Synthesis of functionalized tetracene dicarboxylic imides

Article abstract of DOI:10.1016/j.tetlet.2010.09.116

For the first time, a synthesis of tetracene dicarboxylic imides was established with 1,2,3,4-tetrahydrotetracene (1) instead of tetracene as the starting material. Mono-bromination of 1 by CuBr₂ followed by a Friedel-Crafts reaction, oxidation, and imidization gave the tetrahydrotetracene carboxylic imides 5a-b. Subsequent oxidative dehydrogenation of 5a-b with DDQ afforded the functional tetracene dicarboxylic imide monobromides 6a-b, which can be further functionalized to provide functional materials such as the 'donor-acceptor' type compounds 7a-b.

Full text of DOI:10.1016/j.tetlet.2010.09.116

Tetrahedron Letters 51 (2010) 6313–6315
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of functionalized tetracene dicarboxylic imides
Jun Yin a,b, Kai Zhang a, Chongjun Jiao a, Jinling Li a, Chunyan Chi a, Jishan Wu a,
⇑
a Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
b Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 July 2010
Revised 6 September 2010
Accepted 24 September 2010
Available online 18 October 2010
For the first time, a synthesis of tetracene dicarboxylic imides was established with 1,2,3,4-tetrahydrotet-
racene (1) instead of tetracene as the starting material. Mono-bromination of 1 by CuBr2 followed by a
Friedel–Crafts reaction, oxidation, and imidization gave the tetrahydrotetracene carboxylic imides 5a–
b. Subsequent oxidative dehydrogenation of 5a–b with DDQ afforded the functional tetracene dicarbox-
ylic imide monobromides 6a–b, which can be further functionalized to provide functional materials such
as the ‘donor–acceptor’ type compounds 7a–b.
Keywords:
Acene
Ó 2010 Elsevier Ltd. All rights reserved.
Tetracene
Dicarboxylic imide
Friedel–Crafts reaction
Imidization
Acenes represent a promising class of molecules with applica-
tions in organic electronics.1 Therefore, a large number of function-
alized acenes have been reported and used, for example, for organic
light-emitting diodes, field-effect transistors and solar cells.2
Among derivatives of acene, acene carboximides are attracting
increasing interest due to their important roles in the design of
new semiconductors and organic dyes. For example, 10 years ago,
the smallest acene, naphthalene, was used as a building block for
n-type semiconductors following the introduction of electron-
deficient dicarboxylic imide groups3 and recently naphthalene
carboximides have been widely used for the synthesis of stable
semiconducting materials.4 Anthracene carboximide derivatives
have been used as drugs because of their biological activity.5 In
addition, functionalized naphthalene and anthracene dicarboxylic
imides (Scheme 1) have been applied to the synthesis of rylene
dyes6 and bisanthryl diimide near infrared (NIR) dyes as shown by
us and other groups.7 Compared with other aryl or alkynyl-substi-
tuted acenes, imide substituted acenes usually show high chemical
and photostability, which are crucial for practical applications.
Moreover, they also exhibit high electron affinity due to the elec-
tron-withdrawing character of their carboximide units, and can be
used as building blocks for both n-type semiconductors and stable
NIR dyes. Although naphthalene and anthracene carboximides are
now readily available, the synthesis of carboximide substituted
higher order acenes such as tetracene and pentacene have not been
reported due to synthetic problems. Tetracene as a larger acene is
supposed to exhibit superior optoelectronic properties due to its
extended p-conjugation and many functionalized tetracene deriva-
tives have been reported.8 However, no efficient method for the
introduction of the electron-deficient carboximide to the tetracene
unit has been reported. As imide groups enhance the stabilities
and solubilities and also tune the electronic properties of unstable
and poorly soluble tetracenes, herein, we report the first synthesis
of a series of soluble and stable functional tetracene dicarboxylic
imides.
We first attempted to apply chemistry used for the synthesis of
anthracene dicarboxylic imides7 to the tetracene (Scheme 1). How-
ever, a direct Friedel–Crafts reaction on tetracene using oxalyl
chloride in CS2 in the presence of AlCl3 gave a complicated mixture
and no desired tetracene diketones were observed. Considering the
poor solubility of tetracene, the tetracene monobromide9 which
has good solubility in CS2 was selected as the starting material in
a similar reaction under the same conditions. However, again, no
desired tetracene diketone was obtained. This indicated that it is
not feasible to perform Friedel–Crafts reactions on tetracenes pre-
sumably due to their high reactivity. On the other hand, anthra-
cene and its derivatives can undergo Friedel–Crafts reactions to
give diketones.5,7 Therefore, we revised our synthetic strategy by
using 1,2,3,4-tetrahydrotetracene as the starting material.
As shown in Scheme 2, mono-bromination of 1,2,3,4-tetrahy-
drotetracene (1)10 by CuBr2 in CCl4 gave the mono-bromide 2 in
an 86% yield. Compound 2 was then submitted to a Friedel–Crafts
reaction with oxalyl chloride in CS2 and the two diketone isomers
3a and 3b, in a molar ratio of 3:1, were obtained as a mixture in a
76% yield. This mixture was used directly for the next step, oxida-
tion by H2O2 in dioxane, without further purification due to their
poor solubility and similar polarity, and the two carboxylic
⇑
Corresponding author. Tel.: +65 6516 2677; fax: +65 6779 1691.
E-mail address: chmwuj@nus.edu.sg (J. Wu).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.09.116

6314
J. Yin et al. / Tetrahedron Letters 51 (2010) 6313–6315
R
N
R
N
R
N
R
N
O
O
O
O
O
O
O
O
Napthalene-imide
Anthracene-imide
Tetracene dicarboxylic imides
O
O
O
O
O
O
Cl
Cl
or
Friedel-Crafts
X
X
X
X = H, Tetracene
Tetracene diketones
(NOT detected)
X = Br, 5-Bromotetracene
Scheme 1. Structures of some acene dicarboxylic imides and attempted synthesis of tetracene diketones.
anhydride isomers 4a and 4b were obtained in a total yield of 90%.
Again, the poorly soluble anhydrides were used directly for the
subsequent imidization reaction. A mixture of 4a, 4b and 2-dec-
yltetradecan-1-amine11 in anhydrous toluene and ethanol was
refluxed for four days and the tetrahydrotetracene dicarboxylic
imides 5a and 5b were separated in 8% and 7% yields, respectively.
In view of the low yields, other reaction conditions were used4a
and solvents such as NMP, DMF, or propanoic acid. However, the
results indicated that the current solvent system is the only one
that can give the desired carboximides. This is surprising since,
in many cases, imidization of naphthacene and anthracene carbox-
ylic anhydrides can be achieved in high yields. The reason for such
low yields is still not clear but may be due to the electronic and
steric effects of the additional 1,4-tetramethylene unit. Finally,
oxidative dehydrogenation of 5a and 5b with 2,3-dichloro-5,6-dic-
yanobenzoquinone (DDQ) in refluxing toluene gave the tetracene
dicarboxylic imides 6a and 6b, respectively, both in 37–40% yields.
Obviously, the bromine group in 6a and 6b gives opportunities for
the synthesis of functional tetracene dicarboxylic imides with
tunable properties. As a preliminary test, Sonogashira coupling
reactions of 6a and 6b with 4-ethynyl-N,N-dimethylaniline were
conducted and compounds 7a and 7b, with a ‘donor–acceptor’ type
structure, were obtained in 46% and 52% yields, respectively
(Scheme 2).
While the pure tetracene and 5-bromotetracene are air and
light sensitive materials with an orange-yellow color in solution,
the tetracene dicarboxylic imides 6a and 6b showed violet colors
in chloroform and exhibited very good stability on exposure to
ambient light and air. A detailed photostability test disclosed that
the half life-times of solutions of tetracene, 5-bromotetracene and
6a on irradiation with a 100 W white light bulb were 15, 16 and
660 min, respectively. That is, the introduction of the electron-
withdrawing imide groups into the tetracene improved its photo-
stability significantly. Compounds 6a–b exhibited similar UV–vis
absorption and fluorescence spectra in dichloromethane (CH2Cl2)
although they are structural isomers. A sharp absorption band
centered at 292 nm and a broad band from 450 to 670 nm were
observed for both (Fig. 1a). The optical band gaps were estimated
to be 1.87 eV (670 nm) and 1.88 eV (661 nm) for 6a and 6b,
respectively. For comparison, tetracene and 5-bromotetracene in
CH2Cl2 have optical band gaps of 2.49 eV (498 nm) and 2.39 eV
(518 nm), respectively (Fig. 1a). Accordingly, the fluorescence
O
O
O
O
O
O
CuBr2, CCl4
Cl
Cl
+
86%
AlCl3, CS2
76%
Br
Br
Br
1
2
3a
3b
C24H49
C24H49
O
O
O
O
O
O
O
N
O
O
N
O
C24H49NH2
H2O2, dioxane
rt, 18h, 90%
toluene / EtOH
+
+
+
reflux, 4 d
Br
Br
4a
7%
Br
8%
5a
Br 5b
4b
C24H49
C24H49
O
N
O
O
N
O
C24H49
C24H49
O
N
O
O
N
O
N
DDQ, toluene
PdCl2(PPh3)2, PPh3,
CuI, Et3N / THF
reflux, 8 d
37-40%
Br
Br
6a
C24H49
6b
7a
46%
7b
52%
C10H21
C12H25
=
N
N
Scheme 2. Synthesis of functionalized tetracene dicarboxylic imides.

J. Yin et al. / Tetrahedron Letters 51 (2010) 6313–6315
6315
50000
40000
30000
20000
10000
0
Tetracene
UV-vis 7a
UV-vis 7b
PL 7a
Tetracene
1.0
1.0
0.8
0.6
0.4
0.2
0.0
(a)
(b)
(c)
5-Bromotetracene
5-Bromotetracene
6a
6b
6a
6b
PL7b
0.8
0.6
0.4
0.2
0.0
500
600
700
800
900
300
400
500
600
700
300 400 500 600 700 800 900
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Figure 1. (a) UV–vis absorption spectra of compounds tetracene, 5-bromotetracene, 6a and 6b in CH2Cl2 (1 Â 10À5 M). (b) Normalized fluorescence spectra of compounds
tetracene, 5-bromotetracene, 6a and 6b in CH2Cl2 (1 Â 10À6 M). (c) Normalized UV–vis absorption and fluorescence spectra of compounds 7a and 7b in CH2Cl2 (the
concentration of solutions for absorption and emission spectra is 1 Â 10À5 and 1 Â 10À6 M, respectively, and the excitation wavelength is 294 nm).
spectra of 6a and 6b exhibited significant red-shifts with respect to
tetracene and 5-bromotetracene (Fig. 1b). Such a convergence in
band gap and red-shift of the absorption/emission spectra can be
explained by the intramolecular donor–acceptor interaction in-
duced by the electron-rich tetracene (donor) and electron-deficient
dicarboxylic imide group (acceptor). The fluorescence quantum
yields for tetracene, 5-bromotetracene, 6a and 6b in CH2Cl2 were
determined to be 0.118, 0.007, 0.077 and 0.075, respectively. After
introduction of the strongly electron-donating 4-dimethylamino-
phenylethynyl unit, compounds 7a and 7b had appeared a deep
blue color in CH2Cl2 and displayed significant further red-shifts
in absorption spectra in comparison to 6a and 6b, with optical
band gaps being 1.56 eV (794 nm) and 1.58 eV (784 nm), respec-
can be found, in the online version, at doi:10.1016/j.tetlet.2010.
09.116.
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p-
conjugation and enhanced intramolecular donor–acceptor interac-
tions in 7a and 7b. Compounds 7a and 7b showed quite different
fluorescence spectra from 6a–b in solution, which could be related
to their different symmetries and dipole moments.
In conclusion, stable and functionalizable tetracene dicarboxylic
imides such as 6a and 6b were synthesized for the first time by
using 1,2,3,4-tetrahydrotetracene as the starting material. This will
allow further functionalization on the tetracene dicarboxylic imi-
des in order to prepare soluble and stable tetracene carboximide-
based semiconductors and NIR dyes with tunable optoelectronic
properties. Although the unexpectedly low yield of the imidization
step may limit scale-up work, the new synthetic method provides a
direction for the synthesis of carboximide derivatives of higher or-
der acenes or other polycyclic aromatics in the future.
Acknowledgment
This work was financially supported by Singapore DSTA DIRP
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Supplementary data
Supplementary data associated (detailed synthetic procedure
and characterization data for all new compounds) with this article