Applied Chemistry,
Vol. 15, No. 2, October 2011, 105-108
105
Tetrazine/DPP를 갖는 유기태양전지용 신규 단분자에 관한 연구
김진아ㆍ현지나ㆍ이경균ㆍ이성구ㆍ임은희†
한국생산기술연구원, 호서대학교 (ehlim@kitech.re.kr†)
Small Molecules Based on Tetrazine or DPP for OPV Application
Jin- A KimㆍJina HyunㆍKyeong K. LeeㆍSungkoo LeeㆍEunhee Lim†
Korea Institute of Industrial Technology (KITECH), Cheonan, Chungnam 331- 825, Korea (ehlim@kitech.re.kr†)
Abstract
Organic photovoltaic cells (OPVs) have attracted considerable attention due to their low cost, light- weight and flexible characteristics. Small molecules have advantages of well- defined structure and easy synthesis. In this work, new tetrazine, DPP, and furan- based oligomers for organic solar cell were synthesized by Suzuki coupling reaction. The struc- tures were confirmed by NMR and optical and electronic properties were investigated by UV- vis absorption.
1. Introduction
Organic photovoltaic cells (OPVs) are promising sources of electrical power that have attracted considerable attention because of their efficiency, low cost and potential renewable energy applications than ordinary silicon solar cell. OPV is typically summarizes the third generation of PV technologies which contain at least one organic semiconductor or molecule in the active light absorbing layer. To achieve high- efficiency OPV cells, low- band- gap conjugated materials with appropriate energy levels are required for efficient charge transfer. One successful approach for the synthesis of low band- gap materials is to arrange the electron- donor and electron- acceptor repeating units alternately along the conjugated backbone.
Amongst six- membered heterocyclic systems, tetrazines are of considerable interest not only because of their inherent biological potential but also because of their value as building blocks in organic transformations. Tetrazines (Tz) have demonstrated powerful synthetic utility through their ability to participate in inverse electron demand Diels–Alder reactions providing access to a wide range of other heterocycles and natural products.
Tetrazine has a very high electron affinity and can behave as a strong electron deficient unit in a polymer to lower its HOMO level[1]. On the other hand, DPP is chosen as the core building block for n- type materials development because of extremely good stability, very strong light harvesting covering a wide spectrum of the sun light, strong electron deficiency, and facile coupling ability with various aromatic functional groups[2].
Furthermore, furan is more electron- rich than thiophene. Furan may suppress electron transport behavior and make the hole transport more pronounced. Incorporation of furan
106 김진아ㆍ현지나ㆍ이경균ㆍ이성구ㆍ임은희
broadens band gap and lowers HOMO level. The furan- based polymer showed improved OPV performance than the corresponding thiophene- based polymer[3]. In this study, a series of thiophene- based oligomers based on tetrazine (Tz- DT) or DPP (DPP- DT and DPP- DF) were newly synthesized and their optical properties were investigated.
2. Experiment (Synthesis of oligothiophenes)
4- Hexylthiophene- 2- carbonitrile (1): A solution of 4- hexylthiophene- 2- carbaldehyde (5.00 g, 25.47 mmol) and hydroxylamine hydrochloride salt (2.65 g, 38.20 mmol) in pyridine/ethanol (25.46 mL) was stirred at 80℃ overnight. The residue was dissolved in CHCl3, washed with water and dried over MgSO4. After solvent evaporation, the residue was dissolved in acetic anhydride (12.73 mL) containing potassium acetate (0.08 g) and then refluxed for 3 hrs. The mixture was poured into water and extracted with hexane, washed with 5% aq. NaOH solution and water, dried over MgSO4, and purified by column chromatograph (2.89 g, 73% yield). 1H NMR (400 MHz, CDCl3): δ 7.43 (d, 1H); 7.17 (d, 1H); 2.60 (t, 2H); 1.59 (m, 2H); 1.22- 1.36 (m, 6H); 0.87 (m, 3H).
3,6- Bis(4- hexyl- 2- thienyl)- 1,2,4,5- tetrazine (Tz- DT): To a mixture of 1 (2.89 g, 14.95 mmol) and sulfur (0.34 g, 10.47 mmol) in anhydrous ethanol (9.40 mL) was slowly added fresh hydrazine monohydrate (1.10 g, 22.43 mmol) at rt. The solution was refluxed for 2h. The crystal was collected by filtration and rinsed with cold ethanol before dried under vacuum. A CHCl3 (31 mL) and isoamyl nitrite (3.53 g, 29.90 mmol) was added and the solution was stirred at room temperature overnight. After solvent evaporation, the resulting solid was washed with methanol and purified by column chromatography. (0.92 g, yield: 15%). 1H NMR (400 MHz, CDCl3): δ 8.07 (d, 2H); 7.26 (d, 2H); 2.68 (t, 4H,);
1.63- 1.71 (m, 4H); 1.29- 1.40 (m, 12H); 0.89 (m, 6H).
3,6- Di(furan- 2- yl)pyrrolo[3,4- c]pyrrole- 1,4(2H,5H)- dione (2): Na metal pieces (1.23 g, 53.72 mmol) were progressively added to the warmed solution of t- amyl alcohol (125 mL, 60- 70℃). After complete addition of Na, the mixture was stirred overnight at 120℃.
Furan- 2- carbonitrile (5.00 g, 53.72 mmol) was subsequently added to the hot mixture of sodium alkoxide. Dimethyl succinate (2.75 g, 18.80 mmol) was then added dropwise over 20 min, and the resulting mixture was stirred for 1.5 h. The precipitated sodium salt was filtered and dried under vacuum (4.60 g, 55% yield).
2,5- Bis(2- ethylhexyl)- 3,6- di(furan- 2- yl)pyrrolo[3,4- c]pyrrole- 1,4(2H,5H)- dione (DPP- DF):
2 (4.60 g, 14.79 mmol) and dry DMF (100 mL) were added to a 250 mL round–bottom flask. The mixture was heated to 120℃, stirred for 30 min, and 2- ethylhexylbromide (7.60 g, 42.73 mmol) was then added quickly. The organic phase was extracted with diethyl ether and washed with water. After solvent evaporation, the resulting solid was purified by column chromatography. (0.95 g, 13% yield). 1H NMR (400 MHz, CDCl3): δ (ppm) = 8.33 (d, 2 H), 7.61 (d, 2 H), 6.69 (dd, 2 H), 4.04 (d, 4 H).
107
Tetrazine/DPP를 갖는 유기태양전지용 신규 단분자에 관한 연구
S CHO
H13C6
S H13C6
C=N N N
S N N S
H13C6 C6H13
N X
X N O
X CN
O N
O O
N O
O N
S
S N O
O
DPP-DT DPP-DF
DMF Br
O O
OCH3 H3CO
OH
Tz-DT .O-Na+
1
X=S, O
X=S, O ,
2
Scheme 1. Chemical structures of the Tz- DT, DPP- DT and DPP- DF.
3. Result and Discussion
We have synthesized a series of oligothiophenes, Tz- DT, DPP- DT and DPP- DF, which can be used as good donor materials for OPV application. All oligomers were designed to have the electron- acceptor unit (tetrazine or DPP) at the core and the electron- donor (thiophene) at both ends. Such an arrangement can result in low band gap materials. The synthetic routes to the oligomers are described in Scheme 1.
The normalized UV- vis absorption spectra of the small molecules in dilute chlorobenzene solution and in films are shown in Fig. 1. The absorption spectra of the oligothiophenes in solution showed two absorption bands at around 370 nm and 520 nm, which were assigned to the π- π* transition of the conjugated backbone and intramolecular charge transfer (ICT) interactions between the thiophene donor and the acceptor units (triazole or DPP). The absorption spectra of the oligothiophene films are much more red- shifted and broader than those of the corresponding solution spectra, which can be explained by the increased intermolecular interactions between neighboring molecules in the film state. Interestingly, DPP- DF film showed more red- shifted broad absorption spectrum compared to DPP- DT film, which is advantageous for OPV application.
Fig. 1. UV absorption spectra of oligothiophenes (a) in solution and (b) as film.
108 김진아ㆍ현지나ㆍ이경균ㆍ이성구ㆍ임은희
4. Conclusion
In summary, we have synthesized new thiophene- based oligomers containing electron- transporting unit into the main chain through palladium- catalyzed Suzuki polymerization.
The oligothiophene films showed broad UV absorption extending up to 700 nm.
Acknowledgement
This research was supported by a grant from Green Smart Card Platform Technologies Based on 3D Printed Electronic Devices Project of MKE and the Fundamental R&D Program for Core Technology of Materials funded by MKE of Republic of Korea.
Reference
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2. P. Sonar, G.- M. Ng, T. T. Lin, A. Dodabalapur, Z.- K. Chen, J. Mater. Chem. 20, 3626 (2010).
3. C. H. Woo, P. M. Beaujuge, T. W. Holcombe, O. P. Lee, J. M. J. Frechet, J. Am. Chem. Soc. 132, 15547 (2010).
