Small band gap D-π-A-π-D benzothiadiazole derivatives with low-lying HOMO levels as potential donors for applications in organic photovoltaics: A combined experimental and theoretical investigation

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Small band gap D-p-A-p-D benzothiadiazole

derivatives with low-lying HOMO levels as potential

donors for applications in organic photovoltaics: a

combined experimental and theoretical

investigation†

Cite this: RSC Adv., 2014, 4, 35318

Mahalingavelar Paramasivam,^abAkhil Gupta,^cAaron M. Raynor,^c

c

ad

bd

*

Sheshanth V. Bhosale, K. Bhanuprakash

and V. Jayathirtha Rao

In an attempt to develop small organic molecules with potential applications as donors in organic

photovoltaic (OPV) devices, we have synthesized and characterized four novel benzothiadiazole (A) core

structured D-p-A-p-D dyes featuring carbazole and benzocarbazole as donors (D) and ﬂuorene and

thiophene as spacers (p). The eﬀects of the p-spacer units and variations in donor strength on the

photophysical, electrochemical and thermal properties of the molecules have been investigated in detail.

The replacement of ﬂuorene by thiophene as a p-spacer promotes planarity, resulting in a larger

bathochromic absorption shift, enhanced emission proﬁles and an enhanced intramolecular charge

transfer (ICT) transition. The introduction of the benzocarbazole unit creates a low-lying HOMO level, as

inferred from cyclic voltammetry studies. All the dyes exhibit remarkable thermal robustness. Theoretical

calculations have been carried out to understand the structure–property relationships of the synthesized

materials. The results obtained from the characterization methods reveal that the dyes with thiophene

p-spacers show better optoelectronic properties compared to their ﬂuorene counterparts. Solution-

processable bulk-heterojunction devices with a structure of ITO/PEDOT:PSS (38 nm)/active layer/Ca

(20 nm)/Al (100 nm) were fabricated using the materials investigated in this study as donors and (6,6)-

phenyl C₆₁-butyric acid methyl ester (PC₆₁BM) as an acceptor. A power conversion eﬃciency of 1.62%

for the molecule with thiophene as a spacer and carbazole as donor/PC₆₁BM was achieved for the

preliminary photovoltaic devices under simulated AM 1.5 illumination (100 mW cm^ꢀ2).

Received 27th March 2014

Accepted 21st July 2014

DOI: 10.1039/c4ra02700k

www.rsc.org/advances

polydispersity, variation between batches and complex puri-

cation procedures, thus paving the way for small organic

molecules as emergent alternatives.^3,4Initially, small molecules

Introduction

The quest to produce clean and renewable energy by developing

organic p-conjugated molecules for organic photovoltaic (OPV)

applications has attracted considerable attention from

researchers over the last few decades.¹Early on, organic solar

cells were prepared using polymers as light harvesting electron

donors owing to their high optical density, optimized lm

morphology and other superior properties.²Despite having

these unique features, polymers face the problems of

as electron donors lagged behind the polymers due to their

inferior photovoltaic performance, even though small mole-

cules have several advantages including facile synthesis, well-

dened molecular structures, amenability for large scale

production and so on. Later, the eﬃciency of small molecule

organic solar cells began to gradually evolve, reaching 8.2%

eﬃciency through scrutinized molecular design.⁵Recently,

Heliatek, a German company, reported tandem solar cells

featuring small molecules with the unprecedented cell eﬃ-

ciency of 12% for a standard size of 1.1 cm².⁶In view of this

trend, the day may not be far oﬀ when small-molecule organic

solar cells will outperform their polymer counterparts.

To improve the photovoltaic performance of small mole-

cules, many factors such as light absorption, charge carrier

transport, carrier injection, charge separation, etc. should be

considered.⁷An elegant strategy is to design the backbone

structures of these small molecules by the combination of

^aInorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical

Technology, Hyderabad, 500007, India. E-mail: bhanu2505@yahoo.co.in

^bCrop Protection Chemicals Division, CSIR-Indian Institute of Chemical Technology,

Hyderabad, 500007 India. E-mail: jrao@iict.res.in

^cSchool of Applied Sciences, RMIT University, GPO Box 2476V, Melbourne, Victoria

3001, Australia. E-mail: sheshanath.bhosale@rmit.edu.au

^dCSIR-Network of Institutes for Solar Energy, New Delhi, India

† Electronic supplementary information (ESI) available. See DOI:

10.1039/c4ra02700k

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various p-building blocks, with the following points in mind: (1)

to improve the photocurrent (J_sc), the molecule must have a

small HOMO–LUMO gap without sacricing V_oc; (2) to match

the frontier energy levels, which are mostly dependent on the

chemical structures of individual p-building blocks, with the

electron acceptors such as [6, 6]-phenyl- C₆₁-butyric acid methyl

ester (PC₆₁BM), and perylenediimides (PDI) so as to facilitate

the rate of electron injection eﬃciently.^8,9A very active area of

research involves the fabrication of novel small band gap

molecules possessing low-lying HOMOs with D-p-A-p-D

congurations as donors for OPV.^10–16Recently, Zeng et al.

introduced a cyano group on the spacer of a benzothiadiazole–

triphenylamine system, stabilizing the HOMO level to 5.32 eV.¹⁰

Similarly, Cho et al. introduced a uorine-substituted benzo-

thiadiazole acceptor unit, resulting in an eﬃciency of 2.95%;

this was due to the high V_ocattributed to the low HOMO level

and retention of the band gap.¹¹Tian and co-workers tuned the

band gap of small molecules by the introduction of pyran-4-

ylidenemalononitrile and the electron-donating group TPA

linked by diﬀerent conjugative units; an enhancement in eﬃ-

ciency was obtained from the compounds having low-lying

HOMO values.¹²The same principle was adopted by Sassi

et al. to control the HOMO level by synthesizing a new series of

end-capped diphenylhydrazone derivatives bearing a variety of

electron withdrawing conjugated bridges with diﬀerent substi-

tution patterns.¹³Romero et al. evaluated the band gap of the

molecules by altering the acceptor strength using uorene,

uorenone and benzothiadiazole.¹⁴As a result, the ICT char-

acter of the molecules were improved. Recently, Azoulay et al.

demonstrated the tuning of HOMO–LUMO energy levels by the

incorporation of various bridgehead imine-substituted cyclo-

pentadithiophene derivatives along with the use of benzothia-

diazole as a core unit; this strategy controlled the aromatic

stabilization with unprecedented precision.¹⁵

Based on the above points, it would be of interest to develop

molecules containing chromophores with high optical densi-

ties. A deliberate functional group selection for the modica-

tion of energy levels is essential so as to extend the absorption to

cover the maximum range of the solar spectrum.¹⁷This strategy

also oﬀers the possibility to probe the structure–property rela-

tionships of the molecules and gain insight into the develop-

ment of better candidates for solution-processable solar cells.

Furthermore, optical and electrochemical properties could be

ne-tuned by the appropriate inclusion of diﬀerent p-spacers

and donors into the p-conjugated backbone, which has not

been extensively investigated to date.¹⁸In recent years,

numerous articles have reported small molecule electron

donors containing diﬀerent heterocycle units with inherent

Fig. 1 Molecular structures of the dyes studied in this work.

stability. The incorporation of these kinds of electron decient

moieties into the p-conjugated system stabilizes the frontier

energy levels and aligns them with electron acceptor energy

levels to achieve optimal eﬃciency with minimal loss.

The purpose of this combined experimental and theoretical

study is to explore the absorption tuning and band gap of D-p-A-

p-D conjugated backbones containing diﬀerent inserted p-

conjugated units with varying donor strengths. Our main

design principle is to use benzothiadiazole core derivatives,

which are known to yield small band gaps with benzocarbazole/

carbazole donors that have low-lying HOMO levels. Herein, we

report the synthesis and detailed characterization of four novel

dyes based on the D-p-A-p-D architecture with benzothiadiazole

as the central acceptor unit (A) and the following molecular

combinations: a uorene p-spacer with a carbazole donor

(CFBFC) or a benzocarbazole donor (BFBFB) and a thiophene p-

spacer with a carbazole donor (CTBTC) or a benzocarbazole

donor (BTBTB) (Fig. 1). Quantum chemical calculations have

been used as a guide for the analysis of the ground state and

excited state properties.^23,24The inuence of p-spacers on

diﬀerent donors anked with benzothiadiazole and their eﬀects

on the photophysical and electrochemical properties have been

investigated for these compounds, which could help in the

design of more eﬃcient functional photovoltaic organic mate-

rials. We have compared the eﬀects of the benzocarbazole

moiety with those of its congener carbazole group as donors,

along with the eﬀects of diﬀerent p-spacers. To improve the

solubility and lm morphology, additional ethylhexyl and butyl

groups were attached to the donors and uorene moieties,

respectively.

Experimental

Materials and general procedures

charge transfer nature such as benzothiadiazole,^5,18squar- Unless otherwise specied, all reactions were performed under

aines,¹⁹diketopyrrolopyrrole²⁰and tetrazine.²¹Among these nitrogen atmosphere using standard Schlenk techniques. All

units, benzothiadiazole is considered to be one of the most reagents were reagent/analytical grade and used without further

versatile, and has been extensively used to make enormous purication. THF and toluene were distilled from sodium and

improvements in organic dyes for photovoltaic applications.²²

A

benzophenone, respectively, under nitrogen atmosphere. All

high eﬃciency of 6.8% has been reported due to the good chromatographic separations were carried out on silica gels

electron carrying ability of the thiadiazole moiety. This property (60–120 mesh). ¹H NMR and ¹³C NMR spectra were recorded on

leads to strong p–p interactions, resulting in an ordered a Bruker Avance (300 MHz) spectrometer in CDCl₃and DMSO-

morphology, eﬃcient charge transport and excellent thermal d₆with TMS as a standard in both cases. Mass spectra were

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obtained using electron ionization (EI) mass spectrometry adjacent nitrogen-lled glove box. Samples were placed on a

(Thermonnigan, Sanzox, CA). Gas chromatography-mass shadow mask in a tray. The area dened by the shadow mask

spectrometry (GCMS) was conducted on a VG70–70H, and gave device areas of exactly 0.2 cm². Deposition rates and lm

MALDI-TOF mass spectrometry was carried out on an Axima thicknesses were monitored using a calibrated quartz thickness

performance MALDI-TOF/TOF mass spectrometer (Shimadzu). monitor inside the vacuum chamber. Layers of calcium (Ca;

UV-Vis absorption spectra were measured on a Perkin-Elmer Aldrich) and aluminium (Al; 3 pellets of 99.999%; KJ Lesker)

spectrouorometer, and uorescence spectra were recorded having thicknesses of 20 nm and 100 nm, respectively, were

using a Spex model Fluorolog-3 spectrouorometer. A Perkin- evaporated from open tungsten boats onto the active layer by

Elmer Spectrum BX spectrophotometer was used to obtain IR thermal evaporation at pressures less than 2 ꢃ 10^ꢀ6mbar.

spectra of the dyes at a resolution of 4 cm^ꢀ1. Thermogravimetric Where used, C₆₀(Nano-C) and 2,9-dimethyl-4,7-diphenyl-1,10-

analyses (TGA) were performed with a TGA/SDTA 851e (Mettl_ꢀe₁r phenanthroline (Aldrich) were evaporated from alumina cruci-

ꢁ

Toledo) thermal analyzer using a heating rate of 10 C min

bles. A connection point for the ITO electrode was made by

under nitrogen atmosphere in the temperature range of 33–550 manually scratching oﬀ a small area of the active layers. A small

^ꢁC. The glass-transition temperatures (T_g) of the compounds amount of silver paint (Silver Print II, GC Electronics, part

were measured using diﬀerential scanning calorimetry (DSC) number: 22-023) was then deposited onto all of the connection

under a nitrogen atmosphere at a heating rate of 10 ^ꢁC min^ꢀ1on points of both ITO and Al. The completed devices were then

a DSC Q200 (TA instruments). The T_gvalues were determined encapsulated with glass and a UV-cured epoxy (Summers

from the second heating scan. Melting points were measured Optical, Lens Bond type J-91) by exposure to 365 nm UV light

with an Electro thermal IA 9100 series digital melting point inside the glove box for 10 min. The encapsulated devices were

instrument and are uncorrected. Cyclic voltammetry measure- then removed from the glove box and tested in air within 1 h.

ments were performed on a PC-controlled CH instruments Electrical connections were made using alligator clips. The OPV

model CHI 620C electrochemical analyzer using 1 mM dye devices were tested using an Oriel solar simulator tted with a

solution in dichloromethane (DCM) at a scan rate of 50 mV s^ꢀ1

;

1000 W xenon lamp ltered to give an output of 100 mW cm^ꢀ2at

0.1 M tetra butyl ammonium perchlorate (TBAP) was used as a simulated AM 1.5.²⁵The lamp was calibrated using a standard

supporting electrolyte. Glassy carbon, a standard calomel elec- ltered silicon (Si) cell from Peccell Limited, which was subse-

trode (SCE) and platinum wire were used as the working, quently cross-calibrated with a standard reference cell traceable

reference and auxiliary electrodes, respectively.

to the National Renewable Energy Laboratory. The devices were

tested using a Keithley 2400 source meter controlled by Lab-

view soware.

Device fabrication and characterization of photovoltaic

devices

8-Bromo-11H-benzo[a]carbazole (5)

Indium tin oxide (ITO)-coated glass (Kintek, 15 Ohms per

square) was cleaned by standing in a stirred solution of 5% (v/v)

Deconex 12PA detergent at 90 ^ꢁC for 20 min. The ITO-coated

glass was then successively sonicated for 10 min each in

distilled water, acetone, and isopropanol. The substrates were

then cleaned by UV/ozone at room temperature for 10 min. The

UV/ozone cleaning of glass substrates was performed using a

Novascan PDS-UVT UV/ozone cleaner with the platform set to

maximum height. The intensity of the lamp was greater than 36

mW cm^ꢀ2at a distance of 10 cm. At ambient conditions, the

ozone output of the UV cleaner exceeded 50 parts per million

(ppm). Aqueous solutions of PEDOT/PSS (HC Starck, Baytron P

AI 4083) were ltered (0.2 mm RC lter) and deposited onto glass

substrates in air by spin coating (Laurell WS-400B-6NPP lite

single wafer spin processor) at 5000 rpm for 60 s to give a layer

with a thickness of 40 ꢂ 5 nm. The PEDOT/PSS layer was then

annealed on a hotplate in a glove box at 145 ^ꢁC for 10 min. For

OPV devices, the newly synthesized organic p-type materials and

PC₆₁BM (Nano-C) were separately dissolved in individual vials

by magnetic stirring. The solutions were then combined,

ltered (0.2 mm RC lter), and deposited by spin coating (SCS

G3P spin coater) onto the ITO-coated glass substrates inside a

a-Tetralone (9.16 g, 6.26 mmol) and 4-bromophenylhydrazine

hydrochloride (10 g, 44.7 mmol) were dissolved in 300 mL of

ethanol. A catalytic amount of acetic acid was added to the

reaction mixture, which was then reuxed for 3 h under

nitrogen atmosphere. The reaction mixture was cooled to room

temperature, and the formed product was ltered, dried and

used in the next step without purication. The dried compound

(9.29 g, 30.8 mmol) and tetrachloro-1-benzoquinone (10.6 g,

43.2 mmol) in m-xylene were reuxed under nitrogen atmo-

sphere for 8 h and cooled to room temperature. Subsequently,

NaOH (10%) and water were put into the reaction solution, and

the organic layer was extracted with ethyl acetate and dried over

sodium sulphate. The reaction solution was concentrated and

puried by column chromatography over silica gel. The puried

compound was recrystallised with ethanol to give the desired

product as white crystals (8.8 g, 96% yield). ¹H NMR (500 MHz,

CDCl₃): 8.24 (s, 1H), 8.23–7.99 (m, 3H), 7.68–7.44 (m, 5H). ¹³C

NMR (125 MHz, CDCl₃): 137.5, 132.6, 128.8, 127.0, 125.5, 125.4,

122.3, 121.6, 120.0, 119.1, 112.7. GC/MS: m/z 296 [M]⁺.

3-Bromo-9-(2-ethylhexyl)-9H-carbazole (6a)

glove box (with H₂O and O₂levels both <1 ppm). Film thick- 3-Bromo-9H-carbazole (5 g, 20.3 mmol), 2-ethylhexyl bromide

nesses were determined on identical samples using a Dektak (5.4 mL, 30.5 mmol), and catalytic amount of tetrabutylammo-

6M Prolometer. The coated substrates were then transferred nium iodide (0.75 g, 10 mol %) was taken in 50% aq. NaOH

(without exposure to air) to a vacuum evaporator inside an solution. The reaction mixture was continuously heated to 70 ^ꢁC

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for 8 h and then cooled to room temperature. The reaction 1H, J ¼ 6.9 Hz), 7.92 (d, 1H, J ¼ 7.9 Hz), 7.68 (d, 1H, J ¼ 8.9 Hz),

mixture was extracted with hexane, washed with water and 7.58 (m, 3H), 4.72–4.62 (m, 2H, N–CH₂), 2.28–2.24 (m, 1H, N–

dried over anhydrous sodium sulphate. The solvent was CH₂–CH), 1.41 (s, 12H), 1.37–1.16 (m, 8H), 0.86–0.80 (m, 6H).

removed under vacuum, and the crude product was puried by ¹³C NMR (125 MHz, CDCl₃): 142.4, 133.8, 132.8, 130.1, 128.8,

column chromatography over silica gel with n-hexane as the 126.4, 124.3, 123.6, 121.8, 121.6, 121.3, 120.3, 119, 118.4, 108.4,

eluent to give the desired product as white solid (6.67 g, 92% 82.7, 82.6, 49, 38.8, 29.7, 27.7, 24.3 (4-CH₃), 23, 22.2, 13.2, 10.

yield). ¹H NMR (500 MHz, CDCl₃): 8.29 (s, 1H), 8.10–8.09 (d, 1H, GC/MS: m/z 455 [M]⁺.

J ¼ 7.9 Hz), 7.63–7.57 (d, 2H, J ¼ 8.9 Hz), 7.45–7.44 (d, 1H, d, J ¼

7.9 Hz), 7.35–7.32 (t, 1H, J ¼ 14.9 Hz), 4.08–4.06 (m, 2H, N–CH₂), 3-(7-Bromo-9,9-dibutyl-9H-uoren-2-yl)-9-(2-ethylhexyl)-9H-

2.12–2.09 (m, 1H), 1.46–1.38 (m, 8H), 1.38–0.76 (m, 6H). ¹³C carbazole (8a)

NMR (125 MHz, CDCl₃): 140.7, 138.9, 127.7, 125.8, 124.1, 122.4,

9-(2-Ethylhexyl)-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-

121.3, 119.9, 118.7, 111.1, 109.8, 108.7, 46.8, 38.8, 30.5, 28.5,

9H-carbazole (7a; 4 g, 9.86 mmol), 2,7-dibromo-9,9-dibutyl-9H-

uorene (6; 5.17 g, 11.83 mmol) and tetrakis-

28.4, 23.9, 22.7, 13.7, 10.5. GC/MS: m/z 358 [M]⁺.

(triphenylphosphine) palladium(0) (115 mg, 0.001 mmol) were

dissolved in a mixture of toluene and aqueous 2 M potassium

8-Bromo-11-(2-ethylhexyl)-11H-benzo[a]carbazole (6b)

This compound was synthesized according to a procedure carbonate solution (3 : 1, v/v) in a round-bottomed ask

similar to that of 6a, using 8-bromo-11H-benzo[a]carbazole (5.0 equipped with a reux condenser. The reaction mixture was

g, 16.9 mmol), to give the desired product as a white solid (6.4 g, heated to 80 ^ꢁC for 24 h. Aer cooling, the reaction mixture was

93% yield). ¹H NMR (500 MHz, CDCl₃): 8.39 (d, 1H, J ¼ 8.3 Hz), ltered, poured into water, extracted with chloroform and dried

8.16 (d, 1H, J ¼ 2.3 Hz), 7.99 (d, 2H, J ¼ 8.3 Hz), 7.60–7.44 (m, with anhydrous sodium sulphate. The solvent was removed by

4H), 7.28 (t, 1H, J ¼ 14.6 Hz), 4.41–4.37 (m, 2H, N–CH₂), 2.17– rotary evaporation and puried by column chromatography

2.13 (m, 1H, N–CH₂–CH), 1.52–1.16 (m, 8H), 0.91–0.78 (m, 6H). over silica gel with n-hexane as the eluent to give the desired

¹³C NMR (125 MHz, CDCl₃): 139.6, 134.9, 133.8, 129.5, 127, product as a white foamy solid (4.6 g, 7.2 mmol, 73% yield). ¹H

125.2, 124.8, 124.3, 122.1, 122, 121, 118.8, 118.3, 112.2, 111.2, NMR (500 MHz, CDCl₃): 8.37 (s, 1H), 8.16–8.14 (d, 1H, J ¼ 7.7

49.9, 39.5, 28.4, 23.8, 23, 14, 10.7. GC/MS: m/z 408 [M]⁺.

Hz), 7.72–7.49 (m, 4H), 7.44 (s, 1H), 7.41–7.18 (m, 7H), 4.02–4.00

(m, 2H, N–CH₂), 2.02–1.99 (m, 5H), 1.31–1.24 (m, 12H), 1.11–

1.06 (m, 6H), 0.85–0.78 (m, 8H). ¹³C NMR (125 MHz, CDCl₃):

153.5, 151.4, 141.9, 141.7, 140.7, 140.4, 138.7, 132.7, 130.3,

129.1, 127.6, 127.6, 126.6, 126.4, 126.2, 125.5, 123.7, 123.3,

121.8, 121.3, 121.1, 120.8, 120.5, 119.3, 119.0, 109.5, 55.7, 47.6,

40.6, 39.7, 31.3, 29.2, 26.3, 26.3, 24.7, 23.4, 14.5, 14.2, 11.3. MS

(EI): m/z 635 [M]⁺.

9-(2-Ethylhexyl)-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-

yl)-9H-carbazole (7a)

To a solution of 3-bromo-9-(2-ethylhexyl)-9H-carbazole (6a; 5 g,

14.1 mmol) in anhydrous THF (120 mL) at ꢀ78 ^ꢁC, n-butyl

lithium (2 M in hexane; 7.75 mL, 15.3 mmol) was added. The

mixture was stirred at ꢀ78 ^ꢁC for 2 h followed by the rapid

addition of 3.25 mL of 2-isopropoxy-4,4,5,5-tetramethyl-[1,3,2]-

dioxaborolane (16.7 mmol). The resulting mixture was

warmed to room temperature and stirred overnight. The

8-(7-Bromo-9,9-dibutyl-9H-uoren-2-yl)-11-(2-ethylhexyl)-11H-

benzo[a]carbazole (8b)

mixture was then poured into water and extracted with chlo- This compound was synthesized according to a procedure

roform. The organic extracts were washed with brine and dried similar to that of 8a, using 8-bromo-11-(2-ethylhexyl)-11H-benzo

with anhydrous sodium sulphate. The solvent was removed by [a]carbazole (4.0 g, 8.79 mmol) to give the desired product as a

1

rotary evaporation and puried by column chromatography white foamy solid (4.2 g, 6.1 mmol, 69% yield). H NMR (500

over silica gel with n-hexane : ethyl acetate (95 : 5) as the eluent MHz, CDCl₃): 8.48 (d, 1H, J ¼ 8.3 Hz), 8.38 (d, 1H, J ¼ 1.5 Hz),

to give the desired product as a white solid (5.1 g, 89% yield). ¹H 8.23 (d, 1H, J ¼ 9.1 Hz), 7.98 (d, 1H, J ¼ 8.31 Hz), 7.92 (d, 1H, J ¼

NMR (500 MHz, CDCl₃): 8.64 (s, 1H), 8.12–8.10 (d, 1H, J ¼ 8.3 7.9 Hz), 7.74–7.63 (m, 4H), 7.57–7.42 (m, 5H), 4.60–4.45 (m, 2H,

Hz), 7.95–7.92 (d, 1H, J ¼ 8.3 Hz), 7.38–7.15 (m, 4H), 3.92–3.89 N–CH₂), 2.27–2.22 (m, 1H, N–CH₂–CH), 2.12–1.93 (m, 4H), 1.43–

(m, 2H, N–CH₂), 1.94 (m, 1H), 1.33 (s, 12H), 1.25–1.16 (m, 9H), 1.04 (m, 8H), 0.86–0.80 (t, 6H, J ¼ 7.55), 0.72–0.67 (t, 6H, J ¼

0.82–0.76 (t, 6H, J ¼ 12.1). ¹³C NMR (125 MHz, CDCl₃): 143.2, 8.31). ¹³C NMR (125 MHz, CDCl₃): 153.2, 151.1, 141.6, 140.8,

141.1, 132.4, 127.9, 125.8, 123.3, 122.8, 120.6, 119.4, 109.2, 140.1, 138.5, 135.2, 133.8, 133.1, 130.0, 127.6, 126.4, 126.2,

108.6, 83.6, 47.3, 39.4, 31.1, 28.9, 25.1, 24.5, 23.2, 14.3, 11.1. GC/ 125.2, 124.6, 124.3, 123.4, 122.6, 122.2, 121.6, 121.0, 120.8,

MS: m/z 405 [M]⁺.

120.7, 120.1, 119.7, 119.2, 117.9, 110.2, 55.5, 50.2, 40.3, 39.7,

30.6, 28.6, 26.0, 23.9, 23.1, 14.1, 13.9, 10.8. MS (EI): m/z 684 [M]⁺.

11-(2-Ethylhexyl)-8-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-

yl)-11H-benzo[a]carbazole (7b)

3-(9,9-Dibutyl-7-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-

9H-uoren-2-yl)-9-(2-ethyl hexyl)-9H-carbazole (9a)

This compound was synthesized according to a procedure

similar to that of 7a, using 8-bromo-11-(2-ethylhexyl)-11H-benzo To a solution of 3-(7-bromo-9,9-dibutyl-9H-uoren-2-yl)-9-(2-

[a]carbazole (5.0 g, 12.3 mmol) to give the desired product as a ethylhexyl)-9H-carbazole (8a; 3 g, 4.7 mmol) in anhydrous THF

1

ꢁ

white solid (5.3 g, 95% yield). H NMR (500 MHz, CDCl₃): 8.66 (120 mL) at ꢀ78 C, 2.6 mL of n-butyl lithium (2 M in hexane;

(s, 1H), 8.52 (d, 1H, J ¼ 7.9 Hz), 8.24 (d, 1H, J ¼ 3.9 Hz), 8.02 (d, 5.2 mmol) was added. The mixture was stirred at ꢀ78 ^ꢁC

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for 2 h. Subsequently, 2-isopropoxy-4,4,5,5-tetramethyl-[1,3,2]- CDCl₃): 154.4, 152.0, 151.3, 141.4, 141.3, 141.2, 140.4, 139.1,

dioxaborolane (1.15 mL, 5.64 mmol) was added rapidly to the 135.9, 133.5, 132.6, 128.2, 127.8, 126.2, 125.7, 125.2, 123.8,

solution, and the resulting mixture was warmed to room 123.3, 122.9, 121.6, 120.4, 120.2, 119.6, 118.7, 109.2, 109.1, 55.3,

temperature and stirred overnight. The reaction mixture was 47.5, 40.3, 39.5, 31.0, 28.9, 26.2, 24.4, 23.2, 23.0, 14.1, 13.9, 10.9.

poured into water and extracted with chloroform. The organic MS (Maldi-TOF): (m/z) calcd: 1242.75; found: 1242.43.

extracts were washed with brine and dried with anhydrous

sodium sulphate. The solvent was removed by rotary evapora- 4,7-Bis(9,9-dibutyl-7-(11-(2-ethylhexyl)-11H-benzo[a]carbazol-

tion and puried by column chromatography over silica gel with 8-yl)-9H-uoren-2-yl) benzo[c][1,2,5]thiadiazole (BFBFB)

n-hexane : ethyl acetate (95 : 5) as the eluent to give the desired

This compound was synthesized according to a procedure

similar to that of 10, using 8-(9,9-dibutyl-7-(4,4,5,5-tetramethyl-

1,3,2-dioxaborolan-2-yl)-9H-uoren-2-yl)-11-(2-ethyl hexyl)-11H-

product as a white solid (2.9 g, 4.25 mmol, 90% yield). ¹H NMR

(500 MHz, CDCl₃): 8.59–8.55 (d, 2H, J ¼ 10.6 Hz), 8.37–8.34 (d,

1H, J ¼ 8.3 Hz), 7.66–7.61 (m, 3H), 4.61 (m, 2H, N–CH₂), 2.36–

benzo[a]carbazole (9b; 1.04 g, 1.43 mmol) and 2,7-dibromo

2.27 (m, 5H), 1.53 (s, 12H), 1.46–1.25 (m, 12H), 0.96–0.83 (m,

benzothiadiazole (2) to give the desired product 11 as a yellow

16H). ¹³C NMR (125 MHz, CDCl₃): 151.6, 149.7, 143.8, 141.3,

solid. (600 mg, 0.45 mmol, 62% yield). ¹H NMR (500 MHz,

140.9, 139.9, 134.4, 133.6, 132.2, 128.5, 127.3, 126.8, 125.8,

CDCl₃): 8.51–8.46 (dd, 4H, J ¼ 22.9 Hz), 8.26–8.25 (d, 2H, J ¼ 5.3

125.4, 124.9, 123.0, 122.6, 121.1, 120.2, 120.0, 118.6, 118.3,

Hz), 8.05–8.01 (m, 5H), 7.88–7.80 (m, 5H), 7.78–7.76 (m, 3H),

108.8, 108.7, 83.2, 54.8, 46.9, 40.0, 39.0, 30.6, 28.5, 26.6, 25.7,

7.67–7.66 (d, 2H, J ¼ 6.4 Hz), 7.57–7.55 (d, 2H, J ¼ 6.3 Hz), 7.52–

24.6, 24.0, 22.8, 22.7, 13.8, 13.6, 10.6. ESI-MS: m/z 683 [M]⁺.

7.51 (d, 4H, J ¼ 8.3 Hz), 7.51–7.50 (m, 2H), 7.15 (s, 1H), 4.6–4.56

(m, 2H, N–CH₂), 2.30–2.18 (m, 10H), 1.46–1.38 (m, 24H) 1.29–

1.18 (m, 19H) 0.86–0.76 (m, 12H). ¹³C NMR (125 MHz, CDCl₃):

8-(9,9-Dibutyl-7-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-

9H-uoren-2-yl)-11-(2-ethylhexyl)-11H-benzo[a]carbazole (9b)

154.4, 152.0, 151.3, 141.3, 141.2, 140.7, 139.2, 135.9, 135.1,

133.7, 133.5, 129.6, 128.9, 128.2, 127.8, 126.3, 125.1, 124.5,

124.3, 123.9, 123.3, 122.5, 121.7, 120.7, 120.3, 119.6, 119.2,

117.8, 110.1, 55.3, 50.1, 40.3, 39.7, 30.6, 28.5, 26.2, 23.8, 23.2,

23.0, 14.0, 13.9, 10.8. MS (Maldi-TOF): (m/z) calcd: 1342.78;

found: 1342.43.

This compound was synthesized according to the procedure

similar to that of 9a, using 8-(7-bromo-9,9-dibutyl-9H-uoren-2-

yl)-11-(2-ethylhexyl)-11H-benzo[a]carbazole (3.0 g, 4.38 mmol) to

give the desired product as a white foamy solid. (2.8 g, 3.82 mmol,

87% yield). ¹H NMR (500 MHz, CDCl₃): 8.42–8.38 (d, 2H, J ¼ 10.6

Hz), 8.18–8.17 (d, 1H, J ¼ 8.3 Hz), 7.97–7.62 (m, 10H), 7.59–7.44

(m, 3H), 4.44–4.41 (m, 2H, N–CH₂), 2.12–2.09 (m, 5H), 1.36 (s,

12H), 1.29–1.08 (m, 12H), 0.82–0.79 (t, 6H), 0.70–0.66 (m, 10H).

¹³C NMR (125 MHz, CDCl₃): 151.7, 149.8, 143.8, 141.3, 140.3,

139.1, 134.7, 133.6, 133.4, 132.8, 129.3, 128.6, 126.8, 125.9, 124.7,

124.2, 123.9, 122.9, 122.2, 121.9, 121.3, 120.4, 120.2, 119.3, 118.8,

118.7, 117.5, 109.8, 83.4, 54.9, 49.6, 40.1, 39.3, 30.3, 28.2, 25.7,

24.7, 23.6, 22.9, 22.7, 13.8, 13.6, 10.5. MS (EI): m/z 731 [M]⁺.

4,7-Bis (5-(9-(2-ethylhexyl)-9H-carbazol-3-yl) thiophen-2-yl)

benzo[c][1,2,5]thiadiazole (CTBTC)

A mixture of 9-(2-ethylhexyl)-3-(4,4,5,5-tetramethyl-1,3,2-dioxa-

borolan-2-yl)-9H-carbazole (7a; 0.93 g, 2.3 mmol) and 4,7-bis(5-

bromothiophen-2-yl)benzo[c][1,2,5]thiadiazole (4; 500 mg, 1.1

mmol) was dissolved in a mixture of toluene and aqueous 2 M

potassium carbonate solution (3 : 1, v/v). Subsequently, tetra-

kis(triphenylphosphine)palladium (63 mg, 0.005 mmol) and 18

mol% of 18-crown-6 were added to the solution, followed by

degassing with nitrogen for 15 min. The reaction mixture was

heated to 80 ^ꢁC under nitrogen atmosphere for 24 h. The cooled

crude mixture was poured into water, extracted with chloro-

form, dried over anhydrous sodium sulphate and evaporated to

yield crude product. The residue was puried by column chro-

matography using n-hexane as the eluent to give the desired

product 12 as a black solid (587 mg, 0.69 mmol, 52% yield). ¹H

NMR (500 MHz, CDCl₃): 8.26 (m, 2H), 8.09–8.08 (d, 2H, J ¼ 7.2

Hz), 7.92 (m, 2H), 7.65–7.54 (m, 6H), 7.4–7.2 (m, 8H), 3.98 (m,

4H, N–CH₂), 1.98 (m, 2H), 1.50–1.23 (m, 16H), 0.86–0.83 (m,

12H). ¹³C NMR (125 MHz, CDCl₃): 152.2, 146.6, 141.1, 137.1,

128.3, 125.7, 125.1, 124.9, 124.6, 123.6, 122.9, 122.5, 122.4,

120.2, 118.8, 117.3, 117.2, 109.0, 108.9, 47.2, 30.7, 28.5, 24.1,

22.3, 13.9, 13.8, 10.7. MS (Maldi-TOF): (m/z) calcd: 854.38;

found: 854.

4,7-Bis(9,9-dibutyl-7-(9-(2-ethylhexyl)-9H-carbazol-3-yl)-9H-

uoren-2-yl)benzo[c][1,2,5] thiadiazole (CFBFC)

3-(9,9-Dibutyl-7-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-

9H-uoren-2-yl)-9-(2-ethylhexyl)-9H-carbazole (9a; 0.98 g, 1.43

mmol) and 4,7-dibromobenzo[c][1,2,5]thiadiazole (200 mg, 0.7

mmol) were dissolved in a mixture of toluene and aqueous 2 M

potassium carbonate solution (3 : 1, v/v). Subsequently, tetra-

kis(triphenylphosphine)palladium (48 mg, 0.005 mmol) and 10

mol% of 18-crown-6 were added to the solution followed by

degassing with nitrogen for 15 min. The reaction mixture was

heated at 80 ^ꢁC under nitrogen atmosphere for 24 h. The cooled

reaction mixture was ltered, poured into water, extracted with

chloroform, dried over anhydrous sodium sulphate and evap-

orated to yield the crude product. The residue was puried by

column chromatography using n-hexane as the eluent to give

the desired product 10 as a yellow solid (658 mg, 0.53 mmol,

1

74% yield). H NMR (500 MHz, CDCl₃): 8.42 (s, 2H), 8.22–8.20

4,7-Bis(5-(11-(2-ethylhexyl)-11H-benzo[a]carbazol-8-yl)

thiophen-2-yl)benzo[c][1,2,5] thiadiazole (BTBTB)

(d, 2H, J ¼ 7.7 Hz), 8.11–8.07 (d, 2H, J ¼ 7.9 Hz), 7.99 (s, 2H),

7.93–7.74 (m, 8H), 7.52–7.44 (m, 3H), 7.41–7.29 (m, 6H), 7.27–

7.24 (m, 3H), 4.22–4.20 (m, 2H, N–CH₂), 2.16–2.12 (m, 10H), This compound was synthesized according to a procedure

1.54–4.38 (m, 20H), 0.89–0.76 (m, 12H). ¹³C NMR (125 MHz, similar to that of 12, using 11-(2-ethylhexyl)-8-(4,4,5,5-

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tetramethyl-1,3,2-dioxaborolan-2-yl)-11H-benzo[a]carbazole (7b; benzo[a]carbazole (6b) and the C-dialkylation of 2,7-dibromo-

1.04 g, 2.3 mmol) and 4,7-bis(5-bromothiophen-2-yl)benzo[c] uorene by 1-bromo butane to 2,7-dibromo-9,9⁰-dibutyl uo-

[1,2,5]thiadiazole (4) to give the desired product 13 as a brown rene were carried out according to modied procedures from

solid (632 mg, 0.66 mmol, 59% yield). ¹H NMR (500 MHz, the literature.³⁶The N-alkylated bromo derivative of carbazole

CDCl₃): 8.48–8.40 (m, 4H), 8.19–8.17 (m, 2H), 8.08–8.02 (m, 4H), (6a) and benzo[a]carbazole (6b) was treated with n-BuLi to

7.81–7.46 (m, 12H), 7.43–7.08 (m, 2H), 4.56 (m, 4H, N–CH₂), eliminate the halogen atoms at ꢀ78 ^ꢁC, and 2-isopropoxy-

2.23 (m, 2H), 1.55–1.24 (m, 16H), 0.86–0.84 (m, 12H). MS 4,4,5,5-tetramethyl-1,3,2-dioxaborolane was added to yield the

(Maldi-TOF): (m/z) calcd: 954.38; found: 954.

corresponding boronate derivatives 7a and 7b.³⁷The syntheses

of bromo derivatives 8a and 8b were obtained in good yields by

the Suzuki coupling of 2,7-dibromo-9,9⁰-dibutyl uorene³⁸with

intermediates 7a and 7b in the presence of Pd(Ph₃)₄as catalyst.

The corresponding bromo derivatives 8a and 8b were again

boronated by following the same procedure which is used to

prepare the intermediate 7a and 7b and aﬀord 3-(9,9-dibutyl-7-

(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-uoren-2-yl)-

9-(2-ethylhexyl)-9H-carbazole (9a) and 8-(9,9-dibutyl-7-(4,4,5,5-

tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-uoren-2-yl)-11-(2-ethyl-

hexyl)-11H-benzo[a]carbazole (9b). Finally, the dyes CFBFC and

BFBFB based on the uorene p-spacer were obtained through

the Suzuki coupling of 9a and 9b, respectively, with 4,7-dibromo

benzo[c][1,2,5]thiadiazole (2), and the dyes CTBTC and BTBTB

were obtained from the coupling of 8a and 8b, respectively, with

4,7-bis(5-bromothiophen-2-yl) benzo[c][1,2,5]thiadiazole (4)

under the same reaction conditions.³⁹All these molecules have

been characterized by ¹H NMR, ¹³C NMR, IR, and EI-MS.

MALDI-TOF mass spectrometry showed molecular ions at the

correct mass for the target dyes. The details of the experimental

characterization data are available in the ESI.†

Computational details

Density functional theory (DFT) calculations were performed

using the Gaussian 09 ab initio quantum chemical soware

package.²⁶DFT was used to determine the ground-state prop-

erties, while time-dependent DFT (TDDFT) was employed for

the estimation of ground to excited-state transitions. The opti-

mized geometries for all the molecules in their stable confor-

mations along with their corresponding frequencies calculated

at the B3LYP/6-311G (d,p) level of theory^27–29were used as the

input for further calculations. To expedite the calculations

without compromising the results, methyl groups were intro-

duced instead of the solubilizers such as the 2-ethylhexyl group

in the carbazole and benzocarbazole moieties and the two butyl

groups in uorene. The geometries were then used to obtain the

frontier molecular orbitals (FMOs) and were also subjected to

single-point TDDFT studies (rst 15 vertical singlet–singlet

transitions) to obtain the UV-Vis spectra of the dyes. The inte-

gral equation formalism polarizable continuum model

(PCM)^30,31within the self-consistent reaction eld (SCRF) theory

was used in the TDDFT calculations to describe the solvation of

the dyes in chloroform. The TDDFT calculations were per-

formed with various functionals including B3LYP, PBE and

M06-2X. The soware GaussSum 2.2.5 was employed to simu-

late the major portions of the absorption spectra and to inter-

pret the nature of transitions.³²The contribution percentages of

individual units present in the dyes to the respective molecular

orbitals were calculated.

Molecular geometry and orbitals of the dyes

Theoretical investigation of the molecular geometries and the

electron density distributions of the organic dyes was carried

out to gain insight into the p-conjugated spacers and their

eﬀect on the electronic and spectroscopic properties. An

important aspect to consider when assessing the degree of

p-orbital overlap for eﬃcient electron transport is the planarity

of the backbone.⁴⁰Due to the rotational freedom of the mole-

cule, we obtain many local minima with very little diﬀerence in

energy. This indicates that the molecules may not have single

dominant conformations in both the solution and solid phases.

Results and discussion

Synthesis of dyes

The target dyes CFBFC, BFBFB, CTBTC and BTBTB were To obtain a picture of the molecular orbital (MO) and vertical

synthesized according to a multistep synthetic pathway, as transitions, we froze the molecules in one local minimum and

illustrated in Scheme 1. o-Phenylenediamine was treated with carried out further studies including TDDFT (Fig. 2).⁴¹Variation

thionyl chloride to obtain benzothiadiazole (1), and the subse- in the vertical transition with respect to the change in angle was

quent bromination of 1 with bromine in 47% HBr solution investigated.

provided the key intermediate 4,7-dibromo benzothiadiazole

The electronic energies of the frontier molecular orbitals,

(2).³³Then, 2 was coupled with 2-tributylstannyl thiophene in which are used to assess the nature of charge transfer, and the

the presence of Pd(PPh₃)₄under Stille conditions to get 4,7- isodensity plots of the HOMO and LUMO for the four dyes are

di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (3). Further bromi- presented in Fig. 3. The HOMO is quite delocalized all over the

nation with N-bromosuccinimide gave 4,7-bis(5-bromo thio- spacer and acceptor and, to a certain extent, on the donor part.

phen-2-l)benzo[c][1,2,5]thiadiazole (4).³⁴The synthesis of 8- The electron density on the terminal part of donors such as the

bromo-11H-benzo[a] carbazole (5) was carried out according to phenyl group in carbazole and the naphthyl group in benzo-

35

¨

the Bucherer carbazole synthesis in two steps. The N-alkyl- carbazole is localized to a lesser extent, indicating that these

ation of 3-bromo-9H-carbazole and 8-bromo-11H-benzo[a] moieties are not actively engaged in electron distribution, as

carbazole by 2-ethylhexyl bromide to yield 3-bromo-9-(2-ethyl- reected in the computed excitation energy of the dyes. The

hexyl)-9H-carbazole (6a) and 8-bromo-11-(2-ethylhexyl)-11H- LUMO is mainly located on the central benzothiadiazole

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Scheme 1 Synthetic route for the preparation of target dyes. Reagents and conditions (i) SOCl₂, Et₃N, DCM; (ii) Br₂, 47% HBr, reﬂux; (iii) 2-tributyl-

stannyl thiophene, Pd(Ph₃)₄, toluene; (iv) N-bromosuccinimide, CHCl₃, 48 h; (v) p-bromophenylhydrazine hydrochloride, AcOH (cat), EtOH, 80 ^ꢁC; (vi)

10 mol% tetrabutyl ammonium iodide, 50% NaOH, 80 ^ꢁC; (vii) n-BuLi, 2-isopropoxy 4,4,5,5-tetramethyl-1,3,2-dioxaborolane, THF, ꢀ78 ^ꢁC, 2 h then rt;

(viii) 2,7-dibromo-9,9-dibutyl-9H-ﬂuorene, Pd(Ph₃)₄, toluene/2 M K₂CO₃; (ix) Pd(Ph₃)₄, 18-crown-6, toluene/2 M K₂CO₃.

Fig. 2 Computed interplanar angles in the local ground state between the diﬀerent aryl segments of the dyes.

acceptor moiety and to a lesser extent on the adjacent electron- frontier molecular orbitals by dividing the dyes into three

rich thiophene rings of the thiophene derivatives. A similar segments, donor (D), p-spacer (P), and acceptor (A), using the

pattern is also observed for the uorene dyes. The percentage GaussSum soware. The diagram is illustrated in Fig. S3, ESI.†

contribution of each dye segment has been computed from the

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pronounced eﬀect on the ICT for uorene-based dyes. It is

important to note that, in comparison to the absorption spectra

of uorene-based analogues, compounds based on a thiophene

p-spacer such as CTBTC and BTBTB exhibit two distinct

absorption maxima at 524 nm and 499 nm, respectively. The

substitution of thiophene in place of uorene leads to a 65–90

nm bathochromic shi in both the absorption and emission

proles. Additionally, the dye BTBTB, which contains benzo-

carbazole as a donor, displays hyperchromic and hypsochromic

eﬀects when compared to CTBTC.⁴⁵It has been noted that the

area of the ICT band of thiophene dyes (ꢄ250 nm) is almost two

times as broad as that of uorene dyes (ꢄ136 nm); the intensity

is also comparatively high, which implies that the ICT charac-

ters of the dyes are enhanced by replacing the uorene spacer

units with thiophene. In the lm state, the absorption spectra

exhibit an apparent broadening and bathochromic shi of

absorption bands corresponding to those of the solution

spectra due to the well-structured and intermolecular interac-

tions in the solid state.⁴⁶Thus, the replacement of uorene p-

spacers with thiophene leads to: (1) a considerable reduction in

band gap with a large extension in spectral response, indicating

that thiophene as a p-spacer provides eﬀective electronic

communication between donor and acceptor; and (2) the

extensive delocalization of thiophene dyes, leading to a bath-

ochromic shi due to the more planar conformation and high-

lying p-orbital along with the smaller aromatic resonance

energy of the thiophene entity compared to uorene.^47,48

To further investigate the eﬀect of solvent–solute interac-

tions on the ICT character of the dyes, absorption spectra were

recorded in various solvents. The UV-Vis absorption and emis-

sion spectra of CTBTC recorded in various solvents are shown in

Fig. 5, and the spectra of the remaining dyes are shown in ESI.†

All the dyes showed negligible response to solvent polarity for

intramolecular charge-transfer in the ground state.⁴⁹In

contrast, the emission spectra of the dyes red shied remark-

ably as the solvent polarity increased. Thus, it is clear that in the

presence of polar solvent, the non-polar locally excited (LE) state

observed in absorption is transformed into a polar excited state

as seen in emission.⁵⁰A distinct solvatochromic eﬀect is

observed for the thiophene derivatives in contrast to the

Fig. 3 Computed isodensity (0.02) surfaces of the HOMO and LUMO

of the target molecules.

Optical properties

The evaluation of optical properties is essential to understand

the applicability of small molecules for optoelectronic devices.

Fig. 4 depicts the UV-Visible absorption and emission spectra of

the four dyes CFBFC, BFBFB, CTBTC and BTBTB in chloroform

solution; the corresponding optical properties are compiled in

Table 1. All the compounds exhibited two absorption bands

characteristic of donor–acceptor based systems. Among them,

the higher energy absorption band at 320–370 nm corresponds

to the localized p–p* transition of the conjugated backbone.

The intensity of the p–p* transition peaks for the uorene dyes

are relatively high. Dyes based on uorene p-spacers such as

CFBFC and BFBFB display the lower energy absorption band at

382–516 nm with a molar extinction coeﬃcient of ꢄ3.5 ꢃ 10⁴

M^ꢀ1cm^ꢀ1, which is ascribed to the intramolecular charge

transfer (ICT) transition from the donor segments to the

acceptor group.^42–44The longer wavelength absorption maxima

of 433 nm in BFBFB, which has benzocarbazole as the donor, is

almost identical to that of CFBFC (434 nm), its carbazole

counterpart. This indicates that the introduction of diﬀerent

types of donors such as carbazole and benzocarbazole has a less

Fig. 4 UV-Vis absorption and emission spectra of dyes recorded in chloroform.

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Table 1 Optical properties and thermal analysis data of the dyes^a

b

c

Dye

l_abs(nm)

3 (M^ꢀ1cm^ꢀ1

)

l_u^b(nm)

563

l_abslm

Stokes shi (cm^ꢀ1

)

E_g,opt^d(eV)

2.36

T_d(^ꢁC)

433

T_m(^ꢁC)

T_g(^ꢁC)

121

98

BFBFB

CFBFC

BTBTB

CTBTC

433

337

434

338

499

332

524

356

34 063

107 859

35 653

92 477

40 343

74 029

36 706

53 401

447

521

544

5333

5280

4391

4001

270

215

192

173

563

2.30

435

639

1.95

376

96

663

1.85

372

63

a

T_d– decomposition temperature (corresponding to 5% weight loss). T_m– melting point of the compounds. T_g– glass transition temperature;

values are from the second heating scan. Absorption and emission spectra in chloroform at a concentration of 1 ꢃ 10^ꢀ5M at ambient

b

temperature. ^cMeasured in thin lm state. ^dEstimated from the onset absorption spectra of the compounds in thin lm state.

Fig. 5 Solvatochromism observed from UV-Vis absorption and emission spectra of CTBTC recorded in various solvents (for remaining dyes

see ESI†).

uorene dyes. For example, the emission maximum l_maxof of vertical transitions in terms of molecular orbitals for all the

CTBTC is red-shied from 644 nm in hexane to 712 nm in DMF, dyes. The ground state and transition dipole moments of the

which implies that the excited state of the compound is well dyes computed from DFT and TDDFT calculations also

stabilized upon an increase in solvent polarity, resulting in the corroborate the experimental observations. The intramolecular

red shied emission. The large Stokes shis observed for the charge transfer excited state is clearly formed from the HOMO

uorene dyes reveal that these dyes undergo more structural to LUMO transition. The percentage contribution of thiophene

relaxation during photoexcitation.⁵¹The optical band gap (E_g) as a p-spacer is superior to that of uorene with regard to the

(Table 1) was derived from the onset absorption spectra of the electronic properties of the molecule, which may be attributed

dyes in thin lm state.

to the eﬀective orbital mixing over the whole p-conjugated

Based on the experimental observations, TDDFT studies of system (Fig. S3, ESI†).^8b

these molecules were carried using diﬀerent energy functionals

To ascertain the charge transfer nature of the p-spacers of

(B3LYP, PBE and M06-2X) with the 6-311G (d,p) basis set in the dyes upon photoexcitation, we performed absorption

order to gain a deeper understanding of the excited-state tran- studies as a function of dihedral angle. This was done by varying

sitions.^52,53The transition energies obtained in the gas phase the dihedral angle between the donor vs. p-spacer and p-spacer

using the B3LYP and PBE functionals are highly overestimated vs. acceptor in both the uorene and thiophene derivatives from

compared to the experimental results. On the other hand, the 0^ꢁto 60^ꢁin steps of 5^ꢁusing TDDFT calculations.^54,55The

gas phase absorption spectra results computed using M06-2X structures used for these calculations were obtained from the

show only a small underestimation; thus, this functional was optimized geometries at the B3LYP/6-311G (d,p) level. The

further used in the framework of the polarizable continuum absorption values obtained for these two derivatives with

model (PCM) with chloroform as the solvent. These results are diﬀerent positions are plotted against dihedral angle in ESI

in reasonable agreement with the experimental values. Table 2 (Fig. S5 and S6†). The gures clearly show that the deviation of

contains the computed vertical excitations along with the dihedral angle between the donor and p-spacer has

a

oscillator strengths, frontier orbital energies and compositions pronounced eﬀect on the absorption of BTBTB, leading to a 45

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Table 2 Comparison of the experimental optical properties with the theoretical data

M06-2X (chloroform)

b

c

Dye

l_max^a(nm)

433

l_max(nm)

f (oscillation strength)

Composition

m_gs

m_ge

2.6

2.5

3.1

BFBFB

CFBFC

BTBTB

CTBTC

404 (3.07 eV)

500 (2.48 eV)

503 (2.47 eV)

1.61

1.54

1.37

1.19

HOMO / LUMO (54%), HOMO-2 /

LUMO (25%)

HOMO / LUMO (65%), HOMO-2 /

LUMO (25%)

HOMO / LUMO (90%), HOMO-2 /

LUMO (5%)

HOMO / LUMO (92%), HOMO-2 /

LUMO (4%)

2.3

2.1

2.3

434

499

524

a

Recorded in chloroform (1 ꢃ 10^ꢀ5M solution). ^bDipole moment of the dyes in the ground state (debye units) obtained from the B3LYP/6-311G

(d,p) level. ^cTransition dipole moment of the dyes (debye units) obtained from the M06-2X/6-311G (d,p) level.

nm bathochromic shi; in contrast, only an 11 nm redshi is and CTBTC are estimated to be 1.35 V, 1.27 V, 1.13 V and 0.92 V,

observed in the case of BFBFB. The dye BTBTB exhibits a four- corresponding to HOMO levels of ꢀ5.75 eV, ꢀ5.67 eV, ꢀ5.53 eV,

fold variation in absorption, substantiating the idea that the and ꢀ5.32 eV, respectively. Among these compounds, BFBFB

thiophene as a p-bridge unit promotes eﬃcient electron and BTBTB show more positive shis in oxidation potential,

transport over the entire molecule. The variations in absorption implying that the incorporation of the benzocarbazole moiety

in terms of dihedral angle deviation between the spacer and has a signicant lowering eﬀect on the HOMO level compared

acceptor for these molecules are almost the same (ꢄ97 nm).

to that of the carbazole counterpart. This eﬀect should also

improve the V_ocbecause V_ocis proportional to the energy

diﬀerence between the HOMO of the electron donor and the

LUMO of the electron acceptor in small molecule organic solar

cells.^45bThe onset reduction potentials of BFBFB, CFBFC,

BTBTB and CTBTC are estimated to be ꢀ1.12 V, ꢀ1.17 V, ꢀ0.95

V and ꢀ0.97 V, corresponding to LUMO levels of ꢀ3.28 eV,

ꢀ3.23 eV, ꢀ3.45 eV and ꢀ3.43 eV, respectively. The introduction

of thiophene as a p-spacer in place of uorene leads to a

reduction in the band gap, and the HOMO and LUMO levels

shi upwards and downwards by 0.22 eV and 0.17 eV, respec-

tively, for BTBTB compared to BFBFB and 0.35 eV and 0.19 eV,

respectively, for CTBTC compared to CFBFC (Fig. 7). The band

gap is reduced in the following order: BFBFB (2.47 eV) > CFBFC

(2.44 eV) > BTBTB (2.08 eV) > CTBTC (1.90 eV). The HOMO–

LUMO gaps (DE) derived from cyclic voltammetry (CV) are in

good agreement with the optical band gaps determined from

the onset absorption spectra in the thin lm state. The DFT

calculated HOMO/LUMO energies are ꢀ5.25 eV/ꢀ2.55 eV and

ꢀ5.24 eV/ꢀ2.53 eV for the uorene derivatives and ꢀ5.00 eV/

ꢀ2.67eV and ꢀ4.97/ꢀ2.66 for the thiophene derivatives. The

HOMO–LUMO gaps calculated from the frontier orbital energy

values are estimated to be 2.7 eV for the uorene derivatives and

2.32–2.33 eV for the thiophene derivatives. These values are in

reasonably good agreement with the estimated electrochemical

and optical band gap data. Band gaps calculated using hybrid

functionals with fully periodic boundary conditions (PBC)

usually give better agreement with experimental results.⁵⁶

Electrochemical studies

In order to evaluate the electrochemical properties of these

dyes, the redox behavior was investigated by performing cyclic

voltammetry (CV) using a standard three-electrode congura-

tion.⁵⁴The oxidation potentials in the voltammograms reveal

that all compounds undergo irreversible oxidation (Fig. 6). The

HOMO levels of all the dyes were calculated from the

onset oxidation potential according to the empirical formula

E_HOMO¼ ꢀe(E_ox+ 4.4) (eV), and the LUMO levels were estimated

from the onset reduction potentials⁵⁵according to the empirical

formula E_LUMO¼ ꢀe(E_red+ 4.4) (eV); the calculation results are

compiled in Table 3. The onset oxidation potentials of the u-

orene dyes BFBFB and CFBFC and the thiophene dyes BTBTB

Thermal properties

The TGA analysis depicted in Fig. 8 reveals that the 5% weight

ꢁ

loss temperatures (T_d) of the dyes range from 372–433 C. The

T_gand T_mvalues of the benzocarbazole dyes are comparatively

Fig. 6 Cyclic voltammograms of the dyes recorded in DCM.

higher than those of the carbazole dyes, as seen in Fig. 9 and

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Table 3 Comparison of electrochemical measurements vs. values obtained from DFT

E_onset^a(V)

DFT^d

HOMO^b(eV)

LUMO^b(eV)

E_g,ele(eV)

HOMO (eV)

LUMO (eV)

Band gap^e(eV)

c

Dye

Oxd

Red

BFBFB

CFBFC

BTBTB

CTBTC

1.35

1.27

1.13

0.92

ꢀ1.12

ꢀ1.17

ꢀ0.95

ꢀ0.97

ꢀ5.75

ꢀ5.67

ꢀ5.53

ꢀ5.31

ꢀ3.28

ꢀ3.23

ꢀ3.45

ꢀ3.43

2.47

2.44

2.08

1.90

ꢀ5.25

ꢀ5.24

ꢀ5.00

ꢀ4.97

ꢀ2.55

ꢀ2.53

ꢀ2.67

ꢀ2.66

2.70

2.33

2.32

a

b

Measured in CH₂Cl₂with 0.1 M tetrabutylammonium perchlorate (TBAPC) as a supporting electrolyte with a scan rate of 50 mV s^ꢀ1

.

Deduced

from the formulas HOMO ¼ ꢀ(4.4 + E_ox) and LUMO ¼ ꢀ(4.4 + E_red). ^cBandgap obtained from electrochemical data. ^dComputed values from the

B3LYP/6-311G (d,p) level. ^eBand gap obtained from theoretical data.

Fig. 7 Schematic representation of the band gap reductions esti-

mated from the electrochemical data.

Fig. 8 TGA thermograms measured at a heating rate of 10 ^ꢁC min^ꢀ1

under N₂atmosphere.

Table 1. The T_gvalues determined from the second heating scan

range from 63–124 ^ꢁC. It is apparent that the uorene analogues

exhibit greater thermal stability than their thiophene counter-

parts due to the presence of the rigid structural uorene units.⁵⁷

All the dyes have good thermal stability, which is essential for

device fabrication processes and other types of applications.

Photovoltaic properties

Because of their appropriate optoelectronic features, solubil-

ities and good lm-forming capabilities, all the synthesized

dyes were incorporated as p-type semiconducting components

with the soluble fullerene derivative [6,6]-phenyl-C₆₁-butyric

acid methyl ester (PC₆₁BM) as an n-type semiconductor in bulk-

heterojunction (BHJ) photovoltaic devices. BHJ architectures

typically deliver higher device power conversion eﬃciencies by

maximizing the surface area of the interface between the donor

and acceptor materials in the active layer. For all the

compounds, the device structure used was ITO/PEDOT:PSS (38

nm)/active layer/Ca (20 nm)/Al (100 nm), where the active layer

was a solution-processed blend of one of the donor materials

reported here and the solubilised fullerene PC₆₁BM. The active

layers were prepared by spin coating mixtures of the appropriate

dye with PC₆₁BM in chlorobenzene under ambient conditions

in 1 : 1 ratios without subsequent annealing. The optimum

layer thickness was found to be in the range of approximately 70

nm. The best photovoltaic devices based on CTBTC gave a

Fig.

9 DSC traces of the dyes recorded at a heating rate of

10 ^ꢁC min^ꢀ1

.

power conversion eﬃciency (h) of 1.62%. The respective

current–voltage (J–V) curves are shown in Fig. 10.

The relatively lower eﬃciency achieved with BFBFB, CFBFC

and BTBTB compared to CTBTC is likely due to the weaker

absorption of the visible spectrum. This suggests that the light

35328 | RSC Adv., 2014, 4, 35318–35331

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preferable from a processing point of view. Table 4 presents the

comparative photovoltaic performance data.

This preliminary study was conducted using a C₆₀-derivative

of PCBM; however, reports using a C₇₀-derivative for eﬃciency

enhancement mainly attribute the results to the enhanced

absorption in the lm. These latter derivatives might be useful

for device optimization using the materials reported in this

work; this will be studied in the near future.

Conclusion

In summary, we have reported four benzothiadiazole core-

structured compounds based on D-p-A-p-D architecture

featuring diﬀerent p-spacers (uorene and thiophene) along

with benzocarbazole and carbazole as donors. The signicance

of these modications on the individual segments of the dyes

was thoroughly investigated using a range of experimental

techniques including optical, electrochemical and thermal

methods. The enhanced oxidative and thermal stability of

BFBFB and BTBTB indicate that the benzocarbazole function-

ality can play a vital role as a donor for optoelectronic applica-

tions. The substitution of thiophene as a p-spacer in place of

uorene has a pronounced eﬀect on the photophysical and

electrochemical properties of the dyes. The electrochemical

bandgaps of these dyes are greatly reduced by changing the p-

spacer from uorene to thiophene and are in good agreement

with the band gap data derived from with the band gap data

derived from optical methods. From the electronic structures

calculated by DFT methods, it is clear that the better perfor-

mance of thiophene as a p-spacer is largely due to the planarity

of the molecule. The results obtained from thermal methods

clearly establish that all the dyes have high decomposition

Fig. 10 Characteristic current density versus voltage (J–V) curves for

the best BHJ devices based on BFBFB & CFBFC (upper) and BTBTB &

CTBTC (lower) in blends with PC₆₁BM under simulated sunlight (100

mW cm^ꢀ2, AM 1.5G). The device structure is: ITO/PEDOT:PSS (38 nm)/

active layer/Ca (20 nm)/Al (100 nm).

harvesting ability of the small molecule donors plays an

important role in achieving their optimum photovoltaic

performance. The modest ll factors observed for all devices

suggest that the pathways for charge to be carried to the elec-

trodes are not optimized, a problem that could be addressed by

using donor fragments that encourage stronger intermolecular

interactions. In regard to the processing conditions of the blend

solutions, previous studies⁵⁸indicate that the solvent and

degree of crystallization have a strong eﬀect on cell perfor-

mance. Our attempts to fabricate the devices using a low-

boiling-point solvent such as chloroform resulted in very poor

photovoltaic performance, primarily due to poor lm quality.

The nding that the OPV devices comprised of these novel

donor materials perform better with high-boiling-point solvents

is signicant as the use of high-boiling-point solvents is

ꢁ

temperatures (T_d) in the range of 372–435 C. The signicant

electronic and optical properties demonstrated by these dyes

allowed us to fabricate them in solution-processable BHJ OPV

devices. Aer preliminary device examination, the devices with

CTBTC as the donor material exhibited the best eﬃciency

(1.62%) with PC₆₁BM. The inferior performance observed for

the remaining materials is likely related to their weaker

absorption in the visible region. Our results not only enrich the

molecular library of donors, but also highlight the importance

of considering factors such as (1) light-harvesting, (2) charge

carrier mobility and (3) solvent selection when designing small-

molecule donors. Further examination of device fabrication

using C₇₀-derivatives of PCBM is the subject of on-going work in

our laboratories.

Table 4 Comparative BHJ solar cell performances

Dye

Blend lm thickness^a(1 : 1@3000 rpm)

V_oc(mV)

J_sc(mA cm^ꢀ2

)

FF

Eﬃciency ^b(h%)

BFBFB

CFBFC

BTBTB

CTBTC

64

66

62

63

250 ꢂ 30

430 ꢂ 20

691 ꢂ 30

960 ꢂ 20

5.09 ꢂ 0.4

6.13 ꢂ 0.6

3.30 ꢂ 0.3

4.63 ꢂ 0.2

0.23 ꢂ 0.04

0.26 ꢂ 0.03

0.32 ꢂ 0.02

0.34 ꢂ 0.03

0.30 ꢂ 0.08

0.71 ꢂ 0.15

0.76 ꢂ 0.07

1.62 ꢂ 0.06

a

BHJ devices with specied weight ratios. The device structure is ITO/PEDOT:PSS (38 nm)/active layer/Ca (20 nm)/Al (100 nm) with an active layer

b

thickness of approximately 65 nm. Average value based on 10 devices.

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11 N. Cho, K. Song, J. K. Lee and J. Ko, Chem.–Eur. J., 2012, 18,

11433–11439.

Acknowledgements

12 Z. Li, J. Pei, Y. Li, B. Xu, M. Deng, Z. Liu, H. Li, H. Lu, Q. Li

and W. Tian, J. Phys. Chem. C, 2010, 114, 18270–18278.

13 M. Sassi, M. Crippa, R. Ruﬀo, R. Turrisi, M. Drees,

U. K. Pandey, R. Termine, A. Golemme, A. Facchetti and

L. Beverina, J. Mater. Chem. A, 2013, 1, 2631–2638.

MP thanks UGC for the fellowship. We thank the director of

CSIR-IICT for the encouragement. We acknowledge funding

from the NWP-0054 project. SVB acknowledges the Australian

Research Council under

a

Future Fellowship Scheme

(FT110100152) and the School of Applied Sciences (RMIT

University) for providing the facilities.

´

14 J. R. Romero, L. A. Ixta, M. Rodrıguez, G. R. Ortız,

´

J. L. Maldonado, A. J. Sanchez, N. Farfan and R. Santillan,

Dyes Pigm., 2013, 98, 31–41; S. Tang, B. Li and J. Zhang, J.

Phys. Chem. C, 2013, 117, 3221–3231.

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Article Doi

Small band gap D-π-A-π-D benzothiadiazole derivatives with low-lying HOMO levels as potential donors for applications in organic photovoltaics: A combined experimental and theoretical investigation

DOI: 10.1039/c4ra02700k

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