Photogeneration and thermal generation of pentacene from soluble precursors for OTFT applications

Article abstract of DOI:10.1021/ol8010419

(Graph Presented) A CO adduct of pentacene with an unsymmetrical structure is synthesized; it is soluble and can be spin-coated into thin films. Pentacene is regenerated in near quantitative yield by either thermal or photoinduced elimination of CO. OTFT devices fabricated by this compound exhibit typical FET characteristics.

Full text of DOI:10.1021/ol8010419

ORGANIC
LETTERS
2
008
Vol. 10, No. 13
869-2872
Photogeneration and Thermal
Generation of Pentacene from Soluble
Precursors for OTFT Applications
2
†
Ta-Hsien Chuang, Hsing-Hung Hsieh, Chang-Ken Chen, Chung-Chih Wu,*
‡
‡
,‡
§
Chao-Chen Lin, Pi-Tai Chou,* Ting-Han Chao, and Tahsin J. Chow*
,§
⊥
,†
Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan, Department of Electrical
Engineering and Graduate Institute of Electronics Engineering, National Taiwan
UniVersity, Taipei 106, Taiwan, Department of Chemistry, National Taiwan UniVersity,
Taipei 106, Taiwan, and Department of Chemistry, National Taiwan Normal
UniVersity, Taipei 106, Taiwan
tjchow@chem.sinica.edu.tw
Received May 6, 2008
ABSTRACT
A CO adduct of pentacene with an unsymmetrical structure is synthesized; it is soluble and can be spin-coated into thin films. Pentacene is
regenerated in near quantitative yield by either thermal or photoinduced elimination of CO. OTFT devices fabricated by this compound exhibit
typical FET characteristics.
Designing electronic devices made of organic materials is
currently the subject of intensive interest. The most promising
thin films of pentacene through solution processes. To
4
5
circumvent this problem, scientists at Philips and IBM have
developed a strategy of designing precursors of pentacene
that are soluble in organic solvents and can be used to
p-type semiconductor for use in organic thin-film transistors
1
(
OTFT) is pentacene. The high charge mobility of pentacene
can be attributed to its high degree of crystallinity in the
2
solid state, yet the same property also renders its low
(2) Gundlach, D. J.; Lin, Y. Y.; Jackson, T. N.; Nelson, S. F.; Schlom,
3
D. G. IEEE Electron DeVice Lett. 1997, 18, 87.
solubility in organic solvents. It is rather difficult to prepare
(
(
3) Minakata, T.; Natsume, Y. Synth. Met. 2005, 153, 1.
4) (a) Brown, A. R.; Pomp, A.; de.Leeuw, D. M.; Klaassen, D. B. M.;
†
Academia Sinica.
Department of Electrical Engineering, National Taiwan University.
Department of Chemistry, National Taiwan University.
Department of Chemistry, National Taiwan Normal University.
Havinga, E. E.; Herwig, P.; M u¨ llen, K. J. Appl. Phys. 1996, 79, 2136. (b)
Herwig, P. T.; M u¨ llen, K. AdV. Mater. 1999, 11, 480.
‡
§
(5) (a) Weidkamp, K. P.; Afzali, A.; Tromp, R. M.; Hamers, R. J. J. Am.
Chem. Soc. 2004, 126, 12740. (b) Afzali, A.; Dimitrakopoulos, C. D.; Breen,
T. J. Am. Chem. Soc. 2002, 124, 8812. (c) Afzali, A.; Kagan, C. R.; Traub,
G. P. Synth. Met. 2005, 155, 490.
⊥
(
1) For recent reviews:(a) Zaumseil, J.; Sirringhaus, H. Chem. ReV. 2007,
07, 1206. (b) Murphy, A. R.; Fr e´ chet, J. M. Chem. ReV. 2007, 107, 1066.
1
10.1021/ol8010419 CCC: $40.75 2008 American Chemical Society
Published on Web 05/30/2008


generate pentacene on demand. The structures of these
precursors are usually cycloadducts of pentacene itself (acting
as a diene) with another small volatile fragment (acting as a
dienophile). After a film is produced by spin-coating, pure
pentacene can be regenerated upon heating through a
retrocyclization reaction.
Scheme 1. Synthetic Scheme toward Compound 2
As most of the cycloreversion processes are activated by
thermal energy, photoinduced dissociation is scant. Present
technologies for making integrated electronic circuits rely
largely on photolithography, by which large areas can be
6
patterned. There has been only one earlier report on
photogeneration of pentacene via a cycloreversion process,
7
yet no corresponding device data have been released.
In this study, we report an unprecedented case of photo-
generation of pentacene from both CO-adducts 1 and 2, and
their successful utilization in OTFT devices. Synthesis of
the symmetrical compound 1 has been reported by us
8
previously. Pure pentacene was generated by CO expulsion
at 150 °C, and the device made with it exhibited typical FET
characteristics. Recently we have discovered that the CO
expulsion can also be achieved effectively through photolysis.
To increase the efficiency of photolysis, we have further
designed its isomer 2 with an unsymmetrical structure in
which the position of the CO bridge is shifted off the central
aromatic ring. The absorptivity of compound 2 is expected
to be higher than that of 1 due to its noncentral symmetrical
geometry. The sizes of the major chromophores in these two
compounds are different, i.e., a naphthalene in 1 cf. an
anthracene in 2. The HOMO-LUMO band gap of the
anthracene chromophore is narrower than that of naphthalene;
therefore the low-lying absorption bands of 2 are expected
to be red-shifted. As a result, the activation energy required
for photoelimination is reduced. Comparing the possible
reactivity of 1 and 2, the latter should have the advantages
of lower photodissociation energy along with higher quantum
efficiency. Here we describe the preparation and physical
properties of 2, as well as the preformance of OTFT devices
made with it through either a thermal or a photochemical
process.
9
5 in 72% yield. The C
s
symmetrical geometry of 5 was
1
3
verified by the presence of 14 absorption signals in the
C
1
NMR spectrum. In the H NMR spectrum the two methylene
hydrogens were shown as two separate doublets, at δ 3.27
and 3.57, with a large coupling constant (20 Hz). Aromati-
zation of the fourth ring of 5 was done by oxidation with
dicyanodichloroquinone (DDQ), whereby 6 was obtained in
a quantitative yield. The three types of methinyl hydrogens
appeared as three singlets at δ 1.99, 3.93, and 5.33,
1
respectively, in the H NMR spectrum. Further dehydration/
aromatization under catalysis with toluenesulfonic acid
resulted in a complicated mixture of compounds 7, 8, and
9, with a total yield of 75%. Their relative ratio depended
on the concentration of acid as well as the time of reaction.
The quaternary carbon of 7, to which the hydroxyl group
was attached, was shown as a singlet signal at δ 71.5 in the
1
3
C NMR spectrum. Compounds 8 and 9 were believed to
be secondary products derived from 7. The optimized yield
of 9, as a potential precursor of 2, was less than 5%. To
improve the yield of the desired product, compound 6 was
first treated with m-CPBA to form an epoxide, followed by
an acid-catalyzed rearrangement. The diol 10 was obtained
in 74% yield as the major product. The presence of a 1,2-
diol was evidenced by a strong absorption band at 3450-3550
-
1
cm in the IR spectrum and the two quaternary carbon
1
3
signals at δ 74.3 and 105.5 in the C NMR. Oxidation of
0 by PhI(OAc) cleaved the C-C bond, giving the target
ketone 2 in 76% yield. The carbonyl group shows a strong
1
2
The synthesis (Scheme 1) was started from a [4 + 2]
cycloaddition reaction of the furan derivative 3 with the
benzonorbornadiene derivative 4, which afforded the adduct
-
1
absorption at 1782 cm in IR, and a low field peak at δ
92.2 in C NMR. The optimized overall yield from (3 +
1
3
1
4
) to 2 was 40%.
(
6) (a) Afzali, A.; Dimitrakopoulos, C. D.; Graham, T. O. AdV. Mater.
003, 15, 2066. (b) Weidkamp, K. P.; Afzali, A.; Tromp, R. M.; Hamers,
R. J. J. Am. Chem. Soc. 2004, 126, 12740.
7) (a) Uno, H.; Yamashita, Y.; Kikuchi, M.; Watanabe, H.; Yamada,
H.; Okujima, T.; Ogawa, T.; Ono, N. Tetrahedron Lett. 2005, 46, 1981.
b) Yamada, H.; Yamashita, Y.; Kikuchi, M.; Watanabe, H.; Okujima, T.;
Ogawa, T.; Ohara, K.; Ono, N. Chem. Eur. J. 2005, 11, 6212.
8) (a) Chen, K. Y.; Hsieh, H. H.; Wu, C. C.; Hwang, J. J.; Chow, T. J.
Ketone 2 was made from 9 through ozonolysis in a mixed
solvent (CH Cl :MeOH ) 2:1) at -35 °C, followed by a
2 2
reduction of the ozonide intermediate with dimethyl sulfide.
However, due to the low yield of 9, this method was not
adopted for the production of 2.
2
(
(
(
Chem. Commun. 2007, 1065. (b) Lai, C. H.; Li, E. Y.; Chen, K. Y.; Chow,
T. J.; Chou, P. T. J. Chem. Theory Comput. 2006, 2, 1078.
(9) (a) Garratt, P. J.; Neoh, S. B. J. Org. Chem. 1979, 44, 2667. (b)
Lombardo, L.; Wege, D.; Wilkinson, S. P. Aust. J. Chem. 1974, 27, 143.
2870
Org. Lett., Vol. 10, No. 13, 2008

The UV absorption spectrum of 2 is quite different from
that of 1 (Figure 1). A strong π-π* transition ( A f B) of
pentacene, indicating a very high yield in the thermal
transformation. In the IR region, the strong absorption of
1
1
-
1
the carbonyl group at 1782 cm disappeared completely
after heating. The solubility of 2 was found to be similar to
that of 1, i.e., ∼0.7 mg/mL in either DCM or THF. However,
we have found an effective way to increase the solubility
by mixing 1 and 2 together. The solubility of an equal molar
mixture of 1 and 2 was double that of the individual
compounds, e.g. ∼1.5 mg/mL in DCM. Such a mixture can
be used quite suitably for the fabrication of OTFT devices.
Upon UV irradiation at the absorption peak of ∼366 nm
in degassed THF at room temperature, compound 2 turned
gradually from colorless to purplish. As shown in Figure 3,
Figure 1. UV absorptions of compounds 1 (blue), 2 (red), and the
thermally generated pentacene (black). The long-wavelength bands
are magnified in the center of the graph.
2
gives a strong peak at 268 nm, which is longer than the
corresponding absorption of 1 at 244 nm. This band is known
to be longitudal polarized and is red-shifted with the size of
1
0
1
1
polyacenes. The weaker A f L transition of 2 appears
a
at 387 nm, which is also significantly red-shifted to the
corresponding one of 1 at 300 nm. The latter band is partly
1
1
b
overlapped with the A f L transition at 324 nm.
Compound 2 was stable at ambient temperature yet began
to turn purplish at 128 °C. The fragmentation temperature
was lower than that of 1, which decomposed at 150 °C. A
Figure 3
. Absorption spectra of 2 in degassed THF obtained with
different exposure times, 0 to 40 s, with each increment of 5 s (λ
ex
≈
366 nm). Inset: 1 in degassed THF excited at 310 nm as a
function of exposure times of 0 to 40 s, with each increment of
s.
1
0% weight loss, as indicated by a thermogravimetric curve
5
(
Figure 2), corresponded to the weight ratio of a CO unit.
a new absorption band with a peak wavelength around 575
nm increased along with the time of exposure. An isosbestic
point was observed at ∼395 nm throughout the photolysis,
indicating the existence of only two compounds in the
solution, i.e., the reactant 2 and the product. Since the spectral
feature of the new band, including the vibronic progression
and the associated peak wavelengths, was identical with that
of pentacene (cf. Figures 1 and 3), the CO expulsion from 2
upon UV excitation to form pentacene was well justified.
Note that the UV photolysis had to be performed free from
2 2
O . Under aeration, O -trapped adducts were obtained, as
indicated by the appearance of a broad, diffusive absorption
7
band at <400 nm upon photolysis (not shown here).
Figure 2
(red) at 10 deg/min heating rate. The CO extrusion reaction of 2
started at 128 °C, and the resulting pentacene was stable to 300
C.
.
Comparison of TGA profiles of compounds 1 (blue) and
The yield of pentacene as photoproduct was further
analyzed in a more quantitative manner. In a prototypical
2
2
experiment, after using a 10 mW/cm 385 nm Nd:YAG
°
3
+
pumped Ti :Sapphire laser (LOTIS TII, LT-2211) to
illuminate the 3 mL degassed solution containing compound
-
4
2
(∼1.2 × 10 M in THF) under stirring for 40 s, it was
The pentacene thus produced was stable up to 300 °C. Its
observed that the absorbance at 575 nm increased from near
zero to ∼0.43, corresponding to an increase of ∼4.3 × 10
UV spectrum (Figure 1) showed a strong peak at 300 nm
-
5
1
(
B band for a π-π* transition), along with a characteristic
1
a
long-wavelength absorption at 576 nm ( L band). The
(
10) Jaff e´ , H. H.; Orchin, M. O. Theory and Application of UltraViolet
spectrum was identical with that obtained from authentic
Spectroscopy; John Wiley and Sons, Inc.: New York, 1962.
Org. Lett., Vol. 10, No. 13, 2008
2871

M of pentacene. By taking the ratio of the number of
pentacene being produced versus the number of photons
being absorbed by 2, the yield of pentacene production was
then calculated to be 16.2((1.0)%. As depicted in the inset
of Figure 3, we had also discovered that compound 1
underwent a similar photoinduced reaction when irradiated
7
at 310 nm in degassed THF. On the basis of a similar
photolysis protocol, the yield of pentacene from 1 was
estimated to be 12.6((0.6)%.
An OTFT application based on 2 was first investigated
by using the well-established thermolysis protocol. In this
approach, OTFT devices were fabricated by using a heavily
n-doped Si wafer as the substrate and the gate, and thermally
grown SiO
2
of 2000 Å as a gate insulator. On top of the
SiO , gold was deposited lithographically to 30 nm as the
2
Figure 4
generated pentacene: (a) plot of drain current I
voltage VDS at various gate voltages VGS; (b) plot of log(I
.
Plots a and b are for an OTFT device made of thermal
D
versus drain-source
source and drain electrodes. Compound 2 was spin-coated
repeatedly over the structure to form a thin film, which was
then heated to produce pentacene. The output characteristics
of a device with a channel width of 20 cm and a length of
D
) vs VGS
vs VGS in the saturation region. Plots
1
/2
for VDS ) -80 V, and I
D
c and d are the corresponding ones for a device made of
photogenerated pentacene.
1
0 µm exhibited typical FET characteristics. The OTFT
operated in the p-type enhancement mode and exhibited a
saturation behavior. Plots of drain current (I ) versus drain-
In summary, a new soluble pentacene precursor 2 with a
nonsymmetrical molecular structure was synthesized in 40%
yield from the cycloaddition of a furan derivative 3 and a
benzonorbornadiene 4. Compound 2 expelled a unit of CO
upon heating at 128 °C or by UV irradiation (e.g., 366 nm)
to produce pentacene in high yield. A thin film of pentacene
can be prepared by spin-coating the precursors, followed by
CO expulsion. The devices thus prepared displayed effective
FET characteristics. Although the solubility of precursors
can be doubled by mixing 1 and 2 together, an entirely
contiguous thin film of pentacene is still hard to obtain. Work
focusing on improvements of film morphology and the
performance of OTFT devices is currently in progress.
D
source voltage (VDS) at various gate voltages (VGS) are shown
in Figure 4a. The corresponding transfer characteristics, i.e.,
1
/2
log(I
D
) vs VGS for VDS ) -80 V, and I
D
vs VGS in the
saturation mode, are shown in Figure 4b. The OTFT
4
exhibited an on/off current ratio of about 1.48 × 10 , and
the apparent field-effect mobility was estimated to be 1.3 ×
-
3
2
-1 -1
1
0
cm V s . This relatively low field mobility value
may be ascribed to the limited solubility of 2, which
hampered the formation of an entirely contiguous thin film
of pentacene between the source and the drain.
The photochemical conversions of 1 and 2 to pentacene
films for OTFT devices were examined in a separate
experiment. A solution containing an equal molar ratio of 1
and 2 was spin-coated into a film, which was then irradiated
with UV light in a drybox at ambient temperature to
transform the precursors into pentacene. The resulting device
Acknowledgment. Financial support from the National
Science Council (Taiwan) and a theme project of Academia
Sinica is gratefully acknowledged.
Supporting Information Available: Synthetic experi-
mental and characterization data for compounds 2 and 5-10.
This material is available free of charge via the Internet at
http://pubs.acs.org.
-
3
2
-1 -1
exhibited a field-effect mobility of 2.4 × 10 cm V
s
4
and an on/off current ratio of 3.96 × 10 (Figure 4c,d), which
indeed was better than that with thermally generated penta-
cene. This is the first report on the photogeneration of
pentacene that was used successfully on OTFT devices.
OL8010419
2872
Org. Lett., Vol. 10, No. 13, 2008