/Fcþ). c Estimated vs vacuum level from EHOMO = ꢀ4.80 ꢀ
Eox or ELUMO = ꢀ4.80 ꢀ Ered
.
d Calculated by DFT methods at the
B3LYP/6-31G(d,p) level using the Gaussian 09 program.
The optical and electrochemical properties for isomers 1
and 2 were investigated. The results are summarized in
Figure 3 and in Table 1. The UVꢀvis spectra in solution
showed a typical transition of ADT derivatives. The red
shift of ca. 1 nm (65 cmꢀ1) was observed in isomer 2 (anti-
isomer), and the spectra for each isomer were similar. The
solubility is better for isomer 1 (syn-isomer). Thus, if a
mixture was used for these measurements, the result would
likely include more contributions from the syn-isomer. In
fact, the reported absorption maximum of a mixture was
479 nm which corresponds to that for the syn-isomer.7b
The optical energy gaps estimated from the onset of the
absorption are ca. 2.51 eV. The photooxidative stability
level as determined by a decay in the absorbance was
similar to the previously reported values (see SI).7b
Figure 2. Crystal packing of (a) 1 and (b) 2 along with the b-axis
(thermal ellipsoids 50% probability).
Single crystal structure analyses were carried out to
confirm the molecular geometry and to investigate inter-
molecular interactions in the solid state (Figure 2). The
crystals for isomers 1 and 2 were obtained by sublimation.
Crystal growth was comparatively easier for isomer 2.
Compound 1 crystallizes in the noncentrosymmetric space
group P21, whereby it forms twinned crystals and involves
crystal disorder. On the other hand, the single crystal for
isomer 2 contains centrosymmetric half-molecules (C2h)
in the asymmetric unit without disorder. A herringbone
packing motif was observed for both isomers with a tilt
angle of 52.1° for isomer 1 and 50.8° for isomer 2. As
shown in Figure 2, the molecular orientations are clearly
The absorption spectra in a 50-nm-thick film on a quartz
plate showed clear differences in eachisomer. Bothisomers
showed a large bathochromic shift in the absorption bands
compared to the solution state, indicating the optical en-
ergy gaps of 2.25 eV for isomer 1 and 2.27 eV for isomer 2.
(12) Du, D.; Jiang, Z.; Liu, C.; Sakho, A. M.; Zhu, D.; Xu, L. J.
Organomet. Chem. 2011, 696, 2549.
Org. Lett., Vol. XX, No. XX, XXXX
C

Interestingly, the lowest absorption maximum for isomer 1
(syn-isomer) appeared at an 11 nm longer wavelength
compared to that for isomer 2 (anti-isomer). This is
attributed to differences of intermolecular interactions.
The transition dipoles of molecules, calculated by the
time-dependent density functional theory (TD-DFT) meth-
od, were in the direction of the molecular short axis for
isomer 1, as seen in acenes, and oblique to that for isomer
2.13,14 Since the herringbone crystal packing is exhibited
for both, the aggregation cannot be classified as J-type or
H-type. In practice, the molecular orientations in single
crystals and in thin films are not always identical. However,
the XRD pattern of a thin film of isomer 1 suggested a
d-spacing which is nearly equivalent to the length of the c-
axis for a single crystal, whereas the thin film of isomer 2
exhibited almost the same d-spacing as isomer 1, which alters
the estimation from the single crystal. Therefore, at least
isomer 1, and most likely isomer 2, can be considered to have
a herringbone packing arrangement in a thin-film form. In
this case, the nearest two molecules may lead to the band
splitting into higher energy and lower energy contributions
(Davydov splitting).15 Here, the lowest energy absorption
peak at 520 nm of isomer 1(syn-isomer) exhibited a Davydov
component such as tetracene and pentacene,16 but isomer 2
(anti-isomer) showed a broad peak in the 0ꢀ0 band.
Figure 4. Transfer characteristics of anti-DMADT (blue lines,
circles) and syn-DMADT (red lines, triangles).
0.41 cm2 Vꢀ1 sꢀ1, respectively, which are higher than the
previously reported mobility in a mixture (0.062 cm2 Vꢀ1
s
ꢀ1).7b Interestingly, the anti-isomer 2 exhibited a mobility
that was five times higher than that for syn-isomer 1, which
is attributed to differences in their molecular structures, as
well as to intermolecular interactions in their thin films.
The mobility results for syn-/anti-isomers show the same
tendencies as those for naphthodithiophenes, whereby the
centrosymmetricderivativesshowed highermobilitiesthan
the axisymmetric ones.5a These results clearly suggest the
importance of separating isomers in FET devices.
In summary, pure isomers of anthradithiophene deriva-
tives were successfully synthesized in good yields. The
molecular structures for these isomers have been verified
with X-ray single crystal structure analysis. We observed
only slight differences in photochemical and electrochemi-
cal properties for each isomer in solution. However,
apparent differences were observed in the solid state,
especially in FET devices, whereby the anti-isomer showed
higher device performance than the syn-isomer. Further
investigation to understand these differences in the device
performance is currently underway. This synthetic work
enabled the detailed investigation of physical properties of
ADT derivatives. The synthesis is applicable for pure iso-
mers of unsubstituted-ADTs, dialkyl-ADTs, and dibromo-
ADTs as building blocks for semiconductors.
Differential pulse voltammetry (DPV) in solution for
isomers 1 and 2 showed two oxidation peaks and a
reduction peak (see SI). The first oxidation potential was
0.23 V vs Fc/Fcþ for both isomers, and the reduction
potentials of isomers 1 and 2 were ꢀ2.35 and ꢀ2.34 V,
respectively. The estimated HOMOꢀLUMO values from
these redox potentials are summarized in Table 1. The
HOMOꢀLUMO energy gaps are consistent with the
optical energy gaps derived from UVꢀvis measurements.
Although the redox potentials in solution were nearly the
same for both isomers, those small differences revealed
intermolecular interactions in film, as well as in the
UVꢀvis spectra. The DPV measurements in thin films of
isomers 1 and 2 showed two oxidation peaks and one weak
reduction peak, which are similar in solution. These redox
processes appeared as a positive shift of the results in
solution. Since the shifts in the reduction peaks were larger,
the LUMO levels are located at much deeper levels in the
solid state. The HOMOlevelsfor isomers1 and 2 estimated
from their oxidation potentials were similar.
The FET devices for both isomers were fabricated on a
hexamethyldisilazane (HMDS)-treated substrate at rt with
a top-contact device configuration (Figure 4). The devices
for isomers 1 and 2 showed hole mobilities of 0.084 and
Acknowledgment. This work was supported by the
Japan Regional Innovation Strategy Program by the Ex-
cellence (creating international research hub for advanced
organic electronics) of Japan Science and Technology
Agency (JST), and by the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
(13) DFT calculations were carried out with Gaussian 09 program
package: Frisch, M. J. et al. Gaussian 09, revision C.01; Gaussian, Inc.:
Wallingford, CT, 2010; see SI for full reference.
(14) Lim, S.-H.; Bjorklund, T. G.; Spano, F. C.; Bardeen, C. J. Phys.
Rev. Lett. 2004, 92, 107402.
(15) (a) Davydov, A. S. Theory of molecular excitons; Plenum Press,
New York, 1971. (b) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl.
Chem. 1965, 11, 371.
(16) (a) Philpott, M. R. J. Chem. Phys. 1969, 50, 5117. (b) Maliakal,
A.; Raghavachari, K.; Katz, H.; Chandross, E.; Siegrist, T. Chem.
Mater. 2004, 16, 4980. (c) Miao, Q.; Nguyen, T.-Q.; Someya, T.;
Blanchet, G. B.; Nuckolls, C. J. Am. Chem. Soc. 2003, 125, 10284.
Supporting Information Available. Full experimental
details, characterization data, and CIF files for isomers 1
and 2. This material is available free of charge via the
Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX