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Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 98
4.1 INTRODUCTION
Dye-sensitized solar cells (DSSCs) have attracted attention as noteworthy low-cost
alternatives for the conventional solid p–n junction photovoltaic devices (Gratzel
2001; Gratzel 2004). These solar cells have most dominantly used metal complex
sensitizers involving ruthenium poly-pyridyl complexes especially because of their
high power conversion efficiencies and long term stability (Wang et al. 2003; Liang
et al. 2007). Although such sensitizers have their own advantages, organic dyes
possess wide scope owing to 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.
The photoconversion efficiencies of dye-sensitized solar cells fabricated using
metal-free organic dye molecules commonly contain structural framework involving
donor (D), conjugating π bridge and acceptor (A) groups. This type of arrangement
is important due to the effective photoinduced intramolecular charge transfer
propertyof such systems (Chang et al. 2009; Wu et al. 2012).
Heterocyclic rings prove to be very efficient when incorporated as donors in
molecules for dye-sensitized solar cells. Various nitrogen containing heterocycles
have been explored in the past. These include rings such as tetrahydroquinoline
(Chen et al. 2007a; Chen et al. 2007b), indoline (Ito et al. 2006; Ito et al. 2008),
phenothiazine (Tian et al. 2007), julolidine (Choi et al. 2007), coumarin (Hara et al.
2007; Wang et al. 2007), etc. The nitrogen atom ensures better flow of electrons
from the donor to acceptor units thereby improving the current efficiency (Jsc)
values. Some examples of such systems are shown in the figure below.
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 99
Jsc = 10.9 mA.cm–2
Voc = 0.71 V
Efficiency = 5.5 %
Jsc = 10.6 mA.cm–2
Voc = 0.49 V
Efficiency = 2.3 %
Jsc = 8.5 mA.cm–2
Voc = 0.6 V
Efficiency = 3.2 %
Amongst various heterocyclic donor system, carbazole is a moeity that has been
explored well in DSSCs mainly due to their excellent electron donating and hole-
transporting ability (Ning et al. 2009; Chen et al. 2009; Zhang et al. 2009) . This has
led to its demand even in opto and electroactive materials auch as organic light-
emitting diodes (OLEDs) and solid-state DSSCs (Li et al. 2006 ; Ohkita et al.
2004). Carbazole has been employed in different interesting ways in DSSCs. Wang
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 100
et al. have used it as a donor with several thiophene units attached from the 3-
position of carbazole in D-π-A structural unit (Wang et al. 2008). In another report,
Srinivas et al. and Teng et al. synthesized molecule by extending the linker and
acceptor part from the nitrogen atom of carbazole (Srinivas et al. 2011; Teng et al.
2009).
Another advantage of carbazole is the presence of two active positions (3 and 6)
which leads to di-substituted derivatives (Ramkumar et al. 2012). Ths type of di-
substitution assists in improving the photo-induced intra-molecular charge transfer
(ICT) from donor to acceptor and increases the electron injection ability from the
LUMO level of the dye molecule to conduction band of TiO2 surface (Yang et al.
2010; Ooyama et al. 2011).
Other important nitrogen heterocycles that we have explored includes indole and
julolidene. Although indole has been taken as a donor in many functional
applications such as non-liner optics (Li et al. 2008; Liet al. 2009a), however there
are few reports on its use for solar cells (Li et al. 2009b; Inoue et al. 2010). Similarly
julolidine is another heterocycle which has also been very less explored (Choi et al.
2007a, Choi et al. 2007b).
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 101
Hence, we aimed to utilize these heterocycles as efficient donors for synthesis of
molecules for solar cell applications. The colorants were tested for their photo-
physical and thermal properties. After successful synthesis and purification, these
colorants were successfully used to construct dye-sensitized solar cells. The various
parameters including short-circuit current density (Jsc), short-circuit voltage (Voc)
and cell efficiency (η) were measured.
N
CN
CN
CN
COOH
COOH
NC
N SCOOH
CN
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 102
4.2 RESULTS AND DISCUSSION
4.2.1 Synthesis of styryl dyes for dye sensitized solar cell
The indole based novel styryl colorants [7a-7b] were prepared by classical
Knoevenagel condensation of 1-butyl-1H-indole-3-carbaldehyde [4] with
cyanoacetic acid [5] or 2-cyano-3-(p-tolyl)acrylic acid [6] in ethanol using
piperidine as a solvent. The same method was also used for condensation of other
carbazole based sensitizers [12a-12c]. The intermediates 1-butyl-1H-indole-3-
carbaldehyde [4], 9-hexyl-9H-carbazole-3-dicarbaldehyde [11a] and 3, 6-
dicarbaldehyde [11b] were synthesized in a sequence of two steps including
alkylation and formylation. The alkylation step was performed using phase-transfer
catalysis in toluene wherein butyltriethylammonium chloride was taken as the
quarternary ammonium catalyst. The longer alkyl chains ensure good solubility and
also avoid charge recombination process that could lower the efficiency value (He et
al. 2011). The formylation was done by conventional Vilsmeier Haack method using
dimethylformamide and phosphorus oxychloride. The dicarbaldehyde derivative of
carbazole was synthesized using higher equivalents of formylating agent and longer
refluxing. The intermediates and colorants were characterized by 1H-NMR, 13C-
NMR and Mass spectroscopy.
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 103
Scheme 4.1: Synthesis of N-butyl-3-formylindole [4]
Scheme 4.2: Synthesis of indole based sensitizers [7a-7b]
+ H3C Br PTC,Toluene
40% NaOH solution N
CH3
NH
[1] [2] [3]
N
CH3
DMF
POCl3
H
O
N
CH3
[3] [4]
where PTC = Butyltriethylammonium chloride (BTEAC)
H
O
N
CH3
[4]
+Ethanol
PiperidineReflux
CN
HOOC[6]
CN
COOH
[5]
Dye 7a
Dye 7b
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 104
Scheme 4.3: Synthesis of mono-carbaldehyde [11a] and di-carbaldehyde
derivatives of carbazole [11b]
Scheme 4.4: Synthesis of mono substituted carbazole based sensitizers (12a-12b)
+ BrH3CPTC,Toluene
40% NaOH solution
DMF
POCl3
N
H
O
[8] [9]
N
H
OO
H
N
CH3
NH
[10]
[11a]
[11b]
N
CH3
[10]
N
[11a]
+
Ethanol
PiperidineReflux
CN
HOOC
[6]
CN
COOH
[5]
Dye 12a
Dye 12b
O
H
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 105
Scheme 4.5: Synthesis of di substituted carbazole based sensitizers (12c)
Table 4.1: Physical properties of dyes
Dye No. Molecular formula Molecular
weight
Yield in
%
M.P.
DYE 7a C16H16N2O2 268 92 146
DYE 7b C24H22N2O2 370 61 152
DYE 12a C22H22N2O2 346 95 140
DYE 12b C30H28N2O2 449 60 156
DYE 12c C42H35N3O4 646 49 192
[11b]
+Ethanol
PiperidineRefluxCN
HOOC[6]
Dye 12cN
CH3
OO
H H
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 106
Figure 4.1: Structure of final dyes
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 107
4.2.2 Spectral characteristics of the dyes The linear absorption spectra of the synthesized heterocyclic 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 show a bathochromic shift on changing the donor from indole
to carbazole group and also on extending the conjugation by means of phenyl bridge
(Table 4.2). This could be owing to the better conjugation characteristics
incorporated by use of phenyl linker. The basic spectral characteristics of the dyes
such as the absorption maxima (λmax), emission maxima (λem) and extinction
coefficient (ε) were measured in chloroform and are presented in Table 4.2.
Table 4.2: UV-Visible and emission data of dyes
Dye
No.
Absorption in nm (CHCl3)
Emission in nm
(CHCl3)
Stokes
Shift in nm
DYE 7a 350 385 35
DYE 7b 372 410 38
DYE 12a 392 425 33
DYE 12b 396 436 40
DYE 12c 416 459 43
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 108
4.2.3 Thermal properties of the dyes
The dyes were subjected to the thermogravimetric analysis in order to investigate
their thermal stability. The thermo gravimetric analysis (TGA) was 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 4.3. It
was observed that the incorporation of benzene bridge improved the thermal stability
of the molecules in comparison to their counterparts without linker groups. The
sensitizer 12c showed the best thermal stability amongst all which could be
attributed to the rigidity of the di-substituted system. The higher value of thermal
stability is very much desirable in high-technological applications like dye-
sensitized solar cells.
Table 4.3: Thermal stability of sensitizers
Dye No.
Temperature stability (°C)
(at 4% product decomposition)
7a 242
7b 262
12a 288
12b 312
12c 325
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 109
4.2.4 Application for dye sensitized solar cell The synthesized colorants were applied onto dye-sensitized solar cells. For the
preparation of cells, doctor blading method was employed. After making the films
they were annealed at 450°C for 30 min. For sensitization, the films were
impregnated with 0.5 mM N719 dye in ethanol for 24 h at room temperature. The
samples were then rinsed with ethanol to remove excess dye on the surface and were
air dried at room temperature. This was followed by redox electrolyte addition and
top contact of Pt coated FTO. The electrolyte used was 1 M 1-hexyl-2, 3-dimethyl-
imidazolium iodide, 0.05 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine in
acetonitrile. The other details concerning the construction of solar cell and its
application process is described in chapter 3.
The dyes 7a, 7b and 12b were applied on DSSC and the Photocurrent density vs.
voltage curves were derived as shown in figures 4.2-4.4. These curves indicate that
compound 7a gave the best efficiency amongst other dyes. This was mainly due to
the enhanced values of short-circuit photocurrent density (Jsc). This increase could
be mainly attributed to the improved injection efficiency of electrons into the
conduction band of TiO2 in the case of dye 7a since the donor group is directly
connected to the acceptor group leading to direct passage of electrons. Amongst
benzene bridged moieties 7b and 12b, the dye 12b gave better efficiency and better
Jsc values which could be owing to the better donating ability of carbazole.
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 110
Figure 4.2: Photocurrent density vs. voltage curves for DSSCs based on dye-7a
Name Voc (V) Jsc(mA/cm2) FF (%) η (%)
1st 0.59 2.45 67.9 0.98
2nd 0.59 2.50 66.8 0.99
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 111
Figure 4.3: Photocurrent density vs. voltage curves for DSSCs based on dye-7b
Name Voc (V) Jsc(mA/cm2) FF (%) η (%)
1st 0.49 0.41 68.7% 0.14
N
CH3
COOH
CN
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 112
Figure 4.4: Photocurrent density vs. voltage curves for DSSCs based on dye-12b
Name Voc (V) Jsc(mA/cm2) FF (%) η (%)
1st 0.51 0.6 64.6 0.19
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 113
4.3 EXPERIMENTAL
4.3.1 Materials and equipments
All the solvents and chemicals were procured from S D fine chemicals, Sigma-
Aldrich 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.
4.3.2 Synthesis of key intermediates and compounds
4.3.2.1 Synthesis of N-butyl indole [3]
In a 500 ml round bottomed flask fitted with a mercury sealed stirrer, indole [1] (5g,
42 mmol), 50% aqueous sodium hydroxide solution (17.5 ml) and toluene (10 ml)
was heated to 50-55 ºC for 15 minutes. This was followed by addition of butyl
triethyl ammonium chloride (0.17g, 0.03 mole) to the reaction mixture and heating
was continued at 70-75 ºC for 30 minutes. The addition of 1-bromobutane [2] (8.7 g,
6.9 ml, 64 mmol) was done slowly through an addition funnel and reaction mass was
stirred for 3 hours at 70-75 ºC. The progress of the reaction was monitored by thin
layer chromatography. After completion of reaction, the reaction mass was poured
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 114
into hot water and left overnight. The product was filtered through suction vacuum
pump, washed with water, dried and recrystallised from ethanol with a yield of (6.2
g, 85 %).
4.3.2.2 Synthesis of N-butyl-3-formylindole [4]
In a three necked 500ml 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
(d=0.944, 5.0 g, 5.3 ml, 69.3 mmol) was taken and cooled to 0-5°C with stirring. To
the above solution phosphorous oxychloride (d=1.645, 7.0 g, 4.3 ml, 46.2 mmol)
was added drop wise maintaining the temperature of the reaction mass at 0-5°C. The
DMF - POCl3 complex so formed was stirred for further 15 minutes and N-butyl
indole [3] (4g, 23.1 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 mixture was then vigorously stirred under vigorously stirring and
heated to 75°C for 6 h. This solution was then cooled to room temperature, poured in
to ice water, and neutralized to pH 6-7 by drop wise addition of saturated aqueous
sodium hydroxide solution. The mixture was extracted with dichloromethane. The
organic layer was dried with anhydrous NaSO4 and then concentrated on rotary
evaporator. The crude product on purification by column chromatography (mobile
phase- toluene and silica gel 60-120 mesh) afforded as a yellow powder after drying
with 72% of yield (3.3 g).
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 115
4.3.2.3 Synthesis of N-hexyl-carbazole [10]
In a 500 ml round bottomed flask fitted with a mercury sealed stirrer, carbazole [8]
(5g, 30 mmol), 50% aqueous sodium hydroxide solution (12.5 ml) and toluene (10
ml) was heated to 50-55 ºC for 15 minutes. This was followed by addition of butyl
triethyl ammonium chloride (0.12g, 0.02 mole) to the reaction mixture and heating
was continued at 70-75 ºC for 30 minutes. The addition of 1-bromohexane [9] (7.4 g,
6.3 ml, 45 mmol) was done slowly through an addition funnel and reaction mass was
stirred for 4 hours at 70-75 ºC. The progress of the reaction was monitored by thin
layer chromatography. After completion of reaction, the reaction mass was poured
into hot water and left overnight. The product was filtered through suction vacuum
pump, washed with water, dried and recrystallised from ethanol to get white powder
with a yield of (6.2 g, 82 %); M.P. = 64 °C
4.3.2.4 Synthesis of N-hexyl-3-formyl-carbazole [11a]
In a three necked 500ml 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
(d=0.944, 4.65g, 4.92ml, 63.7 mmol) was taken and cooled to 0-5°C with stirring.
To the above solution phosphorous oxychloride (d=1.645, 7.3g, 4.4 ml, 47.8 mmol)
was added drop wise maintaining the temperature of the reaction mass at 0-5°C. The
DMF - POCl3 complex so formed was stirred for further 15 minutes and N-butyl-
carbazole [10] (4g, 15.9 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 mixture was then vigorously stirred under vigorously stirring and
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 116
heated to 75°C for 8 h. This solution was then cooled to room temperature, poured in
to ice water, and neutralized to pH 6-7 by drop wise addition of saturated aqueous
sodium hydroxide solution. The mixture was extracted with dichloromethane. The
organic layer was dried with anhydrous Na2SO4 and then concentrated on rotary
evaporator. The crude product on purification by column chromatography (mobile
phase- toluene and silica gel 60-120 mesh) afforded as a yellow powder after drying.
Yield = 3 g (69 %); M.P. = 60 °C.
4.3.2.5 Synthesis of N-hexyl-3, 6-diformyl-carbazole [11b]
In a three necked 500ml round bottom flask was 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
(d=0.944, 9.30 g, 9.8 ml, 127 mmol) was taken and cooled to 0-5°C with stirring. To
the above solution phosphorous oxychloride (d=1.645, 14.6 g, 8.89 ml, 95.6 mmol)
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 N-butyl-
carbazole [10] (4g, 15.9 mmol) was added in lots (15-25 minutes) to the complex.
The reaction mixture was stirred at 0-5°C for 2 hrs and then allowed to attain room
temperature. The mixture was then vigorously stirred and heated to 90 °C for 12
hours. This solution was then cooled to room temperature, poured in to ice water,
and neutralized to pH 6-7 by drop wise addition of saturated aqueous sodium
hydroxide solution. The mixture was extracted with dichloromethane. The organic
layer was dried with anhydrous Na2SO4 and then concentrated on rotary evaporator.
The crude product on purification by column chromatography (mobile phase-
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 117
toluene and silica gel 60-120 mesh) afforded a white solid was obtained. Yield = 2.6
g (54 %); M.P. = 138 °C.
4.3.2.6 Synthesis of (Z)-2-cyano-3-(p-tolyl)acrylic acid [6] is described in chapter 2,
section 2.3.2.5
4.3.2.6 Synthesis of (Z)-3-(1-butyl-1H-indol-3-yl)-2-cyanoacrylic acid [7a]
In a three necked 100ml round bottom flask fitted with a mercury sealed stirrer, a
suspension of cyanoacetic acid [5] (0.84 g, 9.9 mmoles) and 1-butyl-1H-indole-3-
carbaldehyde [4] (1.0g, 4.9 mmoles) were heated together in ethanol (10ml, 10 vol)
at reflux in presence of catalytic amount of piperidine for 5 hrs. The completion of
the reaction was monitored by thin layer chromatography. After cooling the reaction
mass, the mixture was poured into water and extracted using ethyl acetate. The ethyl
acetate layer was evaporated under vacuum using rotary evaporator. The obtained
residue was purified by column chromatography (toluene, 60 – 120 mesh silica gel)
to obtain final product [7a]. Yield = 1.2 g (89 %); M.P. = 220 °C.
Analysis of dye [7a]:
A. Mass spectra of the compound showed ion peak at m/z = 269 which
corresponds to molecular weight of [7a]
B. The compound was further confirmed by which showed following signals
[7a].
1H NMR (CDCl3, 300 MHz): δ (ppm) 8.65-8.60 (m, 1H, aromatic CH); 7.84-
7.80 (m, 1H, vinylic CH); 7.43-7.40 (m, 1H, aromatic CH); 7.39-7.32 (m,
2H, aromatic CH); 7.20 (s, 1H, aromatic CH); 4.24-4.20 (m, 2H, aliphatic
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 118
CH2); 2.0-1.86 (m, 2H, aliphatic CH2); 1.45-1.36 (m, 2H, aliphatic CH2), 1.0-
0.96 (t, 3H, aliphatic CH3).
C. 13C NMR (CDCl3, 300 MHz): δ (ppm) 147.3, 136.4, 134.7, 128.5, 124.1,
123.0, 118.6, 110.8, 110.2, 47.7, 31.9, 20.1, 13.6.
4.3.2.7 Synthesis of 3-(4-((E)-2-(1-butyl-1H-indol-3-yl)vinyl)phenyl)-2-
cyanoacrylic acid [7b] was synthesized by the same procedure as that of compound
[7a] except that 2-cyano-3-(p-tolyl)acrylic acid [6] was used instead of cyanoacetic
acid. Yield = 1.1 g (61 %); M.P. = 232 °C.
Analysis of dye [7b]:
A. Mass spectra of the compound showed ion peak at m/z = 297, 269 which
corresponds to molecular weight of [7b] after suitable fragmentation
(removal of alkyl chain and COOH group).
B. The compound was further confirmed by which showed following signals
[7b].
1H NMR (CDCl3, 300 MHz): δ (ppm) 8.51 (s, 1H, aromatic CH); 7.82-7.79
(m, 1H, vinylic CH); 7.43-7.39 (m, 2H, aromatic CH); 7.36-7.26 (m, 6H,
aromatic CH); 7.25-7.16 (m, 2H, vinylic CH); 4.21-4.17 (m, 2H, aliphatic
CH2); 1.90-1.82 (m, 2H, aliphatic CH2); 1.41-1.36 (m, 2H, aliphatic CH2),
1.0-0.93 (t, 3H, aliphatic CH3).
C. 13C NMR (CDCl3, 300 MHz): δ (ppm) 164.1, 145.9, 136.2, 133.8, 129.5,
129.2, 128.5, 123.8, 122.6, 118.6, 118.4, 110.7, 109.9, 93.5, 61.8, 47.5, 31.8,
20.1.
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 119
4.3.2.8 Synthesis of 2-cyano-3-(9-hexyl-9H-carbazol-3-yl)acrylic acid [12a]
In a three necked 100ml round bottom flask fitted with a mercury sealed stirrer, a
suspension of cyanoacetic acid [5] (0.60 g, 7.1 mmoles) and that N-hexyl-3-formyl-
carbazole [11a] (1.0g, 3.5 mmoles) were heated together in ethanol (10ml, 10 vol) at
reflux in presence of catalytic amount of piperidine for 7 hrs. The completion of the
reaction was monitored by thin layer chromatography. After cooling the reaction
mass, the mixture was poured into water and extracted using ethyl acetate. The ethyl
acetate layer was evaporated under vacuum using rotary evaporator. The obtained
residue was purified by column chromatography (toluene, 60 – 120 mesh silica gel)
to obtain final product [12a]. Yield = 1.12 g (90 %); M.P. = 202 °C.
4.3.2.9 Synthesis of 2-cyano-3-(4-(2-(9-hexyl-9H-carbazol-3-
yl)vinyl)phenyl)acrylic acid [12b] was synthesized by the same procedure as that of
compound [12b] except that 2-cyano-3-(p-tolyl)acrylic acid [6] was used instead of
cyanoacetic acid. Yield = 0.97 g (60 %); M.P. = 228 °C.
Analysis of dye [12b]:
A. Mass spectra of the compound showed ion peak at m/z = 444 which
corresponds to molecular weight of [12b]
B. The compound was further confirmed by which showed following signals
[12b].
1H NMR (CDCl3, 300 MHz): δ (ppm) 10.5 (s, 1H, COOH); 8.74 (s, 1H,
aromatic CH); 8.40 (s, 1H, aromatic CH); 8.24-8.12 (m, 2H, vinylic CH);
7.54-7.50 (m, 2H, aromatic CH); 7.46-7.40 (m, 2H, aromatic CH); 7.30-7.24
(m, 6H, aromatic CH); 7.20 (m, 1H, vinylic CH); 4.40-4.26 (m, 2H, aliphatic
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
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CH2); 1.90-1.80 (m, 2H, aliphatic CH2); 1.30-1.20 (m, 2H, aliphatic CH2),
0.90-0.80 (t, 3H, aliphatic CH3).
C. 13C NMR (CDCl3, 300 MHz): δ (ppm) 163.7, 156.0, 143.4, 141.1, 129.0,
126.9, 125.4, 123.5, 122.8, 122.6, 126.9, 126.6, 117.0, 109.5, 104.4, 97.7,
62.3, 43.5, 31.5, 29.7, 28.9, 26.9, 22.5, 14.3, 14.0
4.3.2.10 Synthesis of 9-hexyl-9H-carbazole-3,6-diyl)bis(ethene-2,1-diyl))bis(4,1-
phenylene))bis(2-cyanoacrylic acid) [12c]
In a three necked 100ml round bottom flask fitted with a mercury sealed stirrer, a
suspension of 2-cyano-3-(p-tolyl)acrylic acid [6] (1.55 g, 8.30 mmoles) and 9-hexyl-
9H-carbazole-3,6-dicarbaldehyde [12b] (1.0g, 3.32 mmoles) were heated together in
ethanol (10-15ml) at reflux in presence of catalytic amount of piperidine for 8 hrs.
The completion of the reaction was monitored by thin layer chromatography. After
cooling the reaction mass, the mixture was poured into water and extracted using
ethyl acetate. The ethyl acetate layer was evaporated under vacuum using rotary
evaporator. The obtained residue was purified by column chromatography (toluene,
60 – 120 mesh silica gel) to obtain final product [12c].
Yield = 1.13 g (49 %); M.P. = 242 °C
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 121
4.4 CONCLUSION In this chapter, we had aimed at designing dyes for application in dye-sensitized
solar cells by taking electron rich nitrogen heterocycles as donor groups. In this
respect, we selected indole and carbazole rings which were attached directly or via
phenyl conjugation to the cyanoacetic acid units. The introduction of phenyl bridge
leads to bathochromic shift and also improvement in the thermal stability of the
dyes. Moreover, the scope of di-substitution in moieties such as carbazole gave
further improvement in thermal stability.
These dyes were further applied onto dye-sensitized solar cells to check for their
efficiency values and other parameters. The direct attachment of cyanoacetic acid
unit to these heterocycles improved the electron injection efficiency and gave rise to
higher values of overall efficiency. In terms of donating ability, carbazole can be
considered to be better donating group than indole and therefore gave better
efficiency values. Therefore, nitrogen containing heterocycles possess good potential
and scope of improvement so as to obtain efficient molecules for application in dye-
sensitized solar cells.
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 122
Mass Spectra
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 123
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 124
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 125
1H NMR Spectra
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 126
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 127
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 128
13C NMR Spectra
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 129
Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles
Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 130