Synthesis and characterization of 2,9-bis(perfluorobutyl)pentacene as an n-type organic field-effect transistor

Article abstract of DOI:10.1246/bcsj.81.530

A novel organic semiconductor, 2,9-bis(perfluorobutyl)pentacene, was synthesized in seven steps, and was characterized by NMR, IR, and UV spectroscopy. From differential scanning calorimetry analyses, no liquid crystalline phase was observed, unlike 2,9-dibutylpentacene. The morphology of thin films formed at different substrate temperatures was investigated by atomic force microscopy (AFM) and X-ray diffraction analysis. Organic field-effect transistors (OFETs) fabricated with this compound showed electron mobility of 1.7 × 10^-1cm² V^-1 s^-1 using Ag source-drain electrodes.

Full text of DOI:10.1246/bcsj.81.530

530 Bull. Chem. Soc. Jpn. Vol. 81, No. 4, 530–535 (2008)
ꢀ 2008 The Chemical Society of Japan
Synthesis and Characterization of 2,9-Bis(perfluorobutyl)pentacene
as an n-Type Organic Field-Effect Transistor
ꢀ1
Kazuo Okamoto,1 Kenji Ogino, Mitsuaki Ikari,2 and Yoshihito Kunugi2
1Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology,
Koganei, Tokyo 184-8588
2Department of Applied Chemistry, Faculty of Engineering, Tokai University, 1117 Kitakaname, Hiratsuka 259-1292
Received November 7, 2007; E-mail: kogino@cc.tuat.ac.jp
A novel organic semiconductor, 2,9-bis(perfluorobutyl)pentacene, was synthesized in seven steps, and was charac-
terized by NMR, IR, and UV spectroscopy. From differential scanning calorimetry analyses, no liquid crystalline phase
was observed, unlike 2,9-dibutylpentacene. The morphology of thin films formed at different substrate temperatures was
investigated by atomic force microscopy (AFM) and X-ray diffraction analysis. Organic field-effect transistors (OFETs)
fabricated with this compound showed electron mobility of 1:7 ꢁ 10ꢂ3 cm2 Vꢂ1 sꢂ1 using Ag source–drain electrodes.
Organic field-effect transistors (OFETs) are of great interest
because of their potential applications including low-cost,
large-area, and flexible electronic devices, such as pliable elec-
tronic papers, RFID tags, and flexible OLEDs.1–3 OFETs based
on linear acenes,4–7 oligothiophenes,8 and regioregular poly-
(3-hexylthiophene)9 have been extensively studied. Various
devices showing high-performance exceeding that of amor-
phous silicon based devices, have been developed. Most of
the above-mentioned materials are p-type organic semiconduc-
tors, in which holes are the predominant carriers. Development
of high-performance OFETs based on n-type organic materials
is also very important because the combination of p- and n-
type materials enables the fabrication of complementary logic
circuits10 and ambipolar transistors.11
As n-type materials, C60 derivatives,12 fluorinated copper
phthalocyanine (F16CuPc),13 terthiophene-based quinodimeth-
ane,14 naphthalenetetracarboxamide derivatives,15 perylene
tetracarboxamide,16 thiazole oligomers,17 and perfluoropenta-
cene18 have been reported. It is well known that perfluorination
of p-type materials is an effective way to produce n-type ma-
terials. This is because enhancement of electron injection is as-
sured by the introduction of highly electronegative fluorine
atoms. Alternatively it has also been reported that substitution
of p-type oligothiophene cores with perfluoroalkyl groups19 or
perfluorobenzoyl20 groups affords effective n-type materials.
We have previously reported that liquid crystallinity ap-
peared over a relatively wide temperature range by the intro-
duction of alkyl groups at the 2 and 9 positions of pentacene.21
For example, 2,9-dibutylpentacene shows crystal–liquid crys-
tal phase transition at 115 ꢃC, and is converted to isotropic
phase at 194 ꢃC. In this paper, we report the synthesis and
characterization of 2,9-bis(perfluorobutyl)pentacene as an n-
type analogue of 2,9-dialkylpentacenes. For the synthesis of
unsubstituted or symmetrically tetra-substituted pentacene, a
conventional synthetic scheme has been utilized, where penta-
cenequinone derivatives are prepared by the condensation of
1,4-cyclohexanedione and phthalaldehyde derivatives, and
subsequent reduction gives pentacene derivatives.22 This is a
high yielding reaction and the starting materials are readily
available, but it is not applicable to the synthesis of 2,9-disub-
stituted pentacenes with high purity because of formation of
two isomers. Therefore, a synthetic route starting from pyro-
mellic anhydride and consisting of seven steps is proposed
for a highly pure product with no isomer contamination. It is
crucial that no isomers are produced in the synthesis of organic
transistor materials, because the existence of a trace of isomers
disturbs the regular packing of the crystal.23 The introduction
of perfluoroalkyl groups transforms the originally p-type pen-
tacene derivative to the desired n-type compound. The thermal
properties of the synthesized compound are described in com-
parison with the alkyl analogue. The crystallinity and morphol-
ogy of thin films deposited at different substrate temperatures
are also discussed. n-Type OFETs based on 2,9-bis(perfluoro-
butyl)pentacene with different source and drain electrodes
were fabricated and characterized.
Experimental
General.
All reagents were purchased from Wako Pure
Chemical Industries, Ltd. and were used without further purifica-
tion. Column chromatography was performed on silica gel 60N
(spherical, neutral, 63–210 mm) purchased from KANTO Chemi-
cal Co., Inc. All compounds were characterized by a combination
of MS spectroscopy (Shimadzu, GCMS-QP2010), 1H, 13C, and
19F NMR, spectroscopy (Varian Mercury Plus 400 MHz), and IR
spectroscopy (Shimadzu, FTIR-8300). Chemical shifts are report-
ed as ꢀ values (ppm) relative to internal tetramethylsilane (1H
and 13C NMR,) or hexafluorobenzene (19F NMR). UV–vis spectra
were recorded on a Shimadzu, UV-3101PC for solution and thin
film. Thermal properties were investigated by differential scan-
ning calorimetry (DSC, TA Instruments Q10) under nitrogen flow,
and thermogravimetric analysis (TGA, ULVAC-RIKO, Inc. VAP-
9000). The thin films of 2,9-bis(perfluorobutyl)pentacene were
characterized by X-ray diffraction (XRD) analysis using a Philips
PW1830 X-ray diffractometer. XRD patterns were obtained using
Published on the web April 10, 2008; doi:10.1246/bcsj.81.530

K. Okamoto et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 4 (2008)
531
Br
HOOC
O
Br
MeOOC
O
C4F9
O
O
O
O
O
COOH
Br
O
COOMe
Br
MeOOC
O
O
C4F9I
Cu
MeOH
H+
LiI
AlCl3
O
O
+
COOMe
C4F9
Br
1
2
3
C4F9
C4F9
C4F9
C4F9
HOOC
O
O
HOOC
O
H2
Pd/C
CF3SO3H
Al(O-iPr)3
COOH
C4F9
COOH
C4F9
O
C4F9
C4F9
4
5
6
7
Figure 1. Synthesis of 2,9-bis(perfluorobutyl)pentacene.
Bragg–Brenano geometry (ꢁ–2ꢁ) with Cu Kꢂ radiation as an X-
ray source with an acceleration voltage of 40 kV and a beam cur-
rent of 30 mA. The film morphology was observed by atomic
force microscopy (AFM), using a Digital Instruments NanoScope
IIIa in the tapping mode.
CDCl3, ꢀ): 3.70 (6H, s), 7.64 (8H, s), 8.05 (2H, s). 13C NMR
(100 MHz, CDCl3, ꢀ): 52.9, 129.1, 129.5, 130.8, 132.2, 132.5,
135.2, 142.5, 164.6, 194.2. FT-IR (KBr): 3090, 3059, 3024,
3003, 2949, 2845, 1720, 1682, 1585, 1481, 1437, 1398, 1381,
1308, 1271, 1177, 1113, 1067, 1007, 974, 945, 914, 845, 820, 793,
Synthesis. Figure 1 shows the synthetic route of 2,9-bis(per-
fluorobutyl)pentacene used in this study.
741, 685, 573 cmꢂ1
.
Synthesis of Dimethyl 2,5-Bis(4-perfluorobutylbenzoyl)tere-
phthalate (3): A mixture of 2.0 g (3.5 mmol) of compound 2, and
2.0 g of copper powder in 50 mL of dimethyl sulfoxide (DMSO)
was heated at 120 ꢃC under nitrogen atomosphere. A solution of
27.5 mL (10 mmol) of perfluorobutyl iodide in 70 mL of DMSO
was added over 2.5 h. The reaction mixture was stirred for 2 h at
120 ꢃC. The mixture was cooled to 60 ꢃC, and was poured into
300 mL of chloroform. Precipitate was purified by column chro-
matography (silica gel, dichloromethane), and dried to give 2.5 g
of white powder (85%). 1H NMR (400 MHz, CDCl3, ꢀ): 3.70 (6H,
s), 7.73 (4H, d, J ¼ 8:5 Hz), 7.92 (4H, d, J ¼ 8:5 Hz), 8.11 (2H,
s). 19F NMR (376 MHz, CDCl3, ꢀ): 36.3 (4F, m), 39.3 (2F, q),
50.4 (4F, t, J ¼ 12 Hz), 80.8 (6F, m). FT-IR (KBr): 3050, 2905,
1720, 1684, 1441, 1412, 1354, 1310, 1267, 1246, 1226, 1204,
Synthesis of 2,5-Bis(4-bromobenzoyl)terephthalic Acid (1):
A mixture of 24.0 g (110 mmol) of pyromellic anhydride and
150 mL of bromobenzene was heated at 45 ꢃC under a nitrogen at-
mosphere. To the mixture was added 40.0 g (300 mmol) of alumi-
num chloride in small portions. After stirring for 6 h at 45 ꢃC, the
reaction mixture was poured into a mixture of 400 g of ice and
180 mL of concentrated hydrochloric acid and was stirred 1 h at
room temperature. The organic layer was extracted with 400 mL
of ethyl acetate and 400 mL of tetrahydrofuran (THF). The extract
was washed by three 400 mL portions of water and was dried over
anhydrous magnesium sulfate. After evaporation of solvents, the
light yellow crude product was washed with 150 mL of ethyl ace-
tate, and dried to give 12.5 g of white powder. This was recrystal-
lized from methanol to afford 2,5-bis(4-bromobenzoyl)terephthal-
1192, 1170, 1136, 1113, 947, 868, 824, 795, 750, 710, 687 cmꢂ1
.
1
ic acid as white powder (3.0 g, 5%). H NMR (400 MHz, DMSO,
Synthesis of 2,5-Bis(4-perfluorobutylbenzoyl)terephthalic
Acid (4): A mixture of 5.0 g (5.96 mmol) of compound 3 and
10 g (74.7 mmol) of lithium iodide in 250 mL of dimethyl form-
amide (DMF) was heated at 150 ꢃC for 12 h under nitrogen atmo-
sphere. After cooling to room temperature, the reaction mixture
was poured into 300 mL of 2 M hydrochloric acid. Precipitate
was washed twice with 300 mL of water, and dried to give
4.4 g (5.43 mmol) of white powder (91%). 1H NMR (400 MHz,
DMSO, ꢀ): 7.86 (2H, d, J ¼ 8:4 Hz), 7.97 (2H, d, J ¼ 8:3 Hz),
8.07 (1H, s). 19F NMR (376 MHz, DMSO, ꢀ): 37.4 (4F, t, J ¼
13:3 Hz), 40.3 (4F, q, J ¼ 8:7 Hz), 52.3 (4F, t, J ¼ 11:9 Hz),
82.0 (6F, t, J ¼ 9:2 Hz). FT-IR (KBr): 3418, 3053, 2860, 2671,
2559, 1697, 1493, 1412, 1354, 1306, 1240, 1136, 1111, 1090,
ꢀ): 7.67 (4H, d, J ¼ 8:5 Hz), 7.74 (4H, d, J ¼ 8:5 Hz), 7.95 (2H,
s). 13C NMR (100 MHz, CDCl3, ꢀ): 194.2, 165.6, 141.8, 135.4,
133.4, 131.9, 131.0, 128.9, 127.7. FT-IR (KBr): 3423, 3059, 2654,
2588, 2542, 1697, 1585, 1499, 1398, 1364, 1298, 1252, 1178,
1121, 1070, 1011, 945, 914, 841, 795, 563 cmꢂ1
.
Synthesis of Dimethyl 2,5-Bis(4-bromobenzoyl)terephtha-
late (2): A mixture of 34.0 g (64 mmol) of compound 1, 200 g
of methanol, 850 mL of toluene, and 8.5 mL of sulfuric acid was
refluxed overnight. The reaction mixture was cooled to room tem-
perature, and 1000 mL of methanol was added. Precipitate was
collected and washed with 300 mL of cold methanol, and was
1
dried to give 22.4 g of white powder (63%). H NMR (400 MHz,

532 Bull. Chem. Soc. Jpn. Vol. 81, No. 4 (2008)
2,9-Bis(perfluorobutyl)pentacene
1022, 1005, 987, 953, 914, 887, 868, 843, 824, 800, 787, 748, 717,
584 cmꢂ1
.
Results and Discussion
Synthesis of 2,5-Bis(4-perfluorobutylbenzyl)terephthalic Acid
(5): A mixture of 5.39 g (6.6 mmol) of compound 4 and 8.35 g of
5% palladium on carbon (NX-Type: bought from N.E. Chemcat
Corporation) in 540 mL of THF was heated at 60 ꢃC for 24 h under
an atmosphere of hydrogen at 0.4 MPa. After the reaction, the
mixture was filtered through Celiteꢁ to remove the catalyst. The
filtrate was concentrated in vacuo. The crude product was purified
by washing with 100 mL of hexane, and was dried to give 4.78 g
of white powder (93%). 1H NMR (400 MHz, DMSO, ꢀ): 4.44 (4H,
s), 7.37 (4H, d, J ¼ 8:0 Hz), 7.55 (4H, d, J ¼ 8:0 Hz), 7.77 (2H,
s). 19F NMR (376 MHz, DMSO, ꢀ): 37.4 (4F, m), 40.3 (4F, t,
J ¼ 8:6 Hz), 53.1 (4F, d, J ¼ 13:2 Hz), 82.1 (6F, m). FT-IR
(KBr): 3422, 3017, 2905, 2652, 2540, 2365, 1697, 1616, 1556,
1516, 1499, 1418, 1354, 1271, 1236, 1204, 1132, 1092, 1030,
Synthesis and Molecular Characterization. In order to
synthesize 2,9-disubstituted pentacene, it was necessary to uti-
lize a scheme starting from pyromellic anhydride21 rather than
a conventional method involving the condensation of phthalal-
dehyde with 1,4-cyclohexanedione.22 At first attempt, the syn-
thetic route established for 2,9-dialkylpentacenes21 was direct-
ly applied, where in the first step, Friedel–Crafts acylation of
perfluorobutylbenzene with pyromellitic acid anhydride was
involved. This direct acylation, however, afforded a complicat-
ed mixture of products due to the deactivated nature of per-
fluorobutylbenzene. Therefore, we adopted a modified route
as shown in Figure 1, where the perfluorobutyl groups are in-
troduced via Ulmann reaction of perfluorobutyl iodide with the
bromo-substituted derivative after Friedel–Crafts acylation. In
the first step (Friedel–Crafts acylation of bromobenzene),
ortho-substituted by-products were also generated, but pure
para-substituted product was easily obtained by recrystalliza-
tion. Formation of the ortho-substituted derivative is mainly
responsible for the low yield in this step. Esterification of
the resulting terephthalic acid derivative was essential in order
to avoid unexpected side reactions in the subsequent Ulmann
reaction.
2,9-Bis(perfluorobutyl)pentacene (FBP) was obtained as a
deep blue solid like the dialkyl analogues, and was slightly
soluble in organic solvents such as toluene, tetrahydrofuran,
and chloroform. Chemical structure was confirmed with MS,
NMR, and IR spectroscopies. Improved solubility made it pos-
sible to measure 1H and 19F NMR spectra (in deuterated
1,1,2,2-tetrachloroethane at 100 ꢃC). Signal patterns in the
1H NMR spectrum are very similar to those of the dialkyl ana-
logues, and all signals are shifted toward lower field (0.1–
0.3 ppm), indicating that the strong ꢃ electron-withdrawing
perfluorobutyl substitution deacreases the electron density of
the pentacene ring. In the IR spectrum, characteristic bands
at 1219 and 1200 cmꢂ1 associated with ꢀ (C–F) in CF3 and
CF2, respectively, which are absent in the IR spectrum of 2,9-
dibutylpentacene (BP).21
1005, 987, 920, 872, 851, 797, 743, 689, 590, 532 cmꢂ1
.
Synthesis of 3,10-Bis(perfluorobutyl)pentacene-5(14H),-
12(7H)-dione (6): A mixture of 9.1 g (11.6 mmol) of compound
5 and 45 mL of trifluoromethanesulfonic acid was stirred under ni-
trogen atmosphere overnight at room temperature. The mixture
was poured into 350 g of ice and 350 g of water. The precipitate
was collected by filtration and washed with 350 mL of 5% aqueous
sodium carbonate and then twice with 750 mL of water. The pre-
cipitate was dried to give 7.4 g (9.91 mmol) of dark brown powder
(86%). Due to the insolubility in appropriate organic solvents, no
NMR data were obtained. FT-IR (KBr): 3050, 2905, 1751, 1655,
1616, 1448, 1394, 1354, 1344, 1246, 1205, 1228, 1088, 976,
951, 868, 881, 808, 756, 741, 692, 671, 608, 582 cmꢂ1. MS (EI,
70 eV): m=z (%) = 746 (Mþ, 100), 744 (26), 577 (Mþ ꢂ C3F7,
53), 575 (28), 527 (Mþ ꢂ C4F9, 50), 408 (Mþ ꢂ C6F14, 6), 406
(26), 358 (Mþ ꢂ C8F18, 28), 204 (53), 179 (24).
Synthesis of 2,9-Bis(perfluorobutyl)pentacene (FBP) (7):
A
mixture of 1.0 g (1.34 mmol) of compound 6 and 20 g (0.99 mol)
of aluminum isopropoxide in 200 mL of cyclohexanol was heated
to 160 ꢃC with distilling out isopropanol, and then was refluxed for
24 h under argon atmosphere. After cooling to room temperature,
the resulting precipitate was collected by centrifugal separation.
The precipitate was successively washed with 150 mL of acetic
acid, 150 mL of concentrated hydrochloric acid, 150 mL of water,
150 mL of acetone, 150 mL of THF, and 150 mL of methanol, then
1
was dried to give 0.25 g (26.1%) of dark blue powder. H NMR
Whereas this compound is relatively stable in the solid state,
its solution was rather unstable and like dibutylpentacene
bleached within 30 min when exposed to air and/or light.
DSC analysis revealed that the melting point of FBP is 266
ꢃC, which is higher than that of BP (115 ꢃC)21 suggesting that
not only strong ꢄ–ꢄ interaction between pentacene rings but
different chain–chain interactions between fluoroalkyl and al-
kyl substituents dominate cohesive forces in the solid state.
Whereas the dialkyl analogue shows LC phase over a relative-
ly wide temperature range (smectic; 115–194 ꢃC), the fluori-
nated one shows no crystal–liquid crystal phase transition.
This is probably due to the lack of flexibility in the perfluoro-
alkyl chains.
The introduction of fluoroalkyl groups enhanced the volatil-
ity. Figure 2 shows TGA thermograms measured at 1 ꢁ 10ꢂ4
Pa for FBP (a), BP (b), and unsubstituted pentacene (c). The
onset points of weight loss for BP and pentacene were 282
and 286 ꢃC, respectively. The effect of the alkyl chain is
almost negligible. This result also suggests that volatility is
predominantly determined by the ꢄ–ꢄ interaction between
(400 MHz, C2D2Cl2, 100 ꢃC, ꢀ): 7.40 (2H, d, J ¼ 9:5 Hz), 8.02
(2H, d, J ¼ 9:5 Hz), 8.24 (2H, s), 8.73 (2H, s), 8.81 (2H, s), 9.03
(2H, s). 19F NMR (376 MHz, C2D2Cl2, 100 ꢃC, ꢀ): 37.1 (4F, m),
40.1 (4F, m), 51.5 (4F, m), 81.1 (6F, m). FT-IR (KBr): 3050,
1356, 1261, 1231, 1219, 1200, 1086, 1022, 916, 899, 835, 822,
791, 739, 714, 696, 608, 581, 534, 465 cmꢂ1. MS (EI, 70 eV):
m=z (%) = 714 (Mþ, 100), 545 (Mþ ꢂ C3F7, 48), 376 (Mþ ꢂ
C6F14, 49), 188 (65). mp 266 ꢃC. HRMS (EI, 70 eV): found m=z
(Mþ), 714.0640, calculated for C30H12F18, 714.0652.
Fabrication and Evaluation of OFETs. Top contact OFET
devices with 50 mm channel lengths (L) and 1.5 mm channel
widths (W) were fabricated on thermally oxidized, highly doped
Si substrates. The gate insulator (SiO2) was 210 nm thick. A thin
film (50 nm thick) of FBP was vacuum deposited on the SiO2 lay-
er at a deposition rate of 0.1 nm sꢂ1 under a pressure of 2 ꢁ 10ꢂ3
Pa at different substrate temperatures Ts = rt and 60 ꢃC. Gold or
silver source and drain electrodes were deposited on the semicon-
ductor layer through a shadow mask. The FET characteristics
were measured at rt under vacuum with an Agilent 6155C semi-
conductor parameter analyzer.

K. Okamoto et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 4 (2008)
533
120
1.2
C4F9
FBP
BP
(a)
(a)
192°C
262°C
C4F9
100
1.0
80
60
0.8
0.6
0.4
0.2
0.0
273°C
90.9%
40
20
0
284°C
-20
0
100
200
300
Temperature / °C
400
500
600
120
100
80
60
40
20
0
C4H9
(b)
C4H9
214°C
286°C
400
500
600
700
800
Wavelength / nm
1.2
FBP
BP
96.7%
(b)
300°C
1.0
0.8
0.6
0.4
0.2
0.0
314°C
-20
0
100
200
300
Temperature / °C
400
500
600
120
100
80
60
40
20
0
(c)
223°C
282°C
100.0%
293°C
400
500
600
700
800
Wavelength / nm
Figure 3. UV–vis spectra for 2,9-bis(perfluorobutyl)penta-
cene (FPB) and 2,9-dibutylpentacene (BP) (a) in toluene
solution and (b) on quartz substrate.
304°C
-20
0
100
200
300
Temperature / °C
400
500
600
pear periodically at 435, 470, 503, 539, and 584 nm for FBP,
and this band splitting is probably due to vibronic coupling
with one of the ꢅ(C=C) stretching modes of the pentacene
skeleton (1300–1400 cmꢂ1).24 Each peak shows slight batho-
chromic shifts compared with BP (429, 459, 496, 534, and
577 nm) in solution state. DFT calculations [RB3LYP/6-
31G(d)] revealed that HOMO–LUMO gaps are 2.19 and
2.28 eV for fluoro and nonfluoro derivatives, respectively. Ob-
served red shifts are consistent with the computed energy gap.
The electron-accepting nature of the perfluoroalkyl chain af-
fords a stronger effect on band gap modulation. In a film state,
bathochromic shifts due to molecular aggregation were ob-
served for both BP (646, 578, and 534 nm) and FBP (639,
572, and 531 nm) films compared with a solution state. Smaller
red-shifts for FBP are probably due to loose interplaner stack-
ing of pentacene rings because of the large van der Waals
radius of fluorine.
Figure 2. Thermogravimetric analysis graphs for (a) 2,9-
bis(perfluorobutyl)pentacene, (b) 2,9-dibutylpentacene,
and (c) pentacene. The heating rate is 10 ꢃC minꢂ1 at
1 ꢁ 10ꢂ4 Pa.
pentacene rings. The onset point of weight loss for FBP was
262 ꢃC under the same conditions. The enhancement of the
volatility is also observed for fluorocarbon-substituted thio-
phene oligomers.19a In all cases, smooth TGA plots without in-
flection points were obtained, indicating that no decomposition
occurred during evaporation. TGA under atomospheric pres-
sure of nitrogen revealed that decomposition of 2,9-disubstitut-
ed pentacenes started beyond 350 ꢃC.
Figure 3 shows UV–vis spectra for FBP and BP (a) in tol-
uene solution and (b) on quartz substrate. In the UV–vis spec-
tra in toluene solution, five characteristic absorption peaks ap-

534 Bull. Chem. Soc. Jpn. Vol. 81, No. 4 (2008)
2,9-Bis(perfluorobutyl)pentacene
6x10-7
5x10-7
4x10-7
3x10-7
2x10-7
1x10-7
0
300
(a)
(a)
Vg = 120V
250
200
150
100
50
Vg = 100V
Vg = 80V
Vg = 0~60V
0
0
20
40
60
80
100
0
5
10
15
20
25
30
Vd / V
2
θ
/ deg.
0.0016
0.0014
0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
(b)
10-6
Vd = 100 V
(b)
600
500
400
300
200
100
0
10-7
10-8
10-9
10-10
10-11
-20
0
20
40
Vg / V
60
80 100
0
5
10
15
20
25
30
2
θ
/ deg.
Figure 5. FET characteristics for 2,9-bis(perfluorobutyl)-
pentacene based device fabricated at substrate temperature
of 60 ꢃC with Ag source and drain electrodes. (a) Drain cur-
rent (Id) vs. drain voltage (Vd) characteristics at various
gate voltages (Vg), (b) Id1=2 and log Id vs. Vg at Vd ¼ 100 V.
Figure 4. X-ray diffraction patterns from 2,9-bis(perfluoro-
butyl)pentacene thin films on SiO2 substrate deposited at
different substrate temperature of (a) room temperature
and (b) 60 ꢃC.
Characteristics of Thin Films. AFM measurment of thin
films of FBP deposited on SiO2 at rt and 60 ꢃC were per-
formed. The size of crystal grains were not much different at
different substrate temperatures. XRD measurements of the
thin films afforded crucial information on the molecular orien-
tation on the substrate. Figure 4 shows XRD patterns for FBP
thin films deposited on SiO2 at substrate temperature of (a) rt
and (b) 60 ꢃC. A primary diffraction peak at 2ꢁ ¼ 3:6ꢃ and
higher order peaks were observed in the patterns. At 60 ꢃC,
higher peak intensity was observed indicating that thin film
with a high degree of ordering and crystallinity was obtained
at higher temperature. The interlayer distance (d) estimated
nearly upright structures on the substrate, which is suitable
morphology for electrical transport between source and drain
contacts in an FET.
Top contact configuration OFETs were fabricated with
vapor-deposited FBP thin films, and mobility values were de-
termined in the saturation regime by standard procedures.25
Figure 5a shows drain current (Id) vs. drain voltage (Vd) char-
acteristics at various gate voltages (Vg) for an FBP-based
OFET (Ts ¼ 60 ꢃC) with silver source and drain electrodes.
1=2
The Id
and log Id vs. Vg characteristics of the device at
Vd ¼ 100 V are shown in Figure 5b. The OFET shows a typi-
cal output profile of a metal-oxide-semiconductor field-effect
transistor. When the gate was biased positively, the drain cur-
rent was observed as shown in Figure 5a. These results cer-
tainly indicate that accumulated electrons flow from the source
to the drain through the channel region. Therefore, the OFET
˚
from the primary diffraction was 24.5 A, which is nearly equal
˚
to computed molecular length (24 A, corresponding to the long
axis length of the projection in a plain parallel to a pentacene
ring). This implies that pentacene cores are oriented with

K. Okamoto et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 4 (2008)
535
Table 1. FET Characteristics of Devices Based on 2,9-Bis-
(perfluorobutyl)pentacene
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The mobility and the threshold voltage (Vth) were calculated
from the slope and intercept, respectively, of the linear portion
1=2
of the Id vs. Vg plot. Table 1 summarizes FET characteris-
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vacuum (no FET response was observed in air). When the sub-
strate temperature increased to 60 ꢃC, Vth was extensively re-
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between electrodes and semiconductor. As mentioned above
(Figure 4), the better ordered film with larger grain size was
obtained at 60 ꢃC. When source and drain electrodes were
changed from gold to silver, the electron mobility was im-
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We successfully synthesized novel 2,9-bis(perfluorobutyl)-
pentacene utilizing a modified synthetic scheme established
for the synthesis of 2,9-dialkylpentacenes. It was found that
the substitution of the pentacene core with perfluorobutyl
groups enhances volatility and electron affinity compared with
2,9-dibutylpentacene. We have firstly demonstrated that the
introduction of perfluoroalkyl groups is an effective way to
convert a p-type parent core to n-type material in the pentacene
family. Observed mobility values are still lower than that of
high-performance n-type organic materials like perfluoropen-
tacene and thiazole oligomers, but the efforts including the op-
timization of the length of perfluoroalkyl group, and the sur-
face modification of the dielectric layer are now in progress
in order to improve the performance.
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