Tuning the HOMO-LUMO gap of donor-substituted benzothiazoles

Article abstract of DOI:10.1016/j.tetlet.2014.10.094

A series of push-pull benzothiazoles were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling and [2+2] cycloaddition-retroelectrocyclization reactions. The photonic and electrochemical properties of these systems exhibit strong donor-

Full text of DOI:10.1016/j.tetlet.2014.10.094

Accepted Manuscript
Tuning the HOMO–LUMO gap of donor-substituted benzothiazoles
Prabhat Gautam, Ramesh Maragani, Rajneesh Misra
PII:
S0040-4039(14)01788-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.10.094
TETL 45323
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
23 September 2014
12 October 2014
13 October 2014
Please cite this article as: Gautam, P., Maragani, R., Misra, R., Tuning the HOMO–LUMO gap of donor-substituted
benzothiazoles, Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.2014.10.094
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Graphical Abstract
Tuning the HOMO–LUMO gap of donor-
substituted benzothiazoles
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Prabhat Gautam, Ramesh Maragani, Rajneesh Misra*

1
Tetrahedron Letters
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Tuning the HOMO–LUMO gap of donor-substituted benzothiazoles
Prabhat Gautam, Ramesh Maragani, Rajneesh Misra*
1
Department of Chemistry, Indian Institute of Technology Indore, MP 452017, India.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A series of push–pull benzothiazoles were designed and synthesized by the Pd‐catalyzed
Sonogashira cross‐coupling and [2+2] cycloaddition–retroelectrocyclization reactions. The
photonic and electrochemical properties of these systems exhibit strong donor–acceptor
interaction. The BTs 5–8 show strong intramolecular charge‐transfer in the near‐infrared (NIR)
region. The absorption studies indicate systematic lowering of optical HOMO–LUMO gap with
increase in acceptor strength. The computational studies reveal that incorporation of strong
acceptors TCNE and TCNQ results in substantial stabilization of LUMO level compared to
HOMO level, leading to low HOMO–LUMO gap and bathochromic shift of the absorption
spectrum. The TCBD and DCNQ linked BTs 5–8 exhibit multi‐step redox waves and improved
thermal stability.
Available online
Keywords:
Donor–acceptor
Benzothiazole
Click chemistry
Photophysical
Electrochemical
2
009 Elsevier Ltd. All rights reserved.
In recent years research on π‐conjugated donor–acceptor (D–
A) molecular systems with low HOMO–LUMO gap and strong
intramolecular charge‐transfer (ICT) has gained momentum due
to their application in nonlinear optics (NLO) and organic
The synthesis of D–A benzothiazoles (BTs) 3–8 are shown in
Scheme 1 and 2. The bromo‐BT 2 was synthesized by the
condensation reaction of 2‐aminothiophenol (1) with 4-
bromobenzaldehyde (9) in dimethyl sulfoxide (DMSO) at 190 °C
1
,2,3
14
photovoltaics (OPVs).
The photonic properties of the donor–
for 1 h (Scheme 1).
acceptor system can be tuned either by varying the π-linker
The donor‐substituted BTs 3 and 4 were designed to explore
the D–A interaction of BT with triphenylamine and ferrocene via
acetylene linkage and to facilitate the click type [2+2]
cycloaddition–retroelectrocyclization reaction with TCNE and
TCNQ. The Pd-catalyzed Sonogashira cross-coupling reaction of
between donor and acceptor or by increasing the donor/acceptor
4
strength. Thiazoles are strong acceptor and their derivatives
benzothiazole, benzothiadiazole and thiazolothiazole have been
5
,6,7
attached with
a
variety of donors.
A
variety of
donor‐substituted benzothiazoles (BTs) have been reported in
literature which exhibit ICT. To the best of our knowledge there
are few reports on tuning of the HOMO–LUMO gap and
BT
2
with 4-ethynyl-N,N‐diphenylaniline (10) and
ethynylferrocene (11) resulted donor‐substituted acetylene
7
,8
bridged BTs 3 and 4 in 70 % and 85 % yields respectively
increasing the ICT of D–A BTs.
15
(
Scheme 1).
The cyano-based acceptors tetracyanoethylene (TCNE) and
,7,8,8-tetracyanoquinodimethane (TCNQ) undergo [2+2]
7
NH2
SH
N
cycloaddition–retroelectrocyclization reaction with electron rich
9
,10
alkynes and exhibits ICT.
Our group is engaged in the design
1
0
N
S
1
N
and synthesis of efficient D–A systems for optoelectronic
Br
[
PdCl2(PPh3)2]; CuI;
THF:TEA (1:1); 70 oC;
1
1
O
applications.
9
3
1
5 h
DMSO; 190 oC;
1 h
We have explored triphenylamine and ferrocene as strong
12,13
N
electron donors and have attached with various acceptors.
We
Br
were interested to study the ICT and HOMO–LUMO gap of
triphenylamine and ferrocene substituted BTs and their TCNE
and TCNQ derivatives. In this contribution we wish to report
S
2
Fe
N
1
1
1 2
donor–acceptor BTs of the type D–π–A and D–A –A , where
S
Fe
[
PdCl2(PPh3)2]; CuI;
triphenylamine and ferrocene are donors and BT, TCNE and
TCNQ are acceptors (Chart 1). We have explored the effect of
acceptor by varying its strength on the HOMO–LUMO gap, ICT
and thermal stability in BT‐based molecular system.
THF:TEA (1:1); 70 oC;
4
15 h
Scheme 1. Synthesis of BT chromophores 3 and 4
—
——
∗
Corresponding author. Tel.: +91 7312438710; fax: +91 7312361482; e-mail: rajneeshmisra@iiti.ac.in

2
Tetrahedron
NC
NC
CN
CN
NC
NC
CN
CN
NC
N
S
CN
NC
N
S
CN
12
1
3
N
S
N
N
N
NC
5
o
CN
DCE; 100 C; MW
10 h
DCM; 40 oC;
10 h
NC
3
CN
6
NC
NC
NC
CN
NC
NC
CN
CN
N
S
CN
NC
CN
13
N
S
CN
12
N
S
NC
Fe
CN
Fe
Fe
o
DCM; 40 C;
10 h
DCE; 100 oC; MW
0 h
7
4
1
NC
CN
8
Scheme 2. Synthesis of BTs 5–8
The BTs 5–8 were designed to study the effect of the additional
cyano‐based acceptors (TCNE and TCNQ) on the ICT, HOMO–
LUMO gap and thermal stability. The BTs 5–8 were synthesized
by the [2+2] cycloaddition–retroelectrocyclization reaction of the
acetylene linked BTs 3/4 with TCNE 12 and TCNQ 13.
The optical energy gaps in donor–acceptor BTs were derived
from the onset wavelength of UV/Vis absorption spectra in
dichloromethane. The optical HOMO–LUMO gap values in
BTs follow the order 3 > 4 > 5 > 7 > 6 > 8, which indicate that
the incorporation of TCBD and DCNQ groups lowers the
HOMO–LUMO gap. These results show that acceptor strength is
these BTs are inversely proportional to the HOMO–LUMO gap.
1
9
The [2+2] cycloaddition–retroelectrocyclization reaction of
BTs
3
and 4 with one equivalent of
TCNE 12 in
dichloromethane (DCM) resulted BTs 5 and 7 in 75 % and 80 %
The emission property of BT
3
was studied in
1
6
yield respectively (Scheme 2). The reaction of TCNQ with BTs
and 4 were sluggish in nature. To overcome this barrier the
dichloromethane (see ESI; Figure S20) and the corresponding
data is given in Table 1. The triphenylamine‐substituted BT 3
emits at 522 nm. The ferrocenyl‐substituted BT 4 and TCBD and
DCNQ linked BTs 5–8 are non‐emissive in nature.
3
reactions were carried out under the microwave irradiation. The
reaction of BTs 3 and 4 with one equivalent of TCNQ in 1,2-
dichloroethane (DCE) at 100 ºC under the microwave irradiation
for 10 h resulted BTs 6 and 8 in 60 % and 68 % yields
respectively. BTs 3–8 were purified by silica‐gel column
1
.2
1
.0
3
5
6
1
13
chromatography and well characterized by H, C NMR, and
0.8
HRMS techniques.
0
.6
Thermal stability of the organic chromophores is one of the
essential criteria for practical applications. In order to study the
thermal stability of BTs 3–8 thermogravimetric analysis (TGA)
0.4
0
.2
-
1
0.0
3
were carried out at a heating rate of 10 ºC min , under nitrogen
00
400
500
600
Wavelength (nm)
(a)
700
800
900 1000 1100
atmosphere (Figure S23; see ESI) and the decomposition
1
1
0
0
0
.2
.0
.8
.6
.4
temperatures (T
d
) for 5 % weight loss are listed in Table S1 .
values at 155 ºC
4
7
8
The acetylene linked BTs 3 and 4 exhibit T
d
and 201 ºC, whereas tetracyanobutadiene (TCBD) and
dicyanoquinodimethane (DCNQ) linked BTs 5–8 exhibits
improved T values in the range of 271–472 ºC. The trend in
d
thermal stability follows the order 7 > 5 > 8 > 6 > 4 > 3, which
indicates following: (a) The Ferrocenyl substituted BTs 4, 7 and
0.2
8
exhibit better thermal stability compared to their respective
0
.0
3
00
400
500
600
Wavelength (nm)
b)
700
800
900 1000 1100
triphenylamine analogue BTs 3, 5 and 6. (b) Incorporation of
TCBD and DCNQ units improves the thermal stability of BTs 5–
(
8
.
Figure 1. Normalized electronic absorption spectra of (a) BTs 3, 5
and 6, and (b) BTs 4, 7 and 8 in dichloromethane.
The electronic absorption spectra of the BTs 3–8 were
recorded in dichloromethane at room temperature and the data
are shown in Table 1. The BTs 3–8 exhibit strong absorption
*
7,17
between 300–450 nm, corresponding to π→π transition. The
incorporation of the TCBD and DCNQ acceptor unit in BTs
result in strong intramolecular charge‐transfer (ICT) indicating
1
2d,17,18
strong donor–acceptor interaction in BTs 5–8 (Figure 1).
The ICT band in BTs 5–8 exhibits red-shift, on increasing the
1
6
solvent polarity indicating CT character (see ESI, Figure S19).
The presence of strong CT transition result in intense color
solution of BTs 5–8 in dichloromethane (Figure 2).
−
4
Figure 2. Benzothiazoles 3–8 at 1 × 10 M concentration in
dichloromethane.

3
Table 1. Photophysical data of benzothiazoles 3–8.
4
a
-
2.0
-1.5
-1.0
-1.0
-1.0
-0.5
-0.5
-0.5
0.0
0.0
0.0
0.5
0.5
0.5
1.0
1.0
1.0
BT
Photophysical data
7
−
1
−1
λ
abs
ε
( M cm )
λ
em
Optical Gap
b
(eV)
(
nm)
(nm)
3
4
5
388
337
400
38043
41932
522
2.75
2.53
1.95
-2.0
-1.5
8
–
–
33815
28212
-2.0
-1.5
4
80
+
Potential (V) vs Fc/Fc
6
342
23132
–
1.45
4
6
50
63
sh
20483
Figure 3. Cyclic voltammograms of the BTs 4, 7 and 8.
In order to explore the electronic structure of the D–A
benzothiazoles 3–8, density functional theory (DFT) calculations
were performed at the B3LYP/6-31G** level and the Lanl2DZ
level for Fe. The contours of HOMO and LUMO are shown in
Figure S24. The HOMO orbitals in BTs 3–8 are mainly located
on electron donating triphenylamine and ferrocene unit whereas
the LUMO orbitals are delocalized over BT, TCBD and DCNQ
acceptor units.
7
8
363
27
42297
3589
–
–
1.75
1.35
6
360
37909
sh
24863
7786
4
5
7
26
02
80
a
−5
Absorbance measured in dichloromethane at 1×10
concentration; sh = shoulder; λabs: absorption wavelength; λem
emission wavelength measured in dichloromethane; ε: extinction
coefficient. determined from onset wavelength of the UV/Vis
M
:
The computational HOMO–LUMO gap follows the order 4 > 3 >
7
> 5 > 8 > 6. The trend indicates that BTs 6 and 8 with DCNQ
linkage exhibit lowest HOMO–LUMO gap followed by TCBD
linked BTs 5 and 7, and acetylene linked BTs 3 and 4. The
analysis of HOMO and LUMO energy levels reveal that
incorporation of strong acceptor TCNE and TCNQ in BTs 5–8
results in greater stabilization of the LUMO level compared to
the HOMO level leading to low HOMO–LUMO gap and large
bathochromic shift of the electronic absorption (Figure S25).
b
absorption.
The electrochemical properties of the BTs 3–8 were studied by
the cyclic voltammetric (CV) and differential pulse voltammetric
DPV) analysis in dry dichloromethane (DCM) solution at room
6
temperature (with 0.1 M TBAPF , all potentials vs Fc/Fc ). The
(
+
electrochemical data are listed in Table S1, and the cyclic and
differential pulse voltammograms are shown in Figure 3, S21 and
S22.
A series of D–A benzothiazoles (BTs) 3–8 were synthesized by
the Pd-catalyzed Sonogashira cross‐coupling reaction and [2+2]
cycloaddition–retroelectrocyclization to study the effect of the
acceptor strength on the HOMO–LUMO gap and intramolecular
charge‐transfer (ICT). The single photon absorption properties of
BTs reveal that the incorporation of TCNE and TCNQ groups
result in strong ICT bands and low HOMO–LUMO gap. The
computational studies indicate that the strong acceptors stabilize
the LUMO energy level to greater extent compared to the HOMO
level. The incorporation of cyano-based groups (TCNE and
TCNQ) in D–A BTs improves the thermal stability. The results
ushers the design of D–A BT‐based molecular systems with
strong ICT, low HOMO–LUMO gap and better thermal stability.
The BTs 3–8 are potential candidates for optoelectronic
applications. The study towards their organic photovoltaic
applications is ongoing in our laboratory.
BTs 3, 5 and 6 exhibits irreversible oxidation wave
corresponding to the oxidation of triphenylamine unit in the
2
0
region between 0.52 V to 0.65 V. On the other hand BTs 4, 7
and 8 exhibit reversible oxidation wave corresponding to the
oxidation of ferrocene to ferrocenium ion in the region between
12b
0
.10 V to 0.45 V. The TCBD and DCNQ linked BTs 5–8
exhibit higher oxidation potential compared to BTs 3 and 4
reflecting strong electronic communication.
BTs 5, 6 and 7 exhibit two reversible reduction waves
corresponding to the successive one-electron reductions of the
dicyanovinyl (DCV) groups of the TCBD and DCNQ units. BT 8
undergoes simultaneous electrochemical reduction of the
dicyanovinyl (DCV) groups of DCNQ unit and exhibits one
12d
reduction wave. The comparison of first reduction potential of
BT 5–8 reflects that the DCNQ linked BTs 6 and 8 were easier to
reduce compared to TCBD linked BTs 5 and 7, reflecting strong
Acknowledgments
RM thanks CSIR, and DST, New Delhi for financial support.
1
8
electron accepting nature of the DCNQ unit.
Supplementary Material
Supplementary data (detailed experimental procedures, synthesis,
and characterization of all compounds associated with this article
can be found, in the online version.
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