Photochemical synthesis of tetraaryl-substituted pentacenes

Article abstract of DOI:10.1016/j.tetlet.2010.01.009

The syntheses of 1,4,8,11-tetraphenylpentacene and 1,4,8,11-tetra(2′-thienyl)pentacene are described via photodecarbonylation of the corresponding α-diketone precursors.

Full text of DOI:10.1016/j.tetlet.2010.01.009

Tetrahedron Letters 51 (2010) 1397–1400
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Photochemical synthesis of tetraaryl-substituted pentacenes
a
a
Shuhei Katsuta a, Hiroko Yamada a,b, , Tetsuo Okujima , Hidemitsu Uno
*
a Department of Chemistry and Biology, Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan
b PRESTO, JST, Kawaguchi 332-0012, Japan
a r t i c l e i n f o
a b s t r a c t
The syntheses of 1,4,8,11-tetraphenylpentacene and 1,4,8,11-tetra(20-thienyl)pentacene are described via
Article history:
Received 19 November 2009
Revised 28 December 2009
Accepted 7 January 2010
Available online 13 January 2010
photodecarbonylation of the corresponding
a
-diketone precursors.
Ó 2010 Elsevier Ltd. All rights reserved.
Pentacene is one of the most fascinating molecules for organic
electronic devices because of its high carrier mobility.1,2 Various
pentacene-based organic field-effect transistors (OFETs) have been
reported to show carrier mobilities greater than 1.0 cm2 Vꢀ1 sꢀ1
value of which is comparable with that of amorphous silicon.3
The main drawbacks of pentacene, however, are its poor solubility
in common organic solvents and facile photodegradation in air.4
Introduction of substituents to pentacene increases the solubility
and stability efficiently.5 The most common method for the syn-
thesis of such substituted pentacenes is based on nucleophilic
addition of aryl-lithiums6 or magnesium bromides7 to pentacene
6,13-quinone followed by reductive aromatization. 6,13-Bis(trial-
kylsilylethynyl)-pentacenes8 and 6,13-connected pentacene oligo-
mers9 were successfully obtained and their optical or OFET
properties were exemplified. Although various synthetic methods
for substituted pentacenes have been reported,10 a few methods
for the synthesis of 1,4,8,11-substituted pentacenes have also been
reported.11 This may be due to the inaccessibility of pentacene
1,4,8,11-diquinone.12 Miller et al. reported the formation of
1,4,8,11-tetraphenylpentacene by the reduction of the correspond-
ing quinone, which was, in turn, prepared by the Diels–Alder reac-
tion of benzoquinone with 4,7-diphenylbenzo[c]furan.13 In our
previous studies, we reported the synthesis of pentacene from its
diketone precursor (Scheme 1, 1c to 2c)14 by application of the
photodecarbonylation of dicarbonyl-bridged anthracene.15 We
successfully applied this conversion reaction for solution-pro-
cessed device-fabrication of OFET.16 Recently, Neckers’ group has
reported to prepare the substituted pentacenes and higher acenes
by this method.17 This method is based on the Diels–Alder reaction
of 5,6,7,8-tetramethylenebicyclo[2.2.2]oct-2-ene with arynes. We
planned to prepare soluble pentacene derivatives by the applica-
tion of this preparation method. In this Letter, we report the syn-
thesis of diketone precursors of 1,4,8,11-tetraarylpentacene and
their photochemical conversion.
The synthesis of the diketone precursors is shown in Scheme 2.
3,6-Dibromophthalic anhydride18 (3) was converted to N-hydroxy-
3,6-dibromophthalimide (4) quantitatively and then tosylated to
N-tosyl-3,6-dibromophthalimide (5) with tosyl chloride and trieth-
ylamine in 95% yield. Lossen rearrangement of 5 with NaOH
cleanly afforded 3,6-dibromoanthranilic acid (6) in 85% yield.
Esterification of 6 with diazomethane followed by Suzuki–Miyaura
coupling reaction with phenylboronic acid afforded 3,6-dipheny-
substituted anthranilic ester 8a in 90% yield. Hydrolysis of 8a with
TMSONa afforded anthranilic acid 9a in 90% yield.19 As previously
reported,14 the Diels–Alder reaction of an aryne generated in situ
from 9a with 5,6,7,8-tetramethylenebicyclo[2.2.2]oct-2-ene fol-
lowed by oxidation with DDQ gave tetraphenylethenopentacene
10a in 54% yield. OsO4-mediated dihydroxylation afforded tetraph-
enylethanopentacene diol 11a in 73% yield. Swern oxidation of 11a
afforded the phenyl-substituted diketone precursor 1a in 45%
yield. In a similar manner, 1b was obtained in 22% overall yield
from 8b.
The UV–vis absorption spectra of diketone precursors 1a and 1b
in toluene are shown in Figure 1. The precursors show characteris-
*
tic weak and broad n–p absorptions at ca. 460 nm. The photoreac-
tions of diketone precursors to pentacenes were performed. The
solution of diketone precursor in toluene was degassed by Ar bub-
bling for 20 min in the dark and then the solution was irradiated
with a blue LED. The change in the absorption spectra was mea-
sured at an interval of 30 s during photoreaction.
O
O
11R
8R
R1
13
R
hv
R
in solution
or thin film
R
R4
6
R
diketone precursor
pentacene
1a (R = Ph)
1b (R = 2-thienyl), 1c (R = H)
2a - 2c
* Corresponding author. Tel.: +81 89 927 9613; fax: +81 89 927 9613.
E-mail address: yamada@chem.sci.ehime-u.ac.jp (H. Yamada).
Scheme 1. Photochemical synthesis of pentacenes.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.01.009

1398
S. Katsuta et al. / Tetrahedron Letters 51 (2010) 1397–1400
Ar
Ar
Ar
Ar
Br
Br
Br
Br
Br
Br
Br
Br
O
O
O
e
a
b
c
NH2
f
NH2
NH2
O
N OH
N OTs
COOMe
COOH
COOR
O
O
O
6 (R = H)
d
9a, 9b
8a (Ar = Ph)
3
4
5
7 (R = Me)
8b (Ar = 2-thienyl)
O
OH
OH
O
g
Ar
Ar
Ar
h
Ar
Ar
9a or 9b Ar
i
Ar
Ar
Ar
Ar
Ar
Ar
10a, 10b
11a, 11b
1a (Ar = Ph)
1b (Ar = 2-thienyl)
O
O
Ph
Ph
Ph
12
Ph
Scheme 2. Reagents and conditions: (a) NH2OHꢁHCl, dry-pyridine, rt to 95 °C, 5 h; quant; (b) TsCl, Et3N, THF, rt, 1 h; 95%; (c) NaOH, THF, H2O, 60 °C, 3 h; 80%; (d) DiazaldÒ
,
KOH, THF, 0 °C to rt, overnight; quant (e) ArB(OH)2, Na2CO3, Pd(PPh3)4, DME, H2O, reflux; 8a: 90%; 8b: 92%; (f) TMSONa (in THF), dry-THF, reflux, overnight; 9a: 90%; 9b: 94%;
(g) isoamyl nitrite, dry-THF, 60 °C to reflux, 1.5 h; DDQ, dry-CH2Cl2, rt, overnight; 10a: 54%; 10b: 58%; (h) OsO4, NMO, THF, rt, overnight; 11a: 73%; 11b: 63%; (i) dry-DMSO,
TFAA, Et3N, dry-CH2Cl2, ꢀ60 °C, 1.5 h; 1a: 45%; 1b: 63%.
Figure 1. UV–vis spectra of 1a (blue line) and 1b (red line) in toluene.
The change of UV–vis spectra during the photoreaction of 1a
under Ar or O2 is shown in Figure 2, and the time profiles of the
change are shown in Figure S1. Before irradiation, only the broad
*
n–p peak at 465 nm was observed. As this peak decreased gradu-
ally,the new peaks at 480, 515, 554, and 600 nm assigned to tetra-
phenylpentacene 2a increased. According to the progress, the color
of the solution was changed from yellow to purple. The absorbance
of pentacene became constant after 60-min irradiation. Similarly,
diketone precursor 1b was converted to tetra(20-thienyl)pentacene
2b as shown in Figure S2. The solubility of 2a and 2b is improved
compared to that of 2c, because precipitation of pentacenes was
not observed during the photoirradiation under these condi-
tions.14b The reactivity of the obtained pentacenes with oxygen is
very high since they do not have substituents at 6 and 13-positions
(Fig. S3).
Absorption and fluorescence spectra of the obtained pentacenes
2a and 2b in toluene are shown in Figure 3. The absorbance max-
imums of pentacenes were red shifted to 23 nm for 2a and 32 nm
Figure 2. UV–vis spectra during the photoreaction of 1a in toluene (a) under Ar
(4.5 ꢂ 10ꢀ4 M) or (b) O2 (3.6 ꢂ 10ꢀ4 M).

S. Katsuta et al. / Tetrahedron Letters 51 (2010) 1397–1400
1399
Figure 5. 1H NMR spectra during the photoreaction of 1a in CDCl3 under O2: Top:
before irradiation; Middle: during irradiation; Bottom: after irradiation.
When the photoreaction of diketone precursor 1a was con-
ducted in toluene under O2, peaks at 465 nm gradually decreased
in the same way but peaks for pentacene at 554 and 600 nm first
appeared and then decreased rapidly due to the reaction of the
formed pentacene with oxygen giving 6,13-endoperoxide penta-
cene (Figs. 2b and S1).20
The photoreactions were further monitored by 1H NMR spec-
troscopy under either Ar or O2 condition. Diketone precursor 1a
was placed in degassed CDCl3 and was irradiated with blue LED
under Ar. During the photoreaction, the singlet peak at 5.08 ppm
due to bridgehead protons of 1a gradually decreased while new
singlet peaks due to pentacene 2a emerged at 8.74, 8.67, and
7.30 ppm (Fig. 4). The new singlet peaks correspond to protons
on the 6, 13 carbons, 5, 7, 12, 14 carbons, and 2, 3, 9, 10 (peri)
carbons of 2a, respectively. The TOF-MS spectrum of the reaction
is shown in Figure S4. The parent peak of pentacene 2a was
observed at 582. The peak was also observed at 1164, which corre-
sponded to a pentacene dimer. When the photoreaction was per-
formed under O2 conditions, the diketone singlet at 5.08 ppm
decreased with the emergence of new singlets at 7.90 and
6.05 ppm assignable to the peri and bridgehead protons of penta-
cene 6,13-endoperoxide 12 (Fig. 5).17d,e
Figure 3. Normalized UV–vis spectra (red lines) and fluorescence spectra (blue
lines) of 2a (a) and 2b (b) in toluene.
In summary, the first syntheses of 1,4,8,11-tetraphenylpenta-
cene (2a) and 1,4,8,11-tetra(20-thienyl)pentacene (2b) are described
using a photodecarbonylation method. The newly synthesized pen-
tacenes are difficult to prepare by conventional methods. They were
characterized by UV–vis, fluorescence, and 1H NMR spectroscopy.
The 1,4,8,11 substitution pattern will influence molecular packing
in crystalline thin films making these molecules interesting candi-
dates for further electronic characterization. Preparation and evalu-
ation of solution-processed devices are under way and will be
reported elsewhere.
Acknowledgments
We thank Division of Synthesis and Analysis, Department of
Molecular Science, Integrated Center for Sciences (INCS), Ehime
University for its help in using mass and NMR spectrometer. We
also thank Venture Business Laboratory, Ehime University, for its
help in using TOF-MS spectroscopy. This work was partially sup-
ported by Grants-in-Aid for the Scientific Researches on Innovative
Figure 4. 1H NMR spectra during the photoreaction of 1a in CDCl3 under Ar: Top:
before irradiation; Middle: during irradiation; Bottom: after irradiation.
for 2b in comparison with the unsubstituted pentacene 2c. The
absolute fluorescence quantum yields of pentacenes were 0.19
for 2a and 0.16 for 2b.
Areas (No. 21108517, p-Space, H.U.) and C (No. 20550047 to H.U.)
from the Japanese Ministry of Education, Culture, Sports, Science
and Technology.

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Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.01.009.
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