6,13-Dibromopentacene [2,3:9,10]-Bis(dicarboximide): A versatile building block for stable pentacene derivatives

Article abstract of DOI:10.1021/ol102971a

6,13-Dibromopentacene [2,3:9,10]-bis(dicarboximide) (1) was synthesized for the first time by using in situ generated benzo[1,2-c:4,5-c′]difuran as a key intermediate. Compound 1 exhibits good photostability in comparison to other pentacene derivatives an

Full text of DOI:10.1021/ol102971a

ORGANIC
LETTERS
2011
Vol. 13, No. 5
924–927
6,13-Dibromopentacene [2,3:9,10]-
Bis(dicarboximide): A Versatile Building
Block for Stable Pentacene Derivatives
Hemi Qu, Weibin Cui, Jinling Li, Jinjun Shao, and Chunyan Chi*
Department of Chemistry, National University of Singapore, 3 Science Drive 3,
Singapore, 117543
chmcc@nus.edu.sg
Received December 8, 2010
ABSTRACT
6,13-Dibromopentacene [2,3:9,10]-bis(dicarboximide) (1) was synthesized for the first time by using in situ generated benzo[1,2-c:4,5-c0]difuran as
a key intermediate. Compound 1 exhibits good photostability in comparison to other pentacene derivatives and it can be further functionalized by
Pd-catalyzed coupling reactions to give a series of soluble and stable functional pentacenes.
Acenes, linear fused aromatic hydrocarbons, have been
the subject of extensive studies due to their potential applica-
tions in organic electronics.1 Pentacene, a benchmark mole-
cule in organic field effect transistors, represents the fourth
member of the acene family. Whereas pentacene without any
substituent is unstable under ambient condition and in-
soluble in a variety of organic solvents, chemical modifica-
tion to introduce substituents has been developed to pre-
pare its stable and soluble derivatives.2 A typical approach
toward most functional pentacene derivatives involves the
functionalization of pentacenequinones, i.e., a pentacene-
quinone is treated with aryl or alkyne lithium or Grignard
reagent followed by aromatization of the as-formed diol.3-5
In addition, other strategies such as the ZnI2-mediated reac-
tion of pentacene-6,13-diol with thiols followed by oxidative
dehydrogenation,6 Diels-Alder cycloaddition between iso-
benzofuran derivatives and p-benzoquinone, followed by
reductive aromatization7 and zirconocene-mediated cycliza-
tion of diynes8 were also reported.
(4) (a) Kaur, I.; Jia, W.; Kopreski, R. P.; Selvarasah, S.; Dokmeci,
M. R.; Pramanik, C.; McGruer, N. E.; Miller, G. P. J. Am. Chem. Soc.
2008, 130, 16274. (b) Lehnherr, D.; Murray, A.; McDonald, R.; Tyk-
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McDonald, R.; Tykwinski, R. R. Org. Lett. 2008, 10, 4163. (d) Lehnherr,
D.; Gao, J.; Hegmann, F. A.; Tykwinski, R. R. Org. Lett. 2008, 10, 4779.
(e) Meng, H.; Bendikov, M.; Mitchell, G.; Helgeson, R.; Wudl, F.; Bao,
Z.; Siegrist, T.; Kloc, C.; Chen, C.-H. Adv. Mater. 2003, 15, 1090.
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Org. Chem. 2007, 72, 6584. (b) Jiang, J.; Kaafarani, B. R.; Neckers, D. C.
J. Org. Chem. 2006, 71, 2155.
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Yamaguchi, K. Org. Lett. 2006, 8, 2385.
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Shen, B.; Ohe, T.; Kanno, K.-I. J. Org. Chem. 2006, 71, 7967. (b) Li, S.;
Li, Z.; Nakajima, K.; Kanno, K.-I.; Takahashi, T. Chem. Asian J. 2009,
4, 294.
(1) (a) Anthony, J. E. Chem. Rev. 2006, 106, 5028. (b) Bendikov, M.;
Wudl, F.; Perepichka, D. F. Chem. Rev. 2004, 104, 4891. (c) Anthony,
J. E. Angew. Chem., Int. Ed. 2008, 47, 452.
(2) (a) Li, Y.;Wu, Y.;Liu, P.;Prostran, Z.;Gardner, S.;Ong, B. S.Chem.
Mater. 2007, 19, 418. (b) Maliakal, A.; Raghavachari, K.; Katz, H.;
Chandross, E.; Siegrist, T. Chem. Mater. 2004, 16, 4980. (c) Sakamoto, Y.;
Suzuki, T.; Kobayashi, M.; Gao, Y.; Fukai, Y.; Inoue, Y.; Sato, F.; Tokito, S.
ꢀ
J. Am. Chem. Soc. 2004, 126, 8138. (d) Benard, C. P.;Geng, Z.;Heuft, M. A.;
VanCrey, K.; Fallis, A. G. J. Org. Chem. 2007, 72, 7229. (e) Llorente, G. R.;
Dufourg-Madec, M.-B.; Crouch, D. J.; Pritchard, R. G.; Ogier, S.; Yeates,
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(3) (a) Anthony, J. E.; Brooks, J. S.; Eaton, D. L.; Parkin, S. R. J. Am.
Chem. Soc. 2001, 123, 9482. (b) Anthony, J. E.; Eaton, D. L.; Parkin,
S. R. Org. Lett. 2002, 4, 15. (c) Anthony, J. E.; Gierschner, J.; Landis,
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J. E. Org. Lett. 2004, 6, 1609.
r
10.1021/ol102971a
Published on Web 02/10/2011
2011 American Chemical Society

Scheme 1. Synthetic Route of Stable Pentacene Diimides 1 and 8a-d
An ideal pentacene building block for further functio-
nalizations is brominated pentacene with the Br-groups at
specific sites, e.g., the meso-6,13-positions. However,
synthesis of well-defined brominated pentacenes is challen-
ging and direct bromination by using bromine or N-
bromosuccinimide (NBS) is not feasible due to the high
reactivity and poor solubility of pentacene. Dibromo- and
tetrabromopentacene derivatives have been reported by
several groups and in all cases, the bromine atoms are
attached to the first and fifth ring of the pentacene
framework.9 However, dibromopentacenes with the bro-
mine atoms regioselectively located at the meso-6,13-posi-
tions have never been reported. Another concern is the
relatively poor photostability of most of the pentacene
derivatives due to their high HOMO energy level. Recent
works from our groups and other groups indicate that
attachment of electron-withdrawing groups onto the
highly reactive higher order acenes and peri-fused acenes
is an efficient approach to obtain stable materials.10 Here-
in, we report the firstsynthesisof a 6,13-dibromopentacene
[2,3:9,10]-bis(dicarboximide) (1, Scheme 1), which can be
used as a versatile building block for the synthesis of a
series of soluble and stable pentacene derivatives by Pd-
catalyzed cross-coupling reactions.
A pentacene bis(dicarboximide) analogue of 1 without
the dibromo groups was reported by Takahashi et al.8b but
it tends to dimerize easily. As shown in Scheme 1, our key
synthetic strategy toward the pentacene diimide frame-
work is to utilize the Diels-Alder reaction between the
reactive benzo[1,2-c:4,5-c0]difuran intermediate11 and the
dienophile dicarboxylic imide 3 (see the Supporting In-
formation (SI) for synthesis), followed by stepwise aroma-
tization reactions. The dove-tail alkyl chains are intro-
duced to the imide units to provide sufficient solubility for
the final products. A retro-Diels-Alder reaction of com-
pound 211 upon heating to 190-200 °C generated benzo-
[1,2-c:4,5-c0]difuran in situ, which then reacted with 3 to
give the addition product 4 in 50% yield. Dehydration of 4
with trimethylsilyl triflate in chloroform afforded the
anthracene derivative 5 in 45% yield. Bromination of 5
with 2 equiv of Br2 at 0 °C in the dark selectively provided
the desired meso-dibromoanthracene compound 6 in 85%
yield. Dehydrogenation of 6 by heating together with 2,3-
dichloro-5,6-dicyanobenzoquinone (DDQ) or Pd/C failed
to give the target compound 1 and most of the starting
material was recovered. Alternatively, 6 was brominated
with NBS in refluxing CCl4 and the obtained compound 7
was treated with triethylamine to give the title product 1 in
36% yield for two steps.12
The potential of 1 as a useful synthon for various
pentacene derivatives was then tested by various Pd-cata-
lyzed coupling reactions (Scheme 1). Pd-catalyzed cyana-
tion of 1 with CuCN in dioxane worked smoothly and the
(9) (a) Okamoto, T.; Bao, Z. J. Am. Chem. Soc. 2007, 129, 10308. (b)
Swartz, C. R.; Parkin, S. R.; Bullock, J. E.; Anthony, J. E.; Mayer, A. C.;
Malliaras, G. G. Org. Lett. 2005, 7, 3163. (c) Zhao, Y. W.; Mondal, R.;
Neckers, D. C. J. Org. Chem. 2008, 73, 5506. (d) Zhao, Y. W.; Cai, X. C.;
Danilov, E.; Li, G. F.; Neckers, D. C. Photochem. Photobiol. Sci. 2009, 8,
34.
(10) (a) Qu, H.; Chi, C. Org. Lett. 2010, 12, 3360. (b) Yao, J.; Wu, J.;
Chi, C.; Loh, K. P. Chem.;Eur. J. 2009, 15, 9299. (c) Li, J.; Zhang, K.;
Zhang, X.; Huang, K.; Chi, C.; Wu, J. J. Org. Chem. 2010, 75, 856. (d)
Jiao, C.; Huang, K.; Luo, J.; Zhang, K.; Chi, C.; Wu, J. Org. Lett. 2009,
11, 4508.
(11) Luo, J.; Hart, H. J. Org. Chem. 1988, 53, 1341.
(12) (a) Wang, Z.; Kim, C.; Facchetti, A.; Marks, T. J. J. Am. Chem.
Soc. 2007, 129, 13362. (b) Yin, J.; Qu, H.; Zhang, K.; Luo, J.; Zhang, X.;
Chi, C.; Wu, J. Org. Lett. 2009, 11, 3028.
Org. Lett., Vol. 13, No. 5, 2011
925

Table 1. Summary of Photophysical and Electrochemical Properties of Compounds 1 and 8a-da
opt
λabs (nm)
ε
max (M-1cm-1
)
Eox1 (V)
Eoxonset (V)
Ered1 (V)
Ered2 (V)
Eredonset (V)
HOMO (eV)
LUMO (eV)
Eg
1
352, 622
361, 638
344, 750
355, 620
353, 614
113220
81040
78890
107270
93200
0.76
NA
0.71
NA
-1.17
-0.75
-1.25
-1.36
-1.30
-1.56
-1.28
-1.57
-1.78
-1.77
-1.04
-0.61
-1.15
-1.27
-1.21
-5.51
NA
-3.76
-4.19
-3.65
-3.53
-3.59
1.93
1.88
1.52
1.92
1.95
8a
8b
8c
8d
0.16
0.78
0.79
0.11
0.65
0.70
-4.91
-5.45
-5.50
a Eoxn and Eredn are half-wave potentials for respective redox waves with Fc/Fcþ as reference. HOMO and LUMO energy levels were calculated from
the first oxidation and reduction wave onset according to the equations HOMO = -(4.8 þ Eoxonset) eV and LUMO = -(4.8 þ Eredonset) eV.14
6,13-dicyanopentacene dicarboximide 8a was obtained in
95% yield. The Hagihara-Sonogashira coupling between
1 and 4-dimethylaminophenylacetylene (9a) can even be
carried out at room temperature and gave 8b in nearly
quantitative yield. Such a high reactivity of C-Br bond in
1 can be attributed to the activation effect by the electron-
withdrawing imide groups. Stille coupling reaction be-
tween 1 and tributyl(thiophen-2-yl)stannane (9b) also
afforded the thienyl-substituted pentacene dicarboximide
8c in nearly quantitative yield. Suzuki coupling reaction of
1 with 4-cyanophenylboronic ester (9c) by using the Pd-
(PPh3)4-NaHCO3 system in THF/H2O successfully gave
compound 8d in 90% yield. However, attempts to intro-
duce vinylene substituents through Heck reaction with
styrene have so far been unsuccessful presumably due to
the low stability of the final products and decomposition of
1 at high reaction temperature (100 °C). Further optimiza-
tion of reaction conditions will be carried out in the future.
All the new pentacene bis(dicarboximide)s show good
solubility in normal organic solvents due to the attachment
of the dove-tail alkyl chains. The solutions of compounds 1
and 8a-d in toluene are all in deep color and the UV-vis
absorption spectra are shown in Figure 1a and the data are
toluene. Insert: A picture of the solutions of these compounds in
Figure 1. (a) UV-vis-NIR absorption spectra of 1, 8a-d in
summarized in Table 1. In comparison to the parent
pentacene, which shows absorption maximum at 303 nm
(β-band) and 582 nm (p-band) in trichlorobenzene,13 the
6,13-dibromopentacencene dicarboximide 1 exhibits a si-
milar well-resolved band structure but with significant red-
shift of the whole spectrum, for example, the β-band and p-
band maxima are now shifted to 352 and 622 nm, respec-
tively. Such bathochromic shift can be explained by in-
tramolecular donor (pentacene)-acceptor (dicarboxylic
imide) interactions as observed in many aromatic imides.10
Compounds 8a, 8c, and 8d show similar vibronic band
structures to 1 as they basically hold the same pentacene
chromophore. The absorption spectrum of 8a is red-
shifted in comparison to that of 1 because of extended π-
conjugation along the cyano groups. The absorption
spectra of 8c and 8d are just slightly different from that
of 1, indicating that the π-conjugation between the penta-
cence core and the aryl substituents is relatively weak as
usually observed for other pentacenes. The fluorescence
spectra of 1, 8a, 8c, and 8d are quite similar, with the
emission at 500-750 nm (see Figure S1 in the Supporting
toluene. (b) Cyclic voltammograms of 1 and 8a-d in chloro-
benzene with 0.1 M Bu4NPF6 as supporting electrolyte, AgNO3/
Ag as reference electrode, Au disk as working electrode, Pt wire
as counter electrode, and scan rate at 50 mV/s.
Information (SI)). In contrast, 8b displays a broad absorp-
tion with maximum at 750 nm. Such significant spectral
broadening and red-shift can be explained by the strong
intramolecular donor-acceptor interactions which narrow
the optical energy gap. Compound 8b shows two emission
bands in toluene, one centered at 581 nm and the other
located at 811 nm. The latter emission band arises from the
intramolecular charge transfer character of 8b, which is
further supported by the positive solvatochromism effect
(see Figure S2 and Table S1 in the SI). The fluorescence
quantum yields in toluene are determined as 0.2%, 6.0%,
0.8%, 1.3%, and 8.5% for 1 and 8a-d, respectively.
The electrochemical properties of these new pentacene
dicarboximides were studied by cyclic voltommetry in dry
chlorobenzene (Figure 1b and Table 1). All of them
showed two reversible reduction waves (quasireversible
(13) Clar, E. Polycyclic Hydrocarbons; Academic Press: London, UK,
1964; Vol. 1, p 425.
(14) Chi, C.; Wegner, G. Macromol. Rapid Commun. 2005, 26, 1532.
Org. Lett., Vol. 13, No. 5, 2011
926

The photostability of these pentacene diimides in to-
luene was investigated under ambient light irradiation and
white light (60 W) irradiation in air (Figure 2). For
example, upon irradiation with ambient light, the solutions
of compound 1 gradually decomposed with a decrease of the
optical intensity at the longer wavelength band and the
appearance of a new absorption band in the shorter wave-
lengths (ca. 306 and 410 nm) (Figure 2a). At ambient
condition, the half-life times (t1/2) of around 700, 11000,
90, and 30 min were estimated for compounds 1, 8a, 8c, and
8d, respectively, by plotting the absorption intensity at the
longest wavelength of absorption maximum with the irra-
diation time (Figure 2b). The high photostability of 1 and 8a
can be explained by the further stabilization induced by the
electron-withdrawing Br- and CN-groups. Interestingly,
compound 8b exhibited extremely high photostability
although it owns the highest HOMO energy level. A half-
life time (t1/2) of nearly one month can be estimated by
extrapolation of the degradation curve (Figure 2b)! Such
high stability may be related to the intramolecular charge
transfer character of 8b in excited states. For comparison,
6,13-di(triisopropylsilylethynyl)pentacene as one of the most
stable pentacene derivatives showed a half-life time of about
9 h under ambient conditions .4a
In summary, 6,13-dibromopentacene [2,3:9,10]-bis-
(dicarboximide) 1 was successfully synthesized and used
as a versatile building block to prepare a series of soluble
and stable pentacene derivatives with tunable optical and
electronic properties. Our method opens new opportu-
nities to make useful pentacene or even higher order
acene-based opto-electronic materials with good stability.
Research work on further chemical functionalizations and
exploitation of their material applications are underway in
our laboratory.
Figure 2. (a) Change of the UV-vis-NIR absorption spectra of
1 in toluene (1 ꢀ 10-5 M) with time when exposed to ambient
light irradiation; (b) change of the optical density at the corre-
sponding longest wavelength of absorption maximum of 1 and
8a-d as a function of irradiation time at ambient condition.
reduction waves for 1), indicating that they can be reduced
into the corresponding radical anion and dianion, which
are stabilized by the delocalized pentacene unit and the
electron-withdrawing carboximide groups. For the cya-
nated compound 8a, a significantly higher reduction half-
wave potential (-0.75 V vs Fc/Fcþ) was observed due to
the strong electron-withdrawing character of CN-groups.
A very low-lying LUMO level of -4.19 eV was estimated
based on the reduction onset, indicating that 8a could be a
good candidate for n-type semiconductors.14 Due to its
high electron affinity, oxidation waves were not observed
for 8a in the available electrochemical window. For 1, 8c,
and 8d, one (quasi-)reversible oxidation wave was ob-
served with slightly different half-wave potentials. Accord-
ingly, they have a similar HOMO energy level. Compound
8b exhibited multiple oxidative waves with the first oxida-
tive half-wave potential negatively shifted due to the
existence of electron-donating dimethylamino groups.
Acknowledgment. This work is supported by the Na-
tional University of Singapore under MOE AcRF FRC
grants R-143-000-444-112 and R-143-000-412-112.
Supporting Information Available. Synthetic proce-
dures and characterization data of all new compounds,
fluorescence spectra, solvatochromism effect of 8b, and
photo-oxidative resistance studies. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 5, 2011
927