Synthesis, morphology, and field-effect mobility of anthradithiophenes

Article abstract of DOI:10.1021/ja9728381

The synthesis, thin-film morphology, and hole mobility in thin-film transistors (TFTs) of compounds based on the novel anthradithiophene (ADT) ring system are reported. The parent compound and its 2,8-dihexyl, didodecyl, and dioctadecyl derivatives (DHADT, DDADT, and DOADT, respectively), synthesized via alkylated thiophene dicarboxaldehyde acetals, were investigated. They all form highly ordered polycrystalline vacuum-evaporated films with mobilities as high as 0.15 cm²/(V s), as high as has ever been observed for a polycrystalline organic material. DOADT has a mobility of 0.06 cm²/(V s) even though 70% of its molecular volume is occupied by hydrocarbon chains. DHADT was cast from solution under atmospheric conditions onto a TFT giving a mobility of 0.01-0.02 cm²/(V s). Thus, the alkylated ADTs combine a pentacene-like intrinsic mobility with greater solubility and oxidative stability.

Full text of DOI:10.1021/ja9728381

664
J. Am. Chem. Soc. 1998, 120, 664-672
Synthesis, Morphology, and Field-Effect Mobility of
Anthradithiophenes
Joyce G. Laquindanum, Howard E. Katz,* and Andrew J. Lovinger
Contribution from Bell Laboratories, Lucent Technologies, 600 Mountain AVe.,
Murray Hill, New Jersey 07974
ReceiVed August 13, 1997
Abstract: The synthesis, thin-film morphology, and hole mobility in thin-film transistors (TFTs) of compounds
based on the novel anthradithiophene (ADT) ring system are reported. The parent compound and its 2,8-
dihexyl, didodecyl, and dioctadecyl derivatives (DHADT, DDADT, and DOADT, respectively), synthesized
via alkylated thiophene dicarboxaldehyde acetals, were investigated. They all form highly ordered polycrystalline
2
vacuum-evaporated films with mobilities as high as 0.15 cm /(V s), as high as has ever been observed for a
2
polycrystalline organic material. DOADT has a mobility of 0.06 cm /(V s) even though 70% of its molecular
volume is occupied by hydrocarbon chains. DHADT was cast from solution under atmospheric conditions
2
onto a TFT giving a mobility of 0.01-0.02 cm /(V s). Thus, the alkylated ADTs combine a pentacene-like
intrinsic mobility with greater solubility and oxidative stability.
Thin-film transistors (TFTs), illustrated in Figure 1, are
emerging as key devices for a growing worldwide effort to
1
develop organic-based and printed electronic components, an
2
effort that also encompasses light-emitting diodes (LEDs),
3
4
rectifiers, and capacitors. Applications for TFTs range from
display drivers to identification cards and other memory and
logic elements. The main technological advantage of organic
5
6
or “plastic” electronics is the ability to produce useful circuits
without the most capital- or labor-intensive steps associated with
silicon processing. Secondary advantages are the greater
mechanical flexibility and easier tunability of organic devices
relative to those fabricated on silicon chips. While it is doubtful
that organic materials will rival the inorganics in terms of device
performance, they may be useful in applications where device
density and reliability under extreme conditions can be sacrificed
in favor of ease of processing.
Figure 1. TFT structure showing two different device geometries.
8
9
dithiophene dimer, and pentacene (Chart 1). Other types of
structures that show significant hole-transporting activity in
The most important material properties for the semiconductors
that comprise the active materials in organic TFTs are high
mobility, low “off”-conductivity, stability, and processability.
These attributes have been realized to some extent in p-channel
1
0
11
TFTs include phthalocyanines and poly(alkylthiophenes).
The oxidation potentials of these compounds are near 1 V vs
the standard calomel electrode (SCE), corresponding to a highest
occupied molecular orbital (HOMO) energy of about 5 V below
vacuum. The electronic energy levels provide for facile hole
injection from electrode materials at accessible voltages, while
the molecular shape ensures useful mobility by virtue of the
intermolecular overlap and molecular orientation in a solid film.
For the linear compounds, the mobility appears to increase with
the rigidity of the molecule, with pentacene possessing an
(
hole-transporting) materials, exemplified by a group of linear,
7
conjugated molecules including thiophene oligomers, a benzo-
(
1) (a) Katz, H. E. J. Mater. Chem. 1997, 7, 369-376. (b) Garnier, F.;
Hajlaoui, R.; Srivastava, P. Science 1994, 265, 684. (c) Garnier, F. Pure
Appl. Chem 1996, 68, 1455. (d) Bao, Z.; Feng, Y.; Dodabalapur, A.; Raju,
V. R.; Lovinger, A. J. Chem. Mater. 1997, 9, 1299.
(2) Dodabalapur, A. Solid State Commun. 1997, 102, 259.
(
3) Hide, F.; Greenwald, Y.; Wudl, F.; Heeger, A. J. Synth. Met. 1997,
8
1
8
5, 1255.
(8) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J.; Dodabalapur, A.
AdV. Mater. 1997, 9, 36.
(4) Jeon, N. L.; Clem, P. G.; Nuzzo, R. G.; Payne, D. A. J. Mater. Res.
995, 10, 2996.
(9) (a) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J.; Dodabalapur,
A. Chem. Mater. 1996, 8, 2542. (b) Lin, Y. Y.; Gundlach, D. J.; Jackson,
T. N. Annual DeVice Research Conference Digest, 54th; IEEE: Santa
Barbara, CA, 1996; p 175. See also: Science 1996, 273, 879. (c)
Dimitrakopoulos, C. D.; Brown, A. R.; Pomp, A. J. J. Appl. Phys. 1996,
80, 2501.
(5) (a) Stubb, H.; Punkka, E.; Paloheimo, J. Mater. Sci. Eng. 1993, 10,
5. (b) Jarrett, C. P.; Friend, R. H.; Brown, A. R.; de Leeuw, D. M. J.
Appl. Phys. 1995, 77, 6289.
6) Brown, A. R.; Pomp, A.; Hart, C. M.; de Leeuw, D. M. Science
995, 270, 972.
7) (a) Garnier, F.; Horowitz, G.; Peng, X. Z.; Fichou, D. Synth. Met.
991, 45, 163. (b) Servet, B.; Horowitz, G.; Ries, S.; Lagorsse, O.; Alnot,
(
1
1
(
(10) Bao, Z.; Lovinger, A. J.; Dodabalapur, A. Appl. Phys. Lett. 1996,
69, 3066.
P.; Yassar, A.; Deloffre, F.; Srivastava, P.; Hajlaoui, R.; Lang, P.; Garnier,
F. Chem. Mater. 1994, 6, 1809. (c) Dodabalapur, A.; Torsi, L.; Katz, H. E.
Science 1995, 268, 270.
(11) (a) Bao, Z.; Lovinger, A. J.; Dodabalapur, A. Appl. Phys. Lett. 1996,
69, 4108. (b) Tsumura, A.; Koezuka, H.; Ando, T. Appl. Phys Lett. 1986,
49, 1210.
S0002-7863(97)02838-2 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/16/1998

Synthesis, of Anthradithiophenes
J. Am. Chem. Soc., Vol. 120, No. 4, 1998 665
Chart 1
conclude with a brief study of the mobility of DHADT films
cast from solution.
Experimental Section
All starting materials were purchased from Aldrich and were used
as received unless otherwise specified. THF was distilled from sodium
benzophenone ketyl. NMR spectra were collected with use of a Bruker
3
3
60 MHz spectrometer with CDCl as solvent and tetramethylsilane
(TMS) as an internal standard. Mass spectra were recorded on a
Hewlett-Packard 5989A mass spectrometer. Thermal analyses were
performed with a Perkin-Elmer DSC 7 analyzer. Elemental analyses
were done by Robertson Microlit Laboratories. Molecular modeling
was performed with the Spartan version of the AM1 semiempirical
method and Cerius system from Molecular Simulations, Inc.
TFT devices composed of ADT and its dialkyl-substituted derivatives
were prepared by subliming the materials onto prepared TFT substrates
-
6
at pressures of 3 × 10 Torr or lower, using deposition rates of ca. 5
Å/s. The temperature of the substrates was controlled by heating the
copper block on which they were mounted. Two different substrate
configurations were used to make the TFT devices, “bottom” and “top”
contacts, respectively, illustrated in Figure 1. For both configurations,
a silicon dioxide dielectric layer was thermally grown on n-doped silicon
which acts as the gate. In the bottom-contact geometry, conduction
channels were photolithographically defined with a channel width (W)
of 250 µm and channel lengths (L) between 1.5 and 25 µm. The
semiconductor was sublimed onto the entire substrate above the
dielectric and electrodes. For the top contact geometry, however, the
electrodes were defined after sublimation of the semiconductor by using
shadow masks of W/L of ca. 1.5/1 and 4/1. The active semiconductor
extraordinary TFT mobility of ca. 1 cm /(V s)12 under deposition
conditions that lead to a favorable morphology. Nevertheless,
pentacene suffers from the disadvantages of oxidative instabil-
2
4
2
film area for the two values of W/L was ca. 3-4 × 10 µm and 4
13
ity, extreme insolubility, and, for display applications, a strong
absorbance throughout the visible spectrum. Where photo-
induced decomposition reactions could occur, this absorbance
would make pentacene sensitive to most visible light.
2
mm , respectively. Gold was used as the metal contact for all devices.
The electrical characterizations were performed at room temperature
under vacuum with use of a 4145b Hewlett-Packard semiconductor
parameter analyzer.
For morphological characterizations, the materials were deposited
The closely related fused heterocycles with the anthra-
dithiophene structure have never been reported, although the
onto carbon-coated electron microscope grids and Si/SiO
simultaneously with the TFTs. X-ray diffraction studies were done
with use of films on the Si/SiO chips in the reflection mode at 40 kV
2
chips
14
dione precursors have been prepared. On the basis of the ring
system, anthradithiophene (ADT) would be expected to have a
suitable HOMO energy and solid-state structure for use as a
p-channel TFT semiconductor. In addition, the barrier to
oxidation could be higher than that for pentacene because of
the expected greater loss of aromaticity that would accompany
oxygen addition across the central ring relative to the pentacene
case. Finally, the susceptibility of the C-H bonds R- to the
sulfurs at the ends of anthradithiophene presents an avenue for
end substitution that is not present in pentacene, so that the
solubility, morphology, and adhesion could be better defined.
2
and 25 mA. A 2-kW Rigaku X-ray generator was used as a source of
Ni-filtered Cu KR radiation. The fims on the grids used for electron
-1
microscopy were shadowed with Pt/C at tan 1/2 and lightly carbon-
coated in a vacuum evaporator before examination with a JEOL
transmission microscope operated at 100 kV.
Anthra[2,3-b:6,7-b′]dithiophene-5,11-dione and Anthra[2,3-b:7,6-
b′]dithiophene-5,11-dione (1). Compound 1 was obtained by following
a previously published procedure.14
Anthra[2,3-b:6,7-b′]dithiophene and Anthra[2,3-b:7,6-b′]dithio-
phene (2). Aluminum wire (1.0 g) was reacted in 30 mL of dry
cyclohexanol containing mercuric chloride (0.02 g) and carbon
tetrachloride (0.2 mL) by warming slowly to mild reflux and controlled
by cooling. The reaction was completed by refluxing under a nitrogen
atmosphere overnight. The dione (1.0 g, 3 mmol) was added and the
resulting mixture refluxed under nitrogen for 48 h. The reaction mixture
was cooled to 50 °C and centrifuged to collect the solid product. The
solids were washed with warm cyclohexanol followed by glacial acetic
acid, 10% HCl, and ethanol several times. The crude product was
In this paper, we report the synthesis of the parent compound,
ADT, and the end-substituted derivatives dihexyl-, didodecyl-,
and dioctadecyl-ADT (DHADT, DDADT, and DOADT, re-
spectively, Chart 1). All of these compounds are obtained and
investigated as mixtures of syn and anti isomers. Besides the
usual thermal and analytical characterization, we explore the
crystallinity and morphology of these compounds as evaporated
thin films. Application of these compounds as the semiconduc-
tors in TFTs gives devices with some of the highest mobilities
ever observed for polycrystalline organic films, even in cases
where the film consists predominantly of alkyl groups. We
-4
purified by using vacuum sublimation at pressures of 10 Torr or lower
to give a bright orange solid (0.46 g) in 53% yield. Mp ) 450 °C
Anal. Calcd for C18
4.23; H, 3.49; S, 22.25. MS (m/e) 390 (M , 100).
H
10
S
2
: C, 74.45; H, 3.47; S, 22.08. Found: C,
+
7
2,3-Bis(1,3-dioxolan-2-yl)thiophene (3). 2,3-Thiophene dicarbox-
(
12) (a) Jackson, T. N. Presented at the 1996 Materials Research Society
aldehyde (2 g, 14 mmol), p-toluenesulfonic acid monohydrate (2 mg),
and ethylene glycol (1.96 g, 30 mmol) were added to 25 mL of dry
benzene. The mixture was refluxed and water was collected in a Dean-
Stark trap. After cooling, the reaction mixture was washed with 10%
Fall Meeting, Boston, MA, November 1996; paper D6.1. (b) Lin, Y. Y.;
Gundlach, D. J.; Nelson, S. F.; Jackson, T. N. IEEE Transactions on
Electronic DeVices 1997, 44, 1325.
(13) Yamada, M.; Ikemote, I.; Kuroda, H. Bull. Chem. Soc Jpn. 1988,
NaOH then H
in the succeeding step without purification. NMR (CDCl
4.11 (t), 6.05 (s), 6.33 (s), 7.11 (d, J ) 5.2 Hz), 7.27 (d, J ) 5.55 Hz).
2 4
O and dried over MgSO . The product was used directly
6
1, 1057.
14) De la Cruz, P.; Martin, N.; Miguel, F.; Seoane, C.; Albert, A.; Cano,
H.; Gonzalez, A.; Pingarron, J. M. J. Org. Chem. 1992, 57, 6192.
3
) δ 4.03 (t),
(

666 J. Am. Chem. Soc., Vol. 120, No. 4, 1998
Laquindanum et al.
2
,3-Bis(1,3-dioxolan-2-yl)-5-hexylthiophene (4a). 3 (3 g, 10.3
Scheme 1
mmol) was dissolved in 50 mL of dry THF under N and cooled to
78 °C. n-BuLi (6.8 mL, 2.5 M in hexane) was added dropwise. After
min, 1-iodohexane (2.5 mL, 17 mmol) was added. The solution was
2
-
5
stirred at -78 °C and was left to warm to room temperature overnight.
Ether (30 mL) was added and the organic layer was washed several
times with water. The organic extract was dried over MgSO and
4
concentrated. The crude product was purified by column chromatog-
raphy with use of silica gel (1:2 ethyl acetate:hexane) to give 2.28 g
(
1
71%) of 4a as a yellow oil. NMR (CDCl
.67 (q), 2.74 (t), 3.93 (t), 4.1 (t), 5.96 (s), 6.26 (s), 6.77 (s).
,3-Bis(1,3-dioxolan-2-yl)-5-dodecylthiophene (4b). The same
3
) δ 0.85 (t), 1.27-1.4 (m),
2
procedure as for compound 4a was used with the following amounts
of starting material: 3 (2 g, 9 mmol), n-BuLi (4.2 mL, 2.5 M in hexane),
and 1-iodododecane (3.12 g, 10.5 mmol). NMR (CDCl
1
6
3
) δ 0.88 (t),
.2-1.42 (m), 1.65 (q), 2.74 (t), 4.05 (t), 4.13 (t), 5.98 (s), 6.30 (s),
.79 (s).
,3-Bis(1,3-dioxolan-2-yl)-5-octadecylthiophene (4c). The same
2
a bright orange solid (0.14 g). Mp ) 270 °C Anal. Calcd for
: C, 81.55; H, 10.39; S, 8.06. Found: C, 81.39; H, 10.17; S,
procedure as for compound 4a was used with the following amounts
of starting material: 3 (2 g, 9 mmol), n-BuLi (4.2 mL, 2.5 M in hexane),
54 82 2
C H S
7
.99.
and 1-iodooctadecane (3.84 g, 10 mmol). NMR (CDCl ) δ 0.88 (t),
3
1
6
.2-1.42 (m), 1.65 (q), 2.74 (t), 4.05 (t), 4.13 (t), 5.98 (s), 6.30 (s),
.79 (s).
Results
General Procedure for Deprotection of 5-Alkylthiophene Di-
Synthesis. Anthradithiophene (2) was synthesized according
to Scheme 1. (As already mentioned, all the fused ring
compounds pictured in the schemes, though drawn as anti
isomers, are prepared as syn-anti isomeric mixtures.) The
synthesis is a two-step procedure wherein the first step involves
a cyclization reaction to form the dione. The synthesis of the
acetals. A solution of the diacetal-protected compound in 50 mL of
THF and 3 N HCl was refluxed for at least 15 min. The solution was
cooled to room temperature. Ice was added and the mixture was
extracted with ether. The combined organic extract was washed with
4
dilute sodium bicarbonate and dried over MgSO . The product was
used directly without purification.
1
4
dione was based on a published procedure, starting with
commercially available 2,3-thiophene dialdehyde reacting with
5
-Hexyl-2,3-thiophenedicarboxaldehyde (5a): NMR (CDCl
.92 (t), 1.22-1.45 (m), 1.75 (q), 2.9 (t), 7.34 (s), 10.33 (s), 10.41(s).
-Dodecyl-2,3-thiophenedicarboxaldehyde (5b): NMR (CDCl ) δ
.9 (t), 1.22-1.45 (m), 1.75 (q), 2.9 (t), 7.35 (s), 10.36 (s), 10.40 (s).
-Octadecyl-2,3-thiophenedicarboxaldehyde (5c): NMR (CDCl
δ 0.9 (t), 1.2-1.38 (m), 1.75 (q), 2.85 (t), 7.32 (s), 10.33 (s), 10.4 (s).
,8-Dihexylanthra[2,3-b:6,7-b′]dithiophene-5,11-dione and 2,8-
3
) δ
0
0
1
,4-cyclohexanedione under basic conditions. The resulting
5
3
dione (1) was reduced by using a procedure similar to that used
1
5
to synthesize pentacene. The reducing agent was Al(OCy)3,
which was formed in situ by letting aluminum strips react in a
mixture of mercuric chloride and carbon tetrachloride in
anhydrous cyclohexanol. The dialkyl-substitued derivatives of
ADT were synthesized according to Scheme 2. The synthesis
involved the protection of thiophene dialdehyde as a bis-acetal,
which is stable to basic conditions. The alkylation reaction was
done by lithiating the diacetal-protected thiophene at the
2-position and then reacting with an alkyl halide. Three
different alkyl chains were used in this study, hexyl (4a),
dodecyl (4b), and octadecyl (4c). After the alkyl groups have
been attached to the thiophene ring, the acetal protecting groups
were removed by refluxing the material in acid to obtain the
alkylated dialdehydes 5a, 5b, and 5c. These materials were
then allowed to react with 1,4-cyclohexanedione in a manner
similar to the synthesis of the unsubstituted ADT to obtain the
novel compounds DHADT (7a), DDADT (7b), and DOADT
5
3
)
2
Dihexylanthra[2,3-b:7,6-b′]dithiophene-5,11-dione (6a), 2,8-Dido-
decylanthra[2,3-b:6,7-b′]dithiophene-5,11-dione and 2,8-Didodecyl-
anthra[2,3-b:7,6-b′]dithiophene-5,11-dione (6b), and 2,8-Diocta-
decylanthra[2,3-b:6,7-b′]dithiophene-5,11-dione and 2,8-Diocta-
decylanthra[2,3-b:7,6-b′]dithiophene-5,11-dione (6c). These com-
pounds were synthesized by using a published procedure as for
compound 1. The product in each case was a yellow-green solid
(
yields: 30-80%) that was used in the next step without purification.
No attempt was made to separate the syn and the anti isomers.
,8-Dihexylanthra[2,3-b:6,7-b′]dithiophene and 2,8-Dihexyl-
2
anthra[2,3-b:7,6-b′]dithiophene (7a). The procedure used was the
same as for compound 2 with the following amounts of starting
materials: 6a (0.37 g, 0.7 mmol), as well as Al (0.25 g), HgCl
2
(0.005
g), CCl (0.2 mL) in 25 mL of cyclohexanol. The crude product was
4
-
4
purified by vacuum sublimation at a pressure of ca. 10 Torr to give
(7c).
a bright orange solid (0.1 g). Mp ) 395 °C. Anal. Calcd for
Thermal analyses of ADT, DHADT, DDADT, and DOADT
30 34 2
C H S : C, 78.55; H, 7.47; S, 13.98. Found: C, 78.38; H, 7.20; S,
+
showed melting points decreasing with chain length. The
solubilities of the dialkylated compounds were also tested in
toluene and chlorobenzene. All of the materials were shown
to be soluble in refluxing toluene (0.5 mg/mL) and chloroben-
zene (2 mg/mL) with DHADT showing some slight solubility
in room temperature toluene.
To assess the stability of dissolved ADT compounds, a sample
of DHADT was dissolved in 1,2-dichlorobenzene under ambient
light and atmosphere. No change in color was observed in a
saturated solution over hours at room temperature. Recrystal-
lized material recovered from the solution melted at 395 °C,
identical with the original solid. This is in contrast to pentacene,
1
3.69. MS (m/e) 4580 (M , 100), 386 (10%), 316 (20%).
2,8-Didodecylanthra[2,3-b:6,7-b′]dithiophene and 2,8-Didodecyl-
anthra[2,3-b:7,6-b′]dithiophene (7b). The procedure used was the
same as for compound 2 with the following amounts of starting
materials: 6b (0.9 g, 1.4 mmol), Al (0.4 g), HgCl
2 4
(0.01 g), CCl (0.2
mL) in 50 mL of cyclohexanol. The crude product (0.42 g) was purified
-
4
by vacuum sublimation at a pressure of ca. 10 Torr to give a bright
orange solid (0.23 g). Mp ) 322 °C. Anal. Calcd for C42 : C,
0.45; H, 9.32; S, 10.23. Found: C, 80.33; H, 9.33; S, 10.00.
,8-Dioctadecylanthra[2,3-b:6,7-b′]dithiophene and 2,8-Diocta-
58 2
H S
8
2
decylanthra[2,3-b:7,6-b′]dithiophene (7c). The procedure used was
the same as for compound 2 with the following amounts of starting
2 4
materials: 6c (0.86 g, 1.0 mmol), Al (0.27 g), HgCl (0.006 g), CCl
(
0.2 mL) in 50 mL of cyclohexanol. The crude product (0.22 g) was
(
15) Goodings, E. P.; Mitchard, D. A.; Owen, G. J. Chem. Soc., Perkin
-
4
purified by vacuum sublimation at a pressure of ca. 10 Torr to give
Trans. 1 1972, 11, 1310.

Synthesis, of Anthradithiophenes
J. Am. Chem. Soc., Vol. 120, No. 4, 1998 667
Scheme 2
Table 1. Field-Effect Mobilities of ADT as a Function of
Substrate Deposition Temperature
2
temp (°C)
mobility (cm /V s)
RT
0.02
0.04
0.03-0.06
0.09
6
7
8
1
0
0
5
00
no FET behavior
defined by using a shadow mask. Two different types of
channel width (W) to length (L) ratios were used in the “top”-
contact experiments, W/L of ca. 1.5/1 and 4/1. For the
“
bottom”-contact geometry, sources and drains are photolitho-
graphically defined and the deposition of the semiconductor
material is the final step to complete the device. For all of the
devices, the mobilities were determined in the saturation regime
1
6
(VDS > VGS) by using the equation
WCi
2
IDS
)
µ(V - V )
(1)
G
o
2
L
2
where C, the insulator capacitance, is 10 nF/cm and Vo is the
extrapolated threshold voltage. The materials showed an
enhancement in the current at negative gate-souce voltages
Figure 2. Source-drain current vs voltage (IDS vs VDS) characteristics
of a DHADT TFT with (a) W/L of 250/12 (“bottom”-contact geometry)
and (b) W/L ca. 1.5/1 (“top”-contact geometry) at different gate voltages
deposited at Tsub ) RT. The increasing deviations from zero current at
zero drain-source voltage with higher gate voltages are caused by
leakage through the dielectric.
(VGSs), which indicates that the materials transport holes and
are, therefore, p-channel materials. An example of an I-V curve
is shown in Figure 2.
The mobilities of ADT with “bottom”-contact device geom-
etry at different substrate deposition temperatures, Tsub, are listed
in Table 1. The mobility was shown to increase with increasing
which is virtually insoluble. The small quantity of pentacene
that could be extracted into refluxing 1,2-dichlorobenzene
bleached in less than 5 min.
Electrical Characteristics. The field-effect mobilities (µFET)
of the TFT devices with ADT and its dialkylated derivatives
were determined by using two different device geometries,
namely “top”, and “bottom”-contact geometries. In the former
case, the semiconductor is deposited first and electrodes are then
2
temperature with the highest mobility of 0.09 cm /(V s) achieved
at Tsub ) 85 °C. At temperatures above Tsub ) 85 °C, FET
activity was not observed.
The mobilities of dialkyl-substituted derivatives are listed in
2
Table 2. From the table, mobilities of >0.1 cm /(V s) were
(16) Torsi, L.; Dodabalapur, A.; Katz, H. E. J. Appl. Phys. 1995, 78,
1088.

668 J. Am. Chem. Soc., Vol. 120, No. 4, 1998
Laquindanum et al.
Table 2. Structural and Electrical Characteristics of Alkylated ADTs as a Function of Substrate Deposition Temperatures
2
material
DHADT
temp (°C)
repeat spacing (nm)
morphology
mobility (cm /V s)
RT
2.55
2.6
2.62
smooth, interconnected grains
smooth, interconnected grains
smooth, interconnected grains
0.11 ( 0.01
0.15 ( 0.02
0.12 ( 0.02
8
1
5
25
DDADT
DOADT
RT
3.83
elongated, interconnected grains
elongated, interconnected grains
flat, smooth
0.12 ( 0.02
0.14 ( 0.02
0.04 ( 0.01
8
1
5
25
peak not resolvable
3.68
RT
peak not resolvable
peak not resolvable
4.64
small, elongated grains
large, elongated grains
flat, extended lamellae
0.06 ( 0.01
0.06 ( 0.01
0.04 ( 0.001
8
1
5
25
achieved, comparable to the highest mobilities reported for
polycrystalline organic p-channel materials. The mobilities of
the dialkyl-substituted ADT derivatives were also determined
at different substrate temperatures. For this set of compounds,
“
top”-contact geometries of W/L ca. 1.5/1 were used since the
mobilities calculated with “bottom”-contact geomteries were
irreproducible and were scattered over a wide range of values.
Similar to ADT, the mobilities of the dialkylated derivatives
show a dependence on the substrate deposition temperatures.
The dependence on the deposition temperatures is most apparent
for DDADT, wherein there was an order of magnitude drop in
the mobilities as the substrate deposition temperature was
increased from 85 to 125 °C. The electrical characteristics of
DHADT with use of “top”-contact geometries with channel
dimensions of W/L of 4/1 gave mobilities 30% lower than those
achieved by using smaller channel dimensions. This could be
due to the larger quantity of heat produced in depositing the
larger pads for W/L of 4/1, or the greater probability of finding
a particularly severe defect in the larger active region.
TFT devices using DHADT cast from solution were also
prepared and their mobilities calculated. Dilute solutions (ca.
0.2-1%) of DHADT in hot chlorobenzene were prepared and
cast onto prepared TFT “bottom”-contact substrates. The
solvent was then left to evaporate in a vacuum oven with
different oven temperatures. No field-effect activity was
observed when the solutions were evaporated at room temper-
ature and 50, 150, and 180 °C. However, FET activity was
observed when the solutions were evaporated at oven temper-
2
Figure 3. X-ray diffractogram of ADT films on Si/SiO , deposited
by sublimation at room temperature.
-
4
atures of 70 and 100 °C. Field-effect mobilities of 6 × 10 to
increase in the crystallite sizes from <100 nm at 25 °C to ca.
200 nm at 85 °C. This could explain the higher mobilities
obtained at 85 °C compared to room temperature. The absence
of any FET activity at Tsub ) 100 °C can be attributed to the
formation of large (ca. 400 nm) but widely separated crystals
instead of interconnected crystalline grains. Mobility as a
function of substrate temperature depends on a tradeoff among
factors that enhance mobility, such as chemical and morphologi-
cal homogeneity and domain continuity, all of which are
improved at higher substrate temperature, and interdomain
spacing and intracrystallite fracturing, which also increase at
higher substrate temperatures (or upon cooling therefrom) and
which severely diminish mobiity.
-
3
2
3
°
0
× 10 cm /(V s) were calculated for devices prepared at 70
C while those prepared at 100 °C gave mobilities in the 0.01-
2
.02 cm /(V s) range.
Morphology. X-ray diffraction studies of ADT films
deposited at different substrate temperatures indicate an excep-
tionally high degree of ordering with very sharp reflections at
1
.415, 0.710, 0.470, and 0.356 nm (see Figure 3). The first of
these corresponds exactly to the length of the ADT molecule
and the others are its second, third, and fourth orders, respec-
tively. These results demonstrate that the ADT molecules are
deposited with exceptionally high order and crystallinity in a
perpendicular orientation with respect to their substrates. The
same structure, high order, and perpendicular orientation were
also found for the samples deposited at the higher tem-
peratures.
The morphology of the dialkyl-substituted ADT derivatives
was studied at three different substrate temperatures, room
temperature, 85 °C, and 125 °C. The electron diffraction
patterns of DHADT films at the three temperatures demonstrate
very high crystallinity and orientation in the same manner as
The morphologies of ADT deposited at room temperature,
85 °C, and 100 °C are shown in Figure 4 with their correspond-
14b,17
ADT and R6T,
i.e., with the long axis nearly perpendicular
ing electron-diffraction patterns. The three strongest reflections
are at 0.443, 0.360, and 0.301 nm, repectively. Their absence
in Figure 3 (as well as the total absence of the very intense
peaks from Figure 3 in the electron-diffraction pattern) confirms
the perpendicular orientation of the ADT molecules on the
substrates. As temperature is increased, there is a corresponding
to the substrate. The X-ray patterns at all substrate temperatures
show an extremely sharp and strong peak at 2θ of 3.35° (d )
ca. 2.6 nm), which is in excellent agreement with the molecular
(17) Lovinger, A. J.; Davis, D. D.; Ruel, R.; Torsi, L.; Dodabalapur, A.;
Katz, H. E. J. Mater. Res. 1995, 10, 2958.

Synthesis, of Anthradithiophenes
J. Am. Chem. Soc., Vol. 120, No. 4, 1998 669
Figure 4. Transmission electron micrographs and selected-area diffraction patterns of ADT films deposited at Tsub ) RT (a), 85 °C (b), and 100
C (c).
°
length of DHADT. The basic morphology consists of nodular
nanocrystals, which are no larger than ca. 20 nm even at the
higher substrate temperatures; in corresponding electron-
diffraction patterns, the strongest reflections are at 0.477 and
of DDADT (4.0-4.3 nm depending on conformation), which
indicates that the chains are slightly inclined or interdigitated.
X-ray diffraction studies of DOADT films deposited at RT
and 85 °C show no resolvable peaks at low angles, which is
expected from the findings from DDADT. In this case, excellent
ordering is even less likely because of the greater length of the
molecule. The fact that no interchain peaks are seen at higher
angles suggests that there is again a unique orientation of the
molecules, i.e. close to perpendicular to the substrate. The poor
crystallinity deduced from the X-ray diffractograms is confirmed
by electron diffraction in the orthogonal direction, which yields
a diffuse ring at ca. 0.46 nm for specimens deposited at RT.
However, for those deposited at 85 °C, a sharper peak is seen
at 0.475 nm, signifying higher crystallinity and edge-on mo-
lecular orientation.
Although the underlying structure is still the same as in
DHADT and DDADT, the morphologies at these two substrate
temperatures are much more three-dimensional and rough (see
Figure 5). At RT, the crystallites are poorly defined rodlike
grains ca. 20 × 50 nm. At 85 °C, they are much better defined
with dimensions ca. 30 × 120 nm, as expected by their growth
0
.295 nm, arising from the interchain packing of the molecules
on edge.
The DDADT film morphologies for RT and 85 °C consist
of elongated, three-dimensional grains ca. 20 × 100 nm. At
25 °C, however, the morphology becomes flat and featureless;
1
this change may be related to the drop in mobility. As for
DHADT, DDADT also shows very sharp electron-diffraction
rings at all temperatures, implying very high crystallinity. Their
spacings are the same as we found for DHADT, which indicates
that the interchain packing of these long, alkyl-substituted ADT
molecules is the same. At all temperatures studied, X-ray
diffraction in the reflection mode showed an edge-on orientation,
although in most cases only the first-order peak was seen.
However, for DDADT films deposited at 125 °C, the X-ray
diffraction pattern is composed of six orders of the main
molecular repeat at 2θ of 2.40 ° (d ) 3.68 nm). This repeat
spacing, however, is somewhat less than the molecular length

670 J. Am. Chem. Soc., Vol. 120, No. 4, 1998
Laquindanum et al.
Figure 5. Transmission electron micrographs and selected-area diffraction patterns of DOADT films deposited at Tsub ) RT (a), 85 °C (b), and
25 °C (c).
1
at a higher temperature. This trend increases with substrate
temperature. Films deposited at 125 °C show very high
crystallinity and excellent orientation, with a thin lamellar
morphology similar to what is observed in polyethylene films.
Terraces and screw dislocations are seen in these films, entirely
analogous to what one expects of hydrocarbon chains during
slow growth. These morphological features, seen for the first
time among the many organic semiconductors studied, reflect
the major influence of the long hydrocarbon tails in dictating
molecular packing and crystallization in DOADT.
Discussion
The ring formation and reduction steps that led to ADT and
the dialkyl derivatives are analogous to those reported for ADT
1
4
15
dione and pentacene, respectively. The new preparative
work reported here concerns the method for attachment of the
alkyl groups. This was done by protecting the dialdehyde
presursor with a group stable to lithiation and alkylation of the
5-carbon. The alkylation step would be generalizable to a wide
variety of side chains, including those with some polar
1
8
functionality, provided the functional groups could withstand
the ring closing and reduction reactions. Alternatively, chloro-
alkyl groups could be introduced, which, given the finite
solubility of the alkyl derivatives, could be replaced with
nucleophiles at a later stage of the synthetic sequence. While
polar side chain functionalization has been accomplished in the
The X-ray diffractogram of DOADT deposited at 125 °C is
seen in Figure 6 and depicts exceptionally high order: more
than 10 orders of the main molecular repeat at 1.9° 2θ (d )
4
.65 nm) are discernible (the first order is actually under the
main beam). This is less than the length of an extended planar
molecule of DOADT (Figure 7). Molecular modeling suggests
that some significant molecular inclination is required to provide
the observed d spacing. This might be with the ADT core
perpendicular to the substrate (as indicated by the anti isomer
at the left of Figure 7) or by some highly inclined (35-40°)
orientation.
18
case of the thiophene oligomers, the fused ring systems studied
until now are not amenable to end substitution. Side groups
(
18) (a) Katz, H. E.; Laquindanum, J. G.; Lovinger, A. J. Chem. Mater.
998, accepted for publication. (b) Wei, Y.; Yang, Y.; Yeh, J.-M. Chem.
Mater. 1996, 8, 2659.
1

Synthesis, of Anthradithiophenes
J. Am. Chem. Soc., Vol. 120, No. 4, 1998 671
Figure 7. Molecular models of different DOADT conformations and
hypothetical substrate arrangements.
(
3-octylthiophene),23 which contains a greater volume fraction
of charge transporting material and requires the self-assembly
of fewer molecular units. As a second comparison, regioregular
poly(3-dodecylthiophene) has a very low mobility. The lack
Figure 6. X-ray diffractogram of DOADT films on Si/SiO
by sublimation at 125 °C.
2
, deposited
23
of a strong correlation between structure and mobility in the
ADT series emphasizes that it is the planes of fused rings that
are solely responsible for the mobility, and suggests that further
alterations of the side chains, such as heteroatom substitution,
would be possible.
could be useful in optimizing the self-assembly, processability,
and/or adhesion of the semiconducting films.
The stability of the ADTs, as exemplified by DHADT in
solution under ambient conditions, is significantly greater than
that of pentacene. This is consistent with a molecular modeling
study, which showed that the gas-phase conversion of pentacene
to its bridged dioxygen adduct is more exothermic than the
analogous transformation of ADT.
The observed mobilities are consistent with predictions that
mobility should increase with increasing rigidity and symmetry
along the series of long, conjugated molecules, from R6T to
pentacene. However, it should be borne in mind that the high
mobility of pentacene is achieved because it forms a single
crystal-like film over device dimensions under certain deposition
The morphology and solid-state structures of the ADTs
7
b.17
resemble those of the thiophene oligomers in many ways.
The compounds pack in layers, with their long axes nearly
perpendicular to the planes of the films. This packing motif
has been shown to be favorable for high field-effect mobility
in a TFT, since the two directions of π overlap coincide with
the plane in which the “on” current must flow. The repeat
spacings are consistent with reasonable models of the molecular
structures, except in the case of very long alkyl chains at high
substrate temperatures where the shorter than expected spacings
indicate substantial tilting. We see no evidence of isomer
segregation or other manifestations of the dual isomer composi-
tion of the ADT compounds in the X-ray diffraction patterns.
Molecular models indicate that the S and 3-CH groups differ
only slightly in their electron density and van der Waals radii,
so the two isomers are fairly isostructural. The repeat spacing
9a
conditions. Comparing only polycrystalline films, the mobili-
ties of dialkyl ADTs are as high as that of pentacene. If
substituents and/or deposition conditions are discovered that lead
to formation of a single-crystal film, the mobilities could
increase by an order of magnitude. It is also notable that the
high mobility is achieved in compounds whose visible ab-
sorbance energies (indicative of the “bandgaps”) are not
particularly low compared to the larger thiophene oligomers and
pentacene.
The measured mobilities and on/off ratios reported here are
limited by certain experimental factors. Because of the small
amounts of material prepared, none of the compounds have been
subjected to the slow, ultrahigh vacuum sublimation that
removes the last traces of impurities that are probably respon-
19
of DOADT is one of the longest reported for a molecular solid.
20
sible for the “off” current. The unpatterned nature of the
The observation of a high TFT mobility for this compound is
remarkable, in that the Volume of each molecular layer that
can transport charge is only ca. 30% and is sandwiched between
insulating layers of polyethylene-like chains. DOADT thus
provides the most dramatic demonstration to date of the two-
dimensional transport in organic TFTs. The mobility of
DOADT is significantly higher than that of regioregular poly-
semiconducting film in the TFTs introduces extraneous conduc-
tion and leakage paths around the devices being studied, and
appears to cause the testing of one device to influence others
(
20) Katz, H. E.; Torsi, L.; Dodabalapur, A. Chem. Mater. 1995, 7, 2235.
(21) Torsi. L.; Dodabalapur, A.; Lovinger, A. J.; Katz, H. E.; Ruel, R.;
Davis, D. D.; Baldwin, K. W. Chem. Mater. 1995, 7, 2247.
(22) Brown, A. R.; Pomp, A.; de Leeuw, D. M.; Klaasen, D. B. M.;
Havinga, E. E.; Herwig, P.; Mullen, K. J. Appl. Phys. 1996, 79, 2136.
(23) Bao, Z.; Feng, Y.; Dodabalapur, A.; Raju, V. R.; Lovinger, A. J.
Chem Mater. 1997, 6, 1299.
(19) Fichou, D.; Bachet, B.; Demanze, F.; Billy, I.; Horowitz, G.; Garnier,
F. AdV. Mater. 1996, 8, 500.

672 J. Am. Chem. Soc., Vol. 120, No. 4, 1998
Laquindanum et al.
on the same substrate. The fact that most favorable morphol-
ogies are obtained at temperatures above 70 °C, where the
combined interconnectedness and order in the film appears
optimal, means that a cooling process must occur after deposi-
tion and before testing. Because of the mismatch in coefficients
of thermal expansion between the silicon substrates and the
organic films, microcracks21 can develop which would create
barriers to charge transport and lower the apparent mobility.
Since most of the channel current flows in the first two
monolayers of material,16 such mobility-lowering defects might
be masked by overgrowths and not be readily visible. Moreover,
the series resistance in going through the thick polyethylene-
like layers to inject charge from top contacts to the channel
may be a mobility-lowering factor that is particularly pro-
nounced in DOADT. This may also contribute to the lower
mobility of DDADT deposited at 125 °C, even though films
deposited at those temperatures are flat, smooth, and highly
crystalline. The use of diffraction methods in characterizing
these films is subject to the limitation that the important charge-
carrying region in the TFT is confined to a few molecular layers
near the interface with the dielectric, where the morphology
might differ from that of the bulk of the sample that gives rise
to most of the diffraction signal.
temperatures. This may be because in the absence of single-
crystal morphology, the multiple nucleation that occurs during
growth leads to a more even distribution of the material than at
high temperature especially in the first few monolayers. The
coefficient of thermal expansion, as discussed above, may also
play a role in favoring a lower deposition temperature for
significant mobility. The packing of DHADT films deposited
from solution may be denser than the film obtained from
precursor pentacene, which originates as a highly disordered
material. This density increase could in turn lead to greater
oxidative stability of the film.
In summary, the fully conjugated, nonoxidized ADT ring
system has been synthesized for the first time and the thin film
morphology and hole mobility of the parent and three dialkyl
derivatives have been explored. In most cases, films deposited
by vacuum evaporation are highly crystalline, and the mobilities
are among the highest observed in TFT configurations, espe-
cially for polycrystalline films. Although on balance the
performance of these materials has not proven to be superior to
that of pentacene, the ADTs offer advantages in the areas of
solubility, processability, and tunability through end-group
modification. The results also demonstrate that the high
mobility observed for pentacene is not unique among thin films
of molecular solids, but may be observable in a wider variety
of compounds, including some with lower melting points and
larger HOMO-LUMO gaps.
Taking advantage of the greater solubility and solution
stability of the ADTs relative to that of pentacene, it was
possible to attempt solution-phase deposition of an ADT
derivative, specifically DHADT, as a TFT semiconductor.
While the morphologies are clearly inferior and the mobilities
lower than for vapor-deposited films, this is the first time such
a rigid, high-melting molecule has been found to be processable
Acknowledgment. The authors wish to thank our colleagues
Ananth Dodabalapur, for his help in interpreting some of the
results; Zhenan Bao, for valuable discussions; A. M. Mujsce,
for obtaining mass spectral analyses, and K. Raghavachari, for
his advice on MO calculations.
1
b,22
in this manner without resorting to a precursor route.
It is
surprising that the best results were obtained from dilute
solutions deposited at mildly elevated temperatures, rather than
from the more concentrated solutions available at higher
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