Soluble and easily crystallized anthracene derivatives: precursors of solution-processable semiconducting molecules

Article abstract of DOI:10.1021/ol071135d

New soluble anthracene derivatives containing thlophene and phenylenevlnylene derivatives were synthesized via well-known synthetic routes. TIPS derivatives were added at the 9,10-posltlons of anthracene for the solubility and crystailinlty. Both of the molecules were found to be promising for high charge mobility and stable organic semiconductors. The soluble anthracene core (TIPSAnt) Is a potential precursor for the synthesis of novel semiconducting materials.

Full text of DOI:10.1021/ol071135d

ORGANIC
LETTERS
2007
Vol. 9, No. 13
2573-2576
Soluble and Easily Crystallized
Anthracene Derivatives: Precursors of
Solution-Processable Semiconducting
Molecules
|
Jong-Hwa Park,† Dae Sung Chung,‡ Jong-Won Park,§ Taek Ahn, Hoyoul Kong,†
|
Young Kwan Jung,† Jonghee Lee,† Mi Hye Yi, Chan Eon Park,*,‡
Soon-Ki Kwon,*,§ and Hong-Ku Shim*,†
Department of Chemistry and School of Molecular Sciences (BK21), Korea AdVanced
Institute of Science and Technology, Guseong-Dong, Yuseong-Gu, Daejeon, Korea,
Polymer Research Institute, Department of Chemical Engineering, Pohang UniVersity
of Science and Technology, Pohang, Korea, School of Nano & AdVanced Materials
and Engineering Research Institute, Gyeongsang National UniVersity, Jinju, Korea,
and Information and Electronics Polymer Research Center, Korea Research Institute
of Chemical Technology, Jang-Dong, Yuseong-Gu, Daejeon, Korea
hkshim@kaist.ac.kr; skwon@gnu.ac.kr; cep@postech.ac.kr
Received May 16, 2007
ABSTRACT
New soluble anthracene derivatives containing thiophene and phenylenevinylene derivatives were synthesized via well-known synthetic routes.
TIPS derivatives were added at the 9,10-positions of anthracene for the solubility and crystallinity. Both of the molecules were found to be
promising for high charge mobility and stable organic semiconductors. The soluble anthracene core (TIPSAnt) is a potential precursor for the
synthesis of novel semiconducting materials.
The identification of new organic semiconducting molecules
has been a crucial factor driving improvements in organic
thin-film transistors (OTFTs) and great achievements have
been reported.1 Recently there has been increased interest
in the stability and processability of OTFT devices, in
addition to the conventional focus on device performances.
Organic molecules soluble in organic solvents are promising
materials for inexpensive device fabrication since they can
be processed by simple solution techniques such as spin-
coating, drop-casting, or inkjet-printing.2 To date, most efforts
to synthesize soluble semiconducting molecules have focused
on polymeric conjugated systems.3 Compared to polymers,
however, oligomeric semiconducting molecules have several
advantages including well-defined structure and ease of
synthesis, purification, and structural modification.
† Korea Advanced Institute of Science and Technology
‡ Pohang University of Science and Technology
§ Gyeongsang National University
| Korea Research Institute of Chemical Technology.
(1) (a) Li, Y.; Wu, Y.; Liu, P.; Birau, M.; Pan, H.; Ong, B. S. AdV.
Mater. 2006, 18, 3029. (b) Mcculloch, I.; Heeney, M.; Bailey, C.;
Genevicius, K.; Macdonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.;
Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R. J.; Mcgehee, M. D.;
Toney, M. F. Nat. Mater. 2006, 5, 328. (c) Takimiya, K.; Ebata, H.;
Sakamoto, K.; Izawa, T.; Otsubo, T.; Kunugi, Y. J. Am. Chem. Soc. 2006,
128, 12604.
(2) (a) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int.
Ed. 1998, 37, 402. (b) Murphy, A. R.; Fre´chet, J. M. J.; Chang, P.; Lee, J.;
Subramanian, V. J. Am. Chem. Soc. 2004, 126, 1596.
10.1021/ol071135d CCC: $37.00
© 2007 American Chemical Society
Published on Web 06/02/2007

Among the soluble oligomers reported to date, the func-
tionalized pentacene has one of the greatest potentials as a
semiconducting material.4 Adding bulky triisopropylsilyl-
ethynyl (TIPS) groups at the 6,13-positions of the pentacene
not only improves the π-orbital overlap but also enhances
the solubility and the oxidative stability of the material.
Moreover, solution-deposited OTFT device properties with
use of functionalized pentacene and anthradithiophene were
reported.5 To modify the chemical structure of TIPS penta-
cene, Anthony et al. synthesized various new oligomers based
on tetracene, pentacene ethers, and so on.6
Scheme 1. Synthesis of the Soluble Oligomer Core
(TIPSAnt)
Among the oligomeric systems for OTFTs, anthracene is
one of the most important fused aromatics.7 In 2005 and
2006, Meng et al. reported novel anthracene-oligomer-based
semiconductors that showed high field-effect mobilities with
excellent stability.8 In addition, attempts to develop soluble
oligomers with use of anthracene moieties have also been
reported recently.9
Our approach is to introduce bulky TIPS groups at the
9,10-positions of 2,6-dibromoanthracene molecules to create
a soluble oligomer core. The resulting functionalized dibromo
anthracene can be coupled with various aromatic boronic
acids or esters via the well-known Suzuki coupling reaction.
The target soluble oligomer core 2,6-dibromo-9,10-bis-
(triisopropylsilylethynyl)anthracene (TIPSAnt) was synthe-
sized according to the procedure discribed in Scheme 1.
Commercially available 2,6-diaminoanthraquinone 1 was
converted into 2,6-dibromoanthraquinone 210 by using the
Sandmeyer reaction and then TIPS was introduced at the
9,10-positions of the anthracene via synthetic routes similar
to that used for TIPS pentacene.5a TIPS groups make
anthracene highly soluble in common organic solvents such
as chloroform, chlorobenzene, and toluene. (cf. 2,6-dibro-
moanthraquinone 2 is insoluble in such solvents.)
counterparts of TIPSAnt. 2,6-Bis(5′-hexyl-thiophene-2′-yl)-
9,10-bis(triisopropylsilylethynyl)anthracene (TIPSAntHT)
and 2,6-bis(2′-phenylvinyl)-9,10-bis(triisopropylsilylethynyl)-
anthracene (TIPSAntPV) were synthesized via a Suzuki
coupling reaction as shown in Scheme 2.
Scheme 2. Synthesis of TIPSAntHT and TIPSAntPV
The bromo groups of TIPSAnt enable coupling of the
molecules with various borolanylaryl molecules, for example,
acene, thiophene, and fluorene derivatives. In the present
work we chose hexylthiophene and phenylenevinylene as
(3) (a) Shim, H.-K.; Jin J. I. AdV. Polym. Sci. 2002, 158, 1942. (b) Kim,
Y. M.; Lim, E.; Kang, I.-N.; Jung, B.-J.; Lee, J.; Koo, B. W.; Do, L.-M.;
Shim, H.-K. Macromolecules 2006, 39, 4081.
(4) (a) Anthony, J. E.; Eaton, D. L.; Parkin, S. R. Org. Lett. 2002, 4, 15.
(b) Sheraw, C. D.; Jackson, T. N.; Eaton, D. L.; Anthony, J. E. AdV. Mater.
2003, 15, 2009.
(5) (a) Payne, M. M.; Parkin, S. R.; Anthony, J. E.; Kuo, C.-C.; Jackson,
T. N. J. Am. Chem. Soc. 2005, 127, 4986. (b) Dickey, K. C.; Anthony, J.
E.; Loo, Y.-L. AdV. Mater. 2006, 18, 1721.
(6) (a) Odom, S. A.; Parkin, S. R.; Anthony, J. E. Org. Lett. 2003, 5,
4245. (b) Payne, M. M.; Delcamp, J. H.; Parkin, S. R.; Anthony, J. E. Org.
Lett. 2004, 6, 1609. (c) Swartz, C. R.; Parkin, S. R.; Bullock, J. E.; Anthony,
J. E.; Mayer, A. C.; Malliaras, G. G. Org. Lett. 2005, 7, 3163.
(7) Ito, K.; Suzuki, T.; Sakamoto, Y.; Kubota, D.; Inoue, Y.; Sato, F.;
Tokito, S. Angew. Chem., Int. Ed. 2003, 42, 1159.
(8) (a) Meng, H.; Sun, F.; Goldfinger, M. B.; Jaycox, G. D.; Li, Z.;
Marshall, W. J.; Blackman, G. S. J. Am. Chem. Soc. 2005, 127, 2406. (b)
Meng, H.; Sun, F.; Goldfinger, M. B.; Gao, F.; Londono, D. J.; Marshal,
W. J.; Blackman, G. S.; Dobbs, K. D.; Keys, D. E. J. Am. Chem. Soc.
2006, 128, 9304.
(9) (a) Schmidt, R.; Go¨ttling, S.; Leusser, D.; Stalke, D.; Krausea, A.-
M.; Wu¨rthner, F. J. Mater. Chem. 2006, 16, 3708. (b) Cui, W.; Zhang, X.;
Jiang, X.; Tian, H.; Yan, D.; Geng, Y.; Jing, X.; Wang, F. Org. Lett. 2006,
8, 785.
The products were bright orange, highly crystalline solids.
Similar to TIPSAnt, TIPSAntHT and TIPSAntPV also
show good solubility in common organic solvents due to the
TIPS groups and especially TIPSAntHT shows higher
solubility due to hexyl groups on the thiophenes. The
1
compounds were characterized by H NMR and 13C NMR,
(10) (a) Hodge, P.; Power, G. A.; Rabjohns, M. A. Chem. Commun. 1997,
73. (b) Lee, S. K.; Yang, W. J.; Choi, J. J.; Kim, C. H.; Jeon, S.-J.; Cho,
B. R. Org. Lett. 2005, 7, 323.
as well as elemental analysis. (See the Supporting Informa-
tion.)
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Org. Lett., Vol. 9, No. 13, 2007

The thermal transitions of the oligomers have been studied
by differential scanning calorimetry (DSC) under a nitrogen
atmosphere. We found that TIPSAntHT exhibits a major
melting endotherm at 192 °C and a corresponding crystal-
lization exotherm upon cooling at 160 °C. These two peaks
are very significant and reversible without any extra thermal
transitions. TIPSAntPV exhibits liquid crystalline property
showing the first endothermal peak at 63 °C and the second
endothermal peak at 242 °C, and corresponding exothermal
peaks at 50 and 188 °C. From these thermal responses, we
concluded that TIPSAntHT and TIPSAntPV are highly
crystalline and should form well-ordered thin films when
deposited on a substrate. (TGA and DSC results can be seen
in the Supporting Information.)
Due to their solubility and well-ordered thin film forming
property, we successfully prepared thin films of the com-
pounds on quartz plates by spin-coating a solution of
TIPSAntHT and TIPSAntPV in chloroform (20 mg/1 mL,
1500 rpm for 40 s). We could confirm that simple solution
deposition techniques are feasible for depositing layers of
the oligomers during device fabrication. The UV-vis
absorption and photoluminescence (PL) spectra of the
oligomers were recorded both in solution (chloroform) and
in film form, as shown in Figure 1. Both of the oligomers
show good absorption and emission properties in both
solution and film state. Specifically, they exhibit slightly red-
shifted absorption and significantly red-shifted PL emission
in the film state relative to the solution state. Interestingly,
in the PL spectra, the differences in emission maxima
between the solution and film states of TIPSAntPV and
TIPSAntHT are 70 and 100 nm, respectively, which
correspond to the extremely high intermolecular interactions
in the film state. The existence of strong interactions in
π-conjugated systems between neighboring molecules in the
solid state is desirable for good TFT device performances.
To investigate the electrochemical stability as well as the
charge-transport property of the oligomers, we carried out
cyclic voltammetry (CV) measurements on thin films of the
oligomers, as summarized in the Supporting Information. The
oligomers show reversible waves for both p-doping and
n-doping processes. The oxidation potential Eox (p-doping)
for TIPSAntHT and TIPSAntPV was 0.96 and 0.98 V,
respectively. The relatively high oxidation potentials of the
oligomers compared to TIPS pentacene (380 mV)6c resulted
in better oxidation stability of the materials. The HOMO
levels of the TIPSAntHT and TIPSAntPV were estimated
as -5.36 and -5.38 eV, respectively, using the previously
reported empirical equation.11 It is worth noting that the
HOMO levels of both materials match well with the work
function of gold metal.11c As a result, the hole injection from
the gold source electrode in p-type thin-film transistors is
expected to be efficient.
Figure 1. Optical properties. UV-vis absorption and PL emission
spectra of (a) TIPSAntHT and (b) TIPSAntPV in chloroform
solution (solid line) and film form (dotted line).
by X-ray crystallography. Figure 2 shows π-interactions
between neighboring molecules in the solid state. The
conjugated unit, along the anthracene core, is planar for both
oligomers. (Alkyl chains on the thiophenes are coplanar with
the conjugation unit.) This planar characteristic is favorable
for intermolecular close packing, resulting in a good charge
carrier transport in the solid state of the materials.9b Two
molecules parallel to each other engage in face-to-face
interactions for both oligomers. The interplanar distances for
Single crystals of TIPSAntHT and TIPSAntPV, easily
grown from hexane and methylene chloride, were analyzed
(11) (a) de Leeuw, D. M.; Simenon, M. M. J.; Brown, A. R.; Einerhand,
R. E. F. Synth. Met. 1997, 87, 53. (b) Cervini, R.; Li, X.-C.; Spencer, W.
C.; Holmes, A. B.; Moratti, S. C.; Friend, R. H. Synth. Met. 1997, 84, 359.
(c) Meng, H.; Zheng, J.; Lovinger, A. J.; Wang, B.-C.; Van Patten, P. G.;
Bao, Z. Chem. Mater. 2003, 15, 1778.
Figure 2. X-ray crystal structures of TIPSAntHT (left) and
TIPSAntPV (right): top view (top) and side view (bottom).
Org. Lett., Vol. 9, No. 13, 2007
2575

TIPSAntHT and TIPSAntPV in the single crystal were 3.49
and 3.44 Å, respectively, similar to the value for function-
alized pentacene, which indicates excellent surface overlap
of the adjacent molecules.4a Superior π-stacking is also
confirmed in Figure 3. The oligomers stack in a two-
Figure 3. X-ray crystal structures of TIPSAntHT and TIPSAntPV
stacks: view normal to the plane of the conjugated units. Hexyl
groups are omitted for clarity.
dimensional columnar array with efficient π-orbital overlap
by face-to-face molecular interactions, whereas DHTAnt8a
and DPPVAnt,8b which lack TIPS groups, pack in the
herringbone geometry. Similar to the case of TIPS pentacene,
the bulky TIPS groups improve face-to-face interactions and
discourage edge-to-face molecular interactions for the an-
thracene derivatives.
Figure 4. Characteristics of TIPSAntHT OFET devices.
and introducing other kinds of aromatic counterparts to
TIPSAnt for the synthesis of new soluble oligomers.
In conclusion, we propose TIPSAnt as a precursor of a
new class of semiconducting molecules with desirable
properties: solubility, high crystallinity in single-crystal and
thin-film form, solution-technique processability, good opti-
cal properties, and electrochemical stability. TIPSAntHT
and TIPSAntPV show promising properties and potentiali-
ties for solution-processable semiconductors.
To test the potential of the oligomers as organic semi-
conductors, field-effect transistors (FETs) of TIPSAntHT
and TIPSAntPV were fabricated by solution processing,
preliminarily. Top-contact OFETs were fabricated on a
common gate of highly n-doped silicon with a 300 nm thick
thermally grown SiO2 dielectric layer. Substrates were
modified with octyltrichlorosilane from a toluene solution
for 2 h at room temperature and films of organic semicon-
ductors were spin-coated at 2000 rpm from 0.7 wt %
chloroform solution, with a thickness of 45 nm. As shown
in Figure 4, the FET device of TIPSAntHT shows typical
p-channel responses and the output curve shows very good
saturation behavior. Remarkably, the drain off-current of
TIPSAntHT in the transfer curve was less than 10-12 A,
indicating the electrochemical stability and high purity of
TIPSAntHT. The field-effect mobility of the TIPSAntHT-
based device was 4 × 10-3 cm2/(V‚s) (Ion/Ioff ) 106)
calculated in the saturation regime. In case of TIPSAntPV,
Acknowledgment. This work was supported by a grant
(F0004021-2006-22) from the Information Display R&D
Center, one of the 21st Century Frontier R&D Program
funded by the Ministry of Commerce, Industry, and Energy
of the Korean Government and by the Brain Korea 21
(BK21) project.
Supporting Information Available: Experimental pro-
cedures, characterizations for all compounds, and TGA, DSC,
CV, AFM, and single-crystal X-ray crystallographic data for
TIPSAntHT. This material is available free of charge via
the Internet at http://pubs.acs.org.
the field-effect mobility was 3 × 10-4 cm2/(V‚s) (Ion/Ioff
)
105). We are currently investigating the OFET devices of
TIPSAntHT and TIPSAntPV to improve the performances,
OL071135D
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Org. Lett., Vol. 9, No. 13, 2007