New panchromatic dyes comprising benzothiadiazole units within a donor-acceptor π-conjugated spacer. Synthesis and photophysical properties

Article abstract of DOI:10.1016/j.tet.2012.12.071

Four new organic dyes (D1-D4) having donor-(π-conjugated spacer(donor-acceptor))-acceptor (D-π(D-A)-A) type conjugated systems were synthesized. The dyes were studied for their photophysical and electrochemical properties. The dyes comprised electron donor bis(4-hexyloxy)triphenylamine (D), and electron acceptor cyanoacrylic acid (A) ends connected through a donor-acceptor conjugated spacer group. The conjugated spacer was designed as donor-acceptor system having thiophene to terthiophene and selenophene as donors and benzothiadiazole as acceptor. Thus, optical absorption strengths of the dyes successfully tuned by using the electron accepting property of benzothiadiazole and varying numbers of thiophenes and selenophene. Panchromatic absorption with high extinction coefficient was observed for all four dyes D1-D4.

Full text of DOI:10.1016/j.tet.2012.12.071

Tetrahedron 69 (2013) 2167e2174
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
New panchromatic dyes comprising benzothiadiazole units
within a donoreacceptor
p-conjugated spacer. Synthesis and
photophysical properties
*
Palas Baran Pati, Sanjio S. Zade
Department of Chemical Sciences, Indian Institute of Science Education and Research, Kolkata, PO: BCKV Campus Main Office,
Mohanpur 741252, Nadia, West Bengal, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 October 2012
Received in revised form 16 December 2012
Accepted 28 December 2012
Available online 4 January 2013
Four new organic dyes (D1eD4) having donore(p-conjugated spacer(donoreacceptor))eacceptor
(Dep(DeA)eA) type conjugated systems were synthesized. The dyes were studied for their photo-
physical and electrochemical properties. The dyes comprised electron donor bis(4-hexyloxy)triphenyl-
amine (D), and electron acceptor cyanoacrylic acid (A) ends connected through a donoreacceptor
conjugated spacer group. The conjugated spacer was designed as donoreacceptor system having thio-
phene to terthiophene and selenophene as donors and benzothiadiazole as acceptor. Thus, optical ab-
sorption strengths of the dyes successfully tuned by using the electron accepting property of
benzothiadiazole and varying numbers of thiophenes and selenophene. Panchromatic absorption with
high extinction coefficient was observed for all four dyes D1eD4.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
molecular structure). Their various properties could be finely tuned
by altering independently or matching the different groups of
Conversion of solar energy to electrical energy by using dye-
sensitized solar cells (DSSCs) represents one of the most promis-
ing methods.1 Although silicon based semiconductors dominated
DepeA dyes. Triarylamine and cyanoacrylic acid were often used
as donor and acceptor, respectively, in the conventional dyes for
DSSC. Bezothiadiazole has been extensively studied as acceptor in
donoreacceptor (DeA) polymers.14 Recently, the application to
donoreacceptor type small molecules containing benzothiadiazole
(BDT) structural motifs has been exploited for DSSCs.15
solar cell2 applications for decades, but Gratzel’s idea of DSSCs
€
based on nanocrystalline TiO2 opened up the opportunity to use
organic dyes in DSSC. Some metal based (Ru) dyes, such as N3,3
N719,4 and black dye,5 have achieved remarkable power conver-
sion efficiency upto 10e11%. Recently, porphyrin based dye has
been reported with remarkable 12% efficiency.6 Due to the cost
factor of Ru-metal, use of metal-free dyes as photo sensitizer for
DSSC is more desirable. Organic dyes as photo sensitizer have many
advantages; such as: (i) they have high molar absorption co-
efficients (higher than Ru-dyes) because metal-free dipolar organic
compounds possessing charge-transfer character. (ii) Wide variety
of structures can be designed more easily and the synthesis routes
are also easy. (iii) The most important factor is the low cost of the
organic dyes compared to the Ru-based complexes. Variety of or-
ganic dyes, such as perylene,7 cyanine,8 xanthene,9 merocyanine,10
coumarin,11,8a hemicyaine,10a,12 indoline13 have been reported with
remarkable efficiency in the DSSCs devices.
C6H13
O
CN
COOH
X
N
n
N
N
S
C6H13
O
n = 1 , X = S ; D-1
n = 2 , X = S ; D-2
n = 3 , X = S ; D-3
n = 1 , X = Se ; D-4
Panchromatic optical absorption is a highly desirable property
of the dyes for enhancing the efficiency of DSSCs. Here, we have
designed and synthesized new dyes having donore
p(donore
acceptor)eacceptor (De
p
(DeA)eA) arrangement.16 Triphenyl-
The organic dyes commonly consist of conjugated systems
comprising donor, linker, and acceptor groups (i.e., a DepeA
amine moiety was chosen as donor with bis-p-hexyloxy sub-
stituents. The triphenylamine moiety is known to greatly locate the
cationic charge and efficiently restrict charge recombination be-
tween the photoelectron and the oxidized sensitizer. The two
* Corresponding author. Fax: þ91 33 25873020; e-mail addresses: sanjiozade@
iiserkol.ac.in, sanjiozade@gmail.com (S.S. Zade).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.12.071

2168
P.B. Pati, S.S. Zade / Tetrahedron 69 (2013) 2167e2174
hexyloxy segments in the para-position of the two phenyl groups
will prevent unfavourable dye aggregation also enhance the mo-
lecular rigidity and reduce the vibration induced non-radiative
decays and simultaneously increase solubility in common organic
solvents. The p-hexyloxy substituents additionally enhance the
donor property by mesomeric effect (þM). BDT unit was attached
to the cyanoacrylic acid functionality to improve the electron ac-
ceptance property. Stronger donoreacceptor combination in the
dyes enhances the charge separation ability as well as reduces the
analogues 2aec. Compounds 4aec were prepared from 3aec in
THF by lithiation and treatment with corresponding stannyl re-
agents. Further Stille coupling of compounds 4aec and 7-bromo-
benzo[1,2,5]thiadiazole-4-carbaldehyde in toluene in presence of
Pd(0) catalyst led to the compounds 5aec. Knoevenagel addition
between 5aec and cyanoacetic acid afforded the final dyes D1, D2
and D4. For addition reaction between cyanoacetic acid and 5aec,
chloroform turned out to be better solvent than acetonitrile. To
synthesize D3, we have modified the route used for D1, D2 and D4.
Compound 6 was synthesized by Stille coupling of 4b and 2-
bromothiophene. Stannylation of 6 and further Stille coupling
with 7-Bromo-benzo[1,2,5]thiadiazole-4-carbaldehyde led to the
formation of compound 5d. Condensation reaction of 5d with
cyanoacetic acid gave the dye D3. All the new compounds were
characterized by 1H NMR, 13C NMR, HRMS and other character-
ization methods.
band gap. To study the affect of
tophysical properties of the dyes, we have incorporated thiophene,
bithiophene, terthiophene and selenophene as a part of -conju-
p-conjugated spacer on the pho-
p
gated spacers, respectively. Selenium is more polarizable and less
electronegative than sulfur; as a result photophysical properties of
the dye upon changing thiophene to more conjugated selenophene
will change significantly. Thus, all four dyes absorb in the whole
visible region, which is one of the most important criteria of good
dye for DSSC.
2.2. Photophysical properties
2. Result and discussion
2.1. Synthesis of dyes
The UVevis absorption spectra of D1eD4 were recorded in
chloroform (Fig. 1a) and THF (Fig. 1b). Absorption maxima and
molar absorption coefficients of D1eD4 were listed in Table 1. All
four dyes D1eD4 showed three bands structure in UVevis spec-
trum. Out of these two absorption bands are prominent. A higher
energy band (lmax in the region 368e412 nm) can be attributed to
The organic sensitizers D1, D2 and D4 were synthesized
by the protocol shown in Scheme 1 and dye D3 was synthesized
as shown in Scheme 2. 1-(Hexyloxy)-4-iodobenzene was synthe-
sized from 4-iodophenol by following the reported literature.17
pep transition and a lower energy band (lmax in the region
*
562e595 nm) can be assigned to the intramolecular charge transfer
(ICT). The higher energy absorption band was red-shifted in the
order D1<D2<D3 due to increase of the conjugation by in-
corporation of the additional thiophene ring sequentially
(w20e25 nm for each additional thiophene ring). The lmax of lower
energy ICT bands remained nearly same for all three thiophene-
based dyes D1eD3, however, the intensity was decreased with
Compound
1 was synthesized according to the literature
method.18 2-Tributylstannylthiophene,19 2-tributylstannylseleno-
phene,20 7-bromo-benzo[1,2,5]thiadiazole-4-carbaldehyde,21 and
2-trimethylstannyl-5-thinylthiophne22 were synthesized by re-
ported procedure. Compounds 3aec were synthesized by Stille
coupling of
1 and monostannyl thiophene and selenophene
C6H13
O
C6H13O
Pd(PPh3)4
toluene
reflux, 24 h
X
N
Br
N
H
n
SnR3
H
X
n
3a
3b
3c
;
n = 1, X = S
n = 2, X = S
n = 1, X = Se
2a; X = S, n = 1 R = Bu
2b
C6H13O
C6H13O
1
;
; X = Se, n = 1 R = Bu
;
2c; X = S, n = 2 R = Me
65 - 75 %
THF, -78°C.
-BuLi
Me3SnCl
50 - 60 %
n
Pd(PPh3)4
C6H13
O
C6H13
O
toluene
S
reflux, 24 h
60 - 70 %
N
N
X
X
N
CHO
N
SnMe3
OHC
Br
n
n
N
N
5a
;
n = 1, X = S
4a
;
n = 1, X = S
C6H13O
C6H13O
S
5b
5c
; n = 2, X = S
4b
4c
; n = 2, X = S
;
n = 1, X = Se
;
n = 1, X = Se
CNCH2COOH
piperidine
CHCl3
C6H13
O
65 - 85 %
S
N
N
X
N
CN
COOH
n
D1; n = 1, X = S
C6H13O
D2
; n = 2 X = S
D4; n = 1 X = Se
Scheme 1. Synthetic route for D1, D2 and D4.

P.B. Pati, S.S. Zade / Tetrahedron 69 (2013) 2167e2174
C6H13
2169
O
C6H13O
Pd(PPh3)4
toluene
reflux, 24 h
80%
S
S
S
S
S
N
N
SnMe3
Br
S
6
C6H13O
4b
C6H13O
THF, -78 ° C.
n-BuLi
Me3SnCl
40%
C6H13
O
Pd(PPh3)4
C6H13
O
O
toluene
reflux, 24 h
S
85%
S
SnMe3
N
CHO
N
3
Br
CHO
C6H13
3
N
N
S
N
N
C6H13O
S
7
5d
CNCH2COOH
piperidine
CHCl3
80%
NC
C6H13O
COOH
S
N
3
N
N
S
C6H13O
D3
Scheme 2. Synthetic route of D3.
increasing the number of thiophene rings. This can be explained on
the basis of change in the spacer’s conjugation length.23 In order to
produce the low-energy absorption band, formation of charge-
separated states is important. As the spacer length increased upon
moving from D1 to D3, the ability of the dye to separate charge
decreased due to decrease in the efficiency to transfer the charge
from donor to acceptor. This reduction in ability to separate intra-
molecular charge led to a decrease in the absorption coefficient of
the low-energy band when moving from D1 to D3. Both the ab-
sorption bands of D4 were red-shifted compared to that of D1. It can
(b)
(a)
1.2
1.2
1.0
0.8
0.6
0.4
0.2
0.0
D1
D1
D2
D4
D3
D2
1.0
D4
D3
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
800
300
400
500
600
700
800
λ (nm)
λ (nm)
(c)
1.2
D1
D2
D4
D3
1.0
0.8
0.6
0.4
0.2
300
400
500
λ (nm)
600
700
800
Fig. 1. Normalized absorption spectra of the dyes D1eD4 in (a) chloroform, (b) THF, (c) THFþtriethylamine.

2170
P.B. Pati, S.S. Zade / Tetrahedron 69 (2013) 2167e2174
Table 1
values of the dyes D1, D2 and D4 were red-shifted by 55, 40 and
70 nm compared to that of related dyes without BDT unit C213,
C214 (D21L626) and C215, repsetively.27 Due to enhanced DeA
property, the absorption band-width of the dyes D1, D2 and D4 was
also increased compared to that of C213eC215. The UVevis ab-
sorption spectra of D1eD4 are comparable with that of BzTCA and
BzTMCA dyes.15b
The emission spectra of all dyes were recorded in THF solution
(10ꢀ5 M) (Fig. S21 in Supplementary data). Emission intensities of
all dyes were negligible indicating highly efficient charge-transfer
process from the donor to the acceptors.28
Absorption maxima and molar absorption coefficients of dyes D1eD4a
Dye Absorbance
CHCl3
l1max
THF
THFþtriethylamine
l1max
(
3
)
l2max
(
3
)
l1max
(
3
)
l2max
( 3 )
max
l2max
max
max
max
D1 368 (2.8)
D2 393 (4.4)
D3 412 (2.8)
D4 375 (3.5)
570 (2.5)
562 (4.0)
562 (2.0)
595 (3.8)
362 (3.0)
390 (6.0)
412 (4.2)
372 (3.0)
546 (3.0)
538 (5.0)
520 (3.8)
575 (3.2)
362
389
412
369
526
527
517
538
a
lmax and
3
reported in nm and Mꢀ1 cmꢀ1ꢁ104, respectively.
max
be attributed to the better conjugation of selenophene than that of
thiophene spacer as a result of more polarizability and less elec-
tronegativity of selenium atom.24
2.3. Electrochemical properties
The cyclic voltammetry of dilute solution (10ꢀ5 M) of D1eD4
was measured in dichloromethane containing 0.1 M tetrabuty-
lammonium tetrafluoroborate (TBAF) with 100 mV/s scan rate with
Ag/AgCl as reference and platinum wire as counter electrode
(Fig. 2). The dyes D1, D2 and D4 exhibited two reversible oxidation
peaks, however, for D3 the second oxidation peak showed quasi-
reversible nature. The first oxidation potential was nearly same
(0.67e0.7 V) for all the dyes, which correspond to the removal of
electron from the triphenylamine segment.29 The second oxidation
occurred at higher potentials, which can be assigned to the removal
of an electron from the conjugated backbone of the dyes. The sec-
ond oxidation potential followed the order D1¼D4>D2>D3. As
conjugation length increased from D1 to D3, the second oxidation
potential decreased. The first and second oxidation potentials for
D1 and D4 were nearly same. Thus replacement of thiophene by
selenophene did not produce the significant changes in the oxi-
dation potentials. The first oxidation potentials were more positive
than Iꢀ3 =Iꢀ redox couple, which is an important factor for the effi-
cient dye regeneration in DSSC devices. HOMO energy levels of the
dyes were calculated by following Eq. 1.
When the absorption spectra of dyes D1eD4 were recorded in
solvent of higher dielectric constant (THF (7.5)), lower energy bands
showed solvatochromic changes, while the higher energy bands
remained at the same lmax values (Fig. 1b). Lower energy bands of
D1, D2 and D3 showed blue shift of w15e20 nm by changing sol-
vent chloroform to THF, however, for D3 it was 42 nm. This phe-
nomenon may be attributed to the interaction of polar solvent
molecules with the carboxylic acid, which decreases the electron-
withdrawing strength of the acceptor by coordinating with COOH
group. Further blue shift was observed by the addition of triethyl-
amine to the THF solution of dyes D1eD4. It can be attributed to the
lower strength of acceptor by deprotonation of carboxylic acid
group (Fig. 1c).25 Extinction coefficients increased with increasing
spacer length from D1 to D2, however, it again decreased for D3.
Molar extinction coefficient of D4 was higher than that of D1. Thus
replacement of thiophene by selenophene is beneficial for both
absorption wavelength and molar extinction coefficient.
Due to the incorporation of BDT, the absorption regions of the
dyes penetrated through the whole visible light spectrum. The lmax
(a)
(b)
15
15
10
5
10
5
0
0
-5
-10
-5
-10
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
V
V
(d)
(c)
15
4
3
10
2
5
1
0
0
-5
-10
-1
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
V
V
Fig. 2. CV of 10ꢀ5 M solution of (a) D1, (b) D2, (c) D3 and (d) D3 was measured in DCM containing 0.1 M TBAF with 100 mV/s scan rate with Ag/AgCl as reference and platinum wire
as counter electrode.

P.B. Pati, S.S. Zade / Tetrahedron 69 (2013) 2167e2174
2171
agreement with the experimental values (Table 3). As expected, the
electron density of HOMOs of the dyes is delocalized on the tri-
phenylamine and thiophene conjugated segment, while the LUMOs
are located on acceptor region (BDTecyanoacrylic acid). The
HOMOs and LUMOs are also found to partly overlap at the spacer
(thiophenes and selenophene unit), which indicates appreciable
donoreacceptor interaction between the donors and acceptors. It
could facilitate intramolecular electron transfer from the donor to
the acceptor moiety of the dyes. Trends in the calculated energy
values of HOMO and LUMO of D1eD4 are in good agreement with
the experimentally obtained values from CV (Table 3). TD-DFT
calculations at B3LYP/6-31G(d) on D1 and D2 reproduced the
three-band structure, however, the energy values were under-
estimated (Table 4). Trends in the oscillator strengths from D1 to D2
are in the increasing order, which match well with the trend of
increasing molar extinction coefficients from D1 to D2. Three
transitions correspond to the excitation from HOMO/LUMO,
HOMOꢀ1/LUMO, HOMO/LUMOþ1, respectively, with in-
creasing order of energy.
h
i
HOMO ¼ ꢀ EOX ꢀ E1=2 þ 4:81 eV
(1)
where Eox is the onset oxidation potential and E1/2 is 0.21 for the
ferrocene couple in the above experimental condition.
Optical HOMOeLUMO gaps (Eg) of the dyes were measured from
the onset value of absorption spectra (Eq. 2) and thus LUMO energy
levels were calculated by Eq. 3 (Table 2).
Eg ¼ 1240=lonset
(2)
LUMO ¼ HOMO þ Eg
(3)
Table 2
Electrochemical data of dyes
Dye Eo1x (V) Eo2x (V) lonset (nm) HOMOa (eV) LUMO (eV) Optical band gap (eV)
D1 0.70
D2 0.67
D3 0.67
D4 0.67
1.31
1.10
0.98
1.29
713
730
743
743
ꢀ5.17
ꢀ5.10
ꢀ5.03
ꢀ5.16
ꢀ3.43
ꢀ3.40
ꢀ3.36
ꢀ3.49
1.74
1.70
1.67
1.67
3. Conclusions
a
HOMO energy levels were calculated from the onset oxidation potential.
In summary, a series of new dyes were designed and synthe-
sized. Benzothiadiazole was incorporated as acceptor within the
The HOMO energies of all dyes are in the region of ꢀ5.0 to
ꢀ5.2 eV below the vacuum level, which are comparable with that of
BzTCA and BzTMCA (Fig. 3). The HOMO energy levels of the dyes
must be more positive than the Iꢀ3 =Iꢀ redox couple (4.75 eV) to
ensure a fast and efficient regeneration of the dyes from cation
radical. The HOMO energy levels of D1eD4 are higher (by w0.3 eV)
than iodide/triiodide redox potential. Similarly, the LUMOs of all
dyes D1eD4 are higher by at least 0.5 eV than the energy level of
TiO2 conduction band edge. These observations indicate that the
redox potentials of the dyes in the ground and excited states are
favourable for efficient electron injection to TiO2 conduction band
and the dye regeneration by iodine/iodide redox system.
donoreacceptor
p-conjugated spacer to increase the overall DeA
strength of the dyes. We have shown that by using DeA type
p
-
conjugated spacer optical properties of the dyes can be tuned and
broaden the absorption range. Although with increasing number of
thiophenes within the donoreacceptor
p-conjugated spacer the
lower energy absorption peak is red-shifted, the absorption co-
efficient is decreased. It indicates that with the increase in the
length of
p-conjugated spacer the rate of charge-transfer from
donor to acceptor is decreased. As selenophene enhances the
conjugation and facilitates charge-transfer process due to more
polarizable and less electronegative selenium than sulfur in thio-
phene, dye D4 showed better photophysical properties than that of
D1. As a result D4 exhibited the broader absorption band with
higher molar extinction coefficient than that of D1. On the basis of
photophysical and electrochemical studies, it can be concluded that
(i) the absorption spectra of dyes cover the whole visible region and
(ii) HOMO and LUMO levels are ideal to be promising material for
DSSC devices.
−
3.2
−
3.36
−
3.43
−
3.40
−
3.49
−
4.00
4. Experimental
−
4.2
4.1. General methods
−
4.75
All reactions were performed under nitrogen atmosphere. Dry
toluene and THF was distilled from sodium/benzophenone prior to
use. Anhydrous dichloromethane, thiophene, selenophene, 1.0 M
trimethyltin chloride in hexane, Pd(PPh3)4 and tetrabutylammo-
nium tetrafluoroborate (TBABF4) were purchased from Aldrich and
used without further purification. n-BuLi (1.6 M solution in hexane)
was purchased from Neo-Synth. All the new compounds are
characterized by 1H NMR, 13C NMR and HRMS. 1H NMR and 13C
NMR spectra of the compounds were taken on Bruker Advance
500 MHz and Jeol ECS 400 MHz spectrometers in CDCl3, acetone-
−
5.03
D3
−
−5.2
5.10
D2
−
5.16
D4
−
5.17
D1
-
I-/I3
TiO2
Fig. 3. Energy diagram of the HOMO and LUMO energies of dyes D1, D2, D3 and D4,
conduction band edge of TiO2 and standard potential of the iodide/triiodide redox
couple.
2.4. Theoretical approach
d6 and DMSO-d6 and chemical shifts (d) are reported in parts per
million (ppm) relative to tetramethylsilane as the internal
standard. Electrochemical studies were carried out with
a Princeton Applied Research 263A potentiostat using platinum
(Pt) disk electrode as the working electrode, a platinum wire as
counter electrode, and a AgCl coated Ag wire, which was directly
dipped in the electrolyte solution, as the reference electrode.
Non-aqueous Ag/AgCl wire was prepared by dipping silver wire
in a solution of FeCl3 and HCl. UVevis spectra were recorded on
In order to visualize the geometry, electronic structure, and
estimate the HOMOeLUMO gaps of the dyes, density functional
theory (DFT) calculation at B3LYP/6-31G(d) level was carried out
using Gaussian 09 program.30 For geometry optimization, hexyloxy
substituent of triphenylamine segment was replaced with methoxy
groups. Fig. 4 shows the electron distribution of the HOMO and
LUMO of D1 and D2. The HOMOeLUMO gaps are in good

2172
P.B. Pati, S.S. Zade / Tetrahedron 69 (2013) 2167e2174
Fig. 4. Orbital picture generated by DFT at B3LYP/6-31G(d) level.
4.2.1. Compound 3a. Yield: 66%; 1H NMR (500 MHz, acetone-d6,
ppm) : 0.89 (6H, t, J 6.8 Hz), 1.36 (8H, m), 1.41 (4H, m), 1.75 (4H, m),
Table 3
Calculated values of HOMO and LUMO of D1 and D2
d
3.99 (4H, t, J 6.4 Hz), 6.85 (2H, d, J 8.8 Hz), 6.91 (4H, d, J 8.8 Hz), 7.05
(4H, d, J 8.8 Hz), 7.05 (1H, d), 7.28 (1H, d, J 4.4 Hz), 7.33 (1H, d, J
6.0 Hz), 7.46 (2H, d, J 8.8 Hz).
Dye
HOMO (eV)
LUMO (eV)
Band gap (eV)
D1
D2
D3
D4
ꢀ4.88
ꢀ4.80
ꢀ4.74
ꢀ4.87
ꢀ3.12
ꢀ3.19
ꢀ3.23
ꢀ3.13
1.76
1.62
1.51
1.74
4.2.2. Compound 3b. Yield: 60%; 1H NMR (500 MHz, acetone-d6,
ppm) d: 0.89 (6H, t, J 6.8 Hz), 1.35 (8H, m), 1.41 (4H, m), 1.75 (4H, m),
3.99 (4H, t, J 6.4 Hz), 6.85 (2H, d, J 8.8 Hz), 6.92 (4H, d, J 8.8 Hz), 7.06
(4H, d, J 8.8 Hz), 7.05 (1H, d, J 8.8 Hz), 7.21 (1H, d, J 4.0 Hz), 7.24 (1H,
d, J 4.0 Hz), 7.26 (1H, d, J 4.0 Hz), 7.38 (1H, d, J 4.0 Hz), 7.47 (2H, d, J
Table 4
Calculated low-lying transitions for the dyes D1 and D2
8.8 Hz); 13C NMR (125 MHz, C6D6, ppm)
d: 14.2, 23.0, 26.1, 29.6, 31.9,
Dye
State
Excitation
lcal [eV]
F
68.2, 115.8, 121.2, 122.8, 124.1, 125.1, 125.9, 126.8, 127.1, 127.1, 128.3,
129.2, 135.8, 138.1, 141.1, 148.9, 156.3; HRMS (ESI): Mþ, found
609.2743, C38H43NO2S2 requires 609.2735.
D1
S1
S2
S3
S1
S2
S3
H to L (85.4%)
1.6014
2.4065
2.7313
1.4622
2.1219
2.6183
0.6200
0.3655
0.3752
0.5962
0.6533
0.4496
Hꢀ1 to L (78.4%)
H to Lþ1 (90.5%)
H to L (88.7%)
D2
4.2.3. Compound 3c. Yield: 70%; 1H NMR (400 MHz, acetone-d6,
ppm) d: 0.90 (6H, t, J 6.4 Hz), 1.34 (8H, m), 1.41 (4H, m), 1.75 (4H, m),
Hꢀ1 to L (80.9%)
H to Lþ1 (90.7%)
3.98 (4H, t, J 4.4 Hz), 6.84 (2H, d, J 5.6 Hz), 6.91 (4H, d, J 6.0 Hz), 7.06
(4H, d, J 6.0 Hz), 7.27 (1H, m), 7.41 (2H, d, J 5.6 Hz), 7.42 (1H, d, J
2.4 Hz), 7.95 (1H, d, J 3.6 Hz).
a HITACHI U-4100 UVeviseNIR spectrophotometer. All electro-
chemical potentials were reported against Ag/AgCl taking ferrocene
4.3. Synthesis of compound 4
ferrocene
as external standard, the E
is þ0.21 V.
1=2
n-BuLi (1.6 M in hexane, 0.34 ml, 0.55 mmol) was added drop-
wise to a stirred THF solution of compound 2 (0.5 mmol) and cooled
at ꢀ78 ꢂC. Then the reaction mixture was stirred for 15 min
at ꢀ78 ꢂC, allowed to come at ꢀ50 ꢂC, stirred for 1 h and again
cooled to ꢀ78 ꢂC. Trimethylstannyl chloride (1.0 M in hexane,
0.55 ml, 0.55 mmol) was added slowly at ꢀ78 ꢂC then the mixture
allowed to warm at rt and stirred for overnight. The reaction was
quenched by adding water. Water layer was extracted by diethyl
ether. The organic layer was dried over sodium sulfate; solvent was
removed and the residue was used for the next step without further
4.2. Synthesis of compound 3
A solution of compound 1 (4.30 g, 8.2 mmol), 2-tributyl-
stannylthiophene (or 2-tributylstannylselenophene or 5-trimethyl-
stannylbithiophene) (8.2 mmol) in dry toluene (150 ml) was purged
in N2 for 10 min. Then Pd(PPh3)4 (460 mg, 0.41 mmol) was added
and refluxed for 24 h. After removal of solvent under reduced
pressure, the residue was directly loaded onto a silica gel column
for purification with ethylacetate/hexane (1:5, v/v) as eluent.

P.B. Pati, S.S. Zade / Tetrahedron 69 (2013) 2167e2174
2173
purification. Conversion yields were w50%e60%. Characteristic
peak at
0.28e0.38 ppm (s) was observed in 1H NMR for methyl
4.6. Synthesis of compound 7
d
proton of trimethylstannyl group.
Compound 6 (345 mg, 0.5 mmol) was dissolved in dry THF
(20 ml), cooled to ꢀ78 ꢂC and n-BuLi (1.6 M in hexane, 0.35 ml,
0.55 mmol) was added dropwise and stirred for 15 min then the
reaction mixture was allowed to come at ꢀ50 ꢂC, stirred for 1 h at
this temperature and then again cooled to ꢀ78 ꢂC. Trimethylstannyl
chloride (0.55 ml, 0.55 mmol) was added slowly and allowed the
mixture to warm at rt. The reaction was quenched by adding water.
Water layer was extracted with diethyl ether. The organic layer was
dried over sodium sulfate; solvent was removed under reduced
pressure and residue was used for next step without further puri-
fication. Conversion yield was calculated from the singlet proton for
4.4. Synthesis of compound 5
7-Bromo-benzo[1,2,5]thiadiazole-4-carbaldehyde (243 mg,
1 mmol) was taken in a dry three-necked round bottom flask and
compound 4 in toluene (25 ml) was added. The solution mixture
was purged with nitrogen for 10 min; then Pd(PPh3)4 (60 mg,
0.05 mmol) was added to the reaction mixture. The reaction mix-
ture was heated at 110 ꢂC under nitrogen for 24 h. It was cooled to
rt; toluene was evaporated by rotary evaporator. The crude mixture
was directly loaded to the column. Product was separated by flash
column chromatography using ethylacetate/hexane mixture as
eluent.
trimethylstannyl group at d: 0.38 ppm.
4.7. Synthesis of compound 5d
4.4.1. Compound 5a. Yield: 64%; 1H NMR (500 MHz, CDCl3, ppm)
d
:
7-Bromo-benzo[1,2,5]thiadiazole-4-carbaldehyde (243 mg,
1 mmol) was taken in a dry three-necked round bottom flask
and compound 7 (1.2 mmol) in dry toluene (25 ml) was added.
The solution mixture was purged with nitrogen for 10 min.
Pd(PPh3)4 (110 mg, 0.1 mmol) was added to reaction mixture and
refluxed at 110 ꢂC for 24 h. The reaction mixture was cooled at rt;
toluene was evaporated by rotary evaporator. The crude mixture
was directly loaded to column. The product was purified silica
gel column chromatography using ethylacetate/hexane as elu-
0.86 (6H, t, J 7 Hz), 1.36 (8H, m), 1.45 (4H, m), 1.75 (4H, m), 3.99 (4H,
t, J 6.5 Hz, 4H), 6.86 (4H, d, J 8.5 Hz), 6.93 (2H, d, J 8.5 Hz), 7.08 (4H,
d, J 8.5 Hz), 7.32 (1H, d, J 4.4 Hz), 7.50 (2H, d, J 6.0 Hz), 7.95 (1H, d, J
7.5 Hz), 8.20 (1H, d, J 7.5 Hz), 8.30 (1H, d, J 4 Hz), 10.71 (1H, s); 13C
NMR (125 MHz, CDCl3, ppm) d: 14.0, 22.6, 25.7, 29.3, 31.6, 68.3,
115.3, 119.7, 123.0, 123.1, 125.0, 125.1, 126.6, 126.9, 131.7, 132.8, 133.3,
136.0, 140.0, 149.2, 149.3, 152.3, 153.8, 155.9, 188.4; HRMS (ESI): Mþ,
found 689.2732, C41H43N3O3S2 requires 689.2746.
ent. Yield: 65%; 1H NMR (500 MHz, CDCl3, ppm)
d: 0.92 (6H, t, J
4.4.2. Compound 5b. Yield: 71%; 1H NMR (500 MHz, CDCl3, ppm)
d:
7 Hz), 1.36 (8H, m), 1.46 (4H, m), 1.78 (4H, m), 4.00 (4H, t, J
6.5 Hz), 6.86 (2H, d, J 8.5 Hz), 6.95 (4H, d, J 8.5Hz), 7.11 (4H, d,
J 8.5 Hz), 7.57 (2H, d, J 4 Hz), 7.65 (1H, d, J 4 Hz), 8.28(2H,
d, J 4 Hz), 8.46 (4H, d, J 4 Hz), 10.69 (1H, s); 13C NMR
(125 MHz, CDCl3, ppm): 14.0, 22.6, 25.5, 29.3, 31.5, 68.2, 115.2,
120.0, 122.1, 124.8, 124.4, 123.8, 123.2, 125.1, 125.3, 125.4, 126.1,
126.7, 131.0, 132.4, 132.4, 134.3, 136.7, 137.8, 140.1, 141.5, 144.1,
148.4, 152.0, 153.6, 155.7, 188.2; HRMS (ESI): Mþ, found 853.2512,
C49H47N3O3S4 requires 853.2500.
0.92 (6H, t, J 7 Hz), 1.36 (8H, m), 1.46 (4H, m), 1.78 (4H, m), 3.94 (4H,
t, J 6.5 Hz), 6.87 (4H, d, J 8.5 Hz), 6.91 (2H, d, J 8.5 Hz), 7.08 (4H, d, J
8.5 Hz), 7.12 (1H, d, J 4 Hz), 7.28 (2H, d, J 4.0 Hz), 7.40 (2H, d, J
6.0 Hz), 7.94 (1H, d, J 7.5 Hz), 8.19 (1H, d, J 7.5 Hz), 8.22 (1H, d, J 4 Hz),
10.69 (1H, s); 13C NMR (125 MHz, CDCl3, ppm)
d: 14.0, 22.6, 25.7,
29.3, 31.5, 68.2, 115.3, 120.0, 122.4, 123.2, 124.3, 125.2, 126.3, 126.8,
131.2, 132.6, 132.8, 134.2, 136.4, 140.2, 142.2, 145.1, 148.7, 152.2,
153.8, 155.7, 188.3; HRMS (ESI): Mþ, found 771.2615, C45H45N3O3S3
requires 771.2623.
4.8. Synthesis of compound D1eD4
4.4.3. Compound 5c. Yield: 62%; 1H NMR (500 MHz, CDCl3, ppm)
d:
0.92 (6H, t, J 7 Hz), 1.36 (8H, m), 1.46 (4H, m), 1.78 (4H, m), 4.00 (4H,
t, J 6.5 Hz), 6.86 (2H, d, J 8.5 Hz), 6.95 (4H, d, J 8.5 Hz), 7.11 (4H, d, J
8.5 Hz), 7.57 (2H, d, J 4 Hz), 7.65 (1H, d, J 4 Hz), 8.28 (2H, d, J 4 Hz),
8.46 (1H, d, J 4 Hz), 10.69 (1H, s); 13C NMR (100 MHz, CDCl3, ppm):
14.0, 22.5, 25.7, 29.2, 31.5, 68.2, 115.3, 119.6, 122.2, 124.4, 124.6,
126.8, 126.9, 127.3, 132.6, 132.7, 134.9, 139.8, 149.1, 152.3, 153.5,
155.9,157.4,188.2; HRMS (ESI): Mþ, found 737.2186, C41H43N3O3SSe
requires 737.2190.
Compound 5 was dissolved in dry chloroform in dry three-
necked round bottom flask fitted with condenser. Cyanoacetic acid
and piperidine were added and reaction mixture was refluxed for
12 h, then water added to the reaction mixture and organic layer
was separated by dichloromethane and dried over Na2SO4. The
product was purified by column chromatography dichloro-
methane/methanol (9:1, v/v) as elutant.
4.8.1. Compound D1. Yield: 80%; 1H NMR (500 MHz, DMSO-d6,
4.5. Synthesis of compound 6
ppm) d: 0.86 (6H, t, J 7 Hz), 1.29e1.34 (8H, m), 1.4e1.5 (4H, m),
1.6e1.7 (4H, m), 3.94 (4H, t, J 6.5 Hz), 6.79 (2H, d, J 8.5 Hz), 6.93 (4H,
d, J 8.5 Hz), 7.07 (4H, d, J 8.5 Hz), 7.55 (1H, d, J 8.5 Hz), 7.59 (2H, d, J
4.0 Hz), 8.27 (1H, d, J 5.0 Hz), 8,30 (1H, d, J 5.0 Hz), 8.65 (1H, d, J
A mixture of compound 4b (1.2 mmol) and 2-bromothiophene
(250 mg, 1.2 mmol) was taken in dry toluene (20 ml) and the
mixture was purged with nitrogen for 10 min. Then Pd(PPh3)4
(70 mg, 0.06 mmol) was added and reaction mixture refluxed for
24 h. After removal of solvent under reduced pressure, the res-
idue was directly loaded onto a silica gel column for purification
with ethylacetate/hexane (1:5, v/v) as eluent. Yield: 80%; 1H
5.0 Hz), 8.81 (1H, s); 13C NMR (125 MHz, DMSO-d6, ppm)
d: 13.8,
22.0, 25.1, 28.6, 30.9, 67.6, 113.6, 115.5, 118.7, 122.4, 122.4, 123.2,
123.4, 123.4, 124.2, 124.4, 126.4, 127.1, 129.2, 130.8, 135.4, 139.3,
148.1, 148.7, 150.6, 154.2, 155.6; HRMS (ESI): Mþ, found 756.2798,
C44H44N4O4S2 requires 756.2804.
NMR (400 MHz, CDCl3, ppm) d: 0.90 (6H, t, J 7 Hz), 1.36 (8H, m),
1.46 (4H, m), 1.78 (4H, m), 3.98 (4H, t, J 6.5 Hz), 6.86 (2H, d, J
8.5 Hz), 6.95 (4H, d, J 8.5 Hz), 7.07 (5H, m), 7.26 (2H, d, J 4 Hz),
7.21 (2H, d, J 4 Hz), 7.30 (1H, d, J 4 Hz), 7.44 (1H, d, J 4 Hz), 7.48
4.8.2. Compound D2. Yield:75%; 1H NMR (400 MHz, DMSO-d6,
ppm) d: 0.86 (6H, t, J 7 Hz), 1.29e1.34 (8H, m), 1.4e1.5 (4H, m),
1.6e1.7 (4H, m), 3.94 (4H, t, J 6.5 Hz), 6.76 (2H, d, J 8 Hz), 6.91 (4H, d,
J 8 Hz), 7.04 (4H, d, J 8 Hz), 7.35 (1H, d, J 4 Hz), 7.4e7.5 (4H, m), 8.21
(1H, d, J 4.0 Hz), 8.30 (1H, d, J 8.0 Hz), 8.56 (1H, d, J 8.0 Hz), 8.76 (1H,
(2H, d, J 4 Hz); 13C NMR (125 MHz, C6D6, ppm)
d: 14.2, 23.0, 26.1,
29.6, 31.9, 68.2, 115.8, 121.1, 122.9, 124.0, 124.3, 124.5, 124.8, 125.1,
125.9, 126.8, 127.1, 127.9, 128.2, 128.5, 135.4, 137.6, 141.0, 144.2,
149.0, 156.3; HRMS (ESI): Mþ, found 691.2603, C42H45NO2S3 re-
quires 691.2612.
d, J 4.0 Hz); 13C NMR (100 MHz, CDCl3, ppm)
d: 12.0, 20.2, 23.3, 27.1,
29.1, 65.7, 101.9, 113.4, 114.2, 117.0, 119.8, 121.1, 122.2, 122.4, 122.6,
124.1, 124.9, 128.0, 128.3, 128.8, 131.6, 134.0, 136.8, 137.5, 139.6,

2174
P.B. Pati, S.S. Zade / Tetrahedron 69 (2013) 2167e2174
142.1, 144.2, 146.4, 152.3, 153.6, 161.2; HRMS (ESI): Mþ, found
838.2684, C48H46N4O4S3 requires 838.2681.
8. (a) Hara, K.; Kurashige, M.; Danoh, Y.; Kasada, C.; Shinpo, A.; Suga, S.; Sayama,
K.; Arakawa, H. New J. Chem. 2003, 27, 783e785; (b) Hara, K.; Sato, T.; Katoh, R.;
Furube, A.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Sugihara, H.; Arakawa, H. J.
Phys. Chem. B 2003, 107, 597e606.
9. Hara, K.; Horiguchi, T.; Kinoshita, T.; Sayama, K.; Sugihara, H.; Arakawa, H.
Chem. Lett. 2000, 29, 316e317.
10. (a) Khazraji, A. C.; Hotchandani, S.; Das, S.; Kamat, P. V. J. Phys. Chem. B 1999,
103, 4693e4700; (b) Sayama, K.; Hara, K.; Mori, N.; Satsuki, M.; Suga, S.; Su-
kagoshi, S.; Abe, Y.; Sugihara, H.; Arakawa, H. Chem. Commun. 2000, 1173e1175.
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4.8.3. Compound D3. Yield: 70%; 1H NMR (400 MHz, DMSO-d6,
ppm) d: 0.86 (6H, t, J 7 Hz), 1.29e1.34 (8H, m), 1.4e1.5 (4H, m),
1.6e1.7 (4H, m), 3.94 (4H, t, J 6.5 Hz), 6.76 (2H, d, J 8.5 Hz), 6.91 (4H,
d, J 8.5 Hz), 7.03 (4H, d, J 8.5 Hz), 7.31e7.34 (3H, m), 7.4e7.5 (3H, m),
7.53 (1H, d, J 4.4 Hz), 8.20 (1H, d, J 6.0 Hz), 8.30 (1H, d, J 7.5 Hz), 8.54
(1H, d, J 7.5 Hz), 8.69 (1H, d, J 4 Hz); 13C NMR (100 MHz, CDCl3, ppm)
d: 13.8, 22.0, 25.1, 28.6, 30.9, 67.6, 116.0, 115.5, 118.9, 119.3, 123.1,
12. Sayama, K.; Tsukagoshi, S.; Mori, T.; Hara, K.; Ohga, Y.; Shinpou, A.; Abe, Y.;
Suga, S.; Arakawa, H. Sol. Energy Mater. Sol. Cells 2003, 80, 47e71.
13. Ito, S.; Zakeeruddin, S. M.; Hummphrey-Baker, R.; Liska, P.; Charvet, R.; Comte,
124.6, 124.7, 124.9, 125.6, 125.7, 126.1, 126.9, 124.9, 127.3, 127.6,
129.3, 134.3, 134.4, 136.2, 136.8, 139.2, 139.4, 139.6, 143.1, 148.2,
150.7, 154.0, 155.4, 162.1; HRMS (ESI): Mþ, found 920.2559,
C52H48N4O4S4 requires 920.2558..
€
P.; Nazeeruddin, M. K.; Pechy, P.; Takata, M.; Miura, H.; Uchida, S.; Gratzel, M.
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4.8.4. Compound D4. Yield: 80%; 1H NMR (500 MHz, DMSO-d6,
ppm) d: 0.92 (6H, t, J 7.0 Hz), 1.36 (8H, m), 1.46 (4H, m), 1.78 (4H, m),
4.00 (4H, t), 6.86 (2H, d, J 8.5 Hz), 6.95 (4H, d, J 8.5 Hz), 7.11 (4H, d, J
8.5 Hz), 7.57 (2H, d, J 4.0 Hz), 7.65 (1H, d, J 4.0 Hz), 8.28 (2H, d, J
4.0 Hz), 8.46 (1H, d, J 4.0 Hz); 13C NMR (100 MHz, CDCl3, ppm)
d:
14.0, 22.5, 25.7, 29.2, 31.5, 68.2, 101.4, 115.3, 115.9, 119.3, 121.1, 122.8,
124.5,126.8,127.0,127.1,130.8,132.8,133.9,139.8,147.8,149.2,151.2,
154.8, 155.9, 157.9, 166.2; HRMS (ESI): Mþ, found 804.2240,
C44H44N4O4SSe requires 804.2248.
16. Dye D2 has recently reported in Ref. 15c during our work on this scheme
and D1 very recently reported in: Lin, R. Y.-Y.; Lee, C.-P.; Chen, Y.-C.; Peng, J.-D.;
Chu, T.-C.; Chou, H.-H.; Yang, H.-M.; Lin, J. T.; Ho, K.-C. Chem. Commun. 2012,
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Acknowledgment
PBP thanks University Grant Commission (UGC), India for his
fellowship and we acknowledge Department of Science and Tech-
nology India (DST), India for funding.
Supplementary data
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2012.12.071.
22. Bilge, A.; Zen, A.; Forster, M.; Li, H.; Galbrecht, F.; Nehls, B. S.; Farrell, T.; Neher,
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