Chapter 2: Design and synthesis of Coumarin based sensitizers for dye-sensitized solar cells (DSSCs)
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 39
2.1 INTRODUCTION
Coumarin chromophores have been extensively investigated as suitable materials in
hi-technological fields including electronic or photonic applications (Christie and
Lui 2000; Ayyangar et al.1991; Moylan 1994; Fischer et al. 1995) as fluorescence
probes, nonlinear optical materials, solar energy collectors and charge-transfer
agents. They have also been successfully derivatized to find use as significant
organic fluorescent materials (Christie 1993; Krasovitskii and Bolotin 1988). Their
commercial value and applicability in versatile fields can be mainly attributed to
their inherent photochemical features, efficient light emission properties, relative
ease of synthesis, good stability and solubility. The fluorescent dyes based on
coumarin show absorption in the UV region and emission of blue light (Barton and
Davidson 1974; Moeckli 1980). The nature of these dyes can be well related with
the structural changes in the coumarin moiety. For example, the fluorescent
coumarin dyes usually contain an electron-accepting group in the 3-position and
electron-donating group at the 7th position. The substitution at 7th position mainly
comprise of amino, hydroxy and methoxy groups. The increase in conjugation
results in deepening of colours by means of bathochromic shift whereas
incorporation of a benzo ring fused at the 5,6 position also confers good colour.
Such properties of coumarin based dyes encourage exploration of various structural
features by suitable designing so as to derive a range of colorants with varied
properties.
Dye-sensitized solar cells (DSSCs) have attracted significant attention as low-
cost devices for the photovoltaic conversion of solar energy compared with the
conventional solid p–n junction photovoltaic devices (Gratzel 2001; Gratzel
Chapter 2: Design and synthesis of Coumarin based sensitizers for dye-sensitized solar cells (DSSCs)
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2004). Ruthenium poly-pyridyl complexes are the most widely used sensitizers for
DSSCs, achieving power conversion efficiencies (η) >10% (Wang et al. 2003) and
good long-term stability (Liang et al. 2007). Although the most efficient sensitizers
to date are ruthenium complexes, metal free organic dyes have been attracting
attention because of their ease of synthesis, high molar extinction coefficient,
tuneable absorption spectral response from the visible to the near infrared
(NIR) region, environmental friendliness and inexpensive production
techniques.
Among the metal-free organic dyes studied in DSSCs, coumarin-based dyes are
promising sensitizers owing to their good photo-response in the visible region, good
long-term stability under one sun soaking (Wang et al. 2007), and appropriate
lowest unoccupied molecular orbital (LUMO) levels matching the
conduction band of TiO2. Some coumarin sensitizers as donor groups have reached
efficiencies of upto 8.2 % values comparable to the standard N719 sensitizer (Hara
et al. 2001; Hara et al. 2003; Hara et al. 2003a; Hara et al. 2003b; Hara et al. 2005;
Hara et al. 2005a). The importance of coumarin is understood from the fact that on
replacing the coumarin moiety with N, N-dimethylaminophenyl (DMA) donor
group, (Hara et al. 2005b; Kitamura et al. 2004) a significant hypsochromic shift of
the maximum absorption peak is observed indicating that the coumarin is a
stronger donor than the DMA group.
In order to obtain a red shift, it is important to extend π-conjugation which is usually
done by extending the methine unit (–CH=CH–) of the molecule. However, such an
extension by more double bonds would increase the instability of the dye molecule,
Chapter 2: Design and synthesis of Coumarin based sensitizers for dye-sensitized solar cells (DSSCs)
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 41
owing to the possibility of isomer formation. Also, report by K. Hara (Hara et al.
2001; Hara et al. 2003) indicated that among a series of coumarin dyes with
extended methine units, the highest values of Jsc and efficiency is obtained with two
methine units (n=1 in figure 2.1). The dyes with methine units greater than 2 give
reduced values of Jsc and Voc mainly due to the H-aggregation of the dye due to
strong interactions between dye molecules on the TiO2 surface.
Figure 2.1 Structure of coumarin dyes (n is the no. of methine units)
Another important way of increasing conjugation is by incorporation of benzene and
thiophene rings as the linker unit in the dye structure. This would simultaneously
extend π-conjugation and improve the stability of the dye molecule relative to
the dyes, which have a long methine chain unit. It has been observed that
introduction of thiophene moieties improves the solar cell performance mainly by
broadening the absorption spectra thereby resulting in a large photocurrent and also
relatively lowers the positions of the LUMO levels of the dyes (see figure 2.2).
As the number of thiophene units is increased, the LUMO levels are much lowered
due to increased π-π stacking leading to insufficient driving force for electron
injection (Hara et al. 2003a; Hara et al. 2005a).
N O O
CN
COOHn
Chapter 2: Design and synthesis of Coumarin based sensitizers for dye-sensitized solar cells (DSSCs)
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Figure 2.2 Structure of coumarin dyes (n is the no. of thiophene units)
Herein, we designed molecules based on coumarin moiety so as to be suitable for
application in dye-sensitized solar cells. One of the major requirements of molecules
for dye-sensitized solar cells is their broad absorption in the visible region. In order
to shift the absorption at longer wavelength in coumarin based fluorescent dyes, we
decided to strengthen both the donor and acceptor at 7 and 3- position. At the 7th
position, we took N,N-diethyl amino group as a donor. The acceptor was
incorporated at position 3 by means of conjugating bridge containing halogen atom.
Figure 2.3 Structural features of the synthesized dye
N O O
Cl
H3C
H3C
CN
COOH
LINKER DONOR
ACCEPTOR
N O O
nS
S
COOHNC
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We also prepared coumarin based dyes by replacing N,N-diethylamino group at 7-
position by julolidine group (figure 2.4) which is strong electron donor and can
influence the strength and overlap of a nitrogen donor orbital with a conjugation
system.
Figure 2.4 Coumarin with julolidene
donor group
The concept of lateral anchoring has been explored recently by many scientists
wherein the anchoring carboxylic group is separated from the electron acceptor
groups of the sensitizer. The lateral anchoring provides several alternatives for
conventionally used cyanoacrylic acid groups as the acceptor end. This would not
only result in possibility of various new structures but also can help in overcoming
the restriction of HOMO-LUMO fine tuning. The functioning of these dyes is
expected to occur via injection of electrons from acceptor group rather than
anchoring group to conduction band of titanium dioxide (TiO2). However, further
modifications and also an in-depth study is required so as to make these dyes more
efficient than several other conventional cyanoacrylic acid group containing dyes.
To mention few reports on such type of dyes, Sun et al adopted this strategy for the
first time and were successful in extending the absorption spectra of the sensitizers
(Figure 2.5) towards the NIR region (Hao et al. 2009). Inspite of several other
reports, recently Sun et al reported lower efficiencies with lateral anchoring group
Chapter 2: Design and synthesis of Coumarin based sensitizers for dye-sensitized solar cells (DSSCs)
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(Figure 2.6) wherein they explained attributed this behaviour to the electron
recombination process due to the proximity between donor group and titanium
dioxide surface (Hao et al. 2012). Some examples of such dyes with concept of
lateral anchoring group is provided in figures 2.5 and 2.7.
A (λmax = 610 nm) B (λmax = 638 nm)
Figure 2.5 Structure of near absorbing dyes with lateral anchoring groups
C (Efficiency = 7.0%) D (Efficiency = 2.7%)
Figure 2.6 Example of decrease in efficiency with incorporation of lateral anchoring groups
Hence, we attempted to synthesize dyes with lateral anchoring carboxylic acid group
that is not directly attached on the donor coumarin group. In this regards, chloro
group could be replaced for the incorporation of alkyl chains containing carboxylic
acid end groups (as shown in figure 2.7). After incorporation of anchoring group, we
Chapter 2: Design and synthesis of Coumarin based sensitizers for dye-sensitized solar cells (DSSCs)
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 45
substituted the aldehyde of coumarin group with malononitrile. We aim to improvise
in this direction with further modifications and study, so as to obtain dyes with better
efficiencies.
Figure 2.7 Scope of incorporating anchoring groups in our present dyes
Chapter 2: Design and synthesis of Coumarin based sensitizers for dye-sensitized solar cells (DSSCs)
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2.2 RESULTS AND DISCUSSION
2.2.1 Synthesis of intermediates and coumarin sensitizers for dye-sensitized solar
cells
Keeping in mind all the aspects discussed in introduction section, we synthesized
colorants suitable for application in dye-sensitized solar cell with benzene, thiophene
and furan rings in the methine chain as linker unit as shown in schemes 2.1-2.5. The
sensitizers [10a-10c] were synthesized by condensation of a formylcoumarin
compound [4] with cyanoacetic acid [8] or active methylene compounds containing
cyanocarboxylic acid groups [9a-9c] in ethanol using piperidine. The
formylcoumarin compound [4] was prepared in two steps starting from DEMAP
aldehyde [1] and ethyl cyanoacetate [2] in ethanol using piperidine to obtain acetyl
coumarin intermediate [3]. This was followed by reaction with DMF/POCl3 that
introduces a chloro group in conjugation with the aldehyde group. The active methyl
containing compounds [9a] were synthesized in two steps including formylation of
compounds [6] with DMF/POCl3 and condensation with cyanoacetic acid [8]
whereas, in case of [9b], only the latter step was applied. The fused coumarin
compound was prepared in six steps involving reaction of m-anisidine [11] with
bromochloropropane to form methoxy derivative of julolidene [12]. This was
followed by its demethylation using HI and formylation using DMF/POCl3 which
was finally cyclized with ethylacetoacetate to obtain acetyl fused coumarin
derivative [16]. This acetyl derivative was again subjected to formylation and
knovenagel condensation with cyanoacetic acid to form final compound [18]. The
coumarin aldehyde [4] was further subjected to replacement of chloro group with
side chain thiocarboxylic acid anchoring groups. This was followed by condensation
Chapter 2: Design and synthesis of Coumarin based sensitizers for dye-sensitized solar cells (DSSCs)
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with malononitrile [22] to obtain the final chromophore [23]. The chromophores and
intermediates were characterized by FT-IR, 1H NMR, 13C NMR, mass spectrometry.
The UV-visible absorption and emission spectra were recorded for all these
synthesized derivatives. The physical properties of these colorants are summarized
in Table 2.1.
Scheme 2.1: Synthesis of 3-chloro-3-(7-(diethylamino)-2-oxo-2H-chromen-3-
yl)acrylaldehyde [4]
Scheme 2.2: Synthesis of active methyl compounds
CN
COOH
[5]
Ammonium acetate
Glacial CH3COOH
SH3C
H
O
H3C
H
O
SH3C
SH3C HOOC
CN
DMF / POCl3
SH3C
H
O
+Ethanol
H3C
CN
HOOC
[6]
[7]
[6]
[8]
[9b]
[9a]
HN
O
+0-5 oC
N
CHO
OHH2C
CH3
O
OCH2CH3
O
N O O
O
CH3
Ethanol / Piperidine
Ref lux
+
[1] [2] [3]
N O O
O
CH3DMF / POCl3 N O O
Cl
CHO
[3] [4]
0-5oC
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Scheme 2.3: Synthesis of sensitizers [10a-10c]
Ethanol
Reflux
N O O
Cl
CHO
[4]
NC
COOH
[8]
+N O O
Cl
[10a]
CN
COOHPiperidine
[10b]
H3C
CN
HOOC
[9a]
N O O
Cl
H3C
H3C
CN
COOH
N O O
Cl
CHO
[4]
+
Ethanol
Reflux
Piperidine
SH3C
HOOC
CN
N O O
Cl
H3C
H3C
S
COOH
NC
N O O
Cl
CHO
[4]
+
[9b]
[10c]
Ethanol
Reflux
Piperidine
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Scheme 2.4: Synthesis of (2Z,4Z)-5-chloro-2-cyano-5-(11-oxo-2,3,5,6,7,11-hexahydro-1H-pyrano[2,3-f]pyrido[3,2,1-ij]quinolin-10-yl)penta-2,4-dienoic acid [18]
H2N OCH3 N O
CH3Br Cl2
[11]
+
[12]
N OHN OCH3
[12]
[13]
N OH
CHO
N O
CH3
O
O
CH3
O
OEtO
[14] [16]
Ethanol+
Piperidene[15]
N O O
Cl
CHO
+ C
[17]
POCl3
HN
O
N O
CH3
O
O
[16]
N OH N OH
CHO
[13]
C
[14]
POCl3
HN
O
+
N O O
Cl
CHO
N O O
Cl
CN
COOHCN
COOH
Ethanol+
Piperidene[18][17]
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Scheme 2.5: Synthesis of (Z)-3-((4,4-dicyano-1-(7-(diethylamino)-2-oxo-2H-
chromen-3-yl)buta-1,3-dien-1-yl)thio)propanoic acid [23]
Table 2.1: Physical properties of Coumarin based sensitizers
Dye No. Molecular Formula Molecular weight
Yield (%)
10a C19H17ClN2O4 373 89
10b C27H23ClN2O4 475 59
10c C25H21ClN2O4S 481 45
18 C21H17ClN2O4 397 54
23 C22H21N3O4S 423 64
N O O
ClCHO
H3C
H3CEthanol / Triethylamine
Reflux
+ HSCOOH N O O
SCHO
H3C
H3C
COOH
N O O
S
H3C
H3C
COOH
CN
CN+
[4] [20] [21]
[22] [23]
N O O
SCHO
H3C
H3C
COOH
[21]
Ethanol / Piperidene
RefluxCN
CN
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2.2.2 Photophysical properties of Coumarin dyes
The linear absorption spectra of the newly synthesized sensitizers were measured for
concentrations of 1×10-3 M in chloroform. The path length of the cell was 1 cm
whereby the influences of the quartz cuvette and the solvent have been subtracted.
The dyes 10a-10b show good absorption in visible region especially in the region of
400-500 nm whereas the absorption of dye 10c extends even in the range of 500-600
nm (Figure 2.8). This could be possibly owing to the better conjugation
characteristics incorporated by use of thiophene linker.
In case of fused amino coumarin dye 18, the absorption is mainly observed between
400-600 nm (Figure 2.9). The sensitizer 23 wherein the chloro group was replaced
with carboxylic acid containing thiol group, showed a very broad absorption starting
from 200 nm and extending till 600 nm. The broad nature of absorption could also
be attributed to the better charge separation induced by presence of stronger
electron-withdrawing cyano groups.
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Figure 2.8: UV-Visible spectra of synthesized colorants 10-10c in chloroform
DYE 10a
DYE 10b
DYE 10c
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Figure 2.9: UV-Visible spectra of synthesized colorants 18 and 23 in chloroform
DYE 23
DYE 18
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2.2.3 Thermogravimetric analysis
We also tested the thermal stability of dyes by thermo gravimetric analysis (TGA)
carried out in the temperature range 25-600 °C under nitrogen gas at a heating rate
of 10 °C min-1. The TGA curves revealed that most of the dyes hold extremely good
thermal stability with majority of dyes showing stability above 250 °C as revealed in
Table 2.2. The sensitizer 10a showed the best thermal stability amongst all. The
molecule 10b containing benzene bridge as conjugation bridge showed better
stability than 10c that contains thiophene ring. However, colorant with fused
coumarin ring 18 showed lower stability which might be because presence of
alicyclic ring. The higher value of thermal stability is desirable in high-technological
applications like dye-sensitized solar cells.
Table 2.2: Thermal stability of coumarin based sensitizers
Dye No. Temperature stability (°C)
(at 3% percentage decomposition)
10a 309
10b 295
10c 267
18 275
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2.3 EXPERIMENTAL 2.3.1 Materials and equipments
All the solvents and chemicals were procured from S D fine chemicals (India) and
were used without further purification. The reactions were monitored by TLC using
0.25 mm E-Merck silica gel 60 F254 precoated plates, which were visualized with
UV light. UV – Visible absorption spectra were recorded on Spectronic genesis 2
spectrophotometer instruments from dye solutions (~ 10-3 M) in chloroform. The 1H
NMR spectra were recorded on 400 MHz on Varian mercury plus spectrometer.
Chemical shifts are expressed in δ ppm using TMS as an internal standard. Mass
spectral data were obtained with micromass-Q-Tof (YA105) spectrometer.
Elemental analysis was done on Harieus rapid analyzer. Melting points measured
and thermogravimetric analysis was carried out on SDT Q600 v8.2 Build 100 model
of TA instruments.
The linear absorption spectra of the newly synthesized push pull chromophores were
measured for concentrations of 1×10-3 M in chloroform in a cell of 1 cm path length
whereby the influences of the quartz cuvette and the solvent have been subtracted.
2.3.2 Synthesis of key intermediates and compounds
2.3.2.1 Preparation of 3-acetyl-7-(N, N-diethyl) amino benzopyan-2-one [3]
In a 500ml three-necked round bottomed flask, 4-(N, N-diethyl) amino
salicylaldehyde [1] (19.3 g, 0.1 mole) was dissolved in 25 ml of ethanol. This was
followed by the addition of ethyl acetoacetate [2] (13.0 g, 0.1 mole) and piperidine
(0.5ml) under ice cold conditions. The solution was stirred at room temperature for
half an hour and refluxed for 1 hour. The product separated out as pale yellow
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crystals from the reaction mixture was filtered and washed with ethanol. The
compound was recrystallised from ethanol.
Yield = 22 g (86%); M.P. = 154 °C (lit. 150-152 °C, Lin et al. 2009).
2.3.2.2 Synthesis of 3-chloro-3-(7-(diethylamino)-2-oxo-2H-chromen-3-yl)
acrylaldehyde [4]
In a three necked 100ml round bottom flask fitted with a mercury sealed stirrer,
addition dropping funnel topped by calcium chloride guard tube and reflux
condenser also topped by calcium chloride guard tube. N,N-dimethyl formamide
(2.68g, 2.82ml, 0.036 moles) was taken and cooled to 0-5 °C with stirring. To the
above solution phosphorous oxychloride (3.79g, 2.30ml, 0.024moles) was added
drop wise maintaining the temperature of the reaction mass at 0-5 °C. The Vilsmeier
complex so formed was stirred for further 15 minutes and [3] (4g, 0.015moles) was
added in lots (15-25 minutes) to the complex. The reaction mixture was stirred at 0-5
°C for 3 hrs and then allowed to attain room temperature. After that the contents of
the flask were heated at 80-85 °C in a water bath for 6-7 hrs. Subsequently the
reaction mass was cooled to room temperature and poured in to crushed ice with
stirring and deep external cooling, the clear solution obtained was neutralized with
sodium carbonate to pH 7-8, by keeping the temperature below 10 °C. The product
separated was brick solid. This was filtered, washed with ice cold water and dried at
50oC. The compound was crystallized from ethanol as brick red powder.
Yield = 3.0 g (65%); M.P. = 198 °C; 1H NMR (CDCl3, TMS): ): δ 10.25 (s, 1H),
8.40 (s, 1H), 7.70 (m, 2H), 7.42-7.38 (m, 1H), 6.65-6.60 (m, 1H), 6.45 (s, 1H), 3.45
(m, 4H), 1.30-1.25 (t, 6H); 13C NMR (CDCl3, ppm) δ 192.56, 152.85, 145.36,
144.98, 131.06, 126.28, 113.14, 110.01, 108.45, 96.59, 45.26, 12.54.
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2.3.2.3 Synthesis of 5-methylthiophene-2-carbaldehyde [6]
A three necked 100ml round bottom flask is taken fitted with a mercury sealed
stirrer, addition dropping funnel topped by calcium chloride guard tube and reflux
condenser also topped by calcium chloride guard tube. N, N-dimethyl formamide
(11.0 g, 12 ml, 0.153 moles) was taken and cooled to 0-5°C with stirring. To the
above solution phosphorous oxychloride (23 g, 14 ml, 0.153 moles) was added drop
wise maintaining the temperature of the reaction mass at 0-5 °C. The Vilsmeier
complex so formed was stirred for further 15 minutes and solution of 2-methyl
thiophene [5] (10g, 9.3 ml, 0.102 moles) in DMF (7g, 9 ml, 0.1 moles) was added in
lots (15-25 minutes) to the complex. The reaction mixture was stirred at 0-5 °C for
1.5 hrs and then allowed to attain room temperature. After that the contents of the
flask were heated at 65-70 °C in a water bath for 11 hrs. Subsequently the reaction
mass was cooled to room temperature and poured in to crushed ice with stirring and
deep external cooling, the clear solution obtained was neutralized with sodium
hydroxide to pH 7-8, by keeping the temperature below 10 °C. The reaction mass
was extracted using ethyl acetate. The ethyl acetate layer was washed with water and
then subjected to evaporation under vacuum using rotary evaporator to obtain an oily
viscous brown-colored liquid with 65% yield.
2.3.2.4 Synthesis of (Z)-2-cyano-3-(5-methylthiophen-2-yl)acrylic acid [9a]
In a three-necked 100 mL round bottomed flask, 5-methylthiophene-2-carbaldehyde
[6] (7 g, 0.05 moles) in absolute ethanol (35 ml, 5 vol) was taken to which
cyanoacetic acid [8] (4.72 g, 0.05 moles) and piperidine (2–3 drops) were added
with stirring and then the mixture was refluxed for 6 hrs. The precipitated solid was
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filtered, washed with ethanol and dried give 85% yields and used for further reaction
without purification.
2.3.2.5 Synthesis of (Z)-2-cyano-3-(p-tolyl)acrylic acid [9b] was synthesized by the
same procedure as that for compound [9a]. M.P. = 210 °C
2.3.2.6 Synthesis of 5-chloro-2-cyano-5-(7-(diethylamino)-2-oxo-2H-chromen-3-yl)
penta-2,4-dienoic acid [10a]
In a three-necked 100 mL round bottomed flask, 3-chloro-3-(7-(diethyl amino)-2-
oxo-2H-chromen-3-yl)acrylaldehyde (1g, 3.2 mmol) was taken in absolute ethanol
(10 mL, 10 vol). This was followed by the addition of cyanoacetic acid (0.55 g, 6.5
mmol) and piperidine (3-4 drops) and the reaction mixture was vigorously stirred at
reflux temperature for 4 hrs. The progress of the reaction was monitored by TLC.
After completion of the reaction, the reaction mass was added to cold water and
product was extracted using ethyl acetate. The ethyl acetate layer was washed with
water and then subjected to evaporation under vacuum using rotary evaporator to
obtain product [10a]. The crude product was further purified by silica gel column
chromatography using toluene:ethyl acetate system (6:4) as eluent system.
Yield = 1.0g (89%); M.P. = 242 °C.
Analysis of dye [10a]:
A. Mass spectra of the compound showed molecular ion peak at m/z = 373
which corresponds to molecular weight of [10a] = 372; msms showed ion
peak at m/z = 337 which corresponds to mass of fragment obtained after
removal of chloro group.
B. IR spectra of [10a]
Presence of broad band at 3383cm-1(s) indicating O-H stretching
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Presence of band at 3000-3100cm-1(s) indicating aromatic C-H
Presence of band at 2210 cm-1(s) indicating C=N stretching
Presence of band at 1702 cm-1(m) indicating C=O stretching
Presence of band at 1170 cm-1(s) indicating C-N stretching
Presence of band at 767 cm-1(m) indicating C-Cl bending
C. The compound was further confirmed by which showed following signals
[10a]
1H NMR (CDCl3, 300 MHz): δ (ppm) 8.44 (s, 1H, aromatic CH); 8.18-8.15
(m, 1H, aromatic CH); 8.04-8.01 (d, 1H, vinylic CH); 7.64-7.62 (d, 1H,
aromatic CH); 6.79-6.76 (d, 1H, vinylic CH); 6.59 (s, 1H, aromatic CH2); δ
3.63-3.44 (m, 4H, aliphatic CH2); δ 1.15-1.11 (t, 6H, aliphatic CH3);
13C NMR (CDCl3, 300 MHz): δ (ppm) 172.0, 163.0, 158.2, 155.9, 152.2,
144.5, 144.4, 137.4, 131.3, 122.3, 117.5, 112.4, 110.1, 108.0, 95.7, 62.7,
44.3, 12.3.
2.3.2.7 Synthesis of 3-(4-(4-chloro-4-(7-(diethylamino)-2-oxo-2H-chromen-3-
yl)buta-1,3-dien-1-yl)phenyl)-2-cyanoacrylic acid [10b]
The procedure was same as that for compound 10a except that 2-cyano-3-(p-tolyl)
acrylic acid [9a] (0.76 g, 4.0 mmol) was used instead of cyanoacetic acid. Yield =
0.91g (59%); M.P. = 226 °C.
Analysis of dye [10b]:
A. Mass spectra of the compound showed ion peak at m/z = 422 which
corresponds to molecular weight of [10b] after removal of Cl and OH
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groups; msms showed ion peak at m/z = 337 and 255 which also matches
with fragment ions after suitable fragmentation.
B. The compound was further confirmed by which showed following signals
[10b].
1H NMR (CDCl3, 300 MHz): δ (ppm) 8.04-7.90 (d, 1H, aromatic CH); 7.90-
7.80 (m, 5H, four aromatic CH and one vinylic CH); 7.50-7.42 (m, 4H, three
aromatic CH and one vinylic CH); δ 6.90-6.80 (m, 2H, vinylic CH); δ 3.72-
3.60 (m, 4H, aliphatic CH2); δ 1.50-1.38 (t, 6H, aliphatic CH3)
13C NMR (CDCl3, 300 MHz): δ (ppm) 161.1, 156.8, 151.5, 146.8, 142.7,
142.3, 129.7, 128.8, 123.7, 117.8, 109.5, 108.8, 97.0, 45.1, 12.6
2.3.2.8 Synthesis of 3-(5-(4-chloro-4-(7-(diethylamino)-2-oxo-2H-chromen-3-
yl)buta-1,3-dien-1-yl)thiophen-2-yl)-2-cyanoacrylic acid [10c]
The procedure was same as that for compound 10a except that 2-cyano-3-(5-
methylthiophen-2-yl)acrylic acid [9b] (0.78 g, 4.0 mmol) was used instead of
cyanoacetic acid. Yield = 0.70g, (45%); M.P. = 212 °C.
Mass spectra of the compound showed ion peak at m/z = 199, 244, 407 which
corresponds to molecular weight of [10c] after suitable fragmentation.
2.3.2.9 Synthesis of 8-methoxy-1, 2, 3, 5, 6, 7-hexahydropyrido [3, 2, 1-ij] quinoline
[12]
In a 500ml three-necked round bottomed flask equipped with an overhead
mechanical stirrer, thermometer, and a pressure equalizing addition funnel, 3-
Methoxyaniline (10g, 11ml, 0.10moles) was taken to which 1-bromo-3-
chloropropane (186g, 160ml, 1.5moles) and anhydrous sodium carbonate (42.7g,
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0.4moles) were added. The top of the addition funnel was fitted with a condenser
and the reaction mixture was warmed to 70°C for 1 hr and 100 °C for 2 hrs and then
heated at reflux for 11 hrs. The progress of the reaction was monitored by thin layer
chromatography. The reaction mixture was cooled to room temperature and 150ml
of concentrated HCl and 50ml of water were slowly added. Upon dissolution of all
solids, the phases were separated and the organic layer was washed with 10%HCl to
remove remaining product. This washing was added to the aqueous phase, which
was washed with ether to remove 1-bromo-3-chloropropane. The aqueous phase was
made basic with 50% aqueous sodium hydroxide and extracted with ether until the
organic phase no longer colored. The ethereal solution was dried over MgSO4 and
the solvent was removed. The resulting brown oil was distilled under reduced
pressure (0.5-1mm Hg, 110-114°C) to give yellow oil which turned red on exposure
to air. Yield = 13g (64%).
2.3.2.8 Synthesis of 1,2, 3,5,6,7-hexahydropyrido[3,2,1-ij]quinolin-8-ol [13]
In a 500ml three-necked round bottomed flask equipped with an overhead
mechanical stirrer, 8-Methoxy julolidine (10g, 50 mmol) was dissolved in a solution
consisting of 50 ml of 47% HI, 80ml of concentrated HCl and 200ml of water. This
solution was heated at reflux and the progress of the reaction was monitored by thin
layer chromatography. After 15 hrs, another 50ml portion of concentrated HCl was
added to the reaction mixture. After observing complete consumption of starting
materials (about 60 hrs) by thin layer chromatography, the reflux was stopped. The
solution was cooled in ice bath and neutralized to pH 6 by using NaOH. The
precipitate obtained was filtered to get tan coloured solid.
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Yield = 5.9 g (63%); M.P. = 126-130 °C.
2.3.2.9 Synthesis of 8-hydroxy-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinoline-9-
carbaldehyde [14]
In a 500ml three-necked round bottomed flask equipped with an overhead
mechanical stirrer, compound [13] (5.0g, 26.4 mmol) was dissolved in 8ml of dry
DMF and the resulting solution was added dropwise to cold solution of phosphorus
oxychloride (POCl3) (4.46g, 2.66ml, 29.1mmol) in 10ml of dimethyl formamide
which had been stirring for 15 minutes. The mixture was then stirred for 30 minutes
at room temperature followed by heating the solution at 85-90 °C for 2 hrs. The
reaction mixture was poured in ice-water and neutralized to pH 6-7 using sodium
carbonate solution. The precipitate obtained was filtered, washed with water and air-
dried. The residue was recrystallized from hexane to give yellow crystals.
Yield = 4.5g (78%); M.P. = 70-72°C.
2.3.2.10 Synthesis of 10-acetyl-2,3,6,7-tetrahydro-1H-pyrano[2,3-f]pyrido[3,2,1-
ij]quinolin-11(5H)-one [16]
In a 250ml single necked round bottom flask fitted with reflux condenser, compound
[14] (4.2 g, 0.02moles) was taken along with ethylacetoacetate (2.5g, 0.02moles) in
(42 mL, 10 vol) ethanol and 0.5ml piperidine. The reaction mass was refluxed for 5-
6 hrs and checked for the completion of reaction by thin layer chromatography. The
contents of the flask were cooled to room temperature and the product [15] separated
as orange crystals were filtered, washed with little amount of ethanol and dried.
Yield = 4.1 (76%); M.P. = 181-182 °C.
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2.3.2.11 Synthesis of 3-chloro-3-(11-oxo-2,3,5,6,7,11-hexahydro-1H-pyrano[2,3-
f]pyrido[3,2,1-ij]quinolin-10-yl)acrylaldehyde [17]
In a 100ml three necked round bottom flask fitted with a mercury sealed stirrer,
addition dropping funnel was fitted which was topped by calcium chloride guard
tube and reflux condenser also topped by calcium chloride guard tube. In this flask,
N,N-dimethyl formamide (2.32 g, 2.4 ml, 30 mmol) was taken and cooled to 0-5°C
with stirring. Then, phosphorous oxychloride (2.24g, 2 ml, 0.020 moles) was added
drop wise maintaining the temperature of the reaction mass at 0-5°C. The Vilsmeier
complex so formed was stirred for further 15 minutes and [16] (3g, 10.6 mmol) was
added in lots (15-25 minutes) to the complex. The reaction mixture was stirred at 0-
5°C for 3 hrs and then allowed to attain room temperature. The contents of the flask
were heated at 80-85°C in a water bath for 6-7 hrs. Subsequently the reaction mass
was cooled to room temperature and poured in to crushed ice with stirring and deep
external cooling. The clear solution so obtained was neutralized with sodium
carbonate to pH 7-8, by keeping the temperature below 10°C. The brick solid
colored product separated was filtered, washed with ice cold water and dried at
50°C. Yield = 2.1 g (61%); M.P. = 196-198°C.
2.3.2.10 Synthesis of 5-chloro-2-cyano-5-(11-oxo-2,3,5,6,7,11-hexahydro-1H-
pyrano[2,3-f]pyrido[3,2,1-ij]quinolin-10-yl)penta-2,4-dienoic acid [18]
In a 100ml three necked round bottom flask fitted with a mercury sealed stirrer, a
mixture of 3-chloro-3-(11-oxo-2,3,5,6,7,11-hexahydro-1H-pyrano[2,3-
f]pyrido[3,2,1-ij]quinolin-10-yl)acrylaldehyde [17] (1.5 g, 4.5 mmol) and
cyanoacetic acid (0.60 g, 6.8 mmol) was taken in absolute ethanol (15 mL, 10 vol)
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to which piperidine (0.5 mL) was added. The reaction mass was stirred and refluxed
for 4 hrs. The progress of reaction was monitored by TLC. After completion of the
reaction, the reaction mass was added to cold water and product was extracted using
ethyl acetate. The ethyl acetate layer was washed with water and then subjected to
evaporation under vacuum using rotary evaporator to obtain product [18]. The crude
product was further purified by silica gel column chromatography using toluene:
ethyl acetate system (6:4) as eluent system. Yield = 0.97 g (54%). M.P. = 236 °C.
Analysis of dye [18]:
A. Mass spectra of the compound showed ion peak at m/z = 297, 269, 261
which corresponds to molecular weight of [18] after suitable fragmentation.
B. The compound was further confirmed by which showed following signals
[18].
1H NMR (CDCl3, 300 MHz): δ (ppm) 8.30 (s, 1H, aromatic CH); 8.18-8.09
(m, 2H, vinylic CH); 7.20 (s, 1H, aromatic CH); δ 2.75-2.60 (m, 4H,
aliphatic CH2); 2.50-2.40 (m, 4H, aliphatic CH2); 1.90-1.75 (m, 4H, aliphatic
CH2)
13C NMR (CDCl3, 300 MHz): δ (ppm) 163.2, 158.1, 151.1, 149.2, 148.7,
145.3, 142.5, 127.4, 119.9, 115.4, 109.8, 108.2, 104.5, 49.7, 49.2, 26.7, 20.4,
19.4
2.3.2.11 Synthesis of 5-chloro-2-cyano-5-(11-oxo-2,3,5,6,7,11-hexahydro-1H-
pyrano[2,3-f]pyrido[3,2,1-ij]quinolin-10-yl)penta-2,4-dienoic acid [21]
In a three-necked round bottomed flask, 3-chloro-3-(7-(diethyl amino)-2-oxo-2H-
chromen-3-yl)acrylaldehyde [4] (2g, 6.5 mmol) was taken in absolute ethanol (20
mL, 10 vol). To this reaction mixture, 3-mercaptopropanoic acid (1.38g, 1.1 mL, 13
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mmol)was slowly added followed by addition of triethylamine. The progress of
reaction was monitored by TLC. After maximum consumption of reactants, the
reaction mass was added to cold water and product was extracted using ethyl acetate.
The ethyl acetate layer was washed with water and then subjected to evaporation
under vacuum using rotary evaporator to obtain product [21]. The crude product was
further purified by silica gel column chromatography using toluene:ethyl acetate
system (7:3) as eluent system. Yield = 1.27 g (52%)
2.3.2.12 Synthesis of 5-chloro-2-cyano-5-(11-oxo-2,3,5,6,7,11-hexahydro-1H-
pyrano[2,3-f]pyrido[3,2,1-ij]quinolin-10-yl)penta-2,4-dienoic acid [23]
In a three-necked round bottomed flask, malononitrile [22] (0.3 g, 0.25 mL, 4 mmol)
was taken in absolute ethanol (10 mL, 10 vol) to which piperidene (0.1 mL) was
slowly added. The temperature of the reaction mass was increased to 50 °C. This
was followed by the slow addition of 3-((1-(7-(diethylamino)-2-oxo-2H-chromen-3-
yl)-3-oxoprop-1-en-1-yl)thio)propanoic acid [21] (1g, 2.6 mmol) and the reaction
mixture was vigorously stirred at reflux temperature for 5 hrs. The progress of the
reaction was monitored by TLC. After completion of the reaction, the reaction mass
was added to cold water and product was extracted using ethyl acetate. The ethyl
acetate layer was washed with water and then subjected to evaporation under
vacuum using rotary evaporator to obtain product. [23]. The crude product was
further purified by silica gel column chromatography using toluene:ethyl acetate
system (7:3) as eluent system. Yield = 0.7 g (64%). M.P. = 266 °C. Mass spectra of
the compound showed ion peak at m/z = 199, 244, 407 which corresponds to
molecular weight of [23] after suitable fragmentation.
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2.4 CONCLUSION
In this chapter, we had aimed at designing coumarin based novel colorants suitable
for application in dye-sensitized solar cells. On varying the conjugation bridge in the
colorants, it was observed that UV-visible absorption gets bathochromically shifted.
In this regards, the thiophene bridged dye gave broader absorption in comparison to
other dyes. The fused coumarin dye also gave a bathochromic shift but its absorption
was not very broad. The concept of lateral chain anchoring seemed to improve
charge separation in the dye and gave a broad absorption till 600 nm.
The coumarin based dyes, designed and synthesized in this chapter is further studied
for its application in dye-sensitized solar cells as discussed in chapter 3. The
electronic transfer states of coumarin dye are also known to be affected by the open
chain or fused amino group. This aspect is also covered in chapter 3 by conducting
ultrafast laser studies on some dyes synthesized in this chapter.
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N O O
C l
CN
COOH
Mass spectra
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Chapter 2: Design and synthesis of Coumarin based sensitizers for dye-sensitized solar cells (DSSCs)
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N O O
Cl
H3C
H3C
CN
COOH
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1H-NMR spectra
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with increased intensity
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