Organic Electronics 13 (2012) 2516–2524

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Organic Electronics

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Isomers of dialkyl diketo-pyrrolo-pyrrole: Electron-deficient unitsfor organic semiconductors

Baomin Zhao a, Kuan Sun a, Feng Xue b,⇑, Jianyong Ouyang a,⇑a Department of Materials Science and Engineering, National University of Singapore, 117574 Singapore, Singaporeb Department of Chemistry, National University of Singapore, 117543 Singapore, Singapore

a r t i c l e i n f o

Article history:Received 20 May 2012Received in revised form 17 July 2012Accepted 17 July 2012Available online 31 July 2012

Keywords:Diketo-pyrrolo-pyrrole (DPP)IsomerOrganic solar cellElectron-deficient unitBand gap

1566-1199/$ - see front matter � 2012 Elsevier B.Vhttp://dx.doi.org/10.1016/j.orgel.2012.07.015

⇑ Corresponding authors.E-mail addresses: chmxf@nus.edu.sg (F. Xue

(J. Ouyang).

a b s t r a c t

Diketo-pyrrolo-pyrrole (DPP) is an important electron-deficient unit, and the alkylation ofDPP can gives rise to good solubility of organic and polymer semiconductors in solvent. Inthis paper, we report our observations on the isomers of dialkyl DPP. The alkylation of DPPcan take place on both the nitrogen and oxygen atoms, leading to the formation of threeisomers. The presence of the isomers is evidenced by the nuclear magnetic resonance(NMR) and mass spectra. The yields of the isomers are affected by the experimental condi-tions, including the reaction temperature, reaction time and solvent. The isomers have dif-ferent electronic structures and optical properties as revealed by the UV–Vis absorptionspectroscopy, fluorescent spectroscopy, cyclic voltammetry and theoretical simulationusing Gaussian 03 program with B3LYP/6-31G(d). Small molecules based on the DPP iso-mers with dialkylation at different sites were synthesized and investigated as donor mate-rials of organic solar cells (OSCs). The photovoltaic performances of OSCs are consistentwith the electronic structures of the DPP isomers.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction of this unit with an electron-donating unit can give rise to

Conjugated organic and polymeric materials have beenattracting strong attention due to their interesting elec-tronic structure and optical properties [1,2]. They can beused as the active materials for light-emitting diodes, solarcells and field-effect transistors [3–13]. The band gap andcharge carrier mobility of these materials are two keyparameters affecting the performance of the organic elec-tronic devices [6,9,12–16]. Various conjugated units havebeen adopted in synthesizing conjugated organic and poly-meric materials in order to achieve the target electronicstructure and properties [11,12,14,15]. An electron-defi-cient unit, 3,6-diaryl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) [17,18], has been extensively investigatedfor the development of high-performance organic and poly-meric semiconductors [19–34]. For example, the integration

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), mseoj@nus.edu.sg

molecules or polymers with low band gap. It was reportedthat the band gap of DPP-based materials can be as low as1.3 eV. Materials with low band gap are particularly re-quired for high-performance organic or polymer solar cells,because they can harvest light up to infrared range [5,35].For instance, Loser et al. observed a power conversion effi-ciency (PCE) of >4% for organic solar cells (OSCs) with aDPP-based small molecule as donor and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as acceptor [27]. Liet al. reported a PCE of more than 5% for polymer solar cellswith DPP-based copolymers and [6,6]-phenyl-C71-butyricacid methyl ester (PC71BM) [28]. The DPP unit has also beenadopted for the development of organic and polymericsemiconductors with high charge carrier mobility for field-effect transistors due to its large and planar conjugated pstructure. For instance, a copolymer based on 2,5-bis(2-octyldodecyl)-DPP and 5,50-di(thiophen-2-yl)-2,20-biselen-ophene can have a hole mobility of 1.5 cm2 V�1 s�1 [29].

Alkyl side chains are usually connected to the DPP unitto improve the solubility and the film forming ability of the

Scheme 1. Synthesis routes for the alkylations of 3,6-dithienyl-DPP.

B. Zhao et al. / Organic Electronics 13 (2012) 2516–2524 2517

final product [20–34]. It should be noted that the alkyl-ation step of DPP to make the N,N-dialkyl product lacksgood reproducibility and the yield varies from 12% to 83%as reported in literature [18–24]. It also has been assumedthat the alkylation occurs only on the nitrogen atoms ofDPP [36]. Recently, Frebort et al. reported the alkylationof 3-phenyl-6-(2-thienyl)pyrrolo[3,4-c]pyrrole-1,4-dionewith 1-bromo-2-ethylhexane, and they observed bothN,N-dialkyl DPP and N,O-dialkyl DPP isomers [37]. But theydid not observe the O,O-dialkyl DPP isomer. In addition,the electronic structures of the dialkyl DPP isomers werenot well characterized, and the isomers were not exploitedas units for the development of high-performance smallmolecules and polymers for organic or polymer electronicdevices.

In this paper, we report our observations that the dial-kylation of DPP can take place on not only the nitrogenbut also the oxygen atoms of DPP. The alkylation on thesetwo types of atoms leads to three dialkyl DPP isomers,including the O,O-dialkyl DPP isomer. The dialkyl DPP iso-mers have different electronic structures and optical prop-erties. Small molecules with low band gap based on dialkylDPP isomers were synthesized. They were used as the do-nor materials of OSCs. The open-circuit voltages (Voc) of theOSCs are consistent with the molecular levels of the mole-cules based on dialkyl DPP isomers.

2. Experimental

2.1. Materials and chemicals

All reagents were purchased from Sigma–Aldrich andRegent Chem (Singapore). They were used as receivedwithout further purification unless otherwise stated.Anhydrous dichloromethane was prepared through thedistillation of dichloromethane with CaH2.

2.2. Synthesis of EH-DPP isomers

3,6-dithienyl-DPP was prepared according to the refer-ence [36]. The experimental details were presented in Part1.1 of the Supporting information.

Alkylation of 3,6-dithienyl-DPP with 2-ethylhexyl bro-mide was performed according to Scheme 1. 3,6-dithie-nyl-DPP (1.50 g, 5.0 mmol) and anhydrous potassiumcarbonate (3.45 g, 25 mmol) were added into N,N-dimeth-ylformamide (25 mL) in a two-neck round flask under Arprotection. After the mixture was stirred at 100 �C for1 h, 2-ethylhexyl bromide (2.45 g, 12.5 mmol) was por-tionwise injected with a syringe. After the reaction wasstirred at 100 �C for 5 h, the solution was cooled down toroom temperature. It was then poured into 50 mL icewater, and subsequently extracted with dichloromethane.The organic layer was rinsed successively with water andbrine, and then dried with anhydrous sodium sulfate. Afterthe removal of the solvent by the reduced pressure rotaryevaporation, the product was separated into the three iso-mers, EH-DPP-a, EH-DPP-b and EH-DPP-c, by silica gelchromatography with hexane and ethyl acetate (v/v = 97/3) as eluent.

EH-DPP-a: Purple brown solid (0.95 g, yield 36%). 1HNMR (CDCl3, 500 MHz): d (ppm) 8.87 (d, 2H), 7.62 (d,2H), 7.27 (d, 2H), 4.03 (m, 4H), 1.85 (m, 2H), 1.36–1.22(m, 16H), 0.85 (m, 12H). 13C NMR (CDCl3, 125 MHz): d(ppm) 161.72, 140.39, 135.24, 130.48, 129.80, 128.38,107.89, 45.82, 39.04, 30.18, 30.16, 28.32, 23.50, 23.02,13.98, 10.45. EI-MS(m/z) [M]+: 524.56.

EH-DPP-b: Purple waxy solid (0.74 g, yield 28%). 1HNMR (CDCl3, 500 MHz): d (ppm) 8.48 (d, 1H), 8.21 (d,1H), 7.68 (dd, 1H), 7.70 (d, 1H), 7.23 (dd, 1H), 7.19 (d,1H), 4.51 (m, 2H), 3.93 (m, 1H), 1.77 (m, 2H), 1.36–1.22(m, 16H), 0.85 (m, 12H). 13C NMR (CDCl3, 125 MHz): d(ppm) 166.24, 161.62, 149.48, 142.39, 138.62, 134.86,131.54, 130.02, 129.91, 128.89, 127.90, 114.10, 111.05,72.33, 45.41, 39.25, 39.01, 30.66, 30.17, 29.01, 28.29,24.02, 23.49, 22.97, 14.02, 13.97, 11.20, 10.42. EI-MS (m/z) [M]+: 524.04.

EH-DPP-c: Purple waxy solid (0.29 g, yield 11%). 1H NMR(CDCl3, 500 MHz): d (ppm) 8.03 (dd, 2H), 7.54 (dd, 2H),7.17 (dd, 2H), 4.54 (m, 4H), 1.82 (m, 2H), 1.36–1.22 (m,16H), 0.85 (m, 12H). 13C NMR (CDCl3, 125 MHz): d (ppm)169.13, 152.28, 139.06, 132.06, 131.58, 128.77, 117.96,72.46, 39.38, 30.61, 29.04, 24.01, 22.98, 14.05, 11.23. EI-MS (m/z) [M]+: 524.25.

2.3. Synthesis of Oct-DPP isomers

The alkylation of 3,6-dithienyl-DPP with octyl bromidewas carried out according to Scheme 1. The synthesesand the purification of the final products were performedthrough a similar procedure for EH-DPP isomers. Two iso-mers, Oct-DPP-a and Oct-DPP-b, were collected.

Oct-DPP-a: Purple black needle-like solid (yield 63%). 1HNMR (CDCl3, 500 MHz): d (ppm) 8.92 (dd, 2H), 7.63 (dd,2H), 7.28 (dd, 2H), 4.06 (m, 4H), 1.74 (m, 4H), 1.36–1.22(m, 20H), 0.86 (m, 6H). 13C NMR (CDCl3, 125 MHz): d(ppm) 161.34, 139.99, 135.22, 130.63, 129.75, 128.57,107.66, 42.20, 31.74, 29.92, 29.18, 29.15, 26.84, 22.59,14.05. EI-MS (m/z) [M]+: 524.24.

Oct-DPP-b: Purple black solid (yield 17%). 1H NMR(CDCl3, 500 MHz): d (ppm) 8.45 (d, 1H), 8.25 (d, 1H), 7.70(dd, 1H), 7.50 (d, 1H), 7.26 (dd, 1H), 7.20 (d, 1H), 4.57 (m,2H), 3.97 (m, 2H), 1.86, (m, 2H), 1.74 (m, 2H), 1.41–1.22(m, 18H), 0.85 (m, 6H). 13C NMR (CDCl3, 125 MHz): d(ppm) 166.07, 161.32, 149.37, 142.14, 138.62, 135.08,131.77, 131.45, 130.02, 129.89, 128.92, 128.23, 113.64,

Scheme 2. Synthesis routes for PyT-DPP-a and PyT-DPP-b.

2518 B. Zhao et al. / Organic Electronics 13 (2012) 2516–2524

111.20, 70.20, 41.90, 31.76, 31.73, 29.95, 29.21, 29.19,29.15, 29.14, 28.92, 26.79, 26.01, 22.63, 22.59, 14.09,14.06. EI-MS (m/z) [M]+: 524.36.

2.4. Synthesis of PyT-DPP-a and PyT-DPP-b

Two small organic molecules, PyT-DPP-a and PyT-DPP-b, were synthesized as the donor materials of OSCs interms of the synthesis routes presented in Scheme 2. De-tails are provided below.

2.4.1. Synthesis of 2-(pyren-1-yl)thiopheneIn a two-neck round bottom flask, 1-bromo-pyrene (3 g,

10.6 mmol), thiophene (2.5 g, 31 mmol), Pd(OAc)2 (45 mg,0.2 mmol), pivalic acid (0.34 mL, 3.0 mmol), K2CO3 (3.5 g,26 mmol) and anhydrous dimethylacetamide (18 mL) wereadded. The mixture was stirred at 100 �C under argonatmosphere for 3 h. After cooled down to room tempera-ture, the mixture was poured into an aqueous solution ofethylenediaminetetraacetic acid disodium salt (pH 8). Thecrude product was extracted with chloroform, and the or-ganic phase was successively rinsed with water and brine.It was then dried with anhydrous sodium sulfate. Theproduct, 2-(pyren-1-yl)thiophene (PyT, white solid, 2.2 g,yield = 77%), was obtained by the silica gel chromatogra-phy with hexane as the eluent. 1H NMR (500 MHz, CDCl3):d (ppm) 8.49 (d, 1 H), 8.19 (m, 3 H), 8.11 (m, 4 H), 8.04 (t, 1H), 7.53 (dd, 1 H), 7.39 (dd, 1 H), 7.27 (dd, 1 H). 13C NMR(125 MHz, CDCl3): d (ppm) 142.47, 131.40, 130.94,130.89, 129.73, 129.05, 128.41, 127.89, 127, 85, 127.72,127.44, 127.29, 126.15, 126.08, 125.28, 124.98, 124.96,124.94, 124.71, 124.51. EI-MS (m/z) [M]+ calcd forC20H12S, 284.07 found, 284.29. Anal. calcd for C24H21S: C,84.47 H, 4.25 found: C 84.49 H, 4.22.

2.4.2. Synthesis of 2-bromo-5-(pyren-1-yl)thiopheneIn a dry round bottom flask (100 mL), Py-T (2 g,

7 mmol), chloroform (35 mL), and NBS (1.35 g, 7.8 mmol)were added. The mixture was stirred under argon atmo-

sphere over night. Methanol (2 mL) was then added toquench the reaction. The mixture was successively rinsedwith water and brine. The organic solvent was removedby the reduced rotary evaporation. Pure product, 2-bro-mo-5-(pyren-1-yl)thiophene (PyT-Br, yellow powder,2.34 g, yield = 92%), was obtained through the recrystalli-zation in hexane and ethanol. 1H NMR (500 MHz, CDCl3):d (ppm) 8.46 (d, 1 H), 8.20 (m, 3 H), 8.11 (m, 3 H), 8.04(t, 2 H), 7.20 (d, 1 H), 7.11 (d, 1 H). 13C NMR (125 MHz,CDCl3): d (ppm) 144.10, 131.31, 131.21, 130.77, 130.30,129.00, 128.57, 128.13, 127.97, 127.19, 126.17, 125.47,125.16, 124.88, 124.57, 124.50, 124.46, 112.46. EI-MS (m/z) [M]+ calcd for C20H12SBr, 363.98 found, 363.36. Anal.calcd for C20H12SBr: C, 66.13 H, 3.05 found: C 66.16 H, 3.06.

2.4.3. Synthesis of PyT-DPP-aIn a two-neck round bottom flask, PyT-Br (910 mg,

2.5 mmol), 2,5-diethylhexyl-3,6-dithienyl-DPP (EH-DPP-a)(525 mg, 1 mmol), Pd(OAc)2 (9 mg, 0.04 mmol), pivalicacid (0.11 mL, 1.0 mmol), K2CO3 (1.15 g, 9 mmol) andanhydrous dimethylacetamide (7.5 mL) were added. Themixture was stirred under argon atmosphere at 100 �Cfor 3 h. After cooled down to room temperature, the mix-ture was poured into methanol (100 mL). The precipitatewas collected by filtration, and it was successively rinsedwith water and methanol. The crude product was purifiedwith a silica gel column using chloroform. The pure prod-uct, PyT-DPP-a (black purple solid, 710 mg, yield = 65%),was separated with hexane/dichloromethane (v/v = 70/30) and recrystallized in hexane. Due to the strong inter-molecular interaction, proton NMR and carbon NMR werenot available at room temperature. MALDI-TOF MS (m/z)[M]+ calcd for C70H60N2O2S4, 1088.35 found, 1088.30. Anal.calcd for C70H60N2O2S4: C, 77.17 H, 5.55 N, 2.57 found: C,77.21 H, 5.53 N, 2.56.

2.4.4. Synthesis of PyT-DPP-bPyT-DPP-b was synthesized though a procedure similar

to PyT-DPP-a. PyT-Br (450 mg, 1.25 mmol) and EH-DPP-b

Fig. 1. 1H NMR spectra of the H atoms on the aromatic rings of the DPP-isomers.

B. Zhao et al. / Organic Electronics 13 (2012) 2516–2524 2519

(260 mg, 0.5 mmol) were used. PyT-DPP-b (black purplesolid, 262 mg, yield = 45%) was obtained. Due to the strongintermolecular interaction, proton NMR and carbon NMRwere not available at room temperature. MALDI-TOF MS(m/z) [M]+ calcd for C70H60N2O2S4, 1088.35 found,1088.65. Anal. calcd for C70H60N2O2S4: C, 77.17 H, 5.55 N,2.57 found: C, 77.23 H, 5.54 N, 2.54.

2.5. Characterization of materials

The 1H NMR and 13C NMR spectra were recorded onCDCl3 solutions of the samples using a Bruker DRX 500NMR spectrometer with tetramethylsilane (TMS) as thestandard. EI mass spectra were acquired with an Agilent5975C DIP/MS mass spectrometer. MALDI-TOF mass spec-tra (MS) were taken with a Bruker Autoflex instrumentusing anthracene-1,8,9-triol as matrix. UV–Vis absorptionand fluorescence spectra were collected with a ShimadzuUV-1700 spectrometer and a RF-5301 fluorometer, respec-tively. The cyclic voltammograms (CVs) were taken in0.1 M tetrabutylammonium hexafluorophosphate(Bu4NPF6) in dry dichloromethane with a CHI 620C electro-chemical analyzer. A gold disc with a diameter of 2 mm, aPt wire and an Ag/AgCl electrode were used as the workingelectrode, counter electrode and reference electrode,respectively. The scan rate was 50 mV s�1.

2.6. DFT simulations of DPP isomers

Simulations were carried out using the Gaussian 03 pro-gram. Details are provided in the Supporting information.The initial geometry optimization was performed withthe restricted DFT-B3LYP level of theory, the Handy andcoworkers’ long range corrected version of B3LYP 6-31G⁄,which can accurately calculate the structure and electronicproperties of organic molecules. No symmetry constraintswere imposed during the optimization process. The geom-etries of the molecules were further optimized with a tightthreshold that corresponds to the root mean square (rms)residual forces less than 10�5 au. Frequency calculationswere carried out to ensure that the geometries corre-sponded to minima instead of saddle points.

2.7. Fabrication and characterization of OSCs

Bulk-heterojunction OSCs were fabricated by solutionprocessing [27]. Patterned ITO glass substrates werecleaned sequentially with detergent, de-ionized water,acetone, and isopropanol. They were then treated withUV ozone for 15 min. A layer of poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) with athickness of 30 nm was formed ITO glass by spin coatinga PEDOT:PSS aqueous solution (Clevios P VP Al 4083 fromH.C. Starck) and baked at 120 �C for 60 min. The activelayer was prepared by spin coating a chloroform solutionof 4.5 mg mL�1 PyT-DPP-a or PyT-DPP-b and 15.7 mg mL�1

PC71BM at 2000 rpm. The organic films were annealed at90 �C for 10 min. The cathode of the OSCs was fabricatedby thermally depositing a 40 nm-thick calcium layer andsubsequently a 100 nm-thick Al layer in a vacuum of10�6 mbar. Each OSC had an area of 0.11 cm2.

The devices were encapsulated with epoxy and glasssheets for electrical tests in air. The photovoltaic perfor-mance of the devices was measured with a computer-pro-grammed Keithley 2400 source/meter and a Newport’sOriel class A solar simulator, which simulated theAM1.5G sunlight (100 mW cm�2) and was certified to theJIS C 8912 standard.

3. Results and discussion

3.1. Syntheses and characterizations of dialkyl DPP isomers

Scheme 1 described the synthesis route for the alkyl-ation of 3,6-dithienyl-DPP.[17,22–30] Three products wereobserved by the thin film chromatography (TLC) whenhexane/ethyl acetate (v/v = 97/3) was used as the eluent.Instead, only one tailed spot was observed when hexane/dichloromethane (v/v = 2/1), which was usually adoptedin literature [18,31], was used.

When 2-ethyl-hexyl-1-bromide (EHBr) was used for thealkylation of 3,6-dithienyl-DPP, three different 1H NMRspectra were obtained for the products (Fig. 1). The 1HNMR together with other experimental results suggestthree diethylhexyl-3,6-dithienyl-DPP isomers, whosechemical structures are shown in Scheme 1. The isomerwith three sets of 1H signals at 8.87, 7.61 and 7.27 ppmand another set of 1H signals at 4.04 ppm (Supportinginformation Fig. S1) is EH-DPP-a with the alkylation onthe two nitrogen atoms. The 1H NMR data are consistentto those reported in literature [18,24,26,31]. The formerthree sets of 1H signals arise from H atoms on the aromaticrings, and the latter set of 1H NMR signals is due to the Hatoms of the N–CH2– unit. Quite different 1H NMR signalswere observed for EH-DPP-b. There are six sets of 1H sig-nals at 8.45, 8.24, 7.68, 7.48, 7.24 and 7.19 ppm, arisingfrom the H atoms on the aromatic rings. They are due toasymmetric chemical structure of EH-DPP-b, because onealkylation takes place on a nitrogen atom while anotheralkylation on an oxygen atom of DPP. This chemical struc-ture is further confirmed by the 1H NMR signals at 4.58 and3.97 ppm (Supporting information Fig. S2), which arisefrom O–CH2– and N–CH2–, respectively. Only three setsof 1H NMR signals originating from the H atoms of the aro-

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1.0 EH-DPP-a EH-DPP-b EH-DPP-c

zed

abso

rptio

n

(a)

2520 B. Zhao et al. / Organic Electronics 13 (2012) 2516–2524

matic rings were observed for EH-DPP-c. But they appearat 8.03, 7.54 and 7.17 ppm, respectively, different fromthose of EH-DPP-a. The 1H NMR at 4.54 ppm is from theO–CH2– unit (Supporting information Fig. S3). These re-sults suggest that the third product has a chemical struc-ture of EH-DPP-c with the alkylation occuring on the twooxygen atoms.

The three isomers are confirmed by the 13C NMR spec-tra (Supporting information Figs. S4, S5 and S6). The peakat 161.72 ppm (N–C@O) in Fig. S4 and the peak169.13 ppm in Fig. S6 are the signals of the C atoms ofthe N–C@O unit of EH-DPP-a and ED-DPP-c, respetively.There are only 7 peaks for the C atoms of the aromatic ringsfor EH-DPP-a and EH-DPP-c because of the symmetry ofthe molecules. In contrast, there are 13 peaks for the Catoms of the aromatic rings of EH-DPP-b. More C signalsfor EH-DPP-b than EH-DPP-a and EH-DPP-c is attributedto the lower symmetry of EH-DPP-b. Moreover, the protonintegrations (Supporting information Figs. S1, S2 and S3)and EI-MS results evidence the different structures of thethree isomers.

The yields of the three isomers depend on the experi-mental conditions, particularly the reaction temperatureand reaction time. When the alkylation step was carriedout in N,N-dimethylformamide (DMF) with 5 equivalentof potassium carbonate and 2.5 equivalent of 2-ethyl-hex-yl-1-bromide at 100 �C for 6–8 h. The yields of EH-DPP-a,EH-DPP-b and EH-DPP-c were 36%, 28% and 11%, respec-tively. Instead, when the reaction took place at 140 �C for16 h, EH-DPP-a became the dominant product with a yieldof 30%.

The yields of the three isomers change when a differentalkylbromide is used for the alkylation. When 1-octyl bro-mide was used, the yields of Oct-DPP-a and Oct-DPP-bwere 63% and 17%, respectively, while only trace amountof Oct-DPP-c was obtained. (The 1H and 13C NMR spectra

Scheme 3. Mechanism for the dialkylation of DPP.

of Oct-DPP-a and Oct-DPP-b are shown in Supportinginformation Figs. S7–S10).

Based on the experimental results, we propose the fol-lowing reaction mechanisms for the formation of the threeisomers (Scheme 3). Alkaline can deprotonate a nitrogenatom of 3,6-dithieyl-DPP, 1, into the structure 2. The struc-ture 2 can change into its resonant structure 2-2 with thenegative charge on an oxygen atom. The alkylation as a re-sult of the nucleophilic addition can take place either onthe nitrogen atom of the structure 2 or on the oxygen atomof the structure 2-2. The former gives rise to the formationof the intermediate 3, while the latter leads to the interme-diate 4. The further alkylations on the intermediates 3 and4 produce the three isomer. Since 2-ethylhexyl is morebulky than the linear octyl chain and the steric hindranceto the alkylation on the oxygen atoms is lower than onthe nitrogen atoms, more alkylation takes place on theoxygen atoms of DPP for the 2-ethylhexyl group than forthe 1-octyl group.

3.2. Electronic structure and optical properties of dialkyl DPPisomers

The isomers with the alkyl chains on different sites havedifferent electronic structures and optical properties.Fig. 2a presents the UV–Vis absorption spectra of the three

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550 600 650 700 750 800 850

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EH-DPP-a EH-DPP-b EH-DPP-c

(b)

Fig. 2. (a) UV–Vis absorption spectra and (b) photoluminescence spectraof EH-DPP-a, EH-DPP-b and EH-DPP-c solutions in chloroform.

Table 1Summary of photophysical, electrochemical and electronic parameters of EH-DPP-a, EH-DPP-b, and EH-DPP-c.

Molecule kabs (nm) emax (M�1 cm�1) kem (nm) Eox (V)a Ered (V)a HOMOb (eV) LUMOb (eV) HOMOc (eV) LUMOc (eV) Ebg (eV) Eop

g (eV)

EH-DPP-a 332, 515, 550 78890 579 0.92 �1.14 �5.29 �3.23 �5.15 �2.74 2.06 2.14EH-DPP-b 361, 528 81035 582 0.91 �0.95 �5.28 �3.42 �5.20 �2.86 1.86 2.03EH-DPP-c 374, 502 113225 608 0.89 �0.80 �5.26 �3.57 �5.28 �2.93 1.69 2.02

a Enox and En

red are the onset potentials vs Ag/AgCl.b HOMO and LUMO energy levels were calculated in terms of the oxidation and reduction onset potentials according to the equations HOMO = -�(4.37 + Eonset

ox ) eV and LUMO = �(4.37 + Eonsetred eV.

c HOMO and LUMO energy levels calculated with the Gaussian 03 program with DFT-B3LYP-6-31G⁄.

Scheme 4. Model molecules for three DPP-isomers.

B. Zhao et al. / Organic Electronics 13 (2012) 2516–2524 2521

isomers, EH-DPP-a, EH-DPP-b and EH-DPP-c, in chloro-form. The photophysical data are summarized in Table 1.All the three isomers have absorption bands in the rangesof 280–400 nm and 400–650 nm. In the range of 280–400 nm, the absorption peak appears at 340 nm for EH-DPP-a, whereas it shifts to 370 nm for EH-DPP-c. Theabsorption bands in the range of 400–650 nm arise fromthe DPP chromophore unit [18]. There are two absorptionpeaks at 515 nm and 550 nm for EH-DPP-a, a broad bandwith the peak at 528 nm for EH-DPP-b, and a broad bandwith the peak at 502 nm and two shoulder at 550 nmand 572 nm for EH-DPP-c. The photoluminescent spectraof the three isomers are remarkably different as well(Fig. 2b). There is an intense photoluminescent band atabout 550 nm with a shoulder in the long wavelength forEH-DPP-a, while two strong photoluminescent bands ap-pear for EH-DPP-b and EH-DPP-c.

The electrochemical behaviors of the compounds inanhydrous dichloromethane were examined by CV(Fig. 3). The CVs indicate that the energy gaps of EH-DPP-a, EH-DPPP-b and EH-DPP-c are 2.06, 1.86 and 1.69 eV,respectively (Table 1). The lowest unoccupied molecularorbital (LUMO) and the highest occupied molecular orbital(HOMO) were estimated in terms of the onsets of thereduction and the oxidation [38]. The HOMO/LUMO levelsof the three isomers are �5.29/�3.38, �5.28/�3.42, and�5.26/�3.60 eV for EH-DPP-a, EH-DPP-b, and EH-DPP-c,respectively.

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

EH-DPP-a

EH-DPP-b

EH-DPP-c

Potential vs Ag/AgCl (V)

Fig. 3. CVs of EH-DPP-a, EH-DPP-b and EH-DPP-c in 0.1 M Bu4NPF6

dichloromethane.

DFT simulations were carried out on three model mole-cules, diethyl-3,6-dithienyl-DPP, as shown in Scheme 4.Short side chains were used, because long side chains leadto the non convergence of the simulations and the replace-ment of long alkyl chains with short alkyl chains does notaffect the optimized structures of the molecules. TheHOMO/LUMO levels of the three model molecules by theDFT simulation are presented in Fig. 4. Their HOMO levelsare similar but the LUMO levels increases from DPP-a, DPP-b to DPP-c (Table 1). This can be understood in terms of thedistributions of the electrons for these molecular orbitals.The electron distribution for HOMO is similar for the threeisomers, while it becomes significantly different for LUMO.As shown in Fig. 4, the alkylation on different atoms doesnot remarkably affect the electron distrubition for theHOMOs of the three isomers, while it has a significant ef-fect on the electron distribution for the LUMOs. The differ-ent LUMOs of these isomers may be due to the fact that thealkylation on the oxygen atom(s) can weaken the elec-tronic coupling between the oxygen atom(s) and the azo-ring.

3.3. OSCs with dialky DPP-based molecules as donor materials

The units of dialkyl DPP isomers can be used as elec-tron-deficient units for organic and polymer semiconduc-tors [20–34,39,40]. For example, Lee et al. synthesizedN,N-dialkyl DPP based small molecules with low bandgap and used them as the donor materials for OSCs [40].The PCE of OSCs can be as high as 4.1%. PyT-DPP-a andPyT-DPP-b with two pyrene units as the two ends andtwo bisthiophene units as the linkers were synthesizedand used as the donor materials for OSCs (Scheme 2).PyT-DPP-a and PyT-DPP-b have good solubility in commonsolvents and excellent thermal stability (Fig. 5).

PyT-DPP-a and PyT-DPP-b have similar absorptionspectra. There are two major absorption bands for theirsolutions and films (Fig. 6). The absorption band at around

Fig. 4. HOMO and LUMO of three diethyl-3,6-dithienyl-DPP isomers by the DFT simulations. (a) HOMO and (b) LUMO of DPP-a, (c) HOMO and (d) LUMO ofDPP-b, and (e) HOMO and (f) LUMO of DPP-c.

100 200 300 400 50050

60

70

80

90

100

Wei

ght (

%)

Temperture (oC)

PyT-DPP-a PyT-DPP-b

Fig. 5. TGA curves of PyT-DPP-a and PyT-DPP-b.

2522 B. Zhao et al. / Organic Electronics 13 (2012) 2516–2524

380 nm can be assigned to the p–p⁄ transition of the pyr-ene unit, while another absorption band at 580–800 nmis due to the p–p⁄ transition of the entire conjugated back-bone. Remarkable differences can also be observed on theabsorption spectra of the two molecules. The absorptionband at the short wavelength range is stronger than thatat the long wavelength for PyT-DPP-b, while it is the otherway around for PyT-DPP-a. In addition, there is an absorp-tion shoulder in the absorption spectra of the PyT-DPP-asolution and film, while it is absent for the PyT-DPP-b solu-tion and film. The optical band gap of PyT-DPP-b deter-mined from the absorption onset of the UV–Vis spectraof the solid film is 1.37 eV, slightly smaller than that(1.4 eV) of PyT-DPP-a.

The HOMO/LUMO levels, the band gap and the thermalstability of these two small molecules suggest that theycan be used as the donor materials of OSCs. OSCs were fab-ricated with PyT-DPP-a or PyT-DPP-b as donor and PC71BMas acceptor, and the experimental conditions, including the

300 400 500 600 700 800 900

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

abs

orba

nce

Wavelength (nm)

solution film

PyT-DPP-a

(a)

300 400 500 600 700 800 9000.0

0.2

0.4

0.6

0.8

1.0

1.2

Nom

aliz

ed a

bsor

banc

e

Wavelength (nm)

Solution FilmPyT-DPP-b

(b)

Fig. 6. UV–Vis absorbance spectra of PyT-DPP-a and PyT-DPP-b solutionsin chloroform (black) and films (red). (For interpretation of the referencesto colour in this figure legend, the reader is referred to the web version ofthis article.)

-0.2 0.0 0.2 0.4 0.6 0.8

-4

-3

-2

-1

0

1

2

Cur

rent

den

sity

(m

A c

m-2)

Voltage (V)

PyT-DPP-a:PC71BM PyT-DPP-b:PC71BM

Fig. 7. Current density–voltage curves of PyT-DPP-a:PC71BM and PyT-DPP-b:PC71BM OSCs under AM1.5G illumination (100 mW cm�2).

Table 2Photovoltaic performances of OSCs with PyT-DPP-a and PyT-DPP-b as thedonor materials and PC71BM as the acceptor. The weight ratio of donor toacceptor was 1:3.5.

Donor Voc (V) Jsc (mA cm�2) FF PCE (%) Film thickness (nm)

PyT-DPP-a 0.76 3.19 0.28 0.69 95PyT-DPP-b 0.80 2.98 0.27 0.65 80

B. Zhao et al. / Organic Electronics 13 (2012) 2516–2524 2523

weight ratio of the donor to acceptor and the annealingtemperature were optimized (Supporting informationFigs. S17–S20 and Tables S1–S4). The density (J)–voltage

(V) curves of the optimal PyT-DPP-a:PC71BM and PyT-DPP-b:PC71BM OSCs are presented in Fig. 7. The photovol-taic performances of the two OSCs, including short-circuitcurrent (Jsc), Voc, fill factor (FF) and PCE, are summarizedin Table 2. The two devices have different Voc values. Voc

is 0.76 V for the PyT-DPP-a:PC71BM OSC, whereas it is0.80 V for the PyT-DPP-b:PC71BM OSC. This difference isascribed to the deeper HOMO level of PyT-DPP-b thanPyT-DPP-a.

4. Conclusions

Alkylation of DPP can occur on both N and O atoms, sothat three dialkyl DPP isomers can be produced. The threeisomers have different electronic structures and opticalproperties as revealed by various characterizations. Conju-gate molecules based on dialkyl DPP isomers, PyT-DPP-aand PyT-DPP-b, were synthesized. They have low bandgaps and are investigated as the donor materials of OSCs.The photovoltaic performances of the OSCs with PyT-DPP-a and PyT-DPP-b as the donor materials are consistentwith their chemical and electronic structures.

Acknowledgements

This research work was financially supported by a re-search grant from the Agency for Science, Technologyand Research, Singapore.

Supporting Information. Syntheses, 1H NMR, 13C NMRspectra and mass spectra of chemicals and performanceof OSCs. These materials are available free of charge inthe online version.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.orgel.2012.07.015.

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