Synthesis, crystal structure, and physical properties of 7, 14-disubstituted pentacene-5,12-diones containing a methylenequinoid structure

Article abstract of DOI:10.1021/ol9003838

Title 7, 14-disubsituted pentacene-5, 12-diones were prepared for the first time by a nucleophilic substitution reaction of a pentacene-5, 7, 12, 14- tetraone with aryl- or ethynyllithiums followed by a dehydroxylation reaction. The methylenequinoid struc

Full text of DOI:10.1021/ol9003838

ORGANIC
LETTERS
2009
Vol. 11, No. 8
1813-1816
Synthesis, Crystal Structure, and
Physical Properties of 7,14-Disubstituted
Pentacene-5,12-diones Containing a
Methylenequinoid Structure
Jun-ichi Nishida, Yujiro Fujiwara, and Yoshiro Yamashita*
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science
and Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku,
Yokohama 226-8502, Japan
yoshiro@echem.titech.ac.jp
Received February 23, 2009
ABSTRACT
Title 7,14-disubsituted pentacene-5,12-diones were prepared for the first time by a nucleophilic substitution reaction of a pentacene-5,7,12,14-
tetraone with aryl- or ethynyllithiums followed by a dehydroxylation reaction. The methylenequinoid structures with two conjugated carbonyl
groups were clearly observed in their crystal structures. They showed intense absorptions in the visible region and amphoteric redox properties
with high reduction potentials.
Extended π-conjugated compounds with quinoid systems
have attracted much attention because they have unique
electronic states and small HOMO-LUMO energy gaps.1-3
Methylenequinoid units can serve as useful building blocks
to afford electrochemically active compounds. For example,
when it is linked with electron-withdrawing groups such as
a cyano group, strong electron acceptors, tetracyanoquin-
odimethane (TCNQ) derivatives, can be obtained. Some of
the derivatives show good electron transportation in thin
films.4,5
On the other hand, fused aromatic compounds with
conjugated carbonyl groups are also important candidates
for affording n-type semiconductors because the carbonyl
groups introduced in the π-systems can strengthen the
electron-withdrawing properties leading to facile transporta-
tion of electrons.6 Bearing this in mind, when both the
quinoid unit and carbonyl groups are introduced to a
π-conjugated compound in a rigid system, the compounds
are expected to show novel properties such as high electron
affinity in addition to small HOMO-LUMO energy gaps.
In this paper, we have succeeded in preparing such unusual
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10.1021/ol9003838 CCC: $40.75
Published on Web 03/18/2009
2009 American Chemical Society

systems. Thus, we have found that 7,14-disubstituted pen-
tacene-5,12-dione derivatives 1-3 with a methylenequinoid
structure can be simply obtained from a nucleophilic
substitution reaction of pentacene-5,7,12,14-tetraones with
carbaniones.7-9 We have used three different types of
carbanions like phenyl7,8 or ethynyl9 or thienyl anions to
prepare the diones and investigated their features and physical
properties. We report here the synthesis, structures, intense
absorptions and strong electron-withdrawing properties at-
tributed to the methylenequinoid structure.
Scheme 1
A nucleophilic addition reaction of pentacene-5,7,12,14-
tetraone with 2.5 equiv of 2,6-difluorophenyllithium, triiso-
propylsilylethynyllithium (TIPSLi), or thienyllithium af-
forded the corresponding 7,14-disubstituted diols as shown
in Scheme 1. Dehydroxylation reaction using tin(II) chloride
under acidic conditions gave 7,14-disubstituted pentacene-
5,12-diones 1-3 in 52-67% yields. It is noteworthy that
even when more than 2 equiv (6 equiv) of 2,6-difluorinated
phenyllithium were used, the 7,14-disubstituted diol was
obtained as almost a sole product. For the formation of this
type of compounds, bulky anions are necessary. Thus, when
phenyllithium was used, only a tetrasubstituted derivative
was obtained. In the case of thiophene derivatives, alkyl
groups were introduced to increase the steric repulsion. While
dione 3b with hexyl groups at the 4-position of the thienyl
ring was isolated as a single product in this reaction, 3a with
3-hexyl groups was obtained as a mixture of rotational
regioisomers owing to the restricted free rotation of the C-C
bond between the thienyl ring and pentacenedione unit. These
compounds were purified by either sublimation or column
chromatography. They are stable in both the solid and
solution state in air.
Figure 1. (a) ORTEP drawing of dione 1 containing a methylene-
quinoid structure. (b) π-Stacking structure of 1 with close F-π
contacts. (c) Crystal structure of compound 2. (d) Overlapping mode
of 2. H atoms are omitted for clarification.
Single crystals of diones 1-2 were obtained by slow
sublimation. In dione 1, the pentacene-5,12-dione unit has a
planar geometry with a center of symmetry, and the two
ortho-difluorinated phenyl rings take orthogonal conforma-
tion10 to the planar dione unit affording two crystallographi-
(5) (a) Chesterfield, R. J.; Newman, C. R.; Pappenfus, T. M.; Ewbank,
P. C.; Haukaas, M. H.; Mann, K. R.; Miller, L. L.; Frisbie, C. D. AdV.
Mater. 2003, 15, 1278–1282. (b) Handa, S.; Miyazaki, E.; Takimiya, K.;
Kunugi, Y. J. Am. Chem. Soc. 2007, 129, 11684–11685. (c) Usta, H.;
Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 8580–8581.
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Org. Lett., Vol. 11, No. 8, 2009

cally independent molecules with different dihedral angles
(63.6° for molecule A and 71.0° for molecule B). In both
molecules, the methylenequinoid structures are clearly
observed in the central core in Figure 1a, where the C-C
bondlengths of C1-C11* (1.363 Å) and C9-C10 (1.384
Å) are shorter than that of C1-C10 (1.454 Å). The two
substituents at the 7 and 14 positions can effectively interact
with each other in the neutral state. Dione 1 forms a π-staking
structure with interplanar distances of 3.588 Å (molecule A)
and 3.546 Å (molecule B) with a close intermolecular contact
of 2.879 Å between the C atoms of the carbonyl groups and
F atoms of the fluorinated phenyl rings. On the other hand,
dione 2 containing two TIPS groups also has a planar
structure and the central methylenequinoid structure. A one-
dimentional π-stacking structure is observed in the crystal
with interplanar distances of 3.439 and 3.476 Å as shown in
Figure 1d. This stacking structure may be favorable for
transporting electrons.
absorption maxima and molar extinction coefficients in
dichloromethane are summarized in Table 1. The absorption
maximum of dione 1 appears at 578 nm with a large molar
extinction coefficient. These intense absorptions in the visible
region are considered to be due to charge transfer from the
quinoid unit to the carbonyl groups. Introduction of the
thienyl rings brings about red-shifts of the absorption maxima
to 598 nm in 3a and 600 nm in 3b. The TIPS groups in 2
induce a further red-shift of the absorption maximum to 636
nmowingtotheextendedπ-conjugation.TheHOMO-LUMO
energy gaps were estimated from the end-absorptions and
are listed in Table 1. These end-absorptions are somewhat
dependent on solvents, thus in polar solvents broad peaks
were observed with red-shifts (see the Supporting Informa-
tion). On the other hand, they showed red fluorescence,
whose quantum efficiencies in dichloromethane are shown
in Table 1. Among them, dione 1 with orthogonal difluori-
nated phenyl rings showed the highest efficiency (Φ ) 0.45).
The solvent effects on the fluorescence spectra were also
observed. In ethanol, emission peaks were significantly
weakened with red-shifts (see the Supporting Information).
Table 1. Optical Propertiesa of Diones 1-3
dione
λabs (nm (log ε))
λem (nm (Φf))b
λedge (eV)c
1
2
3a
3b
578 (4.84)
636 (4.75)
598 (4.59)
600 (4.65)
632 (0.45)
687 (0.18)
666 (0.01)
d
1.75
1.61
1.61
1.58
a In CH2CI2. b Determined by using rhodamine B (λex ) 535 nm, Φf )
0.97 in EtOH) as standard. c Optical HOMO-LUMO gaps determined from
the end-absorption in CH2CI2. d Very weak.
Table 2. Redox Potentialsa of Diones 1-3
dione
Ered1 (V)
Ered2 (V)
Epa (V)
Figure 2. Absorption spectra of compounds 1-3 in CH2Cl2.
1
2
3a
3b
-0.28
-0.21
-0.31
-0.31
-0.82
-0.73
-0.83
-0.80
+1.40b
+1.24b
+1.32
+1.06
These pentacene-5,12-diones 1-3 exhibited intense ab-
sorptions in the visible region as shown in Figure 2. The
a 0.1 M n-Bu4NPF6 in DMF, Pt electrode, scanning rate 100 mV s-1, V
versus SCE. b Determined from differential pulse voltammogram.
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These pentacene-5,12-diones 1-3 are expected to show
amphoteric redox properties with low LUMO levels. The
cyclic voltammograms were measured in DMF, and the
redox potentials are summarized in Table 2. All compounds
1-3 showed two clear reversible reduction waves. The first
reduction potential of 1 appeared at -0.27 V, which is
significantly high compared to those of indenofluorenediones
containing two conjugated carbonyl groups (-0.73 V vs
SCE).11 Introduction of the electron-donating thienyl groups
or the acetylene groups did not affect the reduction potentials
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Org. Lett., Vol. 11, No. 8, 2009
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significantly. This result suggests that the reduction potentials
are mainly determined by the pentacenedione core. On the
other hand, the oxidation peaks were observed as irreversible
waves in all of the compounds. The oxidation potentials are
more dependent on the substituents at the 7 and 14 positions
than the reduction potentials. Thus, the oxidation peak of
3b with 4-hexyl groups was observed at a lower potential
(+1.06 V) than that of 3a (+1.32 V) with 3-hexyl groups
as shown in Figure 3. This may be explained by considering
that the thienyl rings in 3b take a more planar geometry to
the core than those in 3a since the rotational regioisomer of
3b was not formed. These results suggest that the HOMO
levels are dependent on the substituents at the 7,14-positions.
vacuum (ca. 240 °C for 1 and 2: 10-6 pa).12 The FET
measurements were carried out in situ. The films of 1 and 2
exhibited n-type semiconducting behavior. The electron
mobilities (on/off ratio) were calculated to be 1.0 × 10-5
cm2V-1s-1 (150) for 1 and 3.4 × 10-5 cm2V-1s-1 (490) for
2, respectively.
In conclusion, we have succeeded in preparing 7,14-
disubstituted pentacene-5,12-diones possessing a methylene-
quinoid structure. The methylenequinoid structure combined
with carbonyl groups led to strong electron-withdrawing
properties and intense absorptions in the visible region. These
structures would be useful for novel n-type semiconductors
and donor-acceptor type chromophores.13 The elongation
of the π-conjugation by using the terminal positions of
thienyl derivatives 3 is currently under way to give new
donor-acceptor type compounds applicable for OPVs.14
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research from the Ministry of Educa-
tion, Culture, Sports, Science and Technology, Japan, and
Mizuho Foundation for the Promotion of Sciences. We also
thank Dr. M. Yamasaki (Rigaku Corp.) for help for the X-ray
structure analysis.
Supporting Information Available: Detailed experimen-
tal procedure, X-ray analysis of 1 and 2, solvent effects on
the optical properties, FET measurements, molecular orbital
calculations, and crystal data (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
OL9003838
(12) The SiO2 (300 nm) surface was treated with hexamethyldisilazane
(HMDS) and kept at rt. Threshold voltages were estimated at +13 V for 1
and +6 V for 2.
Figure 3. Cyclic voltammograms of compounds (a) 3a and (b) 3b.
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Tykwinski, R. R. Org. Lett. 2008, 10, 4163–4166.
Carrier mobilities in the films were estimated from the
field-effect transistor (FET) measurements. Thin films of
diones 1 and 2 were thermally deposited on an SiO2 dielectric
layer with interdigitated Au bottom electrodes at high
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Nguyen, T.-Q.; Dante, M.; Heeger, A. J. Science 2007, 317, 222–225. (c)
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