An improved synthesis of pentacene: Rapid access to a benchmark organic semiconductor

Article abstract of DOI:10.3390/molecules17044625

Pentacene is an organic semiconductor used in a variety of thin-film organic electronic devices. Although at least six separate syntheses of pentacene are known (two from dihydropentacenes, two from 6,13-pentacenedione and two from 6,13-dihydro-6,13-dihydroxypentacene), none is ideal and several utilize elevated temperatures that may facilitate the oxidation of pentacene as it is produced. Here, we present a fast (~2 min of reaction time), simple, high-yielding (≥90%), low temperature synthesis of pentacene from readily available 6,13-dihydro-6,13-dihydroxypentacene. Further, we discuss the mechanism of this highly efficient reaction. With this improved synthesis, researchers gain rapid, affordable access to high purity pentacene in excellent yield and without the need for a time consuming sublimation.

Full text of DOI:10.3390/molecules17044625

Molecules 2012, 17, 4625-4633; doi:10.3390/molecules17044625
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
An Improved Synthesis of Pentacene:
Rapid Access to a Benchmark Organic Semiconductor
Chandrani Pramanik and Glen P. Miller *
Department of Chemistry & Materials Science Program, University of New Hampshire, Durham,
NH 03824, USA
*
Author to whom correspondence should be addressed; E-Mail: glen.miller@unh.edu;
Tel.: +1-603-862-2456; Fax: +1-603-862-4278.
Received: 20 March 2012; in revised form: 6 April 2012 / Accepted: 6 April 2012 /
Published: 20 April 2012
Abstract: Pentacene is an organic semiconductor used in a variety of thin-film organic
electronic devices. Although at least six separate syntheses of pentacene are known (two
from dihydropentacenes, two from 6,13-pentacenedione and two from 6,13-dihydro-6,13-
dihydroxypentacene), none is ideal and several utilize elevated temperatures that may
facilitate the oxidation of pentacene as it is produced. Here, we present a fast (~2 min of
reaction time), simple, high-yielding (≥90%), low temperature synthesis of pentacene
from readily available 6,13-dihydro-6,13-dihydroxypentacene. Further, we discuss the
mechanism of this highly efficient reaction. With this improved synthesis, researchers gain
rapid, affordable access to high purity pentacene in excellent yield and without the need for
a time consuming sublimation.
Keywords: pentacene; acene; semiconductor; organic semiconductor; thin-film;
organic electronic
1
. Introduction
Pentacene is a benchmark organic semiconductor in thin-film organic electronic devices due to its
π-conjugated electronic structure, its relatively low HOMO-LUMO gap and the relatively high charge
carrier mobility of its solid state films. It is the most common p-type organic semiconductor in OFET
devices [1–4]. Extensive efforts have also been carried out to integrate pentacene into OLED [5,6] and
photovoltaic [7–9] devices.

Molecules 2012, 17
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Pentacene is a deep blue, crystalline material of high reactivity that shows only sparing solubility in
organic solvents. Unlike anthracene or tetracene, pentacene cannot be isolated from petroleum; hence
it needs to be synthesized. The first synthesis of pentacene was accomplished by Clar in 1929 via a
transfer dehydrogenation of 6,13-dihydropentacene using phenanthraquinone [10,11]. Thus, successive
Friedel-Craft acylations of m-xylene using benzoyl chloride produces 1,3-dibenzoyl-4,6-dimethylbenzene.
With further heating in the presence of copper, the diketone is transformed to 6,13-dihydropentacene.
Transfer dehydrogenation in the presence of phenanthraquinone and nitrobenzene yields pentacene in
unreported yield (Scheme 1).
Scheme 1. The first synthesis of pentacene by Clar [10,11].
Similarly, pentacene has also been synthesized from 5,14-dihydropentacene via a transition metal
catalyzed dehydrogenation [12]. Thus, Hart reacted benzocyclobutene, an o-quinodimethane precursor,
with anthracene-1,4-endoxide (prepared in a multi-step synthesis involving 2,3-dihydronaphthalene
and furan) to produce a dihydropentacene hydrate that was dehydrated using HCl. A subsequent Pd/C
catalyzed dehydrogenation produced pentacene (59% overall yield from anthracene-1,4-endoxide,
Scheme 2).
Scheme 2. Synthesis of pentacene from 5,14-dihydropentacene via a Pd/C dehydrogenation [12].
A third synthetic pathway to pentacene involves reduction of pentacene-6,13-dione which is itself
synthesized via four successive aldol condensations involving two equivalents of o-phthalaldehyde and
one equivalent of 1,4-cyclohexanedione [13,14]. The reduction of pentacene-6,13-dione using either
Al in cyclohexanol (5–51% yield) [15] or an Al/HgCl
54% yield) [16] requires multiple days to produce pentacene (Scheme 3). The high boiling solvent
conditions may hasten pentacene degradation as it is produced. Moreover, CCl is a regulated chemical
2
mixture in cyclohexanol/carbon tetrachloride
(
4
2
and HgCl is highly toxic.

Molecules 2012, 17
Scheme 3. Synthesis of pentacene from pentacene-6,13-dione using a Hg-Al amalgam [16].
4627
A fourth procedure to prepare pentacene involves the reaction of pentacene-6,13-dione in dry,
boiling THF with LiAlH for 30 min followed by the addition of 6 M HCl and three additional hours of
4
boiling. This procedure yields a crude product mixture that reportedly includes pentacen-6(13H)-one
and its tautomer 6-hydroxypentacene. Following filtration and washing, the crude product is subjected
to the same reaction sequence a second time to produce pentacene in 54% overall
yield [17] (Scheme 4).
Scheme 4. Synthesis of pentacene from pentacene-6,13-dione using successive LiAlH4
reductions followed by acid dehydrations [17].
A fifth procedure to produce pentacene involves the reduction of 6,13-dihydro-6,13-
dihydroxypentacene using potassium iodide and sodium hyposulfite in boiling acetic acid for three
hours (Scheme 5). In this way, pentacene is produced in 67% yield [18].
Scheme 5. Synthesis of pentacene from pentacene-6,13-dihydro-6,13-diol using potassium
iodide, sodium hypophosphite and acetic acid [18].
Finally, a sixth procedure [19] involves a minimum of 30 minutes of stirring of 6,13-dihydro-6,13-
dihydroxypentacene and SnCl (1:2 mole ratio) in acetone. The resulting crude pentacene was isolated
2
by centrifuge, and washed with both methanol and THF. Purification by sublimation resulted in pure
pentacene in 60% overall yield from 6,13-dihydro-6,13-dihydroxypentacene.

Molecules 2012, 17
4628
This latter procedure is closest to the improved synthesis of pentacene reported here. However, the
improved synthesis utilizes HCl as co-reagent, involves shorter reaction times, does not require a
sublimation step, and results in a significantly higher overall yield.
2
. Results and Discussion
Here, we present a simple, fast, high-yielding (≥90%), scalable synthesis of pentacene under mild
conditions. Thus, 6,13-dihydro-6,13-dihydroxypentacene, readily prepared from pentacene-6,13-dione,
is reduced to pentacene at low temperature using SnCl and HCl in an appropriate solvent, preferably
2
DMF or acetone. The low temperature reaction is high yielding (e.g., 90–93% in DMF, Scheme 6) and
is complete within 1–2 min. The reaction begins working within seconds at 0 °C, but it is more
convenient to perform at or near RT. The work-up involves only simple filtration and washing with
water, acetone and hexane. The crude reaction product is high purity pentacene, a slightly soluble
blue solid that exhibits the expected molecular ion [m/z 278 by MALDI-MS (S matrix) and LDI-MS]
8
and UV-Vis max values [o-dichlorobenzene (ODCB) solvent: 501, 537, 582 nm], both in agreement
with the literature [17,20]. Most important, there is no mass spectrometry or UV-Vis evidence for
pentacene-6,13-dione in the reaction product, a consequence of the mild reaction conditions, short
reaction time, and simple work-up. In fact, the crude product prepared in this manner is identical to
a commercial sample purified by sublimation (TCI America, P0030) as determined by UV-Vis
analysis (Figure 1).
2
Scheme 6. Synthesis of pentacene from 6,13-dihydro-6,13-dihydroxypentacene using SnCl /HCl/DMF.
The use of HCl and the choice of solvent are both key to the success of the reaction. Thus, the SnCl
2
reduction of 6,13-dihydro-6,13-dihydroxypentacene is considerably faster in the presence of HCl,
regardless of choice of solvent. All eight solvents tested (i.e., DMF, DMSO, THF, dioxane, acetone,
CH CN, AcOH, MeOH) gave pentacene product but the rates and yields of reactions varied as a
3
function of solvent. Of these solvents, acetone and DMF gave very fast reactions (1–2 min) with high
yields (≥90%). In a typical synthesis, 6,13-dihydro-6,13-dihydroxypentacene is first dissolved in
2
appropriate solvent along with SnCl . Concentrated HCl is added leading to the instant formation of
blue pentacene, most of which immediately precipitates from solution. In acetone, a small amount of
pentacene is observed to form even before the HCl addition. In DMF, pentacene only forms after the
addition of HCl. The HCl addition is exothermic, especially in DMF. Thus, if the reaction is run on a
relatively large scale (e.g., ≥1 g), then the reaction vessel should be submerged in an ice-water bath.
For reactions run on a smaller scale, the reaction vessel can simply be submerged in a water bath held
at room temperature.

Molecules 2012, 17
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Figure 1. UV-Vis comparison of crude product prepared as described (black line) versus
a commercial pentacene sample (red line) that has been purified by sublimation
both samples prepared in o-dichlorobenzene solvent).
(
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
-0.01
300 350 400 450 500 550 600 650 700 750 800 850 900
Wavelength (nm)
Because 6,13-dihydro-6,13-dihydroxypentacene and SnCl
solvent is particularly appropriate for larger scale reactions. Thus, after dissolving 1.0 g of
,13-dihydro-6,13-dihydroxypentacene and 1.5 g of SnCl in 13 mL of DMF, 20 mL of concentrated
2
are both highly soluble in DMF, this
6
2
HCl was added over one minute with stirring while the reaction vessel was submerged in an ice water
bath at 0 °C. Following work-up, 0.83 g of pure pentacene was isolated (93% yield). We see no reason
why the DMF reaction could not be scaled several additional orders of magnitude, although
precautions would need to be taken to properly manage the heat associated with mixing larger
quantities of DMF and HCl (e.g., by submerging the reaction vessel in an ice-water bath).
We propose that the use of concentrated HCl facilitates the conversion of 6,13-dihydro-6,13-
dihydroxypentacene to pentacene via protonation of the hydroxyl groups followed by elimination of
water and formation of short-lived, electron deficient carbocations that act as electron acceptors
(
Scheme 7). It is also possible that HCl converts SnCl into a better electron donor species like, for
2
2
−
4−
example, SnCl
4
6
or SnCl . Additionally, we observe that pentacene precipitates much more rapidly
from solutions that contain HCl as opposed to those that do not. This is critical in that since solution
phase pentacene oxidizes much more rapidly than solid state pentacene. Thus, the use of HCl
minimizes the formation of a common oxidation by-product, pentacene-6,13-dione, and eliminates the
need to perform a rigorous, time consuming separation like a sublimation.

Molecules 2012, 17
4630
Scheme 7. Proposed mechanism for rapid formation of pentacene using a combination of
HCl and SnCl2.
H
H
OH
H
OH2
HCl
-H O
2
HO
H
HO
H
HO
H
SnCl2
H
H
H
H
-
H O
HCl
2
H O
2
H
HO
H
SnCl2
3
. Experimental
.1. General Experimental Methods
Reactions were performed in standard glassware except where noted. Mass spectra were recorded
3
on a Shimadzu Kratos Axima-CFR laser desorption ionization (LDI) time of flight (TOF) mass
spectrometer (Kyoto, Japan). UV-visible spectra were obtained on a Nikolet Evolution 300
spectrometer (Cambridge, UK) using 1 cm quartz cells. Saturated solutions of pentacene were prepared
using spectroscopic grade o-dichlorobenzene. NMR spectra were recorded using Varian Mercury 400 MHz
(
Palo Alto, CA, USA) and Varian Unity INOVA 500 MHz spectrometers (Palo Alto, CA, USA).
Chemicals were used as purchased without further purification: 1,4-cyclohexanedione (Aldrich),
o-phthalaldehyde (TCI America), NaOH (Alfa Aesar), NaBH (Sigma-Aldrich), SnCl (Sigma-Aldrich),
EtOH (Pharmaco-AAPER), THF (EMD Chemicals), CHCl (Fisher Scientific), o-dichlorobenzene
Acros Organics), concentrated HCl (EMD Chemicals).
4
2
3
(
Pentacene-6,13-dione. Aqueous NaOH (10%, 5.96 g, 149 mmol) was slowly added to a solution of
o-phthalaldehyde (10 g, 74.6 mmol) and 1,4-cyclohexanedione (4.18 g, 37.3 mmol) in ethanol
(
460 mL) under a N atmosphere. The solution turned from yellow to golden brown to dark brown
2
before a yellow solid corresponding to pentacene-6,13-dione precipitated. After stirring the reaction
mixture for four hours, the crude reaction mixture was filtered and washed with ethanol, water, and
methanol until the washings were colorless. The solid residue was dried under vacuum to obtain 11.02 g
1
(
96% yield) of bright yellow pentacene-6,13-dione. H-NMR (500 MHz, CDCl
3
) δ (ppm): 8.96

Molecules 2012, 17
4631
1
3
(
s, 4H), 8.14 (m, 4H), 7.72 (m, 4H); C-NMR (100 MHz, CDCl ): δ (ppm): 183.21, 135.45, 130.78,
3
+
+
+
1
30.31, 129.99, 129.67; LDI-MS m/z: [M +O], 324, [M +1] 309, [M ] 308.
6
4
,13-Dihydroxy-6,13-dihydropentacene. Solid NaBH (12.3 g, 324.4 mmol) was slowly added to a
1
,000 mL round bottom flask containing a suspension of pentacene-6,13-dione (5 g, 16.2 mmol) in
THF (600 mL) at 0 °C. After the addition of NaBH was complete, the reaction vessel was purged with
4
2
N and water (20 mL) was added. The reaction mixture was heated to 50 to 60 °C until homogeneous.
After two hours of heating, THF was evaporated at reduced pressure, water was added, and the
reaction mixture was filtered. The solids were washed with copious amounts of water followed by a
small amount of cold CHCl . After drying, 4.31 g of a white solid was recovered (85% yield)
3
1
consisting of only one diastereomer of 6,13-dihydroxy-6,13-dihydropentacene. H-NMR (500 MHz,
1
3
6
DMSO-d ): δ (ppm): 8.12 (s, 4H), 7.95 (m, 4H), 7.48 (m, 4H), 6.63 (s, 2H), 5.81 (s, 2H). C-NMR
(
125.68 MHz, DMSO-d
6
): δ (ppm): 138.19, 131.67, 127.47, 125.44, 120.91, 66.98. LDI-MS m/z:
+
+
+
3
12[M ], 295 [M -OH], 278 [M -2(OH)].
Pentacene. To a N
2
purged 200 mL round bottom flask was added 6,13-dihydroxy-6,13-
(1.44 g, 6.4 mmol) and DMF (15 mL). Twenty mL of
dihydropentacene (1 g, 3.2 mmol), SnCl
2
concentrated HCl was added over ~60 seconds with stirring at 0 °C (ice-water bath) in the dark. The
resulting blue reaction mixture was stirred briefly (~15 s) at 0 °C before an additional 20 mL of
concentrated HCl was added over ~30 seconds followed by the rapid addition of 100 mL water. The
deep blue solids were filtered and washed with copious amounts of water followed by acetone and
finally hexane. All washings were performed in air in the dark. The blue solid was dried under reduced
pressure yielding 0.83 g of pentacene (93% yield). LDI-MS m/z: 278[M+]. UV-Vis (ODCB): max
(
nm): 501, 537, 582; (CHCl
. Conclusions
We have demonstrated a simple, fast, high-yielding, scalable, low temperature synthesis of
3
) max (nm): 497, 533, 577.
4
pentacene from 6,13-dihydro-6,13-dihydroxypentacene. Following a simple work-up, crude product is
isolated and shown by mass spectrometry and UV-Vis spectroscopy to be of equal quality to
commercial samples that have been purified by sublimation. With this synthesis, researchers gain
rapid, affordable access to high purity pentacene in excellent yield and without the need for a time
consuming sublimation.
Supplementary Materials
Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/17/4625/s1.
Acknowledgments
This work was funded by the National Science Foundation through the Nanoscale Science &
Engineering Center for High-rate Nanomanufacturing (grant NSF EEC: 0832785). The authors also
acknowledge Edward Wong for helpful discussions, as well as former and current members of the

Molecules 2012, 17
4632
Miller research group that prepared pentacene under similar conditions, especially Irvinder Kaur,
Jeremy Kintigh and Lei Zhou.
References and Notes
1
2
.
.
Karl, N. Charge carrier transport in organic semiconductors. Synth. Met. 2003, 133, 649–657.
Yamashita, Y. Organic semiconductors for organic field-effect transistors. Sci. Technol. Adv. Mater.
2
009, 10, 1–9.
3.
4.
5.
6.
7.
Lin, Y.; Gundlach, D.J.; Nelson, S.F.; Jackson, T.N. Stacked pentacene layer organic thin-film
transistors with improved characteristics. IEEE Electron Device Lett. 1997, 18, 606–608.
Dimitrakopoulos, C.D.; Malenfant, P.R.L. Organic thin film transistors for large area electronics.
Adv. Mat. 2002, 14, 99–117.
Shi, S.; Ma, D. A pentacene-doped hole injection layer for organic light-emitting diodes.
Semicond. Sci. Technol. 2005, 20, 1213–1216.
Rao, M.V.M.; Huang, T.; Su, Y.; Huang, Y. Fullerene and pentacene as a pure organic connecting
layer in tandem organic light emitting devices. J. Electrochem. Soc. 2010, 157, H69–H71.
Rand, B.P.; Burk, D.P.; Forrest, S.R. Offset energies at organic semiconductor heterojunctions
and their influence on the open-circuit voltage of thin-film solar cells. Phys. Rev. B 2007, 75,
1
15327/1–115327/11.
8
9
1
1
1
.
Linares, M.; Beljonne, D.; Cornil, J.; Lancaster, K.; Brédas, J.; Verlaak, S.; Mityashin, A.;
Heremans, P.; Fuchs, A.; et al. On the interface dipole at the pentacene-fullerene heterojunction:
A theoretical study. J. Phys. Chem. C 2010, 114, 3215–3224.
.
Karak, S.; Ray, S.K.; Dhar, A. Improved photovoltaic properties of pentacene/N,N'-Dioctyl-
3
,4,9,10-perylenedicarboximide-based organic heterojunctions with thermal annealing.
Sol. Ener. Mater. Sol. Cells 2010, 94, 836–841.
0. Clar, E.; John, Fr. Polynuclear aromatic hydrocarbons and their derivatives. V.
Naphthoanthracenes, their oxidation products and a new class of deeply colored hydrocarbons.
Ber. Dtsch. Chem. Ges. 1929, 62, 3021–3029.
1. Clar, E.; John, Fr. Polynuclear aromatic hydrocarbons and their derivatives. VII. A new class of
deeply colored radical hydrocarbons and the supposed pentacene of E. Philippi; also a reply to
remarks of Roland Scholl and Oskar B.ovrddot.ottger. Ber. Dtsch. Chem. Ges. 1930, 63, 2967–2977.
2. Luo, J.; Hart, H. Linear acene derivatives. New routes to pentacene and naphthacene and the first
synthesis of a triptycene with two anthracene moieties. J. Org. Chem. 1987, 52, 4833–4836.
3. Ried, W.; Anthofer, F. Einfache synthese für pentacen-6,13-chinon. Angew. Chem. 1953, 65, 601.
4. Bruckner, V.; Tomasz, J. Weitere vereinfachung der pentacensynthese. Acta Chim. Acad. Sci. Hung.
1
1
1
961, 28, 405–408.
1
1
1
5. Bruckner, V.; Karczag, A.; kormendy, K.; Meszaros, M.; Tomasz, J. Einfache synthese des
pentacenes. Tetrahedron Lett. 1960, 1, 5–6.
6. Goodings, E.P.; Mitchard, D.A.; Owen, G. Synthesis, structure, and electrical properties of
naphthacene, pentacene, and hexacene sulphides. J. Chem. Soc. Perkin Trans. I 1972, 1310–1314.
7. Vets, N.; Smet, M.; Dehaen, W. Synthesis and thermolysis of a Diels-Alder adduct of pentacene
and thiophosgene. Tetrahedron Lett. 2004, 45, 7287–7289.

Molecules 2012, 17
4633
1
8. Sparfel, D.; Gobert, F.; Rigaudy, J. Transformations thermiques des photooxydes méso des
acénes—VI: Cas des photooxydes de pentacenes. Tetrahedron 1980, 36, 2225–2235.
9. Chi, X. Synthesis of acenes and hydroxy-acenes. U.S. Patent US 7,863,488 B2, 2011.
0. Kaur, I.; Jia, W.; Kopreski, R.P.; Selvarasah, S.; Dokmeci, M.R.; Pramanik, C.; McGruer, N.E.;
Miller, G.P. Substituent effects in pentacenes: Gaining control over HOMO-LUMO gaps and
photooxidative resistances. J. Am. Chem. Soc. 2008, 130, 16274–16286.
1
2
Sample Availability: Samples of pentacene prepared as described are available from the authors.
©
2012 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
http://creativecommons.org/licenses/by/3.0/).
(