Isomerically pure syn-anthradithiophenes: Synthesis, properties, and FET performance

Article abstract of DOI:10.1021/ol301503k

The synthesis of isomerically pure syn-anthradithiophene derivatives (syn-ADTs) is described. X-ray crystallography is used to compare the solid-state arrangement of syn-ADT derivatives 2a,b to the analogous mixture of syn- and anti-ADTs. Single-crystal O

Full text of DOI:10.1021/ol301503k

ORGANIC
LETTERS
2012
Vol. 14, No. 14
3660–3663
Isomerically Pure
syn-Anthradithiophenes: Synthesis,
Properties, and FET Performance
Dan Lehnherr,† Andreas R. Waterloo,‡ Katelyn P. Goetz,§ Marcia M. Payne,^
Frank Hampel,‡ John E. Anthony,^ Oana D. Jurchescu,*,§, and Rik R. Tykwinski*,‡,#
Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada,
Department of Chemistry and Pharmacy & Interdisciplinary Center of Molecular
Materials (ICMM), University of Erlangen-Nuremberg, Henkestrasse 42, 91054
Erlangen, Germany, Department of Physics, Wake Forest University, Winston-Salem,
North Carolina 27109, United States, and Department of Chemistry, University of
Kentucky, Lexington, Kentucky 40506, United States
rik.tykwinski@chemie.uni-erlangen.de; jurchescu@wfu.edu
Received June 1, 2012
ABSTRACT
The synthesis of isomerically pure syn-anthradithiophene derivatives (syn-ADTs) is described. X-ray crystallography is used to compare the solid-state
arrangement of syn-ADT derivatives 2a,b to the analogous mixture of syn- and anti-ADTs. Single-crystal OFETs based on isomerically pure syn-ADTs 2a,b
display device performance comparable to those based on a mixture of ADT isomers syn/anti-2a,b with mobilities as high as 1 cm2/(V s).
The synthesis of organic molecules for optoelectronic
applications has seen exponential growth in the past decade
with intense interest in photovoltaics,1 OLEDs, and
OFETs.2 Many promising small molecule semiconductors
are based on a hydrocarbon skeleton, such as pentacene
and its derivatives.2 The incorporation of heteroatoms into
an acene framework, on the other hand, offers an oppor-
tunity to vary physical and electronic properties, and
toward this goal many thienoacene-based small molecule
semiconductors have been recently developed.3 An impor-
tant class of thienoacenes are the anthradithiophenes
(ADTs, Figure 1), and ADTs with well-designed substitu-
tion patterns afford devices with intriguing performance in
organic field-effect transistors (OFETs)4 and organic
photovoltaics (OPVs).4i In specific cases, functionalization
of the ADT skeleton has been exploited, for example, to
control solid-state ordering, improve stability, and facil-
itate thermal vacuum deposition for device fabrication.5 A
common substitution pattern for ADTs features ethynyl
groups in the 5- and 11-positions and incorporation of
alkyl, halogen, or cyano groups at the 2- and 8-positions.
† University of Alberta.
‡ University of Erlangen-Nuremberg.
§ Wake Forest University.
^ University of Kentucky.
Author to whom correspondence for device fabrication and characteriza-
tion should be addressed.
# Author to whom correspondence for molecule synthesis and character-
ization should be addressed.
ꢀ
(1) (a) Beaujuge, P. M.; Frechet, J. M. J. J. Am. Chem. Soc. 2011, 133,
20009–20029. (b) Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Chem. Rev. 2009,
109, 5868–5923.
(2) (a) Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D. Chem. Rev.
2012, 112, 2208–2267. (b) Pron, A.; Gawrys, P.; Zagorska, M.; Djurado,
D.; Demadrille, R. Chem. Soc. Rev. 2010, 39, 2577–2632. (c) Lehnherr,
D.; Tykwinski, R. R. Aust. J. Chem. 2011, 64, 919–929. (d) Murphy,
ꢀ
A. R.; Frechet, J. M. J. Chem. Rev. 2007, 107, 1066–1096. (e) Anthony,
J. E. Angew. Chem., Int. Ed. 2008, 47, 452–483. (f) Anthony, J. E. Chem.
Rev. 2006, 106, 5028–5048. (g) Bendikov, M.; Wudl, F.; Perepichka,
D. F. Chem. Rev. 2004, 104, 4891–4945.
(3) Takimiya, K.; Shinamura, S.; Osaka, I.; Miyazaki, E. Adv. Mater.
2011, 23, 4347–4370.
r
10.1021/ol301503k
Published on Web 07/02/2012
2012 American Chemical Society

An important point of ADTs studied to date, however, is
that they are nearly always formed as an inseparable
mixture of syn- and anti-isomers. This is a result of a
common synthetic approach in which a regiorandom aldol
condensation is used for formation of anthradithiophene-
quinone precursors. During the course of the studies
reported herein, however, two recent examples6,7 of iso-
merically pure ADTs have appeared.8
Liꢀhalogen exchange, and subsequent trapping of aryl-
lithium species with ethyl formate provides the differen-
tially protected dialdehyde 6. An aldol reaction between 7
and monoprotected thiophenealdehyde 6 (2.1 equiv) then
afforded 8 in 89% yield. This reaction also sets the
regiochemistry of the sulfur atoms toward forming syn-
ADTs. The structure of 8 was determined by X-ray crystal-
lography, confirming the desired geometry.10
We hypothesized that the formation and use of isomeri-
cally pure ADT derivatives might improve device perfor-
mance. To test this hypothesis, we chose fluorinated ADTs
2a and 2b because earlier work with the syn/anti mixtures
of these compounds has shown promising hole mobilities
in OFETs, especially for syn/anti-2a.4g,5b We report here the
synthesis of syn-ADTs (syn-2aꢀd) and a comparison of
performance for single-crystal OFET devices of pure syn-
isomers versus the syn/anti-mixture of isomers for 2a and 2b.
The synthesis of halogenated syn-ADT derivatives
(syn-2aꢀd) begins from commercially available 3-thiophe-
necarboxaldehyde (3, Scheme 1). Attempts at regioselec-
tive monobromination at the 5-position of 3 proved to be
difficult because of a significant amount of dibromination.
Thus, thiophene 3 was dibrominated using excess bromine
in the presence of AlCl3 to afford 4.9 Protection of the
aldehyde as an acetal using ethylene glycol provided 5. The
acetal acts as a directing group for the regioselective
Figure 1. Structures of syn- and anti-ADTs 1 and 2aꢀd.
Global deprotection of the three acetal groups of 8 was
successfully achieved using In(OTf)3 in acetone under reflux,
and these conditions effected the subsequent aldol conden-
sation to give dibromo-syn-anthradithiophenequinone 9.
The synthesis of 9 should be amenable to large scale, as the
product precipitates directly from the reaction mixture and
can then be collected in analytical pure form via filtration.
Addition of Liꢀacetylides to 9afforded diols 10a,bin modest
(4) (a) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J. J. Am. Chem.
Soc. 1998, 120, 664–672. (b) Payne, M. M.; Odom, S. A.; Parkin, S. R.;
Anthony, J. E. Org. Lett. 2004, 6, 3325–3328. (c) Payne, M. M.; Parkin,
S. R.; Anthony, J. E.; Kuo, C.-C.; Jackson, T. N. J. Am. Chem. Soc.
2005, 127, 4986–4987. (d) Anthony, J. E.; Subramanian, S.; Parkin,
S. R.; Park, S. K.; Jackson, T. N. J. Mater. Chem. 2009, 19, 7984–7989.
(e) Wang, J.; Liu, K.; Liu, Y.-Y.; Song, C.-L.; Shi, Z.-F.; Peng, J.-B.;
Zhang, H.-L.; Cao, X.-P. Org. Lett. 2009, 11, 2563–2566. (f) Kim, C.;
Huang, P.-Y.; Jhuang, J.-W.; Chen, M.-C.; Ho, J.-C.; Hu, T.-S.; Yan,
J.-Y.; Chen, L.-H.; Lee, G.-H.; Facchetti, A.; Marks, T. J. Org. Electron.
2010, 11, 1363–1375. (g) Jurchescu, O. D.; Subramanian, S.; Kline, R. J.;
Hudson, S. D.; Anthony, J. E.; Jackson, T. N.; Gundlach, D. J. Chem.
Mater. 2008, 20, 6733–6737. (h) Goetz, K. P.; Li, Z.; Ward, J. W.;
Bougher, C.; Rivnay, J.; Smith, J.; Conrad, B. R.; Parkin, S. R.;
Anthopoulos, T. D.; Salleo, A.; Anthony, J. E.; Jurchescu, O. D. Adv.
Mater. 2011, 23, 3698–3703. (i) Jiang, Y.; Okamoto, T.; Becerril, H. A.;
Hong, S.; Tang, M. L.; Mayer, A. C.; Parmer, J. E.; McGehee, M. D.;
Bao, Z. Macromolecules 2010, 43, 6361–6367.
yields. Aromatization of diols 10a,b using SnCl2 2H2O and
3
aqueous H2SO4 afforded functionalized syn-2c,d in near-
quantitative yields (92ꢀ96%). Lithiation of syn-2c,d via
Liꢀhalogen exchange, followed by reaction with N-fluoro-
benzenesulfonimide (NFSI) provided fluorinated syn-ADTs
syn-2a,b. Yields of syn-2a,b were moderate, and the pure
ADTs were isolated from the numerous byproduct via
column chromatography.
The thermal characteristics of syn-2aꢀd as well as syn/
anti-2a,b have been examined by thermal gravimetric
analysis (TGA) and differential scanning calorimetry
(DSC, Table 1). Notably, the isomerically pure syn-ADT
material has a slightly lower melting point (∼10 ꢀC) as
observed by DSC analyses, whereas decomposition is
observed at about the same temperature.
Single crystals of isomerically pure syn-2a,b suitable for
X-ray crystallographic analysis were obtained from slow eva-
poration at 4 ꢀC of dichloromethane solutions layered with
acetone or from slow evaporation at ꢀ15 ꢀC of n-pentane
solutions layered with acetone. Analysis of these data re-
vealed that the packing of syn-2a,b is remarkably similar to
that reported for syn/anti mixtures of syn/anti-2a,b. In
particular, the 2,8-difluoranthra[2,3-b:7,6-b0]dithiophene
(5) See, for example: (a) Facchetti, A.; Mushrush, M.; Katz, H. E.;
Marks, T. J. Adv. Mater. 2003, 15, 33–38. (b) Subramanian, S.; Park,
S. K.; Parkin, S. R.; Podzorov, V.; Jackson, T. N.; Anthony, J. E. J. Am.
Chem. Soc. 2008, 130, 2706–2707. (c) Lee, W.-Y.; Oh, J. H.; Suraru,
€
S.-L.; Chen, W.-C.; Wurthner, F.; Bao, Z. Adv. Funct. Mater. 2011, 21,
4173–4181. (d) Gutzler, R.; Ivasenko, O.; Fu, C.; Brusso, J. L.; Rosei, F.;
Perepichka, D. F. Chem. Commun. 2011, 47, 9453–9455.
(6) Geerts and co-workers reported the synthesis of isomerically pure
anti-ADT derivatives with alkyl groups in the 2- and 8-positions,
although to our knowledge no device data have been reported; see:
Tylleman, B.; Vande Velde, C. M. L.; Balandier, J.-Y.; Stas, S.;
Sergeyev, S.; Geerts, Y. H. Org. Lett. 2011, 13, 5208–5211.
(7) Anthony and co-workers have showed that amide substitution at the
ADT 2- and 8-positions allows for selective crystallization of the syn- and
anti-isomers and that regiochemistry has a significant impact on solar cell
performance; see: Li, Z.; Lim, Y.-F.; Kim, J. B.; Parkin, S. R.; Loo, Y.-L.;
Malliaras, G. G.; Anthony, J. E. Chem. Commun. 2011, 47, 7617–7619.
(8) Formation of structurally related syn-anthrabisbenzothiophenes
has been reported; see: Lehnherr, D.; Hallani, R.; McDonald, R.;
Anthony, J. E.; Tykwinski, R. R. Org. Lett. 2012, 14, 62–65.
(9) The synthesis of compound 4 has been previously reported using
alternate procedures; see: (a) Clarke, J. A.; Meth-Cohn, O. Tetrahedron
Lett. 1975, 16, 4705–4708. (b) Fourani, P.; Guilard, R.; Person, M. Bull.
Soc. Chim. Fr. 1967, 4115–4120. (c) van Beek, R.; Zoombelt, A. P.;
ꢀ
Jenneskens, L. W.; van Walree, C. A.; de Mello Donega, C.; Veldman,
D.; Janssen, R. A. J. Chem.;Eur. J. 2006, 12, 8075–8083.
(10) See the Supporting Information for details.
Org. Lett., Vol. 14, No. 14, 2012
3661

Scheme 1. Synthesis of syn-Anthradithiophenes (syn-2aꢀd)
Table 1. Thermal Properties of ADTs syn-2aꢀd and syn/anti-2a,ba
DSC decomposition
compd
syn-2a
TGA Td/ꢀC
DSC mp/ꢀC
onset/ꢀC
peak/ꢀC
267
222
296
278
295
315
190
200
239
247
273
315
291
267
341
345
298
350
320
321
370
369
313
NA
syn/anti-2a
syn-2b
syn/anti-2b
syn-2c
syn-2d
a Measured under nitrogen atmosphere.
group is disordered about the crystallographic inversion
center (0, 1/2, 1/2), and the two inversion-related orienta-
tions of the molecule are present in equal abundance (both
orientations are shown in Figure 2). The i-Pr3Si derivatives
also have disorder in the position of the alkyl carbons,
whereas the Et3Si derivatives do not.10 Isomerically pure
syn-2a,b pack with 2-D π-stacking, and there are two types
Figure 2. Crystal packing of syn-2a (top) and syn-2b (bottom).
˚
of cofacial interactions with interplanar spacing of 3.30 A
˚
˚
˚
respectively. The identity of silyl group has a more profound
effect on the λmax values of thin film samples of syn-2a,b cast
from CH2Cl2.10 In each case, λmax values of thin films of Et3Si de-
rivatives [543 nm (syn-2a) and 561 nm (syn-2c)] are more red-
shifted in comparison to the i-Pr3Si derivatives [530 nm (syn-
2b) and 555 nm (syn-2d)], which is, in principle, consistent with
solid-state packing observed in the crystallographic analysis.
Overall, however, solution- and solid-state absorption proper-
ties of syn-2a,b are not significantly affected by the isomeric
purity of the material in comparison to syn/anti-2a,b.13,14
Single crystals of isomerically pure syn-2a and syn-2b
and of isomers syn/anti-2a and syn/anti-2b were grown by
physical vapor transport,15 and their electrical properties
were compared using field-effect transistor (FET) mea-
surements. Figure 4 shows the evolution of the drain
and 3.41 A for syn-2a, and 3.30 A and 3.35 A for syn-2b
(Figure 2).11 These values are nearly identical to those
found for the solid-structure of syn/anti-2a,b, which are
˚
˚
˚
determined as 3.29 A and 3.35 A (syn/anti-2a) and 3.29 A
and 3.34 A (syn/anti-2b).12 Thus, isomeric purity has little
˚
effect on solid-state packing, as has been noted by others.7
Solution-state UVꢀvis absorption data (CH2Cl2, Figure 3)
show that the lowest energy absorption maximum (λmax) is
located at 543 nm for Br derivatives syn-2c,d, while F deriva-
tives syn-2a,bare hypsochromically shifted to 525 and 526 nm,
(11) Interplanar distances were calculated as the distance between planes
generated from the atoms of each anthradithiophene moiety. Because of the
positional disorder in the thiophenes, this results in 26 atoms for each plane
instead of 20 atoms found in the anthradithiophene moiety.
(12) Calculated using CIFs of ref 5b.
3662
Org. Lett., Vol. 14, No. 14, 2012

Figure 3. UVꢀvis absorption spectra of syn-2aꢀd in CH2Cl2.
current (ID) with the gate-to-source voltage (VGS) in the
saturation regime (drain-to-source voltage VDS = ꢀ40 V)
for compounds 2a and 2b, respectively. All devices inves-
tigated here presented excellent electrical characteristics,10
with a very sharp turn on and current on/off ratios greater
than 106. We could not detect any significant difference in
performance between devices based on isomerically pure
syn-material compared to mixtures of syn/anti-isomers.
The charge-carrier mobilities determined for the devices
presented in Figure 4 are as follows: 1.07 cm2/(V s) for syn-
2a, 1.01 cm2/(Vs) for syn/anti-2a, 0.30cm2/(V s) for syn-2b,
and 0.41 cm2/(V s) for syn/anti-2b. These mobility values
are typical for each compound and agree well with previous
reports on single crystals of syn/anti-isomers.4g The improved
device performance of the Et3Si versus i-Pr3Si-ADT devices
(syn-2a versus syn-2b, respectively) is due to the improved
π-stacking overlap, as observed in the X-ray data.
In summary, the synthesis of isomerically pure syn-
anthradithiophenes has been developed. The bromide
moieties of syn-2c,d ADTs allow for further synthetic
derivatization to give fluorinated syn-2a,b, and additional
modes of functionalization for syn-2c,d are easily foreseen
through metal-catalyzed coupling reactions. Initial com-
parisons of physical properties of syn-2a,b versus those of
syn/anti-2a,b show that the physical properties of the two
classes are remarkably similar, as established by X-ray
crystallography, UVꢀvis spectroscopy, and thermal analy-
sis. The performance of syn-2a,b and syn/anti-2a,b in single-
crystal OFET devices is comparable, showing mobility
values of ca. 1 cm2/(V s) for 2a and ca. 0.4 cm2/(V s) for
2b. The lack of difference between devices based on isomeri-
cally pure syn-ADTs and syn/anti-ADTs is likely attributed
Figure 4. Drain current (ID) versus gate-to-source voltage (VGS
)
in the saturation regime for a representative single crystal FET
fabricated with (a) syn-2a (L/W = 100/102), (b) syn/anti-2a
(L/W= 100/140), (c) syn-2b (L/W= 100/210), and (d) syn/anti-2b
(L/W = 100/350) in red. The slope of square root of ID (black) was
used to estimate mobility.10
to the fact that both materials suffer from similar positional
disorder of the thiophene moieties in the solid state.
Acknowledgment. This work has been supported by the
University of Erlangen-Nuremberg and NSERC. D.L.
thanks NSERC (PGS-D), the Alberta Ingenuity Fund,
Alberta Heritage, and the Killam Trusts for scholarship
support. O.D.J. acknowledges the National Science Foun-
dation (ECCS-1102275) for support. J.E.A. acknowledges
the Office of Naval Research for support. We thank Dr.
Michael J. Ferguson (University of Alberta) for the crystal
structure determination of compound 8.
Supporting Information Available. Experimental pro-
cedures, spectroscopic data, 1H and 13C NMR spectra for
all new compounds, FET device characteristics; summary
of crystallographic data and CIF file for compounds syn-
2a,b and 8. This material is available free of charge via the
Internet at http://pubs.acs.org.
(13) UVꢀvis data of syn/anti-2a,b, see refs 4g and 5b.
(14) The synthesis of syn/anti-2c,d has been reported for use as a
polymeric building block; see: Okamoto, T.; Jiang, Y.; Qu, F; Mayer,
A. C.; Parmer, J. E.; McGehee, M. D.; Bao, Z. Macromolecules 2008, 41,
6977–6980. No UVꢀvis data are reported.
The authors declare no competing financial interest.
(15) For additional details on the crystal growth technique, see ref 4g.
Org. Lett., Vol. 14, No. 14, 2012
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