Synthesis of new donor-acceptor-donor materials via Au-catalyzed double cascade cyclization

Article abstract of DOI:10.1016/j.tetlet.2011.11.132

A new class of symmetric π-conjugated donor-acceptor-donor (D-A-D) materials, with aryl- or heteroaryl[a]annulated carbazole (AHA[a]C) moieties as the donors and with 2,1,3-benzothiadiazole (BT) as an acceptor, has been synthesized via NaAuCl₄-catalyzed double cascade cyclization of arenyl tetraynes in ethanol in good to high yields. Photophysical and electrochemical properties of the new D-A-D materials were investigated.

Full text of DOI:10.1016/j.tetlet.2011.11.132

Tetrahedron Letters 53 (2012) 914–918
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of new donor–acceptor–donor materials via Au-catalyzed double
cascade cyclization
Giovanni Ferrara a,b, Tienan Jin a, , Kazuaki Oniwa b, Jian Zhao b, Abdullah M. Asiri c,d, Yoshinori Yamamoto a,
⇑
⇑
a Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai 980-8577, Japan
b Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
c Chemistry Department, Faculty of Science, King Abdulaziz University, P. O. Box 80203, Jeddah, Saudi Arabia
d Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, P. O. Box 80203, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
A new class of symmetric
p
-conjugated donor–acceptor–donor (D–A–D) materials, with aryl- or hetero-
Received 21 September 2011
Revised 16 November 2011
Accepted 25 November 2011
Available online 3 December 2011
aryl[a]annulated carbazole (AHA[a]C) moieties as the donors and with 2,1,3-benzothiadiazole (BT) as an
acceptor, has been synthesized via NaAuCl4-catalyzed double cascade cyclization of arenyl tetraynes in
ethanol in good to high yields. Photophysical and electrochemical properties of the new D–A–D materials
were investigated.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Donor–acceptor–donor material
Gold catalysis
Double cascade cyclization
Aryl- and heteroaryl-annulated carbazoles
Benzothiadiazole
Conjugated donor–acceptor–donor (D–A–D) organic semicon-
ductors have attracted increasing interest due to their various
applications in optoelectronic devices such as organic light emit-
ting diodes (OLEDs),1 organic photovoltaic devices (OPVs),2 and or-
ganic thin film transistors (OTFTs).3 Carbazole-based conjugated
polymers and small molecules have been studied extensively for
optoelectronic device applications because of the good hole-
transporting ability of the carbazole moiety which results from
its electron-donating ability.4 In addition, materials with 2,1,3-
benzothiadiazole (BT) as an acceptor are reported to reduce the
HOMO–LUMO band gaps as well as to expand the emission absorp-
tion.5 A key characteristic of the D–A–D materials is their tunable
optical and electrochemical properties by appropriate chemical
modification of the structures of the donors and acceptors, which
controls the HOMO–LUMO energy levels or band gaps associated
with intramolecular donor–acceptor interactions.1–3 The design
RHN
X
NHR
X
S
cat. NaAuCl
4
RN
X
N
N
NR
S
N
N
EtOH, 100 oC, 12 h
ð1Þ
X
double cascade cyclization
3:
4: D-A- Dmaterials
X = C (benzene), S (thiophene, benzothiophene)
⇑
Corresponding authors. Tel.: +81 22 217 6177; fax: +81 22 217 6165 (T.J.); tel.: +81 22 217 6164; fax: +81 22 217 5979 (Y.Y.).
E-mail addresses: tjin@m.tohoku.ac.jp (T. Jin), yoshi@m.tohoku.ac.jp (Y. Yamamoto).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.11.132

G. Ferrara et al. / Tetrahedron Letters 53 (2012) 914–918
915
Table 1
and 3). It is noteworthy that in the case of 2b, compared with
the reported PPh3AuCl/AgOTf catalyst system,10a,b the present
NaAuCl4 catalyst in ethanol produced a much higher yield of 2b.
Optimization of the reaction conditions for the synthesis of 2aa
Ph
Ph
Ph
5 mol% catalyst
solvent
Ph
NaAuCl4 (5mol%)
N
H
S
S
EtOH, 80 oC, 1 h
N
NH2
H
1a
2a
NH2
1b
2b 92%
Entry
Catalyst
Solvent
T (°C)
Time (h)
Yieldb (%)
(ref.9a, 61%)
1
2
3
4
5
6
7
8
PdCl2
PtCl2
AuCl
AuCl3
Toluene
Toluene
Toluene
Toluene
Ethanol
Ethanol
Ethanol
Ethanol
100
100
100
100
100
100
80
20
20
20
20
20
0.3
0.3
1
32
85
ð2Þ
ð3Þ
38c
90
Ph
Ph
AuCl3
68
NaAuCl4 (5mol%)
EtOH, 80 oC, 1 h
NaAuCl4
NaAuCl4
NaAuCl4
94d
91d
87
S
S
rt
N
H
a
b
c
NH2
Reaction conditions: catalyst (5 mol %), 1a (0.2 mmol), solvent (0.2 M, 1 mL).
1H NMR spectroscopic yield determined using CH2Br2 as an internal standard.
The 5-endo cyclization product, substituted indole, was obtained in 35% yield.
Isolated yield of 2a.
1c
2c 85%
d
With these optimized reaction conditions and preliminary
experimental results in hand, we investigated the double cascade
cyclization of tetraynes 3, two diyne moieties symmetrically con-
nected by a BT moiety, for synthesizing the new D–A–D molecules
(Eq. 1 and Table 2).11 Tetraynes 3a–c with a diethynyl benzene
group and different alkyl chains on the aniline moiety afforded
the corresponding products 4a–c in good yields at 100 °C (entries
1–3). The structure of 4c was unambiguously confirmed by X-ray
crystal-structure analysis.12 As expected, tetraynes 3d–f bearing
thienyl and benzothienyl moieties produced the corresponding
D–A–D molecules 4d–f efficiently (entries 4–6). To better under-
stand the photophysical and electrochemical properties of these
D–A–D molecules, another D–A–D compound 4g having a tetraflu-
orobenzene (TFB) acceptor was prepared under the standard
and synthesis of new D–A–D materials having good donor and
acceptor candidates will be helpful to obtain a wide range of band
gaps.
In order to find an effective catalyst, a brief optimization was
carried out on the synthesis of benzo[a]carbazole 2a from ani-
line-substituted diyne 1a as shown in Table 1. The use of PdCl2
as the catalyst in toluene gave a lower yield of 2a, while PtCl2
showed good activity at high temperature (100 °C) (entries 1 and
2). The yield of 2a was low in the case of AuCl as catalyst, because
the reaction stopped at the first 5-endo cyclization step (entry 3). In
BuHN
NHBu
BuN
S
NBu
S
F
F
F
cat. NaAuCl4
F
F
F
F
ð4Þ
EtOH, 100 oC, 12 h
S
S
F
3g
4g 57%
toluene, AuCl3 exhibited high catalytic activity, affording 2a in a
90% yield, although the yield decreased dramatically in ethanol
(entries 4 and 5). NaAuCl4 as catalyst in ethanol showed the high-
est activity, giving 2a in a 94% yield in a much shorter reaction time
(entry 6). The reaction proceeded even at a lower temperature
(80 °C and room temperature) without a significant decrease in
the yield (entries 7 and 8). It should be noted that NaAuCl4 in other
solvents, such as toluene, CH3CN, and dichloroethane led to de-
creased yields of 2a.
The optimized reaction conditions were successfully applied to
heteroarenyl diyne derivatives for the construction of hetero-
aryl[a]annulated carbazoles. Thus, under the optimized conditions
(5 mol % of NaAuCl4 in ethanol at 80 °C) the reactions of thienyl
and benzothienyl diynes 1b and 1c afforded the corresponding
products 2b and 2c in 92% and 85% yields, respectively (Eqs. 2
conditions from tetrayne 3g (Eq. 4). It is noteworthy that the reac-
tions did not proceed without a substituent on the N-atoms in 3.
The absorption spectra of the new D–A–D molecules 4a–g were
measured in dichloromethane and relevant data are shown in
Table 3 and Figure 1a. The absorption bands at 245–290 nm could
be assigned to the AHA[a]Cs-centered transitions.8 For compounds
4a–e, the peaks with moderately strong absorptions at 336–
361 nm are attributed to the
p–
p⁄ transition of the BT acceptor
moiety.1g The bands at about 312–362 nm for compounds 4f and
4g were ascribed to the CF3-substituted benzothieno[a]carbazole
and TFB moieties. The new broad band at the longer wavelength
of 390–412 nm should result from an intramolecular charge trans-
fer between the donor and the acceptor group. Among these
compounds, 4f having an electron-withdrawing group on the ben-

916
G. Ferrara et al. / Tetrahedron Letters 53 (2012) 914–918
Table 2
Synthesis of D–A–D compounds 4a
Entry
Substrate
D–A–D productb
RHN
NHR
S
RN
N
N
NR
S
N
N
1
2
3
R = Me (3a)
R = n-(CH2)3CH3 (3b)
R = n-(CH2)9CH3 (3c)
4a 78%
4b 75%
4c 72%
RHN
NHR
S
H3C(H2C)3N
N
N
N(CH2)3CH3
S
N
N
S
S
S
S
4
R = n-(CH2)3CH3 (3d)
4d 71%
R'
R'
R'
R'
RHN
NHR
S
RN
S
N
N
NR
S
N
N
S
S
R = n-(CH2)7CH3, R0 = H (3e)
R = n-(CH2)7CH3, R0 = CF3 (3f)
S
5
6
4e 68%
4f 75%
a
Reaction conditions: NaAuCl4 (10 mol %), 3 (0.2 mmol), EtOH (0.1 M, 2 mL), 100 °C, 12 h.
Isolated yield of 4.
b
Table 3
Photophysical and electrochemical data of D–A–D compounds 4
a
b
b
c
c
d
e
Compound
UV–vis/kmax,abs (nm)
Eox (V)
Ered (V)
EHOMO (eV)
ELUMO (eV)
Eg(cv) (eV)
Eg(opt) (eV)
4a
4b
4c
4d
4e
4f
256, 283, 343, 360, 400
256, 284, 344, 361, 403
256, 283, 343, 360, 408
245, 272, 290, 336, 351, 412
245, 272, 290, 337, 351, 408
250, 290, 313, 362, 390
250, 288, 312, 358, 398
0.91
0.91
0.91
0.97
0.96
0.94
1.05
ꢀ1.73
ꢀ1.73
ꢀ1.73
ꢀ1.74
ꢀ1.84
ꢀ1.73
—
ꢀ5.71
ꢀ5.71
ꢀ5.71
ꢀ5.77
ꢀ5.76
ꢀ5.74
ꢀ5.85
ꢀ3.07
ꢀ3.07
ꢀ3.07
ꢀ3.04
ꢀ3.07
ꢀ3.06
—
2.64
2.64
2.64
2.71
2.72
2.67
—
2.63
2.67
2.67
2.65
2.55
2.59
2.91
4g
a
Measured in dichloromethane.
b
The onset potentials determined by cyclic voltammetry: 0.1 M TBAPF6 in CH2Cl2, platinum wire working electrode, glassy carbon electrode, and SCE as reference
electrode, scan rate of 100 mV sꢀ1
.
c
Calculated according to EHOMO = ꢀ(Eox + 4.80), ELUMO = ꢀ(Ered + 4.80).
d
e
Band gap was measured from the cyclic voltammogram.
Optical band gaps estimated from the onset position of the UV–vis absorption spectra.
zothieno[a]carbazole moiety exhibited the lowest wavelength for
this new band. Cyclic voltammetry (CV) was performed to investi-
gate the HOMO and LUMO energy levels and band gaps of the new
D–A–D compounds (Table 3 and Fig. 1b–f). Compounds 4a–f hav-
ing a BT acceptor showed a single reversible oxidation and reduc-
tion wave within the span of the solvent electrolyte window,
whereas 4g having a TFB moiety exhibited an irreversible reduc-
tion process. The change of alkyl substituent length in 4a–c had
a negligible influence on the electrochemical properties. The LUMO
energy levels of BT-based compounds 4a–f lie significantly below
that of TFB-based molecule 4g, reflecting the strong electron-
accepting character of the BT fragment. The HOMO–LUMO band
gaps obtained by CV were found to be in good agreement with
the optical band gaps estimated from UV–vis absorption onset.
The small band gaps observed for BT-based compounds and LUMO
energies below ꢀ3 eV suggest the potential application of these
compounds for efficient charge transport semiconductors.13
In conclusion, a new class of D–A–D materials with two
AHA[a]C donors bridged by a BT acceptor has been synthesized
via an efficient NaAuCl4-catalyzed one-step, double cascade cycli-
zation of tetraynes. UV–vis absorption spectra show new broad
bands at a longer wavelength of 390–412 nm indicating effective
donor–acceptor intramolecular interactions. Electrochemical stud-
ies showed that the HOMO–LUMO gaps are affected by the

G. Ferrara et al. / Tetrahedron Letters 53 (2012) 914–918
917
Figure 1. (a) UV–vis spectra in dichloromethane. (b–f) CV of 4a and 4d–g.
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Acknowledgments
We thank the faculty members of the Instrumental Analysis
Center at Tohoku University for the measurement of NMR and
mass spectra. This work was supported by World Premier Interna-
tional Research Center Initiative (WPI), MEXT, Japan.
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