Chapter-3 Design and synthesis of diketopyrrolopyrrole ...shodhganga.inflibnet.ac.in/bitstream/10603/40416/12/12_chapter_3.pdf · The synthetic route for the preparation of intermediates

Chapter-3

Design and synthesis of diketopyrrolopyrrole

(DPP)-based small molecules as promising

donor materials for small molecule bulk

heterojunction solar cell

DPP-based small molecules

95

3.1 Introduction

The increasing global energy demands and concerns over environmental

issues, have led to a greater focus on research to harness solar energy for clean and

economical modes of power generation.1 Due to their low-cost fabrication by solution

processing, lightweight and compatibility with flexible substrates, bulk hetero

junction organic solar cells (BHJOSC) have emerged as potential economical

alternatives to silicon-based solar cells.2-4 Polymer solar cells based on bulk-

heterojunction (BHJ), using materials comprising of π-conjugated polymers as donors

and fullerene derivatives as acceptors have been achieved power conversion

efficiencies (PCEs) over 10%.5-10 Meanwhile, small molecule organic

semiconductors11 are emerging as a competitive alterative to their polymer

counterparts. State-of -the –art power conversion efficiencies (PCEs) of small

molecule bulk hetero junction (SMBHJ) solar cells have shown up to 9%.12-14 In spite

of low PCE, small molecules have several advantages over polymers like synthetic

flexibility, easy purification, less batch-to-batch variation in properties, higher hole

and electron mobility, higher open-circuit voltage (VOC) and intrinsic mono-

dispersity.15-17 Reports on efficient SMBHJ devices have been recently reported in

which merocyanine dyes18,19, squaraine dyes20-23, fused acenes24,25, triphenylamine26-28

and isoindigo29 based chromophores, linear30-32 and star-shaped oligothiophenes33,34

and push–pull organic dyes35 are employed as the light harvesting donor component

and fullerene derivatives as the complementary acceptor. Rational design of narrow

band gap donors to better match the solar spectrum is one of the most successful

strategies to improve the solar cell efficiency. To design these type of materials, the

donor/acceptor (D/A) concept can be utilized, where the π-electron distribution

between electron rich and electron deficient moieties leads to low energy optical

transitions.36

Recently, diketopyrrolopyrrole (DPP) based small molecules have been

emerging as extremely attractive electron donor materials for small molecule bulk

hetero junction solar cells.37-42 Diketopyrrolopyrrole (DPP), with the pyrrolo[3,4-

c]pyrrole-1,4-dione unit as the core chromophore system was first synthesized as a

byproduct by Farnum et al. in 1974.43 Some of the derivatives of DPP have been


96

commercialized as high performance pigments used in plastics, paints and inks

because of their exceptional photochemical, mechanical and thermal stability.44 The

planar conjugated bi-cyclic structure of DPP core unit (promotes strong π−π stacking)

and the electron withdrawing effect of the lactam units make DPP core suitable to use

as the electron withdrawing unit in low band gap donor–acceptor materials. As

solubility is crucial for application in bulk hetero junction organic solar cells

(BHJOSC), the insolubility of DPP in many organic solvents can be solved by

attaching solubilizing groups at the 2, 5-positions of the DPP moiety (N atoms in the

lactam).

Scheme 3.1 Chemical structures of the four new DPP based small molecules

N

N

O

O

S S

R

RN

N

R1

R1

N

N

O

O

O O

R

RN

N

R1

R1

R = 2-ethylhexyl ; R1 = ethyl

N

N

O

O

O O

R

RN

N

R1

R1

N

N

O

O

S S

R

RN

N

R1

R1

CTDP

BCTDP

CFDP

BCFDP


97

In the present work, we report the synthesis and characterization of four new

low band gap molecules CTDP, BCTDP, CFDP and BCFDP (scheme 3.1) with D-

A-D molecular architecture in which, carbazole and benzocarbazole are used as

electron donaing units and DPP as electron accepting unit. Carbazole and

benzocarbazole moieties were chosen as electron donating units to explore their effect

on optical and electro chemical properties. DPP flanked by thiophene unit was widely

utilized45,46 and some studies have reported that charge transport and other properties

could be further improved when the sulfur atom in the thiophene ring was replaced by

oxygen.47 Different electronic states of furan and thiophene could affect not only the

optical and electrochemical characteristics of the relevant compound but also the solar

cell parameters, such as short circuit current density (JSC), open circuit voltage (VOC),

fill factor (FF) and thus the power conversion efficiency (PCE).48-50 Hence we

systematically investigated the structure-property relationship of four new molecules

by changing different donor moieties and difffernt flanking groups of DPP.

3.2 Results and discussion

3.2.1 Synthesis and characterization

The synthetic route for the preparation of intermediates 3, 7, 12 and 13 is

depicted in Scheme 3.2. Carbazole was ethylated and brominated to yield 3-bromo-9-

ethyl-9H-carbazole (2). Compound 2 was treated with n-BuLi and 2-isopropoxy-

4,4,5,5-tetramethyl-[1,3,2]-dioxaborolane to obtain compound 3. 8-bromo-6,11-

dihydro-5H-benzo[a]carbazole (4) was synthesized by condensation of α-tetralone

and 4-bromophenylhydrazine chloride followed by rearrangement. Compound 4 was

aromatized using tetrachloro-1,4-benzoquinone to yield 8-bromo-11H-

benzo[a]carbazole (5). Compound 6, synthesized by ethylation of 5 was treated with

n-BuLi and 2-isopropoxy-4,4,5,5-tetramethyl-[1,3,2]-dioxaborolane to yield 11-ethyl-

8-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-11H-benzo[a]carbazole (7). 3,6-

Di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)- dione (8) was synthesized

through the condensation of thiophene-2-carbonitrile and dibutyl succinate in the

presence of sodium tert-amyl alcohol and FeCl3 at 110 0C.51 The plausible mechanism

for formation of 3,6- Di(thiophen-2-yl)pyrrolo[3,4- c]pyrrole-1,4(2H,5H)- dione is


98

Scheme 3.2 Synthetic route for the preparation of intermediates 3, 7, 12 and 13

HN N

R1

N

R1

Br

N

R1

B

O

O

(i) (ii) (iii)

O N

HN

Br

HN Br HN Br

(v)(iv)

N BO

O

X CN

HN

NH

O

O

X

XN

N

O

O

X

XN

N

O

O

X

X

Br

Br

(iii)

R

R R

R

(vii) (viii) (ix)

(vi)

N Br

R1

1 2 3, R1 = ethyl

4 5

6 7, R1 = ethyl

8 X = S9 X = O

12 X = S, R = 2-ethylhexyl13 X = O, R = 2-ethylhexyl

R1

10 X = S, R = 2-ethylhexyl11 X = O, R = 2-ethylhexyl

Reagents and conditions: (i) bromoethane, KOH, DMF, 60 oC (ii) NBS, DMF, RT (iii) 2M

n-butyllithium, 2-isopropoxy-4,4,5,5-tetramethyl-[1,3,2]-dioxaborolane, THF,-78 °C to RT

(iv) α-Tetralone, 4-Bromophenylhydrazine hydrochloride, acetic acid, reflux (v) tetrachloro-

1,4-benzoquinone, xylene, reflux (vi) bromoethane, KOH, DMF, 60 oC (vii) dibutyl succinate,

Na, tert-amyl alcohol, 90 °C, 20 h (viii) 2-ethylhexyl bromide, DMF, K2CO3, 120 °C, 20 h

(iii) NBS, CHCl3, RT, 48 h

depicted in Scheme 3.3. The succinic ester undergone to pseudo-Stobbe condensation

with an aromatic nitrile in the presence of strong base to afford the desired DPP.52 The

key-step of the mechanism is the formation of pyrrolinone esters from the initially

formed enaminoesters, then it further react with another aromatic nitrile under basic

conditions and subsequent ring closure affords the DPP compounds. Without further

purification, 8 was alkylated in presence of excess K2CO3 and 2-ethylhexyl bromide

to provide compound 10. Bromination of 10 in CHCl3 using NBS yielded compound

12. Furan containing DPP segment 3,6-bis(5-bromofuran-2-yl)-2,5-bis(2-


99

Scheme 3.3 The general mechanism for formation of diketopyrrolopyrrole via the

succinic ester route

XCN

CO2R

RO2C

X

NH

RO2C

CO2RXN

CO2R

O

XN

CO2R

O

NH2

X

HN

NH

O

OX

X

+

XCN

Base -RO

-RO

X = S, O

Scheme 3.4 Synthetic route for the target compounds (CTDP, BCTDP, CFDP and

BCFDP)

N

R1

B

O

O

N

N

O

O

X

X

Br

Br

R

R

12 X = S ; R = 2-ethylhexylR1 = ethyl

13 X = O ; R = 2-ethylhexylR1 = ethyl

N

N

O

O

X X

R

RN

N

R1

R1

N

R1

B

O

O

N

N

O

O

X

X

Br

Br

R

RN

N

O

O

X X

R

RN

N

R1

R1

CTDP X = S, R = 2-ethylhexyl, R1 = ethylCFDP X = O, R = 2-ethylhexyl, R1 = ethyl

(i)

(i)

BCTDP X = S, R = 2-ethylhexyl, R1 = ethylBCFDP X = O, R = 2-ethylhexyl, R1 = ethyl

12 X = S ; R = 2-ethylhexylR1 = ethyl

13 X = O ; R = 2-ethylhexylR1 = ethyl

3

4

Reagents and conditions: (i) Pd(PPh3)4, 2M Na2CO3, EtOH:H2O, Toluene, 100 °C

ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (13) was synthesized using

similar synthetic route as used for synthesis of compound 12 by employing 2-

furylnitrile as starting material. CTDP, BCTDP, CFDP and BCFDP were prepared

by Pd(PPh3)4 assisted Suzuki coupling reaction of 12 with 3 and 4, 13 with 3 and 4,

respectively (Scheme 3.4). The chemical structures and purity of all compounds were

identified by 1H-NMR, 13C-NMR, MALDI-TOF and elemental analysis. The low

solubility of CTDP, BCTDP and CFDP avoided obtaining 13C-NMR spectra.


100

Synthesis of 9-ethyl-9H-carbazole (1)

Carbazole (20 g, 119.6 mmol), potassium hydroxide (20.1 g, 358.8 mmol) and

bromoethane (39.1 g, 358.8 mmol) were dissolved in DMF. The mixture was stirred

during overnight at 60 oC. After completion of reaction monitored using TLC, the

reaction mixture was poured into brine and the mixture was extracted with CHCl3.

The combined organic extracts were dried over anhydrous Na2SO4, filtered and

concentrated by rotary evaporation. Solid residue obtained was purified by

recrystallization in ethanol to afford desired product as white solid.

N

1H NMR (300 MHz, CDCl3) : δ 8.08 (d, J = 7.4 Hz, 2H), 7.44 (d, J = 7.4 Hz,

2H), 7.37 (d, J = 7.4 Hz, 2H), 7.21 (d, J = 7.4 Hz,

2H), 4.31 (q, J = 7.4 Hz, 2H), 1.39 (t, J = 7.4 Hz,

3H) 13C NMR (75 MHz, CDCl3) : δ 139.89, 125.56, 122.89, 120.37, 118.70, 108.38,

37.45, 13.76

MS-ESI (m/z) : 196 (M+H)+

M.p. : 64-66 oC

Yield : 89.9%(10.5 g)

Synthesis of 3-bromo-9-ethyl-9H-carbazole (2)

To a solution of 9-ethyl-9H-carbazole (1) (6 g, 30.7 mmol) in DMF, NBS

(5.47 g, 30.7 mmol) was added slowly. The mixture was stirred overnight at RT under

nitrogen atmosphere and light protection. After completion of reaction monitored

using TLC, the reaction mixture was poured into brine and extracted with CH2Cl2.

The combined organic extracts were dried over anhydrous Na2SO4, filtered and

concentrated under reduced pressure. Purification of solid residue by reprecipitation

with methanol and THF gave desired product as a white powder.


101

N

Br

1H NMR (300 MHz, CDCl3) : δ 8.17 (s, 1H), 8.00 (d, J = 7.4 Hz, 1H), 7.50 (d, J

= 8.5 Hz, 1H), 7.45 (t, J = 7.4 Hz, 1H), 7.35 (d, J

= 8.5 Hz, 1H), 7.21 (t, J = 7.4 Hz, 2H), 4.27 (q, J =

7.4 Hz, 2H), 1.36 (t, J = 7.4 Hz, 3H)

13C NMR (75 MHz, CDCl3) : δ 140.11, 138.42, 128.12, 126.26, 124.55, 123.01,

121.79, 120.50, 119.12, 111.44, 109.78, 108.60,

37.51, 13.63

MS-EI (m/z) : 273 ([M]+)

M.p. : 75-77 oC

Yield : 88% (12.4 g)

Synthesis of 9-ethyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole

(3)

To a stirred solution of 3-bromo-9-ethyl-9H-carbazole (2) (5.0 g, 18.2 mmol)

in anhydrous THF, 2M n-BuLi (1.74 g, 27.3 mmol) was added dropwise at -78 oC

under nitrogen atmosphere, stirring was continued for 1 h at the same temperature. 2-

isopropoxy-4,4,5,5-tetramethyl-[1,3,2]-dioxaborolane (13.5 g, 72.8 mmol) was added

to the reaction mixture and the resultant solution was allowed to reach RT and stirred

at RT for another 3 h. After completion of reaction monitored using TLC, the reaction

mixture was poured into water and extracted with CH2Cl2. The combined organic

extracts were washed with brine, dried over anhydrous Na2SO4 and filtered. The

filtrate was concentrated under reduced pressure and purified using column

chromatography (eluent: 95:5 hexane:ethylaceate) to afford desired product.


102

N

B

O

O

1H NMR (CDCl3, 300 MHz) : δ 8.68-8.59 (m, 1H), 8.20-8.12 (m, 1H), 8.00-7.91

(m, 1H), 7.52-7.38 (m, 3H), 4.40-4.32 (m, 2H),

1.51-1.36 (m, 15H) 13C NMR (CDCl3, 75 MHz) : δ 142.02, 139.96, 132.08, 127.77, 125.57, 123.15,

122.64, 120.56, 119.17, 108.42, 107.78, 83.50,

37.47, 24.89, 13.71

MS-EI (m/z) : 321 [M]+

M.p. : 97-99 oC

Yield : 72% (4.2 g)

Synthesis of 8-bromo-6,11-dihydro-5H-benzo[a]carbazole (4)

To a solution of α-Tetralone (5 g, 34 mmol) and 4-Bromophenylhydrazine

hydrochloride (4.6 g, 20 mmol) in ethanol was added small amount of aceticacid and

refluxed for 2 h under nitrogen atmosphere. After completion of reaction monitored

using TLC, the reaction mixture was cooled to RT and the formed product was

filtered and dried to obtain the desired compound.

HN Br

1H NMR (CDCl3, 300 MHz) : δ 8.20-8.17 (brs, 1H), 7.64 (s, 1H), 7.28-7.15 (m,

6H), 3.04-2.99 (m, 2H), 2.91-2.86 (m, 2H) 13C NMR (CDCl3, 75 MHz) : δ 136.54, 135.43, 134.18, 129.09, 128.52, 128.25,

127.13, 126.62, 124.85, 121.25, 119.96, 112.97,

112.42, 111.93, 29.29, 19.43

MS-EI (m/z) : 297 [M]+


103

M.p. : 120-122 oC

Yield : 85 % (8.7 g)

Synthesis of 8-bromo-11H-benzo[a]carbazole (5)

The solution of 8-bromo-6,11-dihydro-5H-benzo[a]carbazole (4) (5 g, 17

mmol) and tetrachloro-1,4-benzoquinone (5.7 g, 23 mmol) in xylene was refluxed

under nitrogen atmosphere for 2 h. After completion of reaction monitored using

TLC, NaOH (10%) was put into the reaction solution and the mixture was extracted

with ethylaceate. The combined organic extracts were dried over anhydrous Na2SO4,

filtered and concentrated under reduced pressure. The crude product was purified by

recrystallization with EtOH to afford the desired compound.

HN Br

1H NMR (CDCl3, 300 MHz) : δ 8.88-8.79 (brs, 1H), 8.24 (s, 1H), 8.15-7.99 (m,

3H), 7.70-7.44 (m, 5H) 13C NMR (CDCl3, 75 MHz) : δ 136.98, 135.45, 132.67, 129.11, 127.55, 125.94,

125.81, 125.67, 122.64, 120.95, 120.73, 120.45,

119.08, 117.52, 112.91, 112.43

MS-EI (m/z) : 295 [M]+

M.p. : 201-203 oC

Yield : 95% (4.7 g)

Synthesis of 8-bromo-11-ethyl-11H-benzo[a]carbazole (6)

8-bromo-11H-benzo[a]carbazole (5) (3 g, 10 mmol), potassium hydroxide (2.3

g, 41.6 mmol) and bromoethane (1.4 g, 12.5 mmol) were dissolved in DMF and the

mixture was stirred overnight at 60 oC. After completion of reaction monitored using

TLC, the reaction mixture was poured into brine and extracted with CH2Cl2. The

combined organic extracts were dried over anhydrous Na2SO4 and filtered. The

filtrate was concentrated under reduced pressure and purified using column

chromatography (eluent: 98:2 hexane:ethylaceate) to afford desired product.


104

N Br

1H NMR (CDCl3, 300 MHz) : δ 8.50 (d, J = 8.5 Hz, 1H), 8.27-8.24 (m, 1H),

8.09 (d, J = 8.5 Hz, 1H), 8.04 (d, J = 8.7 Hz, 1H),

7.69-7.52 (m, 4H), 7.42 (J = 8.7 Hz, 1H), 4.83 (q,

J = 7.2 Hz, 2H), 1.65 (t, J = 7.2 Hz, 3H) 13C NMR (CDCl3, 75 MHz) : δ 138.47, 134.78, 133.80, 129.64, 127.35, 125.68,

124.94, 124.74, 122.29, 122.06, 121.74, 120.95,

118.95, 118.17, 112.42, 110.30, 40.73, 15.00

MS-EI (m/z) : 323 [M]+

M.p. : 137-139 oC

Yield : 85% (2.7 g)

Synthesis of 11-ethyl-8-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-11H-

benzo[a]carbazole (7)

To a solution of 8-bromo-11-ethyl-11H-benzo[a]carbazole (6) (2 g, 6.2 mmol)

in anhydrous THF at -78 oC, 2M n-butyllithium (0.6 g, 9.2 mmol) was added

dropwise and the resultant mixture was stirred at -78 oC for 2 h. 2-isopropoxy-4,4,5,5-

tetramethyl-[1,3,2]-dioxaborolane (4.6 g, 24.8 mmol) was added at once to the

reaction mixture and the resulting mixture was stirred at -78 oC for another 2 h. The

reaction mixture was warmed to RT and stirred at RT overnight. After completion of

reaction monitored using TLC, the reaction mixture was poured into water and the

mixture was extracted with CH2Cl2. The combined organic extracts were dried over

anhydrous Na2SO4 and filtered. The filtrate was concentrated under reduced pressure

and purified using column chromatography (eluent: 97:3 hexane:ethylaceate) to afford

desired product.

N BO

O


105

1H NMR (CDCl3, 300 MHz) : δ 8.68 (s, 1H), 8.51 (d, J = 8.4 Hz, 1H), 8.23 (d, J

= 8.5 Hz, 1H), 8.04 (dd, J = 8.1, 0.94 Hz, 1H),

7.96 (dd, J = 8.4, 0.7 Hz, 1H) 7.68 (d, J = 8.5 Hz,

1H), 7.64-7.48 (m, 3H), 4.86 (q, J = 7.2 Hz, 2H),

1.65 (t, J = 7.2 Hz, 3H), 1.41 (s, 12H) 13C NMR (CDCl3, 75 MHz) : δ 142.43, 134.70, 134.44, 133.58, 130.97, 129.57,

127.19, 125.34, 124.40, 122.70, 122.20, 121.69,

120.88, 119.61, 119.28, 108.46, 83.53,45.91,

29.61, 14.08

MS-EI (m/z) : 371 [M]+

M.p. : 150-152 oC

Yield : 52% (1.2 g)

Synthesis of 3,6-Di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (8)

A mixture of Na (1.7 g, 73.4 mmol) and a catalytic amount of FeCl3 was

dissolved in tert-amylalcohol (35 mL) by heating at 90 0C for 2 h. After cooling the

mixture to 50 0C, thiophene-2-carbonitrile (4 g, 36.7 mmol) was added and the

temperature was again raised to 90 0C. The solution of dibutylsuccinate (3.4 g, 14.7

mmol) in tert-amylalcohol (17 mL) was added drop wise to this mixture over 2 h and

the mixture was maintained at same temperature for 20 h. The reaction mixture was

cooled to 50 0C, glacial aceticacid was added, heated under reflux for 10 min and

filtered. The solid residue obtained was washed several times with hot methanol and

water and dried under vacuum to yield desired compound. This crude compound

could be used directly in the next step without further purification.

HN

NH

O

O

S

S

Synthesis of 2,5-bis(2-ethylhexyl)-3,6-di(furan-2-yl)pyrrolo[3,4-c]pyrrole-

1,4(2H,5H)-dione (9)

Compound 9 was synthesized according to synthetic pocedure as described for

compound 8 using Na (1.97 g, 86 mmol), catalytic amount of FeCl3, furan-2-


106

carbonitrile (4 g, 43 mmol) and dibutylsuccinate (3.9 g, 17.2 mmol) as synthetic

precursors, which could be used in the next step without further purification.

HN

NH

O

O

O

O

Synthesis of 2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-

1,4(2H,5H)-dione (10)

To the mixture of 8 (6 g, 20 mmol), K2CO3 ( 9.1 g, 66 mmol), and 18-crown-6

(0.5 g, 2 mmol) in DMF (30 mL), solution of 2-ethylhexyl bromide (12.7 g, 66 mmol)

in DMF was added dropwise at 120 0C and the mixture was maintained at 120 0C

overnight. After completion of reaction monitored using TLC, the reaction mixture

was cooled to RT and filtered. The product obtained was dissolved in CHCl3 washed

with water. Then combined organic extracts were dried over anhydrous Na2SO4 and

filtered. The filtrate was concentrated under reduced pressure and purified using

column chromatography (eluent: 97:3 hexane:ethylaceate) to afford desired product.

N

N

O

O

S

S

R

RR = 2-ethylhexyl

1H NMR (CDCl3, 300 MHz) : δ 8.89 (dd, J = 3.9, 0.9 Hz, 2H), 7.63 (dd, J = 4.9,

0.9 Hz, 2H), 7.27 (dd, J = 4.9, 3.9 Hz, 2H), 4.06-

4.00 (m, 4H), 1.92-1.80 (m, 2H), 1.42-1.27 (m,


107.77, 45.72, 38.96, 30.08, 28.24, 23.41, 22.97,

13.94, 10.38

MS-EI (m/z) : 524 [M]+

M.p. 135-137 oC


107

Yield 45% (4.7 g)

Synthesis of 2,5-bis(2-ethylhexyl)-3,6-di(furan-2-yl)pyrrolo[3,4-c]pyrrole-

1,4(2H,5H)-dione (11)

Compound 11 was synthesized according to synthetic procedure as described

for compound 10 using compound 9 (6 g, 22.3 mmol), K2CO3 (10.2 g, 73.6 mmol),

18-crown-6 (0.6 g, 2.2 mmol), and 2-ethylhexyl bromide (14.1 g, 73.6 mmol) as

synthetic precursors.

N

N

O

O

O

O

R

R

R = 2-ethylhexyl

1H NMR (CDCl3, 300 MHz) : δ 8.33 (d, J = 3.7 Hz, 2H), 7.61 (d, J = 1.7 Hz,

2H), 6.69 (dd, J = 3.7, 1.70 Hz, 2H), 4.06-4.01

(m, 4H), 1.79-1.71 (m, 2H), 1.38-1.29 (m, 16H),

0.92-0.86 (m, 12H) 13C NMR (CDCl3, 75 MHz) : 161.18, 144.81, 144.60, 133.86, 120.16, 113.42,

106.39, 46.11, 39.89, 30.52, 28.65, 23.83, 23.01,

14.04, 10.73.

MS-EI (m/z) : 492 [M]+

M.p. : 142-144 oC

Yield : 43% (4.7 g)

Synthesis of 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-

c]pyrrole-1,4(2H,5H)-dione (12)

To the solution of 10 (4 g, 7.62 mmol) in CHCl3 was added NBS (2.84 g, 16

mmol) under nitrogen atmosphere and light protection and stirred at RT for 48 h.

After completion of reaction monitored using TLC, the reaction mixture was poured

into water and extracted with CHCl3. The combined organic extracts were dried over

anhydrous Na2SO4 and filtered. The filtrate was concentrated under reduced pressure

and purified using column chromatography (eluent: 98:2 hexane:ethylaceate) to afford

the desired product.


108

N

N

O

O

S

S

R

R

R = 2-ethylhexyl

Br

Br


2H), 3.98-3.88 (m, 4H), 1.90-1.77 (m, 2H), 1.42-

1.20 (m, 16H), 0.93-0.83 (m, 12H). 13C NMR (CDCl3, 75 MHz) : δ 161.36, 139.37, 135.36, 131.44, 131.11, 119.00,

107.96, 45.97, 39.06, 30.12, 28.27, 23.51, 23.01,

14.00, 10.43.

MS-MALDI (m/z) : 682 [M]+

M.p. : 140-142 oC

Yield : 52% (2.7 g)

Synthesis of 3,6-bis(5-bromofuran-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-

c]pyrrole-1,4(2H,5H)-dione (13)

Compound 13 was synthesized according to synthetic procedure as described

for compound 12 using compound 11 (4 g, 8.1 mmol) and NBS (3.04 g, 17 mmol) as

synthetic precursors.

N

N

O

O

O

O

R

R

R = 2-ethylhexyl

Br

Br


2H), 4.03-3.96 (m, 4H), 1.78-1.69 (m, 2H), 1.40-

1.26 (m, 16H), 0.96-0.86 (m, 12H) 13C NMR (CDCl3, 75 MHz) : 160.89, 146.13, 132.77, 126.29, 122.28, 115.51,

106.24, 46.21, 40.04, 30.49, 28.70, 23.72, 23.17,

14.07, 10.65


109


M.p. : 199-201 oC

Yield : 50% (2.6 g)

Synthesis of 3,6-bis(5-(9-ethyl-9H-carbazol-3-yl)thiophen-2-yl)-2-(2-ethylhexyl)-

5-(2-ethylpentyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (CTDP)

To the mixture of 12 (0.5 g, 0.73 mmol), 3 (0.47 g, 1.46 mmol), Pd(PPh3)4 (16

mg, 2 mol%) and sodium carbonate (1.18 g, 11.2 mmol) in distilled toluene was

added a degassed deionized water:ethanol (12:6 mL, v/v) mixture under nitrogen

atmosphere and the reaction mixture was refluxed for 18 h. After completion of

reaction monitored using TLC, the reaction mixture was cooled to RT and poured into

deionized water and the organic layer was separated. The aqueous layer was extracted

with CH2Cl2 and the combined organic extracts were dried over anhydrous Na2SO4,

filtered, concentrated and purified using column chromatography (eluent: 95:5

hexane:ethylaceate) to afford target compound.

N

N

O

O

SS

R

R

N

N

R1

R1


1H NMR (CDCl3, 300 MHz) : δ 9.05-9.01 (m, 2H), 8.42-8.38 (m, 2H), 8.20-8.15

(m, 2H), 7.84-7.78 (m, 2H),7.55-7.49 (m, 4H)

7.47-7.42 (m, 4H), 7.32-7.28 (m, 2H), 4.44-4.37

(m, 4H), 4.19-4.11 (m, 4H), 2.07-1.98 (brs, 2H),

1.50-1.41 (m, 16H), 1.36-1.29 (m, 6H), 0.99-0.86

(m, 12H)


Elemental Analysis : Anal. Calcd (%) for C58H62N4O2S2: C, 76.45, H,

6.86, N, 6.15. Found: C, 76.50, H, 6.80, N, 6.13

IR (neat) in cm-1 : 2925, 2856, 2312, 1657, 1552, 1428, 1339, 1263,

1229


110

Yield : 76% (0.5 g)

Synthesis of 3,6-bis(5-(11-ethyl-11H-benzo[a]carbazol-8-yl)thiophen-2-yl)-2-(2-

ethylhexyl)-5-(2-ethylpentyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (BCTDP)

BCTDP was synthesized according to synthetic procedure as described for

compound CTDP using 12 (0.5 g, 0.73 mmol), 7 (0.54 g, 1.46 mmol), Pd(PPh3)4 (16

mg, 2 mol%) and sodium carbonate (1.18 g, 11.2 mmol) as synthetic precursors.

N

N

O

O

SS

R

R

N

N

R1

R1



2H), 8.43-8.40 (m, 2H), 8.21 (d, J = 4.5 Hz, 2H ),

8.06 (d, J = 7.4 Hz, 2H), 7.81 (dd, J = 8.5, 1.5 Hz,

2H ), 7.72 (d, J = 8.5 Hz, 2H), 7.66-7.52 (m, 8H),

4.91-4.81 (m, 4H), 4.17-4.08 (m, 4H), 2.09-1.97

(m, 2H), 1.73-1.64 (m, 6H), 1.50-1.29 (m, 16H),

1.01-0.86 (m, 12H)

MS-MALDI (m/z) : 1011 [M]+

Elemental Analysis : Anal. Calcd (%) for C66H66N4O2S2: C, 78.38, H,

6.58, N, 5.54. Found: C, 78.32, H, 6.61, N, 5.49

IR (KBr) in cm-1 : 2925, 2856, 2375, 2311, 1742, 1515, 1461, 1371,

1278, 1219

Yield : 82% (0.6 g)

Synthesis of 3,6-bis(5-(9-ethyl-9H-carbazol-3-yl)furan-2-yl)-2-(2-ethylhexyl)-5-(2-

ethylpentyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (CFDP)

CFDP was synthesized according to synthetic procedure as described for

CTDP using 13 (0.5 g, 0.76 mmol), 3 (0.49 g, 1.53 mmol), Pd(PPh3)4 (16 mg, 2

mol%) and sodium carbonate (1.18 g, 11.2 mmol) as synthetic precursors.


111

N

N

O

O

OO

R

R

N

N

R1

R1


1H NMR (CDCl3, 300 MHz) : δ 8.53 (m, 2H), 8.48-8.46 (m, 2H), 8.13-8.12 (m,

2H), 7.88-7.85 (m, 2H), 7.53-7.49 (m, 2H) 7.46-

7.41 (m, 4H), 7.31-7.28 (m, 2H), 6.98-6.96 (m, 2H),

4.41-4.35 (m, 4H), 4.29-4.26 (m, 4H), 2.09-2.03 (m,

2H), 1.52-144 (m, 16H), 1.30-1.24 (m, 6H), 0.97-

0.93 (m, 6H), 0.84-0.79 (m, 6H)


Elemental Analysis : Anal. Calcd (%) for C58H62N4O4: C, 79.24, H, 7.11,

N, 6.37. Found: C, 79.19, H, 7.16, N, 6.32

IR (KBr) in cm-1 : 2924, 2855, 1659, 1584, 1455, 1397, 1350, 1308,

1260, 1226

Yield : 74% (0.5 g)

Synthesis of 3,6-bis(5-(11-ethyl-11H-benzo[a]carbazol-8-yl)furan-2-yl)-2-(2-

ethylhexyl)-5-(2-ethylpentyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (BCFDP)

BCFDP was synthesized according to synthetic procedure as described for

CTDP using 13 (0.5 g, 0.76 mmol), 7 (0.56 g, 1.53 mmol), Pd(PPh3)4 (16 mg, 2

mol%) and sodium carbonate (1.18 g, 11.2 mmol) as synthetic precursors.

N

N

O

O

OO

R

R

N

N

R1

R1


1H NMR (CDCl3, 300 MHz) : δ 8.54 (d, J = 3.5 Hz, 2H), 8.33-8.27 (m, 2H), 8.08-


112

8.02 (m, 4H), 7.73-7.66 (m, 4H ), 7.55-7.48 (m,

4H), 7.33 (d, J = 8.5 Hz, 2H ), 6.95(d, J = 3.5 Hz,

2H), 4.33-4.19 (m, 4H), 4.09-3.99 (m, 4H), 2.15-

2.06 (m, 2H), 1.34-1.20 (m, 16H), 0.93-0.87 (m,


132.08, 129.58, 125.18, 124.67, 123.05, 122.95,

122.30, 122.09, 121.34, 121.10, 120.99, 119.43,

118.97, 115.36, 110.08, 107.54, 106.12, 49.79,

46.70, 39.47, 39.18, 30.48, 28.48, 28.40, 28.36,

23.78, 23.62, 23.24, 22.98, 14.06, 13.95, 10.76,

10.72, 10.69


Elemental Analysis : Anal. Calcd (%) for C66H66N4O4: C, 80.95, H, 6.79,

N, 5.72. Found: C, 80.91, H, 6.72, N, 5.68

IR (KBr) in cm-1 : 2923, 2854, 1731, 1661, 1546, 1462, 1375, 1215

Yield : 80% (0.6 g)

3.2.2 Photophysical properties

The UV–Visible absorption spectra recorded for the synthesized donor

molecules in CHCl3 (ca. 1x10-5 M) are depicted in Figure 3.1a and the results are

summarized in Table 3.1. All the compounds showed strong absorption around the

peak of the solar radiation spectrum (600 nm), beneficial for the efficient solar light-

harvesting. All the compounds exhibited two absorption peaks, one in ultraviolet (300

to 450 nm) and other in visible (500 to 700 nm) regions. The absorption band

observed at short wavelength is attributed to the π-π* transition of carbazole and

benzocarbazole backbone and absorption band at long wavelength is attributed to the

intra molecular charge transfer (ICT) from the donor units (carbazole,

benzocarbazole) to the DPP core.53 CTDP, BCTDP, CFDP, and BCFDP showed

nearly equal absorption maxima of 621 (62900 M-1cm-1), 619 (26300 M-1cm-1), 625

(58300 M-1cm-1) and 624 (56000 M-1cm-1) nm, respectively. The observed high molar


113

Table 3.1 Absorption maxima ( absλ in nm), molar extinction coefficient ( ε ), emission

maximum ( fluoλ ) of CTDP, BCDP, CFDP and BCFDP

Comp absλ

(nm)a

Molar extinction coefficient

( ε , M-1

cm-1

)

fluoλ

(nm)a

CTDP 621, 427 62900, 16700 656

BCTDP 619, 416 26300, 8300 650

CFDP 625, 577 53800, 30900 647

BCFDP 624, 575 56000, 34000 644

a The absorption and emission spectra were recorded in CHCl3

Figure 3.1 (a) UV-Visible and (b) fluorescence spectra of CTDP, BCDP, CFDP and

BCFDP in chloroform (1x10-5 M)

300 350 400 450 500 550 600 650 700 750 8000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

ε /

ε /

ε /

ε /

(M-1

cm-1

)x10

5

Wavelength (nm)

CTDP

BCTDP

CFDP

BCFDP

650 700 750 800 8500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Flu

oresc

en

ce

inte

nsi

ty

Wavelength (nm)

CTDP

BCTDP

CFDP

BCFDP

(a) (b)

extinction coefficients of these molecules could result in improved light harvesting

capacity. The low molar extinction coefficient of BCTDP compared to CTDP, CFDP

and BCFDP inferred that it has low light harvesting capacity comparably. The UV–

Visible absorption spectra recorded for four new small molecules in different solvents

are depicted in Figure 3.2 and the results are listed in Table 3.2. Fluorescence

emission spectra recorded for CTDP, BCTDP, CFDP and BCFDP (1x10-5 M) are

depicted in Figure 3.1b and the results are summarized in Table 3.1. All the

compounds exhibited maximum emission wavelength in the range of 644-656 nm and


114

Figure 3.2 Normalized absorption spectra of CTDP, BCTDP, CFDP and BCFDP in

different solvents

300 350 400 450 500 550 600 650 700 750 800 8500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

No

rmali

zed

ab

sob

an

ce

Wavelength (nm)

Hexane

THF

CHCl3

CH3OH

CH3CN

CTDP

300 350 400 450 500 550 600 650 700 750 800 8500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

No

rmali

zed

ab

sob

an

ce

Wavelength (nm)

Hexane

THF

CHCl3

CH3OH

CH3CN

BCTDP

300 350 400 450 500 550 600 650 700 750 800 8500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0 Hexane

THF

CHCl3

CH3OH

CH3CN

No

rm

ali

zed

ab

sob

an

ce

Wavelength (nm)

CFDP

300 350 400 450 500 550 600 650 700 750 800 8500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0 Hexane

THF

CHCl3

CH3OH

CH3CN

No

rmali

zed

ab

sob

an

ce

Wavelength (nm)

BCFDP

Table 3.2 Absorption maxima (in nm) of CTDP, BCDP, CFDP and BCFDP

recorded in different solvents.

Comp Hexane THF Chloroform Acetonitrile Methanol

CTDP 608 622 621 613 609

BCTDP 612 621 619 614 599

CFDP 609 620 625 617 623

BCFDP 608 619 624 616 587

same spectral pattern. From the intersection point of absorption and emission spectra,

the optical band gap was calculated to be 1.92, 1.92, 1.94 and 1.92 eV for CTDP,

BCTDP, CFDP and BCFDP, respectively.


115

3.2.3 Electrochemical properties

To investigate the energy levels of the synthesized donor molecules cyclic

voltammetric studies (CV) were performed in a three-electrode cell, using 0.1 M

tetrabutylammonium perchlorate (n-Bu4NClO4) as the supporting electrolyte in

anhydrous CH2Cl2. The HOMO, LUMO levels were calculated using the following

empirical formula.55

HOMO = -( onset

oxE + 4.4) eV

LUMO = -( onset

redE + 4.4) eV

The electrochemical curves for four compounds are depicted in Figure 3.3 and the

oxidation potential, HOMO, LUMO and optical band gap values are summarized in

Table 3.3. CTDP (0.61 V) and BCTDP (0.62 V) were shown almost similar

oxidation potentials whereas, CFDP (0.51 V) and BCFDP (0.53 V) were shown

similar oxidation potentials. These results inferred that carbazole and benzocarbazole

do not affect the oxidation potential. But the replacement of thiophene flanked DPP

with furan flanked DPP affect the oxidation potential modestly and the usage of furan

in place of thiophene linker reduced the oxidation potentials. All the compounds

Table 3.3 Oxidation potential (Eox), reduction potential (Ered), HOMO, LUMO and

optical band gap (E0-0) of CTDP, BCTDP, CFDP and BCFDP.

Comp

onset

oxE

(volts)a

onset

redE

(volts)a

HOMO

(eV)

LUMO

(eV)

CV

gE

(eV)

E0-0

(eV)b

CTDP 0.63 -0.75 -5.03 -3.65 1.38 1.92

BCTDP 0.63 -0.75 -5.03 -3.65 1.38 1.92

CFDP 0.54 -0.75 -4.94 -3.65 1.29 1.94

BCFDP 0.50 -0.78 -4.90 -3.62 1.28 1.92 ameasured in CH2Cl2 with 0.1 M tetrabutylammonium perchlorate (n-Bu4NClO4) as the

electrolyte (working electrode: Glassy carbon; reference electrode: Ag/Ag+; calibrated with

ferrocene/ferrocenium (Fc/Fc+) as an external reference. Counter electrode: Pt wire).b E0–0 was

estimated from the intersection between the absorption and emission spectra.


116

Figure 3.3 Cyclic voltammograms of CTDP, BCTDP, CFDP and BCFDP (5x10-4

M) obtained in CH2Cl2 at a scan rate of 100 mVs-1

-1.5 -1.0 -0.5 0.0 0.5 1.0

Cu

rren

t/µµ µµ

A

CTDP

BCTDP

CFDP

Voltage vs Ag/Ag+ (V)

BCFDP

showed nearly equal reduction potential due to presence of DPP core moiety. To

overcome the exciton binding energy of the small molecule and, thereby, transport

electrons from the small molecule to PC60BM, the LUMO energy level of the small

molecule must be positioned above the LUMO energy level of PC60BM by at least 0.3

eV.56 The HOMO/LUMO levels of CTDP (-5.01/-3.09 eV), BCTDP (-5.02/-3.1 eV),

CFDP (-4.91/-2.97 eV) and BCFDP (-4.93/-3.01 eV) are well matched with PC60BM

(-6.1/-3.7 eV) for photovoltaic applications.

3.2.4 Thermal properties

To evaluate the thermal properties of the synthesized donor molecules

(CTDP, BCTDP, CFDP and BCFDP), thermo gravimetric analysis (TGA)

measurements were performed under nitrogen atmosphere at a heating rate of 10 oC

per minute. The representative TGA plots are shown in Figure 3.4 and the data is

summarized in Table 3.4. Thermal decomposition temperatures (Td) corresponding to

5% weight loss of these compounds are in the range of 382-416 °C representing their

good thermal stability. The melting points of these four compounds are in the range of

262-317 °C. This observation clearly indicates good thermal stability of these

materials even at high temperatures, required for vacuum deposition.


117

Figure 3.4 TGA plots of CTDP, BCTDP, CFDP and BCFDP recorded at a heating

rate of 10 oC min-1

100 200 300 400 500

BCFDP

Wei

gh

t (%

)

Temperature (°°°°C)

100 200 300 400 500

Weig

ht

(%)


CTDP

100 200 300 400 500

BCTDP

Wei

gh

t (%

)


100 200 300 400 500

CFDP


Wei

gh

t (%

)

Table 3.4 Thermal data of the compounds CTDP, BCTDP, CFDP and BCFDP

compound Td a (

oC) Tm b (oC)

CTDP 382 286

BCTDP 396 317

CFDP 397 312

BCFDP 416 262 a Td: decomposition temperature (corresponding to 5% weight loss).

b Tm: melting point

3.3 Experimental details

3.3.1 Measurements and instruments

All chemicals are reagent/analytical grade and used without further

purification. 1H NMR and 13C NMR spectra were recorded on a Bruker Avance (75

MHz) spectrometer in CDCl3 with TMS as the internal standard. Mass spectra were


118

obtained by using electronspray ionization ion trap mass spectrometry

(Thermofinnigan, Sanzox, CA), electron ionization mass recorded on VG70-70H

mass spectrometry. Elemental analyses were performed using a Vario-EL elemental

analyzer. Perkin-Elmer Spectrum BX spectrophotometer was used to obtain IR

spectra at a resolution of 4 cm-1. UV-Vis absorption spectra were measured using a

Jasco V-550 spectrophotometer. Steady state fluorescence spectra were recorded

using a Spex Fluorolog-3 spectrofluorometer. Cyclic voltammetric measurements

were performed on a PC-controlled CHI 620C electrochemical analyzer (CH

instruments). Cyclic voltammetric experiments were performed in 1 mM solution of

degassed dry dichloromethane at a scan rate of 100 mV s-1 using 0.1 M

tetrabutylammoniumperchlorate (TBAP) as the supporting electrolyte. The glassy

carbon was used as the working electrode, Ag/Ag+ as the reference electrode and

platinum wire as the counter electrode. The working electrode surface was first

polished with 1 mm alumina slurry, followed by 0.3 mm alumina slurry on a micro

cloth. It was then rinsed with millipore water and also sonicated in water for 5 min.

The polishing and sonication steps were repeated twice. Thermogravimetric analysis

(TGA) was performed using a TGA/SDTA 851e (Mettler Toledo) in the temperature

range of 30–550 °C under a nitrogen atmosphere at a heating rate of 10 °C min-1.

3.4 Conclusion

In conclusion, we have designed and synthesized four new low band gap

molecules CTDP, BCTDP, CFDP and BCFDP with D-A-D molecular architecture

in which, carbazole and benzocarbazole are used as electron donating units and DPP

is employed as electron accepting unit. The photophysical, electrochemical and

thermal properties of CTDP, BCTDP, CFDP and BCFDP are studied to evaluate

their suitability as donor materials in SMBHJSCs. All the compounds exhibited good

absorption in the range of 300-700 nm where the maximum solar photon flux (300-

800 nm) is available. The HOMO/LUMO energy levels of all the four compounds are

well matched with PCBM to act as donor in SMBHJSC. The high Td values of these

derivatives indicated the high thermal stability, which decreases the possibility of

molecule degradation and morphology deformation in solar cells. Hence we hope the

good optical properties, suitable HOMO/LUMO energy levels and excellent Td values


119

portray them as good candidates as small molecule donors for the realization of highly

efficient SMBHJSC.

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SPECTRA

1H NMR spectrum of compound 1

13C NMR spectrum of compound 1

















MALDI-TOF Mass spectrum of compound 12



MALDI-TOF Mass spectrum of compound 13

1H NMR spectrum of compound CTDP

MALDI-TOF Mass spectrum of compound CTDP

1H NMR spectrum of compound BCTDP

MALDI-TOF Mass spectrum of compound BCTDP

1H NMR spectrum of compound CFDP

MALDI-TOF Mass spectrum of compound CFDP

1H NMR spectrum of compound BCFDP

13C NMR spectrum of compound BCFDP

MALDI-TOF Mass spectrum of compound BCFDP

Documents

Chapter-3 Design and synthesis of diketopyrrolopyrrole ...shodhganga.inflibnet.ac.in/bitstream/10603/40416/12/12_chapter_3.pdf · The synthetic route for the preparation of intermediates