Synthesis and characterization of electron-deficient pentacenes

Article abstract of DOI:10.1021/ol050872b

(Chemical Equation Presented) Halogen functional groups on pentacene can be used both as synthetic handles for further functionalization as well as to tune the π-stacking in these systems. The halogenated pentacene derivatives described here (X = Br, X′ =

Full text of DOI:10.1021/ol050872b

ORGANIC
LETTERS
2
005
Vol. 7, No. 15
163-3166
Synthesis and Characterization of
Electron-Deficient Pentacenes
3
†
†
†
,†
Christopher R. Swartz, Sean R. Parkin, Joseph E. Bullock, John E. Anthony,*
‡
‡
Alex C. Mayer, and George G. Malliaras
Department of Chemistry, UniVersity of Kentucky, Lexington, Kentucky 40506-0055,
and Department of Materials Science and Engineering, Cornell UniVersity,
Ithaca, New York 14853-1501
anthony@uky.edu
Received April 20, 2005
ABSTRACT
Halogen functional groups on pentacene can be used both as synthetic handles for further functionalization as well as to tune the
in these systems. The halogenated pentacene derivatives described here (X Br, X′ ) H, and X ′ ) F) are all stable and soluble, with
reduction potentials significantly lower than that of the parent functionalized pentacene (X ′ ) H). The bromopentacenes could be further
π-stacking
)
) X
)
X
elucidated to pentacene nitriles, further decreasing the acene’s reduction potential, while the charge-carrier mobility in the fluorinated systems
was shown to scale with the degree of fluorine substitution.
One key advantage to the use of organic materials in
electronics is the inherent “tunability” of synthetic organic
semiconductors. Functionalization of organic molecules used
in electronic devices can lead to dramatic changes in
solubility, stability, film-forming ability, and oxidation/
reduction potentials (and thus HOMO and LUMO energy
levels). For example, tetramethyl pentacene has a signifi-
cantly decreased oxidation potential vs pentacene, which may
the pentacene fragment lowered the oxidation potential while
maintaining the high stability typically associated with these
materials. These subtle redox changes led us to consider
3
affecting even more dramatic changes to the silylethynyl-
substituted pentacenes by functionalization. We were specif-
ically interested in the preparation of halogen-containing
derivatives of silylethynylated pentacenes for two reasons:
fluorine-containing pentacenes will provide the ability to alter
1
4
lead to improved charge injection in electronic devices.
π-stacking by exploiting aryl-fluoroaryl interactions, while
Conversely, perfluoropentacene was recently prepared, and
the fluorine substitution altered the LUMO energy level
sufficiently to make this compound an n-type semiconductor.
bromopentacenes will allow the further functionalization of
the pentacene backbone using palladium-based coupling
reactions.
2
The typical approach to most pentacene derivatives
involves the formation of pentacenequinones by an efficient
In our work on silylethynyl-functionalized pentacenes, we
found that the addition of cyclic ether groups to the ends of
(
3) Payne, M. M.; Delcamp, J. H.; Parkin, S. R.; Anthony, J. E. Org.
†
University of Kentucky.
Cornell University.
Lett. 2004, 6, 3325.
‡
(4) See for example: (a) Weck, M.; Dunn, A. R.; Matsumoto, K.; Coates,
G. W.; Lobkovsky, E. B.; Grubbs, R. H. Angew. Chem., Int. Ed. 1999, 38,
2741. (b) Ponzini, F.; Zagha, R.; Hardcastle, K.; Siegel, J. S. Angew. Chem.,
Int. Ed. 2000, 39, 2323. (c) Gillard, R. E.; Stoddart, J. F.; White, A. J. P.;
Williams, B. J.; Williams, D. J. J. Org. Chem. 1996, 61, 4504.
(
1) Meng, H.; Bendikov, M.; Mitchell, G.; Helgeson, R.; Wudl, F.; Bao,
Z.; Siegrist, C.; Kloc, C.-H. AdV. Mater. 2003, 15, 1090.
2) Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Fukai, Y.; Inoue,
Y.; Sato, F.; Tokito, S. J. Am. Chem. Soc. 2004, 126, 8138.
(
1
0.1021/ol050872b CCC: $30.25
© 2005 American Chemical Society
Published on Web 06/30/2005

4
-fold aldol condensation between a phthalaldehyde and 1,4-
tetrabromo quinone 3 was not high (29%), the low cost of
the starting materials and the ease of isolation of this
insoluble compound facilitate its preparation in significant
quantity. The synthesis of the fluorinated pentacenes begins
5
cyclohexanedione, followed by further elaboration of the
quinone to the desired pentacene. For example, Wudl and
co-workers have exploited this approach to form both
tetrachloro and tetramethyl pentacene, and we have used
this method for the formation of the aforementioned penta-
6
1
3
cene ethers. Unfortunately, we had no success in applying
Scheme 2. Synthesis of Fluoropentacenes
this route to the formation of either bromo- or fluoropenta-
cenequinones. In the case of the bromine derivative, the
requisite phthalaldehyde was easily prepared but formed the
corresponding pentacenequinone in very poor yield. In the
case of the fluorinated derivative, we were unable to prepare
the requisite phthalaldehyde.
An alternative approach to pentacenequinones utilizes the
Cava reaction, which converts R,R,R′,R′-tetrabromo o-
xylenes to o-quinodimethanes in situ by elimination of Br
2
7
with sodium or potassium iodide as a nucleophile. The
quinodimethanes react rapidly with terminal quinones (e.g.,
benzoquinone, 1,4-anthraquinone) and aromatize to yield
internal quinones. While the yield of this reaction is often
only moderate, the reaction is simple, and the conditions
tolerate a wide array of functionality. Further, application
of this reaction to commercially available 1,4-anthraquinone
allows the preparation of pentacene derivatives substituted
on only one end.
Starting with readily available 3,4-dibromo-R,R,R′,R′-
tetrabromo-o-xylene 1,8 reaction with either 1,4-benzo-
quinone or 1,4-anthraquinone in a deoxygenated NaI/DMA
solution yielded the desired dibromo- and tetrabromopenta-
cenequinones 2 and 3 (Scheme 1). While the yield of the
Scheme 1. Synthesis of Bromopentacenes
with 2,3,4,5-tetrafluoro phthalic anhydride, which is con-
9
verted to dimethanol 7 following literature procedures and
then elaborated to the corresponding bromomethyl compound
8
in excellent yield. Treating 8 with KI in the presence of
1
,4-anthraquinone at 110 °C yielded tetrafluoropentacene-
quinone 9 in a remarkable 75% yield. A similar treatment
of 8 in the presence of 1,4-benzoquinone did not provide
the desired pentacenequinone, yielding only hydroquinone
10 in 60% yield. Fortunately, this material converted rapidly
(
5) Ried, W.; Anth o¨ fer, F. Angew. Chem. 1953, 65, 601.
(6) Perepichka, D. F.; Bendikov, M.; Meng, H.; Wudl, F. J. Am. Chem.
Soc. 2003, 125, 10190.
7) (a) Cava, M. P.; Shirley, R. L. J. Am. Chem. Soc. 1960, 82, 654. (b)
(
Cava, M. P.; Napier, D. R. J. Am. Chem. Soc. 1957, 79, 1701. For the use
of this reaction in the synthesis of pentacenequinone, see: Miller, G. P.;
Mack, J.; Briggs, J. Proc. Electrochem. Soc. 2001, 11, 202.
(
8) Anthony, I. J.; Wege, D. Aust. J. Chem. 1996, 49, 1263.
(9) Coe, P. L.; Croll, B. T.; Patrick, C. R. Tetrahedron 1967, 23, 505.
3164
Org. Lett., Vol. 7, No. 15, 2005

to anthraquinone 11 on exposure to air; a second Cava
reaction between 11 and 8 yielded the octafluoropentacene-
quinone 12 in 23% yield. Quinones 2, 3, 9, and 12 were all
easily converted to the corresponding triisopropylsilyleth-
ynyl-substituted pentacenes using a standard ethynylation/
Table 1. Redox Properties of Functionalized Pentacenes (All
Values vs (F /F ))
c c
+
a
compound
Ered 1 (mV)
Ered 2 (mV)
Eox (mV)
10
4
5
-1339
-1284
-1303
-1200
-1144
-895
-1838
-1786
-1803
-1715
-1633
-1429
-2267
428
575
598
739
648
954
380
deoxygenation approach. The resulting blue compounds 4,
, 13, and 14 were all stable both in solution and in the solid
5
13
14
15
16
17
state, even when exposed to air and laboratory lighting, and
were easily purified by recrystallization. To explore the redox
-1722
Scheme 3. Synthesis of Pentacene Nitriles
a
Performed in a 0.1 M solution of Bu4NPF6 in tetrahydrofuran using a
Pt electrode, a scan rate of 150 mV/s, and ferrocene as an internal reference.
nm red shift for 13 and 14 compared to 17 and a ∼35 nm
red shift for 15 and 16 compared to 17, UV-vis spectra
presented in Supporting Information), the significant increase
in oxidation potential is accompanied by a similar shift in
reduction potential. All of these substituted pentacenes have
first reductions well below that of 17, with the most
significant shift occurring for the pentacene nitriles: tetra-
cyano TIPS pentacene 16 undergoes a first reduction at a
+
potential of -0.895 V (vs F
c
/F
c
). Such low reduction
potentials in a pentacene derivative may make these penta-
cenenitriles viable candidates as n-type semiconductors.
Our work with functionalized pentacene has shown that a
two-dimensional, π-stacked arrangement in the solid state
1
4
tuning of functionalized pentacene, we wished to substitute
strongly electron-withdrawing groups for the bromine func-
tional groups of 4 and 5. Recent studies of perylene systems
showed that the addition of nitrile groups to an aromatic core
significantly alters the LUMO levels of the material, leading
leads to the highest charge-carrier mobility. We thus
subjected derivatives 13-16 to single-crystal X-ray crystal-
lographic analysis to determine their solid-state arrangements,
1
5
which are represented in Figure 1. Derivatives 13 and 15
were disordered by virtue of their sitting on sites of
crystallographic inversion and pseudoinversion, respectively.
Thus, in Figure 1, the arrangements shown for 13 and
especially for 15 likely represent local ordering. Indeed, we
suspect that 15 may actually be twinned but with domain
sizes too small to be distinguished. In any event, no
satisfactory twin model could be found, whereas the disorder
model refined well. The crystallographic results do show that
while tetracyano 16 adopts a one-dimensional stacking
arrangement, the dicyano compound 15 and both fluorinated
acenes 13 and 14 adopt two-dimensional, π-stacked arrange-
ments very similar to 17 (Figure 1, bottom). Closer inspection
of the packing reveals strong interaction between the
π-surfaces of the fluorinated acenes, likely caused by
interaction between the fluorinated and nonfluorinated rings,
leading to an overall decrease in the spacing between
pentacene planes. Compared to the nonfluorinated 17, with
an average interplanar spacing of 3.43 Å, the tetrafluoro
compound 13 has an average interplanar spacing of 3.36 Å,
while octafluoro 14 has a spacing of 3.28 Å.
11
to facile reduction. A convenient protocol for the conversion
of aryl halides to aryl nitriles involves heating the halide in
DMF with excess CuCN.12 Attempts to perform this substi-
tution on bromopentacenes 4 or 5 led only to decomposition
of the aromatic compounds. There have been several reports
of the conversion of aryl bromides to nitriles mediated by a
palladium catalyst, and these reactions typically occur under
relatively mild conditions.13 In the case of the bromopenta-
cenes, palladium-mediated coupling with CuCN yielded the
desired pentacene nitriles 15 and 16 in reasonable yield.
Electrochemical analysis of pentacenes 4, 5, and 13-16
showed one reversible oxidation and two reversible reduction
waves in the potential window scanned ((1.8 V). In all cases,
these substituted pentacenes had significantly higher oxida-
tion potentials than the parent functionalized acene, 6,13-
bis(triisopropylsilylethynyl)pentacene 17. Because there is
only a small change in the HOMO-LUMO gap in these
substituted materials compared to 17 (as evidenced by a ∼7
(10) (a) Funk, R. L.; Young, E. R. R.; Williams, R. M.; Flanagan, M.
It has been shown that charge-carrier mobility is strongly
dependent on the spacing between aromatic faces in the
F.; Cecil, T. L. J. Am. Chem. Soc. 1996, 118, 3291. (b) Miller, G. P.; Mack,
J.; Briggs, J. Org. Lett. 2000, 2, 3983.
(11) Jones, B. A.; Ahrens, M. J.; Yoon, M.-H.; Facchetti, A.; Marks, T.;
Wasielewski, M. R. Angew. Chem., Int. Ed. 2004, 43, 6363.
(
12) (a) Eberhardt, W.; Hanack, M. Synthesis 1998, 1760.
13) (a) Sakamoto, T.; Ohsawa, K. J. Chem. Soc., Perkin Trans. 1 1999,
(14) Sheraw, C. D.; Jackson, T. N.; Eaton, D. L.; Anthony, J. E. AdV.
Mater. 2003, 15, 2009.
(
2
1
323. (b) Zanon, J.; Klapars, A.; Buchwald, S. L. J. Am. Chem. Soc. 2003,
25, 2890.
(15) Crystallography was also performed on bromopentacenes 4 and 5;
the structural data is presented in Supporting Information.
Org. Lett., Vol. 7, No. 15, 2005
3165

Because optimized deposition conditions would likely be
vastly different for the compounds 13, 14, and 17, it was
decided to compare the field-effect mobilities of all three
derivatives under identical deposition conditions. We evapo-
rated these compounds onto oxidized silicon wafers (the
2
thermally grown SiO served as the gate dielectric) and
deposited Au top contacts, defining channels with a width
of 1 mm and a legnth of 75 µm. The substrates were held at
room temperature. Under these conditions, the parent TIPS
2
pentacene 17 exhibited a hole mobility of 0.001 cm /Vs
2
(
compared with 0.4 cm /Vs under optimized deposition
1
4
conditions ). As expected, the tetrafluoro pentacene 13
showed a significantly higher mobility (0.014 cm /Vs) when
2
deposited under these same conditions, while octafluoro
pentacene 14, the molecule in the series with the smallest
interplanar spacing, yielded devices with a hole mobility of
2
0
.045 cm /Vs. Thus, the field-effect hole mobility scales with
the π-face separation in the acenes, which in turn is
controlled by the degree of fluorine substitution.
We have shown here that the Cava reaction is a viable
alternative to the more common 4-fold aldol condensation
for the preparation of halogenated pentacenequinones. The
addition of bromine functional “handles” allows further
manipulation of the substitution on the pentacene, leading
to the ability to tune the electronic properties of the molecule.
Finally, by exploiting aryl-perfluoroaryl interactions, it is
possible to alter the interplanar spacing between acenes in
the solid state and thus improve charge-carrier mobility.
Optimization of deposition conditions for the fluorinated
pentacenes, as well as device studies of pentacene nitriles,
is currently underway.
Figure 1. Solid-state arrangement of 13 (top) and 17 (bottom).
Frontmost silyl groups have been removed for clarity.
Acknowledgment. The authors are grateful to the Office
of Naval Research for financial support.
16
solid, leading to some elegant studies on methods to control
this parameter.17 We were thus led to measure the hole
mobility of 13, 14, and 17 in a thin-film transistor config-
uration.
Supporting Information Available: Experimental details
and crystallographic CIF files. This material is available free
of charge via the Internet at http://pubs.acs.org.
OL050872B
(
16) For a detailed theoretical treatment, see: Br e´ das, J. L.; Calbert, J.
P.; da Silva Filho, D. A.; Cornil, J. Proc. Nat. Acad. Sci. U.S.A. 2002, 99,
804.
(17) Miao, Q.; Lefenfeld, M.; Nguyen, T.-Q.; Siegrist, T.; Kloc, C.;
Nuckolls, C. AdV. Mater. 2005, 17, 407.
5
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