Introduction of an arylethynyl group onto an anthracene bisimide core for molecular design of new ?-conjugated compounds

Article abstract of DOI:10.1246/cl.130842

Novel ethynylated anthracene bisimide (ABI) derivatives were synthesized by Sonogashira and Stille couplings, and the electronic spectra of the new compounds were compared with those of molecules with related structures. The absorption spectrum and the electrochemical data of the ABI with a 9- anthrylethynyl group suggested weak electronic interactions between the donor anthracene unit and the acceptor ABI unit.

Full text of DOI:10.1246/cl.130842

Received: September 9, 2013 | Accepted: October 7, 2013 | Web Released: October 16, 2013
CL-130842
Introduction of an Arylethynyl Group onto an Anthracene
Bisimide Core for Molecular Design
of New π-Conjugated Compounds
Tetsuo Iwanaga,* Ryo Tanaka, and Shinji Toyota*
Department of Chemistry, Faculty of Science, Okayama University of Science,
1-1 Ridaicho, Kita-ku, Okayama 700-0005
(E-mail: iwanaga@chem.ous.ac.jp, stoyo@chem.ous.ac.jp)
Novel ethynylated anthracene bisimide (ABI) derivatives
were synthesized by Sonogashira and Stille couplings, and the
electronic spectra of the new compounds were compared with
those of molecules with related structures. The absorption
spectrum and the electrochemical data of the ABI with a 9-
anthrylethynyl group suggested weak electronic interactions
between the donor anthracene unit and the acceptor ABI unit.
For the synthesis of the target ABI derivatives, 9-Br-ABI 8
was used as a key intermediate for ethynylation by cross-
coupling reactions. This precursor was prepared by the Diels-
Alder approach, as shown in Scheme 1. Durene was first
brominated to afford compound 5,13 which was then further
brominated by irradiation with bromine to give heptabromide 6
in a manner similar to a previously known method.7,9 The
regioselectivity of the bromination was confirmed by X-ray
structural analysis [see the Supporting Information (SI)].14
Reaction of 6 with N-octylmaleimide in the presence of NaI
afforded the Diels-Alder adduct 7 as a mixture of diastereomers,
which was then aromatized by radical bromination followed by
elimination. The series of reactions gave compound 8 in 26%
overall yield from 6.
As electron-deficient moieties, aromatic bisimides have
been attractive building units for the molecular design of new π-
conjugated functional materials.1 In particular, perylene bis-
imides and naphthalene bisimides have been extensively applied
to the development of new organic devices2 such as organic
light-emitting diodes,3 organic field-effect transistors (OFETs),4
and organic photovoltaics.5,6 Anthracene-2,3:6,7-bisimides
(ABIs) are also promising candidates for the construction of
various functional molecules.7 ABI derivatives with 1,5-diphen-
yl and 9,10-dicyano groups have been utilized as chemical
sensors8 and OFET materials,9 respectively. In general, the
electronic properties of anthracene moieties are considerably
influenced by substitution with arylethynyl groups.10 However,
to the best of our knowledge, no ABI derivatives having
acetylene substituents have yet been reported in the literature.
Recently, we reported that a donor-acceptor array consisting of
a perylene bisimide core and a 9-anthrylethynyl group showed
interesting spectroscopic properties due to charge-transfer
interactions.11 Therefore, we designed a new donor-acceptor-
type ABI derivative 1 having a 9-anthrylethynyl group at the 9
position, in which the ABI unit serves as a terminal acceptor unit
(Figure 1). We herein report the synthesis and spectroscopic data
of ABI derivative 1 and its analogue 2 and the results of a
comparison of their photophysical and electrochemical proper-
ties with those of the related compounds 312 and 49 in order to
evaluate the π-conjugation.
The Sonogashira coupling of 8 with 9-ethynylanthracene15
in a conventional manner gave compound 1 as a red solid
in 95% yield, while the Stille coupling of bromide 8 with
bis(tributylstannyl)ethyne in the presence of [Pd(PPh3)4], as well
as LiCl as additive in dioxane gave compound 2 as a yellow
solid in 58% yield (Scheme 2). These compounds were
reasonably characterized by NMR and mass spectral analyses.14
In the 1H NMR spectrum of 1, three singlets due to the ABI unit
appeared at ¤ 9.06 (C1,8), 8.43 (C10), and 8.26 (C4,5). In the
13C NMR spectra, the alkyne carbons in 1 were observed as two
signals at ¤ 95.4 and 101.3, while those in 2 were observed as
one signal at ¤ 97.0.
The molecular structures of model compounds of 1 and 2
having N-methyl groups in place of the N-octyl chains were
calculated at the B3LYP/6-31(d) level. Compound 1 has a
nearly planar conformation about the acetylene axis (dihedral
Br
Br
Br2
NBS
Br
Br
Br
Br
CH3CN
CCl4
h
ν
r.t.
Durene
Br
6 (66%)
Br
5 (77%)
O
O
C8H17
N
C8H17
N
C8H17
N
C8H17
N
Br
O
O
H
N C8H17
H
H
O
O
O
O
O
O
O
O
NaI
C8H17
N
N
C8H17
DMF
85 oC
H
O
O
O
O
Br
Br
7
1) NBS
Br
Benzoyl Peroxide
9
CCl4
O
O
O
O
O
O
O
O
N
N
N
N
C8H17
N
N C8H17
C8H17
C8H17
C8H17
C8H17
2) Et3N
10
2
3
4
1
O
O
8 (26% from 6)
Figure 1. Target ABI derivatives 1 and 2 and reference
compounds 3 and 4.
Scheme 1. Synthesis of 9-Br-ABI 8.
Chem. Lett. 2014, 43, 105–107 | doi:10.1246/cl.130842
© 2014 The Chemical Society of Japan | 105

C8H17
N
Table 1. UV-vis absorption and fluorescence data for the ABI
derivatives 1 and 2 and the reference compounds 3 and 4
O
O
9-Ethynylanthracene
10mol% [Pd(PPh3)4]
10mol% CuI
-
Calcd -max
/nm ( f )b
-
¸
f
max
em
Compd
/nm (¾)
/nm (Φf)c
/nsd
Toluene, Et3N
reflux
1
2
3
4
531 (7400)a
486 (2400)a
476 (13500)
421 (15000)
603 (0.27)
509 (0.33)
500 (0.44)
397 (0.03)
575 (0.51)
507 (0.65)
489 (0.09)
423 (0.31)
5.0
3.8
C8H17
N
O
O
O
O
O
e
N
®
4.2
C8H17
1 (95%, red solid)
Br
aPeak as shoulder. bTD-DFT calculations (TD/B3LYP/6-
31G(d)) were performed with the use of optimized structures
of N-Me compounds at the B3LYP/6-31G(d) level; f:
oscillator strength. cAbsolute fluorescence quantum yields.
C8H17
N
C8H17
N
O
O
O
O
O
Bu3Sn
SnBu3
N
C8H17
8
5% [Pd(PPh3)4]
LiCl
dFluorescence lifetimes. Not measured due to weak fluores-
e
Dioxane
reflux
cence.
O
O
O
O
C8H17
2 (58%, yellow solid)
N
N
C8H17
Scheme 2. Synthesis of ethynylated ABI derivatives 1 and 2.
LUMO
HOMO
Figure 3. MO plots of calculated structure of 1 (N-Me) using
the B3LYP/6-31G(d) level of theory.
other compounds exhibited absorptions at -max of 486 (2), 476
(3), and 421 nm (4). These data indicate that the bathochromic
effect was very significant for 1, and this effect is attributed to
the intramolecular charge-transfer between the ABI unit (accep-
tor) and the anthracene unit (donor) across the acetylene
linker.11,16 These absorptions were assignable to the HOMO
to LUMO transition on the basis of the results of TD-DFT
calculations using the same model compound as described
above. The molecular orbitals (MOs) of 1 (N-Me) at the HOMO
and LUMO levels were mainly located in the anthracene and
ABI units, respectively (Figure 3). The absorption onset of 2
was bathochromically shifted relative to that of 9-ethynyl-ABI
by ca. 50 nm due to the interactions between the ABI units
through the acetylene axis (Figure S3 in SI).14
The fluorescence spectra were measured in CHCl3
(Figure 2b and Table 1). The emission peaks for compounds 1
and 2 were observed at 575 and 507 nm, respectively. Notably,
the emission maxima of these compounds were bathochromi-
cally shifted compared to that of compound 4 (423 nm). The
absolute fluorescence quantum yields Φf of 1 and 2 were 0.51
and 0.65, respectively, and were also larger than that of
compound 4 (0.31). In addition, the absolute fluorescence
quantum yield of 2 was the highest of all of the ABI derivatives
evaluated in the present study, while the fluorescence lifetime
of 1 was longer than those of the other compounds. These
phenomena are attributed to the introduction of the anthracene
moiety, which extends the π-conjugation.17
Figure 2. (a) UV-vis and (b) FL spectra of ABI derivatives 1
and 2 and reference compounds in CHCl3. ®® 1,
and - - - - 4.
2,
3,
angle 19°), whereas 2 has a nonplanar conformation (dihedral
angle 49°) due to the steric hindrance between the inner carbonyl
oxygen atoms (see SI).14
The absorption spectra of compounds 1 and 2 and reference
compounds 3 and 4 were measured in CHCl3 (Figure 2a). The
spectroscopic data are summarized in Table 1. Compound 1
exhibited a characteristic broad shoulder band extending to
ca. 570 nm in the longest wavelength region, whereas the three
Finally, cyclic voltammetry was conducted in order to
evaluate the redox properties of the ABI derivatives in CH2Cl2
(see SI).14 The values for the reduction potentials are listed in
Table 2. Compound 1 gave pseudoreversible reduction poten-
106 | Chem. Lett. 2014, 43, 105–107 | doi:10.1246/cl.130842
© 2014 The Chemical Society of Japan

Table 2. Electrochemical data for the ABI derivatives 1, 2,
and 4a
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Compd
E1/2red/V
LUMO level/eVb
1
2
4
¹1.50, ¹1.89
¹1.33c
¹1.72, ¹2.02
¹2.94
¹3.25
¹2.82
3
4
5
aMeasured in CH2Cl2 (0.3-1.0 mmol L¹1) with Bu4NBF4
(0.10 mol L¹1) as the supporting electrolyte and Ag/Ag+ as
the reference electrode. Reduction wave of compound 3 was
not observed. bCalculated at the B3LYP/6-31G(d) level for N-
Me derivatives. cOnly the first redox potential is shown owing
to irreversible waves.
tials at ¹1.50 and ¹1.89 V due to the formation of a radical
anion and dianion species, respectively.18 The anodic shifts
of ca. 0.2 V relative to the values for 4 correspond to the
stabilization of the LUMO level as a result of the introduction of
the 9-anthrylethynyl group, as revealed by the calculated MO
levels. These phenomena are attributed to the efficiently
extended π-conjugation in 1, consistent with the red shift in
the absorption spectrum. Compound 2 exhibited multistep
irreversible reduction peaks due to the electronic coupling
between the two ABI units, as observed for perylene bisimide
derivatives.11
6
7
8
9
In conclusion, we synthesized new ABI derivatives having
ethynyl groups by the Sonogashira and Stille coupling reactions.
The ABI derivative 1 exhibited broad absorption bands at long
wavelengths and displayed a strong fluorescence. The photo-
physical properties were influenced by the aromatic ethynyl unit,
which resulted in extended π-conjugation and intramolecular
donor-acceptor interactions. Further studies involving the
synthesis of selected ABI oligomers and their application to
the design of organic electronic devices are in progress.
10 S. Toyota, Chem. Rev. 2010, 110, 5398.
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14 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
This work was partly supported by the Okayama Foundation
for Science and Technology and by the MEXT (Ministry of
Education, Culture, Sports, Science and Technology)-Supported
Program for the Strategic Research Foundation at Private
Universities, 2009-2013.
15 S. Toyota, T. Yamamori, M. Asakura, M. Ōki, Bull. Chem.
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18 In previous report, the pyromellitic diimides exhibited two
reversible one-electron reductions, and the first and second
waves were ascribed to the formation of radical anion and
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Chem. Lett. 2014, 43, 105–107 | doi:10.1246/cl.130842
© 2014 The Chemical Society of Japan | 107