Synthesis of a novel acceptor unit for D-A type polymers:

4,8-Dibromobenzo-[1,2-c;4,5-c′]bis[1,2,5]thiadiazole

SUN Xiaoxia1,a*，Zhuang Xiaoxiao1,b, REN Yali1,c

1Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University,

Nanchang 330013, People’s Republic of China

asunxiaoxia77@126.com;

bzhuangxiaoxiao1987@163.com;

crenyalihi@126.com;

Keywords: organic light-emmitting, Photovoltaic, π-conjugated polymers, acceptor unit

Abstact. A synthetic approach to synthesize 4,8-Dibromobenzo-[1,2-c;4,5-c’] bis[1,2,5]thiadiazole

from a common precursor 2,1,3-Benzothiadiazole is reported. This unit can then be used in the

synthesis of low bandgap materials via palladium-catalyzed coupling reactions.

Introduction

The past decade has seen the rapid development of donor-acceptor systems, which has proven

efficient at tuning the HOMO and LUMO levels of organic semicondectors, let alone their band

gaps.[1-3]

Although acceptors such as benzothiadiazole and related derivations has become popular and

are still the focus of intensive reseach, the rational design of new donor-acceptor systems remains of

synthetic interest. The requirement for full spectral (color) control in materials applicable in both

emissive and nonemissive display technologies is now motivating an expansion of this classical

approach. Inducing a simultaneous bathochromic shift and hypsochromic shift, both peaks tends

towards the λmax of all-donor control homopolyer.[4]

So to generate an effective photocurrent in these

organic solar cells, an appropriate donor-acceptor pair must be selected. One of the most widely applied strategies to make narrow band gap donor polymers is to synthesis

an alternating copolymer from electron-rich (donor) and electron-deficient (acceptor) units in their backbone. For the incorporation of conjugated hetercyclic units can greatly influence the properties of conjugated polymers

[5]. Quinoxaline, thiophene, thieno[3,4-b] pyrazine, and silole have emerged as

useful heterocycles units for conducting a variety of conjugated polymers for photovoltaic application

[6]. As 2,1,3-benzothiadiazole (BT) is an electron-accepting (A) heterocycle that has been

utilized to construct some n-type semiconducting polymers showing high electron mobility, BT has also been recently used as acceptor unit in cooperation with varieties of electron-donating (D) units as low band gap donors in BHJ and PVC. High hole mobility and wide sunlight absorption band could be achieved for the D-A type bt-containing polymers. This category of polymer donors has been extensively studied and has shown outstanding photovoltaic performances. Although the broad utility and popularity of 2,1,3-benzothiazole unit, it is surprising that the synthesis of analogues or derivatives of this heterocyccycle and their successful application to conjugated materials have only rarely been reported

[7]. For instance, the incorporation of the oxygen analogue,

2,1,3-benzothiadiazole, into conjugated materials has emerged until recently[8]

and only been claimed in patents and in accounts of corporate research

[9]. The benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole

(BBT) unit that has potential applications in light-emitting devices (LED), ambipolar organic field-effect transistors (OFETs) and solar cells is a strong electron accepting unit and has been used to make low band gap donor-acceptor materials. As a sign of the effectiveness of BBT in promoting low band-gaps, even small oligomers are reported to possess band-gaps of only 1.5 eV. The small band-gaps observed in these materials can be attributed to the high electron affinity and large quinoid contribution of the BBT unit

[10].

Herein, we report the synthesis and characterization of 4,8-Dibromobenzo-[1,2-c;4,5-c’]

bis[1,2,5]thiadiazole (4), a derivative of 2,1,3-benzothiadiazole (BT). Benzothiadiazole group is also

an electron- deficient moiety.

Advanced Materials Research Vols. 482-484 (2012) pp 1221-1224Online available since 2012/Feb/27 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.482-484.1221

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Experimental Section

General Produres. Unless otherwise noted, all reactions were performed in oven-dried glassware.

All solvents used in the reactions were purified before use. Dry CHCl3 were distilled from CaH2. 1H

NMR and 13

C NMR: Bruker AM-400WB spectrometer in chloroform-d as solvent and

tetramethylsilane (TMS) as the internal standard. The mass spectra were obtained by using a Bruker

Daltonics Autoflex II TOF system. FT-IR spectra were recorded using a Bruker Vertex 70 FT-IR

spectrometer with samples in KBr pellets. Other chemicals were used without further purification.

Unless otherwise stated,all reactions were run under argon.

2.1 Synthesis of 4,7-Dibromobenzo-2,1,3-Benzothiadiazole

Bromine (19.8 g, 110 mmol) was added dropwise over a period of 10 minutes to a vigorously stirred

suspension of benzo[c][1,2,5]thiadiazole (5.0 g, 36.7mmol) in hydrobromic acid (100 mL) at 120°C.

The reaction was left to stir for 8 hours during which a canary yellow precipitate formed. Water (100

mL) was added, the suspension left to stir for 10 minutes, then filtered and washed with a little water.

The yellow solid was dried in vacuo for 24 hours to give the pure product (9.8 g, 91%). 1H NMR (400

MHz, CDCl3, ppm): δ aromatic H, 7.73 (s, 2H).

2.2 Synthesis of 4,7-dibromo-5,6-dinitrobenzothiadiazole

A mixture of concentrated H2SO4 (300 ml) and fuming HNO3 (300 ml) was cooled to 0 °C. To keep

the temperature below 5 °C, 4,7-Dibromo-2,1,3-benzothiadiazole (15.72 g, 53.4 mol) was added in

small portions. After stirring for 6 h at 0°C, the reaction mixture was poured out into ice/water (1.0 l).

The precipitate was filtrated and washed with water. Purification by chromatography on silica gel

(solid deposition) (EtOAc : hexane, 1 : 8) gave 6.76 g 1,4-Di(2-thienyl)-5,6-dinitro-2,1,3-

benzothiadiazole as a beige solid (32.9% yield). 13

C NMR (125 MHz, CDCl3): δ 151.3, 144.9, 110.3.

MS (Mw = 383.9): m/z = 383.9; m.p. = 201 - 203°C.

2.3 Synthesis of 4,7-dibromo-5,6-diaminobenzothiadiazole

To a solution of 4,7-dibromo-5,6-dinitrobenzo[c][1,2,5]thiadiazole 560 mg（1.458 mmol）in acetic

acid (10 mL) was added portionwise Fe powder 1 g (17.8 mmol) at 100°C. After heating at 100°C for

2h, 2 mL of EtOH was added. The mixture was heated for another 1.5 h. After cooling precipitates

were filtrated and entracted with ethyl acetate for 12h (soxhlet). Evaporation of solvent followed by

recrystallization from EtOH gave 5,6-diamino-4,7-Dibromobenzo[c][1,2,5]thiadiazole 195 mg as

pale yellow solid (yield 41.3%) 1H NMR (400 MHz, CDCl3, ppm): δ 5.02 (s, 2H);

13C NMR (100

MHz, CDCl3): δ 138.3, 107.97, 95.3; mp = 256-258 °C.

2.3 Synthesis of 4,8-Dibromobenzo-[1,2-c;4,5-c’] bis[1,2,5]thiadiazole

5,6-Diamino-4,7-Dibromobenzo[c][1,2,5]thiadiazole (0.38 g, 1.2 mmol) was dissolved in anhydrous

chloroform (10 ml) in a round-bottomed flask. The mixture was stirred at 0°C with thionyl chloride

(1.2 mL) was added, then 8.0 ml of pyridine was then added and the reaction was allowed to stir at

room temperature for 20h at room temperature. The reaction mixture was allowed to cool to room

temperature, and pured onto ice water. The precipitate was filtered and washed with water and

methanol. The crude product was black solid (280 mg yield: 68%). MS: m/z = 357.3.

Results and Discussion

To prepare the relatively small amounts of material necessary for property studies compound 4 was

initially synthesized using the route shown (Scheme 1). As Scheme 1 show the deficient acceptor

monomer of was prepared in good yields over four steps starting from the readily commercially

available material, and it was an inexpensive route. First, 2,1,3-Benzothiadiazole was brominated

with 3 molar equivalents of bromine in the presence of hydrobromic acid. The second step,

4,7-Dibromo-2,1,3-benzothiadiazole was nitrated using fuming HNO3 in concentrated H2SO4. This

reaction was highly exothermic, and the addition rate of concentrated H2SO4 and fuming HNO3 to the

reaction mixture was carefully controlled to avoid an uncontrolled exotherm and off-gassing of

nitrogen oxides. Then hydrogenation of4,7-dibromo-5,6-dinitrobenzo[c][1,2,5]thiadiazole to form

the diamine and then cyclization to produce the final compound. Facial reduction of the two nitro

1222 Advanced Composite Materials

group of 4,7-dibromo-5,6-dinitrobenzo[c][1,2,5]thiadiazole using Fe powde in ethanol to gave

diamine 5,6-Diamino-4,7-Dibromobenzo[c][1,2,5]thiadiazole with 41.3% yield. Since the pyridine

moiety can serve as an a cyclization catalyst, refluxing 5,6-Diamino-4,7-Dibromobenzo[c]

[1,2,5]thiadiazole with thionyl chloride in chloroform gave the final product in 68% yield.

Scheme 1. Synthesis of 4,8-Dibromobenzo-[1,2-c;4,5-c’] bis[1,2,5]thiadiazole

Conclusion

In summary, we developed a novel acceptor unit functionalized with electron-rich benzothiadiazole

spacer. The procedure described herein offers several advantages, including middle product yields,

easy purification, operational safety at room temperature. Furthermore, we believe that this method is

a useful addition to the present methodology for the synthesis of this class of compounds. Further

studies on the exact reaction mechanism and applications of low-band-gap polymers and

broad-absorption polymers for high-performance photovoltaic materials are ongoing in our group,

which will be reported in due course.

Acknowledgements

We are grateful for the financial support of the National Natural Science Foundation of China (No.

20862006) and the Science Fund of the Technology Office of Jiangxi, China (2009JZH0033)

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1224 Advanced Composite Materials

Advanced Composite Materials 10.4028/www.scientific.net/AMR.482-484 Synthesis of a Novel Acceptor Unit for D-A Type Polymers: 4,8-Dibromobenzo-[1,2-c;4,5-c′

]bis[1,2,5]thiadiazole 10.4028/www.scientific.net/AMR.482-484.1221