Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from nitrogen heterocycles

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1 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. protocols for heterocyclic synthesis Page 98

2 Jsc = 10.9 ma.cm 2 Voc = 0.71 V Efficiency = 5.5 % Jsc = 8.5 ma.cm 2 Voc = 0.6 V Efficiency = 3.2 % Jsc = 10.6 ma.cm 2 Voc = 0.49 V Efficiency = 2.3 % Amongst various heterocyclic donor system, carbazole is a moeity that has been explored well in DSSCs mainly due to their excellent electron donating and holetransporting 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 lightemitting diodes (OLEDs) and solid-state DSSCs (Li et al ; Ohkita et al. 2004). Carbazole has been employed in different interesting ways in DSSCs. Wang protocols for heterocyclic synthesis Page 99

3 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 disubstitution 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 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). protocols for heterocyclic synthesis Page 100

4 CN COOH NC N S CN COOH COOH CN CN N Hence, we aimed to utilize these heterocycles as efficient donors for synthesis of molecules for solar cell applications. The colorants were tested for their photophysical 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. protocols for heterocyclic synthesis Page 101

5 4.2 RESULTS AND DISCUSSION 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 1 H-NMR, 13 C- NMR and Mass spectroscopy. protocols for heterocyclic synthesis Page 102

6 Scheme 4.1: Synthesis of N-butyl-3-formylindole [4] N H + H 3C Br PTC,Toluene 40% NaOH solution [1] [2] [3] N where PTC = Butyltriethylammonium chloride (BTEAC) CH 3 O DMF H N POCl 3 N [3] [4] CH 3 CH 3 Scheme 4.2: Synthesis of indole based sensitizers [7a-7b] CN O COOH Dye 7a N H + [5] Ethanol Piperidine Ref lux [4] CH 3 [6] HOOC CN Dye 7b protocols for heterocyclic synthesis Page 103

7 Scheme 4.3: Synthesis of mono-carbaldehyde [11a] and di-carbaldehyde derivatives of carbazole [11b] N H + H 3 C Br PTC,Toluene 40% NaOH solution N [8] [9] [10] CH 3 O H N [11a] [10] N DMF POCl 3 H O O H CH 3 N [11b] Scheme 4.4: Synthesis of mono substituted carbazole based sensitizers (12a-12b) H O CN COOH [5] Ethanol Dye 12a N + Piperidine Ref lux [11a] HOOC [6] CN Dye 12b protocols for heterocyclic synthesis Page 104

8 Scheme 4.5: Synthesis of di substituted carbazole based sensitizers (12c) H H O O [11b] N + [6] HOOC CN Ethanol Piperidine Ref lux Dye 12c CH 3 Table 4.1: Physical properties of dyes Dye No. Molecular formula Molecular weight Yield in % M.P. DYE 7a C 16 H 16 N 2 O DYE 7b C 24 H 22 N 2 O DYE 12a C 22 H 22 N 2 O DYE 12b C 30 H 28 N 2 O DYE 12c C 42 H 35 N 3 O protocols for heterocyclic synthesis Page 105

9 Chapter 4: Synthesis of colorants for dye-sensitized solar cells derived from Figure 4.1: Structure of final dyes protocols for heterocyclic synthesis Page 106

10 4.2.2 Spectral characteristics of the dyes The linear absorption spectra of the synthesized heterocyclic sensitizers were measured for concentrations of 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 (CHCl 3 ) Emission in nm (CHCl 3 ) Stokes Shift in nm DYE 7a DYE 7b DYE 12a DYE 12b DYE 12c protocols for heterocyclic synthesis Page 107

11 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 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 dyesensitized solar cells. Table 4.3: Thermal stability of sensitizers Dye No. Temperature stability ( C) (at 4% product decomposition) 7a 242 7b a b c 325 protocols for heterocyclic synthesis Page 108

12 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-dimethylimidazolium 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 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 TiO 2 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. protocols for heterocyclic synthesis Page 109

13 Figure 4.2: Photocurrent density vs. voltage curves for DSSCs based on dye-7a Name Voc (V) Jsc(mA/cm 2 ) FF (%) η (%) 1 st nd protocols for heterocyclic synthesis Page 110

14 Figure 4.3: Photocurrent density vs. voltage curves for DSSCs based on dye-7b COOH CN N CH 3 Name Voc (V) Jsc(mA/cm 2 ) FF (%) η (%) 1 st % 0.14 protocols for heterocyclic synthesis Page 111

15 Figure 4.4: Photocurrent density vs. voltage curves for DSSCs based on dye-12b Name Voc (V) Jsc(mA/cm 2 ) FF (%) η (%) 1 st protocols for heterocyclic synthesis Page 112

16 4.3 EXPERIMENTAL 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 1 H 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 Synthesis of key intermediates and compounds 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 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 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 ºC. The progress of the reaction was monitored by thin layer chromatography. After completion of reaction, the reaction mass was poured protocols for heterocyclic synthesis Page 113

17 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 %) 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 - POCl 3 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 NaSO 4 and then concentrated on rotary evaporator. The crude product on purification by column chromatography (mobile phase- toluene and silica gel mesh) afforded as a yellow powder after drying with 72% of yield (3.3 g). protocols for heterocyclic synthesis Page 114

18 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 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 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 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 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 - POCl 3 complex so formed was stirred for further 15 minutes and N-butylcarbazole [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 protocols for heterocyclic synthesis Page 115

19 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 Na 2 SO 4 and then concentrated on rotary evaporator. The crude product on purification by column chromatography (mobile phase- toluene and silica gel mesh) afforded as a yellow powder after drying. Yield = 3 g (69 %); M.P. = 60 C 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-butylcarbazole [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 Na 2 SO 4 and then concentrated on rotary evaporator. The crude product on purification by column chromatography (mobile phase- protocols for heterocyclic synthesis Page 116

20 toluene and silica gel mesh) afforded a white solid was obtained. Yield = 2.6 g (54 %); M.P. = 138 C Synthesis of (Z)-2-cyano-3-(p-tolyl)acrylic acid [6] is described in chapter 2, section 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, 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]. 1 H NMR (CDCl 3, 300 MHz): δ (ppm) (m, 1H, aromatic CH); (m, 1H, vinylic CH); (m, 1H, aromatic CH); (m, 2H, aromatic CH); 7.20 (s, 1H, aromatic CH); (m, 2H, aliphatic protocols for heterocyclic synthesis Page 117

21 CH 2 ); (m, 2H, aliphatic CH 2 ); (m, 2H, aliphatic CH 2 ), (t, 3H, aliphatic CH 3 ). C. 13 C 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, 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]. 1 H NMR (CDCl 3, 300 MHz): δ (ppm) 8.51 (s, 1H, aromatic CH); (m, 1H, vinylic CH); (m, 2H, aromatic CH); (m, 6H, aromatic CH); (m, 2H, vinylic CH); (m, 2H, aliphatic CH 2 ); (m, 2H, aliphatic CH 2 ); (m, 2H, aliphatic CH 2 ), (t, 3H, aliphatic CH 3 ). C. 13 C 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, protocols for heterocyclic synthesis Page 118

22 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-formylcarbazole [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, mesh silica gel) to obtain final product [12a]. Yield = 1.12 g (90 %); M.P. = 202 C 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]. 1 H NMR (CDCl 3, 300 MHz): δ (ppm) 10.5 (s, 1H, COOH); 8.74 (s, 1H, aromatic CH); 8.40 (s, 1H, aromatic CH); (m, 2H, vinylic CH); (m, 2H, aromatic CH); (m, 2H, aromatic CH); (m, 6H, aromatic CH); 7.20 (m, 1H, vinylic CH); (m, 2H, aliphatic protocols for heterocyclic synthesis Page 119

23 CH 2 ); (m, 2H, aliphatic CH 2 ); (m, 2H, aliphatic CH 2 ), (t, 3H, aliphatic CH 3 ). C. 13 C 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, 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, mesh silica gel) to obtain final product [12c]. Yield = 1.13 g (49 %); M.P. = 242 C protocols for heterocyclic synthesis Page 120

24 4.4 CONCLUSION In this chapter, we had aimed at designing dyes for application in dye-sensitized solar cells by taking electron rich 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 dyesensitized solar cells. protocols for heterocyclic synthesis Page 121

25 Mass Spectra protocols for heterocyclic synthesis Page 122

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28 1 H NMR Spectra protocols for heterocyclic synthesis Page 125

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31 13 C NMR Spectra protocols for heterocyclic synthesis Page 128

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