Photophysical and electrochemical properties of π-extended molecular 2,1,3-benzothiadiazoles

Article abstract of DOI:10.1016/j.tet.2005.08.093

The reaction of 4,7-dibromo-2,1,3-benzothiadiazole with arylboronic acids (phenyl, 1-naphthyl, 4-methoxyphenyl, 4-chlorophenyl and 4- trifluoromethylphenyl) in the presence of catalytic amounts of a NCP-pincer palladacycle affords photoluminescent π-extended 4,7-diaryl-2,1,3- benzothiadiazoles 4a-e in high yields. These 4,7-diaryl-2,1,3-benzothiadiazoles exhibit high fluorescent quantum yields, high electron affinities and adequate band gap values for testing as OLEDs. The 4,7-bis-naphthyl-2,1,3- benzothiadiazole 4b presents two different lifetimes (bi-exponential decay) due to the presence of two atropisomers. The Sonogashira coupling reaction of 4,7-diethynyl-2,1,3-benzothiadiazole 6 with the corresponding halo-aryl compounds (iodobenzene, 1-bromonaphthalene, 4-iodoanisole, 4-bromo-N,N- dimethylaniline and 2-bromopyridine) afforded the photoluminescent π-extended 4,7-bis-alkynylaryl-2,1,3-benzothiadiazoles 7a-e, also in high yields. These 4,7-diethynyl-2,1,3-benzothiadiazoles also present high fluorescent quantum yields, high electron affinities and adequate band gap values for testing as OLEDs. The 4,7-disubstituted-2,1,3-benzothiadiazoles 4a-e and 7a-e exhibit different electrochemical behavior. The presence of two ethynyl spacers in 2,1,3-benzothiadiazoles 7a-e shifts the reduction potentials to less cathodic values and also results in two well-defined and distinct reduction processes.

Full text of DOI:10.1016/j.tet.2005.08.093

Tetrahedron 61 (2005) 10975–10982
Photophysical and electrochemical properties of p-extended
molecular 2,1,3-benzothiadiazoles
Brenno A. DaSilveira Neto,a Aline Sant’Ana Lopes,a Gunter Ebeling,a Reinaldo S. Gonc¸alves,a
Valentim E. U. Costa,a Frank H. Quinab and Jairton Duponta,
*
aLaboratory of Molecular Catalysis, Institute of Chemistry, UFRGS, Av. Bento Gonc¸alves 9500, 91501-970 Porto Alegre, RS, Brazil
bInstitute of Chemistry-USP, CP 26.077, 05513-970 Sa˜o Paulo, SP, Brazil
Received 20 July 2005; revised 22 August 2005; accepted 23 August 2005
Available online 19 September 2005
Abstract—The reaction of 4,7-dibromo-2,1,3-benzothiadiazole with arylboronic acids (phenyl, 1-naphthyl, 4-methoxyphenyl, 4-chloro-
phenyl and 4-trifluoromethylphenyl) in the presence of catalytic amounts of a NCP-pincer palladacycle affords photoluminescent p-extended
4,7-diaryl-2,1,3-benzothiadiazoles 4a–e in high yields. These 4,7-diaryl-2,1,3-benzothiadiazoles exhibit high fluorescent quantum yields,
high electron affinities and adequate band gap values for testing as OLEDs. The 4,7-bis-naphthyl-2,1,3-benzothiadiazole 4b presents two
different lifetimes (bi-exponential decay) due to the presence of two atropisomers. The Sonogashira coupling reaction of 4,7-diethynyl-2,1,3-
benzothiadiazole 6 with the corresponding halo-aryl compounds (iodobenzene, 1-bromonaphthalene, 4-iodoanisole, 4-bromo-N,N-
dimethylaniline and 2-bromopyridine) afforded the photoluminescent p-extended 4,7-bis-alkynylaryl-2,1,3-benzothiadiazoles 7a–e, also in
high yields. These 4,7-diethynyl-2,1,3-benzothiadiazoles also present high fluorescent quantum yields, high electron affinities and adequate
band gap values for testing as OLEDs. The 4,7-disubstituted-2,1,3-benzothiadiazoles 4a–e and 7a–e exhibit different electrochemical
behavior. The presence of two ethynyl spacers in 2,1,3-benzothiadiazoles 7a–e shifts the reduction potentials to less cathodic values and also
results in two well-defined and distinct reduction processes.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
ordered crystal structures; and (b) benzothiadiazole deriva-
tives normally are efficient fluorophores.4 Highly fluor-
escent p-conjugated molecules are also of interest in other
applications such as electroluminescent (EL) devices.5
Surprisingly, the synthesis of benzothiadiazole derivatives
and investigations of their photoluminescent properties are
relatively limited. In fact, most of these studies have been
centered on the preparation of 4,7-diphenyl-2,1,3-benzo-
thiadiazole, 4,7-bis(4-methoxy-phenyl)-2,1,3-benzothia-
diazole and 4,7-bis[(2-pyridyl)-ethynyl]-2,1,3-benzothia-
diazole derivatives, which were obtained in moderate
yields.6 Moreover, only a few photophysical and electro-
chemical properties have been reported so far. In view of the
very promising photoluminescent properties of these
p-conjugated molecules, it is of a great interest to have a
simple synthetic methodology for their preparation.
Fluorescent compounds have found widespread use in
scientific and technological areas, especially as organic
light-emitting diodes (OLEDs).1 Due to their potential as
constituents of OLEDs, much attention has been focused on
p-conjugated molecules, their luminescent properties and
their electronic and optoelectronic functions.2 Only a few
types of molecular building blocks have been exploited in
the organic electronic area,3a such as quinoxalines,3b
benzimidazoles3c and certain polymers.3d Among these
building blocks, benzothiadiazoles are one of the most
important classes due to their relatively high reduction
potential and electron affinity, necessary for utilization in
light-emitting diode (LED) technology. Indeed, the HOMO/
LUMO levels of p-extended conjugated molecules are
defined by their electron affinity (EA) and ionization
potential (IP), which are correlated with electrochemical
reduction and oxidation potentials. 2,1,3-Benzothiadiazole
derivatives have several desirable characteristics: (a)
benzothiadiazole-containing compounds can afford well-
We report here that palladium promoted C–C coupling
reactions of the Suzuki and Sonogashira type are effective
methods for the construction of a new class of more highly
luminescent p-extended 4,7-disubstituted-2,1,3-benzothia-
diazoles. We also present the electrochemical and photo-
luminescent properties of ten such compounds, which
indicate that they are indeed potential candidates for
OLED devices.
Keywords: Sonogashira coupling; Spectrometry; OLED; Photoluminecent;
Benzothiadiazoles.
Corresponding author. Tel.: C55 51 33166321; fax: C55 51 33167304;
e-mail: dupont@iq.ufrgs.br
*
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.08.093

10976
B. A. DaSilveira Neto et al. / Tetrahedron 61 (2005) 10975–10982
2. Results and discussion
at 60 8C with iodobenzene, 1-bromonaphthalene, 4-bromo-
anisole, 4-bromo-N,N-dimethylaniline or 2-bromopyridine,
affording the desired products 7a–e, respectively, in
excellent yields after column purification (Scheme 3).12
Although 7e is a known compound,6 the reported synthesis
via a single Sonogashira coupling step had only a 47% yield
and the electrochemical properties of the compound were
not reported. Our methodology improves the yield to 93% in
the final Sonogashira coupling, with a global yield of 87%
for the synthesis.
Commercially available o-phenylenediamine, 1, was treated
with freshly distilled thionyl chloride in the presence of
triethylamine in CH2Cl2 as solvent, affording 2,1,3-
benzothiadiazole, 2, in 93% yield after steam distillation.7
Upon reaction with molecular bromine (added drop-wise
very slowly) in hydrobromic acid, compound 2 gives
exclusively the 4,7-disubstituted regioisomer 3 in 95%
yield (Scheme 1).8
1
All new compounds were fully characterized by FTIR, H
NMR and 13C NMR, and high-resolution mass spectrometry
(HRMS) and the data are in full accord with the proposed
structures. Differential scanning calorimetry (DSC) was
used to determine the thermal properties of compounds 4a–e
and 7a–e (melting or decomposition temperatures). Table 1
summarizes the results of UV–vis (recorded in acetonitrile
solution), fluorescence and electrochemical analyses (by
cyclic voltammetry)13 of the synthesized photoluminescent
compounds 4a–e and 7a–e and some other known
compounds (Scheme 4).
Scheme 1. Synthesis of benzothiadiazole 3.
The Suzuki coupling reaction of compound 3 with phenyl or
1-naphthyl boronic acid, catalyzed by the NCP pincer
palladacycle,9 produced the desired bis-coupled p-extended
4,7-diarylsubstituted 2,1,3-benzothiadiazoles 4a–e in excel-
lent yields after column chromatographic purification
(Scheme 2). Although compounds 4a and 4c have
previously been prepared by using the classical Suzuki Pd
coupling catalyst,10 the yields were only 48%. With our
methodology, these compounds are obtained in much higher
yields (94–99% yield).
op
For all compounds, for which both the optical (E ) and
gap
el
gap
electrochemical (E ) band gaps could be calculated,17
there is excellent agreement between the two values.
Moreover, the values are in the appropriate range (between
1.5 and 5.5 eV) for application in OLED devices.18 For
example, copolymers with the benzothiadiazole unit has a
band gap values in the range of 2.03–8.61 eV2c and zinc
complex with the 2,1,3-benzothiadiazole moiety has a band
gap of 1.88 eV.19 Many other examples with values in this
range can be found in the literature.20 The electron affinity
(EA) and ionization potential (IP) have been determined
using a known method (EAZEorendsetC4.4 eV and IPZ
EoonxsietC4.4 eV).3a Molecules 4a–c and 7a–e also have high
values of EA (2.72–3.72 eV). This is fundamental in order
to allow electron injection from stable metal cathodes.21
Comparison of 4a and 7a and of 4b and 7b indicates that the
insertion of a triple bond does not significantly affect the
band gap energies in molecules without electron donor or
electron withdrawing groups, although the EA and IP
diminish by about 0.5 eV upon extending the conjugation. A
similar behavior is observed when a MeO group is inserted
into molecules 4c and 7c. The insertion of a N(Me)2 group
in molecule 7d indicates that the presence of an electron
donating group does not significantly affect the electro-
chemical behavior of the compounds. With an electron
withdrawing group, as in molecule 7e (2-pyridyl, in this
case), the reduction process begins at less cathodic values
(K0.68 V) compared to molecules 7c and 7d (OMe and
N(Me)2 groups), which begin at K0.95 and K1.37 V,
respectively. This fact indicates that electron withdrawing
groups facilitate the reduction process. The presence of
the triple bonds (compare compounds 4 and 7) affects the
electrochemical charge transfer processes by shifting the
electrochemical window to more cathodic values.
Scheme 2. Synthesis of p-extend benzothiadiazoles 4a–e.
The Sonogashira cross-coupling reaction of 3 with
trimethylsilylacetylene in the presence of PdCl2(PPh3)2,
CuI, and PPh3 in NEt3 at 90 8C produced the bis-coupling
product 5.11 The de-protection of 5 with KF afforded 6 in
quantitative yield after column chromatographic purifi-
cation. A second Sonogashira cross-coupling reaction was
conducted in the presence of PdCl2(PPh3)2 and CuI, in NEt3
Compounds 4a, 4b, 4c, and 7a exhibit oxidation and
reduction peaks (Fig. 1), which suggests the possibility of
both hole and electron transport, leading to a single-layer
electroluminescent device.22 This fact is very important
since it would not be necessary to employ different
Scheme 3. Synthesis of p-extend benzothiadiazoles 7a–e.

Table 1. UV–vis, fluorescence and some electrochemical data for compounds 4a–e, 7a–e, 8a–b, and 9a–ba
onset
red
onset
oxi
el
gap
op
gap
Compound
E
(V)b
EAc (eV)
E
(V)d
IPe (eV)
E
(eV)f
Log 3
lmabasx (nm)
lmemax (nm)
Stoke’s
shift (nm)
Ffg
tf (Singlet)
(ns)h
tT (Triplet)
(ms)i
E
(eV)j
4a
K1.05
3.35
1.65
6.05
2.70
4.04
402, (380)k, 487, (490)k, 85
0.80, (0.
80)k, (0.74)l
0.17
0.51
0.50
0.22
0.37
0.36
0.29
12.2
—
2.73
(379)l
360
362
368
355
403
363
393
407
(490)l
517
4b
K1.18
K1.23
—
3.22
3.17
—
1.44
1.32
—
5.84
5.72
—
2.62
2.55
—
4.08
3.52
5.30
3.86
4.99
3.56
4.40
4.11
157
185
111
98
94
71
149
31
75
4.96 and 2.38m 14.4
2.68
2.54
2.75
2.74
2.57
2.99
2.37
2.93
2.60
na
4c
547
479
453
497
434
542
438
12.70
10.40
6.31
5.53
8.37
6.42
8.18
—
4dk
4ek
7a
4.18
6.04
21.6
6.05
20.01
23.02
29.50
na
—
—
—
—
—
K1.57
K1.68
K0.95
K1.37
K0.68
na
na
na
na
2.83
2.72
3.45
3.03
3.72
na
na
na
na
1.02
1.23
1.47
1.61
1.93
na
na
na
na
5.42
5.63
5.87
6.01
6.33
na
na
na
na
2.59
2.91
2.42
2.98
2.61
na
na
na
na
7b
7cn
7d
0.15
7e
4.40 (4.48)o 394 (393)o
na
4.07
4.45
4.43
469 (473)o
630
546
0.86 (0.87)p 4.52
8al
8bp
9ao
9bo
481
410
396
388
149
136
83
0.75
0.75
0.80
0.87
na
na
na
na
na
na
na
na
na
na
479
464
76
a Electrochemical experiments in MeCN–CH2Cl2 (3/7 v/v) and potentials against ferrocene (Ref. 3a). Platinum was used as working electrode and as a counter electrode. All potentials were recorded versus Ag/AgCl
(saturated) as a reference electrode.
b Onset cathode potential.
c Electron affinity.
d Onset anode potential.
e Ionization potential.
f
Energy of the band gap (electrochemical).
g Quantum yield of fluorescence (quinine sulfate (Riedel) in 1 M H2SO4, FfZ0.55, as standard).
h Singlet lifetime, by single photon counting.
i
Triplet lifetime, by nanosecond laser flash photolysis.
Energy of the band gap (optical).
j
k The molecule did not form a film to be analyzed by thin film CV (only in solution).
l
Ref. 14.
mBiexponential decay (two lifetimes).
n The monomer properties have not been described (Ref. 16). na, not available.
o Ref. 6a.
p Ref. 15.

10978
B. A. DaSilveira Neto et al. / Tetrahedron 61 (2005) 10975–10982
Scheme 4. Other known benzothiadiazole systems.
Figure 3. CV of compounds 4a–e (1.00 mmol LK1) dissolved in MeCN–
CH2Cl2 (3/7 v/v) recorded at scan rateZ200 mV/s.
probably the first charge transfer process involves a quasi-
reversible reduction of the benzothiadiazole ring (observed
in both series), while the second is associated with reduction
of the triple bond (Figs. 2 and 3). Apparently, the second
charge transfer is an irreversible electrochemical process.
As noted, both series exhibit the first process. This is
indicated by the fact that the current versus potential curves
have the same shape for all compounds (4a–e and 7a–e,
Table 2—see all individual CV curves in the Supporting
information). Since compounds 4a–e have no triple bond,
the well-defined reduction process can only be attributed to
reduction of the benzothiadiazole ring. A plausible
mechanism for this process has been presented by Hirao
et al.23 An exception occurs in benzothiadiazole 4e, which
shows one defined irreversible reduction peaks, but in the
same potential range.
Figure 1. CV (thin film coated onto a Pt wire electrode) of compounds 4a–c
and 7a–b recorded at scan rateZ40 mV/s in MeCN–CH2Cl2 (3/7 v/v).
molecules in the electron-transporting and hole-transporting
layers of organic light emitting devices. Compound 7b did
not show current peaks associated with oxidation/reduction
processes. In contrast, molecules 7c–d (voltammograms not
shown) do not show well-defined oxidation peaks.
Compounds 4a–b and 7a exhibit two oxidation peaks
involving multiple deelectronation processes. All molecules,
however, show well-defined reduction peaks and, in some
cases, two well-defined peaks (4a and 7b). Due to the
impossibility of forming films of molecules 4d and 4e, they
could not be analyzed as a thin film coated onto a Pt wire
electrode, only in solution.
Compounds 7a and 7e show two well-defined and very
distinct reduction processes. The first one is similar to
compounds 4a–e, indicating that the benzothiadiazole ring
is reduced in preference to the triple bond in those systems.
The second process is the reduction of the triple bond, a
process that cannot occur in molecules 4a–e. Comparison of
the reduction behavior of compounds 7a and 7e indicates
that the second reduction process for 7a occurs at much
more cathodic values than that of 7e. The origin of this
difference is still not clear.
Compounds 7a–e exhibit different electrochemical behavior
from that of compounds 4a–e, by comparing both series
dissolved in solution. The former exhibit two well-defined
sets of reduction peaks (from K1.0 to K1.3 V and from
K1.7 to K1.9 V). This suggests the occurrence of two
different processes when the species are reduced. Most
All compounds have rather good fluorescence quantum
yields, especially 4a (FfZ0.80) and 7e (FfZ0.86), and
show large Stoke’s shifts (31–185 nm). The fluorescence
quantum yield of benzothiadiazole 3 (without p-extended
conjugation), which has not been reported before, is very
low (FfZ0.006). The lowest energy absorption bands (in
acetonitrile) are assigned to p–p* transitions by virtue of
their large molar extinction coefficients (log 3 values in the
range of 3.52–5.30). The absorption (lmabasx) and emission
(lmemax) maxima lie between 355–407 nm and 434–547 nm,
respectively (Figs. 4 and 5). Molecules 4c and 7c (with a
MeO group) have large Stoke’s shifts, 185 and 149 nm,
respectively. This indicates a very efficient intramolecular
charge transfer (ICT) in the excited state between the
terminal methoxy group and the benzothiadiazole nucleus.
Figure 2. CV of compounds 7a–e (1.00 mmol LK1) dissolved in MeCN–
CH2Cl2 (3/7 v/v) recorded at scan rateZ200 mV/s.
Compound 4b not only exhibits a large Stoke’s shift
(157 nm), but it is also the only compound whose

B. A. DaSilveira Neto et al. / Tetrahedron 61 (2005) 10975–10982
Table 2. Reduction and oxidation data of compounds 4a–e, 7a–e, 8a–b, and 9a–b
10979
Compound
Eored (V)
First process
Eored (V)
Second process
Eooxi (V)
First process
Eooxi (V)
Second process
Reference
b
4a
4b
4c
4d
4e
7a
7b
7c
7d
7e
8a
8b
9a
9b
K1.43 (K1.36)a
K1.45
K1.55
K1.66
K1.62
—
1.48 (1.82)a
1.91
1.74
4.25 (K)a
4.77
—
This work and 14
This work
This work
This work
This work
This work
This work
This work
This work
This work and 6a
14
b
—
b
—
b
—
c
c
—
—
b
c
c
—
K1.98
—
—
2.50
—
3.77
K1.21
K1.13
K1.10
K1.16
b
d
d
—
—
b
d
—
d
—
—
b
d
d
—
K1.72 (K)e
—
3.54 (K)e
0.92
na
na
na
—
4.12 (K)e
na
na
na
na
K1.11 (K1.18)e
K1.40
na
na
na
na
na
15
6a
6a
K1.08
K1.00
a Ref. 14.
b The molecule has just one process.
c The molecule did not form a film to be analyzed by thin film CV (only in solution).
d The molecule did not show an oxidation peak, only Eonset
.
e Ref. 6a. na, not available.
Figure 6. Compound 4b atropisomers.
Although the presence of the triple bonds in compounds
7a–e extends the p-conjugation compared to 4a–e, this has
only a secondary effect on the photophysical properties of
the compounds, indicating that this structural change is not
fundamental for the fluorescence process itself.
Figure 4. Fluorescence spectra of compounds 4a–e in acetonitrile.
All compounds exhibit strong fluorescence and, with the
exception of 4b, a single fluorescence lifetime. Radiative
decay rate constants (ZFf/tf) are in the range of 5!107 sK1
,
consistent with the observed range of log 3 values for
absorption, and are insensitive to the presence of the triple
bond spacers. With the exception of molecules 4a and 4c,
the remaining molecules show strong triplet–triplet absorp-
tion by laser flash photolysis (355 nm excitation, Nd-YAG
laser) in the absence of oxygen, with triplet lifetimes in the
range of 4–30 ms, indicating efficient intersystem crossing in
these eight compounds. The strong triplet–triplet absorption
and lack of noticeable laser-induced decomposition indicate
high chemical stability also in the excited state. The only
limitation would be the necessity of the absence oxygen,
since the triplet states of all of the compounds are efficiently
quenched by molecular oxygen (presumably via energy
transfer to form singlet oxygen). The motives for the failure
to observe triplet–triplet absorption with 4a and 4c are
unclear and would require more detailed photophysical
studies for clarification.
Figure 5. Fluorescence spectra of compounds 7a–e in acetonitrile.
fluorescence decay exhibits two lifetimes (4.96 and 2.38 ns).
Likewise, the 1H and 13C-{1H} NMR spectra of 4b in
DMSO-d6 at room temperature (see Supporting infor-
mation, also for fluorescence decay) show two sets of
signals, indicating the presence of two isomers. The fact
that these two sets of signals coalesce at 90–100 8C points to
the existence at room temperature of two atropisomers in
which the a-naphthyl groups are in syn or anti geometries
(Fig. 6).
All of the compounds also present fluorescence in the solid
state (Table 3).

10980
B. A. DaSilveira Neto et al. / Tetrahedron 61 (2005) 10975–10982
Table 3. Excitation and emission maxima and Stoke’s shifts for 2,1,3-
benzothiadiazoles 4a–e and 7a–e in the solid state
7a–e was checked by CG analysis on a HP-5890A
chromatograph fitted with a DB17 capillary column
(25 m) with temperature programming from 100 8C up to
250 8C at a heating rate of 15 8C/min. The pressure at the
column head was 10 psi. Cyclic voltammograms (CV) were
recorded on an Autolab PGSTAT 30 Potentiostat. A thin
layer of molecules 4 and 7 was coated from its solution in
CH2Cl2, using similar procedure employed for diphenyl-
anthrazolines.3a UV–vis absorption spectra were taken on a
Cary 50 Varian spectrophotometer or a Shimadzu Model
UV-1601PC. For fluorescence quantum yields (quinine
sulfate (Riedel) in 1 M H2SO4, FfZ0.55, as standard), a
Shimadzu UV-1601PC spectrophotometer and a Hitachi
Model F-4500 spectrofluorometer were employed. Fluor-
escence decays were collected by the time-correlated single
photon counting technique with an Edinburgh Analytical
Instruments FL900 lifetime Spectrometer (H2 lamp exci-
tation source). Lifetimes were determined from the decays
by using the FL900 convolution and fitting routines for
mono- and bi-exponential decay. Nanosecond laser flash
photolysis experiments were performed at 20 8C on air-
equilibrated solutions (MeCN) and on solutions deoxy-
genated by exhaustive purging with solvent-vapor-saturated
argon in cuvettes capped with a rubber septum. The
Edinburgh Analytical Instruments LP900 laser flash
photolysis system is equipped with a 450W Xe high
pressure monitoring lamp and excitation was carried out
with the third harmonic (355 nm) of a Surelite II-10 Nd-
YAG laser. Solutions (MeCN) were stirred between each
laser shot and 10 laser shots averaged to obtain the transient
absorption decays. Solutions were monitored for laser-
induced decomposition by conventional UV–vis absorption
spectroscopy (Hewlett-Packard 8452A diode array spectro-
meter) and replaced by fresh solution at the first signs of
decomposition. The standard exponential decay routines of
the LP900 system software were used to analyze the decays
of the transient and obtain the lifetimes of the excited
species.
Compound
lmabasx (nm)
lmemax (nm)
Stoke’s shift
(nm)
4a
4b
4c
4d
4e
7a
7b
7c
7d
7e
426
427
418
495
430
424
422
522
442
452
493
495
526
544
501
543
500
590
514
543
67
68
108
49
71
119
78
68
72
91
The absorption maxima are red-shifted in all cases and, with
the exception of compounds 4b and 4c, so are the
fluorescence emission maxima. The resultant Stoke’s shifts
in the solid state are smaller for compounds 4a–e and 7c, but
larger for compounds 7a,b,d,e.
3. Conclusion
In summary, the Suzuki and Sonogashira couplings of
4,7-dibromo-2,1,3-benzothiazole with arylboronic acids
and alkynes represent simple and effective methods for
p-extension to obtain 4,7-disubstituted-2,1,3-benzothia-
diazoles. The 4,7-diaryl-2,1,3-benzothiadiazoles 4a–e pos-
sess moderate to high fluorescence quantum yields (0.17–
0.80) and high electron affinities (3.17–3.35 eV). The
fluorescence of 4,7-binaphthyl-2,1,3-benzothiadiazole, 4b,
exhibits bi-exponential decay, attributed to the presence of
two atropisomers. The band gap values of 4,7-biaryl-2,1,3-
benzothiadiazoles 4a–e are in the adequate range (2.54–
2.75 eV) for testing as OLEDs. The presence of ethynyl
spacers in the 2,1,3-benzothiadiazoles 7a–e does not have a
negative impact on the photo-electronic properties of those
compounds and their fluorescent quantum yields are
likewise high (0.15–0.86). These alkynyl benzothiazoles
do, however, possess different electrochemical behavior due
to the presence of the reducible triple bond. The electron
affinities of 4,7-bis-ethynylaryl-2,1,3-benzothiadiazoles
7a–e are also high (2.72–3.72 eV) and their band gap
values in the appropriate range for testing as OLEDs
(2.37–2.99 eV).
4.1.1. General procedure for the synthesis of benzothia-
diazole 2. To a 1000 mL flask were added commercial
o-phenylenediamine 1 (10.00 g, 92.47 mmol), 300 mL of
CH2Cl2 and triethylamine (37.44 g, 369.98 mmol). The
solution was stirred until total dissolution of the diamine 1.
Thionyl chloride was added dropwise very slowly and the
mixture refluxed for 4 h. The solvent was removed in a
rotatory evaporator and 700 mL of water added. Concen-
trated HCl was added to a final pH of 2. The desired
compound was purified by direct steam distillation
following addition of water to the mixture. The steam
distilled mixture was extracted three times with 200 mL of
CH2Cl2, dried over MgSO4 and filtered. The solvent was
removed, affording pure compound 2 in 93% yield (11.71 g,
85.99 mmol).
4. Experimental
4.1. General
All catalytic reactions were carried out under an argon or
nitrogen atmosphere in oven-dried resealable Schlenk tubes.
All substrates were purchased from Acros and used without
further purification. The PCN palladacycle catalyst pre-
cursor was prepared according to the reported method.9 All
new compounds were fully characterized after purification.
NMR spectra were recorded on a Varian Inova 300 MHz or
Varian Gemini 200 MHz spectrometers. Infrared spectra
were registered on a Bomem B-102 spectrometer. Melting
points were measured on a 12000 PL-DSC apparatus at a
heating rate of 5 8C/min or in a Electrothermal IA9000
Melting Point apparatus. The purity of compounds 4a–e and
1H NMR (CDCl3): d ppm 7.99 (dd, 2H, JZ3.3, 4.6 Hz);
7.57 (dd, 2H, JZ3.1, 6.8 Hz). 13C NMR (CDCl3): d ppm
154.6; 129.1; 122.4. FTIR (KBr, cmK1): 1659, 1433, 1264,
1104, 747. Mp 43.6–44.4. Lit.7a,b 44 8C.
4.1.2. General procedure for the synthesis of 4,7-
dibromobenzothiadiazole 3. To a 500 mL two-necked
round bottom flask were added benzothiadiazole 2 (10.00 g,

B. A. DaSilveira Neto et al. / Tetrahedron 61 (2005) 10975–10982
10981
73.44 mmol) and 150 mL of HBr (47%). A solution
containing Br2 (35.21 g, 220.32 mmol) in 100 mL of HBr
was added dropwise very slowly (slow addition is
essential!). If necessary, an additional 100 mL of HBr can
be added to the solution. After total addition of the Br2, the
solution was refluxed for 6 h. Precipitation of an orange
solid was noted. The mixture was allowed to cool to room
temperature and sufficient saturated solution of NaHSO3
added to consume completely any excess Br2. The mixture
was filtered under vacuum and washed exhaustively with
water. The solid was then washed once with cold Et2O and
dried under vacuum for ca. 20 h, affording the desired
dibrominated product 3 in 95% yield (20.51 g, 69.77 mmol).
8.14–7.58 (m, 10H). 13C NMR (CDCl3): d ppm 153.8,
153.7, 152.9, 152.8, 129.5, 129.4, 128.7, 128.4, 125.6,
125.59, 113.8. FTIR (KBr, cmK1): 2924, 1669, 1334, 1169.
Mp 103 8C. HRMS calcd for C20H10F6N2S 424.046890,
found 424.04688.
4.3. General procedure for the Sonogashira coupling
reactions
A mixture of 3 (1.593 g, 5.42 mmol), trimethylsilylacetyl-
ene (1.410 g, 14.35 mmol), Pd(PPh3)2Cl2 (20 mg), cuprous
iodide (20 mg), and triphenylphosphine (70 mg) was
suspended in triethylamine (20 mL), and the resulting
suspension was stirred and heated at 90 8C for 4 h. The
solvent was evaporated and the crude product chromato-
graphed directly with diethyl ether, affording a yellow solid.
The isolated product 5 (air unstable) was dissolved in
methanol (25 mL), treated with potassium fluoride (1.260 g,
21.68 mmol) and stirred at room temperature overnight. The
solvent was evaporated and the crude product chromato-
graphed directly with ether, affording a yellow solid.
Compound 6 (very unstable) was immediately submitted
to a second Sonogashira reaction. A mixture of 6 (0.998 g,
5.42 mmol), the corresponding halogenated compound
(11.38 mmol), Pd(PPh3)2Cl2 (20 mg), and cuprous iodide
(20 mg) was suspended in triethylamine (20 mL), and the
resulting suspension was stirred and heated at 60 8C for
18 h. The solvent was then evaporated and the crude product
chromatographed directly with ether–n-hexane (20/80),
affording the desired products 7a–e.
1H NMR (CDCl3/DMSO-d6—two drops): d ppm 7.73 (s,
2H). 13C NMR (CDCl3/DMSO-d6—two drops): d ppm
152.6; 132.1; 113.6. Mp 187–188 8C. Lit.8 188–189 8C.
4.2. General procedure for the Suzuki coupling reactions
An oven-dried resealable Schlenk tube was evacuated and
back-filled with Ar and charged with CsF (568 mg,
3.74 mmol), arylboronic acid (3.74 mmol) and the NCP
pincer palladacycle (1 mol%).9 Compound 3 (500 mg,
1.70 mmol) was added in 1,4-dioxane (5 mL). The reaction
mixture was stirred at 130 8C for 18 h. The solution was then
allowed to cool to room temperature and the solvent
evaporated under reduced pressure. The crude material was
chromatographed directly on silica gel with Et2O.
4.2.1. Benzothiadiazole 4a. 1H NMR (CDCl3): d ppm 8.23
(d, 2H, JZ6.4 Hz), 7.97–7.24 (m, 10H). 13C NMR (CDCl3):
d ppm 137.3, 136.5, 133.9, 133.2, 113.8, 113.0. FTIR (KBr,
cmK1): 1586, 1463, 1424, 1333. Mp 84 8C HRMS calcd for
C18H12N2S 288.07212, found 288.07212.
4.3.1. Benzothiadiazole 5. Compound 5 is unstable and
1
becomes a dark brown solid in a few minutes. H NMR
(CDCl3): d ppm 7.37 (s, 2H), 0.10 (s, 18H). 13C NMR
(CDCl3): d ppm 154.15, 133.08, 117.21, 103.58, 99.96,
0.11. FTIR (KBr, cmK1): 2152, 1536, 1248, 842.
4.2.2. Benzothiadiazole 4b. 1H NMR (CDCl3): d ppm
8.02–7.25 (m, 16H). H NMR (DMSO-d6, 20 8C): d ppm
1
4.3.2. Benzothiadiazole 6. Compound 6 is unstable and
8.42–8.33 (m, 2H), 8.22–8.02 (m, 2H), 7.94–7.81 (m, 4H),
7.77–7.58 (m, 3H), 7.52–7.41 (m, 5H). H NMR (DMSO-
1
becomes a dark brown solid in a few minutes. H NMR
1
(CDCl3): d ppm 7.76 (s, 2H), 3.69 (s,2H). 13C NMR
(CDCl3): d ppm 154.23, 133.14, 116.65, 85.33, 78.87. FTIR
(KBr, cmK1): 3052, 2988, 2294, 1545, 1426.
d6, 100 8C): d ppm 8.21–8.10 (m, 2H), 7.98–7.86 (m, 2H),
7.63–7.42 (m, 5H), 7.40–7.28 (m, 4H). 13C NMR (DMSO-
d6): d ppm 153.8, 152.7, 135.5, 134.5, 133.2, 132.9, 132.3,
131.9, 130.9, 130.6, 129.1, 128.7, 128.3, 128.2, 128.1,
127.9, 127.7, 127.4, 126.4, 126.0, 125.9, 125.7, 125.5,
125.4, 125.3, 124.9. FTIR (KBr, cmK1): 3037, 1589, 1506,
1483, 1396, 1325, 1265. Mp 184 8C. HRMS calcd for
C26H16N2S 388.10342, found 388.10445.
4.3.3. Benzothiadiazole 7a. 1H NMR (CDCl3): d ppm 8.69
(d, 2H, JZ4.6 Hz), 7.89 (s, 2H), 7.79–7.68 (m, 4H), 7.35–
7.27 (m, 2H). 13C NMR (CDCl3): d ppm 154.26, 132.37,
131.92, 129.02, 128.37, 122.39, 117.08, 97.41, 85.23. FTIR
(KBr, cmK1): 3037, 1537, 840, 750, 688. Mp 159 8C.
HRMS calcd for C22H12N2S 336.07212, found 336.07217.
4.2.3. Benzothiadiazole 4c. 1H NMR (CDCl3): d ppm 3.89
(s, 6H), 7.08 (d, 4H, JZ8.4 Hz), 7.70 (s, 2H), 7.32 (d, 4H,
JZ8.2 Hz). 13C NMR (CDCl3): d ppm 159.6, 154.1, 132.2,
130.3, 129.9, 127.4, 114.0, 55.4. FTIR (KBr, cmK1): 3029,
2954, 1604, 1519, 1284. Mp 207 8C. HRMS calcd for
C20H16N2O2S 348.09325, found 348.09323.
4.3.4. Benzothiadiazole 7b. 1H NMR (CDCl3): d ppm 8.21
(d, 2H, JZ8.2 Hz), 7.80–7.23 (m, 14H). 13C NMR (CDCl3):
d 154.14, 134.47, 133.08, 131.84, 129.78, 128.19, 127.81,
127.21, 126.96, 126.58, 126.07, 122.71, 116.56, 78.85.
FTIR (KBr, cmK1): 2935, 2860,1585, 1265, 844, 742, 615.
Mp 116 8C. HRMS calcd for C30H16N2S 436.10342, found
436.10340.
1
4.2.4. Benzothiadiazole 4d. H NMR (DMSO-d6): d ppm
8.20–7.70 (m, 6H) 7.69–47 (m, 4H). 13C NMR (DMSO-d6):
d ppm 153.1, 152.3, 133.5, 132.6, 130.9, 128.7, 112.8. FTIR
(KBr, cmK1): 3045, 1529, 1469, 1094. Mp 170 8C. HRMS
calcd for C18H10Cl2N2S 355.994176, found 355.99416.
4.3.5. Benzothiadiazole 7c. 1H NMR (CDCl3): d ppm 7.74
(s, 2H), 7.61 (d, 4H, JZ8.8 Hz), 6.92 (d, 4H, JZ8.8 Hz),
3.85 (s, 6H). 13C NMR (CDCl3): d 160.14, 154.30, 133.51,
132.08, 117.02, 114.53, 114.04, 97.60, 84.31, 55.33. FTIR
4.2.5. Benzothiadiazole 4e. 1H NMR (CDCl3): d ppm

10982
B. A. DaSilveira Neto et al. / Tetrahedron 61 (2005) 10975–10982
(KBr, cmK1): 3044, 1599, 1509, 1289. Mp 201 8C. HRMS
calcd for C24H16N2O2S 396.093250, found 396.09407.
C-H. Adv. Funct. Mater. 2004, 14, 83–90. (b) Zhang, X.;
Gorohmaru, H.; Kadowaki, M.; Kobayashi, T.; Ishi-I, T.;
Thiemann, T.; Mataka, S. J. Mater. Chem. 2004, 14,
1901–1904.
4.3.6. Benzothiadiazole 7d. 1H NMR (CDCl3): d ppm 7.76
(d, 2H, JZ1.0 Hz), 7.29 (d, 4H, JZ9.0 Hz), 6.58 (d, 4H,
JZ8.8 Hz) 2.92 (s, 3H) 2.91 (s, 3H). 13C NMR (CDCl3): d
154.31, 149.45, 133.22, 131.63, 116.71, 114.04, 108.44,
85.33, 78.89. FTIR (KBr, cmK1): 3089, 2924, 1574, 1489,
1444, 1239. Mp 178 8C. HRMS calcd for C26H22N4S
422.156519, found 422.15663.
5. Kraft, A.; Grimsdale, C. A.; Homes, B. A. Angew. Chem., Int.
Ed. Engl. 1998, 37, 402–428.
6. (a) Akhtaruzzaman, M.; Tomura, M.; Zaman, M. B.; Nishida,
J.; Yamashita, Y. J. Org. Chem. 2002, 67, 7813–7818.
(b) Akhtaruzzaman, M.; Tomura, M.; Nishida, J.; Yamashita,
Y. Synth. Met. 2003, 137, 873–874.
7. (a) Khaletskii, A. M.; Pesin, V. G.; Chi-Chun, C. Doklady
Akad. Nauk. S.S.S.R. 1956, 106, 88–91. Chem. Abstr. 1956, 50,
13885c. (b) Hinsberg, O. Chem. Ber. 1889, 22, 2895–2902. (c)
Weinstock, L. M.; Davis, P.; Hanclelsman, B.; Tull, R. J. Org.
Chem. 1967, 32, 2823–2829. (d) Carmack, M.; Street, R. W.;
Komim, A. P. J. Org. Chem. 1975, 40, 2749–2752.
8. Pilgran, K.; Zupan, M.; Skiles, R. J. Heterocycl. Chem. 1970,
7, 629–633.
4.3.7. Benzothiadiazole 7e. 1H NMR (CDCl3): d ppm 8.69
(d, 2H, JZ4.6 Hz) 7.89 (s, 2H), 7.79–7.68 (m, 4H), 7.35–
7.27 (m, 2H). 13C NMR (CDCl3): d 154.27, 150.29, 142.70,
136.19, 133.06, 127.96, 123.41, 116.97, 96.39, 84.45. FTIR
(KBr, cmK1): 3049, 2924, 1574, 1489, 1444, 1239. Mp
233 8C. Lit.6aZ232–234. Anal. Calcd For C20H10N4S C,
70.99; H, 2.98; N, 16.56; S, 9.48. Found: C, 70.51; H, 2.94;
N, 16.55.
9. (a) Rosa, G. R.; Ebeling, G.; Dupont, J.; Monteiro, A. L.
Synthesis 2003, 2894–2897. (b) Dupont, J.; Consorti, C. S.;
Spencer, J. Chem. Rev. 2005, 105, 2527–2571.
10. Fang, Q.; Tanimoto, A.; Yamamoto, T. Synth. Met. 2005, 73,
150.
Acknowledgements
Thanks are due to the following Brazilian agencies: CNPq,
CAPES, FAPESP and FAPERGS for partial financial
support and fellowships to B.A.S.N. and A.S.L.
11. Consorti, C. S.; Ebeling, G.; Rodembusch, F.; Stefani, V.;
Livotto, P. R.; Rominger, F.; Quina, F. H.; Yihwa, C.; Dupont,
J. Inorg. Chem. 2004, 43, 530–536.
12. Chen, X.; Wang, K.; Li, H.; Wen, J. Liq. Cryst. 2002, 29,
989–993.
13. Platinum was used as working electrode and as a counter
electrode. All potentials were recorded versus Ag/AgCl
(saturated) as a reference electrode.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tet.2005.08.
093. The complete set of the UV–vis, 1H and 13C NMR and
Fluorescence spectra, and cyclic voltammograms and
HRMS of the new compounds are available.
14. Blanchard, P.; Raimundo, J.-M.; Roncali, J. Synth. Met. 2001,
119, 527–528.
15. Zhang, X.; Gorohmaru, H.; Kadowaki, M.; Kobayashi, T.;
Ishi-i, T.; Thiemann, T.; Mataka, S. J. Mater. Chem. 2004, 14,
1901–1904.
¨
16. Kohler, A.; Wilson, J. S.; Friend, R. H.; Al-Suti, M. K.; Khan,
¨
M. S.; Gerhard, A.; Bassler, H. J. Chem. Phys. 2002, 116,
9457–9463.
17. (a) Agrawal, A. K.; Jenekhe, S. A. Chem. Mater. 1996, 8,
579–589. (b) Yang, C. J.; Jenekhe, S. A. Macromolecules
1995, 28, 1180–1196. (c) Alam, M. M.; Jenekhe, S. A. J. Phys.
Chem. B 2002, 106, 11172–11177. (d) Jegou, G.; Jenekhe,
S. A. Macromolecules 2001, 34, 7926–7928.
References and notes
1. (a) Lee, M. T.; Yen, C. K.; Yang, W. P.; Chen, H. H.; Liao,
C. H.; Tsai, C. H.; Chen, C. H. Org. Lett. 2004, 6, 1241–1244.
(b) Odom, S. A.; Parkin, S. R.; Anthony, J. E. Org. Lett. 2003,
5, 4245–4248. (c) Shen, W. J.; Dodda, R.; Wu, C. C.; Wu, F. I.;
Liu, T. H.; Chen, H. H.; Chen, C. H.; Shu, C. F. Chem. Mater.
2004, 16, 930–934.
´
18. Ortiz, R. P.; Delgado, M. C. R.; Casado, J.; Hernandez, V.;
Kim, O.-K.; Woo, H. Y.; Navarrete, J. T. L. J. Am. Chem. Soc.
2004, 126, 13363–13376.
19. Susumo, K.; Duncan, T. V.; Thierien, M. J. J. Am. Chem. Soc.
2005, 127, 5186–5195.
2. (a) Akhtaruzzaman, M.; Tomura, M.; Nishida, J.; Yamashita,
Y. J. Org. Chem. 2004, 69, 2953–2958. (b) Sun, S.-S.; Lees,
A. J. J. Am. Chem. Soc. 2000, 122, 8956–8967. (c) Hou, Q.;
Zhou, Q.; Zhang, Y.; Yang, W.; Yang, R.; Cao, Y.
Macromolecules 2004, 37, 6299–6305.
20. (a) Yang, R.; Tian, R.; Yan, J.; Zhang, Y.; Yang, J.; Hou, Q.;
Yang, W.; Zhang, C.; Cao, Y. Macromolecules 2005, 38,
244–253. (b) Velusamy, M.; Thomas, K. R. J.; Lin, J. T.; Hsu,
Y-C. ; Ho, K-C. Org Lett. 2005, 7, 1899–1902. (c) Shi, C.;
Wu, Y.; Zeng, W.; Xie, Y.; Yang, K.; Cao, Y. Macromol.
Chem. Phys. 2005, 206, 1114–1125.
3. (a) Tonzola, C. J.; Alam, M. M.; Kaminsky, W.; Jenekhe, S. A.
J. Am. Chem. Soc. 2003, 125, 13548–13558. (b) Cui, Y.;
Zhang, X.; Jenekhe, S. A. Macromolecules 1999, 32,
3824–3826. (c) Gao, Z. Q.; Lee, C. S.; Bello, I.; Lee, S. T.;
Wu, S. K.; Yan, Z. L.; Zhang, X. H. Synth. Met. 1999, 105,
141–144. (d) Kim, D. Y.; Lee, S. K.; Kim, J. L.; Kim, J. K.;
Lee, H.; Cho, H. N.; Hong, S. I.; Kim, C. Y. Synth. Met. 2001,
121, 1707–1708.
21. Raimundo, J. M.; Blanchard, P.; Brisset, H.; Akoudad, S.;
Roncali, J. Chem. Commun. 2000, 939–940.
22. Masui, K.; Mori, A.; Okano, K.; Takamura, K.; Kinoshita, M.;
Ikeda, T. Org. Lett. 2004, 6, 2011–2014.
23. Sakurai, H.; Ritonga, M. T. S.; Shibatani, H.; Hirao, T. J. Org.
Chem. 2005, 70, 2754–2762.
4. (a) Justin, K. R.; Lin, J. T.; Velusamy, M.; Tao, Y-T. ; Chuen,